P. Tripodi

Possibility of high temperature superconducting phases in PdH

Possible new superconducting phases with a high critical transition temperature (Tc) have been found in stable palladium–hydrogen (PdHx) samples for stoichiometric ratio x=H/Pd⩾1, in addition to the well-known low critical transition temperature (0⩽Tc⩽9) when x is in the range (0.75⩽x⩽1.00). Possible new measured superconducting phases with critical temperature in the range 51⩽Tc⩽295 K occur. This Tc varies considerably with every milli part of x when x exceeds unit. A superconducting critical current density Jc⩾6.1×104 A cm−2 has been measured at 77 K with HDC=0 T.

Steady-state concentration profiles of hydrogen in tubular metallic membranes

Abstract This article analyzes the stress-induced diffusion (SID) model for hydrogen transport in thin cylindrical membranes when the planar approximation, usually adopted in the literature, is relaxed. The SID model, explicitly formulated for a cylindrical membrane predicts a non-linear steady-state concentration profile, characterized by a concentration gradient at the inner surface which can be significantly lower than the macroscopic concentration gradient imposed, depending on the initial hydrogen content of the membrane. We provide analytical arguments to show that this effect is due to the fact that concentration-induced stresses do not relax at steady state, and this is a peculiar feature of thin hollow cylinders, not occurring in thin plates.

Irreversibility line and magnetic field dependence of the critical current in superconducting MgB2 bulk samples

The third harmonic components of the ac susceptibility of MgB2 bulk samples have been measured as a function of applied magnetic fields, together with standard magnetization cycles. The irreversibility line (IL) of the magnetic field has been extracted from the onset of the third harmonic components. Using a (1 − t)α glass/liquid best fit where α = 1.27 IL shows a coherent length ξ divergence with exponent ν = 0.63, which indicates a 3D behaviour. Moreover, using the numerical solution of the non-linear magnetic diffusion equation, considering the creep model in a 3D vortex glass, a good description of the vortex dynamics has been obtained. The behaviour of the magnetization amplitude (~Hz) and the ac susceptibility signals (kHz), at different applied magnetic fields, 3.5 T < Hdc < 4.5 T, and at the reduced temperature 0.86 < t < 0.93 (T = 22 K), shows that the superconducting dynamic response of vortices in the MgB2 samples is not evidently dependent on the frequency.

First results about hydrogen loading by means of pulsed electrolysis of Y1Ba2Cu3O7 pellets

Abstract Y 1 Ba 2 Cu 3 O 7 (YBCO) sintered pellets have been loaded at room temperature with hydrogen by means of electrolysis in an aqueous solution (LiOH 0.3 N) with μs current pulses. The superconducting features of loaded samples have been measured as a function of the temperature by the ac magnetic susceptibility. In some conditions the YBCO pellets exhibited an enhancement of the transition temperature up to 95.4 K without a significant weakening of the superconducting grain coupling. These effects appear to be strongly dependent on the loading conditions.

Frequency dependence of susceptibility higher harmonics in YBCO samples by numerical solutions of the non linear magnetic flux diffusion equation

Abstract We present a numerical analysis of the diffusion equation of the magnetic flux in YBCO in presence of ac magnetic field taking into account flux creep and flux flow. Despite critical state models, the results show the dependence of harmonic susceptibilities on the field frequency in agreement with the experimental behavior.

Hysteretic and ac dynamic losses of high T/sub c/ superconductors by numerical solution of the nonlinear magnetic diffusion equation

The complex ac susceptibility (/spl chi/) of high T/sub c/ superconducting materials has been described in terms of hysteretic and dynamic losses. By including both flux creep and flux flow resistivities in the expression of the flux flow resistivities in the expression of the flux diffusivity, induction profiles have been numerically calculated from the nonlinear flux diffusion equation. The imaginary part of /spl chi/ has been evaluated as function of temperature, frequency and amplitude of the applied magnetic field. The role of pinning and dynamic losses occuring in high T/sub c/ materials is discussed.

Frequency dependence of the higher harmonics susceptibilities of hydrogen loaded and unloaded melted YBCO samples

Abstract We measured the first and higher components of ac susceptibilities of melted YBCO samples before and after hydrogenation by μs pulsed electrolysis. First component was measured versus temperature at fixed frequency while higher harmonics were measured versus frequencies at fixed temperature near critical temperature. All measurements were performed at fixed amplitude of ac magnetic field and zero dc magnetic field. The frequency and temperature behavior of the harmonic components before and after hydrogenation gave us information on possible dynamic losses in comparison to the flux pinning mechanism.

Enhancement of critical temperature up to 100 K of YBCO ozone annealed pellets by deuterium absorption

Abstract In this paper we report preliminary results about transition temperatures of YBa 2 Cu 3 O 7 pellets loaded with high pressure deuterium gas. After the gas loading non-superconducting phases were not detected by X-ray diffraction. The complex magnetic ac susceptibility has been measured as a function of the temperature. One D-YBCO pellet exhibited a magnetic critical temperature ( T c ) as high as 100 K and the absence of a significant weakening of the superconducting grain coupling. Another sample, with a T c of 95.8 K in free air, showed a T c onset as high as 102 K under 35 bar of deuterium gas pressure.

Jc(T, B) calculation of BSSCO tape by magnetization loop measurements

Abstract A non-destructive characterization of silver sheathed 2223 BSCCO tapes by magnetization cycles in the temperature range ( 40⩽T⩽80 K ) have been performed. From the amplitude of magnetization cycles, Δm ( H dc ), different J c functional have been calculated using the amplitude of critical current density J c at T =77 K, given by the manufactured company. Many physical considerations with the exponential intra-grain behaviour have been derived. These analyses for H dc =0 T show a clear collective pinning behaviour in the range studied. At low amplitude of the applied magnetic field, to explain higher J c values from the exponential behaviour, the grain boundary effect as power-law in magnetic field with temperature dependence have been considered. At high amplitude of H dc flux flow processes have been considered, but for high temperature also an “enhancement” thermally activated process must be taken into account to explain the shift to lower values from expected exponential behaviour.

RAP: thermoacoustic detection at the DAΦNE beam test facility

In order to investigate the anomalous response at ultra-low temperatures of the resonant-mass gravitational wave detector NAUTILUS, the RAP experiment has been planned to measure the vibrations in a small cylindrical aluminium bar when hit by 10 5 510 MeV electrons from the DANE beam test facility, corresponding to the energies released by typical extensive air showers. The results of the measurement at low temperature and in the superconducting regime are crucial to understand the interaction of ionizing particles with bulk superconductors and to confirm the results on the thermoacoustic model of the past experiments. The first run of RAP experiment is scheduled for the end of June. The scheme of operation and the preliminary results at room temperature will be presented.