Date of Award
Doctor of Philosophy (PhD)
Before utilizing the two phase mixture of β- and α-AI₂O₃ as solid electrolytes in oxygen probes that can monitor very low oxygen potentials, the chemical stability of β-AI₂O₃ (Na₂O, 11Al₂O₃) was studied by determining the sodium oxide activity in α-AI₂O₃, β-Al₂O₃ coexistence using electrochemical cells of the type
Pt⎮W(s), WS₂(s), Na₂S(s)⎮β- and α-Al₂O₃⎮M(s), MᵪO(s)⎮Pt
where M stands for Cu, Ni or Fe. The over-all cell reaction for these cells can be written as
W(s) + 2Na₂S(s) + MᵪO(s) = 2Na₂O(β) + WS₂(s) + xM(s)
By considering the following reaction
Na₂O (β) = 2Na(g) = ½O₂(g)
the measured values of Na₂O activity in α-Al₂O₃, β-Al₂O₃ coexistence were used to evaluate the variation of sodium vapor pressure over this coexistence, PNa(α-β), with temperature and oxygen pressure. The results indicate that PNa (α-β) reaches one atmospheric pressure at 1076°C in environments with AI, Al₂O₃(α) imposed-oxygen potentials, and at 1834°C in atmospheres with Fe,FeO imposed-oxygen potentials.
The activities of Na₂O in Al,Na,β-Al₂O₃ coexistence and β-Al₂O₃, β"-Al₂O₃ equilibria were also determined using the following cells
Ta⎮Al(s or ℓ), Na(ℓ)⎮β-Al₂O₃⎮NiO(s),Ni(s)⎮Pt⎮Ta,
Pt⎮W(s), WS₂(s), Na₂S(s)⎮β- and β"-Al₂O₃⎮Ni(s), NiO(s)⎮Pt,
respectively. The results obtained as well as the available thermodynamic data in the literature for the Na-AI-O system were used to partially represent the equilibrium oxygen pressure diagram of Na-Al-O system at 1000K.
To establish the reliability of using the two phase mixture of β- and α-Al₂O₃ as a solid electrolyte in oxygen probes, galvanic cells using solid electrolyte tubes fabricated from this mixture with electrodes of fixed oxygen chemical potentials were developed. Experimental results indicated that this solid electrolyte responded reversibly to oxygen potentials as high as those of Cu, Cu₂O equilibria between 600°C and 1000°C, and as low as those of Al-α-Al₂O₃, β-Al₂O₃ coexistence between 550°C and 800°C.
The thermodynamics parameters and phase compositions in the Ni-Al-O and Fe-Al-O systems at temperatures in the range 850°C-1150°C have been also investigated using electrochemical technique, electron microprobe analyses, and X-ray powder diffraction studies. Results showed that, about one ppm of Al in Ni or Fe is sufficient to stabilize α-AI₂O₃. Electrochemical cells with calcia stabilized zirconia as solid electrolyte and working electrodes of the type Ni (or Fe)-AI alloy Al₂O₃(α) had shown electrical instability which was not amenable to correction by coulometric disturbance or by temperature cycling. When β- and α-Al₂O₃ solid electrolyte was used in combination with Ni (or Fe)-AI,AI₂O₃(α) electrodes, steady and reproducible potentials were measured at 940°C which enabled the equilibrium oxygen pressures over these electrodes to be calculated. The values obtained for the oxygen pressures were outside the electrolytic domain of calcia stabilized zirconia which explain the instability problems of cells with this electrolyte.
Quantitative results have been obtained for the phase compositions in the ternary systems Ni-AI-O, and Fe-AI-O. These resuIts have been used to construct the 1000°C Ni-AI-O isotherm, and to modify and extend the NiO-Al₂O₃ quasibinary system given previously in the literature. The equilibrium oxygen pressure diagrams of the Ni-AI-O and Fe-AI-O systems have been constructed at 1000°C based on the results of this investigation as well as the available data in the literature.
Elrefaie, Fawzi Abdelkader Abdellatief, "Thermodynamic properties of Na-Al-O, Ni-Al-O, and Fe-Al-O systems" (1979). Open Access Dissertations and Theses. Paper 3253.