Production boron carbides

In laboratory-scale production, boron carbide can also be synthesized in the form of high-purity powders or coatings (e.g., [165]) ... 

Preparation. Boron carbide is most commonly produced by the reduction of boric oxide with carbon in an electric furnace between 1400 and 2300°C. In the presence of carbon, magnesium reduces boric oxide to boron carbide at 1400—1800°C. The reaction is best carried out in a hydrogen atmosphere in a carbon tube furnace. By-product magnesium compounds are removed by acid treatment. 

Boron Triiodide. Boron ttiiodide is not manufactured on a large scale. Small-scale production of BI from boron and iodine is possible in the temperature range 700—900°C (70—72). Excess I2 can be removed as Snl by reaction with Sn, followed by distillation (71). The reaction of metal tetrahydroborates and I2 is convenient for laboratory preparation of BI (73,74). BI can also by synthesized from B2H and HI in a furnace at 250°C (75), or by the reaction of B with excess Agl or Cul between 450—700°C, under vacuum (76). High purity BI has been prepared by the reaction of I2 with mixtures of boron carbide and calcium carbide at elevated temperatures. 

Refractories such as boron nitride, silicon nitride, silicon carbide, and boron carbide are of great importance for the production or protection of systems which can be operated in very high... 

Figure 74 The polyhexenyldecaborane (128) used in the production of nanostructured boron carbide materials by nanoscale templating methods utilizing porous alumina templates.

For the first time we have discovered transparent (painted in various colours) thread-like crystals of carbon among the products of hydrocarbon pyrolysis and during synthesis of silicon and boron carbides (Fig. 3.6) [12]. The X-ray spectral analysis has shown that the transparent threads consist of carbon (Fig. 3.7). 

Most borides are chemically inert in bulk form, which has led to industrial applications as engineering materials, principally at high temperature. The transition metal borides display a considerable resistance to oxidation in air. A few examples of applications are given here. Titanium and zirconium diborides, alone or in admixture with chromium diboride, can endure temperatures of 1500 to 1700 K without extensive attack. In this case, a surface layer of the parent oxides is formed at a relatively low temperature, which prevents further oxidation up to temperatures where the volatility of boron oxide becomes appreciable. In other cases the oxidation is retarded by the formation of some other type of protective layer, for instance, a chromium borate. This behavior is favorable and in contrast to that of the refractory carbides and nitrides, which form gaseous products (carbon oxides and nitrogen) in air at high temperatures. Boron carbide is less resistant to oxidation than the metallic borides. 

Coarse particulate boron carbide is produced by reacting boron/oxygen compounds with carbon. Its worldwide production in 1995 was 600 t/a... 

A. Lipp, Boron Carbide Production, Properties, Application, Technische Rundschau Nos. 14, 28, and 33 (1965) and 7 (1966), Elektroschmelzwerk Kempten GmbH, Munich, 1966. 

In this case, a liquid metal, molten zirconium, can be reacted (oxidized) by a bed of solid oxidant (i.e., B4C) to form products that are different than the bed (Fig 4). Just as described earlier, there is a directed oxidation reaction of a liquid metal and the product may contain some residual metal. In this case, the boron carbide bed is consumed according to the reaction ... 

Carbon as a destabilising agent has received attention from a number of researchers, not least because there are a number of carbides and borocarbides known and these may be potential dehydrogenation end-products. It was hypothesised that boron carbide, B4C, was a potential end-product as the reaction given below would have a A// of 27 kJ mol HH2) (Barkhordarian et al, 2007). However no reaction occurred when ball milled LiH and B4C was attempted to be hydrogenated at 400 °C, under 3 50 bar of hydrogen. 

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