The project included the search and the theoretical analysis of different types of solid-state reactions in metallic based systems and salts based systems. In the selections of the most promising materials all the essential technical, economic and environmental aspects were taken into account discarding toxic, too expensive and low available materials. For the selection, different strategies were followed including, on one side, the analysis of the databases available for thermodynamic properties, on the other side, the use of software of modelling, based on the CALPHAD method, determining the key thermodynamic properties (e.g. enthalpy of reaction, specific heats, densities, etc.) fundamental for the selection criteria.
The results of the theoretical investigation allowed achieving a list of reactions (13 possible candidates) with promising theoretical volumetric energy densities. The second step of the investigation was focused on the experimental study of the systems selected, being the most promising candidate in terms of theoretical energy density and reaction temperature, and the evaluation of their possible use as TES materials.
One of the major challenges of the project was to develop materials with suitable reactivity (yield/kinetic/reversibility). Solid-state reactions are governed by the atomic diffusion in the solid state for this reason a synthesis strategy was defined to maximize the reactivity in the solid state. This included the synthesis of materials with different microstructure (nanostructured materials) to increase the diffusion in the solid state by i) decreasing the atomic diffusion path length (small grain sizes as well as small particle sizes), ii) introducing structural defects (dislocation, grain boundaries step, kink and corner atoms etc) and iii) promoting high intermixing degree (high contact area between the reagent). The effect of the synthesis parameters on the microstructure and reactivity of the powders have been analysed and optimal synthesis conditions have been established. As a result of this study, materials with tailored microstructures were synthesized with the objective to guaranty the maximum yield of the reaction (complete conversion of the reagent into the products and reversibility).
The main results achieved during the project can be summarized as follow:
• The work carried out allowed to identify several promising solid-state reactions that could selected for experimental investigation as possible candidate for the application in thermal energy storage.
• Different solid-state reaction were/are studied experimentally and their performance as TES material evaluated.
• The link between microstructure and reactivity was proven, with the identification of best the microstructural conditions to maximize the reactivity in the solid state.
