(278f) The Effect of Sampler Design on Nanoparticle Sizing

Abstract

Properties and final product performance of flame-made nanoparticles depend on their size and structure characteristics and their composition. However, aerosol characteristics often change during their transport in the sampling lines due to deposition, coagulation and/or fragmentation. These physical mechanisms are strongly affected by the dilution system artifacts (Burtscher, 2005), as the sample stream should be diluted and mixed rapidly early in the sampler, so that the chemistry is effectively quenched. In addition, online measurements such as scanning mobility particle sizer (SMPS) are highly intrusive methods as samplers perturbe the flame altering its dynamics and causing uncertainties in the measured particle concentration and characteristics as well as in the shape and spread of aerosol size distribution.

 Here the effect of different sampler design (one straight-tube and three hole-in-a-tube samplers of varying hole diameter and hole orientation), used to sample, dilute and quench hot, highly concentrated flame-made ZrO₂ nanoparticles on their real-time characterization is investigated at heights above the burner, HAB = 10 – 60 cm . Measurements by differential mobility analyzer (DMA) are compared to the Sauter mean primary particle diameter by off-line diagnostics and the corresponding geometric standard deviation is calculated by scanning transmission electron microscopy (STEM) images obtained by thermophoretic sampling at the flame centerline. The results are in agreement with discrete element modeling simulations (Goudeli et al., 2015) for coagulation by agglomeration in the absence of sintering, coalescence or surface growth. The sampling system does not significantly alter the shape of the aerosol size distribution, however, it does affect the mean particle characteristics: samplers in downstream hole orientation result in larger mobility diameters than in upstream or sidestream orientation, especially at fuel-lean flame conditions. More specifically, these differences become important at low heights above the burner. The straight-tube leads to smaller mobility diameters than the Ø 4 mm hole-in-a-tube sampler due to faster mixing and quenching that suppress further coagulation in the sampling lines.   

Furthermore, combined DMA and aerosol particle mass (APM) measurements are used to determine the mass-mobility exponent, D_fm, and average primary particle diameter by a power law correlation. The particle D_fm evolution from spherical to fractal-like structures is quantified by simple relationships in terms of the number of primary particles per agglomerate and the relative particle density. Experimental results obtained by real-time measurements for Ag (Kim et al., 2009), ZrO₂ (Eggersdorfer et al., 2012) and Cu (Stein et al., 2013) are compared to the above D_fm evolution exhibiting good agreement for low relative densities (fractal-like particles) but considerable deviation for larger ones (compact, spherical-like particles) due to the presence of ZrO₂ hard-agglomerates. These relationships can be used in process design optimization as well as climate dynamics and meteorological models.         

References

Burtscher, H. (2005). J. Aerosol Sci., 36, 896-932.

Eggersdorfer, M. L., Gröhn, A. J., Sorensen, C. M., McMurry, P. H., Pratsinis, S. E. (2012). J. Colloid Interface Sci., 387, 12-23.

Goudeli, E., Eggersdorfer, M.L., and Pratsinis, S.E. (2015). Langmuir31, 1320-1327.

Kim, S. C., Wang, J., Emery, M. S., Shin, W. G., Mulholland, G. W., Pui, D. Y. H. (2009). Aerosol Sci. Technol.43, 344-355.

Stein, M., Kiesler, D., Kruis, F. E. (2013). Aerosol Sci. Technol. 47, 1276-1284.