Breaking the grain size limit of glass-crystallized transparent nanoceramics via multi-phase interface locking

Nanoceramics from sintering routes are challenging to process due to difficulties in deagglomerating nanopowders, suppressing grain growth, and eliminating macroscopic defects. Glass crystallization offers new possibilities by first densifying materials to optical transparency in amorphous form and then fully crystallizing into ceramic form. However, refining the grain size within the nanocrystalline regime is again challenging, due to rapid grain growth under large crystallization driving force and nucleation-controlled kinetics. Here we tackle the problem by decomposing the homogeneous glass precursors into ternary phase composite ceramics, thus coupling the coarsening kinetics to slow cation diffusion required for phase partitioning and hetero-phase boundary motion required for grain growth. The design allows precise adjustment of the evolving microstructure and yields a fine grain size down to ~16 nm in fully crystallized ceramics. We demonstrate that the obtained nanoceramics can deliver high optical transparency, despite the different refractive indices of the cubic La(Al_1/3Ti_2/3)O₃, Y₂(Zr_0.6Ti_0.4)₂O₇ and transitional Al₂O₃ phases, and improved mechanical properties. Our work unlocks the hidden opportunities of designing and producing phase-complex nanoceramics in high-dimensional compositional space beyond the traditional phase-pure solubility constraint.