Bending Strain Tolerance Research Articles

Next-generation highly flexible round REBCO STAR wires with over 580 A mm−2 at 4.2 K, 20 T for future compact magnets

We report the latest developments of next-generation flexible round RE–Ba–Cu–O (REBCO, RE = rare earth) wire, driven by the needs of compact accelerator magnets requiring round isotropic wire with an engineering current density (Je) of 600 A mm−2 at 4.2 K, 20 T at a bend radius of 15 mm. We have developed a Symmetric Tape Round (STAR) REBCO wire using multiple layers of REBCO tapes specifically developed for this architecture, featuring a mechanically symmetric geometry with a 10–18 μm thick substrate wherein the superconductor film is positioned near the tape’s neutral plane for superior bend strain tolerance. Furthermore, each layer of REBCO tape is individually optimized for maximum bend strain tolerance. These ultra-thin substrate REBCO symmetric tapes enabled us to fabricate next-generation isotropic round wires of just 1.3 mm diameter and a critical current equivalent to commercial 12 mm wide REBCO tapes. The in-field performance of STAR wires of several configurations has been tested at National High Magnetic Field Laboratory to identify the most suitable architecture to meet the needs of high-field compact accelerators. At a bend radius of 15 mm, a six-layer STAR wire exhibits critical current of 778 A at 4.2 K in 20 T background field, which equals Je of 586.4 A mm−2 at a Lorentz force (FL) of 15.5 kN m−1 which is the highest reported Je value for REBCO wire in round geometry at this magnetic field. Similarly, a 12-layer STAR wire shows an Ic of 1156 A at 31.2 T, 4.2 K which corresponds to a Lorentz force of 36 kN m−1. Multiple tests of STAR wires at high magnetic field confirm a <0.1% variation in measured Ic. This level of reproducibility of the high performance of STAR wire in high magnetic fields at 4.2 K and small bend radius underscores the potential of STAR REBCO wire for use in compact accelerator magnet and related applications.

Room‐Temperature Fabrication of High‐Quality Lanthanum Oxide High‐κ Dielectric Films by a Solution Process for Low‐Power Soft Electronics

AbstractHigh‐quality dielectric films are indispensable for field‐effect electronic devices to provide high electrical performance. However, deposition of high‐quality dielectric films on flexible substrate remains a challenging task, which greatly limits the electrical performance of flexible or organic field‐effect devices. A room‐temperature deep ultraviolet (DUV) irradiation process is proposed for densification of solution‐processed La2O3 films. It is found that room‐temperature DUV‐La2O3 films not only exhibit leakage and dielectric properties that are comparable to those of high‐temperature annealed‐La2O3 films, but also have very smooth and clean surfaces and excellent bending strain tolerance. The fabricated La2O3 films exhibit a wide band gap (5.78 eV), high dielectric constant (12.68), low leakage density (8.8 × 10−7 A cm−2 at 2 MV cm−1), and high breakdown field strength (3.5 MV cm−1). Organic thin‐film transistors (OTFTs) based on high‐quality solution‐processed La2O3 gate dielectrics show a low operation voltage (≤3 V), low gate leakage current (&lt;10−9 A), and high Ion/Ioff ratio (&gt;105). Based on these room‐temperature‐fabricated high‐performance OTFTs, low‐voltage complementary integrated circuits are successfully fabricated, including complementary inverters and NAND gate circuits. The results demonstrate that room‐temperature DUV‐processed La2O3 films are ideal gate dielectric materials for future flexible and low‐power‐consumption electronic devices.

Bending Behavior of HTS Stacked Tapes in a Cable-in-Conduit Conductor With Twisted Al-Slotted Core

In this paper, we report on the experimental study of the bending behavior, at 77 K, of a cable-in-conduit conductor composed of a stack of REBa 2 Cu 3 O 7-x (REBCO) tapes inserted into a helical duct formed in the extruded aluminum cylindrical core. The investigation was carried out by powering each single tape individually. By the analysis of the single tape I c dependence as a function of the bending radius, R b , the effect of bending strain as a function of tape position inside the stack was investigated for each tape. The results evidence the good bending strain tolerance of all the tapes, showing the onset of degradation at R b ≈ 0.25 m, ascribed to the slippage occurring among tapes within the stack. Interestingly, the decrease in the intertape contact resistance between neighboring tapes with respect to the straight cable condition reveals that the bending stress results in a beneficial additional transverse load on the stacks, which leads to a more uniform compaction of tape stack. The experimental I c behavior with R b has been explained considering that the current transfer mechanism among tapes might mitigate the degradation of the tape I c due to the bending strain. The identification of the mechanisms acting on individual tapes under cable bending conditions is a relevant achievement in perspective of the interpretation of the electromechanical behavior of the conductor with all tapes powered in parallel.

Optimum Copper Stabilizer Thickness for Symmetric Tape Round (STAR) REBCO Wires With Superior Mechanical Properties for Accelerator Magnet Applications

Symmetric Tape Round (STAR) wires are only 1.6-1.9 mm in diameter and are made by winding 2.5-mm-wide REBCO tapes on an AWG 18 (1.02-mm diameter) copper wire former. Such ultra-small diameter wires are possible by the use of symmetric tapes where the REBCO film is positioned at the geometric center, close to the neutral axis. Such symmetric tape architecture is achieved by a strategic selection of the thickness of the copper stabilizer to match the thickness of the Hastelloy substrate. The substrate itself is made thin, to enable good tolerance to bend strain. The optimized STAR wire architecture is found to be mechanically robust and retain nearly 98% of its critical current of over 500 A at 77 K, self-field even when bent to a small radius of 15 mm. The superior bend strain tolerance of STAR REBCO wires make them suitable for use in canted cosine theta coils for future compact accelerator magnet applications.

Measurements on Critical Current and Bending Strain Tolerance for Ex Situ ${{\rm MgB}_{2}}$ Wires and Tapes Under High Field up to 8 T

The critical currents of several configurations of ${{\rm MgB}_{2}}$ conductors from Columbus Superconductors SpA were characterized under high magnetic fields. The measurements were performed at LNCMI Grenoble at a temperature of 4.2 K and under a magnetic field ranging from 3 up to 8 T. The characterization of the different configurations takes into account different parameters, such as the filling factor, number of filaments, shape, and type of matrix. Moreover, in order to define the minimum bending radius, the different samples were prebent at room temperature on different diameters (from 50 to 400 mm) and then straightened again before being tested. This work investigates the critical current density $J_c$ , engineering current density $J_e$ , and the influence of bending strain over the critical current. The different samples are compared according to their critical current densities, engineering densities, and bending strain tolerances.

Bending strain tolerance of MgB2 superconducting wires

This work describes the strain tolerance of MgB2 superconductors subjected to variable bending stresses. Bending of MgB2 wire was done at room temperature in different modes: (i) direct bending of straight annealed samples to variable diameters and by (ii) indirect bending by straightening of bent and annealed samples. Ic–bending strain characteristics of samples made by in situ PIT and by the internal magnesium diffusion (IMD) process were measured at 4.2 K. The results show a good agreement between the direct and indirect bending mode, which allows easier estimation of limits important for the winding process of MgB2 superconductors with brittle filaments. A comparison of MgB2 wires made by in situ PIT and IMD processes showed improved strain tolerance for IMD due to better grain connectivity the low annealing temperature, which does not appear to reduce the mechanical strength of sheath material.

Applicability of Reel-to-Reel RHQ Treatments to $ \hbox{Nb}_{3}\hbox{Al}$ Precursors With a Ta/Cu/Ta Three-Layer Filament-Barrier Structure

A tension-free rapid heating and quenching (RHQ) simulator was previously employed to demonstrate that a ductile and normal-conducting Cu layer can suppress a magnetic field instability and improve bending strain tolerance. However, a molten Cu barrier caused the filament to be shifted from the original location, and excess heating caused the Cu to locally corrode a Ta layer, through which it diffused into the filament, thereby embrittling the resulting wire. In some cases, the swelling molten-Cu tore the Ta skin. Fabrication of a long piece of Ta/Cu/Ta barrier Nb3Al wire is attempted using reel-to-reel RHQ treatment of a modified precursor with a thicker Ta skin that would tolerate both tension forces in a section of the wire between electrodes and pressure from the swelling of molten-Cu. In the optimum RHQ condition (VRHQ: 9.5 V), a 50-m piece of Ta/Cu/Ta barrier precursor was successfully reel-to-reel RHQ treated to form a bcc supersaturated-solid solution (Nb(Al)ss) without bursting of the Ta skin. Microstructural analysis revealed that the molten Cu, into which Ta dissolved, diffused into the jelly-roll filament, mainly through a region of partially melted Al-rich Nb( Al)ss. The critical current density of the filament Jc (18 T, 4.2 K) was optimized by the deformation of Nb(Al)ss and compared favorably with the conventional precursor.

Enhancement of bending strain tolerance and current carrying property of MgB2 based multifilamentary wires

The effect of bending strain on current carrying capacity of MgB2 multifilamentary wires was studied with 4, 8 and 16 multifilamentary wires. The critical current density (JC) of straight wires and bent wires with 5, 10, and 15cm diameter was measured. Both annealed & bent and bent & annealed wires were used for measurement. The JC of annealed & bent wires were found to decrease with decrease in bent diameter and the rate of degradation of JC decreased with increasing number of filaments, while bent & annealed wires almost retained its JC at all diameters studied.

A new RHQT Nb3Al superconducting wire with a Ta/Cu/Ta three-layer filament-barrier structure

To suppress the low-magnetic-field instability (flux jumps in low magnetic fields) of a rapid-heating, quenching and transformation (RHQT) processed Nb3Al superconductor, we had previously modified the cross-sectional design of an RHQT Nb3Al by adopting a Ta filament-barrier structure. Unlike Nb barriers, Ta barriers are not superconducting in magnetic fields at 4.2 K so that they electromagnetically decouple filaments. However, small flux jumps still occurred at 1.8 K, which is a typical operating temperature for the magnets used in high-energy particle accelerators. Furthermore, poor bonding at the Ta/Ta interface between neighboring Ta-coated jelly-roll (JR) filaments frequently caused precursor wires to break during drawing. To overcome these problems, we fabricated a new RHQT Nb3Al wire with a Ta/Cu/Ta three-layer filament-barrier structure for which an internal stabilization technique (Cu rods encased in Ta are dispersed in the wire cross section) was extended. Removing the Ta/Ta interface in the interfilamentary barrier (JR filament/Ta/Cu/Ta/JR filament) allowed precursor wires to be drawn without breaking. Furthermore, the Cu filament barrier electromagnetically decoupled filaments to suppress flux jumps at 1.8 K. The ductile Cu layer also improved the bending strain tolerance of RHQT Nb3Al.

Measurement of Electromechanical Property in Thick-Film EDDC-SmBCO Coated Conductors With High Critical Current

In this paper, in order to ensure its performance in practical applications, the mechanical and electromechanical properties of thick-film evaporation-in-dual chamber (EDDC)-SmBCO coated conductor (CC) tapes with high critical current have been investigated. The effect of CC tape architecture and film thickness on critical current Ic in EDDC-SmBCO CC tapes with thick film under uniaxial tension and bending deformation was measured and compared with the Cu-stabilized sample. By adopting a bridge pattern and by applying a magnetic field and some measures, the electromechanical property of thick-film CC tapes with high Ic could be evaluated, suppressing the burnout during deformation. As a result, increasing the SmBCO film thickness affects the irreversible strain limit and Ic degradation behaviors. In the case of a 5- μm-thick film, an acceptable bending strain tolerance 0.34% was measured and the uniaxial irreversible strain limit of 0.28% can be still improved by an additional Cu stabilizer.

외부보강된 밀봉 상태가 다른 Bi-2223테이프의 가압 LN2하에서 임계전류의 굽힘변형률 의존성

The critical current degradation behaviors of multifilamentary Bi-2223 superconducting tapes under pressurized liquid nitrogen were investigated using a r-shaped sample holder which gives a series of bending strains to tape. Three kinds of externally-reinforced Bi-2223 tapes with different hermeticity were used as samples. The tape with the thicker reinforcement layer had a better bending strain tolerance of , but when the bending strain was calculated at the outermost filament, the degradation behavior became identical. For all samples, decreased with the increase of applied pressure, but the degradation behavior with bending strain at each pressure level was similar. Furthermore, after depressurization from 1 MPa to atmospheric pressure, was completely recovered to its initial values. When the samples were warmed up to room temperature after pressurization tests, the ballooning damage occurred at lower bending strain regions. The region where ballooning was observed was identical to the one where the significant degradation occurred.

Bend strain tolerance of YBa2Cu3O7−x-coated conductors fabricated by inclined substrate deposition

The effect of tensile bend strain on the critical current ofYBa2Cu3O7−x (YBCO)-coated conductors was evaluated as a function of YBCO film thickness. TheYBCO-coated conductors were fabricated on Hastelloy C276 metal substrates templatedwith MgO by inclined substrate deposition. It was observed that a conductor with0.2 µm thick YBCO retained 90% of the unstrained critical current until a strain of≈0.4%, whereas for aconductor with 0.5 µm thick YBCO film the critical strain value was≈0.2%. At strains higher than the critical strain, the critical current dropped precipitously. Theinverse dependence of bend strain tolerance on the YBCO film thickness is in reasonableagreement with analytical prediction. This inverse dependence was used to approximate thefracture toughness of the YBCO film, which was found to be in general agreement with theliterature.

Deformation and I&lt;tex&gt;$_rm c$&lt;/tex&gt;Degradation Behavior in Bent Bi-2223 Tapes Under Pressurized Liquid Nitrogen

The <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ I_ c$</tex> degradation behaviors of Bi-2223 superconducting tapes under pressurized liquid nitrogen were investigated using a newly developed sample holder which gives a series of bending strains to a sample. Four kinds of commercially available multifilamentary Bi-2223 superconducting tapes were used. At atmospheric pressure, the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ I_ c$</tex> degradation behavior depended upon the reinforcement adopted, the tapes externally reinforced or densified by over pressure showed better bending strain tolerance than the Ag alloy-sheathed Bi-2223 tapes. But these tapes showed a larger <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ I_ c$</tex> degradation when pressurized to 1 MPa in liquid nitrogen. For all samples, after depressurization to atmospheric pressure from 1 MPa, the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ I_ c$</tex> was completely recovered to its initial values at atmospheric pressure. When the samples were warmed up to room temperature after being depressurized from 1 MPa, ballooning occurred at some parts of some samples. It was found that the larger degradation of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ I_ c$</tex> occurred at the regions where significant ballooning occurred.

Improvement Mechanism and Bending Strain Tolerance for Reinforced Bi2223 Superconducting Tapes

Comprehensive mechanical characterisation of stainless steel reinforced Bi2223 superconducting tapes was conducted using purpose built rigs. This included the effect of axial (tensile and compressive) and bending strain on the critical current (Ic) of reinforced Bi2223 tape. In addition, the axial tensile and transverse compressive stress properties of the tape were measured and evaluated. It was found that the Ic-bending strain characteristic of the tape was improved as a result of sheath reinforcement, compared to an identical tape without reinforcement. A mechanism for this improvement based on the homogenising effect of the reinforcement on strain is proposed. To avoid Ic degradation, the maximum axial stress on the reinforced tape should not exceed 350 MPa, or the tensile strain should be less than 0.65%. Furthermore, no Ic degradation was observed for transverse compressive stresses up to 100 MPa, while an axial compressive strain of about 0.3% causes about 10% drop in the critical current.

Effect of bending strain on the critical current of Bi-2223/Ag tapes with different structures

We have evaluated the effect of bending strain on the critical current of Bi-2223/Ag tapes with different structures. The measured results were analysed using a symmetric pure bending model. The irreversible degradation indicates that the Ic reduction is caused mainly by crack formation and propagation in the brittle Bi-2223 ceramic core. Experimented results have shown that the number of filaments, as well as the structure of the tapes, could have strong effects on the measured apparent critical values of the tapes. Multifilamentary tapes showed better performance compared with single core tapes. The bending strain tolerance of the tape could be improved by adding a metal strip on the upper side of the tape.

Strain effects of high strength Nb/sub 3/Sn wire reinforced by jelly-rolled Nb/Cu composite and modification of the reinforcement

We are in the process of developing high strength Nb/sub 3/Sn wires reinforced by unique Nb/Cu composites made in a jelly-rolling process to miniaturize high field magnets. A bronze-processed (Nb,Ti)/sub 3/Sn superconducting wire incorporating the composite with a high Nb volume fraction has been made. The wire has shown a high 0.2% proof stress at 4.2 K of 350 MPa. The strain effects such as uni-axial strain dependence and bending strain tolerance on the critical current were examined this time. The influence of the Nb volume fraction in the jelly-rolled composite reinforcement was also investigated to improve the electrical conductivity of the reinforcement.

Effects of axial tensile and bending strains on the critical current of Bi-2223 superconducting tapes

The effects of axial tensile and bending strains on the critical current of Ag and AgMn/Bi-based tapes produced by the powder-in-tube method have been studied at 77 K. The observed degradation of critical current due to tensile strain effects is irreversible. AgMn sheath and multifilament are beneficial to improve the tensile and bending strain tolerance of Bi-2223 superconducting tapes. In the case of Ag/Bi-based tapes, the ε irr≈0.15%, while that of AgMn/Bi-based tapes is 0.34%. The larger tensile and bending strain tolerance of AgMn/Bi-2223 superconducting tape may result from better mechanical properties.

Bend strain tolerance of critical currents for YBa2Cu3O7 films deposited on rolled-textured (001)Ni

Superconducting YBa2Cu3O7/CeO2/YSZ/CeO2 multilayer structures were grown on rolled-textured (001)Ni using pulsed-laser deposition. Critical current densities greater than 1 MA/cm2 were obtained for relatively thick YBa2Cu3O7 films. The compressive and tensile bend strain tolerance of critical currents for the YBa2Cu3O7 films deposited on these rolling-assisted biaxially textured substrates was also determined. These conductors retained up to 80% of their unstrained critical currents for applied compressive bend diameters as small as 1.5 cm and tensile bend diameters of 3.2 cm. The degradation of Jc is caused by the formation and propagation of transverse cracks. The results also suggest a correlation between bend-strain tolerance for these coated conductors and total oxide layer thickness.

Bending strain tolerance of critical current values in Tl-based superconducting tapes

Mono- and multifilamentary silver-sheathed Tl-1223 powder-in-tube (PIT) tapes have been strained by bending at 77 K. The changes in the critical current of the samples due to the bending strain have been related to the tape microstructure. The shapes of the curves are similar to those found in the literature for Bi-2223 PIT samples. In situ reacted samples show a significant improvement in bending strain tolerance compared to samples prepared using pre-reacted powder.

Enhancement of critical current density and strain tolerance in Ag-sheathed BSCCO (2223) tapes by continuous silver reinforcement

We have used a modified powder-in-tube technique in various configurations to demonstrate significant improvement in the transport properties and strain tolerance of Ag-sheathed BSCCO superconducting tapes. In this process, the BSCCO core has been reinforced with a silver rod placed concentrically within the silver tube and then processed into thick tapes by established metal-working techniques. A noticeable improvement in critical current density, , was observed as a result of this modification. This increase is attributed to increased texturing, decrease in thickness of BSCCO layers, increase in the length of the Ag/BSCCO interface, and decrease in the volume fraction of the superconducting core which is less effective in contributing towards the overall critical current of the tape. Further improvements have been made to this configuration by incorporating another ring of superconductor into the core. The improved configurations led to a significant increase in over that of monolithic BSCCO tapes. Critical current densities of at 77 K and self-field have been consistently achieved in thick tapes after 150 h of sintering. Analysis of critical current through various sections of the tape indicates that 80% of the critical current is carried by the centre section of the core within which the Ag rod is located. The bend strain tolerance of the tapes containing a silver core was found to be significantly better than that of the monolithic tapes. At an applied surface bend strain of 1.0%, about 98% of initial was retained in Ag-reinforced tapes, compared with only 54% in monolithic tapes.