‘Illusional’ Nano-Size Effect Due to Artifacts of in-Plane Conductivity Measurements of Ultra-Thin Films
High-Temperature Energy Materials Center, Korea Institute of Science and Technology, Seoul, Republic of Korea.Physical Chemistry Chemical Physics (Impact Factor: 4.49). 02/2011; 13(13):6133-7. DOI: 10.1039/c0cp02673e
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.
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ABSTRACT: Ionic conducting metal oxide thin films for Si-wafer based electroceramic devices are of high relevance to allow for new applications, quicker response times, and higher efficiencies. Metal oxide thin films are deposited via various processes on substrates. Depending on the synthesis method their microstructures differ on an atomistic length scale in their anionic and cationic lattice displacement fields and packing densities. Local changes in ionic bond strength can affect the oxygen migration barriers and ionic diffusion is no longer to be assumed as equal to zero-strained bulk material. This article proposes lattice strain measurements to characterize the defect states of metal oxide thin films that depend on their processing history and to correlate this measure for atomistic disorder. It is suggested to discuss differences in ionic conductivity observed relative to lattice strain and atomistic disorder in addition to other microstructure characteristics such as the grain size. The interplay of grain size, degree of crystallinity, phase changes and ionic conductivity are discussed with respect to lattice-strain for state-of-the-art ionic conducting thin films, i.e. ceria or zirconia solid solutions. Previous findings on the fields of compressive or tensile strain in epitaxial heterolayers or free-standing membrane films are discussed and compared with processing-dependent lattice strain of fully crystalline ceramic thin films of more than 200nm in thickness. Finally, guidelines for processing of highly strained films with high ionic conduction are given for functional oxide thin films in Si-based electroceramic devices.Solid State Ionics 01/2011; 207:1-13. DOI:10.1016/j.ssi.2011.09.009 · 2.56 Impact Factor
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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.Physica Status Solidi (A) Applications and Materials 08/2012; 209(8). DOI:10.1002/pssa.201228248 · 1.62 Impact Factor
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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
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