Publications

In the preparation of superconducting oxide films, one frequently observes crack formation, crystal orientation, phase seperation, and chemical reaction, all of which involve interactions between superconductors and substrate. Quantitative verification of these phenomena is expected to provide useful information for the preparation and microprocessing of high Tc films. Thermal expansion behavior of representative superconductors and substrate materials has been measured by a dilatometric method. The superconductors have far greater mean thermal expansion coefficients than conventional substrate materials and the difference correspond well with the crack size observed in some of the films. The non-linear expansion of BLnCO also contain information on the kinetics of oxygen uptake and secession as well as on the phase transition temperature. The effect of lattice matching has been investigated on Ba-Yb-Cu-O films. The films were deposited by sputtering on (100) and (110) SrTiO3 and some other ceramic substrates and their orientation was measured by SEM. Furthermore, chemical interaction has been examined by heating mixed powders of superconductors and substrate materials at various temperatures. The structural variation and the reduction in superconducting volume fraction were analyzed by X-ray diffraction and ac-susceptibility measurements, respectively.

Fatigue tests were carried out on nanocrystalline Ti- and Cu-based bulk metallic glasses (BMGs) for which there are still no reports on fatigue strength. Test alloy rods with a diameter of 2 mm were prepared in nanocrystalline Ti41.5Zr2.5Hf5Cu42.5Ni7.5Si1 at% and Cu60Zr30Ti10 at% systems by copper mold casting. Nanocrystals were dispersed in the metallic glassy phase in both BMGs. The tensile strength (σB) in the nanocrystalline Ti- and Cu-based BMGs were 2.04 GPa and 2.00 GPa, and Young' s modulus were 95 GPa and 114 GPa, respectively. The test specimens were machined to hourglass shape type (the radius in axial direction; 4.45 mm, the minimum diameter; 0.9 mm), and after machining the specimen surfaces were electropolished by 50~100 μm. The specimens were tested at a stress ratio of 0.1 and a frequency of 5-8 Hz. The fatigue limit (σw) (half a total stress amplitude) and fatigue ratio (σw/σB) in the nanocrystalline Ti- and Cu-based BMGs showed very large values (Ti-based alloy; 806 MPa and 0.40, Cu-based alloy; 490 MPa and 0.25). This experimental result indicated a possibility that the nanocrystal dispersed BMGs have higher fatigue strength than the high strength crystalline alloys with high fatigue strength (e.g. Cr-Mo steel, σw≈500 MPa).