| Phase change thermal storage technology,achieving thermal storage and release by utilizing the latent heat variation in phase transition process,has been extensively applied in energy conservation and emission reduction industries,such as,building energy conservation,solar thermal power generation,and waste heat recycle,etc.In order to meet requirements of engineering application,form stable phase change material(fs-PCM)composites draw attentions and are developing for improving thermal storage capability and thermal conductivity.Graphite as a natural mineral with unique layer structure and excellent thermal conductivity,has been used in preparing PCM composites for thermal energy storage.However,focusing on excellent thermal storage properties,the thermal storage characteristics of graphite should be excavated and promoted further,for enhancing performances of graphite-based PCM composites and building a relationship between structure and performance enhancement.In this thesis,there explored thermal storage characteristics of graphite in the aspects of surface,interlayer,and interface,to obtain supporting materials with evident characteristics.Then,the PCM was absorbed into supports to prepare graphite-based PCM composites for thermal energy storage to achieve the controlling and strengthening of thermal storage performances.(1)It proposed a new method of surface modification of graphite for enhancing thermal storage performances.Flake graphite(FG)was modified surfacely by microwave-H2O2 treatment and then absorbed stearic acid(SA)to prepare form stable PCM composites for thermal energy storage.It proved that surface modification increased special surface area of supporting material to provide more absorption sites for SA through increasing inner defects and surface roughness,the obtained composites(SA/FG3)has maximum loading capability of 32.40%,latent heat in melting and freezing of 61.05 and 61.00 J g-1,respectively,and thermal conductivity of 12.23 times than that of pure SA;After modification,the quantity of surface oxygen functional groups was increased,which could form a stable bridging between SA and supporting materials.The melting latent heat of SA/FG3 was 99.98%after 100 thermal cycling tests.(2)It revealed the mechanism of interlayer stripping of flake graphite for enhancing thermal storage performances.Natural flake graphite was stripped through a coupled ultrasonication-milling process followed by a shear-assisted supercritical CO2 stripping,to obtain untrathin graphite sheets(UGS)with different stripping degree,and then the graphite sheets were used to absorb SA for preparing PCM composites for thermal energy storage.The UGS-5000 samples with higher stripping degree possesses a layer thickness of 3.4-4.2 nm,and loose structure and more defects;it was obtained that composite sample of SA/UGS-5000 has advantages on loadage(63.12%),crystallization(94.02%),and energy storage density(180.14 J g-1).The corresponding thermal conductivity is 10.08 times than that of pure SA.Through investigating the influence of layer stripping on thermal storage performances,it tested that the higher stripping degree can cause a higher loading capability,building a relationship between stripping layer thickness and thermal energy storage performances.(3)It illuminated the synergetic effect of interfacial coupling based on graphite for enhancing thermal storage performances.Carbon nanofibers(CNF)was grew on the surface of flake graphite and then were coupled to bentonite(Bm)through"chemical bonding" to build a ternary structure.The as-prepared supporting material(FG-CNF-Bm)with ternary structure absorbed SA to prepare form stable PCM composites for thermal energy storage.The 3D network constructed by CNF can provide more loading space for SA during impregnation,causing a 41.90%loading capability of fs-PCM3 composite sample.Coupling process can reduce interfacial thermal resistance,so that the coupled supporting material of FG-CNF-Bm(4.595 W m-1 K-1)had a higher thermal conductivity than that of no coupled support of FG-CNF/Bm(4.291 W m-1 K-1);fs-PCM3 had excellent performances including crystallization(98.56%),melting latent heat(79.13 J g-1),and thermal conductivity which is 10.50 times higher than that of pure SA;A synergetic improvement of thermal storage performances was achieved by enlarging loading space and reducing interfacial thermal resistance.It verified the strengthening mechanism of interfacial coupling.Based on the interdiscipline between energy,materials and mineralogy,thermal storage characterizations of graphite were excavated,mineral characteristics were exerted,and the final graphite-based PCM composites were prepared.It built relationships between thermal storage performances and three aspects of graphite including surface,interlayer,and interface for controlling and strengthening thermal storage performances.It also provided new ideas for preparing high performances of form stable PCM composites for thermal energy storage. |
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