Beyond average crystal structures: understanding extended and local environments in proton-conducting Sc-substituted BaTiO3 perovskites
Doktorsavhandling, 2019

Proton conducting ceramics are very promising for applications concerned with energy sourcing with cleaner, safer, more abundant and cheaper alternatives to fossil fuels. These materials are still in development and advances in the field depend on a better understanding of the role of defects, their identification and location in the host framework, and the assessment of their short- and long-range dynamics and kinetics. With that aim, the work included in this thesis focussed on investigations of the effect of Sc substitution on the long- and short-range structure, oxygen vacancies and protons distribution, and their link to proton conductivity, in BaTiO3 materials. The system BaTi1–xScxO3–x/2 with x = ⅙, 20, 50 and 70 was studied with a combination of thermogravimetric, scattering, spectroscopic and computational methods.
 
Neutron powder diffraction (NPD) provided the first representations of hexagonal and cubic members of the solid solution BaTiO3-Sc2O3. They revealed the different ordering of oxygen vacancies, protons and transition metal ions in the two structural types as a function of the Sc concentration and justified the large improvement in proton conductivity from hexagonal to cubic structures, due to the localised nature of protonic defects in the former. The combination of thermogravimetric and NPD methods applied simultaneously to study the dehydration of cubic members of the series suggested that vacancy-vacancy interactions are attenuated by higher Sc levels where the size difference between oxygen vacancy and protonic defect is larger. The Reverse Monte Carlo method revealed the local ordering of Ti in cubic types, a local symmetry-breaking effect that has repercussions on the physical properties of these materials, causing anomalously small volume changes upon hydration in low-Sc phases. Computer simulations, and spectroscopic methods employing radiation (IR, Raman) and neutrons (Inelastic Neutron Scattering) provided further insight into the structural features and offered a detailed characterisation of the proton sites and their dynamics, suggesting that higher Sc levels are associated to weaker hydrogen bonding and to configurations more favourable for proton transport. 
 
The present work contributed further understanding of the factors influencing proton transport in highly defective perovskite-structured materials. It was found that high Sc concentrations in the cubic host lattice of BaTiO3 yield highly stable phases where transport of protonic defects is favoured by a crystal site of high symmetry and multiplicity. Alongside the study of the peculiarities of the BTS system, recommendations for candidate systems identification and doping strategy were provided.

proton conducting oxide

IR

thermogravimetry

perovskite

neutron scattering

Raman

Reverse Monte Carlo

ordering

BaTiO3

Lecture hall KB
Opponent: Dr. Kirsten Marie Ørnsbjerg Jensen, Department of Chemistry, University of Copenhagen

Författare

Nico Torino

Chalmers, Kemi och kemiteknik, Energi och material, Oorganisk miljökemi 2

Torino N., Sławiński W. A., Knee C. S.,Henry P. F., Eriksson S. G. - Reverse Monte Carlo modelling reveals the local ordering in hexagonal and cubic scandium-substituted BaTiO3 protonconducting perovskites

Perrichon A., Torino N., Jedvik Granhed E., Lin, Y.-C., Parker S. F, Jiménez-Ruiz M., Karlsson M., Henry P. F. - Proton sites in hexagonal and cubic Sc-doped BaTiO3 proton-conducting oxides

The third millennium of the Common Era has begun with a number of challenges for humankind which solutions often rely on our mastery of materials engineering. For instance, energy sourcing with cleaner, safer, more abundant and cheaper alternatives to fossil fuels has been on focus for the past five decades. Advances in the field of solid-state ionic conductors have been the result of those instances. This class of materials, crystalline mixed oxides, is known since the late years of the 19th century and their electrical properties related to the transport of ions since the 1940s. These efforts became the foundation of Solid Oxide Fuel Cell technology featuring oxide ion conductors. With the 1980s, research in the new field of solid-state protonics has demonstrated that ceramic fuel cell technology can benefit from materials development focussing on another charge carrier, the proton. To that scope, it is very important to understand where the proton sits in the material, how it moves, at the micrometre and nanometre scale, and what influences its movement.
 
In order to incorporate protons, these materials interact with water vapour absorbing water like a sponge would: they expand when hydrated and contract when dehydrated. The system object of this thesis, Sc-substituted BaTiO3, is alternative to the most known proton conducting materials and was investigated because it behaves differently. The expansion caused by water absorption is remarkably small in one member of the series of compounds, a characteristic that can be crucial for building durable fuel cells. For these studies, I mainly used a combination of light (X-rays) and particles (neutrons) in order to know where the atoms are (the structure) and what they do (their dynamics). Particularly interesting was the combination of neutrons experiments with a scale, so that atoms positions could be followed closely during dehydration.
 
It was found that local structural deviations are present and that the interactions between species at the local level deeply influence the global properties of these materials. Local structures became even more relevant and were studied in conjunction with the global structure. In doing so, this research contributed further understanding of the factors that influence proton transport, from the chemical composition to the immediate vicinities of an atom.

Ämneskategorier

Oorganisk kemi

Materialkemi

Den kondenserade materiens fysik

Drivkrafter

Hållbar utveckling

Styrkeområden

Energi

Materialvetenskap

Fundament

Grundläggande vetenskaper

ISBN

978-91-7905-181-5

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4648

Utgivare

Chalmers tekniska högskola

Lecture hall KB

Opponent: Dr. Kirsten Marie Ørnsbjerg Jensen, Department of Chemistry, University of Copenhagen

Mer information

Senast uppdaterat

2019-10-02