A. M. Baklanov

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.

Homogeneous Nucleation from a Supersaturated Sulfur Vapor in a Laminar Flow Diffusion Chamber

ISSN 00125016, Doklady Physical Chemis try, 2011, Vol. 437, Part 1, pp. 31–34.

Study of morphology of aerosol aggregates formed during co-pyrolysis of C3H8 + Fe(CO)5

Formation of aerosol nanoparticles as well as carbon nanotubes and nanofilaments is studied during co-pyrolysis of iron pentacarbonyl and propane with argon as a carrier gas in a flow reactor. Gaseous intermediates from propane thermal decomposition (CH4, C2H6 and C3H4) and Fe(CO)5 conversion are monitored by gas chromatography and IR-spectroscopy, respectively. The aerosol morphology is studied by transmission electron microscopy (TEM) and high resolution TEM. The aerosol particle concentration and size distribution are measured by an automated diffusion battery. The crystal phase composition of particles is studied by x-ray diffractometry. The decomposition of the Fe(CO)5 + Ar mixture resulted in an iron aggregate formation composed of fine primary particles. In the case of lower pyrolysis temperatures, about 450 K, the primary particle mean diameter is about 10 nm, and consequently, the majority of the primary particles are superparamagnetic, thus forming compact aggregates. At intermediate pyrolysis temperatures in the range 800–1040 K the primary particle diameter is about 20–30 nm, and most of the particles are ferromagnetic in nature. The coagulation of these particles results in a chain-like aggregate formation. Finally, at temperatures higher than the Curie point (1043 K) the ferromagnetic properties vanish and the formation of compact aggregates is observed again. The co-pyrolysis of Fe(CO)5 and C3H8 mixed with Ar carrier gas resulted in aerosol aggregate structures dramatically different from those formed by iron pentacarbonyl pyrolysis. In particular, in the temperature range 1070–1280 K, we observed Fe3C particles connected by long carbon nanotubes (CNTs). The aggregate morphology is described in terms of a fractal-like dimension Df, which is determined from TEM images on the basis of a scaling power law linking the aggregate mass (M) and radius (R), . The Fe3C–CNT aggregate morphology is a function of the inlet ratio between propane and iron pentacarbonyl concentrations [C3H8]0/[Fe(CO)5]0. At the low ratio of [C3H8]0/[Fe(CO)5]0 80 the fractal aggregate dimension is higher for a larger ratio of [C3H8]0/[Fe(CO)5]0, which is explained by the larger thickness of growing nanotubes obtained for a relatively large propane concentration. The aggregate formation mechanism includes consecutive stages of iron aggregate formation due to Fe(CO)5 decomposition, carbon deposition on iron particles from C3H8 pyrolysis intermediates, carbon dissolution in iron particles, nanotube nucleation at the carbon concentration of about 60 at.% in Fe–C solution and disruption of the Fe–C aggregates into small pieces by the growing nanotubes.

Study of Sulfur Heterogeneous Nucleation from Supersaturated Vapor on Tungsten Oxide and Sodium Chloride Seed Particles. Determination of Contact Angle of Critical Sulfur Nuclei

A procedure has been developed for determining the contact angle of a critical nucleus formed on seed particles during the heterogeneous nucleation of a vapor in a flow chamber. The procedure comprises the determination of the fraction of enlarged particles, as well as the selective separation of nanoparticles over sizes to locate the zone of intense nucleation. The concentration and size distribution of aerosol particles have been measured with a diffusion spectrometer of aerosols. Vapor concentration distributions and supersaturation fields have been determined by solving the mass-transfer problem. The calculated supersaturation fields are in good agreement with the location of the intense nucleation zone experimentally found with the help of selective separation. The fractions of enlarged particles have been determined as functions of supersaturation in the chamber. A formula has been derived for calculating the fraction and size distribution function of enlarged particles at known supersaturation and temperature fields and a preset contact angle. The contact angles are selected in a manner such that the calculated fraction of enlarged particles coincides with that measured experimentally. It has been revealed that the contact angle of critical sulfur nuclei formed on tungsten oxide seed particles with average radii 〈Rp〉 ≈ 5.8−4.4 nm is in a range of 21.2−20.5°, while, in the case of sodium chloride seed particles with 〈Rp〉 ≈ 6.0−4.4 nm, the contact angle is 20.4−17.4°. The size of a critical nucleus has been found to be proportional to calculated average radius of a seed particle 〈Rp〉 in both cases.

A study of homogeneous nucleation of ibuprofen in a flow chamber. Determination of the surface tension of critical nuclei

Homogeneous nucleation of ibuprofen vapor is studied in a nucleation flow chamber, a horizontal quartz tube equipped with an external heater. The area of the chamber where the nucleation proceeds most efficiently is determined, and the volume of this area is estimated. The temperature and supersaturation are determined and the homogeneous nucleation rate is calculated for this area. Saturation vapor pressure over liquid ibuprofen is measured in a temperature range of 353–383 K. Using an exact formula that has recently been derived for the nucleation rate based on the works by Kusaka, Reiss, as well as the Frenkel liquid-kinetics theory, surface tension and the radius of surface of tension of a critical nucleus σ= 25.9 mN/m and Rs = 1.6 nm, respectively, are calculated at 318 K. The measurement of the surface tension of an ibuprofen planar surface shows that, at 318 K, σ∞ = 24.38 mN/m; i.e., σ is higher than σ∞ by 6%. A critical nucleus is established as containing nearly 36 ibuprofen molecules.

A study of nucleation in supersaturated ibuprofen vapor in a flow diffusion chamber

Isothermal nucleation of supersaturated ibuprofen racemate vapor has been experimentally studied in a flow diffusion chamber at 293.3 and 301.2 K. Nucleation rates have been measured in the range of 104−104 cm−3 s−1 as functions of supersaturation. According to the first nucleation theorem, the numbers of molecules in critical nuclei have been found and used to determine the nucleation rate and supersaturation values as depending on the sizes of critical nuclei. The comparison of the experimental data with theoretical predictions has shown that the nucleation rates measured as functions of the numbers of molecules in critical nuclei are higher than the rates predicted by the classical theory by six to seven decimal orders of magnitude but, within one order of magnitude, coincide with the rates predicted by a theory previously proposed in a work by one of the authors, in which nucleation clusters were considered to be microscopic objects.

A method of determination of critical nucleus parameters in heterogeneous nucleation of supersaturated vapor in a continuous-flow chamber

Heterogeneous nucleation often plays a key role inatmospheric and technological processes. For example,knowledge of the parameters that determine the efficiency of heterogeneous nucleation is extremely important in design of robust equipment for nanoaerosolanalysis. Heterogeneous nucleation depends on theproperties of the aerosol particle surface where it occurs[1, 2]. If a particle is imperfectly wetted with the condensed substance, the heterogeneous nucleation occursthrough the formation, on the particle surface, of a lensshaped critical nucleus of radius

An experimental method for studying the heterogeneous nucleation in a laminar flow chamber

We developed a method for studying heterogeneous nucleation in a laminar flow chamber, which allows one to determine the relationship between the main parameters of the process, i.e., the critical size of a seed particle, supersaturation, and the temperature. The workability of this method is demonstrated for the heterogeneous nucleation of sulfur vapor on nanoparticles of tungsten oxide.

Size and morphology of the nanooxide aerosol generated by combustion of an aluminum droplet

The increasing recent interest in characteristics ofsubmicron oxide smoke generated by combustion ofaluminum droplets stems from environmental problemsassociated with application and utilization of alumi-nized propellant rocket motors and from the develop-ment of technologies of production of metal nanoox-ides in aerodisperse flames [1, 2]. Oxide particles rang-ing from nanometer to submicrometer size form andgrow in the flame zone surrounding a burning particle,this process being the starting point for their furtherevolution. Rational design of particle combustion pro-cesses in technical devices is based on an understand-ing of physicochemical processes that occur duringcombustion of a particle and on knowledge of how theircharacteristics depend on combustion conditions. Oneof the challenges in studying the aluminum particlecombustion mechanism is to study the formation andproperties of oxide nanoparticles. In this work, we stud-ied how the size distribution and morphology of theoxide aerosol formed by combustion of aluminum par-ticles in atmospheric air depends on the size of a burn-ing droplet. We intend to extend the pressure range inthe future.Burning droplets were produced by combustion of asmall sample (from 2 × 2 × 20 to 2 × 4 × 40 mm in size)of aluminized solid propellant, which was burnt in a20-L container in air filtered from aerosols. Duringcombustion, the sample expelled burning aluminumdroplets (agglomerates), which fell freely under grav-ity. The combustion lasted a few seconds. During thistime, the container was filled with “oxide smoke,” a fineaerosol. After completion of combustion, aerosol parti-cles coagulated, sedimented, and were partially depos-ited on the walls of the container. To study the evolutionof particles after burning, the resulting aerosol wasperiodically sampled. The aerosol sample was fedeither into a thermophoretic precipitator, to collect par-ticles for subsequent dispersion and morphologicalanalysis based on their electron microscopic images, orinto a Millikan cell, to observe aerosol particles andrecord their motion by a video camera with a micro-scope lens and a laser illuminator. The cell constructionimplies that a homogeneous electric field can beapplied. This, in combination with video recording ofparticle motion, makes it possible to determine the elec-tric charge of the particles. The sampling, video record-ing, and image processing techniques were describedelsewhere [1, 3].We carried out four series of experiments with pro-pellants differing in the size of generated burning drop-lets.In series 1, a model solid propellant containing25 wt % ammonium perchlorate (AP), 35 wt %cyclotetramethylenetetranitramine (HMX), 20 wt %binder, and 20 wt % aluminum was used. Inasmuch asaluminum droplets are agglomerated as the propellantburns, the distribution function of burning droplets gen-erated by the propellant was estimated in the followingmanner. The size distribution of agglomerates, as wellas the fraction of the metal involved in agglomeration(≈0.6), was determined using the technique in [4]. If weassume that the metal not involved in agglomerationpasses into the gas phase in the same form as it has inthe propellant, the set of particles generated by combus-tion consists of agglomerates and particles of initialaluminum. The distribution function of initial alumi-num was preliminarily determined on a Malvern 3600Eparticle size analyzer. Table 1 summarizes the charac-teristics of the set of polydisperse particles calculatedtaking into account the weight fractions. Hereinafter,D