Platinum Metals Rev., 1960, 4, (4), 138
Thermal Expansion of Rhodium-Platinum Alloys
This note, communicated from the Johnson Matthey Research Laboratories, records an investigation of the thermal expansion characteristics of a number of rhodium-platinum alloys at temperatures up to 1500°C. The data obtained will be found particularly helpful to users of these alloys in the glass industry.
The glass industry, which now uses large quantities of pure platinum and of rhodium-platinum alloys, is particularly interested in the high temperature physical and mechanical properties of these materials, but detailed information as to the thermal expansion characteristics of the alloys at temperatures above 1000°C has not hitherto been available. To supplement the existing data, linear thermal expansion coefficient determinations at temperatures up to 1500°C have recently been made in these laboratories upon a number of industrially significant alloys.
Method of Measurement
Specimens were in the form of ⅛ inch diameter drawn rod, and were suspended vertically in a split tube resistance furnace. Two telemicroscopes, capable of measuring within limits of ±0.0001 inch, were used to determine the changes in length between reference marks on the specimens originally 3 inches apart. The experiments were carried out in air.
The rods used for these tests were in the hard drawn condition when inserted into the cold furnace; it was found that annealing in situ induced permanent decreases in length. Some typical effects are illustrated in Fig. 1, which relates to the 20 per cent rhodium alloy. The first heating curve was fairly smooth up to 1360°C, after which decreases in length at constant temperature were observed. After one hour at 1500°C the cooling curve was found to be appreciably lower than the first heating curve. Measurements at room temperature showed a permanent decrease in length of approximately 0.003 inch. Subsequent heating and cooling did not cause further appreciable changes in length.
All the alloys studied exhibited this shrinkage effect to some extent, and the expansion curves plotted in Fig. 2 are those obtained during cooling after approximately one hour at 1500°C. All the curves are similar in character and agree fairly well with data for rhodium published by Ebert in 1938 (1) and with data for pure platinum published by Holborn, Scheel and Henning in 1919 (2). Although the expansion coefficients appear to increase slightly with rhodium content the effect is not pronounced, and all the alloys tested expanded 16 to 18 × 10–3 inch per inch over the range o to 1500ºC. The 30 per cent rhodium alloy expands somewhat less than its composition might suggest. The proportional changes in length of the alloys tested over the temperature range o to 1500°C are summarised in the table on the preceeding page.
- 1H. Ebert Phys. Zeit, 1938, 239, 6
- 2L. Holborn,, K. Scheel and F. Henning Warmetabellen, Braunschweig, 1919, p. 54