On nitrogen sorption during high energy milling of silicon powders in ammonia and nitrogen
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I. INTRODUCTION
WHEN a gas is brought into contact with a solid, sorption will take place in a way that some of the gas molecules will be adsorbed on the surface of the solid. Occasionally, this adsorption is accompanied by the absorption, i.e., by solution of the gas into the solid.[1] The sorption of a gas by solids can be substantially enhanced by high energy milling.[2–8] This phenomenon is the underlying basis for synthesizing various compounds through high energy milling of the elemental powder in a desired gas atmosphere.[7,9–14] Compounds formed through this approach include, but are not limited to, TiN, ZrN, VN, BN, Mo2N, Si3N4, AlN, Cu3N, Mg3N2, Fe2N, Fe4N, W2N, and WN.[7,9–14] High energy milling in a gas atmosphere has also been used as an intermediate step to enhance the formation of compounds during subsequent high-temperature reaction. In our laboratory, nanostructured Si3N4[15,16] and CrN[17] have been synthesized through this latter approach. In spite of considerable efforts in using the enhanced sorption by high energy milling, most of the studies aiming at the understanding of the enhanced sorption by mechanical treatment are limited to small numbers of impacts on solid surfaces. Nevertheless, these fundamental studies have provided considerable insights into how mechanical treatment can increase the sorption of gases by solids. For example, enhanced adsorption and absorption of CO2 molecules by various metals such as Al, Sn, Cu, Pt, Au, Ag, and Pb have been studied by repeated single impacts on metal surfaces in a CO2 atmosphere through a total of 150 impacts.[18] It is shown that the CO2 molecules sorbed per unit area increase with the number of impacts; however, when the number of impacts reaches about 40 to 70, a saturation state appears; i.e., the quantity of the CO2 molecules sorbed ceases to ZHENGUO YANG, Research Assistant, and LEON L. SHAW, Assistant Professor, Department of Metallurgy and Materials Engineering, RUIMING REN, Professor, Department of Materials Science and Engineering, Dalian Railway Institute, Dalian, 116028, People’s Republic of China, and Visiting Scientist, Department of Metallurgy and Materials Engineering, and XIANGQUN XIE, Assistant Research Professor, NMR Laboratory, Institute of Materials Science, are with the University of Connecticut, Storrs, CT 06269. Manuscript submitted June 22, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
increase with the number of impacts. At the saturation state, the amounts of the CO2 molecules sorbed are approximately 10 times higher than those calculated from the adsorption of the metal surface.[18] Similar results are also obtained for enhanced sorption of methane, hydrogen, and acetylene gases.[18] These results indicate that most of the sorbed gas is in solid solution, i.e., the absorption, which usually does not occur or requires long times to occur without mechanical treatment, has been greatly enhanced by mechanical treatment. Sorption of inert gases by metals subjected to mechanical treatment has also been investigated.[19
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