Linda E. Jones

The formation and oxidation of BC3, a new graphitelike material

Abstract A study was performed to investigate improved oxidation resistance in a graphitic material containing high concentrations of substitutional boron. The method of producing such samples was the chemical vapor deposition (CVD) reaction between boron trichloride and benzene at temperatures near 973 K. This reaction is known to produce a novel material containing about 25% boron, and having the chemical formula BC3. The material was produced and examined for chemical composition, crystal structure, and microstructure. The compound BC3 has a hexagonal crystal structure similar to that of graphite. Maximum deposition rates were between 0.01 and 0.05 μm/day. Annealing the product of this reaction at 1173 K for 72 hours results in a crystalline material having a layered structure with an interlayer spacing slightly larger than that of graphite. Thermal analysis of the BC3 as a coating on T-300 carbon fibers was conducted. Thermal analysis data suggest that this novel compound has a much greater resistance to oxidation than T-300 carbon fibers in the temperature range 673 to 1073 K.

Reactivity and related microstructure of 3D carbon/carbon composites

Abstract Four carbon/carbon composites fabricated with either PAN fibers or coal tar pitch fibers were examined. Detailed analysis of composite properties and structure included total surface area by Kr adsorption at 77 K, active surface area, porosity, crystallite parameters, as well as SEM and optical microscopic observations. Rates of composite gasification were measured at 1123 K in 3.1 kPa of steam. Under these experimental conditions the composites fabricated with PAN fibers are roughly three times as reactive as those fabricated with pitch fibers. Microscopic examination of the composites provides detail on two different microstructures for each fiber and respective composite. Binder associated with the fibers is influenced by the fiber microstructure, and a continuation of structure is developed throughout the composite body. Even though the fiber fraction is only approximately 50% of the composite by weight, it is clear that fiber microstructure influences overall composite microstructure and, hence, composite physical properties and subsequent composite gasification behavior.

IMPROVED CRYSTALLOGRAPHIC DATA FOR GRAPHITE

Powder diffraction pattern of SP-1 graphite has been obtained using synchrotron X-ray diffraction. Unit cell dimensions were calculated using a least-squares analysis that refined to a |Δ2θ°| of no more than 0.007. A hexagonal cell was determined with a space group of P 6 3 / mmc (194), a =2.4617(2) and c =6.7106 (4) A. The Smith/Synder figure of merit is 167 based upon 11 peaks, which indicates that the quality of this data set is superior to the existing PDF card for graphite, 41-1487. It is also emphasized that the interlayer spacing of graphite should be 3.355(1) A. Using GAS and EXPGUI codes, a new set of calculated powder diffraction data based upon the interlayer spacing of 3.555 A is generated. A comparison with the current calculated card, 75-1621, has also been made.

Differences between surface and bulk properties of glass melts I. Compositional differences and influence of volatilization on composition and other physical properties

The influence of volatilization on the composition, density and surface tension of potassium silicate and soda lime silica melts at 1400°C was examined using the sessile and pendant drop arrangements, diffuse reflectance Fourier transform spectrometry and weight loss measurements. Volatilization of alkali from the melts was modeled as a combined mechanism that included diffusion of volatile species from bulk to surface and a chemical decomposition reaction of alkali oxide on the surface. It was also found that the volatilization of alkali could lead to differences in composition and structure of bulk and surface of the same melt depending upon the rate of diffusion of the alkali through the melt; fast diffusion of alkali results in a surface tension that remains unchanged for long periods whereas blocked diffusion results in a surface tension that changes with time initially rapidly leveling off to an equilibrium surface profile.

Halogenated glass systems for the protection of structural carbon-carbon composites

Abstract Oxidation protection systems or carbon-carbon composites have met limited success, particularly between 900 and 1500 °C where internal glass inhibitors are relied upon to protect the composite. Thi study investigated potential halogenated glass protection systems for the protection of structural carbon-carbon composites. A series of fluoride and fluoroborate glass compositions were selected based on thermodynamic predictions; specifically, to reduce volatility of current B2O3 based protection systems at elevated temperatures. The halogenated glass candidates examined in this study included a series of fluoroborate glasses containing 5 and 10 mol% CaF2, MgF2 and NaF. Since B2O3 is currently used as the matrix inhibitor, the effectiveness of these glasses was compared to B2O3 based upon their high temperature durability and ability to inhibit oxidation of SP-1 and HLM-85 graphite. The high temperature durability of the glasses were compared during two hour isothermal holds in oxygen at 900, 1100, 1300 and 1500 °C. The ability of the halogenated glasses to inhibit the oxidation of SP-1 and HLM-85 graphite was analyzed by examining the oxidation kinetics of compacts containing 33 wt% of the glass inhibitor and graphite. This study is unique in that the oxidation kinetics of inhibited carbon materials were studied in the temperature range of 900 to 1500 °C. This temperature range is of critical importance in the application of these materials.

Oriented microchannel membranes via oxidation of carbon-fiber-reinforced glass composites

Abstract Oriented microchannel membranes were fabricated by the selective oxidation of carbon-fiberreinforced Pyrex glass composites. The oxidation behavior of two PAN-based fibers (T-300R and IM-7) was studied in the as-received condition, and after incorporation into the glass matrix, in order to elucidate differences in oxidation behavior controlled by differences in fiber structure and catalysis by matrix constituents. As-received T-300R fibers oxidized preferentially in an axial fashion from the fiber ends, whereas IM-7 fibers oxidized uniformly on all fiber surfaces. The oxidation rate for the as-received T-300R fiber decreased steadily throughout the oxidation process as a result of a loss of fiber mass, without an increase in the concentration of exposed fiber surface area. The oxidation behavior of the as-received IM-7 fiber was dramatically different than the T-300R fiber. The oxidation rate of the IM-7 fiber increased through 35% burn-off and then steadily decreased, corresponding to the decrease in fiber mass. Incorporation of both fibers in a glass matrix resulted in composites which exhibited similar oxidation behavior. However, the initial oxidation rates for the IM-7 and T-300R fibers in the composite were greater than the initial oxidation rates for the as-received fibers. Interrupted oxidation tests revealed possible catalytic effects of minor constituents in the glass (most notably, sodium) on the oxidation behavior of these fibers embedded in a Pyrex matrix.

Bulk graphite nozzle recession. An analysis based on the carbon-steam reaction

Abstract Bulk graphite gasification rates (Zone I) have been measured between 1123–1323 K at H 2 O H 2 ratios which encompass those found in the exhaust gases resulting from the firing of a typical aluminized solid propellant. These Zone I rates are extrapolated on an Arrhenius plot to a temperature (≈3200 K) which is thought to exist at the surface of a graphite nozzle through which propellant combustion gases pass. The extrapolated rate is higher than that predicted for Zone III kinetics or found experimentally by other investigators. It is concluded that graphite recession under rocket motor conditions is in the intermediate region between Zones II and III. That is, gasification occurs not only at the exterior graphite surface but also in pores close to the exterior surface.

An Auger and XPS Study on CVD and Natural Diamonds

CVD diamond films, ET 100 of Norton Diamond Film, were treateds in ultrahigh purity O 2 from 420 to 575°C at 95 kPa. Auger and x-ray photoelectron spectra were collected from CVD and natural diamond surfaces. The Auger KVV line shapes of the CVD diamond with various surface conditions were compared to those of natural diamonds and sp 2 -bound graphite (HOPG) and glassy carbon. Comparisons were made on the peak shape of A 1 , which is the major satellite peak of carbon KVV. Auger KVV line shaped of oxidized CVD diamonds were more similar to that of natural diamond than either HOPG or glassy carbon. XPS is more sensitive to the change of surface chemisorbed species. The C Is binding energy of oxygenated and oxidized CVD diamonds (287.3 eV) was higher than that of natural diamond (285.7 eV), graphite (283.0 eV) and glassy carbon (284.2 eV). Auger and XPS findings indicated that when treated in O 2 from 420 to 575°C at 95 kPa, direct oxidation of CVD diamond occurred without graphitization