. .

As pointed out earlier, virtual ionic current equilibrium does exist across the iodide film for such a system, so the rate of iodide layer thickening will be controlled by the transport of positive holes, i.e., by the conductivity of positive holes (<Jh·). Then one can write from Eq. 5.67:

(5.68)

The above relation has been experimentally verified for the growth of AgCl, AgBr, and Agl films on silver [20] for film thicknesses exceeding certain values. The electrical field strength (E = VI~ = volt/m) in such cases is very weak and can be neglected. The expression (5.68) is obtained from the relation a = q1c1v" where q1 =charge (in coulomb) on the rate limiting species, v1 =charge mobility in m2 v-• s-1 which remains virutally constant at any temperature (T), and C1 = concentration of the rate-limiting species expressed in number per m3•

One can reexamine the Wagner's theoretical relation (5.63) for Ag-Agl-12(v) system. In this case, '• = t"'+' ==1, 12 = t1-:::.:0 (iodide sublattice remains almost stationary), and t1 = th·· Therefore, one may write, tb·<J = <Jh· = a~P:~ (at constant temperature), where, ab-= hole conductivity at P11 = 1 atm (1.01325 X 10' N. m-2) and Z2 = Zr• = unity.