Characterization of ionic transport in polymer and electronic transport in disordered selenium and ceramic materials.

摘要

In this thesis, the properties of electronic conduction in vanadium (donor) and scandium (acceptor) doped Ba0.7Sr0.3TiO3 ceramics, amorphous Selenium and ionic conduction in polyester polyol based polyurethane have been investigated.;Electrical, thermal and Li transport properties have been measured for polyester polyol and isocyanate-based polyurethanes doped with Lithium trifluoromethanesulfonimide (LiTFSI) and Lithium perchlorate (LiClO4) Electrical conductivities are estimated at 10-5–10-6 S/cm near 300 K. The conductivities show Vogel-Tammann-Fulcher (VTF) behavior over a wide temperature ranges. Differential scanning calorimetry (DSC) shows that the glass temperature Tg, does not significantly depend on doping type or concentration. Room-temperature 7Li diffusivities, measured by pulsed gradient NMR, show an unexpected strong, linear increase with LiTFSI doping, but only a weak increase with LiClO4 content.;Electrical properties for pure and arsenic alloyed amorphous selenium have been measured. The temperature dependence of conductivity shows that at high temperature next nearest hopping in the localized state is dominant. Whereas, at low temperature variable range hopping occurs.;The leakage current of bulk vanadium (donor) and scandium (acceptor) doped Ba0.7Sr0.3TiO3 ceramics structures measured using gold electrical contacts have been characterized and analyzed. Vanadium doping reduces the ohmic leakage current that dominates the transport characteristics up to 5 kV/cm. The Arrhenius activation energy is 0.18, 0.20 and 0.23 eV for 1, 2 and 4 at % V-doped samples, respectively. Above this field, the current-voltage characteristics exhibit discontinuous current transitions associated with trap filling by electronic carriers. At higher fields, trap controlled space charge limited conduction (SCLC) is observed with an effective mobility of 4±1×10-7 cm2/V s, characteristic of electronic transport process that involves quasi equilibrium between conduction in the band and trapping. In contrast, the leakage current of Sc-doped samples increases with impurity concentration and exhibits a 0.60 eV activation energy. In this case, the limiting current conduction mechanism is the transport of holes over the electrostatic barrier at grain boundaries. Comparison of these results to those on similarly-doped homoepitaxial SrTiO3 thin-films deposited on single-crystal and bicrystal substrates helped to identify the characteristics of transport in the bulk and across grain boundaries for this class of materials.