A model for the fast ionic diffusion in alumina-doped LiI

Lithium Iodide shows enhanced ionic conductivity when doped with a powder of the insulator, alumina. We extend Landauer's effective medium model to see if the observations are consistent with a high conductivity layer forming on each non-conducting particle. The predictions are consistent with experiment provided one assumes the layer a few hundred Angstroms thick. At the outside, away from the particle, the enhancement of conductivity should fall off slowly, as in Debye-Huckel screening, whereas it is possible a new phase forms close to the insulator surface

TauFactor: An open-source application for calculating tortuosity factors from tomographic data

TauFactor is a MatLab application for efficiently calculating the tortuosity factor, as well as volume fractions, surface areas and triple phase boundary densities, from image based microstructural data. The tortuosity factor quantifies the apparent decrease in diffusive transport resulting from convolutions of the flow paths through porous media. TauFactor was originally developed to improve the understanding of electrode microstructures for batteries and fuel cells; however, the tortuosity factor has been of interest to a wide range of disciplines for over a century, including geoscience, biology and optics. It is still common practice to use correlations, such as that developed by Bruggeman, to approximate the tortuosity factor, but in recent years the increasing availability of 3D imaging techniques has spurred interest in calculating this quantity more directly. This tool provides a fast and accurate computational platform applicable to the big datasets (>10^8 voxels) typical of modern tomography, without requiring high computational power

Effects of Gallium Doping in Garnet-Type Li7La3Zr2O12 Solid Electrolytes

Garnet-type Li7La3Zr2O12 (LLZrO) is a candidate solid electrolyte material that is now being intensively optimized for application in commercially competitive solid state Li+ ion batteries. In this study we investigate, by force-field-based simulations, the effects of Ga3+ doping in LLZrO. We confirm the stabilizing effect of Ga3+ on the cubic phase. We also determine that Ga3+ addition does not lead to any appreciable structural distortion. Li site connectivity is not significantly deteriorated by the Ga3+ addition (>90% connectivity retained up to x = 0.30 in Li7–3xGaxLa3Zr2O12). Interestingly, two compositional regions are predicted for bulk Li+ ion conductivity in the cubic phase: (i) a decreasing trend for 0 ≤ x ≤ 0.10 and (ii) a relatively flat trend for 0.10 < x ≤ 0.30. This conductivity behavior is explained by combining analyses using percolation theory, van Hove space time correlation, the radial distribution function, and trajectory density

Double perovskite cathodes for proton-conducting ceramic fuel cells: are they triple mixed ionic electronic conductors?

Published by National Institute for Materials Science in partnership with Taylor & Francis. 18 O and 2 H diffusion has been investigated at a temperature of 300 °C in the double perovskite material PrBaCo 2 O 5+δ (PBCO) in flowing air containing 200 mbar of 2 H 2 16 O. Secondary ion mass spectrometry (SIMS) depth profiling of exchanged ceramics has shown PBCO still retains significant oxygen diffusivity (~1.3 × 10 −11 cm 2 s −1 ) at this temperature and that the presence of water ( 2 H 2 16 O), gives rise to an enhancement of the surface exchange rate over that in pure oxygen by a factor of ~3. The 2 H distribution, as inferred from the 2 H 2 16 O − SIMS signal, shows an apparent depth profile which could be interpreted as 2 H diffusion. However, examination of the 3-D distribution of the signal shows it to be nonhomogeneous and probably related to the presence of hydrated layers in the interior walls of pores and is not due to proton diffusion. This suggests that PBCO acts mainly as an oxygen ion mixed conductor when used in PCFC devices, although the presence of a small amount of protonic conductivity cannot be discounted in these materials

The interaction of molecular oxygen on LaO terminated surfaces of La2NiO4

Rare-earth metal oxides with perovskite-type crystal structures are under consideration for use as air electrode materials for intermediate to high temperature electrochemical device applications. The surface chemistry of these materials plays a critical role in determining the kinetics of oxygen reduction and exchange reactions. Among various perovskite-structured oxides, certain members of the Ruddlesden–Popper series, e.g. La2NiO4, have been identified as significantly active for surface oxygen interactions. However, the challenge remains to be the identification of the structure and composition of active surfaces, as well as the influence of these factors on the mechanisms of surface exchange reactions. In this contribution, the changes in the electronic structure and the energetics of oxygen interactions on the surfaces of La2NiO4 are analysed using first principles calculations in the Density Functional Theory (DFT) formalism. As for the surface chemistry, LaO termination rather than NiO2 termination is presumed due to recent experimental evidence of the surfaces of various perovskite structured oxides after heat treatment in oxidizing environments being transition metal free. Our findings substantiate the fact that the LaO-terminated surface can indeed participate in the formation of surface superoxo species. Detailed charge transfer analyses revealed that it is possible for such a surface to be catalytically active owing to the enhanced electronic configurations on the neighbouring La sites to surface species. In addition, positively charged oxygen vacancies, relative to the crystal lattice, can act as active sites and catalyse the O–O bond cleavage

Probing the bulk ionic conductivity by thin film hetero-epitaxial engineering

Highly textured thin films with small grain boundary regions can be used as model systems to directly measure the bulk conductivity of oxygen ion conducting oxides. Ionic conducting thin films and epitaxial heterostructures are also widely used to probe the effect of strain on the oxygen ion migration in oxide materials. For the purpose of these investigations a good lattice matching between the film and the substrate is required to promote the ordered film growth. Moreover, the substrate should be a good electrical insulator at high temperature to allow a reliable electrical characterization of the deposited film. Here we report the fabrication of an epitaxial heterostructure made with a double buffer layer of BaZrO3 and SrTiO3 grown on MgO substrates that fulfills both requirements. Based on such template platform, highly ordered (001) epitaxially oriented thin films of 15% Sm-doped CeO2 and 8 mol% Y2O3 stabilized ZrO2 are grown. Bulk conductivities as well as activation energies are measured for both materials, confirming the success of the approach. The reported insulating template platform promises potential application also for the electrical characterization of other novel electrolyte materials that still need a thorough understanding of their ionic conductivity. © 2015 National Institute for Materials Science

Prediction of the functional properties of ceramic materials from composition using artificial neural networks

We describe the development of artificial neural networks (ANN) for the prediction of the properties of ceramic materials. The ceramics studied here include polycrystalline, inorganic, non-metallic materials and are investigated on the basis of their dielectric and ionic properties. Dielectric materials are of interest in telecommunication applications, where they are used in tuning and filtering equipment. Ionic and mixed conductors are the subjects of a concerted effort in the search for new materials that can be incorporated into efficient, clean electrochemical devices of interest in energy production and greenhouse gas reduction applications. Multi-layer perceptron ANNs are trained using the back-propagation algorithm and utilise data obtained from the literature to learn composition–property relationships between the inputs and outputs of the system. The trained networks use compositional information to predict the relative permittivity and oxygen diffusion properties of ceramic materials. The results show that ANNs are able to produce accurate predictions of the properties of these ceramic materials, which can be used to develop materials suitable for use in telecommunication and energy production applications

Chemistry and structure of homoepitaxial SrTiO3 films and their influence on oxide-heterostructure interfaces

The properties of single-crystal SrTiO3 substrates and homoepitaxial SrTiO3 films grown by pulsed laser deposition have been compared, in order to understand the loss of interfacial conductivity when more than a critical thickness of nominally homoepitaxial SrTiO3 is inserted between a LaAlO3 film and a SrTiO3 substrate. In particular, the chemical composition and the structure of homoepitaxial SrTiO3 investigated by low-energy ion-scattering and surface X-ray diffraction show that for insulating heterointerfaces, a Sr-excess is present between the LaAlO3 and homoepitaxial SrTiO3. Furthermore, an increase in the out-of-plane lattice constant is observed in LaAlO3, indicating that the conductivity both with and without insertion of the SrTiO3 thin film originates from a Zener breakdown associated with the polar catastrophe. When more than a critical thickness of homoepitaxial SrTiO3 is inserted between LaAlO3 and SrTiO3, the electrons transferred by the electronic reconstruction are trapped by the formation of a Sr-rich secondary phase and Sr-vacancies. The migration of Sr towards the surface of homoepitaxial SrTiO3 and accompanying loss of interfacial conductivity can be delayed by reducing the Sr-content in the PLD target