With the exception of thermal storage heat sinks, the term heat sink is a misnomer. Standard heat sinks for electronics cooling are actually heat exchangers, taking the heat from the electronics, and transferring it to a fluid, either air or coolant. Phase Change Material (PCM) heat sinks are the only heat sinks that actually act as a (temporary) sink for heat. They are emerging in the thermal management realm to solve thermal problems in systems where active solutions cannot be used. When there is no place to dissipate the heat generated by electric components, a PCM heat sink is capable of absorbing the generated waste heat [1] .

Figure 1. PCM Heat Sink Example Showing Fins to Enhance Conductivity.

Phase Change Materials (PCMs) store thermal energy by the phase change from solid to liquid. This is an advantage, since the latent heat from melting or freezing is at least 1-2 orders of magnitude higher than the energy stored by the specific heat over a representative 10°C change in temperature. PCM applications in electronics thermal management include:

  • Stabilizing temperature during pulsed operation [2]

  • Short-term thermal storage, where a suitable heat sink is not available [3]

  • Protection from failure during coolant interruptions, when the cooling system is temporarily unavailable.

How PCM works

PCM refers to any material that requires a large amount of energy to undergo a phase change. The energy required to transition between solid and liquid phases is known as the latent heat of fusion. Materials with a high latent heat of fusion can store a significant amount of heat during a phase transition while maintaining a near constant temperature around the material's melting point. This property is advantageous for electronics cooling applications with transient loading. During transient operation, the thermal energy can be stored in the PCM while the heat is generated, without the temperature of the source increasing significantly. While the heat source is off, the PCM can refreeze so it is ready to absorb energy during the next heating cycle. This solution works well provided there is enough PCM to store all of the waste heat and the thermal resistance of the PCM heat sink is low enough to handle the required heat flux.

The thermal benefit of using a phase change material for transient loading is illustrated in Figure 2. The slope represents the temperature rise per unit of energy absorbed. During the melt, a large amount of energy is stored with very little change in temperature. A constant temperature is particularly appealing in applications with ergonomic requirements, but is beneficial in several applications to prevent components from failing during transient power spikes.

Figure 2. Phase Change Material Transient Behavior. The “Latent Heat” region has a slight slope, since it includes the specific heat in the amorphous region.

PCM Selection

The melt temperature and application will dictate the type of PCM that can be used (Table 1). For most electronics applications, paraffin waxes and non-paraffin organics are a good choice because they are relatively inexpensive and stable through many thermal cycles. For high temperature applications, metals and salts (non-hydrated) can be used.

Table 1. PCM Types Include Paraffin Waxes, Non-Paraffin Organics, Hydrated Salts, Non-Hydrated Salts, and Metals.

Paraffin and Non-Paraffin Organics

Paraffin and non-paraffin organics are ideal phase change materials because they melt and freeze congruently (i.e. have the same composition before and after freezing) and can therefore be used for applications requiring material stability through several cycles. The major drawback to these materials is their thermal conductivity, which is typically around 0.2 W/m-K. Paraffins, which are alkanes or saturated hydrocarbons, are inert phase change materials. Non-paraffin organics, other than fatty acids, are mildly corrosive and are more expensive than paraffin materials.

Both paraffin blends and pure component paraffins are commercially available for purchase. Paraffin blends are generally much less expensive and exhibit similar properties to pure component paraffins. The performance can be comparable to pure component paraffins, but when selecting a material, it is important to take a close look at how the latent heat value is reported. Manufacturers will often release the differential scanning calorimetry data of the blend, along with the latent heat value. The Differential Scanning Calorimeter curve (DSC), as seen in Figure 3, will reveal how the latent heat value is determined. The area under the melting peak is used to calculate the latent heat of fusion. Oftentimes, the latent heat value reported includes some of the sensible heating storage outside of the phase change region. This means that in order to see the full amount of energy storage reported, the material would need to be exposed to temperatures below the onset of melt temperature and above the end of melt temperature.

Figure 3. Example Differential Scanning Calorimeter curve (DSC) for a phase change material. The phase change region indicates the latent thermal energy storage.

Hydrated Salts

Salt hydrates are a group of inorganic salts that contain a certain number of water molecules. The hydration level is noted in the chemical formula as “hydrated compound • nH2O”, where n is the number of water molecules per molecule of salt. Salts can have several different hydration levels. For example, CaCl2 has three different hydration levels, CaCl2 ·2H 2O, CaCl2 ·4H2O, and CaCl2 ·6H2O. The lowest water content corresponds to the highest melting point for a hydrated salt series. Hydrated salts have the following advantages when compared with organic PCMs for low temperature applications:

  • High storage capacity with respect to both mass and volume, allowing light and compact thermal energy storage systems,

  • Inexpensive, about $2/kWhthermal to $16/kWhthermal,

  • Relatively high thermal conductivity, more than twice of paraffins, reducing the required structures for thermal enhancement.

Hydrated salts are rarely used in PCM heat sinks, since the designer must overcome several problems: subcooling, phase separation, and corrosion. Recent research has shown that these problems can be mitigated, making hydrated salts suitable for some specialized electronics cooling applications.

Subcooling: Some phase change materials do not solidify immediately upon cooling to its melting temperature, so that solidification begins at a temperature well below the melting temperature. This phenomenon is called sub-cooling or supercooling. The most common way to address the issue is to disperse a nucleating agent in the PCM system. The added nucleating agent provides sites to initiate the crystallization of the PCM with a much lower sub-cooling, allowing a warmer heat sink to be used to regenerate the PCM. The most effective agent is one with a crystal structure similar to the PCM. Take CaCl2·6H2O with a melting point of 29°C as an example. Its subcooling is nearly 20°C without a nucleating agent. ACT's nucleating agent SrCl2·6H2O is able to minimize the subcooling of CaCl2·6H2O to less than 2°C.

Phase Separation: When a PCM has only one component, it will melt congruently, having the same homogeneous composition before and after its phase change. With a eutectic system, the liquid phase transforms completely to its solid phase with the same composition. Non-eutectic mixtures, like some salt hydrates, can separate into multiple phases when heated. When CaCl2·6H2O (melting point 29°C) is heated, instead of melting to a liquid, it forms solid CaCl2.4H2O and a dilute liquid phase first. With further heating, the solid CaCl2.4H2O dissolves into the excess water of the dilute liquid phase, transforming completely to liquid phase CaC 2·6H2O. Phases with different densities can occur in the intermediate step. The heavier CaCl2.4H2O sinks down, with the lighter dilute liquid floating on the top. Potential solutions to handle the situation include adding more water, or reducing the thickness of the PCM by separating it into shallow PCM compartments. A third solution is to add another salt hydride to form a eutectic, e.g., CaCl2·6H2O and MgCl2·6H2O.

Corrosion: While hydrated salts are generally compatible with plastics, metal containers and metallic heat transfer enhancements [4][5] are usually required for electronics cooling. Some salts are very corrosive to certain metals. Tests conducted at ACT with several hydrated salts such as CaCl2·6H2O and CaCl2·6H2O - MgCl2-6H2O) have identified suitable enclosure materials. More than 1-year corrosion life test data has been collected, and the corrosion tests have been run under both isothermal conditions (30°C, 50°C, and 80°C) and cyclic conditions with repeated heating and cooling, Carbon steel and aluminum both show good corrosion resistance with less than 0.5 mil/year corrosion rate.

High-Temperature Phase Change Materials (Salts and Metals)

Phase change material melting from -10 to 100°C is used for thermal management in a variety of commercial and military applications, e.g., building thermal management, electronic cooling, and supplemental cooling for energy weapons, to name a few. Salts, and metals, which are phase change materials with higher melting temperatures, are also attractive. One application is storing high quality heat from concentrated solar radiation and using it to generate electricity. Inorganic salts have been intensively studied, and precursor developments [6] have identified promising storage medium for the high temperature applications. Pilot-scale units have demonstrated its technical and cost feasibility.

PCM Challenges

Figure 4. PCM Heat Sink Schematic. Folded fins are commonly used to increase the effective thermal conductivity. Heat pipes can also be added for larger systems, or systems operating at higher heat flux.

Most PCMs used for electronics cooling have a very low thermal conductivity, effectively insulating a heat source in high heat flux applications. If there is a large temperature gradient through the PCM, the surface in contact with the heat source may reach its maximum temperature before all of the PCM latent heat is utilized (melted). In order to fully utilize the latent heat of the material, thermal enhancements may be required. Aluminum or copper fins are commonly used to improve the heat transfer through the PCM. Other thermal enhancements such as nanoparticle impregnation have been experimented with [7] , but the nanoparticles have not been shown to increase the thermal conductivity significantly enough to make the material viable for most applications. Further, the nanoparticles settle out after very few cycles without a stabilizer [8] . For this reason, finned structures and heat pipes are most commonly used in industry to better distribute the heat into the PCM (Figure 4).

During phase change, the density of a material changes. Depending on the material, different features are used to compensate for the volumetric change. Typically, the PCM volume is controlled during filling so that there is still some void space at the highest expected temperature. If the higher pressure can occur, a pressure relief feature may be required.

PCM Heat Sink Design

Figure 5. Simplified Thermal Resistance Network.

Advanced Cooling Technologies, Inc. (ACT) has developed a simple figure that can indicate whether PCM is suitable for an application and whether thermal enhancements are required. Average PCM heat sink properties were used to generate this figure, based on ACT's experience with PCM heat sink design. The PCM material properties are an average of several paraffins used in practice and the fin material is aluminum. The model also assumes that no more than a 10°C temperature rise to overcome thermal conduction resistance is allowed. A simple energy balance and resistance network, shown in Figure 5, can be iterated to find a goal temperature gradient from the base of the heat sink to the PCM.

Figure 6. PCM heat sink selection guide, showing the PCM/Thermal-Enhancement mass ratio as a function of time and heat flux.

For applications with a known storage time and heat flux, Figure 6 will indicate whether a PCM heat sink solution is a suitable thermal solution. The PCM ratio represents the amount of PCM required relative to the volume of aluminum in the fin enhancement structure. PCM ratios near zero indicate an all metal solution would be required to remove the flux. PCM ratios near unity indicate a PCM structure without any metal enhancement structure would be required.

A closer estimation of the mass, volume, and thermal performance of a PCM heat sink can be obtained using ACT's heat pipe calculator here . The expected transient performance for three different PCM options suitable for your application is plotted. This calculator assumes a generally conservative fin design. High performance custom solutions with heat pipes can be designed upon request.

Conclusions

PCM heat sinks are used in several electronics cooling applications including temperature stabilization during pulsed operation, short term thermal storage when a suitable heat sink is not available, and protection from failure during loss of coolant scenarios, to name a few. If the thermal storage capacity of PCM is suitable for an application, a PCM heat sink can reduce system size, cost, maintenance, and power requirements.

The specific phase change material and enclosure selected is dependent on the specific application requirements. Paraffin waxes and non-paraffin organics are most frequently used, but hydrated salts, non-hydrated salts, and metals can be used in some specialized applications.

PCM heat sink design challenges include the low thermal conductivity of most PCMs. Thermal enhancement such as fin or heat pipes are generally used to improve the thermal conductivity.

This article was written by Rebecca Weigand, Product Development Engineer; Ying Zheng, R&D Engineer II; William G. Anderson, Chief Engineer; and Richard W. Bonner III, Vice President, R&D; Advanced Cooling Technologies, Inc. (Lancaster, PA). For more information, visit here .


Aerospace & Defense Technology Magazine

This article first appeared in the February, 2019 issue of Aerospace & Defense Technology Magazine.

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