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Effect of SiC nanoparticles on in-situ synthesis of SiC whiskers in corundum–mullite–SiC composites obtained by carbothermal reduction
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Effect of SiC nanoparticles on in-situ synthesis of SiC whiskers in corundum–mullite–SiC composites obtained by carbothermal reduction Xinbin Laoa, Xiaoyang Xub,∗, Weihui Jianga,∗∗, Jian Lianga, Lifeng Miaoa, Zhenhong Baoa a b
National Engineering Research Center for Domestic and Building Ceramics, Jingdezhen Ceramic Institute, Jingdezhen, 333000, China Ceramic Intellectual Property Information Center, Jingdezhen Ceramic Institute, Jingdezhen, 333000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbothermal reduction SiC nanoparticles Composites SiC whiskers
Corundum–mullite–SiC composites were synthesised using a carbothermal reduction method. The effects of SiC nanoparticles and sintering temperatures on the phase transformation of the composites and the synthesis of SiC whiskers were studied by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Results indicated that corundum, mullite, and SiC whiskers were produced as final products at 1600–1650 °C. SiC whiskers were formed through the vapor–solid mechanism. The added SiC nanoparticles worked as nucleating agents to facilitate the carbothermal reduction of aluminosilicates and formation of SiC whiskers. The sample with the added SiC nanoparticles exhibited a high yield of β-SiC of 17.1%. Furthermore, the SiC nanoparticles decreased the formation temperature of SiC whiskers from the original 1600 °C to 1500 °C, and the porosity of the composites was increased from 56.7% to 64.7% by increasing the partial pressure of SiO gas. This study provides an insight into the more efficient synthesis of composites with SiC whiskers through the carbothermal reduction of aluminosilicates.
1. Introduction Al2O3, mullite, and their composites have high-temperature structural applications because of their good chemical stability [1,2], high hardness [3], and high strength [4,5]. However, these oxide ceramics have some disadvantages such as low thermal conductivity [6], low fracture toughness [7], and weak thermal shock resistance [8], which limit their applications in critical thermal cycling conditions. Fibrous SiC whiskers, which are characterised by high thermal conductivity, high elasticity modulus, etc., can compensate for the disadvantages of corundum or mullite ceramics. Therefore, research [9–13] on incorporating SiC whiskers into ceramic matrices have been conducted, and the fracture toughness was found to be improved significantly. SiC whiskers are also ideal materials for reinforcing metal [14–16] and other ceramic–matrix composites [17–19]. However, SiC whiskers should be prepared separately before their introduction into ceramic matrices; several methods have been developed to synthesise SiC whiskers, e.g., carbothermal reduction of silica under Ar, N2, or H2 atmosphere [20–24], thermal decomposition of silane compounds [25–27], chemical vapor deposition (CVD) [28–30], and arc discharge [31]. Because of its simplicity and economic efficiency, carbothermal reduction of silica is considered the most acceptable method. The
∗
synthesis mechanisms of SiC whiskers or particles by carbothermal reduction were also revealed in many other research studies [32–35]. Nevertheless, to further prepare monolithic ceramic materials, mixing of the separately prepared SiC whiskers and Al2O3 or mullite powder by conventional powder processing methods is inevitable, thus resulting in the complex production of whisker-reinforced composites. To simplify the processing route for preparing whisker-reinforced composites, the concept of in-situ synthesis of SiC whiskers by carbothermal reduction of natural aluminosilicates has been put forward, as it can give a homogeneous mix of SiC whiskers, Al2O3, and mullite as final products. In the past few decades, notable attempts were made towards the carbothermal reduction of different aluminosilicates, including andalusite [36], kyanite [37], and kaolin [38,39]. Solid wastes, such as waste glass [40], coal gangue [41], kyanite tailing [42], and rice husks [43,44], were also employed as raw materials for reducing manufacturing costs. According to the above research, Ar, N2, or H2 protections and even microwave assistance are needed for the in-situ synthesis of SiC whiskers and some specific devices, such as atmospheric sintering or microwave sintering furnaces, are indispensable for industrial production. Therefore, it is necessary to develop a much simpler method that can be generalised to common sintering devices. Coal-series kaolin, a by-product of seam deposition in coal basins, is
Corresponding author. Corresponding author. E-mail address: [email protected] (X. Xu).
∗∗
https://doi.org/10.1016/j.ceramint.2019.12.175 Received 21 November 2019; Received in revised form 13 December 2019; Accepted 21 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xinbin Lao, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.175
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regarded as a solid waste like coal gangue for decades [45]. However, it should be noted that coal gangue is known as an impurity included in coal minerals, whereas coal-series kaolin in China generally exists as a thick and easy-to-exploit mineral deposit, which is well separated from the coal bed [46,47]. Moreover, coal-series kaolin is detected to have larger reserves (1.67 billion tons) than the soft clay in China, though the chemical characteristics of coal-series kaolin are quite similar to those of coal gangue [48]. Nowadays, coal-series kaolin of high purity is extensively recognised as a valuable non-metallic mineral because of the gradual exhaustion of high-quality clay resources. Hence considerable research on the utilisation of coal-series kaolin has been conducted. For instance, Wang et al. fabricated porous mullite foams from coal-series kaolin and Al2O3 slurry by using a freeze-casting method [49]. Zhao et al. used coal-series kaolin as raw materials for synthesis of cordierite [50]. Liu et al. conducted research on the mineralogy and flotation of coal-series kaolin [51]. Previously, the microstructural evolution and phase transformation of coal-series kaolin during firing [52] and synthesised honeycomb materials for thermal storage by aluminium-assisted carbothermal reduction using coal-series kaolin as a raw material [53] have been studied. These research studies revealed the value of coal-series kaolin as a raw material for ceramic production. Meanwhile, research on in-situ synthesis of SiC whiskercontaining powder by carbothermal reduction of coal-series kaolin has not been reported. In this study, corundum–mullite–SiC whisker composites were synthesised by carbothermal reduction using coal-series kaolin as a raw material. SiC nanowhiskers were in-situ synthesised under a CO-rich atmosphere formed by a carbon-buried method, which can be simplified and easily generalised to conventional furnaces without any protections of inert gases (Ar, H2, and N2). The phase transformation, microstructure evolution, and porosity of coal-series kaolin and carbon black (i.e., used as the reducing agent) powder compact during sintering were studied. The effect of nano-SiC additive on the carbothermal reduction rate of coal-series kaolin was investigated, and the growth mechanism of SiC whiskers under the CO-rich atmosphere was obtained.
Table 2 Formula compositions of CC1 and CC2 (wt%). Coal-series kaolin
Carbon black
Nano-SiC particles (in addition)
CC1 CC2
77.28 77.28
22.72 22.72
3 0
2.2. Preparation Coal-series kaolin powder under 250 mesh and carbon black were mixed for 2 h by a ball-milling method. The obtained powder mix was then pressed to form powder compacts with a 5 wt% polyvinyl alcohol solution by using a 769 YP-30 T type tablet machine. The pressing process was aimed at obtaining a closer contact between kaolin and carbon black, which could accelerate the carbothermal reduction reaction rate. The powder compacts were dried in an electric oven held at 110 °C for 24 h. After drying, powder compacts were pressurelessly heat-treated by using a carbon-buried method in a silicon molybdenum furnace. A schematic of the carbon-buried method is presented in Ref. [54]. The heating rate and holding time at the maximum temperatures were set as 5 °C/min and 2 h, respectively. 2.3. Characterisation In our previous study [55], SiO gas, the intermediate phase for producing SiC whiskers, had a significant effect on the microstructure of the materials, and the porosity increased with the increase in the partial pressure of SiO gas, providing an indirect way to characterise the partial pressure of SiO gas. The water absorption, porosity, and bulk density of the powder compacts were measured according to Archimedes principle by using AUY220-type static electronic balance (Shimadzu Co., Ltd., Japan). X-ray diffraction (XRD) analyses were conducted by a D8 Advance type diffractometer (Bruker Co., Ltd., Germany) using Ni-filtered Cu Kα radiation at a scanning rate of 2°/min to study the phase transformation of powder compacts heat-treated at different temperatures. The relative contents of crystalline phases were calculated semi-quantitatively using the reference intensity ratio (RIR) method [56]. The morphologies of powder compacts heat-treated at different temperatures and the formed SiC whiskers were recorded by SU-8010 type scanning electron microscopy (SEM, Hitachi Co., Ltd., Japan) at a vacuum of 10−5 Pa using an accelerate voltage of 15 kV. The structural characteristics of the SiC whiskers were further studied by high-resolution transmission electron microscopy (HRTEM, JEM 2010, JEOL, Ltd., Japan) at a vacuum of 10−5 Pa.
2. Experimental 2.1. Raw materials Coal-series kaolin of large availability exploited in Shanxi Province, China, was selected as silica (SiO2) and alumina (Al2O3) resources, whilst carbon black (25 nm, Evonik Degussa Co., Ltd., Germany) was employed as a reducing agent. The chemical composition of coal-series kaolin is listed in Table 1. The morphology and mineralogical phase composition had been reported in a previous study [52]. To obtain a higher SiC yield, the content of carbon black was determined to be a factor of 3 greater than the SiO2 content (in mol%) of coal-series kaolin according to the reaction SiO2 + 3C → SiC + 2CO(g)
Formula No.
3. Results and discussion 3.1. Effect of SiC nanoparticles and temperature on phase transformation XRD patterns of samples CC1 and CC2 after heat-treatment at 1400–1650 °C are shown in Fig. 1(a) and (b), respectively; the relative contents of crystalline phases are listed in Table 1. It is worth noting that carbon black is an amorphous carbon material, thus its diffraction peaks cannot be detected [36]. As seen in Fig. 1(a), the detected phases in the XRD patterns of sample CC1 heat-treated at 1400–1500 °C are mullite, cristobalite, and β-SiC. At 1400 and 1450 °C, the diffraction peaks of β-SiC should belong to the introduced SiC nanoparticles. The semi-quantitatively calculated
(1)
As shown in Table 1, the SiO2 content was as high as 48.99 wt%, which was calculated to be 0.81 mol. Therefore, the mass ratio of carbon black should be 29.39 wt%, which became 22.72% in the sum of kaolin and itself (i.e., 100 wt% kaolin plus 29.39 wt% carbon black), as shown in Table 2. In Table 2, only formula CC1 had 3 wt% SiC nanoparticles (99.99% purity, 40 nm) added as nucleating agents for SiC whiskers under a CO-rich atmosphere. Table 1 Chemical compositions of coal-series kaolin (wt%). Oxides
SiO2
Al2O3
TiO2
Fe2O3
CaO
MgO
K2 O
Na2O
Ignition loss
Coal-series kaolin
48.99
35.46
0.4
0.12
0.04
0
0.03
0
14.72
2
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Fig. 1. XRD patterns of samples: (a) CC1 and (b) CC2.
contents of β-SiC (Table 1) are approximate to the added content (i.e., 3 wt%) with small deviations. As shown in Table 1, the carbothermal reduction of coal-series kaolin begins at a lower temperature of 1500 °C, at which the content of β-SiC is improved to 5.3 wt%. After heattreatment at 1550 °C, the intensive peak of cristobalite at 20°–22.5° disappears, indicating its transformation to amorphous silica [52]. Through the reaction described in Eq. (1), partial cristobalite should also be reduced to generate β-SiC, whose content has been increased to 9.5 wt% accordingly. The carbothermal reduction reaction of the mullite phase begins after the complete reduction of free silica (in an amorphous state or cristobalite) at > 1600 °C. This phenomenon corresponds well with Amroune's research [36], which was conducted in an argon atmosphere. When the heat-treatment temperature increases to 1600–1650 °C, mullite is reduced to form corundum (α-Al2O3) and βSiC as final products. At 1650 °C, the highest β-SiC content of 20.1 wt% is obtained for sample CC1, indicating that the carbothermal reduction is accelerated at a higher temperature. As shown in Fig. 1(b), for sample CC2 with no addition of SiC nanoparticles, the intensive peak of cristobalite is not detected until the heat-treatment temperature reaches 1450 °C. Such difference in the formation temperatures of cristobalite formed in CC1 and CC2 can be attributed to the addition of SiC nanoparticles, which also work as nucleating agents in liquid for the crystallisation of cristobalite, whereas cristobalite also transforms into amorphous silica when sample CC2 is heat-treated at 1550 °C. In contrast to sample CC1 heat-treated at 1550 °C, no diffraction peaks of β-SiC are detected in sample CC2. Moreover, only when heat-treatment is in the range of 1600–1650 °C can β-SiC be detected, indicating that the added SiC nanoparticles in sample CC1 not only promote the crystallisation of cristobalite but also decrease the initial synthesis temperature of β-SiC to 1500 °C, which is much lower than that of sample CC2 with a large deviation of 100 °C. It can be concluded that the added SiC nanoparticles increase the reaction rate of carbothermal reduction, because the contents of β-SiC and
corundum in sample CC1 are higher than those of sample CC2. Several mechanisms might be responsible for the remarkable effects of SiC nanoparticles on the synthesis temperature and the reaction rate as follows: SiC nanoparticles (β-type) provide growth interfaces for the deposition of SiO gas and thus lower the energy needed for formation of β-SiC, resulting in the lower synthesis temperature. The efficient deposition of SiO gas also lowers its partial pressure and accelerates carbothermal reduction reactions (see Eq. (2) and Eq. (3)) of free silica and mullite to guarantee the continuous supply of SiO gas. SiO2(free silica) + C → SiO(g) + CO(g)
(2)
3Al2O3·2SiO2 + 2C = 3Al2O3 + 2SiO(g) + 2CO(g)
(3)
3.2. Microstructure evolution The microstructure evolution of powder compacts of samples CC1 and CC2 was characterised by SEM, as shown in Figs. 2 and 3, respectively. It is observed in Fig. 2(a) and (b) that the powder compact of formula CC1 heat-treated at 1400 °C exhibits a loose microstructure, and no columnar mullite crystals are observed on aggregates of coal-series kaolin and carbon black. As the heat-treatment temperature increases to 1500 °C, Fig. 2(c) and (d) reveal that numerous columnar mullite crystals are observed on the surface of aggregates, and some twisted SiC whiskers with a much lower aspect ratio (length/diameter) are formed on the surfaces of aggregates of nanoparticles with a diameter of ~40 nm, which is approximate to the particle size of the added SiC. Therefore, these aggregates should be mainly composed of SiC nanoparticles that are dispersed inhomogeneously. The close connection between SiC nanowhiskers and SiC nanoparticles demonstrates that the nanoparticles work as nucleating agents to provide interfaces for whisker growth, which can also be verified by TEM. However, because of the relatively low heat-treatment temperature and the resultant low 3
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Fig. 2. SEM images of fractured surfaces of samples CC1 sintered at 1400–1650 °C: (a)&(b)-1400 °C. (c)&(d)-1500 °C, (e)&(f)-1650 °C.
effective way to produce SiC nanowhiskers. As shown in Fig. 3(e) and (f), numerous straight or twisted SiC whiskers decorated with numerous beads can be obtained at 1650 °C. This phenomenon implies that the added SiC nanoparticles have no influence on the morphology of the obtained SiC whiskers, though they have immense impact on the amount of SiC whiskers.
partial pressure of SiO gas, the growth kinetics of SiC whiskers cannot be maintained at a high level to obtain a high aspect ratio and a straight morphology. As seen in Fig. 2(c), by heat-treating at a higher temperature of 1650 °C, numerous straight SiC nanowhiskers with a higher aspect ratio are formed and the microstructure composed of SiC nanowhiskers, corundum, and mullite crystals becomes much looser. Apparently, the high heat-treatment temperature improves the SiO partial pressure effectively to endow the whiskers with sufficient growth kinetics, accelerating the reactions for synthesis of SiC whiskers (as described in Eqs. (1) and (4)). Numerous hemispherical beads are also found on the surfaces of the straight whiskers. According to Li's study [57], these beads were formed around the stacking defects inserted in {111} planes, because the energy needed for the deposition of SiO gas around the defects was much lower than along the [111] direction. SiO(g) + 3CO(g) → SiC + 2CO2(g)
3.3. TEM analyses TEM was employed to further study the morphology of the formed SiC whiskers, as presented in Figs. 4 and 5. As shown in Fig. 4(a), which is a high-magnification image of a SiC whisker decorated with hemispherical beads, these hemispherical beads may be the result of depositions of SiO gas, which are amorphous and have not been reduced by CO gas to produce β-SiC. Therefore, there is a quite clear boundary between the well-grown SiC whiskers and the amorphous beads (see the inset of Fig. 4(a), a HRTEM image of a hemispherical bead). Numerous streaks arranged perpendicular to the growth direction can also be illustrated by TEM, as seen in the area highlighted by the yellow circle in Fig. 4(a), whilst a small bead is formed around the streaks. This specific morphology demonstrates the deposition of SiO gas around the stacking faults, because these streaks represent the stacking-fault planes perpendicular to the growth direction [58]. Fig. 4(b) shows that diameters
(4)
For sample CC2, it is observed in Fig. 3(a)–3(d) that the morphology of powder compacts heat-treated at 1400 and 1500 °C is similar to that of sample CC1. However, no SiC nanowhiskers are formed, demonstrating that under a CO-rich atmosphere, SiC nanoparticles are necessary for the synthesis of SiC whiskers at a relatively low temperature of 1500 °C. Increasing the heat-treatment temperature is also an 4
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Fig. 3. SEM images of fractured surfaces of samples CC2 sintered at 1400–1650 °C: (a)&(b-1400 °C. (c)&(d)-1500 °C, (e)&(f)-1650 °C.
nanoparticles and the well-developed whiskers. As can be seen, the connection part exhibits a good periodic lattice structure, and the spacing between lattice fringes is measured to be 0.25 nm, which is consistent with the data reported in Ref. [57].
of the formed SiC whiskers are in the nanoscale range of 9–21 nm. Furthermore, it is observed in areas marked with the square and circle that some SiC whiskers grow with SiC nanoparticles on their tips. Fig. 4(c) shows a HRTEM image of the connection part between SiC
Fig. 4. TEM figures of SiC whiskers in sample CC1 sintered at 1650 °C. (c) is the magnification of the square in (b). 5
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Fig. 5. TEM figures of SiC whiskers in sample CC2 sintered at 1650 °C. (c) is the magnification of the square in (b).
carbothermal reduction reaction on improving the porosity of SiC whisker-containing ceramics by increasing the partial pressure of SiO gas. Therefore, the physical properties of samples CC1 and CC2 were tested to study variations in the partial pressure of SiO gas during heattreatment, which also provided additional evidence for the initial temperature of the carbothermal reduction of coal-series kaolin. Fig. 7 shows the physical properties of samples CC1 and CC2 as functions of heat-treatment temperatures. As shown in Fig. 7, the water absorption and porosity of samples CC1 and CC2 in the temperature range of 1400–1500 °C only exhibit slight increasing tendencies with temperature, whilst sample CC1 possesses higher water absorption and porosity than sample CC2. This phenomenon indicates that sample CC1 obtains a higher partial pressure of SiO gas to loosen the microstructure, thus generating more voids by promoting the carbothermal reduction of free silica with the help of SiC nanoparticles. Abrupt increases can be observed in the water absorption and porosity curves of sample CC1 at 1550–1650 °C, indicating the extensive carbothermal reduction and the generation of SiO gas. The much looser microstructure provides more space for the growth of SiC whiskers in turn, thereby increasing the content of SiC whiskers. The semi-quantitative data collected in Table 3 also show that β-SiC content in sample CC1 increases dramatically at 1550–1650 °C, corresponding well with the variations in porosity or water absorption of sample CC1. Although the water absorption and porosity values of sample CC2 improve remarkably in the same temperature range because of the carbothermal reduction of silica and the increased partial pressure of SiO gas, the generation of β-SiC cannot be verified by XRD or SEM. This result indicates that, without the addition of SiC nanoparticles, the formation of β-SiC nuclei in sample CC2 needs more energy, and only by increasing the heat-treatment temperature to 1600 °C can β-SiC be produced.
Fig. 5 shows TEM images of SiC whiskers formed in sample CC2 heat-treated at 1650 °C. The diameters of SiC whiskers in sample CC2 are not consistent and exhibit a range of 10 to 27 nm. Fig. 5(a) reveals that a SiC whisker with a diameter of 20 nm also exhibits numerous streaks perpendicular to the growth direction, indicating that many stacking faults are inserted in the whisker. Pickard et al. [58] had demonstrated that SiC whiskers with stacking faults had a lower formation energy than those without stacking faults. Because SiC whiskers with the specific morphology can be observed in both CC1 and CC2 and the space between two adjacent lattice fringes of whiskers is consistently 0.25 nm, the main difference in morphology of SiC whiskers formed in samples CC2 and CC1 should be the absence of SiC nanoparticles at the tips in sample CC2. The vapor–solid mechanism can be drawn for the growth of SiC whiskers in samples CC1 and CC2 according to SEM and TEM observations, because no liquid globules are observed on the tips of the asprepared SiC whiskers [36]. However, the growth processes of SiC whiskers in samples with and without addition of SiC nanoparticles are quite different. As indicated in Fig. 6, the schematic of whisker growth processes, the upper route belonging to sample CC1 shows that SiO gas generated from reactions (2) and (3) deposits on the added SiC nanoparticles to react with CO gas, resulting in the whisker growth directly without the SiC nucleation process. For sample CC2 with no nano SiC addition, the SiC nucleation process occurs prior to whisker growth and thus more energy and higher heat-treatment temperatures are demanded for producing SiC whiskers in the lower route.
3.4. Variations in physical properties of the pressed samples Previous research by our group [54] demonstrated the effects of the
Fig. 6. Schematic presentation of different growth process in sample with and with no nano-SiC addition. 6
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Fig. 7. Physical properties of CC1 and CC2 as the functions of sintering temperature: (a) water absorption, (b) open porosity, (c) bulk density.
Declaration of competing interest
Table 3 Semi-quantitative analysis of crystalline phases in samples CC1 and CC2 sintered at different temperatures. Formula No.
Temperature/°C
Mullite
Corundum
β-SiC
SiO2
CC1
1400 1450 1500 1550 1600 1650 1400 1450 1500 1550 1600 1650
81.1 84.8 92.2 90.5 43.5 24.8 100 83.1 63.3 100 62.1 58.3
– – – – 39.9 55.1 – – – – 26.6 27.4
2.9 2.6 5.3 9.5 16.6 20.1 – – – – 11.3 14.3
16 12.6 2.5 –
CC2
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. Acknowledgements We are grateful for the financial support of the National Natural Science Foundation of China (Grant Nos. 51702141 and 51962012) program ‘Doctoral Research Startup Program of Jingdezhen Ceramic Institute’.
– 16.9 36.7 –
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4. Conclusions Corundum–mullite–SiC whisker composites were obtained by the carbothermal reduction of coal-series kaolin. The effects of the added SiC nanoparticles and temperatures on the phase transformation, morphology of SiC whiskers, and physical properties were studied. Several conclusions can be drawn: 1. XRD analyses revealed that corundum, mullite, and β-SiC as final products were obtained by the carbothermal reduction of coal-series kaolin at 1650 °C. With the help of the added SiC nanoparticles, the formation temperature of the β-SiC phase could be decreased to 1500 °C, which was much lower than the 1600 °C for the sample with no addition of SiC nanoparticles. SiC nanoparticles not only worked as nucleating agents to deposit SiO gas, promoting the carbothermal reduction and production of the β-SiC phase, but also lowered the formation temperature of cristobalite. 2. SEM analyses showed that, at 1500 °C, SiC whiskers formed on the surfaces of the added SiC nanoparticles and exhibited a low specific ratio because of the low partial pressure of SiO gas. Straight and twisted SiC whiskers with a high specific ratio could be obtained when heat-treating at 1650 °C and partial straight SiC whiskers with stacking faults were decorated with numerous amorphous beads. The vapor–solid mechanism was responsible for the growth of SiC whiskers. 3. The number of pores in powder compacts increased dramatically by the extensive carbothermal reduction at 1550–1650 °C. The porous structure could provide space for the growth of SiC whiskers. The added SiC nanoparticles improved porosity and lowered the bulk density by accelerating the carbothermal reduction reaction and increasing the partial pressure of SiO gas.
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Effect of SiC nanoparticles on in-situ synthesis of SiC whiskers in corundum–mullite–SiC composites obtained by carbothermal reduction Xinbin Laoa, Xiaoyang Xub,∗, Weihui Jianga,∗∗, Jian Lianga, Lifeng Miaoa, Zhenhong Baoa a b
National Engineering Research Center for Domestic and Building Ceramics, Jingdezhen Ceramic Institute, Jingdezhen, 333000, China Ceramic Intellectual Property Information Center, Jingdezhen Ceramic Institute, Jingdezhen, 333000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbothermal reduction SiC nanoparticles Composites SiC whiskers
Corundum–mullite–SiC composites were synthesised using a carbothermal reduction method. The effects of SiC nanoparticles and sintering temperatures on the phase transformation of the composites and the synthesis of SiC whiskers were studied by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Results indicated that corundum, mullite, and SiC whiskers were produced as final products at 1600–1650 °C. SiC whiskers were formed through the vapor–solid mechanism. The added SiC nanoparticles worked as nucleating agents to facilitate the carbothermal reduction of aluminosilicates and formation of SiC whiskers. The sample with the added SiC nanoparticles exhibited a high yield of β-SiC of 17.1%. Furthermore, the SiC nanoparticles decreased the formation temperature of SiC whiskers from the original 1600 °C to 1500 °C, and the porosity of the composites was increased from 56.7% to 64.7% by increasing the partial pressure of SiO gas. This study provides an insight into the more efficient synthesis of composites with SiC whiskers through the carbothermal reduction of aluminosilicates.
1. Introduction Al2O3, mullite, and their composites have high-temperature structural applications because of their good chemical stability [1,2], high hardness [3], and high strength [4,5]. However, these oxide ceramics have some disadvantages such as low thermal conductivity [6], low fracture toughness [7], and weak thermal shock resistance [8], which limit their applications in critical thermal cycling conditions. Fibrous SiC whiskers, which are characterised by high thermal conductivity, high elasticity modulus, etc., can compensate for the disadvantages of corundum or mullite ceramics. Therefore, research [9–13] on incorporating SiC whiskers into ceramic matrices have been conducted, and the fracture toughness was found to be improved significantly. SiC whiskers are also ideal materials for reinforcing metal [14–16] and other ceramic–matrix composites [17–19]. However, SiC whiskers should be prepared separately before their introduction into ceramic matrices; several methods have been developed to synthesise SiC whiskers, e.g., carbothermal reduction of silica under Ar, N2, or H2 atmosphere [20–24], thermal decomposition of silane compounds [25–27], chemical vapor deposition (CVD) [28–30], and arc discharge [31]. Because of its simplicity and economic efficiency, carbothermal reduction of silica is considered the most acceptable method. The
∗
synthesis mechanisms of SiC whiskers or particles by carbothermal reduction were also revealed in many other research studies [32–35]. Nevertheless, to further prepare monolithic ceramic materials, mixing of the separately prepared SiC whiskers and Al2O3 or mullite powder by conventional powder processing methods is inevitable, thus resulting in the complex production of whisker-reinforced composites. To simplify the processing route for preparing whisker-reinforced composites, the concept of in-situ synthesis of SiC whiskers by carbothermal reduction of natural aluminosilicates has been put forward, as it can give a homogeneous mix of SiC whiskers, Al2O3, and mullite as final products. In the past few decades, notable attempts were made towards the carbothermal reduction of different aluminosilicates, including andalusite [36], kyanite [37], and kaolin [38,39]. Solid wastes, such as waste glass [40], coal gangue [41], kyanite tailing [42], and rice husks [43,44], were also employed as raw materials for reducing manufacturing costs. According to the above research, Ar, N2, or H2 protections and even microwave assistance are needed for the in-situ synthesis of SiC whiskers and some specific devices, such as atmospheric sintering or microwave sintering furnaces, are indispensable for industrial production. Therefore, it is necessary to develop a much simpler method that can be generalised to common sintering devices. Coal-series kaolin, a by-product of seam deposition in coal basins, is
Corresponding author. Corresponding author. E-mail address: [email protected] (X. Xu).
∗∗
https://doi.org/10.1016/j.ceramint.2019.12.175 Received 21 November 2019; Received in revised form 13 December 2019; Accepted 21 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Xinbin Lao, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.175
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regarded as a solid waste like coal gangue for decades [45]. However, it should be noted that coal gangue is known as an impurity included in coal minerals, whereas coal-series kaolin in China generally exists as a thick and easy-to-exploit mineral deposit, which is well separated from the coal bed [46,47]. Moreover, coal-series kaolin is detected to have larger reserves (1.67 billion tons) than the soft clay in China, though the chemical characteristics of coal-series kaolin are quite similar to those of coal gangue [48]. Nowadays, coal-series kaolin of high purity is extensively recognised as a valuable non-metallic mineral because of the gradual exhaustion of high-quality clay resources. Hence considerable research on the utilisation of coal-series kaolin has been conducted. For instance, Wang et al. fabricated porous mullite foams from coal-series kaolin and Al2O3 slurry by using a freeze-casting method [49]. Zhao et al. used coal-series kaolin as raw materials for synthesis of cordierite [50]. Liu et al. conducted research on the mineralogy and flotation of coal-series kaolin [51]. Previously, the microstructural evolution and phase transformation of coal-series kaolin during firing [52] and synthesised honeycomb materials for thermal storage by aluminium-assisted carbothermal reduction using coal-series kaolin as a raw material [53] have been studied. These research studies revealed the value of coal-series kaolin as a raw material for ceramic production. Meanwhile, research on in-situ synthesis of SiC whiskercontaining powder by carbothermal reduction of coal-series kaolin has not been reported. In this study, corundum–mullite–SiC whisker composites were synthesised by carbothermal reduction using coal-series kaolin as a raw material. SiC nanowhiskers were in-situ synthesised under a CO-rich atmosphere formed by a carbon-buried method, which can be simplified and easily generalised to conventional furnaces without any protections of inert gases (Ar, H2, and N2). The phase transformation, microstructure evolution, and porosity of coal-series kaolin and carbon black (i.e., used as the reducing agent) powder compact during sintering were studied. The effect of nano-SiC additive on the carbothermal reduction rate of coal-series kaolin was investigated, and the growth mechanism of SiC whiskers under the CO-rich atmosphere was obtained.
Table 2 Formula compositions of CC1 and CC2 (wt%). Coal-series kaolin
Carbon black
Nano-SiC particles (in addition)
CC1 CC2
77.28 77.28
22.72 22.72
3 0
2.2. Preparation Coal-series kaolin powder under 250 mesh and carbon black were mixed for 2 h by a ball-milling method. The obtained powder mix was then pressed to form powder compacts with a 5 wt% polyvinyl alcohol solution by using a 769 YP-30 T type tablet machine. The pressing process was aimed at obtaining a closer contact between kaolin and carbon black, which could accelerate the carbothermal reduction reaction rate. The powder compacts were dried in an electric oven held at 110 °C for 24 h. After drying, powder compacts were pressurelessly heat-treated by using a carbon-buried method in a silicon molybdenum furnace. A schematic of the carbon-buried method is presented in Ref. [54]. The heating rate and holding time at the maximum temperatures were set as 5 °C/min and 2 h, respectively. 2.3. Characterisation In our previous study [55], SiO gas, the intermediate phase for producing SiC whiskers, had a significant effect on the microstructure of the materials, and the porosity increased with the increase in the partial pressure of SiO gas, providing an indirect way to characterise the partial pressure of SiO gas. The water absorption, porosity, and bulk density of the powder compacts were measured according to Archimedes principle by using AUY220-type static electronic balance (Shimadzu Co., Ltd., Japan). X-ray diffraction (XRD) analyses were conducted by a D8 Advance type diffractometer (Bruker Co., Ltd., Germany) using Ni-filtered Cu Kα radiation at a scanning rate of 2°/min to study the phase transformation of powder compacts heat-treated at different temperatures. The relative contents of crystalline phases were calculated semi-quantitatively using the reference intensity ratio (RIR) method [56]. The morphologies of powder compacts heat-treated at different temperatures and the formed SiC whiskers were recorded by SU-8010 type scanning electron microscopy (SEM, Hitachi Co., Ltd., Japan) at a vacuum of 10−5 Pa using an accelerate voltage of 15 kV. The structural characteristics of the SiC whiskers were further studied by high-resolution transmission electron microscopy (HRTEM, JEM 2010, JEOL, Ltd., Japan) at a vacuum of 10−5 Pa.
2. Experimental 2.1. Raw materials Coal-series kaolin of large availability exploited in Shanxi Province, China, was selected as silica (SiO2) and alumina (Al2O3) resources, whilst carbon black (25 nm, Evonik Degussa Co., Ltd., Germany) was employed as a reducing agent. The chemical composition of coal-series kaolin is listed in Table 1. The morphology and mineralogical phase composition had been reported in a previous study [52]. To obtain a higher SiC yield, the content of carbon black was determined to be a factor of 3 greater than the SiO2 content (in mol%) of coal-series kaolin according to the reaction SiO2 + 3C → SiC + 2CO(g)
Formula No.
3. Results and discussion 3.1. Effect of SiC nanoparticles and temperature on phase transformation XRD patterns of samples CC1 and CC2 after heat-treatment at 1400–1650 °C are shown in Fig. 1(a) and (b), respectively; the relative contents of crystalline phases are listed in Table 1. It is worth noting that carbon black is an amorphous carbon material, thus its diffraction peaks cannot be detected [36]. As seen in Fig. 1(a), the detected phases in the XRD patterns of sample CC1 heat-treated at 1400–1500 °C are mullite, cristobalite, and β-SiC. At 1400 and 1450 °C, the diffraction peaks of β-SiC should belong to the introduced SiC nanoparticles. The semi-quantitatively calculated
(1)
As shown in Table 1, the SiO2 content was as high as 48.99 wt%, which was calculated to be 0.81 mol. Therefore, the mass ratio of carbon black should be 29.39 wt%, which became 22.72% in the sum of kaolin and itself (i.e., 100 wt% kaolin plus 29.39 wt% carbon black), as shown in Table 2. In Table 2, only formula CC1 had 3 wt% SiC nanoparticles (99.99% purity, 40 nm) added as nucleating agents for SiC whiskers under a CO-rich atmosphere. Table 1 Chemical compositions of coal-series kaolin (wt%). Oxides
SiO2
Al2O3
TiO2
Fe2O3
CaO
MgO
K2 O
Na2O
Ignition loss
Coal-series kaolin
48.99
35.46
0.4
0.12
0.04
0
0.03
0
14.72
2
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Fig. 1. XRD patterns of samples: (a) CC1 and (b) CC2.
contents of β-SiC (Table 1) are approximate to the added content (i.e., 3 wt%) with small deviations. As shown in Table 1, the carbothermal reduction of coal-series kaolin begins at a lower temperature of 1500 °C, at which the content of β-SiC is improved to 5.3 wt%. After heattreatment at 1550 °C, the intensive peak of cristobalite at 20°–22.5° disappears, indicating its transformation to amorphous silica [52]. Through the reaction described in Eq. (1), partial cristobalite should also be reduced to generate β-SiC, whose content has been increased to 9.5 wt% accordingly. The carbothermal reduction reaction of the mullite phase begins after the complete reduction of free silica (in an amorphous state or cristobalite) at > 1600 °C. This phenomenon corresponds well with Amroune's research [36], which was conducted in an argon atmosphere. When the heat-treatment temperature increases to 1600–1650 °C, mullite is reduced to form corundum (α-Al2O3) and βSiC as final products. At 1650 °C, the highest β-SiC content of 20.1 wt% is obtained for sample CC1, indicating that the carbothermal reduction is accelerated at a higher temperature. As shown in Fig. 1(b), for sample CC2 with no addition of SiC nanoparticles, the intensive peak of cristobalite is not detected until the heat-treatment temperature reaches 1450 °C. Such difference in the formation temperatures of cristobalite formed in CC1 and CC2 can be attributed to the addition of SiC nanoparticles, which also work as nucleating agents in liquid for the crystallisation of cristobalite, whereas cristobalite also transforms into amorphous silica when sample CC2 is heat-treated at 1550 °C. In contrast to sample CC1 heat-treated at 1550 °C, no diffraction peaks of β-SiC are detected in sample CC2. Moreover, only when heat-treatment is in the range of 1600–1650 °C can β-SiC be detected, indicating that the added SiC nanoparticles in sample CC1 not only promote the crystallisation of cristobalite but also decrease the initial synthesis temperature of β-SiC to 1500 °C, which is much lower than that of sample CC2 with a large deviation of 100 °C. It can be concluded that the added SiC nanoparticles increase the reaction rate of carbothermal reduction, because the contents of β-SiC and
corundum in sample CC1 are higher than those of sample CC2. Several mechanisms might be responsible for the remarkable effects of SiC nanoparticles on the synthesis temperature and the reaction rate as follows: SiC nanoparticles (β-type) provide growth interfaces for the deposition of SiO gas and thus lower the energy needed for formation of β-SiC, resulting in the lower synthesis temperature. The efficient deposition of SiO gas also lowers its partial pressure and accelerates carbothermal reduction reactions (see Eq. (2) and Eq. (3)) of free silica and mullite to guarantee the continuous supply of SiO gas. SiO2(free silica) + C → SiO(g) + CO(g)
(2)
3Al2O3·2SiO2 + 2C = 3Al2O3 + 2SiO(g) + 2CO(g)
(3)
3.2. Microstructure evolution The microstructure evolution of powder compacts of samples CC1 and CC2 was characterised by SEM, as shown in Figs. 2 and 3, respectively. It is observed in Fig. 2(a) and (b) that the powder compact of formula CC1 heat-treated at 1400 °C exhibits a loose microstructure, and no columnar mullite crystals are observed on aggregates of coal-series kaolin and carbon black. As the heat-treatment temperature increases to 1500 °C, Fig. 2(c) and (d) reveal that numerous columnar mullite crystals are observed on the surface of aggregates, and some twisted SiC whiskers with a much lower aspect ratio (length/diameter) are formed on the surfaces of aggregates of nanoparticles with a diameter of ~40 nm, which is approximate to the particle size of the added SiC. Therefore, these aggregates should be mainly composed of SiC nanoparticles that are dispersed inhomogeneously. The close connection between SiC nanowhiskers and SiC nanoparticles demonstrates that the nanoparticles work as nucleating agents to provide interfaces for whisker growth, which can also be verified by TEM. However, because of the relatively low heat-treatment temperature and the resultant low 3
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Fig. 2. SEM images of fractured surfaces of samples CC1 sintered at 1400–1650 °C: (a)&(b)-1400 °C. (c)&(d)-1500 °C, (e)&(f)-1650 °C.
effective way to produce SiC nanowhiskers. As shown in Fig. 3(e) and (f), numerous straight or twisted SiC whiskers decorated with numerous beads can be obtained at 1650 °C. This phenomenon implies that the added SiC nanoparticles have no influence on the morphology of the obtained SiC whiskers, though they have immense impact on the amount of SiC whiskers.
partial pressure of SiO gas, the growth kinetics of SiC whiskers cannot be maintained at a high level to obtain a high aspect ratio and a straight morphology. As seen in Fig. 2(c), by heat-treating at a higher temperature of 1650 °C, numerous straight SiC nanowhiskers with a higher aspect ratio are formed and the microstructure composed of SiC nanowhiskers, corundum, and mullite crystals becomes much looser. Apparently, the high heat-treatment temperature improves the SiO partial pressure effectively to endow the whiskers with sufficient growth kinetics, accelerating the reactions for synthesis of SiC whiskers (as described in Eqs. (1) and (4)). Numerous hemispherical beads are also found on the surfaces of the straight whiskers. According to Li's study [57], these beads were formed around the stacking defects inserted in {111} planes, because the energy needed for the deposition of SiO gas around the defects was much lower than along the [111] direction. SiO(g) + 3CO(g) → SiC + 2CO2(g)
3.3. TEM analyses TEM was employed to further study the morphology of the formed SiC whiskers, as presented in Figs. 4 and 5. As shown in Fig. 4(a), which is a high-magnification image of a SiC whisker decorated with hemispherical beads, these hemispherical beads may be the result of depositions of SiO gas, which are amorphous and have not been reduced by CO gas to produce β-SiC. Therefore, there is a quite clear boundary between the well-grown SiC whiskers and the amorphous beads (see the inset of Fig. 4(a), a HRTEM image of a hemispherical bead). Numerous streaks arranged perpendicular to the growth direction can also be illustrated by TEM, as seen in the area highlighted by the yellow circle in Fig. 4(a), whilst a small bead is formed around the streaks. This specific morphology demonstrates the deposition of SiO gas around the stacking faults, because these streaks represent the stacking-fault planes perpendicular to the growth direction [58]. Fig. 4(b) shows that diameters
(4)
For sample CC2, it is observed in Fig. 3(a)–3(d) that the morphology of powder compacts heat-treated at 1400 and 1500 °C is similar to that of sample CC1. However, no SiC nanowhiskers are formed, demonstrating that under a CO-rich atmosphere, SiC nanoparticles are necessary for the synthesis of SiC whiskers at a relatively low temperature of 1500 °C. Increasing the heat-treatment temperature is also an 4
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Fig. 3. SEM images of fractured surfaces of samples CC2 sintered at 1400–1650 °C: (a)&(b-1400 °C. (c)&(d)-1500 °C, (e)&(f)-1650 °C.
nanoparticles and the well-developed whiskers. As can be seen, the connection part exhibits a good periodic lattice structure, and the spacing between lattice fringes is measured to be 0.25 nm, which is consistent with the data reported in Ref. [57].
of the formed SiC whiskers are in the nanoscale range of 9–21 nm. Furthermore, it is observed in areas marked with the square and circle that some SiC whiskers grow with SiC nanoparticles on their tips. Fig. 4(c) shows a HRTEM image of the connection part between SiC
Fig. 4. TEM figures of SiC whiskers in sample CC1 sintered at 1650 °C. (c) is the magnification of the square in (b). 5
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Fig. 5. TEM figures of SiC whiskers in sample CC2 sintered at 1650 °C. (c) is the magnification of the square in (b).
carbothermal reduction reaction on improving the porosity of SiC whisker-containing ceramics by increasing the partial pressure of SiO gas. Therefore, the physical properties of samples CC1 and CC2 were tested to study variations in the partial pressure of SiO gas during heattreatment, which also provided additional evidence for the initial temperature of the carbothermal reduction of coal-series kaolin. Fig. 7 shows the physical properties of samples CC1 and CC2 as functions of heat-treatment temperatures. As shown in Fig. 7, the water absorption and porosity of samples CC1 and CC2 in the temperature range of 1400–1500 °C only exhibit slight increasing tendencies with temperature, whilst sample CC1 possesses higher water absorption and porosity than sample CC2. This phenomenon indicates that sample CC1 obtains a higher partial pressure of SiO gas to loosen the microstructure, thus generating more voids by promoting the carbothermal reduction of free silica with the help of SiC nanoparticles. Abrupt increases can be observed in the water absorption and porosity curves of sample CC1 at 1550–1650 °C, indicating the extensive carbothermal reduction and the generation of SiO gas. The much looser microstructure provides more space for the growth of SiC whiskers in turn, thereby increasing the content of SiC whiskers. The semi-quantitative data collected in Table 3 also show that β-SiC content in sample CC1 increases dramatically at 1550–1650 °C, corresponding well with the variations in porosity or water absorption of sample CC1. Although the water absorption and porosity values of sample CC2 improve remarkably in the same temperature range because of the carbothermal reduction of silica and the increased partial pressure of SiO gas, the generation of β-SiC cannot be verified by XRD or SEM. This result indicates that, without the addition of SiC nanoparticles, the formation of β-SiC nuclei in sample CC2 needs more energy, and only by increasing the heat-treatment temperature to 1600 °C can β-SiC be produced.
Fig. 5 shows TEM images of SiC whiskers formed in sample CC2 heat-treated at 1650 °C. The diameters of SiC whiskers in sample CC2 are not consistent and exhibit a range of 10 to 27 nm. Fig. 5(a) reveals that a SiC whisker with a diameter of 20 nm also exhibits numerous streaks perpendicular to the growth direction, indicating that many stacking faults are inserted in the whisker. Pickard et al. [58] had demonstrated that SiC whiskers with stacking faults had a lower formation energy than those without stacking faults. Because SiC whiskers with the specific morphology can be observed in both CC1 and CC2 and the space between two adjacent lattice fringes of whiskers is consistently 0.25 nm, the main difference in morphology of SiC whiskers formed in samples CC2 and CC1 should be the absence of SiC nanoparticles at the tips in sample CC2. The vapor–solid mechanism can be drawn for the growth of SiC whiskers in samples CC1 and CC2 according to SEM and TEM observations, because no liquid globules are observed on the tips of the asprepared SiC whiskers [36]. However, the growth processes of SiC whiskers in samples with and without addition of SiC nanoparticles are quite different. As indicated in Fig. 6, the schematic of whisker growth processes, the upper route belonging to sample CC1 shows that SiO gas generated from reactions (2) and (3) deposits on the added SiC nanoparticles to react with CO gas, resulting in the whisker growth directly without the SiC nucleation process. For sample CC2 with no nano SiC addition, the SiC nucleation process occurs prior to whisker growth and thus more energy and higher heat-treatment temperatures are demanded for producing SiC whiskers in the lower route.
3.4. Variations in physical properties of the pressed samples Previous research by our group [54] demonstrated the effects of the
Fig. 6. Schematic presentation of different growth process in sample with and with no nano-SiC addition. 6
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Fig. 7. Physical properties of CC1 and CC2 as the functions of sintering temperature: (a) water absorption, (b) open porosity, (c) bulk density.
Declaration of competing interest
Table 3 Semi-quantitative analysis of crystalline phases in samples CC1 and CC2 sintered at different temperatures. Formula No.
Temperature/°C
Mullite
Corundum
β-SiC
SiO2
CC1
1400 1450 1500 1550 1600 1650 1400 1450 1500 1550 1600 1650
81.1 84.8 92.2 90.5 43.5 24.8 100 83.1 63.3 100 62.1 58.3
– – – – 39.9 55.1 – – – – 26.6 27.4
2.9 2.6 5.3 9.5 16.6 20.1 – – – – 11.3 14.3
16 12.6 2.5 –
CC2
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. Acknowledgements We are grateful for the financial support of the National Natural Science Foundation of China (Grant Nos. 51702141 and 51962012) program ‘Doctoral Research Startup Program of Jingdezhen Ceramic Institute’.
– 16.9 36.7 –
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4. Conclusions Corundum–mullite–SiC whisker composites were obtained by the carbothermal reduction of coal-series kaolin. The effects of the added SiC nanoparticles and temperatures on the phase transformation, morphology of SiC whiskers, and physical properties were studied. Several conclusions can be drawn: 1. XRD analyses revealed that corundum, mullite, and β-SiC as final products were obtained by the carbothermal reduction of coal-series kaolin at 1650 °C. With the help of the added SiC nanoparticles, the formation temperature of the β-SiC phase could be decreased to 1500 °C, which was much lower than the 1600 °C for the sample with no addition of SiC nanoparticles. SiC nanoparticles not only worked as nucleating agents to deposit SiO gas, promoting the carbothermal reduction and production of the β-SiC phase, but also lowered the formation temperature of cristobalite. 2. SEM analyses showed that, at 1500 °C, SiC whiskers formed on the surfaces of the added SiC nanoparticles and exhibited a low specific ratio because of the low partial pressure of SiO gas. Straight and twisted SiC whiskers with a high specific ratio could be obtained when heat-treating at 1650 °C and partial straight SiC whiskers with stacking faults were decorated with numerous amorphous beads. The vapor–solid mechanism was responsible for the growth of SiC whiskers. 3. The number of pores in powder compacts increased dramatically by the extensive carbothermal reduction at 1550–1650 °C. The porous structure could provide space for the growth of SiC whiskers. The added SiC nanoparticles improved porosity and lowered the bulk density by accelerating the carbothermal reduction reaction and increasing the partial pressure of SiO gas.
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