01 Molybdenum disilicide material
The crystal of molybdenum disilicide (MoSi2) has a tetragonal structure. It is an intermediate phase with the highest silicon content in the Mo-Si binary alloy system. It is a Dalton intermetallic compound with a fixed composition. It is gray with metallic luster. It has good electrical and thermal conductivity, excellent mechanical properties and high-temperature oxidation resistance. It can be used as a heating element, and its performance exceeds silicon carbide. Its early application was as a high-temperature and corrosion-resistant coating protection material on the surface of a metal substrate. It was first applied to the coating of gas turbine parts, jet engine combustion chambers and missile combustion chambers. Today, the main industrial applications of molybdenum disilicide materials are in the production of high-temperature heating elements and thermocouple protection tubes.
02 Characteristics of molybdenum disilicide materials
① Crystallographic properties: It belongs to a tetragonal crystal structure with a density of 5.9-6.3g/cm3. Pure molybdenum disilicide has no phase change before reaching the melting point and has good thermal stability. Within the upper limit of its effective temperature range, the rise and fall of temperature will not be destroyed by changes in crystallographic properties.
②Thermodynamic properties: the melting point is as high as 2030℃, the thermal conductivity is high (similar to SiO), and the thermal expansion coefficient is small. These characteristics are the necessary conditions for the production of high-temperature heating elements or thermocouple protection tubes, and help to increase the thermal shock resistance of their products performance.
③Electromagnetic properties: low resistivity (2×10-5Ω·cm), high electric rate, and it is a good conductor. This material is an excellent material for manufacturing heating elements, thermocouple thermoelectrodes and temperature protection tubes (when used as thermocouple protection tubes, please pay attention to insulation).
④ Mechanical properties: The tensile strength and bending strength are not lower than general metal materials and some oxide materials (such as Al2O3), and the hardness and compressive strength are higher than metals. Compared with metal, the biggest disadvantage is poor impact toughness, so it belongs to brittle material like general ceramic products. Therefore, in the actual production process, a small amount of metal bonding agent is added under the condition of not affecting its own high temperature oxidation resistance to make cermet.
⑤Chemical properties: The chemical state is acidic. It is insoluble with most acids and alkalis at room temperature. Except for oxide ceramics, MoSi2 is the most stable of all refractory compounds. Since the solubility of silicon in molybdenum is about 1% (1800°C), it determines the oxidation stability temperature of MoSi2 below 1700°C.
Molybdenum disilicide itself is not resistant to high-temperature oxidation, but at 1300~1600℃, a thin and dense glassy protective layer of silicon dioxide is formed on its surface to inhibit continued oxidation. MoSi2 is not only stable in oxidizing media, but also can be used to high temperatures in reducing, neutral, inert and some strong corrosive atmospheres. For example, in N2, CO2, SO2 and other media, the highest temperature can reach 1600℃.
03 Preparation of molybdenum disilicide
Molybdenum disilicide can be obtained by direct reaction synthesis of Mo powder and Si powder. Or use Mo oxide reduction reaction to synthesize.
Molybdenum disilicide parts can be made by cold pressing sintering or hot pressing. For molding, batter, starch, glycerin, etc. can be used as binders. When forming by extrusion, add 2%~4% dextrin as binder. MoSi2 products are fired at 1500~1900℃. If a small amount of SiO2 is added to the molybdenum disilicide powder, a protective film will be formed on the surface of the molybdenum disilicide product due to the melting of SiO2 at high temperature, which will increase the working temperature of molybdenum disilicide to 1710~1780℃. The mechanical properties of cold-pressed sintered products are poor. During hot pressing, the temperature is 1550~1750℃ and the pressure is 9.8~78.5MPa. Using the mixture of molybdenum disilicide and alumina for hot pressing, molybdenum disilicide products with different properties can be obtained.
04 Application of molybdenum disilicide
At present, the most widely used silicide product in industrial production is molybdenum disilicide, and its main products are high-temperature heating elements and thermocouple protection tubes.
①Apply to electric heating elements:
Although the molybdenum disilicide heating element can be used up to 1700°C for a long time in an oxidizing atmosphere, the furnace temperature of box-type resistance furnaces and industrial tunnel kilns assembled with the heating element is generally only about 1600°C due to factors such as heat dissipation. Molybdenum disilicide rod-shaped "U" heating element resistance furnace is extremely versatile, and it still ranks first among heating elements of this type.
②Apply to thermocouple protection tube:
The molybdenum disilicide-based thermocouple protection tube has excellent characteristics such as high temperature resistance, corrosion resistance, good air tightness, thermal shock resistance, and long service life. The protection tube has been well used in the temperature measurement of high, medium and low pressure heavy oil gasification furnaces in the fertilizer industry in China. Because it replaces the expensive 99.95% alumina corundum tube imported from Japan, it saves a lot of foreign exchange for the country. . In addition, the tungsten-rhenium coupler, which is specially assembled from this tube and can be used for atmospheric temperature measurement, has also been better used in temperature measurement in metallurgy and other industries. The inexpensive tungsten-rhenium thermocouple wire replaces the precious metal thermoelectric Even wire is cheaper to prepare for more than that, so it saves users the cost of temperature measurement. In addition to the above examples, molybdenum disilicide protection tubes have also been widely used in the temperature measurement of high temperature and strong corrosion devices in the petrochemical, coal chemical, carbon black, cement and machinery industries.
③Apply to the sensor:
In March 1986, the Department of Materials Science and Engineering at the University of California, the United States, successfully developed a "molybdenum silicide film potential temperature sensor for turbine engines". According to reports, the molybdenum disilicide temperature sensor is small in size and can be used for long-term temperature measurement at 1200°C. Because the molybdenum disilicide layer in the sensor element is only 3-10μm thick, its resistivity (350μΩ·㎝ at room temperature) is about 2 orders of magnitude greater than the theoretical value, and the temperature coefficient of resistance is 11.95×10-3K. The temperature sensor has the same function as the noble metal thick film resistance temperature sensor, and its outstanding feature is that the upper limit of its working temperature is higher than twice it.