Boron Carbide Shapes

According to Greskovich and Rosolovski [372] and Prochazka [373], the specific surface area during the initial state of sintering is mainly consumed by a coarsening of the particles and pores, which in turn reduces the driving force for densification (local chemical potential). Densification comes to an end before pore closure is obtained. This so-called terminated density has been observed for pure boron carbide, SiC and silicon nitride, when no sintering aids are present. 

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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. 

Naturally occurring boron consists of approximately 20% of 10B and 80% of UB, leading to an average atomic mass of 10.8 amu. Because 10B has a relatively large cross-section for absorption of slow (thermal) neutrons, it is used in control rods in nuclear reactors and in protective shields. In order to obtain a material that can be fabricated into appropriate shapes, boron carbide is combined with aluminum. 

Naturally occurring boron consists of two isotopes 10B, which comprises about 20%, and nB, which makes up the remaining 80%. This results in the average atomic mass being 10.8 amu. 10B has the ability to absorb slow neutrons to a great extent. Therefore, it finds application in reactors as control rods and protective shields. However, because boron itself is very brittle (and, therefore, nonmalleable), it must be combined or alloyed with a more workable material. Boron carbide is often mixed with aluminum and then processed into the desired shape. 

H. C. Longuet-Higgins and M. de V. Roberts, who predicted thereby that the icosahedron of 12 boron atoms familiar from elemental boron, boron carbide, and some borides should be stabilized in molecular hydride form, not as the neutral entity B Hu (which if icosahedral would be a diradical) but as the dianion [B12H12] , which contained the 25 valence shell electron pairs needed for the 12 exo B-H bonds and 13 skeletal bonding MOs. Subsequent MO treatments of the closo deltahedral anions B I 1 and carboranes (AB, 2H, in Figure 3.1 have shown that these are the shapes that make best bonding use of their (n + 1) pairs of electrons available for skeletal bonding. ... 

Very fine boron carbide powders of spherical shape and 20-30 nm in size have been prepared by chemical vapor deposition according to (iii). In an Ar-H2-CH2-BCI3 atmosphere a radio frequency plasma produces stoichiometries between Bi5 gC and B3 9C [33, 166]. Also laser-induced pyrolysis of similar gas mixtures with or without acetylene has been employed for the preparation of nano-sized particles [167]. With similar success, composites of B4C and SiC have been produced by the pyrolysis of boron-containing polysilanes [168]. 

Campbell, J., Klusewitz, M., LaSalvia, J, et at (2008) Novel processing of boron carbide plasma synthesized nano powders and pressureless sintering forming of complex shapes. Proceedings 26th Army Science Conference, December 2008, Orlando, Florida,... 

Media Small pieces of material used inside a mill jar to increase mixing energy and efficiency. Commonly used materials include alumina, stabilized zirconia, flint pebbles, steel, chrome-plated steel, boron carbide, silicon carbide, and plastic-coated versions of these. Common shapes include spherical, cylindrical, radius-end cylinders, and long rods. Sizes range from 1 mm or less to over 5 cm. 

Silicon carbide and boron carbide to a lesser degree are important industrial materials which are produced on a large scale in the form of powders, molded shapes, and thin films. 

Boron is an important material for nuclear applications due to its high neutron absorption cross section (760 bam at neutron velocity of 2200 m/ sec). The cross section of the isotope is considerably higher (3840 bam).l l In addition, boron does not have decay products with long half-life and high-energy secondary radioactive materials. However, pure boron is extremely brittle and difficult to produce in shapes such as control rods. Boron carbide is usually the material of choice since it provides a high concentration of boron atoms in a strong and refractory form and is relatively easy to mold (see Ch. 16). 

The development of ceramic materials for armor since 1970 has been extensive, In addition to alumina and titanium diboride, the most widely used ceramic materials are silicon carbide, boron carbide, and aluminum nitride, as monolithic plates and shapes, which are bonded to a fibrous laminate of fiberglass or Kevlar . A typical impact sequence is shown in Fig. 16.2. On impact, the ceramic plate fractures the projectile core and absorbs a major part of the kinetic energy. The backing material absorbs the residual energy. 

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