Toshitsugu Matsuda

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.

Formation and structures of multiply-twinned nanoparticles with fivefold symmetry in chemical vapor deposited boron nitride

Abstract Formation and structures of multiply-twinned nanoparticles with fivefold symmetry in chemical vapor deposited boron nitride (BN) were investigated by transmission electron microscopy, electron diffraction and molecular orbital calculation. Sizes and nanostructures of the BN nanoparticles were strongly dependent on the deposition temperatures, total gas pressure and synthesis gas. Hexagonal- and rhombohedral-BN were produced at temperatures above 1700 °C and below 1600 °C, respectively. A three-dimensional model for the fivefold BN nanoparticles is proposed; the fivefold symmetry showed some distortion due to the geometrical arrangements along the fivefold axis.

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.

Pressureless sintering of silicon-nitride composites

Abstract A fabrication method for whisker-reinforced silicon-nitride ceramics involving pressureless sintering is reported together with the mechanical properties of the resulting composite materials. Silicon-nitride composites containing up to 20 wt% of silicon carbide whiskers were slip cast to orientate the whiskers two-dimensionally, parallel to the plaster mould surfaces, and then pressureless sintered at 1750 or 1825°C. The sintered body containing 20 wt% of whiskers of average diameter 1·3 μm displayed a relative density of 98%, a mean flexural strength of 950 MPa and a fracture toughness of 7·0 MPa√m . Furthermore, the thermal shock resistance was found to be as high as 1400°C. To date, components successfully produced from this composite material include aluminum diecasting ladles and crucibles.

Sintering anisotropy in slip-cast SiC-whisker/Si3N4-powder compacts

Sintering anisotropy in slip-cast SiC-whisker/Si3N4-powder compacts was studied at 1750°C in 0.1 MPa N2 or at 1825°C in 1.0 MPa N2. It was shown that whiskers oriented parallel to the mould surface and nearly 1.5-dimensionally along the slip flow direction when the whisker content was 10 wt%. Linear shrinkage was largest perpendicular to the mould surface and smallest perpendicular to the whisker alignment. It was shown that the retardation of densification by whiskers is due to the formation of a rigid network along the whisker alignment, which is in accordance with percolation theory. The addition of up to 20 wt% whisker did not affect sintering kinetics but lowered sinterability by 2-dimensional alignment of the whiskers. The anisotropy in fracture toughness is related to the orientation of the whiskers.