Influence of different carbon nanotubes on the mechanical properties of epoxy matrix composites – A comparative study

Carbon nanotubes (CNTs) in general are considered to be highly potential fillers to improve the material properties of polymers. However, questions concerning the appropriate type of CNTs, e.g., single-wall CNTs (SWCNT), double-wall CNTs (DWCNT) or multi-wall CNTs (MWCNT), and the relevance of a surface functionalisation are still to be answered. This first part of the study focuses on the evaluation of the different types of nanofillers applied, their influence on the mechanical properties of epoxy-based nanocomposites and the relevance of surface functionalisation. The nanocomposites produced exhibited an enhanced strength and stiffness and even more important, a significant increase in fracture toughness (43% at 0.5 wt% amino-functionalised DWCNT). The influence of filler content, the varying dispersibility, the aspect ratio, the specific surface area and an amino-functionalisation on the composite properties are discussed and correlated to the identified micro-mechanical mechanisms.     

Impact damage of carbon fiber polymer–matrix composites, studied by electrical resistance measurement

Drop impact damage of continuous carbon fiber epoxy–matrix composite laminates, was studied by electrical resistance measurement, which was shown to be more sensitive than the ultrasonic method. The oblique resistance at an angle between the longitudinal and through-thickness directions was more effective than the surface longitudinal resistance in indicating damage, particularly interior damage. The oblique resistance values from longitudinal segments of a specimen were not additive, but the surface resistance values were. In the case of a unidirectional composite, electrical contacts at 45° from the longitudinal direction in the plane of the laminate were more effective than those at 90°. Even minor damage associated with negligible indentation was sensed. The spatial distribution of damage was also studied.     

Morphology and electrical properties of polyamide 6/polypropylene/multi-walled carbon nanotubes composites

Morphology, electrical properties and conductive mechanisms of polyamide 6/polypropylene/muti-walled carbon nanotubes (PA6/PP/MWNTs) composites with varied compositions and different blending sequences were investigated. The MWNTs were found to be located preferentially in the PA6 phase in the composites, whatever the PA6 was continuous or dispersed phase. While the incorporation of MWNTs changed the dispersed PA6 phase from spherical to elongated or irregular shape. The PA6/PP/MWNTs (20/80/4) composite with a dispersed PA6 phase exhibited a higher electrical conductivity in comparison with the PA6/PP/MWNTs (50/50/4) composite which has a co-continuous phase and exhibits double percolation. This was due to the formation of a conductive MWNTs networks in the PA6/PP/MWNTs (20/80/4) composite as proved by means of field emission scanning electron microscopy and rheological measurements. The morphology and electrical properties of the PA6/PP/MWNTs (20/80/4) composites were significantly influenced by blending sequences. When blending 3.9 phr MWNTs with a pre-mixed PA6/PP/MWNTs (20/80/0.1) composite, the dispersed PA6 phase formed an elongated structure, which was beneficial to the electrical properties.     

Fabrication and mechanical properties of well-dispersed multiwalled carbon nanotubes/epoxy composites

Multiwalled carbon nanotubes (MWCNTs)/epoxy nanocomposites were fabricated by using ultrasonication and the cast molding method. In this process, MWCNTs modified by mixed acids were well dispersed and highly loaded in an epoxy matrix. The effects of MWCNTs addition and surface modification on the mechanical performances and fracture morphologies of composites were investigated. It was found that the tensile strength improved with the increase of MWCNTs addition, and when the content of MWCNTs loading reached 8 wt.%, the tensile strength reached the highest value of 69.7 MPa. In addition, the fracture strain also enhanced distinctly, implying that MWCNTs loading not only elevated the tensile strength of the epoxy matrix, but also increased the fracture toughness. Nevertheless, the elastic modulus reduced with the increase of MWCNTs loading. The reasons for the mechanical property changes are discussed.     

Micromechanics of piezoelectric composites with improved effective piezoelectric constant

This paper is concerned with the derivation of a micromechanics model of a new type of piezoelectric fiber reinforced composite (PFRC) materials. A continuum mechanics approach is employed to determine the effective properties of these composites. The piezoelectric fibers of these composites are considered to be electroded at the fiber–matrix interface such that the electric fields in the fiber and matrix become equal in the direction transverse to the fiber direction. The model has been verified with the existing models. The present model also predicts that the effective piezoelectric coefficient of these PFRC which accounts for the actuating capability in the fiber direction due to the applied field in the direction transverse to the fiber direction improves over the corresponding coefficient of the material of the piezoelectric fibers if the fiber volume fraction exceeds a critical fiber volume fraction.     

Melt mixed nano composites of PA12 with MWNTs: Influence of MWNT and matrix properties on macrodispersion and electrical properties

Nanocomposites containing four different polyamide 12 (PA12) types and three grades of multiwalled carbon nanotubes (MWNTs) were prepared via small-scale melt processing to study the effect of different MWNTs and the influence of polymer properties on the dispersion of the fillers and the electrical properties of the composites. Under the selected mixing conditions the lowest electrical percolation threshold of 0.7 wt.% was found for Nanocyl™ NC7000 in low viscous PA12. Moreover, big influences of the end group functionality (acid or amine excess) and the melt viscosity of the matrix were found. Composites of PA12 with acid excess showed lower percolation thresholds than those based on amine terminated materials. At constant end group ratio low viscous matrices resulted in lower percolation thresholds than high viscous materials. The best MWNT dispersion was obtained in both high viscous PA12 composites. In these systems the mixing speed was varied indicating an optimum concerning electrical conductivity at 150 rpm as compared to 50 and 250 rpm.     

Tribological and mechanical properties of the composites made of carbon fabrics modified with various methods

Carbon fabric (CF) was modified with strong HNO₃ etching, plasma bombardment, and anodic oxidation, respectively. The modified carbon fabric was then used to prepare carbon fabric composites (CFC) by dip-coating in a phenolic resin and the relative mass content of carbon fabric in the carbon fabric composites is 65%. The friction and wear behaviors of the carbon fabric composites were evaluated with a Xuanwu-III high temperature friction and wear tester, and their mechanical properties were evaluated on a Shimadzu™ universal materials testing machine, respectively. The changes in the chemical compositions of the unmodified and modified carbon fabrics were analyzed by means of X-ray photoelectron spectroscopy. The morphologies of the worn surfaces of the unmodified and modified carbon fabric composites were analyzed by means of scanning electron microscopy. It was found that the friction-reduction and anti-wear properties of the carbon fabric composites were improved by anodic oxidation, plasma bombardment, and strong HNO₃ etching, so were the mechanical properties and load-carrying capacity. The composite made of the carbon fabric modified with anodic oxidation showed the best tribological and mechanical properties, and the one made of the carbon fabric etched with HNO₃ had the poorest tribological and mechanical properties among the three kinds of the tested composites. The active groups were produced during the oxidation process, which contributed to strengthen the bonding strength between the carbon fabric and the adhesive and hence to improve the tribological and mechanical properties of the composites made of the modified carbon fabric. The friction and wear properties of the carbon fabric composites were closely dependent on the environmental temperature. Namely, the wear rates of the composites at elevated temperature above 180 °C were much larger than that below 180 °C, which was attributed to the degradation and decomposition of the adhesive resin at excessively elevated temperature. Moreover, the composite made of the carbon fabric modified with anodic oxidation had better thermal stability than the one made of the unmodified carbon fabrics.     

Experimental study of mechanical and electrical properties of carbon nanofiber/epoxy composites

Epoxy nanocomposites of different content of carbon nanofibers up to 1 wt.% have been fabricated under room temperature and refrigerated curing conditions. The composites were studied in terms of mechanical and electrical properties. Flexural modulus and hardness were found to increase significantly in refrigerated samples due to prevention of aggregates of nanofibers during cure condition. Increase and shifting in G-band by Raman spectra of these samples confirmed stress transfer and reinforcement between epoxy matrix and carbon nanofiber. Electrical conductivity improved by 3–6 orders after infusing carbon nanofibers in insulating epoxy. Room temperature samples acquired higher conductivity that was attributed to network formation by aggregates of nanofibers along the fiber alignment direction as revealed by electron microscopic studies.     

树脂浸渍法对炭/炭复合材料力学性能的影响

将炭纤维坯体CVD增密至不同密度,再对其进行树脂浸渍。对自制样品与英国Dunlop公司和美国Ben dix公司产品的力学性能特征进行了对比分析。结果表明:自制样的抗压强度和抗弯强度远远高于国外样品,层间剪切强度也比国外样品高;自制样品在ρCVD不超过1.45g cm3的情况下,随样品中CVD炭含量的增加,样品的抗弯强度和层间剪切强度值都随之增大,抗弯模量在ρCVD为1.06g cm3时达到最大值。同时用扫描电镜(SEM)分析了这几组试样的弯曲与剪切断口,发现除纯浸渍的样品具有明显的脆性断裂特征外,其余材料都呈假塑性断裂,且强度较高,说明树脂由于炭化后产生的树脂炭与纤维粘结太强,不适合在样品增密的初始阶段作浸渍剂。     

Enhanced electrical conductivity and mechanical properties of ABS/EPDM composites filled with graphene

Acrylonitrile–butadiene–styrene (ABS)/ethylene–propylene–diene monomer (EPDM) composites reinforced with graphene nanoplatelets (GN) were fabricated by the direct melt blending, dried premixing and wet premixing process, respectively. The electrical resistivity, tensile strength, impact strength, microstructure, thermal stability, glass transition temperature and morphology of fracture surface of composites were investigated. In case of direct melt blending process, the maximum tensile strength with minimum impact strength is obtained. But this result is reversed while the fabrication of composites by wet premixing process. SEM results show that GN is prior to distributing in the continuous ABS phase. The percolation threshold could be significantly decreased from 11.8 wt% to 6.6 wt% when prepare composites by wet/dried premixing process instead of melt blending.     

Effect of solidification on microstructures and mechanical properties of carbon nanotubes reinforced magnesium matrix composite

A novel approach was successfully developed to fabricate bulk carbon nanotubes (CNTs) reinforced Mg matrix composites. The distribution of CNTs in the composites depends on the solidification rate. When the solidification rate was low, CNTs were pushed ahead of the solidification front and will cluster along grain boundaries. When the solidification rate was high, CNTs were captured by the solidification front, so the CNTs remained inside the grain. Moreover, good interfacial bonding was achieved in the composite under high solidification rate. Meanwhile, compared with the matrix alloy, the ultimate tensile strength (UTS) and yield strength (YS) of the composite were significantly improved. The mechanical properties of the composite under higher solidification rate are better than composite under low solidification rate and the alloy. Moreover, most CNTs on the fracture surfaces were directly pulled out from the matrix. The Kelly–Tyson formula agreed well with the experimental tensile value in the composite under higher solidification rate, and the load-transfer efficiency is almost equal to 1.     

Multi-walled carbon nanotube-filled polyvinyl chloride composites: Influence of processing method on dispersion quality,electrical conductivity and mechanical properties

The influences of dispersion quality and processing conditions on the electrical and mechanical properties of multi-walled carbon nanotube-filled polyvinyl chloride (MWCNT/PVC) composites are examined for potential use in sensor-enabled geosynthetics and other applications involving electrically-conductive polymer composites. Electrical conductivity and mechanical properties of the composite samples made using four different dispersion methods (i.e. probe sonication, bath sonication, mechanical stirring and batch mixing) are measured. Subsurface dispersion in the samples is quantified using laser scanning confocal microscopy and scanning electron microscopy, indicating that MWCNT bundle volumes resulting from all dispersion methods had a log-normal distribution. Dispersion qualities using different mixing methods are compared using the Kolmogorov–Smirnov D-statistic. Findings indicate that samples with higher dispersion quality exhibit greater ultimate strength and failure strain, whereas poorly-dispersed specimens have greater elastic modulus values, which are found to be in good agreement with those predicted by the Halpin–Tsai model.     

Influence of feeding conditions in twin-screw extrusion of PP/MWCNT composites on electrical and mechanical properties

The influence of feeding conditions of multiwalled carbon nanotube (MWCNT) materials, namely Baytubes® C150P and Nanocyl™ NC7000, into polypropylene (PP) was investigated with respect to achieving suitable nanotube dispersion, high electrical conductivity, and good mechanical properties. Both MWCNT materials were fed at selected concentrations either in the hopper of the twin-screw extruder or using a side feeder under otherwise identical extrusion conditions (rotation speed, throughput, temperature profile) using a Berstorff ZE 25 twin-screw extruder. Afterwards, injection molding was performed under identical conditions. The results indicate that the more compact Baytubes® C150P agglomerates should be added into the hopper, as the dispersion assessed by light microscopy is better, electrical resistivities measured on compression and injection molded samples are lower, and elastic modulus, yield strength and impact strength are higher as compared to side feeding. On the other hand, for the more loosely packed Nanocyl™ NC7000 agglomerates, addition using the side feeder leads to better dispersion, lower electrical resistivity, and higher mechanical properties.     

Interface tailoring to enhance mechanical properties of carbon nanotube reinforced magnesium composites

This study highlights the use of a metallic coating of nanoscale thickness on carbon nanotube to enhance the interfacial characteristics in carbon nanotube reinforced magnesium (Mg) composites. Comparisons between two reinforcements were targeted: (a) pristine carbon nanotubes (CNTs) and (b) nickel-coated carbon nanotubes (Ni–CNTs). It is demonstrated that clustering adversely affects the bonding of pristine CNTs with Mg particles. However, the presence of nickel coating on the CNT results in the formation of Mg₂Ni intermetallics at the interface which improved the adhesion between Mg/Ni–CNT particulates. The presence of grain size refinement and improved dispersion of the Ni–CNT reinforcements in the Mg matrix were also observed. These result in simultaneous enhancements of the micro-hardness, ultimate tensile strength and 0.2% yield strength by 41%, 39% and 64% respectively for the Mg/Ni–CNT composites in comparison with that of the monolithic Mg.     

Determining the effect of flake matrix size and Al2O3 content on microstructure and mechanical properties of Al2O3 nanoparticle reinforced Al matrix composites

In this study, Al2024 matrix composites reinforced with Al₂O₃ nanoparticle contents ranging from 1 to 5?wt% were produced via a new method called as flake powder metallurgy (FPM). The effect of flake size and Al₂O₃ nanoparticle content on the reinforcement distribution, microstructure, physical, and mechanical properties of the composites were studied. SEM analysis was performed to investigate the microstructure of metal matrix and the distribution of nanoparticles. The hot-pressed density increased with decreasing the matrix size. The hardness of the Al2024–Al₂O₃ nanocomposites fabricated by using fine matrix powders increased as compared to the Al2024–Al₂O₃ nanocomposites produced by using coarse matrix powders. It has been found that the FPM method proposed in this study revealed to be an effective method for the production of nanoparticle reinforced metal matrix composites.     

A pressure enhanced CVD method for large scale synthesis of carbon microtubes and their mechanical properties

Carbon microtubes (CMTs) are a new morphological form of carbon with micrometer scale internal diameters and thin walls made of a few graphitic layers. However, compared with carbon nanotubes, there is lack of a feasible and reliable synthetic method. Furthermore, the mechanical properties of CMTs have not been reported. In this paper, we report a gas pressure enhanced CVD method for large-scale preparation of high-purity, crystalline and thin-walled CMTs in a gas pressure furnace using urea as raw material in the absence of catalyst. The as-obtained CMTs have highly graphitized structure and have a homogenous morphology with an internal diameter of about 1 μm, a wall thickness of 5 nm and several millimeters in length. The Young's modulus of the CMTs was determined to be 0.652 TPa on average, which is comparable to that of carbon nanotubes reported in previous research.     

Nano-silver modified carbon nanotubes to reinforce the copper matrix composites and their mechanical properties

In ensuring the effective load transfer of carbon nanotubes (CNTs) reinforced copper (Cu)-based composites, good and stable interface contact is a key factor. Powder electrodeposition technology is used in the present study to coat silver (Ag) nanoparticles on CNTs for the first time. Subsequently, by ball milling and spark plasma sintering, uniform distribution of CNTs in the Cu matrix and tight Cu/C interface bonding are successfully achieved. It is found that Ag nanoparticles with a size of 5 nm are evenly embedded in the surface of CNTs. The results reveal that the agglomeration of CNTs is prevented by the addition of Ag nanoparticles and the adhesion between CNTs and Cu matrix is enhanced by the formation of coherent interface. Further, the load transfer of composite materials is effectively realized by the pinning effect of Ag particles on CNTs. The tensile strength, elongation, and conductivity of the 0.75 CNT-Ag/Cu samples were 314 MPa, 24.8%, and 93.6% IACS, respectively, which are 40.1%, 818%, and 3.3% higher than those of the CNT/Cu samples, respectively. The present method provides a new direction for the uniform coating powder materials and the synergistic strengthening of metal matrix composites.     

Nanoscale structure and local mechanical properties of fiber-reinforced composites containing MWCNT-grafted hybrid glass fibers

Carbon nanotubes (CNTs) were grown from the surface of glass fibers by chemical vapor deposition, and these hybrid fibers were individually dispersed in an epoxy matrix to investigate the local composite structure and properties near the fiber surface. High-resolution transmission electron microscopy revealed the influence of infiltration and curing of a liquid epoxy precursor on the morphology of the CNT “forest” region, or region of high CNT density near the fiber surface. Subsequent image analysis highlighted the importance of spatially dependent volume fractions of CNTs in the matrix as a function of distance from the fiber surface, and nanoindentation was used to probe local mechanical properties in the CNT forest region, showing strong correlations between local stiffness and volume fraction. This work represents the first in situ measurements of local mechanical properties of the nano-structured matrix region in hybrid fiber-reinforced composites, providing a means of quantifying the reinforcement provided by the grafted nanofillers.     

Nanoinfiltration behavior of carbon nanotube based nanocomposites with enhanced mechanical and electrical properties

In this work,carbon nanotube (CNT) based nanocomposites with high mass fraction are proposed by in-situ bridging carbon matrix into CNT paper through optimized chemical vapor infiltration (CVI).Nanoinfiltration behavior of CNTs is basically investigated under the CVI process.The contact between each CNT can be strengthened and the conductive pathways can be established,resulting in the better mechanical and electrical properties.Compared with the pristine CNT paper,the CNT/C composite after pyrolysis process confirms a remarkable advance in tensile strength (up to 310 ± 13 MPa) and Young's modulus (up to 2.4 ± 0.1 GPa).Besides,a notable feature of electrical conductivity also shows an improvement up to 8.5 S/cm,which can be attributed to the mass fraction of CNT (41 wt％) breaking the limits of percolation thresholds and the efficient densification of this sample to establish the conductive pathways.This study has a broad application in the development of the multi-functional electrical and engineering materials.