电荷补偿剂对YAG:Ce荧光粉效率的影响

The Luminescent Enhaucement duo to Charge Compensate Effects in YAG:Ce Phosphor Used for White-light-emitting-diodes

在研制传统的YAG:Ce3+荧光粉过程中引入NH4F作为助熔剂,BaF2和MgF2作为电荷补偿剂,使得荧光粉的性能得到有效的改善。助熔剂的引入使YAG:Ce3+荧光粉的合成温度由原来的1600℃降低到1340℃左右,这对于降低生产成本有着十分重要的意义。引入BaF2和MgF2作为电荷补偿剂,荧光粉的发光强度得到有效的提高。尤其是引入MgF2后,光致发光强度提高了42%。从XRD谱可以看出,Mg和Ba进入晶格取代了Y。通过对晶格常数的计算,发现Mg的引入可以部分补偿Ce3+掺杂带来的晶格增大。同时电荷补偿剂的引入使荧光粉达到电荷平衡:2AYYx→2Mg′Y→VO¨、2YYx→2Ba′Y+V¨O减少了烧制过程中产生的氧空位缺陷的影响,从而减少了由于氧空位带来的无辐射跃迁。另外,以杂质离子的半径和原子的电负性为出发点,对发射光谱的峰位相对移动作了定性的解释。

The luminescence properties of Ce^3＋ ions in yttrium aluminum garnet （YAG） powder have been studied. The addition of 5% NH4F flux is equivalent to decrease the preparation temperature by 200 ℃. Since the ionic radius of Ce^3＋ （0. 128 3 nm） is a little larger than that of Y^3＋ （0. 111 59 nm）, one can expect that Ce^3＋ experiences a strong crystal field in YAG： Ce. If all the added Ce^3＋ ions enter into the crystal lattice, substitute the sites of Y^3＋ ions, and then are incorporated as Ce^3＋ at undisturbed Y^3＋ sites, the reagent need to absorb plentiful Joule heat. The content of cerium determines the preparation temperature, more cerium means higher preparation temperature. We consider that the NH4F flux has no effect on the growth of YAG： Ce crystallite because of its low melting point. In order to improve the fluorescence intensity of green-yellow-emitting phosphor based on YAG： Ce for white LED （light-emitting diode）, the effect of BaF2 and MgF2 as charge compensators were studied separa- tely. But the charge compensating mechanism has seldom been reported before. When the charge compensator and NH4F were jointly added, the intensity of emission was enhanced. When the charge compensator is MgF2, the intensity of phosphor YAG： Ce has the strongest brightness. The photoluminescence intensities of YAG： Ce phosphor particles with Ba^2＋ , Mg^2＋ doped were about 135% - 142% of that of YAG： Ce phosphor particles without charge compensator. This result can be explained in terms of the environment of trivalent cerium in the crystal structure of YAG. On one hand, we postulate that the oxygen vacancy formed upon calcining in a re- ductive atmosphere is the most important intrinsic defect. A compensation mechanism expected for YAG： Ce with Mg^2＋ codoped is provided by the association of two Mg^2＋ ions with an oxygen vacancy. In this case, the introduction of Mg^2＋ ions in YAG： Ce takes place in accordance with the scheme： This scheme implies that at certain cerium concentrations a single oxygen vacancy can be compensated by two Mg^2＋ ions. On the other hand, Ce^3＋ and MgF2 dopants have counteracting effects on the lattice parameter of cubic YAG. Ce^3＋ ions, which replace Y^3＋ of YAG to form a substitutional solid solution, cause an increase in the lattice parameter as the ionic radius of Ce^3＋ is larger than that of Y^3＋ , while Mg^2＋ ions leads to a decrease in tile lattice parameter because the ionic radius of Mg^2＋ （0. 086 nm） is smaller than that of Y^3＋. Further more, Mg^2＋ can substitute Y^3＋ easily without disturbing the crystal lattice. The addition of BaF2 as charge compensatoracts as the role of MgF2. Differently, Ce^3＋ and BaF2 dopants don＇t have counteracting effects on the lattice parameter of cubic YAG （the ionic radius of Ba^2＋ is 0. 135 nm, larger than Ce^2＋ and Y^3＋ ）. The lattice parameter expands because of a repulsive interaction between cations. The effect of doping on the optical be- havior of YAG is even more complex. To analyze the role of charge compensator on the luminescent emission property of YAG：Ce, the synthesized products were characterized by XRD, photoluminescence emission and excitation spectra （PLE）.