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Combustion synthesis of rod-like α-SiAlON seed crystals
Materials Letters 58 (2004) 1956 – 1958 www.elsevier.com/locate/matlet
Combustion synthesis of rod-like a-SiAlON seed crystals Renli Fu a,b, Kexin Chen c, Xin Xu b, Jose´ M.F. Ferreira b,* a
School of Mechanical-Electronic and Materials Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221008, People Republic China b Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193, Aveiro, Portugal c State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, People Republic China Received 15 July 2003; accepted 19 December 2003
Abstract Rod-like single-phase crystals of Y-a-SiAlON were synthesized by combustion of Si, Al, a-Si3N4, SiO2 and Y2O3 powders. SEM observation of crystals, separated by chemical etching (HNO3/HF=2:1) followed by ultrasonic agitation in ethanol showed rod-like crystals, assigned to almost pure and well crystallized a-SiAlON. D 2004 Elsevier B.V. All rights reserved.
The a-SiAlON (aV) phase is a solid solution of a-Si3N4, which usually develops as equiaxed grains in the sintered microstructures. It has the advantage of improved hardness relative to the h-SiAlON phase (hV, isostructural with hSi3N4), but its fracture toughness is generally thought inferior to that of hV-based materials, which develops as elongated grains [1]. More recently, it has been proved that a-SiAlON ceramics can also be produced with elongated grain morphologies [2]. Such discovery represented a great breakthrough in state of the art, suggesting that hard and tough a-SiAlON ceramics could be developed. Compared with ceramics based on h-Si3N4, which also have high toughness but a lower hardness, the microstructure of a-SiAlON is more sensitive to the composition, the processing conditions, and the staring powders [3]. As a result, only recently have a-SiAlON ceramics with elongated grains that enable in situ toughening been developed by carefully controlling the nucleation step [4]. In view of the complexity of microstructure control, seeding with elongated particles seems to offer an attractive solution because it has effect on templating growth of elongated grains. Several reports on seeding in h-Si3N4 based ceramics clearly demonstrated the great potential of this method in silicon nitride ceramics [5,6]. Preliminary efforts for seeding a-SiAlON using pulverized ceramic fragments as seeds have also been reported [7,8]. More recently, Chen et al. [9,10] and Zen-
* Corresponding author. Tel.: +351-234-370242; fax: +351-234425300. E-mail address: [email protected] (J.M.F. Ferreira). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.12.014
otchkine et al. [11] separately reported the synthesis of rodlike Ca a-SiAlON crystals [9] and yttrium a-SiAlON crystals [10] using the combustion synthesis method, or by the growing of single crystal seeds from liquid [11]. A dramatic effect of these crystalline seeds on the enhancement of resistance (R-curve) behaviour of yttrium- and calcium-containing a-SiAlON ceramics has been observed [12,13]. Combustion synthesis technology attracted the interest of many researchers as an energy and timesaving process [14]. Compared with the a-SiAlON seed crystals prepared by the liquid method, the rod-like a-SiAlON crystals prepared by combustion synthesis have a lower contamination level and a higher degree of conversion and are easy to disperse. These characteristics make the rod-like a-SiAlON crystal synthesized by the combustion synthesis method more attractive, conferring them a higher potential for industrial applications. The aim of this work is the combustion synthesis of rodlike yttrium a-SiAlON crystalline particles and preparation of well-dispersed a-SiAlON seed crystals. The compositions studied here are located in the socalled yttrium a-SiAlON plane, which is defined as Ym/3 Si12 (m + n)Al(m + n)OnN16 n [1]. The values of m = 1.44 and n = 0.96 were selected for composition of samples. Starting powder mixtures were prepared by using Si (Sigma-Aldrich, 99%, Germany), a-Si3N4 (95% H. C. Stark, Germany), Al (Riedel-de Hae¨n, Germany), Y2O3 (H. C. Stark, Germany), SiO2 (Riedel-de Hae¨n). Powder mixtures were prepared by wet milling with ethanol as liquid medium using planetary ball mill (Restch, PM 400,
R. Fu et al. / Materials Letters 58 (2004) 1956–1958
1957
Fig. 3. SEM micrographs of rod-like yttrium a-SiAlON seed crystals. Fig. 1. XRD patterns of the combusted product before and after chemical treatments and washing.
Germany) for 8 h using an agate jar and agate balls. The milled slurries were dried at 40 jC in an oven with airblower. The mixtures were put into a porous crucible, which was then placed into a high-pressure chamber. The highpressure chamber was illustrated in a previous report [9]. After evacuation was carried out up to a vacuum of 10 3 MPa, high purity nitrogen gas (99.99%) was admitted into the chamber at different pressure values. Then, the powders mixture was ignited by passing a DC electric current of 30 A though a tungsten coil. The combustion reaction temperature was measured through the voltage of W/3% Re – W/ 25% Re thermocouples, which were directly inserted into the samples. XRD (Rigaku Geigerflex D/Mac, C Series, Cu Ka radiation, Japan) and SEM (Hitachi S-4100, 25 kV acceleration voltage, Tokyo, Japan,) with associated facilities for energy dispersive spectroscopy identified the phase’s formation and revealed the microstructural features of the synthesised products.
The as-synthesized samples were then dispersed by chemical treatments using successively the following solutions [11]: (a) a mixture of concentrated HNO3 (68%) and HF (40%) (HNO3:HF = 2:1 v/v) held at room temperature to remove the glass phase between the seed crystal; (b) H2SO4 (95%) at room temperature to remove the Y-containing compounds formed after the first step of washing; (c) 3% HF at room temperature to remove the surface SiO2; (d) 1% NH4OH at room temperature to remove the residual HF. A sample with 5% of NH4F as additive and 20% a-Si3N4 as diluting agent was synthesized under a pressure 4 MPa of nitrogen. The XRD patterns of the combusted product before and after chemical treatments are shown in Fig. 1. It can be noticed that the combusted product of the sample is almost exclusively formed by crystalline yttirum a-SiAlON with only a trace of h-SiAlON. After the chemical treatment as mention above, the phases in the products did not change, merely the intensity of the h phase peaks seem to have been slightly strengthened. This small difference might be due to the removal of the glassy phase attached on the surface of
Fig. 2. Morphologies of the combusted products synthesized under 2.0 MPa nitrogen pressure.
Fig. 4. XRD patterns of the vapors condensed at the wall of the reactor chamber.
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R. Fu et al. / Materials Letters 58 (2004) 1956–1958
rod-like particles by the chemical treatments. Fig. 2 shows the morphologies of the products obtained from a sample combustion synthesized under a nitrogen pressure of 2.0 MPa. It can be seen that the combusted products exhibit well-developed elongated hexagonal crystals with highly agglomerative features. After all the chemical treatments, the powders were washed and dispersed in ethanol with ultrasonic assisted agitation. The elongated hexagonal crystals presented typical dimensions varying in the following ranges: widths from about 0.5 Am to about 3.0 Am, and lengths from about 2.0 Am to about 10.0 Am, as shown as Fig. 3. The experimental results revealed that the amounts of NH4F additive and diluting agent, and the nitrogen pressure have a strong influence on the crystalline phases formed and on the morphology of reaction products, and only the mixture with suitable amounts of HN4F and diluting additives under low nitrogen pressure would benefit the formation a phase. After the combustion synthesis process, a deposited powder layer was found at the inner wall of the reactor chamber, which resulted from the condensation of vapours generated at high temperature. This powder was gathered, analysed by XRD, and identified as (NH4)2SiF6 (Fig. 4). The formation of silicon fluorides is considered to be associated with the catalytic effect of NH4F on the nitridation of silicon. This catalytic effect is possible because fluoride species (i. e., SiFx) can react with nitrogen or NH3 in the gas phase and thus provide an easier route for the nitridation of silicon.
From the results discussed above, rod-like single-phase crystals of Y-a-SiAlON were synthesized by combustion of Si, Al, a-Si3N4, SiO2 and Y2O3 powders. SEM observation of crystals, separated by chemical etching (HNO3/HF = 2:1) followed by ultrasonic agitation in ethanol showed rod-like crystals, assigned to almost pure and well crystallized a-SiAlON.
References [1] S. Hampshire, H.K. Park, D.P. Thompson, K.H. Jack, Nature 274 (1978) 880. [2] I.W. Chen, A. Rosenflanz, Nature 389 (1997) 701. [3] W.W. Chen, W.Y. Sun, Y.W. Li, D.S. Yan, Mater. Lett. 46 (2000) 343. [4] H. Wang, Y.B. Cheng, C. Muddle, L. Gao, T.S. Yen, J. Mater. Sci. Lett 15 (1996) 447. [5] K. Hiro, T. Nagaoka, M.E. Brito, S. Kanzaki, J. Am. Ceram. Soc. 77 (1994) 1857. [6] S.Y. Lee, K.A. Appiagyei, H.D. Kim, J. Mater. Res. 14 (1999) 178. [7] J. Kim, A. Rosenflanz, I.-W. Chen, J. Am. Ceram. Soc. 83 (2000) 1819. [8] W.W. Chen, W.Y. Sun, Y.W. Li, D.S. Yan, J. Mater. Res. 15 (2000) 2223. [9] K. Chen, H. Jin, M. Oliveira, H. Zhou, J.M.F. Ferreira, J. Mater. Res. 16 (2001) 1928. [10] K. Chen, M.E.F.L. Costa, H. Zhou, J.M.F. Ferreira, Mater. Chem. Phys. 75 (2002) 252. [11] M. Zenotchkine, R. Shuba, J.-S. Kim, I.-W. Chen, J. Am. Ceram. Soc. 84 (2001) 1651. [12] M. Zenotchkine, R. Shuba, J.-S. Kim, I.-W. Chen, J. Am. Ceram. Soc. 85 (2002) 1254. [13] R. Shuba, I.-W. Chen, J. Am. Ceram. Soc. 85 (2002) 1260. [14] A. Varma, J.P. Lebrat, Chem. Eng. Sci. 47 (1992) 2179.
Combustion synthesis of rod-like a-SiAlON seed crystals Renli Fu a,b, Kexin Chen c, Xin Xu b, Jose´ M.F. Ferreira b,* a
School of Mechanical-Electronic and Materials Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221008, People Republic China b Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193, Aveiro, Portugal c State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, People Republic China Received 15 July 2003; accepted 19 December 2003
Abstract Rod-like single-phase crystals of Y-a-SiAlON were synthesized by combustion of Si, Al, a-Si3N4, SiO2 and Y2O3 powders. SEM observation of crystals, separated by chemical etching (HNO3/HF=2:1) followed by ultrasonic agitation in ethanol showed rod-like crystals, assigned to almost pure and well crystallized a-SiAlON. D 2004 Elsevier B.V. All rights reserved.
The a-SiAlON (aV) phase is a solid solution of a-Si3N4, which usually develops as equiaxed grains in the sintered microstructures. It has the advantage of improved hardness relative to the h-SiAlON phase (hV, isostructural with hSi3N4), but its fracture toughness is generally thought inferior to that of hV-based materials, which develops as elongated grains [1]. More recently, it has been proved that a-SiAlON ceramics can also be produced with elongated grain morphologies [2]. Such discovery represented a great breakthrough in state of the art, suggesting that hard and tough a-SiAlON ceramics could be developed. Compared with ceramics based on h-Si3N4, which also have high toughness but a lower hardness, the microstructure of a-SiAlON is more sensitive to the composition, the processing conditions, and the staring powders [3]. As a result, only recently have a-SiAlON ceramics with elongated grains that enable in situ toughening been developed by carefully controlling the nucleation step [4]. In view of the complexity of microstructure control, seeding with elongated particles seems to offer an attractive solution because it has effect on templating growth of elongated grains. Several reports on seeding in h-Si3N4 based ceramics clearly demonstrated the great potential of this method in silicon nitride ceramics [5,6]. Preliminary efforts for seeding a-SiAlON using pulverized ceramic fragments as seeds have also been reported [7,8]. More recently, Chen et al. [9,10] and Zen-
* Corresponding author. Tel.: +351-234-370242; fax: +351-234425300. E-mail address: [email protected] (J.M.F. Ferreira). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.12.014
otchkine et al. [11] separately reported the synthesis of rodlike Ca a-SiAlON crystals [9] and yttrium a-SiAlON crystals [10] using the combustion synthesis method, or by the growing of single crystal seeds from liquid [11]. A dramatic effect of these crystalline seeds on the enhancement of resistance (R-curve) behaviour of yttrium- and calcium-containing a-SiAlON ceramics has been observed [12,13]. Combustion synthesis technology attracted the interest of many researchers as an energy and timesaving process [14]. Compared with the a-SiAlON seed crystals prepared by the liquid method, the rod-like a-SiAlON crystals prepared by combustion synthesis have a lower contamination level and a higher degree of conversion and are easy to disperse. These characteristics make the rod-like a-SiAlON crystal synthesized by the combustion synthesis method more attractive, conferring them a higher potential for industrial applications. The aim of this work is the combustion synthesis of rodlike yttrium a-SiAlON crystalline particles and preparation of well-dispersed a-SiAlON seed crystals. The compositions studied here are located in the socalled yttrium a-SiAlON plane, which is defined as Ym/3 Si12 (m + n)Al(m + n)OnN16 n [1]. The values of m = 1.44 and n = 0.96 were selected for composition of samples. Starting powder mixtures were prepared by using Si (Sigma-Aldrich, 99%, Germany), a-Si3N4 (95% H. C. Stark, Germany), Al (Riedel-de Hae¨n, Germany), Y2O3 (H. C. Stark, Germany), SiO2 (Riedel-de Hae¨n). Powder mixtures were prepared by wet milling with ethanol as liquid medium using planetary ball mill (Restch, PM 400,
R. Fu et al. / Materials Letters 58 (2004) 1956–1958
1957
Fig. 3. SEM micrographs of rod-like yttrium a-SiAlON seed crystals. Fig. 1. XRD patterns of the combusted product before and after chemical treatments and washing.
Germany) for 8 h using an agate jar and agate balls. The milled slurries were dried at 40 jC in an oven with airblower. The mixtures were put into a porous crucible, which was then placed into a high-pressure chamber. The highpressure chamber was illustrated in a previous report [9]. After evacuation was carried out up to a vacuum of 10 3 MPa, high purity nitrogen gas (99.99%) was admitted into the chamber at different pressure values. Then, the powders mixture was ignited by passing a DC electric current of 30 A though a tungsten coil. The combustion reaction temperature was measured through the voltage of W/3% Re – W/ 25% Re thermocouples, which were directly inserted into the samples. XRD (Rigaku Geigerflex D/Mac, C Series, Cu Ka radiation, Japan) and SEM (Hitachi S-4100, 25 kV acceleration voltage, Tokyo, Japan,) with associated facilities for energy dispersive spectroscopy identified the phase’s formation and revealed the microstructural features of the synthesised products.
The as-synthesized samples were then dispersed by chemical treatments using successively the following solutions [11]: (a) a mixture of concentrated HNO3 (68%) and HF (40%) (HNO3:HF = 2:1 v/v) held at room temperature to remove the glass phase between the seed crystal; (b) H2SO4 (95%) at room temperature to remove the Y-containing compounds formed after the first step of washing; (c) 3% HF at room temperature to remove the surface SiO2; (d) 1% NH4OH at room temperature to remove the residual HF. A sample with 5% of NH4F as additive and 20% a-Si3N4 as diluting agent was synthesized under a pressure 4 MPa of nitrogen. The XRD patterns of the combusted product before and after chemical treatments are shown in Fig. 1. It can be noticed that the combusted product of the sample is almost exclusively formed by crystalline yttirum a-SiAlON with only a trace of h-SiAlON. After the chemical treatment as mention above, the phases in the products did not change, merely the intensity of the h phase peaks seem to have been slightly strengthened. This small difference might be due to the removal of the glassy phase attached on the surface of
Fig. 2. Morphologies of the combusted products synthesized under 2.0 MPa nitrogen pressure.
Fig. 4. XRD patterns of the vapors condensed at the wall of the reactor chamber.
1958
R. Fu et al. / Materials Letters 58 (2004) 1956–1958
rod-like particles by the chemical treatments. Fig. 2 shows the morphologies of the products obtained from a sample combustion synthesized under a nitrogen pressure of 2.0 MPa. It can be seen that the combusted products exhibit well-developed elongated hexagonal crystals with highly agglomerative features. After all the chemical treatments, the powders were washed and dispersed in ethanol with ultrasonic assisted agitation. The elongated hexagonal crystals presented typical dimensions varying in the following ranges: widths from about 0.5 Am to about 3.0 Am, and lengths from about 2.0 Am to about 10.0 Am, as shown as Fig. 3. The experimental results revealed that the amounts of NH4F additive and diluting agent, and the nitrogen pressure have a strong influence on the crystalline phases formed and on the morphology of reaction products, and only the mixture with suitable amounts of HN4F and diluting additives under low nitrogen pressure would benefit the formation a phase. After the combustion synthesis process, a deposited powder layer was found at the inner wall of the reactor chamber, which resulted from the condensation of vapours generated at high temperature. This powder was gathered, analysed by XRD, and identified as (NH4)2SiF6 (Fig. 4). The formation of silicon fluorides is considered to be associated with the catalytic effect of NH4F on the nitridation of silicon. This catalytic effect is possible because fluoride species (i. e., SiFx) can react with nitrogen or NH3 in the gas phase and thus provide an easier route for the nitridation of silicon.
From the results discussed above, rod-like single-phase crystals of Y-a-SiAlON were synthesized by combustion of Si, Al, a-Si3N4, SiO2 and Y2O3 powders. SEM observation of crystals, separated by chemical etching (HNO3/HF = 2:1) followed by ultrasonic agitation in ethanol showed rod-like crystals, assigned to almost pure and well crystallized a-SiAlON.
References [1] S. Hampshire, H.K. Park, D.P. Thompson, K.H. Jack, Nature 274 (1978) 880. [2] I.W. Chen, A. Rosenflanz, Nature 389 (1997) 701. [3] W.W. Chen, W.Y. Sun, Y.W. Li, D.S. Yan, Mater. Lett. 46 (2000) 343. [4] H. Wang, Y.B. Cheng, C. Muddle, L. Gao, T.S. Yen, J. Mater. Sci. Lett 15 (1996) 447. [5] K. Hiro, T. Nagaoka, M.E. Brito, S. Kanzaki, J. Am. Ceram. Soc. 77 (1994) 1857. [6] S.Y. Lee, K.A. Appiagyei, H.D. Kim, J. Mater. Res. 14 (1999) 178. [7] J. Kim, A. Rosenflanz, I.-W. Chen, J. Am. Ceram. Soc. 83 (2000) 1819. [8] W.W. Chen, W.Y. Sun, Y.W. Li, D.S. Yan, J. Mater. Res. 15 (2000) 2223. [9] K. Chen, H. Jin, M. Oliveira, H. Zhou, J.M.F. Ferreira, J. Mater. Res. 16 (2001) 1928. [10] K. Chen, M.E.F.L. Costa, H. Zhou, J.M.F. Ferreira, Mater. Chem. Phys. 75 (2002) 252. [11] M. Zenotchkine, R. Shuba, J.-S. Kim, I.-W. Chen, J. Am. Ceram. Soc. 84 (2001) 1651. [12] M. Zenotchkine, R. Shuba, J.-S. Kim, I.-W. Chen, J. Am. Ceram. Soc. 85 (2002) 1254. [13] R. Shuba, I.-W. Chen, J. Am. Ceram. Soc. 85 (2002) 1260. [14] A. Varma, J.P. Lebrat, Chem. Eng. Sci. 47 (1992) 2179.