Effect of Co3O4 doping and sintering temperature on

Effect of Co3O4 doping and sintering temperature on optical energy band gap properties in Zn-Bi-Ti-O varistor ceramics Mohd Sabri Mohd Ghazali, Wan Rafizah Wan Abdullah, Azmi Zakaria, Muhamad Azman Zulkifli, Mohd Hafiz Mohd Zaid, and Zahid Rizwan

Citation: AIP Conference Proceedings 1885, 020124 (2017); doi: 10.1063/1.5002318 View online: http://dx.doi.org/10.1063/1.5002318 View Table of Contents: http://aip.scitation.org/toc/apc/1885/1 Published by the American Institute of Physics

Effect of Co3O4 Doping and Sintering Temperature on Optical Energy Band Gap Properties in Zn-Bi-Ti-O Varistor Ceramics Mohd Sabri Mohd Ghazali1,a), Wan Rafizah Wan Abdullah2,b), Azmi Zakaria3,c), Muhamad Azman Zulkifli1,d), Mohd Hafiz Mohd Zaid3,e) and Zahid Rizwan4,f) 1

Advanced Nano-materials Research Group, School of Fundamental Science, Universiti Malaysia Terengganu, 21030 Kuala Nerus, Terengganu, Malaysia 2 School of Ocean Engineering, Universiti Malaysia Terengganu, 21030 Kuala Nerus, Terengganu, Malaysia 3 Department of Physics, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia 4 Department of Applied Sciences, Convener Purchase, National Textile University, Faisalabad (37610) Pakistan Corresponding author: a)[email protected] b) [email protected] c) [email protected] d) [email protected] e) [email protected] f) [email protected]

Abstract. It is necessary to investigate the electronic states of ceramic based ZnO vasristor and effect of doped impurities at different concentration. Band gap (Eg) of the ceramic (99-x) mol% ZnO+0.5 mol% Bi2O3+0.5 mol% TiO2+ xCo3O4 where x = 0, 0.2, 0.4, 0.6 and 0.8 mol%, were determined using UV-Vis spectrophotometer. The samples were prepared via solid-state route and sintered at the sintering temperature at 1110, 1140 and 1170 oC for 45 and 90 min in open air. At no doping of Co3O4, the values of Eg are 2.991 ± 0.001, 2.989 ± 0.001 eV for 45 and 90 min sintering time; respectively. Eg was decreased to 2.368 ± 0.002 and 2.352 ± 0.001 eV at 0.8 mol% Co3O4 for 45 and 90 min sintering time; respectively. XRD analysis indicates that two main phases existed at all concentrations which are ZnO and secondary phases, Bi12TiO20, Zn2Ti3O8, ZnCo2O4 and Co3Ti3O. Relative density decreases with the addition of Co3O4 compared to that of undoped at all doping level. When Co3O4 is added in the ceramics, relative density increases with the increase of doping level at both 45 and 90 min sintering time. The variation of sintering temperatures and XRD findings of steepness factor are correlated with the UV-Vis spectrophotometer results of based ZnO varistor doped with Co3O4 due to the growth of interface states.

INTRODUCTION Zinc oxide (ZnO) varistors are polycrystalline ceramics that consist of ZnO as the base and a couple of dopants as an unique feature of grains and grains boundaries is created in the ceramics during sintering [1, 2]. The varistors are useful for protecting a variety of electrical equipment’s against over voltage and they manage to operate efficiently without damage. Nowadays, rapid developments of micro-electronic technology and large-scale integrated circuits have encouraged the production of this device for low-voltage applications in automobile and small semiconductor electronics application [3, 4].

3rd Electronic and Green Materials International Conference 2017 (EGM 2017) AIP Conf. Proc. 1885, 020124-1–020124-8; doi: 10.1063/1.5002318 Published by AIP Publishing. 978-0-7354-1565-2/$30.00

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They are fabricated with different types of dopants such as Bi 2O3 [5, 6], TiO2 [7], MnO2 [8, 9], Co3O4 [10] and CaMnO3 [11]. The feature of unique grain boundary is responsible for nonlinear current-voltage (I-V) characteristics of the device [2, 12] and thus, it is used to protect electrical surges. I-V studies have been enormously investigated for ZnO based varistor by previous researchers for four decades [7, 13] and it is necessary to study the electronic states of ceramic ZnO and the effect of doped impurities at different processing conditions. The characterization of the absorption spectrum in semiconductors leads to the determination of the optical band-gap energy [14, 15]. The investigation of optical band gap energy of ZnO-Bi2O3-TiO2 doped Co3O4 with different sintering temperatures of 1110, 1140 and 1170 oC for 45 and 90 min in air was studied.

EXPERIMENTAL All oxides precursors’ of 99.9% purity (Alfa Aesar, USA) were used. The composition consists of (98-x) mol% ZnO + 0.5 mol% Bi2O3 + 0.5 mol% TiO2 + x Co3O4 where x = 0, 0.2, 0.4, 0.6 and 0.8 mol%. The powder was ball milled for 24 hours in deionized water. The slurry was dried at 70 °C using hot plate and continuously magnetically stirred to avoid the sedimentation of the heavy particle and then pre-sintered at 800 °C for two hours in open air with heating and cooling rate of 6 °Cmin-1. The pre-sintered mixture was pulverized using an agate mortar/pestle and next after addition of 1.75 wt.% Polyvinyl Alcohol binder, the sample was dried, grinded and granulated by sieving 75 micron mesh screen. The mixture was then pressed into discs of 10 mm in diameter and 1 mm in thickness, each at a pressure of 2 ton/m2. Finally, the discs were sintered at 1110, 1140 and 1170 °C in open air for 45 and 90 min sintering time at heating and cooling rate of 2.66 °C min-1. The disk from each sample was ground for optical and XRD characterizations. The crystalline phases were identified by an XRD (PANalytical X’Pert Pro PW3040/60, Philips) with CuKα radiation and the data were analyzed, using X’Pert High Score software. According to Keskenler and co-researcher [16], structural disordering can be obtained by Urbach’s Rule for the optical absorption (exponential tail) [17]. The density was measured by the geometrical method taking the average of 10 discs [18, 19]. Each of the disk samples was thermally etched at 150 °C in a tube furnace for the microstructure analysis. The microstructure morphology of sintered discs was examined by Scanning Electron Microscopy (JEOL, Model JSM 6460). The average grain size (d) was determined by lineal intercept method [20], given by Equation 1: d = 1.56 L/MN

(1)

where L is the random line length on the micrograph, M is the magnification of the micrograph and N is the number of the grain boundaries intercepted by lines. The UV-Vis spectrophotometer was used to measure the optical bandgap energy of the ceramics. The transmission signal was measured for the wavelength from 200 to 800 nm and then converted to absorption signal for further evaluation [21]. It was assumed that the fundamental absorption edge of the ZnO based varistor ceramics is due to the direct allowed transition [22]. The optical band-gap energy is given by Equation 2 [23]: (Ahυ)2 = C(hυ-Eg)

(2)

where A is the optical absorption coefficient, C is the constant independent of photon energy (hυ), and Eg is the direct allowed optical energy band-gap. The value of Eg is obtained by using Origin Pro 8.0 software within the linear fitted regions at (Ahυ)2=0.

RESULTS AND DISCUSSION Crystalline Phase, Relative Density and Microstructure Figure 1 shows the XRD patterns for ZnO varistors introduced with various concentrations of Co3O4. The patterns indicate the presence of ZnO phase (ICSD code: 067454) and secondary phases in the form of Bi12TiO20 (ref. code: 00-034-0097), Zn2Ti3O8 (ref. code: 00-013-0471) and Co3Ti3O (ICSD code: 029054) spinels in all studied samples. On the other hand, ZnCo2O4 (ref. code: 00-023-1390) spinel was only detected in varistors added with 0.2 and 0.4 mol% of Co3O4. At higher concentration, the formation of this spinel phase was suppressed. The variation in relative density of ZnO varistor ceramics sintered at different temperature and time are revealed in Fig. 2. Generally, the relative density of the ceramics reduced with incorporation of Co 3O4. Undoped ZnO ceramics exhibited higher density value compared to doped ceramics. Application of shorter sintering time (45 min)

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caused the relative density to gradually increase with more addition of Co3O4. No obvious changes in relative density with sintering temperature was observed. This indicates that the Co3O4 component facilitated densification process through pore elimination. Extension of sintering time to 90 min somehow revealed the dominant effect of sintering temperature. The higher the sintering temperature applied, the lower the relative density of the ceramics would be. Prolonged heat treatment and sintering temperature creates or traps more pores inside the ceramics. The varistor ceramic with optimum relative density was obtained by applying sintering at 1140 oC for 45 min or at 1110 o C for 90 min.

i ZnO

x Co3Ti3O

j ZnCo O 2 4

k Bi TiO 12 20

Intensity (a.u.)

h Zn Ti O 2 3 4

0.8 mol%

xk

xh

0.6 mol%

xk

xh

xk

jh

0.4 mol% 0.2 mol%

20

i

30

i

i

xk 40

i

jh

50

i

i

60

i i i i

70

i

80

2T(degree)

FIGURE 1. XRD patterns ZnO based varistor at 1140oC for 45 min sintering time at different Co3O4 dopant concentrations 94.0

94.0 o

1110 C o 1140 C o 1170 C

93.5 93.0 92.5

93.0 92.5

Relative density (%)

Relative density (%)

92.0 91.5 91.0 90.5 90.0 89.5 89.0 88.5

o

1110 C o 1140 C o 1170 C

93.5

92.0 91.5 91.0 90.5 90.0 89.5 89.0

88.0

88.5

87.5 87.0

88.0 0.0

0.2

0.4

0.6

0.8

mol% Co3O4

0.0

0.2

0.4

mol% Co3O4

(b)

(a)

FIGURE 2. Relative density of ZnO varistor ceramics sintered at different temperature for (a) 45 min and (b) 90 min sintering time

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0.6

0.8

Figure 3 shows the variation in average grain size of ZnO in the varistor ceramics with Co 3O4 concentration and sintering condition applied. The incorporation of Co 3O4 together with other known grain growth enhancers such as Bi2O3 and TiO2 has encouraged the grain coarsening process during sintering. In addition, higher sintering temperature has also promoted grain growth of the ZnO in the sintered ceramics. It was observed that the average grain size increased up to 73.8 μm as the Co3O4 concentration was increased from 0.2 mol% to 0.8 mol% and sintered up to 1170 oC. As confirmed by the SEM images in Fig. 4, small and anomalous grains were detected in the microstructure of the ceramics. Meanwhile, pores have developed in both grains and grain boundaries. Elemental analysis using EDX (refer to Fig. 5) depicts the preferential segregation of Co and Ti element at the grain boundaries and at the triple point junctions. The finding supported the formation of secondary phases near these regions in the ceramics.

76 68 64

o

1110 C o 1140 C o 1170 C

72 68 64

60

Average grain size (Pm)

Average grain size (Pm)

76

o

1110 C o 1140 C o 1170 C

72

56 52 48 44 40 36 32 28 24

56 52 48 44 40 36 32 28 24

(a)

20

60

(b)

20 16

16 0.0

0.2

0.4

0.6

0.0

0.8

0.2

0.4

0.6

0.8

mol% Co3O4

mol% Co3O4

FIGURE 3. Average grain size of ZnO varistor ceramics sintered at different temperature for (a) 45 min and (b) 90 min sintering time

FIGURE 4. SEM micrographs of sintered varistor ceramics: at top (45 min) and at bottom (90 min) from left to right is 0.2, 0.4, 0.6 and 0.8 mol% of Co3O4 dopant; respectively at 1140 °C sintering temperature

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FIGURE 5. EDX micrographs and spectra of ZnO based varistor ceramics

Optical Band Gap Energy and Steepness Factor The obtained transmission spectra of the ceramics was analysed in order to obtain the Eg value through transformed spectra, Fig. 6. The value of Eg is decreased from 3.199 eV (pure ZnO) to 2.991 and 2.989 eV for the ceramic combination 0.5 Bi2O3 + 0.5 TiO2 at 1140 oC for 45 and 90 min sintering time, respectively, Fig. 7. This decrement is due to the growth of interface states produced by the combination effect of 0.5 Bi 2O3 and 0.5 TiO2. Further addition of Co3O4 concentration shows the Eg decreases gradually. This decrement goes to 2.368 eV for 45 min sintering time. The drastically decrement in the Eg is due to the interface states produced by the strong combination effect of 0.5 Bi2O3, 0.5 TiO2 and 0.8 Co3O4. Furthermore, the value of Eg decreased slightly from 2.368 to 2.327 eV for this combination of the ceramic with the prolonged of heat treatment. This indicates that the growth of interface states is at near to the saturation limit for the sintering time 90 min and above. It is believed that the growth of interface states is saturated at 0.6 and 0.8 mol% for both sintering time where Eg shows a constant value. The overall trend for Eg is that it decreases and the growth of interface states mechanism occurring in the ceramics when Co3O4 dopant is added.

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o

o

1110 C

1110 C

a = 0 mol% b = 0.2 mol% c = 0.4 mol% d = 0.6 mol% e = 0.8 mol%

(AhQ)

a = 0 mol% b = 0.2 mol% c = 0.4 mol% d = 0.6 mol% e = 0.8 mol%

b

2

20

20

b

c 10

c

d

10

d

a e

a

e

0

0 o

1140 C

a = 0 mol% b = 0.2 mol% c = 0.4 mol% d = 0.6 mol% e = 0.8 mol%

a = 0 mol% b = 0.2 mol% c = 0.4 mol% d = 0.6 mol% e = 0.8 mol%

20

b

(Ahv)

2

20

o

1140 C

b 10

c

c a

a

e

e

0

0

o

o

20

10

d

d

1170 C

1170 C

a = 0 mol% b = 0.2 mol% c = 0.4 mol% d = 0.6 mol% e = 0.8 mol%

a = 0 mol% b = 0.2 mol% c = 0.4 mol% d = 0.6 mol% e = 0.8 mol%

20

(AhQ)

2

b b

10

c

c

10

d

d

a

e

a

e

0

0 1.5

2.0

2.5

3.0

3.5

1.5

2.0

2.5

3.0

3.5

4.0

Photon energy (eV)

Photon energy (eV)

(b) (a) FIGURE 6. Transformed spectra at 45 min (left) and 90 min (right) sintering time 3.1 3.0 2.9

2.9

2.8

2.8

2.7

2.7

2.6

2.6

2.5

2.5

2.4

2.4

2.3

2.3

2.2

o

1110 C o 1140 C o 1170 C

(b)

3.0

Eg (eV)

Eg (eV)

3.1

o

1110 C o 1140 C o 1170 C

(a)

2.2

0.0

0.2

0.4

0.6

0.8

mol% Co3O4

0.0

0.2

0.4

0.6

0.8

mol% Co3O4

FIGURE 7. Band gap of ZnO varistor ceramics sintered at different temperature for (a) 45 min and (b) 90 min sintering time

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It shows that steepness factor σA, Fig. 8 (a), decreases with the doping level of Co3O4 indicating the increase of structural disordering in the varistor ceramics. Thus, this structural disordering introduces the growth of interface states and as a consequence, the Eg value decreases. The steepness factor σB, Fig. 8 (b), decreases with the dopant concentration indicating the increase in the thermal energy of displacement [21], hence increases the structural disordering at all sintering temperatures with slightly decrement of Eg as compared to undoped varistor ceramics. 0.056

0.0165

(a)

0.0160

(b)

0.054

0.0155

0.052

Steepness factor VB (a.u.)

Steepness factor VA (a.u.)

0.0150 0.0145 0.0140 0.0135

45 min

0.0130 0.0125 0.0120

90 min

0.0115

0.050 0.048 0.046

45 min 0.044 0.042

90 min

0.0110

0.040

0.0105 0.0

0.2

0.4

0.6

0.8

0.0

mol% Co3O4

0.2

0.4

0.6

0.8

mol% Co3O4

FIGURE 8. Variation of σA (a) and σB (b) with mol% of Co3O4 at 1140 oC CONCLUSIONS It is observed that steepness factor σA decreases with the doping level of Co3O4 indicating the increase of structural disordering in the varistor ceramics. Thus, this structural ordering gave the growth of interface states and as a consequence, the Eg value decreases. The steepness factor σB, decreases with the sintering temperatures indicating the increase in the thermal energy of displacement, in consequence, increase the structural disordering at high sintering temperature with slightly decrement of Eg as compared to pure ZnO ceramics. ACKNOWLEDGEMENTS The authors would like to thank Ministry of Higher Education Malaysia for providing grant scheme through NicheNiche Research Grant Scheme (Nic-NRGS, Project Number: NRGS/2015/53131/14), Physics Laboratory UPM’s and Fundamental Physics Laboratory UMT’s for facilitating the equipments for the research purpose. REFERENCES 1. 2. 3. 4. 5. 6. 7.

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