Mineral Facts and Problems: 1960 Edition Page: 344
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MINERAL FACTS AND PROBLEMS, ANNIVERSARY EDITION
Controlled purity is essential in germanium
for electronic uses. Less than 1 part per 100
billion of some impurities has an observable
effect. Germanium tetrachloride is purified by
distilling it from strong hydrochloric acid
solution. After distillation it is hydrolyzed in
ice water to obtain pure germanium dioxide,
which is washed completely free of chloride
and is then ready for reduction to the metal.
Germanium dioxide is reduced by heating in
hydrogen. The dioxide is packed in graphite
boats and heated to 6500 C. in an electric fur-
nace in a current of hydrogen until no more
water is evolved. The hydrogen is then re-
placed by helium or nitrogen, and the tempera-
ture is raised to 1,000 C. to melt the resulting
powder.
When germanium is used as a semicon-
ductor, it is necessary to have an ultra-high-
purity which is measurable in parts per billion
and which is achieved by zone refining. A
germanium ingot 12 to 20 inches in length and
1 to 11/2 inches in diameter is passed through
a series of induction-coil heaters each of which
produces a molten zone. Relatively pure ger-
manium crystallizes at the trailing edge of
each molten zone, and impurities remain in
the molten zone and are gradually isolated
toward one end as the bar moves through the
heaters. After the desirable number of passes
are made through the induction stages, the end
of the ingot containing the impurities is cut
off.
The "ultrapure" germanium ingot thus pro-
duced is polycrystalline, and for semiconductor
use it must be in the form of a single crystal.
Two methods of single-crystal growth were
developed for growing germanium crystals.
The first, originated by Czochralski, is called
the vertical pulling method. A crystallo-
graphically oriented seed-crystal is dipped
into molten germanium in a graphite crucible
and slowly withdrawn. Then, by adjusting
the heat input and heat losses, and under
proper conditions and synchronization the ger-
manium will grow on the seed and extend its
crystal lattice until the charge has been con-
verted to one crystal.
The second method, called the horizontal or
zone-levelling technique, is a modification of
zone-melting. The crystal seed is placed at
one end of a horizontal, carbon-coated silica
crucible filled with ultra-pure polycrystalline
germanium. The whole may be converted into
a single crystal with the controlled addition
of a desired impurity by carefully moving a
molten zone toward the seed, melting into it,
and then slowly withdrawing the molten zone.USES
The major use for germanium is in manu-
facturing diodes, transistors, and power recti-
fiers. Other uses for germanium are negligible
by comparison.
Germanium diodes are used primarily for
rectification in electronic circuits including
radio, television, measuring equipment, carrier-
current (telephone and telegraph), multi-
position switching, and voltage multiplier cir-
cuits. Developments of significance in diodes
are the Zener diode and the Tunnel diode.
Germanium transistors are used in a large
number of electrical and electronic devices in
telecommunications, radio, television, comput-
ing, and data processing industries.
Germanium rectifiers are used in electrolytic
metal production and refining, electrochemical
processes, electroplating, anodizing, variable
speed d.c. motor drives, and battery charging.
Germanium has a potential use in the pro-
duction of glass with special properties. Glass,
in which all or part of the silicon dioxide has
been replaced by germanium dioxide, displays
an unusually high index of refraction and is
useful for wide-angle camera lenses and micro-
scope objectives. Glass containing magnesium
germanate has the property of transmitting
infrared radiation over broad regions of the
spectrum and is used in infrared spectroscopes
and other infrared optical instruments. Ger-
manium-magnesium phosphors have been used
in fluorescent lights.
Germanium gives increased hardness and
strength to copper, aluminum, and magnesium
and improves their rolling properties. Ger-
manium-gold alloys have been experimented
with as dental alloys. The alloys of the Au-Ge
series can be used as precision castings as they
expand on cooling. The addition of small
percentages of germanium to gold sharply re-
duces the melting point and the 12-percent
germanium eutectic has a melting point of
356 C. This may be used to advantage as a
gold solder and in jewelry manufacture, as the
gold color remains.
BYPRODUCTS AND SUBSTITUTES
Germanium has a close byproduct relation-
ship with the base metals because it is not
economically feasible at this time to process
germanium-bearing raw materials on a com-
mercial basis solely for germanium. The
major use of germanium as a semiconductor
places it vulnerable to direct substitution by
other semiconductors with comparable per-
formance characteristics such as high-purity
silicon and certain tellurium, selenium, indium,
and gallium bimetallics.344
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United States. Bureau of Mines. Mineral Facts and Problems: 1960 Edition, report, 1960; Washington D.C.. (https://digital.library.unt.edu/ark:/67531/metadc38790/m1/352/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.