Edson Roberto Leite was an undergraduate student of materials engineering when he began to work with research in sintering. At the time, his advisor was Professor José Arana Varela, a distinguished materials scientist who died in 2016 and was founding member and president of the Brazilian Materials Research Society.
Used since antiquity, sintering is a process of agglutination of solid particles that results in a compact material. In this process, the spaces between the particles or grains (the pores) are filled by atoms that detach from the surface of the material.
“I have always been very interested in this subject and curious to know what transport is like at the atomic level during the sintering process”, recalls Edson Leite.
More than thirty years after this first scientific work, Leite, who now is the scientific director of the Brazilian Nanotechnology National Laboratory (LNNano), calmed this curiosity. Together with other researchers at LNNano, he was able to visualize, in real time and with atomic resolution, the process of eliminating pores in a nanometric zirconium oxide ceramic. The work was recently published in Nano Letters.
“Certainly Prof. Varela would be proud and enjoy this work”, says Leite, who received the José Arana Varela Award from B-MRS last year in recognition of his scientific trajectory.
The first step of the work was taken four years ago, when Leite and his team developed a methodology that allowed the preparation of monolayers of self-supported nanoparticles (without substrate). When they took these very thin films to a transmission electron microscope (TEM), the researchers noticed that the electron beam induced the transport of atoms, even at room temperature. The team then saw the possibility of studying the sintering process in situ. That is, being able to follow the changes in pore filling under the microscope, step by step and without having to remove the sample in the middle of the process.
The possibility took shape in LNNano’s high-resolution TEM, an HRTEM, which allows one to visualize atoms. “We put together a team made up of Jefferson Bettini (researcher at LNNano), my postdoc Tanna Rodrigues Fiuza and Marlon Muniz da Silva (intern at LNNano and now a PhD student at LNLS) and started to work hard to visualize sintering at the atomic scale,” says Leite.
Initially, the team prepared ceramic films formed by nanometric grains of zirconium oxide and focused their efforts on studying the process that occurs at the end of sintering: the elimination of isolated pores that remain at the grain boundaries.
The work was as exciting as it was challenging, not only because of the hours spent at the microscope in collecting, processing and analyzing images, but mainly because of the researchers’ effort to understand what they were seeing. “In the end, we were quite happy and demonstrated the transformations that occur during the closing of pores on an atomic scale”, says Leite.
In this video, filmed by the authors of the article using HRTM, it is possible to visualize atoms migrating from adjacent grains and filling the pore:
The work should have an important academic impact, since its results show that the reality of nanometric ceramics does not fit, in some aspects, in the theoretical models that are used to explain sintering processes. “Most existing models involving sinter kinetics consider the surface energy and the energy of isotropic grain boundaries. We showed that this does not occur in nanometric ceramics”, explains Professor Leite. “In addition, we showed that a transition occurs in which a rough surface is eliminated and the appearance of faceted surfaces occurs, indicating that there may be a thermodynamic barrier, in addition to a kinetic barrier for atomic transport”, he adds. “In summary, we can say that it is necessary to modify the existing models to explain the sintering process at the nanoscale”, concludes the scientist.
According to the authors, the work should also have an impact on the ceramic industry, as a better understanding of the sintering process can lead to the development of nanostructured ceramics with controlled porosity and particle size and, therefore, with differentiated mechanical and electronic properties.
Reference of the paper: Visualization of the Final Stage of Sintering in Nanoceramics with Atomic Resolution. Tanna Elyn Rodrigues Fiuza, Marlon Muniz da Silva, Jefferson Bettini, and Edson Roberto Leite. Nano Lett.2022, 22, 1978 – 1985. https://doi.org/10.1021/acs.
Author contact: edson.leite@lnnano.cnpem.br