Er:Sr_(5)(PO_(4))_(3)F透明陶瓷的制备和上转换光学性能

Preparation and Up-Conversion Luminescent Properties of Er:Sr_(5)(PO_(4))_(3)F Transparent Ceramics

随着激光二极管朝着小型化和高度集成化目标的不断发展,包括Sr_(5)(PO_(4))_(3)F (S-FAP)在内的一些热管理性能一般的晶体材料开始重新进入研究者的视野。本研究采用共沉淀法合成了2%(摩尔分数) Er^(3+):S-FAP多晶纳米粉体,通过热压烧结合成出光学性能优异的Er:S-FAP透明陶瓷,并对Er:S-FAP多晶纳米粉体和透明陶瓷进行了表征。结果表明:2%Er^(3+):S-FAP粉体和陶瓷的物相结构都符合六方晶系氟磷灰石结构,没有观察到异常杂质峰。2%Er^(3+):S-FAP粉体整体呈现出分散性优良的短棒状外观,其平均晶粒尺寸为(19.10±1.6) nm。对应陶瓷的平均晶粒尺寸为386.6±20.8 nm,断面和表面微观结构都呈现出均匀致密的特征。2%Er^(3+):S-FAP透明陶瓷在500 nm和1 000 nm波长处的线性光学透过率分别达到了66.14%和84.41%。Er^(3+)在S-FAP陶瓷中的主要的发射峰位于550 nm和661 nm,分别对应绿色光和红色光的发射,红绿光的上转换发光是由双光子吸收过程决定,最后陶瓷在550 nm波长位置的荧光寿命为2.1μs。

Introduction Fluorescent materials have attracted much attention due to their application advantages in solar cells,biological diagnostics,infrared detection and solid-state laser.Up-conversion luminescence process refers to a special anti-stokes process that converts low-energy photons into high-energy photons.Rare-earth element doped fluoride nanoparticles are beneficial to obtaining a high up-conversion luminescence efficiency due to their low phonon energy.However,the lack of particle morphology and stability of the nanoparticles hinders their practical application.From the perspective of application analysis,rare-earth element doped fluoride crystal materials have a lower cost and a better stability,and are more suitable for complex occasions where bulk materials are needed.Er^(3+)ion is widely used in various up-conversion optical materials because it can emit green or red fluorescence under the excitation at980 or 808 nm laser.From the perspective of luminescence,rare-earth element ion has a poor luminescence performance in the crystal environment with a high symmetry due to the prohibition of electric dipole transition.For the luminescence of doped ions,substances with a low symmetry system should have more potential advantages.Strontium fluorophosphates(Sr_(5)(PO_(4))_(3)Freferred to as S-FAP)crystal material is considered as a member of the hexagonal crystalline system fluorapatite family.The synthesis of high-quality single crystal has challenges.The single crystal growth process has frequent performance defects such as bubbles,clouds,cracks and impurity absorption.In this paper,a high quality strontium fluorophosphate transparent ceramic material was synthesized via conventional one-step hot pressing sintering as an economic way.In addition,the up-conversion luminescence properties of Er^(3+)in hexagonal strontium fluorophosphate asymmetric transparent polycrystalline ceramic material were also investigated.Materials and method The phase composition of 2%Er:S-FAP powder and ceramics was analyzed by a model D/Max-RB X-ray diffractometer(XRD)with Cu target at a tube voltage of 60 kV,tube current of 50 m A,scanning angle range of 20°–80°,and scanning step of 0.02°.The Rietveld refinement results of XRD were completed by a software named FullProf.The microstructure of powder and ceramic was determined by a model SU8010 field emission scanning electron microscope(SEM).For the SEM determination,the powder and ceramic samples were treated with gold spraying for 15 s and 20 s,respectively.The grain sizes of powder and ceramic samples were analyzed via softwares named nano Measure 1.2 and Image J.The optical linear transmittance and absorption spectra of 2%Er:S-FAP transparent ceramics were measured by a model UV-3600 UV-Visible-infrared spectrophotometer.The emission spectrum and fluorescence lifetime of ceramic samples at room temperature were measured by a model FLS1000fluorescence spectrometer with excitation laser at 980 nm.All the tests were conducted at room temperature.The diameter and thickness of the ceramic samples were 16 mm and 2.2 mm,respectively.Results and discussion Based on the XRD patterns of 2%Er:S-FAP precursor powder and the XRD Rietveld refinement results of hot-pressing ceramics,the synthesized phase crystal structure is a hexagonal fluorapatite crystal structure.The SEM results of the powder show that the short rod-like fluorapatite nanoparticles with a high sintering activity and a well dispersion can be synthesized by a simple liquid-phase co-precipitation method.The average grain size is(19.10±1.6)nm,which is similar to the calculated value(i.e.,17.0 nm).The existence of compact and uniform surface and sectional structure is a basis of high optical quality 2%Er:S-FAP transparent ceramics,and the average grain size of the ceramics is(386.6±20.8)nm.The linear optical transmittance of the ceramic samples at 500 nm and 1000 nm is 66.14%and 84.41%,respectively.The linear optical transmittance of 2%Er:S-FAP transparent ceramics is not close to the theoretical transmittance possibly due to some factors that reduce the transmittance(i.e.,impurity scattering,porosity,and grain boundary birefringence).The actual average grain size of transparent ceramics is less than 1μm.The scattering caused by coarse grain boundaries is small,and the pore and grain boundary birefringent scattering is a main reason of light scattering loss.The intensity of all emission peaks of 2%Er:S-FAP polycrystalline transparent ceramics increases with the increase of laser power intensity,and the intensity of emission peaks related to red emission at 661 nm is more obvious.The up-conversion luminescence process of Er^(3+)in S-FAP ceramic matrix,including red and green light emission,is dominated by a two-photon absorption process through fitting of excitation power P and up-conversion emission intensity I_(uc).2%of Er^(3+)emits an intense red light in S-FAP matrix when exciting Er^(3+)by a laser at 980 nm after Er^(3+)occupies Sr^(2+)site.The enhanced cross relaxation effect between Er^(3+)–Er^(3+)leads to the relative enhancement of red emission relative to green emission.The specific cross relaxation process is as follows:i.e.,1)^(4)F_(7/2)+^(4)I_(11/2)→^(4)0_(9/2)+^(4)F_(9/2);2)^(4)S_(3/2)+^(4)I_(15/2)→^(4)I_(9/2)+^(4)I_(13/2);and 3)^(4)I_(13/2)+^(4)I_(11/2)→^(4)I_(15/2)+^(4)F_(9/2).Conclusions The XRD and SEM results show that 2%Er:S-FAP nano-powder with a short rod-like morphology and an average grain size of(19.10±1.6)nm was synthesized.The superior linear optical transmittance of the ceramic was due to the high dense and uniform ceramic cross section and surface microstructure.The optical transmittance of the ceramic at 500–1000 nm was 66.14 and84.41%,respectively.The scattering factor of the ceramic was mainly grain boundary birefringent scattering.The absorption spectra at room temperature show that the absorption peak intensity of green light of Er^(3+)in S-FAP transparent ceramic was greater than that of red light.The intensity of the red emission peak of the ceramic was greater than that of the green light in the up-conversion emission spectrum as the laser pump power at 980 nm increases possibly due to the enhancement of cross relaxation phenomenon.Two-photon absorption dominates the up-conversion process of 2%Er^(3+)in S-FAP transparent ceramic matrix by means of the intensity of red-green upconversion light with the excitation power of the laser.It is indicated that Er^(3+):S-FAP transparent ceramic material is a kind of red up-conversion luminescent material with a promising application potential.