Estimation of Agglomeration Degree and

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Part. Part. Syst. Charact. 28 (2011) 13–18

Estimation of Agglomeration Degree and Nanoparticles Shape of Zirconia Nanopowders Igor Danilenko*, Tetyana Konstantinova*, Nikolay Pilipenko*, Galina Volkova*, Valentina Glasunova* (Received: 15 September 2008; in revised form: 11 March 2012; accepted: 12 March 2012; published online: 29 March 2012)

DOI: 10.1002/ppsc.200800041

Abstract The influence of different surfactants (Trilon B, Polyammoniumacrylate and Neonol) and mineralizers (HCl, KOH, CH3COOH, etc.) on agglomeration degree of zirconia nanopowders obtained by precipitation technique was investigated. By adding of surfactants and mineralizers the different phase composition, particle size and agglomeration degree of zirconia nanopowders were obtained. The comparison between experimental specific surface area (SSA) value and calculated surface area

for nonagglomerated spherical particle model show that experimental value of SSA bigger than calculated value. We proposed that low calcining temperatures (less than 700 °C) lead to formation the zirconia particles with cylindrical form. When taking into account the contact area between the particles in this model, we recalculate the surface area of powders and get better fit to experimental value.

Keywords: zirconia, nanoparticles, specific surface area, agglomeration, contact area.

1 Introduction Zirconia nanopowders have a wide range of applications. It’s used for obtaining wear resistant ceramics, ceramics for electrochemical and catalytic devises, biomedicine application and many others [1 – 3]. And in all cases the producer of ceramics parts need to know the optimal pressure-temperature-time regimes for obtaining dense or porous nanostructure ceramics. The advantage of nanopowders is the possibility of low temperature sintering and as a result the ceramic structure homogeneity. It is known that granulometric (size and shape of particles and the size of aggregates and agglomerates), phase and chemical composition of starting powders as well as the same characteristic of agglomeration as agglomerate “hardness” determine the compactions and sintering regime. The agglomeration is condi-

*

Ph.D I. Danilenko (corresponding author), Prof. T. Konstantinova, Ph.D N. Pilipenko, G. Volkova, V. Glasunova, Donetsk Institute of Physics and Engineering NAS of Ukraine R. Luxemburg str, 72, Donetsk, 83114 (Ukraine). E-mail: [email protected]

© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

tioned by van der Waals forces between particles. If these forces are weak the agglomerates are referred to as “soft” agglomerates. These agglomerates can be easily broken in a liquid medium by ultrasonic, or/and dispersants additions. In contrast, strong forces between particles due to high temperature calcinations or incorrect chemical additions result in “hard” agglomerates [4]. In this case is too high to realize the benefits of the nanosized primary crystallites. The high temperature sintering lead to bimodal grain size distribution and phase separation in zirconia ceramics. The prevention of hard agglomeration is a one of the basic aims in nanopowders synthesis process as well as uniform particles shape and narrow size distribution. It is known many technologies based on wet chemicals methods for zirconia nanopowders synthesis which are adopted for semi industrial manufacturing. These are: precipitation methods, mechano-chemicals methods, emulsion methods, hydrothermal and others. These methods aim on nonagglomerated nanopowders obtaining but agglomeration of particles, as show above, is the physical low and therefore the producer obtained the different degree of nanoparticles agglomeration always.

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14 This degree may by very low or very high. It determined by quantity particles in agglomerate and the agglomerate hardness. The agglomeration degree depends on surface properties of nanoparticles, type of dispersant and other. The balance between quality, quantity and price of nanopowders determine the best technology of zirconia nanopowders obtaining. In our early studies [5 – 8] we show that the technology based on precipitation methods with microwave drying, pulse magnetic field treatment and ultrasonication of zirconium hydroxide lead to small size “soft” zirconium hydroxide agglomerates formation. Subsequent calcinations at low temperatures lead to formation of zirconia nanoparticles with predetermined particle size, sharp particle size distribution and “soft” agglomerates. The aims of this work are to estimate the zirconia nanoparticle shape and agglomeration degree. To calculate the theoretical values of specific surface area of each powder were used x-ray analysis and electron microscopy data. The comparison theoretical and experimental SSA data allow estimating the particles shape and agglomeration degree. For obtaining different SSA values and, consequently, different agglomeration degree the addition of different mineralizers and surfactants to amorphous zirconium hydroxide gels were used. For the model material we used the pure monoclinic zirconia nanopowder synthesized by precipitation method.


as mineralizers. All chemicals were chemically grade purity. The concentration of these chemicals was varied from 0,5 to 10 w%. The necessary amount of surfactant or mineralyser in a count on a ZrO2 was added to water – zirconium hydroxide gel mixture. Ultrasound treatment carried out in duration 30 min. After that the slurry was filtered on vacuum pump and drying at heating chamber at 120 °C to constant mass. The calcining at 500 °C at duration 2 hours finalized the zirconia nanopowders obtaining. The low calcining temperature was chosen for elimination from our study the agglomeration connected with necking due to nanoparticles sintering from high-temperature calcinations.

2.2 Characterization XRD analysis was performed on DRON-3 difractometer in CuKa radiation (20kV 30mA). The range of scans was from 20° to 80° for crystalline phase identification and from 25° to 35° for crystalline size measurement. The volume fraction Ci of crystalline phases (tetragonal and monoclinic) was calculated by standard formulas [9]. CM

IM 111 IM 111 ; IM 111 IM 111 IT 111

CM CT 1;

(1) (2)

2 Experimental Procedure 2.1 ZrO2 Nanopowders Preparation The hydrous zirconium hydroxide was prepared by precipitation method. One kilogram of ZrO(NO3)2 was dissolved in distilled water at 40 °C and stirred with propeller stirrer for 1 h. The precipitated gel was obtained by the addition of ZrO(NO3)2 in appropriate NH4OH water solution volumes with stirring. The pH value did not change significantly with final value 10. The agitation was continued a one hour at room temperature. After that the precipitate was recovered by filtration with a vacuum pump. The gel was washed several times by distilled water to pH = 6. This washed gel was used as starting material. Wet gel was divided into several parts for different types of surfactant addition. The 3 small batches were calcined at 1000 °C for determination of ZrO2 content in wet gel. A 200 g of wet zirconium hydroxide gel was stirred in one liter of distilled water in ultrasonic bath, which operate at 18 kHz. The ultrasonic power level in bath was 4 W/cm2. The Trilon B (“Chemservice”, Russia), Polyammoniumacrylate (Japan) and Neonol (Ukraine) were used as surfactants and HNO3, CH3COOH, KOH, KNO3, H2SO4 (“Simbias”, Ukraine)


where IM is the intensity of the monoclinic peaks and IT is the intensity of the tetragonal peak. Crystalline size was estimated using the Debye-Sherer equation. The specific surface area of zirconia powders was calculated from nitrogen adsorption-desorption isotherms measurements at 77K (Sorbi 4N, Meta, Novosibirsk, Russia). Prior to measurements the samples were evacuated at 140 °C. The SSA of the samples was determined from linear part of the BET plots. The particles and agglomerates size and shape was also analyzed by TEM investigation (JEM 200A, Jeol, Japan).

3 Results It is known that the homogeneity of the distribution of small additions of water soluble and insoluble substances in dispersed powder is strongly depend from type and time of mixing processes. The ultrasonic (US) dispersion has undeniable advantages in comparison with mechanical mixing. But the dispersing effect of ultrasound can be replaced by agglomeration in depen-



dence on the type of liquid, the size of the dispersed powder as well as the time of US treatment. Thus, the mode of US treatment was optimized before experiments. The minimum time of dispersing up to the complete disappearing of large agglomerates was 30 minutes for the obtained liquid paste of zirconium hydroxide. The specific surface area of this dried hydroxide was equal to 224 m2/g. The ultrasonic treatment of zirconium hydroxide with the defined quantity of surfactant or mineralizer in distilled water results in the formation of homogeneous suspension. Excess of water was separated by filtration on a vacuum filter. Filtration time depends on the type of surfactant or mineralizers and ranged from 10 minutes to 5 hours. The data which characterized the influence of addition of surfactants and mineralizers with different concentration were obtained by measuring of SSA of powders and particle size by XRD and TEM. The results of the measurement of specific surface for surfactant and for mineralizes addition are presented on Figures 1 and 2, respectively. PAA shows a stable increasing of the specific surface area from the control value 224 m2/g up to 375 m2/g at 5 w% impurity content. The influence of Neonol on SSA value is also observed as an increase of specific surface area of powder but this effect is smaller. The SSA curve has a maximum corresponding to 300 m2/g at 1 w% concentration. An addition of Trilon B yields negative effect with minimum value – 150 m2/g at 5 w% impurity concentrations.

Fig. 1: SBET data of dry zirconium hydroxide with different surfactant addition versus their concentration.

The mineralizers concentration varied from 0,5 to 5w% The samples with addition of hydrochloric (HCl) and sulfuric (H2SO4) acid and potassium hydroxide (KOH) exhibited the values of specific surface of dry zirconium hydroxide not higher than the specific surface of the reference sample (without addition). The values of the


15 specific surface of the zirconium hydroxide with addition of vinegar acid (CH3COOH), ammonium hydroxide (NH4OH), nitric acid (HNO3) and KNO3 were 40 %, 80 %, 100 % and 100 % higher, respectively (Figure 2).

Fig. 2: SBET data of dry zirconium hydroxide with different mineralizer addition versus their concentration.

The samples with vinegar, hydrochloric and sulfuric acid and potassium hydroxide displayed the reduction of specific surface of dry zirconium hydroxide as the amount of the impurity increased within the studied range of concentrations. The samples with KNO3 and NH4OH displayed the increase of specific surface area of dry zirconium hydroxide as the amount of the impurity increased within the studied range of concentrations. The record value between both mineralizers and surfactants belonged to samples with nitric acid (see Figure 2). This fact witnessed to good isolation of the primary particles. The filtration properties of a precipitate of zirconium hydroxide treated by ultrasound and mineralizers were even lower than in the case of surfactants. It was quite explicable because sometimes the specific surface achieved the value 440 and 480 m2/g. This fact confirms our conclusion about good separation of primary particles by addition NO3– ions. The exceptions were the series with the doping by vinegar acid and low concentrations of KOH. The character of aggregates and agglomerates of zirconia powders was analyzed by TEM. The objects were studied immediately after the US treatment, after drying and after calcination. The phase composition, the size of coherent scattering area (CSA) and SBET (in selected cases) of calcined at 500 °C powders are presented in Table 1 for all type of surfactants and in Table 2 for all types of mineralizers which were used in this work. It should be mentioned that the values of specific surface area of oxides obtained by the thermal processing of hydroxide samples with different additions at 500 °C


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(12,4 nm) particle size. It is known that the decrease of a particle size lead to the increase of specific SBET, CS Phase CSA size, CSA N Addition Addition average size, m2/g (D/h = 5) concentration, content,% nm surface area of the powder, if the nm weight% structure of the agglomerate T/M DT / DM changes insignificantly. ConsePri Test 0 44 / 56 10.7 / 9.5 10,0 83 0.64 specimen quently, the information about 10,9 10.9 / 85 / 15 0,3 H2 Neonol the particle size is very important 142.1 0.33 10,9 10.9 / 85 / 15 0,5 H3 for the analysis of SBET data for 134 0.47 9,2 7.7 / 9.7 26 / 74 1 H4 estimation of agglomeration de9,0 8.2 / 9.3 29 / 71 5 H5 gree. The increase of the specific 163 0.41 8,5 8.0 / 8.7 28 / 72 0,5 T05 Trilon B surface area and simultaneous in8,9 9.7 / 8.2 51 / 49 1 T1 crease the average particle size 15,7 15.7 / 100 / 0 5 T2 lead to missing relations between 15,7 15.7 / 100 / 0 10 T3 the oxide particles and their SSA, 153 0.38 9,4 10.6 / 8.9 30 / 70 0,5 P1 Poly but witnessed about formation of 171.2 0.32 9,3 10.1 / 8.0 63 / 37 P2 (ammonium 1 8,1 7.3 / 8.4 30 / 70 2 P4 acrylate) more ramified structure of the 7,9 163 0.45 8.4 / 7.3 54 / 46 5 P3 agglomerate. One of the reasons of the discrepancy between the Table 2: The phase content, CSA size and SBET and contact area CS of the zirconia powder particle size and the calculated obtained at 500°C calcination zirconium hydroxide with mineralizer additions. value of the specific surface area Phase CSA size, CSA average SBET, CS N Addition Addition is that often used the spherical size, nm m2/g (D/h = 5) concentration, content,% nm particles model for calculation. weight% T/M DT / DM We should pay attention to anomPri Test 0 44 / 56 10.7 / 9.5 10,0 83 0.64 alous high values of the tetraspecimen gonal phase CSA in samples with 24 / 76 / 8.4 8,4 176.4 0.41 0,5 N1 HNO3 the sulfuric acid (12,2 – 14,2 nm), 28 / 72 7.0 / 8.1 7,8 1 N2 potassium hydroxide (11,3 – 5 N3 13,1 nm) ammonium hydroxide 120 0.37 12,2 12.2 / S1 H2SO4 94 / 6 0,5 (13,6 nm) and Trilon B (15,7 nm). 13,2 13.2 / S2 100 / 0 1 Probably it is connected with the 14,2 14.2 / S3 100 / 0 5 presence of SO42-, OH– and anion 14 / 86 / 10.3 10,3 124.2 0.37 0,5 Cl1 HCl of Trilon B on the surface of parti38 / 62 14.3 / 10.3 11,8 1 Cl2 5 Cl3 cle. One more interesting result of the present work should be 136.9 0.24 11,3 11.3 / 100 / 0 0,5 K1 KOH 12,9 12.9 / 100 / 0 1 K2 mentioned. Impurities influence 13,1 13.1 / 100 / 0 5 K3 not only the specific surface area and the size of the particles but for 1 hour were higher than the value of specific surface also the phase composition of the powder. The similar area of the reference sample at any kind of impurity and effects were observed in works Nishizawa et al. [10] and at any concentration. The increase of specific surface Benedetti et al. [11] on sodium influence on phase tranarea in the samples of crystal zirconia with surfactants sition in ZrO2. This interesting result will be published ranged from 60 to 100 % in comparison with reference later, now we only use the phase composition for calcusample. The record value of the specific surface area of lation. hydroxides with added nitric acid and KNO3 corre- It is known that in agglomerate same part of particle sursponds to the maximum of this parameter for crystal face (contact area between particles) except from SSA particles. Thus, the different powders particle sizes have measurements. We calculated the SSA from standard no correlation with it SSA values. For example reference equation for spherical particles [12] and got results that sample has a SSA 83 m2/g and average particle size the experimental values of SSA higher for calculated in 10,0 nm. Sample P3 (concentration PAA is 5w %) and supposition that particles are not agglomerated for all KN1 (concentration KNO3 – 0,5w %) has a bigger SSA additions except test specimen. But as we see from picvalue (163 and 183 m2/g, respectively), but sample P3 tures the nanopowders are agglomerated always and has the smaller average particle size (7,9 nm) than real SSA could be smaller than theoretical. This fact was reference sample (10,9 nm) and sample KN1 the bigger the basis of conclusion about the necessity of use of the Table 1: The phase content, CSA size, SBET and contact area CS of the zirconia powder obtained at 500 °C calcination zirconium hydroxide with surfactant additions.




model of plane particles (i.e. cylinders with different ratio of diameter and height: D/h = 5, 4, 3, 2) instead of the model of spheres. The ratio D/h = 5 was used as critical, because in this case the particle was very thin. The value of Stheor calculated within this model at D/h = 1 is coincided with this value in the sphere model. The experimental SBET values obtained at different additions were located between the Stheor curves at D/h = 1 and D/h = 5 (Figure 3). The observed dispersion of values for different additions was caused probably by different area of contacting surfaces.

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Fig. 4: The scheme of contacts of particles. The dashed lines indicated the upper layer of particles.

Fig. 3: The experimental SBET data (points) of the zirconia powders obtained with different additions and calcinated at 500 °C, and calculated specific surface area curves for cylindrical particle with different D/h versus particle size (XRD coherent scattering area).

cylindrical form particles formation at low temperature calcinations confirmed by TEM data. On figure 4 we show the TEM photo of the K3 zirconia powder with addition of KOH and calcined at 750 °C for better resolution. Calculations of this models for D/h = 5 depict on Figure 5. As we can see the SSA calculation with account of contact area of different levels lead to decreasing the theoretical SSA to real values. The low decreasing of calculated SSA caused by lateral contact particles (index m) and high decreasing caused by contact particles with up and down sides (index p). If the calculated curve is close to the experimental SSA value than we may suggests about quantity of contact particles. We think that contacts between particles by lateral sides (in one plane) can not lead to strong particles aggregation at low calcining temperatures, but contacts particles by up and down sides (in a few levels) create pre-conditions for strong powders aggregation. For example, the particle from powder with PAA additions has a 6 contact particles from lateral side and one contact particle on an upper side. The measured SSA

The calculations of Cs = 1 – SBET / Stheor at D/h = 5 allowed us to compare the results of the addition influence on the aggregation level of particles of different sizes. Cs was the relative part of the surface of particles corresponding to their contact area. The Cs values of powders obtained with addition of surfactants and mineralizers after calcination at 500 °C are presented in Table 1 and 2. The analysis showed that all used additions have an increasing of Cs value over the reference specimen. KNO3 and KOH should be separated from mineralizers as additions providing the least contact surface between particles. Polyammoniumacrylate and neonol played the same role in the group of surfactants. The contact area at particle agglomeration basically may be formed by two variants: contacts on lateral side, contacts on up and down sides (Figure 4). The maximal contact area, which can be use for SSA determination, will be formed if the quantity of contact particles with lateral side is six and maximal contact particles with up side are three. We do not show the case if particles contact lateral side with upper side. Our supposition about

Fig. 5: The experimental data (SBET) data (points) of the zirconia powders obtained with different additions and calcinated at 500 °C, and calculated specific surface area curves for cylindrical particle with different D/h, particle size and type of contact area versus particle size (XRD coherent scattering area).



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value for KN1 samples, calcined at 500 °C, is near the theoretical SSA curve with no contact particles (m = 0, p = 0) and calculated contact area equal only 2%. We proposed that this sample consist from nonagglomerated cylinder form particles with diameter near 12 nm and height near 2-3 nm.

4 Conclusions In this work was shown the influence of different types of impurity anions and cations (surfactants and mineralizers) on specific surface area, particle size and phase composition of zirconia nanoparticles. It was found that increasing the specific surface area is not always associated with a decreasing the particle size. Additions of KOH and KNO3, for example, lead to increase of particle size and specific surface area simultaneously. It may be linked with decrease the agglomeration degree of nanoparticles due to changing of the interactions forces between particles due to the influence of surfactants and mineralizers. Also was found that the dopants lead to obtain the powders with SSA bigger than we can calculate from spherical particle model. Consequently, the spherical particle model is incorrect for SSA calculation. More real result can be obtained when we use the cylinder form particle model and take into account the quantity and type of particles contacts. The addition of KNO3 lead to formation of nonagglomerated cylinder form zirconia particles with diameter near 12 nm and height near 2 – 3 nm.

5 Nomenclature Cs CSA D/h PAA SBET

Stheor SSA TEM XRD US w%

contact area between particles coherent scattering area attitude of particle diameter toward its height polyammoniumacrylate specific surface area value measured by method of nitrogen adsorption-desorption isotherms calculation. calculated value of surface area specific surface area transmission electron microscope X-ray diffraction ultrasonic weight percent


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