ORALS
SESSION: CompositeMonPM1-R8
| 6th Intl. Symp. on Composite, Ceramic and Nano Materials Processing, Characterization and Applications |
| Mon Nov, 5 2018 / Room: Grego (50/3rd) | |
| Session Chairs: Raman Singh; Gabor Patonay; Session Monitor: TBA |
15:15: [CompositeMonPM108]
Thermoelectric Properties of the Layered Rhodates K<sub>x</sub>RhO<sub>2</sub> and Na<sub>x</sub>CoO<sub>2</sub> Udo
Schwingenschlogl1 ; Nirpendra
Singh
1 ; Yasir
Saeed
1 ;
1King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia;
Paper Id: 290
[Abstract] We discuss the structural and thermoelectric properties of the layered compounds K<sub>x</sub>RhO<sub>2</sub> [1] in comparison to isostructural and isovalent Na<sub>x</sub>CoO<sub>2</sub>. The optimized structure of K<sub>1/2</sub>RhO<sub>2</sub> exhibits a remarkable deviation of the c/a ratio from the experimental result as well as from c/a ratios of related compounds. This indicates that a hydrated phase of K<sub>x</sub>RhO<sub>2</sub> exists and the experimental structure determination refers to this hydrated phase. The calculated Seebeck coefficient of pristine K<sub>1/2</sub>RhO<sub>2</sub> amounts to 50*10<sup>-6</sup> V/K at 300 K, which is close to the experimental value of 40*10<sup>-6</sup> V/K. More importantly, we observe high values for the Seebeck coefficient and power factor for hydrated K<sub>x</sub>RhO<sub>2</sub> in the whole temperature range from 0 to 700 K. At 100 K we find for hydrated K<sub>7/8</sub>RhO<sub>2</sub> a value of Z = 3*10<sup>-3</sup> K<sup>-1</sup>, which is the highest power factor observed at this temperature. It exceeds also the exceptionally high value of Na<sub>0.88</sub>CoO<sub>2</sub> by more than 50%. Our results, hence, demonstrate that hydration is an effective approach to modify the lattice parameters and, as a result, enhance the thermoelectric performance. The Na<sub>x</sub>RhO<sub>2</sub> oxides [2] are found to form a new class of materials with exciting thermoelectric features, even outperforming the 2H phases of the K<sub>x</sub>RhO<sub>2</sub> system. In the latter the optimal thermoelectric performance is achieved at low temperature, whereas the modified stacking of the atomic layers in the 3R phases of Na<sub>x</sub>RhO<sub>2</sub> results in a reduced interlayer coupling and, in turn, in a dramatically enhanced thermoelectric response in the technologically relevant high temperature range. We find that Na vacancies in Na<sub>x</sub>RhO<sub>2</sub> avoid clustering and that the RhO<sub>6</sub> octahedra are modified depending on the amount of Na deficiency. Analysis of the induced changes in the density of states close to the Fermi level indicates that the Rh<sup>3+d</sup> d<sub>3z2-r2</sub> states control the transport properties of the compounds. A substantial figure of merit of 0.35 at 580 K is found in hydrated Na<sub>0.83</sub>RhO<sub>2</sub> due to the enhanced effective mass of the charge carriers. In general, the figure of merit can be further increased by reduction of the Na vacancy concentration to increase the resistivity.
References:
[1] Y. Saeed, N. Singh, and U. Schwingenschlogl, Adv. Funct. Mater. 22 (2012), 2792-2796.
[2] Y. Saeed, N. Singh, and U. Schwingenschlogl, Sci. Rep. 4 (2014), 4390.
15:40 Break
