Processing and characterization of Ti2AlC, Ti2AlN, and Ti2AlC0.5N0.5

In this article, we report on the fabrication and characterization of Ti2AlC, Ti2AlN, and Ti2AlC0.5N0.5. Reactive hot isostatic pressing (hipping) at ≈40 MPa of the appropriate mixtures of Ti, Al4C3 graphite, and/or AlN powders for 15 hours at 1300°C yields predominantly single-phase samples of Ti2AlC0.5N0.5; 30 hours at 1300°C yields predominantly single-phase samples of Ti2AlC. Despite our best efforts, samples of Ti2AlN (hot isostatic pressed (hipped) at 1400°C for 48 hours) contain anywhere between 10 and 15 vol pct of ancillary phases. At ≈25 μm, the average grain sizes of Ti2AlC0.5N0.5 and Ti2AlC are comparable and are significantly smaller than those of Ti2AlN, at ≈100 μm. All samples are fully dense and readily machinable. The room-temperature deformation under compression of the end-members is noncatastrophic or graceful. At room temperature, solid-solution strengthening is observed; Ti2AlC0.5N0.5 is stronger in compression, harder, and more brittle than the end-members. Conversely, at temperatures greater than 1200°C, a solid-solution softening effect is occurring. The thermal-expansion coefficients (CTEs) of Ti2AlC, Ti2AlN, and Ti2AlC0.5N0.5 are, respectively, 8.2 × 10-6, 8.8 × 10-6, and 10.5 × 10-6 °C-1, in the temperature range from 25°C to 1300°C. The former two values are in good agreement with the CTEs determined from high-temperature X-ray diffraction (XRD). The electrical conductivity of the solid solution (3.1 × 106 (Ω m)-1) is in between those of Ti2AlC and Ti2AlN, which are 2.7 × 106 and 4.0 × 106 Ω-1 m-1, respectively

Effects of varying oxygen partial pressre on molten silicon: ceramic substrate interactions, final report

The objective of the contract is to investigate the interaction of molten silicon with various die and container candidate materials under varying oxygen partial pressures. This has been done by making silicon sessile drop contact angle measurements on the candidate materials to determine the degree to which silicon wets these substances, and subsequently sectioning the post-sessile drop experiment samples and taking photomicrographs of the silicon-substrate interface to observe the degree of surface dissolution and degradation. Several different materials supplied by JPL have been investigated in this manner, i.e., hot pressed silicon nitride (from both Kawecki Berylco, Inc. (KBI) and AVCO), CNTD silicon nitride coated on hot pressed silicon nitride (Chemetal-Eagle Picher), CVD silicon carbide coated on graphite (Ultracarbon), and ..cap omega.. Sialon (Battelle). Results are described. The oxygen concentrations in the EFG silicon ribbon furnace at Mobil Tyco Solar Energy Corp., Waltham, Massachusetts, and in the JPL silicon sessile drop furnace at Pasadena, California, were measured using the portable thoria-yttria solid solution electrolyte oxygen sensor constructed at UMR for this purpose. Oxygen partial pressures of 10/sup -7/ and 10/sup -8/ atm. were obtained for the Mobil Tyco and JPL facilities, respectively

Study program to develop and evaluate die and container materials for the growth of silicon ribbons. Quarterly report No. 5

Die and container material development efforts under the current program are shared among three organizations: Miami Research Laboratories (MRL) - ceramic process development and overall program management; University of missouri-Rolla (UMR) - silicon sessile drop studies with characterization of reaction products and emphasis on atmospheric effects; and Chemetal Corporation, Pacoima, California - special coatings to be applied to test coupons, die shapes, and containers provided by MRL and tested/characterized by UMR. The completion of a major hardware delivery milestone was accomplished with the delivery of three CNTD Si/sub 3/N/sub 4/ coated hot pressed Si/sub 3/N/sub 4/ crucibles to JPL. A limited characterization of the coating was performed at MRL prior to delivery. The coatings were fine grained ..cap alpha.. - Si/sub 3/N/sub 4/. It has been determined that a two piece die design will be required. At UMR the importance of the role of oxygen in influencing the attack of the CNTD materials by molten silicon has been demonstrated. The stability is greatly enhanced by maintaining the oxygen partial pressure near or below the Si + O/sub 2/ = SiO/sub 2/ equilibrium

Effects of varying oxygen partial pressure on molten silicon: ceramic substrate interactions. Quarterly report No. 1

Activity at UMR this quarter has been in three areas: (1) investigation of the devitrification of fused silica under various oxygen partial pressures; (2) construction of a thoriayttria solid solution electrolyte portable oxygen sensor; and (3) theoretical development and application of a method of calculating the surface free energy of ceramic substrates from silicon sessile drop contact angle and oxygen partial pressure measurements. Results are reported. (WHK