Weight boron carbides

The pentaborane cage structure -B5H9- has been used as a side group in the preparation of vinyl-type polymers, but only of relatively low molecular weight. Pyrolysis of this material gives primarily boron carbide, B4C. 

Finally, two other characteristics of hard materials are important their density and their chemical stability. The importance of these properties depends on the application. Oxides are chemically more stable than nitrides, which are in turn more stable than borides or carbides. The susceptibility to oxidation of boron carbide prevents its application at high temperatures, but for ballistic protection, where hardness, rigidity and low density take precedence, boron carbide is unparalleled. Tungsten carbide, on the other hand, is the material of first choice for cutting tools, because of its high hardness and stiffness, and high-temperature oxidation resistance, but is far too dense for application in which weight or inertial forces are important. Its use for ballistic protection is therefore out of the question. 

Active oxidation of B4C at a low partial oxygen pressure leads to the formation of gaseous boron oxides and removal of them from the surface of the specimen. Similar to other materials, porosity increases the reactive surface of boron carbide specimens and weight gain on oxidation [106]. 

Boron carbide in air is immediately, i.e., within seconds, coated by a B2O3 layer, in presence of water by a hydroboric acid layer [172,229]. Oxidation starts at 500-600°C and accelerates significantly above 800-1000°C, depending on the humidity. The weight gain of boron carbide powder being surface-cleaned in glove boxes of 1 ppm O2 and 1 ppm H2O and subsequently Si-sputtered, Ar" " ion implanted, or... 

Boron carbide (B4C) is also surprisingly oxidation-resistant. Its weight decrease after 100 h in air at 1200°C is llmg/cm. It is an abrasive used in sandblasting nozzles, chemical vessels, ignitrons (semiconductor sparkers), thermoelectric energy converters, and nonlinear electronics. It is considered one of the most suitable... 

The oxidation kinetics of the composite materials compared to single-phase boron carbide has been verified by Telle [425] and Narushima et al. [426]. The combined monitoring of weight change and X-ray photoelectron spectroscopy (XPS)... 

Specific Heat. The specific heat (C ) of the covalent carbides as a function of temperature is shown in Fig. 8.1 On a weight basis (J/g K), the specific heat of silicon carbide and particularly boron carbide is higher than that of the other refractory carbides and nitrides listed in Table 8.2 Thermal Conductivity. The thermal conductivity or k (i.e., the time rate of transfer of heat by conduction) of covalent carbides, unlike that of the interstitial carbides, decreases with increasing temperature as shown in Fig. 8.2.P It is highly dependent on the method of formation which is reflected by the large spread in values. The thermal conductivity of silicon carbide... 

Armour. Ceramic armour stops projectiles by absorbing their kinetic energy to generate cracks in the hard, brittle ceramic. Of light weight compared to metal armour, ceramics such as boron carbide and alumina are useful as body armour. 

The light weight, high modulus and high sound velocity of boron carbide (Table 11.1) are utilized in a large variety of military and personnel armor applications (see Section 7.1.4.3). The high thermal neutron capture cross-section of 600 barns (6 X 10 m ) provides neutron absorption capability for wet and dry spent nuclear fuel storage and transport applications, and possibly for the first wall protection of nuclear fusion reactors (Van der Laan et al., 1994 Buzhinskij et al., 2009). 

As indicated above, boron carbide is light in weight, very hard, erosion-resistant, stiff, and an excellent neutron absorber. Selected structural applications include ... 

Chlorine is very corrosive to virtually all metals at high temperatures but not to the binary ceramic compounds tungsten carbide, hexagonal boron nitride, and silicon nitride. WC shows no reaction up to 700°C. BN suffers a weight decrease of 0.55 mg/cm after 40 h at 700°C. Si3N4 remains inert even after 500 h of exposure to chlorine at a temperature of up to 900°C. 

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