The role of interfaces on an apparent grain size effect on the dielectric properties for ferroelectric barium titanate ceramics

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Abstract
We report the effect of interfaces (and thus internal surface area effects) on the value of dielectric constant (K′) calculated from capacitance and geometry data for sub-micron barium titanate (BaTiO3) ceramics prepared with decreasing grain size (and grain volumes). A series model is proposed to explain the decreasing values of apparent K′ obtained for grain sizes below 0.5 μm. A distinction is made between the true dielectric constant (K′) and the apparent dielectric constant (K′) calculated from experimental data. The progressive suppression in K′ is explained in terms of ferroelectric grains of constant dielectric constant (K′1) separated by a lower-K 2 boundary region (i.e., grain boundary) of constant thickness (d 2). The problem is one of an increasing interfacial surface area to grain volume ratio in fine-grain dielectrics. We begin by reporting original dielectric data for high pressure-densified ultrafine-grain BaTiO3 ceramics. Chemically prepared BaTiO3 powder was consolidated at high pressure (8GPa) and low temperature to prepare ultrafine grain microstructures. Specimens pressed at 700°C were substantially dense (≥98% of theoretical density) with an average grain size of 70 nm. Subsequent heat treatment at increasing temperatures (and atmospheric pressure) yielded a series of specimens with increasing grain sizes up to 20 μm. High-pressure consolidation at room temperature, followed by heat treatment at 800°C (and atmospheric pressure), yielded a finer-grain (40 nm) and less dense (≈90%th) microstructure. The temperature dependence of K′ was determined for ceramics ranging in grain size from 40 nm to 20 μm. Curie-Weiss analysis of the dielectric data clearly suggests a series dilution of the composite K′ by the interfacial grainboundary regions. We were able to model the dielectric measurements by considering grain boundary regions of relaxed thickness d 2 = 8 Å and dielectric constant K′2 = 130. Polarization reversal and Curie-Weiss characteristics were observed for all BaTiO3 ceramics studied in this investigation. Thus, the conclusions are made that if a critical grain size exists for ferroelectricity it is less than 40 nm for polycrystalline BaTiO3, and the role of interfaces is extremely important on the calculated values of K′ for fine-grain ceramics below 0.5 μm.