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Abstract
Transition metal oxide NaxCoO2 is one of the best p-type high-temperature thermoelements for power-generation applications due to its superior thermoelectric (TE) properties and stability at high temperature. Nevertheless, efficiency is still the major limiting factor for its common use. Understanding the mechanisms associated with carrier and phonon transports is an essential step towards further enhancing the TE properties of this material. In this dissertation, the correlation of TE properties with intrinsic and extrinsic defects in NaxCoO2-δ system was investigated. It was found that resistivity ρ and thermopower S augment simultaneously with increasing concentration of oxygen vacancies δ due to the recombination of electrons and holes and average Co valence state. The total thermal conductivity κ can be reduced by minimizing δ, which is against the common understanding that oxygen vacancies usually lead to lower κ. Lattice dynamical study reveals that such anomalous κ-δ relation could be attributed to the texturing of Na+ ions stabilized by oxygen vacancies, giving rise to higher κ with δ. The dimensionless figure of merit was strongly affected by δ at lower operating temperature. The experimental results provide guideline for the variation in performance of this material system under different processing and operating environments.
Divalent Zn2+ and Mg2+ impurities with closed-shell electronic configuration have been incorporated into the system and its dependence of TE properties was systematically studied. Experimental results show that doping Zn2+ within the solubility limit leads to simultaneous reduction in ρ and enhancement of S, consequently resulting in a ~ 20% enhancement in power factor. The incorporation of Mg2+, on the other hand, raises ρ and S, yet the ρ begins to decrease above a critical doping level. At certain fraction of Mg2+ doping, a remarkable ~ 50% improvement in power factor at near ambient temperature was achieved. A detailed examination between the two cases reveals that introduction of divalent impurities, which causes a reduction in Co-O-Co angle, with electronically closed-shell nature may be the decisive ingredients for enhancing the TE performance of sodium cobaltates.