Conductivity boron carbides

Diamondlike Carbides. SiUcon and boron carbides form diamondlike carbides beryllium carbide, having a high degree of hardness, can also be iacluded. These materials have electrical resistivity ia the range of semiconductors (qv), and the bonding is largely covalent. Diamond itself may be considered a carbide of carbon because of its chemical stmeture, although its conductivity is low. 

Surfaces of synthetic diamond, doped with boron, are electrically conducting and show promise as very inert elccfrode materials [24]. Boron carbide (B C) has been used as an anode material but tliis cannot be conveniently prepared with a large surface area [25]. 

From Eq. (11), an obviously desirable characteristic for thermoelectric materials is to have low thermal conductivity k. The thermal diffusivity constant, Dt, of ErB44Si2 has been found to have small values of Dt < 1.1 x 10 2 cm 2/s (Mori, 2006c). These values are significantly smaller than what has been observed for boron carbide samples (Wood et ah, 1985). Although no data exists for the sound velocities of ErB44Si2, the velocities are probably high since borides are typically hard materials. Therefore, the small values of Dt indicate extremely short phonon... 

Silicon carbide, widely employed as an abrasive (carborundum), is finding increasing use as a refractory. It has a better thermal conductivity at high temperatures than any other ceramic and is very resistant to abrasion and corrosion especially when bonded with silicon nitride. Hot-pressed, self-bonded SiC may be suitable as a container for the fuel elements in high-temperature gas-cooled reactors and also for the structural parts of the reactors. Boron carbide, which is even harder than silicon carbide, is now readily available commercially because of its value as a radiation shield, and is being increasingly used as an abrasive. 

Included in the term nonoxide ceramics are all non-electrically conducting materials in the boron-carbon-silicon-aluminum system. The industrially most important representatives, apart from carbon (see Section 5.7.4), are silicon carbide (SiC), silicon nitride (Si3N4), boron carbide (B4C), boron nitride (BN) and aluminum nitride (AIN). 

Boron carbide is similar in hardness to diamond, and boron nitride is similar in structure and mechanical properties to graphite, but, unlike graphite, boron nitride does not conduct electricity. -> A1 has widespread use in construction and aerospace industries. Because it is a soft metal, its strength is improved by alloy formation with Cu and Si. 

Such high concentrations of gap states attached to the valence band essentially affect the electronic charge transport in particular, they are responsible for the p-type character and the very low electrical conductivity. Aside from the electric conductivity in extended band states, a hopping-type conduction must be expected in localized gap states. The electronic properties of boron carbide can be consistently described by a band scheme, which highlights deep energy levels in the band gap (2.09 eV) at 0.065, 0.18, 0.47, 0.77, 0.92 and 1.2 eV (values based on optical measurements), related to the valence band edge. This allows the largely consistent description of all reliable experimental results [537]. 

The first examples of catalytic hydroborations were reported in the 1980s. Sneddon published a series of papers on the additions of the B-H bonds in boron clusters to alk5mes catalyzed by transition metal complexes. " An example of these processes is shown in Equation 16.42. These reactions provided precursors to new boron cages and to boron-carbide materials. In 1985, Noth published the hydroboration of olefins with catecholborane catalyzed by Wilkinson s catalyst. One example of this process (Equation 16.43) shows the difference in chemoselectivity between the catalyzed and uncatalyzed processes. This report by Noth led to the development of catalytic hydroboration as a method for organic synthesis. Studies on both early and late transition metal catalysts have been conducted, and these studies included experiments to probe for differences in selectivities between catalyzed and uncatalyzed processes. 

Figure 8.5 Electrical conductivity of boron carbide as a function of temperature,...

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... 

Silicon carbide has self-heating and beta-emitting glow characteristics and as such is a standard material for heating elements (see Ch. 15). The anisotropy of the electrical conductivity of boron carbide is low, between 70 and 700 K.0 1... 

Boron carbide is characterized by a relatively wide gap in its forbidden band, a low thermal conductivity, and a high thermoelectric power. These properties make it a potentially useful material for high-temperature thermoelectric energy conversion. Electrical conductivity and Seebeck coefficient as a function of temperature and composition are shown in Figs. 8.5 and 8.6. 

Werheit, H., and Rospendowski, S., Anisotropy of the Electrical Conductivity of Boron Carbide, itiBoron-Rich Solids, AlP Conf. Proc. 140 (D. Aselage, et al., eds.). Am Inst, of Physics, New York (1986)... 

The TE properties of the RBso-type compounds, RB44Si2, are p-type similar to boron carbide and also exhibit attractive properties with Seebeck coefficients exceeding 200 qV K observed above 1000 K, and low values of thermal conductivity around 0.02 W cm K for crystals. ... 

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