Dynamic chemical expansion of thin-film non-stoichiometric oxides at extreme temperatures.

Actuator operation in increasingly extreme and remote conditions requires materials that reliably sense and actuate at elevated temperatures, and over a range of gas environments. Design of such materials will rely on high-temperature, high-resolution approaches for characterizing material actuation in situ. Here, we demonstrate a novel type of high-temperature, low-voltage electromechanical oxide actuator based on the model material PrxCe1-xO2-δ (PCO). Chemical strain and interfacial stress resulted from electrochemically pumping oxygen into or out of PCO films, leading to measurable film volume changes due to chemical expansion. At 650 °C, nanometre-scale displacement and strain of >0.1% were achieved with electrical bias values <0.1 V, low compared to piezoelectrically driven actuators, with strain amplified fivefold by stress-induced structural deflection. This operando measurement of films 'breathing' at second-scale temporal resolution also enabled detailed identification of the controlling kinetics of this response, and can be extended to other electrochemomechanically coupled oxide films at extreme temperatures.

Role of grain size on redox induced compositional stresses in Pr doped ceria thin films.

In constrained geometries and in varying oxygen partial pressures and operating temperatures, exchange of oxygen ions between non-stoichiometric oxide thin films (for example, doped and undoped ceria systems) and the gas phase can lead to stresses. In this study, these compositional stresses were investigated in thin films of nanocrystalline 10% praseodymium doped ceria (PCO), as a function of average grain size. In situ wafer curvature measurements, along with High Temperature X-Ray Diffraction (HTXRD), were employed to measure stresses and strains, respectively on the PCO films during oxidation-reduction cycling, over the pO2 range of 10-1-10-5 atm at 750 °C. For relatively large grain sizes, the stress values agree well with the amount of expansion induced by oxygen non-stoichiometry (chemical expansion) predicted by a thin film defect equilibria model that was developed previously. The compositional stresses were found to increase with decreasing grain size. The origin of this effect, including the role of space charge effects near surfaces and interfaces are discussed in this paper. To our knowledge, this is the first time that such comparisons are reported by simultaneously employing high temperature in situ wafer curvature and HTXRD measurements on doped ceria systems.

Praseodymium Cuprate Thin Film Cathodes for Intermediate Temperature Solid Oxide Fuel Cells: Roles of Doping, Orientation, and Crystal Structure.

Highly textured thin films of undoped, Ce-doped, and Sr-doped Pr2CuO4 were synthesized on single crystal YSZ substrates using pulsed laser deposition to investigate their area-specific resistance (ASR) as cathodes in solid-oxide fuel cells (SOFCs). The effects of T' and T* crystal structures, donor and acceptor doping, and a-axis and c-axis orientation on ASR were systematically studied using electrochemical impedance spectroscopy on half cells. The addition of both Ce and Sr dopants resulted in improvements in ASR in c-axis oriented films, as did the T* crystal structure with the a-axis orientation. Pr1.6Sr0.4CuO4 is identified as a potential cathode material with nearly an order of magnitude faster oxygen reduction reaction kinetics at 600 °C compared to thin films of the commonly studied cathode material La0.6Sr0.4Co0.8Fe0.2O3-δ. Orientation control of the cuprate films on YSZ was achieved using seed layers, and the anisotropy in the ASR was found to be less than an order of magnitude. The rare-earth doped cuprate was found to be a versatile system for study of relationships between bulk properties and the oxygen reduction reaction, critical for improving SOFC performance.

Oxygen diffusion and surface exchange in the mixed conducting oxides SrTi1-yFeyO3-δ.

Oxygen transport in the mixed ionic-electronic conducting perovskite-oxides SrTi1-yFeyO3-δ (with y = 0.5 and y = 1.0) was studied by oxygen isotope exchange measurements. Experiments were performed on thin-film samples that were grown by Pulsed Laser Deposition (PLD) on MgO substrates. Isotope penetration profiles were introduced by 18O2/16O2 exchanges into the plane of the films at various temperatures in the range 773 < T/K < 973 at an oxygen activity aO2 = 0.5. Isotope profiles were determined subsequently by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), and their analysis yielded tracer diffusion coefficients D* and oxygen surface exchange coefficients k*. Activation energies for oxygen diffusion ΔHD* and surface exchange ΔHk* were obtained. Isothermal values of D* and values of ΔHD* are compared with literature data as a function of Fe content. D* is seen to increase monotonically with Fe content; ΔHD* shows more complex behaviour. D* and ΔHD* are also compared with the predictions of defect-chemical models. Analogous comparisons with literature data for k* and ΔHk* indicate, in contrast to prior studies, no mechanistic difference between electron-poor and electron-rich materials. It is concluded that the single operative mechanism of surface exchange for the entire series of STF compositions requires conduction-band electrons (minority electronic charge-carriers).

Tunable Mixed Ionic/Electronic Conductivity and Permittivity of Graphene Oxide Paper for Electrochemical Energy Conversion.

Graphene oxide (GO) is a two-dimensional graphitic carbon material functionalized with oxygen-containing surface functional groups. The material is of interest in energy conversion, sensing, chemical processing, gas barrier, and electronics applications. Multilayer GO paper has recently been applied as a new proton conducting membrane in low temperature fuel cells. However, a detailed understanding of the electrical/dielectric properties, including separation of the ionic vs electronic contributions under relevant operating conditions, has so far been lacking. Here, the electrical conductivity and dielectric permittivity of GO paper are investigated in situ from 30 to 120 °C, and from 0 to 100% relative humidity (RH) using impedance spectroscopy. These are related to the water content, measured by thermogravimetric analysis. With the aid of electron blocking measurements, GO is demonstrated to be a mixed electronic-protonic conductor, and the ion transference number is derived for the first time. For RH > 40%, conductivity is dominated by proton transport (with a maximum of 0.5 mS/cm at 90 °C and 100% RH). For RH < 40%, electronic conductivity dominates (with a maximum of 7.4 mS/cm at ∼80 °C and 0% RH). The relative permittivity of GO paper increases with decreasing humidity, from ∼10 at 100% RH to several 1000 at 10% RH. These results underline the potential of GO for application not only as a proton conducting electrolyte but also as a mixed conducting electrode material under appropriate conditions. Such materials are highly applicable in electrochemical energy conversion and storage devices such as fuel cells and electrolyzers.

Understanding chemical expansion in perovskite-structured oxides.

In this work, chemical expansion in perovskite oxides was characterized in detail, motivated, inter alia, by a desire to understand the lower chemical expansion coefficients observed for perovskites in comparison to fluorite-structured oxides. Changes in lattice parameter and in local atomic arrangements taking place during compositional changes of perovskites, i.e., stoichiometric expansion, were investigated by developing an empirical model and through molecular dynamics and density functional theory atomistic simulations. An accurate empirical expression for predicting lattice constants of perovskites was developed, using a similar approach to previous reports. From this equation, analytical expressions relating chemical expansion coefficients to separate contributions from the cation and anion sublattices, assuming Shannon ionic radii, were developed and used to isolate the effective radius of an oxygen vacancy, rV. Using both experimental and simulated chemical expansion coefficient data, rV for a variety of perovskite compositions was estimated, and trends in rV were studied. In most cases, rV was slightly smaller than or similar to the radius of an oxide ion, but larger than in the fluorite structured materials. This result was in good agreement with the atomistic simulations, showing contractive relaxations of the closest oxide ions towards the oxygen vacancy. The results indicate that the smaller chemical expansion coefficients of perovskites vs. fluorites are largely due to the smaller change in cation radii in perovskites, given that the contraction around the oxygen vacancy appears to be less in this structure. Limitations of applicability for the model are discussed.

Defining chemical expansion: the choice of units for the stoichiometric expansion coefficient.

Chemical expansion refers to the spatial dilation of a material that occurs upon changes in its composition. When this dilation is caused by a gradual, iso-structural increase in the lattice parameter with composition, it is related to the composition change by the stoichiometric expansion coefficient. In this work, three different approaches to defining the stoichiometric expansion coefficient (αS) are discussed. While all three definitions of αS given here are legitimate, we show that there are advantages to selecting certain ones for comparison across different crystal structures. Examples are provided for changes in oxygen content in fluorite, perovskite, and Ruddlesden-Popper (K2NiF4) phase materials used in solid oxide fuel cells.

Kinetics of Schottky defect formation and annihilation in single crystal TlBr.

The kinetics for Schottky defect (Tl and Br vacancy pair) formation and annihilation in ionically conducting TlBr are characterized through a temperature induced conductivity relaxation technique. Near room temperature, defect generation-annihilation was found to take on the order of hours before equilibrium was reached after a step change in temperature, and that mechanical damage imparted on the sample rapidly increases this rate. The rate limiting step to Schottky defect formation-annihilation is identified as being the migration of lower mobility Tl (versus Br), with an estimate for source-sink density derived from calculated diffusion lengths. This study represents one of the first investigations of Schottky defect generation-annihilation kinetics and demonstrates its utility in quantifying detrimental mechanical damage in radiation detector materials.

Charge localization increases chemical expansion in cerium-based oxides.

In this work, we demonstrate the mechanism by which electronic charge localization increases the chemical expansion coefficient in two model systems, CeO(2-δ) and BaCeO(3-δ). Using Density Functional Theory calculations, we predict that this coefficient is increased by more than 70% when charge is fully localized, consistent with the observation that materials with a smaller degree of charge localization have smaller chemical expansion coefficients. This finding has important consequences for devising materials with smaller chemical expansion coefficients and for the reliability of the widely-used Shannon's ionic radii.

The defect and transport properties of acceptor doped TlBr: role of dopant exsolution and association.

The role of acceptor dopants (S and Se) in controlling the ionic conductivity of single crystal TlBr, grown by the vertical Bridgman method, was examined as a function of temperature with the aid of impedance spectroscopy. Several features in the conductivity were identified and related to acceptor dopant-Br vacancy association, acceptor dopant exsolution, and Br vacancy mobility. The corresponding enthalpies for these processes were extracted from the data and were found to be equal to H(a) = 0.42 ± 0.07 eV, H(sol) = 1.55 ± 0.18 eV and H(m,Br) = 0.31 ± 0.02 eV respectively, the latter consistent with earlier studies on donor doped and undoped TlBr. A long term conductivity decay in the extrinsic region, attributed to S or Se exsolution, was observed. The time constant associated with exsolution was found to be thermally activated with an activation energy of 0.47 ± 0.1 eV. Estimates for Se solubility at different temperatures are provided.

Electrical conductivity and defect equilibria of Pr0.1Ce0.9O(2-δ).

Praseodymium-cerium oxide (PCO) solid solutions exhibit mixed ionic electronic conductivity (MIEC) behavior in a relatively high and readily accessible oxygen partial pressure (P(O(2))) regime and as such serve as a model system for investigating the correlation between thermodynamic and kinetic properties and performance figures of merit in the areas of high temperature energy conversion, automotive control, and gas sensing applications. In this paper, we present measurements on the non-stoichiometry of Pr(0.1)Ce(0.9)O(2-δ) and develop a defect equilibria model to predict the dependence of the concentration of all the dominant charge carriers on temperature, P(O(2)), and Pr fraction. The predictive model is then employed to describe the measured electrical conductivity and oxygen nonstoichiometry whereby pre-exponentials and enthalpies of defect formation and migration are extracted.