Visualization of the Final Stage of Sintering in Nanoceramics with Atomic Resolution

Sintering is a thermally active process, in which densification (or pore elimination) and grain growth can occur. In general, the driving force for densification is lowering the surface free energy by replacing solid–gas interfaces, leading to a new solid-solid interface with a lower system’s free energy [1-6]. The optimization of the ceramic’s densification process lies in a better comprehension of the sintering steps and related mechanisms. Many models satisfactorily describe the sintering at a micrometric scale, however, nanometric effects make the direct extrapolation of these models to the nanoscale more complicated [7-9]. By high-resolution transmission electron microscopy, particularly real-time measurements, we followed the pore elimination process in a self-standing ZrO₂ thin film produced by monoclinic ZrO₂ nanoparticles. The absence of carbon membrane and the surface cleaning by removing the capping ligands produced an ideal scenario to perform the experiments. We found a high anisotropic pore elimination, with a gradual and linear decrease in pore area as a function of time. The pore shrinkage occurs with atoms from a rough surface being redistributed on the solid-gas interface, while atoms are attached to a faceted surface. In the final stage, the pore becomes rounded and shrinks until complete elimination. Also, the pore acts as a pin, reducing GB mobility. In summary, these results can contribute to a better understanding of the sintering process in nanoceramics, and to the improvement of kinetic models that better describe the densification process at the nanoscale.<br/><br/>[1] Bordia, R. K.; Kang, S. L.; Olevsky, E. A. J. Am. Ceram. Soc. 2017, 100 (6), 2314– 2352<br/>[2] Gong, Z.; Zhao, W.; Guan, K.; Rao, P.; Zeng, Q.; Liu, J.; Feng, Z. J. Am. Ceram. Soc. 2020, 103 (10), 5900– 5913<br/>[3] Wollmershauser, J. A.; Feigelson, B. N.; Gorzkowski, E. P.; Ellis, C. T.; Goswami, R.; Qadri, S. B.; Tischler, J. G.; Kub, F. J.; Everett, R. K. Acta Mater. 2014, 69, 9– 16<br/>[4] Mitchell, S.; Qin, R.; Zheng, N.; Pérez-Ramírez, J. Nat. Nanotechnol. 2021, 16 (2), 129– 139<br/>[5] Cao, A.; Lu, R.; Veser, G. Phys. Chem. Chem. Phys. 2010, 12 (41), 13499<br/>[6] Rahaman, M. N. Sintering of Ceramics; CRC Press, 2007<br/>[7] Castro, R. H. R.; Gouvêa, D. J. Am. Ceram. Soc. 2016, 99 (4), 1105– 1121<br/>[8] Chen, I.-W.; Wang, X. Sintering of Nanograin Ceramics. In Ceramics Science and Technology, Riedel, R.; Chen, I., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA, 2012; pp 441– 455<br/>[9] Kang, S.-J. L. Grain Boundary Energy and Sintering. In Sintering: Densification, Grain Growth and Microstructure, Kang, S.-J. L., Ed.; Elsevier Butterworth-Heinemann, Oxford, 2004; pp 139– 143.