Boron nitride Subject

The preparation of boron nitride from orthoboric acid and urea is described by T. E. O Connor [81]. Special measures are required to remove the last traces of boric oxide commercial preparations are likely to be significantly contaminated with 820 and water. Deacon and Goodman [11] found a small loss of volatiles at 373-423 K from boron nitride subjected to thermogravimetric analysis in air and a large increase in weight at 1123-1173 K, which is the temperature range of the active oxidation of BN to B2O2. 

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. 

Boron nitride (BN) exists in two forms. The first is a slippery solid formed from the reaction of BC13 with NH3, followed by heating in an ammonia atmosphere at 750°C. Subjecting the first form of BN to a pressure of 85,000 atm at 1800°C produces a second form that is the second... 

Sodium borohydride is marketed in powdered or pellet form, and in solution, for use in fuel cells. Boron nitride can withstand temperatures of up to 650°C (1,202°E) when subjected to high pressures and temperatures, it forms cubic crystals whose hardness rivals that of diamond. Boron carbide, produced by reacting coke and boric acid at 2,600°C (4,712°E), is a highly refractory material and one of the hardest substances known. It has both abrasive and abrasion-resistant applications, and is used in nuclear shielding, see ALSO Davy, Humphry Gay-Lussac, Joseph-Louis Nuclear Chemistry. 

Unfortunately, not much is known for N2 and CO on graphite or boron nitride with respect to the nature of their commensurate-incommensurate transitions, and the order and mechanism of their melting transition, although various speculations and claims can be found in the literature. Therefore, and in view of the excellent reviews already available, we did not include detailed discussions of the theoretical implications of these topics. Many aspects related to the vast subject of commensurate-incommensurate phase transitions can be found in Refs. 10, 205, 260, and 364 from a theoretical perspective, whereas experimental results for Kr on graphite are described in Refs. 40, 323, and 329, for H2, HD, and D2 on graphite in Refs. 83, 84, 120-123, 243, 377, and 379, and for Kr and Xe on Pt(lll) in Refs. 167 and 168. The possible peculiarities of the melting transition, when restricted to two dimensions, are covered, for example, in Refs. 129, 173, 205, 252, and 332. 

If solid boron nitride is subjected to u.v. irradiation in the presence of a 4 1 mixture of Fg + O2, NO+BF4 is produced. This is a much simpler process than those involved in earlier synthetic routes to this compound, Fluoroborate equilibria have been examined in 1 molar NaCl solution, measuring [F ] with a LaFg electrode and the pH using a hydrogen electrode. The results may be explained in terms of the following reactions and equilibrium quotients ... 

Section 1.2 of this Chapter reviews the crystallization of diamond and cubic boron nitride using high pressure, high temperature techniques. Chapter 4 of Part II deals with the subject of chemical vapor deposition of diamond at low pressure. 

The field of transformation of other BN phases (i.e., a-BN Including rhombohedral and turbostratic BN, y-BN) into cubic boron nitride (P-BN) has been the subject of numerous reviews within the past four years. Perhaps the most comprehensive review of p-BN formation, structure, and properties was compiled by Rapoport [1]. Another review is given in English [2], and there are reviews in French [3], Japanese [4 to 8], Russian [9 to 12], and German [13]. In... 

Boron nitride nanostructures have drawn attention for their storage potential. Hydrogen storage in various nanoporous sorbent materials has been a subject of considerable interest. A primary advantage of these materials is their ease of reversibility [146]. This also creates a disadvantage because hydrogen is only weakly held within these materials, requiring low temperatures to achieve... 

Recent research work by Museur et al. showed UV-laser-assisted nucleation of boron nitride nanorods from hexagonal boron nitride (h-BN). Pallets of h-BN powder were used as the target compound for the synthesis and they were subjected to a UV-laser irradiation in their ablation regime for laser pulse duration in picoseconds domain. The ambient gas pressure was kept relatively high (above 500 bar) and oxygen pressure was below 0.2 mbar. The appearance of the nanorods was noticed typically with the pulse duration of 5 ps or shorter. 

A number of other ceramics have been subjected to biomedical tests for implantation, without ciurently being developed industrially. Among these ceramics, we can cite silicon carbide, titanimn nitrides and carbides, and boron nitride. TiN has been suggested as the friction surface in hip prostheses. While cell culture tests show a good biocompatibility, the analysis of explants shows significant wear, related to a delaminating of the TiN layer [HAR 97]. Silicon carbide is another modem day ceramic which seems to provide good biocompatibihty and can be used as bone implant [SAN 98]. 

Boron and aluminum nitrides are materials suitable for high temperatures but are subject to decomposition and are not used to any extent. 

The experimental and theoretical research activities in the field of materials with hardness comparable to or higher than diamond have been stimulated in recent years by the discovery and postulation of novel carbon aUotropes and novel carbon- and boron-based materials. Comparison of the experimental and theoretical results obtained in the past shows that first principles quantum mechanical and semi-empirical calculations are very useful for the screening of the structural and electronic properties of new materials. The fascinating variety of new possible carbon aUotropes which have been calculated to be stable relative to diamond or can be stabilized for kinetic reasons refiects the high potential and the topicality of modem carbon chemistry. Novel carbon aUotropes and carbon and silicon nitrides are subjects of discussion in Part I in the chapter written by G. Jungnickel et al. and J. E. Lowther. 

---