Alcione Roberto Jurelo

Mechanical properties of Ag-doped top-seeded melt-grown YBCO pellets

In the present work we report the mechanical properties of ca(b)-planes of Ag-doped top-seeded melt-grown YBaCuO (YBCO) pellets at different concentrations. Hardness and elastic modulus were obtained by instrumented indentation and fracture toughness by conventional Vickers indentation. Hardness profiles for both planes indicated values between 7-8 GPa at deep tip penetration. Significant differences in elastic modulus were observed as a function of Ag content for the ab-plane while no difference were seen for the ca(b)-plane. Doping with 5 wt. % Ag2O increases the hardness and elastic modulus for the ab-plane in relation to the undoped sample due to Ag solid-solution hardening. Indentation fracture toughness rises with Ag doping for the ab-plane. Intensive plastic deformation was observed in ca(b) plane for conventional Vickers indentation.

Fluctuation conductivity in melt-textured REBa2Cu3O7−δ superconductors

Abstract We have studied fluctuations on the electrical conductivity in melt-textured REBa 2 Cu 3 O 7− δ superconductors, where RE corresponds to a natural mixture of rare earth elements extracted from xenotime mineral. For this study, melt processed samples were prepared using the top seeding growth technique. To identify power-law divergences of the conductivity, the results were analyzed in terms of the temperature derivative of the resistivity and of the logarithmic temperature derivative of the conductivity. The data revealed the occurrence of a two-stage intragranular–intergranular transition. In the normal phase, different Gaussian and critical regimes were clearly observed to each current direction. Such an observation reveals that there is an evidence for anisotropies effects in studied conductivity regimes. Finally, the results obtained showed that the fluctuation conductivity in melt-textured samples of REBa 2 Cu 3 O 7− δ is very similar to those observed in YBa 2 Cu 3 O 7− δ .

A method to measure fracture toughness using indentation in REBa2Cu3O7-δ superconductor single crystals

A method is proposed to estimate indentation fracture toughness based on the critical load for crack nucleation calculated by the derivative of the loading curve. This method is applied to REBa2Cu3O7-δ superconductor single crystals based on xenotime, where is difficult to observe well defined radial crack lengths due to its lamellar structure. The indentation fracture toughness agrees well with that obtained from the traditional method based on radial crack propagation. The proposed procedure is an alternative technique to estimate indentation fracture toughness without the necessity of measuring radial cracks lengths while keeping the advantages of the traditional method.

Effects of Ag addition on some physical properties of granular YBa2Cu3O7-δ superconductor

The effects of Ag addition on some physical properties of YBa2Cu3O7-δ superconductor has been studied. The samples were produced by different routes and characterized by scanning electron microscopy, differential thermal analysis, thermogravimetric analysis, X-ray diffraction and nanoindentation. The superconducting properties were studied by dc magnetization and electrical resistivity measurements. The X-ray patterns revealed that all samples were polycrystalline and corresponded to the orthorhombic YBa2Cu3O7-δ phase. However, for some samples, silver appears to be present within the grains. The superconducting properties were not significantly influenced by Ag addition. Hardness and elastic modulus were also obtained by instrumented indentation. Ag addition was found to be effective in improving the mechanical properties, probably by filling the pores in the grain boundaries. Hardness profiles indicated values between 3.1 and 3.4 GPa at deep tip penetration depths. The highest elastic modulus of 146 GPa was attained for samples prepared using Y2O3, BaCO3, CuO and Ag2O as precursors, while the lowest modulus (125 GPa) was obtained for the pure YBa2Cu3O7-δ sample.

Mechanical properties of highly oriented FeSe0.5Te0.5 superconductor

We have synthesized highly oriented samples of the superconducting compound FeSe0.5Te0.5 and investigated its mechanical properties. These samples were characterized by scanning electron microscopy (SEM) with energy-dispersive analysis, x-ray diffraction, and electrical resistivity. The measured critical temperature is TC~ 14.5 K. Hardness and elastic modulus on the ab- and a(b)c-plane were obtained by instrumented indentation. The samples’ morphology consists of plate-like crystals with a lamellar structure. Two phases were observed with a typically eutectic microstructure composed by white and gray lamellas when observed with SEM using backscattered electrons. The white phase is richer in Te and poorer in Se than the gray phase. No significant differences in hardness and elastic modulus were observed between ab- and a(b)c-planes. Hardness profiles indicated values in the range 0.6–0.8 GPa at deep tip penetration depths, while the elastic modulus showed values in between 20–25 GPa. Cracking was occasiona...

FLUCTUATION CONDUCTIVITY IN MELT-TEXTURED YBa2Cu3O7-δ

Systematic measurements of the in-plane and out-of-plane fluctuations magnetoconductivity in a melt-textured sample of YBa2Cu3O7-δ are presented. The experiments were performed in fields ranging from 0.02 T to 5 T applied perpendicular to CuO2 atomic (ab) planes. The measurements of the in-plane fluctuations were performed in zero applied field. The method of analysis is based on the determination of the quantity , where Δσ = σ - σR is the fluctuation conductivity. In the normal phase, contributions from Gaussian regimes are clearly evidenced. These regimes are unstable regarding the application of magnetic fields perpendicular to the ab plane. Below TC, in approaching the zero resistance state, our results show a power-law behavior, that is indicative of a phase transition from paracoherent to a coherent state of the granular array.

Fluctuation conductivity of polycrystalline Y1-xPr xBa2Cu3O7-delta superconductors

We studied the effect of superconducting fluctuations on the electrical conductivity of granular samples of Y1 - xPrxBa2Cu3O7-d superconductors, with x = 0.01, 0.03, 0.05, 0.07 and 0.10. Samples were prepared by the standard solid-state reaction technique, with two different types of calcination process, in air at 900 oC ( x < 0.07 ) and in vacuum at 850 oC ( 0.05 < x < 0.10 ). For the samples prepared in air, our results revealed a splitting of the bulk transition, denoted by TC1 and TC2, besides the coherence transition. It was observed fluctuation regimes above the highest transition ( TC1 ) and the lowest transition ( TC2 ). For the samples calcinated in vacuum and high concentrations of Pr, changes were observed in the critical region with chemical substitution of the Pr ion for the Y ion. In the regime of approach to the zero resistance state it was observed an occurrence of a coherence transition for all concentrations of praseodymium.

ANALYSIS OF FLUCTUATION CONDUCTIVITY OF GRANULAR Gd1-xPrxBa2Cu3O7-δ SUPERCONDUCTOR

Single phase polycrystalline Gd1-xPrxBa2Cu3O7-δ (0 ≤ x ≤ 0.10) samples have been prepared by standard solid state reaction technique and characterized by SEM, X-ray diffraction and electrical transport experiments. The samples were granular and homogeneous. It was observed that the critical temperature decreases and that the transition width increases with increasing Pr doping. Systematic measurements of fluctuation conductivity are reported. Special attention is taken above the critical temperature, where Gaussian and critical regimes are observed. Below the critical temperature, on approaching the zero resistance state, our results show a power-law behavior consistent with a phase transition from a paracoherent to a coherent state of the granular array.

Conductivity fluctuations of polycrystalline Ag-doped YBa2Cu3O7-δ superconductor

Systematic conductivity measurements near TC in granular Ag-doped YBa2Cu3O7-δ superconductors are presented. Silver doped YBa2Cu3O7-δ samples were produced by different routes and characterized by X-ray diffraction, dc magnetization and electrical resistivity. The results were analyzed in terms of the temperature derivative of the resistivity and of the logarithmic temperature derivative of the conductivity, that allowed identifying power-law divergences in the conductivity. The results show that the resistive transition proceeds in two stages. In the normal phase, Gaussian and critical fluctuation conductivity regimes were identified. Close to the zero-resistance temperature, a coherence transition related to the connective nature of the granular samples was evidenced. It was not observed significant differences in critical and Gaussian fluctuations between pure and Ag-doped YBa2Cu3O7-δ polycrystalline samples.

FLUCTUATION CONDUCTIVITY OF POLYCRYSTALLINEYbBa2Cu3O7 -δSUPERCONDUCTORS

We studied the effect of superconducting fluctuations on the electrical conductivity of granular samples of Y1-xTbxBa2Cu3O7-δ superconductors. For all samples we observed that the transition proceeds in two stages. Above the critical temperature, the Gaussian and critical regimes were observed, independently of the terbium concentration. In the regime of approach to the zero resistance state, and for low concentrations of terbium, an occurrence of a coherence transition was observed. For high concentrations of Tb, the results show a strong disorder at mesoscopic levels.