A. Eremin

Influence of the bath gas on the condensation of supersaturated iron atom vapour at room temperature

The influence of the kind of bath gas and its pressure on the iron nanoparticle formation and growth was investigated experimentally. Iron nanoparticles were synthesized from supersaturated iron vapour generated by ArF excimer laser pulse photolysis of gaseous Fe(CO)5 at room temperature. The particle size was determined by time-resolved laser-induced incandescence (TiRe-LII) as a function of time after photolysis at different experimental conditions. Additionally, final particles were sampled and analysed by transmission electron microscopy and by energy-dispersive x-ray analysis. The particle growth rate and the final particle size depended on the bath-gas composition and pressure. Increasing the argon bath-gas pressure accelerated the iron nanoparticle growth rate. In contrast to argon, no influence of helium on the particle growth rate was observed. The experimental results are compared with numerical simulations of particle surface growth, based on the model developed in previous investigations. The simulations indicate that the observed differences in the influence of the bath gas on the particle formation are caused by the species-dependent quenching probability of the active atom-particle complexes by the bath gas.

Spectral and structural properties of carbon nanoparticle forming in C3O2 and C2H2 pyrolysis behind shock waves

The diversity of carbon particles, forming durin pyrolysis of C 3 O 2 and C 2 H 2 behind shock waves in thewide temperature range 1200–3800 K, was investigated. The process of condensed carbon particle formation was observed in situ by the multichannel registration of the time profiles of optical properties of media in the UV, visible, and near-IR ranges. Besides that, the probes of postshock materials, deposited on the walls of the shock tube, were analyzed bylow- and high-resolution transition electron microscopy (TEM) and by electron microdiffraction (MDF) measurements. The comparison of extinction properties of young growting particles with the electron microscopic analysis of solidified substance gave a notion about the peculiarities of carbon particle formation process from the diffeerent carbon-bearing gases at various temperatures. particles, forming from both substances at 1500–200 K, look similar to usual soot, and the absence of hydrogen in C 3 O 2 leads to faster formation and graphitization of particles. At the tempratures 2100–2600 K, the decrease of the paticle formation rate and the fall of final particle yield in all mixtures is observed. After C 3 O 2 pyrolysis experiments, gigantic film-like spheres with the size up to 700 nm were observed on the walls. The peculiarity of the high-temperature (2700–3200 K) process of carbon particle formation in C 3 O 2 pyrolysis is the high degree of crystallization of the final particles.

Shock wave induced carbon particle formation from CCL4 and C3O2 observed by laser extinction and by laser-induced incandescence (LII)

Abstract The formation of carbonaceous particles from the hydrogen-free precursors CCl 4 and C 3 O 2 , both diluted in argon was studied behind reflected shock waves in the temperature range 1400 K ≤ T ≤3700 K and at pressures 1.3 bar ≤ p ≤ 4.5 bar. The appearance of particles was measured by laser light extinction (LLE) and by laser induced incandescence (LII). Also, some time and spectrally resolved emission measurements were performed. The LLE experiments are sensitive to the optical density of the post-shock gas-particle mixture and show a time-dependent increase, depending on the detailed reaction conditions. The evaluation of the experiments at a reaction time of t = 1 ms results in a double, bell-shaped temperature dependency of the optical density. The LII-experiments, which are sensitive to the particle size, provide particle growth curves determined from several “identical” shock tube experiments with delayed triggering of the LII heat-up laser. Particle sizing experiments at a reaction time of t = 1 ms after shock-induced heat-up of the initial gas mixtures also clearly yield a double, bell-shaped temperature dependency of the particle diameter and confirm the optical density experiments. The shock tube was also equipped with a molecular beam system allowing supersonic beam probing from the shock-heated gases. Particles were collected on TEM grids and visualized by HR-TEM. The sizes of these images more or less confirm the LII sizing.

Carbon particle formation and decay in two-step pyrolysis of carbon suboxide behind shock waves

The formation and decay of carbon particles following the pyrolysis of C3O2 was investigated behind reflected shock waves toward high temperatures. It is known that in hydrocarbon pyrolysis the temperature range for soot formation extends from about 1300 to 2200 K. That holds also for C3O2. Here, it could be observed that particle formation starts again above 2300 K and increases toward a maximum at around 3000 K, falling off steeply above 3450 K. This maximum is nearly as high as that of the low-temperature particle yield curve at around 1600–1700 K. At temperatures above 3450 K, the process of particle disappearance behind the reflected shock wave was observed, which seems to depend on the history of their formation behind the incident shock wave.

To the temperature dependence of carbon particle formation in shock wave pyrolysis processes

Abstract In this work the results of numerous experiments on carbon particle formation in combustion and pyrolysis of various carbon bearing molecules behind shock waves in the wide temperature range from 1200K to 3500K are analyzed. It is shown, that the discrepancy in the temperatures of the maximum particle yield could be attributed to the differences in the endothermicy of the pyrolysis of various molecules and the maximum optical density at 633nm in all mixtures can be related to the same temperture T=1600K. Based on this consideration, several statements were formulated. First – particle growth in all mixtures can be described by the uniform dependence of optical density D (at 633 nm) on time D~aτ0.4 indicating, that particle formation proceeds via homogeneous condensation. The second – decrease of the optical density at 633nm with the temperature rise is caused not by the decrease of particle yield, but the decrease of their size resulting in the fall of extinction at the given wavelength. Third – the reason of the fall of the final particle size with the temperature rise is the acceleration of the initial cluster formation process and a corresponding increase of the particle number density. And the last statement – the secondary particle growth, observed at T>2200K is completely determined by the primary clusters (nucleus) formed behind the incident wave and the coagulation of small carbon particles formed behind the reflected shock wave using these clusters.

Kinetics of Carbon Cluster Formation in the Course of C3O2Pyrolysis

The kinetics of the formation of condensed carbon particles in the thermal decomposition of C3O2molecules behind shock waves was experimentally studied at temperatures of 1200–2500 K, pressures of ∼20–60 atm, and molar fractions of C3O2in a mixture with argon in the range 0.03–2.00%. The concentration of condensed carbon particles was measured by the absorption of laser radiation at wavelengths of 632.8 and 1064 nm. The experimental results were compared with data calculated in the framework of three different models. Two of these models (analytical) include the kinetics of the formation of the particle-size distribution function and give a simplified description of the kinetics of gas-phase reactions involving C3O2and decomposition fragments. The third model (numerical) combines detailed descriptions of the kinetics and the coagulation dynamics. Several types of condensed carbon particles were considered: carbon clusters, soot particles, and fullerenes. The transitions between various forms of condensed carbon particles were included into the kinetic scheme of the model. All main observed specific features of the growth kinetics of condensed carbon particles during C3O2pyrolysis can be described in terms of these models.

Formation of Nanoparticles by Photolysis from Metal and Carbon Bearing Molecules

Abstract The formation of particles following the photolysis of C3O2, Fe(CO)5 and Mo(CO)6, diluted with Ar or He was registered at room temperature. Particle growth was followed by taking light extinction profiles at 633nm and at 220nm for various mixture compositions and pressures. The particles obtained at different conditions were analyzed using transition electron microscope (TEM) technique. It was found that in the pure undiluted gases at the partial pressures shown in the pictures no light absorption and no particle formation could be observed. Light absorption started for partial pressures of the diluent gas > 10mbar. A comparison of particle size measured here at room temperature with data obtained at elevated temperature shows that the data obtained here fit well to the elevated temperature data.

Formation of carbon nanoparticles by the condensation of supersaturated atomic vapor obtained by the laser photolysis of C3O2

A new technique is suggested for obtaining nanoparticles from highly supersaturated vapor resulting from the laser photolysis of volatile compounds. The growth of carbon nanoparticles resulting from C3O2 photolysis has been studied in detail. Absorbing UV quanta (from an Ar-F excimer laser), C3O2 molecules decompose to yield atomic carbon vapor with precisely known and readily controllable parameters. This is followed by the condensation of supersaturated carbon vapor and the formation of carbon nanoparticles. These processes have been investigated by the laser extinction and laser-induced incandescence (LII) methods in wide ranges of experimental conditions (carbon vapor concentration, nature of the diluent gas, and gas pressure). The current and ultimate particle sizes and the kinetic parameters of particle growth have been determined. The characteristic time of particle growth ranges between 20 and 1000 μs, depending on photolysis conditions. The ultimate particle size determined by electron microscopy is 5–12 nm for all experimental conditions. It increases with increasing total gas pressure and carbon vapor partial pressure and depends on the diluent gas. The translational energy accommodation coefficients for the Ar, He, CO, and C3O2 molecules interacting with the carbon particle surface have been determined by comparing the LII and electron microscopic particle sizes. A simple model has been constructed to describe the condensation of carbon nanoparticles from supersaturated atomic vapor. According to this model, the main process in nanoparticle formation is surface growth through the addition of separate atoms to the nucleation cluster. The nucleus concentrations for various condensation parameters have been determined by comparing experimental and calculated data.

Formation of carbon nanoparticle in carbon suboxide pyrolysis behind shock waves

The process of carbon nanoparticles formation following the pyrolysis of carbon suboxide C3O2 in the wide temperature range 1700-3700 K behind shock waves was investigated by measuring the extinction and emission profiles in the visible and IR ranges of spectra. The temperature dependence of the optical density shows two bell-shaped curves with the maxima at 1600 K and 3200 K. It is remarkable, that the ratio of extinction in the IR range (1.31 µ) to HeNe-laser (633 nm) as well as the ratio of the IR extinction to emissivity show the same temperature behavior. The possible correlation between the observed optical properties of the forming carbon particles and the change of their size and structure is discussed.

Formation of a detonation wave in the thermal decomposition of acetylene

The formation of a condensation detonation wave has been experimentally observed in the shock-induced thermal decomposition of acetylene. The stable detonation wave in the 20% C2H2 + 80% Ar mixture has been obtained at an initial pressure behind the shock wave of no less than 30 atm. The main kinetic characteristics of the pyrolysis of acetylene—the period of the induction of condensation and the growth rate constant of condensed particles—have been determined. The correlation of various stages of the process with the heat release in the condensation has been analyzed. It has been shown that the period of the particle growth induction is not accompanied by noticeable heat release. The subsequent condensation stages characterized by significant heat release occur very rapidly (faster than 10−5 s) in the so-called explosive condensation. The analysis of the results indicates that the reactions leading to the growth of large polyhydrocarbon molecules, which precede the formation of condensed carbon particles, constitute the limiting stage of the process, which determines the possibility of the formation of the condensation detonation wave in acetylene. An increase in the pressure is accompanied by the sharp narrowing of the induction region and the transition of the process to the condensation detonation wave.