Doris Sebold

Synthesis and Characterization of Nonsubstituted and Substituted Proton-Conducting La6–xWO12–y

Mixed proton-electron conductors (MPEC) can be used as gas separation membranes to extract hydrogen from a gas stream, for example, in a power plant. From the different MPEC, the ceramic material lanthanum tungstate presents an important mixed protonic-electronic conductivity. Lanthanum tungstate La(6-x)WO(12-y) (with y = 1.5x + δ and x = 0.5-0.8) compounds were prepared with La/W ratios between 4.8 and 6.0 and sintered at temperatures between 1300 and 1500 °C in order to study the dependence of the single-phase formation region on the La/W ratio and temperature. Furthermore, compounds substituted in the La or W position were prepared. Ce, Nd, Tb, and Y were used for partial substitution at the La site, while Ir, Re, and Mo were applied for W substitution. All substituents were applied in different concentrations. The electrical conductivity of nonsubstituted La(6-x)WO(12-y) and for all substituted La(6-x)WO(12-y) compounds was measured in the temperature range of 400-900 °C in wet (2.5% H2O) and dry mixtures of 4% H2 in Ar. The greatest improvement in the electrical characteristics was found in the case of 20 mol % substitution with both Re and Mo. After treatment in 100% H2 at 800 °C, the compounds remained unchanged as confirmed with XRD, Raman, and SEM.

Improving Atmospheric Plasma Spraying of Zirconate Thermal Barrier Coatings Based on Particle Diagnostics

Lanthanum zirconate (La2Zr2O7) has been proposed as a promising material for thermal barrier coatings. During atmospheric plasma spraying (APS) of La2Zr2O7 a considerable amount of La2O3 can evaporate in the plasma flame, resulting in a non-stoichiometric coating. As indicated in the phase diagram of the La2O3-ZrO2 system, in the composition range of pyrochlore structure, the stoichiometric La2Zr2O7 has the highest melting point and other compositions are eutectic. APS experiments were performed with a TriplexPro™-200 plasma torch at different power levels to achieve different degrees of evaporation and thus stoichiometry. For comparison, some investigations on gadolinium zirconate (Gd2Zr2O7) were included, which is less prone to evaporation and formation of non-stoichiometry. Particle temperature distributions were measured by the DPV-2000 diagnostic system. In these distributions, characteristic peaks were detected at specific torch input powers indicating evaporation and solidification processes. Based on this, process parameters can be defined to provide stoichiometric coatings that show good thermal cycling performance.

Deposition of La1−xSrxFe1−yCoyO3−δ Coatings with Different Phase Compositions and Microstructures by Low-Pressure Plasma Spraying-Thin Film (LPPS-TF) Processes

Perovskite-type materials with the general chemical formula A1−xA′xB1−yB′yO3−δ have received considerable attention as candidates for oxygen separation membranes. Preparation of La1−xSrxFe1−yCoyO3−δ (LSFC) coatings by low-pressure plasma spraying-thin film processes using different plasma spray parameters is reported and discussed. Deposition with Ar-He plasma leads to formation of coatings containing a mixture of cubic LSFC perovskite, SrLaFeO4, FeCo, and metal oxides. Coatings deposited at higher oxygen partial pressures by pumping oxygen into the vacuum chamber contain more than 85% perovskite and only a few percent Fe3−xCoxO4, and/or CoO. The microstructures of the investigated LSFC coatings depend sensitively on the oxygen partial pressure, the substrate temperature, the plasma jet velocities, and the deposition rate. Coatings deposited with Ar-rich plasma, relatively low net torch power, and with higher plasma jet velocities are most promising for applications as oxygen permeation membranes.

Three-Dimensional, Fibrous Lithium Iron Phosphate Structures Deposited by Magnetron Sputtering

Crystalline, three-dimensional (3D) structured lithium iron phosphate (LiFePO4) thin films with additional carbon are fabricated by a radio frequency (RF) magnetron-sputtering process in a single step. The 3D structured thin films are obtained at deposition temperatures of 600 °C and deposition times longer than 60 min by using a conventional sputtering setup. In contrast to glancing angle deposition (GLAD) techniques, no tilting of the substrate is required. Thin films are characterized by X-ray diffraction (XRD), Raman spectrospcopy, scanning electron microscopy (SEM), cyclic voltammetry (CV), and galvanostatic charging and discharging. The structured LiFePO4+C thin films consist of fibers that grow perpendicular to the substrate surface. The fibers have diameters up to 500 nm and crystallize in the desired olivine structure. The 3D structured thin films have superior electrochemical properties compared with dense two-dimensional (2D) LiFePO4 thin films and are, hence, very promising for application in 3D microbatteries.

Temperature and Bias Effects on Sputtered Ceria Diffusion Barriers for Solid Oxide Fuel Cells

The effects of both temperature and applied bias power during the sputtering of gadolinia-doped ceria (GDC) interlayers used as diffusion barriers in anode-supported solid oxide fuel cells (SOFCs) were studied. Scanning electron microscopy analysis revealed that increasing the applied bias power, in the 0-300 W range, increasingly promotes the deposition of continuous and dense interlayers. Such feature was mirrored in the electrochemical performance of single cells that exhibited ∼ 15% enhancement of the power density of an SOFC with bias-assisted sputtered interlayers. In addition, fuel cells having interlayers deposited in the 400-800°C temperature range exhibited similar microstructure and electrochemical performances, indicating that the applied bias allows for the sputtering of GDC protective interlayers at relatively lower temperatures than unbiased depositions. The presented results evidenced that bias-assisted sputtering is an effective technique for the fabrication of high performance anode-supported SOFCs.

Operation of Thin-Film Electrolyte Metal-Supported Solid Oxide Fuel Cells in Lightweight and Stationary Stacks: Material and Microstructural Aspects

In this study we report on the development and operational data of a metal-supported solid oxide fuel cell with a thin film electrolyte under varying conditions. The metal-ceramic structure was developed for a mobile auxiliary power unit and offers power densities of 1 W/cm2 at 800 °C, as well as robustness under mechanical, thermal and chemical stresses. A dense and thin yttria-doped zirconia layer was applied to a nanoporous nickel/zirconia anode using a scalable adapted gas-flow sputter process, which allowed the homogeneous coating of areas up to 100 cm2. The cell performance is presented for single cells and for stack operation, both in lightweight and stationary stack designs. The results from short-term operation indicate that this cell technology may be a very suitable alternative for mobile applications.

Bias-Assisted Sputtering of Gadolinia-Doped Ceria Interlayers for Solid Oxide Fuel Cells

The effects of both temperature and applied bias power during the sputtering of gadolinia-doped ceria (GDC) interlayers used as diffusion barriers in anode-supported solid oxide fuel cells (SOFC) were investigated. Scanning electron microscopy analysis revealed that increasing the applied bias power progressively inhibits the columnar structure typically observed in sputtered films, favoring the deposition of dense interlayers. Such feature was mirrored in the electrochemical tests of single cells that demonstrated enhanced power density for SOFC. The presented results evidenced that bias-assisted sputtering is an effective technique for the fabrication of high-performance anode supported SOFC. Diffusion barrier interlayers, deposited onto the electrolyte, have demonstrated to be an effective way to avoid undesired reactions and preventing the degradation of the SOFC. Ceria-based oxides have been reported as convenient materials for such barriers due to good transport properties and reasonable compatibility with both yttria-stabilized zirconia (YSZ) and LSCF cathode. One of the main challenges is the fabrication of cost effective and reproducible protective interlayers, at sufficiently low temperatures in order to avoid undesired reactions. The effectiveness of the diffusion barrier is closely related to its microstructural properties, and homogeneous interlayers with high density are advantageous. Previous studies, concerning deposition techniques, evidenced that single cells with ceria-based interlayers fabricated by reactive magnetron sputtering (RMS) exhibited enhanced performance when compared to wet ceramic depositions. Such an improved performance is possibly associated with the higher density of such layers. Although sputtering is a scalable industrial process, further investigation is required for the deposition of optimized SOFC protective interlayers. In the present study, the effects of applied bias voltage during sputtering of gadolinia-doped ceria (GDC) diffusion barriers over YSZ electrolytes were studied. Half cells comprised of Ni / YSZ anode support and functional layer, and YSZ electrolyte were used as substrates for the deposition of GDC interlayers. Coatings were carried out in a physical vapor deposition system CS 400ES (Von Ardenne Anlagentechnik). High-frequency bias voltage was applied to the metallic sample holder by controlling a fixed bias power, ranging from 0 to 300 W. Depositions were carried out at different temperatures in the 400-800°C range. Screen-printed LSCF cathodes were applied to the half cells, followed by heat treatment. The microstructure of deposited interlayers was studied by scanning electron microscopy (SEM). The electrochemical properties of single cells were investigated by I-Vcurves under H2/air flow in the 600-900 °C range. Microstructural analyses (Fig. 1) of GDC barriers revealed that increasing the applied bias power progressively inhibits the usual columnar structure of sputtered interlayers, resulting in dense microstructures. Such a feature was reflected in the I-V curves, which showed that the performance of solid oxide fuel cell having bias sputtered interlayers is increased in the whole temperature range studied, as shown in the Fig. 2. 650 700 750 800 850 0.8 1.2 1.6 2.0 2.4