Phase Evolution and Vapor‐Environment Stability of Electrospun Mullite–Zirconia Nanofibrous Ceramic Membranes

ABSTRACT Flexible oxide ceramic nanofibers that maintain structural integrity under extreme thermal and corrosive environments remain fundamentally limited by grain coarsening and interfacial instability at elevated temperatures. Here, we report a mullite–zirconia nanofibrous ceramic membrane synthesized via dual‐template electrospinning followed by controlled thermal treatment (900°C–1300°C). Unlike our previously reported pure mullite fibers, in which Al 2 O 3 nanograins crystallize within a single amorphous SiO 2 matrix, zirconia introduces an independent, earlier nucleation pathway that delays mullite formation and produces a distinct amorphous–crystalline interphase. This interphase suppresses grain coarsening and stabilizes fiber morphology up to 1300°C, and drives strong phonon boundary scattering that yields an ultralow thermal conductivity of 0.032 W m −1 K −1 while retaining fibrous bendability. The membrane also withstands acidic, alkaline, and humid vapor corrosion at 150°C, with corrosion resistance tracking crystallinity across the MZNF‐1100 to 1300 series, a property not evaluated in single‐phase mullite systems. These results identify zirconia‐mediated phase competition, rather than compositional addition alone, as the mechanism governing thermal and chemical durability in electrospun oxide nanofiber membranes. In addition, this study provides a future direction for understanding how interfacial structural engineering can simultaneously enhance thermal stability, mechanical compliance, and environmental resistance.