Molybdenum Silicide, MoSi2
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Molybdenum disilicide (Molybdenumdisilicide, MoSi2) is a kind of silicon molybdenum compounds, because the two atomic radius were similar, electronegativity close to, so it is similar to the nature of the metal and ceramic.With a melting point up to 2030℃ and electrical conductivity, silicon dioxide passivating layer can be formed on the surface at high temperature to prevent further oxidation. Its appearance is a gray metal color, derived from its quadric -type crystal structure, and there are also hexagonal but unstable -modified crystal structure.Insoluble in most acids, but soluble in nitric acid and hydrofluoric acid.
MoSi2 is a kind of mesophase with the highest silicon content in mo-SI binary alloy system.It is a kind of high temperature material with excellent performance.Good high temperature oxidation resistance, oxidation resistance temperature up to 1600℃, equivalent to SiC;Moderate density (6.24g/cm3);Lower thermal expansion coefficient (8.1×10-6K-1);Good electric conductivity;High brittle ductile transition temperature (1000℃) below the ceramic - like hard brittleness.At 1000℃ above the metal like soft plastic.MoSi is mainly used as heating element, integrated circuit, high temperature anti-oxidation coating and high temperature structural material.In MoSi2, molybdenum and silicon are bonded by metallic bonds, silicon and silicon are bonded by covalent bonds, and molybdenum disilicide is a gray quadripartite crystal.Insoluble in general mineral acids (including aqua aqua), but soluble in a mixture of nitric acid and hydrofluoric acid, it has good high temperature antioxidant ability and can be used as high temperature (& LT;Heating element working in an oxidizing atmosphere at 1700℃.In the oxidizing atmosphere, a protective layer is formed on the surface of dense silica glass (SiO2) burned at high temperature to prevent the continuous oxidation of molybdenum disilicate.When the temperature of the heating element is higher than 1700℃, SiO2 protective film is formed, which is thickened at 1710℃ and fused with
SiO2 into molten drops.Due to the movement of its surface extension, it loses its protective ability.Under the action of oxidant, when the element is continuously used, it forms a protective film again.It should be noted that due to strong oxidation at low temperatures, the element cannot be used for long periods at 400-700℃.Molybdenum disilicide is applied in the fields of high-temperature anti-oxidation coating materials, electric heating elements, integrated electrode films, structural materials, reinforcement of composite materials, wear resistant materials, connection materials of structural ceramics, etc. It is distributed in the following industries:
1) Energy chemical industry: electric heating elements, high temperature heat exchangers for atomic reactor installations, gas burners, high temperature thermocouples and their protective tubes, crucible for melting utensils (used for melting sodium, lithium, lead, bismuth, tin and other metals).
2) Microelectronics industry: MoSi2 and other refractory metal silicides Ti5Si3, WSi2 and TaSi2 are important candidates for GATE and interconnection thin films of LSI.
3) Aerospace industry: Widely and deeply researched and applied as high temperature antioxidant coating material.Especially as a turbine engine components, such as blades, impeller, burner, nozzle and sealing device material.Molybdenum disilicide has become the latest hotspot in the research of intermetallic compound structural materials as structural materials used in high temperature components, gas burners, nozzles, high temperature filters and spark plugs for aviation and automobile gas turbines.The biggest obstacle to this application is its large brittleness at room temperature and low strength at high temperature.Therefore, the low temperature toughening and high temperature reinforcing of molybdenum disilicate are the key technologies for its application as structural materials.The results show that alloying and compounding are effective means to improve the toughness and high temperature strength of molybdenum disilicate at room temperature.The components commonly used in molybdenum disilicide alloying are only a few silicides that have the same or similar crystal junction with molybdenum disilicide, such as WSi2, NbSi2, CoSi2, Mo5Si3 and Ti5Si3, among which WSi2 is the most ideal.However, the advantages of molybdenum disilicate in WSi2 alloying were obviously lost, and the application was limited.It has been proved that molybdenum disilicide has good chemical stability and capacitivity with almost all ceramic reinforcing agents (such as SiC, TiC, ZrO2, Al2O3, TiB2, etc.).
Therefore, the most effective way to improve the mechanical properties of molybdenum disilicate is to prepare molybdenum disilicate composite.