Superelastic Subcrystalline Rare‐Earth Ceramic Nanofiber Aerogels Enable 1300°C‐Stable Upconversion Luminescence

Real-time noncontact temperature detection is critical for the reliable operation of specialized robots in extreme thermodynamic environments. Upconversion (UC) luminescent materials, owing to their temperature-sensitive emission, offer a promising solution. However, traditional UC crystals are fundamentally limited by both severe thermal quenching above 250°C and intrinsic brittleness. Here, we break these barriers by developing a subcrystalline upconversion ceramic nanofiber aerogel through an interfacial phonon engineering strategy. Our approach embeds active rare-earth nanocrystals within an amorphous alumina matrix, creating a tensile-strained heterointerface that softens local phonon modes and efficiently scatters high-frequency vibrations. This unique "phonon cage" architecture suppresses nonradiative decay pathways, enabling stable UC emission at the unprecedented temperature of 1300°C. Furthermore, the subcrystalline structure induces a higher-order sinusoidal buckling behavior, endowing the aerogel with thermomechanical superelasticity. The aerogel fully recovers from 95% compressive strain and survives over 1000 fatigue cycles, retaining >80% of its elasticity after 100 rigorous compression cycles under 1300°C thermal load. This work paves the way for noncontact thermal sensing in extreme environments.