Preparation and Characterization of High Quality Single Crystal Refractory Metal Borides and Carbides

摘要

Reproducible techniques were developed to produce single crystals of HfB2, ZrB2, HfC, TaC and ZrC of a higher quality than had been available previously. Single crystal specimens of the above refractory metal-alloys have been prepared in sizes up to 2 inches long x 1/4 inch diameter using an rf-heated floating-zone crystal growth process. Ambient furnace pressures up to 20 atmospheres were used to reduce the loss of volatile constituents by vaporization from the melts, which has improved the control of the stoichiometry of the single crystals. This effect is discussed in terms of a mathematical model that was derived to explain the dependence of vaporization rates on ambient pressure. The purities of the diboride and carbide crystals were in excess of 99.85% and 99.95% respectively. Multiple zone-melting of the diboride specimens did not result in an increased purity over that achieved with a single zone-melting pass and, in fact, degraded the crystals microstructurally. Transmission electron microscopy, laser microprobe and X-ray diffraction analyses were employed to thoroughly characterize the diboride single crystals. Detailed procedures used for the preparation of the uniform, high-density sintered feed rods, as well as crystal growth synthesis, are presented to eliminate much of the art involved in earlier process techniques. (Author)