【作者】 方昕；

【导师】 俞自涛；

【作者基本信息】 浙江大学 ， 工程热物理， 2016， 博士

【摘要】 相变材料利用潜热的方式储存和释放大量热量,温度却几乎不变,这一特性使其能够作为储热介质解决可再生能源供求不平衡的问题。但是由于自身较低的导热系数,传统相变材料的应用范围被大大限制,比较有效的方法是将微纳尺度的高导热颗粒(碳纳米管、石墨烯、纳米银颗粒等)加入到相变材料中,得到具有优异储热性能的复合相变材料。在以往纳米复合相变材料的研究中,由于颗粒种类／形貌、测量方式、制备方法的差异,有关导热系数调控规律与机理的结论众所纷纭。为此,本文通过对比实验,重点分析了银、碳纳米颗粒的形貌、尺寸对相变材料关键热物性和储热性能的影响。结果发现,石墨烯为代表的二维纳米材料(石墨纳米片、六方氮化硼、石墨烯纳米片)具有优异的导热增强效果。当石墨烯纳米片的加载量为10wt.%时,纯二十烷的导热系数最高提升近400%,并且样品的潜热焓仍保持在220 kJ/kg以上。产生这一结果不仅因为颗粒自身的高导热,还和二维结构在相变材料中产生的更小接触热阻有关。固液相变过程往往伴随着相变材料晶体的生长与消失,晶体大小和结晶化程度皆会影响到相变材料的有效导热系数,而纳米颗粒的加入则会让导热系数的影响因素变得更为复杂。因此,本文还结合微观形貌表征、DSC测试等手段,从纳米填料掺杂、相变点温度以及凝固速率等角度,探究了复合相变材料导热系数的调控规律。结果发现,当测试温度接近相变点或者凝固速率更缓慢时,都有助于获得更高的导热系数。针对纳米复合相变材料中分散相易发生团聚沉降的缺点,同时为了在相变材料中主动构建出高效的导热通路,本文利用生物质壳聚糖作为碳源,制备出一种含氮掺杂石墨烯的碳气凝胶。碳气凝胶微纳尺度的开孔结构可以很好地吸附相变材料,得到一种基于3D骨架的定形复合相变材料。热物性测试结果显示,碳气凝胶所提供的导热通路可以显著提升相变材料的导热系数,在大约2wt.%的碳气凝胶含量下,样品导热系数相比纯十六醇提高了107.9%。同时,DSC结果显示了复合相变材料储热性能优异的可逆性,相变焓仍保持在220 kJ/kg以上。更多还原

【Abstract】 With nearly constant temperature, phase change materials (PCMs) absorb and release a large amount of heat by taking advantage of their latent enthalpy upon melting/solidification, These features makes PCMs can serve as an integrated buffer in renewable energy systems. However, a major drawback of traditional PCMs is there relatively low thermal conductivity. Recently, nano-scale or micro-scale nanoparticles (Carbon nanotubes, graphene, Ag nanoparticles, etc.) were used as additives to prepare composite PCMs with high thermal conductivity.Due to the diversity of particles type, measurement instrument and sample preparation method, existing conclusions about the thermal characters of nano-enhanced PCMs are substantially fragmented. As a result, present work focuses on the influences of filler morphology (especially 1-D and 2-D shape), size, loading on the key thermophyscial properties (thermal conductivity, viscosity, latent enthalpy, etc.) of composite PCMs. Results show that the specimens containing 2-D (graphite nanosheets, hexadecanol boron nitride, graphene nanosheets, etc.) possess an remarkable thermal conductivity. The thermal conductivity of eicosane were enhanced 400% by graphene nanosheets at the loading of 10 wt.%, and the latent enthalpy of samples still maintained above 220 kJ/kg. Except for the high intrinsic thermal conductivity, the remarkable performance of 2-D nanofillers on thermal conductivity enhancement were considered to be a result of their unique planer structure that contributes to lower thermal interface resistance.Generally, phase change transition goes through the composition or decomposition of PCM crystals, and the grain size and crystallinity of PCM crystals would determine the heat transfer behavior of composite PCMs, thus some other factors regulating the effective thermal conductivity were investigated, such as hybrid nanofillers, temperature near the melting point and cooling rate of PCMs. The morphological characterization and DSC techniques were also involved to analyze the observed results. It was found that both the temperature close to melting point and lower cooling rate of PCMs would lead to a higher thermal conductivity.Furthermore, a carbon aerogel (CA) containing nitrogen-doped graphene was fabricated using chitosan as raw material. The synthesized CA was infiltrated with 1-hexadecanol to prepare a composite PCM. A thermal conductivity enhancement of 107.9% was achieved by CA compared to the baseline of pure 1-hexadecanol. DSC results showed the promising reversible properties and a satisfied latent heat capacity of around 220 kJ/kg.更多还原