Small Bending Radius Research Articles

Mechanically and Ultraviolet Light Stable Ultrathin Organic Solar Cell via Semi-Embedding Silver Nanowires in a Hydrogen Bonds-Based Polyimide.

Ultrathin organic solar cells (OSCs) with both high power conversion efficiency (PCE) and operational stability are of great significance for the industrial applications but still challenging. Here, a polyimide (PI) substrate for high-performance and stable ultrathin OSCs, which is physically crosslinked via strong hydrogen bonds (denoted as HB-PI) to enhance the mechanical, thermal, solvent-resistant, and UV filtering properties (with a cut-off wavelength of 376nm), is synthesized. An ultrathin flexible transparent composite electrode (FTCE, ≈7µm) is fabricated via semi-embedding AgNWs in the HB-PI substrate. The FTCE possesses excellent optoelectronic property, smooth surface, and high mechanical stability simultaneously. Based on this FTCE, an ultrathin OSC is constructed with a PCE of 13.52% (average of 13.22%). Moreover, the ultrathin OSC shows outstanding mechanical stability (PCE decreased by less than 4% after 1000 bending cycles at a small bending radius of 0.5mm) and superior UV light stability (no evident PCE degradation after irradiation under UV light for 10h). This work will provide a new avenue for fabricating high-performance and stable ultrathin OSCs.

Determination of bend angle and shape imperfection effect on collapse load of pipe bends under in-plane opening moment

This study uses a three-dimensional nonlinear finite-element method to investigate the effects of bend angle ([Formula: see text]) and shape imperfection on the collapse load of pipe bends subjected to in-plane opening bending moment using large displacement analysis. ABAQUS was used to create a pipe bend model, whereas by virtue of symmetry about the longitudinal axis, half of the pipe bend model was built. The bend angle was varied from 30° to 180° with an interval of 30°, and shape imperfections, namely ovality ( Co) and thinning ( Ct), were varied from 0% to 20% each at 5% intervals. Elastic-perfectly plastic is thought to be the material of choice for the pipe bend. The results show that the bend angle has a significant impact on the collapse load of pipe bends between 30° and 120°, after which a significant decline was observed. The ovality impact increases with an increase in bend angle for pipe bends with a large thickness, whereas the opposite effect was found for smaller thicknesses with a small bend radius. The effect of bend radius and thickness was significant for large bend angles. Thinning ( Ct) and thickening ( Cth) have no effect on collapse load. A new mathematical equation has been proposed to predict the integrated effect of bend angle and shape imperfection.

Mechanical-robust and recyclable polyimide substrates coordinated with cyclic Ti-oxo cluster for flexible organic solar cells

With the rapid development of flexible optoelectronic devices, recyclability is highly desirable for alleviating resource waste and environmental pollution, but remains challenging. Here, a fully closed-loop recyclable crosslinked polyimide (RCPI) was fabricated via carboxyl ligand exchange between the CPI with pendent carboxyl groups and the cyclic Ti-oxo cluster (CTOC) with labile carboxyl ligands, which could be reconverted into reprocessable CPI and CTOC solution by pivalic acid. The RCPI-based embedded AgNWs@RCPI electrode shows comparable high conductivity, transparency and low roughness with ITO (indium tin oxide)/glass electrode, and meanwhile outstanding mechanical robustness. The related flexible organic solar cells (FOSCs) provided a high efficiency of 14.78% and maintained ~97% of the initial efficiency after 5000 bending cycles at a small bending radius of 1 mm. Moreover, the recyclability of the RCPI still retains after being manufactured into the FOSCs. This work provides a promising strategy for recyclable flexible optoelectronic devices.

Investigation on the mechanical interface stability of curved high aspect ratio x-ray gratings made by deep x-ray lithography

We describe the rationale for selecting graphite as a substrate material suitable for manufacturing curved high-aspect ratio metallic x-ray gratings and experimentally validate that its properties satisfy requirements relevant for clinical phase-contrast and dark-field x-ray imaging. Selection criteria applied to two candidate materials graphite and polyimide were compliance to bending, mechanical tenacity of the attachment of the lamellar grating structure to the substrate, the substrate material’s x-ray robustness, and the compatibility with the x-ray LIGA process used to manufacture the grating structures. In contrast to other standard materials such as silicon wafers with titanium layer, graphite wafers could be bent to smaller radii and are natively electrically conductive. While polyimide wafers allowed for even smaller bending radii, we found their high risk of grating structure detachment to be a strong detractor. Minimum achievable bending radii were 55 and 70 mm for pure graphite wafers and graphite wafers with mounted grating structure, respectively. Electron microscopy of graphite surface and cross-sections revealed a fine porous structure conducting to a very stable metal–wafer interface. Grating structures with heights of more than 200 μm were bonded to graphite wafers and their integrity confirmed in flat as well as in bent state using microfocus x-ray imaging. We conclude that graphite is a very well-suited substrate material for manufacturing curved x-ray gratings.

Fully implantable wireless batteryless vascular electronics with printed soft sensors for multiplex sensing of hemodynamics.

The continuous monitoring of hemodynamics attainable with wireless implantable devices would improve the treatment of vascular diseases. However, demanding requirements of size, wireless operation, and compatibility with endovascular procedures have limited the development of vascular electronics. Here, we report an implantable, wireless vascular electronic system, consisting of a multimaterial inductive stent and printed soft sensors capable of real-time monitoring of arterial pressure, pulse rate, and flow without batteries or circuits. Developments in stent design achieve an enhanced wireless platform while matching conventional stent mechanics. The fully printed pressure sensors demonstrate fast response times, high durability, and sensing at small bending radii. The device is monitored via inductive coupling at communication distances notably larger than prior vascular sensors. The wireless electronic system is validated in artery models, while minimally invasive catheter implantation is demonstrated in an in vivo rabbit study. Overall, the vascular system offers an adaptable framework for comprehensive monitoring of hemodynamics.

Flexible VO2 Films for In‐Sensor Computing with Ultraviolet Light

AbstractWith their unique advantages in portability, shape adaptability, and human friendly surfaces, flexible electronics pave the way for the implementation of wearable electronic textiles and human–machine interfaces. Although organic materials are promising for flexible devices because of the low‐cost manufacturing and inherent flexibility, they meet challenges in harsh environments such as ultraviolet (UV) irradiation, which limits their applicability in UV sensors. Here, a flexible UV neuromorphic sensor is presented using inorganic vanadium dioxide (VO2) films grown on mica substrates. The flexible device shows UV photoinduced nonvolatile phase transition, and can be reversibly modulated using electrolyte gating. The optical responses remain almost unchanged after 10 000 bending cycles or at small bending radius, exhibiting high tolerance to the bending deformation. Besides, the variations in image recognition accuracy under different bending conditions keep within 1.6%, indicating that the device can be adapted to various deformation conditions. By constructing near‐/in‐sensor computing architectures using the flexible VO2 neuromorphic sensors with photoinduced nonvolatile phase transition, both static image processing and motion detection are realized without redundant and massive information transfer. This result lays the foundation for the development of flexible UV neuromorphic sensors.

Compact and high Q-factor multimode racetrack ring resonator based on transformation optics.

The ring resonator is a versatile and functional component in the silicon-based integrated optical circuit. Most of the previously reported ring resonators work in the single-mode case. With the rapid development of mode division multiplexing technology, a multimode ring resonator (MMRR) has been proposed and the usage beyond the limit of a conventional single mode ring resonator has been explored. However, the reported MMRRs are either large in size or low in quality factor. In this paper, we designed a compact silicon MMRR with a small bending radius of 15µm, in which the three lowest TE modes all have high Q-factors. For suppressing the mode loss and inter-mode crosstalk in MMRR, a multimode waveguide bend (MWB) with mode adiabatic evolution was designed based on transformation optics and waveguide shape optimization. The independent excitation of each order mode of the MMRR is realized by using bending directional coupler and asymmetric directional coupler. We successfully fabricated the device on a silicon-on-insulator (SOI) platform using simple one-step lithography. The measured loaded Q-factors of the three lowest TE modes are 5.9 × 104, 4.5 × 104, and 4.7 × 104, respectively.

Pyrometry with flexible infrared fibers for temperature-controlled laser surgery

Pyrometry is widely used in science, medicine, and industry to measure the surface temperature of objects in a non-contact way. IR fibers are an ideal solution for the flexible delivery of thermal radiation emitted from objects inside a complex structure like internal organs inside the human body. Silver halide polycrystalline infrared fibers (PIR) are transparent in a spectral range of 3 - 18 µm, matching perfectly with the spectra of black body radiation for temperatures ranging from 20°C to 200°C. These fibers are non-toxic and allow small bending radii. They could become critical components in pyrometric systems for temperature-controlled laser surgeries. Here we discuss the ability of the PIR fibers for simultaneous laser power delivery and real-time temperature monitoring in laser surgery applications and demonstrate two different setups for this purpose.

Bending Induced Temperature Increases in Double-Clad Fibers for High-Power Fiber Lasers

Rare-earth-doped optical fibers are one of the most promising solid-state lasers. In these fiber lasers, a cladding-pumping scheme using large-mode-area double-clad fibers (DCFs) is utilized to increase the overall conversion efficiency of pumping light and to overcome the restriction owing to the onset of stimulated Raman scattering. On the other hand, it is extremely challenging to increase the fiber core size while retaining the excellent beam quality because fibers with large core size allow propagation of several higher-order modes (HOMs), except for the fundamental mode (FM). In order to suppress HOMs, DCFs are bent with a relatively small bend radius. Failures at bends in an optical fiber are caused by light leaking from the core when the fiber is accidentally bent tightly with a high power input. For the DCFs with core radii of 10 and 20 mm, the relationship between the bending induced temperature increases and the bend losses in the bent DCFs was investigated theoretically by the explicit finite-difference method using the thermochemical SiO_x production model. The temperature at the boundary between the inner and outer cladding layers increased with increasing optical power <I>P</I> at 1.080 mm and was a large value higher than the softening temperature <I>T</I>_s of the silica glass when <I>P</I> = 5 kW was entered to the bent DCF with a bend radius <I>R</I> of 150 mm and a core radius of 10 mm. On the other hand, the temperature at the boundary was a small value lower than the <I>T</I>_s when <I>P</I> = 10 kW was entered to the bent DCF with a large <I>R</I> of 245-275 mm. Furthermore, it was found that the initiation of the fiber fuse was fairly easy under a certain conditions where the temperature at the boundary was higher than the <I>T</I>_s.

Design of ring-core few-mode multi-core fiber with low crosstalk and low bending loss

Aiming at solving the problems of coupling between modes in a core and mode coupling between cores in few-mode multi-core fiber, a fiber with seven cores each with step index is proposed, and each core can support five modes. Each core has a central low refractive index region and a high refractive index ring to ensure that there is a large refractive index difference between modes in the core, so as to reduce the problem of mode coupling. The bending loss of central core and outer core, the crosstalk characteristics between modes and the influence of core parameters on crosstalk performance are simulated and analyzed by the finite element method. The simulation results show that at a wavelength of 1.55 μm and a bending radius of 50 mm, the bending loss of the proposed multi-core fiber is much lower than its attenuation loss, and the crosstalk between the adjacent cores of the five core modes are less than –20 dB/100 km. Therefore, this multi-core fiber can realize independent transmission of the core modes with long-distance under small bending radius.

Ultralow bending-loss micro-structured fiber with large-mode-area

A novel design of photonic crystal optical fiber device with center-doped capillaries is proposed in this paper. A high leakage loss ratio (3551.41) between the high-order modes (HOMs) and the fundamental mode (FM) can be achieved at wavelength 1.064 µm, which is beneficial for robust single-mode operation and enhancing the transverse mode instabilities. The simulation shows that the effective mode area of the proposed fiber can be reached 806.24 µm2 at bend radius of 10 cm. Besides, the FM bending loss can be reached 6.42 × 10−4 dB/m at the diameter of center-doped capillaries of 3 µm. The proposed fiber has improved single-mode operation at a practically allowable bending radii of 5–10 cm. Compared with recent notable designs, the effective mode area and the FM bending loss of FM are improved, even in smaller bend radius.

Flexible Lead-Free Ba0.5Sr0.5TiO3/0.4BiFeO3-0.6SrTiO3 Dielectric Film Capacitor with High Energy Storage Performance.

Ferroelectric thin film capacitors have triggered great interest in pulsed power systems because of their high-power density and ultrafast charge–discharge speed, but less attention has been paid to the realization of flexible capacitors for wearable electronics and power systems. In this work, a flexible Ba0.5Sr0.5TiO3/0.4BiFeO3-0.6SrTiO3 thin film capacitor is synthesized on mica substrate. It possesses an energy storage density of Wrec ~ 62 J cm−3, combined with an efficiency of η ~ 74% due to the moderate breakdown strength (3000 kV cm−1) and the strong relaxor behavior. The energy storage performances for the film capacitor are also very stable over a broad temperature range (−50–200 °C) and frequency range (500 Hz–20 kHz). Moreover, the Wrec and η are stabilized after 108 fatigue cycles. Additionally, the superior energy storage capability can be well maintained under a small bending radius (r = 2 mm), or after 104 mechanical bending cycles. These results reveal that the Ba0.5Sr0.5TiO3/0.4BiFeO3-0.6SrTiO3 film capacitors in this work have great potential for use in flexible microenergy storage systems.

Cyclic Bending Effects on Resistance of Screen-Printed Silver Conductors

Flexible electronics are becoming more and more widely used in all fields, such as wearable devices, flexible displays, flexible sensors, and so on. Therefore, it is essential to investigate the reliability of flexible electronics under various loading conditions. In this article, the mechanical and electrical behavior of printed conductors was tested under reciprocating adaptive curvature flexure bending. The silver-ink conductors of different widths were screen-printed on polyimide (PI) and polyethylene terephthalate (PET) substrates and served as the samples. Through the tests, the resistance of the conductors was measured by a four-wire method continuously. The tests were carried out under different gap distances and moving ranges. Besides the tests, SEM imaging was also performed to understand the microstructure change of the printed conductor before, during, and after the tests. Finite-element simulations were performed to track the strain history of specific locations of the printed conductor. Finally, a two-stage process of resistance increase was identified in the reciprocating adaptive curvature flexure test. The resistance change of the conductors under different loading conditions was determined. The parameters that affect the resistance change were also figured out. Cracks and delamination in the printed conductor were observed and related to the resistance increase phenomena under small tensile and compressive bending radius, respectively.

High birefringence, single-polarization, low loss hollow-core anti-resonant fibers.

We present a novel hollow-core anti-resonant fiber (HC-ARF) with a cladding ring, two nested resonant tubes and two nested silicon tubes. The cladding ring in the fiber contributes to decrease the fundamental mode (FM) loss of x-polarization and enlarge the polarization-extinction ratio (PER). In addition, the nested silicon tubes can improve birefringence greatly. The combination of cladding ring, nested resonant tubes and nested silicon tubes can make the fiber obtain low FM loss, single-polarization, and high birefringence. Specifically, the proposed HC-ARF exhibits total FM loss of x-polarization, PER, and birefringence of 0.89 dB/km, 4432, 3.07×10-4, respectively, at 1.55 µm. Moreover, the y-bend direction has a great influence on the propagation properties of the fiber. The fiber in the x-bend direction has low total bend-loss of 0.004 dB/m for a small bend radius of 5.8 cm.

Flexibility of Room-Temperature-Synthesized Amorphous CdO-In2O3 Alloy Films and Their Application as Transparent Conductors in Solar Cells.

Due to their low-temperature deposition, high mobility (>10 cm2/V·s), and electrical conductivity, amorphous ionic oxide semiconductors (AIOSs) have received much attention for their applications in flexible and/or organic electro-optical devices. Here, we report on a study of the flexibility of CdO-In2O3 alloy thin films, deposited on a polyethylene terephthalate (PET) substrate by radio frequency magnetron sputtering at room temperature. Cd1-xInxO1+δ alloys with the composition of x > 0.6 are amorphous, exhibiting a high electron mobility of 40-50 cm2/V·s, a low resistivity of ∼3 × 10-4 Ω·cm, and high transmittance over a wide spectral window of 350 to >1600 nm. The flexibility of both crystalline and amorphous Cd1-xInxO1+δ films on the PET substrate was investigated by measuring their electrical resistivity after both compressive and tensile bending with a range of bending radii and repeated bending cycles. Under both compressive and tensile bending with Rb = 16.5 mm, no significant degradation was observed for both the crystalline and amorphous films up to 300 bending cycles. For a smaller bending radius, the amorphous film shows much less electrical degradation than the crystalline films under compressive bending due to less film delamination at the bending sites. On the other hand, for a small bending radius (<16 mm), both crystalline and amorphous films degrade after repeated tensile bending, most likely due to the development of microcracks in the films. To demonstrate the application of amorphous Cd1-xInxO1+δ alloy in photovoltaics, we fabricated perovskite and bulk-heterojunction organic solar cells (OSCs) on glass and flexible PET utilizing amorphous Cd1-xInxO1+δ layers as transparent electrodes. The organic-inorganic hybrid perovskite solar cells (PSCs) exhibit a power conversion efficiency (PCE) of ∼11 to 12% under both front and back illumination, demonstrating good bifacial performance with bifaciality factor >90%. The OSCs fabricated on an amorphous Cd1-xInxO1+δ-coated flexible PET substrate achieve a promising PCE of 12.06%. Our results strongly suggest the technological potentials of amorphous Cd1-xInxO1+δ as a reliable and effective transparent conducting material for flexible and organic optoelectronic devices.

Optimal Design of the Shape of a Non-Ball Mandrel for Thin-Walled Tube Small Radius Cold Bending

In the field of cold bending, it is necessary to use ball mandrels, especially to bend thin-walled tubes with a small radius. However, the bending process with a ball mandrel is complex and expensive, and it is easy to jam the core ball inside the tube. To solve these issues, we designed two kinds of hollow non-ball mandrel schemes with low stiffness that were suitable for the small radius bending of thin-walled tubes. We evaluated the forming quality of cold bending numerically and the influence of the hollow section length and thickness on the forming indices. Our results showed that the thickness of the hollow section has a greater influence on forming quality than the length. As the hollow section’s thickness increased, the wrinkling rate first declined by approximately 40% and then increased by above 50%. When the thickness was 11 mm in scheme 1 and 13 mm in scheme 2, the wrinkling rate reached minimum values of 1.32% and 1.50%, respectively. As the hollow section’s thickness increased, the flattening rate decreased by more than 60% and the thinning rate increased by about 40%. A multi-objective optimization of forming indices was carried out by ideal point method and grey wolf optimizer. By comparing the forming results before and after optimization, the feasibility of using the proposed hollow mandrel was proved, and the hollow mandrel scheme of standard cylinder is therefore recommended.

Wideband, large mode field and single vector mode transmission in a 37-cell hollow-core photonic bandgap fiber

Stable generation and propagation of ultrafast high-order mode beams has become an important research direction. A core diameter not more than 10 times the wavelength is regarded as the upper limit for single mode transmission. However, a high-power laser requires a core diameter 20 to 40 times the wavelength to achieve high-power and stable output, which exceeds the design limit of the traditional fiber. In this paper, a novel 37-cell hollow core photonic bandgap fiber (HC-PBF) that only supports pure TE01 mode over a bandwidth of 50 nm with the lowest loss of 0.127 dB/km is proposed. The HC-PBF has a core diameter of more than 40 μm. Single mode guidance is achieved by adjusting the lattice size in a particular of the cladding. The best single mode performance with a loss ratio as high as 150,000 between TE01 mode and other modes with minimum loss is obtained. The fiber also has low bend-loss and thus can be coiled to a small bend radius of 1 cm having 1.6 dB/km bend loss. The tunability of the single-mode window and the manufacturing feasibility of the proposed fiber are also discussed.

Effect of structural deformations and bend angle on the collapse load of pipe bends subjected to in-plane closing bending moment

PurposeThe purpose of this study is to investigate the effect of structural deformations and bend angle on plastic collapse load of pipe bends under an in-plane closing bending moment (IPCM). A large strain formulation of three-dimensional non-linear finite element analysis was performed using an elastic perfectly plastic material. A unified mathematical solution was proposed to estimate the collapse load of pipe bends subjected to IPCM for the considered range of bend characteristics.Design/methodology/approachABAQUS was used to create one half of the pipe bend model due to its symmetry on the longitudinal axis. Structural deformations, i.e. ovality (Co) and thinning (Ct) varied from 0% to 20% in 5% steps while the bend angle (ø) varied from 30° to 180° in steps of 30°.FindingsThe plastic collapse load decreases as the bend angle increase for all pipe bend models. A remarkable effect on the collapse load was observed for bend angles between 30° and 120° beyond which a decline was noticed. Ovality had a significant effect on the collapse load with this effect decreasing as the bend angle increased. The combined effect of thinning and bend angle was minimal for the considered models and the maximum per cent variation in collapse load was 5.76% for small bend angles and bend radius pipe bends and less than 2% for other cases.Originality/valueThe effect of structural deformations and bend angle on collapse load of pipe bends exposed to IPCM has been not studied in the existing literature.

Numerical investigation of bend insensitive multicore optical fibers

A kind of bending-insensitive multicore fiber (MCF) is proposed. The core adopts a low-mode gradient index structure to form a seven-core few-mode fiber. The finite element method is used to simulate and analyze the bending loss of the central core and the outer core, the crosstalk between the core modes, and the influence of core parameters on the crosstalk performance. Data simulation results show that even for a small bending radius of 5 mm, the bending loss of this MCF is much lower than the attenuation loss of the fiber, and the crosstalk between the adjacent cores is less than −20 dB. Therefore, independent information transmission between cores can still be realized under a small bending radius.

(Invited) Foldable Perovskite Solar Cells Using Carbon Nanotube-Embedded Ultrathin Polyimide Conductor

Recently, foldable electronics technology has become the focus of both academic and industrial research. The foldable device technology is distinct from flexible technology, as foldable devices have to withstand severe mechanical stresses such as those caused by an extremely small bending radius of 0.5 mm. To realize foldable devices, transparent conductors must exhibit outstanding mechanical resilience, for which they must be micrometer-thin, and the conducting material must be embedded into a substrate. Here, a single-walled carbon nanotubes–polyimide composite film with a thickness of 7 μm is synthesized and used as a foldable transparent conductor in perovskite solar cells. During the high-temperature curing of the carbon nanotubes-embedded polyimide conductor, the carbon nanotubes are stably and strongly p-doped using MoOx, resulting in enhanced conductivity and hole transportability. The ultrathin foldable transparent conductor exhibits an sheet resistance of 82 Ω sq.-1 and transmittance of 80% at 700 nm, with a maximum-power-point-tracking-output of 15.2% when made into a foldable solar cell. The foldable solar cells can withstand more than 10,000 folding cycles with a folding radius of 0.5 mm. Such mechanically resilient perovskite solar cells are unprecedented; further they exhibit the best performance among the carbon-nanotube-transparent-electrode-based flexible solar cells. Figure 1