Beizhan Li

MgO buffer-layer-induced texture growth of RE–Ba–Cu–O bulk

A novel cold-seeding approach of top-seeded melt growth of RE–Ba–Cu–O bulk was studied by employing a MgO crystal seed and a buffer pellet. The growth process is divided into two steps. The MgO seed is used to texture-grow the small RE–Ba–Cu–O pellet with high melting point (Tp) and the textured pellet induces the texture growth of the bulk at lower temperature. Undercooling and RE211 content of the pellet are adjusted to avoid the misorientation caused by lattice mismatch between MgO and the RE–Ba–Cu–O matrix. Bulk samples prepared with this method show good growth sections and superconducting performance. One of the promising advantages of this method is to process high Tp RE–Ba–Cu–O bulks with a cold-seeding method, for example Nd–Ba–Cu–O bulk.

Flux pinning properties of Gd–Ba–Cu–O trapped field magnets grown by a modified top-seeded melt growth

We report a modified top-seeded melt-growth technology for processing high performance Gd–Ba–Cu–O trapped field magnets (TFMs) by providing an additional liquid source during the melt-textured growth. The enriched liquid source allows sufficient growth in the a−b plane with an increased growth rate, and depresses the accumulation of Gd2BaCuO5 (Gd211) particles, which largely suppresses the formation of sub-grain boundaries in the a-growth sectors. An average trapped magnet field of 1.2 T is achieved for the 32 mm Gd–Ba–Cu–O TFMs at 77 K. These Gd–Bd–Cu–O TFMs exhibit superior in-field flux trapping performance, thanks to the significant second peak effect of the critical current density (Jc). The additional liquid source intensifies the Gd–Ba substitution, and therefore increases the in-field Jc, especially at the temperature range of 30–70 K, which is meaningful for practical applications.

Process Technology and Superconducting Properties of Bulk HTS With Multi-RE Elements

A series of REBa₂Cu₃O_y (RE123)/Ag bulk superconductors with multi-RE elements at the RE site, were prepared by top-seeded melt growth. By employing the differential thermal analysis, we carefully adjusted the peritectic decomposition temperature (<i>T</i>_p) by changing the ratio of the two RE123 compositions to meet the requirements of the cold-seeding process. The superconducting properties, <i>T</i>_c, <i>J</i>_c and the flux trapping performance were evaluated after achieving the growth of the single grains. The microstructure was also observed. A relatively low growth temperature range depressed the LRE-Ba substitution, which was effective to suppress the probable RE211 coarsening. By employing ZrO₂ chemical doping, the flux pinning property was further improved under both zero-field and in-field, and the <i>J</i>_c was enhanced without reducing <i>T</i>_c.

Doping Effect of Fe-Ni Alloy Particles on Flux Pinning in Gd-Ba-Cu-O Bulk Superconductor

Enhancement of the critical current density (<i>J</i>_C) and trapped flux density (<i>B</i>_T) of bulk high-temperature superconductor are essential for large-scale applications. Introducing artificial pinning centers into the superconducting matrix has been proved to be an effective way to enhance <i>J</i>_C and <i>B</i>_T. It has been suggested that the ferromagnetic particles doping may provide higher pinning potential. The doping effects of Fe-B soft magnetic particles for GdBa₂Cu₃O_{7 - σ} (Gd123) bulk were reported previously. In the present work, we reported the improvements in both <i>J</i>_C and <i>B</i>_T of Gd123 bulk doped by Fe-Ni alloy particles. To make sure a uniform distribution of the dopant, Fe-Ni alloy particles, with amount varied from 0 to 20 mol%, were initially added to prepare Gd123 precursor powders for the bulk processing. X-ray diffraction showed a tetragonal-orthorhombic transition of Gd123 crystal structure with increasing doping amount. The in-field <i>J</i>_C was greatly enhanced by the Fe-Ni particles doping and the optimal doping amount was found to be 0.5 mol%. The microstructure of the bulk samples were also investigated by scanning electron microscopy. The great improvement of trapped flux by Fe-Ni doping was considered to be benefit for practical applications.

Flux Trapping and Field Magnet Stability of Bulk Superconductors

The stability and durability of trapped flux is a focus in investigations of melt-growth (RE)Ba2Cu3O7−δ (RE=rare earth, e.g., Y, Gd, Sm) bulk high-temperature superconductors. In magnetic flux engineering applications, two important issues affect the expected performance of bulk superconductors: flux trapping and flux stability or durability. The introduction of effective pinning with homogeneous spatial distribution is inevitable. The materials process and durability studies have been pursued in the last decade with either a stationary or a momentary applied external field. In machine applications, the magnetic flux stability is influenced by external perturbations. We topically summarize our progress and current status in relation to past and ongoing work.

The Effect of Recrystallization of Fine Inclusions on the Enhancement of Flux Pinning in Textured Gd-Ba-Cu-O Bulk Superconductors

Well textured GdBa2Cu3O7-δ single domain bulks have been prepared successfully. Based on these samples, dilute impurities doping for the enhancement of flux pinning has been investigated. To achieve the fine inclusion particles, except ball milling process, a possible recrystallization mechanism is proposed. BaTiO3 powder was synthesized as dopant. BaO2 doping and mixed BaO2 and TiO2 doping were performed as references. The recrystallization mechanism was studied by means of the evolution of flux pinning behaviors along the growth direction of the successfully textured single domain bulks. BaTiO3 retains a semi-melting state in the harsh ambient of the melting process. The state was assumed like a colloidal droplet, which can be divided into more than one smaller fragment by the anisotropic growth of matrix. This hypothesis was studied in the sample, which was grown at a lower initial growth temperature. The evolution of flux pinning behaviors along the c-axis shows a remarkable increase of the critical current density under low field together with the decreased peak effect. The increased low-field pinning strength is partially attributed to the recrystallization of the BaTiO3 doping related fine precipitations.