Density boron carbides

In general, the purified boron carbide is ultimately obtained as a granular soHd that subsequendy may be molded or bonded into usehil shapes. To achieve high density and strength, it is hot pressed at 1800—2400°C in graphite molds. 

Boron carbide is a non-metallic covalent material with the theoretical stoichiometric formula, B4C. Stoichiometry, however, is rarely achieved and the compound is usually boron rich. It has a rhombohedral structure with a low density and a high melting point. It is extremely hard and has excellent nuclear properties. Its characteristics are summarized in Table 9.2. 

The hardness of boron carbide (carbon hexaboride) is not well defined because it is made as sintered compacts which have variable densities, compositions, and defect densities. It is very hard (up to 4400kg/mm2), and of relatively low density, so it has been used extensively as body-armor (McColm,... 

Properties. Boron carbide has a rhombohedral structure consisting of an array of nearly regular icosahedra, each having twelve boron atoms at the vertices and three carbon atoms in a linear chain outside the icosahedra (3,4,6,7). Thus a descriptive chemical formula would be B12C3 [12075-36 4], Each boron atom is bonded to five others in the icosahedron as well as either to a carbon atom or to a boron atom in an adjacent icosahedron. The structure is similar to that of rhombohedral boron (see Boron, elemental). The theoretical density for B12C3 is 2.52 g/mL. The rigid framework of... 

There are other promising ceramic fibres, e.g. boron carbide and boron nitride. Boron nitride fibre has the same density (2.2 g cm-3) as carbon fibre, but has a greater oxidation resistance and excellent dielectric properties. Boron carbide fibre is a very light and strong material. 

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 ... 

The influence of the laser and plasma parameters (such as wavelength, laser power density, pulse length, plasma temperature, electron and ion density and others) on the physical and chemical processes in a laser induced plasma with respect to the formation of polyatomic and cluster ions has been investigated for different materials (e.g. graphite, boron nitride, boron nitride/graphite mixture, boron carbide, tungsten oxide/graphite mixture and superconductors ). 

Ceramic fibers of the nonoxide variety such as silioon carbide, silicon oxycarbide such as Nicalon, silicon nitride, boron carbide, etc. have become very important because of their attractive combination of high stiffiiess, high strength and low density. We give brief description of some important nonoxide fibers. 

Boron carbide pellets and structures can be produced by cold pressing and sintering (70-80% density) or by hot pressing. In the latter the B4C powder is first cold-pressed into pellet form and then hot-pressed in graphite dies at 2050-2300°C under 10.3 MPa (1500 psi). The density is controlled by varying the temperature and the pressure. 

Boron carbide is rhombohedral (a = 5.163 A a = 65.59°). The stoichiometric composition of boron carbide is Bi3C2(B6.5C), so that B4C is a carbon-saturated solid solution. The phase diagram for the B-C system is shown in Figure 1. The density as a function of stoichiometry is shown in Figure 2. 

Figure 2. Theoretical density of natural boron carbide as a function of stoichiometry. (After Ref 9.)...

Boron carbide (B4C) is one of the hardest known materials with excellent properties of low density, very high chemical and thermal stability, and high neutron absorption cross-section. Bulk B4C is conventionally synthesized by high temperature (up to 2400 °C) reactions, such as the carbothermal reduction of boric acid or boron oxide. Nanocrystalline B4C was solvothermally synthesized in CCI4 at 600 °C (Reaction (32)). 

Organic precursors can be used both polycarbosilane and a small amount of phenolic resin, giving CSi and carbon by in situ pyrolysis the resulting boron-carbide ceramies have high density (> 92 %) and contain no free carbon and a small amount of SiC( 5wt%) . 

The addition of free graphite yields fine-grained compounds near the theoretical density (94-100 %) . Carbon is better added by the in-situ pyrolysis of a phenolfor-maldehyde resin (i 9 wt Pressure-less sintering of boron-carbide is now... 

The high-pressure behavior of boron carbide was studied solely due to the presumption that it belonged to the class of the so-called inverted molecular solids [208], which is revealed through the higher compressibility of BjiC icosahedra than that of space between them [209]. However, the inverted molecular behavior of boron carbide has been disputed recently based on the ab initio density-functional calculation of its vibrational properties [210]. Not much is known about... 

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. 

The only technically important sintering additive for boron carbide is carbon, as discovered almost simultaneously by Schwetz and Vogt [189], Henney and Jones [190], and Suzuki et al. [191], which today allows the routine production of dense parts. An amount ranging from 1 to 6 mass-% is sufficient to obtain almost theoretical density. Schwetz and Grellner [192] added phenolic resin (corresponding to 1-3 mass-% C) to a submicron B4C powder and obtained >98% density at 2150°C. The resins are pyrolized at temperatures up to 1000°C and they leave a... 

Figure 41. Hot-pressing parameters and densities obtained from undoped boron carbide (literature...

---