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Membrane Potentials Across Macor in Mixtures of Molten Bromides

Potentials generated across a Macor membrane in molten NaBr-KBr mixtures were measured. The composition of a melt on one side of the membrane was fixed and the other allowed to vary systematically in the conducted cations. Measurements were POT made with mixtures of Na-Br-KBr and LiBr-KBr. The best and were calculated for the systems from the data points. The measured membrane potentials were plotted as a function of the activity of one of the ions in the indicator melt and compared with a curve of the theoretical potentials .


INTRODUCTION
Many ceramics conduct ionically in molten salt solutions at high temperatures. However most are not exclusive conductors of a single ion. For example mullite conducts Na+, K+, and Li+ ions (1). Such mixed conduction is believed to occur via a glassy phase in the ceramic.
When this type of ceramic forms a separator in an electrochemical cell a membrane potential is generated across the conductor. This potential is predicted by the equation: membrane. K^M is the potential selectivity coefficient which is a measure of the preference of the membrane for M + over N+ as a means of moving ionic current (2). For example, if the potential selectivity coefficient for a mixture of P O T NaBr-KBr, , is less than 1, a preference for Na+ conduction by the membrane is indicated. If the value is small enough the membrane becomes effectively an exclusive conductor of Na+.
The relationship between the value of potential selectivity coefficient and the composition of a membrane has been established for glasses in aqueous solutions (3). It may hold as well for ceramic membranes in molten salts. This investigation proposes to examine that relationship beginning with Macor. Macor is a machinable glass ceramic produced by Corning and containing approximately 60% crystalline and 40% glassy phases. It is approximately 47% Si0 2 , 8.5% B2O 3, 16.7% AI2O 3, 14.5% MgO, 9.5% K2O, and 6.3 fluoride by weight. This material was selected because it is commercially available and is replicated by other manufacturers with only slight modifications in composition. In addition glass ceramics have good resistance to high temperatures and have been shown in this work to resist degradation in molten salts. The indicator melt, the Macor tube containing the reference melt and the electrodes were sealed into the mullite chamber and the temperature of the system raised to 760°C. Equilibration of the temperature required approximately 1-2 hour. After this time the electrodes were lowered into the melts and readings were taken every minute until a stable potential was reached.
To insure that the membrane potential was equal to the measured cell potential the contribution to the potential from the electrodes was fixed at 0.00 volts. Both electrodes were the same silver wire and the mole fraction of AgBr was adjusted in the indicator melt to make the activity of AgBr in that melt the same as in the reference melt. Activities of AgBr in each indicator melt were determined using the equation (4), for a ternary solution where x indicates AgBr and 1 and 2 the alkali metal bromide components of the mixture, yx is the activity coeficient of AgBr, and k is the interaction coefficient of the pair of components indicated by the subscript. Activity coefficients for each system of alkali metal bromides were calculated with the same equation using appropriate interaction coefficients (5-9).

RESULTS AND DISCUSSION
The Macor membrane resisted degradation in the molten salts very well. It lasted, on average several weeks longer than did mullite in earlier experiments. The resistance of the Macor membrane to degradation may be explained by its microstructure and the nature of failure in such membranes. Earlier work with a mullite membrane indicated that failure usually occurs through crack propagation begun through a corrosion process. In the Macor membrane the mica crystalline phase, which makes the ceramic machinable with ordinary tool steel, inhibits crack growth between experimental data and theoretical but the agreement is still not as good as that found for the Na-K system. Since the measured cell potentials were reproducible, and since a very similar problem was observed with the mullite membrane in the Li-K system the error may be due to factors other than experimental procedure. The interaction coefficients and calculated activity coefficients indicate that this system is much more removed from the ideal than is the Na-K system. Any error in the interaction coefficients would lead to a just such systematic error as appears in the experimental data.

P O T
In this system the Ky..K indicates the membrane shows a preference for P O T P O T the conduction of Li+ over K+ this preference being g re a te r, Ky.K < K^^, than that for Na+ over K+. Overall the mullite conducted Na+ to a greater extent than Li+ and Li+ to a greater extent than K+. In the Macor system, even without the results from the third system, the likely preferred order of conduction will be Li+ > Na+>K+. It seems reasonable to assume that the difference in transport numbers is due to the difference in the composition of the conducting phases. Additional work is planned with different glass ceramics to examine this idea further.