Preparation And Properties Of Multifunctional Carbon Aerogel-based Composite Phase Change Materials

As global warming and the energy crisis intensify,the exploitation of solar energy and the improvement of energy utilization are rapidly becoming hot research directions,and the full-spectrum utilization and storage of low-grade solar radiation energy is the most promising solution at present.One of the specific methods is to convert light energy into thermal energy and then store the thermal energy through thermal storage materials.As one of the thermal energy storage materials with good thermal properties,phase change materials have great potential for applications in the fields of waste heat utilization,solar energy and building.However,light energy often cannot output thermal energy continuously and stably due to the influence of region and climate.From a practical application point of view,it is proposed to use electrical energy as an auxiliary energy source to make the heat energy can be continuously and steadily output.This requires the composite material to have not only photothermal conversion capability but also electrothermal conversion capability.In addition,the phase change materials themselves have problems such as easy leakage and low thermal conductivity.In order to solve the above problems,this paper introduces a carbon aerogel-based composite phase change material with good shape-stability,high thermal conductivity,high thermal storage capacity,and good electro-thermal and photothermal conversion performance.The specific research contents are as follows:(1)Carboxymethylcellulose sodium(CMC)was used as the raw material and citric acid(CA)as the cross-linking agent to prepare carboxymethylcellulose hydrogels,and the carboxymethylcellulose-based xerogels were obtained by freeze-drying.The microscopic morphology,chemical composition and mechanical properties of carboxymethylcellulose xerogels obtained after chemical cross-linking and freeze-drying were tested and analyzed by scanning electron microscopy(SEM),Fourier infrared spectroscopy(FT-IR)and universal testing machine.The results showed that the xerogel with a crosslinking agent ratio of 15 wt%showed the best strength.After that,the prepared xerogels were subjected to high-temperature carbonization to obtain carboxymethylcellulose sodium-derived carbon aerogel(CCA).The morphology,composition,structure and specific surface area were systematically characterized by SEM,Raman spectroscopy and specific surface area tests.The optimum cross-linking agent ratio of 15 wt%and the optimum heat treatment temperature of 1000℃were determined by combining all the test results.Firstly,the sample possesses a better degree of graphitization(I_D/I_G=1.47)and lower resistance(resistance is only 1.57Ω),which shows that the material has excellent electrical conductivity.Secondly,the sample has more macro-and mesoporous structures and a very high specific surface area(960.705 m~2/g),which means that it will occupy less mass in the composite,thus maintaining an overall higher enthalpy.(2)The carbon aerogel-derived composite phase change material(CCA/PEG)was prepared by vacuum impregnation of polyethylene glycol(PEG)with carbon aerogel.The morphology,composition,thermal properties and micromechanical properties were characterized by SEM,FT-IR,differential scanning calorimetry(DSC),thermogravimetric analyzer(TGA)and atomic force microscopy(AFM).By leakage experiments with infrared imaging,it was verified that the composites have good conformal properties and high thermal conductivity.By electrothermal conversion experiments,the highest electrothermal conversion efficiency of CCA/PEG with 15 wt%crosslinker ratio can reach 55.6%at 1.4 V.In the photothermal conversion experiments,CCA/PEG with different crosslinker ratios showed high photothermal conversion ability.Under the simulated solar irradiation of 1500 W/m~2,the PEG could be completely phase transformed and warmed up to 70℃after 20 minutes,while the pure PEG could not complete the phase transformation process under the same conditions and only warmed up to 45℃.(3)To further improve the electrothermal energy conversion efficiency,carbon nanotubes(CNTs)treated with strong acid were introduced into the system as highly thermally conductive fillers.As CNTs increase the number and density of highly thermally conductive channels inside the composite,the sample with CNTs doping ratio of 0.75 wt%achieves the highest electrothermal energy conversion efficiency of 87.6%at 1.9V.