# Applicability of Infrared Thermography for the Detection of Phase Transitions in Metal Alloys
## 1 Introduction  
In the frame of energy transition, thermal energy storage (TES) is recognized as one of the key elements to optimize the use of available energy resources, especially renewable ones which are intermittent by nature. Latent heat energy storage provides great energy densities with the smallest temperature differences between storing and releasing heat. This technology relies on Phase Change Materials (PCMs) which can be either inorganic (e.g., metal alloys, salts, and salt hydrates) or organic compounds such as paraffin waves, fatty acids or esters, and polymeric materials (e.g., polyethylene glycol). While PCMs are already used for TES from waste heat and solar energy [1], they could also benefit from numerous applications at low to medium temperatures (\(-10\)\({}^{\circ}\)C to \(200\)\({}^{\circ}\)C) such as thermal comfort in buildings, transport, textile, cooling in electronics, waste heat recovery in industrial processes, etc. To reach this goal, it is necessary to design advanced PCMs with enhanced performance/cost ratio. To explore the phase-change and the thermal behavior of blended PCMs (organic-organic, inorganic-organic) could offer additional degrees of freedoms to tailor thermodynamic as well as kinetic properties and cost-effective performances of PCMs with respect to the applications, i.e., TES applications at low to medium temperatures requiring heating/cooling. The originality of our approach lies in the use of thermal imaging techniques for the study of phase equilibria and melting/crystallization phenomena of multi-component systems. The thermal behavior and melting temperature of PCMs can be tailored using blends instead of single-component PCMs. Finding blends without solidification interval (i.e.,with congruent melting or eutectic), with respect to the performance, is of great importance. Phase diagrams are the road map to select blends with the desired melting temperature and solidification behavior. Unfortunately, the thermal properties of multi-component PCMs and their phase diagrams are not always available. The most common methods used to establish the material phase diagrams (Differential Scanning Calorimetry (DSC), Differential Thermal Analysis (DTA), X-ray powder Diffraction (XRD) [2; 3; 4; 5; 6; 7]) are very time consuming (the samples being characterized one by one successively, the establishment of the phase diagram of a binary system can take weeks to months) and poorly adapted to screening procedures. An innovative infrared thermography (IRT) method has recently been employed for the rapid screening of organic phase change materials in order to find suitable candidates for a latent heat thermal energy storage application [8; 9; 10; 11]. It enabled the estimation of phase diagrams of fatty acid binary systems in a single short experiment. In comparison with other standard methods commonly used for the determination of phase diagrams, this IRT method allows detecting phase transitions in a large number of samples at a time (tested for up to 100 simultaneous samples so far). The method relies on the simultaneous observation using an infrared detector of a multitude of samples deposited on a thermally conductive plate, each with their own composition, submitted to a slow heating ramp. Each pixel records a temperature and emissivity-dependent signal whose behavior can be monitored. An abrupt shift in the trend of the signal of a pixel belonging to a sample can be associated to the occurrence of a phase transition. The detection of transitions can then be made for as many samples as can fit within a frame. The phase change characterization process is consequently reduced by a factor corresponding to the number of samples on the plate in comparison with traditional single-sample methods. Indeed, current standard methods are yet deemed time-consuming and poorly adapted to the fast-paced industrial environment.