Study On Preparation And Properties Of CaF₂ And SrF₂ Ceramics By Cold Sintering Process

Fluoride has the advantages of low phonon energy and wide transmittance range.It is a very important photoelectric device.The preparation of traditional transparent ceramics needs to be above 800℃,because the sintering speed is too fast,the grain size is abnormal,not conducive to the application of ceramics,high sintering temperature will also cause energy loss and environmental pollution,therefore,how to achieve the preparation of transparent ceramics at ultra-low temperature is a big problem.In this paper,CaF₂ powder was synthesized by chemical coprecipitation method,and CaF₂ ceramics with high permeability were obtained by cold sintering process,and the effect of Nd³⁺and Er³⁺doping on CaF₂ ceramics was explored.CaF₂ ceramics were prepared by cold sintering assisted two-step sintering,and the microwave dielectric properties were regulated by TiO₂.In addition,based on the study of CaF₂,the influence of cold sintering process conditions on Sr F₂ceramics was explored.Specific research contents are as follows:（1）Study the influence of different cold sintering conditions on the microstructure and optical property of ceramics.The results show that:CaF₂ ceramics with a density of 97.65%,an average visible light transmittance of 81.99%,a transmittance of 89.00%at 1100 nm and a grain size of less than 1μm were obtained by using 40 wt.%deionized water as solvent and holding temperature at 10℃/min for 1 h under cold sintering condition of 350℃and468 MPa.Compared with conventional sintering temperature reduced 450℃.The effect of Nd³⁺and Er³⁺doping on microstructure and optical property of ceramics was studied.The experiment results show that the transmittance of 1 at.%Nd³⁺:CaF₂ is 73.06%.Due to the charge compensation mechanism,the corresponding intensity of absorption and emission peaks varies with concentration of Nd³⁺ion.Under the same cold sintering condition,Er³⁺doped 1 at.%Nd³⁺:CaF₂ ceramics with different concentrations were prepared.Ca²⁺,Nd³⁺and Er³⁺gathered in large quantities at the grain boundaries of the ceramics,which affected the transparency of the ceramics.CaF₂ ceramics were obtained by cold sintering process assisted two-step sintering.The influences of annealing temperature on microstructure and microwave dielectric properties of CaF₂ were investigated.The results show that the high Qf（66414 GHz）obtained by cold sintering at 200℃,625 MPa and annealing at 950℃is twice as high as that obtained by solid sintering at the same temperature.The 0.1wt.%TiO₂-CaF₂ceramics withτ_f of-3.79 ppm/℃were obtained by TiO₂ regulation,indicating that the ceramics have good temperature stability.（2）Based on the study of CaF₂ ceramics,the influence of cold sintering process on the microstructure and optical property of Sr F₂ ceramics was further explored.The results show that the Sr F₂ ceramics with a particle size of about 100 nm and a permeability of71.86%（1100 nm）are prepared at 250℃and 468 MPa,and the sintering temperature is550℃lower than the traditional sintering temperature The influences of Nd³⁺and Er³⁺on the microscopic and optical properties of Sr F₂ ceramics were investigated.The results show that the transmittance of 3 at.%Nd³⁺:Sr F₂ ceramics and 5 at.%Er³⁺:Sr F₂ ceramics prepared at 350℃and 468 MPa is 62.64%and 76.94%,and the corresponding absorption and emission peaks are observed.Sr F₂ ceramics were prepared by cold sintering assisted two-step sintering.0.1 w.%TiO₂-Sr F₂ ceramics with Qf=9735.13 GHz andτ_f=73.19 ppm/℃were obtained by TiO₂ regulating microwave dielectric properties at 1000℃annealing temperature.The action of the second phase causes the dielectric property to be below 0.1wt.%TiO₂-CaF₂ ceramics.