Browsing by Author "Göbel, Marcus"

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Boundary effects on the electrical conductivity of cerium oxide thin films
(2013) Göbel, Marcus; Maier, Joachim (Prof. Dr.)
Boundary effects in mixed and ionic conductors and in particular space charge layer (SCL) effects are a major field of research in solid state ionics since they are known to strongly affect the electrical transport properties of the materials. In particular, in this study the boundary effects of cerium oxide were investigated which due to its high ionic conductivity is a material of large relevance for a wide range of applications such as solid oxide fuel cells (SOFCs), oxygen membranes and catalysis. Here the thin film geometry was applied which offers the advantage of a usually well defined microstructure allowing to access effects at both the film-substrate interface (FSI) and the grain boundaries (GBs). The ceria thin films were grown with pulsed laser deposition (PLD) on various substrates. Thin films were prepared of (1) different doping contents (nominally pure, acceptor doped and donor doped), (2) different microstructures (epitaxial and nanocrystalline) and (3) different thicknesses (between 20 and 450 nm). Their microstructure was characterized with XRD, SEM, TEM and electron diffraction. The defect chemistry and conductivity properties of the samples were investigated with impedance spectroscopy. Additionally, a software to numerically compute the SCL profiles and conductivity effects in ceria was developed. The numerical approach allowed for the determination of all relevant SCL profile characteristics and for the correlation of them with both the material properties and the resulting conductivity effects. Notably, the numerical approach was observed to yield precise results without the use of further assumptions also for asymmetric and mixed cases in contrast to the well known analytical solutions. This allowed for a test of the assumptions made in the analytical solutions which resulted in the development of improved analytical relationships. Remarkably, the improved analytical approach which is not restricted to ceria but generally applicable was observed to yield very reliable outcomes even for complex situations. In most cases the boundary effects were found to dominate the conductivity of the investigated ceria thin films. Effects at both the film-substrate interface and at the grain boundaries were observed. As expected in the framework of the SCL theory no significant FSI effect was detected in strongly acceptor doped films. In epitaxial, nominally pure ceria films grown on Al2O3 <0001> the conductivity was observed to be reduced at the FSI in agreement with the SCL theory. In nanocrystalline, acceptor doped samples grown on SiO2 <0001> a significant decrease of the conductivity at the GBs was observed in accordance with a SCL potential of 0.32 ± 0.05 V. For this set of samples a thickness dependence of the grain size was detected resulting in a considerable change of the conductivity with film thickness. Therefore, it could be demonstrated that a thickness dependent conductivity in polycrystalline samples can not always be assigned to FSI effects; a finding of relevance for a number of similar studies on interface effects. Nanocrystalline, acceptor doped films grown on Al2O3 <1-102> and MgO <100> substrates were found to exhibit a less significant decrease of the ionic conductivity and, hence, a smaller SCL potential of 0.19± 0.05 V. This is most likely the result of the smaller lattice mismatch between substrate and CeO2 film resulting in less pronounced GB core charges. Thin films prepared at room temperature, characterized by a nanocrystalline microstructure with very small grains, were measured to exhibit very pronounced SCL effects (decrease of the ionic conductivity by 3 orders of magnitude). Most likely here the Mott-Schottky assumption is fulfilled much better resulting in a particularly strong depletion of the oxygen vacancies. Remarkably, for these samples the pO2 of the electrolytic domain boundary, was found to be shifted by 29 orders of magnitude compared with what is expected from an extrapolation of the literature bulk data. Two superimposing effects were found to be the origin of this drastic shift: In addition to (1) the pronounced SCL effects, (2) already in the bulk the electronic conductivity was measured to be strongly increased. Both effects also affected the activation energies. Remarkably, in this case the electronic activation energy fell even below the ionic value resulting in an increase of the electronic conductivity contribution for decreasing temperatures in marked contrast to the properties typically observed in ceria. Notably, in donor doped ceria the electronic conductivity was observed to be decreased at the GBs. The recorded conductivity data indicates that either a negative SCL potential or a change of the electron mobility at the GBs is the cause of this effect. The investigation could also confirm the presence of oxygen interstitial defects in donor doped cerium oxide which was suggested in earlier studies. Also the oxygen insertion enthalpy was measured.