Applications of Sonochemistry to Materials Synthesis

1. Introduction One of the most important recent applications of sonochemistry has been to the synthesis and modification of inorganic materials [1-5]. In liquids irradiated with high intensity ultrasound, acoustic cavitation drives bubble collapse producing intense local heating, high pressures, and very short lifetimes; these transient, localized hot spots drive high energy chemical reactions [5-11]. As described in detail elsewhere in this monograph, these hot spots have temperatures of roughly 5000°C, pressures of about 1000 atmospheres, and heating and cooling rates above 10 10 K/s. Thus, cavitation serves as a means of concentrating the diffuse energy of sound into a unique set of conditions to produce unusual materials from dissolved (and generally volatile) solution precursors. Ultrasonic cavitation in liquid-solid systems produces related phenomena. Cavity collapse near an extended solid surface becomes non-spherical, drives high-speed jets of liquid into the surface, and creates shockwave damage to the surface [11]. This process can produce newly exposed, highly heated surfaces and is responsible for the erosion/corrosion problems associated with hydrodynamic cavitation [12]. Furthermore, during ultrasonic irradiation of liquid-powder slurries, cavitation and the shockwaves it creates can accelerate solid particles to high velocities [13, 14]. As discussed later, the interparticle collisions that result are capable of inducing striking changes in surface morphology, composition, and reactivity [1-5, 14]. There is a wide range of chemical and physical consequences that high intensity can induce, as shown schematically in Figure 1. The chemical effects of ultrasound fall into three areas: homogeneous sonochemistry of liquids, heterogeneous sonochemistry of liquid-liquid or liquid-solid systems, and sonocatalysis (which overlaps the first two). Applications of ultrasound to materials chemistry are found in all of these areas. Chemical reactions are not generally seen in the ultrasonic irradiation of solids or solid-gas systems. To demonstrate the utility of sonochemistry in materials synthesis, we will examine a range of applications discovered at the University of Illinois. Specifically,