Synthetisation and screen printing of NiMn2O4 NTC ...

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Screen printing of coprecipitated NiMn2O4 for production of NTC thermistors

Rainer Schmidt

Supervisors Dipl.Ing. A. Stiegelschmitt Prof.Dr. A. Roosen Dr. A.W. Brinkman Prof.Dr. G. Döhler

Friedrich-Alexander Universität Erlangen-Nürnberg Institut für Werkstoffwissenschaften III Glas und Keramik Institut für technische Physik 2001

Abstract NiMn2O4 is a compound with a typical spinel structure and exhibits a logarithmic dependence of resistance upon temperature, where the resistance decreases with increasing temperature. The electrical conduction relies on a hopping mechanism where the electrons transfer between manganese cations of different valence states. NiMn 2O4 powder was produced by thermal decomposition of co-precipitated nickel and manganese oxalate. A suitable screen printing paste was prepared from the powder and the printing of ceramic thick films of NiMn2O4 via screen printing was studied. The source powder and the sintered films were characterised using X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), Inductive Coupled Plasma spectroscopy (ICP), thermal gravimetric analysis (TGA), differential thermal analysis (DTA), surface imaging and granulometric analysis. The resistance versus temperature characteristic of the films was measured over a wide range of temperatures (150 K – 600 K) and the conduction mechanism was found to be consistent with a variable-range hopping model.

1

Contents 1. Introduction

4

2. Scientific background 2.1. Properties of NiMn2O4 2.1.1. Crystal structure 2.1.2. Phase stability 2.1.3. Synthesis of NiMn2O4

5 9 10

2.2. Electrical conductivity 2.2.1. 2.2.2. 2.2.3. 2.2.4.

Hopping transport in NiMn2O4 Hopping transport in semiconductors Resistance vs. temperature dependence Ageing effects

11 12 17 18

2.3. Production of NiMn2O4 films

19

2.4. Screen printing

20

2.4.1. The screen 2.4.2. Rheology of screen-printing pastes 2.4.3. “Snap off“ distance 2.5. Electrical measurements

21 23 24 27

3. Experimental procedure

29

3.1 Synthesis of NiMn2O4 source powder 3.1.1. Production of stock solutions 3.1.2. Concentration of the stock solutions 3.1.2.1.Nickel titration 3.1.2.2.Oxalate titration 3.1.2.3.Manganese solution 3.1.3. Precipitation of NiMn2O4 mixed oxalate 3.1.4. Characterisation and optimisation of NiMn2O4 powder

30 31 32 32 33 33 34 34

3.2 Paste production and characterisation

37

3.3 The screen printing procedure

39

3.4 Substrates

43

3.5 Heat treatment of screen printed films

43

3.6 Film profile

43

3.7 Electrical characterisation

46

3.7.1. Contacts

46 2

3.7.2. Two-point measurements 3.7.3. Resistance vs. temperature measurements 4.

46 47

Results 4.1.Phase purity and stoichiometry of synthesised NiMn2O4 source powder

50

4.2.Optimisation of the grain size in synthesised NiMn2O4 source powder

51

4.3.Viscosity of the screen printing paste

54

4.4.Conditions for a regular screen printing procedure

56

4.5.Optimisation of the heat treatment of screen printed films

56

4.6.Film profile

58

4.7.Microstructure of screen printed NiMn2O4 films

60

4.8.Resistance vs. temperature dependence 4.8.1. 4.8.2. 4.8.3. 4.8.4.

Contacts Resistance vs. Temperature Deviations from the regular NTC characteristic Effect of the glass phase

5. Discussion

66 67 73 75 76

5.1.Synthesis of NiMn2O4 source powder

76

5.2.Screen printing and thermal treatment

77

5.3.Electrical measurements

78

5.4.Future work

79

6. Conclusion

80

7. References/ Acknowledgement / Declaration

81

8. Appendices

85

8.1.Grain size distributions

86

8.2.X-Ray spectra

91

8.3.Thermal analysis (DTA)

99 3

1. Introduction The aim of this study was to produce a reliable thick film temperature-sensing device based on NiMn2O4 spinel type structure NTC (Negative temperature coefficient) thermistor material. The basic properties of this compound were matter of intense research during the 50ties and 60ties and are well known now. Thin or thick films of NiMn2O4 became interesting during the last 20 years with the fast development of thin and thick film deposition techniques. NiMn2O4 exhibits a logarithmic dependence of resistance upon temperature, which makes it well suited for use in temperature sensing applications such as heaters, boilers, ovens and coolants. The electrical conduction is based on a hopping mechanism where electron transfer takes place between manganese atoms of different valences on neighbouring lattice sites. NiMn2O4 has been widely used in the industry as a temperature sensor in bulk material, but problems with poor stability and reproducibility of the sensor due to high porosity and incomplete intergranular contact occur. In principle, these problems can be minimised in films. In an earlier communication [26] it has been shown that highly controlled thin films of NiMn2O4 can be grown via Electron-Beam-Evaporation. However, NiMn2O4 is a complex compound and the vapour deposition of thin layers without loss of stoichiometry and changes in composition was proved to be difficult. NiMn2O4 dissociates during evaporation and recombines on the substrate. Both processes have not been investigated yet for NiMn2O4 and can be thus not controlled easily. Consequently screen-printing procedures were investigated in this study as an alternative and direct printing technique. NiMn 2O4 powder was produced by the thermal decomposition of co-precipitated nickel manganese oxalate. The particle size distribution of the powder was optimised and then mixed with, in sequence, dispersing agent, glass binder and a so-called “vehicle” containing organic solvents and binders in order to produce a suitable and printable paste. Following, the paste was optimised in terms of dispersion of the powder and viscosity. The screen printing parameters, which in the main are mesh size and design of the screen, snap off distance of the screen and speed of the printing stroke were adjusted, so that dense and even films could be printed onto alumina thick film substrates. Resistance vs. temperature characteristics were measured over a wide range of temperature and the experimental data was compared to theoretical models proposed for variable range hopping. 4

2. Scientific Background 2.1. Properties of NiMn2O4 2.1.1. Crystal structure NiMn2O4 crystallises in a typical cubic spinel structure. In general, spinels can be represented by the expression A(B2)O4 where A is a divalent and B a trivalent cation. The most common spinel is MgAl2O4. In a spinel structure the oxygen atoms form a f.c.c. sublattice in a cubic-closed packed formation. The metal cations are situated at two different types of interstices between the cubic-closed packed oxygen ions, tetrahedral (A) and octahedral (B) sites as can be seen from Fig.1. To the tetrahedral and the octahedral sites it is referred to as the A and B sublattice respectively.

Fig.1 Spinel structure showing the octahedral and tetrahedral interstices (Basu [6])

The cubic unit cell of NiMn2O4 contains 64 tetrahedral and 32 octahedral interstices whereby 8 tetrahedral and 16 octahedral sites are occupied, thus the occupancy is 1/8 for tetrahedral and 1/2 for octahedral interstices. Including 32 oxygen atoms, 16 manganese and 8 nickel cations, the unit cell comprises 56 atoms. As can be seen from Fig.2, 8

5

different layers perpendicular to the (111) direction of the oxygen sublattice can represent the unit cell.

1st Layer

5th Layer

Tetrahedral cation

2nd Layer

3rd Layer

4th Layer

6th Layer

7th Layer

8th Layer

Octahedral cation

Oxygen

Fig.2 Eight layers of the unit cell (Basu [6])

In a regular spinel the divalent cations are situated at the A-sites and the trivalent ones at the B-sites. Contrarily, in a complete inverse spinel structure all divalent cations are present on the B-sites and the trivalent cations on the A- and B-sites by a ratio of 1/2 at each site. NiMn2O4 is an intermediate type and can be represented by the following expression where the brackets [ ] indicate octahedral sites:

Ni1-xMnx[NixMn2-x]O4

(1)

In this expression x is the inversion parameter, which was found to be dependent on the sintering temperature. For temperatures above 200 0C Macklen [1] found: x = 1 – 0.0005(T – 200 0C) ;

(2)

6

where T is the equilibrium temperature in 0C. At this point it has to be stated that by quench cooling of the material from higher temperatures it is possible to “freeze” the cation distribution of higher temperatures and examine different configurations at room temperature [Macklen, 1]. Brabers [2] found from quench cooled samples that cation migration starts to take place by heating to temperatures over 450 0C changing the degree of inversion. As NiMn2O4 is of intermediate type of spinel the manganese cations do not only occur in a trivalent state. Part of the trivalent Mn3+ disproportionates to Mn2+ and Mn4+ leading to the following cation distribution given by Macklen [1] :

Ni12 x Mnx2 ( Ni x2 Mn23

2x

Mnx4 ) O4

(3)

As can be seen from (3) the cation distribution depends on the inversion parameter x and thus from the equilibrium temperature (2). The migration of cations leads to a change of the inversion parameter what can be described by the following expression given by Brabers [2]:

1Ni A2

2MnB3

1Ni B2

1MnA2

1MnB4

E

(4)

The subscripts A and B indicate the occupancy of either A or B sites. The equilibrium constant K for this reaction was found by applying the law of mass action to equation (4) whereby the concentrations of the cations are determined by the subscripts in (3):

K

x3 4(1 x)

3

exp

E kB T

;

(5)

The energy E was found to be about 0.66 eV (Brabers, [2]).

Boucher et al.[2] reported, using Neutron-Diffraction data, that with varying inversion parameter the lattice parameter a is changing, too : 7

a = 8.441 – 0.057 x ;

(6)

As the inversion parameter is temperature dependent this relation modifies the regular thermal expansion behaviour, which was shown to be linear below 450 0C (Brabers, [2]). Above those temperatures, according to Brabers, cation migration starts to take place and therefore a change in the degree of inversion and a deviation from the regular thermal expansion behaviour can be observed. However, at certain temperatures the content of oxygen in NiMn2O4 varies and thus the cation distribution can differ from (3). Macklen [1] found that at atmospheric pressures at temperatures above 900 0C a loss of oxygen g occurs: NiMn2O4

- g.

At lower oxygen

pressures this loss occurs already at lower temperatures. For the reason of charge neutrality a loss of 1 mole oxygen causes the reduction of two moles of tetravalent manganese cations to trivalent ones. Thus the cation distribution is then :

Ni12 x Mnx2 ( Ni x2 Mn23

2x 2g

Mnx4

2g

) O4

(7)

g

Feltz and Töpfer [3] reported about the occurrence of cation defects in NiMn 2O4 at certain circumstances at 350 0C which means that an excess of oxygen d occurs (NiMn2O4 + d) and the cation distribution becomes :

Ni12 x Mnx2 ( Ni x2 Mn(32

2 x) 2d

Mnx4

2d

) O4

d

(8)

Both, an excess and a loss of oxygen in NiMn2O4 are possible and both results in a change of the cation distribution. Therefore knowledge about the oxygen content is important for interpreting the electrical conduction, which depends on the cation distribution. The oxygen content can be determined via the average state of oxidation of the manganese atoms. This can be achieved via titration methods, but as three different valencies of manganese occur in NiMn2O4 those titrations are rather troublesome.

8

2.1.2. Phase stability A phase diagram for the Ni-Mn-O system for temperatures between 500 0C and 1200 0C and for different ratios of manganese and nickel going from NiO to Mn2O3/Mn3O4 was given by Wickham [4]. NiMn2O4 Spinel : Ni 3-y Mn y O4 Rock-salt phase : Ni 1-S Mn S O

Ni1-S MnS O

Tetragonal spinel

Ni 1-x Mn 2+x O4

Ni x Mn 3-x O4

Cubic and Tetragonal spinel

Temp. in

NiO / Ni6 Mn O8

Ni6 MnO8 / NiMnO3 NiMnO 3 / Mn2O3 Ni 2-p Mn p O3 Ni 3-y Mn y O4

Fig.3 Phase diagram for the system NiO-Mn2O3-O2 (Wickham [4]) According to this phase diagram the cubic NiMn2O4 spinel is stable only between 750 0C and 900 0C. It decomposes at intermediate temperatures (< 750 0C) to NiMnO3 and Mn2O3. At higher temperatures (> 900 0C) additionally to NiMn2O4, NiO and Mn3O4 are present. However, the composition of NiMn2O4 at 750 0C to 900 0C can be “frozen” by quench cooling to room temperatures and the spinel does not decompose below 150 0C. The decomposition starting at 150 0C is thought to be very slow and does not become significant up to 300 0C or 350 0C. Wickham [4] stated additionally that for higher manganese contents in NiyMn3-yO4 the cubic and additionally a tetragonal spinel phase with the structure of Mn3O4, occur. The temperature for this phase transition is highly dependent on the manganese content. 9

2.1.3. Synthesis of NiMn2O4 As NiMn2O4 is not a common compound and not available on the free market it has to be synthesised in a laboratory process. NiMn2O4 can be prepared in different ways where the most common production processes are: 1.)NiO and Mn2O3 are mixed (1:1 molar ratio) and fired at 1150 0C. The powder is then tempered at 800 0C for up to 60 hours in order to reincorporate released NiO into the crystal (see also 2.1.2) [Sinha et.al.,5], [Basu, 6]. 2.) NiO and MnO 2 are mixed in a molar ratio of 1:2. After a calcination process at 1300 0C for 3 hours the material is tempered at 800 0C for 24 hours. [Kamiyama, Nara, 7] 3.) NiMn2O4 can be obtained by calcining a mixture of NiSO 4 and MnSO 4 [Gorgeu,8]. Whereas the processes 1,2 and 3 are based on the mixture of precursor powders, other methods are based on wet chemical processes. Nickel and manganese salts are dissolved in water and precipitated. 4.) To a solution of Mn 2 (aq) and Ni 2 (aq) NaOH or NH4Cl is added. At a pH value of 10, both, Ni(OH) 2 and Mn(OH) 2 coprecipitate. A problem with this method is that Na or Cl cause both serious impurities in the hydroxides which have to be removed in a tedious washing process. After co-precipitation, the hydroxides are thermally decomposed and after calcining at 950 0C pure NiMn2O4 can be obtained. More details about this method of coprecipitation are given by Beckham [9]. 5.) To a Ni2+ and Mn2+ containing aqueous solution oxalate ions are added and nickel manganese oxalate precipitates. By thermal decomposition of the co-precipitated powder at about 3500C and sintering at 850 0C phase pure NiMn2O4 can be gained [Feltz et.al.,3]. This route was chosen for this project and is described in more detail in 3.1.

10

2.2. Electrical conduction 2.2.1. Hopping transport in NiMn2O4 In NiMn2O4 the electrical conduction is based on a hopping mechanism where electrons transfer between manganese cations of different valence states according to the mechanism : Mn3+

Mn4+ + e-

(9)

This electron transfer is a thermally activated hopping: the electrons require an activation energy E to hop to a neighbouring lattice site. Macklen [10] found that only trivalent and tetravalent manganese cations on octahedral sites contribute to the conductivity. According to Macklen this electron hopping can only take place between atoms of the same sort, in this case between manganese atoms on octahedral sites and the nickel atoms do not contribute. For hops on tetrahedral interstices the distance of the cations is too large and the occupancy too low. The probability of an electron jump is negligibly low. Therefore the conductivity would be dependent on the Mn3+/Mn4+ ratio on octahedral sites and maximum conductivity would occur if the ratio is 1:1 as the number of hopping paths available would be maximal. Several authors [11,12,13] argued that, as the Mn3+/Mn4+ ratio depends on the degree of inversion, maximum conductivity would occur for x = 2/3. As mentioned before, the degree of inversion is dependent on the sintering temperature, so by sintering at different temperatures and quench cooling the cation distribution and following the conductivity could be adjusted. However, none of the authors took into account the possibility that jumps between divalent and trivalent manganese cations between tetrahedral and octahedral interstices (see (3)) could take place. As can be seen from Fig.2, Mn2+ and Mn3+ cations on A and B sites are only slightly further distanced to each other than octahedral cations what could result in a high hopping probability, especially as electrons jumping from Mn2+ might not been bonded as strongly as electrons from Mn3+. On the other hand the lower occupancy of the

11

tetrahedral sites might lower the hopping probability. This will be discussed in more detail later. However, it is clear that the conduction mechanism strongly depends on the cation distribution. This was confirmed by Macklen [1] who found that with decreasing oxygen content and thus with decreasing Mn4+ concentration the conductivity increases. For a conduction mechanism based only on electron exchange between Mn3+ and Mn4+, one would expect that for x = 2/3 maximum conductivity would be observable. This was not observed by Macklen [1], which would support the proposal that the conduction relies not only on a simple hopping between Mn3+ and Mn4+ cations. Furthermore it is important to consider that the hopping between Mn3+ and Mn4+ cations involves a hop of a lattice distortion, which is caused by Mn3+ cations due to the JahnTeller effect [Tuller, 14]. Those lattice distortions are called polarons and such a polaron hopping is thought to be phonon assisted.

2.2.2. Hopping transport in semiconductors In general, NiMn2O4 shows semiconducting properties such as a negative temperature coefficient with a logarithmic dependence of resistance upon temperature. A thermal energy is required to activate an electron hop between equivalent lattice sites equivalent to the activation of an electron from a valence to a conduction band for regular semiconductors. NiMn2O4 can be regarded as a quasi-semiconductor where conduction takes place via a hopping mechanism between localised electron states. The hopping probability between two localised electron states i and j is not only dependent on the difference in energy

ij

of the localised electron states, but also on their spatial separation

rij. For this kind of network of localised electron states Abram and Miller developed a model which describes the localised electron states and their capability to transfer electrons between them as a random network of resistors (Abram-Miller network) [44]. The resistance of one of those network resistors describes the hopping rate, which is given by the expression

12

ij

ij

exp

2rij a0

ij

k BT

;

(10)

is the hopping rate and a0 is the effective Bohr’s radius of the localised electron states.

For a hopping mechanism to occur throughout the whole sample it is required that sufficiently high and it is clear that only hops between sites with high

ij

ij

is

contribute to the

conduction in the sample. For most thermally activated conduction mechanisms the second term in equation (10) becomes predominant at higher temperatures and the hopping rate is high only if electron states with a small separation rij contribute. This is the case for a “Nearest-Neighbour Hopping” (NNH) where rij is constant (nearest neighbour) and the hopping probability is dominated by the second term in equation (10) with a constant activation energy. The electron hopping then shows the typical Arrhenius behaviour of a thermally activated process :

exp-(E/kBT) ;

(11)

is the conductivity, E the average activation energy for an electron hop, kB Boltzman’s constant and T the temperature. However, if the terms in (10) become comparable, an expression for the hopping probability has to be derived from (10). In this approach not only nearest neighbours of manganese atoms contribute to the conduction. The hopping of an electron to a more distant lattice site can be favourable if the difference in energy of the electron states is sufficiently low. To this mechanism it is referred to as “Variable-Range Hopping” (VRH). For deriving an expression for variable range hopping conductivity Mott considered that only electron states close to the Fermi level

13

contribute.

Fig.4 Electron states around the Fermi level at temperature T = 0, [44]

In Fig.4 the electron states which contribute to the hopping mechanism in the range of are pointed out. In this model the density of states g( constant with energy (→ g( concentration N(

was expected to be

and can be thus expressed in terms of the charge carrier

) (i.e. concentration of localised electron states which contribute to the

hopping mechanism) as follows:

N(

) = 2 g( )

;

(12)

In expression (10) the spatial separation of the contributing localised electron states rij can be substituted with the charge carrier density according to rij = N an average separation

0

1

exp

1

N 3 ( 0 )a0

of the electron states, for the conductivity

0

kT

0

1

exp g( )

0 1

0

3

a0

kT

1/3

(

). Assuming

one can obtain:

;

(13)

As a second step (12) has been substituted in the first part of (13). Numerical factors have been neglected. In the second part of (13) the expression in brackets has a dependence of

in the

denumerator in the first term and in the numerator in the second term. In order to get maximum conductivity, an

can be found by minimising the expression in brackets giving

as follows: 14

(kT )

0 (T )

3

g ( )a0

4

3

1

;

(14)

4

It can be seen that

changes with temperature. Variable range hopping can thus be

regarded as a hopping mechanism with changing activation energy. Substituting (14) back into the second part of (13) one can obtain the well-known Mott law for variable range hopping: exp-(T0/T)1/4 ;

(15a)

The pre-exponential factor was found to be dependent on temperature

[44].

In this model it is assumed that the density of states is constant as expected at the edge of a conduction band. T0 is given by

T0

k B g ( ) a0

The factor

3

;

(15b)

takes into account numerical factors, which have to be determined by

solving the appropriate percolation problem, which fits the conditions in the material of concern. By Monte Carlo simulations

has been determined to be

However, the assumption that the density of states g(

is constant with energy is not a

good approximation for a wide range of materials. This approximation does not hold for NiMn2O4 either. A more general expression for g(

was given by Shklovskii and Efros [44]:

n

g( )

g'

'

;

(16)

15

In this case the density of states vanishes at the Fermi level due to Coulomb interactions (Coulomb gap). Equation (12) has to be replaced as follows :

N( 0)

n

''

0

g'

'

0

n 1

d ''

2 g' 0 ; n n 1 '

(17)

From this point following the same derivation as for the constant density of states one can arrive easily at a similar expression like (15), but the exponential factor p is now connected to the parameter n from equation (16) : p = (n + 1) / (n + 4). For a constant density of states n = 0 is valid and one arrives readily at the exponent ¼. For most crystallic semiconductors the coulomb gap in the density of states is of parabolic shape [44]. The coulomb gap emerges due to non-negligible correlation effects between electrons when the model of single electron states does not hold anymore for electrons close to the Fermi level. The conduction is then given by exp-(T0/T)1/2 ;

(18a)

where the pre-exponential factor was found to be dependent on temperature T and T0 is given by

T0

e2 ; k B a0 1

where

(18b)

describes the strength of the coulomb interaction between the electrons. The

interaction energy

D

at the average distance rD between electrons on localised states is

given by

D

e2 ; rD

(18c)

16

From this it follows that

=4

effective Bohr’s radius and

r

*

r 0

0

and the expression

r *

a0 can be regarded as an

is the microscopic dielectric constant. The microscopic

dielectric constant does not necessarily correspond to the macroscopic value for the bulk material. The numerical factor

1

was found by Monte Carlo simulations to be 2.8. However, in

this project it is suggested that the electrical conduction in NiMn2O4 can be described by (18a) satisfactory.

2.2.3. Resistance vs. temperature dependence In order to fit experimental data for the resistance vs. temperature characteristics of NiMn2O4 several attempts have been made by different other authors to deduce the appropriate expression theoretically. However, theoretical approaches could not fit the experimental data and empirical relations fitted to the experimental findings could not be explained by theoretical models. The main empirical relations assuming an Arrhenius’ thermally activated hopping and fitting the data by introducing empirical correction parameters are : Becker et al. [18]:

0T

Bossom et al. [19]:

0

Feltz et al. [20]:

– 4.83

exp(E/kBT);

E = 0.178 eV

(19)

exp{E/kB(T + )};

E = 0.193 eV;

47.79 K; (20)

0

T1 T

2.91

exp E

1 T

1 T1

; T1 is a fixed temperature (21) dependent on the measurement

As mentioned above (2.2.1.) the hopping transport in NiMn2O4 is supposed to be a phonon assisted polaron hopping. Fritsch et al. [21] suggested that the resistivity can be described by small polaron hopping theory.

0

T

1 E exp NC k BT

(22) 17

with kd 2 0

N oct e 2

(23) 0

where Noct is the concentration of charge carriers on octahedral sites, d the jump distance, 0

the lattice vibrational frequency, N the total number of charge carriers per volume, and

C the probability to find one donor and one acceptor on two neighbouring octahedral sites. NC is the number of charge carriers participating on the hopping mechanism and was suggested to be (Fritsch[21]) :

C

2

[ Mn B3 ][ Mn B4 ] {[ Mn B3 ] [ Mn B4 ]}2

NC

2

[ Mn B3 ][ Mn B4 ] [ Mn B3 ] [ Mn B4 ]

(24)

where the brackets [ ] indicate the concentration of the cations on B sites. A full derivation of the small polaron hopping theory is given by Tuller [14]. However, this model is lacking on experimental verification as the observed resistance vs. temperature characteristics are deviating from the behaviour predicted by (22). A much better agreement of theory and experiment was obtained by Baliga [22] who suggested that the conduction mechanism relies on a variable-range hopping like described above (2.2.2.) for all ranges of temperature and can be explained by expression (18a) which was confirmed by Mansfield [17].

2.2.4. Ageing effects As mentioned in 2.1.1. cation migration takes place at higher temperatures leading to changes in the cation distribution and thus to an altering conductivity. This feature is not desired for temperature sensing applications of NiMn2O4. Rousset et al. [23] observed a slow drift in resistance of NiMn2O4 samples during several days which can be minimised by doping with 1.5% wt. barium. Castelan et al. [24] found by thermo power measurements that for barium doped samples (3.6% wt.) the ageing effects decrease significantly after annealing for 120 hours at 125 0C. However, in order to produce a reliable and stable temperature-sensing device the ageing effects are of severe concern and have to be minimised. 18

2.3. Production of NiMn2O4 films In order to obtain a reliable temperature-sensing device several attempts were made to produce NiMn2O4 in bulk material. Parlak [25] described the production of pellets which were produced by pressing the precursor oxides NiO and Mn2O3 in disc shape and firing the discs at 1200 0C. But despite optimisation of the powder and the sintering process, severe problems with those pellets and with bulk material in general occurred because of poor stability and reproducibility of the sensor due to high porosity and incomplete intergranular contact in the material. In addition, the occurring pores make the device sensitive to effects of the surrounding environment as changing humidity affects the resistivity or varying partial oxygen pressure changes the oxygen content of the sensor material. In principle, these problems can be minimised in thin films. It has been shown that highly controlled films of NiMn2O4 can be grown via electron-beam-evaporation [26]. By this method an electron beam is focused on NiMn2O4 target material which evaporates, transports and condenses on a substrate placed above the source. Thin electron-beam evaporated films have been examined by several authors. Details of the experimental set-up as well as the theory of evaporation were given in a previous study [27]. Beet [28] showed that by annealing procedures in air at 500 0C for 30 min the grains on the film surface align forming a dense surface with no pores observable. This indicates that a small grain size in electron-beam evaporated films can be achieved as desired, because obviously strong diffusion processes of the these small grains occur during annealing forming a close surface. Unfortunately the NiMn2O4 spinel phase is not stable at 500 0C (see Fig.3). Higher annealing temperatures lead to cracks in the film surface due to a mismatch of expansion coefficient of substrate and film. Parlak [29] observed that after annealing procedures in air and oxygen the resistivity decreases as well as the activation energy E. However, NiMn2O4 is a complex compound and the vapour deposition of thin layers without loss of stoichiometry and changes in composition was proved to be difficult. A significant loss of manganese occurred in the films despite assuring stoichiometric target material. It is supposed that the NiMn2O4 target material dissociates into O2, manganese 19

and nickel ions or oxides, when exposed to the electron beam. The evaporated material rises and settles down on the substrate via a nucleation process and recombines to NiMn2O4. These processes have not been investigated yet for NiMn2O4 and can be thus not controlled easily. In order to avoid these problems screen-printing was studied in this work as a direct printing technique of thick films. In addition, other deposition techniques for producing thin and thick NiMn2O4 films have been examined by several authors. Fau et al. [41] reported about RF reactive sputtering. Stoichiometric films could be grown, but the resistivity was affected by a conical columnar structure in the films. Baliga and Jain [30] found that RF reactive sputtered films of similar electrical characteristics as bulk material can be produced by adjusting the oxygen partial pressure during sputtering and by an annealing process after the deposition. Lindner and Feltz [31] produced thin films via electrophoretic deposition.

2.4. Screen-printing Screen printing is a thick film technique which is used for a wide range of applications like printing circuits on board or printing batches and labels on various kinds of materials such as metals, ceramics, textiles or wood [van Duppen, 32]. For this project screenprinting has been chosen for printing NiMn2O4 films, because it is a direct printing technique where the NiMn2O4 powder is mixed with a special carrier material forming a printable paste. The paste is pressed through a screen and deposited directly onto a substrate, which is placed few millimetres beneath the screen ( Fig.4), thus, no problems with loss of stoichiometry can occur. As can be seen from Fig.4, as a first step of the printing process the paste (Ink) is first placed on a framed screen consisting of a gauze and mask. By applying a print stroke with a squeegee blade the screen is pressed on the substrate and the paste in front of the squeegee blade moves into the meshes of the screen. Behind the squeegee the paste is left on the substrate and the screen snaps off the substrate to its original position. The meshes are emptied due to the paste sticking to the substrate [Riemer, 33]. On the substrate the paste is flowing together forming an even layer. For a successfull screen printing the 20

paste must fullfill several specific properties. After printing a heat treatment is required to evaporate the carrier material resulting in a film of the composition of the original source material. In addittion a rigid joint of film and substrate is formed during a sintering process included in this heat treatment.

Fig.4 The principle of screen-printing procedures (DEK [42])

2.4.1. The screen The essential tool for screen-printing is the screen with well-chosen dimensions and parameters. As one mesh of the screen has a certain volume, the meshes of the screen can be regarded as a volumetric measurement container. Assuming that the printing paste will fill the meshes completely, the expected thickness of the printed film is determined by the size of the meshes. For choosing the right mesh size of the screen and thus the film thickness for the application it has to be taken into account that the theoretical film thickness will reduce after printing, because of drying shrinkage due to the evaporating solvents and shrinkage during the sintering process when the organic additives are thermally decomposed. Furthermore, a densification of the particles occurs during 21

sintering. Fig.5 shows the main screen parameter from which the theoretical film thickness hno can be calculated.

Fig.5 Screen parameter (Riemer [33])

df is the thread diameter, hs the fabric thickness, dR the mesh opening, m the mesh size and  the angle of inclination of the screen web. The theoretical film thickness hno can be calculated according to :





 d 2f  hno  2d f    2m cos 



  ; 



(25)

As mentioned before the screen is pressed on the substrate by the squeegee and is elastically strained by this action. Behind the squeegee a restoring force is responsible for the screen to snap off the substrate. Therefore, elastic screen materials have to be chosen. Typical materials are Nylon, stainless steel or Polyester [Franconville, 34]. With a stainless steel screen a higher resolution of the printing pattern can be achieved, but it can be destroyed easily by applying too high forces via the squeegee, so that it does not return 22

to its original state after the strain. This problem does not occur using a polyester screen, which was thus chosen for this project. The resolution of the printing pattern proved to be sufficient.

2.4.2. Rheology of screen-printing pastes The NiMn2O4 powder has to be mixed with a carrier material, called vehicle, in order to form a printable paste. The vehicle consists of organic solvents and organic binder material. Furthermore a dispersing agent and a glass phase can be added for reasons discussed later. The vehicle has to have several well-defined properties [Huber,35]. First, it has to enable the paste to stick on the substrate to ensure that the meshes of the screen are emptied completely. Furthermore the viscosity of the paste has to match several conditions. The viscosity has to be high enough, so that the paste does not drop through the meshes when distributed on the screen before printing (see Fig.4). Contrarily, if any forces are applied to the paste in terms of a shear rate during the print stroke, the paste has to exhibit a lower viscosity in order to move through the screen easily (Fig.6:SCREEN) and “flow” together on the substrate to form an even layer (LEVELING) [Trease et. al. 36]. This means that the paste has to stay liquid after the printing for levelling, but has to regain its original viscosity after a certain time to form a durable and stable film. The printing pattern must not be disfigured. This desired time-dependent behaviour is called thixotropic. Another property desired for the vehicle is a minimised sedimentation effect

Fig.6 Applied shear rates and viscosity changes during screen-printing (Trease et al. [36]) 23

in the paste during storage. These requirements on the viscosity of the vehicle and the resulting paste sound rather contradictory. But the desired rheology can be obtained by the appropriate composition and type of solvents, binder and solid loading. A modern commercial vehicle can be used and the viscosity of the paste can be adjusted carefully by choosing the right ratio of NiMn2O4 source powder, vehicle and a possible glass powder. A glass phase has positive effects on the microstructure of the printed films.

2.4.3. “Snap off” distance In order to gain highly controlled screen-printing procedures the adjustment of the “snap off” distance, the initial distance between substrate and screen, is important. If the snap off distance is chosen too large, the screen will be strained extensively and the restoring force of the screen is higher than the adhesive force of the paste to the substrate. The paste inside a mesh would rip and only a part of the mesh content would be deposited while the other part remains in the mesh causing a non-uniform film thickness. In addition to this, the speed of the squeegee stroke v0 must not be too high in order to ensure the meshes to be emptied completely. Both conditions for a regular printing were expressed by Riemer [33] in the following relation :

l  v0  1560Pa  SR   lA 

 d R2   dF

  

;

(26)

where is the viscosity of the paste, v0 the speed of the squeegee stroke, lA the snap off distance and lSR the distance between squeegee blade and screen frame, which varies during the print stroke. This means that for achieving a regular print during the whole print stroke the lowest value possible for lSR during the print stroke has to be considered for working with relation (26). dR and dF are screen parameters as defined in Fig.5. Contrarily, the snap off distance must not be too small either. This would cause the screen not to snap off immediately behind the squeegee blade but to stick on the substrate. In this case the snapping off would occur later in a certain distance to the blade 24

edge and would be uncontrolled and would cause irregularities in the printed film. This condition was expressed by Riemer [33] as follows :

     256 

v0  

 lA  l    m  d F   S   SR  2m  d F  p A  lA   l SR 

;

(27)

where S is the screen stress and pA the atmospheric pressure assuming working at standard conditions. As the screen stress varies with varying frame- squeegee blade distance, equivalent to (26), the lowest value possible has to be considered. The screen stress S consists of two components, the initial stress of the screen 

0

as

delivered by the manufacturer and the stress  T caused by the strain of the screen while it is pressed onto the substrate surface by the squeegee blade:

S=0+T; The stress 

T

(28)

can be obtained by calculating the strain of the screen knowing the

Young's modulus Ey of the screen material which is defined as :

F stress  Ey   A ; strain l  l

(29)

The stress  = F/A is the applied force per unit area and the strain  = l/l the elongation per initial length of the material. The stress 

T

can be determined by calculating the

strain T which is dependent only on the snap off distance lA , the length of the screen lS and the distance of screen frame and squeegee blade lSR (Riemer [33]):

1  T 

l

2 SR

 l1

 l A2  l A2  l S  l SR  * 2

S

25

;

(30)

The initial stress of the screen  0 is determined by the line stress FL of the screen given by the manufacturer. The line stress FL and the stress  0 are related according to: FL   0

M d 2f  4

;

(31)

where M is the number of meshes per unit length 1/m and dF the thread diameter. By calculating T and 0, S can be determined for testing relation (27). The conditions (26) and (27) given above are contradictory, so the snap off distance, the paste viscosity and the squeegee speed have to be adjusted carefully in order to fullfill both relations and to work at optimum conditions. This optimum region can be seen in Fig.7. where v0 is plotted versus lA/lS for both conditions. A full derivation of the relations (26) and (27) is given by Riemer [33].

Fig.7 Optimum printing parameters

26

2.5. Electrical measurements In order to examine the electrical characteristics of NiMn2O4 films, resistance versus temperature measurements have to be carried out. For measuring the resistance of NiMn2O4 films, two contacts have to be deposited on the films for so-called “twopoint- measurements”. As NiMn2O4 exhibits semiconducting properties, the interface of contacts and a semiconducting material has to be considered. If the work function of the metal contacts M is higher than that of the semiconducting material SC a barrier for electrons forms at the interface. The work function is defined as the energy difference between Fermi- level and free electron state. This barrier can affect resistance measurements significantly, if the applied bias does not ensure that a sufficient rate of electrons overcomes the barrier. Fig.8 illustrates how the metal and the semiconductor are brought together to a decreasing distance d. Electrons transfer to the metal until the fermi-levels  fM and  fS coincide. This is a condition for an equilibrium state. The resulting carrier depletion region at the edge of the semiconductor (see Fig.9) causes a rise of the conduction band level  c about the energy e(M - SC). This band bending at the metal-semiconductor interface works as a barrier for electrons under applied bias and is called Schottky-barrier (Mc Kelvey,[37]).

metal

metal

semiconductor

Fig.9 Metal-semiconductor interface (McKelvey [37])

semiconductor

Fig.8 Metal and semiconductor at a separation d ( McKelvey [37])

27

Conversely, if a contact material with a lower work function than the semiconductor is chosen electrons flow from the metal to the semiconductor (Fig.10) and no barrier for electron flow emerges (Fig.11).

Fig.10 Metal and semiconductor at a separation d (McKelvey [37])

Fig.11 Metal-semiconductor interface (McKelvey [37])

In an earlier work [27] gold was tested as a contact material, but a Schottky-barrier as described above occurred and consequently, an alternative contact material with a lower work function had to be chosen. For aluminium contacts ohmic behaviour of the contacts could be observed, which indicates that the work function of NiMn2O4 is between the work function of gold and aluminium. The work functions of gold (5.1 eV) and aluminium (4.28 eV) were obtained from the literature [38], which determined the work function of NiMn2O4 to be in the range of 4.25 eV - 5.31 eV.

28

29

3. Experimental procedure In order to produce and characterise thick NiMn2O4 screen printed films the experimental route shown in Fig.12 was followed.

Synthesis of the nickel manganese spinel Characterisation

Paste production Characterisation

Screen Printing Drying and thermal treatment Characterisation of the films X-ray diffractometry/ICP phase composition

UBM surface imaging Scanning electron microscopy microstructure of the films

electrical characterisation resistance vs. temperature measurements

Fig.12 Experimental route for producing and characterising screen printed films For producing NiMn2O4 source powder the method of coprecipitation of a mixed oxalate and thermal decomposition of the oxalate to NiMn2O4 was chosen (see 2.1.3., 5.)). The paste was gained by mixing the optimised source powder with vehicle carrier material and a possible dispersing agent and glass phase. Both, the paste and the source powder itself were characterised. In order to produce a thick film of NiMn2O4, the paste was printed via screen-printing onto special thick film substrates which consisted of 96% wt. Al2O3 and 4% wt. glass. The glass acts as a binder and ensures a rigid joint of substrate and film. After printing, the films were dried releasing the organic solvents and the dispersing agent, whereby the organic binder was decomposed and evaporated by a thermal treatment at higher temperatures. For densification and more stable filmsubstrate contact the films were sintered as well. The films produced in this way were characterised in terms of the phase composition (phase purity and stoichiometry), in terms of the microstructure (morphology, thickness and roughness of the films) and the electrical conductivity. 29

3.1. Synthesis of NiMn2O4 source powder NiMn2O4 was produced via coprecipitation of a mixed nickel manganese oxalate NiMn2(C2O4)3. In this compound the nickel manganese ratio is appropriate for forming NiMn2O4 by thermal decomposition. In comparison to firing the mixed oxides NiO and Mn2O3, in coprecipitated NiMn2(C2O4)3 a homogeneous distribution of nickel and manganese atoms in atomic scale can be achieved. Contrarily to the mixed oxide route less heat exposure is required for the atoms to form the NiMn2O4 compound by diffusion processes. At higher temperatures, like 1200 0C for the precursor oxide route, the nickel manganese spinel is not stable (see 2.1.2.) which causes problems with the phase purity. Another problem occurring by heating at high temperatures is a loss of manganese due to sublimation and thus deviations from the desired stoichiometry. These problems could be avoided with the mixed oxalate route. Furthermore, a homogeneous and small average grain size of the powder can be achieved, which is of advantage to form a homogeneous thick film via screen-printing and enables the densification of the films during the heat treatment. Fig.13 shows the steps of the mixed oxalate route :

NiCO3*x H2O, dil. CH3COOH (10%) Mn(CH3COO)2

Mn2+

H2C2O4

C2O42-

Determine concentrations

Ni0.33Mn0.66(C2O4)

Fig.13

Principal of the mixed oxalate route 30

3.1.1. Production of stock solutions As depicted in Fig.13 three stock solutions containing nickel, manganese and oxalate ions were produced. For the nickel stock solution nickel carbonate (NiCO3 * x H2O) (Alfa Aesar Johnson Matthey Ltd., USA, type 012350) was dissolved in diluted acetic acid (10%). The dissolving process was supported by heating the solution on a hot plate and stirring with a magnetic stirrer which was integrated in the hot plate (Janke & Kunkel GmbH, Staufen, IKAMAG RET). After a few hours the solution was filtered using standard filter paper in order to remove undissolved nickel carbonate. For the manganese stock solution manganese (II) acetate tetra-hydrate (Mn(CH3COO)2.4 H2O) (Fluka, Switzerland, type 63537) was dissolved in water and for the oxalate stock solution oxalic acid di-hydrate (H2(C2O4).2H2O) (Merck, Darmstadt, Art.495) was dissolved in water as well. Both, the manganese acetate and the oxalic acid showed a complete dissolution. For producing the three stock solutions, NiCO3, Mn(CH3COO)2 and H2C2O4 were chosen, because of the anions occurring in the solutions. The remaining cations and anions not incorporated in the coprecipitated NiMn2(C2O4)3 or the compounds formed by the residues were all of organic type, so they could be evaporated after the precipitation easily. For the nickel solution OH- anions remained as the carbonate was released as CO2 during the dissolving process what could be observed by rising bubbles. Furthermore acetic acid was present. In the manganese solution acetate anions were present, and for the oxalic acid solution H3O+ cations occurred. The starting materials chosen exhibited all a high solubility which made the coprecipitation process more effective. Before dissolving, the starting materials NiCO3 and Mn(CH3COO)2.4 H2O were both analysed by thermal decomposition according to the reactions : Ni(CO3).xH2O

800 0C, 8h

NiO + CO2 + x H2O

3 Mn(CH3COO)2.y H2O

Mn3O4 + organics + 3y H2O

1200 0C, 8h

(32) (33)

The weight of the samples was measured before and after firing using a Sartorius analytic scale. The amount of water of hydration was determined for nickel carbonate to be x = 0.772. For manganese acetate it was y = 3.96 which is an acceptable range as the manufacturer gave y = 4. 31

3.1.2. Concentrations of the stock solutions The concentration of the nickel and the oxalate stock solutions were determined via titration methods.

3.1.2.1. Nickel titration The determination of the nickel as well as the oxalate concentration was carried out by titration methods as this is the most accurate way to do such work. The titration for nickel was carried out according to the following procedure [39] : From the nickel stock solution an estimated amount was taken and diluted in H2O resulting in a nickel containing solution of 100 ml with a nickel concentration not more than 15 mg per 100 ml. Ammoniac solution was added to form a nickel-ammin complex ([Ni(NH3)6]2+) turning the solution into blue colour. As an indicator murexide solution (Merck, Darmstadt, type 106161, C8H8N6O6) was added until the solution reached a deep yellow colour. The pH value was checked to be 10 using standard pH testing strips. A pH value of 10 had to be reached, caused by an excess of ammoniac what indicated a complete reaction of the nickel ions with ammoniac. After diluting with 300 ml additional H2O the titration solution Titriplex III (Merck, Darmstadt, type 9992 Titrisol, Na2-EDTA) consisting of Ethylenedinitrilo-tetraaceticacid-disodiumsaltdihydrate was added drop-wise using a Dosimat (Metrohm, Switzerland, model 665 ). TitriplexIII was added until the first turning point changed the solution into a redbrownish-lilae colour. The titration was repeated and few drops before the first turning point about 10 ml additional ammoniac was added. By doing this the first turning point did not occur. The second turning point relevant for the determination of the nickel concentration could be seen clearly as a sharp turn of the colour from yellow into deep lilae. This procedure was repeated up to five times and the mean value of the used TitriplexIII solution over all runs was taken. The volume of one drop was measured (0.03 ml) and half of this amount was subtracted from each of the used TitriplexIII volume. This was done because the turning point indicates an excess of TitriplexIII which was half a drop in average over all runs. The nickel concentration was given by 6.0638 mg nickel in the solution per 1 ml TitriplexIII added. The concentration of the TitriplexIII solution was tested by titrating a nickel standard solution (Merck, Darmstadt, type 102640, nickel standard). 32

3.1.2.2. Oxalate titration In order to determine the oxalate concentration the titration was carried out according to [40]: An estimated amount of the oxalic acid solution was taken and mixed with 200 ml H2O giving a 200 ml oxalate solution with a concentration of not more than 40 mg C 2O42per 200 ml H2O. The solution was heated to 75 0C - 85 0C and 10 ml diluted sulfuric acid (1:4) was added to provide an acid solution. 0.1 N potassium permanganate solution (Merck, Darmstadt, KMnO4) was added drop-wise as a titration solution. The permanganate ions were reduced by the oxalate ions which was indicated by the decolourisation of the dark pink/lilae permanganate. After the complete reaction of all oxalate ions the excess of permanganate could be seen by a weak rose colour of the solution. The redox reaction of the permanganate and oxalate ions followed the equation : 2MnO4

5C 2 O42

16 H

2Mn 2

10CO2

8H 2 O

(34)

By the same procedure as described for the nickel titration the oxalate concentration could be obtained according to 4.352 mg C2O42- in the solution per 1 ml KMnO4 added. The concentration of the permanganate solution was tested by titrating a sodium oxalate (Merck, Darmstadt, 1.02407.0060, Na2C2O4) solution of well known concentration. Sodium oxalate was chosen, because it has no water of hydration and can be weighted accurately for producing a solution of well known concentration.

3.1.2.3. Manganese solution Manganese titrations turned out to be troublesome, so other ways of producing a manganese solution of well defined concentration had to be found. It was attempted to determine the amount of water of hydration by Carl Fischer titration and weight a certain amount of manganese acetate (Mn(CH3COO)2. xH2O) resulting in a solution of known manganese content, but Carl Fischer titrations for manganese acetate proved to be difficult. Finally a certain amount was weighted acurately whereby it was assumed that the material exhibited a high purity and that the results of determining the amount of water of hydration by thermal decomposition were accurate (see (33)). 33

3.1.3. Precipitation of NiMn2O4 mixed oxalate The nickel and manganese stock solutions of well known concentrations were mixed together and then the oxalate solution was added. The amounts of added stock solutions were calculated to give a nickel:manganese:oxalate ratio of 1:2:3 moles. After adding the oxalate ions nickel-manganese–oxalate NiMn2(C2O4)3 started to precipitate. After 24 hours the precipitation was expected to be complete and the overstanding solution was evaporated using an evacuated rotating evaporator (Heidolph, Kelheim, model VV 2000) operating at 60 0C and 60 mbar. These conditions were appropriate to evaporate H2O as well as organic impurities .

3.1.4. Characterisation and optimisation of NiMn2O4 powder The precipitated nickel manganese oxalate was thermally decomposed in a furnace (Heraeus, Hanau) equipped with a programmable temperature controller by the following reaction:

3NiMn2 (C 2 O4 ) 3 * 2 H 2 O 2O2

NiMn2 O4

6CO2

2H 2O

(35)

For optimum screen printing procedures it was desired to obtain a minimum average grain size and a homogeneous distribution of the grain sizes. Especially bigger grains or agglomerates in the powder had to be avoided which would prevent a dense homogeneous particle packing and cause irregularities in the printed films. Small grain sizes improve the densification behaviour during the thermal treatment of the films due to higher activity of the grains. Therefore, the thermal exposure of the material was minimised in order to reduce the grain growth. In order to minimise the heat exposure during the decomposition process, the precipitated oxalate was examined by thermal gravimetric analysis (TGA) using a Simultaneous Thermal Analysis device (Netzsch, Selb, STA model 409). With this device differential thermal analysis (DTA) could be carried out as well to indicated the temperature regions at which the reactions of the decomposition take place. Possible reactions are the release of water of hydration in several steps or the main decomposition reaction as represented in (35). In order to minimise the heat exposure the temperature controller was set up according to this data, so that for the temperature 34

regions at which reactions occurred a low temperature gradient during heating up was chosen whereby the heater sweeped through regions with high heating rates where no reactions occurred. The minimum temperature required for a complete formation of NiMn2O4 was determined by analysing the powder with a XRD device (Siemens, Erlangen, model 5000 X-Ray Diffractometer) after different heating times and different heating temperatures. The grain size distributions of the powders were determined by using a granulometer (Quantachrome GmbH, Odelzhausen, Cilas type 715). By this method the laser interference pattern of an assemble of grains is analysed by Fourier analysis and the distribution of the grain sizes can be retrieved. A statistical grain size distribution was displayed by the device. The optimum heating procedure is given in Table 1.

Table1 : Thermal treatment for forming NiMn2O4 Temperature region 0

0

30 C – 90 C

Heating rate in Kelvin per h

Holding time

400

90 0C – 140 0C

100

0

0

400

0

0

100

0

0

400

0

0

270 C – 330 C

100

330 0C – 850 0C

450

30 min

800

quench cooling, ca. 60 min

140 C – 180 C 180 C – 230 C 230 C – 270 C

0

0

850 C – 30 C

In order to further optimise the powder a vibrating tungsten carbide mill (F.Kurt Retsch GmbH, Haan, model MM2000) was used to destroy hard aggregates and grind the powder further down in terms of average grain size. This process was optimised by observing the grain size distribution and the average grain size after milling at different milling times and different forces applied to the powder. According to the granulometric analysis the milling time was chosen to be 8 min and the amplitude was step 8. After the powder processing the optimised powder was checked in terms of the stoichiometry by using a Inductive Coupled Plasma spectroscopy (ICP) (Spectro Analytical Instruments, Kleve, Spectroflame). By this device a quantitative determination of nickel and manganese content could be carried out by analysing the 35

emission of an argon plasma flame exposed to a nickel and manganese containing solution. In order to examine NiMn2O4 by ICP the compound was dissolved in concentrated hydrochloride acid (HCl). Attempts to dissolve the NiMn2O4 spinel in diluted HCl or in nitric acid did not succeed. Few milligrams were dissolved in 10 ml concentrated HCl and left for up to 60 hours allowing a thorough dissolution process. Following, the solution was diluted with H2O to obtain a solution with a manganese or nickel concentration not higher than 25 mg per litre which was required for the ICP apparatus. The nickel:manganese ratio was found to be appropriate what validated the preparation route for NiMn2O4 described.

36

3.2. Paste production and characterisation After optimising the powder synthesis and its processing it was necessary to obtain a printable paste. Several components were included into the system in order to produce an optimum printing medium. A “vehicle”, containing organic solvents and organic binder material, was added. For a part of the samples a dispersing agent and a glass phase were added additionally. As mentioned before (2.4.2.) the main paste parameter is the viscosity which could be adjusted by the amount of vehicle added. The viscosity was measured using a viscosimeter (Paar Physica, Austria, model UDS 200,). The shearing stress was measured for different shearing rates applied to the paste at constant temperature which determined the viscosity. The viscosity is defined as shearing stress/ shearing rate. Furthermore it is important to ensure that the powder is mixed and dispersed thoroughly within the paste. For the part of the samples including a glass phase and a dispersing agent, first the commercial dispersing agent “Hypermer LP1” (ICI Surfanctants, UK) was chosen to be added by an amount of 1% wt Hypermer referred to the NiMn2O4 powder. The dispersing agent was mixed to the powder in order to avoid agglomerates and was evaporated during drying the films after printing. In order to cover the NiMn2O4 grains with a Hypermer containing solution, a certain amount of Hypermer was dissolved in ethanol (0.05g Hypermer in 25 ml ethanol). The dissolving process was difficult, so the mixture was placed in a mill (Willy A. Bachhofen Maschinenfabrik Basel, Switzerland, WAB TURBULA) for 2 hours. Next the NiMn2O4 powder was added. In order to ensure a complete coverage of the grains by the dispersing agent, dissolved in ethanol as a carrier liquid, the mixture was placed again in the mill for 48 hours. Furthermore, possible remained agglomerates were destroyed by holding an ultrasonic probe (KLN GmbH, Heppenheim, Ultraschall 250/101) into the mixture for 15 min. Then, the ethanol was evaporated in the evacuated rotating evaporator mentioned before at 50 0C and 350 mbar. During the evaporation the bottle containing the mixture was covered by a skin foil with little holes in it, so that the dry Hypermer covered powder could not be sucked out by the vacuum pump.

37

As the next step the powder was mixed with 8% wt. glass powder. A thorough mixture, reduction of the glass powder grain size and destruction of agglomerates was achieved by using the tungsten-carbide milling facility. The glass phase was added in order to contract the grains during the thermal treatment after printing resulting in a much denser surface with less holes and pores and to enable a strong adhesion of film and substrate due to bondings with the glass contents in the substrates. 8% wt. glass phase referred to the NiMn2O4 powder was added in form of an glass powder (ESL Europe, UK, code 428). The glass powder exhibited a low viscosity at temperatures above 600 0C during the thermal treatment, so at 850 0C a liquid phase sintering was enabled. A reorientation of the grains occured due to cappilar forces and an optimal particle packing was gained. The dispersing agent and the glass phase were added only to a part of the samples in order to examine their effect on the surface morphology and on the electrical properties by comparison of the samples with and without glass and dispersing agent. The glass phase was of insulating material which was expected to affect the conductivity. As the next step for all samples the vehicle (ESL Europe, UK, type 403 vehicle) was added. The organic solvents in the vehicle enable the paste to be of liquid form rather than a wet powder. The organic binder makes the paste sticking together as well as sticking onto the printing substrate, which is required for a regular printing process (see 2.4.2.). Both components were removed from the material by thermal decomposition during drying and the heat treatment of the films. The vehicle was added drop-wise to the powder and was dispersed immediately in a petri dish using a flat spatula. The vehicle was added until the wet powder turned into a viscous paste. This change occurs at the so-called “oil point”. The turn was very sharp and was determined to occur after the amount of vehicle had reached a percentage of 11% wt. of the whole mixture. Directly after reaching the oil point the paste was dispersed on a drum-mill (Otto Hermann, Norderstedt, model 2/7533). As the viscosity of the paste was very high in this state high forces could be applied to the paste assuring a thorough dispersion and destruction of agglomerates. The possible appearance of agglomerates was tested by using a grindometer (Simex, Haan, type PF 50/2). The milling procedure was repeated until no further improvements in terms of better dispersion and lower agglomeration could be observed using the grindometer. The largest agglomerates were determined to be not bigger than 20 m. This was acceptable as only very few agglomerates of this size could be observed. 38

However, for printing procedures the viscosity of the paste in this state after dispersion was far to high, so additional vehicle had to be added. It turned out that a lack of vehicle resulted in a high viscosity, so that the paste did not flow together on the substrate after printing. In this case the pattern of the screen meshes was observable on the film (see 4.7.). Contrarily, a too high amount of vehicle resulted in a low viscosity causing the paste to disfigure the printing pattern. Therefore it was necessary to adjust the viscosity carefully by optimising the amount of vehicle added which was found to be 0.17 g vehicle per 1g NiMn2O4 or 14.5% wt. vehicle of the whole mixture. The consistence of the printed films before and after the thermal treatment with and without glass phase are given in table2.

Table 2 : consistence of printed films before and after thermal treatment Before

thermal After

thermal Before

thermal After

thermal

treatment

treatment

treatment

with treatment

without glass

without glass

glass

glass

Dispersing agent

--------------

-----------

0.85 %

------------

glass

--------------

-----------

6.8 %

8%

vehicle

15 %

-----------

15 %

------------

NiMn2O4

85 %

100 %

77.35 %

92%

with

powder

3.3. The screen printing procedure For gaining an optimum printing process it was necessary to adjust several parameters of the screen printing apparatus. As mentioned in section 2.4.3. an optimum working region can be found for the screen printer by optimising the snap off distance and the speed of the print stroke. The viscosity of the paste was determined by optimising the amount of vehicle as described before (3.2.). For matching the conditions of a regular printing given by equations (26) and (27) the parameters of the screen had to be considered, too. A screen printer (DEK Ltd., UK, Model 240) was used (Fig.14). The print stroke with the squeegee had to be carried out manually and the snap off distance had to be adjusted manually as well. 39

Fig.14 DEK Model 240 screen printer (DEK,[42])

The screen was a 115 T-mesh manufactured and framed on an aluminium frame by Koenen Siebdruck GmbH, München (Fig.15).

Fig.15 Framed screen with printing pattern of squared and maltese crossing shape 115 T-mesh means 115 meshes per inch, which corresponds to 45 meshes per cm. The mesh opening dR was 140 m, the thread diameter df 70 m, the fabric thickness hS 115 m and the 40

angle of inclination of the screen web thickness was 51

was 45 0. According to (25) the theoretical film

m. The stress of the screen in relaxation state was given by the

manufacturer in terms of the line stress which was 23- 24 N/cm. From this value the initial screen stress was determined by using (31) to be 1.28 *10+8 N/m2. As mentioned in 2.4.3. the screen stress during printing had to be calculated in order to check condition (27). To calculate the screen stress from the screen strain the Young’s modulus of the screen had to be determined. This was done for a piece of a 115 T-mesh polyester screen as used for the printing. An Advanced Force Gauge (MECMESIN, UK, AFG-2500N) linked with a computer which was equipped with appropriate software (MECMESIN, UK, Data Plot) was used. As can be seen from Fig.15 the screen pattern was designed for printing films of squared and maltese crossing shape which was a requirement for the electrical characterisations of the films. This is explained in more detail in section 3.7.2.. In order to use the screen printer, Fig.16 shows how the framed screen (1) was fixed above the substrate tray. The movable unit (4) is able to hold either the squeegee or the paste distributor blade. 4

Fig.16 Installation of the screen (DEK,[42])

41

From Fig.17 it can be seen how the paste (2) was distributed on the screen before printing with a paste distributor blade fixed on the movable unit (1).

Fig.17 Distributing the paste with a distributor blade (DEK,[42])

After distributing the paste the movable unit was in the rear position (Fig.17). The paste distributor blade was removed and the squeegee blade installed. Before carrying out any operation with the printer, the position of the squeegee blade had been adjusted to ensure that screen and squeegee blade were parallel. The print stroke was carried out by moving the movable unit with a slow ponderous action to the front position (Fig.18) pressing the paste through the screen (1) onto the substrate which was placed under the screen. The speed of the squeegee stroke was in the range of 0.11 – 0.15 m/sec. As explained in section 2.4.3. an appropriate snap off distance was essential for reasonable printing. The optimum distance was found empirically to be ~ 1.5 –2.0 mm.

42

Fig.18 After the print stroke (DEK,[42]) After the printing procedure the screen had to be cleaned with a special screen cleaner Pregan C 404 B which was provided by Koenen Siebdruck GmbH, München. The pure cleaning solution was distributed over the screen. Any contact with water had to be avoided, as this would damage the screen. The remaining NiMn2O4 paste was wiped off the screen with a polyester electronic tissue (Bürklin OHG München-Düsseldorf, R.T.S. GmbH, ECS – 454100-000 Electroniktuch 3).

3.4. Substrates Alumina (Al2O3) was chosen as substrate material whereby a special thick film substrate was used. Thick film substrates have a rougher surface than thin film substrates. For thick film techniques a good adhesion of film and substrate is essential what can be achieved by a rough substrate surface. One of the main feature of thick film substrates is that they contain a glass phase. In this case 4% wt. of glass assure a better adhesion of substrate and film. By adding a glass phase to the printing paste strong bondings due to diffussion of the glass phase into the substrate occur. The substrates were placed under the screen in the optimum snap off distance whereby a plastic frame was placed around the substrate in order to obtain an even surface. This was 43

important as the screen was pressed onto the substrate tray by the squeegee whereby the contact line was moving over the substrate with the moving squeegee. If the screen would move over a substrate – substrate tray step this could damage the screen. Before printing, the substrates had to be cleaned thoroughly. This was done by washing the slides with de-ionised water. Next, the samples were placed in a solution of dissolved tartaric acid (Merck, Darmstadt, type 159472, C4H6O6) and exposed to an ultrasonic treatment (Sonorex, Berlin, RK 106) for 30 min. Afterwards they were cleaned again with de-ionised water to remove the acid and dried in a Heraeus drying cupboard for 30 min at 70 0C. One printing procedure included the print of 5 maltese crossings and 5 rectangulars. The substrate had 6 different sections separated by laser fabricated breaking lines. On four of the sections two patterns were printed, on 2 sections, the ones on the edges, only 1 pattern was printed. After the thermal treatment of the printed films the substrate was broken into the 6 sections which were again cut in order to separate all of the patterns from each other. This was done by using a water-cooled diamond saw (Clarke, UK, 20 mm Tile Cutter). As the films got wet from the cooling water of the saw they were dried after cutting in an evacuated drying cupboard (Harvard/LTE, USA, model QUALIVAC).

3.5. Heat treatment of screen printed films The screen-printed films had to be dried and thermally treated in order to remove the ingredients of the vehicle and the dispersing agent by evaporation. The glass phase in a part of the samples exhibited a low viscosity over 600 0C in order to be distributed thoroughly between the grains by a viscous flow leading to an optimum effect of contraction between the grains. To this process it is referred to as liquid phase sintering as mentioned before. The morphology of the films and possible occurring pores and holes were examined by using a scanning electron microscope (Cambridge Instr., UK, model S250 MK3). The drying process and the thermal treatment were carried out in one program and included additionally a sintering process at higher temperatures (850 0C) to gain a strong bonding of the films to the substrate and to densify the material. The contraction effect was determined to be most effective by sintering for 30 min at 850 0C. This was done in a furnace (Thermal Technology GmbH, Bayreuth) capable of fast heating rates equipped with a temperature controller. Equivalent to the decomposition of NiMn2(C2O4)3 (3.1.4.) it was required to minimise the heat treatment of the films concerning temperature and time in order not to form bigger grains on the surface. The formation of bigger grains occurs if smaller grains join together by 44

diffusion processes especially for small grains. A problem with this formation is that holes and pores occur between the bigger grains formed during densification of the films on a rigid substrate. This effect occurs only in films as films can shrink only in one dimension, which is the film thickness. Thus, densification due to sintering processes does not result in denser, but in more porous films. The addition of a glass phase could minimise this effect. Equivalent to the thermal decomposition of the oxalate the heat exposure was minimised by analysing the thermal gravimetry data of the vehicle. At temperature regions where no decomposition reactions were obvious the heating rate was increased. Contrarily, during sweeping through temperature regions where reactions due to the decomposition of vehicle contents occurred the heating rate was chosen much lower. The sintering process in terms of temperature and time was optimised by manually testing the mechanical adhesion of the films on the substrates. The optimum heating procedure was determined to be : Table3 : Thermal treatment of screen-printed NiMn2O4 films Temperature region

Heating rate in Kelvin per h

20 0C – 60 0C

400

Holding time

60 0C – 170 0C

100

170 C – 300 C

400

300 C – 350 C

100

350 0C – 850 0C

3000

30 min

850 0C – 30 0C

3000

quench cooling

0 0

0 0

The phase purity of the sintered films was checked using XRD.

3.6. Film profile The thickness of the films was measured using a 3-dimensional surface imaging facility (UBM Messtechnik GmbH, Ettlingen), which was operated in linear scanning mode. By this device the interference pattern of an emitted laser beam and the reflected beam from the surface was used to display the step in thickness of blank substrate and coated substrate. A resolution power of 6 nm in height and 1 m in vertical direction can be achieved by the device. The roughness of the surface of the films could be assessed as well.

45

3.7. Electrical characterisation In order to characterise the conductivity of the films, the resistance was measured in dependence upon the temperature over a wide range (120K – 600K). The obtained resistance vs. temperature curve was tried to fit theoretical models in order to explain the conduction mechanism in NiMn2O4 films. Measurements with two contacts (twopoint-measurements) and with four contacts in the van-der-Pauw configuration (fourpoint-measurements) were planned, but due to time limitations of this work only twopoint measurements were carried out so far.

3.7.1. Contacts In order to measure the resistance of the films by two-point-measurements two contacts were deposited on the film surface. As mentioned in section 2.5. it was necessary to assure that the work function of the contact material was lower than that of NiMn2O4. Aluminium contacts proved to be appropriate, so two aluminium point contacts of 1 mm in diameter were evaporated onto the film using a coating unit (Edward Ltd., USA, model 6E4). The contacts were covered with quick drying silver paint (Agar Scientific Ltd., UK) immediately after the deposition process in order to avoid oxidation of the aluminium. Two stainless steel wires were fixed to the contacts with solder using a solder iron. The ohmic behaviour of the contacts was tested by measuring the voltage versus current characteristics using a programmable electrometer (Keithley instruments, USA, 617 programmable electrometer) operating in the “ohms V/I “ mode.

3.7.2. Two-point measurements As mentioned in 3.3. different shapes of NiMn2O4 films were printed, rectangular and maltese crossing patterns. For the two-point measurements two contacts with 1 mm in diameter and in a distance of 7 mm were deposited on the rectangular samples as described above. A problem with two-point-measurements is that the resistance of the contacts add to the resistance of the samples as can be seen in Fig. 19. This problem can be avoided by using a four-point van-der-Pauw configuration. By this method four contacts are deposited at the edges of a maltese crossing as depicted in Fig.20. By 46

applying a current on two neighbouring contacts and measuring the voltage drop across the other two contacts the resistance can be determined by varying over all possible configurations and taking a geometrical correction function into account. By calculating the mean value over all configuration the contact resistance vanishes.

Fig. 19 Two-point configuration

Fig.20 van-der-Pauw configuration

The resistance was only measured with the two-point method, but as the samples were very resistive (1*106 – 2*1011 Ohm) the contribution of the contact resistance was thought to be low. It was assumed that the resistance vs. temperature characteristic of the contacts did not influence the resistance vs. temperature dependence of the NiMn2O4.

3.7.3. Resistance vs. temperature measurements For resistance vs. temperature measurements between 77 K and 400 K the samples were placed in a cryostat (Oxford Instruments Ltd., UK, DN1704). The two contacts were connected to the sample holder equipped with four data lines leading out of the sample chamber allowing a maximum of four contacts. The cryostat was linked with a temperature controller (Oxford Instruments Ltd., UK, ITC4) which was able to set up and measure the temperature. The temperature controller was connected to a heater element in the cryostat and two platinum resistance thermo elements, one at the heater block and one right beneath the sample. The cryostat could be cooled with liquid nitrogen, which was kept in a vacuum isolated chamber inside the cryostat. The sample chamber was evacuated before starting the measurements to remove moisture and dust particles and was then flushed with helium gas. This procedure was repeated several times. 47

Fig.21 shows the design of the cryostat :

Fig.21 Cryostat (Oxford Instruments [43]) The resistance of the samples was measured using a programmable electrometer (Keithley, USA, 617 programmable electrometer) operated in the “ohms” mode. A VisualBasic6.0 computer program addressed both, the temperature controller and the electrometer and by communicating with both devices the resistance measurements at different temperatures could be carried out automatically. The temperature controller was linked to the computer via a RS232 port, the electrometer via an IEEE interface. The whole arrangement is depicted in Fig.22 : Cryostat Sample

Temperature controller

Computer

Fig. 22 Principle of the computer controlled resistance versus temperature measurements. 48

Additionally, two-point measurements were carried out at higher temperatures between 293 K and 600 K. The samples were placed in a heat resistive PTFE sample holder with two stainless steal drop down contacts. A chromel-alumel thermocouple was connected to the sample holder allowing the top of the thermocouple to touch the sample surface. The whole arrangement was placed in an insulated calorimeter as can be seen in Fig.23. The heater supply was controlled by the

temperature controller (Eurotherm, UK, Cal 9900) linked to the thermocouple. The

resistance was measured via the drop down contacts that were connected to the programmable electrometer.

Fig.23 Insulated heat calorimeter

49

4. Results 4.1. Phase purity and stoichiometry in NiMn2O4 source powder In order to validate the production route for NiMn2O4 source powder via the thermal decomposition of co-precipitated mixed oxalate the powder gained was analysed in terms of phase purity (XRD) and the stoichiometry (ICP spectroscopy). NiMn2O4 is stable only in a small range of temperatures as described in 2.1.2., so the phase purity is a matter of interest. As can be seen from the XRD spectrum given in Fig.24 a good agreement of the peaks detected with those for NiMn2O4 reference material found in a data base (the JCPDS) is obvious. All significant peaks detected could be assigned to a NiMn2O4 reference peak and no unassigned reference peaks could be seen, either.

1600 1400

intensity

1200 1000 800 600 400 200 0 0

20

40 angle 2-theta

60

80

= NiMn2O4 (1-1110 JCPDS) JPCDS) Fig.24 X-Ray scan of NiMn2O4 powder sintered at 850 0C for 30 min With NiO impurities present the NiMn2O4 peaks at the angle 37.05 degree (two-theta) and 43.01 degree would be overlaid by NiO peaks occurring at 37.28 degree and 43.30 degree. As these NiO and NiMn2O4 peaks occur at very similar angles a high-resolution spectrum is necessary to resolve the peaks as double peaks. However, the high50

resolution spectrum did not give any hint of NiO impurities (see App.8.2.4.). Possible products of a decomposition of NiMn2O4 like Mn3O4, Mn2O3 or NiMnO3 could not be observed either. An ICP spectroscopy was carried out twice to assess the stoichiometry. The desired manganese/nickel ratio of 2 (2 manganese, 1 nickel) was determined to be 2.03 and 2.06. Taking the mean value of 2.045 the source powder can be represented by Ni0.985

0.02

Mn2.015

0.02

O4 . For the nickel and manganese concentrations a possible error of

2% caused by the inaccuracy of the ICP apparatus has to be considered, so the compound is in the range of the desired Ni1.0Mn2.0O4.

4.2. Optimisation of the grain size in NiMn2O4 source powder The average grain size in the NiMn2O4 powder was minimised by reducing the heat exposure during the thermal decomposition of the precipitated oxalate (3.1.4.), so that no bigger grains could be formed. The thermal decomposition of NiMn2(C2O4)3 to NiMn2O4 was analysed by thermal analysis (TGA). The TGA (thermal gravimetric analysis) graph in Fig.25 shows that the decomposition takes place in 4 steps.

Fig.25 Thermal gravimetry analysis of coprecipitated NiMn2(C2O4)3; heating rate : 5K/min 51

The first 3 reactions at 65 0C, 114 0C and 207 0C could be interpreted as the release of remaining un-bonded H2O and water of hydration. The reaction occurring at about 300 0C might well be the main decomposition process of NiMn2O4 oxalate according to the reaction (35). The whole mass loss is 61 %. From the differential thermal ananlysis (DTA) graph given in App.8.3. p.99 it can be seen that the first three reactions are displayed as local minima indicating endothermic reactions which might well be the release of water in three steps. The last reaction at 291.6 0C is a local maximum indicating an exothermic reaction which was interpreted to be reaction (35). The heat exposure for these reactions was minimised as described in 3.1.4. leading to the optimum heat treatment given in Table1 (p.35). By using the granulometer it could be shown that a decreasing calcination temperature reduces the grain size of the powder. The temperature region for the reaction was limited by the stability region of NiMn2O4 (2.1.2.). After heating at 3500C for 6 h the X-ray scan indicated an amorphous powder. By heating at 650 0C for 6 h the occurrence of NiMn2O4, Mn2O3 and NiMnO3 could be observed in agreement to the phase diagram given in 2.1.2. After heating between 750 0C and 850 0C for 6h the Xray spectra indicated phase pure NiMn2O4 . At 9500C, additionally to NiMn2O4, NiO peaks were detectable. The X-ray spectra for different heating temperatures are given in Appendices 8.2.1. – 8.2.5. p.92 – 96 and in Fig.24. Conclusively, the decomposition process of the mixed oxalate had to be optimised for temperatures between 750 0C and 850 0C. As expected, it could be shown that the heating time had a significant effect on the grain size leading to the formation of bigger grains during longer heating times. It can be seen from Fig.26 that the average grain sizes of the powders are increasing with increasing temperature. The grain size of a sample decomposed at 650 0C is displayed as well. The grain size distributions for the samples from Fig.26 are given in Appendices 8.1.1. – 8.1.4. p.86 - 87. The average grain size increases drastically at temperatures over 750 0C. From the XRD spectra given in Appendices 8.2.6. and 8.2.7. p.97 - 98 it can be seen that for 7500C and 770 0C the powder is not phase pure NiMn2O4. Mn2O3 traces are detectable. Fig.26 clearly indicates the start of the formation of NiMn2O4 at 750 0C which is accompanied by a strong grain growth. At temperatures over 800 0C the grain size is

52

increasing more modest with increasing heating time indicating that the formation is complete. The modest increase of grain size displays the grain growth of NiMn2O4.

average grain size in micrometer

6 5 4 3 2 1 0 600

650

700

750

800

850

900

heating temperature in degree celcius

Fig.26 Variation of average grain size in dependence of heating temperature Next the effect of increasing heating time on the average grain size was examined. Three samples were fired at 850 0C for 2, 3,6 and 12 hours. Fig.27 shows the increase of the average grain size with increasing heating time. The grain size of the sample fired for 1 hour from Fig.26 was plotted as well. The grain size distributions of the samples are given in Appendices 8.1.4. – 8.1.8. p.87 – 89.

average grain size in micrometer

16 14 12 10 8 6 4 2 0 0

2

4

6

8

10

12

14

heating time in h

Fig.27 Variation of average grain size in dependence on the holding time at 850 0C

53

From Fig.27 it can be seen that the average grain size can be reduced significantly by reducing the heating time. Therefore it was decided to give priority to the reduction of heating time rather than to the heating temperature. Finally it was decided to heat the powder at 850 0C for 30 minutes. The phase purity for this formation process was proved by the XRD spectrum given in Fig.24 where no impurities could be seen at the expected diffraction angles of possible Mn2O3, NiMnO3 or NiO impurities. Unfortunately the average grain size of this powder before and after milling could not be determined due to mechanical problems with the granulometer and no further optimisation could be carried out. But for the sample heated at 850 0C for 2 hours it could be shown that the average grain size could be reduced from 5.4 m (App.8.1.5. p.88) to 2.8 m by milling with the tungsten- carbide milling machine (see Appendix 8.1.9. p.90). Thus, a reduction of the holding time to half an hour and a milling procedure were supposed to reduce the grain size significantly. This was confirmed by using the SEM facility which suggested an average grain size lower than 1 m. The SEM pictures are presented in 4.7.

4.3. Viscosity of the screen printing paste As described in 2.4.2. it is required that the paste exhibits a lower viscosity at higher shear rates in order to move through the screen by applying the squeegee stroke. After applying high shear rates like during the printing the viscosity of the paste stays low for a while to enable a levelling process on the substrate. After the levelling process the paste has to return into its original state in a hysteresis loop to form a stable film. A plot of the viscosity in dependence of the shear rate is given in Fig.28 (red curve) which reflects this thixotropic behaviour required for a levelling process. The paste was taken for the measurement at the state of the oil point. The viscosity at low shearing rates applied to the paste were measured at the oil point and at the stage ready for printing (see 3.2.) and determined to be ~ 40 Pa*s and ~ 10 Pa*s respectively.

54

Fig.28 : Viscosity vs. shearing rate (red curve)

55

4.4. Conditions for a regular screen printing procedure As mentioned before in 2.4.2. two conditions were given for a regular screen printing process. The condition for a complete deposit of the paste on the substrate and to empty the meshes completely was given by :

v0

1560Pa

d R2 dF

l SR lA

;

(26)

The right side of this relation was determined to be 33.25 N/m by taking the minimum value 13.3 cm of the distance lSR of screen frame and the squeegee blade and substituting the other parameters determined by the screen design. The minimum value of the distance of frame and blade had to be taken as it was varying during a print stroke and the relation had to be validated for the whole print stroke. The viscosity of the paste was determined to be 10,0 ± 0.05 Pa*s and the speed of the squeegee blade was 0.125 m/s which gave a value of 1.25 ± 0.057 N/m for v0. Thus, it can be stated that the condition of complete deposit of the paste was fulfilled for the screen printing processes carried out. A second condition for a regular printing was given for the screen to perform an immediate snap off the substrate right behind the moving squeegee blade. This condition was given by :

v0

256

lA l SR

(m d F )

l SR lA

S

( 2m d F ) p A

;

(27)

or by substituting the known parameter (numbers without standard units) :

v0

4.51 *10 9 *

1 lSR

(1.28 *10 8

T

) 2 *10 4 * lSR

;

(36)

It is clear that this relation makes sense only if the right side is a positive number. This was found to be the case only if

T

> 1.279*10+11 Pa for any value of lSR. This

corresponds to a screen strain of 2594 %. This strain is senseless and can never be

56

achieved with a screen. Equation (27) could thus not be validated and was not applicable to the screen printing procedure used in this project. The screen strain was calculated from the stress divided by the Young's modulus of the screen which was determined using the Mecmesin force gauge facility. The value obtained for the Young’s modulus was 4.9 ± 0.05 *10+9 N/m2.

4.5. Optimisation of the heat treatment of screen-printed films In order to gain a dense and closed film surface of the screen-printed NiMn2O4 films the heat exposure was minimised during drying and the thermal treatment. The samples were first dried in order to remove the organic solvents and the dispersing agent. Then they were thermally treated to decompose the organic binder, densify the films and assure a good adhesion of substrate and film after printing (see 3.5.). Similar to the decomposition of the mixed oxalate powder it could be shown that with less heat exposure a smaller average grain size of the material on the film surface could be observed resulting in a film with less pores and holes. The heating rate was minimised similar as for the coprecipitated oxalates (3.5.). In order to decompose and evaporate the vehicle effectively after printing a thermal gravimetric analysis (TGA) was carried out for the pure vehicle. As can be seen from the graph given in Fig.29 two major steps indicate the decomposition.

Fig.29 Thermal gravimetry analysis (TGA) of the vehicle 57

It is supposed that the first step is due to the evaporation of the organic solvents and the second step because of the organic binder material, which was cellulose. From the spectrum it could be concluded that the vehicle contains 93.96% wt. organic solvents and 5.95% wt. organic binder. The remaining 0.09 % represent not decomposable residues or are caused by the inaccuracy of the apparatus. For the optimum decomposition conditions high heating rates were chosen at temperatures with no reactions occurring and vice versa leading to the optimum heating program given in Table 3 (p.45). The dispersing agent in a part of the samples decomposed at similar temperatures as the organic binder, thus no further adjustment of the heating procedure was necessary. According to the manufacturer, at temperatures above 600 0C the glass phase is not in a rigid phase anymore. By further increase of the temperature the viscosity is decreasing leading to a so-called liquid-phase-sintering. The high heating rates at these temperatures with a sudden melting process did not cause any problems. The phase purity of the films with and without glass phase was proved by XRD analysis from which it could be seen that phase pure NiMn2O4 films were produced. The peaks of the Al2O3 thick film substrates could be seen as well. There was no prove for any possible reaction of the NiMn2O4 source powder with any of the components added or the substrates.

4.6. Film profile As described in 2.4.1. the theoretical film thickness can be calculated in order to determine the thickness of the films in theory directly after printing. However, after the thermal treatment of the printed films the thickness was lowered compared to the theoretical film thickness of 51 m calculated from equation (25) (p.22). In Fig.30 and Fig.31 a scan over a printed film of squared shape, one without glass phase and one including glass, is plotted. The substrate was chosen as zero level. The average film thickness of a film without glass phase was 28.7 m (Fig.30) and 25.0 m for a sample with glass (Fig.31).

58

film thickness in micrometer

45 40 35 30 25 20 15 10 5 0 0

2000

4000

6000

8000

10000

linescan

Fig.30 Line scan for a sample without glass phase sintered at 850 0C for 30 min 40 35 film thickness

30 25 20 15 10 5 0 -5 0

2000

4000

6000

8000

linescan

Fig.31 Line scan for a sample with glass phase sintered at 850 0C for 30 min

As can be seen from Fig.30 and Fig.31 the surface of the glass containing sample is smoother. This might occur, because the glass phase exhibited a low viscosity during the thermal treatment after heating what enabled a rearrangement of the grains. The reduction of the film thickness from the theoretical film thickness of 51 m to 28.7 m or 25

m can be explained by the loss of the including carrier material by

evaporation and due to densification of the film during sintering. Additionally, errors in calculating the theoretical film thickness could have occurred. The meshes might have been filled not completely or the fillings of the meshes did not have a cubic shape as assumed for the calculation. Actually one would suggest that the samples without a 59

glass phase are thinner, because of a higher amount of decomposable components in the printing paste. But the fact that the glass containing film is thinner indicates that the glass phase contracted the grains in the material and achieved a higher density of the films as desired.

4.7. Microstructure of screen printed NiMn2O4 films In Fig.32 and Fig.33 the film surfaces of screen printed films after the thermal treatment are depicted. The films contain a dispersing agent and a glass phase. It can be seen that no major holes and pores are visible and that a small grain size with a dense particle packing in the films was achieved as desired. The sample in Fig.32 was sintered at 850 0C for 10 min and for Fig.33 sintered for 30 min, both including a glass phase:

Fig.32 : SEM micrograph of a sintered surface (850 0C, 10 min holding time, with glass phase; magnification : 9000

60

Fig.33 SEM micrograph of a sintered surface (850 0C, 30 min holding time, with glass phase; magnification : 9000

It can be seen from Fig.32 and Fig.33 that for 30 min sintering time the surface is slightly denser with less pores and holes. On the bottom of the pictures a bar is indicating the size of 1 m. The average grain size can be estimated to be lower than 1 m. In Fig.34 (magnification 2500) and Fig.35 (magnification 9000) a sample without the addition of a glass phase and a dispersing agent is depicted. From both pictures it can be seen that more pores and holes occur and a bigger grain size is exhibited than in the samples depicted in Fig.32 and Fig.33. Therefore, the addition of the glass phase and dispersing agent can be regarded as an improvement in terms of the surface density (glass phase) and average grain size (dispersing agent). The samples shown in Fig.34 and Fig.35 were sintered at 850 0C for 10 min. In general, it is obvious that the glass phase had the desired effect to contract the grains and make the surface denser by allowing a rearrangement of the grains during the heat treatment when the viscosity of the glass is low. The smaller grain size might occur due to the effect of the dispersing agent which might have hindered the formation of agglomerates by the join of smaller grains. 61

Fig.34 SEM micrograph of a sintered surface (850 0C, 10 min holding time, without glass phase; magnification : 2500

Fig.35 SEM micrograph of a sintered surface (850 0C, 10 min holding time, without glass phase; magnification : 9000 62

A problem with the samples fired for 10 min was that the adhesion of substrate and film was poor. Therefore, a sintering time of 30 min is recommended to ensure a stable film by sufficient joining between the paste and the substrate. In section 4.5. it was lined out that the thermal exposure during the heat treatment after printing was minimised in order to minimise the grain size. The efficiency of such a minimisation process is demonstrated in Fig.36 and Fig.37. Fig.36 and Fig.37 show samples which were sintered for 1h and for 12 h. Both samples did not contain a glass phase and no dispersing agent. It can be seen that the grain size is bigger for longer heat exposures and that holes occur between the bigger grains. A strong grain growth of the films without a glass phase results in porous films due to the hindrance of shrinkage in the x/y- direction in films. For both samples the density of the films is not sufficient as the heat exposure neither of the source powder nor of the printed film was minimised. This indicates the importance of the minimisation of the heat treatment.

Fig.36 SEM micrograph of a sintered surface (850 0C, 1 h holding time, without glass phase; magnification : 9000

63

Fig.37 SEM micrograph of a sintered surface (850 0C, 12 h holding time, with glass phase; magnification : 9000

In Fig.38 it is demonstrated that it is important to optimise the amount of vehicle added to the powder before printing. In this case the amount of vehicle was 11 % instead of the required 14.7 %. In 3.2. it was mentioned that a too low amount of vehicle results in the screen pattern occurring on the film surface as can be seen in Fig.38.

64

Fig.38 SEM micrograph of a sintered surface (850 0C, 10 min holding time, without glass phase; magnification : 25

65

4.8. Resistance vs. temperature dependence 4.8.1. Contacts Before conducting electrical measurements the ohmic behaviour of the contacts was tested in order to detect any possible barriers as described in 2.5. This kind of barrier could falsify the resistance measurement. In Fig.39, Fig.40 and Fig.41 the current I vs. voltage V characteristics are depicted at 293.2 K, 350 K and 150 K respectively. Current vs. Voltage characteristic at 293.2K

Current vs. Voltage characteristic at 350 K 250

100 80 60

200 150 6 Current in A *10

Current in A*107

100 50 0 -150

-100

-50 -50 0

50

100

150

-100 -150 -200

-60

-40

-250 -300 Voltage in V

40 20 0 -20 -20 0 -40 -60 -80 -100 Voltage in V

Fig.39 I vs. V at 293.2 K Fig.40 I vs. V at 350 K Current vs. voltage characteristic at 150 K 4

Current in A*1010

3 2 1 0 -60

-40

-20

-1 0

20

-2 -3 -4 voltage in V

Fig.41 I vs. V at 150 K

66

40

60

20

40

60

At 293.3 K and 350 K the contacts show clearly ohmic behaviour. At 150 K slight deviations from a linear behaviour can be seen. This is supposed to be due to small variations of the temperature during the measurement. In general it can be stated that the contacts were appropriate for resistance measurements.

4.8.2. Resistance vs. temperature For the temperature range of 150 K – 350 K the measurements were carried out in the liquid nitrogen cooled cryostat as described in 3.7.3. All the samples clearly showed the expected NTC characteristic. In Fig.42 the resistance vs. temperature characteristic for a sample without a glass phase is depicted. The film was sintered at 850 0C for 10 min. The resistance was measured between 145 K and 345 K during heating up from lower to higher temperatures and during cooling down. Resistance measurements in the range of 5.6*10+5 – 2.0*10+11 Ohm at 50 different temperatures for heating up and at the same temperatures for cooling down were carried out. In order to validate one of the models for the resistivity described in 2.2.3. several ways of plotting the graph were attempted. The best agreement was found for relation (18a) for a variable range hopping with an energy dependent density of states which was given by :

 0T exp(T0/T)½ ;

(18a)

By plotting ln(resistance*temperature-1) versus 1/temperature1/2 (see Fig.42) one would expect a linear graph with the slope T0. For assessing the linearity a linear graph is plotted additionally. From Fig.42 a good linearity of the curve is obvious. Slight deviations from linearity especially for lower temperatures might occur due to the calibration of the thermocouples, which might not have been exactly linear. Two curves were plotted, one for the resistances measured during heating up and one for cooling down, but the two different curves can not be dissolved and no differences in resistance for heating up and cooling down can be observed. This is an indication for reliable resistance measurements and in general a good reproducibility. The slope of the curve resulted in a value of 2.07 105 K for T0.

67

Fig.42 ln(resistance*temperature-1) versus 1/temperature1/2 ----- : experimental curve ----- : linear curve

The characteristic temperature T0 for this model was given by equation (18b) :

T0 

1e 2 ; k B  a0

(18b)

From this expression an estimate for can be made. The value obtained is:

 = 4.73 10-10 C 2 / J m. As  = 4 r0the expression r * a0 can be regarded as an effective Bohr’s radius which was calculated to be 2.25 *10-10 m. This is a reasonable value for an effective Bohr’s radius. Note that for small distances the apparent value for r = 4.25 does not necessarily correspond to the macroscopic value r of the bulk material. Furthermore, dependence (15a) was tested by plotting ln(resistance*temperature-0.5) vs. 1/temperature1/4. This dependence was suggested by Mott [16] for a variable range hopping

68

assuming a constant density of states, but from Fig.43 it is obvious that relation (18a) plotted in Fig.42 shows a better linearity and the density of states can be assumed to be energy dependent.

0 T 1/2 exp(T0/T)1/4 ;

(15a)

For demonstration purposes a linear graph is plotted as well.

Fig.43 ln(resistance*temperature-0.5) vs. 1/temperature0.25 ----- : experimental curve ----- : linear curve In this case T0 would be 2.75 * 109 K. For the variable range model given by (15a) the characteristic temperature T0 was given by equation (15b) : 69

T0 

 3 k B g (  ) a0

;

(15b)

From this equation g() was estimated to be 6.00*1020 cm-3(eV)-1. Other dependencies of resistance and temperature were tested as well, but did not result in a better linearity. In Fig.44 ln(resistance*temperature-1) vs. 1/temperature was plotted to test the relation (22) given by:

   0T

 E  1  exp NC (1  C )  k BT 

(22)

Again, a linear plot is given to demonstrate the deviations from linearity for this model.

Fig.44 : ln(Resistance*Temperature-1) vs. 1/Temperature ----- : experimental curve ----- : linear curve

70

It is obvious that no linear behaviour is exhibited and the model is not valid. However, the activation energy E was determined by attempting to fit a linear line to the data. An activation energy of 0.29 eV was obtained which is in good agreement with the value of 0.37 eV reported by Brabers [2]. Additionally, the empirical relation (19) was tested by plotting ln(resistance*temperature4.83) vs. 1/temperature (Fig.45).

 0T – 4.83 exp(E/kBT);

E = 0.178 eV

(19)

The linearity can be assessed by the linear graph.

Fig.45 : ln(resistance*temperature4.83) vs. 1/temperature ----- : experimental curve ----- : linear curve 71

Dependence (19) was an empirical relation and a linear graph is observable for higher temperatures, but there is no theoretical justification for this dependence. However, the slope of the curve gave an activation energy E = 0.178 eV which is exactly the same value suggested for this relation (Becker et al. [18]). This exact agreement might be by chance, but it shows that the electrical conduction observed in this project corresponds with the electrical characteristics observed by other authors. The value obtained by Becker was found for bulk material, which indicates that in thick screen-printed films the electrical conduction does not exhibit any effect because of the film shape of the material.

72

4.8.3. Deviations from the regular NTC characteristic due to thermally activated cation migration The resistance vs. temperature dependence for higher temperatures (293 K – 590 K) was measured using the calorimeter described in 3.7.3. In Fig.46 the resistance vs. temperature is plotted for heating up from 343 K to 593 K and cooling down to 293 K (room temperature) for a sample without glass phase as depicted in Fig.35. The resistance was in the range of 7.3*104 Ohm – 1.12*108 Ohm. The upper part of the curve was observed for heating up and the lower part for cooling down.

Fig.46 ln(resistance*temperature-1) vs. 1/temperature0.5 heating up and cooling down; sample without glass phase

It can be seen that for heating up the resistance is higher than for cooling down. The linearity of the graph is better for cooling down. The hysteresis effect would suggest that during heating up the cation configuration is changing leading to a lower resistance 73

and a changing slope of the curve due to varying T0. During cooling down the cation distribution does not change anymore and therefore a linear graph occurs. In Fig.47 the equivalent measurement for a sample treated in the same way as in Fig.46 was plotted, but the maximum temperature was only 440 K. The resistance was between 2.2*106 Ohm - 1.04*108 Ohm.

Fig.47 ln(resistance*temperature-1) vs. 1/temperature0.5 Heating up and cooling down; sample without glass phase

From Fig.47 it can be concluded that no migration of cations took place as the curves for heating up and cooling down are very similar. Therefore it is evident that the cation migration starts to take place between 443 K and 593 K (170 0C and 320 0C). This cation migration has serious impact on the stability and reproducibility of the sensor. It is clear that a change in the cation distribution alters the electrical properties of NiMn2O4.

74

4.8.4. Effect of the glass phase As a part of the samples was produced including a glass phase the influence of the glass on the resistance was determined. In Fig.48 the resistance vs. temperature data for a glass containing sample as shown in Fig.33 is given for an equivalent measurement as presented in Fig.42 for a sample without a glass phase. The measurement was carried out between 122 K and 345 K for heating up and cooling down using the liquid nitrogen cooled cryostat giving a resistance of 1.33*106 Ohm – 1.51*1011 Ohm. The curve from Fig.42 for the sample without a glass phase is plotted as well.

355 K

275 K

203 K

155 K

124 K

0.08

0.09

23 21

ln(R*T

-1

)

19 17 15 13 11 9 7 5 0.04

0.05

0.06 1/T

0.07

0.5

in 1/K

0.5

Fig.48 ln(Resistance*Temperature-1) vs. 1/Temperature0.5 Upper curve : sample with glass phase; sintered at 850 0C for 30 min Lower curve : sample without glass phase; sintered at 850 0C for 10 min

A very good linearity for the graph with glass can be observed which indicates that the glass phase does not modify the type of conduction mechanism. The slope of the graph with glass and therefore T0 was determined to be 1.9 *105 K. This is lower than the value T0 = 2,07*105 K obtained for the samples with no glass. The resistance was higher with included glass phase. At room temperature (292.2 K) the resistance for samples with glass was 0.94 * 107 Ohm and without glass it was 0.38 * 107 Ohm. Obviously less hopping paths in the sample with the glass phase are available. 75

5. Discussion In this work it has been shown that NTC thermistor films on the basis of NiMn2O4 could be produced via screen-printing. Phase pure and stoichiometric NiMn2O4 was mixed with several components forming a printable paste. The addition of a glass phase can be regarded as a major improvement in terms of a more even film and especially a denser film surface with less pores and holes. By an appropriate thermal treatment of the dried screen-printed films, dense and phase pure films could be obtained. The films exhibited a good adhesion to the alumina substrates which were chosen in thick film quality. The resistance vs. temperature characteristics were influenced by the glass phase only by a reduction of the conductivity and a decrease of the exponential factor T0. The type of transport mechanism was not affected which was found to rely on a variable range hopping.

5.1. Synthesis of NiMn2O4 source powder The production process of NiMn2O4 via co-precipitated nickel and manganese oxalate can be regarded as successful. Calcination under optimised conditions results in phase pure NiMn2O4 powder. Slight deviations of stoichiometry might well be caused by the inaccuracy of the stoichiometric measurement. For future work this synthesis process for NiMn2O4 is recommended due to the high purity and accuracy of titration methods leading to a precise stoichiometry. The average grain size obtained was sufficiently small and the grain size distribution was appropriate as no major agglomerates occurred. Furthermore by the method of coprecipitation there is additional potential for including dopants into the system. E.g. copper dopants can improve the conductivity and barium doping can minimise ageing effects. A problem with the stoichiometry could be that due to a loss or excess of oxygen the compound has to be represented by NiMn2O4 x. It was reported ([1],[3]) that these deviations do not occur in the temperature range where NiMn2O4 was produced for this study ( 850 0C), but the exact determination of the oxygen content should be carried out for a complete confirmation of the appropriate stoichiometry in NiMn2O4. The oxygen content can be determined if the average oxidation state of the manganese atoms is known. This information can be obtained by titration methods. 76

The grain size of the source powder could be minimised successfully by minimising the thermal exposure concerning temperature and time and by milling procedures of the synthesised powder. It was shown that the average particle size is less than 1 m. By chemical vapour deposition of NiMn2O4 a lower average grain size can be obtained, but nevertheless for screen printing procedures starting from an inorganic powder the achieved particle size is very low.

5.2. Screen-printing It can be stated that highly controlled films of NiMn2O4 can be obtained by screen printing procedures. The paste was optimised in terms of dispersion of the powder and appropriate viscosity for regular printing processes. The resulting films showed a good surface density when dispersing agent and a glass phase were added to the paste. It is supposed that the dispersing agent could further minimise the degree of agglomeration and that the glass phase contracts the particles during heat treatment. The addition of the glass phase also smoothes the surface as the glass was of low viscosity during the heat treatment of the printed films (liquid phase sintering). The glass causes a microscopic flow of the film forming a more dense and even surface. Additionally, the glass phase leads to a better stability between film and substrate by forming bondings with the glass content of the substrates. Due to time limitation of this work only an amount of 8% wt. of glass phase could be tested. It might be interesting to test the effect of different ratios of glass powder and NiMn2O4. The addition of the dispersing agent Hypermer was troublesome, so different agents could be tested to minimise the problems encountered with dissolving the agent and cover the NiMn2O4 particles with the dispersing agent. But in general it can be stated that the addition of a dispersing agent and a glass phase is of great advantage. Furthermore, from the good printing results it can be concluded that the used commercial vehicle was appropriate enabling the paste to exhibit the desired thixotropic behaviour. The screen turned out to be suitable as well, whereby screen material, mesh size and screen tension were appropriate to ensure regular printing procedures. The final sintered film thickness was about 25 m as desired.

77

For a regular printing process two contradictory conditions (26), (27) were given [33], but only one of them (26) was applicable, the second condition (27) was senseless for the printing process used in this project. The snap off distance, the speed of the print stroke and the viscosity of the paste were adjusted empirically what suggests that screen printing is a rather empirical process and the conditions (26) and (27) are of limited use for the praxis. By the thermal treatment after printing the carrier material was decomposed completely and the sintering process resulted in denser films and in a strong joining between film and substrate.

5.3. Electrical measurements The fact that different dependencies of the resistance upon the temperature could be analysed in detail indicates that computer controlled resistance vs. temperature measurements are very accurate. The linearity of the curve obtained for the appropriate dependency was very obvious and the expected NTC characteristic could be examined in detail. The variable range hopping given in equation (18a) was the dependence, which could fit the experimental data best. The value obtained for T0 for NiMn2O4 was 2.07 105 K and

was determined to be

= 4.73 10-10 C 2 / Jm..

The small polaron-hopping model described (Fritsch, [21]) could not be validated, but the constant factor

0

in equation (22) seems to be conclusive.

However, intense theoretical work would be necessary to deduce a complete theoretical explanation for the conduction mechanism in NiMn2O4. By doing this it should be taken into account that hopping events from tetrahedral to octahedral sites could occur. It would also be interesting to carry out impedance measurements. The contribution to the conductivity of hopping over grain boundaries and hopping inside a grain could be separated and analysed in detail. Electrode effects could be displayed as well. However, it could be shown that the glass phase did not influence the type of conduction mechanism as the resistance vs. temperature behaviour followed the same law, but with a 100% higher resistance for glass containing samples. The glass containing layers were slightly thinner than layers without glass, so one could expect an increase of the resistance of only a few percent according to the reduction in thickness. From the observed increase it can be concluded that the insulating glass phase had a 78

perceptible impact on the conductivity. But regarding that the resistance is changing over several orders of magnitude with varying temperature this change is acceptable.

5.4. Future work As described in section 2.1.1. Brabers [2] suggested that a significant cation migration starts to take place at temperatures over 720 K. In contrast to this, deviations from the regular resistance characteristics were observed to start between 440 K and 600 K. For future work it would be a step forward to determine the exact temperature or temperature region when the cation migration starts to take place. For temperature sensing applications it is of vital importance to avoid such resistance deviations from the regular NTC characteristic. It has to be examined if by heating up to 600 K the change in the cation distribution is stable at lower temperatures enabling further accurate measurements up to this temperature. For the electrical measurements two-point-measurements were conducted only. In order to verify the data and obtain even more precise measurements, four-point measurements in the van-der-Pauw configuration should be carried out. It is supposed that the resistivity of the films is affected severe by oxidation or by effects of possible pores. The humidity or other properties of the surrounding area might influence the electrical conduction. Thus, it is planned to cover the films with a thin layer of an appropriate resin to protect them against undesirable effects from the atmosphere. Although a good density of the film surface could be observed, nevertheless few pores and holes are remaining in the film (see Fig.32/Fig.33). In order to examine the effect of such pores in more detail the films could be characterised before and after water absorption through the pores to assess their impact. In addition to the obvious cation migration between 440 K and 600 K, the resistance might be influenced by slow cation migrations between A and B lattice sites even at room temperatures. This ageing effect is a severe problem for applying NiMn 2O4 thermistor devices for accurate temperature measurements. As mentioned above the addition of barium dopants is promising. As a further dopant copper might be used to lower the resistance and increase the number of available hopping paths.

79

6. Conclusion It can be concluded that screen printing is an appropriate deposition technique for producing a reliable thick film temperature sensor on the basis of NiMn2O4. Alternatively to electron beam evaporation or sputtering techniques no problems with the stoichiometry occurred. The printing process proved to be controllable whereby the addition of a glass phase had positive effects on the films. The films showed the desired NTC characteristic and the conduction mechanism was found to rely on a variablerange-hopping mechanism. The conduction mechanism of nickel manganate was matter of controversial discussion in the past and it requires more theoretical effort to explain the conduction satisfactory. In order to produce an applicable temperature sensing device more research is necessary to establish an appropriate procedure to cover the films with a protective cover using a suitable material. The addition of dopants like barium or copper might further improve the temperature sensing performance. Furthermore the effect of different amounts of glass in the films should be investigated. In order to assess and improve the temperature sensing performance of NiMn2O4 thermistors the ageing effects should be examined in detail.

80

7. References [1] E.D. Macklen, J.Phys.Chem.Solids, 47, No.11 (1986) 1073-1079 [2] V.A.M. Brabers, J.C.J.M. Terhell, Phys.Stat.Sol.(a), 69 (1982) 325-332 [3] A. Feltz, J. Toepfer, Z.anorg.allg.Chem., 576 (1989) 71-80 [4] D.G. Wickham, J.Inorg.Nucl.Chem., 26 (1964) 1369-1377 [5] A.P.B. Sinha, N.R. Sarjana, A.B. Biswas, Acta Cryst., 10 (1957) 439 [6] A. Basu, First Year Report of PhD, Department of Physics, University of Durham (2000) [7] M. Kamiyama, Z. Nara, Oyo Butsuri, 21 (1952) 400 [8] A. Gorgeu, Bull. Soc. Chim.France(3), 29 (1903) 1111/7, 1116 [9] M. Beckham, 4H project, Department of Physics, University of Durham (1999) [10] E.D. Macklen, Thermistors, Electrochemical Publications, Glasgow (1979) [11] S. Baliga, A.L. Jain, Materials Letters, 9 (1990) 180-184 [12] M. Suzuki, J.Phys.Chem.Solids, 41 (1980) 1253-1260 [13] B. Gillot, M. Kharroubi, R. Metz, R. Legros, A. Rousset, Solid State Ionics, 44 (1991) 275 [14] H.L. Tuller, A.S. Nowick, J.Phys.Chem.Solids, 18 (1977) 859-867 [15] M.L. Knotek, M. Pollak, Philos.Mag., 35 (1977) 1133 [16] N.F. Mott, J.Non-Cryst.Solids, 1 (1968) 1 [17] R. Mansfield, Hopping Transport in Solids, ed. M. Pollak, B.Shklovskii, p.349375, North Holland, Amsterdam (1991) [18] J.A. Becker, C.B. Green, G.L. Pearson, Bell Syst.Tech.J., 26 (1947) 170 [19] G. Bossom, F. Gutmann, L.M. Simmons, J.Appl.Phys., 21 (1950) 1267 [20] A. Feltz, J. Toepfer, F. Schirrmeister, J.Europ.Ceram.Soc., 9 (1992) 187-191 [21] S. Fritsch, J. Sarrias, M. Brieu, J.J. Couderc, J.L. Baudour, E. Snoeck, A. Rousset, Solid State Ionics, 109 (1998) 229-237 [22] S. Baliga, A.L. Jain, Materials Letters, 11, No.5,6,7 (1991) 226-228 [23] A. Rousset, R. Legros, A. Lagrange, J.Europ.Ceram.Soc., 13 (1994) 185-195 [24] P. Castelan, Bui Ai, A. Loubiere, Sensors and Actuators, 33 (1992) 119-122 81

[25] M. Parlak, Project Report, Department of Physics, University of Durham (1998) [26] R. Schmidt, A.W. Brinkman , International Journal of Inorganic Materials, 3, 8 (2001) 1215-1217 [27] R. Schmidt, First Year Report of PhD, Department of Physics, University of Durham (1999) [28] D. Beet, 4H project, Department of Physics, University of Durham (1999) [29] M. Parlak, T. Hashemi, M.J. Hogan, A.W. Brinkman, Thin Solid Films, 345 (1998) 307 [30] S. Baliga, A.L. Jain, Materials Letters, 8 (1989) 175-178 [31] F. Lindner, A. Feltz, J.Europ.Ceram.Soc., 11 (1993) 269-274 [32] J. v.Duppen, Handbuch fuer den Siebdruck, Verlag Der Siebdruck, Lübeck [33] D.E. Riemer, Ein Beitrag zur Untersuchung der physikalisch-technischen Grundlagen des Siebdruckverfahrens, Dissertation, Technische Universität Berlin (1988) [34] F. Franconville, K. Kurzweil, S.G. Stalnecker, Solid State Technology, (Oct. 1974) 61-68 [35] A. Huber, Das Keramiker-Jahrbuch 1997, (1997) 37-45 [36] R.E. Trease, R.L. Dietz, Solid State Technology, (Jan. 1972) [37] J.P. McKelvey, Solid State and Semiconductor Physics, Harper & Row, New York (1966) [38] CRC Handbook of Chemistry and Physics, ed. R.C. Weast, table E-81, CRC Press, Cleveland (1977-1978) [39] Reagenzien Merck, Komplexometrische Bestimmungsmethoden mit Titriplex, E. Merck, Darmstadt (1988) [40] Jander, Blasius, Lehrbuch der analytischen und präparativen anorganischen Chemie, S. Hirzel Verlag, Stuttgart (1989) [41] P. Fau, J.P. Bonino, J.J. Demai, A. Rousset, Applied Surface Science, 65/66 (1993) 319-324 [42] DEK Model 240, Installation and Operating Instructions, DEK Printing Machines Ltd., No.240M/4M, London (1974) [43] Oxford Laboratory Cryostats, Instruction Manual, Oxford (1987) [44] B.I. Shklovskii, A.L. Efros, Electronic Properties of Doped Semiconductors, Springer-Verlag, Berlin1984 82

Acknowledgement I wish to thank my supervisors Dipl.Ing. A. Stiegelschmitt and Prof.Dr. A. Roosen for their help which was of invalueavble asset. I also wish to thank my supervisor Dr. Brinkman for his great support. Furthermore thanks must be awared to Prof.Dr. Döhler for his advice and help. Thanks to Hanna Strelec for help with the XRD and for providing the SEM facility, Matthias Wagner for conducting UBM measurements, Sabine Brungs for the thermal analysis data, Andreas Thomsen for viscosity measurements and Sieglinde Elsesser and Evelyne Gruber for allowing use and for help with chemical equipment. Thanks to Eva Springer for photos, to Norbert Müller for using computer facilities, to Kurt Sandner and Peter Reinhardt for technical support and to everybody I have forgotten. I want award a special thanks to all my colleagues for help and for the good atmosphere at the department in Erlangen as well as in Durham, Arnab Basu, Peter Cromme, Jürgen Zeschky, Thomas Hassel, Christian Keintzel, Jürgen Bauer and Uli Deisinger. I want to thank my parents for all their support and their help. Last but not least I thank all my friends for helping me and for their friendship, Veit Rössner, Ali Hajighasemi, Thomas Winiecki and Angel Galmiche-Tejeda.

83

Declaration I declare that this report is result of my own work without the help of any other person. Informations or results from others are declared and listed in the references.

Rainer Schmidt

Erklärung Hiermit erkläre ich, dass diese Arbeit Resultat meiner eigenen Bemühungen ist und ohne fremde Hilfe angefertigt wurde. Informationen oder Resultate anderer habe ich dementsprechend deklariert und unter References aufgelistet.

Rainer Schmidt

84

8. Appendices 8.1. Grain size distributions (granulometric analysis)

86

8.2. X-Ray spectra (X-Ray diffraction analysis)

91

8.3. Thermal analysis (DTA)

99

85

8.1. Grain size distributions 8.1.1. Sintered at 750 0C; 1 hour

Average grain size : 1.9 m 8.1.2. Sintered at 770 0C; 1 hour

86

Average grain size : 3.8 m 8.1.3. Sintered at 800 0C; 1 hour

Average grain size : 4.3 m

8.1.4. Sintered at 850 0C; 1 hour

Average grain size : 4.5 m 87

8.1.5. Sintered at 850 0C; 2 h

Average grain size : 5.4 m 8.1.6. Sintered at 850 0C; 3 hours

Average grain size : 7 m 88

8.1.7. Sintered at 850 0C; 6 hours

Average grain size : 9.5 m

8.1.8. Sintered at 850 0C; 12 hours

Average grain size : 13.6 m

89

8.1.9. Sintered at 850 0C; 2 hours ; tungsten-carbide milling

Average grain size : 2.8 m

90

8.2. X-Ray spectra 8.2.1. powder sintered at 650 0C for 6h

92

8.2.2. powder sintered at 750 0C for 6h

93

8.2.3. powder sintered at 800 0C for 6h

94

8.2.4. powder sintered at 850 0C for 30 min, High resolution scan 95 8.2.5. powder sintered at 950 0C for 6 h, High resolution scan

96

8.2.6. powder sintered at 750 0C for 1h

97

8.2.7. powder sintered at 770 0C for 1h

98

91

X-Ray scan NiMn 2O4 powder sintered at 650 0C for 6 hours

250 intensity

200 150 100 50 0 5 = NiMn2O4 (1-1110 JCPDS)

25

45 angle 2-theta

= NiMnO3 (12-269 JCPDS)

= Mn2O3 (10-69 JCPDS)

92

65

X-Ray scan NiMn 2O4 powder sintered at 750 0C for 6 hours

900 intensity

700 500 300 100 -100

5

25

45 angle 2-theta

= NiMn2O4 (1-1110 JCPDS)

93

65

X-Ray scan NiMn 2O4 powder sintered at 800 0C for 6 hours 800 700

intensity

600 500 400 300 200 100 0 0

20

40 angle 2-theta

= NiMn2O4 (1-1110 JPCDS)

94

60

80

High resolution X-Ray scan of NiMn2O4 powder sintered at 850 0C for 30 min 200 180 160 140 120 100 80 60 40 20 0 35,01

37,01

39,01

41,01

= NiMn2O4 ( 1-1110 JCPDS )

95

43,01

45,01

High resolution X-Ray scan of NiMn2O4 powder 0

sintered at 950 C for 6 hours

250

intensity

200 150 100 50 0 35,01

37,01

39,01 41,01 43,01 angle 2-theta

= NiMn2O4 (1-1110 JCPDS) = NiO (4-835 JCPDS)

96

45,01

X-Ray scan NiMn 2O4 powder sintered at 750 0C for 1 hour

700

700 600

600

500

intensity

400

500

300 200

400

100 0 29

31

33

35

37

300 200 100 0 10

30

50 angle 2-theta

= NiMn2O4 (1-1110 JPCDS) = Mn2O3 (10-69 JPCDS)

97

70

X-Ray scan NiMn 2O4 powder sintered at 770 0C for 1 hours

1200

intensity

1000 800 600 400 200 0 25

30

angle 2-theta

= NiMn2O4 (1-1110 JPCDS) = Mn2O3 (10-69 JPCDS)

98

35

40

8.3. Thermal analysis (DTA)

99

Thermal analysis (DTA) of the mixed oxalate

100