References of "Vanderbemden, Philippe"
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See detailMagnetic characterisation of large grain, bulk Y–Ba–Cu–O superconductor–soft ferromagnetic alloy hybrid structures
Philippe, Matthieu ULg; Fagnard, Jean-François ULg; Kirsch, Sébastien ULg et al

in Physica C: Superconductivity (2014), 502

Large grain, bulk Y–Ba–Cu–O (YBCO) high temperature superconductors (HTS) have significant potential for use in a variety of practical applications that incorporate powerful quasi-permanent magnets. In ... [more ▼]

Large grain, bulk Y–Ba–Cu–O (YBCO) high temperature superconductors (HTS) have significant potential for use in a variety of practical applications that incorporate powerful quasi-permanent magnets. In the present work, we investigate how the trapped field of such magnets can be improved by combining bulk YBCO with a soft FeNi, ferromagnetic alloy. This involves machining the alloy into components of various shapes, such as cylinders and rings, which are attached subsequently to the top surface of a solid, bulk HTS cylinder. The effect of these modifications on the magnetic hysteresis curve and trapped field of the bulk superconductor at 77 K are then studied using pick-up coil and Hall probe measurements. The experimental data are compared to finite element modelling of the magnetic flux distribution using Campbell’s algorithm. Initially we establish the validity of the technique involving pick-up coils wrapped around the bulk superconductor to obtain its magnetic hysteresis curve in a non-destructive way and highlight the difference between the measured signal and the true magnetization of the sample. We then consider the properties of hybrid ferromagnet/superconductor (F/S) structures. Hall probe measurements, together with the results of the model, establish that flux lines curve outwards through the ferromagnet, which acts, effectively, like a magnetic short circuit. Magnetic hysteresis curves show that the effects of the superconductor and the ferromagnet simply add when the ferromagnet is saturated fully by the applied field. The trapped field of the hybrid structure is always larger than that of the superconductor alone below this saturation level, and especially when the applied field is removed. The results of the study show further that the beneficial effects on the trapped field are enhanced when the ferromagnet covers the entire surface of the superconductor for different ferromagnetic components of various shapes and fixed volume. [less ▲]

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See detailDesign and fabrication of an electrode array sensor for probing the electric potential distribution at the mesoscopic scale in antistatic felts
Boutaayamou, Mohamed ULg; Lemaire, Philippe; Vanderheyden, Benoît ULg et al

in Measurement Science and Technology (2014), 25

We present an original voltage probe design for measuring the electric potential distribution at the mesoscopic scale (i.e., 1 mm–1 cm) in antistatic felts. The felts are composed of a mixture of non ... [more ▼]

We present an original voltage probe design for measuring the electric potential distribution at the mesoscopic scale (i.e., 1 mm–1 cm) in antistatic felts. The felts are composed of a mixture of non-conductive and metallic fibers and exhibit complex nonlinear electric behavior—including possibly nonlinearity and hysteresis effects—which may be due to localized electrical or electromechanical phenomena. The sensor consists of an array of 8 × 9 needle electrodes (phgr 160 µm at the shaft and less than phgr 50 µm toward the apex), which are mechanically maintained at fixed relative positions while their tips are inserted inside the fabric of the sample. The interelectrode distance is 1.5 mm and the overall active area is 12 × 12 mm². The electrical insulation resistance for nearest neighbor pairs of electrodes was found to be larger than 860 GΩ, thus making the sensor suitable for measuring antistatic felts with an electric resistance that typically does not exceed a few GΩ. The sensor was successfully used for measuring the distribution of the electric potential in a polyester fabric subjected to voltages of up to 6.2 kV, and in a sample containing 2% in weight of metallic fibers, demonstrating the presence of irreversible changes in that felt sample (i.e., with conductive fibers) at high voltages. It is concluded that the developed probe voltage is a promising technique that could be used for the assessment of the conduction mechanisms in the antistatic materials at the mesoscopic scale. [less ▲]

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See detailStructural, magnetic and magneto-transport properties of monovalent doped manganite Pr0.55K0.05Sr0.4MnO3
Thaljaoui, R.; Boujelben, W.; Pękała, M. et al

in Journal of Alloys and Compounds (2014), 611(0), 427-432

Pr0.55K0.05Sr0.4MnO3 sample have been synthesized using the conventional solid state reaction. Rietveld refinements of the X-ray diffraction patterns at room temperature confirm that the sample is single ... [more ▼]

Pr0.55K0.05Sr0.4MnO3 sample have been synthesized using the conventional solid state reaction. Rietveld refinements of the X-ray diffraction patterns at room temperature confirm that the sample is single phase and crystallizes in the orthorhombic structure with Pnma space group; the crystallite size is around 70 nm. The SEM images show that grain size spreads around 1000–1200 nm. DTA analysis does not reveal any clear transition in temperature range studied. The low-temperature DSC indicates that Curie temperature is around 297 K. Magnetization measurements in a magnetic applied field of 0.01 T exhibit a paramagnetic–ferromagnetic transition at the Curie temperature TC = 303 K. A magnetic entropy change under an applied magnetic field of 2 T is found to be 2.26 J kg−1 K−1, resulting in a large relative cooling power around 70 J/kg. Electrical resistivity measurements reveal a transition from semiconductor to metallic phase. The thermal conductivity is found to be higher than that reported for undoped and Na doped manganites reported by Thaljaoui et al. (2013). [less ▲]

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See detailMagnetocaloric effect of monovalent K doped manganites Pr0.6Sr0.4−xKxMnO3 (x=0 to 0.2)
Thaljaoui, R.; Boujelben, W.; Pękała, M. et al

in Journal of Magnetism and Magnetic Materials (2014), 352(0), 6-12

Magnetic and magnetocaloric properties are reported for polycrystalline monovalent potassium doped manganites Pr0.6Sr0.4−xKxMnO3 (x=0, 0.05, 0.1, 0.15 and 0.2) crystallized in orthorhombic structure with ... [more ▼]

Magnetic and magnetocaloric properties are reported for polycrystalline monovalent potassium doped manganites Pr0.6Sr0.4−xKxMnO3 (x=0, 0.05, 0.1, 0.15 and 0.2) crystallized in orthorhombic structure with Pnma space group. The increasing K content shifts the paramagnetic to ferromagnetic transition temperature from 310 K for x=0 to 269 K for x=0.2. The magnetic entropy change under magnetic field variation of 2 T is found to be 1.95, 3.09, 2.89, 3.05 and 3.2 J/kgK for x varying from 0 to 0.2, respectively. The highest relative cooling power of 102 J/kg is observed for the undoped sample. The sensitivity of magnetic entropy change to magnetic field is estimated by a local N(T) exponent exhibiting the characteristic temperature variation. Phenomenological universal curves of entropy change and Arrott plots confirm the second order phase transition. [less ▲]

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See detailYBa2Cu3O7-δ thick films for magnetic shielding: Electrophoretic deposition from butanol-based suspension
Closset, Raphaël ULg; Kumar, Devendra; Wera, Laurent ULg et al

in Materials Letters (2014), 119

Multilayered YBa2Cu3O7-δ (YBCO) thick films were coated on silver substrates by electrophoretic deposition (EPD) followed by heat treatment. A butanol-based YBCO suspension is used instead of the common ... [more ▼]

Multilayered YBa2Cu3O7-δ (YBCO) thick films were coated on silver substrates by electrophoretic deposition (EPD) followed by heat treatment. A butanol-based YBCO suspension is used instead of the common acetone-iodine combination. Tests with several dispersing agents reveal that a branched polyethyleneimine (PEI) dispersant develops large positive surface charge on suspended YBCO particles. As a demonstration of the performance of this new suspension formulation, a 12-layer 100 μm-thick YBCO coating was deposited on an Ag tube. The superconducting transition is sharp with onset critical temperature at 92 K. The sample can shield a magnetic field of ~1.3 mT at 77 K, i.e., the best value so far for an YBCO coating on a metallic substrate. © 2013 Elsevier B.V. [less ▲]

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See detailMeasurement of the magnetic hysteresis cycle of bulk superconductor / ferromagnet hybrids
Philippe, Matthieu ULg; Fagnard, Jean-François ULg; Wera, Laurent ULg et al

Conference (2013, November 07)

Large grain, bulk YBa2Cu3O7-δ (YBCO) high temperature superconductors (HTS) can be potentially used as powerful permanent magnets for magnetic levitation. The magnetic flux distribution above a bulk HTS ... [more ▼]

Large grain, bulk YBa2Cu3O7-δ (YBCO) high temperature superconductors (HTS) can be potentially used as powerful permanent magnets for magnetic levitation. The magnetic flux distribution above a bulk HTS magnet, however, is strongly non-uniform compared to the rather flat distribution above a ferromagnet. In the present work, we study experimentally how cylindrical pieces of FeNi soft ferromagnetic alloys can be combined with a cylindrical, bulk, large grain YBCO superconductor to improve the distribution of the trapped field at the surface or its average value through the volume of the sample. The magnetic properties of each hybrid structure are measured at 77 K under axial magnetic field up to 3 T. The remanent induction distribution near the top and bottom surfaces is determined by miniature Hall probe mapping. Pick-up coils wound around the superconductor are used to measure the average flux density inside the superconductor. This technique relies on low level voltage measurements (typically a few microvolts) and allows non-destructive measurement of the average magnetic hysteresis curve of the entire bulk superconductor. [less ▲]

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See detailSemi-analytical study of AC losses in an infinitely long superconducting cylinder surrounded by a metallic sheath: magnetic field dependent critical current density and generation of harmonics
Kirsch, Sébastien ULg; Fagnard, Jean-François ULg; Vanderbemden, Philippe ULg et al

in Superconductor Science and Technology (2013)

We study the AC losses in an infinitely long cylinder made of a superconducting core surrounded by a non-magnetic metallic sheath and subjected to an axial magnetic field. The losses are computed by ... [more ▼]

We study the AC losses in an infinitely long cylinder made of a superconducting core surrounded by a non-magnetic metallic sheath and subjected to an axial magnetic field. The losses are computed by assuming the Bean–Kim model for the superconductor and Ohmic dissipation for the metal. The time varying magnetic flux crossing the superconductor induces eddy currents in the metal sheath and, due to the nonlinear response of the superconducting material, generates harmonics in the metal current density. In turn, these currents generate distorted magnetic fields acting back on the superconductor. This coupling mechanism is sensitive to the magnetic constitutive law of the superconductor and affects both the waveform of the fields and the total losses. In this paper, we study the importance of the harmonics in the metal on the total losses, as well as their sensitivity to a field dependent critical current density following Kim's law. [less ▲]

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See detailMagnetic hysteresis cycle and remnant field distribution of bulk high temperature superconductor / ferromagnet hybrids
Philippe, Matthieu ULg; Fagnard, Jean-François ULg; Wera, Laurent ULg et al

Poster (2013, September)

Bulk (RE)BCO materials can be used potentially as powerful permanent magnets. The magnetic flux distribution above a bulk HTS magnet, however, is strongly non-uniform (conic profile predicted by the Bean ... [more ▼]

Bulk (RE)BCO materials can be used potentially as powerful permanent magnets. The magnetic flux distribution above a bulk HTS magnet, however, is strongly non-uniform (conic profile predicted by the Bean model) compared to the rather flat distribution above a ferromagnet. In the present work, we study how FeNi soft ferromagnetic alloys of different shapes can be combined with a bulk, large grain (RE)BCO superconductor (RE denotes a rare-earth element) to improve the distribution of trapped field at the surface or its average value through the volume of the sample. A FeNi ferromagnetic alloy was machined into pieces of various shapes (cylinders and rings) and attached to (i) the top surface of the bulk HTS cylinder to form bulk ferromagnet / superconductor (F/S) hybrids and (ii) to the top and bottom surfaces to form bulk F/S/F hybrids. The magnetic properties of each hybrid structure were measured under axial magnetic field at 77 K. Pick-up coils wound around the superconductor were used to measure the average magnetic induction inside the superconductor while the remnant induction distribution near the top and bottom surfaces was determined by miniature Hall probe mapping. The modifications of the hysteresis curves and flux distributions were analyzed by taking into account the ferromagnet intrinsic properties (intrinsic permeability, saturation) and geometrical properties (shape, size and volume). The results show that the effect of the ferromagnet increases with its volume. In presence of a ferromagnet, the superconductor hysteresis curve shows a combination of a diamagnetic and a ferromagnetic behaviour on which it is worth noting that (i) the bulk remnant magnetization increases and (ii) in the magnetic saturation regime of the ferromagnet, the magnetic effects of the superconductor and the ferromagnet are superimposed. The results also give evidence that flux lines curve through the ferromagnetic component, which produces a decrease of the self-demagnetizing field inside the superconductor. [less ▲]

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See detailYBCO SUPERCONDUCTING THICK FILMS: ELECTROPHORETIC DEPOSITION (EPD) ON NON-PLANAR SILVER SUBSTRATES
Namburi, Devendra Kumar ULg; Closset, Raphaël ULg; Fagnard, Jean-François ULg et al

Scientific conference (2013, January 30)

A new formulation of a stable YBa2Cu3O7-δ (YBCO) suspension is proposed in which butanol is preferred to the commonly-used acetone as the suspension medium. Appropriate surfactant has been used to develop ... [more ▼]

A new formulation of a stable YBa2Cu3O7-δ (YBCO) suspension is proposed in which butanol is preferred to the commonly-used acetone as the suspension medium. Appropriate surfactant has been used to develop the superficial charge on the YBCO particles in order to promote migration during the process of electrophoresis. YBCO thick films were deposited on silver tubes and half-tubes by electrophoretic deposition (EPD). The EPD parameters (deposition voltage, deposition time, number of layers etc.) were optimized with respect to the microstructural properties of the YBCO layers after an intermediate heat treatment at 920°C. An essential criterion is the minimization of macrocracks after the 920°C heat treatment, since it was found to favour good superconducting properties after the final heat treatment. This final heat treatment involves (i) partial melting above the peritectic temperature, (ii) peritectic recombination at lower temperature and (iii) reoxygenation at 500°C. Finally, the superconducting properties of the best films are discussed. A uniformly coated 55μm-thick YBCO film on a curved Ag substrate showed excellent superconducting properties with the onset of critical temperature at 92.2 K and an associated sharp resistive transition with transition width < 1 K. [less ▲]

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See detailMagnetic properties and anisotropy of orthorhombic DyMnO3 single crystal
Pękała, M.; Wolff-Fabris, F.; Fagnard, Jean-François ULg et al

in Journal of Magnetism and Magnetic Materials (2013), 335(0), 46-52

An orthorhombic DyMnO3 single crystal has been studied in magnetic fields up to 14 T and between 3 K and room temperature. The field dependent ordering temperature of Dy moments is deduced. The ... [more ▼]

An orthorhombic DyMnO3 single crystal has been studied in magnetic fields up to 14 T and between 3 K and room temperature. The field dependent ordering temperature of Dy moments is deduced. The paramagnetic Curie Weiss behavior is related mainly to the Dy3+sublattice whereas the Mn sublattice contribution plays a secondary role. DC magnetization measurements show marked anisotropic features, related to the anisotropic structure of a cubic system stretched along a body diagonal, with a magnetic easy axis parallel to the crystallographic b axis. A temperature and field dependent spin flop transition is observed below 9 K, when relatively weak magnetocrystalline anisotropy is overcome by magnetic fields up to 1.6 T. [less ▲]

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See detailMagnetic susceptibility and electron magnetic resonance study of monovalent potassium doped manganites Pr0.6Sr0.4−xKxMnO3
Thaljaoui, R.; Pękała, K.; Pękała, M. et al

in Journal of Alloys and Compounds (2013), 580(0), 137-142

The monovalent potassium doped manganites Pr0.6Sr0.4−xKxMnO3 (x = 0.05–0.2) are characterized using the complementary magnetic susceptibility and electron resonance methods. In paramagnetic phase the ... [more ▼]

The monovalent potassium doped manganites Pr0.6Sr0.4−xKxMnO3 (x = 0.05–0.2) are characterized using the complementary magnetic susceptibility and electron resonance methods. In paramagnetic phase the temperature variations of the inverse magnetic susceptibility and the inverse intensity of resonance signal obey the Curie–Weiss law. A similarity in temperature variation of resonance signal width and the adiabatic polaron conductivity points to the polaron mechanism controlling the resonance linewidth. The low temperature limit of the pure paramagnetic phase is determined from the electron resonance spectra revealing the mixed phase spread down to the Curie temperature. [less ▲]

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See detailCharacterization of structures with a complex disposition of YBCO coated conductors for magnetic shielding applications
Wera, Laurent ULg; Fagnard, Jean-François ULg; Levin, G. A. et al

Poster (2013)

An efficient superconducting magnetic shield can be built as an assembly of YBCO 2G coated conductor sections. Each section is milled and placed around a cylindrical support in order to form a joint free ... [more ▼]

An efficient superconducting magnetic shield can be built as an assembly of YBCO 2G coated conductor sections. Each section is milled and placed around a cylindrical support in order to form a joint free superconducting loop where persistent currents can flow and provide a strong attenuation of a magnetic field. Our previous works have shown that this assembly is able to shield an axial quasi static (“DC”) magnetic field and that the shielding performances depend on the aspect ratio and the number of layers. The purpose of the present work is to study experimentally the shielding efficiency of several structures with a more complex orientation and position of YBCO coated conductors. Our aim is to design a magnetic shield that would be able to shield a magnetic field directed at any angle with respect to the superconducting loops. Such a structure can be obtained by placing pairs of coated conductors sections along three orthogonal axes. All experiments are carried out at 77K. The structure is subjected to a quasi-static (“DC”) magnetic field. A Hall probe measures the three components of the local magnetic induction inside the assembly as a function of the applied magnetic induction. The shielding efficiency of the structure is characterized as a function of (i) the magnetic field amplitude, (ii) the position of the Hall probe along the three axes, and (iii) the angles between the applied magnetic field and each axis. The experimental results allow us to determine the shielding efficiency in the central part of the new 3-axes structure. Although the shielding efficiency is lowered with respect to that of the traditional 1-axis-coil geometry, measurements at different field orientations allow us to identify the role played by each of the pairs of coils in screening the external magnetic field. [less ▲]

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See detailThe influence of a grain boundary on the thermal transport properties of bulk, melt-processed Y-Ba-Cu-O
Marchal, C.; Fagnard, Jean-François ULg; Shi, Y. H. et al

in Superconductor Science and Technology (2013), 26

We report the dependence of thermal conductivity, thermoelectric power and electrical resistivity on temperature for a bulk, large grain melt-processed Y-Ba-Cu-O (YBCO) high temperature superconductor ... [more ▼]

We report the dependence of thermal conductivity, thermoelectric power and electrical resistivity on temperature for a bulk, large grain melt-processed Y-Ba-Cu-O (YBCO) high temperature superconductor (HTS) containing two grains separated by a well-defined grain boundary. Transport measurements at temperatures between 10 and 300 K were carried out both within one single grain (intra-granular properties) and across the grain boundary (inter-granular properties). The influence of an applied external magnetic field of up to 8 T on the measured sample properties was also investigated. The presence of the grain boundary is found to affect strongly the electrical resistivity of the melt-processed bulk sample, but has almost no effect on its thermoelectric power and thermal conductivity, within experimental error. The results of this study provide direct evidence that the heat flow in multi-granular melt-processed YBCO bulk samples should be virtually unaffected by the presence of grain boundaries in the material. © 2013 IOP Publishing Ltd. [less ▲]

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See detailMagnetic shielding with YBCO coated conductors: influence of the geometry on its performances
Wera, Laurent ULg; Fagnard, Jean-François ULg; Levin, G. A. et al

in IEEE Transactions on Applied Superconductivity (2012), 23(3),

A superconducting magnetic shield can be built as a stack of several sections of milled 2G coated conductors. Each section consists of a closed loop where persistent currents can flow and provide a strong ... [more ▼]

A superconducting magnetic shield can be built as a stack of several sections of milled 2G coated conductors. Each section consists of a closed loop where persistent currents can flow and provide a strong attenuation of external dc magnetic fields. The purpose of the present work is to study experimentally several geometries of such magnetic shields made out of YBa2Cu3O7 (YBCO) coated conductors from SuperPower. Our aim is to investigate in detail the influence of the aspect ratio and the number of layers of the assembly on the magnetic shielding properties. In order to do so, the magnetic shield is subjected to an axial quasi-static (“dc”) magnetic field ramped slowly at a fixed sweep rate. A Hall probe is used to measure the local magnetic induction inside the assembly as a function of the appliedmagnetic induction. Results show that the shielding factor, SF, (defined as the ratio between the appliedmagnetic induction and the magnetic induction measured inside the shield) is improved for increasing aspect ratios of the global coated conductor assembly and that the threshold magnetic induction (defined for SF = 10) increases with the number of layers. Using a double layer of 18 sections at T = 77 K, dc magnetic fields up to 56 mT can be shielded by a factor larger than 10. Finally, the effect of an air gap of constant width between coated conductor sections is also characterized.(C) 2012 IEEE [less ▲]

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See detailYBa2Cu3O7-x thick films on silver tubes for magnetic shielding applications
Namburi, Devendra Kumar ULg; Closset, Raphaël ULg; Fagnard, Jean-François ULg et al

Scientific conference (2012, December 07)

YBa2Cu3O7-x (YBCO) thick films are investigated for magnetic shielding applications at low frequencies (< 1 kHz). This requires deposition of YBCO on curved substrates such as tubes, half-tubes or even ... [more ▼]

YBa2Cu3O7-x (YBCO) thick films are investigated for magnetic shielding applications at low frequencies (< 1 kHz). This requires deposition of YBCO on curved substrates such as tubes, half-tubes or even more complex shapes. Electrophoretic Deposition (EPD) is an efficient method to achieve this goal: positively-charged YBCO particles suspended in a non aqueous medium drift towards the substrate, used as negative electrode for the application of the electric field. A crucial point is then to optimize the heat treatment of the as-deposited layers in order to achieve suitable superconducting properties. In the present work, we have developed a new suspension formulation in butanol, using a suitable surfactant to stabilize the suspension of YBCO powder (grain size < 2 µm). The EPD parameters (deposition voltage, deposition time, number of layers,...) have been selected to provide uniform layers of YBCO on silver substrates of various shapes. In the proposed communication, we shall discuss in detail the optimization of the heat treatments to achieve densification, peritectic recombination and oxygenation of the YBCO thick films deposited on silver. Scanning electron microscopy coupled with energy dispersive analysis was extensively used to characterize the (i) density / porosity, (ii) presence of macro-cracks, (iii) thickness uniformity, (iv) secondary phase content. We shall then present the characterization of the superconducting properties of the best films. Typically, a uniformly coated 55μm-thick YBCO film on a curved Ag substrate shows a magnetic Tc onset at 92.2 K and sharp resistive transition (< 1K). [less ▲]

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See detailNon-destructive measurement of volume magnetic properties of large, bulk superconductors
Vanderbemden, Philippe ULg; Dennis, T.; Shi, Y. H. et al

Conference (2012, December 07)

The development of large, single grain bulk superconductors by melt processing techniques has generated a need to characterize samples magnetically over large dimensions, exceeding typically 20 mm in ... [more ▼]

The development of large, single grain bulk superconductors by melt processing techniques has generated a need to characterize samples magnetically over large dimensions, exceeding typically 20 mm in diameter. The usual magnetic characterization gives relies on miniature Hall probe mapping and gives information about the field distribution above the sample surface. If volume properties are required (e.g. magnetization hysteresis loops), the sample needs to be cut in smaller pieces, since the DC magnetic characterization systems for measurements at cryogenic temperatures generally accommodate samples of relatively small size (typically < 5-10 mm diameter). In this work we describe how the hysteresis B(H) loops of large bulk superconducting samples exceeding 10 mm diameter can be determined using home-made sensing coils, either in liquid nitrogen or within the experimental chamber of a Quantum Design Physical Property Measurement System (PPMS). Magnetic properties are measured and compared to those given by several Hall probes attached to both faces of sample. The system is used successfully to measure the DC hysteresis loops of entire (RE)BCO bulk superconducting domains. A careful data acquisition and numerical integration of pick-up coil voltage enables the sweep rate of the magnetic field to be varied from 0.5 to 10 mT/s while keeping an excellent signal/noise ratio. A simple model based on demagnetizing field approach is used to emphasize how the hysteresis loops determined by this technique differ from “true” magnetization loops derived from classical magnetic moment measurements (e.g. SQUID or VSM). These differences are supported with numerical modelling of the average magnetization of the bulk sample using the Brandt method. [less ▲]

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See detailMagnetic behaviour of soft ferromagnetic alloys attached to bulk (RE)BCO superconductors
Philippe, Matthieu ULg; Fagnard, Jean-François ULg; Dennis, Anthony et al

Poster (2012, October)

Bulk (RE)BCO materials can be used potentially as powerful permanent magnets. The magnetic field distribution around a bulk HTS magnet, however, is strongly non-uniform (conic profile predicted by the ... [more ▼]

Bulk (RE)BCO materials can be used potentially as powerful permanent magnets. The magnetic field distribution around a bulk HTS magnet, however, is strongly non-uniform (conic profile predicted by the Bean model) compared to the rather flatter distribution around a ferromagnet. In the present work, we study how FeNi soft ferromagnetic alloys of different shapes can be combined with (RE)BCO bulk superconductors to improve the distribution of trapped field or its average value at the surface of the bulk sample. The FeNi ferromagnetic alloys are machined into pieces of various shapes (e.g. cylinder, cone) attached to the bottom surface of the bulk HTS disks. The magnetic properties of each ferromagnetic/superconductor (F/S) assembly are measured under axial magnetic field at 77 K. Small coils and Hall probe mappings provide the average magnetization and field distribution respectively. The results are compared to the intrinsic magnetic hysteresis curves of the magnetic alloys and of the superconductor. Measurements of the average magnetization show that the effects of the ferromagnetic alloys depend mainly on the volume of the ferromagnet and add up to the effect of the superconductor. However, at constant ferromagnetic volume, the shape of the ferromagnetic piece is found to influence the magnetic field penetration and distribution against the top surface of the superconductor. [less ▲]

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See detailInfluence of the shielding currents lengthscale and anisotropy effects on the magnetic flux profiles of high-temperature superconductors
Vanderbemden, Philippe ULg; Lovchinov, Vassil

in Journal of Physics: Conference Series (2012), 398

The so-called "magnetic flux profile" AC inductive technique is a powerful method for determining the critical current density Jc of bulk superconductors. In this work we aim at reporting analytical ... [more ▼]

The so-called "magnetic flux profile" AC inductive technique is a powerful method for determining the critical current density Jc of bulk superconductors. In this work we aim at reporting analytical expressions for magnetic flux profiles of superconducting rectangular samples exhibiting a critical current density anisotropy. The results are used for examining the error resulting from approximating a rectangular cross-section by an "infinite cylinder" or "infinite slab" geometry. It is found that such approximations can lead to an artificial curvature of the flux profiles and errors of 10%–20% in the determination of Jc. Next, the effects of how planar defects (cracks, platelet boundaries,...) affect the magnetic flux profile signal are discussed. It is found that the magnetic flux profiles are much sensitive to the lengthscale of shielding currents, thereby providing means of investigation of the typical size of induced current loops in bulk superconductors. Finally some illustrative flux profile data measured on a bulk, large grain melt-processed YBCO single domain exhibiting Jc anisotropy are presented and discussed in relation with theoretical predictions. [less ▲]

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See detailAn experimental approach for synthesis of Fe-Al-O multiferroic fibrous material
Starbov, N; Starbova, K; Vanderbemden, Philippe ULg et al

in Journal of Physics: Conference Series (2012), 398

Basic principles of the electro-hydrodynamics are applied for synthesis of solid state Fe-Al-O multi-ferroic fibrous material. For that purpose stable blend spinning solutions comprised of a high ... [more ▼]

Basic principles of the electro-hydrodynamics are applied for synthesis of solid state Fe-Al-O multi-ferroic fibrous material. For that purpose stable blend spinning solutions comprised of a high molecular assisting organic polymer and salts of iron and aluminum are developed. These solutions are tested under electrospinning conditions and synthesis of homogeneous as spun non-woven mats characterized by fibre mean diameters in the microand nano-size range is successfully demonstrated. Multi-step thermal procedure is applied for the consecutive solvent evaporation, polymer pyrolisis and final fibre calcination. Electronoptical imaging technique and XRD are applied for revealing the sample morphology and the phase composition correspondingly. The results obtained outline the wide possibilities for fabrication of multi-ferroics fibrous nano-materials on the basis of Fe-Al-O. [less ▲]

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See detailMagnetic shielding properties of a superconducting hollow cylinder containing slits: Modelling and experiment
Fagnard, Jean-François ULg; Elschner, S.; Hobl, A. et al

in Superconductor Science and Technology (2012), 25(10), 104006

This paper deals with the magnetic properties of bulk high temperature superconducting cylinders used as magnetic shields. We investigate, both numerically and experimentally, the magnetic properties of a ... [more ▼]

This paper deals with the magnetic properties of bulk high temperature superconducting cylinders used as magnetic shields. We investigate, both numerically and experimentally, the magnetic properties of a hollow cylinder with two axial slits which cut the cylinder in equal halves. Finite element method modelling has been used with a three-dimensional geometry to help us in understanding how the superconducting currents flow in such a cut cylinder and therefore how the magnetic shielding properties are affected, depending on the magnetic field orientation. Modelling results show that the slits block the shielding current flow and act as an entrance channel for the magnetic flux lines. The contribution of the slits to the total flux density that enters the cylinder is studied through the angle formed between the applied field and the internal field. The modelled data agree nicely with magnetic shielding properties measured on a bulk Bi-2212 hollow cylinder at 77K. The results demonstrate that the magnetic flux penetration in such a geometry can be modelled successfully using only two parameters of the superconductor (constant J c and n value), which were determined from magnetic measurements on the plain cylinder. © 2012 IOP Publishing Ltd. [less ▲]

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