Effect of combined loading on cracks in graphite/epoxy composites

Moiré Interferometry technique has been used to study the effect of combined loading on crack displacement in graphite epoxy plates. Specimens with central cracks of various orientations were subjected to uniaxial tensile loads. Thus, the cracks were under biaxial-stress fields, mode I (tension stress) and mode II (sliding stress). The obtained fringe patterns represented contours of constant-in-plane displacements. Based on these fringe patterns, two types of displacements were determined at the crack center, crack opening displacement (COD) and crack shearing displacement (CSD). The effect of combined loading on COD and CSD as well as the behavior of the material under these conditions was investigated in more detail. Experimental data were compared with the linear elastic fracture mechanics (LEFM) solution.     

Properties of carbon-carbon composites based on exfoliated graphite

We report the first study of the mechanical and electrical properties of carbon-carbon composites based on flexible graphite foils modified with pyrolytic carbon. Our results demonstrate that slight densification with pyrolytic carbon enhances the chemical and thermal stability of the graphite foils, increases their Young’s modulus by a factor of 2.5, and reduces their resistivity by 25%. Decomposition of the polymeric carbon precursors poly(hydridocarbyne) and poly(naphthylhydridocarbyne) in an exfoliated graphite matrix increases the tensile strength of the composite by a factor of 2 compared to the as-prepared graphite foil.     

膨胀石墨/纳米氧化锌/锌复合电极材料的超级电容器性能

以膨胀石墨为原料,与滚压振动磨预处理得到的纳米锌粉混合,超声分散24h制备膨胀石墨-纳米氧化锌及锌的复合电极材料(EG/ZnO/Zn)。采用X射线衍射仪(XRD)、场发射扫描电子显微(SEM)、场发射透射电子显微镜(TEM)、拉曼光谱分析仪(Raman),对材料的微观结构及成分表征。结果表明,复合材料中含锌和氧化锌,纳米锌粉颗粒和氧化锌纳米棒在膨胀石墨表面和层间分散良好,其中氧化锌纳米棒呈现出六方晶系纤锌矿结构,其直径大约为20nm。利用电化学循环伏安和恒电流充放电对材料进行电化学电容性能测试,表明经处理的复合电极材料在0.1A/g的电流密度下有明显的赝电容特性,比电容达147F/g,其赝电容来源不只是欠电位沉积的化学吸附,还有氧化还原反应。     

ZrO2-膨化石墨/焦炭复合材料的结构和制备

运用两种不同技法制备了ZrO2纳米粒子改性的膨化石墨/焦炭复合材料,应用XRD、SEM和TEM对所得ZrO2改性炭材料进行了表征.技法1:在ZrO(NO3)2及NH4OH溶液中反复交替地浸渍低密度膨化石墨/焦炭块体,随后在1200℃氮氛中热处理,以使在炭块自由表面沉积ZrO2纳米粒子的薄层.技法2:将可膨胀石墨、酚醛树脂粉和ZrOC2 O4-改性的纤维素纤维混合物封装于一容器中,使之经受900℃热激震,随后在氮氛中1200℃热处理,获得改性复合材料.结果表明:复合材料中ZrO2纳米粒子呈现三种尺寸:6nm～30nm为单独的纳米粒子和小微粒;200nm～1000nm为长树突状结构物;1μm～40μm为形如纤维素前驱体的杆状物.     

Hierarchical composites of conductivity controllable polyaniline layers on the exfoliated graphite for dielectric application

Hierarchical composites of polyaniline fibers on the surface of exfoliated graphite were synthesized by chemical oxidation method. The conductivity of polyaniline fibers was controlled by doping and dedoping procedures. The morphology, structure and chemical character of doped and dedoped hierarchical composites were systemically investigated by SEM, TEM, XRD, FT-IR, XPS and conductivity measurement. After dedoping procedure, polyaniline/exfoliated graphite composites were used to improve dielectric property of the electroactive polymer poly(vinylidene fluoride). The dielectric constant and loss tangent of composites were 17 and 0.06 (10³ Hz) when the polyaniline/exfoliated graphite loading was 3 wt%. Of great interest was that the dielectric loss tangent of composites showed inherent low loss of poly(vinylidene fluoride) because insulating polyaniline fibers on the surface of exfoliated graphite effectively prevented direct contact of exfoliated graphite flakes in poly(vinylidene fluoride) matrix.     

Effect of graphite nitrate exfoliation conditions on the released gas composition and properties of exfoliated graphite

We have studied the fundamental aspects of the exfoliation of intercalation compounds of natural flake graphite with nitric acid and proposed a technique for qualitative and quantitative in situ analysis of the gases forming during graphite nitrate exfoliation. We have examined the effects of the graphite nitrate exfoliation atmosphere, temperature, and time on the released gas composition and the properties of the resulting exfoliated graphite.     

Nanocomposites based on polystyrene-b-poly(ethylene-r-butylene)-b-polystyrene and exfoliated graphite nanoplates: Effect of nanoplatelet loading on morphology and mechanical properties

Exfoliated graphite nanoplates (xGnPs)/polystyrene-b-poly(ethylene-r-butylene)-b-polystyrene (SEBS) nanocomposites have been prepared by the simple melt-compounding approach. The structural, mechanical and viscoelastic properties of these composites were studied and compared. Wide-angle X-ray diffraction (WAXD) studies indicated that the processing of nanocomposites did not change the original d-spacing of xGnPs. Scanning electron microscopy observation on the fracture surfaces of the composites shows a uniform dispersion of xGnPs throughout SEBS matrix and strong interfacial adhesion between oxidized xGnPs and the matrix, which are responsible for the considerable enhancement of mechanical properties of the composites. It is found that the addition of xGnPs particles improved both the elastic modulus and storage modulus of pure SEBS significantly and the higher the xGnPs content, the higher the modulus of the nanocomposite. Moreover, the effects of dispersed xGnPs on the microphase separation of SEBS have also been investigated using small angle X-ray scattering (SAXS).     

层间韧化的碳纤维复合材料层压板的力学性能

研究了"离位"增韧的双马来酰亚胺复合材料层压板的Ⅰ型和Ⅱ型层间断裂韧性、层间微结构及其形貌变化、冲击分层损伤和冲击后压缩强度,以及典型面内、面外的基本力学性能.结果表明,通过热反应失稳分相、相反转和颗粒粗化等一系列过程,"离位"增韧在复合材料的层间形成了热塑性树脂/热固性树脂双连续的3-3相结构,这种结构大大提高了复合材料层压板的层间韧性、冲击损伤阻抗和损伤容限等性能.     

Electrodeposition of exfoliated graphite nanoplatelets onto carbon fibers and properties of their epoxy composites

Exfoliated graphite nanoplatelet (xGnP)/copper (Cu) coated carbon fibers were fabricated by electrophoretic deposition under different applied voltages. The electrical and mechanical properties of individual fibers and composites made from these fibers and epoxy resin were investigated. The electrical resistivity of xGnP/Cu coated single carbon fiber is lower than that of the uncoated control sample and decreases with increase in the applied voltage. The xGnP and metallic Cu were simultaneously deposited on the carbon fiber surface as a result of the electrochemical cell configuration. The interfacial shear strength decreases with applied voltage up to 30 V but increases with applied voltage of over 30 V. The interfacial shear strength for the coated samples except the 50 V treated sample is lower than that of control sample. The flexural strength and modulus of xGnP/Cu coated carbon/epoxy composites is higher than those of control sample due to the reinforcing effect of xGnP/Cu coated on the carbon fibers.     

鳞片石墨取向对鳞片石墨/聚丙烯、鳞片石墨/尼龙66复合材料导热性能的影响

为了研究鳞片石墨在基体中的取向对复合材料导热性能特别是不同方向导热性能的影响,通过双螺杆挤出混合及注射成型制备了鳞片石墨/聚丙烯（PP）、鳞片石墨/尼龙66（PA66）导热复合材料,并利用扫描电子显微镜和超声波测试对制备的样品进行了分析。结果表明：鳞片石墨的粒径越小,平面取向度越高,平面与垂直方向的热导率差值越大。加工中双螺杆挤出机的过度剪切会破坏鳞片石墨的片层结构,影响鳞片石墨导热网络的形成,降低复合材料的热导率,但提高了材料导热的各向均匀性。适度的剪切可以打开鳞片石墨的片层结构,提高复合材料的热导率,注射成型更多影响到制品导热的各向异性。     

Effect of tungsten addition on thermal conductivity of graphite/copper composites

Graphite/copper composites with high thermal conductivity were fabricated by tungsten addition, which formed a thin tungsten carbide layer at the interface. The microstructure and thermal conductivity of the composite material were studied. The results indicated that the insertion of tungsten carbide layer obviously suppressed spheroidization of copper coating on the graphite particles during the sintering process, and decreased the interfacial thermal resistance of the composites. Compared with the graphite/copper composites without tungsten, the thermal conductivity of the obtained composites was increased by 43.6%.     

Splitting initiation and propagation in notched unidirectional graphite/epoxy composites under tension-tension cyclic loading

The initiation and propagation of splitting damage in notched unidirectional graphite/epoxy composites under tension-tension cyclic loading were investigated experimentally. One-hundred-and-twenty [0°]₈ specimens, notched at the center with a 3·175, 6·350, or 9·525 mm hole or slit, were tested. Thirty specimens were loaded monotonically to failure to determine the static splitting stress. Ninety specimens were cycled at maximum loads corresponding to 80%, 70%, and 60% of their predicted static splitting stress. The stress ratio of the load cycle was maintained at a nominal value of $\text{R}{}_{\mathrm{\sigma }}\text{= 0·1}$, while the frequency was changed with the stress amplitude to maintain the same loading rate. The results show that the static splitting stress is sensitive to both notch type and size and that the split length varies linearly with the natural logarithm of the number of accumulated load cycles. The intercept of this linear relation depends on the notch type, notch size, and stress level while its slope depends only on the notch size and stress level. Empirical functional forms are derived using multiple linear regression analyses of the test data.     

Effect of processing technique on the transport and mechanical properties of graphite nanoplatelet/rubbery epoxy composites for thermal interface applications

Graphite nanoplatelet (GNP)/rubbery epoxy composites were fabricated by mechanical mixer (MM) and dual asymmetric centrifuge speed mixer (SM). The properties of the GNP/rubbery epoxy were compared with GNP/glassy epoxy composites. The thermal conductivity of GNP/rubbery epoxy composite (25 wt.% GNP, particle size 15 μm) reached 2.35 W m⁻¹ K⁻¹ compared to 0.1795 W m⁻¹ K⁻¹ for rubbery epoxy. Compared with GNP/rubbery epoxy composite, at 20 wt.%, GNP/glassy epoxy composite has a slightly lower thermal conductivity but an electrical conductivity that is 3 orders of magnitude higher. The viscosity of rubbery epoxy is 4 times lower than that of glassy epoxy and thus allows higher loading. The thermal and electrical conductivities of composites produced by MM are slightly higher than those produced by SM due to greater shearing of GNPs in MM, which results in better dispersed GNPs. Compression and hardness testing showed that GNPs increase the compressive strength of rubbery epoxy ∼2 times without significantly affecting the compressive strain and hardness. The GNP/glassy epoxy composites are 40 times stiffer than the GNP/rubbery epoxy composites. GNP/rubbery epoxy composites with their high thermal conductivity, low electrical conductivity, low viscosity before curing and high conformability are promising thermal interface materials.     

膨胀石墨负载纳米TiO2光催化剂的制备及对甲基橙溶液的吸附与光降解

以钛酸丁酯为钛源,膨胀石墨为载体,采用溶胶-凝胶法成功地制备了膨胀石墨(EG)负载TiO2光催化剂(TEG),采用X射线衍射分析(XRD)、扫描电镜(SEM)、比表面积等分析技术对样品进行表征,以甲基橙为目标降解物,考察了热处理温度、负载次数、目标降解物浓度等不同条件下催化剂的光催化性能.结果表明,氧化钛以纳米颗粒的形式附着在膨胀石墨薄片表面,具有疏松多孔蠕虫状结构的膨胀石墨为氧化钛提供高浓度的三维降解环境,负载2次在500℃下经过3h热处理得到的催化剂在紫外光激发下对甲基橙溶液表现出较高的光催化活性.     

石墨镀Sn调控对石墨/Cu复合材料组织及力学性能的影响

石墨/Cu自润滑复合材料具有良好的摩擦学性能和耐腐蚀性能,在高速铁路领域具有广阔的应用前景.传统石墨/Cu自润滑复合材料中由于石墨与基体不润湿,复合材料界面结合强度低,在材料承受载荷时容易造成石墨相的剥离、脱落,导致复合材料在高载荷服役条件下性能较差.采用化学镀覆工艺在石墨表面镀覆软金属Sn元素调控石墨/Cu复合材料界...     

掺CCCW的碳纤维石墨水泥基复合材料的导电及压阻特性

利用四电极法研究了内掺水泥基渗透结晶防水材料(CCCW)的碳纤维石墨水泥基复合材料试样(40 mm×40 mm×40 mm)的导电特性及其在循环荷载作用下的压阻特性,分析讨论了碳纤维石墨水泥砂浆的体积电阻率及压阻特性随石墨掺量的变化规律。碳纤维和CCCW的掺量分别为水泥质量的1%和4%;石墨掺量分别为水泥质量的0%、10%、20%、30%、40%和50%。结果表明,添加CCCW的碳纤维石墨砂浆试样的体积电阻率随石墨掺量的增加迅速下降,并存在渗滤现象,渗滤阈值为20%左右。在循环荷载作用下,不同石墨掺量试样的电阻和应力存在一定的对应关系。石墨掺量为水泥质量的20%～30%时,碳纤维石墨水泥砂浆试样的体积电阻率与压应力呈现良好的可重复性,电阻值在应力加载时几乎呈线性下降,而卸载时增加。     

Adsorption of4He,Ar, and N2 on exfoliated graphite and of4He on Ar-coated exfoliated graphite at low temperatures

The results of measurements of adsorption isotherms are given for ⁴ He, Ar, and N ₂ on bare exfoliated graphite (Grafoil) and for ⁴ He on Grafoil covered with a monolayer of argon. The temperatures employed were 77.3 K for the argon and N ₂ , and the range 4.2–18.55 K for the⁴He. The pressure range was from 0.25 to 150 Torr. The Grafoil surface area was calculated from the argon isotherm at 77.3 K, yielding a value of 19.9 m ² /g or a total surface area of 672 m ² for the 33.81 g of adsorbent used in these experiments. From the ⁴ He isotherms the isosteric heats of adsorptionQ _st /R were calculated. In the limit of zero coverage,Q _st /R on the argon monolayer was 82 K, and on the bare GrafoilQ _st /R lies between 130 and 170 K. At theoretical monolayer coverage on the argon-coated GrafoilQ _st /T=68 K. These results are compared with theoretical evaluations and with previous calorimetric data on other graphites.Work supported in part by a grant from the National Science Foundation and by contracts with DOD (Themis Program) and ONR.     

Effect of water removal on the textural properties of resorcinol/formaldehyde gels by azeotropic distillation

The resorcinol/formaldehyde gels containing 5% (w/v) of reactants have been prepared. After curing for seven days, the gel is treated with trifluoroacetic acid to initiate the condensation of hydroxymethyl groups. Drying of the prepared gels is done by removing water with the help of vacuum heterogeneous azeotropic distillation using amyl acetate and n-butanol. The textural properties of the gels thus obtained are compared with the properties of the gel obtained from exchange of water by acetone (control experiment). The pore volume and average pore diameter of the gel obtained from the azeotropic distillation of water with amyl acetate are 0.7663 × 10–3 m3/kg and 0.0145 m, respectively. These values are 1.5 times higher than the values obtained by vacuum heterogeneous azeotropic distillation with n-butanol and about twice than the values obtained in the control experiment. The surface areas in all the three gels are almost constant.     

Enhanced conductivity behavior of polydimethylsiloxane (PDMS) hybrid composites containing exfoliated graphite nanoplatelets and carbon nanotubes

Polydimethylsiloxane (PDMS) hybrid composites consisting of exfoliated graphite nanoplatelets (xGnPs) and multiwalled carbon nanotubes functionalized with hydroxyl groups (MWCNTs-OH) were fabricated, and the effects of the xGnP/MWCNT-OH ratio on the thermal, electrical, and mechanical properties of polydimethylsiloxane (PDMS) hybrid composites were investigated. With the total filler content fixed at 4 wt%, a hybrid composite consisting of 75% × GnP/25% MWCNT-OH showed the highest thermal conductivity (0.392 W/m K) and electrical conductivity (1.24 × 10⁻³ S/m), which significantly exceeded the values shown by either of the respective single filler composites. The increased thermal and electrical conductivity found when both fillers are used in combination is attributed to the synergistic effect between the fillers that forms an interconnected hybrid network. In contrast, the various different combinations of the fillers only showed a modest effect on the mechanical behavior, thermal stability, and thermal expansion of the PDMS composite.