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Rhenium (symbol Re) is one of the last elements
to be discovered. It is right next to tungsten on the periodic
table. Rhenium is known for its high melting point, and its high
density.
Rhenium is scarce in the earth's crust -0.7 parts
per million.
Rhenium is extracted from
flue gases during the roasting of molybdenite concentrates. These
concentrates are commonly found in porphyry ores of copper. Rhenium
is considered a secondary byproduct of copper mining. Major sources
for rhenium are Chile, Russia, Kazakhstan, Ukraine, and the southwestern
United States.
Rhenium, upon extraction,
is treated in an ion exchange system and precipitated in the form
of ammonium perrhenate (APR). This APR is reduced in hydrogen
to form high-purity rhenium metal powder. The powder is pressed
into bars or billets and sintered at high temperature to increase
density. The bars can be rolled into thin sheet, foil, or ribbon.
Rhenium
with a melting point of 3180 ºC, has the second-highest melting
point.. Only osmium, iridium, and platinum exceed its density of
21.04 g/cc. Because of its high melting point, rhenium is a refractory
metal. With this classification, rhenium is unique. It is the only
refractory metal that does not form carbides. Its crystallographic
structure is hexagonal close-packed (hcp), while other refractory
metals have a body-centered cubic (bcc) structure. Rhenium does
not have a ductile-to-brittle transition temperature. In other words,
it maintains its ductility from absolute zero all the way to its
melting point. Rhenium also has a high modulus of elasticity. This
means that structures made of rhenium will have very good stability
and rigidity. Rhenium
offers high electrical resistivity across a wide temperature range.
Its high temperature strength gives it design flexibility. Rhenium
has the third-highest modulus of elasticity of any metallic element.
A high recrystallization temperature is a prerequisite for good
creep resistance. Among refractory metals, rhenium has the highest.
At temperatures up to 2800 ºC and high stresses, the rupture
life of rhenium is longer than tungsten. The metal also accommodates
wide swings in temperature - large thermal expansions and contractions
– without incurring mechanical damage. Rhenium-metal rocket
thrusters, for example, have withstood more than 100,000 thermal
fatigue cycles without any evidence of failure.
Products made from rhenium can be thermally cycled thousands of
times with no ill effect. It can be alloyed with tungsten or molybdenum
and, near the solubility limits, imparts improved ductility to
those materials. High-temperature strength, low friction, ductility
and other unique properties make it the material of choice for
many critical applications.
Rhenium can be welded using inert gas or electron
beam methods when protected against oxidation. ECM (electrochemical
machining), EDM (electrical discharge machining), and abrasive
cutting/grinding methods achieve excellent results for rhenium
and rhenium alloys.
Rhenium has many uses.
One use is small rocket thrusters. These thrusters are used in outer
space to position satellites and help them maintain a geo-stationary
orbit.
Rhenium is also used in medical applications. Radioactive
rhenium is used in prevention and treatment of restenosis, which
develops following balloon angioplasty. Rhenium is also highly effective
in the treatment of liver tumor(s).
Rhenium is a catalyst in the petroleum industry.
Due to its high electrical resistance and low vapor pressure, rhenium
is an excellent choice for filaments.
Rhenium is drawn into wire and rods. Wires made
of rhenium are used in photoflash lamps in photography, high temperature
thermocouples, and thermistors. Rhenium work-hardens after a reduction
in thickness of only 10%. It must be annealed (or stress relieved)
in order to continue rolling. Therefore the time and labor involved
adds considerably to the final cost of the material. A major advantage
of rhenium is that it imparts its good qualities to other metals.
Rhenium in the form of pellets is added to nickel-based
super-alloys in order to raise the operating temperature of turbine
blades in aircraft and gas turbine engines.
Rhenium is also added to molybdenum and tungsten to improve their
qualities especially ductility. These rhenium-alloyed materials
find uses in mass spectrographs, light filaments, and ion gauges.
Common alloys of rhenium and molybdenum are with 41-47.5% rhenium
(Mo- 41Re, and Mo- 47.5Re). These Mo-Re alloys are mostly used in
electronics, space, and nuclear industries. Alloys of rhenium and
tungsten are with 3-5% and 25% W-3 Re, W-5Re, and W-25Re. W-Re alloys
are mostly used in electronics as filaments and thermocouples.
Rhenium was discovered
in Berlin, Germany in 1925 by Ida and Walter Noddack, and Otto Berg.
When Mendeleev developed the Periodic Table in 1869, he left gaps
in his Table for elements that he predicted would be discovered
in the future. He also predicted the properties of some elements
based on the properties of neighboring elements. He made no predictions
for two gaps in the seventh column, which he called eka- and dwi-
manganese. Because these were the last two members of Group VII,
their properties could not be guessed. These gaps were not filled
for more than 50 years. Ida, then 28 years old, along with her husband
Walter, decided to investigate the mystery of these two missing
elements. Their approach was to look for the elements in the ores
of their horizontal neighbors rather than the vertical group on
the Periodic Table. They concentrated on molybdenum, tungsten, ruthenium,
and osmium. In June 1925, with the help of Otto Berg, an x-ray specialist,
they identified a Norwegian columbite ore sample, a new element
which they called rhenium in honor of the Rhine River. A year after
its discovery, they prepared the first gram of the new metal from
660,000 grams of molybdenite ore.
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