Article 4712 of sci.physics Subject Everything you wanted to know about Palladium and were

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Article 4712 of sci.physics: Path: dasys1!cucard!rocky8!cmcl2!rutgers!apple!oliveb!amdahl!nsc!unixprt!paf From: paf@unixprt.UUCP (Paul Fronberg) Newsgroups: sci.chem,sci.physics Subject: Everything you wanted to know about Palladium and were afraid to ask Keywords: Pd Message-ID: <383@unixprt.UUCP> Date: 31 Mar 89 14:08:13 GMT Followup-To: poster Organization: uni-xperts, Inc. - Unix System and Networking Consultants Lines: 200 Xref: dasys1 sci.chem:25 sci.physics:4712 Posted: Fri Mar 31 09:08:13 1989 [ 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 associted 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 make 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-palladium alloy 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 per cent 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. When 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. [ I am a strong sceptic at the moment about the claims of cold fusion. It ] [ seems to me that there may be a possibility that the heat is being ] [ generated by the recombination of atomic hydrogen (102 Kcals/mole) ] [ which is accumulating within the Pd during electrolysis. Given the time ] [ it takes to "charge" the Pd with deuterium, if the D2 is actually being ] [ absorbed as atoms within the metal, then it is possible that all the ] [ energy is not be immediately accounted for during the charge period and ] [ this missing energy is appearing later as recombination energy as the ] [ metal becomes saturated. Very rough estimates show that it would not ] [ take much atomic deuterium recombining into molecular form to produce ] [ the energy output seen including the meltdown. Hopefully the paper will ] [ clarify some of the measurement techniques used, especially those ] [ dealing with the energy surplus claimed. ]


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