Contribution à l’étude des propriétés structurales et de transport électronique des matériaux thermoélectriques.
Loading...
Date
2023
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
université de msila
Abstract
Based on the electronic structure, the physical properties of Zintl alloys 〖Ca〗_(1-x) 〖Yb〗_x 〖Zn〗_2 〖Sb〗_2 and 〖Ca〗_(1-x) 〖Eu〗_x 〖Zn〗_2 〖Sb〗_2 (x=0,0.25,0.5,0.75,1) are studied. We perform ab-initio calculations within the WIEN2K code, which is based on the density functional theory (DFT). The calculations of the transport coefficients such as electrical conductivity, electronic thermal conductivity and Seebeck coefficient were carried out using the BoltzTrap code based on the semi-classical Boltzmann transport theory. The lattice thermal conductivity treated as independent thermoelectric parameter following the Slacks equation, with the help of Gibbs program to determine the Debye temperature, Grüneisen parameter...
The transport properties can be significantly changed by varying the composition x. The increasing of x composition leads the compound to be more metallic behavior and the reducing of x decrease the carrier concentration, and make the material more semiconducting. The thermal and electrical conductivity is found to be significantly increased with increasing the Yb or Eu doping as a result of a shortening of electron’s lifetime. For the two alloys 〖Ca〗_(1-x) 〖Yb〗_x 〖Zn〗_2 〖Sb〗_2 and 〖Ca〗_(1-x) 〖Eu〗_x 〖Zn〗_2 〖Sb〗_2, when 0<x<1, the lattice thermal conductivity k_l take low values compared to the case of x=1 and x=0 due to the disorder that obtain "phonon-glass" properties. With the increase of temperature, the values of k_l decrease until it gets close to 0 at very high temperatures (k_l<1 w/m.K), and the obtained results are in good agreement with the available experimental data. It is found that a more compact structure lead to a small band gap or semi-metal with high electrical and thermal conductivity due to the high carrier concentration, this confirms that why Yb〖Zn〗_2 〖Sb〗_2 (n=〖1.25.10〗^20 cm^(-3)at T=300 K) has a low figure of merit ZT. In other hand a narrow-gap (0.46 eV for Ca〖Zn〗_2 〖Sb〗_2), provide a balance between a high Seebeck coefficient and low electronic thermal conductivity, in addition to the slight increase in the carrier concentration (holes) when the temperature increases (n=〖3.87.10〗^19 cm^(-3)at 600 K) make the material have good performance for thermoelectric applications.
Finally, we developed a formula to estimate the carrier relaxation time τ(T) from the values of the Seebeck coefficient at different temperatures. The relaxation time τ is mainly studied in order to understand the behaviour of the electrical conductivity and the electronic thermal conductivity of material with temperature. The method can be implemented in metals and semiconductors and provides results in agreement with the experimental ones, especially in poor metals and heavily doped semiconductors
Description
Keywords
DFT; Zintl phases; Slack equation; BoltzTrap code; carrier concentration; thermoelectric materials; Figure of Merit; Seebeck coefficient; carrier relaxation time.