Fabrication and spectral properties of Dy:Y2O3 transparent ceramics

Journal of the European Ceramic Society xxx (xxxx) xxx–xxx

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Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc

Fabrication and spectral properties of Dy:Y2O3 transparent ceramics ⁎

⁎

Zongwen Hua, Xiaodong Xua, , Jun Wangb, Peng Liua, , Dongzhen Lia, Xiaodan Wangc, Jian Zhangd, Jun Xue, Dingyuan Tanga a

Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore Department of Physics, Suzhou University of Science and Technology, No. 1, Kerui Road, Suzhou 215009, China d Key Laboratory of Transparent and Opto-Functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 201899, China e School of Physics Science and Engineering, Tongji University, Shanghai 200092, China b c

A R T I C L E I N F O

A B S T R A C T

Keywords: Dy:Y2O3 Transparent ceramic Judd-Ofelt theory Optical spectroscopy SPS sintering

Dy3+-doped Y2O3 ceramics were fabricated by Spark Plasma Sintering method. The effect of sintering temperature on the microstructure and the optical transmittance of the ceramics was studied. The maximum in-line transmittance of Dy:Y2O3 ceramics with thickness of 1 mm was 74.5% at 574 nm when sintering temperature was 1550 ℃. The spectroscopic parameters were determined by Judd-Ofelt theory based on the room temperature absorption spectra. The peak emission cross section for the 4F9/2 → 6H13/2 transition was calculated to be 0.569 × 10−20 cm2. The fluorescence decay curve and quantum efficiency of the 4F9/2 multiplet were measured to be 269.2 μs and 37.1%, respectively.

1. Introduction

microstructure and optical transmittance of the ceramics was studied. The absorption spectra, fluorescence spectra and fluorescence decay of the best sample were measured, and the spectroscopic parameters were calculated based on the Judd–Ofelt theory.

Solid state lasers emitting in the yellow region have many important applications in telecommunication, military, biotechnology and display [1–3]. In recent years, laser materials doped with Dy3+ ion have attracted much attention due to the strong emission in yellow regions, corresponding to the 4F9/2 → 6H13/2 transition of the Dy3+ ions [4–12]. In 2000, yellow lasers were realized in Dy:KY(WO4)2 and Dy:KGd (WO4)2 crystals under Xe-flashlamp pumping for the first time [13]. Since then due to the rapid development of InGaN laser diodes, yellow laser oscillations of the 4F9/2 → 6H13/2 transition in Dy:YAG [14], Dy,Tb:LiLuF4 [15] and Dy:ZnWO4 [16] crystals were also obtained. Sesquioxide Y2O3 single crystal can be easily doped with rare earth ions. It is a promising host material for solid-state lasers due to its high thermal conductivity, chemical stability, and low phonon energy. However, it is extremely difficult to grow Y2O3 single crystal with conventional crystal growth techniques because of its high melting point (∼2420 ℃) [17]. Y2O3 transparent ceramics have attracted much attention because they can be easily fabricated with large size and high doping concentration [18–20]. In addition, fabricating ceramics is less expensive because it is not necessary to use expensive crucible [21]. However, the Dy3+ -doped Y2O3 transparent ceramics have not been reported till now. In this work, Dy:Y2O3 transparent ceramics were fabricated by Spark plasma sintering (SPS). The effect of sintering temperature on the

⁎

2. Experimental procedures Commercial Y2O3, Dy2O3 and LiF powders with purity higher than 99.99% were used as the starting materials. The Dy3+ concentration was 3 at.% with respect to Y, and the LiF content was 0.3 wt.%. The powders were weighted and mixed by ball milling using zirconia ball in ethanol. They were dried at 55 ℃ for 24 h, sieved through a 60 mesh screen, and then calcined at 800 ℃ for 6 h in order to remove residual organic compounds. The calcined powders were pressed and poured into a graphite die with a diameter of 15 mm, and then sintered using an SPS apparatus under the vacuum atmosphere. A pressure of 6 MPa was pre-loaded at room temperature. The sintering temperature was increased to 600 ℃ in 5 min, then increased to 1100 ℃ at a rate of 100 ℃/min and hold at 1100 ℃ for 3 min. The temperature was further increased to the aim temperature at a rate of 10 ℃/min and hold for 30 min. Post-annealing was carried out at 950 ℃ in air for 8 h. Phase composition of the Dy:Y2O3 ceramics was examined with Xray powder diffraction (XRD, Bruker-D2, Germany). XRD patterns of the calcined Dy:Y2O3 powder and Dy:Y2O3 ceramics sintered at 1600 ℃

Corresponding authors. E-mail addresses: [email protected] (X. Xu), [email protected] (P. Liu).

https://doi.org/10.1016/j.jeurceramsoc.2017.12.020 Received 28 August 2017; Received in revised form 7 December 2017; Accepted 12 December 2017 0955-2219/ © 2017 Elsevier Ltd. All rights reserved.

Please cite this article as: Hu, Z., Journal of the European Ceramic Society (2017), https://doi.org/10.1016/j.jeurceramsoc.2017.12.020

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Fig. 3. The in-line transmittance of Dy:Y2O3 bodies sintered at different temperatures. Fig. 1. X-ray diffraction patterns of (a) Dy:Y2O3 ceramics sintered at 1600 ℃, (b) Dy:Y2O3 ceramics sintered at 1400 ℃, (c) Dy:Y2O3 calcined powder, (d) standard pattern of the Y2O3 (PDF#41-1105).

ceramics sintered at different temperatures. A large number of pores distributed in the grain boundary were observed in the ceramics sintered at 1400 ℃. When the sintering temperature increased to 1450 ℃, the number of the intergranular pores decreased dramatically. In addition, as the sintering temperature increased from 1450 ℃ to 1500 ℃, the grain size increased significantly and the pores in the grain boundary disappeared. The gain size became more uniform when the sintering temperature reached 1550 ℃. When the sintering temperature was 1600 ℃, the abnormal growth of the grains appeared and some exaggerated grains (more than 100 μm) could be found. The in-line transmittance spectra of the Dy:Y2O3 ceramics sintered at different temperatures are presented in Fig. 3. The inset image shows the corresponding photographs of the Dy:Y2O3 ceramics. The transmittance increases rapidly with the increase of sintering temperature in the temperature range of 1400–1550 ℃, while the transmittance decreases in the temperature range of 1550–1600 ℃. The highest in-line transmittance of the samples sintered at 1550 ℃ is nearly 75% at 574 nm. When the sintering temperature was 1600 ℃, the samples were crack and the edge of the ceramics became dark together with a significant degradation on the in-line transmittance, which was due to the abnormal growth of the grain in the ceramic sintered at 1600 ℃.

and 1400 ℃ are shown in Fig. 1. The patterns were well indexed to the Y2O3 phase (PDF#41-1105) in all the specimens. There were no other impurity peaks in the patterns, indicating that doping Dy3+ ions did not induce significant changes in the host structure. The polished surfaces of the Y2O3 bodies were etched in phosphoric acid at 70 ℃ for 40 min for morphology measurement. The morphology of the etched surfaces of Sm:Y2O3 ceramics were recorded by a scanning electron microscope (SEM, JSM-6510, JEOL, Kariya, Japan). Samples for spectral measurements were mechanically polished to 1 mm thickness. Optical transmittance of the Sm:Y2O3 ceramics was measured in the range from 200 to 2500 nm using a UV-VIS-NIR spectrophotometer (Lambda 950, Perkin-Elmer, Waltham, MA). The absorption spectra was also obtained by UV–vis-NIR spectrophotometer in the range from 270 nm to 500 nm and 700 nm to 1900 nm with step of 1 nm. The fluorescence spectra, as well as the decay curve at 572 nm, were recorded using Edinburgh Instruments FLS980 spectrophotometer under 447 nm excitation with step of 1 nm. 3. Results and discussion

3.2. Absorption spectra and the judd-ofelt analysis 3.1. Effect of sintering temperature on the microstructure and optical transmittance

The absorption spectra of a Dy:Y2O3 ceramic sintered at 1550 ℃ in the wavelength range of 270–500 nm and 700–1900 nm are shown in Fig. 4. The dominant absorption bands correspond to the 4f electronic

Fig. 2 illustrated SEM images of the etched surfaces of the Dy:Y2O3

Fig. 2. SEM images of the etched surfaces of Dy:Y2O3 ceramics sintered at different temperature: (a)1400 ℃, (b)1450 ℃, (c) 1500 ℃, (d)1550 ℃, (e)1600 ℃.

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Fig. 4. Absorption spectra of Dy:Y2O3 ceramic at room temperature. (a) 270–500 nm; (b) 700–1900 nm.

Where σabs is the absorption cross section at wavelength λ, h and e are

Table 1 The experimental and calculated line 1of Dy3+ in Y2O3 ceramic. Transition 6H15/2→ 4

G11/2 I15/2 F3/2 + 6F1/2 6 F5/2 6 F7/2 6 F9/2 + 6H7/2 6 F11/2 + 6H9/2 6 H11/2 rmsΔS

λ (nm)

Sexp (×10−20 cm2)

424 0.0733 451 0.121 743 0.0352 796 0.204 891 0.432 1082 0.793 1253 12.233 1667 1.184 −20 0.102 × 10 cm2

4 6

—

the Planck constant and electron charge, λ represents the mean wavelength of the absorption band corresponding to the J → J’ transition, J is the total angular momentum of the initial level (J = 15/2 for Dy3+), n is the refractive index of the medium. Root mean square (rms) deviation between the experimental and calculated line strength is determined by

Scal (×10−20 cm2) 0.0209 0.0997 0.0117 0.0663 0.288 0.782 12.176 1.261

n

rmsΔS =

∑

Where N is the number of absorption bands involved in the calculations. After least-square fitting the experimental and the theoretical electric dipole oscillator strength, the J-O intensity parameters for Dy:Y2O3 ceramics were obtained to be Ω2 = 11.89 × 10−2 cm2, Ω4 = 1.11 × 10−20 cm2, Ω6 = 0.19 × 10−20 cm2. The rmΔS together —

with the parameter Sexp, Scal and λ are listed in Table 1. The rmΔS, which usually serves as an indication of the fitting quality, is 0.102 × 10−20 cm2, indicating that the fitting results were in good agreement with the experiments. With the obtained Ωt parameters, the spontaneous transition probability A(J → J’) from an excited manifold J to a lower manifold J’ can be calculated according to the formula below:

ed

Where A is the contribution from the electric-dipole (ED) to the transition probability and Amd is the contribution from the magneticdipole (MD).

Aed (J , J ′) =

∑

Ωt 〈S, L, J U (t) S′, L′J ′〉2

t= 2,4,6

(5)

Amd (J , J ′) =

64π 4e 2n3 S md 3h (2J + 1) λ 3

(6) md

The line strength of the MD transition S does not change with the host material and can be adopted from Ref. [26]. The radiative lifetime τrad of an excited level is given by the inverse of the total radiative emission probability

(1)

∫ σabs (λ)dλ

64π 4e 2 n (n2 + 2)2 × 3 9 3h (2J + 1) λ

Where ∥U (t )∥is the emission transition matrix from Ref. [25]

τrad =

Where 〈∥U (t )∥〉 is the squared reduced matrix elements from Ref. [24], Ωt (t = 2, 4, 6) are the J-O intensity parameters. The experimental line strength Sexp of the transition from ground 6H15/2 manifold to the excited J ′ manifold can be calculated according to the absorption spectrum using the following formula:

3hc(2J + 1) 9n Sexp (J → J ′) = 8π 3e 2λ (n2 + 2)2

(4)

A = Aed + Amd

Ωt | 〈 (S, L) J ∥U (t ) ∥ (S′, L′) J ′〉 |2

t = 2,4,6

(3)

i=1

transitions of the Dy3+ from the ground state 6H15/2 to the various excited states, which are assigned in Fig. 4. The absorption band around 446 nm is assigned to the 6H15/2 → 4I15/2 transition, which coincides to the laser output of commercially available blue laser diodes. The peak absorption cross section and full width at half maximum (FWHM) were calculated to be 0.54 × 10−21 cm2 and 4.6 nm. The absorption cross section is lower than the value of 1.1 × 10−21 cm2 reported for Dy:YAG ceramic [4]. Due to the low absorption cross section, in order to achieve sufficient absorption the crystal length should be in the order of centimeter. Furthermore, compared with the width of the absorption bands in Dy:YAG ceramic [4], that in Dy:Y2O3 was also larger. It can be attributed to the two types of crystallographic sites in the Y2O3 lattice. After doped with Dy3+, the Y3+ lattice site would be substituted by Dy3+ ions. It means that Dy3+ ions were also located on the two types of lattice sites. However, Dy3+ ions only occupied one type Y3+ crystallographic site in YAG structure. The difference of the crystal field around the Dy3+ between Y2O3 and YAG cause the width broadening of the absorption peak. Moreover, the testing of the absorption spectra was carried out at room temperature, which would also induce the homogenous broadening of the absorption bands [4]. The absorption spectra were analyzed on the basis of Judd-Ofelt (JO) theory [22,23]. Eight Dy3+ absorption transitions were used to determine the J-O intensity parameters for the Dy3+ transitions in Y2O3 ceramic. In the Judd-Ofelt theory, the calculated line strength Scal (J , J ′) for electric-dipole transitions can be written as:

Scal (J → J ′) =

∑ (Smea − Scal )2/(N − 3)

1 ∑J ′ A (J ,J ′)

(7)

The fluorescence branching transition ratio is defined by

βJJ ′ =

A (J , J ′) ∑J ′ A (J , J ′)

(8)

Through the above calculation, the spontaneous transition probability, the fluorescence branching ratio and the radiative lifetime of

(2) 3

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Table 2 The calculated spontaneous transition rate, fluorescence branching ratio and radiative lifetime of Dy:Y2O3 ceramic. Transition 4F9/2→

Aed (S−1)

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14.260 2.187 1.664 10.523 8.117 31.522 21.645 118.654 1053.983 31.357

F5/2 F7/2 6 H5/2 6 H7/2 6 F9/2 6 H9/2 6 F11/2 6 H11/2 6 H13/2 6 H15/2 6

Amd (S−1)

5.641 3.368 5.995 53.311 3.137 11.901

β (%)

τrad (μs)

1.04 0.57 0.12 1.01 1.02 6.16 1.80 9.48 76.53 2.28

726.1

the 4F9/2 multiplet of Dy3+ in Y2O3 ceramics were obtained and summarized in Table 2. The radiative lifetime of the 4F9/2 energy level was calculated to be 726.1 μs, which is much lower than that of Dy:YAG ceramic (2.02 ms) [4], but much longer compared to Dy:CaGdAlO4 crystal (501 μs) [6], Dy3+:GdVO4 crystal (411 μs) [12] and Dy:LiNbO3 crystal (387 μs) [27]. The 4F9/2 → 6H13/2 transition at 573 nm represents the largest spontaneous transition probability and the branching ratio value is 76.53%, indicating the higher possibility of yellow laser emission around 573 nm.

Fig. 6. Fluorescence decay curve recorded at 572 nm under excitation of 447 nm.

(0.55 × 10−20 cm2) [6], Dy3+:GdVO4 crystal (0.90 × 10−20 cm2) [12] and Dy:LiNbO3 crystal (0.32 × 10−20 cm2) [27] in a magnitude of 10−20 cm2, indicating that Dy:Y2O3 ceramic could be available for yellow laser oscillation. Fig. 6 presents the fluorescence decay curve of the 4F9/2 multiplet recorded at 572 nm under excitation of 447 nm. The measured decay curve shows single exponential decaying behavior. The fluorescence lifetime of the 4F9/2 → 6F13/2 of Dy3+ was determined to be 269.2 μs. The radiative decay time of the 4F9/2 level of the Dy:Y2O3 ceramics is calculated to be 726.1 μs based on the J-O theory. Thus the fluorescence quantum efficiency of the 4F9/2 level is 37.1%. This indicates that nonradiative processes are operative in Dy:Y2O3 ceramics at the present doping level. The energy gap between the 4F9/2 emitting level and lower level is about 8500 cm−1, while the maximum phonon energy of Y2O3 crystal lattice is about 460 cm−1 [28]. So the multi-phonon relaxation is inefficient. It is well know that Dy3+ is prone to cross-relaxation processes: 4F9/2 + 6H15/2 → (6F3/ 6 6 6 2 + F1/2) + ( H9/2 + F11/2) [6,19], which causes the emission quenching in the Dy:Y2O3 ceramics.

3.3. Fluorescence spectra The fluorescence spectrum of a Dy:Y2O3 ceramic excited by 447 nm in the range of 460–800 nm is shown in Fig. 5. Four emission bands centered at 487, 573, 669, 757 nm corresponding to the 4F9/2 → 6H15/2, 4 F9/2 → 6H13/2, 4F9/2 → 6H11/2, and 4F9/2 → 6H9/2 + 6F11/2 transitions, respectively, are marked. The spectrum is dominated by the 4F9/2 → 6 H13/2 yellow emission, which is in agreement with the calculated results of branching ratio and means that yellow emission is a potential visible laser channel. To estimate the yellow 4F9/2 → 6H13/2 emission cross-section σem , the experimental emission spectrum I (λ ) and Fuchtbauer-Ladenberg equation was used

σem (λ ) =

4. Conclusions

λ5βJJ ′

I (λ ) 8πn2cτR ∫ λI (λ ) dλ

(9)

Transparent Dy:Y2O3 ceramics were fabricated by spark plasma sintering (SPS) with LiF additive. The effect of sintering temperature on the microstructure and the optical transmittance of the ceramics was investigated. The maximum in-line transmittance of the Dy:Y2O3 ceramic samples with thickness of 1 mm was 74.5% at wavelength of 574 nm when sintering temperature was 1550 ℃. The absorption spectra, fluorescence spectra and fluorescence decay curves were recorded at room temperature. The peak absorption cross section was calculated to be 0.54 × 10−21 cm2 at 446 nm and the FWHM is 4.6 nm, which is favorable for InGaN laser diode pumping. The Judd-Ofelt theory was applied to the analysis of the absorption spectra, and the J-O intensity parameters of Ω2, Ω4 and Ω6 were calculated to be 11.89 × 10−20 cm2, 1.11 × 10−20 cm2 and 0.19 × 10−20 cm2, respectively. The peak emission cross section for the 4F9/2 → 6H13/2 transition was calculated to be 0.569 × 10−20 cm2 with FWHM of 4.2 nm. The fluorescence lifetime of the emission line 4F9/2 → 6H13/2 at 572 nm was measured to be 269.2 μs. The results indicate that the Dy:Y2O3 ceramic is a potential candidate for yellow laser operation.

Where βJJ ′ is the branching ratio, τR is the radiative lifetime, I (λ ) represents the experimental emission intensity as a function of the wavelength λ . Thus, for the maximum 573 nm, the estimated emission cross section is 0.57 × 10−20 cm2 with a FWHM of 4.2 nm. The emission cross section is lower than that of Dy:YAG ceramic (1.5 × 10−20 cm2) [4] but similar to that of Dy:CaGdAlO4 crystal

Acknowledgements This work is partially supported by National key Research and Development Program of China (No. 2016YFB1102202), Natural Science Foundation of Shanghai (15ZR1419600) and 333 High-Level Talents Cultivation Project of Jiangsu Province.

Fig. 5. Fluorescence spectra of Dy:Y2O3 ceramic under excitation at 447 nm.

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