**3.4 Thermal conductivity**

Thermal conductivity, λ (W m<sup>−</sup><sup>1</sup> K<sup>−</sup><sup>1</sup> ), is the amount of heat transferred in a unit of time through a material of a surface unit and a unit of thickness when the two opposite sides differ from a unit of temperature. For most materials, thermal conductivity decreases slightly as the temperature rises. Thus, in the liquid state, it is weaker than in the solid state. A high thermal conductivity minimizes temperature differences in the material during melting and crystallization, facilitating heat transfer. Different methods are possible to increase this conductivity, either by inserting fibers or metal matrices, graphite or urea or by increasing the exchange surface.

Thus, the choice of a suitable storage material can only be made by taking into account some of its intrinsic characteristics (melting and crystallization temperatures, enthalpy of phase change, volume expansion and thermal conductivity), but also by knowing its melting behavior and its ability to withstand the thermal cycle, not to mention economic constraints. The last criteria to be taken into account are chemical stability and its non-corrosive aspect to avoid storage problems and material-envelope compatibility.
