Pyrolytic BN

The excellent insulating and dielectric properties of BN combined with the high thermal conductivity make this material suitable for a huge variety of applications in the electronic industry [142]. BN is used as substrate for semiconductor parts, as windows in microwave apparatus, as insulator layers for MISFET semiconductors, for optical and magneto-optical recording media, and for optical disc memories. BN is often used as a boron dopant source for semiconductors. Electrochemical applications include the use as a carrier material for catalysts in fuel cells, electrodes in molten salt fuel cells, seals in batteries, and BN coated membranes in electrolysis cells for manufacture of rare earth metals [143-145]. 

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One of the very few methods of direct observation of the crystal lattice under shock-wave conditions is by means of X-ray diffraction. Johnson and coworkers [68]-[71] make observations of the (200) diffraction line from shock-compressed LiF, aluminum, graphite, and pyrolytic BN. The time resolution for observing the shock-compressed state is 20 ns. 

From shock compression of LiF to 13 GPa [68] these results demonstrate that X-ray diffraction can be applied to the study of shock-compressed solids, since diffraction effects can be observed. The fact that diffraction takes place at all implies that crystalline order can exist behind the shock front and the required readjustment to the shocked lattice configuration takes place on a time scale less than 20 ns. Another important experimental result is that the location of (200) reflection implies that the compression is isotropic i.e., shock compression moves atoms closer together in all directions, not just in the direction of shock propoagation. Similar conclusions are reached for shock-compressed single crystals of LiF, aluminum, and graphite [70]. Application of these experimental techniques to pyrolytic BN [71] result in a diffraction pattern (during compression) like that of wurtzite. 

Pyrolytic BN (pBN) is analogous to pyrolytic graphite, the o-direction layer planes of the hexagonal structure deposit parallel to the substrate surface "". Attention to the substrate crystallography and its temperature is required during deposition. Thicknesses exceeding 25 pm are feasible. Thermal conductivity and infrared transmission are of interest d. 

Pyrolytic BN was deposited on Nicalon NL202 silicon carbide yarns at 1000-1200°C to improve aerodynamic resistance and oxidation behavior of silicon carbide. Yarns were fed into a CVD furnace at a rate of 0.01 m/s. The pBN was made by... 

Additional literature references to pyrolytic BN " include mentions of commercial products such as Boralloy pBN, which exhibits exceptional dielectric strength, chemical inertness, and anisotropic thermal and electrical properties. Applications include crucibles, high temperature insulators, and heating elements T... 

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 schematic representation of the conductivity cell assembly is shown in Figure 8.10. The graphite crucible, which was used as the molten salt holder, was of a 3-cm inner diameter and was also used as the counter electrode connected to a LCR Impedance Meter through a thermocouple Inconel protection sheath. In the upper part of the graphite crucible, a BN holder of 3.8 cm inner diameter was placed, in which the pyrolytic BN tube-type conductivity cell was vertically fastened. The tube cell, made of vapor-deposited pyrolytic BN, was dense, non-conductive, with a consistent inner diameter, and able to resist corrosion attack of molten fluoride. The pyrolytic BN tube was immersed in... 

The upper end of the furnace was covered with a special split water-cooled lid. The furnace was purged with argon to prevent air from burning the graphite crucible and to ensure the accuracy of the measurement. The Model Unidex XI (AeroTech, Pittsburgh, PA) was vertically located on a vibration-free stand. It can move the Pt disc electrode in the center of the pyrolytic BN tube-type conductivity cell up and down to a known position with an accuracy of 0.001 cm. A programmable position controller controlled the position. 

AgGai6Ge3o (A = Sr or Eu) I Pyrolytic BN crucibles, sealed in fused silica ampoules under Ar Resistive furnace 3 days at 1,223 K followed by 4 days at 973 K melts congruently at 1,047 K [36]... 

Pyrolytic BN in the form of a pure, dense, anistropic film can be deposited at high temperatures from a mixture of ammonia and boron halide. It has a higher electrical resistivity than hot-pressed BN (see Fig. 2.24), but its dielectric constant and loss tangent are about the same. The dielectric strength of BN is rated at 4,000 volts/mil at room temperature, which makes it an excellent insulator. As is illustrated in Fig. 2.25, the oxidation rate of pyrolytic BN is significantly less that that of pyrolytic graphite below 2000°C. 

CVD 300-2000. usually 600-1200 Very good Metals, especially refractory TIN and other compounds(a), pyrolytic BN Thin, wear-resistant films on metal and carbide dies, tools, etc. Fiee-standing bodies of refiactory metals and i rolytic C or BN... 

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