Åsa Ekstrand

Preparation of ZnO-based varistors by the sol-gel technique

ZnO-based varistors consisting of 96 mol% ZnO and 4 mol% additive oxides (Bi2O3, Sb2O3, NiO, Co2O3, MnO and Cr2O3) have been prepared. Four different procedures have been applied to obtain the starting powder mixture. Three of the procedures involve sol–gel processing of the additive oxide mixture, and in two cases the surface of the ZnO grains has been covered by the additive oxides. The microstructural and phase composition development during the sintering process has been monitored and the electrical properties of the varistors have been determined. Higher breakdown voltages are obtained for varistors produced by sol–gel-processed starting materials, and the highest values are found for varistors prepared from ZnO grains covered with the additive oxides. Phase analysis revealed that the spinel phase, which retards the grain growth of the ZnO particles, is formed at lower temperatures and in larger amounts in these compacts. The higher breakdown voltages of these compacts thus seem to be related to the content of more finely grained ZnO particles in these compacts.

Oxide/oxide composites in the system Cr2O3-Y2O3-Al2O3

Abstract Ceramic compacts in the systems Al 2 O 3 –Y 2 O 3 , Cr 2 O 3 –Y 2 O 3 and Y 3 (Cr y Al 1- y ) 5 O 12 (Cr-doped YAG) were prepared by solid state reaction in calcined co-precipitated powder mixtures of appropriate compositions. Various solid-solution phases were formed, e.g. Y 3 (Al 1- x Cr x ) 5 O 12 , YAl y Cr 1- y O 3 and Al 2- x Cr x O 3 . Composite materials in the pseudo-binary or ternary systems Al 2 O 3 –Y 3 Al 5 O 12 , Cr 2 O 3 –Y 2 O 3 and Y 3 (Al 1– x Cr x ) 5 O 12 –YAl y Cr 1– y O 3 –(Al z Cr 1− z ) 2 O 3 were obtained by hot-pressing appropriate powder precursors at 1600–1650°C for 1 h. The microstructure of the prepared materials was studied in a scanning electron microscope with element analysis facilities. X-ray diffraction was used to reveal the phases present and their lattice parameters. The chemical compatibility of these phases was investigated. The results are discussed with a special emphasis on the solubility of Cr in the YAG structure, and on the compatibility relationship between Cr-doped YAG and its neighbouring phases. A gel-coating process for preparing Al 2 O 3 –YAG composites with tailored microstructures is also described.

Solution Synthesis of Nano-Phase Nickel as Film and Porous Electrode

Routes to nano-phase nickel in the forms of highly porous sponges and dense films have been developed. Nickel acetate and nitrate were mixed with triethanolamine and methanol, and evaporated to a concentrated liquid. Heat-treatment at 10°C/min to 170°C under N2 atmosphere yielded a highly porous sponge consisting of <10 nm sized Ni crystallites. Deposition of a 1 M nickel precursor solution on polycrystalline Al2O3, SnO2 : F coated glass or titanium substrates spinning at 2700 rpm, followed by heat-treatment at 10°C/min to 400°C yielded smooth and dense nickel films consisting of 3–5 nm sized crystallites. The precursor concentrate was studied by FT-IR and the phase development on heat-treatment was studied by thermogravimetric analysis, powder X-ray diffraction, scanning and transmission electron microscopy.

Processing of ZnO-based varistors using oxide, alkoxide, nitrato-alkoxide and carboxylato precursors

We have investigated the preparation of ZnO varistors by different chemical solution routes with the aim of improving the homogeneity of the phases formed and to obtain a better control of microstructure. One conventional oxide mixing route and four solution chemical routes have been used to prepare the precursor materials. In all cases, the same composition (mol%) was used; ZnO (95.9), Bi2O3 (1), Sb2O3 (1), NiO (1), Co2O3 (0.5), MnO (0.5), Cr2O3 (0.1). The same sintering procedure was also applied. It was found that the precursor materials consisting of ZnO grains covered by a thin film of the additive oxides yielded smaller ZnO grains. Also the microstructure in the final compacts was improved, compared with that of compacts prepared from oxide mixing routes. The smaller ZnO grain size in the final compacts was attributed to the presence of spinel grains. The spinel grains are formed at lower temperatures and, when they reach a size of 1 μm, they hinder the growth of ZnO grains.