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Novel powder-coating solutions to improved micro-structures of ZnO based varistors, WC-Co cutting tools, and Co/Ni nano-phase films and sponges
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry.
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. , p. 45
Series
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 699
Keywords [en]
Chemistry, Solution processing, powder coating, nano-phase materials, sintering process, varistor, WC–Co, cobalt, nickel, films, sponges
Keywords [sv]
Kemi
National Category
Chemical Sciences
Research subject
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-1948ISBN: 91-554-5275-2 (print)OAI: oai:DiVA.org:uu-1948DiVA, id: diva2:161530
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
List of papers
1. Preparation of ZnO-based varistors by the sol-gel technique
Open this publication in new window or tab >>Preparation of ZnO-based varistors by the sol-gel technique
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1994 (English)In: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 4, no 4, p. 615-621Article 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.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-89780 (URN)10.1039/JM9940400615 (DOI)
Available from: 2002-04-03 Created: 2002-04-03 Last updated: 2017-12-14Bibliographically approved
2. Processing of ZnO-based varistors using oxide, alkoxide, nitrato-alkoxide and carboxylato precursors
Open this publication in new window or tab >>Processing of ZnO-based varistors using oxide, alkoxide, nitrato-alkoxide and carboxylato precursors
1997 (English)In: Journal of Sol-Gel Science and Technology, ISSN 0928-0707, E-ISSN 1573-4846, Vol. 8, no 1-3, p. 697-701Article 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.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-89781 (URN)10.1007/BF02436925 (DOI)
Available from: 2002-04-03 Created: 2002-04-03 Last updated: 2017-12-14Bibliographically approved
3. Solution Synthesis of Nano-Phase Nickel as Film and Porous Electrode
Open this publication in new window or tab >>Solution Synthesis of Nano-Phase Nickel as Film and Porous Electrode
2000 (English)In: Journal of Sol-Gel Science and Technology, ISSN 0928-0707, E-ISSN 1573-4846, Vol. 19, no 1-3, p. 353-356Article in journal (Refereed) Published
Abstract [en]

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 nmsized Ni crystallites. Deposition of a 1 M nickel precursor solution on polycrystalline Al2O3, SnO2 : F coated glassor titanium substrates spinning at 2700 rpm, followed by heat-treatment at 10±C/min to 400±C yielded smooth anddense nickel films consisting of 3–5 nm sized crystallites. The precursor concentrate was studied by FT-IR andthe phase development on heat-treatment was studied by thermogravimetric analysis, powder X-ray diffraction,scanning and transmission electron microscopy.

Keywords
solution processing, nano-phase nickel, film, sponge
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-89782 (URN)10.1023/A:1008762405755 (DOI)
Available from: 2002-04-03 Created: 2002-04-03 Last updated: 2017-12-14Bibliographically approved
4. Homogeneous WC–Co-Cemented Carbides from a Cobalt-Coated WC Powder Produced by a Novel Solution-Chemical Route
Open this publication in new window or tab >>Homogeneous WC–Co-Cemented Carbides from a Cobalt-Coated WC Powder Produced by a Novel Solution-Chemical Route
2007 (English)In: Journal of The American Ceramic Society, ISSN 0002-7820, E-ISSN 1551-2916, Vol. 90, no 11, p. 3449-3454Article in journal (Refereed) Published
Abstract [en]

A solution chemical route to cobalt-coated WC-powder is described that allows for the preparation of WC–Co powders and compacts having a carbon content very close to the desired carbon content even under an inert atmosphere. The microstructural homogeneity in the sintered WC–Co composites when using the Co-coated grains was found to be superior as compared with conventionally mill-mixed powders, and the structural changes in the individual WC-grains were found to be much smaller, which is ascribed mainly to the fact that the coated grains do not require a grinding step leading to the formation of a tail of smaller WC grain sizes.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-12096 (URN)10.1111/j.1551-2916.2007.01948.x (DOI)000250762100011 ()
Available from: 2007-11-26 Created: 2007-11-26 Last updated: 2017-12-11Bibliographically approved
5. A New Solution Synthetic Route to Nano-Phase Cobalt Film and Sponge
Open this publication in new window or tab >>A New Solution Synthetic Route to Nano-Phase Cobalt Film and Sponge
Manuscript (Other academic)
Identifiers
urn:nbn:se:uu:diva-89784 (URN)
Available from: 2002-04-03 Created: 2002-04-03 Last updated: 2010-01-13Bibliographically approved

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