We developed a new precursor solution that consists of copper naphthenate, and yttrium- and barium-trifluoroacetates (advanced TFA solution) that can make the heating rate during calcination increase while maintaining a high Jc. In this paper, we investigated the influence of HF gas generation on the calcination process in order to optimize an advanced TFA-MOD process. The high heating rate and/or low copper content in the starting solution lead to a low Jc. In both cases, a high generation rate of HF gas was observed. Consequently, it is clear that rapid HF gas generation leads to the formation of pores that cause a reduction in Jc values.
An electron-beam evaporation technique was used to prepare MgB2 thin films on polished sapphire C-plane single-crystal substrates with and without post-annealing. We maintained the background pressure to less than 1.3×10-7 Pa and the substrate temperature at 247-284°C. The Mg evaporation rate was 2 nm/s and B 0.5 nm/s. The best critical temperature (Tc) for the as-grown films was 34 K and the critical-current density (Jc) was 7.1×106 A/cm2 (4.2 K, self field.). In order to improve the crystallinity of the as-grown MgB2 thin films, we annealed them at 580-620°C for 1 hr under vacuum conditions. The Tc's of the post-annealed films increased about 4 K; however, Jc in the magnetic field decreased proportionate to the as-grown MgB2 thin film. From X-ray diffraction measurements, it was shown that the post-annealing process improved the crystallinity of the post-annealed film. The crystallinity of MgB2 grains may affect the Jc of MgB2 superconductors.
Simple heat-conduction and thermoacoustic self-sustained oscillation branches are discussed employing current thermodynamics and thermoacoustic theory. Supposing a short regenerator where the local entropy production rate is homogeneous, thermodynamic stability is discussed analytically. Minimizing the entropy production rate of the short regenerator gives both simple heat conduction and the thermoacoustic self-sustained branches for a given oscillation mode. Necessary conditions for maintaining the thermoacoustic self-sustained branch restrict the oscillation mode. The conditions required for attaining stable oscillation in the short regenerator are also discussed.