Determination of sodium in molten aluminum and aluminum alloys using a beta alumina probe
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SODIUM beta alumina has the formula NaeO •xA1203, w h e r e x is approximately 11. The material has a hexagonal l a y e r structure with the sodium ions exclusively held in planes which are 11.23A apart. Between these planes are four layers of c l o s e packed oxygen layers with the aluminum ions in octahedral and tetrahedral positions) In 1943, Yamaguchi2 reported a new compound in the sodium oxide-alumina system with a c axis 1.5 t i m e s as l a r g e as in beta alumina. The structure of this compound,/3", which has the approximate formula Na20 • 5A12Os, has been well defined3'4 and contains t h r e e spinel blocks as opposed t o two in ordinary beta alumina. Due to the crystal structure of these compounds the sodium ions do not occupy all the available s i t e s and, a l s o , owing to the open nature of the sodium containing plane, t h e r e is considerable freedom for motion. Many studies 5'6 have shown that the high conductivity of the sodium beta aluminas is totally ionic and this m a k e s it i d e a l for determining sodium activities in m e t a l alloys u s i n g the emf technique. For example, a cell can be set up: Stainless Steel, Na(l)/~-A120,(s)/Na-X(l), Stainless Steel. The potential generated by the cell is related t o the activity by Eq. [1] /~Na - ~t~a =RT
lna=-ZEF ao
[1]
w h e r e #~qa and /ZNa are the chemical potentials of pure sodium and sodium in the alloy Na-X, ao and a are the activities of pure sodium and sodium in the alloy, Z is the c h a r g e number (+1) of the cation, E is the emf and F is the Faraday constant. E a r l i e r experiments 7 had indicated that the operation of this type of cell was possible but the Kovar g l a s s seal between a beta alumina pellet and an alpha alumina tube posed s e v e r e problems. More r e c e n t works using a sodium beta alumina tube and molten sodium as a reference has shown that the activities obtained from this type of cell a r e in excellent agreement with other measurements indicating the suitability for sodium beta alumina cells for the determination of the thermodynamic properties of systems containing sodium.
D. J. FRAY is University Lecturer, Department o fMetallurgy and Materials Science, University of Cambridge, Cambridge CB2 3QZ, England. Manuscript submitted March 1 1 , 1976. METALLURGICAL
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DEVELOPMENT OF A SODIUM PROBE For the thermodynamic measurements, reported e a r l i e r ,8 the cell consisted of a tube of sodium beta alumina, containing molten sodium as a reference, placed in the alloy. Stainless s t e e l leads went from the sodium and the alloy t o a high impedance e l e c trometer (Keithley 610C) for measurement of the emf. In these experiments, the presence of liquid sodium did not pose much of a difficulty but in an industrial environment, it could be considered a hazard and, in some applications, where very low sodium contents need t o be detected, it would be disastrous to m e a s u r e the sodium content only to find that the tube had cracked contaminating the metal. For many years, oxygen probes hav
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