Browsing by Author "Donelson, R"

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    Immobilization of Na ions for substantial power factor enhancement: site-specific defect engineering in Na0.8CoO2
    (American Chemical Society, 2012-02-16) Tsai, PH; Assadi, MHN; Zhang, T; Ulrich, C; Tan, TT; Donelson, R; Li, S
    Simultaneous enhancement of the interdependent Seebeck coefficient and electrical conductivity has been achieved through defect engineering by doping Mg into specific sites of Na0.8CoO2. Results from thermoelectric measurement demonstrate that the power factor was substantially increased by 50% at ambient. Experimental and theoretical analyses show that the occupation of divalent Mg in the disordered Na layer immobilizes the Na ions and thus induces a long-range ordering of Na ions. This phenomenon improves the carrier mobility significantly, giving rise to the observed exotic thermoelectric performance. Moreover, it is predicted that other electronically closed-shell dopants in sodium cobaltate play a similar role in enhancing the thermoelectric conversion efficiency. © 2012, American Chemical Society.
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    Oxygen level dependent lattice dynamics of Na0.73CoO2-δ.
    (American Chemical Society, 2010-11-22) Tsai, PH; Donelson, R; Tan, TT; Avdeev, M; Yu, DH; Strassle, T; Li, S
    The optical and acoustic phonon branches of Na0.73CoO2-δ have been determined using Raman scattering and inelastic neutron scatterings, and their correlation with phononic thermal conductivity kph in terms of oxygen-vacancy concentration δ was investigated. The experimentally observed phonon stiffening of the Raman-active E1g mode suggests that oxygen vacancies may help stabilize texturing of Na ions that gives rise to higher kph with increasing δ. The generalized phonon density of states characterized using inelastic neutron scattering exhibits subtle stiffening of acoustic and optical phonons with δ, which appears to be responsible for the variations in kph(T) profile in the temperature range 323−923 K. © 2010, American Chemical Society