Method of applying phosphor coatings

#The use of mercury in common mass-produced products is declining because of environmental concerns and increased governmental regulation. This trend keenly affects the lighting industry since mercury has been a primary material in the manufacture of lamps for decades, particularly fluorescent lamps.

#In view of this, recent efforts have been made to reduce or eliminate mercury in fluorescent lamps. For example, PCT Patent Application No. WO 02/103748 describes a low-pressure gas discharge lamp based on an indium-containing gas filling. In particular, the lamp contains an indium halide, e.g., indium chloride, and an inert gas. The radiation emitted by the discharge has emission bands around 304, 325, 410 and 451 nm, as well as a continuous molecular spectrum in the visible blue range. A number of phosphors are listed for supplementing the radiation from the discharge in order to obtain white light. PCT Patent Application No. WO 2005/0456881, which is incorporated herein by reference, extends the list of available phosphors to use with the indium halide discharge to nitridosilicate and oxonitridosilicate phosphors.

#In order to apply a phosphor coating to the inner surface of a glass tube to produce a fluorescent lamp, the phosphor powders are mixed into slurries containing a polymer and various other liquids. These liquids can be organic solvents and/or water, depending on which polymer is being used. After the coating is applied to the glass and dried, the coating is generally baked at >500° C. in an air atmosphere in order to decompose the polymer and leave behind only the phosphor layer. However, nitridosilicate and oxonitridosilicate phosphors tend to degrade significantly as a result of the baking process which makes them generally unsuited for use in a conventional fluorescent lamp manufacturing process.

#FIG. 1 is a differential scanning calorimetery (DSC) graph of an ethylcellulose polymer.

#FIG. 2 shows DSC curves for an Ca—SiN phosphor in air and nitrogen showing the higher decomposition temperature of the Ca—SiN phosphor in nitrogen.

#FIG. 3 shows normalized emission spectra of a blend of red- and green-emitting phosphors before and after the baking at 550° C. in air.

#For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings.

#Making use of new phosphor types that have been developed for long-wavelength ultraviolet (UV) and blue/violet excitation assists in generating “white” light from radiation produced in indium halide and other similar Hg-free discharges. These phosphor types include nitridosilicate and oxonitridosilicate phosphors such as red-emitting phosphors (Sr,Ca)2Si5N8:Eu2+ and Ca2Si5N8:Eu2+, yellow-orange-emitting phosphors Ca1.5Al3Si9N16:Eu2+ and CaAl2Si10N16:Eu2+, and green-emitting phosphor SrSi2O2N2:Eu2+. Despite the high quantum efficiencies of these compounds for excitation at the long-UV and blue/violet wavelengths, the phosphors suffer from sensitivity to oxygen-containing environments during preparation as well as application to devices.

#The nitridosilicate and oxonitridosilicate phosphors are reactive at elevated temperatures in the presence of oxygen and if exposed to this environment they will degrade in varying degrees, up to a point of being useless. This may be partly due to the chemical composition of the lattice material and partly due to the relative instability of Eu2+activators in these lattices at elevated temperatures (Eu3+ is typically a more stable configuration). This behavior makes these phosphors unsuited for use in a standard fluorescent lamp manufacturing process wherein the phosphor coating is baked onto the glass envelope in air at a temperature sufficient to decompose the organic binder, typically >500° C.