Preparation,Properties And Applications Of Ce³⁺Doped Garnet Phosphor Ceramics For Solid-state Lighting

Solid-state lighting is the most promising light source due to its high luminous efficacy,environmental friendliness,long lifetime and small size,etc.LED lighting and the state-of-the-art LD lighting are currently the most widely used in solid-state lighting,and white light can be obtained by combining phosphor converters with LED/LD chips.With the increase in market demand for lifetime,brightness and color rendering performance of lighting devices,it is critical to improving the performance of phosphor converters in high-power LED and LD products.As an important part responsible for the conversion of excitation light into the desired visible light,the phosphor converters affect the quality of the obtained white light.Most of the currently used phosphor converters are YAG:Ce phosphor powders combined with organic resins,however,the low thermal conductivity of the phosphor powders makes them difficult to apply in high-power and high-brightness solid state lighting.Garnet based phosphor ceramics have become promising phosphor converters due to their high chemical stability,good mechanical properties,good thermal stability and perfect matchability with blue LED/LD.The color rendering index of classical YAG:Ce phosphor ceramics currently used in the white LED/LD is generally around60-65,which cannot meet the needs of indoor lighting.However,the improvement of color rendering performance is generally accompanied by a decline in luminous efficacy.It is difficult to obtain high luminous efficacy and high color rendering performance simultaneously.The incident power density used in high-power LED/LD lighting is very high,and phosphor ceramics with good thermal stability and high saturation threshold are urgently needed to withstand this high-power excitation.To address the above problems,the following research works are carried out in this dissertation,using the classical garnet system as the phosphor ceramic matrix material:（1）In order to improve the color rendering performance of YAG:Ce phosphor ceramics,（i）Gd³⁺ions are introduced to partially replace Y³⁺in YAG:Ce phosphor ceramics to enhance the crystal field around Ce³⁺thus achieving the spectral redshift of Ce³⁺.In this work,a series of transparent GdYAG:Ce phosphor ceramics with different Gd³⁺contents are prepared by solid-state reaction combined with vacuum sintering,and the microstructure,phase composition,thermal stability,and luminescence properties of the ceramics are systematically investigated.The results show that the addition of Gd³⁺can effectively increase the red component in the emission spectrum of YAG:Ce.With the increase of Gd³⁺content,the main emission peak is red-shifted from 528 nm to 550 nm,which is expected to enhance the color rendering performance and reduce the color temperature of the phosphor ceramics.（ii）Two different composite structure phosphors are designed to further improve the color redering.In the first way,（Sr,Ca）AlSiN₃:Eu red phosphor and BaSi₂N₂O₂:Eu cyan phosphor with high quantum efficiency and high stability are selected to make red and cyan phosphor films,respectively,and then they are combined with YAG:Ce ceramics in different orders to prepare different structures of three-layered composite phosphors.The best performance of the white COB LED is obtained when the concentration ratio of cyan phosphor film is 1:1:0.07（the mass ratio of glue A:glue B:BaSi₂N₂O₂:Eu phosphor）,the concentration ratio of red phosphor film is 1:1:0.01（the mass ratio of glue A:glue B:（Sr,Ca）AlSiN₃:Eu phosphor）,and the concentration of Ce³⁺in the YAG:Ce ceramic is 0.1at.%.The LE of 75lm/W,Rf of 93,Rg of 97 and CCT of 3852 K are achieved in the white COB LED.The LE,CRI and CCT of the obtained white LD light are 120lm/W,90 and 5988 K,respectively.In the second way,the YAG:Ce/YAG:Cr/YAG:Pr composite phosphor ceramics with 2D barcode structure were prepared by tape casting,in which YAG:Ce,YAG:Pr and YAG:Cr are arranged in different thickness ratios in the plane perpendicular to the blue excitation light.After combining the obtained composite ceramics with the blue COB LED,the LE of the white LED based on the composite ceramics with a thickness ratio of 6:6:6 is 106lm/W,the Rg is 77,the Rf is 86,and the CCT is 7642 K.（2）To further enhance the thermal stability and LE of GdYAG:Ce phosphor ceramics,MgAl₂O₄ and Al₂O₃ with higher thermal conductivity are added to the GdYAG:Ce phosphor ceramic matrix as the secondary phase scattering centers,and a series of MgAl₂O₄-GdYAG:Ce and Al₂O₃-GdYAG:Ce composite phosphor ceramics are prepared.The white light based on MgAl₂O₄-GdYAG:Ce phosphor ceramics is closest to the blackbody radiation line with a CRI of 69 and a CCT of 4543 K when the molar ratio of MgAl₂O₄ to GdYAG:Ce is 1:1 and the ceramic thickness is 0.2 mm.When the molar ratio of Al₂O₃ to GdYAG:Ce is 0.8:1,the white LED based on Al₂O₃-GdYAG:Ce composite ceramics can produce natural white light with a LE of 112lm/W and a CRI of 71.The addition of the secondary phase scattering particles has two main purposes.On the one hand,the high thermal conductivity secondary phase can help improve the thermal conductivity of composite ceramics,thus increasing its thermal stability as a phosphor converter to meet the needs of high-power LED/LD lighting.On the other hand,the addition of the secondary phase scattering centers can change the transmission path of incident blue light in the ceramic phosphor,thus increasing the absorption of the incident blue light.（3）To prepare green-emitting phosphor ceramics with high laser saturation threshold suitable for high-power lighting/projection,Al₂O₃-Lu AG:Ce composite phosphor ceramics with different Al₂O₃ contents are prepared.It is demonstrated that the addition of Al₂O₃ effectively improves the thermal stability and internal quantum efficiency of Lu AG:Ce ceramics.After combining with blue COB LEDs,the LE of Al₂O₃-Lu AG:Ce composite ceramics is 113lm/W～155lm/W.After combining with blue LDs,the LE of the composite ceramic is 220lm/W～279lm/W under the excitation of high blue power density,and there is no obvious luminescence saturation until the excitation power density is 11.3W/mm²,which proves that the Al₂O₃-Lu AG:Ce composite ceramics can be applied to high-power LED/LD lighting/projection.