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.     

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 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.     

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 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.     

Effect of Zn substitution on the structural and magnetic properties of nanocrystalline NiFe2O4 ferrites

Nanocrystalline Ni_1?xZn_xFe₂O₄ (where, x = 0.0, 0.2, 0.4, 0.6, 0.8 & 1) samples were synthesized through solution combustion technique using oxylyl de-hydrazide (ODH) as a fuel and the effect of dopant and its concentration on the structural and magnetic properties was investigated. As-prepared samples were characterized using different characterization techniques such as, XRD, SEM-EDS, TEM and Raman spectroscopy for their phase-purity, crystallinity, surface morphology and elemental composition; also magnetic properties were investigated through EPR, Mossbauer spectroscopy and vibrating sample magnetometer (VSM). Rietveld fitted XRD and Raman studies confirm the formation of cubic spinel structured ferrites and substitution of Zn ion at Ni site without formation of impurity phases. No other structural changes were observed and the structure remains in cubic phase with increase of Zn concentration. SEM and TEM micrographs reveal that the particles are agglomerated and the particles size were found in the nano range. Also good stoichiometric composition was observed in all the compositions of Zn substituted Ni ferrite samples. Magnetic measurements (VSM) reveal that pure Ni ferrites exhibits soft magnetic behaviour. Further the ferromagnetic behaviour suppressed with the substitution of diamagnetic Zn ion and with increase of its concentration in Ni ferrites, which was further evidenced in the Mossbauer spectroscopic results. At room temperature, electronic paramagnetic resonance spectra exhibits a broad resonance signal with Lande's g factor varies from 2.23 to 1.95 with increase in Zn content, which is attributed to spin exchange interactions between Fe³⁺, Ni²⁺ and Zn²⁺ ions also asymmetric EPR spectra was observed. The investigated results show that, Zn substitution has greater impact on the magnetic properties of Ni ferrites due to the diamagnetic nature of Zn, which inturn alters the cationic distribution and the exchange interactions between Ni-Fe and Fe-Fe.     

Effect of laser power density on the electrochromic properties of WO3 films obtained by pulsed laser deposition

Pulsed laser deposition (PLD) was used to prepare tungsten trioxide (WO₃) films on ITO substrates with a varying laser power density of 4.0–5.5 W/cm². XPS indicated that when the laser power density decreased, the peak positions of the W 4f and O 1s orbits shifted slightly to low energy due to the difference in oxygen vacancies. As the laser power density decreased, W⁶⁺ gradually replaced the lattice position of O^2?, increasing oxygen vacancies in the lattice. The transmittance modulated values (ΔT) were over 44% at 830 nm, indicating strong absorption by the WO₃ thin films in the near-infrared ray. The switching time of the WO₃ thin films between bleached states and coloured states decreased as the laser power density increased due to the amorphous structure, morphology, and lower oxygen deficiency at a high power density. The high ΔT and very fast switching time of t_b (1.09 s) and t_c (6.01 s) demonstrated the excellent electrochromic (EC) properties of the WO₃ films prepared by PLD.     

A study on the thermoelectric properties of ALD-grown aluminum-doped tin oxide with respect to nanostructure modulations

The thermoelectric properties of aluminum-doped tin oxide (ATO) thin films synthesized by thermal atomic layer deposition (ALD) were studied with respect to the aluminum concentration. The overall aluminum content in each layer was modulated by adjusting the relative number of tin oxide (SnO₂) and aluminum oxide (Al₂O₃) growth cycles, where a sequential process involving n cycles of SnO₂ growth followed by 1 cycle of Al₂O₃ deposition was performed (building up a super-cycle). The electrical conductivity (620 S/cm), free carrier concentration (1.23x10²¹ cm^-3), and power factor (0.49 mW/K²m) increase until their maximum values are reached when the Al content is approximately 1.50 at% of the cations, and decrease as more Al is added in. On the other hand, the Seebeck coefficient decreases monotonically as the Al content increases up to about 2.88 at%, and begins to increase with further Al doping. Here the thermoelectric efficiency is therefore determined primarily by the free carrier concentration, while the Seebeck coefficient appears to be influenced by the overall crystal structure.     

Effect of Si3N4 on Resistance of Magnesia Based Castable

The magnesia based curable specimens with different Si3N4 contents were casted using sintered magnesite （w（MgO）=95%） as starting material, SiO2 micro-powder as binder, 0.3%, 4% and 5% β-Si3N4 powder replacing the equal addition of magnesia powder respectively. The slag resistance test was carried out at 1550℃ for 3h using Baosteel tundish slag and static crucible method. The result indicates that： introducing Si3N4 could obviously improve the slag resistance of MgO based castable, which increased with increasing Si3N4. Dense SiO2 sintered layer formed on the surface of magnesia based castable because of the oxidation of Si3N4 addition, which can prevent the further slag penetration. In the deep inner of castable, the partial-pressure of oxygen was very low, so Si3N4 can exist stably. Meanwhile in reducing atmosphere, Si3N4 was hard to be sintered, which resulted in the loose interior structure of MgO based castable.     

Selective catalytic reduction of NO by alcohols on Co- and Fe-Siβ catalysts

Co–Siβ and Fe–Siβ catalysts prepared by a two-step post-synthesis method were characterised by EPR, diffuse reflectance UV–vis, XRD and N₂-physisorption. Iron and cobalt ions are present as isolated lattice tetrahedral Co^II and Fe^III species for low metal contents (0.7 and 0.9 wt.%, respectively). For higher iron content, FeO_x oligomers appear. Zeolites with isolated Co^II and Fe^III species are active in selective catalytic reduction of NO with ethanol. On FeO_x oligomers the oxidation of NO to NO₂ starts to dominate in the selective catalytic reduction of NO with ethanol at the temperatures higher than 700 K.     

Si3N4对镁质浇注料抗渣性能的影响

以95烧结镁砂为主要原料,以SiO2微粉为结合剂,在配料的细粉部分分别以0、3%、4%、5%的β-Si3N4细粉替代等量的镁砂细粉,搅拌均匀后浇注成氮化硅含量不同的镁质坩埚试样。选用宝钢中间包渣,采用静态坩埚法,在1550℃3h条件下对这些坩埚试样进行了抗渣试验。试验结果表明:加入Si3N4可以明显改善镁质浇注料的抗渣性能,并且随着Si3N4加入量的增加,试样的抗渣性能提高;在含氮化硅的镁质浇注料表面,由于Si3N4被氧化为SiO2而形成了致密烧结层,能阻止渣的进一步渗透;在加入Si3N4的镁质浇注料试样内部深处,由于氧分压非常低,Si3N4稳定存在;由于Si3N4在还原气氛下难以烧结,造成镁质浇注料内部结构疏松。     

An in situ study of the NO + H2 + O2 reaction on Pd/LaCoO3 based catalysts

In situ X-ray diffraction (XRD) and quasi in situ X-ray photoelectron spectroscopy (XPS) measurements were complementary used to investigate structural and surface modifications of a palladium-supported on LaCoO₃ perovskite catalyst under various controlled atmospheres, particularly during the reduction of NO by hydrogen under lean conditions, in the presence of a large excess of oxygen.

An extensive reduction of the perovskite was evidenced during the pre-activation thermal treatment of the palladium-supported catalyst under hydrogen at 773 K leading to the formation of Pd particles in contact with Co⁰ and La₂O₃. In the presence of an excess of oxygen, the catalyst structure changes during the reaction. The reduced solid is progressively transformed into LaCoO₃ in the range of 873–1173 K. However, such a bulk transformation probably occurs at lower temperatures at the surface of the solid according to XPS analyses. At the same time, the binding energy (BE) level of the Pd 3d_5/2 photopeak increases up to 337.5 eV which reveals the stabilisation of oxidic palladium species in a different chemical environment than that corresponding to PdO. Such changes induced different catalytic properties of the catalyst during the reduction of NO by H₂.