'Illusional' nano-size effect due to artifacts of in-plane conductivity measurements of ultra-thin films
ABSTRACT The nano-size effect, which indicates a drastic increase in conductivity in solid electrolyte materials of nano-scale microstructures, has drawn substantial attention in various research fields including in the field of solid oxide fuel cells (SOFCs). However, especially in the cases of the conductivity of ultra-thin films measured in an in-plane configuration, it is highly possible that the 'apparent' conductivity increase originates from electrical current flowing through other conduction paths than the thin film. As a systematic study to interrogate those measurement artifacts, we report various sources of electrical current leaks regarding in-plane conductivity measurements, specifically insulators in the measurement set-up. We have observed a 'great conductivity increase' up to an order of magnitude at a very thin thickness of a single layer yttria-stabilized zirconia (YSZ) film in a set-up with an intentional artifact current flow source. Here we propose that the nano-size effect, reported to appear in ultra-thin single layer YSZ, can be a result of misinterpretation.
SourceAvailable from: Ji-Won Son[Show abstract] [Hide abstract]
ABSTRACT: The reliability of solid oxide fuel cells (SOFCs) particularly depends on the high quality of solid oxide electrolytes. The application of thinner electrolytes and multi electrolyte layers requires a more reliable characterization method. Most of the investigations on thin film solid electrolytes have been made for the parallel transport along the interface, which is not however directly related to the fuel cell performance of those electrolytes. In this work an array of ion-blocking metallic Ti/Au microelectrodes with about a diameter was applied on top of an ultrathin () yttria-stabilized-zirconia/gadolinium-doped-ceria (YSZ/GDC) heterolayer solid electrolyte in a micro-SOFC prepared by PLD as well as an 8- thick YSZ layer by screen printing, to study the transport characteristics in the perpendicular direction relevant for fuel cell operation. While the capacitance variation in the electrode area supported the working principle of the measurement technique, other local variations could be related to the quality of the electrolyte layers and deposited electrode points. While the small electrode size and low temperature measurements increaseed the electrolyte resistances enough for the reliable estimation, the impedance spectra appeared to consist of only a large electrode polarization. Modulus representation distinguished two high frequency responses with resistance magnitude differing by orders of magnitude, which can be ascribed to the gadolinium-doped ceria buffer electrolyte layer with a 200 nm thickness and yttria-stabilized zirconia layer of about . The major impedance response was attributed to the resistance due to electron hole conduction in GDC due to the ion-blocking top electrodes with activation energy of 0.7 eV. The respective conductivity values were obtained by model analysis using empirical Havriliak-Negami elements and by temperature adjustments with respect to the conductivity of the YSZ layers.Journal of the Korean Ceramic Society 09/2012; 49(5). DOI:10.4191/kcers.2012.49.5.404 · 0.27 Impact Factor
[Show abstract] [Hide abstract]
ABSTRACT: The electrical cross-plane conductivity of 8 mol% yttria stabilized zirconia (YSZ) thin films prepared by different deposition techniques, namely aerosol assisted chemical vapor deposition, wet spray pyrolysis (SP), and pulsed laser deposition (PLD), is correlated with their microstructure. Depending on deposition technique and process conditions, microstructures ranging from amorphous to randomly oriented nanocrystalline or columnar with preferred (111) orientation are obtained. Cross-plane AC impedance measurements of these thin films show that the oxygen ion conductivity of randomly oriented nanocrystalline samples is determined by the grain boundaries, which show significantly lower transport properties than the grain interior. In columnar microstructures, the conductivity is determined by ionic transport through the grains only. The same conduction behavior is found for amorphous and randomly oriented microstructures with grain sizes between 3 nm and 9 nm, indicating that no true size effects occur in 8 mol% YSZ. Grain and grain boundary conductivity determined for nanocrystalline 8 mol% yttria stabilized zirconia thin films of different microstructures. In the study by Schlupp et al., cross-plane conductivity measurements are performed on both (111) textured columnar and randomly oriented nanocrystalline 8 mol% yttria stabilized zirconia (8YSZ) thin films with column diameters of 10-20 nm and grain sizes between 3 nm and 13 nm, respectively. Reproducible results are obtained for YSZ samples of 220-600 nm thickness prepared by both physical and chemical thin film deposition methods. The clear microstructure-conductivity relationships established in this study will provide a relevant basis for the engineering of miniaturized low-temperature thin film solid oxide fuel cells.Physica Status Solidi (A) Applications and Materials 08/2012; 209(8). DOI:10.1002/pssa.201228248 · 1.53 Impact Factor
[Show abstract] [Hide abstract]
ABSTRACT: A phenomenological and analytical model for the influence of strain effects on atomic transport in columnar thin films is presented. A model system consisting of two types of crystalline thin films with coherent interfaces is assumed. Biaxial mechanical strain ε0 is caused by lattice misfit of the two phases. The conjoined films consist of columnar crystallites with a small diameter l Strain relaxation by local elastic deformation, parallel to the hetero-interface, is possible along the columnar grain boundaries. The spatial extent δ0 of the strained hetero-interface regions can be calculated, assuming an exponential decay of the deformation-forces. The effect of the strain field on the local ionic transport in a thin film is then calculated by using the thermodynamic relation between (isostatic) pressure and free activation enthalpy ΔG#. An expression describing the total ionic transport relative to bulk transport of a thin film or multilayer as a function of the layer thickness is obtained as an integral average over strained and unstrained regions. The expression depends only on known material constants as Young modulus Y , Poisson ratio ν and activation volume ΔV#, which can be combined as a dimensionless parameter. The model is successfully used to describe own experimental data from conductivity and diffusion studies. In the second part of the paper a comprehensive literature overview on experimental studies on (fast) ion transport in thin films and multilayers along solid-solid hetero-interfaces is presented. By comparing and reviewing the data the observed interface effects can be classified in three groups: i) transport along interfaces between extrinsic ionic conductors (and insulator), ii) transport along an open surface of an extrinsic ionic conductor and iii) transport along interfaces between intrinsic ionic conductors. The observed effects in these groups differ by about five orders of magnitude in a very consistent way. The modified interface transport in group i) is most probably caused by strain effects, misfit dislocations or disordered transition regions.Physical Chemistry Chemical Physics 10/2014; 16(44). DOI:10.1039/C4CP03055A · 4.20 Impact Factor