Thermal Storage Performance And Preparation Of Shape Stabilized Phase Change Material With High Thermal Conductivity Based On Chitin Derived Carbon

Organic phase change material(PCM)is widely used in latent thermal storage technology owing to its high thermal storage density,low supercooling degree,non-toxic and noncorrosiveness.Nevertheless,lower thermal storage capacity and thermal response is existed in organic PCM due to melting leakage and low thermal conductivity.Encapsulation of organic PCM in porous carbon with high thermal conductivity is an efficient method to solve the above problems.Chitin is an N-contained natural polysaccharide mainly in crustaceans,insects,and fungus,which could obtain in situ N-doped carbon after carbonization in nitrogen atmosphere.In this thesis,N-doped carbon derived from chitin and stearic acid(SA)are used to prepare shape stabilized PCM(SSPCM)by the "physical blending-vacuum infiltration" method.Thermal storage performance of SSPCM,including phase change enthalpy,phase change temperature,thermal conductivity,et al.,is evaluated by Differential scanning calorimeter(DSC)and Thermal constant analyzer.Meanwhile,Automatic specific surface area analysis(Nitrogen isothermal adsorption-desorption),Fourier transform infrared spectrometer(FTIR),X-ray powder diffraction analysis(XRD),Raman spectrometer(Raman),Scanning electron microscopy(SEM)are conducted to elucidate the interaction between N-doped carbon and SA,revealing the mechanism of support influence on thermal storage performance.(1)Chitin is carbonized in nitrogen atmosphere to obtain in situ N-doped carbon(CNX).Influence of carbonization temperature on the physicochemical properties of CNX including pore structure,surface functional group,and crystal phase are investigated.Thermal properties(phase change enthalpy,phase change temperature,supercooling degree and thermal storage efficiency),thermal conductivity,and shape stability are used as the index of thermal storage performance,which is analyzed the CNX physicochemical property impact on SA.Results show that synthesized CN800 at a carbonization temperature of 800℃ possesses developed pore structure and abundant N-doped surface,producing capillary force and hydrogen bond for enhanced shape stability of SA.Prepared CN800/SA exhibits the melting/freezing enthalpy are 104.63 J/g and 101.82 J/g,thermal storage efficiency is 92.84%.Supercooling degree of CN800/SA is decreased from 3.52℃ to 1.06℃,and thermal conductivity is elevated from 0.180 W/(m·K)to 0.461 W/(m·K),in comparison to SA.Good thermal stability is observed in CN800/SA without significant mass loss in the range of melting/freezing temperature(66.28℃to 67.34℃),and stable thermal properties,chemical structure,and crystal phase are sustained in CN800/SA even after 200 thermal storage/release cycles.(2)Modified N-doped carbon(ACNX)is constructed by CoCl2/ZnCl2,which is used to prepare SSPCM(ACNX/SA).Evolution of ACNX physicochemical properties under CoCl2/ZnCl2 matrix is explored to analyze ACNX impact on ACNX/SA thermal storage performance.It is concluded that a large amount of micropores is produced in support owing to the ZnCl2 volatilization during carbonization process;meso-macropore ratio and graphitization degree of support is elevated by Co elemental reduced from CoCl2.Specific surface area and average pore diameter of ACN2 are 413.33 m2/g and 9.94 nm,respectively,total pore volume is high as 1.027 cm3/g,which produces enough capillary force and storage space to enhance SA shape stability.ACN2/SA possesses excellent shape stability at 80℃,its melting/freezing enthalpies are 117.49 J/g and 114.26 J/g,respectively.In comparison to SA,the supercooling degree of ANC2/SA is decreased to 0.35℃ about 90%,thermal conductivity is elevated to 0.407 W/(m·K)about 126%.In addition,ACN2/SA also has ideal thermal stability and reliability,which photothermal conversion efficiency is 71.2%,corresponding thermal response time is 78 s faster than SA.(3)N-doped carbon enhanced by expanded graphite(ECNX)is synthesized by physical addition expanded graphite.Then,SSPCM(ECNX/SA)is prepared to further enhance the thermal conductivity of SA.It is found that hierarchical with micro-meso-macropore are generated in ECN15 when doped 15 wt.%expanded graphite,which is recognized as the reason for the enhanced shape stability of SA.ECN15/SA exhibits melting/freezing temperatures of 121.59 J/g and 119.80 J/g,supercooling degree of 0.62℃.Thermal conductivity of ECN15/SA is high as 1.573 W/(m·K)owing to that expanded graphite constructs efficient phonon transfer and decrease phonon scattering.Meanwhile,thermal stability of S A is enhanced by ECN15 that mass loss temperature of ECN15/SA is elevated 4℃ than SA.Chemical structure and crystal phase of ECN15/SA maintain stable when after 200 thermal storage/release cycles.Also,the photothermal conversion efficiency of ENC15/SA is high as 89.2%,corresponding thermal response time is 78 s faster than SA.