10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Effects of Droplet Diameter and Flame Temperature on Nanoparticle Formation Mechanisms in Liquid Aerosol-Fed Non-Premixed Gas Flames
CHRISTOPHER ABRAM, Maksim Mezhericher, Howard A Stone, Yiguang Ju, Princeton University
Abstract Number: 582 Working Group: Combustion-Generated Aerosols: the Desirable and Undesirable
Abstract The formation routes of metal oxide nanoparticles are experimentally investigated in self-sustaining gaseous flames fed by aqueous metal-nitrate solution aerosol droplets. To understand the influence of the precursor droplet diameter on the product particle size and morphology, two different atomisation methods are compared: (1) a novel bubble/gas jet atomisation technique, which generates droplets in the sub-micron range with a number mean diameter of ~200 nm; and (2) conventional ultrasonic atomisation producing droplets with a corresponding diameter of ~3.5 µm. Solution droplets consisting of yttrium and europium nitrate salts dissolved in water are delivered to non-premixed CH4/N2-O2 flames to synthesise Eu-doped Y2O3 phosphor particles. The product particles are characterised using SEM, TEM, BET gas adsorption and luminescence spectroscopy. Flame temperatures are measured using planar laser Rayleigh scattering.
Depending on the synthesis temperature, the results show that product particles are formed via two different mechanisms: droplet-to-particle and gas-to-particle. At low flame temperatures (1150 K), sub-micron droplets produce cubic-phase luminescent Y2O3:Eu3+ nanoparticles via the droplet-to-particle mechanism. The particle size can be controlled from 10 to 100 nm by adjusting the precursor concentration in the 0.01-1 mol/L range. Increasing the flame temperature (1450-2750 K) leads to the formation of nanoparticles with a size 3-5 nm via nucleation and growth in the gas phase. In separate experiments, the aerosol flow was also electrically-preheated to 500 K in order to evaporate the solvent upstream before delivery to the flame. With preheating, under the same high temperature flame conditions, the droplet-to particle formation route dominates instead and very few nanoparticles are formed from the gas phase. This result suggests that even for these non-volatile metal nitrate precursors, vaporisation of the precursor occurs predominantly during the extremely rapid droplet drying when the droplets are exposed to the high temperature gradients in the flame. In such cases, specific surface area measurements of the product powders show that sub-micron droplets form an increased fraction of nanoparticles from the gas phase compared with micron-size droplets, because the greater developed surface area of sub-micron droplets enhances vaporisation of the metal precursor.
Analysis of the relevant timescales indicates that the relative rates of solvent evaporation and bulk solute diffusion are dependent on the droplet size. Electron microscopy confirms that dense particles are formed from sub-micron droplets, even when subjected to a rapid rate of gas temperature increase in the flame (5x104 K/s). For the same synthesis conditions, micron-size droplets tend to form hollow particles because relatively slow solute diffusion causes precipitation on the droplet surface. In this case either slow preheating of the aerosol upstream of the flame or high downstream peak temperatures near the product melting point (>2700 K) are required to promote dense particle formation.