Electret Pattern Fornation in Teflon by Pyroeletric and Photogalvanic Electron Emission From LiNbO3

High field emission of electrons in the c direction of pyroelectric LiNbO₃ has recently been reported [Ref.1]. With modest temperature variations, electron energies are shown to be sufficient to produce the characteristic X-rays from niobium in the crystal and from an adjacent copper foil. We report using these energetic electrons to create the patterned electret state in ETFE (Ethylenetetrafluoroethylene) and in FEP (Tetrafluoroethylene-hexa-fluoropropylene). Temperature sufficient to cause electron emission was established in a LiNbO₃ crystal near or in contact with the polymer films. The persistent negative charge near the surface of the films could then be measured nondestructively with a capacitive probe, which we have developed for other electret applications. Photogalvanic electron emission produced by light illumination of the doped photosensitive LiNbO₃ crystal was also tested for the electret pattern formation. The charge density injected into the polymer inferred from our measurements is limited by the maximum energy of the pyroelectrically and photogalvanically emitted electrons since the electret field opposes the pyroelectric and photogalvanic field. The density and spatial distribution of charges injected by the pyroelectricity of LiNbO₃ into TEFLON electrets are consistent with a selffocusing effect observed in a vacuum by the authors of reference