Optimization of processing to improve critical current density of Ag/Bi-2223 tapes

Abstract

Several recent developments in powder-in-tube (PIT) processing are presented. A cryogenic deformation process has been developed, involving rolling or pressing the wires and tapes in supercold conditions, such as in liquid nitrogen. Cryogenic deformation has been found to improve the density, grain alignment and Ag-oxide core interface and to increase dislocation density, thereby enhancing and flux pinning. By incorporating Pb into Bi-2212 phase the sintering temperature can be raised above , resulting in a significant reduction of total sintering time from several hundred hours to 100 h. Recently, a new process to eliminate the decomposition and recovery of Bi-2223 during cooling and heating has been developed that further reduces the heat treatment time for Ag/Bi-2223 tapes to 20-30 h, with and Bi-2223 volume fraction in the tapes comparable with those in tapes treated for 120 h. A two-stage annealing procedure in the final thermal cycle has been used to eliminate residual amorphous phase and Bi-2201, which has been identified to be one of the major causes of weak links in PIT tapes. By incorporating hot deformation in a two-step process not only can Bi-2201 be eliminated, but also texture and density are improved, resulting in a very high at 77 K in multifilamentary tapes.

Investigation of a multi-set-point first heat treatment methodology for the silver-sheathed (Bi, Pb)2Sr2Ca2Cu3Ox composite conductor

“…Bi-2223 phase purity, grain alignment and connectivity are important for control/optimization of the current transport properties of fully processed Ag/Bi-2223 composite wire because the critical current is transported by both intraand inter-granular mechanisms [3][4][5][6][7][8][9]. The intermediate deformation/size reduction following HT-1 is performed to remove the porosity created in the course of the HT-1 phase conversion process.…”

Effect of post-annealing at extensively varied temperature on phase formation and superconductive properties in Bi-2223/Ag tapes

“…This technique ensures less mechanical deformation problems, such as sausaging and cracking, which severely restrict the J c . This advantage of oxide-layer coating may reveal a large potential to improve the J c , as these deficits are difficult to avoid in the widely used powder-in-tube (PIT) technique [17][18][19][20][21][22][23][24][25][26][27][28][29] due to its mechanically strong deformation characteristics. Two-step heat treatment has been developed for PIT-processed Bi-2223 tapes [25][26][27][28][29].…”

“…This advantage of oxide-layer coating may reveal a large potential to improve the J c , as these deficits are difficult to avoid in the widely used powder-in-tube (PIT) technique [17][18][19][20][21][22][23][24][25][26][27][28][29] due to its mechanically strong deformation characteristics. Two-step heat treatment has been developed for PIT-processed Bi-2223 tapes [25][26][27][28][29]. This process consists of two successive isotherms at different temperatures.…”

Quantitative phase analysis of PBSCCO 2223 precursor powders—an XRD/Rietveld refinement study

“…Long length 'green tapes' are produced by bundling these wires with ensuing hammerpressing, drawing and rolling of these bunches so that the ceramic powder is extended to filaments with ceramic cores of typically less than 20 µm thickness and a diameter of 60-200 µm. Conversion to the desired 2223 phase is obtained by a repeated heat treatment of these tapes, usually between 820 • C and 840 • C in atmospheres between 1% and 20% oxygen for various lengths of time [8][9][10][11]. This thermomechanical process must be controlled carefully to avoid sausaging and to improve grain connectivity.…”