Phase change materials physical properties

Abstract. This section is an introduction into materials that can be used as Phase Change Materials (PCM) for heat and cold storage and their basic properties. At the beginning, the basic thermodynamics of the use of PCM and general physical and technical requirements on perspective materials are presented. Following that, the most important classes of materials that have been investigated and typical examples of materials to be used as PCM are discussed. These materials usually do not fulfill all requirements. Therefore, solution strategies and ways to improve certain material properties have been developed. The section closes with an up to date market review of commercial PCM, PCM composites and encapsulation methods. 

Since volume phase transition of a gel is thought to bring about dramatic changes in physical properties, this phenomenon is expected to be applied to the creation of new types of materials with switching ability. One application is the synthesis of switching-functional membranes. 

Organic molecules have useful optical and electronic functions that can be easily controlled by the structure, substituent, or external fields. Molecular interactions and organized molecular assemblies also can afford much higher functions than isolated or randomly distributed molecules. Photons have many superior properties such as wavelength, polarization, phase, ultrashort pulse, or parallel processability. Through interactions of molecules or molecular assemblies with photons, many properties of photons can be directly converted to changes in physical properties of materials such as fluorescence, absorption,... 

Choi K and Cho G (2011), Physical and mechanical properties of thermostatic fabrics treated with nanoencapsulated phase change materials, Journal of Applied Polymer Science, 121(6), pp. 3238-3245. 

Ferroelectric materials are capable of being polarized in the presence of an electric field. They may exhibit considerable anomalies in one or more of their physical properties, including piezoelectric and pyroelectric coefficients, dielectric constant, and optoelectronic constant. In the latter case, the transmission of light through the material is affected by the electric field, which produces changes in refractive index and optical absorption coefficient. Varying the applied field changes the phase modulation. 

A solid solution is a crystal structure in which two (or more) atom types are arranged at random over the sites normally occupied by one atom type alone. For example, in the comndum structure solid solution formed by Cr2C>3 and AI2O3, a random mixture of Cr3+ and Al3+ ions occupy the cation sites that are only occupied by one of these in the parent phases. The formula of the solid solution materials is written (Al i JCCrJC)203. In this example, x can vary continuously between 0 and 1.0. In some cases, especially when the atoms involved have different sizes, only partial solid solutions are found, characterized by a composition range in which the span of x is smaller than 1.0. Solid solutions are widely exploited as both the chemical and physical properties of the solid can be varied sensitively by changing the relative amounts of the components of the solid solution. 

Under normal conditions, matter can appear in three forms of aggregation solid, liquid, and gas. These forms or physical states are consequences of various interactions between the atomic or molecular species. The interactions are governed by internal chemical properties (various types of bonding) and external physical properties (temperature and pressure). Most small molecules can be transformed between these states (e.g., H2O into ice, water, and steam) by a moderate change of temperature and/or pressure. Between these physical states— or phases—there is a sharp boundary phase boundary), which makes it possible to separate the phases—for example, ice may be removed from water by filtration. The most fundamental of chemical properties is the ability to undergo such phase transformations, the use of which allows the simplest method for isolation of pure compounds from natural materials. 

The effect of slow accumulation of surface-active materials is indicated in Fig. 18, which is a series of photographs of drops suspended in a tapered tube (H9). Tiny amounts of fine solids of colloidal dimensions, as described by Elzinga and Banchero (El), gradually collected at the interface and were swept around to the rear of the drop. Circulation was progressively hindered until it was nearly stopped. Yet no measurable change could be detected in any physical property, including interfacial tension of the separated phases. 

---