A. Emelianov

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.

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.

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 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.

Influence of quantum effects on the initiation of ignition and detonation

A theoretical analysis that allows one to quantify the quantum corrections to the rate constants of endothermic reactions associated with an increase in the high-energy tails of the momentum distribution functions at high pressures due to a manifestation of the uncertainty principle for the energy of the colliding particles at a high collision frequency is performed. The initiation of ignition of hydrogen-oxygen mixtures is investigated and special series of experiments on the initiation of detonation waves of condensation in carbon suboxide and acetylene at elevated pressures near the low-temperature limits have been carried out. The experimentally observed deviations in the Arrhenius dependences of the induction periods of the initiation of hydrogen ignition and detonation waves of condensation are shown to be well described by the proposed quantum corrections.

Comparison of Properties of Carbon Particles Formed by Pyrolysis of C3O2 and C2H2 behind Shock Waves

Various carbon particles formed by the pyrolysis of C3O2 and C2H2 behind shock waves in the temperature range 1200–3800 K are studied. The formation of the condensed carbon particles is observed directly by the multichannel detection of the time profiles of the extinction of the medium in the UV, visible, and near-IR spectral regions. The samples of carbon material deposited on the walls of a shock tube after an experiment are analyzed using transmission electron microscopy with different resolutions and electron microdiffraction. Particles formed from C3O2 and C2H2 at 1500–2000 K are 10–30 nm in size and look like usual soot. The absence of molecular hydrogen in C3O2 only results in faster formation and graphitization. At 2100–2600 K, the formation of particles is retarded, and the yield of the carbon particles decreases for both substances. After experiments on pyrolysis of C3O2 at these temperatures, giant spherical particles up to 700 nm in size are found on the walls of the shock tube. Carbon particles formed at the highest temperatures (2700–3200 K) in C3O2 pyrolysis have the high degree of crystallinity of particles.

Quantum Effects in the Kinetics of the Initiation of Detonation Condensation Waves

The features of the kinetics of the initiation of detonation condensation waves in carbon suboxide and acetylene have been experimentally studied at high pressures near the low-temperature limits. The role of quantum effects in the expansion of detonation limits has been analyzed. Quantum corrections to the endothermic reaction rates, which are caused by an increase in the high-energy tails of the momentum distribution functions at high pressures due to the manifestation of the uncertainty principle for the energy of colliding particles at a high collision frequency, have been quantitatively estimated. It has been shown that experimentally observed deviations in Arrhenius dependences of the induction periods of the initiation of detonation condensation waves are well described by the proposed quantum corrections.

Nonequilibrium Excitation of C2 Radicals during the Thermal Decomposition of C3 O2 behind Shock Waves

C2 concentrations and populations of the ground and the first excited vibrational levels of electronically excited C2(d3Π) radicals during the thermal decomposition of C3O2 behind shock waves were monitored by emission and absorption measurements in the (0-0) and the (1-0) Swan-bands. Experiments were carried out in mixtures of 1% C3O2+Ar and 1% C3O2+He at T=2100–3200 K and p=1.5–7.5 bar. In the whole range of conditions studied an essential nonequlilibrium of electronic and vibrational states of nascent C2 radicals was observed. For analyzing possible formation and consumption reactions of the excited C2 radicals, a convenient kinetic scheme of the C3O2 decomposition was developed, which considers both the recombinative pumping process and exchange and quenching reactions of the C2(d3Π, v=0, 1) states.