Containerless Preparation And Properties Of Rare Earth Doped Near-Mid Infrared Luminescent Glasses

Heavy metal oxide glasses such as La₂O₃-Nb₂O₅-Ga₂O₃（LNG）have the characteristics of low phonon energy,good mechanical and thermal properties.This kind of glass does not contain the network forming body of traditional glass,and has special composition and structure.It has excellent properties in many aspects such as optics,heat and mechanics,and can be used in many applications such as laser gain materials,optical storage materials and special optical fiber materials.However,the glass forming ability of this kind of glass is weak,so it is difficult to prepare this kind of bulk glass by the traditional melting and cooling method,which is limited in various research and application fields.Containerless technology has the unique characteristics of inhibiting heterogeneous nucleation of melt,obtaining deep undercooling and realizing rapid solidification,which provides excellent conditions for preparing new functional glass.In this paper,the gas suspension vessel-free device is used to break through the technical bottleneck that the traditional method is difficult to obtain bulk metal oxide glass,and successfully prepare LNG bulk glass with excellent comprehensive properties.On this basis,Er³⁺/Yb³⁺,Ho³⁺/Yb³⁺and Er³⁺/Pr³⁺were co-doped respectively,and their luminescence,mechanical and thermal properties were discussed.At first,spherical transparent glasses with different diameters（x=0,1,5,10）suchas40La₂O₃-20Nb₂O₅-40Ga₂O₃andx Yb₂O₃-5Er₂O₃-（35-x）La₂O₃-20Nb₂O₅-40Ga₂O₃ were successfully prepared by aerodynamic levitation technology.The research and analysis show that the hardness and density of LNG:Er³⁺/Yb³⁺glass increase with the increase of Yb³⁺concentration,the hardness is above 8.4 MPa,and the density increases from 5.2504 g/cm³ to 5.817 g/cm³.The blank LNG glass has a refractive index of 2.0217 and Abbe number of 85.14,which is an excellent glass with high refractive index and low dispersion.At the same time,this series of glasses has excellent thermal stability,and the glass transition temperature T_g is about 800°C.With the increase of Yb³⁺concentration,T_g does not change obviously,but the crystallization initial temperature T_o andΔT（ΔT=T_o-T_g）increase at first and then decrease.Up-conversion fluorescence spectra show that green and red emission bands of 551 nm,558 nm and 674 nm are obtained under the pump of 980 nm laser,which are two-photon emission processes.At the same time,in the near infrared region,the near infrared emission with the emission center at 1559nm is obtained,which corresponds to the transition from ⁴I_13/2-⁴I_15/2 of Er³⁺.The full width at half maximum（FWHM）of the prepared samples is about 120 nm,which is larger than that of ordinary glass.At the same time,the experimental results show that the change trend of fluorescence lifetime is consistent with that of near infrared emission intensity.Secondly,the Ho³⁺/Yb³⁺co-doped 35La₂O₃-25Nb₂O₅-40Ga₂O₃(LNG:Ho³⁺/Yb³⁺)glass was prepared by the same aerodynamic levitation containerless technology.It is found that Ho³⁺/Yb³⁺co-doping can effectively improve the hardness of LNG glass.When Yb³⁺concentration changes from 0-2 mol%,Vickers hardness changes from7.34 to 8.03 MPa,and fracture toughness changes from 0.96 to 1.20 MPa·m^1/2.The maximum phonon energy of the system glass is 632 cm^-1.In the transmission spectrum,the transmittance of all samples in the mid-infrared region is close to 80%.After calculation,the hydroxyl absorption coefficient and hydroxyl concentration of all samples are small.All samples are high refractive index glasses with refractive index greater than 2.At the same time,the data show that all samples have good thermal stability,and Ho³⁺/Yb³⁺co-doping is beneficial to improve the thermal stability of glass.Effective luminescence at 2059 nm,548 nm,661 nm and 757 nm was obtained in the middle infrared band and visible light band,respectively.When the doping concentration was 2 mol%,the luminescence intensity reached the maximum.At last,we prepared Er³⁺/Pr³⁺co-doped 35La₂O₃-25Nb₂O₅-40Ga₂O₃ glass.It is found that Pr³⁺can effectively improve the hardness of glass.When the concentration of Pr³⁺is from 0 to 0.75 mol%,the Vickers hardness changes from 7.11 MPa to 7.65MPa.The maximum phonon energy of LNG:Pr³⁺/Er³⁺glass is 794 cm^-1.In the transmission spectrum,the transmittance of samples all reached about 80%.The hydroxyl absorption coefficient and hydroxyl concentration of the sample are both low.All samples are high refractive index glasses with refractive index greater than 2.The doping of Pr³⁺is beneficial to improve the thermal stability of glass,and all samples have good thermal stability.Compared with the parent glass,the glass-ceramics after heat treatment has a longer fluorescence lifetime,which is consistent with the conclusion of fluorescence emission spectrum.At the same time,the fluorescence efficiency of energy transfer（ET）process can reach 87%.After heat treatment,La₃Ga_5.5Nb_0.5O₁₄ crystals were formed in the matrix glass.In an appropriate range,the higher the temperature,the more complete the crystal form,the better the surface structure of the glass-ceramics,which is more conducive to improving the luminous intensity.The doping of Pr³⁺can effectively improve the mid-infrared emission intensity of the parent glass sample at 2.7μm,which is generally proportional to the emission intensity.When the concentration is 0.25 mol%,the emission intensity reaches the maximum,and if the concentration is too high,it will cause concentration quenching.Meanwhile,transmission electron microscopy analysis shows that Er and Pr elements are well integrated into La₃Ga_5.5Nb_0.5O₁₄crystal phase.It is proved by atomic force morphology analysis that a good local single crystal environment can effectively reduce the radiation-free relaxation rate between Er³⁺ions,reduce the energy loss and effectively improve the luminous intensity.