Effect of sample morphology on the spectral and spatiotemporal characteristics of laser-induced plasmas from aluminum

• Published in: Applied physics. A, Materials science & processing Materials science & processing, 2020-02, Vol.126 (2), Article 83
• Main Authors: Wainwright, Elliot R., Dean, Steven W., De Lucia, Frank C., Weihs, Timothy P., Gottfried, Jennifer L.
• Format: Article
• Language: English
• Subjects: Aluminum oxide; Applied physics; Bulk density; Characterization and Evaluation of Materials; Condensed Matter Physics; Current State-Of-The-Art in Laser Ablation; Fluid dynamics; Fluid flow; Framing cameras; Hydrodynamics; Laser ablation; Laser plasmas; Lasers; Line spectra; Local thermodynamic equilibrium; Machines; Manufacturing; Materials science; Mathematical analysis; Metal powders; Morphology; Nanotechnology; Optical and Electronic Materials; Oxidation; Physics; Physics and Astronomy; Plasma; Plasma composition; Powder beds; Processes; S.I.: Current State-Of-The-Art in Laser Ablation; Spatial distribution; Surfaces and Interfaces; Thickness; Thin Films
• ISSN: 0947-8396; 1432-0630
• DOI: 10.1007/s00339-019-3201-9

Species stratification and local plasma composition can affect microsecond-timescale oxidation reaction rates of metals such as Al in an oxidizing atmosphere. Here, we utilize fast, gated emission spectroscopy and a high-speed framing camera to determine the intensity and spatiotemporal evolution of various Al ablation products within a laser-induced plasma. Using a high-purity Al plate, micron- and nano-sized Al powders, and inert micron Al 2 O 3 powder, we studied the effect of Al morphology and reactivity on the oxidation characteristics and plasma hydrodynamics in air at 1 atm over two temporal regimes (2–10 μs and 20–100 μs). We observed an increase in the spatial distribution and intensity of emission from vaporized Al within the plasma for powder-based samples compared to plate Al due to enhanced material dispersion. In nano-Al, AlO emission forms at, and propagates along, the surface of the powder bed from 2–10 μs, whereas for micron powder, there is a delay in AlO formation within the bulk of the plasma until tens of microseconds. We measured electron densities from a variety of spectral lines, which can range from ~ 2 × 10 15 to 2 × 10 18  cm −3 , and which scale inversely with the rate of plasma expansion across morphologies. The Al I and Al II species temperatures from 2 to 10 μs calculated via Boltzmann plots are similar (from ~ 10,000 to 14,000 K), and we performed a suite of local thermodynamic equilibrium (LTE) validity calculations to establish that these two species are in LTE, while H is not. Using image co-registration, we calculated the thickness of the AlO layer surrounding the expanding Al cloud at times > 20 μs, which can range from ~ 50 to 300 μm. These results allow us to begin to understand the complexities of laser ablated metal powder reactions at microsecond timescales.