Preparation,Microstructure And Spectroscopic Properties Of Yb:CaF₂?SrF₂ Transparent Ceramics

Fluoride compounds including CaF2 and SrF2 have always been playing important roles in the areas of industry applications and science studies,owing to its excellent properties such as high transparency in a wide wavelength region,low refraction and nonlinear indexes as well as low phonon energy,and high threshold of damage.Particularly,CaF2 and SrF2 attract a great deal of interest as deep ultraviolet optics and laser hosts.CaF2 and SrF2 are very suitable for windows materials in the range of deep ultraviolet region.Almost all the researches focused on the research of single crystal,it is very difficult to fabricate high quality of CaF2 and SrF2 transparent ceramics.As a typical fluorite crystal,CaF2 allows rare earth ions with high concentrations,and has various charge compensation methods giving rise to a rich multisite structure.This unique structure leads to broad absorption and emission spectra.In addition,certain properties of fluoride compounds can be comparable to the oxide crystals such as thermal conductivity,which are suitable for high-energy and high performance laser systems.The gain medium doped with Yb3+ ions is the first choice for high energy efficiency diode pumped systems because it has a simple electronic structure which avoids absorption between excited state,losses by cross relaxation or up-conversion.In addition,the fluorescence lifetime of Yb3+ ions is longer than any other rare earth ions,its broad absorption and emission band are very suitable for ultrashort laser generation and laser wavelength tunablity.The dissertation focused on the research of synthesis of CaF2 and SrF2 nanopowders doped with Yb3+ ions by direct precipitation method and their corresponding transparent ceramics were successfully fabricated by hot pressed method.The fabrication,microstructure and spectroscopic properties of pure CaF2,pure SrF2,and Yb3+(Na+,Y3+):CaF2?SrF2 transparent ceramics were investigated,which was composed of four parts.Yb3+ ions doped Car2 nanoparticles were prepared through the direct precipitation method.The effect of process parameters such as reaction order,reaction speed and time on the particle size,morphology and aggregation of the powders were studied.The results show that well-dispersed Yb:CaF2 nanoparticles with good sintering properties were obtained by the following process:fluorine ions were droped slowly into the solution of cation,the original concentration of Ca(NO3)2 being 1 M and finally the precursors being calcined at 400 ? for 2 h.The crystalline size of the obtained powders was about 30 nm.The Yb:SrF2 nanopowders doped with different Yb3+ ions were also prepared by direct precipitation method.The effect of Yb3+ ions on the unit cell parameter,morphology and size of the particles were studied.Yb:SrF2 nanopowder with high sintering activity were obtained by centrifugation.The prepared powders were sintered by hot-pressed method to obtained Yb:CaF2 transparent ceramics.The effect of sintering temperature,holding time,concentrations of Yb3+ ions on the microstructure,optical transmittance of the Yb:CaF2 transparent ceramics were studied.The transmittance of 5 at.%Yb:CaF2 ceramics(?16 x 2 mm)can reach 88%at 1200 nm,which is about 94%of the theoretical transmittance,when prepared at the optimum sintering temperature of 750? for 2 h.The transmittance of the Yb:CaF2 transparent ceramics was increased from 70%to 83%in the short wavelength of 400 nm by adding NaF as sintering aids to refine the grain size to create more paths for pores' elimination.The transmittance of highly quality of SrF2 transparent ceramics were 75%at the wavelength of 1100 nm using well-dispersed Yb:SrF2 nanoparticles as sintering materials.CaF2 nanoparticles were also prepared through the direct precipitation method.Well-dispersed CaF2 nanoparticles with good sintering properties were obtained using Ca(NO3)2 as reaction raw material,potassium fluoride as the precipitator with the addition of Y(NO3)3.XRD and SEM results showed that the obtained powders were single CaF2 cubic phase with no other impurities and the average diameter was about 30 nm.Highly transparent CaF2 ceramics were fabricated by hot-pressed method at 750 ? for 2 h.The transmittance of the CaF2 transparent ceramics were 77%at the wavelength of 1030 nm,which was improved compared with that of pure CaF2 ceramics significantly.The transmittance of the SrF2 transparent ceramics were 81.5%at the wavelength of 1100 nm,which was about 82%of the theoretical transmittance by using the same fabrication process of CaF2 transparent ceramics.The average grain size of the ceramics was about 450 nm.The spectra properties of Yb:CaF2 transparent ceramics with different Yb3+ ions were investigated.The main absorption bands of Yb:CaF2 transparent ceramics was 976 nm,the FWHM(Full Wave at Half Maximum)was 22 nm,implying that the Yb:CaF2 transparent ceramics are very suitable to be pumped by 980 nm LD.The energy transfer between Yb3+ ions is responsible for the concentration quenching in the ceramics of high concentration of Yb3+?ions.The effect of Na+ ions as charge compensators in Yb:CaF2 was studied,which were discovered to play several important roles.First,transmittance of the samples in the short wavelength was increased with the increasing of Na+ions concentration.Second,Na+ ions effectively prevent Yb3+ ions to form clustering,which increased the fluorescence lifetime.Third,Na+ ions could modulate the absorption and emission spectra properties of Yb:CaF2 in a large scope.According to the absorption and emission spectra,the crystal field splitting of the 2F7/2 and 2F5/2 levels of Yb3+ ions in Yb,Na:CaF2 were determined.Population inversion is easier to achieve in the the Yb:CaF2 ceramics with high concentration of Na+ ions because the matrix crystal field splitting energy of is greater.The trivalent Y3+ ions were also used as regulated ions to modulate the spectral properties of Yb:CaF2 transparent ceramics.The fluorescence lifetime of Yb,Y:CaF2 transparent ceramics increased by 2.8 times than that of Yb:CaF2 ceramics.The spectral properties and thermal conductivity of Yb:SrF2 transparent ceramics were tested,and the fluorescence lifetime of 10 at.%Yb:SrF2 sample is 1.63 ms,which was expected to be used as the gain medium for high energy laser system.