Platinum Metals Review - Volume 31, Issue 1, 1987

Prostheses are devices for restoring artificially some function lost through accident or disease. Neurological prostheses, therefore, are surgically implanted microelectronic devices which seek to ameliorate the results of neurological defect. Examples are: implants for treating incontinence, or for recovering some use of paralysed arms and legs following spinal accident; implants for patients who are blind as a result of damage to the eye or optic nerve; implants for patients who are deaf as a result of damage to the inner ear. In addition, there are some implanted devices which are not strictly prostheses; for example, implants for relieving pain, and implants for correcting curvature of the spine in children. Despite the variety of purposes for which neurological prostheses are built, they have in common that they are all nerve stimulators and can all be realised using, substantially, the same technology. This article discusses the technology for implant-making which has been worked out at this Unit, and shows the essential role played by noble metals in that technology.

Zirconia grain stabilised platinum alloys developed by Johnson Matthey are widely used to contain molten glass. The high temperature strength of these alloys is well in excess of that of similar conventional rhodium-platinum alloys, and zirconia grain stabilised ten per cent rhodium-platinum, the strongest alloy in the range, is used in the most demanding applications where its excellent creep resistance offers prolonged component life. However the present high cost of rhodium, compared with that of platinum, has generated increased interest in alloys with a lower rhodium content suitable for certain applications where the outstanding properties of the ten per cent alloy are not fully utilised. To meet this demand Johnson Matthey have now developed zirconia grain stabilised five per cent rhodium-platinum. This alloy offers a useful range of physical properties for applications where economic considerations are a major factor.

Palladium-based contact materials are replacing gold finishes on the contacts of many separable connectors used in telecommunications systems in the United States of America due to their lower cost. Before this was possible, however, methods had to be developed for: inhibiting the corrosive attack of palladium by certain chlorine-containing compounds in the atmosphere, retarding the formation of insulating frictional polymers on palladium, and improving its wear resistance. Also, improved manufacturing techniques for palladium contact materials were necessary. An alloy, clad 60 per cent palladium-40 per cent silver having a small amount of gold diffused into its near surface region, and a new palladium electroplating process which is used with a thin gold overplate, were developed. The technology behind these developments is described, and an example of the application of the new materials in a major connection system is given.

The complex bis(1,2-cyclohexanedionedioximato)palladium(II) has been prepared and its absorption spectra examined. The preliminary results, which are reported here, show that the colours of thin films of this complex are a function of the applied pressure, a property which may find application as an indicator of pressure and pressure distributions up to perhaps seventy thousand atmospheres.

The addition of platinum to nickel-based alloys can have a profound effect on their oxidation and hot corrosion resistance. Improvements in gas turbine blade performance in aggressive environments are linked with the protective nature of surface oxides and coatings. However the precise role of platinum in promoting and maintaining this protection is still under active investigation and the results of several recent studies are reviewed here.

The use in unmanned space craft of iridium to encapsulate the radioisotope thermoelectric generators, where temperatures of up to 2000°C 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.