The measurement of diffusion coefficients in simple oxides

The diffusion coefficients of cations in metal oxides are usually measured through the use of radioactive isotopes. Because of the friable nature of oxides it is exU emely difficult to use the sectioning technique employed for metal samples. The need for this can be avoided by the application of radioisotopes which emit radiation having a well established absorption law in matter. Isotopes which emit y radiation are very useful when the cation has a relatively high diffusion coefficient because of the long-range peneU ation of y rays. The absorption law is 

For shorter penetration the use of p emitting isotopes is widely made applying much the same absorption law for p radiation as for y rays, with a minor modification. An alternative procedure used by Kingety and Paladino for the study of diffusion in AI2O3 employs two right cylinders, one only containing the radioactive Al isotopic species which emits relatively low energy ( ,nax = 0.511 MeV) p particles. 

If the total amount of radioactivity transfened from one cylinder to another is measured the solution of the diffusion equation is 

For extremely small penetrations a technique may be employed which depends upon the weak penetration of charged atoms with a known absorption law. The a-particles, which are He + ions, have a linear absorption 

The measurement of oxygen diffusion is usually made by the use of as die labelling isotope. If a gas containing an initial concentration C, of in O , and Co is the initial conceiiuation of in a right cylinder oxide sample of thickness 21, and a is the ratio of oxygen atoms in the original gas phase compared widi that in the solid, dieii after a time t, when the concentration in the gas phase is C/ 

This assumes that the gas-solid exchange kinetics at the interface is rapid. When this process affects the O18 exchange kinetics significantly then analysis of O18 concentrations layer by layer in the diffused sample is necessary. This can be done by the use of SIMS (secondary ion mass spectrometry) and the equation used by Kilner, Steele and co-workers for this diffusion study employs a surface exchange component. 

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