PNNL

Abstract

The role of ambient oxygen on molecular and nanoparticle formation and agglomeration was studied in laser ablation plumes. Nanosecond laser ablation of an aluminum alloy (AA6061) target was performed in 750 Torr flowing air and nitrogen environments as a lab-scale surrogate to a high explosion detonation event. Optical emission spectroscopy and mass spectrometry were used to monitor the early to late stages of plasma generation to track the evolution of atoms, molecules, clusters, nanoparticles, and agglomerates. Electron microscopy was performed ex-situ to identify crystal structure and elemental distributions in individual nanoparticles. We find that spectral signatures of laser-produced plasmas vary for air versus nitrogen environments; the presence of $\approx$ 20 \% O$_2$ leads to strong AlO emission, whereas in a flowing nitrogen environment (with trace oxygen), AlN emission is present. Mass spectrometry reveals that as oxygen availability in the environment increases, Al oxide cluster size increases. Nanoparticle agglomerates are formed in both environments, although agglomerates in air are found to be larger. High-resolution transmission electron microscopy demonstrates that nanoparticle agglomerates consist of Al$_2$O$_3$ and AlN nanoparticles in both environments; indicating the presence of trace O$_2$ can lead to Al$_2$O$_3$ nanoparticle formation. The present results highlight that the availability of O$_2$ in the ambient gas significantly impacts spectral signatures, cluster size, and nanoparticle agglomeration behavior, relevant to understanding debris formation in an explosion event and interpreting data from forensic investigations.