A Study On Preparation And Luminescent Properties Of Rare Earth Activated Red-emitting Phosphors

Luminescent materials play an irreplaceable role in the national economy and people’s daily lives because they have been widely used in variety fields such as display, lighting, information storage and amplification, medical diagnosis and so on. Molybdates and tungstates are the most important matrix as luminescent materials. Some rare earth and transition metal ions doped molybdates and tungstates, have high luminescent intensity under UV and N-UV excitation, high stability, non-toxicity, and free pollution. Therefore, the development on molybdates and tungstates phosphors has an important significance.La3+, Gd3+, Y3+, Sm3+ and Eu3+ are main rare earth ions. The transfer type of several ions is identical and f�'f forbidden transfer and sharp line spectra, which is hardly affected by outer crystal field. With molybdates or molybdates-tungstates employed as host materials, a series of red-emitting materials co-doped with La3+, Gd3+, Y3+or Sm3+ ions and Eu3+ were synthesized by solid state reaction and their emission properties were improved by charge compensation. Through XRD, excitation spectra and emission spectra analysis etc, the influences of technological condition, host modified, co-doping and so on on the emission characteristics of luminescent materials were investigated. The main work and research results are as follows:Eu3+ activated CaxSr1-x-1.5yMoO4:yEu3+ and CaxSr1-x₁.5y-zMoO4:yEu3+, zNa+ red-emitting materials were prepared by solid state reaction and the effects of technological conditions on materials structure and emission intensity were investigated. The results are as follows:Firstly, with different kinds and proper amount of fluxing agents, the emission intensity of materials can increase. It is found that the optimum fluxing agent is 0.24 Na2CO3-0.38K2CO3-0.38Li2CO3 and its amount is 6mol%. Secondly, when Eu3+ concentration and Ca2+ concentration are 8mol% and 60mol%, respectively, the luminescent materials prepared at 900℃for 2h can be excited by 311nm,395nm and 465nm wavelength and emit red light at 616 nm.Thirdly, with Na+ ions used as charge compensators, the emission intensity of CaxSr1-x-1.5y-zMoO4:yEu3+, zNa+ materials can be improved and the optimum amount is 6mol%. And when Eu3+ concentration and Ca2+ content are 8mol% and 60mol%, respectively, the luminescent materials prepared at 900℃for 2h can be excited by 311nm,395nm and 465nm wavelength and emit red light at 616nm.By solid state reaction, Eu3+-Gd3+, Eu3+-Y3+, Eu3+-La3+or Eu3+-Sm3+ co-doping luminescent materials was prepared and the influences of Ca2+（Sr2+）, Eu3+, RE3+（RE3+= Y3+、Gd3+、La3+ or Sm3+） and Mo6+（W6+） on emission intensity of luminescent materials were investigated. The results are as follows:Firstly, four kinds of luminescent materials are prepared at 900℃for 2h.Secondly, the amounts of dominant activators are both 8mol% and the amount of Mo6+（W6+） content and co-activators are different in the different co-activators doped luminescent materials and four optimum luminescent materials are obtained, such as Ca0.50Sr0.26 Euo01.08 Y0.08 （MoO4）0.2 （WO4）0.8, Ca0.60Sr0.10 Eu0.08 Gd0.12 （MoO4） 0.2 （WO4）0.8, Ca0.50Sr0.26 Eu0.08 La0.08 （MoO4）0.4 （WO4）0.6 and Ca0.54Sr0.31 Eu0.0s Sm0.02 （MoO4）0.6 （WO4）0.4. In their excitation spectra, under 616 nm monitoring, the broad peaks from 210nm to 350nm are attributed to O�'Eu, O�'Mo and O�'W charge transfer and the peaks from 360nm to 550nm are attributed to 4f-4f electron transfer of Eu3+ ion. The three stronger peaks at 394nm,465nm and 535nm are owed to 7F0�'5L6,7F0�'5D2 and 7F0�'5D1 transfer of Eu3+ ion(the peak at 404nm owed to 6H5/2�'4K11/2 transfer of Sm3+ ion. In the emission spectra, the peaks from 550nm to 710nm attributed to 5D0�'7FJ （J=2,3,4） transfer locate at 616 nm,654nm and 702nm.Thirdly, in luminescent properties and stability, the luminescent materials prepared are better than the commercial red-emitting phosphors. So it acts as the basis to seek high-efficient red-emitting materials and these materials can match ultra-violet chips for LED application. To the four prepared luminescent materials, such as Ca0.50Sr0.26 Eu0.08 Y0.08 （MoO4）0.2 （WO4）0.8, Ca0.60Sr0.10 Eu0.08 Gd0.12 （MoO4） 0.2 （WO4）0.8, Ca0.50Sr0.26 Eu0.08 Lao.08 （MoO4）O4 （WO4）0.6 and Ca0.54Sr0.31 Eu0.08 Sm0.02 （MoO4）0.6 （WO4）0.4, the luminescent properties are improved by charge compensation models and the charge compensators, such as alkali carbonates, NH4Cl or alkali halides. The results are as follows:Firstly, to Ca0.50Sr0.26 Eu0.08 Y0.08（MoO4）0.2 （WO4）0.8, the optimized charge compensators are 2mol% LiCl and the best charge compensating models are 2 Ca2+/Sr2+�'Eu3+/Y3++Li+ and Ca2+/Sr2+�'Eu3+/Y3++Cl- co-action. Its emission intensity can be improved as 15320a.u., up to 90% compared to the one without charge compensation, and its Chromaticity coordinates can be x=0.67, y=0.33, consistent to the international standard for red-emitting materials.Secondly, to Ca0.60Sr0.10Eu0.08 Gd0.12 （MoO4）0.2（WO4）0.8, the optimized charge compensators are 6mol% NaCl and the best charge compensating models are 2 Ca2+/Sr2+�'Eu3+/Gd3++Na+ and Ca2+/Sr2+�'Eu3+/Gd3++Cl- co-action. Its emission intensity can be improved as 8925a.u., up to 300% compared to the one without charge compensation, and its Chromaticity coordinates can be x=0.67, y=0.32, near to the international standard about red-emitting materials.Thirdly, to Ca0.50Sr0.26 Eu0.08 La0.08 （MoO4）0.4 （WO4）0.6, the optimized charge compensators are 4mol% KCl and the best charge compensation model is 2 Ca2+/Sr2+�'Eu3+/La3++K+. Its emission intensity can be improved as 9655a.u., up to 65% compared to the one without charge compensation, and its Chromaticity coordinates can be x=0.66, y=0.33, near to the international standard about red-emitting materials.Fourthly, to Ca0.54Sr0.31 Eu0.08 Sm0.02 （MoO4）0.6 （WO4）0.4, the optimized charge compensators are 2mol% NaCl and the best charge compensating models are 2 Ca2+/Sr2+�'Eu3+/Sm3++Na+ and Ca2+/Sr2+�'Eu3+/Sm3++Cl- co-action. Its emission intensity can be improved as 8764a.u., up to 125% compared to the one without charge compensation, and its Chromaticity coordinates can be x=0.67, y=0.33, consistent to the international standard about red-emitting materials. In a word, the optimized red-emitting materials are Ca0.50Sr0.26 Eu0.08 Y0.08 （MoO4）0.2 （W04）0.8.In emission efficiency and Chromaticity coordinates, the materials is dominant and better than the commercial red-emitting materials and can match ultraviolet/blue chips for LED application.The innovations of this paper are as follows:Firstly, novel red-emitting Ca0.50Sr0.26Eu0.08 Y0.08 （MoO4）0.2 （WO4）0.8, Ca0.60Sr0.10 Eu0.08 Gd0.12 （MoO4） 0.2 （WO4）0.8, Ca0.50Sr0.26 Eu0.08 La0.08 （MoO4）0.4 （WO4）0.6 and Ca0.54Sr0.31 Eu0.08 Sm0.02 （MoO4）0.6 （WO4）0.4 materials were prepared. Their have strong absorption for ultraviolet radiation at 394nm and strongly emit at 616nm. So they could become promising red-emitting materials.Through modifying different kinds of host materials and studying the effect of different kinds and amount of co-activators on emission properties of luminescent materials, it is found that, with different co-activators in same matrix, the effect of structure stability and charge balance on luminescence properties of phosphors are different, which provide references for improving luminescence properties of currently existing phosphors and developing new luminescence materials.Thirdly, studying initially the charge defect by doping in the molybdates-tungstates and the luminescent properties are optimized, by different charge compensators and charge compensation models, to seek one ideal high-efficient luminescent material.