•The electrodes of Bi2Te3 modules with multilayer structure are fabricated.•Lower contact resistance was obtained by inserting Ni layer in Cu/Bi2Te2.7Se0.3.•The epitaxial growth models were proposed based on the XRD results.•The Al/Ni/Cu electrodes showed high anti-oxidation after annealing in air at 473 K.•Formation of dense Al2O3 layer is responsible for the excellent anti-oxidation.Exploring high-performance multilayer electrodes is very important to improve the conversion efficiency of Bi2Te3-based modules. Here, an Al/Cu/Ni multilayer thin-film electrode with low interface resistance and excellent anti-oxidation was designed and fabricated on Bi2Te2.7Se0.3 (BTS) legs through vacuum evaporation method. X-ray diffraction, electron probe microanalyzer, field emission scanning electron microscope, X-ray photoelectron spectroscopy and resistance measurements have been employed to investigate the impacts of Ni transition layer (TL) and Al protective layer (PL) on the phase composition, microstructure, interface resistance and anti-oxidation of the electrode. It was found that the Ni TL can significantly lower the interface resistance through improving the lattice match of interface between Cu and BTS. The Cu monolayer and Cu/Ni bilayer thin-film electrodes have very poor anti-oxidation; while the Al/Cu/Ni multilayer thin-film electrode exhibits excellent anti-oxidation because the dense Al2O3 layer covering on the surface of the Al PL can effectively prevent from the further oxidation.Download high-res image (191KB)Download full-size image

With the growing interest in developing miniaturized thermoelectric devices, there has been a strong demand in preparing thermoelectric thin films with high electrical conductivity and large power factors, hence ensuring the miniaturized devices have large cooling capacity and large output powers. This work demonstrated the preparation of intermetallic YbAl3 thin films through a double-target magnetron co-sputtering technique and a subsequent annealing treatment. It was revealed that the subsequent heat treatment of thin films plays a critical role in achieving crystalline, stoichiometric, and nanostructured YbAl3 thin films. Benefiting from the significantly improved crystallinity and stoichiometry, the optimized YbAl3 thin films exhibit extraordinarily high electrical conductivity reaching 1.7 × 106 S m−1 and large power factors around 7.4 mW m−1 K−2. The figure of merit ZT of the annealed thin films is comparable with that of the bulk materials, showing their potential use in miniaturized thermoelectric devices.

Bismuth telluride alloys are promising thermoelectric materials used for portable␣and wearable cooling devices due to their excellent thermoelectric properties near the ambient temperature. Here, a simple and cost-effective brush-printing technique, together with a subsequent annealing treatment, has been used to prepare Bi2Te3-based thick films and prototype devices. The composition, microstructure, and electrical properties of the brush-printed p-type Bi0.5Sb1.5Te3 thick films at different annealing temperatures are investigated. It is found that annealing temperature plays an important role in promoting densification and preventing the film from cracking, hence improving the electrical transport properties. The maximum power factor of the brush-printed thick films is 0.15 mW K−2 m−1 when annealed at 673 K for 4 h. A prototype thermoelectric device is manufactured by connecting the brush-printed p-type Bi0.5Sb1.5Te3 and n-type Bi2Te2.7Se0.3 thick films with Cu thick-film electrodes on an Al2O3 substrate. The cooling performance of the thermoelectric device is evaluated by measuring the temperature difference produced under applied currents.

The relevance between the fine structure and transport performance of thermoelectric materials can be revealed by X-ray spectroscopy including X-ray absorption and emission spectra as an effective tool. In this paper, the experimental spectra of extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), and valence-band X-ray photoelectron spectroscopy (XPS) of Ba and In double-filled skutterudites (BaxInyCo4Sb12) were analyzed via the first-principles calculation and spectrum simulation. The atomic-scale fine structures indicate that the rectangle Sb4 rings become square when the total filing fraction of Ba and In increases. The transition of Sb4 rings leads to the band convergence and density of states (DOS) increase of the SbSb ppσ bonding and ppπ∗ antibonding states. The enhanced TE performance of BaxInyCo4Sb12 is essentially attributed to the band convergence, the increased DOS near the Fermi level, and the resonant phonon scattering of Ba and In fillers.This paper was to investigate the relevance between the fine structure and transport performance of thermoelectric materials, in which the atomic-scale fine structure and electronic structure of Ba and In double-filled skutterudites were characterized using extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), and valence-band X-ray photoelectron spectroscopy (XPS).Figure optionsDownload full-size imageDownload as PowerPoint slide

A simple, efficient and rapid brush-printing method has been developed for preparation of n-type Bi2Te2.7Se0.3 films approximately 100–150 μm thick. X-ray diffraction, scanning electron microscopy, electron probe microanalysis, and four-point probe measurements were used to characterize the crystal structure, composition, microstructure, and electrical properties of the films. The results showed that all the n-type Bi2Te2.7Se0.3 thick films were composed of single-phase Bi2Te2.7Se0.3; the grains in the films were randomly distributed in the low-temperature-annealed samples and predominantly oriented along the (00l) plane in samples annealed at temperatures >673 K. σ and the absolute value of α first increased substantially with increasing the annealing temperature in the range 573–673 K then decreased when the annealing temperature was increased further. The dependence of σ and α on annealing temperature may be reasonably explained on the basis of the change in the microstructure induced by annealing. The performance of a prototype cooling device containing n-type Bi2Te2.7Se0.3 thick films was evaluated for temperature differences produced by use of different DC currents.

•The interstitial In dopant leads to the local structural perturbations in β-Zn4Sb3.•The simultaneous increases in α and σ are observed in the In-doped Zn4Sb3 compounds.•The In dopant plays different doping behaviors by the dopant contents in the samples.•A maximum ZT of 1.41 at 700 K is achieved for the In-doped Zn4Sb3 compounds.In-doped β-Zn4Sb3 compounds (Zn4−xInxSb3, 0 ⩽ x ⩽ 0.24) were prepared by melt-quenching and spark plasma sintering technology in the work. The resultant samples were systematically investigated by X-ray diffraction, X-ray photoelectron spectroscopy, differential scanning calorimetry and thermoelectric property measurements. The In dopant was identified to preferentially occupy the interstitial site in β-Zn4Sb3 and led to the local structural perturbations near the 12c Sb2 and 36f Zn1 sites. The Auger parameters of Zn and Sb indicated that the increase in the valence of Zn was attributed to the charge transfer from Zn to In atoms. The binding energies of In 3d5/2 core level showed that the interstitial In dopant was n-type dopant (In3+) in slightly In-doped Zn4−xInxSb3, but acted as acceptor and was p-type dopant (In+) in heavily In-doped ones. The discovery provides a reasonable explanation for the puzzled relation between σ and x for Zn4−xInxSb3. Simultaneously increasing the electrical conductivity and Seebeck coefficient of Zn4−xInxSb3 can be realized through the local structural perturbations. The significantly enhanced power factor and the intrinsic low thermal conductivity resulted in a remarkable increase in the dimensionless figure of merit (ZT). The highest ZT reached 1.41 at 700 K for Zn3.82In0.18Sb3 and increased by 68% compared with that of the undoped β-Zn4Sb3.

A series of p-type xBaFe12O19/CeFe3CoSb12 (x = 0, 0.05%, 0.10%, 0.20%, 0.40%) magnetic nanocomposite thermoelectric (TE) materials have been prepared by the combination of ultrasonic dispersion and spark plasma sintering (SPS). The effects of BaFe12O19 magnetic nanoparticles on the phase composition, microstructure, and TE properties of the nanocomposite materials were investigated in this work. x-Ray diffraction analysis shows that all the SPSed bulk samples are composed of main phase skutterudite besides a small amount of FeSb2 and Sb. The TE transport measurements demonstrated that remarkable enhancements in electrical conductivity and Seebeck coefficient can be simultaneously realized by optimizing the doping content of BaFe12O19 magnetic nanoparticles. The lattice thermal conductivity was significantly reduced because of enhanced phonon scattering induced by BaFe12O19 nanoparticles. The highest ZT value reached 0.75 at 800 K for the sample with x = 0.05%, increased by 41.5% as compared with that of p-type CeFe3CoSb12 bulk material without BaFe12O19 magnetic nanoparticles. This work confirms that doping a small amount of BaFe12O19 magnetic nanoparticles can significantly improve the ZT value of p-type xBaFe12O19/CeFe3CoSb12 magnetic nanocomposite TE materials.

A series of Ge-doped and (Ba,In) double-filled p-type skutterudite materials with nominal composition Ba0.3In0.2FeCo3Sb12−xGex (x = 0 to 0.4, Δx = 0.1) have been prepared by melting, quenching, annealing, and spark plasma sintering methods. The effects of Ge dopant on the phase composition, microstructure, and thermoelectric properties of these materials were investigated in this work. A single-phase skutterudite material was obtained in the samples with 0 < x ≤ 0.2, and trace Fe3Ge2 was detected in the samples with x ≥ 0.3. The electrical conductivity increased and Seebeck coefficient decreased with increasing x in the range of 0 to 0.2, while the inverse behaviors of electrical conductivity and Seebeck coefficient were observed in the samples with x ≥ 0.3. The variations of electrical conductivity and Seebeck coefficient are attributed to the significant increase in the carrier concentration in the x range of 0 to 0.2 and the intensive impact of Fe3Ge2 when x ≥ 0.3. The lattice thermal conductivity of all the Ge-doped samples was considerably reduced as compared with the undoped Ba0.3In0.2FeCo3Sb12 sample, and the lowest value of lattice thermal conductivity of the Ba0.3In0.2FeCo3Sb11.8Ge0.2 sample reached 1.0 W m−1 K−1 at 700 K. The highest ZT value of 0.54 was obtained at 800 K for the Ba0.3In0.2FeCo3Sb11.7Ge0.3 sample, increased by 10% as compared with that of Ba0.3In0.2FeCo3Sb12.

The effects of indium impurity on the crystal structure and bonding characteristics of In-doped β-Zn4Sb3 were investigated by powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The XRD Rietveld refinement indicates that the indium impurity preferentially substitutes one of Sb atoms in Sb–Sb dimer at the 12c Sb(2) site and simultaneously leads to the increase of Zn occupancy. The observations of binding energy shift and a new valence state in Sb 3d core-level XPS spectra can be attributed to the charge transfer from In and Zn to Sb. As a result, more electropositive Zn atoms are needed to maintain the charge balance. The reduction of the lattice thermal conductivity is ascribed to the formation of the asymmetric Sb–In bond, resulting in much low lattice thermal conductivity of 0.49 W−1 K−1 of Zn4Sb2.96In0.04.Graphical abstractThe indium impurity substitutes one of Sb atoms in Sb–Sb dimer, resulting the charge transfer from In to Sb, which leads to the binding energy of Sb 3d core level XPS spectra shift to low value.Highlights► The indium impurity preferentially substitutes one of Sb atoms in Sb–Sb dimer at the 12c Sb(2) site. ► The occupancy of Zn increases by the In substitution for Sb, whereas that of Sb keeps constant. ► The binding energy of Sb 3d shifts to low value. ► The charge transfer occurs from In and Zn to Sb.

•Stoichiometric Bi0.5Sb1.5Te3 films are fabricated by in-situ crystallization.•The (000l) orientations and high crystallinity of these films have been realized.•Three parameters of electrical properties (μ, σ, α) are simultaneously increased.•The relationship between the electric properties and orientations are calculated.•A layer-by-layer in-situ growth model is proposed for (000l)-oriented films.The preparation of high-performance Bi2Te3-based films is vitally important for the miniaturization of Bi2Te3 thermoelectric (TE) device. Herein, a series of stoichiometric Bi0.5Sb1.5Te3 films with different preferential orientations have been fabricated through in-situ   crystallization during the co-sputtering process. We discover that the preferential orientation was transformed from (011̅5) to (101̅10) to (000l) orientation with increasing the substrate temperature. The (000l)-oriented films exhibit the best electrical transport properties, which the maximum electrical conductivity of 8.0×104 S·m-1 and power factor of 3.8 mW K-2·m-1 are much more than those of the bulk material. The excellent properties are attributed to the high-crystallinity, well-controlled preferential orientation, and minimized compositional deviation. A layer-by-layer in-situ growth model is proposed to understand the formation mechanism of the (000l)-oriented films. Our work demonstrates that the electrical transport performance of Bi2Te3-based films can be remarkably improved through finely controlling the crystallinity and preferential orientation under the condition of stoichiometric composition.