Applications of Ultrasound to Materials Chemistry
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This article will begin with an introduction to acoustic cavitation, the physical phenomenon responsible for the chemical effects of ultrasound. Some recent applications of sonochemistry to the synthesis of nanophase and amorphous metals, as well as to heterogenous catalysis, will then be highlighted. Finally, we will examine the effects of ultrasound on metal powders in liquid-solid slurries.
compresses this cavity, then another expansion wave re-expands it. So we have an oscillating bubble going back and forth, say, 20,000 times a second. As this bubble oscillates, it grows through several mechanisms, one of which is rectified diffusion. In rectified diffusion, the surface area on expansion is slightly larger than on recompression, so growing processes are kinetically slightly faster than shrinking processes. This oscillating, growing bubble reaches a resonant size determined by the frequency of the sound field. When the
bubble is in resonance, it is well-coupled to the sound field, it can absorb energy efficiently, and it can grow rapidly in a single cycle. Once it has grown, however, it is no longer well-coupled to the sound field. At this point, the surface tension of the liquid combined with the next compression wave implosively collapse the bubble on a submicrosecond time frame. A shock wave can be generated in the gas of the bubble in addition to the simple compressional heating of the gas. When gas is compressed, heating results. When gas is compressed this rapidly, the heating is nearly adiabatic. The heat has no time to flow out, so a very localized, transient hot spot forms, and that hot spot is responsible for the chemistry that is observed. The conditions formed during that transient cavitation are extreme. We have been able to measure temperatures and pressures by comparative rate thermometry and by using sonoluminescence as a spectroscopic probe of the species formed during cavitation. Our current best estimates of the hot-spot conditions give temperatures above 5000 K, pressure of about 1700 atm, and time duration under 100 ns, and the time may be substantially less than that. We therefore have cooling rates associated with this process of more than 101" degrees/s. For calibration purposes, if I thrust a poker of red-hot iron into ice water, I get a cooling rate of a few thou-
Compression
Cavitation The chemical effects of ultrasound do not come from a direct interaction of sound with molecular species. Ultrasound has frequencies from around 15 kilohertz to tens of megahertz. In liquids, this means wavelengths from centimeters down to microns, which are not molecular dimensions. Instead, when sound passes through a liquid, the formation, growth, and implosive collapse of bubbles can occur, as depicted in Figure 1. This process is called acoustic cavitation. More specifically, sound passing through a liquid consists of expansion waves and compression waves. As sound passes through a liquid, if the expansion wave is intense enough (that is, if the sound is loud enough), it can pull the liquid apa
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