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We compare approaches to measure oxygen surface exchange kinetics, by simultaneous optical transmission relaxation (OTR) and AC-impedance spectroscopy (AC-IS), on the same mixed conducting SrTi0.65Fe0.35O3-x film. Surface exchange coefficients were evaluated as a function of oxygen activity in the film, controlled by gas partial pressure and/or DC bias applied across the ionically conducting yttria-stabilized zirconia substrate. Changes in measured light transmission through the film over time (relaxations) resulted from optical absorption changes in the film corresponding to changes in its oxygen and oxidized Fe (~Fe4+) concentrations; such relaxation profiles were successfully described by the equation for surface exchange-limited kinetics appropriate for the film geometry. The kchem values obtained by OTR were significantly lower than the AC-IS derived kchem values and kq values multiplied by the thermodynamic factor (bulk or thin film), suggesting a possible enhancement in k by the metal current collectors (Pt, Au). Long-term degradation in kchem and kq values obtained by AC-IS was also attributed to deterioration of the porous Pt current collector, while no significant degradation was observed in the optically derived kchem values. The results suggest that, while the current collector might influence measurements by AC-IS, the OTR method offers a continuous, in situ, and contact-free method to measure oxygen exchange kinetics at the native surfaces of thin films. [ABSTRACT FROM AUTHOR]

The bulk electrical conductivity of the mixed ionic-electronic conducting perovskite-structured SrSn1-xFexO3-x/2+δ (SSF) was measured to examine how changes in defect chemistry and electronic band structure associated with the substitution of Ti by Sn impact defect charge carrier density and ultimately electrode performance. These results complement a defect chemical model for SSF investigated and reported in Part I of this study. The electrical properties of SSF were found not to differ significantly from the corresponding composition in SrTi1-xFexO3-x/2+δ (STF). It is believed that Fe dominates the character of the valence and conduction bands and thus governs the electronic properties in SSF. Though slightly shifted in energy due to the larger size of Sn, the defect equilibria and therefore the electrical conductivity of SSF were found to be largely dominated by Fe and thus differed only in a limited way from that in STF. Key kinetic parameters obtained include the migration enthalpy of oxygen vacancies (0.772 ± 0.204 eV), the activation energy of area-specific-resistance for oxygen exchange (1.65 ± 0.03 eV) and the magnitudes of electron (0.0002 ± 0.00005 cm2/V∙s) and hole (0.0037 ± 0.0015 cm2/V∙s) mobilities. [ABSTRACT FROM AUTHOR]

While the properties of functional oxide thin films often depend strongly on oxygen stoichiometry, there have been few means available for its control in a reliable and in situ fashion. This work describes the use of DC bias as a means of systematically controlling the stoichiometry of oxide thin films deposited onto yttria-stabilized zirconia substrates. Impedance spectroscopy is performed on the electrochemical cell Pr0.1Ce0.9O2−δ (PCO)/YSZ/Ag for conditions: T = 550 to 700 °C, pO2 = 10−4 to 1 atm, and ΔE = -100 to 100 mV. The DC bias ΔE is used to control the effective pO2 or oxygen activity at the PCO/YSZ interface. The non-stoichiometry ( δ) of the PCO films is calculated from the measured chemical capacitance ( Cchem ). These δ values, when plotted isothermally as a function of effective pO2, established, either by the surrounding gas composition alone, or in combination with applied bias, agree well with each other and to predictions based on a previously determined defect model. These results confirm the suitability of using bias to precisely control δ of thin films in an in situ fashion and simultaneously monitor these changes by measurement of Cchem . Of further interest is the ability to reach effective pO2s as high as 280 atm. [ABSTRACT FROM AUTHOR]

Abstract: SrTi1−xFexO3−δ (STF) model cathodes, with compositions of x=0.05 to 0.80 were deposited onto single crystal yttria stabilized zirconia by pulsed layer deposition as dense films with well defined area and thickness and studied by electrochemical impedance spectroscopy as a function of electrode geometry, temperature and pO2. The STF cathode was observed to exhibit typical mixed ionic-electronic behavior with the electrode reaction occurring over the full electrode surface area rather than being limited to the triple phase boundary. The electrode impedance was observed to be independent of electrode thickness and to the introduction of CGO interlayers and inversely proportional to the square of the electrode diameter, pointing to surface exchange limited kinetics. Values for the surface exchange coefficient, k, were calculated and found to be comparable in magnitude to those exhibited by other popular mixed ionic-electronic conductors such as (La,Sr)(Co,Fe)O3, thereby, confirming the suitability of STF as a model mixed conducting cathode material. The surface exchange coefficient, k, was also found to be insensitive to orders of magnitude change in both bulk electronic and ionic conductivities. [Copyright &y& Elsevier]

Abstract: Dense, photolithographically patterned platinum microelectrodes were created on yttria-stabilized zirconia (YSZ). The electrodes, typically tens of microns across, were orders of magnitude greater in width than that reported for the triple phase boundary width. Impedance spectra exhibited an additional feature at low frequencies that was believed to be due to a constriction resistance, where all of the current had to pass through a narrow triple phase boundary. The resistance of this feature had a similar activation energy to the conductivity of the electrolyte and was as large as nearly 100 times the high frequency resistance. Finite element models describing constricted and unconstricted resistances achieved qualitative agreement with the experimental results but consistently underestimated the constricted resistance. The implications of constriction upon the performance of micro-ionic devices and the potential for future similar measurements to quantify triple phase boundary widths are discussed. [Copyright &y& Elsevier]

CaCu3Ti4O12 exhibits an unusually high dielectric constant on the order of 105 and highly nonlinear I-V characteristics. Impedance spectroscopy measurements carried out in this work point to the crucial role played by grain boundary barriers in controlling the electrical properties of this material. Under dc bias, the grain boundary resistance decreases, followed by a precipitous breakdown at higher applied voltages. The barrier height is estimated to be ∼0.82 eV. The grain conductivity shows a transition from a negative temperature coefficient of resistance with activation energy of ∼0.08 eV to a positive temperature coefficient of resistance at 280 °C suggesting a transition from impurity ionization to scattering controlled mobility in the carrier saturation region. [ABSTRACT FROM AUTHOR]

Abstract: Yttria-doped zirconia (YDZ) thin films with nanometric sized grains were prepared by reactive RF sputtering and their oxygen ion conductivities were systematically measured as a function of yttria doping with levels in the range 0.5–9.1mol.% Y2O3. Enhanced oxygen ion conductivities, as derived from impedance spectra, were observed when compared with values reported for bulk YSZ. Furthermore, the peak conductivity for the YDZ films was observed to occur at considerably reduced yttria levels, i.e., at 6.5mol.% Y2O3 (for T >∼400°C) and at 3.2mol.% Y2O3 (for T <∼300°C) vs. 9mol.% Y2O3 in bulk YSZ. Based on an analysis of the Raman spectra, these results are believed to result from the extended meta-stability of the cubic phase to reduced yttria levels at nanometric grain sizes. [Copyright &y& Elsevier]

Abstract: Dense, nanocrystalline gadolinia doped ceria (CGO) thin films were prepared by sputtering on a variety of substrates. The CGO films were electrochemically characterized with the aid of interdigitated Pt electrodes by impedance spectroscopy as a function of operating temperature (175–340°C), CGO film thickness (157, 233, 315nm), deposition temperature (room temperature, 300, 500°C), and Pt electrode pattern geometry (line pattern spacing 10, 15, 25, 50, 100μm). The different parameter variations enabled the identification of five contributions to the impedance spectra attributed, respectively, to wiring and contacts, grain, grain boundary, current constriction at the triple phase boundary, and electrode effects. The ionic conductivity of the CGO films were characterized by the expression σ =(A/T)exp(−E A/kT), with A =108.66±0.72 Scm−1 K and E A =0.97±0.07eV with magnitude higher than values reported for both bulk and thin film CGO previously reported in the literature. The electrochemical impedance of the Pt electrodes was found to decrease markedly for CGO films grown at reduced temperatures. Explanations for these observations are presented. [Copyright &y& Elsevier]

CaCu3Ti4O12 exhibits an unusually high dielectric constant on the order of 105 and highly nonlinear I-V characteristics. Impedance spectroscopy measurements carried out in this work point to the crucial role played by grain boundary barriers in controlling the electrical properties of this material. Under dc bias, the grain boundary resistance decreases, followed by a precipitous breakdown at higher applied voltages. The barrier height is estimated to be ∼0.82 eV. The grain conductivity shows a transition from a negative temperature coefficient of resistance with activation energy of ∼0.08 eV to a positive temperature coefficient of resistance at 280 °C suggesting a transition from impurity ionization to scattering controlled mobility in the carrier saturation region. [ABSTRACT FROM AUTHOR]

The gas sensor response of tin monoxide micro-disks, functionalized with noble metal nanoparticles (Pd and Ag), to NO 2 , H 2 and CO were studied by monitoring changes in their resistance upon exposure to the various gases. The tin monoxide, with unusually low Sn oxidation state, was synthetized by carbothermal reduction. Surface modification by Pd and Ag catalysts was achieved by coating the micro-disks by metallic nanoparticle dispersions, prepared by the polyol reduction process, followed by thermal treatment. SEM and TEM analysis showed nanoparticles to be well-dispersed over the SnO surfaces. The decorated SnO micro-disks exhibited high sensor response to reducing gases such as H 2 and CO. On the other hand, the catalytic particles tended to reduce the sensor response to oxidizing gases such as NO 2 . The catalytic activity of Pd nanoparticles was tied to chemical sensitization while that of Ag nanoparticles to electronic sensitization. Impedance spectroscopy enabled deconvolution of different contributions to the sensor response with only the Ag-decorated specimens exhibiting two RC time constants. Thus, in contrast to undecorated and Pd-decorated specimens, nearly 80% of Ag modified SnO’s response to H 2 was controlled by changes in the interface between particles and disks. Sensor response to H 2 was optimal at higher temperatures (300 °C), NO 2 at 200 °C while that for Pd-decorated materials; maximum sensor response to CO was observed at lower temperatures (under 150 °C), where CO absorption by metal nanoparticles is favored. [ABSTRACT FROM AUTHOR]

Simultaneous in situoptical absorption and electrochemicalimpedance spectroscopy measurements were performed, for thefirst time, at elevated temperature on a metal oxide thinfilm exhibiting oxygen nonstoichiometry, utilizing Pr0.1Ce0.9O2−δ(10PCO) asa model system. Chemical capacitance measurements, capable of providingexplicit values of δ, were used to determine the optically absorbingcenter (Pr4+) concentration and thereby the optical extinctioncoefficient for Pr4+. The absorption coefficient was foundto exhibit a linear dependence on Pr4+concentration, validatingthe use of optical absorption to examine defect concentration trendsand allowing derivation of the extinction coefficient εPr4+= 5.01 ± 0.14 × 10–18cm2. Values of Pr4+concentration derivedfrom the chemical capacitance and corresponding trends in opticalabsorption were found to be self-consistent, validating the thin filmdefect model for 10PCO, thereby confirming that the oxygen reductionenthalpy in thin film 10PCO is lower than that in the bulk. The non-contactoptical absorption technique thus provides an additional insitumethod for investigating the defect equilibria of thinfilms and is expected to aid in confirming whether and under whatconditions the defect thermodynamics of films differ from that oftheir bulk counterparts. [ABSTRACT FROM AUTHOR]