Extinction particle size

In order to calculate particle size distributions in the adsorption regime and also to determine the relative effects of wavelength on the extinction cross section and imaginary refractive index of the particles, a series of turbidity meas irements were made on the polystyrene standards using a variable wavelength UV detector. More detailed discussions are presented elsewhere (23) > shown here is a brief summary of some of the major results and conclusions. 

The particle size analysis techniques outlined earlier show promise in the measurement of polydispersed particle suspensions. The asumption of Gaussian instrumental spreading function is valid except when the chromatograms of standard latices are appreciably skewed. Calc ll.ation of diameter averages indicate a fair degree of insensitivity to the value of the extinction coefficient. 

Silver nanoparticles can be deposited on Ti02 by UV-irradiation. Deposition of polydisperse silver particles is a key to multicolor photochromism. The nanoparticles with different size have different resonant wavelength. Upon irradiation with a monochromatic visible light, only the resonant particle is excited and photoelectrochemically dissolved, giving rise to a decrease in the extinction at around the excitation wavelength. This spectral change is the essence of the multicolor photochromism. The present photoelectrochemical deposition/dissolution processes can be applied to reversible control of the particle size. 

The extinction is strongly size dependent, and hence wavelength dependent, and for particle sizes of around 100 nm the dependence on the extinction or scattering... 

If Da = 1 is defined as the transition between diffusionally controlled and kinetically controlled regimes, an inverse relationship is observed between the particle diameter and the system pressure and temperature for a fixed Da. Thus, for a system to be kinetically controlled, combustion temperatures need to be low (or the particle size has to be very small, so that the diffusive time scales are short relative to the kinetic time scale). Often for small particle diameters, the particle loses so much heat, so rapidly, that extinction occurs. Thus, the particle temperature is nearly the same as the gas temperature and to maintain a steady-state burning rate in the kinetically controlled regime, the ambient temperatures need to be high enough to sustain reaction. The above equation also shows that large particles at high pressure likely experience diffusion-controlled combustion, and small particles at low pressures often lead to kinetically controlled combustion. 

Light extinction coefficients per unit mass of chemical constituent are constant for growing aerosols for certain forms of the growth laws and particle size distributions. Constant coefficients simplify source allocation analyses for visibility degradat ion. 

The contributions of aerosol chemical species to the extinction coefficient can be estimated from knowledge of their mass distributions, densities, and refractive indices. It is assumed that the particles can be represented as spheres. For a mixture in which the composition is a function of particle size and all particles of a given size have the same composition, defined here as a specific mixture, the contribution of species i becomes (4 ) ... 

Plots of each of these quantities as a function of particle size would look quite different and, therefore, would tell different stories. Except for a scale factor, each of them plotted as a function of wavelength for the same particle size would be identical. In our first example of extinction (Fig. 4.6) we displayed the efficiency Qext, as we shall often do in this chapter. In Chapter 12, however, our preference switches to the extinction cross section per unit particle volume. Unnormalized extinction cross sections (strictly speaking, the differential scattering cross section integrated over the acceptance angle of the detector) are more appropriate in Section 13.5 on particle sizing. 

Extinction features that strongly depend on particle size will be obscured, if not totally obliterated, in a polydispersion. Many analytical expressions for the radius probability distribution have been used in Mie calculations. For purposes of illustration we have chosen the Gaussian distribution, according to which the probability that a sphere has radius between a and a + da is... 

Calculations for a range of particle sizes are shown in Fig. 11.13. Note that the scales have not been shifted for the different sizes extinction increases with size because of scattering. The extinction band for the 0.1-jam particle faithfully reflects the characteristics of the intrinsic absorption band. But asymmetries develop for particles larger than about 0.2 jam indeed, at a radius of 0.3 jam the absorption band looks like an emission band relative to the continuum. The explanation for this strange extinction behavior near an absorption band lies in the preceding section extinction is not a steadily increasing function of bulk absorption. A narrow absorption band is similar to a small absorption edge that falls just as rapidly as it rises, which can thus cause extinction peaks, dips, or both. 

There are some notable differences apparent in Fig. 11.14 between the extinction curves for aluminum spheres and those for water droplets. For example, av is still constant for sufficiently small aluminum particles but the range of sizes is more restricted. The large peak is not an interference maximum aluminum is too absorbing for that. Rather it is the dominance of the magnetic dipole term bx in the series (4.62). Physically, this absorption arises from eddy current losses, which are strong when the particle size is near, but less than, the skin depth. At X = 0.1 jam the skin depth is less than the radius, so the interior of the particle is shielded from the field eddy current losses are confined to the vicinity of the surface and therefore the volume of absorbing material is reduced. 

With the small mean particle sizes (>— 15and less), it was found for underoxidized propellants that pressure increase would cause the burning rate to increase, reach a plateau, decrease, and then possibly extinguish at some critical pressure. Where extinction did not occur, the burning rate would continue to decrease after the plateau, reach a minimum and then again increase monotonically. Further reduction of oxidizer content accentuated the tendency toward plateau behavior. The... 

Pearson (66) found that hot solid surfaces drastically accelerate the ignition of these vapors. In line with our identification of the A/PA reaction zone as the major heat source, it is expected that both burning rate and extinction pressure depend on the total surface area of AP particles exposed to the A/PA reaction occurring in the pores of the ash. Thus, as observed experimentally, both burning rate and extinction pressure depend upon oxidizer particle size. However, this interpretation is obscured by the fact that combustion inefficiency, an important parameter, is also expected to be particle size dependent. [Total AP surface area exposed to A/PA reaction zone = A = na, where n = (number of AP particles exposed), (volume of each AP particle)"1 d 3, a = (exposed surface area of each AP particle) — dr. Therefore, A — d1.]... 

Figure 10.8 Optical extinction of two-dimensional gold nanoparticle arrays with different unit particle sizes.33 (Reprinted with permission from B. Kim et al., J. Am. Chem. Soc. 2001,123, 7955-7956. Copyright 2001 American Chemical Society.)...

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