Thermoelectric efficiency of single crystal semiconducting ruthenium silicide

Thermoelectric efficiency of semiconducting ruthenium silicide Ru₂Si₃ has been systematically studied both experimentally and theoretically. Pure and Mn-doped Ru₂Si₃ single crystals were grown by zone melting with optical heating. Temperature dependences of the resistivity, Hall factor, Seebeck coefficient, and thermal conductivity were studied in the range of 100–900 K. For Mn-doped Ru₂Si₃ crystals, the Seebeck coefficient is positive in the whole temperature range under study, it reaches its maximum value of 400 μV/K at about 500 K. At room temperature, the Seebeck coefficient of these crystals is about 300 μV/K, which is twice as high as in the undoped material. The theoretical study of transport and thermoelectric properties includes the ab initio calculation of band structure, estimation of the carrier effective masses, modeling of the electron and hole mobilities in terms of classical scattering mechanisms, and calculation of the Seebeck coefficient and thermoelectric figure of merit, ZT. The results of theoretical modeling show a good qualitative and quantitative agreement with the experimental data.     

Thermoelectric Properties of Magnesium Silicide Deposited by Use of an Atmospheric Plasma Thermal Spray

The thermoelectric properties of magnesium silicide (Mg₂Si) samples prepared by use of an atmospheric plasma spray (APS) were compared with those of samples prepared from the same feedstock powder by use of the conventional hot-pressing method. The characterization performed included measurement of thermal conductivity, electrical conductivity, Seebeck coefficient, and figure of merit, ZT. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDX) were used to assess how phase and microstructure affected the thermoelectric properties of the samples. Hall effect measurements furnished carrier concentration, and measurement of Hall mobility provided further insight into electrical conductivity and Seebeck coefficient. Low temperature and high velocity APS using an internal-powder distribution system achieved a phase of composition similar to that of the feedstock powder. Thermal spraying was demonstrated in this work to be an effective means of reducing the thermal conductivity of Mg₂Si; this may be because of pores and cracks in the sprayed sample. Vacuum-annealed APS samples were found to have very high Seebeck coefficients. To further improve the figure of merit, carrier concentration must be adjusted and carrier mobility must be enhanced.     

Zintl Phases as Thermoelectric Materials: Tuned Transport Properties of the Compounds CaxYb1–xZn2Sb2

Zintl phases are ideal candidates for efficient thermoelectric materials, because they are typically small‐bandgap semiconductors with complex structures. Furthermore, such phases allow fine adjustment of dopant concentration without disrupting electronic mobility, which is essential for optimizing thermoelectric material efficiency. The tunability of Zintl phases is demonstrated with the series Ca_xYb_1–xZn₂Sb₂ (0 ≤ x ≤ 1). Measurements of the electrical conductivity, Hall mobility, Seebeck coefficient, and thermal conductivity (in the 300–800 K temperature range) show the compounds to behave as heavily doped semiconductors, with transport properties that can be systematically regulated by varying x. Within this series, x = 0 is the most metallic (lowest electrical resistivity, lowest Seebeck coefficient, and highest carrier concentration), and x = 1 is the most semiconducting (highest electrical resistivity, highest Seebeck coefficient, and lowest carrier concentration), while the mobility is largely independent of x. In addition, the structural disorder generated by the incorporation of multiple cations lowers the overall thermal conductivity significantly at intermediate compositions, increasing the thermoelectric figure of merit, zT. Thus, both zT and the thermoelectric compatibility factor (like zT, a composite function of the transport properties) can be finely tuned to allow optimization of efficiency in a thermoelectric device.     

Preparation and Thermoelectric Properties of p-Type Yb-Filled Skutterudites

p-Type Yb _z Fe_4?x Co _x Sb₁₂ skutterudites were prepared by encapsulated melting and hot pressing, and the filling and doping (charge compensation) effects on the transport and thermoelectric properties were examined. The electrical conductivity of all specimens decreased slightly with increasing temperature, indicating that they were in a degenerate state due to high carrier concentrations of 10²⁰ cm^?3 to 10²¹ cm^?3. The Hall and Seebeck coefficients exhibited positive signs, indicating that the majority carriers are holes (p-type). The Seebeck coefficient increased with increasing temperature to maximum values of 100 μV/K to 150 μV/K at 823 K. The electrical and thermal conductivities were reduced by substitution of Co for Fe, which was responsible for the decreased carrier concentration. Overall, the Yb-filled Fe-rich skutterudites showed better thermoelectric performance than the Yb-filled Co-rich skutterudites.     

Nanoscale FeS2 (Pyrite) as a Sustainable Thermoelectric Material

We have synthesized undoped, Co-doped (up to 5%), and Se-doped (up to 4%) FeS₂ materials by mechanical alloying in a planetary ball mill and investigated their thermoelectric properties from room temperature (RT) to 600 K. With decreasing particle size, the undoped FeS₂ samples showed higher electrical conductivity, from 0.02 S cm^?1 for particles with 70 nm grain size up to 3.1 S cm^?1 for the sample with grain size of 16 nm. The Seebeck coefficient of the undoped samples showed a decrease with further grinding, from 128 μV K^?1 at RT for the sample with 70-nm grains down to 101 μV K^?1 for the sample with grain size of 16 nm. The thermal conductivity of the 16-nm undoped sample lay within the range from 1.3 W m^?1 K^?1 at RT to a minimal value of 1.2 W m^?1 K^?1 at 600 K. All doped samples showed improved thermoelectric behavior at 600 K compared with the undoped sample with 16 nm particle size. Cobalt doping modified the p-type semiconducting behavior to n-type and increased the thermal conductivity (2.1 W m^?1 K^?1) but improved the electrical conductivity (41 S cm^?1) and Seebeck coefficient (-129 μV K^?1). Isovalent selenium doping led to a slightly higher thermal conductivity (1.7 W m^?1 K^?1) as well as to an improved electrical conductivity (26 S cm^?1) and Seebeck coefficient (110 μV K^?1). The ZT value of FeS₂ was increased by a factor of five by Co doping and by a factor of three by Se doping.     

Thin-Film Thermoelectric Module for Power Generator Applications Using a Screen-Printing Method

A new process for fabricating a low-cost thermoelectric module using a screen-printing method has been developed. Thermoelectric properties of screen-printed ZnSb films were investigated in an effort to develop a thermoelectric module with low cost per watt. The screen-printed Zn _x Sb_1−x films showed a low carrier concentration and high Seebeck coefficient when x was in the range of 0.5 to 0.57 and the annealing temperature was kept below 550°C. When the annealing temperature was higher than 550°C, the carrier concentration of the Zn _x Sb_1−x films reached that of a metal, leading to a decrease of the Seebeck coefficient. In the present experiment, the optimized carrier concentration of screen-printed ZnSb was 7 × 10¹⁸/cm³. The output voltage and power density of the ZnSb film were 10 mV and 0.17 mW/cm², respectively, at ΔT = 50 K. A thermoelectric module was produced using the proposed screen-printing approach with ZnSb and CoSb₃ as p-type and n-type thermoelectric materials, respectively, and copper as the pad metal.     

Electrical and Thermal Transport Properties of Ge1–xPbxCuySbyTeSe2y

Balancing the contradictory relationship between thermoelectric parameters, such as effective mass and carrier mobility, is a challenge to optimize thermoelectric performance. Herein, the exceptional thermoelectric performance is realized in GeTe through collaboratively optimizing the carrier and phonon transport via stepwise alloying Pb and CuSbSe₂. The formation energy of Ge vacancy is efficiently bolstered by alloying Pb, which reduces carrier density and carrier scattering to maintain superior carrier mobility in GeTe. Additionally, CuSbSe₂, acting as an n-type dopant, further modulates carrier density and validly equilibrates carrier mobility and effective mass. Accordingly, the promising power factor of 45 µW cm⁻¹ K⁻² is achieved at 723 K. Meanwhile, point defects are found to significantly suppress phonons transport to descend lattice thermal conductivity by Pb and CuSbSe₂ alloying, which barely impacts the carrier mobility. A combination with superior carrier mobility and lower lattice thermal conductivity, a maximum ZT of 2.2 is attained in Ge_0.925Pb_0.075Cu_0.005Sb_0.005TeSe_0.01, which corresponds to a 100% promotion compared with that of intrinsic GeTe. This study provides a new indicator for optimizing carrier and phonon transport properties by balancing interrelated thermoelectric parameters.     

Substitutional Atom Influence on the Electronic and Transport Properties of Mn4Si7

Mn₄Si₇ has been substituted with the following elements: Cu, Fe, Re and Cr on the Mn site, and Al, B, Ge, Sn, Ga, In, Na and Tl on the Si site. The Seebeck coefficient, electrical conductivity and power factor of the substitutional alloys have been evaluated by using density functional theory calculations, and the temperature dependence of the electronic transport properties has been investigated at different charge carrier concentrations by solving the semi-classical Boltzmann transport equation. We show that the substitution of Cu for Mn worsens the thermoelectric properties, while that with Fe appears the?most efficient for increasing the power factor. For In and Tl substitutions, the Seebeck coefficient is increased due to the high d-states contribution in the density of states at the Fermi level. Although the power factor is limited by a poor electrical conductivity, Mn₃₂Si₅₅Tl could exhibit a good figure of merit provided that the amount of Tl atoms is low (at most 1 at.%) and the charge carrier concentration is not too high (around 10²⁰ cm^?3).     

Synergistically Improved Molecular Doping and Carrier Mobility by Copolymerization of Donor–Acceptor and Donor–Donor Building Blocks for Thermoelectric Application

In this work, it is demonstrated that random copolymerization is a simple but effective strategy to obtain new conductive copolymers as high‐performance thermoelectric materials. By using a polymerizing acceptor unit diketopyrropyrrole with donor units thienothiophene and oligo ethylene glycol substituted bithiophene (g₃2T), it is found that strong interchain donor–acceptor interactions ensure good film crystallinity for charge transport, while donor–donor type building blocks contribute to effective charge transfers. Hall effect measurements show that the high electrical conductivity results from increased free carriers with simultaneously improved mobility reaching over 1 cm² V^?1 s^?1. The synergistic effect of improved molecular doping and carrier mobility, as well as a high Seebeck coefficient ascribed to the structural disorder along polymer chains via random copolymerization, results in an impressive power factor up to 110 µW K^?2 m^?1 which is 10 times higher than that of solution‐processed polythiophenes.     

Solid-State Synthesis of Te-Doped Mg<Subscript>2</Subscript>Si

Te-doped Mg₂Si (Mg₂Si:Te _m , m = 0, 0.01, 0.02, 0.03, 0.05) alloys were synthesized by a solid-state reaction and mechanical alloying. The electronic transport properties (Hall coefficient, carrier concentration, and mobility) and thermoelectric properties (Seebeck coefficient, electrical conductivity, thermal conductivity, and figure of merit) were examined. Mg₂Si was synthesized successfully by a solid-state reaction at 673 K for 6 h, and Te-doped Mg₂Si powders were obtained by mechanical alloying for 24 h. The alloys were fully consolidated by hot-pressing at 1073 K for 1 h. All the Mg₂Si:Te _m samples showed n-type conduction, indicating that the electrical conduction is due mainly to electrons. The electrical conductivity increased and the absolute value of the Seebeck coefficient decreased with increasing Te content, because Te doping increased the electron concentration considerably from 10¹⁶ cm⁻³ to 10¹⁸ cm⁻³. The thermal conductivity did not change significantly on Te doping, due to the much larger contribution of lattice thermal conductivity over the electronic thermal conductivity. Thermal conduction in Te-doped Mg₂Si was due primarily to lattice vibrations (phonons). The thermoelectric figure of merit of intrinsic Mg₂Si was improved by Te doping.     

Enhanced Thermoelectric Performance in GeTe by Synergy of Midgap state and Band Convergence

The good co-existence of midgap state and valence band degeneracy is realized in Bi-alloyed GeTe through the In-Cd codoping to play different but complementary roles in the valence band structure modification. In doping induces midgap state and results in a considerably improved Seebeck coefficient near room temperature, while Cd doping significantly increases the Seebeck coefficient in the mid-high temperature region by promoting the valence band convergence. The synergistic effects obviously increase the density of state effective mass from 1.39 to 2.65 m₀, and the corresponding carrier mobility still reaches 34.3 cm² V⁻¹ s⁻¹ at room temperature. Moreover, the Bi-In-Cd co-alloying introduces various phonon scattering centers including nanoprecipitates and strain field fluctuations and suppresses the lattice thermal conductivity to a rather low value of 0.56 W m⁻¹ K⁻¹ at 600 K. As a result, the Ge_0.89Bi_0.06In_0.01Cd_0.04Te sample obtains excellent thermoelectric properties of zT_max ≈2.12 at 650 K and zT_avg ≈1.43 between 300 and 773 K. This study illustrates that the thermoelectric performance of GeTe can be optimized in a wide temperature range through the synergy of midgap state and valence band convergence.     

Thermoelectric properties of Al-doped ZnO: experiment and simulation

Advancement in doping other elements, such as Ce, Dy, Ni, Sb, In and Ga in ZnO^[1], have stimulated great interest for high-temperature thermoelectric application. In this work, the effects of Al-doping in a ZnO system on the electronic structure and thermoelectric properties are presented, by experiment and calculation.Nanosized powders of Zn_1-xAl_xO (x=0, 0.01, 0.02, 0.03 and 0.06) were synthesized by hydrothermal method. From XRD results, all samples contain ZnO as the main phase and ZnAl₂O₄ (spinel phase) peaks were visible when Al additive concentrations were just 6 at%. The shape of the samples changed and the particle size decreased with increasing Al concentration. Seebeck coefficients, on the other hand, did not vary significantly. They were negative and the absolute values increased with temperature. However, the electrical resistivity decreased significantly for higher Al content.The electronic structure calculations were carried out using the open-source software package ABINIT^[2], which is based on DFT. The energy band gap, density of states of Al-doped ZnO were investigated using PAW pseudopotential method within the LDA+U. The calculated density of states was then used in combination with the Boltzmann transport equation^[3] to calculate the thermoelectric parameters of Al-doped ZnO. The electronic band structures showed that the position of the Fermi level of the doped sample was shifted upwards in comparison to the undoped one. After doping Al in ZnO, the energy band gap was decreased, Seebeck coefficient and electrical conductivity were increased.Finally, the calculated results were compared with the experimental results. The good agreement of thermoelectric properties between the calculation and the experimental results were obtained.     

Effects of Spark Plasma Sintering Temperature on Thermoelectric Properties of Higher Manganese Silicide

Higher manganese silicide (HMS) is a promising p-type thermoelectric material. HMS samples were synthesized by a vacuum induction melting process and sintered by spark plasma sintering (SPS) at various temperatures to obtain a single phase of HMS and investigate the effect of the SPS temperature on the thermoelectric properties. A single phase of HMS was obtained, and the appearance and the amount of Mn₂O₃ and MnSi as secondary phases could be controlled via the SPS temperature. The effects of the SPS temperature on the electrical conductivity and Seebeck coefficient of the HMS samples were investigated. The changes in the electrical conductivity and Seebeck coefficient were attributed to changes in the density and the amount of Mn₂O₃ secondary phase.     

Transport properties of undoped Cd0.9Zn0.1Te grown by high pressure bridgman technique

The electron drift mobility of undoped Cd_0.9Zn_0.1Te grown by high-pressure Bridgman method is measured by a time-of-flight technique. The sample shows a room temperature mobility and mobility lifetime product of 950 cm²/Vs and 1.6 × 10⁻⁴cm²/V, respectively. The mobility increases monotonically with decreasing temperature to 3000 cm²/Vs at 100 K. The dominant scattering mechanism for the electron transport is discussed by comparing with the theoretical mobility obtained by iterative solution of the Boltzmann equation.     

Crystallographic, Thermoelectric, and Mechanical Properties of Polycrystalline Ba8Al x Si46−x Clathrates

The Al content dependence of crystallographic, thermoelectric, and mechanical properties is reported for polycrystalline Ba₈Al _x Si_46?x (nominal x = 15 to 17) clathrates prepared by combining arc melting and spark plasma sintering methods. The elastic constants and the coefficient of thermal expansion (CTE), which are also important properties for designing thermoelectric devices, are presented. Powder x-ray diffraction, scanning electron microscopy, and energy-dispersive x-ray spectroscopy (EDX) indicate that the type I clathrate is the major phase of the samples but impurity phases (mainly BaAl₂Si₂, Si, and Al) are included in the samples with high Al contents. The actual Al content x determined by EDX ranges from approximately 14 to 15. The absolute value of the Seebeck coefficient increases and the electrical conductivity decreases as the Al content increases. The changes in Seebeck coefficient and electrical conductivity are explained in terms of the dependence of the carrier concentration on the Al content. The elastic constants and the CTE of the samples depend weakly on the Al content. Some of the properties are compared with reported data of single crystals of Ba₈Al₁₆Ge₃₀, Ba₈Ga₁₆Ge₃₀, Sr₈Ga₁₆Ge₃₀, silicon, and germanium as standard references. The effective mass, Hall carrier mobility, and lattice thermal conductivity, which govern the transport properties, are determined to be ~ 2.4m ₀, ~ 7 cm² V^?1 s^?1, and ~ 1.3 W m^?1 K^?1, respectively, for actual Al content x of about 14.77. The thermoelectric figure of merit ZT is estimated to be about 0.35 at 900 K for actual Al content x of about 14.77.     

Theoretical and experimental investigations of the thermoelectric properties of Al-, Bi- and Sn-doped ZnO

In this paper, the thermoelectric properties of ZnO doped with Al, Bi and Sn were investigated by combining experimental and theoretical methods. The average Seebeck coefficient of Bi doped ZnO over the measured temperature range is improved from −90 to −497 μV/K. However, segregation of Bi₂O₃ in ZnO:Bi sample, confirmed by FESEM, lead to enormous grain growth and low electrical conductivity, which makes Bi is not a good dopant to improve ZT value of ZnO. As a 4+ valence cation, Sn doping actually show an increase in carrier concentration to 10²⁰ cm⁻³, further enhancing the electrical conductivity. Unfortunately, the Seebeck coefficient of ZnO:Sn samples is even lower than pure ZnO sample, which lead to a low ZT value. As for ZnO:Al sample, with nearly no change in lattice thermal conductivity, electrical conductivity and Seebeck coefficient were both enhanced. Threefold enhancement in ZT value has been achieved in ZnO:Al sample at 760 °C compared with pure ZnO.     

The Effect of Annealing in Controlled Vapor Pressure on the Thermoelectric Properties of RF-Sputtered Bi2Te3 Film

To investigate the effect of annealing in controlled atmosphere on the thermoelectric properties of Bi-Te film, Te-deficient Bi-Te film was deposited by sputtering, and then annealed with various Bi-Te alloy powders with different Te concentrations in a closed system at 250°C for 24?h. Bi-Te phases other than Bi₂Te₃ in the as-deposited film could be removed when the film was annealed with Bi-Te source powder containing 62?at.% or higher content of Te. At the same time, the values of Seebeck coefficient and carrier concentration of the films approach ?105???V/K and 3?×?10¹⁹?cm^?3 to 6?×?10¹⁹?cm^?3, respectively. This result indicates that mass transport of Te to the film takes place, resulting in the formation of Bi₂Te₃ phase and reduction of the amount of p-type carriers due to compositional change of the film from Te-deficient to stoichiometric. Annealing in controlled Te-vapor atmosphere is an effective method to improve the thermoelectric properties of Bi-Te film by changing the composition and phase of Te-deficient film to stoichiometric Bi₂Te₃ film.     

Effect of Sb Doping on the Thermoelectric Properties of Mg<Subscript>2</Subscript>Si<Subscript>0.7</Subscript>Sn<Subscript>0.3</Subscript> Solid Solutions

This study focuses on Sb-doped Mg₂(Si,Sn) thermoelectric material. Samples were successfully fabricated using a hybrid synthesis method consisting of three different processes: induction melting, solid-state reaction, and a hot-press sintering technique. We found that the carrier concentration increased with Sb content, while the Seebeck coefficient exhibited a decreasing trend. Sb doping was shown to improve the power factor and thermoelectric figure of merit compared with the undoped material, yielding a peak figure of merit (ZT) of ~0.55 at 620 K, while leaving the band gap of Mg₂Si_0.7Sn_0.3 almost unchanged.     

Effects of Ni, Pd, and Pt Substitutions on Thermoelectric Properties of CoSi Alloys

Ni-, Pd-, and Pt-substituted CoSi samples have been prepared by an arc melting and annealing procedure. The x-ray diffraction and scanning electron microscopy results show that Ni and Pd are effective n-type dopants for CoSi, while Pt is immiscible with CoSi and forms an impurity phase with a possible chemical formula of PtCoSi₂. The thermoelectric properties were measured from 80 K to 300 K. For Ni- and Pd-doped samples, the electrical resistivity and Seebeck coefficient decrease simultaneously due to the increasing carrier concentration. For Pt-substituted samples, the electrical resistivity also decreases. However, this reduction is thought to be not due to an increase in carriers but rather to originate from the effect of the Pt-rich impurity phase at the grain boundaries. The Seebeck coefficient is not affected by 1% Pt substitution; however, further increase of the Pt level also causes a decrease in the Seebeck coefficient. The room-temperature power factor is 63 μW K^?2 cm^?1 for pure CoSi and 73 μW K^?2 cm^?1 for the Co_0.99Pt_0.01Si sample. Although the thermal conductivity is reduced for both n-type-doped and Pt-substituted samples around 80 K, the room-temperature values are still close to that of pure CoSi. As a result, ZT of 0.13 is obtained at room temperature for Co_0.99Pt_0.01Si, an 18% increase compared with CoSi.     

Variable temperature hall effect on p-Hg1−xCdxTe grown on CdTe and sapphire substrates by liquid phase epitaxy

Variable temperature Hall effect measurements have been made down to 9–10K on p-type Hg_1−xCd_xTe grown by liquid phase epitaxy on both CdTe and sapphire substrates. Carrier freeze-out was usually observed throughout the measured temperature range. For most samples, the hole mobility was well-behaved and exhibited a maximum at ˜ 35K. Values of acceptor ionization energy E_A and donor concentration N_D were estimated from the data, using a model assuming significant compensation, which provided a good fit to the low temperature data. In addition, values of N_D were also estimated from an analysis of the low temperature mobility using the hole effective mass as a parameter to provide reasonable agreement between the N_D values calculated from the Hall coefficient and mobility data. The measured carrier concentration is a result of close compensation between stoichiometric acceptors and donors, with N_D usually in the low-10¹⁷ cm⁻³ range. Average values of E_A for as-grown, undoped x = 0.32 layers on CdTe and sapphire substrates are 7.4 and 6.6 meV, respectively. An activation energy of 0.84 meV was determined for a Cu-doped x = 0.32 layer that was annealed in Hg vapor to reduce the number of Hg vacancies. The average E_A for undoped Hg-annealed x = 0.22 layers on CdTe substrates is 2.35 meV. Layers with x = 0.32 grown on sapphire substrates have average carrier concentrations of 2.92 (σ = 0.54) × 10¹⁶ cm⁻³, compared with 4.64 (θ = 1.26) × 10¹⁶ cm⁻³ for the same composition layers grown on CdTe substrates.