Mechanical Properties of HTSC at Micro/Nanometric Scale

Abstract

The nanoindentation or indenter testing technique (ITT) is a functional and fast technique that can give us a lot of information about the mechanical properties of different materials at nanometric scale, from soft to brittle materials. The principle of the technique is the evaluation of the response of a material to an applied load. In a composite material, if the size of the residual imprint resulting from a certain load is lower than the size of the studied phase, then it is possible to determine its mechanical properties, and therefore its contribution to the global mechanical properties of the composite. Depending on the tipped indenter used, different equations should be applied to study the response of the material, and thus calculate stress-strain curves, and parameters such as hardness, Young's modulus, toughness, yield strength and shear stress. These equations are related to the different deformation mechanisms (elastic and elasto-to-plastic) that the material undergoes. <br/>In the case of most of the ceramic composites, when a spherical tipped nanoindenter is used, elastic deformation takes place, and Hertz equations can be used to calculate the yield strength, shear stress and the strain-stress curves. On the other hand, when a Berckovich indenter is used, plastic deformation takes place, then Oliver and Pahrr equations must be applied to evaluate the hardness, Young's modulus and toughness. Nevertheless, in the hardness study, Indentation Size Effect (ISE) must be considered.<br/>The mechanical properties of a ceramic superconductor material have been studied. YBa2Cu3O7-ä (YBCO or Y-123) samples textured by Bridgman and Top Seeding Melt Growth (TSMG) techniques have been obtained, and their mechanical properties studied by nanoindentation. This material presents a phase transition from tetragonal to orthorhombic that promotes a change in its electrical properties, from insulating to superconductor, and that can be achieved by partially oxygenating the material. On the other hand, the structure of the textured material is heterogeneous, and two different phases are present: Y-123 as a matrix and Y2BaCuO5 (Y-211) spherical or quasi-spherical inclusions. Moreover, the texture process induces an anisotropic structure, thus being the ab-planes the ones that transport the superconductor properties while the c-axis remains insulating. Nowadays, efforts are focused on the production of long length Superconductor Materials. Silver welding is a technique that allows us to join several TSMG samples thus trying to obtaining larger samples. Nevertheless, one important factor to be accounted for is the mechanical stability across the welding zone, which has been studied in this Thesis. <br/>Another attempt to produce long length superconductor materials is to shape thus as wires or plates. In this case, a metallic sheet, covered with different buffer layers is used as substrate on which YBCO layers can be deposited by PED for example. These structures have been also studied in the present Thesis.