ABSTRACT
The role of ambient oxygen gas (O2) on molecular and nanoparticle formation and agglomeration was studied in laser ablation plumes. As a lab-scale surrogate to a high explosion detonation event, nanosecond laser ablation of an aluminum alloy (AA6061) target was performed in atmospheric pressure conditions. Optical emission spectroscopy and two mass spectrometry techniques were used to monitor the early to late stages of plasma generation to track the evolution of atoms, molecules, clusters, nanoparticles, and agglomerates. The experiments were performed under atmospheric pressure air, atmospheric pressure nitrogen, and 20% and 5% O2 (balance N2), the latter specifically with in situ mass spectrometry. Electron microscopy was performed ex situ to identify crystal structure and elemental distributions in individual nanoparticles. We find that the presence of ≈20% O2 leads to strong AlO emission, whereas in a flowing N2 environment (with trace O2), AlN and strong, unreacted Al emissions are present. In situ mass spectrometry reveals that as O2 availability increases, Al oxide cluster size increases. Nanoparticle agglomerates formed in air are found to be larger than those formed under N2 gas. High-resolution transmission electron microscopy demonstrates that Al2O3 and AlN nanoparticle agglomerates are formed in both environments; indicating that the presence of trace O2 can lead to Al2O3 nanoparticle formation. The present results highlight that the availability of O2 in the ambient gas significantly impacts spectral signatures, cluster size, and nanoparticle agglomeration behavior. These results are relevant to understanding debris formation in an explosion event, and interpreting data from forensic investigations.
ABSTRACT
High-resolution tunable laser absorption spectroscopy is used to measure time-resolved absorption spectra for six neutral uranium transitions in a laser-produced plasma. Analysis of the spectra shows that kinetic temperatures are similar for all six transitions, but excitation temperatures are higher than kinetic temperatures from 10-100â µs, indicating departures from local thermodynamic equilibrium.
ABSTRACT
Laser induced breakdown spectroscopy is a promising, rapid analysis method for the detection and quantification of Li and its isotopes needed in geochemical, nuclear, and energy storage applications. However, spectral broadening in laser produced plasmas, presence of fine and hyperfine structures, and self-reversal effects make Li isotopic analysis via laser induced breakdown spectroscopy challenging. The present study explores the influence of Ar, N2, and He ambient gases over the pressure range of 0.05 - 100 Torr on line broadening and self-reversal of the Li I transition with the greatest isotopic shift in the VIS spectral region (i.e., ≈670.8 nm, ≈15.8 pm isotopic shift). We perform spatially and temporally resolved optical emission spectroscopy of plasmas produced via laser ablation of LiAlO2 substrates. Our results show that the self-reversal and linewidth is reduced at lower pressures for all gases, and using optimized plasma conditions with chemometric methods, the 6Li/7Li isotopic ratios can be predicted.
ABSTRACT
The evolution from gas-phase oxidation to nanoparticle and agglomerate formation was studied in nanosecond laser-produced plasmas of a multi-principal element alloy target in air. Gas-phase oxidation of plasma species was monitored in situ via optical emission spectroscopy, while a custom-built single particle mass spectrometer was used to measure size and compositions of agglomerated nanoparticles formed in laser ablation plumes. Ex situ analysis employing transmission electron microscopy was used to study nanoparticle morphology, crystal structure, and element distribution at the nanoscale. Emission spectra indicate that gas-phase oxidation of elements in the alloy target are formed at varying times during plume evolution, and mass spectrometry results indicate fractal agglomerates contain all principal alloying elements and their oxides. Finally, electron microscopy characterization illustrates that these agglomerates consist of multiple material types: sub-10 nm diameter amorphous, multi-element nanoparticles, ≈10-30 nm diameter Ti-rich crystalline oxide nanoparticles, and ejected base material. Results highlight that the multi-component target composition impacts molecular formation in the gas phase and the morphology, composition, and structure of nanoparticles and agglomerates formed.
ABSTRACT
Predicting microstructure evolution can be a formidable challenge, yet it is essential to building microstructure-processing-property relationships. Yang et al. offer a new solution to traditional partial differential equation-based simulations: a data-driven machine learning approach motivated by the practical needs to accelerate the materials design process and deal with incomplete information in the real world of microstructure simulation.
ABSTRACT
The chemistry evolution in a laser ablation plume depends strongly on its initial physical conditions. In this article, we investigate the impact of plasma generation conditions on the interrelated phenomena of expansion dynamics, plasma chemistry, and physical conditions. Plasmas are produced from a uranium metal target in air using nanosecond, femtosecond, and femtosecond filament-assisted laser ablation. Time-resolved two-dimensional spectral imaging was performed to evaluate the spatio-temporal evolution of atoms, diatoms, polyatomic molecules, and nanoparticles in situ. Emission spectral features reveal that molecular formation occurs at early times in both femtosecond and filament ablation plumes, although with different temporal decays. In contrast, molecular formation is found to occur at much later times in nanosecond plasma evolution. Spectral modeling is used to infer temporal behavior of plasma excitation temperature. We find U atoms and UO molecules co-exist in ultrafast laser-produced plasmas even at early times after plasma onset owing to favorable temperatures for molecular formation. Regardless of irradiation conditions, plume emission features showed the presence of higher oxides (i.e., UxOy), although with different temporal histories. Our study provides insight into the impact of plasma generation conditions on chemistry evolution in plasmas produced from traditional focused femtosecond, nanosecond, and filament-assisted laser ablation.
ABSTRACT
Laser ablation in conjunction with optical emission spectroscopy is a potential non-contact, stand-off detection method for all elements in the periodic table and certain isotopes such as radionuclides. Currently, significant development efforts are on-going to use ultrafast laser filaments for remote detection of materials. The application of filaments is of particular interest in extending the range of stand-off capability associated with elemental and isotopic detection via laser-induced breakdown spectroscopy. In this study, we characterize the expansion dynamics and chemical evolution of filament-produced uranium (U) plasmas. Laser filaments are generated in the laboratory by loosely focusing 35 femtosecond (fs), 6 milli Joule (mJ) pulses in air. Time-resolved, two-dimensional plume and spectral imaging was performed to study hydrodynamics and evolution of U atomic and UO molecular emission in filament-produced U plasmas. Our results highlight that filament ablation of U plasmas gives a cylindrical plume morphology with an appearance of plume splitting into slow and fast moving components at later times of its evolution. Emission from the slow-moving component shows no distinct spectral features (i.e. broadband-like) and is contributed in part by nanoparticles generated during ultrafast laser ablation. Additionally, we find U atoms and U oxide molecules (i.e. UO, UxOy) co-exist in the filament produced plasma, which can be attributed to the generation of low-temperature plasma conditions during filament ablation.
