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Popular Refractory Metals
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Refractory metals refer to metals with a melting point above 1650℃ and with certain reserves (some classify the melting point temperature as above zirconium's melting point of 1852℃) which are different from some common metals, precious metals, and soft metals. The main types of refractory metals include tungsten, tantalum, molybdenum, niobium, hafnium, chromium, vanadium, zirconium, and titanium, among which tungsten, tantalum, molybdenum, and niobium are commonly used. Refractory metals are difficult to process due to their high hardness and melting point, but sheet or wire-shaped tungsten is easy to weld, so it is often used in vacuum systems as an electron-emitting filament, cathode in X-ray tubes, spring components, high-temperature thermocouples, furnace evaporation dishes, and welding electrodes.
Tungsten is a type of refractory metal with a high melting point and a silver-white color, similar to steel. Tungsten also has high hardness, low vapor pressure, and stable chemical properties. Most acids and alkalis only have a slight effect on tungsten, and hydrogen gas, water, or dilute acid have no effect on tungsten. Only in hot aqua regia and mixed hydrofluoric acid and nitric acid solution will tungsten corrode. Tungsten's hardness and high melting point make it difficult to process, but sheet or wire-shaped tungsten is easy to weld. Therefore, it is often used in vacuum systems as an electron-emitting filament, cathode in X-ray tubes, spring components, high-temperature thermocouples, furnace evaporation dishes, and welding electrodes.
Tantalum is a refractory metal with low weight, high strength, and a melting point temperature of 2996℃. It has very good corrosion resistance and does not react with Aqua Regia, chromic acid, nitric acid, sulfuric acid, or hydrochloric acid under cold or hot conditions. However, it is soluble in hydrofluoric acid, fluoride solutions, and oxalic acid, and it can combine with hydrogen to form hydrides, which will destroy its metallic properties and cause brittleness. Tantalum has a significant ability to absorb active gases and can absorb most residual gases in vacuum systems except for inert gases. In vacuum applications, tantalum products are also a good electron emission body (between molybdenum and tungsten). It is often used to produce hollow hot cathode guns in HCD ion coating equipment and can also be used as absorbents, crucibles, and evaporators under high vacuum and high temperature conditions.
Molybdenum is a type of refractory metal with high hardness, non-magnetism, and stable chemical properties. Its melting point is 2620℃. At high temperatures, molybdenum will exhibit oxidizing properties, slowly oxidizing at 520℃ and rapidly oxidizing above 600℃. However, molybdenum is stable in air or water at room temperature or at not too high temperatures. In terms of corrosion resistance, ferro molybdenum is only corroded by hot dilute hydrochloric acid solution and mixed hydrofluoric and nitric acid at 1：1. In vacuum applications, molybdenum is subjected to heat treatment or added with niobium to improve its tensile strength. Molybdenum is commonly used in forms such as rods, cylinders, bolts, nuts, and heating coils, and can also be used as a seal between glass and metal, as an evaporation dish in vacuum electric furnaces, and as an electrode in various vacuum devices.
Ferro niobium is a refractory metal with high strength and low specific gravity, with a melting point of 2468°C. Niobium has various properties similar to tantalum, and you can refer to the properties of tantalum, but it has a low melting point and a higher vapor pressure than tantalum, so it is rarely used as a hot electron emitter. Because of its high affinity, niobium can be used as an aspirant, especially at high temperatures. In vacuum systems, it can also be used as a structural material and is also commonly used in electric or electron beam welding, as an additive to some stainless steels to prevent intergranular corrosion caused by carbon. Also these refractory metals such as niobium are seen in the electric heaters and heat shields of heating devices in vacuum equipment.
