Al₂TiO₅の共析変態の速度論的研究

抄録

The present study was undertaken in an attempt to better understand the decomposition behavior of Al₂TiO₅, known as one of the materials with negative thermal expansion coefficient. Techniques employed are X-ray diffraction analysis, dilatometry and SEM. The eutectoid temperature has been estimated to be 1295±10°C by isothermal heating of sintered β-Al₂TiO₅ with or without preliminary decomposition anneal.
Decomposition isotherms are characterized by sigmoidal shape with a miximum rate at approximately 1150°C, and kinetics has been best described by the Avrami equation ln (1-α)=-ktⁿ(n=2), where α, t, k and n are fractional decomposition, time and proportionality constants respectively. It is shown that the preliminary annealing at low temperatures promotes the decomposition with the maximum-rate temperature increased by approximately 30°C. Arrhenius plot of ln k vs. 1/T yielded an activation energy, of 90±5kcal/mol below the maximum-rate temperature, regardless of the preliminary annealing, and the value has been considered to comprise with the growth or phase boundary process. Results of dilatometry revealed that the effect of preliminary annealing is more pronounced in the cooling run. Summarizing with the results of SEM, it is proposed that nucleation rather than growth plays an important role in the process.