| 摘要 |
1,172,873. Electrographic material. KATSURAGAWA DENKI K.K. 28 Feb., 1967 [7 March, 1966], No. 9405/67. Heading H1K. A photo-sensitive element for use in electrography comprises a thin photo-conductive layer, at least one side of the layer being integrally bonded with a trapping layer and a highly insulating layer on the surface of the trapping layer. The photo-conductive layer changes its volume specific resistivity by at least a factor of 10 upon irradiation and the trapping layer contains a large number of impurity levels which trap charge carriers injected therein sufficiently deep to prevent thermal release of the carriers. A method of electrography comprises applying a D.C. voltage of one polarity across a photo-sensitive element as above and then applying a D.C. voltage of opposite polarity across the element concurrently with imagewise exposure. The conductive layer 1 (Fig. 1) may be of Al, paper, Nesa glass or plastics sheet. The photo-conductive layer 2 may be sintered or vapour deposited films of CdS, CdSe, Se, ZnO, polyvinyl carbazole or mixtures thereof. The trapping layer 3 may be ZnS, ZnCdS 2 activated with Cu, Ag or Pb, anthracene, anthraquinone, S or PbO. The insulating layer 5 may be transparent and of polyester. A combined photo-conductive and trapping layer may be made by diffusing impurities into a layer of photo-conductive material and then performing a second diffusion to give at least one thin layer which has trapping properties. The D.C. voltages may be applied either with an electrode or a corona discharge. A drum 14 (Fig. 4) has a conductive layer 1 thereon, a photo-conductive layer 2, a trapping layer 3 and an insulating layer 5. An object 18 is illuminated by light sources 19 and focused on to the drum by the optical system 20. A corona discharge 15 and if required a light source 16 first apply a voltage of one polarity to the drum and a second corona discharge 17 applies a voltage of opposite polarity. In an alternative embodiment (Fig. 3, not shown), a charging and exposing system using an electrode is disclosed. It is said that the sequential application of voltages of opposite polarities produces an apparent change in the dielectric constant of the trapping layer and that differences in the persistent internal polarization due to exposed and unexposed portions of the layer are thereby emphasized. Development of the electrostatic image may take place under illumination since the charge pattern can only be erased by the application of an electric field. |
