Insulator-Metal Crossover near Optimal Doping in Pr22xCexCuO4: Anomalous Normal-State Low Temperature Resistivity

Normal-state resistivity measurements at high fields and low temperatures in electron-doped Pr2-xCexCuO4 thin films reveal an insulator-metal crossover near a doping level x≈0.15, similar to a previous report on hole-doped La2-xSrxCuO4. The temperature dependence of the resistivity of insulatinglike samples is sublogarithmic, while for metallic samples (with x = 0.17) the resistivity is linear from 40 mK to 40 K. This surprising latter observation suggests an unusual contribution to the scattering processes at low temperature in these materials. We conclude that the ground state at x = 0.15, corresponding to the maximum transition temperature, is equivalent for hole- and electron-doped cuprates

The creep mechanism of ceramic matrix composites at low temperature and stress, by a material science approach

International audienceThis paper deals with the creep mechanism for ceramic matrix composites reinforced by long ceramic fibers in a ceramic or glass-ceramic matrix, tested at low stresses (< 400 MPa) and low temperatures, respectively < 1673 K for the former and < 1373 K for the latter. The macroscopic results give few ideas on the mechanism, but observations at different scales until high resolution evidence brittle damages which lead the authors to use the damage mechanics approach and to demonstrate that a damage creep mechanism is operating for these CMCs in two steps: (1) matrix microcrack development until its saturation, (2) followed by the opening of some of these microcracks enabling under certain conditions creep of the SiC fibers which bridge the microcracks. This was enlightened by precise damage observations on stepping creep tests and by their quantification with automatic image analysis. (C) 2002 Elsevier Science Ltd. All rights reserved