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A simple way to prepare Co:MgAl2O4 transparent ceramics for saturable absorber
Journal of Alloys and Compounds 797 (2019) 1288e1294
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A simple way to prepare Co:MgAl2O4 transparent ceramics for saturable absorber Sha Su a, b, Qiang Liu b, Zewang Hu a, c, Xiaopu Chen a, c, Hongming Pan a, b, Xin Liu a, c, Lexiang Wu a, c, Jiang Li a, c, * a b c
Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China School of Material Science and Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 February 2019 Received in revised form 26 April 2019 Accepted 29 April 2019 Available online 1 May 2019
Co-doped magnesium aluminate (Co:MgAl2O4) transparent ceramics were obtained by pre-sintering in air and hot isostatic pressing (HIP) with commercial spinel and CoO powders as raw materials. The influences of the pre-sintering temperature on the microstructure and optical quality of the 0.05 at% Co:MgAl2O4 ceramics were studied. It is found that the relative density of Co:MgAl2O4 ceramics increases from 96.8% at 1500 C to 98.7% at 1600 C and then decreases with further increasing the pre-sintering temperature. The average grain size of Co:MgAl2O4 ceramics increase from 4.6 mm to 19.2 mm with the increase of pre-sintering temperature from 1500 to 1700 C. For the sample pre-sintered at 1600 C for 5 h and HIP post-treated at 1800 C for 3 h, the average grain size is about 23.0 mm and the near in-line transmittance reaches 75% at the wavelength range of 1700e2400 nm. The ground state absorption cross section at 1540 nm of the optimal 0.05 at% Co:MgAl2O4 ceramics is calculated to be 2.9 1019 cm2. © 2019 Published by Elsevier B.V.
Keywords: Co:MgAl2O4 Transparent ceramics Saturable absorbers Hot isostatic pressing
1. Introduction In recent years, 1.5 mm pulsed laser has been attached much attention due to safety for eyes, high transmission in the air atmosphere and fiber [1]. Passive Q-switching technology using saturable absorber is an effective way to achieve 1.5 mm pulsed laser, because it enables compact and low-cost sources of short pulses to be developed for various applications [2]. At present, the saturable absorbers doped with transition metal are efficient and widely utilized for the passive Q-switching solid-state lasers. And for some special applications, like eye safety field, Co2þ doped MgAl2O4 is a promising material. The cubic space group of spinel is O7h (Fd3m). The Mg2þ occupies tetrahedral coordination, and the Al3þ is in the octahedral environment [3]. Due to the same valence and the similar radii of Mg2þ and Co2þ (the radii of Mg2þ and Co2þ are 0.069 and 0.072 nm, respectively), it is easy for Co2þ to substitute the site of Mg2þ [4,5]. The strong and broad absorption bands of MgAl2O4 doped with
* Corresponding author. Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China. E-mail address: [email protected] (J. Li). https://doi.org/10.1016/j.jallcom.2019.04.322 0925-8388/© 2019 Published by Elsevier B.V.
Co2þ make Co:MgAl2O4 a promising candidate in solid-state lasers for visible and near infrared light [6e10]. So far, Co2þ doped MgAl2O4 crystals, glass-ceramics and transparent ceramics have been fabricated and applied successfully in lasers. However, the large size single crystals (up to several centimeters) with fine quality are difficult to produce due to the high melting point (more than 2100 C) [11], and the cost is also high. For the glass-ceramic, Co2þ may be accommodated in sites of the residual glass phase and cause undesirable absorption (owing to the presence of hexacoordinated Co2þ in glass) [12]. On the contrary, the cost of transparent ceramics is much lower than that of single crystals, and ceramics generally exhibit significantly higher resistance to laser damage, so much attention has been paid to ceramic materials [13,14]. In 2008, Co:MgAl2O4 transparent ceramics were firstly reported and the possible use in the passive laser Qswitching lasers was also suggested by Ikesue [15]. Wajler et al. obtained Co:MgAl2O4 transparent ceramics with the mixed powders of co-precipitated Co:MgAl2O4 powders and commercial high purity MgAl2O4 powders as the starting materials in 2014. The nonlinear absorption of Co:MgAl2O4 transparent ceramics was systematically demonstrated. However, some important information, such as transparency over the visible to mid infrared regions, was not given [16]. In 2016, Goldstein et al. fabricated highly
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transparent Co:MgAl2O4 ceramics for application in pulse laser [17]. However, the poor homogeneity and internal defects in most specimens resulted in the low laser damage threshold. With the mixed powders of the commercial spinel and the lab-made Co:MgAl2O4 powders as the raw materials, Luo et al. fabricated 0.05 at.% Co:MgAl2O4 transparent ceramics by hot pressing and 0.02 at.% Co:MgAl2O4 transparent ceramics by vacuum sintering combined with HIP post-treatment [11,18]. However, the optical transparency of the fabricated Co:MgAl2O4 ceramics in visible wavelength was not good enough. Therefore, further work on Co:MgAl2O4 transparent ceramics is still necessary. Generally, Co:MgAl2O4 transparent ceramics were fabricated using nanopowders obtained by chemical co-precipitation method [19e22], which is known to narrow the particle size distribution and control powders morphology. In our previous work, Co:MgAl2O4 transparent ceramics were fabricated in this way [23]. However, it is hard to control the dispersion-state and the particle size [24]. Instead, using commercial nanopowders as starting materials, solid-state sintering is a relatively simple method to fabricate transparent ceramics with large size, and short preparation period can be realized at the same time [25e27]. In this work, Co:MgAl2O4 transparent ceramics were prepared from the powder mixture of commercial spinel and CoO powders by pre-sintering in air with HIP post-treatment. The effect of presintering temperature on microstructure and optical quality of Co:MgAl2O4 ceramics were systematically studied. And the groundstate absorption cross section of the sample with the optimum optical quality was also discussed. 2. Experimental Commercial powders of MgAl2O4 (99.95%, S30CR, Baikowski, Charlotte, NC, France) and CoO (99.99%, Aladdin, Shanghai, China) were used as raw materials. The higher Co2þ content the lower was the dopant volumetric distribution uniformity [17], and 0.05 at.% was selected here to obtain a uniform structure. Two kinds of powders were mixed in ethanol and then ball-milled with high
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purity corundum. After ball milling for 12 h, the slurry was dried at 70 C for 2 h in an oven and then sieved through a 200-mesh screen. In order to remove the organic ingredients, the mixed powders were calcined at 600 C for 4 h in air. The calcined powders were uniaxially pressed into pellets at 20 MPa pressure followed by cold isostatic pressing (CIP) at 250 MPa. Subsequently, the compacted pellets were pre-sintered in air at 1500e1700 C for 5 h and then HIP post-treated at 1800 C for 3 h in Argon with a pressure of 200 MPa. Finally, the ceramic samples were polished on both sides and thermally etched at 1300 C for 1 h. Phase composition of the ceramics was identified by the X-ray diffraction (XRD, Model D/max 2200 PC, Rigaku, Japan) in the 2q ranged from 10 to 80 with 0.02 step size. The field emission scanning electron microscopy (FESEM, SU8220, Hitachi, Japan) was used to observe the mirror-polished and thermally etched surfaces of samples pre-sintered and HIP post-treated. The transmission and absorption spectra of ceramics pre-sintered and HIP post-treated were tested by a UV-VIS-NIR spectrophotometer (Model Cary5000, Varian, USA). Grain size of the specimens was measured by the linear intercept method and the average grain size was calculated through multiplying the average linear intercept distance by 1.56 [28].
3. Results and discussion Fig. 1 is the field-emission scanning electron microscopy (FESEM) micrographs of CoO, MgAl2O4 powders and the powder mixture after ball milling. As shown in Fig. 1(a) and (b), the starting particles are essentially agglomerated of finer crystallites and the average particle sizes (DSEM) of MgAl2O4, CoO powders are 49.8 and 57.6 nm, respectively. After ball milling for 12 h, large lump of aggregates have been cracked to finer particles and the raw materials are mixed uniformly, which is confirmed in Fig. 1(c). The specific surface area (SBET) of MgAl2O4 and CoO are 27.1 and 15.6 m2/g, respectively. The average particle size (DBET) of MgAl2O4 and CoO could be calculated by the following equation [29]:
Fig. 1. FESEM micrographs of the commercial powders (a) CoO, (b) MgAl2O4 and (c) powder mixture after ball milling.
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DBET ¼ 6=ðr $ SBET Þ
Fig. 2. XRD patterns of 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at different temperatures for 5 h.
(1)
where r is the theoretical density, which is 3.58 and 6.45 g/cm3 for MgAl2O4 and CoO, and the SBET is the specific surface area. The DBET of MgAl2O4 and CoO are 61.8 and 59.6 nm, which are a little bigger than DSEM, and this can attribute to agglomerations in the raw powders. Fig. 2 shows the X-ray diffraction (XRD) patterns of 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at 1500e1700 C. As can be seen, all the characteristic diffraction peaks of the samples can be well indexed as the cubic spinel structure (PDF#21-1152) and no detectable secondary phase is observed. The FESEM (field emission scanning electron microscopy) micrographs of the thermally etched surfaces of Co:MgAl2O4 ceramics pre-sintered at 1500e1700 C are shown in Fig. 3. Images show different gray-scale and we take it that the images gray-scale origins from the different orientation of each grain. The crystal face of each grain is random and the atom density of each crystal face is different, which will lead to the different probability of initial electron scattering and result in the different gray-scale of each grains. This situation was also appeared in other works [11]. It can
Fig. 3. FESEM images of the 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at (a) 1500 C, (b) 1550 C, (c) 1600 C, (d) 1650 C, (e) 1700 C for 5 h.
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Fig. 4. Relative density and average grain size of the 0.05 at.% Co:MgAl2O4 ceramics pre-sintered in air at different temperatures for 5 h as function of the pre-sintering temperature.
be seen that the ceramics show bimodal microstructural patterns, which is usually seen in the dense spinel ceramics [30,31].
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Meanwhile, there are some pores inside the grains or along the grain boundaries of the ceramics. These residual pores may result from the large pores between the agglomerate particles from the powder mixture and have the most adverse effects on ceramics, which are difficult to be removed during the sintering process [32]. In addition, the number of pores decreases gradually from 1500 to 1600 C, however more pores can be found in the Co:MgAl2O4 ceramic samples pre-sintered above 1600 C, which is due to the grain growth rate exceeds the removal rate of the pores at high sintering temperatures [33]. The relative density and average grain size of the Co:MgAl2O4 ceramics sintered in air for 5 h as a function of sintering temperature are shown in Fig. 4. It is observed that the pre-sintering temperature influences the grain size and relative density. The grain size increases with the pre-sintering temperature and the grain growth rate increases greatly at 1600 C, whereas the relative density of ceramic sample shows a peak value of 98.7% at 1650 C. Specifically, the average grain size of the sample increases from 4.6 to 19.2 mm when the pre-sintering temperature increases from 1600 to 1700 C. Fig. 5 demonstrates the FESEM (field emission scanning electron microscopy) micrographs of 0.05 at.% Co:MgAl2O4 ceramics presintered at 1500e1700 C and HIP post-treated at 1800 C for 3 h. It is observed that after HIP post-treatment, the grain size of
Fig. 5. FESEM images of the 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at (a) 1500 C, (b) 1550 C, (c) 1600 C, (d) 1650 C, (e) 1700 C for 5 h and HIP post-treated at 1800 C for 3 h.
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Fig. 6. Average grain size of the 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at different temperatures for 5 h and HIP post-treatment as function of the pre-sintering temperature.
specimens increases rapidly (shown in Fig. 6) and some grains grow obviously, resulting in pores to be occluded inside the ceramics at high temperature [34]. The ceramics pre-sintered at 1600 C shows the least pores and relatively homogenous grain size distribution. Fig. 6 presents the average grain size of the 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at different temperatures for 5 h and HIP post-treatment. As can be seen, in the temperature range of 1500e1650 C, the average grain size increases slowly while it rapidly increases to 38.8 mm when further increasing the pretemperature to 1700 C. Fig. 7 (a) and (b) present the photograph and near in-line
transmittance of the 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at different temperatures for 5 h and HIP post-treated at 1800 C for 3 h. As shown in Fig. 7 (a), the samples pre-sintered at 1500e1650 C exhibit good transparency as the letters under them can be seen clearly while the optical quality of ceramic sample presintered at 1700 C become worse. In addition, the homogeneity of ceramics pre-sintered at 1600 C for 5 h and HIP post-treated is relatively good. In Fig. 7(b), the near in-line transmittance of samples pre-sintered at 1550e1650 C exceed 70.0% at the wavelength range from 1700 to 2400 nm. The ceramic sample pre-sintered at 1600 C and HIP post-treated shows the highest transmittance of 76.3% at 1000 nm. However, the transmittance of ceramics gradually decreases when the temperature exceeds 1600 C, indicating that more defects act as the optical scattering centers and lead to more severe Mie scattering [35e37], which can be confirmed in Fig. 5(d) and (e). Additionally, the absorption peaks located at 500e700 nm and 1200e1600 nm exhibit characteristic absorption bands of Co2þ [38,39], which implying that the Co2þ has dissolved into the lattice of MgAl2O4. In detail, the FESEM (field emission scanning electron microscopy) absorption peaks at 500e700 nm are assigned to the 4A1(4F) /4T2(4P) transition and the broad absorption bands in the 1.2e1.7 mm range will be discussed in the following text. Although the transmittance is still lower than the theoretical value of spinel (87.0%) [40], it is believed that by optimizing the fabrication process, the transmittance can be greatly improved. Notably, an obvious absorption peak can be seen at 407 nm, and it may result from Co3þ dopants [41], which is undesirable for saturable absorber. Fig. 8 illustrates the absorption coefficient spectrum of the 0.05 at.% Co: MgAl2O4 ceramics pre-sintered at 1600 C for 5 h and HIP post-treated at 1800 C for 3 h. The broad absorption band in the 1.2e1.6 mm range is related to the transition from the 4A1(4F) ground state to the 4T2(4F) excited state [11], which is actually the band intended to be used for Q-switching of laser emitting. The broad band is composed of four separate peaks located at 1234, 1347, 1403, and 1527 nm, all relating to the spin-orbit splitting of the parent 4F ionic state (4F/4F9/2 þ4F7/2 þ 4F5/2 þ 4F3/2) [17]. And the ground state absorption cross section of the 0.05 at.% Co:MgAl2O4 ceramics can be estimated from the following equations (2) and (3):
Fig. 8. Absorption coefficient spectrum of 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at 1600 C for 5 h and HIP post-treated at 1800 C for 3 h. Fig. 7. Photograph (a) and the near in-line transmittance (b) of the 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at different temperatures for 5 h and HIP post-treated at 1800 C for 3 h.
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sgas ¼ N¼
aa
(2)
N
r NA M
CS
(3)
where sgas means ground-state absorption cross-section, aa stands for absorption coefficient, N is equivalent to Co2þ ions concentration, NA is Avogadro's number and M is relative molecular mass while CS is molar concentration of Co2þ in the ceramic. For the 0.05 at.% Co:MgAl2O4 ceramics, the value of a at 1540 nm is 2.20 cm1 and the sgas calculated from the above formulas is 2.9 1019 cm2, which is near to the sgas value of other work [11,17], indicating its promising application for passive Q-switching in the 1.3e1.7 mm domain. 4. Conclusion Using the mixture of commercial MgAl2O4 and CoO powders as the starting materials, we report on the fabrication of 0.05 at.% Co:MgAl2O4 transparent ceramics by pre-sintering and HIP posttreatment in this work. All the XRD characteristic diffraction peaks of the samples pre-sintered can be indexed as the cubic spinel structure. The grain size of ceramics pre-sintered at 1500e1700 C ranges from 3.2 to 19.2 nm and then increases to 20.6e38.6 nm after the HIP post-treatment. The near in-line transmittance of 0.05 at.% Co:MgAl2O4 transparent ceramics presintered at 1550e1650 C and HIP post-treated at 1800 C exceeds 70% at the wavelength range from 1700 to 2400 nm. The obtained 0.05 at.% Co:MgAl2O4 ceramics have the typical absorption band of Co2þ, and the ground state absorption cross section of the sample pre-sintered at 1600 C for 5 h and HIP post-treated at 1800 C for 3 h is 2.9 1019 cm2, which is similar to 2.55 1019 [11] and 2.9 1019 cm2 [17] in Luo and Goldstein's works. Moreover, Co:MgAl2O4 transparent ceramics are believed to be a good candidate as the effective saturable absorbers for the solid laser operating at 1.5 mm. Acknowledgements This work was supported by the National Key R&D Program of China (Grant No. 2017YFB0310500), the National Natural Science Foundation of China (Grant No. 61575212) and the key research project of the frontier science of the Chinese Academy of Sciences (No. QYZDB-SSW-JSC022). References [1] J. Mlynczak, N. Belghachem, K. Kopczynski, J. Kisielewski, R. Stepien, M. Wychowaniec, J. Galas, D. Litwin, A. Czyzewski, Performance analysis of thermally bonded Er3þ, Yb3þ: glass/Co2þ:MgAl2O4 microchip lasers, Opt. Quant. Electron. 48 (2016) 247. [2] K.V. Yumashev, Saturable absorber Co2þ:MgAl2O4 crystal for Q switching of 1.34-mm Nd3þ: YAlO3 and 1.54-mm Er3þ:glass lasers, Appl. Opt. 38 (1999) 6343e6346. [3] K.V. Yumashev, I.A. Denisov, N.N. Posnov, P.V. Prokoshin, V.P. Mikhailov, Nonlinear absorption properties of Co2þ:MgAl2O4 crystal, Appl. Phys. B Lasers Opt. 70 (2000) 179e184. [4] K.V. Yumashev, I.A. Denisov, N.N. Posnov, N.V. Kuleshov, R. Moncorge, Excited state absorption and passive Q-switch performance of Co2þ doped oxide crystals, J. Alloys Compd. 341 (2002) 366e370. [5] P.J. Deren, W. Strek, B. Jezowska-Trzebiatowska, I. Trabjerg, The optical spectra of Co2þ in MgAl2O4 spinel, J. Phys. IV Colloq. 1 (1991) 279e283. [6] F.J. Bergin, T.J. Glynn, F.G. Anderson, F.J. Bergin, G.F. Imbusch, Optical and magnetic circular dichroism optically detected magnetic resonance study of the Co2þ ion in LiGa5O8, Phys. Rev. B 45 (1992) 563e573. [7] T. Abritta, F.H. Blak, Luminescence study of ZnGa2O4:Co2þ, J. Lumin. 48e49 (1991) 558e560.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A simple way to prepare Co:MgAl2O4 transparent ceramics for saturable absorber Sha Su a, b, Qiang Liu b, Zewang Hu a, c, Xiaopu Chen a, c, Hongming Pan a, b, Xin Liu a, c, Lexiang Wu a, c, Jiang Li a, c, * a b c
Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China School of Material Science and Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 February 2019 Received in revised form 26 April 2019 Accepted 29 April 2019 Available online 1 May 2019
Co-doped magnesium aluminate (Co:MgAl2O4) transparent ceramics were obtained by pre-sintering in air and hot isostatic pressing (HIP) with commercial spinel and CoO powders as raw materials. The influences of the pre-sintering temperature on the microstructure and optical quality of the 0.05 at% Co:MgAl2O4 ceramics were studied. It is found that the relative density of Co:MgAl2O4 ceramics increases from 96.8% at 1500 C to 98.7% at 1600 C and then decreases with further increasing the pre-sintering temperature. The average grain size of Co:MgAl2O4 ceramics increase from 4.6 mm to 19.2 mm with the increase of pre-sintering temperature from 1500 to 1700 C. For the sample pre-sintered at 1600 C for 5 h and HIP post-treated at 1800 C for 3 h, the average grain size is about 23.0 mm and the near in-line transmittance reaches 75% at the wavelength range of 1700e2400 nm. The ground state absorption cross section at 1540 nm of the optimal 0.05 at% Co:MgAl2O4 ceramics is calculated to be 2.9 1019 cm2. © 2019 Published by Elsevier B.V.
Keywords: Co:MgAl2O4 Transparent ceramics Saturable absorbers Hot isostatic pressing
1. Introduction In recent years, 1.5 mm pulsed laser has been attached much attention due to safety for eyes, high transmission in the air atmosphere and fiber [1]. Passive Q-switching technology using saturable absorber is an effective way to achieve 1.5 mm pulsed laser, because it enables compact and low-cost sources of short pulses to be developed for various applications [2]. At present, the saturable absorbers doped with transition metal are efficient and widely utilized for the passive Q-switching solid-state lasers. And for some special applications, like eye safety field, Co2þ doped MgAl2O4 is a promising material. The cubic space group of spinel is O7h (Fd3m). The Mg2þ occupies tetrahedral coordination, and the Al3þ is in the octahedral environment [3]. Due to the same valence and the similar radii of Mg2þ and Co2þ (the radii of Mg2þ and Co2þ are 0.069 and 0.072 nm, respectively), it is easy for Co2þ to substitute the site of Mg2þ [4,5]. The strong and broad absorption bands of MgAl2O4 doped with
* Corresponding author. Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China. E-mail address: [email protected] (J. Li). https://doi.org/10.1016/j.jallcom.2019.04.322 0925-8388/© 2019 Published by Elsevier B.V.
Co2þ make Co:MgAl2O4 a promising candidate in solid-state lasers for visible and near infrared light [6e10]. So far, Co2þ doped MgAl2O4 crystals, glass-ceramics and transparent ceramics have been fabricated and applied successfully in lasers. However, the large size single crystals (up to several centimeters) with fine quality are difficult to produce due to the high melting point (more than 2100 C) [11], and the cost is also high. For the glass-ceramic, Co2þ may be accommodated in sites of the residual glass phase and cause undesirable absorption (owing to the presence of hexacoordinated Co2þ in glass) [12]. On the contrary, the cost of transparent ceramics is much lower than that of single crystals, and ceramics generally exhibit significantly higher resistance to laser damage, so much attention has been paid to ceramic materials [13,14]. In 2008, Co:MgAl2O4 transparent ceramics were firstly reported and the possible use in the passive laser Qswitching lasers was also suggested by Ikesue [15]. Wajler et al. obtained Co:MgAl2O4 transparent ceramics with the mixed powders of co-precipitated Co:MgAl2O4 powders and commercial high purity MgAl2O4 powders as the starting materials in 2014. The nonlinear absorption of Co:MgAl2O4 transparent ceramics was systematically demonstrated. However, some important information, such as transparency over the visible to mid infrared regions, was not given [16]. In 2016, Goldstein et al. fabricated highly
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transparent Co:MgAl2O4 ceramics for application in pulse laser [17]. However, the poor homogeneity and internal defects in most specimens resulted in the low laser damage threshold. With the mixed powders of the commercial spinel and the lab-made Co:MgAl2O4 powders as the raw materials, Luo et al. fabricated 0.05 at.% Co:MgAl2O4 transparent ceramics by hot pressing and 0.02 at.% Co:MgAl2O4 transparent ceramics by vacuum sintering combined with HIP post-treatment [11,18]. However, the optical transparency of the fabricated Co:MgAl2O4 ceramics in visible wavelength was not good enough. Therefore, further work on Co:MgAl2O4 transparent ceramics is still necessary. Generally, Co:MgAl2O4 transparent ceramics were fabricated using nanopowders obtained by chemical co-precipitation method [19e22], which is known to narrow the particle size distribution and control powders morphology. In our previous work, Co:MgAl2O4 transparent ceramics were fabricated in this way [23]. However, it is hard to control the dispersion-state and the particle size [24]. Instead, using commercial nanopowders as starting materials, solid-state sintering is a relatively simple method to fabricate transparent ceramics with large size, and short preparation period can be realized at the same time [25e27]. In this work, Co:MgAl2O4 transparent ceramics were prepared from the powder mixture of commercial spinel and CoO powders by pre-sintering in air with HIP post-treatment. The effect of presintering temperature on microstructure and optical quality of Co:MgAl2O4 ceramics were systematically studied. And the groundstate absorption cross section of the sample with the optimum optical quality was also discussed. 2. Experimental Commercial powders of MgAl2O4 (99.95%, S30CR, Baikowski, Charlotte, NC, France) and CoO (99.99%, Aladdin, Shanghai, China) were used as raw materials. The higher Co2þ content the lower was the dopant volumetric distribution uniformity [17], and 0.05 at.% was selected here to obtain a uniform structure. Two kinds of powders were mixed in ethanol and then ball-milled with high
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purity corundum. After ball milling for 12 h, the slurry was dried at 70 C for 2 h in an oven and then sieved through a 200-mesh screen. In order to remove the organic ingredients, the mixed powders were calcined at 600 C for 4 h in air. The calcined powders were uniaxially pressed into pellets at 20 MPa pressure followed by cold isostatic pressing (CIP) at 250 MPa. Subsequently, the compacted pellets were pre-sintered in air at 1500e1700 C for 5 h and then HIP post-treated at 1800 C for 3 h in Argon with a pressure of 200 MPa. Finally, the ceramic samples were polished on both sides and thermally etched at 1300 C for 1 h. Phase composition of the ceramics was identified by the X-ray diffraction (XRD, Model D/max 2200 PC, Rigaku, Japan) in the 2q ranged from 10 to 80 with 0.02 step size. The field emission scanning electron microscopy (FESEM, SU8220, Hitachi, Japan) was used to observe the mirror-polished and thermally etched surfaces of samples pre-sintered and HIP post-treated. The transmission and absorption spectra of ceramics pre-sintered and HIP post-treated were tested by a UV-VIS-NIR spectrophotometer (Model Cary5000, Varian, USA). Grain size of the specimens was measured by the linear intercept method and the average grain size was calculated through multiplying the average linear intercept distance by 1.56 [28].
3. Results and discussion Fig. 1 is the field-emission scanning electron microscopy (FESEM) micrographs of CoO, MgAl2O4 powders and the powder mixture after ball milling. As shown in Fig. 1(a) and (b), the starting particles are essentially agglomerated of finer crystallites and the average particle sizes (DSEM) of MgAl2O4, CoO powders are 49.8 and 57.6 nm, respectively. After ball milling for 12 h, large lump of aggregates have been cracked to finer particles and the raw materials are mixed uniformly, which is confirmed in Fig. 1(c). The specific surface area (SBET) of MgAl2O4 and CoO are 27.1 and 15.6 m2/g, respectively. The average particle size (DBET) of MgAl2O4 and CoO could be calculated by the following equation [29]:
Fig. 1. FESEM micrographs of the commercial powders (a) CoO, (b) MgAl2O4 and (c) powder mixture after ball milling.
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DBET ¼ 6=ðr $ SBET Þ
Fig. 2. XRD patterns of 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at different temperatures for 5 h.
(1)
where r is the theoretical density, which is 3.58 and 6.45 g/cm3 for MgAl2O4 and CoO, and the SBET is the specific surface area. The DBET of MgAl2O4 and CoO are 61.8 and 59.6 nm, which are a little bigger than DSEM, and this can attribute to agglomerations in the raw powders. Fig. 2 shows the X-ray diffraction (XRD) patterns of 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at 1500e1700 C. As can be seen, all the characteristic diffraction peaks of the samples can be well indexed as the cubic spinel structure (PDF#21-1152) and no detectable secondary phase is observed. The FESEM (field emission scanning electron microscopy) micrographs of the thermally etched surfaces of Co:MgAl2O4 ceramics pre-sintered at 1500e1700 C are shown in Fig. 3. Images show different gray-scale and we take it that the images gray-scale origins from the different orientation of each grain. The crystal face of each grain is random and the atom density of each crystal face is different, which will lead to the different probability of initial electron scattering and result in the different gray-scale of each grains. This situation was also appeared in other works [11]. It can
Fig. 3. FESEM images of the 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at (a) 1500 C, (b) 1550 C, (c) 1600 C, (d) 1650 C, (e) 1700 C for 5 h.
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Fig. 4. Relative density and average grain size of the 0.05 at.% Co:MgAl2O4 ceramics pre-sintered in air at different temperatures for 5 h as function of the pre-sintering temperature.
be seen that the ceramics show bimodal microstructural patterns, which is usually seen in the dense spinel ceramics [30,31].
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Meanwhile, there are some pores inside the grains or along the grain boundaries of the ceramics. These residual pores may result from the large pores between the agglomerate particles from the powder mixture and have the most adverse effects on ceramics, which are difficult to be removed during the sintering process [32]. In addition, the number of pores decreases gradually from 1500 to 1600 C, however more pores can be found in the Co:MgAl2O4 ceramic samples pre-sintered above 1600 C, which is due to the grain growth rate exceeds the removal rate of the pores at high sintering temperatures [33]. The relative density and average grain size of the Co:MgAl2O4 ceramics sintered in air for 5 h as a function of sintering temperature are shown in Fig. 4. It is observed that the pre-sintering temperature influences the grain size and relative density. The grain size increases with the pre-sintering temperature and the grain growth rate increases greatly at 1600 C, whereas the relative density of ceramic sample shows a peak value of 98.7% at 1650 C. Specifically, the average grain size of the sample increases from 4.6 to 19.2 mm when the pre-sintering temperature increases from 1600 to 1700 C. Fig. 5 demonstrates the FESEM (field emission scanning electron microscopy) micrographs of 0.05 at.% Co:MgAl2O4 ceramics presintered at 1500e1700 C and HIP post-treated at 1800 C for 3 h. It is observed that after HIP post-treatment, the grain size of
Fig. 5. FESEM images of the 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at (a) 1500 C, (b) 1550 C, (c) 1600 C, (d) 1650 C, (e) 1700 C for 5 h and HIP post-treated at 1800 C for 3 h.
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Fig. 6. Average grain size of the 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at different temperatures for 5 h and HIP post-treatment as function of the pre-sintering temperature.
specimens increases rapidly (shown in Fig. 6) and some grains grow obviously, resulting in pores to be occluded inside the ceramics at high temperature [34]. The ceramics pre-sintered at 1600 C shows the least pores and relatively homogenous grain size distribution. Fig. 6 presents the average grain size of the 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at different temperatures for 5 h and HIP post-treatment. As can be seen, in the temperature range of 1500e1650 C, the average grain size increases slowly while it rapidly increases to 38.8 mm when further increasing the pretemperature to 1700 C. Fig. 7 (a) and (b) present the photograph and near in-line
transmittance of the 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at different temperatures for 5 h and HIP post-treated at 1800 C for 3 h. As shown in Fig. 7 (a), the samples pre-sintered at 1500e1650 C exhibit good transparency as the letters under them can be seen clearly while the optical quality of ceramic sample presintered at 1700 C become worse. In addition, the homogeneity of ceramics pre-sintered at 1600 C for 5 h and HIP post-treated is relatively good. In Fig. 7(b), the near in-line transmittance of samples pre-sintered at 1550e1650 C exceed 70.0% at the wavelength range from 1700 to 2400 nm. The ceramic sample pre-sintered at 1600 C and HIP post-treated shows the highest transmittance of 76.3% at 1000 nm. However, the transmittance of ceramics gradually decreases when the temperature exceeds 1600 C, indicating that more defects act as the optical scattering centers and lead to more severe Mie scattering [35e37], which can be confirmed in Fig. 5(d) and (e). Additionally, the absorption peaks located at 500e700 nm and 1200e1600 nm exhibit characteristic absorption bands of Co2þ [38,39], which implying that the Co2þ has dissolved into the lattice of MgAl2O4. In detail, the FESEM (field emission scanning electron microscopy) absorption peaks at 500e700 nm are assigned to the 4A1(4F) /4T2(4P) transition and the broad absorption bands in the 1.2e1.7 mm range will be discussed in the following text. Although the transmittance is still lower than the theoretical value of spinel (87.0%) [40], it is believed that by optimizing the fabrication process, the transmittance can be greatly improved. Notably, an obvious absorption peak can be seen at 407 nm, and it may result from Co3þ dopants [41], which is undesirable for saturable absorber. Fig. 8 illustrates the absorption coefficient spectrum of the 0.05 at.% Co: MgAl2O4 ceramics pre-sintered at 1600 C for 5 h and HIP post-treated at 1800 C for 3 h. The broad absorption band in the 1.2e1.6 mm range is related to the transition from the 4A1(4F) ground state to the 4T2(4F) excited state [11], which is actually the band intended to be used for Q-switching of laser emitting. The broad band is composed of four separate peaks located at 1234, 1347, 1403, and 1527 nm, all relating to the spin-orbit splitting of the parent 4F ionic state (4F/4F9/2 þ4F7/2 þ 4F5/2 þ 4F3/2) [17]. And the ground state absorption cross section of the 0.05 at.% Co:MgAl2O4 ceramics can be estimated from the following equations (2) and (3):
Fig. 8. Absorption coefficient spectrum of 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at 1600 C for 5 h and HIP post-treated at 1800 C for 3 h. Fig. 7. Photograph (a) and the near in-line transmittance (b) of the 0.05 at.% Co:MgAl2O4 ceramics pre-sintered at different temperatures for 5 h and HIP post-treated at 1800 C for 3 h.
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sgas ¼ N¼
aa
(2)
N
r NA M
CS
(3)
where sgas means ground-state absorption cross-section, aa stands for absorption coefficient, N is equivalent to Co2þ ions concentration, NA is Avogadro's number and M is relative molecular mass while CS is molar concentration of Co2þ in the ceramic. For the 0.05 at.% Co:MgAl2O4 ceramics, the value of a at 1540 nm is 2.20 cm1 and the sgas calculated from the above formulas is 2.9 1019 cm2, which is near to the sgas value of other work [11,17], indicating its promising application for passive Q-switching in the 1.3e1.7 mm domain. 4. Conclusion Using the mixture of commercial MgAl2O4 and CoO powders as the starting materials, we report on the fabrication of 0.05 at.% Co:MgAl2O4 transparent ceramics by pre-sintering and HIP posttreatment in this work. All the XRD characteristic diffraction peaks of the samples pre-sintered can be indexed as the cubic spinel structure. The grain size of ceramics pre-sintered at 1500e1700 C ranges from 3.2 to 19.2 nm and then increases to 20.6e38.6 nm after the HIP post-treatment. The near in-line transmittance of 0.05 at.% Co:MgAl2O4 transparent ceramics presintered at 1550e1650 C and HIP post-treated at 1800 C exceeds 70% at the wavelength range from 1700 to 2400 nm. The obtained 0.05 at.% Co:MgAl2O4 ceramics have the typical absorption band of Co2þ, and the ground state absorption cross section of the sample pre-sintered at 1600 C for 5 h and HIP post-treated at 1800 C for 3 h is 2.9 1019 cm2, which is similar to 2.55 1019 [11] and 2.9 1019 cm2 [17] in Luo and Goldstein's works. Moreover, Co:MgAl2O4 transparent ceramics are believed to be a good candidate as the effective saturable absorbers for the solid laser operating at 1.5 mm. Acknowledgements This work was supported by the National Key R&D Program of China (Grant No. 2017YFB0310500), the National Natural Science Foundation of China (Grant No. 61575212) and the key research project of the frontier science of the Chinese Academy of Sciences (No. QYZDB-SSW-JSC022). References [1] J. Mlynczak, N. Belghachem, K. Kopczynski, J. Kisielewski, R. Stepien, M. Wychowaniec, J. Galas, D. Litwin, A. Czyzewski, Performance analysis of thermally bonded Er3þ, Yb3þ: glass/Co2þ:MgAl2O4 microchip lasers, Opt. Quant. Electron. 48 (2016) 247. [2] K.V. Yumashev, Saturable absorber Co2þ:MgAl2O4 crystal for Q switching of 1.34-mm Nd3þ: YAlO3 and 1.54-mm Er3þ:glass lasers, Appl. Opt. 38 (1999) 6343e6346. [3] K.V. Yumashev, I.A. Denisov, N.N. Posnov, P.V. Prokoshin, V.P. Mikhailov, Nonlinear absorption properties of Co2þ:MgAl2O4 crystal, Appl. Phys. B Lasers Opt. 70 (2000) 179e184. [4] K.V. Yumashev, I.A. Denisov, N.N. Posnov, N.V. Kuleshov, R. Moncorge, Excited state absorption and passive Q-switch performance of Co2þ doped oxide crystals, J. Alloys Compd. 341 (2002) 366e370. [5] P.J. Deren, W. Strek, B. Jezowska-Trzebiatowska, I. Trabjerg, The optical spectra of Co2þ in MgAl2O4 spinel, J. Phys. IV Colloq. 1 (1991) 279e283. [6] F.J. Bergin, T.J. Glynn, F.G. Anderson, F.J. Bergin, G.F. Imbusch, Optical and magnetic circular dichroism optically detected magnetic resonance study of the Co2þ ion in LiGa5O8, Phys. Rev. B 45 (1992) 563e573. [7] T. Abritta, F.H. Blak, Luminescence study of ZnGa2O4:Co2þ, J. Lumin. 48e49 (1991) 558e560.
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