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.     

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>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.     

Preparation and Thermoelectric Properties of La-Doped SrTiO<Subscript>3</Subscript> Ceramics

Single-phase polycrystalline La _x Sr_1−x TiO₃ (x = 0, 0.04, 0.06, 0.08, and 0.12) ceramics were prepared by the conventional solid-state reaction method using high-activity hydroxides as the raw materials. The electrical conductivity of all the samples increased with increasing x value and decreased with measurement temperature, while the thermal conductivity decreased with increasing x value and measurement temperature. The La_0.12Sr_0.88TiO₃ sample showed the lowest thermal conductivity of 2.45 W m⁻¹ K⁻¹ at 873 K and the largest ZT of 0.28 at 773 K. The present work revealed that hydroxides with high activity as raw materials are beneficial to improve the thermoelectric properties, especially to decrease the thermal conductivity.     

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.     

Solution Processing Route to Multifunctional Titania Thin Films: Highly Conductive and Photcatalytically Active Nb:TiO2

This paper reports the synthesis of highly conductive niobium doped titanium dioxide (Nb:TiO₂) films from the decomposition of Ti(OEt)₄ with dopant quantities of Nb(OEt)₅ by aerosol‐assisted chemical vapor deposition (AACVD). Doping Nb into the Ti sites results in n‐type conductivity, as determined by Hall effect measurements. The doped films display significantly improved electrical properties compared to pristine TiO₂ films. For 5 at.% Nb in the films, the charge carrier concentration was 2 × 10²¹ cm^?3 with a mobility of 2 cm² V^–1 s^–1 . The corresponding sheet resistance is as low as 6.5 Ω sq^–1 making the films suitable candidates for transparent conducting oxide (TCO) materials. This is, to the best of our knowledge, the lowest reported sheet resistance for Nb:TiO₂ films synthesized by vapour deposition. The doped films are also blue in colour, with the intensity dependent on the Nb concentration in the films. A combination of synchrotron, laboratory and theoretical techniques confirmed niobium doping into the anatase TiO₂ lattice. Computational methods also confirmed experimental results of both delocalized (Ti⁴⁺) and localized polaronic states (Ti³⁺) states. Additionally, the doped films also functioned as photocatalysts. Thus, Nb:TiO₂ combines four functional properties (photocatalysis, electrical conductivity, optical transparency and blue colouration) within the same layer, making it a promising alternative to conventional TCO materials.     

Electrical and Thermal Conductivity and Conduction Mechanism of Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> Alloy

Ge₂Sb₂Te₅ alloy has drawn much attention due to its application in phase-change random-access memory and potential as a thermoelectric material. Electrical and thermal conductivity are important material properties in both applications. The aim of this work is to investigate the temperature dependence of the electrical and thermal conductivity of Ge₂Sb₂Te₅ alloy and discuss the thermal conduction mechanism. The electrical resistivity and thermal conductivity of Ge₂Sb₂Te₅ alloy were measured from room temperature to 823 K by four-terminal and hot-strip method, respectively. With increasing temperature, the electrical resistivity increased while the thermal conductivity first decreased up to about 600 K then increased. The electronic component of the thermal conductivity was calculated from the Wiedemann–Franz law using the resistivity results. At room temperature, Ge₂Sb₂Te₅ alloy has large electronic thermal conductivity and low lattice thermal conductivity. Bipolar diffusion contributes more to the thermal conductivity with increasing temperature. The special crystallographic structure of Ge₂Sb₂Te₅ alloy accounts for the thermal conduction mechanism.     

Thermal Stress Analysis and Structure Parameter Selection for a Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Based Thermoelectric Module

The output power and conversion efficiency of thermoelectric modules (TEMs) are mainly determined by their material properties, i.e., Seebeck coefficient, electrical resistivity, and thermal conductivity. In practical applications, due to the influence of the harsh environment, the mechanical properties of TEMs should also be considered. Using the finite-element analysis (FEA) model in ANSYS software, we present the thermal stress distribution of a TEM based on the anisotropic mechanical properties and thermoelectric properties of hot-pressed materials. By analyzing the possibilities of damage along the cleavage plane of Bi₂Te₃-based thermoelectric materials and by optimizing the structure parameters, a TEM with better mechanical performance is obtained. Thus, a direction for improving the thermal stress resistance of TEMs is presented.     

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.     

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.     

Synthesis and Thermoelectric Properties of “Nano-Engineered” CoSb<Subscript>3</Subscript> Skutterudite Materials

Because of their good electrical transport properties, skutterudites have been widely studied as potential next-generation thermoelectric (TE) materials. One of the main obstacles to further improving their thermoelectric performance has been reducing their relatively high thermal conductivity. To some extent, this hindrance has been partially resolved by filling the voids found in the skutterudite structure with so-called “rattling” atoms. It has been predicted that reducing the dimensionality in a TE material would have a positive effect in enhancing its thermoelectric properties, for example increasing the thermopower and reducing the thermal conductivity. Introducing nanoparticles into the skutterudite materials could therefore have favorable effects on their electrical properties and should also reduce lattice thermal conductivity by introducing extra scattering centers throughout the sample. Nanoparticles may also be used in conjunction with void filling for further reduction of the thermal conductivity of skutterudites. Cobalt triantimonide (CoSb₃) samples with different amounts of embedded nanoparticles have been grown, and the electrical and thermal transport properties for these composites have been measured from 10 K to 650 K. The synthetic techniques and electrical and thermal transport data are discussed in this paper.     

Thermoelectric Characterization of (Ga,In)<Subscript>2</Subscript>Te<Subscript>3</Subscript> with Self-Assembled Two-Dimensional Vacancy Planes

The key properties for the design of high-efficiency thermoelectric materials are a low thermal conductivity and a large Seebeck coefficient with moderate electrical conductivity. Recent developments in nanotechnology and nanoscience are leading to breakthroughs in the field of thermoelectrics. The goal is to create a situation where phonon pathways are disrupted due to nanostructures in “bulk” materials. Here we introduce promising materials: (Ga,In)₂Te₃ with unexpectedly low thermal conductivity, in which certain kinds of superlattice structures naturally form. Two-dimensional vacancy planes with approximately 3.5-nm intervals exist in Ga₂Te₃, scattering phonons efficiently and leading to a very low thermal conductivity.     

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.     

Thermoelectric Characterization of Zone-Melted and Quenched Zn<Subscript>4</Subscript>Sb<Subscript>3</Subscript>

Because of its complex structure, Zn₄Sb₃ exhibits relatively low thermal conductivity. This, in combination with large values of the Seebeck coefficient and moderate to high electrical conductivity, makes the material especially interesting for thermoelectric application in temperatures up to 400°C. The phase purity and thermal stability of Zn₄Sb₃ are major issues for its thermoelectric performance and are strongly dependent on the synthesis method, atmosphere, density, and grain size. Therefore, Zn₄Sb₃ was prepared by both zone melting and quenching in this study, and pressed samples from crushed powders of three different grain sizes were compared. The effect of thermal cycling was studied, along with repeated structural analysis and Seebeck mapping. It was found that zone melting leads to improved thermal stability regarding decomposition via Zn loss, which finally may result in the formation of ZnSb. Larger grain size seems to reduce the degradation, because of lower concentration of grain boundaries, thus hindering diffusion inside the material.     

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.     

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 <Emphasis Type="Italic">p</Emphasis>-Type Mg<Subscript>2.00</Subscript>Si<Subscript>0.25</Subscript>Sn<Subscript>0.75</Subscript> with Li and Ag Double Doping

Mg₂Si_1−x Sn _x -system solid solutions are ecofriendly semiconductors that are promising materials for thermoelectric generators in the middle temperature range. To produce a thermoelectric device, high-performance p- and n-type materials must be balanced. In this paper, p-type Mg_2.00Si_0.25Sn_0.75 with Li and Ag double doping was prepared by the liquid–solid reaction method and hot-pressing. Effects of Li and Ag double doping on thermoelectric properties were investigated in the temperature range from room temperature to 850 K. All sintered compacts were identified as single-phase solid solutions with anti-fluorite structure. The carrier concentration increased with the double doping. The temperature dependence of resistivity of the double-doped samples was similar to that of a metal. The seebeck coefficient increased with temperature to a maximum value and then decreased in the intrinsic region. Thermal conductivity decreased linearly with increasing temperature, reaching a minimum near the intrinsic region, and then increased rapidly because of the contribution of the bipolar component. The dimensionless figure of merit reached 0.32 at 610 K for Mg_2.00Si_0.25Sn_0.75 double-doped with Li-5000 ppm and Ag-20000 ppm.     

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.     

Improved‐Performance Dye‐Sensitized Solar Cells Using Nb‐Doped TiO2 Electrodes: Efficient Electron Injection and Transfer

Well‐crystallized Nb‐doped anatase TiO₂ nanoparticles are prepared by a novel synthetic route and successfully used as the photoanode of dye‐sensitized solar cells (DSSCs). The homogenous distribution of Nb in the TiO₂ lattice is confirmed by scanning transmission electron microscopy (STEM) elemental mapping and line‐scanning analyses. After Nb doping, the conductivity of the TiO₂ powder increases, and its flat‐band potential (V_fb) has a positive shift. The energy‐conversion efficiency of a cell based on 5.0 mol% Nb‐doped TiO₂ is significantly better, by about 18.2%, compared to that of a cell based on undoped TiO₂. The as‐prepared Nb‐doped TiO₂ material is proven in detail to be a better photoanode material than pure TiO₂, and this new synthetic approach using a water‐soluble precursor provides a simple and versatile way to prepare excellent photoanode materials.     

First-Principles and Experimental Studies of Y-Doped Mg<Subscript>2</Subscript>Si Prepared Using Field-Activated Pressure-Assisted Synthesis

The thermoelectric properties of Y-doped (1000 ppm, 2000 ppm, 3000 ppm) Mg₂Si fabricated using field-activated pressure-assisted synthesis (FAPAS) have been characterized using measurements of electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) at temperatures ranging from 285 K to 810 K. The Y-doped Mg₂Si samples were n-type in the measured temperature range. A first-principles calculation revealed that the Y atoms were expected to be primarily located at Mg sites. In sample doped with 2000 ppm Y, which exhibited the best electrical and thermal conductivity, the absolute value of the Seebeck coefficient increased in the temperature range of 320 K to 680 K, being higher than that of undoped Mg₂Si. Moreover, this sample exhibited a higher level of electrical conductivity and a higher power factor. In addition, introduction of Y decreased the thermal conductivity appreciably, indicating that Y dopants are favorable for improving the properties of Mg₂Si.