Hot-press Sintered Transparent KNN-based Ceramics

In this work, KNN based transparent ceramics with perovskite structure were successfully fabricated by hot-press sintering process,using Li and Bi as sintering-aids. The optical and luminescence properties of these ceramics doped with rare earth ions were studied in detail. The effects of doping with Sr2+ and Ba2+ ions on the dielectric and piezoelectric properties of the ceramics were also investigated. The main research work includes:(1) The up-conversion transparent ceramics K0.5(1-y)Na0.5(1-y)Liy Nb1-y Biy O3-Er0.005Yb0.005x(KNNLB:Er/Yb) were fabricated by using hot-press sintering process and Li Bi O3 as additives.The micro-structure,optical transparency and up-conversion fluorescence of the ceramics were investigated.The results show that the ceramics presented an orthorhombic-perovskite crystalline, good optical transparency and were well densified. The grain sizes of the ceramics are smaller than 1μm.The transparency of the ceramics with y = 0.06 and x = 0 reaches 45 % in the visible region and over 95 % in the infrared region(the thickness of the ceramic disks is 0.5 mm).However, the optical transparency declines rapidly in the visible region as the Yb3+ ions content increases owing to the formation of tungsten bronze structure.a Strong up-conversion fluorescence was observed when the ceramics were excited by the 900 nm-wavelength light of an xenon lamp.(2) Both of the undoped and Eu3+-doped down-conversion transparent ceramics K0.47Na0.47Li0.06Nb0.94Bi0.06O3-Eux(KNN:Eu) were fabricated via hot-press sintering process. The scanning electron micrographs of the ceramics cross-section show that the ceramics are well densified. The X-ray diffraction patterns reveal that the crystalline phase of the ceramics presents a pure perovskite structure. Meanwhile, the optical transparency spectra, photoluminescence(PL) excitation and emissionspectra were investigated at room temperature. These ceramics present high optical transmittance: up to 60 % in the visible region and near 100 % in the infrared region. However the ion vacancies and crystal defects of the ceramics with large Eu3+ content will decrease the transparency in the visible region obviously. The down-conversion PL spectra reveal that the ceramics can be excited by near-ultraviolet(NUV) and blue light to achieve very strong reddish luminescence, the red emission at 613 nm is especially intense, which is associated to the electric dipole transition 5D0-7F2 of Eu3+. The PL properties were also analyzed in detail based on the crystal structures, and the Stark splitting was observed as a response to the change of crystal phase. These results indicate the KNN:Eu ceramics may have potential application as host materials for reddish luminescence.(3)The transparent ceramics [(K0.5Na0.5)1-2x(Sr0.75Ba0.25)x]0.93Li0.07Nb0.93Bi0.07O3(KNNLB:Sr/Ba) were fabricated via hot-pressing and using Sr2+, Ba2+ as modifiers. The crystal structure, piezoelectric and dielectric properties of these ceramics with different Sr2+/Ba2+ content were investigated. The results indicate that a small quantity of rear earth ions Sr2+ and Ba2+ could effectively improve the piezoelectric properties of these ceramics. The optimum dielectric and piezoelectric performances of εr=1679, d33=151 p C/N and tanδ=0.026 were obtained for KNNLB: Sr2+/Ba2+ with x=0.025. However, more Sr2+/Ba2+content will result in the formation of the secondary phase and poor electrical performances.