Abstract:
A method of manufacturing Ce:YAG polycrystalline phosphor with the formula (Y 1-x-m A x Ce m ) 3 (Al 1-y B y ) 5 O 12 ; 0≦x≦1, 0≦y≦1, 0≦m≦0.05; wherein A is one of Lu, Tb, Pr, La and Gd; and wherein B is one of Ga, Ti, Mn, Cr and Zr.

Description:
FIELD OF THE INVENTION 
     The invention is directed to the field of LED manufacture, especially to a method of manufacturing Ce:YAG polycrystalline phosphor. 
     BACKGROUND 
     LED is one sort of solid semiconductor device, which is able to directly convert electric energy to light energy. Compared with traditional incandescent and fluorescent light, white LED possesses the advantages of low power consumption, high luminous efficiency, long working life, energy conservation and environmental friendliness, etc. Therefore, not only is it widely used in the field of daily lighting, but it also steps into the field of display equipment. Currently, the techniques of acquiring white LED can be divided into two classes: (1) adopting the combination of three LED chips that emit red, green and blue light; (2) adopting a monochromatic (blue or ultraviolet) LED chip to excite an appropriate fluorescent material. At present, white LED mainly utilizes the combination of a blue LED chip and phosphor powder Ce 3+ :YAG which can be excited effectively by blue light and emit yellow light, and then the lens principle is used to mix complementary yellow and blue light so as to provide white light. 
     For a structure that uses phosphor powder packaging, there are the following disadvantages: 1) phosphor powder excitation efficiency and light conversion efficiency are low; 2) the problem of inhomogeneity of phosphor particle size and dispersion cannot be effectively solved; 3) phosphor powder lacks a red luminescent component, making it hard to manufacture white LED with a low color temperature and a high color rendering index; 4) the light failure of phosphor powder is tremendous, resulting in short working life of white LED; 5) the poor physicochemical properties of phosphor powder fail to meet the development of high-power LED. 
     SUMMARY 
     The technical problem that the invention is intended to solve is to overcome the defects in the prior art by providing a method of manufacturing Ce:YAG polycrystalline phosphor which possesses good optical properties and can be used in producing white LED. 
     To solve the problem above, the invention provides a method of manufacturing Ce:YAG polycrystalline phosphor. This crystal is of the following formula:
 
(Y 1-x-m A x Ce m ) 3 (Al 1-y B y ) 5 O 12  
 
0≦x≦1, 0≦y≦1, 0≦m≦0.05
 
wherein A is one of Lu, Tb, Pr, La and Gd; and wherein B is one of Ga, Ti, Mn, Cr and Zr.
 
     The method includes the following steps: 
     1) proportioned raw materials are mixed well (e.g., homogenously) and compressed into a cake; 
     2) the cake is fired at 1200-1400° C. under reducing or inert atmosphere for 10-20 hours, and placed into a crucible after sintered into a block; 
     3) the crucible is heated to 1970° C. under reducing or inert atmosphere to melt the raw materials by induction heating or resistance heating; the temperature is further increased by 50-100° C. on this basis to make the melt maintain at this temperature for 2-10 hours in a superheated state, allowing the materials to be mixed by convection;
 
4) the temperature is decreased to the melting point of the raw materials, namely 1970° C., then cooled down to room temperature gradually according to a staged cooling down procedure in 10-20 hours in order to obtain Ce:YAG polycrystalline phosphor after cooling down.
 
     In step 4), the staged cooling down procedure includes: 
     Stage 1, the temperature is decreased to 1200° C. from 1970° C. over 4-8 hours; 
     Stage 2, the temperature is decreased to room temperature from 1200° C. over 6-9 hours. 
     In step 4), the staged cooling down procedure includes: 
     Stage 1, the temperature is decreased to 1600° C. from 1970° C. over 6-8 hours; 
     Stage 2, the temperature is decreased to 1000° C. from 1600° C. over 5-7 hours; 
     Stage 3, the temperature is decreased to room temperature from 1000° C. over 5-7 hours. 
     Compared with the prior art, the Ce:YAG polycrystalline phosphor made by the method of the invention has the following advantageous effects: 
     1) this Ce:YAG polycrystalline phosphor possesses good optical efficiency, high growth rate and low cost, making it a promising fluorescent material used in white LED; 
     2) this Ce:YAG polycrystal can be doped with high concentration of cerium ion; the actual doping concentration of cerium ion in Ce:YAG polycrystal can be higher than 1%; 
     3) cracking is not a problem for the polycrystal during the process of its growth which has low requirement and short cycle, significantly reducing the manufacturing cost and improving the economic benefit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
         FIG. 1  is the schematic diagram of the heating device of the invention. 
         FIG. 2  is the relative energy distribution curve of the polycrystal in Example 1 which is excited by blue LED. 
     
    
    
     DETAILED DESCRIPTION 
     In order to make the technical solution of the invention better understood by those skilled in the art, the invention is further illustrated with reference to the embodiments as follows. 
     As shown in  FIG. 1 , the heating device of the invention includes a crucible  3  and an insulating layer  2 . An induction coil  4  is twined around the insulating layer  2  from outside. Polycrystal  6  is produced inside the crucible  3 . The crucible  3  is covered by an insulating hood  1 . There is an observation port  5  through the insulating hood  1 . 
     Example 1 
     The raw materials are weighed according to the proportion based on (Y 0.98 Ce 0.02 ) 3 Al 5 O 12 . The raw materials are then mixed and compressed into a cake. The cake is fired at 1400° C. under inert atmosphere for 15 hours and placed into a crucible after sintered into a block. Induction heating method is applied to heat the crucible to 1970° C. under inert atmosphere to melt the raw materials. Then the temperature is further increased by 50° C. to make the melt maintain at this temperature for 2 hours in a superheated state. The temperature is decreased to 1970° C. and a two-stage procedure of 15 hours in total is used for cooling. The first stage is conducted for 7 hours, from 1970° C. to 1200° C. The second stage is conducted for 8 hours, from 1200° C. to room temperature. After cooling down, yellow (Y 0.98 Ce 0.02 ) 3 Al 5 O 12  polycrystalline phosphor is obtained. 
     Example 2 
     The raw materials are weighed according to the proportion based on (Y 0.79 Gd 0.2 Ce 0.01 ) 3 (Al 0.998 Mn 0.002 ) 5 O 12 . The raw materials are then mixed and compressed into a cake. The cake is fired at 1300° C. under inert atmosphere for 20 hours and placed into a crucible after sintered into a block. Induction heating method is applied to heat the crucible to 1970° C. under inert atmosphere to melt the raw materials. Then the temperature is further increased by 50° C. to make the melt maintain at this temperature for 10 hours in a superheated state. The temperature is decreased to 1970° C. and a three-stage procedure of 20 hours in total is used for cooling. The first stage is conducted for 7 hours, from 1970° C. to 1600° C. The second stage is conducted for 6.5 hours, from 1600° C. to 1000° C. The third stage is conducted for 6.5 hours, from 1000° C. to room temperature. After cooling down, yellow green (Y 0.69 Tb 0.3 Ce 0.01 ) 3 (Al 0.998 Mn 0.002 ) 5 O 12  polycrystalline phosphor is obtained. 
     Example 3 
     The raw materials are weighed according to the proportion based on (Y 0.685 Tb 0.3 Ce 0.015 ) 3 (Al 0.999 Cr 0.001 ) 5 O 12 . The raw materials are then mixed and compressed into a cake. The cake is fired at 1400° C. under reducing atmosphere for 10 hours and placed into a crucible after sintered into a block. Resistance heating method is applied to heat the crucible to 1970° C. under reducing atmosphere to melt the raw materials. Then the temperature is further increased by 80° C. to make the melt maintain at this temperature for 4 hours in a superheated state. The temperature is decreased to 1970° C. and a three-stage procedure of 17 hours in total is used for cooling. The first stage is conducted for 6 hours, from 1970° C. to 1600° C. The second stage is conducted for 6 hours, from 1600° C. to 1000° C. The third stage is conducted for 5 hours, from 1000° C. to room temperature. After cooling down, orange (Y 0.685 Tb 0.3 Ce 0.015 ) 3 (Al 0.999 Cr 0.001 ) 5 O 12  polycrystalline phosphor is obtained.