textfiles/science/metal.txt

Information on Palladium which is one of the metals used in the recent
COLD FUSION experiments................

[ The information below on Palladium is quoted from                       ]
[ "Guide to Uncommon metals" Eric N. Simons                               ]

Palladium, symbol Pd, is a metallic element in the eighth group of the
periodic system, deriving its name from the French Pallas, an asteroid.
It is associated with platinum in the group, and is found in the native
state and in association with gold and silver in certain gold-bearing sands.
Obtained as a by-product in the extraction of platinum, it is produced in a
spongy state by the thermal decomposition of palladium dichlorodiamine.
The metal should be melted in either a zircon or alumina-rich crucible
in a high frequency induction furnace, and cast into moulds made of graphite.
The principle difficulty in this is that palladium absorbs gas to a
considerable degree, so that the metal is always liable to become
brittle at high temperatures, and consequently will lack ductility.

The alternative is to adopt one or other of the numerous processes for
isolating the metal from platinum ore. Bunsen eliminated most of the platinum
as ammonium platinochloride, precipitating the residual metals of the group
by iron. He then heated the resulting precipitate with ammonium chloride,
followed by evaporation with fuming nitric acid. After the residue had been

taken up by water, palladium was precipitated as potassium palladium chloride.
The metal was purified by dissolution in hot water and evaporation of the salt
with oxalic acid, the residue being taken up in potassium chloride, and the
potassium platinochloride present was removed by filtration. The filtrate
deposited potassium palladium chloride, which, heated in a stream of hydrogen
gas left the metal as a residue.

The properties of palladium are as follows: atomic number 46, atomic weight
106.7, density at 20 deg. C. (68 deg. F.) 12.02 g./cu. cm., or 0.4343
lb./cu. in., atomic volume 8.88 cu. cm./g.-atom, melting point 1552 deg. C.
(2826 deg. F.), boiling point 3980 deg. C. (7200 deg. F.), specific heat at
0 deg. C. (32 deg. F.) 0.0584 cal./g./deg. C., heat of fusion 34.2 cal./g.
or 61.6 BTU/lb., coefficient of linear thermal expansion near 20 deg. C.
(68 deg. F.) 11.76 micro-in./in./deg. C. or 6.53 micro-in./in./deg. F.,
thermal conductivity at 18 deg. C. (64 deg. F.) 0.168 cal./sq. cm./cm./sec.,
electrical resistivity 10.8 michohm-cm. at 20 deg. C. (68 deg. F.), at
0 deg. C. (32 deg. F.) 10.0 microhm-cm., modulus of elasticity in tension
16.3 million lb./sq. in. The crystal structure of palladium is face-centered
cubic. The lattice constant at a is 3.8902 Angstrom units at 20 deg. C.
(68 deg. F.), closest approach of atoms 2.750, vapour pressure at 1000 deg. C.
(1832 deg. F.) 1.15 X 10**-5 mm. Hg. at 1500 deg. C. (2732 deg. F.)
6.17 X 10**-2 mm. Hg. and at 1554 deg. C. (2829) deg. F.) 1.18 X 10**-1 mm. Hg,

electrical volume conductivity at 20 deg. C. (68 deg. F.) 16 per cent IACS,
temperature coefficient of electrical resistivity 0.00377/deg. C. between
0 and 100 deg. C. (32 and 212 deg. F.) When palladium is alloyed with other
metals, the resistivity is appreciably increased. Reflectivity in white light
62.8 per cent. This increases somewhat in passing from  blue to red. Emissivity
with a mean wave length of 0.65 mu0 0.33 in the solid state, 0.37 in the liquid
state, magnetic susceptibility at 18 deg. C. (64 deg. F.) about 5.8 X 10**-6
mass units.

The hardness of palladium in the rolled and annealed state is about 37 to 39
Vickers diamond, which is practically identical with that of platinum. As a
wire of 0.050 in. dia., and after annealing at high temperature, the metal may
indicate a tensile strength of as little as 9.5 tons/sq. in., with about 24
per cent elongation. A wire of similar type will when annealed at 800 deg. C.
(1470 deg. F.) posses a tensile strength of about 11.25 tons/sq. in. The
mechanical properties are largely governed by the type and quantity of residual
deoxidizers in the composition, but the variations are not large. For example,
the metal containing deoxidizers of this type may show a tensile strength
ranging from 11.25 to 13 ton/sq. in. as annealed, and about 21 tons/sq. in.
after cold drawing. The best annealing temperature is about 800 deg. C.
(1470 deg. F.).


When deposited by electrolysis, palladium is considerably harder than when in
the wrought state, and may be from 190 Vickers diamond for metal from the
chloride bath to about 400 for the metal deposited by complicated nitrite
baths. Whenever astronger palladium is desired, additional hardness is 
commonly obtained by an addition of the metal ruthenium. The effect of high
temperatures on these properties is as follows: commercial palladium annealed
at 1100 deg. C. (2000 deg. F.) shows a short-time tensile strength of about
12.5 tons/sq. in. at 400 deg. C. (750 deg. F.), about 4.25 tons/sq. in. at
800 deg. C. (1470 deg. F.), and about 3.4 tons/sq. in. at 1000 deg. C.
(1830 deg. F.)

Palladium is akin to platinum in general appearance, ductility and strength.
It has a silvery lustre, is extremely malleable and ductile, and is the most
readily fused of all the platinum metals. It readily distils when heated in
an electric furnace. Its principal sources are South Africa, Japan, Brazil,
Sudbury in Ontario, Canada, and the U.S.S.R. Much of it is extracted from such
ores as chalcopyrite during the production of nickel and copper. In the
electrolytic refining of these metals palladium is found in the deposits in
the electrolytic tanks.

The metal does not oxidize at temperatures up to about 400 deg. C.
(752 deg. F.), and it is not affected by a considerable number of industrial

chemicals. It is, however, not so resistant to corrosion as the metals of the
platinum group in general, and will not withstand corrosion to the degree that
might be expected. On the other hand, it is the lightest and least expensive
of the platinum metals, and ranks second only to platinum itself in industrial
value. It is stable in air at room temperature, but at a low red heat takes
on a violet hue caused by a film of oxide, which decomposes at a higher
temperature so that the metal then regains its lustre.

The main application of palladium is to contacts in electical relays, where
its freedom from tarnish makes it exceptionally trustworthy and gives a
transmission free from noise, highly desirable in voice circuits. It is
also widely used in chemical engineering as a catalyst. In a finely divided
state, dispersed on the surface of an active carrier, it is the most effective
in catalytic action of any in hydrogenating liquids and vapour phase reactions,
being particularly selective either group-wise or stage-wise. Typical processes
in which it forms an admirable catalyst are the production of ethylene from
acetylene, in which palladium on silica-gel causes the catalysis, and the
selective hydrogenation of mthyl butynol to dimethyvinycarbinol, a stage in the synthese of vitamins A and E.

Palladium is also used for removing oxygen from heat treatment atmospheres,
the recombination of hydrongen and oxygen, the hydrogenation of terpines, and

the production of pure gas by the diffusion of hydrogen through a palladium
septum or partition. So applied, however, the gas must be entirely free from
sulphur from the start.

One of the most remarkable properties of the metal is its ferocious absorption
of hydrogen, which it readily takes up, to the extent of about 800 times its
own volume at room temperature. This makes it highly valuable as a diffusion
barrier for the production of small volumes of extremely pure hydrogen. In the
same way septa or membranes of palladium are now embodied in electrolytic
cells for the separation of hydrogen isotopes by electrolytic migration.

For electrical contacts it is not no costly as platinum, nor is it so dense.
It cannot be adapted to a particularly sensitive gear, but is excellent for
light work, and consequently much used in telephone type relays, expecially in
the United States. The alloys of palladium most valuable for contacts are 10
per cent ruthenium palladium, 40 per cent silver palladium, and 40 per cent
copper palladium, the last being used for rubbing contact with nickel chromium
risistance windings in potentiometers where elimination of oxide deposit on the windings is essential, and where long service life in addition to wear
resistance is desired combined with minimum contact resistance. Silver-palladiumalloy gives extrememly low temperature coefficient with fairly high resistivity,and is therefore applied to the windings of those potentiometers requiring high 
precision.

Gold palladium alloys have a restricted melting range of temperature, and
this,
combined with their non-oxidation at tempuratures up to the melting point,
renders them highly satisfactory for temperature-limiting fuses in the
prevention of damage from overheating in electric furnaces. The alloys can be
adapted to melt at suitable intervals, i.e. 50 per cent between 1100 and
1500 deg. C. (2012 and 2732 deg. F.). The gold palladium alloy yields a high
thermal electromotive force against 10 per cent iridium platinum, the
thermocouple embodying it being completely accurate at temperatures up to 1000
deg. C. (1832 deg. F.). It is therefore much used in high frequency
milliammeters and instruments of similar character.

When ruthenium is added to palladium, an `all precious metal' white jewellery
is obtained, and this has been employed to show off diamonds advantageously.
Palladium is also extensively used in dentistry, mainly to make hiat-treatable
casting alloys with up to 30 per cent palladium, the remainder being gold,
silver, and copper. Alloys of this type melt at 1030 deg. C. (1886 deg. F.),
so that there is a limit to the palladium content that can be used in these
alloys. The lustre of palladium, together with its resistance to corrosion and
its ability to accept a high degree of polish, are responsible for its use in
the jewellery trade. Since it is not much more than half the weight of platinum,
and is much like it in appearance and durability, while is costs only about
one fifth as much piece for piece, it is a great competitor of that metal.
The most commonly used alloys of palladium for brooches, tiaras, etc., are
either a 3 percent molybdenum palladium or a 5 per cent ruthenium palladium.

Palladium is also widely used in high temperature solders because it combines
low vapour pressures, satisfactory `wetting' properties and minimum
penetration into austenitic alloys.

For soldering palladium, an oxidizing, oxyacetylene flame is best for those
platinum solders melting between 1100 and 1300 deg. C. (2012 and 2372 deg. F.).
A gas-air torch and lower melting-point white gold solders are used in
soldering palladium jewellery and dental materials. If larger amounts have to
be melted, it is best to employ an induction furnace, using an argon or lean
hydrogen nitrogen gas cover, taking care to prevent silicon contamination,
which produces brittleness at elevated temperatures. The melt is deoxidized
with 0.05 per cent aluminium or calcium boride just before it is poured.

Palladim silver alloys can be used for brazing stainless steel, Inconel and
other heat-resisting alloys. The most popular alloy has 90 per cent silver,
10 per cent palladium, and flows at 1065 deg. C. (1950 deg. F.). This is much
less likely to dissolve or penetrate the base metal than nickel-base brazing
alloys.

Palladium is obtainable in bar, cast, cold rolled, hot rolled, and drawn 
conditions. It is also produced in sheets, rods, tubes and wire, and is usually
sold either as `sponge', or as refined metal, at prices quoted in troy ounces.
It is usually 99.5 per cent pure. The sponge is termed `black'. It is also
obtained in `compact' forms, which are the most resistant to corrosion, being
attacked only by nitric and boiling sulphuric acids. First isolated in 1803, it is only a little heavier than lead. In the form of `leaf' it is sometimes
used for decoration in bookbinding, etc.