Measurement of Thermal Diffusivity at High Temperature by Laser Flash Method

Abstract

Thermal diffusivity of materials is of interest in nuclear applications at temperatures in excess of 2000°C. We developed an experimental system of single laser surface heating, to modify the conventional laser flash method for measurement of thermal diffusivity. The only heating source to raise the sample to high temperatures is the front surface heating, followed by pulse heating and that all of this is done in a small container without the need for a furnace. This way of heating provides easy operation, which are important to certain applications such as radioactive hot cells, but requires extra efforts to determine the sample temperature, besides thermal diffusivity. Three samples (graphite, Inconel 600, and tungsten) were tested in vacuum over the wide temperature range from 500°C to 2100°C. The latter two samples were coated with sprayed graphite on only the front surfaces to achieve unequal surface absorption/emission characteristics, i.e. high absorptivity of the front surface against relatively low emissivity of the rear surface. Thermal diffusivities of graphite measured by our system are within 5% of commercial LFA results at temperatures below 1300°C and agree well with its trend at higher temperatures. Good agreement also exists for Inconel 600 and tungsten when the influence of the coating is neglected. Despite large uncertainty in measuring the sample temperature, the extra uncertainties of thermal diffusivity are less than 3% for all samples at elevated temperatures, owing to weak temperature dependence of thermal diffusivity. The results indicate that single laser surface heating could be very practical for the application of the LFA measurements at the cost of small extra uncertainty, unless the tested material has very strong temperature dependence of thermal diffusivity. Moreover, it is concluded that unequal surface treatment greatly improves the modified LFA measurement in a few aspects: less power consumption of the CW laser, less uncertainty of thermal diffusivity, and more uniform temperature distribution in the sample.