Phosphor thermometry provides noncontact temperature measurements by analyzing the changes of photoluminescence (lifetime decay or intensity ratios) with temperature. Compared with other noncontact temperature measurements, phosphor thermometry is a low-cost technique and provides simple diagnostics, enabling its use in nonplane geometries and in fluids. N. Ishiwada, S. Fujioka, T. Ueda, and T. Yokomori from the Keio University, Japan have investigated the photoluminescence response to the temperature of Tb³⁺/Tm³⁺ co-doped Y₂O₃ phosphors, finding that these materials show a clear intensity ratio response even at high temperatures, not suffering from strong thermal quenching. The researchers have shown that these materials present intensities of two emission lines with different responses to temperature, an emission intensity strong enough to avoid optical noise, and emission of blue or green light, where black-body radiation is relatively weak. The results of this study appeared in the March 1st issue of Optics Letters (DOI: 10.1364/OL.36.000760; p. 760).

The researchers prepared Y₂O₃:Tb³⁺, Y₂O₃:Tm³⁺, and Y₂O₃:Tb³⁺/Tm³⁺ phosphors by a flame spray synthesis method. They dissolved Y(NO₃)₆×6H₂O, Tb(NO₃)₃×6H₂O and Tm(NO₃)₃×6H₂O in distilled water to prepare a 0.3 M precursor solution that was supplied, together with methane and oxygen gases, to the co-axial burner to form a high-temperature diffusion flame where monoclinic Y₂O₃ could be crystallized. The researchers investigated the thermal dependence of photoluminescence by placing the phosphors in a 30 mm diameter ceramic vessel, surrounded by a wire heater, and by monitoring and controlling their temperature with a K-type sheath thermocouple connected to a PID temperature control unit within an accuracy of ±2 K. The researchers excited the phosphors with 355 nm UV light and recorded their emissions with a spectroscope in front of which they used a long-pass filter (>430 nm) to remove any remaining excitation light.

Temperature sensitive visible photoluminescence images of Yb₂O₃:Tb³⁺/Tm³⁺ depending on the concentration of the dopants (Tb³⁺ mol% / Tm³⁺ mol%): (a) 2/1, (b) 1/1 and (c) 0.6/1. Reproduced with permission from Opt. Lett. 36 (2011) DOI: 10.1364/OL.36.000760; p. 760. © 2011 Optical Society of America.

The researchers chose the concentrations of Tb³⁺ (1 mol%) and Tm³⁺ (1 mol%) to obtain the optimum intensity difference between the two dopants. The results demonstrate that the peak line at ~456 nm generated by Tm³⁺ was still strong at more than 1000 K whereas the other peak lines at 489, 543, 588, and 624 nm due to Tb³⁺ decreased due to the thermal quenching effect. By monitoring the intensity ratio between the 543 nm and 456 nm emissions with temperature, the researchers observed that this ratio presented a high gradient with temperature change and linearity over a wide temperature range, representing the most appropriate intensity ratio to be used for thermometry, offering the opportunity for measurement of temperature over a wide range and with high sensitivity.

The researchers also investigated the temperature-sensitive visible photoluminescence of Yb₂O₃:Tb³⁺/Tm³⁺ as a function of dopant concentration. They observed visible photoluminescence color changes from green to blue with increasing temperature for specific dopant concentration (see Figure). The researchers said this property suggests the possibility of measuring the temperature not only by analyzing the intensity ratio but also by observing the color change visually through a visual thermo-sensor (VTS).