No abstract

A transition buffer, consisting of subbuffers grown at different temperatures, was used to enhance the crystallinity and the transport properties of YBa2Cu3O6.8 (YBCO) films on metallic substrates. Films were fabricated using in situ laser deposition and were strongly c-axis oriented. Critical temperature of 87 K and critical current density of 3×104 A/cm2 at 77 K were obtained for a 0.5-μm-thick YBCO film with a 0.2 μm transition yttrium- stabilized ZrO2 buffer layer. Also, the magnetic field dependence of Jc could be enhanced by using Pt-coated metallic substrates.

High-quality Bi2Sr2CaCu2Ox/Ag superconducting tapes were fabricated directly from the nitrate salts of Bi, Sr, Ca, and Cu by the partial-melt-growth technique. Nitrate particles were deposited electrostatically on the Ag substrate and heat treated. After process optimization of the superconducting tapes, the best samples showed typical Tc of 89 K with zero applied field critical current density of 2×104 A/cm2 (Ic=11.2 A) at 77 K and 2.6×105 A/cm2 (Ic=190 A) at 4.2 K. Unlike films grown using solid state reacted precursor powders, these films lacked the typical Bi-free, needlelike defects and possessed a smooth morphology. X-ray diffraction patterns showed the film to be highly c-axis oriented. The processing of these tapes significantly reduces the overall fabrication time from precursor materials preparation to final heat treatment.

Three processing routes that generate uniaxial alignment but otherwise yield very different microstructure and critical current are compared. Fine grain size and c-axis alignment are obtained in magnetically aligned ceramics, pyrolyzed thick films, and in situ deposited thin films. The dense, well-aligned microstructure of the in situ process produces the highest zero field critical current Jc > 104 A/cm2 at 77 K. However, the critical current is suppressed in low magnetic field, suggesting that uniaxial alignment is not sufficient to avoid Josephson-type intergranular coupling. Above 1 T, the critical current of the aligned ceramic dominates in spite of its less ideal microstructure. The critical current in this high field region is one to two orders of magnitude greater than that of nonaligned material. This result implies the existence of a 3-d percolative network of strong links.

The change in superconducting properties due to the presence of a bias electrode in in situ laser deposition was explored. It was found that the bias ring allowed a 50–70 °C reduction in the deposition temperature. This reduction is correlated to an increase in ion current impinging on the substrate.