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In this paper, changes of microstructural characteristics of disperse systems during mechanical activation of zinc oxide (ZnO) have been investigated. ZnO powder was activated by grinding in a planetary ball mill in a continuous regime in air during 300 min at the basic disc rotation speed of 320 rpm and rotation speed of bowls of 400 rpm but with various balls-to-powder mass ratios. During ball milling in a planetary ball mill, initial ZnO powder suffered high-energy impacts. These impacts are very strong, and large amounts of microstructural and structural defects were introduced in the milled powders. The morphology and dispersivity of particles and agglomerates of all powders were investigated by scanning electron microscopy and scanning transmission electron microscopy. The specific surface area of initial ZnO powder was determined as 3.60 m2 g−1 and it increased to 4.42 m2 g−1 in mechanically activated powders. An increase of the ball-to-powder mass ratio led to a decrease of particle dimensions as well as increased the tendency for joining into quite compact agglomerates, that is aggregates, compared with the very loose, soft initial agglomerates. The obtained results pointed out that activation of ZnO powders produces a highly disperse, nano-scaled mixture of small particles, that is crystallites with sizes in the range of 10–40 nm. Most of these particles are in the form of aggregates with dimensions of 0.3–0.1 μm. The crystallite and aggregate size strongly depend on milling conditions, that is ball-to-powder mass ratio, as shown in this investigation. [ABSTRACT FROM AUTHOR]

Abstract: The influence of Al2O3 doping in the range 0.00–0.83mol% on the microstructure and current–voltage characteristics of ZnO-based varistor ceramics sintered at 1200°C for 2h was studied. The threshold voltage V T (V/mm) increased up to a dopant level of about 0.08mol% Al2O3; the nonlinear coefficient α was significantly increased by additions of up to 0.04mol% Al2O3, although larger additions of Al2O3 caused it to decrease; and the leakage current increased sharply with increasing amounts of Al2O3. Doping with Al2O3 up to about 0.12mol% Al2O3 resulted in a significantly decreased ZnO grain size, which is mainly responsible for the significantly increased threshold voltage, V T. No ZnAl2O4 spinel phase was detected in any of the samples, and EDXS and WDXS analyses showed that most of the added Al2O3 distributed between the Zn7Sb2O12 spinel phase and the ZnO phase, while only trace amounts were detected in the Bi2O3-rich phase. The spinel phase incorporates an appropriate amount of Al2O3; however, with an increasing amount of added Al2O3, more of it remains outside the spinel phase in the Bi2O3-rich liquid, where it can incorporate into the growing ZnO grains at the sintering temperature. The amount of Al in the ZnO grains was determined. A mechanism for the grain growth inhibition resulting from the small amounts of Al2O3 in the Bi2O3-rich liquid phase is also proposed. [Copyright &y& Elsevier]

In low-voltage varistor ceramics, the phase equilibrium and the temperature of liquid-phase formation are defined by the TiO2/Bi2O3 ratio. The selection of a composition with an appropriate TiO2/Bi2O3 ratio and the correct heating rate is important for the processing of low-voltage varistor ceramics. The total amount of added Bi2O3 is important as the grain growth is slowed down by a larger amount of Bi2O3-rich liquid phase at the grain boundaries. Exaggerated grain growth in low-voltage varistor ceramics is related to the occurrence of the liquid phase and the presence of TiO2 which triggers the formation of inversion boundaries (IBs) in only a limited number of grains, and as a result the final microstructure is coarse grained. The Zn2TiO4 spinel phase only affects grain growth in compositions with a TiO2/Bi2O3 ratio higher than 1.5. In high-voltage varistor ceramics, just a small amounts of Sb2O3 trigger the formation of IBs in practically every ZnO grain, and in compositions with a Sb2O3/Bi2O3 ratio lower than 1, grain growth that is controlled entirely by an IBs-induced grain growth mechanism results in a fine-grained microstructure. The spinel phase interferes with the grain growth only at higher Sb2O3/Bi2O3 ratios. [Copyright &y& Elsevier]

The preparation of screen-printing inks with a high solids load was studied for the development of thick-film varistors. A high solids load can ensure a higher green density of the films for enhanced densification and microstructure development in order, to obtain good current–voltage ( I–U ) characteristics for the thick-film varistor, even after short firing times at temperatures below 1000 °C. The inks with a solids load of 50, 60 and 70 wt.% enabled the preparation of thick-film varistors on an alumina substrate with a homogenous microstructure, a uniformly distributed Bi 2 O 3 -rich phase and a good varistor film-electrode contact after firing at 900 °C for 15 min. However, the porosity of the films gradually decreased with the increasing solids load and a varistor film prepared from the ink with the highest solids load of 70 wt.% had minimal porosity and improved I–U characteristics in comparison with the other samples. [ABSTRACT FROM AUTHOR]

Abstract: Two ways of application of intensive milling in ZnO varistors processing were compared. First was intensive milling of mixture of previously prepared constituent phases. In this case, intensive milling was applied only to obtain highly activated nanocrystalline varistor powder mixtures. Second application is intensive milling of simple mixture of oxides that could result not only in activation and formation of nanocrystalline powders, but also in mechanochemical reaction and synthesis of constituent phases. Powders and ceramics samples were characterized by XRD and SEM analysis, as well as by dc electrical measurements (nonlinearity coefficients, leakage current and breakdown field). Differences in microstructural and electrical properties of obtained varistors were discussed and optimal milling and processing conditions were recommended. The best electrical characteristics were found in sample Z1-DMCP-m, which exhibited leakage current of 2.5μA/cm2, nonlinear coefficient reaching 58 and breakdown field of 8950V/cm. [Copyright &y& Elsevier]

Abstract: The aim of this work was to reduce number of additives using new method of preparation, refered as “direct mixing of the constituent phases”. The method is based on sintering of a mixture of constituent phases with the following compositions: ZnO phase uniformly doped with 0.1mol% Mn2+ and 0.1mol% Co2+, γ-Bi2O3 stabilized with Mn2+ and spinel phase of composition Zn1.86Co0.46Sb0.67O4 or Zn1.971Ni0.090Co0.030Cr0.247Mn0.090Sb0.545O4. The varistors sintered at 1030°C showed good electrical properties, with nonlinearity coefficient reaching 50 and low values of leakage current. To improve microstructure some powder mixtures were intensively milled and further processed on the same way as unmilled powder mixtures. Electrical and microstructural properties of milled and unmilled samples were compared. [Copyright &y& Elsevier]