吳　疆　張　萍蔣春東　邱澤忠

(四川大學(xué)材料科學(xué)與工程學(xué)院，成都　610065)

綠色熒光粉Ca3Y2Si3O12:Tb3+，Ce3+的制備及發(fā)光特性

吳疆張萍＊蔣春東邱澤忠

(四川大學(xué)材料科學(xué)與工程學(xué)院，成都610065)

采用高溫固相法合成了綠色熒光粉Ca3Y2Si3O12∶Tb3+。XRD檢測(cè)結(jié)果顯示，熒光粉主晶相為Ca3Y2Si3O12，屬單斜晶系。熒光光譜分析表明：Ca3Y2Si3O12∶Tb3+硅酸鹽熒光粉可以被370 nm的近紫外光激發(fā)，發(fā)射綠光，主發(fā)射峰位于490 nm(5D4→7F6)，544 nm (5D4→7F5)，585 nm(5D4→7F4)和621 nm(5D4→7F3)。用544 nm最強(qiáng)峰監(jiān)測(cè)，得到主激發(fā)峰位于370 nm的激發(fā)光譜，此光譜覆蓋了300～450 nm的波長(zhǎng)范圍。研究了煅燒條件、摻雜濃度及Ce3+共摻雜對(duì)熒光粉發(fā)光性能的影響：在1 400℃下經(jīng)二次煅燒6 h得到的樣品的發(fā)光性能最佳，Tb3+離子的最佳摻雜濃度為20mol%，Ce3+離子共摻雜能夠提高熒光粉的發(fā)光強(qiáng)度，其最佳摻雜量為4mol%，說明存在Ce3+→Tb3+的能量傳遞。

發(fā)光；摻雜；固相反應(yīng)；綠色熒光粉

0　Introduction

Compared with the incandescent lamp,fluorescent lamp and high pressure discharge lamp,W-LEDs (white LEDs)are known as the fourth generation lighting source due to their small size,low power consumption,high luminous efficiency,long lifetime and environmentally friendly features,etc[1-4].The most dominant way to create a white LED is by combing a blue InGaN chip with Y3Al5O12∶Ce3+yellow phosphors. However,the device based on this phosphor exhibits a poor color-rendering index and a high correlated colortemperature because of the lack of red light at long wavelengthandlimitsexpansionoftheLED application[5].Current lighting technology employs UV LED chips with red,green and blue phosphors to improve this problem.This approach provides white LEDs with excellent color-rendering indexes and can generate warmly white light[6].Excited by UV or near UV light,the phosphors doped by Tb3+ions can generate a strong green emission,which is considered to be a kind of important activator to synthesize green fluorescent powders[7].Because of a broadband absorption and emission in the UV and near UV region,the Ce3+ion is considered to be an efficient luminescence sensitizer,widely used in several kinds ofinorganicmatrixmaterialsandsensitizedthe luminescence of Tb3+ions[8-9].

Because it has many excellent properties,such as physicalandchemicalpropertiesofstability, resistance to ultraviolet bombardment and so on, alkaline earth silicate is considered as an effective matrix to synthesize fluorescent powders[10-11].In this paper,a series of silicate phosphors doped by the Tb3+ion are synthesized via a high temperature solid state method,and the luminescence properties are studied.

1　Experimental

1.1Sample preparation

The Ca3Y2-x-ySi3O12∶xTb3+,yCe3+phosphors were synthesized by a high temperature solid-state method. According to the stoichiometric ratio,the constituent oxides CaCO3(AR),SiO2(AR),Y2O3(99.99%),Tb4O7(99.99%)and CeO2(99.99%)were weighed,mixed and ground fully.Then the mixture was placed in a muffle furnace at room temperature and calcined in designated temperature to obtain the final samples.

1.2Measurements and characterization

The structure of sintered samples was identified by powder X-ray diffraction(XRD)analysis of DX-1000 with Cu Kα radiation(λ=0.154 18 nm)operating at 40 kV and 40 mA(scanning range of 10°～70°, 0.08° perstep),madeinDandongFangyuan InstrumentCo.,Ltd.Themeasurementof photoluminescence(PL)wasperformedusinga Hitachi F-7000 spectrometer equipped with a 150 W xenon lamp under a working voltage of 350 V.The excitation and emission slits were both set at 10 nm. Allthemeasurementswereperformedatroom temperature.

2　Results and discussion

2.1Phase analysis(XRD)

2.1.1Effects of once sintering on phosphors at different temperatures

TheXRDpatternsofCa3Y1.8Si3O12∶0.2Tb3+phosphors once sintered from 1 100℃ to 1 500℃are shown in Fig.1.The XRD patterns of samples by once sintering for 6 h from 1 100℃ to 1 300℃ are corresponding with the PDF card No.43-1036(Fig.1). The result shows that the main crystalline phase is Y2O3.However,the XRD patterns obtained at 1 400℃or 1 500℃are in accord with the PDF card No. 27-0093.The results prove that the main crystalline phase is Ca4Y6(SiO4)6,and the solid state reactions are notsufficient.Therefore,therequiredcrystalline phase of Ca3Y2Si3O12cannot be obtained by sintering for 6 h from 1 100℃to 1 500℃.

Fig.1　XRD patterns of Ca3Y1.8Si3O12∶0.2Tb3+phosphors by sintering for 6 h at different temperatures

2.1.2Effects of twice sintering on phosphors at different temperatures

TheXRDpatternsofCa3Y1.8Si3O12∶0.2Tb3+phosphors twice sintered from 1 100℃ to 1 500℃are shown in Fig.2.The principal crystalline of the samples is Y2O3by twice sintering for 6 h at 1 100℃or 1 200℃,but it is Ca4Y6(SiO4)6at 1 300℃(Fig.2). However,the XRD patterns obtained at 1 400℃ or1 500℃ are in accord with the PDF card No.87-0453.The results show that the main crystal phase is Ca3Y2Si3O12.Underthesametestcondition,the diffraction peak intensity of the sample obtained at 1 400℃is higher than 1 500℃,indicating that the crystallization of the sample at 1 400℃is better.

Fig.2　XRD patterns of Ca3Y1.8Si3O12∶0.2Tb3+phosphors by twice sintering for 6 h at different temperatures

2.1.3Effect of sintering time on phosphors

TheXRDpatternsofCa3Y1.8Si3O12∶0.2Tb3+phosphors twice sintered for 2 h to 10 h at 1400℃are shown in Fig.3.The principal crystalline for the sample twice sintered for 2 h is Ca4Y6(SiO4)6,but it is Ca3Y2Si3O12in other conditions(Fig.3).The diffraction peaks of(033)and(230)crystal plane are overlapped when calcining for 4 h,8 h and 10 h,and under the same test condition,the diffraction peak intensity for the sample calcined for 6 h is higher than that of others,showing that the sample calcined for 6 h is better.

Fig.3　XRD patterns of Ca3Y1.8Si3O12∶0.2Tb3+phosphors twice sintered for 2 h to 10 h at 1 400℃

The principal crystalline of the sample twice sintered for 6 h at 1 400℃is Ca3Y2Si3O12,with better crystallization.Doping by a few ions has almost no effect on the crystal structure.

2.1.4Effects of co-doped Ce3+ion on phosphors

The XRD patterns of Ca3Y1.8Si3O12∶0.2Tb3+and Ca3Y1.76Si3O12∶0.2Tb3+,0.04Ce3+phosphors are shown in Fig.4.The principal crystalline of two samples is Ca3Y2Si3O12.Ce3+ion co-doping has little effect on the shape and position of the peak,but does have a certain impact on the size of the peak.Again,doping by a few ions has almost no effect on the crystal structure.

Fig.4　XRD patterns of Ca3Y1.8Si3O12∶0.2Tb3+and Ca3Y1.76Si3O12∶0.2Tb3+,0.04Ce3+phosphors

2.2Luminescence properties

2.2.1Effect of Tb3+doping concentration on luminescence properties of Ca3Y2-xSi3O12∶xTb3+phosphors

The excitation and emissionspectraof Ca3Y2-xSi3O12∶xTb3+phosphors doped with different concentrations of Tb3+ion are shown in Fig.5.The excitation spectrum is a broadband peak,covering the wavelength region of 300 nm to 450 nm(monitored at 544 nm),which shows thatCa3Y2-xSi3O12∶xTb3+phosphors can be effectively excited by near ultraviolet LED(350～420 nm)(Fig.4). The excitation spectrum contains three components, having peaks at 316 nm,350 nm and 370 nm, corresponding to the characteristic transitions of the Tb3+ion from7F6to5D0,5L9and5G5,respectively.The main peak is located at 370 nm.Excited by 370 nm, the emission spectrum contains four main emissionpeaks,489 nm(5D4→7F6),544 nm(5D4→7F5),585 nm (5D4→7F4)and 621 nm(5D4→7F3),respectively,where the peak of 544 nm is the maximum.The shape and position of the excitation and emission peaks have littlechangewiththechangingofthedoping concentration,but have a great influence on the peak value.When the concentration of Tb3+ion is lower,the luminous intensity is weak because fewer luminescent center ions are in the matrix.With the increasing of the doping concentration,the luminescence center number increases,so the emission intensity increases gradually,the maximum value is reached when x= 20mol%.The luminous intensity of the fluorescent powders begins to decrease when further increasing the concentration of Tb3+ion,showing the concentration quenching.

Fig.5　Excitation and emission spectra of Ca3Y2-xSi3O12∶xTb3+phosphors

The mechanism of the concentration quenching maybeinteractionsbetweenions.Whenthe concentration of Tb3+ion is increased to a certain value,the mutual distance between the ions is shorter, and the interaction occurs at the same time.The concentrationquenchingoccurswhentheenergy migration among Tb3+ions goes to the concentration quenching centers.The cross relaxation of the5D4level is impossible to emerge because there is no energy cross relaxation pathways[12].

2.2.2Effect of sintering time on luminescence

properties of Ca3Y2-xSi3O12∶xTb3+phosphors

The emission spectra of Ca3Y1.8Si3O12∶0.2Tb3+phosphors twice sintered for 2 h to 10 h at 1 400℃are shown in Fig.6.With the sintering time changing, the shape and position of the emission spectra have little change,but have a great influence on the peak intensity(Fig.6).The peak intensity of the emission spectra is very low when the sintering time is 2 h. With the increasing of the sintering time,the intensity of the peaks increases,reaching the maximum when the sintering time is 6 h.The emission peak intensity decreases with further increase in the sintering time. The results show that the samples twice sintered for 6 h at 1 400℃ are better,which is consistent with the result of XRD analysis.This also shows that the pure crystalline phase of Ca3Y2Si3O12is better to luminescence,while Y2O3or Ca4Y6(SiO4)6is not conducivetoluminescence.Thustheluminous intensity of phosphors can be increased by improving the crystallization effect.

Fig.6　Emission spectra of Ca3Y1.8Si3O12∶0.2Tb3+phosphorstwice sintered for 2 h to 10 h at 1 400℃

2.2.3Effect of Ce3+co-doping on luminescence

properties of Ca3Y2-xSi3O12∶xTb3+phosphors The emission spectra of Ca3Y1.8Si3O12∶0.2Tb3+phosphors doped by the Ce3+ion are shown in Fig.7.When only changing the Ce3+concentration while keeping the Tb3+doping amount at 20mol%,the shape and position of the emission spectra peaks has little change as seen from the Fig.7(a).The range of 300 nm to 450 nm can be ascribed to the broadband emission of the Ce3+ion,and the 450 nm to 650 nm is due to the characteristics emission of the Tb3+ion. Also from the Fig.7(b),with the increasing in Ce3+ion concentration,the peak intensity of the emission spectra gradually increases,reaching the maximum when the doping amount is to 4mol%.When the doping concentration is further increased,the emission peak intensity decreases,showing the concentration quenching,however,the intensity is still greaterthan thatofCa3Y1.8Si3O12∶0.2Tb3+phosphors.This demonstrates the existence of energy transfer from Ce3+to Tb3+in Ca3Y1.8-xSi3O12∶0.2Tb3+,xCe3+phosphors.

Fig.7　Emission spectra of Ca3Y1.8-xSi3O12∶0.2Tb3+, xCe3+phosphors

The transfer process of Ce3+→Tb3+in Ca3Y1.8-xSi3O12∶0.2Tb3+,xCe3+phosphors is shown in Fig.8.Ce3+can be excited from the ground state2FJto the high energy level of 5d band,then to the low state of 5d band by the non-radioactive relaxation.At this time,a part of the excitation energy is used for transition to the ground state2FJ,presenting a broadband emission.At the same time,another part of the energy is transmitted to5D3or5D4levels of Tb3+by non-radioactive relaxation.However,the5D3level of the Tb3+ion has two kinds of transitions.One is direct transition to the ground state from the5D3level,but the other is firstly transferred to the5D4level,and then to the7FJlevels, producing the characteristics emission of the Tb3+ion, and emitting the greenlight[13].Therefore,the emission intensity of the Tb3+ion will be significantly enhanced by the Ce3+ion co-doping.

Fig.8　Energy level structures and the transfer processof Tb3+and Ce3+

2.3Chromatographic analysis

The CIE diagram for different doping concentrations of the Ce3+ion is shown in Fig.9.The color coordinate of the Ca3Y1.8Si3O12:0.2Tb3+phosphor is(0.26, 0.52),in the range of the green color coordinate.With certain amount of Ce3+ions doping,the color coordinate begins to shift towards the short wavelength,and the blue component is increased,although it is still in the green range.The range of color coordinate can be changed by changing the doping concentration of Ce3+ions.This indicates that Ca3Y2Si3O12∶Tb3+silicate phosphors are a kind of fluorescent powders suitable for near ultraviolet LED.

Fig.9　CIE diagram of Ca3Y1.8-xSi3O12∶0.2Tb3+,xCe3+phosphors

3　Conclusions

Insummary,aseriesofgreen-emittingCa3Y2-x-ySi3O12∶xTb3+,Ce3+phosphors have been prepared by a high temperature solid-state method.The principal crystalline of the samples twice sintered for 6 h is Ca3Y2Si3O12.Excited by 370 nm,the emission peaks mainly locate at 490 nm,544 nm(the strongest),585 nm and 621 nm,corresponding to the characteristic transitions of the Tb3+ion from5D4to7F6,7F5,7F4and7F3respectively.Monitored by 544 nm,the excitation spectrum is broadband(300～450 nm).With the changing of the Tb3+ion concentration,the emission intensity of the phosphors firstly increases and then decreases.The optimum doping amount is 20mol%, and the mechanism of the concentration quenching is the cross relaxation between5D3→5D4and7F6→7F0. The sintered conditions on the luminescence proper-ties are studied.The sample twice sintered for 6 h at 1 400℃ shows the best performance.The luminous intensity of phosphors can be increased by the Ce3+ion co-doping,and the optimum is 4mol%,which shows that there exists the transfer process of Ce3+→Tb3+in Ca3Y1.8-xSi3O12∶0.2Tb3+,xCe3+phosphors.The results prove that Ca3Y2Si3O12∶Tb3+,Ce3+silicate phosphors are a kind of fluorescent powders,suitable for near ultraviolet LED.

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Preparation and Luminescence Properties of Green Phosphors Ca3Y2Si3O12:Tb3+,Ce3+

WU JiangZHANG Ping＊JIANG Chun-DongQIU Ze-Zhong

(College of Material Science and Engineering,Sichuan University,Chengdu 610065,China)

Thegreen-emittingCa3Y2-x-ySi3O12∶xTb3+,yCe3+silicatephosphorsweresynthesized by a high temperature solid-state method.The principal crystalline of the samples is Ca3Y2Si3O12,which is belonging to the monoclinic system.All the phosphors can be effectively excited in the range of 300 nm to 450 nm,and emit green light.Excited by 370 nm,the emission peaks mainly locate at 490 nm(5D4→7F6)544 nm(5D4→7F5),585 nm (5D4→7F4)and 621 nm(5D4→7F3).The effect of sintering conditions,doping concentration and Ce3+co-doping was studied on the luminescence properties of the phosphors.The sample twice sintered for 6 h at 1 400℃shows the best performance.The optimum doping content of Tb3+is x(Tb3+)=20mol%.The emission intensity of the phosphors can be increased by the Ce3+ion co-doping,and the test shows that the best doping amount is 4mol%. The result confirms the presence of Ce3+→Tb3+energy transfer.

luminescence;doping;solid-state reactions;green phosphors

O482.31

A

1001-4861(2015)06-1201-06

10.11862/CJIC.2015.136

2014-12-22。收修改稿日期：2015-03-31。

＊通訊聯(lián)系人。E-mail：zhp@scu.edu.cn