Boron nitride layer structure

CVD of boron nitride films on silicon or germanium or on printed circuit boards is now a common practice in the electronic industry [154 to 162]. The high thermal conductivity combined with the excellent electrical insulation properties are most valuable for these applications [163] see additional references in Section 4.1.1.10.8, p. 129. The use of a-BN layers is of particular importance in the manufacture of electrophotographic photoreceptors (such as solar cells) and of X-ray lithographic masks (see Section 4.1.1.10.8, p. 129). In the last mentioned application, structural aspects of the deposited films are of importance. In films still containing hydrogen, (N)H moieties are depleted by annealing at about 600°C, while (B)H moieties are depleted above 1000°C [164]. Also, elastic stiffness and thermal expansion of boron nitride films have to be viewed in connection with the temperature-dependent stress of CVD-deposited boron nitride films [165]. Reviews of properties and electronic applications of boron nitride layers have appeared in Polish [166] and Japanese [167]. 

Electrochemical applications of a-BN include its use as carrier material for catalysts in fuel cells [297], as a constituent of electrodes in molten salt fuel cells [298, 299], as anticracking particles in the electrolyte for molten carbonate fuel cells [300, 301], and in seals for insulating terminals of Li/FeS batteries from the structural case [302], A BN-coated membrane is used in an electrolysis cell for the manufacture of high-purity rare earth metals from salt melts [381]. A porous boron nitride layer is applied to the upper outer surface of the electrolyte tube in sodium-sulfur batteries [303], and ceramic boron nitride separators are used in liquid fuel cells and batteries [304, 305]. Boron nitride powder may be included in the electrolyte of electrolytic capacitors for high-frequency utilization [306]. 

By subjecting boron nitride (a white powder) to high pressure and temperature small crystals of a substance harder than diamond, known as borazon, are obtained. This pressure-temperature treatment changes the structure from the original graphite-like layer structure (p. 163) to a diamond-like structure this hard form can withstand temperatures up to 2000 K. 

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. 

Structures of the lanthanide nitridoborates appear as layered structures with approximate hexagonal arrangements of metal atoms, and typical coordination preferences of anions. As in many metal nitrides, the nitride ion prefers an octahedral environment such as in lanthanum nitride (LaN). As a terminal constituent of a BNx anion, the nitrogen atom prefers a six-fold environment, such as B-N Lns, where Ln atoms form a square pyramid around N. Boron is typically surrounded by a trigonal prismatic arrangement of lanthanide atoms, as in many metal borides (Fig. 8.10). All known structures of lanthanide nitridoborates compromise these coordination patterns. 

FIGURE 13.9 The layered structure of boron nitride. Compare this structure to that of graphite shown in Figure 13.11. 

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. 

The multilayer nanocomposite films containing layers of quasi-spherical Fe nanoparticles (d — 5.8 nm) separated by dielectric layers from boron nitride (BN) are synthesized by the repeated alternating deposition of BN and Fe onto a silicon substrate [54]. In this work the authors managed to realize the correlation in the arrangement of Fe nanoparticles between the layers the thin BN layer deposited on the Fe layer has a wave-like relief, on which the disposition of Fe nanoparticles is imprinted as a result, the next Fe layer deposited onto BN reproduces the structure of the previous Fe layer. Thus, a three-dimensional ordered system of the nanoparticles has been formed on the basis of the initial ordered Fe nanoparticle layer deposited on silicon substrate [54]. The analogous three-dimensional structure composed of the Co nanoparticles layers, which alternate the layers of amorphous A1203, has been obtained by the PVD method [55]. 

Cubic boron nitride (cBN) has a zinc blende-type crystal structure with a lattice constant of 3.615 A, which is very close to that of diamond (3.567 A). The difference is only about 1.3%. According to RHEED measurements with the electron beam parallel to the 111 layer of cBN, a growth of diamond by DC plasma CVD on cBN(lll) [150] using c = 0.5%CH4/H2, T = 900°C, and F=180Torr led to a result that a smooth (111) layer of diamond was epitaxially deposited in such a way that the [110] direction of diamond was parallel to that of cBN. Namely, D 111 //cBN(lll and D[110]//cBN[110]. In the RHEED pattern, however, extra spots were observed, which were presumably due to the twinnings of (111 diamond layers. In the Raman spectra, there were two lines due to cBN at 1054.5 and... 

There are three polytypes of boron nitride (BN), namely those with cubic(c-BN), wurtzite-type(w-BN) and hexagonal(h-BN) structures. Because the ELNES spectrum reflects the difference of the local structure, ELNES for h-BN, which has a graphite-type layered structure, shows considerably different spectrum from the other polytypes " " as shown in Fig.20. Among these polytypes, the near edge structures for c- and w-BN resemble each other, because of the similarity of their stmctures. As mentioned above. 

Figure 1 shows the two models which were calculated. The base structure was considered with another research group for improvement of the thermal conductivity, while three layered structure was adopted on the view point of shielding. The base structure was assumed to be a concentric cylinder of 60 cm height which was formed void, strontium titanate ( "SrTiOs, P 5.12g/cm 0 and boron nitride (BN, p 2.26g/cm3) from the inside. The weight of these two materials was made to be identical each other. 

Boron nitride BN can form two solid structures, one containing hexagonal BN layers similar to graphite, and the other with tetrahedral sp bonding like diamond (see Topic D21. Borazine B- n3h6... 

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