31 research outputs found
Resistivity and Carrier Mobility of the SmBa2 Cu3 O6+x Superconductor with Different Oxygen Doping Levels
Abstract
DC conductivity measurements between 15 and 300 K are reported for SmBa2 Cu3O6+x samples with different oxygen doping amounts (x) produced by annealing under appropriate high temperature and oxygen pressure conditions and quenching. Samples with x≥0.5 are superconductors: Tc ~60 K at x=0.7, Tc >80 K at ; x=0.9. The transition from superconduction to non-superconduction corresponds to the tetragonal to orthorhombic structural tran-sition and to the transition from semiconducting to metallic temperature dependence of the resistivity. Oxygen doping causes a sudden increase of hole mobility near x=0.5. Below this threshold, the be-havior of the carrier mobility is in agreement with an Anderson localization
Correlation between transport properties and lattice effects in the NdCoO3 based catalysts and sensor materials
This study presents correlations between the structural and transport
properties of pure and doped neodymium cobaltate, a compound of great interest
for its foreseen applications as catalyst, sensor and thermoelectric material.
Neutron and x-ray powder diffraction data have been combined to carefully
determine lattice constants and atomic positions and four probe direct current
conductivity and thermoelectric power measurements allowed us to follow the
thermal evolution of the transport properties of these compounds. The dramatic
improvement of the room temperature conductivity of Nd0.8Ca0.2CoO3 with respect
to the pure and the Na-doped compound is explained in terms of a different
spin-state for the Co ions within this structure. The higher conductivity and
the absence of anomalies in the thermal expansion makes the Ca-doped compound
more attractive than the pure NdCoO3 in view of possible applications. The
experimental data and the Co environment analysis here discussed, in particular
bond lengths distortion and bending angles, are fully consistent with a spin
state (low to intermediate) transition in NdCoO3Comment: 32 pages, 9 figure
Cr and Ni doping of Li4Ti5O12: cation distribution and functional properties
Cr- and Ni-doped Li4Ti5O12 compound has been characterized through the combined use of X-ray powder diffraction, electron paramagnetic resonance (EPR), 7Li nuclear magnetic resonance magic-angle spinning (NMR-MAS), micro-Raman, and magnetization measurements. The doping, occurring on the octahedral site of the cubic Li4Ti5O12 spinel lattice, strongly affects both the local and the average structural properties. The glassy character of the observed EPR signals suggests structural disorder in the stable Li4Ti5O12 matrix and the presence of clustering phenomena or nonhomogeneous distribution of the dopant ion, as also supported by 7Li NMR-MAS, micro-Raman, and magnetization results. The computation by numerical method of the complex EPR signal of the Cr-doped sample suggests that both CrTi and CrLi substitutions occur, giving rise to two distinct EPR components, corresponding to opposite axial distortion of the relative octahedral environments. On the basis of the compositional data, defect models involving oxygen or cation vacancies are proposed to explain the conductivity of the doped material
Electrode stability and electrochemical performance of Lamox electrolytes under fuel cell conditions
Electrical Characterization of La 2-x
Abstract
The electrical behaviour of La2-xSrxCuO4-y solid solutions (with x = 0, x = 0.025, x = 0.05, and x = 0.15) at temperatures between 10 and 900 K and under different oxygen partial pressure pO2 = 1 ÷ 10-6 atm) has been investigated. The samples prepared and measured under an O2 flux (i.e., with y = 0) show a superconducting transition with Tc = 46, 29. 37 K for x = 0. 0.05 and 0.15, respectively. The samples with x = 0.025, y = 0, and x = 0, y ≠ 0 exhibit no sign of superconductivity. In the temperature range 100-900 K. La2CuO4 is semiconducting, whereas the electrical resistivity is independent of temperature for the x =0.025 sample, and the x = 0.05 and x = 0.15 are metallic