Enhancing the thermoelectric power factor of nanostructured SnO2 via Bi substitution

Tin dioxide (SnO₂) has recently proved to be a promising material for thermoelectric applications. We have investigated the influence of highly valence Bi doping as an electron donor in oxygenated SnO₂ materials on their thermoelectric properties. We have synthesized the pure and Bi doped SnO₂ nanoparticles (x = 0%, 5%, 10%, and 15%) through a simple hydrothermal approach. The Seebeck coefficient and Hall measurements have been used to determine thermoelectric behaviour. The measured value of the Seebeck coefficient increases from - 56 to - 83 μV/^°C as the Bi content increases. This improvement in the Seebeck coefficient has been attributed to the charge carrier localization (energy filtering effect) caused by the inclusion of the bismuth atoms and the presence of secondary phases based on BiO₂. However, the electrical conductivity measurements show an inverse relation with the Bi doping, increasing the impurities. The Sn_1-xBi_xO₂ sample with x = 15 has achieved the maximum Seebeck value, resulting in the upward trend in power factor of up to 1.97 × 10^?4 Wm^?1C^?2. Further, we have used X-ray diffraction and scanning electron microscopy to determine the effect of Bi on the SnO₂ crystal structure and surface morphology. Which also demonstrates the presence of composites with mixed phases.     

Effect of Gd and Nb co-substitution on enhancing the thermoelectric power factor of nanostructured SrTiO3

Oxide thermoelectric materials have attracted researchers in recent decade due to their attractive features such as low toxicity, low cost and high chemical robustness. Perovskite based oxide thermoelectric are considered as the promising materials, especially for high temperature thermoelectric applications. In the present work, pure SrTiO₃, Sr_1-xGd_xTiO₃ (0 < x < 0.09) and Sr_1-xGd_xTi_1-yNb_yO₃ were prepared by varying Gd concentration (0 < x < 0.09) using hydrothermal method. The XRD analysis confirmed the high crystalline cubic structured nanocomposite with Gd and Nb substitution. The FESEM images revealed cubic morphology of the particles and the size of the cubes varied with the concentration of the dopant. The chemical compositions of the samples were confirmed by EDX analysis. The binding states and elemental composition of the samples were analyzed by XPS. Both the pure SrTiO₃, Sr_1-xGd_xTiO₃ samples show low electrical resistivity and the co-substituted sample exhibited relatively high resistivity. Seebeck coefficient of the samples increased with Gd concentration. The Gd and Nb co-substituted sample shows relatively higher Seebeck coefficient value compared to Gd substituted samples. The power factor of the nanocomposite were calculated from the obtained Seebeck coefficient and resistivity; Gd and Nb co-substituted sample shows relatively high power factor of 311.7 × 10^?6 Wm^?1K^?2 at 550 K compared to other samples.     

Effect of Zn migration on the thermoelectric properties of Zn4Sb3 material

β-Zn₄Sb₃ is interesting as thermoelectric material at moderate temperature due to the extreme low thermal conductivity. Recent success in energy band engineering or nano-engineering led to a significant improvement in the thermoelectric properties of β-Zn₄Sb₃. In this work, we utilize the direct current to drive the migration of Zn by designing of sintering mould. Obvious Zn migration under the direct current applied in the plasma activated sintering (PAS) process is found in Zn₄Sb₃ compounds, and Zn exhibits significantly heterogeneous gradient composition distribution. At the top of sample, the single-phase Zn₄Sb₃ decomposes into ZnSb phase because of the loss of Zn, while Zn originated from lattice and interstitial sites in Zn₄Sb₃ is abundant in the bottom. The temperature-dependent transport measurements are also carried out at 323–673 K. Zn migration has a huge influence on the thermoelectric properties because of the sensitivity of Zn₄Sb₃. The maximum power factor can reach ~ 1.44 mW m⁻¹ K⁻² at 673 K due to the high Seebeck coefficient and low resistivity, which is one of the highest values in the reported results. The resulting peak ZT value of ~ 1.2 at 673 K is obtained. To control the Zn distribution by tuning the current is a feasible approach to improve the thermoelectric properties of Zn₄Sb₃ material.     

Ca3Mn2O7-layered perovskites: Effects of La- and Y-doping on phase stability,microstructure, and thermoelectric transport

We investigate phase stability, microstructure, and thermoelectric transport of polycrystalline bulk Ca_3−xR_xMn₂O₇ samples prepared by standard solid-state reaction, where R = Y or La and 0 ≤ x ≤ 0.33. Ab-initio calculations predict that Y-doping at Ca-sites should reduce the potential energy barrier for electron transport, as opposed to La-doping. We find that Y-doping prompts transformation from Ca₃Mn₂O₇ to Ca₂MnO₄, whereas La-doping is accompanied by no phase transformation. La-doping significantly hinders grain growth, for example, the average grain size decreases from 4.44 ± 0.24 to 1.20 ± 0.03 μm for x = 0 (undoped) and x = 0.33 upon La-doping, respectively. Electrical conductivity and Seebeck coefficients are measured for the temperature range of 300–1300 K, and analyzed in terms of the small polaron hopping model. We find that Y-doping reduces the activation energy for conduction compared to La-doping, for example, 43 and 63 meV, respectively. This suggests that Y reduces the energy barrier for polaron transport, in accordance with computational predictions. This trend is further supported by calculations of selected electronic, structural, and vibrational properties, highlighting the intriguing correlation between electronic transport governed by small polarons and elastic properties, thereby shedding light on charge transport and thermoelectric properties of such layered perovskites.     

Significant enhancement of the thermoelectric performance in Ca3Co4O9 thermoelectric materials through combined strontium substitution and hot-pressing process

This work explores the possibilities for a further enhancement of the thermoelectric properties of Ca₃Co₄O₉ by Sr-doping combined with hot-pressing. Modified hot-pressing process resulted in highly-textured and dense ceramics. Sr-doping significantly improves electrical properties, resulting in extremely large power factor (1.2 mW/K²m at 800 °C) due to simultaneous electrical resistivity decrease and Seebeck coefficient increase. The main effect on cumulative electrical performance is provided by the Seebeck coefficient, reaching 270μV/K at 800 °C. XPS revealed relatively high average cobalt oxidation state at room temperature (+3.3), compared to materials produced by conventional sintering. The results of combined XPS and Auger electron spectroscopy emphasize the importance of high densification in Ca₃Co₄O₉-based ceramics for preventing phase decomposition and interaction with CO₂ and moisture. Still, despite the exceptional electrical performance, the calculated figure-of-merit (estimated as 0.29 at 800 °C) is around the best reported in the literature due to a high thermal conductivity (4.4 W/K m at room temperature).     

Electrical transport and thermoelectric properties of Ca0.8Y0.2−xDyxMnO3−δ (0 ≤ x ≤ 0.2)

Ca_0.8Y_0.2?xDy_xMnO_3?δ (0≤x≤0.2) samples were fabricated by the solid‐state reaction method, and their thermoelectric properties were studied from 500°C to 800°C. Upon the substitution of Dy³⁺ for Y³⁺ in the Ca_0.8Y_0.2?xDy_xMnO_3?δ, the electrical and thermal conductivities gradually decreased with increasing Dy³⁺ concentration, whereas the absolute value of the Seebeck coefficient significantly increased. The Ca_0.8Dy_0.2MnO_3?δ showed the largest value of dimensionless figure of merit (0.180) at 800°C as a result of the combination of the largest absolute value of the Seebeck coefficient and the lowest thermal conductivity. We believe that the Ca_0.8Dy_0.2MnO_3?δ is a promising thermoelectric material at high temperatures.     

Effect of graphite on the power density of selenium doped polyaniline ink based hybrid screen-printed flexible thermoelectric generator

Enhancement in the thermoelectric performance of inorganic/organic hybrid composites and the need for low-cost, flexible thermoelectric generators have motivated this work. The thermoelectric effect on the addition of amorphous polyaniline, crystalline selenium, and layer-structured graphite, with different concentrations on the thermoelectric properties of selenium-doped polyaniline, is reported. Tuning of microstrain, dislocation density, and carrier concentration has improved the Seebeck coefficient by 39.10% and electrical conductivity by 60.22%. The maximum power output and power factor exhibited by the hybrid device are 1.89 nW and 0.42 nW/m²K² at a temperature difference of 100 °C. Replacing 90 wt% of Selenium-doped polyaniline with graphite resulted in a power density of 0.65 mW/m² under external load conditions.     

Effect of CeO2 on the thermoelectric properties of WO3-based ceramics

The thermoelectric properties of tungsten trioxide (WO₃) ceramics doped with cerium dioxide (CeO₂) were investigated. The results demonstrated that the addition of CeO₂ to WO₃ could promote the grain growth and the densification. The magnitude of the electrical conductivity (σ) and the absolute value of the Seebeck coefficient (|s|) depended strongly on the CeO₂ content. The sample doped with 2.0 mol% CeO₂ yielded higher σ and |s|, resulting in a significant increase in the power factor (σs²). In addition, the power factor value of all samples increased abruptly at high temperatures, which revealed that WO₃-based ceramics could have greater thermoelectric properties at high temperatures.     

Enhanced thermoelectric power generation performance of mixed-phase FeS/FeS2 nanostructures by controlling the reaction time duration

Thermoelectric materials can convert heat energy into electricity and could be the future of sustainable energy demand. In the current work, we have successfully improved the thermoelectric performance of hydrothermally synthesized mixed-phase pyrite nanostructures. For this, samples with different FeS/FeS₂ phase ratios were synthesized by controlling the reaction time duration. XRD data confirms the formation of mixed phases of FeS and FeS₂, which increases with the increase in reaction time, while the Raman spectroscopy results verify the argument. Scanning electron microscope images represents the formation of nanoporous morphology. For thermoelectric properties, both the Seebeck coefficient (44–69 μV/^°C) and electrical conductivity (33–48 S/cm) were found to be improved with the increase in reaction time (12–48h). This simultaneous enhancement in Seebeck coefficient and electrical conductivity was associated with the energy filtering effect at the crystal interface leading to the improvement in thermoelectric power factor to 2.28 × 10^?5 Wm^?1C^?2 (at 25^oC) for 48h sample.     

Growth rate effects on the thermoelectric performance of CaMnO3-based ceramics

CaMnO₃–based materials represent a promising family of n-type oxide thermoelectrics. The objective of the present work is assessing the impacts on relevant structural, microstructural and thermoelectric properties of manganites when they are processed by the laser floating zone technique. For this purpose, donor-doped Ca_0.9La_0.1MnO₃, CaMn_0.95Nb_0.05O₃ and undoped CaMnO₃ were used. Different growth conditions have been evaluated through combined studies of structural, microstructural, and thermoelectric characteristics. Despite the presence of secondary phases, electrical resistivity is among the best reported in the literature (9 mΩ.cm at 800 °C for La-doped materials grown at 200 mm/h). Essentially high absolute Seebeck coefficient of 320 μV/K at 800 °C was observed for undoped samples grown at 10 mm/h. Power factor is significantly affected by the growth conditions, reaching the highest values when using the lowest pulling rates. Exceptionally high PF (0.39 mW/K²m at 800 °C) was obtained for undoped CaMnO₃ samples grown at 10 mm/h.     

Enhanced thermoelectric figure of merit in indium and ytterbium double-filled skutterudite bulk materials through simultaneously optimising power factor and reducing thermal conductivity

In this study, Yb_xCo₄Sb₁₂ and In_xYb_0.3Co₄Sb₁₂ bulk materials were fabricated via microwave synthesis combined with spark plasma sintering. Based on ytterbium single filling, indium was further filled into the voids of CoSb₃. Carrier concentration and mobility were regulated as 1.6–1.8 × 10²⁰ cm^-³ and ∼30 cm²V⁻¹S⁻¹, respectively, resulting in an improved power factor of ∼4900 μWm⁻¹K⁻². The phonon-resonant scattering, caused by indium and ytterbium double filling, was combined with other phonon scattering mechanisms such as nano-inclusion, dislocation, and grain boundary segregation, which resulted in a significantly decreased lattice thermal conductivity of 0.72–2.08 Wm⁻¹K⁻¹. Owing to the improvements in carrier concentration and phonon transport, excellent thermoelectric performance, reflected by ZT = 1.38 at 773 K, was achieved in In_0.4Yb_0.3Co₄Sb₁₂.     

Magnetic Ni doping induced high power factor of Cu2GeSe3-based bulk materials

Cu₂GeSe₃ is an eco-friendly material, but pristine Cu₂GeSe₃ has poor thermoelectric properties. Here, the effects of magnetic Ni ion with unpaired 3d electrons on the electrical and thermal transport properties of Cu₂GeSe₃ are reported. Density functional theory (DFT) calculations indicate that the unpaired Ni 3d states cause the hybridization of Ni 3d orbitals with Se 4p orbitals near the Fermi level, giving rise to an increase in density of states (DOS). Combined with the significantly increased carrier concentration, a power factor (S²σ) value of ～8.0 μWcm^?1 K^?2 at 723 K is achieved in Cu₂Ge_0.8Ni_0.2Se₃ sample, seeming to be the highest compared with the other Cu₂GeSe₃ systems. Meanwhile, the substitution of Ni for Ge causes the obvious local distortions in Cu₂GeSe₃ lattice, which yields the large strain fluctuations to suppress the lattice thermal conductivity. Consequently, a peak ZT value of ～0.38 at 723 K is obtained in Cu₂Ge_0.9Ni_0.1Se₃ sample.     

Effect of real working environment/formation of oxide phase on thermoelectric properties of flexible Sb2Te3 films

Flexible Sb₂Te₃ thin films, for thermoelectric generator applications, were deposited by DC magnetron sputtering. As-deposited films were annealed in air to simulated a realistic operating environment. The oxidation behavior of the films was studied by monitoring their phase change on exposure to air at different temperatures between 50 and 300 °C for annealing times from 1 to 15 h. Oxidation of Sb and Te formed Sb₂Te₄ and TeO₂ phases when annealing above 100 °C and Sb₂Te₃ decomposed into oxide phases at an annealing temperature of 250 °C for 15 h. The thermoelectric performance decreased as the content of Sb₂O₄ and TeO₂ phases increased. These findings show the limitations of Sb₂Te₃ films operating in air without vacuum or a protective environment. We propose that the kinetic growth of oxide formation on the Sb₂Te₃ thin films depend on chemical activation energy and oxygen diffusion through the oxide barrier by the variation of annealing temperature and annealing time, respectively.     

Effect of Co Content Upon the Bulk Structure of Sr- and Co-doped LaFeO3

The bulk structure was investigated for Fe-based perovskite-type oxides with the formula La_0.6Sr_0.4Co_yFe_1−yO_3−δ (y = 0.1, 0.2, and 0.3). The materials were confirmed to be stoichiometric with respect to oxygen under ambient conditions and the structural features were then further characterized under different environments as a function of temperature. Under reducing atmospheres, the degree of reduction increased with Co content, suggesting the presence of preferential oxidation of Fe over Co. Under milder conditions, oxygen vacancy formation was not proportional to Co content, which was likely caused by an electronic structure transition. The unit cell parameters were also shown to strongly depend upon Co content, temperature, and environment. A rhombohedral to cubic transition occurred at lower temperatures for higher Co content, but showed less dependence upon environment. A change in the thermal expansion behavior occurred at the temperature where oxygen vacancies formed leading to two regions of linear thermal expansion. The use of lattice parameters compared to dilatometry allowed for the simultaneous monitoring of unit cell symmetry and expansion behavior so the link between thermal properties and unit cell symmetry could be firmly established.     

Influence of Zn doping on microstructures and dielectric properties in CaCu3Ti4O12 ceramic synthesised by semiwet route

Abstract

A single phase of calcium copper titanate [CaCu₃Ti₄O₁₂ (CCTO)] was produced at lower temperature and shorter calcination time via a novel semiwet route. Undoped CCTO and zinc doped CaCu_3?xZn_xTi₄O₁₂ samples with x?=?0·10, 0·20 and 0·30 were prepared by this method for the first time using solid TiO₂ powder in metal nitrate solutions. The CaCu_3?xZn_xTi₄O₁₂ ceramics were characterised by thermogravimetric/differential thermal analysis, X-ray diffraction, SEM and EDX techniques. The SEM images of the sintered CaCu_3?xZn_xTi₄O₁₂ ceramics showed average grain size in the ranges of 2–6, 8–13, 12–16 and 14–20 μm for x?=?0·00, 0·10, 0·20 and 0·30 respectively. Energy dispersive X-ray spectroscopy studies confirm the purity of parent and Zn doped CCTO ceramics. At room temperature, the dielectric constants of Zn doped CCTO are always higher than pure CCTO. CaCu_3?xZn_xTi₄O₁₂ (x?=?0·20) ceramic has the maximum value of ?_r≈4347 along with the minimum value of tan?δ≈0·14 at 1 kHz.     

Effect of Ni substitution on electrical and thermoelectric properties of LaCoO3 ceramics

LaCo_1−xNi_xO₃ (0 ≤ x ≤ 0.2) ceramics were prepared by solid state reaction and their thermoelectric properties were investigated from room temperature (RT) to 400 °C. In the range from RT to 180 °C, LaCoO₃ showed a large negative Seebeck coefficient, but it changed to a positive value above 180 °C. However, the Seebeck coefficient became positive in the whole investigated temperature span due to Ni substitution for Co even for a tiny amount, but its absolute value decreased significantly with increasing Ni content. The LaCo_0.9Ni_0.1O₃ composition showed an enhanced power factor with a maximum value of 1.41 × 10⁻⁴ W m⁻¹ K⁻² at room temperature, which is about 3.5 times higher than that of un-doped LaCoO₃. Because the power factor decreased and the thermal conductivity increased apparently with temperature, the ZT values were not increased at elevated temperatures, in spite of a relatively large ZT value of 0.031 at a low temperature (50 °C) obtained in the composition LaCo_0.9Ni_0.1O₃.     

Growth of Zn2GeO4 thin film by thermal evaporation on ITO substrate for thermoelectric power generation applications

In this paper, we have reported the growth of Zn₂GeO₄ thin film and investigated its potential for thermoelectric power generation applications. Zn₂GeO₄ alloy thin film was grown on Indium coated glass substrate by the evaporation of Zn and Ge metals with constant oxygen gas flow rate of 100 sccm in tube furnace. The grown film was cut into pieces and annealed at various temperatures from 500° to 700°C with a step of 100?°C in a programmable furnace for one hour. The structure of as grown and annealed thin films was verified by XRD and Raman spectroscopy measurements. The XRD data evident that Zn₂GeO₄ alloy hexagonal structure along with GeO₂ and ZnO phases were observed at annealing temperatures 600 and 700?°C but below this temperature no alloy phase was detected by XRD and Raman Spectroscopy. To calculate the thermoelectric properties, temperature dependent Seebeck measurements were performed in the temperature range of 25–100?°C. It was observed that the value of Seebeck coefficient was increased from 91 to 847?μV/K as the annealing temperature increases from 500° to 700°C. This behavior was explained as; high temperature causes stress and cracks in the grown films which may induce electric and thermal discontinues at tips of cracks which cause high thermoelectric concentration. Scanning electron microscope images verified the development of cracks in the samples as annealing temperature increases. The behavior of Seebeck coefficient with the measurement temperature was also observed and explained in detail. The high value of Seebeck coefficient suggested that this material can be a potential candidate for thermoelectric power generation applications in near future.     

Effect of Na-doping on thermoelectric and magnetic performances of textured Bi2Sr2Co2Oy ceramics

Bi₂Sr_2-xNa_xCo₂O_x (x = 0.0, 0.025, 0.050, 0.075, 0.100, and 0.125) samples were prepared through the solid-state route and textured using the laser floating zone technique. Microstructural analysis of as-grown samples showed well oriented grains and a relatively high amount of secondary phases due to their incongruent melting. Annealing procedure has drastically decreased the number and amount of secondary phases. Moreover, Na-doping has further decreased the secondary phases content and improved grain alignment. These modifications have been reflected in a large decrease of electrical resistivity with the annealing procedure. The maximum power factor values have been obtained in 0.075 Na-doped annealed samples, 0.20 mW/K²m, which are much higher than the best values obtained in textured materials through hot uniaxial pressing. Magnetic properties were very similar for all samples, with paramagnetic Curie temperature and effective magnetic moment values of ?48.6 K and ≈2 μ_B, respectively.     

The nature of low temperature deactivation of CoCr2O4 and CrOx/γ-Al2O3 catalysts for the oxidative decomposition of trichloroethylene

The CoCr₂O₄ and CrO_x/γ-Al₂O₃ catalysts were used for the oxidative decomposition of trichloroethylene (TCE). Both catalysts showed an initial deactivation at low temperatures around 280 °C, mainly due to the dissociative adsorption of reactant TCE. This was confirmed by the temperature programmed oxidation of TCE where the carbon oxides were formed up to a temperature below 300 °C. Possible changes in the oxidation state of chromium species were observed with XANES and ESR. During the oxidation reaction at low temperatures, the Cr(VI) species were reduced to Cr(III) species, which seemed to be coupled with TCE adsorption. At higher temperatures, however, the Cr(VI) species appeared again and the catalytic activity was completely recovered.     

Effect of the partial replacement of Fe by Ni and/or Mn on the electrocatalytic activity for oxygen evolution of the CoFe2O4 spinel oxide electrode

The study of the electrochemical behaviour of mixed spinel oxide electrodes, obtained by the partial replacement of Fe by Ni and/or Mn in the cobalt ferrite CoFe₂O₄ is presented. The electrodes were prepared by brush painting of iron substrates with a suspension of the respective oxide, prepared by solid-state reaction. The influence of the substituent on the electrodes electrocatalytic activity towards the OER is analysed in terms of the kinetic parameters obtained by steady state measurements and the cationic distribution proposed for the oxides. The data show that the introduction of Ni brings about the presence of Co³⁺ tetrahedrally coordinated in addition to the Co³⁺/Co²⁺ couple in octahedral sites, giving rise to a better electrocatalyst for the OER. In contrast the presence of Mn produces electrodes with lower catalytic activity.