Fabrication And Optical Characterizations Of Calcium Fluoride Transparent Ceramics

As a new type of optical material with excellent properties,transparent ceramics have been widely used in various applications,such as lasering,lighting,high temperature and resolution optical devices.Compared to their single crystal counterparts,transparent ceramics can be fabricated with high doping level in a convenient way,and easier of fabricating composite material.Transparent ceramics with a fluorite crystal structure have high transparency in a wide wavelength region,low linear and nonlinear refractive index,as well as low phonon energy and so on.Transparent ceramics in this paper were fabricated from fluorite mineral and chemical synthesized CaF₂ powders by a vacuum hot pressing sintering method.In order to improve the thermal conductivities,ceramics with composite structure were also fabricated.Sinterability of particles were improved by adding proper sintering additives.This paper mainly studies fluoride transparent ceramics,and the research content mainly includes the following aspects:（1）Raw fluorite particles were prepared by ball milling fluorite mineral,and transparent ceramic specimens were fabricated by hot pressing sinteing method in a vacuum environment.Influence of powder sizes and sintering temperatures on the optical properties of fluorite ceraimics were studied.The results indicating that,powder sizes were decreased with prolonging the ball-milling duration,and the ceramic presenting a gry performance caused by the introduced impurities.The transparency of the ceramic can be improved with increasing the sintering temperatures,while,the inferior transparency at a high temperature may be caused by the carbon contamination.（2）Y:CaF₂ transparent ceramics were fabricated by a hot pressing method in a vacuum environment from chemical synthesized Y:CaF₂ powders.The effect of Y doping concentration and synthesis temperature on the morphologies of powders were studied.Influence of Y doping concentration and sintering temperatures on the optical properties of transparent ceraimics were also studied,and the fabricating routes were also optimized.（3）Based on the above study,transparent composite ceramics were fabricated by the combination of multistep dry pressing and hot pressing sintering methodology to improve the thermal conductivities.Properties of transparent composite ceramics doped with various ions were studied.The sandwich-like structure of composite ceramics were maintained after hot pressing sintering procedure.Compared to the ceramic with high doping level,the thermal conductivities of ceramics were improved after composite structure designing.The transmittance of the layer with high doping ions level was much higher than that of the layer with low doping ions level.（4）LiF and NaF sintering additives were added into the ceramic to decrease the sintering temperatures of fluorite ceramics and to improve the transparency of CaF₂ceramics.The results indicating that,adding LiF and NaF addidives,sintering temperature of the transparent fluorite ceramic were decreased,and the transmittance of the ceamic were also improved.The average grain size of ceramics were increased significantly after LiF adding.Higly transparent fluoride ceramics were obtained from ball-milled fluorite mineral powders and chemical synthesized CaF₂ powders.The best optical transmissivity of polycrystalline fluorite ceramic was obtained after 1 h ball milling duration and hot-pressed at 1050℃,and the transmittance at 500 nm and 1200 nm were about 60.06%and 73.54%,respectively.The Y（5%）:CaF₂ transparent ceramic sintered at 900℃from chemical synthesized powders exhibited the highest transmission,and the transmittance at 500 nm and 1500 nm were about 73.10%and86.40%,respectively.The transmittance of the Er:CaF₂ composite ceramic hot pressed at 900℃under 30 MPa for 2 h at 500 nm and 1200 nm were about 44.93%and53.61%,respectively.Comparied to the Er（5%）:CaF₂ ceramic,the thermal conductivitiy of the composite ceramic was improved from 3.88 W·m^-1·K^-11 to 4.15W·m^-1·K^-1.Sintering temperatures of transparent fluorite ceramics were decreased from 1050℃to about 800℃after LiF and NaF adding.A highly transparent CaF₂ceramic was fabricated by hot pressing at 800℃after 5 mol%LiF adding,and the transmittance 500 nm and 1200 nm were about 57.16%and 70.34%.In this paper,transparent ceramics were fabricated from fluorite mineral and chemical synthesized CaF₂ powders,the transparent CaF₂ composite ceramics were also fabricated,and the performance of transparent ceramics was improved by adding sintering additives.The results presented here providing a new way for the utilization of fluorite mineral,and also having practical significance in CaF₂ transparent ceramics applications in solid-state laser gain media,window materials and other areas.