Hiroyuki Nakae

Synthesis and structure of chemically vapour-deposited boron nitride

Chemically vapour-deposited boron nitride (CVD-BN) plates have been synthesized on a graphite substrate by the reaction of the BCl3-NH3-H2 gas system in a deposition temperature (Tdep) range from 1200 to 2000° C, with a total gas pressure (Ptot) which was varied from 5 to 60 torr. The effects ofPtot andTdep on the crystal structure and the microstructure of the CVD-BN plate were investigated. Turbostratic BN(t-BN) was deposited above 10 torr, at anyTdep in the range investigated. The interlayer spacing (c0/2), the crystallite size (Lc) and the preferred orientation (PO) were strongly affected byTdep. The t-BN obtained at lowTdep had largec0/2 and smallLc andPO. AsTdep increased,c0/2 tended to decrease whereasLc increased and thec-plane of the crystallites became oriented parallel to the deposition surface. At aPtot of 5 torr, a mixture of t-BN and h-BN (hexagonal BN) was deposited at anyTdep above 1700° C, and two kinds of t-BN different inc0/2 co-deposited at aTdep below 1600° C. Moreover, it was indicated that r-BN (rhombohedral BN) was included in the deposits obtained at aPtot of 5 torr and aTdep of 1500 to 1600° C.

Synthesis of cubic boron nitride from rhombohedral form under high static pressure

The cubic, zincblende-type boron nitride (z-BN) has been synthesized from the rhombohedral form (r-BN) under high static pressures greater than 6 GPa without any planned addition of catalysts. The process of forming z-BN has been delineated from isobaric and isothermal series of data. At 6GPa, r-BN begins conversion to the graphite-type form (g-BN) upon heating to 600 °C. This conversion terminates at 1200 °C forming single-phase g-BN, which in turn transforms into z-BN at temperatures higher than 1300 °C. The appearance of z-BN occurs at lower temperatures when the pressure is raised to 7 or 8 GPa. At pressures beyond 10 GPa the wurtzite-type form (w-BN) is observed between 400 and 1200 °C, whereas z-BN is formed above 1000 °C. The boundary of pressure-temperature conditions for synthesizing z-BN from r-BN runs through 6GPa and 1300 °C, and is located near to the lowest bound hitherto known for non-catalytic z-BN synthesis from g-BN.

Density and deposition rate of chemical-vapour-deposited boron nitride

A study is made on the density and deposition rate characteristics of chemical-vapour-deposited boron nitride (CVD-BN) plates synthesized by use of the BCl3-NH3-H2 system at a deposition temperature (Tdep) of 1200 to 2000°C and a total gas pressure (Ptot) of 5 to 60 torr. At aPtot of 5 torr, all the CVD-BN plates synthesized at eachTdep above 1300°C had a density greater than 2.O g cm−3, and thus showed no noticeable dependence onTdep. Over thePtot range from 10 to 60 torr. on the other hand, the density of the plates reached the maximum of 2.08g cm−3 at aTdep of 2000° C. AsTdep was lowered, the density decreased down to a minimum of 1.40 g cm−3 The deposition rate varied with bothTdep andPtot and showed a maximum value under a certainPtot at a givenTdep. The value ofPtot where the deposition rate becomes maximum changed depending on theTdep. The maximum deposition rate was 0.6 mm h−1 for the CVD-BN plates when the density was less than 2.0 g cm−3, and 0.4 mm h−1 when the density was above 2.0 g cm−3 The effects of deposition conditions on the characteristics of density and deposition rate are discussed in terms of the structure and deposition mechanism.

Behaviour of B-N-Si under high pressure and high temperature

The phase relations of the B-N-Si system have been studied using a quenching method up to 10GPa and 2000 °C using a high-pressure apparatus of the octahedral anvil type. Pressure-temperature conditions for obtaining z-BN (diamond analogue of boron nitride) were delineated for turbostratic BN (t-BN), t-BN/amorphous Si3N4 and t-BN/β-Si3N4. These conditions shift toward higher regimes of temperature as amorphous Si3N4 or β-Si3N4 is incorporated into t-BN. Spontaneous sintering occurringin situ at high pressure yields z-BN-based composite compacts.

Formation of novel sintered composites by high-pressure crystallization of amorphous ceramics

A combination of chemical vapour deposition (CVD) and high-pressure (HP) techniques has achieved the fabrication of the zincblende form of boron nitride (z-BN)-based ceramic composites. The CVD technique provides amorphous B-N-X (X = aluminium, silicon or titanium). The HP technique renders the amorphous samples crystalline and decomposition into z-BN plus the respective nitride (β-Si3N4 for instance) occurs. Compacts of the composites are obtained byin situ sintering under high pressure. The compacts exhibit peculiar microstructures composed of z-BN nano-crystals homogeneously dispersed in the nitride matrices.