Molybdenum silicide is a kind of molybdenum silicon compound. Since the radii of the two atoms are not much different, the electronegativity is relatively close, so it has properties similar to metal and ceramic. It has a melting point of up to 2030°C and is electrically conductive. At high temperatures, the surface can form a silicon dioxide passivation layer to prevent further oxidation. Its appearance is gray metal color, which is derived from its tetragonal α-type crystal structure. It also has hexagonal shape but is unstable. The β-modified crystal structure. Not soluble in most acids, but soluble in nitric acid and hydrofluoric acid.
MoSi2 is a high-temperature material with excellent properties due to its excellent properties of metal and ceramics. It has high melting point, high hardness and excellent electrical conductivity, thermal conductivity and chemical resistance. It has attracted much attention in recent years. Applications in high temperature environments show great vitality, especially in the many fields where metal and ceramics are difficult to perform.
Crystal structure
Molybdenum silicide is an intermetallic compound. Due to the similar atomic radius of Mo and Si, the electronegativity is relatively close, so they form a Daltnide intermetallic compound with a strict chemical distribution ratio. Below 1900℃ is a C11b type ordered body-centered square structure, which is a room temperature stable structure, which is formed by stacking 3 individual heart-shaped cubic cells along the C-axis direction, in which Mo atoms are located at the center node and eight apex angles, and Si The atom is located at other node locations, from 1900 to the melting point of the C40 hexagonal crystal structure, which is a metastable structure. The atomic distribution on the C40 structure is similar to the C11 structure (the atomic distribution on the 1100 plane, except that the stacking order is ABCABC of the C40 structure, not the ABB of the C11b structure. The experimental value of the lattice constant of MoSi2 is a = b = 0.3202 nm, c = 0.7843 nm.
Basic performance
Molybdenum silicide is a mesophase with the highest silicon content in the Mo-Si binary alloy. Its melting point is as high as 2030°C, the density is 6.24 g/cm3 , and the coefficient of thermal expansion is low, 7.8×10﹣6K; and has good electricity. Thermal conductivity, thermal conductivity is 45W/m·K, can be subjected to electrical discharge machining, excellent high temperature oxidation resistance, oxidation resistance temperature up to 1600°C or higher, brittle-to-toile transition temperature is high (BDTT=1400°C), at this temperature it has a hard brittleness of ceramic material and a metal-like soft plasticity above the brittle-to-tough transition temperature. It has the R' characteristic, that is, the strength remains substantially constant with increasing temperature over a certain temperature range, which is the main reason why MoSi2 can be used as a high-temperature structural material.
Molybdenum silicide is a high-temperature material with excellent properties and has the dual characteristics of metal and ceramic. In modern industry, the most widely used molybdenum silicide is the manufacture of heating elements in various resistance heating devices. Electrothermal materials are divided into metal and non-metal. The most widely used metal electrothermal materials are mainly nickel-chromium alloys and iron-aluminum alloys. Non-metal electrothermal materials mainly include silicon carbide, zirconia, molybdenum silicide, etc. Materials, non-metals have the advantages of high temperature resistance, corrosion resistance, oxidation resistance, and high electrothermal conversion efficiency, and are gradually becoming the mainstream of heating elements. Since the advent of the molybdenum molybdenum rod with molybdenum silicide as the main component, through continuous technical improvement, the service life has been significantly improved, and the silicon carbon rod has been gradually replaced, becoming the mainstream of high-temperature heating elements.
With the advancement of science and technology, especially in the fields of aerospace, advanced weapons, electronic equipment, etc., higher requirements have been placed on the performance of materials, that is, it can work safely and reliably in high temperature environments, and it is lightweight. High strength, creep resistance, good toughness, anti-oxidation, and will not pollute the environment. For example, in order to improve the quasi-replacement efficiency of the space shuttle and the steam turbine, it is necessary to increase the operating temperature, and thus structural materials that can be used in the ultra-high temperature field above 1773°C are urgently needed. Nickel-based superalloys, which are currently widely used in aerospace structural materials, are difficult to increase due to melting point limitation (maximum operating temperature of 1100°C), and engineering ceramics are difficult to break due to processing difficulties and poor thermal conductivity. Therefore, intermetallic compounds with excellent performance have gradually become a research hotspot. In this respect, MoSi2 is highly valued for its high melting point, relatively low density, good high temperature oxidation resistance and a series of excellent properties such as mechanics and electricity.
