Crystallization of hafnia and zirconia during the pyrolysis of acetate gels
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Crystallization of hafnia and zirconia during the pyrolysis of acetate gels Masatomo Yashimaa) Department of Materials Science and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama-shi, 226-8502, Japan
Taka-aki Kato, Masato Kakihana, and Mehmet Ali Gulgunb) Materials and Structures Laboratory and Center for Materials Design, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama-shi, 226-8503, Japan
Yohtaro Matsuo Department of Inorganic Materials, Faculty of Engineering, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
Masahiro Yoshimura Center for Materials Design, Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama-shi, 226-8503, Japan (Received 27 January 1997; accepted 25 February 1997)
Hafnia and zirconia gels were prepared by drying hafnyl or zirconyl acetate solutions. Hafnia and zirconia gels contain both hydroxyl group and bidentate acetates which are directly bonded to the metal ions. Thermal decomposition and crystallization behavior of the gels were investigated through XRD, FT-IR, and TEM. Hafnium-containing gels crystallized directly into stable monoclinic hafnia around 500–540 ±C, while zirconium-containing gels first formed metastable tetragonal zirconia around 450 ±C. The dissimilar crystallization behavior of the gels into metastable, tetragonal zirconia or into stable, monoclinic hafnia can be explained through the difference in free-energy changes of the tetragonal-to-monoclinic phase transformation.
I. INTRODUCTION
Hafnia (HfO2 ) and zirconia (ZrO2 ) have similar physical and chemical properties.1–4 Undoped hafnia exhibits the following phase transitions1–5 : monoclinic
–
1510 ±C–2000 ±C 1288 ±C 1800 ±C
tetragonal
2700 ±C
$
cubic
2900 ±C
$
melt .
(1)
The transition temperatures of zirconia are lower than those of hafnia3,6 : monoclinic
–
950±–1250 ±C 740±
1050
±C
2369 ±C
tetragonal $
2710 ±C
cubic $
melt .
(2)
Hafnia, being similar to zirconia in almost every aspect, is used in oxygen ion conductors, catalysts, and high-temperature structural applications.1–4 Just as with any other ceramic material, high purity hafnia powders with uniform particles of submicron size and a)
Author to whom correspondence should be addressed: e-mail: [email protected]; fax: 181-45-924-5630. b) Present address: Max Planck Institute, Seestrasse 92, D-70174 Stuttgart, Germany. J. Mater. Res., Vol. 12, No. 10, Oct 1997
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good sinterability are required.7 There is a special interest in developing simple, inexpensive, and reproducible methods to prepare powders with the above-mentioned characteristics at low temperatures. The acetate gel process8 which utilizes the thermal decomposition of acetates loaded with respective metal ion can fulfill these requirements. The acetate gel process has be
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