Osamu Fukunaga

High temperature cubic boron nitride P-N junction diode made at high pressure

A p-n junction diode of cubic boron nitride was made by growing an n-type crystal epitaxially on a p-type seed crystal at a pressure of 55 kilobars and a temperature of about 1700�C. A temperature-difference solvent method was used for the crystal growth, and beryllium and silicon were doped as acceptors and donors, respectively. Formation of the p-n junction was clearly confirmed at 1 bar by rectification characteristics and by existence of a space charge layer of the junction as observed by electron beam induced current measurement. This diode operated at 530�C.

High-pressure sintering of cBN-TiN-Al composite for cutting tool application

Abstract Microhardness, phase combination and microstructure of the c(cubic)BN–TiN–Al composite sintered under static pressure of 5.8 GPa and at temperatures of 1200 and 1400 °C for 30 min were investigated. Microhardness increased with increasing cBN content of the specimens sintered at both temperatures, and the highest microhardness of 30.7 GPa was achieved for the 75 vol.% cBN–13 vol.% TiN–12 vol.% Al composition specimen sintered at both 1200 and 1400 °C. Most of the samples sintered at 1200 °C had higher microhardness than the same composition specimen sintered at 1400 °C. During high-pressure sintering, reactions occurred between cBN, TiN and Al, then new compounds, TiB2 and AlN, were formed. TEM analysis indicated that all cBN and TiN grains were surrounded with an AlN layer of different thickness, and small TiB2 and AlN grains were co-existed to form clusters.

Phase transformations in ABO 4 type compounds under high pressure

For ABO4 type ternary oxides, high pressure phase transformations known up to the present are reviewed, and an attempt is made to explain and predict crystal structures of their high pressure phases. When ABO4 type compounds are plotted based on the two variables, k=rA/rB and t=(rA+rB)/2rO, where rA, rB, and rO are the ionic radii of A and B cations and divalent oxygen, they can be classified into the major structure types. It is found empirically that a compound basically transforms to the structure type isostructural with a compound lying in a classified area with the same k and larger t values in the diagram.

High pressure sintering of diamond-SiC composite

Sintered polycrystalline compacts in the system diamond-10–50 wt% SiC having average grain size of less than 1 μm were prepared at pressure of 6 GPa and temperature between 1400 and 1600 °C. Knoop indentation hardness of the compacts increased with diamond content and sintering temperature, and specimens with a Knoop indentation hardness greater 40 GPa were obtained. It was found that small amount of Al addition into the starting diamond-SiC powder was effective to improve relative density and Knoop indentation hardness of the compacts. The formation of graphite was also suppressed by the addition of Al. Microstructure observation by SEM and TEM suggested that Al segregated at the grain boundary and promoted the bonding between grains. Thin microtwins were observed in diamond grains, whereas fine wavy structures with slightly different orientations were observed in SiC grains, with or without Al addition.

Growth pressure-temperature region of cubic BN in the system BN-Mg

The growth pressure-temperature region of cBN in the system BN-Mg was determined under the conditions up to 8 GPa and 2300° C. Hexagonal BN with different oxygen contents (1.9 wt % for R-type and 7.9 wt % for N1-type) was used as a starting material. The lower temperature limit of the cBN region obtained from the R-type is about 1380° C under pressures of 6 to 8 GPa. This limit can be compared with the eutectic point in the system hBN-Mg3B2N4. The data suggest that cBN crystals grow through the dissolution and precipitation process from a eutectic liquid. The cBN region obtained from the N1-type is located at higher temperatures than that of the R-type, the lower limit of which is reached at about 1700° C at 6 GPa. MgO and/or Mg3 (BO3)2 are formed as by-products in such a system. The finding implies that Mg3B2N4, a solvent of BN, reacts with oxide impurities (especially B2O3) by the following reaction; Mg3B2N4+3/2O2=3MgO+2BN+N2 or Mg3B2N4+3O2=Mg3 (BO3)2+2N2. It is deduced that the cBN growth region shifts towards higher temperatures depending on the effect of oxygen.

SYNTHESIS OF ZNO BICRYSTALS DOPED WITH CO OR MN AND THEIR ELECTRICAL PROPERTIES

Bicrystals of ZnO without intergranular additives were synthesized to elucidate the effect of Co and Mn doping on the varistor properties of ZnO ceramics. Nonlinear current-voltage (I–V) characteristics due to the formation of an interfacial barrier were observed in bicrystals doped with Co or Mn, while the nondoped bicrystals exhibited linear I–V relationship. Since the charge transport properties of bulk ZnO crystals were not changed, it is indicated that the energy dispersion and/or density of the interfacial states were changed by the Co and Mn doping.

Effect of solvent metals upon the morphology of synthetic diamonds

Abstract Diamond crystals up to 3 mm in diameter were grown from various metal solvents, and the effect of solvent metals upon the morphology was analyzed by the observation of the surface microtopography of as-grown faces and internal morphology. From a Ni solution only {111} and {100} faces develop, whereas {110} and {113} are associated when solvents of Ni-based alloys, Co and Fe, are used. In any metal solvents, {111} and {100} appear as smooth faces on which growth layers thinner than 30 A develop, whereas {110} and {113} are hoppered. The internal morphology of the respective growth sectors indicates that {110} and {113} are transientfaces, whereas {111} and {100} are persistent faces. The order of morphological importance is thus expressed as {111}>{100}a {110} ≈ {113}. The observed characteristics of the {100} faces are interpreted in terms of a surface reconstruction model, and the appearance of the {110} and {113} faces is explained in terms of the adsorption of molecules of metallic nitride bonding with two carbon atoms at third neighbour sites on the diamond surface.

Pressure and temperature control in flat-belt type high pressure apparatus for reproducible diamond synthesis

Abstract This paper describes a technique on the pressure and temperature control in flat-belt (FB) type high pressure apparatus to conduct stable diamond synthesis study. We present results on the reproducibility of the sample pressure and temperature in different sets of diamond synthesis runs. We propose a method of monitoring these variations by the measurements of anvil displacement as a function of press load, and of heater resistance as a function of input power to the heater in every high pressure run. We examined pressure and temperature region of diamond formation in graphite-solvent metal systems, and propose a method to estimate the absolute pressure and temperature of the sample based on the graphite–diamond equilibrium curve. From these data, we conclude that the pressure and temperature of the sample in FB type high pressure apparatus can be estimated and controlled within about 3%.

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.

Sintering behaviour of the diamond-cobalt system at high temperature and pressure

A powder mixture of diamond—8.9 vol % Co was consolidatedin situ on a WC-10 wt % Co base at temperatures of 1300 to 1500° C under a pressure of 5.8 GPa. The sintered body obtained at 1300° C, which is below the diamond—cobalt eutectic point, was not hard, and the surface of the diamond particle was partially graphitized. On the other hand, the sintered body obtained at 1400 to 1500° C was fairly hard. A strong correlation was also observed between hardness and the cobalt content found in the sintered body. The cobalt content in the harder sintered body was clearly lower compared with that of the softer one. The surface graphitization of the diamond particles is necessary to the transfer of cobalt during the sintering of diamond. In sintering the diamond-cobalt system, the sinterability of diamond was closely related to the feasibility of transformation from diamond to graphite.