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What is molybdenum silicide?
is an inorganic compound with the chemical formula MoSi2 and is a gray metallic solid. It is insoluble in most acids but is soluble in nitric acid and hydrofluoric acid. The radii of the two atoms are not very different, the electronegativity is relatively close, and it has properties that approximate those of metals and ceramics. Molybdenum silicide is electrically conductive and forms a silica passivation layer on the surface at high temperatures to prevent further oxidation. It is used in high-temperature antioxidant coating materials, electric heating elements, integrated electrode films, structural materials, reinforcing agents for composite materials, wear-resistant materials, and joining materials for structural ceramics.
Preparation of molybdenum silicide
MoSi2 can be produced by the direct reaction of two elements, Mo and Si. Reaction formula:
Physicochemical properties of molybdenum silicide
MoSi2 is an intermediate phase with the highest silicon content in the Mo-Si binary alloy system and is a Dalton-type intermetallic compound with a fixed composition. It has the characteristics of both metal and ceramic and is a high-temperature material with excellent performance. It has good high-temperature oxidation resistance, the oxidation resistance temperature is above 1600, which is comparable to SiC; moderate density (6.24g/cm3); low coefficient of thermal expansion (8.1×10-6K-1); good electrical and thermal conductivity; high brittle transition temperature (1000) below the ceramic-like hard brittleness. MoSi is mainly used as a heating element, integrated circuit, high-temperature anti-oxidation coating, and high-temperature structural material.
In MoSi2, molybdenum is bonded to silicon by metallic bonds, while silicon is covalently bonded to silicon. It is insoluble in mineral acids (including aqua regia) but soluble in a mixture of nitric acid and hydrofluoric acid, and has good high-temperature oxidation resistance.
In the oxidizing atmosphere, high-temperature combustion dense quartz glass (SiO2) on the surface to form a protective film layer to prevent continuous oxidation of molybdenum silicide. When the temperature of the heating element is higher than 1700 , the formation of SiO2 protective film, thickening at a melting point of 1710 , and SiO2 fusion into molten drops. It loses its protective capacity because of the action of its surface extension. The protective film is formed again in the form of a protective film when the element is used continuously in the presence of an oxidizing agent. It should be noted that due to strong oxidation at low temperatures, the element cannot be used for a long time in the temperature range of 400-700degC.
Applications of molybdenum silicide
Molybdenum silicide is used in the field of high-temperature antioxidant coating materials, electric heating elements, integrated electrode films, structural materials, reinforcing agents for composite materials, wear-resistant materials, joining materials for structural ceramics, etc., distributed in the following industries.
1) Energy and chemical industry: electric heating elements, high-temperature heat exchangers for atomic reactor devices, gas burners, high-temperature thermocouples, and their protection tubes, melting vessel crucibles (for melting sodium, lithium, lead, bismuth, tin, and other metals).
2) Microelectronics industry: MoSi2 and some other refractory metal silicides Ti5Si3, WSi2, TaSi2, etc. are important candidates for the gate of large-scale integrated circuits and interconnecting line films.
(3) Aerospace industry: It has been widely and deeply researched and applied as high-temperature anti-oxidation coating material. Especially as turbine engine components, such as blades, impellers, combustors, tailpipes, and sealing device materials.
(4) Automotive industry: turbo engine supercharger rotor, valve body, spark plug, and engine parts for automobiles.
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