Interface engineered multifunctional oxide thin films with optimized properties
In our world today, energy has become one of the most valuable resources, in particular, renewable and clean energy sources. The research presented here represents an investigation into three separate areas of this topic. In thin film applications, the ordered structures as well as the inherent thinness of the films precludes the normal physics found in bulk materials. Characterizations of films of this type can provide information on molecular level charge transfer processes of the film layer materials since diffusive properties are minimal. With the control given by pulsed laser deposition methods, film and interface structure can be altered allowing for an examination of these effects on the materials properties. For the electrolyte and cathode materials, this equates to finding thermal and PO2 dependencies for electronic and ionic transport. For barium titanate, aside from the effects of oxygen vacancies, the interface quality between the electrodes and the ferroelectric material determines the effectiveness of energy transfer between these boundaries. That is, poor bonding characteristics or the formation of intermediate layers will introduce inconsistencies and (possibly) unwanted piezoelectric response properties of the material which could introduce parasitic dampening (resistance) of the mechanical vibrations of a piezoelectric transducer, altering its resonant characteristics.
The clean reaction products and potential for high power outputs provide a strong impetus into investigations of fuel cell structures to improve their functionality. With conventional applications being dominated by high temperature (>700 °C) cells utilizing YSZ as an electrolyte medium, much gain can be made in efficiency through the lowering of cell operation temperature. The first part of my research focuses on the growth and characterization of a novel multilayered electrolyte structure consisting of alternating layers of GCO and YSZ for use in a medium temperature (400--600 °C) application. Half-cell and whole-cell structures were grown using PLD methods and were characterized using XRD, SEM, TEM and EIS. This multilayered electrolyte structure was found to have a lower activation energy than that previously reported for bulk materials and was found to produce peak power at approximately 600 °C.
The second part of this research focused on the growth and characterization of half-cell structures utilizing the perovskite LBCO as the cathode surface and GCO as the electrolyte material. Characterization was performed with XRD, EIS and TEM. Temperature dependent EIS results indicate that found that the oxygen uptake at the interface is fast with an activation energy of approximately 0.46 eV. It was found that the surface oxygen adsorption process is complicated and possibly the product of several rate limiting processes including possibly dissociative adsorption. It was also found that when combined with gadolinia doped ceria as an electrolyte that the interface grain boundaries are influenced by the deposition oxygen partial pressures. The rapid oxygen uptake and low activation energy makes this material a possible candidate for further testing in SOFC applications.
The final part of the presented research focused on non-standard deposition conditions of BTO on various substrate materials, including flexible carbon fiber fabric, using PLD methods along with a characterization of these films to explore its application in energy harvesting applications. Perovskites are part of the Ruddlesden-Popper family, with the crystal structure A n+1Bn O3n+1, with n = infinity. Barium titanate (BTO) is a member of the perovskite family and has gained notice not only for its excellent dielectric properties but also as a promising, non-lead-containing, ferroelectric material. Dependency on oxygen partial pressure during film growth was found to have a profound effect on the remnant polarization response and ferroelectric response was observed in films grown at temperatures down to 200 °C. Piezoelectric measurements were made with a resulting measured electric field of 0.4kV/m and a calculated d31 component of 2.74 X 10-14 pC/N. The reasons for this very low (2 orders of magnitude lower than in literature) d31 component are found, by considering the relevant contributions, to be related to the properties of the chosen carbon fiber fabric. However, it was shown that low temperature deposition of barium titanate produced films with reasonable ferroelectric and piezoelectric properties allowing for possible use in some energy harvesting applications.