Self sensing carbon nanotube (CNT) and nanofiber (CNF) cementitious composites for real time damage assessment in smart structures

The self sensing properties of cementitious composites reinforced with well dispersed carbon nanotubes and carbon nanofibers were investigated. The electrical resistance of cementitious nanocomposites with w/c = 0.3 reinforced with well dispersed carbon nanotubes (CNTs) and nanofibers (CNFs) at an amount of 0.1 wt% and 0.3 wt% of cement was experimentally determined and compared with resistivity results of nanocomposites fabricated with “as received” nanoscale fibers at the same loading. Results indicate that conductivity measurements, besides being a valuable tool in evaluating the smart properties of the nanocomposites, may provide a good correlation between the resistivity values measured and the degree of dispersion of the material in the matrix. The addition of CNTs and CNFs at different loadings was proven to induce a decrease in electrical resistance, with the nanocomposites containing 0.1 wt% CNTs yielding better electrical properties. Furthermore, conductivity measurements under cyclic compressive loading provided an insight in the piezoresistive properties of selected nanocomposites. Results confirm that nanocomposites, reinforced with 0.1 wt% CNTs and CNFs, exhibited an increased change in resistivity, which is indicative of the amplified sensitivity of the material in strain sensing.     

Mechanical performance of epoxy matrix hybrid nanocomposites containing carbon nanotubes and nanodiamonds

A novel class of epoxy matrix hybrid nanocomposites has been developed containing multiwalled carbon nanotubes (MWCNTs) and nanodiamonds (NDs) to explore the combined effect of nanoreinforcements on the mechanical performance of nanocomposites. Both the nanofillers were functionalized before incorporating into epoxy matrix to promote interfacial interactions. The concentrations of both MWCNTs and NDs in the nanocomposites were increased systematically, i.e. 0.05 wt.%, 0.1 wt.% and 0.2 wt.% while composites containing individual nanoreinforcements were also manufactured for comparison. The developed nanocomposites were characterized microstructurally by scanning electron microscopy (SEM) and mechanically by tensile, flexural, impact and hardness tests. Homogeneous dispersion of MWCNTs and NDs was observed under SEM, which resulted in the enhancement of mechanical properties of nanocomposites. The composites containing 0.2 wt.% MWCNTs and 0.2 wt.% NDs showed 50% increase in hardness while tensile strength and modulus enhanced to 70% and 84%, respectively. Flexural strength and modulus also showed a rise of 104% and 56%, respectively. Interestingly, fracture strain also increased in both the tensile and flexural testing. The impact resistance increased to 161% showing a significant improvement in the toughness of hybrid nanocomposites.     

Crystalline morphology and properties of multi-walled carbon nanotube filled isotactic polypropylene nanocomposites: Influence of filler size and loading

Isotactic polypropylene (PP) nanocomposites with multi-walled carbon nanotubes (MWCNTs) of various diameters (10–50 nm) were fabricated by extrusion and compression-molding techniques and characterized by X-ray diffraction measurements, differential scanning calorimetry, scanning electron microscopy, mechanical test and differential thermal analyses. The pure PP exhibits both the a- and b-axes oriented α-crystal, whereas the MWCNTs induce the b-axis orientation of the α-crystal along with the formation of minor γ-phase crystal in nanocomposites. Crystallinity, long period of lamellae, tensile strength, tensile modulus (TM) and microhardness (H) of PP considerably change by different loading and sizes of MWCNTs. The estimated values H/TM = 0.09–0.10 for all samples approach the predicted value of 0.10 for polymers. The increase in crystallinity has been demonstrated by both XRD and DSC studies. Mathematical models have been invoked to explain the changes in mechanical properties. An increase in thermal stability of polymer matrix occurs with increasing MWCNTs size and loading.     

Strain and damage monitoring in carbon-nanotube-based composite under cyclic strain

The resistive behavior of multi-walled carbon nanotube (MWCNT)/epoxy resins, tested under mechanical cycles and different levels of applied strain, was investigated for specimens loaded in axial tension. The surface normalized resistivity is linear with the strain for volume fraction of MWCNTs between 2.96 × 10⁻⁴ and 2.97 × 10⁻³ (0.05 and 0.5% wt/wt). For values lower than 0.05% wt/wt, close to the electrical percolation threshold (EPT) a non-linear behavior was observed. The strain sensitivity, in the range between 0.67 and 4.45, may be specifically modified by controlling the nanotube loading, in fact the sensor sensitivity decreases with increasing the carbon nanotubes amount. Microscale damages resulted directly related to the resistance changes and hence easily detectable in a non-destructive way by means of electrical measurements. In the fatigue tests, the damage is expressed through the presence of a residual resistivity, which increases with the amount of plastic strain accumulated in the matrix.     

A surfactant dispersed SWCNT-polystyrene composite characterized for electrical and mechanical properties

Single wall carbon nanotubes (SWCNTs) were dispersed in polystyrene (PS) at 0.1, 0.2, 0.3 and 1.0 wt.% (weight percent) concentrations using a surfactant assisted method. The resulting nanocomposites were characterized for their electrical conductivity, mechanical strength and fracture toughness properties. Results show a significant improvement in electrical conductivity with electrical percolation occurring by 0.2 wt.% SWCNT loading and the SWCNT-PS nanocomposite fully conductive at 1.0 wt.%. Three-point bend tests showed a decline in flexural strength and break strain with the addition of 0.1 wt.% SWCNTs. Improvements in the flexural modulus, strength and break strain with increasing SWCNT wt.% content followed The fracture toughness of the SWCNT-PS nanocomposites, in terms of the critical stress-intensity factor K_IC, was reduced relative to the neat material. From optical and high resolution scanning electron microscopy the presence of the carbon nanotubes is shown to have an adverse effect on the crazing mechanism in this PS material, resulting in a deterioration of the mechanical properties that depend on this mechanism.     

Effect of dispersion conditions on the thermo-mechanical and toughness properties of multi walled carbon nanotubes-reinforced epoxy

In this work, multi wall carbon nanotubes (MWCNTs) dispersed in a polymer matrix have been used to enhance the thermo-mechanical and toughness properties of the resulting nanocomposites. Dynamic mechanical analysis (DMA), tensile tests and single edge notch 3-point bending tests were performed on unfilled, 0.5 and 1 wt.% carbon nanotube (CNT)-filled epoxy to identify the effect of loading on the aforementioned properties. The effect of the dispersion conditions has been thoroughly investigated with regard to the CNT content, the sonication time and the total sonication energy input. The CNT dispersion conditions were of key importance for both the thermo-mechanical and toughness properties of the modified systems. Sonication duration of 1 h was the most effective for the storage modulus and glass transition temperature (T_g) enhancement for both 0.5 and 1 wt.% CNT loadings. The significant increase of the storage modulus and T_g under specific sonication conditions was associated with the improved dispersion and interfacial bonding between the CNTs and the epoxy matrix. Sonication energy was the influencing parameter for the toughness properties. Best results were obtained for 2 h of sonication and 50% sonication amplitude. It was suggested that this level of sonication allowed appropriate dispersion of the CNTs to the epoxy matrices without destroying the CNT’s structure.     

Widely dispersed PEI-based nanocomposites with multi-wall carbon nanotubes by blending with a masterbatch

High performance poly(etherimide) (PEI)-based nanocomposites (PNs) with multi-walled carbon nanotubes (MWCNT) were obtained via melt mixing. To achieve this, PEI was mixed with a well-dispersed commercial poly(butylene terephthalate) (PBT)/MWCNT master-batch in an attempt to transfer the dispersed MWCNTs to a PEI matrix. A broad and homogeneous dispersion of MWCNTs throughout the PEI-based matrix was obtained. The electrical percolation threshold (p_c) was reached at only 1 wt.% MWCNT. This p_c showed a power law dependence of conductivity on filler concentration, with a critical exponent of 1.92, which indicates that a three dimensional percolated structure was achieved. The glass transition temperature and the pressure at the output end of the extruder decreased when the master-batch was added, indicating that the processability of PEI was improved. In addition to this, the modified PEI-based PNs presented ductile behaviour and an ameliorated (12% with 5 wt.% MWCNT) elastic modulus compared with pure PEI.     

Preparation,morphology and properties of acid and amine modified multiwalled carbon nanotube/polyimide composite

The precursor of polyimide, polyamic acid, was prepared by reacting 4,4′-oxydianiline (ODA) with 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA). Unmodified, acid-modified and amine-modified multiwall carbon nanotubes (MWCNT) were separately added to the polyamic acid and heated to 300 °C to produce polyimide/carbon nanotube composite. Scanning electron microscopic (SEM) and transmission electron microscopic (TEM) microphotographs reveal that acid-modified MWCNT and amine-modified MWCNT were dispersed uniformly in the polyimide matrix. The effect of the acid and amine-modified MWCNTs on the surface and volume electrical resistivities of MWCNT/polyimide composites were investigated . The surface electrical resistivity of the nanocomposites decreased from 1.28 × 10¹⁵ Ω/cm² (neat polyimide) to 7.59 × 10⁶ Ω/cm² (6.98 wt% unmodified MWCNT content). Adding MWCNTs influenced the glass transition temperatures of the nanocomposites. Modified MWCNTs significance enhanced the mechanical properties of the nanocomposites. The tensile strength of the MWCNT/polyimide composite was increased from 102 MPa (neat polyimide) 134 MPa (6.98 wt% acid modified MWCNT/polyimide composites).     

Reduction of thermal residual stresses of laminated polymer composites by addition of carbon nanotubes

This paper studies the effects of multi-walled carbon nanotubes (MWCNTs) on the thermal residual stresses in polymeric fibrous composites. Reinforced ML-506 epoxy nanocomposites with different amounts of homogeneously dispersed MWCNTs (0.1 wt.%, 0.5 wt.% and 1 wt.%) were fabricated using the sonication technique. Thermo-mechanical analysis and tensile tests of the specimens were carried out to characterize the thermal and mechanical properties of MWCNTs/epoxy composites. Due to the negative thermal expansion and high modulus of MWCNTs, addition of MWCNTs resulted in a great reduction of the coefficient of thermal expansion (CTE) of epoxy. The MWCNTs also moderately increased the Young’s modulus of the epoxy. Then, the effects of adding MWCNTs on micro and macro-residual stresses in carbon fiber (CF)/epoxy laminated composites were investigated using the energy method and the classical lamination theory (CLT), respectively. The results indicated that the addition of low amounts of MWCNTs leads to a considerable reduction in thermal residual stress components in both micro and macro levels.     

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.     

Manufacturing techniques and property evaluations of conductive composite yarns coated with polypropylene and multi-walled carbon nanotubes

This study uses a melt extrusion method, a method for producing wires, to coat polyester (PET) yarns with polypropylene (PP) and multi-walled carbon nanotubes (MWCNTs). The resulting PP/MWCNTs-coated PET conductive yarns are tested for their tensile properties, processability, morphology, melting and crystallization behaviors, electrical conductivity, and applications. The test results indicate that tensile strength of the conductive yarns increases with an increase in the coiling speed that contributes to a more single-direction-orientated MWCNTs arrangement as well as a greater adhesion between PP/MWCNTs and PET yarns. 8 wt% MWCNTs results in an 18 °C higher crystallization temperature (T_c) of PP and an electrical conductivity of 0.8862 S/cm. The test results of this study have proven that this form of processing technology can prepare PP/MWCNTs-coated PET conductive yarns that have satisfactory tensile properties and electrical conductivity, and can be used in functional woven fabrics and knitted fabrics.     

Segregated poly(vinylidene fluoride)/MWCNTs composites for high-performance electromagnetic interference shielding

Conductive polymer composites (CPCs) that contain a segregated structure have attracted significant attentions because of their promising for fulfilling low filler contents with high electromagnetic interference (EMI) properties. In the present study, segregated poly(vinylidene fluoride) (PVDF)/multi-walled carbon nanotubes (MWCNTs) composites were successfully prepared by mechanical mixing and hot compaction. The PVDF/MWCNTs samples with 7 wt% filler content possess high electrical conductivities and high EMI shielding effectiveness (SE), reaching 0.06 S cm⁻¹ and 30.89 dB (in the X-band frequency region), much higher than lots of reported results for CNT-based composites. And the EMI SE greatly increased across the frequency range as the sample thickness was improved from 0.6 to 3.0 mm. The EMI shielding mechanisms were also investigated and the results demonstrated absorption dominating shielding mechanism in this segregated material. This effective preparation method is simple, low-cost, and environmentally-friendly and has potential industrial applications in the future.     

Comparisons of phase morphology and physical properties of PVDF nanocomposites filled with organoclay and/or multi-walled carbon nanotubes

Binary and ternary poly(vinylidene fluoride) (PVDF) nanocomposites filled with organoclay (15A) and/or multi-walled carbon nanotubes (MWNTs) were successfully prepared. MWNTs were dispersed more homogeneously than 15A within PVDF matrix, and the presence of MWNTs facilitated the dispersibility of 15A. The 15A addition induced β-form PVDF crystal formation, but MWNTs hardly changed the α-form crystal development. Both nanofillers facilitated the nucleation of PVDF (up to 12.3 °C increase), and the efficiency of enhancing PVDF crystallization followed the sequence MWNT > 15A/MWNT > 15A. The nanocomposites possess higher T_m° than neat PVDF. In particular, adding 15A led to a T_m° (β-form) increase of no less than 11 °C. A rheological percolation threshold at 1 wt.% MWNT loading was determined. The electrical resistivity dropped by more than 13 orders of magnitude at 5 wt.% MWNT loading. The nanocomposites exhibited enhanced tensile modulus (up to 83% increase with MWNTs inclusion) compared with neat PVDF.     

Preparation and characterization of carbon nanotube/polyetherimide nanocomposite films

Multi-walled carbon nanotube (MWCNT)/polyetherimide (PEI) nanocomposite films have been prepared by casting and imidization. A homogeneous dispersion of MWCNTs throughout the PEI matrix is observed by scanning electron microscopy of fracture surfaces, which shows not only a fine dispersion of MWCNTs but also strong interfacial adhesion with the matrix, as evidenced by the presence of many broken but strongly embedded carbon nanotubes (CNTs) in the matrix and by the absence of debonding of CNTs from the matrix. Differential scanning calorimetry and dynamic mechanical analysis show that the glass transition temperature of PEI increases by about 10 °C by the addition of 1 wt% MWCNTs. Mechanical testing shows that for the addition of 1 wt% MWCNTs, the elastic moduli of the nanocomposites are significantly improved by about 250% while the tensile strength is comparable to that of the matrix. This improvement is due to the strong interfacial interaction between the MWCNTs and the PEI matrix which favors stress transfer from the polymer to the CNTs.     

The role of epoxy matrix occlusions within BaTiO3 nanoparticles on the dielectric properties of functionalized BaTiO3/epoxy nanocomposites

In this work, the effects of controlled nanoparticles aggregations of barium titanate (BaTiO₃) on the dielectric properties of epoxy nanocomposites are investigated in detail with respect to different experimental parameters like frequency, ceramic content and temperature. Dispersing silanized BaTiO₃ nanopowder under ultrasonic and stir, nanocomposites of epoxy-amine matrix with different morphologies are obtained. The nanoparticles silane functionalization containing amine end groups effectively improve the compatibility of the nano-BaTiO₃ and the epoxy matrix. Storage modulus, glass transition temperature, tensile and flexural properties of nanocomposites and dielectric properties are increased until 10% by weight of nano-BaTiO₃ loading, well dispersed in the matrix. Above 10 wt.% of nano-BaTiO₃, scanning electronic microscopy and thermal analysis showed that agglomeration of nanoparticles occurs. Rheological and mechanical nanocomposites properties were evaluated and matrix occlusion behaviors were identified. In light of the specific behavior of the occluded polymer, the dielectric properties, especially dielectric loss are discussed.     

Electrical,mechanical and piezo-resistive behavior of a polyaniline/poly(n-butyl methacrylate) composite

Dodecilbenzene sulfonate doped-polyaniline/poly(n-butyl methacrylate) composites were prepared by cast molding. The electrical, mechanical and morphological behaviors of composite films were studied. The tensile strength of the composite increased by approximately a 66% for 15 wt.% of polyaniline and this increase is attributed to the homogeneous distribution and improved interface properties between the conducting particles and the matrix. Qualitative analysis of tensile fracture surfaces reveals the presence of conductive agglomerates with matrix polymer still coating them after failure, giving an indication of a good interfacial interaction of the conducting particles and the thermoplastic matrix. The piezo-resistive behavior of the films at different compositions was measured under tensile loading and the composite shows higher electromechanical sensitivity for films compositions slightly above the percolation threshold i.e. contents of 4 wt.% and 5 wt.%. The piezo-resistive response had a good reproducibility over five cycles of loading/unloading in the elastic region of deformation. In general, the reversibility of the piezo-resistive behavior revealed that there is a very small hysteresis of the composites, with particle contents close to the threshold and it becomes smaller for higher polyaniline contents, suggesting a potential application of this material in electromechanical devices.     

Strain rate sensitivity of glass/epoxy composites with nanofillers

It is understood that small amount of nanoclay in the neat epoxy and fiber reinforced epoxy composite system improves the mechanical properties. The mechanical properties of most of polymer matrix composites are rate sensitive. Most of the researches have concentrated on the behavior of the polymer composites at high strain rates. The present research work is to study the effect of clay on neat epoxy and glass/epoxy composites, at low strain rates. The clay in terms of 1.5, 3 and 5 wt% are dispersed in the epoxy resin using mechanical stirrer followed by sonication process. The glass/epoxy nanocomposites are prepared by impregnating the glass fiber with epoxy–clay mixture by hand lay-up process followed by compression molding. Characterization of the nanoclay is done by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Tensile stress–strain curves are obtained at strain rates of 10⁻⁴, 10⁻³, 10⁻² and 10⁻¹ s⁻¹ by a servo-hydraulic machine and the variation of modulus, strength and failure strain with strain rate are determined. The results show that, even at low strain rates, the longitudinal strength and stiffness increase as strain rate increases for all clay loadings. It is observed that the tensile modulus increases as the clay loading increases for both epoxy and glass/epoxy nanocomposites. Scanning electron microscopy is used to study the adhesion of composites in fracture surfaces.     

Microstructure and mechanical property of multi-walled carbon nanotubes reinforced aluminum matrix composites fabricated by friction stir processing

Aluminum matrix composites reinforced by different contents of multi-walled carbon nanotubes (MWCNTs) were fabricated by friction stir processing (FSP). The microstructure of nano-composites and the interface between aluminum matrix and MWCNTs were examined using optical microscopy (OM) and transmission electron microscopy (TEM). It was indicated that MWCNTs were well dispersed in the aluminum matrix throughout the FSP. Tensile tests and microhardness measurement showed that, with the increase of MWCNT content, the tensile strength and microhardness of MWCNTs/Al composites gradually increased, but on the contrary, the elongation decreased. The maximum ultimate tensile strength reached up to 190.2 MPa when 6 vol.% MWCNTs were added, and this value was two times more of that of aluminum matrix. Appearances and fracture surface micrographs of failed composite samples indicated that the composites become more and more brittle with the increase of the MWCNT content.     

Chinese ink-facilitated fabrication of carbon nanotube/polyvinyl alcohol composite sheets with a high nanotube loading

Fabricating carbon nanotube-based composites requires high degree of dispersion of carbon nanotubes into a polymer matrix. The widely used approaches reported in open literature for such a purpose are usually complicated and high-cost. Herein, we found that Chinese ink could be used to prepare composites composed of multi-walled carbon nanotubes (MWCNTs) and polyvinyl alcohol (PVA). The Chinese ink acted as a solvent and a dispersant. The MWCNT-ink-PVA ternary composite possessed both high flexibility and high electrical conductivity, with an optimized electrical conductivity of 8.17 S cm⁻¹. This simple method is believed to be applicable to other nanosacle carbon materials.     

Electrically conductive carbon nanotube/polypropylene nanocomposite with improved mechanical properties

Conductive polymer nanocomposites based on carbon nanotubes (CNTs) have wide range of applications in the electronics and energy sectors. For many of these applications, such as the electromagnetic interference (EMI) shielding, high nanofiller loading is typically needed to achieve the desired properties. The high nanofiller concentration deteriorates the composite's tensile strength due to the increase in nanofiller aggregation. In this work, highly conductive CNT/polypropylene (PP) nanocomposite with improved tensile strength was prepared by melt mixing. The effects of CNT content on the processing behavior, microstructure, mechanical and electrical properties of the nanocomposite were investigated. Scanning electron microscopy was used to investigate the composite microstructure. Good level of CNT dispersion with remarkable adhesion at the CNT/PP interface was observed. Based on a theoretical model, the interfacial strength was estimated to be in the range of 36–58 MPa. As a result of this microstructure, significant enhancement in ultimate tensile strength was reported with the increase of CNT content. The tensile strength of the 20 wt.% CNT/PP nanocomposite was 80% higher than that of the unfilled PP. Moreover, and due to the good dispersion of CNT particles, an electrical percolation threshold concentration of 0.93 wt.% (0.5 vol.%) was obtained.