Abstract:
According to the present invention, there are provided a green fluorescent material of the general formula: 
     
       Sr.sub.1-3/2(x+y+z) Ga.sub.2 S.sub.4 :Eu.sub.x, Tb.sub.y, Sm.sub.z 
     
     where, 0.01≦x≦0.2; 0.0001≦y≦0.007; and 0.0001≦z≦0.002, and a blue fluorescent material of the general formula: 
     
       Ba.sub.a Mg.sub.b Al.sub.c O.sub.a+b+1.5c :Eu.sub.x, Tb.sub.v, Sm.sub.w 
     
     where, 0.1≦a≦2.4; 0.1≦b≦5; 8≦c≦64; 0.01≦x≦0.2; 0.00005≦v≦0.002; and 0.00001≦w≦0.001. 
     A fluorescent material of the present invention enhances the luminous efficiency of the active dynamic liquid display device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fluorescent material, and more particularly, to a fluorescent material used in an active dynamic liquid crystal display device and a method for manufacturing the same. 
     2. Description of the Prior Art 
     Generally, in an active dynamic liquid crystal display device, a light emitted from a back light is directed to a liquid crystal layer through a polarizer. The light directed to the liquid crystal layer is transmitted to a fluorescent material or interrupted in accordance with orientations of the liquid crystal molecules. The light transmitted to the fluorescent material excites the material to display an image. 
     As described above, since the active dynamic liquid crystal display device is designed such that the light emitted from the back light directly excites the fluorescent material, there is a need for a fluorescent material with high luminance. 
     In general, SrGa 2  S 4  :Eu which is excited by violet light having a wavelength of about 380 to about 420 nm has been used as a green fluorescent material, and BaMgAl 10  O 17  :Eu has been used as a blue fluorescent material. 
     However, the light emission luminance of these materials are not enough to apply to the active dynamic liquid crystal display device. 
     Therefore, U.S. Pat. Nos. 4,249,108 and 4,733,126 have proposed a method for improving the luminous efficiency by changing composition of the fluorescent material or adding another material to the fluorescent material, and U.S. Pat. No. 5,350,971 has proposed a method for improving the luminous efficiency by adjusting a diameter of each grains of the fluorescent material. 
     However, the fluorescent materials proposed by the above patents are not still enough in light emission luminance. 
     In addition, to increase the luminous efficiency, there has been proposed SrGa 2  S 4  :Eu,Tb and BaMgAl 10  O 17  :Eu,Tb. These materials are not also still enough in luminance. 
     As is well known, the active dynamic liquid display device requires a fluorescent material which is excited in a dark blue color range. The luminous efficiency of SrGa 2  S 4  :Eu and BaMgAl 10  O 17  :Eu is determined by transmission efficiency of light emission energy to Eu. Tb is used to transmit the light emission energy to Eu. 
     SUMMARY OF THE INVENTION 
     Therefore, there is a need for providing a fluorescent material which can enhance the luminous efficiency of the active dynamic liquid display device. 
     It is an object of the present invention to provide a green fluorescent material having a general formula 
     
         Sr.sub.1-3/2(x+y+z) Ga.sub.2 S.sub.4 :Eu.sub.x, Tb.sub.y, Sm.sub.z 
    
     where, 0.0≦x≦0.2; 0.0001≦y≦0.007; and 0.0001≦z≦0.002. 
     Preferably, z is 0.001˜0.002 and Sr 1-3/2 (x+y+z) Ga 2  S 4  :Eu x , Tb y , Sm z  is Sr 0 .96 Ga 2  S 4  :Eu 0 .02, Tb 0 .0005, Sm 0 .002. 
     It is another object of the present invention to provide a blue fluorescent material having a general formula 
     
         Ba.sub.a Mg.sub.b Al.sub.c O.sub.a+b+1.5c :Eu.sub.x, Tb.sub.v, Sm.sub.w 
    
     where, 0.1≦a≦2.4; 0.1≦b≦5; 8≦c≦64; 0.01≦x≦0.2; 0.00005≦v≦0.002; and 0.00001≦w≦0.001. 
     Preferably, w is 0.0001˜0.001, and Ba a  Mg b  Al c  O a+b+1 .5c :Eu x , Tb y , Sm z  is BaMgAl 10  O 17  :Eu 0 .02, Tb 0 .001, Sm 0 .001. 
     It is still another object of the present invention to provide a method for producing a green fluorescent material comprising the steps of: 
     preparing a first compound comprising of strontium salt, gallium salt, europium oxide, terbium salt, and sodium halide as a solvent; 
     preparing a second compound by adding Sm 2  O 3  to the first compound; and 
     calculating the second compound at a temperature of about 700-900° C. for about 1 to about 4 hours, thereby obtaining a green fluorescent material of Sr 1-3/2 (x+y+z) Ga 2  S 4  :Eu x , Tb y , Sm z   
     where, 0.01≦x≦0.2; 0.0001≦y≦0.007; and 0.0001≦z≦0.002. 
     The method further comprises the steps of cooling, crushing, drying, and powdering the calculated material. 
     The calculating step is conducted in a hydrogen sulfide gas atmosphere. 
     It is still another object of the present invention to provide a method for producing a blue fluorescent material comprising the steps of: 
     preparing a first compound comprising of barium salt, magnesium salt, alumina, europium oxide, terbium salt, and sodium halide as a solvent 
     preparing a second compound by adding Sm 2  O 3  to the first compound; and 
     calcining the second compound at a temperature of about 1200 to about 1500° C. for about 1 to about 4 hours, thereby obtaining a blue fluorescent material of Ba a  Mg b  Al c  O a+b+1 .5c :Eu x , Tb v , Sm w   
     where, 0.1≦a≦2.4; 0.1≦b≦5; 8≦c≦64; 0.01≦x≦0.2; 0.00005≦v≦0.002; and 0.00001≦w≦0.001. 
     The calcining step is conducted in N 2  -5% H 2  gas atmosphere. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects and advantages of the present invention will be better understood from the following detailed description when in conjunction with the attached drawings, in which: 
     FIG. 1 is a graph illustrating light emission spectrums of green fluorescent materials obtained by the present invention; 
     FIG. 2 is a graph illustrating relative light emission luminance against amount of Sm of green fluorescent materials obtained by the present examples and comparative examples; 
     FIG. 3 is a graph illustrating light emission spectrums of blue fluorescent materials obtained by the present invention; and 
     FIG. 4 is a graph illustrating relative light emission luminance against amount of Sm of blue fluorescent materials obtained by the present examples and comparative examples. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIRST EXAMPLE 
     A first compound was prepared by mixing strontium carbonate of 0.96 mole, gallium oxide 1 mole, europium oxide 0.01 mole, and terbium oxide of 0.0025 mole with sodium bromide of 1 wt %. 
     Then, Sm 2  O 3  having Sm of 0.001 mole with respect to the host material was added to the first compound, thereby obtaining a second compound. 
     The second compound was calcined at a temperature of about 800° C. for two hours in a hydrogen sulfide gas atmosphere. 
     Finally, the calcined material went through a cooling, crushing, drying, powdering processes, thereby obtaining a green fluorescent material of the present invention having the formula: Sr 0 .96 Ga 2  S 4  :Eu 0 .02, Tb 0 .005, Sm 0 .002. 
     The europium is reduced from trivalent to bivalent in hydrogen sulfide gas, thereby obtaining a luminous quality. The hydrogen sulfide gas provides an atmosphere that allows alkaline earth metal to mix with gallium metal. The powdering was realized using a screen sieving process. 
     SECOND EXAMPLE 
     A first compound was prepared by mixing strontium carbonate of 0.96 mole, gallium sulfate 1 mole, europium oxide 0.01 mole, and terbium nitrate of 0.0025 mole with sodium bromide of 1 wt %. 
     Then, Sm 2  O 3  having Sm of 0.001 mole with respect to the host material was added to the first compound, thereby obtaining a second compound. 
     The second compound was calcined at a temperature of about 800° C. for two hours in a hydrogen sulfide gas atmosphere. 
     Finally, the calcined material went through a cooling, crushing, drying, powdering processes, thereby obtaining a green fluorescent material of the present invention. 
     The powdering process was conducted as in example 1. 
     THIRD EXAMPLE 
     A first compound was prepared by mixing strontium carbonate of 0.96 mole, gallium oxide 1 mole, europium oxide 0.01 mole, and terbium oxide of 0.0025 mole with sodium bromide of 1 wt %. 
     Then, Sm 2  O 3  having Sm of 0.002 mole with respect to the host material was added to the first compound, thereby obtaining a second compound. 
     The second compound was calcined at a temperature of about 800° C. for two hours in a hydrogen sulfide gas atmosphere. 
     Finally, the calcined material went through a cooling, crushing, drying, powdering processes, thereby obtaining a green fluorescent material of the present invention. 
     The powdering process was conducted as in example 1. 
     Comparative Example 1 
     A first compound was prepared by mixing strontium carbonate of 0.96 mole, gallium oxide 1 mole, europium oxide 0.01 mole, and terbium oxide 0.00025 mole with sodium bromide of 1 wt %. 
     The first compound was calcined at a temperature of about 800° C. for two hours in a hydrogen sulfide gas atmosphere. 
     Finally, the calcined material went through a cooling, crushing, drying, powdering processes, thereby obtaining a prior green fluorescent material. 
     The powdering process was conducted as in example 1. 
     Comparative Example 2 
     A first compound was the prepared by mixing strontium sulfate of 0.96 mole, gallium sulfate 1 mole, europium oxide 0.01 mole, and terbium nitrate of 0.0025 mole with sodium bromide of 1 wt %. 
     Then, Sm 2  O 3  having Sm of 0.005 mole with respect to the host material was added to the first compound, thereby obtaining a second compound. 
     The second compound was calcined at a temperature of about 800° C. for two hours in a hydrogen sulfide gas atmosphere. 
     Finally, the calcined material went through a cooling, crushing, drying, powdering processes, thereby obtaining a green fluorescent material. 
     The powdering process was conducted as in example 1. 
     Comparative Example 3 
     A first compound was prepared by mixing strontium carbonate of 0.96 mole, gallium oxide 1 mole, europium oxide 0.01 mole, and terbium carbonate of 0.0025 mole with the sodium bromide of 1 wt %. 
     Then, Sm 2  O 3  having Sm of 0.01 mole with respect to the host material was added to the first compound, thereby obtaining a second compound. 
     The second compound was calcined at a temperature of about 800° C. for two hours in a hydrogen sulfide gas atmosphere. 
     Finally, the calcined material went through a cooling, crushing, drying, powdering processes, thereby obtaining a green fluorescent material. 
     The powdering process was conducted as in example 1. 
     FIG. 1 shows a graph illustrating light emission spectrums of green fluorescent materials obtained by examples 1 to 3. As shown in the graph, the resultant fluorescent materials of the present invention have a relatively high light emission luminance of 550 nm. 
     FIG. 2 shows a graph illustrating relative light emission luminance measured when back light having a 394 nm was emitted to the fluorescent materials obtained by examples 1 to 3 and comparative examples 1 to 3. 
     The measurement of light emission luminance was conducted using MSR 7000 (Products of Opto Research Co.,). 
     When assuming that the light emission luminance of the fluorescent material obtained by comparative example 1 is &#34;100&#34;, the light emission luminance of the fluorescent materials obtained by examples 1, 2 and 3 and comparative examples 2 and 3 are shown in the following table 1. 
     
                       TABLE 1______________________________________  Comp. Comp.   Comp.  Ex. 1 Ex. 2   Ex. 3   Ex. 1 Ex. 2 Ex. 3______________________________________Amount of    0 mole  0.005   0.01  0.001 0.001 0.002Sm               mole    mole  mole  mole  moleRelative 100     91      50    110   112   112Luminance______________________________________ 
    
     As shown in Table 1, when Sm of about 0.001 to about 0.002 mole was added, the luminous efficiency by light of about 380 to about 420 nm was greatly improved. 
     With regard to strontium salt, gallium salt, europium oxide, terbium salt, any materials equivalent to these materials do not affect luminous efficiency. Although the calcination process was conducted for 2 hours, it may be adjusted within a range from 1 to 4 hours. 
     EXAMPLE 4 
     A first compound was prepared by mixing barium carbonate of 1 mole, magnesium carbonate of 1 mole, alumina of 8 mole, europium oxide 0.01 mole, and terbium oxide of 0.00025 mole with sodium fluoride of 0.3 mole. 
     Then, Sm 2  O 3  having Sm of 0.0001 mole with respect to the host material was added to the first compound. 
     The second compound was calcined at a temperature of about 1500° C. for two hours in N 2  -5% H 2  gas atmosphere. 
     Finally, the calcined material went through a cooling, crushing, drying, powdering processes, thereby obtaining a blue fluorescent material of the present invention having the formula: BaMgAl 10  O 17  :Eu 0 .02, Tb 0 .001, Sm 0 .001. 
     N 2  -5% H 2  gas makes the europium be reduced from trivalent to bivalent so that the europium can have a luminous quality. 
     The powdering was realized using a screen sieving process. 
     EXAMPLE 5 
     A first compound was prepared by mixing barium sulfate of 1 mole, magnesium sulfate of 1 mole, alumina of 8 mole, europium oxide of 0.01 mole, and terbium carbonate of 0.00025 mole with sodium chloride of 0.3 mole. 
     Then, Sm 2  O 3  having Sm of 0.0005 mole with respect to the host material was added to the first compound. 
     The second compound was calcined at a temperature of about 1500° C. for two hours in N 2  -5% H 2  gas atmosphere. 
     Finally, the calcined material went through a cooling, crushing, drying, powdering processes, thereby obtaining a blue fluorescent material of the present invention. 
     N 2  -5% H 2  gas makes the europium be reduced from trivalent to bivalent so that the europium can have a luminous quality. 
     The powdering was realized using a screen sieving process. 
     EXAMPLE 6 
     A first compound was prepared by mixing barium carbonate of 1 mole, magnesium sulfate of 1 mole, alumina of 8 mole, europium oxide 0.01 mole, and terbium oxide of 0.00025 mole with sodium fluoride of 0.3 mole. 
     Then, Sm 2  O 3  having Sm of 0.001 mole with respect to the host material was added to the first compound. 
     The second compound was calcined at a temperature of about 1500° C. for two hours in N 2  -5% H 2  gas atmosphere. 
     Finally, the calcined material went through a cooling, crushing, drying, powdering processes, thereby obtaining a blue fluorescent material of the present invention. 
     N 2  -5% H 2  gas makes the europium be reduced from trivalent to bivalent so that the europium can have a luminous quality. 
     The powdering was realized using a screen sieving process. 
     Comparative Example 4 
     A first compound was prepared by mixing barium carbonate of 1 mole, magnesium carbonate of 1 mole, alumina of 8 mole, europium oxide 0.01 mole, and terbium oxide of 0.00025 mole with the aluminum fluoride of 0.3 mole. 
     Then, Sm 2  O 3  having Sm of 0.002 mole with respect to the host material was added to the first compound. 
     The second compound was calcined at a temperature of about 1500° C. for two hours in N 2  -5% H 2  gas atmosphere. 
     Finally, the calcined material went through a cooling, crushing, drying, powdering processes, thereby obtaining a blue fluorescent material. 
     The powdering process was realized using a screen sieving process. 
     Comparative Example 5 
     A first compound was prepared by mixing barium carbonate of 1 mole, magnesium sulfate of 1 mole, alumina of 8 mole, europium oxide 0.01 mole, and terbium oxide of 0.00025 mole with ammonium chloride of 0.3 mole. 
     The first compound was calcined at a temperature of about 1500° C. for two hours in N 2  -5% H 2  gas atmosphere. 
     Finally, the calcined material went through a cooling, crushing, drying, powdering processes, thereby obtaining a prior blue fluorescent material. 
     N 2  -5% H 2  gas makes the europium be reduced from trivalent to bivalent so that the europium can have a luminous quality. 
     The powdering process was realized using a screen sieving process. 
     FIG. 3 shows a graph illustrating light emission spectrums of blue fluorescent materials obtained by examples 4 to 6. As shown in the graph, the resultant fluorescent materials of the present invention have a relatively high light emission luminance of 470 nm. 
     FIG. 4 shows a graph illustrating relative light emission luminance measured when back light having a 394 nm was emitted to the fluorescent materials obtained by examples 4 to 6 and comparative examples 4 and 5. 
     The measurement of light emission luminance was conducted using MSR 7000 (Products of Opto Research Co.,). 
     When assuming that the light emission luminance of the fluorescent material obtained by comparative example 5 is &#34;100&#34;, the light emission luminance of the fluorescent materials obtained by examples 4, 5 and 6 and comparative examples 4 are shown in the following table 2. 
     
                       TABLE 2______________________________________  Comp.  Comp.  Ex. 5  Ex. 4    Ex. 4    Ex. 5  Ex. 6______________________________________Amount of    0 mole   0.002    0.0001 0.0005 0.001Sm                mole     mole   mole   moleRelative 100      86       102    105    108Luminance______________________________________ 
    
     As shown in Table 2, when Sm of about 0.0001 to about 0.0001 mole was added, the light emission luminance effect was greatly improved. 
     Particularly, when adding 0.001 mole, the light emission luminance effect was highest. 
     With regard to barium salt, magnesium salt, europium oxide, terbium salt, any materials equivalent to these materials do not affect quality of luminance. Although the calcination process was conducted for 2 hours, it may be adjusted within a range from 1 to 4 hours. 
     While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.