Search

Toward high-performance thermoelectric energy conversion, the electrons and holes must work jointly like two wheels of a cart: if not longitudinally, then transversely. The bipolar effect — the main performance restriction in the traditional longitudinal thermoelectricity, can be manipulated to be a performance enhancer in the transverse thermoelectricity. Here, we demonstrate this idea in semimetal Mg2Pb. At 30 K, a giant transverse thermoelectric power factor as high as 400 μWcm−1K−2 is achieved, a 3 orders-of-magnitude enhancement than the longitudinal configuration. The resultant specific heat pumping power is ~ 1 Wg−1, higher than those of existing techniques at 10~100 K. A large number of semimetals and narrow-gap semiconductors making poor longitudinal thermoelectrics due to severe bipolar effect are thus revived to fill the conspicuous gap of thermoelectric materials for solid-state applications. Heat pumping is in high demand at cryogenic temperature, but whether thermoelectricity can take on cryogenic heat pumping is an open question. Here, the authors answer this question by leveraging bipolar effect to enhance transverse thermoelectricity in semimetal Mg2Pb for cryogenic heat pumping. [ABSTRACT FROM AUTHOR]

Poly(3-hexylthiophene) (P3HT) films were prepared by using raw P3HT powders with different molecular configurations through solution processing and doped with ferric salt of triflimide anions [Fe(TFSI)], and their thermoelectric (TE) properties were studied. It was found that P3HT with highly regular molecular configuration formed highly ordered chain arrangements in P3HT film (denoted rr-P3HT film), while P3HT with irregular molecular configuration formed random chain arrangements in P3HT film (denoted ra-P3HT film). The ordered chain arrangement in rr-P3HT not only improved the charge carrier mobility but also contributed to produce more carriers, thereby remarkably improving the electrical conductivity. The electrical conductivity of rr-P3HT film was up to 96.1 S/cm, more than two orders of magnitude higher than that of ra-P3HT film. Consequently, the power factor of rr-P3HT film reached 17.10 μW/(m K), about one order of magnitude higher than that of ra-P3HT film and among the best values for pure P3HT TE materials. This study suggests that P3HT with highly regular configuration contributes to form highly ordered molecular chain arrangements, resulting in improved TE properties. [ABSTRACT FROM AUTHOR]

International transport property-measurement round-robins have been conducted by the thermoelectric annex under the International Energy Agency (IEA) Implementing Agreement on Advanced Materials for Transportation (AMT). Two previous round-robins used commercially available bismuth telluride as the test material, with the objectives of understanding measurement issues and developing standard testing procedures. This round-robin extended the measurement temperature range to 773 K. It was designed to meet the increasing demands for reliable transport data for thermoelectric materials used for power-generation applications. Eleven laboratories from six IEA-AMT member countries participated in the study. A half-Heusler ( n-type) material prepared by GMZ Energy was selected for the round-robin. The measured transport properties had a narrower distribution of uncertainty than previous round-robin results. The study intentionally included multiple testing methods and instrument types. Over the full temperature range, the measurement discrepancies for the figure of merit, ZT, in this round-robin were ±11.5 to ±16.4% from the averages. [ABSTRACT FROM AUTHOR]

For bulk thermoelectrics, improvement of the figure of merit ZT to above 2 from the current values of 1.0 to 1.5 would enhance their competitiveness with alternative technologies. In recent years, the most significant improvements in ZT have mainly been due to successful reduction of thermal conductivity. However, thermal conductivity is difficult to measure directly at high temperatures. Combined measurements of thermal diffusivity, specific heat, and mass density are a widely used alternative to direct measurement of thermal conductivity. In this work, thermal conductivity is shown to be the factor in the calculation of ZT with the greatest measurement uncertainty. The International Energy Agency (IEA) group, under the implementing agreement for Advanced Materials for Transportation (AMT), has conducted two international round-robins since 2009. This paper, part II of our report on the international round-robin testing of transport properties of bulk bismuth telluride, focuses on thermal diffusivity, specific heat, and thermal conductivity measurements. [ABSTRACT FROM AUTHOR]

A thermoelectric joint composed of p-type BiSbTe (BiSbTe) material and an antimony (Sb) interlayer was fabricated by spark plasma sintering. The reliability of the thermoelectric joints was investigated using electron probe microanalysis for samples with different accelerated isothermal aging time. After aging for 30 days at 300°C in vacuum, the thickness of the diffusion layer at the BiSbTe/Sb interface was about 30 μm, and SbTe was identified to be the major interfacial compound by element analysis. The contact resistivity was 3 × 10 ohm cm before aging and increased to 8.5 × 10 ohm cm after aging for 30 days at 300°C, an increase associated with the thickness of the interfacial compound. This contact resistivity is very small compared with that of samples with solder alloys as the interlayer. In addition, we have also investigated the interface behavior of Sb layers integrated with n-type BiSeTe (BiSeTe) material, and obtained similar results as for the p-type semiconductor. The present study suggests that Sb may be useful as a new interlayer material for bismuth telluride-based power generation devices. [ABSTRACT FROM AUTHOR]

Recent research and development of high-temperature thermoelectric materials has demonstrated great potential for converting automobile exhaust heat directly into electricity. Thermoelectrics based on classic bismuth telluride have also started to impact the automotive industry by enhancing air-conditioning efficiency and integrated cabin climate control. In addition to engineering challenges of making reliable and efficient devices to withstand thermal and mechanical cycling, the remaining issues in thermoelectric power generation and refrigeration are mostly materials related. The dimensionless figure of merit, ZT, still needs to be improved from the current value of 1.0 to 1.5 to above 2.0 to be competitive with other alternative technologies. In the meantime, the thermoelectric community could greatly benefit from the development of international test standards, improved test methods, and better characterization tools. Internationally, thermoelectrics have been recognized by many countries as a key component for improving energy efficiency. The International Energy Agency (IEA) group under the Implementing Agreement for Advanced Materials for Transportation (AMT) identified thermoelectric materials as an important area in 2009. This paper is part I of the international round-robin testing of transport properties of bulk thermoelectrics. The main foci in part I are the measurement of two electronic transport properties: Seebeck coefficient and electrical resistivity. [ABSTRACT FROM AUTHOR]

The skutterudite/electrode thermoelectric joints were fabricated with the insertion of Ti foil by spark plasma sintering. The interfacial microstructure and reliability of joints were studied during thermal duration test. A multilayer structure, which was composed of intermetallic compounds, was observed at the CoSb3/ Ti interface after thermal aging of 20 days. The interfacial reactions and diffusion path between CoSb3 and Ti were discussed. The contact resistance of CoSb3/electrode junction was measured through four-probe method and the thermal contact resistance was calculated based on multilayer mode measurement. Effects of the contact resistivity on the performance of CoSb3-based device were discussed. [ABSTRACT FROM AUTHOR]

Abstract: In the present study, p-type (Bi,Sb)2Te3 thermoelectric materials have been fabricated through the spark plasma sintering (SPS) method by using powders with various particle sizes. Electrical conductivity (σ), Seebeck coefficient (α), and thermal conductivity (κ) were evaluated in the temperature range of 300–500K. The effect of annealing of the sintered samples on thermoelectric properties was also investigated. The maximum figure of merit ZT (ZT = α2σT/κ) of the sintered samples in the direction perpendicular to the pressing direction (with c-axis preferred orientation) was 1.15 at about 350K, corresponding to be about 115% of that of the zone-melted ingot with the same composition in the same crystallographic orientation; while the bending strength was highly improved from about 10 to 80MPa. [Copyright &y& Elsevier]

Ca3-xDyxCo4O9+δ (x=0, 0.15, 0.3 and 0.45) polycrystalline samples have been prepared using a sol–gel process followed by SPS sintering. Their thermoelectric properties have been carefully studied from room temperature to 1000 K. The substitution of Dy for Ca results in increasing of both thermopower and electrical resistivity, which could be attributed to the decreasing of carrier concentrations. The Dy substituted samples (x=0, 0.3) have lower thermal conductivity than Ca3Co4O9+δ due to their lower electronic and lattice thermal conductivities. The dimensionless figure of merit ZT reaches 0.27 at 1000 K for the sample of Ca2.7Dy0.3Co4O9+δ. [Copyright &y& Elsevier]

The fundamental understanding of the influence of molecular structure on the carrier transport properties in the field of organic thermoelectrics (OTEs) is a big challenge since the carrier transport behavior in conducting polymers reveals average properties contributed from all carrier transport channels, including those through intra-chain, inter-chain, inter-grain, and hopping between disordered localized sites. Here, combining molecular dynamics simulations and experiments, we investigated the carrier transport properties of doped highly oriented poly(3-hexylthiophene) (P3HT) films with different side-chain regioregularity. It is demonstrated that the substitution of side chains can not only take effect on the carrier transport edge, but also on the dimensionality of the transport paths and as a result, on the carrier mobility. Conductive atomic force microscopy (C-AFM) study as well as temperature-dependent measurements of the electrical conductivity clearly showed ordered local current paths in the regular side chain P3HT films, while random paths prevailed in the irregular sample. Regular side chain substitution can be activated more easily and favors one-dimensional transport along the backbone chain direction, while the irregular sample presents the three-dimensional electron hopping behavior. As a consequence, the regular side chain P3HT samples demonstrated high carrier mobility of 2.9 ± 0.3 cm2/V·s, which is more than one order of magnitude higher than that in irregular side chain P3HT films, resulting in a maximum thermoelectric (TE) power factor of 39.1 ± 2.5 μW/mK2 at room temperature. These findings would formulate design rules for organic semiconductors based on these complex systems, and especially assist in the design of high performance OTE polymers. [ABSTRACT FROM AUTHOR]

The low weighted carrier mobility has long been considered to be the key challenge for improvement of thermoelectric (TE) performance in BiTeI. The Rashba-effect-induced two-dimensional density of states in this bulk semiconductor is beneficial for thermopower enhancement, which makes it a prospective compound for TE applications. In this report, we show that intercalation of minor Cu-dopants can substantially alter the equilibria of defect reactions, selectively mediate the donor-acceptor compensation, and tune the defect concentration in the carrier conductive network. Consequently, the potential fluctuations responsible for electron scattering are reduced and the carrier mobility in BiTeI can be enhanced by a factor of two to three between 10 K and 300 K. The carrier concentration can also be optimized by tuning the Te/I composition ratio, leading to higher thermopower in this Rashba system. Cu-intercalation in BiTeI gives rise to higher power factor, slightly lower lattice thermal conductivity, and consequently improved figure of merit. Compared with pristine BiTe0.98I1.02, the TE performance in Cu0.05BiTeI reveals a 150% and 20% enhancement at 300 and 520 K, respectively. These results demonstrate that defect equilibria mediated by selective doping in complex TE and energy materials could be an effective approach to carrier mobility and performance optimization. [ABSTRACT FROM AUTHOR]

Abstract: Polycrystalline (Fe1−x Co x )2VAl (0≤ x ≤0.5) alloys were synthesized through a combined process of arc-melting, annealing and sintering. Thermoelectric properties were investigated from 300 to 850K. Stoichiometric Fe2VAl exhibited p-type conduction, however, it shifted to n-type with slight Co substitution for Fe site. After Co substitution, the electrical conductivity and absolute value of Seebeck coefficient at room temperature increased from 1.2×105 to 5×105 Sm−1 and 30 to 112μV/K, respectively. Meanwhile, the lattice thermal conductivity dropped from 23 to 6W/mK because of the introduction of extra phonon scattering due to alloying effect. The highest dimensionless figure of merit ZT value for (Fe0.7Co0.3)2VAl is 0.09 at 650K. [Copyright &y& Elsevier]

Abstract: TiCoSb-based half-Heusler compounds were prepared by solid-state reaction and their thermoelectric properties were studied. The un-doped TiCoSb compound shows n-type conduction and demonstrates high Seebeck coefficient at high temperatures. The partial substitution of Co by Ni caused great increase in electron concentration and electrical conductivity, while the Seebeck coefficient and the thermal conductivity showed little change when Ni content (x) is below 5%. A maximum power factor of 16μW/K2 cm and a maximum dimensionless figure of merit (ZT) of 0.27 have been obtained for the composition of TiCo0.95Ni0.05Sb at 900K. [Copyright &y& Elsevier]

The intrinsic ultra-low lattice thermal conductivity ( κ L ) of high performance thermoelectric Cu 2 S may arise from its multiform complex crystal structures while the further sharp drop of κ L observed at elevating temperatures close to the phase transition to hexagonal high-chalcocite is of considerable interest. In this work, in-depth in-situ transmission electron microscopy investigations have been performed by either heating or electron irradiation to analyze the structural evolution of Cu 2 S. Multiple including new phase structures have been observed in Cu 2 S. Upon increasing the temperature, drastic structural changes took place including phase transition among different phases, formation of an intermediate phase, switching of crystal orientation and inverse phase transitions, which indicate multiple forms of Cu ordering with high temperature-sensitivity. Such tremendous structural evolution along with formation and movement of phase boundaries are believed to strongly scatter the phonons. [ABSTRACT FROM AUTHOR]

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/Ce y Fe x Co 4 -x Sb 12 , 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/CoSb 3 joints, the growth of the interfacial IMC layers in the p-type Ti/Ce y Fe x Co 4 -x Sb 12 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. [ABSTRACT FROM AUTHOR]

Highly (001)-oriented Cu 2- y Se thin films with tunable thermoelectric performances have been grown by pulsed laser deposition. By using targets with different Cu/Se ratios that further determines the copper deficiency of as-grown films, the carrier concentrations of as-grown films are tuned within a broad range from 10 18 to 10 21 cm −3 . The optimum performance is observed at carrier concentration ~1.58×10 20 cm −3 . The distinct properties of Cu 2- y Se thin films with nearly ideal chemical stoichiometric ratio are observed. In addition, a weak change in the electrical transport during the second-order phase transition was observed in the thin films due to the anisotropic structure of the Cu 2- y Se. [ABSTRACT FROM AUTHOR]

Abstract: P-type compounds Cu2Ga x Sn1 − x Se3 (x =0.025, 0.05, 0.075) with a diamond-like structure were consolidated using hot pressing sintering (HP) and spark plasma sintering (SPS) techniques. High-temperature thermoelectric properties as well as low-temperature Hall data are reported. Microstructural analysis shows that the distribution of Ga is homogeneous in the samples sintered by HP but inhomogeneous in the samples sintered by SPS, even with an electrically isolating and thermally conducting BN layer during the sintering. The Seebeck coefficients of the samples sintered by HP and SPS show similar dependence on the carrier concentration and are insensitive to the composition inhomogeneity. In contrast, the composition inhomogeneity results in lower carrier mobility and thus lower electrical conductivity in the samples sintered by SPS than those sintered by HP. Lattice thermal conductivity is further reduced through Ga doping; however, this effect is weakened by the inhomogeneous distribution of Ga. Due to their larger carrier mobility and lower lattice thermal conductivity, the samples sintered by HP exhibit 15–35% higher thermoelectric figure of merits (ZT) than those SPS samples with a high Ga doping level and without the coated BN layer, in which the composition homogeneity is worse. A ZT value of 0.43 is obtained for the HP Cu2Ga0.075Sn0.925Se3 sample at 700K. [Copyright &y& Elsevier]

Abstract: Composite glass coating was fabricated on thermoelectric (TE) skutterudite (SKD) materials by using the dispersion system of modified nano-silica aerogel and micro glass powder. The organosilane was used as the binder in the slurry-blade process and isolated pores in micrometer size are formed due to the decomposition of polymerization product derived from organosilane and nano-silica aerogel particles at high temperature. These pores can effectively decrease the thermal conductivity of coating layer. The composite glass coating shows good stability at 823K in air. However, significant microstructural changes in the coating were observed after aging at 923K, at which the isolated micro-sized pores grew to sub-millimeter pores while the continuous composite glass coating was still maintained. The glass coated skutterudite materials show good oxidation resistance, i. e. no oxides are observed even after heating at 923K in ambient air. The present glass-coating approach is expected to be applied in fabricating skutterudite-based thermoelectric devices. [Copyright &y& Elsevier]

Abstract: Reversible electrochemical lithium intercalation/deintercalation reactions have been applied to integrally modify the thermoelectric performance of Bi2Se0.3Te2.7 compounds. The carrier concentration of Bi2Se0.3Te2.7 has been precisely controlled experimentally through the adjustment of the residual lithium content. The variations of electronic transport properties of Li x Bi2Se0.3Te2.7, including electrical conductivity, Seebeck coefficient and carrier concentration, as a function of the intercalated lithium amount (x), show a special non-monotonic P-type to N-type doping trend. This variation is caused by the competition of two mechanisms controlling the carrier concentration: (i) decreasing of the Se/Te ratio through a Li–Se reaction that reduces the amount of the TeBi or SeBi antisite defects and therefore the electron concentration; (ii) n-type doping by lithium that provides the electron. Moreover, the nanoparticle exfoliation during lithium intercalation and deintercalation reactions leads to the formation of an internal nanocrystalline composite structure which effectively reduces the lattice thermal conductivity. Through an integral modification of both electrical and thermal transport properties, the maximum ZT value of Li x Bi2Se0.3Te2.7 was improved by nearly 25%. [Copyright &y& Elsevier]

Abstract: Filling two or multiple atoms into the cages of a skutterudite crystal is a successful way to enhance the thermoelectric figure of merit in n-type partially filled skutterudites. Here we use La–Yb double filling to improve the thermoelectric performance of p-type skutterudites Yb x La y Fe2.7Co1.3Sb12 (x + y = 0.85, x = 0, 0.09, 0.17, 0.25, 0.34, 0.42, 0.85). The samples are synthesized by directly melting the raw materials followed by a long time annealing process. X-ray diffraction measurement confirms the skutterudite structure of the samples. By increasing the filling fraction of Yb, the lattice thermal conductivity is gradually depressed and reaches the lowest value of 0.9 W m−1 K−1 at high temperatures. At the same time, the Seebeck coefficient decreases while the electrical conductivity increases. The maximum power factor around 27 μW cm−1 K−2 for samples Yb0.17La0.68Fe2.7Co1.3Sb12 and Yb0.25La0.60Fe2.7Co1.3Sb12 is achieved at 600 K and 700 K, respectively. Sample Yb0.25La0.60Fe2.7Co1.3Sb12 has the maximum thermoelectric figure of merit 0.99 at 700 K, which is among the highest values among p-type filled skutterudites. [Copyright &y& Elsevier]

Abstract: Ab-initio calculations of the resonant modes and frequencies for a number of possible fillers in p-type RFe3CoSb12 and RFe4Sb12 were carried out. The results indicate that, although the exact values of fillers’ resonant frequencies in p-type skutterudites are somewhat different from those in n-type Co-based skutterudites, the Einstein-like resonant modes of the fillers are similar to those in n-type materials. Experimentally, several pairs of the fillers were selected and double-filled p-type skutterudite compounds RxMy Fe3CoSb12 (R, M = Ba, Ce, Nd, and Yb) were successfully synthesized. The reduction in the lattice thermal conductivity was realized by extending the range of resonant frequencies. As a result, enhanced ZT values above unity were achieved in these double-filled p-type skutterudites. [Copyright &y& Elsevier]

Abstract: n-type filled skutterudite Yb0.26Co4Sb12 composites in which p-type GaSb nanostructured inclusions (5–20nm) were dispersed were fabricated by an in situ method involving the introduction of metastable void-filling impurity Ga and enrichment of Sb in the synthesis procedure. With the homogeneously dispersed GaSb nanoinclusions, which probably result from the scattering of low-energy electrons by the GaSb-related boundary energy barriers, the power factor is enhanced due to the significant enhancement of the Seebeck coefficient. The total thermal conductivity was decreased with the depression of electronic thermal conductivity. As a result, the dimensionless thermoelectric figure of merit ZT value was improved over a wide working temperature range of 300–850K, and the expected optimal thermal-electric conversion efficiency ηopt 15.0% was obtained for the Yb0.26Co4Sb12/0.2GaSb nanocomposite. [Copyright &y& Elsevier]

Abstract: Ba8Ga16Ge30 clathrate crystals were grown by Czochralski method. The influence of growth condition on the composition and thermoelectric property of the obtained crystals was investigated. The lower pulling rate and higher pressure resulted in a larger Ga/Ge ratio in the grown crystals, and the elemental composition was homogeneous. The sample with a higher Ga/Ge ratio showed the larger electrical conductivity than the sample with a lower Ga/Ge ratio, but the Seebeck coefficient was not sensitive to the Ga/Ge ratio, which led to a larger power factor. The higher Ga/Ge ratio also resulted in a lower lattice thermal conductivity and as a result led to a higher ZT value, which was measured to be 0.93 at 850K and could reach 1.3 as extrapolated to 1000K. [Copyright &y& Elsevier]

Abstract: Ge-doped TiCoSb-based half-Heusler compounds, TiCoGe x Sb1−x (x =0–0.2), were prepared by arc melting and spark plasma sintering (SPS). The electrical conductivity, Seebeck coefficient and thermal conductivity were measured from room temperature to 850K. The Seebeck coefficient changed from negative to positive by Ge doping, and the electrical conductivity increased with increasing Ge content. The lattice thermal conductivity for TiCoGe x Sb1−x is considerably reduced, because the point defects induced by Ge substitution for Sb intensively scatter the thermal phonons. The mass fluctuation part Γ mass and strain field fluctuation part Γ stra were estimated, and it was found that the reduction of lattice thermal conductivity is mainly attributed to the strain field fluctuation. Ge doping improved the thermoelectric figure of merit ZT of TiCoSb and the maximum ZT value of 0.16 was obtained for TiCoGe0.15Sb0.85 sample at 850K. [Copyright &y& Elsevier]

Abstract: Sr0.29CoO2 crystals have been successfully prepared by a low temperature ion exchange from layered Na x CoO2, and it is isomorphic to its precursor Na x CoO2 with a layered triangular structure. The thermoelectric properties of Sr0.29CoO2 crystal were measured in the temperature of 2.5–773K. A transition has been exhibited in the resistivity, Seebeck coefficient and carrier concentration at approximately 75K. The resistivity, Seebeck coefficient and power factor increase with increasing temperature, and reach the values of 2.78mΩcm, 171μV/K and 7.1×10−4 Wm−1 K−2 at 773K, respectively. [Copyright &y& Elsevier]