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Oxygen plays a key role in various industrial processes, including combustion and energy conversion, which are involved in important fields such as fuel cells, automotive engines, and gas turbines. Thus, a real-time, accurate measurement of oxygen concentration is crucial for the seamless functioning of these industries.

Unfortunately, existing oxygen concentration measurement technologies rely on contact measurements using probes, which cannot withstand high-temperature environments. Moreover, despite the availability of a few optical temperature measurement technologies, the organometallic materials they utilize degrade at temperatures above 120 掳C.

To address this problem, a team of researchers led by Prof. Kyung Chun Kim from Pusan National University, Korea, developed and tested a non-contact technique to measure oxygen concentration under high temperatures. In their study, which was made available online on 19 April 2022 and published in Sensors and Actuators B: Chemical, the team described how a phosphorescent material's glow, or "phosphorescence," can be leveraged to measure oxygen concentration.

The material in question was yttrium oxide doped with europium (Y₂O₃:Eu³⁺)鈥攁 phosphor, i.e., a material that emits light in response to radiation鈥攚hich has a highly temperature-resistant crystalline structure. Like other phosphors, Y₂O₃:Eu³⁺ absorbs light energy and re-emits it at a lower frequency. However, owing to its unique molecular arrangement with oxygen vacancies, its phosphorescence varies depending on the surrounding oxygen. This high sensitivity to oxygen makes Y₂O₃:Eu³⁺ a suitable non-contact luminescent probe.

To investigate this property further, the team set up a two-dimensional (2D) temperature and oxygen concentration adjustable furnace with a quartz window (a window that allows light to pass freely in both directions) and used it to shine an ultraviolent (UV) LED light towards a Y₂O₃:Eu³⁺ tablet. On measuring the resultant phosphorescence using a spectrometer, the team found that it was most sensitive to the oxygen concentration at a temperature beyond 450掳C for a wavelength of 612 nm. Beyond 450掳C, the sensitivity of Y₂O₃:Eu³⁺ to oxygen concentration increased with increasing temperature but decreased with an increase in the oxygen concentration.

Importantly, they also observed two properties of Y₂O₃:Eu³⁺ phosphorescence that could be used to measure oxygen concentration at 550掳C: its intensity and lifetime, i.e., the time it takes for Y₂O₃:Eu³⁺ to stop emitting light. Although measurements using the latter were slightly more accurate, these findings demonstrated the overall applicability of using the phosphorescence of Y₂O₃:Eu³ at high temperatures.

Discussing these findings, Dr. Kim states that their "study is the first to develop a simple, non-contact, 2D method that can provide technical support for the performance improvement of many industrial products at high temperatures."

What are the implications of these findings? Prof. Kim further remarks that "this method can enhance basic mechanism research and industrial production applications, which would help us understand unknown thermophysical phenomena in daily life and engineering."