Rotraut Merkle

How Is Oxygen Incorporated into Oxides? A Comprehensive Kinetic Study of a Simple Solid‐State Reaction with SrTiO3 as a Model Material

The kinetics of stoichiometry change of an oxide--a prototype of a simple solid-state reaction and a process of substantial technological relevance--is studied and analyzed in great detail. Oxygen incorporation into strontium titanate was chosen as a model process. The complete reaction can be phenomenologically and mechanistically understood beginning with the surface reaction and ending with the transport in the perovskite. Key elements are a detailed knowledge of the defect chemistry of the perovskite as well as the application of a variety of experimental and theoretical tools, many of them evolving from this study. The importance of the reaction and transport steps for (electro)chemical applications is emphasized.

Electron and Ion Transport In Li2O2

Bulk Li2O2 is shown to exhibit ionic conductivity via lithium vacancies and electronic conductivity via electron holes (localized as superoxide ions). This is the first systematic study on the charge carrier chemistry of peroxides with high relevance for the performance kinetics of Li-oxygen batteries.

Oxygen tracer diffusion in dense Ba0.5Sr0.5Co0.8Fe0.2O3−δ films

Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) is an interesting material for high temperature oxygen permeation membranes and solid oxide fuel cell cathodes, applications for which oxygen transport properties are crucial. Here oxygen isotope exchange is performed on dense BSCF films prepared by pulsed laser deposition on MgO single crystal substrates. The oxygen isotope profile is analyzed by secondary ion mass spectrometry to extract tracer diffusion coefficients D∗ and effective surface exchange constants k∗. Tracer diffusion has a low activation energy of 0.5 eV. At moderate temperatures, the related oxygen vacancy diffusion coefficient DVO is significantly larger than that for (La,Sr)(Fe,Co)O3−δ perovskites.

Oxygen incorporation into Fe-doped SrTiO3: Mechanistic interpretation of the surface reaction

The surface part of the oxygen incorporation reaction into the model material Fe-doped SrTiO3 is investigated by in situ optical spectroscopy. Experiments close to and far from equilibrium (small and large pO2 changes) and the effect of UV irradiation on the reaction rate allow us to draw the following conclusions with respect to the kinetic processes at the surface: (i) molecular oxygen species participate in or before the rate determining step (RDS), and (ii) a single conduction band electron participates in or before the RDS of the incorporation reaction even though the bulk is p-type conducting. Probable reaction mechanisms comprising chemisorption, charge transfer, dissociation and oxygen ion transfer steps are discussed.

Defect association in acceptor-doped SrTiO3: case study for Fe′TiV˙˙O and Mn″TiV˙˙O

The association of acceptor cations (Fe′Ti and Mn″Ti) with oxygen vacancies in Fe- and Mn-doped SrTiO3 single crystals is investigated using in-situ EPR spectroscopy. Effective association enthalpies ΔassH0eff and entropies ΔassS0eff are determined, and ΔassH0eff is found to depend strongly on the dopant concentration. This dependence can be roughly described by a correction term in the chemical potential that is proportional to the cube root of the defect concentration. Extrapolation to infinite dilution yields an association enthalpy of −26 kJ mol−1. The association of oxygen vacancies can significantly reduce the ionic conductivity of these materials at temperatures up to ca. 200 °C (Fe′Ti) or even higher (Mn″Ti), and must therefore be taken into account in the respective defect chemical models at moderate or low temperatures.

SrTiO3: a model electroceramic

Abstract It is shown that the defect chemistry of acceptor-doped SrTiO3 is understood in great detail, and that this permits the accurate description, not only of the equilibrium electronic and ionic conductivities, but also of non-equilibrium situations generated by electrical or chemical driving forces, and even of the quenched state. Recent investigations on internal and external boundaries provide astonishingly precise insights into the pertinent structure–property relations.

Space charge influenced oxygen incorporation in oxides: in how far does it contribute to the drift of Taguchi sensors?

Abstract The change of resistance of SnO2 upon the change of oxygen partial pressure—a well known sensor effect—is studied theoretically. A linearised drift-diffusion model is used. The boundary conditions are formulated in terms of reaction rate equations. Resistance vs. time curves as well as the evolution of the concentration profiles of electrons and oxygen vacancies are numerically calculated and discussed. Analytical approximations are derived for the corresponding time constants. Relevance of the model for commercial sensors is discussed.

Water uptake of nanocrystalline ceria: weight and conductance effects

Abstract The nature of water adsorption on the surface of nanocrystalline ceria powder (particle size of ∼12 nm) was investigated by means of thermogravimetric techniques as a function of temperature, water partial pressure, doping content and synthesis route. A significant amount of water was found to be adsorbed on the surface of nanocrystalline powder even under nominally dry atmosphere in a broad range of temperatures (300–600 °C), irrespective of doping. The nanocrystalline ceria sinter bodies (grain size of ∼30 nm with ∼90% density) also exhibited water adsorption which can be attributed to open pores rather than to grain boundaries. The impedance measurements clearly indicated, however, that the contribution of the adsorbed water to the measured conductivity of nanocrystalline ceria ceramics is negligible under the condition of the measurements even at high water vapour partial pressures.

CO2 Selective Potentiometric Sensor in Thick-film Technology.

A potentiometric sensor device based on screen-printed Nasicon films was investigated. In order to transfer the promising sensor concept of an open sodium titanate reference to thick film technology, “sodium-rich” and “sodium-poor” formulations were compared. While the “sodium-rich” composition was found to react with the ion conducting Nasicon during thermal treatment, the “sodium-poor” reference mixture was identified as an appropriate reference composition. Screen-printed sensor devices were prepared and tested with respect to CO2 response, reproducibility, and cross-interference of oxygen. Excellent agreement with the theory was observed. With the integration of a screen-printed heater, sensor elements were operated actively heated in a cold gas stream.

Enhanced ionic conductivity in polycrystalline TiO2 by ‘‘one-dimensional doping’’

The influence of line defects (dislocations) on the electrical properties of polycrystalline TiO2 was investigated. Line defects were created in TiO2 during spark plasma sintering at 1000 °C and 400 MPa. TEM characterisation indicates dislocations to be preferably oriented on {110} and {101} planes. The measured electrical conductivity as a function of oxygen partial pressure and temperature revealed that the dislocations play a vital role in modifying the defect chemistry of TiO2. The presence of dislocations enhanced the ionic conductivity over a wide range of oxygen partial pressures. The observed changes can be interpreted in terms of negatively charged dislocation cores and adjacent space charge accumulation layers. The present findings point towards an alternative method to tune the electrical properties of ionic solids.