Particle size, characterization statistical diameters

When a distribufion of particle sizes which must be collected is present, the aclual size distribution must be converted to a mass distribution by aerodynamic size. Frequently the distribution can be represented or approximated by a log-normal distribution (a straight line on a log-log plot of cumulative mass percent of particles versus diameter) wmich can be characterized by the mass median particle diameter dp5o and the standard statistical deviation of particles from the median [Pg.1428]

In most applications, the systems and processes contain large amounts of particles with size distribution each size may also possess a distinguished shape. To describe properly these systems and processes for design and analysis, they need to be adequately characterized to reflect their physical and chemical potentials. In the following sections, different average particle diameter definitions are introduced along with statistical descriptions of particle size distribution. Depending on applications, one definition may be more suitable than others. Thus care must be exercised to select the proper characterization for each process. 

Statistically, the particle size distribution can be characterized by three properties mode, median, and mean. The mode is the value that occurs most frequently. It is a value seldom used for describing particle size distribution. The average or arithmetic mean diameter, d, is affected by all values actually observed and thus is influenced greatly by extreme values. The median particle size, is the size that divides the frequency distribution into two equal areas. In practical application, the size distribution of a typical dust is typically skewed to the right, i.e., skewed to the larger particle size. The central tendency of a skewed frequency distribution is more adequately represented by the median rather than by the mean (see Fig. 9). Mathematically, the relationships among the mean, median, and mode diameter can be expressed as... 

The size of the particles of a monodisperse aerosol is conq)letely defined by a single parameter, the particle diameter. Most aerosols, however, are polydisperse and may have particle sizes that range over two or more orders of magnitude. Because of this wide size range and the fact that the physical properties of aerosols are strongly dependent on particle size, it is necessary to characterize diese size distributions by-statistical means. For the purposes of this chapter, we neglect the effect of particle shape and consider only spherical particles. 

The micro-mechanics studies of the nanostmctured composite are reahzable. Figure 6.24(a) shows the micro-fracture line s deviation and interruption due to cavities inside the tested sample. Figure 6.24(b) illustrates the cross-section of a polymeric thin film with carbon nanotubes. By integrating specific software in the SEM it is possible to perform a statistical analysis of nanostructure size. Figure 6.25 shows an SEM picture representation of the carbon particles with different dimensions. From this micrograph the software calculates area, diameter (max, min and mean) and perimeter of each particle. This is a very important characterization, because in many nanotechnology applications the size of the nano-elements influences strongly the behaviour of apparatus and devices. 

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