Improvement Of ZT Research Articles

Grain recovery facilitated low-angle grain boundaries and texture for high-performance BiSbTe alloys

Low-angle grain boundaries (LAGBs) bring an effective scattering of phonons while maintaining a weak effect on charge carrier transport, which could be utilized for enhancing the thermoelectric performance of solid materials. In the Bi2Te3-based materials fabricated by hot extrusion (HE) technique, however, the formation of the LAGBs is evitably accompanied by severe recrystallization, resulting in texture loss and hindering further zT improvement. Here, we demonstrate a feasible strategy utilizing the grain recovery to maintain dense LAGBs with high grain orientation by the optimized HE technique. As a result, a low thermal conductivity of about 1 W m−1 K−1 and a high power factor of 4.2 mW m−1 K−2 are achieved at 300 K for p-type Bi0.5Sb1.5Te3 alloys, leading to a high room-temperature zT of 1.3. Further, with a decent flexural strength of 25.3 MPa, a 23-pair TE cooling module with the dice dimensions of 0.63 × 0.63 × 1.00 mm3 is assembled, which exhibits a maximum temperature difference of 87.8 K at a hot-side temperature Th of 350 K. These results highlight the important role of grain-recovery manipulation in simultaneously optimizing the thermal and electrical properties toward high-performance Bi2Te3-based TE materials.

Magnetic field-dependent thermopower: Insights into spin and quantum interactions

This study explores the impact of external magnetic fields on thermoelectric properties, focusing on the interplay of spin and quantum effects. Using gadolinium (Gd) as a case study, we observed anomalous magneto-thermopower trends, with a reduction in thermopower at ∼35 K and an enhancement at TC ≈ 293 K under high magnetic fields. Comprehensive temperature and field-dependent measurements, including specific heat capacity, magnetic susceptibility, and Hall effect, were performed to uncover the underlying mechanisms. We derived a relation for the total thermopower of an uncompensated ferromagnetic metal and calculated multi-band carrier characteristics, such as concentration and mobility, using the maximum entropy principle. Our findings reveal a ∼70 % suppression of the magnetic contribution to specific heat capacity under a 12 T field and a positive magnon-drag contribution to the total thermopower. Field-dependent Hall measurements indicate that the anomalous Hall effect is dominated by intrinsic contributions from Berry curvature. Additionally, transverse magnetoresistance data suggest anisotropic Fermi surfaces, domain movement, suppression of spin-flip effects, and Fermi surface modifications. First-principles calculations based on Density Functional Theory (DFT) further support these findings. These calculations reveal significant Berry curvature contributions, leading to an anomalous Hall conductivity of approximately 1260 S/cm at the Fermi level. The enhancement of thermopower near TC is primarily attributed to the suppression of magnon-drag and the imbalance in carrier mobility and relaxation times, driven by spin and quantum effects. These combined effects result in a ∼50 % increase in thermopower and a ∼150 % improvement in zT at 12 T. The notable peak in zT at cryogenic temperatures highlights a potential pathway for designing efficient thermoelectric materials for cryogenic cooling applications. Our results demonstrate the significance of field-dependent spin and quantum effects in enhancing thermoelectric performance, offering new directions for thermoelectric research and material design.

Enhanced thermoelectric performance of Cr-doped ZnSb through lattice thermal conductivity reduction below the phonon glass limit

Zinc antimonide (ZnSb) is a promising thermoelectric material due to its economic viability, elemental abundance, and superior phase stability compared to other Zn-Sb compounds. However, its widespread application is hindered by a low figure-of-merit (zT). Extensive research has explored doping, alloying, and nanostructuring to improve zT. This study investigates the impact of Cr doping in CryZn1-ySb (y = 0.0 – 0.03). Cr doping with y = 0.01 significantly reduces lattice thermal conductivity below the phonon glass limit. Debye-Callaway model calculations suggest a combined effect of point defects and decreased grain size caused by Cr incorporation. This reduction translates to enhanced thermoelectric performance, with the y = 0.01 sample exhibiting a 70 % improvement in zT at 673 K, reaching ∼0.67. Exceeding the Cr substitution limit leads to the formation of a detrimental secondary CrSb2 phase, increasing thermal conductivity. The Single Parabolic Band model calculations predict further zT enhancement in y = 0.01 sample to ∼0.2 at 300 K through optimized carrier concentration (307 % improvement in zT). These findings demonstrate the potential for significant zT improvement in ZnSb via defect engineering and carrier concentration tuning.

Constructing Coated Grain Nanocomposites and Intracrystalline Precipitates to Simultaneously Improve the Thermoelectric and Mechanical Properties of SnTe by MgB2 and Sb Co‐Doping

AbstractThermoelectric materials should be highly efficient and mechanically robust to satisfy the requirements of engineering applications. Herein, an integrated optimization strategy to improve both the thermoelectric performance and mechanical strength of SnTe is proposed. First, grain boundary engineering is applied to SnTe via MgB2 doping. The decomposition of MgB2 results in the Mg‐substituted SnTe solid solution and a special “core–shell” structure of Mg‐B compounds coated SnTe grain remarkably reducing the lattice thermal conductivity. Subsequently, trivalent Sb atoms are introduced to tune the carrier concentration and optimize the electrical performance of the MgB2‐doped sample. Sb‐rich Sn‐Te precipitates inside the grains further diminish the lattice's thermal conductivity. Consequently, a prominent improvement in average ZT of ≈117% is achieved for Sn0.78Sb0.16Te(MgB2)0.09 compared to pristine Sn1.03Te. Moreover, the compressive yield strength and Vickers hardness of Sn0.78Sb0.16Te(MgB2)0.09 are significantly increased by ≈168% and 176% relative to pristine Sn1.03Te, respectively. The quantitative strengthening models including grain boundary, dislocation, solid solution, intergranular, and precipitation strengthening in MgB2‐ and MgB2‐Sb‐doped Sn1.03Te samples are proposed, showing that the dominant strengthening mechanism is precipitation strengthening. This work provides an avenue for designing efficient and robust thermoelectric materials toward commercial application.

Enhancing thermoelectric behavior of Bismuth Selenide crystal via substitution of Sulfur and Tellurium

Sulfur and Tellurium substituted Bismuth Selenide (Bi2Se3-x-ySxTey, x = y = 0, 0.1, 0.15, 0.2) quaternary alloy crystals were synthesized by using vertical Bridgman technique. Energy dispersive analysis of X-rays confirmed elemental composition and purity of the grown crystals. Field emission scanning electron microscopy showed that the growth of crystals has occurred by layer growth mechanism. Rhombohedral crystal structure is validated through analysis of powder X-ray diffraction pattern. Thermoelectric parameters such as electrical conductivity, Seebeck coefficient and thermal conductivity were calculated from 303 K to 773 K and from that power factor as well as figure of merit have been determined for all grown crystals. Electrical conductivity significantly increased due to Sulfur and Tellurium incorporation and maximum value obtained is 147.72 S/cm at 503 K for Bi2Se2.6S0.2Te0.2. Seebeck coefficient and thermal conductivity showed decrement with incorporation of Sulfur and Tellurium. Reduction in thermal conductivity and enhancement in electrical conductivity resulted in the improvement of figure of merit. Sulfur and Tellurium substitution showed considerable improvement in ZT from 0.31 (533 K) for pristine Bi2Se3 to 1.28 (523 K) for Bi2Se2.6S0.2Te0.2.

Rational design and synthesis of Cr1–x Te/Ag2Te composites for solid-state thermoelectromagnetic cooling near room temperature

Materials with both large magnetocaloric response and high thermoelectric performance are of vital importance for all-solid-state thermoelectromagnetic cooling. These two properties, however, hardly coexist in single phase materials except previously reported hexagonal Cr1−x Te half metal where a relatively high magnetic entropy change (−ΔS M) of ∼2.4 J⋅kg−1⋅K−1 @ 5 T and a moderate thermoelectric figure of merit (ZT) of ∼1.2 × 10−2@ 300 K are simultaneously recorded. Herein we aim to increase the thermoelectric performance of Cr1−x Te by compositing with semiconducting Ag2Te. It is discovered that the in-situ synthesis of Cr1−x Te/Ag2Te composites by reacting their constitute elements above melting temperatures is unsuccessful because of strong phase competition. Specifically, at elevated temperatures (T > 800 K), Cr1−x Te has a much lower deformation energy than Ag2Te and tends to become more Cr-deficient by capturing Te from Ag2Te. Therefore, Ag is insufficiently reacted and as a metal it deteriorates ZT. We then rationalize the synthesis of Cr1−x Te/Ag2Te composites by ex-situ mix of the pre-prepared Cr1−x Te and Ag2Te binary compounds followed by densification at a low sintering temperature of 573 K under a pressure of 3.5 GPa. We show that by compositing with 7 mol% Ag2Te, the Seebeck coefficient of Cr1−x Te is largely increased while the lattice thermal conductivity is considerably reduced, leading to 72% improvement of ZT. By comparison, −ΔS M is only slightly reduced by 10% in the composite. Our work demonstrates the potential of Cr1−x Te/Ag2Te composites for thermoelectromagnetic cooling.

Alloying and Doping Control in the Layered Metal Phosphide Thermoelectric CaCuP.

We recently identified CaCuP as a potential low cost, low density thermoelectric material, achieving zT = 0.5 at 792 K. Its performance is limited by a large lattice thermal conductivity, κL, and by intrinsically large p-type doping levels. In this paper, we address the thermal and electronic tunability of CaCuP. Isovalent alloying with As is possible over the full solid solution range in the CaCuP1-xAsx series. This leads to a reduction in κL due to mass fluctuations but also to a detrimental increase in p-type doping due to increasing Cu vacancies, which prevents zT improvement. Phase boundary mapping, exploiting small deviations from 1:1:1 stoichiometry, was used to explore doping tunability, finding increasing p-type doping to be much easier than decreasing the doping level. Calculation of the Lorenz number within the single parabolic band approximation leads to an unrealistic low κL for highly doped samples consistent with the multiband behavior in these materials. Overall, CaCuP and slightly Cu-enriched CaCu1.02P yield the best performance, with zT approaching 0.6 at 873 K.

Impact of resonant state formation and band convergence in In and Sr co-doped SnTe thermoelectric material evaluated via the single parabolic band model

SnTe is a promising thermoelectric material for mid-temperature waste heat recovery. Recently, a high thermoelectric performance (zT) is reported in In and Sr co-doped Sn1−3xInxSr2xTe (x = 0.0 – 0.03). A high zT of 1.075 is achieved when x = 0.02 at 823 K. The In dopant is known to engineer the valence band of the SnTe by forming a resonant state near 300 K. The Sr dopant converges the two valence bands of the SnTe. However, when both In and Sr are doped in SnTe while keeping their atomic ratio constant, their effects on band engineering with increasing are not known. Here, the effects of resonant state formation and band convergence on electronic transport properties of Sn1−3xInxSr2xTe (x = 0.0 – 0.03) are investigated from the estimated temperature-dependent electronic band parameters. Both the resonant state formation and band convergence become most significant at the x = 0.02. According to the single parabolic band model, the zT of the x = 0.02 can be further improved to ∼1.834 (approximately 70 % zT improvement) upon carrier concentration tuning.

Inhibiting the bipolar effect via band gap engineering to improve the thermoelectric performance in n-type Bi2-Sb Te3 for solid-state refrigeration

• Se replacement can enlarge the E g and thus effectively mitigate the bipolar effect. • The manipulation of donor-like effect retains the optimized carrier concentration. • The large strain field and mass fluctuation reduce lattice thermal conductivity. • A state-of-the-art zT = 1.0 at 300 K is attained for n -type Bi 1.5 Sb 0.5 Te 2.8 Se 0.2 . • n -type Bi 2- x Sb x Te 3 becomes a promising candidate for solid-state cooling field. To date, the benchmark Bi 2 Te 3 -based alloys are still the only commercial material system used for thermoelectric solid-state refrigeration. Nonetheless, the conspicuous performance imbalance between the p -type Bi 2- x Sb x Te 3 and n -type Bi 2 Te 3- x Se x legs has become a major obstacle for the improvement of cooling devices to achieve higher efficiency. In our previous study, novel n -type Bi 2- x Sb x Te 3 alloy has been proposed via manipulating donor-like effect as an alternative to mainstream n -type Bi 2 Te 3- x Se x . However, the narrow bandgap of Bi 2- x Sb x Te 3 provoked severe bipolar effect that constrained the further improvement of zT near room temperature. Herein, we have implemented band gap engineering in n -type Bi 1.5 Sb 0.5 Te 3 by employing isovalent Se substitution to inhibit the undesired intrinsic excitation and achieve the distinguished room-temperature zT . First, the preferential occupancy of Se at Te 2 site appropriately enlarges the band gap, thereby concurrently improving the Seebeck coefficient and depressing the bipolar thermal conductivity. In addition, the Se alloying mildly suppresses the compensation mechanism and essentially preserves the already optimized carrier concentration, which maintains the peak zT near room temperature. Moreover, the large strain field and mass fluctuation generated by Se alloying leads to the remarkable reduction of lattice thermal conductivity. Accordingly, the zT value of Bi 1.5 Sb 0.5 Te 2.8 Se 0.2 reaches 1.0 at 300 K and peaks 1.1 at 360 K, which surpasses that of most well-known room-temperature n -type thermoelectric materials. These results pave the way for n -type Bi 2- x Sb x Te 3 alloys to become a new and promising top candidate for large-scale solid-state cooling applications.

Synergistic effect of alloying on thermoelectric properties of two-dimensional PdPQ (Q = S, Se).

Hosts of 2D materials exist, yet few allow compositional and structural tailoring as the MQ2 (M = Mo, W; Q = S, Se) family does, for which various structural superlattices have been synthesized. Using thorough first-principles calculations, we show how bonding hierarchy contributes to the structural resilience of 2D PdPQ and allows for full-range alloying of sulfur and selenium. Within the structural unit of Pd2P2Q2, the covalently-bonded [P2Q2]4- polyanions hold the structure together with their molecular-like P-P bonds while ionically bonded Pd-Qs allow the S/Se substitution. Here, the bonding hierarchy imparts superior electronic and structural features to the PdPQ monolayers. As such, the flat-and-dispersive valence band and the eight degenerate valleys of the conduction band benefit the p-type and n-type thermoelectricity of pristine PdPQ, which can be further enhanced by alloying. The high-entropy alloying synergistically suppresses the lattice heat transport from 75 to 30 W m-1 K-1 and increases the band degeneracy of PdPQ monolayers, resulting in an overall improvement in zT. Combining these features, in a naïve approach, results in a large zT approaching two for both p-type and n-type doping. However, accurate fully-fledged electron-phonon calculations rebut this promise, showing that at high temperatures, the increased electron scattering results in a stagnant power factor in the flat-and-dispersive valence band. Using a realistic first-principles scattering, we finally calculate the thermoelectric efficiency of PdPQ (Q = S, Se) and highlight the importance of an accurate estimation of electron relaxation time for thermoelectric predictions.

Atomic site-targeted doping in Ti2FeNiSb2 double half-Heusler alloys: zT improvement via selective band engineering and point defect scattering

Ti2FeNiSb2 is a promising double half-Heusler thermoelectric compound with intrinsically low thermal conductivity due to low phonon group velocity. Since it is introduced in 2019, many efforts have been focused on further reducing its thermal conductivity via doping. However, the effects of doping on its electronic transport properties have been neglected. Here, we investigate the effects of doping Co and Bi at the Fe- and Sb-sites, respectively, in Ti2FeNiSb2 for the first time. Changes in band parameters due to atomic site-targeted doping are estimated by the Single Parabolic Band model. Bypassing the trade-off relation between the Seebeck coefficient and electrical conductivity is observed when doping Co at Fe-sites. The physics behind the bypass is explained in terms of temperature-dependent reduced chemical potential and non-degenerate mobility. As a result, peak figure of merit zT values of ∼0.69 in Ti2Fe0.9Co0.1NiSb2 is achieved, which is approximately six times higher than that of the pristine Ti2FeNiSb2. The thermoelectric performance of Ti2FeNiSb2 can be improved by selective band engineering to bypass the Seebeck coefficient-electrical conductivity trade-off relation from atomic-site targeted doping.

Thermoelectric properties of nanocrystalline half-Heusler high-entropy Ti2NiCoSn1−xSb1+x (x = 0.3, 0.5, 0.7, 1) alloys with VEC > 18

Powder metallurgy route has been employed to synthesize nanocrystalline Ti2NiCoSn1−xSb1+x (x = 0.3, 0.5, 0.7, 1) alloys for thermoelectric applications. Atom probe analysis confirmed the homogeneous distribution of elements in the half-Heusler phase at a scale of few nanometers. A combination of nanostructuring, lattice distortion and interfacial scattering brings about a reduction in lower thermal conductivity which brings forth an improvement in ZT. Ti2NiCoSb2 exhibited the highest ZT of 0.26 due to the increments effected by higher power factor and lower thermal conductivity.

Electron mean-free-path filtering in n-type SnSe for improved thermoelectric performance at room temperature

Based on the fact that the mean free path of phonon is larger than that of the electron, the improvement of ZT could be obtained by adjusting the size of grain boundary. Pushing the size of grain boundary down to the length ranges stated by the electron MFP, both electron transport and phonon transport are affected strongly, but the reduced scale of phonon transport is larger than that of electron transport, leading to the enhancement of ZT. By combining all electron and phonon transport property from full electron-phonon interactions, the room-temperature thermoelectric property of n-type SnSe under different electron/phonon transport mechanism is evaluated. The h-LO dominates the scattering process of electron transport, while the optic branch controls the scattering process of phonon transport. Compared to the bulk ZT (0.26), the room temperature ZT of 0.62 with the size of 7 nm is obtained, which is nearly three times higher than that of bulk value. Our work not only reveals the enhancement of ZT by the electron filtering effect, but also exhibits the computational framework for accurately calculating the thermoelectric performance under different electron/phonon transport mechanisms.

Broadening temperature plateau of high zTs in PbTe doped Bi0·3Sb1·7Te3 through defect carrier regulation and multi-scale phonon scattering

(Bi, Sb)2Te3 alloys are a promising class of thermoelectrics family for ambient temperature application, which has been widely concerned by the community. However, due to the narrow bandgap, the bipolar excitation limits thermoelectric figure of merit (zT) improvement as temperature rises. We herein combinate extrinsic impurities with intrinsic point defects to regulate carrier concentration and enable multi-scale phonon scattering simultaneously. Multiple types of defects interaction are triggered by PbTe doping to tune the carrier concentration, as well as the band gap is further expanded to suppresses the bipolar excitation at high temperatures. This result leads to the overall enhancement of electrical transport properties. With PbTe addition, the homologous multi-scale defects (including point defects, Te nanoprecipitates and high-density grain boundaries) are produced to strengthen phonon scatterings effectively in BST matrix, resulting a declined in lattice thermal conductivity. Finally, a high zT of ∼1.25 at 425 K and a superior average zT (zTavg) of ∼1.21 (350–500 K) are obtained in BST samples, projecting a maximum conversion efficiency (ηmax) of ∼7.8% at ΔT = 200 K. Importantly, this enhancement of high ranged zTs achieved by rational defect design in BST provides a new insight for promoting high-efficiency thermoelectrics to real applications.

Fabrication of ultrafine powder using processing control agent, and investigation of its effect on microstructure and thermoelectric properties of p-type (Bi, Sb)2Te3 alloys

Low-dimensional materials can significantly enhance the efficiency of thermoelectric devices for power generation and cooling applications. In the present work, ultra-fine powders of p-type (Bi, Sb)2Te3 alloys are fabricated through high energy ball milling using stearic acid as a process control agent (PCA). The influence of the PCA addition on powder characteristics, microstructure and thermoelectric transport properties are studied. Further, the ultra-fine powder is subjected to calcination (Cal-PCA) and subsequently consolidated all powders using spark plasma sintering (SPS). The PCA, Cal-PCA, and non-PCA powder morphological effects on the microstructure and thermoelectric properties are systematically investigated and elucidated thoroughly. The electron beam scattering diffraction (EBSD) results confirmed that the PCA sample exhibited very fine grains (average grain size of ∼800 nm) compared to the non-PCA (average grain size of about 2.6 µm), while the grains were distributed randomly for all samples. Formation of fine grains and partial existence of the stearic acid (carbon and oxygen phases) in the matrix were strongly inhibiting the transport of the carriers that severely decreased the carrier mobility, reflecting the severe reduction in electrical conductivity for PCA sample compared to Cal-PCA and non-PCA. The lowest thermal conductivity (κ) of 0.745 W/mK was achieved for the PCA sample, which is 19%, 12% lower than that of non-PCA, and Cal-PCA samples. The strong reduction in κ was mainly attributed to the dramatic decrease in the phonon thermal conductivity owing to phonon scattering at numerous grain boundaries and oxide phases. The obtained high electrical conductivity with balanced thermal conductivity in Cal-PCA sample is attributed to the significant improvement in ZT of 1.1, which is 27%, and 47% higher than that of the Non-PCA sample at room temperature, and 350 K, respectively.

Enhancing the thermoelectric performance of CoSb3-based skutterudite by decoupling the electrical and thermal properties by embedding Cu nanoparticles

Co0.91Ni0.09Sb3 embedded with Cu nanoparticles (NPs) was synthesized by applying a hydrothermal process followed by spark plasma sintering. The effects of Cu NPs on the electrical and thermal transport properties of the Co0.91Ni0.09Sb3 matrix were investigated in the temperature range of 303–773 K. Cu NPs introduced into the Co0.91Ni0.09Sb3 matrix slightly decreased the electrical conductivity and significantly enhanced the Seebeck coefficient by the energy filtering effect, leading to an improved power factor at 773 K. Furthermore, a significant decrease in the thermal conductivity was realized by increasing the number of grain boundaries. These favorable effects contributed to the improvement in ZT to a value ~57% higher than that of pristine Co0.91Ni0.09Sb3. The highest ZT value of 1.02 was achieved by incorporating 4.5 wt% of Cu NPs in Co0.91Ni0.09Sb3 at 773 K. Thus, decoupling the electrical and thermal transport properties of thermoelectric materials is a viable strategy for optimizing the ZT value.

Improved thermoelectric properties in (1-x)LaCoO3/(x)La0.7Sr0.3CoO3 composite

A high Seebeck coefficient (S), large electrical conductivity (σ), and reduced thermal conductivity (κ) are required to achieve a high figure-of-merit (zT) in an ideal thermoelectric (TE) system, which is challenging in a single system due to the interdependence of TE parameters. However, composite approach is promising to manipulate the TE parameters to achieve enhanced zT. In this study, TE properties of (1− x)LaCoO3/(x)La0.7Sr0.3CoO3 (0.00 = x ≤ 0.05) nanostructured composite is investigated. The structural analysis confirms individual phases in the composite, which is further supported by the electron microscopy analysis. The x-ray photoelectron spectroscopy (XPS) measurement indicates that oxygen vacancies (VO) are present in the parent LaCoO3 system and is found to increase with the addition of La0.7Sr0.3CoO3 (LSCO) in the composite. The increase in VO raises the degenerate states of cobalt and hence improves S in the composites. Temperature variation in S and σ are consistent with the spin-state transition and shows the correlation between these two parameters. The reduction in κ and σ with the addition of ball-milled La0.7Sr0.3CoO3 in the composite is attributed to the enhanced phonon-phonon and charge carrier scattering, respectively. A synergistic effect of enhanced S and reduced κ result in five times improvement in zT of the composite compared to the parent LaCoO3 system at 800 K. This further indicates that composite approach also improves the operating temperature for LaCoO3 based systems.

Selective Enhancement in Phonon Scattering Leads to a High Thermoelectric Figure-of-Merit in Graphene Oxide-Encapsulated ZnO Nanocomposites.

ZnO is a promising candidate for use as an environmentally friendly thermoelectric (TE) material. However, high thermal conductivity leading to a poor TE figure-of-merit (zT) needs to be addressed to achieve a significant TE efficiency for commercial applications. Here, we demonstrate that selective enhancement in phonon scattering leads to an increase in the zT of ZnO because of Al doping and reduced graphene oxide (RGO) encapsulation. These nanocomposites are synthesized via a facile and scalable method. The incorporation of 1 at% Al with 1.5 wt % RGO into ZnO has been found to show significant improvement in zT (0.52 at 1100 K), which is an order of magnitude larger compared to that of bare undoped ZnO. Photoluminescence and X-ray photoelectron spectroscopy measurements confirm that RGO encapsulation significantly quenches surface oxygen vacancies in ZnO along with nucleation of new interstitial Zn donor states. Tunneling spectroscopy performed on bare as well as composite particles reveals that the band gap of ∼3.4 eV for bare ZnO reduces effectively to ∼0.5 eV upon RGO encapsulation, facilitating charge transport. The electrical conductivity also benefits from high densification (>95%) achieved using the spark plasma sintering method, which also aids in reduction of graphene oxide into RGO. The same Al doping and RGO capping synergistically bring about drastic reduction of thermal conductivity, through enhanced interfacial and point-defect-phonon scatterings. These opposing effects on electrical and thermal conductivities lead to enhancement in the power factors as well as the zT value. Overall, a practically viable route has been demonstrated for the synthesis of oxide-RGO TE materials, which could find their potential applications in high-temperature TE power generation.

Enhanced thermoelectric performance of solution-grown Bi2Te3 nanorods

The dominant role of trioctylphosphine (TOP) on the precise crystallite size and enhanced thermoelectric (TE) performance of the solution-grown Bi2Te3 nanorods annealed at 250 °C for 5 h under Ar gas flow and pelletization under 1 GPa pressure at 27 °C only has been demonstrated here. This has resulted in systematic reduction of crystallite size (D), emergence of a secondary phase BiTe with increasing TOP, and enhancement in thermopower but drastic drop in thermal conductivity due to atomic-scale control over the grain size and boundaries. The highest ZT and power factor (PF) obtained for nanorods with D = 40 nm are 1.95 and 1.4 times higher than those of D = 63 nm are due to the optimum intergrain energy barrier height for filtering of the charge/heat carriers. Remarkably, this enhanced ZT/PF is promisingly greater than those of the other more sophisticatedly solution-processed samples reported earlier. Moreover, the IR-active A1u modes in Raman spectra of nanorods of centrosymmetric Bi2Te3 have been observed for the first time with TOP-induced reduction of D confirming the breaking of crystal inversion symmetry due to the formation of sub-quintuples. Thus, this grain size alteration strategy for ZT improvement will open a new vista for further development of Bi2Te3-based efficient TE materials near room temperature.

Evaluation of Thermoelectric Properties of Doped β-Iron Disilicide Prepared by the Powder Metallurgy Technique

Thermoelectric generator (TEG) offers a potential application in harvesting waste-heat energy into usable electrical power. β-iron disilicide (β-FeSi2) thermoelectric is applicable in elevated temperature systems such as hydrogen plants, copper reverberatory furnace and copper refining furnace. In this work, high TE performance Mn doped p-type and Co doped n-type thermoelectric (TE) β-iron disilicide compacts were fabricated for TEG using powder metallurgy route. The effects of variation in type and quantity of dopants [p-type dopant: Mn: stoichiometry Fe1−xMnxSi2 (x = 0.04 − 0.12) or n-type dopant: Co: stoichiometry Fe1−yCoySi2 (y = 0.01 − 0.05)] on thermoelectric properties were studied. Significant improvement in ZT was observed compared to earlier research works for this material system. This result is a positive indication for the potential use of β-FeSi2 in the high temperature TEG. In this work, the working efficiency was also evaluated which was not mentioned in the earlier research works.