Synthesis、Characterization And Kinetics Study For Thermal Process Of Rare Earth Co-Doped Lanthanum Phosphate Luminescent Materials

Rare earth (RE) ions doped compounds is still one of the hot spot of research due to their application in light-emitting devices, laser hosts, catalysts and as markers for biomolecules. This paper has successfully synthesized four precursor:LaPO4:Bi,Sm·4.5H2O (ⅠA)、 LaPO4:Ce,Li,Mn·3.5H2O (ⅡA). LaPO4:Ce,Yb,Tb·3.91H2O (ⅢA). LaPO4:Ce,Tb·3.16H2O (ⅣA) via the low-heating temperature solid-state reaction. After calcining the precursors at800℃in air, four luminescent materials, LaPO4:Bi, Sm (ⅠB), LaPO4:Ce,Li,Mn (ⅡB), LaPO4:Ce,Yb,Tb (ⅢB), LaPO4:Ce,Tb (ⅣB) were obtaind. the precursors and the calcining products were analyzed by XRD. TG/DTG. inductively coupled plasma atomic emission spectrometry, SEM and fluorescence spectrum. The results show that precursors (ⅠA),(ⅡA) and (ⅢA) are hexagonal, with space group P, while precursor (ⅣA) belongs to orthorhombic with space group P. Four precursors contain much non-crystal water. Products (ⅠB),(ⅡB),(ⅢB) and (ⅣB) all are monoclinic, the former three ones belong to P21/a(14), while the last one (ⅣB) is P2i/c(14). Based on the TG/DTG dates, the thermal decomposition of the four precursors were studied by non-isothermal kinetics. The application of various kinetics methods are comprehensively described, and parameters of kinetic triplet (Eα, A, g(α)) are obtained.The experimental results show that:(1)(ⅠB) is an orange-yellow light emitting fluorescent material, which the influence curve of Bi3+on the fluorescent intensity is parabolic shape, and the optimized concentration of Bi3+is0.9mol%, Sm3+is0.9mol%, corresponding to CIE (Commission International de l’Eclairage) chromaticity coordinates of x=0.42and y=0.39. The thermal decomposition of (ⅠA) is single-step kinetic process. Based on the advanced isoconversional procedure, the averge value of the activation energy Eα was determined, which was80.1kJ·mol-1. The most probable reaction mechanism function was estimated as g(a)=[-ln(1-α)]3(No.21) by comparison and nonlinear model-fitting methods, which belonged to the mechanism of assumed random nucleation and its subsequent growth. The results show that the average value of the pre-exponential factor A is2.39×1012s-1.(2)(ⅡB) is a green-yellow fluorescent material. It implies that Mn2+can be changed as spin-allowable transition state from the spin-forbidden transition state by Ce3+and Li+co-dopping. The thermal decomposition of (ⅡA) is single-step kinetic process. The values of Ea were obtained from two isoconversional procedures, and the average value is65.9±4.8kJ·mol-1. The most probable mechanism function g(a) of the thermal decomposition reaction was deduced by masterplots, nonlinear model-fitting and iso-temperature methods, which the most probable reaction mechanism was g(a)=(1-α)-3.99-1(No.6), belonging to the mechanism of chemical reaction. Results of three Comparison methods indicate that the iso-temperature method is a local optimal method, while masterplots and nonlinear model-fitting methods are global optimal methods. The average value of the pre-exponential factor A is2.81×106s-1.(3)(ⅢB) is a novel yellowish-green emitting phosphor. Besides four groups of fluorescent spectra which are assigned as the emission of Tb3+, it is interesting that the emission peak maximum at563is a new one that can not be assigned. The thermal decomposition of (ⅢA) is single-step kinetic process. The values of activation energy Eα associated with the thermal decomposition of the precursor were obtained by using different Ea from OFW, KAS, and Starink methods as initial interval Ea, and the average value of Eα was66.7±4.2kJ·mol-1. It indicates that the values of Ea obtained from above three initial interval of Ea are in agreement with each other. The most probable reaction mechanism function was estimated by mastplosts and nonlinear methods, which is g (α)=(1-a)-3.6-1(No.6)(No.6), and belongs to chemical reaction mechanism. The average value of the pre-exponential factor A is2.81×106s-1.(4)(FVB) is a novel green emitting phosphor. The experimental results indicated that the thermal decomposition of (IVA) had two regions. The Region I is single-step kinetic process and can be described by a unique kinetic triplet:The average value of activation energy Eα is62.6±5.9kJ·mol-1, the averge value of pre-exponential factor A is1.08×106s’1. The most probable reaction mechanism function is determined as g(a)=a2. The Region Ⅱ is a kinetically complex process, and the distributed activation energy model (DAEM) was applied to study the Region Ⅱ.