Kyoko Kawagishi

Approach for the fabrication of MgB2 superconducting tape with large in-field transport critical current density

The letter reports the fabrication of MgB2 superconducting tape using a flexible metallic substrate as well as its strong pinning force, which was verified by direct measurement of transport critical current density. The tape was prepared by depositing MgB2 film on a Hastelloy tape buffered with an yttria-stabilized-zirconia layer. The Jc of the tape exceeds 105 A/cm2 at 4.2 K and 10 T, which is considered as a common benchmark for magnet application. The Jc dependence on magnetic field remains surprisingly very small up to 10 T, suggesting that the tape has much better magnetic field characteristic than conventional Nb–Ti wires in liquid helium.

Oxidation Behavior of Ru-Containing Ni-Base Single-Crystal Superalloys

The 4th and advanced generation Ni-base single crystal superalloys, which contain large amounts of refractory metals for strengthening and platinum group metals, e.g., Ru, for TCP-phase prevention, show excellent high-temperature strengths. However, these alloying elements seem to decrease high-temperature oxidation resistance. In this study, Ni-base superalloys with various amounts of Ta, Re and Ru were examined in isothermal and cyclic exposures at 1373K to investigate the effect on the oxide growth rate and resistance to scale spallation. Structures of the oxide for the alloys were analyzed by XRD, SEM and EDX, and the oxidation kinetics is discussed. Ru and Re were found to degrade the oxidation resistance by the vaporization of their oxide. Ta-rich oxide in the spinel layer affects to stabilize ruthenium and rhenium oxide in the scale and improve the oxidation resistance of Ru-containing Ni-base superalloys.

Factors affecting the microwave surface resistance of crack-free thick YBCO films on sapphire substrates

Abstract We investigated the influence of the composition of YBCO film, deposited on sapphire 〈 R 〉 substrates with CeO 2 buffer layers, on their microwave surface resistance. Y-rich YBCO films, 0.5-μm thick, deposited onto a 250-A thick CeO 2 film showed good crystallinity, a smooth surface and low R s value. Results of the Hall constant measurement and the chemical state analysis infer that the composition deviation of the film from 1-2-3 stoichiometry to Y-rich does not produce defects, such as a second phase, and resulted in a small residual resistance which manifests itself in the microwave surface resistance measurement.

Development of New Bond Coat System in Ni-Base Alloys

Ni-base single crystal (SC) superalloys containing high concentrations of refractory elements are prone to generate a diffusion layer called Secondary Reaction Zone (SRZ) beneath their bond coating during exposure at high temperatures. SRZ causes a reduction of the load bearing cross section and is detrimental to the creep properties of thin-wall turbine airfoils. In this study, a new coating system – “EQ coating”, which is in thermodynamic equilibrium with the substrate, has been proposed and the formation behavior of SRZ beneath bond coat materials was investigated on the 5th generation Ni-base SC superalloy developed by NIMS. Diffusion couples of several alloys were made and were heat treated at 1100°C for 300 h, 1000 h. The concentration profiles were analyzed by EPMA. Also, cyclic oxidation tests were carried out at 1100°C in air.

rf Field dependence of surface resistance for a-b plane-textured YBa2Cu3O7-δ films deposited on copper substrate

Low rf loss at high rf field levels should be realized in order to apply high-Tc films to accelerator cavities. It is well known that c-axis perpendicular to the surface is essential to reduce Rs and its field dependence However, the effect of a–b plane texturing on Rs is not so clear because lack of experiments, especially for films deposited on metallic substrates. We developed a deposition technique that enables a–b plane texturing as well as c-axis orientation of YBCO films on a copper substrate. We prepared four samples with c-axis normal to the surface: two of them were a–b plane well textured films and the other two were a–b plane weakly textured films. The a–b plane well-textured films in low rf field exhibited a Rs higher than that of weakly textured films below 80 K. The rf field dependence measurements showed that the increase of Rs with increasing rf field for the a–b plane well-textured films was slower as compared with that of the weakly textured films.

Development of a Remote Nuclear Quadrupole Resonance Detector

To remotely detect a chemical compound, we have developed a remote nuclear quadrupole resonance (NQR) detector. The detector utilizes 14N which has a nuclear quadrupole moment (I=1) as a probe. At present, the detector can detect 25 g of hexamethylene tetramine located within 25 mm of the sensor head.

Preparation of in-plane textured large-area YBCO films on (100) MgO with CeO2/YSZ buffer layers

Epitaxial YBa2Cu3Oy (YBCO) thin films with smooth surfaces have been successfully grown on large area (100) MgO (30×30 mm2) substrates buffered by CeO2/YSZ (Y2O3-stabilized ZrO2) double layers. The use of the double buffer layers and the employment of a new technique for controlling substrate temperature greatly enhance not only the YBCO film quality, but also the reproducibility of deposition. An excellent in-plane epitaxy was achieved with no evidence of misoriented grains in the YBCO/CeO2/YSZ/MgO structure. Good microwave properties were observed for YBCO films grown on buffered MgO compared with those on bare MgO.

Thickness and composition dependence of microwave properties of YBCO thin films on CeO2-buffered sapphire substrates

Abstract We investigated the influence of the crystallinity and the composition of YBCO film on the formation of cracks in layers deposited on sapphire 〈R〉 substrates with CeO 2 buffer layers. The CeO 2 films and YBCO films were deposited using RF magnetron sputtering. Y-rich YBCO films, 0.5–1 μm thick, deposited onto a 300 A thick CeO 2 film showed good crystallinity and low microwave surface resistance. In high magnification SEM observation, cracks were not observed for films up to 1 μm in thickness.

Strain Controlled Low Cycle Fatigue Behavior of U720Li Disk Superalloy Above 700 °C

The low cycle fatigue behaviors of Udimet 720Li (U720Li) and the related microstructure evolution have been investigated at 725 °C under strain control tests. The interrelationships between microstructure factors and properties were analyzed using transmission electron backscatter diffraction (t-EBSD) in scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For comparison, LCF behaviors at 650 °C were also presented to find the inferior aspects of U720Li alloy at relatively higher service temperatures. The results show that recrystallization occurs during the LCF testing at 725 °C. The extent of recrystallization increases with the strain. The LCF property at 725 °C is weakened to a large extent after recrystallization, which could be the significant factor that causes LCF degradation. Combination t-EBSD and TEM are beneficial for characterizing and analyzing the microstructure evolution in terms of primary γ′ precipitates and dislocations, proposing that gliding dislocations could concentrate into walls to form sub-grain boundaries, with the combination of primary γ′ precipitates to form sub-grains.

Creep Property and Phase Stability of Sulfur-Doped Ni-Base Single-Crystal Superalloys and Effectiveness of CaO Desulfurization

The direct and complete recycling method for Ni-base superalloy is being developed and studied to reduce the material cost for cost-effective operation of gas turbine systems. Understanding the effect of sulfur contamination is important to determine allowable sulfur content after the recycling. However, in the case of single-crystal superalloys, this effect on material properties is not well known except for the detrimental effect on the oxidation resistance. In the present study, creep tests, aging tests, and cyclic oxidation tests have been performed on PWA1484 with varying sulfur content. The increasing sulfur content has been found to correlate with degradation of properties evaluated here. It is observed that the decrease in creep life in PWA1484 due to sulfur doping is primarily due to coarsening of the γ/γ′ interfacial dislocation network, increase in precipitation kinetics of topologically closed-packed phase, and decrease in oxidation resistance. For recycling purposes, a CaO crucible was used in the casting process, which successfully decreased the sulfur level in the alloy, and the resulting material showed comparable or even better properties in comparison to the low sulfur content material.