A History of Iridium OVERCOMING THE DIFFICULTIES OF MELTING AND FABRICATION By L. B. Hunt The Johnson Matthey Group The use in unmanned space craft of iridium to encapsulate the radio- isotope thermoelectric generators, where temperatures of up to 20OO0C have to be withstood over several years of operation, with possible impact velocities of 90 metres per second, has focused greater attention on the remarkable properties of this member of the platinum group of metals. But these same properties of very high melting point and great mechanical strength have been the cause of difficulties in its melting and fabrication over a long period of years. How these problems were tackled and eventually overcome is described in this article. One of the less well-known members of the For some fifty years after the discovery of platinum group, iridium possesses quite platinum in South America and the early remarkable chemical and physical properties. It investigations of its properties by a number of is not only the most resistant of all metals to English, French, German and Swedish scien- corrosion, insoluble in all mineral acids in- tists, it was not realised that the native platinum cluding aqua regia and unattacked by other they were examining also contained other molten metals or by silicates at high tem- elements. The first to recognise that a small in- peratures, but has a very high melting point soluble residue survived the dissolution of and is the only metal to maintain good native platinum in aqua regia was the French mechanical properties in air at temperatures chemist Joseph Louis Proust, working for a above 1600OC.Its great stability can be gauged time in Madrid under the patronage of King from its physical properties, outlined in the Carlos IV, but he failed to grasp that other Table. Its high modulus of elasticity and members of the platinum group were present modulus of rigidity, together with the very low and in 1799 described his black residue as figure for Poisson’s Ratio (the relationship of “Nothing less than graphite or plumbago” (I). longitudinal to lateral strain), clearly indicate the high degree of stiffness and resistance to Discovery and Characterisation deformation that have rendered its fabrication It was this black residue that greatly into useful components a matter of great dif- intrigued Smithson Tennant shortly after his ficulty over the long period since its discovery, entry into partnership with William Hyde but despite these limitations-and despite its Wollaston in 1800. The object of this famous high cost-a number of applications have de- enterprise was the production and marketing of veloped in more recent years where mechanical platinum in quantity and it was soon decided strength is an essential factor in some of the that while Wollaston should pursue the study of extremely severe conditions nowadays en- the aqua regia solution of native platinum (so countered in modern technology. But before discovering palladium and rhodium) Tennant success could be achieved in either the melting should concentrate on the insoluble matter. By or the fabrication of iridium there lies a long the summer of 1803 he had begun his investi- struggle to be recorded. gation, fortunately mentioning to Sir Joseph Platinum Metals Rev., 1987, 31, (l), 32-41 32 Smithson Tennant, who had the advantage of Physical and Mechanical Properties a much greater amount of residue from of Iridium Wollaston’s work on platinum, realised that while the Frenchmen suspected the presence of Atomic number ... 77 one new metal in the black powder, there were Atomicweight. .... 192.22 in fact two, namely osmium and iridium, and Crystal structure ... f.c.c. Lattice constant, A ... 3.8392 on 21st June 1804 he presented his paper, a Melting point, OC ... 2443 masterpiece of clarity and conciseness, to the Specific gravity (2OOC) . 22.65 Royal Society. For this he was awarded the Latent heat of fusion, Jlg. 117 Copley Medal for that year, the highest honour Specific heat (O-lOO°C), in their gift, while a little later the French Jlg. OC ..... 0.134 Thermal conductivity workers fully accepted the priority of his (O-lOO°C),Jlcrns. OC. 1.48 discoveries (4). Vapour pressure at 15OOOC Tennant’s comment on iridium was some- torr ....... 10-8 thing of a forecast of the difficulties to come: Specific electrical resistivit at OOC, rnicrohrn. crn. 4.71 “It appeared of a white colour, and was not Temperature coefficient capable of being melted by any degree of heat I could apply”. of electrical resistivity (O-lOO°C),per OC . 4.27 x 10-3 Electrical conductivity, Oh IACS ..... 36.6 XVI. On two Mrfak,found in tbr black Powder remaining Thermal coefficient of liner after tbe Solution of Platina. By Smithson Tennant, Erq. expansion (O-lOO°C), F. R. S. per OC. ...... 6.8 x Hardness, HV (annealed), Read June si, 1804. kglrnrn’ ..... 200-240 Modulus of elasticity, E, UPONmaking some experiments, last summer, on the black MNlrn’ ..... 516 x 103 powder which remains after the solution of platina, I observed Modulus of rigidity, G, that it did not, as was generally believed, consist chiefly of MNlrn’ ..... 210 x 103 plumbago, but contained some unknown metallic ingdients. Intending to repeat my experiments with more attention during Bulk modulus, K, MN/rn2 . 371 x 103 the winter, I mentioned the result of thein to Sir JOSEPH BANKS, Poisson’s ratio. .... 0.26 together with my intention of communicating to the Royal Typical tensile strength Society, my examination of this substance, as swn as it should (annealed), 2OoC,MNlm 490-740 appear in any degree satisfactory. Two memoirs were after- wards published in France, on the same subject: one of them by M. DESCOTILS.and the other by Messrs. VAUQUELINand FOURCROY.M. DESCOTILS chiefly directs his attention to the effects produced by this substance on the solutions of platina. Banks, President of the Royal Society, his He remarks, that a small portion of it is always taken up by observation that the black powder “did not, as nitro-muriatic acid, during its action on platina : and, principally was generally believed, consist chiefly of plum- from the observations he is thence enabled to make, he infers, that it contains a new metal, which, among other properties, bag0 but contained some unknown metallic in- has that of giving a deep red dour to the precipitates of platina. gredients”. Fortunately, because work on the M. VAUQUELINattempted a more direct analysis of the sub- same problem was being undertaken almost stance, and obtained from it the same metal as that discovered by M. DESCOTILS.But neitherof these chemists have observed, simultaneously in Paris, first by the director of the Ecole des Mines, Hippolyte Victor Collet- Early in the course of their famous partnership Descotils, and a little later by Antoine Franfois designed to yield malleable platinum in commer- cial quantities, it was agreed that while Wollaston de Fourcroy and Nicolas Louis Vauquelin at should pursue the investigation of the portion of the Museum d’Histoire Naturelle (2, 3). All the native metal soluble in aqua regia, Smithson three found that the black residue contained a Tennant would study the insoluble portion. The result was the discovery of both iridium and hitherto unknown element but they neither osmium. This shows the opening page of Ten- isolated it nor gave it a name. nant’s paper given to the Royal Society in 1804 Plarinum Metals Rev., 1987, 31, (1) 33 Fourcroy and Vauquelin remarked similarly: On that historic occasion no less than thirty- “This metal has appeared to us to be of a eight distinguished scientists, including Davy, greyish white colour, difficult of fusion, of oxyda- Wollaston, Hatchett, William Allen, W. H. tion, and of solution in acid”. (5) Pepys, W. T. Brande, Henry Warburton and Some years later in Vauquelin wrote of 1814 William Babington, dined together at Ferox iridium: Hall, spent the night there and in the morning “It appears to be brittle and consequently hard. witnessed the melting of a small piece of I cannot give its specific gravity because I have not yet been able to melt it completely”. (6) iridium. This was achieved by means of “the greatest galvanic battery that has ever been con- First Attempts at Melting structed”, consisting of 20 pairs of copper and But by this time a new and powerful method zinc plates, each 6 feet long by 2 feet 8 inches, of obtaining a high temperature was becoming suspended from the ceiling and then lowered available. Following Volta’s news of his dis- into an enormous tank containing 945 gallons of covery of the electric pile in I 800, a number of dilute nitric and sulphuric acids (9). British scientists turned their attention to the Children was not yet satisfied, however, that construction of large batteries to yield a high he had really achieved the melting of a pure voltage. Foremost among them was John specimen of iridium, and early in I 8 I 5 he again George Children, who began to build large bat- set about its fusion, holding the metal in a teries in his private laboratory at Ferox Hall cavity in a piece of charcoal floating on mer- near Tonbridge in Kent in about 1806, while as cury. He had now, acting on a suggestion made is well known Humphry Davy in October 1807 by Wollaston, added a second copper plate to discovered potassium and sodium by means of each cell so that each zinc plate was now faced a discharge from a battery consisting of 250 by two of copper. This materially increased the plates of zinc and of copper each 6 inches by 4 power of the battery, and he then recorded in inches.