A search for thermal isomerization of olefins to carbenes. Thermal generations of the silicon-nitrogen double bond

Thermal isomerization of an olefin to a carbene is observed only in a few strained olefins. Attempts to observe a 1,2-silyl migration to form a carbene from a simple olefin were not successful. The olefins studied are stable at lower temperature and decomposed to smaller molecules at higher temperature probably via a radical process. The gas-phase thermal behavior of a series of organic and organosilicon compounds was studied by stirred-flow reactor (SFR) pyrolysis. Kinetic parameters were determined in the gas phase for various hydrocarbons and organosilicon compounds. The activation energies and log As of the compounds studied are in the normal range of olefinic cis-trans isomerization proceeding by a singlet biradical mechanism;The substituent effect on the thermal isomerization of olefins was discussed. The substitution of phenyl, t-butyl, and trimethylsilyl groups on the double bond lowers the olefinic cis-trans isomerization Oil, Gas, and Energy;A new reaction, gas-phase thermal isomerization of a silacyclobutene to silylallene, was discovered. The mechanism of the isomerization was studied by deuterium-labeling experiments;Intermediates containing silicon-nitrogen double bonds were generated in the gas phase by [beta]-elimination of trimethylmethoxysilane and retroene elimination of propene. The 1,2-silyl migration of silanimine to aminosilylene was observed. This is the first observation of a silanimine-to-aminosilylene isomerization. The aminosilylene formed was trapped by 1,3-butadiene and the structure of the trapping product was proven by comparison of its spectroscopic data with that of an authentic sample synthesized by an independent route. The isomerization of silanimine to aminosilylene by 2,3-allyl migration was not observed. Dimers of silanimine are formed in the absence of trapping reagents;Different approaches to silicon-nitrogen double bond were also explored. Attempts to obtain evidence for silapyridine, an unknown compound, both theoretically and experimentally, were not successful

Significantly enhanced critical current densities in MgB2 tapes made by a scaleable, nano-carbon addition route

Nanocarbon-doped Fe-sheathed MgB2 tapes with different doping levels were prepared by the in situ powder-in-tube method. Compared to the undoped tapes, Jc for all the C-doped samples was enhanced by more than an order of magnitude in magnetic fields above 9 T. At 4.2 K, the transport Jc for the 5 at% doped tapes reached 1.85x104 A/cm2 at 10 T and 2.8x103 A/cm2 at 14 T, respectively. Moreover, the critical temperature for the doped tapes decreased slightly. Transmission electron microscopy showed a number of intra-granular dislocations and the dispersed nanoparticles embedded within MgB2 grains induced by the C doping. The mechanism for the enhancement of flux pinning is also discussed. These results indicate that powder-in-tube-processed MgB2 tape is very promising for high-field applications.Comment: 13 pages, 5 figures. to be published soo

Enhanced critical current density of MgB2 superconductor synthesized in high magnetic fields

The effect of high magnetic fields on the current carrying properties of both MgB2 bulks and Fe-sheathed tapes was investigated following different thermal sequences. It is found that application of a large magnetic field during processing results in the quite uniform microstructure and the better connectivity between the MgB2 grains. As a result, the Jc of these samples has shown much higher value than that of the MgB2 samples in the absence of magnetic field. The possible mechanism of the Jc enhancement under an external magnetic field is also discussed.Comment: Presented at ISS2005, Tsukuba, 24-26 Oct., 2005; Revised versio