In materials development, various elements are added through a process called sintering-heating and solidifying at temperatures below the melting point-to promote reactions and control microstructures. Ceramics consist of numerous small "crystal grains," which are regions with different atomic arrangement orientations, and it is known that added elements diffuse along interfaces between these grains, namely grain boundaries, during sintering progresses. However, the atomic positions within the boundaries through which the diffusion occurred had not been clarified.
A research group led by Project Associate Professor Bin Feng and Professor Naoya Shibata from the Graduate School of Engineering at the University of Tokyo used atomic-resolution electron microscopy to investigate the atomic positions along the diffusion paths of doped titanium (Ti) at grain boundaries in alumina ceramics, as well as how the grain boundary atomic structure changes at the diffusion front. As a result, they discovered that when Ti atoms diffuse into the alumina grain boundaries with initially asymmetric atomic structures and their concentration increases to a certain level, the atomic structure of the boundaries change from an asymmetric to a symmetric one. Furthermore, when they compared diffusion rates, they found that the diffusion rate of Ti atoms in the symmetric structure reaches more than 10 times that of those in the asymmetric structure. In other words, the researchers demonstrated for the first time the existence of a "two-step diffusion phenomenon" in which diffusion dramatically accelerates at the moment the grain boundary transforms into a symmetric structure.
These findings have the potential to be applied to optimizing the sintering conditions of ceramics and predicting the resultant microstructures. In the future, they may contribute to the efficient development of high-performance polycrystalline materials.
(Article: Masanori Nakajo)
