FORMATION OF DENSE BA0.86CA0.14TIO3 PARTICLES BY AEROSOL DECOMPOSITION

Submicron particles of barium calcium titanate (Ba0.86Ca0.14TiO3) were generated by aerosol decomposition at reactor temperatures ranging from 900 to 1600-degrees-C. X-ray diffraction showed phase-pure tetragonal Ba0.86Ca0.14TiO3 for all reactor temperatures. Particle morphology could be controlled by varying reactor temperature. Scanning electron microscopy and transmission electron microscopy showed that polycrystalline, hollow particles were formed at 900-1100-degrees-C, partially densified particles were formed at 1200-1400-degrees-C, and solid particles were formed at 1500-1600-degrees-C. Specific surface areas decreased from 14 to 4 m2/g, and green densities of dry-pressed pellets increased from 38% to 65% of theoretical density as the reactor temperature was increased from 1000 to 1600-degrees-C. Hollow particles were produced at low temperatures (< 100-degrees-C) as a result of solvent evaporation in the beginning of the reactor tube. The hollow particle morphology persisted as the temperature increased to 1000-degrees-C and the precursors reacted to form Ba0.86Ca0.14TiO3. At higher reactor temperatures, the hollow particles densified in the aerosol phase with residence times on the order of 1 s. Sedimentation velocity measurements showed a narrower size distribution and smaller average size than powders made from the same precursor solution by reaction and milling. Pellets pressed from the powders also showed a lower densification temperature by dilatometry than pellets formed from powders produced by reaction and milling. Sintered ceramics had densities ranging from 89% to 98% of theoretical density and showed a useful positive temperature coefficient of resistance.