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Study On Preparation And Spectroscopy Properties Of A New Kind Of Rare-earth Free Activated Y2O2S-based Phosphor

Posted on:2007-06-04Degree:DoctorType:Dissertation
Country:ChinaCandidate:P Y ZhangFull Text:PDF
GTID:1101360182488874Subject:Materials science
Abstract/Summary:PDF Full Text Request
In the past few years, the problems such as environment pollution and energy crisis have attracted tremendously from many state nations in the world. Long afterglow phosphor with high efficient and various color is one of the hot research topics in the field of materials, biology, energy and informatics because of its intensive and potential application used as luminescent display and energy detector, et al.In this thesis, the preparation and performance of phosphors are summarized;the recent progress and main facing problems of long afterglow phosphors are also reviewed. Recently, a new kind of Rare-earth free Y2O2S:Ti long afterglow phosphor is developed by us. In order to clarify the luminescent and afterglow mechanism of Y2O2S:Ti phosphors, some characteristics such as luminescent property, synthesis condition, crystalline structure, band-gap structure and trap level of defects have been studied in detail. A series of significant conclusions and fruits with theoretical meanings have been obtained.A new kind of rare-earth free Y2O2S:Ti long afterglow phosphors has been successfully synthesized by solid-state reaction under high temperature. The phase -forming evolvement and solid solubility of Y2O2S:Ti phosphor have been investigated, to obtain appropriate synthesis condition for Y2O2S:Ti phosphor with appropriate crystalline grain size, spherical morphology and narrow size distribution.Rietveld analysis result shows Y2O2S:Ti is hexagonal crystalline structure with P/3ml space group, and Ti ion substitutes Y3+ ion where locates a symmetrical C3V site. The luminescent spectra of Y2O2S:Ti phosphors show that there are a strong broad emission peak at 565nm with FWHM about 82nm and two broad excitation peaks at 265nm and 384nm, respectively. Value of CIE is x=0.5443 and y=0.4404. Its afterglow time can last over 5h at the limit of human eye (0.32mcd/m2) in the dark room. In addition, Y2O2S:Ti phosphor has higher concentration quench at 12mol% Ti and temperature quench at 189℃. Furthermore, a series of prior research data about crystallography and spectroscopy of Y2O)2S:Ti phosphor were obtained.The concentration quenching of Y2O2S:xTi(x=00.28) phosphors was investigated in detail. Two value of concentration quenching for Y2O2S:xTi phosphor are 6mol% and 12mol% respectively obtained under 265nm and 384nm wavelength excitation. However, the optimum afterglow property is obtained when x is 0.09. Also, it is confirmed that all of afterglow decay curves of Y2O2S:xTi(x=00.28) phosphors can be fitted based on following equation: I(t)=Al·exp(-t/τl)+ A2-exp(-t/τ2).The XRF and XPS results show that there are three kind of Ti valence states, Ti2+, Ti3+ and Ti4+, co-existing in Y2C>2S:Ti phosphors, and their molar ratio are Ti2+:Ti3+:Ti4+=0.12:1:0.26. In view of intrinsic defect and defect formation energy in Y2O2S host, we consider that beside that two kinds of cation defects of \Tiy+i+\ and \Tiy 3+ j are introduced by Ti4+ and Ti2+ substituting for Y3+ respectivly, two kinds of anion defects of Vs and O^, could be formed in Y2C>2S:Ti phophors.Based on the results of DRS and TL, the band-gap structure and located level of defect induced by doped Ti ion in Y2(>2S:Ti phosphor were carefully studied. The results show that the band-gap of Y2O2S host is about 4.72eV and ionization potential of Oint is about 3.28eV;the trap levels at 0.618eV and 0.858eV which are contributed to Vs2- and [Tiy+}+]' respectively are estimated by experimental formula based on thermoluminescent curve.Based on the comprehensive Analysis of DRS, XRD, XPS, TL, photocurrent and so on, the recombination luminescence mechanism of Y2C>2S:Ti phosphor is brought forward. In this model, 565nm broad emission band corresponds to the recombination of the electrons captured at [Tiyl+]' donor and the holes captured at [O^]" acceptor. Long afterglow is considered to origin from the trap levels of [VS2]" and [Tij+i+]'. According to this model, luminescent behavior such as luminescence process and afterglow origin in Y2O2S:Ti phosphor has been reasonablely clarified.Influence of different kinds of atmosphere on the luminescence properties and crystal defect of Y2C>2S:Ti phosphor is studied. By the order of H2—+N2:H2—>N2 —?C>2 heat treatment atmosphere, the luminescence and afterglow intensity of Y2O2S:Ti phosphors improved synchronously. The results of TL, XPS and XRF show that rising defect concentration of [77*3+ J an(i Oint in Y2C>2S:Ti phosphor caused under oxidized atmosphere is main reason for optimizating synchronously the luminescent and afterglow properties of Y2C>2S:Ti phosphors.Influence of M2+(M2+=Mg2+, Ca2+, Sr2"1" and Ba2+) on the luminescence and afterglow properties of Y2O2S:Ti,M phosphors were studied in detail. The changing rules of luminescent and afterglow property of Y2C>2S:Ti,M phosphors as x increasing are discussed. In addition, the broad emission band is observed in Y2C>2S:Mg or Y2O2S:Ca phosphor, respectivley. This supplys a significant information for spreading other kinds of activations of Rare earth free in Y2O2S phosphors.The XRD result shows that (Yi.xGdx)2O2S:Ti(x=01.0) samples are completely solid solution, and as increasing of Gd3+ doped content cell parameters increase, 384nm broad emission band shifts to short wavelength, while peak site and shape of 565nm broad band change little. When the value of x is 1.0, Gd2O2S:Ti phosphor still possesses obvious afterglow decay phenomenon after the removal of the excitation light. Theafterglow time could maintain above 3h. For luminescent spectrum of phosphor, there is only one strong and broad emission band at 565nm and two broad excitation peaks at 276nm and 371nm along with a series of Gd3+ ion feature peaks.Synthesis condition and flux types have great influence on the crystal structure, luminescence and afterglow properties of Y2C>2S:Ti phosphors. The optimal synthesis condition for Y2O2S:Ti phosphors, Y2O3:S:Na2CO3=l:(22.5):(0.50.7) and 11001250°C/0.51.5h, are obtained in terms of the results from luminescence and afterglow property, XRD and SEM.It was found that as flux type is fixed, the sintering temperature and time could promote formation and crystal growth of Y2O2S phase, further improving luminescence and afterglow properties of Y2O2S:Ti phosphor. Compared with other flux types, such as carbonate and phosphate of K/Na ions, U2CO3 is more effective to promote phase formation and crystal growth of Y2O2S crystal than other fluxes to improve luminescent properties.
Keywords/Search Tags:Y2O2S:Ti, Rare-earth free, Phosphor, Solide state reaction, high temperature, Recombination luminescence
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