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Encyclopedia of Physical Science and Technology EN010b-481 July 14, 2001 18:45

482 Noble Metals (Chemistry)

and thermal conductivity and excellent oxidation resis- sistors can be made by firing platinum complexes onto
tance. The most frequent use of silver and its alloys is glass, mica, or quartz in a variety of forms.
in electrical contact materials. The advantage is that its
contact resistance is not increased by oxidation at normal 4. Palladium
temperatures by arcing. Silver with 15–25% Cd, 7.5–28%
Palladium is extremely important in the construction of
Co, 3–10% Pd, 10% Au, or 3% Pt is used for minimal
electronic relays that are free of tarnish for reliable noise-
demand switching. When greater demands are placed on
freetransmission.Themostcommonlyusedalloyis60Pd–
the contacts, silver alloys with either 0.25–10% C or 10–
40Ag,whichisusedincontactpointsfortelephoneswitch-
40% Ni are used. Heavy-duty electrical contacts utilize
ing equipment. Solid-state switches may diminish this use.
the internally oxided alloys Ag–0.41MgO–0.25Ni or Ag–
Pd–Ag alloys are used also as precision resistance wires.
2.5–15CdO. These two alloys are used in springs in elec-
60Pd–40W is used in electrical contacts in which the cur-
trical equipment such as miniature and micro relays. The
rent is in the milliampere range. Pd–Ag–Cu alloys have
Ag–CdO alloy has good sticking and welding resistance,
good wear resistance and spring properties and are used in
and it does not arc erode easily. Silver with 50–60% Mo,
contacts in which sliding wear can occur. Zinc or platinum
50–70% W, or 10–60% WC is used for heavy switching
can be used to assist in age hardening these alloys.
demands. The Ag–Pd alloys are used as resistance wind-
ings and Ag–Mg is used in relays as spring members.
5. Iridium
Silver paints are available to produce conducting surfaces
on nonconductors. An example is their use in rear window Iridium is used mainly as an additive (e.g., Pt–Ir) that is
defrosters in cars. Paints that require firing are available used in electrical contacts and precision resistance wiring
for ceramics, glass, and mica. Wood, paper, and plastics applications. The alloys Ti 3 Ir, ThIr 2 , ZnIr 2 ,Nb 3 Ir, and
can be coated by air drying paints. Th 7 Ir 3 are superconductors. Iridium is also used in con-
ductive inks and pastes for electronic applications.
3. Platinum
6. Rhodium, Osmium, and Ruthenium
Platinum and its alloys are used in high-frequency con-
ductors where silver corrodes and at high temperatures Rhodium alloys find most of their applications in elec-
where gold might stick. Rhodium and palladium can also trical contacts for radio frequency circuits, precision po-
be used in these applications. Platinum is wetted by, but tentiometers, and spark plug electrodes. The trend is to-
not attacked by, mercury, which makes it useful in special ward lower usage in electronics, mainly because of price.
electrical relays. Platinum with either palladium or ruthe- Osmium has minor uses in electrical equipment, mainly
nium is used in electrical contacts; the ruthenium alloy in contacts. Ruthenium is used in electrical contacts and
◦
is better suited for heavy-duty use. 96Pt–4W was used in high-voltage relays up to 500 C because even RuO x is
aircraft spark plugs because of its high resistance to spark conductive. A ruthenate thick-film paste is used for printed
erosion and to lead. This alloy is also used in electronic circuit resistance elements. The ruthenate is converted to
tubes, such as microwave and X-ray tubes, as a control grid RuO 2 , which has very low resistance drift, ∼0.10%.
because it gives a minimum of electron emissions. Plat-
inum, molybdenum, or tungsten wire has also been used D. Industrial Applications
for this application. The alloy 92Pt–8W is used as po-
This section accents the indirect uses of the noble met-
tentiometer wire because of its low noise and good wear
als in industry. This excludes chemical and catalytic uses
characteristics. This alloy is also used in heater elements
of precious metal compounds where they are part of the
and glow wires for copiers. The alloy Pt–10–4ORh is used
process itself. The majority of the precious metal usage in
in furnace windings. The alloy Pt–Ni is remarkably free
industry is related to the unreactive nature of the metals
of hysteresis and is used in applications such as electric
and the ability to recover the metal after its usefulness has
meters. The alloy Pt–Rh–Ru is very versatile with uses
been exhausted.
in potentiometers, galvanometers, suspension strips, con-
tacts, copier wires, and explosive bridge wires. The alloy
1. Gold
Pt–Ir is used as fuse wire for detonators. The magnetic al-
loy Pt–Co has electrically related uses due to its minimum Gold, because of its corrosion resistance, is used to line
space requirements and stable temperature performance. autoclaves for phosphate handling up to 500 C and in clo-
◦
Its uses include focusing magnets, hearing aid magnets, sure gaskets for obtaining zirconium by the iodine method.
magnetic phonograph cartridges, electric watch magnets, Gold or Au–Pt is used in rupture disks for service with cor-
and digital magneto-optic recording. Platinum-based re- rosive materials such as fluorine. The alloy of 70Au–30Pt