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Hot-pressing and post-HIP treatment of Fe2+:ZnS transparent ceramics from co-precipitated powders
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Hot-pressing and post-HIP treatment of Fe2+ :ZnS transparent ceramics from co-precipitated powders Chaoyu Li a,b , Tengfei Xie a , Huamin Kou a,∗ , Yubai Pan c , Jiang Li a,∗ a Key Laboratory of Transparent and Opto-functional Advanced Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China b University of Chinese Academy of Sciences, Beijing, 100049, China c Department of Physics, Shanghai Normal University, Shanghai, 200234, China
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Article history: Received 3 September 2016 Received in revised form 17 December 2016 Accepted 29 December 2016 Available online xxx Keywords: Fe2+ :ZnS transparent ceramics Mid-IR laser Co-precipitation Hot-pressing HIP treatment
a b s t r a c t Transparent Fe2+ :ZnS polycrystals are 3–5 m mid-IR laser gain materials with great importance. In this paper, a novel route combining wet-chemical co-precipitation, hot-pressed sintering and post hot isostatic pressing (HIP) treatment was proposed for Fe2+ :ZnS transparent ceramics. Fe2+ :ZnS powders were synthesized successfully by the wet-chemical co-precipitation method, which was suitable for particle size and morphology controlling and then ceramics consolidation. Transparent Fe2+ :ZnS ceramics with comparatively high optical quality were successfully fabricated by two step sintering method combining hot pressing and post-HIP treatment. This approach is a promising candidate for fabricating Fe2+ :ZnS ceramics with large size, low cost, short cycle, and flexible concentration distribution design. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Divalent transition metal ions (TM2+ ), mainly Cr2+ and Fe2+ , doped zinc chalcogenides (ZnS and ZnSe) are an important family of mid-IR laser gain medium with ultra-broad tunable range, broad absorption band, high efficiency and room temperature operability [1,2]. In recent years, there is an increasing interest for Fe2+ :ZnS lasers resulting from the combination of spectral characterizations of Fe2+ and mechanical and thermal properties of ZnS substrates. Compared to Cr2+ :ZnSe/S, the emission bands of Fe2+ :ZnSe/S are further shifted to the long wavelength range of 3.5–6 m, which locates in the mid-IR atmospheric window and molecular fingerprint region, simultaneously. As a result, Fe2+ :ZnSe/S lasers arouse great interest due to their plenitude civilian and military applications in spectroscopic, medical, remote sensing, infrared countermeasure and so on [3–6]. On the other hand, ZnS crystal has higher hardness, thermal conductivity, specific heat and lower thermal lensing with respect to ZnSe [7], which make it suitable for high power laser systems, although the absorption and emission
∗ Corresponding authors. E-mail addresses: [email protected] (H. Kou), [email protected] (J. Li).
bands of Fe2+ :ZnS shift towards the short wavelength compared with Fe2+ :ZnSe [6,8,9]. Currently, TM2+ doped ZnSe/S materials with high optical quality are mainly fabricated either by thermal diffusion or vapor growth methods. However, both methods have some “intrinsic” problems based on their fabrication mechanism. For the thermal diffusion method, it takes considerable long time to eliminate the concentration gradient due to the extremely small diffusion coefficient of TM in zinc chalcogenides [10,11]. For the vapor growth method, it also suffers from the long preparation period, inhomogeneous concentration distribution and limited size [12,13]. Aiming at these problems, a promising alternative method is hot-pressed transparent ceramics, which was firstly and successfully demonstrated by Mirov’s group in 2006 [14]. The most prominent advantage of this method is that the concentration gradient can be reduced to the grain size level, which has experimentally proved by our group [15]. What’s more, the flexible preparation process allows fabrication of ceramics with devisable concentration distributions and composite structures to improve the thermo-mechanical properties [16]. In addition, fabrication of hot-pressed ceramics possesses other benefits like better mechanical property, shorter preparation period, and so on. Despite all this, there still exists some challenges for hot-pressed TM2+ :ZnSe/S laser ceramics. First and most important, the optical qualities need to be improved significantly to meet the service demands for high
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Please cite this article in press as: C. Li, et al., Hot-pressing and post-HIP treatment of Fe2+ :ZnS transparent ceramics from co-precipitated powders, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.051
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power lasers. However, optical quality of the both demonstrated samples was low mainly due to the extremely poor sinterability of the powders synthesized by the thermal diffusion method at high temperature for long time. Sintering of pure ZnS ceramics has been investigated for nearly one half century and some breakthroughs have been made due to the great progress in wet chemical powder synthesis technology [17–19]. Recently, Li et al. reported the hot-pressing of Cr2+ :ZnS ceramics from the mixture of wet-chemically synthesized ZnS nanopowders and Cr2 S3 powders [20]. Good optical quality in the Mid-IR region was obtained due to the high sinterability of the nanopowders. Compared with the Cr2+ , absorption and emission of Fe:ZnS further shift into the midIR region, which is convenient to achieve high transmittance in the hot-pressed ZnS ceramics. Consequently, it is promising to fabricate Fe2+ :ZnS ceramics with good optical quality from wet chemically synthesized nanopowders. On the other hand, hot-pressing of the pre-doped Fe2+ :ZnS powders has some advantage in the valence controlling of iron ions and eliminating the scattering of residual dopants in the ceramics. In this work, Fe2+ :ZnS nanopowders were synthesized by the co-precipitated method and transparent ceramics were sintered by two step sintering of hot-pressing and post HIP treatment. To the best of our knowledge, this is the first work on the fabrication of Fe2+ :ZnS ceramics directly from wet chemically synthesized powders.
2. Experimental procedure 2.1. Powder synthesis The co-precipitation procedure of Fe2+ :ZnS powders was based on the following reactions: Fe2+ + S2− → FeS ↓
Zn2+ + S2− → ZnS ↓ Zinc acetate dihydrate Zn(CH3 COO)2 ·2H2 O (≥99.99%, metals basis, Aladdin TM, Shanghai, China), sodium sulfide nonahydrate Na2 S·9H2 O (≥99.99%, metals basis, Aladdin TM, Shanghai, China) and ferrous chloride FeCl2 (≥99.99%, metals basis, Alfa Aesar) were used as raw materials. 0.5 M Sodium sulfide and 0.5 M zinc acetate solutions were purged with nitrogen gas for 2 h to remove the oxygen. Ferrous chloride was dissolved into the zinc acetate solution. The molar ratio of ferrous chloride to zinc acetate was 0.5 at% in the mixed solution. Stoichiometric amount of sodium sulfide solution was drop-wise added into the cationic solution with magnetic stirring. The whole co-precipitation reaction was carried out under nitrogen atmosphere to avoid oxidation of the ferrous ions. After the reaction finished and the suspension aged for 3 h, the precipitates were centrifuged and washed with deionized water for three times, and then dried with a vacuum freeze dryer. The as-obtained powders was referred as “Fe:ZnS-CP-Uncalcined”. The sample was further calcined at 800 ◦ C for 3 h under vacuum and the product was named as “Fe:ZnS-CP-Calcined”. As a comparison, Fe2+ :ZnS powder was also prepared using the thermal diffusion method as in the literature [15]. FeS powders and ZnS pieces were sealed into a quartz tube with high vacuum and the thermal diffusion was carried out at 950 ◦ C for 5 days. After the diffusion finished, the mixture was washed with dilute nitric acid to remove the redundant FeS. Then, the Fe:ZnS pieces were ground into powders in an agate mortar. The obtained product was named as “Fe:ZnS-VD” and used for ceramics sintering.
Fig. 1. XRD patterns of the precipitated powders and the commercial powders.
2.2. Ceramics sintering The Fe2+ :ZnS powders prepared by both methods were mixed with commercial available ZnS powders (≥99.99%, Kojundo Chemical Laboratory Co., Japan) in an agate mortar by radio of 1:9. The mixture was uniaxial pressed into 34 mm pellets with a pressure of 50 MPa and then a pellet was introduced into the tungsten alloy die. The pellets were hot-pressed at 900 ◦ C for 2 h with a uniaxial pressure of 250 MPa. The obtained ceramics were named as “CP-HP” and “VD-HP” for samples from wet-chemically synthesized powders and vacuum diffused powders, respectively. These samples were cut into pieces and parts of them were hot isostatic pressing treated at 950 ◦ C for 5 h under argon pressure of 150 MPa. These samples after HIP treatment were donated as “CP-HP-HIP” and “VD-HP-HIP”. All the samples were mechanically double-sided polished to the final thickness of 0.5 mm. 2.3. Characterization Phase analysis of the samples was determined by X-ray diffraction (D8 Advance, Bruker, Germany). SEM images of powders were obtained by Magellan 400 FESEM (FEI, Hillsboro, OR). Raman spectra of the powders were measured at room temperature by LabRAM HR Evolution spectrometer (HORIBA, France) with excitation with a He-Ne laser at 514 nm. The diffused reflectance spectra of powders in vis-to-infrared range were measured with Cary-5000 spectrophotometer (Varian, USA) equipped with an integrating spheres. The spectral resolution was 2 nm and the step was 1 nm. The diffused reflectance spectra of powders in mid-IR range were measured with Cary 630 FTIR spectroscopy (Agilent, Australia) combining with a diffuse reflectance accessory. Spectral resolution of 8 cm−1 and sampling numbers of 512 were applied during the measurement. The absorption spectra of Fe2+ :ZnS ceramics were measured by VERTEX 70 FT-IR spectrophotometer (Bruker, Ettlingen, Germany). Room temperature electron paramagnetic resonance (EPR) spectra were obtained using a Bruker BioSpin EMX-8/2.7 EPR spectrometer (Bruker BioSpin GmbH, Karlsruhe, Germany) (10 ◦ C, 9.874 GHz, X-band). Doping concentration of the prepared Fe:ZnS powders were measure by ICP analysis (Agilent 725, Agilent, USA). 3. Results and discussion Fig. 1 shows the XRD patterns of the co-precipitated Fe:ZnS powders before (“Fe:ZnS-CP-Uncalcined”) and after (“Fe:ZnS-CPCalcined”) calcination at 800 ◦ C under vacuum. It can be seen that the as-prepared Fe:ZnS powders was of sphalerite phase with the particle size of about 5 nm (estimated using Debye-Scherrer’s formula). However, after calcination, the powder totally transferred
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Fig. 3. Room-temperature Raman spectrum of the calcined Fe2+ :ZnS powder.
Fig. 4. Diffuse reflection spectra of Fe2+ :ZnS powders fabricated by the coprecipitation method and the vacuum diffusion method.
Fig. 2. SEM images of the co-precipitated Fe:ZnS powders before (a) and after (b) calcination, as well as the commercial ZnS powders (c).
into wurtzite phase which was attributed to the size-induced low-temperature phase transition of ZnS nanopowders [21]. XRD pattern of the commercial ZnS powders was also shown in Fig. 1. The phase composition of the commercial powders was almost all sphalerite phase. As mentioned before, the starting powder for Fe:ZnS ceramics was a mixture of the “Fe:ZnS-CP-Calcined” powder and commercial ZnS powder by radio of 1:9. As a result, it can be deduced that the mixture mainly consisted of sphalerite phase with cubic structure, which is favorable for fabrication of transparent ceramics. In addition, small amount of wurtzite nanopowder is acceptable for transparent hot-pressed zinc sulfide ceramics considering the wurtzite-to-sphalerite phase transition and preferred orientation of wurtzite grains under high uniaxial pressure [22]. Consequently, transparent Fe:ZnS ceramics can be expected considering the phase composition. FESEM images of the synthesized Fe:ZnS powders and the commercial ZnS powders are presented in Fig. 2. The “Fe:ZnSCP-Uncalcined” sample consisted of nanoparticle with the size of several nanometers, which was consistent with the XRD results. After calcined at 800 ◦ C for 3 h, the particle grown into the submicrometer level and agglomeration appeared. The commercial ZnS powders were constitute of loose agglomerations with spherical shape. The primary particle size of agglomerations was about 100 nm. Fig. 3 shows the room-temperature Raman spectrum of the “Fe:ZnS-CP-Calcined” powder. The strong first-order scattering at 349 cm−1 is assigned to the LO mode of wurtzite ZnS, while the
weak scattering at 271 cm−1 is assigned to the TO mode. The additional scattering in the range of 320–340 cm−1 is partly attributed to the surface optic (SO) phonon scattering which has been discussed in previous studies on wurtzite ZnS nanowires (at 335 cm−1 ) [23] and nanoribbons (at 339 cm−1 ) [24]. In the spectral region between the TO (271 cm−1 ) and LO (349 cm−1 ) phonons, two obvious additional Raman modes at 300 cm−1 and 312 cm−1 can be observed. According to the literatures [25,26], the additional modes, as well as another one at 332 cm−1 , which was overlapped with the SO mode in our case, are introduced by the incorporation of Fe ions into the ZnS lattices. Consequently, for the obtained “Fe:ZnS-CP-Calcined” powder, Fe2+ ions were successfully doped into the lattice position of Zn2+ ions. The diffuse reflection spectra of the “Fe:ZnS-CP-Calcined” and “Fe:ZnS-VD” powders in the Mid-IR region are shown in Fig. 4. The “Fe:ZnS-VD” powder was used as standard sample in which Fe2+ ions were in the lattice position of Zn2+ ions. It can be seen that “Fe:ZnS-CP-Calcined” powder showed similar broadband absorption with the standard sample in the mid-IR range of 2.0 − 4.5 m. The broad absorption accounts from the energy transfer from 5 E ground state to 5 T2 excited state of Fe2+ in a tetrahedral field, which confirms that Fe2+ was doped into the lattice sites of ZnS by co-precipitated method. In addition, an extra peak centered at ∼2.9 m can be observed for “Fe:ZnS-CP-Calcined” powder, which was attributed to the overlap with stretching vibration of surface-absorbed water. In recent years, there exists an increasing interest in the random lasers based on the TM:ZnS/ZnSe nanocrystals [27,28]. Thus, our demonstration provides a potential method for the synthesis of Fe2+ :ZnS powders. Fig. 5 presents the transmission spectra of hot-pressed Fe2+ :ZnS ceramics from “Fe:ZnS-CP-Calcined” powders before and after post-HIP treatment. Transmittance of the “CP-HP” sample at 6.0 m
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Fig. 5. Transmission spectra of the hot-pressed Fe2+ :ZnS ceramics (0.5 mm thick) before and after post-HIP treatment.
Fig. 7. Absorption spectra of the Fe2+ :ZnS ceramics before and after HIP treatment.
powder were also presented in Fig. 5. The optical quality was extremely low even after the post-HIP treatment, perhaps due to the extremely low sinterability of the initial powders. Obviously, our demonstration presents a novel method for transparent ceramics by applying the wet-chemically synthesized Fe2+ :ZnS powders. By optimizing the sinterability of Fe2+ :ZnS powders and the combination of pre-sintering and HIP post treatment simultaneously, it is promising to obtain Fe2+ :ZnS ceramics with higher optical quality. The broad absorption band in the range of 2.2–4.2 m range corresponds to the transition of Fe2+ from 5 T2 excited state to 5 E ground state. The absorption coefficient ␣ of the Fe2+ :ZnS ceramics was calculated using the flowing equations: T=
Fig. 6. SEM images of the fracture surfaces of the hot-pressed Fe2+ :ZnS ceramics before (a) and after (b) HIP treatment from the precipitated powders.
was about 70%, which was close to the best results of hotpressed ZnS ceramics [18]. However, optical quality of the ceramics sharply dropped to 25% at the wavelength of 2.0 m, which can be attributed to the scattering of residual pores and wurtzite phase grains. In order to further increase the optical quality, a post-HIP treatment of 950 ◦ C − 150 MPa − 5 h was adopted. After the post-HIP treatment, the optical quality of Fe2+ :ZnS ceramics (“CP-HP-HIP”) increased significantly, especially at the short wavelength. A typically increasing of 20% for in-line transmittance at 2.0 m was achieved. The effect of HIP treatment was preliminarily verified via SEM observation of the fracture surfaces (as shown in Fig. 6a and b). For sample “CP-HP”, there existed some residual pores with the size of about several hundred nanometers. According to the scattering theory [29], scattering of residual pores has a significant influence on the transmittance of ZnS ceramics and the pores should be smaller than 100 nm in order to minimize the porosity scattering over the spectral range of 2.5 − 10 m. It can be seen that the residual pore size decreased significantly after the post-HIP treatment, which accounted for the increasing of inline transmittance, especially at the short wavelength. The in-line transmittance spectra of the Fe2+ :ZnS ceramics from “Fe:ZnS-VD”
(1 − R)2 e−˛b 1 − R2 e−2˛b
where T is the measured in-line transmittance, and R = (1n)2 /(1+n)2 is the reflectance, n is the refractive index and b is the thickness of the sample. Here, value of n versus wavelength can be calculated from the Sellmeier relation given in the literature [16]. Baselines of the calculated absorption spectra were subtracted and the absorption coefficients of Fe2+ in the ceramics were shown in Fig. 7. The Fe2+ :ZnS ceramics show the maximum absorption at 2.86 m, which is slightly red-shifted compared with the literature [1]. In addition, it can be seen that the post-HIP sample shows a larger absorption. It may be contributed to the thermal diffusion of the residual FeS precipitator into the lattice of ZnS. Using the maximum absorption cross section at 2.86 m of 0.92 × 10−18 cm2 [1], we roughly evaluated the concentration of Fe2+ ions, which was 8.23 × 1018 cm−3 for the hot-pressed ceramics and 9.66 × 1018 cm−3 for the post-HIP sample. The added Fe2+ ions in the solution was partly oxidized into Fe3+ during the wet chemical synthesis process, which was confirmed by the visible diffuse reflection spectrum [30] and EPR analysis [31] as shown in Fig. 8. As a result, the increasing of the Fe2+ concentration in the postHIP sample can be attributed to the reducing of Fe3+ ions due to the carburizing environment because of the presence of graphite in the HIP chamber [20]. However, the Fe2+ concentrations of both ceramics (0.033 at.% and 0.038 at.% respectively) were smaller than that of the raw mixture powders, which was one-tenth of the calcined Fe2+ :ZnS power with total ion concentration of 0.565 at.% (measured by ICP analysis). Thus, further studies are still needed to control the valence of the iron ions. 4. Conclusion Fe2+ :ZnS powders were synthesized by the co-precipitation method. The Raman and diffuse reflection spectra confirmed that the Fe2+ ions were successfully doped into the lattice position of Zn2+ ions after vacuum calcinations at 800 ◦ C for 3 h, which indicats
Please cite this article in press as: C. Li, et al., Hot-pressing and post-HIP treatment of Fe2+ :ZnS transparent ceramics from co-precipitated powders, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.051
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Fig. 8. Diffuse reflection spectrum (a) and EPR analysis (b) of the as-prepared Fe:ZnS powder.
that the co-precipitation method can be expected for convenient mass production of Fe2+ :ZnS powders used for laser ceramics or random lasers. Transparent Fe2+ :ZnS laser ceramics were achieved by combining the wet chemical co-precipitation method with hot-pressed sintering and post-HIP treatment. In-line transmittance of the ceramics with the best optical quality was about 65% at 4.5 m and 45% at 2.0 m, which was much better than the Fe2+ :ZnS ceramics from vacuum diffused powders. That is, this demonstration supplies a novel method for the fabrication of transparent Fe2+ :ZnS laser ceramics. Acknowledgments This work was supported by 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] S.B. Mirov, V.V. Fedorov, D. Martyshkin, I.S. Moskalev, M. Mirov, S. Vasilyev, Progress in mid-IR lasers based on Cr and Fe-doped II-VI chalcogenides, IEEE J. Sel. Top. Quantum Electron. 21 (2015) 292–310. [2] S. Vasilyev, I. Moskalev, M. Mirov, V. Smolsky, S. Mirov, V. Gapontsev, Recent breakthroughs in solid-State mid-IR laser technology, Laser Technol. J. 13 (2016), http://dx.doi.org/10.1002/latj.201600022. [3] S.D. Velikanov, N.A. Zaretsky, E.A. Zotov, S.Y. Kazantsev, I.G. Kononov, V.K. Yu, A.A. Maneshkin, K.N. Firsov, M.P. Frolov, I.M. Yutkin, Room-temperature 1.2-J Fe2+ :ZnSe laser, Quantum Electron. 46 (2016) 11–12. [4] K.N. Firsov, M.P. Frolov, E.M. Gavrishchuk, S.Y. Kazantsev, I.G. Kononov, V.K. Yu, A.A. Maneshkin, S.D. Velikanov, I.M. Yutkin, N.A. Zaretsky, E.A. Zotov, Laser on single-crystal ZnSe:Fe2+ with high pulse radiation energy at room temperature, Laser Phys. Lett. 13 (2016) 015002.
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Please cite this article in press as: C. Li, et al., Hot-pressing and post-HIP treatment of Fe2+ :ZnS transparent ceramics from co-precipitated powders, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.051
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Hot-pressing and post-HIP treatment of Fe2+ :ZnS transparent ceramics from co-precipitated powders Chaoyu Li a,b , Tengfei Xie a , Huamin Kou a,∗ , Yubai Pan c , Jiang Li a,∗ a Key Laboratory of Transparent and Opto-functional Advanced Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, 200050, China b University of Chinese Academy of Sciences, Beijing, 100049, China c Department of Physics, Shanghai Normal University, Shanghai, 200234, China
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Article history: Received 3 September 2016 Received in revised form 17 December 2016 Accepted 29 December 2016 Available online xxx Keywords: Fe2+ :ZnS transparent ceramics Mid-IR laser Co-precipitation Hot-pressing HIP treatment
a b s t r a c t Transparent Fe2+ :ZnS polycrystals are 3–5 m mid-IR laser gain materials with great importance. In this paper, a novel route combining wet-chemical co-precipitation, hot-pressed sintering and post hot isostatic pressing (HIP) treatment was proposed for Fe2+ :ZnS transparent ceramics. Fe2+ :ZnS powders were synthesized successfully by the wet-chemical co-precipitation method, which was suitable for particle size and morphology controlling and then ceramics consolidation. Transparent Fe2+ :ZnS ceramics with comparatively high optical quality were successfully fabricated by two step sintering method combining hot pressing and post-HIP treatment. This approach is a promising candidate for fabricating Fe2+ :ZnS ceramics with large size, low cost, short cycle, and flexible concentration distribution design. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Divalent transition metal ions (TM2+ ), mainly Cr2+ and Fe2+ , doped zinc chalcogenides (ZnS and ZnSe) are an important family of mid-IR laser gain medium with ultra-broad tunable range, broad absorption band, high efficiency and room temperature operability [1,2]. In recent years, there is an increasing interest for Fe2+ :ZnS lasers resulting from the combination of spectral characterizations of Fe2+ and mechanical and thermal properties of ZnS substrates. Compared to Cr2+ :ZnSe/S, the emission bands of Fe2+ :ZnSe/S are further shifted to the long wavelength range of 3.5–6 m, which locates in the mid-IR atmospheric window and molecular fingerprint region, simultaneously. As a result, Fe2+ :ZnSe/S lasers arouse great interest due to their plenitude civilian and military applications in spectroscopic, medical, remote sensing, infrared countermeasure and so on [3–6]. On the other hand, ZnS crystal has higher hardness, thermal conductivity, specific heat and lower thermal lensing with respect to ZnSe [7], which make it suitable for high power laser systems, although the absorption and emission
∗ Corresponding authors. E-mail addresses: [email protected] (H. Kou), [email protected] (J. Li).
bands of Fe2+ :ZnS shift towards the short wavelength compared with Fe2+ :ZnSe [6,8,9]. Currently, TM2+ doped ZnSe/S materials with high optical quality are mainly fabricated either by thermal diffusion or vapor growth methods. However, both methods have some “intrinsic” problems based on their fabrication mechanism. For the thermal diffusion method, it takes considerable long time to eliminate the concentration gradient due to the extremely small diffusion coefficient of TM in zinc chalcogenides [10,11]. For the vapor growth method, it also suffers from the long preparation period, inhomogeneous concentration distribution and limited size [12,13]. Aiming at these problems, a promising alternative method is hot-pressed transparent ceramics, which was firstly and successfully demonstrated by Mirov’s group in 2006 [14]. The most prominent advantage of this method is that the concentration gradient can be reduced to the grain size level, which has experimentally proved by our group [15]. What’s more, the flexible preparation process allows fabrication of ceramics with devisable concentration distributions and composite structures to improve the thermo-mechanical properties [16]. In addition, fabrication of hot-pressed ceramics possesses other benefits like better mechanical property, shorter preparation period, and so on. Despite all this, there still exists some challenges for hot-pressed TM2+ :ZnSe/S laser ceramics. First and most important, the optical qualities need to be improved significantly to meet the service demands for high
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power lasers. However, optical quality of the both demonstrated samples was low mainly due to the extremely poor sinterability of the powders synthesized by the thermal diffusion method at high temperature for long time. Sintering of pure ZnS ceramics has been investigated for nearly one half century and some breakthroughs have been made due to the great progress in wet chemical powder synthesis technology [17–19]. Recently, Li et al. reported the hot-pressing of Cr2+ :ZnS ceramics from the mixture of wet-chemically synthesized ZnS nanopowders and Cr2 S3 powders [20]. Good optical quality in the Mid-IR region was obtained due to the high sinterability of the nanopowders. Compared with the Cr2+ , absorption and emission of Fe:ZnS further shift into the midIR region, which is convenient to achieve high transmittance in the hot-pressed ZnS ceramics. Consequently, it is promising to fabricate Fe2+ :ZnS ceramics with good optical quality from wet chemically synthesized nanopowders. On the other hand, hot-pressing of the pre-doped Fe2+ :ZnS powders has some advantage in the valence controlling of iron ions and eliminating the scattering of residual dopants in the ceramics. In this work, Fe2+ :ZnS nanopowders were synthesized by the co-precipitated method and transparent ceramics were sintered by two step sintering of hot-pressing and post HIP treatment. To the best of our knowledge, this is the first work on the fabrication of Fe2+ :ZnS ceramics directly from wet chemically synthesized powders.
2. Experimental procedure 2.1. Powder synthesis The co-precipitation procedure of Fe2+ :ZnS powders was based on the following reactions: Fe2+ + S2− → FeS ↓
Zn2+ + S2− → ZnS ↓ Zinc acetate dihydrate Zn(CH3 COO)2 ·2H2 O (≥99.99%, metals basis, Aladdin TM, Shanghai, China), sodium sulfide nonahydrate Na2 S·9H2 O (≥99.99%, metals basis, Aladdin TM, Shanghai, China) and ferrous chloride FeCl2 (≥99.99%, metals basis, Alfa Aesar) were used as raw materials. 0.5 M Sodium sulfide and 0.5 M zinc acetate solutions were purged with nitrogen gas for 2 h to remove the oxygen. Ferrous chloride was dissolved into the zinc acetate solution. The molar ratio of ferrous chloride to zinc acetate was 0.5 at% in the mixed solution. Stoichiometric amount of sodium sulfide solution was drop-wise added into the cationic solution with magnetic stirring. The whole co-precipitation reaction was carried out under nitrogen atmosphere to avoid oxidation of the ferrous ions. After the reaction finished and the suspension aged for 3 h, the precipitates were centrifuged and washed with deionized water for three times, and then dried with a vacuum freeze dryer. The as-obtained powders was referred as “Fe:ZnS-CP-Uncalcined”. The sample was further calcined at 800 ◦ C for 3 h under vacuum and the product was named as “Fe:ZnS-CP-Calcined”. As a comparison, Fe2+ :ZnS powder was also prepared using the thermal diffusion method as in the literature [15]. FeS powders and ZnS pieces were sealed into a quartz tube with high vacuum and the thermal diffusion was carried out at 950 ◦ C for 5 days. After the diffusion finished, the mixture was washed with dilute nitric acid to remove the redundant FeS. Then, the Fe:ZnS pieces were ground into powders in an agate mortar. The obtained product was named as “Fe:ZnS-VD” and used for ceramics sintering.
Fig. 1. XRD patterns of the precipitated powders and the commercial powders.
2.2. Ceramics sintering The Fe2+ :ZnS powders prepared by both methods were mixed with commercial available ZnS powders (≥99.99%, Kojundo Chemical Laboratory Co., Japan) in an agate mortar by radio of 1:9. The mixture was uniaxial pressed into 34 mm pellets with a pressure of 50 MPa and then a pellet was introduced into the tungsten alloy die. The pellets were hot-pressed at 900 ◦ C for 2 h with a uniaxial pressure of 250 MPa. The obtained ceramics were named as “CP-HP” and “VD-HP” for samples from wet-chemically synthesized powders and vacuum diffused powders, respectively. These samples were cut into pieces and parts of them were hot isostatic pressing treated at 950 ◦ C for 5 h under argon pressure of 150 MPa. These samples after HIP treatment were donated as “CP-HP-HIP” and “VD-HP-HIP”. All the samples were mechanically double-sided polished to the final thickness of 0.5 mm. 2.3. Characterization Phase analysis of the samples was determined by X-ray diffraction (D8 Advance, Bruker, Germany). SEM images of powders were obtained by Magellan 400 FESEM (FEI, Hillsboro, OR). Raman spectra of the powders were measured at room temperature by LabRAM HR Evolution spectrometer (HORIBA, France) with excitation with a He-Ne laser at 514 nm. The diffused reflectance spectra of powders in vis-to-infrared range were measured with Cary-5000 spectrophotometer (Varian, USA) equipped with an integrating spheres. The spectral resolution was 2 nm and the step was 1 nm. The diffused reflectance spectra of powders in mid-IR range were measured with Cary 630 FTIR spectroscopy (Agilent, Australia) combining with a diffuse reflectance accessory. Spectral resolution of 8 cm−1 and sampling numbers of 512 were applied during the measurement. The absorption spectra of Fe2+ :ZnS ceramics were measured by VERTEX 70 FT-IR spectrophotometer (Bruker, Ettlingen, Germany). Room temperature electron paramagnetic resonance (EPR) spectra were obtained using a Bruker BioSpin EMX-8/2.7 EPR spectrometer (Bruker BioSpin GmbH, Karlsruhe, Germany) (10 ◦ C, 9.874 GHz, X-band). Doping concentration of the prepared Fe:ZnS powders were measure by ICP analysis (Agilent 725, Agilent, USA). 3. Results and discussion Fig. 1 shows the XRD patterns of the co-precipitated Fe:ZnS powders before (“Fe:ZnS-CP-Uncalcined”) and after (“Fe:ZnS-CPCalcined”) calcination at 800 ◦ C under vacuum. It can be seen that the as-prepared Fe:ZnS powders was of sphalerite phase with the particle size of about 5 nm (estimated using Debye-Scherrer’s formula). However, after calcination, the powder totally transferred
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Fig. 3. Room-temperature Raman spectrum of the calcined Fe2+ :ZnS powder.
Fig. 4. Diffuse reflection spectra of Fe2+ :ZnS powders fabricated by the coprecipitation method and the vacuum diffusion method.
Fig. 2. SEM images of the co-precipitated Fe:ZnS powders before (a) and after (b) calcination, as well as the commercial ZnS powders (c).
into wurtzite phase which was attributed to the size-induced low-temperature phase transition of ZnS nanopowders [21]. XRD pattern of the commercial ZnS powders was also shown in Fig. 1. The phase composition of the commercial powders was almost all sphalerite phase. As mentioned before, the starting powder for Fe:ZnS ceramics was a mixture of the “Fe:ZnS-CP-Calcined” powder and commercial ZnS powder by radio of 1:9. As a result, it can be deduced that the mixture mainly consisted of sphalerite phase with cubic structure, which is favorable for fabrication of transparent ceramics. In addition, small amount of wurtzite nanopowder is acceptable for transparent hot-pressed zinc sulfide ceramics considering the wurtzite-to-sphalerite phase transition and preferred orientation of wurtzite grains under high uniaxial pressure [22]. Consequently, transparent Fe:ZnS ceramics can be expected considering the phase composition. FESEM images of the synthesized Fe:ZnS powders and the commercial ZnS powders are presented in Fig. 2. The “Fe:ZnSCP-Uncalcined” sample consisted of nanoparticle with the size of several nanometers, which was consistent with the XRD results. After calcined at 800 ◦ C for 3 h, the particle grown into the submicrometer level and agglomeration appeared. The commercial ZnS powders were constitute of loose agglomerations with spherical shape. The primary particle size of agglomerations was about 100 nm. Fig. 3 shows the room-temperature Raman spectrum of the “Fe:ZnS-CP-Calcined” powder. The strong first-order scattering at 349 cm−1 is assigned to the LO mode of wurtzite ZnS, while the
weak scattering at 271 cm−1 is assigned to the TO mode. The additional scattering in the range of 320–340 cm−1 is partly attributed to the surface optic (SO) phonon scattering which has been discussed in previous studies on wurtzite ZnS nanowires (at 335 cm−1 ) [23] and nanoribbons (at 339 cm−1 ) [24]. In the spectral region between the TO (271 cm−1 ) and LO (349 cm−1 ) phonons, two obvious additional Raman modes at 300 cm−1 and 312 cm−1 can be observed. According to the literatures [25,26], the additional modes, as well as another one at 332 cm−1 , which was overlapped with the SO mode in our case, are introduced by the incorporation of Fe ions into the ZnS lattices. Consequently, for the obtained “Fe:ZnS-CP-Calcined” powder, Fe2+ ions were successfully doped into the lattice position of Zn2+ ions. The diffuse reflection spectra of the “Fe:ZnS-CP-Calcined” and “Fe:ZnS-VD” powders in the Mid-IR region are shown in Fig. 4. The “Fe:ZnS-VD” powder was used as standard sample in which Fe2+ ions were in the lattice position of Zn2+ ions. It can be seen that “Fe:ZnS-CP-Calcined” powder showed similar broadband absorption with the standard sample in the mid-IR range of 2.0 − 4.5 m. The broad absorption accounts from the energy transfer from 5 E ground state to 5 T2 excited state of Fe2+ in a tetrahedral field, which confirms that Fe2+ was doped into the lattice sites of ZnS by co-precipitated method. In addition, an extra peak centered at ∼2.9 m can be observed for “Fe:ZnS-CP-Calcined” powder, which was attributed to the overlap with stretching vibration of surface-absorbed water. In recent years, there exists an increasing interest in the random lasers based on the TM:ZnS/ZnSe nanocrystals [27,28]. Thus, our demonstration provides a potential method for the synthesis of Fe2+ :ZnS powders. Fig. 5 presents the transmission spectra of hot-pressed Fe2+ :ZnS ceramics from “Fe:ZnS-CP-Calcined” powders before and after post-HIP treatment. Transmittance of the “CP-HP” sample at 6.0 m
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Fig. 5. Transmission spectra of the hot-pressed Fe2+ :ZnS ceramics (0.5 mm thick) before and after post-HIP treatment.
Fig. 7. Absorption spectra of the Fe2+ :ZnS ceramics before and after HIP treatment.
powder were also presented in Fig. 5. The optical quality was extremely low even after the post-HIP treatment, perhaps due to the extremely low sinterability of the initial powders. Obviously, our demonstration presents a novel method for transparent ceramics by applying the wet-chemically synthesized Fe2+ :ZnS powders. By optimizing the sinterability of Fe2+ :ZnS powders and the combination of pre-sintering and HIP post treatment simultaneously, it is promising to obtain Fe2+ :ZnS ceramics with higher optical quality. The broad absorption band in the range of 2.2–4.2 m range corresponds to the transition of Fe2+ from 5 T2 excited state to 5 E ground state. The absorption coefficient ␣ of the Fe2+ :ZnS ceramics was calculated using the flowing equations: T=
Fig. 6. SEM images of the fracture surfaces of the hot-pressed Fe2+ :ZnS ceramics before (a) and after (b) HIP treatment from the precipitated powders.
was about 70%, which was close to the best results of hotpressed ZnS ceramics [18]. However, optical quality of the ceramics sharply dropped to 25% at the wavelength of 2.0 m, which can be attributed to the scattering of residual pores and wurtzite phase grains. In order to further increase the optical quality, a post-HIP treatment of 950 ◦ C − 150 MPa − 5 h was adopted. After the post-HIP treatment, the optical quality of Fe2+ :ZnS ceramics (“CP-HP-HIP”) increased significantly, especially at the short wavelength. A typically increasing of 20% for in-line transmittance at 2.0 m was achieved. The effect of HIP treatment was preliminarily verified via SEM observation of the fracture surfaces (as shown in Fig. 6a and b). For sample “CP-HP”, there existed some residual pores with the size of about several hundred nanometers. According to the scattering theory [29], scattering of residual pores has a significant influence on the transmittance of ZnS ceramics and the pores should be smaller than 100 nm in order to minimize the porosity scattering over the spectral range of 2.5 − 10 m. It can be seen that the residual pore size decreased significantly after the post-HIP treatment, which accounted for the increasing of inline transmittance, especially at the short wavelength. The in-line transmittance spectra of the Fe2+ :ZnS ceramics from “Fe:ZnS-VD”
(1 − R)2 e−˛b 1 − R2 e−2˛b
where T is the measured in-line transmittance, and R = (1n)2 /(1+n)2 is the reflectance, n is the refractive index and b is the thickness of the sample. Here, value of n versus wavelength can be calculated from the Sellmeier relation given in the literature [16]. Baselines of the calculated absorption spectra were subtracted and the absorption coefficients of Fe2+ in the ceramics were shown in Fig. 7. The Fe2+ :ZnS ceramics show the maximum absorption at 2.86 m, which is slightly red-shifted compared with the literature [1]. In addition, it can be seen that the post-HIP sample shows a larger absorption. It may be contributed to the thermal diffusion of the residual FeS precipitator into the lattice of ZnS. Using the maximum absorption cross section at 2.86 m of 0.92 × 10−18 cm2 [1], we roughly evaluated the concentration of Fe2+ ions, which was 8.23 × 1018 cm−3 for the hot-pressed ceramics and 9.66 × 1018 cm−3 for the post-HIP sample. The added Fe2+ ions in the solution was partly oxidized into Fe3+ during the wet chemical synthesis process, which was confirmed by the visible diffuse reflection spectrum [30] and EPR analysis [31] as shown in Fig. 8. As a result, the increasing of the Fe2+ concentration in the postHIP sample can be attributed to the reducing of Fe3+ ions due to the carburizing environment because of the presence of graphite in the HIP chamber [20]. However, the Fe2+ concentrations of both ceramics (0.033 at.% and 0.038 at.% respectively) were smaller than that of the raw mixture powders, which was one-tenth of the calcined Fe2+ :ZnS power with total ion concentration of 0.565 at.% (measured by ICP analysis). Thus, further studies are still needed to control the valence of the iron ions. 4. Conclusion Fe2+ :ZnS powders were synthesized by the co-precipitation method. The Raman and diffuse reflection spectra confirmed that the Fe2+ ions were successfully doped into the lattice position of Zn2+ ions after vacuum calcinations at 800 ◦ C for 3 h, which indicats
Please cite this article in press as: C. Li, et al., Hot-pressing and post-HIP treatment of Fe2+ :ZnS transparent ceramics from co-precipitated powders, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.051
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Fig. 8. Diffuse reflection spectrum (a) and EPR analysis (b) of the as-prepared Fe:ZnS powder.
that the co-precipitation method can be expected for convenient mass production of Fe2+ :ZnS powders used for laser ceramics or random lasers. Transparent Fe2+ :ZnS laser ceramics were achieved by combining the wet chemical co-precipitation method with hot-pressed sintering and post-HIP treatment. In-line transmittance of the ceramics with the best optical quality was about 65% at 4.5 m and 45% at 2.0 m, which was much better than the Fe2+ :ZnS ceramics from vacuum diffused powders. That is, this demonstration supplies a novel method for the fabrication of transparent Fe2+ :ZnS laser ceramics. Acknowledgments This work was supported by 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] S.B. Mirov, V.V. Fedorov, D. Martyshkin, I.S. Moskalev, M. Mirov, S. Vasilyev, Progress in mid-IR lasers based on Cr and Fe-doped II-VI chalcogenides, IEEE J. Sel. Top. Quantum Electron. 21 (2015) 292–310. [2] S. Vasilyev, I. Moskalev, M. Mirov, V. Smolsky, S. Mirov, V. Gapontsev, Recent breakthroughs in solid-State mid-IR laser technology, Laser Technol. J. 13 (2016), http://dx.doi.org/10.1002/latj.201600022. [3] S.D. Velikanov, N.A. Zaretsky, E.A. Zotov, S.Y. Kazantsev, I.G. Kononov, V.K. Yu, A.A. Maneshkin, K.N. Firsov, M.P. Frolov, I.M. Yutkin, Room-temperature 1.2-J Fe2+ :ZnSe laser, Quantum Electron. 46 (2016) 11–12. [4] K.N. Firsov, M.P. Frolov, E.M. Gavrishchuk, S.Y. Kazantsev, I.G. Kononov, V.K. Yu, A.A. Maneshkin, S.D. Velikanov, I.M. Yutkin, N.A. Zaretsky, E.A. Zotov, Laser on single-crystal ZnSe:Fe2+ with high pulse radiation energy at room temperature, Laser Phys. Lett. 13 (2016) 015002.
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Please cite this article in press as: C. Li, et al., Hot-pressing and post-HIP treatment of Fe2+ :ZnS transparent ceramics from co-precipitated powders, J Eur Ceram Soc (2017), http://dx.doi.org/10.1016/j.jeurceramsoc.2016.12.051