Technique of Expanding the Thermal-Indication Capabilities of Standard Thermal Paper for Studying the Field Distribution in a Microwave Chamber

This study reviews the methods of studying a microwave field power density distribution inside microwave heat treatment units and identifies shortcomings of these methods. It proposes a technique of extending the thermal-indication capabilities of standard thermal paper, according to which we provide an example of a study of temperature distribution when a three-layer thermal indicator assembly is heated in a microwave beam-type chamber of approximately 600 W. Temperature variations in the range of 25–400°C with the optical density of standard thermal paper have been experimentally studied by means of reflectance densitometry. Based on the analysis of the temperature dependence on the change in optical reflection density, six regions of the standard thermal paper conversion were identified, namely temperature ranges of 25–70°C, 70–100°C, 100–150°C, 150–210°C, 210–290°C, 290–400°C and optical densities of 0.06–0.07, 0.08–0.90, 0.91–0.99, 0.71–0.91, 0.21–0.70, and 0.20–0.38 B, respectively. In region 1, the optical density of the thermal paper does not change relative to the initial surface, and in region 2, an initial increase in the optical density is observed. In regions 3 and 4, the maximum optical density is achieved due to darkening of a leuco dye, while in region 3, there is a smooth surface of the thermal paper; in region 4, there is a velvety surface of the thermal paper due to formation of microcrystals and clots of the thermosensitive layer material. In region 5, a sharp decrease in optical density is observed as a result of discoloration of a leuco dye, and in region 6, a secondary increase in optical density is observed due to carbonization of the paper backing. The results obtained can be used to design microwave equipment as well as optimize microwave treatment conditions for materials and articles for the food, chemical, electronic, and other industries. The developed technique is also relevant for local measurements of inhomogeneity in the distribution of temperature fields when other thermometry methods cannot be used.