| Ho3+-Yb3+co-doped oxide phosphor12CaO·7Al2O3was prepared by combustion method,with calcium nitrate, aluminum nitrate, holmium nitrate and ytterbium nitrate as rawmaterials, urea, β-alanine as fuel.We improved the Yb3+doping concentration from0.5%to1.0%and annealed the two doped concentrations samples under H2atmosphere at800~1200℃for hours. And further sintered the annealed samples in air. Through thereduction process, the encage O2-were replaced by OH-and H-, and high-energy vibrationgroups were introduced to the matrix cages, at the same time, as the changing of lattice size,makes the symmetry of matrix lattice higher. Through the oxidation process, the OH-and H-anions introduced by reduction process were replaced by O2-again, these changes makesignificant effect on the up-converted emissions as we discussed in details in the article.We used several methods to character the samples,such as XRD, SEM, and pump powerdependent up-converted and temperature dependent up-converted spectrum analysis.XRD spectra shows that we have obtained the single-phase12CaO·7Al2O3:Ho3+,Yb3+crystal without other phases. And it shows that the doped anions, Ho3+and Yb3+, replacedthe Ca2+site in cells. And it can be obtained by SEM spectrum, the particle size ofas-prepared samples is in micron sized, after annealing under hydrogen atmosphere,particle size increased to about4~5um.The up-coverted phosphors were studied in details for their sensitized sinteredatmosphere determined up-converted luminescence characteritic in this paper. Whenunder the NIR980nm exaction, the phosphors yielded relatively bright green up-convertedemission and weakly red up-converted emission assigned to holmium (5S2,5F4)â†'5I8and5F5â†'5I8transitions. When Yb3+doping concentration improved from0.5%to1.0%, theup-converted emission intensity of sample has an obvious enhancement, it shows thatimproving concentration is beneficial to strengthen the energy transfer between the Yb3+and Ho3+. After annealing samples in H2atmosphere, the up-converted emission intensitychanged obviously. First of all, the overall emission intensity decreased, and second, greaterthan the green emission, the red emission intensity quenched almost to zero and thegreen/red ratio increased significantly. In this paper,we due these changes to the changing ofanions inside C12A7cages.After annealling in H2, introduced high-energy vibration groupOH–into the sample cages, and increasing the non-radiative relaxation probability of theintermediate level5I6,5I7of Ho3+.Then we compared the infrared emission peaks of the samples as-prepared and annealedin H2through texting the down-converted spectrum, and through the spectrum, it shows that emission peaks located at1215nm and2021nm from holmium5I6â†'5I8and5I7â†'5I8transitions respectively. And significant intensity decrease also occurred on these emissionpeaks which indicated that the properties intermediate levels5I6,5I7respectively belong tothe green and red emission radiative relaxed to the ground state5I8. The only reason must bethe increase of the non-radiative relaxation which directly made the infrared emissionsdecreased. Meanwhile, the degree which peak intensity at2021nm decreased (13folds) isbigger than the peak at1215nm(5folds) which reflect the degree non-radiative relaxationchanged of5I6and5I7, considered in conjunction with the the changes in the visible lightregion, it shows that, the quenching effect of OH-on longer wavelength emissions is greaterthan on shorter wavelength emissions.Then we annealed the samples in air and test the up-converted emissions.The serieschange for samples annealed in H2all gone back to initial situation.Finally, we texted the temperature dependence up-converted of samples, results show thatthe change of emission intensities conform to the classic theory of thermal quenching theory,but with the increasing of temperature, thermal quenching has more obviously influence onsample annealed after H2.In this paper, we studied the anions and temperature dependent up-converted emissionproperties of12CaO·7Al2O3:Ho3+, Yb3+. The method of green/red emission ratio controllingis different from other literatures, it revealed the up-converted emission characters andemission mechanism of Ho3+, Yb3+co-doped C12A7made by combustion method. |
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