Boron nitride hexagonal

The hexagonal form (h-BN) has considerable crystalline anisotropy resulting in anisotropic properties. It is produced by hot pressing the powder or by chemical vapor deposition (CVD). The processes impart different properties as shown in Tables 13.4 (hot-pressed hBN) and 13.5 

Composition BN Molecular Weight (g/mol) 24.816 Pearson Symbol cF8 Color white to transparent X-ray Density (g/cm3) 2.25 Density (g/cm ) 19.2-29.1 Melting Point 3000°C (sublimes) 

Oxidation Resistance no reaction up to 750 C. Above, oxidizes slowly by the formation of a layer of B2O3 (see Fig. 13.7) 

Chemical Resistance essentially inert to all reagents at room temperature 

F ure 13.7 Oxidation rate of pyrolytic boron nitride and pyrolytic carbon. 

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Lithium Nitride. Lithium nitride [26134-62-3], Li N, is prepared from the strongly exothermic direct reaction of lithium and nitrogen. The reaction proceeds to completion even when the temperature is kept below the melting point of lithium metal. The lithium ion is extremely mobile in the hexagonal lattice resulting in one of the highest known soHd ionic conductivities. Lithium nitride in combination with other compounds is used as a catalyst for the conversion of hexagonal boron nitride to the cubic form. The properties of lithium nitride have been extensively reviewed (66). 

Properties. Under nitrogen pressure hexagonal boron nitride melts at about 3000°C but sublimes at about 2500°C at atmospheric pressure. Despite the high melting point, the substance is mechanically weak because of the relatively easy sliding of the sheets of rings past one another (3). The theoretical density is 2.27 g/mL and the resistivity is about 10 H-cm. 

Hexagonal boron nitride is relatively stable in oxygen or chlorine up to 700°C, probably because of a protective surface layer of boric oxide. It is attacked by steam at 900°C, and rapidly by hot alkaU or fused alkaU carbonates. It is attacked slowly by many acids as well as alcohols (to form borate esters), acetone, and carbon tetrachloride. It is not wetted by most molten metals or many molten glasses. 

Preparation. Hexagonal boron nitride can be prepared by heating boric oxide with ammonia, or by heating boric oxide, boric acid, or its salts with ammonium chloride, alkaU cyanides, or calcium cyanamide at atmospheric pressure. Elemental nitrogen does not react with boric oxide even in the presence of carbon, though it does react with elemental boron at high temperatures. Boron nitride obtained from the reaction of boron trichloride or boron trifluoride with ammonia is easily purified. 

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. 

In Fig. 14.27 we see that the planes in hexagonal boron nitride take positions in which the B atoms are located directly over N atoms, whereas in graphite (Fig. 14.29 ), the carbon atoms are offset. Explain this difference in structure between the two substances. 

Summary of Characteristics and Properties of CVD Hexagonal Boron Nitride... 

The applications of hexagonal boron nitride form an important market, mostly as powder for lubricants and additives. Many of these applications are produced by CVD. 

Cubic boron nitride is obtained from hexagonal boron nitride at high pressure and temperature in the presence of lithium nitride as a catalyst. It is almost as hard as diamond and has superior chemical resistance and a much higher oxidation threshold.Efforts to... 

Molybdenum disulhde (M0S2), graphite, hexagonal boron nitride, and boric acid are examples of lamella materials commonly applied as solid lubricants. The self-lubricating nature of the materials results from the lamella crystalline structure that can shear easily to provide low friction. Some of these materials used to be added to oils and greases in powder forms to enhance their lubricity. Attention has been shifted in recent years to the production and use of nanosize particles of M0S2, WS2, and graphite to be dispersed in liquid lubricants, which yields substantial decreases in friction and wear. 

There is great interest in developing molecular precursors for boron-nitrogen polymers and boron nitride solid state materials, and one general procedure is described in this report. Combinations of B-trichloroborazene and hexamethyldisilazane lead to formation of a gel which, upon thermolysis, gives hexagonal boron nitride. The BN has been characterized by infrared spectroscopy, x-ray powder diffraction and transmission electron microscopy. 

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... 

Scheme 9.10 Conversion of polyborazylene 9.16 into hexagonal boron nitride.

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. 

Hubacek M, Rehak B, Prnka T (1990) Hot pressing of hexagonal boron nitride. In Exner HE, Schumacher V (eds) Adv Mater Processes Proc Eur Conf 1st, p 653... 

HRTEM observations of three differently misoriented interphase boundaries between hexagonal boron nitride (h-BN) and 3C silicon carbide (3C SiC) grains showing an orientation dependence on equilibrium film thickness. In (a) and (b) the (0001) of the highly anisotropic b-BN are parallel to the interface, whereas in (c) they make an angle of 68° with the interphase boundary (reprinted from Ultramicroscopy, Knowles KM and Turan S, The dependence of equilibrium film thickness on grain orientation at interphase boundaries in ceramic-ceramic composites, 83(3/4) 245-259 (2000) with kind permission of Elsevier Science). 

Turan, S. and Knowles, K.M., (1997), Interphase boundaries between hexagonal boron nitride and beta silicon nitride in silicon nitride-silicon carbide particulate composites , J. Eur. Ceram. Soc., 17 (15/16), 1849-1854. 

In retrospect, it is ironic to it that when I met Ernst Schumacher in 1969 (he was then Professor at the University of Bern in Switzerland) we did not talk about the experiments he did at Zurich in the same building where I was at that time. Instead, his interest focussed on our work on borazine transition metal compounds and we discussed in some detail whether it would be possible to incorporate metal atoms like chromium or molybdenum between the layers of hexagonal boron nitride (BN) in a similar way as it can be done with graphite. In the course of these discussions I did not mention that, after I had moved to Zurich, we had begun to investigate the reactivity of nickelocene towards both nucleophilic and electrophilic substrates. The reason was that we were still at the beginning, and while we had been able to prepare a series of monocyclopentadienyl nickel complexes from Ni(C5H5)2 and Lewis bases, our attempts to obtain alkyl- or acyl-substituted nickelocenes by the Friedel-Crafts reaction failed. 

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