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FIGURE 11.75 Measured secondary electron yields from the electron bombardment of 89-nm KC1 particles, 128-nm particles of dioctyl subacate (DOS), and KC1 coated with DOS of film thickness (in nm) shown (adapted from Ziemann and McMurry, 1998).

an electron beam and the change in the charge of the particles is measured using a Faraday cup. The number of secondary electrons emitted per primary electron incident on the particle, defined as the secondary electron yield, is characteristic of the chemical composition and of particle size. Figure 11.75, for example, shows the measured secondary electron yields for an 89-nm KC1 particle, a 128-nm particle of the ester dioctyl sebacate (DOS), and KC1 coated with DOS of various film thicknesses as a function of the energy of the bombarding electron beam (Ziemann and McMurray, 1998). Clearly, the yields are quite different for the organic and KC1 and are a sensitive function of the thickness of the DOS film on the KC1.

Ziemann and McMurray (1998) also measured secondary electron yields and transport efficiencies of NaCl particles coated with octacosane upon exposure to relative humidities above the NaCl deliquescence point and then after drying. These data suggested that upon drying, the original organic film becomes localized on one side of the particle, giving it an irregular shape, in much the same manner as observed for the effect of water on surface nitrate (Figs. 11.63 and 11.64). Similarly, the secondary electron yields indicated that mixtures of NaCl and NaNO-, crystallized heterogeneously whereas those of NaCl and NH4C1 were homogeneously mixed (Ziemann and McMurray, 1997).

5. Generation of Calibration Aerosols

Calibration of the instruments to measure the size distribution and chemical composition requires methods of generating aerosols of well-defined sizes and known composition. Generating aerosols of known

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A monodisperse aerosol is one with a narrow size distribution, which, for log-normal-distributed particles, usually means a geometric standard deviation of about 1.2 or smaller. Monodisperse particles are expected to have simple shapes and uniform composition with respect to size. A polydisperse aerosol, on the other hand, is one containing a wide range of particle sizes, but which may otherwise be homogeneous in terms of the basic physical and chemical properties that are not related to size. The term heterodisperse is also used occasionally; this describes aerosols varying widely in physical and chemical characteristics, as well as size.

As discussed in detail by Raabe (1976), an investigator's use of the terms monodisperse and polydisperse aerosols may depend on the particular properties of importance in the study; thus an aerosol may consist of particles of the same size, that is, be monodisperse with respect to size, but may vary in settling speed due to variations in density, that is, be polydisperse with respect to settling speed.

There are a number of techniques for generating aerosols, and these are discussed in detail in the LBL report (1979) and in volumes edited by Willeke (1980) and Liu et al. (1984). We briefly review here the major methods currently in use; these include atomizers and nebulizers, vibrating orifices, spinning disks, the electrical mobility analyzer discussed earlier, dry powder dispersion, tube furnaces, and condensation of vapors from the gas phase.

a. Atomizers and Nebulizers

Aerosols may be produced by atomizing liquids or suspensions of solids in liquids. Nebulizers are a type of atomizer in which both large and small particles are initially produced but in which the large particles are removed by impaction within the nebulizer. As a result, only particles with diameters < 10 /¿m exit most nebulizers.

There are two basic means of generating particles from liquids in nebulizers: compressed air or ultrasonic vibration. Figure 11.76 shows one relatively simple type of compressed air nebulizer. The compressed air shoots out of a small orifice at high velocity, creating a reduced pressure in the region of the orifice; a feed tube connected to the liquid through a small opening is subjected to this region of lowered pressure (the Ven-turi effect) and hence liquid is drawn up from the reservoir and exits as a thin stream. The flow of highvelocity air striking the liquid stream breaks it up into small droplets and carries this aerosol toward the exit.

Aerosol out

Aerosol out

Coarse spray droplets impact on wall and drain to reservoir

Liquid feed tube Liquid reservoir

Compressed air inlet

FIGURE 11.76 Schematic diagram of compressed air nebulizer (from Hinds, 1982).

Coarse spray droplets impact on wall and drain to reservoir

Liquid feed tube Liquid reservoir

Compressed air inlet

FIGURE 11.76 Schematic diagram of compressed air nebulizer (from Hinds, 1982).

The larger droplets are removed by impaction on the curved wall, and the smaller particles exit the device. Detailed descriptions of other types of compressed air nebulizers that differ somewhat in design are found in Raabe (1976) and Hinds (1982).

These compressed air nebulizers produce polydisperse aerosols. After the aerosol is produced, the size distribution may change due to evaporation of liquid from the droplets. In addition, the particles may be electrically charged due to an ion imbalance in the droplets as they form; if such charges become further concentrated due to evaporation, the particle may break up into smaller particles. Thus electrical neutralization of the aerosol, for example, by exposure to a radioactive source, is usually necessary to prevent electrostatic effects from dominating the particle motion, coagulation, and other behavior.

Nebulization may be used to produce suspensions of liquid droplets in air by using the pure liquid as the fluid or by using liquids with low vapor pressures dissolved in volatile solvents that then evaporate off the particle. Suspensions of solid particles in air may also be generated using the nebulization of suspensions of insoluble materials (insoluble plastic particles suspended in organic solvents, aqueous colloidal suspensions, e.g., of ferric hydroxide, etc.) or of soluble materials dissolved in water (e.g., salts in water). Drying the aerosol after its generation is an important factor in the final aerosol produced since this may alter both the physical and chemical nature of the particles; for example, rapid drying may produce low-density particles that are basically hollow shells formed by crystallization on the surface of the drying droplet.

Although these devices produce polydisperse aerosols, monodisperse aerosols can be generated by following the nebulizer with a size fractionating device such as an aerosol centrifuge or a differential mobility analyzer. Alternatively, nebulizers can be used to produce monodisperse aerosols of solid particles if suspensions of particles of one size are used as the generating fluid; for example, polystyrene and polyvinyltoluene latex beads of uniform size from O.f to 3.5 /¿m are commercially available in water suspensions. However, care must be taken to ensure that the vast majority of droplets formed contain only one sphere; otherwise, when the liquid evaporates, clusters of spheres will be formed. In addition, care must be taken to be sure they do not carry significant electrical charge (an anionic surfactant is added to the suspension to inhibit coagulation of the spheres). Finally, under some conditions, significant concentrations of other small particles may be simultaneously produced due to drying of empty droplets that contain impurities in the suspending liquid.

b. Vibrating-Orifice Generator

Another common method of generating particles with diameters 0.5-50 pm is the Berglund-Liu (1973) vibrating-orifice generator shown schematically in Fig. 11.77. The solution to be aerosolized is pumped through a small orifice 5-20 jun in diameter. The orifice is oscillated by a piezoelectric crystal so that the liquid stream is broken on each oscillation, forming a small liquid particle that is carried away in a stream of air.

Monodisperse droplets

Liquid chamber Dispersion

Orifice plate

Piezoelectric crystal

Orifice plate

Piezoelectric crystal

Electrical signal in

Dispersion — air in

FIGURE 11.77 Schematic diagram of vibrating-orifice aerosol generator (from Hinds, 1982).

Electrical signal in

Dispersion — air in

FIGURE 11.77 Schematic diagram of vibrating-orifice aerosol generator (from Hinds, 1982).

These droplets are then mixed with more air to dry the particles. The rate of droplet formation is equal to the oscillation frequency (/) of the piezoelectric crystal. From the volumetric flow rate of the liquid and the frequency /, the volume of the individual drops and particle diameter can be calculated.

c. Spinning-Disk Generator

A third means of producing aerosols is the spinning-disk aerosol generator. The liquid is fed to the center of the spinning disk and then moves by centrifugal force to the outer edge. It accumulates at the edge until there is sufficient liquid that the centrifugal force exceeds the surface tension forces and a droplet of liquid is thrown off. Some smaller "satellite droplets" are also produced, but these are separated from the primary drops, which are thrown out further by using a flow of air. Monodisperse aerosols with diameters >0.5 /¿m, and more typically in the range ~ 20-30 yu,m, are produced, the size being determined by the radius of the disk and the speed of the rotation (Hinds, 1982).

d. Dry Powder Dispersion

When an aerosol consisting of solid particles is required, they are usually generated by the dispersion of a dry powder. One of the most common devices that is particularly effective for dry, hard materials such as silica is known as the Wright dust feed (Wright, f950). The dust is packed into a plug and a sharp blade is used to scrape dust off the surface of the plug; the particles are then swept away by a stream of air along the outer edge of the scraper blade and exit through a jet onto an impactor to break up particle clusters. The aerosol produced has particle diameters <10 /um. Variations on this type of aerosol generator are described in the LBL report (1979).

Fluidized beds have also been used for generating suspensions of solid particles with diameters in the range of ~ 0.5-40 /¿m. Air flows through the fluidized bed, which contains beads kept suspended by the motion of the air; dust injected into the bed is broken up into small particles and carried out with the air flow (Raabe, 1976).

A device such as an impactor or cyclone is frequently used at the exit of these dry powder dispersion devices to eliminate the large particles. A charge neu-tralizer is usually used to reduce the electrostatic charges on the dispersed particles.

e. Tube Furnaces

Monodisperse particles of salts, metals, metal oxides, and carbon have also been generated using electrically heated tube furnaces (e.g., Scheibel and

Porstendorfer, 1983; Ramamurthi and Leong, 1987). Salt particles, for example, can be generated by inserting the salt in a "boat" inside the tube furnace with a flow of N2 passing over the salt. The salt vaporizes at the high temperatures and is carried downstream where the gas stream is cooled and the salt undergoes condensation into small particles.

Particles consisting of metals and metal derivatives such as the oxides can be generated in a two-step process. Primary aerosol particles are first generated using a vibrating-orifice generator and then dried by passing through a heated tube. This dried aerosol then passes into a tube furnace where the secondary aerosol is generated. For example, particles of nickel or nickel oxide can be generated from a primary aerosol of nickel formate using a tube furnace with N2 or air as the carrier gas, respectively. Similarly, particles of ferric oxide or iron can be obtained from the thermal decomposition of ferrous sulfate heptahydrate in N2 or in a mixture of H2/N2, respectively (Ramamurthi and Leong, 1987).

/. Condensation

When nuclei are present under conditions of supersaturation, condensation occurs on the nuclei, causing them to grow. If the particle radius is much less than the average distance between the particles, the growth rate of the particle with time is uniform, and since the nuclei are so much smaller than the final particles formed, nuclei of different initial sizes all give rise ultimately to particles of the same size, that is, a monodisperse aerosol, with diameters typically in the range 0.003-1.0 /¿m.

In practice then, a liquid having a low vapor pressure such as oleic acid or lubricating oils is carried by an inert gas to a heater where it vaporizes. As it leaves the heated area, it condenses to form the aerosol. Different generator designs based on condensation of a supersaturated vapor are discussed in the LBL report (1979).

However, care must be taken to ensure that seed nuclei are present on which condensation can occur. If a very clean system is used in which nuclei are not present, spontaneous nucleation may occur; this process is such that nuclei do not appear uniformly either in space or in time, and the initial particle growth rate depends on the degree of supersaturation. As a result, a polydisperse aerosol is produced under these conditions.

While condensation from a supersaturated vapor can be used to produce liquid particles, it is not as easily applied to the generation of solid particles except those that can be liquefied at modest elevated temperatures. However, these may be generated using different techniques. Methods used in the past have included the vaporization of wires or of salts fused onto wires, the use of "exploding" wires, and the use of electric arcs. For example, heating metal wires in an inert gas such as N2 to a sufficiently high temperature produces small particles in the condensation nuclei and Aitken nuclei range. Thus tungsten wire at ~1000-f200°C produces tungsten metal particles, whereas nichrome wire gives chromium particles.

Solid salt particles may be produced by fusing the salt onto the wire by immersing the wire in a saturated solution of the salt and passing an electrical current through it. When the wire is heated, salt particles are produced. Theoretical and practical considerations for such generators are described by Fuchs and Sutugin (1970).

The exploding wire method involves putting a large amount of energy into a wire suddenly, causing it to "explode." If 02 is present, a metal oxide aerosol is produced, whereas particles of pure metal are formed in an inert atmosphere such as helium. Exploding wire generators and their size distribution characteristics have been discussed by Phalen (1972).

Finally, electric arcs have also been used in some cases to produce solid particles of the electrode material, such as graphite, as have argon plasmas at high temperatures which produce a finely divided aerosol on rapid cooling.

All these methods generally give (< 1 ^m) polydisperse aerosols of the solid particles and, unless rapid air dilution is provided, coagulation leads to large agglomerates of the small primary particles.

Gas-phase reactions can also be used to produce products of low volatility that condense to give an aerosol. The reaction of gaseous NH-, with HC1 to form particles of solid ammonium chloride and the reaction of gaseous S03 with water vapor to form H2S04 are typical examples. Such methods tend to give submicron particles.

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