Carbon in Platinum and Palladium SOLUBILITY DETERMINATIONS AND DIFFUSION AT HIGH TEMPERATURES BY G. L. Selman, B.SC., P. J. Ellison, and A. s. Darling, Ph.D., M.1.Mech.E. Research Laboratories, Johnson Matthey & Co Limited These workers did not examine the micro- Carbon difluses rapidly through heated structure of their test specimens and assumed membranes of platinum and palladium that all the carbon found was in the dis- even though its solubility in the former solved form. The use of silica heat-treatment metal is less than 0.02 per cent by capsules also introduced a considerable factor weight at 1700°C. Palladium takes up of uncertainty into their determinations. more than 0.4 per cent by weight of The carbon-platinum and carbon-pal- carbon at 1400°C and this interstitial ladium alloys used in the present investigation solubility hardens it considerably and were carefully studied by metallographic and distends the lattice. The efects of X-ray diffraction techniques, and all treat- carbon on the microstructure and ments were carried out in platinum capsules! mechanical properties of platinum and to avoid any possible siliceous contamination. palladium are described in this article, The platinum sponge and sheet used during which dispels some illusions and surveys this investigation for the manufacture of the practical consequences of the con- alloys by melting or solid state diffusion stitutional relationships observed. contained the following impurities : Pd 0.003 per cent, Au 0.0001 per cent, Ca 0.0001 per cent, Fe 0.0005 per cent, Si <O.OOOI per cent, Ag <O.OOOI per cent. Carbon depresses the melting points of all A typical analysis obtained from the pal- the platinum metals, and the eutectic tem- ladium sponge and sheet was as follows: peratures and compositions have been deter- Pt 0.01 per cent, Rh 0.001 per cent, Au 0.0002 mined by several investigators (I, 2, 3, 4). per cent, A1 0.001 per cent, Ca 0.001per cent, Although large quantities of carbon are Cu 0.001 per cent, Fe 0.002 per cent, Pb 0.0003 per cent, Ni 0.001 per cent, Si 0.0003 per cent, dissolved in the molten state most of this is Ag 0.001per cent. thrown out on solidification. Raub (3) con- The graphite crucibles and carbon powder sidered that the eutectiferous structures of employed contained not more than 5 parts platinum- and palladium-carbon alloys were per million of the spectrographically detect- simple mechanical mixtures of the pure metals able elements. Carbon analyses on the binary and carbon, and the solubility of carbon in alloys were carried out on 50 mg samples by a solid platinum and palladium was reported as combustion-conductiometric method to an "vanishingly small". Very different results estimated accuracy at the 0.1 per cent level of have recently been reported by Siller, Oates 55per cent. and McLellan (9, who concluded that both platinum and palladium take up substantial Cast Duplex Alloys quantities of carbon after heating in contact As a first approach to this investigation with graphite powder for a few hours, at 100 g batches of platinum or palladium temperatures as low as ~aoo"C. sponge were melted in a high frequency Platinum Metals Rev., 1970, 14, (l), 14-20 14 Fig. 1 Duplex carbon-platinum alloy Fig. 2 Duplex carbon-palladium alloy produced by melting pure platinum in a produced by melting palladium on graphite. graphite crucible. As cast. x 75 As cast. ~75 furnace in graphite crucibles. The charge The duplex platinum-carbon alloys pre- was held molten for five minutes, following pared by this method were not readily which the crucible, still containing the plati- amenable to cold work and cracked quite num metal, was quenched into water as severely under the forging hammer. The rapidly as possible. Small slices of the palladium alloys although considerably har- quenched ingots were then reduced in thick- der, were, by comparison, relatively ductile. ness approximately 50 per cent by cold Some typical microstructures are illustrated forging and solution treated at various in Figs I and 2. The differing geometry of temperatures for various times before being the primary graphite in the two metals quenched into water. These heat treatments provides a clear explanation for their differing were carried out under a nitrogen atmosphere response to cold work. to avoid oxidation. Lattice parameters and The lattice parameter of the platinum micro-hardness determinations were made on alloys changed little with quenching tem- carefully polished and etched microsections. perature whereas considerable expansion was Platinum Metals Rev., 1970, 14, (1) 15 Fig. 4 Grain boundary porosity in the Fig. 5 Porosity throughout the sheath of a sheath of a platinum capsule containing palladium capsuZe containing graphite, graphite, heated in air for 75 hours at heuted in air for 18 hours at 1200°C. X 20 1200°C. X75. observed when the duplex alloys of palladium and sealed up in pure platinum capsules were quenched from high temperature. under a pressure less than IO-~Torr. These These results, plotted in Fig. 3, indicate that capsules consisted of solid drawn platinum palladium dissolves measurable quantities tubing having an internal diameter of 0.22 of carbon, and platinum very little. inch and a wall thickness of 0.020 inch. The slight changes in lattice parameter For the purposes of the equilibrium observed when carbon was equilibriated studies, these capsules were normally heat- with platinum in this way could well be treated under an atmosphere of nitrogen. caused by the introduction of small quantities Some of the initial specimens were annealed of dissolved impurity - usually of the order in air and in these cases some unusual effects of 30 p.p.m. - during melting and casting. were observed. After IOO hours at 1400°C considerable surface distortion occurred and Diffusion Experiments close examination revealed areas having a To provide specimens containing smaller surface structure quite unlike that normally quantities of carbon than those which were displayed by air annealed platinum. Similar readily obtained by melting and casting, effects were produced by burning carbon on sheets of platinum and palladium 0.020 inch the surfaces of a platinum sheet in air at thick were packed in high purity graphite 14oocCand it was concluded that the surface Fig. 6 Layer of graphite on the outer Fig. 7 Graphite spheroids precipitated surface of a grphite-pocked palladium cap- towards the inner surface of the palladium sule, heated in vacuo for 100 hours at 1200°C sheath illustrated in Fig. 6. x 50 and slowly cooled. x 200 Platinum Metals Rev., 1970, 14, (1) 16 behaviour of the platinum capsules was caused by the combustion of carbon which must have diffused rapidly through the platinum mem- brane. Micro-sections taken through these plati- num capsules showed considerable grain boundary attack as shown in Fig. 4. Here the grains have opened up more than half way through the section of the tube. No attack of this sort was observed when empty platinum capsules were annealed in air, and the experimental observations are consistent with Fig. 8 Porosity in carbon saturated palladium the view that carbon, although sparingly sheet quenchedfrorn 1400°C. x 75 soluble in platinum, diffuses through it DUCUO. Fig. 7 shows that during the furnace very rapidly at high temperatures when cooling period following the annealing treat- surface combustion produces the concentra- ment graphite spherulites had precipitated tion gradients required. This localised out only within the inner half of the sheath intercrystalline attack was characteristic of the section. It seems likely therefore, that the internal oxidation behaviour of platinum. outer layer of graphite which formed initially Palladium capsules, when subjected to the as a consequence of surface volatilisation same treatment showed uniformly distributed acted as a nucleation site for graphite attempt- fine porosity throughout their cross-sections, ing to separate from solid solution within as indicated in Fig. 5. the outer layers of the capsule during the slow Both effects indicate a high rate of carbon cool. These carbon surface deposits were diffusion. Oxygen is of course fairly soluble not obscrvcd on palladium capsules annealed in the palladium lattice and this could account for the uniform porosity observed with the palladium capsules. The lower solubility of oxygen in pure platinum probably confines the effect of the carbonjoxygen reaction to the grain boun- daries. In order to avoid the complicating effects of palladium oxidation some experiments were made on palladium tube capsules which, after packing with carbon, evacuating and sealing, were finally annealed in vucm at 1400°C. These sealed capsules rapidly developed a black surface coating which was identified as pure carbon. The sections through such a palladium capsule shown in Figs 6 and 7 illustrate the thickness of this carbon sheath and also the spherulites of graphite precipitated out within the tube wall. The volume of carbon present on the surface of the capsules was too great to be accounted for entirely in terms of the vola- tilisation of carbon saturated palladium in Platinum Metals Rev., 1970, 14, (1) 17 in argon and rapidly quenched from the geneity of the analytical specimens a series annealing temperature. of hardness impressions were made across At this stage attempts were made to deter- sections through the sheets which had been mine the rate of diffusion of carbon through saturated with carbon by diffusion. platinum and palladium by heating sealed Palladium and platinum specimens satu- carbon containing capsules in air and measur- rated with carbon and quenched from 140oCC ing the change in weight. By carrying out were porous as shown in Fig. 8. No porosity blank runs on empty capsules the losses was detected in platinum or palladium caused by carbon combustion could, it was quenched from IZOOOCor below.