The excessive consumption of fossil fuels, rapid depletion of natural resources, and associated effects like climate change have compelled smart thermal management technologies. Such thermal control technologies are of fundamental importance in reducing energy consumption across different fields like buildings and automobiles. To this end, a number of functional materials have been developed to aid energy saving in their respective areas of applications. Thermochromic materials have attracted significant research attention due to their practical implications. These materials are capable of changing their thermo-optical properties based on the surrounding temperatures, and their potential practical application in thermal management has been demonstrated in the literature. Importantly, most studies have focused on metal-insulator transition (MIT) materials that can significantly change their thermo-optical properties at an arbitrary temperature. However, it has been reported that not only emittance change but also thermal conductivity and enthalpy changes during phase transition.
Previous findings revealed that the temperature of a thermal control device could be effectively controlled by altering its thermophysical properties as a function of the temperature. Notably, developing an effective thermal control device requires accurate measurement of the associated emittance and thermal conductivity changes as well as heat storage characteristics. Despite the extensive research on the thermal conductivity of various materials like vanadium oxide (VO2), reliable thermophysical properties for reliable and effective thermal design have not been identified and verified. This is considered to be of fundamental importance in the design and development of smart thermal-management technologies.
On this account, Dr. Ai Ueno, Mr. Jihoon Kim and Professor Hosei Nagano from Nagoya University studied the thermophysical properties of MIT materials during phase transition and their potential application as multifunctional thermal control devices. The MIT materials were composed of two candidate materials: Vanadium dioxide (VO2) and Perovskite-type manganese oxide (mainly La0.8Pb0.2MnO3 (LPMO) and La0.8Sr0.2MnO3 (LSMO)). VO2 was prepared via spark plasma sintering, while LPMO and LSMO were prepared via solution combustion and simple sintering techniques. The phase-transition temperature, thermal conductivities, total hemispherical emittances, specific heat capacities and heat storage capabilities were measured during the phase transition using calorimetric methods, and their effects were studied and discussed. The work is published in the International Journal of Heat and Mass Transfer.
The heat storage capabilities of these materials were successfully measured during their phase transition. The authors reported that the VO2 exhibited higher heat storage capabilities than both LPMO and LSMO. In contrast, LSMO and LPMO exhibited larger changes in total hemispherical emittances before and after the phase transition. Technically, VO2 is characterized by high transition temperatures despite its larger heat storage capacity. To decrease the transition temperature and its associated impacts, the authors prepared tungsten-doped VWO2 and evaluated its thermophysical properties. The results showed that the doping by tungsten reduced the phase transition temperature significantly and decreased the associated heat storage capacity.
In summary, the authors fabricated MIT materials and evaluated their thermophysical properties as well as their potential application in multifunctional thermal control devices. The thermal control device was proposed based on the emittance, thermal conductivity changes and thermal storage characteristics. The larger changes of the thermal conductivities for VO2 and total hemispherical emittance of both LSMO and LPMO were verified. The study also demonstrated the critical role of tungsten-doped VWO2 in reducing the transition temperature to a value way lower than that of VO2. In a statement to Advances in Engineering, first author, Dr. Ai Ueno pointed out that the study findings would contribute to developing multifunctional thermal control devices to meet the increasing demand for smart thermal control technologies.
Ueno, A., Kim, J., & Nagano, H. (2021). Thermophysical Properties of Metal-Insulator Transition Materials During Phase Transition for Thermal Control Devices. International Journal of Heat and Mass Transfer, 166, 120631.