Papers by Author: Isao Tanaka

Abstract: The phase stability of silicon nitride is examined using a series of first principles phonon calculations. -phase shows slightly higher free energy than  in the temperature range from 0 to 2000K. The difference between  and is only 0.02 eV/Si3N4 or 2 kJ/mol at 300 K. The result is consistent with the experimental report by high-temperature oxide-melt-drop solution-calorimetry. Similar calculations are made for ZrO2, Ga2O3, BN and some perovskites. Using cluster expansion technique combined with a large set of first principles calculations, the cation disordering of II-III spinel oxides and the phase diagram of diamond vs. c-BN are theoretically investigated. Theoretical results on the atomic structures of  and  sialons are also shown.

Abstract: Our recently proposed calculating method reliably predicts the nucleation free energy barrier of the homogeneous and coherent precipitations. Helmholtz free energy change is clearly defined and calculated by the purely enthalpic and entropic contributions between the initial state of the isolated solute atoms scattering around the matrix and the final state of the cluster of size n traveling around the matrix. The enthalpic term is calculated by the reliable first principles method and the entropic term is estimated by the ideal solution model. The vibrational free energy is also included by the quasi-harmonic approximation. The model calculation was performed on bcc Cu precipitations in the Fe-Cu system. The predicted values of the critical number of 12 atoms and the critical free energy barrier of 0.6eV show good agreement with the experimentally estimated ones for the annealing temperature of 773K and the initial concentration of 1.4at%Cu.

Abstract: The formation of Mg vacancy induced by ultra-dilute trivalent impurities in MgO is investigated by a combination of positron lifetime measurements and first-principles calculations. The undoped MgO yields the shortest positron lifetime of 140 ps that is shorter than that of a single crystal sample. The positron lifetime of the doped samples increases with the increase of the Al dopant concentration and is saturated at around 180 ps. This result clearly indicates that the formation of Mg vacancy is induced by Al dopant. The concentration of the other trivalent impurities can be evaluated using the result of component analysis of positron lifetimes. The experimental bulk lifetime of 130 ps, which is obtained by employing trapping model, is well reproduced by the theoretical calculation using the semiconductor model. The calculated defect lifetime is about 20 ps longer than the experimental value. This may be due to the lattice relaxation around Mg vacancy associated with the trapping of positrons.