Saffron is said to be the most expensive and rarest spice of all. Its annual production is around 200 t. The metallic equivalent of saffron is rhenium with indispensable production of 60 t. At a density of 21 g/cm3, this translates into a volume of less than 3 m3 or a sphere of a diameter of less than 2 m.
The metal is contained in the Earth’s crust in extremely sparse amounts and has been found to occur naturally only in a volcano on the Kuril Islands in Russia’s Sakhalin oblast region. It took until 1925 for rhenium to be isolated for the first time. In the periodic table of the elements, rhenium takes on a position between the refractory and noble metals.
What’s so special about rhenium?
No primary sources of rhenium are at hand, but rhenium is extracted as a by-product from molybdenum concentrates, which themselves are a by-product of mining copper ores. Rhenium has a melting point of 3186 °C, which is the third highest melting point among all elements after carbon and tungsten. In terms of the boiling point, it ranks at top positions with 5627 °C.
Rhenium is able to show its superiority in applications that are subject to very hot temperatures and require stable structures. Unfortunately, rhenium
In the early 1950s, molybdenum-rhenium and tungsten-rhenium alloys were developed. The first industrial applications of tungsten-rhenium alloys included high-temperature thermocouples and vacuum tubes used in electronic devices.
For a while, rhenium was also used to produce darts (‘rhenium darts’) to positively influence flight characteristics, although this application arguably uses more of a tungsten alloy and just a small amount of rhenium.
How rhenium became a spice for high-tech alloys
Today, the production of superalloys for jet engines represents the largest single use for the element rhenium. These nickel superalloys contain 3-6 % rhenium and are used in combustion chambers, turbine blades and propelling nozzles of jet plane engines.
Rhenium improves the alloys’ creep resistance, resulting in more durable components that ensure greater fuel efficiency at elevated temperatures – an important contribution toward lowering fuel consumption in the aerospace industry.
The second most frequent use of rhenium is that of a catalyst. Platinum rhenium catalysts are used to produce high-octane lead-free gasoline.
Additionally, rhenium is needed in the form of molybdenum-rhenium alloys in the aviation and electronics industry for reactors, semiconductors, electrical contacts, filaments and ignition wires. Molybdenum-tungsten-rhenium alloys are particularly well suited for use at very high temperatures or applications subject to high wear.
Tungsten-rhenium alloys are also used in X-ray anodes in high-resolution X-ray equipment – for example in mammography, angiography and computed tomography. Luminescent rhenium complexes have been used in intracellular bioapplications because of their efficient cellular internalisation and attractive photophysical properties.
What’s ahead for the
super material rhenium?
Being a relatively ‘young’ element, researchers and developers still see great potential for rhenium in future applications. One of the most promising properties of rhenium alloys is their favorable biocompatibility, making them very attractive for the production of medical products. Specifically, molybdenum-rhenium alloys have performed better in implants than conventional titanium alloys.
Additionally, researchers have examined the properties of rhenium diboride as a material for hard coatings and found that its hardness is almost equivalent to that of cubic boron nitride.
The article is written by Manuel Guzmán Manzo and Heiko Wildner.
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