How phase-change materials function as heat sinks

19 August 2020
Sourcel Krust/Adobe

Overheating is one of the key causes of electronics malfunctions. Therefore, thermal management of electronic systems is of paramount importance. With rapidly advancing technological landscape, the electronic systems are becoming more complex while shrinking in size. Ventilation is not a feasible option for these systems if they have to be sealed. Smartphones, notebooks and other tech gadgets rely on being compact, lightweight and sealed from common debris. In this scenario, it is important to look into suitable methods of cooling that do not depend heavily on ventilation.

Traditional heat sinks used for cooling electronics are not exactly heat "sinks," but are rather heat exchangers. They absorb excess heat and then transfer it to another media like air or a liquid coolant. Phase change material (PCM) heat sinks act as actual heat sinks and are particularly useful where there is a lack of heat exchange media. They absorb the excess heat and store it by changing its physical state from solid to liquid. They have high latent heat capacity, which is the energy absorbed by a material while it transitions from one phase to another without raising the overall temperature of the material. PCM can absorb a large amount of heat without increasing the temperature and this makes them well suited for cooling electronics.

In addition to having a high storage density, PCM can withstand a large number of cycles and retain isothermal nature during phase change at constant temperature. The operation cycle of PCM is shown in figure 1.

PCM cycle. Source: Pazrev/CC BY-SA-4.0PCM cycle. Source: Pazrev/CC BY-SA-4.0

PCM in solid state absorbs heat and its temperature rises to a melting point. The heat absorbed during this stage is the sensible heat. After that, the temperature rise stops and the heat absorbed by the PCM is the latent heat of fusion that results in the phase change of the material. This long duration phase change is what makes PCM a suitable heat sink. During the cooling stage, the opposite happens and the energy is emitted while the phase of the material changes from liquid to solid.

PCM heat sinks are passive heat sinks and no moving parts are involved in their design. They are light weight and reliable, which makes them ideal for compact electronic devices. However, PCM has certain drawbacks. Low conductivity of PCM is a major issue and researchers are continuously looking for new ways to enhance the thermal conductivity of PCMs.

Thermal conductivity enhancers for PCM

Thermal conductivity enhancement can be done by the addition of materials with high conductivity into the PCM heat sinks. These materials are called thermal conductivity enhancers (TCE). Low thermal conductivity means slow heat absorption and reduced effectiveness of the heat sinks. One method to enhance the conductivity of PCM is to add fins to it. Effectiveness of this technique depends on the application and the shape, geometry and arrangement of fins. Generally, the research shows that increasing the number of fins increases the conductivity of PCM heat sinks until the optimal number is reached. This optimal point is the one beyond which the performance of PCM heat sinks starts to decline because the quantity of PCM decreases. As far as the shape of the fins is concerned, fins are usually of two types: pin-fins and plate-fins. Pin-fins outperform the plate-fins in the tests carried out by a number of researchers.

The addition of fins is an effective method to improve thermal conductivity of PCM but it comes at a price: increased weight of the cooling system and reduction in PCM volume. Another thermal conductivity enhancement method that is not burdened with these drawbacks is based on nano particle mixing. PCM is mixed with nano particles that are highly conductive. However, this method compromises the constant melting duration of the PCM. The inherent quality of PCM that makes it a great heat sink is long melting duration at a relatively constant temperature. Since mixing the nanoparticles will obviously decrease the volume of PCM, its melting duration will also be affected. However, a compromise can be reached depending on the application and nanoparticles, which are still a great option for light weight PCM heat sink designs.

PCM filled with metallic foams is another method being explored by the researchers as an effective solution to the low PCM thermal conductivity problem. Metal foams ensure even distribution of heat throughout the PCM. This is an advantage over the fin type TCEs because heat transfer through the fins is not uniform. Metallic foams are also a better option than the nanoparticle mixing mainly because nano particles often have compatibility issues with certain PCMs. However, this method still reduced the melting duration.

PCM materials

The choice of PCM depends on the application but the most commonly used materials are paraffin waxes due to their lower cost and stability. Paraffin is compatible with metals, has a large latent heat and has a wide operation range. Non-paraffin organics are also a good choice for the same reason. For electronics operating at high temperatures, metallic PCMs and PCMs based on non-hydrated salts are a better choice.

PCMs are a great option for designing thermal management systems for electronics. They are perfect for on-chip cooling and high-powered electronic systems. They can save an electronic system in case of loss of the coolant. In short, they are a feasible option for light weight and compact electronic applications. Continuous research into overcoming their only major drawback of low thermal conductivity promises a bright future for PCM heat sinks.

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