High-performance Ba,Ti,O,, ceramics are attracting great attention, but their formation mechanism still is somewhat unclear. The present investigation shows that the formation of Ba,Ti,O,, can be promoted strikingly by the participation of Bi,O, and AI,O,. The effect of Bi,O, on the formation of Ba,Ti,O,, is attributed to the fact that migration of the involved reactants is accelerated by liquid which forms from the melting of Bi,O, above 830°C. This migration, however, is not the only rate-limiting factor. A high potential-energy barrier, resulting from stress that arises along the crystal-structured layers, also heavily restricts the formation of Ba,Ti,O,,. The participation of AI,O,, on the other hand, can reduce the height of this potential-energy barrier and effectively improve the kinetics of the formation of Ba,Ti,O,, by causing the formation of BaAI,Ti,O,, crystals; these crystals intergrow with Ba,Ti,O,, crystals and result in decreased stress. I. IntroductionONOPHASE Ba,Ti,O,,, ceramics possess high dielectric constant, low temperature coefficient of dielectric constant, and low microwave loss, which are excellent properties in such applications as microwave dielectric resonators and filters for satellites.'-* To achieve monophase BaZTi,O,,, first recognized by Jonker and Kwestroo,' fabrication parameters must be chosen carefully. O'Bryan and Thornson, reported, as a prerequisite for the complete transformation of the mixed powders of raw materials into monophase Ba,Ti,O,,, during sintering, that some Ba,Ti,O,, should be created within the mixed powders after calcination. Jaakola et al.' noted that a reaction temperature of 1150°C or higher is necessary for the crystallization of Ba,Ti,O,,, phase within a reasonable time span in a mixture of BaCO, and TiO,. Furthermore, later studies6 revealed a curvature of the Ba,Ti,O,,, equilibrium phase boundary in the system BaO-TiO, at high temperature, which is an additional complication to forming monophase Ba,Ti,O,,.Both O'Bryan and Thornson, and Jaakola et al.' suggested that diffusion was the rate-limiting factor in controlling the formation of Ba,Ti,O,,, which could explain why the reaction kinetics are more rapid for a mixture of BaTiO, and TiO, than for one of BaCO, and TiO,: the BaTi0,-TiO, mixture transforms more easily into Ba,Ti,O,, because of a shorter diffusion distance, since BaTiO, has a Ti content closer to that of Ba,Ti,O,, than does BaCO,. Finally, Wu and Wang' proposed that the low formation rate of Ba,Ti,O,,,, compared with that of BaTi,O, and BaTi,O,,, was attributable to high surface and H. M. O'Bryan, Jr.--contrlhuting editor Manuwipt No. 196023. Received May 15, 1992; approved Decemher7, IY92interface energies. These researchers, however, could offer no convincing evidence to support their thesis. The kinetics of the reaction remain unclear, and investigations continue.The present investigation reveals, for the first time, that, in addition to reactant migration, a higher potential-energy barrier heavily restrains the formation of Ba,Ti,O...
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