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Trapped Field Enhancement of a Thin, High-Jc MgB2 Bulk without Flux Jumps using Pulsed Field Magnetization with a Split-type Coil with a Soft Iron Yoke
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Superconductors with persistent zero-resistance currents serve as permanent magnets for high-field applications requiring a strong and stable magnetic field, such as magnetic resonance imaging. The recent global helium shortage has quickened research into high-temperature superconductors (HTSs)—materials that can be used without conventional liquid-helium cooling to 4.2 K. Herein, we demonstrate that 40-K-class metallic HTS magnesium diboride (MgB2) makes an excellent permanent bulk magnet, maintaining 3 T at 20 K for 1 week with an extremely high stability (<0.1 ppm/h). The magnetic field trapped in this magnet is uniformly distributed, as for single-crystalline neodymium-iron-boron. Magnetic hysteresis loop of the MgB2 permanent bulk magnet was detrmined. Because MgB2 is a simple-binary-line compound that does not contain rare-earth metals, polycrystalline bulk material can be industrially fabricated at low cost and with high yield to serve as strong magnets that are compatible with conventional compact cryocoolers, making MgB2 bulks promising for the next generation of Tesla-class permanent-magnet applications.
Pulsed field magnetization (PFM) was performed at Ts=14 K for the double stacked MgB2 bulks (bulk pair) 55 mm diameter fabricated by a reactive liquid Mg infiltration (Mg-RLI) method, compared with that for the single bulk. The trapped field of Bz=0.80 T was achieved between two bulks and Bz at the bulk surface was enhanced from 0.42 T to 0.50 T by stacking of the bulks. The trapped field characteristics of the bulk pair can be qualitatively explained by the model analyses.
Bulk superconducting MgB2 samples, 20 mm in diameter, were
prepared by hot-pressing of ball-milled Mg and B powders using
fine-grained boron powders. High maximum trapped fields of B0
= 5.4 T were obtained at 12 K in one of the investigated trapped field
magnets (height 8 mm) at the centre of the bulk surface. Investigating
the temperature dependence of the trapped field for short
MgB2 samples (height ≤1.6 mm), trapped fields of up to
B0 = 3.2 T at 15 K were achieved. These high trapped fields
are related to extremely high critical current densities of up to
106 A cm-2 at 15 K, indicating strong
pinning due to nanocrystalline MgB2 grains. Expected trapped
field data for long trapped field magnets prepared from the available
MgB2 material are estimated.
Pulsed field magnetization (PFM) was performed for
the MgB2 bulk 30 mm in diameter and the applied pulsed-field
dependence of the trapped field BC
T, temperature rise ΔT and
time dependence of the local field BC
L (t) were measured on the
bulk. BC
T = 0.71 T was achieved at 14 K after applying the
pulsed field of Bex = 1.55 T, where ΔT of about 10 K was
observed. Numerical simulation of PFM was also performed and
BC
T,ΔT andBC
L (t) were simulated. The results of the simulation
reproduced the experimental ones qualitatively. The flux dynamics
and the heat generation in the MgB2 bulk during PFM were in
clear contrast with those in REBaCuO bulks because of a small
specific heat, large thermal conductivity, and narrow temperature
margin against the transition temperature Tc.
Pulsed field magnetization (PFM) was performed for the first time on a
large MgB2 bulk 50 mm diameter fabricated by a reactive
liquid Mg infiltration (Mg-RLI) method, and the time dependence of the
local field BL{}C(t) and the trapped field
profiles were measured. The trapped field of Bz=0.47 T and
the total trapped flux of Φ=0.50 mWb were achieved at
Ts=23 K and both values decreased with increasing temperature
Ts. The experimental results can be qualitatively reproduced
by numerical simulation using electromagnetic and thermal fields for
PFM. The flux dynamics and the heat generation/propagation in the
MgB2 bulk during PFM were in clear contrast with those in
REBaCuO superconducting bulks because of the large thermal conductivity,
small specific heat, and narrow temperature margin against the
transition temperature Tc.
A trapped field of over 3 T has been measured at 17.5 K in a magnetised stack
of two disc-shaped bulk MgB2 superconductors of diameter 25 mm and thickness
5.4 mm. The bulk MgB2 samples were fabricated by uniaxial hot pressing, which
is a readily scalable, industrial technique, to 91% of their maximum
theoretical density. The macroscopic critical current density derived from the
trapped field data using the Biot-Savart law is consistent with the measured
local critical current density. From this we conclude that critical current
density, and therefore trapped field performance, is limited by the flux
pinning available in MgB2, rather than by lack of connectivity. This suggests
strongly that both increasing sample size and enhancing pinning through doping
will allow further increases in trapped field performance of bulk MgB2.
MgB$_{2}$ superconducting bulks have promising potential as trapped field magnets (TFMs). We have achieved a trapped field of B$_{z}$ = 1.1 T on a high-J$_{c}$ MgB$_{2}$ bulk at 13 K without flux jumps by pulsed field magnetization (PFM) using a split-type coil with a soft iron yoke, which is a record-high trapped field by PFM for bulk MgB$_{2}$ to date. The flux jumps, which frequently took place using a solenoid-type coil during PFM, were avoided by using the split-type coil, and the B$_{z}$ value was enhanced by the insertion of soft iron yoke. The flux dynamics and heat generation/propagation were analyzed during PFM using a numerical simulation, in which the magnetic flux intruded and attenuated slowly in the bulk and tended to align along the axial direction due to the presence of soft iron yoke. The advantages of the split-type coil and the simultaneous use of a soft iron yoke are discussed.
Investigating and predicting the magnetization of bulk superconducting materials and developing practical magnetizing techniques is crucial to using them as trapped field magnets (TFMs) in engineering applications. The pulsed field magnetization (PFM) technique is considered to be a compact, mobile and relative inexpensive way to magnetize bulk samples, requiring shorter magnetization times (on the order of milliseconds) and a smaller and less complicated magnetization fixture; however, the trapped field produced by PFM is generally much smaller than that of slower zero field cooling (ZFC) or field cooling (FC) techniques, particularly at lower operating temperatures. In this paper, the PFM of two, standard Ag-containing Gd-Ba-Cu-O samples is carried out using two types of magnetizing coils: 1) a solenoid coil, and 2) a split coil, both of which make use of an iron yoke to enhance the trapped magnetic field. It is shown that a significantly higher trapped field can be achieved using a split coil with an iron yoke, and in order to explain these how this arrangement works in detail, numerical simulations using a 2D axisymmetric finite element method based on the H-formulation are carried to qualitatively reproduce and analyse the magnetization process from both electromagnetic and thermal points of view. It is observed that after the pulse peak significantly less flux exits the bulk when the iron core is present, resulting in a higher peak trapped field, as well as more overall trapped flux, after the magnetization process is complete. The results have important implications for practical applications of bulk superconductors as such a split coil arrangement with an iron yoke could be incorporated into the design of a portable, high magnetic field source/magnet to enhance the available magnetic field or in an axial gap-type bulk superconducting electric machine, where iron can be incorporated into the stator windings to 1) improve the trapped field from the magnetization process, and 2) increase the effective air-gap magnetic field.
Pulsed field magnetization (PFM) of a high-J
c MgB2 bulk disk has been investigated at 20 K, in which flux jumps frequently occur for high pulsed fields. Using a numerical simulation of the PFM procedure, we estimated the time dependence of the local magnetic field and temperature during PFM. We analyzed the electromagnetic and thermal instability of the high-J
c MgB2 bulk to avoid flux jumps using the time dependence of the critical thickness, d
c(t), which shows the upper safety thickness to stabilize the superconductor magnetically, and the minimum propagation zone length, l
m(t), to obtain dynamical stability. The values of d
c(t) and l
m(t) change along the thermally-stabilized direction with increasing temperature below the critical temperature, T
c. However, the flux jump can be qualitatively understood by the local temperature, T(t), which exceeds T
c in the bulk. Finally, possible solutions to avoid flux jumps in high-J
c MgB2 bulks are discussed.
A melt-processed Sm-Ba-Cu-O was magnetized by pulsed-fields at 30–77 K. The trapped field distribution on the sample was measured by scanning a Hall sensor after magnetization. We found that flux lines penetrated into the sample by flux jump below 70 K. As a result, the trapped field of the sample reached more than 70% of the applied field, which can not be expected in the static zero field cooling. The maximum trapped field of 3.8 Tesla was obtained by the improved IMRA (Iteratively Magnetizing pulsed-field operation with Reducing Amplitude) method.
We have studied the effects of Ti doping on the magnetic properties of MgB2 superconducting bulks. The trapped field, which was obtained by field-cooled magnetization, &${B}_{{\rm{T}}}^{\mathrm{FC}}$;, was about 3.6 T at 13 K at the surface of the single Ti5–20% doped MgB2 bulks, which was about 1.3 times larger than that of the non-doped bulk. The &${B}_{{\rm{T}}}^{\mathrm{FC}}$; of 4.6 T was achieved at 14.1 K in the centre of the doubly stacked Ti-doped MgB2 bulks. The remanent magnetic flux density, which corresponds to the trapped field by zero-field cooled magnetization, &${B}_{{\rm{T}}}^{\mathrm{ZFC}}$;, was comparable with the absolute value of coercive force with a very small vortex creep rate of about 2% over 40 h. These results suggested that the MgB2 bulk was an excellent ‘quasi-permanent’ magnet. The critical current density, &${J}_{{\rm{c}}}$;, under magnetic field was also enhanced by Ti doping; under 3 T at 20 K, the &${J}_{{\rm{c}}}$; of &$4.0\times {10}^{3}$; A cm−2 for the pristine sample was enhanced to that of 1.6–1.8 &$\times \;{10}^{4}$; A cm−2 for the Ti-doped samples. The irreversibility field exceeded 5 T at 20 K for the Ti-doped samples. The existence of nanometric unreacted B and strongly Mg-deficient Mg-B particles and TiB2 layers at the periphery of Ti precipitates was suggested in the Ti-doped bulks by microscopic observation. The improvement of the vortex pinning properties in Ti-doped MgB2 originated from the creation of the nanometric nonsuperconducting particles and TiB2 layers acting as strong vortex pinning centres.
MgB2 in bulk form shows great promise as trapped field magnets (TFMs) as an alternative to bulk (RE)BCO materials to replace permanent magnets in applications such as rotating machines, magnetic bearings and magnetic separation, and the relative ease of fabrication of MgB2 materials has enabled a number of different processing techniques to be developed. In this paper, a comparison is made between bulk MgB2 samples fabricated by the hot isostatic pressing (HIP), with and without Ti-doping, and infiltration growth (IG) methods and the highest trapped field in an IG-processed bulk MgB2 sample, Bz = 2.12 at 5 K and 1.66 T at 15 K, is reported. Since bulk MgB2 has a more homogeneous Jc distribution than (RE)BCO bulks, studies on such systems are made somewhat easier because simplified assumptions regarding the geometry and Jc distribution can be made, and a numerical simulation technique based on the 2D axisymmetric H -formulation is introduced to model the complete process of field cooling (FC) magnetization. As input data for the model, the measured Jc(B,T) characteristics of a single, small specimen taken from each bulk sample are used, in addition to measured specific heat and thermal conductivity data for the materials. The results of the simulation reproduce the experimental results extremely well: (1) indicating the samples have excellent homogeneity, and (2) validating the numerical model as a fast, accurate and powerful tool to investigate the trapped field profile of bulk MgB2 discs of any size accurately, under any specific operating conditions. Finally, the paper is concluded with a numerical analysis of the influence of the dimensions of the bulk sample on the trapped field.
This paper presents a topical review of the current state of the art in modelling the magnetization of bulk superconductors, including both (RE)BCO (where RE = rare earth or Y) and MgB 2 materials. Such modelling is a powerful tool to understand the physical mechanisms of their magnetization, to assist in interpretation of experimental results, and to predict the performance of practical bulk superconductor-based devices, which is particularly important as many superconducting applications head towards the commercialization stage of their development in the coming years. In addition to the analytical and numerical techniques currently used by researchers for modelling such materials, the commonly used practical techniques to magnetize bulk superconductors are summarized with a particular focus on pulsed field magnetization (PFM), which is promising as a compact, mobile and relatively inexpensive magnetizing technique. A number of numerical models developed to analyse the issues related to PFM and optimise the technique are described in detail, including understanding the dynamics of the magnetic flux penetration and the influence of material inhomogeneities, thermal properties, pulse duration, magnitude and shape, and the shape of the magnetization coil(s). The effect of externally applied magnetic fields in different configurations on the attenuation of the trapped field is also discussed. A number of novel and hybrid bulk superconductor structures are described, including improved thermal conductivity structures and ferromagnet–superconductor structures, which have been designed to overcome some of the issues related to bulk superconductors and their magnetization and enhance the intrinsic properties of bulk superconductors acting as trapped field magnets. Finally, the use of hollow bulk cylinders/tubes for shielding is analysed.
The ability to generate a permanent, stable magnetic field unsupported by an electromotive force is fundamental to a variety of engineering applications. Bulk high temperature superconducting (HTS) materials can trap magnetic fields of magnitude over ten times higher than the maximum field produced by conventional magnets, which is limited practically to rather less than 2 T. In this paper, two large c-axis oriented, single-grain YBCO and GdBCO bulk superconductors are magnetized by the pulsed field magnetization (PFM) technique at temperatures of 40 and 65 K and the characteristics of the resulting trapped field profile are investigated with a view of magnetizing such samples as trapped field magnets (TFMs) in situ inside a trapped flux-type superconducting electric machine. A comparison is made between the temperatures at which the pulsed magnetic field is applied and the results have strong implications for the optimum operating temperature for TFMs in trapped flux-type superconducting electric machines. The effects of inhomogeneities, which occur during the growth process of single-grain bulk superconductors, on the trapped field and maximum temperature rise in the sample are modelled numerically using a 3D finite-element model based on the H-formulation and implemented in Comsol Multiphysics 4.3a. The results agree qualitatively with the observed experimental results, in that inhomogeneities act to distort the trapped field profile and reduce the magnitude of the trapped field due to localized heating within the sample and preferential movement and pinning of flux lines around the growth section regions (GSRs) and growth sector boundaries (GSBs), respectively. The modelling framework will allow further investigation of various inhomogeneities that arise during the processing of (RE)BCO bulk superconductors, including inhomogeneous Jc distributions and the presence of current-limiting grain boundaries and cracks, and it can be used to assist optimization of processing and PFM techniques for practical bulk superconductor applications.
Numerical simulations of trapped field Bz and temperature T have been performed for a cryo-cooled superconducting bulk disc after applying a magnetic pulse using a vortex-type coil. Results of the simulation qualitatively reproduced experimental results reported by Ida et al (2004 Physica C 412-414 638). Using the vortex-type coil, the magnetic flux does not intrude into the bulk from the periphery but intrudes from both surfaces of the bulk disc. The temperature rise was less than that obtained when using a conventional solenoid coil. The effect of the trapped field enhancement using a long pulse application is small, which suggests that down-sizing of the condenser bank used for the pulsed field magnetization is possible and that the use of the vortex-type coil is desirable for practical applications.
Various pulsed magnetization experiments employing peak fields of up to 2 T and pulse durations of 30 ms and 3 ms were carried out on YBCO samples at temperatures between 20 and 80 K. Trapped magnetic flux profiles were recorded. The highest remanent magnetizations were obtained for a multi-pulse technique with step-wise cooling. The shape and the absolute values of the trapped flux profile are discussed in terms of the dynamics of such pulsed magnetization processes.
Strong evidence for high intergranular critical current densities and large bulk magnetic flux pinning in superconducting polycrystalline MgB2 has been observed. The presence of strongly-coupled grain boundaries in this material has been confirmed by a dramatic collapse of the magnetic hysteresis loop when a bulk specimen is ground into a fine powder and re-measured under similar conditions. Further evidence for strong intergrain links in polycrystalline MgB2 is provided by the continuous variation of the remanent magnetic moment up to the full penetration field of a bulk sample. The absence of weak-link nature in this material has profound implications for its potential in a wide range of engineering applications.
The trapped field has been realized on the GdBaCuO bulk superconductor by a modified multi pulse technique combined with a stepwise cooling (MMPSC), which surpassed the previous highest record of . At the first stage, a small amount of the magnetic field ∼1 T was trapped at the bulk center with a concave field distribution at a high starting temperature Ts ∼ 45 K by the low pulse field application Bex ∼ 4.5 T. Following the first stage, the higher field of Bex ∼ 6.7 T was applied at a lower Ts ∼ 30 K at the second stage. The concave trapped field profile over the bulk at the first stage and the optimization of the higher applied pulse field at the second stage are key points to enhance BT above 5 T.
Pulsed-field magnetization was studied for field-free cooled high-temperature superconductor (HTS) bulk cylindrical disks of melt-textured Gd–Ba–Cu–O samples at the liquid nitrogen temperature. A bulk sample was inserted in between disks of vortex-type pulsed-field copper coil immersed in the liquid nitrogen. The flux was trapped in the centre of the sample surface under the smaller pulsed peak field than the magnetization with a conventional solenoid coil. With intensifying the pulsed-field, the trapped flux density for the maximum peak remanent value in the field cooling process increases monotonously to the liquid nitrogen temperature. In the samples with strong pinning force, which shows large remanent flux on field cooling, the deviation from the conical profile of trapped field distribution was observed. This is attributed to the transient flux motion, which possibly drives temperature increase resulting in the decrease of the trapped field in the growth sector. However, the subsequent single pulsed-field remarkably compensates the formation of a well-dressed conical field density profile. Employing a couple of vortex-type coils enables us to magnetize the HTS bulk cryo-magnets effectively with reduced electric energy per pulsed-current for the magnetization with a pulsed-field solenoid. The present magnetization geometry is acceptable for application of the HTS bulk to the rotor magnet magnetized with an armature in the synchronous rotating machines.
High trapped fields in bulk MgB2 prepared by hot-pressing of ball-milling precursor powder
- G Fuchs
- W Haber
- K Nenkov
- J Scheiter
- O Perner
- A Handstein
- T Kanai
- L Schultz
- B Holzapfel
G. Fuchs, W. Haber, K. Nenkov, J. Scheiter, O. Perner, A. Handstein, T.
Kanai, L. Schultz and B. Holzapfel, "High trapped fields in bulk MgB2
prepared by hot-pressing of ball-milling precursor powder", Supercond.
Sci. Technol., vol. 26, pp. 122002(1)-(5), 2013.