In order to use superconductors as nonresistive conducting wires, there must exist some mechanisms by which vortices are pinned against the Lorentz force due to a current. Since flux pinning phenomena occur on a microscopic scale, they have been difficult to observe directly, and therefore we can only presume what happens from macroscopic measurements. A new way was revealed to observe the dynamics of vortices in real time together with material defects by using both coherent field emission electrons and detecting the phase of the electrons, thus making it possible to microscopically observe the flux pinning phenomena.
The contact stiffness of a glass fiber-reinforced plastic, EL-850, has been measured at room temperature and in liquid nitrogen. Dimensions of the specimens were 20×20mm2 with three different thicknesses of 2.7, 4 or 8mm. Contact pressures up to 80MPa were applied to the specimens. Results showed how the contact stiffness increased with the applied contact pressure. It seemed that roughness and waviness on the surfaces of the EL-850 were factors which reduced contact stiffness. A computer simulation of contact stiffness, which took account of only the waviness, and measurements of the real contact areas using thin PET films, indicated that the waviness considerably affected contact stiffness. Contact stiffness measured in liquid nitrogen was less than that measured at room temperature when the contact pressure was low. This result was presumably caused by greater waviness of the specimen because of thermal distortion caused by liquid nitrogen.
Ag-sheathed superconductor tape is going to be used widely for many practical applications. However, this type of conductor may suffer damage to its superconducting filaments through production lines or degradation through the winding process, such as coil winding. So we propose a new method for non-destructive inspection at room temperature using ultrasonic waves. This method is a kind of ultrasonic transfer function method using a couple of piezoelectric sensors. One is the driver and the other is the receiver. This time, the authors report the relation between the acoustic characteristic for ultrasonic waves at room temperature and the electrical characteristics at 77K using sample tapes which experienced a bending strain up to 1.0%.
Interlaminar beam tests in the form of three-point flexure were examined both experimentally and analytically. The short-beam shear tests were carried out at room temperature, 77K and 4K to evaluate the interlaminar shear strength of G-10CR glass-cloth/epoxy laminates. These tests were conducted in accordance with ASTM D2344-84. The effects of temperature, specimen width and span-to-thickness ratio on apparent interlaminar shear strength are shown graphically. Photomicrographs (scanning electron micrographs, optical micrographs) of actual failure modes were utilized to verify the failure mechanisms. A three-dimensional finite element analysis was also performed to investigate the effects of specimen width and span-to-thickness ratio on the shear stress distribution in the mid-plane of the beam. Effective elastic moduli were determined under the assumption of uniform strain inside the representative volume element. The numerical findings were then correlated with the experimental results.
Nb3Sn superconductors have been fabricated through a new process starting from Nb6Sn5 intermediate compound powder. The mixed Nb6Sn5 and Nb powder is encased in a Ta tube, and rolled into tapes without annealing and then heat treated. The Nb6Sn5 compound powder can be easily synthesized by melt-diffusion reaction at 900°C between Nb and Sn powders. The Nb3Sn fabricated by this process has a large ρn value and a Bc2 of 24.7T at 4.2K, which is considerably higher than that of bronze-processed Nb3Sn. In this study, the effects of different additional elements on the high-field performance of the new Nb3Sn specimen were studied. The Jc (core) of the specimen with 2at% Ti substitution for Nb is 3.2×104A/cm2 at 20T and 4.2K. The addition of Cu to the specimen decreases the optimum heat-treatment temperature from 900-925°C to 850°C. The addition of a small amount of Ge or the substitution of a small amount of Ta for Nb raises the curvature of the log Jc-B curves of specimens to a convex at high magnetic fields, resulting in significant enhancement in high-field performance. The Nb3Sn specimen with the addition of 1.0wt% Ge and a slightly richer Sn concentration shows a Jc (core) of 3.2×104A/cm2 at 21T and 4.2K. The small amount of Ta substitution for Nb has been found to be most effective to achieve large Jc at high magnetic fields. The optimum amount of Ta substitution is 5-7at%. The JcS (core) of the specimen with 5at% Ta at 22 and 23T at 4.2K are 3.3×104 and 2.4×104A/cm2, respectively. The specimens with Ta substitution are composed of two Nb3Sn phases with different Ta concentrations. The new Nb3Sn superconductors may be quite attractive for high-field applications after multifilamentary-type conductors with Cu stabilizers become available.
The superconducting performance test facility for the 50-kA conductor of a fusion machine was completed at the Japan Atomic Energy Research Institute (JAERI). This facility consists of a 5-kW refrigeration system, 60-kA DC power-supply system, a liquid-helium dewar, and vacuum evacuation and data acquisition systems. The full-size conductor and joint sample for the ITER central solenoid (CS) model coils will be tested in this facility. The samples are cooled by a supercritical helium of from 5 to 16K and tested for an external field of up to 11 T and a 60kA current. According to these results, the operation limits of the model coil will be estimated and the fabrication technique of large coils verified. This facility has completed preliminary tests and its designed performance confirmed.