Study Of Fabrication,optical And Thermal Properties Of YAG:Ce Ceramic Phosphor Plates

The white LED light source has increasingly replaced the traditional lighting source of a wide range of applications due to its ultra-long service life and significant energy saving effect.In order to meet the market's demand for high-brightness and miniaturized light sources,it is an important direction to develop the white LED light source of high power density,light weight and small size at present.The most common,economical and reliable white light LED technology for commercial is the so-called pc-LED(Phosphor-converted LED),which uses phosphor materials to convert part of the blue light emitted by the blue LED chips into yellow light to produce visual white light.The traditional packaging method of white LED is to coat the blue chip with phosphor and silicone or resin.Due to the poor thermal conductivity and heat resistance of organic resin,it is not easy to dissipate heat,but also easy to age and deteriorate at high temperatures,so it is difficult to be used in the occasions for high power density and high brightness;and the phosphor ceramic based on Y₃Al₅O₁₂:Ce(YAG:Ce)is increasingly replacing the solution of silicone+phosphor for high power,high-brightness white LED lighting and laser diode white lighting,due to the thermal conductivity and excellent heat resistance dozens of times that of silicone or resin.In this paper,the preparation,optical and thermal properties of YAG:Ce phosphor ceramic plates were studied.The main research findings are as follows:(1)The(Y_1-xGd_x)₃Al₅O₁₂(x=0.0,0.1,0.2,0.3,0.4)phosphor ceramics were prepared by solid state reaction sintering,and the microstructure changes and grain growth process were studied.The results show that:The density,porosity and grain size of phosphor ceramics are related to the sintering process and the concentration of Gd³⁺.After holding at 1700°C for 45 hours,the porosity is less than 0.22%,the actual density reaches the theoretical density of 99.8%,and the grain size reaches 27?33?m.The analysis of grain growth kinetics shows that the doping of Gd³⁺instead of Y³⁺reduces the activation energy of grain growth of phosphor ceramics.When x increases from 0.0to 0.4,the activation energy decreases from 563.27 k J/mol to 163.18 k J/mol,or decreases 71%.(2)YAG:x Ce(x=0.8 at.%)phosphor ceramics with different grain size were prepared by solid-state reaction,and the effect of grain size on optical properties was studied.The results show that:The grain size has a significant effect on the blue/yellow light conversion efficiency and high temperature stability.When the average grain size increases from 6.0?m to 36.0?m,the yellow light conversion efficiency of the phosphor ceramics excited by the blue light source(peak wavelength of 455 nm,optical power of 1.82 W)increases from 214.14 lm/W to 340.04 lm/W,with an increase of58.8%.When the temperature increased from 296 K to 473 K,the photoluminescence intensity of the samples decreased by 15.97%and 21.87%,respectively.(3)YAG:x Ce(x=0.2?1.0at.%),(Y_1-xGd_x)₃Al₅O₁₂:Ce(x=0.0?1.0)and Y₃(Al_1-xGa_x)₅O₁₂(x=0.0?0.3)phosphor ceramics were prepared by solid state reaction.The effect of different rare earth ion concentration on optical properties was studied.The results show:Doping concentration has a significant effect on blue/yellow light conversion efficiency,high temperature stability,color coordinates and color rendering index(CRI).(a)When the doping concentration of Ce³⁺in YAG:Ce is 0.8 at.%,the maximum blue light/yellow light conversion efficiency is 340.04 lm/W.However,the blue light/yellow light conversion efficiency of(Y_1-xGd_x)₃Al₅O₁₂:Ce(x=1.0)and Y₃(Al_1-xGa_x)₅O₁₂(x=0.3)decreased to 149.22 lm/W and 226.81 lm/W,respectively;(b)YAG:x Ce(x=0.2 at.%)and Y₃(Al_1-xGa_x)₅O₁₂(x=0.1)phosphor ceramics have higher thermal stability.When the temperature rises from 296 K to 473 K,the photoluminescence intensity decreases by 11.29%and 10.5%,respectively.(c)The partial replacement of Y³⁺with Gd³⁺causes the color coordinate of the phosphor ceramic package LED light source moving to red.When x increases from 0.2 to 1.0,the color coordinate changes from(0.3543,0.3600)to(0.4446,0.4129).The CRI reaches the maximum value of 76.4 at x=0.2 and 0.4.(4)The relationship between the thermal properties of YAG:Ce phosphor ceramics and the microstructure(grain size)and the contents of Ce,Gd and Ga were studied.The results show:(a)When the temperature increases from 296 K to 473 K,the thermal conductivity of each sample decrease by 32?37%;(b)The average grain size of phosphor ceramics increased from 6.0?m to 36.0?m.At 296 K and 473 K,the thermal conductivity increased by 3%and 8%,respectively;(c)When the doping concentration of Ce³⁺increases from 0.0 at.%to 1.0 at.%,the thermal conductivity decreases linearly.When the temperature is 296 K and 473 K,the thermal conductivity decreases by 9.7%and 10%,respectively;(d)Ga³⁺replaces Al³⁺from 0.0 to 0.3.When the temperature is296 K and 473 K,the thermal conductivity decreases by 32.74%and 26.90%;(e)When Gd³⁺replaces Y³⁺from 0.0 to 1.0,the thermal conductivity are not linearly dependent on x,but a“U”type.They decrease with x increasing first,reaching a minimum at x between 0.4 and 0.6,and then increase with x as it continues to increase to 1.