Self-assembled multi-layered carbon nanofiber nanopaper for significantly improving electrical actuation of shape memory polymer nanocomposite

This study presents an effective approach to significantly improve the electrical properties of shape memory polymer (SMP) nanocomposites that show Joule heating triggered shape recovery. Carbon nanofibers (CNFs) were self-assembled to form multi-layered nanopaper to enhance the bonding and shape recovery behavior of SMP, respectively. It was found that both glass transition temperature (Tg) and electrical properties of the SMP nanocomposites have been improved by incorporating multi-layers of self-assembled nanopapers. The electrically actuated shape recovery behavior and the temperature profile during the actuation were monitored and characterized at a voltage of 30 V.     

Synergistic effect of siloxane modified aluminum nanopowders and carbon fiber on electrothermal efficiency of polymeric shape memory nanocomposite

The present study reports an effective approach of significantly enhancing electrothermal efficiency and shape recovery performance of shape memory polymer (SMP) nanocomposite, of which shape recovery was induced by electrically resistive heating. Metallic aluminum (Al) nanopowders synthesized from Al³⁺ solution were chemically grafted onto carbon fiber. Siloxane groups were grafted onto surfaces of the Al nanopowders to enhance the interfacial bonding between the carbon fiber and SMP matrix via van der Waals force and covalent bond, respectively. The siloxane modified Al surfaces could improve both the electrically induced shape recovery performance and electrothermal efficiency through facilitating the electrically resistive heating from carbon fiber into the SMP matrix. Effectiveness of the synergistic effect between siloxane modified Al surface and carbon fiber was demonstrated to achieve the electrical actuation for SMP nanocomposites at a low electrical voltage below 4.0 V.     

Effect of processing conditions and electrode characteristics on the electrical properties of structural composite capacitors

Structural capacitors are manufactured from glass fabric/epoxy prepreg dielectrics and metalized polymer film electrodes. The electrical breakdown strengths of these multifunctional materials are investigated across a wide range of electrode constructions and processing parameters. The results show that electrode selection and materials processing have a significant impact on the energy that the device can store. Also, this careful consideration of processing parameters and electrode construction has led to the development of a structural capacitor with an energy density exceeding 0.90 J/cm³, the highest yet reported.     

Shape memory and thermo-mechanical properties of shape memory polymer/carbon fiber composites

In this study, chopped carbon fiber reinforced trans-1, 4-polyisoprene (TPI) was developed via a proposed new manufacturing process with the aim of improving weak mechanical properties of bulk TPI bulk. Specimens of the developed shape memory polymer (SMP) composites were fabricated with carbon fiber weight fraction of 5%, 7%, 9%, 11% and 13%, respectively. Measured are the effects of chopped carbon fiber and temperature on: (a) shape recovery ratio and rate; (b) stress–strain relationship; (c) maximum tensile stress, strain and Young’s modulus; and (d) maximum stress and residual strain under a constant strain cyclic loading. In addition, SEM micrographs were also presented to illustrate the fracture surface. The present experimental results show that the SMP with 7% carbon fiber weight fraction appears to perform best in all the tests. This indicates that the 7% carbon fiber weight fraction could be the optimum value for the SMP developed using the proposed manufacturing process.     

Fabrication of hybrid membrane of electrospun polycaprolactone and polyethylene oxide with shape memory property

Hybrid materials with nanostructure could exhibit a diverse range of applications as advanced functional materials. This research work, composite membranes with shape memory property based on biocompatible polycaprolactone and polyethylene oxide were successfully fabricated by using electrospinning technique. Electrospun fiber configuration is strongly related to the concentration of polymer and electric field strength. The hydrophilic property of hybrid membrane has been improved and water play a critical role in resulting lower its responsive temperature compared with dry membrane. The mechanism of shape memory PCL/PEO hybrid membrane at wet condition has been proposed.     

Electrostatic self-assembled carbon nanotube/nano carbon black composite fillers reinforced cement-based materials with multifunctionality

Electrostatic self-assembled carbon nanotube (CNT)/nano carbon black (NCB) composite fillers are added into cement mortar to fabricate smart cement-based materials. The grape bunch structure of CNT/NCB composite fillers is beneficial for dispersing CNT/NCB in cement mortar matrix and achieving cooperative improvement effect. The mechanical, electrically conductive, and piezoresistive behaviors of the cement mortar are investigated. The CNT/NCB composite fillers can effectively enhance the flexural strength and electrical conductivity of cement mortars, and endow stable and sensitive piezoresistivity to cement mortar at a low filler content. However, they weaken the compressive strength of cement mortar to some extent. The percolation threshold zone of cement mortar with CNT/NCB composite fillers ranges in the amount of 0.39–1.52 vol.%. The optimal content of CNT/NCB composite fillers is 2.40 vol.% for piezoresistivity and the stress and strain sensitivities can reach 2.69% MPa⁻¹ and 704, respectively.     

Instant electrode fabrication on carbon-fiber-reinforced plastic structures using metal nano-ink via flash light sintering for smart sensing

The electrical-resistance-change method (ERCM) is a potential smart-sensing technique for carbon-fiber-reinforced plastic (CFRP) structures. However, a practical way to fabricate electrodes on CFRP structures, such as ink-jet printing with metal nano-inks, is necessary to reduce the time required for the process. As metal nano-inks can be sintered in a few milliseconds under ambient conditions using white-flash-light irradiation from a xenon lamp, the parameters of flash-light sintering such as light energy, duration, and number of pulses were investigated. The light intensity, which is the light energy divided by duration, was found to be an indicator of whether low electrical resistance was attained along with strong adhesion to the CFRP plate. The contact resistance between the electrode and CFRP plate was also examined in a tensile test to confirm the durability. The electrode sintered by flash light with the properly selected parameters exhibited high quality and strain monitoring capability.     

Preparation and characterization of water-borne epoxy shape memory composites containing silica

Shape memory silica/epoxy composites were successfully prepared by hydrolysis of tetraethoxysilane (TEOS) within the epoxy matrix via latex, freeze-drying, and hot-press molding method. The silane coupling agent 3-triethoxysilylpropylamine (KH550) was introduced to improve the interfacial properties between the in-situ generated silica particle and epoxy matrix. The morphology structure and the effect of the content of the in-situ formed silica on the mechanical and shape memory properties of the silica/epoxy composites were studied. The experimental results indicated that the silica particles were homogenously dispersed and well incorporated into the epoxy matrix. Significant improvements were achieved in the mechanical property of the organic–inorganic hybrid materials. The silica/epoxy composites exhibited high shape recovery and fixity ratio approximately 100% even after 10 thermo-mechanical cycles.     

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.     

Significantly improving infrared light-induced shape recovery behavior of shape memory polymeric nanocomposite via a synergistic effect of carbon nanotube and boron nitride

The present work studies the thermomechanical properties and infrared light-induced shape memory effect (SME) in shape memory polymer (SMP) nanocomposite incorporated with carbon nanotube (CNT) and boron nitride. The combination of CNT and boron nitride results in higher glass transition temperature, mechanical strength and thermomechanical strength. While CNTs are employed to improve the absorption of infrared light and thermally conductive property of SMP, boron nitrides facilitate heat transfer from CNTs to the polymer matrix and thus to enable fast response. A unique synergistic effect of CNT and boron nitride was explored to facilitate the heat transfer and accelerate the infrared light-induced shape recovery behavior of the shape memory polymeric nanocomposite.     

Effect of fiber arrangement on shape fixity and shape recovery in thermally activated shape memory polymer-based composites

In the present study, we conducted periodic-cell simulations of the thermomechanical cycle of thermally activated shape memory polymer (SMP)-based composites. The present simulation utilizes a micromechanical model for reproducing the discontinuous fibers and SMP. We analyzed the effect of fiber volume fraction, fiber aspect ratio, and fiber end position on the shape fixity and shape recovery of the composite. The simulated results revealed that fiber elasticity is a key factor for the shape fixity of the composite, while both strain concentration near the fiber ends and fiber elasticity play important roles in the shape recovery properties of the composite.     

Failure detection and monitoring in polymer matrix composites subjected to static and dynamic loads using carbon nanotube networks

In this work, multiwall carbon nanotubes (MWCNTs) have been used as a network of sensors to predict the failure region and to monitor the degradation of mechanical properties in laminated composites subjected to tensile and cyclic fatigue loadings. This is achieved by measuring the electrical resistance change in the semi-conductive MWCNT-fiber glass–epoxy polymer matrix composites. By partitioning the tensile and fatigue samples with electrically conductive probes, it is shown that with both increasing tensile load and number of cycles different resistance changes are detected in different regions and failure happens in the part in which higher resistance change was detected. In cyclic loading, when compared to strain gauge readings, resistance change measurements show more sensitivity in identifying the crack location, which gives this technique a good potential for monitoring damage during fatigue.     

Nanotip-induced ultrahigh pressure-sensitive composites: Principles,properties and applications

Spiky spherical nickel powders with sharp nanotips on their surface are excellent fillers for developing pressure-sensitive composites. The sharp nano-tips are responsible for generating field-assisted tunneling conduction, which leads to the strong responses of electrical conductivity of the composites to external force or deformation. The nanotip-induced ultrahigh pressure-sensitive composites can be used to develop new sensors, switches and controls for a wide range of applications including electronics, transport, space, medicine, defense, textiles, oil and gas, and civil engineering. In this paper, we examine a systematical review of research progress on the nanotip-induced ultrahigh pressure-sensitive composites, with attentions to mechanism of pressure-sensitivity, sensing performances, and applications of the composites. Future challenges in the development and application of the nanotip-induced ultrahigh pressure-sensitive composites are also discussed.     

Actuation of electrochemical, electro-magnetic, and electro-active actuators for carbon nanofiber and Ni nanowire reinforced polymer composites

Actuation of electrochemical, electro-magnetic, and electro-active actuators composed of CNF and/or Ni nanowire/polymer nanocomposites was evaluated with different materials and preparation processes. The actuated strain was compared with frequency, applied voltage, and wave type. The hysteresis of the actuated strain was continuously delayed in electrochemical actuators, whereas the strain was uniformly actuated in the electro-magnetic actuators. The actuated strain decreased with increasing frequency in both electrochemical and electro-magnetic actuators. In magnetic fields the actuated strain increased with increasing Ni nanowire content whereas the current increased with applied voltage. Ni nanowire/cellulose actuators in a magnetic field responded better at high frequencies, compared to the other actuators studied. Actuated strain of cellulose or Ni nanowire/cellulose nanocomposite in air was larger than either PVDF or PVDF/cellulose nanocomposite. In Ni nanowire/cellulose nanocomposite, actuated strain also decreased with increasing frequency and increased with increasing voltage. Electro-active actuators responded well in air when low voltages and high frequencies were applied compared to the other two actuators. Electro-active actuators in this paper have unique advantages for many practical applications, including easy fabrication, lightweight and low application voltages.     

Pullout resistance of straight NiTi shape memory alloy fibers in cement mortar after cold drawing and heat treatment

In our study, we found cold drawing to be an effective method for enhancing the pullout resistance of NiTi shape memory alloy (SMA) fibers in concrete. The pullout resistance was observed to be dependent on the contact pressure and friction coefficient at the interface between the fibers and the mortar matrix. The drawing process increased the stiffness and yield stress of the fibers and consequently increased the contact pressure at the interface between the fibers and the mortar matrix. Moreover, heat treatment of the fibers after cold drawing was found to noticeably recover the fiber diameter, thereby significantly enhancing the pullout resistance. The enhancement of the interfacial bond strength by heat treatment verified the crack-closing capabilities of SMA-fiber-reinforced cement composites.     

Bond behavior of superelastic shape memory alloys to carbon fiber reinforced polymer composites

In this research pull-out specimens were tested to investigate the bond behavior of superelastic NiTi (Nitinol) SMA wires to carbon fiber reinforced polymers (CFRP). A total of 45 pull-out specimens were tested monotonically up to failure. The test parameters considered include the wire diameter and embedment length. A digital image correlation (DIC) system was used to identify the onset and propagation of debonding. Based on the experimental observations two debonding mechanisms were observed: complete debonding after the onset of martensitic transformation of SMA wire, and complete debonding before the onset of wire transformation. The former mechanism predominated, while the latter mechanism governed for larger diameter wires with shorter embedment lengths. A 3-D non-linear finite element model (FEM) was developed to predict the pull-out behavior. A cohesive zone model (CZM) was used to model the interface. A parametric study was conducted using the FEM to quantify the parameters of the cohesive zone model. The results demonstrate that the proposed modeling approach can be used to characterize the bond behavior of superelastic SMA wires embedded in FRP composites.     

An investigation of contact resistance between carbon fiber/epoxy composite laminate and printed silver electrode for damage monitoring

An addressable conducting network (ACN) enables the structural condition to be monitored by the electrical resistance between electrodes on surface of CFRP (carbon fiber reinforced polymer) structure. To improve the reliability of ACN for damage detection, the contact resistance between the electrodes and CFRP laminates needs to be minimized. In this paper, the silver nanoparticles electrodes were fabricated via printed electronics techniques on CFRP composite. The contact resistance between the silver electrodes and CFRP was measured with respect to various fabrication conditions such as the sintering temperature of silver nanoink and the surface roughness of CFRP laminates. The interfaces between silver electrode and carbon fibers were observed using scanning electron microscope (SEM). From the study, it was found that the lowest contact resistance of 0.3664 Ω could be achieved when the sintering temperature of the silver nanoink and surface roughness were 120 °C and 230 nm, respectively.     

A practical structural health monitoring system for carbon fibre reinforced composite based on electrical resistance

A practical structural health monitoring system based on measuring changes in the electrical resistance of a carbon fibre composite structure is presented. Electrical contact with the fibres is provided by flexible printed circuit boards which are interleaved with the carbon fibre plies during the lamination of the composite. The resistance between opposite pairs of contacts was measured before and after an impact load which caused barely visible impact damage (BVID) in the panel. It was found that even low levels of impact damage produced measurable changes in resistance in the vicinity of the damage. Therefore was demonstrated that electrical resistance measurements are a practical means of locating BVID. Various parameters were studied in order to better understand the mechanisms involved and optimise the system for improved sensitivity and accuracy. The location of the contacts in the through thickness direction, the spacing and orientation of the contacts and the residual thermal stress of the laminate were all investigated and recommendations made. A structural health monitoring system for composites based on electrical resistance has several important potential benefits over acoustic, ultrasonic or optical methods; it adds little parasitic mass, causes no reduction in mechanical integrity, can be carried out on structures either in or out of service conditions and is very simple in concept, implementation and data interpretation.     

Thermosetting epoxy reinforced shape memory composite microfiber membranes: Fabrication,structure and properties

The thermosetting epoxy-based shape memory composite microfibers are successfully fabricated by means of coaxial electrospinning. The PCL/epoxy composite fiber shows core/shell structure, in which epoxy as the core layer is for an enhancing purpose. By incorporating epoxy and PCL, the mechanical strength of composite fibers is greatly reinforced. The deformation is via the heating and cooling process, and the shape memory effect can be demonstrated from the micro level to the macro level. The whole shape recovery performance takes only 6.2 s when triggered by the temperature being at 70 °C. The porosity of woven microfibers changes in response to temperature. In addition, the PCL/epoxy composite microfiber membranes are analyzed in an in vitro cytotoxicity test, which proves that PCL as the shell layer provides the composite microfibers potential capabilities in biomedical science.     

Electroactive response of mesoporous silica and its nanocomposites with conducting polymers

Electroactive response of suspensions of mesoporous silica and its nanocomposites with conducting polyaniline and copolyaniline inside its channels were examined under an electric field, mainly focusing on their rheological characteristics. Initially these conducting polymer/mesoporous silica nanocomposites were synthesized and their physical properties were studied by scanning electron microscopy, transmission electron microscopy and N₂-adsorption isotherm. Then, mesoporous silica and its nanocomposites were dispersed in silicone oil as an electrorheological (ER) material. Typical ER behaviors of shear stress and shear viscosity curves as a function of electric field and shear rate were observed. Without an electric field, the suspensions behaved almost like a Newtonian fluid. However, under an electric field, their shear stresses increased with shear rate, demonstrating a yield stress. Compared with mesoporous silica and polyaniline, polyaniline/mesoporous silica-based ER fluid showed enhanced ER performance due to the anisotropic characteristics. In addition, it was found that a suggested shear stress model (Cho–Choi–Jhon model) well described the flow curves.