Processing of Boron Carbide

The mechanical properties of self-bonded boron carbides produced by different densification techniques are summarized in Table 11.2. 

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The vaporization-cold-substrate condensation process of boron carbide melted by electron-gun heating and plasma torch deposition does not give a well-organized and crystalline B4C-type structure. In CH4 [1.33.10 Pa 1 torr)] an electron-gun evaporation of boron carbide contained in a graphite crucible, followed by a condensation on a water-cooled jacket, gives crystals. Sputtering by H is possible . 

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,... 

Reactions of boron ttihalides that are of commercial importance are those of BCl, and to a lesser extent BBr, with gases in chemical vapor deposition (CVD). CVD of boron by reduction, of boron nitride using NH, and of boron carbide using CH on transition metals and alloys are all technically important processes (34—38). The CVD process is normally supported by heating or by plasma formed by an arc or discharge (39,40). 

Janson, U., Chemical Vapor Deposition of Boron Carbides, Materials and Manufacturing Processes, 6(3) 481-500 (1991)... 

A preceramic, carrier polymer route to boron carbide has been reported via the pyrolysis of a polynorbomene that bears decaborane side groups.69 An important feature of this development is the ability to produce nanofibers of boron carbide in the following way. A solution of the poly(norbomenyldecaborane) in THF is subjected to the process of... 

The utilization of the outstanding mechanical and wear properties of cubic BN and related superhard materials has been demonstrated, although further process improvements are necessary before a more general use can be expected. Superhard cubic BN exhibits superb inertness to hot steel surfaces under conditions in which diamond would be severely attacked. Other tetrahedral structures, for instance, BCN and B2O, are anticipated to display similar attractive properties. The high-pressure phase B2O is a semiconductor with a band gap of 0.25 eV and a hardness between that of boron carbide and diamond. 

L. Vandenbuicke, Theoretical and experimental studies on the chemical vapour deposition of boron carbide, Ind. Eng. Chem. Process Des. Dev., 24[4], 568-575(1985). 

Injection molding of boron carbide with 2-5 mass.-% carbon black was developed by Schwetz et al. [210]. Like in conventional processes known for oxide and nitride ceramics, the spray-dried powder blend was mixed with 18 mass-% organic binder and molded at 120°C and 45 MPa. Dewaxing was accomplished by heating in an atmosphere-controlled furnace at 100 mbar. The binder components decomposed thermally by cracking and evaporated within four days and temperatures up to... 

Different processing routes have been applied to synthesize this class of composite material. Besides the classical method of powder mixing using submicron SiC, B4C and carbon or carbon-containing precursors, followed by forming and subsequent heat treatment ]313], the infiltration of boron carbide preforms with organic precursors such as PCS, followed by heat treatment [400, 401] or the pressure-assisted reaction sintering of silicon borides with elemental carbon [423], have been used. 

The boranes nido-B and m do-BioHi4 and carbaboranes n/do-2,3-Et2C2B4H6 and c/oro-l,2-C2BioHi2 have been used as precursors for boron and boron carbide thin films. ISEELS has been used to characterise the molecular precursws at solid surfaces a comparison has been made between the bond lengths determined for the molecular clusters by MNDO calculations and results from X-ray or electron diffraction and from extended EELS fine structure. A study of the deposition of boron carbide films on Si(lll) surfaces from the synchrotron radiation induced decomposition of nido-2,3-diethyldicarbahexaborane has been described. The dissociation of the cluster is the rate determining step in the process. As with the deposition of boron from nido-BioHi4, it is found that thoe is an activation barrier to the dissociation of the boron carbide precursor on Si(l 11) surfaces.43... 

This is a good starting point as a TE material. There are aspects of ffie borosihcide in the processing (significantly lower melting temperatures) that may make it an attractive new replacement of boron carbide. 

Baharvandi H. Hadian A Alizadeh A. (2006). Processing and Mechanical Properties of Boron Carbide-titanium Diboride Ceramic Matrix Composites. Applied Composite Materials, Vol. 13, No. 3, May, 2006, pp. 191-198, ISSN0929189X Basu B. Vleugels J. Biest O. (2005). Processing and Mechanical Properties of Zr02-TiB2 Composites. Journal of the European Ceramic Society, Vol. 25, No.l6, May,2005, pp.3629- 3637, ISSN 09552219... 

However, on the one hand, low ESR signals alone are a weak argument for the assumption of hole bipolarons. On the other hand, several experimental results are in contradiction of this model. For example, (a) the electrical conductivity of boron carbide is maximum at the minimum concentration of BnC icosahedra in the homogeneity range (b) polaron-type effects are restricted to one electron per icosahedron and no corresponding electron-phonon interaction with holes, in particular not with hole pairs in icosahedra, has been proved experimentally (c) the distortion of the icosahedra in boron carbide depends to only a small degree on electron-phonon interaction and (d) the electronic transport in boron-rich solids is due to classical band-type conduction and hopping processes side by side. Hence, the hole bipolaron theory for boron-rich solids can hardly be maintained. 

Synthesis temperature affects the particle size of the boride powder which is extremely important for densification. Powders obtained at lower synthesis temperature have finer size and better sinterability. Particle size of boron carbide also affects the reaction temperature and finally the product quality. As boron carbide grain size increases, the oxide rednction processes and diffusional processes slow... 

A processable polymeric precursor for boride synthesis can be obtained by dispersing a metal oxide in a boron carbide polymeric precursor. On heating, the mixture results in either in situ generation of boron carbide and carbon, followed by reaction to produce tbe boride or direct reaction of the polymer with the metal oxide to give the final boride product. The key requirements of the polymer component are given below ... 

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