Due to the ever-increasing demand for miniaturization and the remarkable advancements in electronic equipment, overheating, decreased performance, efficiency and lifetime have become major issues. Thermal management is crucial in systems because high operating temperatures are the leading cause of electronic component failures. A growing number of electronic boards are being cooled using phase change materials (PCMs) due to their capacity for latent thermal energy storage.

In theory, PCMs can store and release a lot of thermal energy at almost isothermal temperatures by making use of latent heat. To improve thermal management of the system by successfully harnessing latent heat, PCM should be carefully chosen for each unique application. Important criteria for choosing the right PCM include chemical stability, thermal conductivity, phase transition temperature and latent heat of fusion. In fact, thermal management can be negatively affected by including a poorly chosen PCM into a system that either partially or never makes use of latent heat. This can hinder heat storage or rejection.

Selecting a PCM for thermal management

Thermal management systems should only use PCMs that fulfill the following criteria: satisfying the required range for phase change temperature (PCT); having high latent heat, specific heat and thermal conductivity; exhibiting little volume expansion and little to no supercooling when frozen; being devoid of poison, corrosion, fire, explosion and chemical instability; and being inexpensive. Choosing a PCM that meets specific needs in terms of melting point, chemical stability, latent heat, cost and availability is not too difficult, as there is a wide variety of chemical substances that can serve this purpose.

Unfortunately, the low thermal conductivity of the majority of PCMs is a major drawback that prevents them from being used in thermal management systems. However, the following measures have been implemented to enhance PCMs' thermal conductivity: First, incorporating heat-conductive elements such as carbon-fiber chips, aluminum powder or nano-materials into PCMs. Second, including PCMs into composites by absorbing them into porous metal foam or expanded graphite (EG) matrices. Third, incorporating metal screens/spheres or using metal fins.

Types of PCMs

The chemical components allow us to classify common PCM into inorganic and organic groups. Organic PCM typically outperforms inorganic PCM in terms of desirable properties such low corrosivity and absence of supercooling. Due to its desirable thermal properties, paraffin (PA) waxes — an organic PCM that has been the subject of much research — have found widespread application as PCM in thermal energy storage and thermal management. These waxes are composed primarily of straight chain n-alkanes CH3-(CH2)-CH3. For technological applications, the most important features are the phase change temperature and latent heat storage capacity. Users can change the melting point of PA by adjusting the amount of carbon atoms in the main chain.

The need for sealed containers or tanks to contain PA when its temperature rises above its melting point results in a very complicated structure in real-world applications. This problem can be tackled in numerous ways. To begin, shell-core composite materials can be created by encapsulating PA using micro- or nano-encapsulation technology within a polymer or inorganic shell. The shell prevents PCM from leaching and protects it from the outside world. Second, shape-stabilized composites can be made by dispersing PA into porous materials; in these composites, surface tension and capillary force are weak interactions that limit the flow of molten PCM through the pores. The current porous matrix for this method has been composed of carbon materials (e.g., graphite foams, expanded graphite, carbon nanotubes) and silicate minerals (e.g., expanded perlite, diatomite, vermiculite, montmorillonite and halloysite).

By combining PA with a polymeric matrix, foamstable composites can be created. To keep the composite PCM's compact structure even when heated above its phase change temperature, the polymer compound acts as a supporting material to stop the liquid-phase PA from leaking out. It is easy to prepare goods with the appropriate dimensions using the technique for producing polymer-based form-stable PCM, and the process is straightforward. In addition, chemical functions can be easily added into polymers, and composites having a polymeric phase can have unique properties of their own. So, it seems that polymers are a good choice for making form-stable PCM for a lot of different uses.

Advantages of PCMs

  • PCMs help control temperature rise without active cooling, especially during peak loads.
  • PCM absorbs large amounts of energy during melting without a significant temperature increase.
  • Using CMs encourages multidisciplinary design: coupling electrical, thermal and materials engineering.
  • PCMs reduce conduction and switching losses and thus improve efficiency of power converters.
  • PCMs maintain cell-to-cell temperature uniformity and absorb heat during fast charge/discharge of batteries.

Conclusion

Because of their high latent heat, PCMs are among the most promising materials for a wide range of uses. Their ability to regulate the temperature of electrical components, power batteries and solar panels has been well-documented. The highlights of the passive thermal management system based on PCMs are its compactness, high efficiency, simplicity and lack of additional power input, in contrast to the standard active cooling method of forced air/liquid convection, which is complicated and cumbersome.