Fluorescence Detection Of Phase Transition In Li⁺/Er³⁺ Co-Doped BaTiO₃ Ceramics

Barium titanate?BaTiO₃?ceramics with typical perovskite structure are widely used in transducers,multilayer ceramic capacitors,PTC thermistors and ferroelectric random access memory due to their excellent ferroelectric,dielectric and piezoelectric properties.As a typical first-order phase transition material,BaTiO₃ has a simple structure and multiple phase transition points,which is suitable as a matrix material for exploring the relationship between phase transition and fluorescence.As a widely studied rare earth ion,Er³⁺has unique value in the application of luminescent materials.In addition,it is found that Li⁺can effectively improve the luminescence efficiency of Er³⁺ion.The response of fluorescence to trigonometric-orthogonal and orthogonal-tetragonal?ferroelectric-ferroelectric?phase transitions at low temperatures and tetragonal-cubic?ferroelectric-paraelectric?phase transitions at high temperatures of Li⁺/Er³⁺co-doped BaTiO₃?BLET?ceramics was systematically studied.And the relationship between fluorescence and structure changes were discussed.BLET ceramics were synthesized by traditional solid-phase sintering method.The effects of doping concentration and sintering temperature on the optical properties of the samples were investigated.The optimum doping concentration and sintering temperature were determined,and 0.5%of the same proportion of Li⁺/Er³⁺co-doped BaTiO₃(0.5%Li⁺/Er³⁺:BaTiO₃)sample was selected.The structure and morphology of the samples were characterized by X-ray diffraction?XRD?and scanning electron microscopy?SEM?.The density of samples was measured by Archimedes method.And density calculations of samples were performed.The ferroelectric,dielectric and piezoelectric properties of ceramic samples were obtained by ferroelectric measuring instrument,impedance analyzer and quasi-static d₃₃ tester.The dielectric and piezoelectric properties were improved and the ferroelectric properties were slightly reduced after doping compared with pure BaTiO₃ ceramic.The luminescence properties of BLET ceramics in the temperature range of-170¹80? were studied by temperature-dependent fluorescence spectroscopy.The up-conversion fluorescence spectra were studied for the luminescence of ²H_11/2 and ⁴S_3/2states transition to ⁴I_15/2 state.The fluorescence intensities of the two luminescence centers is abruptly changed at the three phase transition points due to the change of the lattice symmetry of the material.And the changes correspond to the temperature-dependence curve of the dielectric constant,which can well measure the phase transition.The down-conversion infrared fluorescence spectra were studied for the luminescence of⁴I_13/2 state transition to ⁴I_15/2 state.The intensity of the spectra in the low temperature region showed a trend similar to the temperature-dependence curve of the dielectric constant,and a peak appeared at the beginning of the dielectric peak.There is a mutation phenomenon consistent with the changes of up-conversion fluorescence intensities in the high temperature region which well corresponds to the temperature dependence of the dielectric constant.Experiments show that a change in fluorescence intensity can be used to realize a phase change measurement method that is non-contact and convenient for micro-area measurement.The structure changes of BLET ceramic were characterized by temperature-dependent XRD.The XRD pattern and the infrared luminescence intensities change were calculated and analyzed in terms of the tetragonal phase ratio.The results of the two experiments were compared,and a consistent trend was obtained in the same temperature range,which confirmed that the spectrum was directly affected by phase structure changes.Therefore,a test method for realizing the characterization of phase structure changes by using spectral measurement is proposed,which can greatly reduce the test cost and time.