Liquid-phase sintering of barium titanate with lithium fluoride
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INTRODUCTION Dielectric ceramics with high permittivity (e'r > 5000) and able to be sintered at low temperatures (t< 1000 °C) are of high interest for realizing less expensive multilayer capacitors: palladium could be then replaced by cheaper metals or alloys for electrodes, e.g., Ag or Pd-Ag. A potential candidate is barium titanate sintered at low temperatures because of the addition of lithium fluoride. More recent studies have shown that the amount of LiF, the Ba/Ti ratio, the temperature, and the time of sintering influence both densification and dielectric properties. '~6 Some authors have pointed out the composition of the ceramic grains to be that of an oxyfluoride, as a consequence of the reaction of BaTiO3 with BaLiF3 that appears intermediarily.3"5 The oxyfluoride perovskite solid solution Ba(Tij _X\AX )O3_3XF3X was actually characterized recently, thanks to investigation of the BaTiO3 -BaLiF 3 binary phase system.7 The fluorine-oxygen substitution brings an increase of the rhombohedral-orthorhombic transition temperature and a decrease of the two higher transition temperatures. The ferroelectric Curie temperature shifts then from tc BaTiO3 = 120 °C to room temperature. This result is in good agreement with earlier research.8 The ceramics obtained by LiF addition (2% in weight) to a mixture of barium titanates (BaTiO 3 + 0.03Ba 2 TiO 4 ) were sintered at 930 °C in an air atmosphere for 1.5 h. They exhibited a permittivity peak between 5° and 20 °C of about 6000. The exact temperature depended on the calcination conditions and on the heating and cooling rates. The grains crystallize with a perovskite-type structure and a cubic symmetry at 20 °C. A weight loss occurs during sintering. Those results have been described in a previous paper.3 The purpose of the present work was to improve our understanding of the sintering process using scanning transmission electron microscopy (STEM) microstrucJ. Mater. Res. 2 (4), Jul/Aug 1987
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tural investigations performed on ceramics sintered at different conditions. II. CERAMICS SYNTHESIS The oxide starting products used were prepared from BaCO3 and TiO 2 (Merck optipur). In the following we will refer to the composition BaTiO3 + 0.03 Ba 2 TiO 4 as BaTiO3 (1.03), taking into account a Ba/Ti ratio of 1.03. The mixture of BaTiO3 (1.03) and LiF (2% in weight) with water was ground for 24 h. After drying at 100 °C, a rhodoviol binder was added; the powder was then pressed as platelets of about 8X8X1 mm 3 . The binder was burnt out at 300 °C. After heating at a rate of 400 °C h " ' , three types of air treatments were carried out: (1) sintering at 930 °C for 1.5 h and then relatively slow cooling (100 °C h ~ ' ) (sample A)—this process is similar to that used previously in Ref. 3; (2) sintering at 930 °C for 1.5 h and then quenching in liquid nitrogen (sample B); (3) sintering at 730 °C for 15 h and then air quenching (sample C). A previous thermogravirrietric analysis showed occurrence of a weight loss above 750 °C. In order to study the microst
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