Thomas M. Ticich

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.

Can soot primary particle size be determined using laser-induced incandescence?

Temporally resolved laser-induced incandescence (LII) signals are obtained from different size primary particles produced by the diffusion flames of methane, ethane, ethylene, and acetylene. These results represent the first direct comparison between primary particle sizes based on optical measurements and those directly measured through transmission electron microscopy (TEM). Analysis of the data at different detected wavelengths as suggested by theory reveals a nonmonotonic relation with primary particle size as measured by TEM. Two alternative measures of the temporal decay of the signal at a single detected wavelength reveal a correlation with primary particle size within soot aggregates produced by the different flames. Comparison between predicted primary particle size based on the calibrations using the temporal analysis of the LII signal and TEM measurements reveals agreement within the growth region (low axial heights) and very late in the oxidation region (high axial heights) within an ethylene gas-jet diffusion flame. Significant differences exist at intermediate positions. These differences are interpreted as representing the effects of cluster-cluster aggregation within the oxidation region.

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.

Cavity ringdown and laser-induced incandescence measurements of soot

Currently laser-induced incandescence (LII) is widely used for the measurement of soot volume fraction. A particularly important aspect of the technique that has received less attention, however, is calibration. The applicability of cavity ringdown (CRD) for measurement of soot volume fraction f(v) is assessed, and the calibration of LII by means of CRD is demonstrated. The accuracy of CRD for f(v) determination is validated by comparison with traditional light extinction and path-integrated LII. By use of CRD, the quantification of LII for parts in 10(9) (ppb) f(v) levels is demonstrated. Results are presented that demonstrate the accuracy of CRD for a single laser pulse to be better than ?5% for measurement of ppb soot volume-fraction levels over a 1-cm path length. By use of CRD, spatially resolved LII signals from soot within methane-air diffusion flames are calibrated for ppb f(v) levels, thereby avoiding the extrapolation required of less sensitive methods in current use.

Comparative flame and furnace synthesis of single-walled carbon nanotubes

Abstract In this Letter, results are reported for flame synthesis of single-walled carbon nanotubes. A pyrolysis flame of CO/H 2 is established with introduction of the nanocatalyst precursor particles as an aerosol created by drying a nebulized solution of iron or iron colloid (in the form of ferrofluid). Results are compared to those produced by entraining the same catalyst aerosol into a tube furnace. Optimum flame gas flows and overall gas composition are reported and interpreted in terms of temperature and particle residence time.

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.