Study On Thermal Conductivity Of Graphene Paper Based On Transient Electro-thermal Technique With Temperature Range From 20 K To 290 K

As a source of scientific and technological development,materials promote the process of human civilization,and have important strategic significance for countries around the world to enhance their core competitiveness.Graphene materials have become one of the hot materials widely researched and applied in many fields with its excellent thermal,electrical and mechanical properties.The self-supporting macroscopic two-dimensional graphene paper is made up of tiny graphene sheet layers and has huge potential applications in flexible electronics,thermoelectric devices,aerospace equipment and other fields.The issue of heat dissipation is one of the most important issues facing the upgrading of electronic products.At the same time,with progress in science and technology such as manned spaceflight,deep space exploration and high power loading,the demand and requirements for high-performance materials have become increasingly high,and the use environment faced by materials has become increasingly harsh.It also puts forward new requirements for the performance of materials.It is necessary for heat exchange equipment to achieve stable and long-term operation in more severe environments,so it is of great significance to explore the thermal conductivity of graphene paper in a wide temperature range.In this paper,the thermal conductivity of graphene paper was studied and analyzed at 17 temperature points in the temperature range of 20～290 K using transient electro-thermal technique（TET）.The effect of current annealing on its thermal conductivity was also investigated.It can provide certain theoretical support for development and application in low-temperature environments such as deep space exploration and deep-sea exploration.The main findings of this paper are as follows:（1）Experimental study of the thermal conductivity of graphene paper in the temperature range of 20～290 K using TET technique.At room temperature,sample 1 had a thermal diffusivity of 1.96×10^-6 m²·s^-1 and a thermal conductivity of 2.31 W·m^-1·K^-1,while sample 2 had a thermal diffusivity of 3.06×10^-6 m²·s^-1 and a thermal conductivity of 6.09 W·m^-1·K^-1.The thermal diffusivity and the thermal conductivity of the graphene paper both decrease with decreasing temperature.When the temperature was reduced to20 K,sample 1 showed a 35.47%reduction in thermal diffusivity and a 97.66%reduction in thermal conductivity,and sample 2 showed a 46.46%reduction in thermal diffusivity and a 96.94%reduction in thermal conductivity.The volume heat capacity gradually decreases with decreasing temperature,while the resistance gradually increases with decreasing temperature,showing a strong negative temperature coefficient thermistor characteristic.（2）After current annealing,the thermal conductivity of graphene paper was effectively improved.At room temperature,the thermal diffusivity of sample 1 increased to 3.21×10^-6 m²·s^-1,an increase of 67.38%,and the thermal conductivity increased to5.47 W·m^-1·K^-1,an increase of 136.80%,while the thermal diffusivity of sample 2increased to 5.02×10^-6 m²·s^-1,an increase of 64.05%,and the thermal conductivity increased to 22.21 W·m^-1·K^-1,an increase of 264.70%.（3）The surface micromorphology of graphene paper material was characterized by Scanning Electron Microscope（SEM）images.The microporous structure resulting from the large number of microfold structures and defects on its surface.The surface of the graphene paper appears smoother,finer and more lustrous after current annealing.The microporous structure on the surface has disappeared,indicating that the defects of the graphene paper were effectively repaired.The elemental composition was quantified by Energy Dispersive Spectroscopy（EDS）.The oxygen element was removed after the current annealing and the atomic percentage content of the carbon element was raised by about 23%relative to that before annealing.It shows that the current annealing can effectively remove the oxygen-containing functional groups inside the material and enhance the carbon content of the material.（4）Graphene paper relies mainly on phonons for heat transport,and the decrease in the average number of phonons involved in heat transport as the temperature decreases is the main factor causing the decrease in thermal conductivity.The intrinsic residual thermal reffusivity of samples 1 and 2 at the limiting temperature of 0 K were fitted by the thermal reffusivity model as 7.84×10⁵ s·m^-2 and 6.33×10⁵ s·m^-2.The Debye temperatures were 159.81 K and 133.67 K respectively,with grain sizes of 0.89 nm and1.10 nm.After the current annealing,the Debye temperature was reduced to 136.56 K and 104.17 K,respectively,and the grain size was increased to 2.01 nm and 2.67 nm,respectively.