L. J. Thompson

Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper nanoparticles

It is shown that a “nanofluid” consisting of copper nanometer-sized particles dispersed in ethylene glycol has a much higher effective thermal conductivity than either pure ethylene glycol or ethylene glycol containing the same volume fraction of dispersed oxide nanoparticles. The effective thermal conductivity of ethylene glycol is shown to be increased by up to 40% for a nanofluid consisting of ethylene glycol containing approximately 0.3 vol % Cu nanoparticles of mean diameter <10 nm. The results are anomalous based on previous theoretical calculations that had predicted a strong effect of particle shape on effective nanofluid thermal conductivity, but no effect of either particle size or particle thermal conductivity.

Enhanced thermal conductivity through the development of nanofluids

Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids required in many industrial applications. To overcome this limitation, a new class of heat transfer fluids is being developed by suspending nanocrystalline particles in liquids such as water or oil. The resulting nanofluids possess extremely high thermal conductivities compared to the liquids without dispersed nanocrystalline particles. For example, 5 volume % of nanocrystalline copper oxide particles suspended in water results in an improvement in thermal conductivity of almost 60% compared to water without nanoparticles. Excellent suspension properties are also observed, with no significant settling of nanocrystalline oxide particles occurring in stationary fluids over time periods longer than several days. Direct evaporation of Cu nanoparticles into pump oil results in similar improvements in thermal conductivity compared to oxide-in-water systems, but importantly, requires far smaller concentrations of dispersed nanocrystalline powder.

Grain-size-dependent thermal conductivity of nanocrystalline yttria-stabilized zirconia films grown by metal-organic chemical vapor deposition

A grain-size-dependent reduction in the room-temperature thermal conductivity of nanocrystalline yttria-stabilized zirconia is reported for the first time. Films were grown by metal-organic chemical vapor deposition with controlled grain sizes from 10 to 100 nm. For grain sizes smaller than approximately 30 nm, a substantial reduction in thermal conductivity was observed, reaching a value of less than one-third the bulk value at the smallest grain sizes measured. The observed behavior is consistent with expectations based on an estimation of the phonon mean-free path in zirconia.

Hydrogen‐induced crystal to glass transformation in Zr3Al

The process of hydrogen‐induced amorphization of the equilibrium intermetallic compound Zr3Al is compared to ion‐irradiation‐induced amorphization of the same compound. In contrast to ion irradiation, where almost complete chemical disordering precedes the onset of amorphization, hydrogenation of Zr3Al causes no appreciable change in long‐range order prior to amorphization. Electron microscopy reveals apparent homogeneous nucleation of the amorphous phase, and striking similarities to martensitic microstructures. The maximum lattice dilation observed prior to amorphization by hydrogen absorption is identical to that found during irradiation, indicating that lattice expansion is a common measure of the crystal instability induced by different solid‐state processing techniques.

Observation of growth modes during metal-organic chemical vapor deposition of GaN

We present real-time surface x-ray scattering measurements during homoepitaxial growth of GaN by metal-organic chemical vapor deposition. We observed intensity oscillations corresponding to the completion of each monolayer during layer-by-layer growth. The growth rate was found to be temperature independent and Ga-transport limited. Transitions between step-flow, layer-by-layer, and three-dimensional growth modes were determined as a function of temperature and growth rate.

Composition effects on the early-stage oxidation kinetics of (001) Cu-Au alloys

An in situ environmental transmission electron microscopy study of the nucleation and growth of oxide islands during the early-stage oxidation of (001) Cu1−xAux alloys (x⩽38at.%) was undertaken in order to investigate the effects of alloying on oxide island nucleation behavior and growth kinetics. The kinetic data reveal that Au enhances the nucleation density of oxide islands and suppresses their growth rate. Our results provide insight into reasons for the decreased passivation properties of Cu when alloyed with Au.

Early-stage suppression of Cu (001) oxidation

In situ synchrotron x-ray studies of the early-stage oxidation behavior of Cu (001) reveal that for Cu2O nanoislands, the Cu–Cu2O equilibrium phase boundary is shifted to larger oxygen partial pressure (pO2) by many orders of magnitude relative to bulk Cu2O. Real-time scattering measurements find that an ordered surface structure appears with increasing pO2, followed by the nucleation of epitaxial Cu2O nanoislands. By adjusting the pO2, it is possible to reversibly grow or shrink these islands and accurately determine the equilibrium phase boundary. These observations provide insight into the general stability of oxide nanoclusters grown by various techniques.

High‐Resolution Studies of Grain Boundary Structure in Yttria‐Stabilized Cubic Zirconia Thin Films Grown by MOCVD

Polycrystalline cubic-phase YSZ films with a colummar grain morphology were sunthesized by MOCVD. Under suitable growth and annealing conditions highly (001) textured films were obtained and used for investigations of [001] tilt grain boundaries (GBs) by TEM, AEM and HREM. Individual primary GB dislocations in low-angle GBs were imaged by Fresnel contrast and HREM techniques. Both low and high-angle GBs were well structured at the atomic scale. No amorphous GB phases were found in any of the GBs. However, yttria segregation to GBs after high-temperature anneals was indicated by EDS. The atomic-scale structures, although in detail quite different from previously investigated oxide GBs, such as the Σ5, (210) GB in NiO, share a number of structural features with other oxide GBs. These include the tendency toward coherence between low-index planes crossing the GB, misfit localization, and the formation of many types of asymmetric facets that include low-index planes. The fact that well-structured GBs free of secondary phases can be produced in YSZ is of technological importance, especially for fast-ion conduction applications, such as in high temperature fuel cells.

Effect of dose rate on ion beam mixing in Nb‐Si

The influence of dose rate, i.e., ion flux, on ion beam mixing in Nb‐Si bilayer samples was measured at room temperature and 325 °C. At the higher temperature, an increase in dose rate of a factor of 20 caused a decrease in the thickness of the mixed layer by a factor of 1.6 for equal total doses. At room temperature, the same change in flux had no effect on mixing. These results are consistent with radiation‐enhanced diffusion theory in the recombination‐limited regime.

Thermally activated step motion observed by high-resolution electron microscopy at a (113) symmetric tilt grain-boundary in aluminium

Grain-boundary migration is demonstrated to proceed by lateral propagation of a small step in a (113), [110] symmetric Al tilt grain-boundary. In-situ high-resolution (transmission) electron microscopy (HREM) at 523K allowed the study of atomic-scale detail at video rates during the migration process. The grain-boundary translational states on both sides of the step are identical, which leads to a step dislocation. This defect can move laterally by a combination of climb and glide. Dynamic HREM images indicate considerable atomic agitation within the dislocation core. A detailed temporal analysis of the step movements shows small random displacements of the dislocation core.Grain-boundary migration is demonstrated to proceed by lateral propagation of a small step in a (113), [110] symmetric Al tilt grain-boundary. In-situ high-resolution (transmission) electron microscopy (HREM) at 523K allowed the study of atomic-scale detail at video rates during the migration process. The grain-boundary translational states on both sides of the step are identical, which leads to a step dislocation. This defect can move laterally by a combination of climb and glide. Dynamic HREM images indicate considerable atomic agitation within the dislocation core. A detailed temporal analysis of the step movements shows small random displacements of the dislocation core.