Synthesis, Characterization And Kinetics Study For Their Thermal Process Of Eu³⁺-Doped Lanthanum Phosphate Phophors For Wled

In recent years, a new white light emitting diode (WLED) project of generation white light with high color rendering index and color reproducibility through the combination of near-ultraviolet (n-UV) LED chip (370-410nm) with red, green and blue (RGB) phosphors has been received a great of attention. But among three trichromatic phosphors excited by near-ultraviolet chip, compared with green or blue phosphors, red phosphors can not effectively absorb near-ultraviolet light, so its red emission intensity can not keep up with that of other two ones, which affects the color rendering, reduces the luminous efficiency, and become one fatal influence factor for restrainting the development of WLED. Therefore, the successful preparation of red phosphors efficiently excited by near-ultraviolet chip has an important significance. In this paper, the main work and research results are as follows:(1) A series of single-phase Eu3+, Tb3+, Bi3+ co-doped LaPO4 phosphors were synthesized by solid-state reaction at 800℃ with corresponding active precursors. Crystal structures of the phosphors were were investigated by X-ray diffraction (XRD), which a monoclinic phase was confirmed. The excitation (PLE) and emission (PL) spectra shows that the phosphors could emit red light centered at 585 nm under the 391 nm excitation, which is in good agreement with the emission wavelength from near-ultraviolet (n-UV) LED chip (370-410 nm). The results of PLE and PL further reveal that the co-doped Tb3+ and Bi3+could enhance emission of Eu3+ and the fluorescent intensities of the phosphors excited at 391 nm could reach to a maximum value when the doping concentration of Tb3+ and Bi3+ is about 2.0 mol% and 2.0 mol%, respectively. In addition, the co-doping Tb3+ and Bi3+ ions can strengthen the absorption of near UV light, lead to be efficient to sensitize the emission of Eu3+-doped LaPO4 phosphors, which indicates that the energy transfer occurs from Tb3+ and Bi3+ to Eu3+ ions. The chromaticity coordinate (x= 0.61, y= 0.39) of La0.94PO4:Eu0.02,Tb0.02,Bi0.02 phosphor shows that it is a red light emission. The UV diffuse reflection spectra explains a low self-absorption efficiency to the emission light and lead to a high fluorescence efficiency. Our research results display that Eu3+, Tb3+, Bi3+ co-doped LaPO4 phosphors could provide a potential red-emitting phosphor for UV-based white LED.(2) A series of single-phased Eu3+, Sm3+ co-doped La0.8Y0.2PO4 phosphors are synthesized by solid-state reaction at 850℃ with corresponding active precursors, in air atmosphere. Crystal structures of the phosphors are investigated by X-ray diffraction (XRD), which a monoclinic phase is confirmed. The excitation (PLE) and emission (PL) spectra show that the phosphors can emits red light centered at 591 nm under the 391 nm excitation, which is in good agreement with the emission wavelength from near-ultraviolet (n-UV) LED chip (370-410 nm), and the optimized molar concentration of Sm3+ is 1.20%. Co-doping Sm3+ions can broaden and strengthen absorption of exciting light at near UV and lead to effectively sensitize the emission of the Eu3+, Sm3+ co-doped La0.8Y0.2PO4 phosphors, which indicates that energy transfer occurs from Sm3+ to Eu3+ ions. The chromaticity coordinate (x= 0.61, y= 0.39) of the (La0.80Y0.2)0.97-xPO4:Eu0.03, Sm0.012 phosphor corresponds to a red light emission. The UV diffuse reflection spectra explains a low self-absorption efficiency to the emission light and lead to a high fluorescence efficiency.The above results indicate that Eu3+, Sm3+ co-doped La0.8Y0.2PO4 phosphors may provide a potential red-emitting phosphor for UV-based white LED.(3) A series of different doping concentration of LaPO4:Eu3+,Al3+ and LaPO4:Eu3+,Li+ phosphors are synthesized by calcining corresponding active precursors under 800℃. XRD pattern shows that no new crystal phase exists in the Al3+-doped or Li+-doped products and it has the same crystal structure as that of monoclinic crystal phase LaPO4 standard material. But compared to the XRD pattern of LaPO4:Eu3+, the doping of Al3+ or Li+ ions results in obvious right shift occurring in the position of the main diffraction peaks and makes the peak intensity stronger. The results indicate that part of doped Al3+/Li+ ions enter into the lattice of matrix. The doping of Al3+/Li+ ions does not introduce impurity phase, but improves the crystallinity of the products and has the effect of the flux. The research for the fluorescent properties shows that the sensitizing ions of Al3+ or Li+ ions can effectively enhance the emission intensity of the strongest peak centred at 591 nm of the LaPO4:Eu3+ phosphors, and the influence law of doping concentration is as follows:(i) Firstly, intensity of emission peak is enhanced and then decreased with increase of doping concentration of Li+ and up to the maximum when the Li+-doping concentration is 3.6mol%, which is 2.7 times of that of phosphor without Li+-doping. (ii) The intensity of emission peak displays a linear relationship with the doping concentration of Al3+ ions and reaches 1.58 times of that of the LaPO4:Eu3+phosphor when the doping concentration of Al3+ ions is 6 mol%. The research results indicate that co-doping of Al3+ or Li+ ions can strengthen the absorption of near UV light and sensitize the red emission of the Eu3+-doped LaPO4 phosphors. So, the as-synthesised LaPO4:Eu3+, Al3+ and LaPO4:Eu3+,Li+ phosphors are a kind of red phosphors with high luminous efficiency and great application prospect for UV-based white LED.In order to understand formation mechanism of phosphor in the process of calcination and provide theoretical guidance for further industrialized production, it is necessary to learn kinetics of thermal decoposition pocess of the precursors. Based on the TG/DTG analysis, this paper use the pyrolysis kinetics theory to study decoposition pocess of the precursors and obtained three parameters of the kinetics—apparent activation energy Ea, most probable mechanism function g(a) and pre-exponential factor lnA or A along with the three thermodynamics functions—△S≠, △H≠ and △G≠.