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Synthesis of BaTiO3 and ZnO varistor precursor powders by reaction spray pyrolysis
Materials Science and Engineering, A168 (1993) 249-252
249
Synthesis of BaTiO 3 and ZnO varistor precursor powders by reaction spray pyrolysis O. M i l o ~ e v i 6 a n d D. U s k o k o v i r *
Institute of Technical Sciences of Serbian Academy of Sciences and Arts, Belgrade (Serbia)
Abstract High-purity, fine, homogeneous ceramic powders with complex composition which are commonly used for electronic materials (ZnO-based non-linear resistors-varistors and piezoelectric BaTiO3) were synthesized by the reaction spray process. The particles obtained were hollow spherical shells and shell fragments with average particle size ranging from 3 to 4/~m, representing the aggregates of small individual crystallites each less than t /~m. Owing to the extremely high heating rate of a salt/droplet particle, mixing on an atomic or nanometre scale in the feeding solution should persist in the resulting powder. It was shown that the chlorides BaCI2 and TiCI 4 are suitable precursors for BaTiO 3 ceramics. Varistor ceramics with a non-linearity coefficient a = 30 and breakdown voltage K ---700 V mm- ~were produced by sintering the reaction spray-derived multicomponent ZnO-based powder. The compositional homogeneity of the resulting powders and the phase content were determined by the processing and solution parameters which affect the dehydration, precipitation and decomposition processes.
I. Introduction The methods of wet chemistry offer many advantages in material processing compared with conventional synthesis [1]. These include better homogeneity, high purity, lower calcination temperatures, the ability to prepare powders with a narrow distribution of particle size and a small mean size, the ability to prepare ceramic materials with novel microstructures and distribution of phases [2]. Investigations into producing stoichiometric high purity submicrometre precursor powders for electronic materials are now performed to an increasing extent by applying chemical methods (e.g. coprecipitation [3-5], sol-gel [6, 7], hydrothermal synthesis [8] etc.). However, it is often difficult to obtain adequate chemical homogeneity, even by the solution techniques, owing to some compositional segregation. Mixing on an atomic (or a nanometre) scale [2], which is provided by wet chemical preparation techniques, is hardly maintained in the resulting powders during calcination. The reaction spray process is one of the dispersion phase techniques and is recognized as an important method for making fine powders and materials [9-12]. It involves atomizing the solution into a heated reaction column. The mist of the solution is dried and subsequently decomposed at elevated temperatures. This
*Correspondence address: Pro Ignis Consultancy, Mr. Miga Ignis, Postbus 82430, NL2508EK Denhaag, Netherlands. 0921-5093/93/$6.00
permits higher surface reaction and prevents any compositional segregation. Therefore, this method is considered to produce powders maintaining compositional homogeneity and to have the advantage of ease of close compositional control for multicomponent powders. We consider here electronic materials where controlled and/or fine grain size and grain boundary chemistry are important or critical to performance. Additionally, the chemistry of the grain boundary and the depletion layer, the defect structure and the dopant concentration are phenomena relating to the lattice structure in electronic materials and are controlled on a nanometer scale. The difficulty of controlling the properties and attaining compositional homogeneity from micrometre size powders is easily appreciated. The goal of this work was to investigate the potential of the reaction spray pyrolysis method for processing of electronic materials (especially multicomponent ZnObased non-linear resistors (varistors) and piezoelectric BaTiO 3 ceramics) when the above mentioned demands are presumed. 2. Experimental procedure
2.1. Laboratory equipment for the reaction spray process The main parts of the equipment were as follows: (i) an atomizing system, including a twin fluid atomizer, which was used in this work; an ultrasonic atomizer, Gapusol 9001 type, 2.5 MHz, RBI was also available; © 1993 - Elsevier Sequoia. All rights reserved
250
O. MilogeviF, D. Uskokovi( /
(ii) a vertical tube furnace (Electron-Ignis Combo Lab), height 1750 m m with plasma generated alumina tube 150 mm in diameter with three independently controllable temperature zones, each of 1473 K maximum; a microcomputer based temperature indicated controller MCM-100 series (Shinko) and AR-201 sixpoint Toshiba recorder. The general flow diagram of the reaction spray process for the production of ceramic powders is given elsewhere [13]. The solution (suspension) of metal salts was atomized into a hot reaction column, where the droplets were dried and decomposed in dispersed phase. The reaction spray temperature was 1173 K. To cause the gas-liquid (droplets) dispersed system to travel through the furnace a pressure drop was maintained at the exhaust by a vacuum pump. Separation of the gas-borne particles from the gas stream (mixture of gases and water vapour) was achieved by deposition onto a collecting surface in gravity settling chambers. After spraying and decomposition of all the suspension, the resulting powder was exposed to flowing hot gases at the exhaust for about 30 min. The process parameters, the gas and liquid flow rates and the temperatures in the reaction zones were adjusted preliminarily. 2.2. Preparation of multicomponent ZnO-based varistor precursor powder In order to obtain powder with complex initial composition (100 - X)ZnO + Y(additives) the following starting components were used: Zn(NOa)2.6H20 (Merck, extra pure), Bi(NO3)2"9H20 (Kemika, p.a.), SbC13 (Kemika, 99%), Co(NO3)2"6H20 (Merck, p.a.), Mn(CH3COO)2"4H20 (Merck, extra pure), Cr(NO3)2" 9H20 (Merck, p.a.) and Ni(NO3)z'6H20 (Merck, p.a.). The starting components were dry mixed and heated to form a homogeneous c o m m o n grey solution [5]. The solution obtained was diluted with distilled water in the proportion of desired composition and mixed thoroughly. The suspension was then fed to an atomizing system and sprayed into the reaction column. The gas pressure (N2) was 0.5 bar and the liquid flow rate was 0.5 1 h -1. The powder was compacted into samples having a diameter of 8 m m and a height of 1-2 mm at a uniaxial pressing pressure of 80 MPa. The samples were sintered in air from 1373 to 1573 K, held for 60 min; the heating and cooling rates were controlled at 5 K rain- 1. 2.3. Preparation of Ba TiO3 precursor powders BaTiO 3 precursor powders were prepared according to the following initial solutions and suspensions: (i) solution of chlorides TiCI4 and BaCI 2 in ethanol-water mixture as solvent, with concentration 0.2 M; based on this solution, two experirnents were
Ba TiO3 and ZnO precursorpowders
performed, after 1 day (powder B-l) and after 4 days (powder B-2); (ii) suspension of TiO2-XH20 + Ba(OH)2 in nitric acid, with concentration 0.2 M (powder B-3); (iii) solution of nitrates TiO(NO3) 3 and Ba(NO3) 2 (powder B-4). The solutions-suspensions were fed to an atomizing system and sprayed after stationary conditions had been established in the system with the parameters denoted in Table 1. 2.4. Characterization Development of the crystal phases in the powders was studied with a Philips PW 1710 diffractometer using a graphite-monochromatized Cu K a radiation. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were performed in air at heating rates of 10 K min-1 (Netzch 409 EP). The distribution of powder size was measured using a Coulter Multisizer. The powder morphology was examined by scanning electron microscopy (SEM, AMR-1600 T). The chemical homogeneity of the microstructures was determined by energy-dispersive spectroscopy (EDS, PGT system IV from Princeton Gamma Tech.). Electrical measurements were registered on sintered varistor ceramics in the interval 1-100 A m - 2 by a d.c. power supply. The non-linearity coefficients were determined within the range 1-10 A m -2 (al) and 10-100 A m -2 (a:); the breakdown field K c was determined at 10 A m-2 and the leakage current JL was measured at a voltage of 0.8 Kc.
3. Results and discussion
A SEM image of ZnO based varistor precursor powder particles is shown in Fig. 1. The particle shape is typical of nitrate-derived powders, consisting of hollow, spherical shells and shell fragments, with the mean particle size measuring approximately 3 /~m in diameter. Their morphology is a remnant of the original dried salt structure, which forms as the droplet dries and the salt precipitates onto the surface to form
TABLE 1. Feed solutions and process parameters during reaction spray pyrolysis Powder
Initial composition
Liquid flow rate 0h -1 )
Gas
B-1 B-2 B-3
TiCI4 + B a C I 2 T i C l 4 + BaCI2 TiO2-XH20 + Ba(OH)2
0.3 0.2 0.3
0.6 0.6 0.6
B-4
T i O ( N O 3 ) 3 + B a (N O 3) 2
--
0.5
pressure (bar)
O. Milogevir, D. Uskokovi6
/
BaTiO 3 and ZnO precursor powders
251
Fig. 2. SEM image of chloride-derived BaTiO3 precursor powder. Fig. 1. SEM image of ZnO based varistor precursor powders obtained by the reaction spray process.
a crust [10]. With the rapid temperature increase, the precipitate salt crystals then decompose to form the oxide crystallites, while the original spherical shape of the particle persist. So the particle surfaces are not smooth and each particle seems to be an aggregate of small individual particles which are less than or equal to 1 p m in diameter. The decomposition curves indicate completed reaction and X-ray diffraction (XRD) implies the presence of a well crytaUized ZnO phase, a poorly crystallized spinel phase, as well as a small amount of an unidentified phase [13]. From the SEM image of chloride-derived BaTiO 3 precursor powder (powder B-l), spherical particles with diameters ranging from 1 to 7 /~m are visible, although there are some irregularly shaped particles (Fig. 2). As in the case of the previous powder, the particles obtained are hollow spherical shells and shell fragments consisting of aggregates of primary particles (crystallites) (Fig. 3). The particle size distribution indicates a mean particle size Ds0 = 4 p m (powder B-1 ) and Ds0 = 4.5 p m (powder B-2). XRD patterns for the chloride-derived powders show the presence of a well crystallized c-BaTiO 3 phase as well as about 10% BaCI2.2t-I20 phase, when the thermal analysis results are considered. The appearance of c-BaTiO3 instead of tetragonal BaTiO 3 may be determined by the size of the crystallites and may occur for crystallite sizes less than 250 nm [14]. EDS analysis of chloride-derived BaTiO3 powder indicates extremely homogeneous composition when the particle surface and the bulk are analysed. This implies that mixing in the feeding solution has persisted in the resulting powder. SEM analysis of powder obtained from the suspension TiO,-XI-teO+Ba(OH)2 in nitric acid (powder B-3) indicates highly agglomerated powder with mostly
Fig. 3. SEM image of the inner structure of one chloridederived BaTiO3particle.
irregularly shaped particles and a mean particle size D50=3.5 pm. XRD powder analysis reveals that decomposition of the salt is not complete for powders B-3 and B-4. The non-linearity coefficient observed for varistor ceramics in the range 10-100 A m -2 realized its maximum value (a 2 = 30) for sintering at 1473 K, while the non-linearity coefficient in the low current region (a l) is relatively low; the leakage current is about 40 /.tA cm -2 and the breakdown field is 700 V mm-1 for these sintering conditions. The relatively higher values of breakdown fields observed in this work, compared with similar systems [13], may be the result of refinement of the microstructure due to the very uniform and spherical particles during preparation of the powder in the dispersion phase.
252
O. Milogevi~, D. Uskokovi~
/
4. Conclusion High purity, fine precursor powders for multicomponent ZnO-based non-linear varistor and BaTiO a ceramics were prepared by the reaction spray process at 1173 K. The particles obtained are hollow spherical shells and shell fragments and seem to be an aggregate of small individual particles less than 1/~m in size. Compositional homogeneity and phase content are determined by the process parameters, by the uniformity of the feed solution and by the characteristics of the starting compounds, which affect the dehydration, precipitation and decomposition processes. The chlorides TiCl4 and BaCI2 are convenient salts for BaTiO 3 precursor powder synthesis. Varistor ceramics with non-linearity coefficient a = 3 0 and breakdown voltage K---700 V mm-~ were produced by sintering of the reaction spray pyrolysis derived multicomponent powder with composition (100 - X)ZnO + Y additives.
Acknowledgments This research was financially supported by the Republic Science Foundation through the project "Physicochemical Processes in Homogeneous and Heterogeneous Systems", and "Metal and Ceramic Matrix Composites" and partly by the National Institute for Standards and Technology, Gaithersburg, MD, through funds made available to the US-Yugoslav Joint Board on Scientific and Technological Cooperation for the project Synthesis and Microanalysis of
Ba Ti03 and ZnO precursor powders
Ceramics. In addition, the authors gratefully acknowledge Professor M. Trontelj, Dr. Lj. Karanovi6, Dr. D. Poleti and Ms. D. Vasovi6 whose contributions were significant in making the work successful.
References 1 D. R. Uhlmann, B. J. J. Zelinski and G. E. Wnek, in C. J. Brinker, D. E. Clark and D. R. Ulrich (eds.), Better Ceramics Through Chemistry, Elsevier, Amsterdam, 1984, p. 59. 2 D. Hennings, in G. de With, R. A. Terpstra and R. Metselaar (eds.), Euro-Ceramics, Vol. 2, Elsevier, Amsterdam, 1989, p. 2273. 3 R. G. Dosch, in L. L. Hench and D. R. Ulrich (eds.), Science of Ceramic Chemical Processing, Wiley, New York, 1986, p. 311. 4 T.T. Fang, H. B. Lin and J. B. Hwang, J. Am. Ceram. Soc., 73 (11)(1990) 3363. 5 0 . Milo]evi~, D. Vasovi~, D. Poleti, Lj. Karanovi6, V. Petrovi~ and D. Uskokovi~,in L. M. Levinson (ed.), Ceramic Transactions: Advances in Varistor Technology, Vol. 3, The American Ceramic Society,Westerville, OH, 1989, p. 395. 6 J. Lanfand W. D. Bond, Ceram. Bull., 63(1984) 278. 7 Ph. Coiomban, Ind. Ceram., 792 (3) (1985) 186. 8 A.K. Maurice and R. C. Buchanan, Ferroelectrics, 74(1987) 61. 9 T. Q. Liu, O. Sakurai, N. Mizutani and M. Kato, J. Mater. Sci., 21 (1986) 3698. 10 D.W. Sproson, G. L. Messing and T. J. Gardner, Ceram. Int., 12(1986)3.
11 E. Ivers-Tiffee and K. Seitz, Am. Ceram. Soc. Bull., 66 (1987) 1384. 12 E. Ivers-Tiffee,Ferroelectrics, 68 ( 1986) 99. 13 O. Milo]evi6, Lj. Karnovi6, M. Tomasevic-Canovic, Lj. Trontelj and D. Uskokovi~,J. Mater. Sci., in press. 14 H. Yamamura, A. Watanabe, S. Shirasaki, Y. Moriyoshi and M. Tanada, Ceram. Int., 11(1)(1985) 17.
249
Synthesis of BaTiO 3 and ZnO varistor precursor powders by reaction spray pyrolysis O. M i l o ~ e v i 6 a n d D. U s k o k o v i r *
Institute of Technical Sciences of Serbian Academy of Sciences and Arts, Belgrade (Serbia)
Abstract High-purity, fine, homogeneous ceramic powders with complex composition which are commonly used for electronic materials (ZnO-based non-linear resistors-varistors and piezoelectric BaTiO3) were synthesized by the reaction spray process. The particles obtained were hollow spherical shells and shell fragments with average particle size ranging from 3 to 4/~m, representing the aggregates of small individual crystallites each less than t /~m. Owing to the extremely high heating rate of a salt/droplet particle, mixing on an atomic or nanometre scale in the feeding solution should persist in the resulting powder. It was shown that the chlorides BaCI2 and TiCI 4 are suitable precursors for BaTiO 3 ceramics. Varistor ceramics with a non-linearity coefficient a = 30 and breakdown voltage K ---700 V mm- ~were produced by sintering the reaction spray-derived multicomponent ZnO-based powder. The compositional homogeneity of the resulting powders and the phase content were determined by the processing and solution parameters which affect the dehydration, precipitation and decomposition processes.
I. Introduction The methods of wet chemistry offer many advantages in material processing compared with conventional synthesis [1]. These include better homogeneity, high purity, lower calcination temperatures, the ability to prepare powders with a narrow distribution of particle size and a small mean size, the ability to prepare ceramic materials with novel microstructures and distribution of phases [2]. Investigations into producing stoichiometric high purity submicrometre precursor powders for electronic materials are now performed to an increasing extent by applying chemical methods (e.g. coprecipitation [3-5], sol-gel [6, 7], hydrothermal synthesis [8] etc.). However, it is often difficult to obtain adequate chemical homogeneity, even by the solution techniques, owing to some compositional segregation. Mixing on an atomic (or a nanometre) scale [2], which is provided by wet chemical preparation techniques, is hardly maintained in the resulting powders during calcination. The reaction spray process is one of the dispersion phase techniques and is recognized as an important method for making fine powders and materials [9-12]. It involves atomizing the solution into a heated reaction column. The mist of the solution is dried and subsequently decomposed at elevated temperatures. This
*Correspondence address: Pro Ignis Consultancy, Mr. Miga Ignis, Postbus 82430, NL2508EK Denhaag, Netherlands. 0921-5093/93/$6.00
permits higher surface reaction and prevents any compositional segregation. Therefore, this method is considered to produce powders maintaining compositional homogeneity and to have the advantage of ease of close compositional control for multicomponent powders. We consider here electronic materials where controlled and/or fine grain size and grain boundary chemistry are important or critical to performance. Additionally, the chemistry of the grain boundary and the depletion layer, the defect structure and the dopant concentration are phenomena relating to the lattice structure in electronic materials and are controlled on a nanometer scale. The difficulty of controlling the properties and attaining compositional homogeneity from micrometre size powders is easily appreciated. The goal of this work was to investigate the potential of the reaction spray pyrolysis method for processing of electronic materials (especially multicomponent ZnObased non-linear resistors (varistors) and piezoelectric BaTiO 3 ceramics) when the above mentioned demands are presumed. 2. Experimental procedure
2.1. Laboratory equipment for the reaction spray process The main parts of the equipment were as follows: (i) an atomizing system, including a twin fluid atomizer, which was used in this work; an ultrasonic atomizer, Gapusol 9001 type, 2.5 MHz, RBI was also available; © 1993 - Elsevier Sequoia. All rights reserved
250
O. MilogeviF, D. Uskokovi( /
(ii) a vertical tube furnace (Electron-Ignis Combo Lab), height 1750 m m with plasma generated alumina tube 150 mm in diameter with three independently controllable temperature zones, each of 1473 K maximum; a microcomputer based temperature indicated controller MCM-100 series (Shinko) and AR-201 sixpoint Toshiba recorder. The general flow diagram of the reaction spray process for the production of ceramic powders is given elsewhere [13]. The solution (suspension) of metal salts was atomized into a hot reaction column, where the droplets were dried and decomposed in dispersed phase. The reaction spray temperature was 1173 K. To cause the gas-liquid (droplets) dispersed system to travel through the furnace a pressure drop was maintained at the exhaust by a vacuum pump. Separation of the gas-borne particles from the gas stream (mixture of gases and water vapour) was achieved by deposition onto a collecting surface in gravity settling chambers. After spraying and decomposition of all the suspension, the resulting powder was exposed to flowing hot gases at the exhaust for about 30 min. The process parameters, the gas and liquid flow rates and the temperatures in the reaction zones were adjusted preliminarily. 2.2. Preparation of multicomponent ZnO-based varistor precursor powder In order to obtain powder with complex initial composition (100 - X)ZnO + Y(additives) the following starting components were used: Zn(NOa)2.6H20 (Merck, extra pure), Bi(NO3)2"9H20 (Kemika, p.a.), SbC13 (Kemika, 99%), Co(NO3)2"6H20 (Merck, p.a.), Mn(CH3COO)2"4H20 (Merck, extra pure), Cr(NO3)2" 9H20 (Merck, p.a.) and Ni(NO3)z'6H20 (Merck, p.a.). The starting components were dry mixed and heated to form a homogeneous c o m m o n grey solution [5]. The solution obtained was diluted with distilled water in the proportion of desired composition and mixed thoroughly. The suspension was then fed to an atomizing system and sprayed into the reaction column. The gas pressure (N2) was 0.5 bar and the liquid flow rate was 0.5 1 h -1. The powder was compacted into samples having a diameter of 8 m m and a height of 1-2 mm at a uniaxial pressing pressure of 80 MPa. The samples were sintered in air from 1373 to 1573 K, held for 60 min; the heating and cooling rates were controlled at 5 K rain- 1. 2.3. Preparation of Ba TiO3 precursor powders BaTiO 3 precursor powders were prepared according to the following initial solutions and suspensions: (i) solution of chlorides TiCI4 and BaCI 2 in ethanol-water mixture as solvent, with concentration 0.2 M; based on this solution, two experirnents were
Ba TiO3 and ZnO precursorpowders
performed, after 1 day (powder B-l) and after 4 days (powder B-2); (ii) suspension of TiO2-XH20 + Ba(OH)2 in nitric acid, with concentration 0.2 M (powder B-3); (iii) solution of nitrates TiO(NO3) 3 and Ba(NO3) 2 (powder B-4). The solutions-suspensions were fed to an atomizing system and sprayed after stationary conditions had been established in the system with the parameters denoted in Table 1. 2.4. Characterization Development of the crystal phases in the powders was studied with a Philips PW 1710 diffractometer using a graphite-monochromatized Cu K a radiation. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were performed in air at heating rates of 10 K min-1 (Netzch 409 EP). The distribution of powder size was measured using a Coulter Multisizer. The powder morphology was examined by scanning electron microscopy (SEM, AMR-1600 T). The chemical homogeneity of the microstructures was determined by energy-dispersive spectroscopy (EDS, PGT system IV from Princeton Gamma Tech.). Electrical measurements were registered on sintered varistor ceramics in the interval 1-100 A m - 2 by a d.c. power supply. The non-linearity coefficients were determined within the range 1-10 A m -2 (al) and 10-100 A m -2 (a:); the breakdown field K c was determined at 10 A m-2 and the leakage current JL was measured at a voltage of 0.8 Kc.
3. Results and discussion
A SEM image of ZnO based varistor precursor powder particles is shown in Fig. 1. The particle shape is typical of nitrate-derived powders, consisting of hollow, spherical shells and shell fragments, with the mean particle size measuring approximately 3 /~m in diameter. Their morphology is a remnant of the original dried salt structure, which forms as the droplet dries and the salt precipitates onto the surface to form
TABLE 1. Feed solutions and process parameters during reaction spray pyrolysis Powder
Initial composition
Liquid flow rate 0h -1 )
Gas
B-1 B-2 B-3
TiCI4 + B a C I 2 T i C l 4 + BaCI2 TiO2-XH20 + Ba(OH)2
0.3 0.2 0.3
0.6 0.6 0.6
B-4
T i O ( N O 3 ) 3 + B a (N O 3) 2
--
0.5
pressure (bar)
O. Milogevir, D. Uskokovi6
/
BaTiO 3 and ZnO precursor powders
251
Fig. 2. SEM image of chloride-derived BaTiO3 precursor powder. Fig. 1. SEM image of ZnO based varistor precursor powders obtained by the reaction spray process.
a crust [10]. With the rapid temperature increase, the precipitate salt crystals then decompose to form the oxide crystallites, while the original spherical shape of the particle persist. So the particle surfaces are not smooth and each particle seems to be an aggregate of small individual particles which are less than or equal to 1 p m in diameter. The decomposition curves indicate completed reaction and X-ray diffraction (XRD) implies the presence of a well crytaUized ZnO phase, a poorly crystallized spinel phase, as well as a small amount of an unidentified phase [13]. From the SEM image of chloride-derived BaTiO 3 precursor powder (powder B-l), spherical particles with diameters ranging from 1 to 7 /~m are visible, although there are some irregularly shaped particles (Fig. 2). As in the case of the previous powder, the particles obtained are hollow spherical shells and shell fragments consisting of aggregates of primary particles (crystallites) (Fig. 3). The particle size distribution indicates a mean particle size Ds0 = 4 p m (powder B-1 ) and Ds0 = 4.5 p m (powder B-2). XRD patterns for the chloride-derived powders show the presence of a well crystallized c-BaTiO 3 phase as well as about 10% BaCI2.2t-I20 phase, when the thermal analysis results are considered. The appearance of c-BaTiO3 instead of tetragonal BaTiO 3 may be determined by the size of the crystallites and may occur for crystallite sizes less than 250 nm [14]. EDS analysis of chloride-derived BaTiO3 powder indicates extremely homogeneous composition when the particle surface and the bulk are analysed. This implies that mixing in the feeding solution has persisted in the resulting powder. SEM analysis of powder obtained from the suspension TiO,-XI-teO+Ba(OH)2 in nitric acid (powder B-3) indicates highly agglomerated powder with mostly
Fig. 3. SEM image of the inner structure of one chloridederived BaTiO3particle.
irregularly shaped particles and a mean particle size D50=3.5 pm. XRD powder analysis reveals that decomposition of the salt is not complete for powders B-3 and B-4. The non-linearity coefficient observed for varistor ceramics in the range 10-100 A m -2 realized its maximum value (a 2 = 30) for sintering at 1473 K, while the non-linearity coefficient in the low current region (a l) is relatively low; the leakage current is about 40 /.tA cm -2 and the breakdown field is 700 V mm-1 for these sintering conditions. The relatively higher values of breakdown fields observed in this work, compared with similar systems [13], may be the result of refinement of the microstructure due to the very uniform and spherical particles during preparation of the powder in the dispersion phase.
252
O. Milogevi~, D. Uskokovi~
/
4. Conclusion High purity, fine precursor powders for multicomponent ZnO-based non-linear varistor and BaTiO a ceramics were prepared by the reaction spray process at 1173 K. The particles obtained are hollow spherical shells and shell fragments and seem to be an aggregate of small individual particles less than 1/~m in size. Compositional homogeneity and phase content are determined by the process parameters, by the uniformity of the feed solution and by the characteristics of the starting compounds, which affect the dehydration, precipitation and decomposition processes. The chlorides TiCl4 and BaCI2 are convenient salts for BaTiO 3 precursor powder synthesis. Varistor ceramics with non-linearity coefficient a = 3 0 and breakdown voltage K---700 V mm-~ were produced by sintering of the reaction spray pyrolysis derived multicomponent powder with composition (100 - X)ZnO + Y additives.
Acknowledgments This research was financially supported by the Republic Science Foundation through the project "Physicochemical Processes in Homogeneous and Heterogeneous Systems", and "Metal and Ceramic Matrix Composites" and partly by the National Institute for Standards and Technology, Gaithersburg, MD, through funds made available to the US-Yugoslav Joint Board on Scientific and Technological Cooperation for the project Synthesis and Microanalysis of
Ba Ti03 and ZnO precursor powders
Ceramics. In addition, the authors gratefully acknowledge Professor M. Trontelj, Dr. Lj. Karanovi6, Dr. D. Poleti and Ms. D. Vasovi6 whose contributions were significant in making the work successful.
References 1 D. R. Uhlmann, B. J. J. Zelinski and G. E. Wnek, in C. J. Brinker, D. E. Clark and D. R. Ulrich (eds.), Better Ceramics Through Chemistry, Elsevier, Amsterdam, 1984, p. 59. 2 D. Hennings, in G. de With, R. A. Terpstra and R. Metselaar (eds.), Euro-Ceramics, Vol. 2, Elsevier, Amsterdam, 1989, p. 2273. 3 R. G. Dosch, in L. L. Hench and D. R. Ulrich (eds.), Science of Ceramic Chemical Processing, Wiley, New York, 1986, p. 311. 4 T.T. Fang, H. B. Lin and J. B. Hwang, J. Am. Ceram. Soc., 73 (11)(1990) 3363. 5 0 . Milo]evi~, D. Vasovi~, D. Poleti, Lj. Karanovi6, V. Petrovi~ and D. Uskokovi~,in L. M. Levinson (ed.), Ceramic Transactions: Advances in Varistor Technology, Vol. 3, The American Ceramic Society,Westerville, OH, 1989, p. 395. 6 J. Lanfand W. D. Bond, Ceram. Bull., 63(1984) 278. 7 Ph. Coiomban, Ind. Ceram., 792 (3) (1985) 186. 8 A.K. Maurice and R. C. Buchanan, Ferroelectrics, 74(1987) 61. 9 T. Q. Liu, O. Sakurai, N. Mizutani and M. Kato, J. Mater. Sci., 21 (1986) 3698. 10 D.W. Sproson, G. L. Messing and T. J. Gardner, Ceram. Int., 12(1986)3.
11 E. Ivers-Tiffee and K. Seitz, Am. Ceram. Soc. Bull., 66 (1987) 1384. 12 E. Ivers-Tiffee,Ferroelectrics, 68 ( 1986) 99. 13 O. Milo]evi6, Lj. Karnovi6, M. Tomasevic-Canovic, Lj. Trontelj and D. Uskokovi~,J. Mater. Sci., in press. 14 H. Yamamura, A. Watanabe, S. Shirasaki, Y. Moriyoshi and M. Tanada, Ceram. Int., 11(1)(1985) 17.