Search

Mn alloying in thermoelectrics is a long-standing strategy for enhancing their figure-of-merit through optimizing electronic transport properties by band convergence, valley perturbation, or spin-orbital coupling. By contrast, mechanisms by which Mn contributes to suppressing thermal transports, namely thermal conductivity, is still ambiguous. A few precedent studies indicate that Mn introduces a series of hierarchical defects from the nano- to meso-scale, leading to effective phonon scattering scoping a wide frequency spectrum. Due to insufficient insights at the atomic level, the theory remains as phenomenological and cannot be used to quantitatively predict the thermal conductivity of Mn-alloyed thermoelectrics. Herein, by choosing the SnTe as a case study, aberration-corrected transmission electron microscopy (TEM)/scanning transmission electron microscopy (STEM) to characterize the lattice complexity of Sn1.02−xMnxTe is employed. Mn as a “dynamic” dopant that plays an important role in SnTe with respect to different alloying levels or post treatments is revealed. The results indicate that Mn precipitates at x = 0.08 prior to reaching solubility (≈10 mol%), and then splits into MnSn substitution and γ-MnTe hetero-phases via mechanical alloying. Understanding such unique crystallography evolution, combined with a modified Debye-Callaway model, is critical in explaining the decreased thermal conductivity of Sn1.02−xMnxTe with rational phonon scattering pathways, which should be applicable for other thermoelectric systems.

Thermoelectric performance of Ca3Co4O9 system has been enhanced through the misplaced substitution with Cr doping at Ca-sites. The study of Cr doping at Co-sites has also been performed for comparing. Base on the analysis of the structural data, we conclude that the doped Cr ions should be in the form of Cr+6, and enter into Ca-sites of Ca2CoO3 layers in Ca3−xCrxCo4O9 lattice and Co-sites of CoO2 layers in Ca3Co3.90Cr0.10O9 lattice, respectively. As Cr ions are doped into Ca-sites in Ca3−xCrxCo4O9 lattice, both resistivity and thermopower increase obviously, while thermal conductivity decreases. Among all samples, Ca2.95Cr0.05Co4O9 shows the maximum ZT value, which is enhanced by 12% than that of Ca3Co4O9. The results show that the misplaced substitution with a proper Cr doping at Ca-sites is beneficial to the enhancement of the thermoelectric performance in Ca3Co4O9 system compared to the traditional idea: Cr doping at Co-sites, which is suggested to originate from the combined action of the carrier concentration, the electronic correlation, and the lattice disharmony.

The structural, magnetic, electrical and thermal transport properties of the polycrystalline samples Ca3Co4−xInxO9 (x = 0, 0.05, 0.10, 0.15, and 0.20) have been investigated systematically. The results indicate that the In-substitution has a negative effect on the low-temperature ferrimagnetic transition and the propagation of spin-density-wave. All In-doped samples have a much larger resistivity than that of the un-doped sample. And the temperature region where the Fermi liquid transport mechanism dominates is remarkably widened via In-doping. However, for the In-doped samples, as the In-doped content increases, the value of resistivity ρ decreases monotonously and the electronic correlation in the system enhances. With the addition of In-ions in Ca3Co4O9 system, the thermopower S and the ZT value decrease. The results are suggested to originate from the variations of carrier concentration and lattice disharmony in this system induced by In-doping.

BiFeO3 films are prepared directly on n-type Si (100) substrates using chemical solution deposition method. The effects of individual-layer thickness via controlling the solution concentration on the texture, morphology, magnetic properties as well as the magnetoelectric properties are investigated. It was first observed that the derived films are the highly preferred c-axis oriented on Si substrates. With decreasing individual-layer thickness, the c-axis texture is improved due to preferred nucleation at the interfaces. The magnetization and the dielectric constant is increased and decreased with decreasing individual-layer thickness, respectively. It is interesting to observe that the magnetoelectric coupling is obviously enhanced with decreasing individual-layer thickness, which is beneficial for potential applications.