Characterisation of the sol–gel process in the superconducting NdBa2Cu3O7−y system

Abstract

The objective of this paper is to obtain a better insight into the sol–gel mechanism of water-based precursors for the development of thin NdBa₂Cu₃O_7−y (NBCO) superconducting films. The influence of metal complexation behaviour on the formation of transparent and homogenous gels after the combination of different metal salts and ligands has been studied for several metal salts (Cu²⁺, Nd³⁺ and Ba²⁺). Two inorganic aqueous sol–gel precursors have been studied: a metal nitrate–citric acid-based and a metal acetate–triethanolamine-based solution. The characteristics of the precursor solution are based on the determination of the stability constants by the computer program Superquad. The prediction of the complex stability in this solution was related to the complexation of the three metal ions (Cu²⁺, Nd³⁺ and Ba²⁺) with a certain ligand. IR-spectroscopy was used for the determination of the gel. This resulted in a better understanding of the composition of the solution and could be used for preparation of more stable sol–gel precursors for the synthesis of homogeneous end products. These sol–gel systems were used for the deposition of highly textured superconducting thin films on SrTiO₃ substrates by dip-coating. Using detailed thermal analysis, it is shown that the morphology of the films can be optimised by adjusting the parameters during thermal treatment, resulting in dense and highly textured thin films. Special attention is given to the microstructure of the thin film because of its relevance to the superconducting transport properties of the coated conductor system.