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A controlled introduction of second-phase Y2BaCuO5 (211) nanoparticles into YBa2Cu3O7−δ (123) thin films was achieved for the first time for the purpose of increasing flux pinning. The island-growth mode of 211 on 123 was utilized to obtain an area particle density >1011 cm-2 of 211 thick-disk-shaped nanoparticles in individual layers. Composite layered structures of (211y nanoparticles/123z)×N were deposited by pulsed laser deposition on LaAlO3 substrates, with N bilayers = 24 to 100, y thickness = 1 to 2 nm, and z thickness = 6 to 15 nm (assuming continuous layer coverage). With 211 addition, the critical current densities at 77 K were higher at magnetic fields as low as 0.1 T and increased as much as approximately 300% at 1.5 T. The superconducting transition temperature was reduced by approximately 2 to 4 K for 211 volume fraction <20%. Reinitiation of 123 growth after every 211 layer resulted in a smooth and flat surface finish on the films and also greatly reduced surface particulate formation especially in thicker films (∼ μm).
Pulsed-laser ablation was used to deposit YBa2Cu3O7-x (YBCO) films on LaAlO3 and metallic substrates in an effort to understand and develop the processing of coated conductors with enhanced properties. Doping of YBCO films with Nd was utilized as an approach for increasing their flux pinning properties. Separate targets of Nd2O3 and YBCO were used instead of a pre-mixed Nd2O3-YBCO target. The critical current density (Jc) of the films was measured by whole body dc transport measurements and the transition temperature (Tc) by ac susceptibility. The composition vs. depth profiles of the films were obtained by Secondary Ion Mass Spectrometry. The critical current of a 5000 Å thick Nd-doped YBCO film on a LaAlO3 substrate was measured at 77 K and found to be 57 A (Jc = 1.1×106 A/cm2).
Yba2Cu3OT7−x (Y123) and NdBa2Cu3O7−x (Nd123) thin films were deposited by PLD using a 248 nm wavelength KrF excimer laser. The emission spectra PLD process control techniques developed to grow reproducibly high quality Y123 thin films were also applied to the Nd123 depositions. The time-resolved spectral components of plumes generated from Y123 and Nd123 targets at the same deposition conditions were compared. When the two targets were mounted at the same distance from the substrate heater, the spectral emissions at 327 ± 5 nm (Cu*) were essentially identical for similar laser pulse energies and other deposition conditions (150 mTorr oxygen pressure, 760°C substrate heater temperatures, etc.). High quality Nd 123 films (Tc ˜93 K, Jc > 2x 106 A/cm2) are considerably more difficult to produce than those of Y123. Deposition conditions associated with PLD of high quality Y123 films produce very poor Nd 123 films. This observation is consistent with the belief that the nucleation and growth of Nd 123 are significantly different than that of Y 123.
The pulsed-laser deposition (PLD) technique utilizes one of the most energetic beams available to form thin films of the superconducting oxide YBa2Cu3O7 (YBCO). IN this study we examine the growth of YBCO at very high laser fluences (25 to 40 J/cm2); a more typical fluence for PLD would be nearer to 3 J/cm2. the use of high fluences leads to unique film microstructures which, in some cases, appear to be related to the correspondingly higher moveabilities of the adatoms. Films grown on vicinal substrates, using high laser fluences, exhibited well-defined elongated granular morphologies (with excellent transition temperature, Tc, and critical current density, Jc). Films grown on vicinal substrates using off-axis magnetron sputtering, plasma-enhanced metal organic chemical vapor deposition (PE-MOCVD), or PLD at more typical laser fluences showed some similar morphologies, but less well-defined. Under certain growth conditions, using high laser fluences with (001) oriented substrates, the YBCO films can exhibit a mixture of a- and c-axis growth where both crystallographic orientations nucleate on the substrate surface at the same time, and grow in concert. the ratio of a-axis oriented to c-axis oriented grains is strongly affected by the pulse repetition rate of the laser.
The surface morphology of Yba2Cu3O7 thin films formed on (001)-oriented LaAlO3 substrates by pulsed-laser deposition has been examined using electron and scanning probe microscopies. The observed surface features can be divided into two types: particles formed as a result of material (often molten) ejected from the target and outgrowths formed as a result of nucleation and growth processes on the substrate and/or the film surface. Where both types of surface feature occur on a particular film the outgrowths are always more numerous. The density of outgrowths is strongly related to the deposition parameters and, as a consequence, with the film growth mechanism.
Ac susceptibility measurements have been used to monitor the changes in the superconductive properties of sintered and uniaxially pressed samples of Pb-stabilized 2223-phase bismuth cuprate ceramic as the structural damage was annealed in air in a sequence of steps in temperature between 500 and 850 °C. It is concluded that below 600 °C a relaxation of residual stresses is responsible for a 2% shrinkage in sample volume and a small improvement in bulk superconductive transition temperature, Tc. Above 700 °C, a recovery of the original properties occurs through grain regrowth governed by an activation energy of ∼200 kJ/mol. However, in the region between 600 and 700 °C, a decrease in Tc of ∼40 K appears to be the result of plastic flow and amorphization associated with local decomposition of 2223 which reduces the effective grain size and weakens the intergranular superconductive coupling.
Superconducting composite samples were prepared by sintering a mixture of metallic silver and Bi(Pb)–Sr–Ca–Cu–O powder in air. For Ag contents between 0 and ∼15 vol. %, the bulk critical temperature was depressed as much as 40 K, and the relative proportion of the 2223 phase with respect to the 2212 phase was decreased, compared to the pure sample. However, as the Ag content was increased to between 19 and 70 vol. %, critical temperatures above 100 K were measured by transport and ac susceptibility techniques, and the relative proportions of the phases were restored to that of the pure sample. The limit of bulk superconductivity was reached at 73 vol. % Ag where the bulk critical temperature was ∼85 K. For higher Ag contents bulk superconductivity was not observed. Our results also show that sintering in air of silver-clad tapes containing Bi(Pb)–Sr–Ca–Cu–O powder, either by itself or with silver powder, need not be detrimental to the final superconductive properties.
To assess a Ni-clad melt-processed YBa2Cu3Oy (YBCO) conductor approach, a systematic study was made on commercially available YBCO powder mixed and co-sintered with nickel oxide (NiO). Our research effort has focused on the effect of nickel on the critical current density in the YBCO/Ni composites. Four series of YBCO samples mixed with 0.3, 0.7, 1, 1.3, 2 and 3 wt% of high purity NiO were prepared, and optimized conditions for the melt process were established. These samples were characterized by resistivity, AC susceptibility and DC magnetization measurements. The highest magnetization critical current density (Jcm∼3×105 A/cm2 at T=4.2K and H=0 was observed in melt-processed samples with ∼ 1 wt% Ni (or -1.3 wt% NiO) subjected to a cooling rate of 2°C/hr from 1040°C to 980°C. This Jcm was found near the middle of the Ni-solubility range (about 2 wt% Ni is the solubility limit of Ni in YBCO) which may be attributed to the local Ni/Cu compositional fluctuation leading to an increased pinning force density in these composites.
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