Abstract:
The invention discloses new luminescent materials for use as UV-A phosphors in suntanning lamps. The luminescent materials are of general formula Ca 3-x B 2 P 2 O 11+0.5x :xCe 3+  and Ca 2.5-x B 5.5 P 2 O 15.75+0.5x :xCe 3+ .

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a luminescent material, more in particular to a luminescent material that is suitable for use as a UV-A phosphor in suntanning lamps. The invention relates also relates to a discharge lamp. 
     An example of a luminescent material that is widely used as UV-A phosphor in suntanning lamps is barium silicate activated with lead (further indicated as BSP). This material combines a high quantum efficiency with a spectral power distribution which is suitable to obtain suntanning lamps with a UV-B content of about 1%. The main drawbacks are a low maintenance and sensitivity towards water. In order to overcome these disadvantages a phosphor particle coating can be applied. However, such a coating is normally applied making use of a wet chemical process and BSP is soluble to a certain extent even at relatively high pH-values. For this reason the coated material exhibits a lower light output than the starting material. 
     SUMMARY OF THE INVENTION 
     The invention aims to provide a luminescent material that is suitable for use as a UV-A phosphor in a suntanning lamp and has a low solubility in water. 
     A luminescent material in accordance with the invention includes an alkaline earth borate activated with trivalent cerium of a general formula belonging to the group formed by Ca 3-x B 2 P 2 O 11+0.5x :xCe 3+  and Ca 2.5-x B 5.5 P 2 O 15.75+0.5x :xCe 3+ . 
     The luminescent materials according to the invention are stable in water, show a high quantum efficiency (&gt;80%) and can easily be excited with radiation having a wavelength of 254 nm (the wavelength of most the UV radiation generated by mercury in a fluorescent lamp). The maximum of the emission band is situated between 360 and 365 nm. For these reasons the luminescent materials are very suitable to be used in the luminescent screen of a fluorescent suntanning lamp. 
     In case x divalent calcium ions are replaced by x monovalent ions chosen from the group formed by Na + , K +  and Li + , the resulting luminescent material has no net electrical charge. It has been found that the presence of these monovalent ions improves the quantum efficiency and the absorption of a luminescent material according to the present invention. 
     Good results have been obtained for luminescent materials according to the invention, wherein 0.025≦×≦0.100. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The luminescent materials according to the invention were prepared by thoroughly grinding CaCO 3 , H 3 BO 3 , (NH 4 ) 2 HPO 4  and CeO 2  (and, if desirable, Li 2 CO 3 , K 2 CO 3  and/or Na 2 CO 3 ) in an agate mortar. The powder was subsequently heated in a reducing atmosphere containing N 2 , H 2  and H 2 O in an alundum boat at 700 C for an hour. The resulting product was ground into a powder and heated at 1050 C in the same reducing atmosphere for an hour. The obtained material was ground and sieved (30 μm sieve). 
     For instance Ca 2.45 B 5.5 P 2 O 15.775 :0.05Ce 3+  was prepared using 24.523 gram (245 mmol) CaCO 3 , 30.915 gram (550 mmol) H 3 BO 3 , 26.412 gram (200 mmol) (NH 4 ) 2 HPO 4  and 0.861 gram (5.0 mmol) CeO 2  as starting materials. Similarly Ca 2.9 B 2 P 2 O 11.05 :0.1Ce 3+  was prepared using 29.027 gram (290 mmol) CaCO 3 , 12.366 gram (200 mmol) H 3 BO 3 , 26.412 gram (200 mmol) (NH 4 ) 2 HPO 4  and 1.721 gram (10 mmol) CeO 2  as starting materials. 
     Table I shows the properties of a number of luminescent materials of general formula Ca 3-x B 2 P 2 O 11+0.5x :xCe 3+  for different values of x. Similarly Table II shows the properties of a number of luminescent materials of general formula Ca 2.5-x B 5.5 P 2 O 15.75+0.5x :xCe 3+  for different values of x. In both tables λ max  is the wavelength where the emission of the luminescent material is highest. FWHM is the full width at half maximum of the emission peak. QE is quantum efficiency and Abs is the absorption of 254 nm radiation of the luminescent material. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 x 
                 λ max (nm) 
                 FWHM(nm) 
                 QE(%) 
                 Abs(%) 
               
               
                   
               
             
             
               
                 0.025 
                 360 
                 58 
                 84 
                 62 
               
               
                 0.050 
                 362 
                 57 
                 91 
                 74 
               
               
                 0.075 
                 363 
                 57 
                 91 
                 79 
               
               
                 0.100 
                 363 
                 56 
                 90 
                 86 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                 x 
                 λ max (nm) 
                 FWHM(nm) 
                 QE(%) 
                 Abs(%) 
               
               
                   
               
             
             
               
                 0.025 
                 364 
                 55 
                 82 
                 64 
               
               
                 0.050 
                 365 
                 53 
                 89 
                 75 
               
               
                 0.075 
                 365 
                 52 
                 88 
                 82 
               
               
                 0.100 
                 365 
                 53 
                 86 
                 87 
               
               
                   
               
             
          
         
       
     
     From these tables it can be concluded that both the quantum efficiency as well as the absorption of a luminescent material according to the invention are relatively high. The same optical properties that are listed in the tables were also measured for a luminescent material of formula Ca 2.4 B 5.5 P 2 O 15.75 :0.05Ce 3+ ;0.05Li + . This material is comparable with the material in the second row of table I, the difference being that in addition to trivalent cerium the material comprises an equimolar amount of monovalent lithium ions. The λ max  of this material was 365 nm and its FWHM 53 nm. The emission peak of this material is thus very similar to the emission peak of the material in the second row of table II. However, the quantum efficiency and the absorption of the lithium containing material were 92% and 78% respectively and therefore higher than the quantum efficiency and the absorption of the material in the second row of table II.