Carrier concentration optimization for thermoelectric performance enhancement in n-type Bi2O2Se

The Bi₂O₂Se-based compounds with an intrinsically low thermal conductivity and relatively high Seebeck coefficient are good candidates for thermoelectric application. However, the low electrical conductivity resulted from carrier concentration of only 10¹⁵?cm^?3 for pristine material, which is too low for optimized thermoelectrics. As a result, the carrier concentration optimization of Bi₂O₂Se is important and useful to achieve higher power factor. In this work, the effect of Ge-doping at the Bi site has been investigated systematically, with expectations of carrier concentration optimization. It is found that Ge doping is an efficient method to increase carrier concentration. Due to the largely increased carrier concentration via Ge doping, the room temperature electrical conductivity rises rapidly from 0.03?S/cm in pristine sample to 133?S/cm in x?=?0.08 sample. Combined with the intrinsically low thermal conductivity, a maximum ZT value of 0.30 has been achieved at 823?K for Bi_1.92Ge_0.08O₂Se, which is the highest ZT value for Bi₂O₂Se-based thermoelectric materials.     

Enhanced thermoelectric properties in Ge-doped and single-filled skutterudites prepared by unique melt-spinning method

In this paper, we determined an effective substitution of Ge at Sb sites in Nd-filled p-type skutterudites, a series of Nd_.9Fe₂Co₂Sb_12-xGe_x compounds with compositions of x?=?0, 0.1, 0.2, 0.3 and 0.4 were synthesized by home-made induction melting, assembled inside the glove box and followed by spark plasma sintering process (SPS). The thermoelectric properties are investigated as a function of Ge doping content with fixed Nd-filler at 0.9 and the formation of skutterudite phase is characterized by X-ray diffraction. All samples possess positive Seebeck coefficients, representing effective p-type doping. It is observed that the electrical conductivity decreases with decreasing Ge doping concentrations while increased with temperature due to bipolar effect. The lightly doped samples (x?=?0.1 and 0.2) have lower lattice thermal conductivity over the entire temperature range, in which x?=?0.2 sample shows the highest ZT value of 0.82 at 700?K, which is 30% higher than that of the Ge-free sample. The improvement in ZT can be attributed to the optimized carrier concentration and reduced thermal conductivity. The enhancement of ZT through Ge doping, coupled with drastically reduced processing time, shows that these compounds may have great potential in application as p-type segments of the thermoelectric devices.     

Thermoelectric properties of Mo-doped bulk In2O3 and prediction of its maximum ZT

In a search for new thermoelectric materials, indium oxide (In₂O₃) was selected as a candidate for high-temperature thermoelectric oxide materials due to its intrinsically low thermal conductivity (<2 W/mK) and ZT values around 0.05. However, low electrical conductivity is a factor limiting the thermoelectric performance of this oxide, and was addressed in this study by Mo doping. It was found that Mo is soluble in In₂O₃ but forms secondary phases at a fraction near x = 0.06 and higher. Mo was found to be unsuitable for heavy n-type doping necessary to improve the thermoelectric performance of the oxide to the desired level (ZT = 1). However, the experimental data enabled us to analyze the electrical conductivity behavior and the Seebeck coefficient of doped In₂O₃ with different carrier concentrations, predicting a theoretically achievable maximum power factor value of 1.77 × 10^?3 W/mK² at an optimum carrier concentration. This estimation predicts the highest ZT value of 0.75 at 1073 K, assuming the lattice thermal conductivity value remaining at an amorphous level.     

Mechanical Alloying and Spark Plasma Sintering of BiCuSeO Oxyselenide: Synthesis Process and Thermoelectric Properties

BiCuSeO is a promising thermoelectric material but needs a complicated synthesis process. In this work, fine‐grained BiCuSeO samples with high phase purity were synthesized by a combination of mechanical alloying (MA) and spark plasma sintering (SPS). It is found that BiCuSeO compounds can be formed by the reaction between BiSe and CuO, after BiSe appeared firstly when using Bi, Se, and CuO as raw materials during MA process. The electrical conductivity of the corresponding bulk attains a modest enhancement due to the significantly increased carrier concentration although the carrier mobility was reduced compared with the sample prepared by classical solid‐state reaction. The total thermal conductivity is below 0.7 W/mK in the whole measured temperature range, which is attributable to the increased defects and gain boundaries. As a result, a relatively high ZT value of 0.50 was obtained for the pristine BiCuSeO sample. In addition, when Bi was replaced by Ca element, the electrical transport property of the samples was improved, resulting in an enhancement of ZT in the whole measured temperature range.     

The effect of Er3+ doping on the structure and thermoelectric properties of CdO ceramics

The effect of Er³⁺ doping on the structure and thermoelectric transport properties of CdO ceramics was investigated. The solubility limit of Er³⁺ in CdO was very small and that additions of more than about 0.5 at% Er³⁺ resulted in the presence of Er₂O₃. With the addition of Er³⁺, the average grain size of Cd_1?xEr_xO (0 ≤ x ≤ 0.015) decreased and the carrier concentration as well as mobility increased at room temperature. A small amount of Er³⁺ doping resulted in a marked increase of electric conductivity and a moderate decrease of Seebeck coefficient. Although Er³⁺ doping also leaded to an increase in thermal conductivity, a large ZT of 0.2 was achieved in x = 0.005 sample at 723 K due to the obvious improvement of power factor. The results demonstrate that CdO:Er is a new promising n-type thermoelectric material.     

Enhanced Thermoelectric Performance of <Emphasis Type="Italic">c</Emphasis>-Axis-Oriented Epitaxial Ba-Doped BiCuSeO Thin Films

We reported the epitaxial growth of c-axis-oriented Bi_1?xBa_xCuSeO (0?≤ x ≤?10%) thin films and investigated the effect of Ba doping on the structure, valence state of elements, and thermoelectric properties of the films. X-ray photoelectron spectroscopy analysis reveal that Bi³⁺ is partially reduced to the lower valence state after Ba doping, while Cu and Se ions still exist as +?1 and ??2 valence state, respectively. As the Ba doping content increases, both resistivity and Seebeck coefficient decrease because of the increased hole carrier concentration. A large power factor, as high as 1.24 mWm^?1 K^?2 at 673 K, has been achieved in the 7.5% Ba-doped BiCuSeO thin film, which is 1.5 times higher than those reported for the corresponding bulk samples. Considering that the nanoscale-thick Ba-doped films should have a very low thermal conductivity, high ZT can be expected in the films.     

One-pot fabrication of Ag–SrTiO3 nanocomposite and its enhanced thermoelectric properties

Ag–SrTiO₃ ceramic nanoparticles were fabricated by doping SrTiO₃ with various contents (0.5, 1, 3, and 5%, in mass ratio) of Ag. Composite samples were prepared through a one-pot solvothermal method and sintering process. The temperature-dependent thermoelectric properties of these sample were measured from 300 K to 500 K. The maximum power factor (843.3 μ·W/m·K²) at 500 K, which is ∼3.96 times higher than that of the pristine SrTiO₃ ceramics, was obtained for the Ag–SrTiO₃ composite sample with 1% of Ag. In addition, the thermal conductivity of the composites decreased due to the phonon scattering effect. The maximum thermoelectric figure of merit (ZT), i.e., ∼0.09, which was achieved with 1% of Ag at 500 K, yielded an enhanced power factor and a reduced thermal conductivity. This ZT value was ∼4.27 times larger than that of pristine SrTiO₃ at the same temperature.     

Improvement of thermoelectric performance of oxygen reduced Sr0.7Ba0.3Nb2O6-δ ceramics by Lanthanides doping

Thermoelectric properties of lantanide doped Sr_0.7Ba_0.3Nb₂O₆ ceramics were investigated in the temperature range from 323 K to 1073 K. A better electrical conductivity is obtained in the sample with larger ionic radius of doping element. Thus, the highest PF value is achieved in La-doped sample. The NbO₂ second phase is counterproductive to reduce the lattice thermal conductivity, so La-doped sample without impurity shows low thermal conductivity. Thus, La-doped sample shows an excellent thermoelectric performance ZT ∼ 0.35. The small average grain size and the nano-sized phases are observed in Gd and Dy doped samples, both of which contribute to scattering phonons, resulting in low thermal conductivities.     

Achieving weak anisotropy in N-type I-doped SnSe polycrystalline thermoelectric materials

SnSe is a very strong anisotropic material; sometimes, strong anisotropy is unenviable for producing parts of thermoelectric (TE) devices. In order to study the efficient preparation of high-performance n-type polycrystalline SnSe with weak anisotropy, in this work, we combine mechanical alloying at 450 RPM for 10 h and spark plasma sintering at 773 K under 50 MPa pressure for the preparation of polycrystalline SnSe _0.95-xI_x (x = 0,0.01,0.02,0.03) samples, and investigate the TE properties. The prepared samples show very weak anisotropy. With iodine doping, increased carrier concentration is observed, in agreement with DFT calculations. A peak ZT ≈ 1.02 at 723 K is observed with I-doping of x = 0.02, which is about 225% higher than that of undoped sample with ZT ≈ 0.31 at 723 K in parallel direction, mainly attributed to the enhanced power factor and about 56% reduced thermal conductivity from 0.68 Wm^?1K^?1 to 0.30 Wm^?1K^?1. TE properties in both directions are not much different, and the ratios of electrical and thermal conductivities in both directions are very close to unity.     

Fabrication and high-temperature thermoelectric properties of Ti-doped Sr0.9La0.1TiO3 ceramics

Ti-doped Sr_0.9La_0.1TiO₃ ceramics with high density were successfully prepared in argon atmosphere by conventional solid state reaction. The influences of titanium doping content on the microstructure and thermoelectric properties were investigated. The results showed that titanium was oxidized during the calcination procedure. TiO₂ phase survived and coexisted with Sr_0.9La_0.1TiO₃ phase in the sintered ceramics. The Seebeck coefficients were increased from −163 to −259 μV/K as the temperature increased from 350 K to 1073 K. The thermal conductivity can be significantly reduced by doping Ti. Thermoelectric figure of merit (ZT) first decreased and then increased with increasing Ti doping content. Ceramics showed the best thermoelectric properties when Ti doping amount was 5 wt%, the maximum PF was 7.13 μW/K²/cm, and ZT value was 0.144 at 1073 K.     

Effect of Ag+ doping and Ag addition on the thermoelectric properties of KSr2Nb5O15

The electron and phonon thermal transport behavior of Ag ⁺ doped KSr₂Nb₅O₁₅ were discussed by using the first-principles calculations. The band gap was reduced after Ag⁺ doping, and the electrons near the Fermi level had stronger transition capability, which effectively increased the carrier concentration and electrical conductivity and reduced the thermal conductivity, thereby improving the ZT of the doped KSr₂Nb₅O₁₅ from 0.6298 to 0.7214 (1200 K) under ideal conditions. In addition, the solid-state reaction method was used to prepare Ag nanoparticle added KSr₂Nb₅O₁₅ samples, and their thermoelectric performance was tested. The experimental results and the calculated results showed a good consistent trend in which Ag improved the thermoelectric properties of KSr₂Nb₅O₁₅. When the amount of addition of nanosized Ag was 20 wt%, the power factor and ZT of the material were the highest at 1073 K, which were 0.228 mW/(K²·m) and 0.1090, respectively. This research shows how to improve the thermoelectric performance of KSr₂Nb₅O₁₅ ceramics and broaden their temperature range for application.     

Thermoelectric Properties of Pb‐Doped BiCuSeO Ceramics

A high‐performance thermoelectric BiCuSeO oxyselenides has been prepared by a simple fabrication approach. Phase composition and microstructure analysis indicate that the obtained ceramic samples are almost BiCuSeO phase with plate structure. Our results show that Pb‐doped BiCuSeO bulks have good electrical conductivity, large Seebeck coefficient, and low thermal conductivity. A large power factor ~672 μ/Wm/K² at 573 K can be observed in the BiCuSeO ceramic by the 10% Pb doping, and the dimensionless figure of merit (ZT) can reach 0.95 at 873 K, which makes them promising candidates for thermoelectric applications.     

Synergetic improvement strategy on thermoelectric performance of CuAlO2 compacts

We fabricated Sr and Ca-doped CuAlO₂ compacts from the mixed powder of CuO, Cu₂O, Al₂O₃, and SrO or CaO by multi-pass hot pressing followed by spark plasma sintering (SPS). The XRD results showed the reaction of CuO, Cu₂O and Al₂O₃ was rather complete and only CuAlO₂ single phase was formed when the additive amount of SrO or CaO was appropriate. Multi-pass hot pressing increased the relative density and the usage of plate-like Al₂O₃ as reaction raw material enhanced the orientation degree. In spite of performance improvement conflicts through elemental doping, density increase and orientation improvement, the measurement on the thermoelectric performance indicated that the electrical resistivity, Seebeck coefficient, power factor, dimensionless figure of merit (ZT) were improved to varying degrees through the synergetic action of elemental doping, density increase, and orientation control. The highest power factor and ZT of the Ca-doped CuAlO₂ compacts reached 1.54?×?10^?4 W?m^?1 K^?2 and 0.015 at 973?K, respectively. The thermoelectric performance was higher than those of CuAlO₂ at the same temperature in published work before.     

Realizing n-type BiCuSeO through halogens doping

Since the p-type BiCuSeO has been well developed in 2010, the maximum ZT value > 1.5 was achieved. Recently, the extensive research on n-type BiCuSeO is of interest to match its p-type counterpart. In this work, we found that n-type BiCuSeO can be successfully obtained in Bi_1.04Cu_1.05Se_0.99X_0.01O by using halogens (X = Cl, Br, I) doping. The first-principle calculations on the electronic band structure of BiCuSeO show that the Fermi levels were shifted into the conduction band by halogens doping at Se site. Both electrical transport properties and Hall measurements indicate that halogens are effective electron-dopants to realize n-type BiCuSeO. Interestingly, the results of temperature-dependent Seebeck coefficients elucidate a p-n-p type transport behavior, which experience less and less pronounced with increasing heating-cooling measurement cycles until n-type transports disappeared. Our results indicate that even though a large negative Seebeck coefficient (– 500 μV/K) can be realized in n-type BiCuSeO, but these n-type behaviors are reversible and unstable due to the fragile bonding in Cu-X (X = Cl, Br, I), and thus halogens losing upon several measurement cycles for heating and cooling. Even though our results indicate that the obtained n-type BiCuSeO is not stable through halogens doping, the calculated projected density of states elucidate that a high-performance n-type BiCuSeO can be expected by electron-doping at Bi sites.     

Effect of La3+, Ag+ and Bi3+ doping on thermoelectric properties of SrTiO3: First-principles investigation

First principles calculations were applied to study the thermoelectric properties of La³⁺-, Ag⁺- and Bi³⁺- doped SrTiO₃. With the exception of Sr_0.9La_0.1TiO₃, the band gaps of Sr_0.8La_0.1Ag_0.1TiO₃, Sr_0.8La_0.1Bi_0.1TiO₃ and Sr_0.7La_0.1Ag_0.1Bi_0.1TiO₃ were higher than that of SrTiO₃. La³⁺, Ag⁺, and Bi³⁺ doping can cause an increase in electrical conductivity and power factor, and a decrease in thermal conductivity, which improves the ZT. The thermal conductivities of SrTiO₃, Sr_0.9La_0.1TiO₃, Sr_0.8La_0.1Ag_0.1TiO₃, Sr_0.8La_0.1Bi_0.1TiO₃ and Sr_0.7La_0.1Ag_0.1Bi_0.1TiO₃ successively decreased, while power factor and ZT increased. Sr_0.7La_0.1Ag_0.1Bi_0.1TiO₃, in particular, has the smallest thermal conductivity (2.237 W/m/K), the highest power factor (1.18 mW/(mK²)) and ZT (0.597) at 1200 K, 2.195 times larger than that of SrTiO₃ (0.272). In addition, the solid state reaction method was applied to prepare dense ceramics of 10 wt% Bi-doped and (Bi, Ag)-codoped Sr_0.9La_0.1TiO₃. It is demonstrated that (Bi, Ag)-codoped Sr_0.9La_0.1TiO₃ have improved power factors, thermal conductivities and ZT values. The calculations and experimental results are consistent. This work demonstrates a method of co-doping Bi³⁺, Ag⁺, and La³⁺ to enhance the thermoelectric performance of SrTiO₃.     

Thermoelectric properties and thermal stability of metal-doped cuprous sulfide thermoelectrics

Mn doping and S-evaporation are strategies used to improve the thermoelectric properties and thermal stability of cuprous sulfide thermoelectric materials. Cu_1.8S and Mn-alloyed Cu_1.8S powders were prepared via ball milling, and different samples were obtained via current-assisted sintering at different times. It was found that Mn and S-evaporation optimized the carrier concentration and thus improved the figure of merit (ZT) of the samples. The introduction of pore defects induced by S-evaporation also improved the ZT. The maximum ZT of the optimized sample reached 0.89 at 500 °C. Mn in the samples reacted with oxygen to form an oxide film on the surface of the block, which inhibited the kinetic process of Cu_1.8S decomposition and improved the thermal stability of the samples. However, the reaction between Mn and oxygen led to a continuous loss of metal cations in the material, resulting in changes in the thermoelectric properties.     

Enhanced thermoelectric properties of Cu2Se via Sb doping: An experimental and computational study

In this study, Cu₂Se_1?xSb_x (x = 0.000, 0.005, 0.010, and 0.015) thermoelectric materials were synthesised using a solid-state reaction technique. A first-principles calculation indicated that the formation energy of the substitution of antimony (Sb) on the Se site is negative and more stable than those of copper (Cu) sites. Sb doping enhanced the lamellar orientation, decreased the grain size, and created an acceptor impurity level. The electrical resistivity and Seebeck coefficient decreased with increasing Sb doping. A minimum reduction in the thermal conductivity by approximately three times that of the undoped sample was obtained at x = 0.005 with a value of 0.40 W/m K at 523 K. The maximum figure of merit (ZT) was obtained at x = 0.005 with a value of 0.47 at 523 K. These findings indicate that substituting Sb into Se sites is an efficient approach for improving copper selenide (Cu₂Se) thermoelectric materials.     

High-performance copper selenide nanocomposites for power generation

Cu_1.97Se-x wt.% In₂O₃ (x = 0–1) bulk composites are fabricated by combining high-temperature melting and spark plasma sintering technology. The doping of In³⁺in Cu vacancies tunes the carrier concentration and modifies the band structure of Cu_1.97Se. Additionally, excessive In₂O₃ particles benefit the construction of multiple lattice defects to strengthen phonon scattering and suppress the long-range migration of Cu ions, resulting in obviously reduced thermal conductivity and improved service stability. Ultimately, a peak ZT value of 1.43 is obtained at 873 K for the Cu_1.97Se-0.5 wt.% In₂O₃ bulk specimen, which is almost 150% higher than that of pristine sample. The thermoelectric conversion efficiency and mechanical properties of the composites sample are also measured. The results suggest that Cu_1.97Se-based nanocomposites offer potential for use in power generation device assembly.     

Enhanced thermoelectric properties of CNT dispersed and Na-doped Bi2Ba2Co2Oy composites

The thermoelectric properties of Bi₂Ba₂Co₂O_y and Bi_1.975Na_0.025Ba₂Co₂O_y+x wt% carbon nanotubes (CNT; x=0.00, 0.05, 0.10, 0.15, 0.5, and 1.0) ceramic samples synthesised by the solid-state reaction method were investigated from 300K to 950K. Na doping with a small amount played an important role in reducing resistivity and slightly reduced the Seebeck coefficients and the thermal conductivity. The CNT dispersant increased resistivity, but the thermal conductivity was reduced remarkably. In particular, the Bi_1.975Na_0.025Ba₂Co₂O_y+1.0wt% CNT sample exhibited an ultralow thermal conductivity of 0.39 W K⁻¹ m⁻¹ at 923K. This was attributed to the point defects caused by Na doping and the interface scattering caused by the CNT dispersant. The combination of Na doping and CNT dispersion had better effects on thermoelectric properties. The Bi_1.975Na_0.025Ba₂Co₂O_y+0.5wt% CNT sample exhibited a better dimensionless figure of merit (ZT) value of 0.2 at 923K, which was improved by 78.2%, compared with the undoped Bi₂Ba₂Co₂O_y sample.     

Highly enhanced thermoelectric performance in BiCuSeO ceramics realized by Pb doping and introducing Cu deficiencies

In recent years, BiCuSeO oxyselenides have been developed as a promising thermoelectric material. In this article, Pb_xBi_1−xCu_1−ySeO (x = y = 0, 0.02, 0.04, 0.06, and 0.08) are prepared by solid-state reaction method and spark plasma sintering (SPS), and the combinatorial effects of Pb doping and Cu deficiencies on thermoelectric properties are investigated systematically. The transport properties are significantly enhanced due to the optimized carrier density, majorly contributing to the promotion of ZT values. As a result, the maximum ZT of 0.77 at 873 K and average ZT (from 300 to 873 K) of 0.50 are obtained for Pb_0.06Bi_0.94Cu_0.94SeO sample. The values are 0.4 and 1.2 times, respectively, higher than that of pristine sample.