Platinum Metals Review - Volume 50, Issue 4, 2006

In order to provide reliable data on the high-temperature deformation behaviour of iridium, the high-temperature material properties such as stress-rupture strength, high-temperature tensile strength and creep behaviour are determined for pure iridium in the temperature range 1650–2300°C. Analyses of the stress-rupture curves and the creep behaviour of pure iridium samples at 1650°C, 1800°C and 2000°C imply that the fracture behaviour is controlled by two different fracture mechanisms depending on test conditions, in particular applied load and test temperature. The existence of the different fracture modes is confirmed by SEM examination of the fracture surface of samples ruptured at high temperatures. Anomalies in the creep curves and the results of high-temperature tensile tests indicate that dynamic recrystallisation plays an important role in the high-temperature deformation behaviour of pure iridium.

Work to extend the catalytic chemistry of rhodium and iridium, with particular emphasis on the great versatility of the former, is outlined and summarised. Topics addressed include the design of chelating N-heterocyclic carbene ligands, and the cyclisation of alkynes using rhodium and iridium phosphine compounds as reagents or catalysts. The work was carried out by Ph.D. students sponsored through the prize awarded by Johnson Matthey to the winner of their Rhodium Bicentenary Competition.

Under the use of super-dry dimethylformamide containing tetraalkylammonium salts (TAAX with X = Cl, Br, I, ClO, BF4, etc.) platinised platinum layers may exhibit a reversible charging process that occurs at quite negative potentials (more negative than –2.2 V vs. saturated calomel electrode (SCE)). The mode of reactivity of the electrolyte and the reversibility of the platinum charging of platinised layers (whatever the type of conducting substrate – gold and glassy carbon can also be used successfully) are discussed in terms of the nature of the salt. After cathodic charge and oxidation by air of samples removed from the cell, a huge change of morphology of the original platinised layer was observed. During repeated reduction/oxidation stages, the original amorphous platinised layer was progressively transformed, with a noticeable swelling of the original layer. This transformation, based essentially on cathodic swelling due to the peculiar reactivity of platinum in the presence of bulky tetraalkylammonium salts, is the precondition for a new kind of platinum interface.

A series of articles in this Journal by Wilkinson on platinum-based thermocouples and their use (1–4) addressed most of the potential problems and performance-limiting factors. The author referred to the possibility of deterioration of thermocouple performance through contamination. This article expands on this, demonstrating that the prevailing atmosphere in which the thermocouple is operating can have a profound effect on its life and accuracy.