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Show being "formed" from the parent platinum-silicon structure as this structure (porous non-uniform globules) outgasses an excess of a certain component (note the pores) and a eutectic composition is reached. Being liquid at the use temperature, these spheres migrate readily and join to form larger spheres (Figure 9), some readily visible to Fig. 9 - Large sphere of platinumsilicon the naked eye. The formation of these spheres, while retaining the platinum within the system (as opposed to the evaporation of Pt02 ), significantly decreases the surface area of the platinum available for catalysis, and would probably eliminate the ability of the tube to reach a radiant condition over time. Finally, at the tube exit, the deposition of a visually bright and well-dispersed coating occurs. This coating appears to be platinum-like in appearance, but during operation in the test facility this end of the tube possessed reduced catalytic activity. SEM reveals that this surface has a porous, blotchy appearance of little uniformity. EDAX analysis showed that the surface contains platinum and a small amount of silicon, much less than seen in the formation of the low melting point eutectic at the intermediate area. In viewing the SEM it appears that this surface was deposited onto the end of the tube during operation. Possibly evaporated Pt02 condensed back onto the tube surface at this point. While Pt02 is catalytic, its activity is less than that of 35 platinum itself, which would account for the reduced radiance observed at this end of the tube. A better accounting of the generation of this surface would be obtainable through analysis using Auger spectroscopy, which can be used to identify specific oxides within a surface. This technique will be utilized during future work. CONCLUSIONS AND RECOMMENDATIONS During the radiant operation of a platinum catalyzed silicon carbide tube, a variety of combustion conditions may be present inside the tube. These conditions range from high temperature oxidizing to high temperature reducing conditions, with the former being the most likely initial condition. The test data suggested the existence of a relationship between catalytic combustion stability and combustion conditions (i.e., firing rate and stoichi-ometry). This relationship is illustrated in Figure 10, which is a PCtO ATING+ 1 TEMP. --_ .. REACTANT FLOW RATE Pt LOSS THRESHOLD Fig. 10 - Dependence of catalyst stability on operating conditions qualitative representation of platinum temperature versus flow conditions for an oxidizing atmosphere. This figure shows that, at a given stoichiometry, the reactant flow rate might control the stability of the Pt coating. For a given platinum catalyst, a threshold temperature exists above which the catalyst will oxidize and subsequently be lost from the substrate surface through evaporation or sublimation. This threshold temperature is eventually attained as the reactant flow rate is increased from an initial low |
