Dul Chun Cinn Taighde ar Ábhair Sheoltaí Teirmeacha Graphene

      1. Research status of graphene thermal film  

      Graphene is a carbon-containing six-membered ring structure with sp2 hybrid structure, and various physical and chemical properties are very stable. Compared with traditional metal materials such as copper and aluminum, graphene has a higher in-plane thermal conductivity. Because of its special structure, graphene has lower density, good thermal stability, ultra-high electrical conductivity, excellent light transmittance and better mechanical properties. Graphene is an ideal choice as an additive component of thermally conductive materials. At the same time, the graphene film can be used as a heat sink in electronic components, attached to the surface of electronic components that are easy to generate heat, and evenly disperses the heat generated by the heat source. Among them, the highest thermal conductivity and best heat dissipation effect is the artificial graphite thermal conductive film obtained from polyimide film through graphitization process. The thermal conductivity in the plane direction can reach 7001950W∙m-1∙K-1, and the thickness is 10100 μm, with good thermal conductivity.

      Wang Wen et al. added graphene nanosheets to polypropylene, which can increase its thermal conductivity by as much as 14 times. Chen et al. compounded graphene and nanocellulose through vacuum filtration, which greatly improved the thermal conductivity and mechanical properties of the graphene/cellulose nanofiber composite film. Song et al. combined graphene and nanocellulose into a cellulose/reduced graphene oxide hybrid film through layer-by-layer self-assembly, showing a highly ordered layered structure. However, the combination of the matrix and graphene consumes a lot of energy and cannot achieve large-scale production.  

      In this context, the study of high thermal conductivity graphene films has two important meanings. One is that due to the high cost of artificial graphite films and the difficulty of preparing high-quality polyimide films, the industry hopes that high thermal conductivity graphene films can be used as alternative plan. Second, due to the increasing demand for heat dissipation in electronic products, new heat dissipation solutions require not only high thermal conductivity of the thermally conductive film, but also a certain thickness of the thermally conductive film to increase the heat conduction flux in the plane direction. In the artificial graphite film, due to the degree of polyimide molecular orientation, the graphitized polyimide thermally conductive film has higher thermal conductivity only when the thickness is small. The graphene thermal film is easy to be made into a thick thermal film (100 μm), which has good application prospects in the thermal management system of new electronic devices.

      2.Technical Research on Graphene Film Thickness 

      The preparation of thicker graphene thermal conductive film is currently a hot research topic. Theoretically speaking, to increase the thickness of the graphene film, only a thicker graphene oxide film can be applied. However, there are the following problems in actual operation:        (1) The quality of the thick film formed by knife coating is not high. Due to the low concentration of the graphene oxide dispersion (less than 10 percent ), the rest of the graphene oxide is water, which requires a long time to evaporate. The graphene oxide flakes interact with water molecules through hydrogen bonds, and the water molecules escape when they evaporate, so that the graphene oxide flakes form cross-links through hydrogen bonds, forming a "milk skin"-like film on the surface. This thin film slows down the evaporation of water inside the graphene oxide dispersion, and causes the graphene oxide sheet to have inconsistent orientation, which reduces the quality of the film.  

      (2) It is difficult to obtain a thick film through a one-step method. Due to the low concentration of the graphene oxide dispersion, it is impossible to prepare a graphene oxide film with a thickness of 100μm at a time regardless of methods such as knife coating, spin coating or spraying.  Luo et al. found that the graphene oxide film can still be bonded to each other under the condition of deionized water infiltration after being evaporated to dryness. This phenomenon occurs because the graphene oxide layers are connected to each other through hydrogen bonds under the action of water. So that the graphene oxide film can be pasted like paper.  Zhang et al. used a similar method to swell the prepared graphene oxide film in water and paste it layer by layer. After drying, hot pressing, graphitization, and cold pressing, an ultra-thick graphene film with a thickness of 200 μm was obtained. It is 1224 W∙m-1∙K-1, and the heat dissipation effect measured by the infrared camera is better than that of copper, aluminum and thin-layer graphene thermal film. At present, there are relatively few studies on the preparation of 100-micron thick high thermal conductivity graphene films. In addition to swelling and bonding, the overlap of graphene oxide films can also be achieved by methods such as electric heating and metal ion bonding. Graphene film with micron thickness and high thermal conductivity provides new ideas.    

      3. Outlook  

      With the development of large-scale graphene preparation technology, the thermal conductivity of the highly thermally conductive graphene film prepared based on the graphene oxide method can reach 2000W∙m-1∙K-1. The thermal conductivity of the high thermal conductivity graphene film is equivalent to that of the high-quality graphitized polyimide film for industrial applications, and it has lower cost and better thickness controllability. At the same time, graphene, as a two-dimensional thermally conductive filler, is easy to construct a three-dimensional thermally conductive network in a polymer matrix, and has good application prospects in thermal interface materials. By improving the dispersion of graphene in the polymer matrix and building a three-dimensional graphene thermal conductivity network, the thermal conductivity of graphene-filled thermal interface composites is several times higher than that of polymers, and the filler ratio is lower than that of traditional thermal conductive fillers. Graphene, whether used as a self-supporting thermally conductive film or as a thermally conductive filler for thermal interface materials, will play an important role in the next generation of electronic component heat dissipation applications.