Designand Luminescence Mechanism Of Rare Earth Doped 12CaO·7Al₂O₃ Based Conductive Phosphorsfor Low-voltage FEDs

Field emission displays?FEDs?is consideredas one of the most promisingflat plate display technologies for its advantages of high brightness,high colorpurity,wide view angle,low cosuption and fast response and so on.The key that determine the display quality lies on the properties of phosphors.Generally,it is essential for FEDs phosphors possess properties of high brightness,excellent color saturation,wide viewing angle display and good conductive ability.Currently,sulfur-based conductive phosphors are mainly adopted for FEDs because of their high luminescence intensity and good conductive ability.However,when they are used under the excitation of high-current beam,the sulfur-based conductive phosphors are easily decomposed and release sulfur vapor.The existence of superabundant sulfur vapor will pollute the electron cathode that decrease the luminescent intensity.For other FEDs devices that employ oxide-based phosphors,the existence of charge accumulation effect due to their poor conductivity will bring down the luminescent intensity.Therefore,it is a vital task for researchers to find a novel conductive phosphors that possess high luminescent intensity and resist the bombardment of high energy density electron beam.The work in this thesis based on that the insulating 12CaO·7Al₂O₃ could be converted to semiconductor through heat treatment in reduced atmosphere.Sr²⁺ and Gd³⁺ ions co-doped 12CaO·7Al₂O₃ condcutive phosphors have been prepared by high temperature solid state sintering method in combination with subsequent heat treatment.A Gd³⁺ bi-functional probe with optical and magnetism properties has been employed to study the nanocage size,influences of different encaged anions and the doped Sr²⁺ on the Gd³⁺ emission properties.New design strategy and though have been put forward to prepare novel 12CaO·7Al₂O₃ conductive phosphors for low voltage FEDs.In addition,V⁵⁺ doped red-emitting 12CaO·7Al₂O₃ conductive phosphors have been prepared through self-propagating combustion method.The results have been listed as follows:Sr²⁺?0-5%?and Gd³⁺?0.1%?ions doped12CaO·7Al₂O₃ based powders are prepared through solid state sintering method.It can be observed from the X-ray patterns that single-phased Sr²⁺ ions doped 12CaO·7Al₂O₃ powders can be obtained when the Sr²⁺ substitute for Ca²⁺ amount is less than 3%.The emission intensity of Gd³⁺ ions increase as a function of the Sr²⁺ ion doping amount based on the emission spectra upon 273 nm excitation.After the samples are heat treated in different atmosphere?O2,H₂ and air?,emission spectraunder 271 nm excitation show the emission properties of Gd³⁺ ions near different encaged anions.When the Gd³⁺ ions are surrounded mainly by encaged O2-anions,the emission intensity is stronger due to the highest cage deformation.When they are surrounded by encaged H-and OH-anions,the emission intensity decreased remarkable due to the quenching effect of OH-?high-energy vibration group?.When they are surrounded by encaged electrons,its weak emission is caused by the energy transfer between the stimulated rare earth ions and cage conduction band.A solid state sintering in H₂ atmosphere incombination with subsequent UV-irradiationmethod is adoped to prepared Sr²⁺?0-2%?and Gd³⁺?0.1%?ions co-doped 12CaO·7Al₂O₃ based conductive powders.The obtained samples sintered at 1300 oC in H₂ atmosphere are all single-phased12CaO·7Al₂O₃.The encaged electron concentration is tuned through Sr²⁺ doping.The results are further identified through means of electron spin resonance?ESR?.Sample possesses maximum encaged electron concentration is chosen with further heat treatmeng in air to remove encaged OHanions.Through comparion of samples with and without encaged OH-anions,it can be observed that the local environment species around the doped Gd³⁺ ions is simplex in the as-prepared sample.Meanwhile,for the heat-treated sample,its irregular ESR signals suggest that the local environments around the Gd³⁺ ions become complex.In particular,for the heat treated samples,its CL intensity increased markedly because of the decreased encaged OH-anions concentration.Encaged-OH--free Sr²⁺?0-1.5%?and Gd³⁺?0.1%?ion co-doped 12CaO·7Al₂O₃conductive phosphors had been prepared through melt-solidification processincarbon crucibles.Influence of encaged electrons and O2-anions on the doped Gd³⁺ ions isinvestigated.Absorption spectra showed that the maximum encaged electron concentration of the conductive powders increased to 1.08×1021 cm-3 through optimizing the amount of Sr²⁺?x=0.005?.Conductive phosphor with the maximum encaged electron concentration is further heat-treated in air to tune its encaged electron concentration.The heat treatment time of the conductive phosohors increased from 0 to 60 min,while the encaged electron concentration decreased.The decreased encaged electron concentration led to a decreased conductive ability,enhanced PL intensity.The increased decay time verified that the doped Gd³⁺ ions surrounded by the encaged O2-anions have negative concentration to the UV emission due to the existence of the energy transfer process.Under low voltage electron beam excitation?3 Kv?,the enhanced cathodluminescence?CL?of the conductive phosphors could be achieved at the concentration of 8.7×1020 cm-3 by tuning encaged electron concentrations.In particular,for the encaged-OH--free conductive phosphor,the emission intensity of the CL was about one order of magnitude higher than that of the conductive phosphor containing encage OH-anions.Red emitting V⁵⁺doped12CaO·7Al₂O₃:Sm³⁺ conductive phosphors have been prepared through self-propagating combustion method.The enhanced emission intensity can be obtained when the V⁵⁺ doping concentration researches 60%.It can be concluded that the doped V⁵⁺ ions can optimize the emission intensity of 12CaO·7Al₂O₃ based phosphors.The enhanced emission can be attributed to energy transfer between the luminescence center and the generated VO4³⁺.Subsequently,the doped amount of Sm³⁺ is further optimized and the enhanced emission under low voltage excitation can be obtained.A portion of V⁵⁺ can be reduced to V4+ during the preparing process and further increase the conductivity of the samples.The optimized luminescent property and conductivity further increase the CL property of the red emitting V⁵⁺ doped 12CaO·7Al₂O₃:Sm³⁺ conductive phosphors.