Preparation And Properties Of Aramid Nanofiber-based Phase Change Composite Materials

Phase change materials can store and release energy through the phase change process,holding advantages of high energy density,reversible phase change,and small temperature fluctuations.They have broad application prospects in solar energy utilization,electric power "peak to valley",waste heat recovery,building energy conservation,thermal management of electronic components,etc.Among various phase change materials,organic solid-liquid phase change materials have attracted widespread attention due to moderate phase change temperature,small volume change,and high energy storage density.However,phase change leakage,low thermal conductivity,and limited energy resource greatly restrict their application and energy storage efficiency.Encapsulation is an effective means to solve the leakage problem.Pickering emulsion is a common method to prepare microcapsules.Herein,the phase change material-loaded microcapsules were prepared by Pickering emulsion,where aramid nanofibers(ANF)were adopted as Pickering emulsion stabilizers and shell materials.The microcapsules were further freeze-dried to prepare three-dimensional porous phase change materials,and the encapsulation methods with ANF as the substrate were established.Then,phase change films with flexibility,leakage-proof,high strength,and good temperature control ability could be efficiently prepared through optimization.Finally,functional fillers with thermal conductivity and light absorption were introduced into the encapsulating system to construct phase change films with high photothermal,electro-thermal properties and high thermal conductivity.The specific contents include the following points:In view of the leakage problem of PCM,aramid nano fibers with a high aspect ratio and moderate hydrophilicity were used as stabilizers to encapsulate phase change materials(hexadecanol,HD),and HD@ANF Pickering emulsions were successfully obtained.Then,the bulk HD/ANF porous phase change materials were prepared by freeze-drying the HD@ANF emulsions,benefiting from the strong hydrogen bonds and excellent film-forming capacity of ANF fibers.The results showed that ANF had a good emulsifying and stabilizing capacity to phase change material(HD).HD@ANF emulsions could keep their original microcapsule structure after being placed at room temperature for 10 days.The three-dimensional network skeleton of ANF could effectively prevent leakage during the melting of hexadecanol.The HD/ANF porous phase change materials with 50%～70%HD content showed good shape stability during solid-liquid phase transition.The melting enthalpy and the freezing enthalpy reached 139.7 J/g,and 139.5 J/g with HD core content(70.2%).HD/ANF porous phase change materials showed excellent temperature regulation and cycle stabilities,which had great potential in hermal management and infrared thermal camouflage.Then,the flexible phase-change energy films were further prepared via directly filtered HD@ANF emulsions and drying at high temperatures because of the excellent film-forming performance,high strength and modulus of aramid nanofibers.As a result,the preparation efficiency and thermal conductivity of HD/ANF composites were significantly increased.It is demonstrated that HD/ANF phase change films with hexadecanol content of 50%～70%exhibited good shape stability and leak resistance during solid-liquid phase transition.HD/ANF phase change films had good heat storage capacity and thermal reliability.The melting enthalpy and freezing enthalpy could reach 109.1 J/g and 106.6 J/g with HD core content(68.6%).The retention rates of melting enthalpy and freezing enthalpy were as high as 97.3%and 99.0%after 50 heating and cooling cycles,respectively.Meanwhile,the HD/ANF phase change film with a core content of 70%had excellent temperature regulation properties,which could significantly improve the surface temperature fluctuations.In addition,the HD/ANF films with high HD content have good mechanical properties and flexibility.The tensile strength was 24.9 MPa,the modulus was 457.1 MPa,and the strain at break was 34.1%,which could adapt well to different application scenarios.To expand energy sources and increase thermal conductivity,we firstly attampted to introduce carbon nanotubes(CNT)with electrothermal,photothermal,and high thermal conductivity to the ANFs/HD composite.Simultaneously constructing conductive and thermal channels,and expanding heat sources to improve the energy storage/release rate of phase change films.The results showed that the enthalpy of melting and freezing of 70%HD/ANF/CNT composite films were 128.0 J/g and 130.2 J/g.CNT endowed the phase change films with excellent photothermal properties.The surface temperature of films could reach 80℃ after 1 minute of irradiation with a 100 W infrared lamp,and could reach around 120℃after 5 minutes.However,due to the inferior dispersion of CNT in ANF,the electrothermal performance of composite film was unsatisfactory.We further adopted MXene as a filler for electrothermal and photothermal conversion.Fortunately,the obtained HD/ANF/MXene composite phase change films exhibited excellent photothermal absorption ability and cyclic stability performance.The film with only 3 wt%MXene could reach 81.8℃ within 60 s under simulated sunlight irradiation,and could maintain a stable temperature after 20 cycles.At the same time,when the MXene content was higher than 6 wt%,the HD/ANF/MXene phase change films exhibited excellent Joule thermal performance and electrothermal stability at a safe voltage of 15-25 V.Moreover,MXene improved the thermal conductivity of the composite films.The maximum endothermic enthalpy and exothermic enthalpy of HD/ANF/MXene films reached 115.0 J/g and 112.6 J/g respectively.The DSC curve of the films did not change during 50 cycles of heating and cooling,verifying the excellent cycle stability of the HD/ANF/MXene phase change films.The final product has great potential application in comprehensive utilization of waste heat,and off-peak power energy storage.