Fabrication And The Spectral Modulation Of Nd³⁺/Pr³⁺ Doped CaF₂ Transparent Ceramics

Fluoride transparent ceramics are transparent structural materials with excellent physical and chemical properties.It is possible to realize the multifunctional applications by the doping modification of activator ions.CaF₂ transparent ceramic is an ideal matrix material because of its wide range of transparency,strong corrosion resistance and low refractive index.Rare earth ions Nd³⁺/Pr³⁺used as activator ions in fluorescent materials have good luminescent potential and spectral diversity.Nd³⁺/Pr³⁺doped transparent ceramic materials with the potential applications in solid-state laser,optical communication,medical detection,data storage and other fields have attracted extensive attention.However,the strong luminescence quenching effect of Nd³⁺and Pr³⁺ions in fluoride matrix has brought great difficulties to the related research.In this paper,a series of transparent Nd:CaF₂ and Pr:CaF₂ ceramics with different doping concentrations were prepared by chemical co-precipitation method and hot-pressing method.Owing to its serious concentration quenching effect,Gd³⁺was co-doped in Nd:CaF₂ and Pr:CaF₂ transparent ceramics to change the coordination center and local crystal field environment of Nd³⁺/Pr³⁺luminescent ions.As a result,the photoluminescent properties were greatly improved.Furthermore,the microstructure,optical properties and luminescent properties of the presented ceramics were investigated in detail.Nd:CaF₂ transparent ceramics with Nd³⁺doping concentrations of 0.1-5.0 at.%were prepared by hot-pressing method.The effect of Nd³⁺contents on the phase compositions,microstructure and optical properties of Nd³⁺-doped calcium fluoride transparent ceramics were investigated.It is clear that the obtained transparent ceramics are of single-phase composition and compact microstructure.The doping of rare-earth ions promotes the sintering of CaF₂ transparent ceramics.Nd:CaF₂ transparent ceramics with high Nd³⁺concentrations exhibit excellent optical properties.Furthermore,the photoluminescence characterizations of the Nd:CaF₂ transparent ceramics were discussed under the excitation of 792 nm.The optimal concentration,quenching mechanism and energy transfer of Nd³⁺ions in the Nd:CaF₂ transparent ceramics were performed.Because of the luminescence quenching behavior of Nd:CaF₂ transparent ceramics,the Nd³⁺,Gd³⁺co-doped CaF₂ transparent ceramics were prepared according to the concept of local structure design and spectral regulation.The phase compositions,microstructure,transparency and luminescence characteristics of the ceramics were analyzed in detail.The results show that the co-doping of Nd³⁺and Gd³⁺will not cause obvious changes in CaF₂ lattice,but increase the lattice constants.The incorporation of Gd³⁺ions can effectively reduce the size of grains and changes the fracture mode of microstructure.The investigation of luminescent properties shows that the co-doping of Gd³⁺ions greatly improves the luminescent intensity and lifetime of Nd:CaF₂transparent ceramics.At the same time,high quality(Ca_0.94Gd_0.06)F_2.06 transparent ceramics doped with different concentrations of Nd³⁺ions were fabricated.The optical properties of Nd:(Ca_0.94Gd_0.06)F_2.06 transparent ceramics were compared with the Nd³⁺-single-doped samples.Moreover,the thermal properties of the Nd³⁺,Gd³⁺co-doped CaF₂ transparent ceramics were calculated and discussed.The structural and spectroscopic properties of Pr:CaF₂ transparent ceramics with Pr³⁺doping concentrations of 0.2-5.0 at.%were performed.XRD analysis shows that the doping of rare earth ions do not change the lattice structure of CaF₂ transparent ceramics.However,the lattice parameters increase along with the doping of Pr³⁺ions.SEM analysis shows that the grain boundaries of the ceramics are clean and clear,while a small amount of residual pore still exist.Pumped by the blue light of 444 nm,the visible emission of Pr³⁺ions in the CaF₂ transparent ceramics were measured.The dependence of luminescence characterizations on the Pr³⁺contents were studied in detail.According to the energy level structure and band difference of Pr³⁺ions,the concentration quenching mechanism of Pr³⁺ions was analyzed.On the basis of Pr:CaF₂ transparent ceramics,Pr³⁺,Gd³⁺:CaF₂transparent ceramics with different Gd³⁺concentrations were prepared.The effects of sintering temperature on the densification and optical properties of Pr³⁺,Gd³⁺:CaF₂ transparent ceramics were studied.The optimal transmittance is of 89%at 1200 nm when the sintering temperature is 780 ^oC.Furthermore,the effects of Gd³⁺concentrations on the microstructure and spectral properties of Pr:CaF₂ transparent ceramics were studied.The co-doping of Gd³⁺ions refines the grain size and improves the transmittance of ceramics,as well as the visible emission intensity.The shape of emission bands are changed because of the variation of the luminous centers.In addition,the effect of Pr³⁺contents on the luminescence properties of Pr:(Ca_0.94Gd_0.06)F_2.06 transparent ceramics were examined.It is proved that the quenching concentration of Pr³⁺ions is greatly increased after co-doping with Gd³⁺ions.