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

Abstract

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 T_d (tetrahedral crystal filed) or O_h (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.