Critical Bending Radius Research Articles

Niobium Oxide Thin Films Grown on Flexible ITO-Coated PET Substrates

Niobium oxide thin films were grown on both rigid and flexible substrates using DC magnetron sputtering for electrochromic applications. Three experimental series were conducted, varying the oxygen to argon flow rate ratio and deposition time. In the first series, the oxygen to argon ratio was adjusted from 0 to 0.32 while maintaining a constant growth time of 30 min. For the second and third series, the oxygen to argon ratios were fixed at 0.40 and 0.56, respectively, with deposition times ranging from 15 to 60 min. A structural transition from crystalline to amorphous was observed at an oxygen to argon flow rate ratio of 0.32. This transition coincided with a change in appearance, from non-transparent with metallic-like electrical conductivity to transparent with dielectric behavior. The transparent niobium oxide films exhibited thicknesses between 51 nm and 198 nm, with a compact, dense, and featureless morphology, as evidenced by both top-view and cross-sectional images. Films deposited on flexible indium tin oxide (ITO)-coated polyethylene terephthalate (PET) substrates displayed a maximum surface roughness (Sq) of 9 nm and a maximum optical transmission of 83% in the visible range. The electrochromic response of niobium oxide thin films on ITO-coated PET substrates demonstrated a maximum coloration efficiency of 30 cm2 C−1 and a reversibility of 96%. Mechanical performance was assessed through bending tests. The ITO-coated PET substrate exhibited a critical bending radius of 6.5 mm. Upon the addition of the niobium oxide layer, this decreased to 5 mm. Electrical resistance measurements indicated that the niobium oxide film mitigated rapid mechanical degradation of the underlying ITO electrode beyond the critical bending radius.

Optimization of low-loss, high birefringence parameters of a hollow-core anti-resonant fiber with back-propagation neural network assisted hyperplane segmentation algorithm

In engineering, optimizing parameters often involves computationally expensive tasks, especially when dealing with multi-dimensional variables and multiple performance metrics. This falls under the category of multi-objective black-box optimization. To address this, we propose two optimization algorithms for low and medium-dimensional spaces, incorporating relaxation conditions for hyperplane segmentation. For the specific parameter optimization of HC-ARF, we employed a two-stage approach. It combines a BP neural network as a surrogate model with a hyperplane separation optimization algorithm. This method efficiently optimizes both confinement loss (CL) and birefringence, using a weighted sum approach to identify their Pareto sets. We validate the effectiveness and stability of the surrogate model by comparing it with traditional optimization algorithms. Exhaustive experiments confirm the superiority of this algorithm and the results show that our optimized structure achieves impressive performance metrics, including a loss of 0.8 dB/m, a birefringence of 2.2×10−4, and a critical bending radius of 0.5 cm under optimal parameters.

Deep Learning-Based Simultaneous Temperature- and Curvature-Sensitive Scatterplot Recognition.

Since light propagation in a multimode fiber (MMF) exhibits visually random and complex scattering patterns due to external interference, this study numerically models temperature and curvature through the finite element method in order to understand the complex interactions between the inputs and outputs of an optical fiber under conditions of temperature and curvature interference. The systematic analysis of the fiber's refractive index and bending loss characteristics determined its critical bending radius to be 15 mm. The temperature speckle atlas is plotted to reflect varying bending radii. An optimal end-to-end residual neural network model capable of automatically extracting highly similar scattering features is proposed and validated for the purpose of identifying temperature and curvature scattering maps of MMFs. The viability of the proposed scheme is tested through numerical simulations and experiments, the results of which demonstrate the effectiveness and robustness of the optimized network model.

Highly flexible and transparent amorphous indium doped tin oxide on bio-compatible polymers for transparent wearable sensors

Highly transparent and flexible amorphous Sn-doped In2O3 (a-ITO) films deposited on flexible and bio-compatible cyclic olefin polymer (COP) substrate using a direct-current magnetron sputtering at room temperature were used as flexible and transparent electrodes for transparent wearable sensors. The figure of merits (FoM) value was calculated to determine the optimal sputtering process for the a-ITO electrodes by varying the direct current power, working pressure, oxygen flow rate, and a-ITO thickness. The optimized a-ITO/COP with a high FoM value of 8.9 exhibited a low sheet resistance of 36 Ohm/square, average transmittances of 89.5 % in the visible wavelength region, and a small critical bending radius of 7 mm, which are acceptable transparent electrodes for fabrication of wearable and transparent wearable sensors. To demonstrate the feasibility of the a-ITO/COP substrate as a promising wearable sensor, we examined the performance of wearable temperature sensors, wearable heaters, and wearable glucose sensors. The successful operation of a-ITO/COP-based temperature sensors with high sensitivity, transparent heaters with a saturation temperature of 87.8 °C at 6 V, and glucose sensors with high sensitivity indicates that a-ITO/COP is a promising bio-compatible electrode for next-generation wearable bionic electronics.

Bending characteristics of a round strand made by stacked HTS tapes

Although the development of REBCO coated conductor has proved its superiority of critical current in high-field background, mechanical issues caused by electromagnetic forces are still big challenges. Large-scale magnets call for robust and high-Je cables for simplicity of the system. To meet the requirements, here we report a round cable comprised of a stack of 2 mm wide high-temperature superconducting (HTS) tapes inserted into a copper tube and all soldered with Sn63Pb37. The two prepared samples include five HTS tapes and four copper strips in untwisted manner. Self-field critical current at 77 K was analyzed both numerically and experimentally. The characteristics of in-plane (hard) and out-of-plane (easy) bending were analyzed by finite element method (FEM) and compared to experimental measurements. Good agreement was found between the analysis and experimental results on critical bending radius, which is about 200–215 mm. Our results indicate the robustness of the round strand and its potential for use in large-scale magnets.

Nonzero spontaneous electric polarization in metals: novel predictive methods and applications

Ferroelectricity in metals has advanced since the initial discovery of nonmagnetic ferroelectric-like metal LiOsO_3, anchored in the Anderson and Blount prediction. However, evaluating the spontaneous electric polarization (SEP) of this metal has been hindered by experimental and theoretical obstacles. The experimental challenge arises from difficulties in switching polarization using an external electric field, while the theoretical limitation lies in existing methods applicable only to nonmetals. Zabalo and Stengel (Phys Rev Lett 126:127601, 2021, https://doi.org/10.1103/PhysRevLett.126.127601) addressed the experimental obstacle by proposing flexoelectricity as an alternative for practical polarization switching in LiOsO_3, which requires a critical bending radius similar to BaTiO_3. In this study, we focus on resolving the theoretical obstacle by modifying the Berry phase and Wannier functions approaches within density functional theory plus modern theory of polarization. By employing these modifications, we calculate the SEP of LiOsO_3, comparable to the polarization of BaTiO_3. We validate our predictions using various ways. This study confirms the coexistence of ferroelectricity and metallicity in this new class of ferroelectric-like metals. Moreover, by addressing the theoretical limitation and providing new insights into polarization properties, our study complements the experimental flexoelectricity proposal and opens avenues for further exploration and manipulation of polarization characteristics. The developed approaches, incorporating modified Berry phase and Wannier function techniques, offer promising opportunities for studying and designing novel materials, including bio- and nano-ferroelectric-like metals. This study contributes to the advancement of ferroelectricity in metals and provides a foundation for future research in this exciting field.

Induced delamination in REBCO coated-conductor tape by a scratch line and bending

The aim of this work is to study delamination promotion of the coated conductor (CC) tape under the influences of the scratches on the Ag surface when bending. For this purpose, the microstructural and superconducting properties of the CC tape bent in helical fashion with different former diameters were analyzed. Such CC tape that exhibits visible damage along its length in the form of “scratch line” caused on the final product was used. The CC tape was bent at a constant lay angle of 45° with a superconducting layer positioned inward to the former core experiencing compression. Transport current measurements in liquid nitrogen were performed to study superconducting properties of the CC tape.The results showed that delamination of the REBCO (Rare-Earth Barium Copper Oxide) layer started to form exclusively on the scratch location at tube diameter of 7 mm (corresponding to strain ε = −0.71 %) while the rest of the REBCO surface started to delaminate at tube diameter of 5 mm (ε = −0.99 %). As the superconducting properties of the CC tape are related with the REBCO delamination, the critical current degraded when first signs of REBCO delamination appeared as expected. Thus, it was observed that the scratches induce REBCO delamination during the bending process resulting in increase of critical bending radius.

Bending performance of the CORC cable with flexible interlocked stainless steel former

The high temperature superconducting cable on round core (CORC) is a kind of cable that could be used in fusion projects. Nevertheless, conventional copper former CORC cables require a large external force to allow the cable to endure plastic deformation and be tightly wound into solenoids. In this case, the superconducting tape will be affected by concentrated stress, resulting in a risk of critical current degradation. Therefore, this paper proposes a new CORC cable with flexible interlocked stainless steel former, which can be wound into a solenoid by applying a small external force. To verify the bending performance of this interlocked former CORC cable, a double-layer and a ten-layer interlocked stainless steel former CORC cable, as well as a double-layer traditional copper former CORC cable, are fabricated. And these three CORC cables are used to wind solenoids of various radius sizes respectively. The experimental results show that the critical bending radius of the double-layer interlocked stainless steel former CORC cable is less than 20 mm, the critical bending radius of the ten-layer interlocked stainless steel former CORC cable is less than 50 mm, and the critical bending radius of the double-layer traditional copper former CORC cable is larger than 55 mm. A self-consistent finite element model for the critical current of the CORC cable solenoid is also established. And the critical current experimental results are in good agreement with the simulation results. The results of this paper verify the excellent bending performance of the interlocked former CORC cable, which provides a good option for the preparation of insert magnets for future fusion projects.

Investigation of double-cladding heterogeneous step-index 2LP-mode multicore fiber with a two-ring layout

This paper investigates the design of C-band double-cladding 2LP mode multicore fiber (2LP-MCF) with 125-µm cladding diameter by numerical simulation, in which a two-ring layout is used to improve crosstalk and critical bend radius. The numerical results show that the crosstalk can be less than −60 and −35dB/km for 2LP mode eight-core and 10-core fibers, respectively, and it is also shown that the crosstalk can be suppressed compared with that in heterogeneous 2LP-MCFs without a double-cladding layer.

Laser‐Assisted Nanotexturing for Flexible Ultrathin Crystalline Si Solar Cells

Ultrathin (UT) crystalline Si wafers, which are more flexible than conventional ones, can apply to curved surfaces, enabling a wide range of applications such as building‐integrated photovoltaics, vehicle‐integrated photovoltaics, and wearable devices. Thinner wafers require more effective light trapping; thus, surface texturing in microscale is a common approach to compensate for the reduced thickness by enhancing the light pathlength. Microscale textures, however, deteriorate the mechanical flexibility due to stress concentration in the valley of the microtextures. In this study, a laser‐assisted nanotexturing process is proposed for enhanced flexibility of the UT Si solar cells with a 50 μm thickness while maintaining light‐trapping performances. A nanolens array is used to focus laser onto the Si wafers, inducing the formation of nanoparticle etch masks for nanopyramid texturing in an alkaline solution. The origin of the enhanced flexibility of the nanotextured Si wafers is discussed by a micromechanics simulation study. Herein, nanotexturing technique is applied to UT Si‐based passivated emitter rear cells and the enhanced flexibility of the cells with a 26 mm critical bending radius is demonstrated. Also, it is shown that the nanotextured Si wafer provides a higher efficiency of 18.68%, whereas the microtextured one exhibits 18.10%.

Investigation of heterogeneous step-index 2LP-mode multi-core fibers based on a two-ring layout with standard cladding diameter

The numerical results of the step-index 2LP-mode 6-core and 8-core multi-core fibers (MCFs) with 125-µm cladding diameter and two-ring layout for C-band, show that the crosstalk less than −51 and −35 dB/km can be obtained respectively. It is revealed that the two-ring layout is adapted for improving the crosstalk and critical bending radius, also effective for few-mode MCFs as well as single-mode MCFs.

Flexible Organic Photodetectors with Mechanically Robust Zinc Oxide Nanoparticle Thin Films.

Zinc oxide nanoparticle (ZnO-NP) thin films have been intensively used as electron transport layers (ETLs) in organic optoelectronic devices, but their moderate mechanical flexibility hinders their application to flexible electronic devices. This study reveals that the multivalent interaction between ZnO-NPs and multicharged conjugated electrolytes, such as diphenylfluorene pyridinium bromide derivative (DFPBr-6), can significantly improve the mechanical flexibility of ZnO-NP thin films. Intermixing ZnO-NPs and DFPBr-6 facilitates the coordination between bromide anions (from the DFPBr-6) and zinc cations on ZnO-NP surfaces, forming Zn2+-Br- bonds. Different from a conventional electrolyte (e.g., KBr), DFPBr-6 with six pyridinium ionic side chains holds the Br--chelated ZnO-NPs adjacent to DFP+ through Zn2+-Br--N+ bonds. Consequently, ZnO-NP:DFPBr-6 thin films exhibit improved mechanical flexibility with a critical bending radius as low as 1.5 mm under tensile bending conditions. Flexible organic photodetectors with ZnO-NP:DFPBr-6 thin films as ETLs demonstrate reliable device performances with high R (0.34 A/W) and D* (3.03 × 1012 Jones) even after 1000 times repetitive bending at a bending radius of 4.0 mm, whereas devices with ZnO-NP and ZnO-NP:KBr ETLs yield >85% reduction in R and D* under the same bending condition.

Typical electrical, mechanical, electromechanical characteristics of copper-encapsulated REBCO tapes after processing in temperature under 250 °C

Heat treatments are inevitable not only in the production of rare earth barium copper oxide (REBCO) tapes, but also in their post-processing for applications, typically, in soldering and epoxy/wax impregnation during the fabrication of REBCO coils. In general, the heat treatment of REBCO tapes should be carried out at lower temperature for a shorter time, but the specific safe boundary of heat-treatment temperature and time for REBCO tapes is still unknown. Therefore, a comprehensive study on the typical electrical, mechanical, and electromechanical characteristics of REBCO tapes after heat treatments under temperature of 250 °C is necessary. This work focus on the copper-encapsulated REBCO tapes, which are more robust (while with much lower engineering current density) to be processed in application systems than the tapes without encapsulation. The critical current degradation, stress–strain characteristic, and electromechanical properties of REBCO tapes were measured after heat treatments at different temperatures in argon and oxygen atmosphere. A 2D finite element (FE) analysis model was established for detailed stress/train analyzes under tension and bending based on the analysis of residual stress/strain. The results indicate that the critical current of the copper-encapsulated REBCO tapes decreases with increasing heat-treatment temperature and dwell time, and is of no evident relation to atmosphere. In addition, increased temperature of heat treatment leads to an obvious decrease in the yield strength and critical tensile stress. This effect is mainly attributed to the degradation of mechanical properties of the encapsulated copper layer, which is demonstrated by the combination of our FE simulation and the experiments results. Interestingly, the change in the critical bending radius due to heat treatments was slight, because the bending axial strain of the REBCO layer remained almost unchanged after heating. It is also worthy to note that all the properties tested in this study were irrelevant to the external oxygen partial pressure during the heating process. As a practical conclusion for the application systems, an upper and atmosphere-irrelevant limit of processing temperature of 130 °C or 150 °C (2 h dwell time) was proposed for copper-encapsulated REBCO tapes, under which the critical current, yield strength, critical tensile stress/strain and critical bending radius of the copper-encapsulated REBCO tapes decay by <1% or 3%, respectively.

Ultrabend resistant circular photonic crystal fiber design for cancer diagnosis by Raman spectrometer

One of the ways to diagnose cancer in its early stages, which can lead to timely treatment, is to use Raman spectroscopy. The Raman spectrum of a material is created by the interaction of light with the material; it determines the exact characteristics and properties of the material, which can be used to distinguish cancerous cells from healthy ones. However, the intensity of this spectrum is weak, and the optical fibers used in these devices must have very little loss and thus have low sensitivity to bending. This paper proposes a circular photonic crystal fiber with a structure in which the losses in different bending radii are significantly reduced. The modified state of the fiber at the wavelength of 1.55 μm supports the transmission of 10 modes propagated in the core. The loss for the fundamental mode at this wavelength and bending radius of 2 mm equals 3.82 × 10 − 16 dB / m, which is considered a low value. As the bending radius decreases further, low-order modes diminish, which are more sensitive to bending than high-order modes. However, their losses are lower than in high-order modes. The critical bending radius for this fiber, with the loss reaching 0.1 dB / m, is equal to 0.096 mm. This low critical bending radius shows the low sensitivity of the proposed fiber to bending. The fiber was examined in different bending radii and wavelengths, and the parameters of this structure were presented for the lowest bending losses, which can be used in Raman spectrometers as spectrum collecting fibers and improve cancer diagnosis devices with Raman spectrum.

Studies on optical properties of multimode Core-Only silver halide fibres under bending

In this paper, we investigated the influence of bending on some functional characteristics of core-only multi-mode mid-infrared fibres placed into a polymer protecting shell. As a test fibre, we chose AgBr0.87I0.13 polycrystalline fibre of 525 µm in diameter with in-plane bends. Using an analytical equation, we calculated the critical bending radius of the fibre within the entire operating wavelength range of 3–27 µm with the increment of 1 µm. Then, we estimated total energy output through numerical simulations by COMSOL Multiphysics over the bend radii range of 1.5–52.5 mm. Further, we acquired mid-infrared transmission spectra of the fibre bent to various radii of curvature and compared the data to the analytical results. Moreover, we validated the simulations by measuring power output from the bent fibre guiding the radiation of a CO2 laser. It was revealed that both analytical and modelling results are in good agreement with the experiments. Then, we determined cut-off values of the bending radius which allow for maintaining optimum working conditions for the fibres and showed that stripping out higher-order modes due to bending leads to significant improvement of the quality of the laser beam outgoing from the test fibre.

Evaluation of encapsulation strategies for solution-processed flexible organic light-emitting diodes

Owing to the sensitivity of organic materials to ambient conditions, encapsulation of organic light-emitting diodes (OLEDs) is critical. In this work, Al2O3 film, deposited by atomic layer deposition (ALD) technique, on flexible polyethylene terephthalate (PET) substrate is used as a barrier film. Al2O3 is also deposited directly on OLED as thin-film encapsulation. Electrical Ca tests are used to measure water vapor transfer rate (WVTR) values, and an optimum WVTR value of 9 *10−4 g/m2/day using 43 nm Al2O3 film has been obtained. The related results on five different encapsulation structures for solution-processed OLEDs: (i) glass to glass encapsulation (G2G) as a reference, (ii) glass to barrier film encapsulation (G2B), (iii) glass to direct thin-film encapsulation (G2D), (iv) barrier to barrier film encapsulation (B2B), and (v) barrier to direct thin-film encapsulation (B2D), are evaluated. The operating half-lifetime values of OLEDs with G2B- and G2D-encapsulation are 54% and 82% of the half-lifetime with G2G encapsulation, respectively. The lifetime of flexible OLED devices with B2D encapsulation is also 34% more than that with B2B encapsulation. Based on the bending stress analysis, the estimated critical bending radius for B2B and B2D encapsulation of flexible OLEDs are 16.83 and 7.07 mm, respectively.

Effects of bending and torsion behavior on Ic degradation and microstructure of ReBCO coated conductors

The performance degradation caused by bending and torsion is crucial for the application of superconducting coated conductors. In this study, we investigated the critical current (Ic) degradation of four kinds of conductors under bending and torsion. With 1 mm resolution, the critical bending radius of STT236μm (Shanghai Superconductor Technology) conductors is 6–7 mm, and that of STT380μm conductors is 4–5 mm, and that of SAMRI (Suzhou Advanced Materials Research Institute) conductors is 4 mm. With better bending tolerance, SAMRI conductors also have a less critical twist pitch of 82–87 mm. Considering the residual thermal stress during fabrication, the longitudinal strain corresponding to 5 % Ic degradation is 0.484 %. Further, to figure out the failure mechanism of coated conductors, the micromorphology of the ReBCO layer was observed after chemical etching. The crack distribution not only verified the cause of Ic degradation under mechanical behaviors, but also helps to develop a crack-based Ic prediction method.

Finite Element Analysis for Safe Design of a Flexible Microelectronic System under Bending Deformation

We modeled flexible microelectronic systems and analyzed the stress and strain distribution assuming an international standard bending test evaluating flexible electronics. The flexible microelectronic system consisted of a flexible substrate, a thin silicon die bonded to the substrate using bumps, along with an underfill and molding layer. The highest stress values were found at the square silicon die corners under bending deformation. After rounding the die corners, the copper bumps were the weakest component in the system. When polymer bumps replaced copper bumps to improve the mechanical stability, the critical bending radius reduced from 13.5 mm to 8.0 mm, and the silicon die was again the most vulnerable component. When the die thickness decreased from 50 to 30 μm, the critical bending radius of the flexible system even decreased to 6.9 mm. Therefore, it is advantageous for mechanically reliable flexible microelectronic systems to bond thin silicon dies with rounded corners to a flexible substrate using polymer bumps.

Ga and Ti co-doped In2O3 films for flexible amorphous transparent conducting oxides

We designed Ga and Ti co-doped In2O3 (IGTO) films to use as a flexible and transparent amorphous conducting oxide electrode in thin film heaters (TFHs) and flexible touch screen panels (FTSPs) for automobiles. The properties of the IGTO electrodes deposited on cyclic olefin copolymer (COP) at room temperature were investigated as a function of the O2/(Ar + O2) flow ratio, to confirm the best sputtering condition for transparent and flexible electrode. Depending on the oxygen flow ratio, the IGTO/COP electrodes showed sheet resistance of (39.3 – 1.57) × 104 Ohm/sq, an average transmittance of (84.90 – 87.12) % at visible wavelength area, and a surface roughness of (0.95 – 3.23) nm. In addition, IGTO/COP samples exhibited good mechanical flexibility with critical bending radius of 3 mm, which is enough to be used as FTSPs. From the previously mentioned results, we found the amorphous IGTO/COP to be a promising flexible and transparent electrode for curved TFHs and FTSPs. The flexible IGTO/COP TFHs demonstrated a saturated temperature of 78.6 °C when applied with low operating direct current (DC) of 8 V, due to its low sheet resistance. In addition, the IGTO/COP FTSPs showed very stable touch sensitivity, even at a bent state. We found that the optimized IGTO/COP is a promising flexible and transparent electrode for next-generation automobiles.

A Novel Core Allocation in Heterogeneous Step-Index Multi-Core Fibers With Standard Cladding Diameter

We numerically reveal the feasibility of designing multi-core fibers (MCFs) with the simple step-index (SI) profile within the standard 125- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m cladding diameter. We show that the standard cladding diameter enables us to incorporate five identical cores for C-band operation, and the number of cores can be further increased with the heterogeneous core arrangement. We then propose a novel method of allocating the non-identical cores in heterogeneous MCFs (Hetero-MCFs), where the crosstalk (XT) and critical bending radius ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pk</sub> ) are improved comparing to that of Hetero-MCFs designed by the conventional method. These new types of step-index MCFs (SI-MCFs) with standard cladding diameter have potential application in the space-division multiplexing (SDM) systems.