thin films ; interfaces ; first-principles molecular dynamics ; high temperature proton conductors; pulsed laser deposition ; strain
Kahle Leonid, Musaelian Albert, Marzari Nicola, Kozinsky Boris (2019), Unsupervised landmark analysis for jump detection in molecular dynamics simulations, in Physical Review Materials
, 3(5), 055404-055404.
Gilardi Elisa, Fluri Aline, Lippert Thomas, Pergolesi Daniele (2019), Real-time monitoring of stress evolution during thin film growth by in situ substrate curvature measurement, in Journal of Applied Physics
, 125(8), 082513-082513.
Kahle Leonid, Marcolongo Aris, Marzari Nicola (2018), Modeling lithium-ion solid-state electrolytes with a pinball model, in Physical Review Materials
, 2(6), 065405-065405.
Fluri Aline, Pergolesi Daniele, Wokaun Alexander, Lippert Thomas (2018), Stress generation and evolution in oxide heteroepitaxy, in Physical Review B
, 97(12), 125412-125412.
Fluri A., Schneider C.W., Pergolesi D. (2018), In situ stress measurements of metal oxide thin films, Elsevier, The Netherlands, 109-132.
Fluri Aline, Marcolongo Aris, Roddatis Vladimir, Wokaun Alexander, Pergolesi Daniele, Marzari Nicola, Lippert Thomas (2017), Enhanced Proton Conductivity in Y-Doped BaZrO 3 via Strain Engineering, in Advanced Science
, 4(12), 1700467-1700467.
Fluri Aline, Gilardi Elisa, Karlsson Maths, Roddatis Vladimir, Bettinelli Marco, Castelli Ivano E., Lippert Thomas, Pergolesi Daniele (2017), Anisotropic Proton and Oxygen Ion Conductivity in Epitaxial Ba 2 In 2 O 5 Thin Films, in The Journal of Physical Chemistry C
, 121(40), 21797-21805.
Proton-conducting oxides have been investigated for decades, especially for their application as electrolyte materials in solid-oxide fuel cells (SOFC). Current SOFC technology is based on oxygen-ion conducting electrolytes, such as Y2O3-stabilized ZrO2, and requires very high operating temperatures (above 800°C) to achieve suitable ionic conductivity. Such high temperatures require stringent and challenging material specifications that up to now have represented the key drawback for this technology. The use of proton-conducting electrolytes, as alternative to oxygen-ion conductors, shows great potential for a significant reduction of the SOFC operating temperature, due to the much smaller activation energies for charge transport. Among proton conducting oxides, Y-doped BaZrO3 (BZY) is a particularly interesting material due to its chemical stability in the fuel-cell operating environments and its good proton conductivity in the grains’ interior. However, difficult sintering and poor conducting properties for its grain-boundary regions have precluded so far the use of BZY in the form of sintered ceramic membranes. Only in 2009 a relatively large average-grain size was achieved for BZY sintered pellets, while in 2010 highly textured grain-boundary-free BZY thin films were grown, showing the largest proton conductivity ever reported for solid electrolytes. These recent results have revitalized the search for materials displaying fast protonic conduction pathways at low temperatures (below 500°C); in addition, very recent literature reports that the lattice strain of proton conductors seems to have a large influence on their conductivity. In particular, it has been observed that compressive strains significantly decrease the conductivity by increasing the activation energy for charge transfer, whereas the opposite effect, i.e. a lower activation energy, had been expected from theoretical considerations, with a tensile strain leading to enhanced conductivity in these lower-temperature range. In this project we will combine state-of-the-art experimental synthesis, thin-film growth and characterization, and first-principles calculations to determine the driving force and the microscopic origin for enhanced conductivities, and engineer optimal materials and microstructures for deployment in lower-temperature SOFCs. In particular, biaxially textured thin films of BZY will be fabricated by pulsed laser deposition, where different strain states of the films can be tuned by suitable insulating buffer layers with the same crystalline symmetry but different lattice parameter. Moreover, the growth of highly ordered epitaxial multi-layered heterostructures comprising a proton conducting and an insulating phase will allow investigating the potential contribution of the interfaces to the total conductivity. The conducting properties of the films will be measured and correlated with the microstructural characteristics and the residual strain state of the depositions. In close feedback with these experimental results, extensive simulations will be performed, to elucidate the mechanisms of proton migration in distorted lattices. These will involve the determination of reaction pathways, activation energies, and diffusion coefficients for the protons using a combination of static transition-state finding methods and first-principles Car-Parrinello molecular dynamics; these latter also able to take into account nuclear quantum effects via non-Markovian Langevin equations of motion. Last, an accelerated-dynamics framework will also be developed to rapidly screen diffusion of light nuclei into the solid matrix. The main target of this investigation is the discovery and identification of low-temperature superprotonic conductivity resulting from strain-engineering. The existence of such a superprotonic state has been strongly suggested as an extrapolation of reported experimental results, but has never been observed to date.