Measurement of the thermal diffusivity of optical materials and products by a new thermographic express method that does not require cutting samples from bulk

Abstract

Thermal diffusivity a and thermal conductivity λ are important for many building, structural and functional material applications. They determine the intensity of heat transfer, the quality of thermal insulation, the rate of heating / cooling, reaching a stationary mode, and the efficiency of power equipment. In laser technology, the radiation strength of the optical components of the system depends upon them, and in laser technologies with material removal they determine the speed and quality of processing. Most methods for measuring a and λ in solid materials require cutting out samples of a certain geometry, which makes them unsuitable for testing finished products. The paper proposes and describes an express method for determining a and λ in translucent materials, which does not require cutting a sample from a controlled object. It consists in the analysis of a non-stationary temperature field on the surface of the test object using a high-speed thermal imaging camera. The unsteady heating spot was created by a focused laser beam. It was switched on abruptly and operated in the mode of continuous irradiation with a constant intensity during the entire time of measurements. Heat propagated from this spot to the periphery, creating a non-stationary temperature field containing information about a and λ. The a value was extracted from the primary data using original algorithms and software. A thermal imager, as a recorder of a dynamic temperature field, provides a number of advantages – non-contact, high speed and a large amount of information (each of the many hundreds of thousands of pixels of a professional thermal imager matrix is a temperature sensor in a small surface area). Measurements of a and λ in semitransparent materials of laser optics have their own specifics. The low radiation absorption coefficient and the possible curvature of the surface (for example, in lenses) require special measures, which are described in the article. Due to the large amount of information contained in the dynamic patterns of the thermal field and the possibility of averaging over a large data array, the RMS of the thermal diffusivity measurement does not exceed 2 %.