Optimizing Up-conversion Luminescence Thermometry for EBCs: Composition-Tailored Er3+-Doped Ytterbium-Yttrium Orthosilicate Phosphors via Yb/Y Ratio Engineering

Non-contact measurement techniques based on fluorescence intensity ratio (FIR) and high-sensitivity upconversion optical pyrometry offer a viable solution for real-time temperature monitoring of environmental barrier coatings (EBCs) under extreme service conditions. In this study, monoclinic phase (Er0.02YbxY0.98-x)2SiO5 upconversion phosphors were synthesised using the sol-gel method, employing modulation of the Yb/Y composition ratio. Enhanced upconversion luminescence was achieved through efficient energy transfer from Yb3+ to Er3+, combined with the low phonon energy characteristics of the Y2SiO5 matrix, which effectively suppresses nonradiative transitions. Spectral analysis indicated that under 980 nm laser excitation, the phosphors exhibited a typical two-photon upconversion mechanism, showcasing temperature-dependent green emissions from the 2H11/2 → 4I15/2 (510-540 nm) and 4S3/2 → 4I15/2 (540-570 nm) transitions, as well as red emission from the 4F9/2 → 4I15/2 (630-710 nm) transition. Optimal luminescence performance was observed at a concentration of 20 mol% Yb3+, which served as a sensitiser. Within the temperature range of 298-1073 K, the maximum absolute sensitivity (Sa) and relative sensitivity (Sr) of FIR, based on the thermally coupled 2H11/2 and 4S3/2 sublevels, reached 0.760% K-1 (423K) and 1.324% K-1 (298K), respectively. Furthermore, thermal cycling tests confirmed exceptional luminescence stability and measurement repeatability across the broad temperature range. These findings conclusively demonstrate that Er3+-doped (Yb, Y)2SiO5 phosphors, prepared through Yb/Y composition tuning, not only function as high-performance optical temperature sensing materials but also hold promise for applications in self-sensing temperature monitoring within next-generation environmental barrier coatings.