Einfluss fehlpassungsinduzierter Spannungsfelder auf den Transportprozess bei ionischer Leitfähigkeit

Abstract

Over the last two decades a new route for tailor made functional ceramics has been found in elastic strain engineering. The aim of this thesis was the investigation of the effect of mismatch induced interface strain on the oxygen ion conductivity in yttria stabilized zirconia YSZ. Therefore an analytic model is proposed to describe the elastic strain and stress in columnar thin films. Hereby it is possible to describe the ionic conductivity and the mean lattice plane distance from XRD strain measurements as a function of the layer thickness, and therefore, an estimate of the width of the strained interface region can be given. As a modell system (111)-oriented multilayers of YSZ and a rare earth sesquioxide SE$_{2}$O$_{3}$ (SE =Dy, Y, Er und Sc) have been prepared on (0001) Al$_{2}$O$_{3}$-substrates by pulsed laser deposition. By using different SE$_{2}$O$_{3}$ the interface strain and by changing the deposition parameters the texture of the films can be varied. By measuring the mean lattice distance as a function of the layer thickness using X-Ray diffraction the interface strain release can be nicely visualized. It was able to show that the width of the strained interface region strongly depends on the texture of the films. Values between 3 nm and 10 nm have been determined. The width of the interface increases with increasing degree of texture. The ionic conductivity was determined using electrochemical impedance spectroscopy. However, it needs to be pointed out that the ionic conductivity is both sensitive to the interface strain as well as texture effects and cannot be unequivocally deconvoluted. Samples with only one azimuthal variant showed an unexpected anisotropic conductivity. Samples with a fiber texture did not show this due to a geometric averaging. These highly textured samples exhibited the highest increase in ionic conductivity of up to 400 % when moving down to a film thickness of 100 nm, however the ionic conductivity did not increase monotonousely upon a further decrease of the film thickness. Films with a smaller degree of texture showed the expected monotone change of the ionic conductivity with decreasing film thickness. The width of the strained interface region from these measurements is in accordance with results from the XRD strain measurements. The development of the modell in combination with the texture dependent measurements build a foundation for a better understanding of interfacial strain and its impact on the physicochemical properties of functional ceramics. It was shown that besides the material’s properties the preparation conditions play a crucial role for the strain relaxation and the ionic conductivity, namely inform of the texture and the mean diameter of the columnar crystallites l