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.     

Thermoelectric Properties of Co<Subscript>4</Subscript>Sb<Subscript>12</Subscript> Skutterudite Materials with Partial In Filling and Excess In Additions

The properties of Co₄Sb₁₂ with various In additions were studied. X-ray diffraction revealed the presence of the pure δ-phase of In_0.16Co₄Sb₁₂, whereas impurity phases (γ-CoSb₂ and InSb) appeared for x = 0.25, 0.40, 0.80, and 1.20. The homogeneity and morphology of the samples were observed by Seebeck microprobe and scanning electron microscopy, respectively. All the quenched ingots from which the studied samples were cut were inhomogeneous in the axial direction. The temperature dependence of the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) was measured from room temperature up to 673 K. The Seebeck coefficient of all In-added Co₄Sb₁₂ materials was negative. When the filler concentration increases, the Seebeck coefficient decreases. The samples with In additions above the filling limit (x = 0.22) show an even lower Seebeck coefficient due to the formation of secondary phases: InSb and CoSb₂. The temperature variation of the electrical conductivity is semiconductor-like. The thermal conductivity of all the samples decreases with temperature. The central region of the In_0.4Co₄Sb₁₂ ingot shows the lowest thermal conductivity, probably due to the combined effect of (a) rattling due to maximum filling and (b) the presence of a small amount of fine-dispersed secondary phases at the grain boundaries. Thus, regardless of the non-single-phase morphology, a promising ZT (S ² σT/κ) value of 0.96 at 673 K has been obtained with an In addition above the filling limit.     

Thermoelectric Properties of NdGd<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>3</Subscript> Prepared by CS<Subscript>2</Subscript> Sulfurization

Ternary rare-earth sulfides NdGd_1+x S₃, where 0 ≤ x ≤ 0.08, were prepared by sulfurizing Ln₂O₃ (Ln = Nd, Gd) with CS₂ gas, followed by reaction sintering. The sintered samples have full density and homogeneous compositions. The Seebeck coefficient, electrical resistivity, and thermal conductivity were measured over the temperature range of 300 K to 950 K. All the sintered samples exhibit a negative Seebeck coefficient. The magnitude of the Seebeck coefficient and the electrical resistivity decrease systematically with increasing Gd content. The thermal conductivity of all the sintered samples is less than 1.9 W K⁻¹ m⁻¹. The highest figure of merit ZT of 0.51 was found in NdGd_1.02S₃ at 950 K.     

Influence of Doping on Structural and Thermoelectric Properties of AgSbSe<Subscript>2</Subscript>

A series of samples with nominal compositions of AgSb_1−x Sn _x Se₂ (with x = 0.0, 0.1, 0.2, and 0.3) and AgSbSe_2−y Te _y (with y = 0.0, 0.25, 0.5, 0.75, and 1.0) were prepared. The crystal structure of both single crystals and polycrystalline samples was analyzed using x-ray and neutron diffractometry. The electrical conductivity, thermal conductivity, and Seebeck coefficient were measured within the temperature range from 300 K to 700 K. In contrast to intrinsic AgSbSe₂, samples doped with Sn and Te exhibit apparent semiconducting properties (E _g = 0.3 eV to 0.5 eV), lower electrical conductivity, and higher values of the Seebeck coefficient for a small amount of Sn (x = 0.1). Further doping leads to decrease of the thermoelectric power and increase of the electrical conductivity. In order to explain electron transport behavior observed in pure and doped AgSbSe₂, electronic structure calculations were performed by the Korringa–Kohn–Rostoker method with coherent potential approximation (KKR–CPA).     

Thermoelectric Properties of Sb-Doped Mg<Subscript>2</Subscript>Si<Subscript>0.3</Subscript>Sn<Subscript>0.7</Subscript>

Mg₂(Si_0.3Sn_0.7)_1−y Sb _y (0 ≤ y ≤ 0.04) solid solutions were prepared by a two-step solid-state reaction method combined with the spark plasma sintering technique. Investigations indicate that the Sb doping amount has a significant impact on the thermoelectric properties of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y compounds. As the Sb fraction y increases, the electron concentration and electrical conductivity of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y first increase and then decrease, and both reach their highest value at y = 0.025. The sample with y = 0.025, possessing the highest electrical conductivity and one of the higher Seebeck coefficient values among all the samples, has the highest power factor, being 3.45 mW m⁻¹ K⁻² to 3.69 mW m⁻¹ K⁻² in the temperature range of 300 K to 660 K. Meanwhile, Sb doping can significantly reduce the lattice thermal conductivity (κ _ph) of Mg₂(Si_0.3Sn_0.7)_1−y Sb _y due to increased point defect scattering, and κ _ph for Sb-doped samples is 10% to 20% lower than that of the nondoped sample for 300 K < T < 400 K. Mg₂(Si_0.3Sn_0.7)_0.975Sb_0.025 possesses the highest power factor and one of the lower κ _ph values among all the samples, and reaches the highest ZT value: 1.0 at 640 K.     

Thermoelectric Properties of Zr<Subscript>3</Subscript>Mn<Subscript>4</Subscript>Si<Subscript>6</Subscript> and TiMnSi<Subscript>2</Subscript>

The Seebeck coefficient, electrical resistivity, and thermal conductivity of Zr₃Mn₄Si₆ and TiMnSi₂ were studied. The crystal lattices of these compounds contain relatively large open spaces, and, therefore, they have fairly low thermal conductivities (8.26 Wm⁻¹ K⁻¹ and 6.63 Wm⁻¹ K⁻¹, respectively) at room temperature. Their dimensionless figures of merit ZT were found to be 1.92 × 10⁻³ (at 1200 K) and 2.76 × 10⁻³ (at 900 K), respectively. The good electrical conductivities and low Seebeck coefficients might possibly be due to the fact that the distance between silicon atoms in these compounds is shorter than that in pure semiconductive silicon.     

Thermoelectric Properties of Ca-Filled CoSb<Subscript>3</Subscript>-Based Skutterudites Synthesized by Mechanical Alloying

Ca _z Co_4−x (Fe/Mn) _x Sb₁₂ skutterudites were prepared by mechanical alloying and hot pressing. The phases of mechanically alloyed powders were identified as γ-CoSb₂ and Sb, but they were transformed to δ-CoSb₃ by annealing at 873 K for 100 h. All specimens had a positive Hall coefficient and Seebeck coefficient, indicating p-type conduction by holes as majority carriers. For the binary CoSb₃, the electrical conductivity behaved like a nondegenerate semiconductor, but Ca-filled and Fe/Mn-doped CoSb₃ showed a temperature dependence of a degenerate semiconductor. While the Seebeck coefficient of intrinsic CoSb₃ increased with temperature and reached a maximum at 623 K, the Seebeck coefficient increased with increasing temperature for the Ca-filled and Fe/Mn-doped specimens. Relatively low thermal conductivity was obtained because fine particles prepared by mechanical alloying lead to phonon scattering. The thermal conductivity was reduced by Ca filling and Fe/Mn doping. The electronic thermal conductivity was increased by Fe/Mn doping, but the lattice thermal conductivity was decreased by Ca filling. Reasonable thermoelectric figure-of-merit values were obtained for Ca-filled Co-rich p-type skutterudites.     

Effect of Cyclic Thermal Loading on the Microstructure and Thermoelectric Properties of CoSb<Subscript>3</Subscript>

The effect of cyclic thermal loading on the microstructure and thermoelectric properties of CoSb₃ was investigated. The microstructures of the samples were characterized by x-ray diffractometry, scanning electron microscopy, energy dispersive x-ray spectrometry and density measurements. The electrical conductivity, the Seebeck coefficient and the thermal conductivity were measured from room temperature to 800 K. Under cyclic thermal loading, antimony partially volatilized from the surface of the sample, and the density obviously decreased. After 2000 cycles, the phase composition of the sample remained stable, and the average grain size did not change significantly. Moreover, the electrical conductivity varied only slightly, except in the low temperature region. The Seebeck coefficient decreased slightly. However, the thermal conductivity changed remarkably with increasing numbers of thermal cycles.     

Thermoelectric Properties of In<Subscript>0.3</Subscript>Ga<Subscript>0.7</Subscript>N Alloys

We report on the experimental investigation of the potential of InGaN alloys as thermoelectric (TE) materials. We have grown undoped and Si-doped In_0.3Ga_0.7N alloys by metalorganic chemical vapor deposition and measured the Seebeck coefficient and electrical conductivity of the grown films with the aim of maximizing the power factor (P). It was found that P decreases as electron concentration (n) increases. The maximum value for P was found to be 7.3 × 10⁻⁴ W/m K² at 750 K in an undoped sample with corresponding values of Seebeck coefficient and electrical conductivity of 280 μV/K and 93␣(Ω cm)⁻¹, respectively. Further enhancement in P is expected by improving the InGaN material quality and conductivity control by reducing background electron concentration.     

Thermoelectric Characteristics of a Commercialized Mg<Subscript>2</Subscript>Si Source Doped with Al,Bi, Ag,and Cu

The thermoelectric characteristics of commercial polycrystalline Mg₂Si doped with Bi, Al + Bi, Ag, and Cu were examined. The samples for the thermoelectric measurements were prepared using the plasma-activated sintering (PAS) technique. The measured values of the Seebeck coefficient were compared with values calculated using the all-electron band-structure calculation package (ABCAP) based on a full-potential augmented-plane-wave (FLAPW) band-structure calculation in a local density approximation (LDA). For the Bi + Al-co-doped samples, the observed values of the dimensionless figure of merit, ZT, were higher than those of solely Bi-doped samples. The maximum value obtained for Bi + Al-doped Mg₂Si was 0.77 at 862 K. For the Ag-doped samples, ZT was significantly lower than that of the Bi + Al-doped samples, with the maximum value being about 0.11 at 873 K.     

Thermoelectric Properties of Co<Subscript>4</Subscript>Sn<Subscript>6</Subscript>Se<Subscript>6</Subscript> Ternary Skutterudites

A ternary ordered variant of the skutterudite structure, the Co₄Sn₆Se₆ compound, was prepared. Polycrystalline samples were prepared by a modified ceramic method. The electrical conductivity, the Seebeck coefficient and the thermal conductivity were measured over a temperature range of 300–800 K. The undoped Co₄Sn₆Se₆ compound was of p-type electrical conductivity and had a band gap E _g of approximately 0.6 eV. The influence of transition metal (Ni and Ru) doping on the thermoelectric properties was studied. While the thermal conductivity was significantly lowered both for the undoped Co₄Sn₆Se₆ compound and for the doped compounds, as compared with the Co₄Sb₁₂ binary skutterudite, the calculated ZT values were improved only slightly.     

Synthesis and Characterization of <Emphasis Type="Italic">p</Emphasis>-Type Pb<Subscript>0.5</Subscript>Sn<Subscript>0.5</Subscript>Te Thermoelectric Power Generation Elements by Mechanical Alloying

A mechanical alloying (MA) process to transform elemental powders into solid Pb_0.5Sn_0.5Te with thermoelectric functionality comparable to melt-alloyed material is described. The room-temperature doping level and mobility as well as temperature-dependent electrical conductivity, Seebeck coefficient, and thermal conductivity are reported. Estimated values of lattice thermal conductivity (0.7 W m⁻¹ K⁻¹) are lower than some reports of functional melt-alloyed PbSnTe-based material, providing evidence that MA can engender the combination of properties resulting in highly functional thermoelectric material. Though doping level and Sn composition have not been optimized, this material exhibits a ZT value >0.5 at 550 K.     

Thermoelectric Properties of Chevrel-Phase Sulfides M<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mo<Subscript>6</Subscript>S<Subscript>8</Subscript> (M: Cr,Mn, Fe,Ni)

Chevrel-phase sulfides M _x Mo₆S₈ (M, Cr, Mn, Fe, Ni; x: 1.3, 2.0) were prepared by reacting appropriate amounts of M, Mo, and MoS₂ powders. The samples were then consolidated by pressure-assisted sintering to fabricate dense compacts. While Cr_1.3Mo₆S₈ crystallized in a triclinic structure, Mn_1.3Mo₆S₈, Fe_1.3Mo₆S₈, and Ni_2.0Mo₆S₈ crystallized in a hexagonal structure. The Seebeck coefficient, electrical resistivity, and thermal conductivity of the sintered samples were measured over the temperature range of 300 K to 973 K. All the samples exhibited a positive Seebeck coefficient. The Seebeck coefficient, electrical resistivity, and thermal conductivity of M_1.3Mo₆S₈ (M: Cr, Mn, Fe) were almost identical and increased with temperature. However, the corresponding values and temperature dependent behavior of Ni_2.0Mo₆S₈ were different from those of M_1.3Mo₆S₈ (M: Cr, Mn, Fe). For Ni_2.0Mo₆S₈, as temperature increased, the Seebeck coefficient and thermal conductivity increased while the electrical resistivity decreased. The highest value of the thermoelectric figure of merit (0.17) was observed in Cr_1.3Mo₆S₈ at 973 K.     

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.     

Thermoelectric Properties of Ag-doped Mg<Subscript>2</Subscript>Ge Thin Films Prepared by Magnetron Sputtering

Silver doped p-type Mg₂Ge thin films were grown in situ at 773 K using magnetron co-sputtering from individual high-purity Mg and Ge targets. A sacrificial base layer of silver of various thicknesses from 4 nm to 20 nm was initially deposited onto the substrate to supply Ag atoms, which entered the growing Mg₂Ge films by thermal diffusion. The addition of silver during film growth led to increased grain size and surface microroughness. The carrier concentration increased from 1.9 × 10¹⁸ cm⁻³ for undoped films to 8.8 × 10¹⁸ cm⁻³ for the most heavily doped films, but it did not reach saturation. Measurements in the temperature range of T = 200–650 K showed a positive Seebeck coefficient for all the films, with maximum values at temperatures between 400 K and 500 K. The highest Seebeck coefficient of the undoped film was 400 μV K⁻¹, while it was 280 μV K⁻¹ for the most heavily doped film at ∼400 K. The electrical conductivity increased with silver doping by a factor of approximately 10. The temperature effects on power factors for the undoped and lightly doped films were very limited, while the effects for the heavily doped films were substantial. The power factor of the heavily doped films reached a non-optimum value of ∼10⁻⁵ W cm⁻¹ K⁻² at 700 K.     

High-Performance (Ag<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>SbTe<Subscript><Emphasis Type="Italic">x</Emphasis>/2+1.5</Subscript>)<Subscript>15</Subscript>(GeTe)<Subscript>85</Subscript> Thermoelectric Materials Prepared by Melt Spinning

A new preparation process combining melt spinning and hot pressing has been developed for the (Ag _x SbTe _x/2+1.5)₁₅(GeTe)₈₅ (TAGS-85) system. Compared with samples prepared by the traditional air-quenching and hot-pressing method, electrical conductivity and thermal conductivity are lowered. The thermoelectric performance of the TAGS-85 samples varied with changing Ag content and reached the highest ZT of 1.48 when x was 0.8 for the melt-spun sample, compared with the maximum ZT of 1.36 for the air-quenched sample. The Seebeck coefficient of the melt-spun TAGS-85 alloys was improved, while both the electrical conductivity and thermal conductivity were decreased. The net result of this process is to effectively enlarge the temperature span of ZT > 1, which will benefit industrial application.     

Preparation and Thermoelectric Characterization of SiC-B<Subscript>4</Subscript>C Composites

SiC-B₄C composites with various values of SiC-to-B₄C ratio and grain size were fabricated by pressureless sintering. This paper presents the results of current investigations of this composite material. This includes the parameters of manufacture (shrinkage, density, and open porosity), thermoelectric properties (electrical and thermal conductivity, and thermopower), and material characterization (x-ray diffraction, scanning electron microscopy, oxidation resistance, and thermal expansion). The results indicate high potential of this composite as an alternative material for thermoelectric applications at high temperatures. The Seebeck coefficient of the composite was higher than that of the single-component materials B₄C and SiC and reached 400 μV/K at 500°C.     

Thermal Stability of Barium and Indium Double-Filled Skutterudite Ba<Subscript>0.3</Subscript>In<Subscript>0.2</Subscript>Co<Subscript>3.95</Subscript>Ni<Subscript>0.05</Subscript>Sb<Subscript>12</Subscript> Coated by SiO<Subscript>2</Subscript> Nanoparticles

Filled skutterudite thermoelectric (TE) materials have been extensively studied to search for better TE materials in the past decade. However, there is no detailed investigation about the thermal stability of filled skutterudite TE materials. The evolution of microstructure and TE properties of nanostructured skutterudite materials fabricated with Ba_0.3In_0.2Co_3.95Ni_0.05Sb₁₂/SiO₂ core–shell composite particles with 3 nm thickness shell was investigated during periodic thermal cycling from room temperature to 723 K in this work. Scanning electronic microscopy and electron probe microscopy analysis were used to investigate the microstructure and chemical composition of the nanostructured skutterudite materials. TE properties of the nanostructured skutterudite materials were measured after every 200 cycles of quenching in the temperature range from 300 K to 800 K. The results show that the microstructure and composition of Ba_0.3In_0.2Co_3.95Ni_0.05Sb₁₂/SiO₂ nanostructured skutterudite materials were more stable than those of single-phase Ba_0.3In_0.2Co_3.95Ni_0.05Sb₁₂ bulk materials. The evolution of TE properties indicates that the electrical and thermal conductivity decrease along with an increase in the Seebeck coefficient with increasing quenching up to 2000 cycles. As a result, the dimensionless TE figure of merit (ZT) of the nanostructured skutterudite materials remains almost constant. It can be concluded that these nanostructured skutterudite materials have good thermal stability and are suitable for use in solar power generation systems.     

Effects of Cooling Rate on Thermoelectric Properties of <Emphasis Type="Italic">n</Emphasis>-Type Bi<Subscript>2</Subscript>(Se<Subscript>0.4</Subscript>Te<Subscript>0.6</Subscript>)<Subscript>3</Subscript> Compounds

A series of Bi₂(Se_0.4Te_0.6)₃ compounds were synthesized by a rapid route of melt spinning (MS) combined with a subsequent spark plasma sintering (SPS) process. Measurements of the Seebeck coefficient, electrical conductivity, and thermal conductivity were performed over the temperature range from 300 K to 520 K. The measurement results showed that the cooling rate of melt spinning had a significant impact on the transport properties of electrons and phonons, effectively enhancing the thermoelectric properties of the compounds. The maximum ZT value reached 0.93 at 460 K for the sample prepared with the highest cooling rate, and infrared spectrum measurement results showed that the compound with lower tellurium content, Bi₂(Se_0.4Te_0.6)₃, possesses a larger optical forbidden gap (E _g) compared with the traditional n-type zone-melted material with formula Bi₂(Se_0.07Te_0.93)₃. Our work provides a new approach to develop low-tellurium-bearing Bi₂Te₃-based compounds with good thermoelectric performance.     

Increase in the Thermoelectric Efficiency of the Disordered Phase of Layered Antiferromagnetic CuCrS<Subscript>2</Subscript>

The thermoelectric figure of merit (ZT) of the layered antiferromagnetic compound CuCrS₂ is further improved with increase in the Cr-vacancy disorder on sintering above 900°C. X-ray photoelectron spectroscopy and x-ray diffraction refinement results for different samples show that the chromium atoms are transferred from the filled layers to the vacant sites between the layers. This atomic disorder increases the electrical conductivity (σ) due to self-doping of the charge carriers and reduces thermal conductivity (κ) due to increase in phonon scattering. The Seebeck coefficient (S) is p-type and remains nearly temperature independent with values between 150 μV/K and 450 μV/K due to electronic doping in different samples.