SELF-DIFFUSION IN MOLTEN ALKALI HALIDES
A new method for the measurement of self-diffusion coefficients in molten salts is described. Determinations of D/sub i/ for i = Na/sup +/ and Cl/ sup -/ in NaCl; Rb/sup +/ and Cl/sup -/ in RbC l; Cs/sup +/ and Cl/sup -/ in CsCl; and Na/sup +/ and I/sup -/ in NaI as a function of temperature, are reported. The heats of activation for self. diffusion of cations and anions in the same salts are almost equal and in the range 4 to 8 kcal mole/sup -1/. The pre-exponential factors in D = A exp - E/RT are about 10/sup -3/ cm/sup 2/ sec/ sup -l/. The D values are about 10/sup 4/ greater than those in the corresponding solid at the melting point. The ratio of D/sub cat/D/sub an/.is about the same as in the solid at the mp. The results yield friction coefficients on the basis of an electrolyte consisting of two ions with no uncharged reference species; these show trends in which physical consistence is unclear. Sources of deviation from the Nernst-Einstein equation are summarized. If the principal cause of deviation is attributed to a non-charge transporting mode of transport, activation energies are obtained for chargecarrying transport which are consistent ( plus or minus 0.2 kcal mole/sup -1/) with those experimentally determined for electrical conductance; derived D are consistent with other tests of the approximation. The mean observed heat of activation is 3.5 RT/sub mp/, which is the theoretical value expected if the diffusive movements are jumps into thermally distributed holes. Ealculations of the pre- exponential factor are consistent with the coupled requirements of entropy of activation and activation volume. They support transport by nondiameter jumps into ion-sized holes. (auth)
- Research Organization:
- Univ. of Pennsylvania, Philadelphia
- NSA Number:
- NSA-17-004513
- OSTI ID:
- 4753863
- Journal Information:
- Discussions Faraday Soc., Vol. Vol: No. 32; Other Information: Orig. Receipt Date: 31-DEC-63
- Country of Publication:
- Country unknown/Code not available
- Language:
- English
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