- Email: [email protected]
Control of pore structure during freeze casting of porous SiC ceramics by different freezing modes
Ceramics International 45 (2019) 11558–11563
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Control of pore structure during freeze casting of porous SiC ceramics by different freezing modes
T
Ning Wanga, Yongsheng Liub, Ying Zhanga,∗, Yi Dua, Junzhan Zhanga a b
College of Materials Science and Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi, 710055, China State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, Shaanxi, 710072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Freeze casting Silicon carbide Freezing modes Finite element analysis
The frozen moulds, including homogeneous, unidirectional and bidirectional freezing, were designed using materials with different thermal conductivities, and the temperature variations of the moulds and samples during the freezing process were simulated by finite element analysis. Highly porous SiC ceramics with significant differences in pore structure were fabricated by using the SiC/water slurries prepared via uniform or oriented freeze casting with various freezing modes, and porosity and compressive strength of the as-fabricated ceramics were investigated. The results showed that the pore structure of ceramics prepared by homogeneous freezing was relatively intricate and inconsistent, and had a higher compressive strength. In contrast, the pore structure of ceramics fabricated using bidirectional freezing mode was more ordered and higher porosity was observed. Moreover, porous ceramics prepared by unidirectional freezing mode exhibited a typical gradient structure with increased pore size from tens of micrometers in the bottom to hundreds of micrometers in the top.
1. Introduction The unique performance of porous SiC ceramics with high porosity, permeability, specific surface area, corrosion resistance, oxidation resistance, thermal conductivity and excellent mechanical properties, makes it to be the key candidate in the fields of separation, heat exchange, catalyst carrier, and it also has great prospects for filtering applications [1–5]. Various fabrication methods of porous SiC ceramics have been performed in the past years, including addition of pore forming agent, sacrificial template, direct foaming, gel-casting and freeze casting [6–10]. Notably, freeze casting, or ice templating technique, which is using a simple physical sublimation principle for the preparation of porous ceramics, is particularly attractive as an alternative to other methods due to its low cost, precise and adjustable pore structure, and environmental friendliness without any pollution [11–15]. So far, much work has focused on the factors affecting the preparation of porous ceramics by freeze casting, such as liquid vehicle, solid contents, additives, sintering temperature [16–20]. For example, Wang et al. [21] produced highly porous SiC ceramic with macropores and micropores by adding PVA as binder and using water-based slurries as pore morphology controller. The open porosity, flexural strength and linear shrinkage were controlled by adjusting the PVA addition and
sintering temperature. Singh et al. [22] adopted a camphene-based freeze casting method to fabricate scaffold with micro sized particles, in which the pore size, porosity and compressive strength were controlled through varying freezing condition (constant freezing temperature and constant freezing rate) and solid loading. Hu et al. [23] fabricated porous YSZ ceramics with unidirectionally aligned channels by the freeze casting method. The pore channel size of YSZ ceramics were controlled by adjusting the freezing temperature in the freezing process. Obviously, the liquid vehicle which is frozen and solidified under its solidification point acts as a porous template in freeze casting, thus the freezing temperature and freezing rate play a vital role in nucleation and growth of the solidified crystals. Furthermore, freezing modes contribute significantly to microstructures and performances of porous ceramics. However, single freezing mode, mostly unidirectional freezing, is only used in the current study and little attention has been devoted to the effects of different freezing modes due to the mould limitations or other reasons. Herein, it is of great significance to systematically study the effects of freezing modes on the structure and properties of porous ceramics. In the current work, a processing route, using different freezing modes during freeze casting process to control the pore size in porous SiC ceramics with high porosity and sufficient compressive strength, is reported. In order to achieve freeze casting with different freezing
∗ Corresponding author. College of Materials Science and Engineering, Xi'an University of Architecture and Technology, Yanta Road 13, Beilin District, Xi'an, 710055, Shaanxi, China. E-mail address: [email protected] (Y. Zhang).
https://doi.org/10.1016/j.ceramint.2019.03.025 Received 26 December 2018; Received in revised form 28 February 2019; Accepted 5 March 2019 Available online 08 March 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 11558–11563
N. Wang, et al.
modes, moulds are designed utilizing materials with different thermal conductivity, which achieved different temperature field distribution. However, the cooling rate of each position inside the samples cannot be regulated accurately during the experiment, so its crystallization behavior at low temperatures is also hard to forecast. To illustrate the effectiveness of the moluds design and simulate the cooling curves of different positions inside the sample, finite element analysis is used. Subsequently, the effects of the freezing modes on the microstructures and performances of porous SiC ceramics are further investigated. The pore structure of porous ceramics prepared by freeze casting can be designed and improved according to the research. 2. Experimental procedure 2.1. Sample preparation The starting powder used in the experiment was commercially available SiC powder (99.42 wt% purity, d50 = 44 μm, Kaihua Silicon Carbide Powders Co., Ltd., Shandong, China). Poly(vinyl alcohol) (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and sodium carboxymethyl cellulose (CMC, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) were used as the organic binder and dispersant agent, in which 5 wt% and 0.2% in relation to the total amount of powders were added, respectively. Al2O3 powder (99.55 wt% purity, d50 = 7 μm, Xinyuan Ceramic Material Co., Ltd., Henan, China) and Guangxi white clay were used as the sintering additives in the sintering process. The chemical composition of Guangxi white clay was shown in Table 1. The designed mould can be divided into three parts: top cover, mould sleeve and bottom cover. The three parts of the molud with different materials were shown in Table 2. SiC suspensions with initial solid mass fraction of 50 wt% were prepared by mixing SiC powder with dispersant agent and sintering additives in the deionized water. The suspensions were ball milled for 2 h, and vacuum defoaming was performed subsequently because bubbles were generated in the ball milling process. Then, the suspensions were poured into the above moulds and placed inside a lyophilizer (LGJ-10C, Sihuan Scientific Instrument Factory Co., Ltd., Beijing, China) with the temperature of −65 °C for 2 h. The moluds are erected to prevent direct contact with the bottom of the lyophilizer which ensures a consistent environment around the molud. The frozen samples after demoulding were put into the lyophilizer and dried at −65 °C and 1 Pa for more than 24 h to remove the ice crystals. Before sintering, the green bodies were heated up to 600 °C for 1 h to remove the organic additives and subsequently sintered at 1400 °C for 2 h in air. The obtained porous ceramics were evenly divided into two parts of top and bottom and hereafter referred to as HT (homogeneous top), HB (homogeneous bottom), UT (Unidirectional top), UB (Unidirectional bottom), BT (bidirectional top) and BB (bidirectional bottom), respectively. 2.2. Characterization The temperature field distributions of different moulds and samples were simulated by finite element analysis through ANSYS. Before solving the problem, a model with the same size as the actual mould was established, and different material properties, such as thermal conductivity, density and specific heat capacity, were defined. It is assumed that the initial temperature of the mould and slurry was 25 °C and the ambient temperature was - 65 °C achieved by the lyophilizer. After the Table 1 Chemical composition of Guangxi white clay. Chemical composition
SiO2
Al2O3
Fe2O3
K2O + Na2O
Fraction/wt%
49.9
30.7
1.2
0.9
Table 2 Mould materials for different freezing modes. Freezing modes
Top cover
Mould sleeve
Bottom cover
Homogeneous freezing Unidirectional freezing Bidirectional freezing
Nylon Nylon Aluminum
Nylon Nylon Nylon
Nylon Aluminum Aluminum
solution, the temperature distribution of the model and the curve of a node temperature with time were displayed. The XRD pattern were acquired by using X-ray diffraction (XRD, RIGAKU D/max 2000 PC, Japan, Cu Ka, λ = 0.15406 nm). Scanning electron microscopy (SEM, JEOL JSM-6460, Japan) was applied to investigate the morphology of as-prepared porous SiC ceramics. Pore size distributions were characterized by the mercury porosimetry (AutoPore Iv 9510, Micromeritics, USA). The open porosity was measured by the Archimedes method. Specimens were machined into Φ30 mm × 20 mm to test the compressive strength via universal testing machine (WDW-5, Changchun Kexin tester Co., Ltd. Jilin, China). 3. Results and discussion 3.1. Finite element analysis of the moulds and samples Fig. 1 shows the designed moulds and temperature distributions, which are directly showed by the finite element analysis, of different moulds and samples during the freezing process of 1 s and 2000 s. As showed in Fig. 1(d)–(i), the temperature distributions in these freezing modes are significantly diverse. Owing to the homogeneous freezing molud constituted using the same material, the surrounding temperature distribution is uniform and evenly changed over time. The bidirectional freezing molud has larger thermal conductivity due to the aluminum at both top and bottom where the temperature decreases rapidly, while the thermal conductivity of the molud sleeve is so small that the temperature drops slowly. Similarly, the bottom of the unidirectional freezing molud cools faster, and the top and mould sleeve cools slower regardless of freezing times (1 s or 2000 s). 3.2. Phase analysis of porous ceramics Fig. 2 only displays the X-ray diffraction (XRD) pattern of porous SiC ceramics prepared by unidirectional freezing sintered at 1400 °C because the freezing modes have little effect on the phase compositions of porous ceramics with the same sintering temperature. According to the standard XRD pattern of these substances (JCPDS card no. 82-0512 and no. 88-0826), besides the strong diffraction peaks at 35.6° originating from the 6HeSiC, the diffraction peaks observed at 21.7° and 43.3° can be assigned to cristobalite and α-Al2O3 which are the remnants of the raw materials, respectively. It is noted that the XRD pattern showed in Fig. 2 indicates the presence of mullite after sintering at 1400 °C. It can be deduced that Al2O3 powder (sintering additives) and Guangxi white clay in the raw materials interreact at high temperatures to form mullite, which can significantly reduce the sintering temperature of the porous SiC ceramics owing to its low sintering temperature. 3.3. Microstructure of porous ceramics The morphologies of the as-obtained porous SiC ceramics are obtained by SEM and the corresponding results are showed in Fig. 3(a)–(f). It can be seen that the pore structure of porous ceramics varies obviously with the change of freezing modes. The lamellar channels of HT and HB exhibit an intricate and inconsistent distribution. Moreover, there are some intersecting points of the pore channels. Different from homogeneous freezing, it can be distinctly observed from unidirectional freezing that pore sizes of the orientated channels and
11559
Ceramics International 45 (2019) 11558–11563
N. Wang, et al.
Fig. 1. The designed moulds and the temperature distributions of different moulds with different freezing times (a) (d) (g) homogeneous freezing; (b) (e) (h) unidirectional freezing; (c) (f) (i) bidirectional freezing (d) (e) (f) 1 s; (g) (h) (i) 2000 s.
Fig. 2. XRD pattern of porous SiC ceramics.
the thickness of the wall are gradually increased from tens of micrometers in UB to hundreds of micrometers in UT while the number of channels is reduced. As to bidirectional freezing, the pore channels of samples are better ordered and connected. Meanwhile, there is little difference in the pore sizes and the thickness of the wall between BT and BB. The formation mechanisms of different pore structures are further illustrated combining the schematic illustration in Fig. 3(g)–(i). It is generally accepted that in addition to the change in temperature ΔT (or
supercooling) what really matters to the nucleation of ice is the cooling rates. The difference in physical properties of the mould itself allows the samples to freeze with different freezing rates despite the same ambient temperature. Specifically, there is the same cooling rate around the slurry under homogeneous freezing process, which produces little difference in the supercooling. The diversification of the crystallization direction of the crystals leads to the diversification of the pore direction of the porous ceramics. In unidirectional freezing, the nucleation rate increases in UB, resulting in more severe supercooling due to its faster cooling rate. On the contrary, the crystal growth rate is larger in UT with slower cooling rate and lower supercooling, and channels with larger size are inclined to form after sublimation of crystals. The difference of supercooling between two ends will cause the bottom crystals to grow along the temperature gradient to the top and eventually form a typical gradient pore structure. Pore structure obtained by bidirectional freezing could account for that there is little difference in the supercooling at both ends, and the crystals grow from both ends to the middle with lower supercooling. As the height of the sample is limited, the pore sizes at the top and bottom have little difference. Besides the top and the bottom, SEM images of the middle of porous ceramics prepared by different freezing modes are showed in Fig. 4. It can be seen that pore size and wall thickness of the samples prepared by bidirectional freezing are smaller while that of the samples prepared by the homogeneous freezing is larger. In order to clarify the possible reasons for the varieties of pore size as well as wall thickness, the cooling curve at the middle of the samples were emulated by finite
11560
Ceramics International 45 (2019) 11558–11563
N. Wang, et al.
Fig. 3. SEM images and schematic illustration of the microstructure evolutions of porous SiC ceramics prepared with different freezing modes (a) (d) (g) homogeneous freezing; (b) (e) (h) unidirectional freezing; (c) (f) (i) bidirectional freezing (a) (b) (c) top; (d) (e) (f) bottom.
element analysis under different freezing modes as showed in Fig. 4. The average cooling rates of homogeneous freezing, unidirectional freezing and bidirectional freezing, which can be calculated through cooling curves when the temperature reduced from 25 °C to 0 °C, are 0.25 °C/min, 0.35 °C/min, 0.47 °C/min, respectively. The corresponding pore sizes decrease in turn, and the effect of cooling rates on the pore structure is consistent with that described above. It is worth noting that the position of middle part has the highest temperature in the process of homogeneous freezing and bidirectional freezing, while in unidirectional freezing, the position with the highest temperature during the freezing process will appropriately move up due to the asymmetry of freezing rates at top and bottom. The fact can be seen from the figure that the time, which temperatures in the middle of samples prepared by homogeneous freezing, unidirectional freezing and bidirectional freezing drop below the freezing points, is 99 min, 72 min and 53 min, respectively. In other words, bidirectional freezing has the shortest overall freezing time, followed by unidirectional freezing and homogeneous freezing. 3.4. Porosity and compressive strength of porous ceramics The variation of porosity and compressive strength of the top and bottom in as-fabricated porous ceramics under different freezing modes is showed in Fig. 5. It can be seen that the porosity of porous ceramics prepared by various freezing modes are higher than 70%. Contrary to unidirectional freezing, the porosity of the top and bottom of the samples under other two freezing modes is very close. The slight decrease in porosity is probably due to little sedimentation in the slurry during freezing process. The porosity of UT is as high as 76.5%, while
that of UB is only 74.4%. The prepared samples have enough compressive strength to meet the requirements for conveying and basic use. The variation tendency of compressive strength is opposite to the porosity and it can be explained by RICE formula [24] which is commonly known. According to the SEM images, the same supercooling on BT and BB results in more ordered lamellar structure with better pore connectivity and closer porosity. Although the homogenous freezing forms a relatively messy pore structure, however, the supercooling and the cooling rate of HT and HB are so consistent that porosity is also very close. Compared with the other two kinds of freezing modes, a typical gradient type pore structure is formed under unidirectional freezing because of the difference in supercooling, resulting in porosity difference. The pore distribution of unidirectional freezing is further studied. The results of mercury intrusion porosimetry test, which is used to recorded the large changes of pore size distribution in UT and UB, are showed in Fig. 6. It can be seen that both cases exhibit a bimodal pore size distribution. Among them, the pore distribution peak near 20 μm in UB (26 μm in UT) shows a narrow half width, and more than 95% of the pore diameter is distributed within this range. In addition, a peak with a wider half width, which can be called a steamed bun peak resulted from both burn out of binders and grain packing, appears near 0.35 μm. By comparison, it can be found that the positions of the steamed bun peak of UB and UT are almost the same, but the pore size distribution of lamellar pore structure formed by the sublimation of ice crystals of UB is more concentrated around 20 μm, which is smaller than 26 μm of the distribution of UT.
11561
Ceramics International 45 (2019) 11558–11563
N. Wang, et al.
Fig. 5. Porosity and compressive strength of porous SiC ceramics prepared with different freezing modes.
Fig. 4. SEM images and cooling curves of the middle of porous ceramics prepared by different freezing modes (a) homogeneous freezing, (b) unidirectional freezing, (c) bidirectional freezing.
4. Conclusions In summary, mould design using materials with different thermal conductivity offers possibility for controlling the pore structure of porous SiC ceramics which are prepared by freeze casting under different freezing modes. Finite element analysis is used to predict the effectiveness of the moluds design. It is found that the main phase in the porous ceramics was 6HeSiC and the bonding phases were mullite and
Fig. 6. Pore size distribution and cumulative pore volume of porous SiC ceramics with different freezing modes (a) UT, (b) UB.
cristobalite. The porosity of porous ceramics prepared by various freezing modes are higher than 70%. The pore structure of samples prepared by homogeneous freezing is relatively intricate and inconsistent and has higher compressive strength. In contrast, the pore structure of as-prepared ceramics using bidirectional freezing is more ordered and possess higher porosity. Moreover, porous ceramics prepared by unidirectional freezing mode exhibited a typical gradient structure with increased pore size from tens of micrometers in the
11562
Ceramics International 45 (2019) 11558–11563
N. Wang, et al.
bottom to hundreds of micrometers in the top. Acknowledgement This work was supported by the fund of the State Key Laboratory of Solidification Processing in NWPU (Grant No: SKLSP201854) and the fund of Key Laboratory for Equipment Pre-research (Grant No: 614291104011317). References [1] F.D. Xue, K.C. Zhou, N. Wu, H. Luo, X.F. Wang, X.F. Zhou, Z.N. Yan, I. Abrahams, D. Zhang, Porous SiC ceramics with dendritic pore structures by freeze casting from chemical cross-linked polycarbosilane, Ceram. Int. 44 (2018) 6293–6299. [2] C. Ferraro, E. Garcia-Tunon, S. Barg, M. Miranda, N. Ni, R. Bell, E. Saiz, SiC porous structures obtained with innovative shaping technologies, J. Eur. Ceram. Soc. 38 (2018) 823–835. [3] S. Dong, X.H. Zhang, D.Y. Zhang, B.Q. Sun, L.W. Yan, X.G. Luo, Strong effect of atmosphere on the microstructure and microwave absorption properties of porous SiC ceramics, J. Eur. Ceram. Soc. 38 (2018) 29–39. [4] P. Barick, B.P. Saha, S.V. Joshi, R. Mitra, Spray-freeze-dried nanosized silicon carbide containing granules: properties, compaction behaviour and sintering, J. Eur. Ceram. Soc. 36 (2016) 3863–3877. [5] K.H. Zuo, Y.P. Zeng, D.L. Jiang, Mechanical properties of solid-sintered porous silicon carbide ceramics, Adv. Eng. Mater. 15 (2013) 491–495. [6] J.J. Liu, W.L. Huo, B. Ren, K. Gan, Y.J. Lu, X.Y. Zhang, X.Y. Tang, J.L. Yang, A novel approach to fabricate porous alumina ceramics with excellent properties via poreforming agent combined with sol impregnation technique, Ceram. Int. 44 (2018) 16751–16757. [7] N. Hedayat, Y.H. Du, H. Ilkhani, Review on fabrication techniques for porous electrodes of solid oxide fuel cells by sacrificial template methods, Renew. Sustain. Energy Rev. 77 (2017) 1221–1239. [8] S. Bhaskar, J.G. Park, K.S. Lee, S.Y. Kim, I.J. Kim, Thermal and mechanical behavior of ZrTiO4-TiO2 porous ceramics by direct foaming, Ceram. Int. 42 (2016) 14395–14402. [9] X. Dong, M.C. Wang, A.R. Guo, Y.X. Zhang, S. Ren, G.F. Sui, H.Y. Du, Synthesis and properties of porous alumina ceramics with inter-locked plate-like structure through the tert-butyl alcohol-based gel-casting method, J. Alloy. Comp. 694 (2017) 1045–1053.
[10] H. Park, H.H. Cho, K. Kim, K. Hong, J.H. Kim, H. Choe, D.C. Dunand, Surfaceoxidized, freeze-cast cobalt foams: microstructure, mechanical properties and electrochemical performance, Acta Mater. 142 (2018) 213–225. [11] K.L. Scotti, D.C. Dunand, Freeze casting-A review of processing, microstructure and properties via the open data repository, Freeze Casting. Net, Prog. Mater. Sci. 94 (2018) 243–305. [12] S. Deville, The lure of ice-templating: recent trends and opportunities for porous materials, Scripta Mater. 147 (2018) 119–124. [13] R.P. Liu, T.T. Xu, C.A. Wang, A review of fabrication strategies and applications of porous ceramics prepared by freeze-casting method, Ceram. Int. 42 (2016) 2907–2925. [14] T. Wu, W.Q. Zhang, B. Yu, J. Chen, A novel electrolyte-electrode interface structure with directional micro-channel fabricated by freeze casting: a mini review, Int. J. Hydrog. Energy 42 (2017) 29900–29910. [15] Y.H. Du, N. Hedayat, D. Panthi, H. Ilkhani, B.J. Emley, T. Woodson, Freeze-casting for the fabrication of solid oxide fuel cells: a review, Materialia 1 (2018) 198–210. [16] Y. Liu, W.Y. Zhu, K. Guan, C. Peng, J.Q. Wu, Freeze-casting of alumina ultra-filtration membranes with good performance for anionic dye separation, Ceram. Int. 44 (2018) 11901–11904. [17] N. Soltani, U. Simon, A. Bahrami, X.F. Wang, S. Selve, J.D. Epping, M.I. Pech-Canul, M.F. Bekheet, A. Gurlo, Macroporous polymer-derived SiO2/SiOC monoliths freezecast from polysiloxane and amorphous silica derived from rice husk, J. Eur. Ceram. Soc. 37 (2017) 4809–4820. [18] L. Li, Q.G. Li, J. Hong, M.Y. Sun, J. Zhang, S.M. Dong, Effect of Si3N4 solid contents on mechanical and dielectric properties of porous Si3N4 ceramics through freezedrying, J. Alloy. Comp. 732 (2018) 136–140. [19] W.L. Li, K. Lu, J.Y. Walz, Freeze casting of porous materials: review of critical factors in microstructure evolution, Int. Mater. Rev. 57 (2012) 37–60. [20] S. Deville, Freeze-casting of porous biomaterials: structure, properties and opportunities, Materials 3 (2010) 1913–1927. [21] F. Wang, D.X. Yao, Y.F. Xia, K.H. Zuo, J.Q. Xu, Y.P. Zeng, Porous SiC ceramics prepared via freeze-casting and solid state sintering, Ceram. Int. 42 (2016) 4526–4531. [22] G. Singh, S. Soundarapandian, Effect of freezing conditions on β-Tricalcium Phosphate/Camphene scaffold with micro sized particles fabricated by freeze casting, J. Mech. Behav. Biomed. Mater. 79 (2018) 189–194. [23] L.F. Hu, C.A. Wang, Y. Huang, C.C. Sun, S. Lu, Z.J. Hu, Control of pore channel size during freeze casting of porous YSZ ceramics with unidirectionally aligned channels using different freezing temperatures, J. Eur. Ceram. Soc. 30 (2010) 3389–3396. [24] R.W. Rice, Comparison of stress concentration versus minimum solid area based mechanical property-porosity relations, J. Mater. Sci. 28 (1993) 2187–2190.
11563
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Control of pore structure during freeze casting of porous SiC ceramics by different freezing modes
T
Ning Wanga, Yongsheng Liub, Ying Zhanga,∗, Yi Dua, Junzhan Zhanga a b
College of Materials Science and Engineering, Xi'an University of Architecture and Technology, Xi'an, Shaanxi, 710055, China State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, Shaanxi, 710072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Freeze casting Silicon carbide Freezing modes Finite element analysis
The frozen moulds, including homogeneous, unidirectional and bidirectional freezing, were designed using materials with different thermal conductivities, and the temperature variations of the moulds and samples during the freezing process were simulated by finite element analysis. Highly porous SiC ceramics with significant differences in pore structure were fabricated by using the SiC/water slurries prepared via uniform or oriented freeze casting with various freezing modes, and porosity and compressive strength of the as-fabricated ceramics were investigated. The results showed that the pore structure of ceramics prepared by homogeneous freezing was relatively intricate and inconsistent, and had a higher compressive strength. In contrast, the pore structure of ceramics fabricated using bidirectional freezing mode was more ordered and higher porosity was observed. Moreover, porous ceramics prepared by unidirectional freezing mode exhibited a typical gradient structure with increased pore size from tens of micrometers in the bottom to hundreds of micrometers in the top.
1. Introduction The unique performance of porous SiC ceramics with high porosity, permeability, specific surface area, corrosion resistance, oxidation resistance, thermal conductivity and excellent mechanical properties, makes it to be the key candidate in the fields of separation, heat exchange, catalyst carrier, and it also has great prospects for filtering applications [1–5]. Various fabrication methods of porous SiC ceramics have been performed in the past years, including addition of pore forming agent, sacrificial template, direct foaming, gel-casting and freeze casting [6–10]. Notably, freeze casting, or ice templating technique, which is using a simple physical sublimation principle for the preparation of porous ceramics, is particularly attractive as an alternative to other methods due to its low cost, precise and adjustable pore structure, and environmental friendliness without any pollution [11–15]. So far, much work has focused on the factors affecting the preparation of porous ceramics by freeze casting, such as liquid vehicle, solid contents, additives, sintering temperature [16–20]. For example, Wang et al. [21] produced highly porous SiC ceramic with macropores and micropores by adding PVA as binder and using water-based slurries as pore morphology controller. The open porosity, flexural strength and linear shrinkage were controlled by adjusting the PVA addition and
sintering temperature. Singh et al. [22] adopted a camphene-based freeze casting method to fabricate scaffold with micro sized particles, in which the pore size, porosity and compressive strength were controlled through varying freezing condition (constant freezing temperature and constant freezing rate) and solid loading. Hu et al. [23] fabricated porous YSZ ceramics with unidirectionally aligned channels by the freeze casting method. The pore channel size of YSZ ceramics were controlled by adjusting the freezing temperature in the freezing process. Obviously, the liquid vehicle which is frozen and solidified under its solidification point acts as a porous template in freeze casting, thus the freezing temperature and freezing rate play a vital role in nucleation and growth of the solidified crystals. Furthermore, freezing modes contribute significantly to microstructures and performances of porous ceramics. However, single freezing mode, mostly unidirectional freezing, is only used in the current study and little attention has been devoted to the effects of different freezing modes due to the mould limitations or other reasons. Herein, it is of great significance to systematically study the effects of freezing modes on the structure and properties of porous ceramics. In the current work, a processing route, using different freezing modes during freeze casting process to control the pore size in porous SiC ceramics with high porosity and sufficient compressive strength, is reported. In order to achieve freeze casting with different freezing
∗ Corresponding author. College of Materials Science and Engineering, Xi'an University of Architecture and Technology, Yanta Road 13, Beilin District, Xi'an, 710055, Shaanxi, China. E-mail address: [email protected] (Y. Zhang).
https://doi.org/10.1016/j.ceramint.2019.03.025 Received 26 December 2018; Received in revised form 28 February 2019; Accepted 5 March 2019 Available online 08 March 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 11558–11563
N. Wang, et al.
modes, moulds are designed utilizing materials with different thermal conductivity, which achieved different temperature field distribution. However, the cooling rate of each position inside the samples cannot be regulated accurately during the experiment, so its crystallization behavior at low temperatures is also hard to forecast. To illustrate the effectiveness of the moluds design and simulate the cooling curves of different positions inside the sample, finite element analysis is used. Subsequently, the effects of the freezing modes on the microstructures and performances of porous SiC ceramics are further investigated. The pore structure of porous ceramics prepared by freeze casting can be designed and improved according to the research. 2. Experimental procedure 2.1. Sample preparation The starting powder used in the experiment was commercially available SiC powder (99.42 wt% purity, d50 = 44 μm, Kaihua Silicon Carbide Powders Co., Ltd., Shandong, China). Poly(vinyl alcohol) (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and sodium carboxymethyl cellulose (CMC, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) were used as the organic binder and dispersant agent, in which 5 wt% and 0.2% in relation to the total amount of powders were added, respectively. Al2O3 powder (99.55 wt% purity, d50 = 7 μm, Xinyuan Ceramic Material Co., Ltd., Henan, China) and Guangxi white clay were used as the sintering additives in the sintering process. The chemical composition of Guangxi white clay was shown in Table 1. The designed mould can be divided into three parts: top cover, mould sleeve and bottom cover. The three parts of the molud with different materials were shown in Table 2. SiC suspensions with initial solid mass fraction of 50 wt% were prepared by mixing SiC powder with dispersant agent and sintering additives in the deionized water. The suspensions were ball milled for 2 h, and vacuum defoaming was performed subsequently because bubbles were generated in the ball milling process. Then, the suspensions were poured into the above moulds and placed inside a lyophilizer (LGJ-10C, Sihuan Scientific Instrument Factory Co., Ltd., Beijing, China) with the temperature of −65 °C for 2 h. The moluds are erected to prevent direct contact with the bottom of the lyophilizer which ensures a consistent environment around the molud. The frozen samples after demoulding were put into the lyophilizer and dried at −65 °C and 1 Pa for more than 24 h to remove the ice crystals. Before sintering, the green bodies were heated up to 600 °C for 1 h to remove the organic additives and subsequently sintered at 1400 °C for 2 h in air. The obtained porous ceramics were evenly divided into two parts of top and bottom and hereafter referred to as HT (homogeneous top), HB (homogeneous bottom), UT (Unidirectional top), UB (Unidirectional bottom), BT (bidirectional top) and BB (bidirectional bottom), respectively. 2.2. Characterization The temperature field distributions of different moulds and samples were simulated by finite element analysis through ANSYS. Before solving the problem, a model with the same size as the actual mould was established, and different material properties, such as thermal conductivity, density and specific heat capacity, were defined. It is assumed that the initial temperature of the mould and slurry was 25 °C and the ambient temperature was - 65 °C achieved by the lyophilizer. After the Table 1 Chemical composition of Guangxi white clay. Chemical composition
SiO2
Al2O3
Fe2O3
K2O + Na2O
Fraction/wt%
49.9
30.7
1.2
0.9
Table 2 Mould materials for different freezing modes. Freezing modes
Top cover
Mould sleeve
Bottom cover
Homogeneous freezing Unidirectional freezing Bidirectional freezing
Nylon Nylon Aluminum
Nylon Nylon Nylon
Nylon Aluminum Aluminum
solution, the temperature distribution of the model and the curve of a node temperature with time were displayed. The XRD pattern were acquired by using X-ray diffraction (XRD, RIGAKU D/max 2000 PC, Japan, Cu Ka, λ = 0.15406 nm). Scanning electron microscopy (SEM, JEOL JSM-6460, Japan) was applied to investigate the morphology of as-prepared porous SiC ceramics. Pore size distributions were characterized by the mercury porosimetry (AutoPore Iv 9510, Micromeritics, USA). The open porosity was measured by the Archimedes method. Specimens were machined into Φ30 mm × 20 mm to test the compressive strength via universal testing machine (WDW-5, Changchun Kexin tester Co., Ltd. Jilin, China). 3. Results and discussion 3.1. Finite element analysis of the moulds and samples Fig. 1 shows the designed moulds and temperature distributions, which are directly showed by the finite element analysis, of different moulds and samples during the freezing process of 1 s and 2000 s. As showed in Fig. 1(d)–(i), the temperature distributions in these freezing modes are significantly diverse. Owing to the homogeneous freezing molud constituted using the same material, the surrounding temperature distribution is uniform and evenly changed over time. The bidirectional freezing molud has larger thermal conductivity due to the aluminum at both top and bottom where the temperature decreases rapidly, while the thermal conductivity of the molud sleeve is so small that the temperature drops slowly. Similarly, the bottom of the unidirectional freezing molud cools faster, and the top and mould sleeve cools slower regardless of freezing times (1 s or 2000 s). 3.2. Phase analysis of porous ceramics Fig. 2 only displays the X-ray diffraction (XRD) pattern of porous SiC ceramics prepared by unidirectional freezing sintered at 1400 °C because the freezing modes have little effect on the phase compositions of porous ceramics with the same sintering temperature. According to the standard XRD pattern of these substances (JCPDS card no. 82-0512 and no. 88-0826), besides the strong diffraction peaks at 35.6° originating from the 6HeSiC, the diffraction peaks observed at 21.7° and 43.3° can be assigned to cristobalite and α-Al2O3 which are the remnants of the raw materials, respectively. It is noted that the XRD pattern showed in Fig. 2 indicates the presence of mullite after sintering at 1400 °C. It can be deduced that Al2O3 powder (sintering additives) and Guangxi white clay in the raw materials interreact at high temperatures to form mullite, which can significantly reduce the sintering temperature of the porous SiC ceramics owing to its low sintering temperature. 3.3. Microstructure of porous ceramics The morphologies of the as-obtained porous SiC ceramics are obtained by SEM and the corresponding results are showed in Fig. 3(a)–(f). It can be seen that the pore structure of porous ceramics varies obviously with the change of freezing modes. The lamellar channels of HT and HB exhibit an intricate and inconsistent distribution. Moreover, there are some intersecting points of the pore channels. Different from homogeneous freezing, it can be distinctly observed from unidirectional freezing that pore sizes of the orientated channels and
11559
Ceramics International 45 (2019) 11558–11563
N. Wang, et al.
Fig. 1. The designed moulds and the temperature distributions of different moulds with different freezing times (a) (d) (g) homogeneous freezing; (b) (e) (h) unidirectional freezing; (c) (f) (i) bidirectional freezing (d) (e) (f) 1 s; (g) (h) (i) 2000 s.
Fig. 2. XRD pattern of porous SiC ceramics.
the thickness of the wall are gradually increased from tens of micrometers in UB to hundreds of micrometers in UT while the number of channels is reduced. As to bidirectional freezing, the pore channels of samples are better ordered and connected. Meanwhile, there is little difference in the pore sizes and the thickness of the wall between BT and BB. The formation mechanisms of different pore structures are further illustrated combining the schematic illustration in Fig. 3(g)–(i). It is generally accepted that in addition to the change in temperature ΔT (or
supercooling) what really matters to the nucleation of ice is the cooling rates. The difference in physical properties of the mould itself allows the samples to freeze with different freezing rates despite the same ambient temperature. Specifically, there is the same cooling rate around the slurry under homogeneous freezing process, which produces little difference in the supercooling. The diversification of the crystallization direction of the crystals leads to the diversification of the pore direction of the porous ceramics. In unidirectional freezing, the nucleation rate increases in UB, resulting in more severe supercooling due to its faster cooling rate. On the contrary, the crystal growth rate is larger in UT with slower cooling rate and lower supercooling, and channels with larger size are inclined to form after sublimation of crystals. The difference of supercooling between two ends will cause the bottom crystals to grow along the temperature gradient to the top and eventually form a typical gradient pore structure. Pore structure obtained by bidirectional freezing could account for that there is little difference in the supercooling at both ends, and the crystals grow from both ends to the middle with lower supercooling. As the height of the sample is limited, the pore sizes at the top and bottom have little difference. Besides the top and the bottom, SEM images of the middle of porous ceramics prepared by different freezing modes are showed in Fig. 4. It can be seen that pore size and wall thickness of the samples prepared by bidirectional freezing are smaller while that of the samples prepared by the homogeneous freezing is larger. In order to clarify the possible reasons for the varieties of pore size as well as wall thickness, the cooling curve at the middle of the samples were emulated by finite
11560
Ceramics International 45 (2019) 11558–11563
N. Wang, et al.
Fig. 3. SEM images and schematic illustration of the microstructure evolutions of porous SiC ceramics prepared with different freezing modes (a) (d) (g) homogeneous freezing; (b) (e) (h) unidirectional freezing; (c) (f) (i) bidirectional freezing (a) (b) (c) top; (d) (e) (f) bottom.
element analysis under different freezing modes as showed in Fig. 4. The average cooling rates of homogeneous freezing, unidirectional freezing and bidirectional freezing, which can be calculated through cooling curves when the temperature reduced from 25 °C to 0 °C, are 0.25 °C/min, 0.35 °C/min, 0.47 °C/min, respectively. The corresponding pore sizes decrease in turn, and the effect of cooling rates on the pore structure is consistent with that described above. It is worth noting that the position of middle part has the highest temperature in the process of homogeneous freezing and bidirectional freezing, while in unidirectional freezing, the position with the highest temperature during the freezing process will appropriately move up due to the asymmetry of freezing rates at top and bottom. The fact can be seen from the figure that the time, which temperatures in the middle of samples prepared by homogeneous freezing, unidirectional freezing and bidirectional freezing drop below the freezing points, is 99 min, 72 min and 53 min, respectively. In other words, bidirectional freezing has the shortest overall freezing time, followed by unidirectional freezing and homogeneous freezing. 3.4. Porosity and compressive strength of porous ceramics The variation of porosity and compressive strength of the top and bottom in as-fabricated porous ceramics under different freezing modes is showed in Fig. 5. It can be seen that the porosity of porous ceramics prepared by various freezing modes are higher than 70%. Contrary to unidirectional freezing, the porosity of the top and bottom of the samples under other two freezing modes is very close. The slight decrease in porosity is probably due to little sedimentation in the slurry during freezing process. The porosity of UT is as high as 76.5%, while
that of UB is only 74.4%. The prepared samples have enough compressive strength to meet the requirements for conveying and basic use. The variation tendency of compressive strength is opposite to the porosity and it can be explained by RICE formula [24] which is commonly known. According to the SEM images, the same supercooling on BT and BB results in more ordered lamellar structure with better pore connectivity and closer porosity. Although the homogenous freezing forms a relatively messy pore structure, however, the supercooling and the cooling rate of HT and HB are so consistent that porosity is also very close. Compared with the other two kinds of freezing modes, a typical gradient type pore structure is formed under unidirectional freezing because of the difference in supercooling, resulting in porosity difference. The pore distribution of unidirectional freezing is further studied. The results of mercury intrusion porosimetry test, which is used to recorded the large changes of pore size distribution in UT and UB, are showed in Fig. 6. It can be seen that both cases exhibit a bimodal pore size distribution. Among them, the pore distribution peak near 20 μm in UB (26 μm in UT) shows a narrow half width, and more than 95% of the pore diameter is distributed within this range. In addition, a peak with a wider half width, which can be called a steamed bun peak resulted from both burn out of binders and grain packing, appears near 0.35 μm. By comparison, it can be found that the positions of the steamed bun peak of UB and UT are almost the same, but the pore size distribution of lamellar pore structure formed by the sublimation of ice crystals of UB is more concentrated around 20 μm, which is smaller than 26 μm of the distribution of UT.
11561
Ceramics International 45 (2019) 11558–11563
N. Wang, et al.
Fig. 5. Porosity and compressive strength of porous SiC ceramics prepared with different freezing modes.
Fig. 4. SEM images and cooling curves of the middle of porous ceramics prepared by different freezing modes (a) homogeneous freezing, (b) unidirectional freezing, (c) bidirectional freezing.
4. Conclusions In summary, mould design using materials with different thermal conductivity offers possibility for controlling the pore structure of porous SiC ceramics which are prepared by freeze casting under different freezing modes. Finite element analysis is used to predict the effectiveness of the moluds design. It is found that the main phase in the porous ceramics was 6HeSiC and the bonding phases were mullite and
Fig. 6. Pore size distribution and cumulative pore volume of porous SiC ceramics with different freezing modes (a) UT, (b) UB.
cristobalite. The porosity of porous ceramics prepared by various freezing modes are higher than 70%. The pore structure of samples prepared by homogeneous freezing is relatively intricate and inconsistent and has higher compressive strength. In contrast, the pore structure of as-prepared ceramics using bidirectional freezing is more ordered and possess higher porosity. Moreover, porous ceramics prepared by unidirectional freezing mode exhibited a typical gradient structure with increased pore size from tens of micrometers in the
11562
Ceramics International 45 (2019) 11558–11563
N. Wang, et al.
bottom to hundreds of micrometers in the top. Acknowledgement This work was supported by the fund of the State Key Laboratory of Solidification Processing in NWPU (Grant No: SKLSP201854) and the fund of Key Laboratory for Equipment Pre-research (Grant No: 614291104011317). References [1] F.D. Xue, K.C. Zhou, N. Wu, H. Luo, X.F. Wang, X.F. Zhou, Z.N. Yan, I. Abrahams, D. Zhang, Porous SiC ceramics with dendritic pore structures by freeze casting from chemical cross-linked polycarbosilane, Ceram. Int. 44 (2018) 6293–6299. [2] C. Ferraro, E. Garcia-Tunon, S. Barg, M. Miranda, N. Ni, R. Bell, E. Saiz, SiC porous structures obtained with innovative shaping technologies, J. Eur. Ceram. Soc. 38 (2018) 823–835. [3] S. Dong, X.H. Zhang, D.Y. Zhang, B.Q. Sun, L.W. Yan, X.G. Luo, Strong effect of atmosphere on the microstructure and microwave absorption properties of porous SiC ceramics, J. Eur. Ceram. Soc. 38 (2018) 29–39. [4] P. Barick, B.P. Saha, S.V. Joshi, R. Mitra, Spray-freeze-dried nanosized silicon carbide containing granules: properties, compaction behaviour and sintering, J. Eur. Ceram. Soc. 36 (2016) 3863–3877. [5] K.H. Zuo, Y.P. Zeng, D.L. Jiang, Mechanical properties of solid-sintered porous silicon carbide ceramics, Adv. Eng. Mater. 15 (2013) 491–495. [6] J.J. Liu, W.L. Huo, B. Ren, K. Gan, Y.J. Lu, X.Y. Zhang, X.Y. Tang, J.L. Yang, A novel approach to fabricate porous alumina ceramics with excellent properties via poreforming agent combined with sol impregnation technique, Ceram. Int. 44 (2018) 16751–16757. [7] N. Hedayat, Y.H. Du, H. Ilkhani, Review on fabrication techniques for porous electrodes of solid oxide fuel cells by sacrificial template methods, Renew. Sustain. Energy Rev. 77 (2017) 1221–1239. [8] S. Bhaskar, J.G. Park, K.S. Lee, S.Y. Kim, I.J. Kim, Thermal and mechanical behavior of ZrTiO4-TiO2 porous ceramics by direct foaming, Ceram. Int. 42 (2016) 14395–14402. [9] X. Dong, M.C. Wang, A.R. Guo, Y.X. Zhang, S. Ren, G.F. Sui, H.Y. Du, Synthesis and properties of porous alumina ceramics with inter-locked plate-like structure through the tert-butyl alcohol-based gel-casting method, J. Alloy. Comp. 694 (2017) 1045–1053.
[10] H. Park, H.H. Cho, K. Kim, K. Hong, J.H. Kim, H. Choe, D.C. Dunand, Surfaceoxidized, freeze-cast cobalt foams: microstructure, mechanical properties and electrochemical performance, Acta Mater. 142 (2018) 213–225. [11] K.L. Scotti, D.C. Dunand, Freeze casting-A review of processing, microstructure and properties via the open data repository, Freeze Casting. Net, Prog. Mater. Sci. 94 (2018) 243–305. [12] S. Deville, The lure of ice-templating: recent trends and opportunities for porous materials, Scripta Mater. 147 (2018) 119–124. [13] R.P. Liu, T.T. Xu, C.A. Wang, A review of fabrication strategies and applications of porous ceramics prepared by freeze-casting method, Ceram. Int. 42 (2016) 2907–2925. [14] T. Wu, W.Q. Zhang, B. Yu, J. Chen, A novel electrolyte-electrode interface structure with directional micro-channel fabricated by freeze casting: a mini review, Int. J. Hydrog. Energy 42 (2017) 29900–29910. [15] Y.H. Du, N. Hedayat, D. Panthi, H. Ilkhani, B.J. Emley, T. Woodson, Freeze-casting for the fabrication of solid oxide fuel cells: a review, Materialia 1 (2018) 198–210. [16] Y. Liu, W.Y. Zhu, K. Guan, C. Peng, J.Q. Wu, Freeze-casting of alumina ultra-filtration membranes with good performance for anionic dye separation, Ceram. Int. 44 (2018) 11901–11904. [17] N. Soltani, U. Simon, A. Bahrami, X.F. Wang, S. Selve, J.D. Epping, M.I. Pech-Canul, M.F. Bekheet, A. Gurlo, Macroporous polymer-derived SiO2/SiOC monoliths freezecast from polysiloxane and amorphous silica derived from rice husk, J. Eur. Ceram. Soc. 37 (2017) 4809–4820. [18] L. Li, Q.G. Li, J. Hong, M.Y. Sun, J. Zhang, S.M. Dong, Effect of Si3N4 solid contents on mechanical and dielectric properties of porous Si3N4 ceramics through freezedrying, J. Alloy. Comp. 732 (2018) 136–140. [19] W.L. Li, K. Lu, J.Y. Walz, Freeze casting of porous materials: review of critical factors in microstructure evolution, Int. Mater. Rev. 57 (2012) 37–60. [20] S. Deville, Freeze-casting of porous biomaterials: structure, properties and opportunities, Materials 3 (2010) 1913–1927. [21] F. Wang, D.X. Yao, Y.F. Xia, K.H. Zuo, J.Q. Xu, Y.P. Zeng, Porous SiC ceramics prepared via freeze-casting and solid state sintering, Ceram. Int. 42 (2016) 4526–4531. [22] G. Singh, S. Soundarapandian, Effect of freezing conditions on β-Tricalcium Phosphate/Camphene scaffold with micro sized particles fabricated by freeze casting, J. Mech. Behav. Biomed. Mater. 79 (2018) 189–194. [23] L.F. Hu, C.A. Wang, Y. Huang, C.C. Sun, S. Lu, Z.J. Hu, Control of pore channel size during freeze casting of porous YSZ ceramics with unidirectionally aligned channels using different freezing temperatures, J. Eur. Ceram. Soc. 30 (2010) 3389–3396. [24] R.W. Rice, Comparison of stress concentration versus minimum solid area based mechanical property-porosity relations, J. Mater. Sci. 28 (1993) 2187–2190.
11563