Germanium (pronounced /dʒərˈmeɪniəm/, jər-MAY-nee-əm) is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid
in the carbon group, chemically similar to its group neighbors tin and silicon.
Germanium has five naturally occurring isotopes
ranging in atomic mass number from 70 to 76. It forms a large number of organometallic compounds, including tetraethylgermane and isobutylgermane
Detailed description
Germanium was
discovered comparatively late because very few minerals contain it in high
concentration. Germanium ranks near fiftieth in relative abundance of the elements in the
Earth's crust.
In 1869, Dmitri
Mendeleev predicted its existence and some of its properties
based on its position on his periodic table and called the element eka-silicon. Nearly two decades later, in 1886, Clemens Winkler found it in the mineral argyrodite. Winkler found that experimental
observations agreed with Mendeleev's predictions and named the element after
his country, Germany.
Germanium is an
important semiconductor material used in transistors and various other electronic devices.
Its major end uses are fiber-optic systems and infrared optics, but it is also used for polymerization catalysts, and in electronics and solar cell applications. It is finding a new use
in nanowires.
Germanium is
mined primarily from sphalerite, though it is also recovered from silver, lead, and copper ores. Some germanium compounds, such as germanium
chloride and germane, can irritate the eyes, skin, lungs, and throat.
Characteristics
Under standard
conditions
germanium is a brittle, silvery-white, semi-metallic element.[25] This form constitutes an allotrope technically known as α-germanium,
which has a metallic luster and a diamond cubic crystal structure, the same as diamond.[23] At pressures above 120 kbar, a different allotrope known as β-germanium
forms, which has the same structure as β-tin.[26]
Along with silicon, gallium, bismuth, antimony, and water, it is one of the few substances that expands
as it solidifies (i.e. freezes) from its molten state.[26]
Germanium is a semiconductor. Zone refining techniques have led to the production
of crystalline germanium for semiconductors that has an impurity of only one
part in 1010,[27] making it one of the purest materials
ever obtained.[28] The first metallic material
discovered (in 2005) to become a superconductor in the presence of an extremely
strong electromagnetic
field was an alloy
of germanium with uranium and rhodium.[29]
Pure germanium is
known to spontaneously extrude very long screw dislocations, referred to as germanium whiskers.
The growth of these whiskers is one of the primary reasons for the failure of
older diodes and transistors made from germanium; depending on what they
eventually touch, they may lead to an electrical short.[30]
Natural abundance
Germanium is
created through stellar
nucleosynthesis,
mostly by the s-process in asymptotic
giant branch
stars. The s-process is a slow neutron capture of lighter elements inside
pulsating red giant stars.[46] Germanium has been detected in the
atmosphere of Jupiter[47]
and in some of the most distant stars.[48] Its abundance in the Earth's crust is approximately 1.6 ppm.[49] There are only a few minerals like argyrodite, briartite, germanite, and renierite that contain appreciable amounts of
germanium, but no minable deposits exist for any of them. Nonetheless, none is
mined for its germanium content.[23][50] Some zinc-copper-lead ore bodies
contain enough germanium that it can be extracted from the final ore
concentrate.[49] An unusual enrichment process causes
a high content of germanium in some coal seams, which was discovered by Victor
Moritz Goldschmidt
during a broad survey for germanium deposits.[51][52] The highest concentration ever found
was in the Hartley coal ash with up to 1.6% of
germanium.[51][52] The coal deposits near Xilinhaote, Inner Mongolia, contain an estimated 1600 tonnes of germanium.[49]
Production
Worldwide
production in 2006 was roughly 100 tonnes of germanium.[23] Currently, it is recovered as a
by-product from sphalerite zinc ores where it is concentrated in amounts of
up to 0.3%,[53] especially from sediment-hosted,
massive Zn–Pb–Cu(–Ba) deposits and carbonate-hosted Zn–Pb
deposits.[49] Figures for worldwide Ge reserves are
not available, but in the US it is estimated to be around 500 tonnes.[49] In 2007 35% of the demand was met by
recycled germanium.[49]
Applications
The major end
uses for germanium in 2007, worldwide, were estimated to be: 35% for fiber-optic systems, 30% infrared optics, 15% for polymerization catalysts, and 15% for electronics
and solar electric applications.[23] The remaining 5% went into other uses
such as phosphors, metallurgy, and chemotherapy.[23]
Optics
The most notable
physical characteristics of germania (GeO2) are its high index of refraction and its low optical
dispersion.
These make it especially useful for wide-angle
camera lenses,
microscopy, and for the core part of optical fibers.[57][58] It also replaced titania as the silica dopant for silica fiber, eliminating the need for
subsequent heat treatment, which made the fibers brittle.[59] At the end of 2002 the fiber optics
industry accounted for 60% of the annual germanium use in the United States,
but this use accounts for less than 10% of world wide consumption.[58] GeSbTe is a phase change alloy used for its optic properties,
such as in rewritable DVDs.[60]
Electronics
Silicon-germanium alloys are rapidly becoming an
important semiconductor material, for use in high speed integrated circuits.
Circuits utilizing the properties of Si-SiGe junctions can be much faster than
those using silicon alone.[63] Silicon-germanium is beginning to
replace gallium
arsenide (GaAs) in
wireless communications devices.[23] The SiGe chips, with high-speed
properties, can be made with low-cost, well-established production techniques
of the silicon
chip industry.[23]
The recent rise
in energy cost has improved the economics of solar panels, a potential major new use of
germanium.[23] Germanium is the substrate of the
wafers for high-efficiency multijunction photovoltaic cells for space applications.
Gallium arsenide germanium solar cell
Because germanium
and gallium
arsenide have very
similar lattice constants, germanium substrates can be used to make gallium
arsenide solar cells.[64] The Mars
Exploration Rovers
and several satellites use triple junction gallium arsenide on germanium cells.[65]
Germanium-on-insulator
substrates are seen as a potential replacement for silicon on miniaturized
chips.[23] Other uses in electronics include phosphors in fluorescent lamps,[27] and germanium-base solid-state
light-emitting diodes (LEDs).[23] Germanium transistors are still used
in some effects
pedals by musicians who
wish to reproduce the distinctive tonal character of the "fuzz"-tone from the early rock and roll era, most notably the Dallas Arbiter Fuzz Face.[66]
Other uses
Germanium dioxide
is also used in catalysts for polymerization in the production of polyethylene
terephthalate
(PET).[67] The high brilliance of the produced
polyester is especially used for PET bottles marketed in Japan.[67] However, in the United States, no
germanium is used for polymerization catalysts.[23] Due to the similarity between silica
(SiO2) and germanium dioxide (GeO2), the silica
stationary phase in some gas
chromatography
columns can be replaced by GeO2.[68]
In recent years
germanium has seen increasing use in precious metal alloys. In sterling silver alloys, for instance, it has been
found to reduce firescale, increase tarnish resistance, and
increase the alloy's response to precipitation hardening. A tarnish-proof
sterling silver alloy, trademarked Argentium, requires 1.2% germanium.[23]
Dietary supplements
Germanium has
gained popularity in recent years for its reputed ability to improve immune
system function in cancer patients. It is available in the U.S. as a
nonprescription dietary supplement in oral capsules or tablets, and has also
been encountered as an injectable solution. Earlier inorganic forms, notably
the citrate-lactate salt, led to a number of cases of renal dysfunction,
hepatic steatosis and peripheral neuropathy in individuals using it on a
chronic basis. Plasma and urine germanium concentrations in these individuals,
several of whom died, were several orders of magnitude greater than endogenous
levels. The more recent organic form, beta-carboxyethylgermanium sesquioxide (propagermanium), has not exhibited the same spectrum
of toxic effects.[73]
Precautions
As early as 1922,
doctors in the United States used the inorganic form of germanium to treat
patients with anemia.[74] It was used in other forms of
treatments, but its efficiency has been dubious. Its role in cancer treatments has been debated.[75] U.S. Food and Drug Administration research has concluded that
germanium, when used as a nutritional
supplement,
"presents potential human health hazard".[43]
Germanium is not
thought to be essential to the health of plants or animals. Some of its compounds
present a hazard to human health, however. For example, germanium chloride and
germane (GeH4) are a liquid and gas, respectively, that can be very
irritating to the eyes, skin, lungs, and throat.[76] Germanium has little or no impact on
the environment because it usually occurs only as a trace element in ores and carbonaceous materials, and is used in very small
quantities in commercial applications.
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