Fabrication And Properties Of Rare Earth Ions Doped SrF₂ Transparent Ceramics

Strontium fluoride（SrF₂）material has the advantages of wide light transmission range,low phonon energy,negative thermo-optic coefficient and low refractive index.It is an excellent laser gain medium matrix.Nd³⁺has a four-level system which is easy to achieve the inversion of the population.Nd:SrF₂ is easy to achieve continuous tuning in the wavelength range of 1000-1100 nm,and has the potential to achieve ultra-fast laser output,which is suitable for nuclear fusion energy.Er³⁺has a very rich energy level structure.Er:SrF₂ has a wide absorption band in the visible light and 1.5μm bands.It is easy to find an effective pump source.Er:SrF₂ has a good application prospects in the fields of medical surgery,optical communications and laser ranging.The laser emission wavelength of Ho:SrF₂ is around 2μm band,which is a laser gain medium for medical treatment and detection to be developed.The research on RE:SrF₂ mainly focuses on single crystal.However,the shortcomings of single crystals,such as long preparation period,small size,and low doping concentration,limit its performance and application range.Laser ceramics have incomparable advantages over single crystals in terms of preparation technology and material properties,which have attracted widespread attention.The preparation technology of high-quality RE:SrF₂ transparent ceramics has been difficult to break through,and it is urgent to conduct in-depth research.In this paper,SrF₂ is used as the matrix,and rare earth ions such as Nd³⁺,Er³⁺and Ho³⁺are doped ions.RE:SrF₂ nano-powders were synthesized by the co-precipitation method,and RE:SrF₂ transparent ceramics were prepared by air pre-sintering combined with hot isostatic pressing（HIP）post-treatment.The effects of doping concentration and air pre-sintering temperature on the microstructure and spectral characteristics of the ceramics were systematically studied.The specific research content is as follows:（1）The pure-phase of Nd:SrF₂ nano-powders were prepared by the co-precipitation method.The influences of Nd³⁺doping concentration on the phase,degree of agglomeration,average grain size of the powders were studied.The results showed that with the increase of Nd³⁺doping concentration,the average grain size of Nd:SrF₂powders showed a decreasing trend.The average grain sizes of 1at.%,3at.%,5at.%,7at.%Nd:SrF₂ nano-powders were 36.3 nm,36.2 nm,35.3 nm,and 31.9 nm,respectively.Nd³⁺plays a role in refining the grain size of the powders.However,the morphology of the prepared powders is poor.The influences of the reactant concentration,titration rate and aging time on the characteristics of the powders were studied in order to improve the morphology of the powders.The results show that the reactant concentration of Sr（NO₃）₂ solution is 1 mol/L,KF solution is 2.5 mol/L,the titration rate is 2 m L/min,and then the powders whose morphology behave best were obtained after aging for 12 hours.（2）Nd:SrF₂ transparent ceramics were prepared by air pre-sintering combined with HIP post-treatment.The effects of air pre-sintering temperature on the densification and optical quality of ceramics doped with different rare earth ions were studied.The results show that with the increase of the doping concentration of Nd³⁺,the pre-sintering temperature required for getting transparent Nd:SrF₂ ceramics also increased.As the doping concentration of Nd³⁺increases,the degree of agglomeration of powders increases,and the driving force required for ceramics densification during the sintering process also increases.Among the series of Nd:SrF₂ ceramics with different concentrations,3at.%Nd:SrF₂ ceramics have the best optical quality,which pre-sintered in air at 575℃combined with HIP post-treatment at 600℃has an in-line transmittance of 88.0%at 1064 nm.Inductively Coupled Plasma Optic Emission Spectrometry（ICP-OES）analysis on the obtained ceramics showed that the actual doping concentrations of 1at.%,3at.%,5at.%,7at.%Nd:SrF₂ were 0.88at.%,2.56at.%,4.17at.%,5.68at.%respectively.The Nd³⁺doping concentrations increase from 1at.%to 7at.%,the absorption cross sections at 797 nm of SrF₂ transparent ceramics increase from 0.34×10^-20 cm² to 4.98×10^-20 cm².The absorption cross section is greatly increased.The number of emission peaks and the intensity of the fluorescence spectra gradually decreases and the fluorescence lifetime also decreases from 2.03 ms to 13.5μs.The main reason for these changes is the increase of Nd³⁺doping concentration and the formation of Nd³⁺-Nd³⁺clusters.（3）The pure-phase of 1at.%Er:SrF₂ and 1at.%Ho:SrF₂ nano-powders were prepared by the co-precipitation method.Then the transparent ceramics were prepared by air pre-sintering combined with HIP post-treatment.The effects of air pre-sintering temperature on the densification and optical quality of ceramics were studied.The 1at.%Er:SrF₂ ceramics pre-sintered at 650℃combined with HIP post-treatment has the best optical quality.The in-line transmittance of a 2.5 mm-thick sample at 2500 nm and1500 nm reaches 83.4%and 73.6%,respectively.For 1at.%Ho:SrF₂ ceramics,the optical quality of the ceramic sample pre-sintered in air at 650℃and HIP post-treatment is the best,and the sample with a thickness of 2.5 mm has an in-line transmittance of 85.1%at 2500 nm.Through the ICP analysis,the actual doping concentrations of 1at.%Er:SrF₂ and 1at.%Ho:SrF₂were 0.95at.%and 0.97at.%,respectively.1at.%Er:SrF₂ ceramics has the largest absorption cross section at 1530nm.After calculation,the absorption cross section is 0.61×10^-20cm².After being tested and fitted,the 1at.%Er:SrF₂ ceramics shows the fluorescence lifetime of 56.4 ms at1530 nm,which demonstrates that there is no obvious rare earth ion cluster phenomenon in ceramics.The 1at.%Ho:SrF₂ ceramics have the largest absorption cross section at 1904 nm,and the value is 2.26×10^-21 cm².