G. Desgardin

French National Centre for Scientific Research, Lutetia Parisorum, Île-de-France, France

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Publications (113)138.39 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We report point contact spectroscopy measurements on YBa2Cu3O7−δ thin films and textured bulk ceramics. The I(V) curves measured along the Copper-Oxygen (ab) plane direction are characteristic of the Andreev reflection phenomenon, with a sharp break at the gap edge. This allows a rather precise determination of the (in plane) gap, Δ = 20 mev. Subgap structure suggests the existence of a smaller gap for some orientations in the (ab) plane, of the order of 12 mev. The data also enabled us to set a lower bound to the inplane Fermi velocity in YBa2Cu3O7−δ: vF ≥ 7 × 107cm/sec. The value of vF has a crucial importance in the determination of the theoretical explanation for superconductivity in the High Temperature Superconductors. For out of plane measurements, only a weak zero bias anomaly is observed at energies up to 5 mev.
    International Journal of Modern Physics B 01/2012; 07(01n03). · 0.46 Impact Factor
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    ABSTRACT: 010 Figure 3: Laue diagrams (top images) measured a) along the c axis and b) along the a/b axes of YBCO. Insets show the three main orientation components. Red, blue and white crosses in the bottom centre image show the c-axis orientation of all the domains and their misorientation along an elongated-like dispersion, to be compared to the {001} pole.
    01/2005;
  • M. Pollet, S. Marinel, G. Desgardin
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    ABSTRACT: The aim of this work is to obtain CaZrO3 powders that are co-sinterable with nickel electrodes. Since CaZrO3 is often sintered at temperatures higher than 1550 °C to achieve high sample densities, obtaining a powder co-sinterable with Ni would require lower sintering temperatures. The co-sintering also requires that the reaction takes place in a reducing atmosphere. After optimizing the thermal cycle for the phase synthesis, several approaches were investigated to decrease the CaZrO3 sintering temperature. First, the grain size effect on the densification was studied to enhance the calcined powder's reactivity before sintering. In addition, the Ca/Zr ratio was investigated, and the effect of doping with lithium salts was examined. Similarly, in order to allow co-sintering with base-metal inner electrodes, the sinterability and the properties of the dielectric were examined in a reducing atmosphere. Finally, a co-sintered material was synthesized, resulting in good dielectric properties. The samples were analyzed in terms of structure and microstructure. The electric and dielectric properties were also measured on sintered samples.
    Journal of the European Ceramic Society 01/2004; 24(1):119–127. · 2.36 Impact Factor
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    ABSTRACT: The NdBa2Cu3O7−δ compound exhibits better critical current densities under high magnetic field than YBa2Cu3O7−δ. As it has been shown that the use of dopants such as CeO2, PtO2 or SnO2 can increase the superconducting properties of YBa2Cu3O7−δ, the effects of neodymium oxide additions as a dopant are envisaged. The SEM study of the microstructure of the samples shows that no refinement of the Y211 phase occurs due to neodymium additions but without cancelling the refinement due to CeO2 additions. No secondary phases containing neodymium have been observed. EDS analyses show that neodymium is present in both the Y123 matrix and the Y211 particles, about twice more in the Y211 particles than in the Y123 matrix. The YBa2Cu3O7−δ usual annealing process (430 °C—150 h in flowing oxygen for a 16 mm diameter pellet) leads to Tc(onset) of 91 K with large transition and poor critical current densities. A new annealing process at lower temperatures (between 430 and 300 °C) gives samples with sharper transitions (Tc(onset) of 91.4 K) and good critical current densities (around 50,000 A cm−2 at 77 K and 0 T). However these critical current densities are similar to those usually obtained for neodymium undoped compositions.
    Physica C Superconductivity 08/2002; · 0.72 Impact Factor
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    ABSTRACT: The effects of Nd2O3 doping (from 0.25 to 1% wt of Nd2O3) of top-seeding-melt-texture-growth-processed YBa2Cu3O7−δ (Y123) ceramics are investigated. The texturing thermal cycle of the pellets is similar to the one used to texture neodymium-free Y123 samples. The SEM study of the microstructures of the doped samples does not show significant differences compared to those of textured neodymium-free Y123 samples; the size of the Y2Ba1Cu1O5 particles (Y211) does not change and no secondary phase appears. From EDS analysis, neodymium is found in both the Y123 matrix and the Y211 particles (about 1.80% atom of neodymium in the Y211 particles and about 0.7% atom of neodymium in the Y123 matrix for 1% wt of Nd2O3). After the usual oxygen annealing process used for neodymium-free Y123 samples, the samples show poor superconducting properties. A second similar annealing does not improve these properties. A new annealing process at lower temperatures (between 430 and 300 °C) leads to samples with good superconducting properties, but only similar to those usually observed for neodymium undoped compositions.
    Superconductor Science and Technology 01/2002; 15(2):206. · 2.76 Impact Factor
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    ABSTRACT: The NdBa2Cu3O7-delta compound exhibits better critical current densities under high magnetic field than YBa2Cu3O7-delta. As it has been shown that the use of dopants such as CeO2, PtO2 or SnO2 can increase the superconducting properties of YBa2Cu3O7-delta, the effects of neodymium oxide additions as a dopant are envisaged. The SEM study of the microstructure of the samples shows that no refinement of the Y211 phase occurs due to neodymium additions but without cancelling the refinement due to CeO2 additions. No secondary phases containing neodymium have been observed. EDS analyses show that neodymium is present in both the Y123 matrix and the Y211 particles, about twice more in the Y211 particles than in the Y123 matrix. The YBa2Cu3O7-delta usual annealing process (430 °C-150 h in flowing oxygen for a 16 mm diameter pellet) leads to Tc(onset) of 91 K with large transition and poor critical current densities. A new annealing process at lower temperatures (between 430 and 300 °C) gives samples with sharper transitions (Tc(onset) of 91.4 K) and good critical current densities (around 50,000 Acm-2 at 77 K and 0 T). However these critical current densities are similar to those usually obtained for neodymium undoped compositions.
    Physica C-superconductivity and Its Applications - PHYSICA C. 01/2002; 372:1127-1130.
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    ABSTRACT: According to recent studies, (Nd0.33Eu0.33Gd0.33)Ba2Cu3O7-delta (NEG123) material presents very attractive physical properties owing to an important fishtail effect. However, this three light rare earth (LRE=Nd, Eu and Gd) elements have ionic radius closed to the barium's one leading to the solid solution formation between the LRE and the Ba sites. These substitutions are detrimental for the superconducting properties and up to now, the synthesis under 1% of O2 in Ar is required to achieve interesting properties. In this work, it is shown that, using the Bridgman vertical configuration, single-domains of stoichiometric NEG123 with an optimal critical temperature of 94 K is obtained under air. Satisfying critical current density of 55 kA/cm2 is also measured at 77 K under self field and a fishtail effect can be observed at 2 T and 77 K.
    Physica C Superconductivity 01/2002; 372:1099-1102. · 0.72 Impact Factor
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    ABSTRACT: Conductors cut from large YBCO and NdBCO single domain crystals, textured by the top seeding technique, are measured in transport current. For YBCO, the magnetic field angular dependence of critical current densities along the ab planes and along the c-axis is measured as a function of temperature. A special accent is put on measurements with a configuration of the field in the ab planes and the current along the c-axis for which vortex pinning is strongly influenced by twin planes.
    Physica C Superconductivity 01/2002; 372:1843-1846. · 0.72 Impact Factor
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    C. Harnois, X. Chaud, I. Laffez, G. Desgardin
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    ABSTRACT: The multi-seeding and welding techniques are employed to join top seeded melt textured domains of YBa2Cu3O7−δ along the a- or b-direction. Silver doped Y123 is used as a welding compound. In the case of the dual seeded sample, micro-structural features and trapped field maps reveal that cracks existence and large non-superconducting agglomerations at the frontier between the two domains lead to a bad junction quality. On the contrary, the results obtained on the welded sample show that the spacer material has effectively grown from the textured domains and that the junction is clean enough to allow superconducting current to flow from one domain to the other.
    Physica C Superconductivity 01/2002; 372:1103-1106. · 0.72 Impact Factor
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    ABSTRACT: The addition of silver to YBa2Cu3O7−δ (Y123) is well known to decrease its decomposition temperature. This study demonstrates the success in welding large Y123 textured samples with Ag doped Y123 junctions. Attempts to weld two melt textured domains of Y123 by a 1 mm thick layer of Ag doped Y123 are reported. The crucial importance of the initial Y211 excess content in the spacer material is pointed out. Microstructural observations, trapped field measurements and transport critical current density measurements prove that for a given Y211 amount a very high joint quality can be obtained. A transport critical current density as high as 10 kA/cm2 is registered through the junction at 77 K and 5 T when the applied field is parallel to the (a,b) planes.
    Physica C Superconductivity 01/2002; 383(3):269-278. · 0.72 Impact Factor
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    E. Guilmeau, B. Andrzejewski, G. Desgardin
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    ABSTRACT: The phase equilibria between Bi2201, Bi2212 and Bi2223 were established relative to the sintering temperature. By improving the sintering conditions (temperature, intermediate grindings), a procedure was determined to prevent the formation of Bi2201 phase and also to obtain nearly pure Bi2223 powder. It was found that the residual Bi2201 phase is an important limiting factor of superconducting properties—samples rich in the Bi2201 phase exhibited almost no intergranular critical currents and fields, while the highest superconducting values were found in the samples free of the phase. Magnetic and transport measurements showed that the transport behaviour of the grain boundaries becomes semiconducting in samples sintered at high temperature, in which an increase of the Bi2201 phase content is observed.
    Physica C Superconductivity 01/2002; 377(3):304-312. · 0.72 Impact Factor
  • V. Garnier, A. Ambrosini, G. Desgardin
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    ABSTRACT: In this work, we report the secondary phase evolution that occurs during calcination of Bi-2223 precursor powder prepared by the polymer-matrix method. The effect of the calcination conditions (time, temperature, and intermediate milling) on the formation of secondary phases has been studied. The samples were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis. The calcined powder phase assemblage is sensitive to the calcination conditions, and control of the calcination parameters is necessary to form the adequate reactive secondary phases (Bi-2212, Ca2PbO4, Ca2CuO3, CuO). These phases are necessary for the proper synthesis of Bi-2223 during the sintering step. The optimized calcination conditions necessary to obtain these phases are 24 h at 820 °C.
    Journal of Materials Science Materials in Electronics 01/2002; 13(6):353-356. · 1.49 Impact Factor
  • V. Garnier, S. Marinel, G. Desgardin
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    ABSTRACT: The effect of the addition of SnO2 nano-particles on Bi-2223 phase formation has been studied. Tin oxide was added at different intervals of the synthesis process. It is shown that the addition of SnO2 inhibits the Bi-2223 phase formation regardless of when it is introduced to the system. SnO2 partially reacts with calcium to form the Ca2SnO4 phase. This phase is stable throughout the synthesis process. As a consequence, the formation of the Bi-2223 phase is compromised for the benefit of secondary phases such as Bi-2212, Ca2SnO4, Sr0.15Ca0.85CuO2 and Cu2SrO2. The c parameter of the Bi-2223 phase increases when tin is added. A possible explanation is a partial substitution of Cu2+ by Sn4+.
    Journal of Materials Science 01/2002; 37(9):1785-1788. · 2.16 Impact Factor
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    ABSTRACT: According to recent studies, (Nd0.33Eu0.33Gd0.33)Ba2Cu3O7−δ (NEG123) material presents very attractive physical properties owing to an important fishtail effect. However, this three light rare earth (LRE=Nd, Eu and Gd) elements have ionic radius closed to the barium's one leading to the solid solution formation between the LRE and the Ba sites. These substitutions are detrimental for the superconducting properties and up to now, the synthesis under 1% of O2 in Ar is required to achieve interesting properties. In this work, it is shown that, using the Bridgman vertical configuration, single-domains of stoichiometric NEG123 with an optimal critical temperature of 94 K is obtained under air. Satisfying critical current density of 55 kA/cm2 is also measured at 77 K under self field and a fishtail effect can be observed at 2 T and 77 K.
    Physica C-superconductivity and Its Applications - PHYSICA C. 01/2002; 372:1099-1102.
  • S. Marinel, M. Pollet, G. Desgardin
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    ABSTRACT: CaZrO3 exhibits attractive dielectric properties for applications as type I capacitors and mixed resonators. The relatively high permittivity combined with the low dielectric losses of this ceramic are convenient for processing such passive components. However, CaZrO3 is generally sintered at high temperature (about 1550 °C), which is detrimental from a technological point of view. Thus, the aim of this work is to investigate the possibility of processing CaZrO3 at lower temperature and a short processing time while maintaining a high density. To obtain this goal, microwave sintering has been developed in a single-mode microwave cavity. Microwave sintering of CaZrO3 was tested versus the resonance mode and by using various susceptors for combining microwave and conventional heating. Parallel to the process improvements, investigations of the effect of Zr non-stoichiometry and the addition of a sintering agent (lithium fluoride) were undertaken. By combining the optimized composition with the improved process, dense CaZrO3-based ceramics have been processed in a few minutes at relatively low temperature (lower than 1100 °C). Moreover, such samples exhibit satisfactory dielectric properties.
    Journal of Materials Science Materials in Electronics 01/2002; 13(3):149-155. · 1.49 Impact Factor
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    V. Garnier, S. Marinel, G. Desgardin
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    ABSTRACT: We report bulk Bi-2223 transport measurements for samples of various synthesis conditions. The measurements were carried out at 77 K in self-field and the critical current density, JC, reaches up to 12000 A/cm2 depending on the sample's preparation. The samples were prepared using a polymer matrix method. The same calcination procedure was performed on all the samples. After milling and pelletizing, calcined pellets were either directly sinter-forged (838 °C/20 h/10 Mpa) or sintered at 838 °C for various times (25–50–75–100–150–200 h) before being sinter-forged under the same conditions as the just-calcined pellets. The resulting hot-forged discs were analysed in terms of texture quality (50% of the Bi-2223 grains are aligned within an angle ranging from 6.2° to 3.9°) and Bi-2223 phase percentage (ranging from 26% to 95%). The discs were cut into bars which were either directly annealed (825 °C/100 h+1 °C per hour to 800 °C under 7.5% O2 balance N2) or subjected to various (1–3) sintering steps (838 °C/50 h) with or without intermediate cold pressing (1 GPa) before annealing. Transport measurements were performed on these samples, and the results are discussed in terms of Bi-2223 phase percentage, texture quality, grain boundaries, effect of cold intermediate pressing and post sintering. This novel type of bulk synthesis process is similar to the one used for tapes, and it allows the preparation of more reproducible bulk samples with high JC values and better grain connectivity.
    Physica C Superconductivity 01/2002; 372:1107-1110. · 0.72 Impact Factor
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    ABSTRACT: Conductors cut from large YBCO and NdBCO single domain crystals, textured by the top seeding technique, are measured in transport current. For YBCO, the magnetic field angular dependence of critical current densities along the ab planes and along the c-axis is measured as a function of temperature. A special accent is put on measurements with a configuration of the field in the ab planes and the current along the c-axis for which vortex pinning is strongly influenced by twin planes.
    Physica C-superconductivity and Its Applications - PHYSICA C. 01/2002; 372:1843-1846.
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    ABSTRACT: TSMTG (top seeding melt texture growth)-processed YBa2Cu3O7-δ ceramics have reached such critical current densities (Jc) that it is now possible to envisage technical applications such as magnetic bearings or fault current limiters. Most authors believe that in this material, defects (such as Y-211 inclusions, oxygen vacancies, dislocations, stacking faults, etc) can act as pinning centres and hence increase critical current densities. The effects on the critical current density of a uniaxial pressure, applied along a direction perpendicular to the c-axis of Y-123, at 400 °C under an air atmosphere are investigated. These conditions were chosen to avoid oxygen loss in the samples, because oxygen content is the most critical parameter for superconducting properties. Thus this allows a sample with the same oxygen content to be compared directly before and after experiment. TSMTG-processed Y-123 ceramics exposed to thermomechanical treatment show a complex behaviour. For low pressures (lower than 2 MPa) a significant increase in the critical current density (up to 1.37 times at H = 0, for a pressure of 1.38 MPa applied for 22 h at 400 °C under an air atmosphere) is observed and attributed to an increase in the dislocation density. For higher pressures, the critical current densities under low magnetic fields are always lower but sometimes slightly higher at moderate magnetic fields (between 1-3 T).
    Superconductor Science and Technology 10/2001; 14(11):973. · 2.76 Impact Factor
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    ABSTRACT: Nd123 bars were textured using the melt texturing growth process in a vertical furnace by the floating zone method. Different applied growth rates were tested, from 1 to 4 mm/h, combined with different secondary phases additions (Nd4Ba2Cu2O5 and Nd3.6Ba2.2Cu1.9Oy). Nd123 single-domains were obtained independently of the tested conditions and correlation between process conditions (composition, growth rate), microstructure and superconducting properties were investigated. An optimized Tc (96 K) with a sharp transition is obtained without Ar annealing at high temperature generally necessary to recover the optimal Tc. This attractive result is linked with the process used. Jc curves were measured and results were discussed in relation with process conditions.
    Physica C Superconductivity 10/2001; 361(4):274–282. · 0.72 Impact Factor
  • V Garnier, R Caillard, G Desgardin
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    ABSTRACT: Bi-2223 ceramic has been prepared by a polymer matrix method using polyethylenimine (PEI, 50 wt.% H2O). This route leads to an intimate and a homogeneous mix of the starting metal acetates, and allows a rapid Bi-2223 phase formation compare to the solid state method. Nevertheless, the grain size and the kinetics of formation of the Bi-2223 phase depends on the calcination conditions and thus on the phase assemblage formed at the end of the calcination, which determined the reactivity during sintering. The sintering time, temperature and intermediate milling have also been optimized to reach the Bi-2223 powder purity up to 95%.
    Journal of the European Ceramic Society 09/2001; 21(9):1139–1142. · 2.36 Impact Factor

Publication Stats

452 Citations
138.39 Total Impact Points

Institutions

  • 1998–2001
    • French National Centre for Scientific Research
      • Institut Néel
      Lutetia Parisorum, Île-de-France, France
  • 1995–1997
    • Université de Caen Basse-Normandie
      Caen, Lower Normandy, France
  • 1992
    • Tel Aviv University
      • Department of Physics and Astronomy
      Tel Aviv, Tel Aviv, Israel