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Studies on the synthesis of α-Al2O3 nanopowders by the polyacrylamide gel method
Powder Technology 191 (2009) 91–97
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Powder Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p o w t e c
Studies on the synthesis of α-Al2O3 nanopowders by the polyacrylamide gel method M. Tahmasebpour a, A.A. Babaluo a,b,⁎, S. Shafiei a, E. Pipelzadeh a a b
Nanostructure Materials Research Center (NMRC), Sahand University of Technology, P.O. Box 51335/1996, Tabriz, I.R. Iran Institute of Polymeric Materials, Sahand University of Technology, P.O. Box 51335/1996, Tabriz, I.R. Iran
a r t i c l e
i n f o
Article history: Received 7 November 2007 Received in revised form 31 August 2008 Accepted 23 September 2008 Available online 10 October 2008 Keywords: Polyacrylamide gel Al2O3 Powders Nanostructure Crystallisation Morphology
a b s t r a c t The polyacrylamide sol–gel method is a simple, fast and cheap method used for the synthesis of a wide variety of nanopowders. However, the effects of various experimental conditions on the nanopowders' properties have not been reported. In this paper, the effects of atmospheric conditions, heating rate, type of precursor, monomer-to-salt ratio and solution concentration on the structural properties of synthesized αAl2O3 nanopowder were investigated. The results show that better stabilized networks were obtained using two-stage atmospheres. Finer nanoparticles were obtained at lower heating rates as a result of delay in phase transformations. The type of precursor had no significant effect on particle size of the final product, as illustrated by scanning electron microscopy images, but it directly affected nanoparticle crystallization and thermal degradation of the polymeric network. In addition, there was an inverse relationship between the particle size and the ratio of monomer to the precursor salt. The decrease in particle size was due to higher thermal stability of the polymeric network. The concentration of the starting solution had no effect on the structure of the final product. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In advanced ceramic technology, synthesis of ceramic powders on the nanometer scale is critical because the properties of the starting powder play an important role in determining the quality of the finished product [1]. Thus, in recent decades, increasing attention has been given to identifying better synthesis processes and to the structural characterization of ceramic nanoparticles [1–4]. Nanopowder synthesis by classical methods has its own shortcomings, such as uncontrolled crystalline growth, composition inhomogeneities, nonuniformity of grain size and high operating costs. Wet chemical methods such as sol–gel and modified sol–gel (Pechini), due to their atomic scale of operation, are more commonly used to avoid these shortcomings and to achieve better control of nanoparticle properties [5]. Alumina has been chosen for synthesis for its ability to produce high-strength materials and its wide range of applications such as electronic ceramics and catalysts [1]. Alumina is a structurally complex oxide having several different metastable phases (boehmite → γ → δ → θ-Al2O3) that are eventually converted to stable α-Al2O3 after controlled calcination at high temperatures, making its synthesis a challenging process. Two major problems are encountered during nanoparticle alumina synthesis, namely, calcination
and dehydration, where the former causes grain growth of the particles and the later promotes aggregation [2,6,7]. An auxiliary three-dimensional (3D) tangled polyacrylamide gel network using a polymer carrier has recently been used in solutionpolymerization techniques [7–10]. It is a wet chemical process that is fast, cheap, reproducible and easily scaled up to obtain a number of fine nanopowders [9]. This method is time-saving in relation to the Pechini method, and it provides ultra fine powders at relatively low temperatures [5]. The polyacrylamide gel method was introduced in 1989 by Odier to produce YBa2Cu3O7 − x [11]. Although this method has been used for the preparation of ultra fine powders, the optimal synthesis conditions on particle size distribution have not been systematically studied. The aim of the present work was to clarify the effects of synthesis conditions on the particle size distribution of alumina nanoparticles using the polyacrylamide gel method. The effects of atmospheric conditions, heating rate, type of precursor, monomer-to-salt ratio and solution concentration on the properties of the final products were tested. In order to achieve this aim, X-ray Diffraction (XRD), Fourier Transform Infra-Red (FTIR), Thermal analysis (TG-DTA), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) techniques were employed. 2. Experimental
⁎ Corresponding author. Nanostructure Materials Research Center (NMRC), Sahand University of Technology, P.O. Box 51335/1996, Tabriz, I.R. Iran. Tel.: +98 412 3459081; fax: +98 412 3444355. E-mail address: [email protected] (A.A. Babaluo). 0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.09.016
2.1. Materials The characteristics of the materials used are given in Table 1.
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Table 1 Characteristics of materials used Materials
Function
Acrylamide (AM)
Monofunctional monomer N,N′-Methylene bis acrylamide Difunctional monomer (MBAM) (crosslinker) Ammonium persulfate Initiator N,N,N′,N′-Tetramethyl ethylene Accelerator diamide (TEMED) Aluminum nitrate Initial salt Aluminum chloride Initial salt Water Solvent a b
Molecular formula
Supplier
C2H3CONH2
Mercka
(C2H3CONH2)2CH2 Mercka (NH4)2S2O8 C6H16N2
Mercka Mercka
Al(NO3)3.9H2O AlCl3.6H2O H2O
Mercka Mercka Ghazib
E. Merck, D 6100 Darmstadt, Germany. Shahid Ghazi Pharmaceutical Co, Tabriz, Iran.
2.2. Synthesis procedure
Fig. 2. FTIR absorption spectra of the polymeric gels prepared in different temperatures and atmospheres (▲: C–O–C group).
The precursors prepared from Al(NO3)3.9H2O and AlCl3.6H2O are denoted as AN and AC, respectively. Synthesis of nanoparticles was carried out by dissolving the appropriate amount of these precursors, followed by the addition of acrylamide (AM) and N,N′-methylene bis acrylamide (MBAM) monomers in a molar ratio of 22:1 (AM:MBAM), producing a transparent solution after stirring [12]. Freshly made 10% (w/v) ammonium persulfate (APS) and 1% (v/v) N,N,N′,N′-tetramethyl ethylene diamide (TEMED) were added to the premixed solution to act
as an initiator and an accelerator, respectively. A rapid polymerization was observed, forming a transparent polymeric gel without any precipitation, followed by homogenization in a ceramic mortar. Different samples were obtained after subsequent thermal treatment using an Ex.1200-2LA laboratory furnace. 2.3. Characterization XRD data were collected from the synthesized powders for phase identification and determination of crystallite size by SIEMENS D5000 and Philips TW3710 X'Pert diffractometers using CuKα radiation in the range of 2θ = 10°–80°. The polymeric gel was examined by differential thermal and thermogravimetric analysis with a thermal analyzer (RAS Diamond TG-DTA high temperature). FTIR spectra were obtained on a UNICAM Matson 1000 spectrophotometer using the KBr pellet method. Structure and morphology of the synthesized powders were analyzed by a CamScan MV2300 scanning electron microscope and a LEO-906 transmission electron microscope. 3. Results and discussion 3.1. Effect of atmosphere It has been reported that, in the polyacrylamide gel method, the presence of the polymeric network can effectively inhibit the
Fig. 1. TG (top) and DTA (bottom) curves under different atmospheres at a heating rate of 5 °C/min: (a) air (b) argon (c) two-stage: argon-air.
Fig. 3. TG curves of pure polyacrylamide gel at heating rates of 5, 10 and 20 °C/min under two-stage atmospheres.
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aggregation of powders at high temperatures [9]. Therefore, due to the high calcination temperatures of α-Al2O3 (1100–1200 °C), protecting the polymeric network from thermal degradation is critical to maintaining good control over particle size and morphology. Polyacrylamide network degradation was studied under air and argon atmospheres. A total degradation of the polymeric network was observed at an undesirable temperature of 600 °C under the air atmosphere (Fig. 1a). A considerable 26% polymeric decomposition was observed between 290 °C and 310 °C under an inert atmosphere, which can be confirmed by the presence of an exothermic peak at about 295 °C in the DTA curve. Despite degradation of the sample at lower temperatures under the argon atmosphere, approximately 15%
Fig. 5. XRD patterns of Al2O3 powders calcined at 900, 1000 and 1100 °C using two heating rates of 5 and 20 °C/min.
of the original mass of the network remained after heating to temperatures up to 600 °C (Fig. 1b). Increasing the thermal stability of the network under an air atmosphere and at low temperatures (∼ 300 °C) is probably associated with the fact that oxygen molecules can act as crosslinkers, as has been previously reported for the spinning of polyacrylonitrile fibres to produce carbon fibers [13,14]. FTIR data at 4000–400 cm− 1 for polymeric gels heated to different temperatures and in different atmospheres are shown in Fig. 2. Absorption at 1000–1300 cm− 1 is consistent with stretching vibration of the C–O–C group, which can be attributed to the crosslinked oxygen [6]. No peak was observed for the untreated gel above this wave number range using the FTIR analysis. This absorption band was observed for samples heated up to 250 °C under an air atmosphere, but it was not detected under an argon atmosphere. The characteristic peak disappears when the sample with C–O–C groups was heated to 400 °C under an air atmosphere, indicating thermal decomposition of the crosslinked oxygen. These results clearly show that the presence of oxygen has a strong effect on the thermal stability of the polyacrylamide network, in good agreement with the literature [13,14]. Using air initially, followed by an inert atmosphere, enabled an increase in polymeric network strength via the formation of C–O–C groups in air (b280 °C) and preservation of the network with an argon atmosphere at higher temperatures. Fig. 1c shows the TG-DTA patterns for a polymeric gel under two-stage atmospheres, indicating slow degradation of the polymeric gel, with about 25% of the original polymer mass preserved up to 600 °C. Products were further heated to 1100 °C under an inert atmosphere, and the remaining polymeric network was removed by calcination under an air atmosphere to obtain the final powders. 3.2. Effect of heating rate
Fig. 4. SEM images of α-Al2O3 powders prepared at different heating rates (a) 20 (b) 10 and (c) 5 °C/min under two-stage atmospheres.
Thermal decomposition of polyacrylamide gel was carried out at 5, 10 and 20 °C/min under two-stage atmospheres, and the results are shown in Fig. 3. Lower heating rates allow more time for the polymer to reach a given temperature, resulting in greater thermal
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decomposition, which would be expected to yield larger average nanoparticles. However, SEM images from the synthesized powders using different heating rates, shown in Fig. 4, demonstrate that a lower heating rate produces finer particles. In order to support this observation, further investigation was carried out using XRD analysis. Fig. 5 displays the XRD patterns of alumina powders calcinated at 900, 1000 and 1100 °C using two heating rates of 5 and 20 °C/min. γ-Al2O3 synthesis was poor for both heating rates at 900 °C. Further heating up to 1000 °C revealed the presence of θ-Al2O3 and α-Al2O3. The latter is dominant using the 20 °C/min heating rate, indicating acceleration in the phase transformation from θ-Al2O3 to α-Al2O3. α-Al2O3 was the main product observed when the synthesized particles were heated to 1100 °C during calcination, and the intensity of the pattern was greater for the higher heating rate. Due to its low transformation temperatures and more easily detectable XRD peaks, zirconia was used for evaluation of the effect of heating rate. Zirconia was reported to have three well-known crystalline forms, monoclinic (m), tetragonal (t) and cubic (c), at atmospheric pressure. The tetragonal phase is mainly seen at low calcination temperatures, but, as the temperature increases to 400 °C or higher, both monoclinic and tetragonal phases are detected. Pure monoclinic phase is observed after calcination above 800 °C [15,16]. Zirconium oxynitrate and zirconium oxychloride were used as precursors for producing the zirconia nanopowders. Fig. 6 displays the XRD patterns of the synthesized zirconia powders calcinated up to 600 °C using two heating rates of 5 and 20 °C/min. The results show that the tetragonal phase had a more intense XRD peak at 2θ 30° at the
Fig. 6. XRD patterns of ZrO2 prepared by oxynitrate salt (top) and oxychloride salt (bottom) calcined at 600 °C with 5 and 20 °C/min heating rates.
Fig. 7. TG curves of the polymeric gels prepared by nitrate (AN) and chloride (AC) salts.
higher rate of heating for both precursors. The monoclinic phase is also clearly observed at higher rate of heating, demonstrating a delay in tetragonal-to-monoclinic phase transformation at lower heating rates. These results suggest that lower heating rates for both alumina and zirconia cause a delay in the phase transformations, supporting the SEM results obtained for alumina synthesis.
Fig. 8. XRD patterns of Al2O3 powders prepared by AN and AC salts calcined at 800, 1000 and 1100 °C.
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Fig. 9. SEM images and SD curves of α-Al2O3 powders synthesized by various salts calcined at 1100 °C: (a) AN (b) AC.
3.3. Effect of precursor salt Fig. 7 illustrates the weight loss associated with AN and AC base polymeric gels, and the results indicate an acceleration in polymeric network degradation caused by the presence of nitrate ions, leading to nanoparticles of greater size than those obtained using chloride-based precursors. The X-ray patterns (Fig. 8) of both precursors after 2 h of calcination at 800 °C showed poor crystalline γ-Al2O3 synthesis. Furthermore, X-ray patterns of the products after heating to 1000 °C revealed the presence of θ-Al2O3 and α-Al2O3. The intensity of the α-Al2O3 pattern was greater in the AC-based sample than in the AN sample. These results suggest that the presence of nitrate ions delayed the θ-Al2O3 to α-Al2O3 phase transformation, and that the nucleation of α-Al2O3 occurs more heterogeneously in the AN sample. By increasing the calcination temperature to 1100 °C, the crystallinity of α-Al2O3 was completed, and the phase in both samples was mainly α-Al2O3. The above observations suggest two competing mechanisms: crystallization and polymeric network degradation. As shown by SEM images and size distribution (SD) curves taken from the products using different salt precursors at 1100 °C (Fig. 9), no significant difference was observed in particle size distribution, with an average size of 65 nm. This reveals that these two mechanisms eliminate each
others' effects. Although the size of the particles was in the same range, a narrower size distribution was achieved for the chloridebased precursor.
Fig. 10. TG curves of the polymeric gels prepared with different monomers to salt ratio.
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Fig. 11. TEM images and SD curves of α-Al2O3 powders obtained with different conditions: (a) M/S = 2, Concentration = 80 wt.% (b) M/S = 1, Concentration = 80 wt.% and (c) M/S = 2, Concentration = 50 wt.%.
3.4. Effect of monomer-to-salt ratio Monomer-to-salt ratio (M/S) plays a key role in the thermal stability of the polyacrylamide gel network, as demonstrated in Fig. 10, where TG curves are drawn for various M/S ratios (0.5, 1.0 and 2.0). The results show an increase in thermal stability with an increase in M/S ratio. At 0.5 M/S ratio, 92% loss of the original mass was observed when heated to 200 °C, but, as this ratio was increased, the rate of polymeric decomposition was reduced, resulting in less aggregation and producing finer nanoparticles. TEM images and SD curves (Fig. 11a
and b) support the above claims, indicating an increase from 65 nm to 100 nm particle size using M/S ratios of 2 and 1, respectively. 3.5. Effect of solution concentration TG curves of polymeric gels prepared with various solution concentrations (20, 50 and 80 wt.%) reveal that initial solution concentration had no significant effect on weight loss of the polymeric network (Fig. 12). Fig. 11a and c correspond to TEM and SD curve images of the synthesized powders using 80 and 50 wt.% solution concentration, respectively. As can be seen, both powders had an average size of ∼65 nm, indicating that solution concentration has no significant effect on particle size in the polyacrylamide gel method. 4. Conclusion In this study, the effects of the atmospheric condition, heating rate, monomer-to-salt ratio, choice of precursor, and concentration of the solution on the structural properties of synthesized α-Al2O3 nanoparticles using the polyacrylamide gel method were investigated. Conclusions are as summarized with the following remarks
Fig. 12. TG curves of the polymeric gels prepared with various solution concentrations (20, 50 and 80 wt.%).
• The presence of polymeric network can effectively inhibit the aggregation of powders at high temperatures. Heating the polymeric gel under an air atmosphere at low temperatures (b280 °C) followed by an inert atmosphere (two-stage atmospheres) results in a thermally stable polyacrylamide network. • Phase transformation delay is observed when low heating rates are applied, resulting in finer particles on average. • The choice of precursor affects the particle size via crystallization and polymeric network degradation mechanisms. These opposing mechanisms negate one another's effect, resulting in similarly sized particles for either precursor.
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• Higher monomer-to-precursor salt ratio promotes production of smaller particles. • Solution concentration has no effect on particle size in the polyacrylamide gel method. Acknowledgments The Authors wish to thank Dr. M.H. Pipelzadeh for editorial contributions, Sahand University of Technology (SUT) for the financial support of this work. We also, thank the co-workers and technical staff in the Institute of Polymeric Materials and Nanostructure Materials Research Center of SUT for their help during the various stages of this work. References [1] V.S. Giri, R. Sarathi, S.R. Chakravarthy, C. Venkataseshaiah, Mater. Lett. 58 (2004) 1047–1050. [2] Y.K. Park, E.H. Tadd, M. Zubris, R. Tannenbaum, Mater. Res. Bull. 40 (2005) 1506–1512.
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[3] V.V. Srdic, M. Winterer, J. Eur. Ceram. Soc. 26 (2006) 3145–3151. [4] J.A. Wang, M.A. Valenzuela, J. Salmones, A. Vazquez, A. Garcia-Ruiz, X. Bokhimi, Catal. Today. 68 (2001) 21–30. [5] A. Tarancon, G. Dezanneau, J. Arbiol, F. Peiro, J.R. Morante, J. Power Sources 118 (2003) 256–264. [6] J. Li, Y. Pan, Ch. Xiang, Q. Ge, J. Guo, Ceram. Int. 32 (2006) 587–591. [7] H. Wang, L. Gao, W. li, Q. li, Nanostruct. Mater. 11 (1999) 1263–1267. [8] A. Douy, Int. J. Inorg. Mater. 3 (2001) 699–707. [9] X. Fu, H. Zhang, S. Niu, Q. Xin, J. Solid State Chem. 178 (2005) 603–607. [10] N. Liu, Y. Yuan, P. Majewski, F. Aldinger, Mater. Res. Bull. 41 (2006) 461–468. [11] A. Douy, P. Odier, Mater. Res. Bull. 24 (1989) 1119–1126. [12] A.A. Babaluo, M. Kokabi, A. Barati, J. Eur. Ceram. Soc. 24 (2004) 635–644. [13] R. Moreton, The Spinning of Polyacrylonitrile Fibres for the Production of Carbon Fibers, Technical Report, Ministry of Technology Farnborough Hants, U.K., 1970. [14] W. Watt, The pyrolysis of polyacrylonitrile fibres, Technical Report, Ministry of Technology Farnborough Hants, U.K., 1970. [15] M. Tahmasebpour, A.A. Babaluo, M.K. Razavi Aghjeh, J. Eur. Ceram. Soc. 28 (2008) 773–778. [16] G.-Y. Guo, Y.-L. Chen, J. Solid State Chem. 178 (2005) 1675–1682.
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / p o w t e c
Studies on the synthesis of α-Al2O3 nanopowders by the polyacrylamide gel method M. Tahmasebpour a, A.A. Babaluo a,b,⁎, S. Shafiei a, E. Pipelzadeh a a b
Nanostructure Materials Research Center (NMRC), Sahand University of Technology, P.O. Box 51335/1996, Tabriz, I.R. Iran Institute of Polymeric Materials, Sahand University of Technology, P.O. Box 51335/1996, Tabriz, I.R. Iran
a r t i c l e
i n f o
Article history: Received 7 November 2007 Received in revised form 31 August 2008 Accepted 23 September 2008 Available online 10 October 2008 Keywords: Polyacrylamide gel Al2O3 Powders Nanostructure Crystallisation Morphology
a b s t r a c t The polyacrylamide sol–gel method is a simple, fast and cheap method used for the synthesis of a wide variety of nanopowders. However, the effects of various experimental conditions on the nanopowders' properties have not been reported. In this paper, the effects of atmospheric conditions, heating rate, type of precursor, monomer-to-salt ratio and solution concentration on the structural properties of synthesized αAl2O3 nanopowder were investigated. The results show that better stabilized networks were obtained using two-stage atmospheres. Finer nanoparticles were obtained at lower heating rates as a result of delay in phase transformations. The type of precursor had no significant effect on particle size of the final product, as illustrated by scanning electron microscopy images, but it directly affected nanoparticle crystallization and thermal degradation of the polymeric network. In addition, there was an inverse relationship between the particle size and the ratio of monomer to the precursor salt. The decrease in particle size was due to higher thermal stability of the polymeric network. The concentration of the starting solution had no effect on the structure of the final product. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In advanced ceramic technology, synthesis of ceramic powders on the nanometer scale is critical because the properties of the starting powder play an important role in determining the quality of the finished product [1]. Thus, in recent decades, increasing attention has been given to identifying better synthesis processes and to the structural characterization of ceramic nanoparticles [1–4]. Nanopowder synthesis by classical methods has its own shortcomings, such as uncontrolled crystalline growth, composition inhomogeneities, nonuniformity of grain size and high operating costs. Wet chemical methods such as sol–gel and modified sol–gel (Pechini), due to their atomic scale of operation, are more commonly used to avoid these shortcomings and to achieve better control of nanoparticle properties [5]. Alumina has been chosen for synthesis for its ability to produce high-strength materials and its wide range of applications such as electronic ceramics and catalysts [1]. Alumina is a structurally complex oxide having several different metastable phases (boehmite → γ → δ → θ-Al2O3) that are eventually converted to stable α-Al2O3 after controlled calcination at high temperatures, making its synthesis a challenging process. Two major problems are encountered during nanoparticle alumina synthesis, namely, calcination
and dehydration, where the former causes grain growth of the particles and the later promotes aggregation [2,6,7]. An auxiliary three-dimensional (3D) tangled polyacrylamide gel network using a polymer carrier has recently been used in solutionpolymerization techniques [7–10]. It is a wet chemical process that is fast, cheap, reproducible and easily scaled up to obtain a number of fine nanopowders [9]. This method is time-saving in relation to the Pechini method, and it provides ultra fine powders at relatively low temperatures [5]. The polyacrylamide gel method was introduced in 1989 by Odier to produce YBa2Cu3O7 − x [11]. Although this method has been used for the preparation of ultra fine powders, the optimal synthesis conditions on particle size distribution have not been systematically studied. The aim of the present work was to clarify the effects of synthesis conditions on the particle size distribution of alumina nanoparticles using the polyacrylamide gel method. The effects of atmospheric conditions, heating rate, type of precursor, monomer-to-salt ratio and solution concentration on the properties of the final products were tested. In order to achieve this aim, X-ray Diffraction (XRD), Fourier Transform Infra-Red (FTIR), Thermal analysis (TG-DTA), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) techniques were employed. 2. Experimental
⁎ Corresponding author. Nanostructure Materials Research Center (NMRC), Sahand University of Technology, P.O. Box 51335/1996, Tabriz, I.R. Iran. Tel.: +98 412 3459081; fax: +98 412 3444355. E-mail address: [email protected] (A.A. Babaluo). 0032-5910/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2008.09.016
2.1. Materials The characteristics of the materials used are given in Table 1.
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Table 1 Characteristics of materials used Materials
Function
Acrylamide (AM)
Monofunctional monomer N,N′-Methylene bis acrylamide Difunctional monomer (MBAM) (crosslinker) Ammonium persulfate Initiator N,N,N′,N′-Tetramethyl ethylene Accelerator diamide (TEMED) Aluminum nitrate Initial salt Aluminum chloride Initial salt Water Solvent a b
Molecular formula
Supplier
C2H3CONH2
Mercka
(C2H3CONH2)2CH2 Mercka (NH4)2S2O8 C6H16N2
Mercka Mercka
Al(NO3)3.9H2O AlCl3.6H2O H2O
Mercka Mercka Ghazib
E. Merck, D 6100 Darmstadt, Germany. Shahid Ghazi Pharmaceutical Co, Tabriz, Iran.
2.2. Synthesis procedure
Fig. 2. FTIR absorption spectra of the polymeric gels prepared in different temperatures and atmospheres (▲: C–O–C group).
The precursors prepared from Al(NO3)3.9H2O and AlCl3.6H2O are denoted as AN and AC, respectively. Synthesis of nanoparticles was carried out by dissolving the appropriate amount of these precursors, followed by the addition of acrylamide (AM) and N,N′-methylene bis acrylamide (MBAM) monomers in a molar ratio of 22:1 (AM:MBAM), producing a transparent solution after stirring [12]. Freshly made 10% (w/v) ammonium persulfate (APS) and 1% (v/v) N,N,N′,N′-tetramethyl ethylene diamide (TEMED) were added to the premixed solution to act
as an initiator and an accelerator, respectively. A rapid polymerization was observed, forming a transparent polymeric gel without any precipitation, followed by homogenization in a ceramic mortar. Different samples were obtained after subsequent thermal treatment using an Ex.1200-2LA laboratory furnace. 2.3. Characterization XRD data were collected from the synthesized powders for phase identification and determination of crystallite size by SIEMENS D5000 and Philips TW3710 X'Pert diffractometers using CuKα radiation in the range of 2θ = 10°–80°. The polymeric gel was examined by differential thermal and thermogravimetric analysis with a thermal analyzer (RAS Diamond TG-DTA high temperature). FTIR spectra were obtained on a UNICAM Matson 1000 spectrophotometer using the KBr pellet method. Structure and morphology of the synthesized powders were analyzed by a CamScan MV2300 scanning electron microscope and a LEO-906 transmission electron microscope. 3. Results and discussion 3.1. Effect of atmosphere It has been reported that, in the polyacrylamide gel method, the presence of the polymeric network can effectively inhibit the
Fig. 1. TG (top) and DTA (bottom) curves under different atmospheres at a heating rate of 5 °C/min: (a) air (b) argon (c) two-stage: argon-air.
Fig. 3. TG curves of pure polyacrylamide gel at heating rates of 5, 10 and 20 °C/min under two-stage atmospheres.
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aggregation of powders at high temperatures [9]. Therefore, due to the high calcination temperatures of α-Al2O3 (1100–1200 °C), protecting the polymeric network from thermal degradation is critical to maintaining good control over particle size and morphology. Polyacrylamide network degradation was studied under air and argon atmospheres. A total degradation of the polymeric network was observed at an undesirable temperature of 600 °C under the air atmosphere (Fig. 1a). A considerable 26% polymeric decomposition was observed between 290 °C and 310 °C under an inert atmosphere, which can be confirmed by the presence of an exothermic peak at about 295 °C in the DTA curve. Despite degradation of the sample at lower temperatures under the argon atmosphere, approximately 15%
Fig. 5. XRD patterns of Al2O3 powders calcined at 900, 1000 and 1100 °C using two heating rates of 5 and 20 °C/min.
of the original mass of the network remained after heating to temperatures up to 600 °C (Fig. 1b). Increasing the thermal stability of the network under an air atmosphere and at low temperatures (∼ 300 °C) is probably associated with the fact that oxygen molecules can act as crosslinkers, as has been previously reported for the spinning of polyacrylonitrile fibres to produce carbon fibers [13,14]. FTIR data at 4000–400 cm− 1 for polymeric gels heated to different temperatures and in different atmospheres are shown in Fig. 2. Absorption at 1000–1300 cm− 1 is consistent with stretching vibration of the C–O–C group, which can be attributed to the crosslinked oxygen [6]. No peak was observed for the untreated gel above this wave number range using the FTIR analysis. This absorption band was observed for samples heated up to 250 °C under an air atmosphere, but it was not detected under an argon atmosphere. The characteristic peak disappears when the sample with C–O–C groups was heated to 400 °C under an air atmosphere, indicating thermal decomposition of the crosslinked oxygen. These results clearly show that the presence of oxygen has a strong effect on the thermal stability of the polyacrylamide network, in good agreement with the literature [13,14]. Using air initially, followed by an inert atmosphere, enabled an increase in polymeric network strength via the formation of C–O–C groups in air (b280 °C) and preservation of the network with an argon atmosphere at higher temperatures. Fig. 1c shows the TG-DTA patterns for a polymeric gel under two-stage atmospheres, indicating slow degradation of the polymeric gel, with about 25% of the original polymer mass preserved up to 600 °C. Products were further heated to 1100 °C under an inert atmosphere, and the remaining polymeric network was removed by calcination under an air atmosphere to obtain the final powders. 3.2. Effect of heating rate
Fig. 4. SEM images of α-Al2O3 powders prepared at different heating rates (a) 20 (b) 10 and (c) 5 °C/min under two-stage atmospheres.
Thermal decomposition of polyacrylamide gel was carried out at 5, 10 and 20 °C/min under two-stage atmospheres, and the results are shown in Fig. 3. Lower heating rates allow more time for the polymer to reach a given temperature, resulting in greater thermal
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decomposition, which would be expected to yield larger average nanoparticles. However, SEM images from the synthesized powders using different heating rates, shown in Fig. 4, demonstrate that a lower heating rate produces finer particles. In order to support this observation, further investigation was carried out using XRD analysis. Fig. 5 displays the XRD patterns of alumina powders calcinated at 900, 1000 and 1100 °C using two heating rates of 5 and 20 °C/min. γ-Al2O3 synthesis was poor for both heating rates at 900 °C. Further heating up to 1000 °C revealed the presence of θ-Al2O3 and α-Al2O3. The latter is dominant using the 20 °C/min heating rate, indicating acceleration in the phase transformation from θ-Al2O3 to α-Al2O3. α-Al2O3 was the main product observed when the synthesized particles were heated to 1100 °C during calcination, and the intensity of the pattern was greater for the higher heating rate. Due to its low transformation temperatures and more easily detectable XRD peaks, zirconia was used for evaluation of the effect of heating rate. Zirconia was reported to have three well-known crystalline forms, monoclinic (m), tetragonal (t) and cubic (c), at atmospheric pressure. The tetragonal phase is mainly seen at low calcination temperatures, but, as the temperature increases to 400 °C or higher, both monoclinic and tetragonal phases are detected. Pure monoclinic phase is observed after calcination above 800 °C [15,16]. Zirconium oxynitrate and zirconium oxychloride were used as precursors for producing the zirconia nanopowders. Fig. 6 displays the XRD patterns of the synthesized zirconia powders calcinated up to 600 °C using two heating rates of 5 and 20 °C/min. The results show that the tetragonal phase had a more intense XRD peak at 2θ 30° at the
Fig. 6. XRD patterns of ZrO2 prepared by oxynitrate salt (top) and oxychloride salt (bottom) calcined at 600 °C with 5 and 20 °C/min heating rates.
Fig. 7. TG curves of the polymeric gels prepared by nitrate (AN) and chloride (AC) salts.
higher rate of heating for both precursors. The monoclinic phase is also clearly observed at higher rate of heating, demonstrating a delay in tetragonal-to-monoclinic phase transformation at lower heating rates. These results suggest that lower heating rates for both alumina and zirconia cause a delay in the phase transformations, supporting the SEM results obtained for alumina synthesis.
Fig. 8. XRD patterns of Al2O3 powders prepared by AN and AC salts calcined at 800, 1000 and 1100 °C.
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Fig. 9. SEM images and SD curves of α-Al2O3 powders synthesized by various salts calcined at 1100 °C: (a) AN (b) AC.
3.3. Effect of precursor salt Fig. 7 illustrates the weight loss associated with AN and AC base polymeric gels, and the results indicate an acceleration in polymeric network degradation caused by the presence of nitrate ions, leading to nanoparticles of greater size than those obtained using chloride-based precursors. The X-ray patterns (Fig. 8) of both precursors after 2 h of calcination at 800 °C showed poor crystalline γ-Al2O3 synthesis. Furthermore, X-ray patterns of the products after heating to 1000 °C revealed the presence of θ-Al2O3 and α-Al2O3. The intensity of the α-Al2O3 pattern was greater in the AC-based sample than in the AN sample. These results suggest that the presence of nitrate ions delayed the θ-Al2O3 to α-Al2O3 phase transformation, and that the nucleation of α-Al2O3 occurs more heterogeneously in the AN sample. By increasing the calcination temperature to 1100 °C, the crystallinity of α-Al2O3 was completed, and the phase in both samples was mainly α-Al2O3. The above observations suggest two competing mechanisms: crystallization and polymeric network degradation. As shown by SEM images and size distribution (SD) curves taken from the products using different salt precursors at 1100 °C (Fig. 9), no significant difference was observed in particle size distribution, with an average size of 65 nm. This reveals that these two mechanisms eliminate each
others' effects. Although the size of the particles was in the same range, a narrower size distribution was achieved for the chloridebased precursor.
Fig. 10. TG curves of the polymeric gels prepared with different monomers to salt ratio.
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Fig. 11. TEM images and SD curves of α-Al2O3 powders obtained with different conditions: (a) M/S = 2, Concentration = 80 wt.% (b) M/S = 1, Concentration = 80 wt.% and (c) M/S = 2, Concentration = 50 wt.%.
3.4. Effect of monomer-to-salt ratio Monomer-to-salt ratio (M/S) plays a key role in the thermal stability of the polyacrylamide gel network, as demonstrated in Fig. 10, where TG curves are drawn for various M/S ratios (0.5, 1.0 and 2.0). The results show an increase in thermal stability with an increase in M/S ratio. At 0.5 M/S ratio, 92% loss of the original mass was observed when heated to 200 °C, but, as this ratio was increased, the rate of polymeric decomposition was reduced, resulting in less aggregation and producing finer nanoparticles. TEM images and SD curves (Fig. 11a
and b) support the above claims, indicating an increase from 65 nm to 100 nm particle size using M/S ratios of 2 and 1, respectively. 3.5. Effect of solution concentration TG curves of polymeric gels prepared with various solution concentrations (20, 50 and 80 wt.%) reveal that initial solution concentration had no significant effect on weight loss of the polymeric network (Fig. 12). Fig. 11a and c correspond to TEM and SD curve images of the synthesized powders using 80 and 50 wt.% solution concentration, respectively. As can be seen, both powders had an average size of ∼65 nm, indicating that solution concentration has no significant effect on particle size in the polyacrylamide gel method. 4. Conclusion In this study, the effects of the atmospheric condition, heating rate, monomer-to-salt ratio, choice of precursor, and concentration of the solution on the structural properties of synthesized α-Al2O3 nanoparticles using the polyacrylamide gel method were investigated. Conclusions are as summarized with the following remarks
Fig. 12. TG curves of the polymeric gels prepared with various solution concentrations (20, 50 and 80 wt.%).
• The presence of polymeric network can effectively inhibit the aggregation of powders at high temperatures. Heating the polymeric gel under an air atmosphere at low temperatures (b280 °C) followed by an inert atmosphere (two-stage atmospheres) results in a thermally stable polyacrylamide network. • Phase transformation delay is observed when low heating rates are applied, resulting in finer particles on average. • The choice of precursor affects the particle size via crystallization and polymeric network degradation mechanisms. These opposing mechanisms negate one another's effect, resulting in similarly sized particles for either precursor.
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• Higher monomer-to-precursor salt ratio promotes production of smaller particles. • Solution concentration has no effect on particle size in the polyacrylamide gel method. Acknowledgments The Authors wish to thank Dr. M.H. Pipelzadeh for editorial contributions, Sahand University of Technology (SUT) for the financial support of this work. We also, thank the co-workers and technical staff in the Institute of Polymeric Materials and Nanostructure Materials Research Center of SUT for their help during the various stages of this work. References [1] V.S. Giri, R. Sarathi, S.R. Chakravarthy, C. Venkataseshaiah, Mater. Lett. 58 (2004) 1047–1050. [2] Y.K. Park, E.H. Tadd, M. Zubris, R. Tannenbaum, Mater. Res. Bull. 40 (2005) 1506–1512.
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