Bernhard Obst

Growth of thick YBa2Cu3O7−x single crystals from Al2O3 crucibles

Abstract The growth of large, isometric and free-standing single crystals of YBa 2 Cu 3 O 7− x using a CuOBaO self-flux is reported. Within an optimum concentration range in the phase diagram centered around 4 at% Y, 30 at% Ba and 66 at% Cu, isometric crystals with aspect ratios between 1–10 and sizes of 5×5×2 mm 3 were grown. Outside this range of favourable flux compositions thin platelets up to 8×8 mm 2 with aspect ratios ≫ 10 were obtained. The thickness of the isometric crystals is essentially determined by the cooling rate: 500μm, 1 mm and 2 mm thick crystals were obtained for cooling rates of 0.5, 0.3 and 0.1° C/h, respectively. For cooling rates 30° C/h a dendritic non equilibrium morphology with a zigzag-like habit of the edge of the (001) face can be observed. Aluminum impurities arising from the Al 2 O 3 crucible have a strong influence on the crystal growth: they considerably reduce the soak time and the viscosity of the melt. The presence of Al also favours the formation of yellow-greenish needles of a new quaternary phase, YBa 3 Al 2 O 7.5 .

Superconductivity at 40 K in La1.8Sr0.2CuO4

High-Tc superconductivity withTc onsets up to 42 K (midpoint: 37 K, zero resistance: 34 K) is observed in X-rays homogeneous single phase La1.8Sr0.2CuO4. The quarternary compounds La2−xBaxCuO4 and La2−xSrxCuO4 (0≤x≤0.3 for Ba and 0≤x<1 for Sr, depending on the heating conditions) are mixed-valence oxygen defect oxides, characterized by the presence of Cu2+ and Cu3+ simultaneously. These oxides have a tetragonal symmetry (space group:I 4/mmm) similar to that of K2NiF4. We report the synthesis, characterization, and superconducting properties of various high-temperature superconducting La−Ba−Cu−O and La−Sr−Cu−O compounds. Through the substitution of Sr for Ba in these oxygen defect compounds an increasing superconducting transition temperature onset from 28 K to 35 K for La1.8Sr0.1Ba0.1CuO4 was observed. A positive initial pressure coefficient ofdTc/dp=290 (mK/kbar) has been found for La1.8Sr0.2CuO4 with a magnetic susceptibility change consistent with the 100% diamagnetic expectation value.

3D-XY CRITICAL FLUCTUATIONS OF THE THERMAL EXPANSIVITY IN DETWINNED YBA2CU3O7-DELTA SINGLE CRYSTALS NEAR OPTIMAL DOPING

The strong coupling of superconductivity to the orthorhombic distortion in YBa2Cu3O7-d makes possible an analysis of the superconducting fluctuations without the necessity of subtracting any background. The present high-resolution capacitance dilatometry data unambiguously demonstrate the existence of critical, instead of Gaussian, fluctuations over a wide temperature region (+/- 10 K) around Tc. The values of the amplitude ratio A+/A-=0.9-1.1 and the leading scaling exponent |alpha|<0.018, determined via a least-squares fit of the data, are consistent with the 3D-XY universality class. Small deviations from pure 3D-XY behavior are discussed.

Cube-textured nickel and Ni alloy substrates for YBCO coated conductors

Abstract This paper deals with the texturing of nickel and NiCr, NiW and NiCrW alloys. Concurrently with measurements of the primary recrystallization kinetics, the recrystallization texture of these materials was investigated in dependence on the annealing temperature and time. An increase of the recrystallization temperature or time sharpens the texture due to grain growth, limited however by the occurrence of secondary recrystallization when a sample specific temperature is exceeded. The most favorable recrystallization temperature was found to correlate with the recrystallization kinetics, varying with the sample composition. The reduced stacking fault energy (SFE) in binary Ni alloys in comparison with pure Ni favors the formation of recrystallization twins {1 2 2}〈2 1 2〉 . The effect of a ternary alloy on the SFE is not easy to predict. So we tried to find, experimentally, a ternary alloy with medium to high SFE to produce the cube texture.

Superconductivity above 100 K in multi-phase Y−Ba−Cu−O

We report the synthesis, characterization and superconducting temperature of high-temperature superconducting Y0.3Ba0.7CuO3, Y0.4Ba0.6CuO3 and Y1.2Ba0.8CuO4. The volume fraction of the superconducting phase is estimated to be of the order of 10%.HighTcsuperconductivity withTc-onsets up to 125 K (midpoint 102 K, zero resistance: 90 K) is observed in multi-phase Y−Ba−Cu−O oxygen defect compounds with the nominal composition of Y1.2Ba0.8CuO4−y (0≦y≦0.4).

Structure and superconductivity of La1.8Sr0.2CuO4 and multi-phase Y1.2Ba0.8CuO4 under high pressure

Superconducting La1.8Sr0.2CuO4 has been investigated under pressure. Its resistiveTc-midpoint was found to be 37 K at ambient pressure. It inductively determinedTcis 33 K monotonically shifting up to 38.4 K at 19.2 kbar. The initial linear increase is 0.295 K/kbar. X-ray investigations in a diamond anvil revealed no phase transformation of the K2NiF4 structure up to about 50 kbar at 300 K, 60 K and 15 K. The bulk modulus of La1.8Sr0.2CuO4 was 1070 kbar at 300 K and 1500 kbar at 15 K. Thec/a ratio decreases and shows pronounced differences between room temperature and low temperature data under pressure. Multi-phase Y−Ba−Cu-Oxide with resistiveTc-onset of 90 K was investigated under pressure, too. The inductive transition starts at 70 K and is completed at 20 K. The superconducting volume fraction was estimated to be about 2% at 10 K.

The effect of chemical doping and hydrostatic pressure on Tc of Y1−yCayBa2Cu3Ox single crystals

We performed susceptibility measurements on Y1-yCayBa2Cu3Ox single crystals under high He pressure. For each Ca content various samples with different oxygen contents have been prepared to probe the influence of Ca on Tc(x), dTc/dp(x) and Tc,max. Starting from the parabolic Tc(nh) behavior we calculated nh values from Tc and Tc,max for each sample. It is shown that in the overdoped region dTc/dp can be described by a pressure induced charge transfer with dnh/dp ≈ 3.7⋅10 -3 GPa -1 and a dTc,max/dp value of 0.8 K/GPa, irrespective of the Ca content. In the underdoped region additional pressure effects lead to a peak in dTc/dp at ≈0.11 holes/CuO2 plane. However, with increasing Ca content this peak is strongly depressed. This is explained in terms of an increasing disorder in the CuO chain system due to doping. Deviations in dTc/dp at very low nh values can be assigned to the ortho II ordering in the CuO chain system.

Misorientations and geometrically necessary dislocations in deformed copper crystals: A microstructural analysis of X-ray rocking curves

Abstract In this study, the deformation-induced misorientations that are typically found in face-centred cubic single crystals deformed in single slip into stage II (and early stage III) of the work-hardening curve are discussed with respect to the experimentally observed broadening of X-ray rocking curves. By making use of well-established empirical relationships between characteristic features of the microstructure and the flow stress, some of the ambiguities of earlier interpretations of rocking curves could be avoided, and relationships between the half-widths of the rocking curves, the density of geometrically necessary dislocations, and the flow stress could be derived for both the tilt misorientations due to the kink bands lying perpendicular to the primary Burgers vector and the twist misorientations originating from the dislocation networks (grids, sheets) lying parallel to the primary glide plane. An evaluation of largely unpublished experimental rocking-curve data obtained on different crystall...

HIP synthesized Nb/sub 3/Sn bulk materials with extraordinary homogeneity

The authors present a simple powder metallurgical approach to synthesizing Nb/sub 3/Sn bulk materials by heat treatment under hot isostatic pressure (HIP) conditions that yields very homogeneous and fully densified large Nb/sub 3/Sn samples. The total width of the superconducting transition obtained from a specific heat measurement was only 0.4 K, corresponding to a variation of the Sn content of less than +or-0.1 at.% over the macroscopic sample size. The physical and superconducting properties of such samples are characterized, and possible application of the resulting materials are given.<<ETX>>

Influence of the preparation process on microstructure, critical current density and T/sub c/ of MgB/sub 2/ powder-in-tube wires

MgB/sub 2/ wires and tapes suffer from a limitation of the transport current densities, especially at low field, due to an unsufficient thermal stabilization caused by hot spots from a nonhomogeneous microstructure, the presence of secondary phases and voids. For the preparation of MgB/sub 2/ wires and tapes two different precursor routes, using MgB/sub 2/ (ex-situ process) or Mg + B (in-situ process) are applied. The in-situ approach allows variation of many parameters in the wire preparation and the improvement of this approach is the central topic of this paper. We present an improved in-situ approach, the characterization of the filament microstructure, effects from stoichiometry and the corresponding superconducting properties. The transport current capability of such in-situ wires could be improved to the level of the reference ex-situ wires.