Valérie Buissette

New luminescent rare earth activated oxynitridosilicates and oxynitridogermanates with the apatite structure

New luminescent oxynitrides based on the apatite structure doped with Eu2+ and Ce3+ have been prepared by ammonolysis of oxysilicate or oxygermanate precursors. The host compounds have the general formula La10−xSrx(Si/Ge)6O27−x/2−3y/2Ny. When doped with divalent europium or trivalent cerium they present blue, green, yellowish green or orange luminescence under UV/blue excitation. Remarkable long wavelength emissions centred beyond 500 nm are observed for the Ce3+ activated oxynitridosilicates and oxynitridogermanates, with compositions La8.9Ce0.1SrSi6N0.37O25.95, La8.9Ce0.1SrSi6N1.06O24.91 (λem = 545 nm) and La9.9Ce0.1Ge6N2.46O23.31 (λem = 590 nm). The treatment of precursor oxides with the required cationic ratios in NH3 gas was used as a convenient general method to prepare the oxynitride materials at moderate temperatures. This allowed decreasing the nitriding temperature from 1600–1400 °C to 1050–600 °C with respect to the method generally used for the synthesis of luminescent (oxy)nitride silicates, the solid state reaction between oxides, nitrides and carbonates under a N2 or N2–H2 flow.

Modeling the role of phosphor grain packing in compact fluorescent lamps

Compact fluorescent lamps contain mercury gas which generates ultraviolet radiation. A thin powder layer constituted of rare-earth oxides is coated inside the glass tube. The role of this layer is to convert the inside ultraviolet radiation into outside visible radiation. We focus here on a particular powder layer, constituted by phosphor grains. The phosphor layer has to achieve two distinct goals. On the one hand the grains have to absorb the maximum amount of ultraviolet radiation in order to generate visible light, and on the other hand the transmission of visible light has to be maximized in order to optimize the efficiency of the compact fluorescent lamp. Here, we study the influences of grain size, grain shape, density of packing powder, and thickness of the phosphor coating. Such a study is a first step towards a better understanding of the conversion efficiency of ultraviolet radiation into visible radiations, and can eventually, help to improve the production line of compact fluorescent lamps. All the presented simulations were performed with the commercial software LightTools® using a ray tracing method.