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Processing of ZnO-based varistors using oxide, alkoxide, nitrato-alkoxide and carboxylato precursors
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry.
1997 (English)In: Journal of Sol-Gel Science and Technology, ISSN 0928-0707, E-ISSN 1573-4846, Vol. 8, no 1-3, 697-701 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
1997. Vol. 8, no 1-3, 697-701 p.
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:uu:diva-89781DOI: 10.1007/BF02436925OAI: oai:DiVA.org:uu-89781DiVA: diva2:161526
Available from: 2002-04-03 Created: 2002-04-03 Last updated: 2013-05-24Bibliographically 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.
Series
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 699
Keyword
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
Identifiers
urn:nbn:se:uu:diva-1948 (URN)91-554-5275-2 (ISBN)
Public defence
2002-04-24, Häggsalen, Ångström-laboratoret, Uppsala, 10:15
Opponent
Available from: 2002-04-03 Created: 2002-04-03 Last updated: 2014-01-24Bibliographically approved

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