PREPARATION OF NEW METAL NITRIDE-FLUORIDES AND OTHER HIGH-TEMPERATURE STUDIES
ASHCRAFT, RICHARD HENRY
Doctor of Philosophy
Various solid-solid, solid-gas, and gas-gas reactions were explored for preparation of new refractory metal-nitrogen-fluorine compounds. The gas-gas reactions represent a new method for preparing this type of compound. The reaction of metal fluoride vapors with ammonia yielded two new nitride-fluorides, Y(,2)NF(,3) and TiNF, as confirmed by electron microprobe analysis and x-ray powder diffraction. Using computer methods in indexing x-ray powder patterns, crystal systems were found for both. Y(,2)NF(,3) crystallizes with nearly the same hexagonal unit cell as TcNF giving lattice constants of a(,0) = 5.93 (ANGSTROM) and c(,0) = 4.79 (ANGSTROM). TiNF was indexed with a low-symmetry triclinic unit cell, a(,0) = 9.76 (ANGSTROM), b(,0) = 5.88 (ANGSTROM), c(,0) = 5.47 (ANGSTROM), (alpha)(,0) = 77.52(DEGREES), (beta)(,0) = 136.24(DEGREES), (gamma)(,0) = 112.27(DEGREES). Evidence for the formation of Sc(,2)NF(,3) and Al(,2)NF(,3) is also presented. These compounds are amorphous but give elemental analyses consistent with theoretical percentages. Calculations of estimated heats of formation for nitride-fluorides have been made, assuming ionic crystals and using the Kapustinsky approximation, the third electron affinity of nitrogen, and other physical and thermodynamic data for the components in Born-Haber cycle relationships. These results confirm the refractory nature of the nitride-fluorides and their resistance to attack by HF and F(,2) at high temperatures. An estimation of the third electron affinity of nitrogen was made using thermodynamic data for the nitrides and crystal energy approximations in Born-Haber cycle calculations. The estimated value is 600 (+OR-) 50 kcal/mole. The gas-phase reaction apparatus was also used to investigate the high-temperature carbon-silicon dioxide reaction which is used to form SiC whiskers. By using a quartz-crystal microbalance to measure condensable species from pyrolized samples, it was found that the amount of elemental silicon vaporized during pyrolysis increased with decreasing carbon content of the samples, so for the desired formation of silicon carbide crystals, high carbon samples were favored.