两种石墨烯修饰的复合热界面材料的制备及热性能的研究

采用化学氧化还原法制备的石墨烯和化学气相沉积法制备的三维网状石墨烯共同作为导热填料改性环氧树脂,研究导热填料质量分数的变化对环氧树脂热导率的影响,并进一步测定复合热界面材料的热导率在高温下的稳定性。结果表明:当石墨烯-三维网状石墨烯的质量分数为0.2(石墨烯和三维网状石墨烯的比例为1∶9)时,可使环氧树脂的热导率提高2 400%;三维网状石墨烯的三维网状结构和石墨烯的表面官能团对复合热界面材料的热性能具有显著地影响;三维网状石墨烯为声子提供了快速传输通道,而石墨烯的表面官能团能促进环氧树脂与石墨烯之间形成良好的接触,降低界面热阻,在石墨烯和三维网状石墨烯的协同作用下可提高热界面材料的热导率。此外,可以通过优化导热填料的尺寸,提高复合热界面材料热导率的稳定性。     

两种石墨烯修饰的复合热界面材料的制备及热性能研究

采用化学氧化还原法制备的石墨烯和化学气相沉积法制备的三维网状石墨烯共同作为导热填料改性环氧树脂,研究导热填料质量分数的变化对环氧树脂热导率的影响,并进一步测定复合热界面材料的热导率在高温下的稳定性。结果表明:当石墨烯-三维网状石墨烯的质量分数为0.2(石墨烯和三维网状石墨烯的比例为1∶9)时,可使环氧树脂的热导率提高2 400%;三维网状石墨烯的三维网状结构和石墨烯的表面官能团对复合热界面材料的热性能具有显著地影响;三维网状石墨烯为声子提供了快速传输通道,而石墨烯的表面官能团能促进环氧树脂与石墨烯之间形成良好的接触,降低界面热阻,在石墨烯和三维网状石墨烯的协同作用下可提高热界面材料的热导率。此外,可以通过优化导热填料的尺寸,提高复合热界面材料热导率的稳定性。     

强化导热相变材料对PV/PCM热控特性影响研究

文章建立了光伏/相变材料(PV/PCM)太阳能热控系统二维模型,并根据模拟结果研究了相变材料热导率对太阳电池热控特性的影响。模拟结果表明,当PCM热导率由0.3 W/(m·K)逐渐增加至1.1 W/(m·K)时,相变材料对太阳电池的热控效果越来越好。此外,文章设计了PCM热导率分别为0.8,1.1 W/(m·K)的PV/PCM太阳能热控系统实验装置,在模拟光源和自然光条件下,对太阳能热控系统实验装置的输出功率以及太阳电池的温度进行测试。实验结果表明:在模拟光源下,与无PCM太阳电池相比,PCM热导率分别为0.8,1.1 W/(m·K)的太阳电池的最高温度分别降低了4.6,10.8℃,平均输出功率分别提高了2.2%,4.1%;在自然光条件下,与无PCM太阳电池相比,PCM热导率分别为0.8,1.1 W/(m·K)的太阳电池的最高温度分别降低了9.7,12℃,平均输出功率分别提高了3.1%,5.98%。     

显热储热材料的制备及性能研究

采用水泥作为材料的胶凝剂,添加热容、热导率大的物质作为骨料来制备混凝土储热材料。研究表明:当铝酸盐水泥含量为10%时,材料的抗压、抗折强度能满足工业需求;材料的比热容随温度的升高先增大,在500℃时达到最大,后随着温度的升高反而降低;材料的热导率随着石墨粉含量的增加几乎成直线上升,当石墨含量为5%时材料的热导率大于1.7W/(m·K)。     

Thermal conductivity of metal-phase change materials--A review

基于相变储热材料存在导热系数低的问题,研究者利用金属导热系数高的特点,与相变储热材料复合,通过理论分析,数值模拟,实验研究三方面来研究复合相变材料导热性的变化,金属与相变储热材料复合后,显著提高了相变储热材料的热导率,但同时对储热装置的储热能力以及自然对流等性能又具有一定的抑制作用.文章综述了近年来国内外学者对金属复合相变储热材料导热性的研究进展,分析了多孔泡沫金属,金属颗粒,纳米颗粒,金属矩阵等几种主要强化方法,并探讨了今后相关方面的研究重点,提出泡沫金属和纳米颗粒对相变储热材料热导率强化的优点,同时应加强这两方面的理论研究,开拓其在市场上的应用.     

GaAs纳米线晶格热导率温度和尺度效应及声子非弹性散射

为了深入理解砷化镓(GaAs)纳米线的微观热输运机理,采用平衡分子动力学的方法模拟其晶格热导率。基于声子态密度的频域特性和热流自相关函数的时域特性,分别研究温度、尺寸对纳米线晶格热导率的影响规律。结果表明,纳米线的热导率有明显的温度与尺度效应。随温度升高,其晶格热导率先逐渐增大后减小,300 K左右达到最大值1.6 W/(m·K);晶格热导率随长度、直径的增大均先增加后趋于稳定。砷化镓纳米线晶格热导率的温度和尺度效应均可用声子非弹性散射解释。模拟的热导率比体材料的值小一个数量级,模拟结果可为改善半导体材料的热电性能提供指导。     

石墨烯-PFA复合材料换热器与金属换热器的传热性能对比

氟塑料换热器以其耐腐蚀、耐磨损等优点而备受关注,但氟塑料热导率较低,换热能力差,限制了其广泛应用。石墨烯-PFA复合材料兼具石墨烯优异的导热性和可熔性聚四氟乙烯(PFA)良好的耐酸碱腐蚀性,是新一代的换热器材料。搭建了余热回收测试实验台,对石墨烯-PFA复合材料换热器和金属换热器的传热性能进行对比。研究了不同烟气流速、不同进口烟气温度以及不同石墨烯配比对复合材料传热性能的影响。结果表明:对于金属换热器和复合材料换热器,当烟气流速从2.0增加到4.0 m/s时,传热系数分别增加到原来的1.19和1.34倍;随着进口烟温的升高,两种材质的传热系数分别降低了15.6%和14.7%;随着石墨烯含量增加,复合材料的导热系数以及传热系数均增加。     

保温用天然生物质材料的热湿特性

实验分析一些廉价天然材料(椰壳和花生壳)的导热特性。利用同心球稳态测量方法测量确定椰壳和花生壳的导热系数,以及导热系数随温度的变化规律,同时以硅酸铝纤维材料为标准试样,与天然材料的导热性能进行对比分析。还对这些材料的低温吸湿作了初步测试,分析此类材料作为低温绝热材料的吸湿特性。研究结果表明,所有材料的导热系数均随温度的升高而增大,且增大速率都近似相等。影响天然生物质材料导热性质的因素主要有：纤维或多孔固体材料中的导热、孔隙中空气的对流换热,如果温度足够高的话,还有辐射换热。     

石墨烯粉末导热性能的研究

采用瞬态电热技术测量了5~6层纯石墨烯粉末中石墨烯的热扩散率,其值为1.15×10-5 m2/s,相应的导热系数为18.000 W/(m·K)。借助导热仪研究了不同密度下石墨烯粉末导热系数的变化情况,发现其导热系数与密度成正比。密度由0.02增加到0.22g/cm3时,导热系数总体提升了8.09%。另通过实验得到了含水率对石墨烯粉末导热性能的影响。实验结果显示,石墨烯粉末含水率由0.0%增加到99.8%的过程中,导热系数先是上升随后下降最终直线降至最低点(约为0.765~1.030 W/(m·K))。其中当含水率达到96.7%时,混合物(石墨烯与水)的导热系数提高了62.80%。该研究为石墨烯热应用及热管理提供了理论支撑。     

基于热辨识技术的硅基防热材料高温热导率测量

硅基防热材料是高超声速飞行器防热系统用重要材料之一,但由于硅基防热材料在高温条件下存在着复杂的物理化学变化,使得高温热导率的获取变得困难,这已成为飞行器防热系统精细化设计的主要制约瓶颈。基于热导率辨识方法,设计了一种能够实现硅基防热材料高温热导率测量的试验测量装置,对硅基防热材料在常温~800℃热导率进行了测量,并将测得的热导率外推应用到其他试验状态。结果表明,测得的硅基防热材料高温热导率合理可靠,具有很高的工程精度。该试验测量装置可实现不同温度下热导率的同步测量,测量成本低,效率高,这对其他防热材料的高温热导率测量具有重要的参考价值。     

Molecular Dynamics Study on Thermal Conductivity of Carbon Nanotubes

The thermal conductivity of a (5, 5) carbon nanotube at room temperature is studied by non‐equilibrium molecular dynamics simulations. The thermal conductivity increases from 30 W/(m·K) to 1000 W/(m·K) as the tube length increases from 6 nm to 4 µm. It is proportional to the tube length when the tube length is less than 40 nm, which indicates that the heat conduction is in the ballistic transport regime. The thermal conductivity relates to the tube length by an exponential function as the tube length increases, and the length dependence exponent decreases and approaches zero, which indicates that the phonon transport changes from the ballistic regime to the diffusive regime. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; 39(7): 455–459, 2010; Published online in Wiley Online Library ( wileyOnlinelibrary.com ). DOI 10.1002/htj.20311     

Conduction paths of backbones for thermal enhancement of nanocomposite phase change materials

Thermal energy storage (TES) based on phase change materials (PCMs) has become a research hot spot due to its high energy storage density and maintained operating temperature during the phase change. However, as PCM has a poor thermal conductivity that can be as low as 0.2~0.5 W/m· K, the charging/discharging processes of PCM modules are significantly restrained, which severely affects the application of the TES technology in industrial sectors. This study concerns the improvement of the effective thermal conductivity of composite PCM formed by adding nanoparticles with high thermal conductivity into different PCMs. A theoretical model is established to reveal the intrinsic mechanism for the promotion of thermal conductivity of composite PCM consisting of nanoparticles. The results show that aggregation and interfacial thermal resistance are the main reasons for the change of the thermal conductivity. By forming effective conduction paths composed of backbones in the composite PCM, the average thermal conductivity can be improved significantly, which can be as high as 10~50 W/m· K with a wide range of volume fraction of the additives.     

不同加热功率下充液率对无芯环路热管性能的影响

设计了以铝为管材、丙酮为传热工质的无芯环路热管。其蒸发段采用加热带加热,冷凝段用风冷降温。热管依靠蒸发压头使工质循环,并依靠重力作用,使冷凝液回流到蒸发段。搭建试验台并研究了不同加热功率下充液率对无芯环路热管的传热温差、传热量、热效率、热阻和当量导热系数的影响。结果表明:加热功率为150.00 W、充液率为30%时,无芯环路热管的均温性最好;传热温差和热阻均最小,分别为6.75℃、0.045 K/W。传热量132.00 W、热效率0.88、当量导热系数168 125 W/(m·K),均达到最大值。所以,该无芯环路热管在本实验研究范围内的最佳工作条件为加热功率150.00 W、充液率30%。     

Hydrate salt/self-curing acrylic resin form-stable phase change materials with enhanced surface stability and thermal properties via the incorporation of graphene oxide

This work presents a novel eutectic hydrate salt/self-curing acrylic resin form-stable phase change materials (PCMs) composite (EHS/SCR) with favorable form-stable performances for heat energy storage. Further, to improve the surface stability, latent heat and thermal conductivity of the EHS/SCR particles, graphene oxide (GO) used as cladding materials is incorporated onto the surface of the EHS/SCR particles to prepare the GO modified EHS/SCR phase change composite (EGO). The obtained results indicate that the GO-targeted absorption model has achieved the enhancements in stability and thermal properties of EHS/SCR while making use of GO in an efficient and economical way. To be specific, with the introduction of GO of only 1.07 wt%, the thermal conductivity of 0.508 W/m·K is achieved, the value shows a significant rise of 128.6% compared with the EHS/SCR of 0.222 W/m·K. Additionally, the maximum latent heat of EGO-6 is up to 90.4 J/g, which exhibits a 3.19-fold increase compared against that of the uncoated EHS/SCR. Moreover, the prepared EGO composite PCM remain a good thermal cycling reliability after 300 thermal cycles. This work provides a novel opportunity to improve the performance of form-stable PCM composites with an intelligent manufacture-oriented pattern.     

含二氧化硅气凝胶和相变材料三层玻璃窗热性能分析

为了研究含二氧化硅气凝胶和相变材料三层玻璃窗对严寒地区建筑能耗的影响,建立了相变材料层与其他透明壁层结合发生的传热数值模型。分析了含二氧化硅气凝胶和相变材料三层玻璃窗在不同二氧化硅气凝胶厚度、导热系数和不同保温材料下的动态热调节性能,得到了含二氧化硅气凝胶和相变材料三层玻璃窗内表面热流密度和液相率随时间的变化规律。结果表明:随着二氧化硅气凝胶厚度增加,总传热量降低和液相率增加,当二氧化硅气凝胶厚度为20～30 mm时,可以实现有效的利用太阳能;随着二氧化硅气凝胶导热系数增加,总传热量升高和液相率降低;当二氧化硅气凝胶的导热系数从0.022降低到0.014 W/(m·K)时,最大液相率从0.83增加到1.00。二氧化硅作为保温层比相变材料作为保温层具有更好的保温隔热作用。     

A study on thermal conductivity of a mixture of magnesia particles and molten nitrate by a transient hot wire method

In this study, the effective thermal conductivity of a mixture of magnesia particles and molten nitrate used in a high temperature thermal energy storage system was investigated by a transient hot wire method. The effective thermal conductivity of the mixture was around 2.0W/(m·K) in the temperature range of 423 K to 703 K, although it decreased about 5% with increasing temperature. This value was about 10 times larger than that of the packed bed of magnesia particles including air. The effective thermal conductivity increased about 3% with a 1% increase in the volume ratio of magnesia particles in the mixture with molten nitrate. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(4): 245–253, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20116     

Evidence of the illegitimacy of the additive approach to the determination of the thermophysical properties of coal-water fuel with glycerol

For the fuel ignition, the thermal conductivity and heat capacity are the key properties that determine the pre-ignition behavior of the drop of the fuel. The classic monophase fuels, such as natural gas, liquid propellants, or solid one-component fuels, have been investigated for a long time; and their thermophysical properties are well known in most of the cases. Composite fuels, which have recently attracted the attention of the researchers, have complex contents. In many cases, composite fuel is a mixture of solid and liquid components in the form of a slurry. Coal-water fuel and its derivatives with different additives are examples of such type fuels. For those fuels, the thermophysical properties are usually unknown. Nowadays, researchers use simple additivity theory for the calculation of the thermophysical properties of complex fuels for the first approach. Authors of this research believe that the simple additivity approach is not correct and can lead to the wrong results in the case of the numerical research of the ignition and burning processes of such a fuel. In the present research, the thermophysical properties of coal-water fuel with glycerol additives were experimentally obtained. It was found that the coefficient of thermal conductivity increases with temperature and varies in the range of 0.45 to 0.53 W/(m·K). The heat capacity of the fuel also increases with the temperature and varies from 4.7 to 5.5 kJ/kg·K. The higher the glycerol content, the lower the thermal conductivity and heat capacity of the composite fuel in the investigated temperature range. The results confirm the failure of the approach of the additivity law usage. Neither, thermal conductivity coefficient or heat capacity of the coal-water fuel with the addition of up to 20% glycerol complies with the additivity law. Differences between real values of the thermophysical properties and calculated ones are more than 30% to 50%. Empirical expressions for calculation of the thermophysical properties of coal-water fuel with the addition of up to 20% glycerol are presented.     

Study on reduction of thermal conductivity of composite phase change material using Cu nanoparticles

A composite phase change material (PCM) containing Cu nanoparticles (NPs) was prepared. XRD and TEM results showed that the Cu NPs were pure metal with irregular morphology. The diameter of Cu NPs was in the range of 5–15 nm. The composite PCM melted at the temperature of 310 K and the total phase change enthalpy was 122 J/g. The thermal conductivity of the composite PCM was reduced by Cu NPs from 0.130 W/mK to 0.102 W/mK. The as-prepared composite PCM might be an appropriate candidate for thermal protection where PCM with lower thermal conductivity is needed.     

带有空穴子模型的石墨烯气凝胶复合相变材料导热系数的分形研究

在分形理论的基础上,确定了基于石墨烯气凝胶(graphene aerogel,GA)骨架复合相变材料的分形维数,并基于改进的Sierpinski地毯建立了带有空穴的导热系数预测模型。预测结果表明,对于基于GA骨架的复合相变材料,在所制得的材料孔隙率为0.7的条件下,无论空穴尺寸如何,均可将导热系数从相变材料本身的0.250 W/(m·K)提升至10.900 W/(m·K),增长幅度达40倍以上。结果显示,复合相变材料的导热系数随着孔隙率的减小而增加,且在低孔隙率下,导热系数随空穴尺寸的减小而增加。