Boron Nitride, BN

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. 

The traditional method for the preparation of boron nitride is by the fusion of urea with boric acid in an atmosphere of ammonia at 750 °C.54 The product from these reactions is hexagonal boron nitride with a layer structure like that of graphite. Unlike graphite, it is colorless and is not an electronic conductor. Conversion of the hexagonal form to a cubic modification requires heating at 1,800 °C at 85,000 atmospheres pressure. 

Alternative routes to boron nitride have been examined that do not require those challenging reaction conditions. These fall into three categories, which are (1) the pyrolysis of borazines (2) pyrolysis of organic or inorganic polymers that bear borazine groups as side units and (3) pyrolysis of polyhedral borane derivatives. 

Unfortunately, borazine itself is quite volatile and any thermolysis reactions must be conducted in a pressure vessel with provision for removal of hydrogen as the pyrolysis proceeds. 

A decrease in volatility can be achieved by the linkage of borazine rings to form a cyclolinear oligomer or polymer of the type shown in 9.19. Oligomers that approximate to this structure have been produced by the controlled dehydrogenolysis of borazine (reaction (15)) and these yield /-boron nitride when heated to 1,000 °C.55 

Boron nitride may be added to Si3N4 to improve shock and electrical resistance [158]. BN is covalently bonded and resistant to sintering without the addition of liquid forming additives, such as Si3N4, Si02, Y2O3 and CaO, which will allow sintering between 

Thermal conductivity is eight times higher than AI2O3 and is second only to beryllia. The thermal expansion matches Si and at 200°C, exceeds that of Cu. 

It exceeds other ceramics in thermal shock resistance and strength is retained at elevated temperature. If can be sintered, reaction bonded or hot pressed. 

Silicon nitride has been reinforced by carbon [159] and Suzuki et al [160] have investigated fiber pullout mechanisms of carbon fiber reinforced SisN4 ceramic composites. Carbon fiber composites are stable in N2 at high temperatures [161,162]. At present, interest has been generated in SiC-SiC composites. Lundberg et al [163,164] successfully HIPed carbon fiber reinforced Si3N4 composites and Guo et al [165] used carbon fiber to reinforce Si3N4. Silicon nitride will react with carbon  

The temperature resistance of carbon fiber reinforced ceramics is limited by the oxidation of carbon fiber at temperatures above 500°C and it is necessary to provide some form of protective coating. However, the attributes of increased toughness, strength and modulus are worthwhile considerations. Continuous fiber gives improved tensile properties, but may be more difficult to impregnate with the matrix material than discontinuous fiber. 

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Next to Cr C2, TiC is the principal component for heat and oxidation-resistant cemented carbides. TiC-based boats, containing aluminum nitride, AIN, boron nitride, BN, and titanium boride, TiB2, have been found satisfactory for the evaporation of metals (see Boron compounds, refractory boron compounds Nitrides). 

Figure 2.36 A shows a typical low-loss spectrum taken from boron nitride (BN). The structure of BN is similar to that of graphite, i. e. sp -hybridized carbon. For this reason the low-loss features are quite similar and comprise a distinct plasmon peak at approximately 27 eV attributed to collective excitations of both n and a electrons, whereas the small peak at 7 eV comes from n electrons only. Besides the original spectrum the zero-loss peak and the low-loss part derived by deconvolution are also drawn. By calculating the ratio of the signal intensities hot and Iq a relative specimen thickness t/2 pi of approximately unity was found. Owing to this specimen thickness there is slight indication of a second plasmon.

When boron is heated to white heat in ammonia, boron nitride, BN, is formed as a fluffy, slippery powder ... 

FIGURE 14.27 (a) The structure of hexagonal boron nitride, BN, resembles that of graphite, consisting of flat planes of hexagons of alternating B and N atoms (in place of C atoms but, as shown for two adjacent layers in part (b), the planes are stacked differently, with each B atom directly over an N atom and vice-versa (compare with Fig. 14.29). Note that (to make them distinguishable) the B atoms in the top layer are red and the N atoms blue. 

Other useful refractory nitrides for corrosion protection are silicon nitride (Si3N4) and boron nitride (BN). Silicon nitride has good corrosion resistance and is not attacked by most molten metals as shown in Table 17.6 (see Ch. 10). 

C21-0047. Boron nitride (BN) is a planar covalent solid analogous to graphite. Write a portion of the Lewis structure and describe the bonding of boron nitride, which has alternating B and N atoms. 

Non-oxide ceramics such as silicon carbide (SiC), silicon nitride (SijN ), and boron nitride (BN) offer a wide variety of unique physical properties such as high hardness and high structural stability under environmental extremes, as well as varied electronic and optical properties. These advantageous properties provide the driving force for intense research efforts directed toward developing new practical applications for these materials. These efforts occur despite the considerable expense often associated with their initial preparation and subsequent transformation into finished products. 

Boron-containing nonoxide amorphous or crystalline advanced ceramics, including boron nitride (BN), boron carbide (B4C), boron carbonitride (B/C/N), and boron silicon carbonitride Si/B/C/N, can be prepared via the preceramic polymers route called the polymer-derived ceramics (PDCs) route, using convenient thermal and chemical processes. Because the preparation of BN has been the most in demand and widespread boron-based material during the past two decades, this chapter provides an overview of the conversion of boron- and nitrogen-containing polymers into advanced BN materials. 

Lithium Iron Sulfide (High Temperature). High-temperature molten salt Li—Al/LiCl— KCl/FeS - cells are known for their high energy density and superior safety. At one point they were being actively pursued for electric vehicle and pulse-power applications. Historically, boron nitride (BN) cloth or felt has been used as the separator in flooded-electrolyte cells, while MgO pressed-powder plaques have been used in starved-electrolyte cells. 

So far, all efforts to generate, isolate and characterize heterofuUerenes via Kratsch-mer-Huffman vaporization of graphite in the presence of hetero-element-containing compounds such as boron nitride (BN) or cyanogen (CN)2 have failed. An alternative route for the direct formation of heterofuUerenes is cluster rearrangement within exohedral fullerene derivatives such as iminofullerenes and azafuUeroids. The first hints of success by this approach were obtained from mass spectrometry investigations of the cis-l-diazabishomo[60]fullerene 3 [12], the n-butylamine adduct 4 [12] the 1,2-epiminofullerene 5 [11] and the cluster opened ketolactam 6 [2]. 

Table 3 summarizes the properties of the so-called nonmetallic hard materials, including diamond and the diamondlike carbides B4C, SiC, and Be2C. Also included in this category are comndum, A1203, cubic boron nitride, BN, aluminum nitride, AIN, silicon nitride, S N and silicon boride, SiB6 (12). 

Development of practical and low cost separators has been an active area of cell development. Cell separators must be compatible with molten lithium, restricting the choice to ceramic materials. Early work employed boron nitride. BN, but a more desirable separator has been developed using magnesium oxide, MgO, or a composite of MgO powder-BN fibers,... 

Nitride Boron nitride, BN, white solid, insoluble, reacts wiLh steam Lo form NHj and boric acid, formed by heating anhydrous sodium borate with ammonium chloride, or by burning boron in air. 

When boron is heated to high temperatures with carbon, it forms boron carbide, B12C3, a solid with a high melting point that is almost as hard as diamond. The solid consists of B12 groups that are pinned together by C atoms. When boron is heated to white heat in ammonia, boron nitride, BN, is formed as a fluffy, slippery powder ... 

Boron nitride, BN, is a covalent network solid with a structure similar to that of graphite. Sketch a small portion of the boron nitride structure. 

PI5.9 Boron nitride (BN) is isoelectronic with carbon and the B, C, and N atoms are about the same size. The result is that BN forms crystal structures similar to those of carbon, in that it crystallizes in a hexagonal (graphite-like hBN) and a cubic (diamond like cBN) structure. The data summarized at the end of the problem are available for the two forms of BN.17... 

The decabome cage structure has also been used with some comonomers, specifically diamines, to give relatively high molecular weight polymers. Fibers formed from these chains can be pyrolyzed to give products that consist largely of boron nitride, BN. 

Metals and ceramics (claylike materials) are also used as matrices in advanced composites. In most cases, metal matrix composites consist of aluminum, magnesium, copper, or titanium alloys of these metals or intermetallic compounds, such as TiAl and NiAl. The reinforcement is usually a ceramic material such as boron carbide (B4C), silicon carbide (SiC), aluminum oxide (A1203), aluminum nitride (AlN), or boron nitride (BN). Metals have also been used as reinforcements in metal matrices. For example, the physical characteristics of some types of steel have been improved by the addition of aluminum fibers. The reinforcement is usually added in the form of particles, whiskers, plates, or fibers. 

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