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In this work, the effect of multi-walled carbon nanotubes (MWCNT1, MWCNT2, and MWCNT3) with different outer diameters and specific surface areas on the mechanical and electrical properties of cement mortar have been investigated. Various concentrations of MWCNTs were used (0.05, 0.10, and 0.15%), the effective dispersion of which was carried out by an Ultrasonic machine (for 40 min with 160 W power and a 24 kHz frequency) using a surfactant. Composites have been processed with a biosilica content of 10% by weight of cement and without it. Compressive strength tests were carried out on days 7 and 28 of curing. The 7-day compressive strength of samples prepared without biosilica increased compared to the result of the control sample (6.4% for MWCNT1, 7.4% for MWCNT2, and 10.8% for MWCNT3), as did those using biosilica (6.7% in the case of MWCNT1, 29.2% for MWCNT2, and 2.1% for MWCNT3). Compressive strength tests of 28-day specimens yielded the following results: 21.7% for MWCNT1, 3.8% for MWCNT2, and 4.2% for MWCNT3 in the absence of biosilica and 8.5%, 12.6%, and 6.3% with biosilica, respectively. The maximum increase in compressive strength was observed in the composites treated with a 0.1% MWCNT concentration, while in the case of 0.05 and 0.15% concentrations, the compressive strengths were relatively low. The MWCNT-reinforced cement matrix obtained electrical properties due to the high electrical conductivity of these particles. The effect of MWCNT concentrations of 0.05, 0.10, and 0.15 wt% on the electrical properties of cement mortar, especially the bulk electrical resistivity and piezoresistive characteristics of cement mortar, was studied in this work. At a concentration of 0.05%, the lowest value of resistivity was obtained, and then it started to increase. The obtained results show that all investigated specimens have piezoresistive properties and that the measurements led to a deviation in fractional change in resistivity. [ABSTRACT FROM AUTHOR]

Development of smart nanocomposites by reinforcing carbon nanomaterials into polymer composite materials has gained significant attention due to their extensive potential applications, especially in the field of monitoring of mechanical, aerospace and infrastructure systems. These smart nanocomposites exhibit excellent electro-mechanical sensing capability. Carbon nanotubes (CNTs) with outstanding electrical, mechanical, and thermal properties can be reinforced into polymer composite materials by appropriate dispersion to produce polymer/CNTs-based smart nanocomposites with excellent sensitivity. Understanding the processing, characterization and properties of the nanocomposites will help in yielding a deeper insight into the effect of incorporation of carbon nanotubes (CNTs) into the matrix of the polymer composites. This study provides a comprehensive overview (primarily in the last 2 decades) of the major influencing factors on the underlying mechanisms involved in the fabrication and performance of nanocomposites which can cause a paradigm shift in instrumentation and sensing for structural health monitoring (SHM). Therefore, this review article presents the recent advancement in the development of CNTs-reinforced polymer nanocomposites-based sensors encompassing the mechanism for electrical conductivity/piezo properties, fabrication, applications for various types of sensing, and the futuristic application for smart sensing through appropriate tailoring of nanocomposites using CNTs. [ABSTRACT FROM AUTHOR]

A three-dimensional theory has been established for the piezoresistivity of carbon nanotube (CNT) polymer composites. Based on the Mori–Tanaka method in meso-mechanics theory and considering quantum tunneling effect between CNTs, an approach to calculate equivalent electrical conductivity of composites was proposed. On this basis, a piezoresistive theory, which incorporates the effect of composites' geometric nonlinearity, was developed for CNT polymer composites. The theory is dependent only on some basic physical parameters of the materials. A finite element formula of the theory for the numerical calculation of piezoresistivity was presented from the analysis of both elastic and electric fields. Numerical simulations demonstrated that the results predicted by the theory were in good agreement with those of the experimental tests. Parameter sensitivity analysis revealed that when both the potential barrier height of the matrix and the initial average separation distance between CNTs increased, the piezoresistivity obviously increased. However, with the increase in aspect ratio and CNT conductivity, the piezoresistivity decreased gradually. A practical engineering application of this theory is also provided. [ABSTRACT FROM AUTHOR]

The gauge factor of strain gauges depends on several factors such as the geometry of the gauge and the piezoresistive properties of the materials used. Herein, the piezoresistive response of a strain gauge comprising a network of carbon nanotubes (CNTs) deposited over a polyimide (Kapton) substrate is theoretically and experimentally investigated. Two computational approaches are used, namely, a continuous finite element model and a multiscale model comprising two dimensional scales. The finite element model is a simplified model which simulates the CNT grid as a continuous solid. The multiscale approach utilizes a resistor network model to simulate the change in electrical resistance of the CNT grid with strain and uses such effective electrical resistance as an input of the finite element model. It is found that the gauge factor increases with increased number of end loops, decreasing grid width, and decreasing the end loop height. Both modeling approaches agree with the observed experimental tendencies. The multiscale approach is more realistic and accounts for effects occurring at the micro and nanoscales, such as electron tunneling. The gauge factors predicted by this model were moderately higher than the measured values, given some idealizations of the model. [ABSTRACT FROM AUTHOR]

Cement mortars (CM) containing different contents and sizes of multiwalled carbon nanotubes (MWCNTs) were prepared using surfactants in combination with ultrasonic dispersion. The effects of the content, diameter and length of MWCNTs on the electrical conductivity of cement mortar and the self-inductive piezoresistive rate under single and cyclic loading were studied using the four-electrode method. The results show that the conductive percolation thresholds of CMs containing MWCNTs with different length–diameter ratios occur at an approximate MWCNT content of 0.1 wt%. The electrical conductivity of cement mortars with MWCNT diameters of 10–20 nm was the largest for a content of 0.1 wt%, and the values were 112% and 128.3% of the conductivity of the MWCNTs/CM with diameters of 20–40 nm and 40–60 nm, respectively. In addition, the fractional change in resistivity of the cement mortar specimens varied most significantly at a content of 0.75 wt% of MWCNTs; the fractional change in resistivity reached 14.27%. Adding small-diameter MWCNTs to cement mortar can maximise the sensitivity of electrical resistance of cement mortar to compressive stress. [ABSTRACT FROM AUTHOR]

Carbon nanotube polymer composites exhibit piezoresistivity that can be utilized for flexible strain sensors. Piezoresistive modeling is critical for improving sensor performance, but remains a challenging subject due to the complex CNT network microstructure and the various effects occurring at both the micro and nano scales. Tunneling conduction between CNTs is the primary mechanism underlying piezoresistivity in carbon nanotube polymer composites. Existing models have employed a myriad of assumptions on the CNT network kinematics and the tunneling conduction behavior, but their physical basis and effects have not been thoroughly studied. In this paper, a detailed evaluation on these modeling assumptions is presented with the goal of establishing an approach without ad hoc assumptions and improved predictive accuracy. A computational model that is capable of tracking the history of tunneling separation of each CNT junction is first constructed. The effects of CNT extensibility, tunneling cross-section, conduction channels are studied via statistical analysis of the tunneling conduction and comparison of computed piezoresistive response. It is found that CNT inextensibility leads to higher predicted piezoresistivity, the tunneling cross-section area in the Simmons approximation is a highly influential parameter, and the Landauer formula provides a more physically based approach that accounts for CNT properties. [ABSTRACT FROM AUTHOR]

The current paper presents the results of an experimental study of carbon nano-, micro-, and hybrid-modified cementitious mortar to evaluate mechanical performance, energy absorption, electrical conductivity, and piezoresistive sensibility. Three amounts of single-walled carbon nanotubes (SWCNTs), namely 0.05 wt.%, 0.1 wt.%, 0.2 wt.%, and 0.3 wt.% of the cement mass, were used to prepare nano-modified cement-based specimens. In the microscale modification, 0.05 wt.%, 0.5 wt.%, 1.0 wt.% carbon fibers (CFs) were incorporated in the matrix. The hybrid-modified cementitious specimens were enhanced by adding optimized amounts of CFs and SWCNTs. The smartness of modified mortars, indicated by their piezoresistive behavior, was investigated by measuring the changes in electrical resistivity. The effective parameters that enhance the composites' mechanical and electrical performance are the different concentrations of reinforcement and the synergistic effect between the types of reinforcement used in the hybrid structure. Results reveal that all the strengthening types improved flexural strength, toughness, and electrical conductivity by about an order of magnitude compared to the reference specimens. Specifically, the hybrid-modified mortars presented a marginal reduction of 1.5% in compressive strength and an increase in flexural strength of 21%. The hybrid-modified mortar absorbed the most energy, 1509%, 921%, and 544% more than the reference mortar, nano-modified mortar, and micro-modified mortar, respectively. The change rate of impedance, capacitance, and resistivity in piezoresistive 28-day hybrid mortars improved the tree ratios by 289%, 324%, and 576%, respectively, for nano-modified mortars and by 64%, 93%, and 234%, respectively, for micro-modified mortars. [ABSTRACT FROM AUTHOR]

The importance of proper CNT dispersion is still the main challenge in CNTs doped epoxy nanocomposites. Therefore, this study was aimed to investigate the effect of toroidal stirring-assisted sonication on final mechanical, electrical and electromechanical properties of the nanocomposites. Two different samples were produced i.e. one with just sonication (M1 batch) and the other was produced using a combination of sonication and high toroidal stirring in an iterative approach (M2 batch). While piezoresistivity performance of the CNT based nanocomposites were mainly investigated in the literature for tensile mode and less attempts were conducted in presence of a pre-crack, both tensile and fracture tests were performed in this study to measure mechanical and electromechanical properties of the nanocomposites. SEM and FESEM were used for the microstructural characterizations. Results showed that M2 batch resulted in a better mechanical, electrical, and piezoresistivity performance than the M1 batch resulting from a better CNT dispersion and less amount of voids in the former compared to the latter. In fact, tensile strength and fracture toughness was increased by 70% and 17%, respectively for M2 batch with respect to M1 batch. Moreover, piezoresistive-sensitivity of the M2 batch increased 14%, compared to M1 batch. Finally, different trends in piezoresistivity was revealed in the fracture test before the occurrence of macroscopic damage, attributed to state of CNT dispersion and manifesting as a negative and positive trend for the M2 and M1 batches, respectively. [ABSTRACT FROM AUTHOR]

In this study, the temperature-dependent piezoresistivity of hybrid carbon black (CB)/carbon nanotube (CNT) nanocomposites is studied using a percolation model with a Monte Carlo simulation approach. With this approach, the temperature-dependency of electrical resistivity and the strain sensitivity of the nanocomposite are investigated. In addition, other parameters such as the aspect ratio, dispersion state and dimensions' effect on piezoresistivity of the compensated stretchable nanocomposite are investigated. By developing a temperature-dependent percolation model, the temperature sensitivity response of nanocomposite is investigated based on defining the temperature coefficient of resistance of CNT and thermal expansion coefficient of polymer. It is found that the dimensional aspects and dispersion state affected the percolation threshold and resistivity. The obtained results also indicated that the piezoresistivity increased with the decrease in the Poisson's ratio and intrinsic electrical conductivity. Moreover, the predicted results showed high prediction accuracy for temperature-dependence resistivity compared with the existing experimental data. [ABSTRACT FROM AUTHOR]

The electrical and mechanical properties of carbon nanotube/polymer nanocomposites depend strongly upon several factors such as CNT volume fraction, CNT alignment, CNT dispersion and CNT waviness among others. This work focuses on obtaining estimates and distribution for the effective electrical conductivity, elastic constants and piezoresistive properties as a function of these factors using a stochastic approach with numerous CNT/polymer realizations coupled with parallel computation. Additionally, electrical percolation volume fraction and percolation transitional behavior is also studied. The effective estimates and percolation values were found to be in good agreement with experimental works in the literature. It was found that with increasing CNT volume fraction, the mechanical properties improved. However, due to the interaction of CNTs with one another through electrical tunneling, the conductivity and piezoresistivity properties evolved in a more complex manner. While the degree of alignment played a strong role in the effective properties making them anisotropic, the effect of waviness was found to be insubstantial. [ABSTRACT FROM AUTHOR]

Electrical conductivity and piezoresistivity of carbon nanotube (CNT) nanocomposites are analyzed by nodal analysis for aligned and random CNT networks dependent on the intrinsic CNT conductivity and tunneling barrier values. In the literature, these parameters are assigned with significant uncertainty; often, the intrinsic resistivity is neglected. We analyze the variability of homogenized conductivity, its sensitivity to deformation, and the validity of the assumption of zero intrinsic resistivity. A fast algorithm for simulation of a gauge factor is proposed. The modelling shows: (1) the uncertainty of homogenization caused by the uncertainty in CNT electrical properties is higher than the uncertainty, caused by the nanocomposite randomness; (2) for defect-prone nanotubes (intrinsic conductivity ~104 S/m), the influence of tunneling barrier energy on both the homogenized conductivity and gauge factor is weak, but it becomes stronger for CNTs with higher intrinsic conductivity; (3) the assumption of infinite intrinsic conductivity (defect-free nanotubes) has strong influence on the homogenized conductivity. [ABSTRACT FROM AUTHOR]

High-performance compression sensors have been playing an increasingly important role in human motion detection, health monitoring and human-machine interfaces over recent years. However, it remains a great challenge to develop theoretical models providing practical guidance to the sensor design. Herein, carbon black (CB), carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) were respectively incorporated into porous melamine sponges by a facile approach of dip-coating to fabricate compression sensors. Uniaxial compression-resistance tests show that the compressibility, stability and piezoresistive sensitivity of sensors could be tailored by the filler type and concentration. A model considering the number of conductive pathways (NCP) is given to describe the relationship between the resistance change and applied compression, showing extremely good agreement with the experimental data. Also, the correlation between the equivalent filler volume fraction and conductivity is described by the other two models proposed by McLachlan and Kirkpatrick, revealing the electrical percolation thresholds (Φc) for the conductive systems under compression. Among the three fillers, CB particles endowed the composite with the best piezoresistive sensitivity but the largest Φc due to its small size and aspect ratio. A combination of experimental study and theoretical model opens up a way of further understanding the piezoresistive sensing behavior as well as optimizing the electrical property and piezoresistivity of compressive conductive polymer composite. [ABSTRACT FROM AUTHOR]

Composites made from multi-walled carbon nanotubes and polydimethylsiloxane exhibit an excellent piezoresistivity at elastic limits where strain sensitivity reaches 0.93–0.96 and resistance changes due to stretching and compressing matches with loading histograms. Resistance change becomes pulse-like as the samples are bent and/or twisted, attributed to the rapid increase in intertube spacing. Straining induces movements and segmentations of tube aggregates with tube mobility ranging at 0.1–90 nm/s. Intertube barrier driven sensing mechanism is viscoelasticity related and is therefore interpreted on the basis of spring-dashpot models.

Piezoresistive and capacitive cement-based smart composites are promising candidates for stress monitoring applications in buildings. However, the drawbacks of designing cement-based smart composites with a piezoresistive mechanism sensitive to temperature variations or a capacitive mechanism sensitive to vibration can be solved with sensor designs based on two principles. To address this issue, cement-based smart composites with piezoresistive and capacitive stress sensitivity and significantly improved compressive strength were designed using carbon nanotubes (CNT) doped with graphene nanopowder. A maximum fractional change in electrical resistivity of 90.08% with a high linearity of 98% was obtained with a blend of 0.01 wt%, and the maximum fractional change in capacitance was 163.11% with a high linearity of 97% at 0.05 wt%. Cement-based smart composites have absolute piezoresistive stress sensitivities that vary from 1.38% MPa−1 to 4.16% MPa−1 and 2.54% MPa−1 to 2.57% MPa−1 for blended and impregnated low doses of 0.01 wt% to 0.05 wt% CNT doped with graphene. In addition, capacitive stress sensitivities of 22.74% MPa−1 and 5.1% MPa−1 were achieved by blending and impregnating 0.01 wt% CNT doped with graphene in cementitious composites. Furthermore, the conductive path, overlapping, and tunneling effect created by CNT doped with graphene in cementitious composites and the porous structure in the dielectric layer are responsible for the increased stress sensitivity. [Display omitted] • The piezoresistive and capacitive stress sensitivity were assessed. • A novel CNT doped with graphene cement-based sensors was produced. • CNT doped with graphene cement pastes exhibits remarkable self-sensing ability. • CNT doped with graphene improved the compressive strength. • The sensitivity mechanism of CNT doped with graphene cement pastes was evaluated. [ABSTRACT FROM AUTHOR]

Recent research has increasingly focused on the potential applications of carbon nanotube (CNT) hybrid composites in wearable sensor technologies. Piezoresistivity, which is characterized by the ability to detect alterations in electrical resistance in response to external forces, is a pivotal attribute of resistive sensors. Numerous studies have attempted to improve this performance by incorporating secondary fillers. Despite extensive efforts to comprehend the influence of the dimensions of secondary fillers on electrical conductivity under static and dynamic conditions, notable confusion persists in the literature regarding the comparative analysis of the effects of nano- and microscale secondary fillers. In this study, two distinct sizes of silica particles were introduced as secondary fillers in CNT/polymer composites, followed by a rigorous comparative analysis of their mechanical and electrical properties under static conditions. Furthermore, this study assessed the influence of the silica particle size on the electrical resistance under dynamic tensile conditions, elucidating its impact on the conductive network. [Display omitted] [ABSTRACT FROM AUTHOR]

The rapidly increased interest in multifunctional nanoelectronic devices, such as wearable monitors, smart robots, and electronic skin, motivated many researchers toward the development of several kinds of sensors in recent years. Flexibility, stability, sensitivity, and low cost are the most important demands for exploiting stretchable or compressible strain sensors. This article describes the formation and characteristics of a flexible, low-cost strain sensor by combining a commercial melamine sponge and a graphene/carbon nanotubes hybrid. The composite that emerged by doping the highly elastic melamine sponge with a highly conductive graphene/carbon nanotubes hybrid showed excellent piezoresistive behavior, with low resistivity of 22 kΩ m. Its function as a piezoresistive material exhibited a high sensitivity of 0.050 kPa−1 that combined with a wide detection area ranging between 0 to 50 kPa. [ABSTRACT FROM AUTHOR]

Printed and flexible sensors are in the focus of recent efforts to establish the advantages of low-cost manufacturing techniques such as screen printing or inkjet printing for printed electronical applications. Devices based on conductive carbon nanotube (CNT) networks within polymeric matrices such as polydimethylsiloxane (PDMS) are already exceeding mere technological demonstrations. Therefore, we investigate the application-oriented behaviour of fully inkjet-printed CNT/PDMS strain sensors under different conditions such as short- and long-term performance. The sensors exhibit a quasi-linear piezoresistive behaviour with vanishing hysteresis to tensile strain. Significant differences in the resistive response between compressive and tensile strain suggest complex re-orientation mechanisms of CNTs inside the matrix. No clear indication for this phenomenon could be observed in the evolution of the CNT network resistance during fatigue measurements within an uncured or cured PDMS matrix, where both scenarios exhibit no visual degradation. However, these measurements over thousands of cycles show different permanent changes in the overall device resistance exhibiting damages but also recovery in the network. Considering these findings facilitates the development of printed sensor devices.

Piezoresistive strain sensing of multiwalled carbon nanotube (CNT)‐reinforced polymer nanocomposites considering the effect of agglomeration is experimentally investigated. Thermoplastic polyurethane is used as composite matrix with melt mixing method for fabricating the nanocomposite. The electrical resistance of the nanocomposite specimens is measured by a designed electrical circuit in conductivity and piezoresistivity tests. The electrical resistance of the nanocomposite decreases by a nonlinear relation when CNT is added up to 1.5 wt% and has an inverse behavior after this weight fraction. Comparison between scanning electron microscope images of the nanocomposite specimen fracture surfaces shows that the CNT agglomeration is the reason for decreasing the nanocomposite electrical properties. Also, decrease of the mechanical properties of the nanocomposite in higher filler content than 1.5 wt%, confirms the agglomeration effects. In addition, it is illustrated that the CNT agglomeration decreases strain sensing in the nanocomposite. Moreover, different piezoresistive behaviors of the nanocomposite in low and large strains are explained, according to the piezoresistivity mechanisms. Change of the piezoresistivity mechanism type, causes a nonlinear growth of the nanocomposite electrical resistance, in large strain. [ABSTRACT FROM AUTHOR]

The rapidly increased interest in multifunctional nanoelectronic devices, such as wearable monitors, smart robots, and electronic skin, motivated many researchers toward the development of several kinds of sensors in recent years. Flexibility, stability, sensitivity, and low cost are the most important demands for exploiting stretchable or compressible strain sensors. This article describes the formation and characteristics of a flexible, low-cost strain sensor by combining a commercial melamine sponge and a graphene/carbon nanotubes hybrid. The composite that emerged by doping the highly elastic melamine sponge with a highly conductive graphene/carbon nanotubes hybrid showed excellent piezoresistive behavior, with low resistivity of 22 kΩ m. Its function as a piezoresistive material exhibited a high sensitivity of 0.050 kPa−1 that combined with a wide detection area ranging between 0 to 50 kPa.

The electrical and electromechanical responses of ~200 µm thick extruded nanocomposite films comprising of 4 wt.% and 5 wt.% multiwall carbon nanotubes mixed with polypropylene are investigated under an alternating current (AC) and compared to their direct current (DC) response. The AC electrical response to frequency (f) and strain (piezoimpedance) is characterized using two configurations, namely one that promotes resistive dominance (resistive configuration) and the other that promotes the permittivity/capacitive contribution (dielectric configuration). For the resistive configuration, the frequency response indicated a resistive–capacitive (RC) behavior (negative phase angle, θ), with a significant contribution of capacitance for frequencies of 104 Hz and above, depending on the nanotube content. The piezoimpedance characterization in the resistive configuration yielded an increasing impedance modulus (|Z|) and an increasing (negative) value of θ as the strain increased. The piezoimpedance sensitivity at f = 10 kHz was ~30% higher than the corresponding DC piezoresistive sensitivity, yielding a sensitivity factor of 9.9 for |Z| and a higher sensitivity factor (~12.7) for θ. The dielectric configuration enhanced the permittivity contribution to impedance, but it was the least sensitive to strain.

Strain sensing capability of carbon nanotubes (CNTs) doped epoxy under tensile and flexural tests were broadly studied, while the piezoresistive behavior of nanocomposites in precracked specimen, that is, in fracture toughness tests, have not been addressed deeply in the literature. Therefore, in the current paper, both the strain and crack sensing capabilities of single wall carbon nanotubes‐double wall carbon nanotubes (SWCNTs‐DWCNTs) doped epoxy subjected to tensile and mode I fracture tests were investigated. Different CNT loadings including 0.5 and 0.75 wt% were used to compare the effect of various states of CNTs dispersion, including uniformly dispersed CNTs and aggregates, on electromechanical properties. Results showed that specimens loaded at 0.5 wt% of CNTs possessed proper tensile strength, fracture toughness, piezoresistivity, and sensitivity. A nonlinear trend in normalized resistance change with respect to strain was noticed under tensile test resulting from tunneling effect which induced nonlinearity in the electrical signals. During fracture tests, prior to crack propagation, different trends in piezoresistivity as a function of displacement were observed, depending on the CNT loading, that is, linear and nonlinear behavior at CNT content of 0.5 and 0.75 wt%, respectively. The nanocomposite could simultaneously monitor damage initiation and extension with the onset of crack growth by showing abrupt increase in normalized resistance, and two different failure modes can be distinguished including abrupt and crack extensions. It was concluded that any abrupt jump in normalized resistance could be used as an indicator for damage initiation in the specimen. [ABSTRACT FROM AUTHOR]

Multi-walled carbon nanotube (MWCNT)/elastomer composites exhibit a piezoresistive behavior, i.e. their resistivity changes when they are subjected to mechanical loading. Thus, these materials can be used as strain or pressure sensors. In this paper, the effect of carbon nanotube weight fraction on the sensitivity and repeatability of the electrical response of multi-walled carbon nanotube/ethylene-propylene-diene monomer composites is investigated, under quasi-static and dynamic compression (using split Hopkinson pressure bars). It was found that multi-walled carbon nanotube weight fraction and the strain rate have a major influence on the piezoresistivity of such composites. Although all samples exhibited a good repeatability of their electrical response under quasi-static cyclic compression, those with a lower multi-walled carbon nanotube weight fraction had a higher sensitivity to strain. An increase in the electrical resistance during compression was observed under both quasi-static and dynamic compression. Reversible movements of multiwalled carbon nanotube in the transverse direction of compression increased the average inter-multi-walled carbon nanotube distance under quasi-static compression, leading to higher values of resistance. After the dynamic tests, the Young's modulus of the composites decreased by about 45% and the electrical resistance increased a hundredfold, indicating damage induced by dynamic loading. [ABSTRACT FROM AUTHOR]

Improving mechanical properties and decreasing costs have significantly increased the use of fiber composites in automotive, aerospace, and civil engineering applications. Structural composites are bonded to traditional metallic materials in a variety of applications, and mechanical fasteners often cannot be used due to the low bearing strength of composites. With the increasing use of adhesives in load-bearing structures, novel techniques are required for monitoring the structural integrity of adhesive joints. Previously, carbon nanotubes (CNTs) have been added to adhesives and resins to create in-situ sensors, but the increased viscosity and potential for galvanic corrosion remains a challenge. In this research, a piezoresistive carbon nanotube-based sensing layer is embedded in a composite/steel adhesive joint for damage sensing. The use of a thin sensing layer with low-fiber volume fraction enables the use of existing adhesives without causing any major changes in the physical properties of the adhesives or the curing cycle and reduces the chances of galvanic corrosion. Different approaches of using an adhesive layer and a nonconductive fabric are investigated for insulation of the sensing layer. The nonconductive fabric approach for insulating the specimen yields better mechanical properties as the there are no weak interfaces in the adhesive bondline. Additionally, it is more convenient for scaling up for field applications as the adhesive is cured in one stage. The sensing layer can not only be used to detect incipient damage in the joint, but also identify different modes of failure. [ABSTRACT FROM AUTHOR]

It has been known that the carbon nanotube (CNT)/cement composites can be used to reflect the external loading information and detect premature damage; however, the agreement on whether the piezoresistivity as a function of external loading is positive or negative has yet to be achieved, and the innate electrically conductive mechanism remains unclear. In this study, to minimize the application cost, small-size CNT/cement composites were embedded into the surface of the concrete as sensing elements, and the piezoresistivity of the CNT/cement composites under external loading along with/without salt attacks was tested. The results indicate that the piezoresistivity significantly depends on the percolation backbone density and the critical exponent of the CNT networks near the percolation threshold. Without salt environments, an "M" shape was observed in the curve of the fractional electrical resistivity (FCR) as a function of cyclic loading applied on the concrete samples. However, when the concrete samples were exposed to a salt environment, the "M" shape in the FCR curve disappeared. In addition, based on the percolation theory, the electrical resistivity changes as a function of strain fits well with an exponential function. The microstructure analysis demonstrates that the pore structure of the CNT/cement composites can be divided into spherical pores and cracks with layered structures. This study not only provides a new insight into the electrical conduction mechanism in CNT/cement composites systems but also sheds light on how to accurately monitor and analyze concrete structures with external loads, especially along with salt environment. [ABSTRACT FROM AUTHOR]

Mechanical strain sensing is ubiquitous, found in applications such as heart rate monitoring, analysis of body part motion, vibration of machines, dilatation in buildings and large infrastructure, and so forth. Piezoresistive materials and sensors based on those offer versatile and robust solutions to measure strains and displacements and can be implemented even in acceleration and pressure analyses. In this paper, we overview the most prominent piezoresistive materials, and present a case study on carbon foams as well as on their hierarchical hybrid structures with carbon nanotubes/nanofibers. Our results show highly non-linear electrical resistance and mechanical stress dependence on uniaxial strain in both types of materials up to 50% compression. The Young's moduli increase with compressive strain between 1–65 and 0.1–92 kPa for the foam and hierarchical structure, respectively. The foams have giant gauge factors (up to −1000) with large differential gauge factors (on the scale of −10) and differential pressure sensitivity of 0.016 Pa−1 (at 10% strain) making them suitable for detecting both small and large displacements with excellent accuracy of electrical readout as we demonstrate in detecting building wall displacement as well as in monitoring heart rate and flexing of fingers.

Electrical monitoring of strain and damage in multiscale hierarchical composites comprising unidirectional aramid fibers modified by multiwall carbon nanotubes and polypropylene as matrix is investigated. The key factor for electrical self-sensing in these thermoplastic composites is the formation of a multiwall carbon nanotube network, which is achieved by using two material architectures. In the first architecture, the multiwall carbon nanotubes are dispersed within the polypropylene matrix, while aramid fibers remain unmodified. The second architecture uses also multiwall carbon nanotube-modified polypropylene matrix, but the aramid fibers are also modified by depositing multiwall carbon nanotubes. Under tensile loading, the electrical response is nonlinear with strain (ε), and the piezoresistive sensitivity was quantified by gage factors corresponding to low (ε < 0.25%) and high (ε > 0.3%) strain regimes. Such gage factors were 4.83 (for ε < 0.25%) and 13.2 (for ε > 0.3%) for composites containing multiwall carbon nanotubes only in the polypropylene matrix. The composites containing multiwall carbon nanotubes in the matrix and fibers presented higher piezoresistive sensitivity, with average gage factors of 9.24 (ε < 0.25%) and 14.0 (ε > 0.3%). The higher sensitivity to strain and damage for a specific material architecture was also evident during cyclic and constant strain loading programs and is attributed to the preferential localization of multiwall carbon nanotubes in the hierarchical composite. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • CNTs were systematically fragmented by high density ultrasonic energy. • PMMA composites with fragmented CNTs exhibited lower electrical conductivity. • Electromechanical sensitivity was higher for composites with fragmented CNTs. • Contributions of piezoresistive mechanisms are discussed. Abstract Multiwall carbon nanotubes (MWCNTs) were systematically fragmented using high density ultrasonic energy and used as fillers of a poly (methyl methacrylate) matrix (PMMA). MWCNT fragmentation was confirmed by scanning electron microscopy and atomic force microscopy. Dedicated MWCNT length measurements yielded a log-normal statistical distribution, with mean value of 1.33 μm for the as-received MWCNTs and 0.66 μm for the fragmented MWCNTs. Higher electrical percolation threshold and lower electrical conductivity were obtained for MWCNT/PMMA composites (0.01–1 wt.% MWCNT concentration) using the fragmented MWCNTs. Higher electro-mechanical (piezoresistive) sensitivity under tensile tests was measured for PMMA nanocomposites with fragmented MWCNTs, which was explained by a competition of mechanisms related to the MWCNT length. [ABSTRACT FROM AUTHOR]

Self-sensing ultra-high performance concrete (UHPC) features superior mechanical capacity, excellent erosion resistance, long life cycle, and broad range of stress sensing, and is expected to be a pathway towards next-generation smart cementitious composites. This study presents a novel strategy to achieve electrical percolation and elevate the piezoresistive sensing capability of UHPC through the incorporation of multiphase and multiscale functional additives including graphene (G) and carbon nanotube (CNT). The workability, compressive strength, microstructural characteristics, and alternating current (AC) impedance response are investigated. The effect of electrical percolation on the piezoresistive behaviour is studied and discussed using various ratios of G/CNT. Results indicate that as the G/CNT ratio decreases from 4:0 to 0:4, the compressive strength is reduced from 194.7 MPa to 166.3 MPa due to the certain increase in the plastic viscosity of fresh mixture and the proportion of harmful pores, although the basic requirement of 150 MPa is met. Moreover, the AC impedance response significantly moves left and the radius of high-frequency arc reduces from 72910 Ω to 1295 Ω, suggesting electrical percolation. Bounded by percolation threshold, the fractional change of resistance curves can be divided into a two-stage pattern (i.e., linear and nonlinear stages) and a three-stage mode (i.e., linear decrease, balance, and abrupt increase stages). An underlying mechanism is proposed to explain the tremendous change in piezoresistive behaviour of self-sensing UHPC, considering the tunneling-percolation theory and electromechanical coupling behaviour. Additionally, the gauge factors range from 11 to 28, which is higher than most of the existing reported values, demonstrating the great potential of using hybrid functional nano-additives in self-sensing UHPC. [ABSTRACT FROM AUTHOR]

A 3D Monte Carlo method paired with a percolation model is carried out to predict the electrical resistivity and piezoresistive sensitivity of carbon nanotube (CNT)-polymer piezoresistive sensors. CNTs are randomly dispersed into the insulating layer of a polymer matrix having agglomerated morphologies in some parts. The electrical resistivity of nanocomposite with a defined agglomeration state is determined by considering the tunneling effect between each connected pair of CNTs. The influence of various parameters such as agglomeration radius, percentage, intrinsic properties and geometries of CNTs are investigated. A comparison was made between the analytical results and experimental data. Considering the tunneling behavior in the vicinity of the percolation transition, a good agreement is obtained between the analytical results and experimental data. Monte Carlo simulations indicated that the tunneling resistance with a random distribution of CNTs depends on the level of agglomeration. Results revealed that piezoresistive sensitivity was diminished by larger agglomerations. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract Herein, we report strain- and damage-sensing performance of biocompatible smart CNT/UHMWPE nanocomposites for the first time. CNT/UHMWPE nanocomposites are fabricated by solution mixing followed by compression molding. The surface morphology, microstructural properties, thermal decomposition and stability, glass transition temperature and thermal conductivity of the nanocomposites are characterized. The degree of crystallinity of CNT/UHMWPE nanocomposites is found to have a maximum value of 52% at 0.1 wt% CNT loading. The degree of crystallinity influences the mechanical properties of the CNT/UHMWPE nanocomposites. The electrical percolation threshold is achieved at 0.05 wt% of CNT and it follows a two dimensional conductive network according to percolation theory. The piezoresistive response of CNT/UHMWPE nanocomposites is demonstrated with a gauge factor of ~2.0 in linear elastic regime and that in the range of 3.8–96.0 in inelastic regimes for 0.05 wt% of CNT loading. A simple theoretical model is also developed to predict the resistivity evolution in both elastic and inelastic regimes. High sensitivity of CNT/UHMWPE nanocomposites coupled with linear piezoresistive response up to 100% strain demonstrates their potential for application in artificial implants as a self-sensing material. Graphical abstract Unlabelled Image Highlights • Self-sensing performance of CNT/UHMWPE nanocomposites is reported for the first time. • The electrical percolation threshold is achieved at 0.05 wt% of CNT loading. • CNT/UHMWPE nanocomposites exhibits a gauge factor of ~2.0 in linear elastic regime. • A simple theoretical model is developed to predict the resistivity evolution with stretch. • Strong piezoresistivity up to 100% strain demonstrates their candidacy for knee implants. [ABSTRACT FROM AUTHOR]

Within the novel approach for structural health monitoring of civil engineering structures based on smart self-sensing structural materials, Carbon NanoTube (CNT)-reinforced cement-based composites, often termed “smart concretes”, have drawn rising interest. These composites exhibit self-sensing capabilities resulting in measurable variations of their electrical properties under applied mechanical deformations. This, along with the similarity between these composites and conventional structural concrete, makes it possible to devise cost-efficient distributed monitoring systems for large-scale Reinforced Concrete (RC) structures. While several papers in the literature have focused on pilot applications of smart concrete strain sensors embedded into large RC components, the response of these sensors is not yet fully understood at the fundamental physical level and, as a consequence, their output is not properly interpreted. In order to shed some light on this issue, previous work by the authors focused on laterally unconstrained uni-axial loading conditions. This work extends the previous studies by presenting a mixed micromechanics and finite element approach for the analysis of CNT-reinforced composites subjected to arbitrary strain states. The two mechanisms that contribute to the electrical conductivity of CNT-reinforced composites, namely electron hopping and conductive networking, are contemplated within a percolation framework in the micromechanics model. On the basis of the micromechanics model, the 3D piezoresistivity matrix is determined, for the first time in the literature, by means of virtual dilation and distortion tests. Afterwards, the electro-mechanical modeling of three-dimensional composite elements is conducted by a multi-physics finite element code. The potential of the presented approach is illustrated by extensive parametric analyses, as well as a comparison against experimental data, including application of the mixed micromechanics-finite-element multi-physics formulation for electro-mechanical modeling of three-dimensional elements. [ABSTRACT FROM AUTHOR]

Highlights • This paper focuses on the strain sensitivity of electrical conductivity of MWCNT/ PVDF conductive polymer composite. • An optimal combination of ductility and piezoresistivity is found slightly above the electrical percolation threshold. • An improved model of piezoresistivity is established to explain the experimental measurements. Abstract Conductive polymer composites are prepared with a polyvinylidene fluoride (PVDF) matrix and multiwalled carbon nanotube (MWCNT) as filler. The mechanical properties, electrical conductivity and piezoresistivity of the composites are investigated as a function of MWCNT volume fraction. An optimal combination of ductility and piezoresistivity is found in the composite slightly above the electrical percolation threshold, showing a maximum fracture strain ∼ 4 times higher than the pure PVDF samples, as well as an outstanding sensitivity of piezoresistive response. Increasing further the MWCNT volume fraction leads to deterioration of both ductility and piezoresistivity. The effects of MWCNT content on piezoresistivity and fracture strain can be understood based on the microstructure observations. In particular, by incorporating two characteristic microstructural features identified in the materials, i.e., the filler random distribution and the filler agglomerate, into a simple statistical network model, the experimental measurements of piezoresistivity can be successfully fitted and understood in a semi-quantitative manner. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • We developed a data acquisition bridge simultaneously collecting signals from MCNTCC, strain gauge, and transducer. • We studied effects of MCNT’s fraction, moisture, stress/strain level on resistivity and piezoresistivity of MCNTCC. • We developed an intrinsic sensor with favorable stress/strain sensitivity and linearity for SHM. Abstract Multi-walled carbon nanotube (MCNT) cement-based composites (MNTCCs) with seven different volume additions of MCNT (V) were prepared with surfactant ultrasonic dispersion process. A Wheatstone-based data acquisition technique was developed to simultaneously obtain the electrical resistances, the uniaxial stresses (σ c), and the longitudinal strains (ε l) of the cured MNTCCs under compressive loading. The percolation threshold, humidity impact on initial resistivity (ρ in), and self-sensing piezoresistivity under mono- and cyclic-loading of the MNTCCs were correspondingly investigated. Results reveal that, the ρ in of MNTCC successively conforms to tunneling transition and percolation effect along with V increment; Fraction change in resistivity (Δ ρ) of MNTCCs with 1.31 vol.% MCNT (round percolation threshold) after excluding humidity impact can steadily and linearly alter with σ c or ε l , and effectively reflect the microstructure changes and crack emerging; The corresponding strain sensitivity, reproducibility of the Δ ρ -ε l curve can maintain above 132, less than 5%. These consequences favor the MNTCCs to be intrinsic sensors applied in infrastructure for real-time monitoring of cyclic loadings with high sensitivity, stable and fast responses. [ABSTRACT FROM AUTHOR]

To develop the differential pressure sensor made of carbon nanotube-filled silicone rubber composites with a simplified structure, an electrode-shared differential configuration is designed. The configuration includes the top and bottom sandwich layers. The top sandwich layer is composed of the top film, the intermediate film, and the composite film with low carbon nanotube content between them. The bottom sandwich layer is composed of the intermediate film, the bottom film, and the composite film with high carbon nanotube content between them. The electrode in the intermediate film is shared by the top and bottom sandwich layers, which can make the sensor probe more concise. The electrical resistance of the composite film with high/low carbon nanotube content increases/decreases with the increase of the pressure, and increases with the increase of the temperature. The top and bottom sandwich layers are connected to the neighboring arms of an electrical bridge to construct a differential system. The experiments verify the feasibility of using the electrode-shared configuration to construct the differential piezoresistive sensor made of the composite to decrease the temperature-induced drift and increase the sensitivity. [ABSTRACT FROM AUTHOR]

A new method for monitoring 3D braided composite structure health in real time by embedding the carbon nanotube yarn, based on its piezoresistivity, in the composite axially has been designed. The experimental system for piezoresistive effect detection of the carbon nanotube yarn in the 3D braided composite was built, and the sensing characteristics has been analyzed for further research. Compared with other structural health monitoring methods, the monitoring technique with carbon nanotubes yarns is more suitable for internal damage detection immediately, in addition the strength of the composite can be increased by embedding carbon nanotubes yarns. This method can also be used for strain sensing, the development of intelligent materials and structure systems. [ABSTRACT FROM AUTHOR]

The mechanical properties, electrical conductivity and piezoresistive response of thermosetting polymer nanocomposites comprising carbon nanotubes (CNTs, one-dimensional topology, 1D), few-layer thermally reduced graphene oxide (FLG, two-dimensional topology, 2D), cubic-shaped few-layer graphene shells (CGSs, three-dimensional topology, 3D), and hybrid combinations of them (1D-2D and 1D-3D) are investigated. It is observed that the most electro-conductive materials are formed when CGSs, CNTs or a hybrid combination of both are used, likely because of the lower defect density of CGSs and the higher aspect ratio of CNTs. The mechanical properties and piezoresistive sensitivity are higher for composites comprising CNTs or a 1D-2D hybrid combination of CNTs and FLGs. While the increased mechanical properties for these two groups of composites are attributed to the higher aspect ratio/lateral size, higher number of dangling bonds and functionalities, and higher specific surface area (for the case of FLGs) of their fillers, the increased piezoresistive sensitivity is explained in terms of their higher excluded volume within the composite. [ABSTRACT FROM AUTHOR]

A theoretical model is developed to predict the electrical conductivity of polymer composites containing carbon nanotubes. It incorporates the effect of two conductivity mechanisms, electron hopping between adjacent nanotubes at the nanoscale and the transmission of electrons through conductive networks formed by nanotubes at the microscale. Based on the model, analytical expressions of both electrical conductivity and piezoresisitivity under stretching are established. The expressions are simple in form, without resorting to heavy computation, and able to give accurate estimations for both electrical conductivity and piezoresistivity. [ABSTRACT FROM AUTHOR]

A hybrid additive manufacturing process is proposed to incorporate piezoresistive sensors into a polyamide matrix for real-time structural health monitoring. 3D-printed samples were tested under tensile, flexural, and indentation testing using various sizes, numbers, and locations of the sensors, while the electrical resistance was recorded in real-time. Under uniaxial tensile loading, strength reached 36 MPa, and the resistance values linearly increased by up to 105% at ∼150% of strain. During flexural testing, the sensor had higher resistance when it was located above the supports rather than under the loading roller. The conductive paths were placed near the plate's top, middle and bottom sections to localise the damage during indentation testing. The energy absorbed during indentation testing reached 34–36 J. Resistance readings of sensors with 1 cm width showed better linearity and sensitivity than their wide (10 cm) counterparts. Three 2D sensors were interconnected to make a 3D sensor. The resistance values gradually increased after each strike during repeated indentation, but the overall strength remained similar. In summary, this work demonstrates a novel approach toward customisable 3D-printed piezoresistive sensors for detecting global or local damage. • Piezoresistive sensors were embedded inside a polymer shell by additive manufacturing. • The method allows to customize sensors and detect global or local damage. • Change in resistance recorded in real-time during tensile, flexural and indentation testing. • Able to detect multiple low-energy indentation strikes. [ABSTRACT FROM AUTHOR]

Smart multi-walled carbon nanotube (MWCNT)-coated cellulose fibers with a unique sensing ability were manufactured by a simple dip coating process. The formation of electrically-conducting MWCNT networks on cellulose mono- and multi-filament fiber surfaces was confirmed by electrical resistance measurements and visualized by scanning electron microscopy. The interaction between MWCNT networks and cellulose fiber was investigated by Raman spectroscopy. The piezoresistivity of these fibers for strain sensing was investigated. The MWCNT-coated cellulose fibers exhibited a unique linear strain-dependent electrical resistance change up to 18% strain, with good reversibility and repeatability. In addition, the sensing behavior of these fibers to volatile molecules (including vapors of methanol, ethanol, acetone, chloroform and tetrahydrofuran) was investigated. The results revealed a rapid response, high sensitivity and good reproducibility for these chemical vapors. Besides, they showed good selectivity to different vapors. It is suggested that the intrinsic physical and chemical features of cellulose fiber, well-formed MWCNT networks and favorable MWCNT-cellulose interaction caused the unique and excellent sensing ability of the MWCNT-coated cellulose fibers, which have the potential to be used as smart materials.

Compared with traditional conductive fillers, carbon nanotubes (CNTs) have unique advantages, i.e., excellent mechanical properties, high electrical conductivity and thermal stability. Nanocomposites as piezoresistive films provide an interesting approach for the realization of large area strain sensors with high sensitivity and low manufacturing costs. A polymer-based nanocomposite with carbon nanomaterials as conductive filler can be deposited on a flexible substrate of choice and this leads to mechanically flexible layers. Such sensors allow the strain measurement for both integral measurement on a certain surface and local measurement at a certain position depending on the sensor geometry. Strain sensors based on carbon nanostructures can overcome several limitations of conventional strain sensors, e.g., sensitivity, adjustable measurement range and integral measurement on big surfaces. The novel technology allows realizing strain sensors which can be easily integrated even as buried layers in material systems. In this review paper, we discuss the dependence of strain sensitivity on different experimental parameters such as composition of the carbon nanomaterial/polymer layer, type of polymer, fabrication process and processing parameters. The insights about the relationship between film parameters and electromechanical properties can be used to improve the design and fabrication of CNT strain sensors.

It is known that carbon nanotubes (CNT’s) modify the properties of cement based composites in fresh and hardened state; it is also known that these modifications are positive when appropriate dispersion of CNT’s within the matrix is achieved. Traditional experimental approaches used for fiber reinforced composites have been widely applied to study CNT’s-cement composites; nevertheless, high statistical dispersions and conflicting reports have been found to be a common issue due to the nanometric nature of CNT’s. This review presents a critical analysis of the most commonly used techniques to test CNT’s-cement composites, opening the discussion of the necessity of specific testing standardization for the development of the technology. Topics such as CNT’s dispersion and measurement of mechanical performance, electromagnetic properties and durability of CNT’s-cement composites are addressed. It is concluded that the benefits of CNT’s in cement composites have been fairly identified, but the mechanisms by which nanotubes maintain dispersion within the matrix, interact with hydrating cement, and modify properties of composites are not yet fully understood. Understanding these mechanisms is considered of most importance to identify the limitations of CNT’s-cement composites and define which applications are achievable with the use of CNT’s. Better characterization of the interaction between CNT’s and hydration products at the nanoscale is required to develop more efficient composites, targeting the enhancement of multiple properties at the same time. [ABSTRACT FROM AUTHOR]

The concept of lightweight design is widely employed for designing and constructing aerospace structures that can sustain extreme loads while also being fuel-efficient. Popular lightweight materials such as aluminum alloy and fiber-reinforced polymers (FRPs) possess outstanding mechanical properties, but their structural integrity requires constant assessment to ensure structural safety. Next-generation structural health monitoring systems for aerospace structures should be lightweight and integrated with the structure itself. In this study, a multi-walled carbon nanotube (MWCNT)-based polymer paint was developed to detect distributed damage in lightweight structures. The thin film's electromechanical properties were characterized via cyclic loading tests. Moreover, the thin film's bulk conductivity was characterized by finite element modeling. [ABSTRACT FROM AUTHOR]