E-Mobility Engineering | January/February 2024 35 Phase change materials | Focus grease, with the lower value providing more effective thermal transfer. In one example, a drive system design had a higher power density than the previous version and so needed a reliable, efficient mechanism to remove the additional heat. Previous experience with conventional thermal grease suggested that, due to inherent material migration (also known as ‘pump out’), a grease thermal interface would not deliver the reliability and performance required. The manufacturing process also needed to be improved. A phase change thermal interface material with high thermal conductivity of 3.0/W-mK provided higher performance over common thermal greases across two important metrics. In active power cycle testing, one thermal grease experienced a fatal result after 600 hrs of cycling between temperatures of -50 C to 150 C, while another lasted 800 hrs. The PCM did not fail and the temperature of the chip did not exceed 125 C even after 1,000 hours. Once applied, the material remains solid at room temperature until the device’s operational heat causes it to melt and spread out across the interface. The PCM can also be applied via stencil printing, allowing an automated, multidevice material application process. Once the PCM material is dried, the PCM layers can be cut to size and stored until needed for system integration. PCM for battery packs Using a PCM for cooling a battery pack supports faster charging, where the higher charging rates create higher temperatures in the pack which can be a major issue for the health of the system. It also supports safety, as it can limit thermal propagation. Thermal propagation is the effect where one cell malfunctions and overheats. This can cause neighbouring cells to overheat, leading to a chain reaction and, in the worst case, a battery pack fire. A cooling system based on a PCM can provide enhanced thermal management over air cooled, particularly with pouch cells. Moreover, as it is a passive system it has a lower energy requirement than a liquid cooled system and potentially higher reliability as there are less moving parts. As more heat is generated in the cells the phase change carries away the initial thermal energy, and then the liquid phase carries heat away through convection rather than the traditional conduction paths of cooling plates. The cooling in the battery pack can be implemented with a PCM in a number of different ways. The PCM can be contained in cooling units that are interspersed with the battery cells. This is particularly helpful with pouch cells that are stacked together. Cooling modules, often built from lightweight, thermally conducting composite materials such as carbon fibre, can be inserted between the cells to carry the thermal energy away. Another approach is to have the PCM in the same cylindrical cell format used for the battery cell. This allows the cooling cells to be placed in the battery pack without additional engineering being required. Thermal management PCMs are increasingly used for thermal management in power electronics. The materials are solid at room temperature which allows them to be easily cut and formed, and are then applied between a power device such as an electronic control unit (ECU) and a heat sink more easily than a thermal grease. This simplifies the steps in the assembly process with less waste and higher reliability as there is no danger of a grease dispenser blocking. When the ECU heats up, the PCM melts and provides a more efficient thermal pathway from the device to the heatsink. When the vehicle stops, the ECU cools and the PCM solidifies. The phase change material also reacts faster to a change in temperature than greases and pastes, changing from a solid to a liquid to efficiently release and absorb heat for better dissipation and stronger thermal performance. This comes from the lower thermal impedance value for the PCM, which can be as low as 0.062 compared to a typical value of 0.160 for a thermal The melt zone for a phase change material (Image courtesy of 1-Act)
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