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Dispersion curves for phonons propagating in the trigonal direction in bismuth at room temperature and at 7S oK have been obtained in a neutron inelastic scattering experiment. Observed frequencies (units 10 1S rod/sec> at 7S oK are as follows: at the zone center, WTO 1,40 ± 0.02, (tJLO = 1.89 ± 0.02; at the zone boundary in the trigonal direction, WTA 0.73 0.01, Wr,A = 1.12 ± 0.02, WTO = 1.91 ± 0.02, UiLo = 2.03 ± 0.02. At room temperature, the observed frequencies were about 1.5 percent lower. Data were also obtained for longitudinal phonons propagating in the binary direction at 7S oK. It is interesting to note that the splitting between the zone boundary frequencies for the optical and acoustic branches for each polarization is quite large. This splitting is difficult to understand if bismuth is thought of as a slightly distorted simple cubic laltice. The experimental results may be qualitatively understood if bismuth is considered to be made up of a series of double layers normal to the trigonal axis. The atoms in each double layer form a crinkled hexagonal net with strong, probably covalent, bonds between atoms. The forces between atoms on adjacent double layers are relatively weak. This model is consistent with the easy cleavage of bismuth normal to the trigonal axis. Analysis of the trigonal dispersion curves in terms of a linear chain model indicates that there are significant forces connecting a given atom with atoms situated on the four planes on either side of it. 234 ffiM JOURNAL • JULY 1964 235 " -,

No abstract

A steady fluid flow, called microstreaming, can be generated in the vicinity of a microbubble excited by ultrasound. In this article, we use this phenomenon to assemble and power a microfabricated rotor at rotation speeds as high as 625 rpm. The extractible power is estimated to be on the order of a few femtowatts. A first series of experiments with uncontrolled rotor shapes is presented, demonstrating the possibility of this novel actuation scheme. A second series of experiments with 65 m rotors micromanufactured in SU-8 resin are presented. Variables controlling the rotation speed and rotor stability are investigated, such as the bubble diameter, the acoustic excitation frequency and amplitude, and the rotor geometry. Finally, an outlook is provided on developing this micro-rotor into a MEMS-based motor capable of delivering tunable, infinitesimal rotary power at the microscale.

Recent years have seen advancements in the development and use of life-cycle assessment (LCA) analytic techniques. Although these techniques have highlighted the power of LCA to identifj, the environmental consequences of a product system through its entire life cycle, they have also highlighted a major shortcoming ofLCA-the lack of cost information. Because companies make daily decisions that involve trade-ors between economics and the environment, including cost information in LCA is critical for advancing its use as an overall environmental decision-making tool. This article outlines the current state of LCA methodology development, defines key life-cycle cost assessment terms and concepts, and evalriates existing cost assessment techniques with the objective of building an integrated life-cycle cost assessment tool.