On the origins of prompt features in time-resolved laser-induced incandescence measurements of metal and carbonaceous nanoparticles

Abstract

Synthetic nanoparticles have become highly beneficial in many applications, including, for example, catalytic conversion, enhancing the functionality of electronic devices, targeted drug delivery in medicine, and purifying water through the removal of bacteria and heavy metals. Nanoparticles are often synthesized through gas-phase synthesis, where the nanoparticles are formed in a bath gas, resulting in a nanoparticle aerosol. Such aerosols are also unintentionally emitted through processes such as welding or combustion. The benefits and negative impacts of nanoaerosols significantly depend on their properties, such as particle size and concentration. Laser and optical-based characterization techniques can provide such information in an in situ and time-resolved manner.

Time-resolved laser-induced incandescence (TiRe-LII) is a widely used laser-based diagnostic for soot characterization and is increasingly being applied to non-carbonaceous nanoparticles. The technique involves heating nanoparticles in an aerosol to incandescent temperatures with a laser and recording their radiative emissions with a photodetector, as they cool to the temperature of the bath gas. Nanoparticle properties such as size and concentration are obtained from temporally- and spectrally-resolved measurements through inference techniques that involve regressing a TiRe-LII instrument model to the data. Accurate and reliable inference of the nanoaerosol properties relies on the robustness of the instrument model. Unfortunately, previously developed models do not fully describe experimental observations, and the reported discrepancies need to be reconciled to improve fundamental understanding, modeling capabilities, and ultimately measurements. These discrepancies include effects previously termed excessive absorption and anomalous cooling. The effect of excessive absorption is observed when the measured particle temperatures exceed the values predicted by the related model, and the anomalous cooling was related to the effect that occurred when the measured particle cooling rate, immediately following the peak-temperature phase, is faster than what is predicted based on the model.

This thesis work addresses these reported data–model discrepancies, providing insight into laser–nanoparticle interactions in the context of TiRe-LII and the impact of certain experimental conditions. In particular, it is shown that for metal nanoparticle aggregates, radiative properties are enhanced compared to isolated metal nanoparticles, and laser energy absorption becomes spatially nonuniform within aggregates. Under laser heating, the primary nanoparticle may melt; subsequently, the aggregates may partially sinter or coalesce, which further alters their radiative properties as a function of time, phenomena that do not occur in the case of soot. Furthermore, the existence of nanoparticles of different sizes within the aerosol and spatial energy variations across the irradiating laser sheet influence the data in ways that are not accounted for in current TiRe-LII instrument models. The investigations combined theoretical modeling and experimental work. The modeling utilized several light absorption models to explore the light–matter interactions between nanoaerosols and the electromagnetic field of the laser. The experimental component employed calibrated, time- and spectrally-resolved detection techniques to observe the radiative emissions from irradiated particles within the aerosol. The findings of this thesis contribute to a deeper understanding of laser–nanoparticle interactions and open new avenues for further research in this area.