The invention relates generally to phosphors, specifically phosphors for use in fluorescent lamps. More particularly, the invention relates to europium and manganese activated aluminate phosphors doped with rare earth elements.
A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Phosphors of one important class are crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. With the right combination of activators and inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range.
Aluminate phosphors such as barium-magnesium-aluminate (BAM) are widely used as the blue-emitting component of the phosphor blends in most fluorescent lamps intended for white light generation. Such phosphors generally have the formula (Ba, Ca, Sr)MgAl10O17. These phosphors may contain various activator ions, which impart the phosphor property. For example, a divalent europium (Eu2+) activated phosphor absorbs ultraviolet (UV) emission (i.e., exciting radiation) from the mercury plasma in a fluorescent lamp and emits blue visible light. Furthermore, a divalent manganese (Mn2+) activated BAM phosphor produces blue-green emission in fluorescent lamps.
Despite its wide use, BAM is notorious for its shortcomings in brightness and maintenance, particularly in those applications involving exposure to high ultraviolet (UV) and vacuum ultraviolet (VUV) fluxes. These phosphors suffer from poor efficacy and lumen maintenance, specifically under high wall load conditions, which is usually found in compact fluorescent lamps (CFLs), and linear fluorescent lamps. Efficacy is the luminosity per unit of input electric power (measured in units of lumens/watt). Lumen maintenance is the ability of the phosphor to resist radiation damage over time. Because of these shortcomings, the blue BAM emission is reduced at a significantly faster rate over time than the emissions of the other color components in the blends or pixels. This results in a loss of lumens and a color shift in the overall light output.
It is believed that the poor efficacy and lumen maintenance are caused by UV-induced visible absorption centers, such as “color centers” and other lattice defects. Color centers are believed to be caused by lattice defects in the lattice that trap an electron or a hole, as described on pages 79-80 of K. H. Butler, Fluorescent Lamp Phosphors, Penn State University Press, 1980, incorporated herein by reference. It has been established that in many fluorescent lamp phosphors, the color centers are created by the 185 nm radiation emitted by the mercury plasma. The color centers induce absorption of the exciting radiation anywhere from the deep UV to the infrared region of the spectrum. Thus, these centers can degrade phosphor brightness by either absorbing the visible phosphor emission or by absorbing a part of the 254 nm mercury exciting radiation.
Therefore, it would be desirable to obtain a BAM phosphor with an improved efficacy and lumen maintenance.