Dielectric and microstructural behaviour of BaTiO3 sintered in the presence of a crystallizable glass

Abstract

Barium titanate is an important material used for making ceramic capacitors. It is normally sintered at temperatures greater than 1300°C [1]. The use of titanate-based ceramics in multilayer capacitors necessitates the use of either a higher temperature sintering electrode material combination of platinum, palladium or gold [2], or the addition of a small amount of fluxing agent or low-melting glass powder to promote densification by liquid phase sintering at lower temperatures. It was found that the addition of 0.3 to 0.5 wt % LiF yielded dense BaTiO3 at temperatures around 800 to 900°C [3, 4]. BaLiF 3 [5], various oxides B203, CuO, PbGeO3 [6, 7] and various glasses containing metal oxides such as PbO, BaO, Bi203, CdO and ZnO [8] have been used to lower the sintering temperature. The addition of the fluxing agents results in a decrease in the dielectric constant, mainly due to the dilution of the high dielectric constant ceramic with a low dielectric flux or glass. To obtain a high dielectric constant, a variety of glass compositions in the system CdO-Bi203-PbO-B203 were examined for their ability to cause grain growth during sintering and in the sintering process to be homogeneously assimilated so that they do not remain as diluents. In the present work we have chosen a crystallizable glass in the system PbO-BaO-TiO2-B203-SiO2 as an additive for the sintering of BaTiO3. It was reported that this glass can be ceramized by suitable heattreatment to form glass ceramics [10]. During ceramization BaTiO 3 crystallites precipitate out whose nucleation and growth rate depend on the composition of glass and temperature of heat-treatment [11]. We thought that during heat-treatment for sintering either the glass will help the growth of BaTiO3 grains or will crystallize itself. The crystallization will proceed from the surface where the glass touches the BaTiO3 grains. The BaTiO3 and glass mixture was subjected to various types of heat-treatment schedules for sintering and crystallization. The dielectric properties and microstructure of the resulting samples have been determined to correlate the dielectric behaviour with heat-treatment schedules, i.e. processing parameters. BaTiO3 powder was prepared using reagent-grade BaCO3 and TiO~, and its dielectric behaviour characterized. The behaviour was similar to thermochemically prepared BaTiO3 reported in the literature. The glass having the composition 21PbO-21 BaO-42TiO210B203-6SIO2 (mol %) was prepared by melting it in an alumina crucible at 1200°C for 1 h and quenching the melt into distilled water. The glass frit was powdered and mixed with BaTiO3 in an agate morter. The mixture had the composition 30% glass and 70% BaTiO 3 (wt %). Disc shaped pellets were prepared by pressing in a three-piece steel die. A few drops of 2% solution of PVA were added to the powdered mixture as binder. The crystallisation temperature is reported to be 625°C [10]. We chose the following heat-treatment schedules for sintering these pellets. 1. The pellets were placed in an alumina crucible in an electrical furnace and were heated at a constant rate to the sintering temperature. After a given soaking time these pellets were furnace cooled. 2. A two-stage heat treatment was given. Initially the pellets were heated to the crystallization temperature and kept there for long durations, then the temperature was raised to the sintering temperature and after a short duration the pellets were cooled in the furnace. 3. In another two-stage heat-treatment schedule,