Charging corona

Dielectrofiltration by the corona-charging, electrophoresis-dominated Cottrell technique is now widely used. 

It is apparent from the values of ps that diffusional charging is primarily effective with very small particles, as might be expected. With particles as large as 10 microns the charge level produced in any reasonable time is small compared with what can be achieved with corona charging. For the 0.1-micron particle, however, the charge level attained by diffusion is of the same order and may be larger if the mean free path correction is allowed for. 

A promising detection principle utilizes similar nebulization procedure as applied in ELS, namely the corona-charged aerosol detectors, CAD. In CAD, the aerosol particles interact with an ionized gas (usually nitrogen). The particles become charged and electrically detected [294]. It has been shown that the response of CAD does not depend on the nature of analyte. On the other hand, the size of the aerosol depends on the mobile phase composition and it has to be calibrated. 

In xerographic measurements, as illustrated in Fig. 5.3, the sample is corona-charged to a voltage Vq and then exposed to a short wavelength (absorption depth S L) step illumination. At the end of the illumination, there is a measurable surface potential, termed the residual potential V because of the bulk trapped charges. If positive charging is used, then is due to trapped holes in the bulk of the specimen. 

The powder is supplied to the gun by a powder pump at a uniform and consisfent rate. There are two types of powder pumps ejector type, using the venturi principle, and auger pumps, which use positive displacement. A corona charging system is most widely used to electrostatically charge the powder. The electrostatic spray gun has several functions ... 

Figure 9.11 Corona charged aerosol detector (CAD). (Courtesy of ESA)...

One of the important characteristics of gas-solid multiphase flows is concerned with the electrostatic effect. Particles can be charged by surface contact in a collision, by corona charging and scattering in an ionized gas, by thermionic emission in a high-temperature environment, and by other charging mechanisms such as colloidal propulsion... 

A variation of the experimental methods described above is the xerographic discharge technique which is gaining wide acceptance in the study of polymeric systems. The polymer film deposited on a metallic substrate is corona charged and... 

Figure 7 A schematic of a process in which the various steps are arranged around the circumference of a photoreceptor drum. Here, (1) is the corona charging apparatus, (2) the exposure system, (3) the development system, (4) the transfer corona, (5) the fusing apparatus, (6) the cleaning system, and (7) the erase exposure system. Processes of this kind are widely used for printer and low-volume copier applications.

A photoinduced discharge apparatus includes a corona charging unit, an exposure system, and a noncontacting voltmeter. A grid is usually positioned between the corona wires and the sample to control the corona current. The apparatus is normally provided with a beam splitter and radiometer. The surface potentials are determined by a capacitively coupled probe and FET amplifier. The time derivative of the voltage can be obtained electronically or by numerical differentiation on a computer. When a time resolution greater than 10 ms is adequate, commercial voltmeters, for example, Monroe Electronics Isoprobe or Trek Electrostatic voltmeters, can be used. These are feedback devices, based... 

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