Refractory metals
Some of their applications include tools to work metals at high temperatures, wire filaments, casting molds, and chemical reaction vessels in corrosive environments.
Most definitions of the term 'refractory metals' list the extraordinarily high melting point as a key requirement for inclusion.
By one definition, a melting point above 4,000 °F (2,200 °C) is necessary to qualify, which includes iridium, osmium, niobium, molybdenum, tantalum, tungsten, rhenium, rhodium, ruthenium and hafnium.
The physical properties of the refractory elements vary significantly because they are members of different groups of the periodic table.
This gives stiff, highly stable bonds to neighboring atoms and a body-centered cubic crystal structure that resists deformation.
Moving to the right in the periodic table, more d electrons increase this effect, but as the d subshell fills they are pulled by the higher nuclear charge into the atom's inert core, reducing their ability to delocalize to form bonds with neighbors.
[9][10] The refractory metals show a wide variety of chemical properties because they are members of three distinct groups in the periodic table.
[6] Refractory metals, and alloys made from them, are used in lighting, tools, lubricants, nuclear reaction control rods, as catalysts, and for their chemical or electrical properties.
Powders of the pure metal are compacted, heated using electric current, and further fabricated by cold working with annealing steps.
Its strength at high temperatures, resistance to wear and low coefficient of friction are all properties which make it invaluable as an alloying compound.
The high melting point and the wear resistance against the electric arc makes tungsten a suitable material for the electrode.
[21][22] Tungsten's high density and strength are also key properties for its use in weapon projectiles, for example as an alternative to depleted Uranium for tank gun rounds.
[23] Its high melting point makes tungsten a good material for applications like rocket nozzles, for example in the UGM-27 Polaris.
Tungsten can be found in printing inks, x-ray screens, in the processing of petroleum products, and flame proofing of textiles.
It is unique in that it can be worked through annealing to achieve a wide range of strength and ductility, and is the least dense of the refractory metals.
An alloy used for liquid rocket thruster nozzles, such as in the main engine of the Apollo Lunar Modules, is C103, which consists of 89% niobium, 10% hafnium and 1% titanium.
Many important uses have been found for tantalum owing to this property, particularly in the medical and surgical fields, and also in harsh acidic environments.
Tantalum films provide the second most capacitance per volume of any substance after Aerogel,[citation needed] and allow miniaturization of electronic components and circuitry.
[28] The strength and high-temperature stability of refractory metals make them suitable for hot metalworking applications and for vacuum furnace technology.
The refractory metal alloys of molybdenum, niobium, tantalum, and tungsten have been applied to space nuclear power systems.
