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Development of ethylene glycol-based gelcasting for the preparation of highly porous SiC ceramics
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Development of ethylene glycol-based gelcasting for the preparation of highly porous SiC ceramics Yicheng Jina, Biao Zhanga,b,∗∗, Feng Yea,∗, Haoqian Zhanga, Zhaoxin Zhonga, Qiang Liua, Zhiguo Zhangb a b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China Department of Physics, Harbin Institute of Technology, Harbin, 150001, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Gelcasting Ethylene glycol Porous ceramics SiC
Aqueous gelcasting is inappropriate for the preparation of highly porous ceramics, due to the large drying shrinkage of green bodies caused by the high surface tension of water. To solve this problem, non-aqueous gelcasting using organic solvents with much lower surface tension was developed. However, for most organic solvents, the precipitation polymerization of gels led to the low strength of green bodies, which was inconvenient for the fabrication of large size workpieces. In this work, a novel ethylene glycol-based gelcasting was developed to prepare highly porous SiC ceramics. Ethylene glycol induced the solution polymerization of gels and increased the strength of green bodies effectively. In addition, the high flexibility of the ethylene glycol-based gels could release the inner stress in the drying process. Highly porous SiC ceramics with large size were successfully prepared by the optimized gelcasting method.
1. Introduction Porous ceramics have been widely applied in many fields such as filters, catalyst supports and reinforcements of composites [1–3]. In particular, porous SiC has been considered as one of the most promising materials for high temperature filtration and catalysis owing to its great chemical and thermal stability [4,5]. High open porosity is beneficial to the permeability of filters and supports [6]. Several techniques have been developed to prepare highly porous ceramics such as replication, foaming, freeze casting and gelcasting [7–10]. Among these techniques, gelcasting is a net-shape method and can be used to fabricate complexshaped green bodies [11]. Aqueous gelcasting can prepare green bodies with high mechanical properties, which can be machined easily before debinding [12]. Because of the high surface tension of water (72.1 mN/m), the drying of solvents caused large shrinkage, limiting the porosity of green bodies [13]. Thus, other pore-forming methods, such as foaming [14] and sacrificial template [10], were combined with aqueous gelcasing to increase the porosity. The obtained ceramics presented multi-modal pore distribution. Sub-micro pores were caused by the sintering of ceramic particles while large pores (> 15 μm) were generated by poreforming agents. High fluid permeability and adequate mechanical
∗
properties are desirable for filters and supports. However, the sub-micro pores limited the permeability and the large pores decreased the mechanical properties of ceramics. Moreover, the pore-forming agents could not be removed completely and closed pores are unavoidable in green bodies [15]. Non-aqueous gelcasting has been developed to prepare highly porous ceramics with single-mode pore distribution [16]. Due to the low surface tension of organic solvents, little shrinkage was occurred after drying, which preserved the highly porous skeleton of green bodies [16–19]. In addition, the porosity of ceramics could be tailored freely (about 60% to 90%) by changing the solid loading of slurries. The uniform pore structure and controllable porosity make it an ideal candidate for filters and supports. Idle time is defined as the time between the addition of initiator and the commencement of polymerization, during which the evaporation of solvents and the sedimentation of ceramic powders occur [20]. Thus, high polymerization rate with short idle time are favorable for the uniformity of green bodies [11]. Unfortunately, due to the low polymerization rate in most organic solvents, the idle time of non-aqueous gelcasting might be too long. Moreover, the low reaction rate could induce defects in gels and decrease the mechanical properties of green bodies [21]. The compressive strength of the highly porous green bodies was lower than 2 MPa [16].
Corresponding author. Corresponding author. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China. E-mail addresses: [email protected] (B. Zhang), [email protected] (F. Ye).
∗∗
https://doi.org/10.1016/j.ceramint.2019.12.009 Received 2 November 2019; Received in revised form 29 November 2019; Accepted 2 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Yicheng Jin, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.009
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Though small samples could be handled and processed, it might be unreliable to fabricate large size green bodies with high porosity. The strength of green bodies is mainly supported by gels. However, few research has been focused on the gelation behavior of non-aqueous gelcasting. In our tentative experiments, the gelation behaviors of AM/MBAM in water and various organic solvents were explored. From these experiments, we got a crucial information, that is the gelation behaviors of AM/MBAM in ethylene glycol (EG) were similar to that in water, which performed short idle time and high gel strength. In addition, the surface tension of EG (46.5 mN/m) is much lower than that of water, which can reduce the shrinkage and inner stress in drying. Compared with other organic solvents and water, EG provides a modest condition for gel synthesis, though it has seldom been applied in gelcasting. This work aims to prepare highly porous SiC ceramics with large size by EG-based gelcasting. The gelation behaviors of AM/MBAM in EG were explored. The reactivity of monomer and cross-linker in EG was researched. The microstructures and mechanical properties of the EG-based gels were compared with those in other solvents. Base on the research, the gelcasting technique was optimized to promote the drying process and the gel strength. The porosity, pore diameter and compressive strength of the sintered SiC ceramics were also tested.
Table 1 Idle time of AM/MBAM in different solvents. Temperature
TBA
Water
EG
25 °C 60 °C
– 20min 12s
3min 15s 1min 42s
3min 37s 1min 48s
3. Results and discussion Water and TBA are two most widely used solvents in gelcasting [11,19]. To better understand the gelation behavior of AM/MBAM in EG, water-based and TBA-based gels were synthesized as contrast samples. AM and MBAM with weight ratio of 30:1 were applied to prepare pure gels (without ceramic powders). Table 1 lists the idle time of AM/MBAM in different solvents with the same content of initiator and catalyst. The polymerization of AM/MBAM in TBA could not be initiated at room temperature [16] and its idle time at 60 °C was 20min 12s. It was still longer than 15 min while the initiator and catalyst contents were excessively increased, indicating the low polymerization rate of AM/MBAM in TBA. On the other hand, AM/MBAM performed high reactivity in water, with idle time as short as 3min 15s at 25 °C. The polymerization rate highly depends on the reactivity of initiator (APS). The high reactivity of initiator in water may attribute to the strong dissociation and ionization capability of water, which is extremely low for most organic solvents. However, the idle time of AM/ MBAM in EG was similar to that in water and even got closer when the temperature was raised from 25 °C to 60 °C. The dipole moment of EG and TBA is 7.34 × 10-30 C m and 5.54 × 10-30 C m, respectively. The high polarity of EG may account for the high reactivity of initiator in EG, although the precise mechanism need to be further studied. By decreasing the catalyst content, the idle time for EG was adjusted to 5–10 min, which was convenient for casting and prevented the sedimentation of slurries. Fig. 1 shows the macroscopic morphologies of the wet gels with 0.1 g/mL AM/MBAM in different solvents. The water-based and EGbased gels were transparent, revealing that their gel network was complete and homogeneous [22]. However, the TBA-based gels were opaque, indicating that the gelation behavior in TBA was different from that in water and in EG. The wet gels were pressed by a glass rod. The water-based gels showed high stiffness and only a little deformation could be observed while being pressed. The EG-based gels were elastic and flexible, which could be folded easily without generating any cracks (Fig. 1c). After pressed, clear indentation and cracks were observed on the TBA-based gels, indicating their low strength and elasticity. The physical state and mechanical properties of the gels are closely
2. Experimental α-SiC (98% grade, 2.29 μm, Kaihua SiC Co., Ltd., China) was used as the starting material. Al2O3 (grade A16SG, Alcoa, China) and Y2O3 (99.99% grade, Rare Metallic Co., Ltd., Japan) with molar ratio of 5:3 were added as sintering additive. Acrylamide (AM, AR, Standard Science and Technology Co., Ltd., China) and N,N′-methylenebisacrylamide (MBAM, AR, Guangfu Fine Chemical, China) were used as monomer and crosslinker, respectively. Ammonium persulfate (APS, AR, Tianli Chemical Reagents Co., Ltd., China) and N,N,N′,N'-tetramethylethylenediamine (TEMED, AR, Guangfu Fine Chemical, China) were used as initiator and catalyst, respectively. Deionized water, ethylene glycol (AR, Hengxing Chemical Preparation Co., Ltd., China), tert-butyl alcohol (TBA, AR, Fuchen Chemical, China) and ethanol (AR, Hengxing Co., Ltd., China) were used as solvents for gelcasting. Polyvinyl pyrrolidone (PVP, MW 58000, AR, Guangfu Fine Chemical, China) was used to disperse the SiC slurries. α-SiC powders with 5 wt% sintering additive were ball milled in ethanol for 24 h. The slurry was dried and sieved through a 200 mesh sieve. Then, the slurries including the premixed ceramic powders, PVP, AM and MBAM were ball milled in different solvents for 12 h. After degassed, the obtained slurries were mixed with initiator and catalyst and subsequently casted into cylindrical molds. The water-based and EG-based slurries were solidified at room temperature while the TBAbased ones were solidified at 60 °C. After demolding and drying, the polymer in green bodies was removed at 600 °C for 2 h, with heating rate of 0.5 °C/min. Green bodies were then sintered at 1850 °C for 2 h in Ar atmosphere, with heating rate of 10 °C/min. Idle time was recorded from the addition of initiator to the commencement of solidification. After the addition of initiator and stirring for 1 min, a glass rod was dipped in solvents once a second. The time when the solvents suddenly turned to solid was regarded as the commencement of solidification. The microstructures of specimens were observed by scanning electron microscopy (HELIOS NanoLab 600i, FEI Co., US). The molecular weight of polyacrylamide (PAM) was conducted by gel permeation chromatograms (Waters 2690, Waters, US) with tetrahydrofuran as the eluent and polystyrene as the standard. The porosity was measured by Archimedes’ method. The pore size distribution of ceramics was investigated on a mercury porosimetry (Autopore 9500, Micrometics, US). The compressive strengths of green bodies and ceramics were tested by electronic universal testing machine (Instron5569, Instron, US) on Φ 8 mm × 12 mm cylindrical specimens with a cross-head speed of 0.5 mm/min.
Fig. 1. Macroscopic morphologies of the wet gels in (a) water, (b), (c) EG and (d) TBA. 2
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insoluble in most organic solvents, including TBA. Thus, as the crosslinking degree of the gels increased to a certain value, the gels precipitated from the solvents and the chain propagation terminated (precipitation polymerization). Although the absence of α-hydrogen in TBA could reduce the chain transfer reaction and slightly promote the chain propagation [18], the precipitation polymerization in TBA played the dominant role, leading to low cross-linking degree and weak gel strength. In precipitation polymerization, the propagation time (from chain initiation to chain termination) for each chain was very short, so the MBAM content showed little effect on the strength of the TBA-based gels (Fig. 2). The insolubility of the AM/MBAM gels in TBA may also be responsible for the opacity of the TBA-based wet gels (Fig. 1d). Particularly, the AM/MBAM gels could be dissolved in EG. Despite an organic solvent, EG could induce the solution polymerization of AM/ MBAM and promote the gel strength. Fig. 3 shows the microstructures of the dried gels synthesized in different solvents. Regardless of the solvent, all the gels exhibited uniform microstructure when the AM/MBAM weight ratio was 30:1. Comparing Fig. 3a and 3d, the water-based gels became disordered when the AM/MBAM weight ratio was decreased from 30:1 to 5:1. Large pores and aggregation of gel walls can be observed in Fig. 3d. These results confirmed that excessive MBAM in water would lead to the inhomogeneity of gel network. Due to the low reactivity of MBAM in EG, no obvious change could be observed in the microstructures of the EG-based gels when the AM/MBAM ratio was decreased from 30:1 to 5:1 (Fig. 3b and 3e). Comparing Fig. 3a with Fig. 3b, the gel wall thickness of the EG-based gels was a little lower than that of the waterbased gels, which was attributed to the relatively low reactivity of AM in EG as mentioned above. The gel wall thickness directly affected the gel strength, so the compressive strength of the EG-based gels was also slightly lower than that of the water-based gels (Fig. 2). Because of the precipitation polymerization of AM/MBAM in TBA, small gel particles, instead of continuous gel walls, were formed in the TBA-based gels, shown in Fig. 3c and 3f. The diameter of the TBA-based gel particles (about 4 μm) was much lower than the thickness of the EG-based gel walls (about 11 μm), which could explain the low strength of the TBAbased gels (Fig. 2). It could be concluded that the solution polymerization produced continuous gel walls with high strength while the precipitation polymerization generated small gel particles with low strength. Different from the gels prepared in other organic solvents, the EGbased gels performed a water-like gelation behavior with solution polymerization and high polymerization reactivity. The high strength of the EG-based gels and the low surface tension of EG make it a promising solvent for non-aqueous gelcasting. However, to realize its application, the EG-based gelcasting should be further developed. SiC slurries with 15 vol% solid loading were applied to prepare green bodies. The volatilization of EG took a long time due to its low saturated vapor pressure. In general, the drying rate of ceramic green bodies is higher than that of pure gels, because the formation of ceramic skeleton provides pore channels for the volatilization of solvents. However, the drying rate of the EG-based SiC green bodies was even lower than that of the pure gels. Moreover, bloating was emerged on the surface of the green bodies after drying, indicating that the volatilization of solvents was hindered by some mechanism. Fig. 4a shows the microstructure of the EG-based SiC green body. It can be seen that most pore channels were covered by gels, although the gel content was only 8 wt% and the theoretical porosity of the green body was 85%. In the drying process, the EG-based gels clogged the pore channels and prevented the volatilization of solvents, leading to the bloating in green bodies. Water-based SiC green bodies with 8 wt% gels were also prepared and their microstructures before and after debinding is shown in Fig. 4d and 4e, respectively. The water-based green body exhibited interconnected pore structure. After debinding, the SiC particles emerged and presented a different appearance. Comparing Fig. 4d with 4e, it can be seen that all the SiC particles were covered by gels before
related to their polymerization degree and cross-linking degree. AM (without the cross-linking of MBAM) with concentration of 0.1 g/mL was used to prepare PAM in different solvents. The weight average molecular weights of the PAM prepared in water, EG and TBA were 6.1 × 104, 5.3 × 104 and 8.6 × 103, respectively. The results demonstrated that the polymerization reactivity of AM in EG was slightly lower than that in water and was much higher than that in TBA, which was coincident with the results of idle time in Table 1. The effects of cross-linker content on the properties of water-based gels have been researched in other papers in detail [11,22]. Increasing the cross-linker content could increase the strength of gel network. For aqueous gelcasting, however, the polymerization reactivity of cross-linker was much higher than that of monomer. Thus, excessive cross-linker caused it to react with other cross-linker molecules rather than monomer chains, resulting in the local gathering of cross-linking points, in other words, the inhomogeneity of gel network. To avoid the self-polymerization of cross-linker, the content of cross-linker was always much lower than that of monomer. For the AM/MBAM system in water, the proper AM/MBAM weight ratio was about 30:1 to 20:1 [11]. Further increasing of MBAM caused the inhomogeneity of gels and turned them from transparent to opaque [22]. However, the EG-based gels kept transparent and flexible (same as Fig. 1b and1c) when the AM/MBAM ratio was decreased from 30:1 to 5:1, revealing that the reactivity of MBAM was sharply decreased in EG. Fig. 2 shows the compressive strengths of the dried gels with different AM/MBAM ratio. With the decreasing of AM/MBAM ratio, the compressive strength of the water-based gels first increased and then decreased while that of the EG-based gels increased slightly. The compressive strength of the TBA-based gels was much lower than the other two gels and was independent on the AM/MBAM ratio. The uniformity and cross-linking degree are two main factors affecting the gel strength. Owing to the high reactivity of MBAM in water, the uniformity of the water-based gels could only be preserved when the AM/ MBAM weight ratio was higher than 30. Further increasing MBAM content would facilitate the self-polymerization of MBAM molecules, disturbing the gel network and decreasing the gel strength. For the EGbased gels, the uniformity of gels could not be influenced by AM/ MBAM ratio because of the low reactivity of MBAM in EG. Thus, the increasing of MBAM increased the cross-linking degree and improved the gel strength. Also because of the low reactivity of MBAM in EG, the strengthening effect of MBAM content on the EG-based gels was not obvious. The AM/MBAM gels can be dissolved (swelled) in water, so the polymer chains propagated continually in solvent until all the AM and MBAM molecules were consumed (solution polymerization). The solution polymerization of aqueous gelcasting led to high cross-linking degree and strong gel strength. The solubility of the AM/MBAM gels in various organic solvents were also examined. The AM/MBAM gels were
Fig. 2. Compressive strengths of the dried gels. 3
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Fig. 3. Microstructures of the gels synthesized in (a), (d) water, (b), (e) EG and (c), (f) TBA. The AM/MBAM weight ratio of (a–c) were 30:1 while that of (d–f) were 5:1.
film-like gels clogged the pores and hindered the volatilization of solvents. Ethanol is an organic solvent with low surface tension (22.3 mN/m) and can be soluble with EG at any proportion. The mixture of EG and ethanol was used as solvent and the microstructures of the as-prepared green bodies are shown in Fig. 4b and 4c. Comparing Fig. 4a, b and 4c, it can be seen that the pore quantity increased with the increasing of ethanol and most of the clogging gels were cleared when the ethanol content reached 50 vol%. The precipitation polymerization in ethanol partially destroyed the gel network in EG, which played as crack sources while the gels were pulled by the adsorption force. Thus, the gels were easier to be tore and their deformation mode was turned from “EG-type” to “water-type” (Fig. 5b to a). Organic solvents, such as TBA and N,N′-dimethylformamide (DMF), can play the same role as ethanol because they are also soluble with EG and able to induce precipitation polymerization. However, the viscosity of ethanol is relatively low and it has been proved to be an excellent solvent for SiC slurries [24], so the mixed solvents of EG and ethanol were adopted in the following research. Fig. 6 shows the compressive strengths of the SiC green biodies prepared in different solvents. When the ethanol content was lower
debinding. Besides, the particle size in Fig. 4d and 4e was similar, revealing that the gels were closely absorbed on the SiC particles and the gel layer was very thin. Comparing Fig. 4a with 4d, it can be seen that the gels performed different deformation modes in EG and in water during drying. In the gelation process, some AM monomers was absorbed on SiC particles through the hydrogen bond between the carbonyl group of AM and the silanol group on SiC surface [23]. These AM monomers polymerized with other AM and MBAM molecules to form gel network, connecting the SiC particles. In the drying process, the surface tension of solvents caused the shrinkage of gels. The adsorption between gels and SiC particles could be considered as an external force against shrinkage. Thus, the deformation of gels was caused by the co-action of the drying shrinkage and the adsorption force. Fig. 5 shows the schematic illustration of the deformation of the gels in different green bodies. Due to their high stiffness, the water-based gels were torn directly by the adsorption force. Then the gel layer became thinner and thinner until all the solvent was volatilized. On the other hand, the EGbased gels released the stress through deformation because of their high flexibility. Besides, owing to the low surface tension of EG, the deformation of the EG-based gels was relatively small. In consequence, the
Fig. 4. Microstructures of the SiC green bodies prepared in (a) EG, (b) 75 vol% EG + 25 vol% ethanol, (c) 50 vol% EG + 50 vol% ethanol and (d), (e) water. Only the gels in (e) were burnout. 4
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Fig. 5. Schematic illustration of the deformation of gels in (a) water-based and (b) EG-based green bodies. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Compressive strengths of the SiC green bodies.
than 30 vol% (EG content > 70 vol%), the obtained green bodies were difficult to be tested due to their large bloating. The compressive strengths of the green bodies prepared in TBA and ethanol were extremely low (< 2 MPa) because of the precipitation polymerization of the gels. For the green bodies prepared in the mixture of EG and ethanol, the compressive strength slightly increased when the ethanol content increased from 30 vol% to 50 vol%, which could be attributed to the elimination of bloating in the green bodies. Further increasing of ethanol severely destroyed the gel network, so the strength of the green bodies decreased sharply when the ethanol content increased from 80 vol% to 100 vol%. For ease of handling, an ethanol content of 50–70 vol% was adopted to promise the strength of green bodies. The SiC green bodies mentioned in Fig. 6 were about 80 mm in diameter and no defect could be observed on these samples (Fig. 7a and 7c). To examine the reliability, cylindrical samples with 180 mm in diameter were prepared and their macroscopic morphologies are shown in Fig. 7b and 7d. The linear drying shrinkage of the TBA-based and the EG-based samples were 0.4% and 0.9%, respectively. Despite the relatively low shrinkage, the TBA-based green body showed small cracks on the edge (Fig. 7b). In the drying process, the shrinkage of green bodies took place from periphery to interior. The asynchronous shrinkage caused inner stress which increased with the size of green bodies. Although the TBA-based sample performed extremely low drying shrinkage (0.4%), its weak strength (1.41 MPa) could not endure the increasing inner stress while the sample diameter increased from 80 mm to 180 mm. Thus, inner stress in outer circles firstly reached the
Fig. 7. Macroscopic morphologies of the SiC green bodies prepared in (a), (b) TBA and (c), (d) 50 vol% EG + 50 vol% ethanol. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
ultimate strength of the TBA-based green body and generated cracks. On the other hand, the strength of the EG-based green body (9.72 MPa) was high enough to resist the drying stress. In addition, due to their high flexibility, the EG-based gels could also release the inner stress in the green bodies by deformation. Fig. 8a shows the microstructure of the porous SiC ceramic prepared from the slurry with 15 vol% solid loading. It presented a typical microstructure of non-aqueous gelcasting, with uniformly distributed ceramic particles and interconnected pore structure [16–19]. Its pore diameter distribution is shown in Fig. 8b. The narrow-single-peak pore distribution indicated the uniform microstructure of the porous ceramic, which could be attributed to the fine gelation and drying processes. Table 2 lists the properties of the porous SiC ceramics with different solid loadings. The porosity of the sintered SiC ceramics could be tailored from 58.2% to 74.1% by changing the solid loading of the slurries. Since the porosity and pore size control in non-aqueous gelcasting has been researched extensively [16–18], it will not be discussed in this work. The compressive strength of the sintered SiC ceramic was 20.3 MPa even the porosity was as high as 74.1%. The
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Fig. 8. (a) Microstructure and (b) pore diameter distribution of the porous SiC ceramic.
References
Table 2 Properties of the porous SiC ceramics. Solid loading, vol%
Porosity, %
Average pore diameter, μm
Compressive strength, MPa
15 25 35
74.1 66.8 58.2
3.48 3.31 3.08
20.3 ± 0.7 38.5 ± 1.0 56.8 ± 1.2
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uniform microstructure and small pore size may account for its high strength. 4. Conclusion The gelation behaviors of the AM/MBAM gels in various solvents were researched. Different from the gels prepared in other organic solvents, the EG-based gels performed a water-like gelation behavior with high polymerization reactivity and solution polymerization. The high polymerization reactivity led to short idle time and the solution polymerization endowed the green bodies with high strength. The low reactivity of cross-linker in EG resulted in the high flexibility of the EGbased gels. In the drying process, the flexible gels were deformed under the co-action of the drying shrinkage and the adsorption between gels and SiC particles. In consequence, the pore channels of the EG-based green bodies were clogged and the volatilization of solvents were hindered. The addition of ethanol partially destroyed the gel network and cleared the clogging in the pore channels. The asynchronous drying caused inner stress in the green bodies, which increased with sample size. The precipitation polymerization in most organic solvents led to the low strength of the green bodies. The as-prepared green bodies could not endure the inner stress while the sample diameter was larger than 180 mm. On the other hand, owing to the solution polymerization in EG, the strength of the EG-based green bodies was high enough to resist the increasing stress. Highly porous SiC ceramics with large size were successfully prepared by the EG-based gelcasting. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was financially supported by the National Natural Science Foundation of China with Grant No. 51772062. 6
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Development of ethylene glycol-based gelcasting for the preparation of highly porous SiC ceramics Yicheng Jina, Biao Zhanga,b,∗∗, Feng Yea,∗, Haoqian Zhanga, Zhaoxin Zhonga, Qiang Liua, Zhiguo Zhangb a b
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China Department of Physics, Harbin Institute of Technology, Harbin, 150001, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Gelcasting Ethylene glycol Porous ceramics SiC
Aqueous gelcasting is inappropriate for the preparation of highly porous ceramics, due to the large drying shrinkage of green bodies caused by the high surface tension of water. To solve this problem, non-aqueous gelcasting using organic solvents with much lower surface tension was developed. However, for most organic solvents, the precipitation polymerization of gels led to the low strength of green bodies, which was inconvenient for the fabrication of large size workpieces. In this work, a novel ethylene glycol-based gelcasting was developed to prepare highly porous SiC ceramics. Ethylene glycol induced the solution polymerization of gels and increased the strength of green bodies effectively. In addition, the high flexibility of the ethylene glycol-based gels could release the inner stress in the drying process. Highly porous SiC ceramics with large size were successfully prepared by the optimized gelcasting method.
1. Introduction Porous ceramics have been widely applied in many fields such as filters, catalyst supports and reinforcements of composites [1–3]. In particular, porous SiC has been considered as one of the most promising materials for high temperature filtration and catalysis owing to its great chemical and thermal stability [4,5]. High open porosity is beneficial to the permeability of filters and supports [6]. Several techniques have been developed to prepare highly porous ceramics such as replication, foaming, freeze casting and gelcasting [7–10]. Among these techniques, gelcasting is a net-shape method and can be used to fabricate complexshaped green bodies [11]. Aqueous gelcasting can prepare green bodies with high mechanical properties, which can be machined easily before debinding [12]. Because of the high surface tension of water (72.1 mN/m), the drying of solvents caused large shrinkage, limiting the porosity of green bodies [13]. Thus, other pore-forming methods, such as foaming [14] and sacrificial template [10], were combined with aqueous gelcasing to increase the porosity. The obtained ceramics presented multi-modal pore distribution. Sub-micro pores were caused by the sintering of ceramic particles while large pores (> 15 μm) were generated by poreforming agents. High fluid permeability and adequate mechanical
∗
properties are desirable for filters and supports. However, the sub-micro pores limited the permeability and the large pores decreased the mechanical properties of ceramics. Moreover, the pore-forming agents could not be removed completely and closed pores are unavoidable in green bodies [15]. Non-aqueous gelcasting has been developed to prepare highly porous ceramics with single-mode pore distribution [16]. Due to the low surface tension of organic solvents, little shrinkage was occurred after drying, which preserved the highly porous skeleton of green bodies [16–19]. In addition, the porosity of ceramics could be tailored freely (about 60% to 90%) by changing the solid loading of slurries. The uniform pore structure and controllable porosity make it an ideal candidate for filters and supports. Idle time is defined as the time between the addition of initiator and the commencement of polymerization, during which the evaporation of solvents and the sedimentation of ceramic powders occur [20]. Thus, high polymerization rate with short idle time are favorable for the uniformity of green bodies [11]. Unfortunately, due to the low polymerization rate in most organic solvents, the idle time of non-aqueous gelcasting might be too long. Moreover, the low reaction rate could induce defects in gels and decrease the mechanical properties of green bodies [21]. The compressive strength of the highly porous green bodies was lower than 2 MPa [16].
Corresponding author. Corresponding author. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China. E-mail addresses: [email protected] (B. Zhang), [email protected] (F. Ye).
∗∗
https://doi.org/10.1016/j.ceramint.2019.12.009 Received 2 November 2019; Received in revised form 29 November 2019; Accepted 2 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Yicheng Jin, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.009
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Though small samples could be handled and processed, it might be unreliable to fabricate large size green bodies with high porosity. The strength of green bodies is mainly supported by gels. However, few research has been focused on the gelation behavior of non-aqueous gelcasting. In our tentative experiments, the gelation behaviors of AM/MBAM in water and various organic solvents were explored. From these experiments, we got a crucial information, that is the gelation behaviors of AM/MBAM in ethylene glycol (EG) were similar to that in water, which performed short idle time and high gel strength. In addition, the surface tension of EG (46.5 mN/m) is much lower than that of water, which can reduce the shrinkage and inner stress in drying. Compared with other organic solvents and water, EG provides a modest condition for gel synthesis, though it has seldom been applied in gelcasting. This work aims to prepare highly porous SiC ceramics with large size by EG-based gelcasting. The gelation behaviors of AM/MBAM in EG were explored. The reactivity of monomer and cross-linker in EG was researched. The microstructures and mechanical properties of the EG-based gels were compared with those in other solvents. Base on the research, the gelcasting technique was optimized to promote the drying process and the gel strength. The porosity, pore diameter and compressive strength of the sintered SiC ceramics were also tested.
Table 1 Idle time of AM/MBAM in different solvents. Temperature
TBA
Water
EG
25 °C 60 °C
– 20min 12s
3min 15s 1min 42s
3min 37s 1min 48s
3. Results and discussion Water and TBA are two most widely used solvents in gelcasting [11,19]. To better understand the gelation behavior of AM/MBAM in EG, water-based and TBA-based gels were synthesized as contrast samples. AM and MBAM with weight ratio of 30:1 were applied to prepare pure gels (without ceramic powders). Table 1 lists the idle time of AM/MBAM in different solvents with the same content of initiator and catalyst. The polymerization of AM/MBAM in TBA could not be initiated at room temperature [16] and its idle time at 60 °C was 20min 12s. It was still longer than 15 min while the initiator and catalyst contents were excessively increased, indicating the low polymerization rate of AM/MBAM in TBA. On the other hand, AM/MBAM performed high reactivity in water, with idle time as short as 3min 15s at 25 °C. The polymerization rate highly depends on the reactivity of initiator (APS). The high reactivity of initiator in water may attribute to the strong dissociation and ionization capability of water, which is extremely low for most organic solvents. However, the idle time of AM/ MBAM in EG was similar to that in water and even got closer when the temperature was raised from 25 °C to 60 °C. The dipole moment of EG and TBA is 7.34 × 10-30 C m and 5.54 × 10-30 C m, respectively. The high polarity of EG may account for the high reactivity of initiator in EG, although the precise mechanism need to be further studied. By decreasing the catalyst content, the idle time for EG was adjusted to 5–10 min, which was convenient for casting and prevented the sedimentation of slurries. Fig. 1 shows the macroscopic morphologies of the wet gels with 0.1 g/mL AM/MBAM in different solvents. The water-based and EGbased gels were transparent, revealing that their gel network was complete and homogeneous [22]. However, the TBA-based gels were opaque, indicating that the gelation behavior in TBA was different from that in water and in EG. The wet gels were pressed by a glass rod. The water-based gels showed high stiffness and only a little deformation could be observed while being pressed. The EG-based gels were elastic and flexible, which could be folded easily without generating any cracks (Fig. 1c). After pressed, clear indentation and cracks were observed on the TBA-based gels, indicating their low strength and elasticity. The physical state and mechanical properties of the gels are closely
2. Experimental α-SiC (98% grade, 2.29 μm, Kaihua SiC Co., Ltd., China) was used as the starting material. Al2O3 (grade A16SG, Alcoa, China) and Y2O3 (99.99% grade, Rare Metallic Co., Ltd., Japan) with molar ratio of 5:3 were added as sintering additive. Acrylamide (AM, AR, Standard Science and Technology Co., Ltd., China) and N,N′-methylenebisacrylamide (MBAM, AR, Guangfu Fine Chemical, China) were used as monomer and crosslinker, respectively. Ammonium persulfate (APS, AR, Tianli Chemical Reagents Co., Ltd., China) and N,N,N′,N'-tetramethylethylenediamine (TEMED, AR, Guangfu Fine Chemical, China) were used as initiator and catalyst, respectively. Deionized water, ethylene glycol (AR, Hengxing Chemical Preparation Co., Ltd., China), tert-butyl alcohol (TBA, AR, Fuchen Chemical, China) and ethanol (AR, Hengxing Co., Ltd., China) were used as solvents for gelcasting. Polyvinyl pyrrolidone (PVP, MW 58000, AR, Guangfu Fine Chemical, China) was used to disperse the SiC slurries. α-SiC powders with 5 wt% sintering additive were ball milled in ethanol for 24 h. The slurry was dried and sieved through a 200 mesh sieve. Then, the slurries including the premixed ceramic powders, PVP, AM and MBAM were ball milled in different solvents for 12 h. After degassed, the obtained slurries were mixed with initiator and catalyst and subsequently casted into cylindrical molds. The water-based and EG-based slurries were solidified at room temperature while the TBAbased ones were solidified at 60 °C. After demolding and drying, the polymer in green bodies was removed at 600 °C for 2 h, with heating rate of 0.5 °C/min. Green bodies were then sintered at 1850 °C for 2 h in Ar atmosphere, with heating rate of 10 °C/min. Idle time was recorded from the addition of initiator to the commencement of solidification. After the addition of initiator and stirring for 1 min, a glass rod was dipped in solvents once a second. The time when the solvents suddenly turned to solid was regarded as the commencement of solidification. The microstructures of specimens were observed by scanning electron microscopy (HELIOS NanoLab 600i, FEI Co., US). The molecular weight of polyacrylamide (PAM) was conducted by gel permeation chromatograms (Waters 2690, Waters, US) with tetrahydrofuran as the eluent and polystyrene as the standard. The porosity was measured by Archimedes’ method. The pore size distribution of ceramics was investigated on a mercury porosimetry (Autopore 9500, Micrometics, US). The compressive strengths of green bodies and ceramics were tested by electronic universal testing machine (Instron5569, Instron, US) on Φ 8 mm × 12 mm cylindrical specimens with a cross-head speed of 0.5 mm/min.
Fig. 1. Macroscopic morphologies of the wet gels in (a) water, (b), (c) EG and (d) TBA. 2
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insoluble in most organic solvents, including TBA. Thus, as the crosslinking degree of the gels increased to a certain value, the gels precipitated from the solvents and the chain propagation terminated (precipitation polymerization). Although the absence of α-hydrogen in TBA could reduce the chain transfer reaction and slightly promote the chain propagation [18], the precipitation polymerization in TBA played the dominant role, leading to low cross-linking degree and weak gel strength. In precipitation polymerization, the propagation time (from chain initiation to chain termination) for each chain was very short, so the MBAM content showed little effect on the strength of the TBA-based gels (Fig. 2). The insolubility of the AM/MBAM gels in TBA may also be responsible for the opacity of the TBA-based wet gels (Fig. 1d). Particularly, the AM/MBAM gels could be dissolved in EG. Despite an organic solvent, EG could induce the solution polymerization of AM/ MBAM and promote the gel strength. Fig. 3 shows the microstructures of the dried gels synthesized in different solvents. Regardless of the solvent, all the gels exhibited uniform microstructure when the AM/MBAM weight ratio was 30:1. Comparing Fig. 3a and 3d, the water-based gels became disordered when the AM/MBAM weight ratio was decreased from 30:1 to 5:1. Large pores and aggregation of gel walls can be observed in Fig. 3d. These results confirmed that excessive MBAM in water would lead to the inhomogeneity of gel network. Due to the low reactivity of MBAM in EG, no obvious change could be observed in the microstructures of the EG-based gels when the AM/MBAM ratio was decreased from 30:1 to 5:1 (Fig. 3b and 3e). Comparing Fig. 3a with Fig. 3b, the gel wall thickness of the EG-based gels was a little lower than that of the waterbased gels, which was attributed to the relatively low reactivity of AM in EG as mentioned above. The gel wall thickness directly affected the gel strength, so the compressive strength of the EG-based gels was also slightly lower than that of the water-based gels (Fig. 2). Because of the precipitation polymerization of AM/MBAM in TBA, small gel particles, instead of continuous gel walls, were formed in the TBA-based gels, shown in Fig. 3c and 3f. The diameter of the TBA-based gel particles (about 4 μm) was much lower than the thickness of the EG-based gel walls (about 11 μm), which could explain the low strength of the TBAbased gels (Fig. 2). It could be concluded that the solution polymerization produced continuous gel walls with high strength while the precipitation polymerization generated small gel particles with low strength. Different from the gels prepared in other organic solvents, the EGbased gels performed a water-like gelation behavior with solution polymerization and high polymerization reactivity. The high strength of the EG-based gels and the low surface tension of EG make it a promising solvent for non-aqueous gelcasting. However, to realize its application, the EG-based gelcasting should be further developed. SiC slurries with 15 vol% solid loading were applied to prepare green bodies. The volatilization of EG took a long time due to its low saturated vapor pressure. In general, the drying rate of ceramic green bodies is higher than that of pure gels, because the formation of ceramic skeleton provides pore channels for the volatilization of solvents. However, the drying rate of the EG-based SiC green bodies was even lower than that of the pure gels. Moreover, bloating was emerged on the surface of the green bodies after drying, indicating that the volatilization of solvents was hindered by some mechanism. Fig. 4a shows the microstructure of the EG-based SiC green body. It can be seen that most pore channels were covered by gels, although the gel content was only 8 wt% and the theoretical porosity of the green body was 85%. In the drying process, the EG-based gels clogged the pore channels and prevented the volatilization of solvents, leading to the bloating in green bodies. Water-based SiC green bodies with 8 wt% gels were also prepared and their microstructures before and after debinding is shown in Fig. 4d and 4e, respectively. The water-based green body exhibited interconnected pore structure. After debinding, the SiC particles emerged and presented a different appearance. Comparing Fig. 4d with 4e, it can be seen that all the SiC particles were covered by gels before
related to their polymerization degree and cross-linking degree. AM (without the cross-linking of MBAM) with concentration of 0.1 g/mL was used to prepare PAM in different solvents. The weight average molecular weights of the PAM prepared in water, EG and TBA were 6.1 × 104, 5.3 × 104 and 8.6 × 103, respectively. The results demonstrated that the polymerization reactivity of AM in EG was slightly lower than that in water and was much higher than that in TBA, which was coincident with the results of idle time in Table 1. The effects of cross-linker content on the properties of water-based gels have been researched in other papers in detail [11,22]. Increasing the cross-linker content could increase the strength of gel network. For aqueous gelcasting, however, the polymerization reactivity of cross-linker was much higher than that of monomer. Thus, excessive cross-linker caused it to react with other cross-linker molecules rather than monomer chains, resulting in the local gathering of cross-linking points, in other words, the inhomogeneity of gel network. To avoid the self-polymerization of cross-linker, the content of cross-linker was always much lower than that of monomer. For the AM/MBAM system in water, the proper AM/MBAM weight ratio was about 30:1 to 20:1 [11]. Further increasing of MBAM caused the inhomogeneity of gels and turned them from transparent to opaque [22]. However, the EG-based gels kept transparent and flexible (same as Fig. 1b and1c) when the AM/MBAM ratio was decreased from 30:1 to 5:1, revealing that the reactivity of MBAM was sharply decreased in EG. Fig. 2 shows the compressive strengths of the dried gels with different AM/MBAM ratio. With the decreasing of AM/MBAM ratio, the compressive strength of the water-based gels first increased and then decreased while that of the EG-based gels increased slightly. The compressive strength of the TBA-based gels was much lower than the other two gels and was independent on the AM/MBAM ratio. The uniformity and cross-linking degree are two main factors affecting the gel strength. Owing to the high reactivity of MBAM in water, the uniformity of the water-based gels could only be preserved when the AM/ MBAM weight ratio was higher than 30. Further increasing MBAM content would facilitate the self-polymerization of MBAM molecules, disturbing the gel network and decreasing the gel strength. For the EGbased gels, the uniformity of gels could not be influenced by AM/ MBAM ratio because of the low reactivity of MBAM in EG. Thus, the increasing of MBAM increased the cross-linking degree and improved the gel strength. Also because of the low reactivity of MBAM in EG, the strengthening effect of MBAM content on the EG-based gels was not obvious. The AM/MBAM gels can be dissolved (swelled) in water, so the polymer chains propagated continually in solvent until all the AM and MBAM molecules were consumed (solution polymerization). The solution polymerization of aqueous gelcasting led to high cross-linking degree and strong gel strength. The solubility of the AM/MBAM gels in various organic solvents were also examined. The AM/MBAM gels were
Fig. 2. Compressive strengths of the dried gels. 3
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Fig. 3. Microstructures of the gels synthesized in (a), (d) water, (b), (e) EG and (c), (f) TBA. The AM/MBAM weight ratio of (a–c) were 30:1 while that of (d–f) were 5:1.
film-like gels clogged the pores and hindered the volatilization of solvents. Ethanol is an organic solvent with low surface tension (22.3 mN/m) and can be soluble with EG at any proportion. The mixture of EG and ethanol was used as solvent and the microstructures of the as-prepared green bodies are shown in Fig. 4b and 4c. Comparing Fig. 4a, b and 4c, it can be seen that the pore quantity increased with the increasing of ethanol and most of the clogging gels were cleared when the ethanol content reached 50 vol%. The precipitation polymerization in ethanol partially destroyed the gel network in EG, which played as crack sources while the gels were pulled by the adsorption force. Thus, the gels were easier to be tore and their deformation mode was turned from “EG-type” to “water-type” (Fig. 5b to a). Organic solvents, such as TBA and N,N′-dimethylformamide (DMF), can play the same role as ethanol because they are also soluble with EG and able to induce precipitation polymerization. However, the viscosity of ethanol is relatively low and it has been proved to be an excellent solvent for SiC slurries [24], so the mixed solvents of EG and ethanol were adopted in the following research. Fig. 6 shows the compressive strengths of the SiC green biodies prepared in different solvents. When the ethanol content was lower
debinding. Besides, the particle size in Fig. 4d and 4e was similar, revealing that the gels were closely absorbed on the SiC particles and the gel layer was very thin. Comparing Fig. 4a with 4d, it can be seen that the gels performed different deformation modes in EG and in water during drying. In the gelation process, some AM monomers was absorbed on SiC particles through the hydrogen bond between the carbonyl group of AM and the silanol group on SiC surface [23]. These AM monomers polymerized with other AM and MBAM molecules to form gel network, connecting the SiC particles. In the drying process, the surface tension of solvents caused the shrinkage of gels. The adsorption between gels and SiC particles could be considered as an external force against shrinkage. Thus, the deformation of gels was caused by the co-action of the drying shrinkage and the adsorption force. Fig. 5 shows the schematic illustration of the deformation of the gels in different green bodies. Due to their high stiffness, the water-based gels were torn directly by the adsorption force. Then the gel layer became thinner and thinner until all the solvent was volatilized. On the other hand, the EGbased gels released the stress through deformation because of their high flexibility. Besides, owing to the low surface tension of EG, the deformation of the EG-based gels was relatively small. In consequence, the
Fig. 4. Microstructures of the SiC green bodies prepared in (a) EG, (b) 75 vol% EG + 25 vol% ethanol, (c) 50 vol% EG + 50 vol% ethanol and (d), (e) water. Only the gels in (e) were burnout. 4
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Fig. 5. Schematic illustration of the deformation of gels in (a) water-based and (b) EG-based green bodies. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Compressive strengths of the SiC green bodies.
than 30 vol% (EG content > 70 vol%), the obtained green bodies were difficult to be tested due to their large bloating. The compressive strengths of the green bodies prepared in TBA and ethanol were extremely low (< 2 MPa) because of the precipitation polymerization of the gels. For the green bodies prepared in the mixture of EG and ethanol, the compressive strength slightly increased when the ethanol content increased from 30 vol% to 50 vol%, which could be attributed to the elimination of bloating in the green bodies. Further increasing of ethanol severely destroyed the gel network, so the strength of the green bodies decreased sharply when the ethanol content increased from 80 vol% to 100 vol%. For ease of handling, an ethanol content of 50–70 vol% was adopted to promise the strength of green bodies. The SiC green bodies mentioned in Fig. 6 were about 80 mm in diameter and no defect could be observed on these samples (Fig. 7a and 7c). To examine the reliability, cylindrical samples with 180 mm in diameter were prepared and their macroscopic morphologies are shown in Fig. 7b and 7d. The linear drying shrinkage of the TBA-based and the EG-based samples were 0.4% and 0.9%, respectively. Despite the relatively low shrinkage, the TBA-based green body showed small cracks on the edge (Fig. 7b). In the drying process, the shrinkage of green bodies took place from periphery to interior. The asynchronous shrinkage caused inner stress which increased with the size of green bodies. Although the TBA-based sample performed extremely low drying shrinkage (0.4%), its weak strength (1.41 MPa) could not endure the increasing inner stress while the sample diameter increased from 80 mm to 180 mm. Thus, inner stress in outer circles firstly reached the
Fig. 7. Macroscopic morphologies of the SiC green bodies prepared in (a), (b) TBA and (c), (d) 50 vol% EG + 50 vol% ethanol. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
ultimate strength of the TBA-based green body and generated cracks. On the other hand, the strength of the EG-based green body (9.72 MPa) was high enough to resist the drying stress. In addition, due to their high flexibility, the EG-based gels could also release the inner stress in the green bodies by deformation. Fig. 8a shows the microstructure of the porous SiC ceramic prepared from the slurry with 15 vol% solid loading. It presented a typical microstructure of non-aqueous gelcasting, with uniformly distributed ceramic particles and interconnected pore structure [16–19]. Its pore diameter distribution is shown in Fig. 8b. The narrow-single-peak pore distribution indicated the uniform microstructure of the porous ceramic, which could be attributed to the fine gelation and drying processes. Table 2 lists the properties of the porous SiC ceramics with different solid loadings. The porosity of the sintered SiC ceramics could be tailored from 58.2% to 74.1% by changing the solid loading of the slurries. Since the porosity and pore size control in non-aqueous gelcasting has been researched extensively [16–18], it will not be discussed in this work. The compressive strength of the sintered SiC ceramic was 20.3 MPa even the porosity was as high as 74.1%. The
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Fig. 8. (a) Microstructure and (b) pore diameter distribution of the porous SiC ceramic.
References
Table 2 Properties of the porous SiC ceramics. Solid loading, vol%
Porosity, %
Average pore diameter, μm
Compressive strength, MPa
15 25 35
74.1 66.8 58.2
3.48 3.31 3.08
20.3 ± 0.7 38.5 ± 1.0 56.8 ± 1.2
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uniform microstructure and small pore size may account for its high strength. 4. Conclusion The gelation behaviors of the AM/MBAM gels in various solvents were researched. Different from the gels prepared in other organic solvents, the EG-based gels performed a water-like gelation behavior with high polymerization reactivity and solution polymerization. The high polymerization reactivity led to short idle time and the solution polymerization endowed the green bodies with high strength. The low reactivity of cross-linker in EG resulted in the high flexibility of the EGbased gels. In the drying process, the flexible gels were deformed under the co-action of the drying shrinkage and the adsorption between gels and SiC particles. In consequence, the pore channels of the EG-based green bodies were clogged and the volatilization of solvents were hindered. The addition of ethanol partially destroyed the gel network and cleared the clogging in the pore channels. The asynchronous drying caused inner stress in the green bodies, which increased with sample size. The precipitation polymerization in most organic solvents led to the low strength of the green bodies. The as-prepared green bodies could not endure the inner stress while the sample diameter was larger than 180 mm. On the other hand, owing to the solution polymerization in EG, the strength of the EG-based green bodies was high enough to resist the increasing stress. Highly porous SiC ceramics with large size were successfully prepared by the EG-based gelcasting. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was financially supported by the National Natural Science Foundation of China with Grant No. 51772062. 6
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