Preparation of Fine-grained BaTiO 3 Ceramics by Spark Plasma Sintering
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Claudio Capiglia Department of Chemistry, Faculty of Engineering, Mie University, Kamihama-cho 1515, Tsu, Mie, 514-8507 Japan
Nalini Balakrishnan National Institute of Advanced Industrial Science and Technology, AIST, Midorigaoka 1-8-31, Ikeda, Osaka, 563-8577 Japan
Yasuo Takeda Department of Chemistry, Faculty of Engineering, Mie University, Kamihama-cho 1515, Tsu, Mie, 514-8507 Japan
Hiroyuki Kageyama National Institute of Advanced Industrial Science and Technology, AIST, Midorigaoka 1-8-31, Ikeda, Osaka, 563-8577 Japan (Received 13 June 2001; accepted 19 December 2001)
Dense BaTiO3 ceramics consisting of fine grains were prepared using fine powder (average grain size of 0.06 m; BT006) as a starting material and the spark plasma sintering (SPS) method. The powder was densified to >95% of theoretical x-ray density by the SPS process, and the average grain size of the resulting ceramics was 1000, and will continue to be used in the manufacture of thermistors, multiplayer capacitors, electro-optic devices, and dynamic random access memories (DRAM) well into the future.1 Improved performance, high permittivities, and miniaturization of BaTiO3-based devices can be achieved by controlling aspects of the ceramic microstructure such as grain size and distribution, porosity, and homogeneity. These features depend on the properties of the starting BaTiO3 powder and the sintering method.1–3 In particular, grain size can have a dramatic influence on dielectric properties and crystal structure for BaTiO3 ceramics; 4 it has been widely reported that roomtemperature permittivity shows an apparent maximum at a grain size of approximately 1 m.1–12 Therefore, obtaining an approximately 1 m grain size is conducive to the production of high-permittivity BaTiO3-based devices. a)
Address all correspondence to this author. J. Mater. Res., Vol. 17, No. 3, Mar 2002
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It is well known that BaTiO3 undergoes exaggerated grain growth during conventional sintering in a muffle furnace at elevated temperatures and over long sintering periods.3 Frey et al.12 and Oonishi et al.13 have prepared dense BaTiO3 ceramics (>98% of theoretical x-ray density) with fine grains (0.07–0.6 m) by high-pressure consolidation or hot isostatic pressing (HIP). They applied a pressure of 8 GPa or 500 MPa to the BaTiO3 powders before or during sintering. This was sufficient to generate internal stress leading to a change in the c/aratio of the tetragonal unit cell, the Curie temperature, and permittivity of the resulting BaTiO3 ceramics.14,15 Spark plasma sintering (SPS) is a process that makes use of a microscopic electrical discharge between particles under mild pressure (30–50 MPa).16 Although SPS is commonly used to produce dense metal and engineering ceramics, there are relatively few reports on the application of this technique to produce dense oxide-based electroceramics. The SPS process enables a compact powder to be sintered to high density at relatively lower temperatures and in much shorter s
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