Randall L. Vander Wal

Substrate–support interactions in metal-catalyzed carbon nanofiber growth

Catalyst–support interactions are critical in CVD processes for nanotube synthesis. In this article, the relative contributions of the catalyst electronic structure and support chemical composition are evaluated with Cu, Fe and Ni as catalysts and Al2O3, CaO, SiO2 and TiO2 as support media. The impact of the interaction is judged qualitatively based on nanotube growth and structure. Results are interpreted in terms of electron charge donation to the metal nanoparticle enabled by either strong-metal support interaction (SMSI) or by interaction of the catalyst nanoparticle with exposed Lewis base sites on the support material. The role of the physical structure of the support medium is explored by comparison of nanotubes grown upon powdered and fumed phases of the support oxides. Carbon nanotubes catalyzed by metal nanoparticles generated in-situ or preformed illustrate the advantage of presynthesized particles for size uniformity with attendant greatly lessened dependence upon catalyst preparation conditions. Catalyst retention and dispersion under rapid heating conditions is evaluated for the same support-catalyst systems listed above as a preliminary test for flame synthesis. Results show that SMSI interaction is critical to using the supported catalyst method in a flame.

Diffusion flame synthesis of single-walled carbon nanotubes

Abstract Flame synthesis is demonstrated for the synthesis of single-wall nanotubes via a simple laboratory-scale diffusion flame. Results using different hydrocarbon reagents, the effects of dilution with an inert, the role of hydrogen and processes likely accountable for the deactivation of the catalyst particles are illustrated and discussed. Finally, a plot of the integrated residence time–temperature history of a fluid parcel along the flame axis indicates carbon nanotube growth occurring within 20 ms, thus demonstrating the great potential of flame synthesis for large-scale commercial production of nanotubes.

Trace Metal Detection by Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been widely pursued for trace elemental determination in gases, solids, and liquids. Application to liquids has proved problematic due to high spatial confinement of the LIBS plasma and rapid quenching of the excited-state emission. This work presents an alternative approach to trace metal determination in liquids in which 1.0 mL of liquid is deposited onto a carbon planchet and then evaporated, thus transforming the liquid analysis to a solid surface analysis. Using optimized excitation and detection conditions, we have identified spectral regions for sensitive detection of 15 metals (Mg, Al, Si, Ca, Ti, Cr, Fe, Co, Ni, Cu, Zn, As, Cd, Hg, Pb). The limit of detection (LOD) for the technique ranged from 10 ppb to 10 ppm for these elements. A 100 ppb LOD represents detection of 130 picograms of metal (approximately 2 picomoles) in a single measurement calculated from the laser spot size on the sample. Scanning electron microscopy (SEM) images and energy-dispersive X-ray (EDX) spectra of the samples provide insight into the observed reproducibility and linearity of the technique for several of the metals studied.

Flame synthesis of Fe catalyzed single-walled carbon nanotubes and Ni catalyzed nanofibers: growth mechanisms and consequences

Abstract Flame synthesis of single-walled carbon nanotubes and nanofibers is demonstrated using a pyrolysis flame configuration. Fe reacts preferentially with CO/H 2 /He mixtures to produce single-walled nanotubes (SWNTs). In contrast, Ni reacts preferentially with C 2 H 2 /H 2 /He mixtures to yield nanofibers. Both catalyst metals exhibit a marked size dependent reactivity towards these reactant gas mixtures. The yarmulke mechanism and a carbon solvation/diffusion/precipitation account for the different catalyzed products; SWNTs and nanofibers, respectively. Consequences of the size dependent reactivities of Fe and Ni nanoparticles and the respective growth mechanisms for the SWNTs and nanofibers are discussed.

Flame synthesis of Ni-catalyzed nanofibers

Abstract Flame synthesis of Ni-catalyzed nanofibers is illustrated. Ni nanoparticles are formed by thermal decomposition of a nebulized Ni nitrate solution entrained into a reactive fuel mixture. Although at the earliest stages of growth, the Ni nanoparticles are sufficiently small to catalyze single-wall nanotubes (SWNTs), only the larger particles appear catalytically active yielding only nanofibers. Using different reactive gas mixtures consisting of CO or C2H2 or their combination, Ni nanoparticles exhibited a high preferential reactivity towards C2H2 to form nanofibers. These results are interpreted as a result of CO enhancing the dissociative adsorption of C2H2 through electronic charge donation mediated by the Ni nanoparticle and physically protecting active sites against blockage by aromatic molecules.

Laser-induced incandescence applied to metal nanostructures

Laser-induced incandescence is both characterized and demonstrated for the measurement of metal nanoparticle concentration. Reported are the results of an initial characterization of the spectral and temporal signature of the laser-induced incandescence as a function of the excitation laser fluence and wavelength. Validation of the incandescence as a measure of the concentration is demonstrated by absorption measurements. Fluence dependence measurements are also presented. Double-pulse measurements determine the fluence for the onset of vaporization-induced mass loss. Comparisons between the present observations and those for carbon nanostructures are also made. Metals tested include (in order of increasing vaporization temperature) Fe, Ti, Mo, and W.

NUMERICAL AND EXPERIMENTAL STUDIES OF SPLASHING DROPLETS

The present paper reports ongoing work to develop numerical, modeling, and experimental tools used to investigate the physical mechanism leading to splashing modes and to determine characteristic parametric non-dimensional dependence for impinging droplets representative of practical fluids and conditions. In particular, we present data that will delineate and ultimately control the impingement dynamics of droplets upon a heated substrate germane to practical situations.