Randy L. Vander Wal

Soot oxidation: dependence upon initial nanostructure

Although the relation between carbon structure and reactivity is well-known from thermal and oxidative studies of coal, char, and graphite, the relation for soot remains unstudied. In this article, the dependence of the soot oxidation rate upon the length and curvature of the graphene segments, which depicts the nanostructure, is shown. Reflecting different ratios of edge to basal plane sites or amounts of ring strain imposed by curvature, burnout rates are found to differ by greater than 400% for the soots studied here. Surprisingly, the different soot nanostructures are readily produced by using different fuels and pyrolysis conditions.

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.

The effects of rapid heating of soot: Implications when using laser-induced incandescence for soot diagnostics

Recent experimental efforts have exploited the high temporal and spatial resolution of laser-induced incandescence (LII) as both a qualitative and quantitative measure of soot volume fraction. As a relatively new diagnostic technique, issues remain as to appropriate excitation laser intensities and the potential intrusive characteristics of LII. The high temperatures to which the soot is heated may accelerate heterogeneous reactions between the soot and flame gases. Vaporization of soot by high energy pulsed laser light has been theoretically modeled and experimentally observed. Potential physical and/or chemical changes in the laser-heated soot raises the question of how the LII signal depends upon these changes as well as the inferred soot volume fraction. Thus the authors investigated the effects of high energy pulsed laser light on the soot particles. The results caution use of laser-induced incandescence without careful consideration of excitation laser intensity and possible variation in soot composition at different measurement locations.

Carbon Nanostructure Examined by Lattice Fringe Analysis of High Resolution Transmission Electron Microscopy Images

The dimensions of graphitic layer planes directly affect the reactivity of soot towards oxidation and growth. Quantification of graphitic structure could be used to develop and test correlations between the soot nanostructure and its reactivity. Based upon transmission electron microscopy images, this paper provides a demonstration of the robustness of a fringe image analysis code for determining the level of graphitic structure within nanoscale carbon, i.e., soot. Results, in the form of histograms of graphitic layer plane lengths, are compared to their determination through Raman analysis.

XPS analysis of combustion aerosols for chemical composition, surface chemistry, and carbon chemical state.

Carbonaceous aerosols can vary in elemental content, surface chemistry, and carbon nano-structure. Each of these properties is related to the details of soot formation. Fuel source, combustion process (affecting formation and growth conditions), and postcombustion exhaust where oxidation occurs all contribute to the physical structure and surface chemistry of soot. Traditionally such physical and chemical parameters have been measured separately by various techniques. Presented here is the unified measurement of these characteristics using X-ray photoelectron spectroscopy (XPS). In the present study, XPS is applied to combustion soot collected from a diesel engine (running biodiesel and pump-grade fuels); jet engine; and institutional, plant, and residential oil-fired boilers. Elemental composition is mapped by a survey scan over a broad energy range. Surface chemistry and carbon nanostructure are quantified by deconvolution of high-resolution scans over the C1s region. This combination of parameters forms a distinct matrix of identifiers for the soots from these sources.

Soot nanostructure : Definition, quantification and implications

Although the relation between carbon structure and reactivity is well-known from thermal and oxidative studies of coal, char and graphite, the relation for soot remains unstudied. In this article, the dependence of the soot oxidation rate upon nanostructure, namely the length, separation distance and curvature of the graphene segments is shown. Soots possessing graphitic, fullerenic or amorphous nanostructure are used for this comparison. Surprisingly, the different soot nanostructures are readily produced by using different fuels and pyrolysis conditions. Reflecting different ratios of edge to basal plane sites or amounts of ring strain imposed by curvature, burnout rates are found to differ by nearly 500% for the soots studied here. Using high resolution transmission electron images as input data, this paper interprets the varied oxidation rates in terms of differences in nanostructure between the soots. In addition to fringe length, the fringe analysis algorithm can also provide fringe separation distance and tortuosity. Results are shown in the form of histograms for each of these quantities. The combination of the three measurements can give a better indication of the graphitic structure within nanoscale carbons and can distinguish carbon nanostructure based upon fullerenic, graphitic and amorphous content.

Synthesis Methods, Microscopy Characterization and Device Integration of Nanoscale Metal Oxide Semiconductors for Gas Sensing

A comparison is made between SnO2, ZnO, and TiO2 single-crystal nanowires and SnO2 polycrystalline nanofibers for gas sensing. Both nanostructures possess a one-dimensional morphology. Different synthesis methods are used to produce these materials: thermal evaporation-condensation (TEC), controlled oxidation, and electrospinning. Advantages and limitations of each technique are listed. Practical issues associated with harvesting, purification, and integration of these materials into sensing devices are detailed. For comparison to the nascent form, these sensing materials are surface coated with Pd and Pt nanoparticles. Gas sensing tests, with respect to H2, are conducted at ambient and elevated temperatures. Comparative normalized responses and time constants for the catalyst and noncatalyst systems provide a basis for identification of the superior metal-oxide nanostructure and catalyst combination. With temperature-dependent data, Arrhenius analyses are made to determine activation energies for the catalyst-assisted systems.

Impact of rail pressure and biodiesel fueling on the particulate morphology and soot nanostructures from a common-rail turbocharged direct injection diesel engine

An investigation of the impact of rail pressure and biodiesel fueling on exhaust particulate agglomerate morphology and primary particle (soot) nanostructure was conducted with a common-rail turbocharged direct injection diesel engine. The engine was operated at steady state on a dynamometer running at moderate speed with both low (30%) and medium–high (60%) fixed loads, and exhaust particulate was sampled for analysis. The fuels used were ultra-low sulfur diesel and its 20% v/v blends with soybean methyl ester biodiesel. Fuel injection occurred in a single event around top dead center at three different injection pressures. Exhaust particulate samples were characterized with transmission electronic microscopy imaging, scanning mobility particle sizing, thermogravimetric analysis, Raman spectroscopy, and X-ray diffraction analysis. Particulate morphology and oxidative reactivity were found to vary significantly with both rail pressure and biodiesel blend level. Higher biodiesel content led to an increase in the primary particle size and oxidative reactivity but had no impact on nanoscale disorder in the as-received samples. For particulates generated with higher injection pressures, the initial oxidative reactivity increased, but there was no detectable correlation with primary particle size or nanoscale disorder.

Chemical Sensors Based on Metal Oxide Nanostructures

This paper is an overview of sensor development based on metal oxide nanostructures. While nanostructures such as nanorods show significan t potential as enabling materials for chemical sensors, a number of s ignificant technical challenges remain. The major issues addressed in this work revolve around the ability to make workable sensors. This paper discusses efforts to address three technical barriers related t o the application of nanostructures into sensor systems: 1) Improving contact of the nanostructured materials with electrodes in a microse nsor structure; 2) Controling nanostructure crystallinity to allow co ntrol of the detection mechanism; and 3) Widening the range of gases that can be detected by using different nanostructured materials. It is concluded that while this work demonstrates useful tools for furt her development, these are just the beginning steps towards realizati on of repeatable, controlled sensor systems using oxide based nanostr uctures.

Application of laser-induced incandescence to the detection of carbon nanotubes and carbon nanofibers

Laser-induced incandescence applied to a heterogeneous, multielement reacting flow is characterized by temporally resolved emission spectra, time-resolved emission at selected detection wavelengths, and fluence dependence. Two-pulse laser measurements are used to further probe the effects of laser-induced changes on the optical signal. Laser fluences above 0.6 J/cm2 at 1064 nm initiate laser-induced vaporization, yielding a lower incandescence intensity, as found through fluence-dependence measurements. Spectrally derived temperatures show that values of excitation laser fluence greater than this value lead to superheated plasmas with temperatures well above the vaporization point of carbon. The temporal evolution of the emission signal at these fluences is consistent with plasma dissipation processes, not incandescence from solidlike structures. Two-pulse laser experiments reveal that other material changes are produced at fluences below the apparent vaporization threshold, leading to nanostructures with different optical and thermal properties.