Thermoelectric Micromodules for Small Heat Flows

Single phase <TEX>$Zn_4Sb_3$</TEX> with <TEX>$98.5\%$</TEX> of theoretical density was successfully produced by direct hot pressing of elemental powders containing <TEX>$1.2 at\%$</TEX> excess Zn for compensating the evaporation during the process. Temperature dependences of thermoelectric properties were investigated from 4 K to 300 K. Seebeck coefficient, electrical conductivity, thermal conductivity as well as thermoelectric figure of merit showed the discontinuity in variation at 242K, indicating the <TEX>$\alpha-\beta$</TEX>, phase transformation. Interestingly, it was found that lattice thermal conductivity by phonons is dominant in total thermal conductivity of <TEX>$\alpha-\beta$</TEX>. Therefore, it is expected that thermoelectric properties can be improved by reduction of lattice thermal conductivity inducing lattice scattering centers by doping and solid solution.

The optimal conditions for synthesizing a pure chalcopyrite CuFeS2 phase were thoroughly investigated through the combination of mechanical alloying (MA) and hot pressing (HP) processes. The MA process was performed at a rotational speed of 350 rpm for durations ranging from 6 to 24 h under an Ar atmosphere, ensuring proper mixing and alloying of the starting materials. Afterward, MA-synthesized chalcopyrite powder was subjected to HP at temperatures between 723 K and 823 K under a pressure of 70 MPa for 2 h in a vacuum. This approach aimed to achieve phase consolidation and densification. A thermal analysis via differential scanning calorimetry (DSC) revealed distinct endothermic peaks at the range of 740-749 K and 1169-1170 K, corresponding to the synthesis of the chalcopyrite phase and its melting point, respectively. An X-ray diffraction (XRD) analysis confirmed the successful synthesis of the tetragonal chalcopyrite phase across all samples. However, a minor secondary phase, identified as Cu1.1Fe1.1S2 (talnakhite), was observed in the sample hot-pressed at the highest temperature of 823 K. This secondary phase could result from slight compositional deviations or local phase transformations at elevated temperatures. The thermoelectric properties of the CuFeS2 samples were evaluated as a function of the HP temperatures. As the HP temperature increased, the electrical conductivity exhibited a corresponding rise, likely due to enhanced densification and reduced grain boundary resistance. However, this increase in electrical conductivity was accompanied by a decrease in both the Seebeck coefficient and thermal conductivity. The reduction in the Seebeck coefficient could be attributed to the higher carrier concentration resulting from improved electrical conductivity, while the decrease in thermal conductivity was likely due to reduced phonon scattering facilitated by the grain boundaries. Among the samples, the one that was hot-pressed at 773 K displayed the most favorable thermoelectric performance. It achieved the highest power factor of 0.81 mWm-1K-1 at 523 K, indicating a good balance between the Seebeck coefficient and electrical conductivity. Additionally, this sample achieved a maximum figure-of-merit (ZT) of 0.32 at 723 K, a notable value for chalcopyrite-based thermoelectric materials, indicating its potential for mid-range temperature applications.

A method for the synthesis of ZnSb and β-Zn4Sb3 compounds has been developed, using rapid crystallization of the melt on a rotating disk (melt spinning) to obtain powders. The microstructure and thermoelectric properties of samples obtained by hot pressing of powders prepared by this method are investigated. The microstructure, chips, and composition of the samples were studied using optical and scanning electron microscopes. The nanoscale structure of the grains of the materials obtained has been established. Thermoelectric parameters: Seebeck coefficient, electrical conductivity and thermal conductivity, in the temperature range 300 − 700 K were measured. The coefficient of thermoelectric figure of merit is calculated. The hot-pressed β-Zn4Sb3 samples from granules, prepared by melt spinning had the highest figure of merit ZТ = 1,0 at 600 K.

The single-phase InSb compounds have been prepared by novel melt spinning (MS) technique combined with spark plasma sintering (SPS) method, and the effects of melt spinning process on their microstructure and thermoelectric transport properties have been investigated. The results show that the free surface of ribbon obtained by MS consists of cubic grains with the size of 03—20 μm, and the contact surface of ribbon obtained by MS amorphous-like phases or finer nanostructures have formed, and after SPS the highly dense bulk material with lots of fine layered nanostructure has been obtained of about 40 nm in dimensions. By comparing the bulk InSb material prepared by melting method combined with SPS (Melt+SPS sample) with the bulk InSb material obtained by melting method combined with MS and SPS (Melt+MS+SPS sample)， we see that the MS process leads to a slight decrease in electrical conductivity, and an obvious increase in Seebeck coefficient, as well as a remarkable decrease in thermal conductivity and lattice thermal conductivity for bulk InSb in the testing temperature range of 300—700 K. At 300 K and 700 K, the lattice thermal conductivities of Melt+SPS sample and Melt+MS+SPS sample decrease by the scopes of 106% and 1664%, respectively. As a result, the maximum dimensionless figure of merit ZT of 0.49 is obtained at 700 K for the Melt+MS+SPS sample. Compared with that of Melt+SPS sample, it is increased by 29% at the same temperature.

Preparation and thermoelectric properties of La filled skutterudites by mechanical alloying and hot pressing

Thermoelectric generator is expected as an independent source, an energy converter for co-generation with Refuse Derived Fuel (RDF) and so on. Thermoelectric materials were required high Seebeck coefficient, low electrical resistivity and low thermal conductivity. Thermoelectric oxides are suitable at the high temperature range because of chemical stability. Authors focus attention on Aurivillius compounds. The Aurivillius compounds consist of Perovskite layers and Bi-O layers. It is expected that nano-layered structure shows high Seebeck coefficient due to the quantum confinement of electron in Perovskite layers. It was reported that the Seebeck coefficient of Aurivillius phase Bi2VO5.5 was high value of -28.3 mVK-1 at 1010 K, and the electrical resistivity of the one was also high value of 0.033 Ωm at 1010 K. We investigated about element substitution effects at V site on thermoelectric properties. Bi2V1-xMxO5.5 (M=Cu, Cr, x=0, 0.05, 0.1, 0.2) were prepared by solid-state reaction and hot pressing. From the results of the electrical resistivities and the Seebeck coefficients, Cu and Cr behaved as acceptor to Bi2VO5.5. Cr was effective for reducing the thermal conductivity of Bi2VO5.5. The maximum value of dimensionless figure of merit for Bi2VO5.5 was 0.06 at 910 K.

Mn-doped FeVSb half-Heusler alloys were synthesized via a mechanical alloying process and consolidated by vacuum hot pressing. The microstructure and phase transformation of all the samples were examined by XRD and SEM. Thermoelectric properties such as the Seebeck coefficient, electrical conductivity and thermal conductivity were investigated in the moderate temperature range from 300 to 973 K. The negative value of both the Seebeck and Hall coefficients confirms the presence of n-type conductivity. The Seebeck coefficient increased with an increasing doping amount, but the electrical conductivity decreased, owing to decreasing carrier concentration. The thermal conductivity found in this experiment was quite high, possibly due to bipolar diffusion of the electronic band energy. The maximum value of the dimensionless figure of merit was achieved using a relatively high value of the Seebeck coefficient and a significantly higher value of electrical conductivity. The maximum value of ZT was observed for Fe0.996Mn0.004VSb at 468 K.

Thermoelectric properties of NaCo 2O 4

Famatinite Cu3SbS4 is considered to be a promising p-type thermoelectric material that consists of earth-abundant and nontoxic elements. In this study, this material was prepared by using mechanical alloying (MA) as a solid-state route and was consolidated using hot pressing (HP). The effects of MA–HP conditions on the phase synthesis (transformation) and thermoelectric properties were examined. Thermogravimetric and differential scanning calorimetric analyses confirmed that severe mass loss and endothermic reactions occurred at temperatures above approximately 750 K. This was possibly due to the melting of famatinite and the volatilization of the constituent elements. All the specimens exhibited p-type conduction and nondegenerate semiconductor characteristics. It was determined that the electrical and thermal conductivities decreased with an increase in the HP temperature, while the Seebeck coefficient increased. The thermal conductivity was lower than 0.74 Wm−1 K−1 at 623 K, and there was a small contribution of the electronic thermal conductivity to the thermal conductivity due to the intrinsically low electrical conductivity. The dimensionless figure of merit increased with increasing temperature, and the highest value was 0.14 at 623 K.

In this study, tetrahedrite compounds doped with Sn were prepared by mechanical alloying and hot pressing, and their charge transport and thermoelectric properties were analyzed. X-ray diffraction analysis revealed that both the synthetic powders and sintered bodies were synthesized as a single tetrahedrite phase without secondary phases. Densely sintered specimens were obtained with relatively high densities of 99.5%-100.0% of the theoretical density, and the component elements were distributed uniformly. Sn was successfully substituted at the Sb site, and the lattice constant increased from 1.0348 to 1.0364 nm. Positive signs of the Hall and Seebeck coefficients confirmed that the Sn-doped tetrahedrites were p-type semiconductors. The carrier concentration decreased from 1.28 × 1019 to 1.57 × 1018 cm-3 as the Sn content decreased because excess electrons were supplied by doping with Sn4+ at the Sb3+ site of the tetrahedrite. The Seebeck coefficient increased with increasing Sn content, and Cu12Sb3.6Sn0.4S13 exhibited maximum values of 238-270 µVK-1 at temperatures of 323-723 K. However, the electrical conductivity decreased as the amount of Sn doping increased. Thus, Cu12Sb3.9Sn0.1S13 exhibited the highest electrical conductivity of (2.24-2.40) × 104 Sm-1 at temperatures of 323-723 K. A maximum power factor of 0.73 mWm-1K-2 was achieved at 723 K for Cu12Sb3.9Sn0.1S13. Sn substitution reduced both the electronic and lattice thermal conductivities. The lowest thermal conductivity of 0.49-0.60 Wm-1K-1 was obtained at temperatures of 323-723 K for Cu12Sb3.6Sn0.4S13, where the lattice thermal conductivity was dominant at 0.49-0.57 Wm-1K-1. As a result, a maximum dimensionless figure of merit of 0.66 was achieved at 723 K for Cu12Sb3.9Sn0.1S13.

We report on the preparation and thermoelectric properties of the quaternary system AgPbmBiTe2+m (Bismuth-Lead-Silver-Tellurium, BLST-m) that were nanostructured by mechanical alloying. Nanopowders of various compositions were compacted by three different methods: cold pressing/annealing, hot pressing, and short term sintering. The products are compared with respect to microstructure and sample density. The thermoelectric properties were measured: thermal conductivity in the temperature range from 300 K to 800 K and electrical conductivity and Seebeck coefficient between 100 K and 800 K. The compacting method and the composition had a substantial impact on carrier concentration and mobility as well as on the thermoelectric parameters. Room temperature Hall measurements yielded carrier concentrations in the order of 1019 cm−3, slightly increasing with increasing content of the additive silver bismuth telluride to the lead telluride base. ZT values close to the ones of bulk samples were achieved. X-ray diffraction and transmission electron microscopy (TEM) showed macroscopically homogeneous distributions of the constituting elements inside the nanopowders ensembles, indicating a solid solution. However, high resolution transmission electron microscopy (HRTEM) revealed disorder on the nanoscale inside individual nanopowders grains.

The Fe-Si compound with atomic ratio Fe:Si = 1:2.0, 1:2.3 and 1:2.5 were synthesized by thermal method. All samples were annealed at 820 ℃ or 950℃ for 3,6 and 12 h in Ar atmosphere. Longer anneal time increased Seebeck coefficient but ε-FeSi phase still presented in FeSi [subscript 2.0] and FeSi [subscript 2.3] sample. Based on standard deviation of Seebeck coefficient measured by hot-probe technique, it would imply that non-homogenity in FeSi [subscript 2.0] and FeSi [subscript 2.3] samples was higher than that in FeSi [subscript 2.5] sample. Influences of excess Si on thermoelectric properties were investigated. It was shown that the Seebeck coefficient increased and thermal conductivity decreased due to the phonon and carriers scattering by exess Si, especially at lower temperatures than 400℃. Thermoelectric properties, Seebeck coefficient, thermal conductivity and electrical conductivity were measured as a function of temperature (30℃ &lt; T&lt;700℃). The dispersion of excess Si in FeSi [subscript 2.3] and FeSi [subscript 2.5] samples increased the Seebeck coefficient and decreased the thermal conductivity while the present of metallic ε-phase in in FeSi [subscript 2.0] and in FeSi [subscript 2.3] samples increased the electrical conductivity. The highest dimensionless figure of merit ZT for in FeSi [subscript 2.0] and in FeSi [subscript 2.5] were annealed at 950℃ for 12 h were 0.44, 0.48 and 0.20 at 700℃, respectively.

MXenes, two-dimensional transition metal carbides/nitrides, have gained substantial interest owing to their distinctive properties. This study utilizes density functional theory (DFT) calculations to study the electronic, magnetic, and thermoelectric properties of pristine, molybdenum (Mo), and Te-doped Nb3C2 monolayer MXenes. Both doped structures exhibit metallic characteristics with indirect band gaps, as revealed by band structure and density of states (DOS) analysis. This fulfills a crucial requirement for electrode applications in lithium-ion batteries (LIBs). Pristine Nb3C2 displays diamagnetic, while Mo doping induces ferromagnetism and Te doping leads to ferrimagnetism behavior. Notably, doping significantly impacts electronic and thermoelectric properties, Seebeck coefficient, electrical conductivity, and thermal conductivity, which demonstrably depend on the chosen structure. Te-doped Nb3C2 consistently exhibits a larger bandgap, less Seebeck coefficient, and lower thermal conductivity compared to Mo-doped Nb3C2 attributed to a narrow bandgap, exceptionally high Seebeck coefficient, and high thermal and electrical conductivity. Additionally, positive open-circuit voltage (OCV) values suggest favorable lithium-ion intercalation for all materials. Theoretical capacities of 592, 745, and 668 mAh/g are computed for pristine, Mo-doped, and Te-doped Nb3C2, respectively, comparable to reported values for pristine V3C2 (606.42 mAh/g). These results suggest that Mo- and Te-doped Nb3C2 MXenes exhibit potential as anode materials for LIBs due to their improved electronic conductivity, reduced operating voltage, and comparable theoretical lithium storage capacity.

Single crystalline Bi-Te thermoelectric materials have a rhombohedral structure with van der Waals bonding along c-axis direction. This structure gives intrinsic anisotropy in thermoelectric properties like electrical resistivity, thermal conductivity, Seebeck coefficient, as well as electron/hall mobility. Recent research works report that nano structures in polycrystalline thermoelectric materials improve thermoelectric performance by reducing thermal conductivity or by enhancing Seebeck coefficient. We investigated the effect of adjusting crystal orientation in the microstructures containing sub micro-sized grains on the thermoelectric properties for polycrystalline Bi-Te materials. Bi-Te powder, prepared through the conventional pulverization process, was sufficiently dispersed in an appropriate solvent, and then was formed into c-axis aligned green body under a designated high magnetic field. The green bodies were sintered with spark-plasma-sintering machine. The degree of crystal alignment of sintered bodies was examined with the electron-back-scatter-diffraction SEM and the X-ray diffraction patterns. It was observed that for p-type thermoelectric Bi-Te materials, aligning crystal orientation properly made electrical resistivity in the preferred direction decreased with keeping Seebeck coefficient and thermal conductivity remained unchanged.

Solid solution Mg2Si1−xGex for various concentration of germanium, x, is successfully prepared in single phase by the bulk mechanical alloying (BMA) and the hot pressing (HP). Both BMA and HP process conditions were optimized to yield high dense samples with fine, homogeneous microstructure. The electrical conductivity, the Seebeck coefficient and the thermal conductivity are measured from room temperature up to about 700 K. The Seebeck coefficient is much sensitive to the germanium content, x in Mg2Si1−xGex. The pn-transition takes place at x=0.35 where the Seebeck coefficient drastically changes its sign. The measured band gap of Mg2Si1−xGex decreases with x from 0.71 to 0.54 eV. The figure of merit at 613 K of Mg2Si0.6Ge0.4 reaches 0.34×10−3 K−1 in the case of BMA for N=600 and HP at 773 K by 1 GPa for 3.6 ks.