Controlling the axis orientation in REBa2Cu3Oy (RE123; RE meaning Nd, Sm, Gd, Y and Yb) thin films is an important problem for many superconducting applications since RE123 materials have anisotropic properties (e.g., irreversible field, critical current density and so on) along their crystallographic axes. To obtain the RE123 thin films with excellent superconducting properties, we systematically studied the orientation, mechanism of orientation control and superconducting properties in RE123 thin films. We found that the preferred orientation of RE123 thin film on MgO and SrTiO3 is c-axis orientation. RE123 thin films with RE/Ba substitution (RE=Sm, Gd) showed that the substrate temperature corresponding to the boundary line between the c-axis and a, c-axis mixed orientation decreased as RE/Ba substitution increased. These facts imply that the orientations of RE123 thin film are principally determined by their peritectic temperatures. The highest critical current densities of RE123 thin films so far are 7.93 MA/cm2 for Sm123, 3.03 MA/cm2 for Gd123 and 1.85 MA/cm2 for Yb/Nd123.
The micro-miniature JT cooler is a promising cooling device as an element of the micro-thermal system for semiconductor devices, which dissipate an incredible amount of heat. In particular, the thermal management of laser diodes is a serious problem because they require a large amount of power but must maintain a fixed temperature. Considering such a situation, the performance of a micro-miniature JT cooler was calculated under the assumption of a countercurrent heat exchanger operating with a laminar gas flow, and the guidelines of a micro-miniature JT cooler for optimizing pump power and size were designed from the viewpoint of minimizing the cost. The authors present the results in this paper. A micro-miniature JT cooler was fabricated from Si wafer using a photolithographic process, which is available for operation with C2H6 from 3 MPa to 0.1 MPa. It is shown that a cooling power of 2.1 W was obtained at 239 K with a mass flow of 51.7 mg/s.
A small, low-magnetic-noise nondestructive inspection (NDI) system that uses a high-Tc superconducting quantum interference device (SQUID) cooled by a coaxial pulse tube cryocooler (PTC) has been developed. The key components for realizing the compact, low-noise system developed in this study, include the following: a high-Tc SQUID gradiometer, the coaxial PTC, and a SQUID-mount stage separated from the cold head of the cryocooler. The high-Tc SQUID gradiometer contributes not only to stable system operation when subjected to the environmental noise without magnetic shielding, but also a decrease in magnetic noise caused by the mechanical vibration of the SQUID, which is transmitted from the cryocooler. By using the coaxial PTC, which was constructed of non-magnetic materials, both magnetic noise and the mechanical vibration are well suppressed compared to conventional two-axial PTCs. In addition, the coaxial PTC is designed to be compact (50 mm diameter, 400 mm height, and 4 kg weight) for practical use. The SQUID-mount stage separation contributes to stable operation by reducing the transmitted vibration to the SQUID, and thermal stability at the SQUID-mount stage. The magnetic flux noise spectrum of the cryo-cooled system was measured and compared with that of a liquid-nitrogen-cooled system using the same SQUID. From the comparison, no increase in magnetic noise due to the cryocooler was observed, although the magnetic flux noise level of the cryo-cooled system was dominated by SQUID operation noise, which was rather high, (i.e., around 100 μΦ0/Hz1/2 at 100 Hz). The relation between the mechanical vibration at the SQUID and magnetic noise is discussed.
The vibrations of a Gifford-McMahon (GM) and pulse-tube (PT) cryocoolers were measured and analyzed. The vibrations of the cold stage and cold head were measured separately to investigate their vibration mechanisms. The measurements were performed while maintaining the thermal conditions of the cryocoolers at a steady state. We found that the vibration of the cold head for the 4 K PT cryocooler was two orders of magnitude smaller than that of the 4 K GM cryocooler. On the other hand, the vibration of the cold stages for both cryocoolers was of the same order of magnitude. From a spectral analysis of the vibrations and a simulation, we concluded that the vibration of the cold stage is caused by elastic deformation of the pulse tubes (or cylinders) due to the pressure oscillation of the working gas.