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Effect of CaO on the optical quality and microstructure of transparent MgO·1.5Al2O3 spinel ceramics prepared by reactive sintering
Journal of the European Ceramic Society xxx (xxxx) xxx–xxx
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Original Article
Effect of CaO on the optical quality and microstructure of transparent MgO·1.5Al2O3 spinel ceramics prepared by reactive sintering ⁎
Dan Hana,b, Jian Zhanga,c, , Peng Liud, Gui Lia, 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 b
A R T I C LE I N FO
A B S T R A C T
Keywords: CaO Spinel Transparent ceramic Reactive sintering
Transparent MgO·1.5Al2O3 spinel ceramics were successfully prepared via reactive sintering of Al2O3 and MgO raw powders followed by hot isostatic pressing (HIP) using CaO as the sintering additive. The effects of CaO on the densification process, microstructure and optical quality of samples were investigated. It was found that the amount of CaO played an important role in the sintering process. By adding 0.05 wt% CaO, the sample with high transmittance (82.3% at 400 nm), small grain size (< 5 μm) and high strength (228 ± 15 MPa) was obtained after HIPing at 1550 °C. However, when the amount of CaO increased to 0.1 wt%, non-cubic and columnarshaped grains generated at low HIP temperatures (1550–1650 °C), which severely reduced the optical quality of resulting samples. The grains were calcium aluminates, whose formation was closely related to the molar ratio of Al2O3/MgO, CaO amount and sintereing temperature.
1. Introduction
It is reported that a small amount of CaO can markedly improve the densification of spinel compacts in pressureless sintering [12,19]. Bratton [8] firstly used CaO as the sintering additive to prepare translucent spinel ceramic. It was found that even as little as 0.1 wt% CaO could significantly increase the densification rate. Some residual glassy phase was found at the grain boundary, indicating that the promotion of densification was mainly benefited from the formation of liquid phase. Krell et al [9] showed that 0.2–0.3 wt% CaO could decrease the sintering temperature by more than 100 °C and eliminate visible flaws formed by residual pores. But the scattering of grain boundary with Ca segregations severely dereriorated the transparency of samples. Kim et al [10] also reported an improved transmittance of spinel ceramics, especially in the UV range, by adding 0.08 wt% CaO. However, when the pre-sintering temperature was above 1500 °C, severe abnormal grain growth and CaAl4O7 precipitates appeared in the samples, which led to the rapid decrease of optical quality. In the above studies, stoichiometric spinel powder was used as the raw powder. In this case, CaO need to capture Al2O3 from spinel phase to form calcium aluminates during sintering process. A relatively large amount of CaO was necessary to form enough liquid phase to promote the densification process. Meanwhile, to avoid the precipitation of calcium aluminates, the sintering process must be operated at low
Transparent magnesium aluminate spinel ceramincs are widely used as transparent armors, IR windows and domes, owing to their excellent optical quality, mechanical strength and other functional properties such as erosion resistance [1–4]. During the past 50 years, many methods have been explored to prepare high-quality transparent spinel ceramics. However, spinel is inherently difficult to attain the neartheoretical density required for transparency due to its low oxygen diffusion rate [5]. Many studies have been devoted to promoting the densification process of spinel using sintering additive, such as florides (e.g. LiF, MgF2, AlF3, NaF) [6,7], CaO [8–10], B2O3 [11], SiO2 [12] and Y2O3 [13]. Among them, LiF is the most commonly used sintering additive [5]. The liquid phase of LiF and vacancies (generated from the substitution of Li and F to Mg and O, respectively) can effectively promote the densification process and reduce the sintering temperature of spinel compacts. However, the use of LiF is normally combined with hot pressing or spark plasma sintering [14–16], and it is easy to cause grain coarsensing and grain-boundary microcracks, resulting in a low fracture strength (< 200 MPa) [17,18], which severely limits the widely application of transparent spinel ceramics.
⁎ Corresponding authors at: Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, PR China. E-mail addresses: [email protected] (D. Han), [email protected] (J. Zhang), [email protected] (S. Wang).
https://doi.org/10.1016/j.jeurceramsoc.2018.03.032 Received 27 November 2017; Received in revised form 17 March 2018; Accepted 19 March 2018 0955-2219/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Han, D., Journal of the European Ceramic Society (2018), https://doi.org/10.1016/j.jeurceramsoc.2018.03.032
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the mean intercept. The in-line transmittances of the transparent samples at 190–1100 nm were tested using a UV-VIS-NIR spectrometer (Carry 5000 spectrophotometer, Varian, Seattle, USA). Optical microscope (BX51 system microscope, Olympus, Tokyo, Japan) was used to detect defects existing in the resulting transparent ceramics, such as pores and second phases. X-ray diffraction (D8, Bruker, Berlin, Germany) was used to test the phase compositions of products of reaction between mixed raw powders and CaO. The flexure strength of asprepared transparent ceramics was measured through a 3-point test using the mechanical universal testing machine (Instron-5566, Instron Co., Norwood, USA). The specimen’s dimensions were 3 × 4 × 36 mm, and 10 specimens were tested and averaged for one data. Hardness tests were performed under a 3 kg loading through the Vickers harness instrument (TUKON-2100B, Instron Co., Norwood, USA).
temperatures. This requires spinel powders to have a high sintering ability, i.e., it can be completely densified below 1500 °C. In fact, synthesis of such powder is difficult, and the cost is quite high. In this study, cheap γ-Al2O3 and MgO powders instead of the pure spinel powder were used to prepare transparent spinel ceramics by reactive sintering, using CaO as the sintering additive. The free Al2O3 in the mixed powders is much easier to react with CaO than with spinel. The liquid phase of low melting calcium aluminates can obviously promote the densification process and achieve the low-temperature sintering of transparent spinel ceramics. In our previous work [20], transparent Al-rich spinel ceramics with different compositions (MgO﹒ nAl2O3, n = 1.05–2.5) have been successfully prepared in the absence of sintering additive. However, when n is above 1.5, the transmittance of resulting samples began to decrease, especially at high temperatures, due to the appearance of second-phase grains. The second phase was metastable spinel, which had different compositions from the matrix, and it was easy to precipate during the cooling process. Here, CaO was used as the sintering additive to lower sintering tenperature and the MgO·1.5Al2O3 spinel was chosen as the example to study whether CaO can effectively inhibite the generation of second phase and improve the transmittance of spinel ceramics with high n value. The effects of CaO on the densification process, microstructure and properties of resulting samples were investigated. Similar to the sintering of spinel powder using CaO as the sintering additive, possible calcium aluminate precipitates generated during the reactive sintering process were examined.
3. Results and discussions 3.1. Densification process To investigate the effect of CaO on the densification process, the samples with different amounts of CaO additive were pre-sintered at 1400–1550 °C. The densities of as-prepared samples were displayed as a function of temperature (Fig. 1). For the samples without CaO, the density firstly decreased at 1450 °C, and then increased as the temperature increased. The decrease of density was mainly caused by the reaction between residual alumina and spinel (completed at about 1500 °C), which was a volume expansion process. When the pre-sintering temperature was near 1500 °C, large amount of residual Al2O3 began to dissolve into the spinel lattice, leading to obvious volume expansion that can compete with the shrinkage caused by densification. For samples with different amounts of CaO, the densities of samples pre-sintered at 1400 °C were lower than that of sample without additive; and the larger amount of CaO, the lower density. This may be because of the grain-boundary pinning effect of calcium aluminates generated from the reaction between CaO and alumina. According to the previous report [21], the lowest melting point of calcium aluminates was at about 1400 °C (CaO·Al2O3). Therefore, at low temperatures (≤1400 °C), the resulting compounds mainly existed at the grain boundaries in the form of solid phase, and played a hinder role on the densification process. When the pre-sintering temperature was above 1400 °C, the densities of samples obviously increased with the increasing amount of CaO, which was ascribed to the appearance of liquid phase. It is interesting to find that the decrease of density at 1450 °C was offset via adding a certain content of CaO (≥0.03 wt%). The fracture surfaces of samples pre-sintered at 1450 °C were observed to reveal the effect of CaO on the microstructure of pre-sintered
2. Experimental procedures High-purity commercial γ-Al2O3 (99.99%; 50 nm; Dalian Hiland Pothoelectric Material Co., Ltd) and MgO (99.99%; 150 nm; Konoshima Chemical Co., Ltd) powders were selected as raw powders. 0.01–0.1 wt % CaO was added as the sintering additive in the form of CaCO3, which can be decomposed into CaO at 950 °C. The raw powders were weighted in a molar ratio of Al2O3 : MgO = 1.5:1, and mixed with CaCO3 (99.999%; Alfa Aesar) by wet ball milling in ethanol. The mixed powders were milled in a nylon jar, using 3-mm diameter alumina balls as the milling medium. The solid loading of the slurry was about 23 wt %, and the weight ratio of powder and alumina balls was 1:5. No other dispersants or organic additives were added. After milling at a speed of 250 rpm/min for 12 h, the mixed slurry was dried at 60 °C for 24 h in an air dry oven. After that, the mixed powders were sieved through an 80mesh screen to obtain homogeneous powders. In order to remove residual organic impurities, the mixed powders were heated at 2 °C/min to 800 °C, holding at this temperature for 6 h, and naturally cooling to room temperature. Prior to sintering, green bodies were shaped through bidirectional dry pressing at 15 MPa, using a 20-mm diameter metal mold. Then they were cold isostatic pressed for 5 min under a pressure of 200 MPa. The green bodies were firstly pre-sintered in air for 3 h in the range of 1400–1550 °C, and the heating and cooling rate were both 5 °C/min. To obtain the transparent ceramics, HIPing treatment of samples with proper densities were done at 1550-1800 °C for 3 h under 200 MPa in Argon to further remove residual pores. A rapid heating and cooling of 10 °C/min was applied in this process. The resulting samples were annealed at 1200 °C for 6 h in air. Finally, they were both-side ground and then mirror polished using the diamond slurry, and all the transparent ceramics were polished to 4 mm thick for further tests. The traditional Archimedes method was employed to measure the bulk densities of the pre-sintered samples. The wet weight and buoyant weight were measured after boiling samples in deionized water to make sure that all the open pores have been filled with water. Scanning electron microscopy (SEM, JSM-6390, JEOL, Tokyo, Japan) and EDS (SwiftED3000, Hitachi, Tokyo, Japan) were used to analyze the thermally etched surfaces and fracture surfaces of pre-sintered and HIPed samples. The average grain sizes were measured by the common linear intercept analysis according to the equation GS = 1.56 L , where L is
Fig. 1. Densities of samples with different amounts of CaO as a function of presintering temperature. 2
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Fig. 2. Fracture surfaces of samples pre-sintered at 1450 °C. (a–d: adding 0, 0.01, 0.05 and 0.1 wt% CaO, respectively).
tendency of transmittance with HIPing temperature was quite different from those of samples with 0.01 wt% and 0.05 wt%. The samples exhibited hazy appearances and quite low transmittances at 400 nm after HIPing at 1550 °C (37.2%) and 1650 °C (55.1%). The curves of transmittance spectra were parallel with each other, and increased with HIP temperature (Fig. 3c). The flat curve in visible-infrared range indicated that the poor optical quality was caused by scattering of second phases rather than residual pores. This was similar to Kim’s result [10], revealing that too much CaO could cause the precipitation of calcium aluminates. Besides, the second-phase grains magically vanished at 1800 °C, which was beneficial for the improvement of transmittance. The optimal transmittance of samples with and without CaO was compared (Fig. 3d). It can be clearly seen that adding 0.01–0.1 wt % CaO all can effectively improve the optical quality of transparent ceramics, even though the required HIP temperature varied with the amount of additive. In brief, the optimal amount of CaO was 0.05 wt%, which could simultaneously eliminate residual pores and prevent the precipitation of second phase at low temperatures. As mentioned above, the variation of transmittance with HIP temperature was closely related to the amounts of CaO. The microstructures of resulting transparent ceramics were observed to study the factors that affected the optical quality. The thermally etched surfaces and fracture surfaces of samples with 0.05 wt% CaO were shown in Fig. 4. The average grain sizes of samples varied from 18 μm to 132 μm when the HIPing temperature increased from 1650 °C to 1800 °C. The calcium aluminate precipitates with radial shape, which often appeared above 1500 °C [9,10], cannot be observed in the samples (Fig. 4a and c). This demonstrated that reducing the amount of CaO can effectively avoid the precipitation of calcium aluminates at high temperatures. No obviously residual pores were observed in the samples HIPed at 1650 °C (Fig.4b). However, a few pores of hundreds of nanometers appeared in the large grains of the sample HIPed at 1800 °C, leading to the decrease of transmittance as shown in Fig. 3b. Meanwhile, the second-phase precipitates of spinel with high molar ratio n and anisotropic optical properties, which were easily generated for the Al-rich spinel during the cooling process, were also prevented by CaO sintering additive. However, many small grains with the columnar shape were
samples (Fig. 2). For the sample without additive, residual alumina was easy to aggregate together to form large grains, which dispersed in the spinel matrix (Fig. 2a). During the solid reaction process, alumina diffused outside and dissolved into the surrounding spinel grains, leading to the formation of porous alumina grains. However, the microstructures of samples containing different amounts of CaO were uniform and no large alumina grains can be observed (Fig. 2b–c). This indicated that CaO could effectively prevent the aggregation of alumina in the presence of liquid phase, which was beneficial to the homogeneity of pre-sintered samples. In addition, the degree of densification and average grain size of samples increased with the increasing amount of CaO. 3.2. Optical quality and microstructure The transmittances and pictures of samples with different amounts of CaO were shown in Fig. 3. The sample without additive that sintered under the optimal condition (HIPed at 1600 °C) was chosen for comparison. When the amount of CaO was below 0.05 wt%, ceramics with transparent appearances can be obtained after HIPing at as low as 1550 °C (Fig. 3a and b). In details, the transmittance of sample with 0.01 wt % CaO, which was HIPed at 1550 °C, rapidly decreased in UV–visible range owing to the existence of small pores based on the Mie scattering theory. This phenomenon was obviously improved by increasing HIP temperature to 1650 °C. It clearly revealed that high HIP temperature was necessary to provide enough driving force to eliminate residual pores. However, when the amount of CaO increased to 0.05 wt %, the optimal HIP temperature was reduced to 1550 °C. This indicated that adding more CaO can generate enough liquid phase to promote the densification process and eliminate residual pores at a lower temperature. Another noticeable feature was that too high HIP temperature such as 1800 °C was easy to cause the decrease of transmittance for both samples with 0.01 and 0.05 wt% CaO. With the assist of liquid phase, the rate of grain boundary movement was significantly larger than that of pore migration, which may resulting in the generation of residual pores. When the amount of CaO increased to 0.1 wt%, the variation 3
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Fig. 3. In-line transmittance curves and pictures of 4-mm thick samples: (a–c) transmittance curves of ssamples with different amounts of CaO as a function of HIPing temperature (a: 0.01 wt%, b: 0.05 wt%, c: 0.1 wt%); (d) Transmittance comparation of samples with and without CaO, all the samples were sintered in the optimal conditions.
Fig. 4. Thermally etched surfaces and fracture surfaces of transparent ceramics with 0.05 wt% CaO. (a and b: HIPed at 1650 °C; c and d: HIPed at 1800 °C). 4
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Fig. 5. Optical microscope images of samples with 0.1 wt% CaO observed in the transmitted mode (a, c, e) and orthogonal polarization mode (b, d, f), respectively. (a and b: HIPed at 1550 °C; c and d: HIPed at 1650 °C; e and f: HIPed at 1800 °C).
aluminate such as CaAl12O19 (CA6) was unable to form because of the limited amount of Al2O3 that captured from the spinel phase. The Al2O3 in our starting samples was free and surplus, which may sufficiently react with CaO to generate a compound containing more Al2O3. In order to study the formation process of calcium aluminates, the amount of CaO was increased to 1 wt%. The XRD patterns of the samples heated at 1300–1600 °C were shown in Fig. 7a. It can be seen that the CA2 was firstly generated at the low temperature (1400 °C). As raising the heating temperature, part of CA2 continuously reacted with the residual Al2O3 to form CA6, which was the intermediate compound containing the most Al2O3 in the CaO–Al2O3 system. The morphology of CA6 is normally shown as columnar [21,22]. Due to the limited amount of Al2O3, CA2 cannot be completely transformed into CA6, and CA2 and CA6 co-existed in the sample heated at 1500 °C and 1600 °C. To verify whether the type of generated compounds was related to the alumina contents of mixed raw powders, samples containing less Al2O3 (n = 1 and 1.1) were selected for comparison. The mixed powders were also added 1 wt% CaO and heated at 1500 °C. Fig. 7b was the XRD patterns of the resulting samples. The species of generated calcium aluminate obviously varied with the molar ratios of Al2O3/MgO. For the stoichiometric sample (n = 1), a Ca-rich phase (Ca5Al6O14) phase and MgO was detected. This was different from Kim et al’s results [12], where only CA2 was found in the sample with 3 wt% CaO. With the increase of n, CA2 appeared in sample with n = 1.1, indicating that
observed in the sample with 0.1 wt% CaO via the optical microscope (Fig. 5). In the orthogonal polarization mode, the grains were bright, indicating that they were non-cubic phase. As temperature increased from 1550 °C to 1650 °C, the columnar-shaped grains gradually decreased and eventually disappeared at 1800 °C, corresponding with the increase of transmittance shown in Fig. 3c. The composition of grains was analyzed by SEM / EDS mapping analysis (Fig. 6). The results showed that the columnar-shaped grains were only composed of Al, Ca and O three elements, indicating that the non-cubic phase was calcium aluminates. In conclusion, the transmittance of resulting transparent ceramics was mainly affected by residual pores and second-phase grains, which were closely related to the sintering temperatures and amounts of CaO. When adding 0.01–0.05 wt% CaO, the amounts of calcium aluminates generated during the sintering process were very small, which will not cause obvious light scattering. The residual pores were the main reason that affected the transmittance of samples. So the sintering process should be carefully controlled to prevent the existence of pores in the samples. As the amount of CaO increased to 0.1 wt%, the generated calcium aluminates were easy to aggregate together to form large grains, leading to severe light scattering, which required high HIPing temperatures to eliminate.According to Krell et al’s [9] and Kim et al’s studies [10], the stable calcium aluminate phase existing in the stoichiometric spinel system was normally CaAl4O7 (CA2). The calcium 5
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Fig. 6. SEM / EDS mapping analysis of the sample with 0.1 wt% CaO and HIPed at 1650 °C. (The arrow was pointed to the columnar-shaped grain).
Fig. 8. Average grain sizes of samples with different amounts of CaO as a function of HIPing temperature.
increasing the amount of alumina was beneficial for the formation of calcium aluminate with high content of Al2O3. For the samples with n = 1.5, both CA2 and CA6 existed and their eutectic melting point was about 1754 °C [23]. At 1800 °C, they were melt and uniformly distributed inside the sample, which was helpful to the improvement of optical quality. The average grain sizes of samples with different amounts of CaO were displayed in Fig. 8. At the low temperature (1550 °C), the average grain sizes of samples were almost the same, which were smaller than 5 μm. However, at 1800 °C, the average grain size of sample with 0.05 wt% CaO was 132 μm, much larger than that of other samples (34 and 47 μm for the samples with 0 wt% and 0.01 wt% CaO, respectively). This indicated that the promotion of CaO on grain growth obviously increased with the HIPing temperature. The effect of CaO on the mechanical strength of samples was measured (Table 1). The flexure strength and HV3 hardness of sample with 0.05 wt% CaO were slightly higher than those of samples without additive at the same HIPing temperature, although the average grain size was much larger. This may be attributed to the reinforcement of grain
Fig. 7. XRD patterns of samples with 1 wt% CaO. (a) n = 1.5, reactive sintered at 1300–1600 °C. (b) n = 1 and 1.1, reactive sintered at 1500 °C.
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Table 1 Flexure strength and hardness of samples without and with 0.05 wt% CaO as a function of HIPing temperature. HIPing temperature (°C)
Flexure strength (MPa)
HV3 hardness (GPa)
Without CaO
0.05 wt% CaO
Without CaO
0.05 wt% CaO
1550 1650 1800
220 ± 17 189 ± 19 172 ± 16
228 ± 15 200 ± 22 177 ± 12
11.98 ± 0.09 11.58 ± 0.08 11.49 ± 0.15
12.30 ± 0.07 11.67 ± 0.10 11.56 ± 0.20
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boundary by the second phase. 4. Conclusions Excellent transparent MgO·1.5Al2O3 spinel ceramics with high optical quality and mechanical strength were prepared by reactive sintering and HIPing treatment in the presence of CaO. The amount of CaO played an important role on the densification process and final properties of samples. A certain amount of CaO (about 0.05 wt%) was necessary to generate enough liquid phase to promote the densification process and eliminate residual pores. However, as the amount of CaO increased to 0.1 wt%, columnar-shaped grains composed of CA2 and CA6 appeared in transparent ceramics at low HIP temperatures (1550–1650 °C) and disappeared at 1800 °C. The appearance of columnar-shaped grains severely decreased the optical quality of resulting samples. Overall, the optimal amount of CaO was 0.05 wt%, which can simultaneously eliminate residual pores and prevent precipitation of calcium aluminates. On the other hand, CaO can lead the rapid grain growth at high temperatures, but the mechanical strength of samples was not redued due to the reinforcement of grain boundary by second phase. The sample with 0.05 wt% CaO that HIPed at 1550 °C exhibited an excellent transmittace of 82.3% at the short wavelength of 400 nm (4 mm thick), close to the theoretical value. The flexure strength and HV3 hardness of resulting samples were 228 ± 15 MPa and 12.30 ± 0.07 GPa, respectively. Acknowledgements The authors gratefully acknowledge financial supports from National Key R&D Program of China (No. 2017YFB0310501) and Key Development Program of Chinese Academy of Sciences (No. Y62YZ4140G). References [1] M.C.L. Patterson, J.E. Caiazza, D.W. roy, Transparent spinel development, Inorg.
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Original Article
Effect of CaO on the optical quality and microstructure of transparent MgO·1.5Al2O3 spinel ceramics prepared by reactive sintering ⁎
Dan Hana,b, Jian Zhanga,c, , Peng Liud, Gui Lia, 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 b
A R T I C LE I N FO
A B S T R A C T
Keywords: CaO Spinel Transparent ceramic Reactive sintering
Transparent MgO·1.5Al2O3 spinel ceramics were successfully prepared via reactive sintering of Al2O3 and MgO raw powders followed by hot isostatic pressing (HIP) using CaO as the sintering additive. The effects of CaO on the densification process, microstructure and optical quality of samples were investigated. It was found that the amount of CaO played an important role in the sintering process. By adding 0.05 wt% CaO, the sample with high transmittance (82.3% at 400 nm), small grain size (< 5 μm) and high strength (228 ± 15 MPa) was obtained after HIPing at 1550 °C. However, when the amount of CaO increased to 0.1 wt%, non-cubic and columnarshaped grains generated at low HIP temperatures (1550–1650 °C), which severely reduced the optical quality of resulting samples. The grains were calcium aluminates, whose formation was closely related to the molar ratio of Al2O3/MgO, CaO amount and sintereing temperature.
1. Introduction
It is reported that a small amount of CaO can markedly improve the densification of spinel compacts in pressureless sintering [12,19]. Bratton [8] firstly used CaO as the sintering additive to prepare translucent spinel ceramic. It was found that even as little as 0.1 wt% CaO could significantly increase the densification rate. Some residual glassy phase was found at the grain boundary, indicating that the promotion of densification was mainly benefited from the formation of liquid phase. Krell et al [9] showed that 0.2–0.3 wt% CaO could decrease the sintering temperature by more than 100 °C and eliminate visible flaws formed by residual pores. But the scattering of grain boundary with Ca segregations severely dereriorated the transparency of samples. Kim et al [10] also reported an improved transmittance of spinel ceramics, especially in the UV range, by adding 0.08 wt% CaO. However, when the pre-sintering temperature was above 1500 °C, severe abnormal grain growth and CaAl4O7 precipitates appeared in the samples, which led to the rapid decrease of optical quality. In the above studies, stoichiometric spinel powder was used as the raw powder. In this case, CaO need to capture Al2O3 from spinel phase to form calcium aluminates during sintering process. A relatively large amount of CaO was necessary to form enough liquid phase to promote the densification process. Meanwhile, to avoid the precipitation of calcium aluminates, the sintering process must be operated at low
Transparent magnesium aluminate spinel ceramincs are widely used as transparent armors, IR windows and domes, owing to their excellent optical quality, mechanical strength and other functional properties such as erosion resistance [1–4]. During the past 50 years, many methods have been explored to prepare high-quality transparent spinel ceramics. However, spinel is inherently difficult to attain the neartheoretical density required for transparency due to its low oxygen diffusion rate [5]. Many studies have been devoted to promoting the densification process of spinel using sintering additive, such as florides (e.g. LiF, MgF2, AlF3, NaF) [6,7], CaO [8–10], B2O3 [11], SiO2 [12] and Y2O3 [13]. Among them, LiF is the most commonly used sintering additive [5]. The liquid phase of LiF and vacancies (generated from the substitution of Li and F to Mg and O, respectively) can effectively promote the densification process and reduce the sintering temperature of spinel compacts. However, the use of LiF is normally combined with hot pressing or spark plasma sintering [14–16], and it is easy to cause grain coarsensing and grain-boundary microcracks, resulting in a low fracture strength (< 200 MPa) [17,18], which severely limits the widely application of transparent spinel ceramics.
⁎ Corresponding authors at: Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, PR China. E-mail addresses: [email protected] (D. Han), [email protected] (J. Zhang), [email protected] (S. Wang).
https://doi.org/10.1016/j.jeurceramsoc.2018.03.032 Received 27 November 2017; Received in revised form 17 March 2018; Accepted 19 March 2018 0955-2219/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Han, D., Journal of the European Ceramic Society (2018), https://doi.org/10.1016/j.jeurceramsoc.2018.03.032
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the mean intercept. The in-line transmittances of the transparent samples at 190–1100 nm were tested using a UV-VIS-NIR spectrometer (Carry 5000 spectrophotometer, Varian, Seattle, USA). Optical microscope (BX51 system microscope, Olympus, Tokyo, Japan) was used to detect defects existing in the resulting transparent ceramics, such as pores and second phases. X-ray diffraction (D8, Bruker, Berlin, Germany) was used to test the phase compositions of products of reaction between mixed raw powders and CaO. The flexure strength of asprepared transparent ceramics was measured through a 3-point test using the mechanical universal testing machine (Instron-5566, Instron Co., Norwood, USA). The specimen’s dimensions were 3 × 4 × 36 mm, and 10 specimens were tested and averaged for one data. Hardness tests were performed under a 3 kg loading through the Vickers harness instrument (TUKON-2100B, Instron Co., Norwood, USA).
temperatures. This requires spinel powders to have a high sintering ability, i.e., it can be completely densified below 1500 °C. In fact, synthesis of such powder is difficult, and the cost is quite high. In this study, cheap γ-Al2O3 and MgO powders instead of the pure spinel powder were used to prepare transparent spinel ceramics by reactive sintering, using CaO as the sintering additive. The free Al2O3 in the mixed powders is much easier to react with CaO than with spinel. The liquid phase of low melting calcium aluminates can obviously promote the densification process and achieve the low-temperature sintering of transparent spinel ceramics. In our previous work [20], transparent Al-rich spinel ceramics with different compositions (MgO﹒ nAl2O3, n = 1.05–2.5) have been successfully prepared in the absence of sintering additive. However, when n is above 1.5, the transmittance of resulting samples began to decrease, especially at high temperatures, due to the appearance of second-phase grains. The second phase was metastable spinel, which had different compositions from the matrix, and it was easy to precipate during the cooling process. Here, CaO was used as the sintering additive to lower sintering tenperature and the MgO·1.5Al2O3 spinel was chosen as the example to study whether CaO can effectively inhibite the generation of second phase and improve the transmittance of spinel ceramics with high n value. The effects of CaO on the densification process, microstructure and properties of resulting samples were investigated. Similar to the sintering of spinel powder using CaO as the sintering additive, possible calcium aluminate precipitates generated during the reactive sintering process were examined.
3. Results and discussions 3.1. Densification process To investigate the effect of CaO on the densification process, the samples with different amounts of CaO additive were pre-sintered at 1400–1550 °C. The densities of as-prepared samples were displayed as a function of temperature (Fig. 1). For the samples without CaO, the density firstly decreased at 1450 °C, and then increased as the temperature increased. The decrease of density was mainly caused by the reaction between residual alumina and spinel (completed at about 1500 °C), which was a volume expansion process. When the pre-sintering temperature was near 1500 °C, large amount of residual Al2O3 began to dissolve into the spinel lattice, leading to obvious volume expansion that can compete with the shrinkage caused by densification. For samples with different amounts of CaO, the densities of samples pre-sintered at 1400 °C were lower than that of sample without additive; and the larger amount of CaO, the lower density. This may be because of the grain-boundary pinning effect of calcium aluminates generated from the reaction between CaO and alumina. According to the previous report [21], the lowest melting point of calcium aluminates was at about 1400 °C (CaO·Al2O3). Therefore, at low temperatures (≤1400 °C), the resulting compounds mainly existed at the grain boundaries in the form of solid phase, and played a hinder role on the densification process. When the pre-sintering temperature was above 1400 °C, the densities of samples obviously increased with the increasing amount of CaO, which was ascribed to the appearance of liquid phase. It is interesting to find that the decrease of density at 1450 °C was offset via adding a certain content of CaO (≥0.03 wt%). The fracture surfaces of samples pre-sintered at 1450 °C were observed to reveal the effect of CaO on the microstructure of pre-sintered
2. Experimental procedures High-purity commercial γ-Al2O3 (99.99%; 50 nm; Dalian Hiland Pothoelectric Material Co., Ltd) and MgO (99.99%; 150 nm; Konoshima Chemical Co., Ltd) powders were selected as raw powders. 0.01–0.1 wt % CaO was added as the sintering additive in the form of CaCO3, which can be decomposed into CaO at 950 °C. The raw powders were weighted in a molar ratio of Al2O3 : MgO = 1.5:1, and mixed with CaCO3 (99.999%; Alfa Aesar) by wet ball milling in ethanol. The mixed powders were milled in a nylon jar, using 3-mm diameter alumina balls as the milling medium. The solid loading of the slurry was about 23 wt %, and the weight ratio of powder and alumina balls was 1:5. No other dispersants or organic additives were added. After milling at a speed of 250 rpm/min for 12 h, the mixed slurry was dried at 60 °C for 24 h in an air dry oven. After that, the mixed powders were sieved through an 80mesh screen to obtain homogeneous powders. In order to remove residual organic impurities, the mixed powders were heated at 2 °C/min to 800 °C, holding at this temperature for 6 h, and naturally cooling to room temperature. Prior to sintering, green bodies were shaped through bidirectional dry pressing at 15 MPa, using a 20-mm diameter metal mold. Then they were cold isostatic pressed for 5 min under a pressure of 200 MPa. The green bodies were firstly pre-sintered in air for 3 h in the range of 1400–1550 °C, and the heating and cooling rate were both 5 °C/min. To obtain the transparent ceramics, HIPing treatment of samples with proper densities were done at 1550-1800 °C for 3 h under 200 MPa in Argon to further remove residual pores. A rapid heating and cooling of 10 °C/min was applied in this process. The resulting samples were annealed at 1200 °C for 6 h in air. Finally, they were both-side ground and then mirror polished using the diamond slurry, and all the transparent ceramics were polished to 4 mm thick for further tests. The traditional Archimedes method was employed to measure the bulk densities of the pre-sintered samples. The wet weight and buoyant weight were measured after boiling samples in deionized water to make sure that all the open pores have been filled with water. Scanning electron microscopy (SEM, JSM-6390, JEOL, Tokyo, Japan) and EDS (SwiftED3000, Hitachi, Tokyo, Japan) were used to analyze the thermally etched surfaces and fracture surfaces of pre-sintered and HIPed samples. The average grain sizes were measured by the common linear intercept analysis according to the equation GS = 1.56 L , where L is
Fig. 1. Densities of samples with different amounts of CaO as a function of presintering temperature. 2
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Fig. 2. Fracture surfaces of samples pre-sintered at 1450 °C. (a–d: adding 0, 0.01, 0.05 and 0.1 wt% CaO, respectively).
tendency of transmittance with HIPing temperature was quite different from those of samples with 0.01 wt% and 0.05 wt%. The samples exhibited hazy appearances and quite low transmittances at 400 nm after HIPing at 1550 °C (37.2%) and 1650 °C (55.1%). The curves of transmittance spectra were parallel with each other, and increased with HIP temperature (Fig. 3c). The flat curve in visible-infrared range indicated that the poor optical quality was caused by scattering of second phases rather than residual pores. This was similar to Kim’s result [10], revealing that too much CaO could cause the precipitation of calcium aluminates. Besides, the second-phase grains magically vanished at 1800 °C, which was beneficial for the improvement of transmittance. The optimal transmittance of samples with and without CaO was compared (Fig. 3d). It can be clearly seen that adding 0.01–0.1 wt % CaO all can effectively improve the optical quality of transparent ceramics, even though the required HIP temperature varied with the amount of additive. In brief, the optimal amount of CaO was 0.05 wt%, which could simultaneously eliminate residual pores and prevent the precipitation of second phase at low temperatures. As mentioned above, the variation of transmittance with HIP temperature was closely related to the amounts of CaO. The microstructures of resulting transparent ceramics were observed to study the factors that affected the optical quality. The thermally etched surfaces and fracture surfaces of samples with 0.05 wt% CaO were shown in Fig. 4. The average grain sizes of samples varied from 18 μm to 132 μm when the HIPing temperature increased from 1650 °C to 1800 °C. The calcium aluminate precipitates with radial shape, which often appeared above 1500 °C [9,10], cannot be observed in the samples (Fig. 4a and c). This demonstrated that reducing the amount of CaO can effectively avoid the precipitation of calcium aluminates at high temperatures. No obviously residual pores were observed in the samples HIPed at 1650 °C (Fig.4b). However, a few pores of hundreds of nanometers appeared in the large grains of the sample HIPed at 1800 °C, leading to the decrease of transmittance as shown in Fig. 3b. Meanwhile, the second-phase precipitates of spinel with high molar ratio n and anisotropic optical properties, which were easily generated for the Al-rich spinel during the cooling process, were also prevented by CaO sintering additive. However, many small grains with the columnar shape were
samples (Fig. 2). For the sample without additive, residual alumina was easy to aggregate together to form large grains, which dispersed in the spinel matrix (Fig. 2a). During the solid reaction process, alumina diffused outside and dissolved into the surrounding spinel grains, leading to the formation of porous alumina grains. However, the microstructures of samples containing different amounts of CaO were uniform and no large alumina grains can be observed (Fig. 2b–c). This indicated that CaO could effectively prevent the aggregation of alumina in the presence of liquid phase, which was beneficial to the homogeneity of pre-sintered samples. In addition, the degree of densification and average grain size of samples increased with the increasing amount of CaO. 3.2. Optical quality and microstructure The transmittances and pictures of samples with different amounts of CaO were shown in Fig. 3. The sample without additive that sintered under the optimal condition (HIPed at 1600 °C) was chosen for comparison. When the amount of CaO was below 0.05 wt%, ceramics with transparent appearances can be obtained after HIPing at as low as 1550 °C (Fig. 3a and b). In details, the transmittance of sample with 0.01 wt % CaO, which was HIPed at 1550 °C, rapidly decreased in UV–visible range owing to the existence of small pores based on the Mie scattering theory. This phenomenon was obviously improved by increasing HIP temperature to 1650 °C. It clearly revealed that high HIP temperature was necessary to provide enough driving force to eliminate residual pores. However, when the amount of CaO increased to 0.05 wt %, the optimal HIP temperature was reduced to 1550 °C. This indicated that adding more CaO can generate enough liquid phase to promote the densification process and eliminate residual pores at a lower temperature. Another noticeable feature was that too high HIP temperature such as 1800 °C was easy to cause the decrease of transmittance for both samples with 0.01 and 0.05 wt% CaO. With the assist of liquid phase, the rate of grain boundary movement was significantly larger than that of pore migration, which may resulting in the generation of residual pores. When the amount of CaO increased to 0.1 wt%, the variation 3
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Fig. 3. In-line transmittance curves and pictures of 4-mm thick samples: (a–c) transmittance curves of ssamples with different amounts of CaO as a function of HIPing temperature (a: 0.01 wt%, b: 0.05 wt%, c: 0.1 wt%); (d) Transmittance comparation of samples with and without CaO, all the samples were sintered in the optimal conditions.
Fig. 4. Thermally etched surfaces and fracture surfaces of transparent ceramics with 0.05 wt% CaO. (a and b: HIPed at 1650 °C; c and d: HIPed at 1800 °C). 4
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Fig. 5. Optical microscope images of samples with 0.1 wt% CaO observed in the transmitted mode (a, c, e) and orthogonal polarization mode (b, d, f), respectively. (a and b: HIPed at 1550 °C; c and d: HIPed at 1650 °C; e and f: HIPed at 1800 °C).
aluminate such as CaAl12O19 (CA6) was unable to form because of the limited amount of Al2O3 that captured from the spinel phase. The Al2O3 in our starting samples was free and surplus, which may sufficiently react with CaO to generate a compound containing more Al2O3. In order to study the formation process of calcium aluminates, the amount of CaO was increased to 1 wt%. The XRD patterns of the samples heated at 1300–1600 °C were shown in Fig. 7a. It can be seen that the CA2 was firstly generated at the low temperature (1400 °C). As raising the heating temperature, part of CA2 continuously reacted with the residual Al2O3 to form CA6, which was the intermediate compound containing the most Al2O3 in the CaO–Al2O3 system. The morphology of CA6 is normally shown as columnar [21,22]. Due to the limited amount of Al2O3, CA2 cannot be completely transformed into CA6, and CA2 and CA6 co-existed in the sample heated at 1500 °C and 1600 °C. To verify whether the type of generated compounds was related to the alumina contents of mixed raw powders, samples containing less Al2O3 (n = 1 and 1.1) were selected for comparison. The mixed powders were also added 1 wt% CaO and heated at 1500 °C. Fig. 7b was the XRD patterns of the resulting samples. The species of generated calcium aluminate obviously varied with the molar ratios of Al2O3/MgO. For the stoichiometric sample (n = 1), a Ca-rich phase (Ca5Al6O14) phase and MgO was detected. This was different from Kim et al’s results [12], where only CA2 was found in the sample with 3 wt% CaO. With the increase of n, CA2 appeared in sample with n = 1.1, indicating that
observed in the sample with 0.1 wt% CaO via the optical microscope (Fig. 5). In the orthogonal polarization mode, the grains were bright, indicating that they were non-cubic phase. As temperature increased from 1550 °C to 1650 °C, the columnar-shaped grains gradually decreased and eventually disappeared at 1800 °C, corresponding with the increase of transmittance shown in Fig. 3c. The composition of grains was analyzed by SEM / EDS mapping analysis (Fig. 6). The results showed that the columnar-shaped grains were only composed of Al, Ca and O three elements, indicating that the non-cubic phase was calcium aluminates. In conclusion, the transmittance of resulting transparent ceramics was mainly affected by residual pores and second-phase grains, which were closely related to the sintering temperatures and amounts of CaO. When adding 0.01–0.05 wt% CaO, the amounts of calcium aluminates generated during the sintering process were very small, which will not cause obvious light scattering. The residual pores were the main reason that affected the transmittance of samples. So the sintering process should be carefully controlled to prevent the existence of pores in the samples. As the amount of CaO increased to 0.1 wt%, the generated calcium aluminates were easy to aggregate together to form large grains, leading to severe light scattering, which required high HIPing temperatures to eliminate.According to Krell et al’s [9] and Kim et al’s studies [10], the stable calcium aluminate phase existing in the stoichiometric spinel system was normally CaAl4O7 (CA2). The calcium 5
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Fig. 6. SEM / EDS mapping analysis of the sample with 0.1 wt% CaO and HIPed at 1650 °C. (The arrow was pointed to the columnar-shaped grain).
Fig. 8. Average grain sizes of samples with different amounts of CaO as a function of HIPing temperature.
increasing the amount of alumina was beneficial for the formation of calcium aluminate with high content of Al2O3. For the samples with n = 1.5, both CA2 and CA6 existed and their eutectic melting point was about 1754 °C [23]. At 1800 °C, they were melt and uniformly distributed inside the sample, which was helpful to the improvement of optical quality. The average grain sizes of samples with different amounts of CaO were displayed in Fig. 8. At the low temperature (1550 °C), the average grain sizes of samples were almost the same, which were smaller than 5 μm. However, at 1800 °C, the average grain size of sample with 0.05 wt% CaO was 132 μm, much larger than that of other samples (34 and 47 μm for the samples with 0 wt% and 0.01 wt% CaO, respectively). This indicated that the promotion of CaO on grain growth obviously increased with the HIPing temperature. The effect of CaO on the mechanical strength of samples was measured (Table 1). The flexure strength and HV3 hardness of sample with 0.05 wt% CaO were slightly higher than those of samples without additive at the same HIPing temperature, although the average grain size was much larger. This may be attributed to the reinforcement of grain
Fig. 7. XRD patterns of samples with 1 wt% CaO. (a) n = 1.5, reactive sintered at 1300–1600 °C. (b) n = 1 and 1.1, reactive sintered at 1500 °C.
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Table 1 Flexure strength and hardness of samples without and with 0.05 wt% CaO as a function of HIPing temperature. HIPing temperature (°C)
Flexure strength (MPa)
HV3 hardness (GPa)
Without CaO
0.05 wt% CaO
Without CaO
0.05 wt% CaO
1550 1650 1800
220 ± 17 189 ± 19 172 ± 16
228 ± 15 200 ± 22 177 ± 12
11.98 ± 0.09 11.58 ± 0.08 11.49 ± 0.15
12.30 ± 0.07 11.67 ± 0.10 11.56 ± 0.20
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boundary by the second phase. 4. Conclusions Excellent transparent MgO·1.5Al2O3 spinel ceramics with high optical quality and mechanical strength were prepared by reactive sintering and HIPing treatment in the presence of CaO. The amount of CaO played an important role on the densification process and final properties of samples. A certain amount of CaO (about 0.05 wt%) was necessary to generate enough liquid phase to promote the densification process and eliminate residual pores. However, as the amount of CaO increased to 0.1 wt%, columnar-shaped grains composed of CA2 and CA6 appeared in transparent ceramics at low HIP temperatures (1550–1650 °C) and disappeared at 1800 °C. The appearance of columnar-shaped grains severely decreased the optical quality of resulting samples. Overall, the optimal amount of CaO was 0.05 wt%, which can simultaneously eliminate residual pores and prevent precipitation of calcium aluminates. On the other hand, CaO can lead the rapid grain growth at high temperatures, but the mechanical strength of samples was not redued due to the reinforcement of grain boundary by second phase. The sample with 0.05 wt% CaO that HIPed at 1550 °C exhibited an excellent transmittace of 82.3% at the short wavelength of 400 nm (4 mm thick), close to the theoretical value. The flexure strength and HV3 hardness of resulting samples were 228 ± 15 MPa and 12.30 ± 0.07 GPa, respectively. Acknowledgements The authors gratefully acknowledge financial supports from National Key R&D Program of China (No. 2017YFB0310501) and Key Development Program of Chinese Academy of Sciences (No. Y62YZ4140G). References [1] M.C.L. Patterson, J.E. Caiazza, D.W. roy, Transparent spinel development, Inorg.
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