RESUMO
A very large thermoelectric figure of merit ZT = 6 at 380 K has recently been reported for Fe2V0.8W0.2Al in the thin-film form (B. Hinterleitner et al., Nature, 2019, 576, 85-90). In this form, Fe2V0.8W0.2Al experimentally crystallizes in a disordered A2 crystal structure, different from its bulk-form structure (L21). The first-principles calculations of the electronic structure performed in A2-Fe2V0.8W0.2Al supercells generated by the special quasirandom structure (SQS) method are thus reported here. These calculations unambiguously indicate that A2-Fe2V0.8W0.2Al is a ferromagnetic metal at 0 K, displaying a small Seebeck coefficient at 400 K (<30 µV K-1). The present results contradict the scenario of the occurrence of a deep pseudo-gap at the Fermi level, previously invoked to justify ZT = 6 in Fe2V0.8W0.2Al thin films.
RESUMO
We report a combined theoretical and experimental search for thermoelectric materials based on semiconducting zinc antimony. Influence of three new doping elements (sodium, potassium and boron) on the electronic properties was investigated as well as the carrier concentration and temperature dependence of the thermoelectric coefficients obtained through density-functional calculations and Boltzmann transport theory. Distortion of the electron arrangement caused by the doping elements is displayed as a deformation charge density around the atoms. Based on the band structures, the density of states, and the transport properties, we found that the presence of Na and K in the ZnSb matrix leads to a slightly improved p-type conductivity, whereas the B substitution leads to a n-type doping. Because of the stronger need for obtaining n-type ZnSb-based material, the B(0.01)Zn(0.99)Sb structure has been transferred to the laboratory to be synthesized by direct melting. The sample was investigated using x-ray diffraction and scanning electron microscopy.
RESUMO
Stretched exponential relaxation [exp-(t/tau)(beta(K))] is observed in a large variety of systems but has not been explained so far. Studying random walks on percolation clusters in curved spaces whose dimensions range from 2 to 7, we show that the relaxation is accurately a stretched exponential and is directly connected to the fractal nature of these clusters. Thus we find that in each dimension the decay exponent beta(K) is related to well-known exponents of the percolation theory in the corresponding flat space. We suggest that the stretched exponential behavior observed in many complex systems (polymers, colloids, glasses...) is due to the fractal character of their configuration space.
