Ultrasonic Milling and Dispersing Technology for Nano-Particles
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Ultrasonic Milling and Dispersing Technology for Nano-Particles Kathrin Hielscher1 1 Hielscher Ultrasonics GmbH, Warthestr. 21, 14513 Teltow, Germany [email protected]; www.hielscher.com ABSTRACT Ultrasonically generated forces are well known for dispersing and deagglomeration of small volumes in laboratory and bench-top scale. By the evaluation and optimization of the most important ultrasonic parameters and the development of large scale ultrasonic machinery, ultrasound forces can be applied also for particle size reduction and wet-milling of nano-particles in industrial scale. Keywords: acoustic, nanoscale, nano-structure, ultrasound, dispersion, particle size reduction, ink INTRODUCTION The dispersing and deagglomeration of solids into liquids is an important application of ultrasonic devices. If powders are wetted, the individual particles build agglomerates and are held together by attraction forces of various physical and chemical natures, including van der Waals forces and liquid surface tension. This effect is stronger for higher viscosity liquids, such as polymers or resins. The attraction forces must be overcome on order to deagglomerate and disperse the particles into liquid media. An even dispersion and deagglomeration is important to use the full potential of the particles. Especially nano-particles offer extraordinary characteristics, which can only be exploited in highly even dispersed state. The application of mechanical stress – e.g. generated by ultrasonic cavitation - breaks the particle agglomerates apart. Also, liquid is pressed between the particles. Different technologies are commonly used for the dispersing of powders into liquids. This includes high pressure homogenizers, agitator bead mills, impinging jet mills and rotor-stator-mixers. High intensity ultrasonication is an interesting alternative to these technologies and particularly for the particle treatment in the nano-size range the only effectual method to achieve the required results. ULTRASONIC CAVITATION By high-power/ low-frequency ultrasound high amplitudes can be generated. Thereby, high-power/ low-frequency ultrasound can be used for the processing of liquids such as mixing, emulsifying, dispersing and deagglomeration, or milling. When sonicating liquids at high intensities, the sound waves that propagate into the liquid media result in alternating highpressure (compression) and low-pressure (rarefaction) cycles, with rates depending on the frequency. During the low-pressure cycle, high-intensity ultrasonic waves create small vacuum bubbles or voids in the liquid. When the bubbles attain a volume at which they can no longer absorb energy, they collapse violently during a high pressure cycle. This phenomenon is termed cavitation. Cavitation, that is "the formation, growth, and implosive collapse of bubbles in a liquid. Cavitational collapse produces intense local heating (~5000 K), high pressures (~1000
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atm), and enormous heating and cooling rates (>109 K/sec)" and liquid jet streams (~400 km/h)”. [5] There are differe
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