Preparation of Ni-doped ZnO ceramics for thermoelectric applications

Zn_1−xNi_xO dense ceramics were prepared from Zn_1−xNi_xO nanoparticles with x varying from 0 to 0.06. These nanoparticles were synthesized by liquid route. In the sintered samples, the solubility limit of Ni in the Zn_1−xNi_xO wurtzite structure was found to be 0.03. The increase of x until 0.03 led to a significant raise in both electrical conductivity (σ) and absolute value of Seebeck coefficient (|S|). Ni-richer samples (x > 0.03) contained in addition a small amount of Ni rich secondary phase (Zn_yNi_zO) with a cubic structure similar to NiO. The thermoelectric properties of all samples were investigated from room temperature to 1000 K. All doped samples showed a n-type semiconducting conductivity. For Ni contents higher than x = 0.03, the increase of the secondary phase content induced a decrease in σ and |S|. The highest power factor (0.6 mW m⁻¹ K⁻²) and ZT (0.09) were found for Zn_0.97Ni_0.03O at 1000 K.     

A thermoelectric generator comprising selenium-doped bismuth telluride on flexible carbon cloth with n-type thermoelectric properties

Carbon cloth was used as a flexible substrate for bismuth telluride (Bi₂Te₃) particles to provide flexibility and improve the overall thermoelectric performance. Bi₂Te₃ on carbon cloth (Bi₂Te₃/CC) was synthesized via a hydrothermal reaction with various reaction times. After over 12 h, the Bi₂Te₃ particles showed a clear hexagonal shape and were evenly adhered to the carbon cloth. Selenium (Se) atoms were doped into the Bi₂Te₃ structure to improve its thermoelectric performance. The electrical conductivity increased with increasing Se-dopant content until 40% Se was added. Moreover, the maximum power factor was 1300 μW/mK² at 473 K for the 30% Se-doped sample. The carbon cloth substrate maintained its electrical resistivity and flexibility after 2000 bending cycles. A flexible thermoelectric generator (TEG) fabricated using the five pairs of 30% Se-doped sample showed an open-circuit voltage of 17.4 mV and maximum power output of 850 nW at temperature difference ΔT = 30 K. This work offers a promising approach for providing flexibility and improving the thermoelectric performance of inorganic thermoelectric materials for wearable device applications using flexible carbon cloth substrate for low temperature range application.     

Experimental estimation of the Lorenz number and scattering parameter for p-type bismuth antimony telluride via multiple doping under constant temperature conditions

The effects of the addition of lead as one of the multiple dopants in p-type Bi_0.3?xSb_1.7Te_3.0+0.01Pb_x (x = 0, 0.001, 0.002, 0.003) fabricated by mechanical alloying followed by hot pressing were investigated. Measurements by X-ray diffraction (XRD), differential thermal analysis (DTA), and scanning electron microscopy (SEM) showed the same matrix morphology. The second phase by doped elements was not confirmed by transmission electron microscopy (TEM). By using the lead addition results and previously studied tellurium doping effects, the Lorenz number L was evaluated to be 0.73–1.18 × 10^?8 W S^?1 K^?2. The scattering parameter γ and reduced Fermi energy η were estimated by using expressions on the basis of a one-electron approximation, measured Seebeck coefficients, and the estimated L at room temperature. The γ ranged approximately from ?1.06 to ?0.60 and showed a mutual effect of acoustic and optical phonon scattering. The relationship between a dimensionless figure of merit ZT and η was clarified. The optimum η was determined as ?1.25 at ZT = 1.26. From these results, multi-doped Bi_0.3Sb_1.7Te_3.0 could be applied to evaluate L, γ, and η at a constant temperature.     

Shift of tellurium solid-solubility limit and enhanced thermoelectric performance of bismuth antimony telluride milled with yttria-stabilized zirconia balls and vessels

As a thermoelectric material, Bi_0.3Sb_1.7Te_3.0+x (x = 0‒0.05) was fabricated by mechanical alloying using yttria-stabilized zirconia (YSZ) ceramic balls and vessels, followed by hot pressing. The effects of the added tellurium on the thermoelectric properties of Bi_0.3Sb_1.7Te_3.0 fabricated with YSZ milling media were investigated. All sintered samples were isotropic and showed p-type conduction. The tellurium solid-solubility limit for Bi_0.3Sb_1.7Te_3.0 was determined to be x = 0.01 by differential thermal analysis (DTA). The solid-solubility limit of the sample fabricated using YSZ was narrower than that of the congener prepared with Si₃N₄ balls and stainless-steel metal vessels. Among the evaluated compositions, the Bi_0.3Sb_1.7Te_3.01 sintered disk had the highest dimensionless figure of merit, ZT = 1.30, at room temperature. This value was superior to that of Bi_0.3Sb_1.7Te_3.0+x fabricated using metal vessels. Thus, selection of the milling media affected the optimum doping amount and maximum ZT.     

Nanocomposites of CuO/SWCNT: Promising thermoelectric materials for mid-temperature thermoelectric generators

Nanocomposites of ultra-thin copper oxide nanosheets (CuO NSs) and single-wall carbon nanotubes (SWCNTs) were produced and studied for their thermoelectric (TE) properties. Incorporating a small amount of SWCNTs into the matrix of CuO NSs enhanced the TE properties. This might be due to good band alignment between the two phases with large contact areas. The nanocomposite [CuO]_99.9[SWCNT]_0.1 showed a rapid increase with the increasing temperature in both the Seebeck coefficient and power factor. At 673 K, they reached 882 μV/K and 2500 μW/mK², respectively. The phonon and charge transport properties were attributed to the CuO NS/SWCNT interfaces. A small module of 2 p-n pairs based on CuO/SWCNT nanocomposites (p-type) and SnO₂ nanoparticles (n-type) was constructed and worked in a temperature range of 573–673 K with good stability and reusability. The measured output power was ˜1200 μW, which can power small electronic devices.     

The effect of nano-twins on the thermoelectric properties of Ga2O3(ZnO)m (m = 9, 11, 13 and 15) homologous compounds

Ga₂O₃(ZnO)_m (m = integer) homologous compounds are naturally occurring nanostructured materials. Their intrinsically low thermal conductivity makes them attractive for thermoelectric applications. High density Ga₂O₃(ZnO)_m (m = 9, 11, 13, and 15) single phase ceramics were prepared by solid-state reaction. Nano-sized, twin-like V-shaped boundaries parallel to b-axis (apex angle ∼ 60°) were observed for all compositions. Atomic resolution Z-contrast imaging and EDS analysis for m = 15 showed segregation of Ga ions at the interface of V-shaped twin boundaries. Thermal and charge transport properties depend on the value of m. Compositions with m = 9 exhibited very low lattice thermal conductivity of 2 to 1.5 W/m.K at 300 K–900 K; compositions with m=15 showed improved power factor of 140 μW/m. K² at 900 K leading to a thermoelectric figure of merit (ZT value) of 0.055. This study explores the structural variants and routes to improve the thermoelectric properties of these materials     

Synthesis and thermoelectric performance of titanium diboride and its composites with lead selenide and carbon

The exploration of new thermoelectric material is the current area of research in energy conversion and storage technologies, in that nanocomposite approach is a promising root to get desirable thermoelectric properties. The present study demonstrates a composite containing highly conductive titanium diboride (TiB₂), polyvinyl alcohol (PVA) as binder and lead selenide (PbSe) as semiconductor. The synthesis and transport physics are studied with an intention to increase the power factor and figure of merit (ZT) of TiB₂ by reducing thermal conductivity through creating inhomogeneity in microstructures. Sol gel method and carbothermal reduction reaction have been used to synthesize TiB₂. More than 95% of thermal conductivity is reduced due to the phonon scattering, which is desirable to achieve a high power factor and ZT. TiB₂/PVA composite possesses a very low Seebeck coefficient and exhibits three order of magnitude reduction in electrical conductivity, which hinders in achieving a good power factor and ZT. Power factor of 25.3?µW/mK², Seebeck coefficient of 36.3 μV/K at 550?K and electrical conductivity of 2.5?×?10⁴ S/m at ~300?K and ZT of 0.064 at 550?K are worth to report in this study. Finally, the synthesized TiB₂ is incorporated into PbSe to evaluate thermoelectric properties. Maximum ZT of 0.12 at 495?K, Seebeck coefficient of ?342?µV/K at 550?K, electrical conductivity of 2.8?×?10³ S/m at 400?K, thermal conductivity of 1.03?W/mK at 550?K and highest power factor of 280.2?µW/mK² at 495?K have been achieved in this composite.     

Fabrication of bismuth telluride nanoparticles using a chemical synthetic process and their thermoelectric evaluations

Bismuth telluride nanoparticles for thermoelectric applications were successfully prepared via a water-based chemical reaction. In this process, we used both a complexing agent (ethylenediaminetetraacetic acid) and a reducing agent (ascorbic acid) to stabilize the bismuth precursor (Bi(NO₃)₃) in water and to favor the reaction with the reduced source of tellurium. The resulting powder was confirmed to range in size below ca. 100 nm with the crystalline structure corresponding to the rhmobohedral Bi₂Te₃. We sintered the nanocrystalline powder via a spark plasma sintering process, thus we obtained the sintered body composed of nano-sized grains. Then, we measured some important transport properties (electrical resistivity, Seebeck coefficient, and thermal conductivity) of the sintered body to calculate its thermoelectric performance, the figure of merit. Finally, we discussed the effect of the nanostructure in the sintered body on the thermal conductivity.     

Oxide materials for high temperature thermoelectric energy conversion

Thermoelectric energy conversion can be used to capture electric power from waste heat in a variety of applications. The materials that have been shown to have the best thermoelectric properties are compounds containing elements such as tellurium and antimony. These compounds can be oxidized if exposed to the high temperature air that may be present in heat recovery applications. Oxide materials have better stability in oxidizing environments, so their use enables the fabrication of more durable devices. Thus, although the thermoelectric properties of oxides are inferior to those of the compounds mentioned above, their superior stability may expand potential the high temperature application of thermoelectric energy conversion.In this paper, the thermoelectric properties of promising oxide materials are reviewed. The different types of oxides used for thermoelectric applications are compared and approaches for improving performance through doping are discussed.     

Enhanced thermoelectric performance in aluminum-doped zinc oxide by porous architecture and nanoinclusions

As a promising thermoelectric material, aluminum-doped zinc oxide (AZO) exhibits high thermal conductivity, which limits its use in high-temperature thermoelectric applications. Here, we report an effective route for forming a porous architecture and nanoinclusions by introducing nano-SiC to reduce the thermal conductivity. The simultaneous formation of a porous architecture and nanoinclusions promotes enhanced phonon scattering, resulting in fairly low thermal conductivity of approximately 3.3 W? m^?1? K^?1. Meanwhile, the Seebeck coefficient shows the significant improvement due to energy filtration effect caused by porous architecture and nanoinclusions. The resultant dimensionless figure of merit of approximately 0.2 at 1050 K was 1.5 times higher than that of AZO ceramic without nano-SiC, which is attributed to the combined factors of increased Seebeck coefficient and reduced thermal conductivity.     

Effects of sulfur substitution for oxygen on the thermoelectric properties of Bi2O2Se

In the present work, the thermoelectric properties of S-doped Bi₂O_2-xS_xSe at the temperatures from 320 to 793 K have been studied. The results show that the solubility limit of S is around x = 0.01 and S-doping is helpful to the sintering and grain growth of Bi₂O₂Se. Moreover, S-doping reduces the band gap of Bi₂O_2-xS_xSe remarkably as x rises. As a result, a thousand times promotion of electrical conductivity at x = 0.02 is obtained, leading to a nearly 3 times increase of power factor at 787 K. By virtue of the intrinsically low thermal conductivity, a peak ZT of 0.29 at 793 K with an average of 0.21 has been achieved for Bi₂O_1.98S_0.02Se, which is nearly 3 and 6 times larger than that of the pristine one. This study indicates that a small amount of S substitution for O could improve the thermoelectric properties of Bi₂O₂Se effectively.     

High thermoelectric performance achieved in Bi0.4Sb1.6Te3 films with high (00l) orientation via magnetron sputtering

To obtain p-type Bi–Sb–Te-based thin films with excellent thermoelectric performance, the Bi_0.4Sb_1.6Te₃ target is prepared by combining mechanical alloying with the spark plasma sintering technique. Afterward, Bi_0.4Sb_1.6Te₃ thin films are deposited via magnetron sputtering at variable working pressures. With an increasing working pressure, the frequency of collisions between the argon ions and sputtered atoms gradually increases, the preferred orientation of (00l) increases, and the sputtering rate decreases. The Seebeck coefficient increases from ∼140 μV/K to ∼220 μV/K as the carrier concentration decreases along with an increasing working pressure. Furthermore, the decrease in carrier concentration and acceleration of carrier mobility also affect the change in electrical conductivity. The maximum power factor of the p-type Bi_0.4Sb_1.6Te₃ thin film deposited at 4.0 Pa and at room temperature exceeds 20.0 μW/cm K² and is higher than that of most p-type Bi–Sb–Te-based films.     

Single-layer XBi2Se4 (X = Sn Pb) with multi-valley band structures and excellent thermoelectric performance

Using the first-principle simulations and the Boltzmann transport equation, our study investigated the properties of single-layer SnBi₂Se₄ and PbBi₂Se₄, including stability, elasticity, electronic and thermoelectric transport properties. We discovered that both 2D materials have acceptable cleavage energies ranging from 0.27 to 0.28 J/m² and that they are indirect semiconductors with narrow band gaps of 0.68 eV and 0.94 eV, respectively. Interestingly, the valence band maximum exhibits ‘multi-valley’ energy dispersion. Furthermore, SnBi₂Se₄ and PbBi₂Se₄ have comparable electron and hole mobility of about ∼10² cm²/Vs and ∼10³ cm²/Vs, respectively resulting in high conductivity and a high thermoelectric power factor. Owing to low group velocities and strong phonon–phonon scattering rates, the materials exhibit low lattice thermal conductivities of 2.59 W/mK (SnBi2Se₄) and 1.73 W/mK (PbBi₂Se₄). Thus, they demonstrate high thermoelectric figures of merit, namely 0.31 (SnBi2Se₄) and 0.37 (PbBi₂Se₄) at 300 K, which rise further to 1.22 and 1.82, respectively, at 700 K. Our results suggest that these two single-layer materials are promising candidates for use in nanoelectronics and thermoelectric appliances.     

Synergistic effects of zirconium- and aluminum co-doping on the thermoelectric performance of zinc oxide

This work aims to explore zirconium as a possible dopant to promote thermoelectric performance in bulk ZnO-based materials, both within the single-doping concept and on simultaneous co-doping with aluminum. At 1100–1223 K mixed-doped samples demonstrated around ～2.3 times increase in ZT as compared to single-doped materials, reaching ～0.12. The simultaneous presence of aluminum and zirconium imposes a synergistic effect on electrical properties provided by their mutual effects on the solubility in ZnO crystal lattice, while also allowing a moderate decrease of the thermal conductivity due to phonon scattering effects. At 1173 K the power factor of mixed-doped Zn_0.994Al_0.003Zr_0.003O was 2.2–2.5 times higher than for single-doped materials. Stability tests of the prepared materials under prospective operation conditions indicated that the gradual increase in both resistivity and Seebeck coefficient in mixed-doped compositions with time may partially compensate each other to maintain a relatively high power factor.     

A sandwich structure assisted by defect engineering for higher thermoelectric performance in ZnO-based films

Gallium (Ga) doping together with low dimensionality has been a promising approach to improve thermoelectric performance of zinc oxide (ZnO) materials, due to the increase of carrier concentration and suppression of phonon transport. So far, the highest power factor of Ga-doped ZnO (GZO) thin films has reached 280 μW m⁻¹ K⁻², which is still limited for practical applications. In this work, we have simultaneously optimized the electrical conductivity and Seebeck coefficient of GZO thin films using the combination of oxygen defects and sandwich structure (GZO-ZnO-GZO). Benefiting from energy filtering effect at the interface between GZO and ZnO layers and high oxygen vacancy concentration, the density of states (DOS) effective mass has been increased together with a relatively high carrier concentration. As a result, an improved power factor value of 434 μW m⁻¹ K⁻² at 623 K has been achieved, which is comparable to the best values reported for ZnO-based films. This method of combining defect engineering and sandwich structure design shows great potential in enhancing the thermoelectric performance of ZnO-based thin films or other oxide materials.     

Enhanced thermoelectric performance based on special micro-configuration of graphene-ZnO induced by high-pressure and high-temperature

Zinc oxide (ZnO) is a promising high-temperature thermoelectric material. Graphene is typically a two-dimensional material, and its development and application have attracted wide attention due to its excellent thermal stability and mechanical properties. To the best of our knowledge, the graphene-ZnO (C–ZnO) composite has never been studied in the field of thermoelectric conversion. The high-pressure and high-temperature (HPHT) technique has unique advantages in improving the thermoelectric properties of ZnO. In this study, for the first time, C–ZnO bulk energy materials with novel micro-configuration were prepared by rapid sintering using the HPHT method. Observation under a microscope revealed that as the doping amount of graphene increased, a large number of graphene nanowires formed connected between the ZnO grains, and with the excess amount of graphene introduced the morphology of the ZnO grains changed and their size became smaller. This novel micro-configuration of the 0.1C–ZnO sample showed an ultrahigh electrical conductivity of 2.8 × 10⁴ S/m with a significantly lower lattice thermal conductivity of 4.3 Wm^?1K^?1 at 973 K. Ultimately, at 973 K, the zT value of the 0.1C–ZnO sample was 129 times higher than that of pure ZnO. Therefore, the high-temperature thermoelectric material C–ZnO prepared by the HPHT method can be used in automobile exhaust systems and industrial boilers to effectively recover and reuse the waste heat.     

A novel high-entropy perovskite ceramics Sr0.9La0.1(Zr0.25Sn0.25Ti0.25Hf0.25)O3 with low thermal conductivity and high Seebeck coefficient

In this work, a novel high-entropy n-type thermoelectric material Sr_0.9La_0.1(Zr_0.25Sn_0.25Ti_0.25Hf_0.25)O₃ with pure perovskite phase was prepared using a conventional solid state processing route. The results of TEM and XPS show that various types of crystal defects and lattice distortions, such as oxygen vacancies, edge dislocations, in-phase rotations of octahedron and antiparallel cation displacements coexist in this high-entropy ceramic. At 873 K, the high-entropy ceramics showed both a low thermal conductivity (1.89 W/m/K) and a high Seebeck coefficient (393 μV/K). This work highlights a way to obtain high-performance perovskite-type oxide thermoelectric materials through high-entropy composition design.     

Formation, Characterization and Electrical Conductivity of Polycarbosilazane-Cu(II), -Ni(II) and -Cr(III) Chloride Metallopolymers

A series of transition metal-polycarbosilazane complexes have been prepared by the reaction of poly(N,N-bis(dimethylsilyl)ethylenediamine), [–Si(CH₃)₂NHCH₂CH₂NH–] _n , with Cu(II), Ni(II), and Cr(III) chloride. The resulting complexes were characterized by infrared (FT-IR) and UV-visible spectroscopy, magnetic susceptibility measurements, thermogravimetric analysis (TGA), and powder X-ray diffraction (XRD). The average chain-chain spacing in these materials were estimated from XRD data and found to be 6.88, 7.91, 7.09, and 6.33 Å in metal-free, Cu(II)-, Ni(II)-, and Cr(III)-containing polycarbosilazanes, respectively. DC electrical conductivity measurements showed that all these metal-polycarbosilazane complexes exhibit semiconductor behavior while the metal-free matrix is an insulator.     

The effects of clay on the thermal degradation behavior of poly(styrene-co-acrylonitirile)

The thermal degradation of poly(acrylonitrile-co-styrene) (SAN) and its clay nanocomposites were studied using TGA/FTIR and GC/MS. Virgin SAN degrades by chain scission followed by β-scission, producing monomers, dimers and trimers. The degradation pathway of SAN in clay nanocomposites contains additional steps; extensive random chain scission, evolving additional compounds having an odd number of carbons in the chain backbones, and radical recombination, producing head-to-head structures. Since acrylonitrile-butadiene-styrene copolymer (ABS) has butadiene rubber incorporated as a grafted phase in a SAN matrix, ABS follows a similar degradation pathway as that of SAN. The effect of butadiene rubber is similar to that of clay, leading to extensive random scission and an increase in thermal stability, but as not effective as clay due to its shorter duration. Eventually, the butadiene rubber phase degrades to small aliphatic molecules.     

Enhancing thermoelectric properties of p-type (Bi,Sb)2Te3 via porous structures

Bismuth telluride is a widely used commercial thermoelectric material with excellent thermoelectric performances near room temperature. Reducing thermal conductivity is one of the most effective ways to improve performances of thermoelectric materials. In this study, the thermal conductivity of the material was reduced by fabricating porous structures. Highly dense NaCl-(Bi,Sb)₂Te₃ composites were fabricated by a high-pressure technology. The NaCl phase was then removed from the composites by ultrasonic washing to produce porous structures. The produced (Bi,Sb)₂Te₃ porous materials possessed excellent thermoelectric properties. The porosity and pore size of the (Bi,Sb)₂Te₃ porous materials increased with the increasing NaCl content, decreasing the thermal conductivity significantly. An ultra-low lattice thermal conductivity of 0.21 Wm^?1K^?1 at 493 K was achieved when the porosity was 39%, almost the lowest lattice thermal conductivity reported for (Bi,Sb)₂Te₃ bulk materials. The figure of merit ZT value was enhanced to 1.05 at 493 K when the porosity was 25%. Compared with the most compacted samples (ZT = 0.79 and porosity of 10%) prepared under the same conditions, the ZT value of the porous samples increased by 33%. This study indicated that porous thermoelectric materials can be prepared simply, quickly and efficiently by high-pressure/ultrasonication washing to improve thermoelectric performances, which has evident reference values for preparing other thermoelectric pore materials with enhancing behaviors.