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In this work, we investigate the flame aerosol deposition of TiO 2 nanoparticles in a well-defined premixed stagnation flame based on two-dimensional phase-selective laser-induced breakdown spectroscopy (PS-LIBS). The deposited nanoparticles are in anatase phase with an average size of ∼10 nm. In PS-LIBS measurement, atomic emissions of titanium near the wavelength of 500 nm are extracted by spectral filter lenses and imaged for demonstrating the particle volume fraction distributions. The PS-LIBS signals can well depict the whole process of particle formation and transport, including the rapid gas-to-particle conversion near the flame front, dilution and concentrating of the nanoparticles induced by gas density variations, and dropping of particle volume fractions near the substrate. A quantitative model has been proposed to describe convection, thermophoresis, and diffusion of the nanoparticles. Combining the PS-LIBS measurement and the numerical simulation, it is found that, the nanoparticles concentrate outside the boundary layer at low substrate temperatures due to a joint effect of gas compression and thermophoresis. Further parametric analysis indicates that substrate temperatures and precursor loading rates can strongly affect the particle concentration boundary layer and the particle deposition rate.

Key effects of solvent formulations on the structure and morphology of optical-quality yttrium-aluminum nanocomposites using liquid-fed aerosol flame synthesis are investigated. Employing a temperature-controlled flat flame burner with inexpensive nitrates as multi-component precursors, three different solvent formulations, i.e., ethanol, ethanol/2-ethylhexanoic acid (EHA), and butanol, are studied. Adding EHA into ethanol in a 1:1 volume ratio dramatically changes the flame-made yttrium-aluminum oxides from hollow inhomogeneous powders that contain non-uniform large particles to homogeneous nanopowders around 10 nm. As characterized by in-situ phase Doppler anemometry, droplet size with increasing burner height for the EHA/ethanol case remains constant at the beginning, whereas those for both ethanol and butanol cases reduce immediately. EHA likely causes a shift from the droplet-to-particle precipitation route to the gas-to-particle route because of the formed low-boiling-point 2-ethylhexanoates from nitrates via ligand exchange. By replacing ethanol with butanol, hollow particles are produced with better crystallinity because of its high calorific value that helps to heat precursors at the droplet surface. In-situ diagnostics using phase-selective laser-induced breakdown spectroscopy, which tracks only atomic emission from the nanoparticle phase. The result shows that the Al atomic emissions in the EHA/ethanol mixture case gradually increase along the burner height, while those for both ethanol and butanol cases fluctuate, further verifying the favoring of the gas-to-particle route for producing uniform, ultrafine solid multi-oxide particles by adding EHA in solvents.