Preparation And Optical Properties Of Nd:SrF₂ Transparent Ceramics

Alkaline-earth fluoride is a kind of laser material with excellent properties,such as wide transmittance range,relatively high thermal conductivity,low phonon energy,low refractive index and low nonlinear coefficient.Compared with their single crystals,the rare-earth doped alkaline-earth fluoride(RE:Ca F₂ and RE:SrF₂)transparent ceramics have many advantages,such as low cycle and low cost of preparing,heavily and uniform of RE³⁺doping and can be prepared to large size and complex shape.Therefore,RE:Ca F₂ and RE:SrF₂ transparent ceramics are considered as promising laser materials.SrF₂ and Ca F₂ are belongs to face-center cubic fluorite structure.There are many interstitial sites in this kind of structure,which ensures that high concentration of RE³⁺ions can be doped into these materials.However,so far the researches on RE³⁺doped alkaline-earth fluoride transparent ceramics are mostly on Ca F₂,and for the isostructural SrF₂ transparent ceramics,the reports are really rare.Among the rare-earth ions,Nd³⁺benefits from its low pumping threshold,large absorption cross-section and emission cross-section,and is widely used as activated ion in laser materials.Based on this background,Nd:SrF₂ transparent ceramic was taken as the research subject,the preparation and the optical properties of this material were investigated.In this thesis,we first synthesized the Nd:SrF₂ precursor nanopowders via a chemical co-precipitation method,and then used a hot-pressing technique to fabricate the Nd:SrF₂ transparent ceramics.The reactant concentration,Nd³⁺doping,fluoride source and titration order as factors influencing the microstructure of Nd:SrF₂ particles were investigated.It was found that the desired SrF₂ nanoparticles with approximately spherical morphology and dozens of nanometers in size were prepared when the reactant concentration was 0.5 M and 1.0 M.When the Nd³⁺doping concentration reached 20 mol%,XRD results indicated that the Nd:SrF₂ powder products kept pure SrF₂ phase,however,it was different from Nd:Ca F₂ that the XRD peak positions almost remained unchange with the increasing of Nd³⁺doping concentration.FESEM and HRTEM micrographs showed that the shape and size of SrF₂ nanoparticles was changed and the lattice fringe became discontinuous after Nd³⁺doping,suggesting Nd³⁺was successfully introduced into the SrF₂ lattice.The morphology and size of Nd:SrF₂ nanoparticles obtained from different fluoride sources almost remained unchanged.Different titration orders influenced the size,morphology and agglomeration degree of Nd:SrF₂nanopowder.The Nd:SrF₂ nanopowders obtained from dropping the fluoride ionic solution into the strontium ionic solution were finer,less agglomerated,and more uniform in Nd³⁺doping than those obtained from dropping the strontium ionic solution into the fluoride ionic solution.The size distribution of precursor powder,Nd³⁺doping concentration as factors influencing the optical quality of Nd:SrF₂ transparent ceramic were investigated.The optical quality of the Nd:SrF2 transparent ceramic was improved and the spontaneous cracking of the ceramic was solved by separating and sintering the Nd:SrF2nanopowders with uniform size distribution and good dispersion.Nd³⁺doping concentration significantly influenced the optical quality of Nd:SrF₂ transparent ceramic.With increasing the doping concentration of Nd³⁺from 1 mol%to 5 mol%,the transmittance of Nd:SrF₂transparent ceramic at 1?m was increased from less than5%to 78%.The optical quality of 1 mol%Nd:SrF₂ transparent ceramic can be tuned by Y³⁺codoping.When codoping with 5 mol%Y³⁺,the transmittance of 1 mol%Nd:SrF₂ at 1?m increased from less than 5%to 88%.The effects of Nd³⁺doping concentration and Y³⁺codoping on the spectroscopic properties of Nd:SrF₂ transparent ceramic were studied and the mechanisms were analyzed.With increasing Nd³⁺doping concentration from 1 mol%to 5 mol%,the emission lifetime of Nd:SrF₂ transparent ceramic decreased from 1156.2?s to 16.4?s,and the luminescence intensity was first increased and then decreased,the maximum luminescence was achieved at 2 mol%.The luminescence spectra suggested that the quenching concentration of Nd³⁺in SrF₂ transparent ceramic is 1–2 mol%.In the case that Nd³⁺was highly clustered,codoping Y³⁺in Nd:SrF₂ transparent ceramic significantly enhanced the luminescence and emission lifetime.In the case that Nd³⁺was not clustered,codoping Y³⁺significantly enhanced the luminescence but decreased the emission lifetime.The radiative properties of Nd:SrF₂ and Nd:Ca F₂ transparent ceramics were evaluated and compared by using J-O theory.The Nd:SrF₂ transparent was found to have smaller line strengths,?₂ and larger spectroscopic quality,radiative lifetime,indicating the local environment of Nd³⁺in SrF₂ transparent ceramic has higher symmetry and stronger iconicity,and Nd³⁺is more easier to realize population inversion in SrF₂ matrix.Besides,the energy transfer mechanism of Nd³⁺in Nd:SrF₂and Nd:Ca F₂ transparent ceramics was studied.The cluster centers in Nd:SrF₂ and Nd:Ca F₂ contained at least 12 and 20 Nd³⁺ions,respectively,suggesting more serious clustering of Nd³⁺in Ca F₂ transparent ceramic.This thesis systematically investigated the preparation and optical properties of Nd:SrF₂ transparent ceramic.This work is helpful to identify the differences between Nd:SrF₂ and Nd:Ca F₂ transparent ceramics in their preparations and optical properties.