Structure and enhanced thermoelectric properties of InGaO3(ZnO)m (m=1, 2, 3, 4, and 5) ceramics

InGaO₃(ZnO)_m (m = 1, 2, 3, 4, and 5) ceramics are a series of n-type oxide thermoelectric materials with layered structures and low thermal conductivities. Herein, InGaO₃(ZnO)_m (m = 1, 2, 3, 4, and 5) ceramics were fabricated by spark plasma sintering (SPS). Two different trends in the thermoelectric properties of the InGaO₃(ZnO)_m (m = 1, 2, 3, 4, and 5) ceramics were observed depending on the odevity of the m value. The InGaO₃(ZnO) sample exhibited a relatively high electrical conductivity and was therefore selected for vacuum annealing to further improve the electrical transport performance. Oxygen vacancy defects were introduced to the matrix during the annealing procedure, which improved the thermoelectric performance. A maximum ZT of 0.45 was obtained at 973 K for the InGaO₃(ZnO) sample with a 96 h vacuum annealing treatment, which is 30 times higher than that of the pristine sample.     

Microstructure and thermoelectric properties of La0.1Dy0.1SrxTiO3 ceramics

La_0.1Dy_0.1Sr_xTiO₃ (x=0.80, 0.78, 0.75, 0.70) powders were synthesized via a sol-gel method, followed by sintering at 1550 °C in a reducing atmosphere of 5 vol% hydrogen in nitrogen. The microstructure and thermoelectric properties of the Sr-deficient La and Dy co-doped SrTiO₃ were investigated. The result of XRD revealed that La_0.1Dy_0.1Sr_xTiO₃ consisted of SrTiO₃ with a cubic crystal structure as the main phase and of a small amount of Dy₂Ti₂O₇ as the second phase. All the Sr-deficient samples exhibited a step-like microstructure. As the nominal Sr deficient content increased, the electrical conductivity of the Sr-deficient La_0.1Dy_0.1Sr_xTiO₃ ceramics enhanced due to the increasing Sr and oxygen vacancies, the absolute value of the Seebeck coefficient increased a little, and the thermal conductivity decreased to ~3.0 W m⁻¹ K⁻¹, leading to a high ZT value of 0.19 for La_0.1Dy_0.1Sr_0.75TiO₃ at 500 °C.     

The effect of nano-twins on the thermoelectric properties of Ga2O3(ZnO)m (m = 9, 11, 13 and 15) homologous compounds

Ga₂O₃(ZnO)_m (m = integer) homologous compounds are naturally occurring nanostructured materials. Their intrinsically low thermal conductivity makes them attractive for thermoelectric applications. High density Ga₂O₃(ZnO)_m (m = 9, 11, 13, and 15) single phase ceramics were prepared by solid-state reaction. Nano-sized, twin-like V-shaped boundaries parallel to b-axis (apex angle ∼ 60°) were observed for all compositions. Atomic resolution Z-contrast imaging and EDS analysis for m = 15 showed segregation of Ga ions at the interface of V-shaped twin boundaries. Thermal and charge transport properties depend on the value of m. Compositions with m = 9 exhibited very low lattice thermal conductivity of 2 to 1.5 W/m.K at 300 K–900 K; compositions with m=15 showed improved power factor of 140 μW/m. K² at 900 K leading to a thermoelectric figure of merit (ZT value) of 0.055. This study explores the structural variants and routes to improve the thermoelectric properties of these materials     

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.     

Effects of microstructure evolution on transport properties of thermoelectric nickel-doped zinc oxide

We investigate the effects of microstructure evolution on transport properties of nickel-doped ZnO for thermoelectric waste heat recovery at high temperatures. A 3 at.% supersaturated Ni-alloyed ZnO solid solution was prepared by sintering at 1400 °C followed by controlled nucleation and growth of sub-micrometer size NiO-precipitates by aging at 750, 800, and 900 °C for different durations. Minimum thermal conductivity as low as 8.0 W m⁻¹ K⁻¹ at 700 °C is obtained for samples aged at 750 °C for 16 h due to precipitates with high number density of 1.3·10²⁰ m⁻³, which initiate phonon scattering. In turn, as-quenched samples exhibit the highest electrical conductivity, ca. 17.9 S cm⁻¹ at 700 °C. Further nucleation and growth of precipitates taking place for longer annealing durations reduce electrical conductivity and increase Seebeck coefficients, which is associated with dilution of the ZnO-matrix from Ni-atoms. This study provides us with guidelines for optimization of thermoelectric Ni-doped ZnO.     

Densification,microstructure, and thermoelectric properties of AZO/SrTiO3 composites

Al-doped ZnO (AZO) has emerged as a potential high-temperature thermoelectric material with an appropriate Seebeck coefficient and high thermal stability, and hence is considered as a promising material for power generation applications. Herein, we report the fabrication of AZO/SrTiO₃ composites with improved thermoelectric performance. The densification, microstructure, and thermoelectric properties of the AZO/SrTiO₃ composites were investigated. The significant increase in the relative density of AZO from 89.1 to 98.0% after the addition of SrTiO₃ indicates that SrTiO₃ promoted the densification of the composites. Furthermore, the electrical conductivity of AZO increased after the addition of SrTiO₃, which can mainly be attributed to its enhanced relative density. The AZO/SrTiO₃ composite with 2.0 wt% SrTiO₃ showed the highest power factor at 1000 K because of its highest electrical conductivity. In addition, the composite showed the highest ZT value, which was 1.8 times higher than that of pure AZO.     

Phase formation,microstructure development,and mechanical properties of kaolin-based mullite ceramics added with Fe2O3

The effects of Fe₂O₃ on phase evolution, density, microstructural development, and mechanical properties of mullite ceramics from kaolin and alumina were systematically studied. X-ray diffraction results suggested that the ceramics consisted of mullite, sillimanite, and corundum, in the sintering range of 1450°C–1580°C. However, as the sintering was raised to 1580°C, mullite is the main phase with a content of 94%, and the corundum phase content is 5.9%. Simultaneously, high-temperature sintering had a positive effect on the densification of the mullite ceramics, where both the bulk density and flexural strength could be optimized by adjusting the content of Fe₂O₃. It was found that 6 wt% Fe₂O₃ was optimal for the formation of rod-shaped mullite after sintering at 1550°C for 3 h. The sample's maximum bulk density was 2.84 g/cm³, with a flexural strength of 112 MPa. Meanwhile, rod-shaped mullite grains with an aspect ratio of ~9 were formed. As a result, a dense network structure was developed, thus leading to mullite ceramics with excellent mechanical properties. The effect of Fe₂O₃ on the properties might be attributed to the fact that Al³⁺ ions in the [AlO₆] octahedron were replaced by Fe³⁺ ions, resulting in lattice distortion.     

Synthesis of single-phase Ca3Co4O9 ceramics and their processing for a microstructure-enhanced thermoelectric performance

Single-phase Ca₃Co₄O₉ with a high porosity, having a ZT of 0.09 at 627 °C, was successfully prepared by a simple solid-state reaction using fine powders of CaCO₃ and Co₃O₄ after a calcination at 760 °C for 12 hours. It was excellent either for the preparation of the starting powder for the processing of the Ca₃Co₄O₉ ceramics or for direct processing. The influence of already-reported processing methods (classic sintering, hot pressing, free-edge spark-plasma sintering (SPS) of a sintered pellet) and new methods, such as free-edge SPS of a pellet from just-compacted powder, cold pressing of a sintered pellet without post annealing, and free-edge cold pressing of a sintered pellet with post-annealing, on the preparation of Ca₃Co₄O₉ ceramics was studied. The results showed how the density, the grain morphology and the microstructural anisotropy can all influence the thermoelectric characteristics of Ca₃Co₄O₉ ceramics measured in directions parallel and perpendicular to the applied pressure. While the fully dense (99%) and perfectly textured ceramics prepared by the free-edge SPS of a pellet from just-compacted powder had a ZT of 0.17, the highest ZT of 0.31 was obtained for the free-edge cold-pressed and annealed ceramics with modest texturing and low density (65%), having a very low κ of 0.47 W/mK. The results also showed that thin and irregular-shaped plate-like grains with sharp edges are preferred, while their thickening accompanied by rounding of their shape resulted in a reduced thermoelectric performance. The study revealed both the possibilities and the limitations for enhancing the TE characteristics of Ca₃Co₄O₉ ceramics just through microstructure optimisation.     

Structural features and thermoelectric properties of Al‐doped (ZnO)5In2O3 homologous phases

In this work, we investigated the influence of Al doping on the structure of the (ZnO)₅In₂O₃ homologous phase and the thermoelectric characteristics of (ZnO)₅(In_1?xAl_x)₂O₃ ceramics for x=0, 0.01, 0.03, 0.05, 0.1, and 0.2, prepared using a classic ceramic procedure and sintering at 1500°C for 2 hours. The Al substituted for In on both the primary sites in the Zn₅(In_1?xAl_x)₂O₈ homologous phase, the octahedral sites in the basal‐plane inversion boundaries and the trigonal bi‐pyramidal sites in the zig‐zag inversion boundaries, which resulted in a uniformly increased shrinkage of the unit cell with the additions of Al. The a and c parameters were reduced for x=0.2 by a maximum 0.8%. All the samples had similar microstructures, so the differences in the TE characteristics mainly resulted from the effects of the substitution of Al for In, decreasing the charge‐carrier concentration and affecting their mobility. Slightly improved TE characteristics were only observed for Al additions with x=0.01‐0.05, while larger additions of Al only resulted in a reduced electrical conductivity and decreased ZT values in comparison to the un‐doped composition.     

Influence of Ca-doping on the nonlinear properties of novel ZnO-Cr2O3-based varistor ceramics

The influence of Ca-doping on the microstructure development and electrical characteristics was studied in the novel, ZnO-Cr₂O₃-based, varistor ceramics with the nominal composition (99.47-x) mol% ZnO + 0.1 mol% Cr₂O₃ + 0.33 mol% Co₃O₄ + 0.1 mol% La₂O₃ + x mol% CaCO₃, with x ranging from 0 to 6, sintered at 1200 °C. The results showed that, within the limits of the solid solubility of CaO in ZnO, the addition of CaO greatly enhanced the diffusion processes and hence the grain growth, thus broaden the range of breakdown voltages. Moreover, the Ca-doping simultaneously increases the height of the electrostatic Schottky barrier, resulting in a higher nonlinear coefficient of 40 and a lower leakage current of 2.8 μA/cm². The results are important for the potential application of these novel ZnO-Cr₂O₃-based varistors, having different nominal voltages and yet suitable dimensions, in overvoltage protection across a broad range of voltages.     

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.     

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.     

Low thermal conductivity and thermoelectric properties of Si80Ge20 dispersed Bi2Sr2Co2Oy ceramics

Bi₂Sr₂Co₂O_y is a thermoelectric material with low thermal conductivity. The Bi₂Sr₂Co₂O_y/Si₈₀Ge₂₀ composite samples were prepared by solid phase sintering at high temperature to investigate the effects of Si₈₀Ge₂₀ alloys as the second phase on the microstructure and thermoelectric properties of the fabricated composites. An appropriate amount of the dispersed Si₈₀Ge₂₀ in the Bi₂Sr₂Co₂O_y matrix can reduce the resistivity of the composite successfully. In particular, the increase in phonon scattering caused by the second phase leads to a significant decrease in thermal conductivity, which improves the thermoelectric properties of the material significantly. At 923 K, the thermal conductivity of the Bi₂Sr₂Co₂O_y + 0.2 wt% Si₈₀Ge₂₀ sample achieves an ultralow value of 0.58 W/K·m. Its corresponding optimal dimensionless thermoelectric figure of merit value is 0.36, which is 56% higher than that of the pure Bi₂Sr₂Co₂O_y sample.     

Low temperature thermoelectric properties of K-substituted Bi2Sr2Co2Oy ceramics prepared via laser floating zone technique

In the present study, Bi₂Sr_2-xK_xCo₂O_y (x = 0.0, 0.050, 0.075, 0.100, and 0.15) ceramic precursors have been produced using the classical ceramic route, followed by texturing through the laser floating zone technique. XRD results show that the thermoelectric phase is the major one in all cases. Moreover, K-substitution decreases the secondary phases content, when compared to the undoped sample. SEM observations indicate that grain orientation is significantly enhanced when K-content is increased. K-doping decreases electrical resistivity from 32 10⁻⁵ Ω m (in undoped samples) to 20–22 10⁻⁵ Ω m at 300 K, while increasing Seebeck coefficient from 55 μV/K to 100–117 μV/K at 300 K. On the other hand, thermal conductivity is slightly lower in undoped samples (0.93 W/K m, compared to 1.10–1.28 W/K m for doped ones at 300 K), due to their lower electrical conductivity. Finally, ZT values are higher when the K-content increases up to x = 0.10, reaching 0.029 at around 400 K, and slightly decreasing for higher doping levels.     

Synthesis,structure, and thermo‐physical properties of Fe2O3.Al2O3 and polyethylene nanocomposites

In this study, Fe₂O₃.Al₂O₃/polyethylene composites were prepared using a two‐step process. In the first step, PE is synthesized using titanium based metallocene catalyst system. While the synthesized PE was subsequently purified, hydrated alumina filled PE (MHFP) composites was formed by the hydrolysis of methylaluminoxane (MAO). In the second step, Fe₂O₃.Al₂O₃/PE was prepared via thermal decomposition of ferric formate in a high temperature solution of MHFP composite. The structure, morphology, and thermal properties of the composites were characterized using the XRD, FTIR, SEM‐EDX, TGA, and DSC analytical techniques. Results showed that the incorporation of a suitable amount of Fe₂O₃.Al₂O₃ into the composites enhances the thermal stability. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Microwave dielectric properties,microstructure, and bond energy of Zn3-xCoxB2O6 low temperature fired ceramics

Low temperature co-fired ceramics (LTCCs) technology plays an important role in modern wireless communication. Zn_3-xCo_xB₂O₆ (x = 0–0.25) low temperature fired ceramics were synthesized via traditional solid-state reaction method. Influences of Co²⁺ substitution on crystal phase composition, grain size, grain morphology, microwave dielectric properties, bond energy, and bond valence were investigated in detail. X-ray diffraction analysis indicated that the major phase of the ceramics was monoclinic Zn₃(BO₃)₂. Solid solution was formed with Co²⁺ substituted for Zn²⁺ because no individual phase that contained Co was observed. An increase in the amount of Co²⁺ substitution changed average grain sizes, and regrowth of grains were observed with Co²⁺ substitution. Appropriate amount of Co²⁺ substitution improved densification. With changes in Co²⁺ substitution, bond energy of major phase and average bond valence of B–O were positively correlated to temperature coefficient of resonant frequency. The Zn_2.927Co_0.075B₂O₆ ceramic sintered at 875 °C for 4 h exhibited excellent microwave properties with ε_r = 6.79, Q × f = 140,402 GHz, and τ_f = −87.42 ppm/°C. This ceramic is regarded as candidate for LTCC applications.     

Improvement of microstructure,mechanical properties and cutting performance of Ti(C,N)-based cermets by ultrafine La2O3 additions

Ti(C,N)-WC-Mo₂C-TaC-Co-Ni cermets with various content of La₂O₃ were prepared by gas-pressure sintering at 1450 °C. The effects of ultrafine La₂O₃ additions (0, 0.05, 0.1 and 0.2 wt%) on the microstructure, mechanical properties, wear resistance and cutting performance of cermets were explored. In the microstructure of cermets, the La₂O₃ particles and dissolved La element in binder phases were observed, which could inhibit the dissolution-precipitation process of ceramics phases during liquid-sintering. Furthermore, the La₂O₃ could absorb and react with the impurity Al element with low melting point from raw powders, avoiding the appearance of liquid phase at the low temperature and partial overheating during sintering process. These mechanisms could inhibit the abnormal growth of Ti(C,N) core-(Ti,W,Mo,Ta)(C,N) rim structures effectively, leading to the thinning of brittle rim phases and coarsening of wear-proof Ti(C,N) particles. The decrease of proportion of brittle rim phase and ultrafine Ti(C,N) particles promoted the fracture toughness. The increase of proportion and grain size of Ti(C,N) improved the hardness, wear resistance and cutting performance significantly. However, the excessive addition of La₂O₃ would result in the agglomeration of La₂O₃, causing the sharp decline of mechanical properties and cutting performance. The cermet with 0.1 wt% La₂O₃ addition possessed the optimal mechanical properties with Vickers hardness, transverse rupture strength and fracture toughness of 1710 (HV30) Kgf/mm², 2480 MPa and 11.7 MPa m^1/2, respectively.     

Synthesis and post-annealing effects on the transport properties of thermoelectric oxide (ZnO)mIn2O3 ceramics

The synthesis and transport properties of n-type thermoelectric oxide (ZnO)_mIn₂O₃ (Z_mIO) ceramics prepared by conventional solid-state reaction method have been reported. It is found that the transport properties of Z_mIO ceramics are very sensitive to the post-annealing temperature as well as the zinc content m. The resistivity of Z₅IO annealed at 1400 °C decreases by more than 2 orders of magnitude in comparison with that of Z₅IO annealed at 1200 °C, while the resistivities of Z₆IO compounds annealed at 1250 and 1350 °C are more than 3 orders of magnitude larger than that of Z₆IO annealed at 1300 °C. All the Z_mIO compounds annealed at 1300 °C show electron-type conduction with a lowest resistivity at m = 6. It is suggested that the oxygen defects or vacancies in the InO₂ layers play a major role on the carrier scattering mechanism, and the observed temperature-dependent resistivity for Z₅IO and Z₆IO compounds can be satisfactorily described by the variable-range hopping conduction. Furthermore, it is found that the values of Seebeck coefficient for Z_mIO are also very sensitive to the zinc content m. The dimensionless figure of merit of 0.0045 at 300 K for m = 6 has been obtained.     

Densification,microstructure evolution,and microwave dielectric properties of Mg1-xCaxZrTa2O8 ceramics

In this study, the effects of the Mg²⁺ ions replaced by Ca²⁺ ions on the microwave dielectric properties of newly developed MgZrTa₂O₈ were investigated. Mg_1-xCa_xZrTa₂O₈ (x = 0–1.0) ceramics were prepared via a solid-state reaction method. Calcination of the mixed powders was performed at 1200 °C and sintering of the powder compacts was accomplished at temperatures from 1200 to 1550 °C. The substitution of Ca²⁺ significantly inhibited the densification of Mg_1-xCa_xZrTa₂O₈, led to the expansion of the unit cells, and triggered the formation of a second phase, CaTa₂O₆. The porosity-corrected relative permittivity increased almost linearly with the x value because of the replacement of the less polarizable Mg²⁺ ions by the more polarizable Ca²⁺ ions. The variation in the Q × f values followed a similar trend as that of the sintered density, and the change trend in the τ_f values was in accordance with that of relative permittivity. The best composition appeared to be Mg_0.9Ca_0.1ZrTa₂O₈, which showed excellent microwave dielectric properties of ε_r = 22.5, Q × f = 231,951 GHz, and τ_f = −32.9 ppm/°C. The Q × f value obtained is the highest among the wolframite dielectric ceramics reported in literature.     

Densification,microstructure, and mechanical properties of V-substituted HfB2-based ceramics

This study firstly developed Hf_1-xV_xB₂ (x = 0, 0.01, 0.02, 0.05) powders, which were derived from borothermal reduction of HfO₂ and V₂O₅ with boron. The results revealed that significantly refined Hf_1-xV_xB₂ powders (0.51 μm) could be obtained by solid solution of VB₂, and x ≥ 0.05 was a premise. However, as the content of V-substitution for Hf increased, Hf_1-xV_xB₂ ceramics sintered by spark plasma sintering at 2000 °C only displayed a slight densification improvement, which was attributed to the grain coarsening effect induced by the solid solution of VB₂. By incorporating 20 vol% SiC, fully dense Hf_1-xV_xB₂-SiC ceramics were successfully fabricated using the same sintering parameters. Compared with HfB₂-SiC ceramics, Hf_0.95V_0.05B₂-20 vol% SiC ceramics exhibited an elevated and comparable value of Vickers hardness (23.64 GPa), but lower fracture toughness (4.09 MPa m^1/2).