SynopsisThe papers on the oxidation kinetics are reviewed to clarify the rate-controlling steps and the micromechanisms of oxidative attack. In the first part of the report, the general oxidation behavior of c/c composites is presented and then the detailed discussion is given on the relation of the oxidation rate to total surface area, to gas flow rate, to partial pressure of oxidizing gases and to the mechanical stresses. Finally, the oxidation rate of c/c composites with antioxidation treatments is discussed.
The carbon fiber/carbon matrix composites (C/C composites) are widely used in such systems as rocket nozzles, reentry shields of space vehicles, disk brakes, and heating elements. However, the unprotected C/C composites have very poor oxidation resistance even at temperature as low as 500~ In this work, the oxidation behavior of C/C composites without and with antioxidation treatments was studied in order to improve the oxidation resistance of C/C composites. The specimens were embedded in the mixture of chromium powder, alumina powder, and/or a small quantity of the activator of NH4C1. All the mixture was contained in a graphite crucible of $ 25 • ~b 15 • 35 mm and reacted at 1000~ under argon atmosphere for several hours. The addition of silicon carbide into the matrix of the C/C composites was also tried, It decreased the oxidation rate of the C/C composites under 600~ but had no effect on the oxidation rate over 600~ When the composites were oxidized without any antioxidation treatment, the matrix was easily oxidized compared with the fibers. The chromium coating by pack-cementation with the activator was very useful in preventing the oxidation of the C/C composites. The micromechanism of the coatiflg reactions is discussed on the basis of thermodynamic ~tabilit$ diagram of the Cr-C-C12 system. The chromium powder would produce chromium carbides by the reaction with the C/C composites. The HC1 dissociated from the NH4C1 would react with chromium powder and then produce chromous halide. This liquid chromous halide would react with the C/C composites and yield chromium carbide, Also, this chromous halide would further permeate the openings between the chromium carbide producing chromium carbides in deep sites of the composites. The dense layer of Cr203 would be formed by the oxidation of the chromium coatings, and thus it would prevent the oxidation of the C/C composites.
In this paper, we present the fabrication and test of a carbon-nanotube (CNT)-based field emission ion source for a micro time-of-flight mass spectrometer (-TOF MS). The -TOF MS is composed of two parts, i.e., a field emission ion source and a mass analyzer. Molecules are ionized by the impact of electrons emitted from CNTs. We calculated the ion beam path using a commercial electrodynamics simulation tool (Simion ver. 7). The results of the ion beam path simulation show that the ions generated in the effective ionization region can pass only between the first acceleration electrode pair. We fabricated a CNT-based field emission ion source for the -TOF MS. The electron current of the field emitter and the ion current of the ion source were measured. Through the characteristic test of a field emission ion source, we confirmed that the fabricated ion source is feasible for the -TOF MS. #
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