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

Noble Metals (Chemistry) 481

electromechanical, or electronic connections and may not 3. Palladium
be relied on for mechanical strength. Solders are low melt-
Palladium alloys in general are high-temperature solder
ing point alloys that can be tailored to wet the metals of
or brazing alloys that have low vapor pressures, excel-
interest with the proper heat and flux. Brazing alloys are
lent wetability, and minimum penetration into austenitic
used where bonding strength is required, and they are de-
alloys. The alloy components are usually silver, copper,
signed to create a bond below the melting point of the
manganese, nickel, and/or chromium. The Pd–Ag alloy
metals to be joined. The alloy is drawn into the close fit-
is used to braze stainless steel, Inconel, and other heat-
ted joint by capillary action. Alloying of the brazing agent
resistant alloys. A Pd–Ag–Mn brazing alloy was used to
and the metal can occur, which may result in an unfilled
join nickel tubing for the liquid hydrogen engines used in
joint.
the Saturn rocket. Engines on the Apollo spacecraft were
Soldering and brazing alloys containing lead, zinc, or
joined with a Pd alloy. Use of the other PGMs in brazing
cadmium should be used with proper ventilation. The no-
and soldering is almost nonexistent.
ble metals themselves present no direct problems because
the reactive osmium is not used in these alloys.
C. Electrical and Electronic Applications
1. Gold The precious metals are important in electrical applica-
tions because of their durability, conductivity, and tarnish
Gold, silver, and palladium are the most frequently used
resistance. Tarnish-free conductors and contacts are es-
precious metals in solder and brazing alloy formulations.
pecially important where low voltages and currents are
A large number of gold alloys are available; they are well
used. The alloy mixtures provide a method of varying the
suited for applications in which good conductivity under
resistance and the mechanical and thermal properties.
oxidizingconditionsisnecessary.Goldalloyswithcopper,
silver, indium, silicon, silver, tin, cadmium, zinc, nickel,
chromium, molybdenum, tantalum, palladium, and plat- 1. Gold
inum are known mainly as two- and three-component al-
Gold is widely used in electronics in contacts, connectors,
loy systems, with gold the major component. Low-melting
and printed circuits, mainly in alloyed form. Gold is al-
alloys, such as 73Au–27In, 94Au–6Si, 88Au–12Ge, and
loyed with silicon, germanium, tin, or indium to yield the
80Au–20Sn, have good corrosion resistance and are used
low-melting eutectics used to make semiconductor con-
in transistor technology. Alloys of Au–Cu, Au–Ni, and
nections. The Au–Sb alloys are used for n-type and Au–In
Au–Cu–Ni have low metal vapor pressures and have been
for p-type semiconductors. The alloy mixture 94Au–6Si,
used in high-vacuum applications. The Au–Pd–Ni brazing
◦
which has a eutectic at 370 C, can be brittle and cause
alloys are used to join tungsten, molybdenum, nickel, and
p-type semiconductivity, which may require countermea-
stainless steel parts. The 35Au–35Ni–30Mo and 60Au–
sures. The Au–Pd alloys form a continuous series of so-
10Ni–30Tablendsareusedinbrazingwithgraphite.These
lutions that offers a wide range of resistivities for resistor
two alloys are resistant to molten fluorides. One of the
and potentiometer applications. An advantage of these al-
most oxidation-resistant alloys is 92Au–8Pd. Gold alloys
loys is their minimum temperature coefficient of resistiv-
have found applications in the electronics, nuclear, and
ity. The alloy Au–0.4Mn is recommended for resistance
aerospace industries. Gold brazing alloys are used to join
thermometry at cryogenic temperatures. The Au–Pd al-
some jet engine components.
loys are useful as thermal limit fuses for electric furnaces
because of their narrow melting range. Au–Ag–Cu alloys
are used as brazing agents in electrical applications and
2. Silver
as slip rings and brushes for electrical instruments. Gold,
Silver alloys provide the ability to wet and join base met- electroplated to a thickness of 0.05-mil, has been used as
als at low temperatures while providing good strength. stop-offs in ferric chloride and chromic acid etching of
Because low temperatures can be used for silver soldering printed circuits. Powders involving Au–Pd–Pt alloys with
and brazing, the risk of surface oxidation and heat dis- high surface areas and small particle size are used in inks
tortion is reduced. Silver alloys usually comprise eutectic for hybrid circuits. The Au–Pd–Ag based alloys are used
mixtures of tin, zinc, or copper, with lead and cadmium in electronic applications where a vacuum is required.
being used in some applications. The Ag–Cu alloys have
been used with ceramics to produce electrical parts, and
2. Silver
they have been used to braze carbide inserts to drills. The
carbide inserts can easily be adjusted or replaced by melt- Use of silver in electronics is second only to its use in pho-
ing the alloy. tography. Silver is important because it has high electrical