Synthesis of hollow metallic particles via ultrasonic treatment of a metal emulsion Dierk Raabe * and Dennis Hessling Max-Planck-Institut fu ¨ r Eisenforschung, Max-Planck Str. 1, 40237 Du ¨ sseldorf, Germany Received 4 December 2009; revised 13 January 2010; accepted 14 January 2010 Available online 18 January 2010 We introduce a synthesis method in which liquid metals in an emulsion mix and solidify under the influence of ultrasound radiation. We use it to produce two types of metallic nano- and microparticles. The first group, formed by rapid droplet solidifica- tion, is characterized by a nearly perfect spherical shape. The second group consists of hollow particles which were probably formed by near-adiabatic cooling of the metal inside an expanding cavitation bubble. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Eutectic solidification; Rapid solidification; Amorphous materials; Metallic glass; Ultrasonic synthesis This report describes a new synthesis method which uses metal drop emulsion sonication. In this approach, used to produce metallic micro- and nanopar- ticles, liquid metal immersed in an emulsion mixes and solidifies when exposed to intense ultrasound. The approach is inspired by sonochemistry, where acoustic cavitation drives chemical reactions under extreme conditions [1–5]. Cavitation describes the formation, growth, and implosion of bubbles in liquids [6,7]. In the vicinity of collapsing bubbles extreme temperatures (>5000 K), pressures (>20 MPa), and cooling rates (>10 7 Ks 1 ) occur [1–3,6–9]. Such an environment can be used to synthesize amorphous Fe, Pd, and Ni by decomposition of organometallic precursors in low-vol- atility solvents [1–3]. Suh et al. synthesized hollow particles via sonochemistry [10]. Abramov [11] applied ultrasonic radiation directly to bulk metallic melts during solidification with the aim of refining grain size and reducing segregation. Our new approach is based on an ultrasound-assisted solidification route where liquid metal droplets that are immersed in an emulsion are exposed to intense ultra- sound radiation and solidify as micro- and nanoscaled spherical particles. The method differs from sonochemis- try [1–10] as it does not directly use the extreme temper- ature or pressure of collapsing cavities for driving chemical reactions. Instead, it exploits the stirring, jet, and Joule–Thomson effects to create metallic spheres of different morphology. Also, it differs from Abramov’s approach [11] as ultrasound is, in the current work, applied to a water–metal emulsion rather than to bulk liquid metal. The method is demonstrated using an eutectic solder consisting of 51 wt.% indium, 32.5 wt.% bismuth, and 16.5 wt.% tin (Field’s metal, melting point: 62 °C). The pre-alloyed metal was charged in bulk form and melted immediately in hot deionized water (73 °C, no additives) to form a coarse emulsion. Turning on the ultrasonic horn increased the dispersion drastically, rendering the emulsion a fine grayish material. The mixture was slowly cooled (1 K min 1 ) to the melting point of the solder under the influence of ultrasound radiation. A Bandelin Sonopuls ultrasonic homogenizer HD 3200, coupled to the sonotrode VS 70 T operated at 20 kHz and a power of 113 W, was used. The horn was dipped to a depth of 80 mm, corresponding to two-thirds depth, into the cru- cible (Fig. 1). Sonication time was 5 min (energy input: 33.98 kJ) at a metal weight of 0.55 g immersed into 120 ml water at 73 °C. Characterization of the synthe- sized particles was conducted using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD). Three main observations were made. First, most of the particles have dimensions between a few nanometers and several micrometers (Fig. 2). The size distribution has in part fractal character: at increased microscopy resolution, similar log-normal size distributions appear at different scales. We observed large clusters of particles with diameters above 1 lm as well as nanosized 1359-6462/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2010.01.028 * Corresponding author. E-mail: d.raabe@mpie.de Available online at www.sciencedirect.com Scripta Materialia 62 (2010) 690–692 www.elsevier.com/locate/scriptamat