REBa_(2)Cu_(3)O_(x)(REBCO)coated conductors exhibit irreversible critical current(I_(c))degradation under tensile strain beyond the irreversible strain(ε_(irr)).While this degradation is attributed to cracking,the sp...REBa_(2)Cu_(3)O_(x)(REBCO)coated conductors exhibit irreversible critical current(I_(c))degradation under tensile strain beyond the irreversible strain(ε_(irr)).While this degradation is attributed to cracking,the specific crack evolution process and its quantitative relation to I_(c)degradation remain unclear.Here,the multiplication of cracks in REBCO tapes under tensile is characterized.It was found that surface particles disrupt continuous crack propagation,causing fragmentation of the REBCO layer.A shear lag model describes how the stress distribution evolves with fragmentation spacing.A probabilistic fracture model then predicts the two-regime spacing reduction as a function of applied strain.The discontinuous crack network acts as the residual flow channel for current once strain exceeds𝜀ε_(irr).Crack spacing determines the remaining I_(c)in this regime.An analytical model is thus proposed to capture the I_(c)degradation behavior based on the quantified fragmentation process.Additionally,this study suggests approaches to obtain a wider safe operating strain range through tailoring of the fracture behavior,expanding the practical application limits.This work provides new insights into the progressive cracking mechanisms governing irreversible I_(c)degradation in strained REBCO tapes.展开更多
The integration of REBCO superconducting coils into compact tokamak toroidal field magnets represents a transformative advancement in fusion technology.In the fabrication of these magnets,electrical connections betwee...The integration of REBCO superconducting coils into compact tokamak toroidal field magnets represents a transformative advancement in fusion technology.In the fabrication of these magnets,electrical connections between pancake coils are typically established through a transverse bridging joint,the performance of which critically impacts device efficiency.To address the demand for enhanced connectivity,this study presents a novel bridge-type"stacked joint"utilizing a superconducting-copper composite structure,optimized for inter-coil connections of stacked REBCO cables.Leveraging a modular stacking approach and low-temperature soldering,this design achieves a resistance as low as 1 nΩover a 10 cm length alongside a soldering resistivity below 25 nΩcm^(2)in a liquid nitrogen(77 K)self-field environment.The study further investigates the impact of tape length and quantity on joint electrical performance under this configuration.Rigorous testing,including 10 thermal cycles(77 K to room temperature)and pressure loads up to 30 MPa,confirms the joint’s structural reliability and the consistency of the fabrication process,underscoring its potential for fusion applications.展开更多
Enhancing the mechanical strength of highly conductive pure metals usually causes significant reduction in their electrical conductivity.For example,introducing phase/matrix interfaces or more grain boundaries,are com...Enhancing the mechanical strength of highly conductive pure metals usually causes significant reduction in their electrical conductivity.For example,introducing phase/matrix interfaces or more grain boundaries,are common and effective methods to strengthen metals.But it simultaneously increases the electron scattering at the interface,thus reducing the electrical conductivity.In this study,we demonstrate that pure aluminum(Al)/carbon nanotubes(CNTs)nanocomposites prepared by friction stir processing have successfully broken through these limitations.The yield strength and tensile strength of Al/CNTs nanocomposites have improved by 104.7%and 51.8%compared to pure Al,while the electrical conductivity remained comparable to that of pure Al.To explore the potential mechanisms,the interface between CNTs and Al was examined and characterized by transmission electron microscopy(TEM)and Raman spectroscopy.Little interfacial reaction compounds were present and no visible physical gaps were observed at CNTs and Al interfaces.We defined it as a clean and tightly bonded interface.Although the quantity of phase interface has increased,the electrical conductivity of the nanocomposite remains approximately unchanged.We attribute the preserved electrical conductivity to the clean and tightly bonded CNTs/Al interface in the nanocomposite.展开更多
The slitting process for manufacturing REBa2Cu3O7δ(REBCO,RE=Rare earth)tapes of required width significantly improves the production efficiency and reduces production costs.However,edge cracks induced by the slitting...The slitting process for manufacturing REBa2Cu3O7δ(REBCO,RE=Rare earth)tapes of required width significantly improves the production efficiency and reduces production costs.However,edge cracks induced by the slitting process of wide REBCO tapes may cause premature degradation under high tensile stress in highfield magnets.Therefore,it is necessary to evaluate the effect of edge cracks of REBCO tapes on the critical current(Ic)degradation.Firstly,Ic degradation under artificial cracks was measured to validate the applicability of linear elastic fracture mechanics for the REBCO layer.The maximum circumferential stress criterion was used to derive the mixed-mode stress intensity factor of multiple oblique edge cracks.A semi-analytical model considering edge crack properties such as angleβ,spacing d,and length a,was built to evaluate the critical load and critical crack.We found that when the stress intensity factor at the crack tip is below KIC?2:3 MPa ffiffiffiffim p,edge cracks did not propagate.We examined commercial REBCO tapes manufactured by two different processes,concluding that edge cracks in these tapes will not cause premature degradation.展开更多
REBa_(2)Cu_(3)O_(7−x)(REBCO)coated conductors,owing to its high tensile strength and current‐carrying ability in a background field,are widely regarded a promising candidate in high‐field applications.Despite the gr...REBa_(2)Cu_(3)O_(7−x)(REBCO)coated conductors,owing to its high tensile strength and current‐carrying ability in a background field,are widely regarded a promising candidate in high‐field applications.Despite the great potentials,recent studies have highlighted the challenges posed by screening currents,which are featured by a highly nonuniform current distribution in the superconducting layer.In this paper,we report a comprehensive study on the behaviors of screening currents in a compact REBCO coil,specifically the screeningcurrent‐induced magnetic fields and strains.Experiments were carried out in the self‐generated magnetic field and a background field,respectively.In the self‐field condition,the full hysteresis of the magnetic field was obtained by applying current sweeps with repeatedly reversed polarity,as the nominal center field reached 9.17 T with a maximum peak current of 350 A.In a background field of 23.15 T,the insert coil generated a center field of 4.17 T with an applied current of 170 A.Ultimately,a total center field of 32.58 T was achieved before quench.Both the sequential model and the coupled model considering the perpendicular field modification due to conductor deformation are applied.The comparative study shows that,for this coil,the electromagnetic–mechanical coupling plays a trivial role in self‐field conditions up to 9 T.In contrast,with a high axial field dominated by the background field,the coupling effect has a stronger influence on the predicted current and strain distributions.Further discussions regarding the role of background field on the strains in the insert suggest potential design strategies to maximize the total center field.展开更多
基金supported by the National Natural Science Foun-dation of China under Grant No.52407029,No.52277026,and No.12327901.
文摘REBa_(2)Cu_(3)O_(x)(REBCO)coated conductors exhibit irreversible critical current(I_(c))degradation under tensile strain beyond the irreversible strain(ε_(irr)).While this degradation is attributed to cracking,the specific crack evolution process and its quantitative relation to I_(c)degradation remain unclear.Here,the multiplication of cracks in REBCO tapes under tensile is characterized.It was found that surface particles disrupt continuous crack propagation,causing fragmentation of the REBCO layer.A shear lag model describes how the stress distribution evolves with fragmentation spacing.A probabilistic fracture model then predicts the two-regime spacing reduction as a function of applied strain.The discontinuous crack network acts as the residual flow channel for current once strain exceeds𝜀ε_(irr).Crack spacing determines the remaining I_(c)in this regime.An analytical model is thus proposed to capture the I_(c)degradation behavior based on the quantified fragmentation process.Additionally,this study suggests approaches to obtain a wider safe operating strain range through tailoring of the fracture behavior,expanding the practical application limits.This work provides new insights into the progressive cracking mechanisms governing irreversible I_(c)degradation in strained REBCO tapes.
基金National MCF Energy R&D Program under Grant 2022YFE03150103in part by CTRFR project(Compact-Tokamak based repetitive Reconnection-heated Fusion Reactor)from Shaanxi Startorus Fusion Technology Co.,Ltd.
文摘The integration of REBCO superconducting coils into compact tokamak toroidal field magnets represents a transformative advancement in fusion technology.In the fabrication of these magnets,electrical connections between pancake coils are typically established through a transverse bridging joint,the performance of which critically impacts device efficiency.To address the demand for enhanced connectivity,this study presents a novel bridge-type"stacked joint"utilizing a superconducting-copper composite structure,optimized for inter-coil connections of stacked REBCO cables.Leveraging a modular stacking approach and low-temperature soldering,this design achieves a resistance as low as 1 nΩover a 10 cm length alongside a soldering resistivity below 25 nΩcm^(2)in a liquid nitrogen(77 K)self-field environment.The study further investigates the impact of tape length and quantity on joint electrical performance under this configuration.Rigorous testing,including 10 thermal cycles(77 K to room temperature)and pressure loads up to 30 MPa,confirms the joint’s structural reliability and the consistency of the fabrication process,underscoring its potential for fusion applications.
文摘Enhancing the mechanical strength of highly conductive pure metals usually causes significant reduction in their electrical conductivity.For example,introducing phase/matrix interfaces or more grain boundaries,are common and effective methods to strengthen metals.But it simultaneously increases the electron scattering at the interface,thus reducing the electrical conductivity.In this study,we demonstrate that pure aluminum(Al)/carbon nanotubes(CNTs)nanocomposites prepared by friction stir processing have successfully broken through these limitations.The yield strength and tensile strength of Al/CNTs nanocomposites have improved by 104.7%and 51.8%compared to pure Al,while the electrical conductivity remained comparable to that of pure Al.To explore the potential mechanisms,the interface between CNTs and Al was examined and characterized by transmission electron microscopy(TEM)and Raman spectroscopy.Little interfacial reaction compounds were present and no visible physical gaps were observed at CNTs and Al interfaces.We defined it as a clean and tightly bonded interface.Although the quantity of phase interface has increased,the electrical conductivity of the nanocomposite remains approximately unchanged.We attribute the preserved electrical conductivity to the clean and tightly bonded CNTs/Al interface in the nanocomposite.
基金the Strategic Priority Research Program of Chinese Academy of Sciences under Grant No.XDB25000000the National Natural Science Foundation of China under Grant No.52007089.
文摘The slitting process for manufacturing REBa2Cu3O7δ(REBCO,RE=Rare earth)tapes of required width significantly improves the production efficiency and reduces production costs.However,edge cracks induced by the slitting process of wide REBCO tapes may cause premature degradation under high tensile stress in highfield magnets.Therefore,it is necessary to evaluate the effect of edge cracks of REBCO tapes on the critical current(Ic)degradation.Firstly,Ic degradation under artificial cracks was measured to validate the applicability of linear elastic fracture mechanics for the REBCO layer.The maximum circumferential stress criterion was used to derive the mixed-mode stress intensity factor of multiple oblique edge cracks.A semi-analytical model considering edge crack properties such as angleβ,spacing d,and length a,was built to evaluate the critical load and critical crack.We found that when the stress intensity factor at the crack tip is below KIC?2:3 MPa ffiffiffiffim p,edge cracks did not propagate.We examined commercial REBCO tapes manufactured by two different processes,concluding that edge cracks in these tapes will not cause premature degradation.
基金supported by the National MCF Energy R&D Program under Grant No.2022YFE03150103the National Natural Science Foundation of China(NSFC)under Grant No.52277026the BK21 FOUR program of the Education and Research Program for Future ICT Pioneers,Seoul National University in 2023.
文摘REBa_(2)Cu_(3)O_(7−x)(REBCO)coated conductors,owing to its high tensile strength and current‐carrying ability in a background field,are widely regarded a promising candidate in high‐field applications.Despite the great potentials,recent studies have highlighted the challenges posed by screening currents,which are featured by a highly nonuniform current distribution in the superconducting layer.In this paper,we report a comprehensive study on the behaviors of screening currents in a compact REBCO coil,specifically the screeningcurrent‐induced magnetic fields and strains.Experiments were carried out in the self‐generated magnetic field and a background field,respectively.In the self‐field condition,the full hysteresis of the magnetic field was obtained by applying current sweeps with repeatedly reversed polarity,as the nominal center field reached 9.17 T with a maximum peak current of 350 A.In a background field of 23.15 T,the insert coil generated a center field of 4.17 T with an applied current of 170 A.Ultimately,a total center field of 32.58 T was achieved before quench.Both the sequential model and the coupled model considering the perpendicular field modification due to conductor deformation are applied.The comparative study shows that,for this coil,the electromagnetic–mechanical coupling plays a trivial role in self‐field conditions up to 9 T.In contrast,with a high axial field dominated by the background field,the coupling effect has a stronger influence on the predicted current and strain distributions.Further discussions regarding the role of background field on the strains in the insert suggest potential design strategies to maximize the total center field.
