DEVELOPMENT OF A NOVEL SIMS TECHNIQUE FOR OXYGEN SELF-DIFFUSION AND SURFACE EXCHANGE COEFFICIENT MEASUREMENTS IN OXIDES OF HIGH DIFFUSIVITY

The determination of D* and k provide key information about technologically important oxides in applications such as electrode and electrolyte materials for high-temperature electrochemical devices, e.g., the solid oxide fuel cell. A high oxygen self-diffusion coefficient, D*, of approximately 10(-6) cm2/s and surface exchange coefficient, k, of approximately 10(-4) cm/s are typical requirements for these applications. Measurement of D* and k may be performed by the isotopic exchange/diffusion profile technique with secondary ion mass spectrometry (SIMS) used to determine the O-18 stable isotope depth distribution. In the case of oxides of high diffusivity the penetration depth-at the chosen anneal temperature, approximately (D*t)0.5, is of the order of hundreds of micrometers from the surface into the bulk of the sample for the shortest practicable anneal times, t. SIMS depth profiles are generally limited to tens of micrometers due to various considerations including the time required for sputtering and roughening at the base of the SIMS crater. Thus the sputter depth profiling approach must be abandoned in favour of a new SIMS technique described in this paper. Crater base roughening is particularly severe for polycrystalline bulk samples which also have a high defect density. Results from polycrystalline cobaltite perovskite solid solutions and YBCO single crystals are used to demonstrate the technique and precautions required for its successful application.