YBaCuO Grain Research Articles

“C” axis alignment of YBaCuO grains by creep sintering

Abstract The possibility of aligning the “c” axis of YBaCuO grains by deforming them during sintering has been investigated for two different commercial powders. It is shown that when sintering is performed in the presence of a liquid phase, the ceramic deforms substantially at very low and practically constant stress before exhibiting a regular strain hardening stage. When there is just a small amount of liquid phase, the misorientation between the “c” axis of the various grains can be as low as 8 °, whereas when the liquid phase is too abundant, the 123 phase starts decomposing and the material is not textured. At temperatures below that of appearance of the liquid phase, deformation directly starts with a long strain hardening stage and the material is not textured. The liquid phase originates from carbon pollution and the carbon content of the creep sintered specimens is dependant on the treatment atmosphere.

High temperature “in situ” growth bu cathodic sputtering of fully oxygenated YBaCuO thin films

The structure and oxygen content of YBaCuO thin films were measured at room temperature combining XRD analysis of individual YBaCuO grains with recently developed IBA techniques. As a function of the conditions of the sample cooling (dT/dt, pO 2), the ∼500nm thick YBaCuO samples grown on LaAlO 3 single crystal substrates can exhibit either orthorhombic (ORT) or quasitetragonal (TETR) structure. The former is obtained by low cooling rates at high oxygen pressure (0.2–1 atm) whereas the later needs a high cooling rates (≥ 100 °C/min) and can be performed at low oxygen pressure. Both type of samples are fully oxygenated (∼O 7) YBaCuO compounds with Tc onset of 91 K and ΔTc ≈ 1K. The resistivity of the (TETR) compound is about 3 times higher than that of the (ORT) one and the intercept of the ϱ(T) curve crosses the axis of the resistivity at a large positive value, as expected for the tetragonal compound of the 1-2-3 family. From these observations and following kinetics considerations, we postulate that the formation of YBaCuO thin films with fully oxygenated (∼O 7) tetragonal structure takes place during the “in situ” high temperature growth by cathodic sputtering.

Weak link effects on the electrical properties of YBaCuO-CuO random composites

We report experimental data on the resistive transition of granular superconductive composites made of YBaCuO grains sintered in a CuO matrix as a function of temperature and grain concentration. The grain concentration dependence of the sample resistivity at constant temperature may exhibit two percolation transitions: a poor conductor-to-good conductor transition, which is present at every temperature, and a good conductor-to-superconductor transition below the grain superconducting transition temperature. We show that the temperature dependence of the superconductive critical concentration gives informations on the probability function describing the weak superconductive link transition to the normal state.

An experimental implementation of high-Tc-based RSFQ set-reset trigger at 4.2 K

A rapid single-flux quantum (RSFQ) set-reset trigger circuit has been implemented using YBaCuO grain boundary junctions on an asymmetric 32 degrees Y-ZrO2 bicrystal. The circuit contains 18 Josephson junctions aligned along the grain boundary and required six film layers: a YBaCuO base electrode, two amorphous SrTiO3 insulating layers, a superconducting Pb/Au/Cr ground plane and Au/Cr wiring. It consists of two DC-to-SFQ converters, two Josephson transmission lines and an SFQ-to-DC converter. Two pulse trains, 'signal' and 'clock', were applied to change the state of the trigger from '0' to '1' and vice versa, and registered at the SFQ-to-DC: output. All experiments were performed at 4.2 K.

X-ray diffraction studies elucidating high temperature superconducting properties of YBa2Cu3O7−x thin films

Abstract A quantitative evaluation of the influence of the amount of the crystallographically unoriented grains of YBa2 Cu3O7−x epitaxial films on the major high temperature superconducting (HTS) properties such as microwave surface resistance Rs and critical current Jc has been analyzed. The correlation is quite complex but it is clear that a very high percentage of 〈100〉 basal plane orientation, as determined by the X-ray (108) phi-scan, is essential to obtain the optimal HTS properties. The role of (005) rocking curves width and percentage of crystallographic orientation of YBa2Cu3O7 − x grains decomposed into Y2BaCuO5 and BaCuO2 are also discussed. The optimal deposition temperature for thin films is analyzed in terms of the structural and HTS properties.

Structure of grain boundaries in YBa2Cu3O7 thin-film step-edge junctions

The atomic structure of 90° [100] (or [010]) tilt grain boundaries in YBa2Cu3O7 thin-film step-edge junctions and, for comparison, in the interface between a-axis and c-axis oriented YBa2Cu3O7 grains is investigated by means of high-resolution transmission electron microscopy. For (100)(001)-type boundaries two different structures are found. In the first a (001) CuO2 plane of one grain faces a (100) Y−Ba−O plane of the other grain, in the second a (001) BaO plane faces a (100) Cu−O plane. In the former structure an incomplete unit cell of YBa2Cu3O7 terminates at the boundary and a smaller strain in the adjacent CuO2 planes is detected in comparison with the latter. It is found that a combination of a partial dislocation with a “124” stacking fault is a way to accommodate the lattice mismatch between c and 3a of YBa2Cu3O7 in the boundary. For a symmetric (103)(103)-type boundary a displacement of the Cu-atoms of the CuO2 planes is found near the boundary plane. From this a redistribution of the oxygen atoms around the Y-atoms located right in the boundary plane is inferred. The possible effect of the boundary structure on the superconducting properties of YBa2Cu3O7 films is discussed.

Characterization of epitaxial SrTiO3/YBa2Cu3O7 layers deposited on (001) MgO by laser ablation

We discuss the results obtained for SrTiO3/YBa2Cu3O7 layers deposited on (001) MgO substrates by UV pulsed laser ablation. Different samples were prepared to study both the growth of a thin (55 nm) layer of SrTiO3 on MgO and the successive epitaxy of a 220 nm YBa2Cu3O7 (YBCO) film on the SrTiO3 layer. An x-ray diffraction (XRD) texture analysis is reported for the bilayers together with resistivity versus temperature and critical current density (Jc) measurements of the superconducting films. The results show that YBCO grains grow with c-axis normal to the surface; the main in-plane orientations are [100] MgO // [100] SrTiO3 // [100] YBCO ([010] YBCO). The XRD line broadening analysis suggests that YBCO columnar grains grow along the whole thickness of the film, also evidencing dislocations and/or faulting separated by a mean distance of 80 nm. The values obtained for the critical current of the superconductor demonstrate the effectiveness of the SrTiO3 intermediate layer in improving the structural quality of the YBCO film.

Effective-medium approach to the microwave properties of the high-T c superconductor-insulator composites

We study microwave transmission through composites consisting of small YBaCuO (BiSrCaCuO) grains embedded in a polymer host. For these composites we evaluate: (a) the effective dielectric constant ζ, (b) the effective magnetic permeability μ, and (c) the losses. We demonstrate that ζ almost does not change upon transition to the superconducting state while μ changes considerably. The dependence of μ on the volume fraction of superconductor is well described by the Clausius-Mossotti approximation.

High-T/sub c/ Josephson junctions and DC SQUIDs

Step edge Josephson junctions and DC superconducting quantum interference devices (SQUIDs) made of laser ablated and sputtered YBaCuO (YBCO) films and grain boundary junctions in polycrystalline TlBaCaCuO (TBCCO) films have been investigated. Bridges on the micrometer scale have been structured by laser or ion beam etching as well as by the inhibit-layer technique. The Josephson junctions are characterized comparing I-V measurements under microwave irradiation to calculations within the resistively shunted junction (RSJ) model with finite capacitance and white noise. The dependence of the I/sub C/R/sub N/ product on temperature can be fitted to the microscopic theory of superconductor-normal insulator-normal superconductor (SNINS) or superconductor-normal conductor-superconductor (SNS) junctions with additional temperature-independent pair breaking. Flux modulation and noise properties of the DC SQUIDs at 77 K are shown. >

Phenomenological approach to the resistive transition of YBaCuO-Ag superconducting random composites

We report experimental data on the resistive transition of granular superconductive composites made of YBaCuO grains sintered in a silver matrix. As the grain concentration is decreased, the transition region broadens, eventually resulting in the disappearance of the zero-resistance state for the lowest concentrations. A percolation model involving weak superconductive links, whose critical temperatures of transition to the normal state are distributed at random, is shown to account for the experimental behaviors. We show that the junction critical temperature distribution function can be deduced from the experimental data: it fits with a log-normal function.

Conversion loss of a YBa2Cu3O7 grain boundary mixer at 20 GHz

We have measured the conversion gain of a YBa2Cu3O7 annealed grain boundary microbridge mixer by operating the device as a lower side-band down-converter from an 18.5 GHz radio frequency to a 1.5 GHz intermediate frequency under the action of a 20.0 GHz local oscillator. We have found the conversion gain to be relatively independent of bias up to a current density which marks the onset of the flux flow resistance. Above the onset of flux flow resistance, the gain decreased dramatically. A maximum conversion gain of −8±4 dB was measured at 25 K. The mixer was operated with a local oscillator power of only −60 dBm. The results demonstrate that high Tc superconductor mixers are capable of delivering comparable conversion gains to Schottky diode mixers while requiring substantially lower power local oscillators.

MOCVD superconducting oxide films

Preparation of high- T c superconducting oxide films by MOCVD, their films structure and superconducting properties are reviewed from the standpoint of “nano-composites” and “fine-composites”. Y-Ba-Cu-O (YBCO) films formed on SrTiO 3 (100) at 850°C showed a superconducting transition temperature with zero resistivity above 90 K. The maximum critical current density was 2.0×10 6 A/cm 2 at 77.3 K and 0 T, and 6.5×10 4 A/cm 2 at 77.3 K and 27 T. CuO and a -axis oriented YBCO grains were contained in the matrix of c -axis oriented YBCO. A transmission electron microscope observation revealed that inclusions of about 10–30 nm were embedded in the a - b plane of YBCO. MOCVD-YBCO films prepared on MgO(100) were used for superconducting devices. Some studies on the MOCVD films of Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O are also reviewed.

Current transport across grain boundaries in superconducting YBa2Cu3O7 films

Superconducting transport properties of YBa2Cu3O7 grain boundaries have been investigated. The results of these experiments are summarized.

Epitaxy and texture analysis of Bi-Sr-Ca-Cu-O thin films on (100) MgO using Transmission Electron Microscopy

Investigators observe large critical currents for superconducting thin films deposited epitaxially on single crystal substrates. The orientation of these films is often characterized by specifying the unit cell axis that is perpendicular to the substrate. This omits specifying the orientation of the other unit cell axes and grain boundary angles between grains of the thin film. Misorientation between grains of YBa2Cu3O7−δ decreases the critical current, even in those films that are c axis oriented. We presume that these results are similar for bismuth based superconductors and report the epitaxial orientations and textures observed in such films.Thin films of nominally Bi2Sr2CaCu2Ox were deposited on MgO using molecular beam epitaxy (MBE). These films were in situ grown (during growth oxygen was incorporated and the films were not oxygen post-annealed) and shuttering was used to encourage c axis growth. Other papers report the details of the synthesis procedure. The films were characterized using x-ray diffraction (XRD) and transmission electron microscopy (TEM).

Structural and chemical stability in nonrandom silver-YBa2Cu3O7 composites in the presence of water

Accelerated water and moisture degradation of YBa2Cu3O7(YBCO)-silver nonrandom composites is studied. Oxygen evolution for these composites immersed in water at 65 °C was compared with that for regular YBCO ceramic material prepared under similar sintering conditions. Time evolution of the decay of crystalline structure was followed by x-ray diffraction for the composite material and for the regular YBCO ceramic. We find that the nonrandom Ag-YBCO composite exhibits high chemical and structural stability in the presence of water; this advantageous property is probably due to the encapsulation of the YBCO grains with silver in this composite material.

Spatially resolved observation of charge transfer across single grain boundaries in YBaCuO films

Transport properties of a single YBaCuO grain boundary have been investigated with a spatial resolution of 1 μm for voltages up to 70 meV. The conductance versus voltage characteristic is found to increase linearly for voltages between 6 and 40 mV. Below 2 mV this characteristic shows periodic oscillations. The origin of these features is discussed.

Superconducting Property and Structure Studies of YBa2Cu3O7–Ag2O Composites

We have studied the superconducting property and the structure of YBa2Cu3O7 added with Ag2O, with the weight ratios of YBa2Cu3O7 to Ag equal to 4, 6, 8, and 10. Resistance and magnetization measurements show that the normal-state transport behavior and the superconducting transition temperature, Tc, are insensitive to the amount of Ag added. The normal-state resistance decreases linearly with temperature, reaching a large value of \\cong 3 for the ratio R(300 K)/R(100 K). This value is evidently larger than that (≤ 2.5) generally observed in polycrystalline YBa2Cu3O7, and is ascribed to the improved grain growth behavior resulting from Ag addition. X-ray diffraction and scanning electron microscopy studies have also been performed. X-ray diffraction study indicates the Ag peaks appear as a separate phase. Scanning electron microscopy study reveals large YBa2Cu3O7 grains (\\cong20–60 µm). Finally, we point out that the microstructures of the samples depend much on the subtle preparation conditions.

Percolation and electrical resistivity in the YBa2Cu3O7?Ag composite system

We have measured the resistivity of the YBa2Cu3O7−Ag composite system with Ag volume fraction,pAg ranging from 0–100%. The percolation behavior and the normal-state resistivity are studied. We find a three-dimensional threshold for the Ag matrix occurring atpAg ≅20% and a threshold for the superconducting matrix occurring at 1−pAg≅30%. The electrical resistivity is interpreted in terms of a resistivity model for granular YBa2Cu3O7 proposed by Halbritter et al. [Z. Phys. B —Condensed Matter71, 411 (1988)]. Based on this model, we show that the resistivities between the YBa2Cu3O7 grains and flaws in the grains are greatly diminished in the samples, indicating significantly improved grain growth behavior of the YBa2Cu3O7 phase resulting from the addition of Ag. With the diminishing of the grain boundary resistivities, an “intrinsic” value for the temperature coefficient of resistivity, (1/gr)(dρ/dT), can be inferred, which is 3.3×10−3/K at 300 K. This value is very close to that observed in the Cu−O2 planes in single crystals and epitaxial films of YBa2Cu3O7.

Magnetic flux pinning properties of oxide superconductors produced by melt processes

The flux pinning mechanism of YBaCuO superconductors produced using a melt process was investigated by measuring the magnetic field dependence of transport critical current densities ( J c) at 77 K. Superconductors produced by the melt process had high transport J c values and the magnetic field dependence of the transport J c was quite small compared to that of bulk sintered materials. This suggests that, in this case, J c in a high field region is determined by flux pinning within the bulk of the material. Using procedures based on conventional flux pinning theory, the J c- B properties were analysed to estimate dominant pinning sites in solidified specimens. According to the theoretical analysis and microstructural observations, the expected site of dominant pinning is the normal precipitates dispersed in the YBaCuO grains, and the transport J c may be determined by their average size and density.

Microwave absorption of aligned crystalline grains of YBa2Cu3O7−x

We investigate here the anisotropic magnetic and electric properties of aligned crystalline grains of YBa2Cu3O7−x. The microwave absorption is indeed consistent with the expected temperature behaviors of the resistivity in the normal state along the c axis and in the a-b plane. In the superconducting state, both the absorptions in pure rf electric and magnetic fields are high compared to the conventional superconductors; for the magnetic case activation energies are deduced and the dispersion is related to the diamagnetic susceptibility. These results are discussed qualitatively in relation to similar data obtained on single crystals and ceramic samples.