Sintering boron carbides

The mechanical strength of sintered boron carbide is 15-20 % lower than that of hot-pressed samples this may be owing to intercrystalline defects in the former case, and transcrystalline ones in the latter case. 

Single crystals a few mm long are obtained by chemical vapor deposition (cf. 5.3.2.2.3), or by reduction of B2O3 by graphite in an electrical arc . A 6-mm diameter sintered boron carbide rod can be zone melted under Ar... 

The sintered boron carbide-aluminum control material has good physical properties, is a good heat transfer media and can be fabricated to close tolerances. The flat rod tips require these properties. The powdered boron carbide used in the round tips is not required to have the attributes listed above. A sufficient quantity of boron must be contained in the rod tip to compensate for that burned up during the tiime the rod is performing its control function. Since boron gives off helium after capturing a neutron, the poison volume of the rod tip must be vented to prevent a pressure buildup. 

Schwetz, K. A., Grellner, W., and Lipp, A., Mechanical Properties of HIP-treated Sintered Boron Carbide, Inst. Phys. Conf. Series No. 75, Ch. 5, Adam Hilger Ltd. Bristol, UK (1984)... 

Figure 71. BF, Sintered boron carbide after etching. Uniform grain structure.

Boron carbide was found to couple with 2.45 GHz radiation almost four times better than water on a Joule mo1 of energy absorbed basis. More recently, Katz et al sintered boron carbide disks, 1 cm diameter by 1 cm thick, to greater than 95Z density in only 12 minutes. This compares extremely well with the 1-2 hours required using hot pressing. No sintering aids were used and the sintering process appeared to follow a similar pattern to that obtained when conventional techniques are used. 

Sdicon carbide, hot pressed Alumina, dense sintered Boron nitride, hot pressed Sdicon nitride, hot pressed Boron carbide, hot pressed... 

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,... 

Manufactured abrasives include silicon carbide, fused aluminum oxide, sintered aluminum oxide, sol-gel sintered aluminum oxide, fused zirco-nia-alumina, synthetic diamond, cubic boron nitride, boron carbide, slags, steel shot, and grit. 

It is technically easier to manufacture a molded component from SiC-powder by slip casting or dry pressing, working it mechanically and then sintering pressureless at 1950 to 2000°C. Due to the low sintering activity of silicon carbides, such processes have only been recently successfully carried out with the advent of fine particulate SiC-powders (specific surface area > 5 m /g) with low oxygen-contents (< 0.2%). Boron or aluminum and free carbon or boron carbide are added as sintering aids. 

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 powders are preferred (reaction sintering gives porous material). 

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... 

Mi) Hot Pressing of Boron-Rich Carbides (2273-2373 K, 20- 0 MPa, vacuum or inert atmosphere, 30-60 min). The hot pressing of these compounds is a delicate operation. Extensive carbon diffusion takes place from the mold toward the sample, and the sintering of pure boron produces only boron carbide . The diffusion diagram in the B-C system is established . Refractory metallic foils (Ta, Mo, W, etc.) can be used for protection " . Diffusion of the Mo barrier is noted (1.3-1.9 wt %) . 

The Si-B-C system was mainly investigated with view to the understanding of the sintering mechanisms of SiC with boron in combination with carbon and the sintering of boron carbide with silicon [4]. The silicon solubility of about 2.5 at.% in B4C at 2323 K and the comparatively low temperatures of liquid phase formation in the ternary system enhance the sintering of boron carbide. 

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