PhD Thesis abstract
Institut des Sciences de la Matiere et du RAyonnement (ISMRA).
LERMAT and CRISMAT
Elaboration and thermomechanical characterization of sintered or textured "YBaCuO"-type superconducting ceramics.
Introduction
Since the discovery in 1987 of superconductivity above the liquid nitrogen temperature in YBa2Cu3O7-x, a lot of studies aimed to improve its superconducting properties. However, the main problems impeding its industrial application are its high brittleness and its poor thermomechanical properties (in particular its resistance to thermal shock in liquid nitrogen).
In this aim, bulk superconducting YBa2Cu3O7-x ceramics have been elaborated by conventional sintering and by the "melt-textured-growth" technique. The latter yields single-domain ceramics with a reduced number of poorly desorientated large grains (several millimeters or centimeters). These materials are well known for their good superconducting properties, such as their very high critical current density.
Their mechanical properties have been measured (Young's modulus, hardness, toughness) using conventional techniques and the Vickers indentation technique. The thermal shock resistance has been investigated using the stage of stable extension of Vickers indentation cracks submitted to thermal shock.
An improvement of the thermomechanical properties was searched by the realization of composite microstructures: porosity and secondary phases (a brittle Y2BaCuO5 phase or a ductile silver phase).
Results and conclusions
The measurement of toughness by the Vickers indentation technique has been calibrated with respect to conventional SENB technique. In the case of porous YBa2Cu3O7-x, anomalies in the elastic-plastic and fracture behaviors have been detected. They have been interpreted and modellized in terms of a splitting of the indentation "plasticity" between the plastic deformation of the grains and a densification of the porous body.
Otherwise, a new thermal shock resistance parameter, based on the stable propagation of Vickers indentation cracks submitted to thermal shock, has been proposed and calculated. It represents theoretically an extension of the fourth Hasselman's thermal shock resistance parameter R'''' for quenching temperature differences lower than the critical one ?Tc. This point has been experimentally verified on alumina. Moreover, it should be noted that the use of this technique allows an evaluation of the heat transfer coefficient in the quenching medium, h.
The thermomechanical properties of the sintered materials have been measured, and correlations between composite microstructures (porosity, secondary phases) and properties have been established.
The influence of porosity on the Young's modulus has been compared to theoretical models. These models have been extended to the case of toughness, departing from fracture energy considerations, and assuming a constancy of the fracture micromechanisms with porosity.
However, porosity induces a negligible effect on thermal shock resistance, the competition between toughness and thermal transient stress (mainly governed by the Young's modulus) being approximately the same over the whole range of porosity.
The addition of brittle inclusions Y2BaCuO5 to the sintered materials does not affect any of the investigated thermomechanical properties. This is due to the fact that Y2BaCuO5, because of its thermoelastic properties, does not introduce additional residual stresses in the microstructure compared to pure YBa2Cu3O7-x.
The introduction of a metallic silver phase yields improved toughness and damage tolerance (R-curve effect). The combination of these properties with a lower Young's modulus and a higher thermal conductivity (inducing lower transient thermal stresses) induces a significantly increased thermal shock resistance.
All the fracture and toughening micromechanisms have been studied and identified by Scanning Electron Microscopy: microcracking, crack branching and kinking in YBa2Cu3O7-x and YBa2Cu3O7-x/Y2BaCuO5 ceramics (inducing small R-curve effects), crack bridging by plastically deformed silver particles in YBa2Cu3O7-x/Ag cermets (inducing a strong R-curve effect).
Concerning the melt-textured samples, their main problem is a great thermal anisotropy combined with large grains (several mm or cm) containing Y2BaCuO5 inclusions, which provokes severe spontaneous microcracking. This is harmful for the current transport properties. This phenomenon has been modellized, and a theoretical calculation of a critical inclusion size under which cracking should be avoided has been performed. This result has been successfully compared to the observations.
The addition of Y2BaCuO5 slightly toughens the melt-textured materials by crack attraction and deviation, due to intense residual stresses around particles. The melt-textured ceramics exhibit a much higher thermal shock resistance than the sintered ones, with a decrease with increasing Y2BaCuO5 content in the investigated domain. This high thermal shock resistance is due to the multi-cracked microstructure, which leads to a very low thermal stress during quenching. The Y2BaCuO5 particles, however, act as "pinning" centres between the cracked layers. They increase the stiffness of the material, thus the thermal stress, and decreases the thermal shock resistance
As in the case of sintered ceramics, the addition of metallic silver induces an increase in thermal shock resistance, because of the improved toughness and thermal conductivity.
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