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Fabrication and spectroscopic properties of Co:MgAl2O4 transparent ceramics by the HIP post-treatment
Optical Materials 69 (2017) 152e157
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Fabrication and spectroscopic properties of Co:MgAl2O4 transparent ceramics by the HIP post-treatment Wei Luo a, b, c, Peng Ma a, Tengfei Xie a, Jiawei Dai a, b, Yubai Pan d, Huamin Kou a, Jiang Li a, * a
Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China University of Chinese Academy of Sciences, Beijing 100049, China Department of Material Science and Technology, Luoyang Institute of Technology, Luoyang, Henan 471023, China d Department of Physics, Shanghai Normal University, Shanghai 200234, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 February 2017 Received in revised form 21 March 2017 Accepted 23 March 2017
Cobalt-doped magnesium aluminate spinel (Co:MgAl2O4) is one of the most important saturable absorbers for the passive Q-switching of solid-state lasers operating at eye-safe wavelength of 1.5 mm. In this work, highly transparent Co:MgAl2O4 ceramics were fabricated by vacuum sintering combined with hot isostatic pressing (HIP) post-treatment, using the mixture of the commercial spinel and the lab-made Co:MgAl2O4 powder as the raw materials. The densification mechanism of Co:MgAl2O4 transparent ceramics was discussed. The microstructure and optical properties of the samples were investigated. The ground state absorption cross section (sGSA) was calculated from the fitted curve of the absorption coefficient spectrum. The results show that Co:MgAl2O4 ceramics fabricated by vacuum sintering at 1500 C for 5 h and then HIP post-treatment at 1650 C for 3 h perform good transparency, whose in-line transmittance exceeds 80% at 2500 nm. Moreover, the ground state absorption cross section of 0.02 at.% Co:MgAl2O4 ceramics is calculated to be 3.35 1019 cm2 at the wavelength of 1540 nm, which is promising for the application to the passive Q-switching of solid-state laser operating in the near infrared region (NIR). © 2017 Published by Elsevier B.V.
Keywords: Co:MgAl2O4 Saturable absorber Transparent ceramics HIP post-treatment
1. Introduction Co:MgAl2O4 is an important and effective saturable absorber (SA) for the passive Q-switching solid-state lasers operating at the near infrared region, such as 1.34 mm Nd:YAlO3 lasers, and 1.54 mm Er3þ glass lasers [1e7]. Especially, in the Er3þ/Yb3þ microchip glass lasers with the eye-safe wavelength of 1.5 mm, the highest peak power generated by the microchip laser was 7.68 KW with the pulse width of 2.9 ns, and the pulse energy was up to 22 mJ [8]. Until recently, only single crystal produced by Verneuil-method or glassceramic SA doped with Co:MgAl2O4 has been employed in the majority of researches. However, the spinel single crystal with fine quality and large dimension is difficult to produce and has the high cost due to its high melting point (about 2150 C). Meanwhile, the thermal conductivity of glass ceramics is rather low, and the laser damage threshold of glass-ceramic SA is quite low because of the
* Corresponding author. E-mail address: [email protected] (J. Li). http://dx.doi.org/10.1016/j.optmat.2017.03.036 0925-3467/© 2017 Published by Elsevier B.V.
thermal effect. On the other hand, the ceramic SA can be produced with lower cost than the single crystal and shows significantly higher laser damage threshold than glass-ceramic SA. Therefore, it is believed that Co:MgAl2O4 transparent ceramics for saturable absorber can overcome the drawbacks discussed above. In the pioneering works, Iksue first claimed the fabrication of the Co:MgAl2O4 transparent ceramics [9]. In his work, Co:MgAl2O4 ceramics exhibited a characteristic absorption of tetradentate Co2þ, indicating possible application for the passive Q-switching of the laser cavity. However, further researches are scarce. Recently, Wajler et al. reported that Co:MgAl2O4 ceramics with sub micrometer grain were fabricated by spark plasma sintering (SPS) [10]. The relevant non-linear optical parameters were estimated. However, in this work, the in-line transmittance of Co:MgAl2O4 ceramics was not reported, and the inefficiency equipment of SPS limited the application of industrial production. Goldstein et al. had fabricated Co:MgAl2O4 transparent ceramics by pre-sintering in air combined with HIP post treatment [11]. The main spectral properties were tested, the parameters exhibited suitable values for the
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NIR laser passive Q-switch application. However, the optical properties of Co:MgAl2O4 ceramics doped with different amounts of Co2þ were instable, and the microstructure of the ceramics were inhomogeneous. It suggested that further improvement and research of the optical transmittance was necessary for the Co:MgAl2O4 ceramics. Note-worthily, the technology of pre-sintering combined with HIP post treatment was the most promising method to produce highly transparent MgAl2O4 ceramics [12e14]. Krell et al. fabricated pure MgAl2O4 ceramics using commercial spinel powder as raw material by this technology. Highly transparent spinel ceramics with fine grain size and good mechanical performance were successfully fabricated [15e17]. The in-line transmittance of 4 mm thick ceramics exceeded 82%, and the optical loss factor was lower to 0.05/cm. In order to obtain highly transparent Co:MgAl2O4 ceramics, a deep understanding of correlation between Co:MgAl2O4 ceramics microstructure and optical properties was necessary. In this work, highly transparent Co:MgAl2O4 ceramics were fabricated by vacuum pre-sintering combined with HIP post treatment. The densification of Co:MgAl2O4 transparent ceramics was investigated, and the microstructure and optical properties of the ceramics were studied. The ground state absorption cross section of Co:MgAl2O4 transparent ceramics was also discussed. 2. Experimental procedure In present work, 0.02 at.% Co:MgAl2O4 powder was prepared by a mixture of pure spinel powder and lab-made Cobalt doped spinel powder which was synthesized by reverse dropping coprecipitation. The pure spinel powder was commercial available high purity Magnesium Aluminate Spinel powder (Baikalox S30CR, Baikowski, Charlotte, NC, USA). The purity of the commercial spinel powder was 99.95%, and the powder had a specific surface area of 26 m2/g. Lab-made Co:MgAl2O4 powder was synthesized by the aluminum, magnesium and cobalt nitrates (99% purity, Sinopharm Chemical Reagent Co. Ltd., ShangHai, China) dropped to ammonium carbonate solution (99% purity, Sinopharm Chemical Reagent Co. Ltd., ShangHai, China). The detail procedure was described in previous studies [18e21]. Firstly, the aqueous solutions of 0.20 mol aluminum nitrate, 0.9995 mol magnesium nitrates and 0.0005 mol cobalt nitrates were prepared. 400 ml of both solutions were prepared. Then the mixture solution was titrated to 800 ml of 1.5 mol/L ammonium carbonate solution with 4 ml/min speed, and the ammonium carbonate solution was added with 25 ml ammonium hydroxide to adjust PH value. After the precipitation, the suspension was aged for 24 h. The resultant slurry was washed twice by deionized water, rinsed twice with ethanol. Then the precursor powder was dried at 70 C and sieved. Finally, the dried precursor
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powder was calcined at 1100 C for 4 h, and lab-made 0.5 at.% Co:MgAl2O4 powder was obtained. After that, the two kinds of powders described above were mixed into 0.02 at.% Co:MgAl2O4 powder, then the mixture was milled on a planetary ball mill for 12 h with 10 mm-diameter alumina balls in 99.99% ethanol. The rotation speed was 130 rpm/ min. After that, the mixture was dried and sieved through a 200mesh screen, and uniaxial pressed into pellets in a F 20 mm stainless steel mold under 60 MPa, following by cold isotactic pressed in 250 MPa for 1 min. Finally, all the compacted green bodies were vacuum pre-sintered at a series of temperatures (1450 C, 1500 C, 1550 C, 1600 C, 1650 C, 1700 C) for 5 h following by HIP post treatment at 1600e1700 C for 3 h in 200 MPa Ar atmospheres. By the procedure, the flake-like ceramics were prepared, and the samples were ground and polished to 2 mm-thick disks with different grades of the diamond pastes. The density of the ceramics was measured by the Archimedes method using deionized water as the immersion medium. The inline transmittance and the absorption spectrum of 2 mm-thick Co:MgAl2O4 ceramic samples were tested with a UVeViseNIR spectrophotometer (Cary 5000 spectrophotometer, Varian Inc., USA) as a function of wavelength. Microstructures of the polished and thermally etched surfaces of the ceramics were observed with a field emission scanning electron microscope (FESEM, SU8220, Hitachi, Japan). The polished ceramics were etched at 1300 C for 1 h to reveal the grain boundaries. In addition, the average grain size was determined by the multiplying the average linear intercept distance by 1.56. 3. Results and discussion Fig. 1 shows morphologies of the commercial pure spinel powder and the Co:MgAl2O4 powder synthesized by coprecipitation method. Both powders present quasi-spherical particle microstructure. The specific surface area of the commercial pure spinel powder is 26 m2/g, and the particle size calculated from Formula (1) [22] is 64 nm.
d¼
6
r$S
1000
(1)
where d stands the calculated particle size with unit of nm, r is the theory density of spinel, which is 3.58 g/cm3, and the S is the specific surface area value. The average particle size of the commercial spinel powder observed from SEM is 48 nm, which is just a litter smaller than the calculated grain size from specific surface area due to weak agglomeration. Meanwhile, lab-made Co:MgAl2O4 powder performs a similar microstructure with pure spinel powder, which is
Fig. 1. SEM micrographs of (a) the commercial spinel powder and (b) the lab-made Co:MgAl2O4 powder.
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benefit for homogeneity of the powder mixture. The specific surface area of Co:MgAl2O4 powder is 32 m2/g, and the calculated particle size is 52 nm. The average particle size is 39 nm observed from the SEM photograph. The agglomeration of powder is also slight. It indicates that both powders have good sintering activity. Fig. 2 is the density curves of Co:MgAl2O4 ceramics fabricated by vacuum pre-sintering at the temperatures range from 1450 C to 1700 C before and after HIP post-treatment, and the HIP posttreatment is implemented at the temperatures of 1600 C, 1650 C and 1700 C for 3 h in 200 MPa Ar atmosphere. As shown in the figure, the densification of the vacuum pre-sintering Co:MgAl2O4 ceramics is enhanced with the increase of the temperature, and the relative density increase from 93% to 96% of the theoretical value (3.58 g/cm3). For the Co:MgAl2O4 transparent ceramics, the densification degree is not enough, because there are many scattering centers in the ceramics. When the pre-sintering temperatures are range from 1500 C to 1700 C, the relative densities of the samples exceed 95.5%. It indicates that there are still few close pores in Co:MgAl2O4 ceramics, which can be removed by HIP post
Fig. 2. Density curves of the Co:MgAl2O4 ceramics fabricated by different HIP posttreatments.
treatment [23]. The relative densities of all ceramics treated by HIP post treatment exceed 98.3%, which supports the above assumption. When the post-treatment temperature is higher than 1500 C, the density of the Co:MgAl2O4 ceramics is enhanced remarkably, and approaches to the theoretical value of Co:MgAl2O4. It indicates that the pores formed in the pre-sintering of Co:MgAl2O4 ceramics are eliminated by the effect of HIP post-treatment, which is favorable to improve the optical transmittance of Co:MgAl2O4 transparent ceramics. SEM micrographs of the Co:MgAl2O4 ceramics pre-sintering at different temperatures for 5 h under the vacuum ambience are presented in Fig. 3. It can be seen that the observed bimodal microstructural pattern is similar to what is usually seen in the dense spinel ceramics sintered at the 1500e1600 C temperatures range [24,25], and the grain size of the pre-sintered ceramics increases with the increase of the pre-sintering temperature. When the temperature is higher than 1550 C, the crystal grain size increases from ~1 mm to 10e20 mm, significantly. It indicates that grain growth rate increases when the temperature is up to ~1550 C. In the previous works, during the pressure-less sintering of spinel, the abnormal grain growth becomes observable when the grain size exceeds the critical value [26,27], and the growth rate of grain size is variation owing to enhancement of abnormal grain growth because of the grain boundary separation. For the spinel ceramics, the critical value is about 10e20 mm [26]. In our study, the existence of abnormal grain is in accordance with the results as reported. However, the presence of abnormal grain would be harmful to the optical homogeneity for Co:MgAl2O4 transparent ceramics. In addition, there are a few of the pores and second phases captured at the interior of the grain and between the grain boundaries, which can be removed by the HIP post-treatment. All the Co:MgAl2O4 ceramics pre-sintered without HIP posttreatment are opaque, because the relative densities of the Co:MgAl2O4 ceramics sintered in vacuum ambience are less than 96%, indicating that there are many scattering centers in the samples. However, the Co:MgAl2O4 ceramics pre-sintered at the temperature of higher than 1500 C become translucent after the HIP posttreatment. Because the HIP post-treatment promotes the densification of Co:MgAl2O4 ceramics, and the samples reach fully dense. As shown in Fig. 4, the transmission curves of the Co:MgAl2O4 ceramics with the thickness of 2 mm after the HIP post-treatment at
Fig. 3. SEM micrographs of the Co:MgAl2O4 ceramics pre-sintered at different temperatures for 5 h (a) 1450 C; (b) 1500 C; (c) 1550 C; (d) 1600 C; (e) 1650 C; (f) 1700 C.
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different temperatures are present. After the HIP post-treatment, the in-line transmittance of the sample pre-sintered at 1450 C is almost zero in whole wavelength range. However, the Co:MgAl2O4 ceramics with the pre-sintering temperatures higher than 1450 C have the optical transmittance. The in-line transmittance of the Co:MgAl2O4 ceramics pre-sintered at 1500 C exceeds 70% in the wavelength range from 1000 to 2500 nm, and the best transmittance exceeds 80% at 2500 nm. Meanwhile, the transparency of the Co:MgAl2O4 ceramics after the HIP post-treatment decreases with the decrease of the wavelength. After subtracting the Fresnel reflection loss, the theoretical transmittance of MgAl2O4 ceramics is about 87% [17]. The optical transmission loss of the Co:MgAl2O4 ceramics can be explained by light scattering derived from Mie regime of scattering center [28]. Moreover, it can be seen that the optical transmittance of the Co:MgAl2O4 ceramics is not sensitive to the temperatures of HIP post-treatment, the transmission curves of the samples treated at different temperatures perform the similar features. With the increase of pre-sintering temperature, the in-line transmittance of the Co:MgAl2O4 ceramics decreases. The results will be discussed in detail based on the microstructures of the samples. The structure details of transmission curves are similar with the crystal samples doped Co2þ ion [1,4]. The absorption spectra peaks
show typical for Co:MgAl2O4 that are assigned to the energy levels transition of the tetrahedral coordinated Co2þ ion [29,30]. In detail, the absorption peak at 587 nm is assigned to the 4A2(4F) / 4T1(4P) transition. The broad absorption band in the 1.2e1.6 mm range can be ascribed to the transition from the 4A2(4F9/2) ground state to the 4 T1(4F) excited state. It indicates that Co:MgAl2O4 ceramics can be used for Q-switching of lasers operating in the same range. The absorption band composes of four separate peaks located at about 1230, 1347, 1403, and 1527 nm, all related to the spin-orbit splitting of the 4F state (4F / 4F9/2 þ 4F7/2 þ 4F5/2 þ 4F3/2 in an increasing energy order). The absorption peaks at 547, 582, and 626 nm relate to excitation to the 4T1(4P) excited multiplet. The structure is due to the spin-orbit splitting of the 4P state (4P / 4P5/2 þ 4P3/2 þ 4P1/2). In addition, the UV absorption edge of Co:MgAl2O4 ceramics locates at ~200 nm. Fig. 5 shows the micrographs of the Co:MgAl2O4 ceramics presintered at different temperatures for 5 h and HIP post-treated at 1600 C for 3 h. It can be seen that after the HIP post-treatment, the crystal grains are coarsen, especially for the samples pre-sintered at the temperature above 1600 C, the grains grow significantly. The abnormal grains become much more and bigger with the increase of the pre-sintering temperature. And the average grain sizes of the Co:MgAl2O4 ceramics before and after the HIP post-treatment at 1600 C for 3 h with different vacuum sintering temperatures are shown in Fig. 6. The average grain size of the samples pre-sintered at 1450 Ce1550 C increases from 3 to 4 mm to about 8 mm, and a few of abnormal grains occur. It demonstrates that the HIP posttreatment has a limited impact on grain growth whereas it improves significantly the densification process of the Co:MgAl2O4 ceramics pre-sintered at the low temperatures [31]. As discussed above, the density of Co:MgAl2O4 ceramics changes slightly when the pre-sintering temperature is higher than 1550 C. However, the grains of the samples grow rapidly, and the abnormal grains become much larger when the pre-sintering temperature is higher than 1600 C. It will cause the pores to be occluded inside the ceramics [26], and the properties of Co:MgAl2O4 transparent ceramics deteriorate. So, the density of the Co:MgAl2O4 ceramics presintered at 1700 C is smaller than that of the samples presintered at 1650 C after the HIP post-treatment, and the transparency of the samples also worsens, which supports these claims as well. Fig. 7 illustrates the absorption coefficient spectrum fitted by the optical transmittance of 0.02 at.% Co:MgAl2O4 ceramic presintered at 1500 C for 5 h and HIP post-treated at 1650 C for 3 h under 200 MPa Ar atmosphere and the energy level schematic of Td-site Co2þ ion. The broad absorption band range from 1200 to 1600 nm is attributed to 4A2(4F) / 4T1(4F) multiplet transitions. And the ground state absorption cross section of the Co:MgAl2O4 ceramics can be estimated from the following equations:
sgas ¼ N¼
Fig. 4. In-line transmittance of the Co:MgAl2O4 ceramics (2 mm thickness) presintered at different temperatures for 5 h and HIP post-treated at (a) 1600 C and (b) 1650 C for 3 h.
155
aa
(2)
N
r NA M
Cs
(3)
where sgas stands the ground state absorption cross section, aa stands the absorption coefficient, N is the concentration of Co2þ ions, NA is Avogadro's number and M is atomic molar mass, r is the density of the Co:MgAl2O4 ceramics and Cs is the molar concentration of Co2þ ions in the ceramic samples. For the 0.02 at.% Co:MgAl2O4 ceramic, the absorption coefficient aa is 0.76 cm1 at the wavelength of 1540 nm as shown in Fig. 7. So the ground state absorption cross section of Co:MgAl2O4 ceramics at 1540 nm can be calculated to be 3.35 1019 cm2, which is in agreement with the
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Fig. 5. SEM micrographs of Co:MgAl2O4 ceramics pre-sintered at different temperatures for 5 h and HIP post-treated at 1600 C for 3 h (a) 1450 C; (b) 1500 C; (c) 1550 C; (d) 1600 C; (e) 1650 C; (f) 1700 C.
sgas value of (3.5 ± 0.6) 1019 cm2 for the 0.8 at.% Co2þ in MgAl2O4 single crystal [1]. And the difference is caused by the experimental uncertainty, for example, the accurate molar concentration of Co2þ ions is difficult to be determined. Moreover, the ground state absorption cross section of Co:MgAl2O4 ceramics varies with the different sintering methods. Compared with the result of our work, the ground state absorption cross-section of the Co:MgAl2O4 ceramics produced by spark plasma sintering (SPS) is 2.6 1019 cm2 [10], and the value of the Co:MgAl2O4 ceramics obtained by the combination of pre-sintering in air with HIP post-treatment is 2.9 1019 cm2 [11]. It indicates that HIP post-treatment is an effective technology to produce Co:MgAl2O4 transparent ceramics for saturable absorber. 4. Conclusions
Fig. 6. The average grain sizes of the Co:MgAl2O4 ceramics before and after HIP posttreatment at 1600 C for 3 h with different vacuum sintering temperatures.
In this study, fully dense and highly transparent Co:MgAl2O4 ceramics were successfully fabricated by the vacuum pre-sintering combined with HIP post-treatment. The optical transmittance of Co:MgAl2O4 transparent ceramics pre-sintered at 1500 C and HIP
Fig. 7. Absorption coefficient spectra of Co:MgAl2O4 ceramic pre-sintered at 1500 C for 5 h and HIP post-treated at 1650 C for 3 h and the energy level schematic of Td-site Co2þ ion.
W. Luo et al. / Optical Materials 69 (2017) 152e157
post-treated at 1600 C and 1650 C exceeds 70% at the wavelength range from 1000 to 2500 nm. The optical properties of ceramics reduce remarkably with increase of the pre-sintering temperature, because the grain size coarsens rapidly and the abnormal grains are present at the high sintering temperature. 0.02 at.% Co:MgAl2O4 ceramics have the typical absorption band of Td-site Co2þ, and the ground state absorption cross section at the wavelength of 1540 nm is calculated to be 3.35 1019 cm2. It is predictable that Co:MgAl2O4 transparent ceramics are the good candidate for the effective saturable absorber for the solid laser operating at 1.5 mm.
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Acknowledgments [16]
This work was supported by the National Natural Science Foundation of China (Grant No. 61575212), the Key research project of the frontier science of the Chinese Academy of Sciences (Grant No. QYZDB-SSW-JSC022) and the open project of the Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences (Grant No. FCLT201504). References
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Fabrication and spectroscopic properties of Co:MgAl2O4 transparent ceramics by the HIP post-treatment Wei Luo a, b, c, Peng Ma a, Tengfei Xie a, Jiawei Dai a, b, Yubai Pan d, Huamin Kou a, Jiang Li a, * a
Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China University of Chinese Academy of Sciences, Beijing 100049, China Department of Material Science and Technology, Luoyang Institute of Technology, Luoyang, Henan 471023, China d Department of Physics, Shanghai Normal University, Shanghai 200234, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 February 2017 Received in revised form 21 March 2017 Accepted 23 March 2017
Cobalt-doped magnesium aluminate spinel (Co:MgAl2O4) is one of the most important saturable absorbers for the passive Q-switching of solid-state lasers operating at eye-safe wavelength of 1.5 mm. In this work, highly transparent Co:MgAl2O4 ceramics were fabricated by vacuum sintering combined with hot isostatic pressing (HIP) post-treatment, using the mixture of the commercial spinel and the lab-made Co:MgAl2O4 powder as the raw materials. The densification mechanism of Co:MgAl2O4 transparent ceramics was discussed. The microstructure and optical properties of the samples were investigated. The ground state absorption cross section (sGSA) was calculated from the fitted curve of the absorption coefficient spectrum. The results show that Co:MgAl2O4 ceramics fabricated by vacuum sintering at 1500 C for 5 h and then HIP post-treatment at 1650 C for 3 h perform good transparency, whose in-line transmittance exceeds 80% at 2500 nm. Moreover, the ground state absorption cross section of 0.02 at.% Co:MgAl2O4 ceramics is calculated to be 3.35 1019 cm2 at the wavelength of 1540 nm, which is promising for the application to the passive Q-switching of solid-state laser operating in the near infrared region (NIR). © 2017 Published by Elsevier B.V.
Keywords: Co:MgAl2O4 Saturable absorber Transparent ceramics HIP post-treatment
1. Introduction Co:MgAl2O4 is an important and effective saturable absorber (SA) for the passive Q-switching solid-state lasers operating at the near infrared region, such as 1.34 mm Nd:YAlO3 lasers, and 1.54 mm Er3þ glass lasers [1e7]. Especially, in the Er3þ/Yb3þ microchip glass lasers with the eye-safe wavelength of 1.5 mm, the highest peak power generated by the microchip laser was 7.68 KW with the pulse width of 2.9 ns, and the pulse energy was up to 22 mJ [8]. Until recently, only single crystal produced by Verneuil-method or glassceramic SA doped with Co:MgAl2O4 has been employed in the majority of researches. However, the spinel single crystal with fine quality and large dimension is difficult to produce and has the high cost due to its high melting point (about 2150 C). Meanwhile, the thermal conductivity of glass ceramics is rather low, and the laser damage threshold of glass-ceramic SA is quite low because of the
* Corresponding author. E-mail address: [email protected] (J. Li). http://dx.doi.org/10.1016/j.optmat.2017.03.036 0925-3467/© 2017 Published by Elsevier B.V.
thermal effect. On the other hand, the ceramic SA can be produced with lower cost than the single crystal and shows significantly higher laser damage threshold than glass-ceramic SA. Therefore, it is believed that Co:MgAl2O4 transparent ceramics for saturable absorber can overcome the drawbacks discussed above. In the pioneering works, Iksue first claimed the fabrication of the Co:MgAl2O4 transparent ceramics [9]. In his work, Co:MgAl2O4 ceramics exhibited a characteristic absorption of tetradentate Co2þ, indicating possible application for the passive Q-switching of the laser cavity. However, further researches are scarce. Recently, Wajler et al. reported that Co:MgAl2O4 ceramics with sub micrometer grain were fabricated by spark plasma sintering (SPS) [10]. The relevant non-linear optical parameters were estimated. However, in this work, the in-line transmittance of Co:MgAl2O4 ceramics was not reported, and the inefficiency equipment of SPS limited the application of industrial production. Goldstein et al. had fabricated Co:MgAl2O4 transparent ceramics by pre-sintering in air combined with HIP post treatment [11]. The main spectral properties were tested, the parameters exhibited suitable values for the
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NIR laser passive Q-switch application. However, the optical properties of Co:MgAl2O4 ceramics doped with different amounts of Co2þ were instable, and the microstructure of the ceramics were inhomogeneous. It suggested that further improvement and research of the optical transmittance was necessary for the Co:MgAl2O4 ceramics. Note-worthily, the technology of pre-sintering combined with HIP post treatment was the most promising method to produce highly transparent MgAl2O4 ceramics [12e14]. Krell et al. fabricated pure MgAl2O4 ceramics using commercial spinel powder as raw material by this technology. Highly transparent spinel ceramics with fine grain size and good mechanical performance were successfully fabricated [15e17]. The in-line transmittance of 4 mm thick ceramics exceeded 82%, and the optical loss factor was lower to 0.05/cm. In order to obtain highly transparent Co:MgAl2O4 ceramics, a deep understanding of correlation between Co:MgAl2O4 ceramics microstructure and optical properties was necessary. In this work, highly transparent Co:MgAl2O4 ceramics were fabricated by vacuum pre-sintering combined with HIP post treatment. The densification of Co:MgAl2O4 transparent ceramics was investigated, and the microstructure and optical properties of the ceramics were studied. The ground state absorption cross section of Co:MgAl2O4 transparent ceramics was also discussed. 2. Experimental procedure In present work, 0.02 at.% Co:MgAl2O4 powder was prepared by a mixture of pure spinel powder and lab-made Cobalt doped spinel powder which was synthesized by reverse dropping coprecipitation. The pure spinel powder was commercial available high purity Magnesium Aluminate Spinel powder (Baikalox S30CR, Baikowski, Charlotte, NC, USA). The purity of the commercial spinel powder was 99.95%, and the powder had a specific surface area of 26 m2/g. Lab-made Co:MgAl2O4 powder was synthesized by the aluminum, magnesium and cobalt nitrates (99% purity, Sinopharm Chemical Reagent Co. Ltd., ShangHai, China) dropped to ammonium carbonate solution (99% purity, Sinopharm Chemical Reagent Co. Ltd., ShangHai, China). The detail procedure was described in previous studies [18e21]. Firstly, the aqueous solutions of 0.20 mol aluminum nitrate, 0.9995 mol magnesium nitrates and 0.0005 mol cobalt nitrates were prepared. 400 ml of both solutions were prepared. Then the mixture solution was titrated to 800 ml of 1.5 mol/L ammonium carbonate solution with 4 ml/min speed, and the ammonium carbonate solution was added with 25 ml ammonium hydroxide to adjust PH value. After the precipitation, the suspension was aged for 24 h. The resultant slurry was washed twice by deionized water, rinsed twice with ethanol. Then the precursor powder was dried at 70 C and sieved. Finally, the dried precursor
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powder was calcined at 1100 C for 4 h, and lab-made 0.5 at.% Co:MgAl2O4 powder was obtained. After that, the two kinds of powders described above were mixed into 0.02 at.% Co:MgAl2O4 powder, then the mixture was milled on a planetary ball mill for 12 h with 10 mm-diameter alumina balls in 99.99% ethanol. The rotation speed was 130 rpm/ min. After that, the mixture was dried and sieved through a 200mesh screen, and uniaxial pressed into pellets in a F 20 mm stainless steel mold under 60 MPa, following by cold isotactic pressed in 250 MPa for 1 min. Finally, all the compacted green bodies were vacuum pre-sintered at a series of temperatures (1450 C, 1500 C, 1550 C, 1600 C, 1650 C, 1700 C) for 5 h following by HIP post treatment at 1600e1700 C for 3 h in 200 MPa Ar atmospheres. By the procedure, the flake-like ceramics were prepared, and the samples were ground and polished to 2 mm-thick disks with different grades of the diamond pastes. The density of the ceramics was measured by the Archimedes method using deionized water as the immersion medium. The inline transmittance and the absorption spectrum of 2 mm-thick Co:MgAl2O4 ceramic samples were tested with a UVeViseNIR spectrophotometer (Cary 5000 spectrophotometer, Varian Inc., USA) as a function of wavelength. Microstructures of the polished and thermally etched surfaces of the ceramics were observed with a field emission scanning electron microscope (FESEM, SU8220, Hitachi, Japan). The polished ceramics were etched at 1300 C for 1 h to reveal the grain boundaries. In addition, the average grain size was determined by the multiplying the average linear intercept distance by 1.56. 3. Results and discussion Fig. 1 shows morphologies of the commercial pure spinel powder and the Co:MgAl2O4 powder synthesized by coprecipitation method. Both powders present quasi-spherical particle microstructure. The specific surface area of the commercial pure spinel powder is 26 m2/g, and the particle size calculated from Formula (1) [22] is 64 nm.
d¼
6
r$S
1000
(1)
where d stands the calculated particle size with unit of nm, r is the theory density of spinel, which is 3.58 g/cm3, and the S is the specific surface area value. The average particle size of the commercial spinel powder observed from SEM is 48 nm, which is just a litter smaller than the calculated grain size from specific surface area due to weak agglomeration. Meanwhile, lab-made Co:MgAl2O4 powder performs a similar microstructure with pure spinel powder, which is
Fig. 1. SEM micrographs of (a) the commercial spinel powder and (b) the lab-made Co:MgAl2O4 powder.
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benefit for homogeneity of the powder mixture. The specific surface area of Co:MgAl2O4 powder is 32 m2/g, and the calculated particle size is 52 nm. The average particle size is 39 nm observed from the SEM photograph. The agglomeration of powder is also slight. It indicates that both powders have good sintering activity. Fig. 2 is the density curves of Co:MgAl2O4 ceramics fabricated by vacuum pre-sintering at the temperatures range from 1450 C to 1700 C before and after HIP post-treatment, and the HIP posttreatment is implemented at the temperatures of 1600 C, 1650 C and 1700 C for 3 h in 200 MPa Ar atmosphere. As shown in the figure, the densification of the vacuum pre-sintering Co:MgAl2O4 ceramics is enhanced with the increase of the temperature, and the relative density increase from 93% to 96% of the theoretical value (3.58 g/cm3). For the Co:MgAl2O4 transparent ceramics, the densification degree is not enough, because there are many scattering centers in the ceramics. When the pre-sintering temperatures are range from 1500 C to 1700 C, the relative densities of the samples exceed 95.5%. It indicates that there are still few close pores in Co:MgAl2O4 ceramics, which can be removed by HIP post
Fig. 2. Density curves of the Co:MgAl2O4 ceramics fabricated by different HIP posttreatments.
treatment [23]. The relative densities of all ceramics treated by HIP post treatment exceed 98.3%, which supports the above assumption. When the post-treatment temperature is higher than 1500 C, the density of the Co:MgAl2O4 ceramics is enhanced remarkably, and approaches to the theoretical value of Co:MgAl2O4. It indicates that the pores formed in the pre-sintering of Co:MgAl2O4 ceramics are eliminated by the effect of HIP post-treatment, which is favorable to improve the optical transmittance of Co:MgAl2O4 transparent ceramics. SEM micrographs of the Co:MgAl2O4 ceramics pre-sintering at different temperatures for 5 h under the vacuum ambience are presented in Fig. 3. It can be seen that the observed bimodal microstructural pattern is similar to what is usually seen in the dense spinel ceramics sintered at the 1500e1600 C temperatures range [24,25], and the grain size of the pre-sintered ceramics increases with the increase of the pre-sintering temperature. When the temperature is higher than 1550 C, the crystal grain size increases from ~1 mm to 10e20 mm, significantly. It indicates that grain growth rate increases when the temperature is up to ~1550 C. In the previous works, during the pressure-less sintering of spinel, the abnormal grain growth becomes observable when the grain size exceeds the critical value [26,27], and the growth rate of grain size is variation owing to enhancement of abnormal grain growth because of the grain boundary separation. For the spinel ceramics, the critical value is about 10e20 mm [26]. In our study, the existence of abnormal grain is in accordance with the results as reported. However, the presence of abnormal grain would be harmful to the optical homogeneity for Co:MgAl2O4 transparent ceramics. In addition, there are a few of the pores and second phases captured at the interior of the grain and between the grain boundaries, which can be removed by the HIP post-treatment. All the Co:MgAl2O4 ceramics pre-sintered without HIP posttreatment are opaque, because the relative densities of the Co:MgAl2O4 ceramics sintered in vacuum ambience are less than 96%, indicating that there are many scattering centers in the samples. However, the Co:MgAl2O4 ceramics pre-sintered at the temperature of higher than 1500 C become translucent after the HIP posttreatment. Because the HIP post-treatment promotes the densification of Co:MgAl2O4 ceramics, and the samples reach fully dense. As shown in Fig. 4, the transmission curves of the Co:MgAl2O4 ceramics with the thickness of 2 mm after the HIP post-treatment at
Fig. 3. SEM micrographs of the Co:MgAl2O4 ceramics pre-sintered at different temperatures for 5 h (a) 1450 C; (b) 1500 C; (c) 1550 C; (d) 1600 C; (e) 1650 C; (f) 1700 C.
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different temperatures are present. After the HIP post-treatment, the in-line transmittance of the sample pre-sintered at 1450 C is almost zero in whole wavelength range. However, the Co:MgAl2O4 ceramics with the pre-sintering temperatures higher than 1450 C have the optical transmittance. The in-line transmittance of the Co:MgAl2O4 ceramics pre-sintered at 1500 C exceeds 70% in the wavelength range from 1000 to 2500 nm, and the best transmittance exceeds 80% at 2500 nm. Meanwhile, the transparency of the Co:MgAl2O4 ceramics after the HIP post-treatment decreases with the decrease of the wavelength. After subtracting the Fresnel reflection loss, the theoretical transmittance of MgAl2O4 ceramics is about 87% [17]. The optical transmission loss of the Co:MgAl2O4 ceramics can be explained by light scattering derived from Mie regime of scattering center [28]. Moreover, it can be seen that the optical transmittance of the Co:MgAl2O4 ceramics is not sensitive to the temperatures of HIP post-treatment, the transmission curves of the samples treated at different temperatures perform the similar features. With the increase of pre-sintering temperature, the in-line transmittance of the Co:MgAl2O4 ceramics decreases. The results will be discussed in detail based on the microstructures of the samples. The structure details of transmission curves are similar with the crystal samples doped Co2þ ion [1,4]. The absorption spectra peaks
show typical for Co:MgAl2O4 that are assigned to the energy levels transition of the tetrahedral coordinated Co2þ ion [29,30]. In detail, the absorption peak at 587 nm is assigned to the 4A2(4F) / 4T1(4P) transition. The broad absorption band in the 1.2e1.6 mm range can be ascribed to the transition from the 4A2(4F9/2) ground state to the 4 T1(4F) excited state. It indicates that Co:MgAl2O4 ceramics can be used for Q-switching of lasers operating in the same range. The absorption band composes of four separate peaks located at about 1230, 1347, 1403, and 1527 nm, all related to the spin-orbit splitting of the 4F state (4F / 4F9/2 þ 4F7/2 þ 4F5/2 þ 4F3/2 in an increasing energy order). The absorption peaks at 547, 582, and 626 nm relate to excitation to the 4T1(4P) excited multiplet. The structure is due to the spin-orbit splitting of the 4P state (4P / 4P5/2 þ 4P3/2 þ 4P1/2). In addition, the UV absorption edge of Co:MgAl2O4 ceramics locates at ~200 nm. Fig. 5 shows the micrographs of the Co:MgAl2O4 ceramics presintered at different temperatures for 5 h and HIP post-treated at 1600 C for 3 h. It can be seen that after the HIP post-treatment, the crystal grains are coarsen, especially for the samples pre-sintered at the temperature above 1600 C, the grains grow significantly. The abnormal grains become much more and bigger with the increase of the pre-sintering temperature. And the average grain sizes of the Co:MgAl2O4 ceramics before and after the HIP post-treatment at 1600 C for 3 h with different vacuum sintering temperatures are shown in Fig. 6. The average grain size of the samples pre-sintered at 1450 Ce1550 C increases from 3 to 4 mm to about 8 mm, and a few of abnormal grains occur. It demonstrates that the HIP posttreatment has a limited impact on grain growth whereas it improves significantly the densification process of the Co:MgAl2O4 ceramics pre-sintered at the low temperatures [31]. As discussed above, the density of Co:MgAl2O4 ceramics changes slightly when the pre-sintering temperature is higher than 1550 C. However, the grains of the samples grow rapidly, and the abnormal grains become much larger when the pre-sintering temperature is higher than 1600 C. It will cause the pores to be occluded inside the ceramics [26], and the properties of Co:MgAl2O4 transparent ceramics deteriorate. So, the density of the Co:MgAl2O4 ceramics presintered at 1700 C is smaller than that of the samples presintered at 1650 C after the HIP post-treatment, and the transparency of the samples also worsens, which supports these claims as well. Fig. 7 illustrates the absorption coefficient spectrum fitted by the optical transmittance of 0.02 at.% Co:MgAl2O4 ceramic presintered at 1500 C for 5 h and HIP post-treated at 1650 C for 3 h under 200 MPa Ar atmosphere and the energy level schematic of Td-site Co2þ ion. The broad absorption band range from 1200 to 1600 nm is attributed to 4A2(4F) / 4T1(4F) multiplet transitions. And the ground state absorption cross section of the Co:MgAl2O4 ceramics can be estimated from the following equations:
sgas ¼ N¼
Fig. 4. In-line transmittance of the Co:MgAl2O4 ceramics (2 mm thickness) presintered at different temperatures for 5 h and HIP post-treated at (a) 1600 C and (b) 1650 C for 3 h.
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aa
(2)
N
r NA M
Cs
(3)
where sgas stands the ground state absorption cross section, aa stands the absorption coefficient, N is the concentration of Co2þ ions, NA is Avogadro's number and M is atomic molar mass, r is the density of the Co:MgAl2O4 ceramics and Cs is the molar concentration of Co2þ ions in the ceramic samples. For the 0.02 at.% Co:MgAl2O4 ceramic, the absorption coefficient aa is 0.76 cm1 at the wavelength of 1540 nm as shown in Fig. 7. So the ground state absorption cross section of Co:MgAl2O4 ceramics at 1540 nm can be calculated to be 3.35 1019 cm2, which is in agreement with the
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Fig. 5. SEM micrographs of Co:MgAl2O4 ceramics pre-sintered at different temperatures for 5 h and HIP post-treated at 1600 C for 3 h (a) 1450 C; (b) 1500 C; (c) 1550 C; (d) 1600 C; (e) 1650 C; (f) 1700 C.
sgas value of (3.5 ± 0.6) 1019 cm2 for the 0.8 at.% Co2þ in MgAl2O4 single crystal [1]. And the difference is caused by the experimental uncertainty, for example, the accurate molar concentration of Co2þ ions is difficult to be determined. Moreover, the ground state absorption cross section of Co:MgAl2O4 ceramics varies with the different sintering methods. Compared with the result of our work, the ground state absorption cross-section of the Co:MgAl2O4 ceramics produced by spark plasma sintering (SPS) is 2.6 1019 cm2 [10], and the value of the Co:MgAl2O4 ceramics obtained by the combination of pre-sintering in air with HIP post-treatment is 2.9 1019 cm2 [11]. It indicates that HIP post-treatment is an effective technology to produce Co:MgAl2O4 transparent ceramics for saturable absorber. 4. Conclusions
Fig. 6. The average grain sizes of the Co:MgAl2O4 ceramics before and after HIP posttreatment at 1600 C for 3 h with different vacuum sintering temperatures.
In this study, fully dense and highly transparent Co:MgAl2O4 ceramics were successfully fabricated by the vacuum pre-sintering combined with HIP post-treatment. The optical transmittance of Co:MgAl2O4 transparent ceramics pre-sintered at 1500 C and HIP
Fig. 7. Absorption coefficient spectra of Co:MgAl2O4 ceramic pre-sintered at 1500 C for 5 h and HIP post-treated at 1650 C for 3 h and the energy level schematic of Td-site Co2þ ion.
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post-treated at 1600 C and 1650 C exceeds 70% at the wavelength range from 1000 to 2500 nm. The optical properties of ceramics reduce remarkably with increase of the pre-sintering temperature, because the grain size coarsens rapidly and the abnormal grains are present at the high sintering temperature. 0.02 at.% Co:MgAl2O4 ceramics have the typical absorption band of Td-site Co2þ, and the ground state absorption cross section at the wavelength of 1540 nm is calculated to be 3.35 1019 cm2. It is predictable that Co:MgAl2O4 transparent ceramics are the good candidate for the effective saturable absorber for the solid laser operating at 1.5 mm.
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Acknowledgments [16]
This work was supported by the National Natural Science Foundation of China (Grant No. 61575212), the Key research project of the frontier science of the Chinese Academy of Sciences (Grant No. QYZDB-SSW-JSC022) and the open project of the Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences (Grant No. FCLT201504). References
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