Abstract:
Disclosed herein are phosphor compositions comprising a BAM phosphor BaMgAl 10 O 17 :Eu 2+  and at least one other hexaaluminate that may also be europium activated. The BAM may be mixed with any number of different kinds of heaxaaluminates having a similar crystal structure. The aluminum ratio may also be adjusted to alter the defect structure or to produce a second phase. Addition of another hexaaluminate to BAM enhances emission intensity and resistance to degradation, which is beneficial to applications such as plasma display panels.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 60/830,269, filed Jul. 11, 2006, and titled “An improved BAM phosphor.” U.S. Provisional Application Ser. No. 60/830,269 is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments of the present invention are directed to phosphor compositions that comprise a BAM phosphor BaMgAl 10 O 17 :Eu 2+  and at least one other hexaaluminate that may also be europium activated. 
         [0004]    2. Description of the Related Art 
         [0005]    Owing to its high light output and excellent display of color (as represented, for example, on a CIE color diagram), the known BAM phosphor BaMgAl 10 O 17  doped with divalent europium (Eu 2+ ) has been widely used as the blue phosphor component in applications such as fluorescent lamps, light emitting diode (LED) and plasma display panels (PDP). In plasma display panel applications, BAM:Eu is conventionally adopted as the blue-emitting component under vacuum ultraviolet (VUV) excitation; however, considerable degradation in light output and color shift (toward the green) are known to be problematic. This is thought to be due to an annealing procedure during the manufacturing process, as well as to plasma radiation and/or sputtering damage that occurs during day-to-day use. 
         [0006]    BAM has a β-alumina structure, and belongs to the family of hexaaluminates. Hexaaluminates have a column-like structure and consists of blocks of cubic closed packed (CCP) oxygen layers with cations in tetrahedral and octahedral interstices. Because the blocks are quite similar to the MgAl 2 O 4  spinel structure, hexyluminates are often referred to as “spinel blocks.” The blocks are separated by mirror planes, which contain the large cations. 
         [0007]    The magnetoplumbite structure is similar to the β-alumina structure as both have identical spinel blocks. The difference between them lies in the mirror planes, which are loosely-packed in the magnetoplumbite case, and tightly-packed in the β-alumina structure. LaMgAl 11 O 19  is a typical magnetoplumbite. Structurally, its formula can be rewritten as [LaAlO 3 ][(Al 3 Mg)A 7 O 16 ], where [LaAlO 3 ] are the ions on the mirror plane, and the [(Al 3 Mg)Al 7 O 16 ] portion of the structure exists as the above-mentioned spinel blocks. In the formula (Al 3 Mg) are groups of ions having a 4-fold coordination, and the aluminum as part of the Al 7  oxide has a six-fold coordination. In the same manner, β-alumina BaMgAl 10 O 17  can be rewritten as [BaO][(Al 3 Mg)A 7 O 16 ], where just two ions Ba and O are presented on the mirror plane. 
         [0008]    Diagrams of the atomic arrangements (and hence crystal structure) of three hexaaluminates are shown in  FIGS. 1A-1C .  FIG. 1A  is β-alumina as represented by the formula NaAl 11 O 17 , which may be re-written as [NaO][Al 4 Al 7 O 16 ];  FIG. 1B  is the BAM compound BaMgAl 10 O 17 , which may be written as [BaO][(Al 3 Mg)Al 7 O 16 ]; and  FIG. 1C  is the magnetoplumbite LaMgAl 11 O 19 , which may be written as [LaAlO 3 ][(Al 3 Mg)Al 7 O 16 ]. 
         [0009]    What is needed in the art are phosphor compositions that enhance the emission intensity and degradation resistance of the conventional BAM phosphor (BaMgAl 10 O 17 :Eu 2+ ), utilizing properties afforded by mixing the conventional BAM phosphor with other hexaaluminates. 
       SUMMARY OF THE INVENTION 
       [0010]    Embodiments of the present invention are directed to a phosphor composition comprising a europium activated BAM phosphor and a hexaaluminate other than the BAM phosphor, the composition represented by the formula (x) hexaaluminate+(1−x) BaMgAl 10 O 17 :Eu 2+ , where the hexaaluminte is selected from the group consisting of a β-alumina, a β′-alumina, and a magnetoplumbite, and wherein x ranges from about 0.001 to about 0.999. 
         [0011]    In another embodiment, the phosphor composition comprises a europium activated BAM phosphor and a hexaaluminate other than the BAM phosphor, the composition represented by the formula (x) LnMAl 11 O 19 +(1−x) BaMgAl 10 O 17 :Eu 2+ , where Ln is a trivalent lanthanide, M is a divalent cation, and x ranges from about 0.001 to about 0.5. Alternatively, the phosphor composition comprises a europium activated BAM phosphor and a hexaaluminate other than the BAM phosphor, the composition represented by the formula (x) Ln u Al v O w +(1−x) BaMgAl 10 O 17 :Eu 2+ , where Ln is a trivalent lanthanide, x ranges from about 0.001 to about 0.5, u ranges from about 0.67 to about 1, v ranges from about 11 to about 12, and w ranges from about 18 to about 19. 
         [0012]    In another embodiment, the phosphor composition comprises a europium activated BAM phosphor and a hexaaluminate other than the BAM phosphor, the composition represented by the formula (x) M′ 1.5 Al 10.5 O 16.5 +(1−x) BaMgAl 10 O 17 :Eu 2+ . Alternatively, the phosphor composition comprises a europium activated BAM phosphor and a hexaaluminate other than the BAM phosphor, the composition represented by the formula (x) M′ 1.5 Al 10.5 O 16.5 +(1−x) BaMgAl 10 O 17 :Eu 2+ , where M′ is a monovalent cation, and x ranges from about 0.001 to about 0.5. 
         [0013]    In another embodiment, the phosphor composition comprising a europium activated BAM phosphor and a hexaaluminate other than the BAM phosphor, the composition represented by the formula (x) M 0.75 Al 11 O 17.25 +(1−x) BaMgAl 10 O 17 :Eu 2+ , where M is a divalent cation, and x ranges from about 0.001 to about 0.5. Alternatively, the phosphor composition comprising a europium activated BAM phosphor and a hexaaluminate other than the BAM phosphor, the composition represented by the formula x (yMO.6Al 2 O 3 )+(1−x) BaMgAl 10 O 17 :Eu 2+ , where M is a divalent cation, and x ranges from about 0.001 to about 0.5, and y ranges from about 1.28 to about 1.32. 
         [0014]    The present phosphor compositions may be synthesized by a method selected from the group consisting of liquid processing methods, co-precipitation methods, and sol-gel methods. Exemplary steps include: (a) dissolving the precursor metal salts in an aqueous based solution; (b) co-precipitating an intermediate product; (c) removing at least a portion of the water the intermediate product of step (b); (d) calcining the product of step (c); and (e) sintering the product of step (d). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIGS. 1A-1C  are diagrams showing the atomic arrangements (and hence crystal structure) of three hexaaluminates:  FIG. 1A  is β-alumina as represented by the formula NaAl 11 O 17 , which may be re-written as [NaO][Al 4 Al 7 O 16 ];  FIG. 1B  is the BAM compound BaMgAl 10 O 17 , which may be written as [BaO][(Al 3 Mg)Al 7 O 16 ]; and  FIG. 1C  is the magnetoplumbite LaMgAl 11 O 19 , which may be written as [LaAlO 3 ][(Al 3 Mg)Al 7 O 16 ]; 
           [0016]      FIG. 2  is a graph of emission intensity vs. fraction of alumina (denoted by “x”), comparing heated and unheated samples; 
           [0017]      FIG. 3  is an x-ray diffraction (XRD) pattern of the compound (Na 0.2 Ba 0.75 Eu 0.05 Mg 0.8 O 1.7 )x(Al 10.2 O 15.3 ), where x is greater than or equal to 100 percent (x≧100%); and 
           [0018]      FIG. 4  is an x-ray diffraction (XRD) pattern of the compound (Na 0.2 Ba 0.75 Eu 0.05 Mg 0.8 O 1.7 )x(Al 10.2 O 15.3 ), where x is less than or equal to 100 percent (x≦100%). 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Disclosed herein are phosphor compositions comprising a BAM phosphor BaMgAl 10 O 17  activated with divalent europium (Eu 2+ ), and at least one other hexaaluminate having a similar crystal structure. The similarity among hexaaluminte structures has led the present inventors to synthesize new compositions by mixing various hexaaluminates. 
       Exemplary Formulations 
       [0020]    According to embodiments of the present invention, a BAM:Eu 2+  is mixed with at least one other type of hexaaluminate, as represented by the general formula: 
         [0000]      (x)hexaaluminate+(1−x)BaMgAl 0 O 17 :Eu 2+   
         [0021]    where the (x) hexaaluminte includes but is not limited to a β-alumina, a β′-alumina, and a magnetoplumbite, and is not BaMgAl 10 O 17 . The phosphor composition mixture may be in the form of a solid solution of the hexaaluminate and the BAM:Eu 2+ , or it may contain distinct phases of the hexaaluminate and the BAM:Eu 2+ . The value of x in this general formula ranges from about 0.001 to about 0.999. 
         [0022]    More specifically, and according to one embodiment of the present invention, the phosphor composition is generated by mixing the BAM phosphor with a magnetoplumbite compound, the phosphor composition represented by the formula: 
         [0000]      (x)LnMAl 11 O 19 +(1−x)BaMgAl 10 O 17 :Eu 2+ , 
         [0000]    where Ln is a trivalent lanthanide, M is a divalent cation, and x ranges from about 0.001 to about 0.5. In some embodiments, M may be an alkaline earth metal from group IIA of the periodic table, the alkaline earth metal selected from the group consisting of Mg, Ca, Sr, and Ba. 
         [0023]    According to another embodiment of the present invention, the BAM phosphor may be mixed with a lanthanide-containing hexaaluminate compounds according to the formula: 
         [0000]      (x)Ln u Al v O w +(1−x)BaMgAl 10 O 17 :Eu 2+ , 
         [0000]    where Ln is a trivalent lanthanide, x ranges from about 0.001 to about 0.5, u ranges from about 0.67 to about 1, v ranges from about 11 to about 12, and w ranges from about 18 to about 19. 
         [0024]    While not wishing to be bound by any particular theory, it may be noted that the lanthanide hexaaluminate has a magnetoplumbite framework of the type AB 12 O 19 , with vacancies in the structure. Ideally, the structure would have the formula Ln 0.67 Al 12 O 19 , in accordance with a “true” magnetoplumbite structure. However, such structures apparently do not exist as the AB 12 O 19  framework cannot accommodate a sufficient number of vacancies at the A sites. Therefore, it is believed the actual composition of a lanthanide hexaaluminate lies somewhere between the stoichiometric LnAl 11 O 18 , and the ideal stoichiometry of Ln 0.67 Al 12 O 19 . An example of such a lanthanide hexaaluminate is La 0.85 Al 11.6 O 18.7 . 
         [0025]    In another embodiment of the present invention, the BAM phosphor may be mixed with a hexaaluminate comprising one or more β-alumina compounds such that the phosphor composition has the formula: 
         [0000]      (x)M′Al 11 O 17 +(1−x)BaMgAl 10 O 17 :Eu 2+ , 
         [0000]    where M′ is a monovalent cation from group IA of the periodic table (an alkali metal), and x ranges from about 0.001 to about 0.5. The M′ cation is selected from the group consisting of Li, Na, K, Rb, and Cs. 
         [0026]    In another embodiment of the present invention, the BAM phosphor may be mixed with a hexaaluminate comprising one or more of the so-called β′-alumina compounds such that the phosphor composition has the formula: 
         [0000]      (x)M′ 1.5 Al 10.5 O 16.5 +(1−x)BaMgAl 10 O 17 :Eu 2+ , 
         [0000]    where M′ is a monovalent cation from group IA of the periodic table (an alkali metal), and x ranges from about 0.001 to about 0.5. The M′ cation is selected from the group consisting of Li, Na, K, Rb, and Cs. The assumed structure of β′-alumina has been reported before in the literature; however, whether it is a new phase other than non-stoichiometric β-alumina remains unclear. 
         [0027]    In another embodiment of the present invention, the BAM phosphor may be mixed with one or more alkaline-earth-poor (or alkaline earth deficient, at least relative to previous embodiments) hexaaluminate compounds, such that the phosphor composition has the formula: 
         [0000]      (x)M′ 0.75 Al 11 O 17.25 +(1−x)BaMgAl 10 O 17 :Eu 2+ , 
         [0000]    where M is a divalent cation, and x ranges from about 0.001 to about 0.5. The alkaline-earth-poor hexaaluminates have a β-alumina structure with about 75 percent of the group IA alkali metal ions being replaced by group IIA alkaline-earth ions and about 25 percent by oxygen ions. 
         [0028]    In another embodiment of the present invention, the BAM phosphor may be mixed with one or more alkaline-earth-rich hexaaluminate compounds, such that the phosphor composition has the formula: 
         [0000]      x(yMO.6Al 2 O 3 )+(1−x)BaMgAl 10 O 17 :Eu 2+ , 
         [0000]    where M is a divalent cation, and x ranges from about 0.001 to about 0.5, and y ranges from about 1.28 to about 1.32. These alkaline-earth-rich hexaaluminates are assumed to have a β′-alumina structure with about 75 percent of the group IA alkali metal ions being replaced by group IIA alkaline-earth ions and about 25% by oxygen ions. This embodiment provides a composition with the ideal structural formula M 1.125 Al 10.5 O 16.875 . 
         [0029]    For certain mixtures of BAM:Eu 2+  and some other hexaaluminte, according to the embodiments outlined above, the ratio of the aluminum to the other cations may be varied to enhance luminescence output and oxidation stability. Furthermore, it is believed that by mixing BAM:Eu 2+  with other hexaaluminates according to the present embodiments, oxidative stability and light emission output are enhanced due to changes in the crystal field properties of the overall phosphor composition. 
       Processing Considerations 
       [0030]    The present phosphor compositions may be synthesized by mixing the BAM phosphor BaMgAl 10 O 17  with one or more hexaaluminates other than the BAM having a similar crystal structure. 
         [0031]    In one embodiment of the present invention, the phosphor composition may be synthesized by mixing the BAM phosphor BaMgAl 10 O 17  with the β-alumina (NaAl 11 O 17 ). The fraction of the β-alumina contained within the composition may be adjusted to vary the light emission behavior and degradation resistance of the overall composition. In this embodiment a composition is mixed having a content of about 20 percent of the β-alumina NaAl 11 O 17  and about 80 percent of the BAM. The formula of the composition may be represented by the formula: 
         [0000]      (Na 0.2 Ba 0.75 Eu 0.05 Mg 0.8 O 17 )+x(Al 10.2 O 15.3 ), 
         [0000]    where x ranges from about 0.7 to about 1.30. 
         [0032]    In one example of the synthesis of the present phosphor composition, the starting materials comprised the appropriate metal nitrates in the desired mole ratios. In one embodiment, a flux may be added during processing, for example, a 5 mole percent addition of a flux such as aluminum fluoride. 
         [0033]    Such BAM/hexaaluminate compositions have been synthesized by the present inventors using liquid mixing, co-precipitation, and/or sol-gel techniques. In accordance with these processes, metals that included sodium, barium, magnesium, aluminum, and europium, along with salts of halogens such as aluminum fluoride, were first dissolved in hot water. An aqueous solution of ammonia water was added to facilitate co-precipitation of the mixed nitrates. The solution was then heated to remove water, and the partially dried mixture was calcined at about 800° C. for about two hours. Finally, the calcined powders were sintered at about 1500° C. for about 6 hours in an atmosphere comprising nitrogen gas mixed with about 1 to 5 percent by volume hydrogen. After sintering, the powders were milled and sieved with a 25 μm sieve. 
       Physical Properties and Optical Performance 
       [0034]    To perform a thermal degradation test, the powders were heated at about 510° C. for about one hour in air. The emission intensity of un-heated and heated samples were then measured using a 147 nm plasma lamp as the excitation source. Sample compositions and measurement data are shown in Table 1: 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Compositions and photoemission intensities 
               
               
                 of samples (Na 0.2 Ba 0.75 Eu 0.05 Mg 0.8 O 1.7 ) + (Al 10.2 O 15.3 ) 
               
             
          
           
               
                   
                   
                   
                   
                   
                 Emission 
               
               
                   
                   
                   
                 Emission 
                 Emission 
                 change 
               
               
                 Sample # 
                 x 
                 Compositions 
                 (unheated) 
                 (heated) 
                 ratio 
               
               
                   
               
             
          
           
               
                 Control 
                   
                 (Ba 0.95 Eu 0.05 )MgAl 10 O 17  (BAM) 
                 1097 
                 1060 
                 −4.6% 
               
               
                 1 
                 70% 
                 Na 0.2 Ba 0.75 Eu 0.05 Mg 0.8 Al 7.14   
                  952 
                  975 
                 +2.4% 
               
               
                 2 
                 80% 
                 Na 0.2 Ba 0.75 Eu 0.05 Mg 0.8 Al 8.16   
                 1202 
                 1191 
                 −0.91%  
               
               
                 3 
                 90% 
                 Na 0.2 Ba 0.75 Eu 0.05 Mg 0.8 Al 9.18   
                 1106 
                 1102 
                 −0.36%  
               
               
                 4 
                 95% 
                 Na 0.2 Ba 0.75 Eu 0.05 Mg 0.8 Al 9.69   
                 1133 
                 1102 
                 −2.7% 
               
               
                 5 
                 100% 
                 Na 0.2 Ba 0.75 Eu 0.05 Mg 0.8 Al 10.2   
                 1116 
                 1058 
                 −5.2% 
               
               
                 6 
                 105% 
                 Na 0.2 Ba 0.75 Eu 0.05 Mg 0.8 Al 10.71   
                 1091 
                 1090 
                 −0.1% 
               
               
                 7 
                 110% 
                 Na 0.2 Ba 0.75 Eu 0.05 Mg 0.8 Al 11.22   
                 1067 
                 1069 
                 +0.2% 
               
               
                 8 
                 120% 
                 Na 0.2 Ba 0.75 Eu 0.05 Mg 0.8 Al 12.24   
                 1059 
                 1044 
                 −1.4% 
               
               
                 9 
                 130% 
                 Na 0.2 Ba 0.75 Eu 0.05 Mg 0.8 Al 13.26   
                 1064 
                 1036 
                 −2.6% 
               
               
                   
               
             
          
         
       
     
         [0035]    These experiments show that of the data presented, the compositions comprising mixtures of the conventional BAM phosphor (Ba 0.95 Eu 0.05 )MgAl 10 O 17  with hexaaluminates other than that BAM in all cases demonstrate either an increase in intensity, or at least less of an intensity decrease from heating than the control. 
         [0036]      FIG. 2  is a graph of emission intensity versus x, the fraction of alumina in the composition. In the graph, the square symbols represent unheated samples, and the circles heated samples. Data in the figure shows that the emission intensity increases dramatically as the fraction of the alumina in the composition is increased from 0.7 to 0.8, and whereas it decreases somewhat from that highest value, the emission intensity is still greater for x fractions of 0.9 to 1.3 than the emission intensity is when x is 0.7. 
         [0037]      FIG. 3  is an x-ray diffraction (XRD) pattern of the compound (Na 0.2 Ba 0.75 Eu 0.05 Mg 0.8 O 1.7 )x(Al 10.2 O 15.3 ), where x is greater than or equal to 100 percent (x≧100%). The diffraction pattern shows a comparison of samples with increasing ratios of aluminum. The data shows that as a second phase of α-alumina became evident as the value of x was increased above 100 percent. The larger the value of x, the more prominent the diffraction peak(s) of the α-alumina became. 
         [0038]      FIG. 4  is an x-ray diffraction (XRD) pattern of the compound (Na 0.2 Ba 0.75 Eu 0.05 Mg 0.8 O 1.7 )x(Al 10.2 O 15.3 ), where x is less than or equal to 100 percent (x≦100%). In  FIG. 4 , the diffraction pattern shows a comparison of samples with smaller ratios of aluminum than found in stoichiometrical composition. It was found that a second phase, BaAlO 2 , was formed when x was decreased to about 70 percent of the stoichiometrical ratio. When x was equal to or less than about 80 percent, no second phase was formed. Furthermore, the β-alumina could not be distinguished from the BAM in the diffraction pattern. Therefore, it could not be determined in this case whether or not β-alumina existed in the form of a second phase.