Hexagonal a-BN

The range of applications of a-BN (for instance, for crucibles or fusion equipment) is to an increasing extent determined by a combination of its resistance to physical attack (extreme heat, radiation) and chemical inertness. Hence, both aspects have to be considered. 

Although an Ni/Mo alloy melt does not wet a-BN, slow interface reactions are observed [20]. On the other hand, mutual wettability of materials sometimes is a first indication for chemical affinity. Thus, the wettability of a-BN by aluminium and aluminium alloys increased with increasing temperature a content of rare earth metals in the aluminium melt leads to a decrease of the wettability [21]. Reaction-bonded a-BN is completely eroded by liquid steel at 1650°C in an Ar atmosphere [22]. The contact angles formed on graphite substrates by molten lead di-chloride/alkali metal chloride mixtures do not change when the Ar atmosphere is replaced by CI2. However, when air is introduced complete wetting is observed after about five minutes. This is not the case with a boron nitride substrate [23]. 

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Figure 4.12. Two modifications of boron nitride, which is isoelectronic with carbon (a) layered hexagonal a-BN, and (b) cubic ) -BN with a sphalerite structure. Both are electrical insulators, but like carbon they are either very soft or very hard.

Boron nitride has two crystalline forms, hexagonal (h-BN) and cubic (c-BN), with much different properties. Hexagonal BN is the more important and has many industrial applications. Its structure is similar to that of graphite which it resembles in many ways. It has a very large anisotropy in the crystal with resulting anisotropic properties. 

Similar to the carbon system, BN exists in a soft hexagonal (h-BN) modification, a hard cubic (c-BN) one, and many others which are not very well crystallized, or amorphous. The properties of h-BN and c-BN are summarized in Table 1 [2-17], and the crystal structures of c-BN, w-BN (wurtzitic-BN), and h-BN are illustrated in Fig. 1. 

The last example we would like to discuss is a lattice of holes formed in stoichiometric hexagonal (h) BN double layers on Rh(lll), see Fig. 5(c) and [99]. The lattice is composed of holes in the BN-bilayer with a diameter of 24 2 A, and an average distance of 32 2 A. The holes in the upper layer are offset with respect to the smaller holes in the lower layer. We note that well-ordered superstructures with a large period have already been observed some time ago by means of LEED for borazine adsorption onto Re(0001) [102], while borazine adsorption onto other close-packed metal surfaces, such as Pt(lll), Pd(lll), and Ni(lll), leads to the self-limiting growth of commensurate ABN monolayers [103,104]. For BN/Rh(lll) it is not clear at present whether the Rh(lll) substrate is exposed at the bottom of the holes. If this was the case the surface would not only be periodic in morphology but also in chemistry, and therefore would constitute a very useful template for the growth of ordered superlattices of metals, semiconductors, and molecules. 

Resonant x-ray spectra and dangling bond density Hexagonal boron nitride Resonant x-ray emission or resonance x-ray Raman scattering has recently been often studied using synchrotron radiation facilities. A successful example of the application of the DV-ATa method to the x-ray spectra is this resonance x-ray emission spectra of hexagonal boron nitride (A-BN). 

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 12. A small portion of the (001) two-dimensional hp lattice of micas, e and t (exaggerated) are the angular and linear deviations from hexagonality. A, A2. hexagonal axes (e = 0. T] = 0) Qh, bn- orthohexagonal axes (s = 0. r] = 0) of the C cell (bfj = 0 -3 ) a, b pseudo-orthohexagonal axes (e O. r 0). The figure is drawn for the case b > bn. Black circles lattice nodes of the ciystal lattice dashed lines H cell of the twin lattice dotted lines Q cell built on the hexagonal and pseudo-hexagonal meshes (modified after Nespolo et al. 2000a).

The oxidation of boron nitride as well as boron carbide is distinguished by an oxide layer remaining in a liquid state and vaporizing considerably over the whole temperature range. However, the study of the mechanisms of oxidation of boron nitride materials is complicated by the existence of several BN modifications with considerable differences in their structure and properties. The oxidation of a stable hexagonal a-modification of BN with a graphite lattice has been studied most thoroughly [104]. 

Boron nitride exists in many different structures due to the special bonding behaviors of boron and nitrogen. Although the most well-defined crystallographic structures are hexagonal BN (h-BN), cubic BN (c-BN), and wurtzitic BN (w-BN), other crystalline structures, such as explosion boron nitride (e-BN) and ion beam-deposited boron nitride (i-BN) [124—135] and amorphous BN (a-BN) [136,137] also exist. 

Hexagonal Boron Nitride with Graphite-Type Structure (a-BN) and Other Structures of Normal Density... 

Physical Adsorption on Hexagonal Graphitic Boron Nitride (a-BN)... 

Highly crystalline hexagonal boron nitride layers can be formed on graphite layers which have been obtained from the CVD of benzene [128]. Low-pressure CVD originating from 2,4,6-trichloroborazine on graphite, metallic, and oxide ceramic substrates at 1050°C leads to dense, amorphous boron nitride deposits [129] see also [130]. Ceramics are frequently coated or infiltrated with a-BN by the different chemical vapor deposition (CVD) methods already... 

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