Temperature is a parameter of fundamental importance because it affects nearly every aspect of our environment and the technology we use. Temperature strongly influences heat transfer, the rates of chemical reactions, and the thermodynamic state of matter. Knowing the temperature is therefore critical both for basic scientific research and to understand and optimise industrial processes. However, measuring the temperature is challenging, because the processes are either delicate, for example, the metabolism of a living cell; harsh and chaotic, for example combustion in a gas turbine engine; or highly dynamic and transient, for example in natural convection flows in the Earth’s mantle, ocean or atmosphere. The problem is that established measurement techniques often cannot perform under these adverse conditions.
To address this problem, the objective of this project is to produce new optical thermometers based on phosphors. Phosphors are nano- to micro- scale particles consisting of a solid crystalline compound ‘doped’ with specific chemical elements, lending the material useful luminescence (i.e. light-emitting) properties. These materials are encountered everywhere in our daily lives: in efficient LEDs, display panels, medical equipment and so on. Nearly always though, the luminescence properties are temperature-dependent, and so ‘thermographic’ phosphors can also be used as temperature sensors. In this way, phosphor particles can be embedded in or coated onto solid surfaces, added to liquids, or seeded into gas flows. Following excitation of the particles using, for instance, laser light, the luminescence signal of the particles is detected using cameras, to allow remote optical thermometry [1, 2]. These materials possess several remarkable advantages making their application both relatively simple and well-suited to measurements in these environments of interest: they are chemically inert; their luminescence properties typically depend only on the temperature; and using additional optical techniques it is straightforward to measure other vital parameters such as the fluid velocity at the same time.
This phosphor thermometry technique has been used for temperature measurements of solid surfaces for decades. Now it is also attracting considerable attention for measurements in gases and liquids [3]. However, though an uncountable number of potentially useful phosphor materials exist, these are nearly exclusively produced for the other applications cited above. Therefore, the goal of this work is to produce phosphors specifically for fluid temperature measurements: new particles with optimised structure and composition that improve their temperature range, sensitivity and overall functionality.
For this, several material synthesis methods are proposed, in particular aerosol synthesis, a method to produce particles with structures on the nano- to micro-scale. It is currently used to produce industrial quantities of many different materials also encountered in our everyday lives: for pigments, structural reinforcement of other materials, optical fibers, and for pharmaceuticals. In brief, a liquid solution containing the required chemical elements is atomised into fine droplets and delivered to a high temperature zone such as a flame, where the droplets undergo various transformational processes and ultimately form the desired product particles. The method is relatively environmentally clean and allows some adjustment of the size, morphology and chemical composition of the particles, and so is suitable to produce the phosphor particles needed for this project.
The project consisted of an outgoing phase conducted at Princeton University in New Jersey, USA, where aerosol synthesis methods were developed. During the return phase at Otto-von-Guericke Universität Magdeburg in Germany, these methods are currently being applied to produce and characterise phosphor particles with improved luminescence properties, and use these new materials to advance temperature measurement capabilities.
