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The in-situ determination of small mass changes of thin films became feasible with the availability of high temperature stable microbalances. With this technique, changes of the mechanical properties of thin films deposited on piezoelectric resonators are investigated at temperatures above 500 °C by monitoring the resonance behavior of the resonators. The results are valuable for fundamental understanding of the ionic and electronic transport processes in ceramic materials and for applications such as high temperature gas sensors.
This work correlates the electrical and the mechanical properties of TiO2-x at different oxygen partial pressures. TiO2-x films are deposited onto high temperature resonators by laser ablation and characterized by the high temperature microbalance technique as well as electrical impedance spectroscopy at 600 °C.
The oxygen partial pressure dependent resonance behavior cannot be attributed solely to mass changes of the TiO2-x film. Changes of the film's mechanical stiffness have to be taken into consideration to explain the resonance behavior. The simultaneous electrical impedance measurements indicate a n-type conduction behavior of the TiO2-x films.
SOFC anodes have to combine various tasks. In anode supported single cells a thick anode substrate is used for current collecting and gas distribution whereas a thin functional layer adjacent to the electrolyte is the electrochemically active part of the anode. This functional anode layer is cofired together with the thin film electrolyte to obtain an enhanced interface with low polarisation losses. This multilayer structure was transferred to an electrolyte supported single cell. The electrochemical active Ni/8YSZ anode layer was screen printed onto a 8YSZ electrolyte green tape and subsequently cofired at 1350 °. Mechanical stresses during cofiring due to shrinkage mismatch of anode and electrolyte were avoided by changing the geometry of the anode layer from a continuous layer to a large number of small sized individual areas. Simulations by finite element modeling indicated that a hexagonal pattern similar to honeycombs is preferable. The second layer which adjoins to the fuel gas channels and which is responsible for current collecting and gas distribution was later on screen printed on top and sintered together with the cathode. Single cells with a multilayer anode and different functional layers were electrochemically characterised under realistic operation conditions. The performance and reduction/oxidation stability of this type of anode was investigated. The electrochemically active layer showed only small degradation during redox cycling and long term operation at high fuel utilisation. In contradiction to single layer anodes Nickel agglomeration was not observed in the functional layer.
In this work, composite polymer electrolytes based on a thermoplastic polyurethane/LiClO4 amorphous system and on bare and functionalized zirconia nanoparticles as a filler are reported. The ceramic nanoparticles were synthesized via the sol-gel route using zirconium butoxide as the precursor for zirconium oxide nanoclusters and methacrylic acid as an organic modifier group. The salt concentration in the polymer phase was 17 wt% and fillers were added in the range between 2 and 10wt%. Scanning electron microscopy (SEM) was used to characterize the average size and the homogeneity of the nanoparticles in the polymer matrix, while impedance spectroscopy (IS) was used to evaluate the ionic conductivity of the composites. The addition of zirconia fillers results in an increase in ionic conductivity for all filled systems. The results also show that the functionalization of the zirconia nanoparticles promotes a significant increase in conductivity, suggesting that the interaction of the metracrylate-functionalized fillers with the polyurethane matrix was greatly improved. These results raise interest in the study of organically modified ceramic clusters as fillers for electrolyte polymers.
The layered compounds of formula LiNi0.4Mn0.6-yCoyO2, for y=0.2, 0.3, and 0.4 and LiNi0.7-yMn0.3CoyO2 for y=0.3, and 0.1 were synthesized at 800°C. X-ray powder diffraction indicates layered structure of R 3m symmetry similar to a-NaFeO2. Rietveld refinement data shows that Mn and Ni increase the tendency of transition metal ions to migrate into the interlayer sites relative to LiCoO2. Both magnetic susceptibility and XPS data support a 2+ oxidation state for Ni and 4+ and 3+ for Mn and Co, respectively. The layered compound LiNi0.4Mn0.4Co0.2O2 shows a high initial capacity of about 200mAh/g when cycled between 2.5V and 4.3 V at 20°C.
New emissions regulations will increase the need for compact, inexpensive sensors for monitoring and control of automotive exhaust gas pollutants. Species of interest include hydrocarbons, carbon monoxide, and oxides of nitrogen (NOx). The current work is directed towards the development of fast, high sensitivity electrochemical NOx sensors for automotive diesel applications. We have investigated potentiometric NO sensors with good sensitivity and fast response when operated in 10% O2. The sensors consist of yttria-stabilized zirconia substrates attached with NiCr2O4 sensing electrodes and Pt reference electrodes. A composite NiCr2O4:Rh sensing electrode is shown to give significantly faster response than NiCr2O4 alone. The exact role of the Rh in enhancing the response speed is not clear at present. However, the Rh appears to accumulate at the contacts between the NiCr2O4 particles and may enhance the inter-particle electronic conduction. Ongoing testing of these sensors is being performed to elucidate the sensing mechanisms and to quantify cross sensitivity to, for example, NO2.
Metallic interconnects in the solid oxide fuel cell (SOFC) are oxidized on the cathode side by air and carburized on the anode side by natural gas. Metallic alloys can be attacked by metal dusting corrosion in carbonaceous gases of high carbon activity in the temperature range of 350–1000°C. Under these conditions, pits form on the alloy surface and can become large holes through the alloy plate, with subsequent disintegration into a powdery mixture composed of carbon, fine particles of metal, and carbide. Fe and Ni-base alloys were tested in carbonaceous gases around the SOFC operating temperature. It was found that the oxide scales on the alloy surface prevent metal dusting corrosion. If the major phase in the oxide scale is chromic oxide, the alloys have good resistance to metal dusting corrosion. However, the alloys are easily attacked if the major phase is spinel.
In this study the electrical properties of thin films of Ysubstituted zirconia were investigated. The films were prepared using a polymer precursor technique and investigated in the temperature region 250 to 900°C. It was shown, that impedance spectroscopy (IS) and direct current (DC) conductivity measurements results are in good agreement for the films measured in plane for temperatures greater than 400°C. Due to the high resistance resulting from a planar geometry, the DC measurements were found preferable at temperatures <600°C.
Since in planar geometry the films represent a high resistance to the measurement circuit, it is important to minimize sources of electrical leakage, so different sample holders and substrates were investigated. A sapphire substrate and sample holder design using separated alumina single bore tubing for each electrode provided the lowest electrical leakage.
The experimental results showed that electrical behavior of all of the films produced at low annealing temperatures (less than 400°C) was similar regardless of Y content. These films have relatively low conductivity and an activation energy of about 1.5eV. The influence of different Y content started to appear after annealing above 600°C.
The results of the film conductivity measurements were compared with those for the bulk samples of Y stabilized zirconia prepared from 200nm powder by tape casting. These samples were measured by IS in plane and through the tape. It was shown that electrical properties of bulk and thin film material were similar.
Apatite-type oxides, La10-x(Si/Ge)6O26+z, have been attracting significant interest recently due to their high oxide ion conductivities. Most of the work so far has focused on the Si based systems, since the Ge based systems suffer from problems attributed to Ge loss. In this paper we show that doping divalent cations on the La site or B on the Ge site helps to stabilise the hexagonal apatite lattice for these Ge based systems. These doped phases show high oxide ion conductivities, although results from extended sintering studies suggest that Ge loss is still a problem. In order to limit Ge loss, we have also examined Bi doping to lower the sintering temperature and preliminary results for the novel Bi containing apatite-type phases, La6Bi2M2Ge6O26 (M=Mg, Sr, Ba) and La8-xBi2Ge5GaO26+y, are also reported.
Highly active catalysts for high-temperature partial oxidation reactions have been synthesized based on a microemulsion-templated sol-gel synthesis. The catalysts were tested with the direct catalytic oxidation of methane to synthesis gas and showed excellent selectitivites towards syngas combined with very high activity and low ignition temperatures. Furthermore, a surprisingly high long term stability was observed at these high-temperature conditions of T > 900°C. The catalyst therefore seem very promising candidates for high-temperature partial oxidation and hydrogen production from hydrocarbon fuels.
The effect of space charge layers in polycrystalline cerium oxide was analyzed by comparing experimental results of grain size-dependent electrical conductivity with theoretical models. Modeling included the calculation of space charge segregation of acceptor ions and of the effective electrical conductivity of polycrystalline cerium oxide in both the macroscopic and mesoscopic range of grain sizes. It is shown that an L-3 power law for the electronic conductivity in the nm-regime is characteristic for the equilibrium space charge model and different from the scaling behavior of alternative models. The origin of space charge potential was investigated by numerical calculation of the electrical potential in a two-phase model. It was found, that a positive excess charge at grain boundaries of cerium oxide is caused by an enhanced oxygen deficiency at the grain boundary core. The influence of acceptor ion doping in the dilute limit and of non-equilibrium distribution of acceptor ions on electrical conductivity was also studied.
High purity H2 gas streams are increasingly important for a variety of applications including feed gases for fuel cells. The potential of hydrogen gas as primary energy source has generated considerable interest in hydrogen separation technologies. We have been investigating ion beam irradiation as a method to modify polymeric membranes to enhance both hydrogen permeability and permselectivities. Combined high permeabilities and permselectivities are required to give high recoveries of high purity hydrogen. Ion irradiation typically results in the formation of numerous crosslinks within the polymer matrix that should enable these materials to maintain selectivities at high temperatures and to resist chemical attack. Helium separations over a range of temperatures of irradiated polyimides were used as a model of the hydrogen system. Finally, the impact of irradiation conditions on gas separations in these materials will be addressed.
Processing-induced residual stresses in the component layers of Solid Oxide Fuel Cells (SOFC's) can lead to fracture or to cell curvature which impedes stack assembly. Reducing or eliminating residual stresses to improve the mechanical behavior of the cells becomes increasingly important as the area of the cells is increased to increase the power of the fuel cell stack. Residual stresses in SOFC's result primarily from differential thermal expansion and sintering shrinkage between the component layers, such as the electrolyte and the anode support in a planar cell. This work investigates the impact of anode composition on each of these factors, with the ultimate goal of designing a flat, large-area cell. A range of anode compositions is investigated to determine the effect of different additives on the sintering behavior, and on the thermal and mechanical properties. Dilatometry, sintering shrinkage, scanning electron microscopy, and reduction studies are performed to correlate the microstructure, thermomechanical behavior, and composition. The experimental results are used to select an anode composition that leads to low overall cell curvature and improved mechanical behavior with respect to standard SOFC anode cermet materials.
Ammonia formation in autothermal reforming process was studied in Nuvera's Modular Pressurized Flow Reactor facility. Experiments were conducted to study and compare different catalysts for their ammonia formation characteristics. Different hydrocarbon fuels were reformed and effects of fuel structure and operating conditions on ammonia formation were investigated. Reformate generated was analyzed for ammonia contamination by using FTIR spectroscopy.
Relationships between lattice parameters of manganese dioxides (γ/ε-MD) and their surface properties at the solid-aqueous solution interface were investigated. The studied series ranged from orthorhombic ramsdellite to tetragonal pyrolusite and encompassed disordered MD samples. The structural model used takes into account two structural defects which affect the orthorhombic network of ramsdellite: Pr (rate of pyrolusite intergrowth) and Tw (rate of microtwinning). Water adsorption isotherms showed that the cross sectional surface area of water molecules is linearly correlated to Pr: from 6.3 Å2 (Pr=0.2) to 13.1 Å2 (Pr=1). Titration of their surface charge evidenced a linear relationship between PZC and Pr starting from ramsdellite (Pr = 0, Tw = 0, PZC = 1) to pyrolusite (Pr = 1, Tw = 0, PZC = 7.3). γ-MD with intermediate values of Pr (0.2 to 0.45) have increasing PZC values. For similar Pr values (0.45), high Tw percentage (0.3 and 1) makes the PZC to increase. The experimental results are compared with data collected in the literature for dioxides of transition elements with tetragonal structure. Surface titration leads to the determination of electrochemically active surface area at alkaline pH.
New ionoconducting composite membranes to be used as an interface between the skin and the actual electrical instrumentation used to produce an electroencephalogram (EEG) have been developed. The gels are based on lithium salts and PMMA (polymethyl methacrylate) and have been doped with nanometric titanium oxide. The samples have been electrochemically characterized by means of impedance spectroscopy and their structure studied by ATR-FTIR and MAS NMR. Spectroscopic studies indicate interactions between the polymer and oxide dopant. The polymeric electrolytes allowed the registration of good electrophysiological cortical signals either spontaneous or stimulus-related.
The structural and chemical stabilities of layered Li1-xCoO2-δ, Li1-xNi0.85Co0.15O2-δ and Li1-xNi0.5Mn0.5O2-δ (0 ≤ (1-x) ≤ 1) cathodes have been investigated by chemically extracting lithium from the corresponding LiMO2 with the oxidizer NO2BF4 in acetonitrile medium. While Li1-xCoO2-δ and Li1-xNi0.85Co0.15O2-δ begin to form a P3-type and a new O3-type (designated as O3') phases, respectively, for (1-x) < 0.5 and (1-x) < 0.3, Li1-xNi0.5Mn0.5O2-δ maintains the initial O3-type structure without forming any second phase. Chemical analysis with a redox titration indicates that the Li1-xCoO2-δ, Li1-xNi0.85Co0.15O2-δ, and Li1-xNi0.5Mn0.5O2-δ systems begin to lose oxygen from the lattice, respectively, for (1-x) < 0.5, < 0.3 and < 0.4, which is accompanied by an onset of a decrease in the c parameter. The oxygen loss signals chemical instability and the trend in instability correlates with the charging voltage profiles of the cathodes.
Ruthenium sulfide samples were prepared by flowing pure hydrogen sulfide into an aqueous solution of ruthenium chloride followed by further sulfidation in hydrogen sulfide. The final products were characterized by X-ray diffraction and crystallite-sizes were estimated from line broadening. The specific surface areas of catalysts were measured using the multipoint BET method and compositions were determined by thermogravimetric analysis. Ruthenium sulfide loaded gas diffusion electrodes were fabricated by a spraying technique and their electrochemical behavior studied. The electrochemical oxidation of hydrogen was investigated in a three -electrode cell using a ruthenium sulfide loaded gas diffusion electrode as the working electrode with humidified hydrogen containing small amounts of carbon monoxide. Results on the activity and the effects of carbon monoxide with reference to a standard platinum electrode measured at the same conditions show that ruthenium sulfide has a lower activity for hydrogen oxidation but is not susceptible to CO poisoning.
The semiconductor properties of a praseodymium-cerium oxide solid solution with composition Pr0.15Ce0.85O2-x (PCO) were investigated by d.c. current-voltage and bias-dependent impedance measurements in aqueous solution. The solution data were compared with impedance values of dry cells in air. A Mott-Schottky analysis of the PCO-solution interface capacitance showed p-type semi conductivity, a flat-band potential Efb = (2.0 ± 0.1) V/NHE and an ionized acceptor density NA = 3 10 cm-3. Using these data, an electron hole mobility μh ∼ 10-5 cm2 V-1 s-1 was calculated pointing to a small polaron conduction mechanism with a hopping energy (Eh = 0.4 eV).
Current limiting electrochemical pumping cells (amperometric sensors) based on zirconia are commonly used for engine control applications. Fast resistive-type sensors adapted from semiconducting metal oxides are a promising alternative for future exhaust gas monitoring systems. Therefore among the interesting characteristics of the materials system Sr(Ti0.65Fe0.35)O3, including high sensitivity and temperature independence at high oxygen partial pressures (pO2 > 10-4 bar), a short response time (t90 = 30 ms) is obviously the most salient.
The latter is determined by the kinetics of the oxygen surface transfer and subsequent diffusion of oxygen vacancies V‥O. For thin samples and low temperatures the surface transfer is dominant, since bulk diffusion usually occurs very fast. The presented model is based on the frequency-domain analysis of amplitude and phase shift of the response signal obtained from a pO2 modulation in a fast kinetic measurement setup. This method allows both the measurement of response times in the sub-millisecond range as well as the distinction of the behaviour either controlled by volume diffusion or by surface transfer reaction in Sr(Ti0.65Fe0.35)O3 ceramics.
Atomic force microscopy was used to characterize the structural evolution of the V2O5(001) surface during the electrochemical cycling of lithium. With Li insertion, nanometer-scale pits develop at the V2O5(001) surface. The pits first appear as the composition of the crystal approaches Li0.0006V2O5. Pit nucleation and growth continue through further discharge, resulting in a micro-porous (001) surface morphology. During subsequent Li extraction, cracks develop along the V2O5 <010> axis. Surface regions in the vicinity of these cracks “swell” during ensuing lithiation reactions, suggesting that the cracks locally facilitate Li uptake.