Doctor of Philosophy
Department of Materials Engineering
Vo, Nghia Van, Development of high-Tc superconducting coils and magnets through processing optimization and characterization of Bi(Pb/Ag and Ag-alloy composite tapes, Doctor of Philosophy thesis, Department of Materials Engineering, University of Wollongong, 1997. http://ro.uow.edu.au/theses/1490
The extensive research effort in the development of high-Tc superconducting (HTSC) class-II (Bi,Pb)2Sr2Ca2Cu3Ox+10(Bi(Pb)-2223) coils, and magnets, through processing optimization, and characterization of silver, and silver-alloy composite tapes, has been conducted at the Center for Superconducting and Electronic Materials (part of the Institute of Materials Technology and Manufacturing), Department of Materials Engineering, University of Wollongong.
Studies leading up to the production of Bi(Pb)-2223 class-II coils, and magnets include the following investigations in materials processing, and science of Bi(Pb)-2223 composite tapes:-
The processing of Bi(Pb)-2223 short, and long length composite tapes (from the standard procedure of PIT, to the conventional method of CTFF, and the novel format of WIT , or wire-in-tube), and the characterization on their physical, chemical, electrical, and magnetic properties by means of measurements - implemented techniques include; Vickers microhardness tests (HyT), particle size analysis (PSA), Xray diffraction (XRD), differential thermal and thermo-gravimetric analysis (DTA/TG), scanning electron microscopy with energy-dispersive spectrometry (SEM/EDS), electrical transport, ac-susceptibility, and the SQUID (superconducting quantum interference device) magnetometry.
The effect of powder packing density, using the conventional method of powder-intube (PIT) by various means, on the transport critical current density, Jc, mechanical properties, and high-Tc phase (2223) formation of Bi(Pb)-2223 single filamentary (SF) superconducting tapes - higher initial packing density of PIT monocore short tapes has been found to give higher Jc values, as well as higher rate of 2223 formation, while lower initial packing density samples gave better mechanical performance. Critical strains for bend tests showed a dependence on the core thickness, and silver/oxide volume ratio.
The effect of sintering periods on the microstructure, phase development, and electrical transport and magnetic properties of Bi(Pb)-2223 SF tapes - comparisons have been made between three batches of tapes with the same thermomechanical deformation, and sintering conditions, but with the starting precursor powders being different in their synthesis. It has been found that the volume density of the macroscopic pinning force in fields up to IT is greater for a particular sintering period, viz. ~ 60 h in duration. Resistivity measurements further showed that the pinning activation energy also reaches a higher value for the same sintering period chosen. Scanning electron microimages of the fractured surfaces indicated that better core morphology, and density can be achieved in SF tapes from using this particular sintering period. Prolong sintering has been found to lead to greater porosity, while short duration sintering inhibits grain growth, which is undesirable for not providing sufficient time for the healing of cracks, induced during the intermediate deformation between sintering periods. The results also showed that longer sintering periods (of duration >= 100 h) tend to give better grain growth (in terms of grain size), but greater grain misalignment. Prolong sintering however, has been found to facilitate the increase in porosity of the core due to random grain growth. Critical currents in tapes treated with long sintering periods also have been measured to be lower in comparison with the values obtained for samples treated with slightly shorter sintering periods. The tapes made using 'threeto- four-sinter-period' (each period ~ 60 h) showed superior electrical transport properties over the 'two-sinter-period' (each period >= 100 h) tapes, and revealed less microcracks in comparison with the 'five-to-six-sinter-period' (each period ~ 40 h) tapes. These attributes are the result of higher core density with better connected and aligned grains, owing to sufficient sintering time for each period of heat treatment.
The effect of intermediate rolling reduction on the transport property, microstructure, phase formation and fill factor of Bi(Pb)-2223 composite tapes - well controlled intermediate deformation between sintering stages of Bi(Pb)-2223 composite tapes is crucial in achieving high critical current density. This deformation whether by rolling or pressing, depends on factors such as the compositional phase, the degree of microcracks, sintering periods or duration, and sintering temperature. It has been investigated that there is indeed an optimal 'window' of reduction for intermediate rolling (at room temperature) between sintering stages. For a particular sintering temperature, this window has been determined to vary depending on the time of sintering and phase composition after a certain sintering period. It has been found that the amount of intermediate deformation controls the transformation rate of 2223, as well as determining the tape's final microstructure and fill factor.
Mechanical property, and filamentary configuration of Bi(Pb)-2223 composite tapes - mutifilamentary (MF) tapes prepared by both methods of oxide-powder-in-tube (OPIT), and continuous-tube-forming-filling (CTFF) technique with reproducible, and scalable transport critical current densities between 15 and 20 kA/cm2 (77 K, self field, 1μ V/cm) by rolling have been achieved. It has been found that the filament distribution (geometry of filaments), the fill factor (ratio of superconducting ceramic core to the total cross-section of the conductor), and the stacking factor (number of times the filaments are stacked) play a crucial roll in determining the overall mechanical and electrical performance of these tapes.
The determination of stability parameters for Bi(Pb)-2223 composite tapes - with high upper critical fields, HTSC have been recognized as candidate materials for coil and magnet applications. High field devices at one time or another, when operated close to their rated limits, are often faced with instability problems which normally are of electrical, magnetic or mechanical nature. The determination of stability parameters therefore are of interest to the conductor designer which assists in the processing aspect of long length production of wires and tapes. It has been found that Bi(Pb)-2223/Ag tapes are cryostabled, as well as flux-jump stabled. Processing parameters such as intermediate deformation and fill factor, have found to have a direct effect on the stability outcome of these tapes.
Thermal stability of Bi(Pb)-2223 coils at 77 K - it has been found that the degree of thermal stability of Bi(Pb)-2223/Ag pancake-shaped coils at 77 K can be determined by controlling the amount of matrix and superconducting materials during processing. The intermediate deformation step between sintering stages has been found to be crucial in optimizing the performance of the processed composite tapes as well as governing the thermal stability of the subsequently produced pancake-shaped coils. Results obtained from numerical analysis by the finite element method has shown that monolayer coils produced from Bi(Pb)-2223/Ag composite tapes are thermally very stable, with high values of fill factor. Increasing the number of co-wound tapes however, for reasons of achieving higher current carrying capacity, and improving in mechanical integrity of the coil or magnet system, has been found to require either a reduction in the fill factor or an increase in cooling rate for thermal stability to be sustained as would otherwise be achieved with the metallurgically same single tape.
Development of Bi(Pb)-2223 class-II coils and magnets - once the production of high quality long length Bi(Pb)-2223/Ag tapes have been achieved considerations such as how to fabricate them into practical applications without degrading the performance of the tape becomes the major issues. The main problems concerned are usually related to the winding procedures (e.g. wind-react (W&R) and react-wind (R&W)), the insulating material, the construction or stacking of coils if they are pancake-shaped, and the study of quench propagation parameters.
A novel W&R solenoidal coil (reaching ~ 973 ampere-turns) wound on an alumina ceramic tube generates a dc-field of ~ 19 m T at 77 K has been produced together with five pancake-shaped coils, each generates an average of ~ 5 m T at 77 K, destined for magnet construction with a possible combined calculated field of - 0.04 T at 77 K (with liquid nitrogen as a coolant). Critical currents of superconducting silver, and silver-alloy composite double pancake coils of Bi(Pb)-2223 have been measured at liquid nitrogen temperature. A critical current density of - 12.5 kA/cm2 has been measured for a coil fabricated from Bi(Pb)-2223/Ag composite tape of length ~ 2.2 m (~ 342 Ampere-turns) using the W I T method. For coils made from alloys of silver (e.g. ~ 0.02% magnesium), higher critical currents (~ 17 A ) have been achieved over those fabricated with tapes processed with pure silver matrix. Transport measurements of normalised Jc vs B in fields up to IT showed encouraging results for WIT , and CTFF composite tapes prepared from intermediate rolling. New methods of joining between double pancake-shaped coils have also been investigated, together with the determination of some quench propagation parameters. The implementation of the superconducting joint with strap has produced a laboratory test magnet constructed from two double pancake-shaped coils (total length of ~ 20 m , or ~ 552 Ampere-turns) giving a self dc-field of ~ 21 m T , at 77 K.