1、 Tungsten

It is a commonly used refractory metal with a melting point of up to 3410 ± 20 ℃, appearing silver white and resembling steel in appearance. Tungsten not only has high hardness, low vapor pressure, and low evaporation rate, but also has stable chemical properties. Most acids and bases have only a slight effect on tungsten, while hydrogen, water, or dilute acids have no effect on tungsten at all. Corrosion only occurs in hot aqua regia and a 1:1 mixture of hydrofluoric acid and nitric acid. Tungsten has high hardness and melting point, making it difficult to process and shape, but the welding of flake or filamentous tungsten is easy. Therefore, tungsten is often used in vacuum systems for electron emission filaments, cathodes of X-ray tubes, spring components, high-temperature thermocouples, evaporating dishes for furnaces, and welding electrodes.

2、 Tantalum

A lightweight and high-strength refractory metal with a melting point temperature of 2996 ℃ and a low vapor pressure. In terms of chemical properties, tantalum has excellent corrosion resistance. It does not react with aqua regia, chromic acid, nitric acid, sulfuric acid, and hydrochloric acid under cold and hot conditions, but is soluble in hydrofluoric acid, fluoride solutions, and oxalic acid. At the same time, it can combine with hydrogen to generate hydrides, damaging metal properties and causing brittleness. Tantalum has obvious suction performance for active gases and can absorb most residual gases in vacuum systems except for inert gases. In vacuum applications, tantalum is also a good electron emitter (between molybdenum and tungsten), often used in HCD ion coating equipment to produce electron beams through hollow cathode guns. Other materials can also be used as getters, crucibles, and evaporators under high vacuum and high temperature conditions.

3、 Molybdenum

It is a refractory metal with high hardness, no magnetism, and stable chemical properties, with a melting point temperature of 2620 ℃. At high temperatures, molybdenum exhibits oxidizing properties and is slowly oxidized at 520 ℃. Rapid oxidation begins above 600 ℃, while molybdenum is stable in air or water at room temperature or not too high. In terms of corrosion resistance, molybdenum is only corroded by a heated dilute hydrochloric acid solution and a 1:1 mixture of hydrofluoric acid and nitric acid. In vacuum applications, molybdenum can be heat treated or niobium added to improve its tensile strength. Molybdenum is commonly used in forms such as rods, cylinders, bolts, nuts, and heating coils. It can also be used for sealing glass to metal, evaporating dishes in vacuum furnaces, and electrodes for various vacuum devices.

4、 Niobium

Melting point 2468 ℃, a refractory metal with high strength and low specific gravity. The various properties of niobium are similar to tantalum, and you can refer to the properties of tantalum. However, its low melting point and higher vapor pressure make it rarely used as a hot electron emitter. Due to its strong affinity, niobium can be used as a getter, especially at high temperatures. In vacuum systems, it can also be used as a structural material and is commonly used in electric welding or electron beam welding. It is added as an additive to some stainless steels to prevent intergranular corrosion caused by carbon. In addition, refractory metals such as niobium can also be seen in the heating element and insulation screen of the heating device in vacuum equipment.

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