Highly flexible and deformable electrically conductive materials are vital for the emerging field of wearable electronics. To address the challenge of flexible materials with a relatively high ...electrical conductivity and a high elastic limit, we report a new and facile method to prepare porous polydimethylsiloxane/carbon nanofiber composites (denoted by p-PDMS/CNF). This method involves using sugar particles coated with carbon nanofibers (CNFs) as the templates. The resulting three-dimensional porous nanocomposites, with the CNFs embedded in the PDMS pore walls, exhibit a greatly increased failure strain (up to ∼94%) compared to that of the solid, neat PDMS (∼48%). The piezoresistive response observed under cyclic tension indicates that the unique microstructure provides the new nanocomposites with excellent durability. The electrical conductivity and the gauge factor of this new nanocomposite can be tuned by changing the content of the CNFs. The electrical conductivity increases, while the gauge factor decreases, upon increasing the content of CNFs. The gauge factor of the newly developed sensors can be adjusted from approximately 1.0 to 6.5, and the nanocomposites show stable piezoresistive performance with fast response time and good linearity in ln(R/R 0) versus ln(L/L 0) up to ∼70% strain. The tunable sensitivity and conductivity endow these highly stretchable nanocomposites with considerable potential for use as flexible strain sensors for monitoring the movement of human joints (where a relatively high gauge factor is needed) and also as flexible conductors for wearable electronics (where a relatively low gauge factor is required).
Abstract
The predicted strong piezoelectricity for monolayers of group IV monochalcogenides, together with their inherent flexibility, makes them likely candidates for developing flexible ...nanogenerators. Within this group, SnS is a potential choice for such nanogenerators due to its favourable semiconducting properties. To date, access to large-area and highly crystalline monolayer SnS has been challenging due to the presence of strong inter-layer interactions by the lone-pair electrons of S. Here we report single crystal across-the-plane and large-area monolayer SnS synthesis using a liquid metal-based technique. The characterisations confirm the formation of atomically thin SnS with a remarkable carrier mobility of ~35 cm
2
V
−1
s
−1
and piezoelectric coefficient of ~26 pm V
−1
. Piezoelectric nanogenerators fabricated using the SnS monolayers demonstrate a peak output voltage of ~150 mV at 0.7% strain. The stable and flexible monolayer SnS can be implemented into a variety of systems for efficient energy harvesting.
The present paper demonstrates that multi-scale fillers such as carbon nanofibres (CNFs) and short carbon fibres (SCFs) can significantly improve the mode I fracture toughness of epoxy composites by ...various toughening mechanisms. A comparative assessment on the toughening performance promoted by CNFs and SCFs is presented along with the effects of aligning the filler normal to the crack growth using an applied alternating current (AC) electric field. For SCF concentrations of up to 1.5 wt%, with a concentration of CNFs of 1.0 wt%, the multi-scale, hybrid reinforcements additively toughen the epoxy polymer, with the measured fracture toughness being up to about fourteen times the value of the unmodified epoxy polymer. When subjected to an external AC electric field, these two reinforcements rapidly align along the direction of the electrical field in epoxy resin, with the CNFs concentrating between the ends of, and depositing on, the SCFs. For the same concentrations of SCFs and CNFs, the electric field induced alignment of the CNFs and the SCFs further increases the fracture toughness of the multi-scale toughened or hybrid epoxy polymer by up to twenty times that of the unmodified epoxy polymer. The intrinsic and extrinsic toughening mechanisms spanning the nano-to-millimetre length scale have been identified, based upon which an analytical model has been proposed.
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Through-thickness reinforcements using z-pins is a highly effective method for increasing the delamination resistance of fibre-polymer laminates. Here we report a hierarchical approach of ...incoporating carbon nanofillers and z-pins to further improve delamination toughness. The nanofillers include one-dimensional (1D) carbon nanofillers (e.g. carbon nanotubes, carbon nanofibres) or two-dimensional (2D) nanofillers (i.e. graphene nanoplatelets). Results of the interlaminar fracture toughness reveal that 1D nanofillers induce synergistic (i.e. greater-than-additive) improvements to both modes I and II delamination resistance of z-pinned laminates, whereas the 2D nanofiller show virtually no effect. Observations of the z-pin pull-out surfaces and the fracture surfaces reveal the main mechanisms why 1D nanofillers create a synergistic toughening effect, but not the 2D nanofillers.
An electric field is used to align carbon nanofibres (CNFs) in the matrix of a glass-fibre reinforced-polymer (GFRP) composite to simultaneously improve the (a) delamination toughness, (b) electrical ...conductivity, and (c) damage-sensing capability. The CNFs are added to the epoxy resin prior to the manufacture of the GFRP composites. To align the CNFs, an alternating current (AC) electric field of 30 V/mm at 10 kHz is applied across the GFRP sheet throughout the matrix-curing process. The electromechanical force induced by the electric field, applied in the through-thickness direction of the composite sheet, rotates and aligns the CNFs in the direction of the applied electric field prior to the gelation of the epoxy matrix. After curing, the resultant aligned, ‘chain-like’, microstructure of the CNFs in the epoxy matrix significantly enhances both the interlaminar fracture toughness and the through-thickness electrical conductivity of the GFRP composite. Specifically, the addition of 0.7 vol% of randomly-orientated CNFs in the GFRP composite yielded an ∼50% and 25% increase in the values of the mode I fracture toughness pertinent to the initiation, GIci, and steady-state growth, GIcss, of delamination crack, respectively, compared to the control GFRP composite. The alignment of the CNFs, in the transverse direction to the direction of the crack growth, increases the mode I toughness values of GIci and GIcss by ∼100% and ∼80%, respectively, compared to the control GFRP composite. These significant increases are attributable to multiple toughening mechanisms, including debonding of the CNFs from the matrix, void growth of the epoxy matrix, pull-out and rupture of the CNFs. Further, the electric-field induced alignment of the CNFs, in the through-thickness direction, increases the out-of-plane electrical conductivity of the GFRP by about twenty-six times, compared to the GFRP composite containing randomly-orientated CNFs. Of particular interest, the damage-sensing capacity is enhanced for the GFRP composite with aligned CNFs in the epoxy matrix, which stems from the greatly increased out-of-plane electrical conductivity, as confirmed by a modelling study. Therefore, this present work has identified a new strategy to develop GFRP composites with greatly improved delamination toughness, electrical conductivity, and higher crack-detection sensitivity.
•Shows improvements in an epoxy by using CNF or GNP or CNF/GNP hybrid nanofillers.•Fracture and fatigue properties improved.•Toughening mechanisms have been identified.•Successful model developed.
...The present paper describes improvements in the fracture resistance of epoxy polymers due to the addition of either (a) one-dimensional (1-D) carbon nanofibres (CNFs), or (b) two-dimensional (2-D) graphene nanoplatelets (GNPs), or (c) hybrid combinations of these carbon nanofillers (i.e. using both CNFs and GNPs). The effects of the dimensional shape and concentration (i.e. 0.0, 0.5, 1.0, 1.5 and 2.0 wt%) of the nanoscale carbon fillers are considered. The addition of CNFs, GNPs or hybrid combinations of CNFs and GNPs increases greatly the quasi-static fracture energy, GIc, of the epoxy due to these nanofillers inducing multiple intrinsic (e.g. interfacial debonding and void growth) and extrinsic (e.g. pull-out and bridging) toughening mechanisms. A mechanistic model is presented to quantify the contributions from the different toughening mechanisms induced by the CNF and the GNP fillers which result in the improvements observed in the fracture energy. The resistance of the epoxy, modified with either the GNPs or the CNFs, to cyclic-fatigue loading is also increased by the presence of the carbon nanofillers.
Improving the interlaminar fracture toughness of fibre-reinforced composites based on thermosetting polymeric matrices is of significant interest to a broad range of applications. In the present work ...we report a multi-scale approach to synergistically toughen composites by combining nano- and macro-scale reinforcements inspired by natural composite materials. Carbon reinforcements with two different length scales are used: nano-scale carbon nanofibres (∼100 nm diameter) and macro-scale carbon z-pins (∼280 μm diameter) to reinforce continuous carbon-fibre composites in the through-thickness direction. The resultant composite, featuring three-dimensional reinforcement architecture, possesses triple toughening mechanisms at three different scales, thus yielding a synergistic effect. At the nano-scale, the carbon nanofibres alone promote high mode I delamination resistance (∼70% increase in interlaminar fracture energy) by multiple intrinsic and extrinsic toughening processes around the crack tip. The macro-size carbon z-pins, together with the crossover continuous fibres, promote a strong extrinsic toughening mechanism (∼200% increase in the interlaminar fracture energy) behind the crack tip and over a larger length-scale via both the z-pins and crossover fibres bridging the crack faces. When used concurrently, the nanofillers and z-pins promote a higher toughness under quasi-static loading (∼400% increase in fracture energy) than when used separately due to a multiplicative effect from the interplay between intrinsic and extrinsic toughening processes operative ahead of, and behind, the crack tip. Under mode I interlaminar cyclic-fatigue loading, the multi-scale laminates show a strong improvement in resistance against fatigue delamination growth. Similar to the synergistic increase in fracture energy, a greater increase in the delamination fatigue resistance occurs when both are active together. However, the results indicate that the synergistic effect of the multi-scale toughening is statistically significant under quasi-static loading but not under fatigue loading. A very small reduction (∼2%) in the tensile strength is observed for the multi-scale reinforced laminates.
A liquid metal synthesis process provides a new low energy pathway avenue to manufacture various low-dimensional nanomaterials in order to improve the mechanical properties of polymer composites. ...This paper presents an investigation of the strengthening and toughening performances of two-dimensional platelets of boehmite (γ-AlO(OH)) and alumina (γ-Al2O3). Using a liquid metal alloy reaction process, two-dimensional metal oxide hydroxide and oxide platelets were synthesised and then used for reinforcing epoxy polymer composites at different weight fractions up to 10%. Both boehmite and alumina platelets increased the tensile modulus, yield stress and fracture toughness of the epoxy composite by up to 40%, 35% and 320%, respectively. Of the two materials, the boehmite platelets were more effective than the alumina platelets in increasing the tensile modulus (up to 27%) and ultimate strength (up to 14%) of the epoxy. In contrast, the alumina platelets promoted a 50% greater improvement to the mode I fracture energy when compared to using boehmite platelets. The primary mechanisms responsible for the measured property improvements are identified.
Fused deposition modelling (FDM) is an additive manufacturing process for the 3D printing of continuous fibre-thermoplastic composite laminates. The electrical conductivity of continuous carbon fibre ...(CF)-nylon filaments and laminates printed with a Markforged MarkTwo® FDM machine is experimentally investigated. The axial electrical conductivity of single filaments measured before, during and following the FDM process reveal a large reduction (∼40%) due mostly to breakage of carbon fibres. The electrical conductivity of 3D printed laminates made by the layer-by-layer deposition of the single filaments were determined in the longitudinal, transverse and through-thickness directions, and compared to the electrical properties of a hot moulded composite with a near-equivalent carbon fibre content. The longitudinal conductivity of the 3D printed laminate was only ∼50% that of the hot moulded composite, and this was due mostly to fibre breakages caused by the FDM process. However, the transverse and through-thickness electrical conductivities of the 3D printed laminate were higher (∼13 times and ∼3 times, respectively) than the hot moulded composite due to higher fibre waviness causing increased fibre-to-fibre contact which aids the flow of electrical current in these two directions.
Carbon fibres and their composites are well known for their excellent tensile properties and light weight characteristics which, amongst other advantages continues to drive demand from new structural ...applications across a wide range of weight sensitive industries. However, since their inception, their inferior performance under compression loading has remained a major impediment to even wider usage. In this review, a systematic discussion highlights the reasons for this deficiency from a microstructural and macroscale perspective and provides suggestions on how to improve the compressive properties for both carbon fibre and carbon fibre composites. In this work, structure-property, property-property and defect-property correlations from fibre to composite are developed from an analysis of the literature and other available technical information. This review covers the most important issues that require addressing by carbon fibre and composite manufacturers if new applications are to emerge in industries where not only tensile strength or stiffness to weight matters, but so too does compression.
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