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High temperature superconductors (HTSs) are enablers of extensive electrification for aircraft propulsion. Indeed, if used in electrical machines, HTS materials can drastically improve their performance in terms of the power-to-weight ratio. Among the different topologies of superconducting electrical machines, a flux modulation machine based on HTS bulks is of interest for its compactness and light weight. Such a machine is proposed in the FROST (Flux-barrier Rotating Superconducting Topology) project led by Airbus to develop new technologies as part of their decarbonization goals driven by international policies. The rotor of the machine will house large ring-segment-shaped HTS bulks in order to increase the output power. However, the properties of those bulks are scarcely known and have barely been investigated in the literature. In this context, the present work aims to fill out partially this scarcity within the framework of FROST. Thus, a thorough characterisation of the performances and homogeneity of 11 large REBaCuO bulks was carried out. Ten of the bulks are to be utilized in the machine prototype, originally keeping the eleventh bulk as a spare. A first set of characterisation was conducted on the eleven bulks. For this set, the trapped field mapping and the critical current were estimated. Then, a series of in-depth characterisations on the eleventh bulk followed. It included critical current measurement, X-ray diffraction, and scanning electron microscopy on different millimetre-size samples cut out from the bulk at various locations. The X-ray diffraction and scanning electron microscopy showed weakly oxygenated regions inside the bulk explaining the local drop or loss in superconducting properties. The objective was to determine the causes of the inhomogeneities found in the trapped field measured on all the bulks, sacrificing one of them, here the spare one. To help obtain a clearer picture, a numerical model was then elaborated to reproduce the field map of the eleventh bulk using the experimental data obtained from the characterisation of its various small samples. It is concluded that further characterisations, including the statistics on various bulks, are still needed to understand the underlying reasons for inhomogeneity in the trapped field. Nonetheless, all the bulks presented enough current density to be usable in the construction of the proposed machine. [ABSTRACT FROM AUTHOR]

High temperature superconductors (HTSs) are enablers of extensive electrification for aircraft propulsion. Indeed, if used in electrical machines, HTS materials can drastically improve their performance in terms of the power-to-weight ratio. Among the different topologies of superconducting electrical machines, a flux modulation machine based on HTS bulks is of interest for its compactness and light weight. Such a machine is proposed in the FROST (Flux-barrier Rotating Superconducting Topology) project led by Airbus to develop new technologies as part of their decarbonization goals driven by international policies. The rotor of the machine will house large ring-segment-shaped HTS bulks in order to increase the output power. However, the properties of those bulks are scarcely known and have barely been investigated in the literature. In this context, the present work aims to fill out partially this scarcity within the framework of FROST. Thus, a thorough characterisation of the performances and homogeneity of 11 large REBaCuO bulks was carried out. Ten of the bulks are to be utilized in the machine prototype, originally keeping the eleventh bulk as a spare. A first set of characterisation was conducted on the eleven bulks. For this set, the trapped field mapping and the critical current were estimated. Then, a series of in-depth characterisations on the eleventh bulk followed. It included critical current measurement, X-ray diffraction, and scanning electron microscopy on different millimetre-size samples cut out from the bulk at various locations. The X-ray diffraction and scanning electron microscopy showed weakly oxygenated regions inside the bulk explaining the local drop or loss in superconducting properties. The objective was to determine the causes of the inhomogeneities found in the trapped field measured on all the bulks, sacrificing one of them, here the spare one. To help obtain a clearer picture, a numerical model was then elaborated to reproduce the field map of the eleventh bulk using the experimental data obtained from the characterisation of its various small samples. It is concluded that further characterisations, including the statistics on various bulks, are still needed to understand the underlying reasons for inhomogeneity in the trapped field. Nonetheless, all the bulks presented enough current density to be usable in the construction of the proposed machine.

We present the results of a study of the superconducting and paramagnetic properties of polycrystalline Y1–xGdxBa2Cu3O7–δ samples. The critical current density and critical temperature of YBCO were weakly decreased by the Gd doping. A peak effect, which is a nonmonotonic dependence of the critical current density on magnetic field, was detected for all samples. The peak position shifted to higher magnetic fields with increasing Gd content. This behavior is opposite to the shift of the peak effect observed for other YBCO compounds doped by magnetic ions. This unusual behavior is apparently related to the realized granular structure instead of the type of doping ion. A correlation between the peak position and the granule size was found in the investigated samples and other polycrystalline YBCO compounds. [ABSTRACT FROM AUTHOR]

We present the results of a study of the superconducting and paramagnetic properties of polycrystalline Y1–xGdxBa2Cu3O7–δ samples. The critical current density and critical temperature of YBCO were weakly decreased by the Gd doping. A peak effect, which is a nonmonotonic dependence of the critical current density on magnetic field, was detected for all samples. The peak position shifted to higher magnetic fields with increasing Gd content. This behavior is opposite to the shift of the peak effect observed for other YBCO compounds doped by magnetic ions. This unusual behavior is apparently related to the realized granular structure instead of the type of doping ion. A correlation between the peak position and the granule size was found in the investigated samples and other polycrystalline YBCO compounds. [ABSTRACT FROM AUTHOR]

Bulk high-temperature superconducting materials can trap magnetic fields up to an order of magnitude larger than conventional permanent magnets. Recent advances in pulsed field magnetization (PFM) techniques now provide a fast and cost-effective method to magnetize bulk superconductors to fields of up to 5 T. We have developed a portable, desktop bulk high-temperature superconducting magnet system by combining advanced PFM techniques with state-of-the-art cryocooler technology and single-grain, REâ€"Baâ€"Cuâ€"O [(RE)BCO, where RE is a rare-earth element or yttrium] bulk superconducting materials. The base temperature of the system is 41 K and it takes about 1 h for the system to cool down to 50 K from room temperature. A capacitor bank, combined with easily-interchangeable, solenoid- or split-type copper magnetizing coils and an insulated bipolar gate transistor acting as a high-speed switch, allows magnetic pulses to be generated with different pulse profiles. The system is capable of trapping magnetic fields of up to ∼3 T. In this work, we report the results of the magnetization of a range of single-grain Yâ€"Baâ€"Cuâ€"O, Euâ€"Baâ€"Cuâ€"O and Gdâ€"Baâ€"Cuâ€"O (GdBCO), bulk superconducting discs using this system. A higher trapped field was recorded using a split coil incorporating iron yokes at temperatures of 65 K and above, whereas at lower temperatures, a higher trapped field was obtained using the solenoid coil. The GdBCO sample achieved the highest trapped field for both single-pulse (SP) and two-stage-multi-pulse (TSMP) methods using the solenoid coil. Maximum trapped fields of 2.26 T at 55 K and 2.85 T at 49 K were recorded at the centre of the top surface of the GdBCO sample for the SP and TSMP methods, respectively. The PFM process is substantially an adiabatic process so, therefore, the thermal contact between the sample and sample holder is of critical importance for cooling the bulk sample during application of the pulse. The design of the sample holder can be modified easily to enhance the thermal stability of the sample in order to achieve a higher trapped field. [ABSTRACT FROM AUTHOR]

The effect of ball milling on the microstructure and some superconducting parameters such as flux pinning force (Fp) and critical current density (Jc) of the oleic acid (C18H34O2) added MgB2 bulk samples produced by two different methods, was analysed in this article. In the first method, ball milling was applied to the samples produced by using boron (B) powders coated with carbon (C) released from oleic acid. In the second method, oleic acid was mixed with magnesium (Mg) and B powders at the same time and then the same ball milling process used in the first method was applied to the powder mixture. The structural, magnetic and electrical properties of the produced samples were analysed. The results showed that ball milling process enhances the homogeneity of the structure, decreases the grain size and improves the grain connectivity of MgB2. Also, C substitution into MgB2 lattice resulting in an increase in electron scattering and disorders, enhances after ball milling process. It supplies a significant increase in Jc at high fields and causes a slightly decrease in transition temperature (Tc), especially for the samples produced by using the first method. Because the first method supports the ball milling effect on the homogenous dispersion of oleic acid addition in the MgB2 structure and the C entrance to MgB2 lattice, acting as pinning centres, the best Jc and Fp values at high fields was obtained for the produced samples with second method. [ABSTRACT FROM AUTHOR]

Gd1212 (GdSr2RuCu2O8-y) is a superconductor, isostructural to Y123 (YBa2Cu3O7-x), whose high interest is due to the coexistence of superconducting and magnetic ordered phases in the same unit cell. Up to now the best superconducting properties for Gd1212 have been achieved by TSG (Top-Seeded Melt-Textured growth), a well-known technique used to grow high quality samples from High Temperature Superconductors characterized by a peritectic melting reaction. Due to the relevant production of liquid during the melting of Y123-like materials, the standard TSG procedure involves the addition of secondary phases in the starting powders mixture, generally the peritectic solid phase or one of its precursors, to enhance the structural stability of pellets during the process. This practice also provides additional pinning centers in the final samples, improving their Superconducting features. Unfortunately, Gd1212 peritectic reaction produces an insufficient amount of liquid phase, so preventing from obtaining wide crystallographic domains, and the addition of the peritectic solid phase Gd1210 (GdSr2RuO6), although beneficial on the superconducting properties, even worsen this problem. In this work we tried to grow larger Gd1212 domains by inverting the usual approach: i.e., by adding in the precursor powders some peritectic liquid phase (CuO) instead of Gd1210. Anyway, previous analyses suggested us that the chemistry of Gd1212 melting may change for excess CuO mixtures. We have characterized the obtained samples by means of Raman spectroscopy and mapping, data were compared with those of Gd1210-doped and pure Gd1212 samples, results confirm our hypothesis of different melting reaction for CuO rich samples. [ABSTRACT FROM AUTHOR]

In this contribution, the usage of a pyramid-like buffer stack for optimization of the top-seeded melt growth process of GdBCO (GdBa 2 Cu 3 O 7-δ) single-domain superconductors is demonstrated. The process was optimized by the addition of SmBCO into GdBCO/Ag into the buffer precursor powder, which effectively increased the peritectic temperature of the final mixed powder in the pyramid-like buffer stack. As such, this approach can be also used for other REBCO systems by selecting rare earth with a higher peritectic temperature and adding it to the buffer pellets. The thermal behavior of the powders was studied by STA. This enables fast and effective crystal growth through the buffer stack and the formation of large effective seeds. This approach also allows for reliable and precise tuning of growth angle and together with effective seed size facilitates tuning of a - b to c sector ratio. The phase composition of precursors and prepared bulks were studied by XRD. The microstructure was also studied by SEM and OM in detail. Subsequently, levitation force and trapped field were measured. Results confirmed that the addition of at least a single buffer pellet improved the melt growth process significantly by acting as a large effective seed. Superconducting properties of the samples grown by pyramid-like buffer stack are superior in comparison to reference TSMG samples. [ABSTRACT FROM AUTHOR]

Superconducting and paramagnetic contributions to the magnetization of polycrystalline Y1−xHoxBa2Cu3O7−δ samples were investigated. The superconductivity is responsible for a partial screening of magnetic ions from an external magnetic field and for a possible sinking of antiferromagnetic correlations between these ions. Magnetic moments of Ho ions influence on a peak effect induced by the order–disorder transition of the Abrikosov vortex lattice. The critical current density and the critical temperature of YBCO are not changed by the Ho doping. [ABSTRACT FROM AUTHOR]

High temperature superconducting (HTS) magnetic levitation (maglev) has the potential as a high-speed passenger transportation mode owing to its passive stabilization. Our group is constructing a 1500 m-long HTS maglev high speed test line. As a kind of passenger transport, safety and comfort are primary factors for its operation quality. However, the weak damping in HTS maglev system is not enough to suppress the vibration. As a preliminary study of the test line, this article investigates an additional suspension system aiming to improve the dynamic characteristics of HTS maglev through experiments and simulation. We first built an HTS maglev model vehicle composed of a levitation frame and a car body. The dynamic responses were tested based on a dynamic measurement system. Second, a dynamic model of this kind of maglev device was established and the influence of suspension parameters on its vibration performance is analyzed theoretically. Third, this suspension system was applied to an HTS maglev engineering prototype through numerical simulations. Experimental and simulation results manifested that the suspension system can effectively reduce the vibration, especially with high frequencies. This article provides a reference for the design of HTS maglev vehicles. [ABSTRACT FROM AUTHOR]

Induced currents in the bulk superconducting cylinder of finite length placed in a transverse magnetic field are calculated by the partial and complete separation of variables. The Fourier transform over the angular and axial coordinates reduces the problem to the solution of one-dimensional integral equations. The harmonic balance method is applied to account for the highly nonlinear constitutive relation between the voltage and current. The current and field distribution is compared with the results of FEA. The approximation of the current distribution by a set of cylindrical and disc layers is presented. The magnetic moment of induced currents is analyzed in the increasing applied field when the cylinder is magnetized up to the state of saturation. [ABSTRACT FROM AUTHOR]

A high-temperature superconducting (HTS) magnetic levitation (maglev), based on YBaCuO bulk superconductors and permanent magnet guideways, shows the potential to be applied in the future rail transportation. The maglev forces (levitation and guidance force) of the HTS maglev system, which bond the vehicle and the guideway, are the key factors of its dynamics studies. In practical operation, the HTS maglev vehicle moves in both the vertical and lateral directions, but the previous force models are in single degree of freedom. In this article, an experiment was designed to study the vertical–lateral coupled effect of the levitation and guidance force. A 2-D coupling model was proposed to fit the experimental data. Furthermore, the motion stability and the dynamical characteristics of the HTS maglev system in 2-D space were studied. The simulation results show that the vertical–lateral coupling effect in maglev forces cannot be neglected, and one direction motion can affect the motion in the other direction. It also shows that the proposed force model is suitable for HTS maglev dynamics’ calculations. [ABSTRACT FROM AUTHOR]

The electromagnetic behaviour of the Maglev systems should be predicted to figure out the induced current and trapped flux distribution inside the high temperature superconductors (HTS) and thus to optimize the design of Maglev systems. In the present study, firstly the magnetic flux density distributions of single-PM and double-PM permanent magnetic guideways (PMGs) have been obtained numerically by using magneto-static solution in AC/DC Module in COMSOL Multiphysics package. Then the trapped magnetic flux and induced current inside the HTS have been determined under the non-uniform magnetic field via the Partial Differential Equation (PDE) Mode in COMSOL by using H-formulation. Obtained numerical results in this study showed that different PMG configurations have various magnetic field characteristic and this situation can improve the magnetic and supercurrent properties of the HTS in Maglev system. [ABSTRACT FROM AUTHOR]

This paper discusses some mechanical results from experiments on Superconducting Magnetic Bearings (SMBs) consisting of HTS tapes wound in a ring and a permanent magnet as could be applied to larger rotating machines and ultra-low temperature submerged motors. The wound-ring superconductors are made of 12 mm-width HTS tapes. The authors estimated the spring constant, k , after measuring the thrust force, as 3.85 × 103 N/m, for three-layered wound HTS tapes, and 2.44 × 103 N/m, for the single-wound HTS tape, respectively. The rotating losses for speeds of 500–4500 rpm in the constructed SMB system were also estimated and they were less than 10 W in the constructed measurement system. [ABSTRACT FROM AUTHOR]

• We developed a novel calculation method to account for the reflection of the fluxoid in magnetic field calculations for superconducting bulk magnets. • The error in the B z calculation using the proposed method was below 0.05 T at distances of more than 13 mm from the bulk center. • We expect the proposed technique to be more readily applicable and precise than existing models for magnetic field calculations of superconducting bulk magnets. Highly precise magnetic field calculations must be conducted for superconducting bulk magnets at least 3 mm above the surface for cryocooler-cooling applications; in practice, calculations must be conducted for those at a distance of 5 mm from the surface. However, the reflection of the fluxoid affects the precision of the magnetic field calculations for superconducting bulk magnets but this effect has yet to be accounted for. Here, this effect was approximated and incorporated into the magnetic field calculations for superconducting bulk magnets. The numerical results were then compared with the experimental results. The data shows that the error was less than 4% at distances above 3 mm from the bulk surface in the magnetic field calculations performed by the new method. It is expected that the calculation error can be corrected in future studies by comparing the extrapolative semi-empirical model, like that presented here, with mesoscopic calculation models. These findings will be useful in the design of devices containing highly homogeneous superconducting magnets. [ABSTRACT FROM AUTHOR]

Taking a technology from the laboratory to industry is a long and resource-consuming process. Discovered more than a century ago, the phenomenon of superconductivity is testament to this process. Despite the promise of this technology, currently the only major use of superconductors outside the laboratory is in MRI machines. The advent of high-temperature superconductors in 1986 heralded a new dawn. Machines which do not require cooling with liquid helium are a very attractive target. A myriad range of different superconductors were rapidly discovered over the next decade. This process of discovery continues to this day with, most recently, a whole new class, the pnictides, being discovered in 2006. Many different usages have been identified, including in motors, generators, wind turbines, fault current limiters, and high-current low-loss cables. This Special Issue looks at some of the different factors which will help to realise these devices and thereby bring about a superconducting world

The mechanical stress of the electromagnetic hoop stress, σFCM θ, during field-cooled magnetization (FCM) from 20 T, and the thermal hoop stress, σcool θ, under cooling, in REBaCuO disk and ring bulks reinforced by stainless steel (SUS) ring with various widths, were calculated using numerical simulations. The reinforcement effect of the SUS ring was investigated for each bulk. For the ring bulk, themaximum of σFCM θ was reduced with increasing SUS ring width, from 216 MPa for without SUS ring reinforcement, down to 93 MPa for using SUS ring with 20 mm in width, was less than that of the disk bulk. The numerical results showed that the stress concentration in the ring bulk could be alleviated by the SUS ring reinforcement. We suggest that the width of SUS ring needs to be at least 10 mm to prevent the fracture behavior of the bulk. The optimal design of the reinforcement ring should be constructed so that the total hoop stress, σtotal θ (= σFCM θ + σcool θ ), could be less than the fracture strength of the bulk during FCM. [ABSTRACT FROM AUTHOR]

Polycrystalline Y 1− x Nd x Ba 2 Cu 3 O 7−δ ( x =0.02, 0.11, and 0.25) superconductors are synthesized. Nd atoms are uniformly distributed over grains. The magnetization loops of the samples have a pronounced second peak in a wide temperature range. The magnetization data are analyzed using the extended critical state model. It is found that the order-disorder transition of the vortex lattice is affected by doping with neodymium and temperature; the second-peak field and width decrease monotonically with increasing x value. The undoped polycrystalline YBa 2 Cu 3 O 7−δ compound is assumed to exhibit the peak effect in higher magnetic fields. [ABSTRACT FROM AUTHOR]

The fabrication of (RE)-Ba-Cu-O bulk superconductors, where RE is a rare-earth element such as Y, Gd and Sm, is both time consuming and expensive due to the complexity of the melt process and the slow growth rate of large, single grains. In this study, different approaches to the fabrication of bar-shaped, bulk YBCO superconductors are investigated and compared using single- and multiple-seeding techniques via top-seeded melt growth (TSMG). Both the microstructural and superconducting properties of the bulk samples are investigated, including trapped field, critical current density, critical temperature and levitation force. The results of this study indicate that, in general, the superconducting properties of YBCO fabricated by a single-seeded process are significantly better than those of samples fabricated by a four-seeded process for non-bridge seeds. The differences between the samples are less pronounced in the levitation force measurements, however. In this paper, we attempt to explain the reasons for the similarities and differences observed between bulk samples fabricated by the different single- and multi-seeded processes. [ABSTRACT FROM AUTHOR]

Large single-grain bulk high-temperature superconducting materials can trap high magnetic fields in comparison with conventional permanent magnets, making them ideal candidates to develop more compact and efficient devices, such as actuators, magnetic levitation systems, flywheel energy storage systems, and electric machines. However, macrosegregation of Y-211 inclusions in melt-processed Y–Ba–Cu–O (YBCO) limits the macroscopic critical current density of such bulk superconductors, and hence, the potential trapped field. A new fabrication technique using graded precursor powders has recently been developed by our research group, which results in a more uniform distribution of Y-211 particles, in order to further improve the superconducting properties and trapped field capability of such materials. In this paper, experimental results on the pulsed field magnetization of a graded single-grain YBCO bulk sample are presented. Pulsed fields of magnitude up to 5.5 T were applied to the sample, at temperatures of 40 and 65 K. The trapped field profiles indicate that magnetic flux enters the sample in a more uniform manner than standard YBCO samples, resulting in a higher overall trapped field and indicating the success of this processing technique to produce a more homogeneous sample. [ABSTRACT FROM PUBLISHER]

We report on significant correlations between the levitation-suspension forces and critical current densities in bulk YBCO/Ag composite superconductors fabricated by infiltration and growth processing (IGP) technique. Based on our studies, we have found that there is a strong correlation between various parameters obtained from levitation and suspension force curves and the magnetic Js calculated from M-H loops using the standard Bean's model-type approaches. Significantly, the levitation force gap and the critical current density of the bulk sample are in direct correlation. We also find that it is possible to qualitatively predict the Js of samples from the levitation and suspension force curves by using this cost-effective measurement technique as a preliminary tool. These correlations provide a novel approach to rapid selection and characterization of bulk superconducting samples for their employment in practical applications. [ABSTRACT FROM AUTHOR]

High temperature superconducting (HTS) maglev technology is becoming more and more mature, and many key technologies have been deeply studied. However, the transition curve plays a key role in HTS maglev system, and related studies have not been carried out. In this paper series of simulations were conducted to test the lateral and vertical vibration of HTS maglev when passing through curves. Two magnetic guideways, of which one has transition curves but the other does not, are designed to test the vibration characteristics of a mini HTS maglev model running though curves. Results show that after adding transition curves between straight line and circular curve the vibration of HTS maglev model in lateral and vertical directions are all weakened in different degrees. It proves that adding transition curve into HTS maglev system is favorable and necessary. [ABSTRACT FROM AUTHOR]

Flux pinning and thermally assisted flux flow are studied in a (Ba $$_{0.6}$$ K $$_{0.4})$$ Fe $$_{2}$$ As $$_{2}(T_\mathrm{c}$$ =38.3 K) bulk samples in magnetic fields up to 18 T via ac susceptibility measurements. Ac susceptibility curves shift to higher temperatures as the frequency is increased from 75 to 1,997 Hz in all fields. The frequency ( $$f)$$ shift of the susceptibility curves is modeled by the Anderson-Kim Arrhenius law $$f = f_{0}\mathrm{exp}(-E_\mathrm{a}{ /kT})$$ to determine flux activation energy $$E_\mathrm{a}/k$$ as a function of ac field $$H_\mathrm{ac}$$ and dc magnetic flux density $$\mu $$ $$_\mathrm{0} H_\mathrm{dc}$$ . $$E_\mathrm{a}/k$$ ranges from 8,822 K (761 meV) at $$\mu $$ $$_{0} H_{dc}$$ = 0 T to 1,100 K (95 meV) at 18 T for $$H_\mathrm{ac}=$$ 80 A/m (1 Oe). The energies drop very quickly in a non-linear manner as $$\mu $$ $$_{0} H_\mathrm{dc}$$ increases from 0 to 1 T, and more gradually, in a linear-like manner, as $$\mu $$ $$_{0} H_\mathrm{dc}$$ increases further to 18 T, suggesting some kind of vortex transition. For ac fields of 400 A/m (5 Oe) and higher, the Arrhenius model starts breaking down, at around $$\mu $$ $$_{0} H_{ \mathrm dc}$$ = 2 T. As the dc magnetic flux density increases further, this breakdown becomes significant for $$\mu _{0} H_\mathrm{dc}$$ = 15 and 18 T at ac fields of 400 A/m and higher. Extensive mapping of the de-pinning, or irreversibility, lines shows broad dependence on the magnitude of the ac field, frequency, in addition to the dc magnetic flux density. [ABSTRACT FROM AUTHOR]

Small-sized hybrid magnet (HM) operated in liquid nitrogen was constructed. It consists of two superconducting Y-Ba-Cu-O annuli, placed on soft magnetic cores made of permenduire. The magnetization of the superconducting annuli was realized by a pulsed method using two copper coils installed near the annuli. The trapped field value of 2.2 T with the radial ( r ≈ 5 mm) homogeneity of 0.1 % can be achieved in the gap of 2 mm between magnetic cores. The magnetization procedure and time evolution of the trapped field are also discussed. The magnets of similar type are promising for application in compact electron spin resonance (ESR) spectrometers. [ABSTRACT FROM AUTHOR]