Dual mode temperature sensing through luminescence lifetimes of F- and O-coordinated Cr3+ sites in fluorosilicate glass-ceramics

Luminescence lifetime based temperature sensing has an intrinsic immunity to the influence of external conditions, and dual mode thermometry is highly accurate due to its “self-calibration” merit. To develop thermometry with both features, we investigated the phase and microstructural evolution of Cr3+-doped calcium-fluorosilicate glass and glass-ceramics, which revealed different luminescent behavior relating to the different Cr3+ sites in the materials. From the photoluminescence (PL) spectra, the emission at 717 nm was derived from the O-coordinated octahedral sites, while the 1 μm super-broad emission was assigned to the F-coordinated octahedral sites. After an annealing treatment, cubic CaF2 nanocrystals were homogeneously precipitated in the glass-ceramics; thus, both the O-coordination in the residual glass phase and F-coordination in the CaF2 crystalline phase were strengthened. This led to the enhancement of both the emissions at 717 nm and 1 μm. The O-coordinated sites were relatively strong-field sites in which the fluorescence of Cr3+ originated from the radiative transitions of the two thermally coupled energy levels, 2E and 4T2, while the F-coordinated sites were relatively weak-field sites. Hence, the Cr3+ exhibits only one excited state 4T2, which is inactivated by radiative transitions and non-radiative transitions from the thermal quench. Based on the obtained results, the maximum relative temperature sensitivity coefficients are 0.76% K−1 at 498 K for the 717 nm emission and 0.47% K−1 at 351 K for the 1 μm emission. This provides the possibility of developing a dual mode temperature sensor with high precision only using a single material.