Conventional and microwave sintering studies of SrTiO 3
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I-Nan Lin Materials Science Center, National Tsing-Hua University, Hsinchu 30043, Taiwan, Republic of China (Received 27 June 1994; accepted 26 April 1995)
Using the nonconventional sintering technique, such as microwave sintering, it is observed to enhance the densification rate of SrTiO3 materials as effectively as employing the highly active powders prepared by the chemical route. Although the chemically derived powders demonstrate better sinterability than the mixed oxide powders, the thermal analysis indicates that the segregation of Ti4+-containing clusters during decomposition of precursors in the direct pyrolysis (DP) process induces the occurrence of TiO2 particles (anatase phase) prior to the formation of SrTiCb phase. These particles retard the necking process required to sinter the materials. The spray pyrolysis (SP) process can circumvent the preferential nucleation of TiO2 phase and, therefore, produce powders exhibiting superior sintering behavior to the DP-derived powders. The microwave sintering technique, on the other hand, substantially enhances the rate of diffusion of the ions in the materials such that even the mixed oxide powders can be sintered at a temperature about 200 °C lower than that needed to achieve the same density in a conventional sintering process. Fine grain (~4 fim) microstructure is obtained for the materials microwave sintered at 1220 °C for 10 min. The migration of grain boundaries requires higher temperature to initiate than the formation of neckings between the grains. The grain growth occurs only when the material was sintered at temperatures higher than 1250 °C.
I. INTRODUCTION SrTiO3 (STO) materials are one of the family of the ferroelectric perovskites that possess paraelectric characteristics with dielectric constant e r = 285-320 at room temperature, since the tetragonal to cubic phase transformation of these materials occurs at Tc = -168 "C.1'2 The advantage of having cubic structure is that there is no sudden change in the electrical behavior during operation such that the electrical components made of these materials possess high stability with respect to temperature. They are, therefore, widely used as materials for high voltage capacitors,3 grain boundary barrier capacitors (GBBL), 45 low voltage non-ohmic thermistors (varistor),6'7 and voltage-controlled microwave dielectrics.8'9 Preparation of STO ceramics via conventional mixed oxide (MO) route is difficult due to the high sintering temperature (>1400 °C) needed to densify the materials. The low sinterability of the powders can be ascribed to (i) the large and nonuniform particle size resulting from the mechanical pulverization process, and (ii) low activity caused by the high calcination temperature adopted to convert the raw materials into prevoskite phase. Enhancing the densification rate of the STO ceramics such as to increase the sinterability of ceramic powders 2052 http://journals.cambridge.org
J. Mater. Res., Vol. 10, No. 8, Aug 1995 Downloaded: 18 Mar 2015
using the chemical method or to promote
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