Chronopotentiometric methods showed that, in melts for which the equivalent fraction of AlF3 was 0.136, the diffusivity at temperatures of between 1128 and 1273K could be described by the expression:

D (m2/s) = exp[-3980/T - 15.904]

In melts for which the equivalent fraction of AlF3 was 0.25, the diffusivity could be described by the expression:

D (m2/s) = exp[-5532/T - 14.609]

The Fick's law values of the diffusion coefficient were very high, and were of the order of 10-7m2/s at a temperature of 1270K and an equivalent fraction of 0.685. This was attributed to the existence of a strong thermodynamic driving force in the system. Suitable modification of Fick's law, to take account of the density changes caused by electrolysis, gave thermodynamic diffusivities having more usual values (table 198).

E.W.Dewing: Journal of the Electrochemical Society, 1984, 131[5], 1073-8

Table 198

Interdiffusion Data for LiF-AlF3 Melts

AlF3 (equivalent fraction) | Temperature (C) | D (m2/s) |

0.500 | 860 | 8.4 x 10-10 |

0.500 | 900 | 1.2 x 10-9 |

0.500 | 950 | 1.7 x 10-9 |

0.500 | 1000 | 2.2 x 10-9 |

0.500 | 1020 | 1.6 x 10-9 |

0.562 | 860 | 7.2 x 10-10 |

0.562 | 900 | 1.3 x 10-9 |

0.562 | 950 | 1.7 x 10-9 |

0.562 | 1000 | 4.2 x 10-9 |

0.562 | 1020 | 2.7 x 10-9 |

0.618 | 860 | 5.1 x 10-10 |

0.618 | 900 | 1.2 x 10-9 |

0.618 | 950 | 5.1 x 10-10 |

0.685 | 860 | 1.0 x 10-9 |

0.685 | 950 | 1.3 x 10-9 |

0.685 | 1000 | 2.6 x 10-9 |

0.685 | 1020 | 2.3 x 10-9 |