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Influence of bonding phases on properties of in-situ bonded porous SiC membrane supports
Journal Pre-proof Influence of bonding phases on properties of in-situ bonded porous SiC membrane supports Zhi-Yong Luo, Wei Han, Kai-Qi Liu, Wen-Qing Ao, Kai-kai Si PII:
S0272-8842(19)33571-0
DOI:
https://doi.org/10.1016/j.ceramint.2019.12.082
Reference:
CERI 23735
To appear in:
Ceramics International
Received Date: 7 October 2019 Revised Date:
4 December 2019
Accepted Date: 6 December 2019
Please cite this article as: Z.-Y. Luo, W. Han, K.-Q. Liu, W.-Q. Ao, K.-k. Si, Influence of bonding phases on properties of in-situ bonded porous SiC membrane supports, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2019.12.082. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Influence of bonding phases on properties of in-situ bonded porous SiC membrane supports Zhi-Yong Luoa, Wei Hana,*, Kai-Qi Liub,c,* *, Wen-Qing Aoa, Kai-kai Sib,c a
Beijing Key Laboratory of Advanced Ceramic and Refractories, Central Iron & Steel Research Institute, Beijing 100081, China; b State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China c University of Chinese Academy of Sciences, Beijing, 100190, China
*Corresponding author: Prof. Wei Han, Tel.: +86-10-62182468; Fax.: +86-10-62184553. E-mail: [email protected]. ∗∗ Corresponding author: Prof. Kai-qi Liu, Tel.: +86-10-82545059; Fax.: +86-10-82545059. E-mail: [email protected]. Abstract: Porous SiC ceramic membrane supports are widely employed in a wide variety of hightemperature applications, such as hot flue gas filtration, porous burners and molten metal filters. Herein, SiC supports, with a porosity of ~37%, were prepared by using low-temperature bonding techniques and the influence of different bonding phases, such as mullite, cordierite and glass, on ambient-temperature flexural strength, hot modulus of rupture (HMOR), thermal shock resistance and oxidation resistance were systematically investigated. The results reveal that the glass-bonded SiC (GBSC) support exhibited the highest ambient-temperature flexural strength of 33.6 MPa, whereas the flexural strength of mullitebonded SiC (MBSC) and cordierite-bonded SiC (CBSC) supports ranged from 22 to 25 MPa. However, the presence of glass phase deteriorated the high-temperature properties of the support. MBSC support rendered superior mechanical strength at high temperature and self-strengthening in a certain temperature range, such as HMOR improved 47.5% at 900 oC, but HMOR of glass-bonded support was only 57.4% of the ambient-temperature strength. Moreover, MBSC and CBSC supports exhibited better thermal shock resistance than GBSC supports and the critical temperature difference of water quenching for MBSC supports was ~200 oC higher than GBSC supports. In addition, MBSC support rendered superior oxidation resistance and exhibited a weight gain rate of ~0.1% at 1150 oC for 24 h, which is 54.4% and 42.2% lower than CBSC and GBSC supports, respectively. Keywords: Porous SiC ceramics; membrane support; bonding phase; strength; thermal shock resistance; oxidation resistance
1. Introduction Porous SiC ceramics, with superior strength, wear resistance, corrosion resistance, thermal stability and thermal shock resistance, render promise in high-temperature applications, such as molten metal filters, diesel particulate filters, hot gas filters and porous burners [1-4]. However, a temperature of 2000 o
C or higher temperature and an inert or vacuum environment are required to sinter pure SiC ceramics
due to highly covalent characteristics of Si–C bonds [5,6], limiting the practical applications of porous SiC ceramics. Therefore, low-temperature fabrication techniques are being developed to sinter porous
SiC ceramics, where making use of bonding materials is an effective method to reduce the sintering temperature. For instance, the in-situ reaction bonding process can realize the low-temperature fabrication of porous SiC ceramics. Mullite [7], cordierite [8], silica [9], silicon nitride [10], silicon [11], silicon oxycarbide [12] and glass phase [13] have been investigated as bonding materials for the fabrication of porous SiC ceramics. In particular, mullite, cordierite and glass phase bonding systems are easy to obtain and considered viable for industrial production and applications. Mullite is an ideal bonding phase for porous SiC ceramic membrane support due to its excellent mechanical properties, thermal shock resistance, corrosion resistance, oxidation resistance, temperature stability and similar thermal expansion coefficient to SiC [14]. The mullite-bonded SiC support can be obtained by in-situ reaction of Al2O3 deriving from added aluminum-containing powder, such as Al, AlN, Al2O3, Al (OH)3, bauxite and kaolin, with oxidation-derived SiO2 at the surface of SiC [15]. The sintering temperature of mullite-bonded SiC support ranges from 1350 oC to 1550 oC [16-20]. Cordierite is chosen as a bonding phase to fabricate porous SiC ceramics due to its low thermal expansion coefficient [21] and low synthesis temperature [22]. The sintering mechanism of cordierite-bonded SiC is similar to mullite-bonded SiC support, which implies that cordierite phase is obtained by in-situ reaction of magnesium-containing and aluminum-containing powders with oxidation-derived SiO2 [8]. In general, MgO and talc powders are mainly employed as magnesium-containing raw materials, where Al2O3 and kaolin are utilized as aluminum-containing raw materials. One should note that the sintering temperature of porous cordierite-bonded SiC ceramics is ~1350 oC [8,23]. Glass-bonded SiC porous ceramics are fabricated from SiC and low melting point raw materials such as clay, potassium feldspar and glass powder, at a lower sintering temperature of ~1300 oC [13, 24, 25]. The sintering mechanism is as follows: the raw materials with low melting point are transformed into viscous substance during the sintering process, where SiC particles are immersed and wrapped. After cooling, the as-formed glass phase results in the strength of SiC particles. For instance, clay-assisted bonding is carried out to prepare Schumalith DIA series of SiC products (Schumacher Company, Germany) [26]. It is worth noting that the SiC ceramic membrane support relies on the bonding phase formed during high-temperature sintering to produce strength. Hence, the mechanical properties and service life of SiC support are dictated by the composition of bonding phase. However，the influence of different bonding phases on the performance of porous SiC ceramics has not been studied yet. In addition, SiC support is considered as one of key materials for high-temperature industrial flue gas cleaning. During the application, the SiC support must withstand stresses induced by mechanical clamping, vibration and support structures, as well as thermal stresses imparted from pulse cleaning and flue gas temperature fluctuation [26]. Moreover, it is subjected to oxidation from high-temperature gases. Therefore,
excellent high temperature performances such as hot modulus of rupture (HMOR), thermal shock resistance, and oxidation resistance are important criteria in the selection of a viable, durable filter material. However, only a few reports can be found on the high temperature performances of porous SiC ceramic supports. Herein, we aim to explore the influence of different bonding phases, including mullite, cordierite and glass phase, on properties of porous SiC ceramic membrane supports. The effects of bonding phase on ambient-temperature properties, high-temperature mechanical properties, thermal shock resistance and oxidation resistance of SiC support have been systematically investigated for the first time. The
present study serves as a baseline for the design and utilization of in-situ bonded SiC support for hightemperature flue gas filtration. 2. Experimental procedure 2.1. Raw materials Industrial black SiC particles were used as aggregate for the support. SiC powder, active ρ-Al2O3, basic magnesium carbonate, talc, kaolin and potassium feldspar were utilized as part of materials to form bonding phases. Charcoal powder was employed as a pore-forming agent. 10% weight of polyvinyl alcohol (PVA) solution was used as a low-temperature binder. The main characteristics of raw materials are summarized in Table 1. Table 1. Characteristics of the raw materials. Raw materials SiC particles SiC powder Active ρ-Al2O3 Potassium feldspar Kaolin
Characterization Size range:150-180 µm; SiC (wt%): 98.89 D50 = 1.46 µm, D90 =2.80 µm; SiC (wt%): 98.89 D50 = 2.83 µm, D90 =4.85 µm; Al2O3 (wt%): 86.2 Average particle diameter: ≤45 µm Average particle diameter: ≤15 µm
Talc
Average particle diameter: ≤15 µm
Basic magnesium carbonate
Average particle diameter: 5.8 µm; MgO (wt%): 40-44.5%
Polyvinyl alcohol
10 wt% solution in water
Charcoal powder
D50 = 25.44 µm, D90 = 93.34 µm
Resource Shandong Jinmeng New Materials Co., Ltd., China Shandong Jinmeng New Materials Co., Ltd., China Zhengzhou Light Metal Research Institute, Henan, China Lingshou Lihua Mineral Products Processing Plan, China Yuejiang New Materials (Guangzhou) Co., Ltd., China Tianjin Guangfu Fine Chemical Research Institute, China Tianjin Zhiyuan Chemical Reagent Co., Ltd., China Sinopharm Chemical Reagent Beijing Co., Ltd., China
2.2. Samples preparation It is worth noting that mechanical properties and thermal shock resistance of the support are significantly influenced by the porosity. Therefore, we aimed to obtain a similar level of porosity for SiC supports with different bonding phases to reasonably compare the performance parameters. Hence, the volume ratio of SiC aggregate to the bonding phase was set at 90:10. Then, the mass ratio of SiC aggregate to the bonding phase was correspondingly calculated from the volume ratio. The density of SiC, mullite (3Al2O3·2SiO2), cordierite (2MgO·2Al2O3·5SiO2) and glass phase was 3.20 g/cm3, 3.16 g/cm3 and 2.50 g/cm3, respectively. The theoretical mass ratio of the SiC skeleton particles to the bonding phase in the as-sintered support, with a volumetric ratio of 90:10, is shown in Table 2. According to the theoretical chemical composition of SiC supports with different bonding phases, their formulation of the samples can be obtained. Table 2. The volumetric and mass ratios of SiC supports with different bonding phases. Phase composition SiC:mullite
Volumetric ratio
Mass ratio
90:10
90.1:9.9
SiC:cordierite
90:10
91.7:8.3
SiC:glass
90:10
92.0:8.0
SiC and active ρ-Al2O3 powders were used as raw materials to prepare mullite-bonded SiC support, where basic magnesium carbonate was employed as a sintering aid. The calculated original composition of the mullite-bonded SiC support (MBSC) was 91 wt% of SiC particles, 8 wt% of active ρ-Al2O3 powder and 1 wt% of SiC powder. In addition, 1 wt% of basic magnesium carbonate and 6 wt% of charcoal powder were added with respect to the composition of the abovementioned mixture. The SiC powder, active ρ-Al2O3 powder and talc powder were utilized as raw materials to form cordierite bonding phase for SiC support. The calculated original composition of cordierite-bonded SiC support (CBSC) was 92 wt% of SiC particles, 3.6 wt% of talc powder, 3.4 wt% of active ρ-Al2O3 powder and 1 wt% of SiC powder. Moreover, 6 wt% of charcoal powder was added with respect to the composition of the abovementioned mixture. Herein, the chemical composition of glass binder, aluminum and potassium sources, and their ratios are the same as reported elsewhere [27]. However, the silica source is replaced by SiC. The calculated original composition of the glass-bonded SiC support (GBSC) was 93 wt% of SiC particles, 5 wt% of potassium feldspar powder, 1 wt% of washed kaolin and 1 wt% of SiC micron-sized powder. Furthermore, 6 wt% of charcoal powder was added with respect to the composition of the abovementioned mixture. The micron-sized powders were thoroughly mixed by using a cement mixer (NJ-160A, Wuxi Jianyi Instrument Machinery Co., Ltd., China). Then, added SiC particles to the mixed powders and mixed for 3 min, followed by the addition of charcoal powder and mixed for another 3 min. Then, 6 % weight of PVA binder solution (10 wt% in water) was added and mixed for additional 3 min to obtain a semi-dry mixed aggregate. The aggregate was placed in a rectangular steel mold, and uniaxially pressed to green bodies with dimensions of ~10 mm × 20 mm × 120 mm under a pressure of 50 MPa. The green bodies were naturally dried for 24 h and, then, baked in an oven at 110 oC for 24 h. The dried green bodies were heated to a predetermined sintering temperature at a heating rate of 3 oC/min, and then, the sintering temperature was maintained for 3 hours. The sintering regimes of the mullite-, cordierite- and glass-bonded SiC supports were 1400 oC × 3 h, 1360 oC × 3 h and 1300 oC × 3 h, respectively. Then, the as-sintered samples were machined into the rectangular specimens with dimensions of ~60 mm × 10 mm × 8 mm. 2.3. Characterization The apparent porosity was measured by the Archimedes method using distilled water as the immersion medium. Flexural strengths were characterized via the three-point bending test (CTM6005, Shanghai Xieqiang Instrument Technology Co., Ltd., China) with a support distance of 40 mm, and a cross-head speed of 0.5 mm/min. The fracture morphology was investigated by field-emission scanning electron microscopy (FESEM, Hitachi SU8220/S4800, Japan), equipped with an energy dispersive spectrometer (EDS). The phase composition was analyzed by using X-ray diffractometer (XRD, Xpert Powder, Panaco, Netherlands), equipped with Cu Kα radiations (λ = 0.1541 nm).
The hot modulus of rupture (HMOR) was assessed at the temperature of 900 oC and 1200 oC by using a high-temperature stress-strain gauge (Model 5928, Instron, USA). Briefly, the rectangular sample was placed on a fixture with a span of 40 mm and the temperature was increased to a predetermined temperature at a heating rate of 5 oC/min. After insulation for 15 minutes, the sample was loaded at a cross-head speed of 0.5 mm/min and the peak pressure was recorded at the breakage point. The HMOR was calculated by using the given relationship:
σf =
3FL 2bh2
(1)
where σf refers to the HMOR (MPa), F represents the peak pressure at the breakage point (N), L corresponds to the distance between supporting cylinders of the folding clamp (mm), b represents the sample width (mm), and h denotes the sample height (mm). The thermal shock resistance of SiC supports was evaluated by a water-quenching technique. The sample was transferred from the air at 20 oC into the furnace at the preset temperature and held for 20 min. Then, the as-heated sample was quickly taken out from the furnace and dropped by free fall into a water bath with a constant temperature of 20 oC. The residual flexural strength of quenched sample was measured. For comparison, the flexural strength of the sample before the thermal shock was also measured under the same conditions and thermal shock resistance was assessed by calculating the strength retention rate. The oxidation resistance was assessed by a weighing method, where mass change is measured due to oxidation at predetermined temperatures and times. Briefly, the sample, with dimensions of ~10 mm × 20 mm × 120 mm, was heated to the preset temperature (950~1150 oC) at a heating rate of 5 oC/min in air and held at this temperature for 24 h. The sample was weighed with a precision electronic balance before and after heat treatment and the oxidation rate was calculated from the given relationship: m − m1 Os = 2 ×100% m1
(2)
where Os refers to the oxidation rate of the sample (%), and m1 and m2 denote the mass before and after heat-treatment (g). 3. Results and discussion 3.1. Phase composition and microstructure The phase composition of mullite-, cordierite- and glass-bonded SiC supports is presented in Fig.1. MBSC support is mainly composed of SiC, mullite and cristobalite phases. Semi-quantitative analysis reveals that the mass fraction of SiC, mullite and cristobalite phases is 90%, 7% and 3%, respectively. The SiC phase formed a skeleton-like structure, where a large proportion of cristobalite phase is distributed at the surface and most of the mullite phase is distributed in the bonding phase region. Thus, the relative content of mullite in the neck region of the bonding phase is higher than 70%, which indicates that the bonding region is dominated by the mullite phase. CBSC support is mainly composed of SiC, cordierite and cristobalite phases with mass fraction of 89%, 8% and 3%, respectively. If SiC particles are ignored, the relative content of cordierite and cristobalite phases was found to be 72.7% and 27.3%, respectively. Moreover, the relative content of
cordierite in the neck region exceeded 72.7%, which indicates that the bonding phase region is a cordierite-rich area. In addition to the main crystalline phase, i.e., SiC, GBSC support contains a trace amount of cristobalite phase (<1 %) and the diffraction peaks, corresponding to the glass phase, have not been observed in the 2θ range of 20-30o, which can be attributed to the high intensity of SiC diffraction peaks and low content of glass phase. Similar to MBSC and CBSC, the bonding region is mainly composed of the glass phase. XRD analysis confirmed that the phase composition of in-situ bonded SiC supports is consistent with the designed experimental conditions.
S: SiC M: Mullite C: Cordierite Cr: Cristobalite
Intensity/a.u.
S
S M C
Cr C+Cr Cr
10
20
S
M M
S
C CC
S
S
S M S S MBSC S
S
S
CBSC GBSC
Cr
30
S
S
40 2 θ/°
50
60
70
Fig. 1. XRD pattern of mullite-, cordierite- and glass-bonded porous SiC supports.
Fig. 2 shows the fracture morphology of the in-situ bonded SiC supports. In general, in-situ bonded SiC supports exhibited a mixture of transgranular and intergranular fracture, however, significant differences have been observed in terms of surface roughness and micropores in the bonding region. Micropores are formed which can be attributed to the fact that the bonding matrix is densified during the initial stage of the sintering, however, uncompleted oxidation SiC powder and SiC particles, encapsulated in the dense bonding strut, continued to oxidize in the later stages of sintering due to the diffused oxygen. The gaseous products (SiO, CO) are entrapped in the dense bonded phases and lead to the formation of closed pores [28]. CBSC support exhibited the maximum fracture roughness and the largest number of internal micropores, followed by MBSC support. On the other hand, GBSC support rendered a smooth fracture surface and a minimum number of internal micropores. One should note that MBSC and CBSC supports are prepared by in-situ reaction sintering, where the reaction temperature of CBSC support is lower than MBSC support. Hence, the preliminary sintering for the densification of the bonded matrix is performed at a lower temperature, resulting in lesser oxidation of SiC powder in the bonded phase. However, the oxidation of SiC powder in dense bonded phase is accelerated with increasing sintering temperature and time. In addition, the cordierite bonding system promotes the oxidation of SiC due to the presence of alkali metal oxides [29]. Therefore, a large amount of gaseous products is entrapped in the bonded neck region, forming a large number of closed micropores. On the other hand, MBSC support is sintered at a relatively higher temperature for an extended time. Hence, SiC powder is sufficiently oxidized before densification.
Thus, the degree of continued oxidation after densification is correspondingly reduced, resulting in a small number of micropores in the bonded neck region. Furthermore, GBSC support is sintered in liquid phase at lower temperature, resulting in low oxidation degree of SiC. Therefore, a smooth and pore-free fracture morphology has been observed (Fig. 2c). It is worth emphasizing that micropores in the bonded phases are responsible for the inferior strength of in-situ bonded SiC supports. However, the presence of micropores hinders crack propagation and therefore improves thermal shock resistance.
(a) MBSC
(b) CBSC
(c) GBSC
Fig. 2. Fracture morphology of (a) mullite-, (b) cordierite- and (c) glass-bonded SiC supports.
3.2 Ambient temperature flexural strength and apparent porosity The apparent porosity and ambient-temperature flexural strength values of mullite-, cordierite- and glass-bonded SiC supports are summarized in Table 3. The apparent porosity of MBSC, CBSC and GBSC supports was found to be 36.8%, 36.2% and 37.2%, respectively. The standard deviation of 0.5% indicates that the apparent porosity of mullite-, cordierite- and glass-bonded SiC supports are quite close, which is designed to compare the properties of in-situ bonded SiC supports with different bonding phases. The lower porosity of CBSC support can be ascribed to the accelerated oxidation of SiC in the presence of MgO-Al2O3-SiO2 system, which results in an increase of bonded phases. As mentioned earlier, the total content of bonded phase in CBSC support is ~11%, which is higher than the experimentally targeted bonded phase (8.3%) and explains the lower porosity of CBSC support. The GBSC support exhibited a slightly higher porosity, which can be ascribed to the lower degree of SiC oxidation. Table 3. Mechanical and physical properties of in-situ bonded SiC supports with different bonding phases. Name
Apparent porosity / %
Bulk density / g·cm-3
Ambient-temperature flexural strength / MPa
MBSC
36.8
1.93
22.5
CBSC
36.2
1.93
24.5
GBSC
37.2
1.92
33.6
Furthermore, GBSC support rendered the maximum ambient-temperature flexural strength of 33.6 MPa, whereas the flexural strength of MBSC and CBSC supports ranged from 22 to 25 MPa. Theoretically, the strength of mullite- and cordierite-bonded SiC supports should be higher than the glass-bonded SiC support due to the higher mechanical strength of mullite and cordierite. However, the solid-state sintering of mullite- and cordierite-bonded SiC supports results in incomplete sintering process, incomplete structure and porosity defects, leading to inferior flexural strength. On the other hand, glass-bonded SiC support possesses fewer defects in the neck region due to liquid phase sintering
and renders higher strength. 3.3. High-temperature mechanical properties Fig. 3 presents the HMOR of mullite-, cordierite- and glass-bonded SiC supports at 900 oC and 1200 oC. The HMOR of MBSC support, at 900 oC, was found to be 33.2 MPa, which is 147.5% higher than the ambient-temperature flexural strength. Moreover, the HMOR of CBSC support, at 900 oC, was found to be 27.5 MPa, which is close to the corresponding ambient-temperature flexural strength. However, the HMOR of GBSC support significantly decreased to 19.3 MPa, with the strength retention rate of 57.4%, which can be ascribed to the decrease in viscosity of glass phase. At 1200 oC, MBSC support exhibited maximum flexural strength of 10 MPa, whereas GBSC support showed a negligible strength value of only 0.2 MPa. The significant improvement in HMOR of mullite-bonded SiC support can be mainly attributed to mechanically enhanced properties of the mullite bonding phase at high temperature. In addition to mullite and cristobalite crystalline phases, there is also small amount of MgO-Al2O3-SiO2 glassy phase distributed in the mullite crystal boundary of the binding neck [29]. When temperature is increased to 900 oC, these grain-boundary glassy phases exhibit viscoelastic behavior, resulting in the relief of the crack-tip stress in the plastic deformation zone through viscoelastic relaxation [30-32]. Thus, the mullite-bonded SiC support was strengthened at 900 oC. However, when the temperature is further increased to 1200 oC, the decreased viscosity of the low-melting glass phase, at the grain boundaries, results in the slip of grain boundaries and, thereby, lowers the HMOR of the in-situ bonded SiC support. Therefore, the high-temperature bending strength of MBSC and CBSC supports is decreased at 1200 oC. In the case of glass-bonded SiC support, high-temperature softening of the bonding phase resulted in a decrease in strength and a negligible flexural strength of 0.2 MPa at 1200 oC. It can be seen that the mullite bonding phase imparted excellent high temperature mechanical properties to the SiC support, whereas the presence of the glass phase deteriorated the high temperature mechanical properties of the support. Thus, the content of glass phase should be minimized to improve the high-temperature mechanical behavior of in-situ bonded SiC support. 60 Ambient temperatrue 900oC
Flexural strength/MPa
50
1200oC
40 33.6
33.2
30 24.5
22.5
27.5 19.3
20 10 0
10.5
8.5 0.2
MBSC
CBSC
GBSC
Fig. 3. Flexural strength of mullite-, cordierite- and glass-bonded SiC supports at different temperatures.
3.4. Thermal shock resistance
Fig. 4 shows the strength retention rate of mullite-, cordierite- and glass-bonded SiC supports after 1 cycle of instantaneous heating-water quenching at different temperatures. It can be readily observed that the strength retention rate of each support decreased with the increase of quenching temperature. Under the same quenching temperature, the strength retention rate of MBSC support is slightly higher than CBSC support and much higher than GBSC support. The critical temperature difference is defined at a strength retention rate of 70%. And the critical temperature difference of MBSC, CBSC and GBSC supports can be determined to be ~570 oC, 500 oC and 370 oC, respectively, from Fig. 4. One should note that the critical temperature difference of mullite-bonded SiC support is 200 oC higher than the glass-bonded SiC support, indicating the superior thermal shock resistance of mullite-bonded SiC support.
Strength retention rate/%
120 MBSC CBSC GBSC
100 80
470oC
570oC
60 40 20 0
200
400
600
800
1000
o
Quenching temperature/ C
Fig. 4. Strength retention rate vs. water quenching temperature of 8 mm thick mullite-, cordierite- and glass-bonded SiC supports.
Fig. 5 shows the microstructure of mullite-, cordierite- and glass-bonded SiC supports after water quenching at 950 oC. It can be readily observed that the microcracks appeared in the neck region of each support and the number, width and length of cracks in MBSC and CBSC supports are much lower than GBSC support. Both mullite- and cordierite-bonded SiC supports rendered excellent thermal shock resistance due to the similar thermal expansion coefficients of mullite and cordierite to SiC, which resulted in low thermal stress at the bonding interface and less cracking. At the same time, the micropores are formed in both bonding phases during reaction sintering, which contribute to the stress relaxation at the crack tip and hinder the further propagation of crack in the bonding phases. Therefore, under the same thermal shock conditions, MBSC and CBSC supports exhibit lesser, shorter and narrower cracks, resulting in superior thermal shock resistance. On the other hand, the thermal physicochemical properties of glass phase are largely different from the SiC and its thermal expansion coefficient is significantly high, resulting in a large thermal stress inside the bonding phase and at the glass/SiC interface. Therefore, the cracks are easily initiated and rapidly propagated in the dense structure of glass phase, leading to poor thermal shock resistance GBSC support.
(a) MBSC
(b) CBSC
(c) GBSC
Fig. 5. SEM images of (a) mullite-, (b) cordierite- and (c) glass-bonded SiC supports after water quenching at 950 oC.
3.5. Oxidation resistance The weight gain of mullite-, cordierite- and glass-bonded SiC supports support after oxidation for 24 h at various temperatures is shown in Table 4. After continuous oxidation at 950 oC for 24 h, the MBSC and CBSC supports did not exhibit any significant weight gain, whereas GBSC support exhibited slight oxidation with a weight gain rate of 0.003 %. At 1050 oC, MBSC, CBSC and GBSC supports exhibited weight gain rate of 0.01 %, 0.023 % and 0.032 %, respectively. When the temperature was further increased to 1150 oC, MBSC, CBSC and GBSC supports exhibited significant oxidation with weight gain rate of 0.118 %, 0.204 % and 0.259 %, respectively, which is an order of magnitude higher than the oxidation at 1050 oC. Even though MBSC support is oxidized at >1000 oC, the weight gain rate is only 0.1 % after oxidation at 1150 oC, which is 54.4% and 42.2% lower than CBSC and GBSC supports, respectively. These results confirm that mullite-bonded SiC support possesses superior oxidation resistance. One should note that a dense mullite coating is formed on the surface of the SiC skeleton after sintering. Moreover, the structure of protective coating remained intact during high-temperature oxidation due to the excellent thermal stability of mullite, and the absence of stress-induced cracks because of matching thermal expansion coefficient with SiC at the abovementioned temperatures. The morphology of mullite-bonded SiC support after continuous oxidation at 1150 oC for 24 h is shown in Fig. 6a. A dense, smooth and crack-free protective layer can be readily observed on the SiC particles. Moreover, the diffusion coefficient of oxygen in mullite is extremely small (1.5×10-11 cm2/s) [33] and mullite renders stable performance at high temperature. Therefore, mullite-bonded SiC support exhibited excellent oxidation resistance at 1150 oC. Compared with MBSC support, CBSC and GBSC supports exhibited inferior oxidation resistance. Despite the fact that cordierite-bonded SiC support exhibits excellent thermal shock resistance, the porous structure of bonding phase, as shown in Fig.6b, resulted in a high diffusion coefficient of oxygen. Moreover, MgO-Al2O3-SiO2 system promoted the oxidation of SiC. Hence, cordierite-bonded SiC support rendered inferior oxidation resistance. In the case of glass-bonded SiC support, the surface coating on SiC particles is easy to soften and it melted at high temperature. Moreover, the volumetric changes during the crystallization process and crystal transformations generated cracks, as shown in Fig.6c, enhancing the penetration capacity of oxygen and resulting in low oxidation resistance of glassbonded SiC support.
Table 4. Weight gain rate of mullite-, cordierite- and glass-bonded SiC supports after oxidation at different temperatures for 24 h. Weight gain rate / % Samples 950 ℃
1050 ℃
1150 ℃
MBSC
0
0.01
0.118
CBSC
0
0.023
0.259
GBSC
0.003
0.032
0.204
(a) MBSC
(b) CBSC
(c) GBSC
Fig. 6. The interface between bonding phase and SiC particles in (a) mullite-, (b) cordierite- and (c) glass-bonded SiC supports after oxidation at 1150 oC for 24 h.
4. Conclusions In summary, we have successfully prepared mullite-, cordierite- and glass-bonded SiC supports with similar porosity (~37%) by controlling the volumetric ratio of bonding materials and studied the influence of bonding phases on ambient-temperature flexural strength, hot modulus of rupture, thermal shock resistance and oxidation resistance. The results revealed that the choice of bonding phase significantly influenced the ambient-temperature and high-temperature properties of SiC support. The mullite-bonded SiC support rendered superior high-temperature mechanical properties, thermal shock resistance and oxidation resistance, which can be attributed to the intrinsic characteristics of the mullite phase and its thermal and chemical compatibility with SiC. The combination of cordierite and SiC can also impart relatively superior ambient-temperature and high-temperature flexural strength, and desirable thermal shock resistance, however, the oxidation resistance of cordierite-bonded SiC support is compromised due to the presence of alkali metal ions. The glass-bonded SiC support exhibited maximum ambient-temperature flexural strength and a smooth pore structure due to the liquid phase sintering, however, the high-temperature flexural strength, thermal shock resistance and oxidation resistance cannot meet the practical requirements. In high-temperature applications, such as hot flue gas filtration, porous burners and molten metal filters, the mullite-bonded SiC support can render superior performance and extended service life.
Acknowledgments
This work was supported by the Open Research Fund of State Key Laboratory of Multiphase Complex Systems (Grant no. MPCS-2019-D-07) and the National Key R&D Program of China (Grant no. 2016YFB0601100). References [1] J. Adler, Ceramic Diesel Particulate Filters, Int. J. Appl. Ceram. Technol. 2 (6) (2005) 429-439. [2] U.F. Vogt, L. Györfy, A. Herzog, T. Granule, G. Plesch, Macroporous silicon carbide foams for porous burner applications and catalyst supports, J. Phys. Chem. Solids 68 (5) (2007) 1234-1238. [3] W. Susie, A.T. Harris, Porous burners for lean-burn applications, Prog. Energ. Combust. Sci. 34 (5) (2008) 667-684. [4] A.G. Martín, M.P. Orihuela, J.A. Becerra, J.M. Fernández , J.R. Rico, Permeability and mechanical integrity of porous biomorphic SiC ceramics for application as hot-gas filters, Mater. Des. 107 (2016) 450-460. [5] R. Riedel, G. Passing, H. Schönfelder, R.J. Brook, Synthesis of dense silicon-based ceramics at low temperatures, Nature 355 (6362) (1992) 714-717. [6] Y. Li, H. Wu, X. Liu, Z. Huang, D. Jiang, Microstructures and properties of solid-state-sintered silicon carbide membrane supports, Ceram. Int. 45(16) (2019)19888-19894. [7] J. She, Z. Deng, J.D. Doni, T. Ohji, Oxidation bonding of porous silicon carbide ceramics, J. Mater. Sci. 37 (17) (2002) 3615-3622. [8] S. Liu, Y. Zeng, D. Jiang, Fabrication and characterization of cordierite-bonded porous SiC ceramics, Ceram. Int. 35 (2) (2009) 597-602. [9] Y.S. Chun, Y.W. Kim, Processing and mechanical properties of porous silica-bonded silicon carbide ceramics, Met. Mater. Int. 11 (5) (2005) 351-355. [10] Y. Zhang, Microstructures and mechanical properties of silicon nitride bonded silicon carbide ceramic foams, Mater. Res. Bull. 39 (6) (2004) 755-761. [11] I.H. Song, M.J. Park, H.D. Kim, Y.W. Kim, J.S. Bae, Microstructure and permeability property of Si bonded porous SiC with variations in the carbon content, J. Korean Ceram. Soc. 47 (6) (2010) 546-552. [12] J.H. Eom, Y.W. Kim, Low-Temperature processing of silicon oxycarbide-bonded silicon carbide, J. Am. Ceram. Soc. 93 (9) (2010) 2463-2466. [13] S.H. Chae, Y.W. Kim, I.H. Song, H.D. Kim, J.S. Bae, S.M. Na, S.I. Kim, Low-temperature processing and properties of porous frit-bonded SiC ceramics, J. Korean Ceram. Soc. 46 (5) (2009) 488-492. [14] J.H. Eom, Y.W. Kim, S. Raju, Processing and properties of macroporous silicon carbide ceramics: A review, J. Asian Ceram. Soc. 1 (4) (2013) 220-242. [15] S. Ding, S. Zhu, Y. Zeng, D. Jiang, Effect of Y2O3 addition on the properties of reaction-bonded porous SiC ceramics, Ceram. Int. 32 (4) (2006) 461-466. [16] S. Ding, S. Zhu, Y. Zeng, Fabrication of mullite-bonded porous silicon carbide ceramics by in situ reaction bonding, J. Eur. Ceram. Soc. 27 (4) (2007) 2095-2102. [17] Y.H. Choi, Y.W. Kim, I.S. Han, Effect of alkaline earth metal oxide addition on flexural strength of porous mullitebonded silicon carbide ceramics, J. Mater. Sci. 45 (24) (2010) 6841-6844. [18] C. Bai, X. Deng, J. Li, W. Wang, Preparation and properties of mullite-bonded porous SiC ceramics using porous alumina as oxide, Mater. Charact. 90 (4) (2014) 81-87. [19] C. Bai, Y. Li, Z. Liu, P. Liu, X. Deng, J. Li, J. Yang, Fabrication and properties of mullite-bonded porous SiC
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Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0272-8842(19)33571-0
DOI:
https://doi.org/10.1016/j.ceramint.2019.12.082
Reference:
CERI 23735
To appear in:
Ceramics International
Received Date: 7 October 2019 Revised Date:
4 December 2019
Accepted Date: 6 December 2019
Please cite this article as: Z.-Y. Luo, W. Han, K.-Q. Liu, W.-Q. Ao, K.-k. Si, Influence of bonding phases on properties of in-situ bonded porous SiC membrane supports, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2019.12.082. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Influence of bonding phases on properties of in-situ bonded porous SiC membrane supports Zhi-Yong Luoa, Wei Hana,*, Kai-Qi Liub,c,* *, Wen-Qing Aoa, Kai-kai Sib,c a
Beijing Key Laboratory of Advanced Ceramic and Refractories, Central Iron & Steel Research Institute, Beijing 100081, China; b State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China c University of Chinese Academy of Sciences, Beijing, 100190, China
*Corresponding author: Prof. Wei Han, Tel.: +86-10-62182468; Fax.: +86-10-62184553. E-mail: [email protected]. ∗∗ Corresponding author: Prof. Kai-qi Liu, Tel.: +86-10-82545059; Fax.: +86-10-82545059. E-mail: [email protected]. Abstract: Porous SiC ceramic membrane supports are widely employed in a wide variety of hightemperature applications, such as hot flue gas filtration, porous burners and molten metal filters. Herein, SiC supports, with a porosity of ~37%, were prepared by using low-temperature bonding techniques and the influence of different bonding phases, such as mullite, cordierite and glass, on ambient-temperature flexural strength, hot modulus of rupture (HMOR), thermal shock resistance and oxidation resistance were systematically investigated. The results reveal that the glass-bonded SiC (GBSC) support exhibited the highest ambient-temperature flexural strength of 33.6 MPa, whereas the flexural strength of mullitebonded SiC (MBSC) and cordierite-bonded SiC (CBSC) supports ranged from 22 to 25 MPa. However, the presence of glass phase deteriorated the high-temperature properties of the support. MBSC support rendered superior mechanical strength at high temperature and self-strengthening in a certain temperature range, such as HMOR improved 47.5% at 900 oC, but HMOR of glass-bonded support was only 57.4% of the ambient-temperature strength. Moreover, MBSC and CBSC supports exhibited better thermal shock resistance than GBSC supports and the critical temperature difference of water quenching for MBSC supports was ~200 oC higher than GBSC supports. In addition, MBSC support rendered superior oxidation resistance and exhibited a weight gain rate of ~0.1% at 1150 oC for 24 h, which is 54.4% and 42.2% lower than CBSC and GBSC supports, respectively. Keywords: Porous SiC ceramics; membrane support; bonding phase; strength; thermal shock resistance; oxidation resistance
1. Introduction Porous SiC ceramics, with superior strength, wear resistance, corrosion resistance, thermal stability and thermal shock resistance, render promise in high-temperature applications, such as molten metal filters, diesel particulate filters, hot gas filters and porous burners [1-4]. However, a temperature of 2000 o
C or higher temperature and an inert or vacuum environment are required to sinter pure SiC ceramics
due to highly covalent characteristics of Si–C bonds [5,6], limiting the practical applications of porous SiC ceramics. Therefore, low-temperature fabrication techniques are being developed to sinter porous
SiC ceramics, where making use of bonding materials is an effective method to reduce the sintering temperature. For instance, the in-situ reaction bonding process can realize the low-temperature fabrication of porous SiC ceramics. Mullite [7], cordierite [8], silica [9], silicon nitride [10], silicon [11], silicon oxycarbide [12] and glass phase [13] have been investigated as bonding materials for the fabrication of porous SiC ceramics. In particular, mullite, cordierite and glass phase bonding systems are easy to obtain and considered viable for industrial production and applications. Mullite is an ideal bonding phase for porous SiC ceramic membrane support due to its excellent mechanical properties, thermal shock resistance, corrosion resistance, oxidation resistance, temperature stability and similar thermal expansion coefficient to SiC [14]. The mullite-bonded SiC support can be obtained by in-situ reaction of Al2O3 deriving from added aluminum-containing powder, such as Al, AlN, Al2O3, Al (OH)3, bauxite and kaolin, with oxidation-derived SiO2 at the surface of SiC [15]. The sintering temperature of mullite-bonded SiC support ranges from 1350 oC to 1550 oC [16-20]. Cordierite is chosen as a bonding phase to fabricate porous SiC ceramics due to its low thermal expansion coefficient [21] and low synthesis temperature [22]. The sintering mechanism of cordierite-bonded SiC is similar to mullite-bonded SiC support, which implies that cordierite phase is obtained by in-situ reaction of magnesium-containing and aluminum-containing powders with oxidation-derived SiO2 [8]. In general, MgO and talc powders are mainly employed as magnesium-containing raw materials, where Al2O3 and kaolin are utilized as aluminum-containing raw materials. One should note that the sintering temperature of porous cordierite-bonded SiC ceramics is ~1350 oC [8,23]. Glass-bonded SiC porous ceramics are fabricated from SiC and low melting point raw materials such as clay, potassium feldspar and glass powder, at a lower sintering temperature of ~1300 oC [13, 24, 25]. The sintering mechanism is as follows: the raw materials with low melting point are transformed into viscous substance during the sintering process, where SiC particles are immersed and wrapped. After cooling, the as-formed glass phase results in the strength of SiC particles. For instance, clay-assisted bonding is carried out to prepare Schumalith DIA series of SiC products (Schumacher Company, Germany) [26]. It is worth noting that the SiC ceramic membrane support relies on the bonding phase formed during high-temperature sintering to produce strength. Hence, the mechanical properties and service life of SiC support are dictated by the composition of bonding phase. However，the influence of different bonding phases on the performance of porous SiC ceramics has not been studied yet. In addition, SiC support is considered as one of key materials for high-temperature industrial flue gas cleaning. During the application, the SiC support must withstand stresses induced by mechanical clamping, vibration and support structures, as well as thermal stresses imparted from pulse cleaning and flue gas temperature fluctuation [26]. Moreover, it is subjected to oxidation from high-temperature gases. Therefore,
excellent high temperature performances such as hot modulus of rupture (HMOR), thermal shock resistance, and oxidation resistance are important criteria in the selection of a viable, durable filter material. However, only a few reports can be found on the high temperature performances of porous SiC ceramic supports. Herein, we aim to explore the influence of different bonding phases, including mullite, cordierite and glass phase, on properties of porous SiC ceramic membrane supports. The effects of bonding phase on ambient-temperature properties, high-temperature mechanical properties, thermal shock resistance and oxidation resistance of SiC support have been systematically investigated for the first time. The
present study serves as a baseline for the design and utilization of in-situ bonded SiC support for hightemperature flue gas filtration. 2. Experimental procedure 2.1. Raw materials Industrial black SiC particles were used as aggregate for the support. SiC powder, active ρ-Al2O3, basic magnesium carbonate, talc, kaolin and potassium feldspar were utilized as part of materials to form bonding phases. Charcoal powder was employed as a pore-forming agent. 10% weight of polyvinyl alcohol (PVA) solution was used as a low-temperature binder. The main characteristics of raw materials are summarized in Table 1. Table 1. Characteristics of the raw materials. Raw materials SiC particles SiC powder Active ρ-Al2O3 Potassium feldspar Kaolin
Characterization Size range:150-180 µm; SiC (wt%): 98.89 D50 = 1.46 µm, D90 =2.80 µm; SiC (wt%): 98.89 D50 = 2.83 µm, D90 =4.85 µm; Al2O3 (wt%): 86.2 Average particle diameter: ≤45 µm Average particle diameter: ≤15 µm
Talc
Average particle diameter: ≤15 µm
Basic magnesium carbonate
Average particle diameter: 5.8 µm; MgO (wt%): 40-44.5%
Polyvinyl alcohol
10 wt% solution in water
Charcoal powder
D50 = 25.44 µm, D90 = 93.34 µm
Resource Shandong Jinmeng New Materials Co., Ltd., China Shandong Jinmeng New Materials Co., Ltd., China Zhengzhou Light Metal Research Institute, Henan, China Lingshou Lihua Mineral Products Processing Plan, China Yuejiang New Materials (Guangzhou) Co., Ltd., China Tianjin Guangfu Fine Chemical Research Institute, China Tianjin Zhiyuan Chemical Reagent Co., Ltd., China Sinopharm Chemical Reagent Beijing Co., Ltd., China
2.2. Samples preparation It is worth noting that mechanical properties and thermal shock resistance of the support are significantly influenced by the porosity. Therefore, we aimed to obtain a similar level of porosity for SiC supports with different bonding phases to reasonably compare the performance parameters. Hence, the volume ratio of SiC aggregate to the bonding phase was set at 90:10. Then, the mass ratio of SiC aggregate to the bonding phase was correspondingly calculated from the volume ratio. The density of SiC, mullite (3Al2O3·2SiO2), cordierite (2MgO·2Al2O3·5SiO2) and glass phase was 3.20 g/cm3, 3.16 g/cm3 and 2.50 g/cm3, respectively. The theoretical mass ratio of the SiC skeleton particles to the bonding phase in the as-sintered support, with a volumetric ratio of 90:10, is shown in Table 2. According to the theoretical chemical composition of SiC supports with different bonding phases, their formulation of the samples can be obtained. Table 2. The volumetric and mass ratios of SiC supports with different bonding phases. Phase composition SiC:mullite
Volumetric ratio
Mass ratio
90:10
90.1:9.9
SiC:cordierite
90:10
91.7:8.3
SiC:glass
90:10
92.0:8.0
SiC and active ρ-Al2O3 powders were used as raw materials to prepare mullite-bonded SiC support, where basic magnesium carbonate was employed as a sintering aid. The calculated original composition of the mullite-bonded SiC support (MBSC) was 91 wt% of SiC particles, 8 wt% of active ρ-Al2O3 powder and 1 wt% of SiC powder. In addition, 1 wt% of basic magnesium carbonate and 6 wt% of charcoal powder were added with respect to the composition of the abovementioned mixture. The SiC powder, active ρ-Al2O3 powder and talc powder were utilized as raw materials to form cordierite bonding phase for SiC support. The calculated original composition of cordierite-bonded SiC support (CBSC) was 92 wt% of SiC particles, 3.6 wt% of talc powder, 3.4 wt% of active ρ-Al2O3 powder and 1 wt% of SiC powder. Moreover, 6 wt% of charcoal powder was added with respect to the composition of the abovementioned mixture. Herein, the chemical composition of glass binder, aluminum and potassium sources, and their ratios are the same as reported elsewhere [27]. However, the silica source is replaced by SiC. The calculated original composition of the glass-bonded SiC support (GBSC) was 93 wt% of SiC particles, 5 wt% of potassium feldspar powder, 1 wt% of washed kaolin and 1 wt% of SiC micron-sized powder. Furthermore, 6 wt% of charcoal powder was added with respect to the composition of the abovementioned mixture. The micron-sized powders were thoroughly mixed by using a cement mixer (NJ-160A, Wuxi Jianyi Instrument Machinery Co., Ltd., China). Then, added SiC particles to the mixed powders and mixed for 3 min, followed by the addition of charcoal powder and mixed for another 3 min. Then, 6 % weight of PVA binder solution (10 wt% in water) was added and mixed for additional 3 min to obtain a semi-dry mixed aggregate. The aggregate was placed in a rectangular steel mold, and uniaxially pressed to green bodies with dimensions of ~10 mm × 20 mm × 120 mm under a pressure of 50 MPa. The green bodies were naturally dried for 24 h and, then, baked in an oven at 110 oC for 24 h. The dried green bodies were heated to a predetermined sintering temperature at a heating rate of 3 oC/min, and then, the sintering temperature was maintained for 3 hours. The sintering regimes of the mullite-, cordierite- and glass-bonded SiC supports were 1400 oC × 3 h, 1360 oC × 3 h and 1300 oC × 3 h, respectively. Then, the as-sintered samples were machined into the rectangular specimens with dimensions of ~60 mm × 10 mm × 8 mm. 2.3. Characterization The apparent porosity was measured by the Archimedes method using distilled water as the immersion medium. Flexural strengths were characterized via the three-point bending test (CTM6005, Shanghai Xieqiang Instrument Technology Co., Ltd., China) with a support distance of 40 mm, and a cross-head speed of 0.5 mm/min. The fracture morphology was investigated by field-emission scanning electron microscopy (FESEM, Hitachi SU8220/S4800, Japan), equipped with an energy dispersive spectrometer (EDS). The phase composition was analyzed by using X-ray diffractometer (XRD, Xpert Powder, Panaco, Netherlands), equipped with Cu Kα radiations (λ = 0.1541 nm).
The hot modulus of rupture (HMOR) was assessed at the temperature of 900 oC and 1200 oC by using a high-temperature stress-strain gauge (Model 5928, Instron, USA). Briefly, the rectangular sample was placed on a fixture with a span of 40 mm and the temperature was increased to a predetermined temperature at a heating rate of 5 oC/min. After insulation for 15 minutes, the sample was loaded at a cross-head speed of 0.5 mm/min and the peak pressure was recorded at the breakage point. The HMOR was calculated by using the given relationship:
σf =
3FL 2bh2
(1)
where σf refers to the HMOR (MPa), F represents the peak pressure at the breakage point (N), L corresponds to the distance between supporting cylinders of the folding clamp (mm), b represents the sample width (mm), and h denotes the sample height (mm). The thermal shock resistance of SiC supports was evaluated by a water-quenching technique. The sample was transferred from the air at 20 oC into the furnace at the preset temperature and held for 20 min. Then, the as-heated sample was quickly taken out from the furnace and dropped by free fall into a water bath with a constant temperature of 20 oC. The residual flexural strength of quenched sample was measured. For comparison, the flexural strength of the sample before the thermal shock was also measured under the same conditions and thermal shock resistance was assessed by calculating the strength retention rate. The oxidation resistance was assessed by a weighing method, where mass change is measured due to oxidation at predetermined temperatures and times. Briefly, the sample, with dimensions of ~10 mm × 20 mm × 120 mm, was heated to the preset temperature (950~1150 oC) at a heating rate of 5 oC/min in air and held at this temperature for 24 h. The sample was weighed with a precision electronic balance before and after heat treatment and the oxidation rate was calculated from the given relationship: m − m1 Os = 2 ×100% m1
(2)
where Os refers to the oxidation rate of the sample (%), and m1 and m2 denote the mass before and after heat-treatment (g). 3. Results and discussion 3.1. Phase composition and microstructure The phase composition of mullite-, cordierite- and glass-bonded SiC supports is presented in Fig.1. MBSC support is mainly composed of SiC, mullite and cristobalite phases. Semi-quantitative analysis reveals that the mass fraction of SiC, mullite and cristobalite phases is 90%, 7% and 3%, respectively. The SiC phase formed a skeleton-like structure, where a large proportion of cristobalite phase is distributed at the surface and most of the mullite phase is distributed in the bonding phase region. Thus, the relative content of mullite in the neck region of the bonding phase is higher than 70%, which indicates that the bonding region is dominated by the mullite phase. CBSC support is mainly composed of SiC, cordierite and cristobalite phases with mass fraction of 89%, 8% and 3%, respectively. If SiC particles are ignored, the relative content of cordierite and cristobalite phases was found to be 72.7% and 27.3%, respectively. Moreover, the relative content of
cordierite in the neck region exceeded 72.7%, which indicates that the bonding phase region is a cordierite-rich area. In addition to the main crystalline phase, i.e., SiC, GBSC support contains a trace amount of cristobalite phase (<1 %) and the diffraction peaks, corresponding to the glass phase, have not been observed in the 2θ range of 20-30o, which can be attributed to the high intensity of SiC diffraction peaks and low content of glass phase. Similar to MBSC and CBSC, the bonding region is mainly composed of the glass phase. XRD analysis confirmed that the phase composition of in-situ bonded SiC supports is consistent with the designed experimental conditions.
S: SiC M: Mullite C: Cordierite Cr: Cristobalite
Intensity/a.u.
S
S M C
Cr C+Cr Cr
10
20
S
M M
S
C CC
S
S
S M S S MBSC S
S
S
CBSC GBSC
Cr
30
S
S
40 2 θ/°
50
60
70
Fig. 1. XRD pattern of mullite-, cordierite- and glass-bonded porous SiC supports.
Fig. 2 shows the fracture morphology of the in-situ bonded SiC supports. In general, in-situ bonded SiC supports exhibited a mixture of transgranular and intergranular fracture, however, significant differences have been observed in terms of surface roughness and micropores in the bonding region. Micropores are formed which can be attributed to the fact that the bonding matrix is densified during the initial stage of the sintering, however, uncompleted oxidation SiC powder and SiC particles, encapsulated in the dense bonding strut, continued to oxidize in the later stages of sintering due to the diffused oxygen. The gaseous products (SiO, CO) are entrapped in the dense bonded phases and lead to the formation of closed pores [28]. CBSC support exhibited the maximum fracture roughness and the largest number of internal micropores, followed by MBSC support. On the other hand, GBSC support rendered a smooth fracture surface and a minimum number of internal micropores. One should note that MBSC and CBSC supports are prepared by in-situ reaction sintering, where the reaction temperature of CBSC support is lower than MBSC support. Hence, the preliminary sintering for the densification of the bonded matrix is performed at a lower temperature, resulting in lesser oxidation of SiC powder in the bonded phase. However, the oxidation of SiC powder in dense bonded phase is accelerated with increasing sintering temperature and time. In addition, the cordierite bonding system promotes the oxidation of SiC due to the presence of alkali metal oxides [29]. Therefore, a large amount of gaseous products is entrapped in the bonded neck region, forming a large number of closed micropores. On the other hand, MBSC support is sintered at a relatively higher temperature for an extended time. Hence, SiC powder is sufficiently oxidized before densification.
Thus, the degree of continued oxidation after densification is correspondingly reduced, resulting in a small number of micropores in the bonded neck region. Furthermore, GBSC support is sintered in liquid phase at lower temperature, resulting in low oxidation degree of SiC. Therefore, a smooth and pore-free fracture morphology has been observed (Fig. 2c). It is worth emphasizing that micropores in the bonded phases are responsible for the inferior strength of in-situ bonded SiC supports. However, the presence of micropores hinders crack propagation and therefore improves thermal shock resistance.
(a) MBSC
(b) CBSC
(c) GBSC
Fig. 2. Fracture morphology of (a) mullite-, (b) cordierite- and (c) glass-bonded SiC supports.
3.2 Ambient temperature flexural strength and apparent porosity The apparent porosity and ambient-temperature flexural strength values of mullite-, cordierite- and glass-bonded SiC supports are summarized in Table 3. The apparent porosity of MBSC, CBSC and GBSC supports was found to be 36.8%, 36.2% and 37.2%, respectively. The standard deviation of 0.5% indicates that the apparent porosity of mullite-, cordierite- and glass-bonded SiC supports are quite close, which is designed to compare the properties of in-situ bonded SiC supports with different bonding phases. The lower porosity of CBSC support can be ascribed to the accelerated oxidation of SiC in the presence of MgO-Al2O3-SiO2 system, which results in an increase of bonded phases. As mentioned earlier, the total content of bonded phase in CBSC support is ~11%, which is higher than the experimentally targeted bonded phase (8.3%) and explains the lower porosity of CBSC support. The GBSC support exhibited a slightly higher porosity, which can be ascribed to the lower degree of SiC oxidation. Table 3. Mechanical and physical properties of in-situ bonded SiC supports with different bonding phases. Name
Apparent porosity / %
Bulk density / g·cm-3
Ambient-temperature flexural strength / MPa
MBSC
36.8
1.93
22.5
CBSC
36.2
1.93
24.5
GBSC
37.2
1.92
33.6
Furthermore, GBSC support rendered the maximum ambient-temperature flexural strength of 33.6 MPa, whereas the flexural strength of MBSC and CBSC supports ranged from 22 to 25 MPa. Theoretically, the strength of mullite- and cordierite-bonded SiC supports should be higher than the glass-bonded SiC support due to the higher mechanical strength of mullite and cordierite. However, the solid-state sintering of mullite- and cordierite-bonded SiC supports results in incomplete sintering process, incomplete structure and porosity defects, leading to inferior flexural strength. On the other hand, glass-bonded SiC support possesses fewer defects in the neck region due to liquid phase sintering
and renders higher strength. 3.3. High-temperature mechanical properties Fig. 3 presents the HMOR of mullite-, cordierite- and glass-bonded SiC supports at 900 oC and 1200 oC. The HMOR of MBSC support, at 900 oC, was found to be 33.2 MPa, which is 147.5% higher than the ambient-temperature flexural strength. Moreover, the HMOR of CBSC support, at 900 oC, was found to be 27.5 MPa, which is close to the corresponding ambient-temperature flexural strength. However, the HMOR of GBSC support significantly decreased to 19.3 MPa, with the strength retention rate of 57.4%, which can be ascribed to the decrease in viscosity of glass phase. At 1200 oC, MBSC support exhibited maximum flexural strength of 10 MPa, whereas GBSC support showed a negligible strength value of only 0.2 MPa. The significant improvement in HMOR of mullite-bonded SiC support can be mainly attributed to mechanically enhanced properties of the mullite bonding phase at high temperature. In addition to mullite and cristobalite crystalline phases, there is also small amount of MgO-Al2O3-SiO2 glassy phase distributed in the mullite crystal boundary of the binding neck [29]. When temperature is increased to 900 oC, these grain-boundary glassy phases exhibit viscoelastic behavior, resulting in the relief of the crack-tip stress in the plastic deformation zone through viscoelastic relaxation [30-32]. Thus, the mullite-bonded SiC support was strengthened at 900 oC. However, when the temperature is further increased to 1200 oC, the decreased viscosity of the low-melting glass phase, at the grain boundaries, results in the slip of grain boundaries and, thereby, lowers the HMOR of the in-situ bonded SiC support. Therefore, the high-temperature bending strength of MBSC and CBSC supports is decreased at 1200 oC. In the case of glass-bonded SiC support, high-temperature softening of the bonding phase resulted in a decrease in strength and a negligible flexural strength of 0.2 MPa at 1200 oC. It can be seen that the mullite bonding phase imparted excellent high temperature mechanical properties to the SiC support, whereas the presence of the glass phase deteriorated the high temperature mechanical properties of the support. Thus, the content of glass phase should be minimized to improve the high-temperature mechanical behavior of in-situ bonded SiC support. 60 Ambient temperatrue 900oC
Flexural strength/MPa
50
1200oC
40 33.6
33.2
30 24.5
22.5
27.5 19.3
20 10 0
10.5
8.5 0.2
MBSC
CBSC
GBSC
Fig. 3. Flexural strength of mullite-, cordierite- and glass-bonded SiC supports at different temperatures.
3.4. Thermal shock resistance
Fig. 4 shows the strength retention rate of mullite-, cordierite- and glass-bonded SiC supports after 1 cycle of instantaneous heating-water quenching at different temperatures. It can be readily observed that the strength retention rate of each support decreased with the increase of quenching temperature. Under the same quenching temperature, the strength retention rate of MBSC support is slightly higher than CBSC support and much higher than GBSC support. The critical temperature difference is defined at a strength retention rate of 70%. And the critical temperature difference of MBSC, CBSC and GBSC supports can be determined to be ~570 oC, 500 oC and 370 oC, respectively, from Fig. 4. One should note that the critical temperature difference of mullite-bonded SiC support is 200 oC higher than the glass-bonded SiC support, indicating the superior thermal shock resistance of mullite-bonded SiC support.
Strength retention rate/%
120 MBSC CBSC GBSC
100 80
470oC
570oC
60 40 20 0
200
400
600
800
1000
o
Quenching temperature/ C
Fig. 4. Strength retention rate vs. water quenching temperature of 8 mm thick mullite-, cordierite- and glass-bonded SiC supports.
Fig. 5 shows the microstructure of mullite-, cordierite- and glass-bonded SiC supports after water quenching at 950 oC. It can be readily observed that the microcracks appeared in the neck region of each support and the number, width and length of cracks in MBSC and CBSC supports are much lower than GBSC support. Both mullite- and cordierite-bonded SiC supports rendered excellent thermal shock resistance due to the similar thermal expansion coefficients of mullite and cordierite to SiC, which resulted in low thermal stress at the bonding interface and less cracking. At the same time, the micropores are formed in both bonding phases during reaction sintering, which contribute to the stress relaxation at the crack tip and hinder the further propagation of crack in the bonding phases. Therefore, under the same thermal shock conditions, MBSC and CBSC supports exhibit lesser, shorter and narrower cracks, resulting in superior thermal shock resistance. On the other hand, the thermal physicochemical properties of glass phase are largely different from the SiC and its thermal expansion coefficient is significantly high, resulting in a large thermal stress inside the bonding phase and at the glass/SiC interface. Therefore, the cracks are easily initiated and rapidly propagated in the dense structure of glass phase, leading to poor thermal shock resistance GBSC support.
(a) MBSC
(b) CBSC
(c) GBSC
Fig. 5. SEM images of (a) mullite-, (b) cordierite- and (c) glass-bonded SiC supports after water quenching at 950 oC.
3.5. Oxidation resistance The weight gain of mullite-, cordierite- and glass-bonded SiC supports support after oxidation for 24 h at various temperatures is shown in Table 4. After continuous oxidation at 950 oC for 24 h, the MBSC and CBSC supports did not exhibit any significant weight gain, whereas GBSC support exhibited slight oxidation with a weight gain rate of 0.003 %. At 1050 oC, MBSC, CBSC and GBSC supports exhibited weight gain rate of 0.01 %, 0.023 % and 0.032 %, respectively. When the temperature was further increased to 1150 oC, MBSC, CBSC and GBSC supports exhibited significant oxidation with weight gain rate of 0.118 %, 0.204 % and 0.259 %, respectively, which is an order of magnitude higher than the oxidation at 1050 oC. Even though MBSC support is oxidized at >1000 oC, the weight gain rate is only 0.1 % after oxidation at 1150 oC, which is 54.4% and 42.2% lower than CBSC and GBSC supports, respectively. These results confirm that mullite-bonded SiC support possesses superior oxidation resistance. One should note that a dense mullite coating is formed on the surface of the SiC skeleton after sintering. Moreover, the structure of protective coating remained intact during high-temperature oxidation due to the excellent thermal stability of mullite, and the absence of stress-induced cracks because of matching thermal expansion coefficient with SiC at the abovementioned temperatures. The morphology of mullite-bonded SiC support after continuous oxidation at 1150 oC for 24 h is shown in Fig. 6a. A dense, smooth and crack-free protective layer can be readily observed on the SiC particles. Moreover, the diffusion coefficient of oxygen in mullite is extremely small (1.5×10-11 cm2/s) [33] and mullite renders stable performance at high temperature. Therefore, mullite-bonded SiC support exhibited excellent oxidation resistance at 1150 oC. Compared with MBSC support, CBSC and GBSC supports exhibited inferior oxidation resistance. Despite the fact that cordierite-bonded SiC support exhibits excellent thermal shock resistance, the porous structure of bonding phase, as shown in Fig.6b, resulted in a high diffusion coefficient of oxygen. Moreover, MgO-Al2O3-SiO2 system promoted the oxidation of SiC. Hence, cordierite-bonded SiC support rendered inferior oxidation resistance. In the case of glass-bonded SiC support, the surface coating on SiC particles is easy to soften and it melted at high temperature. Moreover, the volumetric changes during the crystallization process and crystal transformations generated cracks, as shown in Fig.6c, enhancing the penetration capacity of oxygen and resulting in low oxidation resistance of glassbonded SiC support.
Table 4. Weight gain rate of mullite-, cordierite- and glass-bonded SiC supports after oxidation at different temperatures for 24 h. Weight gain rate / % Samples 950 ℃
1050 ℃
1150 ℃
MBSC
0
0.01
0.118
CBSC
0
0.023
0.259
GBSC
0.003
0.032
0.204
(a) MBSC
(b) CBSC
(c) GBSC
Fig. 6. The interface between bonding phase and SiC particles in (a) mullite-, (b) cordierite- and (c) glass-bonded SiC supports after oxidation at 1150 oC for 24 h.
4. Conclusions In summary, we have successfully prepared mullite-, cordierite- and glass-bonded SiC supports with similar porosity (~37%) by controlling the volumetric ratio of bonding materials and studied the influence of bonding phases on ambient-temperature flexural strength, hot modulus of rupture, thermal shock resistance and oxidation resistance. The results revealed that the choice of bonding phase significantly influenced the ambient-temperature and high-temperature properties of SiC support. The mullite-bonded SiC support rendered superior high-temperature mechanical properties, thermal shock resistance and oxidation resistance, which can be attributed to the intrinsic characteristics of the mullite phase and its thermal and chemical compatibility with SiC. The combination of cordierite and SiC can also impart relatively superior ambient-temperature and high-temperature flexural strength, and desirable thermal shock resistance, however, the oxidation resistance of cordierite-bonded SiC support is compromised due to the presence of alkali metal ions. The glass-bonded SiC support exhibited maximum ambient-temperature flexural strength and a smooth pore structure due to the liquid phase sintering, however, the high-temperature flexural strength, thermal shock resistance and oxidation resistance cannot meet the practical requirements. In high-temperature applications, such as hot flue gas filtration, porous burners and molten metal filters, the mullite-bonded SiC support can render superior performance and extended service life.
Acknowledgments
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Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: