【作者】 张磊；

【导师】 朱教群；

【作者基本信息】 武汉理工大学 ， 材料学， 2012， 博士

【摘要】 相变储热技术具有储热密度大和储热过程近似恒温的优点,在太阳能热利用、电子器件热保护以及建筑节能等方面具有广阔的应用前景。聚乙二醇(PEG)是目前最受关注的相变材料之一,具有相变潜热大、相变温度范围广、物化性能稳定和安全环保等优点,但其在实际应用中仍存在导热性能差和液相泄漏问题。本文提出采用材料复合技术对其进行改性,筛选两类不同结构特征的石墨材料为导热强化相,分别制备了PEG/膨胀石墨(EG)和PEG/纳米石墨片(GnPs)复合相变材料；研究了复合材料组成、结构与热物性能之间的关系及其动力学机理；在此基础上设计并制备了新型复合定形相变材料；同时对所制备复合材料的相变传热过程进行了实验测量研究,基于传热性能评价结果反馈指导了复合相变材料的组分优化设计。以多孔结构EG为导热强化相,采用真空浸渗工艺制备了PEG/EG复合相变材料,综合运用FE-SEM、XRD、FT-IR、POM和DSC等手段研究了复合相变材料的形貌、结构和热物性能。结果表明：EG丰富的孔结构对液态PEG具有较强的吸附固定作用,当EG含量为8wt.%时可以获得定形相变材料；随着EG含量的增加,PEG基体中逐步形成了完善的导热网络结构,复合相变材料的导热系数逐渐增大,EG含量为10wt.%的复合相变材料导热系数较纯PEG提高了19.4倍；但是当EG含量超过6wt.%后,其微孔结构对PEG分子链段热扩散运动的阻碍作用加剧,导致复合相变材料相变温度和相变焓的显著下降,EG含量为10wt%的复合相变材料熔点(Tm)和凝固点(Tf)分别由纯PEG的50.9和35.7℃降低至41.8和21.5℃,而其融化焓(ΔHm)和凝固焓(ΔHf)则分别仅为理论值的67.4%和73.6%。超声粉碎处理EG获得了形状比为300～800倍的GnPs,以之为导热强化相,采用真空浸渗工艺制备了PEG/GnPs复合相变材料,并对其形貌、结构和热物性能进行了研究。结果表明：超声粉碎处理对石墨物相结构及表面化学特性基本没有影响；GnPs较大的形状比令其较易均匀分散在聚合物基体中并形成导热网络,GnPs含量为10wt.%的复合相变材料导热系数较纯PEG提高了10.8倍；与PEG/EG复合相变材料相比,PEG/GnPs复合体系的相变温度变化更小,相变焓更接近其理论计算值。针对不同结构特征石墨材料对PEG相变参数影响的差异性,采用DSC研究了纯PEG及复合相变材料的非等温相变过程动力学。结果显示：复合相变材料的表观活化能均高于纯PEG,表明EG与GnPs对PEG分子链段热扩散运动均具有一定的限制作用；与GnPs相比,相同质量分数的EG使复合体系表观活化能的增加幅度更大,表明EG微孔结构较GnPs片层结构对PEG分子链段热扩散运动的阻碍作用更强。基于相变动力学分析结果,选取GnPs为导热强化相,以聚甲基丙烯酸甲酯(PMMA)为结构支撑材料,采用超声辅助原位聚合工艺制备了新型PEG/PMMA/GnPs复合定形相变材料。FE-SEM与POM结果显示,PEG被均匀吸附固定在PMMA网络结构中,该复合方式在保证复合定形相变材料相变过程中无液态PEG泄露的同时为其提供了一定的力学性能。XRD与FT-IR结果表明,各组分在复合工艺过程中以物理形式结合。超声辅助原位聚合工艺能使GnPs有效分散在聚合物基体中并形成导热网络,GnPs含量为8wt%的复合定形相变材料导热系数较PEG/PMMA提高了8.4倍。所制备复合定形相变材料均具备可观的储热能力和定形性能,当GnPs含量为8wt%时,复合定形相变材料的ΔHm和ΔHf分别达到114.7kJ·kg-1和97.0kJ·kg-1,55℃时其抗压强度达到3.7MPa,75次相变循环后质量损失率仅为2.5%。采用时间-温度法对纯PEG及PEG/EG复合相变材料的相变传热过程进行了实验研究,基于实测结果评价了复合改性对相变材料使用性能的影响。结果表明：纯PEG在相变传热过程中存在显著的自然对流效应；多孔结构EG能有效吸附固定液态PEG,限制自然对流作用；EG能显著提高复合相变材料的导热系数,降低相变传热过程中的热阻,有效提高复合相变材料的相变传热速率；PEG/EG复合相变材料中,EG的优化含量约为6～8wt.%。综上所述,本论文分别开展了PEG基复合相变材料的设计与制备,复合材料的微观结构、热物性能和相变动力学机理等方面的研究。研究结果对改善PEG使用性能并促进其实际应用具有一定的参考价值；其中关于PEG/石墨复合相变材料相变动力学机理的研究对理解复合体系中的非常规相变机制具有较大的帮助；另外,关于复合材料相变传热过程的实验研究能有效评价复合改性的效果,可用于指导储热材料的性能调控与优化设计。更多还原

【Abstract】 Phase change thermal energy storage technique has great potential in many fields such as solar thermal application, thermal management of electronic equipment and building energy conservation owing to its superior advantages of large heat storage capacity and nearly isothermal phase change behavior. Polyethylene glycol (PEG) is one of the most preferential phase change materials during the current research and applications due to its superior advantages such as high latent heat, wide range of phase change temperatures, stable in physical and chemical property, security and environmental protection. In this work the technology of materials compositization was proposed to overcome the disadvantages of PEG including low thermal conductivity and liquid leakage during the phase change process. Expanded graphite (EG) and graphite nanoplatelets (GnPs), with different types of structural characteristics, were employed as thermal conductive filler, and the PEG/EG and PEG/GnPs composite phase change materials were prepared, respectively. The relationship between the component, structure and thermo-physical property of composite phase change material and its kinetic mechanism were investigated. On this basis, a new type of composite form-stable phase change material was designed and prepared. Moreover, the phase change heat transfer process of prepared composite phase change material was studied experimentally, and the result was employed to guidance the components optimization design of composite phase change materials.Using the method of vacuum infiltration, the porous structure EG serving as conductive filler was combined with PEG to obtain the PEG/EG composite phase change material. The morphology, structure and thermo-physical properties of composite phase change materials were investigated by several means such as FE-SEM, XRD, FT-IR, POM and DSC. The obtained results show that, EG with porous structure can effectively absorb and embed the liquid PEG, and the composite phase change material with EG content of8wt.%can maintain its shape during the phase change process. With the increase of EG content, the thermal conductivity network is gradually formed inside the PEG matrix, and the thermal conductivity of the composite phase change material increased gradually. The thermal conductivity of composite phase change material with EG content of10wt.%changes up to19.4times over that of pure PEG. When the EG content exceeds6wt.%, the diffusion of PEG molecular chain segments is obviously inhibited by the porous structure of EG, which leads to the obvious decrease of phase change temperature and phase change enthalpy of composite phase change material. The melting point (Tm) and the solidification point (Tf) of composite phase change material with EG content of10wt.%decrease from50.9℃and35.7℃of pure PEG to41.8℃and21.5℃, respectively. Moreover, the melting enthalpy (ΔHm) and the solidification enthalpy (ΔHf) are only67.4%and73.6%of their theoretical values, respectively.GnPs with large aspect ratios (300～800times) were obtained by sonicating the EG. Using the method of vacuum infiltration, GnPs serving as conductive filler was combined with PEG to obtain the PEG/GnPs composite phase change material, and the morphology, structure and thermo-physical properties of composite were investigated. Results show that, the ultrasonic fragmentation nearly does not impact the phase and chemical surfactant of graphite. The GnPs with large aspect ratio possess advantage of easier to be dispersed in polymer matrix to form conducting network. The thermal conductivity of the composite phase change material with GnPs content of10wt.%changs up to10.8times over that of pure PEG. Compared with PEG/EG composite phase change material, the PEG/GnPs composite possesses more stable phase change temperature, and its phase change enthalpy is more closer to the theoretical value.Aim at the difference between the impacts of graphite structures on the phase change parameters of PEG, the non-isothermal phase change kinetics of pure PEG and composite phase change materials were studied by means of DSC. Results show that, the apparent activation energy of composite phase change material is higher than that of pure PEG, which indicates that the EG and GnPs are both inhibiting the diffusion of PEG molecular chain segments at certain limitations. Compared with GnPs, the same mass fraction of EG leads to the greater increase of apparent activation energy of the composite phase change material, which indicates that the inhibition of lamellar structure of GnPs on the diffusion of PEG molecular chain segments is relatively smaller than the porous structure of EG.Based on the phase change kinetics results, GnPs selecting as conductive fillers and polymethyl methacrylate (PMMA) acting as supporting material were combined with PEG to obtain a new-type of PEG/PMMA/GnPs composite form-stable phase change material by using the method of in situ polymerization upon ultrasonic irradiation. XRD and FT-IR results indicated that all the components are physically combined with each other during polymerization process. FE-SEM and POM results show that the PEG is uniformly dispersed and embedded inside the micro-level network structure of PMMA, which contributed to the well package and self-supporting properties of composite form-stable phase change material. Ultrasonic-assisted in situ polymerization process could effectively disperse the GnPs into the polymer matrix to build the thermal conductivity network. The thermal conductivity of composite form-stable phase change material with content of8wt.%changs up to8.4times over that of PEG/PMMA composite. It is also confirmed that all the prepared specimens possess available thermal storage density and form-stable performance. When the content of EG is8wt.%, the ΔHm, ΔHf, compressive strength at55℃and mass loss rate after75cycles of composite form-stable phase change material are114.7kJ-kg-1,97.0kJ-kg-1,3.7MPa and2.5%, respectively.The phase change heat transfer processes of pure PEG and composite phase change materials were experimentally studied by time-temperature method. According to the experimental results, the influence of composite modification on the usability of phase change material was evaluated. Results show that there is a significant natural convection effect during the phase change heat transfer process of pure PEG. The porous structure of the EG is able to effectively absorb and embed the liquid PEG to limit natural convection effect. The EG is able to significantly increase the thermal conductivity of the composite phase change material and reducing the thermal resistance of phase change heat transfer process. Then the phase change rate of composite phase change material is able to be increased effectively. The optimize content of EG among PEG/EG composite phase change material is approximately6～8wt.%.In this paper, the design and preparation of PEG based composite phase change materials, the microstructure, the thermo-physical property and the kinetic mechanism of composite phase change materials are together investigated. The research results may be helpful to improve the usability of PEG and promote its practical application. One point deserved mentioning is that the research on phase change kinetics of PEG/graphite composite phase change materials may be helpful to understand the unconventional phase transition mechanism inside the composite system. Moreover, the experimental measurement of phase change heat transfer processes is able to effectively evaluate the influence of composite modification on the usability of phase change material, and it has a great guiding significance on the performance control and optimum design of thermal energy storage materials.更多还原