High Temperature Thermoelectric Properties Research Articles

Enhancing Thermoelectric Performance of CrN Ceramics by Optimizing Sintering Temperature

Chromium nitride (CrN) ceramics were sintered at various temperatures by plasma activated sintering, and their thermoelectric properties were explored. The formation of Cr2N phase can be suppressed by venting N2 gas into the chamber before sintering and increasing the sintering temperature to accelerate the densification process. Sintering temperature significantly impacted the thermoelectric properties. Higher sintering temperature led to an increase in the N content, reducing the electrical and thermal conductivity, and increasing the Seebeck coefficient. The single-phased CrN ceramic sintered at 1573K exhibited excellent thermoelectric performance with a maximum power factor of 1002 μW·m-1·K-2, a thermal conductivity of 4.78W·m-1·K-1, and a zT value of 0.20 at 973K. Furthermore, this CrN ceramic also exhibited a high relative density of 97% and a hardness of ~11.9GPa. Enhanced high-temperature thermoelectric properties and excellent mechanical properties make CrN ceramics suitable for thermoelectric applications in extreme environments.

Transport properties of mechanochemically synthesized copper (I) selenide for potential applications in energy conversion and storage

This work studied the thermal stability, electrical, and thermoelectrical properties of copper(I) selenide, Cu2Se synthesized by high-energy milling in a planetary ball mill. The phase composition was investigated by X-ray powder diffraction analysis and scanning electron microscopy. The conversion of the precursors during mechanochemical synthesis and the stability of the product was monitored by thermal analysis. The dependence of electrical properties on the product porosity was observed. For the densification of Cu2Se, the method of spark plasma sintering was applied to prepare suitable samples for thermoelectric characterization. High-temperature thermoelectric properties of synthetic Cu2Se were compared to its natural analogue-mineral berzelianite in terms of its potential application in energy conversion. Based on the results a relatively high figure-of-merit, ZT parameter (~ 1.15, T = 770 K) was obtained for undoped Cu2Se, prepared by rapid mechanochemical reaction (5 min). Cyclic voltammetry measurements of Na/NaClO4/Cu2Se cell implied that mechanochemically synthesized Cu2Se could be used as a promising intercalation electrode for sodium-ion batteries.

Oxygen vacancy-activated thermoelectric properties of ZnO ceramics

Understanding the structural and thermoelectric (TE) properties of a pure material is essential for developing a practical approach to improve its TE performance. This study focuses on the high-temperature TE properties of ZnO ceramics synthesized by the spark plasma sintering (SPS) technique. The analyses of crystallinity and microstructure along with photoluminescence and Raman spectroscopy indicated an increase in oxygen vacancy in the ZnO ceramics with SPS temperature, which resulted in unit-cell shrinkage, lattice modification, grain densification, and TE modification. Specifically, oxygen vacancies were found to be a crucial factor affecting the TE performance of the spark-plasma-sintered ZnO ceramics at temperatures exceeding 773 K. Oxygen vacancies can act as carrier donors when they are ionized, contributing to an increase in electrical conductivity and a decrease in thermal conductivity. Additionally, this study proposes a potential way to engineer native defects in ZnO ceramics by controlling the SPS process.

Study on Enhancing the Thermoelectric Stability of the β-Cu2Se Phase by Mn Doping.

Cu2Se is a promising thermoelectric (TE) material due to its low cost, Earth abundance, and high thermoelectric properties. However, the biggest problem of Cu2Se is its unstable chemical properties. In particular, under the action of an electric field or gradient temperature field, the chemical potential of copper ions inside the material increases. When the external field is strong enough, the chemical potential of copper ions at the negative end of the material reaches the chemical potential of elemental copper. Under these conditions, copper ions must precipitate out, causing Cu2Se to be unstable, and making it unsuitable for use in applications. In this study, we prepared Cu2-xMnxSe (x = 0, 0.02, 0.04 and 0.06) series bulk materials by vacuum melting-annealing and sintered by spark plasma sintering (SPS). We investigated the effects of Mn doping on the composition, microstructure, band structure, scattering mechanism, thermoelectric properties, and stability of Cu2Se. The results show that Mn doping can adjust the carrier concentration, promote the stabilization of the β-phase structure and improve the electrical properties of Cu2Se. When x = 0.06, the highest power factor (PF) value of Cu1.94Mn0.06Se at 873 K was 1.62 mW m-1 K-2. The results of carrier scattering mechanism analysis based on the conductivity ratio method show that the sample doped with Mn and pure Cu2Se had the characteristics of ionization impurity scattering, and the scattering factor was 3/2. However, the deterioration in thermal conductivity was large, and a superior zT value needs to be obtained. The cyclic test results of high-temperature thermoelectric properties show that Mn doping can hinder Cu+ migration and improve its thermoelectric stability, which preliminarily verifies the feasibility of using the stable zirconia mechanism to improve the thermoelectric stability of Cu2Se.

Improved High Temperature Thermoelectric Properties in Misfit Ca3Co4O9 by Thermal Annealing

Ca3Co4O9, a p-type thermoelectric material based on transition-metal oxides, has garnered significant interest due to its potential in thermoelectric applications. Its unique misfit-layered crystal structure contributes to low thermal conductivity and a high Seebeck coefficient, leading to a thermoelectric figure of merit (zT) of ≥1 at 1000 K. Conventionally, it has been believed that thermopower reaches its upper limit above 200 K. However, our thermopower measurements on polycrystalline Ca3Co4O9 samples have revealed an unexpected increase in thermopower above 380 K. In this study, we investigate the effects of high oxygen pressure annealing on Ca3Co4O9 and provide an explanation based on the mixed oxide states of cobalt and carrier hopping. Our results demonstrate that annealing induces modifications in the defect chemistry of Ca3Co4O9, leading to a decrease in electron hopping probability and the emergence of a thermal activation-like behavior in thermopower. These findings carry significant implications for the design and optimization of thermoelectric materials based on misfit cobaltates, opening new avenues for enhanced thermoelectric performance.

LiNbCoX (X = Al, Ga) quaternary Heusler compounds for high-temperature thermoelectric properties: a computational approach

LiNbCoX (X = Al, Ga) quaternary Heusler compounds for high-temperature thermoelectric properties: a computational approach

Electronic structure, phonon stability, mechanical and high-temperature thermoelectric properties of Li-based quaternary Heusler alloys

This paper presents a detailed discussion of thermoelectric and electronic properties of newly designed Li-based Heusler compounds (LiScPtGe, LiYPtSn, LiYPdPb) using Boltzmann transport theory alongside the first-principles calculations. Our investigations predict that these materials exhibit band gaps 0.76 eV (for LiScPtGe), 0.67 (for LiYPtGe) and 0.21 eV (for LiYPdPb), respectively. All the reported materials are indirect band gap semiconductors; mechanical and dynamical stability is also confirmed. At 300 K, the lowest value of lattice thermal conductivity is observed in LiYPdPb (21.64 Wm-1 K−1), which is very small as compared to the other two materials. The perceived value of the figure of merit (ZT) is 0.61 (for LiScPtGe), 0.52 (for LiYPtSn) and 0.35 (for LiYPdPb) respectively, and probably ensure a considerable thermoelectric efficiency of these newly designed materials.

Ultrafast high-temperature sintering and thermoelectric properties of n-doped Mg2Si

Ultrafast high-temperature sintering (UHS) is a recently proposed technique able to synthesize and sinter dense materials within seconds. Although UHS has already proved its effectivity with a large set of materials, spanning from refractory ceramics to complex metal alloys, any application to thermoelectric materials is today still lacking. Mg2Si is a well-established thermoelectric material. It is based on wide available non-critical raw materials, it is non-toxic, lightweight and it expresses its best thermoelectric performances in the intermediate temperature range (up to about 600 °C). Mg2Si is typically produced with powder processing by Spark Plasma Sintering (SPS), partially limiting its widespread diffusion also due to the costly production technique. Here we present a simple route to sinter Mg2Si pressed powders by UHS. The process allowed to obtain dense samples (with relative densities >95%) with 20 s heating up to about 1080 °C followed by a rapid free cooling, a total thermal history below 1 min, and with energy demand at the Wh scale. The high process rate proved its efficacy in preventing grain growth and in avoiding any significant Mg evaporation. A full thermoelectric functional characterization is presented for Mg2Si and Bi-doped Mg2Si, together with a comparison with SPS-produced properties.

Tuning optical and thermoelectric properties of LaCoO3: Partial substitution of La by isovalent Gd

In this study, we report the structural, hardness, optical, and high temperature thermoelectric properties of Gd-doped lanthanum cobaltite La1−xGdxCoO3 (x = 0.0, 0.2, 0.4, 0.6). The samples have been prepared by solid-state reaction technique. The room temperature XRD studies revealed that the pristine sample (x = 0.0) possesses a distorted rhombohedral structure with an R-3c space group. The samples with x ≥ 0.2 show a structural phase transition and possess an orthorhombic structure with the Pnma space group. Structural and optical studies confirm the formation of the La1-xGdxCoO3 compound. The electrical resistivity for all the studied compositions is found to be inversely proportional to temperature, which is a typical semiconductor behavior. However, the electrical resistivity was found to increase with increased Gd content in the material. The pure and Gd substituted samples show a crossover from negative to positive thermopower, thereby indicating the presence of both electrons and holes as charge carriers. The substitution of Gd has enhanced the thermopower at high temperatures. The highest thermopower of –345 μV/K was obtained at 310 K for the pristine sample. A significant drop in electrical resistivity (T > 400 K) has caused the power factor to rise, resulting in a higher power factor of 141 µW/mK2 for the sample x = 0.4 at 755 K.

Optimizing electrical and thermal transport properties of Ca3Co4O9 based thermoelectric materials by Ag and Fe co-addition

Ca 3-x Ag x Co 4-y Fe y O 9 (x=0, 0.05; y=0, 0.1, 0.2, 0.4) polycrystalline materials were prepared by sol-gel method with spark plasma sintering. The phase composition, microstructure and high temperature thermoelectric property of the bulk samples were systematically studied. The sintered Ca 3-x Ag x Co 4-y Fe y O 9 ceramics exhibit fine lamellar grain and dense morphology. Compared with single addition of Ag or Fe, appropriate co-additive amount of Ag and Fe can further increase electrical conductivity due to higher carrier concentration and mobility, while decreasing thermal conductivity by stronger phonon scattering. The coordinated regulation of electron and phonon transport is realized in Ca 2.95 Ag 0.05 Co 3.8 Fe 0.2 O 9 with the maximum ZT value of 0.25 at 1000 K, which is 34% larger than that of pure Ca 3 Co 4 O 9 . • Appropriate co-doping content of Ag and Fe in Ca 3 Co 4 O 9 can bring higher carrier concentration and mobility, while enhancing phonon scattering. • Simultaneously enhanced power factor and suppressed thermal conductivity can be achieved by Ag and Fe co-doping with appropriate doping contents. • Excessive doping of Fe would generate secondary phase FeO and deteriorate thermoelectric property.

Investigation of near-room and high-temperature thermoelectric properties of (Bi0.98In0.02)2Se2.7Te0.3/Bi2Te3 composite system

Polycrystalline composite samples of (Bi0.98In0.02)2Se2.7Te0.3/Bi2Te3 with different concentrations of Bi2Te3 such as 5%,10%,15% and 20% were prepared by the solid-state reaction technique. The X-Ray diffraction analysis has shown the hexagonal composite crystal structure with space group of Roverline{3 }m. Field emission scanning electronic microscope shows secondary particles on the surface of the samples. All the samples have shown the usual semi-conducting behaviour throughout the temperature range. It is observed that bismuth has been co-ordinated with 6 selenium atoms in (Bi0.98In0,02)2Se2.7Te0.3 compound and it has enormous selenium vacancies. The electrical resistivity represents the noteworthy result of the grain boundaries leading to the higher content of scattering centres in the polycrystalline composite samples. It is found that the electronegativity differences of In and Te, In and Se are less than Bi and Se, Bi and Te is the reason for the decrease in Seebeck coefficient in the compound containing 15% and 20% of Bi2Te3.

Unlocking the thermoelectric potential of the Ca14AlSb11 structure type.

Yb14MnSb11 and Yb14MgSb11 are among the best p-type high-temperature (>1200 K) thermoelectric materials, yet other compounds of this Ca14AlSb11 structure type have not matched their stability and efficiency. First-principles computations show that the features in the electronic structures that have been identified to lead to high thermoelectric performances are present in Yb14ZnSb11, which has been presumed to be a poor thermoelectric material. We show that the previously reported low power factor of Yb14ZnSb11 is not intrinsic and is due to the presence of a Yb9Zn4+xSb9 impurity uniquely present in the Zn system. Phase-pure Yb14ZnSb11 synthesized through a route avoiding the impurity formation reveals its exceptional high-temperature thermoelectric properties, reaching a peak zT of 1.2 at 1175 K. Beyond Yb14ZnSb11, the favorable band structure features for thermoelectric performance are universal among the Ca14AlSb11 structure type, opening the possibility for high-performance thermoelectric materials.

Altering the high-temperature stability and thermoelectric properties of Cu1.8S thermoelectric materials by Se incorporation

Thermoelectric materials based on Cu 1.8 S are not hampered by electric-field-driven degradation in real applications, but their high-temperature applications are problematic owing to their tendency to oxidize or spontaneously decompose. Here, we reveal that Cu 1.8 S powder is gradually oxidized to form a mixture of Cu 2 O, Cu(SO 4 ), and Cu 2 O(SO 4 ) at high temperatures under air. Although a dense oxide film is formed on the surface of the sintered Cu 1.8 S block at high temperature, hindering the oxidation process, the brittle oxide film fails to inhibit the spontaneous decomposition of the Cu 1.8 S block into the Cu 1.96 S phase and S vapor at high temperatures. Thus, Cu 1.8 S thermoelectric materials are unsuitable for applications above 300 °C in air. To address this issue, we partially replaced S atoms with Se; the sintered Cu 1.8 S 0.8 Se 0.2 blocks exhibited significantly increased anti-oxidation and anti-decomposition capacities in air. In addition, we found that the thermoelectric figure of merit of the sintered Cu 1.8 S 0.8 Se 0.2 block was 47% higher than that of traditionally prepared, pure Cu 1.8 S at 450 °C. • The Cu 1.8 S block cannot function stably at temperatures over 300 °C. • The anti-oxidation and anti-decomposition capacities of the Cu 1.8 S 0.8 Se 0.2 are improved. • The ZT of the Cu 1.8 S 0.8 Se 0.2 is 117% higher than that of pure Cu 1.8 S at 450 °C.

First-Principles Study of Silicon-Tin Alloys as a High-Temperature Thermoelectric Material.

Silicon–germanium (SiGe) alloys have sparked a great deal of attention due to their exceptional high-temperature thermoelectric properties. Significant effort has been expended in the quest for high-temperature thermoelectric materials. Combining density functional theory and electron–phonon coupling theory, it was discovered that silicon–tin (SiSn) alloys have remarkable high-temperature thermoelectric performance. SiSn alloys have a figure of merit above 2.0 at 800 K, resulting from their high conduction band convergence and low lattice thermal conductivity. Further evaluations reveal that Si0.75Sn0.25 is the best choice for developing the optimum ratio as a thermoelectric material. These findings will provide a basis for further studies on SiSn alloys as a potential new class of high-performance thermoelectric materials.

Experimental and computational approaches to study the high temperature thermoelectric properties of novel topological semimetal CoSi

Here, we study the thermoelectric properties of topological semimetal CoSi in the temperature range 300–800 K by using combined experimental and density functional theory (DFT) based methods. CoSi is synthesized using arc melting technique and the Rietveld refinement gives the lattice parameters of a = b = c = 4.445 Å. The measured values of Seebeck coefficient (S) shows the non-monotonic behaviour in the studied temperature range with the value of ∼−81 μV K−1 at room temperature. The |S| first increases till 560 K (∼−93 μV K−1) and then decreases up to 800 K (∼−84 μV K−1) indicating the dominating n-type behaviour in the full temperature range. The electrical conductivity, σ (thermal conductivity, κ) shows the monotonic decreasing (increasing) behaviour with the values of (12.1 W m−1 K−1) and (14.2 W m−1 K−1) Ω−1 m−1 at 300 K and 800 K, respectively. The κ exhibits the temperature dependency as, κ ∝ T 0.16. The DFT based Boltzmann transport theory is used to understand these behaviour. The multi-band electron and hole pockets appear to be mainly responsible for deciding the temperature dependent transport behaviour. Specifically, the decrease in the |S| above 560 K and change in the slope of σ around 450 K are due to the contribution of thermally generated charge carriers from the hole pockets. The temperature dependent relaxation time (τ) is computed by comparing the experimental σ with calculated σ/τ and it shows temperature dependency of 1/T 0.35. Further this value of τ is used to calculate the temperature dependent electronic part of thermal conductivity (κ e) and it gives a fairly good match with the experiment. Present study suggests that electronic band-structure obtained from DFT provides a reasonably good estimate of the transport coefficients of CoSi in the high temperature region of 300–800 K.

Improved High-Temperature Thermoelectric Properties of Dual-Doped Ca3Co4O9.

Layered structured Ca3Co4O9 has displayed great potential for thermoelectric (TE) renewable energy applications, as it is nontoxic and contains abundantly available constituent elements. In this work, we study the crystal structure and high-temperature TE properties of Ca3–2yNa2yCo4–yMoyO9 (0 ≤ y ≤ 0.10) polycrystalline materials. Powder X-ray diffraction (XRD) analysis shows that all samples are single-phase samples and without any noticeable amount of the secondary phase. X-ray photoelectron spectroscopic (XPS) measurements depict the presence of a mixture of Co3+ and Co4+ valence states in these materials. The Seebeck coefficient (S) of dual-doped materials is significantly enhanced, and electrical resistivities (ρ) and thermal conductivities (κ) are decreased compared to the pristine compound. The maximum thermoelectric power factor (PF = S2/ρ) and dimensionless figure of merit (zT) obtained for the y = 0.025 sample at 1000 K temperature are ∼3.2 × 10–4 W m–1 K–2 and 0.27, respectively. The zT value for Ca2.95Na0.05Co3.975Mo0.025O9 is about 2.5 times higher than that of the parent Ca3Co4O9 compound. These results demonstrate that dual doping of Na and Mo cations is a promising strategy for improving the high-temperature thermoelectric properties of Ca3Co4O9.

Enhancement of high temperature thermoelectric performance of cobaltite based materials for automotive exhaust thermoelectric generators

Thermoelectric (TE) generators can directly convert exhaust waste heat into electricity in vehicles. However, the low conversion efficiency of TE generators is the main obstacle to their commercialization in automotive. Their efficiency mainly depends on the performance of the used materials which is quantified by the figure of merit (ZT value). In the present study, single- and co-doped calcium cobaltites (CCO) with rare-earth (Tb) and transition metals (Cu, Fe, Ni, Mn, Cr) were produced using sol–gel technique in order to improve their high temperature TE properties for heat recovery in exhaust manifold applications. By the combined effect of doping approach and the production technique used in this study, a remarkable decrease in the grain size of CCO was obtained, and thus its thermal conductivity dramatically decreased. Besides, thermopower values were improved significantly. The reduction in thermal conductivity and the increase in thermopower led to an enhancement in ZT value of CCO ceramics. Among all the co-doped samples, Tb–Cu co-doped CCO displayed the maximum ZT value of 0.116 at 873 K which is 2.5 times larger than that of pure CCO. The high thermal stability and the enhanced TE performance make Tb–Cu co-doped CCO material a potential candidate for heat recovery in automotive exhaust TE generators.

Role of Al Substitution in the Enhancement of High-Temperature Thermoelectric Properties of ZnO Compound

ZnO thermoelectric material has shown a high Seebeck coefficient and low thermal conductivity. Nevertheless, the dimensionless figure of merit (ZT) value is quite low because of its high resistivity. We proposed a way to reduce electrical resistivity by substituting Al into ZnO. The Al-substituted ZnO samples were synthesized by solid-state reaction method and pressed into pellets by hot-pressing (HP) method. The Seebeck coefficient, electrical resistivity, and thermal conductivity (κ) are investigated. We have found that the electrical resistivity and the thermal conductivity significantly decreased with increasing Al content into the increases ZT = 0.28 (Al = 0.02) at 1073 K. The ZT values were higher than four times when compared with the unsubstituted sample.

High-temperature thermoelectric performance of (W1−xTix)18O49

High-temperature thermoelectric properties of tungsten-based Magnéli phase oxide (W1-xTix)18O49 (0 ≤ x ≤ 0.25) prepared by solid state reaction followed by densification via spark plasma sintering (SPS) were studied. The Ti substitution increased the Seebeck coefficient, the power factor, and decreased both the electronic and lattice thermal conductivity. The synergistic substitution effect on the electrical and thermal properties and inherently low total thermal conductivity of 1.46 ± 0.08 WK−1 m−1 originating from the tunnel-like crystal structure of the oxide led to a significantly high ZT of 0.50 ± 0.07 at 1073 K for the sample with x = 0.2.

High temperature thermoelectric properties of the tetradymite Bi2−xPbxTe2Se (0 ≤ x ≤ 0.03)

We report on the synthesis, characterizations, and high-temperature thermoelectric properties of polycrystalline samples of Bi2−xPbxTe2Se for 0 ≤ x ≤ 0.03. Powder x-ray diffraction and scanning electron microscopy confirm the successful insertion of Pb into the hexagonal unit cell of the tetradymite Bi2Te2Se. Pb acts as an acceptor-like impurity, gradually shifting the lightly doped, n-type electronic properties of Bi2Te2Se toward heavily doped, p-type properties in Pb-containing samples, which is additionally confirmed by electronic structure calculations. The increased degenerate character results in high power factors while maintaining low thermal conductivity values. This favorable combination of thermoelectric properties yields a peak dimensionless figure of merit ZT of 0.70 at 425 K for x = 0.02 in the direction perpendicular to the pressing direction.