Preparation of BaTiO 3 /SrTiO 3 composite dielectric ceramics with a flat temperature dependence of permittivity
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Composite dielectric ceramics of BaTiO3/SrTiO3 were prepared using the spark plasma sintering (SPS) method. Relatively dense (>92% of the theoretical x-ray density) composite ceramics were obtained for a relatively short sintering time (∼3 min). Such a short sintering time was advantageous in suppressing the formation of a complete solid solution (Ba,Sr)TiO3. Fixed-frequency (100 kHz), room-temperature permittivity measurements of the BaTiO3/SrTiO3 ⳱ 9/1 composites showed a flat temperature dependence within the range 30–110 °C. X-ray diffractometry measurements showed that the current SPS-BaTiO3/SrTiO3 composites consisted of BaTiO3 and small amounts of (Ba1−xSrx)TiO3 with various x values having different phase transition temperatures. Such multicomponent microstructures would be responsible for the flat temperature dependence of permittivity.
I. INTRODUCTION
Barium titanate (BaTiO3) is a well-known ferroelectric material with a high room-temperature permittivity value (approximately >1000) and will continue to be used in the manufacture of multilayer capacitors, thermistors, and electro-optic devices well into the future.1 For use in disk or multilayer ceramic capacitors, several properties such as higher permittivity with lower temperature and frequency dependences and lower dielectric losses are required, particularly at room temperature. Because pure BaTiO3 shows large changes in permittivity around the phase transition temperatures (at –90, 0, and 130 °C), many kinds of dopants, such as Ca2+, Mg2+, or Zr4+ cations, have been added to depress and/or broaden the permittivity maximum, leading to a lower temperature dependence of permittivity around room temperature.1–3 In addition to the above doped dielectric ceramics where homogeneous cation distributions were achieved, compositionally graded dielectric ceramics have been recently reported. 4–8 Fujii et al. prepared laminated Ba(Zr1−xTix)O3 ceramics with varying x values from one to the other side of the pellet and found a flat temperature dependence of permittivity in the range 20–120 °C.6 Ota et al. prepared similar (Ba1−xSrx)TiO3 laminated ceramics and reported a flat permittivity profile (4000–5500) in the temperature range 30–110 °C.7 In these composite dielectric ceramics, the observed apparent permittivity is the summation of the permittivity of each component, a)
Address all correspondence to this author. J. Mater. Res., Vol. 18, No. 8, Aug 2003
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and it is controllable by changing its volume fraction.1 Therefore, the appropriate lamination of the selected components having various phase transition temperatures produces a desirable permittivity profile versus temperature. Because the expansion coefficients of each layered component in these laminated ceramics are different, some improvements in the production process are required for preventing cracks between the layers. For example, Fujii et al. applied the hot isostatically pressing method to produce compositionally graded Ba(Zr1−xTix)O3 cera
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