Preparation And Properties Of Erythritol Confined In Carbon Nanotubes Reinforced Silica Aerogel As Novel Form-Stable Phase Change Materials

Erythritol(ME)has ultrahigh latent heat and can be applied as medium temperature phase change material(PCM).It has broad applications in the fields of solar thermal utilization,peak loading shifting and waste heat recovery.However,it also has some problems,such as high and unstable supercooling,potential leakage and low thermal conductivity.Silica aerogel has high porosity and can be applied as a good supporting material to adsorb ME to prepare form-stable PCMs(FS-PCMs)to solve its leakage problem.However,the preparation conditions of silica aerogels are harsh,while the prepared aerogels are generally fragile and the thermal conductivity is ultralow.In order to solve the above problems,in this thesis,multi walled carbon nanotubes(MWCNTs)were used to reinforce the backbone and enhance the thermal conductivity of silica aerogels.A series of silica aerogels reinforced by MWCNTs with different surface properties were prepared by sol-gel method and freeze drying,and FS-PCMs were the obtained by vacuum impregnation of ME.In addition,the properties of the prepared aerogels and FS-PCMs were comprehensively characterized,and the effects of the MWCNTs surface characteristics on the properties of the aerogels and FS-PCMs were explored.The research work of the thesis mainly includes the following three parts:(1)Pristine MWCNTs(MCNTs)enhanced FS-PCMs.MCNTs/Silica composite aerogels were prepared by in-situ sol-gel and freeze drying with commercially obtained MCNTs as filler and tetraethyl orthosilicate(TEOS)as silicon source.FS-PCMs were then obtained by vacuum impregnation of ME.The characterization results showed that when the amount of MCNTs was not more than 30 mg(3.3 wt%of total aerogel mass),the loading of ME in the FS-PCMs increased with the increasing of the MCNTs dosage,while further increasing the MCNTs dosage would reduce the loading of ME in the FS-PCMs.The aerogel doped with 30 mg MCNTs could adsorb 1.5 times ME of its own mass,and the melting enthalpy of the prepared FS-PCM(MFPCM-3)was 197.8 J/g,which is significantly higher than that of the FS-PCMs without MCNTs(50.1 J/g).Meanwhile,the solidification temperature also increased to 49.3℃.In addition,its thermal conductivity attained 0.50 W/mK,which is 55%higher than that of pure me(0.32 W/mK).(2)Mixed acid modified MCNTs(ACNTs)enhanced FS-PCMs.MCNTs was treated by mixed acid to prepare ACNTs with surface-anchored polar groups such as hydroxyl group and carboxyl group.Then ACNTs enhanced silica aerogels and FS-PCMs were prepared.The polar groups on the surface of ACNTs enhanced the dispersion of ACNTs and the interaction with silica particles,which resulted in that the loading of ME in the prepared aerogels was significantly increased.The optimum dosage of ACNTs was 60 mg(6.6 wt%of aerogel mass)while the obtained aerogel could adsorb 5 times ME of its own mass.The corresponding FSPCM(AFPCM-6)possessed a very high latent heat of 297 J/g,a further increased solidification temperature of 56℃,and good long-term melting/solidification stability.The thermal conductivity of AFPCM-6,however,was 0.44 W/mK,which is lower than that of MFPCM-3,due to the defects of ACNTs.(3)Silane modified MCNTs(Si-CNTs)enhanced FS-PCMs.HNO3 treated MCNTs were silane modified by TEOS to obtain Si-CNTs,and then Si-CNTs enhanced silica aerogels and FS-PCMs were prepared.The results showed that the interactions between Si-CNTs and silica particles were further enhanced,thus effectively increasing the porosity of the prepared aerogels and increasing their adsorption capacity to ME.When only 20 mg Si-CNTs was added,the prepared silica aerogel could adsorb nearly 4 times ME of its own mass,and the Si-CNTs dosage had no obvious effect on the adsorption capacity of ME.The latent heat of the FS-PCM with 20 mg Si-CNTs(SiFPCM-2)attained 270 J/g while the solidification temperature was raised to 61℃,and could be maintained basically stable during 100 melting/solidification cycles.In addition,its thermal conductivity attained 0.49 W/mK.Based on the above results,it can be concluded that the better the dispersibility and the less the defects of MWCNTs,and the stronger the interactions between MWCNTs and silica particles,the easier the formation of effective interpenetrating carbon nanotubes network in the composite aerogels,the better the enhancement of the skeleton stability of the composite aerogels and the higher the thermal conductivity of the FS-PCMs.At the same time,stable aerogel skeleton is also conducive to the stable and long-term suppression of the supercooling degree of ME in the FS-PCMs.