Grain growth and phase transformation of nano-sized titanium dioxide powder during heat treatment and spark plasma sintering

Abstract

The control of nanostructures is crucial for enhancing the thermoelectric properties of materials. In this work, the grain growth and phase transformation of different nano-sized titanium dioxide (TiO2) powders were investigated during heat treatment and spark plasma sintering. First, the thermal behaviors of TiO2 powders with different particle sizes during heat treatment, such as grain growth, phase transformation, and adsorption properties, were studied. The results showed that the phase transformation of 7 nm anatase TiO2 occurred more easily, and the rutile phase content increased with increasing heat treatment temperature. The grain growth of 7 nm anatase TiO2 occurred at low temperature (i.e., 873 K), even the grain size increased by 13-fold at 1123 K (i.e., the phase transformation temperature). Thermogravimetry results suggested that the 7 nm anatase TiO2 powder had a greater adsorption characteristic owing to its large specific surface area. Moreover, TiO2 compacts with and without graphite were fabricated using spark plasma sintering. The results indicated that a lower sintering temperature was beneficial for maintaining the nanostructure, whereas the addition of graphite powder promoted grain growth. Furthermore, the thermal conductivity was reduced owing to the nanostructure, and the phonon thermal conductivity accounted for a dominant proportion of the total thermal conductivity. Consequently, the lowest thermal conductivity was 1.1 Wm−1K−1 at 973 K for the nanostructured compact sintered at 1073 K, which is approximately 50% lower than that of the microstructured compact sintered at 1273 K.