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Synthesis of SrBi2Ta2O9 by solution combustion and its characterization
Powder Technology 225 (2012) 239–243
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Synthesis of SrBi2Ta2O9 by solution combustion and its characterization F.F. Oliveira a,⁎, S. Da Dalt a, V.C. Sousa b, C.P. Bergmann a a b
Laboratory of Ceramic Materials (LACER), Department of Materials, Federal University of Rio Grande do Sul, Av Osvaldo Aranha 99, Porto Alegre, Brazil Laboratory of Biomaterials and Advanced Ceramics (Labiomat), Department of Materials, Federal University of Rio Grande do Sul, Brazil
a r t i c l e
i n f o
Article history: Received 18 December 2011 Received in revised form 18 March 2012 Accepted 12 April 2012 Available online 21 April 2012 Keywords: Powder Ferroelectric materials Electrical resistivity
a b s t r a c t Solution combustion synthesis is an inexpensive technique to obtain high purity, homogeneous nanostructured materials. Today, ferroelectric materials are employed in electronic devices such as ferroelectric random access memories (FeRAM). This paper describes the preparation of strontium bismuth tantalate (SBT) by solution combustion synthesis (SCS), using strontium nitrate, bismuth nitrate pentahydrate, and tantalum pentachloride as oxidation reagents and urea as fuel. The influence of fuel on the electrical properties of the ferroelectric material was investigated by adding different amounts of fuel (stoichiometric (AS), 300% enriched (AS300) and 500% enriched (AS500). The AS500 powder presented a surface area of approximately 16.2 m2/g and a crystallite size of about 38 nm and its ferroelectric phase was obtained after calcination at 800 °C for 2 h. The electrical resistance increased substantially in response to an increase in the amount of fuel. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Growing interest in energy production and storage has led to the development of synthesis methods and the application of numerous materials. In this context, the high surface energy of nanomaterials shows a promising potential in the search for clean and renewable fuels. Nanostructured materials are good candidates for use as catalysts in photocatalytic processes. Li et al. [1] produced SrBi2Ta2O9 and observed its properties in the water dissociation process in the formation of hydrogen gas. The development of fuel cells requires the production of hydrogen gas as an alternative fuel source. SrBi2Ta2O9 possesses an orthorhombic structure better known as pseudoperovskite (aurivillius), whose crystalline structure is composed of a Ta+ 5 ion located at the center of an octahedron, Sr+ 2 ions located at the vertices of the perovskite structure, and bismuth in the form of layers [Bi2O2] + 2 involving two consecutive perovskite structures (SrTa2O6) [2]. The conventional way to synthesize SBT, based on the solid-state reaction at high temperatures, is not suitable for ferroelectric applications, since the high-temperature synthesis can lead to the formation of an unwanted non-ferroelectric bismuth-deficient pyrochlore phase resulting from the severe loss of the bismuth component at high temperatures. Moreover, through this method it is difficult to obtain a compositionally homogeneous product [3]. In most cases SBT is prepared by alkoxide based sol–gel processes in organic solvents. An important disadvantage of alkoxides, however, is that they are scarcely available and quite expensive. Moreover, they are extremely reactive
⁎ Corresponding author. Tel.: + 55 51 3308 3637; fax: + 55 51 3308 3405. E-mail address: [email protected] (F.F. Oliveira). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2012.04.023
in humidity, making them difficult to characterize and requiring great care during processing. An alternative is aqueous sol–gel chemistry, starting with stable, rather cheap and easily available inorganic salts. Aqueous sol–gel synthesis of ceramics containing metals (like Ta in SBT) is, however, very complicated, since very few salts are water soluble. Although Ta oxalate is water soluble, it is not suitable for gel formation [4]. Combustion synthesis technology attracted the interest of many researchers as an energy and time saving process. This simple technique has many advantages: homogeneous mixing, good stoichiometric control, production of active submicron-size particles in a relatively short processing time, and it involves a combustion process initiated at low temperatures, which makes use of the heat energy liberated by the exothermic reaction between fuel and nitrate ions [3]. Combustion synthesis allows for the production of nanostructured powders with a variety of applications, ranging from catalysts [5] to fuel cells [6], biomaterials [7], electronic devices such as varistors [8], ferroelectric materials [9], etc. A similar composite with a layered structure of bismuth oxide was obtained by Zanetti et al. [9], who reported that as-synthesized SrBi2Nb2O9 powder presented a surface area of 13.25 m 2/g and that the ferroelectric phase was obtained by heat treatment at 750 °C for 2 h. The present work presents the synthesis of SrBi2Ta2O9 by solution combustion and studies the influence of the concentration of the fuel on some physical and electrical properties of the powder obtained. 2. Materials and methods The SrBi2Ta2O9 was synthesized using strontium nitrate (Sr(NO3)2, Vetec 99%), bismuth nitrate pentahydrate (Bi(NO3)3.5H2O, Vetec 98%) and tantalum pentachloride (TaCl5, Aldrich 99%) as precursors, and urea (CO(NH2)2, nuclear 99%) as fuel.
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The proportion of ions used in the synthesis of SrBi2Ta2O9 was 1:2:2 of Sr + 2:Bi + 3:Ta + 5, respectively. Based on propellant chemistry [10], a stoichiometric reaction requires 6.67 moles of urea. However, the proportion of urea was varied as follows: stoichiometric (AS), 300% enriched (AS300) and 500% enriched (AS500). A tantalum pentachloride solution was prepared and ammonium hydroxide was added to trigger the precipitation of Ta(OH)5, after which the suspension was washed to remove the chlorine. The nitrates and urea were then added and the mixture was stirred for 2 h. The solution was then poured into a metal container, placed in an oven preheated to 650 °C, and left there for about 10 min, until ignition occurred. The resulting samples were subjected to different heat treatments varying from 650 to 800 °C for 2 h. The powders were characterized using a TA Instruments TGA 2050 thermogravimetric analyzer, a Perkin Elmer Spectrum 1000 FTIR spectrometer, and a Philips X'Pert MPD X-ray diffractometer. The specific surface area was measured at multiple nitrogen absorption points (BET method) using an Autosorb Quantachrome 1200 surface area and pore size analyzer. SEM images were recorded with a JEOL JSM-6060 scanning electron microscope and TEM images with a JEOL JEM-2010 transmission electron microscope. An AGILENT 4294A impedance spectrometer was used for the electrical characterization of the powders, varying the frequency from 40 Hz to 1 MHz and a temperature of 900 °C. The powder was placed inside an alumina support with a ring shape (with 4.0 mm of internal diameter, 5.0 mm of external diameter and 1.5 mm of thickness),after which a platinum sheet was placed on the two surfaces of the support. 3. Results and discussion Fig. 1 shows the infrared transmission (FTIR) curves of the assynthesized samples: stoichiometric (AS), 300% enriched (AS300) and 500% enriched (AS500). Note the presence of bands at 854 and 1469 cm − 1. According to Lu and Saha [11], these bands correspond to carbonate groups. An absorption band at 1617 cm − 1 corresponds to OH vibrations, indicating the presence of hydroxyl groups [11,12]. In addition, note the absorption band in the range of 520–690 cm − 1, albeit not clearly defined. The behavior of the infrared absorption spectra of the as-synthesized samples indicates that the formation of bands is similar, regardless of the quantity of urea in the reaction. Fig. 2 depicts the X-ray diffraction patterns of the as-synthesized materials AS, AS300 and AS500. The reactions produced the phases: bismuth oxide (Bi2O3), strontium carbonate (SrCO3 - strontianite) and bismuth oxichloride (BiOCl) was also found. The presence of SrCO3 was identified in XRD and is in agreement with the carbonate
Fig. 1. Infrared transmission curves of the as-synthesized samples: AS, AS300 and AS500.
Fig. 2. X-ray diffraction patterns of the as-synthesized samples: AS, AS300 and AS500.
group found in FTIR, while the presence of BiOCl may be due to the incomplete removal of chlorine in the Ta(OH)5 precipitation and washing procedure. Fig. 3 presents the TGA and DTA curves of the stoichiometric (AS), 300% enriched (AS300) and 500% enriched (AS500) as-synthesized samples. The TGA shows an increasing mass loss as the amount of fuel is increased compared to stoichiometric amount of fuel, especially above 650 °C. The AS sample presents only one mass loss while the AS300 and AS500 present two mass losses. This behavior can be explained because the amount of unreacted fuel started its decomposition at 600 °C. DTA curves for the same reactions confirm the increase of temperature according to the burning of the remaining, excess fuel. All samples present the mass loss observed between 800 and 850 °C. Frost et al. studied the decomposition of strontianite. They observed a mass loss between 709 and 869 °C that can be attributed to the decarbonization of strontianite [13]. One of the reactions was selected to study the phase transformations that took place in the as-synthesized powder subjected to heat treatment. Since all the reactions with AS, AS300 and AS500 presented chemically and structurally similar characteristics, sample AS500 was chosen. Fig. 4 shows the infrared absorption spectrum of the AS500 powder after heat treatment at temperatures of 650, 750 and 800 °C for 2 h. Note the decline in the bands corresponding to the carbonate groups in all the samples. Conversely, in the sample heat treated at 800 °C,
Fig. 3. Thermogravimetric analysis (TGA) and thermodifferential analysis (DTA) of the as-synthesized samples: AS, AS300 and AS500.
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Table 1 Values of specific surface area (S) of the as-synthesized powders and after heat treatment.
Fig. 4. Infrared transmission curves of sample AS500: as-synthesized and heat treated at 650, 750 and 800 °C.
note the formation of two infrared absorption bands at 550 and 623 cm− 1, indicating the formation of SrBi2Ta2O9 oxide [11,14]. Also note that the presence of carbonate groups is visible even at a temperature of 800 °C, corroborating the TGA results discussed earlier. Fig. 5 shows the X-ray diffraction analysis of sample AS500 heat treated at 650, 750 and 800 °C. At 650 °C, the formation of a strontium tantalate hydrate phase might be observed. In addition, the disappearance of the crystal planes corresponding to the SrCO3 phase is observed. Probably, the start of decarbonization occurs at 650 °C. Infrared spectroscopy analysis confirms the reduction of bands corresponding to carbonate groups with increasing temperature, but they do not disappear. Moreover, it is also possible to note the presence of BiOCl, which was eliminated when the sample was heat treated at 750 °C. Increasing the temperature to 800 °C promoted the formation of SrBi2Ta2O9 in the orthorhombic phase, which is responsible for the ferroelectric characteristics. This result confirms the infrared spectroscopy analysis at the same temperature. There is also a residual presence of bismuth oxide and strontium tantalate hydrate as secondary phases at 800 °C. The TGA analysis showed earlier that there was a residual amount of mass at 800 °C and the presence of bands of carbonate groups suggests that the formation of the SBT is incomplete, unlike other routes noted in the literature [11,12,14]. It can be suggested that the high stability of the strontium carbonate powder
Fig. 5. X-ray diffraction patterns of sample AS500: as-synthesized and heat treated at 650, 750 and 800 °C for 2 h.
Sample
Temperature (°C)
Time (h)
S (m2/g)
AS AS AS300 AS300 AS500 AS500 AS500 AS500
As-synthesized 800 As-synthesized 800 As-synthesized 650 750 800
— 2 — 2 — 2 2 2
13.1 5.6 21.2 9.7 16.2 11.4 5.1 8.8
formed in the as-synthesized samples influences the subsequent stages of the reactions for the formation of SBT. The results obtained in this study suggest that the decomposition reaction of SrCO3 and its subsequent reaction with Ta(OH)5 promotes the formation of strontium tantalate. The formation of the bismuth oxide might be observed separately. The temperature between 750 and 800 °C promotes the reaction between Bi2O3 and SrTa2O6, forming the ferroelectric phase SrBi2Ta2O9. The BET results (Table 1) show as-synthesized powders with high surface area. The surface area decreased considerably after heat treatment at 800 °C due to the reduction of free energy at the particle surfaces, causing interparticle coalescence and thus reducing the total specific surface area. The S values are very close to those of powders used as catalysts in photocatalytic processes to produce hydrogen gas [1,7]. Fig. 6a shows the SEM image of the as-synthesized AS500 sample. Note the formation of particle agglomerates due to van der Waals forces. The magnified image reveals the typical porosity of powders obtained by the SCS technique, resulting from the emission of gases during combustion. Fig. 6b, however, shows the AS500 sample after heat treatment at 800 °C. It is possible to observe a morphology
Fig. 6. (a) SEM micrograph of the as-synthesized AS500 sample and (b) AS500 heat treated at 800 °C for 2 h under × 25000 magnification.
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F.F. Oliveira et al. / Powder Technology 225 (2012) 239–243 Table 2 Comparison of the average crystallite size of prepared SrBi2A2O9 (A = Nb, Ta) powders. Author
Compound
Technique
Temperature Average crystallite (°C) size (nm)
Zanetti et. al. [7] Lu and Saha [8] Lu and Saha [9] Yu et. al. [10] Panda et. al. [11] This work
SrBi2Nb2O9 SrBi2Ta2O9 SrBi2Ta2O9 SrBi2Ta2O9 SrBi2Ta2O9 SrBi2Ta2O9
SCS 700 Colloid gel 800 Colloid emulsion 800 Sol–gel 800 Chemical route 800 SCS 800
15.7* 23* 28 50–70** 21* 38**
*The average crystallite size calculated using single line. **The average crystallite size observed by images of TEM.
Fig. 7. TEM image of AS500 sample heat treated at 800 °C for 2 h.
without the presence of porosity, corroborating with BET measurements. The formation of aggregates in this sample may suggest the coalescence of particles, and can be better analyzed by TEM. Fig. 7 depicts a TEM image of sample AS500 heat treated at 800 °C for 2 h, showing the nanostructured formation of the ferroelectric powder, with crystallite sizes varying from 20 to 50 nm. A histogram of the crystallite sizes was built from the TEM images, which revealed that the average crystallite size was 38 nm, with a standard deviation of 12 nm (Fig. 8). It is possible to observe the coalescence of particles due to the heat treatment temperature as suggested by SEM morphology observed in Fig. 6b. Table 2 lists some studies in which powders with the same crystalline structure were obtained by other methods of synthesis, and compares the sizes obtained by different routes with those obtained by the solution combustion synthesis (SCS) technique. As can be seen, the crystallite size obtained in this study is smaller than those obtained by the sol–gel route [15]. Figs. 9–11 show the electrical characterization of the AS, AS300 and AS500 powders after heat treatment at 800 °C for 2 h. The impedance results reveal the semicircles on the Nyquist diagram (Z″ as a function of Z′). Note the poorly defined semicircle for the 500% enriched powder, suggesting the high resistivity of the material whose combustion was performed with the highest content of urea.
Fig. 8. Histogram of crystallize size distribution found in TEM images.
High resistivity is important for the maintenance of poling efficiency at high temperatures [16] and therefore suitable for application in metal ferroelectric–(insulator)–semiconductors field effect transistors (MFSFETs), for instance. The equivalent electrical circuits were modeled with the ZView software. The electrical resistivities of the AS, AS300 and AS500 powders after heat treatment at 800 °C for 2 h were determined by adjustment of the experimental data. The values for the resistance of the powders AS, AS300 and AS500 at 900 °C were 166 KΩ, 500 KΩ and 3.9 MΩ, respectively.
Fig. 9. Nyquist diagram of the AS powder after heat treatment at 800 °C for 2 h.
Fig. 10. Nyquist diagram of the AS300 powder after heat treatment at 800 °C for 2 h.
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the equipment for impedance spectroscopy. The author Felipe Fernandes de Oliveira is grateful to the CNPq for the financial support. References
Fig. 11. Nyquist diagram of the AS500 powder after heat treatment at 800 °C for 2 h.
4. Conclusions Solution combustion synthesis yielded a satisfactory SrBi2Ta2O9 ferroelectric material with nanostructured particles. The ferroelectric phase of the SBT obtained after heat treatment at 800 °C for 2 h presented a high surface area. Increasing the amount of urea in the reaction led to a substantial increase in the electrical resistivity of the resulting powder, illustrating the influence of the amount of fuel on the properties of the material. However, no significant changes were observed in the specific surface area and in the crystallization of the phases. Acknowledgements The authors are grateful to the staff of CME/UFRGS (Electron Microscopy Center of the Federal University of Rio Grande do Sul), who made the SEM and TEM analyses possible. Moreover, we are also grateful to the Institute de Cerámica y Vidrio for having given
[1] Y. Li, G. Chen, H. Zhang, Z. Li, J. Sun, Electronic structure and photocatalytic properties of ABi2Ta2O9 (A=Ca, Sr, Ba), Journal of Solid State Chemistry 181 (2008) 2653–2659. [2] H.S. Nalwa, Handbook of thin film materials, Ferroelectric and Dielectric thin films, vol. 3, Academic Press, 2002, p. 78, p313-314. [3] R.Q. Chu, Z.J. Xu, Z.G. Zhu, G.R. Li, Q.R. Yin, Synthesis of SrBi4Ti4O15 powder and ceramic via auto-combustion of citrate–nitrate gel, Materials Science and Engineering B 122 (2005) 106–109. [4] D. Nelis, K. Van Werde, D. Mondelaers, G. Vanhoyland, M.K. Van Bael, J. Mullens, L.C. Van Poucke, Synthesis of SrBi2Ta2O9 (SBT) by means of a soluble Ta(V) precursor, Journal of the European Ceramic Society 21 (2001) 2047–2049. [5] S. Schuyten, P. Dinka, A.S. Mukasyan, E. Wolf, A novel combustion synthesis preparation of CuO/ZnO/ZrO2/PB for oxidative hydrogen production from methanol, Catalysis Letters 121 (2008) 189–198. [6] L.D. Jadhav, M.G. Chourashiya, K.M. Subhedar, A.K. Tyagi, J.Y. Patil, Synthesis of nanocrystalline Gd doped ceria by combustion technique, Journal of Alloys and Compounds 470 (2009) 383–386. [7] J. Zhang, J. Guo, S. Li, B. Song, K. Yao, Synthesis of β-tricalcium phosphate using sol–gel combustion method, Frontiers of Chemistry in China 3 (2008) 451–453. [8] V.C. Sousa, M.R. Morelli, R.H.G. Kiminami, Combustion process in the synthesis of ZnO – Bi2O3, Ceramics International 26 (2000) 561–564. [9] S.M. Zanetti, E.I. Santiago, L.O.S. Bulhões, J.A. Varela, E.R. Leite, E. Longo, Preparation and characterization of nanosized SrBi2Nb2O9 powder by the combustion synthesis, Materials Letters 57 (2003) 2812–2816. [10] D.A. Fumo, M.R. Morelli, A.M. Segadães, Combustion synthesis of iron substituted strontium titanateperovskltes, Materials Research Bulletin 32 (n.10) (1996) 1459–1470. [11] C.H. Lu, S.K. Saha, Synthesis of ultrafine strontium bismuth tantalate powder by colloid-emulsion technique, Materials Letters 42 (2000) 150–154. [12] H. Wang, J. Liu, M. Zhu, B. Wang, H. Yan, Hydrothermal synthesis of strontium bismuth tantalate powder, Materials Letters 57 (2003) 2371–2374. [13] R.L. Frost, M.C. Hales, W.N. Martens, Thermogravimetric analysis of selected group (II) carbonate minerals – implication for the geosequestration of greenhouse gases, Journal of Thermal Analysis and Calorimetry 95 (n 3) (2009) 999–1005. [14] -C.H. Lu, S.K. Saha, Synthesis of ferroelectric nanocrystalline SrBi2Ta2O9 powder by the colloid-gel process, Materials Research Bulletin 35 (2000) 1235–2143. [15] Z. Yu, W. Wen, J. Dechang, Y. Feng, Synthesis of SrBi2Ta2O9 nanocrystalline powder by a modified sol–gel process using bismuth subnitrate as bismuth source, Materials Chemistry and Physics 77 (2002) 60–64. [16] I. Coondoo, S.K. Agarwal, A.K. Jha, Ferroelectric and piezoelectric properties of tungsten substituted SrBi2Ta2O9 ferroelectric ceramics, Materials Research Bulletin 44 (2009) 1288–1292.
Contents lists available at SciVerse ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Synthesis of SrBi2Ta2O9 by solution combustion and its characterization F.F. Oliveira a,⁎, S. Da Dalt a, V.C. Sousa b, C.P. Bergmann a a b
Laboratory of Ceramic Materials (LACER), Department of Materials, Federal University of Rio Grande do Sul, Av Osvaldo Aranha 99, Porto Alegre, Brazil Laboratory of Biomaterials and Advanced Ceramics (Labiomat), Department of Materials, Federal University of Rio Grande do Sul, Brazil
a r t i c l e
i n f o
Article history: Received 18 December 2011 Received in revised form 18 March 2012 Accepted 12 April 2012 Available online 21 April 2012 Keywords: Powder Ferroelectric materials Electrical resistivity
a b s t r a c t Solution combustion synthesis is an inexpensive technique to obtain high purity, homogeneous nanostructured materials. Today, ferroelectric materials are employed in electronic devices such as ferroelectric random access memories (FeRAM). This paper describes the preparation of strontium bismuth tantalate (SBT) by solution combustion synthesis (SCS), using strontium nitrate, bismuth nitrate pentahydrate, and tantalum pentachloride as oxidation reagents and urea as fuel. The influence of fuel on the electrical properties of the ferroelectric material was investigated by adding different amounts of fuel (stoichiometric (AS), 300% enriched (AS300) and 500% enriched (AS500). The AS500 powder presented a surface area of approximately 16.2 m2/g and a crystallite size of about 38 nm and its ferroelectric phase was obtained after calcination at 800 °C for 2 h. The electrical resistance increased substantially in response to an increase in the amount of fuel. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Growing interest in energy production and storage has led to the development of synthesis methods and the application of numerous materials. In this context, the high surface energy of nanomaterials shows a promising potential in the search for clean and renewable fuels. Nanostructured materials are good candidates for use as catalysts in photocatalytic processes. Li et al. [1] produced SrBi2Ta2O9 and observed its properties in the water dissociation process in the formation of hydrogen gas. The development of fuel cells requires the production of hydrogen gas as an alternative fuel source. SrBi2Ta2O9 possesses an orthorhombic structure better known as pseudoperovskite (aurivillius), whose crystalline structure is composed of a Ta+ 5 ion located at the center of an octahedron, Sr+ 2 ions located at the vertices of the perovskite structure, and bismuth in the form of layers [Bi2O2] + 2 involving two consecutive perovskite structures (SrTa2O6) [2]. The conventional way to synthesize SBT, based on the solid-state reaction at high temperatures, is not suitable for ferroelectric applications, since the high-temperature synthesis can lead to the formation of an unwanted non-ferroelectric bismuth-deficient pyrochlore phase resulting from the severe loss of the bismuth component at high temperatures. Moreover, through this method it is difficult to obtain a compositionally homogeneous product [3]. In most cases SBT is prepared by alkoxide based sol–gel processes in organic solvents. An important disadvantage of alkoxides, however, is that they are scarcely available and quite expensive. Moreover, they are extremely reactive
⁎ Corresponding author. Tel.: + 55 51 3308 3637; fax: + 55 51 3308 3405. E-mail address: [email protected] (F.F. Oliveira). 0032-5910/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2012.04.023
in humidity, making them difficult to characterize and requiring great care during processing. An alternative is aqueous sol–gel chemistry, starting with stable, rather cheap and easily available inorganic salts. Aqueous sol–gel synthesis of ceramics containing metals (like Ta in SBT) is, however, very complicated, since very few salts are water soluble. Although Ta oxalate is water soluble, it is not suitable for gel formation [4]. Combustion synthesis technology attracted the interest of many researchers as an energy and time saving process. This simple technique has many advantages: homogeneous mixing, good stoichiometric control, production of active submicron-size particles in a relatively short processing time, and it involves a combustion process initiated at low temperatures, which makes use of the heat energy liberated by the exothermic reaction between fuel and nitrate ions [3]. Combustion synthesis allows for the production of nanostructured powders with a variety of applications, ranging from catalysts [5] to fuel cells [6], biomaterials [7], electronic devices such as varistors [8], ferroelectric materials [9], etc. A similar composite with a layered structure of bismuth oxide was obtained by Zanetti et al. [9], who reported that as-synthesized SrBi2Nb2O9 powder presented a surface area of 13.25 m 2/g and that the ferroelectric phase was obtained by heat treatment at 750 °C for 2 h. The present work presents the synthesis of SrBi2Ta2O9 by solution combustion and studies the influence of the concentration of the fuel on some physical and electrical properties of the powder obtained. 2. Materials and methods The SrBi2Ta2O9 was synthesized using strontium nitrate (Sr(NO3)2, Vetec 99%), bismuth nitrate pentahydrate (Bi(NO3)3.5H2O, Vetec 98%) and tantalum pentachloride (TaCl5, Aldrich 99%) as precursors, and urea (CO(NH2)2, nuclear 99%) as fuel.
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The proportion of ions used in the synthesis of SrBi2Ta2O9 was 1:2:2 of Sr + 2:Bi + 3:Ta + 5, respectively. Based on propellant chemistry [10], a stoichiometric reaction requires 6.67 moles of urea. However, the proportion of urea was varied as follows: stoichiometric (AS), 300% enriched (AS300) and 500% enriched (AS500). A tantalum pentachloride solution was prepared and ammonium hydroxide was added to trigger the precipitation of Ta(OH)5, after which the suspension was washed to remove the chlorine. The nitrates and urea were then added and the mixture was stirred for 2 h. The solution was then poured into a metal container, placed in an oven preheated to 650 °C, and left there for about 10 min, until ignition occurred. The resulting samples were subjected to different heat treatments varying from 650 to 800 °C for 2 h. The powders were characterized using a TA Instruments TGA 2050 thermogravimetric analyzer, a Perkin Elmer Spectrum 1000 FTIR spectrometer, and a Philips X'Pert MPD X-ray diffractometer. The specific surface area was measured at multiple nitrogen absorption points (BET method) using an Autosorb Quantachrome 1200 surface area and pore size analyzer. SEM images were recorded with a JEOL JSM-6060 scanning electron microscope and TEM images with a JEOL JEM-2010 transmission electron microscope. An AGILENT 4294A impedance spectrometer was used for the electrical characterization of the powders, varying the frequency from 40 Hz to 1 MHz and a temperature of 900 °C. The powder was placed inside an alumina support with a ring shape (with 4.0 mm of internal diameter, 5.0 mm of external diameter and 1.5 mm of thickness),after which a platinum sheet was placed on the two surfaces of the support. 3. Results and discussion Fig. 1 shows the infrared transmission (FTIR) curves of the assynthesized samples: stoichiometric (AS), 300% enriched (AS300) and 500% enriched (AS500). Note the presence of bands at 854 and 1469 cm − 1. According to Lu and Saha [11], these bands correspond to carbonate groups. An absorption band at 1617 cm − 1 corresponds to OH vibrations, indicating the presence of hydroxyl groups [11,12]. In addition, note the absorption band in the range of 520–690 cm − 1, albeit not clearly defined. The behavior of the infrared absorption spectra of the as-synthesized samples indicates that the formation of bands is similar, regardless of the quantity of urea in the reaction. Fig. 2 depicts the X-ray diffraction patterns of the as-synthesized materials AS, AS300 and AS500. The reactions produced the phases: bismuth oxide (Bi2O3), strontium carbonate (SrCO3 - strontianite) and bismuth oxichloride (BiOCl) was also found. The presence of SrCO3 was identified in XRD and is in agreement with the carbonate
Fig. 1. Infrared transmission curves of the as-synthesized samples: AS, AS300 and AS500.
Fig. 2. X-ray diffraction patterns of the as-synthesized samples: AS, AS300 and AS500.
group found in FTIR, while the presence of BiOCl may be due to the incomplete removal of chlorine in the Ta(OH)5 precipitation and washing procedure. Fig. 3 presents the TGA and DTA curves of the stoichiometric (AS), 300% enriched (AS300) and 500% enriched (AS500) as-synthesized samples. The TGA shows an increasing mass loss as the amount of fuel is increased compared to stoichiometric amount of fuel, especially above 650 °C. The AS sample presents only one mass loss while the AS300 and AS500 present two mass losses. This behavior can be explained because the amount of unreacted fuel started its decomposition at 600 °C. DTA curves for the same reactions confirm the increase of temperature according to the burning of the remaining, excess fuel. All samples present the mass loss observed between 800 and 850 °C. Frost et al. studied the decomposition of strontianite. They observed a mass loss between 709 and 869 °C that can be attributed to the decarbonization of strontianite [13]. One of the reactions was selected to study the phase transformations that took place in the as-synthesized powder subjected to heat treatment. Since all the reactions with AS, AS300 and AS500 presented chemically and structurally similar characteristics, sample AS500 was chosen. Fig. 4 shows the infrared absorption spectrum of the AS500 powder after heat treatment at temperatures of 650, 750 and 800 °C for 2 h. Note the decline in the bands corresponding to the carbonate groups in all the samples. Conversely, in the sample heat treated at 800 °C,
Fig. 3. Thermogravimetric analysis (TGA) and thermodifferential analysis (DTA) of the as-synthesized samples: AS, AS300 and AS500.
F.F. Oliveira et al. / Powder Technology 225 (2012) 239–243
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Table 1 Values of specific surface area (S) of the as-synthesized powders and after heat treatment.
Fig. 4. Infrared transmission curves of sample AS500: as-synthesized and heat treated at 650, 750 and 800 °C.
note the formation of two infrared absorption bands at 550 and 623 cm− 1, indicating the formation of SrBi2Ta2O9 oxide [11,14]. Also note that the presence of carbonate groups is visible even at a temperature of 800 °C, corroborating the TGA results discussed earlier. Fig. 5 shows the X-ray diffraction analysis of sample AS500 heat treated at 650, 750 and 800 °C. At 650 °C, the formation of a strontium tantalate hydrate phase might be observed. In addition, the disappearance of the crystal planes corresponding to the SrCO3 phase is observed. Probably, the start of decarbonization occurs at 650 °C. Infrared spectroscopy analysis confirms the reduction of bands corresponding to carbonate groups with increasing temperature, but they do not disappear. Moreover, it is also possible to note the presence of BiOCl, which was eliminated when the sample was heat treated at 750 °C. Increasing the temperature to 800 °C promoted the formation of SrBi2Ta2O9 in the orthorhombic phase, which is responsible for the ferroelectric characteristics. This result confirms the infrared spectroscopy analysis at the same temperature. There is also a residual presence of bismuth oxide and strontium tantalate hydrate as secondary phases at 800 °C. The TGA analysis showed earlier that there was a residual amount of mass at 800 °C and the presence of bands of carbonate groups suggests that the formation of the SBT is incomplete, unlike other routes noted in the literature [11,12,14]. It can be suggested that the high stability of the strontium carbonate powder
Fig. 5. X-ray diffraction patterns of sample AS500: as-synthesized and heat treated at 650, 750 and 800 °C for 2 h.
Sample
Temperature (°C)
Time (h)
S (m2/g)
AS AS AS300 AS300 AS500 AS500 AS500 AS500
As-synthesized 800 As-synthesized 800 As-synthesized 650 750 800
— 2 — 2 — 2 2 2
13.1 5.6 21.2 9.7 16.2 11.4 5.1 8.8
formed in the as-synthesized samples influences the subsequent stages of the reactions for the formation of SBT. The results obtained in this study suggest that the decomposition reaction of SrCO3 and its subsequent reaction with Ta(OH)5 promotes the formation of strontium tantalate. The formation of the bismuth oxide might be observed separately. The temperature between 750 and 800 °C promotes the reaction between Bi2O3 and SrTa2O6, forming the ferroelectric phase SrBi2Ta2O9. The BET results (Table 1) show as-synthesized powders with high surface area. The surface area decreased considerably after heat treatment at 800 °C due to the reduction of free energy at the particle surfaces, causing interparticle coalescence and thus reducing the total specific surface area. The S values are very close to those of powders used as catalysts in photocatalytic processes to produce hydrogen gas [1,7]. Fig. 6a shows the SEM image of the as-synthesized AS500 sample. Note the formation of particle agglomerates due to van der Waals forces. The magnified image reveals the typical porosity of powders obtained by the SCS technique, resulting from the emission of gases during combustion. Fig. 6b, however, shows the AS500 sample after heat treatment at 800 °C. It is possible to observe a morphology
Fig. 6. (a) SEM micrograph of the as-synthesized AS500 sample and (b) AS500 heat treated at 800 °C for 2 h under × 25000 magnification.
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F.F. Oliveira et al. / Powder Technology 225 (2012) 239–243 Table 2 Comparison of the average crystallite size of prepared SrBi2A2O9 (A = Nb, Ta) powders. Author
Compound
Technique
Temperature Average crystallite (°C) size (nm)
Zanetti et. al. [7] Lu and Saha [8] Lu and Saha [9] Yu et. al. [10] Panda et. al. [11] This work
SrBi2Nb2O9 SrBi2Ta2O9 SrBi2Ta2O9 SrBi2Ta2O9 SrBi2Ta2O9 SrBi2Ta2O9
SCS 700 Colloid gel 800 Colloid emulsion 800 Sol–gel 800 Chemical route 800 SCS 800
15.7* 23* 28 50–70** 21* 38**
*The average crystallite size calculated using single line. **The average crystallite size observed by images of TEM.
Fig. 7. TEM image of AS500 sample heat treated at 800 °C for 2 h.
without the presence of porosity, corroborating with BET measurements. The formation of aggregates in this sample may suggest the coalescence of particles, and can be better analyzed by TEM. Fig. 7 depicts a TEM image of sample AS500 heat treated at 800 °C for 2 h, showing the nanostructured formation of the ferroelectric powder, with crystallite sizes varying from 20 to 50 nm. A histogram of the crystallite sizes was built from the TEM images, which revealed that the average crystallite size was 38 nm, with a standard deviation of 12 nm (Fig. 8). It is possible to observe the coalescence of particles due to the heat treatment temperature as suggested by SEM morphology observed in Fig. 6b. Table 2 lists some studies in which powders with the same crystalline structure were obtained by other methods of synthesis, and compares the sizes obtained by different routes with those obtained by the solution combustion synthesis (SCS) technique. As can be seen, the crystallite size obtained in this study is smaller than those obtained by the sol–gel route [15]. Figs. 9–11 show the electrical characterization of the AS, AS300 and AS500 powders after heat treatment at 800 °C for 2 h. The impedance results reveal the semicircles on the Nyquist diagram (Z″ as a function of Z′). Note the poorly defined semicircle for the 500% enriched powder, suggesting the high resistivity of the material whose combustion was performed with the highest content of urea.
Fig. 8. Histogram of crystallize size distribution found in TEM images.
High resistivity is important for the maintenance of poling efficiency at high temperatures [16] and therefore suitable for application in metal ferroelectric–(insulator)–semiconductors field effect transistors (MFSFETs), for instance. The equivalent electrical circuits were modeled with the ZView software. The electrical resistivities of the AS, AS300 and AS500 powders after heat treatment at 800 °C for 2 h were determined by adjustment of the experimental data. The values for the resistance of the powders AS, AS300 and AS500 at 900 °C were 166 KΩ, 500 KΩ and 3.9 MΩ, respectively.
Fig. 9. Nyquist diagram of the AS powder after heat treatment at 800 °C for 2 h.
Fig. 10. Nyquist diagram of the AS300 powder after heat treatment at 800 °C for 2 h.
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the equipment for impedance spectroscopy. The author Felipe Fernandes de Oliveira is grateful to the CNPq for the financial support. References
Fig. 11. Nyquist diagram of the AS500 powder after heat treatment at 800 °C for 2 h.
4. Conclusions Solution combustion synthesis yielded a satisfactory SrBi2Ta2O9 ferroelectric material with nanostructured particles. The ferroelectric phase of the SBT obtained after heat treatment at 800 °C for 2 h presented a high surface area. Increasing the amount of urea in the reaction led to a substantial increase in the electrical resistivity of the resulting powder, illustrating the influence of the amount of fuel on the properties of the material. However, no significant changes were observed in the specific surface area and in the crystallization of the phases. Acknowledgements The authors are grateful to the staff of CME/UFRGS (Electron Microscopy Center of the Federal University of Rio Grande do Sul), who made the SEM and TEM analyses possible. Moreover, we are also grateful to the Institute de Cerámica y Vidrio for having given
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