Minoru Iwata

The synthesis of cBN using Ca3B2N4

The phase relation of the system Ca-B-N was investigated at a pressure of 2.5 GPa by both differential thermal analysis (DTA) and a quenching method. When BN reacted with Ca at temperatures higher than 1150° C, Ca3B2N4 was formed together with small amounts of Ca3N2 and CaB6. The system of Ca3B2N4-BN has a eutectic relationship at 1316° C and 2.5 GPa. The synthesis of cBN was established by using Ca3B2N4 under the thermodynamic stable conditions of cBN. It could be seen that cBN crystals grew repeatedly in Ca3B2N4 melt, which advanced into the hBN layer. The cBN crystals obtained in this way were of outstanding purity and of good quality. This indicated that hBN dissolved and precipitated as cBN in Ca3B2N4 melt without any appreciable change of molar boron/nitrogen ratio. In the system of Ca-B-N, the low-temperature limit of cBN formation was closely related to the eutectic relationship between BN and Ca3B2N4.

Formation mechanism of cBN crystals under isothermal conditions in the system BN-Ca3B2N4

Cubic BN was synthesized by treating hexagonal BN(hBN) in the presence of Ca3B2N4 which acted as a flux for BN. After the high pressure and temperature treatments, cBN was obtained in a part of hBN layer initially charged, but hardly in the flux layer. The grains of cBN were precipitated accompanied with the advance of Ca3B2N4 flux into hBN zone. From these observations, it can be deduced that hBN dissociated beneath the thin layer of flux advanced, and crystallized as cBN under an isothermal condition. In this system, the mechanism was basically the same as that of the diamond growth in the metal flux such as nickel and cobalt. Namely, the crystal growth of cBN was governed by the difference of solubility between hBN and cBN in a melted Ca3B2N4, because of the eutectic relation between BN and Ca3B2N4. For the nucleation process, two types of features were observed; one was the preferential nucleation at the boundary of hBN-flux, and the other was the homogeneous nucleation in the part of hBN layer initially charged.

Effect of oxygen on the growth of cubic boron nitride using Mg3N2 as catalyst

Cubic boron nitride (cBN) was synthesized from hexagonal boron nitride (hBN) under high pressure and high temperature using Mg3N2 as catalyst. The yield and morphology of cBN were investigated in relation to the oxygen impurity of the BN-Mg3N2 system. Magnesium oxide precipitated as a by-product in this system and the amount of the precipitate increased with an increase in the oxygen content of the starting materials. The morphology and surface patterns of cBN crystals synthesized using an hBN which contained oxygen showed unusual features. It was confirmed that the precipitation of MgO interfered with the free growth of cBN crystals. Purification of starting materials and addition of zirconium powder to the catalyst as an oxygen getter increased the yield of cBN crystals showing smooth surfaces.

Electron-phonon interactions in layered hexagonal boron nitride

Abstract The characteristics of electron-phonon coupling in layered hexagonal boron nitride are studied for vibronic transitions in typical defects, in view of the strong anisotropy inherent in its crystal structure. Participation of low-frequency vibrational modes is explained in connection with an approximation in which the motions of atoms in a layer containing a defect can be separated from low-frequency vibrations of atoms in surrounding layers. Two kinds of vibronic transitions, formerly ascribed to carbon acceptors and nitrogen-site vacancies, are examined.