Synthesis and pulsed laser deposition of lead-based cuprate superconducting materials.

摘要

The search for the synthesis, and the characterisation of new superconducting materials such as the Y-Ba-Cu-O, Tl-Ba-Ca-Cu-O, Bi-Sr-Ca-Cu-O, Pb-Sr-Y-Ca-Cu-O systems have initiated an almost unparalleled degree of activity during the past several years because of their various possible technological applications. Different existing techniques of synthesis have been applied or modified for the fabrication of these superconducting copper oxide materials (cuprate), with the motive of obtaining single phase and better superconducting properties. The bulk synthesis and reproducibility of lead-cuprates are not straightforward because of the formation of complex co-existing phases, the volatile nature of lead and the required oxygen adjustment in the structure to obtain good superconducting properties. Usually, different superconducting phases give rise to different superconducting transition temperatures which are also modified by the cationic ratios (Pb:Sr:(Y,Ca);Cu) of the constituent elements of lead-cuprates. The synthesis of various cationic ratios of lead-based materials have been performed through a two- step ceramic technique or in a single-step with some modifications in synthesis procedure and optimisation, and characterised using EDX, XRD, resistive and diamagnetic transition temperature measurement. The different cationic ratios of Pb:Sr:(Y,Ca):Cu such as Pb-2213, (Pb,Cu)-1212, Pb-1213, Pb-1313, and Pb-3324 have been proposed, and those are shown to exhibit superconducting properties at various temperatures up to 83K. The pulsed laser deposition technique (PLD) has been used for the synthesis and fabrication of cuprate superconducting thin layers since the discovery of the Y-Ba-Cu- O system. This technique enables very precise and stoichiometric evaporation from an irradiated target due to its pulsed nature. It also allows thin layers of complex materials to be produced. This thesis also reports on the growth and synthesis of single phase Pb2Sr2(Y0.5Ca0.5)Cu3O8 layers using PLD and the highest transition temperature achieved so far in this material by any technique. The synthesis of (Pb,Cu)-1212 phase layers on MgO substrates has also been demonstrated using PLD multi-layering of nonsuperconducting Sr2(Y0.5Ca0.5)Cu3O8, and PbO targets. Ex-situ annealing of these structures in air provided superconducting (Pb,Cu)-1212 phase layers. Ex-situ and in-situ thin film growth of the Pb-2213 phase on MgO substrate by PLD are also reported, whereby the growth parameters such as annealing temperature and duration are optimised. The superconducting properties of the thin films are found to be sensitive to the annealing period at certain temperatures in a nitrogen ambient. In-situ oxygen-adjustment and minimisation of lead loss with high substrate temperature are shown to be excessively difficult task. In-situ adjustment of the oxygen in nitrogen ambient after deposition has been demonstrated with maximum onset transition temperature of 79K and zero resistance at 63K. Layers prepared by both annealing routes are found to be highly c-axis oriented. Uniformity and large area deposition are inherently serious problems with PLD. A simple approach to understanding and alleviating these difficulties using a four sided substrate holding geometry is presented. A substrate holder placed inside, mid way, and along the forward motion of plume at certain inclinations enable the main problem of droplets, non-uniformity and large-area deposition without any scanning of laser beam, target or substrate rotation to be minimised. A slight variation in superconducting transition temperature in 7X7mm samples deposited at distances from 3 to 12 cm from the target is demonstrated after ex-situ annealing. The critical current density is an important parameter for the potential use of any superconducting material. This was found to vary with oxygen content in the thin Pb2Sr2(Y0.5Ca0.5)Cu3O8 layers. Different reduction periods provided variations in the critical currents. The critical current density in Pb2Sr2(Y0.5Ca0.5)Cu3O8 was found to be as high as 0.32xl06A/cm2 in zero field and 0.5x 104A/cm2 in 1Tesla applied field at 55K. These values are much higher than for the bulk where a critical current density value of typically 5.5A/cm2 in zero field and 0.6A/cm2 in 2 Tesla field at 65K was found. The bulk (Pb, Cu)-1212 compositions with some impurity phases showed its highest onset transition temperature of 82K and TC, zero of 76K without the requirement of high pressure oxygen anneal. The critical current density was found to be as high as 130A/cm2 at 50K in zero field. The bulk composition Pb3Sr3(Y0.5Ca0.5)2Cu4Oz has shown maximum TC, onset 81.5K, TC, zero and a critical current density as high as 75A/cm2 in zero field and 1.2A/cm2 in 1 Tesla at 60K.