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Pressureless sintering of highly transparent AlON ceramics with CaCO3 doping
Scripta Materialia 157 (2018) 148–151
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Regular article
Pressureless sintering of highly transparent AlON ceramics with CaCO3 doping Yingchun Shan a, Xiannian Sun a,⁎, Binglin Ren a, Haokai Wu a, Xialu Wei b, Eugene A. Olevsky b, Jiujun Xu a,⁎, Jiangtao Li c a b c
Department of Materials Science and Engineering, Dalian Maritime University, Dalian 116026, China College of Engineering, San Diego State University, San Diego, CA 92182, USA Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China
a r t i c l e
i n f o
Article history: Received 20 June 2018 Accepted 13 August 2018 Available online xxxx Keywords: AlON CaCO3 addtitive Transparent ceramics Densification Grain growth
a b s t r a c t Employing CaCO3 as sintering additive, highly transparent aluminum oxynitride (AlON) ceramics were pressurelessly fabricated from AlON powder at 1870 °C during150 min. The transmittance of the AlON doped with 0.3–0.4 wt% CaCO3 is up to 83–85% at ~3700 nm for 2 mm thickness samples, and their transmittances are consistently higher than that of the AlON doped with the ideal amount of Y2O3 at wavelength ranging from 200 nm to 6000 nm. The AlON doped with CaCO3 exhibits the transmittance of 71% at 4800 nm (typical band for infrared targeting), which is higher by 6% than that of the doped Y2O3. © 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Transparent aluminum oxynitride (AlON) ceramics are one of the most promising transparent ceramic materials for infrared/visible windows used in severe environments, such as high temperature, abrasion, corrosion, etc., due to their excellent optical transparency properties, high strength and hardness, and good chemical inertness [1–4]. AlON ceramics can be sintered by pressureless sintering, hot pressing sintering, hot isostatic pressing, spark plasma sintering, etc. [5–8]. Among those methods, pressureless sintering is a preferred technology due to its significant advantage in fabricating large size and complicated components. Generally, sintering additive is always required to fabricate transparent AlON ceramics. Currently, Y2O3, La2O3 and MgO have been chosen as sintering additives to accelerate the densification process of AlON ceramics [5,9–13]. Among these additives, Y3+ is believed to enhance the mobility of grain boundaries and accelerate grain growth, and 0.5 wt% Y2O3 was reported to be the ideal doping amount to obtain AlON ceramics with high transparency [9,14]. At the same time, La3+ and Mg2+ can inhibit the abnormal grains growth [9,13,15]. Consequently, La2O3 and MgO are usually selected as grain growth inhibiters to co-dope with Y2O3 to fabricate transparent AlON ceramics [13,15]. It is commonly recognized that high densification and reducing the amount of scattering and refracting sources are all a must to ensure the optical quality of fabricated AlON ceramics. To further improve the
⁎ Corresponding authors. E-mail addresses: [email protected] (X. Sun), [email protected] (J. Xu).
https://doi.org/10.1016/j.scriptamat.2018.08.023 1359-6462/© 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
properties of transparent AlON ceramics, it is necessary to find more sintering additive candidates. Recently, it was reported that proper amount of CaO dopant can promote densification and grain growth of MgO·1.5Al2O3 and YAG ceramics at a suitable sintering temperature [16,17]. It is understandable that it is Ca2+ that is being used as dopant for fabrication of MgO·1.5Al2O3 and YAG ceramics, which implies that it is highly possible that Ca2+ can also induce positive properties in the densification process of AlON ceramics. It is well known that CaO is easy to react with H2O to form Ca(OH)2 at room temperature, which means a nonignorable small amount of Ca(OH)2 may exist in CaO additive during fabrication process. This Ca(OH)2 is decomposed into CaO and H2O at ~1200 °C, where H2O is incompatible with sintering furnace environment. To avoid the risk induced by unexpected H2O, CaCO3 was mixed with AlON powder to fabricate transparent AlON ceramics in this study. As a matter of fact, CaCO3 decomposes into CaO and CO2 at ~825 °C. More importantly, the decomposition temperature of CaCO3 is much lower than the starting temperature of AlON ceramics sintering (~1300 °C) [6]. Therefore, it is still CaO that is employed to be a sintering additive, i.e., Ca2+ is being used as a dopant for sintering AlON ceramics.
Table 1 Effects of doping amount of CaCO3 on the relative density of AlON ceramics. Doping amount of CaCO3 (wt%) Relative density (%)
0.2 99.70
0.3 99.92
0.4 99.94
0.5 99.62
Y. Shan et al. / Scripta Materialia 157 (2018) 148–151
Fig. 1. Transmittance of the transparent AlON ceramics doped with 0.2–0.5 wt% CaCO3 and 0.5 wt% Y2O3 at 1500–6000 nm (a), doped with 0.4 wt% CaCO3 and 0.5 wt% Y2O3 at 190–3300 nm (b). Insert: photographs of AlON ceramics doped with 0.2–0.5 wt% CaCO3.
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The pure AlON powder was firstly synthesized by carbothermal reduction and nitridation (CRN) method (detailed fabrication process is described in Ref. [18]), then 0.2 wt%, 0.3 wt%, 0.4 wt% and 0.5 wt% CaCO3 (99.99%; Macklin, China) were added into the obtained AlON powder, respectively. Using Si3N4 ball as milling media, mixture of the powders of AlON and CaCO3 were grinded in absolute ethyl alcohol at 170 rpm for 24 h. The obtained slurry was fully dried and sieved to obtain the starting mixed powder. Then, 1.4 g of mixed AlON powder was packed into pellets of 13 mm in diameter under 50 MPa. The pellets were pressureless sintered within a graphite furnace in an atmosphere of 0.1 MPa N2. All samples were heated to 1870 °C at a heating rate of 40 °C/min, the heating system was shut down after holding for 150 min. The sintered specimens were then grinded and polished at both sides to a thickness of 2 mm for the optical transmittance measurement. The phase assemblage of the sintered samples was characterized by X-ray diffractometry (XRD; D/Max-ULtima1, Rigaku, Tokyo, Japan) using Co Kα1 radiation. The microstructure of the sintered samples was observed by field-emission scanning electron microscopy (FESEM; supra 55, Zeiss, Jena, Germany). Micrograph observation of the polished samples hot etched at 1640 °C for 40 min was performed using a metallurgical microscope (GX51, OLYMPUS, Japan). The grain area and the average grain size of AlON were statistically calculated, where the average grain size was proportional to the average value of diameters passing through the objects' centroid. Then, based on the calculated average grain size, the grain number was counted for every 36 μm as a group to analyze the grain size distribution. The bulk density of the sintered samples was measured by the Archimedes method. Optical transmittance of the samples in the wave range of 1500–6000 nm was recorded by the Fourier transform infrared spectroscopy (FTIR; Frontier, PE, USA). The transmittance of AlON ceramics at the wave range of 190–3300 nm was measured with a spectrophotometer (Cary 5000, Varian, USA). The XRD pattern of all the samples after holding for 150 min at 1870 °C showed that only the AlON crystalline phase was detected. The absence of secondary phases means that the AlON grains were below its solubility limit [12]. Relative densities of all the sintered samples measured by the Archimedes principle are ≥99.62%, as listed in Table 1. It indicates that the AlON powder was fast densified to achieve a high
Fig. 2. SEM images of the fracture surfaces of the AlON ceramics with CaCO3 doping: (a) 0.2 wt%, (b) 0.3 wt%, (c) 0.4 wt% and (d) 0.5 wt%.
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Y. Shan et al. / Scripta Materialia 157 (2018) 148–151
Fig. 3. Microstructures of the hot etched samples doped with different amounts of CaCO3 and their statistical results of grain size vs. area fraction: (a, e) 0.2 wt%, (b, f) 0.3 wt%, (c, g) 0.4 wt% and (d, h) 0.5 wt%.
relative density by using CaCO3 as the sintering additive. It also can be seen from Table 1 that the samples doped with 0.3 wt% and 0.4 wt% CaCO3 have a higher relative density of ≥99.92%. As shown in Fig. 1 (inset), all the 2 mm samples doped with CaCO3 are transparent. For the convenience of comparison, the measured transmittances of all the samples are depicted in Fig. 1a together with the reported transmittances of AlON ceramics doped with 0.5 wt% Y2O3 sintered at 1880 °C for 150 min [18]. Apparently, both additives, i.e., CaCO3 and Y2O3, lead to a similar transmittance vs. wavelength pattern of AlON ceramics at wavelength range from 1500 nm to 6000 nm under the almost identical sintering conditions, except for 10 °C difference in sintering temperature. Moreover, the transmittance of the
present AlON ceramics doped with 0.3 wt% and 0.4 wt% CaCO3 is consistently higher than that of the reported AlON ceramics doped with 0.5 wt % Y2O3 within the wavelength range of 1500–6000 nm, as shown in Fig. 1a. Their maximum transmittances are respectively up to 83% and 85% at wavelength of ~3700 nm, which is slightly higher than that of ceramics doped with Y2O3 (82%). It should be noted that the transmittance of the AlON ceramics at 4800 nm is a vital property for Mid-IR targeting [19]. From Fig. 1a, the transmittance of the obtained AlON ceramics doped with 0.3–0.4 wt% CaCO3 is 71% at 4800 nm, which is 6% higher than that of the AlON ceramics doped with 0.5 wt% (ideal amount) Y2O3. Further measurement of the transmittance of the obtained AlON ceramics from ultraviolet (≥200 nm) to visible range is
Y. Shan et al. / Scripta Materialia 157 (2018) 148–151
Fig. 4. Effects of doping amount of CaCO3 on the grain numbers of fabricated AlON ceramics.
depicted in Fig. 1b. The result in Fig. 1b indicates that the sintered AlON ceramics with 0.3 wt% and 0.4 wt% CaCO3 doping has a higher transmittance than that of the AlON ceramics doped with 0.5 wt% Y2O3. Therefore, it is promising to use CaCO3 as an additive for sintering highly transparent AlON ceramics. Fig. 1a also reveals that the doping amount of CaCO3 has a significant effect on the transmittance of the as-fabricated AlON ceramics. The sample doped with 0.4 wt% CaCO3 demonstrates the highest transmittance in the measured wavelength range, followed by the samples doped with 0.3 wt% and 0.2 wt% CaCO3. The sample doped with 0.5 wt% CaCO3 has the lowest transmittance. Further observation of fracture surfaces of all the obtained samples shown in Fig. 2 reveals that a few pores can be detected in the samples doped with 0.2 wt% and 0.5 wt% CaCO3 (Fig. 2a and d) and no obvious porosity can be found in the samples doped with 0.3 wt% and 0.4 wt% CaCO3 (Fig. 2b and c). Therefore, it is reasonable to attribute the low transmittance of the samples doped with 0.2 wt% and 0.5 wt% CaCO3 to the presence of pores and consequently low relative density. It is worth noting that the pores in the sample doped with 0.2 wt% CaCO3 are located within grains, while the pores in the sample doped with 0.5 wt% CaCO3 are located on the grain boundaries. Fig. 3 shows the hot etched images and area fraction distributions of different grain size groups for all the fabricated AlON ceramics doped with CaCO3. In Fig. 3a-d, the grains of ≥36 μm dominate in all the sintered samples doped with different quantities of CaCO3. More importantly, even the smallest grain size is much larger than the starting powder size (~1–5 μm as shown in Ref. [18]). Apparently, the enlarged grain size is the result of grain coarsening during sintering. For the convenience of analysis, grains shown in Fig. 3a-d were classified into five groups according to their measured grain sizes, i.e., 0–36 μm, 36–72 μm, 72–108 μm, 108–144 μm and N144 μm, respectively. Correspondingly, the area fractions of each grain size group for all the samples are categorized as shown in Fig. 3e-h. It is evident that an increase in wt% of CaCO3 (from 0.2% to 0.5%) tends to prompt area fraction of small size grains, which was verified by counting the total grain numbers of all the samples shown in Fig. 4. It is apparent that grain growth is a major phenomenon occurring at the final sintering stage for all the fabricated AlON samples. For all the doping amounts of CaCO3 (0.2–0.5 wt%), the mixtures have significant grain growth for the grains of a similar large size (N144 μm), as observed
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in Fig. 3 in all the samples. However, the presence of the residual pores within grains (Fig. 2a) for the sample doped with 0.2 wt% CaCO3 implies that the grain growth for this sample during sintering was not sufficiently uniform to eliminate the pores among particles before they grew into each other. The non-uniform grain growth may be caused by un-even additive distribution or inadequate additives in the starting powders. On the other hand, the sample doped with 0.5 wt% CaCO3 has pores on the grain boundaries (Fig. 2d) and has much more small size grains, which implies that over doping of CaCO3 inhibits grain growth during sintering. Even worse, the redundant grain boundary resulted in remaining pores resided on the boundary. Therefore, it is necessary to control grain growth by adjusting the wt% of CaCO3 to eliminate pores and other defects during sintering. It should be pointed out that, compared with CaO, CaCO3 is a more universal, feasible and cheap powder, which is easier for storage. Therefore, using CaCO3 as sintering additive may be more favorable for industrial applications. In summary, CaCO3 was found to be an effective and feasible sintering additive for the fabrication of transparent AlON ceramics. By adding 0.3–0.4 wt% CaCO3 as a sintering additive into the single phase AlON starting powder, highly transparent AlON ceramics were pressurelessly fabricated at 1870 °C for 150 min. The transmittance of the fabricated AlON ceramics is up to 83–85% at ~3700 nm for 2 mm thickness samples, and their transmittances are consistently higher than that of the AlON doped with the ideal amount of Y2O3 at the wavelength ranging from 200 nm to 6000 nm. The measured transmittances revealed that the proposed CaCO3 doping can promote AlON grain growth and effectively eliminate pores during sintering. However, an improper amount of CaCO3 doping may lead to residual pores located within a grain or on the boundary of grains, which degrades the transmittance of the AlON ceramics. This work has been supported by the National Key R&D Program of China (2017YFB0310300) and the Project of Postgraduation Education (YJG2018603).
References [1] C.T. Warner, T.M. Harnett, D. Fisher, W. Sunne, Proc. SPIE 5786 (2005) 95. [2] J.W. Macauley, P. Patel, M. Chen, G. Gilde, E. Strassburger, B. Paliwal, K.T. Ramesh, D.P. Dandekar, J. Eur. Ceram. Soc. 29 (2009) 223–236. [3] T.M. Hartnett, S.D. Bernstein, E.A. Maguire, R.W. Tustison, Infrared Phys. Technol. 39 (1998) 203–211. [4] J.J. Swab, R. Pavlacka, G. Gilde, S. Kilczewski, J. Wright, D. Harris, J. Am. Ceram. Soc. 97 (2013) 592–600. [5] X.H. Jin, L. Gao, J. Sun, Y.Q. Liu, L.H. Gui, J. Am. Ceram. Soc. 95 (2012) 2801–2807. [6] F. Chen, F. Zhang, J. Wang, H.L. Zhang, R. Tian, J. Zhang, Z. Zhang, F. Sun, S.W. Wang, Scr. Mater. 81 (2014) 20–23. [7] X.B. Li, J.M. Luo, Z.J. Feng, S.F. Zhou, J.T. Huang, Ceram. Int. 42 (2016) 17382–17386. [8] Y.C. Shan, X.L. Wei, X.N. Sun, J.J. Xu, Q.H. Qin, E.A. Olevsky, J. Mater. Res. 32 (2017) 3279–3285. [9] Y. Wang, X.M. Xie, J.Q. Qi, J. Wang, D. Wu, X.F. Guo, N. Wei, T.C. Lu, J. Mater. Res. 29 (2014) 2325–2331. [10] J. Wang, F. Zhang, F. Chen, H.L. Zhang, R. Tian, M.J. Dong, J. Liu, Z. Zhang, J. Zhang, S.W. Wang, J. Am. Ceram. Soc. 97 (2014) 1353–1355. [11] X.L. Li, J.M. Luo, Y. Zhou, J. Eur. Ceram. Soc. 35 (2015) 2027–2032. [12] L. Miller, W.D. Kaplan, J. Am. Ceram. Soc. 91 (2008) 1693–1696. [13] N. Jiang, Q. Liu, T.F. Xie, P. Ma, H.M. Kou, Y.B. Pan, J. Li, J. Eur. Ceram. Soc. 37 (2017) 4213–4216. [14] C. Martin, B. Cales, Proc. SPIE 1112 (1989) 20–24. [15] M.Y. Su, Y.F. Zhou, K. Wang, Z.F. Yang, Y.G. Cao, M.C. Hong, J. Eur. Ceram. Soc. 35 (2015) 1173–1178. [16] D. Han, J. Zhang, P. Liu, G. Li, S.W. Wang, J. Eur. Ceram. Soc. 38 (2018) 3261–3267. [17] L. Zhang, T.Y. Zhou, F.A. Selim, H. Chen, J. Am. Ceram. Soc. 101 (2018) 703–712. [18] Y.C. Shan, J.X. Xu, G. Wang, X.N. Sun, G.H. Liu, J.J. Xu, J.T. Li, Ceram. Int. 41 (3) (2015) 3992–3998. [19] M. Rubat Du Merac, H. Kleebe, M.M. Muller, I.E. Reimanis, J. Am. Ceram. Soc. 96 (2013) 3341–3365.
Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Regular article
Pressureless sintering of highly transparent AlON ceramics with CaCO3 doping Yingchun Shan a, Xiannian Sun a,⁎, Binglin Ren a, Haokai Wu a, Xialu Wei b, Eugene A. Olevsky b, Jiujun Xu a,⁎, Jiangtao Li c a b c
Department of Materials Science and Engineering, Dalian Maritime University, Dalian 116026, China College of Engineering, San Diego State University, San Diego, CA 92182, USA Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100080, China
a r t i c l e
i n f o
Article history: Received 20 June 2018 Accepted 13 August 2018 Available online xxxx Keywords: AlON CaCO3 addtitive Transparent ceramics Densification Grain growth
a b s t r a c t Employing CaCO3 as sintering additive, highly transparent aluminum oxynitride (AlON) ceramics were pressurelessly fabricated from AlON powder at 1870 °C during150 min. The transmittance of the AlON doped with 0.3–0.4 wt% CaCO3 is up to 83–85% at ~3700 nm for 2 mm thickness samples, and their transmittances are consistently higher than that of the AlON doped with the ideal amount of Y2O3 at wavelength ranging from 200 nm to 6000 nm. The AlON doped with CaCO3 exhibits the transmittance of 71% at 4800 nm (typical band for infrared targeting), which is higher by 6% than that of the doped Y2O3. © 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Transparent aluminum oxynitride (AlON) ceramics are one of the most promising transparent ceramic materials for infrared/visible windows used in severe environments, such as high temperature, abrasion, corrosion, etc., due to their excellent optical transparency properties, high strength and hardness, and good chemical inertness [1–4]. AlON ceramics can be sintered by pressureless sintering, hot pressing sintering, hot isostatic pressing, spark plasma sintering, etc. [5–8]. Among those methods, pressureless sintering is a preferred technology due to its significant advantage in fabricating large size and complicated components. Generally, sintering additive is always required to fabricate transparent AlON ceramics. Currently, Y2O3, La2O3 and MgO have been chosen as sintering additives to accelerate the densification process of AlON ceramics [5,9–13]. Among these additives, Y3+ is believed to enhance the mobility of grain boundaries and accelerate grain growth, and 0.5 wt% Y2O3 was reported to be the ideal doping amount to obtain AlON ceramics with high transparency [9,14]. At the same time, La3+ and Mg2+ can inhibit the abnormal grains growth [9,13,15]. Consequently, La2O3 and MgO are usually selected as grain growth inhibiters to co-dope with Y2O3 to fabricate transparent AlON ceramics [13,15]. It is commonly recognized that high densification and reducing the amount of scattering and refracting sources are all a must to ensure the optical quality of fabricated AlON ceramics. To further improve the
⁎ Corresponding authors. E-mail addresses: [email protected] (X. Sun), [email protected] (J. Xu).
https://doi.org/10.1016/j.scriptamat.2018.08.023 1359-6462/© 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
properties of transparent AlON ceramics, it is necessary to find more sintering additive candidates. Recently, it was reported that proper amount of CaO dopant can promote densification and grain growth of MgO·1.5Al2O3 and YAG ceramics at a suitable sintering temperature [16,17]. It is understandable that it is Ca2+ that is being used as dopant for fabrication of MgO·1.5Al2O3 and YAG ceramics, which implies that it is highly possible that Ca2+ can also induce positive properties in the densification process of AlON ceramics. It is well known that CaO is easy to react with H2O to form Ca(OH)2 at room temperature, which means a nonignorable small amount of Ca(OH)2 may exist in CaO additive during fabrication process. This Ca(OH)2 is decomposed into CaO and H2O at ~1200 °C, where H2O is incompatible with sintering furnace environment. To avoid the risk induced by unexpected H2O, CaCO3 was mixed with AlON powder to fabricate transparent AlON ceramics in this study. As a matter of fact, CaCO3 decomposes into CaO and CO2 at ~825 °C. More importantly, the decomposition temperature of CaCO3 is much lower than the starting temperature of AlON ceramics sintering (~1300 °C) [6]. Therefore, it is still CaO that is employed to be a sintering additive, i.e., Ca2+ is being used as a dopant for sintering AlON ceramics.
Table 1 Effects of doping amount of CaCO3 on the relative density of AlON ceramics. Doping amount of CaCO3 (wt%) Relative density (%)
0.2 99.70
0.3 99.92
0.4 99.94
0.5 99.62
Y. Shan et al. / Scripta Materialia 157 (2018) 148–151
Fig. 1. Transmittance of the transparent AlON ceramics doped with 0.2–0.5 wt% CaCO3 and 0.5 wt% Y2O3 at 1500–6000 nm (a), doped with 0.4 wt% CaCO3 and 0.5 wt% Y2O3 at 190–3300 nm (b). Insert: photographs of AlON ceramics doped with 0.2–0.5 wt% CaCO3.
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The pure AlON powder was firstly synthesized by carbothermal reduction and nitridation (CRN) method (detailed fabrication process is described in Ref. [18]), then 0.2 wt%, 0.3 wt%, 0.4 wt% and 0.5 wt% CaCO3 (99.99%; Macklin, China) were added into the obtained AlON powder, respectively. Using Si3N4 ball as milling media, mixture of the powders of AlON and CaCO3 were grinded in absolute ethyl alcohol at 170 rpm for 24 h. The obtained slurry was fully dried and sieved to obtain the starting mixed powder. Then, 1.4 g of mixed AlON powder was packed into pellets of 13 mm in diameter under 50 MPa. The pellets were pressureless sintered within a graphite furnace in an atmosphere of 0.1 MPa N2. All samples were heated to 1870 °C at a heating rate of 40 °C/min, the heating system was shut down after holding for 150 min. The sintered specimens were then grinded and polished at both sides to a thickness of 2 mm for the optical transmittance measurement. The phase assemblage of the sintered samples was characterized by X-ray diffractometry (XRD; D/Max-ULtima1, Rigaku, Tokyo, Japan) using Co Kα1 radiation. The microstructure of the sintered samples was observed by field-emission scanning electron microscopy (FESEM; supra 55, Zeiss, Jena, Germany). Micrograph observation of the polished samples hot etched at 1640 °C for 40 min was performed using a metallurgical microscope (GX51, OLYMPUS, Japan). The grain area and the average grain size of AlON were statistically calculated, where the average grain size was proportional to the average value of diameters passing through the objects' centroid. Then, based on the calculated average grain size, the grain number was counted for every 36 μm as a group to analyze the grain size distribution. The bulk density of the sintered samples was measured by the Archimedes method. Optical transmittance of the samples in the wave range of 1500–6000 nm was recorded by the Fourier transform infrared spectroscopy (FTIR; Frontier, PE, USA). The transmittance of AlON ceramics at the wave range of 190–3300 nm was measured with a spectrophotometer (Cary 5000, Varian, USA). The XRD pattern of all the samples after holding for 150 min at 1870 °C showed that only the AlON crystalline phase was detected. The absence of secondary phases means that the AlON grains were below its solubility limit [12]. Relative densities of all the sintered samples measured by the Archimedes principle are ≥99.62%, as listed in Table 1. It indicates that the AlON powder was fast densified to achieve a high
Fig. 2. SEM images of the fracture surfaces of the AlON ceramics with CaCO3 doping: (a) 0.2 wt%, (b) 0.3 wt%, (c) 0.4 wt% and (d) 0.5 wt%.
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Y. Shan et al. / Scripta Materialia 157 (2018) 148–151
Fig. 3. Microstructures of the hot etched samples doped with different amounts of CaCO3 and their statistical results of grain size vs. area fraction: (a, e) 0.2 wt%, (b, f) 0.3 wt%, (c, g) 0.4 wt% and (d, h) 0.5 wt%.
relative density by using CaCO3 as the sintering additive. It also can be seen from Table 1 that the samples doped with 0.3 wt% and 0.4 wt% CaCO3 have a higher relative density of ≥99.92%. As shown in Fig. 1 (inset), all the 2 mm samples doped with CaCO3 are transparent. For the convenience of comparison, the measured transmittances of all the samples are depicted in Fig. 1a together with the reported transmittances of AlON ceramics doped with 0.5 wt% Y2O3 sintered at 1880 °C for 150 min [18]. Apparently, both additives, i.e., CaCO3 and Y2O3, lead to a similar transmittance vs. wavelength pattern of AlON ceramics at wavelength range from 1500 nm to 6000 nm under the almost identical sintering conditions, except for 10 °C difference in sintering temperature. Moreover, the transmittance of the
present AlON ceramics doped with 0.3 wt% and 0.4 wt% CaCO3 is consistently higher than that of the reported AlON ceramics doped with 0.5 wt % Y2O3 within the wavelength range of 1500–6000 nm, as shown in Fig. 1a. Their maximum transmittances are respectively up to 83% and 85% at wavelength of ~3700 nm, which is slightly higher than that of ceramics doped with Y2O3 (82%). It should be noted that the transmittance of the AlON ceramics at 4800 nm is a vital property for Mid-IR targeting [19]. From Fig. 1a, the transmittance of the obtained AlON ceramics doped with 0.3–0.4 wt% CaCO3 is 71% at 4800 nm, which is 6% higher than that of the AlON ceramics doped with 0.5 wt% (ideal amount) Y2O3. Further measurement of the transmittance of the obtained AlON ceramics from ultraviolet (≥200 nm) to visible range is
Y. Shan et al. / Scripta Materialia 157 (2018) 148–151
Fig. 4. Effects of doping amount of CaCO3 on the grain numbers of fabricated AlON ceramics.
depicted in Fig. 1b. The result in Fig. 1b indicates that the sintered AlON ceramics with 0.3 wt% and 0.4 wt% CaCO3 doping has a higher transmittance than that of the AlON ceramics doped with 0.5 wt% Y2O3. Therefore, it is promising to use CaCO3 as an additive for sintering highly transparent AlON ceramics. Fig. 1a also reveals that the doping amount of CaCO3 has a significant effect on the transmittance of the as-fabricated AlON ceramics. The sample doped with 0.4 wt% CaCO3 demonstrates the highest transmittance in the measured wavelength range, followed by the samples doped with 0.3 wt% and 0.2 wt% CaCO3. The sample doped with 0.5 wt% CaCO3 has the lowest transmittance. Further observation of fracture surfaces of all the obtained samples shown in Fig. 2 reveals that a few pores can be detected in the samples doped with 0.2 wt% and 0.5 wt% CaCO3 (Fig. 2a and d) and no obvious porosity can be found in the samples doped with 0.3 wt% and 0.4 wt% CaCO3 (Fig. 2b and c). Therefore, it is reasonable to attribute the low transmittance of the samples doped with 0.2 wt% and 0.5 wt% CaCO3 to the presence of pores and consequently low relative density. It is worth noting that the pores in the sample doped with 0.2 wt% CaCO3 are located within grains, while the pores in the sample doped with 0.5 wt% CaCO3 are located on the grain boundaries. Fig. 3 shows the hot etched images and area fraction distributions of different grain size groups for all the fabricated AlON ceramics doped with CaCO3. In Fig. 3a-d, the grains of ≥36 μm dominate in all the sintered samples doped with different quantities of CaCO3. More importantly, even the smallest grain size is much larger than the starting powder size (~1–5 μm as shown in Ref. [18]). Apparently, the enlarged grain size is the result of grain coarsening during sintering. For the convenience of analysis, grains shown in Fig. 3a-d were classified into five groups according to their measured grain sizes, i.e., 0–36 μm, 36–72 μm, 72–108 μm, 108–144 μm and N144 μm, respectively. Correspondingly, the area fractions of each grain size group for all the samples are categorized as shown in Fig. 3e-h. It is evident that an increase in wt% of CaCO3 (from 0.2% to 0.5%) tends to prompt area fraction of small size grains, which was verified by counting the total grain numbers of all the samples shown in Fig. 4. It is apparent that grain growth is a major phenomenon occurring at the final sintering stage for all the fabricated AlON samples. For all the doping amounts of CaCO3 (0.2–0.5 wt%), the mixtures have significant grain growth for the grains of a similar large size (N144 μm), as observed
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in Fig. 3 in all the samples. However, the presence of the residual pores within grains (Fig. 2a) for the sample doped with 0.2 wt% CaCO3 implies that the grain growth for this sample during sintering was not sufficiently uniform to eliminate the pores among particles before they grew into each other. The non-uniform grain growth may be caused by un-even additive distribution or inadequate additives in the starting powders. On the other hand, the sample doped with 0.5 wt% CaCO3 has pores on the grain boundaries (Fig. 2d) and has much more small size grains, which implies that over doping of CaCO3 inhibits grain growth during sintering. Even worse, the redundant grain boundary resulted in remaining pores resided on the boundary. Therefore, it is necessary to control grain growth by adjusting the wt% of CaCO3 to eliminate pores and other defects during sintering. It should be pointed out that, compared with CaO, CaCO3 is a more universal, feasible and cheap powder, which is easier for storage. Therefore, using CaCO3 as sintering additive may be more favorable for industrial applications. In summary, CaCO3 was found to be an effective and feasible sintering additive for the fabrication of transparent AlON ceramics. By adding 0.3–0.4 wt% CaCO3 as a sintering additive into the single phase AlON starting powder, highly transparent AlON ceramics were pressurelessly fabricated at 1870 °C for 150 min. The transmittance of the fabricated AlON ceramics is up to 83–85% at ~3700 nm for 2 mm thickness samples, and their transmittances are consistently higher than that of the AlON doped with the ideal amount of Y2O3 at the wavelength ranging from 200 nm to 6000 nm. The measured transmittances revealed that the proposed CaCO3 doping can promote AlON grain growth and effectively eliminate pores during sintering. However, an improper amount of CaCO3 doping may lead to residual pores located within a grain or on the boundary of grains, which degrades the transmittance of the AlON ceramics. This work has been supported by the National Key R&D Program of China (2017YFB0310300) and the Project of Postgraduation Education (YJG2018603).
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