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The BiCuSeO based ceramics have been prepared by mechanical alloying (MA) and resistance pressing sintering (RPS) process. The effects of the parameters of annealing, ball milling and sintering on the microstructure, thermoelectric properties, and mechanical properties are investigated systematically. The results indicates that the samples prepared by MA exhibit higher carrier concentration due to higher Cu vacancies concentration, smaller grain size resulting in stronger phonon scattering, and correspondingly higher thermoelectric properties than those prepared by annealing. By prolonging milling time and increasing ball milling strength, it can further refine grains, increase the carrier concentration, enhance phonon scattering and further improve thermoelectric properties. Coupled with Ca/Pb doping, the electrical conductivity at room temperature increased to 445 Scm −1 . The total thermal conductivity is below 0.90 Wm −1 K −1 in the whole measured temperature range, which is attributable to the increased defects and gain boundaries. The maximum power factor of 0.75 mWm −1 K −2 and ZT value of 1.15 were obtained for the Bi 0.84 Ca 0.08 Pb 0.08 CuSeO with BM time ( t ) = 16 h at 873 K. [ABSTRACT FROM AUTHOR]

As the best oxide based thermoelectric materials, the BiCuSeO compounds with very low lattice thermal conductivity and high figure of merit have been widely investigated because of the good thermal/chemical stability, low price and environmental friendliness. In this paper, the Bi 1-x Sb x CuSeO ( x = 0, 0.005, 0.01, 0.02, 0.05, 0.08) oxychalcogenides were prepared by mechanical alloying (MA) and resistance pressing sintering (RPS) process in order to improve the thermoelectric properties. The effect of Sb doping on the microstructure and thermoelectric properties of p-type BiCuSeO ceramic has been investigated in detail. To analysis the change rule of the thermoelectric properties of Bi 1-x Sb x CuSeO due to Sb replacing Bi site, the lattice structure and electronic behavior of Bs CuSeO ( Bs = Bi or Sb) were calculated and simulated, respectively. The band gap of Bs CuSeO has been reduced from 0.478 eV to 0.329 eV, and the bond length of Cu/Bi-Se has been decreased, the hole pocket in the band structure has been deeper, and the peaks of Cu/Se orbitals in the decomposed partial density of states have been broadened after Bi site was replaced by element Sb, which resulted in the significant increase of carrier concentration and electrical conductivity, while still keep a relatively high Seebeck coefficient and low thermal conductivity. The maximum power factor of 0.36 mWm −1 K −2 and ZT value of 0.73 were obtained for the Bi 0.95 Sb 0.05 CuSeO at 873 K, which was 65% higher than that of the undoped stoichiometry BiCuSeO ceramic. [ABSTRACT FROM AUTHOR]

Thermoelectric materials have great application prospects in the fields of micro-refrigeration, waste heat utilization and catalysis, and BiCuSeO is one of the oxide thermoelectric materials with the best comprehensive thermoelectric properties at present. Herein, the dispersion of 20-nm SiC in BiCuSeO matrix has greatly increases the thermoelectric and mechanical properties of the system. SiC nanoparticles dispersed in BiCuSeO matrix can act as energy filters to increase the Seebeck coefficient, on the other hand, as phonon scattering centers to scatter the low- and mid-frequency phonons effectively to reduce the lattice thermal conductivity, combined with point defects such as Pb Bi + scattering the high-frequency phonons, thus achieving the scattering enhancement of full-frequency phonons. In addition, the mechanical strength and machinability of BiCuSeO have been improved significantly due to the dispersion strengthening effect of SiC nanoparticles. Image 1 • Nano-SiC is dispersed in BiCuSeO matrix by mechanical alloying and spark plasma sintering. • Nano-SiC can enhance the Seebeck coefficient by energy filtering effect, and enhance the power factor. • SiC dispersion achieves the scattering enhancement of full-frequency phonons and decreases thermal conductivity. • The highest ZT reached 1.32 at 873 K for 8% SiC compositing. [ABSTRACT FROM AUTHOR]

A new lightweight magnesia refractory for working linings of high-temperature furnaces was prepared by using microporous magnesia aggregates instead of conventional dense magnesia aggregates. The median pore size of the microporous magnesia aggregates was 2.97 μm, while the apparent porosities of the microporous and dense aggregates were 33.3% and 5.5%, respectively. The phase composition, microstructures, fracture behaviors and strengths at room and high temperature of the lightweight magnesia refractory in comparison with those of the dense one were measured by means of XRD, SEM and three-point bending instrument. After the dense aggregates were replaced by the microporous ones, a better aggregate/matrix interface bonding was observed and the fracture mode changed from intergranular to transgranular fracture, which significantly improved the mechanical properties. Thus, the lightweight magnesia refractories had a lower bulk density of 11.8%, a higher apparent porosity of 56.2% as well as a higher flexural strength at room and high temperature of 48.4% and 160.0% comparing with the dense one, respectively. • A new lightweight refractory has been prepared by using microporous aggregates. • The properties of the lightweight refractories were compared to the dense ones. • The lightweight refractories had a lower bulk density and a higher apparent porosity. • The fracture mode changed from intergranular fracture to transgranular fracture. • The better microporous aggregate/matrix interface improved the strength. [ABSTRACT FROM AUTHOR]

Lightweight design and preparation of refractories can promote the energy saving and emissions reduction for the working linings of high temperature furnaces. In this work, two lightweight periclase-magnesium aluminate spinel refractories were prepared by using microporous aggregates under the industrial production condition and compared with commercial dense refractories. The effects of the microporous aggregates and compaction pressure on their microstructures and properties were studied. A comparison of the results indicates that the flexural strengths of the lightweight specimens were enhanced from 3.7 MPa to 6.8 MPa, and the thermal conductivities were reduced significantly, better microporous aggregate/matrix interface structures were formed in the lightweight specimens due to the pore structures in microporous aggregates. Meanwhile, increasing compaction pressure from 120 MPa to 160 MPa negatively affected the thermal shock resistance and refractoriness under load of the lightweight specimens. In summary, the lightweight specimen made using a compaction pressure of 120 MPa exhibited higher strength, excellent thermal shock resistance, lower thermal conductivity(3.47 W/m·K at 1200 ℃) and high refractoriness under load (1661 ℃), which can meet the requirements for the upper transition zone and burning zone of cement rotary kilns. • Lightweight refractories for cement rotary kilns with high strength were prepared. • Lightweight refractories from industry were compared with commercial dense ones. • Better aggregate/matrix interfaces were formed in lightweight refractories. • Higher compacting pressure deteriorated the service performance of lightweight ones. • An application framework of lightweight refractories with low thermal conductivity. [ABSTRACT FROM AUTHOR]

In the present work, the thermal stabilities of p-type polycrystalline Bi 0.44 Sb 1.56 Te 3 bulks have been investigated at different temperatures in vacuum environment. The effect of service temperature on the microstructure, thermoelectric and mechanical properties of Bi 0.44 Sb 1.56 Te 3 bulks were investigated in detail. The results showed that the service temperature below 473 K had no obvious influence on the grain size, the crystal lattice constant, the density and the corresponding thermoelectric properties, and p-type Bi 0.44 Sb 1.56 Te 3 based bulks could maintain good thermal stability below 473 K. Nevertheless, when the service temperature was over 523 K, the volume expansion began to accelerate, the increasingly obvious mass loss presented due to drastic Te volatilization, and the density of the bulks decreased notably, which resulted in the obvious increase of resistivity and a serious decline in the power factor as service temperature increased, although the Seebeck coefficient presented a slight increase. More seriously, the mechanical properties of the Bi 0.44 Sb 1.56 Te 3 bulks degraded seriously when the service temperature was above 523 K. All evidences implied that the Bi 0.44 Sb 1.56 Te 3 bulks could be stabilized at service temperature of 473 K, the thermoelectric and mechanical properties would get worse obviously when the service temperature was over 523 K. Therefore, the highest service temperature of Bi 2 Te 3 based thermoelectric power generation modules could not exceed 473 K. [ABSTRACT FROM AUTHOR]

In this paper, fine-grained and highly dense n-type Bi 2 Te 2.7 Se 0.3 alloys were prepared via mechanical alloying and a new molding technology named microwave activated hot pressing (MAHP) with relatively low sintering temperature and short holding time. The effects of sintering temperature and holding time on the microstructure and thermoelectric properties of sintered samples were investigated in detail. The results indicated that the sintered alloys could achieve nearly complete densification. Laminated microstructure dominated in the as-MAHPed samples and their size increased gradually with increasing sintering temperature. At the same time, although Seebeck coefficients decreased slightly, their power factors increased obviously due to the drastic decrease of electrical resistivities. And a maximum power factor of 1.81 × 10 − 3 W m − 1 K − 2 was achieved at room temperature for the sample sintered at 693 K for 10 min. A minimum thermal conductivity of 0.61 W m − 1 K − 1 was obtained for the sample sintered at 533 K for 10 min owing to the drastic boundary-phonons scattering effect. A highest ZT value of 0.71 was obtained around 423 K for the Bi 2 Te 2.7 Se 0.3 sample sintered at 613 K without holding time. It confirms that the MAHP technology introduced here can achieve low temperature and rapid sintering process well in preparations of Bi 2 Te 3 -based alloys, and has widely potential applications in other functional materials. [ABSTRACT FROM AUTHOR]

The Fe–6.5 wt.%Si/SiO 2 core–shell composite powders and compacts have been prepared by simple ball milling coating and spark plasma sintering process. The microstructure, magnetic and electrical properties of the compact samples obtained at different sintering temperatures were investigated by differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrometry, vibrating sample magnetometer, and impedance analyzer. The relative densities of Fe–6.5 wt.%Si/SiO 2 composite compacts first increase with sintering temperature from 800 °C to 1150 °C, and then keep nearly unchanged with higher temperature, while the coercivity and electrical resistivity change in the opposite direction. The saturation magnetization fluctuates around an identical value with increasing the sintering temperature from 800 °C to 1250 °C. The Fe–6.5 wt.%Si/SiO 2 composite compact sintered at 1150 °C displays excellent soft magnetic properties: saturation magnetization of 153.9 emu/g, coercivity of 14 Oe, relative permeability of 8.6 × 10 3 at 50 Hz, respectively. The Fe–6.5 wt.%Si/SiO 2 core–shell composite compacts clearly exhibit much higher electrical resistivity and lower core loss (1.4 × 10 −5 Ω m and 18.1 W/kg ( P 5/400 )) than those of Fe–6.5 wt.%Si compacts without core–shell structures (9.6 × 10 −7 Ω m and 37.9 W/kg ( P 5/400 )). The core–shell structures introduced here is a good potential method to increase electrical resistivity and reduce core loss of soft magnetic materials, which can obviously improve energy conversion efficiency of electric–magnetic switching device. [ABSTRACT FROM AUTHOR]

Highlights: [•] Resistance pressing sintering introduced firstly into thermoelectric materials. [•] The locally oriented lamellar structures were obtained in Bi0.44Sb1.56Te3 alloys. [•] The maximum ZT value of 1.17 was obtained for the Bi0.44Sb1.56Te3 alloys. [•] Resistance pressing sintering was more suitable for industrial application. [ABSTRACT FROM AUTHOR]

• A novel strategy was proposed to control the microstructures of microporous spinel aggregates and improve their strengths. • In-situ decomposition reaction process and pore structure evolution mechanism were studied. • Using Al(OH) 3 and calcined magnesite as raw materials was conducive to reducing pore size and improving strength. • Microporous spinel aggregate with an appropriate apparent porosity (58.2%), a small median pore size (6.8 µm) and a high compressive strength (15.2 MPa) was obtained. In the present work, a novel strategy is proposed to control the microstructures of microporous spinel aggregates and improve their strengths. This strategy controls the in-situ decomposition behavior of raw material particles and the Kirkendall effect through preheating magnesite and Al(OH) 3 , then milling with balls and water. Thus, the expected microstructures of microporous spinel aggregates can be obtained. Four microporous magnesium aluminate spinel aggregates of stoichiometric ratio were prepared using magnesite, Al(OH) 3 , calcined magnesite, and calcined Al(OH) 3. The microstructures and strengths of microporous spinel aggregates were analyzed by XRD, SEM, image analysis method, etc. The microporous spinel aggregates prepared from magnesite and Al(OH) 3 had slightly lower apparent porosity (58.2%) than the one (59.2%) synthesized from Al(OH) 3 and calcined magnesite. In addition, the median pore size significantly reduced from 9.5 to 6.8 µm, and the compressive strength dramatically increased from 7.3 to 15.2 MPa. These results verify the reliability of this strategy. [ABSTRACT FROM AUTHOR]

In this paper, Fe-6.5 wt%Si powders were oxidized with different temperatures, and Fe-Si soft magnetic composites with Fe 2 SiO 4 coating layer were transformed successfully by high temperature solid state reaction sintering between the surface oxides and Si element in Fe-6.5 wt%Si powders. The influence of the oxidation temperature on the microstructure and electromagnetic performance of the Fe-Si soft magnetic composites has been studied systematically. The analysis results show that when the oxidation temperature increases from 40 °C to 70 °C, the formed surface oxides are mainly γ-Fe 2 O 3 and become more, but are more likely to fall off the surface of the Fe-6.5 wt%Si powders owing to the accelerated oxidation educed by higher oxidation temperature. Hence, the uniformity of the transformed Fe 2 SiO 4 coating layer is getting worse, resulting in a downward trend of the resistivity and the deteriorating dynamic loss. And the saturation magnetization decreases slowly, while the coercivity exhibits an opposite trend. In addition, it is worth noting that, due to the generation of ferromagnetic Fe by the reaction between Si and γ-Fe 2 O 3 , all the Fe-Si soft magnetic composites exhibit very high saturation magnetization above 188.5 emu/g, which is benefit for the miniaturization of electromagnetic devices. ga1 • Fe-Si/Fe 3 O 4 powders and Fe-Si/Fe 2 SiO 4 SMCs were obtained successfully via water oxidation and SPS process. • The influence of oxidation temperature on performance of Fe-Si SMCs was studied. • Higher oxidation temperature would lead to poor uniformity of Fe 2 SiO 4 coating layer. • All the Fe-Si SMCs exhibit very high saturation magnetization above 188.5 emu/g. [ABSTRACT FROM AUTHOR]

In this work, Fe–Si/Fe 2 SiO 4 soft magnetic composites (SMCs) are prepared successfully via sintering Fe-6.5 wt%Si/α-Fe 2 O 3 core-shell composite powders obtained by ball milling. The transformation mechanism from Fe-6.5 wt%Si/α-Fe 2 O 3 to Fe–Si/Fe 2 SiO 4 is revealed in detail. The effect of α-Fe 2 O 3 coating content on microstructure and magnetic properties for the Fe–Si/Fe 2 SiO 4 SMCs is also investigated. During the high temperature sintering process, α-Fe 2 O 3 coating will in situ react with diffused Si from Fe–Si alloy cores, and result in the generation of Fe 2 SiO 4 insulating layer and Fe atoms. When the α-Fe 2 O 3 coating content increases from 0.0 wt% to 7.5 wt%, the insulating layer becomes thicker. Nevertheless, with further increasing the α-Fe 2 O 3 coating content from 7.5 wt% to 12.5 wt%, the generation of excessive Fe breaks the continuity of the insulation layer. As a result, the saturation magnetization decreases gradually, while the coercivity exhibits an opposite trend. The core loss decreases firstly and then increases. Therefore, the Fe–Si/Fe 2 SiO 4 SMCs coated with 7.5 wt% α-Fe 2 O 3 exhibit superior comprehensive performance such as very low core loss (39.6 mW/cm3 at 50 kHz), very high saturation magnetization (191.4 emu/g) and stable permeability (44) from 50 Hz to 200 kHz. Image 1 • Fe–Si/Fe 2 SiO 4 SMCs transformed from Fe-6.5 wt%Si/α-Fe 2 O 3 composites were prepared. • The transformation mechanism was revealed in detail. • Effect of α-Fe 2 O 3 coating content on magnetic properties was studied. • The Fe–Si SMCs coated with 7.5 wt% α-Fe 2 O 3 exhibited very low P cv (39.6 mW/cm3) and high Ms (191.4 emu/g). [ABSTRACT FROM AUTHOR]