Preparation And Optical Properties Of Eu²⁺ Doped 12CaO·7Al₂O₃ Blue Emitting Conductive Phosphors Based On D-f Transition

Low-voltage field emission display?FED?has attracted extensive research interest in the field of flat panel display due to its low energy consumption and fast response.Especially,the choice of phosphors for FED affects the imaging quality of FED device.On one hand,for the phosphors with poor conductivity,the occurred charge accumulation effect under the excitation of low-voltage electron beam will greatly reduce the luminescence intensity.On the other hand,for sulfide-based conductive phosphors,the electron beam bombardment leads to their decomposition and volatilization,which will pollute the environment.Therefore,the preparation of oxide-based conductive phosphors with good stability become a hotspot in the field of FED application.Among them,for rare earth doped 12CaO·7Al₂O₃?C12A7?conductive phosphors,which conductivity originates from the generation of encaged electrons,the UV,yellow and red emissions originating from f-f transition were obtained.It is well known that the 5d-4f transition of Eu²⁺is an electric dipole allowed transition and the luminescence efficiency of the phosphors containing Eu²⁺is high.However,it was found that,for Eu²⁺doped C12A7 phosphors prepared by a combustion method in combination with a thermal annealing in H₂ ambient,Eu³⁺can be reduced to Eu²⁺,but encaged electrons can not be generated,i.e.,the sample was not conductive.Thus,it is still an urgent issue on how to develop C12A7:Eu²⁺conductive phosphors based on d-f optical transition.In this dissertation,C12A7:x%Eu²⁺conductive ceramics were prepared by a melt-solidification method.Firstly,the initial reactants were poured into the semi-sealed graphite crucible,then the graphite crucible embeded with carbon powders was heattreated at1600^oC,finally the calcined ceramics were grinded to obtain conductive powders.The effects of different air humidity conditions?relative humidity of 30%and 70%?on the microstructure and optical properties of the prepared C12A7:x%Eu²⁺conductive phosphors are investigated.When the relative air humidity is 70%,the initial C12A7:0.5%Eu²⁺phosphors have very weak photoluminescence intensity under the 254 nm UV excitation.After the phosphors were annealed in air,we can observe several peaks in the range of 530-750 nm,which originate from the ⁵D₁-⁷F_j?j=1,2?and ⁵D₀-⁷F_j?j=0,1,2,3,4?transitions of Eu³⁺anions,in addition,we can also observe a broad blue emitting band peaked at 444 nm originated from the 4f⁶5d¹-4f⁷ transition of Eu²⁺anions.The reason of the weak photoluminescence intensity may be that the high air humidity leads to more water in the environment,and the water makes the sample produce OH^-trapped in the cage,which reduces the photoluminescence intensity of the phosphors.FTIR further demonstrates the presence of OH^-trapped in the cage.The calculated encaged electron concentration of C12A7:0.5%Eu²⁺phosphors is⁷.33×10¹⁹ cm^-3,which suits FED's needs.When the relative air humidity is 30%,C12A7:0.5%Eu²⁺phosphors were synthesized by the same method.The comparative tests are carried out on the samples prepared under the two conditions,and the results show that:after optimizing the air humidity,the concentration of encaged OH^-is decrease,and several distinct emission peaks can be observed in the emission spectrum.It indicates that reduce the humidity of the air during the preparation process,the concentration of encaged OH^-will decrease,and the photoluminescence intensity of the phosphors will increase.When the relative air humidity is 30%,Eu²⁺doped C12A7 blue emitting conductive phosphors with different Eu²⁺doping concentrations(C12A7:x%Eu²⁺?x=0.1,0.5,1.0,1.5?)were synthesized by a melting solidification method.Under 254 nm UV excitation,we can observe a blue emitting band peaked at 444 nm originated from the 4f⁶5d¹-4f⁷ transition of Eu²⁺anions.And when the Eu²⁺doping concentration is 1.0%,the sample achieve strongest emission intensity.The calculated encaged electron concentration of C12A7:1.0%Eu²⁺powders is about 3.40×10¹⁹ cm^-3.After annealing in air with different conditions,its photoluminescence intensity will increase with decreasing the amount of encaged electrons(i.e.increasing the amount of encaged O^2-anions).Under the excitation of 5 kV low-voltage electron beam?filament current is 224?A?,the phosphors heat treated at 225°C for 10minutes have the maximum CL intensity.It is mixed with yellow emitting C12A7:0.5%Dy³⁺and red emitting C12A7:1.0%Eu³⁺phosphors in different ratios to adjust the colour of CL spectrum,and the colour can be adjusted from blue to near white.Our results suggest that C12A7:Eu²⁺blue emitting conductive phosphors have a potential application value in FED.