| OCR Text |
Show platinum coating activity occurs. With the naked eye, roughly circular areas that appear to be transparent are noted on the tube surface. In low magnification SEM (Figure S) these areas appear as bright spots. A high magnification SEM (Figure 6) shows that these bright areas have a much finer structure and consist of globules on top of the original structure, which appears to be slightly smoothed out. Figure 7 shows a closeup of the sharp boundary between the two areas, the reacted Fig. 6 - Fine structure of platinum silicon on top of original platinum structure Fig. 7 - Closeup of boundary line between platinum, and platinumsilicon coating area on the right showing its characteristic finer structure. 34 EDAX analysis indicates that the platinum structure at this pO,l n t c' ot n -r-tains significant silicon. The 11 t e ature (References 9 and 10) sugges s that low melting point platinum~ silicon compounds (Pt sSi 2 , Pt 3S1 , Pt 7Si , or Pt 2Si) can be readIly , formed at these temperatures in aIr; also their formation is highly favorable under slightly reduc~ng conditions. At various times 1n testing the tube, temperatures may have exceeded 11000C (2000 0F). This is above the formation temperature for any of the eutectics found on the platinum-silicon phase diagram. The above evidence suggests that silica from the silicon carbide tube wall and the platinum coating react with each other to form a low melting point platinum-silicon compound that deposits itself on top of the remaining original dispersed platinum structure. Surface tension then draws this liquid together to form the fine "globules" seen at high magnifications. It is possible that reducing conditions may be found within this surface, especially within the highly porous coating, where oxygen will be used up quickly. Reducing conditions (an atmosphere of CH 4 , CO2 and CO) could then exist at the interface between the platinum coating and the tube wall. When conditions for the formation of this eutectic are highly favorable, rapid generation apparently occurs in the form of small spheres, as shown in Figure 8. Here these spheres are Fig. 8 - Formation of spherical platinum-silicon |
