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Title: Suppression of thermopower of Na xCoO 2 by an external magnetic field

Abstract

We calculate the thermopower in Na{sub x}CoO{sub 2} using the standard Boltzmann transport theory and first principles electronic structures with spin polarization taken into account. The thermopower is found to be smaller when the system is polarized, which thereby provides an alternative reasonable explanation for the suppression of thermopower in a magnetic field. The role of the spin-orbit coupling on the thermoelectricity is also discussed.

Authors:
 [1];  [2]
  1. North Carolina State University
  2. ORNL
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Energy Efficiency and Renewable Energy (EERE)
OSTI Identifier:
968239
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review Letters; Journal Volume: 76; Journal Issue: 19
Country of Publication:
United States
Language:
English
Subject:
30 DIRECT ENERGY CONVERSION; 36 MATERIALS SCIENCE; COBALT OXIDES; SODIUM OXIDES; BOLTZMANN EQUATION; DENSITY FUNCTIONAL METHOD; ELECTRONIC STRUCTURE; MAGNETIC FIELDS; POLARIZATION; SPIN; THERMOELECTRICITY; TRANSPORT THEORY; L-S COUPLING

Citation Formats

Xiang, H. J., and Singh, David J. Suppression of thermopower of NaxCoO2 by an external magnetic field. United States: N. p., 2007. Web. doi:10.1103/PhysRevB.76.195111.
Xiang, H. J., & Singh, David J. Suppression of thermopower of NaxCoO2 by an external magnetic field. United States. doi:10.1103/PhysRevB.76.195111.
Xiang, H. J., and Singh, David J. Mon . "Suppression of thermopower of NaxCoO2 by an external magnetic field". United States. doi:10.1103/PhysRevB.76.195111.
@article{osti_968239,
title = {Suppression of thermopower of NaxCoO2 by an external magnetic field},
author = {Xiang, H. J. and Singh, David J},
abstractNote = {We calculate the thermopower in Na{sub x}CoO{sub 2} using the standard Boltzmann transport theory and first principles electronic structures with spin polarization taken into account. The thermopower is found to be smaller when the system is polarized, which thereby provides an alternative reasonable explanation for the suppression of thermopower in a magnetic field. The role of the spin-orbit coupling on the thermoelectricity is also discussed.},
doi = {10.1103/PhysRevB.76.195111},
journal = {Physical Review Letters},
number = 19,
volume = 76,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • Cells using polyethylene oxide as a sodium ion conducting electrolyte, P2 phase Na[sub x]CoO[sub 2] as the positive electrode and either sodium or sodium/lead alloy as the negative electrode were assembled, discharged, and cycled. Na[sub x]CoO[sub 2] intercalates sodium over a range of x = 0.3--0.9, giving theoretical energy densities of 1,600 Wh/liter (for sodium) or 1,470 Wh/liter (for sodium/lead alloy). Cells could be discharged at rates up to 2.5 mA/cm[sup 2] corresponding to 25% depth of discharge and typically were discharged and charged at 0.5 mA/cm[sup 2] (100% depth of discharge) or approximately 1--2 C rate. Over one hundredmore » cycles to 60% utilization or more, and two hundred shallower cycles at this rate have been obtained in this laboratory. Experimental evidence suggests that the cathode is the limiting factor in determining cycle life and not the Na/PEO interface as previously thought. Estimates of practical energy and power densities based on the cell performance and the following configuration are presented: 30--45 w/o electroactive material in the positive electrode, a twofold excess of sodium, 10 [mu]m separators, and 5 [mu]m current collectors composed of metal coated plastic. On the basis of these calculations, practical power densities of 335 W/liter for continuous discharge at 0.5 mA/cm[sup 2] and up to 2.7 kW/liter for short periods of time should be attainable. This level of performance approaches or exceeds that seen for some lithium/polymer systems under consideration for electric vehicle applications, but with a lower anticipated cost.« less
  • The Raman spectra of the parent compound Na{sub x}CoO{sub 2} (x = 0.75) and the superconducting oxyhydrates Na{sub x}CoO{sub 2} {center_dot} yH{sub 2}O with different superconducting temperatures (T{sub c}) have been measured. Five Raman active phonons around 195 cm{sup -1} (E{sub 1g}), 482 cm{sup -1}, 522 cm{sup -1}, 616 cm{sup -1} (3E{sub 2g}), and 663 cm{sup -1} (A{sub 1g}) appear in all spectra. These spectra change systematically along with the intercalation of H{sub 2}O and superconducting properties. In particular, the Raman active phonons (A{sub 1g} and E{sub 1g}) involving the oxygen motions within the Co-O layers show up monotonous decreasemore » in frequency along with superconducting temperature T{sub c}. The fundamental properties and alternations of other active Raman phonons in the superconducting materials have also been discussed.« less
  • The puzzle of three-dimensional magnetic interactions in the structurally two-dimensional layered-oxide Na{sub x}CoO{sub 2} is addressed using first-principles calculations and an analysis of the exchange mechanisms. The calculations agree with recent neutron results, favoring the antiferromagnetic stacking of ferromagnetic planes. The superexchange via direct O-O hopping and through intermediate Na sp{sup 2} hybrids couples each Co to its nearest and six next-nearest interplanar neighbors via equivalent paths. The individual exchange constants are rather two-dimensional, like the lattice itself, but due to multiple c-axis exchange paths, the magnetism becomes effectively three-dimensional.
  • The results of dc magnetization measurements under hydrostatic (heliumgas) pressure are presented for an ambient pressure superconductor Na{sub 0.35}CoO{sub 2}1.4D{sub 2}O and its precursor compound Na{sub 0.75}CoO{sub 2} known to combine metallic conductivity with an unusual magnetic state below {approx}22K.
  • A comparative study on the oxidation and charge compensation in the A{sub x} CoO{sub 2-} {sub {delta}} systems, A=Na (x=0.75, 0.47, 0.36, 0.12) and Li (x=1, 0.49, 0.05), using X-ray absorption spectroscopy at O 1s and Co 2p edges is reported. Both the O 1s and Co 2p XANES results show that upon removal of alkali metal from A{sub x} CoO{sub 2-} {sub {delta}} the valence of cobalt increases more in Li {sub x} CoO{sub 2-} {sub {delta}} than in Na {sub x} CoO{sub 2-} {sub {delta}} . In addition, the data of O 1s XANES indicate that charge compensationmore » by oxygen is more pronounced in Na {sub x} CoO{sub 2-} {sub {delta}} than in Li {sub x} CoO{sub 2-} {sub {delta}} . - Graphical abstract: The valence of cobalt increases more upon removal of alkali metal from Li {sub x} CoO{sub 2} than from Na {sub x} CoO{sub 2}.« less