Nanometer-sized silver particle measurements by low pressure differential mobility analyzer and its classification performance

Size distribution measurements and classification performance tests under low pressure conditions ranging from 123 to 300 Torr were systematically carried out for nano-meter sized silver particles formed in a nitrogen gas flow at atmospheric pressure using a new type of differential mobility analyzer (LPDMA) based on the Vienna type design concept which has a short classification zone. Although the values of the volumetric average diameter, D-V,D-DMA, are approximately 7% larger than those of the volumetric average diameter, D-V,D-TEM, obtained from the transmission electron microscope (TEM) observation results of the silver particles collected at the LPDMA inlet, both the DMA and TEM size measurements are found to have good correlation in the range from 6 to 15 nm. When the ratio of the sheath gas flow rate and aerosol gas flow rate is set at 5:1, the theoretical sizes calculated from the classification voltage applied to the LPDMA at 160 Torr are also found to be in good correlation with those of D-V,D-TEM obtained from the TEM observation results of the silver particles collected after the classification by the LPDMA in the range from 6 to 25 nm, although the theoretical sizes set by the LPDMA are approximately 14% larger than those of D-V,D-TEM measured from the TEM observation results after the classification by the LPDMA. The geometric standard deviations of the size distributions obtained from the TEM images after classification range from 1.08 to 1.17, and they are approximately equal to the theoretical value (1.093) of the particles classified by the LPDMA, which is derived from the assumption that the broadening effects by Brownian diffusion are ignored, whereas the geometric standard deviations of the size distributions obtained from the TEM images before the classification range from 1.34 to 1.50 and are greater than those after classification. Thus, we experimentally demonstrate the validity of our LPDMA system in size measurements and classification of the nm-sized particles under low pressure conditions.