Sintering of nanometer scaled powders

Contact: Eva Jud

Nanometer sized ceramics such as CeO₂(ss) can be sintered at low temperatures to dense ceramics with nm scaled grain size by the additions of transition metal oxides. The rapid densification of the CoO doped CeO₂(ss) may be attributed to a transient liquid phase that melts unusually low due to size dependent melting of the CoO in the neck region of the particle contacts. The transient nature of the grain boundary layer allows to tailor the electrical properties of CeO ₂(ss) from electronic to pure ionic. This material offers new possibilities for electrochemical devices such as solid oxide fuel cells and gas separation membranes. The low sintering temperatures also open up new preparation sequences for electrode-electrolyte sandwich structures.

In undoped nano-powders (Ce_0.8Gd_0.2O_1.9 (CGO)), the activation energy increases during densification from ~ 4.5 to 9.2 eV. This proofs that after grain boundary diffusion at low densities and temperatures also volume diffusion during the final stage of sintering becomes active. Higher temperatures lead however to grain growth.

In case of CoO-doped CGO, one activation energy (~ 6 eV) has been observed and therefore concluded that only grain boundary diffusion predominates during the entire densification process. This allows reducing the sintering temperature and thereby reducing grain growth resulting in nm grain sized microstructures in the dense sintered material.

We are convinced that other nano-scaled oxides powders may show similar low sintering temperatures when properly doped with small MeO additions. The usual grain coarsening during the later stage of sintering of ceramics may be kept at minimum and nano-scaled microstructures will be possible by conventional sintering methods.

For further information, consult the papers from Kleinlogel and Jud in our literature database.