The BN Phases

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. 

Crystallographic data Crystal structure Space group Lattice constant 

Colorless B excess changes to yellow, orange, black [8] 

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Parts prepared of h-BN as well as c-BN are of great interest for industrial applications but also for materials science. The thermodynamic data for c-BN and the BN-phase diagrams found in literature are not in agreement. After the first high pressure experiments the B-N phase diagram was designed, and after some modifications c-BN was described as metastable phase at room temperature. Contrary to this opinion in 1988 it was reported that c-BN is the stable phase. Many experiments have confirmed this result, but exact thermodynamic data are still not available. 

The transformation of h-BN into c-BN (at 6.5 GPa) and the reverse transformation of c-BN to h-BN (from 0.6 to 2.1 GPa) were investigated in a Li3N-BN catalyst system [40], Synchrotron radiation was used to check the phases and to examine reactions between the BN-phases and the catalyst. 

As in the case for precursor-derived Si-C-N ceramics (see Sect. 5.3, Fig. 19) reaction (2) (see Sect. 5.3 and reaction Dg in Fig. 26) appears at 1757 K. Si3N4 reacts completely with graphite to form SiC and gas phase. Excess graphite remains. The BN phase does not take part in reaction (2 Dg) and its amount remains stable up to a temperature of 2586 K. From this result a thermal decomposition of the PHBS(p)-derived material with significant mass losses is expected. However, thermogravimetric analysis shows remarkable thermal... 

Reinke et al. [33] analyzed numerous deposition methods with respect to film structure (cBN or hBN) on the one hand and ion bombardment on the other hand. To describe the ion bombardment quantitatively they used the ion energy and the ratio of fluxes of impinging ions (/) and deposited boron atoms (a). Figure 1 shows the results for the ratio i/a against the ion energy. The total number of deposited atoms can be estimated taking into account the deposition rate and the mass densities of the BN phases. As can be seen from that presentation. 

Laser-assisted CVD of BN can originate from a gaseous reactant [58], or the plasma can be formed by irradiating a target consisting mainly of BN [59 to 63, 139]. The cluster distribution of boron nitride in a laser plasma and the structure of the BN phases in the case of laser-induced plasma deposition have been studied [64]. It is also reported that a combination of an electron cyclotron plasma with laser irradiation produces a coating which consists of p-BN and y-BN [65]. 

The wetting of the boron nitride phases by molten metal mixtures containing Ag, Cu, Sn, Pb is not only affected by the BN phase (a, p, y) but also by the texture (e.g., the grain size) of the boron nitride samples. From 900°C the dense phases start to form a-BN and three-phase solid surfaces can be studied [42]. 

In addition to the Hquid-phase -butyl nitrite (BN) process, UBE Industries has estabHshed an industrial gas-phase process using methyl nitrite (50—52). The oudine of the process is described in Eigure 4 (52). This gas-phase process is operated under lower reaction pressure (at atmospheric pressure up to 490 kPa = 71 psi) and is more economical than the Hquid-phase process because of the foUowing reasons owing to the low pressure operation, the consumption of electricity is largely reduced (—60%) dimethyl oxalate (DMO) formation and the methyl nitrite (MN) regeneration reaction are mn... 

Eosinophil infiltration is a major feature of asthma and allergic reactions [203], These cells are not abundant during the acute phase of the response, but increase in number and account for 10-80% of the total cell infiltrate during the late phase. Furthermore, major basic protein (MBP), which is released from eosinophil granules, causes respiratory epithelial damage [204]. Since PAF is a potent activator of eosinophil functions [205], BN 52021 may interfere with the late phase response. 

Cubic Phase of Boron Nitride c-BN. The cubic phase of boron nitride (c-BN) is one of the hardest materials, second only to diamond and with similar crystal structure. It is the first example of a new material theoretically predicted and then synthesized in laboratory. From automated synthesis a microcrystalline phase of cubic boron nitride is recovered at ambient conditions in a metastable state, providing the basic material for a wide range of cutting and grinding applications. Synthetic polycrystalline diamonds and nitrides are principally used as abrasives but in spite of the greater hardness of diamond, its employment as a superabrasive is limited by a relatively low chemical and thermal stability. Cubic boron nitride, on the contrary, has only half the hardness of diamond but an extremely high thermal stability and inertness. 

The c-BN phase was first obtained in 1957 [525] by exposing hexagonal boron nitride phase (h-BN) to high pressures and low temperatures. A pressure of more than 11 GPa is necessary to induce the hexagonal to cubic transformation, and these experimental conditions prevent any practical application for industrial purposes. Subsequently, it has been found that the transition pressure can be reduced to approximately 5 GPa at very high temperature (1300-1800°C) by using catalysts such as alkali metals, alkali metal nitrides, and Fe-Al or Ag-Cd alloys [526-528]. In addition, water, urea, and boric acid have been successfully used for synthesis of cubic boron nitride from hexagonal phase at 5-6 GPa and temperature above 800-1000°C [529]. It has been... 

The photochemistry of borazine delineated in detail in these pages stands in sharp contrast to that of benzene. The present data on borazine photochemistry shows that similarities between the two compounds are minimal. This is due in large part to the polar nature of the BN bond in borazine relative to the non-polar CC bond in benzene. Irradiation of benzene in the gas phase produces valence isomerization to fulvene and l,3-hexadien-5-ynes Fluorescence and phosphorescence have been observed from benzene In contrast, fluorescence or phosphorescence has not been found from borazine, despite numerous attempts to observe it. Product formation results from a borazine intermediate (produced photochemically) which reacts with another borazine molecule to form borazanaphthalene and a polymer. While benzene shows polymer formation, the benzyne intermediate is not known to be formed from photolysis of benzene, but rather from photolysis of substituted derivatives such as l,2-diiodobenzene ... 

Finally, three examples are reported in which iminoboranes as intermediates do not react with trapping agents but stabilize themselves intramolecularly in the gas phase during a hot tube procedure [Eqs. (17)-(19)]. A prerequisite is the steric availability of side groups with respect to the BN bonds (9, 21). The products are well established either by solvolytic degradation (9) or by X-ray analysis (21). Note that... 

A complex nanostructured catalyst for ammonia synthesis consists of ruthenium nanoclusters dispersed on a boron nitride support (Ru/BN) with barium added as a promoter (33). It was observed that the introduction of barium promoters results in an increase of the catalytic activity by 2—3 orders of magnitude. The multi-phase catalyst was first investigated by means of conventional HRTEM, but this technique did not succeed in identifying a barium-rich phase (34). It was even difficult to determine how the catalyst could be active, because the ruthenium clusters were encapsulated by layers of the boron nitride support. By HRTEM imaging of the catalyst during exposure to ammonia synthesis conditions, it was found that the... 

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