Search

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]

The surface oxygen exchange kinetics of mixed ionic-electronic conducting (MIEC) oxides play a crucial role in a variety of applications, including solid oxide fuel/electrolysis cells (SOFCs/SOECs), permeation membranes and sensors. To date, a variety of methods, including isotope exchange, electrical conductivity, optical absorptivity and thermogravimetry relaxation, and impedance spectroscopy have been used to measure the oxygen exchange coefficient of MIEC oxides. Each of these methods have their advantages and limitations, depending on the physical and chemical properties of the investigated materials and their sample dimensions and densities. Here, we demonstrate the ability to precisely measure the surface oxygen exchange coefficient (k chem) of MIEC model material Pr 0.1 Ce 0.9 O 2-δ (PCO) by use of dilatometric relaxation measurements, particularly of interest for materials that exhibit larger chemical expansion coefficients. This is achieved by use of porous bulk specimens to ensure that the contribution of oxygen exchange to the overall kinetics is dominant. We demonstrate that k chem values extracted from chemical expansion relaxation measurements on PCO are nearly identical to those derived from electrical conductivity relaxation measurements. This provides the opportunity to precisely investigate the oxygen exchange and chemical expansion kinetics of a wide range of materials used in high-temperature applications, particularly where more conventional methods are difficult or inappropriate to apply. [Display omitted] [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]

An impedimetric NOx dosimeter based on the NOx sorption material KMnO4 is proposed. In addition to its application as a low level NOx dosimeter, KMnO4 shows potential as a precious metal free lean NOx trap material (LNT) for NOx storage catalysts (NSC) enabling electrical in-situ diagnostics. With this dosimeter, low levels of NO and NO2 exposure can be detected electrically as instantaneous values at 380°C by progressive NOx accumulation in the KMnO4 based sensitive layer. The linear NOx sensing characteristicsare recovered periodically by heating to 650°C or switching to rich atmospheres. Further insight into the NOx sorption-dependent conductivity of the KMnO4-based material is obtained by the novel eTPD method that combines electrical characterization with classical temperature programmed desorption (TPD). The NOx loading amount increases proportionally to the NOx exposure time at sorption temperature. The cumulated NOx exposure, as well as the corresponding NOx loading state, can be detected linearly by electrical means in two modes: (1) time-continuously during the sorption interval including NOx concentration information from the signal derivative or (2) during the short-term thermal NOx release. [ABSTRACT FROM AUTHOR]

We report on the facile synthesis of highly conductive RuO2/Mn3O4 composite nanofiber mats by electrospinning of mixed ruthenium and manganese precurosr solution and their potential suitability as an active electrode in electrochemical capacitors. Electrospun RuO2/Mn3O4 fiber mats calcined at 300°C exhibit multiple fiber networks composed of nanoparticles in a size range of 5-10 nm (average 7.5 nm). Partially crystalline Mn3O4 phases are well mixed with the amorphous RuOx (a-RuO2) phases while pure Mn3O4 fibers exhibit only crystalline phase. Electrochemical capacitors utilizing a-RuO2/Mn3O4 fiber mats show a high specific capacitance of 293 Fg-1 at 10 mVs-1, high rate capability with a capacity loss of only 45% from 10 to 2000 mVs-1, and superior cycle performance of over 30,000 cycles as compared to those characteristics of Mn3O4 fiber mats (216 Fg-1 at 10 mVs-1, capacity loss of 94% at 2000 mVs-1, poor cycle performance of 2000 cycles). These results are correlated with the improved mixed ionic/electronic conductivity of the a-RuO2/Mn3O4 fiber mats as well as to its morphological evolution, i.e., highly porous flower-like-fiber networks, during cycling tests. [ABSTRACT FROM AUTHOR]

Abstract: Determination of the ionic and electronic contributions to the total conductivity in mixed ionic–electronic conductors (MIEC) is central to understanding their properties, particularly in nanostructured ionic solids. The Hebb–Wagner blocking technique, commonly used to deconvolute ionic and electronic contributions in bulk MIECs, is susceptible to misinterpretation when applied to thin films. In this work, microfabricated electronic blocking electrodes consisting of porous Pt on dense thin yttria-stabilized zirconia (YSZ) films were applied to nanocrystalline CeO2 thin films. The validity of the blocking structure was expressly considered with respect to alternate current and gas phase reaction pathways, with criteria developed to aid in identifying spurious effects. The ionic partial conductivity in nanocrystalline CeO2 thin films was confirmed to be pO2-independent while the electronic partial conductivity was found to be pO2 dependent with a power dependence of −0.31±0.02. These results are compared with theoretical predictions of extrinsically-compensated ceria and previous results on bulk nanocrystalline ceria. [Copyright &y& Elsevier]

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 The effects of lanthanum doping on the transport properties and oxygen exchange kinetics of La x (Ce 0.9 Pr 0.1) 1−x O 2−δ (x = 0, 0.1 and 0.2) have been investigated by electrical conductivity on bulk samples and optical relaxation on thin films respectively. Lanthanum substitution was observed to increase the magnitude of ionic conductivity to values comparable to that of single substituted La x Ce 1−x O 2−δ , with a maximum for a La content of x = 0.1, reaching σ = 9.5 × 10−3 S·cm−1 at 600 °C and a decrease in activation energy from ≈1 eV for Ce 0.9 Pr 0.1 O 2−δ down to ≈0.75 eV for the lanthanum substituted compositions. At the same time, a significant electronic contribution to the total conductivity, estimated to be on the order of 10%, supports significant mixed ionic-electronic conductivity. Substitution with lanthanum increases the absolute values of the oxygen surface exchange coefficient from k chem = 6 × 10−7 cm·s−1 for Ce 0.9 Pr 0.1 O 2−δ to k chem = 1 × 10−6 cm·s−1 for La 0.2 (Ce 0.9 Pr 0.1) 0.8 O 2−δ at 550 °C. Moreover, one observes significant increases in activation energy from 0.7 eV for Ce 0.9 Pr 0.1 O 2−δ to ≈1 eV for La substituted compositions, suggesting a change in the rate-limiting step. Possible sources for the change in the rate-limiting step are suggested. The composition with the highest La content of x = 0.2, although not exhibiting the highest ionic conductivity, shows the highest surface exchange rates, is suggested to result from enhanced segregation of catalytically active lanthanum to the surface. Highlights • La doping in PCO increases ionic conductivity. • La doping in PCO pin praseodymium into its 4+ state. • Oxygen surface exchange coefficient is measured by optical transmission relaxation. • La doping enhances the oxygen surface exchange coefficient. • La doping changes the rate limiting step of the oxygen surface exchange. [ABSTRACT FROM AUTHOR]

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]

Abstract: The widely used electrical conductivity relaxation technique for extracting kinetic data such as diffusivity and surface reaction rates typically relies on monitoring relaxations following changes in pO2 rather than changes in temperature. In this study, we demonstrate the utility in examining conductivity relaxation upon rapid changes in temperature to aid in deconvoluting defect migration and formation energies. This is achieved by the use of a low thermal mass heater stick enabling rapid variations in sample temperature. We apply this technique to a specimen of Pr0.1Ce0.9O2−δ to determine the reduction (H r) and carrier mobility (H m,i) energies. We further illustrate the ability to extract activation energies, with high precision, over narrow temperature ranges, by sinusoidal variation in temperature. [Copyright &y& Elsevier]

Abstract: High temperature stable piezoelectric crystals open a wide range of new sensor applications due to the ability to monitor environmentally dependent mechanical properties of sensor films deposited onto piezoelectric resonators. An understanding of the defect chemistry of piezoelectric single crystals like langasite (La3Ga5SiO14) aids in establishing their application limits and enables the creation of monolithic structures by appropriate doping. Key defect chemical properties of langasite are outlined with respect to dopants which increase the electrical conductivity or lower the electromechanical losses. We focus here on electronic and ionic transport in heavily Sr-doped langasite since spatially localized doping can be used to create monolithic electrodes. The total electrical conductivity (impedance measurements), the transport of oxygen and dopants (diffusion runs using stable tracers and subsequent secondary ion mass spectrometry) as well as the electromechanical properties (network analysis) are correlated. Strontium doped areas exhibit an increased conductivity by about 4 orders of magnitude, largely governed by oxygen ions. For demonstration of the concept, langasite bulk acoustic wave resonators with Sr-doped monolithic electrodes are operated up to 800°C. The resonator quality factors are found to be sufficiently high to use such resonators as high sensitivity sensors. [Copyright &y& Elsevier]

Abstract: Praseodymium doped cerium oxide (Pr x Ce1− x O2− δ ; PCO) exhibits mixed ionic and electronic conductivity at relatively high oxygen partial pressures (e.g. air) and enhanced oxygen storage capacity resulting from the ready reduction/oxidation of tri- and tetra-valent Pr. A change in the redox state of Pr results in a color change of PCO, and so serves as a means of investigating the Pr oxidation state, oxygen non-stoichiometry, and position of the Pr energy level within the ceria band gap. The wavelength dependent optical absorptivity was examined as a function of the Pr content (x) and oxygen partial pressure. Based on an analysis of the optical absorption spectra, the Pr level was found to lie approximately 1.6eV below the ceria conduction band for x =0.1 and 0.2, consistent with a defect model derived from electrical and thermogravimetric measurements on PCO. [Copyright &y& Elsevier]