Localized doping and apparent fast interdiffusion in bulk nanocrystalline ceria and zirconia obtained by Field Assisted Sintering

We investigated the interdiffusion process in nanostructured fluoritic oxides sintered by Field Assisted or Spark Plasma Sintering (FAST/SPS). Ceria and zirconia nanopowders were surface decorated with dopants, using simple solution techniques and then sintered at temperatures of 600-900 °C for very short times, under a pressure of 575 MPa. Remarkable ionic conductivity increase and phase composition alteration were observed after only 5 min of sintering. This rapid modification cannot be justified with regular bulk diffusion models, as cationic bulk diffusivity in these oxides is very limited at these temperatures. We propose an alternative model, involving only surface diffusion processes, justifying the apparently anomalous fast interdiffusion. This model helps clarifying some fundamental aspects of diffusion at the nanoscale when fast sintering is involved. We also evidenced as surface decoration coupled with FAST/SPS represents a simple alternative approach to the doping of oxides, which can be applied when other approaches are inconvenient.