Doping boron carbides

Telle R (1990) Structure and properties of Si-doped boron carbide. In Freer R (ed) The Physics and Chemistry of Carbides, Nitrides and Borides. Kluwer Academic Publishers, Dordrecht, p 249... 

Figure 43. Hot-pressed B-doped boron carbide with strain-induced polysynthetic twinning.

H Werheit, U Kuhlmann, M Laux, T Lundstrom. Structural and optical properties of Si-doped boron carbide. Proceedings of the 11th International Symposium on Boron, Borides and Related Compounds, Tsukuba, 1993. JJAP Series 10 86, 1994. 

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]. 

Doping has been investigated extensively for compounds like /3-boron and boron carbide to try to modify their thermoelectric properties (e.g., Werheit et al., 1981 Slack et al., 1987 Aselage and Emin, 2003). There have not been as many attempts to dope the rare earth higher borides, we are only aware of transition metal doping into YB66 (Tanaka et al., 2000,2006 Mori and Tanaka, 2006). 

Interesting problems remain to be solved. Although the figure of merit is still quite low due to the poor density, can the homologous R-B-C(N) borides be den-sified and/or doped to become a viable n-type counterpart to boron carbide which is an exemplar p-type high temperature thermoelectric compound ... 

Acceptor dopants are introduced in the crucible either in elemental form or in the form of carbides. If a dopant is introduced in elemental form, it is placed in a special internal crucible with carbon or silicon carbide powder. This is required to prevent the dissolution of the crucible, in the case of aluminium doping, and to reduce the boron vapour pressure to the equilibrium value for the SiC-C system, in the case of boron doping. If elemental boron is placed in the vicinity of the substrate, this results in the formation of boron carbide on the crystal faces of SiC [46]. For moderate doping of crystals, grown at high temperatures, doped SiC sources also can be employed. 

In this paper, to meet the different requirements in self-healing CMCs, typical boron carbide and boron doped carbon was prepared by CVD ftom BCla-CH -Ha-Ar and BClj-CaH -Ha-Ar mixture, respectively. Microstructures, phases and chemical bonding characters of the deposits were systematically analyzed and the relationship between microstmctures and compositions was discussed. 

All the above primarily refers to materials containing relatively small amounts of impurities. However, boron carbide ceramics usually contain 7-20% A1 or AI2O3 to increase hardness and 6-13% silicon to improve mechanical characteristics [98]. Aluminum- and silicon-doped materials often exhibit structural nonuniformity. They have areas of pure boron carbide, and silicon- and aluminum-doped areas [107]. 

The doping of semiconductors is important for tailoring their electronic properties in applications. Boron carbide is p-type because of its high concentration of largely... 

Figure 83. Pressureless-sintered silicon carbide doped with boron carbide, BF. Etched by the Murakami method. Shown here are pores and elongated SiC grains and twins.

Various results to furtiier develop these materials as TE materials have been obtained such as the densification of these materials through usage of sintering additives, doping with transition metals,and modification of the [B]/[C] composition, which previously had led to significant improvement in the TE properties of boron carbide. 

Additives used in final products Fillers barium and strontium ferrites, boron carbide, calcinated clays, calcium carbonate, carbon black, carbon-silica dual phase filler, clays, dolomite, fumed silica, iron oxide, magnesium aluminum silicate, magnesium carbonate, mica, montmorillonite, nickel zinc ferrite, nylon fibers, pulverized polyurethane foam, quartz, silica carbide, soapstone, talc, zinc oxide Plasticizers naphthenic oil, polybutene, aromatic oil, esters of dicarboxylic acid Plasticizers adipates, aromatic mineral oil, paraffin oil, phosphates, phthalates, polyethylene glycol, processing oil, sebacates Antistatics dIhydrogen phosphate of 8-amlnocaprolc add. Iodine doping Antistatics carbon black, quaternary ammonium salt, zinc oxide whisker Antiblocking diatomaceous earth Release propylene wax Slip erucamide+stearamide ... 

FIGURE 20.18 Boron carbide nanorods (a) TEM image (arrow A shows the tubular structures of the boron-doped CNTs, arrow B shows a large catalyst ball with a diameter of 90 nm). (Adapted from Wei, J. et ah, J. Mater. Chem., 2002,12, 3121-3124.) (b) Higher-magnification SEM micrograph. (Adapted from Jazirehpour, M. and Alizadeh, A. J. Phys. Chem. C, 2009,113, 1657-1661.)... 

H Werheit, U Kuhlmann, FH Werheit, G Krach. Optical absorption of carbon-doped P-rhombohedral boron and boron carbide in the absorption edge range. In D Emin, T Aselage, AC Switendick, B Morosin, CL Beckel, eds. Boron-Rich Solids, AIP Conference Proceedings 231. Albuquerque AlP, 1990, p 144. 

Materials made of siHcon nitride, siHcon oxynitride, or sialon-bonded siHcon carbide have high thermal shock and corrosion resistance and may be used for pump parts, acid spray nozzles, and in aluminum reduction ceUs (156—159). A very porous siHcon carbide foam has been considered for surface combustion burner plates and filter media. It can also be used as a substrate carrying materials such as boron nitride as planar diffusion source for semiconductor doping appHcations. 

Sihcon carbide can be doped using boron [7440-42-8] to provide acceptor levels within the band gap (0.3 eV above the valence band), thus making it a -type conductor, or nitrogen can be added to provide donor levels and n-ty e conduction (0.07 eV) below the conduction band. 

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