Preparation And Spectral Control Of Eu²⁺ Activated Polyphosphate Phosphors

As is known to all,white LEDs have many prominent advantages such as energy saving,environmental protection and long lifetime and so on.The combination of chip and phosphor is the main way to produce white LEDs,which provides a broad space for the development of phosphor.Besides,phosphate-based luminescent materials have the advantages of low synthesis temperature,stable physicochemical properties,etc.Therefore,rare earth ions doped phosphate phosphors attract more and more attention for fabricating white LEDs.In this paper,four kinds of polyphosphate phosphors were prepared by the traditional high temperature solid-state method: Sr_5-5x?PO₄?₃F: 5xEu²⁺,Ba_5-xCl?PO₄?₃: xEu²⁺,BaCdP₂O₇: xEu²⁺ and Ba_3?1-x?P₄O₁₃: 3xEu²⁺.The structure,luminescence properties,concentration quenching and thermal stability of these phosphors were studied.In the first chapter,the development history of white LEDs,the application status of rare earth luminescent materials,the existing problems and the development prospects were introduced,which provides a theoretical basis for the graduation thesis.The second chapter mainly introduces the chemical reagents and related instruments involved in the preparation and characterization of the samples.In the third chapter,the crystal structure,microstructure and luminescence properties of the blue phosphor Sr_5-5x?PO₄?₃F: 5xEu²⁺ were studied.The XRD patterns show that the synthesized samples are pure phase and the doping of a small amount of Eu²⁺ ions does not have a significant effect on the spatial structure of the crystal.The FE-SEM image shows that the sample has a good particle growth and slight sintering.The surface of particles is smooth the particle size is about 2-4 ?m.As can be seen from the fluorescence spectra,the phosphor can be excited by near-UV LED chips?350-420 nm?.The mechanism of concentration quenching can be interpreted as electron dipole-dipole interaction mechanism.Fluorescence spectra and related theoretical calculations show that the phosphor has a moderate thermal stability,the activation energy Ea is 0.159 eV.With the increase of Eu²⁺ concentration,CIE chromaticity coordinates have almost no obvious change,which means that the sample has good color stability.In addition,LED device test shows that Sr₅?PO₄?₃F phosphor has a good application prospects.In the fourth chapter,a blue phosphor Ba_5-xCl?PO₄?₃: xEu²⁺ was prepared by high temperature solid-state method at 875? and the related characterization was carried out.Ba₅Cl?PO₄?₃: Eu²⁺ phosphors exhibit a broad excitation spectrum ranging from 250 to 420 nm,which is well matched to the emission of n-UV LED chips.At 334 nm excitation,the phosphor showed strong emission centered at 436 nm.The optimum doping concentration of Eu²⁺ was determined to be 1.0 mol%.The main concentration quenching mechanism of Ba₅Cl?PO₄?₃: xEu²⁺ phosphor is the dipole-dipole interaction mechanism.In addition,the phosphor exhibits a high thermal stability,which is comparable to that of commercial phosphors.These results reveal that Ba₅Cl?PO₄?₃: Eu²⁺ blue phosphor has great potential in n-UV w-LED applications.Chapter 5 describes the correlation analysis of a blue-green phosphor BaCdP₂O₇: xEu²⁺.XRD pattern characterization and structural refinement show the prepared phosphor has the same structure with BaCdP₂O₇ and its space structure contains two different cationic sites.The resulting phosphor has a wide excitation band in the range of 250 to 420 nm,which is well matched to the n-UV LED chip.Under the excitation of n-UV light,BaCdP₂O₇: Eu²⁺ showed strong blue-green emission band peak at 445 nm.The optimum concentration of Eu²⁺ in the BaCdP₂O₇ host is 3.0 mol%.The quenching mechanism of Eu²⁺ ions is calculated as the quadrupole-quadrupole interaction mechanism.By adjusting the Eu²⁺ content,the CIE chromaticity coordinates can vary within a certain range.Temperature-dependent PL spectra show that BaCdP₂O₇: Eu²⁺ has relatively good thermal stability.The above results show that BaCdP₂O₇: Eu²⁺ is a promising blue-green phosphor for n-UV-based w-LED.The structure design and spectral control of Ba_3?1-x?P₄O₁₃: 3xEu²⁺ phosphors are introduced in Chapter 6.We have found that the crystal structure of Ba₃P₄O₁₃: Eu²⁺ phosphor has been transformed from low-temperature?LT?phase to high-temperature?HT?phase by changing the doping concentrations of Eu²⁺ at the same preparation temperature.Due to the change of the crystal structure of the phosphor,the emission spectrum of Ba₃P₄O₁₃: Eu²⁺ phosphor has been adjusted.Based on this property,a full-color phosphor Ba_3?1-x?P₄O₁₃: 3xEu²⁺ was designed and synthesized.By controlling the doping concentration of Eu²⁺,white light emission were realized in single-host Ba₃P₄O₁₃: Eu²⁺ phosphor.The CIE coordinates of Ba_3?1-x?P₄O₁₃: 3xEu²⁺?0.003 ? x ? 0.027?and the photographs of the as-prepared phosphors under the 365 nm UV lamp show that the luminescent color of the phosphor can vary from blue to white and yellow by adjusting the doping concentration of Eu²⁺.