Effect of SiC Particle Size on Properties of SiC Porous Ceramics

XU Xiaohong, LIU Xing, WU Jianfeng, MA Sitong, LIU Shaoheng, CHEN Tiantian

(State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China)

Abstract: We used different SiC particle size as raw materials and via reaction bonding technique to prepare porous SiC membrane supports. The phase composition, microstructure, bending strength, open porosity, and pore size distribution were investigated as a function of SiC particle size and firing temperature.It is found that the reduction of SiC particle size not only dramatically enhances the bending strength of porous SiC membrane supports, but also slightly reduces the firing temperature duo to smaller SiC particle with higher specific surface area and higher reaction activity. Besides, the open porosity and pore size distribution are dependent on the firing temperature, but insensitive to the SiC particle size due to the pore related characters mainly controlled by the binder. The bending strength increases with the increasing of the firing temperature and with the decreasing of SiC particle size. When the firing temperature was 1 500 ℃ and SiC average particle size was 447.75 μm, the optimal performance were achieved, the bending strength was 15.18 MPa, the open porosity was 36.02 %, the pore size distributed at 3.09-112.47 μm, and the mean pore size was 14.16 μm.

Key words: membrane supports; SiC; particle size; bending strength

1 Introduction

Air is a key element in our daily life, however,as industry and economy developing, air pollution(mainly generated from coal-fired power station,household waste combustion,etc) has become more and more serious[1]. Ceramic membrane filters for air treatment is one of the most promising means for cleaning gas, especially for high temperature gas[2-5].Porous SiC ceramic membrane filters have attracted much attention due to their series unique properties such as excellent mechanical strength, good chemical resistance, low thermal expansion coefficient, high thermal conductivity, excellent thermochemical and thermomechanical stability,etc[6-10]. To get better performance, the structure of SiC ceramic membrane filters are usually designed as two or more parts:membrane supports (porous media), surface layer(membranes) and so on. Thereinto, the quality of SiC ceramic membrane filters is strongly dependent on the strength, porosity and microstructure (pore shape, pore size,etc) of the membrane supports. Hence, preparation of porous SiC membrane supports with well-designed microstructural characteristics is a key factor for producing high quality SiC membrane filters[11-14].

Recently, various processing methods, including recrystallization[15], partial sintering[16-18], and bonding techniques[19-23]etc, have been reported for producing SiC membrane supports. Due to the strong covalent of the Si-C bond, recrystallization and partial sintering techniques require higher firing temperature (normally higher than 1 600 ℃) if SiC powder is used as a starting material, which greatly limits the practical production and application of porous SiC ceramics[15-18]. Possible approaches to reduce the firing temperature are to use preceramic polymers as a precursor for SiC and the application of bonding materials[7,20]. Some of bonding materials reported in literatures include silica (SiO2)[21,24], cordierite[12,23,25], silicon oxycarbide (SiOC[26,27],glass frit[22,28], mullite (3Al2O3·2SiO2)[13,29-33]and so on. However, open-pore SiC supports produced by preceramic polymers showed lower mechanical strength compared to powder-produced SiC supports. In contrast,porous SiC supports prepared with bonding materials presented comparatively higher mechanical strength[7,20].Among the above bonding materials, mullite has attracted much attention because it has a close thermal expansion match and good chemical compatibility with SiC which ensures mullite bonded porous SiC ceramics with the excellent high-temperature strength and thermal shock resistance[13,29-34].

However, most of previous research on mullite bonded porous SiC ceramics mainly focused on (1)the effect of content of binder (e g, Bauxite[13,35],kaolin[30], clay[19],etc), (2) the effect of different aluminum source[16], (3) the effect of other additives,such as Y2O3[14,36,37], CeO2[38], V2O5[39]and so on. The effect of SiC particle size on the properties of mullite bonded porous SiC supports has not been investigated extensively, although SiC is the main raw materials for mullite bonded porous SiC supports. Only a few previous works were found in the literature and are summarized as follow:

(1) Li[40]et alresearched the effect of molding pressure, bonding phase contents, and SiC particle size on the bending strength of porous SiC-based ceramics.It was observed that the flexural strength increased with increasing of the molding pressure on the green body,and decreasing of the porosity. In addition, it was also reported that both porosity and mechanical strength increased with decreasing of SiC particle size.

(2) Baitalik[41]et alresearched the influence of SiC particle size on the bonding phase content,microstructure, SiC oxidation degree, flexural strength,porosity and pore size distribution of mullite bonded porous SiC ceramics. It was found that increasing of SiC particle size effectively enhanced the porosity and decreased the strength. The porosity decreased with the decreasing of SiC particle size from 36% at 99 μm to 25% at 4.47 μm. In addition, bimodal pore size distributions were obtained for mullite-bonded porous SiC ceramics and the average pore diameter varied in the range of 2-30 μm with different SiC particle sizes.

From above two studies, we can found that the effect of SiC particle size on porosity of mullite bonded porous SiC supports is indistinct. In the present work,porous SiC membrane supports were fabricated by SiC, kaolin and α-Al2O3at 1 400-1 550 ℃ for 2 h in air. Graphite and charcoal powder were used as poreformer. The effects of SiC particle size and firing temperature on the phase composition, microstructure,bending strength, porosity, and pore size distribution of porous SiC membrane supports were investigated.

2 Experimental

2.1 Starting materials

Commercially available α-SiC particles (purity＞99 %, Gongyi Mingyue Corundum Co., Henan,China) with different particle size (marked as SiC(1),SiC(2), SiC(3), respectively) were used as the starting materials, and their morphology pictures and particle size distribution curves were presented in Fig.1 and Fig.2, respectively. It could be seen that the major particle size distribution (about 80 %) of SiC(1),SiC(2), SiC(3) are 286.54 - 590.25 μm, 412.43 - 1 243.65 μm, 812.55 - 1 552.51 μm, respectively, and their average particle sizes were 447.75, 677.70,1 352.18 μm, respectively. Kaolin (250 mesh, XingZi County, Jiangxi, China) and α-Al2O3powders (250 mesh, Aluminum Industry, Shandong, China) were selected as the binder which generated mullite and vitrified substance to bond SiC particles at relatively lower temperature, and their chemical composition were listed in Table 1. Moreover, graphite powders(17.07 μm, Tsingtao Pingdu Fukang Graphite Co.,Shandong, China) and charcoal powders (14.33 μm,Dongguan Daang Charcoal Co., Guangdong, China)were employed as the pore-former.

Table 1 The chemical compositions of raw materials/wt%

2.2 Specimen preparation

Firstly, kaolin, α-Al2O3, graphite and chaocoal were mixed by ball milling in ethanol for 24 h to obtain homogeneous slurries, after being dried in a drying oven at 90 ℃ and sieving for 250 meshes. The mixed powder was used to surround the SiC particles which were covered with a small amount of polyvinyl alcohol(PVA, average polymerization degree of 1 750±50, 2.0 wt% concentration) as a temporary binder. Then, the final mixture was uniaxially pressed into the rectangular samples with dimensions of 6.5 mm×6.5 mm×30 mm under a 30 MPa pressure using a steel die. Table 2 listed the compositions of the samples, where the weight ratio of kaolin to α-Al2O3was to make the molar ratio of Al2O3to SiO2fixed to the value of 1.5, which was equal to the molar ratio of Al2O3to SiO2in the stoichiometric composition of mullite, and the weight ratio of graphite+ charcoal to SiC + kaolin + α-Al2O3was fixed as 0.30.The weight ratio of graphite to charcoal was 1. After that, the samples were pressureless heated to burn out graphite and chaocoal before 1 000 ℃ at a heating rate of 3 ℃/min, and then fired in air at 1 400-1 550 ℃ for 2 h with a heating rate of 5 ℃/min, and finally cooled in a seggar at molybdenum disilicide furnace (SX 19/16, Yingshan State-owned Experimental Equipment Factory, Hubei, China).

Table 2 Compositions of the samples used in this study

2.3 Assessment methods

Open porosity of the fired samples was determined by the Achimedes method using distilled water as the liquid medium (AUY120, Shimazu, Japan).Bending strength was measured by the three-point bending test ( Shenzhen Reger Instrument Co., Ltd.,Guangdong, China) with a 28 mm span and a crosshead speed of 1 mm/min. Five fired samples were measured to obtain the average bending strength value.The bending strength was calculated by Eq.(1):

whereFis the force at which fracture occurs (N),lis the length of span (28 mm),bis the width of the samples (mm),his the height of the samples (mm), andσis the bending strength (MPa).

Phase composition was detected by X-ray diffractometer (XRD, D/MAX-ⅢA; Rigaku Corporation, Japan) equipped with CuKα radiation(λ=1.54 Å), operated at 15 mA, 40 kV, and a step width of 0.02° with a scanning range of 10°-80°. The microstructure and morphology of the raw materials and macroporous SiC ceramics were observed by using scanning electron microscopy (SEM, JEOL Ltd., Japan). At least 200 pores on SEM micrographs were used to calculate the average pore size, and those distributions of porous samples were done by using“Image-Pro Plus 6.0” software.

3 Results and discussion

3.1 Effect of firing temperature on the phase composition and microstructure

Fig.3 shows the XRD patterns of A1 specimen fired at different temperatures for 2 h in air. It was found that moissanite (6H-SiC, 4H-SiC), mullite(3Al2O3· 2SiO2), quartz (SiO2) and cristobalite (SiO2)existed at all the firing temperatures, and mullite diffraction peak intensity changed slightly with increasing of firing temperature, which indicates the amounts of mullite crystalline phases almost unchange with firing temperature after 1 400 ℃. The mullitization has almostly been completed before 1 400 ℃.According to the previous studies[29-34], the raw materials greatly influence the reactions in the Al2O3-SiO2system and mullite is formed from SiO2and α-Al2O3around 1 400 ℃ via the solution-precipitation mechanism, and the mullite formation is diffusioncontrolled. Moreover, the undetection of α-Al2O3peaks at all the five firing temperature also illustrated that α-Al2O3in the raw material almost completely react with viscous SiO2glass to form mullite before 1 400 ℃.In addition, the peak intensity of cristobalite increase continuously as the temperature increased from 1 400 to 1 550 ℃ due to the significant oxidation of SiC. The peak intensity of SiC decreased dramatically.

Fig.4 presented a typical microstructure of A1 specimen fired at different temperatures for 2 h in air.It could be found that an increase in firing temperature led to a clear change in the microstructure of A1 specimen. When the firing temperature was 1 400℃, SiC particles were surrounded by binder formed by kaolin andα-Al2O3powders (Fig.4(a1)), and welldeveloped necks appeared between the kaolin or α-Al2O3powders. Furthermore, two types of pores with different pore size were observed in Fig.4(a2).It can be speculated that small pores are produced by the stacking of binder particles and large pores are developed by burning out pore former (graphite and charcoal powders) according to the particle size of kaolin, α-Al2O3, graphite and charcoal powders.Besides, it was strange that pores formed by the stacking of SiC particles were not observed, which was different with the results of other researchers[24,30].The most likely reason is that SiC particles used in this study are much larger than other researchers’, and the pores derived by SiC particles are almostly filled by binder because SiC particle size (D50= 447.75 μm) is far larger than binder particle size (less than 60 μm).With the firing temperature increasing to 1 450 ℃, as shown in Fig.4 (b1) and (b2), SiC particles morphology had no significant change, binder generated undesired densification, and pores among the binder were clustered together which formed channel with large pore size between SiC and relatively compact binder. With the firing temperature further increasing to 1 500 ℃, the channel between SiC and binder disappeared, as shown in Fig.4(c1), due to the decreasing of the viscosity of binder and wetting on the surface of SiC particles. The bond between SiC and binder can be improved, which enhance the mechanical strength of porous membrane supports. In addition, comparing to the binder presented in Fig.4(b2), the binder observed in Fig.4(c2) was uncompacted instead of further densification. A possible reason is that the oxidation of SiC increases with the increasing of the temperature and a lot of gaseous oxidation products (SiO, CO) exhaust. Meanwhile,the decreasing of the viscosity of binder make it easier for the gaseous oxidation products to escape which is in favour of the formation of connecting pores among binder and improves the permeation performance of porous SiC membrane supports to a certain extent. With further increasing of the firing temperature, especially when the temperature was 1 550 ℃, as shown in Fig.4(d1) - (e2), the binder was densified again because of the further decrease of viscosity. Besides, SiC particles were completely wrapped by binder and larger pores produced by the stacking of SiC particles. Furthermore,the connected pores existed in binder disappeared which may be unbeneficial to the permeation performance of porous SiC membrane supports. Hence, from above analysis, the optimum firing temperature of A1 sample is 1 500 ℃.

3.2 Effect of SiC particle size on the phase composition and microstructure

The XRD patterns of samples with different SiC particle sizes fired at 1 500 ℃ were shown in Fig.5.It could be clearly seen from Fig.5 that the phases of samples all include SiC, mullite, quarz and cristobalite,which states that SiC particle size has less effect on the phase composition. Moreover, corundum phase was undetected which can speculate that α-Al2O3in the raw material nearly completely react with viscous SiO2glass to form mullite or exist in the glass phase with ionic state. In addition, the peaks intensity of SiC, mullite and quarz were nearly unchanged with the increasing of SiC particle size. The peaks height referring to cristobalite decreasedversusSiC particle size. It can be explained that the oxidation of smaller SiC particle is faster due to the higher specific surface area and lower packing density. Similar results were also reported by Anchita Baitaliket al[41], who observed that the SiC oxidation degree decreased with increasing of SiC particle size. Sheet al[45]also observed an increase of silica content with decreasing of SiC particle size.

Fig.6(a1-c1) shows a typical fracture surface of samples with different SiC particle sizes fired at 1 500 ℃ for 2 h in air, and Fig.6(a2) - (c2) were the larger version of Fig.6(a1) - (c1) respectively. For all the three samples, morphologies with SiC, binder and pores were observed on the fracture surface. Through their morphologies, we can found that SiC particle size have great influence on the fracture behavior of samples. From Fig.6(a1), we can observe that there are three kinds of fracture areas,i e, binder phase fracture area, self-bonding phase fracture area and SiC transcrystalline fracture area. With increasing of SiC particle size, only two kinds of fracture areas can be observed,i e, binder phase fracture area and selfbonding phase fracture area. As seen from Fig.6(b1) and(b2), it can be explained that smaller SiC particles with higher specific surface area is easier oxidized, which promotes the bond of binder phase and SiC particles.In the meantime, smaller SiC particles are more prone to breakage, so samples with smaller SiC particles more easily emerge SiC transcrystalline fracture area.Furthermore, the morphology features of binder phase are affected by SiC particle size to some extent. As seen from Fig.6 (a2), (b2), (c2), the size of pores in binder phase decrease with increasing of SiC particle size.According to the analysis of the above content and the researches of predecessors, the smaller the SiC particle is, the higher the SiC oxidation degree is, and the more gaseous oxidation products are, which produced more pores and wider channels in the process of discharging gas.

3.3 Effect of SiC particle size and firing temperature on the mechanical strength and porosity

Bending strength and open porosity are two main properties of porous membrane supports. Fig.7 shows relationship between SiC particle size, firing temperature and bending strength of porous SiC membrane supports. It was interesting to found that SiC particle size not only had effect on the bending strength of samples, but also on the firing temperature.The bending strength decreased with increasing of SiC particle size from 425.3 to 1 352.2 μm when firing temperature was a constant value, and the decreasing of SiC particle size can reduce the firing temperature to a certain extent. Similar result was also reported by Junfeng Liet al[40], who observed that SiC particle size had a great influence on the bending strength of samples. The reason why the strength deceased with increasing of SiC particle size was the fraction of the SiC particle fracture point area in the minimum solid area of fracture surface with decreasing of SiC particle size. However, we have another viewpoint that samples with small SiC particle have higher strength are not only attributed to the more fracture area on the fracture surface of porous SiC membrane supports, but also to the higher oxidation degree of SiC with small particle size.For instance, when the temperature was 1 500 ℃,the bending strength of specimen A1, A2 and A3 were 6.4 MPa, 10.52 MPa, and 15.18 MPa, respectively.

In addition, the firing temperature have different effects on the bending strength of samples with different SiC particle size. When SiC particle size is 447.75 μm, the bending strength of samples increased slightly as the firing temperature rises from 1 400 to 1 450 ℃. However, it increased significantly with further increase in firing temperature to 1 500 ℃, and then the strength slowly increased again with the increase of firing temperature from 1 500 to 1 550 ℃. Based on the analysis of Fig.4, the bond of SiC particles and binder became tighter on account of the rapidly decrease of binder viscosity at 1 450 - 1 500 ℃. Besides, when the starting SiC particle size increased to 677.70 μm,the bending strength of samples also increased with increasing of firing temperature, but it was interesting to note that the firing temperature range of samples with strength rapidly increase was 1500 - 1520 ℃,which was higher than that of samples with smaller SiC particle. It could be speculated that SiC particle with smaller particle size is easier to be oxidized at lower firing temperature on account of higher specific surface area, and the oxidation of SiC particle has a positive effect on the bending strength of samples to a certain degree. As a further increase of SiC particle size to 1 352.18 μm, no obvious change of the bending strength was observed with the increase of firing temperature. The reason is that the bending strength of samples mainly depends on the strength of binder and the bonding tightness degree between binder and SiC particles, and the larger the SiC particles are, the harder to be oxidized, so the firing temperature has less effect on the oxidation of larger SiC particle relatively,and the bond between SiC particles and binder was almost not affected by firing temperature. In a word,comparing to the firing temperature, SiC particle size has more influence on the bending strength of porous SiC membrane supports.

Fig.8 shows the relationship between SiC particle size, firing temperature and open porosity of porous SiC membrane supports. It was clear to observe that the open porosity was more easily affected by firing temperature than SiC particle size,and the open porosity decreased with increasing of firing temperature. It can be explained that the liquid viscosity of binder decreases with increasing of firing temperature, and pores produced by burning out of pore formers are filled gradually, which directly leads to the decrease of open porosity. When SiC particle size was 447.75 μm, the open porosity decreased from 41.01%to 28.85% as firing temperature increased from 1 450 to 1 550 ℃. Nevertheless, there was no obvious change in open porosity with increasing of SiC particle size.The reason may be that, as shown in the Fig.4 and Fig.6 above, pores existing in samples are unrelated with SiC particles and the open porosity mainly depends on the content of pore former. When firing temperature was 1 500 ℃, the open porosity of sample A1, A2 and A3 were 36.02 %, 37.02 %, and 38.44 %, respectively.

In addition, according to the minimum solid area models proposed by Rice[43], the equation between bending strength and open porosity can be approximated as follows:

where σ0is the strength of nonporous structure (MPa),σ is the strength of corresponding porous structure with the same composition at open porosityp(%).Meanwhile,bis an empirical parameter which depends on the pore characteristics. On the basis of Eq.(1), the strength of porous material has an exponential increase with the decline of the open porosity. As shown in Fig.9, the relationship between bending strength and open porosity was in roughly accordance with Eq.(1).The bending strength of the sample composed of SiC and binder without open porosity, σ0, was obtained as 54.93 MPa, and the value ofbwas 3.784, with a correlation factorR2=0.898. Surprisingly, thebvalue was different with other researcher’s results. A value ofb= 4.4 was obtained by Sheet alfor mullite-bonded porous SiC ceramics with the open porosity of 28 vol%- 44 vol%. Yani Jinget al[30]obtained the value ofb=4.26 for the SiC/mullite composite porous ceramics.And the most likely explanation for this difference is that each sample has a different SiC particle size distribution and/or different firing temperature,resulting to the production of a different solid area for each sample irrespective of the material open porosity.Besides, on the basis of the research of Gary Paul kennedyet al[44]for porous self-bonded SiC ceramics,two other possible explanations for the difference could be the following: (1) incomplete data to represent a good relationship between open porosity and strength and (2) the open porosity range of the obtained data is too narrow. In a word, the values ofbchanged with the preparation process and the characteristics of raw material.

3.4 Open porosity size distribution

Fig.10 shows the pore size distribution of porous SiC membrane supports with different SiC particle size fired at 1 500 ℃ for 2 h in air. It was obvious to find that there was less difference in pore distribution of porous SiC membrane supports with different SiC particle size, and the pore size distribution were mainly about 1.03-85.32 μm, and a few pore size of A1 sample distributes at 97.96 - 112.47 μm. Interestingly, the mean pore sizes of A series samples slightly decreased with increasing of SiC particle size, and A1 specimen had a bimodal pore size distribution. Nevertheless, A3 specimen present a unimodal distribution. According to the above analysis of SEM (Fig.6), the A series samples have different types of pores, and the pores of A1 sample were mainly produced by stacking of SiC particles, burning out of pore former and the gaseous products emission during SiC oxidation. However, that of A3 sample were mainly produced by burning out of pore forming materials due to the bigger SiC particle size, which may be the reason for the slightly difference of mean pore size between A1 and A3 sample. In short,the pore size distribution of the samples is insensitive to the SiC particle size.

Fig.11 shows the pore size distribution of A1 specimen fired at different temperatures for 2 h in air.It can be obviously observed that, comparing to SiC particle size, firing temperature had a signally impact on pore size distribution of porous SiC membrane supports. When the firing temperature was 1 400℃, the pore size of A1 sample presented bimodal distribution, the pore size distribution was 0.68 - 74.31 μm, and the corresponding position of two peaks were about 3.56 and 21.43 μm respectively, which illustrate that there were two kinds of pores in A1 sample. On the basis of the analysis of SEM (Fig.4), smaller pores were generated by stacking of binder powders and bigger pores were produced by burning out of pore forming materials. With increasing of temperature to 1 500 ℃, the sample showed unimodal pore size distribution which was 1.55 - 129.13 μm, and the mean pore size was 10.74 μm. It can be explained that the pores produced by stacking of binder powders and burning out of pore former were filled by binder due to the decline of liquid phase viscosity. With the further increasing of firing temperature to 1 520 ℃, the unimodal pore size distribution became narrow, and the mean pore size tinily declined from 14.15 to 9.36 μm on account of the further decrease of the binder viscosity. Furthermore, it was interesting to found that the sample presented bimodal pore size distribution again when the temperature was 1 550 ℃, and the reasons for this were explained earlier in the discussion.The size of closed pore existing in binder was mainly distributed at 0.25-10.74 μm and the mean pore size is 2.35 μm. Moreover, the size of pore produced by stacking of SiC particles was distributed at 42.76-85.32 μm.

4 Conclusions

Porous SiC membrane supports were successfully produced by a reaction bonding method using different SiC particle size. The effects of SiC particle size and firing temperature on the phase composition,microstructure, bending strength, open porosity, and pore size distribution have been investigated:

a) The firing temperature and SiC particle size had little impact on the phase composition of porous SiC membrane supports, and the final phase composition of fired products were determined to SiC, mullite,cristobalite and quartz. The phase content of cristobalite increased with increasing of firing temperature and decreasing of SiC particle size.

b) The reduction of SiC particle size not only dramatically enhanced the bending strength but also somewhat reduced the firing temperature of porous SiC membrane supports due to the increasing of specific surface area. The bending strength was effected by the firing temperature when the SiC particle size was small,but the effect was limited when the SiC particle size was large.

c) The open porosity of porous SiC membrane supports, chiefly produced by burning out of poreformer and stacking of binder powders, were mainly dependent on the firing temperature and insensitive to the SiC particle size. The open porosity decreased with increasing of firing temperature.

d) The pore size distribution of porous SiC membrane supports was virtually impervious to SiC particle size, but the mean pore size slight decreased as the SiC particle size increasing. Besides, the pore size began with a bimodal distribution to a unimodal distribution and to a bimodal distribution again as the increasing of firing temperature.