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Microwave-assisted combustion synthesis of AlN–SiC composites using a solid source of nitrogen
Powder Technology 249 (2013) 181–185
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Microwave-assisted combustion synthesis of AlN–SiC composites using a solid source of nitrogen Zahra Abbasi ⁎, Mohammad Hossein Shariat, Sirus Javadpour Department of Materials Science and Engineering, Shiraz University, Shiraz, Iran
a r t i c l e
i n f o
Article history: Received 27 March 2013 Received in revised form 13 July 2013 Accepted 10 August 2013 Available online 19 August 2013 Keywords: Microwave processing Ceramics Composites X-ray diffraction
a b s t r a c t In this paper, the synthesis of Aluminum Nitride–Silicon Carbide composites through a combustion reaction among aluminum, silicon nitride (as the solid source of nitrogen) and carbon powders was investigated by Xray diffraction and scanning electron microscopy. The combustion reaction was initiated through the ignition of the powder mixture by microwave heating in a domestic oven, thereby led to the synthesis of the desired ceramic. To achieve a higher-quality product, some parameters were altered during the synthesis. These included the mixing time, the amount of carbon in the reactant mixture and the weight of the reactant compacts. The product was not affected dramatically by the alteration in the first two parameters; however, the increase in the weight of the samples made it possible to achieve a more complete combustion and a higher degree of conversion to the products. It can be concluded that the procedure used in this work successfully leads to the synthesis of the product with no need to apply an electric field or utilize a nitrogen atmosphere. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Aluminum nitride is an outstanding industrial material, due to its unique properties like high thermal conductivity, high electrical resistivity, low thermal expansion coefficient, desirable dielectric properties and good resistance to thermal shock [1]. In addition, it possesses a high mechanical strength, low density and good oxidation resistance [2]. Thus, AlN has been considered for many applications, such as a substrate material in electrical applications and hightemperature structural components [2–4]. Other applications include rf/microwave packages, heat sinks and cutting tools [2]. On the other hand, silicon carbide is a refractory ceramic which has a higher toughness, hardness and creep resistance relative to aluminum nitride ceramics [5]. A good thermal conductivity and an excellent chemical resistivity are other desirable properties of this material [6]. SiC has been developed for severe environmental applications, such as hightemperature heat exchangers and cylinder liners in combustion engines [7]. Solid solutions of AlN and SiC have desired mechanical properties, mainly due to the similar crystal structure and high-temperature properties of both the ceramics [8]. The formation of homogenous solid solutions is reported to lead to improvements in flexural strength and fracture toughness [9]. Likewise, composites of these ceramics have attractive mechanical properties. These composites are considered as suitable materials for high-temperature electronic ceramics and possess
⁎ Corresponding author. Tel.: +98 9177377710; fax: +98 7112307293. E-mail address: [email protected] (Z. Abbasi). 0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.08.012
an excellent microwave attenuator performance [10]. AlN–SiC ceramics are prepared by using a variety of methods, including the carbothermal reduction of alumina and silica in a nitrogen atmosphere [8], hot pressing of SiC and AlN mixtures [11] and combustion synthesis [12]. Combustion synthesis or self-propagating high-temperature synthesis (SHS) has attracted the interest of many researchers, due to the costeffective production of AlN–SiC ceramics. SHS has many advantages in comparison to solid-state reaction synthesis, such as the self generation of the energy required for the process, lower cost, the simplicity of the process and the higher purity of the products [13]. Two experimental approaches are utilized in the SHS method. In the first approach, the source of nitrogen is solid and the synthesis is accomplished via reactions among silicon nitride, aluminum and carbon powders under an electric field. The reaction of these powders is highly exothermic, but without the application of an external voltage, no self-propagating combustion wave can be initiated in this system. Therefore, the solid solution can be formed only when the magnitude of the external field is relatively high, typically higher than 25 V/cm. At the lower values of the field, the product is a composite consisting of AlN-rich and SiCrich phases, where their compositions are dependent on the applied field [12,14]. In the second approach, the source of nitrogen is gaseous. Powder mixtures of aluminum, silicon and carbon or aluminum and silicon carbide are ignited in a nitrogen atmosphere at pressures in the range of 0.1–10 MPa. Under these conditions, the solid solution can be produced within a matter of seconds. In this method, the nitridation of aluminum generates the heat which ensures the subsequent formation of the silicon carbide phase. However, the combustion synthesis should be performed under a high nitrogen pressure (about 6.0 MPa) to obtain homogeneous solid solutions [15].
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Other reports have been presented on the basis of these approaches to modify and optimize the accomplished synthesis. For instance, a synthesis by the first approach has been performed under the influence of an electric field and also in a conventional furnace. The results showed that the field-activated combustion synthesis is a very successful technique for making AlN–SiC ceramics, compared to furnace heating under vacuum [16]. The products have been also prepared under the obligatory presence of a nitrogen atmosphere with no need of an electric field [17]. Alterations in the different parameters of the second approach have been also studied by several researchers [18–20]. To ignite a sample of SHS, usually one surface of the sample is exposed to a heat source. The heat source may be a heating element, a spark generator or a laser. However, there is another method to ignite the sample, ignition using microwave energy. In this method, internal volumetric heating results in thermal gradients which are the reverse of gradients in conventional heating. Heat wave, therefore, moves coercively outwards and converts the reactants to the final products on its path. In combustion synthesis ignition using microwave, energy is absorbed constantly within the materials. As the temperature of the materials increases, they may even absorb more energy [21]. Since heat is generated by microwave as well as exothermic reactions, higher combustion temperatures will be reached [21]. The purpose of this study was to produce AlN–SiC ceramics by the microwave-assisted combustion synthesis using a solid source of nitrogen. During this work, no electric field was applied. Afterwards, the influences of alteration in the milling time, the amount of carbon in the reactant mixture and the weight of the reactant compacts on the products were investigated.
2. Materials and methods The raw materials used in this study include aluminum, silicon nitride and carbon powders. The reactant powders were weighed out according to the stoichiometric ratio of Reaction (1) and then attrition milled in ethanol for 1 h using ZrO2 balls to mix them. The powders were dried at 150 °C for 2 h and then sieved through a 60 mesh screen after granulating by the polyvinyl alcohol (PVA) binder. The characteristics of the powders are given in Table 1. The role of PVA was significant in producing strong green pellets. The weight of PVA in a gram of the powder mixture was about 0.75 × 10−3 g which was expected to evaporate at temperatures below the ignition temperature of the pellets. 4A1 þ Si3 N4 3C ¼ 4A1N þ 3SiC
ð1Þ
3. Results and discussions The powder mixtures of Al, C and Si3N4 reacted together after ignition. It was expected to produce AlN–SiC ceramics via the microwaveassisted combustion synthesis without the application of the gaseous source of nitrogen in this step. The XRD pattern of the reaction products of the system is shown in Fig. 1. This pattern includes the sharp peaks of AlN and a few peaks of SiC. The products consisted of AlN and SiC, with cubic and hexagonal structures, respectively, residual Si3N4, Al and C. The JCPDS card numbers of all phases are listed in Table 2. As the expectation was partly fulfilled, it was necessary to find some ways to eliminate the residual reactants. Accordingly, effective synthesis parameters were altered and their effects on the reaction products were investigated. These parameters include the attrition milling time, the amount of carbon in the reactant mixtures and the weight of the powder compacts. Fig. 2 shows the XRD patterns of the reaction products of the samples milled for different times. Initially, the attrition milling time was 60 min and then the time was halved and doubled. It is noteworthy that attrition milling is a high energy milling which in addition to mixing, reduces the size of particles. As the milling time increases, the particles size decreases and consequently particles have better contact with each other. It has been reported that the reduction of the particle size results in a better combustion [22]. However, contrary to what was thought, the change in the milling time from 30 min to 120 min had negligible effects on the products, as can be seen in Fig. 2. Therefore, in the present work, it can be inferred that the alteration in the milling time, albeit in this limit, does not have the desired effects on the completion of the combustion and the increase in the conversion degree. Note that in the previous reports, pure elements, as the starting materials, directly react to synthesize products; thus, the particle size reduction could affect the combustion reaction. However, in this work, Si3N4 as the source of Si should be initially decomposed, as the controlling step, to proceed the reaction; and according to the results, the milling time and size reduction do not affect the decomposition and thereby reaction. According to Juang et al. [20], adding extra carbon to the starting powders results in a nearly pure AlN–SiC ceramics and also the best homogeneity of the ceramics. In this work, the amount of carbon in the reactant mixtures was increased to twice and four times of its stoichiometric weight. The influences on the XRD patterns of the products are shown in Fig. 3. As can be seen, increasing the amount of carbon did not affect the products. This disagreement is due to the difference in the starting materials used in Ref. [20] and the present work. In Ref. [20], the starting materials included pure Si, where Si is available to react
The reactant powder mixtures were uniaxially pressed into 0.5 g cylindrical pellets. The combustion synthesis reactions of the samples were conducted in a 900 W domestic microwave oven (LF-570NR, LG, Korea). The reaction temperature was monitored by a two-color pyrometer (VF-3000, Optex, Japan) that was focused on the sample. The chemical composition and microstructure of the combustion products were identified by X-ray diffraction (XRD, Advance D8, Bruker, Germany) and scanning electron microscopy (SEM, S-360, Oxford, UK), respectively.
Table 1 Characteristics of the commercial powders. Material
Average particle size (μm)
Purity (%)
Source
Aluminum Si3N4 Graphite
45 0.5–1.0 90
99 99.5 99.9
K.M.P., Iran SkySpring Nanomaterials, Inc, USA –
Fig. 1. XRD pattern of the product obtained from the combustion reaction between the stoichiometric mixture of Al, Si3N4 and C powders according to Reaction (1).
Z. Abbasi et al. / Powder Technology 249 (2013) 181–185
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Table 2 JCPDS card numbers of the phases. Phases
JCPDS card numbers
Al C Si3N4 Si AlN (cubic) AlN (hexagonal) SiC
00-004-0787 01-075-2078 00-041-0360 00-027-1402 00-046-1200 00-025-1133 00-029-1131
with C and to produce SiC from the start. However, in this work, the reaction of Si and C originally needs the decomposition of Si3N4 as the source of Si. Thus, adding extra carbon cannot be effective on the desired phase evolution in this circumstance. Fig. 4 shows the XRD patterns of the products obtained after changing the weight of the reactant compacts to four, sixteen and thirty two times of their original weight. When the weight of the compact was quadrupled, no significant changes were observed, except the increase in the amount of AlN. However, in the case of the 8 g samples, the Si3N4 peaks completely disappeared and the intensities of Al and C peaks decreased. In addition to the presence of new SiC peaks, the existing peaks were intensified. Furthermore, the position and intensity of AlN peaks were generally changed. When the weight of the compact was increased to 16 g, the Al peaks disappeared too and the intensity of the AlN and SiC peaks continued to increase. When the compact weight is increased, its surface area also increases. A larger sample results in a higher heat production per unit surface area of the sample, and thereby a higher temperature of the sample [23]. The temperature rise of the 2 g sample results in a slight increase in the amount of the Si3N4 decomposition and AlN formation. On the other hand, the larger amount of heat remains in the 8 g sample, due to the increase in the mass-to-surface ratio. The generated heat can entirely decompose Si3N4; hence, there is no trace of this phase in the product. From the study of the AlN characterization in the XRD analysis, it can be realized that the AlN produced in the previous cases has the cubic crystal structure, whereas the crystal structure of AlN in the present case (8 g sample) is hexagonal, which results in changes in the positions and intensities of the AlN peaks in the pattern. The formation of AlN and SiC has led to the reduction in the amount of residual Al and C in the samples. In the case of the 16 g samples, the conversion is to such an extent that leads to the complete consumption of Al and a more increase in the amount of AlN and SiC. It would be worth mentioning that in the samples experiencing the complete decomposition of Si3N4
Fig. 2. XRD patterns of the products obtained after alteration in milling time.
Fig. 3. XRD patterns of the products obtained after adding extra carbon to the starting powders.
(8 and 16 g samples), as well as the desired phases, there is some remained Si which has not reacted with C to produce SiC. This is due to the fact that in used heating regime, the sample does not remain at the high temperatures for a sufficient duration. Kexin et al. [19] reported that increasing and keeping the combustion temperature at a relatively high level are a key to decrease the unreacted Si in the combustion products. Accordingly, it is noteworthy that as the sample weight is increased, the productivity of the reaction is improved and only traces of the reactants remain in the reaction products. Furthermore, applying this approach can lead to the production of AlN and SiC, both with the hexagonal structure. The adiabatic temperature is an appropriate measure of the exothermicity of reactions. Providing the self-propagating mode was initiated at room temperature (298 K) without any preheat, the values of the adiabatic temperature (Tad) can be calculated based on the following equation [22]: Z ΔHð298Þ þ
Tadð298Þ X 298
njCpðPjÞdT þ ΣnjLðPjÞ ¼ 0
ð2Þ
where ΔH(298) is the reaction enthalpy at 298 K, nj is the stoichiometric coefficient of the product, CP(Pj) and L(Pj) are the heat capacity and the latent heat of the phase change (if the product goes through a phase change) of the product, respectively. The reaction enthalpy can
Fig. 4. XRD patterns of the products obtained after increasing the compact weights.
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be calculated using the formation enthalpy of each component [24]. In the present case, the obtained value of ΔH(298) for Reaction (1) is equal to −729.5 kJ. Now, considering that the product includes 4 moles of AlN and 3 moles of SiC, Equation (2) is converted to the following relationship: Z ‐ΔHð298Þ ¼
Tad h 298
i 4Cp ðAINÞ þ 3Cp ðSiCÞ dT
ð3Þ
Using thermodynamic data [24], the adiabatic temperature calculated from Equation (3), based on the complete combustion of the reactants to the products, is equal to 2043.2 °C. For this system, the maximum temperature measured using the pyrometer is 1027 °C (for 0.5 g sample). The difference between the actual combustion temperature and theoretical adiabatic temperature is an expected fact, due to heat loss [22] and the incomplete conversion of the reactants to the products. The maximum measured temperature for the 16 g sample is 1889 °C, which indicates that a more complete combustion is conducted and possibly a more microwave heat is absorbed under these conditions. According to the AlN–SiC phase diagram [6], there is a critical temperature around 1960 °C which separates the stable solid solution region and the two-phase region that tends to separate into AlN-rich and SiC-rich phases. At temperatures above 1960 °C, the AlN and SiC system is located in the hexagonal stable solid solution region. Since the maximum measured temperature in this study is 1889 °C and this value is lower than the critical temperature, it can be concluded that the obtained product is a composite of AlN and SiC. In the 8 g sample, AlN and SiC have a similar crystal structure; thus, they have the necessary condition for the formation of a solid solution. It is expected that increasing the sample size and therefore increasing the temperature result in the formation of a hexagonal stable solid solution between these two ceramics. According to previous papers [14,15,17], to achieve AlN–SiC ceramics (composites or solid solutions) through the reaction between Al, Si3N4 and C, it is essential to apply a strong electric field or conduct the reaction under a nitrogen atmosphere. In this study, the AlN–SiC composite were synthesized using microwave heating without applying an electric field or utilizing a nitrogen atmosphere. The cross section areas of the 0.5 g to 8 g pellets were constant (1.13 cm2), while their heights were changed with increasing the weight of the powders. The desired results were achieved in the 8 g sample. In the 16 g samples, the cross section areas of the pellets were increased to 3.14 cm2, which improved results. The SEM images of the 0.5 g, 2 g and 8 g samples are shown in Fig. 5a–c. The needle-like phase is Si3N4 which has remained unreacted in the products. As the sample size has been increased, the amount of the needle-like phase has decreased until this phase has been completely eliminated in the 8 g sample products. Increasing the sample size also has increased the particle size, possibly due to grain growth associated with the temperature rise. It is expected that the temperature rise associated with increasing sample size results in the achievement of a homogeneous and uniform morphology of a hexagonal phase via the formation of a solid solution. 4. Conclusions The feasibility of synthesizing AlN–SiC composite ceramics by a combustion synthesis reaction was investigated through igniting the powder mixtures of Al, Si3N4 and C in a domestic microwave oven. The synthesis was successfully conducted with no need to apply an electric field or to utilize a nitrogen atmosphere. Alteration in the milling time and the amount of carbon over its stoichiometry had no significant effect on the formation of the product, but in the case of increasing the sample size, the result was more desirable and a better product quality was obtained.
Fig. 5. SEM images of the products related to the effects of increasing the compact weight: a) 0.5 g, b) 2 g and c) 8 g (arrows show the needle-like Si3N4 phase).
References [1] T.J. Mroz Jr., Annual materials review: aluminum nitride, Am. Ceram. Soc. Bull. 71 (1992) 782–786. [2] L.M. Sheppard, M. Laurel, Aluminum nitride: a versatile but challenging material, Am. Ceram. Soc. Bull. 69 (1990) 1801–1812. [3] Y. Kurokawa, K. Utsumi, H. Takamizawa, Development and micro-structural characterization of high-thermal-conductivity aluminum nitride ceramics, J. Am. Ceram. Soc. 71 (1988) 588–594. [4] V. Virkar, T.B. Jackson, R.A. Cutler, Thermodynamic and kinetic effects of oxygen removal on the thermal conductivity of aluminum nitride, J. Am. Ceram. Soc. 72 (1989) 2031–2042. [5] C.K. Unni, D.E. Gordon, Mechanical properties of monolithic AlN and SiCw/AlN composites, J. Mater. Sci. 30 (1995) 1173–1179. [6] A. Zangvil, R. Ruh, Phase relationship in the silicon carbide-aluminum nitride system, J. Am. Ceram. Soc. 71 (1988) 884–890. [7] R.N. Katz, High temperature structural ceramics, Science 208 (1980) 841–847. [8] 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 (London) 275 (1978) 434–435. [9] J.F. Li, R. Watanabe, Preparation and mechanical properties of SiC-AlN ceramic alloy, J. Mater. Sci. 26 (1991) 4813–4817.
Z. Abbasi et al. / Powder Technology 249 (2013) 181–185 [10] W.B. Bu, T. Qui, J. Xu, Preparation and microwave attenuation performance of AlN-SiC composites, J. Chin. Ceram. Soc. 31 (2003) 828–831. [11] R. Ruh, A. Zangvil, Composition and properties of hot-pressed SiC-AlN solid solution, J. Am. Ceram. Soc. 65 (1982) 260–265. [12] H. Xue, Z.A. Munir, The synthesis of composites and solid solutions of α-SiC-AlN by field-activated combustion, Scr. Mater. 35 (1996) 979–982. [13] J. Subrahmanyam, M. Vijaykumar, Review, self-propagating high-temperature synthesis, J. Mater. Sci. 27 (1992) 6249–6273. [14] H. Xue, Z.A. Munir, Synthesis of AlN-SiC composites and solid solution by field-activated self-propagating combustion, J. Eur. Ceram. Soc. 17 (1997) 1787–1792. [15] M. Ohyanagi, N. Balandina, K. Shirai, M. Kiozumi, Z.A. Munir, Synthesis of AlN-SiC solid solution by combustion nitridation, in: N.P. Bansal, J.P. Singh (Eds.), Ceramic Transactions 94. Innovative Processing and Synthesis of Ceramics, Glasses, and Composites II, American Ceramic Society, Westerville, OH, 1999, pp. 3–12. [16] E.M. Carrillo-Heian, H. Xue, M. Ohyanagi, Z.A. Munir, Reactive synthesis and phase stability investigation in the aluminum nitride-silicon carbide system, J. Am. Ceram. Soc. 83 (2000) 1103–1107.
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[17] D. Kata, K. Shirai, M. Ohyanagi, Z.A. Munir, Formation mechanism of AlN-SiC solid solution by combustion nitridation in Si3N4-Si-Al-C system, J. Am. Ceram. Soc. 84 (2001) 726–732. [18] M. Ohyanagi, K. Shirai, N. Balandina, M. Hisa, Z.A. Munir, Synthesis of aluminum nitride-silicon carbide solid solution by combustion nitridation, J. Am. Ceram. Soc. 83 (2000) 1108–1112. [19] C. Kexin, J. Haibo, H.P. Zhou, J.M.F. Ferreira, Combustion synthesis of AlN-SiC solid solution particles, J. Eur. Ceram. Soc. 20 (2000) 2601–2606. [20] R.C. Juang, C.C. Chen, J.C. Kuo, T.Y. Huang, Y.Y. Li, Combustion synthesis of hexagonal AlN-SiC solid solution under low nitrogen pressure, J. Alloys Compd. 480 (2009) 928–933. [21] D.E. Clark, I. Ahmad, R.C. Dalton, Microwave ignition and combustion synthesis of composites, Mater. Sci. Eng., A 144 (1991) 91–97. [22] J.J. Moore, H.J. Feng, Combustion synthesis of advanced materials: part I. reaction parameters, Prog. Mater. Sci. 39 (1995) 243–273. [23] 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. [24] O. Knacke, O. Kubaschewski, K. Hesselman, Thermochemical Properties of Inorganic Substances, second ed. Springer-Verlag, 1991.
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Microwave-assisted combustion synthesis of AlN–SiC composites using a solid source of nitrogen Zahra Abbasi ⁎, Mohammad Hossein Shariat, Sirus Javadpour Department of Materials Science and Engineering, Shiraz University, Shiraz, Iran
a r t i c l e
i n f o
Article history: Received 27 March 2013 Received in revised form 13 July 2013 Accepted 10 August 2013 Available online 19 August 2013 Keywords: Microwave processing Ceramics Composites X-ray diffraction
a b s t r a c t In this paper, the synthesis of Aluminum Nitride–Silicon Carbide composites through a combustion reaction among aluminum, silicon nitride (as the solid source of nitrogen) and carbon powders was investigated by Xray diffraction and scanning electron microscopy. The combustion reaction was initiated through the ignition of the powder mixture by microwave heating in a domestic oven, thereby led to the synthesis of the desired ceramic. To achieve a higher-quality product, some parameters were altered during the synthesis. These included the mixing time, the amount of carbon in the reactant mixture and the weight of the reactant compacts. The product was not affected dramatically by the alteration in the first two parameters; however, the increase in the weight of the samples made it possible to achieve a more complete combustion and a higher degree of conversion to the products. It can be concluded that the procedure used in this work successfully leads to the synthesis of the product with no need to apply an electric field or utilize a nitrogen atmosphere. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Aluminum nitride is an outstanding industrial material, due to its unique properties like high thermal conductivity, high electrical resistivity, low thermal expansion coefficient, desirable dielectric properties and good resistance to thermal shock [1]. In addition, it possesses a high mechanical strength, low density and good oxidation resistance [2]. Thus, AlN has been considered for many applications, such as a substrate material in electrical applications and hightemperature structural components [2–4]. Other applications include rf/microwave packages, heat sinks and cutting tools [2]. On the other hand, silicon carbide is a refractory ceramic which has a higher toughness, hardness and creep resistance relative to aluminum nitride ceramics [5]. A good thermal conductivity and an excellent chemical resistivity are other desirable properties of this material [6]. SiC has been developed for severe environmental applications, such as hightemperature heat exchangers and cylinder liners in combustion engines [7]. Solid solutions of AlN and SiC have desired mechanical properties, mainly due to the similar crystal structure and high-temperature properties of both the ceramics [8]. The formation of homogenous solid solutions is reported to lead to improvements in flexural strength and fracture toughness [9]. Likewise, composites of these ceramics have attractive mechanical properties. These composites are considered as suitable materials for high-temperature electronic ceramics and possess
⁎ Corresponding author. Tel.: +98 9177377710; fax: +98 7112307293. E-mail address: [email protected] (Z. Abbasi). 0032-5910/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.08.012
an excellent microwave attenuator performance [10]. AlN–SiC ceramics are prepared by using a variety of methods, including the carbothermal reduction of alumina and silica in a nitrogen atmosphere [8], hot pressing of SiC and AlN mixtures [11] and combustion synthesis [12]. Combustion synthesis or self-propagating high-temperature synthesis (SHS) has attracted the interest of many researchers, due to the costeffective production of AlN–SiC ceramics. SHS has many advantages in comparison to solid-state reaction synthesis, such as the self generation of the energy required for the process, lower cost, the simplicity of the process and the higher purity of the products [13]. Two experimental approaches are utilized in the SHS method. In the first approach, the source of nitrogen is solid and the synthesis is accomplished via reactions among silicon nitride, aluminum and carbon powders under an electric field. The reaction of these powders is highly exothermic, but without the application of an external voltage, no self-propagating combustion wave can be initiated in this system. Therefore, the solid solution can be formed only when the magnitude of the external field is relatively high, typically higher than 25 V/cm. At the lower values of the field, the product is a composite consisting of AlN-rich and SiCrich phases, where their compositions are dependent on the applied field [12,14]. In the second approach, the source of nitrogen is gaseous. Powder mixtures of aluminum, silicon and carbon or aluminum and silicon carbide are ignited in a nitrogen atmosphere at pressures in the range of 0.1–10 MPa. Under these conditions, the solid solution can be produced within a matter of seconds. In this method, the nitridation of aluminum generates the heat which ensures the subsequent formation of the silicon carbide phase. However, the combustion synthesis should be performed under a high nitrogen pressure (about 6.0 MPa) to obtain homogeneous solid solutions [15].
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Other reports have been presented on the basis of these approaches to modify and optimize the accomplished synthesis. For instance, a synthesis by the first approach has been performed under the influence of an electric field and also in a conventional furnace. The results showed that the field-activated combustion synthesis is a very successful technique for making AlN–SiC ceramics, compared to furnace heating under vacuum [16]. The products have been also prepared under the obligatory presence of a nitrogen atmosphere with no need of an electric field [17]. Alterations in the different parameters of the second approach have been also studied by several researchers [18–20]. To ignite a sample of SHS, usually one surface of the sample is exposed to a heat source. The heat source may be a heating element, a spark generator or a laser. However, there is another method to ignite the sample, ignition using microwave energy. In this method, internal volumetric heating results in thermal gradients which are the reverse of gradients in conventional heating. Heat wave, therefore, moves coercively outwards and converts the reactants to the final products on its path. In combustion synthesis ignition using microwave, energy is absorbed constantly within the materials. As the temperature of the materials increases, they may even absorb more energy [21]. Since heat is generated by microwave as well as exothermic reactions, higher combustion temperatures will be reached [21]. The purpose of this study was to produce AlN–SiC ceramics by the microwave-assisted combustion synthesis using a solid source of nitrogen. During this work, no electric field was applied. Afterwards, the influences of alteration in the milling time, the amount of carbon in the reactant mixture and the weight of the reactant compacts on the products were investigated.
2. Materials and methods The raw materials used in this study include aluminum, silicon nitride and carbon powders. The reactant powders were weighed out according to the stoichiometric ratio of Reaction (1) and then attrition milled in ethanol for 1 h using ZrO2 balls to mix them. The powders were dried at 150 °C for 2 h and then sieved through a 60 mesh screen after granulating by the polyvinyl alcohol (PVA) binder. The characteristics of the powders are given in Table 1. The role of PVA was significant in producing strong green pellets. The weight of PVA in a gram of the powder mixture was about 0.75 × 10−3 g which was expected to evaporate at temperatures below the ignition temperature of the pellets. 4A1 þ Si3 N4 3C ¼ 4A1N þ 3SiC
ð1Þ
3. Results and discussions The powder mixtures of Al, C and Si3N4 reacted together after ignition. It was expected to produce AlN–SiC ceramics via the microwaveassisted combustion synthesis without the application of the gaseous source of nitrogen in this step. The XRD pattern of the reaction products of the system is shown in Fig. 1. This pattern includes the sharp peaks of AlN and a few peaks of SiC. The products consisted of AlN and SiC, with cubic and hexagonal structures, respectively, residual Si3N4, Al and C. The JCPDS card numbers of all phases are listed in Table 2. As the expectation was partly fulfilled, it was necessary to find some ways to eliminate the residual reactants. Accordingly, effective synthesis parameters were altered and their effects on the reaction products were investigated. These parameters include the attrition milling time, the amount of carbon in the reactant mixtures and the weight of the powder compacts. Fig. 2 shows the XRD patterns of the reaction products of the samples milled for different times. Initially, the attrition milling time was 60 min and then the time was halved and doubled. It is noteworthy that attrition milling is a high energy milling which in addition to mixing, reduces the size of particles. As the milling time increases, the particles size decreases and consequently particles have better contact with each other. It has been reported that the reduction of the particle size results in a better combustion [22]. However, contrary to what was thought, the change in the milling time from 30 min to 120 min had negligible effects on the products, as can be seen in Fig. 2. Therefore, in the present work, it can be inferred that the alteration in the milling time, albeit in this limit, does not have the desired effects on the completion of the combustion and the increase in the conversion degree. Note that in the previous reports, pure elements, as the starting materials, directly react to synthesize products; thus, the particle size reduction could affect the combustion reaction. However, in this work, Si3N4 as the source of Si should be initially decomposed, as the controlling step, to proceed the reaction; and according to the results, the milling time and size reduction do not affect the decomposition and thereby reaction. According to Juang et al. [20], adding extra carbon to the starting powders results in a nearly pure AlN–SiC ceramics and also the best homogeneity of the ceramics. In this work, the amount of carbon in the reactant mixtures was increased to twice and four times of its stoichiometric weight. The influences on the XRD patterns of the products are shown in Fig. 3. As can be seen, increasing the amount of carbon did not affect the products. This disagreement is due to the difference in the starting materials used in Ref. [20] and the present work. In Ref. [20], the starting materials included pure Si, where Si is available to react
The reactant powder mixtures were uniaxially pressed into 0.5 g cylindrical pellets. The combustion synthesis reactions of the samples were conducted in a 900 W domestic microwave oven (LF-570NR, LG, Korea). The reaction temperature was monitored by a two-color pyrometer (VF-3000, Optex, Japan) that was focused on the sample. The chemical composition and microstructure of the combustion products were identified by X-ray diffraction (XRD, Advance D8, Bruker, Germany) and scanning electron microscopy (SEM, S-360, Oxford, UK), respectively.
Table 1 Characteristics of the commercial powders. Material
Average particle size (μm)
Purity (%)
Source
Aluminum Si3N4 Graphite
45 0.5–1.0 90
99 99.5 99.9
K.M.P., Iran SkySpring Nanomaterials, Inc, USA –
Fig. 1. XRD pattern of the product obtained from the combustion reaction between the stoichiometric mixture of Al, Si3N4 and C powders according to Reaction (1).
Z. Abbasi et al. / Powder Technology 249 (2013) 181–185
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Table 2 JCPDS card numbers of the phases. Phases
JCPDS card numbers
Al C Si3N4 Si AlN (cubic) AlN (hexagonal) SiC
00-004-0787 01-075-2078 00-041-0360 00-027-1402 00-046-1200 00-025-1133 00-029-1131
with C and to produce SiC from the start. However, in this work, the reaction of Si and C originally needs the decomposition of Si3N4 as the source of Si. Thus, adding extra carbon cannot be effective on the desired phase evolution in this circumstance. Fig. 4 shows the XRD patterns of the products obtained after changing the weight of the reactant compacts to four, sixteen and thirty two times of their original weight. When the weight of the compact was quadrupled, no significant changes were observed, except the increase in the amount of AlN. However, in the case of the 8 g samples, the Si3N4 peaks completely disappeared and the intensities of Al and C peaks decreased. In addition to the presence of new SiC peaks, the existing peaks were intensified. Furthermore, the position and intensity of AlN peaks were generally changed. When the weight of the compact was increased to 16 g, the Al peaks disappeared too and the intensity of the AlN and SiC peaks continued to increase. When the compact weight is increased, its surface area also increases. A larger sample results in a higher heat production per unit surface area of the sample, and thereby a higher temperature of the sample [23]. The temperature rise of the 2 g sample results in a slight increase in the amount of the Si3N4 decomposition and AlN formation. On the other hand, the larger amount of heat remains in the 8 g sample, due to the increase in the mass-to-surface ratio. The generated heat can entirely decompose Si3N4; hence, there is no trace of this phase in the product. From the study of the AlN characterization in the XRD analysis, it can be realized that the AlN produced in the previous cases has the cubic crystal structure, whereas the crystal structure of AlN in the present case (8 g sample) is hexagonal, which results in changes in the positions and intensities of the AlN peaks in the pattern. The formation of AlN and SiC has led to the reduction in the amount of residual Al and C in the samples. In the case of the 16 g samples, the conversion is to such an extent that leads to the complete consumption of Al and a more increase in the amount of AlN and SiC. It would be worth mentioning that in the samples experiencing the complete decomposition of Si3N4
Fig. 2. XRD patterns of the products obtained after alteration in milling time.
Fig. 3. XRD patterns of the products obtained after adding extra carbon to the starting powders.
(8 and 16 g samples), as well as the desired phases, there is some remained Si which has not reacted with C to produce SiC. This is due to the fact that in used heating regime, the sample does not remain at the high temperatures for a sufficient duration. Kexin et al. [19] reported that increasing and keeping the combustion temperature at a relatively high level are a key to decrease the unreacted Si in the combustion products. Accordingly, it is noteworthy that as the sample weight is increased, the productivity of the reaction is improved and only traces of the reactants remain in the reaction products. Furthermore, applying this approach can lead to the production of AlN and SiC, both with the hexagonal structure. The adiabatic temperature is an appropriate measure of the exothermicity of reactions. Providing the self-propagating mode was initiated at room temperature (298 K) without any preheat, the values of the adiabatic temperature (Tad) can be calculated based on the following equation [22]: Z ΔHð298Þ þ
Tadð298Þ X 298
njCpðPjÞdT þ ΣnjLðPjÞ ¼ 0
ð2Þ
where ΔH(298) is the reaction enthalpy at 298 K, nj is the stoichiometric coefficient of the product, CP(Pj) and L(Pj) are the heat capacity and the latent heat of the phase change (if the product goes through a phase change) of the product, respectively. The reaction enthalpy can
Fig. 4. XRD patterns of the products obtained after increasing the compact weights.
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be calculated using the formation enthalpy of each component [24]. In the present case, the obtained value of ΔH(298) for Reaction (1) is equal to −729.5 kJ. Now, considering that the product includes 4 moles of AlN and 3 moles of SiC, Equation (2) is converted to the following relationship: Z ‐ΔHð298Þ ¼
Tad h 298
i 4Cp ðAINÞ þ 3Cp ðSiCÞ dT
ð3Þ
Using thermodynamic data [24], the adiabatic temperature calculated from Equation (3), based on the complete combustion of the reactants to the products, is equal to 2043.2 °C. For this system, the maximum temperature measured using the pyrometer is 1027 °C (for 0.5 g sample). The difference between the actual combustion temperature and theoretical adiabatic temperature is an expected fact, due to heat loss [22] and the incomplete conversion of the reactants to the products. The maximum measured temperature for the 16 g sample is 1889 °C, which indicates that a more complete combustion is conducted and possibly a more microwave heat is absorbed under these conditions. According to the AlN–SiC phase diagram [6], there is a critical temperature around 1960 °C which separates the stable solid solution region and the two-phase region that tends to separate into AlN-rich and SiC-rich phases. At temperatures above 1960 °C, the AlN and SiC system is located in the hexagonal stable solid solution region. Since the maximum measured temperature in this study is 1889 °C and this value is lower than the critical temperature, it can be concluded that the obtained product is a composite of AlN and SiC. In the 8 g sample, AlN and SiC have a similar crystal structure; thus, they have the necessary condition for the formation of a solid solution. It is expected that increasing the sample size and therefore increasing the temperature result in the formation of a hexagonal stable solid solution between these two ceramics. According to previous papers [14,15,17], to achieve AlN–SiC ceramics (composites or solid solutions) through the reaction between Al, Si3N4 and C, it is essential to apply a strong electric field or conduct the reaction under a nitrogen atmosphere. In this study, the AlN–SiC composite were synthesized using microwave heating without applying an electric field or utilizing a nitrogen atmosphere. The cross section areas of the 0.5 g to 8 g pellets were constant (1.13 cm2), while their heights were changed with increasing the weight of the powders. The desired results were achieved in the 8 g sample. In the 16 g samples, the cross section areas of the pellets were increased to 3.14 cm2, which improved results. The SEM images of the 0.5 g, 2 g and 8 g samples are shown in Fig. 5a–c. The needle-like phase is Si3N4 which has remained unreacted in the products. As the sample size has been increased, the amount of the needle-like phase has decreased until this phase has been completely eliminated in the 8 g sample products. Increasing the sample size also has increased the particle size, possibly due to grain growth associated with the temperature rise. It is expected that the temperature rise associated with increasing sample size results in the achievement of a homogeneous and uniform morphology of a hexagonal phase via the formation of a solid solution. 4. Conclusions The feasibility of synthesizing AlN–SiC composite ceramics by a combustion synthesis reaction was investigated through igniting the powder mixtures of Al, Si3N4 and C in a domestic microwave oven. The synthesis was successfully conducted with no need to apply an electric field or to utilize a nitrogen atmosphere. Alteration in the milling time and the amount of carbon over its stoichiometry had no significant effect on the formation of the product, but in the case of increasing the sample size, the result was more desirable and a better product quality was obtained.
Fig. 5. SEM images of the products related to the effects of increasing the compact weight: a) 0.5 g, b) 2 g and c) 8 g (arrows show the needle-like Si3N4 phase).
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