Effect of microwave frequency on hydrothermal synthesis of nanocrystalline tetragonal barium titanate

Nanocrystalline tetragonal barium titanate (BaTiO3) with particle sizes ranging from 30 to 100 nm were synthesized via microwave-hydrothermal routes at various fixed microwave frequencies and also using variable frequency with 1-5 s sweep times. The effects of microwave frequency, microwave bandwidth sweep time, and aging time on the microstructure, particle sizes, phase purity, surface areas, and porosities of the as- prepared BaTiO3 were investigated systematically. The crystallized BaTiO3 powders were characterized by X-ray diffraction, Raman spectroscopy, thermal analysis, infrared spectroscopy, field emission scanning electron microscopy, and transmission electron microscopy with an EDS analyzer. The results show that the particle sizes, morphologies, and surface areas of the products are influenced by the microwave frequency and bandwidth sweep time. High microwave frequency (5.5 GHz) and variable frequency (3-5.5 GHz to 1 s) led to spherical particles with narrow and more uniform particle size distributions. BaTiO3 prepared using the standard 2.45 GHz yielded particles with a cubic microstructure. The surface areas of the prepared powders decreased with aging time using 4.0 and 5.5 GHz, but increased gradually with extended aging time in variable frequency (3-5.5 GHz to 1 s) processing. The dependence of properties of barium titanate on microwave frequency could be due to different transverse magnetic modes at different frequencies. For comparison purposes, conventional hydrothermal experiments were also performed under similar conditions as in microwave hydrothermal routes.