Conductivity boron nitrides

PolarTherm Thermally Conductive Boron Nitride Fillers For Polymeric Materials. Advanced Ceramics Co., www.advceramics.com 2001. accessed July 28, 2010. 

The isotope boron-10 is used as a control for nuclear reactors, as a shield for nuclear radiation, and in instruments used for detecting neutrons. Boron nitride has remarkable properties and can be used to make a material as hard as diamond. The nitride also behaves like an electrical insulator but conducts heat like a metal. 

Lithium Nitride. Lithium nitride [26134-62-3], Li N, is prepared from the strongly exothermic direct reaction of lithium and nitrogen. The reaction proceeds to completion even when the temperature is kept below the melting point of lithium metal. The lithium ion is extremely mobile in the hexagonal lattice resulting in one of the highest known soHd ionic conductivities. Lithium nitride in combination with other compounds is used as a catalyst for the conversion of hexagonal boron nitride to the cubic form. The properties of lithium nitride have been extensively reviewed (66). 

The interelectrode insulators, an integral part of the electrode wall stmcture, are required to stand off interelectrode voltages and resist attack by slag. Well cooled, by contact with neighboring copper electrodes, thin insulators have proven to be very effective, particularly those made of alumina or boron nitride. Alumina is cheaper and also provides good anchoring points for the slag layer. Boron nitride has superior thermal conductivity and thermal shock resistance. 

Fig. 16. Insulator wall designs (a) peg wall (b) conducting bar wall and (c) segmented bar wall. The gas-side materials are tungsten and tungsten—copper composite, the base material, copper, and the insulators, boron nitride. Slagging grooves are shown.

Its structure resembles that of graphite, but the latter s flat planes of carbon hexagons are replaced in boron nitride by planes of hexagons of alternating B and N atoms (Fig. 14.27). Unlike graphite, boron nitride is white and does not conduct... 

Cubic boron nitride (c-BN) is a different material altogether from h-BN, with a structure similar to that of diamond, which is characterized by extremely high hardness (second to diamond) and high thermal conductivity.As such, it is a material of great interest and a potential competitor to diamond, particularly for cutting and grinding applications. Its characteristics and properties are shown in Table 10.3... 

Boron nitride, in view of its unique properties, namely absence of electrical conductivity, oxidation resistance, optical transparency, and high neutron capture cross-section for special applications, offers advantages over other ceramics. Thus, for the... 

In experiments run over a number of cycles, the activity was observed to increase after the first cycle, unlike the y-A Os counterpart which deactivated. Using BN, no Pt sintering occurred and this was ascribed to the high thermal conductivity of BN, ensuring that no local hot-spots were formed. On the basis of XPS, the locus of Pt particle attachment was proposed to be surface boron oxide impurities. Taylor and Pollard have compared the activities of silica (194 m g ) and boron nitride (7 m g ) supported vanadium oxide catalysts for propane oxidation. The use of boron nitride was reported to significantly... 

Its structure resembles that of graphite, but the latter s flat planes of carbon hexagons are replaced in boron nitride by planes of hexagons of alternating B and N atoms (Fig. 14.31). Unlike graphite, it is white and does not conduct electricity. Under high pressure, boron nitride is converted to a very hard, diamondlike crystalline form called Borazon. In recent years, boron nitride nanotubes similar to those formed by carbon have been synthesized (Section 14.18), and they have been found to be semiconducting (see Box 14.2). 

Boron nitride is a ceramic with outstanding properties. It is thermally stable at temperatures up to 2,730 °C, is a good electrical insulator, and has a high thermal conductivity coupled with excellent thermal-shock resistance. It is also chemically inert. 

Thermal Evaporation The easiest way of evaporating metal is by means of resistance evaporators known commonly as boats . Boats, made of sintered ceramics, are positioned side by side at a distance of approximately 10 cm across the web width (Fig. 8.1). Titanium boride TiB2 is used as an electrically conductive material with boron nitride BN (two-component evaporator) or BN and aluminum nitride AIN (three-component evaporator) as an insulating material [2]. By combination of conductive and insulating materials, the electrical properties of evaporators are adjusted. 

Some applications, however, must conduct heat but not electricity. In these applications the adhesive must permit high transfer of heat plus a degree of electrical insulation. Fillers used for achieving thermal conductivity alone include aluminum oxide, beryllium oxide, boron nitride, and silica. Table 9.9 lists thermal conductivity values for several metals as well as for beryllium oxide, aluminum oxide, and several filled and unfilled resins. 

Theoretically, boron nitride is an optimum filler for thermally conductive adhesives. However, it is difficult to fill systems greater than 40 percent by weight with boron nitride. Beryllium oxide is high in cost, and its thermal conductivity drops drastically when it is mixed with organic resins. Therefore, aluminum, aluminum oxide, and copper fillers are commonly used in thermally conductive adhesive systems. 

Stronger van der Waals forces hold the sheets in line with each other so that boron nitride is not as good a lubricant as graphite. However, research is being conducted on the use of boron nitride as a high-temperature lubricant because of its chemical stability. 

In graphite, which can be considered as a giant two-dimensional molecule from the series of condensed rings, the bonding between the separate layers is very weak, being due, as in molecular lattices, to Van der Waals-London interaction. The now infinite system of n electrons results in metallic conduction, only, however, in the plane of the rings Boron nitride has perhaps also a diamond-like form as well as the common graphite-like modification (p. 235). 

Samples were exposed to synchrotron radiation through a gold-on-boron nitride mask and then developed using various mixtures of isopropyl alcohol (IPA) and methyl isobutyl ketone (NIBK). Development was conducted under stirred conditions using a constant temperature bath at 23°C and was terminated by blow drying with Freon gas. Film thicknesses were measured using a Tencor Alpha Step. 

In the first approach, the temperature-time profiles of the combustion wave are measured using thin thermocouples (Zenin et al, 1980 Dunmead et al, 1992a,b,c). In the work of Zenin et al (1980), 7-/im-diameter microthermocouples protected by a layer of boron nitride were used. The temperature-time profiles were analyzed using the one-dimensional heat conduction equation with heat generation (see Section IV,A, 1) taking numerical derivatives of the spatial temperature distribution. An implicit assumption in the analysis is that the combustion wave is stable and planar at both the macro- and microscopic scales. 

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,... 

Conductivity of (BN) iSO F and comparison with Cg SO F. In our early studies (12), a four-probe technique was employed, in which four platinum wires were used for electrical contact, and the samples were prepared by pressing powdered polycrystalline material into pellets. Because the platinum wires and the pellet surface are not ideally flat, a uniform intimate contact could not be assured between the wires and the pellet. The boundary effects due to the polycrystalline nature of the pellet sample also render such conductivity measurements unreliable. Attempts to use a contactless radio frequency inductive technique described by Zeller et al. (22) failed because this technique is not sensitive to low conductivities. A four-point probe measurement (21) on an intercalated highly oriented boron nitride sample was used in the present set of conductivity measurements. The <7295k 1.5Scm . The specific conductivity increased with decreasing temperature (see Fig. 1), it having nearly twice the room temperature value at 77 K. This indicates metallic behavior. 

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