M_xBa_1-xAl_1.4Si_0.6O_3.4N_0.6: Eu²⁺(M=Sr,Ca) Green Phosphor Preparation By Molten Salt Method And Research On Its Luminescence Performance

In order to acquire high color rendering index?CRI?and low color chromaticity temperature?CCT?,the green and red phosphors is needed to add for white LEDs.At present,the conventional green phosphor systems cannot be satisfied White LEDs.Recent years,oxynitride phosphors have been extensively investigated due to advantages of high luminous efficiency and thermal stability.However,the preparation of these phosphors is very difficult,due to the fact that the prolonged high temperature and high nitrogen pressure are required during the reaction,which limits the applications of them.Therefore,it is urgent to find a novel synthetic route for the preparation of oxynitride.Molten salt synthesis?MSS?method,possesses low sintering temperature,simple process and dispersive particle,is expected to overcome the disadvantages of solid-state reaction?SSR?method.In this paper,based on the BaAl_2-x-x Si_xO_4-xN_x:Eu²⁺phosphors synthesized via SSR method,we try to further prepare Ba_1-x-x Al_1.4Si_0.6O_3.4N_0.6:Eu_x ²⁺and M_xBa_0.95-x.95-x Al_1.4Si_0.6O_3.4N_0.6:Eu²⁺_0.05?M=Sr,Ca?green-emitting phosphors by MSS method.Simultaneously,the systematic investigations about the influence of preparation technology and alkaline earth metal cation doping on morphology and luminescent properties of this type of phosphor were discussed.The main works and results are as follows:?1?A series of BaAl_2-x-x Si_xO_4-xN_x:Eu²⁺green phosphors was synthesized by SSR method.The substitution of Ba²⁺by Eu²⁺ions and the replacement of Al-O bonds with Si-N bonds have no obvious influences on Ba Al₂O₄ host structure.These phosphors have irregular crystal morphology,obvious aggregation and uneven size distribution and average particle size of approximately 7-10?m.The excitation spectra of samples cover broad band of 200-500 nm and under 365 nm excitation,the optimal replacement content of Si-N bonds is found to be x=0.6.With the increase of Si-N bonds content,the luminescence spectrum of samples exhibits the red-shift because the covalency and crystal field splitting of host lattice are enhanced.Thus,the emission peak shifts from 498 nm?x=0?to 514 nm?0.6-1.0?.?2?A series of Ba_1-x-x Al_1.4Si_0.6O_3.4N_0.6:Eu_x²⁺phosphors which based on green emitting BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺phosphor was prepared by MSS method using NaNO₃ as the reaction medium.The quenching concentration of Eu²⁺is found to be x=0.05.In this type of phosphor,the critical energy transfer distance between Eu²⁺and Eu²⁺ions is 15.84?and the concentration quenching mechanism is mainly due to dipole-dipole interaction.Under the optimal condition of molten salt content,sintering temperature and holding time?mass ratio of molten salt and reactants is R=2,T=900?and t=6 h?,the sphere-like particles with better dispersibility and higher crysta1lization are acquired.?3?Compared with SSR method,The MSS method could prepare pure products with sphere-like morphology and better dispersibility,at lower temperature and the sintering temperature is decreased by 300?.In MSS method,the growth process of powder particles is controlled by diffusion mechanism,and is divided into four steps as follow:?I?the diffusion of the reactants in molten salt,?II?the reaction of the reactants in molten salt,?III?the formation of the small products crystal nucleus,?IV?the growth of the products particles.?4?A series of M_xBa_0.95-xAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺_0.05?M=Sr,Ca?green phosphors was prepared via MSS method.The alkaline earth cation Sr²⁺or Ca²⁺substitution will improve photoluminescence properties by adjusting crystal structure and changing crystal field strength.Under the wavelength excitation at 365 nm,the emission intensities of samples reach a maximum at Sr=0.05 and Ca=0.10,respectively.The samples show low thermal quenching behavior,and the emission intensities maintain 71%and 80%of their original intensities when the temperature reaches 150?,respectively.Simultaneously,the emission peaks show a tendency of red-shift from 507.2 nm?x=0?to 514.2 nm?x=0.20?while increasing Sr content because of large crystal field splitting and stokes shift.With the increase of Ca content,the emission peaks exhibit a red-shit from 507.0 nm?x=0?to 509.6 nm?x=0.20?due to the fact that the red-shift effect caused by crystal field splitting is stronger than the blue-shift caused by stokes shift.