Creixement de capes superconductores tipus nanocompòsit assistit per líquids transitoris

Resumo

High temperature superconducting materials have unique properties which have been under investigation for many years, mainly involved with their zero resistance properties at high temperatures or at high magnetic fields. Currently, one of the main interest in the superconducting community is to demonstrate the applicability of these materials, in order to achieve the widespread use of their applications. As such, there is a big international effort on optimizing performances and developing growth methodologies compatible with big-scale production at low cost. In this context, the results presented in this thesis are an important step forward, reporting for the first time the possibility to use the increased superconducting properties of nanocomposite technology together with a low-cost and high throughput liquid-based methodology based on chemical solution deposition. The growth of YBa2Cu3O(7-x) } is performed by the newly reported method of transient liquid assisted growth (TLAG), which combines the inexpensive chemical solution deposition with the presence of a transient liquid that provides ultra-high growth rates. We have been successful in combining this liquid-based growth with the presence of nanoparticles through the understanding of nucleation, microstructure and defect landscape of our films. We have chosen BaZrO3, BaHfO3 and LaF3 pre-formed nanoparticles stabilized in alcoholic media for these studies. The results are divided by the different processing routes, presenting the efforts on optimizing the nucleation, growth and superconducting properties of nanocomposites in two chapters. The two different paths consist of the temperature route (heating at constant PO2), and PO2-route (heating at very low PO2 and then increasing PO2 to reach growth conditions). In the T-route, several parameters were optimized in order to achieve epitaxial nanocomposite films, such as heating ramp and the thickness of a PLD-YBCO buffer layer. Also different liquid stoichiometries were tested, revealing the importance of supersaturation control to achieve epitaxy. Jc is 1MA/cm2, and we demonstrated that introducing pre-formed nanoparticles to TLAG-YBCO creates a defect structure with a lot of potential towards improved vortex pinning. The growth of YBCO nanocomposites through the PO2-route with BaZrO3 and BaHfO3 molar percentages ranging from 6% to 32% was studied through XRD techniques which allow the quantification of different YBCO crystalline orientations. Introduction of a seed layer accomplished a better reproducibility and fully c-axis oriented epitaxial films for 6% and 12% nanocomposites. We demonstrated Jc self-field up to 2.2MA/cm2 at 77K, a very promising result, which led to the evaluation of Jc under applied magnetic fields through dc-SQUID and electrical transport measurements. Thus, we could show the increased performance of nanocomposites with magnetic field in comparison with pristine samples, necessary for high magnetic field applications of coated conductors. This dissertation also includes a preliminary study on the growth of YBCO thick films (1um) and buffer layer compatibility with TLAG. We demonstrated the successful elimination of barium carbonate in films up to 1um thick and fully epitaxial YBCO layers could be processed by the PO2-route. CSD methodologies were used to grow thin films of Ba2342, Nd2CuO4, LaMnO3 and La0.8Sr0.2MnO3, in order to evaluate the reactivity of the transient liquid with these materials as well as the nucleation of YBCO on top. LaMnO_3 (LMO) was found to be a very promising material and was further investigated by using PLD-grown LMO and SuNAM commercial tape with LMO as the last layer of their architecture. We achieved epitaxial YBCO films with good Tc on top of these buffers layers, demonstrating that TLAG is compatible with commercial coated conductors architecture.