Preparation And Spectral Properties Of Red Luminescent Materials

Materials science, information technology and energy sources are the three pillars industry in the 21st century. The development of every department in national economy and high-tech field is unavoidable to be promoted and restricted by the development of materials science. With the development of the science and technology, luminescence materials have been already widely applied to every fields,such as national defenses, space navigation, industry and agriculture. Red, blue and green are considered as the tricolor of luminescent materials. Nowadays, blue and green fluorescent powder whose categories have been widely developed and performances have been greatly improved has been extensively applied. However, red phosphor, which has a single system and lower brightness, is unable to achieve practical requirements. Just as T. Welker from the Philips Research Laboratory said that:"For red light phosphor, only one kind of phosphor has been being used all over the world, it is Y₂O₃:Eu³⁺. Despite its higher prices, but it has been close to the ideal tricolor phosphor. Its quantum efficiency is nearly 100, and the excitation band overlaps very well with 254nm of mercury. The sharp line which is located at 613nm is the main emission peak. It can be said that in recent years, there's no red phosphor whose performance is beyond Y₂O₃: Eu³⁺.Especially for red long-lasting phosphor, according to tricolor principle, if materials whose long-lasting color are red, green and blue were mixed at a certain proportion, any color one demand would be obtain. But the chemical properties, afterglow intensity and decay time of the three kinds of material must be similar; otherwise the afterglow color of the mixed material willchange in the decay process. At present, the blue and green long-lasting materials with best performance are SrAl₂O₄:Eu²⁺,Dy³⁺ and CaAl₂O₄: Eu²⁺,Nd³⁺,however, the red phosphor has been in the stage of research and development, mainly because of its intensity and duration of the afterglow do not meet the practical requirements. Therefore, the search for high performance red fluorescent material became the problem that needs urgent soluted in the field of luminous materials.Firstly, CaAl₂O₄, SrAl₂O₄ and BaAl₂O₄ were prepared by ceramics method in this chapter. The structure were determined through XRD technology and compared with JCPDS, the optimal synthetic conditions of investigated as follows: CaAl₂O₄:sintering temperature is 1200℃, sintering time is 3h, concentration of the fluxing agent is 3%; SrAl₂O₄: sintering temperature is 1250℃, sintering time is 4h, concentration of the fluxing agent is 3%; BaAl₂O₄:sintering temperature is 1150℃, sintering time is 4h, concentration of the fluxing agent is 3%.Secondly, the effects of activated ion ratio and auxiliary activated ion species on the luminescent properties were investigated. The results show that activated ion ratio are 3%,1.5%,2% and when auxiliary activated ion is Li+, the luminescent intensity of CaAl₂O₄:Eu³⁺,Li+,SrAl₂O₄: Eu³⁺,Li+å’ŒBaAl₂O₄:Eu³⁺,Li+ is the strongest. The excitation, emission spectral and fluorescence lifetime demonstrate that,a broad band located at 200³00nm in the excitation spectral is attributed to the charge transfer of Eu³⁺→O2-;the sharp peaks between 350～500nm are due to the transition of the 4f electron shell of Eu³⁺. In the emission spectra of the three phosphors, there are sharp peaks located at 615nm and 612nm, respectively, ascribed to⁵_D0→⁷F₂ transition of Eu³⁺; the less sharp peak at 590nm is due to ⁵D₀→⁷F₁ transition. Because the intensity of CaAl₂O₄ system is stronger, so take it as an example to measure the fluorescence lifetime. The results show that when the concentration of Li⁺ is 3%, the fluorescence lifetime is the shortest, 1.47ms.Thirdly,α,βandγ-Zn₃(PO₄)₂: Mn²⁺, Ga³⁺ were prepared by ceramics method and the effects of concentration of Mn²⁺, quenching and annealing and temperature on the phase conversion were explored. The results show that when there was no or very little MnCO3 (0.3mol%) added into the raw materials, the product isαform either the sintering temperature is at 850℃or 970℃. When the concentration of Mn²⁺ is up to 0.3mol%, the phase of the sample ZPMG changed when sintered at 970℃. Due to the reduction of the experimental conditions, quenching has been used. Under this condition, the mixed samples calcined at 970℃can getβ-Zn₃(PO₄)₂: Mn²⁺, Ga³⁺, calcined at 850℃can getγ-Zn₃(PO₄)₂: Mn²⁺, Ga³⁺.Finally, the afterglow emission spectra ofγ-Zn₃(PO₄)₂:Mn²⁺, Al³⁺ andγ-Zn₃(PO₄)₂:Mn²⁺, Ga³⁺ were measured. The results show that the introduction of Al³⁺and Ga³⁺ can greatly enhanced the phosphorescence intensity. The excitation, emission and long-lasting phosphorescence emission spectra and decay curves ofα,βandγ-Zn₃(PO₄)₂:Mn²⁺,Ga³⁺ have been measured. The results show that the emission peaks at 507nm occurs in the three forms ofα,βandγ. Another sharp peak at both 616nm occurs in the two forms ofβandγ. Both the two peaks are attributed to the ⁴T₁(⁴G)→⁶A_1g(⁶S)transition of Mn²⁺. The emission color is strongly dependent on the coordination environment of Mn²⁺ in the hostlattice: the Mn²⁺ ion emits green light when it is tetrahedrally coordinated, whereas it emits red light in octahedral coordination. Inα-Zn₃(PO₄)₂:Mn²⁺, both unique cation sites are tetrahedrally coordinated, leading the only green emitting occurs at 507nm in the emission spectra. Inβandγ-Zn₃(PO₄)₂:Mn²⁺,Ga³⁺,both tetrahedrally coordinated and octahedral coordinated Zn²⁺ exist in the lattice, so the emission peaks locate at 507nm and 616nm. After irradiated by UV lamp for 5min, the red phosphorescence of Mn²⁺,Ga³⁺ co-dopedβandγ-Zn₃(PO₄)₂ are still observed in the dark for no less than 30 and 15min, respectively.