References of "Vanderbemden, Philippe"
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See detailMagnetic shielding of short thick GdBCO tube fabricated by the buffer aided TSIG method
Yang, Pengtao; Fagnard, Jean-François ULiege; Vanderbemden, Philippe ULiege et al

in Superconductor Science and Technology (in press)

Melt-textured (RE)Ba2Cu3O7 superconductors (where RE is a rare earth element like Y, Gd or Dy) have a significant potential for passive magnetic shielding. Their critical current density Jc, however, is ... [more ▼]

Melt-textured (RE)Ba2Cu3O7 superconductors (where RE is a rare earth element like Y, Gd or Dy) have a significant potential for passive magnetic shielding. Their critical current density Jc, however, is not uniform throughout the sample and this fact needs to be taken account to design a large magnetic shield made of these materials. In this work, magnetic shielding properties have been investigated at 77 K in an axial magnetic field for short GdBCO tubes fabricated by the top-seeded infiltration and growth (TSIG). The tubes, having ~ 10 mm thick walls, were studied with or without caps made of the same material. First, the shielding factor measured at several positions along the axis of the open tube was used to assess the non-uniformity of the shielding performance along the z axis. Second, four different configurations were brought when closing a tube with a disk, and their shielding performances were measured and compared. Third, we investigated the magnetic shielding efficiency for the tube closed with caps of different geometrical characteristics. The thickness of the cap used to close one side of the tube was shown to be an important parameter; experimental results showed that the same efficiency of a given cap could be obtained by stacking up two thinner caps with the same total height. Finally, the shielding behaviour of the open tube was modelled in order to investigate the flux penetration and to analyse the possible distribution of the critical current density Jc in the non-uniform GdBCO tube. The field dependence of the shielding factors measured at five elevations along the axis of the tube could be reproduced qualitatively by considering the GdBCO tube made of 3 layers, each of them with a constant Jc. This gives evidence that a simple model ignoring the field dependence of the critical current density can be used to reproduce the experimental magnetic shielding data. The model was also used to show the field penetration into the tube walls. These results are useful for getting better understanding of the properties of bulk superconductors synthesized by the TSIG process and to identify, in a non-destructive manner, which parts of the samples could be improved to obtain a good shielding performance. [less ▲]

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See detailCharacterization of superconductor magnetic properties in crossed magnetic fields
Vanderbemden, Philippe ULiege

in Larbalestier, David; Cardwell, David (Eds.) Handbook of Superconducting Materials (2nd edition) (in press)

This chapter deals with the characterization of the magnetic properties of superconductors which are subjected to magnetic fields that have been applied along two orthogonal directions, which is commonly ... [more ▼]

This chapter deals with the characterization of the magnetic properties of superconductors which are subjected to magnetic fields that have been applied along two orthogonal directions, which is commonly referred to as a “crossed” magnetic field configuration. The purpose of this chapter is to describe the techniques that are useful to perform crossed field experiments, with an emphasis placed on practical aspects that are useful for designing the system and for understanding the measured data. This chapter is organized as follows. In Section 2, the key terms involved in the literature dealing with crossed field effects are defined. Section 3 deals with experimental methods and some key parameters will be outlined. In Section 4, practical conclusions will be drawn and next challenges in this area will be discussed. [less ▲]

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See detailMagnetic shielding of various geometries of bulk semi-closed superconducting cylinders subjected to axial and transverse fields
Fagnard, Jean-François ULiege; Vanderheyden, Benoît ULiege; Pardo, Enric et al

in Superconductor Science and Technology (2019), 32(7), 074007

We consider the properties of bulk superconductors to be used as low-frequency passive magnetic shields. Although remarkable shielding properties have been recently achieved using high-temperature ... [more ▼]

We consider the properties of bulk superconductors to be used as low-frequency passive magnetic shields. Although remarkable shielding properties have been recently achieved using high-temperature superconductors of various kinds, one current issue is to assemble medium-size superconducting parts to obtain large superconducting volumes. The aim of the present work is to understand how hollow, semi-closed superconductors can be combined to improve the shielding properties over sizeable volumes. In axisymmetric superconducting geometries subjected to an axial field, 2D modelling can be used to understand important features of the shielding properties. When finite-size superconductors are subjected to a transverse field, 3D modelling must be used. In this work, we use 3D finite-element modelling with an A-phgr formulation to investigate various geometries in which a tube is closed by a superconducting element shaped like a disk, a cup, or another cup-shaped superconductor that is coaxial with the first. The simulations help in revealing the most performant configurations to use as a function of the geometry of the applied field. Under an axial field, the type of closing is found to be irrelevant and the key ingredient to improve the shielding factor is to reduce the average field in the opening plane, e.g. by using a thicker superconductor near the open end. Under a transverse field, the difference between the shielding properties arise from the different routes taken by flux lines to penetrate the shield. In particular, the presence of flux lines channelled through the gap between a tube and a cup-shaped sample surrounding the tube are detrimental to the shielding properties. The configurations where the tube surrounds the cup-shaped sample are found to yield much higher shielding factors, whose field dependence is further improved when the tube extends slightly beyond the end of the cup. The values of the shielding factors that can be reached under a transverse field of low amplitude are discussed by comparing them to those predicted for an ideal perfectly diamagnetic superconductor of similar dimensions. [less ▲]

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See detailMagnetic shielding of open and semi-closed bulk superconductor tubes: the role of a cap
Wéra, Laurent; Fagnard, Jean-François ULiege; Hogan, Kevin et al

in IEEE Transactions on Applied Superconductivity (2019), 29(3), 6801109

In this paper we investigate the magnetic shielding of hollow and semi-closed bulk superconducting tubes at 77 K. We first consider the properties of a commercial Bi-2223 tube closed by a disk-shaped cap ... [more ▼]

In this paper we investigate the magnetic shielding of hollow and semi-closed bulk superconducting tubes at 77 K. We first consider the properties of a commercial Bi-2223 tube closed by a disk-shaped cap placed against its extremity. The results are compared with those obtained on a bulk large grain Y-Ba-Cu-O (YBCO) tube produced by buffer-aided top seeded melt growth. In this process, the disk-shaped pellet and the tubular sample are grown together, resulting in a tube naturally closed at one extremity. The field to be shielded is either parallel or perpendicular to the main axis of the tube. The experimental results are compared with the results of finite element numerical modeling carried out either in two dimensions (for the axial configuration) or three dimensions (for the transverse configuration). In the axial configuration, the results show that the shielded volume can be enhanced easily by increasing the thickness of the cap. In the transverse configuration, the results show the critical role played by the superconducting current loops flowing between the tube and the cap for magnetic shielding. If the tube and the cap are separated by a non-superconducting joint or air gap, the presence of a cap leads only to a small improvement of the transverse shielding factor, even for a configuration where the gap between the cap and the tube contains a 90° bend. The cap leads to a significant increase in the transverse shielding when the cap and the tube are naturally grown in the same process, i.e., made of a continuous superconducting material. The experimental results can be reproduced qualitatively by 3-D numerical modeling. [less ▲]

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See detailIn situ measurements of partial discharge patterns on porous YSZ pellets pressed between planar platinum electrodes used for flash sintering
Fagnard, Jean-François ULiege; Gajdowski, Caroline; Boilet, Laurent et al

Poster (2019, March)

Flash sintering of prismatic or cylindrical samples placed between planar electrodes allows processing larger volumes of material, compared to dogbone-shaped samples. The contact surface with platinum ... [more ▼]

Flash sintering of prismatic or cylindrical samples placed between planar electrodes allows processing larger volumes of material, compared to dogbone-shaped samples. The contact surface with platinum electrodes are usually coated with Pt paste in this setup, in order to improve the galvanic contact with the sample, and obtain a better current homogeneity. This coating is however too expensive for industrial applications whenever large surfaces are involved. Nevertheless, a homogeneous contact resistance remains in all cases essential to control the uniformity of the electrical current through the sample, hence the importance of an accurate characterization of the quality of the contact between electrodes and sample in the non-coated case. We propose to analyse the Partial Discharges (PDs) patterns obtained by Phase Resolved Partial Discharge (PRPD) method as an in situ measurement technique to study this problem. The setup consists in placing a ceramic sample in a furnace in the same configuration as flash sintering experiments but at a temperature lower than the onset temperature of the material. In this work, porous cylindrical samples of stabilised zirconia (average dimensions of 9 mm in diameter and 5 mm in height) are studied between 200°C and 400°C. A 50 Hz AC voltage is therefore applied to the sample and increased gradually up to 1.7 kV. The sample is connected in parallel with a 1 nF coupling capacitor in series with a quadrupole used to separate the high frequency current of the partial discharge signals from the AC current at 50 Hz. At a given amplitude of the applied voltage, we record for 60 s the patterns of partial discharges, their number and their phase with respect to the applied voltage. We compare the partial discharge signals to the net AC current at the power frequency measured simultaneously with a current probe at the primary winding of the low noise transformer. At a given temperature, we notice that well-defined patterns of low amplitude partial discharges appear when the applied voltage is increased above 300 V. At much higher voltages, partial discharges of larger amplitudes initiate and grow rapidly as well as the net 50 Hz current which increases non-linearly resulting from the temperature rise of the sample and its associated resistivity drop (similarly to what happens in (pre-)flash sintering conditions). Cross examination of the sample surface by optical microscopy shows physical traces related to the occurrence of partial discharges. For comparison, a YSZ sample coated with Pt paste is also analysed in the same conditions. In that case, at the same temperature and for applied voltages up to 1 kV, partial discharge patterns are almost inexistent. These results show that the partial discharge measurement is an efficient technique that can be used in situ prior to flash sintering experiments in order to assess the quality of the contact between the electrodes and the sample when no Pt paste is used. This study is supported by ERDF and the Walloon Region, in the frame of IMAWA-FLASHSINT research project (program 2014-2020). [less ▲]

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See detailBulk superconducting tube subjected to the stray magnetic field of a solenoid
Hogan, Kevin ULiege; Fagnard, Jean-François ULiege; Wera, Laurent ULiege et al

in Superconductor Science and Technology (2018), 31

Hard type-II hollow superconductors are well suited for low frequency magnetic shielding. The properties and performances of superconducting magnetic shields subjected to homogeneous magnetic fields have ... [more ▼]

Hard type-II hollow superconductors are well suited for low frequency magnetic shielding. The properties and performances of superconducting magnetic shields subjected to homogeneous magnetic fields have been extensively discussed in the literature. In the present work, we investigate the magnetic shielding and the penetration of magnetic flux in a bulk high temperature superconducting tube subjected to the inhomogeneous fringe field of a solenoidal coil. Thanks to a bespoke microdisplacement measurement system, we measure the magnetic field distribution around the tube. We develop a full 3D finite element model based on an H formulation to understand the flux penetration mechanisms and predict the shape of the current loops. Using constitutive law parameters obtained from previous independent experiments, our model is found to be in excellent agreement with the measurements. We discuss how to assess the degree of inhomogeneity of the magnetic field and show that, in our case study, the field can be treated as the magnetic field of an equivalent magnetic dipole. We also show that some features of the flux penetration in inhomogeneous field can be also observed when the tube is subjected to an oblique homogeneous magnetic field, which offers a better understanding of the shielding current density distribution inside the shield. Finally, we discuss the magnetic field concentration occurring around the shield for different magnetic field configurations. In particular, we show that the extremities of the tube on the side not facing the magnetic field source experience the highest flux concentration. [less ▲]

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See detailMesures magnétiques : comment concevoir un magnétomètre adapté aux échantillons de grande taille ?
Vanderbemden, Philippe ULiege; Philippe, Matthieu; Wéra, Laurent et al

Conference (2018, October 18)

Les magnétomètres commerciaux traditionnels sont adaptés pour mesurer le moment magnétique d'échantillons de taille relativement réduite, en général < 1cm³. Pour certains matériaux cependant, il est ... [more ▼]

Les magnétomètres commerciaux traditionnels sont adaptés pour mesurer le moment magnétique d'échantillons de taille relativement réduite, en général < 1cm³. Pour certains matériaux cependant, il est désirable de pouvoir déterminer le moment magnétique d'échantillons plus volumineux de manière non-destructive; c'est le cas notamment des supraconducteurs massifs utilisés comme aimant permanents. Dans ce cas, il est nécessaire de revoir la conception de l'appareillage dans son ensemble, et notamment ta taille des bobines de détection. Dans cet exposé, nous montrerons comment dimensionner les bobines de détection d'un magnétomètre à extraction et comment traiter le signal mesuré de manière à déterminer aussi précisément que possible le moment magnétique d'un échantillon, potentiellement de grande taille. Nous envisagerons également le problème lié à la forme particulière (rapport d'aspect) de l'échantillon sur le signal mesuré. Nos propos seront illustrés par les résultats expérimentaux obtenus sur un magnétomètre conçu sur mesure pour les échantillons de taille importante. Reference : R. Egan et al., "A flux extraction device to measure the magnetic moment of large samples; application to bulk superconductors." Rev Sci Instrum. (2015) 86(2), 025107. doi: 10.1063/1.4907903. [less ▲]

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See detailBulk superconductors: a roadmap to applications
Durrell, John H.; Ainslie, Mark D.; Zhou, Difan et al

in Superconductor Science and Technology (2018), 31

Progress in superconducting bulk materials has been somewhat overshadowed by the considerable effort required to produce practical long-length conductors. There has, however, been steady progress in both ... [more ▼]

Progress in superconducting bulk materials has been somewhat overshadowed by the considerable effort required to produce practical long-length conductors. There has, however, been steady progress in both the materials science of bulk superconducting materials and the technologies required to use them effectively in engineering applications. In particular, magnetised bulk superconductors are capable of acting as quasi-permanent magnets with the potential of providing magnetic fields of several tesla or greater from a small volume of material, they can act as magnetic shields and they can provide self-stabilised levitation. This roadmap, based on a workshop which involved the participation of a wide range of academic and industrial participants (see doi: <A href="https://doi.org/10.17863/CAM.586">10.17863/CAM.586</A> for details of the workshop methodology), aims to explore some of the key potential domains of application of bulk superconductors. Detailed technological roadmaps are presented for four key applications that were identified as providing both good market opportunity and feasibility. These are: portable systems for bulk superconductivity; portable, high-field magnet systems for medical devices; ultra-light superconducting rotating machines for next-generation transport & power applications; and magnetic shielding applications for electric machines, equipment and other high-field devices. [less ▲]

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See detailDesign and real-time test of a hybrid energy storage system in the microgrid with the benefit of improving the battery lifetime
Li, Jianwei ULiege; Xiong, Riu; Mu, Hao et al

in Applied Energy (2018), 218

This study proposes a hybrid energy storage system (HESS) composed of the superconducting energy storage system (SMES) and the battery. The system is designed to compensate power fluctuations within a ... [more ▼]

This study proposes a hybrid energy storage system (HESS) composed of the superconducting energy storage system (SMES) and the battery. The system is designed to compensate power fluctuations within a microgrid. A novel control method is developed to share the instantaneous power between the SMES and the battery. The new control scheme takes into account the characteristics of the components of the HESS, and the battery charges and discharges as a function of the SMES current rather than directly to the power disturbances. In this way, the battery is protected from the abrupt power changes and works as an energy buffer to the SMES. An new hardware-in-loop experiment approach is introduced by integrating a real-time digital simulator (RTDS) with a control circuit to verify the proposed hybrid scheme and the new control method. This paper also presents a battery lifetime prediction method to quantify the benefits of the HESS in the microgrid. A much better power sharing between the SMES and the battery can be observed from the experimental results with the new control method. Moreover, compared to the battery only system the battery lifetime is quantifiably increased from 6.38 years to 9.21 years. [less ▲]

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See detailFEM Simulations of Crossed-field Effects in Bulk Superconducting Trapped-field Permanent Magnets
Vanderheyden, Benoît ULiege; Fagnard, Jean-François ULiege; Hogan, Kevin ULiege et al

Conference (2018, April 30)

Bulk (RE)BCO samples can be used as efficient permanent magnets achieving trapped flux densities of a few teslas at 77 K. However, their magnetization decays over time when subjected to crossed fields ... [more ▼]

Bulk (RE)BCO samples can be used as efficient permanent magnets achieving trapped flux densities of a few teslas at 77 K. However, their magnetization decays over time when subjected to crossed fields (with a direction perpendicular to the initially trapped flux density). Such parasitic field may be generated in synchronous motors as stray fields. In this talk, I summarize the recent work carried out in our group at the University of Liege to better understand the crossed-field effects. Under a magnetic field perturbation with a non-vanishing crossed field, the superconductor magnetization is modified due to a rearrangement of currents. We have developed finite-element models predicting the time evolution of the current distribution, on the basis of an A-V formulation implemented in the open-source software GetDP and Gmsh. The construction of the models is discussed, including the time-step scheme used, the choice for the mesh and convergence issues. It is demonstrated how the redistribution of currents under a transverse field can be studied with either 3D models or computationally simpler 2D models. The crossed-field effects are illustrated as a function of the amplitude of the perturbations, their waveform, and the variation of their direction over time (purely transverse vs rotation in space). Hybrid structures made of superconductors and soft ferromagnetic materials are also investigated. For the case of a soft ferromagnetic disk put on top of a superconducting pellet, it is demonstrated that the ferromagnetic material helps in both increasing the initial trapped field and in reducing the crossed-field effects. Different hybrid structures are discussed in order to find a compromise between the maximum level of induction which can be reached and the improved protection against crossed-field effects. Potential extensions of the numerical models are also discussed. [less ▲]

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See detailSpecific heat and magnetocaloric effect in Pr0.6Sr0.4−xAgxMnO3 manganites
Szewczyk, Daria; Thaljaoui, Rachid; Mucha, Jan et al

in Intermetallics (2018), 102

The detailed investigation on the specific heat in manganites of Pr0.6Sr0.4−xAgxMnO3 (x=0.05 and x=0.1) has been reported. The experimental conditions were: the temperature range 1.8–330 K and applied ... [more ▼]

The detailed investigation on the specific heat in manganites of Pr0.6Sr0.4−xAgxMnO3 (x=0.05 and x=0.1) has been reported. The experimental conditions were: the temperature range 1.8–330 K and applied magnetic fields of 0, 1, 2 and 3 T. Specific heat results confirmed two phase transitions, a structural one at Tstr = 43 K for both samples and a paramagnetic–ferromagnetic one near TPM–FM=288 K and 284 K for the x=0.05 and x=0.1 sample, respectively. For the more doped sample the magnetic field causes the shift of the PM–FM transition temperature towards higher temperatures. A thorough analysis of particular contributions to the specific heat was presented. The indirect magnetocaloric effect was deduced for the more doped sample (x=0.1). Under anapplied magnetic field of 3 T magnetic entropy changes and adiabatic temperature exhibits a maximum. The absolute ΔSmax reaches the value of 1.74 J/mol K, resulting in adiabatic cooling of ΔTad=0.95 K. Such result onfirms presented samples as promising candidates for the magnetic cooling technology at room temperature. [less ▲]

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See detailThe influence of an initial trapped field on the magnetic shielding performance of bulk high-temperature superconducting tubes
Wéra, Laurent; Fagnard, Jean-François ULiege; Hogan, Kevin et al

in Miryala, Muralidhar; Koblischka, Michael (Eds.) High-Temperature Superconductors: Occurrence, Synthesis and Applications (2018)

Type-II superconducting hollow cylinders can act as efficient passive magnetic shields. Unlike in ferromagnetic materials, the magnetic shielding mechanism in a superconductor occurs through persistent ... [more ▼]

Type-II superconducting hollow cylinders can act as efficient passive magnetic shields. Unlike in ferromagnetic materials, the magnetic shielding mechanism in a superconductor occurs through persistent currents flowing on the macroscopic scale. When a superconducting tube is used to shield a sequence of magnetic fields with different orientations, magnetic shielding performance levels are likely to be degraded because of the superconducting currents that are trapped in the superconductor. In this chapter we study experimentally the magnetic shielding configurations where an external magnetic field is applied first along one direction and a second field is subsequently applied along another (perpendicular) direction. In particular, we focus on the effect of an axial trapped field on the transverse shielding performance and on the effect of a transverse trapped field on the axial or the transverse shielding performance. Finally, we show how the pristine state of the tube can be restored by an appropriate magnetic field sequence, i.e., without heating up the shield above its critical temperature Tc. [less ▲]

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See detailQuantitative magneto-optical investigation of superconductor/ferromagnet hybrid structures
Shaw, Gorky ULiege; Brisbois, Jérémy ULiege; Pinheiro, L.B.G.L. et al

in Review of Scientific Instruments (2018), 89

We present a detailed quantitative magneto-optical imaging study of several superconductor/ ferromagnet hybrid structures, including Nb deposited on top of thermomagnetically patterned NdFeB, and ... [more ▼]

We present a detailed quantitative magneto-optical imaging study of several superconductor/ ferromagnet hybrid structures, including Nb deposited on top of thermomagnetically patterned NdFeB, and permalloy/niobium with erasable and tailored magnetic landscapes imprinted in the permalloy layer. The magneto-optical imaging data is complemented with and compared to scanning Hall probe microscopy measurements. Comprehensive protocols have been developed for calibrating, testing, and converting Faraday rotation data to magnetic field maps. Applied to the acquired data, they reveal the comparatively weaker magnetic response of the superconductor from the background of larger fi elds and field gradients generated by the magnetic layer. [less ▲]

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See detailStudy of buoyancy driven heat transport in silicone oils and in liquid nitrogen in view of cooling applications
Satpathy, Kamalakanta ULiege; Duchesne, Alexis; Dubois, Charles et al

in International Journal of Heat and Mass Transfer (2018), 118

Motivated by applications for cooling superconducting pellets with liquid nitrogen, we consider a source with a fixed heating rate per unit volume, immersed in a liquid pool and cooled through natural ... [more ▼]

Motivated by applications for cooling superconducting pellets with liquid nitrogen, we consider a source with a fixed heating rate per unit volume, immersed in a liquid pool and cooled through natural convection. In one recent experimental investigation (Dubois et al., 2016) carried on silicone oils and liquid nitrogen, we have demonstrated that the velocity field satisfies specific scaling laws with respect to the temperature increase in the liquid pool. In this work, we pursue the analysis by modeling the heat transfer in a parallelepiped enclosure for a steady laminar flow regime. The Navier-Stokes equations are solved using a finite volume approach to obtain the detailed three-dimensional flow and heat transfer characteristics. A quantitative analysis of the velocity field over the temperature field shows that the experimental power laws are reproduced in simulations. Following Dubois and Berge (1978), a theoretical law originally introduced in the context of the classical Rayleigh-Bénard experiment is shown to be satisfied in the simulations over a wide range of Rayleigh numbers (Ra), assuming the definition of the characteristic convection length is adapted to the investigated geometry. Moreover, the simulations are shown to correctly reproduce the main features of the flow, including the characteristic convection length, for different heater lengths. [less ▲]

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See detailMagnetic shielding of open and semi-closed tubes made of bulk superconductors : the role of a cap
Vanderbemden, Philippe ULiege; Wera, Laurent ULiege; Hogan, Kevin ULiege et al

Conference (2017, December 11)

Amongst the range of prospective applications for bulk superconductors, the shielding of high magnetic fields at low frequency is extremely promising. Unlike conventional high permeability ferromagnetic ... [more ▼]

Amongst the range of prospective applications for bulk superconductors, the shielding of high magnetic fields at low frequency is extremely promising. Unlike conventional high permeability ferromagnetic materials which exhibit a relatively low saturation magnetization (e.g. µ0Msat ~ 0.75 T for mumetal), superconductors do not suffer from this drawback. They are therefore well suited to shield magnetic flux densities above 1 tesla. This can be achieved by hollow bulk superconductors of cylindrical geometry. The shielding efficiency and shielded volume are both strongly improved when the hollowed tube is closed at one extremity. As an example, we showed recently [1] that YBa2Cu3O7 (YBCO) tubes of ~ 10 mm inner diameter closed at one extremity and engineered by a buffer-aided top seeded melt growth fabrication process were able to attenuate axial flux densities of 1.5 T by a factor of 100 at 20 K. Such record performances are partly due to the presence of a cap at one extremity. The aim of the present work is to examine further and understand the influence of a cap on the shielding properties for various configurations. In this communication we investigate the shielding properties of hollow and semi-closed bulk superconducting tubes at 77 K. For the semi-closed configuration, we compare situations the cap is either placed against the extremity (with a small controllable gap between them) or welded naturally. The field to be shielded is either parallel or perpendicular to the main axis of the tube. Both YBCO tubes produced by buffer-aided top seeded melt growth and commercial Bi-2223 tubes are studied. The experimental results are compared to finite element numerical modelling carried out either in 2D (for the axial configuration) or 3D (for the transverse configuration). In the axial configuration, the results show that a small gap between the tube and the cap is not detrimental to the shielding properties. We also show that the shielded volume can be easily enhanced by increasing the thickness of the cap or its critical current density. In the transverse configuration, the results point out the crucial role of the superconducting joint between the tube and the cap. The need for a superconducting joint depends therefore on the main direction of the magnetic field to be shielded. References: [1] L. Wéra. et al., IEEE. Trans. Appl. Supercond. 27 (2017) 6800305 [less ▲]

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See detailInvestigation of the shielding properties of a Bi-2212 bulk superconducting tube subjected to an inhomogeneous magnetic field at various temperatures
Hogan, Kevin ULiege; Wera, Laurent ULiege; Fagnard, Jean-François ULiege et al

Poster (2017, December 11)

Magnetic shielding with bulk superconductors is commonly studied under homogeneous magnetic fields. This configuration simplifies the analysis of the magnetic reaction of the superconductor as the applied ... [more ▼]

Magnetic shielding with bulk superconductors is commonly studied under homogeneous magnetic fields. This configuration simplifies the analysis of the magnetic reaction of the superconductor as the applied magnetic field is known precisely everywhere. In practical applications, however, the magnetic field to be screened is seldom homogeneous. Magnetic shields experience magnetic field gradients and flux lines curvatures that might lead to a deterioration of the shielding properties. In the present work, we investigate experimentally the magnetic shielding properties of a bulk superconducting Bi-2212 tube subjected to an inhomogeneous magnetic field. This study is carried out at various temperatures thanks to a bespoke variable temperature measurement system based on a cryocooler. A superconducting pancake coil, made of 1G superconducting tape (Bi-2223), generates the inhomogeneous magnetic field to be screened. The bulk superconducting tube is anchored thermally to a sample holder fixed in the vicinity of the cold head. A cryogenic Hall probe is placed at the center of the bulk Bi-2212 tube to characterize the shielding properties of the superconductor. With this system, we characterize the threshold induction, i.e. the maximal applicable magnetic field below which the shielding factor (ratio between the measured and applied field) drops below a threshold value, at various temperatures ranging from 60 K to 92 K. Then we show and discuss the effect of the sweep rate of the applied magnetic field on the threshold induction value. Our experimental results are confronted to experimental results obtained at 77 K in an axial homogeneous magnetic field. This allow us to emphasize the impact of the inhomogeneity of the applied magnetic field on the shielding properties. We finally point out the portions of the magnetic field shield experiencing the highest magnetic field. [less ▲]

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See detailStudies on convective cooling of cryogenic fluids towards superconducting applications
Satpathy, Kamalakanta ULiege; Dubois, Charles; Duchesne, Alexis et al

in WIT Transactions on Engineering Sciences (2017), 118

To understand the cooling aspect through natural convection in a cryogenic fluid interacting with a constant heat source, numerical simulations are carried out in a parallelepiped enclosure. The 3D form ... [more ▼]

To understand the cooling aspect through natural convection in a cryogenic fluid interacting with a constant heat source, numerical simulations are carried out in a parallelepiped enclosure. The 3D form of N-S equations is solved to obtain the detailed flow features through path line profiles, isotherm contours and velocity vectors. The effect of heater aspect ratio (x/L) on the rate of heat transfer is studied in terms of the average Nusselt number (Nuave). The results indicate that effective heat transfer enhancement occurs for a small heater length, resulting in an efficient cooling. Increasing the heater length will favor heat transfer through conduction over convection. The maximum temperature difference across the fluid and the velocity magnitude are found to decrease with heater length. 3D and 2D results are in agreement for short heater lengths, but vary for higher heater lengths, presumably due to the essential effect of the heater width. Further analysis on different types of coolant reveals a constant correlation between Nuave and the Rayleigh number (Ra), with Nuave ~ Ra^0.374. Benchmark validation for natural convection in a square enclosure is found to be satisfactory against the reported results. [less ▲]

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See detailModel for the response of superconducting/ferromagnetic structures subjected to crossed fields
Vanderheyden, Benoît ULiege; Fagnard, Jean-François ULiege; Hogan, Kevin ULiege et al

Poster (2017, September 20)

Bulk (RE)BCO samples can be used as efficient permanent magnets with trapped magnetic flux densities of a few teslas at 77 K. However, their magnetization may decay over time when subjected to time ... [more ▼]

Bulk (RE)BCO samples can be used as efficient permanent magnets with trapped magnetic flux densities of a few teslas at 77 K. However, their magnetization may decay over time when subjected to time-varying parasitic fields perpendicular to the initial trapped flux density. In this work, we consider hybrid structures made of superconducting pellets and soft ferromagnetic rings or disks and investigate numerically how their magnetization varies under such crossed-field excitations. We consider a finite-element model in the A-V formulation, which is developed for both the full 3D configuration of the system and for a computationally lighter 2D approximation. The models provide the volumic distribution of current densities, the field lines, and the magnetization of the system, and the latter is compared to experimental measurements obtained with a bespoke magnetometer. A critical assessment is made outlining the advantages and limitations of the 2D approach. It is demonstrated that the ferromagnetic material helps in increasing the initial trapped field and in reducing the crossed-field effects. For the case of a single ferromagnetic disk put on top of the superconducting pellet, we study the dependence of the initial magnetization with respect to the disk thickness and the saturation field of the ferromagnetic material, and discuss the resulting optimal thickness. We also suggest different hybrid structures made of one superconducting pellet and one or several ferromagnetic disks or rings, and show numerically that a compromise is to be found between the maximum level of induction which can be reached and the improved protection against crossed-field effects. [less ▲]

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See detailNon-destructive investigation of hybrid of ferromagnet /(RE)BCO large grain bulks by flux extraction magnetometry and levitation force
Vanderbemden, Philippe ULiege; Philippe, Matthieu; Wera, Laurent ULiege et al

Conference (2017, July 10)

This work deals with bulk, large grain superconductors used as permanent magnet for rotating machines or levitation applications. It has recently been shown that the magnetic properties of bulk large ... [more ▼]

This work deals with bulk, large grain superconductors used as permanent magnet for rotating machines or levitation applications. It has recently been shown that the magnetic properties of bulk large grain superconductors can be improved easily by attaching a short section of a soft ferromagnetic material (F) to one of the faces of the bulk superconductor (S), thereby producing a hybrid F/S structure [1]. Here we investigate the contactless determination of the magnetic behavior of such structures using a recently constructed bespoke magnetometer based on the flux extraction technique [2]. This device allows magnetic moments as large as 1 Am² to be measured at 77 K and accommodates large bulk samples up to 20 mm diameter. This extends significantly the accessible measurement range of “off-the shelf” magnetometers. Unlike techniques based on recording the distribution of flux at the surface of the sample, the measured signal is representative of the superconducting currents flowing across the entire volume of the sample. In the present work we examine the properties of permanently magnetized superconductors and hybrid structures, and measure the irreversible demagnetization of these structures when they are subjected to magnetic field cycles that are not parallel to their magnetization. We also investigate the levitation behavior of hybrid structures subjected to the non-uniform field of a permanent magnet or a combination of permanent magnets used as guideway for levitation applications, and compare the results to those obtained with a bulk superconductor alone. This abstract is for the special session on bulk superconductivity. References: [1] Egan R. et al., Rev. Sci. Instrum. 86 (2015) 025107 [2] M. P. Philippe et al., Supercond. Sci. Technol. 28 (2015) 095008 Acknowledgments: We greatly acknowledge Nippon Steel & Sumitomo Metal Corporation for providing bulk large grain GdBa2Cu3O7 (GdBCO) samples. [less ▲]

Detailed reference viewed: 42 (11 ULiège)
See detailInstrumentation, magnetic and electrical characterization of superconductors
Vanderbemden, Philippe ULiege

Conference (2017, June 27)

Detailed reference viewed: 19 (2 ULiège)