Study On Bamboo Hydrothermal Carbon Based Photo-thermal Composite Phase Change Materials

With the development of society,the issue of energy shortage is becoming increasingly prominent,and there is an urgent need to develop new technologies for efficient energy utilization.Solar energy is a renewable and clean energy source,which is mainly converted into thermal or electrical energy for utilization through light-heat,light-electricity,and light-chemical conversion.Phase change materials(PCMs)can store and release a large amount of thermal energy through their phase change behavior which is used for the thermal conversion and storage of solar energy.However,the defects of solid-liquid phase change materials such as liquid leakage,low thermal conductivity,and low photo-thermal conversion efficiency still need to be further studied and resolved.To solve the above problems,in this thesis,bamboo was used as a direct raw material,and hydrothermal carbon nanospheres(HCS)with mesoporous structures were prepared through a hydrothermal carbonization process.Composite PCMs with high thermal conductivity were prepared by vacuum impregnation using carboxylated carbon nanotubes(CNTs)as thermal conductivity reinforcing materials and polyethylene glycol(PEG)as phase change media.Finally,the composite PCMs were electrochemically treated with metallic silver using a high-voltage electrostatic field,and nano-Ag particles were loaded on the surface of the PCMs to improve the photothermal conversion efficiency of the composite PCMs.The specific research content is as follows:(1)In order to solve the liquid leakage problem of PCMs,it is necessary to develop excellent skeleton materials to encapsulate it.Different from traditional high-temperature tube furnace carbonization,this article used a relatively mild hydrothermal carbonization technology,utilizing the characteristics of bamboo parenchyma cells(PC)that are rich in cellulose and hemicellulose,and have thin walls that are easily soluble.The acidic Li Br was used as a solvent,the hydrothermal carbonization temperature was controlled below 200~oC to prepare porous carbon spheres for packaging PEG.The results showed that the yield of carbon spheres increased with the increase of hydrothermal time and Li Br solution concentration,and showed a trend of first increasing and then decreasing with the increase of hydrothermal temperature and raw material quality.Mesoporous carbon spheres with an average particle size of 410.62 nm and a yield of 39.75%were obtained by hydrothermal reaction of 4 g of bamboo PC in a 65 wt%Li Br solution at 200~oC for 5 h.The BET test showed that the average pore diameter of HCS was about 8 nm,and the specific surface area increased with the extension of hydrothermal time.The specific surface area of HCS-5h was 702.254 cm~2/g,and the surface was rich in free hydroxyl groups.It is easy to bind with PEG through hydrogen bonds,which is conducive to improving the shape stability of composite PCMs.(2)In this paper,the reaction formation process of bamboo hydrothermal carbon has been deeply studied.The experimental results showed that the presence of Li Br can play a role in destroying the crystal matrix in the raw material.Cellulose and hemicellulose were successfully dissolved under the action of Li Br and were hydrolyzed to glucose and xylose under acidic conditions.Lignin was removed in the form of solid precipitation.Subsequently,the monosaccharide was completely hydrolyzed to produce 5-HMF,which ultimately undergoes intermolecular dehydration and aggregation of furan compounds to form the target product HCS.(3)Bamboo-based HCS were used as substrates to prepare a high thermal conductivity composite skeleton material for packaging PEG by loading carboxylated CNTs.Through SEM observation,carboxylated CNTs formed a densely interconnected network structure on the surface of HCS.After vacuum impregnation of PEG 10000,PEG successfully filled the pore structure of HCS.The loading of CNTs was through the formation of hydrogen bonds between O in CNTs-COOH and H in the free hydroxyl groups on the surface of HCS.The main interaction between PEG10000 and the skeleton material was the physical binding force.HCS-CNTs bind to PCMs through hydrogen bonding force,capillary force,surface tension,etc.(4)The leakage test results showed that the encapsulation efficiency of HCS and HCS-CNTs for PEG can reach 85%,and the shape remained stable after 1 h of heating treatment.HCS-CNTs-3%/PEG-85%had excellent thermal properties,with melting and crystallization latent heats of 159.1 J/g and 144.0 J/g,respectively.The thermal conductivity was 0.585 W/m K,which was 127%higher than pure PEG 10000.TGA results showed that both HCS/PEG-85%and HCS-CNTs-3%/PEG-85%had excellent thermal stability and were suitable for thermal energy storage at 0-100~oC.The heating/cooling cycle experiment showed that after 1000 heating/cooling cycles,the melting and cooling latent heat of HCS-CNT-3%/PEG-85%decreased by 1.13%and0.97%,respectively,meeting the application requirements.(5)By treating HCS-CNTs/PEG with a high-voltage electrostatic field and using a metal Ag sheet as the negative electrode,nano-Ag particles were successfully loaded on the surface of composite PCMs to improve the photothermal properties of the composite.EDS scanning results showed that the 80 k V/24 h treatment group had the highest Ag element content on the surface of the sample,with a mass fraction of 7.04%of nano-Ag particles.During the in-situ migration process,metal Ag mainly interacted with HCS and CNTs COOH in composite PCMs,and the adsorption sites were mainly oxygen-containing active groups such as hydroxyl,hydroxymethyl,and carboxyl groups.The photothermal conversion experiment showed that the photothermal conversion efficiency of Ag-HCS-CNTs/PEG PCMs prepared by the 80 k V/24 h treatment group was as high as 110.6%,the thermal conductivity was 0.701 W/mK.