Ultra‐Uniform Nanocrystalline Materials via Two‐Step Sintering. (30th September 2020)

• Record Type: Journal Article
• Authors: Dong, Yanhao; Yang, Hongbing; Zhang, Lin; Li, Xingyu; Ding, Dong; Wang, Xiaohui; Li, Ju; Li, Jiangong; Chen, I‐Wei
• Is Part Of: Advanced functional materials. Volume 31:Number 1(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-09-30
• Subjects: ceramics -- grain growth -- microstructure -- nanocrystalline materials -- refractory metals -- sintering; Materials -- Periodicals; Chemical vapor deposition -- Periodicals; 620.11
• DOI: 10.1002/adfm.202007750
• Languages: English
• ISSNs: 1616-301X

Abstract

Abstract: Nanocrystalline materials with superior properties are of great interest. Much is discussed about obtaining nanograins, but little is known about maintaining grain‐size uniformity that is critical for reliability. An especially intriguing question is whether it is possible to achieve a size distribution narrower than what Hillert theoretically predicted for normal grain growth, a possibility suggested—for growth with a higher growth exponent—by the generalized mean‐field theory of Lifshitz, Slyozov, Wagner (LSW), and Hillert but never realized in practice. Following a rationally designed two‐step sintering route, it has been made possible in bulk materials by taking advantage of the large growth exponent in the intermediate sintering stage to form a uniform microstructure despite residual porosity, and freezing the grain growth thereafter while continuing densification to reach full density. The bulk dense Al2 O3 ceramic thus obtained has an average grain size of 34 nm and a size distribution much narrower than Hillert's prediction. Bulk Al2 O3 with a grain‐size distribution narrower than the particle‐size distribution of starting powders is also demonstrated, as are highly uniform bulk engineering metals (refractory Mo and W‐Re alloy) and complex functional ceramics (BaTiO3 ‐based alloys with superior dielectric strength and energy capacity). Abstract : The possibility of producing ultra‐uniform nanocrystalline materials beyond Hillert's limit of size uniformity is theoretically analyzed and experimentally demonstrated. Bulk dense oxides and metals including alumina, zirconia, two complex perovskite oxides, and two bcc refractory metal/alloys, are successfully obtained via a two‐step sintering process. Practical guidelines for free‐sintering general structural and functional engineering materials are provided.