Search for Isotope Effect in Superconducting Y-Ba-Cu-O

An isotope effect has been searched for in the high-Tc, superconductor YBa2Cu307 —b through substitution of 180 for 16O. No shift in the superconducting transition temperature T, is observed by electrical resistivity or magnetic susceptibility measurements. We discuss the implications of this result for mechanisms of superconductivity in the high-T, oxides

Dielectric Response and Conductivity of Poly(Propylene Oxide) Sodium Poly-Iodide Complexes. Discussion of Charge Transport by an Ion Relay Mechanism

The addition of iodine to poly(propylene oxide), PPO, with or without Nal results in the formation of polyiodides as evidenced by a resonance Raman band at 170 cm 1. The conductivities of these complexes, measured with ac and dc methods, show both ohmic and nonohmic responses characteristic of electronic and ionic conductors, respectively. The conductivity rises with both increasing iodine and salt concentrations. Low-temperature conductivity data showed a very small inflection in the vicinity of Tg for the host polymer, indicating that dynamics of the host polymer are only weakly coupled to the mechanism for conductivity in the polyiodide system. An ion relay along polyiodide chains is consistent with these observations. For comparison purposes, Raman spectra and conductivities were studied for structurally characterized metal compounds containing infinite I3 species. In these structures the conductivity is very low, and this is attributed to structural pinning of the polyiodides, which would block ion relay or carrier hopping charge transport

Lewis Number Effects on Turbulent Premixed Flame Structure

The influence of the Lewis number on turbulent flame front geometry is investigated in a premixed turbulent stagnation point flame. A laser tomography technique is used to obtain the flame shape, a fractal analysis of the multiscale flame edges is performed and the distribution of local flame front curvature is determined. Lean H{sub 2}/Air and C{sub 3}H{sub 8}/Air mixtures with similar burning rates were investigated with Lewis numbers of 0.33 and 1.85 respectively. At the conditions studied the laminar H{sub 2}/Air mixture is unstable and a cellular structure is observed. Turbulence in the reactant is generated by a perforated plate and the turbulent length scale (3mm) and intensity (7%) at the nozzle exit are fixed. The equivalence ratio is set so that the burning velocity is the same for all the cases. Results show clearly that the turbulent flame surface area is dependent on the Lewis number. For a Lewis number less than unity surface area production is observed. The shape of the flame front curvature distribution is not found to be very sensitive to the Lewis number. For the H{sub 2}/Air mixture the distribution is skewed toward the positive values indicating the presence of cusps while for the C{sub 3}H{sub 8}/Air mixture the distribution is more symmetrical. In both cases the average curvature is found to be zero, and if the local burning speed varies linearly with curvature, the local positive and negative burning velocity variations due to curvature will balance