碳纤维增强环氧树脂基复合材料湿热残余应力的微Raman光谱测试表征

采用微Raman光谱仪对碳纤维增强环氧树脂复合材料CF/EP(纤维体积分数为30%)的湿热残余应力进行了研究。实验结果表明：湿热残余应力能够使碳纤维Raman光谱发生频移,根据频移可对纤维所受湿热残余应力进行表征;选择合适的试验点是复合材料湿热残余应力Raman测试成功的关键;在湿热环境下长期吸湿,纤维所受轴向残余应力由吸湿前的热残余压应力转变成吸湿后的湿热残余拉应力;由吸湿后碳纤维所受湿热残余拉应力减去吸湿前热残余压应力获得的吸湿拉应力非常大,平均为2272 MPa,接近所用碳纤维的拉伸强度(2800 MPa);适当的加工热残余压应力有利于降低吸湿导致的应力。     

Evaluation of interfacial shear stress between multi-walled carbon nanotubes and epoxy based on strain distribution measurement using Raman spectroscopy

This study investigated the stress recovery of aligned multi-walled carbon nanotubes (MWCNTs) embedded in epoxy using Raman spectroscopy, and evaluated interfacial shear stress between MWCNTs and epoxy using shear-lag analysis. To this end, ultralong aligned MWCNTs (3.8 mm long) were embedded in epoxy to obtain Raman spectra at multiple points along the MWCNTs. Downshift of the G′-band due to tensile strain was measured from the nanotube end to the center, and the strain distribution of embedded MWCNTs was evaluated successfully. Interfacial shear stress was then estimated by minimizing the error between the shear-lag analysis and measured strain distribution. The maximum interfacial shear stress between the embedded MWCNTs and epoxy was 10.3–24.1 MPa at the failure strain of aligned MWCNT-reinforced epoxy composites (0.46% strain). Furthermore, the interfacial shear stress between an individual MWCNT and epoxy was investigated.     

Effect of oxygen plasma treatment of PPTA and PBO fibers on the interfacial properties of single fiber/epoxy composites studied by Raman spectroscopy

The interfacial micromechanics of single poly(p-phenylene terephthalamide (PPTA) and poly(p-phenylene benzobisoxazole (PBO) fibers embedded in an epoxy resin has been investigated by determining the interfacial shear stress distributions along the fiber length. The effects of an oxygen plasma treatment on the interfacial shear stress of the fiber-epoxy systems are analyzed. Raman spectroscopy was used to map the stress distributions along the fiber when the composite is subjected to a small axial tensile strain (3.5% for PPTA and 2.5% for PBO). The quality of the interface or adhesion was improved after the surface treatment, supporting the ability of plasma oxidation to enhance the adhesion of high-performance fibers to epoxy resins. The tensile behavior of fiber-reinforced systems was different in each case. PPTA reinforcements underwent fragmentation, likely by fiber microfailure, whereas debonding or bridging is the most probable fragmentation mechanism in the case of PBO.     

Controlling of the interfacial shear strength between thermoplastic resin and carbon fiber by adsorbing polymer particles on carbon fiber using electrophoresis

In order to control the interfacial adhesion between carbon fibers and thermoplastic resins, poly(methyl methacrylate) (PMMA) particles have been adsorbed on the carbon fiber surfaces using an electrophoresis process. The amount of PMMA particles adsorbed on the modified carbon fibers was varied using the electrophoresis technique performed in polymer colloids for a short time. Additionally, the interfacial shear strength between the modified carbon fiber and the resin was controlled by a modification of the present process. An improved interaction and a strengthened surface adhesion between the carbon fiber coated with particles and the PMMA resin were observed.     

A study of mechanisms of stress transfer in continuous- and discontinuous-fibre model composites by laser Raman spectroscopy

The micromechanics of stress transfer in single-fibre/epoxy-resin model composites has been investigated. Two specimen geometries are examined incorporating both continuous and discontinuous fibres in epoxy resin blocks. In both cases, the point-by-point strain in the fibre is measured from the fibre Raman spectrum and its strain dependence. The continuous-fibre model composites (CFMC) are subjected to incremental tensile loading and the fibre fragmentation process is continuously monitored. The short-fibre model composites (SFMC) are loaded incrementally to levels of stress of sufficient magnitude to cause interfacial failure and the fibre strain profiles are obtained at each level of applied stress.

In addition to fibre strain measurements, the interfacial shear stress distribution is derived at each increment of applied stress by means of a balance-of-forces argument. The effects of fibre surface treatment and fibre modulus on the strain transfer profile and the distribution of the interfacial shear stress along the fibre are examined. The importance of parameters such as fibre/matrix debonding and interphase yielding in the vicinity of fibre breaks or fibre ends is discussed.     

Strain-sensitive Raman spectroscopy and electrical resistance of carbon nanotube-coated glass fibre sensors

This paper presents the development of glass fibres coated with nanocomposites consisting of carbon nanotubes (CNTs) and epoxy. Single glass fibres with different CNT content coating are embedded in a polymer matrix as a strain sensor for composite structures. Raman spectroscopy and electrical response of glass fibres under mechanical load are coupled for in situ sensing of deformation in composites. The results show that the fibres with nanocomposite coating exhibit efficient stress transfer across the fibre/matrix interface, and these with a higher CNT content are more prone to fibre fragmentation at the same matrix strain. A relationship between the fibre stress and the change in electrical resistance against the fibre strain is established. The major finding of this study has a practical implication in that the fibres with nanocomposite coating can serve as a sensor to monitor the deformation and damage process in composites.     

Electrical conductivity and Raman imaging of double wall carbon nanotubes in a polymer matrix

Raman spectroscopy is used to access the dispersion state of DWNTs in a PEEK polymer matrix. The interaction of the outer tube with the matrix can be determined from the line shape of the Raman G band. This allows us to distinguish regions where the nanotubes are well dispersed and regions where the nanotubes are agglomerated. The percolation threshold of the electrical conductivity of the double wall carbon nanotubes (DWNTs)/PEEK nanocomposites is found to be at 0.2-0.3 wt%. We find a maximum electrical conductivity of 3 × 10⁻² S cm⁻¹ at 2 wt% loading. We detect nanotube weight concentrations as low as 0.16 wt% by Raman spectroscopy using a yellow excitation wavelength. We compare the Raman images with transmission electron microscopy images and electrical conductivity measurements. A statistical method is used to find a quantitative measure of the DWNTs dispersion in the polymer matrix from the Raman images.     

Control of interfacial adhesion in continuous carbon and kevlar fiber reinforced polymer composites

Carbon fiber surfaces were treated by cold plasmas of oxygen, nitrogen, argon, ammonia, and propylene. A two-component bismaleimide, an epoxy, and a model thermoplastic resin polypropylene were used as the matrices for composites. The effectiveness of various plasmas in improving the interfacial adhesion between carbon fibers and matrix resins was demonstrated. Predominant adhesion promotion mechanisms as influenced by various plasma treatments were determined. Oxygen and argon plasmas were found to promote mechanical keying by increasing the level of fiber surface roughness and porosity. The wettability of carbon fiber surface by the matrix resin was also enhanced by oxygen plasmas and argon plasmas (to a lesser extent), as evidenced by the increased total surface energies and their polar components. These surface energy increases are mainly due to the various oxygen-containing functional groups observed on the oxygen plasma-treated surface. For the cases of ammonia and combined ammonia/argon plasma treatments, possible chemical bonding between bismaleimide and the plasma-deposited amine groups is one important promoter of interfacial bonding. In these cases increased wettability was also observed. Ammonia and ammonia/argon plasmas appear to be the more appropriate treatments for carbon-fiber/thermoset resin composites considering that they generally do not induce any appreciable reduction in fiber strength. In contrast, excessively prolonged exposure of carbon fibers to oxygen, nitrogen or argon plasma could lead to a significant reduction in fiber strength. The plasma-polymerized polypropylene deposited on the fiber surface was capable of improving the compatibility and adhesion between the fiber and the polypropylene matrix.     

Analysis of the fragmentation test for carbon-fibre/epoxy model composites by means of Raman spectroscopy

The interfaces between high-modulus PAN-(T50) and mesophase pitch-based (P55) carbon fibres and an epoxy matrix have been studied by using the conventional fragmentation test in conjunction with polarised-light optical microscopy. Raman spectroscopy has also been used to follow stress transfer from the matrix to the fibres for the same fragmentation geometries. The level of fibre/matrix adhesion and mechanisms by which the stress is transfered from the matrix to the fibres has been determined from both the stress birefringence patterns and strain-induced Raman band shifts in the fibres. The values of interfacial shear strength have been determined by means of both the conventional analysis and the Raman technique. It is found that the Raman method gives a much more detailed picture of stress transfer in the test specimens and that the two methods give somewhat different values of the interfacial shear strength. The values of interfacial shear stress have been discussed with respect to fibre surface energy, surface chemistry and surface morphology. It was found that the surface chemical functional groups appear to have no direct correlation with interfacial shear strength. Furthermore, it appears that mechanical interlocking due to surface roughness could contribute to the higher values of interfacial shear strength determined for the PAN-based fibre.     

Influence of ultrasonic treatment on the characteristics of epoxy resin and the interfacial property of its carbon fiber composites

To enhance the interfacial property between a carbon fiber and epoxy matrix, an ultrasonic technique was used to treat the resin liquid and the impregnated fibers respectively. The effects of the treatments on the characteristics of the resin system and the fiber surface, as well as fiber/matrix interfacial bonding strength, were analyzed and discussed. The results indicated ultrasonic treatments significantly decreased the viscosity and surface tension of the resin system, and increased the wettability and the oxygen content of the fiber surface due to the ultrasonic cavitation effects. Microbond tests revealed much more increase of interfacial shear strength when the ultrasound was applied to the impregnated fibers, and combination failures of interface and matrix layer were observed from the pulled-out fiber surface.     

Plasma treatment of the carbon fiber bipolar plate for PEM fuel cell

A Polymer Electrolyte Membrane Fuel Cell or Proton Exchange Membrane (PEM) fuel cell stack is composed of a Gas Diffusion Layer (GDL), an Membrane Electrode Assembly (MEA), and bipolar plates. The electrical resistance of the bipolar plates for a PEM fuel cell should be very low, so that the electricity generated in the fuel cell is conducted through the bipolar plates with minimum electrical loss.     

纳米TiO2粉末的结构表征:X射线衍射、透射电镜及高温Raman光谱分析

采用沉淀-胶溶-絮凝法，以偏钛酸为反应物．制备出锐钛矿型纳米TiO2粉末．用X射线衍射(XRD)、透射电镜(TEM)和高温Raman光谱对其结构进行表征。并采用高温Raman光谱仪对所制备的纳米TiO2从25～1200℃进行了原住Raman光谱研究。温度较低时，锐钛矿相的Raman谱峰明显可见．由于高温下Raman信号减弱，随着温度升高，Raman特征谱峰逐渐减弱以至于全部消失，仅显示出几个宽宽的凸起。同时，高温Raman光谱存在明显的温度效应，因而导致了随温度升高144cm^-1和196cm^-1谱峰的蓝移,640cm^-1谱峰的红移以及谱峰宽化。     

Mechanical characterization of epoxy composite with multiscale reinforcements: Carbon nanotubes and short carbon fibers

Carbon nanotubes (CNT) and short carbon fibers were incorporated into an epoxy matrix to fabricate a high performance multiscale composite. To improve the stress transfer between epoxy and carbon fibers, CNT were also grown on fibers through chemical vapor deposition (CVD) method to produce CNT grown short carbon fibers (CSCF). Mechanical characterization of composites was performed to investigate the synergy effects of CNT and CSCF in the epoxy matrix. The multiscale composites revealed significant improvement in elastic and storage modulus, strength as well as impact resistance in comparison to CNT–epoxy or CSCF–epoxy composites. An optimum content of CNT was found which provided the maximum stiffness and strength. The synergic reinforcing effects of combined fillers were analyzed on the fracture surface of composites through optical and scanning electron microscopy (SEM).     

Mixed resin and carbon fibres surface treatment for preparation of carbon fibres composites with good interfacial bonding strength

The objective of this work is to improve the interlaminar shear strength of composites by mixing epoxy resin and modifying carbon fibres. The effect of mixed resin matrix’s structure on carbon fibres composites was studied. Anodic oxidation treatment was used to modify the surface of carbon fibres. The tensile strength of multifilament and interlaminar shear strength of composites were investigated respectively. The morphologies of untreated and treated carbon fibres were characterized by scanning electron microscope and X-ray photoelectron spectroscopy. Surface analysis indicates that the amount of carbon fibres chemisorbed oxygen-containing groups, active carbon atom, the surface roughness, and wetting ability increases after treatment. The tensile strength of carbon fibres decreased little after treatment by anodic oxidation. The results show that the treated carbon fibres composites could possess excellent interfacial properties with mixed resins, and interlaminar shear strength of the composites is up to 85.41 MPa. The mechanism of mixed resins and treated carbon fibres to improve the interfacial property of composites is obtained.     

Magnetic field enhances the graphitized structure and field emission effect of carbon nanotubes

This study uses a low temperature thermal chemical vapor deposition with an applied external magnetic field to grow carbon nanotubes (CNTs) on Ni/Ag-printed glass substrates. A mixture of C₂H₂ and H₂ gas was used for the growth of the CNTs. A Ni catalyst layer was deposited on the Ag-printed glass substrate by pulse electroplating. Scanning electron micrographs as well as the presence of two sharp peaks at 1320 cm⁻¹ (D band) and 1590 cm⁻¹ (G band) in the Raman spectra indicate that the graphitized structure of CNTs synthesized under a magnetic field has higher quality (i.e., a D-band to G-band intensity ratio of 0.303) than CNTs synthesized without a magnetic field. Transmission electron micrographs show a fine Ni catalyst at the tip of the tube for CNTs synthesized under a magnetic field, exhibiting a CNT “tip-growth” model. The synthesis of CNTs in the presence of a magnetic field also generates better field emission properties and better lighting morphology than without a magnetic field.     

Comparing single-walled carbon nanotubes and samarium oxide as strain sensors for model glass-fibre/epoxy composites

The study of the interfacial stress transfer for glass fibres in polymer composites through the fragmentation test requires certain assumptions, such as a constant interfacial shear stress. In order to map the local interfacial properties of a composite, both Raman spectroscopy and luminescence spectroscopy have been independently used. Unlike other polymer fibre composites, the local strain state of a glass fibre cannot be obtained using Raman spectroscopy, since only very broad and weak peaks are obtainable. This study shows that when single-walled carbon nanotubes (SWNTs) are added to the silane sizing as a strain sensor, it becomes possible to map the local fibre strain in glass fibres using Raman spectroscopy. Moreover, if this model glass fibre contains a small amount of Sm₂O₃, as one of the components, luminescence spectroscopy can be simultaneously used to confirm this local fibre strain. A combined micromechanical properties study of stress transfer at the fibre–matrix interface using luminescence spectroscopy, together with Raman spectroscopy, is therefore reported. The local strain behaviour of both Sm³⁺ doped glass and SWNTs in the silane coating are shown to be consistent with a shear-lag model. This indicates that Sm³⁺ dopants and SWNTs are excellent sensors for the local deformation of glass fibre composites.     

Synthesis and characterization of carbon nanotube-reinforced epoxy: Correlation between viscosity and elastic modulus

We report the synthesis and characterization of nanocomposite thin films consisting of single-walled carbon nanotubes with different functionalization schemes dispersed in an epoxy matrix. The thermal, rheological, and mechanical properties of nanocomposite thin films were experimentally characterized to establish a relationship between processing and performance. The results from the rheological analysis confirmed that the nanotube type and functionalization strongly affect the resin viscosity during cure. A correlation between the rheological behaviour and the measured elastic properties was established. Nanotubes produced by plasma and functionalized with carboxyl group had the lowest influence on viscosity and led to the highest improvement in elastic properties. The measured increase in elastic modulus was consistent with predictions based on Mori–Tanaka micromechanics.     

Effect of γ-ray irradiation grafting on the carbon fibers and interfacial adhesion of epoxy composites

A special technique using γ-ray irradiation-induced graft-polymerization was applied to carbon fibers. Epoxy resin and chloroepoxy propane reacted with carbon fibers by a co-irradiation grafting method and acrylic acid was graft-polymerized onto the fiber surface via a pre-irradiation grafting method. The roughness, amount of containing-oxygen functional groups and surface energy were all found to increase significantly after irradiation grafting. Gamma-ray irradiation grafting improved marginally the tensile strength of carbon fibers, which was evaluated by statistical analysis using the Weibull distribution. The interlaminar shear strength of treated carbon fiber/epoxy was enhanced by at least 17.5%, compared with that of untreated carbon fiber/epoxy. The mechanisms of irradiation grafting are proposed by radical reactions. The γ-ray co-irradiation grafting and pre-irradiation grafting are both an effective method for modifying the physicochemical properties of carbon fibers and improving the interfacial adhesion of composites.     

Probing deformation of double-walled carbon nanotube (DWNT)/epoxy composites using FTIR and Raman techniques

Two of the limitations of carbon nanotube (CNT) polymer composites have been the low volume fraction of nanotubes and inadequate load transfer from the polymer to the stiff CNT. Here, we have utilized functionalized mats of double-walled nanotubes (DWNT) to obtain 10 wt.% DWNT in an epoxy matrix, with strength approaching those of quasi-isotropic carbon fiber composites. We used the transmission FTIR technique with in situ loaded specimens to monitor spectral shift per unit applied stress for understanding load transfer behavior at the nanotube–epoxy interface. Tests show that in most cases a tensile stress causes negative FTIR peak shift in the neat epoxy, but this behavior is not always observed for the epoxy matrix in the composite. The FTIR data can be used successfully to estimate the average matrix stress in the composite and thereby the average stress in the nanotubes. In situ Raman studies using the G′ peak are also conducted to obtain complementary information on average tensile stress in the DWNT in the loading direction. The shift response is found to be ∼37 cm⁻¹/GPa.     

Characterization of sol-gel derived TiO2/ZrO2 films and powders by Raman spectroscopy

The effect of zirconium dioxide addition on crystal structure of sol-gel TiO₂ mesoporous films and powders has been investigated by means of Raman spectroscopy, X-Ray diffraction, and Atomic force microscopy. Zirconium incorporation (up to 30 mol%) into TiO₂ lattice resulted in the formation of Ti_1 − xZr_xO₂ solid solution with anatase structure for the binary powders has been proved. Appearance of tetragonal ZrO₂ phase was observed for the samples with high zirconium content.