Study On Preparation And Properties Of Cellulose-based Photothermal Conversion Materials

Solar desalination technology has provided new opportunities for solving the global water problem.The key to solar desalination technology lies in photothermal conversion materials(PCMs).The application of PCMs to desalination can greatly improve the efficiency of desalination and provide favorable conditions for the production of freshwater.At the same time,this process uses clean and efficient solar energy which is environmentally friendly.At present,scholars have made great achievements in the research of PCMs.However,due to the limitations of structural rationality,such materials have not been able to achieve large-scale application.Therefore,this project have prepared PCMs with cellulose nanofiber(CNF)as raw materials and resin as adhesives.Several kinds of PCMs with different structures,including carbon-based PCMs with CNF as carbon precursor,heterogeneous CNF-based PCMs,carbon black/CNF composite PCMs and carbon black/CNF/hardwood fiber composite PCMs,were designed and fabricated to optimize structure and increase the efficiency of photothermal conversion and water evaporation.The specific research contents are as follows:First,carbon-based PCMs with CNF as carbon precursor were prepared by using CNF and the mixture of epoxy and polyamide resin as raw materials through a series of processes such as mixing,stirring,injection molding,curing,and carbonation.It was found that the material had a highly porous structure,excellent light absorption performance,a total solar energy absorptivity of 94.12%,good thermal insulation performance,and a thermal conductivity coefficient of 0.045 W·m-1·K-1.In addition,the material was irradiated at one sun illumination for 30 min,and the evaporation rate could reach 1.20 kg·m-2·h-1.The evaporation efficiency was up to 84.61%,which indicated that the material had good stability and can be reused for a long time.Then,in order to achieve better thermal management,the method of surface carbonization was used to prepare heterogeneous CNF-based PCMs.The material has the heterogeneous structure with double-layer(the light absorbing layer and the carrier layer)with the same source(CNF).Meanwhile,the material had a porous structure that penetrated up and down,and there was almost no clear dividing line between the upper and lower layers.The total solar energy absorptivity could reach 92.15%.The lower uncarbonized material was very hydrophilic and could reasonably transport water.It had excellent thermal insulation performance and a thermal conductivity of 0.052 W·m-1·K-1.The material was irradiated at one sun illumination for 30 min,the evaporation rate could reach 1.27 kg·m-2·h-1,and the evaporation efficiency could reach 85.42%.Next,the carbon black/CNF composite PCMs was prepared,using carbon black as the light absorbent.The influence of the amount of carbon black on the structure and properties of the PCMs was investigated.With the increasing of carbon black,the pore density of the material increased first and then decreased,the adiabatic performance became better and then became worse,the hydrophilicity and absorption property of the material kept increasing,and the evaporation rate of the material increased rapidly first and then slowed down.When the amount of carbon black reached 1.0 wt.%,the carbon black/CNF PCMs had the best comprehensive performance.The pore density of carbon black/CNF composite PCM was the largest,and the total solar energy absorptivity was 92.21%.Meanwhile,the hydrophilicity was kept,and the thermal conductivity was 0.053 W·m-1·K-1.Moreover,the evaporation rate was the largest,and the evaporation efficiency was 79.23%.Finally,in order to further reduce the cost and optimize the structure of the PCMs,carbon black/CNF/hardwood fiber composite PCM was prepared by mixing hardwood fibers and CNF.By investigating the influence of the ratio of the two fibers on the structure and performance of the PCM,it was found that with the increase of the amount of hardwood fiber,the pore diameter of the material decreased first and then increased,the pore density increased first and then decreased,and the light absorption performance gradually decreased,the hydrophilicity became worse,the adiabatic performance became worse first and then became better,the evaporation rate increased first and then decreased.When the amount of hardwood fiber was 50%,the carbon black/CNF/hardwood fiber composite PCM had the larger pore density,the total solar energy absorptivity could reach 90.08%,the hydrophilicity was relatively good,and the thermal conductivity was 0.051 W·m-1·K-1,the evaporation rate was the largest,and the evaporation efficiency was 81.34%.At this time,the comprehensive properties of the material were better.In this paper,several different PCMs were prepared by different methods to solve the problems of reasonable structure of PCMs,which provided a new idea for the research of PCMs and the development of solar desalination technology.