Preparation And Properties Of NOx Phosphors For Rare Earth Doped LEDs

White Light-emitting diodes?LEDs?,as a new type of solid-state light source,show an outstanding potential in illumination and display applications owing to their high luminous efficiency,low energy consumption,environment friendly,long life time.As one of the most important part of white LEDs,Phosphors directly decides the quality of White-LEDs such as color rendering,color temperature,life time and luminescent efficiency.Oxynitride phosphor,a thermally and chemically stable compound,led to intensive research into this new class of materials during the past decade.They have strong absorption in the near-UV to blue light region,well matching the near-UV LED chips and blue LED chips,thus making them excellent candidates for White-LEDs.This thesis focused on the preparation and luminescence properties of SrSi₂O₂N₂:Eu²⁺ phosphors,We systematically researched the preparation process of SrSi₂O₂N₂:Eu²⁺,luminous characteristics of host lattice cation ions co-doped SrSi₂O₂N₂:Eu²⁺ and multiple activator ions co-doped SrSi₂O₂N₂,thermal stability of luminescence of SrSi₂O₂N₂:Eu²⁺.The main research work and the results are as follows:1.SrSi₂O₂N₂:Eu²⁺ phosphors were respectively prepared by a high temperature solid-state reaction route from one step method with Si-Si3N4 system as starting materials and a two step method with Sr2SiO4 as a precursor.The formation of an impurity phase in the synthesis of SrSi₂O₂N₂:Eu²⁺ was caused by the extra oxygen in starting material.Single-phase SrSi₂O₂N₂:Eu²⁺ phosphors were synthesized successfully using the Si-Si3N4 as silicon source,avoiding the formation of an unknown phase,which usually exists from SiO2-Si3N4 system.The impurity phase appeared by the precursor method,which result from the high oxygen content of Sr2SiO4 precursor.2.It was found that the SrSi₂O₂N₂ had an oxygen-rich phase and nitrogen-rich phase which are reflected with the diffraction intensity of?020?plane?2?=25.2°?and?220?plane?2?=31.6°?,respectively.The comparison of different composition of starting materials indicates that the different Si content in starting materials plays a part in phase and luminescence of SrSi₂O₂N₂:Eu²⁺.With the increasing of Si content in starting material,the relative diffraction intensity ratio of?020?plane/?220?plane is increasing,following with the increasing of emission intensity.3.Sintering temperature and flux are important for the preparation of SrSi₂O₂N₂:Eu²⁺.The samples can be prepared at 1350?,properly increasing the soaking temperature can improve the luminescence performance of the phosphors.Flux also has an important effect on the preparation of SrSi₂O₂N₂:Eu²⁺.The addition of flux is beneficial to the formation of samples at 1350?,but caused the decrease of luminescence performance at 1425 ?.Experiments show that BaF2 is a better flux compared to NH4 Cl and H3BO3.there exits the optimum soaking temperature for various fluxes,1350?for H3BO3,1400? for BaF2 and NH4 Cl.4.The effect of different Eu valence(Eu²⁺/Eu³⁺)and partial substitution of Sr²⁺ by Ba²⁺ in Sr2SiO4 precursor on the luminescence properties of SrSi₂O₂N₂:Eu²⁺ was studied.There exist charge defects originated from the replacement of Sr²⁺ by Eu³⁺ in Sr2SiO4:Eu³⁺ and anion vacancies which likely occur in Sr1.95Ba0.05SiO4:Eu²⁺ due to partial substitutions of Sr²⁺ by Ba²⁺.The defect in precursor plays a role on the PL properties.The emission intensity of SrSi₂O₂N₂:Eu²⁺ can be obviously enhanced based the substitution of Sr²⁺ by Eu³⁺(Sr2SiO4:Eu³⁺)or Sr²⁺ by Ba²⁺(Sr1.95Ba0.05SiO4:Eu²⁺)while degraded in Sr1.95Ba0.05SiO4:Eu³⁺ precursor at 0.5-1%mol Eu²⁺ concentration.When the Eu²⁺ concentration exceeds 3mol%,the concentration quenching occurs,which plays main part in luminescence property.5.Host lattice cation ions co-doping has an important effect on color point tuning and performance improvement of SrSi₂O₂N₂:Eu²⁺.The emission color of SrSi₂O₂N₂:Eu²⁺ can be tuned by substitution of the host lattice cation Sr²⁺ by Ca²⁺.The luminescence measurements show that for the solid solutions Sr_1-xCa_xSi₂O₂N₂:Eu²⁺ the emission band continuously shifts from 530 nm to 540 nm upon increasing the Ca content.Luminescence performance can be significantly improved by co-doping Re³⁺?Re=Y,La,Gd?in Sr_1-xCa_xSi₂O₂N₂:Eu²⁺ phosphors.The effect of Y³⁺ doping on the luminescence properties of the samples was studied.The results revealed that 1%Y³⁺ ion doping can significantly improve the photoluminescence properties of MSi₂O₂N₂:Eu²⁺?M=Ca,Sr?because the point defect result from the substitution of Y³⁺ ions for M²⁺ could inhibit the oxidization of Eu²⁺ to Eu³⁺.Besides,the Y³⁺ ions has smaller radius which could relax the expansion caused by the larger Eu²⁺ in CaSi₂O₂N₂:Eu²⁺.The structure of Sr_1-xCa_xSi₂O₂N₂:Eu²⁺/Y³⁺ can solve more Eu²⁺ ions at an equal distortion degree.With the increasing of x values in Sr_1-xCa_xSi₂O₂N₂:Eu²⁺/Y³⁺,the luminescence intension ratio of Sr_1-xCa_xSi₂O₂N₂:Eu²⁺/Y³⁺ to Sr_1-xCa_xSi₂O₂N₂:Eu²⁺ raises?1.2¹.8?.Concentration quenching occurred in Sr0.99-xSi₂O₂N₂: Eu²⁺0.01/xY³⁺ and Sr0.95-xSi₂O₂N₂: Eu²⁺0.05/xY³⁺ as Y³⁺ content exceed 1%,co-doping Y³⁺ has better effect for Sr0.95Si₂O₂N₂: Eu²⁺0.05 than Sr0.99Si₂O₂N₂: Eu²⁺0.01.Luminescence performance can be significantly improved by co-doping Re³⁺?Re=Y,La,Gd?in SrSi₂O₂N₂:Eu²⁺.6.Ce³⁺ sensitizer proved to be a good candidate for multiple activator ions doped SrSi₂O₂N₂.The effective energy transfer from Ce³⁺ to Eu²⁺ in SrSi₂O₂N₂:Ce³⁺/Eu²⁺ and from Ce³⁺ to Tb³⁺ in SrSi₂O₂N₂:Ce³⁺/Tb³⁺ were observed.The efficiency of energy transfer between Ce³⁺ and Eu²⁺ was calculated and the energy transfer mechanism among them is due to dipole-dipole interactions.The luminescence intensity was greatly enhanced by doping Li+/Na+/K+ ions with charge compensation,and Li+ ions enhanced the luminescence intensities more obviously than Na+ and K+.7.Thermal stability of luminescence of SrSi₂O₂N₂:Eu²⁺ was studied.The samples demonstrate a high thermal quenching temperature above 200 ?.The luminescence intensity of Sr0.95CaxSi₂O₂N₂:Eu²⁺0.05 decreased faster compared to Sr0.99CaxSi₂O₂N₂:Eu²⁺0.01 at 300°C.Furthermore,the quenching effect of SrBa0.05Si1.95O2N2:Eu²⁺ was slightly risen than SrSi₂O₂N₂:Eu²⁺.The reason is that the substitution of small Sr²⁺ ions by larger Ba²⁺ ions in Sr0.95Ba0.05Si₂O₂N₂:Eu²⁺ gived rise to a lattice expansion,and it results in increased Eu-?O/N?bond lengths,in which the thermal quenching properties can be improved.The samples were holding time for 10 h at 150-300? and the luminescent properties were tested once every one hour.The luminescent intensity of the samples increase with increasing holding time at 300 ?,while decreased slightly with increasing holding time below 300 ??150²50??.