This paper reviews the current status of electrical metrology standards using cryogenic engineering; i.e., Josephson voltage standards (JVSs) and quantum Hall resistance standards (QHRSs). The Josephson effect and the quantum Hall effect are widely used throughout the world for realizing the national standard systems for DC voltage and DC resistance, respectively. These effects provide intrinsic, essentially stable and accurate reference values based on the quantum theory. Recently, much effort has been made to expand the functions of the conventional systems to AC and other applications. In addition, in order to resolve the recent situation related to the worldwide liquid-Helium (LHe) shortage, we are promoting the development of LHefree JVS systems using closed-cycle mechanical coolers. These attempts are now reaching the practical stage and will have much importance for various fields.
We prepared polycrystalline Tl-1223 films without impurity phases and with a c-axis orientation by heat treatment and intermediate pressing of precursor films using a laser ablation process on Ag substrates. The critical temperature (Tc) of Tl1223 films on Ag was 120 K and the critical current density (Jc) was 15 kA/cm2 at 77 K, 0 T. The strong field dependence of Jc of the Tl-1223 films in a low magnetic field showed that the grain boundaries of the Tl-1223 films behaved like a weak link. The temperature dependence of Jc of the Tl-1223 films was proportional to (1-T/Tc)n with n=2.0 for 0.08 ?? 1-T/Tc ?? 0.38. These results confirm that grain boundaries in Tl-1223 films act as superconductor-normal metal-superconductor (SNS) junctions.
Low AC loss Ag-sheathed Bi2223 tapes with oxide barriers among the twisted filaments were successfully fabricated using the powder-in-tube method. SrZrO3 was selected as the barrier material for its compatibility with Bi2223 oxide. Moreover, a small amount of Bi2212 was mixed with SrZrO3 to improve its ductility for cold working. By controlling barrier thickness and reducing the final tape width down to 2.7 mm, transport critical current densities Jc at 77 K and self-field attained 17-23 kA/cm2 for twisted barrier tapes with twist lengths Lt between 4 and 9 mm. For barrier tape with the shortest Lt = 4 mm, a coupling frequency fc , which is related to inverse of coupling time constant τc, of 270 Hz was attained in an AC transverse field perpendicular to a broader face of the tape. To our best knowledge, fc = 270 Hz under a perpendicular field is the highest ever reported value for Bi2223 tapes with self-field Jc > 15 kA/cm2 at 77 K. According to these achievements, perpendicular field losses around 50 Hz for barrier tape were 70-80% lower than those for conventional 4 mm-width tape with fully coupled filaments.
We are currently developing an external-heating-type superconducting magnesium diboride (MgB2) level sensor for a liquid hydrogen tank. The heating effect of an external heater on an 800-mm-long MgB2 level sensor was previously reported. However, the heating effect of an external heater on the MgB2 level sensor has not yet been clarified in the case of changing the sensor length. In this study, we examined the temperature distribution and level-detecting characteristics of a 200- mm-long MgB2 level sensor. It was found that the temperature of the sensor above the liquid surface was higher than Tc at a heater input of 6 W or more. Regarding the level-detecting characteristics of the MgB2 sensor, the linear correlation coefficient was 0.99 or more at a heater input of 3 W and above. Moreover, the ratio of the effective level-detecting length to the total length remained constant at a heater input of 6 W or more. Similar results were observed for different sensor lengths. Therefore, the optimum heater input was 6 W regardless of the sensor length.