Abstract:
This invention relates to a CeYMg Aluminate fluorescent lamp phosphor which emits ultraviolet radiation in a preferred region of the electromagnetic spectrum proven useful for the treatment of psoriasis. The phosphor in fluorescent lamps shows improved performance for this application over other phosphors known to the art.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a cerium yttrium magnesium aluminate fluorescent lamp phosphor possessing the hexagonal magneto plumbite structure and to fluorescent lamps incorporating the phosphor. 
     Philips Electronics and Associated Industries in British Pat. No. 1,452,083 disclosed an ultraviolet emitting fluorescent lamp phosphor having the composition CeMgAl 11  O 19  and the magneto plumbite structure. H.F. Ward of Thorn Industries, in British Patent 1,194,014 disclosed a phosphor having the composition CeAl 11  O 18  and the magnetoplumbite structure. Although CeAl 11  O 18  and CeMgAl 11  O 19  have similar crystal structures, incorporation of magnesium shifts the UV excited emission peak from about 460 nm in CeAl 11  O 19  to about 370 nm in CeMgAl 11  O 19 . These phosphors are self-activated by the Ce 3  + ion. 
     SUMMARY OF THE INVENTION 
     It has been found that certain hexagonal magneto plumbite aluminate solid solutions of cerium yttrium magnesium aluminate composition show superior performance for use in fluorescent lamps used in the treatment of psoriasis. Phosphor compositions of this invention may be defined by the following formula: 
     
         Ce.sub.x Y.sub.y Mg.sub.z Al.sub.11 O.sub.16.5+3/2(x+y)+z 
    
     Where: 
     X may lie between 0.375 and 0.750 
     Y may lie between 0.050 and 0.350 
     X + y less than or equal to 1.0 
     Z may lie between 0.500 and 2.000 
     In accordance with a preferred embodiment, 
     X may lie between 0.550 and 0.750 
     Y may lie between 0.050 and 0.250 
     Z may lie between 0.500 and 1.500 
     The invention also includes fluorescent lamps incorporating the phosphor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 in connection with the above description of some of the aspects of the invention. 
     Phosphors of this invention may be prepared easily by blending the required amounts of appropriate salts and firing in a reducing atmosphere at an elevated temperature for a modest time interval. Time, temperature, and reducing atmosphere are not critical, but a mixture of nitrogen and hydrogen or hydrogen are preferred, a temperature range from 1450° C. to 1750° C. is preferred, and a time interval from 1 to 5 hours is preferred. The particular salt of the metal ions to be used is not critical. Preferred salts are Al(OH) 3 , MgO, Y 2  O 3 , and CeO 2  or CeF 3 . The currently especially preferred composition has the approximate molar formula 
     
         Ce.sub.0.60± .05 Y.sub.0.15± .05 Mg.sub.0.60±.15 Al.sub.11 O.sub.18.2± .3 
    
     example 
     preparation of Ce.sub..611 Y.sub..150 Mg.sub..611 Al 11  O 18 .252 
     10.516 grams of CeO 2 , 2.500 grams of MgO, and 1.694 grams of Y 2  O 3  are mixed with 87.46 grams of Al(OH) 3 . The mixture is fired at 1550° C. for four hours in an atmosphere consisting of 75 volume percent N 2 , 25 volume percent H 2 . The resulting phosphor having the approximate molar composition 
     
         Ce.sub..611 Y.sub..150 Mg.sub..611 Al.sub.11 O.sub.18.252, 
    
     will consist essentially of hexagonal magneto plumbite phase. When the phosphor is fabricated into a 40 watt fluorescent lamp, the emission maximum will be 349 nanometers. The energy output after 100 hours operating time will be 94.3 percent of the initial energy output and the energy output will be distributed over the range 380 nanometers to below 320 nanometers as follows: 
     Watts output less than or equal to 320 nanometers -0.13 
     Watts output between 320 nanometers and 380 nanometers -7.4 
     Table 1 lists lamp data obtained with the phosphors of this invention compared to previous state of the art phosphors. 
     
                                           TABLE 1__________________________________________________________________________LAMP DATA FROM 40 WATT FLUORESCENT LAMPS                     Energy in WattsMolar               Emission                     at 100 Hrs                               Maintenance (%)Source Composition   Peak (nm)                     320nm                         320-380nm                               100 Hrs                                    250 Hrs__________________________________________________________________________Invention Ce.sub..611 Y.sub..150 Mg.sub..611 Al.sub.11 O.sub.18.252               349   .13 7.4   94.3 93.5 Ce.sub..750 Y.sub..250 Mg.sub.1.0 Al.sub.11 O.sub.19               353   .11 6.9   92.2 --StandardPsoriasisPhosphor BaSi.sub.2 O.sub.5 :Pb               352   .18 7.2   72.3 63.8U.S. Pat.4,088,922 Ce.sub..611 Ba.sub..20 Mg.sub..611 Al.sub.11 O.sub.18.228               344   .36 7.2   88.9 88.5&#34;     Ce.sub..786 Ba.sub..05 Mg.sub..786 Al.sub.11 O.sub.18.515               348   .20 6.7   --   87.9&#34;     Ce.sub..611 Sr.sub..2 Mg.sub..611 Al.sub.11 O.sub.18.228               341   .61 6.9   --   90.1&#34;     Ce.sub..611 Ca.sub..3 Mg.sub..611 Al.sub.11 O.sub.18.328               338   .57 5.6   --   90.4British Pat.1,452,083 Ce.sub.1.0 Mg.sub.1.0 Al.sub.11 O.sub.19               365   .06 4.2   --   88.2__________________________________________________________________________ 
    
     There are several points from the table that warrant discussion. 
     First, the phosphor prepared according to this invention shows comparable or superior energy output in the 320 to 380 nanometers range after 100 hours operation relative to the currently used (standard) BaSi 108  2 O 5  : Pb phosphor. In addition, the energy output in the erythermal region (less than 320 nanometers) is significantly less than that produced by the standard phosphor. Finally, the maintenance of the phosphor according to this invention is much greater than standard. Thus, the output of the cerium yttrium magnesium aluminates will increase relative to that of the BaSi 2  O 5  : Pb with operating time. Furthermore, the need to adjust the time interval of exposure due to lamp aging will be a much less serious problem with phosphors of this invention compared to BaSi 2  O 5  Pb. 
     Second, the phosphor claimed in U.S. Pat. No. 4,088,922 shows a similarly improved performance over BaSi 2  O 5  : Pb. However, the material has higher energy output in the erythermal region and therefore is not as well suitable for use in psoriasis fluorescent lamps as phosphors of this invention. 
     Finally, while the CeMg Aluminate of British Patent No. 1,452,083 has low energy output in the erythermal region, it also has significantly lower output in the desired range of 320 to 380 nanometers and lower maintenance than the compositions of the invention. 
     While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.