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Abstract: Graphene has the highest thermal conductivity among known materials, and has good application prospects in electronic devices, information technology, national defense and other fields. The theoretical and experimental study of graphene thermal conductivity is of great significance, and it has made great progress in the last decade. This paper summarizes the progress and application of the intrinsic thermal conductivity of graphene. First, the micro-and nano-scale heat transfer techniques applied to graphene thermal conductivity measurements are introduced, including Raman spectroscopy, suspended thermal bridge method, and time-domain thermal reflection method. Then we show the theoretical research results of graphene thermal conductivity and summarize the influencing factors of the intrinsic thermal conductivity of graphene. Subsequently, the application of graphene in thermal conductive materials is introduced, including high thermal conductivity graphene film, graphene fibers and graphene in thermal interface materials. Finally, the results of the thermal conductivity research of graphene are summarized, and the opportunities and challenges existing in the current research of graphene heat conduction are proposed, and the possible development direction in the future is discussed.
Key words: graphene; thermal conductivity; phonon; thermal interface materials; suspended thermal bridge method; size effect
1 Introduction
Graphene is a two-dimensional material with a single atomic layer thickness, which has attracted much attention because of its unique electrical, optical, mechanical, and thermal properties. Relative to the study of the electrical properties, the study of the thermal properties of graphene started relatively late. In 2008, Balandin's team measured the thermal conductivity of monolayer graphene for the first time by Raman spectroscopy, and observed that the thermal conductivity of graphene was up to 5,300 W?m−1? K 1, higher than the graphite block and diamond, is the highest thermal conductivity in known materials, which has attracted extensive attention from researchers. With the deepening of theoretical research and the progress of measurement technology, it is found that the thermal conductivity of single-layer graphene is higher than that of graphite block is related to its special phonon scattering mechanism, which has become an important research object to verify and develop the theory of phonon thermal conductivity theory.
The study of the thermal conductivity of graphene soon inspired the application of graphene in the field of thermal conductivity. With the development of the large-scale graphene preparation technology, the thermal conductivity of the highly thermal conductive graphene film prepared based on the GO method can reach ~2000 W?m−1? K−1. The thermal conductivity of highly thermal conductive graphene films is comparable to the high-quality graphitized polyimide films used in industrial applications, with a lower cost and better thickness controllability.
On the other hand, graphene, as a two-dimensional thermal conductivity filler, is easy to construct a 3-dimensional thermal conductivity network in the polymer matrix, which has good application prospects in thermal interface materials. By improving the dispersion of graphene in the polymer matrix and constructing the three-dimensional graphene thermal conductivity network, the thermal conductivity of the graphene-filling thermal interface composite is several times higher than that of the polymer, and the packing ratio is lower than that of the traditional thermal conductivity filler. Whether graphene is used as a self-supporting thermal conductivity film or as a thermal conductivity filler of the thermal interface material, it will play an important role in the heat dissipation applications of the next generation of electronic components.
This paper summarizes the measurement methods, research results and application of graphene thermal conductivity in graphene. First, three measurement methods of graphene are introduced: Raman spectroscopy, suspended thermal bridge method and time-domain thermal reflection method. Then we introduce the measurement results of the thermal conductivity of graphene, including the size dependence, thickness dependence of the thermal conductivity and the method of thermal conductivity regulation through defects and grain size. Subsequently, the application of graphene thermal conductivity is introduced, mainly including the application of high thermal conductivity graphene film, graphene fiber and graphene thermal conductivity filler in thermal interface materials. Finally, the development of thermal conductivity in graphene is discussed.
2 Measurement method of the thermal conductivity of graphene
Because the thickness of graphene is nanoscale, the commercial measurement equipment (laser flash method, flat plate heat source method, etc.) cannot accurately measure its thermal conductivity, so the micro-nano scale thermal measurement method is needed. Common micro-nano scale heat transfer measurement techniques include Raman spectroscopy, suspended thermal bridge method, 3 method, time-domain thermal reflection method and so on. The following will focus on thermal conductivity measurements suitable for graphene.
3 Progress in studying the thermal conductivity of graphene
The thermal conduction of graphene is mainly contributed by the phonons. Similar to diamond, graphene is composed of strong chemical bonds C-C bonds in the plane direction, and because of the carbon atom's relatively light and extremely high sound speed, it has a comparable thermal conductivity to diamond in the plane direction (~2000W?m−1?K−1) 。Regarding the main phonon contribution source of graphene heat conduction, the academic cognition has changed with the update of the research. Initially, it was expected that graphene heat transfer was mainly contributed by the longitudinal acoustic branch (LA) and the transverse acoustic branch (TA), and the vibration plane of both phonons was oriented along the ab plane of graphite. Such an expectation is reasonable, because the vibration plane of another transverse acoustic branch (ZA) phonon is perpendicular to the ab plane, while for graphene, as a single atom layer material, the vibration in the vertical plane is difficult. Moreover, the dispersion relationship of ZA phonons is ~ ω 2, and the sound velocity rapidly decreases to 0 at q 0, thus making almost no contribution to the thermal conductivity of graphene. Later, Lindsay et al 7 numerically solved the Boltzmann equation and found that due to the two-dimensional material characteristics of the phonons in the scattering of monolayer graphene, this rule is called the "selection rule (Selection rule)". For this reason, the phase space of ZA phonon scattering decreases by 60%. Meanwhile, considering the large number of ZA phonons, ZA phonon actually becomes the largest thermal conductivity contribution in single-layer graphene, accounting for about 70%. With the progress of computational methods, the researchers' understanding of phonon conduction in graphene has gradually deepened. Ruan's group calculated the thermal conductivity of monolayer graphene under the condition of considering four-phonon scattering. Due to the large number of ZA phonons, there are many four-phonon scattering processes involved by ZA phonons. By solving the Boltzmann transport equation (BTE), it is found that the contribution of ZA phonons to the thermal conductivity of single-layer graphene is actually about 30%. The molecular dynamics calculations found that the contribution of ZA phonons to the thermal conductivity of graphene when considering higher-order phonon scattering will be reduced, Cao et al. In addition, first-principles calculations show the existence of hydrodynamic thermal transport and second sound phenomena in graphene, as well as the thermal rectification phenomenon found in experimental measurements and molecular dynamics calculations, which make the study of phonon transport in graphene constantly updated. The thermal conductivity dependence is discussed below for the ideal monolayer graphene single crystal material.
4 Application of the thermal conductivity of graphene
The intrinsic high thermal conductivity of graphene is described in the previous section, which is verified from both theoretical calculations and experimental measurements. In the above experimental measurements, researchers often use mechanical stripping method and CVD method to prepare graphene. The samples prepared by these two methods have the characteristics of high quality and strong controllability, which are suitable for studying the intrinsic properties of graphene. However, because the preparation of graphene by mechanical stripping method and CVD method has the characteristics of low yield, long preparation cycle and difficult to scale, it is not suitable for the macro preparation of graphene. Correspondingly, graphene sheets can be prepared in large quantities by wet chemical methods such as reduced GO and electrochemical stripping. The graphene sheets can form graphene film, graphene fiber and graphene macrobody through the chemical bonding between the lamells, so that they can be actually applied in thermal conductivity scenarios.
4.1 Application of high thermal conductivity graphene films
Graphene film can be used as a radiator in electronic components, which are usually attached to the surface of electronic components that are prone to heat, evenly dispersing the heat generated by the heat source. Radiator is usually made of high thermal conductivity materials, common radiator copper, aluminum, graphite and so on. Among them, the highest thermal conductivity and the best heat dissipation effect is the artificial graphite thermal conductivity film obtained by the polyimide film by the graphitization process, and the plane direction thermal conductivity can reach 700~1950 W?m−1? K 1, with a thickness of 10~100 μ m, has a good thermal conductivity effect, which is the most ideal choice for the thermal conductivity film for a long time in the past. In this context, there are two important meanings to study highly conductive graphene film. One is that due to the high cost of artificial graphite film and the difficulty of preparing high-quality polyimide film, the industry hopes that highly conductive graphene film can be used as an alternative. Second, due to the increasing demand for heat dissipation of electronic products, the new heat dissipation scheme not only requires the thermal conductivity film to have a high thermal conductivity, but also requires the thermal conductivity film to have a certain thickness, in order to improve the thermal conductivity flux in the plane direction. In artificial graphite films, the graphitized polyimide thermal conductivity is high only high at small thickness due to the molecular orientation of the polyimide. The graphene thermal film is easy to make a large thick thermal film (~100 μ m), which has a good application prospect in the thermal management system of new electronic devices. Therefore, the study of graphene thermal film also mainly follows two directions. One is to improve the in-plane thermal conductivity of graphene thermal film to approach or exceed the level of artificial graphite film. The second is to improve the thickness of the graphene thermal conduction film, expand the thermal conduction flux, while maintaining good thermal conduction performance. These two aspects are discussed separately below.
Preparation and characterization of highly thermal conductive graphene films
4.2 Application of high thermal conductivity graphene fibers
Highly thermal conductive graphene fiber is a new type of carbonaceous fiber, which is orderly assembled by the wet spinning method of graphene dispersion liquid. Its main advantage is that it has good mechanical, electrical and thermal properties at the same time, and can be prepared by a large number of wet spinning method, easy to achieve scale, combined with the textile process, can reach a kilometer level output.
Similar to the raw materials of graphene fiber and graphene fiber, it is usually a graphene oxide dispersion or a functional graphene dispersion. Therefore, the main influencing factors of its thermal conductivity are also in common. The size of the graphene sheet and the interface strength between the graphene sheet play an important role. It is worth noting that Xin et al. found that the use of two graphene dispersions of different sheet sizes for assembling graphene fibers had the best physical properties. They spun a large layer (~23 μ m) with a small layer (~0.8 μ m) of graphene dispersion, and obtained the thermal conductivity of up to 1290 W?m−1? The graphene fibers of K 1 have better thermal conductivity than that of the graphene fibers prepared from a single component. Large layers of graphene provide heat transfer space for long average free range phonons. Small layers of graphene play a bond cooperation between large layers of graphene, improving the interfacial density between graphene sheets, and thus improving the thermal conductivity of graphene fibers.
4.3 Application of graphene in thermal interface materials
As a high thermal conductivity material, graphene can be used as a thermal conductivity filler in the thermal interface materials (Thermal interface material, TIM). Thermal interface material is a kind of material used in chip packaging, the main role is to fill the air gap in the chip, playing the role of providing mechanical support, electromagnetic shielding and auxiliary heat dissipation to the chip. Traditional thermal interface materials use resin-based composite materials filled with ceramics, metal, carbon materials as thermal conductive fillers, which use the mechanical properties of polymer materials to provide protection, and improve the heat dissipation ability by adding thermal conductive fillers. Because the resin's thermal conductivity is very low (less than 0.5 W?m−1? K 1), and the thermal conductivity of the commercial thermal packing is also low (alumina thermal conductivity ~35 W?m−1? K 1), the thermal conductivity of the overall thermal interface material is mostly 110 W?m−1? Between the K 1. The researchers have tried to use graphene with high thermal conductivity as a thermal filler to improve the thermal conductivity of thermal interface materials. The following focuses on the main factors affecting the thermal conductivity of graphene-reinforced resin-based composites.
5. Summary and outlook
Since the thermal conductivity of monolayer graphene was measured experimentally, the study of the thermal conductivity of graphene has made great progress. This paper summarizes the measurement methods of the graphene thermal conductivity, focusing on the Raman spectroscopy, the suspended thermal bridge method, and the time-domain thermal reflection method. The factors affecting the thermal conductivity of graphene are explored, and the application of graphene in thermal conductivity devices is introduced. While the thermal conductivity study of graphene is on the rise, we note that challenges remain in theoretical research, experimental measurements, and practical applications.
First, it is a phononic interpretation of the high thermal conductivity of graphene. In 2010, Lindsay proposed that ZA phonon is the phonon mode with the largest thermal conductivity contribution in monolayer graphene. This theory successfully explains that the thermal conductivity of monolayer graphene is higher than that of graphite block. However, when four-phonon scattering is considered, the contribution of ZA mode phonons is lower than LA and TA. How to understand the ZA phonon vibrations in single atomic layers and how to predict the contribution of high-order phonon scattering to the thermal conductivity of graphene still requires in-depth theoretical calculations.
Second, it is the length dependence and thickness dependence of the thermal conductivity of graphene. With the progress of measurement techniques, Raman spectroscopy and the suspended thermal bridge method are able to accurately measure the thermal conductivity of monolayer graphene. However, there are still some technical challenges of how to transfer the specified thickness of graphene and how to realize the placement of suspended graphene samples at a large scale. This part of the study is the most difficult, most meaningful and most interesting, and it is expected that the micro-nano scale heat transfer measurement methods will continue to improve in the future to verify the results of the theoretical predictions.
Finally, it is the process factor for the thermal conductivity applications of graphene. Currently, the thermal and mechanical properties of graphene thermal conductive films have been comparable to those of graphitized polyimide films and can achieve commercial applications in specific fields. In this subject, the preparation of highly thermal conductive graphene materials is closely related to the technology. How to realize the double target optimization of the close overlap of the high thermal conductivity of the graphene sheet and the graphene layer of the graphene sheet, and how to build the graphene three-dimensional thermal conductivity network on a large scale at low cost, to answer these questions still requires in-depth exploration and continuous improvement of the graphene preparation process. With the deepening of graphene thermal conductivity research in theoretical calculation and experimental measurement, we believe that highly thermal conductivity graphene materials will play a greater value in electronic devices, energy storage, biomedicine, national defense and other fields.
Article source: Carbontech
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