Sonofragmentation – The Effect of Power Ultrasound on Particle Breakage

Sonofragmentation describes the breakage of particles into nano-sized fragments by high power ultrasound. In contrast to the common ultrasonic deagglomeration and milling – where particles are mainly grinded and separated by inter-particle collision – , sono-fragementation is distinguished by the direct interaction between particle and shock wave. High power/ low frequency ultrasound creates cavitation and thereby intense shear forces in liquids. The extreme conditions of cavitational bubble collapse and interparticular collision grind particles to very fine size material.

Ultrasonic Production and Preparation of Nano Particles

The effects of power ultrasound for the production of nano materials are well-known: Dispersing, Deagglomeration and Milling & Grinding as well as Fragmentation by sonication are often the only effective method to treat nano particelle. This is especially true when it comes to very fine nano materials with especial funcionalities as with nano size unique particle characteristics are expressed. To create nano material with specific functionalities, an even and reliable sonication process must be ensured. Hielscher supplies ultrasonic equipment from lab scale to full commercial production size.

Sono-Fragmentation by Cavitation

The input of powerful ultrasonic forces into liquids creates extreme conditions. When ultrasound propagates a liquid medium, the ultrasonic waves result in alternating compression and rarefaction cycles (high pressure and low pressure cycles). During the low pressure cycles, small vacuum bubbles arise in the liquid. These cavitazione bubbles grow over several low pressure cycles until they achieve a size when they cannot absorb more energy. At this state of maximum absorbed energy and bubble size, the cavitation bubble collapse violently and creates locally extreme conditions. Due to the implosion of the cavitazione bubbles, very high temperatures of approx. 5000K and pressures of approx. 2000atm are reached locally. The implosion results in liquid jets of up to 280m/s (≈1000km/h) velocity. Sono-fragmentation describes the use of these intense forces to fragment particles to smaller dimensions in the sub-micron and nano range. With a progressing sonication, particle shape turns from angular to spherical, which makes the particles more valuable. The results of sonofragmentation are expressed as fragmentation rate which is described as a function of power input, sonicated volume and the size of the agglomerates.
Kusters et al. (1994) investigated the ultrasonically assisted fragmentation of agglomerates in relation to its energy consumption. The researchers’ results „indicate that the ultrasonic dispersion technique can be as efficient as conventional grinding techniques. The industrial practice of ultrasonic dispersion (e.g. larger probes, continuous throughput of suspension) may alter these results somewhat, but over-all it is expected that the specific energy consumption is not the reason for the selection of this comminutron technique but rather its ability to produce extremely fine (submicron) particles.“ [Kusters et al. 1994] Especially for eroding powders such as Silice or zirconia, the specific energy required per unit powder mass was found to be lower by ultrasonic grinding than that of conventional grinding methods. Ultrasonication affects the particles not only by milling and grinding, but also by polishing the solids. Thereby, a high sphericity of the particles can be achieved.

Sono-fragmentation for the Crystallization of Nanomaterials

„While there is little doubt that interparticle collisions do occur in slurries of molecular crystals irradiated with ultrasound, they are not the dominant source of fragmentation. In contrast to molecular crystals, metal particles are not damaged by shock waves directly and can be affected only by the more intense (but much rarer) interparticle collisions. The shift in dominant mechanisms for sonication of metal powders versus aspirin slurries highlights the differences in properties of malleable metallic particles and friable molecular crystals.“ [Zeiger/ Suslick 2011, 14532]

Sonofragmentation of aspirin particles [Zeiger/ Suslick 2011]

Gopi et al. (2008) investigated the fabrication of high-purity submicrometer alumina ceramic particles (predominantly in sub-100 nm range) from micrometer-sized feed (e.g., 70-80 μm) using sonofragmentation. They observed a significant change in color and shape of alumina ceramic particles as a result of sono-fragmentation. Particles in micron, submicron and nano sized range can be easily obtained by high power sonication. The sphericity of the particles increased with increasing retention time in the acoustic field.

Dispersion in Surfactant

Due to the effective ultrasonic particle breakage, the use of surfactants is essential to prevent deagglomeration of the sub-micron and nano-sized particles obtained. The smaller the particle size, the higher the apect ratio of surface area, which must be covered with surfactant to keep them in suspension and to avoid particles’ coagualation (agglomeration). The advantage of ultrasonication lays in the dispersing effect: Simultaneously to the grinding and fragmentation, ultrasounds dispersed the grinded particle fragments with the surfactant so that agglomeration oft he nano particles is (almost) completely avoided.

Industrial Production

To serve the market with high quality nano material that expresses extraordinary functionalities, reliable processing equipment is required. Ultrasonicators with up to 16kW per unit which are clusterizable allow fort he processing of virtually unlimited volume streams. Due to the fully linear scaleability of ultrasonic processes, ultrasonic applications can be risk-free tested in laboratory, optimized in bench-top scale and then implemented without problems into the production line. As the ultrasonic equiment does not require a large space it can be even retrofitted into existing process streams. The operation is easy and can be monitored and run via remote control, whilst maintenance of an ultrasonic system is almost neglectable.

Particle-size distribution and SEM images of Bi2Te3-based alloy before and after ultrasonic milling. a – Particle-size distribution; b – SEM image before ultrasonic milling; c – SEM image after ultrasonic milling for 4 h; d – SEM image after ultrasonic milling for 8 h.
source: Marquez-Garcia et al. 2015.

Letteratura / Riferimenti