Fermi-Level Mediated Acceleration of Flash Sintering of Oxide Ceramics

"Flash" processing is a powerful versatile method for quick synthesis of a wide variety of materials, which is hard to achieve by any other methods [1,2]. This ultrafast process can swiftly realize in practice what is identified through high-throughput materials screening as a target for practical use. Flash sintering of functional metal oxide ceramics is one of the best examples. The similarity in flash-processing conditions possibly underlies a common mechanism, yet the mechanisms of the reactions and the physics behind remain speculative. Equipped by first-principles calculations, we unravel [3] how charge compensation and equilibrium of a range of defects in the prototypical yttria-stabilized cubic ZrO₂ (YSZ) lead to Fermi level shifts at different stages of flash, thereby accelerating cation migration for fast mass transport and sintering kinetics. The charge transition of the Zr vacancy, V_Zr^q, reduces its volume diffusion barrier by 2 eV, in V_Zr^-4 state during flash, relative to V_Zr⁰ before the surging of flash current. The V_Zr^q charge state is changed by the presence of nonstoichiometric defects, Y substitutions on Zr as electron acceptor favor V_Zr⁰ before flash, whereas excessive oxygen vacancies thermally generated at the onset of flash sintering donate electrons to conduction band edge, leading to Fermi level upshift and thus favoring V_Zr^-4. The proposed mechanism of Fermi-level driven cation diffusion scheme for YSZ has considerable bearing on the general theory of flash sintering techniques in oxide ceramics and other functional materials. This work is supported by the Office of Naval Research (N00014-22-1-2788).<br/><br/>[1] C. Wang <i>et. al.</i> Science 368, 521 (2020).<br/>[2] D. X. Luong <i>et. al.</i> Nature 577, 647 (2020).<br/>[3] Q.-K. Li, E. S. Penev, B. I. Yakobson, submitted (2024).