Nonlinear resistance of polymer composites with carbon nanotube additives in the percolation state

The electrical properties of a polymer composite with carbon nanotube additives have been analyzed. The state of the system near the percolation threshold, when charge is transferred along a single percolation path, has been considered. For this state, the current–voltage characteristics of a percolation chain made up of carbon nanotubes have been calculated under the assumption that the contact resistance between neighboring nanotubes is much higher than the intrinsic resistance of the nanotubes. According to recent data, the distance between neighboring (contacting) nanotubes has been assumed to be randomly distributed. It has been shown that, under the given conditions, the current–voltage characteristic is essentially nonlinear. This indicates the nonohmic conductivity of the composites. The dependence of the current–voltage characteristic on the spread of the contact distribution over distances has been discussed.     

The repetitive microwave discharge as a catalyst for a chemical reaction

Results are presented from studies on plasma catalysis of the decomposition of methane into hydrogen and carbon in a repetitive microwave discharge. The dependence of the propagation velocity of a plasma channel on the sort of gas is determined; from this dependence, a preliminary conclusion can be drawn about both the mechanism for the development of the discharge and the ion composition of the discharge plasma. The measurements of the electron temperature in the discharge show that the rate at which active particles are produced is high enough to explain the acceleration of a chemical reaction by chain processes with the participation of these particles.     

Quantum critical point, scaling and universality in high Tc(CaxLa1−x)(Ba2−c−xLac+x)Cu3Oy

Using charge transport in sintered ceramic samples it is observed that at all doping, including non superconducting overdoped samples, there exists a temperature in which below it dR/dT < 0. This suggests that either the quantum critical point is not necessarily inside the superconducting dome or that the CuO₂ plane is never overdoped. Data relating experimental Cooper pair density, conductivity and T_c suggest that Homes’ relation might need a more specific definition of the conductivity σ.     

Ballistic transfer of spin momentum in multiferroic tunnel junction

The field dependence of critical voltages of switching of magnetic states of a synthetic multiferroic structure is studied based on a bifurcation analysis of Landau–Lifshitz–Gilbert equations with the torques caused by a tunneling spin transport, taking into account the voltage dependence of transferred spin momenta at the variations in the magnitude and direction of electric polarization. The voltage dependences of transferred spin momenta are determined based on the free electron model, taking into account exchange splitting of electron energy subbands in magnetic beaches and the effect of changing the tunnel barrier height at the variations in the polarization magnitude and state.     

Conformational structure of divinylacetylene

Ab initio quantum chemical calculations of the divinylacetylene molecule with different mutual orientations of vinyl groups are carried out using the 6–31G*/MP2 basis set. The torsional potential is approximated by a Fourier series. It is shown that the second term of the series dominates. The enthalpies of the cis- and trans-isomers are nearly equal; the maximum corresponds to the gosh-orientation (180.4 cm−1 with respect to the cis-form). The vibrational spectra of the compounds with cis-, gosh-, and trans-orientations of vinyl groups are analyzed based on the ab initio calculations. It is concluded that the experimental data available in the literature agree with the hypothesis that divinylacetylene exists as cis- and trans-isomers. Translated fromZhurnal Struktumoi Khimii, Vol. 39, No. 4, pp. 610–617, July–August, 1998.     

Organoelemental metal-containing paramagnetic and ferromagnetic polymers 1. Polyferro- and polycobaltsiloxanes,structure and magnetic properties

Polyferro- and polycobaltorganosiloxanes with a different silicon/metal ratio were synthesized. A study of the magnetic properties of these polymers showed that they are natural paramagnets. The low magnetic moment of iron ions is the consequence of the antiferromagnetic interaction of the iron atoms in the metalloxane fragments. It was found that the iron atoms in polyferrophenylsiloxanes have an octahedral oxygen environment and differ with respect to the degree of distortion of the coordination node.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2512–2518, November, 1990.     

Mechanism of the formation of palladium complexes serving as catalysts in hydrogenation reactions : I. Reactions of complexes with molecular hydrogen

The interaction of Ph₃PPD(OAc)₂₂ with molecular H₂ yields a binuclear complex of zero-valent palladium, (Ph₃P)₂Pd₂. This complex interacts reversibly with H₂ in CH₂Cl₂, yielding (Ph₃P)₂Pd₂H₂. In argon atmosphere (Ph₃P)₂Pd₂ reacts with [Ph₃PPd(OAc)₂₂ to form a binuclear complex of Pd^I with a metal—metal bond. These data, as well as the results of kinetic studies of the reactions between [Ph₃PPd(OAc)₂₂ and H₂, are in agreement with an autocatalytic mechanism for the process, including catalysis of the reduction of Pd^II complexes by the Pd⁰ compounds. It has been established that the synthesized compound of Pd^II, Pd^I and Pd⁰ with the ratio P/Pd?1, are inactive in the hydrogenation of unsaturated compounds. The catalytically active complex (PPh)₂Pd₅ is formed when palladium acetate reacts with (Ph₃P)₂Pd₂ in the presence of H₂. The same compound is formed when a solution of (Ph₃P)₂Pd₂ is treated with a mixture of H₂ and O₂ (or H₂O₂ in an atmosphere of H₂). (PPh)₂Pd₅ is an effective catalyst for the hydrogenation of olefins, dienes, acetylenes, aldehydes, organic peroxides, quinones, O₂, Schiff bases, and nitro, nitroso, and azo compounds.