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Preparation of ZnO-based varistors by the sol-gel technique
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry.
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1994 (English)In: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 4, no 4, 615-621 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
1994. Vol. 4, no 4, 615-621 p.
National Category
Natural Sciences
URN: urn:nbn:se:uu:diva-89780DOI: 10.1039/JM9940400615OAI: oai:DiVA.org:uu-89780DiVA: diva2:161525
Available from: 2002-04-03 Created: 2002-04-03 Last updated: 2013-05-16Bibliographically approved
In thesis
1. Novel powder-coating solutions to improved micro-structures of ZnO based varistors, WC-Co cutting tools, and Co/Ni nano-phase films and sponges
Open this publication in new window or tab >>Novel powder-coating solutions to improved micro-structures of ZnO based varistors, WC-Co cutting tools, and Co/Ni nano-phase films and sponges
2002 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Solution chemistry is a versatile and powerful tool in the synthesis of designed, complex nano-level high-tech materials. Normally, the technique is considered too expensive for large-scale production of complex multi-component ceramic materials. This thesis describes the expansion of the useful area of solution processing to multi-component bulk materials such as ZnO-based high-field varistors and WC–Co cutting tools, by developing novel techniques for solution-based coating of conventionally prepared metal and ceramic powders. The chemistry and microstructure development in the preparation of coatings, and the sintering of the coated powders to compacts, were studied in detail by SEM-EDS, TEM-EDS, XRD, IR-spectroscopy, dilatometry, TGA and DSC chemical analysis.

ZnO powder with a ca 20 nm thick, homogeneous oxide coat of Bi–Sb–Ni–Co–Mn–Cr–Al oxide was prepared. After sintering to dense varistor bodies, much improved microstructures with much reduced ZnO-grain sizes were obtained. This shows that the oxides added as liquid sintering aid and grain-growth inhibitor become much more active when added homogeneously as a skin on the ZnO powder.

After sintering of cobalt-coated WC, much improved micro-structures were obtained with a much more narrow WC grain-size distribution than that obtained from starting powders mixed by a conventional milling route. Coated powders also obviate the need for the extensive milling of WC and Co powders used in conventional mixing.

The novel solution route was also applied to preparation of porous sponges and thin films on metal, glass and Al2O3 of sub 20 nm sized Co- or Ni-particles.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2002. 45 p.
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 699
Chemistry, Solution processing, powder coating, nano-phase materials, sintering process, varistor, WC–Co, cobalt, nickel, films, sponges, Kemi
National Category
Chemical Sciences
Research subject
Inorganic Chemistry
urn:nbn:se:uu:diva-1948 (URN)91-554-5275-2 (ISBN)
Public defence
2002-04-24, Häggsalen, Ångström-laboratoret, Uppsala, 10:15
Available from: 2002-04-03 Created: 2002-04-03 Last updated: 2014-01-24Bibliographically approved

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