Repeated Bending Cycles Research Articles

Silver Nanowire-Based Flexible Transparent Composite Film for Curvature Measurements

A transparent, flexible, and conductive epoxy resin film embedded with silver nanowires is fabricated through a simple process. The effect of silver nanowire areal mass density on the electrical and optical properties of films is investigated. It is found that resistance of the transparent composite film will change regularly during the bending process. The influence mechanism of bending behavior on the electrical property is explored. On the basis of this mechanism, the curvature in the bending process can be acquired immediately by measuring the resistance of the composite film. The high elastic modulus (800 MPa) of the composite film is a benefit for distinguishing the factor of resistance change caused by bending or stretching with low force. Additionally, high stability of the bending detector is demonstrated with 500 repeated bending cycles and a 6-month durability test.

Integrated flexible supercapacitor based on poly (3, 4-ethylene dioxythiophene) deposited on Au/porous polypropylene film/Au

Thin layers of continuous gold have been deposited on the both sides of porous polypropylene films (PPF) via electroless plating method to form a sandwich structure with two conductive surfaces and an intermediate insulating layer (Au/PPF/Au). Then poly (3, 4-ethylenedioxy thiophene) (PEDOT) layers are electrodeposited on the two conductive sides via cyclic voltammetry method to form the sandwich-structured PEDOT-Au/PPF/Au-PEDOT. Gel electrolyte of H3PO4/polyvinyl alcohol (H3PO4/PVA) is then added to the sandwich-structured PEDOT-Au/PPF/Au-PEDOT to form the integrated flexible supercapacitor of Au-PEDOT|H3PO4/PVA|PEDOT-Au. The PEDOT-Au/PPF/Au-PEDOT sandwiched structures are characterized and the capacitive performances of the integrated flexible cells are investigated in detail. The results show that the optimum integrated flexible cell exhibits excellent cycling stability (91.8% capacitance retention after 20000 cycles) and relatively high areal specific capacitance. Furthermore, the cells of Au-PEDOT|H3PO4/PVA|PEDOT-Au not only exhibit excellent bending endurance (99.1% capacitance retention after 5000 repeated bending cycles), but also can be conveniently connected in series without additional wires. This integrated flexible cell may exhibit potential application in portable electronics.

Capacitive Properties of the Binder‐Free Electrode Prepared from Carbon Derived from Cotton and Reduced Graphene Oxide

The binder‐free composite films of reduced graphene oxide (rGO) and activated carbon derived from cotton (aCFC) have been fabricated and used as electrodes for electrochemical capacitors (ECs) to avoid the decrease of capacitive performance in traditional process caused by the additional binder. The optimal aCFC is prepared at 850 °C when the mass ratio of carbon and potassium hydroxide is 1 to 4. The optimal composite film prepared from the mass ratio of aCFC/GO=2/1 exhibits porous structure, and has a specific surface area of 849.6 m2•g−1 and a total pore volume of 0.61 mL•g−1. Based on the two‐electrode system testing in 6.0 mol/L KOH electrolyte, the optimal composite has specific capacitance of about 202 F•g−1, 374 mF•cm−2 and 116 F•cm−3 in terms of mass, area and volume, and shows excellent rate capability and good cyclic stability (91.7% retention of the initial capacitance after 5000 cycles). Furthermore, the assembled solid‐state ECs by using KOH/polyvinyl alcohol as electrolyte show good mechanical stability and capacitive performances after repeated bending cycles. It is proved that this method is effective to fabricate binder‐free electrodes for ECs and will open up a novel route for the reuse of waste cotton.

Area-Selective Lift-Off Mechanism Based on Dual-Triggered Interfacial Adhesion Switching: Highly Facile Fabrication of Flexible Nanomesh Electrode.

With the recent emergence of flexible and wearable optoelectronic devices, the achievement of sufficient bendability and stretchability of transparent and conducting electrodes (TCEs) has become an important requirement. Although metal-mesh-based structures have been investigated for TCEs because of their excellent performances, the fabrication of mesh or grid structures with a submicron line width is still complex due to the requirements of laborious lithography and pattern transfer steps. Here, we introduce an extremely facile fabrication technique for metal patterns embedded in a flexible substrate based on submicron replication and an area-selective delamination (ASD) pattern. The high-yield, area-specific lift-off process is based on the principle of solvent-assisted delamination of deposited metal thin films and a mechanical triggering effect by soft wiping or ultrasonication. Our fabrication process is very simple, convenient, and cost-effective in that it does not require any lithography/etching steps or sophisticated facilities. Moreover, their outstanding optical and electrical properties (e.g., sheet resistances of 0.43 Ω sq-1 at 94% transmittance), which are markedly superior to those of other flexible TCEs, are demonstrated. Furthermore, there is no significant change of resistance over 1000 repeated bending cycles, with a bending radius of 5 mm, and immersion in various solvents such as salt water and organic solvents. Finally, we demonstrate high-performance transparent heaters and flexible touch panels fabricated using the nanomesh electrode, confirming the long-range electrical conduction and reliability of the electrode.

Preparation of Bismuth Telluride Films with High Thermoelectric Power Factor.

Highly conductive n-type Bi2Te3 films on a flexible substrate were prepared via electrodeposition followed by a transfer process using an adhesive substrate. The growth of the Bi2Te3 crystals was precisely controlled by an electrochemical deposition potential (Vdep), which was critical to the preferred orientation of the crystal growth along the (110) direction and thus to the properties of a flexible thermoelectric generator (FTEG). A Bi2Te3 film prepared under Vdep of 0.02 V showed high electrical conductivity (691 S cm-1) with a maximum power factor of 1473 μW m-1 K-2, which is the highest among the Bi2Te3 films prepared by the electrodeposition methods. As-prepared FTEG was bendable, showing only a small resistance change after 300 repeated bending cycles. Combined with the n-type Bi2Te3 FTEG, a prototype p-n-type flexible thermoelectric (pn-FTEG) was prepared using p-type poly(3,4-ethylene dioxythiophene)s. The pn-FTEG (5-couples) generated an output voltage of 5 mV at ΔT = 12 K with high output power of 56 nW (or 105 nWg-1). These results indicate that the FTEG can reproducibly work well in a bent state and has high application potential for harvesting thermal energy from curved sources such as human body temperature.

Electrical characterization of benzenedithiolate molecular electronic devices with graphene electrodes on rigid and flexible substrates

We investigated the electrical characteristics of molecular electronic devices consisting of benzenedithiolate self-assembled monolayers and a graphene electrode. We used the multilayer graphene electrode as a protective interlayer to prevent filamentary path formation during the evaporation of the top electrode in the vertical metal–molecule–metal junction structure. The devices were fabricated both on a rigid SiO2/Si substrate and on a flexible poly(ethylene terephthalate) substrate. Using these devices, we investigated the basic charge transport characteristics of benzenedithiolate molecular junctions in length- and temperature-dependent analyses. Additionally, the reliability of the electrical characteristics of the flexible benzenedithiolate molecular devices was investigated under various mechanical bending conditions, such as different bending radii, repeated bending cycles, and a retention test under bending. We also observed the inelastic electron tunneling spectra of our fabricated graphene–electrode molecular devices. Based on the results, we verified that benzenedithiolate molecules participate in charge transport, serving as an active tunneling barrier in solid-state graphene–electrode molecular junctions.

Electrical evaluation of crack generation in SiNx and SiOxNy thin-film encapsulation layers for OLED displays

By measuring leakage current density, we detected crack generation in silicon nitride (SiNx) and silicon oxynitride (SiOxNy) thin-film encapsulation layers, and correlated with the films’ water vapor permeability characteristics. After repeated bending cycles, both the changes in water vapor transmission rate and leakage current density were directly proportional to the crack density. Thick SiNx films had better water vapor barrier characteristics in their pristine state, but cyclic loading led to fast failure. Varying the atomic concentration of the SiOxNy films affected their bending reliability. We attribute these differences to changes in the shape of the crack tip as the oxygen content varies.

Extremely Stable Current Emission of P-Doped SiC Flexible Field Emitters.

Novel P-doped SiC flexible field emitters are developed on carbon fabric substrates, having both low Eto of 1.03-0.73 Vμm-1 up to high temperatures of 673 K, and extremely high current emission stability when subjected to different bending states, bending circle times as well as high temperatures (current emission fluctuations are typically in the range ±2.1%-3.4%).

Flexible Multifunctionalized Carbon Nanotubes‐Based Hybrid Nanowires

In this work, flexible multifunctionalized carbon nanotube (CNT)‐based hybrid nanowires are synthesized through surface modification processes. The good dispersability of the hybrid nanowire in polar solvents facilitates directly making fine patterns with a minimum width of 40 μm for applications of flexible and stretchable circuits (FSCs). The hybrid nanowire possesses a flexible and highly conductive structure which demonstrates stable electro‐mechanical properties on polydimethylsiloxane (PDMS) substrates under large structural deformation. FSCs fabricated from the hybrid nanowires show a constant resistance of 0.096 Ω □−1 (equivalent of a resistivity 0.96 Ω μm) under repeated bending cycles. The FSCs also have a low and stable sheet resistance of 0.4 Ω □−1 for strains up to 30%, which is almost four orders of magnitude lower than that of pure CNT samples (1316 Ω □−1). Further improved stretchability and electro‐mechanical properties (0.1 Ω □−1, at the strain of 100%) are achieved with a prestrain PDMS substrate. Repeated deformation tests demonstrate the high reliability of FSCs. The observed stable and reliable electro‐mechanical performance of FSCs suggests the potential use of the material in wearable and portable electronics.

Large-scale plasma patterning of transparent graphene electrode on flexible substrates.

Graphene, a two-dimensional carbon material, has attracted significant interest for applications in flexible electronics as an alternative transparent electrode to indium tin oxide. However, it still remains a challenge to develop a simple, reproducible, and controllable fabrication technique for producing homogeneous large-scale graphene films and creating uniform patterns with desired shapes at defined positions. Here, we present a simple route to scalable fabrication of flexible transparent graphene electrodes using an oxygen plasma etching technique in a capacitively coupled plasma (CCP) system. Ascorbic acid-assisted chemical reduction enables the large-scale production of graphene with solution-based processability. Oxygen plasma in the CCP system facilitates the reproducible patterning of graphene electrodes, which allows controllable feature sizes and shapes on flexible plastic substrates. The resulting graphene electrode exhibits a high conductivity of 80 S cm(-1) and a transparency of 76% and retains excellent flexibility upon hard bending at an angle of ±175° and after repeated bending cycles. A simple LED circuit integrated on the patterned graphene film demonstrates the feasibility of graphene electrodes for use in flexible transparent electrodes.

Highly flexible and robust N-doped SiC nanoneedle field emitters

Flexible field emission (FE) emitters, whose unique advantages are lightweight and conformable, promise to enable a wide range of technologies, such as roll-up flexible FE displays, e-papers and flexible light-emitting diodes. In this work, we demonstrate for the first time highly flexible SiC field emitters with low turn-on fields and excellent emission stabilities. n-Type SiC nanoneedles with ultra-sharp tips and tailored N-doping levels were synthesized via a catalyst-assisted pyrolysis process on carbon fabrics by controlling the gas mixture and cooling rate. The turn-on field, threshold field and current emission fluctuation of SiC nanoneedle emitters with an N-doping level of 7.58 at.% are 1.11 V μm−1, 1.55 V μm−1 and 8.1%, respectively, suggesting the best overall performance for such flexible field emitters. Furthermore, characterization of the FE properties under repeated bending cycles and different bending states reveal that the SiC field emitters are mechanically and electrically robust with unprecedentedly high flexibility and stabilities. These findings underscore the importance of concurrent morphology and composition controls in nanomaterial synthesis and establish SiC nanoneedles as the most promising candidate for flexible FE applications. Spiky silicon carbide (SiC) ‘nanoneedles’ can improve light emission from e-paper and other bendable electronic devices. Flexible field-emission displays are an emerging technology in which tiny conductive tips grown on lightweight, rollable surfaces generate intense light. Significant manufacturing- and materials-related obstacles, however, have limited their application. Now, a team led by Tom Wu from King Abdullah University of Science and Technology in Saudi Arabia and Weiyou Yang from Ningbo University of Technology in China investigated how SiC — a compound with notable stiffness and stablity — performed as a field emitter by catalytically synthesising this material into nanoscale needles with ultrasharp tips and controllable doping levels on a carbon fabric surface. Their experiments showed that the SiC nanoneedles had low ‘turn-on’ field requirements and minimal emission fluctuations even after repeated bending cycles, thanks to their impressive mechanical robustness. We demonstrated for the first time highly flexible N-doped SiC nanoneedle field emitters with low turn-on fields and excellent emission stabilities. The characterizations of their field emission properties under repeated bending cycles and different bending states confirmed that such emitters are mechanically and electrically robust. These findings underscore the importance of concurrent morphology and composition controls in nanomaterial synthesis and establish SiC nanoneedles as the most promising candidate for flexible FE applications.

Bending fatigue characteristics of corroded wire ropes

The corrosion of a wire rope reduces its life expectancy. In this study, repeated bending tests were conducted using a bending fatigue tester by changing the tensile load and corrosion time of wire ropes, which were the same type as those used in elevators. The number of broken wires was studied, and a tensile test was conducted for cases in which the fracture was severe. The effect of corrosion fatigue on life expectancy was considered by comparing fracture strength values and observing fracture surfaces. The results indicate that an increase in accumulated corrosion fatigue, a greater tensile load, and repeated bending cycles may yield a rapid decrease in fracture strength and an increase in the number of broken wires. Therefore, it is concluded that corrosion fatigue is an important factor that decreases the life expectancy of wire rope.

Flex Fatigue of Side Emitting Optical Fiber

Side emitting plastic optical fibers can be used in textile structures for some special optical activities due to the light leaking out from their surface. One significant characteristic of plastic optical fibers is their tendency to weaken side emitting capability under mechanical deformation, which can be simulated as repeated bending cycles under prescribed pretension. The principle of evaluation of flex fatigue was based on the repeated bending cycles until break. Q-Q plot and three-parameter Weibull distribution were used for estimation of numbers of bending cycles of plastic optical fibers with different diameters. The relationship between flex fatigue and flexibility were analyzed and compared with logarithmic values.

Textile electrodes woven by carbon nanotube–graphene hybrid fibers for flexible electrochemical capacitors

Functional graphene-based fibers are promising as new types of flexible building blocks for the construction of wearable architectures and devices. Unique one-dimensional (1D) carbon nanotubes (CNTs) and 2D graphene (CNT/G) hybrid fibers with a large surface area and high electrical conductivity have been achieved by pre-intercalating graphene fibers with Fe3O4 nanoparticles for subsequent CVD growth of CNTs. The CNT/G hybrid fibers can be further woven into textile electrodes for the construction of flexible supercapacitors with a high tolerance to the repeated bending cycles. Various other applications, such as catalysis, separation, and adsorption, can be envisioned for the CNT/G hybrid fibers.

Imperfection analysis of flexible pipe armor wires in compression and bending

The work presented in this paper is motivated by a specific failure mode known as lateral wire buckling occurring in the tensile armor layers of flexible pipes. The tensile armor is usually constituted by two layers of initially helically wound steel wires with opposite lay directions. During pipe laying in ultra deep waters, a flexible pipe experiences repeated bending cycles and longitudinal compression. These loading conditions are known to impose a danger to the structural integrity of the armoring layers, if the compressive load on the pipe exceeds the total maximum compressive load carrying ability of the wires. This may cause the wires to buckle in the circumferential pipe direction, when these are restrained against radial deformations by adjacent layers.In the present paper, a single armoring wire modeled as a long and slender curved beam embedded in a frictionless cylinder bent into a toroid will be studied in order to gain further understanding of this failure mode. In order to study the compressive behavior, both perfect beams as well as beams with small geometrical imperfections are studied. The mathematical formulation of the problem is based on curved beam equilibrium and allows large deflections to be taken into calculation.

On modelling of lateral buckling failure in flexible pipe tensile armour layers

In the present paper, a mathematical model which is capable of representing the physics of lateral buckling failure in the tensile armour layers of flexible pipes is introduced. Flexible pipes are unbounded composite steel–polymer structures, which are known to be prone to lateral wire buckling when exposed to repeated bending cycles and longitudinal compression, which mainly occurs during pipe laying in ultra-deep waters. On the basis of multiple single wire analyses, the mechanical behaviour of both layers of tensile armour wires can be determined. Since failure in one layer destabilises the torsional equilibrium which is usually maintained between the layers, lateral wire buckling is often associated with a severe pipe twist. This behaviour is discussed and modelled. Results are compared to a pipe model, in which failure is assumed not to cause twist. The buckling modes of the tensile armour wires can be obtained by the presented method.

Flexible ZnSnO3/Ag/ZnSnO3 multilayer electrodes grown by roll-to-roll sputtering on flexible polyethersulfone substrates

The authors report on the characteristics of flexible ZnSnO3 (ZTO)/Ag/ZTO multilayer electrodes sputtered on a flexible polyethersulfone substrate using roll-to-roll sputtering at room temperature. The sheet resistance and optical transmittance of the flexible ZTO/Ag/ZTO multilayer were mainly affected by the thickness and morphology of the Ag layer inserted between the ZTO layers. Dependence of photoluminescence peak intensity on the Ag thickness indicates that the high transmittance of the flexible ZTO/Ag/ZTO electrode can be explained by surface plasmon resonance as well as antireflection effects. At the optimized Ag thickness (10 nm), the flexible ZTO/Ag/ZTO exhibited a resistivity of 4.15 × 10−5 Ω-cm, an optical transmittance of 86.03%, and a figure of merit value of 42.83 × 10−3 Ω−1 comparable to those of conventional crystalline ITO electrodes. In addition, the optimized ZTO/Ag/ZTO sample showed constant resistance change (ΔR/R) during repeated bending cycles, indicating the robustness of the ZTO/Ag/ZTO electrode as a flexible electrode for cost efficient-flexible displays and photovoltaics.

Thin Ag Layer Inserted GZO Multilayer Grown by Roll-to-Roll Sputtering for Flexible and Transparent Conducting Electrodes

We prepared a nanoscale Ag-layer-inserted Ga-doped ZnO (GZO) multilayer on a flexible poly(ether sulfone) (PES) substrate using a continuous roll-to-roll (RTR) sputtering system at room temperature for the fabrication of highly flexible and transparent conducting oxide electrodes. When the thickness of the Ag interlayer was optimized (12 nm), the resulting GZO/Ag/GZO (GAG) multilayer showed a resistivity of and a transparency of 87.2% without in situ or ex situ annealing processes. However, although it exhibited lower resistivity, the GAG multilayer showed decreased optical transparency above the critical Ag interlayer thickness of 12 nm due to severe light scattering caused by the Ag layer. The results of bending tests demonstrated that the GAG multilayer resistance varies depending on the bending radius of the sample during repeated bending cycles. The constant surface morphology and resistance of the GAG multilayer even after the performance of bending tests indicate that RTR sputter grown GAG multilayers are promising as flexible transparent conducting electrodes for use in cost-efficient flexible displays and photovoltaics.

Degradation Due to Bending Fatigue Strain in YBCO Coated Conductors

A YBCO coated conductor is expected to be a high performance superconductor. To design applied superconducting machines with YBCO conductors, evaluation of not only the superconducting property but also the mechanical property is very important. In particular, the performance against fatigue cycles should be investigated for practical use of the machines. Hence, we measured degradation of YBCO conductors due to bending strain including fatigue. First, we show dependence of the bending strain on critical currents of the conductor. And next, we give the bending strain to the conductor and then release the strain to zero to estimate whether or not the critical currents changes before and after the bending. This test is so called the reversible/irreversible property test. Finally, we show the dependence of the critical currents of the conductor on repeated bending cycles, namely the fatigue test.

Mechanical Degradation of High Voltage Rotating Machine Insulation

Several kinds of insulation systems, which were developed for high voltage coil insulation for motors and generators, are examined for insulation damage caused by mechanical stresses. Tests have been carried out on electrical properties under bending stress, i. e. the critical deformation characteristics which are defined as the maximum deformations able to withstand a fixed proof voltage, the fatigue characteristics under repeated bending stresses, and the voltage life characteristics under bending stress. Breakdown voltage, BDV, decreases with an increase of bending stress. Moreover, a remarkable increase of the base tan6 value, tan6o, and tan6 tip-up can be seen due to the delamination of mica-splittings, especially in the case of the insulation using mica splitting. An insulation system in which a flexible binder is used has a far greater critical deformation. The decrease of residual breakdown voltage RBDV, and the increase of tan6o and tan6 tip-up are seen at less repeated bending stress in the insulation system with mica-splitting tapes than in the insulation system with only mica-paper, because of the weakness of mica-splittings under repeated bending stress. Effects of the temperature on the repeated bending fatigue characteristics from -55 to 1550C are greatly influenced by the binder of the mica tape and/or the impregnating resin. Moreover, voltage life under a static bending stress becomes extremely short beyond a certain stress. Lastly, experimental equations are proposed for estimating the relationship between RBDV and the numbers of repeated bending cycles, and the voltage life under a static bending stress.