变密度预制体C/C复合材料压缩性能的研究

采用平均密度不同的预制体制备变密度预制体C/C复合材料,并对应制备了常用的恒密度预制体C/C复合材料.研究了不同结构和不同平均密度的预制体对C/C复合材料压缩性能的影响.实验结果表明,变密度预制体C/C复合材料的压缩强度远远大于相同平均预制体密度的恒密度预制体C/C复合材料,并且随预制体平均密度的增大呈先增大后下降的趋势.由于预制体内部纤维含量的不同分布状态,变密度预制体C/C复合材料的压缩破坏同时呈现出压溃和剪切破坏模式.     

针刺+缝合预制体C/C复合材料的制备及性能研究

以无纬布/碳纤维网胎为原料,采用针刺+接力缝合的工艺流程制备预制增强体,并经化学气相渗透(CVI)联合沥青浸渍/高压碳化(HPIC)方法得到新型厚壁C/C复合材料。对该材料进行了力学和热物理性能测试,并观测其微观断口形貌。结果表明:此预制体制备方法能够突破传统缝合材料在厚度方向上的尺寸限制,工艺过程简便,不易变形且利于致密。所得C/C材料厚度方向的纤维密度较高,具有优异的机械和良好的热物理性能,能够满足固体火箭发动机等极端环境下的应用需求。     

针刺炭纤维预制体结构单元对C/C复合材料性能的影响

采用6K炭纤维无纬布/网胎交替叠层及12K炭纤维无纬布/网胎交替叠层,在针刺工艺,致密化、热处理工艺完全相同的情况下,制备了密度为1.8g/cm3的热解炭/树脂炭双元基体的两种C/C复合材料产品,考察了针刺预制体结构单元对C/C复合材料性能的影响.结果表明,两种C/C复合材料的热学（垂直方向导热系数）、电学性能及石墨化度基本相当;而针刺6K炭纤维无纬布/网胎预制体C/C复合材料的拉伸、弯曲、压缩、层间剪切强度分别为127MPa,189MPa,263MPa,24.6MPa;其平行方向导热系数为54.6W/m＆#183;K,比常规针刺12K炭纤维无纬布/网胎预制体C/C复合材料相应提高了38％,32.2％,32.8％,38.9％,21％,彰显了细化针刺预制体结构单元对C/C复合材料力学性能的显著影响.     

高温热处理对C/C多孔体显微结构的影响

以三维针刺碳毡作为预制体,先采用树脂单向加压浸渍结合热压固化制备了CFRP复合材料,然后通过树脂热解碳化制备出C/C多孔体。文章重点研究了高温热处理对C/C多孔体显微结构的影响。通过扫描电子显微镜观察了材料的显微结构,使用阿基米德方法测定材料的密度和气孔率,并利用压汞仪分析了材料的孔隙分布,利用X射线衍射分析碳基体的石墨化程度。结果显示,高温热处理后材料的密度降低,孔隙率增大;高温热处理没有改变材料中孔隙的类型,但使材料中三类孔隙尺寸均增大;经过高温热处理材料的石墨化度提高,部分块状碳基体转变为片层状石墨碳结构。     

液相法合成高容量LiFePO4/C复合正极材料

采用液相共沉淀法合成了纯相橄榄石型LiFePO4和LiFePO4/C复合正极材料。利用原子吸收(AAS)、X射线衍射(XRD)、扫描电镜(SEM)、振实密度测定等方法对其进行表征,并组装成电池研究其电化学性能。结果表明:LiFePO4和LiFePO4/C都具有单一的橄榄石型晶体结构,且前者的振实密度可达1.67 g/cm2,掺碳后制成的LiFePO4/C振实密度有所降低,但充放电平台非常平稳。与纯相LiFePO4相比,LiFePO4/C具有更高的放电比容量和循环性能,室温下以0.2 mA/cm2和0.4 mA/cm2电流密度充放电,首次放电比容量分别达到158.1 mA.h/g、150.0 mA.h/g。充放电循环20次后放电比容量仍分别保持在154.2 mA.h/g,137.2 mA.h/g。     

碳含量对LiFePO4/C复合正极材料性能的影响

Olivine-type LiFePO4/C composite cathode materials were synthesized by a solid-state reaction method in an inert atmosphere. The glucose was added as conductive precursors before the formation of the crystalline phase. The effects of glucose content on the properties of as-synthesized cathode materials were investigated. The crystal structure and the electrochemical performance were characterized by X-ray diffraction （XRD）, scanning electron microscopy （SEM）, laser particle-size distribution measurement and electrochemical performance testing. The material has a single crystal olivine structure with grain-sizes ca. 100-200 nm. SEM micrographs and the corresponding energy dispersive spectrometer （EDS） data confirm that the carbon particulates produced by glucose pyrogenation are uniformly dispersed among the LiFePO4 grains, ensuring a good electronic contact. Impedance spectroscopy was used to investigate the ohmic and kinetic contributions to the cell performance. It is found that increasing the carbon content leads to a reduction of the cell impedance due to the reduction of the charge transfer resistance. The galvanostatically charge and discharge tests show that the material obtained by adding 10% C （by mass） gives a maximum discharge capacity of 140.8mA·h·g^-1 at the same rate （C/10）. The material also displays a more stable cycle-life than the others.     

前驱体FePO4的制备工艺及其对流变相法合成LiFePO4/C正极材料的影响

以FeSO4·7H2O,NH4H2PO4,H2O2和NH3·H2O为原料,采用均相沉淀法制备前驱体FePO4·2H2O,再通过流变相法制得LiFePO4/C复合材料,研究了反应温度、搅拌速度和pH值等反应条件对合成LiFePO4/C的影响。采用XRD、SEM和恒流充放电方法表征了材料的结构、形貌和电化学性能。结果表明:当反应温度为60℃,搅拌速度为800 r/min,pH值为2.5时,合成的LiFePO4/C为纯相,且粒度均匀,粒径约为200 nm,在0.1 C充放电倍率下,其首次放电比容量达137 mAh/g。     

LiFePO4/C复合材料的制备和性能

采用高温固相合成法二次灼烧工艺制备锂离子电池正极复合材料LiFePO4/C。经300℃和650℃二次灼烧,得到了从纳米到亚微米尺寸的LiFePO4和LiFePO4/C复合材料。X射线衍射(XRD)结果表明,所得到的LiFePO4和LiFePO4/C样品具有单一的橄榄石型晶体结构,且具高纯度。在多种碳源(如乙炔黑、Vulcan XC-72碳黑、鳞状石墨、各向异性石墨和葡萄糖)制备的LiFePO4/C复合材料中,以葡萄糖为碳源合成的样品具有最好的电化学性能。在电池工作温度由室温提高到40℃时,由于复合材料的电子电导率增大和锂离子在材料中的扩散速度加快,电池的充放电循环性能明显提高。     

LiFePO4/C正极材料的制备及性能表征

以LiH2PO4、Fe2O3及葡萄糖为原材料,采用高温高能球磨法(HTHEBM)制备了性能优良的碳包覆磷酸铁锂(LiFePO4/C)正极材料.在该法中,高能球磨将机械能转变为热能,有效降低了烧结温度且减少了烧结时间,在600℃下9 h烧结后获得纯相的LiFePO4/C正极材料.利用X射线衍射、扫描电镜、透射电镜、电化学...     

LiFePO4／C正极材料中试生产工艺及产品性能研究

采用模板-高温固相技术中试生产LiFePO4／C复合正极材料,并用该材料组装成品电池。通过XRD、SEM及粒度分析对材料进行物理性能分析,通过循环性能、倍率放电性能和过充性能对电池进行电化学及安全性能研究。结果表明,该材料组装的成品电池循环600次后放电容量仍保持在标准容量的90％以上,在大电流放电下,容量减小不大。     

Flexural behavior of fiber and nanoparticle reinforced concrete at high temperatures

In this study, the flexural tests were conducted to investigate the effects of temperature, steel fiber, nano‐SiO₂, and nano‐CaCO₃ on flexural behavior of concrete at high temperatures. The load‐deflection curves of fiber and nanoparticle reinforced concrete (FNRC) were measured both at room and high temperatures. Test results show that the load‐deflection curves become flatter, and the flexural strength, peak deflection, and energy absorption capacity decrease seriously with the increase of temperature. Both steel fiber and nanoparticles could significantly improve the flexural behavior of the concrete at room and high temperatures. The energy absorption capacity of FNRC before the peak point increases with the increase of steel fiber volume fraction. The improvement of nano‐SiO₂ on flexural strength of FNRC at high temperature is better than that at room temperature, but the enhancement on energy absorption capacity is reverse. Nano‐SiO₂ is more effective than nano‐CaCO₃ in improving flexural behavior of concrete both at room and high temperatures.     

预制体结构对炭／炭复合材料力学性能影响

四种用于制备炭／炭（C／C）复合材料的预制体，即1K发布叠层坯体（1#坯体），3K发布叠层坯体（4#坯体），发市 炭纸叠层坯体（2#坯体），特殊炭毡 发布叠层坯体（3#坯本），并探索了预制体结构对C／C复合材料力学性能影响．研究表明：用1#坯体制备的C／C复合材料弯曲强度最高，2#坏体制备的材料弯曲强度最低，随著炭纤维（CF）体积含量的增加，用四种坯体制备的材料弯曲强度增大。确定了弯曲强度的优化配方．     

Densification rate and mechanical properties of carbon/carbon composites with layer-designed preform

In this work, carbon fiber needle-punching preforms were designed into two structure according to the density change along the thickness direction. One structure is designed to two layers with low-density layer and high-density layer, and the other is to three layers with low-density exterior layer and high-density interior layer. Then the effect of the preform with different structure on the densification rate and compressive properties of C/C composites was investigated. The results show that both two designed preforms can effectively avoid surface blocking, and lead to the faster densification rate of C/C composites during the chemical vapor infiltration processes. These results are attributable to the change of pore size distribution and pyrocarbon thickness, which was caused by fiber architecture designs. Meanwhile, these structural changes can improve the compressive strength of C/C composites, especially for the three-layer preforms. When the density of preforms is 0.45?g/cm³, C/C composites with three-layer preform has the highest compressive strength. The damage of most C/C composites with two and three-layer preforms is caused by shear or delamination failure, while that of C/C composites with common preforms usually caused by matrix collapse. Cracks in C/C composites with two and three-layer preforms always happened on the low-density layer, and consequently ceased or changed propagation direction in the interface between two layers.     

Preparation and properties of SiOC ceramic modified carbon fiber needled felt preform composites

SiOC ceramic modified carbon fiber needled felt preform composites (C_f/SiOC) with densities of 0.4 and 0.7 g/cm³ were prepared by precursor infiltration and pyrolysis (PIP) method using methyltrimethoxysilane (MTMS) and dimethyldimethoxysilane (DMDMS) as precursors. The densification behavior was investigated through scanning electron microscopy (SEM) analysis of microstructure of C_f/SiOC composites undergoing different PIP times. The results indicate that with increase of PIP times, a great amount of SiOC ceramic was introduced into the preform, completely covering on the carbon fibers and occupying the open pores. The thermal performance, mechanical properties, and oxidation resistance of the composites were studied via various tests. The results illustrate that after two-time PIP procedure, thermal conductivities of the composites are 0.41–2.54 and 1.28–4.04 W/(m·K) in z direction and x/y plane, respectively, at RT-1500 °C. The compressive strengths of the composite arrive at 2.1 MPa in z direction and 7.8 MPa in x/y plane, which are almost 3.5 times and 6.5 times, respectively, counterparts of the raw preform. The incorporation of SiOC ceramic can remarkably improve anti-oxidation ability of the composites at 600 °C. The oxidation weight loss is merely 2.1 wt% after 60-min oxidation at 600 °C.     

碳纤维增强碳化硅基复合材料的研究

碳纤维增强碳化硅基复合材料(Cf/SiC)具有良好的力学性能,作为特殊结构的功能材料,是航空航天领域和新能源领域的研究热点之一。本文主要阐述了增强相碳纤维的发展,复合材料的基体复合技术,以及复合材料界面相的研究,并展望了碳纤维复合材料在高新技术领域中的应用与发展前景。     

Microstructure of carbon/carbon composites reinforced with pitch-based ribbon-shape carbon fibers

Carbon/carbon composites were prepared with ribbon-shape pitch-based carbon fibers serving as reinforcement and thermosetting PFA resin and thermoplastic pitch as matrix precursors. The composites were heat treated to 1000, 1600 and 2700 °C. Microstructural transformations taking place in the reinforcement, carbon matrix, and the interface were studied using polarized optical and scanning electron microscopy. The fiber/matrix bond and ordering of the carbon matrix in heat-treated composites was found to vary depending on the heat treatment temperature of the fibers. Stabilized fiber cleaved during carbonization of resin-derived composites. In contrast, fibers retain their shape during carbonization of pitch matrix composites. Optical activity was observed in composites made with carbonized fibers; the extent decreases with increased heat treatment of the fibers. Studies at various heat treatment temperatures indicate that ribbon-shape fibers developed ordered structure at 1600 °C when co-carbonized with thermosetting resin or thermoplastic pitches.     

Oxidation behavior of SiC/glaze-precursor coating on carbon/carbon composites

In order to improve the oxidation behavior of carbon/carbon composites in a wide range of temperature, a new SiC/glaze-precursor coating was developed.The SiC layer was produced by slurry and sintering, while the glaze precursor layer was prepared by slurry and drying. The microstructures and phase compositions of the coating were analyzed by SEM and XRD, respectively. The oxidation resistance of the coated composites was investigated using both isothermal and temperature-programmed thermogravimetric analysis in the temperature range from room temperature to 1600 °C. The results showed that the oxidation behavior of the coating was mainly controlled by the diffusion of oxygen during the test.The coating showed excellent oxidation resistance and self-healing ability in a wide range of temperature.     

Electromechanical behavior of hybrid carbon/glass fiber composites with tension and bending

To understand the smart (i.e., good memory) characteristics of hybrid composites of carbon fibers (CFs) and glass fibers (GFs) with epoxy resin as a matrix, the changes in the electrical resistance of composites with tension and on bending were investigated. The electrical resistance behavior of composites under tension changed with the composition of the CF/GF, as well as with the applied strain. The fractional electrical resistance increased slowly with increasing strain within a relatively low strain region. However, with further loading it increased stepwise with the strain according to the fracture of the CF layers. The strain sensitivity of the samples increased with increasing CF weight percentage, and the samples incorporating more than 40 wt % CF showed a strain sensitivity higher than 1.54 for a single CF. The changes in the fractional electrical resistance with bending were not so dominant as those with tension. This difference was attributed to the action of two cancelling effects, which are the increasing and decreasing fractional electrical resistance due to tension and compression with bending, respectively. On recovery from a large applied bending, the fractional electrical resistance decreased slowly with unloading because of the increase of contacts between the fibers that resulted from the reorganization of ruptured CFs during the recovery. Even the composites incorporating a relatively small CF content showed an irreversible electrical resistance with both tension and bending. However, the strain sensitivity being larger with tension than with bending is ascribed to the difference in their mechanical behaviors. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2447–2453, 2002     

Effect of carbon nanotubes on the strength of adhesive lap joints of C/C and C/C-SiC ceramic fibre composites

Similar substrates of carbon/carbon (C/C) and carbon/carbon-silicon carbide (C/CSiC) composites were bonded with pure epoxy resin and the one containing 3% multiwall carbon nanotubes (MWCNTs). The results show that MWCNT/filled epoxy resin bonded C/CC/C and C/CSiCC/CSiC substrates have a higher adhesive joint strength than those bonded with epoxy resin alone. MWCNTs increase the toughness and strength of the epoxy resin, which increases the interface bond strength between two similar matching surfaces.