Synthesis And Regulation Of Luminescence Properties Of Two Ce³⁺/Eu²⁺ Activated Blue Phosphors

Inorganic phosphors have applications in many fields,such as illumination,display,agricultural light conversion,biological and medical analyses,etc.Different applications require phosphors with different luminescent properties.When they are used in white light emitting diodes?LED?for the general illumination,for example,phosphors should have proper excitation band matching the emission band of LED chip,broad emission band,high quantum efficiency,and good thermal stability,etc.When the LED is used as the backlight of the display device,the related phosphors should have a narrow emission band to achieve a wide color gamut.And the phosphors should convert green light to red,or near UV to blue/red with special wavelength range if they are utilized in plant growth.These luminescent properties are related to the crystal structures.Therefore,as a key component,the development of new phosphors and the study on the relationship between crystal structure and luminescence properties are critical for such applications.This thesis mainly includes the following two parts.The first one is to investigate the influence of adding H₃BO₃ on the crystal structure and the luminescence properties of hydroxyapatite Ca₅?PO₄?₃OH realted phosphor.The second one is to study the evolution of phase compositions and to discover Eu²⁺-activated blue phosphors through phase segregation and related analyses,which uses Na₂Al₂B₂O₇ as starting host structure.The specific research contents and results are as follows:1.Ca₅?PO₄?₃OH:Ce³⁺samples synthesized by a traditional high-temperature solid-state reaction exhibited no light under excitation at a UV light.While strong blue emission at 450 nm appears after adding H₃BO₃ into Ce³⁺-doped hydroxyapatite under excitation of a UV light.Analyses suggest that B atoms enter into the host structure,which lead to the modification of crystal structure and the formation of vacancies of O and H to compensate charge mismatch.The decrease of OH^-groups around Ce³⁺ion on Ca?3?site is responsible for the appearance of strong blue emission.The absolute quantum efficiency?QE?of the best blue-emitting phosphor is?92%,and the emission intensity at 150°C remains 81%of that at room temperature.The emission peak and International Commission on Illumination?CIE?coordinates hardly change upon increasing temperature.These results suggest that the phosphor has a good thermal stability.A warm white pc-LED is fabricated by combining a 380 nm chip with a blue phosphor and commercial green and red phosphors,whose CRI value is higher than 90.These results suggest that boron-modified hydroxyapatite phosphor could be a candidate for UV-LED-pumped white pc-LEDs.This strategy may provide a new insight into the exploration of phosphors'hosts and other functional materials.2.Phosphors with nominal composition Na_2-xAl₂B₂O₇:xEu were synthesized by traditional high temperature solid-state reaction.XRD,BSE and SEM-EDS analyses suggest that phase segregation occurred upon increaseing Eu content,and Eu concentrated in Eu BO₃ phase mainly,leaving a small amount of Eu²⁺in NaAl₁₁O₁₇ phase.Analyses results suggest that EuBO₃ phase segregates with the gradual replacement of Na by Eu atoms,which is accompanied by phase transformation from Na₂Al₂B₂O₇:Eu²⁺to NaAl₁₁O₁₇:Eu²⁺and dramatic enhancement of blue emission.Furthermore,SEM-CL analyses for Na_2-xAl₂B₂O₇:xEu sample with x=1.6,which indicates the broad blue emission peaking at around470 nm from Eu²⁺and the weak emission of Eu³⁺at 612 nm all originate from the NaAl₁₁O₁₇ phase.Eu BO₃ phase has no red emission due to high concentration of Eu³⁺.The internal quantum efficiencies of blue emission for nominal Na_0.4Al₂B₂O₇:1.6Eu and Na_0.2Al₂B₂O₇:1.8Eu are 58.6%and60.2%,respectively.The emission intensity for these two samples at150°C remains 96%?x=1.6?and 83%?x=1.8?of the room-temperature values,respectively.