- Email: [email protected]
Preparation of high-quality transparent Al-rich spinel ceramics by reactive sintering
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Preparation of high-quality transparent Al-rich spinel ceramics by reactive sintering ⁎
Dan Hana,b, Jian Zhanga,c, , Peng Liud, Gui Lia, Liqiong Ane, Shiwei Wanga,c,
⁎
a
Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China c The State Key Lab of High Performance Ceramics and Surperfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China d School of Physics and Electronics Engineering, Jiangsu Normal University, Xuzhou 221116, PR China e College of Ocean Science and Engineering, Shanghai Maritime University, Shanghai 201306, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Spinels Reactive sintering Optical properties Grain size
Transparent Al-rich spinel ceramics (MgO·nAl2O3, n = 1.05–2.5) were prepared by reactive sintering in air followed by the hot isostatic press (HIP). Commercial MgO and γ-Al2O3 powders were used as the raw materials, and the effects of composition and HIP temperature on the transmittance and microstructure of resulting samples were investigated. To obtain the high optical quality, extra alumina (n ≥ 1.1) was used to help eliminate residual pores and suppress abnormal grain growth during the sintering process. The appropriate HIP temperature was also critical to realize the single-phase formation and prevent the generation of second-phase precipitates. The resulting samples with n = 1.1 and 1.3 exhibited excellent optical quality and fine grains below 5 µm after HIPed at 1550 °C.
1. Introduction Transparent magnesium aluminate spinel ceramics have been studied for more than 50 years since the first translucent sample was produced by the General Electric Company in the 1960s [1]. With the emergence of high-quality powders and advanced sintering methods such as hot press (HP) [2], HIP [3], spinel ceramics with high optical quality and large size have been prepared and widely used as transparent armors, high-temperature windows, IR domes and laser hosts [4–6]. Nowadays, the main issues that affect the preparation of transparent spinel ceramics are how to lower costs and further improve the optical quality and mechanical strength of samples [7]. Spinel (MgO·nAl2O3) is the only compound in the MgO-Al2O3 phase system, and it has a broad solubility for extra Al2O3 at high temperatures [8,9]. However, most studies about transparent spinel ceramics are mainly focused on the near-stoichiometric range. This is because that Al-rich spinel powders are very difficult to be synthesized through traditional wet-chemical methods, i.e. sol-gel [10–12], isopropoxide hydrolysis [13] and co-precipitation [14]. Versus stoichiometric spinel, Al-rich samples generally exhibit superior optical quality [15] and high mechanical strength [16,17]. Transparent Al-rich spinel ceramics with a wide composition range (1 < n < 3) have been successfully prepared
by reactive sintering of magnesium and aluminum compounds [18–21]. In these studies, high purity MgO and Al2O3 powders are the most commonly used raw materials because of their low prices, high chemical purity and wide sources. Waetzig et al. [22] prepared transparent spinel ceramics with n = 1–2.5 through pressureless reactive sintering in air and HIP treatment using high-purity MgO and α-Al2O3 powders as raw materials. The transmittances of Al-rich samples were much higher than that of stoichiometric sample. However, the HIP treatment was applied at 1750 °C, which caused large average grain sizes of 23–622 µm. Dericioglu et al. [16] hot pressed MgO and α-Al2O3 powder mixtures in vacuum at 1400 °C followed by HIP treatment at 1900 °C. The sample with n = 2 exhibited high transmittance and fracture toughness. In general, the preparation of high-quality transparent Al-rich spinel ceramics through reactive sintering of MgO and α-Al2O3 powders usually requires high HIP temperatures. This leads to severe grain growth, which is harmful to the mechanical strength and optical quality of resulting samples. The reaction between MgO and Al2O3 occurs by mutual diffusion of magnesium and aluminum ions [23]. The crystal structure of γ-Al2O3 was similar to that of spinel. This may be beneficial for the formation of spinel at the low temperature. Watzig et al. demonstrated that γ-Al2O3 started to react with MgO at 800 °C, much lower than α-Al2O3 (1000 °C)
⁎ Corresponding authors at: The State Key Lab of High Performance Ceramics and Surperfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China. E-mail addresses: [email protected] (J. Zhang), [email protected] (S. Wang).
https://doi.org/10.1016/j.ceramint.2017.11.089 Received 15 September 2017; Received in revised form 13 November 2017; Accepted 13 November 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Han, D., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.11.089
Ceramics International xxx (xxxx) xxx–xxx
D. Han et al.
Table 1 Pre-sintering temperature, phases after pre-sintering and minimum HIP temperature of Al-rich samples. n
1.05
1.1
1.3
1.5
2
2.5
Pre-sintering temperature (°C) Phases after presintering Minimum HIP temperature (°C)
1500
1500
1500
1450
1400
1350
spinel
spinel
spinel
1500
1500
1500
spinel + α-Al2O3 1550
spinel + α-Al2O3 1650
spinel + α-Al2O3 1800
[24]. Moreover, compared to α-Al2O3, the sintering temperature of sample can be obviously decreased because of the higher sintering activity and larger specific surface area of γ-Al2O3 [24–26]. Here, transparent Al-rich spinel ceramics (MgO·nAl2O3, 1.05 ≤ n ≤ 2.5) were prepared by reactive sintering in air followed by HIP treatment using high-purity commercial γ-Al2O3 and MgO powders as raw materials. The effects of composition (Al2O3 / MgO ratio, n) and HIP temperature were intensively studied to find optimal strategies for preparation of transparent spinel ceramics with high optical transmittance and fine grains.
Fig. 2. Thermally etched surface of the transparent spinel sample with n = 1.05, which HIPed at 1600 °C.
2. Experimental procedures High-purity MgO (99.99%, 150 nm) and γ-Al2O3 (99.99%, 100 nm) powders were used as raw materials. The raw powders were weighed according to different compositions of n = 1.05, 1.1, 1.3, 1.5, 2 and 2.5 (MgO·nAl2O3). Then they were mixed via wet ball milling for 12 h in ethanol using Al2O3 balls as the milling medium. After drying at 60 °C for 24 h in oven, the mixed powders were sieved through an 80-mesh screen and calcined at 800 °C to remove the residual organic impurities. The green bodies were shaped through dry pressing at 20 MPa followed by cold isostatic press at 200 MPa for 5 min. To eliminate open pores, the green bodies were pre-sintered in air between 1350 and 1500 °C for 3 h, which varied with the composition of samples. Finally, they were HIPed at 1550–1800 °C for 3 h in argon under a pressure of 200 MPa to obtain transparent samples. After annealing at 1200 °C for 6 h, the samples were double-side mechanical polished to 3 mm thick for further tests. The in-line transmittances of polished samples were tested with a UV–VIS–NIR spectrometer (Carry 5000 spectrophotometer, Varian, Seattle, USA) in the range of 190–1100 nm. Scanning electron microscopy (JSM-6390, JEOL, Tokyo, Japan) was used to observe the microstructure of samples. The average grain sizes of samples were
Fig. 3. In-line transmittances and pictures of the 3 mm thick transparent spinel samples with n = 1.1 and 1.5 (a: n = 1.1, b: n = 1.5).
measured by common linear intercept analysis from the SEM images. Optical microscopy (BX51 system microscope, Olympus, Tokyo, Japan) and EDS (SwiftED3000, Hitachi, Tokyo, Japan) were used to analyze the homogeneity of the resulting transparent samples. 3. Results and discussions 3.1. Effect of composition Table 1 shows the effects of composition on the pre-sintering and HIP temperatures. We can see that the pre-sintering temperature of samples with n = 1.05–1.3 was 1500 °C, much higher than the phase
Fig. 1. In-line transmittances of the transparent alumina-rich spinel samples sintered under optimal conditions. (3 mm thick, 1.05 ≤ n ≤ 2.5).
2
Ceramics International xxx (xxxx) xxx–xxx
D. Han et al.
Fig. 4. Optical microscopic images of the transparent spinel samples with n = 1.5 (a and b: sample HIPed at 1600 °C; c - f: sample HIPed at 1800 °C, where, e and f were rotated about 45° relative to c and d).
samples after pre-sintering. So HIP should be applied, not only enhancing the densification process, but also supplying the high enough temperature to complete the phase formation process. The elimination of second phases and residual pores was critical for obtaining high optical quality of samples. To exclude the influence of sintering temperature, samples sintered in the optimal conditions were used to investigate the effect of composition on optical quality of samples (Fig. 1). The transmittance of the sample with n = 1.05 was much lower than that of others, especially in the UV–visible range. The poor optical quality was mainly due to the residual pores, which existed both inside the large grains and around the grain boundaries (Fig. 2). The highest transmittance appeared in samples with n = 1.1 and 1.3, which was higher than 84% at the short wavelength of 400 nm, close to the theoretical value. The optical
formation temperature of spinel (below 1400 °C). It is clear that pure spinel phase was obtained after the pre-sintering process. With n increased from 1.5 to 2.5, the pre-sintering temperature of samples decreased from 1450 °C to 1350 °C due to the rapid densification rate of alumina. This was opposite to the variation trend of phase formation temperature, which increased from 1500 °C to 1800 °C according to the phase diagram of Al2O3-MgO [7]. So the pre-sintering temperature was far below the phase formation temperature of spinel, leading to the mixed compositions of spinel and α-Al2O3 existed in the pre-sintered samples. For the HIP process, it can be clearly seen that the minimum HIP temperature continuously increased with the increasing n. When n = 1.05–1.3, the purpose of HIP process was mainly to provide a sufficient driving force for the elimination of residual pores in pre-sintered samples. With n increasing above 1.5, residual alumina existed in the 3
Ceramics International xxx (xxxx) xxx–xxx
D. Han et al.
qualities slightly decreased with increasing alumina content (n = 1.5–2.5). This may be caused by small grain precipitates, and it will be illustrated in the following part. In general, the extra alumina must reach a certain content to eliminate residual pores and inhibit abnormal grain growth. 3.2. Effect of HIP temperature As mentioned above, HIP treatment can effectively eliminate the residual pores and alumina phase of pre-sintered samples. It usually plays a decisive role in the final properties of transparent ceramics. For a given composition, a wide range of HIP temperature was employed here to further investigate its effect on the optical quality of resulting spinel ceramics by correlating with microstructure. The transmittance spectras and pictures of transparent samples with n = 1.1 and 1.5 are displayed in Fig. 3. When the samples were HIPed at the minimum temperatures (1500 °C for n = 1.1 and 1550 °C for n = 1.5), the transmittances rapidly decreased as the wavelength decreased due to the serious scattering caused by intergranular pores. Therefore, the high HIP temperature was necessary for the completely elimination of residual pores. However, the variations of transmittances with HIP temperature increasing were closely related to the composition of samples. For example, samples with n = 1.1 all exhibited clear visual appearances and high transmittances when the HIP temperature increased from 1550 to 1700 °C. However, with n increased to 1.5, only the sample HIPed at 1600 °C exhibited the excellent optical quality. When applied HIP temperature increased to above 1600 °C, a hazy ring appeared in the outer edge of the sample. The thickness of this hazy ring would increase with temperature until the entire sample was hazy. The transmittance at 400 nm also rapidly decreased from 80.1% for the sample HIPed at 1600 °C to 57.1% for the sample HIPed at 1800 °C (Fig. 3b). Normally, the haze in transparent ceramics is caused by scattering centers, such as pores or second phases. Optical microscopy is an effective instrument to detect the type of defects in transparent ceramics. Pores are usually shown as black dots in the transmission mode, and an anisotropic crystal phase exhibits obvious light and dark changes in the orthogonal polarization mode. To illustrate the mechanism of haze formation, samples with n = 1.5, which HIPed at 1600 and 1800 °C, were observed through an optical microscopy (Fig. 4). For the sample HIPed at 1600 °C, the microstructures in the transmission and orthogonal polarization modes were both uniform and no defect was found. However, many small and needle-shaped grains were observed in the sample HIPed at 1800 °C in the transmission mode (Fig. 4c and e). Meanwhile, the small grains displayed obvious bright and dark changes in the orthogonal polarization mode (Fig. 4d and f) while rotating the sample. This indicated that the small grains exhibited an anisotropic optical property. In order to verify the composition of the small needle-shaped grains, the fracture surface of the sample with n = 1.5, which HIPed at 1800 °C, was observed by SEM (Fig. 5). A small needle-shaped grain was observed between two large grains (Fig. 5a), and it was similar to the grains observed via optical microscopy. This area was enlarged to analyze the specific composition (Fig. 5b) via EDS analysis (Table 2). To our surprise, the molar ratio of Al2O3 / MgO in grain II was 2.27, much higher than that in the surrounding grains (grains I and III). According to previous TEM studies [27–29], the meta-stable spinel phases with high n values firstly appeared before alumina precipitating from Al-rich spinel phase. During the HIP process, defects such as dislocations can be easily induced by the high temperature and high pressure treatment. This will provide enough nucleation positions for the generation of needle-shaped spinel grains. The physical properties of spinels with different compositions usually exhibit some difference, which can lead to heterogeneous property of samples. For example, the different coefficient of thermal expansion between the grains with n = 1.45 and n = 2.27 may cause stress-induced birefringence. Thus, the grain with
Fig. 5. Fracture surface of the transparent spinel sample with n = 1.5, which HIPed at
Table 2 EDS analysis of the grains in Fig. 5.
Grain I Grain II Grain III
Mg (%)
Al (%)
O (%)
n
10.32 7.72 10.40
29.92 35.11 30.25
59.76 57.17 59.35
1.45 2.27 1.45
Fig. 6. Average gain size as a function of HIP temperature for the transparent spinel samples with n = 1.1, 1.3 and 1.5.
high n might exhibit obvious anisotropic optical properties in spite of having a cubic crystal structure. Furthermore, the meta-stable spinel with a high n value may transform into alumina and matrix spinel if the cooling rate is sufficiently slow. 4
Ceramics International xxx (xxxx) xxx–xxx
D. Han et al.
Fig. 7. Thermally etched surfaces of the transparent spinel samples with n = 1.1 and 1.5, which HIPed at 1550 °C and 1700 °C, respectively (a and b: n = 1.1; c and d: n = 1.5).
of the People's Republic of China (No. 2017YFB0310501); Chinese Academy of Sciences (No. Y62YZ4140G); and Science and Technology Commission of Shanghai Municipality (No. 15ZR1419600).
The influence of HIP temperatures on the grain growth of Al-rich samples is shown in Fig. 6 and Fig. 7. As the HIP temperature increased, the average grain sizes of samples continuously increased, and the grain growth rate increased with n (Fig. 6). At 1550 °C, the average grain sizes of samples with different compositions were almost identical and they were below 5 µm (Fig. 7a and c). However, at 1700 °C, the average grain size of sample with n = 1.5 rapidly grew up to 31 µm (Fig. 7d), much larger than that of sample with n = 1.1 (Fig. 7b). Therefore, low HIP temperatures were beneficial to prevent rapid grain growth.
References [1] Mark C.L. Patterson, Jenni E. Caiazza, Don W. Roy, transparent spinel development, Inorg. Opt. Mater. 4102 (2000) 59–68. [2] L. Esposito, A. Piancastelli, P. Miceli, S. Martelli, A thermodynamic approach to obtaining transparent spinel (MgAl2O4) by hot pressing, J. Eur. Ceram. Soc. 35 (2015) 651–661. [3] A. Krell, T. Hutzler, J. Klimke, A. PotthoffJens, Nano-processing for larger finegrained windows of transparent spinel, Adv. Ceram. Armor. VIII (2010) 167–182. [4] A. Rothman, S. Kalabukhov, N. Sverdlov, M.P. Dariel, N. Frage, The effect of grain size on the mechanical and optical properties of spark plasma sintering-processed magnesium aluminate spinel MgAl2O4, Int. J. Appl. Ceram. Technol. 11 (2014) 146–153. [5] L. Lallemant, J. Petit, S. Lalanne, S. Landais, S. Trombert, L. Vernhet, R. Viroulaud, Modeling of the green body drying step to obtain large size transparent magnesiumaluminate spinel samples, J. Eur. Ceram. Soc. 34 (2014) 791–799. [6] A. Krell, T. Hutzler, J. Klimke, Physics and Technology of Transparent Ceramic Armor: Sintered Al2O3 vs Cubic Materials. 〈http://www.rto.nato.int/abstracts〉. asp. [7] M. Rubat du Merac, H.J. Kleebe, M.M. Müller, I.E. Reimanis, Fifty years of research and development coming to fruition; Unraveling the complex interactions during processing of transparent magnesium aluminate (MgAl2O4) spinel, J. Am. Ceram. Soc. 96 (2013) 3341–3365. [8] P.C. Panda, R. PAJ, Kinetics of precipitation of α-Al2O3 supersaturated MgO.2A12O3 spinel solid solution, J. Am. Ceram. Soc. 69 (1986) 365–373. [9] K.R. Wilkerson, J.D. Smith, J.G. Hemrick, Metastability in the MgAl2O4-Al2O3 system, Int. J. Appl. Ceram. Tec. 11 (2014) 1020–1024. [10] S. Sanjabi, A. Obeydavi, Synthesis and characterization of nanocrystalline MgAl2O4 spinel via modified sol–gel method, J. Alloy. Compd. 645 (2015) 535–540. [11] S.S. Balabanov, R.P. Yavetskiy, A.V. Belyaev, E.M. Gavrishchuk, V.V. Drobotenko, I.I. Evdokimov, A.V. Novikova, O.V. Palashov, D.A. Permin, V.G. Pimenov, Fabrication of transparent MgAl2O4 ceramics by hot-pressing of sol-gel-derived nanopowders, Ceram. Int. 41 (2015) 13366–13371. [12] R.W. Tustison, R. Cook, M. Kochis, I. Reimanis, H.J. Kleebe, A new powder production route for transparent spinel windows: powder synthesis and window
4. Conclusions Transparent Al-rich spinel ceramics with high transmittance and fine grains were prepared by reactive sintering followed by HIP treatment using MgO and γ-Al2O3 powders as the raw materials. Extra alumina (n ≥ 1.1) effectively helped eliminate residual pores and restrain abnormal grain growth. In addition, the HIP temperature played an important role in the optical quality and microstructure of resulting samples. The transmittance and average grain size of sample with n = 1.1 were not effectively influenced by the HIP temperature. However, for the sample with n = 1.5, the high HIP temperature generated needle-shaped spinel grains with higher n. This led to a severe deterioration of optical transmittance. In summary, transparent Al-rich spinel ceramics with excellent properties were successfully prepared by optimizing the composition and sintering conditions. The n = 1.1 and 1.3 samples HIPed at 1550 °C exhibited transmittance values over 84% at 400 nm and fine grains below 5 µm. Acknowledgements This work was supported by the Ministry of Science and Technology 5
Ceramics International xxx (xxxx) xxx–xxx
D. Han et al.
[20] M. Vu, R. Haber, H. Gocmez, Preparation and sintering of A12O3-doped magnesium aluminate spinel, Adv. Ceram. Armor. VIII (2013) 93. [21] A.F. Dericioglu1, A.R. Boccaccini, I. Dlouhy, Y. Kagawa, Effect of chemical composition on the optical properties and fracture toughness of transparent magnesium aluminate spinel ceramics, Mater. Trans. 46 (2005) 996–1003. [22] K. Waetzig, A. Krell, The effect of composition on the optical properties and hardness of transparent Al-rich MgO·nAl2O3 spinel ceramics, J. Am. Ceram. Soc. 99 (2016) 946–953. [23] R.C. Rossi, M. Fulrath, Epitaxial growth of spinel by reaction in the solid state, J. Am. Ceram. Soc. 46 (1963) 145–149. [24] K. Watzig, A. Krell, Transparent spinel by reactive sintering of different alumina, Process Eng. 86 (2009) 47–49. [25] Z. Zhihui, L. Nan, R. Guozhi, Effect of polymorphism of Al2O3 on sintering and grain growth of magnesia aluminate spinel, Sci. Sinter. 39 (2007) 9–15. [26] D. Han, J. Zhang, P. Liu, S. Wang, Effect of polymorphism of Al2O3 on the sintering and microstructure of transparent MgAl2O4 ceramics, Opt. Mater. 71 (2017) 62–65. [27] W.T. Donlon, T.E. Mitchell, A.H. Heuer, Precipitation in non-stoichiometric spinel, J. Mater. Sci. 17 (1982) 1389–1397. [28] N. Doukhan, J.C. Doukhan, B. Escaig, T.E.M. study of high temperature precipitation in (Al2O3)n MgO spinels, Mat. Res. Bull. 11 (1976) 125–134. [29] M.H. Lewis, Precipitation in non-stoichiometric spinel crystals, Philos. Mag. 20 (1969) 985–998.
properties, Window Dome Technol. Mater. IX 5786 (2005) 41–47. [13] S.S. Balabanov, V.E. Vaganov, E.M. Gavrishchuk, V.V. Drobotenko, D.A. Permin, A.V. Fedin, Effect of magnesium aluminum isopropoxide hydrolysis conditions on the properties of magnesium aluminate spinel powders, Inorg. Mater. 50 (2014) 830–836. [14] J. Rufner, D. Anderson, K. van Benthem, R.H.R. Castro, R. Hay, Synthesis and sintering behavior of ultrafine (< 10 nm) magnesium aluminate spinel nanoparticles, J. Am. Ceram. Soc. 96 (2013) 2077–2085. [15] A. Krell, K. Waetzig, J. Klimke, Influence of the structure of MgO·nAl2O3 spinel lattices on transparent ceramics processing and properties, J. Eur. Ceram. Soc. 32 (2012) 2887–2898. [16] A.F. Dericioglu, A.R. Boccaccini, I. Dlouhy, Y. Kagawa, Effect of chemical composition on the optical properties and fracture toughness of transparent magnesium aluminate spinel ceramics, Mater. Trans. 46 (2005) 996–1003. [17] C.B. Huang, T.C. Lu, L.B. Lin, M.Y. Lei, C.X. Huang, A Study on toughening and strengthening of Mg-Al spinel transparent ceramics, Key Eng. Mater. 336–338 (2007) 1207–1210. [18] M. Shimada, T. Saito, T. Sato, Fabrication of transparent spinel polycrystalline materials, Mater. Lett. 28 (1996) 413–415. [19] A.C. Sutorik, C. Cooper, G. Gilde, M. Cinibulk, Visible light transparency for polycrystalline ceramics of MgO·2Al2O3 and MgO·2.5Al2O3 spinel solid solutions, J. Am. Ceram. Soc. 96 (2013) 3704–3707.
6
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Preparation of high-quality transparent Al-rich spinel ceramics by reactive sintering ⁎
Dan Hana,b, Jian Zhanga,c, , Peng Liud, Gui Lia, Liqiong Ane, Shiwei Wanga,c,
⁎
a
Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China c The State Key Lab of High Performance Ceramics and Surperfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China d School of Physics and Electronics Engineering, Jiangsu Normal University, Xuzhou 221116, PR China e College of Ocean Science and Engineering, Shanghai Maritime University, Shanghai 201306, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Spinels Reactive sintering Optical properties Grain size
Transparent Al-rich spinel ceramics (MgO·nAl2O3, n = 1.05–2.5) were prepared by reactive sintering in air followed by the hot isostatic press (HIP). Commercial MgO and γ-Al2O3 powders were used as the raw materials, and the effects of composition and HIP temperature on the transmittance and microstructure of resulting samples were investigated. To obtain the high optical quality, extra alumina (n ≥ 1.1) was used to help eliminate residual pores and suppress abnormal grain growth during the sintering process. The appropriate HIP temperature was also critical to realize the single-phase formation and prevent the generation of second-phase precipitates. The resulting samples with n = 1.1 and 1.3 exhibited excellent optical quality and fine grains below 5 µm after HIPed at 1550 °C.
1. Introduction Transparent magnesium aluminate spinel ceramics have been studied for more than 50 years since the first translucent sample was produced by the General Electric Company in the 1960s [1]. With the emergence of high-quality powders and advanced sintering methods such as hot press (HP) [2], HIP [3], spinel ceramics with high optical quality and large size have been prepared and widely used as transparent armors, high-temperature windows, IR domes and laser hosts [4–6]. Nowadays, the main issues that affect the preparation of transparent spinel ceramics are how to lower costs and further improve the optical quality and mechanical strength of samples [7]. Spinel (MgO·nAl2O3) is the only compound in the MgO-Al2O3 phase system, and it has a broad solubility for extra Al2O3 at high temperatures [8,9]. However, most studies about transparent spinel ceramics are mainly focused on the near-stoichiometric range. This is because that Al-rich spinel powders are very difficult to be synthesized through traditional wet-chemical methods, i.e. sol-gel [10–12], isopropoxide hydrolysis [13] and co-precipitation [14]. Versus stoichiometric spinel, Al-rich samples generally exhibit superior optical quality [15] and high mechanical strength [16,17]. Transparent Al-rich spinel ceramics with a wide composition range (1 < n < 3) have been successfully prepared
by reactive sintering of magnesium and aluminum compounds [18–21]. In these studies, high purity MgO and Al2O3 powders are the most commonly used raw materials because of their low prices, high chemical purity and wide sources. Waetzig et al. [22] prepared transparent spinel ceramics with n = 1–2.5 through pressureless reactive sintering in air and HIP treatment using high-purity MgO and α-Al2O3 powders as raw materials. The transmittances of Al-rich samples were much higher than that of stoichiometric sample. However, the HIP treatment was applied at 1750 °C, which caused large average grain sizes of 23–622 µm. Dericioglu et al. [16] hot pressed MgO and α-Al2O3 powder mixtures in vacuum at 1400 °C followed by HIP treatment at 1900 °C. The sample with n = 2 exhibited high transmittance and fracture toughness. In general, the preparation of high-quality transparent Al-rich spinel ceramics through reactive sintering of MgO and α-Al2O3 powders usually requires high HIP temperatures. This leads to severe grain growth, which is harmful to the mechanical strength and optical quality of resulting samples. The reaction between MgO and Al2O3 occurs by mutual diffusion of magnesium and aluminum ions [23]. The crystal structure of γ-Al2O3 was similar to that of spinel. This may be beneficial for the formation of spinel at the low temperature. Watzig et al. demonstrated that γ-Al2O3 started to react with MgO at 800 °C, much lower than α-Al2O3 (1000 °C)
⁎ Corresponding authors at: The State Key Lab of High Performance Ceramics and Surperfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China. E-mail addresses: [email protected] (J. Zhang), [email protected] (S. Wang).
https://doi.org/10.1016/j.ceramint.2017.11.089 Received 15 September 2017; Received in revised form 13 November 2017; Accepted 13 November 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Han, D., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.11.089
Ceramics International xxx (xxxx) xxx–xxx
D. Han et al.
Table 1 Pre-sintering temperature, phases after pre-sintering and minimum HIP temperature of Al-rich samples. n
1.05
1.1
1.3
1.5
2
2.5
Pre-sintering temperature (°C) Phases after presintering Minimum HIP temperature (°C)
1500
1500
1500
1450
1400
1350
spinel
spinel
spinel
1500
1500
1500
spinel + α-Al2O3 1550
spinel + α-Al2O3 1650
spinel + α-Al2O3 1800
[24]. Moreover, compared to α-Al2O3, the sintering temperature of sample can be obviously decreased because of the higher sintering activity and larger specific surface area of γ-Al2O3 [24–26]. Here, transparent Al-rich spinel ceramics (MgO·nAl2O3, 1.05 ≤ n ≤ 2.5) were prepared by reactive sintering in air followed by HIP treatment using high-purity commercial γ-Al2O3 and MgO powders as raw materials. The effects of composition (Al2O3 / MgO ratio, n) and HIP temperature were intensively studied to find optimal strategies for preparation of transparent spinel ceramics with high optical transmittance and fine grains.
Fig. 2. Thermally etched surface of the transparent spinel sample with n = 1.05, which HIPed at 1600 °C.
2. Experimental procedures High-purity MgO (99.99%, 150 nm) and γ-Al2O3 (99.99%, 100 nm) powders were used as raw materials. The raw powders were weighed according to different compositions of n = 1.05, 1.1, 1.3, 1.5, 2 and 2.5 (MgO·nAl2O3). Then they were mixed via wet ball milling for 12 h in ethanol using Al2O3 balls as the milling medium. After drying at 60 °C for 24 h in oven, the mixed powders were sieved through an 80-mesh screen and calcined at 800 °C to remove the residual organic impurities. The green bodies were shaped through dry pressing at 20 MPa followed by cold isostatic press at 200 MPa for 5 min. To eliminate open pores, the green bodies were pre-sintered in air between 1350 and 1500 °C for 3 h, which varied with the composition of samples. Finally, they were HIPed at 1550–1800 °C for 3 h in argon under a pressure of 200 MPa to obtain transparent samples. After annealing at 1200 °C for 6 h, the samples were double-side mechanical polished to 3 mm thick for further tests. The in-line transmittances of polished samples were tested with a UV–VIS–NIR spectrometer (Carry 5000 spectrophotometer, Varian, Seattle, USA) in the range of 190–1100 nm. Scanning electron microscopy (JSM-6390, JEOL, Tokyo, Japan) was used to observe the microstructure of samples. The average grain sizes of samples were
Fig. 3. In-line transmittances and pictures of the 3 mm thick transparent spinel samples with n = 1.1 and 1.5 (a: n = 1.1, b: n = 1.5).
measured by common linear intercept analysis from the SEM images. Optical microscopy (BX51 system microscope, Olympus, Tokyo, Japan) and EDS (SwiftED3000, Hitachi, Tokyo, Japan) were used to analyze the homogeneity of the resulting transparent samples. 3. Results and discussions 3.1. Effect of composition Table 1 shows the effects of composition on the pre-sintering and HIP temperatures. We can see that the pre-sintering temperature of samples with n = 1.05–1.3 was 1500 °C, much higher than the phase
Fig. 1. In-line transmittances of the transparent alumina-rich spinel samples sintered under optimal conditions. (3 mm thick, 1.05 ≤ n ≤ 2.5).
2
Ceramics International xxx (xxxx) xxx–xxx
D. Han et al.
Fig. 4. Optical microscopic images of the transparent spinel samples with n = 1.5 (a and b: sample HIPed at 1600 °C; c - f: sample HIPed at 1800 °C, where, e and f were rotated about 45° relative to c and d).
samples after pre-sintering. So HIP should be applied, not only enhancing the densification process, but also supplying the high enough temperature to complete the phase formation process. The elimination of second phases and residual pores was critical for obtaining high optical quality of samples. To exclude the influence of sintering temperature, samples sintered in the optimal conditions were used to investigate the effect of composition on optical quality of samples (Fig. 1). The transmittance of the sample with n = 1.05 was much lower than that of others, especially in the UV–visible range. The poor optical quality was mainly due to the residual pores, which existed both inside the large grains and around the grain boundaries (Fig. 2). The highest transmittance appeared in samples with n = 1.1 and 1.3, which was higher than 84% at the short wavelength of 400 nm, close to the theoretical value. The optical
formation temperature of spinel (below 1400 °C). It is clear that pure spinel phase was obtained after the pre-sintering process. With n increased from 1.5 to 2.5, the pre-sintering temperature of samples decreased from 1450 °C to 1350 °C due to the rapid densification rate of alumina. This was opposite to the variation trend of phase formation temperature, which increased from 1500 °C to 1800 °C according to the phase diagram of Al2O3-MgO [7]. So the pre-sintering temperature was far below the phase formation temperature of spinel, leading to the mixed compositions of spinel and α-Al2O3 existed in the pre-sintered samples. For the HIP process, it can be clearly seen that the minimum HIP temperature continuously increased with the increasing n. When n = 1.05–1.3, the purpose of HIP process was mainly to provide a sufficient driving force for the elimination of residual pores in pre-sintered samples. With n increasing above 1.5, residual alumina existed in the 3
Ceramics International xxx (xxxx) xxx–xxx
D. Han et al.
qualities slightly decreased with increasing alumina content (n = 1.5–2.5). This may be caused by small grain precipitates, and it will be illustrated in the following part. In general, the extra alumina must reach a certain content to eliminate residual pores and inhibit abnormal grain growth. 3.2. Effect of HIP temperature As mentioned above, HIP treatment can effectively eliminate the residual pores and alumina phase of pre-sintered samples. It usually plays a decisive role in the final properties of transparent ceramics. For a given composition, a wide range of HIP temperature was employed here to further investigate its effect on the optical quality of resulting spinel ceramics by correlating with microstructure. The transmittance spectras and pictures of transparent samples with n = 1.1 and 1.5 are displayed in Fig. 3. When the samples were HIPed at the minimum temperatures (1500 °C for n = 1.1 and 1550 °C for n = 1.5), the transmittances rapidly decreased as the wavelength decreased due to the serious scattering caused by intergranular pores. Therefore, the high HIP temperature was necessary for the completely elimination of residual pores. However, the variations of transmittances with HIP temperature increasing were closely related to the composition of samples. For example, samples with n = 1.1 all exhibited clear visual appearances and high transmittances when the HIP temperature increased from 1550 to 1700 °C. However, with n increased to 1.5, only the sample HIPed at 1600 °C exhibited the excellent optical quality. When applied HIP temperature increased to above 1600 °C, a hazy ring appeared in the outer edge of the sample. The thickness of this hazy ring would increase with temperature until the entire sample was hazy. The transmittance at 400 nm also rapidly decreased from 80.1% for the sample HIPed at 1600 °C to 57.1% for the sample HIPed at 1800 °C (Fig. 3b). Normally, the haze in transparent ceramics is caused by scattering centers, such as pores or second phases. Optical microscopy is an effective instrument to detect the type of defects in transparent ceramics. Pores are usually shown as black dots in the transmission mode, and an anisotropic crystal phase exhibits obvious light and dark changes in the orthogonal polarization mode. To illustrate the mechanism of haze formation, samples with n = 1.5, which HIPed at 1600 and 1800 °C, were observed through an optical microscopy (Fig. 4). For the sample HIPed at 1600 °C, the microstructures in the transmission and orthogonal polarization modes were both uniform and no defect was found. However, many small and needle-shaped grains were observed in the sample HIPed at 1800 °C in the transmission mode (Fig. 4c and e). Meanwhile, the small grains displayed obvious bright and dark changes in the orthogonal polarization mode (Fig. 4d and f) while rotating the sample. This indicated that the small grains exhibited an anisotropic optical property. In order to verify the composition of the small needle-shaped grains, the fracture surface of the sample with n = 1.5, which HIPed at 1800 °C, was observed by SEM (Fig. 5). A small needle-shaped grain was observed between two large grains (Fig. 5a), and it was similar to the grains observed via optical microscopy. This area was enlarged to analyze the specific composition (Fig. 5b) via EDS analysis (Table 2). To our surprise, the molar ratio of Al2O3 / MgO in grain II was 2.27, much higher than that in the surrounding grains (grains I and III). According to previous TEM studies [27–29], the meta-stable spinel phases with high n values firstly appeared before alumina precipitating from Al-rich spinel phase. During the HIP process, defects such as dislocations can be easily induced by the high temperature and high pressure treatment. This will provide enough nucleation positions for the generation of needle-shaped spinel grains. The physical properties of spinels with different compositions usually exhibit some difference, which can lead to heterogeneous property of samples. For example, the different coefficient of thermal expansion between the grains with n = 1.45 and n = 2.27 may cause stress-induced birefringence. Thus, the grain with
Fig. 5. Fracture surface of the transparent spinel sample with n = 1.5, which HIPed at
Table 2 EDS analysis of the grains in Fig. 5.
Grain I Grain II Grain III
Mg (%)
Al (%)
O (%)
n
10.32 7.72 10.40
29.92 35.11 30.25
59.76 57.17 59.35
1.45 2.27 1.45
Fig. 6. Average gain size as a function of HIP temperature for the transparent spinel samples with n = 1.1, 1.3 and 1.5.
high n might exhibit obvious anisotropic optical properties in spite of having a cubic crystal structure. Furthermore, the meta-stable spinel with a high n value may transform into alumina and matrix spinel if the cooling rate is sufficiently slow. 4
Ceramics International xxx (xxxx) xxx–xxx
D. Han et al.
Fig. 7. Thermally etched surfaces of the transparent spinel samples with n = 1.1 and 1.5, which HIPed at 1550 °C and 1700 °C, respectively (a and b: n = 1.1; c and d: n = 1.5).
of the People's Republic of China (No. 2017YFB0310501); Chinese Academy of Sciences (No. Y62YZ4140G); and Science and Technology Commission of Shanghai Municipality (No. 15ZR1419600).
The influence of HIP temperatures on the grain growth of Al-rich samples is shown in Fig. 6 and Fig. 7. As the HIP temperature increased, the average grain sizes of samples continuously increased, and the grain growth rate increased with n (Fig. 6). At 1550 °C, the average grain sizes of samples with different compositions were almost identical and they were below 5 µm (Fig. 7a and c). However, at 1700 °C, the average grain size of sample with n = 1.5 rapidly grew up to 31 µm (Fig. 7d), much larger than that of sample with n = 1.1 (Fig. 7b). Therefore, low HIP temperatures were beneficial to prevent rapid grain growth.
References [1] Mark C.L. Patterson, Jenni E. Caiazza, Don W. Roy, transparent spinel development, Inorg. Opt. Mater. 4102 (2000) 59–68. [2] L. Esposito, A. Piancastelli, P. Miceli, S. Martelli, A thermodynamic approach to obtaining transparent spinel (MgAl2O4) by hot pressing, J. Eur. Ceram. Soc. 35 (2015) 651–661. [3] A. Krell, T. Hutzler, J. Klimke, A. PotthoffJens, Nano-processing for larger finegrained windows of transparent spinel, Adv. Ceram. Armor. VIII (2010) 167–182. [4] A. Rothman, S. Kalabukhov, N. Sverdlov, M.P. Dariel, N. Frage, The effect of grain size on the mechanical and optical properties of spark plasma sintering-processed magnesium aluminate spinel MgAl2O4, Int. J. Appl. Ceram. Technol. 11 (2014) 146–153. [5] L. Lallemant, J. Petit, S. Lalanne, S. Landais, S. Trombert, L. Vernhet, R. Viroulaud, Modeling of the green body drying step to obtain large size transparent magnesiumaluminate spinel samples, J. Eur. Ceram. Soc. 34 (2014) 791–799. [6] A. Krell, T. Hutzler, J. Klimke, Physics and Technology of Transparent Ceramic Armor: Sintered Al2O3 vs Cubic Materials. 〈http://www.rto.nato.int/abstracts〉. asp. [7] M. Rubat du Merac, H.J. Kleebe, M.M. Müller, I.E. Reimanis, Fifty years of research and development coming to fruition; Unraveling the complex interactions during processing of transparent magnesium aluminate (MgAl2O4) spinel, J. Am. Ceram. Soc. 96 (2013) 3341–3365. [8] P.C. Panda, R. PAJ, Kinetics of precipitation of α-Al2O3 supersaturated MgO.2A12O3 spinel solid solution, J. Am. Ceram. Soc. 69 (1986) 365–373. [9] K.R. Wilkerson, J.D. Smith, J.G. Hemrick, Metastability in the MgAl2O4-Al2O3 system, Int. J. Appl. Ceram. Tec. 11 (2014) 1020–1024. [10] S. Sanjabi, A. Obeydavi, Synthesis and characterization of nanocrystalline MgAl2O4 spinel via modified sol–gel method, J. Alloy. Compd. 645 (2015) 535–540. [11] S.S. Balabanov, R.P. Yavetskiy, A.V. Belyaev, E.M. Gavrishchuk, V.V. Drobotenko, I.I. Evdokimov, A.V. Novikova, O.V. Palashov, D.A. Permin, V.G. Pimenov, Fabrication of transparent MgAl2O4 ceramics by hot-pressing of sol-gel-derived nanopowders, Ceram. Int. 41 (2015) 13366–13371. [12] R.W. Tustison, R. Cook, M. Kochis, I. Reimanis, H.J. Kleebe, A new powder production route for transparent spinel windows: powder synthesis and window
4. Conclusions Transparent Al-rich spinel ceramics with high transmittance and fine grains were prepared by reactive sintering followed by HIP treatment using MgO and γ-Al2O3 powders as the raw materials. Extra alumina (n ≥ 1.1) effectively helped eliminate residual pores and restrain abnormal grain growth. In addition, the HIP temperature played an important role in the optical quality and microstructure of resulting samples. The transmittance and average grain size of sample with n = 1.1 were not effectively influenced by the HIP temperature. However, for the sample with n = 1.5, the high HIP temperature generated needle-shaped spinel grains with higher n. This led to a severe deterioration of optical transmittance. In summary, transparent Al-rich spinel ceramics with excellent properties were successfully prepared by optimizing the composition and sintering conditions. The n = 1.1 and 1.3 samples HIPed at 1550 °C exhibited transmittance values over 84% at 400 nm and fine grains below 5 µm. Acknowledgements This work was supported by the Ministry of Science and Technology 5
Ceramics International xxx (xxxx) xxx–xxx
D. Han et al.
[20] M. Vu, R. Haber, H. Gocmez, Preparation and sintering of A12O3-doped magnesium aluminate spinel, Adv. Ceram. Armor. VIII (2013) 93. [21] A.F. Dericioglu1, A.R. Boccaccini, I. Dlouhy, Y. Kagawa, Effect of chemical composition on the optical properties and fracture toughness of transparent magnesium aluminate spinel ceramics, Mater. Trans. 46 (2005) 996–1003. [22] K. Waetzig, A. Krell, The effect of composition on the optical properties and hardness of transparent Al-rich MgO·nAl2O3 spinel ceramics, J. Am. Ceram. Soc. 99 (2016) 946–953. [23] R.C. Rossi, M. Fulrath, Epitaxial growth of spinel by reaction in the solid state, J. Am. Ceram. Soc. 46 (1963) 145–149. [24] K. Watzig, A. Krell, Transparent spinel by reactive sintering of different alumina, Process Eng. 86 (2009) 47–49. [25] Z. Zhihui, L. Nan, R. Guozhi, Effect of polymorphism of Al2O3 on sintering and grain growth of magnesia aluminate spinel, Sci. Sinter. 39 (2007) 9–15. [26] D. Han, J. Zhang, P. Liu, S. Wang, Effect of polymorphism of Al2O3 on the sintering and microstructure of transparent MgAl2O4 ceramics, Opt. Mater. 71 (2017) 62–65. [27] W.T. Donlon, T.E. Mitchell, A.H. Heuer, Precipitation in non-stoichiometric spinel, J. Mater. Sci. 17 (1982) 1389–1397. [28] N. Doukhan, J.C. Doukhan, B. Escaig, T.E.M. study of high temperature precipitation in (Al2O3)n MgO spinels, Mat. Res. Bull. 11 (1976) 125–134. [29] M.H. Lewis, Precipitation in non-stoichiometric spinel crystals, Philos. Mag. 20 (1969) 985–998.
properties, Window Dome Technol. Mater. IX 5786 (2005) 41–47. [13] S.S. Balabanov, V.E. Vaganov, E.M. Gavrishchuk, V.V. Drobotenko, D.A. Permin, A.V. Fedin, Effect of magnesium aluminum isopropoxide hydrolysis conditions on the properties of magnesium aluminate spinel powders, Inorg. Mater. 50 (2014) 830–836. [14] J. Rufner, D. Anderson, K. van Benthem, R.H.R. Castro, R. Hay, Synthesis and sintering behavior of ultrafine (< 10 nm) magnesium aluminate spinel nanoparticles, J. Am. Ceram. Soc. 96 (2013) 2077–2085. [15] A. Krell, K. Waetzig, J. Klimke, Influence of the structure of MgO·nAl2O3 spinel lattices on transparent ceramics processing and properties, J. Eur. Ceram. Soc. 32 (2012) 2887–2898. [16] A.F. Dericioglu, A.R. Boccaccini, I. Dlouhy, Y. Kagawa, Effect of chemical composition on the optical properties and fracture toughness of transparent magnesium aluminate spinel ceramics, Mater. Trans. 46 (2005) 996–1003. [17] C.B. Huang, T.C. Lu, L.B. Lin, M.Y. Lei, C.X. Huang, A Study on toughening and strengthening of Mg-Al spinel transparent ceramics, Key Eng. Mater. 336–338 (2007) 1207–1210. [18] M. Shimada, T. Saito, T. Sato, Fabrication of transparent spinel polycrystalline materials, Mater. Lett. 28 (1996) 413–415. [19] A.C. Sutorik, C. Cooper, G. Gilde, M. Cinibulk, Visible light transparency for polycrystalline ceramics of MgO·2Al2O3 and MgO·2.5Al2O3 spinel solid solutions, J. Am. Ceram. Soc. 96 (2013) 3704–3707.
6