Decoupling grain growth from densification during sintering of oxide nanoparticles

When external pressure is exerted on oxide nanoparticles (NPs), they densify without exhibiting significant grain growth at temperatures lower than half their melting temperature. This type of densification behavior contradicts the usual sintering behavior observed during densification, which is inevitably accompanied by grain growth. It has been found that NPs of various oxides, including ZnO, CuO, TiO2, SnO2, Fe2O3, and BaTiO3, show slight low-temperature densification (LTD) at temperatures much lower than half their melting temperature, even when an external pressure is not applied. Here we report that LTD is crucial for the densification of NPs during pressure sintering: without LTD, densification does not progress sufficiently even when a pressure as high as 2 GPa is applied. The phenomenon of LTD can be ascribed to surface and/or boundary diffusion in the NPs because of the low thermal activation energy of LTD as well as its sensitivity to changes in the NP surface morphology. It is likely that the decoupling of grain growth from densification in oxide NPs is related to LTD-assisted yield deformation, that is, the migration of surface atoms, which is not accompanied by significant lattice diffusion.