Preparation And Characterization Of Luminescent Materials For WLED Lighting And For Temperature Sensing

The research content of this thesis mainly consists of two parts. The first part is the synthesis and characterization of single-host white light emitting phosphors for white LED (WLED) lighting, which is discussed in Chapter3. The other part is about the thermal properties of the luminescence of the related phosphors, in view of their application in temperature sensing, which is discussed in Chapter4.Chapter1is the general introduction about the basic knowledge and the research background. The basic concepts involved in the luminescent phenomenon and its physical mechanism are introduced, along with some important application directions of the luminescent materials. At the end of this chapter, the mechanism of color vision of human eye is briefly addressed. Chapter2presents the major sample preparation methods and the luminescence measurement equipments used in our research.In the part of WLED phosphors, the research is focused on preparing and characterization of ultraviolet exciting single-phase white phosphors. Firstly in Chapter3, the status and the current situation of phosphors converting LED lighting research is given. Some important characters of LED lighting phosphors are also discussed. Secondly, some famous lighting phosphors and their properties are introduced. Finally the expectations for single-phase white light emitting phosphors are proposed. In section3.2,3.3and3.4, three new single-phase white light emitting phosphors (Li, Ba, Sr)2Si04:Ce3+, Eu2+, Mn2+, NaSrPO4:Eu2+, Tb3+, Mn2+and KCaPO4:Eu2+, Tb3+, Mn2+are discussed in detail. The results are as following:1. The (Li, Ba, Sr)2SiO4:Ce3+, Eu2+, Mn2+powder samples were synthesized via high temperature solid-state reaction. The emission spectrum contains three bands:the370—470nm blue band, the470—570nm green band and the570—700nm red band, which arise from5dâ†'4f transitions of Ce3+and Eu2+, and4T1â†'6A1transition of Mn2+, respectively. Under345nm excitation, white light emission is obtained from the tri-doped sample of appropriate doping concentration. The color of the sample emission keeps in white light region when the excitation wavelength shifts from310nm to370nm. Further investigation about the temperature dependent emission spectra shows that the color of the phosphor emission maintains white in a certain temperature range. However, the blue emission band locates in a violet-blue region where the human eye response falls off. Therefore, further studies are needed to achieving a better blue emission with wavelength near440nm.2. The NaSrPO4:Eu2+, Tb3+, Mn2+powder samples were synthesized via high temperature solid-state reaction. White light emitting was observed upon the excitation of a wide range of ultraviolet (UV) wavelengths. The color shift is insignificant when altering the excitation wavelength from260nm to400nm. This indicates that the phosphor could exhibit good color stability when used in combination with a near ultraviolet (NUV) LED. The energy distribution of the emission spectrum is better than (Li, Ba, Sr)SiO4:Ce3+, Eu2+, Mn2+phosphor. The emission color of the sample keeps in the white region below120℃, which is about the maxium working temperature limit of modern high power LED. However, the thermal quenching of NaSrPO4:Eu2+, Tb3+, Mn2+is observed.3. The KCaPO4:Eu2+, Tb3+, Mn2+powder samples were synthesized via high temperature solid-state reaction. The influence of doping concentration on spectrum is discussed. White light emission is obtained under the excitation of ultraviolet between255nm and405nm. The color is stable. The absorption of near ultraviolet is efficient for KCaPO4:7%Eu2+,7%Tb3+,2%Mn2+powder sample. The intensity of the excitation spectrum at395nm is about92%of the maxium. As a result, KCaPO4:7%Eu2+,7%Tb3+,2%Mn2+white light phosphor fits well with the emission of modern high power NUV LEDs. The thermal stability is studied in detail. The color of the sample KCaPO4:7%Eu2+,7%Tb3+,2%Mn2+keeps in white region with temperature below200℃. The quenching of the emission intensity is not very serious during the temperature range of a working LED.Chapter4focuses on the thermal quenching of the related phosphors. Although it requires good thermal stability for LED phosphors, it is better for a temperature sensing phosphor to be sensitive to temperature. The advances of luminescent temperature sensor are discussed with their advantages and significance. The research content of Chapter4mainly consists of two parts, including the trivalent Eu3+activated Y2MoO6:Eu3+and the divalent Eu2+activated NaSrPO4:Eu2+and M2Si04:Ce3+, Eu2+(M=Ba, Sr). The results are as follows:1. Temperature dependent emission spectra of Y2MOO6:20%Eu3+phosphor were investigated over a temperature range from20to500K, and the experimental results show that the emission intensity decreases dramatically with the temperature increasing. With the help of the Eu3+lifetimes, the possible mechanisms for the thermal quenching behavior of the Eu3+luminescence were discussed. The thermal quenching of Mo-O CTS and the quenching caused by transferring excitation energy to quenching centers during the energy migration among Eu3+ions and Mo-O CTS are proposed to be responsible for the thermal quenching of the emission intensity of the sample. The relative sensitivity of temperature can reach as high as7%K-1at500K, and is about2%K-1at367K. Considering the acute change of the emission intensity with temperature, Eu3+doped Y2MoO6could be a promising candidate for temperature sensors.2. Eu2+doped NaSrPO4phosphors were synthesized via high temperature solid-state reaction. The emission intensity and the lifetime of Eu2+decrease with temperature increasing from room temperature to400℃. The relative sensitivities of intensity or lifetime versus temperature are calculated. The relative sensitivities of NaSrPO4:Eu2+maintain among0.6—0.9%K-1in300—500K.3. Samples of Ba2Si04:Ce3+, Eu2+and BaSrSiO4:Ce3+, Eu2+were synthesized by solid state reaction. The emission intensity of Eu2+decreases quickly with temperature increases, while the emission of Ce3+quenches a little comparing with the thermal quenching of Eu2+emission. Therefore, the emission intensity ratio of Eu2+versus Ce3+could be a good indicate of temperature. Ratio monitoring is easier than the intensity or the lifetimes measurements, which requires repetitive temperature initializing, so Eu2+/Ce3+intensity ratio monitoring provides a better alternative for temperature sensing. With a integration interval of Ce3+:385—425nm, Eu2+:505—525nm, a maximum relative sensitivity of3%K-1can be reached for Ba2Si04:Ce3+, Eu2+. With a integration interval of Ce3+:393—433nm, Eu2+:515—535nm, a maximum relative sensitivity of2%K-1was observed for BaSrSiO4: Ce3+, Eu2+phosphor.