To utilize liquid and slush hydrogen at temperatures between 15 to 20 K as the refrigerant of HTS and MgB2 superconductors with a clean energy carrier without exhaust of CO2, a few methods are suggested. The boil-off gas of para-hydrogen must be pure and can also be fed to PEFC for highly efficient electric power generation. If it were possible to construct long and thermally well-insulated para-hydrogen transfer lines, for example, along highways, it would be possible to transfer high-current electric power through refrigerated HTS and MgB2 cables without loss. The technologies of combining liquid hydrogen and HTS, MgB2 superconductors will give rise to new solutions for clean energy use in communities around the world.
The behaviors of bubble nucleation and growth in liquid 3He were investigated visually using the shadowgraph method. We have constructed two types of cryostats and sample cells for measurements between 0.5 and 1K. The 3He bubble that formed on the heated copper surface was spheroid-like due to a low surface tension and small buoyancy force. The time for required for the bubble to grow and separate from the surface was 14ms at a heat flux of q = 8=10-5 W/cm2 and bulk liquid 3He temperature of 0.7K. This means that the bubble growth rate is high as compared to other low-temperature liquids. The bubble size, number of bubbles separating from the heated surface in a specific time and the rising velocity of the bubbles were analyzed using photographs obtained from a high-speed camera as a function of heat flux and liquid 3He temperature. In the region of large heat flux, large bubbles formed by bubble accumulation covered the copper surface and the increase in bubble number as q increased became insignificant. In this region, the amount of heat flux carried by bubbles as latent vaporization heat was approximately 60% of the applied heat flux. The characteristic heat transfer curve of liquid 3He measured previously by our group could be explained qualitatively by these boiling behaviors.
We investigated the temperature dependence of the trapped magnetic field and levitation force for c-axis-oriented single-grain Gd-Ba-Cu-O bulk superconductors 48 mm in diameter. The trapped magnetic fields of the samples were 2.1-2.2 T at 77 K and increased with decreasing temperature, reaching 4.1 T at 70 K. However, the samples fractured during the measurements at lower temperatures due to a large electromagnetic force. Reinforcement using a metal ring was effective in improving the mechanical strength. The sample encapsulated in an Al ring could trap a very high magnetic field of 9.0 T at 50 K. In liquid O2(90.2 K), the Gd-Ba-Cu-O bulk superconductor exhibited a trapped magnetic field of 0.4 T and a levitation force about half the value of that in liquid N2.