Passive Aerosol Generator

Piezoelectric Ultrasonic Transducer Theory
The PAG utilizes focused parabolic ultrasonic transducers to generate the encapsulating aerosol. Transducers generate the aerosol as described below.

Sound waves above the frequency normally detectable by the human ear, that is, above 16 to 20 KHz are referred to as ultrasonic waves. The molecules of matter transmitting a longitudinal wave move back and forward about mean positions in a direction parallel to the path of the wave. Alternate compressions and refractions in the transmitting material exist along the wave propagation direction. In shear waves, the molecules move perpendicularly to the direction of wave propagation. In most applications of ultrasonics, pulses or packets containing a number of oscillation cycles are sent through the liquid. A longitudinal wave pulse, when incident on the boundary between two materials having different sound velocities, is transformed into reflected and refracted shear and longitudinal waves. Snell's law governs the angles of reflection and refraction for both types of waves: sin q/V = Constant where q is the angle the beam makes with a plane normal to the intervening surface and V is the sound velocity. The practical application of ultrasonics requires effective transducers to change electrical energy into mechanical vibrations and vice versa. The application of a voltage across a piezoelectric crystal causes it to deform with an amplitude of deformation proportional to the voltage. Reversal of the voltage causes reversal of the mechanical strain.

A focused parabolic piezoelectric ultrasonic transducer is a ceramic chip that acts as a capacitor while connected to a continuous wave power amplifier. The power amplifier is driven through a sine wave cycle by a high frequency oscillator. The frequency of the oscillator is set by the known resonance frequency of the transducer. The transducer is vibrated at the frequency set by the oscillator and when this frequency is equivalent to the resonance vibration of the ceramic transducer, positive displacement of the transducer occurs.

This movement or displacement of the transducer sets up a high frequency pressure wave. This pressure (sound) wave is focused to a focal point defined by the parabolic shape of the transducer. Nominal frequency varies based on the individual transducer. The range of frequencies useful for the PAG range from 0.05 to 2.3 MHz. The mechanical energy, focused at a given point, with a specific coating or encapsulation material, causes the coating material to be "sheared" off while remaining in the liquid phase. The size of the droplet "sheared" off is dependent upon the frequency of the transducer. The droplet size is such that the coating material assumes the properties of a gas, slowly and evenly coating the process area. By using ultrasonic technology, the aerosol droplets formed have the same chemical properties of the liquid. The various chemical constituents of the liquid are NOT separated by physical properties such as boiling point, density differences, or by vapor pressure. The process of submerging the transducers in a coating liquid gives the PAG its unique feature. The liquid "acts like a gas" without being generated by heating or boiling of a liquid. Thus the chemical properties of the liquid are retained by the aerosol droplets.

Aerosol Size-to-Frequency Ratio
The size of the aerosol droplets can be defined by a variety of parameters. Examples are the Number Median Diameter, Weight Mean Diameter, Number Mean Diameter, and Sauter Mean Diameter. For purposes of the PAG, the Number Median Diameter (NMD) is the most common method of describing aerosol size. The NMD indicates that the physical diameter of 50% of the aerosol droplets are less than the NMD and that 50% are greater than the NMD. Attachment 1 describes the frequency to droplet size ratio.

The size of the droplets formed by the PAG are a function of the frequency of the transducer. Although described by a power function, in general, the aerosol droplet sizes decrease as the frequency of the transducer increases. The ideal droplet size meets two objectives. One, it is small enough to allow the aerosol to behave like a gas; it moves from areas of higher concentration to areas of lower concentration and does not "rain out" of the atmosphere in the process area. Secondly, the droplet size is large enough to "carry" the molecules in the solution; an adequate amount of the compound can be deposited on the surfaces within the process area and form a film or coating. The PK-2000 operates at a frequency of 2.3 megahertz and produces droplets with a NMD of 2 microns.

Function of Fluid Level to Aerosol Generation
The transducers are submerged in the coating fluid. The distance from the center of the surface of the transducers to the surface of the solution is another factor in efficiently generating the aerosol. The transducers are concave and parabolic and are referred to as "focused" transducers. This concave shape deposits the energy from the entire surface of the transducer into a much smaller area, thereby intensifying the energy at the focus point. The point where the energy will be focused is a geometrical function of the size and shape of any transducer. Generally, the point of focus will be about equal to the diameter of the transducer. The PK-2000 uses 1 inch diameter transducers. That distance between the center of the surface of the transducer to the ideal point of focus for a given liquid will be referred to as the distance to the point of focus.

In addition to the size and shape of the transducer, other factors affecting the distance to the point of focus such as solution viscosity and surface tension will be discussed later.

The distance to the point of focus must be precisely controlled. The actual distance to the point of focus is dependent upon the chemistry of the coating material, the power applied to the transducers, the temperature of the coating fluid, and the frequency of the transducers. A variance of one or two thousandths of an inch can greatly reduce the efficiency of the process. The PAG uses a slow recirculation system spilling over a weir dam to maintain the distance to the point of focus at a precise value during operation. In addition, the PK-2000 has a drive mechanism for the transducers that allows for fine adjustment of the distance to the point of focus.

Function of Leveling System to Aerosol Generation
It is imperative that the six ultrasonic transducers arranged in a hexagon within the PK-2000 all maintain the same distance to the point of focus during operations. The leveling system in the PK-2000 allows the machined plated that the six transducers are mounted in to be leveled relative to the liquid in the tank.

Coating Viscosity and Surface Tension
The viscosity and surface tension of the solution in the PAG affect the efficiency of aerosol generation and the power requirements for the system. Fluid surfaces exhibit features resembling the properties of a stretched elastic membrane; hence the term surface tension. The value of the surface tension at any interface is determined by the nature and the physical condition of the two substances in contact. The surface tension of a liquid (against air) decreases with rising temperature. An empirical formula known as the Eotvos-Ramsey law expresses surface tension as proportional to the temperature of the liquid.

The effect of surface tension on aerosol generation is a complex function. Using distilled water as a reference point, the efficiency of aerosol generation increases as surface tension increases. This relationship continues until the surface tension approaches the critical point for a given liquid. This critical point is empirically derived for each coating formula. After this critical point, further increase in surface tension decreases the efficiency for aerosol generation.

Surface tension works in a symbiotic relationship with viscosity. Viscosity is a property of the entire solution. As viscosity increases, aerosol generation decreases.

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