Gary M. Crosbie

Application of Fourier-Based Transforms to Impedance Spectra of Small-Diameter Tubular Solid Oxide Fuel Cells

Recent demonstrations of direct utilization of hydrocarbon fuels have stimulated an automotive interest in solid oxide fuel cells for reformerless auxiliary power units with high power density, high chemical-to-electrical efficiency, and low exhaust emissions. Furthermore, recent designs with small-diameter oxide tubes appear to be well-suited to accommodate repeated cycling under rapid changes in electrical load and in cell operating temperatures. To understand the limiting transient processes in these small-tube fuel cell designs, we applied an analysis approach which requires no a priori equivalent circuit model assumptions. This approach was applied to the electrochemical impedance spectroscopy (EIS) data measured from such cells in the temperature range from 585 to 888°C In this way, the complex, overlapping arc EIS details (seen in Cole-Cole plots) were transformed in a network-model-independent way into a spectrum of relaxation times. We extended the deconvolution method to allow peak fitting and integration to calculate the resistances of individual processes within the cathode polarization, which becomes limiting in comparison to either anode or electrolyte at temperatures below about 700°C. With the new results, the process with the highest apparent activation energy can be targeted to improve cathode development.

Transmission electron microscopy structure and platinum-like temperature coefficient of resistance in a ruthenate-based thick film resistor with copper oxide

As an alternative to thin-film platinum temperature sensor elements, thick film resistor ones are of interest for circuits which can withstand a near-engine environment. From a pyrochlore paste (DuPont 5091D), a close match is obtained (after firing) to the positive temperature coefficient of resistance (TCR) of Pt. Within the glassy matrix during 850 °C firing, needle-like RuO2 grains grow by a mechanism consistent with periodic bond chain theory. The acicular growth habit is attributed to a Cu2O additive, which is assumed to oxidize upon firing. The needles provide direct paths for metallic conduction and a characteristic positive TCR to the thick film in spite of having a low RuO2 volume fraction.

The effects of processing conditions on the resistivity and microstructure of ruthenate-based thick film resistors

Elevated temperature processing parameters affect the microstructure and electrical behaviour of thick film resistors on alumina substrates. Blended resistors (DuPont QS87 series) with a nominal sheet resistivity of 56 kΩ/□ and temperature coefficient (TCR) less than ±100 p.p.m K-1 were fired in a laboratory process that simulated production ramp rates and atmosphere.Resistances were measured in situ during firing in a three-factor, replicated experiment with two levels and centrepoints for peak temperature, firing time and probe current. Room temperature resistance values after firing show a strong correlation to temperature and time, which both increase resistance and flatten the R(T) curve around room temperature. In situ resistance during firing shows a weaker correlation, inverse with temperature because the thermally activated glass conduction has a greater share of the composite conduction at firing temperature.X-ray diffration (XRD) shows lead ruthenate, alumina, and zirconium silicate present in the resistors. The ruthenate lattice parameters increase with increasing firing temperature and time. Qualitative particle coarsening is observed with increasing firing temperature and time by transmission electron microscopy (TEM). Energy dispersive spectroscopy (EDX) shows lead ruthenate, CuBi ruthenate and zirconium silicate crystallites dispersed in a lead silicate glass matrix, without much particle chaining. Resistance changes are attributed to increased separation of ruthenate particles by coarsening.

Processing factor dependence of resistivity parameters of ruthenate-based thick film resistors with low temperature coefficients

Many analog sensor and control circuits take advantage of the low ( 95% confidence) from 1.56 to 1.15 meV with increasing firing temperature (from 845 to 855 °C) and time (from 8 to 11 min). Model-based estimation of parameters is a means to provid...

CANDIDATE MATERIALS FOR THE POSITIVE CURRENT COLLECTOR IN SODIUM-SULFUR CELLS - I. CERAMIC OXIDES

Abstract Chromium oxides doped with various metal oxides and strontium doped lanthanum chromite perovskites were prepared, physically characterized and tested for their suitability as coating materials for positive current collectors in sodium-sulfur cells. All of the materials studied are corrosion resistant. Perovskites show the highest conductivities, whereas only the chromium oxides doped with lower valence metal cations indicated acceptably low resistivities. However, upon exposure to the melt the resistivity of all materials studied increased in time. This has been shown to be caused by the loss of electron hole carriers during sample equilibration with the low oxygen partial pressure in the melt. The rate of the loss of conductivity is sufficiently slow to make the perovskites and chromium oxides doped with lithia or magnesia usable as coating materials for positive current collectors in sodium-sulfur cells.

Effects of Composition and Surface Finish of Silicon Nitride Tappet Inserts on Valvetrain Friction

Abstract In order to build more fuel efficient engines, new materials and lubricant formulations are being sought to reduce frictional losses. The valvetrain contributes about 6-10% of the total frictional losses in an engine. To reduce valvetrain frictional losses, polished silicon nitride tappet inserts were evaluated for their friction reduction potential. Silicon nitrides obtained from three sources were polished using three different processes: the suppliers conventional diamond polishing technique and two non-diamond polishing techniques-“Ford Finish” and chemo-mechanical polishing. The valvetrain friction torque was measured in a laboratory apparatus using a single cam lobe rotating against a direct acting mechanical bucket tappet with production engine hardware. The friction torque values obtained with surfaces prepared by all three processes differed significantly although their initial centerline average surface roughnesses were similar. All three silicon nitride surfaces prepared by “Ford Finish” showed lower friction torque than the production steel surface when an engine oil containing no friction modifier was used. With a low friction oil containing a friction reducing additive, friction torque values were significantly lower and polished silicon nitride surfaces did not offer additional friction reduction benefit relative to the production steel surface. A simple calculation showed a maximum of about 0.5 % fuel economy benefit can be gained due to polishing with the engine oil without friction modifier but very little, if any, with low friction oil. However, silicon nitride inserts may be useful for weight reduction and increased durability.

Liquid Phase Reaction Processes

Liquid phase processing routes hold considerable promise to meet the growing demands for more reliable advanced non-oxide ceramics. These routes to new products bring the advantages that are the norm for the present, large scale chemical process industry (CPI). Indeed, most of the examples given in a well-known survey of industrial processes (Lowenheim and Moran, 1975) describe reactions with a liquid phase present at the reaction site. Only a few of these are examples with solids as the product, rather than another liquid.

Processing, X-Ray, and TEM Studies of QS87 Series 56 KΩ/Square Thick Film Resistors

Thick film resistors are glass/metal oxide nanocomposites used in hybrid microcircuits. These components have a small temperature coefficient of resistance that is useful in systems that experience a wide range of service temperatures. Test samples were produced by printing, drying, and firing resistor pastes in a laboratory process that simulated production conditions. The process parameters of peak firing temperature, time at peak temperature, and probe current were factors in a 2 3 factorial experiment that measured in-situ resistance (resistance during processing), as-fired resistance, and the temperature coefficients of resistance. As-fired resistance is shown to increase with firing time and temperature. In-situ resistance exhibited a small decrease with increasing firing temperature due to thermally-activated glass conduction at firing temperatures. The temperature coefficient of resistance measurements show that R[T] curve flattens with increasing firing time and temperature. X-ray diffraction revealed Pb-ruthenate, alumina, and Zr-silicate phases to be dispersed in the glass. Transmission electron microscopy in conjunction with energy dispersive x-ray spectroscopy revealed that the conductive phases, Pb- and CuBi-ruthenate particles, increased in size with increasing firing time and temperature. Lattice parameter measurements revealed only a small increase in the ruthenate structure. Resistance changes are attributed to increased separation of the conductive ruthenate particles by coarsening.

Lowered Diffusivity in TiO 2 with a Nanophase Dispersion of SiO 2

In materials that are subject to environmental reactions, one way to increase durability is to decrease the chemical interdiffusion rate. In metal systems, mass transport by diffusion is enhanced – if any change – with finer grained microstructures, because of the greater number of high diffusion interface and boundary paths. In non-metals, increased transport is not necessarily the case, as other mechanisms may control the overall diffusivity. C. Wagner [1] showed the theoretical basis of transport in certain nanocomposite cases based on space charge layers at interfaces. In an oxide-in-oxide nanocomposite, one can use the tools of bulk analysis to study the near-interface effects on transport. The immiscible pair, TiO 2 -SiO 2 , was used as a model system. Nanophase powders (50 and 200 m 2 /g, respectively) from flame hydrolysis were dispersed with high shear, then freeze-dried and hot-pressed to near theoretical density. Transmission electron microscopy was used to show 2 diffusivity for bars of about 10 times different square cross-section areas confirmed the interpretation of the conductance rate changes to be the result of bulk interdiffusion. The interdiffusion rate in the nanoscale dispersoid composites was more than 5 times lower than for same SiO 2 -fraction composites after coarsening which were like TiO 2 with no second-phase. Results are in qualitative agreement with models for impurity segregation to interfaces and for space charge layers near the dispersed nanoparticles.