Abstract
An ultrafine BaTiO3 powder of 80 nm diameter was consolidated to powder compacts with 58% TD and uniform structures by filtration of 2-methoxyethanol, 2-ethoxyethanol or methanol suspension and subsequent isostatic pressing under a pressure of 294MPa. These compacts were sintered into dense BaTiO3 ceramics with average grain sizes of 1-2μm (>95% TD) within 2h at 1200-1250°C. Increasing the heating rate to the sintering temperatures of 1000-1300°C was effective in increasing the density. Grain growth from 0.5 to 2μm caused significant increase of the c-parameter of BaTiO3, resulting in the increase of tetragonality (c/a ratio). The phase transition from tetragonal to cubic form at room temperature was estimated to occur below 0.4μm average grain size. The relative dielectric constant (ε′) at room temperature was highest (≈6000 at 1kHz) in the high-density BaTiO3 with an average grain size of 1.4μm, sintered at 1200°C. Increase of the grain size to 2.3μm in dense BaTiO3 caused the decrease of the ε′ value. The low-density and small-grained (0.5μm) BaTiO3 also showed a low ε′ value (1400 at 1kHz). The ε′ values of all samples were almost independent of frequency in the range of 100Hz to 500kHz. The dielectric loss was strongly affected by the porosity and decreased at higher density. The decrease of grain size reduced the ε′ values below and above the Curie temperature, leading to the broadening of ε′ in a wide temperature range (18-220°C).
