Preparation of silica-molybdenum carbide nanocomposite by in situ reduction
01 Apr 2003-Transactions of The Indian Ceramic Society (Indian Ceramic Society)-Vol. 62, Iss: 2, pp 92-96
TL;DR: In this paper, the average particle size of molybdenum carbide is in the range of 5-15 nm and the detailed XRD analyses coupled with thermodynamic arguments show that reduction of ammonium moly bdate is predominantly by in situ generated hydrogen.
Abstract: Molybdenum carbide has been formed in the silica gel matrix by the in situ reduction of ammonium molybdate. The average particle size of molybdenum carbide is in the range of 5-15 nm. The detailed XRD analyses coupled with thermodynamic arguments show that reduction of ammonium molybdate is predominantly by in situ generated hydrogen.
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TL;DR: The mechanism of isothermal leaching kinetics of tungstic acid in ammonium hydroxide is mainly of contracting geometry type as mentioned in this paper, it is observed that initially the dissolution rate is very fast followed by decrease in rate with time.
Abstract: Leaching of tungstic acid is carried out in ammonium hydroxide solution at two different pH (8 and 10.5), at three different temperatures of 40°, 50° and 60°C. It Is observed that initially the dissolution rate is very fast followed by decrease in rate with time. It also increases with increase of temperature and pH. The mechanism of isothermal leaching kinetics of tungstic acid in ammonium hydroxide is mainly of contracting geometry type. Ammonium tungstate has been used as a precursor to synthesize nano tungsten carbide in the silica gel matrix by in situ generation of hydrogen and extremely fine carbon. The size of tungsten carbide is found to be in the nm range.
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TL;DR: In this article, the authors summarize the basic physical concepts and the microstructural features of equilibrium and non-equilibrium nanostructured materials (NsM) and make an attempt to summarize their properties.
2,629 citations
"Preparation of silica-molybdenum ca..." refers background in this paper
...(1) This carbon and water vapour reacts to form hydrogen, uniformly throughout the nanometric pores of gel....
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TL;DR: A review of the current status of research and development on the structure and properties of nanocrystalline materials can be found in this paper, where a critical analysis of this aspect and grain growth is presented.
Abstract: The present article reviews the current status of research and development on the structure and properties of nanocrystalline materials. Nanocrystalline materials are polycrystalline materials with grain sizes of up to about 100 nm. Because of the extremely small dimensions, a large fraction of the atoms in these materials is located at the grain boundaries, and this confers special attributes. Nanocrystalline materials can be prepared by inert gas-condensation, mechanical alloying, plasma deposition, spray conversion processing, and many other methods. These have been briefly reviewed. A clear picture of the structure of nanocrystalline materials is emerging only now. Whereas the earlier studies reasoned out that the structure of grain boundaries in nanocrystalline materials was quite different from that in coarse-grained materials, recent studies using spectroscopy, high-resolution electron microscopy, and computer simulation techniques showed unambiguously that the structure of the grain boundaries is the same in both nanocrystalline and coarse-grained materials. A critical analysis of this aspect and grain growth is presented. The properties of nanocrystalline materials are very often superior to those of conventional polycrystalline coarse-grained materials. Nanocrystalline materials exhibit increased strength/hardness, enhanced diffusivity, improved ductility/toughness, reduced density, reduced elastic modulus, higher electrical resistivity, increased specific heat, higher thermal expansion coefficient, lower thermal conductivity, and superior soft magnetic properties in comparison to conventional coarse-grained materials. Recent results on these properties, with special emphasis on mechanical properties, have been discussed. New concepts of nanocomposites and nanoglasses are also being investigated with special emphasis on ceramic composites to increase their strength and toughness. Even though no components made of nanocrystalline materials are in use in any application now, there appears to be a great potential for applications in the near future. The extensive investigations in recent years on structure-property correlations in nanocrystalline materials have begun to unravel the complexities of these materials, and paved the way for successful exploitation of the alloy design principles to synthesize better materials than hitherto available.
207 citations
"Preparation of silica-molybdenum ca..." refers background in this paper
...(4) Water gas reaction is feasible above 667°C for the in situ generation of H2 ....
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TL;DR: Balluffi et al. as mentioned in this paper reviewed the state-of-the-art knowledge of nanophase materials on a variety of length scales and provided some future research needs and opportunities.
97 citations
TL;DR: In this article, a glass-metal nanocomposites involving copper and nickel, respectively, have been synthesized in bulk form by hot pressing sol-gel derived silica-metal nanoparticle composite powders.
Abstract: Glass-metal nanocomposites involving copper and nickel, respectively, have been synthesized in bulk form by hot pressing sol-gel derived silica-metal nanoparticle composite powders. The particle diameters range from 9 to 17.5 nm. The specimens exhibit the characteristic behavior of the metallic species in their nanocrystalline forms.
33 citations
"Preparation of silica-molybdenum ca..." refers background in this paper
...(6) Reduction by carbon is feasible when the temperature is greater than 52....
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