High performance optical temperature sensing via selectively partitioning Cr4+ in the residual SiO2-rich phase of glass-ceramics.

Quadrivalent Cr4+ theoretically exhibits great potential to achieve higher photo-luminescence (PL) lifetime based temperature sensitivity than the commonly utilized trivalent Cr3+, but the problem is how to stabilize the anomalous quadrivalent chemical state of Cr4+. Here we propose a type of glass-ceramic phase structure with a precipitated ZnAl2O4 crystalline sub-phase and a residual ZnO-SrO-SiO2 glassy sub-phase, where Cr4+ can be well stabilized in the residual glassy sub-phase. From PL spectra, Cr4+ or Cr3+ was found to be located at Td (tetrahedral crystal filed) or Oh (octahedral crystal filed) sites with a relatively high crystal field strength. The thermally coupled 1E(1D)/3T2(3F) states of Cr4+ or the 2E(2G)/4T2(4F) states of Cr3+ were revealed as competitive energy level pairs suitable for PL lifetime based temperature sensing. Quadrivalent Cr4+ had a particular PL lifetime ratio of 1E(1D)/3T2(3F) up to 103, which was much higher than that (101) of trivalent Cr3+:2E(2G)/4T2(4F). This supported Cr4+ to eventually achieve a higher temperature sensitivity (1.72% K-1) one order of magnitude higher than that of Cr3+ (0.83% K-1). This provides the possibility of utilizing Cr4+-doped glass to develop a type of temperature sensor with high precision and sensitivity.