Anomalous Surface Heating Rates

Metals, semiconductors, and insulators, when exposed to the high irradiance produced by an arc image furnace, appear hot much sooner than predicted by heat theory. For an irradiance of 1500 W/cm2, the surface of the samples tested reached a temperature of 2000°K or more in a few milliseconds whereas the mathematical theory of heat conduction suggests that several seconds should be required. The existence of anomalously fast heating is confirmed by a variety of experiments. The observations are consistent with the results expected for a surface region of very low thermal conductivity. Tests using a high-brightness flash tube (to obtain higher irradiances) show a ``saturation'' of reflectivity for smoked MgO, tantalum, and water-cooled platinum samples. The ``saturation'' can be qualitatively explained on the basis of thermal equilibrium of the surface region with the irradiance.