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Fabrication and characterisation of AlN-SiC porous composite ceramics by nitridation of Al4SiC4
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Fabrication and characterisation of AlN-SiC porous composite ceramics by nitridation of Al4SiC4 Guangchao Xing, Chengji Deng, Jun Ding, Hongxi Zhu, Chao Yu∗ The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, 430081, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Al4SiC4 AlN-SiC Porous ceramics Foaming Nitridation
AlN-SiC porous composite ceramics were fabricated by heat-treating Al4SiC4 porous ceramic green body prepared using the foaming method under flowing nitrogen. The porous structure could provide increased nitrogen diffusion channels, leading to a decrease in N2 diffusion resistance and an increasing degree of nitridation. The micro-morphology of AlN was in the form of particles or whiskers and influenced by heating temperature and time parameters. AlN has good dispersibility with SiC particles, resulting in local homogeneous chemical composition of prepared AlN-SiC composite ceramics. Furthermore, the potential reaction mechanism responsible for the synthesis of AlN-SiC porous composite ceramics during the nitridation was revealed. This simple approach may be used for large-scale synthesis of AlN-SiC porous composite ceramics.
1. Introduction Composites of porous AlN-SiC combine the advantages of AlN and SiC and exhibit high thermal conductivity, good mechanical properties and excellent resistance against oxidation. These materials are promising non-oxide ceramics that can be used for applications such as hotgas and molten-metal filters, metal-ceramic composite preforms and structural microwave-absorbing materials [1–7]. Generally, AlN-SiC composites can be prepared using a series of methods such as hotpressing of SiC and AlN mixtures, combustion synthesis, and carbothermal reduction and nitridation. Jiang et al. [8] synthesised SiCwAlN composites using AlN powder and SiC whiskers as raw materials and added 8 wt% Y2O3 as a sintering additive during hot-pressing in nitrogen. Abbasi et al. [9] synthesised AlN-SiC composites by combustion reaction using silicon nitride, aluminium and carbon powder as raw materials. Cutler et al. [10] prepared AlN-SiC ceramics through carbothermal reduction of alumina and silica under flowing nitrogen. However, the aforementioned methods normally aim to prepare dense polycrystalline materials and require two or more raw materials. The main drawback of these methods is poor dispersion and distribution between the different materials [6,8–10]. In terms of efficiency and simplicity, a heat-treating ternary carbide process is a promising synthesis method for composite powders due to the requirement for only a single raw material and homogeneous chemical composition [6,11–13]. Ternary carbide Al4SiC4 is an attractive compound for refractory and high-temperature applications due to its high melting
∗
point, high chemical stability, and excellent oxidation and hydration resistances [14–19]. Itatani et al. [6] reported the synthesis of AlN-SiC composite powders by direct nitridation of Al4SiC4 powders, however, the reaction mechanism for the synthesis of AlN-SiC ceramics remains inconclusive. Furthermore, gas phase formation during the decomposition of ternary carbides may play an important role in the nitridation of Al4SiC4. However, the preparation of AlN-SiC porous composite ceramics with homogeneous chemical composition using this kind of method has not yet been successful. The foaming method has the advantages of being a simple process, low preparation cost, environmental friendliness, and its suitability for preparing porous ceramics with complex shapes [20,21]. In this work, AlN-SiC porous composite ceramics were successfully fabricated via a one-step process of direct nitridation of Al4SiC4 porous ceramics prepared by the foaming method. The prepared AlN-SiC porous composite ceramics had local homogeneous chemical composition. SiC particles constituted the skeleton structure and had good dispersibility with AlN particles/whiskers. Furthermore, the mechanism underpinning the synthesis of aluminium nitride-silicon carbide composite ceramics was investigated. 2. Experimental methods 2.1. Materials Al4SiC4 powders (prepared using a published method [14], 99 wt%,
Corresponding author. E-mail address: [email protected] (C. Yu).
https://doi.org/10.1016/j.ceramint.2019.10.234 Received 15 August 2019; Received in revised form 23 October 2019; Accepted 24 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Guangchao Xing, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.234
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≤74 μm) were used as the raw materials. Levelling Agent O-25 (Analytical Reagent, Tianjin Damao Chemical Reagent Factory), hydroxyethyl cellulose (Shanghai Aladdin Bio-Chem Technology Co., Ltd), and polyethylene glycol 400 (Chemically Pure, Sinopharm Group Chemical Reagent Co., Ltd) were used as the surfactant, foam stabilising agent, and foaming agent, respectively. 2.2. Preparation of AlN-SiC porous ceramics Al4SiC4 powders were first mixed in 20 wt% deionised water by stirring and wetting treatment. 5 wt% of additive was added to 60 wt% deionised water (all with respect to the Al4SiC4 powder) under stirring to produce foam, and the weight ratio (the additive) of levelling agent O-25: hydroxyethyl cellulose: polyethylene glycol 400 was 1: 2: 1. Subsequently, the foam was added to the wet Al4SiC4 powders under mechanical stirring. The slurry was then cast in a cylindrical mould measuring Ø 25 mm × H 4 mm and retained at 298 K for 2 h, to stabilise the foam followed by drying at 333 K for 24 h. After demoulding, specimens were dried at 383 K for 12 h. The green bodies were placed in a graphite crucible and moved into an electric furnace, and the furnace was initially purged with nitrogen (99.999%) multiple times. The specimens were then heated to 1473 K, 1573 K, 1673 K, and 1773 K for 1–10 h at 5 K/min under flowing nitrogen (99.999%, 300 ml/min) before cooling to room temperature. The experimental flow chart is illustrated in Fig. 1. 2.3. Specimen characterisation Crystalline phases of the specimens were identified using X-ray powder diffraction with Cu Kα radiation, at a scanning speed of 10°/ min (XRD, X'Pert-Pro-MPD, 40 kV and 40 mA). High resolution X-ray photoelectron spectroscopy with Al Kα radiation (XPS, Axis-Ultra-DLD600W) was used to elucidate the bonding states of Al, Si, C and N elements of the prepared specimens. Microstructures of specimens were observed using scanning electron (SEM, Nova-400-Nano) and transmission electron microscopes (TEM, JEM2000F) with an energy-dispersive spectroscopy detector (EDX, Penta FETx3, Oxford). Pore size distributions of specimens were determined by a mercury intrusion porosimeter method (MIP) (Autopore IV 9500). Brunauer-EmmettTeller (BET) specific surface area analysis was determined using a fully automatic surface area and porosity analyzer (Quantachrome, Autosorb-1-MP/LP).
Fig. 2. X-ray diffractometric patterns of the specimens before and after treatment.
specimens was significantly increased [11–13]. Upon increasing the temperature to 1773 K, the relative intensity of AlN and SiC peaks increased, whilst that of the Al4SiC4 peaks decreased rapidly. XPS analysis was undertaken to clarify binding of valence bonds in the nitrided specimens. Fig. 3a shows the Al 2p spectrum consisted of two peaks at 73.50 and 73.60 eV from Al-N in the AlN and Al-C in the Al4SiC4 [12,23]. In the high-resolution Si 2p spectrum (Fig. 3b), the typical peak at 100.80 eV arose from the Si-C bond in SiC [24]. Three peaks at 282.6 eV, 284.9 eV, and 287.0 eV in the C 1s spectrum corresponded to the binding energies of Si-C (SiC), free carbon (graphite), and CNx, respectively [6,25,26]. The peak of N 1s centred at 397.30 eV was attributed to N-Al bonds (AlN), as shown in Fig. 3d. These results were consistent with the XRD analysis, as shown in Fig. 2. Fig. 4 demonstrates the micrographs of the specimens after being nitrided for 1 h at 1473 K, 1573 K, 1673 K, and 1773 K. Fig. 4a shows a high density of AlN whiskers were formed at 1473 K. Many AlN whiskers that had a diameter below 100 nm grew randomly and became mutually entangled. SiC was present in the form of nanoparticles as
3. Results and discussion The XRD patterns of the specimens before and after heat-treatment at 1473 K, 1573 K, 1673 K, and 1773 K for 1 h are demonstrated in Fig. 2. AlN and SiC peaks were detected at 1473 K and 1573 K, and carbon phase peaks appeared at 1673 K. The presence of pores was conducive to the rapid diffusion of nitrogen when the porous ceramic body was made using the foaming method [22]. Compared with the dense ternary carbide reaction, the degree of nitridation of the
Fig. 1. Experimental flow chart. 2
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Fig. 3. XPS spectra of a specimen treated at 1773 K for 1 h: (a) Al 2p peak, (b) Si 2p peak, (c) C 1s peak, (d) N 1s peak.
temperature applications by the carbothermal reduction reaction [29,30]. The EDS compositional mapping of Al, Si, C, and N elements further revealed the forms of AlN, SiC, and graphite present. In previous studies, Itatani et al. [6] prepared AlN-SiC composite powders by directly nitriding Al4SiC4 powders. Chu et al. [31] fabricated SiC-AlN solid solution powders by a two-step carbothermal method in which Al4SiC4 was formed as a reaction intermediate phase. In their study, the mechanism underpinning the synthesis was attributed to a gas-solid reaction with the resulting irregular-shaped composite powders. In addition, it has been reported that Al4SiC4 can be regarded as a unit in which Al4C3-type and hexagonal SiC-type structures were alternately stacked along the [0001] growth direction, and the bond strength of Al-C was lower than the Si-C bonds in the Al4SiC4 substrate [23,32], and this low-strength bonding of Al-C caused the high activity of Al. Consequently, Al escaped from the substrate to the surface of Al4SiC4 as the preferred mode of decomposition, and SiC and carbon phases remained. The thermal decomposition behaviour of Al4SiC4 under vacuum can be described by reaction (1) [32].
shown in Fig. 4b. As shown in Fig. 4c and d, AlN whiskers exhibited a short bar morphology in the specimens heated at 1573 K. At 1673 K, the SiC particles grew denser, the grainy crystal particle size became larger, and the diameter and length of the AlN whiskers increased with increasing temperature (Fig. 4e and f). As shown in Fig. 4g and h, the diameter of the AlN whiskers at 1773 K was approximately 200 nm. The surface and inside of the specimens were nitrided (Fig. 4a, c, e, and g), which may be related to the space available for the growth of AlN in the specimens, as well as the diffusion of N2 [24]. The dense specimen may react with nitrogen to form a dense surface layer that inhibited nitrogen diffusion into the specimen [27]. However, with pores acting as nitrogen diffusion channels, gas can easily diffuse into the specimens, promoting the diffusion of nitrogen, which is believed to be conducive to accelerating the nitridation of Al4SiC4. Combined with the XRD analysis, the main phases of the specimens were AlN, SiC, and C nitrided at 1773 K. Fu et al. [28] reported that different sites allow for different spaces available for growth, and that whiskers have different growth patterns which result in different morphologies. However, in our study, due to the introduction of a porous structure, whiskers have sufficient space for growth and so the morphologies of the AlN whiskers formed at different sites were similar. Fig. 5 shows the micro-morphology of the specimens under TEM. Fig. 5a illustrates the prepared AlN whiskers (treated at 1473 K for 1 h) that were mainly present in straight or curved forms. The SAED pattern of AlN whiskers in area A indicated that the whiskers were singlecrystalline AlN (ICDD card No. 01-070-0779) (Fig. 5a). The whiskers were probably grown through a vapour-vapour-solid (VVS) process as no droplets were observed at the tip. Fig. 5b shows TEM images of a specimen heated at 1773 K for 1 h. HR-TEM micrographs presented in Fig. 5c and d showed that an AlN whisker formed in situ a SiC nanoparticle, and graphite with a clean surface was implanted and wellbonded rather than merely located on their surface. These findings indicated that interfacial bonds were well developed and the growth of AlN whiskers occurred along the [01¯0] growth direction. These graphite layers could play a positive role as an antioxidant during high-
Al4SiC4(s) = 4Al(g) + SiC(s) + 3C(s)
(1)
According to SEM and TEM-imaged morphologies, the VVS process was used to explain the growth mechanism of AlN whiskers as no droplets were observed at the tip. The thermal decomposition of aluminium-based compounds depended on the partial pressure of aluminium vapour [33]. The Al vapour (thermally decomposed from Al4SiC4) would be consumed immediately via the reaction with nitrogen to generate aluminium nitride. This resulted in a low partial pressure of Al vapour and accelerated decomposition of Al4SiC4 in the nitridation process. Therefore, Al continuously evaporated from Al4SiC4 and then reacted with N2 to form AlN nuclei (reactions (1) and (2)) [13,34]. The AlN whiskers then grew due to the continuous reaction between Al atoms and N2 near the nuclei, therefore, the formation of AlN whiskers was attributed to the joint action of two independent reactions (1) and (2) after introducing the foaming method. The mechanism 3
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Fig. 4. SEM micrographs of the specimens treated at (a, b) 1473 K, (c, d) 1573 K, (e, f) 1673 K, and (g, h) 1773 K.
which accelerated the decomposition of Al4SiC4. The SiC nanoparticles in the substrate gradually agglomerated into large particles, which constituted the skeleton structure of the composites, and finally, the AlNw-SiCp composite ceramics were formed.
underpinning their formation was proposed as shown in Fig. 6. In the initial stage, Al vapour thermally decomposed from Al4SiC4, and SiC nanoparticles and graphite phases remained in the substrate. The Al vapour was consumed to generate AlN whiskers via a VVS mechanism, 4
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Fig. 7. Thermodynamic evaluation of the formation of AlN-SiC composites.
Fig. 8. X-ray diffractometric patterns of the specimens treated at 1773 K with different holding times (1 h, 5 h and 10 h). Fig. 5. TEM and HR-TEM images of specimens: (a) TEM images with inset showing the SAED pattern of the A part of the whiskers (1473 K for 1 h), (b) TEM images of AlN whiskers (1773 K for 1 h), (c) HR-TEM image of area B in (b), (d) HR-TEM image of area C in (b), (e, f, g, h) EDS compositional mapping of Al, Si, C and N in (b).
2Al(g) + N2(g) = 2AlN(s)
(2)
It is reported that the foaming method is easy to operate and applicable to highly-porous, large-scale ceramics with open or closed pores [35]. The porous structures can provide more space for growth of
Fig. 6. The formation mechanism of the AlN-SiC composites. 5
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Fig. 9. SEM micrographs of the specimens treated at 1773 K and held for (a, b) 5 h and (c, d) 10 h.
Fig. 11. Pore size distributions of the specimens before and after treatment at 1773 K for 10 h: (a) MIP method and (b) BJH method.
Fig. 10. Nitrogen adsorption-desorption isotherms: (a) the specimens before treatment, (b) the specimens after treatment at 1773 K for 10 h.
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Fig. 12. TEM and HR-TEM images of the specimens treated at 1773 K held for 10 h: (a) Typical TEM images of bulks, (b) HR-TEM image of area A of the bulks in (a), (c, d, e,f) EDS compositional mapping of Al, Si, C and N in (a).
whiskers, reduce resistance of N2 diffusion and increase the degree of nitridation [22,24,36]. According to the XRD and SEM analyses, a large amount of AlN whiskers also grew inside the specimen cavity. Combined with the corresponding reactions (1) and (2), the overall nitridation of the Al4SiC4 porous body after using the foaming method can be written as reaction (3). The correlation between Gibbs free energy (△G) and temperature (K) of reaction (3) could be calculated based on the thermodynamic data [16,31,37,38], as shown in Fig. 7. The △G for reaction (1) was less than zero between 1473-1773 K, implying that both AlN and SiC could be generated within the temperature range.
Al4SiC4(s) + 2N2(g) = 4AlN(s) + SiC(s) + 3C(s)
(3)
Fig. 8 shows the XRD results of the specimens treated at 1773 K with different holding times. When the specimens were heated at 1773 K for 5 h, the relative intensity of the AlN, SiC and graphite peaks increased, whereas the relative intensity of the Al4SiC4 peak decreased. Upon increasing the nitridation time to 10 h, only AlN, SiC, and graphite were observed in the prepared specimens. It has been reported that AlN and SiC can form a solid solution over a wide chemical compositional range [39,40], however, no solid solution was found in the diffraction patterns in this study. These observations may be attributed to the low 7
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degree of nitridation. The micro-morphology of the resulting AlN was shown to be influenced by the heating temperature and time. A VVS mechanism was revealed to explain the formation of AlN whiskers. When heat-treated at 1773 K for 10 h, the Al4SiC4 porous ceramics were completely nitrided, The AlN and SiC particles were integrated and the resulting AlN-SiC composite ceramics had local homogeneous chemical composition.
sintering temperature resulting in fewer atoms diffusing [8]. The degree of supersaturation is known to determine the preferred growth morphology from vapour, and a low supersaturation is necessary to generate a 1-d structure [41–44]. Initially, the supersaturation of the system was believed to be very low due to the limited amount of Al vapour decomposed from the Al4SiC4. Thus, the nucleation rate was lower than the growth rate of the AlN whiskers. However, the presence of nitrogen facilitated the decomposition of Al4SiC4 with increasing temperature and the extension of holding time. This resulted in a high degree of supersaturation in the system and so the particle grains may be generated under homogeneous nucleation. Fig. 9 demonstrates the micro-morphology of a specimen obtained at 1773 K after different holding times. For the specimens treated at 1773 K for 5 h, most AlN whiskers had diameters of 400–600 nm (Figs. 9a and b). Fig. 9c and d shows the micro-morphology of the specimens obtained at 1773 K for 10 h. The AlN whiskers had completely disappeared, and the specimen consisted of irregular bulky formations. The specific surface area and pore size distribution of porous ceramics have a significant influence on gas diffusion. Fig. 10 shows the nitrogen adsorption-desorption isotherm curves of the specimens before and after treatment at 1773 K for 10 h. It has been reported that specific surface area can significantly influence gas permeability by surface adsorption and diffusion [45]. However, the specific surface area (SBET) of the specimens had no obvious change before and after heating treatment (Fig. 10a and b). The pore size distribution of porous ceramics therefore had more influence on gas diffusion in the present study [46]. As shown in Fig. 11, the micron-sized and nanometer-sized pore distributions of the specimens before and after treatment at 1773 K for 10 h were measured by MIP and BJH methods, respectively. In contrast to the specific surface area, both the sizes of micron- and nanometersized pores in the specimens decreased after heating treatment. This was associated with the generation of AlN, SiC and graphite, as well as the sintering process narrowing the pore diameter (Fig. 11a and b) [47]. Therefore, gas diffusion was restricted with decreasing pore diameter in pore structures [47,48]. Combined with the MIP and BET analyses, the decrease in pore size of the specimens after heating could explain the requirement for high temperature and prolonged reaction time for complete nitridation of the specimens. Fig. 12 shows the TEM results of the specimens formed at 1773 K and held for 10 h. Fig. 12a shows that the morphologies of the bulk components are mainly irregular. Fig. 12b illustrates the HR-TEM of a grain boundary between AlN/SiC, AlN/graphite, and SiC/graphite. Although no AlN-SiC solid solution diffraction peak was observed using XRD, the (100) lattice plane of AlN, the (102) lattice plane of SiC, and the (002) lattice plane of graphite were concatenated to each other at their grain boundaries. The EDS compositional mapping further revealed the even distribution of the four elements, resulting in homogeneous chemical composition. The fabrication of AlN-SiC porous composite ceramics by heattreating Al4SiC4 porous ceramic green body prepared using the foaming method under flowing nitrogen is a simple method. Appropriate heating temperature and time parameters are important for the growth of such composites, however, the foaming method also suffers from problems such as uneven pore size of the prepared porous ceramics. Other methods can be used to optimise pore structure, such as the template, pore-former and freeze-drying methods, and extrusion moulding technology. This work provides a new idea for the preparation of AlN-SiC porous composite ceramics, and lays a foundation for preparing porous composite ceramics with uniform chemical composition.
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4. Conclusions AlN-SiC porous composite ceramics were fabricated by heat-treating Al4SiC4 porous ceramic green body prepared using the foaming method under flowing nitrogen. The porous structure can provide more diffusion channels, leading to reduced N2 diffusion resistance and increasing 8
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Fabrication and characterisation of AlN-SiC porous composite ceramics by nitridation of Al4SiC4 Guangchao Xing, Chengji Deng, Jun Ding, Hongxi Zhu, Chao Yu∗ The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, 430081, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Al4SiC4 AlN-SiC Porous ceramics Foaming Nitridation
AlN-SiC porous composite ceramics were fabricated by heat-treating Al4SiC4 porous ceramic green body prepared using the foaming method under flowing nitrogen. The porous structure could provide increased nitrogen diffusion channels, leading to a decrease in N2 diffusion resistance and an increasing degree of nitridation. The micro-morphology of AlN was in the form of particles or whiskers and influenced by heating temperature and time parameters. AlN has good dispersibility with SiC particles, resulting in local homogeneous chemical composition of prepared AlN-SiC composite ceramics. Furthermore, the potential reaction mechanism responsible for the synthesis of AlN-SiC porous composite ceramics during the nitridation was revealed. This simple approach may be used for large-scale synthesis of AlN-SiC porous composite ceramics.
1. Introduction Composites of porous AlN-SiC combine the advantages of AlN and SiC and exhibit high thermal conductivity, good mechanical properties and excellent resistance against oxidation. These materials are promising non-oxide ceramics that can be used for applications such as hotgas and molten-metal filters, metal-ceramic composite preforms and structural microwave-absorbing materials [1–7]. Generally, AlN-SiC composites can be prepared using a series of methods such as hotpressing of SiC and AlN mixtures, combustion synthesis, and carbothermal reduction and nitridation. Jiang et al. [8] synthesised SiCwAlN composites using AlN powder and SiC whiskers as raw materials and added 8 wt% Y2O3 as a sintering additive during hot-pressing in nitrogen. Abbasi et al. [9] synthesised AlN-SiC composites by combustion reaction using silicon nitride, aluminium and carbon powder as raw materials. Cutler et al. [10] prepared AlN-SiC ceramics through carbothermal reduction of alumina and silica under flowing nitrogen. However, the aforementioned methods normally aim to prepare dense polycrystalline materials and require two or more raw materials. The main drawback of these methods is poor dispersion and distribution between the different materials [6,8–10]. In terms of efficiency and simplicity, a heat-treating ternary carbide process is a promising synthesis method for composite powders due to the requirement for only a single raw material and homogeneous chemical composition [6,11–13]. Ternary carbide Al4SiC4 is an attractive compound for refractory and high-temperature applications due to its high melting
∗
point, high chemical stability, and excellent oxidation and hydration resistances [14–19]. Itatani et al. [6] reported the synthesis of AlN-SiC composite powders by direct nitridation of Al4SiC4 powders, however, the reaction mechanism for the synthesis of AlN-SiC ceramics remains inconclusive. Furthermore, gas phase formation during the decomposition of ternary carbides may play an important role in the nitridation of Al4SiC4. However, the preparation of AlN-SiC porous composite ceramics with homogeneous chemical composition using this kind of method has not yet been successful. The foaming method has the advantages of being a simple process, low preparation cost, environmental friendliness, and its suitability for preparing porous ceramics with complex shapes [20,21]. In this work, AlN-SiC porous composite ceramics were successfully fabricated via a one-step process of direct nitridation of Al4SiC4 porous ceramics prepared by the foaming method. The prepared AlN-SiC porous composite ceramics had local homogeneous chemical composition. SiC particles constituted the skeleton structure and had good dispersibility with AlN particles/whiskers. Furthermore, the mechanism underpinning the synthesis of aluminium nitride-silicon carbide composite ceramics was investigated. 2. Experimental methods 2.1. Materials Al4SiC4 powders (prepared using a published method [14], 99 wt%,
Corresponding author. E-mail address: [email protected] (C. Yu).
https://doi.org/10.1016/j.ceramint.2019.10.234 Received 15 August 2019; Received in revised form 23 October 2019; Accepted 24 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Guangchao Xing, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.234
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≤74 μm) were used as the raw materials. Levelling Agent O-25 (Analytical Reagent, Tianjin Damao Chemical Reagent Factory), hydroxyethyl cellulose (Shanghai Aladdin Bio-Chem Technology Co., Ltd), and polyethylene glycol 400 (Chemically Pure, Sinopharm Group Chemical Reagent Co., Ltd) were used as the surfactant, foam stabilising agent, and foaming agent, respectively. 2.2. Preparation of AlN-SiC porous ceramics Al4SiC4 powders were first mixed in 20 wt% deionised water by stirring and wetting treatment. 5 wt% of additive was added to 60 wt% deionised water (all with respect to the Al4SiC4 powder) under stirring to produce foam, and the weight ratio (the additive) of levelling agent O-25: hydroxyethyl cellulose: polyethylene glycol 400 was 1: 2: 1. Subsequently, the foam was added to the wet Al4SiC4 powders under mechanical stirring. The slurry was then cast in a cylindrical mould measuring Ø 25 mm × H 4 mm and retained at 298 K for 2 h, to stabilise the foam followed by drying at 333 K for 24 h. After demoulding, specimens were dried at 383 K for 12 h. The green bodies were placed in a graphite crucible and moved into an electric furnace, and the furnace was initially purged with nitrogen (99.999%) multiple times. The specimens were then heated to 1473 K, 1573 K, 1673 K, and 1773 K for 1–10 h at 5 K/min under flowing nitrogen (99.999%, 300 ml/min) before cooling to room temperature. The experimental flow chart is illustrated in Fig. 1. 2.3. Specimen characterisation Crystalline phases of the specimens were identified using X-ray powder diffraction with Cu Kα radiation, at a scanning speed of 10°/ min (XRD, X'Pert-Pro-MPD, 40 kV and 40 mA). High resolution X-ray photoelectron spectroscopy with Al Kα radiation (XPS, Axis-Ultra-DLD600W) was used to elucidate the bonding states of Al, Si, C and N elements of the prepared specimens. Microstructures of specimens were observed using scanning electron (SEM, Nova-400-Nano) and transmission electron microscopes (TEM, JEM2000F) with an energy-dispersive spectroscopy detector (EDX, Penta FETx3, Oxford). Pore size distributions of specimens were determined by a mercury intrusion porosimeter method (MIP) (Autopore IV 9500). Brunauer-EmmettTeller (BET) specific surface area analysis was determined using a fully automatic surface area and porosity analyzer (Quantachrome, Autosorb-1-MP/LP).
Fig. 2. X-ray diffractometric patterns of the specimens before and after treatment.
specimens was significantly increased [11–13]. Upon increasing the temperature to 1773 K, the relative intensity of AlN and SiC peaks increased, whilst that of the Al4SiC4 peaks decreased rapidly. XPS analysis was undertaken to clarify binding of valence bonds in the nitrided specimens. Fig. 3a shows the Al 2p spectrum consisted of two peaks at 73.50 and 73.60 eV from Al-N in the AlN and Al-C in the Al4SiC4 [12,23]. In the high-resolution Si 2p spectrum (Fig. 3b), the typical peak at 100.80 eV arose from the Si-C bond in SiC [24]. Three peaks at 282.6 eV, 284.9 eV, and 287.0 eV in the C 1s spectrum corresponded to the binding energies of Si-C (SiC), free carbon (graphite), and CNx, respectively [6,25,26]. The peak of N 1s centred at 397.30 eV was attributed to N-Al bonds (AlN), as shown in Fig. 3d. These results were consistent with the XRD analysis, as shown in Fig. 2. Fig. 4 demonstrates the micrographs of the specimens after being nitrided for 1 h at 1473 K, 1573 K, 1673 K, and 1773 K. Fig. 4a shows a high density of AlN whiskers were formed at 1473 K. Many AlN whiskers that had a diameter below 100 nm grew randomly and became mutually entangled. SiC was present in the form of nanoparticles as
3. Results and discussion The XRD patterns of the specimens before and after heat-treatment at 1473 K, 1573 K, 1673 K, and 1773 K for 1 h are demonstrated in Fig. 2. AlN and SiC peaks were detected at 1473 K and 1573 K, and carbon phase peaks appeared at 1673 K. The presence of pores was conducive to the rapid diffusion of nitrogen when the porous ceramic body was made using the foaming method [22]. Compared with the dense ternary carbide reaction, the degree of nitridation of the
Fig. 1. Experimental flow chart. 2
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Fig. 3. XPS spectra of a specimen treated at 1773 K for 1 h: (a) Al 2p peak, (b) Si 2p peak, (c) C 1s peak, (d) N 1s peak.
temperature applications by the carbothermal reduction reaction [29,30]. The EDS compositional mapping of Al, Si, C, and N elements further revealed the forms of AlN, SiC, and graphite present. In previous studies, Itatani et al. [6] prepared AlN-SiC composite powders by directly nitriding Al4SiC4 powders. Chu et al. [31] fabricated SiC-AlN solid solution powders by a two-step carbothermal method in which Al4SiC4 was formed as a reaction intermediate phase. In their study, the mechanism underpinning the synthesis was attributed to a gas-solid reaction with the resulting irregular-shaped composite powders. In addition, it has been reported that Al4SiC4 can be regarded as a unit in which Al4C3-type and hexagonal SiC-type structures were alternately stacked along the [0001] growth direction, and the bond strength of Al-C was lower than the Si-C bonds in the Al4SiC4 substrate [23,32], and this low-strength bonding of Al-C caused the high activity of Al. Consequently, Al escaped from the substrate to the surface of Al4SiC4 as the preferred mode of decomposition, and SiC and carbon phases remained. The thermal decomposition behaviour of Al4SiC4 under vacuum can be described by reaction (1) [32].
shown in Fig. 4b. As shown in Fig. 4c and d, AlN whiskers exhibited a short bar morphology in the specimens heated at 1573 K. At 1673 K, the SiC particles grew denser, the grainy crystal particle size became larger, and the diameter and length of the AlN whiskers increased with increasing temperature (Fig. 4e and f). As shown in Fig. 4g and h, the diameter of the AlN whiskers at 1773 K was approximately 200 nm. The surface and inside of the specimens were nitrided (Fig. 4a, c, e, and g), which may be related to the space available for the growth of AlN in the specimens, as well as the diffusion of N2 [24]. The dense specimen may react with nitrogen to form a dense surface layer that inhibited nitrogen diffusion into the specimen [27]. However, with pores acting as nitrogen diffusion channels, gas can easily diffuse into the specimens, promoting the diffusion of nitrogen, which is believed to be conducive to accelerating the nitridation of Al4SiC4. Combined with the XRD analysis, the main phases of the specimens were AlN, SiC, and C nitrided at 1773 K. Fu et al. [28] reported that different sites allow for different spaces available for growth, and that whiskers have different growth patterns which result in different morphologies. However, in our study, due to the introduction of a porous structure, whiskers have sufficient space for growth and so the morphologies of the AlN whiskers formed at different sites were similar. Fig. 5 shows the micro-morphology of the specimens under TEM. Fig. 5a illustrates the prepared AlN whiskers (treated at 1473 K for 1 h) that were mainly present in straight or curved forms. The SAED pattern of AlN whiskers in area A indicated that the whiskers were singlecrystalline AlN (ICDD card No. 01-070-0779) (Fig. 5a). The whiskers were probably grown through a vapour-vapour-solid (VVS) process as no droplets were observed at the tip. Fig. 5b shows TEM images of a specimen heated at 1773 K for 1 h. HR-TEM micrographs presented in Fig. 5c and d showed that an AlN whisker formed in situ a SiC nanoparticle, and graphite with a clean surface was implanted and wellbonded rather than merely located on their surface. These findings indicated that interfacial bonds were well developed and the growth of AlN whiskers occurred along the [01¯0] growth direction. These graphite layers could play a positive role as an antioxidant during high-
Al4SiC4(s) = 4Al(g) + SiC(s) + 3C(s)
(1)
According to SEM and TEM-imaged morphologies, the VVS process was used to explain the growth mechanism of AlN whiskers as no droplets were observed at the tip. The thermal decomposition of aluminium-based compounds depended on the partial pressure of aluminium vapour [33]. The Al vapour (thermally decomposed from Al4SiC4) would be consumed immediately via the reaction with nitrogen to generate aluminium nitride. This resulted in a low partial pressure of Al vapour and accelerated decomposition of Al4SiC4 in the nitridation process. Therefore, Al continuously evaporated from Al4SiC4 and then reacted with N2 to form AlN nuclei (reactions (1) and (2)) [13,34]. The AlN whiskers then grew due to the continuous reaction between Al atoms and N2 near the nuclei, therefore, the formation of AlN whiskers was attributed to the joint action of two independent reactions (1) and (2) after introducing the foaming method. The mechanism 3
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Fig. 4. SEM micrographs of the specimens treated at (a, b) 1473 K, (c, d) 1573 K, (e, f) 1673 K, and (g, h) 1773 K.
which accelerated the decomposition of Al4SiC4. The SiC nanoparticles in the substrate gradually agglomerated into large particles, which constituted the skeleton structure of the composites, and finally, the AlNw-SiCp composite ceramics were formed.
underpinning their formation was proposed as shown in Fig. 6. In the initial stage, Al vapour thermally decomposed from Al4SiC4, and SiC nanoparticles and graphite phases remained in the substrate. The Al vapour was consumed to generate AlN whiskers via a VVS mechanism, 4
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Fig. 7. Thermodynamic evaluation of the formation of AlN-SiC composites.
Fig. 8. X-ray diffractometric patterns of the specimens treated at 1773 K with different holding times (1 h, 5 h and 10 h). Fig. 5. TEM and HR-TEM images of specimens: (a) TEM images with inset showing the SAED pattern of the A part of the whiskers (1473 K for 1 h), (b) TEM images of AlN whiskers (1773 K for 1 h), (c) HR-TEM image of area B in (b), (d) HR-TEM image of area C in (b), (e, f, g, h) EDS compositional mapping of Al, Si, C and N in (b).
2Al(g) + N2(g) = 2AlN(s)
(2)
It is reported that the foaming method is easy to operate and applicable to highly-porous, large-scale ceramics with open or closed pores [35]. The porous structures can provide more space for growth of
Fig. 6. The formation mechanism of the AlN-SiC composites. 5
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Fig. 9. SEM micrographs of the specimens treated at 1773 K and held for (a, b) 5 h and (c, d) 10 h.
Fig. 11. Pore size distributions of the specimens before and after treatment at 1773 K for 10 h: (a) MIP method and (b) BJH method.
Fig. 10. Nitrogen adsorption-desorption isotherms: (a) the specimens before treatment, (b) the specimens after treatment at 1773 K for 10 h.
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Fig. 12. TEM and HR-TEM images of the specimens treated at 1773 K held for 10 h: (a) Typical TEM images of bulks, (b) HR-TEM image of area A of the bulks in (a), (c, d, e,f) EDS compositional mapping of Al, Si, C and N in (a).
whiskers, reduce resistance of N2 diffusion and increase the degree of nitridation [22,24,36]. According to the XRD and SEM analyses, a large amount of AlN whiskers also grew inside the specimen cavity. Combined with the corresponding reactions (1) and (2), the overall nitridation of the Al4SiC4 porous body after using the foaming method can be written as reaction (3). The correlation between Gibbs free energy (△G) and temperature (K) of reaction (3) could be calculated based on the thermodynamic data [16,31,37,38], as shown in Fig. 7. The △G for reaction (1) was less than zero between 1473-1773 K, implying that both AlN and SiC could be generated within the temperature range.
Al4SiC4(s) + 2N2(g) = 4AlN(s) + SiC(s) + 3C(s)
(3)
Fig. 8 shows the XRD results of the specimens treated at 1773 K with different holding times. When the specimens were heated at 1773 K for 5 h, the relative intensity of the AlN, SiC and graphite peaks increased, whereas the relative intensity of the Al4SiC4 peak decreased. Upon increasing the nitridation time to 10 h, only AlN, SiC, and graphite were observed in the prepared specimens. It has been reported that AlN and SiC can form a solid solution over a wide chemical compositional range [39,40], however, no solid solution was found in the diffraction patterns in this study. These observations may be attributed to the low 7
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degree of nitridation. The micro-morphology of the resulting AlN was shown to be influenced by the heating temperature and time. A VVS mechanism was revealed to explain the formation of AlN whiskers. When heat-treated at 1773 K for 10 h, the Al4SiC4 porous ceramics were completely nitrided, The AlN and SiC particles were integrated and the resulting AlN-SiC composite ceramics had local homogeneous chemical composition.
sintering temperature resulting in fewer atoms diffusing [8]. The degree of supersaturation is known to determine the preferred growth morphology from vapour, and a low supersaturation is necessary to generate a 1-d structure [41–44]. Initially, the supersaturation of the system was believed to be very low due to the limited amount of Al vapour decomposed from the Al4SiC4. Thus, the nucleation rate was lower than the growth rate of the AlN whiskers. However, the presence of nitrogen facilitated the decomposition of Al4SiC4 with increasing temperature and the extension of holding time. This resulted in a high degree of supersaturation in the system and so the particle grains may be generated under homogeneous nucleation. Fig. 9 demonstrates the micro-morphology of a specimen obtained at 1773 K after different holding times. For the specimens treated at 1773 K for 5 h, most AlN whiskers had diameters of 400–600 nm (Figs. 9a and b). Fig. 9c and d shows the micro-morphology of the specimens obtained at 1773 K for 10 h. The AlN whiskers had completely disappeared, and the specimen consisted of irregular bulky formations. The specific surface area and pore size distribution of porous ceramics have a significant influence on gas diffusion. Fig. 10 shows the nitrogen adsorption-desorption isotherm curves of the specimens before and after treatment at 1773 K for 10 h. It has been reported that specific surface area can significantly influence gas permeability by surface adsorption and diffusion [45]. However, the specific surface area (SBET) of the specimens had no obvious change before and after heating treatment (Fig. 10a and b). The pore size distribution of porous ceramics therefore had more influence on gas diffusion in the present study [46]. As shown in Fig. 11, the micron-sized and nanometer-sized pore distributions of the specimens before and after treatment at 1773 K for 10 h were measured by MIP and BJH methods, respectively. In contrast to the specific surface area, both the sizes of micron- and nanometersized pores in the specimens decreased after heating treatment. This was associated with the generation of AlN, SiC and graphite, as well as the sintering process narrowing the pore diameter (Fig. 11a and b) [47]. Therefore, gas diffusion was restricted with decreasing pore diameter in pore structures [47,48]. Combined with the MIP and BET analyses, the decrease in pore size of the specimens after heating could explain the requirement for high temperature and prolonged reaction time for complete nitridation of the specimens. Fig. 12 shows the TEM results of the specimens formed at 1773 K and held for 10 h. Fig. 12a shows that the morphologies of the bulk components are mainly irregular. Fig. 12b illustrates the HR-TEM of a grain boundary between AlN/SiC, AlN/graphite, and SiC/graphite. Although no AlN-SiC solid solution diffraction peak was observed using XRD, the (100) lattice plane of AlN, the (102) lattice plane of SiC, and the (002) lattice plane of graphite were concatenated to each other at their grain boundaries. The EDS compositional mapping further revealed the even distribution of the four elements, resulting in homogeneous chemical composition. The fabrication of AlN-SiC porous composite ceramics by heattreating Al4SiC4 porous ceramic green body prepared using the foaming method under flowing nitrogen is a simple method. Appropriate heating temperature and time parameters are important for the growth of such composites, however, the foaming method also suffers from problems such as uneven pore size of the prepared porous ceramics. Other methods can be used to optimise pore structure, such as the template, pore-former and freeze-drying methods, and extrusion moulding technology. This work provides a new idea for the preparation of AlN-SiC porous composite ceramics, and lays a foundation for preparing porous composite ceramics with uniform chemical composition.
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. Acknowledgements Authors acknowledge the financial support from the National Natural Science Foundation of China (51602232, 51502215 and 51574187), and the Natural Science Foundation of Hubei Province (2018CFA022). References [1] D.H.A. Besisa, E.M.M. Ewais, Y.M.Z. Ahmed, F.I. Elhosiny, T. Fend, D.V. Kuznetsov, Investigation of microstructure and mechanical strength of SiC/AlN composites processed under different sintering atmospheres, J. Alloy. Comp. 756 (2018) 175–181. [2] Q. Wang, W.B. Cao, J.L. Kuang, J. Peng, Spherical AlN particles synthesized by the carbothermal method: effects of reaction parameters and growth mechanism, Ceram. Int. 44 (2018) 4829–4834. [3] H.A. Toutanji, D. Friel, T. El-Korchi, R.N. Katz, G. Wechsler, W. Rafaniellod, Room temperature tensile and flexural strength of ceramics in AlN-SiC system, J. Eur. Ceram. Soc. 15 (1995) 425–434. [4] J.F. Li, R. Watanabe, Pressureless sintering and high-temperature strength of SiCAlN ceramics, J. Ceram. Soc. Jpn. 102 (1994) 727–731. [5] J. Kuang, X. Hou, T. Xiao, Y. Li, Q. Wang, P. Jiang, W. Cao, Three-dimensional carbon nanotube/SiC nanowire composite network structure for high-efficiency electromagnetic wave absorption, Ceram. Int. 45 (2019) 6263–6267. [6] K. Itatani, R. Tsukamoto, A.C.A. Delsing, H.T. Hintzen, I. Okada, Preparation of aluminum nitride-silicon carbide nanocomposite powder by the nitridation of aluminum silicon carbide, J. Am. Ceram. Soc. 85 (2002) 1894–1896. [7] E. Hagen, T. Grande, M.A. Einarsrud, Preparation and properties of porous aluminum nitride-silicon carbide composite ceramics, J. Am. Ceram. Soc. 87 (2004) 1200–1204. [8] X. Jiang, Y. Chen, X.W. Sun, L.P. Huang, Mechanical property improvement and microstructure observation of SiCw-AlN composites, J. Eur. Ceram. Soc. 19 (1999) 2033–2038. [9] Z. Abbasi, M.H. Shariat, S. Javadpour, Microwave-assisted combustion synthesis of AlN-SiC composites using a solid source of nitrogen, Powder Technol. 249 (2013) 181–185. [10] I.B. Cutler, P.D. Miller, W. Rafaniello, H.K. Park, D.P. Thompson, K.H. Jack, New materials in the Si-C-Al-O-N and related systems, Nature 275 (1978) 434–435. [11] C. Yu, K.R. Cheng, J. Ding, C.J. Deng, Z.L. Xue, X.X. Wu, H.X. Zhu, Synthesis and characterisation of Al4O4C nanorod/CNT composites, Ceram. Int. 43 (2017) 11415–11420. [12] K.R. Cheng, C. Yu, J. Ding, C.J. Deng, H.X. Zhu, Z.L. Xue, Synthesis and characterization of AlN whiskers by nitridation of Al4O4C, J. Alloy. Comp. 719 (2017) 308–313. [13] X.X. Wu, C.J. Deng, J. Ding, H.X. Zhu, C. Yu, Synthesis of AlN-SiC microrods by the Co-catalysed nitridation of Al4SiC4, Ceram. Int. 45 (2019) 2680–2683. [14] C. Yu, W.J. Yuan, C.J. Deng, H.X. Zhu, J. Li, Synthesis of hexagonal plate-like Al4SiC4 from calcined bauxite, silica and carbon black, Powder Technol. 247 (2013) 76–80. [15] S.W. Zhang, A. Yamaguchi, Effect of Al4SiC4 addition to carbon-containing refractories, J. Ceram. Soc. Jpn. 103 (1995) 235–239. [16] K. Inoue, A. Yamaguchi, Synthesis of Al4SiC4, J. Am. Ceram. Soc. 86 (2003) 1028–1030. [17] J.S. Lee, S.H. Lee, T. Nishimura, H. Tanaka, Hexagonal plate-like ternary carbide particulates synthesized by a carbothermal reduction process: processing parameters and synthesis mechanism, J. Am. Ceram. Soc. 92 (2009) 1030–1035. [18] J.H. Chen, Z.H. Zhang, W.J. Mi, E.H. Wang, B. Li, K.C. Chou, X.M. Hou, Fabrication and oxidation behavior of Al4SiC4 powders, J. Am. Ceram. Soc. 100 (2017) 3145–3154. [19] C. Yu, C.J. Deng, H.X. Zhu, Z.L. Xue, J. Ding, Slag corrosion resistance of Al4SiC4, Int. J. Mater. Res. 108 (2017) 249–252. [20] X.G. Deng, J.K. Wang, J.H. Liu, H.J. Zhang, F.L. Li, H.J. Duan, L.L. Lu, Z. Huang, W.G. Zhao, S.W. Zhang, Preparation and characterization of porous mullite ceramics via foam-gelcasting, Ceram. Int. 41 (2015) 9009–9017. [21] Y. Hangaia, K. Takadaa, R. Endoa, H. Fujiib, Y. Aokib, T. Utsunomiyac, Foaming of
4. Conclusions AlN-SiC porous composite ceramics were fabricated by heat-treating Al4SiC4 porous ceramic green body prepared using the foaming method under flowing nitrogen. The porous structure can provide more diffusion channels, leading to reduced N2 diffusion resistance and increasing 8
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[22]
[23] [24]
[25]
[26]
[27]
[28]
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