Electrical and Thermal Properties of Mixed Conductors and Superconductors

The electrical and thermal properties of three types of electronic materials have been analyzed. Variable -temperature conductivity and thermoelectric power were measured on the following materials: rm V _2O_5 polymer electrolyte nanocomposites, oxygen-deficient rm YBa_2Cu_3O _{x} single crystals, and quaternary chalcogenides, rm K_2Cu_2CeS _4 and rm CsCuCeS_3. Theoretical models were employed to advance the understanding of the conduction mechanisms present in these materials. The sol-gel process has been used to intercalate rm V_2O_5 xerogels with the polymer electrolyte, oxymethylene linked poly(ethylene oxide)-lithium triflate ((a-PEO)_ {n}(LiCF_3SO _3)). The molar ratios of vanadium and lithium to ethylene oxide were varied to examine the effect on the charge transport properties. The V^ {+4} concentration was also increased in an effort to enhance the electrical properties. The conductivity and thermoelectric power data measured parallel to the planes exhibited semiconductor behavior. The thermoelectric power was negative, indicating electrons as the majority charge carriers. Both the conductivity and thermoelectric power data fit a variable-range hopping transport model. Thermoelectric power measurements were performed on various stoichiometries of twinned, rm YBa_2Cu_3O_{x} single crystals. The data were collected along both the ab-plane and c-axis directions to investigate the anisotropic properties of these materials. Several crystals were reoxygenated to either the same x values or different x values. The reoxygenation study was conducted to extend the knowledge of the effect of oxygen content on transport behavior. Theoretical models previously described in the literature were used to examine the conduction mechanisms in the rm YBa_2Cu_3O_{x} single crystals. The quaternary chalcogenides, rm K_2Cu_2CeS_4 and CsCuCeS _3, were also investigated. The amount of Cu present was found to have an effect on the crystal structure of both materials. As a result, previously unreported chalcogenides were prepared by systematically varying the amount of Cu in the synthesis reactions. The conductivity and thermopower data for both rm K_2Cu _2CeS_4 and CsCuCeS_3 indicate non-metallic behavior which is directly related to the crystal structure. The range of varying behavior in the electrical and thermal properties of both mixed conductors and superconductors has been better identified using different material systems than could have been obtained from just examining a single material system.