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Emulsion Combustion Synthesis
Emulsion Combustion Synthesis Osamu Odawara
An emulsion combustion method (ECM) has been proposed by K. Takatori [1] for making fine oxide powders with an emulsion of aqueous solution of precursor compounds of the desired powder product by following certain steps: (i) atomizing the w/o (water in oil) type emulsion in which metal nitrates are dissolved in an aqueous phase, (ii) burning the oil in the atomized emulsion droplet, and (iii) oxidizing the metal salts in the microspheres of aqueous solution. An aqueous solution of the metal nitrates is stirred with kerosene and a small amount of emulsifier to obtain w/o type emulsion. In many cases, the aqueous phase is 65 vol.% of the emulsion, and the dispersed droplets are about 1–2 μm in diameter. The constituent metal ions in the aqueous droplets are rapidly oxidized at high temperature by combustion of the surrounding kerosene. Most of combustion synthesis is carried out at combustion temperatures higher than 1000 K. Air is introduced into the flame so as to achieve complete combustion. The residence time of the powder at high temperatures is estimated to be less than one-half second. Hollow alumina fine powders are synthesized by the EMC using w/o type emulsions with different aqueous microsphere sizes [2]. The powders are spherical and hollow with a very thin shell from 10 to 20 nm in thickness when synthesized with large aqueous microspheres from 200 nm to 1 mm. Large particles are obtained from small aqueous microspheres of 6 nm, which cause the adhesion of particles during the firing process with the short distance between microspheres. The emulsion process and the combustion technology are combined so that the reaction fields are of submicron size and the reaction times a fraction of second. The characteristic feature of the synthesized powder by the ECM process is a thinner shell than those obtained by the other processes. The rapid heating of aqueous microspheres by combustion of the oil phase causes immediate supersaturation, precipitation, and oxidation of aluminum nitrate solution at the surface of the aqueous microsphere. The inner substances of the microspheres are consumed in the formation of the dense shell in a short time. The ECM process is characterized by (1) an isolated small reaction field in which constituent metal ions are mixed homogeneously in the aqueous phase, (2) a short reaction period achieved by the combustion of the thin kerosene film surrounding each aqueous droplet, (3) a continuous fabrication procedure that contributes to lower production costs. The ECM process is classified in chemical synthesis processes such as coprecipitation, sol-gel process, and flame spray pyrolysis (FSP). The ECM process 114
Copyright © 2017 Elsevier Inc. All rights reserved.
Concise Encyclopedia of Self-Propagating High-Temperature Synthesis http://dx.doi.org/10.1016/B978-0-12-804173-4.00050-8
Emulsion Combustion Synthesis
differs from the standard organic solution-fed FSP, because the higher amount of aqueous phase (e.g., 65 vol.%) in the precursor emulsion decreases the flame temperature, favoring the precipitation of precursor in the liquid phase rather than precursor evaporation, and oxidation at the gas phase for particle formation during spray pyrolysis. By comparing the ECM process and the FSP process for synthesis of pure and mixed SiO2 and ZnO nanoparticles, lower flame temperatures in the ECM process than in the FSP process result in mixed gas and liquid phase reaction, forming ZnO particles with inhomogeneous sizes [3]. The significant differences between ECM and the conventional spray pyrolysis are from the size of the reaction field and the reaction period. An isolated small reaction field of about 1 μm is easily prepared using the emulsion process. The reaction period of the conventional spray pyrolysis is longer than that of the ECM. In the ECM process, each dispersed droplet of emulsion is heated by the combustion of a kerosene film surrounding the droplet, which substantially reduces the reaction period of the ECM. The ECM process is capable of producing a wide variety of fine powders. It is particularly well suited for making metal oxides for which water-soluble precursors are widely available. The aqueous droplets in the emulsion contain precursor compounds in the ratios needed to produce the desired composition of the powder product [4]. Since the precursors are dissolved and mixed in the aqueous solution, the chemical composition (e.g., ceramic stoichiometry) can be more exactly controlled, and the method is able to provide highly reliable products of unsurpassed compositional homogeneity. The powders made by the ECM process are inherently spherical solid and/or hollow particles. Spherical solid particles having a narrow size distribution are considered to be ideal for obtaining high packing densities and are desired for making dense ceramic and metal films in the electronics industry. Hollow particles can be attractive as insulating and lightweight filler materials as well as catalyst carriers because of their small size, very thin shell (10 nm), and high specific area (50 m2/g). The ECM process is a promising method for the synthesis of hollow particle of oxides used as fillers (e.g., ZnO) and catalysts (e.g., TiO2 and ZrO2) [5].
REFERENCES [1] [2] [3] [4] [5]
Takatori K. R&D Rev Toyota CRDL 1997;32:1–12 [in Japanese]. Takatori K, Tani T, Watanabe N, Kamiya N. J Nanopart Res 1999;1:197–204. Tani T, Watanabe N, Takatori K. J Nanopart Res 2003;5:39–46. Tani T, Watanabe N, Takatori K, Pratsinis SE. J Am Ceram Soc 2003;86:898–904. Chandradass J, Balasubramanian M, Kim KH. Emulsion combustion synthesis. In: Lackner M, editor. Combustion synthesis: novel routes to novel materials. Sharjah: Bentham Science Publishers Ltd.; 2010. p. 25–33
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An emulsion combustion method (ECM) has been proposed by K. Takatori [1] for making fine oxide powders with an emulsion of aqueous solution of precursor compounds of the desired powder product by following certain steps: (i) atomizing the w/o (water in oil) type emulsion in which metal nitrates are dissolved in an aqueous phase, (ii) burning the oil in the atomized emulsion droplet, and (iii) oxidizing the metal salts in the microspheres of aqueous solution. An aqueous solution of the metal nitrates is stirred with kerosene and a small amount of emulsifier to obtain w/o type emulsion. In many cases, the aqueous phase is 65 vol.% of the emulsion, and the dispersed droplets are about 1–2 μm in diameter. The constituent metal ions in the aqueous droplets are rapidly oxidized at high temperature by combustion of the surrounding kerosene. Most of combustion synthesis is carried out at combustion temperatures higher than 1000 K. Air is introduced into the flame so as to achieve complete combustion. The residence time of the powder at high temperatures is estimated to be less than one-half second. Hollow alumina fine powders are synthesized by the EMC using w/o type emulsions with different aqueous microsphere sizes [2]. The powders are spherical and hollow with a very thin shell from 10 to 20 nm in thickness when synthesized with large aqueous microspheres from 200 nm to 1 mm. Large particles are obtained from small aqueous microspheres of 6 nm, which cause the adhesion of particles during the firing process with the short distance between microspheres. The emulsion process and the combustion technology are combined so that the reaction fields are of submicron size and the reaction times a fraction of second. The characteristic feature of the synthesized powder by the ECM process is a thinner shell than those obtained by the other processes. The rapid heating of aqueous microspheres by combustion of the oil phase causes immediate supersaturation, precipitation, and oxidation of aluminum nitrate solution at the surface of the aqueous microsphere. The inner substances of the microspheres are consumed in the formation of the dense shell in a short time. The ECM process is characterized by (1) an isolated small reaction field in which constituent metal ions are mixed homogeneously in the aqueous phase, (2) a short reaction period achieved by the combustion of the thin kerosene film surrounding each aqueous droplet, (3) a continuous fabrication procedure that contributes to lower production costs. The ECM process is classified in chemical synthesis processes such as coprecipitation, sol-gel process, and flame spray pyrolysis (FSP). The ECM process 114
Copyright © 2017 Elsevier Inc. All rights reserved.
Concise Encyclopedia of Self-Propagating High-Temperature Synthesis http://dx.doi.org/10.1016/B978-0-12-804173-4.00050-8
Emulsion Combustion Synthesis
differs from the standard organic solution-fed FSP, because the higher amount of aqueous phase (e.g., 65 vol.%) in the precursor emulsion decreases the flame temperature, favoring the precipitation of precursor in the liquid phase rather than precursor evaporation, and oxidation at the gas phase for particle formation during spray pyrolysis. By comparing the ECM process and the FSP process for synthesis of pure and mixed SiO2 and ZnO nanoparticles, lower flame temperatures in the ECM process than in the FSP process result in mixed gas and liquid phase reaction, forming ZnO particles with inhomogeneous sizes [3]. The significant differences between ECM and the conventional spray pyrolysis are from the size of the reaction field and the reaction period. An isolated small reaction field of about 1 μm is easily prepared using the emulsion process. The reaction period of the conventional spray pyrolysis is longer than that of the ECM. In the ECM process, each dispersed droplet of emulsion is heated by the combustion of a kerosene film surrounding the droplet, which substantially reduces the reaction period of the ECM. The ECM process is capable of producing a wide variety of fine powders. It is particularly well suited for making metal oxides for which water-soluble precursors are widely available. The aqueous droplets in the emulsion contain precursor compounds in the ratios needed to produce the desired composition of the powder product [4]. Since the precursors are dissolved and mixed in the aqueous solution, the chemical composition (e.g., ceramic stoichiometry) can be more exactly controlled, and the method is able to provide highly reliable products of unsurpassed compositional homogeneity. The powders made by the ECM process are inherently spherical solid and/or hollow particles. Spherical solid particles having a narrow size distribution are considered to be ideal for obtaining high packing densities and are desired for making dense ceramic and metal films in the electronics industry. Hollow particles can be attractive as insulating and lightweight filler materials as well as catalyst carriers because of their small size, very thin shell (10 nm), and high specific area (50 m2/g). The ECM process is a promising method for the synthesis of hollow particle of oxides used as fillers (e.g., ZnO) and catalysts (e.g., TiO2 and ZrO2) [5].
REFERENCES [1] [2] [3] [4] [5]
Takatori K. R&D Rev Toyota CRDL 1997;32:1–12 [in Japanese]. Takatori K, Tani T, Watanabe N, Kamiya N. J Nanopart Res 1999;1:197–204. Tani T, Watanabe N, Takatori K. J Nanopart Res 2003;5:39–46. Tani T, Watanabe N, Takatori K, Pratsinis SE. J Am Ceram Soc 2003;86:898–904. Chandradass J, Balasubramanian M, Kim KH. Emulsion combustion synthesis. In: Lackner M, editor. Combustion synthesis: novel routes to novel materials. Sharjah: Bentham Science Publishers Ltd.; 2010. p. 25–33
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