Xerography

The photocopiers with which we are all familiar are based on a process known as either electrophotography or xerography. The latter is derived from the 

Greek for dry (xeros) and writing (graphos). The process was invented in 1938 and patented in 1940 by Chester F. Carlson and involves several different physical phenomena. At the time the invention was first made these processes were poorly understood and the first demonstration copies were not of the quality we are used to today. Carlson demonstrated his invention to twenty American companies and generated an enthusiastic lack of interest . Nevertheless, development was undertaken initially by the Battelle Memorial Institute and subsequently by the Haloid Company, now known as the Xerox Corporation. The first commercial xerographic copier appeared on the market in 1959 and is today a worldwide multi-billion pound industry. 

Electrical charge is deposited on the surface of the photoreceptor which has a high enough dark resistivity to prevent the charge leaking away. 

An image of the document to be copied is projected on to the photoreceptor which becomes conductive in the illuminated regions, so that the charge leaks away there. This leaves the photoreceptor with a charge pattern that is a positive copy of the original document. 

The toner neutralises the charge on the photoreceptor, so that the image can be transferred to a charged piece of paper. 

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Selenium is used in Xerography for reproducing and copying documents, letters, etc. It is used by the glass industry to decolorize glass and to make ruby-colored glasses and enamels. It is also used as a photographic toner, and as an additive to stainless steel. 

Amorphous (vitreous) selenium, vacuum-deposited on an aluminum substrate such as a dmm or a plate, was the first photoconductor commercially used in xerography (6). It is highly photosensitive, but only to blue light (2). Its light absorption falls off rather rapidly above 550 nm. Because of the lack of photoresponse in the red or near infrared regions, selenium photoreceptors caimot be used in laser printers having He—Ne lasers (632.8 nm), or soHd-state lasers (680—830 nm). 

J. Mort, TheA.natomy of Xerography McEadand Co., Pubhshers, Jefferson, N. C., 1989. 

C. F. Carlson, in J. H. Dessauer and H. E. Clark, eds.. History of Electrostatic Recording in Xerography and Kelated Processes Focal Press, New York, 1965, Cliapt. 1. 

J. H. Dessauer and H. Clark, Xerography and Related Processes Focal Press, London, 1965. 

However, the discovery that exposure to light could increase substantially the electrical conductivity of this polymer, i.e. it is said to be photoconductive, has led to important new applications, particularly in xerography. 

Returning to Mort s survey, we learn that the first widely recognised version of xerography was demonstrated by an American physicist, Chester Carlson, in 1938 it was based on amorphous sulphur as the photosensitive receptor and lycopodium powder. It took Carlson 6 years to raise 3000 of industrial support, and at last. 

Just as the growth of xerographic copying and laser-printing, which derives from xerography, was a physicists triumph, the development of fax machines was driven by chemistry, in the development of modern heat-sensitive papers most of which have been perfected in Japan. 

The elegance, cheapness and convenience of xerography for document copying has led to rapid commercial development on a colossal. scale throughout the world. 

To prepare the charge generation material of photoreceptor used in xerography, the crude VOPc synthesized at 150 °C for 4 h in the microwave synthesis was acid-treated, and then recrystallized. As shown in Fig. 4, the amorphous VOPc can be obtainol from crude VOPc by acid-treatment and the fine crystal VOPc can he obtained fixim amorphous VOPc by recrystallization. From XRD results, it can be calculated that the crystallite size of fine crystal VOPc is about 18 nm. As shown in Fig. 5, the fine crystal VOPc is well dispersed with uniform size. It indicates that this fine crystal VOPC can be probably used as the chaige generation material of photoreceptor. Thus, further research will be required to measure the electrophotographic properties of fine crystal VOPc. 

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