**3.1 Melting temperature: Tm**

It is the first criterion for selecting a product, as it must be appropriate for the application. Its determination by differential calorimetric analysis is relatively easy. However, depending on the origin or purity (and the nature of the impurities) of the materials, it can vary by a few degrees. There are two types of fusion, one ideal called "congruent," the other called "incongruent." The ideal fusion is isothermal fusion, where when the material is at the melting temperature, the liquid and solid phases present are in equilibrium and have the same composition. In this case, the change of state occurs reversibly. Thus all the energy stored during melting is fully restored during crystallization, promoting the material's resistance during the cycles. This type of fusion is generally found in pure substances. The melting can also be incongruous. In this case, two phases are formed beforehand, one liquid and the other solid, and then a fully liquid phase is obtained. Generally, the material decomposes at a temperature below its melting temperature into liquid and crystals. A two-phase mixture with a solid compound of a different composition from the defined compound is obtained in the same way during the cooling. During the cycles, the initial material gradually changes, all the more quickly as variations in density can lead to phase segregation, which accentuates the degradation phenomenon.

#### **3.2 The crystallization temperature/supercooling problem**

The crystallization temperature does not necessarily coincide with that of the liquid to solid transition. There is sometimes a slight delay related to the crystallization kinetics which is dependent both on the crystal growth of the material and on nucleation phenomena. This phenomenon also called supercooling, corresponds

**71**

surface.

*Phase Change Materials for Textile Application DOI: http://dx.doi.org/10.5772/intechopen.85028*

for a certain period of times.

**3.3 The enthalpy of phase change**

few are those to exceed 350 MJ m<sup>−</sup><sup>3</sup>

Thermal conductivity, λ (W m<sup>−</sup><sup>1</sup>

material-envelope compatibility.

**3.4 Thermal conductivity**

melting enthalpy, ∆Hm, higher than 180 MJ m<sup>−</sup><sup>3</sup>

to the fact that a body temporarily remains in a liquid state at a temperature below its crystallization point. This is a relatively common phenomenon since even water has a supercooling of a few degrees varying with its impurity level. From a microscopic point of view, supercooling is related to nucleation and the rate of growth of germs. The probability of germ formation is related to the creation of a solid/liquid interface requiring a certain amount of energy. The germ thus created must have a radius more significant than a critical radius, rc, in order not to be dissolved in the medium. The rate of growth of germs corresponds to the change in the size of the crystal over time. This velocity is zero at the melting point and increases rapidly as the temperature drops to a maximum value. Beyond this value, crystallization will depend mainly on the mobility of the molecules, linked to the viscosity of the medium. Thus, the growth rate decreases with increasing supercooling. Adding a nucleating agent, such as a saline hydrate, stable in the exploited thermal domain and chemically inert towards the storage medium, can eliminate this delay in crystallization. Another solution is to play on the roughness of the container walls to promote nucleation. Supercooling is also related to the mass of the product used; indeed, it decreases considerably with the use of large amounts. Thus, the results obtained by DSC, with masses in the order of mg, must be used with caution. In some cases, supercooling can be considered an asset, since it delays crystallization

The enthalpy of fusion corresponds to the energy absorbed by the material during the solid/liquid transition. The liquid/solid transition is achieved by fully restoring this energy. Except in the case of significant supercooling, this is the enthalpy of crystallization. These enthalpies can be determined experimentally by DSC analysis, and like temperatures, differ by a few J g–1 depending on the purity and origin of the products. The literature considers that a suitable storage material must have a

transition, and for most materials, the volume expansion ∆V/V at fusion is positive except for ice and gallium and its alloys. Depending on the nature of the materials, it can vary from a few percent to 50%, and in particular from 5 to 15% for paraffin.

K<sup>−</sup><sup>1</sup>

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

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

. Even if some reach 580 MJ m<sup>−</sup><sup>3</sup>

. A change in volume accompanies a first-order

), is the amount of heat transferred in a

,

### *Phase Change Materials for Textile Application DOI: http://dx.doi.org/10.5772/intechopen.85028*

*Textile Industry and Environment*

*Q* = *m*.∫

*Tinitial Ttr*

The absence of any liquid helps the use of the materials.

**3. Properties of phase change materials**

tioning of the application in question [19, 20].

**3.1 Melting temperature: Tm**

storing by sensible heat. It should be noted that in practice, not only the latent heat of state change but also the sensible heat of the liquid and the corresponding solid are exploited, which significantly increases the stored energy (Q ) (Eq. (3)) [16].

*Cp*(*T*). *dT* + *m*.Δ*H* + *m*.∫

The phase changes required for this type of storage are first-order transitions. However, only solid-solid and solid-liquid transitions are used [17]. Indeed, the liquid-gas transition is technologically unusable since it leads to too high a variation in volume, and the liquid-liquid transition is vigorously too weak to generate any interest. The use of the solid-solid transition is interesting only in the case where the transition is relatively energetic, that is, for plastic crystal-crystal transitions [18].

The use of latent heat from a phase change material is not in itself a new technique. At the end of the nineteenth century, to overcome the inconvenience of changing hot water bottles too often to heat railway cars, water was replaced by sodium acetate. This salt can store a large amount of heat which it releases entirely after a few hours. While a few patents were filed before 1973, it was not until the first oil shock that many specialized laboratories began research in this field. Taking into account the thermodynamic can only make the choice of good storage material, kinetic, chemical and economic criteria considered essential for the proper func-

It is the first criterion for selecting a product, as it must be appropriate for the application. Its determination by differential calorimetric analysis is relatively easy. However, depending on the origin or purity (and the nature of the impurities) of the materials, it can vary by a few degrees. There are two types of fusion, one ideal called "congruent," the other called "incongruent." The ideal fusion is isothermal fusion, where when the material is at the melting temperature, the liquid and solid phases present are in equilibrium and have the same composition. In this case, the change of state occurs reversibly. Thus all the energy stored during melting is fully restored during crystallization, promoting the material's resistance during the cycles. This type of fusion is generally found in pure substances. The melting can also be incongruous. In this case, two phases are formed beforehand, one liquid and the other solid, and then a fully liquid phase is obtained. Generally, the material decomposes at a temperature below its melting temperature into liquid and crystals. A two-phase mixture with a solid compound of a different composition from the defined compound is obtained in the same way during the cooling. During the cycles, the initial material gradually changes, all the more quickly as variations in density can lead to

phase segregation, which accentuates the degradation phenomenon.

The crystallization temperature does not necessarily coincide with that of the liquid to solid transition. There is sometimes a slight delay related to the crystallization kinetics which is dependent both on the crystal growth of the material and on nucleation phenomena. This phenomenon also called supercooling, corresponds

**3.2 The crystallization temperature/supercooling problem**

*Ttr Tend*

*Cp*(*T*). *dT* (3)

**70**

to the fact that a body temporarily remains in a liquid state at a temperature below its crystallization point. This is a relatively common phenomenon since even water has a supercooling of a few degrees varying with its impurity level. From a microscopic point of view, supercooling is related to nucleation and the rate of growth of germs. The probability of germ formation is related to the creation of a solid/liquid interface requiring a certain amount of energy. The germ thus created must have a radius more significant than a critical radius, rc, in order not to be dissolved in the medium. The rate of growth of germs corresponds to the change in the size of the crystal over time. This velocity is zero at the melting point and increases rapidly as the temperature drops to a maximum value. Beyond this value, crystallization will depend mainly on the mobility of the molecules, linked to the viscosity of the medium. Thus, the growth rate decreases with increasing supercooling. Adding a nucleating agent, such as a saline hydrate, stable in the exploited thermal domain and chemically inert towards the storage medium, can eliminate this delay in crystallization. Another solution is to play on the roughness of the container walls to promote nucleation. Supercooling is also related to the mass of the product used; indeed, it decreases considerably with the use of large amounts. Thus, the results obtained by DSC, with masses in the order of mg, must be used with caution. In some cases, supercooling can be considered an asset, since it delays crystallization for a certain period of times.
