Abstract:
A core-shell structured silicate luminescent material and a preparation method thereof. The molecular formula of the luminescent material is: MLn 1-x SiO 4 :xRE@SiO 2 ; where @ represents a coating, where M is one or two elements among Li, Na, and K, where Ln is one or two elements among Y, Sc, Lu and La, where the value of x is 0&lt;x≦0.6; and where RE is one, two, or three elements among Tb, Gd, Sm, Eu, Dy, Ce and Tm. The compositions of the luminescent material are all chemicals of increased chemical stability, and, when subjected to electron beam bombardment for an extended period, provide a stable matrix and do not decompose easily.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the field of luminescent material, in particular to a core-shell structured silicate luminescent material and preparation method thereof. 
       BACKGROUND ART 
       [0002]    In recent years, field emission devices due to their low operating voltage, low power consumption, while deflection coil, X-ray radiation, radiation and magnetic interference are not involved, are of great concern. Among them, the preparation of high-performance luminescent materials is one of the key factors in the manufacture of high-performance field emission devices. 
         [0003]    Conventional sulfide-based phosphor powder includes: blue powder ZnS:Ag, Cl, SrGa 2 S 4 :Ce, green powder SrGa 2 S 4 :Eu and red powder Y 2 O 2 S:Eu. 
         [0004]    For the sulfide-based phosphor powder, which the phosphor powder gets damp very readily, and that the matrix is generally unstable in a long-term electron beam bombardment, and thus got decomposed very readily to generate gases, such as H 2 S, which the gases are not only poisonous to the cathode which results in the reduction of the ability of the cathode to emit electrons, but also results in the reduction of the luminous efficiency of the phosphor powder perse which shorten its life. 
       DISCLOSURE OF THE INVENTION 
       [0005]    On this basis, for the unstable problem of matrix and the associated problem of ease to decompose caused by the long-term electron beam bombardment suffered by the conventional sulfide-based phosphor powder, it is necessary to provide a silicate luminescent material which the matrix is stable, and not decompose readily under long-term electron beam bombardment, and also a preparation method thereof. 
         [0006]    A core-shell structured silicate luminescent material having the molecular formula of: 
         [0000]      MLn 1-x SiO 4   :x RE@SiO 2 ; 
         [0007]    where, @ represents a coating; 
         [0008]    M is one or two elements selected from Li, Na, and K; 
         [0009]    Ln is one or two elements among Y, Sc, Lu and La, x is 0&lt;x≦0.6; and 
         [0010]    RE is one, two or three elements selected from Tb, Gd, Sm, Eu, Dy, Ce and Tm. 
         [0011]    In one embodiment, x is 0.01≦x≦0.3. 
         [0012]    A method of preparing a core-shell structured silicate luminescent material comprising the step of: 
         [0013]    mixing 10 to 20 parts by volume of water, 15 to 50 parts by volume of anhydrous ethanol and 1 to 7 parts by volume of ammonia water, followed by dropwisely adding 0.5 to 3 parts by volume of tetraethylorthosilicate under stirring, stirring to react for 2 h to 6 h further, removing impurities after being separated by centrifugation, taking the precipitate and redispersing the same to 10 to 20 parts by volume of anhydrous ethanol to give a suspension of SiO 2  microsphere; 
         [0014]    according to the stoichiometric ratio of the formula of MLn 1-x SiO 4 :xRE, taking by measurement the nitrate solution of RE and nitrate solution of M, mixing the same homogenously, and subjecting the same to a water bath at 50° C. to 90° C., which the pH of the resulting mixture was adjusted to 1 to 6, followed by dropwisely adding tetraethylorthosilicate to give the resulting mixture, where M is one or two elements selected from Li, Na and K, Ln is one or more elements selected from Y, Sc, Lu and La, x ranges from 0&lt;x≦0.6, RE is one, two or three elements selected from Tb, Gd, Sm, Eu, Dy, Ce and Tm; 
         [0015]    according to the stoichiometric ratio of the formula of MLn 1-x SiO 4 :xRE@SiO 2 , adding said suspension of SiO 2  microsphere into said resulting mixture in a water bath at 50° C. to 90° C., stirring sufficiently to give a gel, drying the gel to give a precursor; 
         [0016]    pre-baking said precursor, and subjecting the same to grinding, repeating the operation of grinding after pre-baking 1 to 4 times, followed by calcined in air or under a reducing atmosphere, to give the core-shell structured silicate luminescent material after cooling, having the molecular formula of: Ln 1-x SiO 4 :xRE@SiO 2 , where @ represents a coating. 
         [0017]    In one embodiment, the step of removing impurities after separation by centrifugation comprises subjecting said suspension of SiO 2  microsphere containing impurities to centrifugal separation spinning at 12000 rpm, taking the precipitate, followed by washing the precipitate with deionized water three times. 
         [0018]    In one embodiment, the concentration of said corresponding nitrate solution of RE and nitrate solution of M ranges from 0.1M to 5M. 
         [0019]    In one embodiment, x ranges from 0.01≦x≦0.3. 
         [0020]    In one embodiment, the step of drying the gel to give the precursor comprises completely drying the gel at 80° C. to 150° C. to give the precursor. 
         [0021]    In one embodiment, the step of pre-baking said precursor and subjecting the same to grinding comprises pre-baking the precursor at 500° C. to 800° C. for 2 h to 7 h, followed by subjecting the same to grinding after cooling. 
         [0022]    In one embodiment, the temperature of calcined in air or under a reducing atmosphere is 900° C. to 1600° C., and the time of calcined is 2 h to 10 h. 
         [0023]    In one embodiment, said reducing atmosphere is a mixed atmosphere comprised of 95 vol % of N 2  and 5 vol % of H 2 . 
         [0024]    The core-shell structured silicate luminescent material thus prepared has the molecular formula of MLn 1-x SiO 4 :xRE@SiO 2 , of which the components are chemically stable, which the matrix is stable and not easily to decompose under long-term electron beam bombardment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0025]      FIG. 1  shows a flowchart of the process of preparing the core-shell structured silicate luminescent material of one embodiment. 
           [0026]      FIG. 2  shows a comparative plot of the luminescent spectrum of the core-shell structured silicate luminescent material prepared in Example 5 and that of the luminescent material having the formula of NaY 0.8 SiO 4 :0.18Gd 3+ , 0.02Tb 3+  being excited with an electron beam at 5 kV. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0027]    To facilitate the understanding of the present invention, the present invention will be described in further details accompanying the drawings. The drawings give the preferable embodiment of the present invention. However, it should be understood that the detailed description of the embodiment is merely illustrative of the invention and is not to be construed a limiting the invention. The present invention can be achieved in many different ways. 
         [0028]    The core-shell structured silicate luminescent material of one embodiment having the molecular formula of: 
         [0000]      MLn 1-x SiO 4   :x RE@SiO 2 ; 
         [0029]    where, @ represents a coating; 
         [0030]    M is one or two elements selected from Li, Na, and K; 
         [0031]    Ln is one or two elements among Y, Sc, Lu and La, x is 0&lt;x≦0.6; and 
         [0032]    RE is one, two or three elements selected from Tb, Gd, Sm, Eu, Dy, Ce and Tm. 
         [0033]    The core-shell structured silicate luminescent material according to the present invention is not easily decomposed due to the relatively high stability of the matrix under long-term electron beam bombardment. As compared with the non-core-shell structured luminescent material, the core-shell structured silicate luminescent material according to the present invention has a higher luminous efficiency and can be preferably used in the field emission light source device. 
         [0034]    In a preferable embodiment, x is 0.01≦x≦0.3. 
         [0035]    As depicted in  FIG. 1 , a method of preparing a core-shell structured silicate luminescent material comprises the step of: 
         [0036]    S 10 : mixing 10 to 20 parts by volume of water, 15 to 50 parts by volume of anhydrous ethanol and 1 to 7 parts by volume of ammonia water, followed by dropwisely adding 0.5 to 3 parts by volume of tetraethylorthosilicate under stirring, stirring to react for 2 h to 6 h further, removing impurities after being separated by centrifugation, taking the precipitate and redispersing the same to 10 to 20 parts by volume of anhydrous ethanol to give a suspension of SiO 2  microsphere. 
         [0037]    Said stirring may be conducted by magnetic stirring. 
         [0038]    In the present embodiment, said suspension of SiO 2  microsphere containing impurities is subjected to centrifugal separation spinning at 12000 rpm, where the precipitate is taken out, and then washed with deionized water three times to remove the surplus ammonia water and residual TEOS to give the SiO 2  microsphere. 
         [0039]    Said precipitation and redispersion may be conducted by ultrasonic dispersion. 
         [0040]    S 20 : according to the stoichiometric ratio of the formula of MLn 1-x SiO 4 :xRE, taking by measurement the corresponding nitrate solution of RE and nitrate solution of M, mixing the same homogenously, and subjecting the same to a water bath at 50° C. to 90° C., which the pH of the resulting mixture was adjusted to 1 to 6, followed by dropwisely adding tetraethylorthosilicate to give the resulting mixture, where M is one or two elements selected from Li, Na and K, Ln is one or more elements selected from Y, Sc, Lu and La, x ranges from 0&lt;x≦0.6, RE is one, two or three elements selected from Tb, Gd, Sm, Eu, Dy, Ce and Tm. 
         [0041]    The concentration of said corresponding nitrate solution of RE and nitrate solution of M ranges from 0.1M to 5M. 
         [0042]    In one embodiment, x is 0.01≦x≦0.3. 
         [0043]    In the present embodiment, the order of S 10  and S 20  can be interchanged with each other without affecting the method of preparing the core-shell structured silicate luminescent material. 
         [0044]    S 30 : according to the stoichiometric ratio of the formula of MLn 1-x SiO 4 :xRE@SiO 2 , adding said suspension of SiO 2  microsphere into said resulting mixture in a water bath at 50° C. to 90° C., stirring sufficiently to give a gel, drying the gel to give a precursor. 
         [0045]    MLn 1-x SiO 4 :xRE and SiO 2  are mixed in a stoichiometric ratio of 1:1. 
         [0046]    In the present embodiment, the gel is completely dried in an oven at 80° C. to 150° C. to give the precursor. 
         [0047]    S 40 : the precursor thus prepared from S 30  is pre-baked and then ground, the operation of grinding after pre-baking is repeated 1 to 4 times, followed by subjecting the same to calcined in air or under a reducing atmosphere, to give the core-shell structured silicate luminescent material after cooling, having the molecular formula of: MLn 1-x SiO 4 :xRE@SiO 2 , where @ is a coating. 
         [0048]    The precursor is placed in a high temperature furnace and pre-baked at 500° C. to 800° C. for 2 h to 7 h, cooled to room temperature and then ground. 
         [0049]    In order for increasing the coating thickness of MLn 1-x SiO 4 :xRE, the process of grinding after pre-baking is repeated 1 to 4 times. 
         [0050]    The ground precursor is placed in a box typed high temperature furnace or tube furnace, and then subjected to calcined at 900° C. to 1600° C. for 2 h to 10 h, to afford the desired core-shell structured silicate luminescent material. 
         [0051]    In the present embodiment, the reducing atmosphere is a mixed atmosphere comprised of 95 vol % of N 2  and 5 vol % of H 2 . 
         [0052]    The method of preparing this core-shell structured silicate luminescent material produces the small sized core-shell structured powder having a uniform morphology by sol-gel method without milling. Traditional commercial luminescent material is mainly prepared by high-temperature solid-phase process, which the high temperature reaction is usually energy consuming, and that particles are unevenly distributed and presented in different morphologies, where milling is required to obtain the luminescent material having a particle size of 2 to 8 micron, by which the luminous intensity of the luminescent material will be reduced. In the present method of preparing the core-shell structured silicate luminescent material, the shell thickness of the luminescent material can be controlled by the process to prepare a core-shell luminescent material having a uniform and single morphology, and that the luminous intensity of the luminescent material thus prepared by the present method is stronger than that of the traditional commercially available luminescent material. 
         [0053]    The specific embodiment of the present invention will now be given below. 
       Example 1 
     Preparation of NaY 0.99 SiO 4 :0.01Tb 3+ @SiO 2  by Sol-Gel Method 
       [0054]    1) Preparation of SiO 2  microsphere: 10 mL of water was placed in a beaker, followed by sequentially added therein 15 mL of anhydrous ethanol and 1 mL of ammonia water, and subjected to magnetic stirring to achieve homogenous, and dropwisely added therein 0.5 mL of tetraethylorthosilicate (TEOS) under magnetic stirring; after addition, reaction was stirred for 6 h to give a suspension of SiO 2  microsphere containing impurities. The suspension of SiO 2  microsphere containing impurities was then subjected to centrifugal separation spinning at 12000 rpm, rinsed with deionized water three times to remove surplus ammonia water and residual TEOS to give the SiO 2  microsphere as precipitate. The thus prepared SiO 2  microsphere was subjected to ultrasonic dispersion and redispersed in 10 mL of anhydrous ethanol to give a suspension of SiO 2  microsphere. 
         [0055]    2) Preparation of NaY 0.99 SiO 4 :0.01Tb 3+ @SiO 2  core-shell luminescent material: according to the chemical formula, 4.95 ml of 2 mol/L of Y(NO 3 ) 3  solution, 1 ml of 0.1 mol/L of Tb(NO 3 ) 3  solution and 5 ml of 2 mol/L of NaNO 3  solution were accurately weighed and placed in a beaker, heated with a water bath at 50° C., which the pH of the resulting mixture was adjusted to pH 1 using ammonia water and diluted nitrate solution. According to the required amount of a silicon source, 2.3 mL of tetraethylorthosilicate (TEOS) was measured and added dropwisely to the above solution. Into the solution was added said suspension of SiO 2  microsphere under stirring in a water bath at 50° C., sufficient stirring was provided until a gel was obtained. The resulting gel was placed and completely dried in an oven at 80° C. to obtain a precursor. The precursor was placed in a high temperature furnace, and calcined at 500° C. for 7 h, cooled to room temperature and then ground; the ground precursor was then placed and calcined in a tube furnace at 900° C. in a reducing atmosphere comprised of 95% N 2 +5% H 2  for 10 h, which was then allowed to cool, and taken out to give the desired core-shell luminescent material. 
       Example 2 
     Preparation of LiLa 0.7 SiO 4 :0.3Ce 3+ @SiO 2  by Sol-Gel Method 
       [0056]    1) Preparation of SiO 2  microsphere: 20 mL of water was placed in a beaker, followed by sequentially added therein 50 mL of anhydrous ethanol and 7 mL of ammonia water, and subjected to magnetic stirring to achieve homogenous, and dropwisely added therein 3 mL of tetraethylorthosilicate (TEOS) under magnetic stirring; after addition, reaction was stirred for 2 h to give a suspension of SiO 2  microsphere containing impurities. The suspension of SiO 2  microsphere containing impurities was then subjected to centrifugal separation spinning at 12000 rpm, rinsed with deionized water three times to remove surplus ammonia water and residual TEOS to give the SiO 2  microsphere as precipitate. The thus prepared SiO 2  microsphere was subjected to ultrasonic dispersion and redispersed in 20 mL of anhydrous ethanol to give a suspension of SiO 2  microsphere. 
         [0057]    2) Preparation of LiLa 0.7 SiO 4 :0.3Ce 3+ @SiO 2  core-shell luminescent material: according to the chemical formula, 3.5 ml of 2 mol/L of La(NO 3 ) 3  solution, 1.5 ml of 2 mol/L of Ce(NO 3 ) 3  solution and 5 ml of 2 mol/L of LiNO 3  solution were accurately weighed and placed in a beaker, heated with a water bath at 90° C., which the pH of the resulting mixture was adjusted to pH 6 using ammonia water and diluted nitrate solution. According to the required amount of a silicon source, 2.3 mL of tetraethylorthosilicate (TEOS) was measured and added dropwisely to the above solution. Into the solution was added said suspension of SiO 2  microsphere under stirring in a water bath at 90° C., sufficient stirring was provided until a gel was obtained. The resulting gel was placed and completely dried in an oven at 150° C. to obtain a precursor. The precursor was placed in a high temperature furnace, and calcined at 800° C. for 2 h, cooled to room temperature and then ground; the whole operation above may be repeated four times in order for increasing the coating thickness of LiLa 0.7 SiO 4 :0.3Ce 3+ @SiO 2 . The ground precursor was then placed and calcined in a tube furnace at 1600° C. in a reducing atmosphere comprised of 95% N 2 +5% H 2  for 2 h, which was then allowed to cool, and taken out to give the desired core-shell luminescent material. 
       Example 3 
     Preparation of KLu 0.4 SiO 4 :0.5Gd 3+ , 0.05Eu 3+ , 0.05Tm 3+ @SiO 2  by Sol-Gel Method 
       [0058]    1) Preparation of SiO 2  microsphere: 15 mL of water was placed in a beaker, followed by sequentially added therein 20 mL of anhydrous ethanol and 3 mL of ammonia water, and subjected to magnetic stirring to achieve homogenous, and dropwisely added therein 1.5 mL of tetraethylorthosilicate (TEOS) under magnetic stirring; after addition, reaction was stirred for 3 h to give a suspension of SiO 2  microsphere containing impurities. The suspension of SiO 2  microsphere containing impurities was then subjected to centrifugal separation spinning at 12000 rpm, rinsed with deionized water three times to remove surplus ammonia water and residual TEOS to give the SiO 2  microsphere as precipitate. The thus prepared SiO 2  microsphere was subjected to ultrasonic dispersion and redispersed in 15 mL of anhydrous ethanol to give a suspension of SiO 2  microsphere. 
         [0059]    2) Preparation of KLu 0.4 SiO 4 :0.5Gd 3+ , 0.05Eu 3+ , 0.05Tm 3+ @SiO 2  core-shell luminescent material: according to the chemical formula, 2 ml of 2 mol/L of Lu(NO 3 ) 3  solution, 2.5 ml of 2 mol/L of Gd(NO 3 ) 3  solution and 1 ml of 0.5 mol/L of Eu(NO 3 ) 3  solution, 1 ml of 0.5 mol/L of Tm(NO 3 ) 3  solution and 5 ml of 2 mol/L of KNO 3  solution were accurately weighed and placed in a beaker, heated with a water bath at 70° C., which the pH of the resulting mixture was adjusted to pH 3 using ammonia water and diluted nitrate solution. According to the required amount of a silicon source, 2.3 mL of tetraethylorthosilicate (TEOS) was measured and added dropwisely to the above solution. Into the solution was added said suspension of SiO 2  microsphere under stirring in a water bath at 80° C., sufficient stirring was provided until a gel was obtained. The resulting gel was placed and completely dried in an oven at 100° C. to obtain a precursor. The precursor was placed in a high temperature furnace, and calcined at 600° C. for 4 h, cooled to room temperature and then ground; the whole operation above may be repeated twice in order for increasing the coating thickness of KLu 0.4 SiO 4 :0.5Gd 3+ , 0.05Eu 3+ , 0.05Tm 3+ @SiO 2 . The ground precursor was then placed and calcined in a box typed high temperature furnace or a tube furnace at 1200° C. in air for 3 h, which was then allowed to cool, and taken out to give the desired core-shell luminescent material. 
       Example 4 
     Preparation of Na 0.91 Li 0.1 Y 0.7 Sc 0.1 SiO 4 :0.15Gd 3+ , 0.05Sm 3+ @SiO 2  by Sol-Gel Method 
       [0060]    1) Preparation of SiO 2  microsphere: 15 mL of water was placed in a beaker, followed by sequentially added therein 30 mL of anhydrous ethanol and 5 mL of ammonia water, and subjected to magnetic stirring to achieve homogenous, and dropwisely added therein 2 mL of tetraethylorthosilicate (TEOS) under magnetic stirring; after addition, reaction was stirred for 4 h to give a suspension of SiO 2  microsphere containing impurities. The suspension of SiO 2  microsphere containing impurities was then subjected to centrifugal separation spinning at 12000 rpm, rinsed with deionized water three times to remove surplus ammonia water and residual TEOS to give the SiO 2  microsphere as precipitate. The thus prepared SiO 2  microsphere was subjected to ultrasonic dispersion and redispersed in 20 mL of anhydrous ethanol to give a suspension of SiO 2  microsphere. 
         [0061]    2) Preparation of Na 0.91 Li 0.1 Y 0.7 Sc 0.1 SiO 4 :0.15Gd 3+ , 0.05Sm 3+ @SiO 2  core-shell luminescent material: according to the chemical formula, 3.5 ml of 2 mol/L of Y(NO 3 ) 3  solution, 0.5 ml of 2 mol/L of Sc(NO 3 ) 3  solution and 3 ml of 0.5 mol/L of Gd(NO 3 ) 3  solution, 1 ml of 0.5 mol/L of Sm(NO 3 ) 3  solution, 4.5 ml of 2 mol/L of NaNO 3  solution and 0.5 ml of 2 mol/L of LiNO 3  solution were accurately weighed and placed in a beaker, heated with a water bath at 80° C., which the pH of the resulting mixture was adjusted to pH 2 using ammonia water and diluted nitrate solution. According to the required amount of a silicon source, 2.3 mL of tetraethylorthosilicate (TEOS) was measured and added dropwisely to the above solution. Into the solution was added said suspension of SiO 2  microsphere under stirring in a water bath at 80° C., sufficient stirring was provided until a gel was obtained. The resulting gel was placed and completely dried in an oven at 100° C. to obtain a precursor. The precursor was placed in a high temperature furnace, and calcined at 700° C. for 4 h, cooled to room temperature and then ground. The ground precursor was then placed and calcined in a box typed high temperature furnace or a tube furnace at 1100° C. in air for 4 h, which was then allowed to cool, and taken out to give the desired core-shell luminescent material. 
       Example 5 
     Preparation of NaY 0.8 SiO 4 :0.18Gd 3+ , 0.02Tb 3+ @SiO 2  by Sol-Gel Method 
       [0062]    1) Preparation of SiO 2  microsphere: 15 mL of water was placed in a beaker, followed by sequentially added therein 30 mL of anhydrous ethanol and 5 mL of ammonia water, and subjected to magnetic stirring to achieve homogenous, and dropwisely added therein 1.5 mL of tetraethylorthosilicate (TEOS) under magnetic stirring; after addition, reaction was stirred for 4 h to give a suspension of SiO 2  microsphere containing impurities. The suspension of SiO 2  microsphere containing impurities was then subjected to centrifugal separation spinning at 12000 rpm, rinsed with deionized water three times to remove surplus ammonia water and residual TEOS to give the SiO 2  microsphere as precipitate. The thus prepared SiO 2  microsphere was subjected to ultrasonic dispersion and redispersed in 15 mL of anhydrous ethanol to give a suspension of SiO 2  microsphere. 
         [0063]    2) Preparation of NaY 0.8 SiO 4 :0.18Gd 3+ , 0.02Tb 3+ @SiO 2  core-shell luminescent material: according to the chemical formula, 4 ml of 2 mol/L of Y(NO 3 ) 3  solution, 3.6 ml of 0.5 mol/L of Gd(NO 3 ) 3  solution, 0.4 ml of 0.5 mol/L of Tb(NO 3 ) 3  solution and 5 ml of 2 mol/L of NaNO 3  solution were accurately weighed and placed in a beaker, heated with a water bath at 80° C., which the pH of the resulting mixture was adjusted to pH 3 using ammonia water and diluted nitrate solution. According to the required amount of a silicon source, 2.3 mL of tetraethylorthosilicate (TEOS) was measured and added dropwisely to the above solution. Into the solution was added said suspension of SiO 2  microsphere under stirring in a water bath at 80° C., sufficient stirring was provided until a gel was obtained. The resulting gel was placed and completely dried in an oven at 120° C. to obtain a precursor. The precursor was placed in a high temperature furnace, and calcined at 600° C. for 5 h, cooled to room temperature and then ground. The ground precursor was then placed and calcined in a box typed high temperature furnace or a tube furnace at 1100° C. in a reducing atmosphere comprised of 95% N 2 +5% H 2  for 4 h, which was then allowed to cool, and taken out to give the desired NaY 0.8 SiO 4 :0.18Gd 3+ , 0.02Tb 3+ @SiO 2  core-shell luminescent material. 
         [0064]    NaY 0.8 SiO 4 :0.18Gd 3+ , 0.02Tb 3+  luminescent material was prepared under the same condition except that a suspension of SiO 2  microsphere was not added. 
         [0065]    In the comparative plot as shown in  FIG. 2 , curve a and b, represents, respectively, the luminescent spectrum of the NaY 0.8 SiO 4 :0.18Gd 3+ , 0.02Tb 3+ @SiO 2  core-shell luminescent material of the present example and that of NaY 0.8 SiO 4 :0.18Gd 3+ , 0.02Tb 3+  luminescent material being excited with an electron beam at 5 kV. 
         [0066]    From the figure, it can be seen that as comparing with the NaY 0.8 SiO 4 :0.18Gd 3+ , 0.02Tb 3+  luminescent material, NaY 0.8 SiO 4 :0.18Gd 3+ , 0.02Tb 3+ @SiO 2  core-shell luminescent material has a stronger luminous intensity, which shows an increasement of 33%. 
       Example 6 
     Preparation of NaLa 0.95 SiO 4 :0.05Dy 3+ @SiO 2  by Sol-Gel Method 
       [0067]    1) Preparation of SiO 2  microsphere: 15 mL of water was placed in a beaker, followed by sequentially added therein 30 mL of anhydrous ethanol and 5 mL of ammonia water, and subjected to magnetic stirring to achieve homogenous, and dropwisely added therein 2 mL of tetraethylorthosilicate (TEOS) under magnetic stirring; after addition, reaction was stirred for 4 h to give a suspension of SiO 2  microsphere containing impurities. The suspension of SiO 2  microsphere containing impurities was then subjected to centrifugal separation spinning at 12000 rpm, rinsed with deionized water three times to remove surplus ammonia water and residual TEOS to give the SiO 2  microsphere as precipitate. The thus prepared SiO 2  microsphere was subjected to ultrasonic dispersion and redispersed in 20 mL of anhydrous ethanol to give a suspension of SiO 2  microsphere. 
         [0068]    2) Preparation of NaLa 0.95 SiO 4 :0.05Dy 3+ @SiO 2  core-shell luminescent material: according to the chemical formula, 4.75 ml of 2 mol/L of La(NO 3 ) 3  solution, 1 ml of 0.5 mol/L of Dy(NO 3 ) 3  solution and 5.0 ml of 2 mol/L of NaNO 3  solution were accurately weighed and placed in a beaker, heated with a water bath at 80° C., which the pH of the resulting mixture was adjusted to pH 5 using ammonia water and diluted nitrate solution. According to the required amount of a silicon source, 2.3 mL of tetraethylorthosilicate (TEOS) was measured and added dropwisely to the above solution. Into the solution was added said suspension of SiO 2  microsphere under stirring in a water bath at 80° C., sufficient stirring was provided until a gel was obtained. The resulting gel was placed and completely dried in an oven at 100° C. to obtain a precursor. The precursor was placed in a high temperature furnace, and calcined at 650° C. for 5 h, cooled to room temperature and then ground. The ground precursor was then placed and calcined in a box typed high temperature furnace or a tube furnace at 950° C. in air for 5 h, which was then allowed to cool, and taken out to give the desired core-shell luminescent material. 
         [0069]    Although the preferable embodiments of the present invention has been described and illustrated in detail, it is clearly understood that the same is not to be taken by way of limitation, it should be understood that various changes, substitutions, and alterations could be made hereto by an ordinary skilled person in the art without departing from the spirit and scope of the invention as defined by the appended claims.