Preparation And Luminescence Properties Of Eu²⁺ Activated Borate Phosphors

Owing to their low power consumption,superior long lifetimes,long operating time,energy efficiencies and environmental friendliness,white light-emitting diodes（WLEDs）have been regarded as lighting sources of the next generation,compared with conventional incandescent and fluorescence lamps.The way to achieve white light emission can be roughly divided into three categories.The combination of chip and phosphor is the governing way to produce white LEDs,which provides a broad space for the development of phosphor.Therefore,borate-based luminescent materials are considered as a luminescent matrix with practical value because of its low sintering temperature,low raw material cost and simple synthesis process.In this paper,three kinds of borate phosphor systems were successfully prepared by the traditional high-temperature solid-phase method:Ba_（1-x）Zn₂（BO₃）₂:xEu²⁺、Ba_（1-x）Na（B₃O₅）₃:xEu²⁺、Ba_2（1-x）Zn（BO₃）₂:2xEu²⁺.And the luminescence properties of these three kinds of phosphor systems,such as their spatial structure,luminescent properties,concentration quenching mechanism,and thermal stability were studied.In the first chapter,briefly introduces the luminescent mechanism,classification,synthesis and application of rare earth luminescent materials,and briefly introduces the implementation,advantages,status quo and prospects of white LEDs,which provides a theoretical basis for the graduation thesis.The second chapter mainly introduces the related instruments and chemical reagents involved in the preparation and characterization of the samples.The third chapter,Ba_（1-x）Zn₂（BO₃）₂:xEu²⁺phosphors were synthesized by solid-state reaction method at 750°C and the luminescent properties of the compounds were investigated.The results of XRD and Rietveld refinement indicate that there is only one type of Ba²⁺site and,with high probability,Ba²⁺is substituted by Eu²⁺in the BaZn₂（BO₃）₂ host.The broad PLE band,extending from 250 to 420 nm,matches well with the n-UV LED chip emission.The critical quenching concentration of Eu²⁺in BaZn₂（BO₃）₂:Eu²⁺phosphor is determined as 6 mol%.The mechanism of concentration quenching of BaZn₂（BO₃）₂:Eu²⁺is determined to be the dipole-dipole interaction.The activation energy was calculated to be 0.301 eV.The relatively high activation energy suggests that the phosphor can serve as a potential phosphor in w-LEDs.With the increasing of Eu²⁺concentration,CIE chromaticity coordinates have almost no obvious change,which means that the sample has good color stability,The CIE was calculated to be（x,y）=（0.174,0.318）.In addition,LED device test shows that BaZn₂（BO₃）₂ phosphor has a good application prospects.In the fourth chapter,a series of deep-blue Ba_（1-x）Na（B₃O₅）₃:xEu²⁺phosphors have been synthesized by solid state reaction method.The crystal structure and photoluminescence properties were investigated by X-ray diffraction（XRD）,photoluminescence excitation（PLE）and photoluminescence emission（PL）spectra.The obtained phosphors show a broad excitation band ranging from 200 to 380 nm when monitored at 405 nm and a strong deep-blue emission band peaked at 405 nm when excited at 285 or 310 nm.The optimal concentration of Eu²⁺in BaNa（B₃O₅）₃:Eu²⁺phosphor is about 7 mol%.The concentration quenching mechanism of Eu²⁺ions in the BaNa（B₃O₅）₃ host was verified to be quadrupole-quadrupole interaction.The chromaticity coordinates of Ba_0.93Na（B₃O₅）₃:0.07Eu²⁺is located in the deep-blue light region and Ba_（1-x）Na（B₃O₅）₃:xEu²⁺phosphors show high color purity and good color stability.In the fifth chapter,a series of green phosphors Ba_2（1-x）Zn（BO₃）₂:2xEu²⁺have been synthesized by the traditional high-temperature solid-state method.The crystal structure and photoluminescence properties were investigated by X-ray diffraction（XRD）,photoluminescence excitation（PLE）and photoluminescence emission（PL）spectra.The matrix contains two different coordination environments of Ba²⁺,one is six-coordinate and the other is seven-coordinate.Based on the radius and valence,we can judge that Eu²⁺randomly occupies the Ba²⁺site.The Ba₂Zn（BO₃）₂:Eu²⁺phosphor exhibits a wide excitation spectrum in the range from 250 to 400 nm,which matches the emission of the n-UV LED chip well.At 368 nm excitation,the phosphor showed green emission centered at 494 nm.The optimal doping concentration of Eu²⁺was determined to be 3 mol%.The main concentration quenching mechanism in the Ba₂Zn（BO₃）₂:Eu²⁺phosphor is the electronic dipole-dipole interaction mechanism.In addition,the phosphor also has good color stability.These results indicate that the Ba₂Zn（BO₃）₂:Eu²⁺green phosphor has a certain application potential in the application of n-UV w-LEDs.