三维高导热炭/炭复合材料的制备、结构及性能（英文）

采用连续沥青基炭纤维与商业PAN基炭纤维的混编制备了三维炭/炭复合材料预制体,通过多次化学气相渗透(CVI)、液压浸渍(LPI)工艺对其进行增密处理和一系列的炭化和石墨化处理获得高导热三维炭/炭复合材料。在此典型结构中,沥青基炭纤维沿x,y方向水平正交排布,而商业PAN基炭纤维沿z方向双向贯通排布。研究了炭/炭复合材料的显微结构以及炭纤维和热解炭对炭/炭复合材料热导率和力学性能的相对贡献。CVI热解炭具有高结晶度并且沿纤维轴高度择优取向。通过3CVI和3CVI+4LPI工艺制备的炭/炭复合材料的密度分别达到了1.58和1.84 g/cm3。所制备的炭/炭复合材料沿x,y方向分别具有115.9 W/m·K (3CVI)和234.7 W/m·K (3CVI+4LPI)的高热导率,沿z方向的热导率分别只有18.6(3CVI)和41.5 W/m·K (3CVI+4LPI)。热扩散和热导率主要依赖于炭/炭复合材料中的连续性沥青基炭纤维。通过PAN基炭纤维的引入和后续增密过程,三维炭/炭复合材料的力学性能相对于一维炭/炭复合材料和二维炭/炭复合材料显著提高。     

热处理温度对热解炭及炭/炭复合材料力学性能的影响

以丙烷为气源,采用等温等压化学气相渗透技术制备了炭/炭复合材料,利用X射线衍射、偏光显微镜、扫描电镜、纳米压痕仪、三点弯曲法研究了热处理温度对热解炭以及炭/炭复合材料微观结构和力学性能的影响.微观结构观察显示随着热处理温度的升高,热解炭层间距减小,同时石墨化度提高;由于发生了局部应力石墨化,热解炭出现同心微裂纹,并且随热处理温度的升高裂纹的数量和宽度增加.纳米压痕测试表明,热解炭的纳米压痕行为是完全的弹性形变,完全卸载后热解炭表面没有残余压痕,但加载和卸载曲线没有重合而是存在一定的能量耗散,随着热处理温度的升高,热解炭的弹性模量增大.热处理后纤维强度降低,并且纤维与基体炭界面脱离,导致炭/炭复合材料的弯曲强度和模量下降.     

高温处理对中间相沥青基炭纤维结构与热导率的影响（英文）

采用扫描电子显微镜(SEM)、透射电子显微镜(TEM)、拉曼光谱仪(Raman)及X射线衍射仪(XRD)考察了中间相沥青基炭纤维在不同热处理温度下的结构及形貌变化,并采用3ω法对经不同温度处理后的纤维热导率进行了表征。结果表明,中间相沥青基炭纤维的石墨化度与热导率随着热处理温度的升高而增大,经3000℃处理后纤维的热导率最高可达518W/m·K。此外,还探讨了中间相沥青基炭纤维结构、热导率及热处理温度之间的相互关系,发现中间相沥青炭纤维的石墨化过程存在3个阶段,在不同温度区间内分别对应石墨微晶的生长和取向。     

一维高导热C/C复合材料的制备研究

以三种沥青作为基体前驱体, 实验室自制的AR中间相沥青基纤维为增强体, 通过500℃热压成型, 随后经炭化和石墨化处理制备出一维炭/炭(C/C)复合材料。研究了前驱体沥青种类和热处理温度对复合材料导热性能的影响, 并采用扫描电子显微镜和偏光显微镜对其石墨化样品的形貌和微观结构进行表征。结果表明; C/C复合材料在沿纤维轴向的室温热扩散系数和导热率均随热处理温度的升高而逐渐增大; 由AR沥青作为基体前驱体所制备的C/C复合材料具有更加明显的沿纤维轴向取向的石墨层状结构以及最好的导热性能, 其3000℃石墨化样品沿纤维轴向的室温热扩散系数和导热率分别达到594.5 mm²/s和734.4 W/(m·K)。     

沥青基炭／炭复合材料的弯曲断裂特征

以1KPAN基高强度炭纤维为增强体、以调制中温煤沥青为基体前躯体,采用压力浸渍－炭化工艺制备出了不同密度二维沥青基炭／炭复合材料。经过对复合材料试样进行的弯曲试验表明,其弯曲断裂特征与材料密度具有密切的联系。根据弯曲强度－位移曲线,高密度复合材料表现为脆性断裂,而低密度复合材料表现为韧性断裂。从弯曲断面的SEM图片来看,脆性断裂时的断面比较平整,韧性断裂时断面上有大量炭纤维拔出。炭／炭复合材料的断裂破坏过程实质上就是基体裂纹在材料内的扩展过程,其扩展的途径与界面结合状况有关。裂纹沿界面的扩展将引起基体与纤维的脱粘,脱粘又导致纤维与基体之间的相对滑动,这种相对滑动将吸收相当一部分能量,从而可以延缓材料的断裂过程,起到韧化作用。     

高压浸渍-炭化制备炭/炭复合材料的组织结构

为研究高压浸渍-炭化制备的炭/炭复合材料的组织结构,以1 K PAN基高强度炭纤维为增强体,以调制中温煤沥青为基体前驱体,采用超高压浸渍-炭化工艺制备出2.5D沥青基炭/炭复合材料.采用偏光显微镜及SEM电镜对材料内部的组织形态进行了观察.研究表明:以中温沥青为基体前驱体所制备的炭/炭复合材料,在纤维束内,由于纤维之间的孔隙较小,形成的基体组织主要为镶嵌组织;而在纤维束之间,由于空间较大,出现的基体组织既有镶嵌型组织,也有域型组织.在沥青基炭基体中,有孔洞、裂纹、沟槽等缺陷.     

中间相沥青基石墨中缺陷的原子尺度

含有大量液晶分子的中间相沥青是制备轴向高导热炭/炭复合材料的重要原料,以中间相沥青基炭纤维为增强体与中间相沥青基体经液相浸渍法制备高导热炭/炭复合材料。中间相沥青基体随着热处理温度不断升高逐渐转化为包含诸多缺陷的石墨晶体,这些缺陷对复合材料的热传导有较大影响。利用透射电子显微镜研究中间相沥青基石墨晶体中的缺陷结构,尤其是不完整石墨晶体[11 0]晶带轴衍射谱中(10)列衍射斑点拉线的成因。结果表明,中间相沥青基体经3 000℃热处理后主要形成六角石墨,但六角石墨主体中夹杂数层菱形石墨,进一步发现,够成(10)列衍射拉线的缺陷主要包括两类:层错(基面间不同程度位移形成的位移缺陷)和晶界(相邻晶粒间相互旋转形成的旋转缺陷以及非共格晶界)。     

热解炭过渡层-中间相沥青基炭/炭复合材料的微观结构

借助偏光显微镜、扫描电镜、透射电镜对具有热解炭过渡层的中间相沥青基炭/炭复合材料的微观结构进行了研究。结果表明:材料的基体由热解炭和中间相沥青炭组成,在偏光显微镜下均呈现出光学各向异性。材料内部形成了多层次的界面结构,热解炭与纤维的界面连续,界面层内的石墨微晶择优取向度较高,晶格条纹排列规整;中间相沥青炭与热解炭界面不连续,为"裂纹型"界面,界面层内主要为非晶态碳。材料中炭纤维、热解炭、中间相沥青炭的石墨微晶大小逐渐增大,择优取向度逐渐增高,晶格条纹的排列逐渐规整。片层条带状结构的中间相沥青炭以及材料内的微裂纹平行于炭纤维轴向。     

短切炭纤维-炭复合材料的制备及传导性能和微观结构的研究

以中间相沥青基短切炭纤维和中间相沥青为原料,采用模压成型、炭化、致密化、高温石墨化等一系列常规工艺,制备了传导性能良好的炭/炭复合材料.主要考察了中间相沥青与中间相沥青基炭纤维质量配比对材料密度及传导性能的影响,并进一步研究了材料微晶参数的变化与材料性能的相关性.结果表明中间相沥青与纤维质量配比对材料的导热、导电性能以及微晶参数有很大影响.随着中间相沥青用量的增大,材料导热、导电性能均提高,石墨层间距d002减小,石墨微晶尺寸La、Lc增大;当中间相沥青与炭纤维质量比为 0.8时,制备出的炭/炭复合材料石墨微晶尺寸最大,常温传导性能最佳(垂直于压制方向的面向热导率为385W/(m·K),电阻率为2.85μΩ·m);进一步提高中间相沥青用量,石墨微晶尺寸La、Lc减小,材料的传导性能降低.     

炭/炭复合材料纤维束界面层的形成过程

通过对炭/炭复合材料纤维束界面不同成型阶段结构和性能的研究, 探索束界面在制备过程中的形成规律. 采用顶出实验、SEM、Micro-CT、XRD以及Raman对不同成型阶段的炭/炭复合材料中纤维束/基体界面剪切强度、界面层结构进行了分析. 结果发现材料密度较低时, 石墨化程度增加不利于束界面剪切强度的提高; 随着材料密度的增大, 束界面剪切强度明显升高. 通过对其界面结构进行分析, 可以看出在沥青浸渍、炭化和石墨化的制备过程中, 炭基体优先在束内形成, 然后逐步向束界面层及束间空间发展, 最后束界面层组织结构趋于完善. 随着热处理温度的升高, 其界面层组织结构的石墨化程度逐渐增强, 其结晶程度也不断增强.     

高导热炭纤维及其炭基复合材料

主要介绍了国际上近十年来高导热炭纤维（CF）及其炭基复合材料发展的新动向。     

高导热炭材料的研究进展

炭材料具有高热导率、低密度、优异的机械性能,是近年来最具发展前景的一类散热材料.本文结合炭材料结构和传导机理主要介绍目前国内外在提高炭材料热导率方面的研究工作及发展趋势.     

高导热中间相沥青基炭纤维的微观结构分析

中间相沥青基炭纤维具有低电阻、高导热特性,是目前最具发展前景的功能型导热、散热材料,但是国内对其微观结构和性能的研究报道较少。对国外高导热中间相沥青基炭纤维的微观结构和形貌进行了分析,同时将实验室研发的不同截面形状中间相沥青基炭纤维进行比较。研究结果表明中间相沥青基炭纤维具有的高导热特性源于其内部三维有序堆积的类石墨层状结构和较为完整生长的石墨晶体。热处理温度越高,其类石墨晶体生长越完善,层片取向程度越高。与圆形截面辐射状结构中间相沥青炭纤维相比,带状截面炭纤维有效解决了劈裂问题,其石墨片层间距为0.337nm,层片堆积高度达到26.77nm,轴向热导率高于800W/（m.K）。     

炭布叠层/热解炭复合材料导热系数与石墨化度的关系

采用脉冲激光闪光法和XRD，分析了一种炭布叠层／粗糙层结构热解炭复合材料的导热系数与石墨化度之间的关系，建立了二者之间的定量数学模型，并运用声子导热机制对其机理进行了探讨。结果表明，复合材料在平行于层面方向的室温导热系数约为垂直方向的2倍，但均随石墨化度的升高、石墨微晶尺寸的增大而逐渐升高．两个方向导热系数与石墨化度之间关系可分别表示为：λ=31．22 8．62exp(g／27．10)及λ=6．08 892exn(g／33.99)。     

Effect of halogen purification and heat treatment on thermal conductivity of high porosity carbon/carbon composite thermal insulation

Thermal conductivity of a highly porous carbon/carbon composite, known as carbon bonded carbon fiber (CBCF) and used as thermal insulation, was measured and related to the structure investigated by optical microscopy, x-ray diffraction and Raman spectroscopy. It was found that halogen purification of CBCF, that involves heat treatment in chlorine atmosphere, did not result in a greater extent of structural development than heat treatment at the same temperature for the same time in inert atmosphere (unpurified sample). The thermal conductivity of CBCF, both halogen purified and unpurified, was found to increase with temperature in the measured range 1000°C to 2200°C. The experimental thermal conductivity values were in good agreement with those calculated from a model that indicated that in CBCF solid heat transfer was dominant, compared to radiation heat transfer, even at 2200°C. The matrix in CBCF was found to be relatively graphitic as a result of stress orientation on carbonization and as matrix was observed along the fiber length it was tentatively suggested that it may contribute to the effective axial conductivity of the fibers.     

Paraffin/carbon aerogel phase change materials with high enthalpy and thermal conductivity

Two kinds of carbon aerogels, graphene aerogels (GA) and carbon nanotubes-graphene aerogels (CGA), were prepared by modified hydrothermal method. The form-stable phase change materials (PCMs) were fabricated by adsorbing paraffin into carbon aerogels. Morphology, structure, form stability and thermal property were characterized by scanning electron microscope (SEM), in situ X-ray diffraction (in situ XRD) and differential scanning calorimeter (DSC). The results showed that GA presented wrinkled surface textures with curling edges, and carbon nanotubes (CNTs) were interspersed or attached to GA sheets. The phase transition temperature and the phase change enthalpy of the GA/paraffin PCM composite were 48.7 °C and 223.2 J/g, respectively. Thermal and mechanical properties of PCM composites achieved a qualitative leap with the adding of carbon aerogels. The PCM composites had a thermal conductivity of about 2.182 W/m K at the carbon aerogels loading fraction of 2 wt%. The form-stable PCM composites with high thermal conductivity and high enthalpy could be promising for thermal energy storage applications in construction field.     

The thermal conductivity of carbon fibre-reinforced composites

Measurements of the thermal conductivity between approximately 80 and 270 K of a series of unidirectional and bidirectional specimens of epoxy resin DX210/BF₃400 reinforced with Morganite high modulus (HMS) and high strength (HTS) carbon fibres are reported for in-plane and out-of-plane directions. The main features of the results conform with expectations based upon known structural properties of the fibres and predictions based upon current theoretical models. Employing the results for the composites in association with results for the pure resin, the account concludes with an assessment of some of the heat transmission characteristics of the fibres.     

Effects of carbon nanotubes and carbon fiber reinforcements on thermal conductivity and ablation properties of carbon/phenolic composites

The ablation properties and thermal conductivity of carbon nanotube (CNT) and carbon fiber (CF)/phenolic composites were evaluated for different filler types and structures. It was found that the mechanical and thermal properties of phenolic-polymer matrix composites were improved significantly by the addition of carbon materials as reinforcement. The concentrations of CF and CNT reinforcing materials used in this study were 30 vol% and 0.5 wt%, respectively. The thermal conductivity and thermal diffusion of the different composites were observed during ablation testing, using an oxygen–kerosene (1:1) flame torch. The thermal conductivity of CF mat/phenolic composites was higher than that of random CF/phenolic composites. Both CF mat and CNT/phenolic composites exhibited much better thermal conductivity and ablation properties than did neat phenolic resin. The more conductive carbon materials significantly enhanced the heat conduction and dissipation from the flame location, thereby minimizing local thermal damage.     

Effect of aspect ratio on thermal conductivity of high density polyethylene/multi-walled carbon nanotubes nanocomposites

This study aims to investigate experimentally the effects of aspect ratio (length/diameter ratio) and concentration of multiwalled carbon nanotubes (MWCNTs) on thermal properties of high density polyethylene (HDPE) based composites. The aspect ratios of two types of MWCNT fillers are in the range of 200–400 and 500–3000. Composite samples were prepared by melt mixing up to weight fraction of 19% filler content, followed by a compression molding. Measurements of density, specific heat and thermal diffusivity (by modulated photothermal radiometry, PTR) were performed and effective thermal conductivities k_e of nanocomposites were calculated using these values. The results show that the composites containing MWCNTs with higher aspect ratio have higher thermal conductivities than the ones with lower aspect ratio. In terms of conductivity enhancement k_e/k_m − 1, the results indicate that MWCNTs with higher aspect ratio provide three to fourfold larger enhancement than the ones with lower aspect ratio, at low filler concentrations.