AC Magnetic Measurements on Superconductors: Design of a Device for Magneto-Thermal Measurements

This work describes the design and realisation of an apparatus to measure simultaneously the AC magnetic properties and the temperature distribution on the top surface of bulk superconducting samples (up to 32 mm in diameter) in cryogenic conditions (temperature range 78-120 K). First we describe the experimental set-up used for simultaneous thermal and magnetic characterization of the sample. Next we describe the practical considerations required for generating the large AC magnetic fields, possibly in the presence of DC fields. In the third section we present a custom-made high speed data acquisition system for replacing the laboratory devices (DC voltmeter and AC lock-in amplifiers) when both temperature and magnetic data need to be recorded at high a sampling rate. The performances and limitations of the system are discussed.Peer reviewe

Development of a microcontroller-based phase resolved partial discharge measurement system with application to the monitoring of flash sintering discharge patterns

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Finite element analysis of hot spots in flash sintering

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Electrical transport and magnetic properties of Mn3O4-La0.7Ca0.3MnO3 ceramic composites prepared by a one-step spray-drying technique

peer reviewedLa0.7Ca0.3MnO3/Mn3O4 composites can be synthesized in one step by thermal treatment of a spray-dried precursor, instead of mixing pre-synthesized powders. Another advantage of this composite system is that a long sintering step can be used without leading to significant modification of the manganite composition. The percolation threshold is reached at ~ 20 vol% of manganite phase. The 77 K low field magnetoresistance is enhanced to ~ 11 % at 0.15 T when the composition is close to the percolation threshold. (c) 2007 Elsevier Ltd. All rights reserved

Magnetic shielding properties of high- Tc superconducting hollow cylinders: model combining experimental data for axial and transverse magnetic field configurations

peer reviewedMagnetic shielding efficiency was measured on high- Tc superconducting hollow cylinders subjected to either an axial or a transverse magnetic field in a large range of field sweep rates, dBapp/dt. The behaviour of the superconductor was modelled in order to reproduce the main features of the field penetration curves by using a minimum number of free parameters suitable for both magnetic field orientations. The field penetration measurements were carried out on Pb-doped Bi-2223 tubes at 77 K by applying linearly increasing magnetic fields with a constant sweep rate ranging between 10 uT s[?]1 and 10 mT s[?]1 for both directions of the applied magnetic field. The experimental curves of the internal field versus the applied field, Bin(Bapp), show that, at a given sweep rate, the magnetic field for which the penetration occurs, Blim, is lower for the transverse configuration than for the axial configuration. A power law dependence with large exponent, n', is found between Blim and dBapp/dt. The values of n' are nearly the same for both configurations. We show that the main features of the curves Bin(Bapp) can be reproduced using a simple 2D model, based on the method of Brandt, involving a E(J) power law with an n-exponent and a field-dependent critical current density, Jc(B), (following the Kim model: Jc = Jc0(1+B/B1)[?]1). In particular, a linear relationship between the measured n'-exponents and the n-exponent of the E(J) power law is suggested by taking into account the field dependence of the critical current density. Differences between the axial and the transverse shielding properties can be simply attributed to demagnetizing fields

In situ measurements of partial discharge patterns on porous YSZ pellets pressed between planar platinum electrodes used for flash sintering

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)

Use of phase resolved partial discharges for studying the incubation period of room temperature flash sintering of zinc oxide52

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Low Thermal Conductivity Adhesive as a Key Enabler for Compact, Low-Cost Packaging for Metal-Oxide Gas Sensors

peer reviewedMetal-oxide (MOX) gas sensors commonly rely on custom packaging solution. With an ever-increasing demand for MOX gas sensors, there is a clear need for a low cost, compact and high-performance package. During normal operation, MOX sensors are heated up to a temperature in the typical range of 200-300°C. However, the generated heat must not damage or degrade any other part of the assembly. Using 3D finite elements modelling, we developed an optimal package configuration. To thermally insulate the assembly from the heated MOX sensor we have developed in-house a low thermal conductivity xerogel-epoxy composite with 22.7% by weight xerogel and a thermal conductivity of 107.9 mW m-1 K-1 which is a reduction exceeding 30% compared to commercially available epoxy. Based on the low thermal conductivity xerogel-epoxy composite, we have developed a novel packaging approach that can suit the large family of MOX sensors. The developed alternative packaging solution includes a small number of assembly steps and uses standard processes and techniques. The assembled MOX sensor is low cost and has a low power consumption, while all thermally sensitive assembly parts remain at low temperature during the system’s lifetime.Microsystème_ULg Microsys Project funded by Wallonia, Belgium; and in part by the MicroCProject co-funded by the European Regional Development Fund (ERDF), Wallonia, Belgium, under Grant 675781-642409

Low-field magnetoresistance in La0.7Ca0.3MnO3 manganite compounds prepared by the spray drying technique

peer reviewedCalcium-substituted lanthanum manganite compounds were synthesized by the spray drying technique. This method - whose main advantages are versatility, high reproducibility and scalability - yields small grain materials of high homogeneity and displaying low-field magnetoresistance effects. We report about the physical and chemical characterizations of these samples in order to investigate the potential interest of spray drying for the production of materials for low-field magnetoresistance applications. We have studied the dependence of the low-field magnetoresistance on the temperature and duration of the thermal treatment applied to the pelletized powders. The issue of the shape anisotropy (demagnetisation effects) influence on the magnetoresistance properties has also been dealt with. (C) 2005 Springer Science + Business Media, Inc

Silver paint as a soldering agent for DyBaCuO single-domain welding

peer reviewedSilver paint has been tested as a soldering agent for DyBaCuO 4 single-domain welding. Junctions have been manufactured on Dy-Ba-Cu-O single domains cut either along planes parallel to the c-axis IT or along the ab-planes. Microstructural and superconducting characterizations of the samples have been performed. For both types of junctions, the microstructure in the joined area is very clean: no secondary phase or Ag particle segregation has been observed. Electrical and magnetic measurements for all configurations of interest are reported (rho(T) curves, and Hall probe mapping). The narrow resistive superconducting transition reported for all configurations shows that the artificial junction does not affect significantly the measured superconducting properties of the material