Simone Sanna

High proton conduction in grain-boundary-free yttrium-doped barium zirconate films grown by pulsed laser deposition

Reducing the operating temperature in the 500-750 °C range is needed for widespread use of solid oxide fuel cells (SOFCs). Proton-conducting oxides are gaining wide interest as electrolyte materials for this aim. We report the fabrication of BaZr(0.8)Y(0.2)O(3-δ) (BZY) proton-conducting electrolyte thin films by pulsed laser deposition on different single-crystalline substrates. Highly textured, epitaxially oriented BZY films were obtained on (100)-oriented MgO substrates, showing the largest proton conductivity ever reported for BZY samples, being 0.11 S cm(-1) at 500 °C. The excellent crystalline quality of BZY films allowed for the first time the experimental measurement of the large BZY bulk conductivity above 300 °C, expected in the absence of blocking grain boundaries. The measured proton conductivity is also significantly larger than the conductivity values of oxygen-ion conductors in the same temperature range, opening new potential for the development of miniaturized SOFCs for portable power supply.

Enhancement of the chemical stability in confined [delta]-Bi2O3

Bismuth-oxide-based materials are the building blocks for modern ferroelectrics, multiferroics, gas sensors, light photocatalysts and fuel cells. Although the cubic fluorite δ-phase of bismuth oxide (δ-Bi2O3) exhibits the highest conductivity of known solid-state oxygen ion conductors, its instability prevents use at low temperature. Here we demonstrate the possibility of stabilizing δ-Bi2O3 using highly coherent interfaces of alternating layers of Er2O3-stabilized δ-Bi2O3 and Gd2O3-doped CeO2. Remarkably, an exceptionally high chemical stability in reducing conditions and redox cycles at high temperature, usually unattainable for Bi2O3-based materials, is achieved. Even more interestingly, at low oxygen partial pressure the layered material shows anomalous high conductivity, equal or superior to pure δ-Bi2O3 in air. This suggests a strategy to design and stabilize new materials that are comprised of intrinsically unstable but high-performing component materials.

Fast mass interdiffusion in ceria/alumina composite

Gadolinium-doped ceria (CGO) presents unique processes at low oxygen partial pressure (pO2 800 °C) such as faster mass diffusion, which are not observed in conventional sintering under ambient air conditions. In CGO/Al2O3 composites the resulting effects driven by such mass diffusion are low viscosity flows and high reactivity between phases, indicated by the formation of CeAlO3. This reaction is promoted by the high content of oxygen defects and the chemical reduction of Ce4+ cations to Ce3+ in CGO/Al2O3 composites under low temperature and low pO2. In this work, a comparison is made between sintering CGO/Al2O3 under ambient air conditions and under low pO2, focusing on densification, viscosity and the evolution of the microstructure.

Effects of surface finish and mechanical training on Ni-Ti sheets for elastocaloric cooling

Elastocaloric cooling has emerged as a promising alternative to vapor compression in recent years. Although the technology has the potential to be more efficient than current technologies, there are many technical challenges that must be overcome to realize devices with high performance and acceptable durability. We study the effects of surface finish and training techniques on dog bone shaped polycrystalline samples of NiTi. The fatigue life of several samples with four different surface finishes was measured and it was shown that a smooth surface, especially at the edges, greatly improved fatigue life. The effects of training both on the structure of the materials and the thermal response to an applied strain was studied. The load profile for the first few cycles was shown to change the thermal response to strain, the structure of the material at failure while the final structure of the material was weakly influenced by the surface finish.

High ionic conductivity in confined bismuth oxide-based heterostructures

Bismuth trioxide in the cubic fluorite phase ( δ - Bi 2 O 3 ) exhibits the highest oxygen ionic conductivity. In this study, we were able to stabilize the pure δ - Bi 2 O 3 at low temperature with no addition of stabilizer but only by engineering the interface, using highly coherent heterostructures made of alternative layers of δ - Bi 2 O 3 and Yttria Stabilized Zirconia (YSZ), deposited by pulsed laser deposition. The resulting [ δ - Bi 2 O 3 / YSZ ] heterostructures are found to be stable over a wide temperature range (500-750 °C) and exhibits stable high ionic conductivity over a long time comparable to the value of the pure δ - Bi 2 O 3 , which is approximately two orders of magnitude higher than the conductivity of YSZ bulk.

Releasing cation diffusion in self-limited nanocrystalline defective ceria thin films

Acceptor-doped nanocrystalline cerium oxide thin films are mechanically constrained nano-domains, with film/substrate interfacial strain and chemical doping deadlock mass diffusion. In contrast, in this paper we show that chemical elements result in highly unstable thin films under chemical reduction, with unexpected diffusion-driven effects such as fast migration of grain boundaries, porosity nucleation, and interdiffusion at low temperatures.

Ceria-Based Thin Film Hetero-structure Growth and Characterization for SOFC Applications

Electrolyte thin films of Ce0 8Gd0 2O 1 9-I´; (GDC20) were deposited by pulsed laser deposition (PLD) for the fabrication of micro solid oxide fuel cells (SOFCs). The crystal structure and morphology of the films deposited on different substrates were investigated as a function of the growth parameters, using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) analysis. The film conductivity was measured in parallel and perpendicular directions of the film substrate using electrochemical impedance spectroscopy (EIS). Pt or La 0 8Sr0 2Co0 8Fe0 2O 3-I´ (LSCF) films were used as electrodes in symmetrical cells. For parallel conductivity measurements, two electrodes were deposited by PLD on the surface of the film, while for perpendicular conductivity, thin film hetero-structure were grown sandwiching the electrolyte film between the two electrodes. ©The Electrochemical Society.

Effects of accelerated degradation on metal supported thin film-based solid oxide fuel cells

A thin film-based solid oxide fuel cell is deposited on a Ni-based metal porous support by pulsed laser deposition with a multi-scale-graded microstructure design. The fuel cell, around 1 μm in thickness, is composed of a stabilized-zirconia/doped-ceria bi-layered dense electrolyte and nanostructured Ni-stabilized zirconia and La0.6Sr0.4CoO3 electrodes as the anode and cathode, respectively. The cell is tested at intermediate temperatures (600–650 °C) with the aim to discern the degradation mechanisms occurring in the cell under accelerated conditions. Under open circuit conditions, electrochemical performances are steady, indicating the stability of the cell. Under electrical load, a progressive degradation is activated. Post-test analysis reveals both mechanical and chemical degradation of the cell. Cracks and delamination of the thin films promote a significant nickel diffusion and new phase formation. Signs of elemental distribution at low temperature are detected throughout the cell, indicating a combination of low energy surface elemental interdiffusion and electromigration effects.