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Chemical and physical synthesis routes were combined to prepare macroporous CaCu(3)Ti(4)O(12) thin films by pulsed laser deposition onto poly(methyl methacrylate) (PMMA) microsphere templated substrates. These films showed remarkably enhanced gas sensitivity compared with control films deposited on untreated substrates, demonstrating the virtues of combining thin film physical vapor deposition (PVD) techniques in concert with colloidal templates to produce macroporous structures of inorganic films with enhanced surface activity for applications in chemical sensors, catalysts, and fuel cells.

Enhanced ionic mobility in mixed ionic and electronic conducting solids contributes to improved performance of memristive memory, energy storage and conversion, and catalytic devices. Ionic mobility can be significantly depressed at reduced temperatures, for example, due to defect association and therefore needs to be monitored. Measurements of ionic transport in mixed conductors, however, proves to be difficult due to dominant electronic conductivity. This study examines the impact of different levels of quenched‐in oxygen deficiency on the oxygen vacancy mobility near room temperature. A praseodymium doped ceria (Pr0.1Ce0.9O2–δ) film is grown by pulsed laser deposition and annealed in various oxygen partial pressures to modify its oxygen vacancy concentration. Changes in film non‐stoichiometry are monitored by tracking the optical absorption related to the oxidation state of Pr ions. A 13‐fold increase in ionic mobility at 60 °C for increases in oxygen non‐stoichiometry from 0.032 to 0.042 is detected with negligible changes in migration enthalpy and large changes in pre‐factor. Several factors potentially contributing to the large pre‐factor changes are examined and discussed. Insights into how ionic defect concentration can markedly impact ionic mobility should help in elucidating the origins of variations seen in nanoionic devices. [ABSTRACT FROM AUTHOR]

Solid‐state mixed ionic–electronic conductors (MIECs) in which ionic transport is commonly accompanied by predominant electronic conductivity underpin key technologies and require universal characterization methods for monitoring transport at the nanoscale, at both high and near ambient temperatures, the latter being especially challenging. In this study, a novel dynamic current–voltage analysis technique is utilized to decouple ionic and electronic transport properties from each other. The versatility of the method is demonstrated by enabling measurement of the oxygen vacancy mobility in Pr0.1Ce0.9O2−δ thin films, across an unusually wide temperature range, from 35 to 500 °C. Despite the presence of predominant electronic conduction, the oxygen vacancy mobility in Pr0.1Ce0.9O2−δ is measured, being 6.8 × 10−6 cm2 V−1 s−1 at 500 °C, decreasing by seven orders of magnitude down to 35 °C, and following a single thermal activation energy of 0.82 ± 0.02 eV. A comparison with previous reports on oxygen vacancy transport and with the one derived in this study from impedance spectroscopy, interpreted with the Jamnik–Maier model, further confirms the dynamic current–voltage analysis results. This method can more generally be applied to other types of MIECs, thereby enabling deeper insights into mobile ionic defect transport and accompanying thermodynamic properties. [ABSTRACT FROM AUTHOR]

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]

Plastic strain engineering was applied to induce controllable changes in electronic and oxygen ion conductivity in oxides by orders of magnitude, without changing their nominal composition. By using SrTiO3 as a model system of technological importance, and by combining electrical and chemical tracer diffusion experiments with computational modeling, it is revealed that dislocations alter the equilibrium concentration and distribution of electronic and ionic defects. The easier reducibility of the dislocation cores increases the n-type conductivity by 50 times at oxygen pressures below 10−5 atm at 650 °C. At higher oxygen pressures the p-type conductivity decreases by 50 times and the oxygen diffusion coefficient reduces by three orders of magnitude. The strongly altered electrical and oxygen diffusion properties in SrTiO3 arise because of the existence of overlapping electrostatic fields around the positively charged dislocation cores. The findings and the approach are broadly important and have the potential for significantly impacting the functionalities of electrochemical and/or electronic applications such as thin film oxide electronics, memristive systems, sensors, micro-solid oxide fuel cells, and catalysts, whose functionalities rely on the concentration and distribution of charged point defects. [ABSTRACT FROM AUTHOR]

In this paper, we focus on the effect of processing-dependent lattice strain on oxygen ion conductivity in ceria based solid electrolyte thin films. This is of importance for technological applications, such as micro-SOFCs, microbatteries, and resistive RAM memories. The oxygen ion conductivity can be significantly modified by control of lattice strain, to an extent comparable to the effect of doping bulk ceria with cations of different diameters. The interplay of dopant radii, lattice strain, microstrain, anion-cation near order and oxygen ion transport is analyzed experimentally and interpreted with computational results. Key findings include that films annealed at 600 °C exhibit lattice parameters close to those of their bulk counterparts. With increasing anneal temperature, however, the films exhibited substantial compaction with lattice parameters decreasing by as much as nearly 2% (viz, Δd600-1100 °C: -1.7% (Sc+3) > -1.5% (Gd+3) > -1.2% (La+3)) for the annealing temperature range of 600-1100 °C. Remarkably 2/3rd of the lattice parameter change obtained in bulk ceria upon changing the acceptor diameter from the smaller Sc to larger La, can be reproduced by post annealing a film with fixed dopant diameter. While the impact of lattice compaction on defect association/ordering cannot be entirely excluded, DFT computation revealed that the main effect appears to result in an increase in migration energy and consequent drop in ionic conductivity. As a consequence, it is clear that annealing procedures should be held to a minimum to maintain the optimum level of oxygen ion conductivity for energy-related applications. Results reveal also the importance to understand the role of electro-chemo-mechanical coupling that is active in thin film materials. [ABSTRACT FROM AUTHOR]

We demonstrate the impact on thermal conductivity of varying the concentration of oxygen vacancies and reduced cations in Pr0.1Ce0.9O2-δ thin films prepared by pulsed laser deposition. The oxygen vacancy concentration is controlled by varying the oxygen partial pressure between 1×10-4 and 1 atm at 650 °C. Corresponding changes in the oxygen non-stoichiometry (δ) are monitored by detecting the lattice parameters of the films with high-resolution X-ray diffraction, while the thermal properties are characterized by time-domain thermoreflectance measurements. The films are shown to exhibit a variation in oxygen vacancy content, and in the Pr3+/Pr4+ ratio, corresponding to changes in δ from 0.0027 to 0.0364, leading to a reduction in the thermal conductivity from k=6.62±0.61 to 3.82±0.51 W/m-K, respectively. These values agree well with those predicted by the Callaway and von Baeyer model for thermal conductivity in the presence of point imperfections. These results demonstrate the capability of controlling thermal conductivity via control of anion and cation defect concentrations in a given reducible oxide. [ABSTRACT FROM AUTHOR]

Nonstoichiometric mixed ionic and electronic conductors (MIECs) find use as oxygen permeation membranes, cathodes in solid oxide fuel cells, oxygen storage materials in three-way catalysts, and chemoresistive gas sensors. Praseodymium–cerium oxide (PrxCe1−xO2−δ) solid solutions exhibit MIEC behavior in a relatively high and readily accessible oxygen partial pressure ($P_{{\rm{O}}_2 } $) regime and as such serve as model systems for investigating the correlation between thermodynamic and kinetic properties as well as exhibiting high performance figures of merit in the above applications. In this paper, we extend recently published results for Pr0.1Ce0.9O2−δ to include values of x = 0, 0.002, 0.008, 0.1, and 0.20 (in PrxCe1−xO2−δ) to test how both defect and transport parameters depend on Pr fraction. Important observed trends with increasing x include increases in oxygen ion migration energy and MIEC and reductions in vacancy formation and Pr ionization energies. The implications these changes have for potential applications of PrxCe1−xO2−δ are discussed. [ABSTRACT FROM PUBLISHER]

Oxygen and gallium diffusivities in langasite were experimentally determined by analysis of diffusion profiles of 18O and 71-Ga tracers by SIMS analysis as functions of temperature and doping. Strontium-enhanced diffusivities and activation energies of ∼1.2 ± 0.2 eV confirm the predominant role of oxygen vacancies in controlling the electrical conductivity of langasite at elevated temperature and oxygen partial pressure. The potential impact of high levels of porosity and the use of an oxygen primary ion beam on the accuracy of some of the data is discussed. The gallium diffusivity, with activation energy of 3.13 eV, was found to be more than two orders of magnitude lower than that of oxygen. Surface exchange measurements enabled estimation of gallium loss at elevated temperatures and oxygen partial pressure; the level is not believed to be of major concern for resonator performance. [ABSTRACT FROM AUTHOR]

The electrical and defect properties of langasite (La3Ga5SiO14) were studied as a function of temperature, oxygen partial pressure and dopants in order to characterize its electrical behavior in relation to its performance as a bulk acoustic wave (BAW) gas sensor operating at elevated temperatures. Undoped, 5%–Nb donor doped, and 1%–Sr acceptor doped langasite specimens were studied. A defect model, shown to be consistent with experimental observations, was used to extract key defect parameters for the system. Implications for device operation are discussed. [Copyright &y& Elsevier]

Abstract: SrTi0.65Fe0.35O3−δ (STF35) is a potential candidate for oxygen sensors in lean burn engines due to its strong sensitivity to oxygen partial pressure variations and negligible cross-sensitivity to temperature fluctuations. To understand the origin of this unique phenomenon, the temperature coefficient of resistance (TCR) of SrTi1−x Fe x O3−δ (STF) solid solutions with varying compositions between SrTiO3 and SrFeO3−δ was systematically analyzed. Changes in TCR from negative values at low to positive values at high iron concentrations were found to be correlated to systematically decreasing bandgap energy with increasing Fe/Ti ratio. At an intermediate composition of x =0.35 (STF35), the bandgap energy is such that the Fermi energy lies just far enough above the valence band to compensate for the temperature-dependence of the mobility, yielding a zero TCR from the product of the free carrier (holes) concentration and mobility terms. We propose that the variations in the bandgap energy of STF solid solutions are due to additional bands derived largely from Fe3+/Fe4+ and Fe2+/Fe3+ states. These bands lie between the O 2p valence and Ti 3d conduction bands of SrTiO3, and due to variations in their position and width, the bandgap energy of STF solid solutions decreases with increasing iron concentration. [Copyright &y& Elsevier]

• Oxygen removal from solar thermochemical water-splitting cycles is inefficient and costly. • Hybrid schemes combining thermochemical pumping, vacuum pumps and inert sweep are compared. • Direct coupling of the redox reactors with thermochemical pumping material has lowest energy penalty at p less than 10 Pa. • Challenges with high gas flowrates at p less than 4 Pa are quantified for vacuum and inert sweep systems. Solar thermochemical hydrogen (STCH) is a promising route for renewable fuel production, but efficiency and cost concerns must be addressed before it can contribute meaningfully to decarbonization efforts. The oxygen removal (reduction) step of two-step STCH redox cycles needs high temperature (1300–1500 °C) and low oxygen partial pressure (10-5-10-3 bar). This consumes significant energy by way of redox reheating, re-radiation losses and pumping work. Thermochemical oxygen pumping (TcOP) has been shown to be more efficient than mechanical pumps in the medium vacuum range, enabling higher heat-to-fuel efficiency and potentially lower reduction temperatures. We have previously proposed a novel Reactor Train System for STCH and reported significant efficiency and productivity improvements by replacing mechanical pumps with TcOP. The current work is a comparative analysis of different implementations of TcOP and its hybridization with conventional oxygen removal schemes like mechanical vacuum pumps and inert gas sweep. The integration of these hybrid TcOP systems with different STCH reactor systems using redox monoliths or particles is analyzed in terms of energy use along with mass and heat transfer considerations. Our results show that direct oxygen transfer between STCH and TcOP reactors results in the best performance for reduction oxygen partial pressures below 10 Pa. Furthermore, we show that achieving reduction pressures of 1 Pa and lower is challenging with existing reactor systems due to a combination of low gas density and relatively high molar flowrates. This challenge also applies to other oxygen removal systems like vacuum pumps and inert sweep. [ABSTRACT FROM AUTHOR]

The mixed ionic and electronic conductor Sr(Ti,Fe)O 3 - α (STF) exhibits fast oxygen surface exchange kinetics, with electron concentration potentially playing a key role. The effect of La donor doping on electron concentration, Fermi level, and overall defect chemistry of STF is investigated on Sr 1 - y La y Ti 0.65 Fe 0.35 O 3 - α (LSTF, 0 ≤ y ≤ 0.5) thin films. Defect chemical modeling, optical absorption, and electrical conductivity measurements in oxidizing conditions indicate compensation of donors by an increase in oxygen and electron concentrations, increase in Fermi level, decrease in oxygen vacancy and hole concentrations, and formation of cation vacancies (for [La] > [Fe]). The surface exchange coefficient, measured by impedance spectroscopy, decreased with increasing donor concentration, suggesting that oxygen exchange kinetics in LSTF are limited by low oxygen vacancy and/or hole concentrations, rather than electron transfer. [ABSTRACT FROM AUTHOR]

The oxygen nonstoichiometry of mixedconducting perovskite-structuredBaxSr1–xTi1–yFeyO3–y/2+δ(BSTF) (x= 0, 0.1, 0.5 and y= 0.05, 0.35) wasmeasured by means of thermogravimetry as a function of oxygen partialpressure, pO2, in a temperature rangetypical for solid oxide fuel cell (SOFC) cathode applications. Withincreasing Ba content, the nonstoichiometry curve was shifted to higher pO2, indicating enhanced reducibility. Basedon a defect chemical analysis of thermogravimetry derived nonstoichiometrydata, the substitution of Ba for Sr on the A-site of STF was foundto result in a decrease in both the reduction enthalpy and band gapenergy, consistent with expectations that substitution of Sr by thelarger Ba leads to a reduction in bond strength. A consequent increasein minority electron density and oxygen vacancy concentration areexpected to result in enhancements in oxygen surface exchange kineticsand diffusivity and thereby cathode performance. The nonstoichiometrydata obtained in this study also brought to light a previous underestimationof the minority electron density, by approximately a factor of 103, a key parameter believed to impact cathodic performance. [ABSTRACT FROM AUTHOR]