With a statistically designed experiment, it is demonstrated that gravimetric determinations of the oxygen-to-metal ratio of (U,Pu)O2 depend strongly on the day of analysis, slightly on the particular sintering run, and not detectably on position in either the sintering furnace or the analysis furnace. The average oxygen-to-metal ratio of a single sintering run can be determined to a precision of ±0.005 at the 95% confidence level.
Erosion‐ and wear‐resistant coatings are needed for numerous applications, including valve and pump components for coal liquefaction and gasification plants. Titanium diboride is unusually hard and stable and offers considerable promise for use in highly erosive and corrosive environments. We deposited coatings by hydrogen reduction of and . Our objective was to correlate process variables with coating structure and properties, with emphasis on obtaining coatings having high erosion resistance. We varied deposition temperatures from 750° to 1050°C and the and flow rates. Commercial cemented carbides and experimental nickel‐bonded , were used as substrates. After structural characterization, the resistance of the deposited coatings to erosion was determined by a hot coal‐oil slurry impingement test. The deposition rate was very temperature dependent, ranging from 0.1 to 2.9 μm/min between 750° and 950°C. The surface of the coatings showed nodules increasing in size with temperature. The coatings were dense and adherent. The grain size varied with deposition conditions. Often, the grains were too small to resolve optically, but transmission electron microscopy showed the grain size to be very small (2–300 nm) and to increase with distance from the substrate. Electron microscopy and x‐ray diffraction showed only single‐phase , and ion microprobe analysis revealed a constant Ti:B ratio across the coating thickness. Energy‐dispersive fluorescence analyses showed more chlorine in coatings deposited at 800°C than in coatings deposited at higher temperatures. The Knoop microhardnesses of coatings deposited at 800° and 900°C were 15 and 33 GPa, respectively. Coatings deposited at 850°C or below eroded extensively (up to 30 μm deep craters during a 1h test), while those deposited at 900°C showed very little or no erosion (0–3 μm).
The chemical vapor infiltration (CVI) process is increasingly recognized as a practical method for fabrication of fiber-reinforced ceramic matrix composites. The isothermal and thermal gradient-forced flow techniques are the two most frequently used variants of CVI processing. Both are used for fabrication of carbon-carbon and SiC-SiC composites which possess high fracture toughness compared to monolithic ceramics. A pulse CVI process, where the reagents are forced to flow in and out of the preform because of cyclical evacuation and backfilling, may be of value for densification of preforms which have high initial density.
Thermal stresses induced during cooling from the fabrication temperature of a fiber with a noncubic superconducting ceramic coating are analyzed theoretically. The coating considered has a preferred crystallographic orientation such that the c axis of the coating material is perpendicular to the fiber surface which, in turn, results in different thermal expansion coefficients in the radial and the tangential directions in the coating. The thermal stresses are shown to result both from the mismatch between the fiber and the coating, and from the thermal expansion anisotropy of the coating. Stresses due to bending of the fiber/coating to form a superconducting component are also addressed. A critical radius of curvature of bending is obtained below which segmentation of the coating is predicted.
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Thermodynamic modeling of the Ti-Si-C-H-CI-Ar system is presented in order to better understand the conditions surrounding chemical vapor deposition of Ti3SiC2. This ternary compound is classified as a soft, mechanically tough ceramic, and as such, is a strong candidate as a matrix material or fiber-matrix interface coating in fiber reinforced composites synthesized using forced flow-thermal gradient chemical vapor infiltration (FCVI). The computer program SOLGASMIX was used to calculate deposition diagrams for the TiCl4-SiC4-CCl4-H2 reagent system. The effects of hydrogen to reagent concentration, temperature, and pressure were explored with a “box” type study, surrounding a middle condition of 1300K, 760 torr, and hydrogen to reagent concentration ratio of 20:l. Results suggest that Ti3SiC2 prefers to deposit at lower hydrogen to reagent concentrations, and lower temperatures but higher pressures.
YBa 2 Cu 3 O 7– x films were deposited by chemical vapor deposition (CVD) onto single-crystal MgO, single-crystal Al 2 O 3 , and polycrystalline Al 2 O 3 substrates, characterized before and after annealing, and tested for their superconducting properties. The preferred orientation in the films was analyzed (i) with pole figures and (ii) by comparison of experimental x-ray powder diffraction patterns with those calculated for the material using the March–Dollase function to model the degree of preferred orientation. Preferred orientation was significant in as-deposited films, with March coefficients ranging from 0.1–0.5 (random orientation would have a coefficient of 1.0). The (006) pole figures of the films on single crystal substrates exhibited uniquely symmetric patterns. On single-crystal MgO before annealing, a minor secondary orientation of (006) poles in the film was observed in a pattern consistent with the symmetry of major crystallographic directions of MgO. On single-crystal Al 2 O 3 after annealing, a “dual orientation” phenomenon was observed. The high-temperature anneal destroyed the orientation and superconducting properties of the CVD films deposited at high temperatures.
About 60 Biso-coated particle batches with coatings deposited in either 0.13- or 0.24-m dia coaters were studied in this work. These batches were carefully characterized for permeability by neon-helium intrusion, long-term chlorination followed by radiography, and fission gas release. These methods of permeability measurement were compared and correlated with deposition conditions as well as pyrocarbon properties. The results from several irradiation tests were also used to evaluate the validity of the permeability measurements. The neon-helium and long-term chlorination techniques correlated very clearly. Coatings with neon-to-helium ratios below 0.3 were gastight by the chlorination procedure, whereas those with ratios above 0.4 were permeable. The fission gas release technique was unable to distinguish between slightly permeable coatings and gastight ones. Therefore, neon-helium and long-term chlorination procedures are preferred over the fission gas release technique. Results from several irradiation tests verified that coatings with neon-to-helium ratios below 0.3 were gastight, whereas those with ratios above about 0.4 were permeable. 10 figures, 2 tables.
Silicon carbide coatings on HTGR microsphere fuel act as the barrier to contain metallic fission products. Silicon carbide coatings were applied by the decomposition of CH/sub 3/SiCl/sub 3/ in a 13-cm-diam (5-in.) fluidized-bed coating furnace. The effects of temperature, CH/sub 3/SiCl/sub 3/ supply rate and the H/sub 2/:CH/sub 3/SiCl/sub 3/ ratio on coating properties were studied. Deposition temperature was found to control coating density, whole particle crushing strength, coating efficiency, and microstructure. Coating density and microstructure were also partially determined by the H/sub 2/:CH/sub 3/SiCl/sub 3/ ratio. From this work, it appears that the rate at which high quality SiC can be deposited can be increased from 0.2 to 0.5 ..mu..m/min.
