Abstract
Abstract Thermal energy storage is defined as the temporary holding of thermal energy in the form of hot or cold substances for later utilization. It is a significant technology in systems involving renewable energies as well as other energy resources as it can make their operation more efficient, particularly by bridging the period between periods when energy is travestied and periods when it is needed. The thermal storage materials store the heat energy in different forms, such as sensible heat, latent heat and Thermo-chemical storage. In the sensible and latent heat of storage heat energy, that is mainly depending on the temperature of the materials and the accompanied phase change occurring during the charge and discharge of the heat storage. So that it is necessary to study the effect of the temperature on the physical properties for thermal storage material, to clearing and understanding this affect on the phase change or phase transformation for these material during this affect, which is mainly related to the efficiency for the charge and discharge process in these materials. The temperature dependence of the physical properties for matrix systemxi (Na0.6K0.4)NO3 storage material is deeply studied her in the case solid-solid phase transfer motion by various experimental investigations of the thermal (DSC&DTA), electrical �, �, � and Microstructure by using X-Ray and SEM &EDAX analysis. These analyses are clearing the structure and dynamics of the isostructur phase transition of the solid-solid phase for the eutectic (Na0.6K0.4)NO3 thermal storage system and it is nature behaviour for the dependence on the temperature for this transformation. According to the thermal analysis, the phase transformation during heating cycle occurs at ~ 114 ℃ , however during cooling run the transformation occurs at ~ 106 ℃ . This lead to the presence of thermal hysteresis phenomenon and the magnitude of the thermal hysteresis does not exceed 8℃. Also the electrical properties �, �, � for this matrix (Na0.6K0.4)NO3 during the heating and cooling runs supported the occurrence of this transformation accompanied by the thermal hysteresis. The hysteresis phenomenon may arise due to some internal friction (strain energy) due to the impurities and vacancies formation inside the system and that leading to expansion during the transformation process for this system. The effect of heating rates on the mechanism for the transformation process and the variation in the thermal conductivity (K) and the heat capacity (Cp) in the (Na0.6K0.4)NO3 thermal storage system is mainly related to the thermal interaction potential of ��+��� �+ ions and the formation rate of polarizability arising from the cationic atoms sphere around the (��3)− group inside the (Na0.6K0.4)NO3 system. From the microstructure analysis (X-Ray, SEM and EDAX) it's clear that , this phase transformation is isostructure (i.e.) doesn't change with the variation temperature for this matrix system and the X-Ray supported the present of thermal hysteresis through the expansion of the unit cell for this structure and that indicate, this transformation is displacive structure modified by orientation switching and finally the solid-solid phase transformation for this thermal storage materialxii (Na0.6K0.4)NO3 system is mainly related to the role of the (�+)ion in this system, which is producing a random filed effects on the lattice mode vibration,
