Rhenium is a hard metal (HV 250), with the highest-density crystal structure—hexagonal. Rhenium oxidizes at a fairly low temperature (300°C) to create volatile rhenium heptoxide (Re2O7), making processing under heat nearly impossible. Any warm processing needs to be performed in a vacuum or under inert gas. Rhenium is a byproduct of refining molybdenum from copper ores. In the periodic system, rhenium lies between tantalum (a refractory metal), and osmium, iridium and platinum (platinum group metals). It displays typical characteristics of both classes of metals, including a very high melting point, high density and an extremely high elastic modulus (466 GPa). Rhenium occurs with valences between +1 and +7 in compounds, as well as -1 and -3, but the oxidation states +7, +6, +4, +2 and -1 are the most common. Rhenium also possesses valuable catalytic properties, and is particularly resistant to poisoning from nitrogen, sulfur and phosphorus.Rhenium is an important alloying element in many materials for high-temperature applications. The highest-performance turbine blades in modern engines and gas turbines are made of monocrystal nickel superalloys, which contain 3–6% rhenium. Heraeus produces high-purity rhenium tablets for alloy production. Adding rhenium improves ductility and creep resistance in molybdenum and tungsten. For example, rotation anodes on medical x-ray systems are often made of molybdenum and tungsten alloys because of their extreme exposure to stress and heat. The use of platinum-rhenium catalysts in the reforming process for the petrochemical industry is equally important, especially in the production of high-octane fuel. Rhenium bands from Heraeus are used as filaments for mass spectrographs. Thermocouples made of tungsten-rhenium alloys can withstand temperatures up to 2,300°C under vacuum or an H2 atmosphere. Heraeus also produces sintered rhenium crucibles for growing hard laser crystals.
