Pyrolytic boron nitride

H-BN is produced by hot-pressing the powder or by CVD. The processes impart different properties. The hot-pressed material shows less anisotropy than the CVD BN, since the powder grains are randomly oriented. CVD BN is usually a turbostratic boron nitride with warped basal planes and lattice defects. It is also known as pyrolytic boron nitride or PBN.1 11 " ]... 

Boralloy Pyrolytic Boron Nitride, Technical Brochure, Union Carbide, Cincinnati, OH (1994)... 

Uses. Hot-pressed hBN is useful for high temperature electric or thermal insulation, vessels, etc, especially in inert or reducing atmospheres, and for special materials such as III-V semiconductors (qv). Its low thermal expansion makes it resistant to thermal shock. The powder can be used as a mold release agent or as thermal insulation. Boron nitride is also available in fiber form (19). BN deposited pyrolytically on refractory substrates at 1200—1800°C has a turbostratic structure and low porosity it has greater chemical resistance and is impervious to helium. 

Finally, an important form of boron nitride should be mentioned, pyrolytic boron nitride. It is manufactured by reacting ammonia and a boron halogenide at about 2000°C and depositing the BN vapor on a graphite substrate or mandrel. The characteristic feature of pyrolytic boron nitride is the high degree of crystal orientation with the hexagonal basal plane parallel to the mold surface and the c-direction perpendicular to the substrate. 

Reagents. A-BN powder (Ventron, Beverly, Ma) was used in the synthesis of powder samples of (BN). 3S03F, Highly oriented pyrolytic boron nitride (14) (Union Carbide), HOPBN, was used to prepare (BN) 3S03F for the conductivity measurement. The A-BN powder and HOPBN were degassed under a dynamic vacuum of lO" Torr,... 

In earlier studies, we prepared [1] the salt Cg SOgF from pyrolytic graphite and the first salt of layer boron nitride by exploiting the powerfully oxidizing fluorosulfate radical which derives from peroxydisulfuryldifluoride,... 

Figure 8.9. Schematic representaion of the boron nitride (BN) capillary conductivity cell. 1 - steel tube, 2 - molybdenum contact rod, 3 - thermocouple, 4 - pressed BN cell holder, 5 - tungsten electrode, 6 - melt, 7 - pyrolytic BN tube, 8 - graphite crucible.

A common means of facilitating sample removal consists of lining the inside of the graphite mold with (1) graphite foils -, (2) pyrolytic carbon ", (3) hexagonal boron nitride ... 

In view of its similarity to graphite in crystal structure and in the lamellar properties of the crystallites, it was expected that boron nitride would function effectively as a solid lubricant. However, it has been found that the friction of boron nitride is characteristically higher than that of graphite under the same circumstances. D. H. Buckley [82] observed a value of v = 1.0 for the friction of pyrolytic boron nitride in high vacuum (10 torr) at 296 K G. W. Rowe [54] obtained v... 

Prior to the matrix infiltration one ore more layers of pyrolytic carbon or boron nitride are usually applied to the fibers to provide a means of fiber debonding and toughening by pull-out and crack bridging. 

Fig. 4-3. Thermal conductivity and of a pyrolytic boron nitride sample [19].

Layers of pure pyrolytic boron nitride are generally prepared by thermally induced chemical vapor deposition (CVD) on graphite substrates. In most cases the reaction is performed using BCI3, NH3, and N2 (sometimes at about 0.5 to 50 Torr) and working at deposition temperatures of 1800 to 2000°C [97 to 111]. The same process is used at lower temperatures (600 to 800°C) for the coating of nuclear fuel pellets with boron nitride [112]. The thermally induced CVD of BN in such systems is not restricted to the formation of boron nitride layers but can be used to prepare compact boron nitride articles such as crucibles [113 to 121]. 

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