half-Heusler Thermoelectric Materials Research Articles

The effect of thermoelectric augmentation dramatically increased the specific capacity for electrochemical energy storage

Conventional methods to enhance the performance of electrode materials include morphological modification and defect engineering, but the limited number of active sites for the electrode remains a huge obstacle to achieving high-performance supercapacitors. In this work, we proposed a strategy to improve the electrode performance under the condition of limited active sites through the thermoelectric effect, and designed thermoelectric electrodes of the p-type Fe0.3Co0.7Sb3 cathode and the n-type Cu0.3Ag1.7Se anode for supercapacitors, respectively. In particular, the charge carriers generated by the thermoelectric effect accelerate the electrochemical reaction process at the interface between the thermoelectric electrode and the electrolyte under the temperature difference (ΔT) condition, thus increasing the specific capacity of the electrode. The p-Type Fe0.3Co0.7Sb3 cathode and the n-Type Cu0.3Ag1.7Se anode exhibit a 150% and 208% increase in the specific capacity, respectively. Furthermore, electrodes with different conduction types and various thermoelectric properties, such as n/p-type bismuth telluride and n/p-type half-Heusler thermoelectric materials are also studied, confirming the universality of the above phenomenon in thermoelectric electrodes. This work provides a universal route for the thermoelectric effect to promote the energy density of supercapacitors used in high temperature difference environments.

Design and optimization of three segmented thermoelectric generator for nuclear reactor application

With the scientific exploration in deep space and deep sea, traditional fossil energy and solar energy cannot supply energy for deep space aircrafts and deep sea vehicles continuously, new energy power systems with the features of long endurance, high reliability and high energy density are imperative. Small nuclear reactor has become one of the best solutions for deep space energy demand in the future due to its strong environmental adaptability and long-life time. To improve the reliability of the static heat pipe reactor, the thermoelectric generators (TEGs) are utilized to convert fission heat into electrical energy. Focusing on the demands of improving the conversion efficiency, a new three segmented TEG is proposed based on the Bi2Te3, CoSb3 and Half-Heusler thermoelectric materials in this paper. Combined with the application requirements of heat pipe reactor, the geometric configuration such as different heights and the cross-sectional area of the three different materials in the PN legs is designed and optimized under different heat flux boundary conditions. The influences of contact electrical resistance and contact thermal resistance on the conversion efficiency have been investigated. When the contact electrical resistivity increases from 0.01 μΩ cm2 to 1000 μΩ cm2, the conversion efficiency loss increases from 2.6% to 80.1%. As the surface roughness rises from 0.1 μm to 100 μm, the conversion efficiency loss increases from 0.9% to 42.7%. Based on the heat flux boundary conditions provided by the heat pipes, the thermoelectric conversion performances of three segmented TEG under different external loads are investigated. Through the geometric optimization of the three segmented TEG with total height 20 mm, the thermoelectric conversion efficiency improves from 6.9% to 15.34%. For the simulation heat flux density conditions of 70 kW m−2, 75 kW m−2, 80 kW m−2, 85 kW m−2 and 90 kW m−2, the maximum output power of a thermoelectric generator can reach 8.56W, 9.71W, 10.85W, 12.00W, and 13.11W, respectively. The output characteristics of the three segmented TEGs meet the design demands of the static conversion system.

Strong electron-phonon coupling and high lattice thermal conductivity in half-Heusler thermoelectric materials.

Traditional half-Heusler thermoelectric materials, identified as 18-electron compounds, are characterized by the high power factor and the high lattice thermal conductivity. Interestingly, the emerging 19-electron half-Heusler compounds were also found to be promising thermoelectric materials, but with a 5-10 times lower lattice thermal conductivity. Since the two kinds of compounds have similar chemical and physical structures, such as TiCoSb and VCoSb, the large difference in lattice thermal conductivity is a puzzling question. Here, we present a theoretical study to clarify the lattice thermal transport in half-Heusler thermoelectric materials. Based on electronic band structure analysis, we show that the two transition-metal elements in half-Heusler compounds form the strong and direct d-d interaction that is responsible for the high lattice thermal conductivity of 18-electron compounds. In 19-electron half-Heusler compounds, however, the extra valence electron enters the d-d antibonding states, which significantly weakens the atomic bond strength, leading to a large decrease in the cohesive energy. The resulting softened acoustic phonons enhance the phonon-phonon scattering, and thus reduce the lattice thermal conductivity significantly. By constructing an artificial 18-e compound V0.5Sc0.5CoSb, it is proved that the one less electron relative to VCoSb increases the lattice thermal conductivity significantly.

Thermoelectric properties of Co doped TiNiCo<sub><i>x</i></sub>Sn alloys fabricated by melt spinning

Although TiNiSn-based half-Heusler thermoelectric materials obtain high power factors, their high lattice thermal conductivity greatly hinders the improvement of thermoelectric properties. In this work, TiNiCo<sub><i>x</i></sub>Sn (<i>x</i> = 0–0.05) samples are prepared by melt spinning combined with spark plasma sintering method, and their phase, microstructure and thermoelectric properties are studied. The XRD results show that the main phase of all samples is TiNiSn phase, and no any other impurity phases are found, indicating that the high purity single phase can be prepared by rapid quenching process combined with SPS process. In the solidification process, the large cooling rate (10<sup>5</sup>–10<sup>6</sup> K/s) is conducive to obtaining the uniform nanocrystalline structure. The grains are closely packed, with grain sizes in a range of 200–600 nm. The grain sizes decrease to 50–400 nm for the Co-doping samples, which indicates that Co doping can reduce the grain size. For the <i>x</i> = 0 sample, the thermal conductivity of the rapid quenching sample is significantly lower than that of bulk sample, with an average decrease of about 17.8%. Compared with the TiNiSn matrix, the Co-doping sample has the thermal conductivity that decreases significantly, and the maximum decrease can reach about 38.9%. The minimum value of lattice thermal conductivity of TiNiCo<sub><i>x</i></sub>Sn samples is 3.19 W/(m·K). Therefore, Co doping can significantly reduce the <i>κ</i><sub>l</sub> values of TiNiCo<sub><i>x</i></sub>Sn (<i>x</i> = 0.01–0.05) samples. With the increase of Co doping amount <i>x</i>, n/p transition is observed in the TiNiCo<sub><i>x</i></sub>Sn samples, resulting in gradually reducing the conductivity and the power factor, and finally deteriorating the electrical transport performance, of which, the TiNiSn sample obtains the highest power factor of 29.56 W/(m·K<sup>2</sup>) at 700 K. The <i>ZT</i> value decreases with the Co doping amount <i>x</i> increasing, and the maximum <i>ZT</i> value of TiNiSn sample at 900 K is 0.48. This work shows that the thermal conductivity of TiNiSn can be effectively reduced by using the melt spinning process and magnetic Co doping.

Enhanced Thermoelectric Properties of Nb-Doped Ti(FeCoNi)Sb Pseudo-Ternary Half-Heusler Alloys Prepared Using the Microwave Method.

Pseudo-ternary half-Heusler thermoelectric materials, which are formed by filling the B sites of traditional ternary half-Heusler thermoelectric materials of ABX with equal atomic proportions of various elements, have attracted more and more attention due to their lower intrinsic lattice thermal conductivity. High-purity and relatively dense Ti1-xNbx(FeCoNi)Sb (x = 0, 0.01, 0.03, 0.05, 0.07 and 0.1) alloys were prepared via microwave synthesis combined with rapid hot-pressing sintering, and their thermoelectric properties are investigated in this work. The Seebeck coefficient was markedly increased via Nb substitution at Ti sites, which resulted in the optimized power factor of 1.45 μWcm-1K-2 for n-type Ti0.93Nb0.07(FeCoNi)Sb at 750 K. In addition, the lattice thermal conductivity was largely decreased due to the increase in phonon scattering caused by point defects, mass fluctuation and strain fluctuation introduced by Nb-doping. At 750 K, the lattice thermal conductivity of Ti0.97Nb0.03(FeCoNi)Sb is 2.37 Wm-1K-1, which is 55% and 23% lower than that of TiCoSb and Ti(FeCoNi)Sb, respectively. Compared with TiCoSb, the ZT of the Ti1-xNbx(FeCoNi)Sb samples were significantly increased. The average ZT values of the Nb-doped pseudo-ternary half-Heusler samples were dozens of times that of the TiCoSb prepared using the same process.

Recent trends of ternary, quaternary half-Heusler thermoelectric materials and contact resistance effect on their power output efficiency

Climate change isone of the most pressing global issues, owing to the high carbon footprint generated during conventional power generation via coal consumption and gasoline-fuelled exhaust. Furthermore, nearly half of thetotal energy is lost as heat. Suitable thermoelectric (TE) devices can convert waste heat into green/clean energy. Moreover, these devices have no moving parts, require no maintenance, and are environmentally friendly. A better TE device's thermoelectric performance (zT) should be highly cost-effective for process scaling. Because of their low cost, narrow band gap, better contact engineering, decent electrical properties, and high mechanical performance, Half-Heusler (HH) compounds (space group Fm-3 m) have been considered the best candidate materials worldwide. However, these materials have a high thermal conductivity. The high thermoelectric performance of HH compounds was achieved through iso-electronic doping and nanostructuring, which reduced thermal conductivity significantly. The current report briefly overviews17, 18, 19, and 21 valence electron count (VEC) HH materials and their effects on thermal conductivity and zT. Finally, using cumulative temperature dependence (CTD) and constant property models (CPM), the theoretical estimation of power density and efficiency was calculated by considering the materials' thermoelectric performanceand contact resistance.

First-principles research on the thermoelectric properties of NbCoGe based on the scattering mechanisms

ABSTRACT Half-Heusler compounds have excellent power generation performance at high temperatures. The scattering mechanism is an essential factor affecting the electrical transport properties of thermoelectric materials. In computational simulations, only the role of acoustic phonon scattering on the thermoelectric properties of materials is considered, whereas other scattering mechanisms are neglected. In this work, we investigate the thermoelectric properties of NbCoGe compounds with different combinations of acoustic deformation potential, polar optical phonon, and ionised impurity scattering mechanisms. The calculated results show that the ZT values of n-type and p-type NbCoGe compounds reach 8 and 2.8, respectively, when only acoustic deformation potential scattering is considered, indicating that this sole scattering is insufficient. The calculated ZT values of p-type NbCoGe compounds reach 1.8 at 1200 K and more than 1 at 800 K for p- and n-type NbCoGe compounds under the combined effect of the three scattering mechanisms due to their high–power factor. This provides solid theoretical guidance for the search for potentially high–temperature half-Heusler thermoelectric materials.

High thermoelectric performance in ZrNiSn through electron injection and boosting carrier mobility

Inert insulating oxides have been used as phonon scattering centers to reduce the lattice thermal conductivity of half-Heusler thermoelectric materials, but they often cause a decline in electrical transport performance. In this work, the in-situ conductive semi-metal ZrTe2 makes 15% decreased thermal conductivity and 42% increased electrical conductivity in ZrNiSn simultaneously. When the phase boundary scatters the mid-long wavelength phonons, it is not an obstacle for the electrons. Thanks to the decreased interfacial energy barrier and high mobility electron injection from ZrTe2, even when the charge carrier concentration increases by 56% due to the in-situ ZrTe2 induced bandgap decreasing, 190% improved mobility is also obtained. High zT ∼0.9 is achieved at 873 K in 4 at.% Te composited ZrNiSn through electron injection and boosting carrier mobility. These results have suggested a promising way to decouple the electron and phonon transport behaviors via forming the in-situ conductive secondary phase.

Thermoelectric properties of the aliovalent half-Heusler alloy Zn0.5Ti0.5NiSb with intrinsic low thermal conductivity

Aliovalent substitutions lead to bond disorder and low lattice thermal conductivities in half-Heusler thermoelectrics.

Synchronously enhanced thermoelectric and mechanical properties of Ti doped NbFeSb based half-heusler alloys by carrier regulation and phonon engineering

NbFeSb based half-heusler thermoelectric materials have great potential in high-temperature energy harvesting. However, their conversion efficiency is limited by the low figure of merit zT, which is attributed to poor electronic performance and high thermal conductivity. Here, it is demonstrated that enhanced power factor of 46 μW cm−1 K−2 and zT value of 0.9 at 973 K is achieved in spark plasma sintered Nb0.8Ti0.2FeSb. It is depicted that a tremendous increase in the hole concentration owing to Ti substitution, results in a significant promotion of electrical performances. In addition, numerous phonon scattering centers, such as point-defects, nano precipitates as well as electro-acoustic coupling, collectively suppress the lattice thermal conductivity from 7.4 W m−1 K−1 to 3.3 W m−1 K−1 at 973 K. Furthermore, NbFeSb based alloys exhibit excellent mechanical property and the Vickers hardness reaches 10.8 GPa, which would be beneficial for TE devices fabrication.

Thermoelectric Properties of Ta-doped Zr0.6-xTi0.4TaxNiSn n-type Half-Heusler Materials

Half-Heusler (HH) thermoelectric materials are promising for mid- to high-temperature applications, and MNiSn (M = Ti, Zr, Hf) is a representative n-type HH alloy. In general, the M sites are mixed with isoelectronic elements Ti, Zr, and Hf, to lower the lattice thermal conductivity, and the Sn sites are doped with Sb to adjust the electron concentration. However, Hf is a rare element in earth’s crust, and the volatility of Sb makes it difficult to maintain the initial Sb amount during material synthesis. In this study, as an alternative, Ta was added in the M sites along with the host elements Ti and Zr to produce Hf-free Zr0.6-xTi0.4TaxNiSn alloys (0 ≤ x ≤ 0.04), and the effects of Ta doping on the thermoelectric properties were analyzed. The electrical conductivity of Zr0.6-xTi0.4TaxNiSn increased with Ta content, and the electron concentration increased almost linearly, reaching 5.6 × 1020 cm -3 at x = 0.04. By adding Ta, the maximum power factor also increased by approximately 17% to 3.94 × 10-3 W m-1K-2 at x = 0.02. The lowest lattice thermal conductivity (κlat) was observed at x = 0.02, reaching approximately 1.9 W m-1K-1 at 723 K, but overall, a dramatic decrease in κlat was not observed with Ta doping in Zr0.6-xTi0.4TaxNiSn. This is probably due to the existing effect of Zr/Ti mixing at the M sites, which enhances phonon scattering. A maximum figure of merit (zT) of 0.91 was obtained in Zr0.58Ti0.4Ta0.02NiSn at 873 K, which is a high value for ZrNiSn-based Hf-free HH materials. In conclusion, Ta doping is a viable method to replace Sb doping in ZrNiSn-based HH alloys.

Unsupervised machine learning for discovery of promising half-Heusler thermoelectric materials

Thermoelectric materials can be potentially applied to waste heat recovery and solid-state cooling because they allow a direct energy conversion between heat and electricity and vice versa. The accelerated materials design based on machine learning has enabled the systematic discovery of promising materials. Herein we proposed a successful strategy to discover and design a series of promising half-Heusler thermoelectric materials through the iterative combination of unsupervised machine learning with the labeled known half-Heusler thermoelectric materials. Subsequently, optimized zT values of ~0.5 at 925 K for p-type Sc0.7Y0.3NiSb0.97Sn0.03 and ~0.3 at 778 K for n-type Sc0.65Y0.3Ti0.05NiSb were experimentally achieved on the same parent ScNiSb.

Thermodynamic approaches to determine the vacancy concentration in defective Nb1-xCoSb half-Heusler thermoelectric materials

The defective half-Heusler compound Nb1-xCoSb has been identified to be a promising thermoelectric material via modification of vacancies. Herein, we combined the experimental phase diagram with CALPHAD (CALculation PHAse Diagram) method to determine the vacancy concentration of Nb1-xCoSb in the equilibrium state, which is 0.17 ≤ x ≤ 0.22 at 1173 K and 0.17 ≤ x ≤ 0.2 at 1323 K and extrapolated to the whole composition and temperature range computationally. The calculated homogeneous region of the half-Heusler phase increases first and then decreases with increasing temperature, reaching a maximum Δx = 0.042 at ∼1123 ± 20 K. The stoichiometric NbCoSb alloy is proved to locate at the binary-phase region of Nb1-xCoSb/Nb3Sb. This work opens a new avenue for understanding, design and preparation of thermoelectric materials.

Inhibition of Lattice Thermal Conductivity of ZrNiSn-based Half-Heusler Thermoelectric Materials by Entropy Adjustment

Inhibition of Lattice Thermal Conductivity of ZrNiSn-based Half-Heusler Thermoelectric Materials by Entropy Adjustment

Synthesis of Nb0.8Hf0.2FeSb0.98Sn0.02 and Hf0.25Zr0.25Ti0.5NiSn0.98Sb0.02 Half-Heusler Materials and Fabrication of Thermoelectric Generators

In this study, half-Heusler (HH) thermoelectric materials Nb0.8Hf0.2FeSb0.98Sn0.02 (p-type) and Hf0.25Zr0.25Ti0.5NiSn0.98Sb0.02 (n-type) were synthesized using induction melting and spark plasma sintering. For alloying, a conventional induction melting technique was employed rather than arc melting, for mass production compatibility, and the thermoelectric properties of the materials were analyzed. The maximum dimensionless figures of merit (zTmax) were 0.75 and 0.82 for the p- and n-type material at 650 oC and 600 oC, respectively. These materials were then used to fabricate generator modules, wherein two pairs of p- and nlegs without interfacial metal layers were brazed on direct bonded copper (DBC)/Al2O3 substrates using a Zrbased alloy. A maximum power of 0.57 W was obtained from the module by applying a temperature gradient of 476 oC, which corresponds to a maximum power density of 1.58 W cm -2 when normalized by the area of the material. The maximum electrical conversion efficiency of the module was 3.22% at 476 oC temperature gradient. This value was negatively affected by the non-negligible contact resistivity of the brazed interfaces, which ranged from 6.63 × 10 -9 Ωm2 to 7.54 × 10 -9 Ω m2 at hot-side temperatures of 190 oC and 517 oC, respectively. The low electrical resistivity of the HH materials makes it especially important to develop a brazing technique for ultralow resistance contacts.

Synthesis and Thermoelectric Properties of ZrxTi1−xNiSn0.98Sb0.02 n-Type Half-Heusler Materials

Hf-free ZrxTi1−xNiSn0.98Sb0.02 (x = 0.25, 0.5, 0.75) n-type half-Heusler (HH) thermoelectric materials were synthesized by a serial processing method including induction melting (IM), annealing, ball milling, and spark plasma sintering (SPS). For comparison, a Hf-containing half-Heusler Hf0.25Zr0.25Ti0.5NiSn0.98Sb0.02 ingot was also alloyed by arc melting, and the effects of Hf on the thermoelectric properties were estimated. The ZrxTi1−xNiSn0.98Sb0.02 HH materials were nearly pure according to the x-ray diffraction analysis, but microscopic investigation revealed impurity phase inclusions of unalloyed Sn, Zr, and Ti. The power factor (PF) of the Hf-free HH materials reached the maximum value of 4.31 mWm−1 K−2 at 823 K in Zr0.75Ti0.25NiSn0.98Sb0.02, which was higher than Hf0.25Zr0.25Ti0.5NiSn0.98Sb0.02 (4.01 mWm−1 K−2 at 773 K) in this study. However, the thermal conductivity of the Hf-free samples was significantly higher, by which the maximum dimensionless figure of merit was slightly lower (ZTmax = 0.92 in Zr0.75Ti0.25NiSn0.98Sb0.02 at 873 K) than that of Hf0.25Zr0.25Ti0.5NiSn0.98Sb0.02 (ZTmax = 1.03 at 873 K). The thermal conductivity was decomposed into lattice and electronic contributions, and the possible correlation with Ni off-stoichiometry is discussed.

Half-Heusler thermoelectric materials

Semiconducting half-Heusler compounds with the valence electron count of 18 have been identified as a class of promising high-temperature thermoelectric materials. Recently, nominal 19-electron half-Heusler compounds, traditionally regarded as metals, have gained reacquaintance and popularity due to their unexpected high thermoelectric performance and fascinating defective structure. In this Perspective, we summarize the current progress of 18-electron half-Heusler thermoelectric materials and focus on the discovery and challenge of the cation-deficient 19-electron half-Heusler compounds with the vacancy-related short-range order. Further outlook on the discovery of promising half-Heusler thermoelectrics and the insightful understanding of the defect-tailored thermoelectric properties are offered.

Origin of low thermal conductivity in Nb1-xTixFe1.02Sb half-Heusler thermoelectric materials

Nb1-xTixFe1.02Sb half-Heusler thermoelectric materials were synthesized trough arc melting and subsequent spark plasma sintering (SPS). Doping effect of Ti at Nb-site results in high power factor due to the optimization of hole carrier concentration. Further, the lattice thermal conductivity reduced as the Ti content increased, which mainly resulted from point defects and mass fluctuation by Ti doping as well as coherent nano-scale second phase, Ti-rich and Fe-deficient Ti1.18Fe0.57Sb. For the sample of Nb0.6Ti0.4Fe1.02Sb, a minimum lattice thermal conductivity of ∼2.08 W/mK was obtained. This is attributed to the enhanced phonon scattering at the alternating white and dark lamellar interphase boundaries of (Nb0.6,Ti0.4)FeSb half-Heusler and Nb-doped Ti1.18Fe0.57Sb. A maximum thermoelectric figure of merit, ZT, of ∼0.81 was obtained at 973 K, which is comparable with values previously reported for this system.

Pressure-Induced Enhancement of Thermoelectric Figure of Merit and Structural Phase Transition in TiNiSn.

Half-Heusler thermoelectric materials are potential candidates for high thermoelectric efficiency. We report high-pressure thermoelectric and structural property measurements, density functional theory calculations on the half-Heusler material TiNiSn, and an increase of 15% in the relative dimensionless figure of merit, ZT, around 3 GPa. Thermal and electrical properties were measured utilizing a specialized sample cell assembly designed for the Paris-Edinburgh large-volume press to a maximum pressure of 3.5 GPa. High-pressure structural measurements performed up to 50 GPa in a diamond-anvil cell indicated the emergence of a new high-pressure phase around 20 GPa. A first-principles structure search performed using an ab initio random structure search approach identified the high-pressure phase as an orthorhombic type, in good agreement with the experimental results.

High thermoelectric performance of half-Heusler compound BiBaK with intrinsically low lattice thermal conductivity

Half-Heusler compounds usually exhibit relatively higher lattice thermal conductivity that is undesirable for thermoelectric applications. Here we demonstrate by first-principles calculations and Boltzmann transport theory that the BiBaK system is an exception, which has rather low thermal conductivity as evidenced by very small phonon group velocity and relaxation time. Detailed analysis indicates that the heavy Bi and Ba atoms form a cage-like structure, inside which the light K atom rattles with larger atomic displacement parameters. In combination with its good electronic transport properties, the BiBaK shows a maximum n-type ZT value of 1.9 at 900 K, which outperforms most half-Heusler thermoelectric materials.