Phase-transformation kinetics of TiO₂ in TiO₂/ (O′+β′)-sialon multi-phase ceramics

TiO₂/(O′+β′)-Sialon multiphase ceramics with different phase composition of TiO₂ were prepared by pressureless sintering under high-purity N₂ atmosphere with (O′+β ′)-Sialon powder and nano TiO₂ (anatase) powder as raw materials, Yb₂O₃ or Tb₂O₃ as additive. For each sample, the weight percentage of anatase in TiO₂ was calculated from XRD data and the kinetics of anatase-rutile transformation was investigated, wherein the emphasis was placed on the influence of Yb ₂O₃ and Tb₂O₃. The results indicate that the added Tb₂O₃ and Yb₂O₃ serve the significant function of inhibition and promotion on the phase transformation, and the effects are enhanced and attenuated with increasing additive content, respectively. For the sample without additive, the transformation process follows apparent first-order kinetic model. The addition of Yb₂O₃ or Tb₂O₃ results in completely different transformation kinetic law. For the samples with Yb ₂O₃ added, the transformation is an apparent second-order reaction, whereas a unique kinetic model, C_A=kt^1/2+C, is valid for the samples containing Tb₂O₃. In the two cases, the effect of the additive content on the transformation can be perfectly reflected by the apparent rate constant.