Preparation And Properties Of Phosphor In Glass/Ceramic For High Power LED

As a key material in solid-state lighting technology,the main function of the luminescence convertor is absorbing the excitation light of the chip and converting it to the required light.The performance of the convertor directly determines the color rendering index（CRI）,lumen efficiency（LE）and reliability of the solid-state lighting device.At present,the luminescence convertor obtained by encapsulating yellow phosphor with silicone or epoxy resin（PiS）is widely used,which is assembled with a blue chip to obtain the white light-emitting-diode（WLED）.With the development of WLEDs towards high power and integration,the disadvantages of PiS as aging and yellowing at high temperature are gradually prominent.In order to solve the problems as color coordinate drift and shortened life in the use of high power WLED,it is urgent to explore a luminescence convertor with excellent thermal stability,high light conversion efficiency and high thermal conductivity.Therefore,glass/ceramic materials with high thermal conductivity and excellent stability have become ideal substrates for luminescence convertors.Although the research on fluorescent glass/ceramics has made significant progress in recent years,these inorganic luminescence convertors have not been mass-produced and widely used in practice.Due to the poor thermal stability of the red phosphor,it is difficult to obtain a high-performance red phosphor in glass/ceramic,which causes the problems as low CRI and high correlated color temperature（CCT）of WLEDs.In addition,due to the existence of the second phase（phosphor powders）and grain boundaries,the transmittance of fluorescent ceramic prepared by encapsulating phosphors in ceramic is extremely low and cannot be adopted to prepare WLEDs in transmission mode.In this project,by using commercial phosphors and mesoporous nanopowders as raw materials,phosphor in glass（PiG）and phosphor in ceramic（PiC）with high light conversion efficiency and tunable emission spectra are obtained at low sintering temperature and extremely short preparation time through spark plasma sintering（SPS）.WLED with high CRI and LE was obtained by adopting the PiG or PiC as color convertor.The details of these works are as follows:（1）Preparation of red PiGs:Firstly,mesoporous silica powder（FDU-12）with cage channel structure was prepared by hydrothermal method.The luminescent glass with high performance was prepared by SPS technology at 950?C,using the mixture of FDU-12 and commercial nitride red phosphor(CaAlSiN₃:Eu²⁺)as raw material.Analysis techniques such as X-Ray Diffraction（XRD）,Scanning Electron Microscope（SEM）,Transmission Electron Microscope（TEM）,Ultra-violet and Visible spectrophotometer（UV-Vis）,Fluorescence spectroscopy（PL）and so on were used.Then the effects of phosphor morphology,sintering procedure and phosphor concentration on the microscopic morphology,crystalline structure and luminescence properties of the red PiGs were explored.Finally,the optimum preparation process of the silica-based red PiG was clarified.After being embedded in the glass,the micro-morphology of the red phosphor and the coordination environment of the internal Eu²⁺were well retained.Furthermore,no interfacial reaction occurred between the matrix and the phosphor.When excited by the blue light,the PiG containing CASN:Eu²⁺phosphors emitted red light of 600 nm,which presents a same shape of emission spectra as the original phosphor with a 10 nm blue-shift.From the results,it could be concluded that by avoiding the melting process,reducing the sintering temperature and shortening the sintering time,the thermal quenching and interface reaction of the red phosphor can be effectively conquered and the luminous performance of the red phosphor was well retained.Finally,the red PiG with high light conversion efficiency was obtained.（2）Preparation of multi-color PiGs:In order to solve the problems as low CRI and high CCT of WLED assembled by PiG containing YAG:Ce phosphor,multi-color PiGs were prepared in this chapter.Firstly,monocolor PiGs were prepared by encapsulating phosphors such as CASN:Eu²⁺red phosphor,YAG:Ce³⁺yellow phosphor and LuAG:Ce³⁺green phosphor in silica glass by SPS,insuring the phosphors were suitable for preparing PiGs.Subsequently,a series of multi-color PiGs was prepared:a)yellow&red dual PiG;b)green&red dual PiG;c)green&red double-layer PiGs.Through XRD,SEM,PL,confocal laser scanning microscope（LSCM）,laser thermal conductivity and other characterization methods,the phase composition,microstructure and element distribution of the multi-color PiG were analyzed in depth.The results show that the phosphors are evenly dispersed in the glass matrix with no micro-morphology change and the luminescence characteristics of different phosphors are effectively protected with no reabsorption.Moreover,by changing the mass fraction of various phosphors and the thickness of the PiG,the luminous performance of the PiG can be flexibly adjusted,and the final assembled WLED showed adjustable chromaticity.In addition,the double-layer PiG prepared by the one-step method presents a clear boundary between each layer without cracks.Moreover,the phosphors are evenly distributed in the respective layers with no deposition.The performance of the WLED assembled with layered PiG also has been effectively improved.（3）Preparation of transparent Hydroxyapatite（HA）ceramics:In order to explore an encapsulation substrate with a lower sintering temperature and a better refractive index matching with the phosphor,we prepared a dense HA ceramic by using mesoporous HA nanorod as raw material.The phase composition,microscopic morphology,optical and mechanical properties of HA powder and ceramic block were studied by XRD,TEM,SEM,UV-Vis and other methods,and the optimum preparation condition of the HA ceramic with high transmittance was determined.As the research results show,the prepared mesoporous HA nanorods have high thermal stability and specific surface area.Fully dense HA ceramic with an average grain size of 100 nm can be prepared by sintering at 850?C（density～100%）.More importantly,by using the high aspect ratio of the HA nanorods and the growth rate difference between lattice planes in the HA crystal grain,the grain orientation was achieved by SPS.Benefitting from the nano wave plate formed between HA crystals,the transmittance of HA ceramics in the visible region is as high as 88%,which is basically consistent with the theoretical value.（4）Preparation of HA-based PiCs:HA-YAG:Ce transparent PiC with ultra-high light conversion efficiency was prepared at a low temperature of 850?C by using as-synthesized HA nanorods as matrix material.In addition,by using HA-YAG:Ce PiC as the shell and HA-CASN:Eu red PiC as the interlayer,we prepared a multi-layer yellow&red PiC.The XRD,TEM,SEM,XPS,LSCM and other methods were used to explore the changes of phase composition,microscopic structure,fluorescence properties,physical properties and thermal conductivity of HA-based fluorescent PiCs.The study found that,due to the lower sintering temperature of the HA ceramic substrate,the structure of the nano-wave plate and the matching refractive index between the substrate and the phosphor,the quantum yield of the HA-based fluorescent ceramic reaches 90%of the raw phosphor,which retained the luminous characteristics of YAG:Ce phosphors.The introduction of the Rayleigh scattering improves the utilization of incident light and further improves the light conversion efficiency of HA-YAG:Ce PiC.The LE of the assembled WLED is as high as 170 lm·W^-1.Finally,using HA-YAG:Ce PiC as the outer layer,HA-CASN:Eu red PiC as the inner layer,a multi-layer yellow&red PiC was prepared through one-step sintering.Benefiting from the multi-layer mechanism design,we successfully embedded red phosphor into HA-YAG:Ce PiC,which increased the CRI of WLED by 20%.