Ceramics are valuable because they can withstand heat and chemical attack. They have a critical drawback--they are brittle, do not deform under load and therefore crack or break easily. Flaws in their structure cause this cracking, therefore much effort in ceramic research is aimed at developing new processing techniques that minimize microscopic flaws. When the material is produced in the form of fine fibers, the brittleness properties are greatly improved because the probability that a sample of material will contain a flaw large enough to cause brittle failure deceases as the sample size is reduced. Also, if one fiber in a bundle fails, the crack cannot propagate further and the other fibers remain intact. Composites harness the fiber's attractive properties and eliminate their drawbacks by imbedding them in the matrix of another material. For use in high temperature applications, these fibers are often imbedded in metal matrices along with the temperature resistance, reinforcing fiber and the metal's ductility lends added usefulness to the composite. Light metals--aluminum, magnesium and titanium--are common matrices. Silicon carbide (SiC) fibers are promising for reinforcing because of their high intrinsic strength, stiffness, high temperature stability, and excellent oxidation resistance. Boron carbide (B4C), titanium-diboride (TiB2), titanium carbide (TiC), and titanium bromide (TiB) fibers also have potential to use chemical vapor deposition (CVD) to manufacture SiC, B4C, TiC and TiB fibers. The CVD process involves the information of a solid by a heated substrate. Typically, a small-diameter substrate wire is run through a glass reaction tube, and suitable gases are introduced. The substrate is resistance heated causing the gas to react and deposit in the heating wore. A key problem of efficient manufacturing batch to continuous flow systems, scale-up of the laboratory continuous system to a pilot plant unit, modeling and simulation of the CVD reactions and securing cheap sources of precursors necessary for the CVD process. The PI will do both experimental and modeling work to study the effect of: (1) Pretreatment of the substrate fiber by hydrogen, steam and nitric acid on deposition rate, strength, toughness and adhesivity. (2) The purity of the material on the mechanical quality of the fiber. (3) Gas flow rate on the rate of deposition. (4) Hydrogen to silane ratio and temperature on the formation of stoichiometric SiC. (5) BC13, hydrocarbon (CH4/CC14) and H2 concentrations on the deposition of B4C. (6) The ratio of C to Ti fed for production of TiC. Detailed kinetic study experiments will be carried out in a batch reactor to determine kinetic parameters such as reaction control regimes, activation energy, etc. This will be used in the design of the larger scale continuous units.
