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Molybdenum Disilicides: Production, Properties and Applications (pp. 41-75) $100.00
Authors:  (Peizhong Feng, Xiaohong Wang, School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou, P. R. China)
In order to meet an ever-increasing demand on the operating temperature capability of structural components, a considerable amount of research has been focused on developing new materials. Silicides of molybdenum are well known for their oxidation resistance. The equilibrium between molybdenum and silicon results in the formation of three silicide compounds MoSi2, Mo5Si3, and Mo3Si. The MoSi2 compound is dimorphous. The tetragonal lattice structure (Cllb type) is stable up to 1900C, above which it transforms to the hexagonal phase (C40 type) and then melts at 2030C. The Mo5Si3 compound has a tetragonal lattice structure (D8m type) and melts congruently at 2180C. The Mo3Si compound has a cubic lattice structure (A15 type), and decomposes by a peritectic reaction at 2025C.
MoSi2 with tetragonal Cllb structure has been regarded as the most promising candidate for high temperature structural purposes. It is used in oxidizing environments at 1200-1600C, which is a much higher working range than Ni-base and Co-base super alloys (applying temperature 800-1000C) and structure ceramics (<1300C). MoSi2 has attractive properties like high melting point (2030C), relatively low density (6.24 g/cm3), excellent resistance to elevated temperature oxidation, high thermal conductivity and electrical conductivity (thermal resistivity = 25 W.m-1.K-1 and electrical resistivity = 21.610-6 Ω.cm). It exhibits thermodynamic compatibility with a wide variety of potential ceramic reinforcements for composites and can be alloyed with other silicides. MoSi2 and MoSi2 matrix composites can undergo electron discharge machining (EDM). MoSi2 is a cheaper, non-toxic, and environmentally sustainable raw material.
MoSi2 has been used as a heating element at temperature as high as 1900C in air atmosphere, which is probably the predominant commercial application. MoSi2 heating elements are electric heating elements used in industrial and laboratory furnaces. When the heating elements are heated to high temperature in oxidizing atmosphere, a self-healing, thin, adhesive and dense layer of glass is formed on the surface. The furnace installed MoSi2 heating elements have the advantages of saving energy, good product quality, long life, resistance to thermal shock, etc.
Mo5Si3 has a somewhat higher melting point than MoSi2 and appears to be more creep resistant than MoSi2. In fact, compressive creep rate of Mo5Si3 was measured to be an order of magnitude lower than that of MoSi2. However, its oxidation resistance is significantly inferior to MoSi2. Recent work has shown that boron additions to Mo5Si3 greatly improve the oxidation resistance of Mo5Si3. This is due to the formation of a protective borosilicate glass as a result of the boron additions. This approach has been extended to the development of oxidation-resistant MoSiB alloys.
Mo3Si is a Daltonide compound (a line compound) with negligible solubility range. Since its crystal structure is retained up to near melting point (2025C), Mo3Si seems to have excellent phase stability. However, the systematic investigation on Mo3Si has not been yet conducted. 

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Molybdenum Disilicides: Production, Properties and Applications (pp. 41-75)