•Binary Ti/CoSb3 & p-type Ti/p-SKD TE joints were made and aged at 550 °C in vacuum.•Diffusion mechanism controls the growth of the interfacial IMC layers.•The p-type joints exhibit less interfacial diffusion than the Ti/CoSb3 joints.•No TiCoSb layer forms at the interfaces of the p-type joints.•Absence of the TiCoSb layer leads to good stability of the contact resistivity.The interfacial stability of thermoelectric (TE) joints is among the key issues determining the durability of thermoelectric generators (TEGs) in practical operation. For skutterudite (SKD) based TEGs, the inter-diffusion between the metal electrode and the SKD material at high temperature plays a vital role on the interfacial stability of the joints. In this work, we report the evolution of the interfacial microstructure and the interfacial contact resistivity for a series of p-type SKD TE joints (Ti/CeyFexCo4-xSb12, x = 1.5–4, y = 0.35–1) at high temperature. The growth of the intermetallic compound (IMC) layers at the interface of all the joints was found to be controlled by the diffusion process. In contrast to the binary Ti/CoSb3 joints, the growth of the interfacial IMC layers in the p-type Ti/CeyFexCo4-xSb12 joints was considerably depressed. Moreover, no brittle TiCoSb phase was formed at the interface of the p-type joints, which resulted in good stability of the interfacial contact resistivity at high temperature. Related mechanisms were discussed.

Nanostructures and nano-composites have been shown to be effective in depressing the lattice thermal conductivity and improving the performance of thermoelectric materials. However, ZT enhancement by nano-particle dispersion is limited only to a restricted level due to the difficulty in increasing the particle contents while maintaining a uniform and narrow size distribution. In the present work, YbyCo4Sb12-based nano-composites with reduced graphene oxide (rGO) layers of several nanometers intercalated on the grain boundary matrix forming a 3D network have been prepared through a simple in situ reduction approach using graphene oxide (GO) as the precursor. The 3D-rGO network wrapping architecture dramatically reduced the lattice thermal conductivity due to enhanced interparticle and intraparticle phonon scattering effects, and simultaneously enhanced the Seebeck coefficient due to the energy filtering effect of the grain boundary semiconductive rGO layer with nanometer thickness. The maximum ZT value of 1.51 was achieved for the Yb0.27Co4Sb12/rGO (0.72 vol%) composite at 850 K, outperforming all single-filled skutterudites and their nanocomposites ever reported.

•Ce0.9Fe3Co1Sb12 exhibits “pesting”-like oxidation phenomenon at high temperature.•The highest oxidation rate of Ce0.9Fe3Co1Sb12 appears around 800 K.•Severe periodically oxide layer peeling-off behavior is observed around 800 K.•The co-existence of Fe and Co is responsible for the poor oxidation resistance.Oxidation behavior of p-type filled skutterudite Ce0.9Fe3CoSb12 in air was investigated and the oxidation mechanism was discussed in this study. Ce0.9Fe3CoSb12 exhibits interesting “pesting”-like oxidation phenomenon around 800 K. The bulk sample completely disintegrates into a crowd of plate-like particles under this temperature range after only 24 h exposure in air. However, this abnormal oxidation phenomenon is not observed at temperature below 750 K or above 850 K. This result is consistent with the thermogravimetry and derivative thermogravimetry measurements which show that the oxidation rate for Ce0.9Fe3CoSb12 around 800 K is the highest among 650–900 K. Microstructure observations suggest that this “pesting”-like oxidation is related with the severe periodically oxide layer peeling-off behavior around 800 K, which makes the Ce0.9Fe3CoSb12 samples are easy to be oxidized because the fresh substrate surface is always exposed to high concentration oxygen atmosphere. X-ray diffraction and X-ray photoelectron spectroscopy measurements indicated that in the oxide scale the direct contact of Fe3+-oxide and CoSb2O4 which possess different formation/growth rate and volume expansion coefficient should be responsible for this peculiar oxide layer peeling-off behavior around 800 K. This work can serve as an important reference for the designation of MyFe4−xCoxSb12-based skutterudite thermoelectric device.

Carbon nanotube (CNT)/Polyaniline (PANI) thermoelectric composite nanofibers were prepared by a combination of in situ polymerization and electro-spinning processes. The obtained composite nanofibers were macroscopically aligned, while the CNTs were embedded in the nanofibers and oriented along the fiber axis. Polarized Raman spectra and X-ray diffraction (XRD) studies verified that the PANI backbone chains were orientated along the CNT axis and therefore along the fiber axis due to the strong chemical interactions between PANI and CNTs. The highly ordered arrangement of the PANI backbone chains not only reduces the π–π conjugated defects in the polymer backbone, but also increases the effective degree of electron delocalization and therefore enhances the carrier mobility in PANI, which results in the anisotropic thermoelectric properties especially with more than a doubled improvement of power factor in the orientation direction. This study not only provides an effective route to orient the conducting polymer chains at the molecular level but also demonstrates a possibility to design a conducting polymer with high thermoelectric performance through constructing a highly extended and orderly aligned backbone chain structure by chemical routes.

Electrical and thermal transportation properties of a novel structured 3D CNT network have been systematically investigated. The 3D CNT net work maintains extremely low thermal conductivity of only 0.035 W/(m K) in standard atmosphere at room temperature, which is among the lowest compared with other reported CNT macrostructures. Its electrical transportation could be adjusted through a convenient gas-fuming doping process. By potassium (K) doping, the original p-type CNT network converted to n-type, whereas iodine (I2) doping enhanced its electrical conductivity. The self-sustainable homogeneous network structure of as-fabricated 3D CNT network made it a promising candidate as the template for polymer composition. By in situ nanoscaled composition of 3D CNT network with polyaniline (PANI), the thermoelectric performance of PANI was significantly improved, while the self-sustainable and flexible structure of the 3D CNT network has been retained. It is hoped that as-fabricated 3D CNT network will contribute to the development of low-cost organic thermoelectric area.Keywords: CNT network; nanocompositing; thermal conductivity; thermoelectric;

Silica-based composite coatings with dispersion of glass or alumina granular particles were fabricated on n-type Yb0.3Co4Sb12 and p-type CeFe3CoSb12 skutterudite (SKD) thermoelectric (TE) materials using hybrid silica sol as raw material which was synthesized through hydrolysis and condensation under acid condition process. The dispersion of either glass particles or alumina particles restrained the formation of micro-cracks during solidification due to relaxing tremendous cohesive stress in the gel. The high temperature stability for both the coating materials and the coated skutterudite matrix were investigated through thermal aging at 873 K in vacuum. Inter-diffusion of Sn from glass toward SKDs and Sb from SKDs toward coating layer was observed in the glass particle dispersed silica composite coating, which resulted in the failure of protecting function of the coating layer. In the alumina particle dispersed silica coating, neither diffusion nor reaction between the coating and SKD materials was observed after thermal aging at 873 K. The sublimation of antimony is suppressed in the SKDs overlaid by the alumina dispersed silica composite coating.Highlights► Silica-based composite coatings were fabricated on thermoelectric skutterudites. ► Nano-silica particles were modified by organosilane. ► The addition of glass or alumina particles made the coatings thicker and stronger. ► The sublimation of antimony was prohibited.

p-Type Bi0.45Sb1.55Te3 thermoelectric (TE) thin films have been prepared at room temperature by a magnetron cosputtering process. The effect of postannealing on the microstructure and TE properties of Bi0.45Sb1.55Te3 films has been investigated in the temperature range from room temperature to 350°C. x-Ray diffraction analysis shows that the annealed films have polycrystalline rhombohedral crystal structure, and the average grain size increases from 36 nm to 64 nm with increasing annealing temperature from room temperature to 350°C. Electron probe microanalysis shows that annealing above 250°C can cause Te reevaporation, which induces porous thin films and dramatically affects electrical transport properties of the thin films. TE properties of the films have been investigated at room temperature. The hole concentration shows a trend from descent to ascent and has a minimum value at the annealing temperature of 200°C, while the Seebeck coefficient shows an opposite trend and a maximum value of 245 μV K−1. The electrical resistivity monotonically decreases from 19.8 mΩ cm to 1.4 mΩ cm with increasing annealing temperature. Correspondingly, a maximum value of power factor, 27.4 μW K−2 cm−1, was obtained at the annealing temperature of 250°C.

Previous reports about the thermal conductivities of VO2 showed various temperature dependences across metal-insulator transition (MIT) temperature. In this work, polycrystalline VO2 samples were fabricated by spark plasma sintering of VO2 powder. Temperature dependences of their thermal conductivities were investigated using laser flash technique, and the thermal conductivity showed a significant decrease trend from metal-phase to insulator phase. Electrical transport properties were investigated to confirm both carrier and lattice contribution to the thermal conductivity. It is found that the lattice thermal conductivity decreased significantly across MIT point, which may be caused by soft phonon mode in metal phase of VO2.

Heavily doped compounds Mo3Sb7−xTex (x = 0, 1.0, 1.4, 1.8) were synthesized by solid state reaction and sintered by spark plasma sintering. Both X-ray diffraction and electron probe microanalysis indicated the maximum solubility of Te was around x = 1.8. The trends in the electrical transport properties can generally be understood using a single parabolic band model, which predicts that the extremely high carrier concentration of Mo3Sb7 (∼1022 cm−3) can be reduced to a nearly optimized level (∼2 × 1021 cm−3) for thermoelectric figure of merit (zT) by Te-substitution with x = 1.8. The increased lattice thermal conductivity by Te-doping was found to be due to the decreased Umklapp and electron–phonon scattering, according to a Debye model fitting. The thermoelectric figure of merit (zT) monotonously increased with increasing temperature and reached its highest value of about 0.51 at 850 K for the sample with x = 1.8, making these materials competitive with the state-of-the-art thermoelectric SiGe alloys. Evidence of significant electron–phonon scattering is found in the thermal conductivity.

Lithium intercalation and de-intercalation processes have been used to fabricate bulk Bi2Se0.3Te2.7 with internal nanostructures. The doped Li content can be precisely controlled through this method. It provides a chance to directly optimize electrical properties when preparing nano-structured materials, leading to the optimum carrier concentration for improved thermoelectric figure of merit.

Nanostructured bismuth selenide thin films have been successfully fabricated on a silicon substrate at low temperature by rational design of the precursor solution. Bi2Se3 thin films were constructed of coalesced lamella in the thickness of 50–80 nm. The nucleation and growth process of Bi2Se3 thin films, as well as the influence of solution chemistry on the film structure were investigated in detail. As one of the most promising thermoelectric materials, the thermoelectric properties of the prepared Bi2Se3 thin films were also investigated. The power factor increased with increasing carrier mobility, coming from the enlarged crystallites and enhanced coalesced structure, and reached 1 μW cm−1 K−1.

Hybrid nanocomposites containing carbon nanotubes (CNTs) and ordered polyaniline (PANI) have been prepared through an in situ polymerization reaction using a single-walled nanotube (SWNT) as template and aniline as reactant. TEM, SEM, XRD, and Raman analyses show that the polyaniline grew along the surface of CNTs forming an ordered chain structure during the SWNT-directed polymerization process. The SWNT/PANI nanocomposites show both higher electrical conductivity and Seebeck coefficient as compared to pure PANI, which could be attributed to the enhanced carrier mobility in the ordered chain structures of the PANI. The maximum electrical conductivity and Seebeck coefficient of composites reach 1.25 × 104 S m−1 and 40 μV K−1, respectively, and the maximum power factor is up to 2 × 10−5 W m−1 K−2, more than 2 orders of magnitude higher than the pure polyaniline. This study suggests that constructing highly ordered chain structure is a novel and effective way for improving the thermoelectric properties of conducting polymers.Keywords: carbon nanotubes; hybrid; nanocomposites; polyaniline; thermoelectric

Interfacial structure plays a critical role for the reliability of thermoelectric (TE) device. This study investigated the interfacial evolution behavior and the reliability of CoSb3/Ti/Mo–Cu TE joints during accelerated thermal aging. After thermal aging, three-layer intermetallic compound (IMC) structure was observed at the CoSb3/Ti interface, and EDS analysis confirmed that the IMC layers were composed of TiCoSb, TiSb2 and TiSb. The total thickness of IMC layers increased linearly with the square root of aging time. The growth kinetics of IMC layers was studied. The theoretic life of CoSb3/Ti/Mo–Cu TE device was predicted using the growth rate of IMC layers. The shear strength of aged CoSb3/Ti/Mo–Cu TE joints was also investigated and the results showed the joints had sufficient strength after aging at 575 °C for 720 h. SEM showed that, in all the aged samples, the fracture occurred inside of the IMC layers, while the fracture occurred in the Ti layer or Ti/TiSb interface in the un-aged samples.

Homogeneously dispersed nano-TiO2 was introduced into the matrix of n-type barium-filled cobalt antimony (Ba0.22Co4Sb12) via sol–gel method successfully. The Seebeck coefficients are improved and the lattice thermal conductivities are depressed with a proper addition of nano-TiO2 not more than 0.8 vol.%, while the electrical conductivities nearly remain unchanged. The effects of n-type semiconductive nanoscale inclusions on the electrical and thermal transport characteristics are discussed. Microstructure analyses indicate that the fine nanoinclusions locate on the grain boundaries as well as inside grains, which serve as extra phonon scattering mode and new type energy filtering. As a result, the optimal ZT value was achieved for the sample of 0.4 vol.% TiO2 composition.

The CoSb3/electrode thermoelectric (TE) joints were fabricated with the insertion of Ti foil by spark plasma sintering (SPS). The interfacial microstructure evolution and reliability of joints were investigated during accelerated thermal aging. After thermal aging, a multi-layer structure, which was composed of intermetallic compounds (IMCs), was observed at the CoSb3/Ti interface. The growth kinetics of IMC layers was studied. The theoretic life of CoSb3/electrode joints was predicted using the growth rate of IMC layers. The shear strength of aged CoSb3/electrode joints was also tested and the results showed the joints had sufficient strength for keeping the reliability of TE device. Scanning electron microscopy (SEM) showed that, in the aged sample, all fracture occurred inside of the IMC layers, while the fracture occurred in the Ti layer or Ti/TiSb interface in the un-aged sample. The results of four-probe tests showed that the electrical contact of CoSb3/electrode joint was good.

Polycrystalline skutterudites Ba0.32PdxCo4−xSb12 (nominally x from 0 to 0.1) were synthesized by a melting method and sintered by spark plasma sintering. The electrical and thermal transport properties were investigated from room temperature to 850 K. Pd substitution for Co in Ba0.32Co4Sb12 affected both electron concentration and transport processes, enhanced the thermoelectric power factors and depressed the thermal conductivities. Both the dimensionless figure of merit (ZT) and the thermoelectric compatibility factor (CF) were improved over the whole temperature region.

Nanostructured bismuth selenide thin films have been successfully fabricated on a silicon substrate at low temperature by rational design of the precursor solution. Bi2Se3 thin films were constructed of coalesced lamella in the thickness of 50–80 nm. The nucleation and growth process of Bi2Se3 thin films, as well as the influence of solution chemistry on the film structure were investigated in detail. As one of the most promising thermoelectric materials, the thermoelectric properties of the prepared Bi2Se3 thin films were also investigated. The power factor increased with increasing carrier mobility, coming from the enlarged crystallites and enhanced coalesced structure, and reached 1 μW cm−1 K−1.

Nanostructures and nano-composites have been shown to be effective in depressing the lattice thermal conductivity and improving the performance of thermoelectric materials. However, ZT enhancement by nano-particle dispersion is limited only to a restricted level due to the difficulty in increasing the particle contents while maintaining a uniform and narrow size distribution. In the present work, YbyCo4Sb12-based nano-composites with reduced graphene oxide (rGO) layers of several nanometers intercalated on the grain boundary matrix forming a 3D network have been prepared through a simple in situ reduction approach using graphene oxide (GO) as the precursor. The 3D-rGO network wrapping architecture dramatically reduced the lattice thermal conductivity due to enhanced interparticle and intraparticle phonon scattering effects, and simultaneously enhanced the Seebeck coefficient due to the energy filtering effect of the grain boundary semiconductive rGO layer with nanometer thickness. The maximum ZT value of 1.51 was achieved for the Yb0.27Co4Sb12/rGO (0.72 vol%) composite at 850 K, outperforming all single-filled skutterudites and their nanocomposites ever reported.

Lithium intercalation and de-intercalation processes have been used to fabricate bulk Bi2Se0.3Te2.7 with internal nanostructures. The doped Li content can be precisely controlled through this method. It provides a chance to directly optimize electrical properties when preparing nano-structured materials, leading to the optimum carrier concentration for improved thermoelectric figure of merit.