SPI Precious Metals Properties
Each of the precious metals has its own unique set of properties
Gold is the best known, since historical times, it has been used as a
medium of currency. It is also a surprise to many, that ancient
civilizations learned the art and technology for the making of gold at
purities of 0.99995! Today, the applications for gold are quite diverse,
but the ancient coinage application has carried over to this millennium
and gold coins are still valued and collected worldwide. And in some
countries, the strength of the national currency is measured by the amount
of accumulated gold in their central banking system.
The jewelry application is still important and gold jewelry is prized by
everyone worldwide. The dental applications have declined over the years
with the development of alloys of base metals that are either equivalent to
or in some cases better than gold. And in these days of tight budgets, the
fact that the alternatives are significantly lower in price make them even
more attractive. And of course there are applications ranging from
electronics to medicine for this most interesting of metals.
Iridium is the most inert of the precious metals. Its chemical resistance
is legend, and is superior in that respect to gold and platinum, while
featuring a far higher melting point. As a result, it is used for making
crucibles and other apparatus for use at high temperatures. It is also used
in certain electrical contacts and also, a hardening agent for platinum.
Osmium and iridium are the densest known solids, 143g/cm3
which is about twice the density of lead. We have been told that in the
past, osmium metal was used for phonograph needles because of its
outstanding resistance to chipping and wear.
Today, the uses for osmium are somewhat different. The main uses for osmium
seem to be divided approximately equally between the applications in
electron microscopy (used as
osmium tetroxide) applications as a homogeneous catalyst
(as soluble osmium complexes) in the pharmaceutical industry. Some amounts
of osmium are used to produce hardened alloys with other metals of the
platinum group, for example, for fountain pen tips, and electrical
contacts. Certain platinum/osmium alloys are used as biomaterials
and implants.
Palladium is the least dense and lowest melting of the "platinum group"
metals. When annealed, it is soft and ductile, but cold working increases
its strength and hardness. Historically, it has been used for watch springs.
Another interesting fact: At room temperature, palladium has the unusual
property of absorbing up to 900 times its own volume of hydrogen, which
readily diffuses through heated palladium, providing a means of purifying
the gas (e.g. hydrogen).
Both ruthenium and rhodium are rarely used alone, because of their softness
and are almost always alloyed with other metals.
Platinum is perhaps best known for its jewelry applications (in the making
of "white gold"). For those in chemistry laboratories, platinum crucibles
are a familiar site. Its mix of properties result in a number of
electronics applications, ranging from contacts and thermocouple wires.
Like gold, there were previously large applications in dentistry, but with
the rising cost of the metal as well as new technology involving base metals,
this application is on the decline. In colloidal form, and finely divided
on a substrate such as alumina, it functions as a highly effective catalyst,
from oil refineries as a "cracking" catalyst to exhaust catalysts in the
automotive industry.
While technically speaking, not a "platinum group metal", no discussion of
these metals would be complete without saying at least something about
silver. To the lay person, perhaps "sterling silver" is the most familiar,
it is used in jewelry, silverware, silver service appliances, and other
applications where appearance is most important. Sterling silver contains
92.5% silver, most of the remainder being copper. There are numerous other
high volume uses for silver, one very familiar one being photography, but
that application is on the decline because the two reasons: a)environmental
regulations that greatly restrict the amount of silver that can be
discharged into public sewer systems, and b)the trend toward digital
photography that of course uses no silver. It is still used in dentistry,
in electrical contacts and solder and brazing alloys. It was discovered
many years ago that silver is about the best reflector of visible light
known when freshly deposited, but without protection, it quickly tarnishes
(oxidizes or sulfides) and loses most of its high reflectance. Early
civilizations, as was the case with gold, learned the art of purification
and forming into the earliest coins.
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Tuesday December 02, 2008
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