Diffusion of ^110mAg Tracer in Polycrystalline and Single-Crystal Lead-Containing Piezoelectric Ceramics

We have conducted diffusion measurements of radioactive 110mAg tracer in single-crystal PbMgNbO3-PbTiO3 (PMN-PT) and in polycrystalline 50Pb(Ni1/3Nb2/3)O3·35PbTiO 3·15PbZrO3 (PNN-PT-PZ) piezoelectric ceramics. Both materials measured belong to the perovskite family. Diffusion in PMN-PT is characterized by an activation energy of 277 kJ/mol and pre-exponential factor of 0.0034 m2/s and compares well with cation diffusion in high-temperature superconducting YBa2Cu3O7-δ. Diffusion in polycrystalline PNN-PT-PZ, on the other hand, is many orders of magnitude faster and is attributed to grain boundaries. PNN-PT-PZ has a lower activation energy, 168 kJ/mol, and a combined pre-exponential factor (sδ(Db)o, where s is the segregation factor of silver, δ the thickness, and (Db)o the pre-exponential factor for grain boundaries) of 3.7 x 10-9 m3/s. The unusually large combined pre-exponential factor infers large segregation of silver at the grain boundaries and small solid solubility within the grains. It is possible, using a semiempirical model, to compute metal- (silver-) ceramic interface energies as a function of temperature, from which values of 90 kJ/mol and 0.9R are obtained for enthalpy and entropy, respectively, for grain-boundary segregation.