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Synthesis of [email protected]2O3 core–shell nanoparticles for dense SiC sintering
Particuology 44 (2019) 80–89
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Synthesis of SiC@Al2 O3 core–shell nanoparticles for dense SiC sintering Yiteng Liu, Rongzheng Liu, Malin Liu ∗ , Jiaxing Chang Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China
a r t i c l e
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Article history: Received 1 July 2018 Received in revised form 28 July 2018 Accepted 30 August 2018 Available online 18 February 2019 Keywords: Core–shell nanoparticles Dense sintering SiC Coating Hot-pressing
a b s t r a c t Owing to the difficulty for dense SiC sintering, high sintering temperatures and pressures are usually needed. Lowering the sintering temperature by adding Al2 O3 as a sintering additive has previously been shown to be beneficial. However, traditional addition methods limit the effect of the Al2 O3 owing to inhomogeneous mixing at the nanoscale. A SiC@Al2 O3 composite nanoparticle with a core–shell structure is designed and prepared using the slow co-precipitation method. The differences between this method and the traditional mechanical ball milling method are interpreted by different experimental parameters, such as temperature, pressure, amount of additive, and mixing type. It is found that the method of slow co-precipitation enables homogeneous mixing of Al2 O3 and SiC at a smaller scale, and makes the sintered SiC much denser and more homogeneous, when compared with the traditional method. The parameters of sintering at 1900 ◦ C and 30 MPa for 30 min are recommended. The conclusions here are also beneficial for the sintering research of other ceramic materials. © 2019 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Introduction Silicon carbide (SiC) has been widely applied in high temperature applications because of its physical and chemical properties, such as high strength and oxidation resistance. For example, SiC has been used in high temperature applications (Chen, Gao, Zhou, & Wang, 2002; Zapata-Solvas, Bonilla, Wilshaw, & Todd, 2013), cutting tools (Eblagon, Ehrle, Graule, & Kuebler, 2007), and armor (Flinders, Ray, Anderson, & Cutler, 2005). Among its many applications in different fields, SiC is also used widely for nuclear applications, especially in advanced nuclear fabrication. Fully ceramic micro-encapsulated (FCM) fuel, based on SiC matrix sintering together with TRISO (tristructural-isotropic) coated particles (Luby & Turner, 1972; Petti, Buongiorno, Maki, Hobbins, & Miller, 2003; Liu et al., 2017), is proposed and studied for nuclear reactor safety in many countries (Carmack, Goldner, Bragg-Sitton, & Snead, 2013; Braun et al., 2017). However, it is very difficult to obtain a high performance FCM fuel because a high pressure and temperature are usually required to sinter the SiC matrix (Yamamoto et al., 2010). Under such sintering conditions, TRISOtype nuclear fuel coated particles will be broken (Shinoda, Nagano, & Wakai, 1999), so it is an essential requirement to reduce the sin-
∗ Corresponding author. E-mail address: [email protected] (M. Liu).
tering temperature and pressure to maintain the integrity of the TRISO particles, while not damaging the density of the SiC matrix. SiC sintering is also a hot topic in traditional ceramic research, and has attracted much attention for many years. In previous studies, an effective method to optimize the sintering process of SiC was by adding different sintering additives (Parish, Terrani, Kim, Koyanagi, & Katoh, 2017; Asadikiya, Rudolf, Zhang, Boesl, & Zhong, 2016). Alumina (Al2 O3 ) is an indispensable ingredient in many effective original materials for SiC sintering (Zhang, Iwasa, & Jiang, 2005; Candelario, Nieto, Guiberteau, Moreno, & Ortiz, 2013). Al2 O3 enhances SiC sintering by diffusing the liquid phase at a reasonably low temperature, which promotes atomic diffusion of SiC, and forms compounds with SiC at the interface (Shimoda, Hinoki, Terrani, Snead, & Katoh, 2012; Gomez, Echeberria, Iturriza, & Castro, 2004). To improve this process while adding the same amount of Al2 O3 , it is essential to mix Al2 O3 with SiC at a smaller scale. The traditional addition method has been to grind the Al2 O3 and SiC together to obtain the mixture. A disadvantage of this method arises from the high degree of hardness of SiC, so that the effect of mixing is limited by the size of the original SiC particles (Kim, Blomgren, & Kumta, 2004). Additionally, the agglomeration of Al2 O3 in the powders influences the final sintering effect (Duran, Göc¸mez, & Yılmaz, 2008). In the present study, Al2 O3 was used as the coating layer to construct a SiC@Al2 O3 core–shell structure. This structure allows the surface of raw SiC particles to be coated by a layer of Al2 O3 with different thickness. Using the slow co-precipitation method,
https://doi.org/10.1016/j.partic.2018.08.010 1674-2001/© 2019 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
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Table 1 Sintering parameters of SiC@Al2 O3 composites.
Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Case 7 Case 8 Case 9 Case 10 Case 11 Case 12 Case 13 Case 14 Case 15 Case 16
Al2 O3 addition (wt%)
Temperature (◦ C)
Pressure (MPa)
Raw materials
10 10 10 10 30 30 30 30 10 10 10 10 10 20 20 20
2000 2000 2000 2000 2000 2000 2000 2000 1900 1900 1800 1800 1800 1800 1800 1800
30 30 0 0 30 30 0 0 30 30 30 0 0 30 0 0
C M C M C M C M C M C C M C C M
C stands for the slow co-precipitation method of SiC@Al2 O3 nanoparticles preparation; M stands for the mechanical milling method for the mixture of SiC and Al2 O3 powders.
Materials and methods Coating of Al2 O3 by slow precipitation
Fig. 1. Coating process of SiC@Al2 O3 nanoparticles.
SiC@Al2 O3 nanoparticles with a core–shell structure were successfully prepared at first. After coating, the two phases realized an atomic contact and the layer thickness could be well tuned. Then, the different parameters, such as temperature, pressure, amount of additive and mixing type were tested for sintering SiC. It was determined that the raw material with a core–shell structure achieved a better performance at different sintering conditions compared with the traditional mixing method. The SiC was more uniform by using this core–shell structure as the raw material and experimental parameters of the temperature (1900 ◦ C), pressure (30 MPa), and amount of additive (10%) were recommended.
The slow precipitation method was used for coating Al2 O3 on SiC nanoparticles. In a typical procedure, SiC powder (0.2 g, GR, Shanghai Macklin Biochemical Co., Ltd., China) and polyethylene glycol (9 mL, AR, Tianjin Kemel Chemical Reagent Co., Ltd., China) were added to water (75 mL), and the mixture was dispersed evenly in the solution by ultrasonic oscillation. Then, aluminum nitrate (2 mmol, AR, Shanghai Macklin Biochemical Co., Ltd., China) and urea (4 g, GR, Tianjin Kemel Chemical Reagent Co., Ltd., China) were added to the solution and dissolved. The coating layer was formed slowly after stirring for 48 h at 65 ◦ C, and was further aged by allowing to stand at room temperature for 12 h. The prepared powder was centrifuged from the solution and dried in an oven. Three amounts of Al2 O3 , 10%, 20%, and 30%, were prepared. The samples containing 30% Al2 O3 were prepared to reveal the effect of the different addition methods on the sintering process, which were rare owing to the extremely high amount of additive. In the process of coating, the hydroxyl groups, which were from the urea, were released at a controlled rate and reacted with Al ions slowly to form hydroxides by controlling the urea content and
Fig. 2. A schematic view of the hot-press sintering furnace.
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Fig. 3. Sintering temperature and pressure curves of SiC@Al2 O3 nanoparticles.
Fig. 4. SEM images of the raw materials: (a) SiC nanoparticles and (b) Al2 O3 nanoparticles.
Fig. 5. Mixing effect of SiC particles and Al2 O3 powders prepared by two methods. (a) The sample prepared by the co-precipitation method; (b) the sample prepared by mechanical milling. (c) and (d) represent the microstructure after sintered by slow precipitation and mechanical mixing respectively. The small images in the upper right corners represent the distribution of aluminum.
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Fig. 6. XRD patterns of samples at different stages of the preparation process.
Fig. 7. EDS results of SiC@Al2 O3 particles prepared by the co-precipitation method. (a) TEM image of SiC@Al2 O3 particles; (b) Si element distribution area; (c) O element distribution area; (d) Al element distribution area; (e) C element distribution area. Al element exists in the area of Si element by comparing Fig. 7(b) and (d).
reaction temperature (Sohn, Kwon, Kim, & Kim, 2004). Al ions were first hydrolyzed from Al(NO3 )3 , and were suspended around the dispersed SiC particles. By controlling the hydrolysis rate of urea, Al(OH)3 could be slowly grown on the surface of the SiC particles at a controlled rate. Then the final Al2 O3 coating was prepared by heat treatment at 600 ◦ C for 1 h in an argon atmosphere, as shown in Fig. 1. Sintering of SiC@Al2 O3 nanoparticles To test the different parameters, such as temperature, pressure, additive amount, and mixing type, cases 1–16 were conducted, as shown in Table 1. To reduce the volatilization of Al2 O3 at high temperature, argon (high purity, 99.999%) was used as the sintering atmosphere, and two groups of comparative experiments of no-pressure sintering and pressure sintering were performed to verify the effect. In the no-pressure sintering process, the nanopowders were pressed into a tablet for sintering. A schematic of
the hot-pressing sintering furnace and the sintering process curve of temperature and pressure used are shown in Figs. 2 and 3, respectively. Characterizations The crystal structure of the samples was examined by X-ray diffraction (XRD) on a diffractometer using Cu K˛ radiation with = 1.5418 Å (D8-Advance, Bruker, USA). TEM images and the corresponding selected area electron diffraction (SAED) patterns of the samples were obtained on a high-resolution transmission electron microscope (HRTEM, JEM2010, JEOL, Japan; acceleration voltage: 100 kV). Microstructures of the samples were observed by a fieldemission scanning electron microscope (SEM, JSM7401, JEOL). The hardness and density of the sintered SiC sample were also measured. The sintered SiC sample was mounted by glue and grinded. Then the hardness was measured using Vickers microhardness test, and a 130◦ diamond quadrangular pyramid was used as the
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Fig. 8. EDS results of SiC@Al2 O3 particles prepared by mechanical milling method. (a) TEM image of SiC@Al2 O3 particles; (b) Si element distribution area; (c) O element distribution area; (d) Al element distribution area; (e) C element distribution area.
press-in head. The value was calculated as follows: HV = 18.18P/d2 (HV: Vickers hardness; P: load(N); d: Average indentation diagonal length (mm)). The density of the SiC samples was measured using the liquid suspension method. By adding a heavy liquid (diiodomethane, 3.32 g/cm3 ) to change the density of the original liquid (tribromomethane, 2.89 g/cm3 ), a series of different density liquids was obtained. Then the SiC sample was added into the series of liquids, and if the sample could be suspended, the density was determined to be the same as that of the liquid. Results and discussion To reduce the sintering temperature and pressure of the SiC matrix, Al2 O3 was added as the sintering additive. The core–shell structure, where a thin Al2 O3 layer was coated on a SiC surface, was designed and used as the raw material. The structural differences and sintering advantages resulting from the core–shell structure composites, compared with the raw materials prepared by the mechanical milling method, will be discussed in detail below. Coating process of SiC@Al2 O3 nanoparticles Fig. 4 shows the SEM images of the SiC and Al2 O3 particles. The SiC particles exhibited a diameter of approximately 40 nm. The diameter of the Al2 O3 particles was approximately 200 nm. One of the samples was prepared by adding the SiC particles shown in Fig. 4(a) to an Al(NO3 )3 solution using the slow co-precipitation method. The other sample was prepared by directly mechanically mixing SiC and the Al2 O3 nanoparticles shown in Fig. 4(b). The microstructure differences of the two samples could be obtained by comparing the SEM images. The composites prepared by the slow co-precipitation method exhibited obvious differences in structure compared with the raw material SiC particles. The surface of the SiC particles were coated with a layer of Al2 O3 , as can be seen from Fig. 5(a). This result was verified from the energy dispersive X-ray spectroscopy (EDS) and
XRD results, as shown in Figs. 6 and 7. The XRD results showed that the co-precipitation method successfully introduced Al2 O3 onto the SiC particles (Fig. 6). The EDS results showed that the Al element distribution area basically coincided with the Si element distribution area. Since the amount of added Al element was less than that of SiC, therefore, Al was uniformly coated on the surface of the SiC particles in the form of Al2 O3 . According to Fig. 5(b), it was found that the composites prepared by the mechanical mixing method had more Al2 O3 agglomerates on the structure. The Al2 O3 nanoparticles were not uniformly mixed with the SiC particles. After sintering, there were also many Al agglomerates. This result was verified by the EDS results shown in Fig. 8. The structural differences had a large impact on the sintering behavior of the two as-prepared raw materials.
Effect of temperature on the sintered SiC matrix To verify the differences in the sintering performance between the materials prepared by the two methods, sintering experiments were carried out under various conditions. The three process parameters of cases 1, 9, and 11 were selected for studying the effect of temperature on the sintering performance. The mass fraction of Al2 O3 was controlled to be 10%. Under the conditions of a sintering atmosphere of Ar and 30 MPa, the materials obtained by the slow co-precipitation method were sintered at 1800, 1900, and 2000 ◦ C for 30 min, which resulted in three different microstructures, as shown in Fig. 9. The SiC matrix shown in Fig. 9(a) and (b) was relatively dense. The SiC crystal grains did not grow significantly. The crystal grains in Fig. 9(b) were relatively small and uniform. The sintered SiC structure was relatively loose, basically still granular with many porous holes when sintered at 1800 ◦ C. Therefore, the temperature is a basic factor that influences the sintering behavior, as has already been indicated in a previous study (Wu, Huang, Yang, Zhao, & Xu, 1996).
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Fig. 9. SEM images of SiC sintered at different temperatures. (a) 10% Al2 O3 was added by slow co-precipitation, sintered at 2000 ◦ C and 30 MPa, and in Ar; (b) 10% Al2 O3 was added by slow co-precipitation, sintered at 1900 ◦ C and 30 MPa, and in Ar; (c) 10% Al2 O3 was added by slow co-precipitation, sintered at 1800 ◦ C and 30 MPa, and in Ar.
Effect of pressure on the sintered SiC matrix Six cases were investigated to show the effect of pressure, as shown in Fig. 10. In the process of non-pressure sintering, no nucleation and recrystallization occurred. The obtained structure was relatively porous and the SiC particles were basically granular. When the amount of added Al2 O3 was increased, the obtained structure became less dense. As the sintering temperature was increased, the structure also became less dense. This was attributed to the volatilization of Al2 O3 at high temperatures. The degree of volatilization was regulated by the temperature and the amount of added Al2 O3 . This led to a greater difficulty in forming the desired liquid phase sintering (Yan, Huang, Dong, & Jiang, 2006). Similarly, this also accounted for the structure shown in Fig. 10(c) and (d)
being less dense than that of Fig. 10(a) and (b). It was shown that a high pressure is needed to obtain a dense SiC matrix, otherwise, the additives are unable to exert their effect. Effect of amount of added Al2 O3 on the sintered SiC matrix When the Al2 O3 content was controlled as an independent variable, the effect of this change on the sintered structure was studied. From the images of Fig. 11(a) and (b), as the Al content increased, the structure of the SiC substrate was still dense, but fine Al2 O3 particles appeared at the grain boundaries. This was consistent with a previous study on the additive content (Zhang, Zhang, & Hu, 2014). This indicated that when the temperature is increased, the amount of added Al2 O3 can be reduced to obtain a dense SiC matrix.
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Fig. 10. SEM images of samples sintered under a pressure condition or not. (a), (b) 10% Al2 O3 was added by slow co-precipitation, sintered at 1800 ◦ C and in Ar; (c), (d) 20% Al2 O3 was added by slow co-precipitation, sintered at 1800 ◦ C and in Ar; (e), (f) 10% Al2 O3 was added by slow co-precipitation, sintered at 2000 ◦ C in Ar. (a), (c), (e) sintered at 30 MPa; (b), (d), (f) sintered with no pressure.
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Fig. 11. SEM images of sintered samples with different amounts of Al2 O3 . (a) 10% Al2 O3 was added by slow co-precipitation, sintered at 2000 ◦ C and 30 MPa, and in Ar; (b) 30% Al2 O3 was added by slow co-precipitation, sintered at 2000 ◦ C and 30 MPa, and in Ar; (c) 10% Al2 O3 was added by slow co-precipitation, sintered at 1800 ◦ C and 30 MPa, and in Ar; (d) 20% Al2 O3 was added by slow co-precipitation, sintered at 1800 ◦ C and 30 MPa, and in Ar.
The increase in Al2 O3 content resulted in no significant changes at 1800 ◦ C. Effect of Al2 O3 addition method on the sintered SiC matrix According to the results shown in Fig. 12, the effects of two addition methods of Al2 O3 on the microstructure were compared. In general, the sintering matrix prepared by the slow precipitation (SP) method was denser and more homogenous than that prepared by the mechanical mixing (MM) method. Regardless of the presence of pressure, the hollow volume of the microstructure prepared by mechanical milling was more than that prepared by the slow co-precipitation method after sintering. This was related to the uniformity of the Al2 O3 distribution (Kamrani, Razavi Hesabi, Riedel, & Seyed Reihani, 2011). The hollow volume was controlled by the uniformity of the alumina distribution in the raw material. Density and hardness measurement of the sintered SiC matrix The four samples shown in Fig. 13(a) all had an alumina content of 10%, sintering pressure and sintering atmosphere of 30 MPa and Ar, and samples 1 and 2 were sintered at 2000 ◦ C, and samples 3 and 4 were sintered at 1900 ◦ C. The other two samples shown Fig. 13(b) were sintered at 1800 ◦ C in Ar. Samples 5 and 6 had an alumina content of 10% and 20%, respectively. The alumina addition for samples 1 and 3 was by mixing method (MM) and for samples 2, 4, 5, and 6 was by slow precipitation method (SP). The density of samples sintered at 2000 ◦ C was slightly higher than the samples
sintered at 1900 ◦ C. The relative density was from 92% to 96%, as shown in Fig. 13(a). When sintered at 1800 ◦ C, the relative density and hardness were both lower. The samples shown in Fig. 13 were all sintered at 30 MPa, and the other samples which are not shown in this figure were sintered without pressure, because the samples were broken and incomplete at the sample preparation process, which made it difficult to measure the density and hardness.
Conclusions (1) The slow co-precipitation method allows for Al2 O3 to be evenly coated around SiC particles to form a core–shell structure. Uniform mixing of Al2 O3 and SiC can be achieved at the nanometer scale by this addition method, compared with the traditional mechanical mixing method, which results in a denser and uniform sintering silicon carbide matrix. (2) The core–shell composites sintered at 1900 ◦ C and under 30 MPa can result in a dense SiC matrix. When the sintering temperature was 2000 ◦ C, as the alumina content increased, the structure of the SiC substrate was still dense, but fine particles appeared at the grain boundaries. (3) As a result of the volatilization of Al2 O3 at high temperature, the condition of sintering at 30 MPa makes it easier to obtain a dense SiC matrix, compared with sintering without pressure. Comparing the sintering results (morphology, density, hardness) and time/energy consumption, the optimized parameters (1900 ◦ C, 30 MPa, and amount of additive 10%) are recommended to obtain a dense sintered SiC matrix.
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Fig. 12. SEM images of sintered samples with different methods of Al2 O3 addition. (a) 10% Al2 O3 was added by slow co-precipitation, sintered at 1800 ◦ C and in Ar; (b) 10% Al2 O3 was added by mechanical milling, sintered at 1800 ◦ C and in Ar; (c) 10% Al2 O3 was added by slow co-precipitation, sintered at 1900 ◦ C and 30 MPa, and in Ar; (d) 10% Al2 O3 was added by mechanical milling, sintered at 1900 ◦ C and 30 MPa, and in Ar; (e) 10% Al2 O3 was added by slow co-precipitation, sintered at 2000 ◦ C and 30 MPa, and in Ar; (f) 10% Al2 O3 was added by mechanical milling, sintered at 2000 ◦ C and 30 MPa, and in Ar.
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Fig. 13. Results of hardness and density. (All results were calculated by the average of three measurements on the same sample.)
Acknowledgements This work was supported by National Natural Science Foundation of China (NSFC, Nos. 91634113, 21771116). References Asadikiya, M., Rudolf, C., Zhang, C., Boesl, B., & Zhong, Y. (2016). The role of calphad approach in the sintering of B4 C with SiC as a sintering aid by spark plasma sintering technique. pp. 185–191. Additive manufacturing and strategic technologies in advanced ceramics: Ceramic transactions series (Vol. 258). Braun, J., Guéneau, C., Alpettaz, T., Sauder, C., Brackx, E., Domenger, R., et al. (2017). Chemical compatibility between UO2 , fuel and SiC cladding for LWRs. Application to ATF (accident-tolerant fuel). Journal of Nuclear Materials, 487, 380–395. Candelario, V. M., Nieto, M. I., Guiberteau, F., Moreno, R., & Ortiz, A. L. (2013). Aqueous colloidal processing of SiC with Y3 Al5 O12 liquid-phase sintering additives. Journal of the European Ceramic Society, 33(10), 1685–1694. Carmack, J., Goldner, F., Bragg-Sitton, S. M., & Snead, L. L. (2013). Overview of the US DOE accident tolerant fuel development program. In Proceedings of the 2013 LWR Fuel Performance Meeting/TopFuel 2013 (pp. 15–19). Chen, M. H., Gao, L., Zhou, J. H., & Wang, M. (2002). Application of reaction sintering to the manufacturing of a spacecraft combustion chamber of SiC ceramics. Journal of Materials Processing Technology, 129(1–3), 408–411. Duran, C., Göc¸mez, H., & Yılmaz, H. (2008). Dispersion of mechanochemically activated SiC and Al2 O3 powders. Materials Science and Engineering: A, 475(1-2), 23–26. Eblagon, F., Ehrle, B., Graule, T., & Kuebler, J. (2007). Development of silicon nitride/silicon carbide composites for wood-cutting tools. Journal of the European Ceramic Society, 27(1), 419–428. Flinders, M., Ray, D., Anderson, A., & Cutler, R. A. (2005). High-toughness silicon carbide as armor. Journal of the American Ceramic Society, 88(8), 2217–2226. Gomez, E., Echeberria, J., Iturriza, I., & Castro, F. (2004). Liquid phase sintering of SiC with additions of Y2 O3 , Al2 O3 and SiO2 . Journal of the European Ceramic Society, 24(9), 2895–2903. Kamrani, S., Razavi Hesabi, Z., Riedel, R., & Seyed Reihani, S. M. (2011). Synthesis and characterization of Al–SiC nanocomposites produced by mechanical milling and Sintering. Advanced Composite Materials, 20(1), 13–27. Kim, I. S., Blomgren, G. E., & Kumta, P. N. (2004). Si–SiC nanocomposite anodes synthesized using high-energy mechanical milling. Journal of Power Sources, 130, 275–280.
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Particuology journal homepage: www.elsevier.com/locate/partic
Synthesis of SiC@Al2 O3 core–shell nanoparticles for dense SiC sintering Yiteng Liu, Rongzheng Liu, Malin Liu ∗ , Jiaxing Chang Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 1 July 2018 Received in revised form 28 July 2018 Accepted 30 August 2018 Available online 18 February 2019 Keywords: Core–shell nanoparticles Dense sintering SiC Coating Hot-pressing
a b s t r a c t Owing to the difficulty for dense SiC sintering, high sintering temperatures and pressures are usually needed. Lowering the sintering temperature by adding Al2 O3 as a sintering additive has previously been shown to be beneficial. However, traditional addition methods limit the effect of the Al2 O3 owing to inhomogeneous mixing at the nanoscale. A SiC@Al2 O3 composite nanoparticle with a core–shell structure is designed and prepared using the slow co-precipitation method. The differences between this method and the traditional mechanical ball milling method are interpreted by different experimental parameters, such as temperature, pressure, amount of additive, and mixing type. It is found that the method of slow co-precipitation enables homogeneous mixing of Al2 O3 and SiC at a smaller scale, and makes the sintered SiC much denser and more homogeneous, when compared with the traditional method. The parameters of sintering at 1900 ◦ C and 30 MPa for 30 min are recommended. The conclusions here are also beneficial for the sintering research of other ceramic materials. © 2019 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
Introduction Silicon carbide (SiC) has been widely applied in high temperature applications because of its physical and chemical properties, such as high strength and oxidation resistance. For example, SiC has been used in high temperature applications (Chen, Gao, Zhou, & Wang, 2002; Zapata-Solvas, Bonilla, Wilshaw, & Todd, 2013), cutting tools (Eblagon, Ehrle, Graule, & Kuebler, 2007), and armor (Flinders, Ray, Anderson, & Cutler, 2005). Among its many applications in different fields, SiC is also used widely for nuclear applications, especially in advanced nuclear fabrication. Fully ceramic micro-encapsulated (FCM) fuel, based on SiC matrix sintering together with TRISO (tristructural-isotropic) coated particles (Luby & Turner, 1972; Petti, Buongiorno, Maki, Hobbins, & Miller, 2003; Liu et al., 2017), is proposed and studied for nuclear reactor safety in many countries (Carmack, Goldner, Bragg-Sitton, & Snead, 2013; Braun et al., 2017). However, it is very difficult to obtain a high performance FCM fuel because a high pressure and temperature are usually required to sinter the SiC matrix (Yamamoto et al., 2010). Under such sintering conditions, TRISOtype nuclear fuel coated particles will be broken (Shinoda, Nagano, & Wakai, 1999), so it is an essential requirement to reduce the sin-
∗ Corresponding author. E-mail address: [email protected] (M. Liu).
tering temperature and pressure to maintain the integrity of the TRISO particles, while not damaging the density of the SiC matrix. SiC sintering is also a hot topic in traditional ceramic research, and has attracted much attention for many years. In previous studies, an effective method to optimize the sintering process of SiC was by adding different sintering additives (Parish, Terrani, Kim, Koyanagi, & Katoh, 2017; Asadikiya, Rudolf, Zhang, Boesl, & Zhong, 2016). Alumina (Al2 O3 ) is an indispensable ingredient in many effective original materials for SiC sintering (Zhang, Iwasa, & Jiang, 2005; Candelario, Nieto, Guiberteau, Moreno, & Ortiz, 2013). Al2 O3 enhances SiC sintering by diffusing the liquid phase at a reasonably low temperature, which promotes atomic diffusion of SiC, and forms compounds with SiC at the interface (Shimoda, Hinoki, Terrani, Snead, & Katoh, 2012; Gomez, Echeberria, Iturriza, & Castro, 2004). To improve this process while adding the same amount of Al2 O3 , it is essential to mix Al2 O3 with SiC at a smaller scale. The traditional addition method has been to grind the Al2 O3 and SiC together to obtain the mixture. A disadvantage of this method arises from the high degree of hardness of SiC, so that the effect of mixing is limited by the size of the original SiC particles (Kim, Blomgren, & Kumta, 2004). Additionally, the agglomeration of Al2 O3 in the powders influences the final sintering effect (Duran, Göc¸mez, & Yılmaz, 2008). In the present study, Al2 O3 was used as the coating layer to construct a SiC@Al2 O3 core–shell structure. This structure allows the surface of raw SiC particles to be coated by a layer of Al2 O3 with different thickness. Using the slow co-precipitation method,
https://doi.org/10.1016/j.partic.2018.08.010 1674-2001/© 2019 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
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Table 1 Sintering parameters of SiC@Al2 O3 composites.
Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Case 7 Case 8 Case 9 Case 10 Case 11 Case 12 Case 13 Case 14 Case 15 Case 16
Al2 O3 addition (wt%)
Temperature (◦ C)
Pressure (MPa)
Raw materials
10 10 10 10 30 30 30 30 10 10 10 10 10 20 20 20
2000 2000 2000 2000 2000 2000 2000 2000 1900 1900 1800 1800 1800 1800 1800 1800
30 30 0 0 30 30 0 0 30 30 30 0 0 30 0 0
C M C M C M C M C M C C M C C M
C stands for the slow co-precipitation method of SiC@Al2 O3 nanoparticles preparation; M stands for the mechanical milling method for the mixture of SiC and Al2 O3 powders.
Materials and methods Coating of Al2 O3 by slow precipitation
Fig. 1. Coating process of SiC@Al2 O3 nanoparticles.
SiC@Al2 O3 nanoparticles with a core–shell structure were successfully prepared at first. After coating, the two phases realized an atomic contact and the layer thickness could be well tuned. Then, the different parameters, such as temperature, pressure, amount of additive and mixing type were tested for sintering SiC. It was determined that the raw material with a core–shell structure achieved a better performance at different sintering conditions compared with the traditional mixing method. The SiC was more uniform by using this core–shell structure as the raw material and experimental parameters of the temperature (1900 ◦ C), pressure (30 MPa), and amount of additive (10%) were recommended.
The slow precipitation method was used for coating Al2 O3 on SiC nanoparticles. In a typical procedure, SiC powder (0.2 g, GR, Shanghai Macklin Biochemical Co., Ltd., China) and polyethylene glycol (9 mL, AR, Tianjin Kemel Chemical Reagent Co., Ltd., China) were added to water (75 mL), and the mixture was dispersed evenly in the solution by ultrasonic oscillation. Then, aluminum nitrate (2 mmol, AR, Shanghai Macklin Biochemical Co., Ltd., China) and urea (4 g, GR, Tianjin Kemel Chemical Reagent Co., Ltd., China) were added to the solution and dissolved. The coating layer was formed slowly after stirring for 48 h at 65 ◦ C, and was further aged by allowing to stand at room temperature for 12 h. The prepared powder was centrifuged from the solution and dried in an oven. Three amounts of Al2 O3 , 10%, 20%, and 30%, were prepared. The samples containing 30% Al2 O3 were prepared to reveal the effect of the different addition methods on the sintering process, which were rare owing to the extremely high amount of additive. In the process of coating, the hydroxyl groups, which were from the urea, were released at a controlled rate and reacted with Al ions slowly to form hydroxides by controlling the urea content and
Fig. 2. A schematic view of the hot-press sintering furnace.
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Fig. 3. Sintering temperature and pressure curves of SiC@Al2 O3 nanoparticles.
Fig. 4. SEM images of the raw materials: (a) SiC nanoparticles and (b) Al2 O3 nanoparticles.
Fig. 5. Mixing effect of SiC particles and Al2 O3 powders prepared by two methods. (a) The sample prepared by the co-precipitation method; (b) the sample prepared by mechanical milling. (c) and (d) represent the microstructure after sintered by slow precipitation and mechanical mixing respectively. The small images in the upper right corners represent the distribution of aluminum.
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Fig. 6. XRD patterns of samples at different stages of the preparation process.
Fig. 7. EDS results of SiC@Al2 O3 particles prepared by the co-precipitation method. (a) TEM image of SiC@Al2 O3 particles; (b) Si element distribution area; (c) O element distribution area; (d) Al element distribution area; (e) C element distribution area. Al element exists in the area of Si element by comparing Fig. 7(b) and (d).
reaction temperature (Sohn, Kwon, Kim, & Kim, 2004). Al ions were first hydrolyzed from Al(NO3 )3 , and were suspended around the dispersed SiC particles. By controlling the hydrolysis rate of urea, Al(OH)3 could be slowly grown on the surface of the SiC particles at a controlled rate. Then the final Al2 O3 coating was prepared by heat treatment at 600 ◦ C for 1 h in an argon atmosphere, as shown in Fig. 1. Sintering of SiC@Al2 O3 nanoparticles To test the different parameters, such as temperature, pressure, additive amount, and mixing type, cases 1–16 were conducted, as shown in Table 1. To reduce the volatilization of Al2 O3 at high temperature, argon (high purity, 99.999%) was used as the sintering atmosphere, and two groups of comparative experiments of no-pressure sintering and pressure sintering were performed to verify the effect. In the no-pressure sintering process, the nanopowders were pressed into a tablet for sintering. A schematic of
the hot-pressing sintering furnace and the sintering process curve of temperature and pressure used are shown in Figs. 2 and 3, respectively. Characterizations The crystal structure of the samples was examined by X-ray diffraction (XRD) on a diffractometer using Cu K˛ radiation with = 1.5418 Å (D8-Advance, Bruker, USA). TEM images and the corresponding selected area electron diffraction (SAED) patterns of the samples were obtained on a high-resolution transmission electron microscope (HRTEM, JEM2010, JEOL, Japan; acceleration voltage: 100 kV). Microstructures of the samples were observed by a fieldemission scanning electron microscope (SEM, JSM7401, JEOL). The hardness and density of the sintered SiC sample were also measured. The sintered SiC sample was mounted by glue and grinded. Then the hardness was measured using Vickers microhardness test, and a 130◦ diamond quadrangular pyramid was used as the
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Fig. 8. EDS results of SiC@Al2 O3 particles prepared by mechanical milling method. (a) TEM image of SiC@Al2 O3 particles; (b) Si element distribution area; (c) O element distribution area; (d) Al element distribution area; (e) C element distribution area.
press-in head. The value was calculated as follows: HV = 18.18P/d2 (HV: Vickers hardness; P: load(N); d: Average indentation diagonal length (mm)). The density of the SiC samples was measured using the liquid suspension method. By adding a heavy liquid (diiodomethane, 3.32 g/cm3 ) to change the density of the original liquid (tribromomethane, 2.89 g/cm3 ), a series of different density liquids was obtained. Then the SiC sample was added into the series of liquids, and if the sample could be suspended, the density was determined to be the same as that of the liquid. Results and discussion To reduce the sintering temperature and pressure of the SiC matrix, Al2 O3 was added as the sintering additive. The core–shell structure, where a thin Al2 O3 layer was coated on a SiC surface, was designed and used as the raw material. The structural differences and sintering advantages resulting from the core–shell structure composites, compared with the raw materials prepared by the mechanical milling method, will be discussed in detail below. Coating process of SiC@Al2 O3 nanoparticles Fig. 4 shows the SEM images of the SiC and Al2 O3 particles. The SiC particles exhibited a diameter of approximately 40 nm. The diameter of the Al2 O3 particles was approximately 200 nm. One of the samples was prepared by adding the SiC particles shown in Fig. 4(a) to an Al(NO3 )3 solution using the slow co-precipitation method. The other sample was prepared by directly mechanically mixing SiC and the Al2 O3 nanoparticles shown in Fig. 4(b). The microstructure differences of the two samples could be obtained by comparing the SEM images. The composites prepared by the slow co-precipitation method exhibited obvious differences in structure compared with the raw material SiC particles. The surface of the SiC particles were coated with a layer of Al2 O3 , as can be seen from Fig. 5(a). This result was verified from the energy dispersive X-ray spectroscopy (EDS) and
XRD results, as shown in Figs. 6 and 7. The XRD results showed that the co-precipitation method successfully introduced Al2 O3 onto the SiC particles (Fig. 6). The EDS results showed that the Al element distribution area basically coincided with the Si element distribution area. Since the amount of added Al element was less than that of SiC, therefore, Al was uniformly coated on the surface of the SiC particles in the form of Al2 O3 . According to Fig. 5(b), it was found that the composites prepared by the mechanical mixing method had more Al2 O3 agglomerates on the structure. The Al2 O3 nanoparticles were not uniformly mixed with the SiC particles. After sintering, there were also many Al agglomerates. This result was verified by the EDS results shown in Fig. 8. The structural differences had a large impact on the sintering behavior of the two as-prepared raw materials.
Effect of temperature on the sintered SiC matrix To verify the differences in the sintering performance between the materials prepared by the two methods, sintering experiments were carried out under various conditions. The three process parameters of cases 1, 9, and 11 were selected for studying the effect of temperature on the sintering performance. The mass fraction of Al2 O3 was controlled to be 10%. Under the conditions of a sintering atmosphere of Ar and 30 MPa, the materials obtained by the slow co-precipitation method were sintered at 1800, 1900, and 2000 ◦ C for 30 min, which resulted in three different microstructures, as shown in Fig. 9. The SiC matrix shown in Fig. 9(a) and (b) was relatively dense. The SiC crystal grains did not grow significantly. The crystal grains in Fig. 9(b) were relatively small and uniform. The sintered SiC structure was relatively loose, basically still granular with many porous holes when sintered at 1800 ◦ C. Therefore, the temperature is a basic factor that influences the sintering behavior, as has already been indicated in a previous study (Wu, Huang, Yang, Zhao, & Xu, 1996).
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Fig. 9. SEM images of SiC sintered at different temperatures. (a) 10% Al2 O3 was added by slow co-precipitation, sintered at 2000 ◦ C and 30 MPa, and in Ar; (b) 10% Al2 O3 was added by slow co-precipitation, sintered at 1900 ◦ C and 30 MPa, and in Ar; (c) 10% Al2 O3 was added by slow co-precipitation, sintered at 1800 ◦ C and 30 MPa, and in Ar.
Effect of pressure on the sintered SiC matrix Six cases were investigated to show the effect of pressure, as shown in Fig. 10. In the process of non-pressure sintering, no nucleation and recrystallization occurred. The obtained structure was relatively porous and the SiC particles were basically granular. When the amount of added Al2 O3 was increased, the obtained structure became less dense. As the sintering temperature was increased, the structure also became less dense. This was attributed to the volatilization of Al2 O3 at high temperatures. The degree of volatilization was regulated by the temperature and the amount of added Al2 O3 . This led to a greater difficulty in forming the desired liquid phase sintering (Yan, Huang, Dong, & Jiang, 2006). Similarly, this also accounted for the structure shown in Fig. 10(c) and (d)
being less dense than that of Fig. 10(a) and (b). It was shown that a high pressure is needed to obtain a dense SiC matrix, otherwise, the additives are unable to exert their effect. Effect of amount of added Al2 O3 on the sintered SiC matrix When the Al2 O3 content was controlled as an independent variable, the effect of this change on the sintered structure was studied. From the images of Fig. 11(a) and (b), as the Al content increased, the structure of the SiC substrate was still dense, but fine Al2 O3 particles appeared at the grain boundaries. This was consistent with a previous study on the additive content (Zhang, Zhang, & Hu, 2014). This indicated that when the temperature is increased, the amount of added Al2 O3 can be reduced to obtain a dense SiC matrix.
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Fig. 10. SEM images of samples sintered under a pressure condition or not. (a), (b) 10% Al2 O3 was added by slow co-precipitation, sintered at 1800 ◦ C and in Ar; (c), (d) 20% Al2 O3 was added by slow co-precipitation, sintered at 1800 ◦ C and in Ar; (e), (f) 10% Al2 O3 was added by slow co-precipitation, sintered at 2000 ◦ C in Ar. (a), (c), (e) sintered at 30 MPa; (b), (d), (f) sintered with no pressure.
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Fig. 11. SEM images of sintered samples with different amounts of Al2 O3 . (a) 10% Al2 O3 was added by slow co-precipitation, sintered at 2000 ◦ C and 30 MPa, and in Ar; (b) 30% Al2 O3 was added by slow co-precipitation, sintered at 2000 ◦ C and 30 MPa, and in Ar; (c) 10% Al2 O3 was added by slow co-precipitation, sintered at 1800 ◦ C and 30 MPa, and in Ar; (d) 20% Al2 O3 was added by slow co-precipitation, sintered at 1800 ◦ C and 30 MPa, and in Ar.
The increase in Al2 O3 content resulted in no significant changes at 1800 ◦ C. Effect of Al2 O3 addition method on the sintered SiC matrix According to the results shown in Fig. 12, the effects of two addition methods of Al2 O3 on the microstructure were compared. In general, the sintering matrix prepared by the slow precipitation (SP) method was denser and more homogenous than that prepared by the mechanical mixing (MM) method. Regardless of the presence of pressure, the hollow volume of the microstructure prepared by mechanical milling was more than that prepared by the slow co-precipitation method after sintering. This was related to the uniformity of the Al2 O3 distribution (Kamrani, Razavi Hesabi, Riedel, & Seyed Reihani, 2011). The hollow volume was controlled by the uniformity of the alumina distribution in the raw material. Density and hardness measurement of the sintered SiC matrix The four samples shown in Fig. 13(a) all had an alumina content of 10%, sintering pressure and sintering atmosphere of 30 MPa and Ar, and samples 1 and 2 were sintered at 2000 ◦ C, and samples 3 and 4 were sintered at 1900 ◦ C. The other two samples shown Fig. 13(b) were sintered at 1800 ◦ C in Ar. Samples 5 and 6 had an alumina content of 10% and 20%, respectively. The alumina addition for samples 1 and 3 was by mixing method (MM) and for samples 2, 4, 5, and 6 was by slow precipitation method (SP). The density of samples sintered at 2000 ◦ C was slightly higher than the samples
sintered at 1900 ◦ C. The relative density was from 92% to 96%, as shown in Fig. 13(a). When sintered at 1800 ◦ C, the relative density and hardness were both lower. The samples shown in Fig. 13 were all sintered at 30 MPa, and the other samples which are not shown in this figure were sintered without pressure, because the samples were broken and incomplete at the sample preparation process, which made it difficult to measure the density and hardness.
Conclusions (1) The slow co-precipitation method allows for Al2 O3 to be evenly coated around SiC particles to form a core–shell structure. Uniform mixing of Al2 O3 and SiC can be achieved at the nanometer scale by this addition method, compared with the traditional mechanical mixing method, which results in a denser and uniform sintering silicon carbide matrix. (2) The core–shell composites sintered at 1900 ◦ C and under 30 MPa can result in a dense SiC matrix. When the sintering temperature was 2000 ◦ C, as the alumina content increased, the structure of the SiC substrate was still dense, but fine particles appeared at the grain boundaries. (3) As a result of the volatilization of Al2 O3 at high temperature, the condition of sintering at 30 MPa makes it easier to obtain a dense SiC matrix, compared with sintering without pressure. Comparing the sintering results (morphology, density, hardness) and time/energy consumption, the optimized parameters (1900 ◦ C, 30 MPa, and amount of additive 10%) are recommended to obtain a dense sintered SiC matrix.
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Fig. 12. SEM images of sintered samples with different methods of Al2 O3 addition. (a) 10% Al2 O3 was added by slow co-precipitation, sintered at 1800 ◦ C and in Ar; (b) 10% Al2 O3 was added by mechanical milling, sintered at 1800 ◦ C and in Ar; (c) 10% Al2 O3 was added by slow co-precipitation, sintered at 1900 ◦ C and 30 MPa, and in Ar; (d) 10% Al2 O3 was added by mechanical milling, sintered at 1900 ◦ C and 30 MPa, and in Ar; (e) 10% Al2 O3 was added by slow co-precipitation, sintered at 2000 ◦ C and 30 MPa, and in Ar; (f) 10% Al2 O3 was added by mechanical milling, sintered at 2000 ◦ C and 30 MPa, and in Ar.
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Fig. 13. Results of hardness and density. (All results were calculated by the average of three measurements on the same sample.)
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