References of "Fagnard, Jean-Francois"
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See detailOrigin of dc voltage in type II superconducting flux pumps: field, field rate of change, and current density dependence of resistivity
Geng, J.; Matsuda, K.; Fu, L. et al

in Journal of Physics : D Applied Physics (2016), 49(11), 1101

Superconducting flux pumps are the kind of devices which can generate direct current into superconducting circuit using external magnetic field. The key point is how to induce a dc voltage across the ... [more ▼]

Superconducting flux pumps are the kind of devices which can generate direct current into superconducting circuit using external magnetic field. The key point is how to induce a dc voltage across the superconducting load by ac fields. Giaever (1966 IEEE Spectr . 3 [http://dx.doi.org/10.1109/MSPEC.1966.5217702] 117 ) pointed out flux motion in superconductors will induce a dc voltage, and demonstrated a rectifier model which depended on breaking superconductivity. van de Klundert et al (1981 Cryogenics 21 [http://dx.doi.org/10.1016/0011-2275(81)90195-8] 195, 267 ) in their review(s) described various configurations for flux pumps all of which relied on inducing the normal state in at least part of the superconductor. In this letter, following their work, we reveal that a variation in the resistivity of type II superconductors is sufficient to induce a dc voltage in flux pumps and it is not necessary to break superconductivity. This variation in resistivity is due to the fact that flux flow is influenced by current density, field intensity, and field rate of change. We propose a general circuit analogy for travelling wave flux pumps, and provide a mathematical analysis to explain the dc voltage. Several existing superconducting flux pumps which rely on the use of a travelling magnetic wave can be explained using the analysis enclosed. This work can also throw light on the design and optimization of flux pumps. [less ▲]

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See detailInfluence of crossed fields in structures combining large grain, bulk (RE)BCO superconductors and soft ferromagnetic discs
Philippe, Matthieu ULg; Fagnard, Jean-François ULg; Wera, Laurent ULg et al

in Journal of Physics: Conference Series (2016), 695

Bulk (RE)BCO superconductors are able to trap record magnetic fields and can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings. When ... [more ▼]

Bulk (RE)BCO superconductors are able to trap record magnetic fields and can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings. When such superconducting (SC) “trapped field magnets” are combined to a ferromagnetic (FM) disc, the total magnetic moment is increased with respect to that of the superconductor alone. In the present work, we study experimentally the magnetic behaviour of such hybrid FM/SC structures when they are subjected to cycles of applied field that are orthogonal to their permanent magnetization, i.e. a “crossed-field” configuration. Experimental results show that the usual “crossed-field demagnetization” caused by the cycles of transverse field is strongly reduced in the presence of the ferromagnet. [less ▲]

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See detailCritical analysis of the paramagnetic to ferromagnetic phase transition in Pr0.55K0.05Sr0.4MnO3
Thaljaoui, R.; Pękała, M.; Fagnard, Jean-François ULg et al

in Physica B: Condensed Matter (2016), 482

The critical properties of monovalent doped manganite Pr0.55K0.05Sr0.4MnO3 around the paramagnetic to ferromagnetic phase transition were investigated through various methods: the modified Arrott plots ... [more ▼]

The critical properties of monovalent doped manganite Pr0.55K0.05Sr0.4MnO3 around the paramagnetic to ferromagnetic phase transition were investigated through various methods: the modified Arrott plots (MAP), the Kouvel-Fisher method and the critical isotherm analysis. Data obtained near Tc were examined in the framework of the mean field theory, the 3D–Heisenberg model, the 3D–Ising model, and tricritical mean field. The deduced critical exponents values obtained using MAP method were found to be β = 0.44(4) with TC ~ 303 K and γ = 1.04(1) with TC ~ 302 K. Kouvel-Fisher method supplies the critical values to be β = 0.41(2) with TC ~ 302 K and γ = 1.09(1) with TC ~ 302 K. The obtained critical parameters show a tendency towards the mean-field behavior, suggesting the existence of long-range ferromagnetic order in the compound studied. The exponent δ deduced separately from isotherm analysis at T= 303 K was found to obey to the Widom scaling relation δ = 1+ γ/ β. The reliability of obtained exponents was confirmed by using the universal scaling hypothesis. The itinerant character of ferromagnetism in the present system was also tested by using Rhodes-Wohlfarth’s criterion. [less ▲]

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See detailThermal properties of Ti-doped Cu–Zn soft ferrites used as thermally actuated material for magnetizing superconductors
Stachowiak, Piotr; Mucha, Jan; Szewczyk, Daria et al

in Journal of Physics : D Applied Physics (2016), 49

A great majority of widely used ferrite ceramics exhibit a relatively high temperature of order–disorder phase transition in their magnetic subsystem. For applications related to the magnetization process ... [more ▼]

A great majority of widely used ferrite ceramics exhibit a relatively high temperature of order–disorder phase transition in their magnetic subsystem. For applications related to the magnetization process of superconductors, however, a low value of Tc is required. Here we report and analyze in detail the thermal properties of bulk Ti-doped Cu–Zn ferrite ceramics Cu0.3Zn0.7Ti0.04Fe1.96O4 and Mg0.15Cu0.15Zn0.7Ti0.04Fe1.96O4. They are characterized by a Curie temperature in the range 120–170 K and a maximum DC magnetic susceptibility exceeding 20 for the Cu0.3Zn0.7Ti0.04Fe1.96O4 material. The temperature dependence of both the specific heat Cp and of the thermal conductivity κ, determined between 2 and 300 K, are found not to exhibit any peculiar feature at the magnetic transition temperature. The low-temperature dependence of both κ and the mean free path of phonons suggests a mesoscopic fractal structure of the grains. From the measured data, the characteristics of thermally actuated waves are estimated. The low magnetic phase transition temperature and suitable thermal parameters make the investigated ferrite ceramics applicable as magnetic wave producers in devices designed for magnetization of high-temperature superconductors. [less ▲]

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See detailMagnetic Shielding with Bulk High Temperature Superconductors: Improvement of the Shielded Volume in Hollow Cylinders
Wera, Laurent ULg; Fagnard, Jean-François ULg; Hogan, Kevin ULg et al

in Muralidhar, Miryala (Ed.) Superconductivity: Applications Today and Tomorrow (2016)

The superconducting HTS hollow cylinder has been one of the first geometries to demonstrate its utility as an efficient passive magnetic shield, and particularly for DC magnetic shielding. The shielding ... [more ▼]

The superconducting HTS hollow cylinder has been one of the first geometries to demonstrate its utility as an efficient passive magnetic shield, and particularly for DC magnetic shielding. The shielding performances can be characterized by two parameters: (i) the shielding factor SF, defined as the ratio between the applied magnetic induction Bapp and the magnetic induction inside the shield Bin and (ii) the threshold induction Blim for which the shielding factor becomes lower than a given level. The main drawback of the hollow cylinder geometry is that the shielding is only effective in the central region of the tube and decreases towards the tube extremities because of the field penetration through the open ends. In order to improve the performances at tube extremities and hence increase the shielded volume, the tube should be closed. This can be achieved by using a superconducting vessel or by closing one or both extremities with a cap. This chapter describes the experimental and the numerical analysis of the DC shielding performances of a Bi-2223 tube closed by a superconducting Bi-2223 cap, for an axial configuration. The first part is dedicated to the experimental study of two tubes of different lengths when they are closed at one extremity or closed at both extremities. We also study the effect of the gap size between the cap and the tube. In the second part, we use numerical simulations to highlight the possible improvements and to optimize the design. In particular we study the influence of the critical current density and of the thickness of the cap. [less ▲]

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See detailDesign of the superconducting end caps of a tubular magnetic shield.
Wera, Laurent ULg; Fagnard, Jean-François ULg; Hogan, Kevin ULg et al

Poster (2015, September 09)

Shielding a sensitive device against DC or AC stray magnetic fields can be realized very efficiently with high-Tc superconductors. A simple magnetic shield can be obtained with a tube enclosing the device ... [more ▼]

Shielding a sensitive device against DC or AC stray magnetic fields can be realized very efficiently with high-Tc superconductors. A simple magnetic shield can be obtained with a tube enclosing the device, which is protected against external fields until the magnetic flux diffuses either across the tube wall or through the end openings. A better shielding vessel is obtained by using superconducting end caps. In the simplest design, the end caps, consisting of superconducting disks placed at the extremities of the tube, are making a 90 degree angle with the tube wall. Because of demagnetization effects, this arrangement results in a 'weak spot' for the penetrating magnetic flux. In this work, we investigate numerically different shapes for the end caps so as to avoid sharp angles and obtain a smoother magnetic flux penetration. The external magnetic induction is applied parallel to the shield axis. The magnetic flux distribution and the shielding currents are calculated with a finite element approach. The shielded volume is defined as the region where the local magnetic flux density is attenuated below a given fraction of the external field. It is studied as a function of the shape of the cap, its superconducting properties, the aspect ratio of the assembly, and the presence of a cabling hole. Curved and elongated caps, which could in principle be obtained by melt cast processing or different coating techniques, are shown to substantially increase the shielded volume. [less ▲]

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See detailMagnetic shielding properties of a bulk Bi-2223 superconducting hollow cylinder subjected to the magnetic stray field of a nearby magnetic source
Hogan, Kevin ULg; Wera, Laurent ULg; Fagnard, Jean-François ULg et al

Poster (2015, September 09)

Superconducting materials act as efficient magnetic shields thanks to their intrinsic magnetic properties. At low frequencies, their efficiency even surpasses that of conventional ferromagnetic materials ... [more ▼]

Superconducting materials act as efficient magnetic shields thanks to their intrinsic magnetic properties. At low frequencies, their efficiency even surpasses that of conventional ferromagnetic materials. Therefore, they can be very useful for many applications which require low or very low magnetic field in intense quasi-static magnetic environments. The vast majority of studies on superconducting screens has been conducted in homogeneous magnetic field configurations. Few works have been done concerning inhomogeneous magnetic field configurations while those are representative of practical situations. Moreover, those configurations are non-trivial given the magnetic hysteretic behaviour of type-II superconductors. In the present work, we aim at investigating and understanding the shielding properties of a superconducting hollow cylinder made of Bi-2223 subjected to the magnetic stray field of a coil placed in the vicinity. All experiments are carried out at 77K. [less ▲]

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See detailCharacterization of the magnetic properties of a continuously coated YBCO coated conductor cylinder in persistent current mode
Wera, Laurent ULg; Fagnard, Jean-François ULg; Hogan, Kevin ULg et al

Poster (2015, September 09)

YBCO coated conductors are good candidates for magnetic shielding applications because of (i) their high current density and (ii) their lower weight compared to magnetic shields made of bulk HTS materials ... [more ▼]

YBCO coated conductors are good candidates for magnetic shielding applications because of (i) their high current density and (ii) their lower weight compared to magnetic shields made of bulk HTS materials. In particular, an efficient superconducting magnetic shield can be built as an assembly of milled YBCO 2G coated conductor tape sections, or, as in this case, a continuously coated cylinder. The purpose of the present work is to characterize experimentally the DC magnetic properties of a continuous band of YBCO film (about 1 cm width) deposited around a hastelloy cylinder of 6.4 cm in diameter. In particular, we characterize the distribution and the time dependence of the trapped field inside the cylinder due to persistent current loops. We also investigate the magnetic shielding properties at several sweep rates when the applied magnetic field is parallel to the main axis of the cylinder. All measurements are carried out at 77 K. The measurement of the trapped field of the cylinder gives evidence that a persistent current of about 80 A can be induced in the YBCO band. Magnetic shielding measurements show that, thanks to the persistent current flowing in the band, the cylinder is able to attenuate by a factor of 2 the applied magnetic field. These results open interesting perspectives for a further development of magnetic shields made of coated conductor cylinders. We investigate how the shielding performances could be further improved by increasing the number of layers and by increasing the height of the cylinder. The results are compared to a multilayer coated conductor cylinder with joints and to persistent current loops obtained with milled coated conductor tape sections. [less ▲]

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See detailMagnetic shielding properties of tubular bulk superconductors subjected to stray magnetic fields
Hogan, Kevin ULg; Fagnard, Jean-François ULg; Wera, Laurent ULg et al

Conference (2015, September 04)

Superconducting materials act as efficient magnetic shields thanks to their intrinsic magnetic properties. The vast majority of studies on superconducting shields has been conducted in homogeneous ... [more ▼]

Superconducting materials act as efficient magnetic shields thanks to their intrinsic magnetic properties. The vast majority of studies on superconducting shields has been conducted in homogeneous magnetic field configurations. Only few works have been done concerning inhomogeneous magnetic field configurations while those are representative of practical situations. Moreover, those configurations are non-trivial given the magnetic hysteretic behaviour of type-II superconductors. In the present work, we aim at investigating and understanding the magnetic response of a superconducting hollow cylinder made of Bi-2223 subjected to the magnetic stray field of various magnetic sources. Thanks to a bespoke 3D mapping system and a three-axis Hall probe, we measure the magnetic flux density distribution in the vicinity of the cylinder when the latter is subjected to those stray fields. All experiments are carried out in liquid nitrogen. Our experimental data allow us to apprehend the magnetic field penetration inside the superconductor in inhomogeneous field configurations and to compare those to results obtained with homogeneous fields. Then we compare our results to a finite element model of an infinite tube subjected to an inhomogeneous magnetic field in order to gather information on the magnetic field penetration and the current density distribution inside the superconductor. [less ▲]

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See detailInfluence of soft ferromagnetic materials on the magnetic flux density above large grain, bulk high temperature (RE)BCO superconductors: measurements and modelling
Philippe, Matthieu ULg; Ainslie, Mark D; Fagnard, Jean-François ULg et al

Poster (2015, September)

Due to their ability to trap high magnetic fields, bulk (RE)BCO superconductors can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic ... [more ▼]

Due to their ability to trap high magnetic fields, bulk (RE)BCO superconductors can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings. We combine soft ferromagnetic pieces with bulk, large grain (RE)BCO high temperature superconductors to form superconductor/ferromagnet (SC/FM) hybrid structures. We study how the ferromagnetic pieces influence (i) the profile shape of the trapped magnetic flux density at the surface of each structure and (ii) the decrease of the trapped field under an applied field that is orthogonal to the main magnetization (crossed field configuration). Hall probe mappings of the trapped magnetic flux density profile above the hybrid structures at 77 K are compared to modelled profiles using a 2D finite element method. Modelling results are in excellent qualitative and quantitative agreement with the measurements. The model is then used to study the magnetic flux distribution and predict the behaviour for other constitutive laws and ferromagnet geometries. For the modelled configurations, the magnetic flux density is enhanced on the face opposite to the ferromagnet. Both thickness and saturation magnetization of the ferromagnetic material are found to be important parameters. The saturation regime of the ferromagnet can be predicted using a simple analytical model. We show that thin ferromagnetic discs can be driven to full saturation even though the superconductor magnetic field is much smaller than their saturation magnetization. Remarkably, the beneficial influence of the ferromagnet on the trapped field can be observed even if the trapped flux density above the superconductor is much larger than the ferromagnet saturation magnetization. Finally, we show that the ferromagnetic material acts as a magnetic shield and lowers the relative demagnetization effect caused by the application of transverse (crossed) magnetic field cycles. [less ▲]

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See detailTrapped magnetic flux density and influence of crossed fields in structures combining large grain, bulk (RE)BCO superconductors and soft ferromagnetic discs
Philippe, Matthieu ULg; Ainslie, Mark D; Fagnard, Jean-François ULg et al

Conference (2015, September)

Bulk (RE)BCO superconductors are able to trap record magnetic fields and can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings. We ... [more ▼]

Bulk (RE)BCO superconductors are able to trap record magnetic fields and can be used as powerful permanent magnets in various engineering applications such as rotating machines and magnetic bearings. We combine soft ferromagnetic pieces with bulk, large grain (RE)BCO high temperature superconductors to form superconductor/ferromagnet (SC/FM) hybrid structures. We study how a ferromagnetic disc influences the trapped flux density at the surface of a SC/FM structure at 77 K and the magnetic moment of the whole structure. The flux density generated by the superconductor is shielded above the ferromagnet. The saturation of the ferromagnet by the flux of the superconductor is investigated with the help of a finite element model and a simple analytical formula is suggested to determine the saturation threshold. Finally, we show that the ferromagnetic disc lowers the relative demagnetization effect caused by the application of magnetic field cycles orthogonal to the main magnetization (crossed field configuration). [less ▲]

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See detailInfluence of soft ferromagnetic sections on the magnetic flux density profile of a large grain, bulk Y-Ba-Cu-O superconductor
Philippe, Matthieu ULg; Ainslie, Mark D; Wera, Laurent ULg et al

in Superconductor Science and Technology (2015), 28

Bulk, high temperature superconductors have significant potential for use as powerful permanent magnets in a variety of practical applications due to their ability to trap record magnetic fields. In this ... [more ▼]

Bulk, high temperature superconductors have significant potential for use as powerful permanent magnets in a variety of practical applications due to their ability to trap record magnetic fields. In this paper, soft ferromagnetic sections are combined with a bulk, large grain Y-Ba-Cu-O (YBCO) high temperature superconductor to form superconductor/ferromagnet (SC/FM) hybrid structures. We study how the ferromagnetic sections influence the shape of the profile of the trapped magnetic induction at the surface of each structure and report the surface magnetic flux density measured by Hall probe mapping. These configurations have been modelled using a 2D axisymmetric finite element method based on the H-formulation and the results show excellent qualitative and quantitative agreement with the experimental measurements. The model has also been used to study the magnetic flux distribution and predict the behaviour for other constitutive laws and geometries. The results show that the ferromagnetic material acts as a magnetic shield, but the flux density and its gradient are enhanced on the face opposite to the ferromagnet. The thickness and saturation magnetization of the ferromagnetic material are important and a characteristic ferromagnet thickness d* is derived: below d*, saturation of the ferromagnet occurs, and above d*, a weak thickness-dependence is observed. The influence of the ferromagnet is observed even if its saturation magnetization is lower than the trapped flux density of the superconductor. Conversely, thin ferromagnetic discs can be driven to full saturation even though the outer magnetic field is much smaller than their saturation magnetization. [less ▲]

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See detailA comparative study of triaxial and uniaxial magnetic shields made out of YBCO coated conductors
Wera, Laurent ULg; Fagnard, Jean-François ULg; Levin, George et al

in Superconductor Science and Technology (2015), 28

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See detailInvestigation of Demagnetization in HTS Stacked Tapes Implemented in Electric Machines as a Result of Crossed Magnetic Field
Baghdadi, M.; Ruiz, H. S.; Fagnard, Jean-François ULg et al

in IEEE Transactions on Applied Superconductivity (2015), 25(3), 6602404

This paper investigates the practical effectiveness of employing superconducting stacked tapes to superconducting electric machinery. The use of superconducting bulks in various practical applications has ... [more ▼]

This paper investigates the practical effectiveness of employing superconducting stacked tapes to superconducting electric machinery. The use of superconducting bulks in various practical applications has been addressed extensively in the literature. However, in practice, dramatic decrease in magnetization would occur on superconducting bulks due to the crossed field effect. In our study, we employed the superconducting stacked tapes in a synchronous superconducting motor, which was designed and fabricated in our laboratory, aiming to lessen demagnetization due to crossed field effect in comparison with superconducting bulks. Applying the transverse AC field, the effects of frequency, amplitude, and number of cycles of the transverse magnetic field are discussed. Furthermore, a stack of 16 layers of superconducting tapes is modelled and the consequences of applying the crossed magnetic field on the sample are evaluated. The confrontation between experiments and simulation allows us to thoroughly understand the crossed field effects on stacked tapes. At the end, a preventive treatment, based on the shielding characteristic of superconductor and materials with high permeability, i.e. $mu$-metal and metalic glass, is suggested. On the other hand, the shielding feature of aforementioned materials will hinder the penetration of magnetic field and, consequently, reduction of the demagnetization will be attained. [less ▲]

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See detailMeasurements on magnetized GdBCO pellets subjected to small transverse ac magnetic fields at very low frequency: Evidence for a slowdown of the magnetization decay
Fagnard, Jean-François ULg; Kirsch, Sébastien; Morita, Mitsuru et al

in Physica C: Superconductivity (2015), 512

Due to their ability to trap large magnetic inductions, superconducting bulk materials can be used as powerful permanent magnets. The permanent magnetization of such materials, however, can be ... [more ▼]

Due to their ability to trap large magnetic inductions, superconducting bulk materials can be used as powerful permanent magnets. The permanent magnetization of such materials, however, can be significantly affected by the application of several cycles of a transverse variable magnetic field. In this work, we study, at T = 77 K, the long term influence of transverse ac magnetic fields of small amplitudes (i.e. much smaller than the full penetration field) on the axial magnetization of a bulk single grain superconducting GdBCO pellet over a wide range of low frequencies (1 mHz–20 Hz). Thermocouples are placed against the pellet surface to probe possible self-heating of the material during the experiments. A high sensitivity cryogenic Hall probe is placed close to the surface to record the local magnetic induction normal to the surface. The results show first that, for a given number of applied triangular transverse cycles, higher values of dBapp/dt induce smaller magnetization decays. An important feature of practical interest is that, after a very large number of cycles which cause the loss of a substantial amount of magnetization (depending on the amplitude and the frequency of the field), the rate of the magnetization decay goes back to its initial value, corresponding to the relaxation of the superconducting currents due to flux creep only. In the amplitude and frequency range investigated, the thermocouples measurements and a 2D magneto-thermal modelling show no evidence of sufficient self-heating to affect the magnetization so that the effect of the transverse magnetic field cycles on the trapped magnetic moment is only attributed to a redistribution of superconducting currents in the volume of the sample and not to a thermal effect. [less ▲]

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See detailEffect of nanocrystalline structure on magnetocaloric effect in manganite composites (1/3)La0.7Ca0.3MnO3/(2/3)La0.8Sr0.2MnO3
Pękała, M; Pękała, K; Drozd, V et al

in Journal of Alloys & Compounds (2015), 629

Poly- and nanocrystalline manganite composites (1/3)La0.7Ca0.3MnO3 and / (2/3)La0.8Sr0.2MnO3 prepared by the sol-gel method are studied by magnetic and transport measurements. The Arrott plots and ... [more ▼]

Poly- and nanocrystalline manganite composites (1/3)La0.7Ca0.3MnO3 and / (2/3)La0.8Sr0.2MnO3 prepared by the sol-gel method are studied by magnetic and transport measurements. The Arrott plots and universal curve of magnetic entropy change confirm that magnetic transition is of the second order. The magnetocaloric effect of polycrystalline composite is found to be roughly twice smaller as compared to the polycrystalline Ca- and Srbased parent phases. The nanocrystalline composite of the same composition exhibits only 8 % reduction of magnetic entropy change. Due to the large temperature spread of magnetocaloric effect the relative cooling power RCP of nanocrystalline composite is about three times larger as compared to the nanocrystalline Ca- and Sr- based parent phases. The maximum magnetic entropy change DSMAX and relative cooling power RCP are found to be more sensitive to magnetic field strength for the nano- than for polycrystalline composites studied. [less ▲]

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See detailA flux extraction device to measure the magnetic moment of large samples; application to bulk superconductors
Egan, Raphael ULg; Philippe, Matthieu ULg; Wera, Laurent ULg et al

in Review of Scientific Instruments (2015), 86(2), 025107

We report the design and construction of a flux extraction device to measure the DC magnetic moment of large samples (i.e., several cm3) at cryogenic temperature. The signal is constructed by integrating ... [more ▼]

We report the design and construction of a flux extraction device to measure the DC magnetic moment of large samples (i.e., several cm3) at cryogenic temperature. The signal is constructed by integrating the electromotive force generated by two coils wound in series-opposition that move around the sample. We show that an octupole expansion of the magnetic vector potential can be used conveniently to treat near-field effects for this geometrical configuration. The resulting expansion is tested for the case of a large, permanently magnetized, type-II superconducting sample. The dimensions of the sensing coils are determined in such a way that the measurement is influenced by the dipole magnetic moment of the sample and not by moments of higher order, within user-determined upper bounds. The device, which is able to measure magnetic moments in excess of 1 Am2 (1000 emu), is validated by (i) a direct calibration experiment using a small coil driven by a known current and (ii) by comparison with the results of numerical calculations obtained previously using a flux measurement technique. The sensitivity of the device is demonstrated by the measurement of flux-creep relaxation of the magnetization in a large bulk superconductor sample at liquid nitrogen temperature (77 K). [less ▲]

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See detailMagnetic shielding of an inhomogenous magnetic field source by a bulk superconducting tube
Hogan, Kevin ULg; Fagnard, Jean-François ULg; Wera, Laurent ULg et al

in Superconductor Science and Technology (2015), 28(3), 035011

Bulk type-II irreversible superconductors can act as excellent passive magnetic shields, with a strong attenuation of low frequency magnetic fields. Up to now, the performances of superconducting magnetic ... [more ▼]

Bulk type-II irreversible superconductors can act as excellent passive magnetic shields, with a strong attenuation of low frequency magnetic fields. Up to now, the performances of superconducting magnetic shields have mainly been studied in a homogenous magnetic field, considering only immunity problems, i.e. when the field is applied outside the tube and the inner field should ideally be zero. In this paper, we aim to investigate experimentally and numerically the magnetic response of a high-Tc bulk superconducting hollow cylinder at 77 K in an emission problem, i.e. when subjected to the non-uniform magnetic field generated by a source coil placed inside the tube. A bespoke 3-D mapping system coupled with a 3-axis Hall probe is used to measure the magnetic flux density distribution outside the superconducting magnetic shield. A finite element model is developed to understand how the magnetic field penetrates into the superconductor and how the induced superconducting shielding currents flow inside the shield in the case where the emitting coil is placed coaxially inside the tube. The finite element modelling is found to be in excellent agreement with the experimental data. Results show that a concentration of the magnetic flux lines occurs between the emitting coil and the superconducting screen. This effect is observed both with the modelling and the experiment. In the case of a long tube, we show that the main features of the field penetration in the superconducting walls can be reproduced with a simple analytical 1D model. This model is used to estimate the maximum flux density of the emitting coil that can be shielded by the superconductor. [less ▲]

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See detailNumerical modelling and comparison of MgB 2 bulks fabricated by HIP and infiltration growth
Zou, J.; Ainslie, M. D.; Fujishiro, H. et al

in Superconductor Science and Technology (2015), 28(7), 075009

MgB 2 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 ... [more ▼]

MgB 2 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 MgB 2 materials has enabled a number of different processing techniques to be developed. In this paper, a comparison is made between bulk MgB 2 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 MgB 2 sample, B z = 2.12 at 5 K and 1.66 T at 15 K, is reported. Since bulk MgB 2 has a more homogeneous J c distribution than (RE)BCO bulks, studies on such systems are made somewhat easier because simplified assumptions regarding the geometry and J c 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 J c ( 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 MgB 2 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. [less ▲]

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See detailMagnetization of 2-G Coils and Artificial Bulks
Coombs, Tim; Fagnard, Jean-François ULg; Matsuda, Koichi

in IEEE Transactions on Applied Superconductivity (2014), 24(5), 8201005

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