J. Beille

Bulk textured BiPbSrCaCuO (2223) ceramics by solidification in a magnetic field

Abstract Bulk textured BiPbSrCaCuO (2223) ceramics have been prepared by solidification in a magnetic field. We have determined the optimal temperature required in the melt in order to maximize the effect of the magnetic field. X-ray pole figures and scanning electron microscopy have been used to characterize the nature and degree of texture. Magnetic and electrical transport measurements in the superconducting state are also reported. Transport critical current densities as high as 1450 A/cm 2 at 77 K and 0 tesla have been reached. This value is significantly greater than that, 900 A/cm 2 , obtained following the same heat treatment in the absence of an applied magnetic field.

Pressure effects, anisotropic transport properties and upper critical fields of Bi2Sr2CaCu2O8+x single crystals - characterization of needle-like BiSrCaCuO crystals

Abstract We study the physical properties of Bi 2 Sr 2 CaCu 2 O 8+ x platelet-like single crystals with a zero resistance superconducting transition T c of ≅ 80 K. Normal state electrical resistivity and upper critical fields are more anisotropic than in the YBa 2 Cu 3 O 7 single crystals. The normal state resistivity along the a-b plane linearly increases with the temperature as in the YBa 2 Cu 3 O 7 compounds. Along the c -axis, the resistivity is very high and exhibits a metallic behaviour which can be induced by defects instead to be intrinsic. The superconducting transition temperature, T c , increases under pressure at a rate of ≅ 0.2 K/kbar. Superconducting needle-like crystals are also studied. They are not single crystals but are composed of two intergrowing phases. One of them is insulating, the other one superconducting. This last phase could be different from the “2212” one. Similarities between the BiSrCaCuO compounds and La 2 CuO 4 compounds are underlined.

Optimization of texturing process by hot pressing of Bi-Pb-Sr-Ca-Cu-O (2223) ceramics

Abstract We have optimized a texturing process of Bi-Pb-Sr-Ca-Cu-O (2223) superconducting ceramics by hot pressing. The Hoechst precursor powder has been first pressurized at 1 GPa and then heated under uniaxial stress up to 35 MPa, just below the melting temperature between 850°C and 857°C. We have obtained large textured samples having critical transport current densities J c as large as 2500 A/cm 2 . In this work, we have optimized the stress value as well as the temperature treatment on the basis of characterizations such as magnetization and electrical-transport measurements.

Temperature dependence of the resistivity and its anisotropy in n-type Nd1.85Ce0.15CuO4 single crystal

Abstract Measurements of the in-plane and c -axis normal-state resistivity in a superconducting Nd 1.85 Ce 0.15 CuO 4−δ single crystal are reported. The resistivity anisotropy of this n-type material is ≤250, much smaller than BiSCCO and comparable to YBaCuO. Both, ϱ ab ( T ) and ϱ c ( T ) displays a metallic-like positive temperature coefficient of resistivity with a basic T 2 dependence. We discuss some possible origins of this peculiar temperature dependence.

Magnetic melt texturing combined with hot pressing applied to superconducting (2223) Bi-Pb-Sr-Ca-Cu-O ceramics

We describe a new texturing process for superconducting ceramics which combines the effects of magnetic melt texturing (MMT) and hot pressing (HP), referred to in the following as the MMHPT process. This process essentially involves three steps: (i) a hot pressing at a temperature Tp, (ii) a partial melting in a magnetic field at a slightly higher temperature TM, (iii) a second hot pressing at the same temperature Tp. Optimization of the thermomechanical treatment in a magnetic field applied to the (2223) Bi-Pb-Sr-Ca-Cu-O cuprates led to highly textured and dense samples carrying transport current density Jc of 3800 A cm-2 at 77 K in zero field.

Transport properties of Ln2-xCexCuO4-y single crystals under high pressure

Abstract The transport properties of the electron-doped cuprates Ln 2- x Ce x CuO 4 (Ln z.bbnd; Pr, Nd and Sm) single crystals are investigated under hydrostatic pressure up to 2 GPa. The effects of pressure on both the c -axis (ϱ c and the in-plane (ϱ ab resistivity are measured from room temperature down to the superconducting state. It is found that at 300 K, dln ϱ ab /d P ≈dln ϱ c /d P ≈ −3% GPa −1 . The pressure does not change the temperature dependence of the normal state resistivity, which at high temperature in both directions follows a ϱ ∼ A ( P ) T 2 law, but the coefficient A ( P ) decreases as pressure is increased. Moreover, we have found that for any lanthanide host (Pr, Sm and Nd) the onset of superconductivity does not change with pressure ( P

Upper critical field anisotropy and dissipative flux motion in NdCeCuO single crystals

Abstract We report resistivity and AC susceptibility measurements under external magnetic field in NdCeCuO single crystals. From resistivity data we have estimated the upper critical fields, coherence lengths and anisotropy factor; AC susceptibility has allowed the extraction of the activation energy values, which turn out to be higher than the p-type HTSC ones.

Susceptibility and antiferromagnetic order under hydrostatic and quasihydrostatic pressure in sintered La2CuxO4−y

In this work we study a sintered La2CuxO4−y sample corresponding to the initial stoichiometry x=1.02 which is anti ferromagnetic with a Neel temperature TN=240 K and superconductor below Tc=37 K. Antiferromagnetism coexists with superconductivity in the bulk as deduced from susceptibility and magnetization measurements. Antiferromagnetic order is characterized by an anomaly of resistivity, a maximum in DC susceptibility and a metamagnetic transition. TN decreases under hydrostatic pressure (H.P.). Under quasihydrostatic pressrure (Q.H.P.), the superconducting transition becomes incomplete and the antiferromagnetic ordering is suppressed. The superconducting transition temperature (Tc(p)) passes through a maximum for P⋍40 kbar. Under Q.H.P. the sample exhibits a resistivity behaviour, p ∝ exp (T0Tα with 0.25<α<0.5. The evaluated normal-pressure magnetic resistivity is comparable to the one of heavy-fermion systems with a small number of carriers. The variation of TN under pressure is understood in terms of the decrease of the number of holes in the Cu-O planes.

New apparatus for ceramic texturing working under uniaxial stress and in a magnetic field

We have developed a new versatile apparatus combining the effect of hot pressing and the effect of magnetic field to texture HTSC (high-temperature superconductor) ceramics. The experimental set-up allows application of uniaxial stress up to 1100 degrees C under a maximum pressure of 60 MPa, in a controlled atmosphere and a magnetic field of 8 T. We have optimized the cycle for stress application along with the thermal cycle to produce highly-textured ceramics. In this paper, we describe the stress and thermal cycles used to texture Bi-Pb-Sr-Ca-Cu-O (2223 phase) ceramics. We present the physical and chemical characterizations of the samples which show a strong degree of crystallite orientation.

Combined effects of melting under magnetic field and hot pressing to texture superconducting ceramic Bi-Pb-Sr-Ca-Cu-O (2223)

Abstract Magnetic melt texturation has been applied to ceramics of Bi-Pb-Sr-Ca-Cu-O (2223), including critical current density, Jc of 1090 A/cm 2 at 77 K. Hot pressing of such ceramics increases Jc up to 1667 A/cm 2 . The best Jc of 2500 A/cm 2 is obtained for ceramics hot pressed in the solid state, just below the melting temperature. The effects of the different processes are discussed.