Measurements of thermophysical materials properties at very high temperatures need to overcome a number of challenges related to the chemical and physical stability of the sample. Several of these challenges are addressed by containerless techniques, the use of suitable buffer gases, high ambient pressures, and pulse-heating methods that shorten the dwell time at the highest temperatures. Still, the significant challenge of an accurate temperature measurement remains. Radiation thermometry in its various forms, an inherently high-speed and non-contact technique, lends itself naturally to such measurements. However, it is only able to provide a measure of the so-called radiance temperature, a sort of apparent temperature, which requires correction to thermodynamic temperature by means of an independent (prior or in-situ) measurement of the appropriate type of emittance or the use of emittance-enhancing techniques.

At JRC Karlsruhe we employ a laser-based technique for the study of high-temperature phase diagrams, in particular melting and solidification. Materials of interest include conventional and advanced nuclear fuels and claddings, ultra-high-temperature ceramics, and targets for the production of medical radioisotopes for cancer theragnostic applications. High-power Nd:YAG laser beams heat the disk-shaped sample from either one or both sides. Complex power/time profiles are programmed to adapt the heating and cooling rate to the needs of each experiment. Temperatures exceeding 4000 K can be reached in less than a second. Experiments are performed in an autoclave under medium to high pressure of a buffer gas (up to hundreds of MPa), in particular when the goal is to decrease the evaporation rate from highly volatile materials and to study the pressure dependence of melting. Zirconia-based oxygen pump/gauge systems measure and control the oxygen content of the buffer gas. When required, radioactive samples are handled inside a glovebox, which provides α-shielding and prevents contamination. The surface (radiance) temperature of the sample is measured by means of fast radiation thermometers and infrared cameras. The near-normal spectral emittance of the sample is estimated based on radiance spectrometry. Detection of melting and solidification, as well as solid-solid phase transitions, is supported by the use of a low-power laser that is reflected off the sample and is sensitive to changes of its effective reflectance of the sample (‘Oscillating Directional Reflectance’ technique).

In this presentation, the measurement setup, its strengths and weaknesses will be discussed in detail and examples of measurements on nuclear and non-nuclear materials will be shown.