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Fabrication of (Ca + Yb)- and (Ca + Sr)-stabilized α-SiAlON by combustion synthesis
Materials Research Bulletin 41 (2006) 547–552 www.elsevier.com/locate/matresbu
Fabrication of (Ca + Yb)- and (Ca + Sr)-stabilized a-SiAlON by combustion synthesis Guanghua Liu a,b, Kexin Chen a,*, Heping Zhou a, Kegang Ren a, C. Pereira b, J.M.F. Ferreira b a
Department of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, PR China b Department of Ceramics and Glass Engineering, University of Aveiro, CICECO, 3810-193, Aveiro, Portugal Received 26 June 2005; received in revised form 17 August 2005; accepted 16 September 2005 Available online 5 October 2005
Abstract In this paper, (Ca + Yb)- and (Ca + Sr)-stabilized a-SiAlON powders were fabricated by combustion synthesis. The influence of Ca2+ incorporation on the phase composition and grain morphology of combustion products was discussed. The experimental results showed that with the incorporation of Ca2+ well-developed rod-like (Ca + Yb) a-SiAlON crystals could be produced. It was also found that, when only Sr2+ was used as stabilizing cation, the reaction product was (a + b)-SiAlON composite, in which bSiAlON was the predominant phase and the relative content of a-SiAlON was low. With the incorporation of Ca2+, however, both the relative content and the lattice parameters of a-SiAlON were clearly increased. These results indicated that the incorporation of Ca2+ could assist Sr2+ into the a-SiAlON lattice structure. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Ceramics; A. Nitrides; B. Crystal growth; C. X-ray diffraction; C. Electron microscopy
1. Introduction Owing to the high hardness, good wear resistance, excellent thermal shock resistance, and reduced grain-boundary phases, a-SiAlON has been regarded as an ideal candidate for tribological and high temperature structural materials [1–5]. However, further application of a-SiAlON is generally limited by its poor toughness associated with the usually observed isotropic grain morphology. In this way, a variety of efforts have been made to fabricate new tough aSiAlON ceramics. It has been revealed that, by controlling the nucleation and grain growth carefully, a-SiAlON can also develop into rod-like crystals, and thus a-SiAlON ceramics with improved fracture toughness can be prepared [6–9]. Seeding has been proved to be an effective way for fabricating tough a-SiAlON ceramics toughened by coarse rodlike grains [10–12]. Two methods for preparing a-SiAlON seed crystals have been reported: liquid-phase growth and combustion synthesis [13–16]. Because of the low cost and easily pulverized products, combustion synthesis has great potential for industry applications. Recently, novel functional properties of a-SiAlON have been also noticed and the
* Corresponding author. Tel.: +86 10 62772548; fax: +86 10 62771160. E-mail address: [email protected] (K. Chen). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.09.013
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photoluminescence of a-SiAlON has been well studied [17–21]. As an economic and facile technique, combustion synthesis can probably provide a practical route to produce a-SiAlON phosphor powders in a large quantity. Up to now, a-SiAlON powders stabilized by Ca2+, Y3+, and Yb3+ cations have been prepared by combustion synthesis [14,16,22,23]. Compared with those stabilized by one cation, a-SiAlONs co-stabilized by two cations have more complex chemical compositions, thus having higher flexibility in tailoring the phase composition, grain morphology, and functional properties. This paper describes (Ca + Yb)- and (Ca + Sr)-stabilized a-SiAlON powders prepared by combustion synthesis. The effect of Ca2+ incorporation on the phase composition and grain morphology of reaction products is discussed. 2. Experimental procedure The compositions studied here were selected according to the general formula of Mm/zSi12 (m+n)Alm+nOnN16 n for a-SiAlON, where M stands for the stabilizing cation and z represents its chemical valence. Starting powders were prepared by using Yb2O3 (99.9%, General Research Institute for Nonferrous Metals, China), CaCO3 (A.R., Beijing Chemical Co., China), SrCO3 (A.R., Beijing Chemical Co., China), Si (99.0%, Fushun Al Factory, China), Al (99.5%, Gaizhou Al Co., China), a-Si3N4 (94% a, 1.7 wt.% O, Fangda High-Technology Ceramics Co., China), and SiO2 (A.R., Beijing Chemical Co., China). The surface oxygen of silicon nitride particles was considered when calculating the composition. The chemical formulas and starting compositions of all the samples are listed in Table 1. The raw materials were mixed by agate balls in a plastic jar for 24 h with absolute ethanol used as medium. The obtained slurry was dried in an oven at 70 8C for 8 h and then sieved. The reactant powder was poured into a porous graphite crucible and then placed in a special reaction chamber for combustion synthesis. The reaction chamber was evacuated to a vacuum of 10 4 MPa and then inflated with high-purity N2 at a pressure of 2 MPa. The sample was ignited by passing an electric current through a tungsten coil, and the reaction temperature was measured by a W-Re3/ W-Re25 thermocouple. Once the combustion reaction was triggered, it took place very quickly and the temperature was elevated drastically. The whole combustion synthesis process lasted no longer than 1 min, and the apex of reaction temperature was higher than 1800 8C. The phase assemblage was identified by X-ray diffraction (XRD; Cu Ka, Rigaku, Japan). The microstructure was examined by scanning electron microscopy (SEM; JSM-6460LV, JEOL, Japan) equipped with an energy dispersive spectroscopy detector (EDS; INCA, Oxford Instrument). 3. Results and discussion Fig. 1 shows XRD patterns of all the reaction products. In the samples YS, CYS1, and CYS2, a-SiAlON is the predominant crystalline phase, with a small amount of Si and the trace of b-SiAlON, AlN, and Yb2SiO5. These results are expected because the starting composition is located in the single-phase area of Yb a-SiAlON [24] and Ca2+ also has a high solid solution limitation [25]. Fig. 2 shows the SEM images of as-synthesized Yb and (Ca + Yb) a-SiAlON. In the sample YS, the aspect ratios of grains are low, as shown in Fig. 2(a). But in the samples CYS1 and CYS2, most grains are rod-like with higher aspect ratios, as shown in Fig. 2(b) and (c). These results reveal that the incorporation of Ca2+ enhances the anisotropic growth of rod-like grains. Fig. 2(d) shows the EDS result for a rod-like crystal in the sample CYS1, confirming that both Ca2+ and Yb3+ cations have entered the a-SiAlON lattice. Table 1 Chemical formulas and starting compositions of the samples Sample code
YS CYS1 CYS2 SS CSS
Chemical formula
Yb0.5Si9.5Al2.5O1.0N15 Yb0.4Ca0.15Si9.5Al2.5O1.0N15 Yb0.3Ca0.3Si9.5Al2.5O1.0N15 Sr0.8Si8.8Al3.2O1.6N14.4 Sr0.4Ca0.4Si8.8Al3.2O1.6N14.4
Starting composition (wt.%) Yb2O3
CaCO3
19.59 15.82 11.98
3.01 6.08 7.99
Ca/M
SrCO3
Si
a-Si3N4
Al
SiO2
22.71 11.79
32.65 32.96 33.27 29.64 30.76
32.65 32.96 33.27 27.56 28.61
13.68 13.81 13.94 16.63 17.26
1.43 1.44 1.46 3.46 3.60
Ca/Yb = 0.375 Ca/Yb = 1 Ca/Sr = 1
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Fig. 1. XRD patterns of the combustion products: (a) YS, CYS1, and CYS2; (b) SS and CSS.
Compared with a-Si3N4 and a-SiAlON, the anisotropic growth of rod-like b-Si3N4 and b-SiAlON crystals has been noticed much earlier and widely studied [26–28]. It has been reported that the anisotropic grain growth of bSi3N4 and b-SiAlON crystals is caused by the preferential interfacial segregation mechanism [29]. More recently, direct observation of rare-earth segregation in silicon nitride ceramics at subnanometre dimensions [30,31] has confirmed that the anisotropic grain growth of silicon nitride derives from preferential segregation, which suppresses diametrical growth involving the prismatic surfaces. Usually, the formation of a-SiAlON or b-SiAlON is fulfilled by the aid of a co-existing liquid phase, and the grain growth is related with the property of this liquid such as its relative amount and viscosity. Therefore, the final grain morphology strongly depends on the existence of grain-boundary glassy phase and its interaction with the crystal surface. Because of similar grain growth process of a-SiAlON and b-SiAlON, the above-mentioned reports on the anisotropic growth of b-Si3N4 and b-SiAlON should be partially valid for a-SiAlON. During the formation of rod-like (Ca + Yb) a-SiAlON crystals, two different growth modes are observed. In the first mode, many crystals grow simultaneously from the central liquid towards various directions, leading to a radiant flower-like morphology, as shown in Fig. 2(c). In the second mode, based on the smooth surface of as-existing rod-like crystals, new smaller a-SiAlON crystals can form and grow up. This growth mode usually results in a branchy shape, as indicated by arrows in Fig. 2(b). In previous study about the phase stability of a-SiAlON, it has been demonstrated that the stability as well as the single-phase area of a-SiAlON is related with the ionic radius of stabilizing cations [24]. Generally speaking, it is
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Fig. 2. SEM micrographs and EDS result of as-synthesized Yb and (Ca + Yb) a-SiAlON: (a) YS; (b) CYS1; (c) CYS2; (d) EDS result for a rod-like (Ca + Yb) a-SiAlON crystal in CYS1.
difficult for the cations such as Ce3+, La3+, and Sr2+ to enter the a-SiAlON crystal structure. However, it has been reported that a-SiAlONs doped by Eu, Pr, and Ce can be synthesized by combustion synthesis (also known as selfpropagating high-temperature synthesis) [32]. To the authors’ knowledge, combustion synthesis of Sr-stabilized aSiAlON has not been reported, although the effort at preparing Sr a-SiAlON ceramics by sintering has been made [33] and some Sr-containing SiAlON phases have been studied [34]. In this study, we attempt to prepare Sr a-SiAlON powders by combustion synthesis. Fig. 1(b) shows XRD patterns of the samples SS and CSS. Different with almost single-phase a-SiAlON obtained in YS, the reaction product in SS is (a + b)-SiAlON composite. But in CSS with incorporation of Ca2+, the phase content of a-SiAlON is remarkably increased with minor b-SiAlON found. It is also noticed that, in the XRD patter of SS, other than the diffraction peaks of a-SiAlON and b-SiAlON, there are another two small diffraction peaks. These two peaks have not been determinedly indexed (referred to as ‘‘un-indexed’’ in Fig. 1(b)), which may be caused by an intermediate phase. At the same time, in the XRD pattern of SS, the baseline slightly deviates from zero around the strongest peaks, which suggests the existence of a glassy phase. Since Sr content is relative high in the starting composition of SS but only minor Sr2+ enters the a-SiAlON lattice in the product, the above-mentioned un-indexed phase and glassy phase should be rich in Sr element, so as to balance the overall composition. Based on the XRD patterns, the lattice parameters of a-SiAlON in the samples SS and CSS are calculated. The ˚ , c = 5.703 A ˚ ) are much larger than those in results show that the lattice parameters of a-SiAlON in CSS (a = 7.853 A ˚ ˚ SS (a = 7.808 A, c = 5.665 A). From the obviously smaller lattice parameters of a-SiAlON in SS, it seems that not all
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Fig. 3. SEM micrographs of the combustion products of (a) SS and (b) CSS.
but only partial Sr2+ has entered the a-SiAlON lattice. The increased a-SiAlON lattice parameters in CSS reveal that the incorporation of Ca2+ has greatly assisted Sr2+ into a-SiAlON lattice and increased the solid solution of Sr2+. Fig. 3 shows images of the reaction products of SS and CSS. In both samples, rod-like grains are observed, but their aspect ratios in SS are clearly higher than in CSS. This difference may be attributed to stronger interfacial segregation effect of Sr2+ compared with Ca2+. Of course, it should be noticed that the product of SS is (a + b)-SiAlON composite and the majority of rod-like crystals are b-SiAlON. In this case, further investigations (such as TEM study including selected area electronic diffraction pattern and EDS) are required in the future for further understanding of the nucleation and growth character of Sr a-SiAlON crystals. 4. Conclusion (Ca + Yb) and (Ca + Sr) stabilized a-SiAlON powders with rod-like crystals were fabricated by combustion synthesis. The effect of Ca2+ incorporation on the phase composition and grain morphology of the combustion products was studied. It was found that Ca2+ incorporation was helpful to enhance the anisotropic growth of rod-like crystals in (Ca + Yb) a-SiAlON. At the same time, Sr (a + b)-SiAlON powder composite was synthesized, where bSiAlON was the predominant product and the phase content of a-SiAlON was low. With the incorporation of Ca2+, more Sr2+ could enter a-SiAlON lattice and the phase content as well as the lattice parameters of a-SiAlON could be increased.
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Acknowledgements This work is supported by National Natural Science Foundation of China (Grant No. 50102002), and by the Foundation for Science and Technology of Portugal, Project FCT SAPIENS—Reference: POCTI/CTM/39419/2001. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
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Fabrication of (Ca + Yb)- and (Ca + Sr)-stabilized a-SiAlON by combustion synthesis Guanghua Liu a,b, Kexin Chen a,*, Heping Zhou a, Kegang Ren a, C. Pereira b, J.M.F. Ferreira b a
Department of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, PR China b Department of Ceramics and Glass Engineering, University of Aveiro, CICECO, 3810-193, Aveiro, Portugal Received 26 June 2005; received in revised form 17 August 2005; accepted 16 September 2005 Available online 5 October 2005
Abstract In this paper, (Ca + Yb)- and (Ca + Sr)-stabilized a-SiAlON powders were fabricated by combustion synthesis. The influence of Ca2+ incorporation on the phase composition and grain morphology of combustion products was discussed. The experimental results showed that with the incorporation of Ca2+ well-developed rod-like (Ca + Yb) a-SiAlON crystals could be produced. It was also found that, when only Sr2+ was used as stabilizing cation, the reaction product was (a + b)-SiAlON composite, in which bSiAlON was the predominant phase and the relative content of a-SiAlON was low. With the incorporation of Ca2+, however, both the relative content and the lattice parameters of a-SiAlON were clearly increased. These results indicated that the incorporation of Ca2+ could assist Sr2+ into the a-SiAlON lattice structure. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Ceramics; A. Nitrides; B. Crystal growth; C. X-ray diffraction; C. Electron microscopy
1. Introduction Owing to the high hardness, good wear resistance, excellent thermal shock resistance, and reduced grain-boundary phases, a-SiAlON has been regarded as an ideal candidate for tribological and high temperature structural materials [1–5]. However, further application of a-SiAlON is generally limited by its poor toughness associated with the usually observed isotropic grain morphology. In this way, a variety of efforts have been made to fabricate new tough aSiAlON ceramics. It has been revealed that, by controlling the nucleation and grain growth carefully, a-SiAlON can also develop into rod-like crystals, and thus a-SiAlON ceramics with improved fracture toughness can be prepared [6–9]. Seeding has been proved to be an effective way for fabricating tough a-SiAlON ceramics toughened by coarse rodlike grains [10–12]. Two methods for preparing a-SiAlON seed crystals have been reported: liquid-phase growth and combustion synthesis [13–16]. Because of the low cost and easily pulverized products, combustion synthesis has great potential for industry applications. Recently, novel functional properties of a-SiAlON have been also noticed and the
* Corresponding author. Tel.: +86 10 62772548; fax: +86 10 62771160. E-mail address: [email protected] (K. Chen). 0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2005.09.013
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photoluminescence of a-SiAlON has been well studied [17–21]. As an economic and facile technique, combustion synthesis can probably provide a practical route to produce a-SiAlON phosphor powders in a large quantity. Up to now, a-SiAlON powders stabilized by Ca2+, Y3+, and Yb3+ cations have been prepared by combustion synthesis [14,16,22,23]. Compared with those stabilized by one cation, a-SiAlONs co-stabilized by two cations have more complex chemical compositions, thus having higher flexibility in tailoring the phase composition, grain morphology, and functional properties. This paper describes (Ca + Yb)- and (Ca + Sr)-stabilized a-SiAlON powders prepared by combustion synthesis. The effect of Ca2+ incorporation on the phase composition and grain morphology of reaction products is discussed. 2. Experimental procedure The compositions studied here were selected according to the general formula of Mm/zSi12 (m+n)Alm+nOnN16 n for a-SiAlON, where M stands for the stabilizing cation and z represents its chemical valence. Starting powders were prepared by using Yb2O3 (99.9%, General Research Institute for Nonferrous Metals, China), CaCO3 (A.R., Beijing Chemical Co., China), SrCO3 (A.R., Beijing Chemical Co., China), Si (99.0%, Fushun Al Factory, China), Al (99.5%, Gaizhou Al Co., China), a-Si3N4 (94% a, 1.7 wt.% O, Fangda High-Technology Ceramics Co., China), and SiO2 (A.R., Beijing Chemical Co., China). The surface oxygen of silicon nitride particles was considered when calculating the composition. The chemical formulas and starting compositions of all the samples are listed in Table 1. The raw materials were mixed by agate balls in a plastic jar for 24 h with absolute ethanol used as medium. The obtained slurry was dried in an oven at 70 8C for 8 h and then sieved. The reactant powder was poured into a porous graphite crucible and then placed in a special reaction chamber for combustion synthesis. The reaction chamber was evacuated to a vacuum of 10 4 MPa and then inflated with high-purity N2 at a pressure of 2 MPa. The sample was ignited by passing an electric current through a tungsten coil, and the reaction temperature was measured by a W-Re3/ W-Re25 thermocouple. Once the combustion reaction was triggered, it took place very quickly and the temperature was elevated drastically. The whole combustion synthesis process lasted no longer than 1 min, and the apex of reaction temperature was higher than 1800 8C. The phase assemblage was identified by X-ray diffraction (XRD; Cu Ka, Rigaku, Japan). The microstructure was examined by scanning electron microscopy (SEM; JSM-6460LV, JEOL, Japan) equipped with an energy dispersive spectroscopy detector (EDS; INCA, Oxford Instrument). 3. Results and discussion Fig. 1 shows XRD patterns of all the reaction products. In the samples YS, CYS1, and CYS2, a-SiAlON is the predominant crystalline phase, with a small amount of Si and the trace of b-SiAlON, AlN, and Yb2SiO5. These results are expected because the starting composition is located in the single-phase area of Yb a-SiAlON [24] and Ca2+ also has a high solid solution limitation [25]. Fig. 2 shows the SEM images of as-synthesized Yb and (Ca + Yb) a-SiAlON. In the sample YS, the aspect ratios of grains are low, as shown in Fig. 2(a). But in the samples CYS1 and CYS2, most grains are rod-like with higher aspect ratios, as shown in Fig. 2(b) and (c). These results reveal that the incorporation of Ca2+ enhances the anisotropic growth of rod-like grains. Fig. 2(d) shows the EDS result for a rod-like crystal in the sample CYS1, confirming that both Ca2+ and Yb3+ cations have entered the a-SiAlON lattice. Table 1 Chemical formulas and starting compositions of the samples Sample code
YS CYS1 CYS2 SS CSS
Chemical formula
Yb0.5Si9.5Al2.5O1.0N15 Yb0.4Ca0.15Si9.5Al2.5O1.0N15 Yb0.3Ca0.3Si9.5Al2.5O1.0N15 Sr0.8Si8.8Al3.2O1.6N14.4 Sr0.4Ca0.4Si8.8Al3.2O1.6N14.4
Starting composition (wt.%) Yb2O3
CaCO3
19.59 15.82 11.98
3.01 6.08 7.99
Ca/M
SrCO3
Si
a-Si3N4
Al
SiO2
22.71 11.79
32.65 32.96 33.27 29.64 30.76
32.65 32.96 33.27 27.56 28.61
13.68 13.81 13.94 16.63 17.26
1.43 1.44 1.46 3.46 3.60
Ca/Yb = 0.375 Ca/Yb = 1 Ca/Sr = 1
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Fig. 1. XRD patterns of the combustion products: (a) YS, CYS1, and CYS2; (b) SS and CSS.
Compared with a-Si3N4 and a-SiAlON, the anisotropic growth of rod-like b-Si3N4 and b-SiAlON crystals has been noticed much earlier and widely studied [26–28]. It has been reported that the anisotropic grain growth of bSi3N4 and b-SiAlON crystals is caused by the preferential interfacial segregation mechanism [29]. More recently, direct observation of rare-earth segregation in silicon nitride ceramics at subnanometre dimensions [30,31] has confirmed that the anisotropic grain growth of silicon nitride derives from preferential segregation, which suppresses diametrical growth involving the prismatic surfaces. Usually, the formation of a-SiAlON or b-SiAlON is fulfilled by the aid of a co-existing liquid phase, and the grain growth is related with the property of this liquid such as its relative amount and viscosity. Therefore, the final grain morphology strongly depends on the existence of grain-boundary glassy phase and its interaction with the crystal surface. Because of similar grain growth process of a-SiAlON and b-SiAlON, the above-mentioned reports on the anisotropic growth of b-Si3N4 and b-SiAlON should be partially valid for a-SiAlON. During the formation of rod-like (Ca + Yb) a-SiAlON crystals, two different growth modes are observed. In the first mode, many crystals grow simultaneously from the central liquid towards various directions, leading to a radiant flower-like morphology, as shown in Fig. 2(c). In the second mode, based on the smooth surface of as-existing rod-like crystals, new smaller a-SiAlON crystals can form and grow up. This growth mode usually results in a branchy shape, as indicated by arrows in Fig. 2(b). In previous study about the phase stability of a-SiAlON, it has been demonstrated that the stability as well as the single-phase area of a-SiAlON is related with the ionic radius of stabilizing cations [24]. Generally speaking, it is
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Fig. 2. SEM micrographs and EDS result of as-synthesized Yb and (Ca + Yb) a-SiAlON: (a) YS; (b) CYS1; (c) CYS2; (d) EDS result for a rod-like (Ca + Yb) a-SiAlON crystal in CYS1.
difficult for the cations such as Ce3+, La3+, and Sr2+ to enter the a-SiAlON crystal structure. However, it has been reported that a-SiAlONs doped by Eu, Pr, and Ce can be synthesized by combustion synthesis (also known as selfpropagating high-temperature synthesis) [32]. To the authors’ knowledge, combustion synthesis of Sr-stabilized aSiAlON has not been reported, although the effort at preparing Sr a-SiAlON ceramics by sintering has been made [33] and some Sr-containing SiAlON phases have been studied [34]. In this study, we attempt to prepare Sr a-SiAlON powders by combustion synthesis. Fig. 1(b) shows XRD patterns of the samples SS and CSS. Different with almost single-phase a-SiAlON obtained in YS, the reaction product in SS is (a + b)-SiAlON composite. But in CSS with incorporation of Ca2+, the phase content of a-SiAlON is remarkably increased with minor b-SiAlON found. It is also noticed that, in the XRD patter of SS, other than the diffraction peaks of a-SiAlON and b-SiAlON, there are another two small diffraction peaks. These two peaks have not been determinedly indexed (referred to as ‘‘un-indexed’’ in Fig. 1(b)), which may be caused by an intermediate phase. At the same time, in the XRD pattern of SS, the baseline slightly deviates from zero around the strongest peaks, which suggests the existence of a glassy phase. Since Sr content is relative high in the starting composition of SS but only minor Sr2+ enters the a-SiAlON lattice in the product, the above-mentioned un-indexed phase and glassy phase should be rich in Sr element, so as to balance the overall composition. Based on the XRD patterns, the lattice parameters of a-SiAlON in the samples SS and CSS are calculated. The ˚ , c = 5.703 A ˚ ) are much larger than those in results show that the lattice parameters of a-SiAlON in CSS (a = 7.853 A ˚ ˚ SS (a = 7.808 A, c = 5.665 A). From the obviously smaller lattice parameters of a-SiAlON in SS, it seems that not all
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Fig. 3. SEM micrographs of the combustion products of (a) SS and (b) CSS.
but only partial Sr2+ has entered the a-SiAlON lattice. The increased a-SiAlON lattice parameters in CSS reveal that the incorporation of Ca2+ has greatly assisted Sr2+ into a-SiAlON lattice and increased the solid solution of Sr2+. Fig. 3 shows images of the reaction products of SS and CSS. In both samples, rod-like grains are observed, but their aspect ratios in SS are clearly higher than in CSS. This difference may be attributed to stronger interfacial segregation effect of Sr2+ compared with Ca2+. Of course, it should be noticed that the product of SS is (a + b)-SiAlON composite and the majority of rod-like crystals are b-SiAlON. In this case, further investigations (such as TEM study including selected area electronic diffraction pattern and EDS) are required in the future for further understanding of the nucleation and growth character of Sr a-SiAlON crystals. 4. Conclusion (Ca + Yb) and (Ca + Sr) stabilized a-SiAlON powders with rod-like crystals were fabricated by combustion synthesis. The effect of Ca2+ incorporation on the phase composition and grain morphology of the combustion products was studied. It was found that Ca2+ incorporation was helpful to enhance the anisotropic growth of rod-like crystals in (Ca + Yb) a-SiAlON. At the same time, Sr (a + b)-SiAlON powder composite was synthesized, where bSiAlON was the predominant product and the phase content of a-SiAlON was low. With the incorporation of Ca2+, more Sr2+ could enter a-SiAlON lattice and the phase content as well as the lattice parameters of a-SiAlON could be increased.
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Acknowledgements This work is supported by National Natural Science Foundation of China (Grant No. 50102002), and by the Foundation for Science and Technology of Portugal, Project FCT SAPIENS—Reference: POCTI/CTM/39419/2001. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
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