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Contact force is related to the mechanical response of superconducting strands under a large electromagnetic body force, which is important for the safety of the international thermonuclear experimental reactor (ITER) magnet structure. Due to the complex structure of the cable-in-conduit conductor (CICC), the component unit of the ITER magnet, and the extreme operating environment, the research on the strand contact force caused by the electromagnetic force has been progressing slowly. In this study, a two-dimensional (2D) theoretical model based on the granular element method is constructed to compute the contact forces among some opaque and non-photoelastic ferromagnetic particles which are placed in a non-uniform magnetic field. In the experiment, the contact deformations of these particles may be obtained by the digital image correlation method. We also propose a method, which is similar to the least-squares method, to calculate the electromagnetic body forces of different particles. Subsequently, the distributional and statistical characteristics of the contact force chains and contact angles are presented. It is considered that the method proposed in this paper is suitable for the contact force analysis of the cross section of superconducting strands in the ITER CICC that is subjected to a transverse electromagnetic force. In the end, this 2D theoretical model is generalized to the three-dimensional (3D) case, and the concise mathematical framework is presented.

YBa2Cu3O7-x (YBCO) is a kind of high-temperature superconducting material that has important application in information, energy, medical treatment, etc., and the superconducting properties of YBCO are closely related to its internal microstructure. In this study, the microwave heating method was adopted to prepare the YBCO materials. The internal 3D online evolution observation based on the synchrotron radiation computed tomography technology shows that there was directional grain growth phenomenon of YBCO during microwave sintering process. In local regions with special microstructure, these particles grew to the same point. Here, the theoretical models of single and multiple particles in the microwave electromagnetic fields were established. Based on these theoretical models and finite element analysis, it shows that the YBCO particles can modulate the distribution of electromagnetic fields, resulting in the significantly higher electric field intensity at the particle junctions than other regions. Moreover, there were very high electric field intensity and temperature gradients in the directions of particle growth. These factors were crucial in directional sintering. These results will provide theoretical basis and technical guidance for the controllable preparation and performance optimization of the internal microstructure of superconducting materials in the preparation process.

Lithium dendrites cause battery failures and safety hazards in liquid‐electrolyte lithium‐based batteries. To address this problem, each component of the battery, such as cathode, anode, electrolyte and separator, should be well matched and engineered for the integrated battery system. The separator plays an increasingly important role owing to its gradual transformation from an inert to an active component in the battery, especially for resolving Li dendrite issues in Li‐metal batteries. Armed with advanced nanotechnology solutions, separator engineering presents a formidable strategy to suppress dendrite growth dynamics by modifying the electrolyte chemistry, regulating/trapping unwanted ionic species and redirecting dendrite growth direction. This review summarizes recent advances in separator engineering for effective dendrite suppression of Li‐metal anodes. We first introduce the challenges that the Li‐metal anode faces and the irreplaceable role of separators in the battery system. The characterization parameters and design principles of separators are also discussed. Then, the mainstream techniques for separator functionalization and their impacts on dendrite suppression are scrutinized and highlighted. Lastly, the conclusion is drawn and the future outlook for separator engineering is presented.

Lithium-ion batteries (LIBs) are considered as one of the most successful energy storage technologies due to the high energy density, long cyclability and no memory effect. With the ever-increasing energy density of LIBs in practical applications, operational safety becomes more critical because unintentional release of energy can lead to devastating accidents related to fire and explosions of LIBs. The flammable polymer separators and liquid electrolytes are the root causes of such safety hazards. For the advancement of safe LIB design, this review summarizes the recent developments towards highly flame-retardant separators and solid-state electrolytes. The composition, structure and the performance of flame-retardant separators for liquid LIBs are firstly discussed, which include inorganic particle blended polymer separators, ceramic material coated separators, inherently nonflammable polymer separators, and separators with flame-retardant additives. Then much attention is paid on the solid-state electrolytes including inorganic solid electrolytes, solid polymer electrolytes, and inorganic/organic composite electrolytes. Future outlooks for the challenges of flame-retardant separators to practical applications are also discussed.

The second generation HTS wires have been used in many superconducting components of electrical engineering after they were fabricated. New challenge what we face to is how the damages occur in such wires with multi-layer structure under both mechanical and extreme environment, which also dominates their quality. In this work, a macroscale technique combined a real-time magneto-optical imaging with a cryogenic uniaxial-tensile loading system was established to investigate the damage behavior accompanied with magnetic flux evolution. Under a low speed of tensile strain, it was found that the local magnetic flux moves gradually to form intermittent multi-stack spindle penetrations, which corresponds to the cracks initiated from substrate and extend along both tape thickness and width directions, where the amorphous phases at the tip of cracks were also observed. The obtained results reveal the mechanism of damage formation and provide a potential orientation for improving mechanical quality of these wires., Damage that occurs in second generation high-temperature-superconducting wires is problematic. Here, the authors present real-time magnetic flux behaviour in these wires under tensile strain and reveal damage evolution, including the amorphous phase in the superconducting layer acting in crack blunting during tension

The residual stress is formed in $$\hbox {YBa}_{\mathrm {2}}\hbox {Cu}_{\mathrm {3}}\hbox {O}_{{7\text {-}\updelta }}$$ (YBCO) bulk superconductors, which undergoes a complex fabrication process and experienced a huge temperature change about $$1000^{\circ }\,\hbox {C}$$ . The residual stress may result in micro-cracks in YBCO bulks and cause degradation of its superconducting and mechanical properties. It will be greatly helpful to reform the structural designs if the distribution of residual stress in YBCO bulk can be investigated. The traditional method of measuring residual stress does not apply to oxide ceramics as they are brittle materials. In this paper, a YBCO bulk was sliced into pieces of several millimeters in height, which could thus be treated as a plane-stress problem. The residual stress and principal stress angle for YBCO bulk have been obtained by an artificial hole-drilling method with very low speed.

Superconducting thin films are widely used in superconducting quantum interferometers, microwave devices, etc. The electrical performance of a superconducting thin film is often affected by structural deformation or stress. Based on four-point bending of a Cu-Be beam, we constructed a device that could apply uniaxial, uniform, compressive strain to a superconducting thin film at both room temperature and the temperature of liquid nitrogen. The thin film was placed into a slot carved in the Cu-Be beam. We optimized the size of this slot via numerical simulation. Our results indicated that the slot width was optimal when it was same as the width of the Cu-Be beam. Notably, the sample bended hardly after machining two slits along width direction on both sides of the slot. A YBa2Cu3O7-δ-SrTiO3 (YBCO-STO) film was used as an example. It was loaded by the aforementioned device to determine its electrical characteristics as functions of the uniaxial-uniform-compressive strain. The optimized design allowed the sample to be compressed to a larger strain without breaking it.

We fabricated a porous ZrW2O8/graphene hybrid meta-material (ZGHM) via a one-step in-situ hydrothermal process with the mixed suspension solution of graphene oxide (GO) and ZrW2O8 as the precursors. The porous ZrW2O8 accretion covers over the GO sheets to form the interconnected microstructure with two components assembled together strongly. After subsequent freeze-drying and thermal annealing processes, the as-obtained ZGHM presents a porous structure and lightweight density of 4.35 mg/cm3. Due to the synergy interaction and interfacial strengthening between GO and ZrW2O8, ZGHM demonstrates significantly enhanced negative thermal expansion (NTE) effect over a large temperature range 293–473 K. The maximum thermal expansion coefficient (−44.7 × 10−6 K−1) of ZGHM is three times over that of pure ZrW2O8. Moreover, in situ observation by scanning electron microscope reveals NTE by microstructural shrink evolution under controlled heating schemes as well as finite element simulation. A concomitant sharp displacement was triggered by thermal strain-induced buckling of micro-elements. Such NTE suggests smart modulating performance on thermal strain distribution in graphene-based ceramic composites for engineering.

Rare earth barium copper oxide (REBCO) type materials are widely used in second-generation high-temperature superconductor tapes to carry high current volumes. These tapes can provide high critical currents along with good tolerance to perpendicular magnetic fields. However, REBCO tapes generally use a laminate structure that suffers from low peeling strength among its inner interfaces. Previously, we proposed a bending-peeling (BP) method and applied it to measurement of the interfacial strength of REBCO tapes at room temperature. All samples failed at the REBCO layer at room temperature. In this paper, we used the BP method to perform low-temperature interfacial strength measurements at liquid-nitrogen temperatures. There are two types of failure, where one involves peeling of the REBCO layer and the other involves peeling at the interface between the tape and the epoxy resin. The errors in the BP method were discussed in detail. The superconducting current was monitored during the BP experiment and showed that the effect of out-of-plane tensile stress on the superconducting current was not significant before the breaking of the tape. The degradation in the superconductivity is mainly caused by the structural failure of the REBCO layer.

The theoretical and technological achievements in the damage mechanism and evaluation model obtained through the national basic research program “Key Fundamental Scientific Problems on Mechanical Equipment Remanufacturing” are reviewed in this work. Large centrifugal compressor impeller blanks were used as the study object. The materials of the blanks were FV520B and KMN. The mechanism and evaluation model of ultra-high cycle fatigue, erosion wear, and corrosion damage were studied via theoretical calculation, finite element simulation, and experimentation. For ultra-high cycle fatigue damage, the characteristics of ultra-high cycle fatigue of the impeller material were clarified, and prediction models of ultra-high cycle fatigue strength were established. A residual life evaluation technique based on the “β-HV-N” (where β was the nonlinear parameter, HV was the Vickers hardness, and N was the fatigue life) double criterion method was proposed. For erosion wear, the flow field of gas-solid two-phase flow inside the impeller was simulated, and the erosion wear law was clarified. Two models for erosion rate and erosion depth calculation were established. For corrosion damage, the electrochemical and stress corrosion behaviors of the impeller material and welded joints in H2S/CO2 environment were investigated. KISCC (critical stress intensity factor) and da/dt (crack growth rate, where a is the total crack length and t is time) varied with H2S concentration and temperature, and their variation laws were revealed. Through this research, the key scientific problems of the damage behavior and mechanism of remanufacturing objects in the multistrength field and cross-scale were solved. The findings provide theoretical and evaluation model support for the analysis and evaluation of large centrifugal compressor impellers before remanufacturing.

We designed a transverse loading device that can apply a more uniform compressive force perpendicular to superconducting tape than previously designed equipment. The effective compressive length and width reached up to 100 and 20 mm, respectively. The critical current (IC) and/or transport AC losses of the superconducting tape during transverse compression could be measured using this equipment. We used YBa2Cu3O7−x-coated conductor (YBCO CC) specimens with copper stabilizing layers of different thickness to investigate the characteristics of the critical current and transport AC losses under transverse compression loading. For comparison, several Bi-2223/Ag samples were also tested. A critical stress was observed for the compression process (the same as that for axial tension), and beyond this stress, IC degraded irreversibly. For the transport AC losses behavior of the YBCO CC and Bi-2223/Ag specimens, below the critical stress value, the AC losses remained almost unchanged with the increase of the compressive stress at the same current; while above the critical stress value, at increasing compressive stress, the AC losses increased rapidly and the normalized AC losses for normalized current decreased.

The purpose of the experiments presented here is to explore the transport alternating current (AC) losses in YBa2Cu3O7−x coated conductor (YBCO CC) that contains a transverse crack. The experimental tests are performed using an improved local calorimetry method with high accuracy. We define a factor called c to describe the ratio between the AC losses of the specimen with and without the crack in terms of unit length. It is found that a power law curve between c and the crack length L coincides perfectly with the experimental results. In addition, we also find that the AC losses in the crack region are hysteretic losses.

In this study, Cu alloy joints were fabricated with a Ni-foam reinforced Sn-based composite solder with the assistance of ultrasonic vibration. Effects of ultrasonic soldering time on the microstructure and mechanical properties of Cu/Ni-Sn/Cu joints were investigated. Results showed that exceptional metallurgic bonding could be acquired with the assistance of ultrasonic vibration using a self-developed Ni-foam/Sn composite solder. For joint soldered for 5 s, a (Cu,Ni)6Sn5 intermetallic compound (IMC) layer was formed on the Cu substrate surface, Ni skeletons distributed randomly in the soldering seam and a serrated (Ni,Cu)3Sn4 IMC layer was formed on the Ni skeleton surface. Increasing the soldering time to 20 s, the (Ni,Cu)3Sn4 IMC layer grew significantly and exhibited a loose porous structure on the Ni skeleton surface. Further increase the soldering time to 30 s, Ni skeletons were largely dissolved in the Sn base solder, and micro-sized (Ni,Cu)3Sn4 particles were formed and dispersed homogeneously in the soldering seam. The formation of (Ni,Cu)3Sn4 particles was mainly ascribed to acoustic cavitations induced erosion and grain refining effects. The joint soldered for 30 s exhibited the highest shear strength of 64.9 ± 3.3 MPa, and the shearing failure mainly occurred at the soldering seam/Cu substrate interface.

The purpose of the current study is to explore the frictional behavior of a micro-sized superconducting fiber at the low-temperature condition. At first, a highly precise tribometer composed of a superconducting fiber wrapping around a cylinder made of pure Cu was immersed in liquid nitrogen. The force and displacement resolutions of the experimental system were as high as $$0.01\, \hbox {mN}$$ and $$0.03\,\upmu \hbox {m}$$ , respectively. The NbTi fibers with diameters ranging from 22.9 to $$115\,\upmu \hbox {m}$$ were used in the experiments, and their frictional behaviors in three media, i.e., liquid nitrogen, air and water, were systemically investigated. It was found that the frictional force in air showed a remarkable size effect. The existence of water medium could significantly reduce the frictional force, but could not eliminate the size effect. For the samples with the same diameter, the frictional force in liquid nitrogen was about 1.4 times of that in air, accompanied with remarkable stick-slip phenomenon. Notably, the fiber’s frictional behavior in liquid nitrogen showed no dependence on diameter. In order to interpret these phenomena, the frictional behaviors of the fibers in air, water and liquid nitrogen were simulated using a modified spring-slider model, by taking into account the influence of hydrophilicity on surface roughness, and the influence of surface roughness on the fiber’s frictional behavior. The simulation results were consistent with the experimental data qualitatively.

Twist structures have diverse applications, ranging from dragline, electrical cable, and intelligent structure. Among these applications, tension deformation can't be avoided during the fabrication and working processes, which often leads to the twist structure rotation (called untwisting effect) and twist pitch increasing. As a consequence, this untwisting behavior has a large effect on the effective Young's modulus. In this paper, we present an improved model based on the classical Costello's theory to predict the effective Young's modulus of the basic structure, twisted by three same copper strands under cyclic loading. Series of experiments were carried out to verify the present model taking into account the untwisting effect. The experimental results have better agreements with the presented model than the common Costello's model. Keywords: Twist structure, Young's modulus, Cyclic loading, Untwisting effect

The central solenoid (CS) is one of the key components of the International Thermonuclear Experimental Reactor (ITER) tokamak and which is often considered as the heart of this fusion reactor. This solenoid will be built by using $$\hbox {Nb}_{3}\hbox {Sn}$$ cable-in-conduit conductors (CICC), capable of generating a 13 T magnetic field. In order to assess the performance of the $$\hbox {Nb}_{3}\hbox {Sn}$$ CICC in nearly the ITER condition, many short samples have been evaluated at the SULTAN test facility (the background magnetic field is of 10.85 T with the uniform length of 400 mm at 1% homogeneity) in Centre de Recherches en Physique des Plasma (CRPP). It is found that the samples with pseudo-long twist pitch (including baseline specimens) show a significant degradation in the current-sharing temperature (Tcs), while the qualification tests of all short twist pitch (STP) samples, which show no degradation versus electromagnetic cycling, even exhibits an increase of Tcs. This behavior was perfectly reproduced in the coil experiments at the central solenoid model coil (CSMC) facility last year. In this paper, the complex structure of the $$\hbox {Nb}_{3}\hbox {Sn}$$ CICC would be simplified into a wire rope consisting of six petals and a cooling spiral. An analytical formula for the Tcs behavior as a function of the axial strain of the cable is presented. Based on this, the effects of twist pitch, axial and transverse stiffness, thermal mismatch, cycling number, magnetic distribution, etc., on the axial strain are discussed systematically. The calculated Tcs behavior with cycle number show consistency with the previous experimental results qualitatively and quantitatively. Lastly, we focus on the relationship between Tcs and axial strain of the cable, and we conclude that the Tcs behavior caused by electromagnetic cycles is determined by the cable axial strain. Once the cable is in a compression situation, this compression strain and its accumulation would lead to the Tcs degradation. The experimental observation of the Tcs enhancement in the CS STP samples should be considered as a contribution of the shorter length of the high field zone in SULTAN and CSMC devices, as well as the tight cable structure.

The second generation high-temperature superconducting coated conductors (CCs) have excellent electrical and mechanical properties, and are extensively used in superconducting devices such as fault current limiters, magnets and motors. During the operation of these superconducting devices, superconducting CCs inevitably bear the combination of electromagnetic force and thermal mismatch stress, resulting in straining of YBa2Cu3O7− δ (YBCO) layer along the tape length. It is well known that the strains of superconducting CCs cause degradation of critical current (I c). Generally, the irreversible strain limit ( ε irr ) is used to characterize the phenomenon that I c of superconducting CCs degrades with axial strain. When the axial strain of superconducting CCs is less than ε irr , I c can be reversibly recovered by over 99% after being unloaded. Therefore, ε irr is a key parameter for the design and application of superconducting CC devices. For this reason, to carry out a practical engineering method for improving ε irr of superconducting CCs has become a challenge and aroused interests among researchers. This study is based on the idea of precompression. A 316LN stainless steel tape was pretensioned at 77 K to improve its elastic strain limit. Then, two superconducting CCs were soldered onto both surfaces of pretensioned stainless steel tape respectively. As a result, ε irr of the superconducting CCs can be controlled manually with different precompressions. Taking YBCO CCs produced by SuperPower Inc. as an example, the measurement results show that the ε irr of the YBCO CCs increased from 0.39% to 0.73%. Meanwhile, the thickness of the sample did not increase more than once.

In this study, damage evolution of Nb3Sn/Cu superconducting strand was investigated under asymmetric strain cycling. The results of the fatigue tests indicate that higher strain ranges and peaks can accelerate damage evolution, and damage evolution occurs seriously at the early fatigue stage. Based on the stiffness degradation rule of composites, two independent plastic damage evolution equations of the Nb3Sn sub-elements and Cu matrix were derived to obtain stiffness degradation and fatigue life by considering the effect of strain range and strain peak, respectively, and the accumulated plastic strain of Nb3Sn/Cu superconducting strands was obtained by deriving the total damage evolution equation. The proposed model was verified and validated by studying the fatigue behavior of Nb3Sn/Cu superconducting strands under asymmetric strain cycling. A good agreement between the analytical model and experimental data irrespective of the strain range and strain peak shows that the analytical model can not only predict the stiffness degeneration and accumulated plastic strain, but also describe the fatigue life of Nb3Sn/Cu superconducting strands under asymmetric strain cycling.

In this article, by composing three functional constituents into a multilayered structure with well-bonded interfaces, a conductive graphene-papers (GPs)/glass-fibers (GFs) reinforced epoxy composite (GPs-GE) with outstanding mechanical robustness, high electrical conductivity and sensitive stimuli-responsive performance is highlighted. Thereinto, GPs serves as a stimuli-responsive and Joule heating chip due to its superiorities on both of mechanical and electrical properties, while GFs possessing mechanically protective and strengthening functionality. The stimuli-responsive characterizations by electrical resistance change reveal the synchronous sensing properties of GPs-GE to different stimuli inputs such as mechanical deformation, temperature fluctuation and humidity. Subsequently, Joule heating and de-icing/anti-icing properties of GPs-GE are investigated symmetrically, indicating its large heating rate, high efficiency of energy conversion and low cost. Such superior performance of GPs-GE confirms that the design of multilayer microstructure to achieve multi-functionalization paves a novel way to scale-up fabrication of advanced graphene composites, indicating promising applications as mechanically reinforcing elements, stimuli-responsive sensors and electrical Joule heating chips in intelligent engineering monitoring and icing-induced disaster prevention at low-temperature environment.

Anvil tests are effective for determination of the mechanical delamination strength (MDS) of 2G high temperature superconducting coated conductors (CCs), which has been widely used. However, the discrete property of measurement data makes the MDS of CCs hard to evaluate just from the average and variance. To properly provide an analysis of these discrete experimental data, the three-parameter Weibull distribution analysis along with its reliability function has been adopted, from the criterion of 99% reliability, the obtained MDS is conveniently referred for both engineering test and design. Nevertheless, the stable and reliable Weibull distribution analysis should be based on enough sample capacity and proper anvil size. In this work, the influences of anvil size and sample capacity on the reliability function of Weibull distribution analysis for MDS determination were systematically investigated at 77K. It is found that the minimum of sample capacities for Weibull distribution analysis are different with different anvil sizes. From the aspects of energy dissipation and cost consumption, we recommend the full width size of anvils to be utilized to obtain the justified MDS of CCs without being slitted.

Recent experiments on the international thermonuclear experimental reactor Nb3Sn cable-in-conduit conductor have shown degradation in current-sharing temperature ( ${T_{cs}}$ ) over several thousand electromagnetic (EM) cycles. The destructive investigations on these specimens have found that the Nb3Sn strand buckled and cracked on the surface of the long twist pitch cable at the low EM load side. In order to discuss the strand buckling behavior, we propose a theoretical model to analyze the influences of twisting pitch, stainless steel tape width (wrap width), coverage rate, wrap stiffness, and friction factor on the buckling behavior of the Nb3Sn strand. The cases of Nb3Sn strand with and without EM loading are discussed, respectively. Moreover, a slip of the cable within jacket, which often reinforces the plastic deformation of the strand, is also considered in this study.

Interstrand force is related to the understanding of the mechanical response of superconducting strand under large electromagnetic force, which is important to the safety of the ITER magnet structure. A two-dimensional discrete element method and relevant simulations to quantitatively characterize the contact force behavior on the superconducting strands in the ITER cable-in-conduit-conductor cross-section has been performed by Zhu et al. (2012, Supercond. Sci. Technol. , 25 , 125011). Here, we report direct measurements of the contact forces inside a two-dimensional system of photoelastic disks that are subjected to a single-direction compression, and obtained the contact force (including force chain) and its direction. Then, we can conclude that the contact force distribution among strands is strongly anisotropic. We use a probability density function to analyze this inhomogeneity. The influences of a helium channel on the contact force behavior are also taken into account. Notably, the energy loss during this compression is also calculated based on the experimental results, which are considered to be suitable for providing qualitative predictions of the contact force behavior among the superconducting strands under electromagnetic force.

The high temperature superconducting YBa 2 Cu 3 O 7 − x films are widely used in various classes of microwave devices and these structures often undergo pulse magnetic field in applications. The stress behavior is of relevance to understand the mechanical response and important for assessing the safety of these superconducting structures. In this paper, we construct a theoretic model to analyze the stress behavior of a superconducting thin film-substrate system under a pulse loading (magnetic field), which is different with the well-known theoretic framework based on the Stoney formula. In the case of radial symmetry, the relationship between the stress of thin film and the electromagnetic body force is derived analytically. When a pulse loading is exerted on this film-substrate system, an analytical expression of stress dependent on the film curvature is presented, which can be considered as the foundation for the further experimental measurement. Moreover, we extend the coherent gradient sensor to kinetic measurement of the full field curvature. Using the presented model and the experimental technique, the full field stress and its evolution with time of a superconducting thin film-substrate system under a pulse magnetic field are obtained.

Experimental determination of the frictional properties of a microsized fiber wrapped around a cylinder has been of long-standing interest to the academic community. The purpose of the current experiments was to explore the diameter dependence of such microsized fibers in nonlubricated friction using a highly accurate tribometer. For this work, NbTi superconducting material was used for the fibers and polyvinyl chloride (PVC) was used as the cylinder material. Significant effects were observed in the kinetic friction coefficient for different fiber diameters, normal forces, and sliding speeds. Moreover, the effects of fiber diameter on the frictional stability were measured. Smaller fiber diameters and low sliding speeds both produced poor frictional stability. The most likely explanation for the observed stick–slip phenomena is hypothesized to be a combination of creep mechanics and plastic deformation of the junctions on the contacting surfaces.

High-speed measurements, particularly close to the velocity of light, have always been a great challenge in the field of experimental measurement. Although traditional high-speed measurement methods based on imaging technology can achieve millions of frames per second, they face the problem that the field of view decreases with an increase in the frame rate, which is difficult to overcome in a short period. In this study, a closed multi-exposure optical path is designed first based on a 600 ps pulse laser that realizes the continuous measurement of sub-light velocity and is not subject to the field of view. The path can also be adjusted with respect to time, accurate up to sub-nanoseconds. Second, it is found that the accuracy and resolution of the present method are related to the used pulse laser and camera. Once the performances of the pulse laser and the camera are improved, the corresponding range of measurement of the velocity can be improved further. Compared with traditional pumping technologies, the proposed technology achieves continuous velocity measurement with the utilization rate of laser energy as high as 100%. Finally, we use this novel optical system to determine the flux avalanche velocity of the YBa2Cu3O7-x superconducting thin film, and a highest speed of 323.5 Km/s is obtained.

Superconducting YBa2Cu3O7−x (YBCO) bulks have demonstrated promising applications in quasi-permanent magnets, levitation, etc. Recently, the applications of bulk superconductors have been proposed, such as in portable, ultralight superconducting devices. These applications require bulk superconductors with lightweight, complex morphology, and good processability. However, the traditional fabrication method of YBCO bulks requires prolonged supplemental oxygenation and produces limited geometries with many randomly distributed cracks. This, combined with the inherent brittleness of the ceramic material, seriously impedes its wide application. In this study, a YBCO bulk with a multiscale hierarchical geometric configuration is constructed by a direct-ink-writing-based 3D-printing process. The 3D-printed YBCO green bodies exhibit a robust structure after directional freezing, which promotes intimate contact between the precursor particles in the deposited layers. This technology offers the advantage of a low shrinkage of 13.6 vol% after sintering at 920 °C. The 3D-printed bulks show lightweight, highly crystalline, and good electromagnetic properties compared with those produced by traditional cold-pressed sintering. Importantly, the multilevel void structure with a large specific surface area significantly reduces the oxygenation time and retains superconductivity. The proposed 3D-printing process can be adopted for the industrial production of superconducting bulk with complex geometries for novel applications. © 2021 Wiley-VCH GmbH

Epoxy-impregnated pancake coil is observed to delaminate locally when its temperature is cooled from room temperature to 77 K. The delaminations cause the degradation of the critical current of the coil. Besides, the delamination may degrade the thermal conductivity and affect the thermal conductive path. Thus, in this study, the delamination behavior of epoxy-impregnated pancake coil during the cooling process is analyzed through a coupled thermal–mechanical cohesive zone model. The measured transverse tensile strength of coated conductors has shown considerable scatter and Weibull distribution has been adopted to fit the transverse tensile strength. The simulations are able to capture the main characterizations of the observed delaminations with the cohesive strength of the cohesive element following a Weibull distribution. After the cooling process, a heat spot is applied to the degraded positions to analyze the temperature field and the thermal conductive path in the damaged coil. Compared to the original undamaged coil, the delaminations indeed degrade the thermal conductivity and increase the thermal resistance. What’s more, the effects of the variation of the thickness of the epoxy and different interfacial cohesive strength distributions of the coated conductor are considered. The results show that reduction of the thickness of the epoxy can lower the radial stress and release the damage, and the random distribution of the cohesive strength inside the coated conductor dominantly determines the delamination pattern of the coil.

Recently, YBa2Cu3O7−x coated conductor (YBCO CC) has been developed intensively for different applications including power cables and high-filed magnets. Of all its physical properties, the thermal conductivity of the YBCO CC is considered as one of the most important parameters in guiding the temperature distribution, heat flux, and prediction of quench propagation. To accurately predict this property, a thermometry technique of high-speed fluorescent thermal imaging is introduced to monitor heat diffusion of commercial YBCO CCs in real time based on the Europium tris[3-(trifluoromethylhydroxymethylene)-(+)-camphorate] (EuTFC) in this study. We propose a new imaging process to eliminate the influences of background intensity and non-uniform illumination on the calibration results accompanying with good accuracy of measurement. And the fluorescence performances are evaluated by static and dynamic calibration experiments. The experimental results show that the photoluminescence of EuTFC has excellent photostability and temperature dependence, and there is no hysteresis in the temperature response when comparing with the PT100 measurements. Subsequently, four kinds of commonly used theoretical models of thermal conductivity and the corresponding calculation curves of the YBCO CC are presented. Finally, the numerical simulation based on the theoretical models has been conducted to reproduce the transient heat conduction process. The simulation results show that the transient heat conduction predicted by the Maxwell’s equivalent model show the best agreement compared with experimental results.

Superconducting multifilamentary N b 3 S n strand and cables have complex microstructure and macrostructure which are used in the International Thermonuclear Experimental Reactor (ITER) superconducting magnet system. In a generic form, the superconducting strand is made of copper, multifilamentary N b 3 S n and bronze which is a composite. The twisted composite strands form the superconducting cables with typical multi-stage structures. Here we propose a numerical model focusing on the problem of characterizing the effective properties of composite superconducting strand and cables at microscopic and macroscopic scales. The effective elastic constants of the composite superconducting strand can be derived by involving the combination of the representative volume element (RVE) and the Mori-Tanaka method. Subsequently, the proposed method is applied to capture the effects of temperature on the effective elastic constants of the superconducting multi-stage cables. Results show the elastic constants predicted by this model agree well with existing theoretical predictions and available experimental data. This numerical model may be utilized to optimize the mechanical properties of the composite superconducting strand by tuning the constituent fractions and to control the tensile stiffness of superconducting cables by tuning the winding laps of the cables.

Haze incorporating abundant charged aerosol has become a main air problem in various metropolitan cities worldwide. Studying the propagation characteristics of electromagnetic wave (EMW) in charged haze aerosol is conducive to calibrate instruments designed based on energy attenuation of EMW and is critical to providing guidance for deployment of network base stations. Here, we simulated haze conditions by quantitatively controlling the concentration, water content, and charge quantity of aerosols in laboratory and first found that attenuation coefficient of EMW changes linearly with these factors. In addition, we proposed a method to determine the complex dielectric constant of haze aerosols. Upon this, a new model, which combines the surface conductivity of a single particle at micro level with the charge density of the aerosol at macro level, was developed to quantitatively evaluate the contributions of aerosol concentration, water content and charges on the attenuation of EMW. The results show that when EMW propagates in charged haze aerosol, its attenuation coefficient can be quantitatively characterized by mass concentration, particle moisture and charge-to-mass ratio of aerosol. And the simulated results are in good agreement with the experiments.

Superconducting cable-in-conduct-conductors (CICC) are multi-stage structures with complex geometries. We study the contact behavior and tensile stiffness for a new design of CICC. The geometrical configurations of the new design of CICC is first formulated. Based on the superposition principle, a simple theoretical model is proposed to study the contact behavior of the new design of CICC. A wide range of numerical simulations have been performed by using finite element method. The contact force and tensile stiffness are analyzed by changing the winding laps of C u strand. Further, an extensive parametric study is conducted in which the effect of tensile stiffness on the contact properties are also considered. The results show the trend of the contact properties and tensile stiffnesses in the new design of CICC which can be used to optimize the superconducting CICC. This analytical and computational studies provide a guide for new design of CICC to improve the mechanical properties of CICC by controlling the winding patterns of each strand.

The experimental determination of the AC losses in a superconducting material or structure has been of long-standing interest to the academic community. The purposes of the current experiments are to explore the influences of the position of temperature sensor heating time and limit temperature rise on the measurement results with the calorimetric method. The optimization heating time and the limit temperature rise of this method are obtained. The present results are beneficial for the more precise measurements of the AC losses in hightemperature superconducting materials and structures.

AC loss is often considered one of the most important design parameters that guide the fabrication of superconducting devices. Thus, the electrometric method to determine AC loss of superconductors is crucial. Usually, the obtained voltage contains the resistive part and the inductive part. If the induced voltage is large, it will make the result very sensitive to the phase error, and hence, a compensating method is usually adopted. However, phase determination was still needed in the previous studies. In this work, we present an approach to precisely eliminate the unknown induced voltage of the sample just by alternating the inductance of the compensating coil. By determining the minimum of the sample voltage, where the induced voltage of the sample is precisely compensated, the AC loss is then able to be calculated without phase control. The principle of the presented method is strictly certified with the mathematic derivation, along with experimental proof. Current and frequency dependent inductance and loss resistance are obtained naturally. The result reveals that the ratio of induced voltage to resistive voltage decreases exponentially with an increase in current. We also found that the resistance of the CC is determined by both applied currents and frequencies. To universally describe these relations, the cycle resistance r is defined by an exponential function, allowing calculating AC loss with different frequencies just through the data of one frequency, which greatly facilitates the testing process. This method is also expected to be utilized in AC loss measurements for superconducting cables and coils.

Cracks caused by tensile stress in YBa2Cu3O7-x-coated conductors (YBCO-CCs) can cause irreversible degradation to their superconducting properties. Understanding the initiation and propagation modes of cracks can assist in preventing conductor failure and enhancing their mechanical properties in further. In this study, we used a chemical etching method and scanning electron microscopy to investigate crack morphology in the YBCO layer of conductors where the protective metal layer had been removed. For YBCO-CCs that experienced no deformation, many non-superconducting phase particles were observed and their grain size distribution corresponded to a Gaussian distribution. Energy dispersive x-ray spectroscopy identified these as Y-Cu-O particles. For the YBCO-CCs that experienced axial tension at 77 K, different propagation modes of cracks in the YBCO layer, including transgranular fracture, branching, deflection and pinning were observed for the first time. Statistical analysis demonstrated that transgranular fracture occurred in ∼ 95% of the crack modes. We analysed the reason for this phenomenon considering the thermal stresses stored inside and around the non-superconducting phase particles. The coefficient of thermal expansion of the Y2Cu2O5 particles is less than that of the YBCO superconducting matrix, and therefore, the hoop tensile stress generated near the boundary of the Y2Cu2O5 particle accelerates the bottom-up propagation of the crack. The other crack propagation modes such as crack branching, deflection, pinning and bridging in the YBCO layer can be considered mechanisms of blocking crack propagation that can increase the fracture toughness of the YBCO layer.

International Thermonuclear Experimental Reactor (ITER) hopes to achieve controllable thermonuclear fusion by constraining the motion of high temperature plasma by a strong magnetic field which is generated by superconducting magnets wound by the cable-in-conduit conductors (CICCs). The CICC structure is complex, usually with thousands of NbTi/Cu or Nb3Sn/Cu twisted in four or five stages. It operates in a complex environment of extremely low temperature, high current and strong magnetic field. The electromagnetic force generated by the superconducting material will cause deformation, reduce its characteristic parameters and bring hidden trouble to the safety of the device. Traditional studies on the contact force of the cross-section strand of CICC mostly adopt non-actual object photoelastic model in the experiment, and numerically adopt discrete element method (DEM) without particle deformation. However, it is difficult to give the contact force of the cross-section strand closer to the real working condition. In this experiment, the deformation characteristics were calculated between strands of the prototype of CICC by spraying speckle on the cross-section and using digital image correlation (DIC). Theoretically, taking the strand deformation obtained by DIC as the input parameter of granular element method (GEM), the distribution characteristics of the contact force of the CICC conductor under the action of transverse pressure are calculated, and the size and direction of the contact force chain are statistically analyzed.

Of crucial importance for the application of high temperature superconducting coated conductors (CCs) is the delamination strength due to thermal stress and Lorentz force during their operation. This paper reports the mechanical delamination strengths of YBCO CCs at room temperature and 77 K, as well as the electro-mechanical delamination strength form a modified anvil tensile method. Compared to previous measurement setups, our anvil device only has one degree of freedom of transverse tension. Moreover, we use a vertical soldering technique to ensure the data reliability. Since we eliminate the influence of the measurement method on the experimental results, a naturally discrete property of the delamination strength is obtained. To describe these experimental data, a three-parameter Weibull distribution function has been suggested and a new criterion for determining delamination strength is proposed from the reliability function, which can be conveniently referred to for engineering design.

The conductors used in large fusion reactors, e.g. ITER, CFETR and DEMO, are made of cable-in-conduit conductor (CICC) with large diameters up to about 50 mm. The superconducting and copper strands are cabled around a central spiral and then wrapped with stainless-steel tape of 0.1 mm thickness. The cable is then inserted into a jacket under tensile force that increases with the length of insertion. Because the cables are long and with a large diameter, the insertion force could reach values of about 40 kN. The large tensile force could lead to significant rotation forces. This may lead to an increase of the twist pitch, especially for the final one. Understanding the twist pitch variation is very important; in particular, the twist pitch of a cable inside a CICC strongly affects its properties, especially for Nb3Sn conductors. In this paper, a simplified numerical model was used to analyze the cable rotation, including material properties, cabling tension as well as wrap tension. Several rotation experiments with tensile force have been performed to verify the numerical results for CFETR CSMC cables. The results show that the numerical analysis is consistent with the experiments and provides the optimal cabling conditions for large superconducting cables.

We now report the delamination properties of soldered lap joints in YBa2Cu3O7-x coated conductors (YBCO CCs) without removing the metallic stabilizer. The delamination strength, current, and resistance behaviors in the lap joints under transverse tension are investigated at 77 K. It is found that the current shows no degradation (95% of the critical current $I_{c0} $ in original YBCO CC sample as a criterion) before a visible delamination, whereas there is no remarkable change in resistance of the lap joints. Moreover, the lap joint shows significant edge effects on the critical anvil strength, which can be raised as high as the original CC samples by adding solder fillets at the edge of the joint. In addition, the delamination of the YBCO layer (including Buffer layer) and Hastelloy in YBCO CC samples presented in literature is successfully simulated by using a 3-D ANSYS, and the enhancement of delamination strength in the lap joint reported in this paper is also interpreted by the simulation.

In this paper, we present a direct measurement of the transport AC loss in solder joint of the YBa 2Cu3O7−x -coated conductor (YBCO CC) at liquid nitrogen temperature. The method is based on measuring the temperature rise of the thermally insulated joint which is fabricated by soldering the YBCO CC samples face to face without removing the metallic stabilizer. The contact resistances in these joints are in the range of 10 to 40 n Ω. It is found that (1) the AC losses in the sample with lap joint dependence of I/I c can be described by use of a third-order power law, where I and I c denote the transport current peak and critical current of the YBCO CC sample, respectively. (2) The loss in sample with joint is greater than that of the original YBCO CC sample in case of I/I c0.7, the loss in sample with joint is less than that in the original tape.

The Coherent Gradient Sensor (CGS) system has many merits, such as full field, real time, nondestructive, vibration insensitivity, etc., which is developed to measure the curvatures and nonuniform curvatures changes of film-substrate systems in multiple media with different refractive indexes. The general calculation formula for the curvatures in multiple media are obtained. Take the CGS applied in two types of medium as an example, the presented theoretical analysis has been verified by experimental measurements when the specimen is immerged into water or silicone oil. Moreover, the influences of wave length on the measurement error of this system are reported.

In this paper, we present a solder method for the lap joint of the YBa2Cu3O7-x coated conductor (YBCO CC) without removing the metallic stabilizer. Several solder joints with the same overlap of 80 mm were manufactured. In the case of self-field and at liquid nitrogen temperature, the resistances of solder joints are equal to about 10 n Omega. In addition, the resistance is not influenced by the axial tension force and external magnetic field. The critical current degradation caused by the axially applied strain and external magnetic field exhibits consistent behavior of the original YBCO CC. At room temperature, the mechanical property of the specimen with joint is determined by the original tape. Moreover, at 77 K, the tension characteristic of the sample with and without joint exceeds that at room temperature.

Ultrasound-assisted soldering of Cu/Cu was conducted with a self-developed Ni-Sn composite solder. The microstructure and mechanical properties of Cu/Ni-Sn/Cu joints were investigated in terms of ultrasonic vibration time. Results indicated that, in the joint ultrasonically soldered for 5 s, (Ni,Cu) 3 Sn 4 was formed along the Ni metal foam and got contact with each other in local area while (Cu,Ni) 6 Sn 5 was fabricated at the interface of Cu substrate and Ni-Sn composite solder. Ni metal foam distributed randomly and a large amount of Sn remained in solder seam. Increasing the ultrasonic vibration time to 20 s, the amount of (Ni,Cu) 3 Sn 4 increased and a stripe layered structure composed of (Ni,Cu) 3 Sn 4 and Ni metal foam was obtained. The thickness of (Cu,Ni) 6 Sn 5 increased slightly and the amount of Sn decreased a little. Further increasing the ultrasonic vibration time to 30 s leaded to the volume increase of the stripe layered structure, which is composed of (Ni,Cu) 3 Sn 4 and Ni metal foam. The thickness change of (Cu,Ni) 6 Sn 5 was not obvious compared with its counterpart fabricated in 20 s while there still remained a small amount of Sn in solder seam. The joint ultrasonically soldered for 30 s exhibited the highest shear strength of 64 MPa and the fracture occurred in the solder seam near the (Cu,Ni) 6 Sn 5 intermetallic compound layer.

Ultrasound-assisted fluxless brazing of Zr based Bulk metallic glasses (Zr-BMG) joint using Zn-3Al filler metal was performed in this study. The effect of ultrasonic vibration time on the microstructure and mechanical properties of Zr-BMG joints were investigated. Results showed that excellent metallurgic bonding could be obtained in ultrasonically brazed Zr-BMG joints. The interfacial reaction between liquid Zn-3Al filler metal and Zr-BMG substrate showed a mutation characteristic, which could be distinguished into incubation period and acceleration period. In the incubation period, Zn50Zr25Al25 intermetallic compounds (IMCs) with small ellipsoidal shape were slowly formed and distributed randomly on Zr-BMG surface. However, in the acceleration period, Zn50Zr25Al25 ellipsoids developed rapidly into a wavy-structured IMCs layer with a thickness of 17 μm, which was comprised of alternate Zn50Zr25Al25 and Zn22Zr sublayers. The microstructure evolution of Zn-3Al/Zr-BMG interface was ascribed to the combined effects of acoustic cavitations and Al element controlled interfacial metallurgic reactions. The average shear strength of joint was increased firstly then decreased slightly with increasing ultrasonic vibration time, and a highest strength value of approximately 100 MPa was obtained for joints brazed for 96 s. The shearing failure was inclined to occur at the Zn-3Al/Zr-BMG interface then transferred into the interfacial IMCs layer with increasing ultrasonic vibration time.

Digital speckle correlation method (DSCM) is used to investigate fatigue evolution in the YBa2Cu3O7−x coated conductors (YBCO CCs) at room temperature. Damage analysis based on the DSCM has been performed to detect the damaged areas on the experimental samples. Two important parameters including stiffness variation (E) and dissipated energy per cycle (H) have been calculated for every fatigue test. Moreover, the superconducting specimens are subdivided into five sections with a distance of 1 cm between each of them, and the local critical currents (77 K and self-field) of fatigue and fatigue free are measured. The nonuniform current distributions are observed and can be interpreted qualitatively.

We have investigated the effect of the current sweep rate (CSR) on the vortex dynamic in the YBa 2 Cu 3 O 7– x coated conductors (YBCO CCs). It is found that the CSR has several effects on vortex motion in that it gives rise to enhancement of dissipation as the CSR decreases, significant time effects and instabilities are observed in current–voltage ( I – V ) and voltage–time ( V – t ) curves. Thus, the CSR on practical applications of the YBCO CCs can be optimized, and relevant CSR which is designing superconducting devices made by the YBCO CCs should be considered in future.

Magneto-optical microscopy for superconducting films is always involved with extreme environments, such as high vacuum and cryogenic temperature. In these applications, devices of high spatial resolution and compact focusing are often required. Besides, the calibration process is also of importance for quantitative measurement of magnetic fields. In this paper, we present a comprehensive module design for magneto-optical microscopy based on a combination of an infinity optical system and dynamic sealing technique. This technique gets rid of the limit of distance between the sample and the microscope, and also guarantees no influences of windows on image resolution. In addition, for the problem of uneven illumination, a practical calibration method is presented and verified through experimental results. As a result, the novel module and its calibration method make the technology of magneto-optical microscopy have wide applications.

When a material is immersed in a low-temperature medium, e.g., liquid nitrogen, its Young’s modulus and surface roughness will change as the temperature of the material decreases as a result of different friction behaviors. In this study, a high-precision friction test device was constructed to perform a detailed comparative study on the friction properties of a pure copper strand immersed in liquid nitrogen, air, and water. The force and displacement resolutions of the experimental system were as high as 0.01mN and 0.03μm, respectively. It was found that the stick-slip phenomenon in the liquid nitrogen was significant, while the slope of the stick-slip was larger than those observed in the air and water media. These experimental results were simulated using a spring-slider model that considered the influence of hydrophilicity on surface roughness. The roughness was shown to change the amplitude of the friction curve with time, while the slope of the stick-slip was dominated by the modulus’ magnitude variety.

Pinning induction in REBa2Cu3O7−x (RE = rare earth) films and coated conductors improves their critical current density (Jc). In general, however, interstitial elements, e.g., C and N, cannot act as effective pinning centers in REBa2Cu3O7−x materials because they typically lead to degeneration of the superconducting properties. In this work, the influence of graphene oxide (GO) on YBa2Cu3O7−x (YBCO) films was studied by measuring the critical temperature and angle-dependent critical current density. The results revealed that the concentration of GO in YBCO films modulated the critical temperature. The decay rate of Jc in H∥c field orientations decreased with decreasing temperature. At 35 K and 4.2 K, the YBCO/GO film exhibited a lower decay of Jc and less anisotropic behavior than the YBCO-BaMO3 (M = Zr, Hf) film. GO doping could, therefore, improve the pinning property of YBCO films below 35 K. This improvement could be due to the presence of nanosized carbon structures, which served as isotropic pinning centers at low temperature, as revealed by high-resolution transmission electron microscopy.Pinning induction in REBa2Cu3O7−x (RE = rare earth) films and coated conductors improves their critical current density (Jc). In general, however, interstitial elements, e.g., C and N, cannot act as effective pinning centers in REBa2Cu3O7−x materials because they typically lead to degeneration of the superconducting properties. In this work, the influence of graphene oxide (GO) on YBa2Cu3O7−x (YBCO) films was studied by measuring the critical temperature and angle-dependent critical current density. The results revealed that the concentration of GO in YBCO films modulated the critical temperature. The decay rate of Jc in H∥c field orientations decreased with decreasing temperature. At 35 K and 4.2 K, the YBCO/GO film exhibited a lower decay of Jc and less anisotropic behavior than the YBCO-BaMO3 (M = Zr, Hf) film. GO doping could, therefore, improve the pinning property of YBCO films below 35 K. This improvement could be due to the presence of nanosized carbon structures, which served as isotropic pinning...

We have measured the thermal stresses of the large grain bulk YBaCuO superconductors which had been cooled in a zero magnetic field. During the cooling process from room temperature to 30 K, the axial thermal stress, radial stress, and hoop stress have been obtained by using the strain gauges which had been directly mounted onto the sample surface. It has been found that the maximum tension stress is over 30 MPa along the c axis and the maximum compression stress along the ab axis is over 700 MPa, which should not be neglected when the mechanical properties of these superconductors are considered. The multiphysics analysis software COMSOL has been used to simulate the thermal stresses during the cooling process; it has been confirmed that the large thermal stresses of the superconductors are mainly governed by the thermal expansion coefficients.