Barium titanate (BaTiO3) powders of varying particle
sizes were grown under hydrothermal
conditions at a temperature of 240 °C. This was accomplished by
varying the hydroxide
concentration of the reaction medium as well as the time required for
synthesis. The
spectroscopic, thermal, and microscopic characteristics of three
BaTiO3 powders with average
particle sizes 0.09, 0.3, and 0.5 μm were examined. Transmission
electron microscopy
indicated that all the particles were of single domain. The powder
with smallest particle
sizes (0.09 μm) contained unreacted titanium dioxide, whereas the
larger particles (0.3, 0.5
μm) were pure barium titanate. The Raman spectra showed that all
these crystals were
tetragonally distorted. Infrared spectra showed primarily Frohlich
modes, whose intensities
also decreased with particle size. Analysis of the broadening of
the powder diffraction
patterns suggested that the BaTiO3 powders exhibited small
strains. Differential scanning
calorimetry showed a decrease in enthalpy of transition
(ΔH) values with particle size. The
particle size dependence on ΔH is attributed to the
transition from a polar to a nonpolar
state which occurs due to the bulk dipoles disordering due to
interaction with the surface
dipoles.
The sintering behavior of hydrothermally synthesized cubic and tetragonal BaTiO3 powders was studied as a function of temperature. In addition, the dielectric properties of the sintered samples were examined from room temperature to 200°C at a frequency of 1 kHz. The influence of powder characteristics and sintering temperature on the microstructure development and dielectric properties were investigated. It was found that tetragonal powders showed limited grain growth and higher final density upon sintering, whereas the cubic powder exhibited abnormal grain growth and lower density. This was attributed to more disperse and homogeneous particles in the tetragonal powders with fewer defects, as compared to the cubic powder. Correlations between the microstructure and the dielectric properties were developed. As expected, samples with finer grain size and higher density exhibited higher dielectric constant and lower loss.
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