Bulk Superconductors Research Articles

Stability model of bulk HTS field pole of a synchronous rotating machine under load conditions

High-temperature superconducting (HTS) bulks act as magnets with trapped magnetic flux and potentially exhibit high magnetic field performance, superior to that of permanent magnets. In the fourth industrial revolution, HTS magnets could potentially facilitate the development of highly efficient and lightweight motors and generators for propulsion. To evaluate and verify the durability of HTS rotating machines, we developed a radial-gap-type rotating machine whose field-pole modules comprised HTS bulks. We examined the stability of the trapped magnetic flux of the field poles during the machine’s operation. The rotating machine was operated with different loads, and the magnetic field associated with the trapped flux of the HTS bulks was analyzed. The analysis was performed for different loads and revolution speeds, and the long-term operation for a constant load was investigated. The trapped flux of the bulk field poles and the output power were observed to be almost constant for over 360 h, and the HTS bulk magnet’s surface temperature hardly changed below 40 K during the machine’s operation. Furthermore, the magnetic flux decay determined from magnetic flux measurements before and after a load test was less than 1%. An analysis of the magnetic field distribution around the HTS bulk revealed that the transverse magnetic flux flowed through the magnetic guide of the rotor core. These results indicate the potential synchronous machine application of the high trapped magnetic flux of the field-pole modules accumulated by the appropriate positioning the HTS bulks three-dimensionally.

Relationship between charge density wave, filamentary superconductivity and bulk superconductivity in ZrTe3

We investigated the relationship between a charge density wave (CDW) with a transition temperature of , filamentary superconductivities (SCs) along the a-axis and the b-axis with transition temperatures (’s) of , and a bulk SC with a below 2.5 K in ZrTe3, by measuring electrical resistance and magnetization for several crystals synthesized at different growth temperatures. As the growth temperature increased, the decreased, the of the bulk SC increased, and the ’s of the filamentary SCs did not show a clear change. The anisotropy of the upper critical field was different between the filamentary SCs and the bulk SC. These differences in the growth temperature dependence of and the anisotropy of indicate that the filamentary SCs are unrelated to the bulk SC. Moreover, the growth temperature dependence of the transition temperatures indicates that the CDW has a competitive relationship with the bulk SC, but a non-competitive coexistence with the filamentary SCs. Based on the anisotropies of , we also infer that the filamentary SCs and the bulk SC occur on quasi-one-dimensional Fermi surfaces and a three-dimensional Fermi surface, respectively, and that the relationship between the three states in ZrTe3 cannot be explained solely in terms of conventional competition for a Fermi surface.

Investigation of Superconducting Properties and Possible Nematic Superconductivity in Self‐Doped BiCh2‐Based Superconductor CeOBiS1.7Se0.3

The superconducting properties and possible nematic superconductivity of the self‐doped BiCh2‐based (Ch: S, Se) superconductor CeOBiS1.7Se0.3 are investigated through measurements of the in‐plane anisotropy of magnetoresistance (MR). Single crystals of CeOBiS1.7Se0.3 are grown using a flux method. The single‐crystal structural analysis reveals that the crystal structure at room temperature is tetragonal (P4/nmm). Bulk superconductivity with a transition temperature of 3.3 K is observed through electrical resistivity and magnetization measurements. Investigation of the anisotropy of the upper critical field suggests relatively low anisotropy in the crystal compared to that of other BiCh2‐based superconductors. In the superconducting states of CeOBiS1.7Se0.3, a twofold symmetric in‐plane anisotropy of MR is observed, which indicates the in‐plane rotational symmetry breaking in the superconducting states of CeOBiS1.7Se0.3.

Reliable 4.8 T trapped magnetic fields in Gd–Ba–Cu–O bulk superconductors using pulsed field magnetization

A robust and reliable in-situ magnetization method is essential for exploiting the outstanding magnetic flux trapping ability of bulk superconductors in practical applications. We report a 4.8 T peak trapped magnetic field, B T, achieved at 30 K in a 36 mm diameter GdBa2Cu3O7-δ –Ag bulk superconductor using pulsed field magnetization (PFM). To realize this, we have developed a reliable two-step multi-pulse PFM process based on understanding and exploiting the avalanche-like flux jump phenomenon observed in these materials. The magnitude of the applied pulsed magnetic field (B a) necessary to trap 4.8 T was merely 5.29 T, corresponding to a remarkable magnetization efficiency (B T/B a) of 90%.

Use of Bulk Superconductors in the Excitation System of Low-Speed Synchronous Generators

This work focuses on implementing HTS bulks in the excitation circuit of low-speed synchronous generators as permanent magnets. In addition to the implementation study, it is also addressed to remagnetize HTS bulks in electric generators. To reduce the maintenance costs and time required for the magnetization process, an in-loco remagnetization is used. The HTS bulks are magnetized in their position inside the machine's magnetic circuit. A pulse-field magnetization (PFM) technique is used, using the stator coils of a linear generator. YBCO and GdBCO samples are tested. The experimental procedure starts with the in-loco PFM to magnetize the HTS bulks and, by applying an oscillatory movement to the linear generator's moving part, measuring the magnetic flux density induced voltage is done. Experimental tests show that, for the FeSi core used in the generator, the YBCO material is most suitable for the excitation circuit.

A conceptual study of a high gradient trapped field magnet (HG-TFM) toward providing a quasi-zero gravity space on Earth

In this work, we propose a new concept of a high gradient trapped field magnet (HG-TFM). The HG-TFM is made from (RE)BaCuO bulk superconductors, in which slit ring bulks (slit-TFMs) are tightly stacked with TFM cylinders (full-TFMs), and state-of-the-art numerical simulations were used to investigate the magnetic and mechanical properties in detail during and after magnetization. A maximum value of the magnetic field gradient product of = 6040 T2 m−1 was obtained after conventional field cooled magnetization (FCM) with an applied field, B app, of 10 T of the HG-TFM with 60 mm in outer diameter and 10 mm in inner diameter. This value may be the highest value ever reported compared to any other magnetic sources. The value increased with decreasing inner diameter of the HG-TFM and with increasing B app during FCM. The electromagnetic stress in the HG-TFM during the FCM process mainly results from the hoop stress along the circumferential direction. The simulations suggested that there is no fracture risk of the bulk components during FCM from 10 T in a proposed realistic configuration of the HG-TFM where both TFM parts are mounted in Al-alloy rings and the whole HG-TFM is encapsulated in a steel capsule. A quasi-zero gravity space can be realized in the HG-TFM with a high value in an open space outside the vacuum chamber. The HG-TFM device can act as a compact and cryogen-free desktop-type magnetic source to provide a large magnetic force and could be useful in a number of life/medical science applications, such as protein crystallization and cell culture.

Higgs-like pair amplitude dynamics in superconductor–quantum-dot hybrids

We consider a quantum dot weakly tunnel coupled to superconducting reservoirs. A finite superconducting pair amplitude can be induced on the dot via the proximity effect. We investigate the dynamics of the induced pair amplitude after a quench and under periodic driving of the system by means of a real-time diagrammatic approach. We find that the quench dynamics is dominated by an exponential decay towards equilibrium In contrast, the periodically driven system can sustain coherent oscillations of both the amplitude and the phase of the induced pair amplitude in analogy to Higgs and Nambu-Goldstone modes in driven bulk superconductors.

Improved mechanical properties through recycling of Y-Ba-Cu-O bulk superconductors

The success rate of the growth of single grain Y–Ba–Cu–O (YBCO) bulk superconductors, applied typically as trapped field magnets, by melt processing techniques is often rather low. To ensure that the rare earth elements used in these materials are not wasted, and to improve the economics of production, therefore, an effective way of recycling such “failed” samples is essential for the long-term sustainability of the bulk process. In the present paper, a detailed study of the microstructural and mechanical properties of recycled YBCO samples has been made and their properties compared to those of primary grown samples. Recycled YBCO samples exhibit trapped magnetic fields of ∼ 70 – 80 % of those observed in primary grown samples. Microstructural investigations reveal a significant reduction in the porosity and a simultaneous improvement in the distribution of Y2BaCuO5 second phase inclusions throughout the microstructure of recycled, single grain YBCO samples. This means that, while the superconducting properties of the recycled samples are generally inferior to those of primary grown samples, somewhat surprisingly the mechanical properties such as flexural strength, hardness and tensile strength of the recycled samples can be improved. The recycled YBCO samples studied here exhibited an average flexural strength of 75 MPa, which is almost 50 % higher compared to primary grown YBCO, which has significant implications for their use in practical applications that involve large mechanical forces, such as levitation. Good correlations were observed between the microstructural parameters and measured mechanical properties in both primary grown and recycled YBCO samples.

Numerical Simulations on the Vertical Dynamic Characteristics of High-Temperature Superconducting Bulk

High-temperature superconducting (HTS) maglev, owing to its unique self-stability characteristic, has a wide range of application prospect in flywheel energy storage, magnetic levitation bearing, rail transportation, and other fields. As the important foundation of the engineering application, researches on the dynamic characteristics of HTS maglev have attracted more and more attention. As most of existing models adopted the power exponential function to approximately fit the relationship between the force and displacement, which can only qualitatively analyze the dynamic characteristics of HTS maglev, this paper employs the flux flow and creep model to deduce a 2D H formulation of the HTS maglev system to make up for this deficiency. Moreover, related experiments were also carried out to validate the accuracy of this simulation model and comparisons with other previous models have been achieved. The results prove that the simulation model can reduce the calculation time still with a good convergence. Then, an electromagnetic-thermal-force multiphysics coupling model was established to analyze the dynamic characteristics, especially the levitation height drift of an HTS bulk above the permanent magnet guideway. Results indicate that when the superconductor has an initial velocity that causes disturbance at the working position, vibration and drift phenomena will occur, and the vertical levitation drift also grows as the velocity increases. The simulation results also show that resonance will occur if the excitation frequency is close to the HTS maglev system’s resonance frequency, and a strong “beat” phenomenon will occur if the excitation frequency is close to twice the main vibration frequency of the system. Additionally, the HTS maglev system shows good anti-vibration ability on the relatively low-frequency region as well as the high-frequency region, which proves that it can be well applied to the rail transportation field. All results could be supported as references for the design of HTS maglev systems and its future application.

Физические принципы создания магнитолевитационных систем на основе высокотемпературных сверхпроводящих композитов второго поколения (Обзор)

An overview of experimental and theoretical studies of the characteristics of maglev systems using high-temperature superconductors (HTSC) is presented. Materials used in maglev technologies, namely bulk superconductors and HTSC tape composites, are considered. The main experimental data obtained on both bulk and tape superconductors assembled in stacks of various configurations are demonstrated. The factors influencing the magneto-force characteristics are analyzed: geometric parameters, the influence of external alternating magnetic fields, temperatures, relaxation phenomena. A significant part of the review is devoted to the description of various methods for calculating maglev systems, including those based on stacks of HTSC composites. The features of thermal processes in maglev systems with cryocooler and nitrogen cooling are considered. General recommendations for the creation of optimal maglev systems based on tape HTSC composites are given.

Fabrication process and superconducting properties of recycling multi-domain GdBCO bulk superconductors using improved infiltration technique

High temperature superconductor has become one of the hotspots of research, because of its high critical temperature, strong trapped flux density, stable suspension characteristics and large magnet levitation force. The single domain REBa<sub>2</sub>Cu<sub>3</sub>O<sub>7–<i>δ</i></sub> (REBCO) superconductors have the wide and potential applications in the high-tech fields, such as micro-magnet superconducting maglev train, superconducting motor and superconducting magnetic separation system. However, a large number of multi-domain samples are easy to produce in the preparation process, which leads the success rate to decrease significantly and the cost to increase considerably, which restricts its practical application process. Inspired by the top seeded infiltration growth method, we develop a reliable method of recycling failed GdBCO sample by re-supplementing the liquid phase lost in the primary growth process and pretreating the failed sample as solid phase source billets. We recycle a series of GdBCO samples by using this new technique successfully. The growth morphology, superconducting properties, and microstructures of the recycled GdBCO bulk superconductors are investigated in detail in this study. The results show that the magnetic levitation forces of the recycled GdBCO samples are all greater than 30 N, their magnetic flux densities are all above 0.3 T, and their capture efficiencies are above 60%. These results provide the scientific basis and new ideas for developing the low cost and high efficient yield of fabrication of the REBCO bulk superconductors.

Unconventional superconductivity as a quantum Kuramoto synchronization problem in random elasto-nuclear oscillator networks

We formulate the problem of unconventional d − wave superconductivity, with phase fluctuations, pseudogap phenomenon, and local Cooper pairs, in terms of a synchronization problem in random, quantum dissipative, elasto-nuclear oscillator networks. The nodes of the network correspond to localized, collective quadrupolar vibrations of nuclei-like, elastic inhomogeneities embedded in a dissipative medium. Electrons interacting with such vibrations form local Cooper pairs, with a superfluid d − wave pseudogap Δ PG , due to an effective, short range attractive interaction of character. Phase coherent, bulk superconductivity, with a d − wave gap Δ, is stabilized when the oscillator network is asymptotically entangled in a nearly decoherence-free environment. Phase coherence will in turn be destroyed, at T c , when the thermal noise becomes comparable to the coupling between oscillators, the superfluid density K. The 2Δ/k B T c ratio is a function of Kuramoto’s order parameter, , for the loss of synchronization at K c , and is much larger than the nonuniversal 2Δ PG /k B T * ratio, where T * is the temperature at which Δ PG is completely destroyed by thermal fluctuations. We discuss our findings in connection to the available data for various unconventionally high-temperature superconductors.

Synthesis and characterization of the bulk YBCO-target of superconducting material

Superconducting devices have emerged as promising candidates, due to their outstanding sensitivity, such as wideband and high frequency operation, and Josephson tunnelling junction detectors with high sensitivity which have been developed and applied to THz imaging. This paper presents the details of synthesizing the d-wave superconductor. We applied a standard method of solution-gel combustion to form the high-temperature yttrium barium copper oxide (YBCO) of superconductor materials. To realize bulk superconductor materials, it was used the calcination temperature of 850 °C, and the sintering temperature about 950 °C for 5 h. For characterizing YBCO, we could be realized the critical transition temperature ( T c ) of about 93 K and conventional R-T dependencies. The formation of the characterizations structural showed that YBCO samples have an orthorhombic crystal structure. As well as the morphology and structure samples tested according to characterize the X-ray diffraction (XRD) analysis data and SEM (scanning electron microscopy).

Exotic Superconducting States in Transition Metal Tellurides

Nb2Pd3Te5 was found to show bulk superconductivity at 3.3 K. In contrast, Ta2Pd3Te5, a 5d analogue of Nb2Pd3Te5, showed nonmetallic electronic conduction, which was changed to show bulk superconductivity by chemical doping.

Angle-Dependence of the Levitation Force From a Frustum-Shaped Magnet and Recessed Superconducting Bulk

The widespread application of superconducting magnetic levitation bearings is limited by their relatively low stiffness. Recently we investigated a novel thrust bearing geometry comprised of a conical frustum (or truncated cone) shaped permanent magnet levitating inside a matching tapered hole machined into a high-temperature superconductor bulk. This configuration was found to produce superior restoring forces and stiffness compared to the conventional cylindrical magnet and superconductor arrangement. Here, using H-formulation finite-element simulations, we evaluate the angle-dependence of the frustum on the levitation force. We find that the optimal angle is not universal, but depends on the mode of displacement as well as the frustum dimensions. Correlations with the incident magnetic flux are identified for estimating the angle best suited to the operating regime of the bearing.

Quasi-one-dimensional TaSe3: A New Topological Superconductor Candidate

Topological superconductors are a new type of quantum matter with the combination of superconductivity and nontrivial electronic topology. Such a unique class of material has become one of the hottest research topics in condensed matter physics, due to its rich scientific implications (predicted to host exotic emergent particles such as Majorana bound states) and considerable application potentials in fault-tolerant quantum computation. Up to date, only very few materials are experimentally verified as candidates of topological superconductors. Searching for new material system is always the focus in this intriguing field. Recently, Chen et al., for the first time, observed both nontrivial topological surface states and bulk superconductivity in a quasi-one-dimensional compound TaSe₃, suggesting a promising new candidate of topological superconductors. Moreover, unlike other existing candidates, the simultaneously combination of stoichiometric, exfoliable, and air-stable properties in TaSe₃ makes it an ideal platform for both fundamental studies and further applications.

CsV_{3}Sb_{5}: A Z_{2} Topological Kagome Metal with a Superconducting Ground State.

Recently discovered alongside its sister compounds KV_{3}Sb_{5} and RbV_{3}Sb_{5}, CsV_{3}Sb_{5} crystallizes with an ideal kagome network of vanadium and antimonene layers separated by alkali metal ions. This work presents the electronic properties of CsV_{3}Sb_{5}, demonstrating bulk superconductivity in single crystals with a T_{c}=2.5 K. The normal state electronic structure is studied via angle-resolved photoemission spectroscopy and density-functional theory, which categorize CsV_{3}Sb_{5} as a Z_{2} topological metal. Multiple protected Dirac crossings are predicted in close proximity to the Fermi level (E_{F}), and signatures of normal state correlation effects are also suggested by a high-temperature charge density wavelike instability. The implications for the formation of unconventional superconductivity in this material are discussed.

Application of Hot Press Bending for Shaping a Stack of HTS Tapes Operating as a Trapped Flux Magnet

Trapped-flux magnets made of stacks of superconducting tapes are an interesting and important alternative to the application of rare-earth permanent magnets and trapping flux in bulk superconductors. Many applications, such as electric motors, can require the complex shapes of magnetic elements to combat issues such as demagnetization during the operation. Therefore, a reliable production technique of high temperature superconductor (HTS) stacks with different geometries is required to meet the needs of industry. In this article, a method of shaping the epoxy-glued stacks of superconducting tapes is proposed and investigated. The application of hot press allows to easily change the shape of the stack. The influence of shaping the stack on the density of trapped magnetic flux as well as critical current are presented. Experimental and numerical analyses are performed and their results are matched to find the change in the superconducting properties of the tapes. They show that with the proper process parameters the stacks can be shaped with only minor decrease of performance. However, strong degradation is observed if the applied force is too high. The observed problems are discussed and the possible solutions are proposed.

Unconventional Bulk Superconductivity in YFe_{2}Ge_{2} Single Crystals.

Sharp superconducting transition anomalies observed in a new generation of single crystals establish that bulk superconductivity is intrinsic to high purity YFe_{2}Ge_{2}. Low temperature heat capacity measurements suggest a disorder and field dependent residual Sommerfeld coefficient, consistent with disorder-induced in-gap states as expected for a sign-changing order parameter. The sevenfold reduction in disorder scattering in these new crystals to residual resistivities ≃0.45 μΩ cm was achieved using a new liquid transport growth technique, paving the way for multiprobe experiments investigating the normal and superconducting states of YFe_{2}Ge_{2}.

Relativistic magnetic lensing of electron beams using superconducting spheres

In this paper, the theoretical design of magnetic electron lens made of macroscopic superconducting spheres in rotation is proposed. The relativistic quantum behavior of an electron in the magnetic field induced by rotating bulk superconductors is described based on the Dirac equation, and electron beam transport is manifested in specific cases. Based on this approach, the focal length of magnetic electron lens made of macroscopic superconductors is theoretically calculated. Subsequently, the characterization of relativistic electron beam manipulating capability of newly designed magnetic electron lens is carried out by variation of the system parameters. Owing to the ability to focus the relativistic electron beam in versatile and precise manner, this approach offers a new way to study a macroscopic system of superconductors based on special relativity, and it sets a milestone in the construction of a new generation of magnetic electron lenses with superconducting spheres. The model is inherited and developed based on previous calculations of the field induced by spherical and cylindrical superconductors (L.B. Holdeman, NASA TR R-443 (1975); R.G. Rystephanick, Can. J. Phys. 51 (1973) 789–794) and the quantum theory of electron propagation through an electron lens (R. Jagannathan, Phys. Rev. A 42 (1990) 6674).