石墨烯气凝胶复合相变材料的热物性研究

相变材料利用其相变潜热能力可吸收储存和释放利用热量,同时在相变过程中其温度浮动小,能够实现温度控制从而用于热管理.但是其低热导率和易泄露问题严重制约了其性能.石墨烯气凝胶因其丰富的多孔结构而具有较大的比表面积,可吸附相变材料解决其泄露问题,同时石墨烯的高导热系数可提高相变材料的热导率.这里选取正十八烷为相变材料,制备了不同质量分数的石墨烯气凝胶复合相变材料.测得石墨烯气凝胶含量为13.99 wt%的样品,其导热系数比纯正十八烷高出306.2%,熔化潜热和凝固潜热分别下降了13.8%和10.8%.分子动力学模拟结果表明,石墨烯气凝胶的引入会在一定程度上增强正十八烷分子的有序性和一致性,即在同一温度下复合相变材料中的正十八烷分子比纯正十八烷分子拥有更集中分布的末端距和扭转角,径向分布函数和自扩散系数都相对较低,说明石墨烯材料的引入可以提升正十八烷的导热系数.     

掺杂石墨烯纳米片对硝酸钠相变特性的影响及机理

纳米增强剂通常被用来提升相变材料的导热性能,但这种方式通常伴随着复合后材料相变焓的降低.虽然这种降低难以避免,但其微观机理乃至影响规律却始终未能明晰.为深入探究纳米复合相变材料相变焓降低的机理,本文以熔融硝酸钠(太阳盐的重要组成成分)为相变材料,制备了石墨烯纳米片质量分数为0%,0.5%, 1%, 1.5%, 2%的复合相变材料.通过实验测量与分子动力学模拟的方法深入分析了石墨烯纳米片的掺杂导致熔融硝酸钠产生团簇以及复合材料熔点和相变焓非依数性降低的影响机理.结果表明,石墨烯纳米片质量分数为1.5%时,硝酸钠致密层和石墨烯纳米片间的质心等效距离最接近他们相互作用势的势阱位置,此时二者之间相互吸引作用最强,熔盐分子的运动受限最为严重,难以发生熔化,从而导致相变焓降低最为显著.为了最大限度地避免纳米复合相变材料相变焓的损失,应根据相变材料与纳米增强剂的类型及其相互作用类型,合理选择纳米增强剂的质量分数.在实际应用中,恰当的质量分数还将在一定程度上降低复合相变材料的制备成本.     

膨胀石墨基十二烷复合相变蓄冷材料的性能研究

本文以十二烷为相变材料,利用膨胀石墨多孔吸附特性制备出十二烷/膨胀石墨复合相变蓄冷材料。通过扫描电镜(SEM)、差示扫描量热仪(DSC)和热常数分析仪(HotDisk)对复合相变材料的结构及热性能进行了表征。实验结果表明,十二烷被膨胀石墨均匀吸附,吸附后膨胀石墨保持了原来的疏松多孔的蠕虫状形态。复合相变材料的相变焓值随着添加的膨胀石墨质量分数增加而降低,导热系数随着添加的膨胀石墨质量分数增加而增加。相变材料质量分数为80%,复合相变材料熔化焓和凝固焓分别为132.8 J/g和132.5 J/g,表观密度为700 kg/m~3时,导热系数为2.64 W/(m·K)。复合相变材料具有良好的热循环稳定性能。     

低温纳米复合相变蓄冷材料热物性研究

低温相变蓄冷的关键是蓄冷材料的开发.本文针对啤酒工艺对冷源温度的要求,选择BaCl2共晶盐水溶液为相变蓄冷材料基液,在基液中添加粒径为20nm的TiO2纳米粒子,形成具有蓄冷功能的纳米复合材料.对其热物性如导热系数、相变潜热、相变温度、过冷度及粘度进行了实验测量和分析.结果表明,在TiO2纳米粒子的质量分数为1%的情况下,纳米复合蓄冷材料的导热系数比基液提高了11.28%,过冷度从3.97℃降为1.21℃,同时粘度增加了21.7%,相变潜热略有降低.     

改性CaCl2·6H2O混合相变蓄热材料的制备与特性

选用碳素材料(C)石墨(Gr)、石墨烯纳米片(GnPs)、多壁碳纳米管(MWCNTs)和SrCl_2·6H_2O (SCH)为改性剂添加至无机相变材料CaCl_2·6H_2O (CCH)中,制备出不同质量分数碳素材料的无机混合相变蓄热材料C/0.2%SCH/CCH,并表征混合相变材料性能。结果表明:混合相变材料液态导热系数随碳素材料质量分数增加均呈线性提高,其中2.0%石墨烯纳米片可将导热系数提高69.35%;XRD谱图表明碳素材料、SrCl_2·6H_2O和CaCl_2.6H_2O之间的混合都为物理作用;DSC结果显示随碳素材料质量分数增加,相变潜热逐渐降低;质量分数4.0%Gr、2.0%GnPs和2.8%MWCNTs将0.2%SCH/CCH 的放热时间分别减少了 18.0%、41.9%和 12.8%;4.0%Gr/0.2%SCH/CCH、2.0%GnPs/0.2%SCH/CCH 和2.8%MWCNTs/0.2%SCH/CCH在经历100次加速热循环实验后,相变潜热分别降至186 J/g、189 J/g和183 J/g左右,仍满足蓄热要求,混合相变蓄热材料具有良好的热循环稳定性。     

石墨烯/石蜡复合材料的热物理性能研究

为改善相变蓄热用石蜡的导热性能,通过向石蜡基材中掺杂微量的石墨烯制备石墨烯/石蜡复合材料。采用扫描电子显微镜、热传导分析仪和差式扫描量热仪等获取了复合材料的表观形貌、热导率、熔点和相变潜热等关键热物性参数,讨论了石墨烯质量分数对这些参数的影响。通过动态热响应实验,揭示了石墨烯质量分数和热源温度对复合材料热响应速率的影响规律。结果表明,与纯石蜡相比,石墨烯/石蜡复合材料的热导率得到显著提高。当石墨烯的质量分数由0.2%升至2.0%,复合材料的热导率由0.266 W·m~(-1)·K~(-1)提高到0.346 W·m~(-1)1·K~(-1)。复合材料的相变潜热与纯石蜡相比并未减小,且其热响应速率高于纯石蜡。     

管壳式相变蓄热系统性能实验研究

相变蓄热(LHTES)系统利用相变材料(PCM)的潜热可有效地提高系统的储热能力。通常,PCM的导热系数低,使LHTES系统的应用受到了限制。本文通过向十五烷中添加膨胀石墨提高导热系数,分别采用纯十五烷和含有质量分数为30%膨胀石墨的复合十五烷,在不同水流量下,对管壳式LHTES系统的储热特性进行了实验研究。实验结果发现:当储能介质为质量分数为30%膨胀石墨的复合十五烷时,放冷过程中有效储热系数和相对储热率在Re=4298时出现明显峰值,换热效率与储热能力达到相对平衡。并且,当Re=4298,采用含有质量分数为30%膨胀石墨的复合十五烷与纯十五烷相比,系统的有效储热系数可最高提升336.84%。     

石墨烯制备方法及粒径对复合材料热导率的影响

石墨烯是由碳原子构成的单原子厚度的二维层状材料,具有许多优异的性能,是当前国内外研究热点之一,受到物理、化学、材料、电子、能源、生物和信息技术等领域的广泛关注。石墨烯是目前所测得导热系数最高的材料,在强化传热领域具有潜在的应用价值。较为系统地研究了石墨烯的制备方法及石墨的粒径对环氧树脂复合材料热导率的影响。实验发现,所采用的三种插层剂(硫酸、十四烷基胺及FeCl3)中,添加量较少时(如体积分数为1%),十四烷基胺插层法最为有效。而添加量较高时,硫酸插层制备的石墨烯纳米片效果最佳。制备石墨烯纳米片所采用的石墨粒径较大时,石墨烯/环氧树脂(Epoxy)复合材料的热导率越高。通过优化石墨烯的制备方法,石墨烯纳米片在体积分数为3.9%时,其热导率可达0.94 W·m~(-1)·K~(-1)),比基体材料提高了2.6倍。     

碳纳米管与石墨烯协同提高导热硅脂热性能

以甲基硅油为基础油,碳纳米管、石墨烯或碳纳米管/石墨烯混杂物为导热填料,制备复合导热硅脂.研究结果表明:以碳纳米管为单一导热填料,碳纳米管管径越小,管长越长,越有利于导热硅脂的导热性能提升。当总填充量在6%、碳纳米管和石墨烯配比为2:1时,导热硅脂的热导率提高19%。碳纳米管对石墨烯纳米片起到了分隔和桥连的作用,提高了石墨烯纳米片的分散性,有利于三维热传导网络的形成,进而提高导热硅脂的热传导性能。     

石蜡/膨胀石墨/石墨片复合相变材料导热性能研究

相变材料的导热性能对蓄热能力和能量转化速率具有重要影响。本文以58号全精炼石蜡为基底相变材料,利用真空吸附法将石蜡与膨胀石墨结合,再通过搅拌将石墨片与材料均匀混合,以模压法制备了定形复合相变材料,并通过实验测试了复合相变材料不同方向导热系数。实验结果表明:制备的复合相变材料导热性能具有明显的各向异性,垂直于压缩方向的导热性能远高于压缩方向的导热性能;同时也发现石墨片的添加可以有效的增强复合相变材料的导热性能,复合相变材料压缩方向导热性能随石墨片的添加先增大后减少;垂直于压缩方向的导热性能随石墨片的添加线性增强。     

A study on preparation and properties of carbon materials/myristic acid composite phase change thermal energy storage materials

Microscale graphite (Gr) and nanoscale multi-walled carbon nanotubes (MWCNTs) were chosen to modify the organic phase change material (PCM) of myristic acid (MA). The Gr/MA and MWCNTs/MA composite PCMs were prepared by adding the carbon materials at different mass fractions into MA. The experimental results indicated that both Gr and MWCNTs could enhance the thermal conductivity of MA. For the 3?wt% loading, the solid thermal conductivity of MA increased by 37.42% with Gr and 62.26% with MWCNTs. The FT-IR spectra showed that the reactions between carbon materials and MA were physical. The DSC results illustrated that the phase change latent heats of the composite PCMs decreased gradually with the additives increasing. Gr and MWCNTs strengthened the thermal stability of MA. The heat release rates of the composite PCMs accelerated. Three hundred thermal cycles of the chosen composite PCMs revealed that the prepared composite PCMs presented good thermal cycling stability.     

带环状翅片管式相变储热器的数值模拟

相变材料的低导热性是相变储热器传热过程的主要障碍。在相变材料侧进行强化,是目前提升相变储热器蓄热速率的主要方法。本文采用有限容积法对带有环状翅片的管式相变储热器的蓄热过程进行了数值模拟,得到了温度场、相界面随时间的变化。在此基础上,本文对翅片导热系数、翅片厚度和翅片间距等影响储热速率的因素进行了计算和分析。为相变储热器的优化设计提供了一定的参考。     

空调蓄冷材料研究现状及其新进展

分析了空调蓄冷材料研究现状及存在的一些问题 ,提出研制一种新型空调复合蓄冷材料 ,通过实验 ,分析该蓄冷材料的融点、融解热等热学性能。并通过实验研究寻找到了一种新型空调蓄冷材料 ,测试结果表明该蓄冷材料具有较高的相变潜热、适宜的相变温度和较好的热稳定性 ,因此可被应用于蓄冷空调系统中。     

Synthesis and characteristics of form-stable n-octadecane/expanded graphite composite phase change materials

N-octadecane/expanded graphite composite phase-change materials were prepared by absorbing liquid n-octadecane into the expanded graphite. The n-octadecane was used as the phase-change material for thermal energy storage, and the expanded graphite acted as the supporting material. Fourier transformation infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal diffusivity measurement were used to determine the chemical structure, crystalline phase, microstructure and thermal diffusivity of the composite phase-change materials, respectively. The thermal properties and thermal stability were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The DSC results indicated that the composite phase-change materials exhibited the same phase-transition characteristics as the n-octadecane and their latent heat increased with the n-octadecane content in composite phase-change materials. The SEM results showed that the n-octadecane was well absorbed in the porous network of the expanded graphite, and there was no leakage of the n-octadecane from the composites even when it was in the molten state.     

Study on the properties of a novel shape-stable epoxy resin sealed expanded graphite/paraffin composite PCM and its application in buildings

In this study, a novel phase change material (PCM) of epoxy resin sealed expanded graphite/paraffin composite was developed as an independent attachment for building applications. A relatively high thermal energy storage density and a high thermal conductivity (2.141?W/(m·K)) were obtained in the composite PCM. The mass fraction of paraffin in the composite PCM could reach 94% without leakage of liquid paraffin when being heated at 50°C for more than 3 h, and the thermal cycle stability was good. Moreover, the thermal storage and release properties of this composite PCM with different thicknesses were studied by numerical simulation. The results showed that the thermal storage and release time are proportional to the thickness of the composite PCM, and there was almost no temperature gradient during the thermal storage and release process, which indicated that the thermal conductivity of this kind of composite PCM was high enough for building applications.     

Experimental study on the phase change and thermal properties of paraffin/carbon materials based thermal energy storage materials

In order to enhance the thermal energy storage efficiency of phase change materials, in this paper, expanded graphite (EG), multi-layer graphene nanoplatelet (MGN), graphite powder (GP) and multi-walled carbon nanotube (MWCNT) as the effective heat transfer promoters in different mass fraction (0.1, 0.5, 1.0, 1.5 and 2.5 wt.%) were added into the paraffin. The chemical properties, latent heat capacities, thermal conductivities and heat storage performances of paraffin and the composites were investigated. The results showed that the addition of EG, MGN, GP and MWCNT could increase the thermal conductivity of paraffin. At 20 °C, the thermal conductivity of the paraffin was increased by 61.04%, 51.2%, 12.18% and 10.22% with 2.5 wt.% EG, MGN, GP and MWCNT, respectively. In addition, with the same mass fraction, the heat storage and release time of the composite were 56.03% and 54.26%, respectively, shorter than that of paraffin when the additive was EG.     

Melting temperature and enthalpy variations of phase change materials (PCMs): a differential scanning calorimetry (DSC) analysis

Differential scanning calorimetry (DSC) analysis is a standard thermal analysis technique used to determine the phase transition temperature, enthalpy, heat of fusion, specific heat and activation energy of phase change materials (PCMs). To determine the appropriate heating rate and sample mass, various DSC measurements were carried out using two kinds of PCMs, namely N-octadecane paraffin and calcium chloride hexahydrate. The variations in phase transition temperature, enthalpy, heat of fusion, specific heat and activation energy were observed within applicable heating rates and sample masses. It was found that the phase transition temperature range increased with increasing heating rate and sample mass; while the heat of fusion varied without any established pattern. The specific heat decreased with the increase of heating rate and sample mass. For accuracy purpose, it is recommended that for PCMs with high thermal conductivity (e.g. hydrated salt) the focus will be on heating rate rather than sample mass.     

Lattice Boltzmann simulation on thermal performance of composite phase change material based on Voronoi models

Phase change materials(PCMs) are important for sustaining energy development. For the thermal performance enhancement, the composite PCM with metal foam reconstructed by the Voronoi method is investigated in this work. The lattice Boltzmann method(LBM) is used to analyze the melting process on a pore scale. The melting interface evolution and temperature contour of the composite PCM are explored and compared with those of pure PCM. Moreover, structure parameters including the pore density, porosity and irregularity are investigated comprehensively, indicating that the additive of metal foam strengthens the melting performance of PCM obviously. Compared with pure PCM, the composite PCM has quick rates of the melting front evolution and heat transfer. The heat conduction plays a great role in the whole melting process since the convection is weakened for the composite PCM. To improve the melting efficiency, a larger pore density and smaller irregularity are recommended in general. More significantly, a suitable porosity is determined based on the requirement for the balance between the melting rate and heat storage capacity in practical engineering.     

热管式相变蓄热换热器储/放能过程中传热特性的实验研究

将热管作为换热元件应用于相变蓄热系统中,研制了一套热管式相变蓄热换热器。采用石蜡作为蓄热材料,对其储、放能过程即内部石蜡的融化与凝固过程进行了实验研究。测定了储、放能过程中不同时刻换热器内石蜡的温度分布; 改变供、取热流体参数,分析了供/取热流体的入口温度与流量对换热器储/放能过程的影响;分析了储、放能过程中能量随时间的变化情况。结果表明,热管在本换热器内极好地发挥了换热元件的作用,换热器运行状况良好,各项功能均能较好地实现。