Aqueous-Solution Synthesized p-Type Cu2Se0.96Te0.04-xIx/Cu2O Composite and Thermoelectric Performance of TEG Made of 6 Pairs of p-Leg Cu2Se0.96Te0.02I0.02/Cu2O Composite and n-Leg InSb0.94Bi0.06.

Cu2Se-based thermoelectric materials exhibit high dimensionless figure of merit (zT) values at elevated temperatures (900-1000 K) but relatively lower zT values at intermediate temperatures, approximately 500 K. We synthesized a series of polycrystalline Cu2Se0.96Te0.04-xIx/Cu2O composites (where x = 0.00, 0.01, 0.02, and 0.03) using an energy-efficient synthesis method conducted at room temperature, followed by heat treatment at 923 K for 6 h. X-ray diffraction (XRD) analysis confirmed the monoclinic crystal structure of the α phase. The introduction of iodine doping at Te sites introduced electron carriers to p-type Cu2Se0.96Te0.04, reducing the hole carrier concentration. Consequently, the electrical resistivity increased, and the thermopower exhibited a significant increase. The incorporation of electron carriers into the p-type Cu2Se0.96Te0.04/Cu2O composites resulted in an enhanced power factor within the medium-temperature range. Specifically, at 500 K, the Cu2Se0.96Te0.02I0.02/Cu2O (x = 0.02) composites demonstrated the highest power factor among the series of Cu2Se0.96Te0.04-xIx/Cu2O composites, measuring 9.1 µW cm-1 K-2. According to the weighted mobility analysis, it is clear that the x = 0.02 composite possesses the optimal carrier concentration, which accounts for its superior power factor compared to the other composites in the series. Furthermore, the Cu2Se0.96Te0.02I0.02/Cu2O composites and Cu2Se0.96Te0.04/Cu2O composites displayed zT values of 0.49 and 0.33, respectively, at 550 K. Additionally, iodine doping led to an enhancement in the average zT values between 450 and 550 K. Therefore, electron doping in p-type materials presents itself as a viable strategy for shifting the operating temperature of a thermoelectric device from high to medium temperature. We successfully fabricated a thermoelectric generator comprising 6 pairs of p-leg Cu2Se0.96Te0.02I0.02/Cu2O composites and n-leg InSb0.94Bi0.06. This TEG achieved impressive results, including a maximum output voltage, power output, power density, and efficiency of 0.115 V, 10.6 µW, 35.1 µW cm-2, and 1.74% at a temperature difference (ΔT) of 120 K.

Enhanced Thermoelectric Performance of p-Type Mg3-xZnxSb2/Sb Composites: The Role of ZnSb/Sb Composites.

Mg3Sb2-based Zintl compounds have garnered recent attention as promising materials for thermoelectric applications due to their low thermal conductivity and high zT values as n-type materials. However, the zT values of p-type materials are lower compared to their n-type counterparts. Through a straightforward process involving cold pressing and evacuating-and-encapsulating sintering, we have successfully synthesized a variety of p-type Mg3-xZnxSb2/Sb composites by adding the ZnSb-4%Sb composite into the Mg3Sb2 host material. Structural analyses have provided insights into the role of the ZnSb-4%Sb composite, demonstrating its significance in Zn doping on the Mg sites and Sb acting as an additive in the composite. The introduction of Zn on the Mg tetrahedral sites enhances the concentration of carriers, while the presence of highly conductive Sb grains facilitates the movement of charge carriers between adjacent Mg3-xZnxSb2 grains, thereby promoting mobility. Consequently, the electrical resistivity of the Mg3-xZnxSb2/Sb composites decreases as the Zn content increases. At 710 K, the Mg1.91Zn1.09Sb2/Sb composite exhibits the lowest resistivity, measuring 5.1 mΩ-cm, which is 46 times lower than that of the Mg3Sb2 host. Furthermore, the zT value of the Mg3-xZnxSb2/Sb composites increases with higher Zn content (x), benefiting from a combination of an improved power factor and reduced thermal conductivity. Significantly, our straightforward fabrication process enables us to achieve a maximum zT value of 0.58 at 710 K for the Mg1.91Zn1.09Sb2/Sb composite. This achievement can primarily be attributed to the 8-fold enhancement in power factor compared to the Mg3Sb2 host.

Investigation of the electronic structure, mechanical, and thermoelectric properties of novel semiconductor compounds: XYTe (X = Ti/Sc; Y = Fe/Co).

We report the structural, mechanical, electronic, phonon, and thermoelectric properties of new XYTe (X= Ti/Sc; Y = Fe/Co) half-Heusler compounds by employing first principles based DFT computation and Boltzmann transport equations. At their equilibrium lattice constants, these alloys exhibit a crystal structure with a space group (#216) of F4̄3m and adhere to the Slater Pauling (SP) rule, while being non-magnetic semiconductors. The Pugh's ratio of TiFeTe shows that it is a ductile material, which makes it suitable for use in thermoelectric applications. On the other hand, ScCoTe's brittleness or fragility makes it less promising as a potential thermoelectric material. The dynamical stability of the system is investigated using the phonon dispersion curves obtained from its lattice vibrations. The band gaps of TiFeTe and ScCoTe are 0.93 eV and 0.88 eV, respectively. The electrical conductivity (σ), Seebeck coefficient (S), thermoelectric power factor (PF), and electronic thermal conductivity are calculated at various temperatures ranging from 300 K to 1200 K. At 300 K, TiFeTe has a Seebeck coefficient of 1.9 mV K-1 and a power factor of 136.1 mW m-1 K-2. The highest S value for this material is obtained through n-type doping. The optimal carrier concentration for achieving the highest Seebeck coefficient in TiFeTe is 0.2 × 1020 cm-3. Our study indicates that the XYTe Heusler compounds exhibit n-type semiconductor behavior.

An algorithm of calculating transport parameters of thermoelectric materials using single Kane band model with Riemann integral methods.

We develop an algorithm called SKBcal to conveniently calculate within minutes the thermoelectric transport parameters such as reduced Fermi level (η), electronic thermal conductivity (κe), lattice thermal conductivity (κl), Hall factor (A), effective mass (m*), quality factor (ß) and theoretical zT within the framework of single Kane band (SKB) model. The generalized Fermi-Dirac integral for SKB model is integrated by left Riemann integral method. A concept of significant digits of relative error is involved to determine the accuracy of calculation. Furthermore, a combined program of "For" and "While" is coded to set up an iteration for refining the reduced Fermi level. To easily obtain the quality factor, we re-derive the expression into a formula related to carrier mobility. The results calculated by SKBcal are consistent with the data reported in the literatures.

High Performance of Post-Treated PEDOT:PSS Thin Films for Thermoelectric Power Generation Applications.

Thermoelectric materials capable of converting waste heat energy into electrical energy are enchanting for applications in wearable electronics and sensors by harvesting heat energy of the human body. Organic conducting polymers offer promise of thermoelectric materials for next-generation power sources of wearable devices due to their low cost in preparation, easy processing, low toxicity, low thermal conductivity, mechanical flexibility, light weight, and large area application. Generally, the pristine PEDOT:PSS film has low electrical conductivity, small Seebeck coefficient, and low thermal conductivity. The thermoelectric power factors of conducting polymers of p-type PEDOT:PSS films are considerably improved via synergistic effect by using ethylene glycol and reductants of EG/NaBH4 or EG/NaHCO3. As such, the charge carrier concentration of PEDOT:PSS films is tuned. The synergistic effect might lead to enhanced variation of density of states at the Fermi level and hence enhanced Seebeck coefficient. The resulting PEDOT:PSS films were characterized by atomic force microscopy (AFM), Raman spectroscopy, and XPS spectroscopy. The electrical conductivity and Seebeck coefficient were measured between 325 and 450 K. The carrier concentration and mobility were obtained by Hall measurements. The pristine thin film treated with 0.05 M EG/NaHCO3 solution exhibits the highest power factor of 183 µW m-1 K-2 at 450 K among these two series of films due to its significant enhanced Seebeck coefficient of 48 µV/K. The maximum output power of 121.08 nW is attained at the output voltage of 6.98 mV and the output current of 17.45 µA. The corresponding maximum power density is 98 µW/cm2 for a power generation device made of four pairs of p-leg (EG/NaHCO3 post-treated PEDOT:PSS) and n-leg (Cu0.6Ni0.4) on the polyamide substrate with the size of 4 mm × 20 mm for each leg.

Energy-Efficient Synthesis and High Thermoelectric Performance of α-Cu2-y Se1-x Tex.

The high-temperature phase of ß-Cu2 Se always appears as the major phase for the reaction carried out using chemical solution methods. Here, a procedure was developed that could fabricate a single phase of α-Cu2 Se1-x Tex (x=0.02 and 0.04) by room-temperature aqueous synthesis using NaBH4 as reducing agent followed by cold pressing and sintering at 650 °C for 6 h in a flowing gas mixture of 20 % H2 and 80 % N2 . The energy-efficient synthesis carried out at room temperature abides by the 6th principle for green chemistry with less energy consumption. The reaction mechanism was studied, and evidence was provided of α-Cu2 Se being formed via the reaction between elemental Cu and Se atoms at room temperature. The resulting materials were characterized by powder X-ray diffraction, field-emission scanning electron microscopy, thermoelectric transport measurements, and Hall measurements. Cu1.96 Se0.96 Te0.04 had the highest power factor of 11 µW cm-1 K-2 at 818 K, and Cu2 Se0.96 Te0.04 had the maximum zT≥1.4 at T≥920 K among this series of materials.

A facile energy-saving route of fabricating thermoelectric Sb2Te3-Te nanocomposites and nanosized Te.

A facile energy-saving route is developed for fabricating Sb2Te3-Te nanocomposites and nanosized Te powders. The fabrication route not only avoids using organic chemicals, but also keeps the energy consumption to a minimum. The fabrication procedure involves two steps. Energetic precursors of nanosized powders of Sb and Te are produced at room temperature followed by hot pressing at 400°C under 70 MPa for 1 h. The resulting Sb2Te3-Te nanocomposite exhibits enhanced power factor. The dimensionless figure of merit zT value of the Sb2Te3-Te nanocomposite is 0.29 at 475 K.

Enhanced thermoelectric properties of hydrothermally synthesized Bi0.88-x Zn x Sb0.12 nanoalloys below the semiconductor-semimetal transition temperature.

Bi0.88-x Zn x Sb0.12 alloys with x = 0.00, 0.05, 0.10, and 0.15 were prepared using hydrothermal synthesis in combination with evacuating-and-encapsulating sintering. The effects of partial Zn substitution for Bi and different sintering temperatures on the thermoelectric properties of Bi0.88-x Zn x Sb0.12 alloys were investigated between 25 K and 425 K. Both the electrical conductivity and absolute thermopower are enhanced for the set of alloys sintered at 250 °C. The maximum power factor of 57.60 µW cm-1 K-2 is attained for the x = 0.05 alloy sintered at 250 °C. As compared with Zn-free Bi0.88Sb0.12, both the total thermal conductivity and lattice component are reduced upon Zn doping. Bipolar conduction is observed in both electronic and thermal transport. The maximum zT of 0.47 is attained at 275 K for the x = 0.05 alloy sintered at 250 °C.

Enhanced ZT of InxCo4Sb12-InSb Nanocomposites Fabricated by Hydrothermal Synthesis Combined with Solid-Vapor Reaction: A Signature of Phonon-Glass and Electron-Crystal Materials.

A rapid route of synthesizing pristine Co4Sb12 at relatively low temperature was previously developed. However, filling the voids using the same procedure is not successful. We develop a new route to fabricate In-filled cobalt skutterudites with InSb nanoinclusions InxCo4Sb12-(InSb)y via solid-vapor reaction between hydrothermally synthesized Co4Sb12 powder and the indium chunk. The nanocomposites are characterized using powder X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and inductively coupled plasma mass spectrometry (ICP-MS). With the success of partial filling of In into the voids and InSb nanoinclusions, the power factor of the InxCo4Sb12-(InSb)y nanocomposites is significantly enhanced, and the thermal conductivity is lowered as compared with the pristine Co4Sb12. As a result, ZT with its highest value of 1.0 is attained for the hierarchical structured In0.04Co4Sb12-(InSb)0.05 nanocomposite at 575 K. The attained ZT value is among the highest ever reported value at T ≤ 575 K for In-filled cobalt skutterudites.

Optimization and Analysis of Thermoelectric Properties of Unfilled Co(1-x-y)Ni(x)Fe(y)Sb3 Synthesized via a Rapid Hydrothermal Procedure.

A series of nanostructured co-doped Co(1-x-y)Ni(x)Fe(y)Sb3 were fabricated using a rapid hydrothermal method at 170 °C for a duration of 12 h, followed by evacuated-and-encapsulated heating at 580 °C for a short period of 5 h. The resulting samples were characterized using powder X-ray diffraction, field emission scanning electron microscopy, bulk density, electronic and thermal transport measurements. The power factor of Co(1-x-y)Ni(x)Fe(y)Sb3 is significantly enhanced in the high-temperature region due to significant enhancement of the electrical conductivity and absolute value of thermopower. The latter arises from the onset of bipolar effect being shifted to higher temperatures as compared with the non-doped CoSb3. The room temperature thermal conductivity falls in the range between 1.22 and 1.67 W m(-1) K(-1) for Co(1-x-y)Ni(x)Fe(y)Sb3. The thermal conductivity of both the (x,y) = (0.14,10) and (0.14,12) samples is measured up to 600 K and found to decrease with increasing temperature. The thermal conductivity of the (0.14,10) sample goes down to ∼1.02 W m(-1) K(-1). As a result, zT = 0.68 is attained at 600 K. The lattice thermal conductivity is analyzed to gain insight into the contribution of various scattering processes that suppress the heat transfer through the phonons in Co(1-x-y)Ni(x)Fe(y)Sb3. The effect of the simultaneous presence of Co, Ni, and Fe elements on the electronic structure and transport properties of Co(1-x-y)Ni(x)Fe(y)Sb3 is described using the quantum mechanical tunneling theory of electron transmission among the potential barriers.

Thermoelectric properties of Ca(1-x)Gd(x)MnO(3-δ) (0.00, 0.02, and 0.05) systems.

Polycrystalline samples of Ca(1-x)Gd(x)MnO(3-δ) (x = 0.00, 0.02, and 0.05) have been studied by X-ray diffraction (XRD), electrical resistivity (ρ), thermoelectric power (S), and thermal conductivity (κ). All the samples were single phase with an orthorhombic structure. The Seebeck coefficient of all the samples was negative, indicating that the predominant carriers are electrons over the entire temperature range. The iodometric titration measurements indicate that the electrical resistivity of Ca(1-x)Gd(x)MnO(3-δ) correlated well with the average valence of Mn(v+) and oxygen deficiency. Among the doped samples, Ca0.98Gd0.02MnO(3-δ) had the highest dimensionless figure of merit 0.018 at 300 K, representing an improvement of about 125% with respect to the undoped GaMnO(3-δ) sample at the same temperature.

Preparation of Superconducting Na(x)(H2O)yCoO2 using NaMnO4 as the deintercalation and oxidation agent.

We have used aqueous NaMnO4 solution as the deintercalation and oxidation agent to treat gamma-Na0.7CoO2 powders and to successfully obtain superconducting sodium cobalt oxyhydrates, Nax(H2O)yCoO2, with onset Tc approximately 4.6 K without using highly toxic Br2/CH3CN solution. Chemical analyses indicate that the sodium content x decreases with increasing concentration of NaMnO4 solution and depends slightly on the immersion time. Unlike using a high concentration of aqueous KMnO4 as the deintercalation and oxidation agent, all the hydrated products are the c approximately 19.6 A phase with bilayers of water molecules intercalated between the CoO2 layers and sodium layers because of the absence of K+ in the Na+ layers.