Stoichiometric boron carbide

Starting from fine boron and carbon powders, the direct synthesis of stoichiometric boron carbide is possible -, either under vacuum at 2073K in an electric furnace, or at 2273 K by hot pressing under Ar. This method is inefficient economically and finds no practical application. The compositions in the phase homogeneity range B10.4C-B4C can be obtained by hot pressing mixtures of B C with boron . The diffusion diagram between B and C is established. ... 

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. 

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. 

The highest, conversion of BClj to boron carbide was obtained at low BCI3 flow rate 20 g/min, high H2/BCI ratio of 8, stoichiometric BCyCH, ratio of 4. 

Boron carbide is among the hardest materials yielding only to diamond and boron nitride. It is also one of the most corrosion-resistant compounds at room or moderate temperatures. When considering the corrosion resistance of boron carbide materials, it is important to remember that they are rarely stoichiometric, with the carbon content varying from 9.88 to 23.4% [96] Many of them contain free carbon or sintering aids. Thus their behavior depends on the chemical composition. 

Joly [21] reported the preparation of boron carbide in 1883, labeling the product as B3C, whilst in 1899 Moisson [22] labeled the compound as BeC. Yet, another 50 years passed until Ridgeway [23] suggested the stoichiometry to be 4 to 1. Today, it is well established that the composition of boron carbide has no exact stoichiometric composition but ranges from B4 3C to B10.4C. The composition of commercially produced boron carbide, using the carbothermal reduction of boron oxide in arc furnaces, is usually close to B4C, which corresponds to the stoichiometric limit on the high-carbon side. 

When the pyrolysis of PCS precursor fibers is carried out in the presence of hydrogen, or when boron or aluminum doped PCS precursor fibers are pyrolyzed, it is possible to obtain quasi-stoichiometric silicon carbide fibers. Alternatively, quasi-stoichiometric fibers are also obtained from precursor fibers consisting of SiC powder reinforced polymers. 

Figure 11.3 Results of pressureless sintering of boron carbide of different stoichiometric compositions (green density 60%, 1 h soaking time at final temperature, 1 bar argon atmosphere). The numbers in italics refer to percentage theoretical density.

The process is effectively stimnlated by argon heated up inside of the RF-ICP discharge and takes place downstream from the thermal discharge, where reactive gases are injected into argon (Tumanov, 1981 see Fig. 7-92). The yield achieved by the process is 93-95%, the composition of the boron chloride is close to a stoichiometric value Bs gC, the size of the produced particles is 200-300 nm, and the particles color varies from gray to black depending on the fraction of carbon in the carbide. 

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