Study On The Synthesis And Luminescent Properties Of Ca₂GdZr₂Al₃O₁₂ Based Phosphors

Ca₂GdZr₂Al₃O₁₂（CGZA）is an aluminate compound belonging to the garnet structure.It is chemically stable and has a plurality of sites for activating ion doping,and an ideal matrix material for the study of white LED phosphors.Most of the white LED phosphors are conventional phosphors,some contain both N or S elements and need to be synthesized under severe conditions（high temperature,high pressure）,while some synthetic processes containing F phosphors can pollute the environment.Oxide phosphors can avoid these disadvantages.and thus become the current research hotspot.In this paper,the luminescence properties of the CGZA oxide based phosphors doping various activation ions were studing,the main research contents are as follows（1）The Ca₂GdZr₂Al₃O₁₂:Mn⁴⁺phosphor was synthesized by high temperature solid phase method.The matrix structure analysis showed that Mn⁴⁺could replace Zr⁴⁺and be located at the octahedral point,which can be effectively excited to emit light.Comparing XRD and luminescence intensity of Ca₂GdZr₂Al₃O₁₂:Mn⁴⁺phosphor synthesized at different temperatures,the optimum temperature for synthesis is intended to be 1500°C,when the doping concentration of Mn⁴⁺is 0.005（molar concentration）,in which the luminescence intensity of the phosphor is the strongest. The emission peak at 703 nm belongs to the²E_g→⁴A_2gg energy level transition of Mn⁴⁺,and the excitation peaks at 356 nm and 491 nm correspond to ⁴A_2g→⁴T_1gg and ⁴A_2g→⁴T_2gg energy level transitions,respectively.Since the 3d electron of Mn⁴⁺is exposed to the outermost layer,it is easily affected by the environment,and the excitation and emission peaks appear as a broad band.The quenching mechanism of Mn⁴⁺concentration can be explained as electric dipole-electric dipole interaction by calculation of the concentration quenching formula.The phosphor shows off-white powder in daylight and emits deep red light under fluorescent light（about 365 nm）,corresponding（0.7249,0.2751）value of the CIE coordinate point.Crystal field strength Dq and Racah parameters B and C of Mn⁴⁺in the matrix CGZA were calculated,indicating that Mn⁴⁺is in a strong crystal field.When Mn⁴⁺is co-doped with Bi³⁺,the strongest emission of Bi³⁺is at 430 nm,which partially overlaps with the excitation peak of Mn⁴⁺at 450-550 nm range,illustrating there may be energy transfer from Bi³⁺to Mn⁴⁺.The emission spectrum shows that the fluorescence intensity of Ca₂YZr₂Al₃O₁₂:Mn⁴⁺is weaker than that of Ca₂Gd_1-yZr_2-xAl₃O₁₂:Mn⁴⁺,Bi³⁺,when the doping concentration of Bi³⁺is 0.02,the luminescence intensity of Mn⁴⁺is the strongest,and the energy transfer effect at this condition is the best.Fluorescence lifetime test showed that the fluorescence lifetime of Mn⁴⁺in Ca₂Gd_1-yZr_{2- x}Al₃O₁₂:Mn⁴⁺,Bi³⁺was slightly longer than that of Mn⁴⁺single doped phosphor. Ca₂YZr₂Al₃O₁₂:Mn⁴⁺phosphor was prepared by replacing Gd with Y in the matrix Ca₂GdZr₂Al₃O₁₂.The luminescence intensity of Ca₂YZr₂Al₃O₁₂:Mn⁴⁺phosphor was weak,and the strongest peak was at 717 nm.Co-doping Bi³⁺and Gd³⁺to prepare Bi³⁺,Mn⁴⁺co-doped and Gd³⁺,Bi³⁺,Mn⁴⁺three-doped phosphors,and the luminescence intensity of phosphor can be enhanced,suggesting there may be energy transfer of Gd³⁺→Bi³⁺→Mn⁴⁺.（2）Ca₂GdZr₂Al₃O₁₂:Eu³⁺phosphor can be both synthesized by the high temperature solid phase method and the sol-gel method,and the different properties is compared.Firstly,the thermal analysis of dry gel obtained by the sol-gel method corresponds to a second-order phase transition process at 900-1100°C range, suggesting the lowest synthesis temperature.XRD patterns of phosphors synthesized at different temperatures（1100¹400°C）show that the pure phase can be formed at1200°C,in which the fluorescence intensity is the strongest.Compared with the phosphor synthesized by the high temperature solid phase method at 1500°C, electron microscopy（SEM for high temperature solid phase method,TEM for sol-gel method）,diffuse reflectance,fluorescence spectroscopy,fluorescence lifetime were tested separately.For the phosphor prepared by high temperature solid phase method,the minimum particle size is about 10μm,and the agglomeration is obvious.The spectra show that the light absorption rate is lower,but the luminescence intensity is stronger,the most suitable Eu³⁺doping concentration is 0.4,and the fluorescence lifetime at this time is 1.357 ms.The phosphor particles prepared by the sol-gel method is about 200 nm,the spectra of Ca₂GdZr₂Al₃O₁₂:Eu³⁺show a higher absorption of light,and a weaker luminescence intensity,the most suitable Eu³⁺ doping concentration is 0.6,and the fluorescence lifetime is 0.524 ms.The Bi³⁺and Eu³⁺co-doped phosphors were prepared respectively by two methods,and their excitation and emission were slightly changed.When Ca₂GdZr₂Al₃O₁₂:Eu³⁺,Bi³⁺synthesized by the high temperature solid phase method excited from 278 nm to 311nm,the blue light of Bi³⁺is gradually enhanced,while the red light of Eu³⁺is gradually weakened.According,the coordinate point moves from the red region to the violet region,and to the blue region.When Ca₂GdZr₂Al₃O₁₂:Eu³⁺,Bi³⁺synthesized by the sol-gel method under excitation from 283 nm to 312 nm,the change of blue light intensity of Bi³⁺is not obvious,and the other changes are similiar as that of the high-temperature solid phase method.（3）Ca₂GdZr₂Al₃O₁₂:M(M=Tb³⁺,Sm³⁺,Bi³⁺)phosphors were synthesized by high temperature solid phase method.Ca₂GdZr₂Al₃O₁₂:Tb³⁺emits the strongest peak at 547 nm under the excitation at 262 nm,and the optimum doping concentration is 0.09.When the concentration of Tb³⁺changes from 0.01 to 0.13,the phosphor color changes from blue-green region to green region.The lower-concentration Tb³⁺（0.01）and Eu³⁺（0.01）were co-doped to obtain a near-white luminescent phosphor with a CIE coordinate point of（0.3379,0.3601）.Ca₂GdZr₂Al₃O₁₂:Sm³⁺emit the 605 nm lightunder the excitation of 410 nm,and 619 nm light underthe excitation of 278 nm,when the Sm³⁺doping concentration is 0.05,the luminescence intensity is the strongest.When Ca₂GdZr₂Al₃O₁₂:Sm³⁺,Bi³⁺under excitation from273nm to 310 nm,and the luminescent color moves from the orange region to the blue-violet region. Analogousy,Ca₂GdZr₂Al₃O₁₂:Tb³⁺,Sm³⁺can be excited under the lights from 262nm to 278 nm the luminescent color moves from the blue-green region to the orange region,corresponding.