Jean-Yves Genoud

Large transport critical currents in dense Fe- and Ni-clad MgB2 superconducting tapes

We report on the preparation of dense monofilamentary MgB2/Ni and MgB2/Fe tapes with high critical current densities. In annealed MgB2/Ni tapes, we obtained transport critical current densities as high as 2.3×105 A/cm2 at 4.2 K and 1.5 T, and for MgB2/Fe tapes 104 A/cm2 at 4.2 K and 6.5 T. An extrapolation to zero field of the MgB2/Fe data gives a critical current value of ∼1 MA/cm2, corresponding to a critical current value well above 1000 A. The high jc values obtained after annealing are a consequence of sintering densification and grain reconnection. Fe does not react with MgB2 and is thus an excellent sheath material candidate for tapes with self-field jc values at 4.2 K in excess of 1 MA/cm2.

Variation of the superconducting properties of Bi2Sr2CaCu2O8+x with oxygen content

Abstract We have mapped the critical temperature of Bi 2 Sr 2 CaCU 2 O 8+δ (2212) in the ( p, T ) plane for 10 −5 ≤ p ≤ 80 bar (O 2 ) and 300 ≤ T ≤ 840°C. Equilibrium conditions are frozen by quenching the samples into liquid gallium. The DC diamagnetic transitions remain sharp, and span an unusual temperature range for the 2212 phase (51.5 K to 94.4 K). This variation is attributed to a measured 2.5% change of the oxygen content ( Δδ ⋍ 0.2 ) in the structure. At the same time, we do not observe any significant variation of the lattice parameters. The Meissner effect, measured in an external magnetic field of 20 Oe, increases with T c . The slope of the normal-state susceptibility ∂ χ / ∂T becomes markedly less negative with decreasing oxygen concentration. It vanishes for the optimum doping that leads to T c ⋍ 94 K .

Variation of the superconducting and structural properties of Y2Ba4Cu7Oz with oxygen content (14.1 < z < 15.3, 30 K ⩽ Tc ⩾ 95 K)

A great number of pure and homogeneous Y2Ba4Cu7Oz samples were investigated by AC susceptibility, field cooling magnetization measurements, X-ray diffraction, microprobe analysis and microstructural examinations in a light microscope and a scanning electron microscope. The oxygen content z between 14.1 and 15.3 was determined by the reduction of the samples in a hydrogen flow. To reach the (z-T-p) equilibrium, well-crystallized samples were annealed in pure oxygen (10−6 ⩽p(O2)⩽102 bar) between 300 and 900°C and subsequently quenched. The specimens produced in this way show sharp χAC transitions in the entire range of critical temperatures (30 K ⩽ Tc ⩽ 95 K). The correlation Tc versus the orthorhombicity 2(b−a)/(b+a) is almost linear, similar to that observed in YBa2Cu3Ox. This is contrast to the distinct behaviour of the functions Tc versus z and Tc versus x in these two compounds. We present a stability-decomposition map 0for Y2Ba4Cu7Oz over a wide range of (T, p) coordinates.

Preparation, resistivity, magnetic properties and specific heat of the 95 K superconductor YBa2Cu3.5O7.5+x (“247”)

Abstract Under suitable preparation conditions, Y2Ba4Cu7O15 (“247”) has the highest critical temperature of the YBCO superconductor family. We present an extensive characterization of 247 samples with Tc onsets as high as 95K in bulk ceramic form. The experiments performed include metallographic examinations, X-ray diffraction, microprobe analysis, resistivity, AC susceptibility, Meissner and normal-state susceptibility, magnetic determinations of the slope of the critical field at Tc and of the critical current density, and specific heat from 1 to 330 K. Several properties (e.g., normal-state susceptibility, specific heat jump at Tc, slope of Hc2(T) at Tc, resistivity) may be considered as an approximate average between those of YBa2Cu4O8 (“124”) and those of YBa2Cu3O7 (“123”). The signature of fluctuations in magnetic properties is enhanced with respect to both 124 and 123. The behavior of the specific heat close to T = 0 is similar to that of the other members of the family, with the presence of a quasi linear T-term and the absence of any upturn in C/T. The maximum Tc appears to be related to optimal doping, consistent with the behavior observed in 123 as a function of the oxygen concentration.

Specific heat peaks observed up to 16 T on the melting line of the vortex lattice in YBa2Cu3O7

Abstract Adiabatic specific heat experiments on YBa2Cu3O7.00 are performed in magnetic fields from 0 to 16 T (B||c and B ⊥ c The twinned, 99.995% pure 18 mg crystal is grown in an inert BaZrO3 crucible, and oxidized in high oxygen pressure to suppress the magnetic “fishtail” effect. First-order-like specific heat peaks rising up to 1.1% above the background are observed on the melting line of the vortex lattice from 6 to 16 T (B||c). The entropy under these peaks is 0.6 ± 0.1 kB/vortex/layer. The fields Bm and peak temperatures Tm obey the relation Bm and peak temperatures Tm obey th relation B m = Bo(1− T n m T c ) 4 3 with B0 = 133 T. The order of the transition is investigated. Only faint anomalies are observed for B||ab. Thermodynamically incompatible specific heat steps and magnetization jumps are observed below 6 T (B||c), suggesting irreversible glassy behaviour. No such features are seen in an YBa2Cu3O6.92 crystal that is characterized by strong pinning.

Reversible magnetization below Tc in high-quality superconducting ceramics

We have investigated the reversible magnetization below Tc in high-quality YBa2Cu307_d (Y-123), YBa2Cu4Os (Y-124), Y2Ba4Cu7O15+x (Y-247) and Bi2Sr2CaCu2O8+x (Bi-2212), Tl2Ba2Cu106+d (Tl-2201) and Tl2Ba2CalCu2O8+d (T1-2212) ceramics. Except for the stoichiometric Y-124 phase, the oxygen concentration was optimized in order to obtain the highest value of the critical temperature for which the normal-state susceptibility becomes temperature independent. Using the simple London model, we are able to fit the reversible magnetization M(T, H) outside the region near Tc with good accuracy for the nearly three-dimensional YBaCuO phases. For the very anisotropic BiSrCaCuO and TlBaCaCuO phases, we have to include an additional term to take into account the fluctuations of vortices. An important result is that Y-123 exhibits a critical field clearly higher than those of the BiSrCaCuO or TlBaCaCuO phases. We obtain for the Y-123 phase a slope at Tc μodHC2,C/dT = -4.3 T/K and an extrapolated μoHC2,C (0) = 280 T.

Preparation and characterization of (100) cube textured Ag substrates for in-plane oriented HTS tapes

A systematic study of texturing mechanisms in pure Ag is presented, the goal being to obtain a cube textured {100}001 tape that can be used as a substrate for superconducting coated conductors. It is shown that a sharp cube texture can be produced after pre-heated deformation and recrystallization. Optimal parameters are detailed. 99.99% pure Ag powder is used as starting material. Fine grained (15 µm) Ag ingots are pre-heated at 100 °C before standard cold rolling. The effect of pre-heating on deformation texture is to change the main texture from the brass orientation {110}112 to the Goss {110}001 one. The presence of a copper-type texture with relatively high values of the ODF (oriented distribution function) after the 100 °C pre-heated deformation can be understood as a slight increase of the stacking fault energy of Ag during the pre-heated deformation. The optimal annealing is achieved at 700 °C for 30 min in a primary vacuum. This leads to a sharp cube texture. The FWHM values of the three x-ray pole figures are, at no more than 10°, the smallest ever reported for cube textured Ag tapes. The formation of cube texture is interpreted in terms of orientation nucleation and orientation growth theories that play a common role during the texturing process. The stability of cube texture at high temperature up to 900 °C indicates that the tapes are suitable for epitaxial growth of superconducting oxides.

Improvements in crystal growth and crystal homogeneity and its impact on physics

With the development of the novel crucible material BaZrO3 in Geneva a new class of 123 — single crystals has become available, the purity of which reaches now semiconductor grade (> 99.995%). Due to the exceptional purity the experiments are not longer obscured by residual impurities and the influence of different oxygen content and distributions could be studied in detail. This has given way to a set of new findings and basic understanding of the physics of high Tc superconductivity. Examples for the impact of this new crystal generation on physics are the reversible suppression of the so called fishtail effect, which is accompanied with the absence of the Schottky anomaly in low temperature specific heat and the calorimetric observation of the vortex lattice melting. On such samples it is possible to observe the vortex lattice by STM without any surface conditioning and to study the influence of pinning centers such as twin boundaries, micro-structural inhomogeneities of the metallic sub-lattice and oxygen vacancies on pinning of vortices.

Preparation and superconducting properties of high-quality Bi-2212 ceramics

Abstract High-quality Bi-2212 ceramic samples were obtained by a solid-state reaction method. At the starting composition Bi 2.12 Sr 1.90 Ca 1.02 Cu 1.96 O y , X-ray diffraction, optical micrographs and scanning electron microscopy investigations show that the level of impurity phases is less than 1% by volume. Synthesis at a few degrees below the melting point favours grain growth. A final annealing treatment at controlled temperature and oxygen pressure, followed by rapid quenching, yields sharp superconducting transitions, as observed by measurement of the a.c. susceptibility and Meissner effect, and specific heat experiments. The Meissner fraction in 2 mT approaches 50%.

Stable (110) textured Ag ribbons for biaxially aligned YBa2Cu3O7-d coated tapes

Texture development in pure Ag was investigated in order to obtain {110}uvw textured Ag ribbons that can be used as a substrate for YBa2Cu3O7 superconducting coated tapes without any buffer layer. The starting material was 99.95% pure commercial Ag foils. A 20% deformation reduction was used at each step of the cold rolling process after which an optimal annealing was achieved at 800 °C for 4 h in a primary vacuum. This process leads to large-grain ribbons with the {110}112 orientation. A sharp and pure texture is obtained with a FWHM smaller than 6° in the three x-ray pole figures that were used to calculate the orientation distribution function. Crystallographic orientation maps achieved by electron back-scattering diffraction show a misorientation from the normal direction of less than 4°. A twinning mechanism is used to explain the formation of the {110}112 texture. The stability of the {110}112 texture is confirmed by further annealings up to 900 °C, a practical temperature for coated tapes preparation. Lengths up to 25 cm with a uniform {110}112 texture have been obtained.