Fabrication And Spectral Conversion Of Rare-earth Doped Novel Transparent Glass-ceramics

Rare-earth doped transparent glass-ceramics?GCs?are nanocryatalline phase about several nanometers evenly distributed within the glass phase with high transparency due to low scattering.Moreover,the doped rare earths are concentrated into the nanocrystalline with low phonon energy to avoiding the effect of the glass matrix of high phonon energy.So the luminescence properties of rare earth ions in the GCs are much better than that in the oxyfluoride glass of high energy(?930 cm^-1).However,the mininum phonon erergy of fluoride can achieve about 230 cm^-1.Thus,oxyfluride glass ceramics combine the high luminescence efficiency of fluoride and the stability of oxide.Up to now,there are three kinds of oxyfluoride glass cermics:fluosilicate,fluogermanate and fluotellurite.The main compositions of fluosilicate GCs glass matrix are SiO2 and Al2O3 which makes the GCs higher chemical stability and mechanical strength.Therefore,the fluosilicate oxyfluride GCs is a potential upconversion material with higher optical damage threshold compare to other type of oxide glass and crystal.Hence,the fabrication of novel GCs has great importance and significance.Oxyfluride GCs has many aspects application like white light emiiting diode display,scintillating medium,long afterglow,thermoluminescence and fiber optical fluorescence temperature measurement.To improve the properties for temperature sensing,searching for novel UC materials with good stability,efficient luminescence and improved sensitivity is still on the way.In order to find materials with excellent temperature sensing properties,rare earth ion doped diverse luminescence materials were characterized as optical temperature sensors,such as metal-organic frameworks?MOFs?,fluoride nanocrystals,ceramics and glass.Among these material systems,the temperature sensing ability varies notably.MOFs might have some advantages sensivity more systematically and compatibility with biological systems,but their insufficient stability at relatively high temperature restricts their operationg temperature to the physiological temperature range.Fluorides with low phonon energy provide efficient upconversion?UC?luminescence,which favors the accuracy of spectrum measurement and sensitivity.However,their poor stability at high temperature probably leads to phase transformation,which will affect the repeatability in heating-cooling cycles.Transparent ceramics still have lots of challenges including complicated synthesis procedures and high cost of preparation.For glass,it is difficult to produce the desired luminescence intensity in silica or in the alumino-silicate glass matrix due to their high phonon energy and abundant defects acting as quenching centers,hindering their performance for temperature sensing.Hence,to improve the properties of optical thermometers,the search for novel Er³⁺ ion doped UC materials with good stability,efficient luminescence and improved sensitivity is still on the way.The first chapter is the introduction part of this paper,we first introduced the fundamantal and classification of luminescence,luminescence properties of rare earth ions.Secondly,we also introduce the definition,classification,fabrication and characterization of GCs.At last,the principle of temperature sensing based on fluorenscence intensity ratio technique with thermally coupled energy levels was given.In chapter two,we studied fabrication of novel Eu³⁺-doped TbPO₄ GCs and energy transfer from Tb³⁺ to Eu³⁺ ions.The Eu³⁺-doped TbPO₄ GCs was elaborated through high temperature melt quenching method for the first time.The X-ray diffraction,transmittance electron microscopy,selected-area electron diffraction and photo luminescence emission spectra confirm the formation of Eu³⁺-doped TbPO₄ GCs.The obvious Stark splitting of ⁵D₀?7FJ?J = 1,2,4?transitions of Eu³⁺ and the increase of internal quantum efficiency indicate the incorporation of Eu³⁺ into TbPO₄ nanocrystals.Energy transfer from Tb³⁺ ions to Eu³⁺ ions was investigated using excitation and emission spectra at room temperature.The GCs obtained have more efficient Tb³⁺ to Eu³⁺ energy transfer than the glass,and so serve as good hosts for luminescence materials.In chapter three,Tb³⁺ and Yb³⁺ ions co-doped transparent phosphate GCs monoclinic GdPO₄ nanocrystals were successfully synthesized using a conventional high temperature melting quenching method in air atmosphere.The structure and morphology properties were systematically analyzed by recording X-ray diffraction patterns and transmission electron microscopy images,which indicate the GdPO₄ GCs was well formed.The luminescent properties of the GdPO₄:Yb³⁺,Tb³⁺ were investigated based on excitation,emission spectra and decay curves.All samples exhibited the emission of Tb³⁺ ions from ⁵D₄?FJ?J = 6,5,4 and 3?and 5D3?7FJ?J= 6,5 and 4?under irradiation with ultraviolet and near infrared light.Specially,strong green up-conversion emission of Tb³⁺ at 543 nm?⁵D₄ ? ⁷F₅?is observed.And the decay curves illustrate more efficient UC process in GdPO₄ GCs rather than precursor glass.Energy transfer from Gd³⁺ to Tb³⁺ as well as cooperative energy transfer from Yb³⁺ pair to Tb³⁺ gives a dual mode luminescence.Laser power dependence of up-conversion shows that two-photon process is responsible for the green emission as expected.Additionally,the paramagnetic property of this material was discussed.The hysteresis plot?M-H?analysis results at room temperature indicate their good paramagnetic property.Based on the above results,the block GdPO₄ GCs have potentials to serve as multifunctional materials applied in laser field materials.In chapter four,novel Er-doped transparent NaYb₂F₇ GCs were successfully fabricated for the first time by a conventional melt-quenching technique with subsequent heat treatment.The formation of NaYb₂F₇ nanocrystals was confirmed by X-ray diffraction,transmission electron microscopy,high-resolution transmission electron microscopy,selected-area electron diffraction,and photoluminescence emission spectra.Moreover,the appearance of Stark level splitting of Er³⁺ emission bands and the variation of the decay curves demonstrate the accumulation of active centers into the NaYb₂F₇ NCs lattice.Hence,the photoluminescence emission intensities of Er³⁺ doped GC680 are greatly enhanced relative to those in precursor glass.Furthermore,the temperature dependent fluorescence intensity ratio?FIR?of thermally coupled emitting states?4S3/2,2H11/2?in Er³⁺ doped GCs was studied under 980 nm laser excitation with a very low power density of 13 mW 112 to avoid the possible laser induced heating.A high temperature sensitivity of FIR of 1.36%K^-1 is obtained at 300 K and the corresponding effective energy difference is 852 cm^-1.Besides,laser induced heating at several excitation power densities was measured to evaluate the laser induced heating effect and the accuracy of temperature sensing for our sample.The GCs with relatively high sensitivity under low excitation power density are promising for temperature sensing.Moreover,the study on down-conversion?DC?spectra of the GCs samples shows their ability to convert a high energy photon into two low energy photons,implying that they may also have important application as DC materials.In chapter five,Tb³⁺/Eu³⁺ co-doped transparent GCs containing CaF₂ nanocrystals were successfully synthesized by high temperature melt-quenching method and subsequent heating.The structure and morphology of the samples were investigated by X-ray diffraction,transmittance electron microscopy,high resolution TEM and selected area electron diffraction.The photoluminescence properties and energy transfer process from Tb³⁺ to Eu³⁺ of CaF₂:Tb³⁺,Eu³⁺ phosphors were also investigated through excitation spectra and decay curves.In addition,the emission spectra of the GCs in a wide temperature range from 21 to 320 K were recorded under the excitation of 485 nm.It is found that the fluorescence intensity ratios of Tb³⁺ at 545 nm?⁵D₄ ? ⁷F₅?to Eu³⁺ at 615 nm?⁵D₀ ? ⁷F₂?is highly temperature-dependent with an approximate linear relationship,and the relative temperature sensitivity is about 0.4%K^-1.It is expected that the investigated Tb³⁺/Eu³⁺ co-doped CaF₂ GCs may have prospective application in optical thermometry.