Cubic zirconia nanocrystals with interfaces ‘healed’ by rare earth (e.g. Gd) segregation. On the right is an electron energy loss spectroscopy (EELS) map showing rare earth segregation to the grain boundary.Materials made up of nanoscale grains have some unique properties – but excess energy associated with all those grain boundaries brings thermal instability, which can cause collapse during processing or operation.
Researchers have for many years wondered if it is possible to create a nanomaterial with zero excess energy – in other words, nanocrystalline materials with the same energy and stability as bulk materials. Such a material would have no intrinsic driving force to increase grain size (an effect known as coarsening) enabling nano-stability even at high temperatures. Ricardo H. R. Castro and his colleague Nazia Nafsin at the University of California-Davis think the answer to this question is yes [Nafsin and Castro, J. Mater. Res. 32 (2017) 166].
The process of adding dopants to nanocrystalline materials is a well-recognized way of mitigating the effect of coarsening. Dopant atoms act as ‘pinning’ agents, migrating to grain boundaries and neutralizing the free energies.
“We were able to lower the grain boundary energy of a nanomaterial until it was essentially zero,” explains Castro, “creating a material with basically no excess energies compared with its bulk counterpart and hence no driving force for growth.”
Although the idea of an essentially zero-energy grain boundary is not new, this is the first time that it has been demonstrated directly, says Castro. The researchers used the ceramic oxide yttria-stabilized cubic zirconia (YSZ), which shows relatively high interfacial energies, for the demonstration. Into this material, they introduced the rare-earth metal gadolinium (Gd) as a dopant. Since yttrium forms a stable solid solution with zirconia, the larger ionic radius Gd can segregate along the grain boundaries.
“We show that a rare-earth doping can ‘heal’ bonds at the interfaces of the nanocrystal, enabling a systematic lowering of the excess energy to the point at which it is effectively zero,” says Castro.
The researchers employed microcalorimetry to measure the heat released during grain growth in a sample of Gd-doped YSZ. As Gd accumulates along the grain boundaries, the team observed a reduction in the released heat, consistent with their predictions. The Gd atoms appear to block the movement of the boundaries, increasing the activation energy and therefore decreasing the driving force for coarsening. When the grains reach around 50 nm in size, growth ceases altogether.
“This represents one of the greatest advances in the manufacturing of nanomaterials that can withstand processing and operating conditions without degrading in terms of nano-features,” Castro says. “If materials can be designed to have zero excess energies, the excellent mechanical properties of nanoceramics can be utilized in extreme environments where they are so needed.”