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XXII Session of the Russian Acoustical Society Moscow, June 15-17, 2010
Session of the Scientific Council of Russian Academy of Science on Acoustics
А.А. Novik
LLC "Ultrasonic technique - INLAB" 20 Chugunnaja st., Saint-Petersburg, 194044, Russia Tel.: (7-812) 329-4961; Fax: (7-812) 329-4962 Using of power ultrasound for producing of nanomaterials is quickly developing and promising area of scientific researches. Applying of ultrasonic radiation offers significant advantages in many cases and sometimes it’s the only effective solution of problems related with synthesis and further application of nano-particles. In this work some existing technologies that uses power ultrasonic oscillations for producing of nano-particles and nanomaterials and ultrasonic equipment used for these purposes are considered. Power ultrasonic oscillations are means of active effect on heat- and mass-exchange in liquids, on structure and properties of solids, also on velocity and character of chemical reactions. Sonochemistry is the research area which studies behavior of chemical reaction under action of powerful ultrasound. The application of power ultrasound to production of nanomaterials has manifold effects. The first area of application is particle synthesis and precipitation, the second is dispersing of materials in liquids in order to break particle agglomerates. Let us first address the question of how ultrasonic radiation can rupture chemical bonds, accelerate chemical reactions and diffusion, disperse effectively solids in liquids and then describe the unique products obtained when ultrasound radiation is used in materials science, particularly in production of nanomaterials. Influence of ultrasonic radiation generally relates with development of acoustic cavitation effect, arising in medium under propagation of ultrasound. Acoustic cavitation represents an effective mean of concentration of sound wave low energy density to high energy density, related with pulsations and collapse of cavitational bubbles [1]. General pattern of cavitational bubble formation represents in the following way. In phase of acoustic wave depression a cavity appears, which is filled up by saturated vapor of this liquid. In phase of compression cavity collapses under the action of increased pressure and forces of surface tension and vapor condenses at the interphase boundary. Trough the cavity walls dissolved in liquid gas diffuses into it and then is subjected to high adiabatic compression [2]. In moment of collapse gas pressure and temperature reach significant values – according to some data up to 100 MPa and 5000-25000 K [3]. After collapse of cavity in surrounding liquid sphere impact wave propagates which rapidly damps. Since this collapse occurs in less than a nanosecond [4,5], very high cooling rates, in excess of 1011 K/s, are obtained. Also collapses of bubbles cause liquid jets with velocity up to 150 m/s. Returning to the production of nanomaterials, it’s obvious that this high cooling rate hinders the organization and crystallization of the products. For this reason, in all cases dealing with volatile precursors where gas phase reactions are predominant, amorphous nanoparticles are obtained [3]. While the explanation for the creation of amorphous products is well understood, the reason for the nanostructured products is not clear. One explanation is that the fast kinetics does not permit the growth of the nuclei, and in each collapsing bubble a few nucleation centers are formed whose growth is limited by the short collapse. If, on the other hand, the precursor is a non-volatile compound, the reaction occurs in a 200 nm ring surrounding the collapsing bubble [6]. In this case, the sonochemical reaction occurs in the liquid phase. The products are sometimes nanoamorphous particles, and in other cases, nanocrystalline. This depends on the temperature in the ring region where the reaction takes place. The temperature in this ring is lower than inside the collapsing bubble, but higher than the temperature of the bulk. In paper [6] the temperature in the ring region has been estimated as 1900 °С. In short, in almost all the sonochemical reactions leading to inorganic products, nanomaterials were obtained. They varied in size, shape, structure, and in their solid phase (amorphous or crystalline), but they were always of nanometer size [3]. Many methods have been developed to make nanoparticles. There are, however, four topics related to materials science and nanotechnology in which the sonochemical method is superior to all other techniques. These areas are: • Preparation of amorphous products. Although amorphous metals can be obtained by the cold quenching of bulk metals, when this is extended to metal oxides the cooling rate required for many oxides is well beyond that which can be obtained using the cold quenching method. This is why glass-former materials XXII Session of the Russian Acoustical Society Moscow, June 15-17, 2010
Session of the Scientific Council of Russian Academy of Science on Acoustics

are added to the mixture to form the amorphous products [7,8]. When sonochemistry is applied for the
synthesis of amorphous metal oxides (or sulfides or other chacogenides) there is no need to add these glass
formers, and as a bonus the amorphous products are obtained in nanometer size.
• Insertion of nanomaterials into mesoporous materials. Ultrasonic waves are used for the insertion of amorphous nanosized catalysts into the mesopores [9,10]. A detailed study demonstrates that the nanoparticles are deposited as a smooth layer on the inner mesopores walls, without blocking them. When compared to the other methods such as impregnation or thermal spreading, sonochemistry shows better properties. • Deposition of nanoparticles on ceramic and polymeric surfaces. Sonochemistry is used to deposit various nanomaterials (metals, metal oxides, semiconductors) on the surfaces of ceramic [11,12] and polymeric materials. A smooth homogeneous coating layer is formed on the surface. The nanoparticles are anchored to the surface by forming chemical bonds or chemical interactions with the substrate and cannot be removed by washing. • The formation of proteinaceous micro- and nanospheres. It have been demonstrated that any protein (e.g., polyglutamic acid) can be converted into a sphere by sonication [13]. It have been also illustrated that one can encapsulate a drug, such as tetracycline, in such sphere [14]. Studies have shown that the spherical protein is biologically active, although its biological activity is reduced. The sonochemical spherization process is only 3 min shorter than any other process [3]. Company Nano-Size Ltd. on the base of ultrasonic system with power 4 kW of LLC «Ultrasonic technique - INLAB» production (fig. 1) have developed sonochemistry reactor (USA patent №7,157,058 B2) for production of nano-particles. To produce nanosized metal oxides and hydrates, a metal salt solution (generally a chloride) is subjected to powerful ultrasound in the presence of basic solution such as for example an alkali hydroxide. According to information that this patent contains a 10-liter reactor providing energy 0.6 W/cm3 is suitable for this purposes (authors accentuate that it is magnetostrictive transducer that is used). Under such conditions highly active radicals are rapidly created inside cavitational bubbles that collapse rapidly, leaving nuclei of nanoparticles. In such sonoreaction one mole of metal salt yields up to several hundred grams of nano-powder, 5 to 60 nm crystallite size, in a short reaction time, about 3 – 6 minutes [15]. Examples of compounds which can be derived as nano-particles by this method described by patent authors are oxides: FeO, Fe2O3, Fe3O4, NiO, Ni2O3, CuO, Cu2O, Ag2O, CoO, СO2O3 and hydroxide crystal hydrates: Fe(OH)3, Co(OH)3, NiO(OH). BaTiO3 can be produced by sonochemical method as well. Metal nano-particles also can be produced sonochemically, for example, nano-particles of Fe, Co, Cu, Ag, Ni, Pd and so on. The reactor is an effective unit for accelerating of chemical reactions, for example, the reduction of metal salts or oxides to a metallic powder in relatively high amounts (1 mole) is completed in 5-10 minutes. Such powder consists of ultra-fine metallic or non-metallic particles in nano-scale range (5-100 nm) [15]. Fig 1. Ultrasonic system with power 4 kW: generator, magnetostrictive transducer and changeable
XXII Session of the Russian Acoustical Society Moscow, June 15-17, 2010
Session of the Scientific Council of Russian Academy of Science on Acoustics
As mentioned above another application of ultrasound is dispersion. Nanomaterials, for example metal oxides or carbon nanotubes tend to be agglomerated when mixed into a liquid, while production of nanomaterials requires effective dispersion and obtaining of uniform distribution of nanoparticles in liquid. Effective means of deagglomerating and dispersing are needed to overcome the bonding forces after wettening the powder. The ultrasonic breakup of the agglomerate structures in aqueous and non-aqueous suspensions allows utilizing the full potential of nanosize materials. Investigations at various dispersions of nanoparticulate agglomerates with a variable solid content have demonstrated the considerable advantage of ultrasound when compared with other technologies, such as rotor stator mixers, piston homogenizers, or wet milling methods, e.g. bead mills or colloid mills. For example, carbonnanotubes are strong and flexible but very cohesive. They are difficult to disperse into liquids, such as water, ethanol, oil and so on. Ultrasound is an effective method to obtain discrete - single-dispersed – carbonnanotubes in few minutes. LLC «Ultrasonic technique - INLAB» develops and produces ultrasonic equipment for realizing of all above-mentioned technologies, both specialized, for example ultrasonic physicochemical reactor (Russian patent № 744540), and universal – laboratory ultrasonic units (universal source of ultrasonic oscillations) by Russian patent № 43785. These units could be used in scientific and laboratory investigations, in industrial and semi-industrial applications. To provide application flexibility a series of laboratory units is produced: from IL100-6/1 with power 630 W to IL100-6/6 with power 5 kW. Unit consists of laboratory stand, ultrasonic generator, high-effective high-Q magnetostrictive transducer and three sonotrodes-emitters (concentrators) for transducer. Ultrasonic generator of IL10 series has stepped tuning of power output – 50%, 75%, 100% from nominal power output. Power tuning possibility and three different sonotrodes-emitters (with gain factors 1:0.5, 1:1 and 1:2) allow deriving different amplitude of ultrasonic oscillations in liquids and elastic mediums under investigation, approximately from 0 to 80 µm at the frequency 22 kHz. Using of power ultrasound for producing of nanomaterials is quickly developing and promising area of scientific researches, that is confirmed by increase in quantity of published works on this subject. It have been shown that applying of ultrasonic radiation offers significant advantages in many cases and sometimes it’s the only effective solution of problems related with synthesis and further application of nano-particles. R E F E R E N C E S:
1. Flinn G. Physics o acoustic cavitation in liquids // Physic acoustic / Edited by Y. Meson. – Moscow.: Mir, 1967. – v.1, P. B, p. 7 2. Promtov M. A. Cavitation, (in Russian). 3. Geganken A. Using sonochemistry for the fabrication of nanomaterials // Ultrasonics Sonochemistry, 2004. - vol. 11. - 47. 4. Hiller R., Putterman S.J.,. Barber B.P. Spectrum of synchronous picosecond sonoluminescence // Phys. Rev. Lett., 1992. - 69. - 5. Barber B.P., Putterman S.J. Observation of synchronous picosecond sonoluminescence // Nature, 1991. - vol. 352. - 414. 6. Suslick K.S., Hammerton D.A., Cline R.E. Sonochemical hot spot // J. Am. Chem. Soc., 1986. - vol. 108. - 5641. 7. Livage J. Amorphous transition metal oxides // J. Phys., 1981. - vol. 42. - 981. 8. M. Sugimoto. Amorphous characteristics in spinel ferrites containing glassy oxides // J. Magn. Magn. Mater., 1994. - vol. 133. - 9. Landau M.V., Vradman L., Herskowitz M., Koltypin Y., Gedanken A. Ultrasonically Controlled Deposition–Precipitation: Co– Mo HDS Catalysts Deposited on Wide-Pore MCM Material // J. Catal., 2001. - vol. 201. - 22. 10. Perkas N., Wang Y., Koltypin Yu., Gedanken A., Chandrasekaran S. Mesoporous iron-titania catalyst for cyclohexane oxidation 11. Ramesh S., Koltypin Y., Prozorov R., Gedanken A. Ultrasound Driven Deposition and Reactivity of Nanophasic Amorphous Iron Clusters with Surface Silanols of Submicrospherical Silica // Chem. Mater., 1997. - vol. 9. - 546. 12. Pol V.G., Reisfeld R., Gedanken A. Sonochemical synthesis and optical properties of europium oxide nanolayer coated on titania // Chem. Mater., 2002. - vol. 14. - 3920. 13. S. Avivi Levi, A. Gedanken. Are S-S Bonds Responsible for the Sonochemical Formation of Proteinaceous Microspheres? The Case of Streptavidin // Biochem. J., 2002. - vol. 366. - 705. 14. S. Avivi Levi, Y. Nitzan, R. Dror, A. Gedanken. An easy sonochemical route for the encapsulation of tetracycline in bovine serum albumin microspheres // J. Am. Chem. Soc., 2003. - vol. 125. - 15712. 15. United States Patent No.: US 7,157,058 B2: «High power ultrasonic reactor for sonochemical applications».


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