Yi-Yuan Xie

Enhanced and uniform in-field performance in long (Gd, Y)?Ba?Cu?O tapes with zirconium doping fabricated by metal?organic chemical vapor deposition

The influence of Zr doping in (Gd, Y)?Ba?Cu?O ((Gd, Y)BCO) tapes made by metal?organic chemical vapor deposition has been studied with a specific objective of uniform and reproducible enhancement in in-field critical current (Ic) over long lengths. 50?m long tapes with 7.5 and 10?at.% Zr doping in 1? ?m thick (Gd, Y)BCO films have been found to exhibit a sharply enhanced peak in Ic in the orientation of field parallel to the c-axis and retain 28% of their self-field Ic value at 77?K and 1?T. BaZrO3 (BZO) nanocolumn density in the cross-sectional microstructure was found to increase with increasing Zr addition. The end segments of the 50?m long tapes were found to display nearly identical angular dependence of critical current at 77?K and 1?T, indicative of the uniformity in in-field performance over this length. A 610?m long tape was fabricated with 10% Zr doping and a 130?m segment showed a 3.2% uniformity in critical current measured every meter in the orientation of -axis. A retention factor of 36% of the zero-field Ic value measured at 0.52?T over the 130?m is consistent with that obtained in short samples.

Cation exchange: A scheme for synthesis of mercury-based high-temperature superconducting epitaxial thin films

A cation-exchange process has been developed for growth of high-quality epitaxial thin films of highly volatile mercury-based high-temperature superconductors. By selecting epitaxial precursor matrices of TlyBa2CaCu2Ox (y=1,2) and annealing them in Hg vapor, c-axis-oriented epitaxial HgBa2CaCu2O6+δ thin films with superior quality were formed through Tl and Hg-cation exchange. At 110 K, these films can carry supercurrent density close to 1 MA/cm2 which is nearly an order of magnitude higher than the best reported previously. This cation-exchange technique also provides a general scheme for synthesis of other volatile compounds with predesigned structure and composition.A cation-exchange process has been developed for growth of high-quality epitaxial thin films of highly volatile mercury-based high-temperature superconductors. By selecting epitaxial precursor matrices of TlyBa2CaCu2Ox (y=1,2) and annealing them in Hg vapor, c-axis-oriented epitaxial HgBa2CaCu2O6+δ thin films with superior quality were formed through Tl and Hg-cation exchange. At 110 K, these films can carry supercurrent density close to 1 MA/cm2 which is nearly an order of magnitude higher than the best reported previously. This cation-exchange technique also provides a general scheme for synthesis of other volatile compounds with predesigned structure and composition.

High critical current density in epitaxial HgBa2CaCu2OX thin films

High quality superconducting HgBa2CaCu2Ox (Hg-1212) thin films have been reproducibly fabricated using cation-exchange method. The thin films have pure Hg-1212 phase and have smooth surface morphology. The superconducting transition temperatures of these films are in the range of 120–124 K. The critical current density Jc is up to 3.2×106 A/cm2 at 77 K and drops only by a factor of 2 at 100 K and self field. At 110 K, a Jc of 7.8×105 A/cm2 has been obtained. X-ray diffraction pole figures show that these films are epitaxially grown on LaAlO3(001) substrates, which is consistent with a χmin of 19% obtained using Rutherford backscattering/channeling analysis.

Microwave surface resistance of HgBa2CaCu2O6+δ thin films

Microwave surface resistance (Rs) has been measured on c-axis-oriented superconducting HgBa2CaCu2O6+δ (Hg-1212) films. A cavity perturbation method was employed using a high-Q Nb cavity cooled at 4.2 K. For the best film, an Rs as low as ∼0.3 mΩ was observed at 10 GHz up to ∼120 K on Hg-1212 films that have smooth surface morphology and high critical current density near 2 MA/cm2 at 100 K and self-field. This result suggests that Hg-1212 films are very promising for microwave applications above 100 K.

Microwave-power handling capability of HgBa2CaCu2O6+δ superconducting microstrip lines

Measurement of microwave-power handling capability, performed on high-temperature superconducting HgBa2CaCu2O6+δ (Hg-1212) microstrip lines, was carried out at a microwave frequency of 1 GHz and temperatures ranging from 77 to 120 K. These samples were prepared using a different route in which direct patterning on Hg-1212 was avoided to minimize their moisture exposure. Microwave input power was swept to high levels to drive the samples from a linear (less dissipative) state into nonlinear (more dissipative) state. Critical input power (Pc) was determined to be the crossover between the two states. The Pc values observed for Hg-1212 microstrip lines were 29.3, 25.8, 23, and 16 dBm at 80, 90, 100, and 110 K, respectively. These Pc values, comparable with the best obtained so far on other superconductors at lower temperatures, are orders of magnitude better at 100 K or higher, suggesting that Hg-1212 is promising for microwave devices and circuits operating above 100 K or higher temperatures.

Fabrication and physical properties of large-area HgBa2CaCu2O6 superconducting films

Epitaxial c -axis-orientated HgBa2 CaCu2 O6 (Hg-1212) superconducting thin films with their dimensions as large as ½ inch × ½ inch have been recently achieved using a cation-exchange process. Uniformity of the film was studied by slicing samples into eight pieces, on which uniform zero-resistance Tc values above 120 K were observed. In self-field, Jc values of the film range from 14.6 × 106 to 23.7 × 106 A cm-2 at 5 K, 1.77 × 106 to 3.85 × 106 A cm-2 at 77 K and 0.54 × 106 to 1.38 × 106 A cm-2 at 100 K. The scattering of Jc is found to be within 44% across the film and could be smaller with improvement of processing.

Epitaxy of HgBa2CaCu2O6 superconducting films on biaxially textured Ni substrates

Superconducting HgBa2CaCu2O6 (Hg-1212) films have been coated on biaxially textured Ni substrates buffered with CeO2/YSZ/CeO2 trilayers. A two-step cation-exchange process was employed for Hg-1212 coatings, whereby epitaxial Tl2Ba2CaCu2O8 precursor films grown in a fast temperature ramping annealing process were converted to Hg-1212 via diffusive Hg–Tl cation exchange. This minimizes chemical diffusion/reaction of Hg with the substrates and allows Hg-1212 films to inherit the epitaxy and smooth surface morphology of the Tl2Ba2CaCu2O8 precursor films. The superconducting transition temperature (Tc) of these Hg-1212 coated conductors is up to 124 K. Self-field critical current densities (Jc) of 2.2×106 A/cm2 at 77 K have been obtained, with Jc values of 0.7×106 A/cm2 achieved at 100 K. These results show Hg-1212 coated conductors may hold promise for electrical power applications above 77 K.

Synthesis of Hg-1223 superconductors using a cation-exchange process

Abstract The newly developed cation-exchange process was applied to convert Tl 2 Ba 2 Ca 2 Cu 3 O 10 (Tl-2223) superconductors to HgBa 2 Ca 2 Cu 3 O 8+ δ (Hg-1223) superconductors via Tl–Hg cation-exchange in Hg-vapor at processing temperatures ranging from 700 to 800°C for 6–24 h. Oxygen annealing process at 350°C for 10 h was followed to optimize the content of oxygen in the Hg-1223 samples. The Tl-2223 phase has been successfully converted to Hg-1223 phase with a negligible amount of non-superconducting impurity phase. T c values up to 135 K have been obtained in Hg-1223, which are ∼15 K higher than that of the original Tl-2223 compounds. Since the cation-exchange process is a kinetic process, it is very promising for fabrication of high-quality Hg-1223 superconductor at large scales.

Stability of Hg-based high-Tc superconducting thin films

Abstract Stability of Hg-based superconducting thin films has been studied in air, acetone, alcohol, water, and thermal cycling. T c , J c , and surface microstructures were analyzed as functions of exposure period. In dry air, fully reacted samples did not show degradation for a period of over one year, while samples which were not fully reacted due to air contamination or other reasons degraded within a few weeks. In humid air, samples were totally degraded within a few minutes. It has been found that the films degraded considerably in alcohol and acetone after they were immersed for a few hours, while this degradation was much severe in water. The mechanism of sample degradation has been discussed as well.

Diffusion mechanism of cation-exchange process for fabrication of HgBa2CaCu2O6+δ superconducting films

In a cation-exchange process, epitaxial Hg-based high-temperature superconducting (HTS) films are obtained from epitaxial Tl-based HTS precursor films by diffusing Tl cations out of, and Hg cations into the crystal lattice. In order to understand the cation diffusion mechanism, we have carried out a systematic study on conversion of Tl2Ba2CaCu2O8 precursor films to HgBa2CaCu2O6 films. Such a conversion was found to be composed of two steps. In the first step, Tl2Ba2CaCu2O8 collapses structurally into the (Tl,Hg)Ba2CaCu2O6 lattice, while in the second step, Tl cations diffuse out while Hg cations diffuse in, resulting in pure HgBa2CaCu2O6 phase. A time constant τ1 of about 45–60 min was found for the first stage, while the second stage τ2 is beyond 2τ1. This provides potential to optimize the cation exchange processing parameters for Hg-based HTS films and devices.