TEION KOGAKU (Journal of Cryogenics and Superconductivity Society of Japan)

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When EuBa₂Cu₃O_7-δ (EuBCO) thin films of more than 3,000 Å in thickness were deposited on R-Al₂O₃ substrates with a CeO₂ buffer layer, there was a tendency for micro-cracks to be generated due to the difference in thermal expansion coefficient. If a thick CeO₂ buffer layer with a flat surface is used, it is believed that microcrack generation can be suppressed as the result of reducing the influence of thermal distortion. In this study, in order to improve the surface morphology of a thick CeO₂ buffer layer, we focused on the effect of sputtering gas pressure. Sputtering gas pressure was changed from 0.5 to 7 Pa when a CeO₂ buffer layer of 3,000 Å in thickness was prepared, and the effects of sputtering gas pressure on deposition rate (R_d), surface morphology and crystallinity were examined. The effect of sputtering gas pressure on the superconducting characteristics of the EuBCO thin film grown on the CeO₂ buffer layer was also examined. Smooth CeO₂ buffer layers were obtained at 3 Pa, and the EuBCO thin films deposited on the CeO₂ buffer layer showed a critical temperature (T_ce) of 90 K and a critical current density (J_c) values of about 4.0 MA/cm² at 77.3 K.

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We developed a helium circulating system that re-liquefies all the evaporating helium gas and consumes far less power than conventional systems, because warm helium gas at about 40 K collected high above the surface of the liquid helium in the Dewar is used to keep the Dewar cold, and because cold helium gas just above the liquid helium surface is collected and re-liquefied while still cold. A special transfer tube with multi-concentric pipes was developed to make the system operate efficiently. The system can produce up to 35.5 l/D of liquid helium from the evaporated helium using two 1.5W@4.2K GM cryocoolers. (Translation of the article originally published in Cryogenics 48 (2008) 6-11)

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