Allan J. Jacobson

Rapid oxygen ion diffusion and surface exchange kinetics in PrBaCo2O5+x with a perovskite related structure and ordered A cations

As part of an investigation of new cathode materials for intermediate temperature solid oxide fuel cells, we have investigated particular perovskite oxides with ordered A cations which, in turn, localize the oxygen vacancies into layers. The oxygen exchange kinetics of polycrystalline samples of the oxygen-deficient double perovskite PrBaCo2O5+x (PBCO) have been determined by electrical conductivity relaxation (ECR) and by oxygen-isotope exchange and depth profiling (IEDP). The ECR and IEDP measurements reveal that PBCO has high electronic conductivity and rapid oxygen ion diffusion and surface exchange kinetics. Both techniques demonstrate that the oxygen kinetics in this structure type are significantly faster than in corresponding disordered perovskites.

Oxygen permeation studies of SrCo0.8Fe0.2O3 − δ

Abstract Oxygen permeation fluxes through dense SrCo 0.80 Fe 0.20 O 3 − δ membranes have been measured in the temperature range 620–;920 °C under various oxygen partial pressure gradients. The permeation results are compared with the previous measurements. Below 770 °C, the apparent activation energy for the overall permeation is 22 ± 4 kcal/mol. Above 770 °C, a change in the activation energy is observed associated with an order-disorder transition of the oxygen atom vacancies. The permeation mechanism is discussed in relation to the phase diagram of SrCo 0.80 Fe 0.20 O 3 − δ . Based on permeation measurements using discs with different thicknesses, we propose that the surface exchange process is the rate-limiting step in the overall permeation mechanism.

Field-induced resistive switching in metal-oxide interfaces

We investigate the polarity-dependent field-induced resistive switching phenomenon driven by electric pulses in perovskite oxides. Our data show that the switching is a common occurrence restricted to an interfacial layer between a deposited metal electrode and the oxide. We determine through impedance spectroscopy that the interfacial layer is no thicker than 10nm and that the switch is accompanied by a small capacitance increase associated with charge accumulation. Based on interfacial I–V characterization and measurement of the temperature dependence of the resistance, we propose that a field-created crystalline defect mechanism, which is controllable for devices, drives the switch.

A thermogravimetric study of the phase diagram of strontium cobalt iron oxide, SrCo0.8Fe0.2O3 − δ

Abstract The phase diagram of the nonstoichiometric oxide SrCo 0.8 Fe 0.2 O 3 − δ has been determined as a function of temperature (550 °C ≤ T ≤ 900 °C) and oxygen partial pressure (0.015 atm ≤ pO 2 ≤ 1 atm) by thermogravimetric analysis. Two phases are observed, perovskite and brownmillerite, separated by a two phase region. Above 770 °C only the phase with the perovskite structure is stable. Over the range of conditions studied, the perovskite phase has maximum and minimum oxygen contents of 3 − δ = 2.65 (550 °C, pO 2 = 1 atm) and 3 − δ = 2.43 (850 °C, pO 2 = 5 × 10 −4 atm), respectively.

Impedance Studies of Oxygen Exchange on Dense Thin Film Electrodes of La0.5Sr0.5CoO3 − δ

Solid-state electrochemical cells with dense oriented thin film electrodes of La 0.5 Sr 0.5 CoO 3-δ (LSCO) were prepared on (100) surfaces of single-crystal disks of yttria-stabilized zirconia (YSZ) by the pulsed laser deposition technique. Oxygen exchange at the electrodes was studied with alternating current impedance spectroscopy under various temperature and oxygen partial pressure conditions. Three distinctive features were observed in the impedance spectra from high to low frequency corresponding to contributions from the ionic conduction of the YSZ electrolyte, ionic transfer at the LSCO/YSZ interface, and the oxygen exchange on the LSCO electrode surface. An equivalent circuit model of the electrode process is used to fit the impedance data. The time constant for the oxygen surface exchange was derived from the impedance simulation. The surface chemical exchange coefficients, K chem , were calculated from the time constants as a function of temperature and pO 2 . k chem is 7 X 10 -4 cm/s at T = 700°C and pO 2 = I atm. The activation energy at pO 2 = 1 atm is 1.1 eV. The interfacial conductivity data were also derived from the impedance simulations as a function of temperature and pO 2 . The activation energy for the interfacial transport at pO 2 = 1 atm is 1.6 eV.

Epitaxial growth of dielectric CaCu3Ti4O12 thin films on (001) LaAlO3 by pulsed laser deposition

High dielectric CaCu3Ti4O12 (CCTO) thin films were epitaxially grown on (001) LaAlO3 (LAO) substrates by pulsed laser deposition. Microstructural studies by x-ray diffraction, pole figure measurements, and transmission electron microscopy show that the as-grown films are good single crystalline quality with an interface relationship of (001)CCTO//(001)LAO and [100]CCTO//[100]LAO. Dielectric property measurements show that the films have an extremely high dielectric constant with value of 10 000 at 1 MHz at room temperature. It is interesting to note that the twinned substrate results in the formation of twinning or dislocations inside the CCTO film.

Oxygen exchange kinetics of epitaxial PrBaCo2O5+δ thin films

The oxygen exchange kinetics of thin films of the oxygen-deficient double perovskite PrBaCo2O5+δ (PBCO) have been determined by electrical conductivity relaxation (ECR) and by oxygen-isotope exchange and depth profiling (IEDP). Microstructural studies indicate that the PBCO films, prepared by pulsed laser deposition, have excellent single-crystal quality and epitaxial nature. The ECR and IEDP measurements reveal that the PBCO films have high electronic conductivity and rapid surface exchange kinetics, although the ECR data indicate the presence of two distinct kinetic pathways. The rapid surface kinetics compared with those of other perovskites suggest the application of PBCO as a cathode material in intermediate-temperature solid oxide fuel cells.

Oxygen permeation in dense SrCo0.8Fe0.2O3-δ membranes: Surface exchange kinetics versus bulk diffusion

Abstract Oxygen permeation measurements were made on dense discs of SrCo 0.8 Fe 0.2 O 3 − δ as a function of pressure gradient, 0.01 to 1 atm, temperature, 775 to 870 °C and disc thickness, 1 to 2.6 mm. Two different measuring configurations were used: with the pressure gradient applied parallel to the disc axis and with the pressure gradient applied between the edge and the two flat surfaces of the disc. This enabled us to take into account leakage effects due to the edge. A model is proposed for the dependence of the surface exchange current on the chemical potential drop at the gas-solid interface. Analysis of the pressure gradient and membrane thickness dependence of the permeation shows that the permeation is controlled by both bulk diffusion of oxide ions as well as exchange kinetics between gas-phase oxygen molecules and oxide ions. Values for the surface exchange rate coefficient k i 0 and the ambipolar diffusion coefficient D a are reported.

Oxygen Surface Exchange in Mixed Ionic Electronic Conductors: Application to La0.5Sr0.5Fe0.8Ga0.2 O 3 − δ

We propose an oxygen surface exchange model in which the effect of vacancies at the gas-mixed ionic electronic conductor interface are included and apply the model to isotope exchange, oxygen permeability, and electrical conductivity relaxation. We deduce relationships between the surface-exchange coefficients associated with these phenomena and extend the treatment of the conductivity relaxation to large changes in oxygen partial pressure, where the commonly used assumption of first order reaction rate breaks down We apply the model to interpret the permeation and electrical conductivity relaxation measurements in La 0.5 Sr 0.5 Fe 0.8 Ga 0.2 O 3-δ . Transport in the material is almost completely surface limited, and data were interpreted in terms of a single surface-exchange coefficient.

Thermally robust and porous noncovalent organic framework with high affinity for fluorocarbons and CFCs

Metal-organic and covalent organic frameworks are porous materials characterized by outstanding thermal stability, high porosities and modular synthesis. Their repeating structures offer a great degree of control over pore sizes, dimensions and surface properties. Similarly precise engineering at the nanoscale is difficult to achieve with discrete molecules, since they rarely crystallize as porous structures. Here we report a small organic molecule that organizes into a noncovalent organic framework with large empty pores. This structure is held together by a combination of [N-H···N] hydrogen bonds between the terminal pyrazole rings and [π···π] stacking between the electron-rich pyrazoles and electron-poor tetrafluorobenzenes. Such a synergistic arrangement makes this structure stable to at least 250 °C and porous, with an accessible surface area of 1,159 m(2) g(-1). Crystals of this framework adsorb hydrocarbons, CFCs and fluorocarbons-the latter two being ozone-depleting substances and potent greenhouse species-with weight capacities of up to 75%.