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Novel high-entropy perovskite-type (Ca0.2Sr0.2Ba0.2La0.2Pb0.2)TiO3 (CSBLP) ceramics with cubic structure of Pm-3m space group were successful prepared by solid-state reaction method. Results of XRD, SEM-EDS, HRTEM confirmed a homogeneous distribution and equimolar arrangement of multicomponent cations on the A-site. The high-entropy CSBLP ceramics showed long-range structural order and short-range chemical disorder with widely dispersed nanoscale grains varying sizes of 4-6 nm in the microstructure. Because of increased configurational entropy, the CSBLP ceramic has an enhanced Seebeck coefficient ( | S | = 272 μV/K at 1073 K) and low thermal conductivity (κ=1.75 W/m•K at 1073 K) when annealed at 1300 °C. This work demonstrates the possibility of effectively reducing thermal conductivity and improving the performance of thermoelectric oxides through high-entropy composition design.

The thermoelectric properties of perovskite CaMnO3 with A-site defects were synergistically optimized and the lowest κt value of 0.94 W m−1 K−1 was found at 800 °C.

Either CoxSy transition metal sulfide or reduced graphene oxide (RGO) has become major electrode material for supercapacitor. Herein, nanostructured RGO@CoxSy composites were optimized by different sulfur sources and a one-step solvothermal method. The nano-sesame sphere RGO@Co4S3 (NSS-RGO@Co4S3) and nano-crispy stuffed nori RGO@Co3S4 (NCSN-RGO@Co3S4) were formed from thiourea and TAA, respectively. The synergistic effect of RGO and CoxSy promoted the electrochemical properties of RGO@CoxSy electrode material for energy storage. The synergistic effect between RGO and CoxSy and the structure–function relationship of RGO@CoxSy improve the electrochemical energy storage of nanostructured RGO@CoxSy. NSS-RGO@Co4S3 was given to priority to energy storage than NCSN-RGO@Co3S4 due to the crack between NSS-RGO@Co4S3 nanoparticles could shorten the ion diffusion path. A high specific capacitance of 1085 F·g−1 with 0.5 A·g−1 and energy density of 37.67 Wh⋅kg−1 at a power density of 0.125 KW⋅kg−1 for nanostructured NSS-RGO@Co4S3 was obtained, whose specific capacitance is nearly 1.3 times as large as NCSN-RGO@Co3S4 and about 96 times larger than one of RGO. It is realized to improve the electrochemical performance of electrode materials with nanostructural modulation. This study showed the nanostructured RGO@CoxSy can effectively control and broaden the other electrode material to electrochemical energy storage.

(Ca0.87Ag0.1La0.03)3Co4O9 ceramics along [00l]c crystallographic orientation were prepared by using tape casting combined with template grain growth method, which were regulated bywhich the sintering temperature and the soaking time. The (La, Ag) co-doping not only can balance the contradiction of the electrical resistivity and Seebeck coefficient, but also generat the point-defects to increase phonon scattering, which can lower thermal conductivity. The effect of grain orientation evolution on thermoelectric properties was investigated and the mechanism of epitaxial growth was discussed through the analysis of microstructures. The results show that the optimized sintering parameters led to great changes in the degree of texturing, which was closely related to thermoelectric performance. A large Seebeck coefficient of 232 μV/K was obtained for the 40 h sample and the high Lotgering factor of 92.7% for the 20 h sample reached, giving rise to the power factor of 0.38 mW/(m·K2) at 1073 K. The morphologic orientation and epitaxial growth give a possible route of preparing highly textured ceramics and achieving high thermoelectric properties.

La-doped strontium titanite (Sr0.9La0.1TiO3) is a promising candidate for n-type oxide thermoelectric materials. However, the ZT values of this material are low, leading to low conversion efficiency. Improvements in this efficiency are required. In this work, a high ZT value of 0.50 was obtained for Sr0.9La0.1TiO3 ceramic samples by adding 10 wt % Bi2O3 sintering aids and 20 wt % nanosized Ti powders to the matrix material. Although Ti was oxidized to TiO2 during the sintering process, nanoscale phase interfaces were beneficial for phonon scattering and thermal conductivity reduction. Nanosized metallic Bi and Bi2O3 particles were observed. These two factors played an important role in reducing the thermal conductivity from 2.5 W/(m K) at room temperature to 1.31 W/(m K) at 1073 K. Nanostructure control using nanosized metal powders as additives combined with the Bi2O3 sintering aid paves a way for enhancement of thermoelectric properties of oxide thermoelectric materials.

Misfit-layered Ca3Co4O9 as a p-type semiconductor is difficult to commercialize because of its relatively poor performance. Here, Ca2.7-xLaxAg0.3Co4O9/Ag composites prepared by spark plasma sintering were systematically investigated in terms of La3+ dopant levels and nano-sized Ag compacts. Multiscale microstructures of stacking fault, dislocation, and oxygen vacancy-linked defects could be recognized as an effective strategy for tuning the transport of charge carriers and phonon scattering. An increasing concentration of charge carriers was caused by the introduction of nano-sized Ag particles at the grain boundary. The multiscale structural defects served as phonon scattering centers to reduce the thermal conductivity. Finally, the Ca2.61La0.09Ag0.3Co4O9/Ag sample exhibited a maximum ZT of 0.35 at 1073 K. The results suggest that the interplay of structural defects provides an impetus for a huge improvement in thermoelectric performance.

Sr0.9La0.1TiO3/20 wt%Ti ceramics with enhanced thermoelectric properties were prepared by spark plasma sintering (SPS) and nanosized Ti metal powders were used as additive. In the sintering process, a large proportion of Ti was oxidized, promoting the volatilization of lattice oxygen and creating more $$V_{{\text{O}}}^{ \cdot \cdot }$$ . Part of sintering aid Bi2O3 was reduced to metal Bi particles which facilitate carriers transport by forming an electron transport network. These two factors contribute to electrical resistivity reduction. The lowest electrical resistivity of ρ389K = 4.0 × 10–4 Ω cm and ρ1120K = 1.5 × 10–3 Ω cm and the maximum PF value PF389K = 1.08 mW/m/K2 were obtained. This work demonstrates that nanosized Ti addition together with Bi2O3 sintering aid in SPS process enhances the thermoelectric performance of Sr0.9La0.1TiO3.

Porous (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (BCZT) piezoelectric ceramics with an oriented directional hole structure were prepared by using the tertbutyl alcohol (TBA)-based freeze-casting method. The influences of sintering temperatures on the microstructure and piezoelectric properties of porous BCZT ceramics were investigated both perpendicular and parallel to the freezing direction. With the increase in sintering temperatures and the porosities decreased from 58% to 42%, the compressive strength increased from 14.0 MPa to 25.0 MPa. In addition, the d33 value of 407 pC/N for the sample sintered at 1400 °C was obtained parallel to the freezing direction, which was 1.40 times that of the other direction.

The 3YSZ/40 wt% Al2O3 composites were prepared by flash sintering at a low furnace temperature (700 °C). The effects of the current density on the relative density and Vickers hardness of the composites were systematically investigated. The results showed that the relative densities and Vickers hardness of the samples increased gradually with the increasing of the current densities, and the relative density was as high as 94.2%. The Vickers hardness of 11.3 GPa was obtained under a current density of 102 mA/mm2. Joule heating and defects generation are suggested to be the main causes of rapid densification in flash sintering. The microstructure of the molten zone showed the formation of eutectic structures in the composite, suggesting that grain boundary overheating may have contributed to the formation of the molten zone.

High entropy perovskite oxides (HEPOs) were a class of advanced ceramic materials, which had attracted much scientific attention in recent years. However, the effect of factors affecting the phase stability of high entropy perovskite oxides was still controversial. Herein, 17 kinds of A-site HEPOs were synthesized by solid-state methods, and several criteria for the formation of HEPOs and phase stability were investigated. Single-phase solid solutions were synthesized in 12 kinds of subsystems. The results show that the phase stability of a single-phase solid solution was affected by the size disorder and configurational entropy. The electronegativity difference was the key parameter to predict the evolution of the cubic/tetragonal phase, rather than the tolerance factor. Cubic HEPOs were easily formed when the electronegativity difference was

Two-step pressureless sintering processing was used to prepare high density (97.6%) Ca3Co4O9 thermoelectric ceramics. A duplex structure was observed and channel paths for charge carriers were formed between the grains. Oxygen vacancies were identified by fitting of XPS spectra, and the effect of the oxygen vacancies on thermoelectric properties was investigated. The electrical resistivity of samples decreased from 17.5 to 11.7 mΩ·cm and Seebeck coefficient decreased from 197.5 to 181.4 μV/K at 1073 K with increasing of the first-step sintering temperature. Furthermore, a low thermal conductivity of 0.94 W/(m·K) and the peak ZT of 0.30, which was 1.3 times larger than previous work, were obtained at 1073 K when the first-step sintering temperature was 1373 K.

The commercial application of Ca3Co4O9 ceramics at high temperatures is greatly restricted due to poor thermoelectric properties. One of the approaches to enhance the performance of oxide thermoelectric ceramics is to control the microstructure of the ceramics to provide texturing. Herein, a high figure of merit of ZT = 0.43 was reported at 1073 K for the highly grain-oriented (Ca0.87La0.03Ag0.1)3Co4O9 ceramics, which were fabricated by tape casting combined with templated grain growth. Multi-scale interfaces, such as the parallel grain boundary and regular zigzag edge, the “core–shell” interface inside the stripe-like grain orientation, and lattice stack default, were observed in the “brick-wall” microstructure. The electron transport properties and thermal conductivity in different directions were investigated during the texture evolution, which was regulated by increasing the sintering temperature. By adjusting the sintering temperature, the sample parallel to the tape-casting direction showed a maximum power factor of 0.64 mW (m−1 K−2) as a result of effective interfacial effects, which was ∼3 times higher than that in the perpendicular direction. The ultralow thermal conductivity of 0.366 W (m−1 K−1) at 1073 K was obtained because phonon scattering was strengthened through the “core–shell” interface and zigzag interface. This work provides a new insight to achieving high-performance thermoelectric oxides using an interesting interface modulation and anisotropic properties through texture engineering.

A two–step processing method, spark plasma sintering (SPS) combined with a heat–treatment, was used to fabricate (Ca0.9Ag0.1)3Co4O9/nano–sized Ag composites. Sliver within the lattice generated hole carriers, and silver along the grain boundaries improved the transport path of the charge carriers. Samples sintered using SPS at different temperatures had very different thermoelectric properties. The results showed that when the sample was sintered at 1233 K, the maximum power factor reached 0.43 mW/(m·K2) along with an electrical resistivity of 8.61 mΩ·cm and a Seebeck coefficient of 196.90 μV/K, and the corresponding lattice thermal conductivity was 1.86 W/(m·K). This study shows how to improve the properties and broaden the application of Ca3Co4O9 thermoelectric ceramics.

Ca-Co-O ceramics is typically p-type thermoelectric materials and possesses positive Seebeck coefficient. In this work, n-type Ca-Co-O ceramics with negative Seebeck coefficients were fabricated by sintering and annealing in a reducing atmosphere. The microstructures and thermoelectric properties of the ceramics were investigated. The results show that the carrier concentration and the carrier mobility dramatically increase after the samples were annealed in the reducing atmosphere. The electrical resistivity increases from 0.0663 mΩ·cm to 0.2974 mΩ·cm, while the negative Seebeck coefficients varies from −24.9 μV/K to −56.3 μV/K as the temperature increases from 323 K to 823 K, and the maximum power factor (PF, 1.536 mW/m·K2) is obtained at 623 K. The samples have n-type thermoelectric properties with large PF values and ZT value (ZT = 0.39, 823 K). The unusual results will pave a new way for studying Ca-Co-O thermoelectric ceramics. Keywords: Thermoelectric ceramic, Negative Seebeck coefficient, Annealing, Reducing atmosphere

Strontium titanate (SrTiO3 ) based materials are promising for high-temperature thermoelectric applications. In order to enhance their performance, annealing is usually required and carried out under various atmospheres. Annealing with graphite is quite effective, but the mechanism is not yet clear. In this work, we use IR spectroscopy and gas chromatography (GC) to monitor the chemical environment under the annealing conditions (1350 °C for 8 h under 16.9 mL/min N2 with graphite) and quantify the various gases evolved in the process. It is shown that reducing agents, H2 and CO (concentrations peaked at ca. 0.4-0.5 %), are generated from graphite in the annealing process. H2 is produced in carbon gasification reaction, which also generates CO. Additional CO is produced from incomplete combustion of carbon. In the annealing of a La-doped SrTiO3 -based ceramic with graphite, higher levels of H2 O and CO2 are detected, which is resulted from the reduction of the ceramic by H2 and CO, respectively. About 67 % of the oxygen vacancies were created by CO reduction while about 33 % by H2 reduction. The conclusions are well supported by direct weight loss measurements with a difference of less than 6 %.

As a well-known n-type perovskite thermoelectric oxides, SrTiO3 has always captured attention to find strategies for achieving high thermoelectric performance. In this paper, a series of reduced Sr0.9La0.1TiO3/TiO2 biphase composite ceramics were prepared via the solid-state reaction method. The effects of TiO2 on the microstructure and thermoelectric properties of the ceramics were investigated. The addition of TiO2 enhanced the carrier mobility via the formation of A-site cation vacancies and increased the carrier concentration probably through introducing oxygen vacancies in rutile TiO2 during the reduction process. Moreover, the additional phonon scattering provided by the interface between the second phase and matrix and oxygen vacancies decreased the thermal conductivity. When the TiO2 addition ratio was optimized to 25 wt%, a large power factor of 1.66 mW/K2/m and the ZT value of 0.24 were achieved at 447 K. The present strategy in this work may be feasible to broaden the range of operating temperature of SrTiO3-based oxide thermoelectric materials.

Large-size Ca3Co4O9 microplates in the range of 10 μm to 18 μm, were prepared by using molten salt synthesis method. The piezoelectric properties of Ca3Co4O9 were reported in the first time. The crystal structure was characterized by X-ray diffractometry along with Rietveld refinement of lattice parameters. In situ piezoresponse force microscopy (PFM) measurements demonstrated that a maximum piezoelectric strain coefficient d*33 of 95.1 pm/V was obtained, rooting in the non-centrosymmetric and anisotropic crystalline structure. This work suggests that the misfit-layered cobaltite oxides can be considered to be a promising alternative lead free piezoelectric material.

This paper has quantitatively investigated strain-rate sensitivity (SRS) coefficients m for hot processing with different dynamic softening (DS) behavior. Isothermal compression experiment of TC18 titanium alloy has been conducted for verification and relevant sensitivity coefficients have been obtained from Arrhenius constitutive function. Kocks–Mecking model has been employed to estimate SRS coefficients for dynamic recovery (DRV) process and SRS values maintain stable at 0.2. SRS coefficients for dynamic recrystallization (DRX) process have been calculated from JMAK kinetic model and the values have been obtained over 0.3 with obvious sensitivity to parameter variation. Dependence of strain-rate sensitivity on dynamic softening behavior has been demonstrated in comparison with dislocation evolution map. Microstructure evolution incorporating different SRS values has been analyzed under deformation parameter variation. Power dissipation efficiency distribution map for hot deformation of TC18 alloy has been constructed from SRS values in quantized characterization on DS behavior.