Mass-mobility characterization of flame-made ZrO2 aerosols: Primary particle diameter and extent of aggregation

Abstract

Gas-borne nanoparticles undergoing coagulation and sintering form irregular or fractal-like structures affecting their transport, light scattering, effective surface area and density. Here, zirconia (ZrO 2 ) nanoparticles are generated by scalable spray combustion, and their mobility diameter and mass are obtained nearly in-situ by differential mobility analyzer (DMA) and aerosol particle mass (APM) measurements. Using these data, the density of ZrO 2 and a power law between mobility and primary particle diameters, the structure of fractal-like particles is determined (mass-mobility exponent, prefactor and average number and surface area mean diameter of primary particles, d va ). The d va determined by DMA-APM measurements and this power law is in good agreement with the d va obtained by ex-situ nitrogen adsorption and microscopic analysis. Using this combination of measurements and above power law, the effect of flame spray process parameters (e.g. precursor solution and oxygen flow rate as well as zirconium concentration) on fractal-like particle structure characteristics is investigated in detail. This reveals that predominantly agglomerates (physically-bonded particles) and aggregates (chemically-or sinter-bonded particles) of nanoparticles are formed at low and high particle concentrations, respectively.

Improved sizing of soot primary particles using mass-mobility measurements

“…As discussed by Eggersdorfer et al (2012a), the average projected area (a a ) of fractal-like aggregates can be related to the number of their constituent primary particles n va by the following power-law (Medalia 1967) correlation:…”

“…1994;Park et al 2004aPark et al , 2004b; here D a;opt D 1800 kg/m 3 is used. The mobility diameter (d m ) of aggregates in the free molecular (Meakin et al 1989) and transition (Rogak et al 1993) regimes is approximately equal to the projected area equivalent diameter (d a ) of the aggregates (d a D 2 ffiffiffiffiffiffiffiffiffiffi a a =p p ) (Eggersdorfer et al 2012a). Park et al (2004a,b) showed that this approximation is accurate for diesel aggregates of up to D a;meas nm.…”

“…Park et al (2004a,b) showed that this approximation is accurate for diesel aggregates of up to D a;meas nm. Considering this approximation, and by combining Equations (1) and (2), D a;opt can be written in terms of m and d m (Eggersdorfer et al 2012a):…”

“…In an another approach, Eggersdorfer et al (2012a,b) proposed an indirect method for the measurement of the primary particle diameter using particle mobility, particle mass and two constants, projected area exponent D a and prefactor k a , estimated from either numerical simulations (Medalia 1967;Meakin et al 1989;Pierce et al 2006;Al Zaitone et al 2009;Eggersdorfer et al 2012a) or experiments (Megaridis and Dobbins 1990;K€ oyl€ u and Faeth 1992;K€ oyl€ u et al 1995). Average primary particle diameters of flame-made zirconia particles estimated by this model have been shown to be in good agreement with those obtained from standard TEM analysis.…”

“…Considering this size variation, it is shown in the online supplementary information (SI) of this article that using the model constants D a and prefactor k a determined by Eggersdorfer et al (2012a) results in inaccurate estimation of the primary particle size for carbon soot. Here, it is shown for the first time that their power-law correlation is also true for clusters grown by surface growth (instead of sintering) if correct values of parameters D a and k a are used.…”