Strain effects on ion conduction in heterogeneous oxide electrolytes

Shen, Weida
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University of Delaware
The high operating temperature (> 800 °C) of solid oxide fuel cells (SOFCs) is required to a large degree by insufficient low temperature oxygen ion conductivity of the electrolyte materials such as Y2 O 3 -stabilized ZrO2 (YSZ). This extremely high operating temperature decreases cell durability and increases the component cost of SOFCs. Finding means to increase the conductivity of the electrolytes and thereby decrease the operating temperature to below 500 °C plays a decisive role in wider commercialization of this clean power technology. Thin film processing methods offer a number of means to investigate and engineer ion conduction in solid electrolytes. Numerous recent reports have been focused on the study of nanostructured multilayers or heterostructures for solid electrolytes. The driving force behind this trend is the notion that the interfaces between two oxides may have significantly altered--ideally improved--ionic transport behavior in fluorite-structured materials. In this dissertation, we take advantage of a recent developed film deposition method to systematically study the interfacial effects on oxygen ion conduction in ZrO 2 -and CeO2 -based materials. This fabrication technique allows us to create mesoscale compositional heterogeneity by using films with composition Ce1-x-z Zrx DzO2-z/2 (D = Y, Gd, or La) where x or z can vary through the thickness of the film at the single nanometer level. Interfacial lattice mismatch strain has been controversially suggested as a means to alter ionic conductivity in solid ion conductors. The first type of multilayered films prepared in this dissertation was composed of Ce 1-x Zrx O2 (CZO) with Y2 O3 doped CeO2 (YDC) or with YSZ. The aim of this study was to systematically quantify the effects of biaxial compressive or tensile strains on oxygen ion conductivity in doped CeO2 and doped ZrO2 materials. Since the lattice parameter of CZO is highly dependent on the Ce/Zr atomic ratio, its use enables precise control of the strain magnitude in neighboring lattice planes of YDC or YSZ. The compressive strain in the YDC layers caused fairly drastic reductions in the ionic conductivity. Each 1% increase in compressive strain of the YDC yielded a 1.6-fold reduction in interfacial conductivity at 650 °C and 3-fold reduction at 450 °C. On the other hand, when decreasing the individual layer thickness form 35 nm to 5 nm, all of the YSZ/CZO multilayers with lattice mismatch ranged between +2.9% and 5.2% exhibited little change of the conductivity, with values consistently near that of bulk YSZ. X-ray diffraction results indicated that the interfacial tensile strains in YSZ layers were largely relaxed. The interaction between dopants and oxygen vacancies is one of the major causes for the decreased conductivity at higher dopant concentration in ZrO 2 - and CeO2 -based ion conductors. The second type of multilayers prepared in this dissertation was composed of alternating highly doped and undoped CeO2 layers. At the limit, the modulated films consisted of very thin layers of pure dopant oxide (e.g., Y2 O3 , Gd2 O3 or La2 O3 ) within thicker layers of pure CeO2 . The three dopant oxides examined, Y2 O3 , Gd2 O3 and La2 O 3 , adopt the cubic bixbyite crystal structure with "pseudo-fluorite" lattice parameters that are, relative to CeO2 , smaller (Y 2 O3 ), larger (La2 O3 ), and nearly equal (Gd2 O3 ). Due to the dopant concentration gradient, vacancies created in the layers with high concentration were expected to diffuse into the pure CeO2 layers with low concentration, forming space charge regions at the interfaces. This study aimed to investigate the effect of vacancy trapping on the overall oxygen ion conduction in heterogeneously doped CeO 2 films, since the oxygen vacancies which trapped in the space charge regions located in the nominally pure CeO2 layers were expected to be highly mobile. An electrostatic Gouy-Chapman model was implemented to give the distribution of the oxygen vacancies in the space charge regions of pure CeO2 layers. Conduction was found to occur predominantly by the vacancies trapped in the CeO2 space charge regions in films composed of pure dopant oxide and pure CeO2 layers. Moreover, the total conductivity of these films increased with increasing lattice parameter of the dopant oxides. The Gouy-Chapman model was not sufficient to explain this behavior, since all of the dopants theoretically contributed identical interfacial oxygen vacancy concentrations. Therefore, an extended Gouy-Chapman model was proposed that considered the effects of interfacial strain on the concentration and mobility of vacancies from dopant oxides in the space charge regions.