Electroluminescence Cells Based on the Lamellar Solid Hydrogen Uranyl Phosphate

Abstract

Electroluminescence (EL) cells have been constructed with the layered, ionically conducting solid, hydrogen uranyl phosphate, HUO2PO4 9 4 H20 (HUP), as the emissive medium. With ac excitation, both uranyl emission and molecular nitrogen plasma emission are observed, with the latter appearing to excite the former; the+ uranyl EL spectrum matches the photoluminescence spectrum of the solid. Similar results were obtained with fully substituted sodium (NaUP), magnesium (Mg0.sUP), and pyridinium (pyHUP) derivatives of HUP. For all of these solids, the dependence ~f the EL intensity on sample thickness, ac frequency, and applied voltage has been determined. Typical operating conditions are 1.5-3.0 kV at 0.2-4 kHz. Impedance measurements permitted acquisition of dielectric constants and ionic conductivities for these solids, both of which decrease in the order HUP > NaUP > Mg0.sUP > pyHUP. A model describing the dependence of EL intensity on cell parameters is presented.Display devices involving electroluminescence (EL) have traditionally been based on semiconductors (1, 2) and gas plasma discharges (3, 4). As part of an ongoing investigation of solid-state electro-optical properties, we have studied the photoluminescence (PL) of the lamellar solid, hydrogen uranyl phosphate, HUO2PO4-4H20 (HUP) (5). Since this highly emissive compound is also known to be a good proton conductor (6), we sought to determine whether HUP could serve as the basis for an EL cell.In this paper we report that ac-induced EL can be observed from cells containing HUP and several fully-substituted derivatives of HUP. Characteristic nitrogen plasma emission, arising from the gaseous ambient, is observed and appears to excite the uranyl emission; the uranyl EL spectrum matches its PL counterpart. The dependence of EL intensity on sample thickness, ac frequency, and applied voltage has been determined and impedance measurements have yielded dielectric constants and ionic conductivities for HUP and its derivatives. We present a model based on these data that accounts for the dependence of EL intensity on cell parameters. ExperimentalMaterials.--Samples of HUO2PO4 9 4H20 (HUP), NaUO2PO+. 3 H20 (NaUP), Mg(UO2PO4)2 9 9 H20 (Mg0 sUP), and (CsHsNH)UO=PO4 (pyHUP) were prepared as previously described (5,7,8), except for NaUP and Mg0 sUP, which were slurried in a more dilute 1M solution of the appropriate cation. After isolation, the samples were thoroughly air-dried before use in EL experiments. X-ray powder diffraction patterns were obtained for each solid, using a Nicolet I2/V powder diffractometer, to confirm the identity and single-phase nature of each material. Nitrogen gas ("boil off" from liquid N~) and helium (Badger Welding Supply, Incorporated, 99.99%) were used as received.EL experiments.--The cell shown in Fig. 1 was used for the majority of EL experiments. The cell consists of a cylindrical acrylic holder (3.8 cm diam x 2.8 cm) containing a 0.64 cm diam hole at its center. A cylindrical A1 rod (0.64 cm diam • 2.7 cm) can be inserted into the hol...