V.N. Panfilov

Evolution of structure and charge of soot aggregates during and after formation in a propane/air diffusion flame

Abstract Evolution of soot aggregate morphology, size and concentration is investigated during and after formation of soot in propane/air diffusion flame. Monitoring of gaseous intermediates in the flame is done by gas chromatography. Soot aggregate size and morphology are analyzed by a transmission electron microscope; soot number concentration is determined by an automated diffusion battery. Aggregate–aggregate collisions and aggregate structural transformations are observed in real time using a video system. It is determined that soot aggregates formed in flame are charged. The electric charge per aggregate is determined by video observation of aggregate movement in electric field. Both positively and negatively charged aggregates are formed. Typical net charge per aggregate is a few elementary units. An effect of soot aggregate restructuring from chain-like to compact structures is observed. It is determined that the driving force for this restructuring is Coulomb interactions between different parts of the aggregate. It is demonstrated that Coulomb interactions between aggregates can affect considerably coagulation process and the final aggregate shape.

AEROSOL FORMATION UNDER HETEROGENEOUS/HOMOGENEOUS THERMAL DECOMPOSITION OF SILANE: EXPERIMENT AND NUMERICAL MODELING

Abstract Experimental and numerical study of aerosol formation under heterogeneous/homogeneous silane thermal decomposition is carried out. Experimental exploration included monitoring of the following parameters during silane decomposition: silane conversion degree and concentrations of disilane and trisilane; aerosol concentration; size and morphology of aerosol particles; number of monohydride SiH and polyhydride (SiH 2 ) n groups in aerosol particles; number of dangling bond active centers in aerosol particles. To explain the experimental results, a numerical model was developed. It covered homogeneous reactions (involving 11 gaseous species), heterogeneous reactions of silicon wall deposition from gaseous species, aerosol formation and aerosol particle wall deposition. The modeling results are in reasonable agreement with the experimental data.

On the pathways of aerosol formation by thermal decomposition of silane

The mechanism of aerosol formation during thermal decomposition of silane is investigated. To this end a simultaneous analysis of gas-phase products of silane decomposition (disilane, trisilane, hydrogen) and the parameters of the forming aerosol particles of amorphous hydrogenated silicon is carried out. The silane loss and gaseous product concentrations are analyzed by mass-spectrometer; particle size and morphology are analyzed by transmission electron microscope. The total amount of bonded hydrogen and the relative amounts of monohydride and polyhydride groups contained in the particles are analyzed by the methods of hydrogen evolution and IR-spectroscopy. It is concluded that during the initial stages of aerosol formation, particles are mainly formed from gaseous products with a stoichiometry Si n H 2n . At these stages the hydrogen in particles is mainly contained as a constituent of polyhydride groups. During later stages the particles are formed from hydrogen-depleted intermediates, and the hydrogen in particles is mainly bound in monohydride groups.

STUDYING OF SILANE THERMAL DECOMPOSITION MECHANISM

The mechanism of silane thermal decomposition is investigated in a flow reactor. The time dependencies of silane consumption and disilane formation were compared with those parameters of solid product (aerosol particles) such as concentration, total hydrogen content in solid product, and fraction of hydrogen contained in solid product as polyhydride groups (SiH2)n. Silane loss and gaseous product formation were analyzed using a mass spectrometer. The hydrogen content in solid product was analyzed by the methods of IR-spectroscopy and hydrogen evolution. Based on a simple kinetic scheme we qualitatively explained the experimental dependencies of silane conversion and disilane formation, the effective activation energy of the decomposition process, and the amount of polyhydride groups in the solid product on reaction time and initial silane concentration.

Chemical composition and bond structure of aerosol particles of amorphous hydrogenated silicon forming from thermal decomposition of silane

Abstract Aerosol particles of amorphous hydrogenated silicon resulting from thermal decomposition of silane were investigated by hydrogen evolution, IR-, EPR-, NMR spectroscopy, and transmission electron microscopy. The experimental data show that aerosol particles contain to a various extent {SiH2}n polymer structures and two types of monohydride groups SiH- “clustered” and “dilute” monohydride groups. The hydrogen atoms of the “clustered” monohydride groups are located close to each other. The “clustered” monohydride groups are inaccessible to the ambient because they are embedded in the amorphous network. The “dilute” monohydride groups are relatively isolated from each other. The majority of “dilute” monohydride groups are open to the ambient. They are located on the surface of preferentially interconnected microchannels and microvoids. Interaction between the “dilute” SiH groups and atmospheric oxygen results in formation of OSiH groups in which hydrogen and oxygen are bonded to a common silicon atom. Evidently, the interaction occurs throw the oxygen reaction with weak bonds associated with “dilute” monohydride groups. There is no interaction between oxygen and both “clustered” SiH groups and {SiH2}n chain because the former are inaccessible to atmospheric oxygen and the latter has presumably no weak bonds in the chains.

AGGREGATE FORMATION UNDER HOMOGENEOUS SILANE THERMAL DECOMPOSITION

A complete model of aerosol particle formation by thermal decomposition of silane is presented, which includes all steps from aerosol precursor formation in the homogeneous reactions to particle coagulation. The model predicts silane conversion, concentrations of gaseous intermediates (disilane, trisilane, and others), size and concentration of aerosol particles, chemical composition of aerosol particles (number of SiH and SiH2 groups in the particles). Additionally, we report measurements of aggregate fractal dimensions on the basis of electron microscopy micrographs. These data are required to link the chemical kinetic steps of the model with predictions about the particle size. Calculated time dependencies of gaseous concentrations, particle size and concentration, particle chemical composition are in reasonable agreement with the experimental data obtained in the present work as well as in our earlier papers (Onischuk et al., 1997a,b, 1998a). The numerical simulations showed that the relative contribution of vapor and clusters (containing not more than 10 silicon atoms) to the rate of particle growth is in the range of 20–30%. The contribution from the particles containing more than 106 silicon atoms is in the range 70–80%.

Formation of electrical dipoles during agglomeration of uncharged particles of hydrogenated silicon

Abstract A new phenomenon of the formation of agglomerates having electric dipoles by agglomeration of originally uncharged aerosol particles is reported. The aerosol particles of hydrogenated silicon are formed via silane pyrolysis in a flow reactor. The size and morphology of agglomerates were analyzed by a transmission electron microscope. The agglomerate radius increased from 0.3 to 6 μm with the coagulation time increasing from 0.1 to 200 s (for T=873 K ). The mass M of these agglomerates is connected with the radius R via the equation M ( g )=8.5×10 −14 ×[ R (μm)] 1.65 . An imaging system coupled with a TV setup was used for direct observation of agglomerate coagulation and investigation of agglomerate movement in the electric field. The experimental results testify that the agglomerates are electric dipoles. It is suggested that this dipole moment arises due to the difference in Fermi energy of primary particles. The dipole moment is ∼3.5×10 −12 (units of CGSE) for the agglomerates with the radius R∼0.5 μm (T=873 K ) . This dipole moment corresponds to the Fermi energy difference for coagulating particles ∼0.08 V .