Parametric integral equations for the steady-state terminal voltage and external current of a solid electrolyte energy conversion device are derived under the assumption that the anode and cathode chemical potentials remain invariant. Plots of terminal voltage, power delivered, and efficiency vs. ex-* Electrochemical Society Student Member. ** Electrochemical Society Active Member.

Steady‐state chemical potential profiles in a solid electrolyte under open‐ circuit and ion‐blocking conditions, may be obtained by balancing the steady‐ state flux at points inside the electrolyte. Steady‐state oxygen chemical potential profiles inside a scaling layer of thoria and inside yttria‐doped thoria specimens under open‐circuit and ion‐blocking (d‐c polarization) conditions are illustrated in special cases.

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phase at 1400°C and that their strength differences reflect the volume content of the liquid phase. Although eutectic temperatures were not measured during this study, observations made during densification indicate that the lowest eutectic temperature may lie close to the Si,N,-Mg,SiO, tie line and that it may be as low as 1300°C (i.e. rapid densification could be achieved at temperatures as low as 1300°C for compositions on the Mg,SiO, side of the Si,N4-Mg,Si04 tie line; liquidus temperature of these same compositions was -1 500°C). Therefore, compositions approaching the Si,N4-Mg,Si04 tie line will contain a larger volume fraction of liquid and thus have a lower strength relative to other compositions.This idea is consistent with the observations reported in Fig. 2 and suggests the need for determining eutectic temperatures in this and other Si,N4 systems. (3) OxidationAs pointed outby Langeetal. 16forthe system Si,lN,-SiO,-Y,O,, the oxidation resistance of hot-pressed Si,N, must be viewed in terms of the compatibility of SiO, and Si2N20 (the oxidation products of Si:,N4) with the secondary phases. For oxidation, the most desirable secondary phase would be the binary compound closest to SiO, in the Si0,-metal oxide system. Such a compound would not react with SiOB until the eutectic temperature between these two phases is reached.MgSiO, is the silicate closest to SiO, in the system Si0,-MgO. Since the Si,,N,-MgSiO, tie line was not observed (Fig. l), MgSiO, is excluded as an equilibrium secondary phase in Si,N, materials hot-pressed with the aid of MgO. The oxidation product of Si,N4, e.g. SiO,, can react in the solid state with the secondary phases, viz. Mg,SiO, and MgO, by Si:,N, + 30,-3Si0, + 2N,SiO, + Mg2Si0,-+2MgSi0,and/or to eventually form polyphase SiO, and MgSiO, surface scales by solid state diffusion. Thus, although other factors must be considered, e.g. reaction kinetics of Eq. (l), (21, and (3), low eutectic temperatures in the system Si,N,-SO,-MgO and the ability of the SiO,+ MgSiO:, scale to act as a diffusional barrier for oxygen, the noncompatibility of SiO, with the secondary phase($ must be considered as an important driving force to decrease the oxidation resistance of Si3N4 densified with the aid of MgO by promoting the diffusivity of oxygen by solid-state reaction. (4) Optimization of Strength and Oxidation ResistanceThe data presented in Figs. 2 and 4 show that optimum strength and oxidation resistance can be obtained with compositions fabricated with high mole fractions of Si,N, (20.91) and an MgO/SiO, molar ratio between 0.25 and 0.50. Material with a better oxidation resistance, but a slightly lower strength, can be fabricated with an MgO/SiO, molar ratio of -0.17. It should be noted that, when such small amounts of MgO are used for densification, knowledge of the SiO, content in the Si3N4 powder and its loss or gain during fabrication are keys to the control of composition and, thus, properties.

Open-circuit emf measurements have been made to demonstrate that a two-phase, polycrystalline mixture of /3-alumina and a-alumina could be used as a solid electrolyte in galvanic cells ,. with reversible' electrodes fixing oxygen or aluminum chemical potentials.-These measurements indicate that such a two-phase solid electrolyte can be used to monitor oxygen chemical 4? 2 potentials as low as that corresponding to Al, AUO, coexistence (10 N/m 47 10 atm at 1000 K). The activity of by emf measurements.