Magnetic phase transitions in nanostructures induced by shear stress under high pressure

Magnetic phase transitions of the first and second order were revealed by Mössbauer spectroscopy in nanosystems of - and -ferric oxides and metallic europium subjected to shear stress (240°) under high pressure (20 kbar). For - and -ferric oxide nanoclusters, the Curie (Neel) points decreased to 300 K, whereas for nanostructured europium the Neel point increased from 90 to 100 K. The thermodynamic model of magnetic phase transitions predicting a change in the character of magnetic phase transitions and a decrease (increase) in the critical Neel (Curie) points in nanoclusters was developed. The type of magnetic phase transitions and the change in the critical points were caused by defects in nanoclusters, whose maximum concentration was observed for the clusters with the 20—50 nm size range.     

Mechanism of Hexafluoropropylene Formation in Gas-Phase Pyrolysis of Tetrafluoroethylene

The consecutive formation of short-lived intermediates during tetrafluoroethylene conversion into hexafluoropropylene by gas-phase pyrolysis under conditions of pulsed adiabatic compression was studied by kinetic spectroscopy. The process mechanism was determined in detail. The ways for improving the pyrolysis procedure were considered.     

New method for determination of heats of thermal gas-phase reactions yielding intermediates

A new procedure for the determination of the heats of thermal gas-phase reversible reactions leading to intermediates in the framework of the adiabatic compression method is proposed. A model problem is solved, and the error of the method is evaluated (-1 %). The validity of the method was confirmed using the reversible formation of the I,2-biradical from tetrafluoroethylene as an example. Translated from Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 64–67, January, 1996.     

Spectral features of 1,2-biradicals isomeric to C2–C4 perfluoroolefins

Absorption spectra of 1,2-biradicals were obtained by kinetic spectroscopy under adiabatic compression of mixtures of perfluoroolefins with argon.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2048–2050, October, 1995.     

Kinetics and mechanism of the stereospecific dimerization of trifluoromethylfluorocarbene

Experimental data on the kinetics of the formation of the singlet CFCF₃ carbene and its stereospecific dimerization into cis-perfluorobutene-2 during the homogeneous pyrolysis of CF₃CF₂SiF₃ are obtained. Quantum-chemical interpretation of the mechanism and the mathematical modeling of the kinetics of these processes explain why practically only cis-perfluorobutene-2 is formed.     

Effect of the Calorific Intensity of Combustion Chamber on Production of Synthesis Gas in Partial Oxidation of Methane–Oxygen Mixtures in the Combustion Mode

Optimal design of a flow-through chemical reactor with increased calorific intensity was experimentally sought for in partial oxidation of natural gas by oxygen at oxidant excess factors in the range 0.27 < α < 0.4. It was shown that this reactor with a chamber for additional turbulent mixing of the starting components, turbulizer, and supercritical pressure difference at the outlet from the combustion chamber can provide a combustion mode close to the process in the plug-flow reactor. The increase in the calorific intensity of the combustion chamber of the reactor as a result of a decrease in its volume leads to full conversion of the starting reagents and to lower carbon-black formation.     

Magnetic Phase Transitions in Nanosystems: Role of Size Effects,Intercluster Interactions and Defects

Shear stress under high pressure loaded nanoclusters of α, γ-ferric oxides and metallic europium showed first- and second-order magnetic phase transitions. For nanoclusters of α, γ-ferric oxides and metallic europium 57Fe and 151Eu, Mossbauer spectroscopy revealed a considerable change in the Curie (Neel) points. The thermodynamic model predicts first-to-second-order (Eu) and second-to-first-order (Fe oxides) changes in the character of magnetic phase transitions and a decrease or increase of critical Curie (Neel) points. The model defines the critical cluster size and concentration of defects in nanosystems responsible for the change in magnetic properties. For nanoclusters of Fe oxides and metallic Eu, the critical (and maximum) concentration of defects corresponds to a cluster size 20–50nm.     

Renner-teller effect at the initial acetylene thermal transformation stages

It is shown that the vibrational excitation of the acetylene molecule is accompanied by the transformation of its linear structure into a transoid structure because of the Renner-Teller effect. The electron density distribution in transoid acetylene was calculated by nonempirical quantum-chemical methods. It was shown that this structure corresponded to a 1,2-biradicaloid. The intense light absorption in the acetylene spectrum at λ _max = 240 nm and temperature above 800 K was found to be related to the S₂ ? S₀ electron transition. Transoid acetylene 1,2-biradicaloids were shown to participate in the dimerization reaction with the formation of trans-1,4 biradicals. The enthalpies and energy barriers were determined for some elementary stages of the primary thermal transformations of acetylene.