Preparation And Thermal Characterization Of Carbon Wood-based Composite Phase Change Materials With Shape-stabilized And High Thermal Conductivity

Organic phase change materials(PCMs)as a strategy for thermal energy storage has attracted widespread attention in recent years by virtues of its high latent storage capacity,suitable phase change temperature,chemical and thermal stability,non-toxicity non-toxic and non-corrosive,low cost,and chemical stability properties.They can store and release energy in the form of phase change latent heat,and play an important role in the field of new energy development and utilization and thermal energy storage.However,the leakage problem and low conductivity of organic phase-change materials during the phase-change process hinder their practical application.Currently,polyethylene glycol(PEG)is the most widely produced and used organic phase change material,but also suffers from the problems of leakage and low thermal conductivity.Therefore,preparing the shape-stabilized composite PCMs with high thermal conductivity have become a research hotspot.Wood is commonly characterized by virtues of sustainable,hierarchical porous structure,renewable,and environmentally friendly,thereby important research significance for increasing its added value and functionalization utilization.In this study,poplar wood was used as the raw material and the supporting material was obtained by simple high temperature carbonization,the PEG was used as the phase change matrix.The carbon wood-based composite PCMs were prepared by using the vacuum impregnation method.Firstly,adjusting the conditions of carbonized temperature and carbonized rate to understand the influence of the structure of the supporting material on the composite PCMs;Then,the nano-sized carbon dots(CDs)are commonly possessed of multiple functional groups,which is beneficial to form interaction with other materials and afford unexpected performances of CDs-contained composites,thus the effect of CDs-contained carbonized wood as supporting materials for constructing composite PCMs was analyzed.Finally,to further obtain a high thermal conductivity and morphological stability of the composite PCMs,the raw poplar woods were immersed in the Cu Cl₂ 2H₂O solution,and the carbon wood supporting material loaded with copper particles was obtained by high temperature carbonization,which can enhance the thermal conductivity of composite PCMs because copper itself has a high thermal conductivity.The results are as follow:(1)The structure of the carbonized wood was easily affected by the temperature and rate of carbonization.Typically,increasing carbonization temperature and decreasing carbonization rate can obtain larger specific surface area and pore size suitable for PEG encapsulation.The three-dimensional porous carbonized wood had the largest specific surface area(598.19 m²/g)and the average pore size was 3.25 nm with strong capillary force when the temperature was slowly heated up to 250°C at a heating rate of 3°C/min,after incubating for 2 h,the temperature was raised to1000°C at 5°C/min and then maintained for 2 h.At these conditions,the supporting material not only reduces the amount of mass loss of PEG,but also improves the thermal conductivity of the composite phase change material.Finally,the minimum mass loss percentage of the composite phase change material was 6.9%after leakage test,while the composite exhibited high thermal conductivity(0.434 W/m K),showing 107%higher than that of pristine PEG.Other physical and chemical properties confirmed that the carbonized wood was excellent supporting material,which was beneficial for PEG encapsulation.Meanwhile,the carbonized wood-based composite phase change material exhibited excellent latent heat and high energy storage efficiency(99.8%),the melting and crystallization enthalpy were 130.5 J/g and 126.3 J/g,respectively.(2)Further,the CDs-contained carbonized wood was used as the supporting materials for constructing composite PCMs with shape-stability and excellent thermal performances.The surface of CDs contained abundant oxygen functional groups and with an average diameter of about 2.4 nm,and the effects of CDs loading on the pore structure and chemical components of carbonized wood were discussed.The results showed that the concentration of CDs was 3.8 mg/m L,the prepared composite possessed excellent shape-stabilized property even after long period utilization.It is reasonable to deduce that PEG molecules can be anchored well into the CW-based framework via H-bonding interactions with the surface hydroxyl and amine functional group on the CDs.Meanwhile,the porous structure also can prevent the leakage of PEG in the melting process through capillarity and surface tension.In addition,the thermal conductivity value was0.449 W/m K,which was 114%higher than that of the pure PEG.The thermal properties showed that the melting and crystallization enthalpy of CDs-contained phase change material respectively were 112.4 J/g and 110.8 J/g,which was beneficial to the storage and release of thermal energy.(3)In order to further improve the thermal conductivity of composite PCMs,poplar wood was soaked with different concentrations of Cu Cl₂·2H₂O solution.Then,the high thermal conductivity carbonized wood modified with copper particles was obtained by high temperature reduction,the chemical composition showed that the copper particles were mainly in the form of copper monomers or copper oxides,which could be uniformly distributed in the pore structure of carbonized wood.The composite phase change material obtained by using Cu-loaded carbonized wood as supporting material had not only good morphological stability but also a high thermal conductivity(0.991 W/m K),which 338%higher than that of the pure PEG.Meanwhile,the supercooling temperature of composite was lower than that of the pure PEG(by 7.8°C).The thermal properties showed that the melting and crystallization enthalpy of Cu-loaded phase change material respectively were 97.3 J/g and 96.5 J/g,and the energy storage efficiency was 100%.Moreover,the composite PCMs also exhibited good thermal cycling and thermal stability properties.