Diborides processing

Ti02/Na2C02/Na2AlF2/NaCl/Na2B40, at 1050°C (20). Very fine titanium diboride may be made by a gas-phase plasma process in which titanium tetrachloride and boron trichloride are reacted in a hydrogen gas heated by a d-c plasma (21). 

Similar results are obtained for dre deposition of the carbides of these metals using methane as a source of carbon, atrd silicon tetrahalides for the preparation of silicides. These reactions are more complex than dre preparation of the diborides because of the number of carbides atrd silicides that the tratrsition metals form, some of which have wide ranges of non-stoichiometry. The control of the ratio of the partial pressures of dre ingoing gases is therefore important as a process variable. 

The volatile BjOt is formed in this borothermic process for preparing diborides, tetraborides or hexaborides " . This method is a means of preparing pure borides. 

Chemical vapor deposition processes are complex. Chemical thermodynamics, mass transfer, reaction kinetics and crystal growth all play important roles. Equilibrium thermodynamic analysis is the first step in understanding any CVD process. Thermodynamic calculations are useful in predicting limiting deposition rates and condensed phases in the systems which can deposit under the limiting equilibrium state. These calculations are made for CVD of titanium - - and tantalum diborides, but in dynamic CVD systems equilibrium is rarely achieved and kinetic factors often govern the deposition rate behavior. 

Reaction (3.106) illustrates the primary advantage of a CVD process. Solid titanium diboride, TiB2, melts at 3325°C, yet it can be produced via reaction (3.106) at 1027°C or lower on a suitable substrate by the CVD process. Additionally, the diffusion of gas... 

The borothermic process for diborides proceeds in vacuo at high temperature (>1300 K) according to the equation ... 

Chemical Engineering Progress, Process improves titanium diboride materials. 91,25 (1995). 

Subrahmanyam, J., Vijaykumar, M., and Ranganath, S., Thermochemistry of self propagating high temperature synthesis of titanium diboride composites. Metals Mater. Processes, 1, 105 (1989). 

The high resistance of titanium diboride, TiB2, to many metal melts, particularly aluminum, has led to its use as an electrode and crucible material for electrometallurgical processes. 

Fig. 4. A schematic illustration of the processing used to produce zirconium diboride reinforced zirconium carbide materials by directed metal oxidation.

Different methods used to prepare titanium diboride have been reviewed by Samsonov et al. (1975). At present, it is mainly produced as a powder by thermochemical reduction of boron and titanium oxides followed by hot pressing and sintering to process the final product. The less costly alternative appears to be to coat suitable substrate materials with TiB2 or TiB2-based composites by hot pressing, plasma spraying, chemical vapor deposition, etc. 

F. Monteverde, A. Bellosi, et al.. Processing and Properties of Zirconium Diboride-based Composites, J. European Ceram. Soc., 22 3, 279-288 (2002). 

F. Monteverde, S. Guicciardi, et al.. Advances in Microstructure and Mechanical Properties of Zirconium Diboride Based Ceramics, Mater. Sci. Eng. A-Struct. Mater. Prop. Microstruct. Process., 346(1-21,310-319(2003). 

M. Gasch, D. Ellerby, E. liby, S. Beckman, M. Gusman, and S. Johnson, Processing, Properties and Arc Jet Oxidation of Hafnium Diboride/Silicon Carbide Ultra High Temperature Ceramics, J. Mater. Sci., 39, 5925-5937 (2004). 

Gasch, M., Ellerby, D., Irby, E., Beckman, S., Gusman, M. and Johnson, S., Processing and Properties of Hafnium Diboride/Silicon Carbide Ultra High Temperature Ceramics, To be published in J. of Materials Science (2003). 

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