Cage structure materials for n-type transparent conducting oxides

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The transport properties of mayenite-based materials, Ca12Al14O33, were investigated. Systems studied included H-doped/ UV-irradiated Ca12Al14O33, Ca(12-x)MgxAl14O33 (x = 0.1, 0.3, 0.5, 0.8, 1) and Ca12Al(14-x)SixO(33+x/2) (x = 1, 2, 3, 4). The as-prepared Ca12Al14O33 exhibited a room temperature electronic conductivity on the order of 10^-10 S/cm, and a high temperature (600 C) ionic conductivity approximately an order of magnitude less than that of yttria-stabilized zirconia. After H-doping/UV-irradiation Ca12Al14O33 displayed an electronic conductivity of 0.3 S/cm at room temperature rising reversibly to 1 S/cm at 130C. Beyond this temperature hydrogen loss is evident and with it the carrier concentration decreases. Below room temperature the system exhibits a variable range hopping and a small polaron hopping above room temperature. H-treated/UV-irradiated Mg-substituted compounds present inferior electrical properties compared to Ca12Al14O33, with conductivities in the range of 0.15 - 0.28 S/cm at room temperature. Magnesium is suggested to be responsible for the overall decrease in conductivity, as theoretical calculations suggest that it acts as a blocking agent. On the other hand, the proton-implanted/ UV-irradiated Ca12Al(14-x)SixO(33+x/2) shows an increase in conductivity, consistent with the aliovalent substitution. Conductivities of 0.68 S/cm were found for the higher Si-substitution level (in H-implanted specimens) suggesting that the amount of free oxygen ions inside the structural cavities plays a significant role in determining the electrical properties of the compound. Results are in good agreement with density functional calculations performed on all these systems. The highly reduced [Ca24Al28O62]^(+4)+4e^- system presents a wide range of conductivities dependent on the reduction time. The maximum value obtained was 1400 S/cm for Ca12Al14O33 reduced in the presence of titanium shot for 4 days of treatment. A reversal in the sign of the thermopower was observed for the high conductivity samples. The temperature dependance of the conductivity shows degenerate behavior above room temperature, and a linear relation between the resistivity and temperature below room temperature. This degenerate behavior suggests an insulator-to-metal conversion, however, it can also be explained by the small polaron mechanism observed for the H-treated/UV-activated mayenite-based samples. Ultimately, mobility data will define the type of mechanism governing this compound's behavior.

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  • 05/10/2018
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