Jianqing Feng

Kinetics of stabilized cubic zirconia formation from MnO2–ZrO2 diffusion couple

Microstructure and kinetics of the cubic phase formation are studied in ZrO2–MnO2 diffusion couple. During annealing in Ar, no other compounds are detected except the zirconia solution. With diffusion of Mn ions from manganese oxide to ZrO2, the cubic zirconia are found at the phase interface. The growth kinetics of the cubic zirconia solution layer in the submicrometer scale are analyzed. The growth behavior of the cubic zirconia layer is better described by the grain boundary diffusion model, and our experiment data exhibit a t1/4 dependence on time. The deviation from the experiment data to the theoretical value is also explained.

Optimization of High-Energy Ball Milling Aided Sintering Process for FeSe Superconductors

Superconducting FeSe bulks with high superconducting content of PbO-type tetragonal β-FeSe phase were prepared with high-energy ball milling (HEBM) aided sintering process. During the HEBM process, different ball-to-powder ratios of 6:1, 8:1, and 10:1 were adopted. The influences of ball-to-powder ratio on the phase composition of both ball milled powders and sintered bulks, morphology, and superconducting properties of final bulks were systematically investigated. It was noticed that, with the increase of ball-to-powder ratio, the energy input into the ball milling system increased, and the high-energy collision times increased. Therefore, more Fe atoms entered into the Fe-Se matrix, and the ball milling products changed from Fe3Se4 to Fe7Se8. On the other hand, the decreasing total collision times led to the increase of final particle size. After the same sintering process, FeSe bulks with the major phase of β-FeSe were all obtained. However, with the increasing ball-to-powder ratio, the contents of both hexagonal δ-FeSe phase and residual Fe contents increased. Due to the different Fe/Se ratio in the final β-FeSe phase, the superconducting transition behavior varied obviously. Higher critical temperature of 8.0 K and larger superconducting phase content were achieved with the optimal ball to powder ratio of 6:1.

Development of Low-Fluorine Solution for YBCO Film Growth

We developed a simple and stable precursor solution containing low fluorine content to fabricate YBa2Cu3Oy (YBCO) films on CeO2/YSZ/CeO2 buffered NiW substrates. The fluorine content in the precursor solution could be controlled at 23.1% by tuning the ratio of barium trifluoroacetate to barium acetate. Fast pyrolysis process and smooth surface of YBCO precursor films could be acquired by using the low-fluorine solution. The phase, texture, microstructure, and superconducting properties of YBCO film were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscope, and four-probe method, respectively. The crystallized YBCO film showed good out-of-plane and in-plane textures. A high critical current density Jc of ~3 MA/cm 2 (77 K, 0 T) could be obtained in the YBCO/CeO2/YSZ/CeO2/NiW sample derived from the low-fluorine solution. The results indicate that the controllable fluorine content solution could be useful for epitaxial growth of YBCO films on buffered NiW substrates.