Fabrication and Characterization of Dy,Tb∶LuAG Transparent Ceramics for Yellow Lasers

LIU Qiang，WU Long-fei，，LI Xiao-ying，LIU Zi-yu，CHEN Peng-hui，TIAN Feng，XIE Teng-fei，PIRRI Angela，TOCI Guido，LI Jiang*

（1.School of Material Science and Engineering，Jiangsu University，Zhenjiang 212013，China；

2.Transparent Ceramics Research Center，Shanghai Institute of Ceramics，Chinese Academy of Sciences，Shanghai 201899，China；3.Center of Materials Science and Optoelectronics Engineering，University of Chinese Academy of Sciences，Beijing 100049，China；4.Istituto di Fisica Applicata“N.Carrara”，Consiglio Nazionale delle Ricerche，CNR-IFAC，Via Madonna del Piano 10C，50019 Sesto Fiorentino（Fi），Italy；5.Istituto Nazionale di Ottica，Consiglio Nazionale delle Ricerche，CNR-INO，Via Madonna del Piano 10C，50019 Sesto Fiorentino（Fi），Italy）

Abstract：3%Dy，1%Tb∶LuAG（Dy，Tb∶LuAG）nanopowders with good dispersibility were synthesized by the coprecipitation method using NH4HCO3 as a precipitant.The thermal decomposition behavior of the precursor，and the phase and microstructure of powders were studied.Dy，Tb∶LuAG ceramics with high transparency were fabricated by vacuum pre-sintering and hot isostatic pressing（HIP）post-treatment without any sintering additives for the first time.The influences of pre-sintering temperature on the microstructure and the optical quality of the ceramics were investi⁃gated.When the pre-sintering temperature is 1 600℃，the in-line transmittance of the annealed Dy，Tb∶LuAG ceramics（1.5 mm in thickness）reaches 83.6% at 578 nm，and the average grain size of the annealed ceramics is 0.9 μm.In addition，the absorption cross section of the 3%Dy，1%Tb∶LuAG ceramics at 447 nm is calculated to be 1.3×10-21 cm2，with a full width at half maximum（FWHM）of 3.0 nm，which matches that of commercial GaN blue laser diodes.This study shows that Dy，Tb∶LuAG transparent ceramics have potential application value in the yellow lasers.

Key words：Dy，Tb∶LuAG；transparent ceramics；co-precipitation method；hot isostatic pressing

1 Introduction

A yellow laser at 578 nm plays an important role in the treatment of retinal maculopathy[1]and excitation of the Yb lattice clock[2],which can be directly obtained by pumping Dy3+doped gain media at about 450 nm with a GaN or InGaN laser diode（LD）,corresponding to the4F9/2→6H13/2transition of Dy3+[3-4].

Previous reports on Dy3+doped gain media mainly focused on single crystals such as Dy,Tb∶Li⁃LuF4,Dy,Tb∶YAG and Dy,Tb∶Na2Gd4（MoO4）7[5-7].Compared with single crystals,transparent ceramics can be easily fabricated with large size and high dop⁃ing concentration,which are beneficial to improve the absorption efficiency of Dy3+at 450 nm[8].In re⁃cent years,Dy∶YAG,Dy∶Y2O3and Dy∶LuAG trans⁃parent ceramics prepared by the solid-state reaction method have been reported one after another[9-11].However,no laser oscillation was demonstrated due to the poor optical quality of available ceramics.Currently,the preparation methods of laser ceramics mainly include the solid-state reaction method[12-15]and the non-reactive sintering method[16-17].While the latter usually uses the co-precipitation nano-pow⁃ders with more uniform mixing of various atoms and higher sintering activity[18],which can prepare trans⁃parent ceramics at lower temperatures.In addition,the residual pores inside the ceramics should be completely eliminated to improve the optical quality of the ceramics at 578 nm.The hot isostatic pressing（HIP）technology has been widely used in the prepa⁃ration of YAG,Y2O3and CaF2transparent ceram⁃ics[19-21],which is beneficial to eliminate the intergran⁃ular pores and obtain fully dense ceramics.

When Dy3+and Tb3+were co-doped,LuAG trans⁃parent ceramics have smaller lattice distortions than YAG transparent ceramics,because the radius of Lu3+is closer to the radius of the dopant ions than that of Y3+[22-24].Besides,gain media with high pho⁃non energies are prone to non-radiative transitions that reduce laser efficiency.However,the lower en⁃ergy level6H13/2of Dy3+has a significantly long life⁃time.LuAG transparent ceramics with high phonon energies are beneficial to quench the lifetime of the lower energy level6H13/2[8].Therefore,Dy,Tb∶LuAG transparent ceramics have important research value in the yellow lasers.

In this work,3%Dy,1%Tb∶LuAG transparent ceramics with high optical quality were fabricated by vacuum pre-sintering at different temperatures and HIP post-treatment for the first time.Moreover,the influences of pre-sintering temperature on the micro⁃structure and optical quality of the Dy,Tb∶LuAG ce⁃ramics were investigated in detail.Finally,the ab⁃sorption coefficient of the annealed Dy,Tb∶LuAG ce⁃ramics was also calculated.

2 Experiments

Dy,Tb∶LuAG nanopowders were synthesized by the reverse-strike co-precipitation method.Dy（NO3）3,Tb（NO3）3and Lu（NO3）3solutions were obtained by dissolving the Dy2O3（99.995%,Aladdin,China）,Tb4O7（99.995%,Zhongkai New Materials Co.,Ltd.,Jining,China）,Lu2O3（99.995%,Jingyun Ma⁃terials Technology Co.,Ltd.,Shanghai,China）in the hot nitric acid,respectively.while Al（NO3）3so⁃lution was prepared by dissolving Al（NO3）3·9H2O（99.0%,Sinopharm Chemical Reagent Co.,Ltd.,China）in the deionized water.NH4HCO3（99.0%,Aladdin,China）solution with a concentration of 1.5 mol/L was used as a precipitant and（NH4）2SO4（99.0%,Sinopharm Chemical Reagent Co.,Ltd.,China）solution as a dispersant.In addition,extra 1.0% Lu was added to the mixed metal ion solution in order to avoid the generation of the alumina sec⁃ondary phase,which may be caused by the inconsis⁃tent dissolution of the precipitate during the washing process[25].Firstly,the metal ion solutions were mixed according to the stoichiometric ratio of 3%Dy,1%Tb∶Lu3（1+x）Al5O12（x=1.0%）and then added into the NH4HCO3solution containing（NH4）2SO4solu⁃tion.After aging for 1 h,the precipitates were washed three times with deionized water and twice with etha⁃nol.The precursor was then dried at 70℃for 36 h,sieved through a 200-mesh screen and finally cal⁃cined at 1 100℃for 4 h in air to obtain the Dy,Tb∶LuAG powders.Next,the obtained powders were dry pressed at 46 MPa and then cold isostatically pressed at 250 MPa.The green bodies were vacuum pre-sintered at different temperatures for 3 h and hot isostatically pressed（HIPed）at 1 600℃for 3 h under 176 MPa in Ar atmosphere.Post-annealing was carried out at 1 200℃for 10 h in air to elimi⁃nate oxygen vacancies.Finally,the pre-sintered ce⁃ramics and the annealed ceramics were mirror-pol⁃ished on both surfaces into 1.5 mm thickness and the polished Dy,Tb∶LuAG ceramics were then ther⁃mally etched at 1 200℃for 3 h in air for further tests.

The thermogravimetry and differential thermal analysis（TG-DTA）curves of the precursor were measured by a thermal analyzer（Thermoplus EVOⅡ,Rigaku,Japan）at a heating rate of 10℃/min in the flowing air.The phase composition of the pow⁃ders was identified by the X-ray diffraction system（XRD,D/max 2200PC,Rigaku,Japan）with CuKα1radiation in the range of 2θ=10°-80°.The morpholo⁃gy of the powders and microstructures of the ceram⁃ics were observed by a field emission scanning elec⁃tron microscope（FESEM,SU8220,Hitachi,Japan）.The in-line transmittance and the absorption spec⁃troscopy of mirror-polished Dy,Tb∶LuAG ceramics were measured by a UV-VIS-NIR spectrophotometer（Cary-5000,Varian,USA）.

3 Results and Discussion

Fig.1 shows the TG-DTA curves of Dy,Tb∶LuAG precursor.The weight loss on the TG curve is a con⁃tinuous process,mainly divided into three stages.The first stage occurs before 300℃with the weight loss of 24.7%.An endothermic peak is located at 122℃on the DTA curve,which is mainly assigned to the removal of adsorbed water,crystallization water and residual ammonium.In addition,the hydroxycarbon⁃ate is also decomposed at this stage.The weight loss of 8.2% in the second stage occurred between 300℃and 900℃is mainly caused by the decompo⁃sition of lutetium carbonate,which corresponds to the endothermic peak located at 830℃on the DTA curve.The third stage ranges from 900℃to 1 100℃with a weight loss of 4.3%.The exothermic peak at 936℃on the DTA curve corresponds to the phase formation of garnet（Lu3Al5O12）.The endothermic peak at 970℃corresponds to the decomposition of sulfates,while the exothermic peak at 1 070℃is caused by the grain growth[26].Furthermore,the weight loss of the precursor is little after 1 100℃and the total weight loss is about 37.2%.Therefore,the precursor was calcined at 1 100℃for 4 h in or⁃der to obtain nanopowders with pure phase and good crystallinity.

Fig.1 TG-DTA curves of the Dy，Tb∶LuAG precursor

Fig.2 shows the FT-IR spectra（a）and the XRD patterns（b）of the precursor and the Dy,Tb∶LuAG powders calcined at 1 100℃for 4 h.The wide ab⁃sorption band at 3 420 cm-1is related to the stretch⁃ing vibrations of O—H bond including crystal water,aluminum hydroxide and hydroxycarbonate groups.The strong peak at 1 520 cm-1can be attributed to the bond-stretching of NH4+from the precipitant and the dispersant.The absorption peaks at 1 420 cm-1and 850 cm-1correspond to C—O bond stretching and bending.In addition,the peak located at 1 090 cm-1corresponds to the vibration of SO42-.These in⁃dicate that the precursor contains OH-,NH4+,CO32-and SO42-groups.However,no visible absorption of NH4+,CO32-or SO42-groups is observed in the pow⁃ders calcined at 1 100℃.The weak absorption peaks at 3 420 cm-1and 1 630 cm-1demonstrate that the existence of residual OH-groups,which may be ascribed to the molecular water absorbed from the air.Moreover,the bands resulting from the stretching of Lu—O and Al—O bonds in the 400-800 cm-1region are characteristics of LuAG,which are similar to the garnet structure of YAG[27].These show that the pre⁃cursor is completely decomposed after calcination at 1 100℃.It can be seen from Fig.2（b）that the precur⁃sor is amorphous and the diffraction peaks of the powders calcined at 1 100℃for 4 h are identified as the LuAG phase（PDF#73-1368）without any impurity phase.This indicates that the precursor has been completely converted into the powders with the pure LuAG phase after calcination at 1 100℃for 4 h.

Fig.2 FT-IR spectra（a）and XRD patterns（b）of the precursor and the Dy，Tb∶LuAG powders calcined at 1 100℃for 4 h

Fig.3 shows the FESEM micrographs of the Dy,Tb∶LuAG precursor（a）and the powders calcined at 1 100℃for 4 h（b）.The precursor is severely ag⁃glomerated,which is related to the strong force of hy⁃drogen bonds.However,the Dy,Tb∶LuAG powders have good dispersibility,which indicates that the hy⁃drogen bonds inside the precursor have been de⁃stroyed after calcination at 1 100℃.Moreover,it can be observed from Fig.3（b）that the primary particle size of the powders is about several tens of nanometers.

Fig.3 FESEM micrographs of the Dy，Tb∶LuAG precursor（a）and the powders calcined at 1 100℃for 4 h（b）

Fig.4 shows the FESEM images of the thermally etched surfaces of the Dy,Tb∶LuAG ceramics vacu⁃um pre-sintered at different temperatures for 3 h.With the increase of the pre-sintering temperature,the submicron-scale intergranular pores gradually transform into fine intergranular pores,and the num⁃ber of pores is also decreasing.The relative densi⁃ties of the ceramics pre-sintered at 1 550，1 600,1 650，1 700℃were measured to be 88.9%,92.8%,94.2% and 95.9% by the Archimedes method,respectively.The relative density change of the pre-sintered ceramics is consistent with the mi⁃cromorphology of the ceramics pre-sintered at differ⁃ent temperatures.In addition,the average grain sizes of the ceramics pre-sintered at 1 550,1 600,1 650，1 700℃were calculated to be 0.6,0.7,0.8，1.1 μm by the intercept method,respectively.When the pre-sintering temperature increases to 1 700℃,the average grain size of the ceramics increases signifi⁃cantly.

Fig.4 FESEM images of the thermally etched surfaces of the Dy，Tb∶LuAG ceramics vacuum pre-sintered at different tempera⁃tures for 3 h.（a）1 550℃.（b）1 600℃.（c）1 650℃.（d）1 700℃.

Fig.5 shows the FESEM images of the thermally etched surfaces of the ceramics after the HIP posttreatment.When the pre-sintering temperature is in the range of 1 550-1 650℃，no pores and second⁃ary phases exist inside the ceramics.The average grain sizes of the HIPed ceramics are 0.7,0.9，1.0 μm,respectively.However,a small number of resid⁃ual pores marked by the red circle can be observed in the Fig.5（d）.When the pre-sintering tempera⁃ture increases to 1 700℃,the average grain size of the pre-sintered ceramics becomes larger,which is harmful to the exclusion of pores during the HIP pro⁃cess.

Fig.5 FESEM images of the thermally etched surfaces of the Dy，Tb∶LuAG ceramics vacuum pre-sintered at different tempera⁃tures for 3 h and HIPed at 1 600℃for 3 h under 176 MPa in Ar.（a）1 550℃.（b）1 600℃.（c）1 650℃.（d）1 700℃.

Fig.6（a）shows the in-line transmittance of the Dy,Tb∶LuAG transparent ceramics annealed at 1 200℃for 10 h in air and the inset image shows the pho⁃tograph of the annealed ceramics.The in-line trans⁃mittance of the annealed ceramics exceeds 83.0% at 578 nm in the pre-sintering temperature ranging from 1 550℃to 1 650℃,which is attributed to the effective elimination of intergranular closed pores.As the pre-sintering temperature increases to 1 700℃,a dramatic decrease of the in-line transmittance is observed.This is due to the small number of inter⁃granular pores acting as optical scattering centers which are still retained inside the ceramics.In addi⁃tion,when the pre-sintering temperature is 1 600℃,the annealed ceramics obtain the optimum optical quality.The in-line transmittance reaches 83.6% at 578 nm（theoretical transmittance 83.9%@578 nm）,which has been greatly improved than that of the Dy∶LuAG transparent ceramics（～75%@578 nm）in the previous reports[11].Fig.6（b）shows the absorption spectrum of the annealed ceramics vacuum pre-sin⁃tered and HIP-treated at 1 600℃.The strongest ab⁃sorption peak in the 420-490 nm region is located at 447 nm with the FWHM of 3.0 nm.Considering the output wavelength of the commercial GaN blue LD is 447 nm with the line width of about 2.0 nm,this in⁃dicates the GaN blue LD can be used as an effective pumping source for the Dy,Tb∶LuAG transparent ce⁃ramics.Moreover,the larger FWHM favors the de⁃crease of the temperature dependence of a GaN blue LD pumping source and improving the pumping effi⁃ciency[28].Concerning the absorption cross sectionσabs,it can be estimated by the formula:

Fig.6 In-line transmittance of the Dy，Tb∶LuAG transparent ceramics（1.5 mm thick）vacuum pre-sintered at different tempera⁃tures for 3 h，HIPed at 1 600℃for 3 h under 176 MPa in Ar，and annealed at 1 200℃for 10 h in air（a）and the absorp⁃tion spectrum of the annealed ceramics vacuum pre-sintered and HIP-treated at 1 600℃（b）.

whereNcis the concentration of Dy3+in the ceramics which is about 4.26×1020cm-3,α（λ）is the absorption coefficient which is calculated with the equation:

whereLexpresses the thickness of the ceramics,lg（I0/I）is the optical density obtained by the spec⁃trophotometer.According to it,the absorption cross section of the Dy,Tb∶LuAG ceramics at 447 nm was calculated to be 1.3×10-21cm2,which is larger than that of Dy∶Y2O3and Dy∶YAG transparent ceramics.However,it is lower than that of Dy,Tb∶LiLuF4crys⁃tals,as listed in Tab.1.The difference of the absorp⁃tion cross section is mainly caused by the difference of host materials in the Dy3+doped gain media.

Tab.1 Absorption cross section of Dy3+-doped materials at 447 nm

4 Conclusions

3%Dy,1%Tb∶LuAG ceramics with high trans⁃parency were prepared from the nanopowders syn⁃thesized by the co-precipitation method without any sintering additives.After calcination at 1 100℃for 4 h,the total weight loss of the precursor was 37.2% and Dy,Tb∶LuAG nanopowders with high sintering activity were obtained.In addition,the in⁃fluences of pre-sintering temperature on the micro⁃structure and the optical transmittance of the ceram⁃ics were studied.When the pre-sintering tempera⁃ture of the ceramics is in the range of 1 550-1 650℃,the average grain size of pre-sintered ce⁃ramics does not exceed 1 μm,which is conducive to the exclusion of intergranular closed pores inside the ceramics during the HIP post-treatment.When the pre-sintering temperature is 1 600℃,the in-line transmittance of the annealed ceramics reaches 83.6%.Finally,the absorption spectrum of the an⁃nealed ceramics pre-sintered at 1 600℃was calcu⁃lated.The strongest absorption peak is at 447 nm with the FWHM of 3.0 nm,the absorption cross sec⁃tion is 1.3×10-21cm2.The results indicate that Dy,Tb∶LuAG transparent ceramics are a potential candidate for the yellow laser pumped by the GaN blue LD.

Response Letter is available for this paper at:http://cjl.lightpublishing.cn/thesisDetails#10.37188/CJL.20220153.