February/March 2021

6 RESEARCH Issue 1 2021 Power Electronics Europe www.power-mag.com Graphene Heat Pipe Increases Thermal Conductivity Heat pipes are usually made of copper or aluminum. However, due to their relatively high density and limited heat transmission capacity, such heat pipes are facing challenges in power electronics. The newly researched graphene enhanced heat pipe featuring graphene assembled film with nanostructure enhanced inner surfaces improve heat dissipation capacity about 3.5 times higher. Involved were researchers in Sweden, China and Italy. “Heat pipes are one of the most efficient tools for this because of their high efficiency and unique ability to transfer heat over a large distance,” says Johan Liu, Professor of Electronics Production, at the Department of Microtechnology and Nanoscience at Swedish Chalmers University. And, compared to metal materials, graphene shows overwhelming advantages such as lightweight, good stability and superior in-plane thermal conductivity (up to 5000 W m-1 K-1 at room temperature), far better than that of copper (402 W m -1 K -1 ) and aluminum (237 W m -1 K -1 ). Furthermore, compared with single layer graphene film, graphene assembled films (GF) reach a good compromise between thermal conductivity, scalable preparation and applicable mechanical strength. GF with thickness at 1–2 microns has exhibited thermal conductivities up to 2500 W m -1 K -1 and tensile strength of 78 ± 6 MPa. More importantly, GF has little corrosion risks even under acid, alkali and moisture exposure than metal based materials. Taken all of these advantages into consideration, graphene possesses promising potential to boost heat dissipation performance of heat pipes. Design of graphene heat pipe A recently demonstrated graphene heat pipe (GHP) at Chalmers University of Technology in Sweden exhibits a cooling capacity up to 7230 W m -2 K -1 g -1 under a 10 W heat loading, which is about 3.5 times better than that of commercial copper based heat pipes with the same geometry (2053 W m -2 K -1 g -1 ). Besides, simulation results reveal that the GF contributes over 30 % to the total heat dissipation ability of the heat pipe due to its outstanding thermal conductivity. Heat transfer modeling suggests that increasing thermal conductivity of wall material (container) can significantly improve its heat dissipation capacity. The GHP consists of three key components, including container, wick structure and working fluid (Figure 1A, B). To improve the mechanical strength of the GHP container, a copper spring was inserted. On one hand, the ultra-high in- plane thermal conductivity of graphene film endows heat conduct from the evaporator section to the condenser section rapidly. On the other hand, the working fluid is evaporated when heat enters GHP at its evaporator section. The evaporated fluid creates a pressure gradient to force the vapor to move towards the condenser section. While vapor reaches the condenser section, the vapor condenses and releases its latent heat of evaporation by external devices that is, heat sink. After that, the working fluid is relatively or passively pumped to the evaporator section for re-evaporation by the capillary force of wick structure (Figure 1C). Thus, by using this liquid-vapor phases changes, heat transport in GHP becomes extremely fluent and efficient. Heat dissipation performance GHPs with various lengths (90, 130, 150 mm) were fabricated to investigate the relationship between length and specific thermal transfer coefficient. When the length of the 6 mm outer- diameter GHP decreases from 150 to 90 mm, temperature on the evaporator section decreases from 77°C to 42°C (Figure 2 A-C) with 10 W heat loading. Meanwhile, the g increases with shortened length of GHP and reaches 7230 W m- 2 K-1 g-1 with a heat loading of 10 W (Figure 2D). It is demonstrated that the heater temperature decreases with shortened pipe length (Figure 2E), consistent with the trends of g. With a 10 W heat loading, temperature of the heater containing a 90 mm GHP goes to 42°C with a 15.4 mins start-up time in steady state (Figure 2F, T4 is the temperature at thermal couple 4), far lower than that (86°C) of the independent heater without graphene pipe. These provide evidence that shorter GHP carries more power than longer pipe since capillary limit is an inverse function of the length. On the other hand, the start?up time of GHP significantly shorts with higher heat loading. Such tendency is because higher heat loading provides sufficient pressure difference between inlet and outlet of evaporator section to activate the GHP. The contributions from the container’s thermal conductivity and phase change process were assessed numerically by COMSOL Multiphysics solver. Models of 6 mm outer diameter GHPs with length of 90, 130, and 150 mm have been “The graphene-enhanced heat pipe exhibits a specific thermal transfer coefficient which is about 3.5 times better than that of copper-based heat pipe“, states Chalmers-University Prof. Johan Liu Design and image of the GHP (A, image of a real GHP; B, schematic designing of the GHP; C, working principle of the GHP)