氧化石墨烯/聚乙烯醇 复合材料导热性能研究

高分子材料作为一种新型导热材料,弥补了传统导热材料如金属、金属合金等抗腐蚀性弱、力学性能差等缺陷,但其导热系数的低下导致了应用范围的局限性,而将高分子材料复合后则能很好的解决这个问题。本文以聚乙烯醇为复合材料基体,氧化石墨烯为无机填充物,经反复冷冻解冻法制备出不同体积分数梯度的氧化石墨烯/聚乙烯醇复合材料。实验采用瞬态电热测试技术对其进行导热性能测试,得出该材料达到导热系数最优点时对应的填料体积分数为25%。在实验的基础上,利用软件拟合得出导热系数值曲线图并建立导热公式,加以验证与分析。     

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。     

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。     

Paraffin based composite phase change materials for thermal energy storage: Thermal conductivity enhancement

为提高石蜡作为固-液相变储热材料的导热性能,在石蜡(PW)中掺加高导热系数的碳纳米管(CNTs),制备了碳纳米管-石蜡复合相变材料(PW-CNTs).为进一步增强PW-CNTs的传热性能,通过内置金属网结构,利用金属网的高导热性,加快PW-CNTs作为相变材料的充放热速率.测试了PW-CNTs的熔点和相变潜热,导热系数以及置入金属网前后的充放热时间.结果显示,PW-CNTs的导热系数较石蜡得到显著提高,其中掺加10%(质量分数)CNTs的复合材料的固态,液态导热系数平均分别提高31.4%,40.2%.置入金属网结构后,PW-CNTs的充放热时间至少分别缩短了40.3%和30.2%.此外,碳纳米管在石蜡中易发生团聚沉积,针对这一特点,对PW-CNTs进行了多次热循环,研究了热循环对PW-CNTs导热系数的影响.     

石墨/十四酸复合相变材料的热性能研究

利用超声解离的方法以膨胀石墨(EG)制得微米级片层石墨(MSGF),将MSGF添加到十四酸(MA)基体中,制备出MSGF质量分数分别为0.1%、0.5%、1.0%和2.0%的石墨/十四酸复合相变材料,采用实验的手段对该复合材料的热性能进行表征。结果表明:该复合材料的导热系数较纯MA有明显提高,随着MSGF质量分数的增加,固态体系的导热系数增加得越快,液态体系的增加则基本呈线性趋势;相变潜热随MSGF含量增加逐渐降低,但相变温度变化不大,熔化时的相变温度略高于凝固时的温度;储(放)热时间与纯MA相比明显减少。     

激光闪射法对石墨烯薄膜导热性能的研究

石墨烯是目前发现的导热系数最高的材料,其理论导热系数值可达5 300 W/(m·K),成为新一代最具潜力的高导热材料。文中采用激光闪射法研究了石墨烯薄膜横向和法向的热扩散系数,并根据测试的密度和比热计算得出导热系数。研究表明:横向热扩散系数随着薄膜厚度的增加而不断减小,当MCT检测器在电压为260 V,脉冲宽度为100μs,信号高度为1 V,Inplane+各向同性计算模型下,高温烧制薄膜横向热扩散系数高达740.16 mm~2/s,是法向的238倍,导热系数为1 252.28 W·(m·K)~(-1);压片法制得的石墨烯薄膜的横向热扩散系数为7.58 mm2/s,是法向的19倍,导热系数为9.43 W·(m·K)~(-1)。     

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。     

石蜡基碳纳米管复合相变材料的热物性研究

以多壁碳纳米管为填料,制备了不同质量分数(1%～5%)的石蜡基纳米复合相变材料。采用差示扫描量热技术对所制备复合相变材料的相变特性进行了表征,其导热性能则通过瞬态热线法导热仪进行了测量。实验结果发现,虽然复合相变材料的相变温度几乎不变,但其相变焓则随碳纳米管的加载量的增加而近似线性下降。在质量分数为5%时,相变焓较纯石蜡下降了约15%。复合相变材料的导热系数大致随温度的升高而降低,而在30和50℃时分别由于固固和固液相变的作用,导热系数测量值出现了较大程度的突增。此外,导热系数随质量分数呈线性增长的趋势,在质量分数为5%时,最大的相对提升率接近40%,展现了良好的导热强化效果。     

纳米金属/石蜡复合相变蓄热材料的实验研究

以石蜡为相变材料基体、纳米金属铜、镍、铝、铁和锌为导热增强剂、油酸为分散剂,采用超声波震荡法制备纳米金属/石蜡复合相变蓄热材料体系。通过复合蓄热体系的步冷曲线分析,结果显示纳米铁为有效导热增强剂。对不同质量分数纳米铁/石蜡复合相变蓄热体系进行DSC和导热系数测试分析,结果表明:随着纳米铁质量分数的增加,复合材料的导热系数逐渐增大,相变潜热值逐渐降低,相变温度变化不大;纳米铁质量分数为0.1%时,复合材料的固态导热系数可增大2.8倍,相变潜热值下降1.1%。     

Thermal Conductivity Prediction of Thermally Conductive,Electrically Insulating Materials under van der Waals Interactions

The thermal transport phenomenon in small-scale heterogeneous composites is essentially controlled by van der Waals interactions. In this article, thermal conductivity of nanocomposites with 33 wt% crystallized silicon dioxide is four times higher than that of epoxy (EP) resin composites. Nanocomposites with 33 wt% boron carbide exhibit seven times higher thermal conductivity than pure EP. Pal and Lewis-Nielsen multiscale models were used to infer that distance-associated van der Waals interactions vary between composites with different weight fractions. Such variation consequently affects the thermal conductivity of the composites. Scanning electron microscope images of crystallized silicon dioxide/EP composites provide evidence of our reasonable and accurate inferences with regard to the thermal conduction mechanism. Experimental values confirm that the Pal model is superior to the Lewis-Nielsen model. The observed enhancement in thermal conductivity indicates important implications for the development of highly and thermally conductive electrically insulating materials. Results of this study can also be considered to improve modeling for thermal conductivity under van der Waals interactions.     

Effect of carbon fillers on properties of polymer composite bipolar plates of fuel cells

Bipolar plates are major components of fuel cell (FC) stacks and they make up a large portion of the stack volume and cost. In order to reduce their weight and fabrication cost, polymer composite materials with various carbon conducting fillers are tested for use as composite bipolar plates for FCs. The composite materials are prepared by using graphite with a small vol.% of carbon black (CB), multi-walled carbon nanotubes (MWNTs) or carbon fibres (CF) in an epoxy resin. The electrical conductivity and flexural properties of the composites are measured as a function of the carbon conductive filler content. The highest electrical conductivity is observed at a total conducting filler content of 75 vol.%. The addition of a small amount of hybrid conducting filler enhances the electrical conductivity up to certain threshold, viz. 5 vol.% of CB, 2 vol.% of MWNTs, and 7 vol.% of CF. Above these thresholds, the electric conductivity decreases with increasing filler content, due to the lack of sufficient resin to bind the fillers tightly. The hybrid filler system has better properties than the single filler system. The experimental results indicate that there is an optimum composition range with respect to electrical conductivity and mechanical properties.     

Synergy effects of hybrid carbon system on properties of composite bipolar plates for fuel cells

A hybrid carbon system of graphite powder (GP) and continuous carbon fibre fabric (CFF) is used for an epoxy composite to improve the electrical conductivity, mechanical properties and mouldability of a composite bipolar plate. These improvements are achieved simultaneously by inserting several layers of CFF into the GP/epoxy composite to enhance the mechanical properties and in-plane conductivity. The electrical properties, flexural strength and mouldability of the composite plates are measured as a function of conducting filler content and number of CFF layers. The composites show improved electrical conductivity, flexural properties and mouldability. Composites with 70-75 vol.% carbon fillers have the highest electrical conductivity with reasonable flexural properties. These results suggest that the poor mouldability and low through-plane electrical conductivity of the continuous fibre composite bipolar plate, as well as the weak flexural properties of GP composites, can be overcome by incorporating a GP/CFF hybrid system.     

Experimentally measured thermal transport properties of aluminum–polytetrafluoroethylene nanocomposites with graphene and carbon nanotube additives

Reactive materials such as aluminum (Al) and polytetrafluoroethylene (Teflon) are used for energy generation applications and specifically in ordnance technologies. With the advent of nanotechnology various nano-scale additives have become incorporated into reactive material formulations with the hope of enhanced performance. An important component to the study of energy generation is an examination of energy transport through a reactant matrix. This study examines an experimental approach to quantifying thermal properties of an Al/Teflon nanocomposite reactant matrix that has been impregnated with carbon additives. Various structures of carbon are investigated and include amorphous nanoscale carbon spheres (nano C), graphene flakes and unaligned multiwalled carbon nanotubes (CNTs). The additives were selected based on their completely different structures with the hypothesis that the structure of the additive will influence the thermal transport properties of the matrix. Results show graphene has the greatest influence on the thermophysical properties. For example, thermal conductivity of the composites containing graphene increased by 98%. Graphene similarly enhanced the thermal diffusivity and specific heat of the Al/Teflon matrix. Conversely, nano C and CNTs decreased the thermal conductivity and thermal diffusivity of the samples significantly.     

The effects of various carbon derivative additives on the thermal properties of paraffin as a phase change material

The storage of thermal energy in phase change materials (PCMs) has found wide applications that enable energy conservation and management. Paraffin is a major PCM with its low cost, wide availability, and relatively high latent heat, yet its low thermal conductivity may become a drawback in high‐power applications. In this study, composites of paraffin were prepared with multiwalled carbon nanotubes and activated carbon by a dispersion technique to overcome these drawbacks. Thermal, chemical, and physical influences of incorporating carbon additives with varying structures in paraffin composites on thermal storage capacity were determined. Results indicated that the thermal conductivities of paraffin‐activated carbon composites (PACC) and paraffin multiwalled carbon nanotube composites (PCNC) were improved by a factor of 39.1 and 34.1%, respectively, compared with the conductivity of pure paraffin. As a bonus, the thermal energy storage capacities of PCNCs were enhanced by 9.6%, whereas this remained unchanged for PACCs. Copyright © 2015 John Wiley & Sons, Ltd.     

High performance polyvinylidene fluoride/graphite/multi-walled carbon nanotubes composite bipolar plate for PEMFC with segregated conductive networks

Composite bipolar plates (BPs) are preferred to graphite BPs and metal BPs, in proton exchange membrane fuel cells (PEMFC), due to their pronounced advantages. However, facile and high-efficiency fabrication of high performance composite BPs, remains a challenge. In this study, high performance polyvinylidene fluoride (PVDF)/graphite/multi-walled carbon nanotubes (MWCNTs) composite BPs with segregated conductive network are prepared by structural design and compression molding. Due to the “brick-mud” structure formed in composite BPs by structural manipulation, its conductivity of low filler content is greatly improved. In addition, segregated synergistic conductive networks are observed in composite BPs after adding MWCNTs. The composite BP (5 wt% MWCNTs and 35 wt% graphite) exhibited electrical conductivity of 161.57 S/cm and area specific resistances of 7.5 mΩ cm². Moreover, the composite BPs have good flexural strength, excellent hydrophobicity and corrosion resistance. In summary, our work provides a simple and feasible strategy for manufacturing high performance composite BPs with low fillers.     

Effective thermal conductivity of isotropic polymer composites

The effective thermal conductivity of tin powder filled high density polyethylene composites is investigated experimentally as a function of filler concentration and the measured values are compared with the existing theoretical and empirical models. Samples are prepared by compression molding process, up to 16% volumetric concentration of tin particles. The thermal conductivity is measured by a modified hot wire technique in a temperature range from about 0°C to 70°C. Experimental results show a region of low particle content, up to about 10% volume concentration, where the increase in thermal conductivity is rather slow. The filler particles are dispersed in the matrix material in this region, the thermal conductivity is best predicted by Maxwell's model and Nielsen's model with A=1.5, φ_m=0.637. Whereas, at high filler concentrations, the filler particles tend to form agglomerates and conductive chains in the direction of heat flow resulting in a rapid increase in thermal conductivity. A model developed by Agari and Uno estimates the thermal conductivity in this region, using two experimentally determined constants.     

Thermal performance of stearic acid/carbon nanotube composite phase change materials for energy storage prepared by ball milling

In this work, stearic acid/carbon nanotubes composite phase change materials (SA/CNTs composite PCMs) were fabricated by ball milling for the first time to enhance the heat conduction of SA and prevent the delamination of SA and CNTs components. The results of suspension stability study conducted using a gravity sedimentation method showed that polyvinylpyrrolidone (PVP) used as dispersant has the best effect on the stability of composite PCMs. Then, the thermal cycling test further proved the stability of prepared composite. The SEM and FT‐IR results revealed that ball milling led to the formation of highly homogeneous composites. The thermal properties of the fabricated SA/CNTs composites with CNTs contents of 2, 6, and 10 wt.% characterized by differential scanning calorimetry (DSC) demonstrated that their phase change temperatures varied slightly while the latent heat decreased with the increased CNTs content. Furthermore, the thermal conductivity of the SA/CNTs composites were greater than that of pure SA by 61.5%, 92.3%, and 119.2%, respectively. The addition of CNTs also increased the thermal release rates of the prepared PCMs and decreased their storage rates. Therefore, the produced materials can be potentially used in thermal management.