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The aim of this communication is to show how a very simple stagnation plate positioned transversally to the of an air-ethylene premixed flame ([C]/[O]=1.38) does introduce significant changes in both flame temperature, soot 2D distribution and, moreover, does produces a dramatic increase of the soot particle residence time about a factor 6-8 with respect to the identical flame without the stagnation plate. This has crucial effect of the possibility to modulate the permanence time of the carbon particle in the high-reactive flow according to the design of the burner system.

A Monte Carlo model is here considered to account for phonon heat transfer in 1D nanostructures (nanowires and nanorods) based on the Boltzmann equation, Bose-Einstein statistics and Debye parameters. We focus attention on metal oxide nanostructures obtained in our NLSX lab via CVD experiments and demonstrate by the simulations that the heat conductivity is expected to be size-depending upon the dimension of the transverse size of these nanomaterials. The model is tested for silicon to compare with previous literature and, therafter, applied to zinc oxide 1D nanostructures, which were reported in previous experimental work [1]. The role of boundary scattering is discussed. Thermal conductivity ?T is found out to be lower by a factor 2-3, with respect to the bulk ZnO, depending on transverse size of the 1D nanopillar.

The Debye model for heat capacity of crystals is used to study the phonon heat transfer properties of quasi-1D structures such as nanowires and nanorods. According to the results reported in previous literature, we schematize a nanowire by a stack of 40 cells, total length 400 nm, with zdirection for heat flux coincident with the axis of nanowire and initial temperature 300 K. Transient temperature (time step 2 ps) field is studied after that at t=0 the wall of the first cell is set at 500 K. The nanostructure dimensions considered in the plane perpendicular to the z-axis are 2-50 nm. Calculations are performed by Monte Carlo simulation for the phonon drift movements and scattering process. The main result is the following: the smaller sizes Lx, Ly, of the simulation cell in the plane perpendicular to the heat flux (x-, y-axis) does influences the rate of temperature transient response in the neighbouring of hot boundary (~50 nm) at short times (200 ps). Moreover, smaller Lx, Ly , yield to predicted longer times (~10 ns) for settling of linear temperature regime and to smaller heat conductivity within 10% at parity of heat flux.

Several experimental studies performed by ex-situ, in-situ and on-line techniques suggest that the carbon units born in combustion systems the so-called primary particles (20-60 nm), which constitute the fractal soot aggregates (~0.1 micron), are actually constituted by clusters of smaller sub-units, termed sub-primary particles (typically in the size range 3 to 8 nm). These last undergo, at the very early stage of soot formation, an agglomeration process that occurs contemporarily to the addition on the surface of fresh condensed matter. The net result is that graphitic nuclei are detected at relatively low residence times. Thus, sub-primary particles are included in compact clusters with the incoming fresh matter acting as a glue to tight them and form the primary particles. The present contribution is aimed to demonstrate that a simple Brownian coagulation algorithm is able to simulate and predict the clustering process of sub-primaries with the formation of rather compact entities (that later on are observed as primary particles which agglomerate to fractal soot). Both the coagulation constant and the fractal dimension are found out in simulations to critically depend on the size of the smaller carbon bricks which are considered in the agglomeration process at typical combustion aerosol conditions. In particular, simulations predict the tendency of smaller particles (20 nm) aggregate more frequently in clusters with higher number of bricks (6-8). The net result is that the mean gyration radius is skewed towards lower and higher values in the case of smaller and larger monomers, respectively. This results seems to be coherent with the SAXS observation that sub-primaries with radius 3-4 nm undergo a fast agglomeration process to form larger units with the addition of condensable matter which constitute the primary particles of fractal soot aggregates.

The morphological analysis of flame generated carbon clusters is object of this work. Some methods to characterize this particulate on the basis of the fractal approach are briefly reported. Experiments are performed by sampling on the axis of ethylene-air diffusion flame. Analysis by Scanning Electron Microscope is worked out by application of the Nesting Square technique. Results show that the fractal dimension is not constant in correspondence of different residence times within the flame, in accord with previous works from the same authors performed by static light scattering techniques. Finally, a comparison is performed between the of aggregate morphology observed in flame experiments at temperatures about 1400 °C (Diffusion Limited Cluster-Cluster Aggregation) with respect to the shape of clusters generated by a slower agglomeration process at 70°C temperature (Reaction Limited Cluster-Cluster Aggregation).

Aerosol particles produced by combustion or thermal decomposition, referred to as smokes, are frequently composed of numerous individual primary particles forming an aggregate. Their characterization is usually restricted mainly by measuring a size distribution expressed in terms of the equivalent diameters. As a rule, the equivalent diameter measured by a particular tool reects some physical properties like diffusion coefficient, mobility in the electric field, sedimentation velocity, inertia deposition efficiency, etc. In some cases, the properties of such particles differ tremendously from those of rigid and isometric aerosol particles. To be able to predict particle mobility, the coagulation rate constant, and optical properties one should study the mechanism of particle morphology evolution. In this paper, the structure of aerosol particles formed by pyrolysis of hydrocarbons + metalorganics was investigated. The main results are obtained for the C3H8 + Fe(CO)5 system. The mechanism of particle formation which determines the particle morphology was shown to be completely different in the cases of Ar + C3H8, Ar+Fe(CO)5, and Ar+C3H8 + Fe(CO)5 pyrolysis

Flame generated soot nanoparticles, sized 10-50 nm and called primary particles, are widely recognized in the literature (Samson et al, 1987; Megaridis and Dobbins, 1990; Sorensen and Feke, 1996; di Stasio, 2001; Sorensen, 2001) to undergo agglomeration process in flame and to form at longer residence times chain-like or fractal type aggregated structures. Some recent experimental evidences (di Stasio, 2001; Hessler, et al, 2001) has put in some doubt the standard combustion scheme, which retained the combustion as the developing of the separate phases of nucleation, surface growth and agglomeration. In particular, evidence has been reported both from ex-situ TEM and in-situ X-ray scattering measurements, that the primary particles of the soot fractals agglomerates are, in turn, formed through a coagulation process of smaller nanoparticles (size below 8 nm) called sub-primary particles. The laser experiments at visible wavelengths are not adapted to allow the coming into existence of so fine structures. The aim of this communication is to demonstrate the feasibility of a synchrotron X-ray scattering technique to probe in-situ the early stage of growth of primary particles born in a diffusion flame.

Zinc oxide has attracted recently the attention of several investigators for the wide range of possible technological applications, such as solar cells, acoustic wave devices, photoluminescence phosphor for panel display. Several methods for the deposition of ZnO films (spray pyrolysis, laser ablation, arc discharge, sol-gel processes) have been proposed in the scientific literature of 1990s (see f.i. Studenikin et al., 1998). Synthesis of zinc monoxide nanobelts by an evaporation method has been also recently reported (Pan et al., 2001). In this communication we describe some results on nanocrystalline zinc/zinc oxide fibers fabricated by an aerosol route and tested for gas sensing of nitrogen dioxide in very low concentrations (sub-ppm). The extensive description of such a study is reported elsewhere (di Stasio et al., 2003).

Nitric Oxide (NO) and Nitrogen Dioxide (NO2) are particularly harmful gaseous pollutants in urban and industrial areas, owing to the large use of thermal engines and power plants. In the case of engine exhausts, in spite of the fact that NO2 is a minority gas pollutant species, it does yield the major damage to vegetation and living beings because of the fact that NO2 reacts in the troposphere and forms acid rains. The decomposition rate of the nitrogen oxides is relatively low, thus it is necessary to use noble metals (Rh, Pd, Co, etc.) to catalyze this process (DeNOx process). Nanostructured metallic Zn and ZnO could be retained as catalyst candidates to the reduction of NO2 in the future (Shimokawabe et al., 1997). This study reports some IR-Spectroscopy results about the surface chemistry of nanostructured Zn/ZnO fabricated by evaporation/condensation routes and preventively characterized by SEM/TEM and XRD.

Soot aerosol particles generated in hydrocarbon flames are investigated in this paper by laser light scattering techniques. The contribution of co-polarized and cross-polarized scattered light, each for vertical and horizontal linear polarization states of the incident laser light at 514.5 nm are measured at variable polar angles theta. Main results here reported are the following: (1) the vertical depolarization ratio rho_v measured against the scattering angle is very low and flat in the case of smaller chain-like (aggregate fractal dimension Df~1.3), whereas, in the case of larger branched-chain aggregates (Df~1.7), rho_v exhibits a maximum at about 90°. (2) in contrast to the reciprocity theorem, the measured ratio I_HV / I_VH between the two depolarized contributes for each of the two possible linear (Vertical and Horizontal) polarization states of the incident light, is found to be different than unity when first sticking of the primary particles occurs. Therefore, the measurement of the reciprocity ratio I_HV / I_VH seems a powerful and reliable tool to establish the onset of the early aggregation mechanism between nano-size soot particles formed in a flame.

The agglomeration kinetics of growing soot generated in a diffusion atmospheric flame are here studied in-situ by light scattering technique to infer cluster morphology and size (fractal dimension Df and radius of gyration Rg). SEM analysis is used as a standard reference to obtain primary particle size Dp at different residence times. The number Np of primary particles per aggregate and the number concentration n_A of clusters are evaluated on the basis of the measured angular patterns of the scattered light intensity. The major finding is that the kinetics of the coagulation process that yields to the formation of chain-like aggregates by soot primary particles (size 10 to 40 nm), can be described with a constant coagulation kernel beta_c,exp=2.37×10^-9 cm^3/s (coagulation constant 0.28 ms). This result is in nice accord with the Smoluchowski coagulation equation in the free molecular regime, and, vice versa, it is in contrast with previous studies conducted by invasive (ex-situ) techniques, which claimed the evidence in flames of coagulation rates much larger than the kinetic theory predictions. Thereafter, a number of numerical simulations is implemented to compare with the experimental results on primary particle growth rate and on the process of aggregate re-shaping that is observed by light scattering at later residence times. The restructuring process is conjectured to occur, for not well understood reasons, as a direct consequence of the atomic rearrangement in the solid phase carbon due to the prolonged residence time within the flame. Thus, on one side, it is shown that the numerical simulations of primary size history compare well with the values of primary size from SEM experiment with a growth rate constant of primary diameter about 1 nm/ms. On the other side, the evolution of aggregate morphology is found to be predictable by the numerical simulations when the onset of a first-order "thermal" restructuring mechanism is assumed to occur in the flame at about 20 ms residence time leading to aggregates with an asymptotic fractal dimension Df~2.5.