Liège, Belgium
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Fagnard J.-F.,SUPRATECS | Elschner S.,Mannheim University of Applied Sciences | Hobl A.,Nexans SuperConductors GmbH | Bock J.,Nexans SuperConductors GmbH | And 2 more authors.
Superconductor Science and Technology | Year: 2012

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.

Koutzarova T.,Bulgarian Academy of Science | Kolev S.,Bulgarian Academy of Science | Grigorov K.,Bulgarian Academy of Science | Ghelev C.,Bulgarian Academy of Science | And 6 more authors.
Solid State Phenomena | Year: 2010

Thin hexagonal barium hexaferrite particles synthesized using the microemulsion technique were studied. A water-in-oil reverse microemulsion system with cetyltrimethylammonium bromide (CTAB) as a cationic surfactant, n-butanol as a co-surfactant, n-hexanol as a continuous oil phase, and an aqueous phase were used. The microstructural and magnetic properties were investigated. The particles obtained were mono-domain with average particle size 280 nm. The magnetic properties of the powder were investigated at 4.2 K and at room temperature. The saturation magnetization was 48.86 emu/g and the coercivity, 2.4 × 105 A/m at room temperature. The anisotropy field Ha and magneto-crystalline anisotropy K1 were 1.4 × 106 A/m and 2.37 × 105 J/m3, respectively. © (2010) Trans Tech Publications.

Peekala M.,University of Warsaw | Wolff-Fabris F.,Helmholtz Center Dresden | Wolff-Fabris F.,Los Alamos National Laboratory | Fagnard J.-F.,University of Liège | And 5 more authors.
Journal of Magnetism and Magnetic Materials | Year: 2013

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. © 2013 Elsevier B.V.

Marchal C.,SUPRATECS | Fagnard J.F.,SUPRATECS | Shi Y.H.,University of Cambridge | Cardwell D.A.,University of Cambridge | And 5 more authors.
Superconductor Science and Technology | Year: 2013

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.

Egan R.,SUPRATECS | Philippe M.,SUPRATECS | Wera L.,SUPRATECS | Fagnard J.F.,SUPRATECS | And 5 more authors.
Review of Scientific Instruments | Year: 2015

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 A m2 (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). © 2015 AIP Publishing LLC.

Vanderbemden P.,SUPRATECS | Laurent P.,SUPRATECS | Fagnard J.-F.,Royal Military Academy of Belgium | Ausloos M.,SUPRATECS | And 2 more authors.
Superconductor Science and Technology | Year: 2015

In the present work we study, both theoretically and experimentally, the temperature increase in a bulk high temperature superconductor subjected to applied AC magnetic fields of large amplitude. We calculate analytically the equilibrium temperatures of the bulk sample as a function of the experimental parameters using a simple critical state model for an infinitely long type-II superconducting slab or cylinder. The results show the existence of a limit heat transfer coefficient (AUlim) separating two thermal regimes with different characteristics. The theoretical analysis predicts a 'forbidden' temperature window within which the temperature of the superconductor can never stabilize when the heat transfer coefficient is small. In addition, we determine analytical expressions for two threshold fields Htr1 and Htr2 characterizing the importance of magneto-thermal effects and show that a thermal runaway always occurs when the field amplitude is larger than Htr2. The theoretical predictions of the temperature evolution of the bulk sample during a self-heating process agree well with the experimental data. The simple analytical study presented in this paper enables order of magnitude thermal effects to be estimated for simple superconductor geometries under applied AC magnetic fields and can be used to predict the influence of experimental parameters on the self-heating characteristics of bulk type-II superconductors. © 2010 IOP Publishing Ltd 1 Printed in the UK & the USA.

Laurent P.,SUPRATECS | Fagnard J.-F.,Royal Military Academy of Belgium | Babu N.H.,Brunel University | Cardwell D.A.,University of Cambridge | And 2 more authors.
Superconductor Science and Technology | Year: 2010

In this work we study, both experimentally and numerically, the self-heating of a bulk, large YBCO pellet of aspect ratio (thickness/diameter) ∼0.4 subjected to a large AC magnetic field. To ensure accurate temperature measurements, the sample was placed in an experimental vacuum chamber to achieve a small and reproducible heat transfer coefficient between the superconductor and the cryogenic fluid. The temperature was measured at several locations on the sample surface during the self-heating process. The experimentally determined temperature gradients are found to be very small in this arrangement (<0.2 K across the radius of the superconductor). The time-dependence of the average temperature T (t) is found to agree well with a theoretical prediction based on the one-dimensional (1D) Bean model, assuming a uniform temperature in the sample. A 2D magneto-thermal model was also used to determine the space and time-dependent temperature distribution T (r, z, t) during the application of the AC field. The losses in the bulk pellet were determined using an algorithm based on the numerical method of Brandt, which was combined with a heat diffusion algorithm implemented using a finite-difference method. The model is shown to be able to reproduce the main trends of the observed temperature evolution of the bulk sample during a self-heating process. Finally, the 2D model is used to study the effect of a non-uniform distribution of critical current density Jc(r, z) on the losses within the bulk superconductor. © 2010 IOP Publishing Ltd.

Pompeo N.,Third University of Rome | Rogai R.,Third University of Rome | Ausloos M.,SUPRATECS | Cloots R.,University of Liège | And 3 more authors.
Physica C: Superconductivity and its Applications | Year: 2011

We report on microwave measurements on DyBa2Cu3O 7-δ monodomains grown by the top-seeded melt-textured technique. We measured the field increase of the surface resistance R s(H) in the a-b plane at 48.3 GHz. Measurements were performed at fixed temperatures in the range 70 K-Tc with a static magnetic field μ0H < 0.8 T parallel to the c-axis. Low field steep increase of the dissipation, typical signature of the presence of weak links, is absent, thus indicating the single-domain behavior of the sample under study. The magnetic field dependence of Rs(H) is ascribed to the dissipation caused by vortex motion. The analysis of Xs(H) points to a free-flow regime, thus allowing to obtain the vortex viscosity as a function of temperature. We compare the results with those obtained on RE-BCO systems. In particular, we consider strongly pinned films of YBa2Cu 3O7-δ with nanometric BaZrO3 inclusions. © 2011 Elsevier B.V. All rights reserved.

Kirsch S.,SUPRATECS | Fagnard J.-F.,SUPRATECS | Vanderbemden P.,SUPRATECS | Vanderheyden B.,SUPRATECS
Superconductor Science and Technology | Year: 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 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. © 2013 IOP Publishing Ltd.

Wera L.,SUPRATECS | Fagnard J.F.,SUPRATECS | Levin G.A.,Air Force Research Lab | Vanderheyden B.,SUPRATECS | Vanderbemden P.,SUPRATECS
IEEE Transactions on Applied Superconductivity | Year: 2013

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 YBa2Cu 3O7 (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 applied magnetic induction. Results show that the shielding factor, SF, (defined as the ratio between the applied magnetic 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 = 77K , 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. © 2002-2011 IEEE.

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