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Negative thermal expansion materials

Finite element analysis is used to explore composites of negative thermal expansion materials with positive thermal expansion materials (ZrW₂O₈ in Cu and ZrO₂ in ZrW₂O₈) and evaluate how thermal and mechanical properties, rates of cooling/heating, and geometry and packing fraction influence the overall expansion and thermal stress. During rapid temperature changes, the transient short-time thermal expansion can be considerably larger than the steady-state value. Furthermore, thermal stress in the composite can be large, especially at the interface between the materials, and can exceed the material strength.     

Chemical and electronic properties of sulfur-passivated InAs surfaces

Treatment with ammonium sulfide ((NH₄)₂S_x) solutions is used to produce model passivated InAs(0 0 1) surfaces with well-defined chemical and electronic properties. The passivation effectively removes oxides and contaminants, with minimal surface etching, and creates a covalently bonded sulfur layer with good short-term stability in ambient air and a variety of aqueous solutions, as characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and Hall measurements. The sulfur passivation also preserves the surface charge accumulation layer, increasing the associated downward band bending.     

Propagation of acoustic waves scattered in a lossy three-dimensional channel

Abstract

In previous papers we discussed the scattering of acoustic waves by random sound-speed fluctuations in lossless two- and three-dimensional channels. Here we include the effect of bottom loss in a three-dimensional channel. The bottom loss is included by replacing the rigid bottom condition by one allowing for loss. We find an asymptotic solution for the angular distribution of the scattered acoustic energy and the characteristic propagation distance at which the asymptotic solution is valid. Beyond this characteristic distance the angular distribution saturates and is no longer dependent on the propagation distance. Before the mathematical development is given a simple physical argument is presented which shows why a saturation region is expected.     

Cascade system for rapid on-line dilutions in flow-injection analysis

A system for on-line dilutions in flow-injection analysis (FIA) is described. The system requires two flow-through pumps (peristaltic) and a conventional rotary injection valve. No precision timing or computer-controlled valves are required. The "cascade" dilution technique is demonstrated by the determination of 0-1.7M chloride. Viscosity effects are studied for samples with viscosities of 1-150 relative to water. Sample-zone volume reduction by splitting streams is combined with dilution by merging streams to achieve effective dilution factors of nearly 500. The reciprocal mole fraction is proposed as a variable for the characterization of open flow-injection systems and compared with the dispersion coefficient. An equation is included to predict this factor as a function only of the flow-rates used in the system. The cascade dilution system yields reproducible dilution factors with relative standard deviations of less than 3%, with sampling rates up to 100/hr.