Describe the effect of graphene microchip size on thermal conductivity of graphene paper

Summary:

The thermal conductivity of graphene is much higher than that of thermal conductive materials such as traditional metal thin films and can be used as thermal diffusion materials. Graphene paper is composed of graphene micro sheets. The size of graphene micro sheets has an important influence on the micro structure of its assembly method and its macroscopic thermal conductivity. The dispersed and uniform graphene oxide paper was prepared by the solution filtration self-assembly method, and then graphene oxide paper was thermally reduced in the ar/h2 atmosphere to obtain graphene paper. The results showed that the structure of graphene paper made up of large graphene micro tablets was denser and more crystalline. The thermal conductivity of graphene paper prepared from graphene oxide of 0.5 μm to 3 μm and 50 μm to 100 μm was 632.8 w/mk and 683.7 w/mk, respectively, and the thermal conductivity of graphene paper composed of large graphene micro tablets was increased. 8 %.

Key words:

Graphene paper; Graphene microchip size; Micro structures; Thermal conductivity

Based on the improvement of the current level of industrial manufacturing technology, the goals of miniaturization, high integration, and high-energy in the fields of electronics, communications, and energy are gradually being achieved and will continue to be pursued. This trend will inevitably lead to a continuous increase in the energy density of equipment installations in related fields. Therefore, the problem of high heat flow density heat dissipation has become an urgent problem[1-2]. Under the above background, the high-efficiency two-dimensional thermal diffusion materials have begun to receive extensive attention. The main working principle is to use the high thermal conductivity of the materials in the two-dimensional plane to rapidly spread the heat in the hot spot areas of the system. In order to reduce the temperature of the hot spot area, the temperature gradient and internal thermal stress in the system structure are reduced, so as to eliminate the resulting structural thermal deformation and reduce the negative impact of high temperature concentration on the operation of the system. Graphene materials have high thermal conductivity, and ballandin et al.[3] measured the surface thermal conductivity of graphene at room temperature by non-contact optical technology of about 5200w/mk. The surface thermal conductivity of graphene materials is much higher than that of traditional metal films such as copper and aluminum(200-400w/mk), and graphene materials have lower density and good thermal stability[4]. It has an important and broad application prospect in the field of two-dimensional heat dissipation materials[5-7]. Although the nanoscale graphene micro tablets have excellent thermal conductivity, they are difficult to apply directly to the industrial field. Nanoscale graphene microtablets need to be assembled into macro graphene paper. Since graphene micro tablets are difficult to disperse in solvents, the current preparation of graphene paper is mainly based on the reduction of graphene oxide paper. The preparation process of graphene oxide paper(GOP) includes solution filtration self-assembly method[8], solution evaporation method[9], spin coating method[10] and so on. The GOP based on self-assembly process has good mechanical and electrochemical properties. The graphene paper is formed by stacking graphene micro sheets. The properties of graphene micro sheets have an important influence on the macroscopic properties of graphene papers. The size of graphene micro sheets not only affects the thermal conductivity of graphene micro sheets themselves[13], The effect on the assembly of graphene micro sheets into graphene paper, etc., affects the macroscopic thermal conductivity of graphene paper. At present, there is no public report on the study of the influence of the size of graphene micro tablets on the thermal conductivity of graphene paper, and it is important to explore this rule for the preparation of high-heat conductivity graphene paper.

1 Experiment

1.1 The main experimental material selects two sizes of graphene oxide micro tablets. The specific parameters are shown in table 1.

1.2 Graphene oxide paper prepared to dissolve 20mg graphene oxide powder to 40mln, n-dimethyl formamide liquid, ultrasonic oscillation 30 min, assisted mechanical stirring, prepared colloidal solution, oxide graphene concentration is 0.5 mg/ml. The polyvinylidene membrane with an aperture of 0.2 μm was selected, and the graphene oxide paper was prepared by using shz-d(III) cyclic pump filtration. The prepared graphene oxide paper is stripped and fully dried, removing the dmf contained therein. For ease of presentation, the GOP prepared by 0.5 μm to 3 μm graphene oxide micro tablets is marked as sgop, and the GOP prepared by 50 μm to 100 μm graphene oxide micro tablets is labeled as lgop.

1.3 Thermal reduction of graphene oxide paper can improve the structure and regularity of graphene oxide paper by applying constraints. In order to improve the quality of the product, GOP is sandwiched between two pieces of quartz glass and placed in a tubular quartz furnace for heat treatment. Treatment atmosphere: the mixture of H2 and AR is introduced at a rate of 400 SCCM and 500 SCCM. Heating reduction system as shown in Figure 1(initial warming rate 5 °C / min, temperature at 230 °C, temperature at 30 min, continued at 5 °C / min to 800 °C, 120 min); After the heating is completed, the quartz furnace is cooled to room temperature and the reduced graphene paper(ergop) product is removed for performance testing and characterization. For ease of expression, the graphene paper prepared by the reduction of sgop is marked as s<UNK> GOP, and the graphene paper prepared by the reduction of lgop is marked as l<UNK> GOP.

1.4 The performance characterization test of the sample was performed by Hitachis 4800 field emission scanning electron microscopy of Hitachi Co., Ltd.. The microscopic layered structure of the GOP and ergop samples was measured using the D 8 X-ray diffractometer of braker, Germany. The radioactive sources used cuk α, tube voltage 40v, tube current 100mA, and scanning rate 2 ° / min. The surface thermal diffusivity of graphene paper was measured using nix's lfa447 laser conductor. Calculate the thermal conductivity of the sample according to formula(1) where the CP is the specific heat of the sample, the graphite specific heat is used, and the value is 0.709 j/g · K, and <UNK> is the sample density. It can be calculated by testing the mass M and volume V of the sample, as shown in equation(2). Hey = m/v(2), where the volume V is calculated after measuring the radius R and thickness of the sample(obtained by scanning electron microscopy), as shown in equation(3). V = π × R2 × Δ(3).

2 Outcomes and discussions

2.1 Factographic analysis Figure 2(a) is a photo of sgop, which is dark brown and has poor light permeability; Figure 2(b) is an lgop photograph, a paper made up of large sized graphene oxide microchips, with a smooth appearance; Figures 2(c) and 2(d) are optical photographs of ergop, both of which have metallic luster. During the thermal reduction process, oxygen-containing functional groups in graphene oxide are removed by emitting small molecules of gas. The emitted gas may destroy the micro structure of the GOP, so the microchip folds deepen and the surface roughness of the microchip increases. Relatively speaking, the GOP surface consisting of large-size graphene oxide is relatively flat in quality, and the corresponding ergop is also more polished. Fig. 3 is a scanning electron microscope photograph of two types of graphene oxide paper and the corresponding heat-reduced graphene paper. Figure 3(a) has a different light and darkness on the surface, indicating that the size distribution of the graphene oxide micro tablets is relatively large, and the microchip stacks are messy and not dense enough to reduce the surface flatness of the graphene oxide paper; Figure 3(b) shows a more uniform distribution of surface folds, relatively small inter-laminated folds, and a more flat spread of micro sheets, indicating that the surface flatness of graphene oxide micro sheets increases with the increase in the size of graphene oxide micro sheets. In Figure 3(c), there were many deeper gullies on the surface of the reduced graphene paper, indicating that its smoothness was poor; As can be seen in Figure 3(d), graphene microchips are completely stacked together, and the micro sheets are relatively flat, and the scanning electron microscope(SEM) diagram of the reduced graphene paper can also reflect this information. The layered structure of graphene paper can be seen in Figure 3(E) and Figure 3(f), in which the snaps are loosely stacked and graphene micro sheets are warped; The graphene microchips in lop are stacked densely and have a high degree of smoothness. The above experimental phenomena can be explained by the self-assembly process of the graphene solution, as shown in Figure 4. Graphene micro tablets are subjected to various forces during the self-assembly process, such as gravity, electrostatic force, inter-molecular force, and diffusion. In low viscosity liquid, it is mainly affected by gravity; Large size graphene oxide micro tablets have a large ratio of long thickness and are easy to form regular stacks; In the small size of graphene oxide in the long thickness, the carbon oxygen ratio of the small graphene is high, the degree of functionalization is high, and the force is more complex, making it difficult to achieve orderly accumulation. During the filtration process, the small oxide graphene solution is quickly filtered and the entire process only takes about 30 min; Large amounts of graphene oxide solution are filtered slowly, and the complete filtration of the solution requires 24h. This shows that the sgop structure is loose, the stack of graphene oxide micro tablets is scattered, and there are more liquid circulation channels in sgop; The tissue structure in lgop is dense, graphene oxide accumulates densely, and liquid circulation channels in lgop are few.

2.2 x <UNK> D analysis of the X-ray diffraction maps of GOP and ergop, respectively, according to the Prague equation 2dsin θ = nλ,(D is the crystal plane spacing, θ is the diffraction angle, N is the diffraction series, λ is the wavelength of the X-rays), According to the 2θ value of the GOP or ergop(002) crystal surface, their layer spacing D can be calculated, as shown in table 2. The peak width and smoothness of the GOP are corresponding. The 2θ values of the GOP prepared by the graphene oxide micro tablets with dimensions of 0.5 μm to 3 μm and 50 μm to 100 μm are 10.456 ° and 9.607 °, respectively. The layer spacing is 0.845 nm and 0.920 nm, respectively, which means that the Moene micro tablets in the GOP are stripped due to the presence of oxygen-containing functional groups, forming a large layer spacing. When graphene oxide is reduced to graphene, the 2θ values of the two ergop are 26.522 ° and 26.460 °, respectively, and the layer spacing is 0.336 nm and 0.337 nm, respectively, close to the graphite crystal spacing(0.335 nm). The size of graphene oxide is basically reduced to graphene. The peak shape of the lergop diffraction peak became more acute and increased in strength, indicating that during the reduction of graphene to graphene in large size oxidation, oxygen-containing functional groups distributed on the surface of graphene oxide were removed, lattice restoration, and micro structures were improved. The peak shape of the S ergo diffraction peak became more gentle and its strength decreased, indicating that during the reduction of graphene to graphene in small sizes, the structure became more chaotic and the regularity decreased. Because small pieces of graphene are more likely to be pushed by the gas emitted during the reduction process, this disrupts the originally relatively orderly structure.

2.3 Thermal conductivity analysis The densities of ergop prepared by two different sizes of graphene oxide micro tablets are 1.79 g/cm3 and 2.04 g/cm3, respectively. The thermal diffusion coefficient and thermal conductivity test results are shown in table 3. As can be seen from table 3, the thermal conductivity of srago and lrago were 632.8 w/mk and 683.7 w/mk, respectively, and the thermal conductivity of lrago increased by 8 %. Among them, lergop's density increased by 14 %, and lergop's thermal diffusion coefficient did not increase. The graphene paper prepared by large-size graphene micro tablets has a higher thermal conductivity, mainly due to the more regular structure of graphene paper assembled by large-size graphene micro sheets, which can produce more dense materials. Eventually improve the thermal conductivity of the material.

3 Conclusion

Through the solution filtration assembly method, the graphene oxide micro tablets that are evenly dispersed in the solution are sequentially assembled to obtain evenly dispersed graphene oxide paper, and the graphene paper with apparent formation is prepared by heat treatment reduction. The effect of graphene size on the micro structure and thermal conductivity of graphene paper was studied, and the following conclusions were obtained:

1) The size of the graphene oxide microchip increased, the structure of the graphene oxide paper became denser, and the surface roughness decreased. After thermal reduction, the corresponding graphene paper micro structure is more orderly.

2) The high-temperature reduction treatment significantly reduced the layer spacing of graphene oxide paper. The spacing of graphene micro sheets of different sizes was not significant, and they were all close to 0.335 nm, but the graphene paper micro structures with large micro sizes were more orderly.

3) The thermal conductivity of the reduced graphene paper prepared by the two microfilm sizes of graphene oxide was 632.8 w/mk and 683.7 w/mk, respectively, and the thermal conductivity of graphene paper composed of large micro tablets increased by 8 %. This is due to the fact that large graphene piles up into graphene paper structures that are more orderly and have greater density.

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