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absolute number densities of neutral atom and ion a t the ground states by using the absorption or fluorescence technique (1 0, 51, 52). Registry No. Ar, 7440-37-1; Mg, 7439-95-4. LITERATURE CITED (1) Demers, D. R.; Allemand, C. 0. Anal. Chem. 1981, 53, 1915. (2) Fassel. V. A.; Knlseley, R. N. Anal. Chem. 1974, 46, 1llOA. (3) Houk, R. S.; Fassel, V. A.; Flesch, G. D.; Svec, H. J.; Gray, A. L.; Taylor, C. E. (IO) Nojlri, Y.; Tanabe, K.; Uchida, H.; Haraguchl, H.; Fuwa, K.; Wlnefordner, J, D.The use o f aolkhtate %I NMR spectroscopy to analyze q N 4 powders b demonstrated for a series of commercial and research samples. I n particular, two dlfferent commercial powders contaln 20% and 30% amorphous Si3N4 content, as identtfied by the applicatlon of methods described herein.These same materlals had been analyzed by X-ray dmtaction as only crystalline SiaNp NYR spectroscopy is capable of dlstlngulohlng among dlfferent amorphous silicon specks normally found In preparatlons of S13N4 powders. These Include amorphous S13N4, sHlcon oxynltrldes, sllicates, and elemental slllcon. The measurement of concentration of these species Is made for correlatlon wHh properties of powder sinterability.Sinterable silicon nitride (Si3N4) has been intensely investigated since 1974 because of potential applications as a tough, refractory ceramic material (1). The production of reliable and cost-effective structural Si3N, ceramic by sintering of powders must begin with powders that have, among other qualities, an a-phase content in excess of 85% (2). Excessive @-Si3N4 in the powder interfers with microstructure changes that accompany sintering. On the other hand, some a-Si3N4 (amorphous) can aid densification of the final material (3).Oxygen and elemental silicon can also aid sintering, although concentrations in excess of 2% can deteriorate mechanical properties at elevated temperatures. Oxygen normally appears as amorphous silicon oxynitrides and silicates.In view of what is known about the effects of the powder's phase and purity on sinterability and fiial ceramic mechanical properties, it is necessary to have rapid, reliable methods for the determination of the crystallinity and purity of batches of Si3N4 powder before carrying out final product formation and sintering. Traditionally, X-ray powder diffraction has been used to determine the presence of @-Si3N4, silicon oxynitrides, silicates, and silicon in the a-Si,N4 powders. The powder diffraction technique fails, however, in the identification of amorphous species. All amorphous species contribute to the background diffraction signal whose intensity is difficult

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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 ͑EIS͒ 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 plots͒ 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.Solid oxide fuel cells ͑SOFCs͒ are established as electrochemical devices for stationary electrical power generation. 1 Because of their high power conversion efficiency and low exhaust emissions, 2,3 these energy conversion devices have also been suggested for use in automotive auxiliary power units ͑APUs͒ to meet the increasing demands for on-board electrical power. Futhermore, recent laboratory-based demonstrations have given evidence of direct oxidation of hydrocarbon fuels 4,5 at the SOFC anode, without carbon deposition, at sufficiently low temperatures. Consequently, the likelihood of developing a compact, reformerless APU that can operate on the same fuel as the internal combustion engine now seems to be more realistic.The development of practical SOFCs for automotive use can benefit from fundamental knowledge about ac impedance related interfacial processes and associated microstructures. Since a SOFC is a complicated electrochemical system consisting of heterogeneous phases of the electrodes 6,7 and the electrolyte, 8 improvements in cell performance and manufacturing technology can be better focused if there is a means to separate the individual impedance-related processes within a single cell. These processes include the transport of ions and electrons through ͑and reacting at͒ the interphase boundaries. Each of these processes may depend upon temperature.Therefore, the main objective of the present work is to estimate the influence of temperature ͑580-888°C͒ on the separate component impedances in fast-thermal-response cells of potential automotive interest. ApproachWe adopt and extend an approach that does not require any a priori assumptions of an equivalent network model to rep...