Rapid fabrication of Se-modified skutterudites obtained via self-propagating high-temperature synthesis and pulse plasma sintering route

Selenium is an effective dopant in skutterudite-based thermoelectric materials. It strongly influences thermal transport properties due to effective phonon scattering. This study proposes a short-term fabrication route to Se-modified CoSb₃-based materials. Alloy synthesis was conducted via self-propagating high-temperature synthesis. Subsequently, pulse plasma sintering consolidated all materials. As a result, thermoelectric materials with high electrical properties homogeneity were obtained. Seebeck potential mapping showed the measured deviation of the Seebeck coefficient for all fabricated samples was between 5 and 7%. A very low thermal conductivity (1.59 W m^?1 K^?1, at 573 K) was achieved for the highest doped sample, and one of the lowest reported results obtained for bulk skutterudite-based thermoelectric materials ever. This resulted in a low lattice thermal conductivity (1.51 W m^?1 K^?1, at 573 K). This led to the highest ZT (0.27 at 623 K) for the highest doped sample.     

Synchronously enhanced thermoelectric and mechanical properties of Ti doped NbFeSb based half-heusler alloys by carrier regulation and phonon engineering

NbFeSb based half-heusler thermoelectric materials have great potential in high-temperature energy harvesting. However, their conversion efficiency is limited by the low figure of merit zT, which is attributed to poor electronic performance and high thermal conductivity. Here, it is demonstrated that enhanced power factor of 46 μW cm^?1 K^?2 and zT value of 0.9 at 973 K is achieved in spark plasma sintered Nb_0.8Ti_0.2FeSb. It is depicted that a tremendous increase in the hole concentration owing to Ti substitution, results in a significant promotion of electrical performances. In addition, numerous phonon scattering centers, such as point-defects, nano precipitates as well as electro-acoustic coupling, collectively suppress the lattice thermal conductivity from 7.4 W m^?1 K^?1 to 3.3 W m^?1 K^?1 at 973 K. Furthermore, NbFeSb based alloys exhibit excellent mechanical property and the Vickers hardness reaches 10.8 GPa, which would be beneficial for TE devices fabrication.     

Giant room-temperature electrocaloric effect within wide temperature span in Sn-doped Ba0.85Ca0.15Zr0.1Ti0.9O3 lead-free thin films

The electrocaloric (EC) effect cooling technique of environmentally friendly lead-free thin film materials driven by electric fields has recently gained tremendous attention due to the urgent demand for microelectronic and integrated circuit refrigeration devices. However, the widespread use of lead-free materials in EC devices is seriously hindered due to the small electrocaloric temperature change (ΔT) within a narrow operation temperature span (T_span) near room temperature. Here, lead-free Ba_0.85Ca_0.15Zr_0.1Ti_0.9-xSn_xO₃ (BCZT-xSn, 0 ≤ x ≤ 0.03) thin films were prepared on substrates (Pt/Ti/SiO₂/Si) via a sol-gel route. The BCZT-0.02Sn thin film presents an excellent EC effect (ΔT = 32.74 K, ΔS = 37.18 J kg^?1 K^?1) and large EC strength (ΔT/ΔE = 0.033 K cm kV^?1, ΔS/ΔE = 0.037 J cm K^?1 kg^?1 kV^?1) over a wide T_span (～26 K) under 1000 kV cm^?1 near room temperature. The giant ΔT is mainly attributed to the emergence of an intermediate O phase and the formation of a multiphase (R, O and T phases) coexistence structure at room temperature, while the diffuse phase transition behavior is responsible for the wide T_span. Our study provides a new idea for developing environment-friendly EC materials with an excellent room-temperature ΔT over a broad operational temperature region.     

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.     

Enhancements of thermoelectric performance in n-type Bi2Te3-based nanocomposites through incorporating 2D Mxenes

Bi₂Te_2.7Se_0.3 compound has been considered as an efficient n-type room-temperature thermoelectric (TE) material. However, the large-scale applications for low-quality energy harvesting were limited due to its low energy-conversion efficiency. We demonstrate that TE performance of Bi₂Te_2.7Se_0.3 system is optimized by 2D Ti₃C₂T_x additive. Here, a 43% reduction of electrical resistivity is obtained for the nanocomposites at 380 K, originating from the increased carrier concentration. Consequently, the g = 0.1 sample shows a maximum power factor of 1.49 Wmm^?1K^?2. Meanwhile, the lattice thermal conductivity for nanocomposite samples is reduced from 0.77 to 0.41 Wm^?1K^?1 at 380 K, due to the enhanced phonon scattering induced by the interfaces between Ti₃C₂T_x nanosheets and Bi₂Te_2.7Se_0.3 matrix. Therefore, a peak ZT of 0.68 is achieved at 380 K for Bi₂Te_2.7Se_0.3/0.1 wt% Ti₃C₂T_x, which is enhanced by 48% compared with pristine sample. This work provides a new route for optimizing TE performance of Bi₂Te_2.7Se_0.3 materials.     

3D printing of cordierite materials from raw reactive mixtures

Porous cordierite materials are 3D printed by robocasting from two kaolin containing raw materials mixtures. Water suspensions of both mixtures at variable solid concentrations (40–67 wt%) are characterized by rheological measurements, showing good printability for concentrations >60 wt% without the need of any printing additive. The mixtures react during sintering (at 1250 °C) giving indialite as the main phase in the structures, which differ in minor phases. Three types of lattices are printed for both compositions with a logpile inner structure. Properties of interest like the coefficient of thermal expansion (CTE), the thermal conductivity (K_T) and the compression strength (σ) of the printed cordierites are determined and compared with published data. Results evidence that printing of clay containing reactive mixtures is a straightforward and cost-effective way to achieve porous complex shaped cordierite with CTE～ 2–3 x10^?6 K^-1, K_T ～ 0.4–0.6 W m^?1 K^?1 and maximum σ of 24 MPa.     

Fiber Reinforced Polyimide Aerogel Composites with High Mechanical Strength for High Temperature Insulation

Glass fiber/polyimide aerogel composites are prepared by adding glass fiber mat to a polyimide sol derived from diamine, 4,4′‐oxydianiline, p‐phenylene diamine, and dianhydride, 3,3′,4,4′‐biphenyltetracarboxylic dianhydride. The fiber felt acts as a skeleton for support and shaping, reduces aerogel shrinkage during the preparation process, and improves the mechanical strength and thermal stability of the composite materials. These composites possess a mesoporous structure with densities as low as 0.143–0.177 g cm^?3, with the glass fiber functioning to improve the overall mechanical properties of the polyimide aerogel, which results in its Young's modulus increasing from 42.7 to 113.5 MPa. These composites are found to retain their structure after heating at 500 °C, in contrast to pure aerogels which decompose into shrunken ball‐like structures. These composites maintain their thermal stability in air and N₂ atmospheres, exhibiting a low thermal conductivity range of 0.023 to 0.029 W m^?1 K^?1 at room temperature and 0.057to 0.082 W m^?1 K^?1 at 500 °C. The high mechanical strengths, excellent thermal stabilities, and low thermal conductivities of these aerogel composites should ensure that they are potentially useful materials for insulation applications at high temperature.     

Microstructure and Thermal Conductivity of Silicon Carbide with Yttria and Scandia

A fully dense SiC ceramic with high thermal conductivity was obtained by conventional hot pressing, with 1 vol% Y₂O₃–Sc₂O₃ additives. The ceramic had a bimodal microstructure consisting of large and small equiaxed SiC grains. Observation with high‐resolution transmission electron microscopy (HRTEM) showed two kinds of homophase (SiC/SiC) boundaries, that is crystallized and clean boundaries, and a fully crystallized junction phase. The thermal conductivity of the SiC ceramic was 234 W (m·K)^?1 at room temperature. The high thermal conductivity was attributed to a clean SiC lattice and good contiguity between SiC grains.     

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

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

Enhanced thermoelectric properties and electronic structures of p-type BiCu1−xAgxSeO ceramics

The thermoelectric properties and electronic structures were investigated on p-type BiCu_1-xAg_xSeO (x=0, 0.02, 0.05, 0.08) ceramics prepared using a two-step solid state reaction followed by inductively hot pressing. All the samples consist of single BiCuSeO phase with lamella structure and no preferential orientation exists in the crystallites. Upon replacing Cu⁺ by Ag⁺, maximum values of electrical conductivity of 36.6 S cm⁻¹ and Seebeck coefficient of 350 μV K⁻¹ are obtained in BiCu_0.98Ag_0.02SeO and BiCu_0.92Ag_0.08SeO, respectively. Nevertheless, a maximum power factor of 3.67 μW cm⁻¹K⁻² is achieved for BiCu_0.95Ag_0.05SeO at 750 K owing to the moderate electrical conductivity and Seebeck coefficient. Simultaneously, this oxyselenide exhibits a thermal conductivity as low as 0.38 W m⁻¹ K⁻¹ and a high ZT value of 0.72 at 750 K, which is nearly 1.85 times as large as that of the pristine BiCuSeO. The enhancement of thermoelectric performance is mainly attributed to the increased density of states near the Fermi level as indicated by the calculated results.     

Thermoelectric properties of layered oxyselenides with 3d transition metal ions

Layered oxychalcogenides have received extensive attention in the fields of magnetism, superconductivity, lithium battery, and luminescence due to their unique electronic, magnetic properties, and layered structure, among which layered oxyselenides have excellent and promising thermoelectric performance, such as BiCuSeO and Bi₂LnO₄Cu₂Se₂. Here, we successfully synthesized Sr₂MO₂Cu₂Se₂ (M = Co, Ni, Zn) and Sr₂FeO₃CuSe and investigated the thermoelectric properties at a wide temperature range (298–923 K). They have a relatively high Seebeck coefficient (>300 μV K⁻¹) in medium to high temperature range and possess a low thermal conductivity. The power factor and ZT reach 65 μW m⁻¹ K⁻¹ and 0.07 at 923 K for intrinsic Sr₂NiO₂Cu₂Se₂, and a higher performance is expected to be achieved by strategies like carrier concentration optimization and band structure engineering.     

Super-insulating,ultralight and high-strength mullite-based nanofiber composite aerogels

Ceramic nanofiber aerogel is one of the most attractive insulation materials in recent years. However, its practical application ability is limited at high temperature due to high radiation heat transfer. Herein, we constructed a novel closed-cell/nanowire structured mullite-based nanofiber composite aerogel via electrospinning technology and solvothermal synthesis method. Hollow TiO₂ spheres were used as pore-making material and infrared opacifier to reduce fiber solid heat conduction and high temperature radiation heat transfer simultaneously. In addition, TiO₂ nanowires grown in-situ on the fiber surface further decrease the radiation heat transfer of aerogel and improve the mechanical properties. The unique structure endows the aerogel with high mechanical robustness (0.32–0.35 MPa, 10% strain), low density (39.2–47.5 mg/cm³) and ultralow thermal conductivity (~0.017 W m^?1 K^?1 at 25 ℃ and ~0.056 W m^?1 K^?1 at 1000 ℃). This work offers a novel strategy for the development of ceramic nanofiber aerogel at high temperature.     

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.     

Thermoelectric properties of In- and Ga-doped spark plasma sintered ZnO ceramics

In this study, indium (In)- and gallium (Ga)-doped zinc oxide (ZnO) ceramics, [Zn_(1?x?y)Ga_xIn_y]O (x = 0, 0.02; y = 0, 0.005, 0.01, 0.02), were fabricated via spark plasma sintering (SPS) at 1423 K. Crystal structure and microstructural analyses were conducted to confirm the solubility of the dopants and understand the correlations between the crystallographic phases and the various compositions. It was confirmed that the solubility of Ga (x = 0.02; y = 0.005) was promoted by doping with In and Ga, and the highest power factor of 0.99 mW K^?2 m^?1 was acquired at 1046 K. Furthermore, the thermal conductivity at 340–530 K was reduced by doping with In and Ga.     

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.     

Effects of mechanical milling on preparation and properties of CuAl1−xFexO2 thermoelectric ceramics

CuAl_1?xFe_xO₂ (x = 0, 0.1, and 0.2) thermoelectric ceramics produced by a reaction-sintering process were investigated. Pure CuAlO₂ and CuAl_0.9Fe_0.1O₂ were obtained. Minor CuAl₂O₄ phase formed in CuAl_0.8Fe_0.2O₂. Addition of 10 mol% Fe lowered the sintering temperature obviously and enhanced the grain growth. At x = 0.1, electrical conductivity = 3.143 Ω^?1 cm^?1, Seebeck coefficient = 418 μV K^?1, and power factor = 5.49 × 10^?5 W m^?1 K^?2 at 600 °C were obtained. The reaction-sintering process is simple and effective in preparing CuAlO₂ and CuAl_0.9Fe_0.1O₂ thermoelectric ceramics for applications at high temperatures.     

Synthesis,structure, and characterization of the thorium zirconolite CaZr1-xThxTi2O7 system

A series of zirconolite ceramics with composition CaZr_1-xTh_xTi₂O₇ (Δx = 0.10) were reactively sintered at 1350°C for 20 h, in air (0 ≤ x ≤ 0.60) and 5% H₂/N₂ (0 ≤ x ≤ 0.40). A sample with composition corresponding to x = 0.20 was also produced by hot isostatic pressing (HIP) at 1300°C and 100 MPa for 4 hours. Th⁴⁺ immobilization was most readily achieved under oxidizing conditions, with Th⁴⁺ preferentially incorporated within a pyrochlore-structured phase in the range 0.10 ≤ x ≤ 0.50, yet formation of the zirconolite-4M polytype was not observed. We report the novel synthesis of single-phase pyrochlore with nominal composition CaZr_0.40Th_0.60Ti₂O₇ when targeting x = 0.60. Th⁴⁺ incorporation under reducing conditions produced a secondary Th-bearing perovskite, comprising 24.2 ± 0.6 wt% of the phase assemblage when targeting x = 0.40, alongside 8.8 ± 0.3 wt% undigested ThO₂. Under reducing conditions, powder XRD data were consistent with zirconolite adopting the 3T polytype structure. The sample produced by HIP presented a nonequilibrium phase assemblage, yielding a major phase of zirconolite-2M alongside accessory Th⁴⁺-bearing phases ThTi₂O₆, ThO₂, and perovskite. These data highlight the efficacy of Th⁴⁺ as a Pu⁴⁺ surrogate, with implications for the formation of Zr-stabilized Th-pyrochlore phases as matrices for waste with elevated Th⁴⁺ content.     

Enhanced pyroelectric properties in (Bi0.5Na0.5)TiO3–BiAlO3–NaNbO3 ternary system lead‐free ceramics

High pyroelectric performance and good thermal stability of pyroelectric materials are desirable for the application of infrared thermal detectors. In this work, enhanced pyroelectric properties were achieved in a new ternary (1?x)(0.98(Bi_0.5Na_0.5)(Ti_0.995Mn_0.005)O₃–0.02BiAlO₃)–xNaNbO₃ (BNT–BA–xNN) lead‐free ceramics. The effect of NN addition on the microstructure, phase transition, ferroelectric, and pyroelectric properties of BNT–BA–xNN ceramics were investigated. It was found that the average grain size decreased as x increased to 0.03, whereas increased with further NN addition. The pyroelectric coefficient p at room temperature (RT) was significantly increased from 3.87 × 10^?8Ccm^?2K^?1 at x = 0 to 8.45 × 10^?8Ccm^?2K^?1 at x = 0.03. The figures of merit (FOMs), F_i, F_v and F_d, were also enhanced with addition of NN. Because of high p (7.48 × 10^?8Ccm^?2K^?1) as well as relatively low dielectric permittivity (~370) and low dielectric loss (~0.011), the optimal FOMs at RT were obtained at x = 0.02 with F_i = 2.66 × 10^?10 m/V, F_v = 8.07 × 10^?2 m²/C, and F_d = 4.22 × 10^?5 Pa^?1/2, which are superior to other reported lead‐free ceramics. Furthermore, the compositions with x ≤ 0.03 exhibited excellent temperature stability in a wide temperature range from 20 to 80°C because of high depolarization temperature (≥110°C). Those results unveil the potential of BNT–BA–xNN ceramics for infrared detector applications.     

Filtering low-energy carriers to enhance thermoelectric performance of Cd-doped SnSe via incorporation of MXene sheets

SnSe-based materials have attracted widespread attention in thermoelectrics due to their outstanding thermoelectric performance. However, the pristine and unmodified polycrystalline SnSe reveals poor electrical properties. Doping and constructing nanostructured composite architectures to produce energy filtering effect proved to be an effective method to strengthen thermoelectric performance. In this study, Ti₃C₂/Sn_0.98Cd_0.02Se composites are successfully fabricated by the solvothermal method combined with the electrostatic self-assembly method and spark plasma sintering. The phase interface introduced by incorporating Ti₃C₂ into Sn_0.98Cd_0.02Se can effectively filter low-energy carriers due to its generation of energy barriers, thereby the Seebeck coefficient of x wt% Ti₃C₂/Sn_0.98Cd_0.02Se x = (0.05, 0.5, 1) samples is better than that of the pristine Sn_0.98Cd_0.02Se over the whole temperature range. Meanwhile, high conductivity was also obtained in 1 wt% Ti₃C₂/Sn_0.98Cd_0.02Se sample so that the high power factor of 3.31 μWcm⁻¹K⁻² was acquired at 773 K. Ultimately, a peak ZT value of 0.41 was obtained at 773 K, compared with pristine Sn_0.98Cd_0.02Se, and the thermoelectric performance improved by 24%. This study offers an available approach to efficiently enhance the thermoelectric properties of polycrystalline SnSe-based materials.     

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.