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Synthesis of Novel Phosphor Sensor Particles for Advanced Flame Diagnostics

Periodic Reporting for period 2 - PHOSPHOR (Synthesis of Novel Phosphor Sensor Particles for Advanced Flame Diagnostics)

Reporting period: 2018-08-01 to 2019-07-31

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.
In terms of sensor development, an aerosol reactor has been installed in Magdeburg and this has been used to produce new phosphor particles (cerium-doped phosphates) which extend the measurement temperature range in gas flows to 1200 K, more than 200 K higher than was previously possible. Using an alternative synthesis method, phosphor particles (bismuth-doped vanadates) have also been produced which allow a novel fluid temperature imaging technique to be employed that improves the measurement precision to better than one degree, a fivefold improvement over that previously possible.

In the course of the project, quantitative spectroscopic tools to characterise dispersed phosphor particles have also been developed, that (1) provide a proven platform to extract essential luminescence signal data, and (2) demonstrate that the nature of phosphor-specific laser-particle interactions are fundamental to understanding and predicting the useful luminescence properties of phosphor materials in any given application [4].

Additionally, in aerosol synthesis the various complex and coupled mass transfer, chemical and fluid mechanical processes are not fully understood, and therefore using aerosol synthesis methods to flexibly and controllably produce a very wide range of new materials remains challenging. In this context, using a new liquid atomisation method that produces unusually small droplets in the sub-micron range, it was possible to experimentally evaluate how the droplet size affects the way in which particles form in high temperature environments [5, 6], which in turn has a profound influence on the particle properties. Building on these findings it should become possible to predict whether the product particles will be on the scale of nanometres or micrometres, and whether they will be hollow or dense. Also in this project, for example, the effect of synthesis parameters on aerosol production of particles for lithium-ion battery cathodes was investigated [7].
New temperature-sensitive luminescent sensor particles have been developed in this project. These materials advance the capabilities of fluid thermometry using phosphors to higher temperatures and to detect smaller temperature differences, thereby providing new measurement tools to researchers in basic science and industry. Quantitative spectroscopic tools to characterise dispersed optically-active particles have been developed, which is relevant for research on thin films, biomedicine and aerosols. The project results also further the understanding of how particles form from solution droplets in high temperature environments, and how aerosol synthesis can be controlled to produce particles with different morphologies and functionalities. Future sustained research efforts should enable the synthesis of a wide variety of functional particles with a broad range of pre-designed electrical, chemical, mechanical, thermal and optical properties.

[1] S.W. Allison, G.T. Gillies. Rev. Sci. Instrum. 1997
[2] J. Brübach et al. Prog. Energ. Combust. 2013
[3] C. Abram et al. Prog. Energ. Combust.2018
[4] B. Fond et al. Opt. Mat, 2019.
[5] C. Abram et al. Proc. Combust Inst.2018.
[6] C. Abram et al. International Aerosol Conference 2018, St. Louis, Missouri, 2018.
[7] C. Abram et al. ACS Applied Energy Materials, 2019.