NORMAL SPECTRAL EMISSIVITY CHANGES IN NIOBIUM UNDER PULSE-HEATING CONDITIONS

摘要

The pulse-heating reflectometric technique developed at the IMGC has been recently improved to bring the strip specimen to its melting point in one experiment. Repeated heating pulses to the melting point modify the specimen structure, with the appearance of a molten surface layer and of large grains. At the same time the normal spectral emissivity at 900 nm of the strip specimen changes by large amounts. The experimental results obtained on niobium strips with this technique are presented and discussed. A correlation with electrical resistivity measurements performed at the same time indicates that the normal spectral emissivity changes are slow with respect to the bulk heating rate. Accurate measurements of thermophysical properties are possible only when the specimen surface has been fully stabilised.