S. Le Floch

Enhancement of Tc of CyCu1−yBa2Ca2Cu3Ox from 67 K to 120 K by reduction treatments

Abstract The T c of the C y Cu 1− y Ba 2 Ca 2 Cu 3 O 8+δ ( y ≈ 0.5) phase, synthesized under high pressure, has been raised from 67 K to about 120 K by heat-treatments under reducing conditions. An increase of both a and c lattice parameters occurs concomitantly with that of T c (before: a = 3.858 (1), c = 14.766 (9) A , after: a ≈ 3.862(1), c ≈ 14.825 (3) A ). Subsequent heat-treatments under oxygen atmosphere lead, reversibly, to a decrease of T c and of the lattice parameters. Consequently, the increase of T c has been associated with an oxygen loss. Measurements of the resistivity under high pressure indicate that after reduction the overdoped as-prepared samples go towards the optimal doping. A structural model is proposed about the position of the mobile oxygen in the basal plane of the structure.

Crystallization of Y3Al5O12 garnet from deep undercooled melt effect of the Al-Ga substitution

Abstract Melts of garnet Y 3 Al 5 O 12 and compositions lying on both sides of this compound were rapidly cooled in contactless conditions by using an aerodynamic levitation device coupled with a CO 2 laser. The crystallized phases were analyzed at room temperature by XRD and 27 Al MAS NMR. As the temperature reached by the supercooled melt decreases, the solidification was shown to lead successively to a mixture of YAlO 3 and Al 2 O 3 , a monophasic garnet type structure and a glass. The formation of Y 3 Al 5 O 12 starts at about 920 °C under the melting temperature and only 50 °C above the glass transition temperature. The solidification behaviour of liquid Y 3 Al 5− x Ga x O 12 ( x = 0.5, 1) was similar. Results clearly show the difficulties of garnet to nucleate. They are consistent with a rapid increase of the melt viscosity in the temperature range just above the formation of glass.

The importance of the precursor in high-pressure synthesis of Hg-based superconductors

Abstract The formation process of high-pressure synthesized Hg-based superconductors is characterized in terms of the average copper valence (〈V Cu 〉) of the precursor. This parameter is varied by oxygen intake of non-stoichiometric phases during the precursor synthesis at specific temperature, time and cooling process treatment. The average copper valence is shown to be a controllable parameter that determines both the overall composition of the precursor and the proportion of the different phases present in it. More important, the average copper valence of the precursor seems to control the formation of a given member n of the HgBa 2 Ca n −1 Cu n O 2 n +2+δ homologous series. We find that higher precursor copper valences favor lower members of the series while higher members are only obtained with lower 〈V Cu 〉.

Metastable solidification process of Ln3Al5O12 (Ln≡Dy, Y, Lu) garnets

Abstract The solidification process of liquid Ln 3 Al 5 O 12 (Ln ≡ Dy, Y, Lu) garnets was investigated by differential thermal analysis (DTA) and a critical temperature T cl was found. Cooling from temperatures below T cl leads to the crystallization of the single garnet phase. Cooling from temperatures above T cl leads to the solidification of a mixture of LnAlO 3 perovskite and α-Al 2 O 3 . For Ln ≡ Y, the results are in agreement with previously proposed stable and metastable phase diagrams. The variation of T cl with composition was investigated. Rapid cooling of liquid yttrium aluminium garnet in contactless conditions was performed, using an aerodynamic levitation device coupled with a CO 2 laser. The results are consistent with those obtained by DTA. In addition, cooling from temperatures above around 2400 °C leads to the crystallization of a metastable form of garnet.

Gold substitution in mercury cuprate superconductors

Abstract By using the high-pressure high-temperature technique, we succeeded in substituting in the mercury cuprate superconductors up to about 40% of mercury by gold. The samples were studied by X-ray and neutron powder diffraction, electron microscopy, energy dispersive spectroscopy. AC suceptibility measurements were also performed. The experimental data obtained for the Au substituted Hg-1223 compound indicate that the substitution leads to overdoping of the CuO 2 layers, with a progressive decrease of cell parameters and T c with increasing gold content. Taking into account these results, we used gold substitution to increase T c of a Hg-1245 sample up to 110 K. The corresponding unsubstituted samples are normally obtained by the high-pressure high-temperature technique in an underdoped state with a T c around 100 K.

Permanently densified SiO2 glasses: a structural approach.

Densified silica can be obtained by different pressure and temperature paths and for different stress conditions, hydrostatic or including shear. The density is usually the macroscopic parameter used to characterize the different compressed silica samples. The aim of our present study is to compare structural modifications for silica glass, densified from several routes. For this, densified silica glasses are prepared from cold and high temperature (up to 1020 °C) compressions. The different densified glasses obtained in our study are characterized by micro-Raman spectroscopy. Intertetrahedral angles from the main band relative to the bending mode decrease and their values are larger for densified samples from high temperature compression than those samples from cold compression. The relative amount of 3-membered rings deduced from the D2 line area increases as a function of density for cold compression. The temperature increase during the compression process induces a decrease of the 3 fold ring population. Moreover, 3 fold rings are more deformed and stressed for densified samples at room temperature at the expense of those densified at high temperature. Temperature plays a main role in the reorganization structure during the densification and leads to obtaining a more relaxed structure with lower stresses than glasses densified from cold compression. The role of hydrostatic or non-hydrostatic applied stresses on the glass structure is discussed. From the Sen and Thorpe central force model, intertetrahedral angle average value and their distribution are estimated.

Vascular imaging modulography: an experimental in vitro study

The major cause of thrombosis, occlusion and acute coronary syndrome is the result of atherosclerotic plaque rupture (Virmani et al. 2000). It is now recognised that the prediction of the vulnerable coronary plaque rupture requires not only an accurate quantification of fibrous cap thickness, Capthick (minimum distance between the necrotic core and the vessel lumen) and necrotic core morphology (Virmani et al. 2000), but also a precise estimation of the mechanical properties of plaque components. Indeed, such knowledge allows a precise evaluation of the peak capstress amplitude that appears to be responsible for plaque rupture (Ohayon et al. 2008). Several studies have been performed to reconstruct a Young’s modulus (YM) map (or modulograms) from intravascular strain elastograms (Baldewsing et al. 2008). In a previous work, our group proposed an original theoretical imaging modulography approach (iMOD), combining a dynamic segmentation method with an optimisation procedure to extract both plaque morphology and modulograms of the atherosclerotic plaque (Le Floc’h et al. 2009). In the present study, an in vitro experiment was designed to investigate the performance and robustness of our algorithm iMOD.

Synthesis of mercury cuprates

Mercury cuprates have very interesting potential applications that have not been thoroughly explored until now because of the complexity of their synthesis. This paper presents an overview of recent results concerning their processing. At first, a simple sol-gel technique is described that permits one to easily and intimately mix the precursors. The method uses the gelification of an inorganic solution of the cations by acrylamide polymerization. Mercuration of the precursor at moderate pressures (<2-5 MPa) is then discussed. The control of the total pressure during the synthesis by a simple method is shown, and this enables one to quantify some important parameters of the synthesis and to optimize the superconducting properties. This method has been also used successfully to incorporate mercury into layers of precursors and then to form thick layers of superconducting (Hg, Re)-1223, c-axis oriented. Finally, mercuration at higher pressures (up to 6 GPa) is considered and the case of the double mercury layer Hg-2212 is discussed in connection with the oxygen content of the reactants.

A control architecture with stabilizer for 3D stable dynamic walking of SHERPA biped robot on compliant ground

This paper deals with design and implementation of a control architecture for 3D dynamic walking with foot/ground compliant contact. This architecture includes a ZMP-based pattern generator, a computed torque controller based on the reduced order dynamics of the system, and a stabilizer to enhance the stability robustness of the control architecture. The effectiveness of the proposed control architecture is shown through numerical simulations.

The effect of Sr substitution on superconductivity in Hg2(Ba1-ySry)2YCu2O8-δ: I. A neutron powder diffraction study

The effect of Sr chemical pressure on superconductivity was investigated in Hg2(Ba1?ySry)2YCu2O8??. The samples were synthesized at high pressure and high temperature from y = 0.0 to full substitution, y = 1.0. These Sr substituted compounds are superconducting, without Ca doping on the Y site, and show a Tc increasing with Sr, reaching 42?K for y = 1.0. A detailed neutron powder diffraction study compares the structural changes induced by this chemical Sr/Ba substitution and the mechanical pressure effects in the Hg-2212 system. It shows the strong decrease of the three Ba/Sr?O distances and consequently the shrinkage of Cu?O 1in?plane and Cu?O 2apical bonds. These structural changes, by affecting the charge transfer which occurs between the charge reservoir and the superconducting block, are responsible of the Tc enhancement with Sr content.