A new diagnostic for volume fraction measurement of metal-oxide nanoparticles in flames using phase-selective laser-induced breakdown spectroscopy

Abstract

A new diagnostic has been developed for measuring nanoparticle volume fraction by using phase-selective laser-induced breakdown spectroscopy (LIBS) for TiO2 nanoparticles generated during flame synthesis. The volume fraction is determined from Ti atomic spectra by exciting all the nanoparticle-phase matter to the plasma state, while keeping the gas-phase unexcited, based on the selectivity of breakdown thresholds between gas phase and particle phase. The measurement is performed with no detection delay, as no Bremsstrahlung background is detected. The nonvisible nanoplasmas are likely confined around each nanoparticle, differing from the continuously expanding plasmas produced in conventional LIBS. The intensity of the atomic spectra increases proportionally with laser fluence and reaches a saturation regime above 35J/cm2, facilitating the measurement of volume fraction. It is found that smaller nanoparticles decrease in absorption efficiency due to the size-dependent band gap. Nevertheless, this effect diminishes for nanoparticle sizes above 8nm. In the saturation regime, a fairly-good linear relation exists between signal intensity and nanoparticle volume fraction, for volume fractions above 140ppb. Benefitting from locally-confined nanoplasmas without detectable sparks, two-dimensional planar measurements of TiO2 nanoparticle volume fraction is accomplished, resulting in the visualization of the rapid formation of nanoparticles across the flame sheet and conservation of their volume fraction in the post-flame region. When an impinged-upon substrate is added, the two-dimensional phase-selective LIBS imaging shows good resolution of nanoparticle volume fraction variation in the boundary layer, influenced by diffusion and thermophoresis.