Determination of volume, scaling exponents, and particle alignment of nanoparticle agglomerates using tandem differential mobility analyzers

Tandem Differential Mobility Analyzers (TDMA) were used along with TEM analysis to determine agglomerate volume, scaling exponents for both mass-mobility diameter (D_fm) and friction coefficient-number of primary particles (η) for the mobility diameter in the range 30-300. nm. The larger agglomerates with d_m=250 and 300. nm require a temperature of 800 °C and a sintering time of 0.7. s to form a spherical shape compared to 600 °C for a mobility diameter of 150. nm. It is shown that the 3% decrease in mobility size of the 250 and 300. nm agglomerates with increasing sintering temperature (600-800 °C) is a result of a morphology change from an ellipsoid to a sphere during the sintering process. The effect of sublimation on the sintered particle size is negligible with less than a 0.5% decrease in diameter for a 300. nm mobility diameter agglomerate at 800 °C. The TDMA results show that D_fm is not dependent on mobility size range and that η is dependent on the size range. Both results are counter to predictions based on free molecular models. These results confirm previous results obtained using a DMA together with an aerosol particle mass analyzer (APM) and are shown to have about a factor of two smaller uncertainty. It is also experimentally demonstrated that the agglomerate particles with d_m=300. nm are partially aligned in the electric field of DMA. The correction for a random orientation results in a significant decrease in D_fm by 3.5% and a significant increase in η by 3%.