Rhodium-platinum alloys, work

For general work at higher temperatures, several different types of couples are employed in this country. Up to 360°C. for extreme precision or to 500°C. for a precision of 5 to 10°C. the couple may consist of one wire of copper and the other wire of constantan. Iron-constantan or nichrome-constantan may be employed for technical processes below 900°C. For operation below 1,100°C. special patented alloys of chromium and nickel and of aluminum and nickel, chromel-alumek or nichrome-alumel are very satisfactory even for continuous service. For the temperature range 300 to 1,500°C. the Le Chatelier couple should be employed. This couple consists of one wire of platinum and the other wire an alloy containing 90 per cent platinum and 10 per cent rhodium. Other alloys and metals may be employed for special work but the above combinations are sufficient for almost all technical processes carried on in the temperature range 0 to 1,500°C. No satisfactory couple has been developed for operation much above 1,500°C. 

A platinum-10% rhodium to platinum thermocouple was operated with hot and cold junctions, respectively, in the alloy and condensed cadmium regions. Reference tables (16) were used to convert e.m.f. to absolute temperature. However, in the tables, temperatures are rounded to the nearest 0.1° C., which is too rough for our temperature differences. The tables are based on earlier work (13) with the values adjusted to the 1948 international temperature scale. To evaluate the difference between alloy and cadmium condensate temperatures, we have used without adjustment two equations from the earlier work... 

F6r crucibles an alloy of platinum and rhodium, with 3 to 5 per cent of the latter, is recommended for high temperature work. Iridium stiffens platinum but increases its volatility above 900° C whereas rhodium not only stiffens the platinum but also reduces its volatility. Iridio-platinum is used successfully in sparking-plug electrodes of aero-engines, best all-round results being obtained with 20 per cent iridium. 

The form of SThM most relevant to the subject of this discussion is carried out using near-field electrical resistance thermometry, and this method has been adopted in the work reported in this chapter. This is because miniaturized resistive probes have the considerable advantage that they can be used both in passive mode as a thermometer and as an active heat source. This enables local thermal analysis (L-TA see text below) as well as SThM to be carried out. At present the most common type of resistive probe available is the Wollaston or Wollaston Wire probe, developed by Dinwiddle et al. (1994) and first used by Balk et al. (1995) and Hammiche et al. (19%a) The construction details of this probe are illustrated in Fig. 7.3. A loop of 75-pm-diameter coaxial bimetallic Wollaston wire is bent into a sharp V-shaped loop. The wire consists of a central 5-pm-diameter platinum/10% rhodium alloy core surrounded by silver. The loop is stabilized with a small bead of epoxy resin deposited approximately 500 pm from its apex. The probe tip or sensor is made... 

The binary alloy of platinum and rhodium is especially known for the catalytic properties of its surfaces [123-126]. Therefore, various publications have already attended to this system for example. Refs [125-130] discuss the relevancy of Pt-Rh in three-way catalysts, and some empirical models about the catalytic process are developed in Refs [131-133]. Here, we take two steps back first, because the whole alloy Pt-Rh will be brought down to its atomic constituents, and second, because not the dynamics of the catalytic process itself wiU be under investigation but, more fundamentally, the equilibrium properties of a Pt-Rh surface. This approach joins other atomistic works on Pt-Rh surfaces, experimental papers such as Refs [1, 134], as well as numerous theoretical surveys such as Refs [2,106, 135-137]. 

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