Development Of New Red And White Long Afterglow Materials And Their Luminescence Properties

This dissertation includes four chapters. Chapter 1 gives the background of the subject - the long-lasting phosphors. Chapter 2 to chapter 4 deals with the preparation, crystal structure and luminescence properties of several long-lasting phosphors.In chapter 1, the basic principles of photo-luminescence are introduced at first. Then the luminescence properties of rare-earth ions and transition metal ions are given. The recent research progress of long-lasting phosphors and the correlative theory of thermo-luminescence (TL) are also presented.Chapter 2 is the main part of the dissertation. The preparation and luminescence properties of two red long-lasting phosphors: Mg₂SiO₄:Dy³⁺,Mn²⁺ and MgSiO₃:Eu²⁺,Dy³⁺,Mn²⁺ are discussed in detail in this chapter. The contents consist of three parts:1. The comparison of the crystallization and luminescence properties of the two series: Mn²⁺-doped MgSiO₃ and Mg₂SiO₄. The presintering temperature (450°C) and sintering temperature (1300°C) of the two series are chosen through the TGA (Thermo-gravimetry analysis) and DTA (Differential thermal analysis) of the gel precursor for them. According to the XRD patterns, the major diffraction peaks of the MgSiO₃ and Mg₂SiO₄ series are consistent with a proto-enstatite structure (No.11-0273) and a forsterite structure (No.85-1364) respectively. There are two nonequivalent Mg²⁺ sites: inversion symmetric octahedron (M₁) and mirror symmetric octahedron (M₂) in the structure of Mg₂SiO₄.The emission spectra of Mg₂SiO₄:Mn²⁺ obviously consist of two bands, different from MgSiO₃:Mn²⁺, which are attributed to the emissions of Mn²⁺ ions occupying two nonequivalent Mg²⁺ sites in Mg₂SiO₄, respectively. The emission band of MgSiO₃.'Mn²⁺ shows a red shift with increasing Mn²⁺ concentration, which may be due to the exchange interaction between Mn²⁺ ions. Compared with Mg₂SiO₄:Mn²⁺ samples, for the same Mn²⁺ concentration, MgSiO₃:Mn²⁺ samples exhibit stronger luminescence intensity and higher quenching concentration. The reasonable explanation is that the edge-sharing (SiO₄) tetrahedrons form a one-dimensional chain in MgSiO₃ host.2. The luminescence properties of Mg₂SiO-4:Dy³⁺,Mn²⁺ prepared by solid-state reaction. The details of excitation and emission spectra of Mn²⁺-doped samples are analyzed at first. The excitation bands longer than 300nm are assigned using the T-S diagram. And then the investigation of the effects of Dy³⁺ ions in co-doped samples was conducted, indicating that Dy³⁺ ions serve as trap centers. A novel phenomenon is the sensitizing effect of Dy³⁺ to the emission of Mn²⁺, which has not been reported in other long-lasting phosphors. However, the real mechanism of the sensitizing effect of Dy³⁺ is not clear now, and further investigation is needed. The results may be due to the energy transfer from Dy³⁺ to Mn²⁺.3. The luminescence properties of MgSiO₃:Eu²⁺, Dy³⁺, Mn²⁺ synthesized by sol-gel method. The function of three doped ions is discussed in this part. Mn²⁺ ions serve as luminescent centers, which is based on its strong emission in the phosphorescence spectra detected in different decay time. In three-doped samples, the excitation band peaked at 305nm is originated from Mn²⁺, and the emission intensity of Eu²⁺ under 305nm excitation is much lower than that in Eu²⁺-doped samples. The experimental results proved that the function of Eu²⁺ ions is transferring energy to Mn²⁺ ions efficiently by cross relaxation in three-doped samples. The TL curves of samples indicate that Dy³⁺ ions serve as trap centers in three-doped samples. The two overlapped bands peaked at 365K and 489K in the TL curves of MgSiO₃:Dy³⁺ are separated. The fitting results show that they are both second-order bands, the trap depths of them are 0.733eV and 1.26eV, respectively. The orders of the frequency factors are also obtained. By comparison between the TL curves of MgSiO₃ and MgSiO₃:Dy³⁺, we tentatively put forward that the origin of the two TL bands of the Dy³⁺-doped sample is Dy_Mg(?) defects but not V_?^?.In chapter 3, the luminescence properties of two white long-lasting phosphors: Ca₂MgSi₂O₇:Dy³⁺ and Y₂O₂S:Tb³⁺,Sm³⁺ prepared by solid-state reaction are represented. The strong band peaked at 260nm in the excitation spectra of Ca₂MgSi₂O₇:Dy³⁺ is attributed to the host absorption. This excitation band is helpful to increase the luminescence intensity of Dy³⁺ ions under the UV excitation. The afterglow of Ca₂MgSi₂O₇:Dy³⁺ can be seen after 3h. The phosphorescence spectra measured in different decay time show that the color of afterglow is in the white range, but tend to yellowish, which is accordance with the eye observation. Two bands are separated from the TL curves of Ca₂MgSi₂O₇:Dy³⁺, the trap depths and frequency factors of the two bands are also calculated.The white long-lasting phosphor Y₂O₂S:Tb³⁺,Sm³⁺ is an improvement of Y₂O₂S:Tb³⁺ reported by our group. The function of co-doped Sm³⁺ ions in this phosphor is as follows: 1. the orange-red emission of Sm³⁺ can adjust the emission color of co-doped samples to daylight. 2. The co-doped Sm³⁺ ions result in new trap centers in the proper range.Our research about a potential new-type red long-lasting phosphors: Mn⁴⁺-activated aluminate phosphors is introduced in chapter 4. The experimentresults indicate that LiAl₅O₈:Mn⁴⁺ and Li₅AlO₄:Mn⁴⁺ have stronger luminescence intensity and red afterglow lasting several minutes under near ultraviolet excitation. The phosphorescence of LiAl₅O₈:Mn⁴⁺ may derive from the Mn⁴⁺ ions occupying octahedron Al³⁺ sites. In Li₅AlO₄:Mn⁴⁺ case, we suggest that the phosphorescence is due to the mixed LiAl₅O₈ phase. However, further investigation is needed.