Optimization Of Photoluminescence Through Non-Equivalence Substitution In Phosphors

In recent years,with the continuous consumption of non-renewable energy resources,energy crisis has become the focus of world.With high efficiency,long life,white lighting emitting LED is becoming the mainstream of the lighting and display area,which could ease the energy crisis.Thus,the phosphors for white LEDs have gained great attention.In this thesis,we mainly studied the crystal structure and photoluminescent properpies of two green phosphors,namely,KMg₄?PO₄?₃:Ce³⁺,Tb³⁺ and Ca_<sup>2+xLa_8-x?SiO₄?₆O_^2-0.5x:Eu²⁺,and their potential applications in white emitting LEDs.This paper is made up of four chapters.In the first chapter,the development history of lighting technology,white LEDs technology,the luminescent mechanism of phosphors and phosphors for white LEDs are briefly introduced.Based on the above background knowledge,the experiment ideas are put forward.The chapter two studied the preparation and energy transfer mechanism of KMg₄?PO₄?₃:Ce³⁺,Tb³⁺ phosphors.As an example of non-equivalence substitution,the substitution of Ce³⁺ and Tb³⁺ in the lattice of KMg₄?PO₄?₃ is thoroughly discussed.By analyzing the crystal structure of KMg₄?PO₄?₃ in detail and the XRD and EPR tests,it is proposed that both Ce³⁺ and Tb³⁺ take the position of K+ ions while interstitial O^2-ions exist in the tunnels form in the KMg₄?PO₄?₃ crystal to balance the excess of positive charge.It is also found that Ce³⁺ can improve the luminescence intensity of Tb³⁺ to a great extent.The excitation spectra,emission spectra and decay curves all prove the existence of energy transfer between Ce³⁺ and Tb³⁺.The caclulations show that the energy transfer efficiency of the phosphors can be as high as 95%.Through theoretical analysis,it is found that the mechanism of energy transfer is dipole-quadrupole interactions.²⁺The chapter three introduces the research on Ca_<sup>2+xLa_8-x?SiO₄?₆O_2-0.5x.yEu²⁺phosphors,including synthetic method,crystal structure,luminescence properties and reduction mechanism of Eu³⁺ in the crystal lattice.By increasing the proportion of Ca_<sup>2+ ions and reducing the proportion of La³⁺ ions in the crystal,we reduce Eu³⁺ions to Eu²⁺ successfully,greatly enhance the intensity of green emitting located at 510 nm.Since the substitution of Ca_<sup>2+ for La³⁺ is not equivalent,this leads to the reduction of the number of O^2-ions in the crystal and thus makes it easier for Eu³⁺ to be reduced.XANES test results show that with the increase of x value,the number of Eu²⁺ ions in the phosphors increases.Rietveld refinement analysis shows that the Ca_<sup>2+xLa_8-x?SiO₄?₆O_2-0.5x?x=0,1,2?crystals all belong to hexagonal crystal system,the space group is P6/3m?176?and the amount of oxygen ions in the crystal decreases with the increase of x value.The EPR data also prove that there are more oxygen vacancies in the crystal after Ca_<sup>2+ ions replacing La³⁺ ions.All these results provethat the proposed reduction mechanism of Eu³⁺ is correct.Furthermore,using Ca₆La₄?SiO₄?₆:0.04Eu²⁺ as green phosphor combined with 365 nm n-UV LED and commercial blue and red phosphors,a white LED lamp with good color rendering is successfully prepared.This indicates that Ca₆La₄?SiO₄?₆:0.04Eu²⁺ green phosphor has commercial potential in the field of white LEDs.The chapter four is a summary of the whole theis.We point out the deficiencies in the research work,and put forward some feasible suggestions for further research.