John B. Vander Sande

Fullerenic carbon in combustion-generated soot

Soot samples collected as bulk solids and by thermophoretic sampling at different residence times in a fullerene-forming premixed benzene/oxygen flat flame (C/O=0.96, P=5.34 kPa, 10% argon, v=25 cm/s) were analyzed by high resolution electron microscopy. The samples contained soot particles that were composed to some extent of amorphous and fullerenic carbon (e.g., curved layers and fullerene-molecule-sized closed-shell structures). Qualitative and quantitative analyses of residence time-resolved samples showed that the length of curved layers increases and their radius of curvature decreases with increasing residence time in the flame. The number of closed-shell structures in the soot as well as the concentration of fullerene molecules in the gas phase increase with increasing residence time, consistent with fullerenes concentration increasing with residence time and with the consumption of fullerenes by reaction with soot. The data suggest that the formation of amorphous and fullerenic carbon occurs in milliseconds, with the fullerenic carbon becoming more curved as a soot particle traverses the length of the flame. Conversely, the formation of highly ordered carbon nanostructures, such as tubes and onions, appears to require much longer residence times, perhaps seconds or minutes depending on the temperature, in the flame environment.

Combustion synthesis of fullerenes and fullerenic nanostructures

Abstract Samples of condensable material collected from low-pressure premixed and diffusion benzene/oxygen/argon flames were analyzed chemically to determine fullerene yield and by high-resolution transmission electron microscopy to characterize the fullerenic material on and within the soot particles. Results show that fullerene formation is sensitive to changes in operating conditions, such as fuel/oxygen ratio, chamber pressure, and inert gas dilution, and that the formation of amorphous and fullerenic carbon occurs early in the flame, with the structures becoming more curved with greater residence time. All flames exhibit a fullerene maximum with the premixed flame showing two distinct regions of formation. Additionally, the fullerene maximum in the diffusion flames is always just above the stoichiometric flame surface and a maximum is observed with increasing dilution due to competing dilution effects. Image analysis data show that the curvatures and diameters of the structures are consistent with the chemical analysis and that nanostructures, found at greater residence times than fullerenes, are formed directly from curved structures in the soot. These data complement previous fullerene studies and shed light on several proposed mechanisms for fullerene formation in combustion.

Soot morphology: An application of image analysis in high‐resolution transmission electron microscopy

Interest in the fine structure of soots and carbon blacks is motivated by a variety of possible applications. The structure provides information on the origins of the particles and on their adsorptive and reactive properties. This paper describes a method for quantification of the structure of soots and carbon blacks based on direct electron microscopic observation followed by image analysis of these materials. High‐resolution transmission electron microscopy (HRTEM) provides a very detailed observation of particle structure. The differences in soot structure, because of its complexity, may not be easily quantifiable with the human eye; therefore, high‐level computer software has been used to manipulate HRTEM images. This technique involves the application of fast Fourier transforms (FFT) to single particles and the measurement of characteristic parameters such as interplanar spacings and crystallite sizes from these particles. The methodology and application of this characterization technique are presented here. Results are shown for different samples obtained from soot and carbon black particles selected to illustrate the capabilities of the methodology. Quantitative information can be obtained on structural characteristics, e.g., interplanar spacing, circularity, orientation, elongation, and length distribution of lattice fringes, as well as on the fractional coverage of the extracted pattern.

Reversible Clustering of pH- and Temperature-Responsive Janus Magnetic Nanoparticles

Janus nanoparticles have been synthesized consisting of approximately 5 nm magnetite nanoparticles coated on one side with a pH-dependent and temperature-independent polymer (poly(acrylic acid), PAA), and functionalized on the other side by a second (tail) polymer that is either a pH-independent polymer (polystyrene sodium sulfonate, PSSNa) or a temperature-dependent polymer (poly(N-isopropyl acrylamide), PNIPAM). These Janus nanoparticles are dispersed stably as individual particles at high pH values and low temperatures, but can self-assemble at low pH values (PSSNa) or at high temperatures (>31 degrees C) (PNIPAM) to form stable dispersions of clusters of approximately 80-100 nm in hydrodynamic diameter. The Janus nanoparticle compositions were verified using FTIR and XPS, and their structures observed directly by TEM. Their clustering behavior is analyzed by dynamic light scattering and zeta potential measurements.

Rapid solidification of a droplet-processed stainless steel

Individual powder particles of a droplet-processed and rapidly solidified 303 stainless steel are characterized in terms of microstructure and composition variations within the solidification structure using scanning transmission electron microscopy (STEM). Fcc is found to be the crystallization phase in powder particles larger than about 70 micron diameter, and bcc is the crystallization phase in the smaller powder particles. An important difference in partitioning behavior between these two crystal structures of this alloy is found in that solute elements are more completely trapped in the bcc structures. Massive solidification of bcc structures is found to produce supersaturated solid solutions which are retained to ambient temperatures in the smallest powder particles. Calculated liquid-to-crystal nucleation temperatures for fcc and bcc show a tendency for bcc nucleation at the large liquid supercoolings which are likely to occur in smaller droplets. The importance of small droplet sizes in rapid solidification processes is stressed.

Controlled Assembly of Nanoparticle Structures: Spherical and Toroidal Superlattices and Nanoparticle-Coated Polymeric Beads

The emulsion droplet solvent evaporation method has been used to prepare nanoclusters of monodisperse magnetite nanoparticles of varying morphologies depending on the temperature and rate of solvent evaporation and on the composition (solvent, presence of polymer, nanoparticle concentration, etc.) of the emulsion droplets. In the absence of a polymer, and with increasing solvent evaporation temperatures, the nanoparticles formed single- or multidomain crystalline superlattices, amorphous spherical aggregates, or toroidal clusters, as determined by the energetics and dynamics of the solvent evaporation process. When polymers that are incompatible with the nanoparticle coatings were included in the emulsion formulation, monolayer- and multilayer-coated polymer beads and partially coated Janus beads were prepared; the nanoparticles were expelled by the polymer as its concentration increased on evaporation of the solvent and accumulated on the surfaces of the beads in a well-ordered structure. The precise number of nanoparticle layers depended on the polymer/magnetic nanoparticle ratio in the oil droplet phase parent emulsion. The magnetic nanoparticle superstructures responded to the application of a modest magnetic field by forming regular chains with alignment of nonuniform structures (e.g., toroids and Janus beads) that are in accord with theoretical predictions and with observations in other systems.

On producing high-spatial-resolution composition profiles via scanning transmission electron microscopy

Abstract Measurement of the concentration variation of a grain boundary segregant near a grain boundary has been chosen as a system in which to investigate the spatial resolution attainable by X-ray microanalysis in the scanning transmission electron microscope (STEM). In this paper extensive experimental work on Fe-doped MgO is compared with a theoretical model which examines the effect of incident probe size and electron beam broadening in the sample on concentration profiles measured using standard analysis of X-ray data. It is shown that the spatial extent of segregation can be determined to a resolution dependent on the incident probe size. The magnitude of the peak concentrations determined at the boundary are, however, strongly dependent on beam broadening and hence foil thickness. Comparing experimental and calculated results suggests that the extent of beam broadening may not be as great as current theoretical estimates would predict.

Synthesis of fullerenes and fullerenic nanostructures in a low-pressure benzene/oxygen diffusion flame

Samples of condensable material from laminar low-pressure benzene/argon/oxygen diffusion flames were collected and analyzed by high-performance liquid chromatography to determine the yields of fullerenes and by high-resolution transmission electron microscopy (HRTEM) to characterize the fullerenic material (i.e., curved-layer nanostructures) on and within the soot particles. The highest concentration of fullerenes was always detected just above the visible stoichiometric surface of a flame. The percentage of fullerenes in the condensable material increases with decreasing pressure. The overall highest amount of fullerenes was found for a surprisingly high dilution of fuel with argon. A comparison of the flames with the same cold gas velocity of fuel and oxygen showed a strong dependence of fullerene content on flame length. A shorter flame, resulting from higher dilution or lower pressure, favors the formation of fullerenes rather than soot, and the amount of soot and precursors of both soot and fullerenes is less at lower pressure and higher dilution. This behavior indicates a stronger correlation of fullerene consumption to the total amount of soot than of fullerene formation to precursor concentration. The maximum flame temperature seems to be of minor importance in fullerence formation. The HRTEM analysis of the soot showed an increase of the curvature of the carbon layers, and hence increased fullerenic character, with increasing distance from the burner up to the point of maximum fullerence concentration. After this maximum, where soot and fullerences are consumed by oxidation, the curvature decreases. In addition to the soot, the samples included fullerenic nanostructures such as tubes and spheroids including highly ordered multilayered or onionlike structures. The soot itself shows highly ordered regions that appear to have been cells of ongoing fullerenic nanostructure formation.

Precipitate Crystal Structure Determination in Melt Spun Mg-1.5wt%Ca-6wt%Zn Alloy

A melt-spun Mg-1.5%wtCa-6wt%Zn alloy was analyzed by means of transmission electron microscopy, energy dispersive X-ray spectroscopy, and scanning transmission electron microscopy. The as-solidified alloy exhibited both spherical matrix precipitates and elongated precipitates at the grain boundaries (grain-boundary films). After heat treatment, the alloy showed faceted precipitates (cuboidal shape), mostly on dislocations. It was found that the observed precipitates are the same compound, Ca2Mg6Zn3. As there was no crystallographic data for this compound in the literature, its crystal structure was investigated by comparison of experimental and simulated selected-area electron-diffraction patterns and high-resolution electron microscopy images. This study indicated that Ca2Mg6Zn3 is a trigonal compound with space group P 3 1c and lattice parameters a = 0.97 nm, c = 1.0 nm.

The oriented growth of precipitates on dislocations in Al-Zn-Mg—part I. Experimental observations

Abstract The weak-beam transmission electron microscope technique was employed to examine the precipitation of η phase laths on dislocation lines in Al-3.87 wt.% Zn-1.79 wt.% Mg. The use of this technique allowed the observation of fine-scale particles which were still in intimate contact with their catalyzing dislocations, and so would presumably most strongly reflect the role of dislocations during particle growth. The lath-like particles generally formed with their long axes (preferred growth directions) in the dislocation glide planes and parallel to the 〈110〉 matrix directions closest to the estimated initial orientations of the catalyzing defects. During these early stages of growth, the randomly oriented dislocations maintained contiguity with the lengthening particles by bending in their glide planes to accommodate the preferred growth directions of the particles. The misorientation angle between the 〈110〉 growth direction of a lath and the initial orientation of its associated dislocation was found to affect the particle growth kinetics: Laths growing at larger misorientation angles were found to be significantly shorter than those growing at smaller angles for the same aging time.