Refractory metals - a class of chemical elements (metals), which have very high melting point and resistance to abrasion. Expression of refractory metals most commonly used in disciplines such as materials science, metallurgy, and engineering sciences. Determination of refractory metals applies to each element of the group in different ways. The main representatives of this class of elements - niobium and molybdenum, tantalum, tungsten, and rhenium. They all have a melting point above 2000 ° C, chemically relatively inert and has a high density exponent.
Most definitions of refractory metals define them as metals with high melting points. By this definition, it is necessary that the metal would have a melting point above 2200 ° C. It is necessary to define them as refractory metals. The five elements - niobium, molybdenum, tantalum, tungsten, and rhenium. The list includes both core while a broader definition of these metals can be included in this list are the elements and having a melting point of 2123K (1850 ° C) - titanium, vanadium, chromium, zirconium, hafnium, ruthenium and osmium.
The melting point of these elements is the highest, excluding carbon and osmium. This property does not depend only on their properties, but the properties of the alloys. Metals have a cubic crystal system, except rhenium. Most of the physical properties of the elements in this group varies widely because they are members of different groups.
Resistance to creep deformation is the defining property of refractory metals. In ordinary metals deformation begins with the melting point of the metal, and hence the creep deformation in aluminum alloys starts at 200 ° C, while in refractory metals, it starts at 1500 ° C. This resistance to deformation and high melting refractory metals can be used, such as jet engines or parts in forging various materials.
In the open air refractory metals oxidized. This reaction is slowed down due to the formation of the passivated layer. Rhenium oxide is very unstable, because when passing dense flow of oxygen to the oxide film evaporates. All of them are relatively resistant to acids.
Refractory metals are used as light sources, parts, lubricants, in the nuclear industry as the ARC, as the catalyst. Because they have high melting points, they are never used as a material for smelting in the open air. In the powdered material is compacted by melting furnaces. Refractory metals can be processed into wire, bar, rod, sheet, foil, tube.
Tungsten was discovered in 1781 by Swedish chemist Carl Wilhelm Scheele. Tungsten has the highest melting point of all metals - 3422 ° C.
Rhenium is used in alloys with tungsten in concentrations up to 22%, which can increase refractoriness and resistance to corrosion. Thorium is used as an alloying element tungsten. This increases the wear resistance of materials. In powder metallurgy components can be used for sintering and subsequent use. To obtain tungsten heavy alloy using nickel and iron or nickel Imedi. Tungsten content in these alloys are generally above 90%. Mixing doping material with it low even during sintering.
Tungsten used in high temperatures, where high hardness and high density can be neglected. Filament, consisting of tungsten, used in the instrumentation. Lamps more efficiently convert electricity to light with increasing temperature. The high melting point of tungsten allows it to be used for welding. The high density and hardness of tungsten can be used in artillery shells. Its high melting point is used in the structure of rocket nozzles. Sometimes it finds its application due to its density.
Molybdenum is the most frequently used refractory metals. The most important thing is to use it as an amplifier of steel alloys. Used to manufacture pipes with stainless steel. The high melting point of molybdenum, its resistance to wear and low friction coefficient makes it a very useful material for doping. His excellent friction coefficients lead him to use as a lubricant where you want reliability and performance.
Niobium is almost always found together with tantalum. Niobium finds many ways to use some of it common with refractory metals. Its uniqueness lies in the fact that it can be developed by annealing in order to achieve a wide range of hardness and elasticity, and its density index is the smallest compared to the other metals of the group. It can be used in electrolytic capacitors. Niobium can be used in gas turbine aircraft, lamps and nuclear reactors.
Niobium is used to create nozzles in liquid rocket engines. Alloys are used prevents oxidized niobium, as the reaction occurs at a temperature of 400 ° C.
Tantalum is the most corrosion-resistant metal of all refractory metals.
An important property of tantalum was found due to its use in medicine - it is able to withstand the acidic environment (the body). It is sometimes used in electrolytic capacitors. Capacitors used in cell phones and computers.
Rhenium is the latest open refractory element. It is located in low concentrations in ores of other metals of this group - platinum and Copper. Can be used as an alloying element with other metals and alloys gives good performance - toughness and tensile strength increases. Rhenium alloys can be used in the components of electronic devices, gyroscopes and nuclear reactors. The most important application is found as a catalyst. Can be used in the alkylation, dealkylation, hydrogenation and oxidation. His presence so rare in nature, making it the most expensive of all refractory metals.
Refractory metals and alloys attract the attention of researchers because of their unusual properties and future prospects in the application.
The physical properties of refractory metals such as molybdenum, tantalum and tungsten, their hardness and stability at high temperatures, make them the material used in many industries. Many details are based on their unique properties.
However, their resistance to oxidation up to 500 ° C makes it one of the major drawbacks of this group. Exposure to air can significantly affect their high-temperature performance. That is why they are used in the materials isolated from oxygen.
Refractory metals - molybdenum, tantalum and tungsten - used in the details of space nuclear technology. These components have been specifically designed as a material capable of withstanding high temperature. As stated above, they do not have contact with oxygen.