Data-Driven Design of Ecofriendly Thermoelectric High-Entropy Sulfides.

High-entropy compounds with compositional complexity can be designed as new thermoelectric materials. Here a data-driven model was developed, which chose suitable elements to reduce the enthalpy of formation and hence to increase the chance of single phase formation. Using this model, two high-entropy sulfides were designed, metallic Cu5SnMgGeZnS9 and semiconducting Cu3SnMgInZnS7. They were then successfully fabricated as single-phase dense ceramics with homogeneously distributed cations, and their phase stability and atomic local structures were investigated using density functional theory calculations. Finally, a zT value of 0.58 at 773 K was obtained for Cu5Sn1.2MgGeZnS9, where additional Sn was used to tune the carrier concentration. This work provides a simple approach to find new high-entropy functional materials in the largely unexplored multielement chemical space.

Effect of the Processing Route on the Thermoelectric Performance of Nanostructured CuPb18SbTe20.

The quaternary AgPb18SbTe20 compound (abbreviated as LAST) is a prominent thermoelectric material with good performance. Endotaxially embedded nanoscale Ag-rich precipitates contribute significantly to decreased lattice thermal conductivity (κlatt) in LAST alloys. In this work, Ag in LAST alloys was completely replaced by the more economically available Cu. Herein, we conscientiously investigated the different routes of synthesizing CuPb18SbTe20 after vacuum-sealed-tube melt processing, including (i) slow cooling of the melt, (ii) quenching and annealing, and consolidation by (iii) spark plasma sintering (SPS) and also (iv) by the state-of-the-art flash SPS. Irrespective of the method of synthesis, the electrical (σ) and thermal (κtot) conductivities of the CuPb18SbTe20 samples were akin to those of LAST alloys. Both the flash-SPSed and slow-cooled CuPb18SbTe20 samples with nanoscale dislocations and Cu-rich nanoprecipitates exhibited an ultralow κlatt ∼ 0.58 W/m·K at 723 K, comparable with that of its Ag counterpart, regardless of the differences in the size of the precipitates, type of precipitate-matrix interfaces, and other nanoscopic architectures. The sample processed by flash SPS manifested higher figure of merit ( zT ∼ 0.9 at 723 K) because of better optimization and a trade-off between the transport properties by decreasing the carrier concentration and κlatt without degrading the carrier mobility. In spite of their comparable σ and κtot, zT of the Cu samples is low compared to that of the Ag samples because of their contrasting thermopower values. First-principles calculations attribute this variation in the Seebeck coefficient to dwindling of the energy gap (from 0.1 to 0.02 eV) between the valence and conduction bands in MPb18SbTe20 (M = Cu or Ag) when Cu replaces Ag.

Optimization of thermoelectric properties of carbon nanotube veils by defect engineering.

Carbon nanotubes (CNTs), with their combination of excellent electrical conductivity, Seebeck coefficient, mechanical robustness and environmental stability are highly desired as thermoelectric (TE) materials for a wide range of fields including Internet of Things, health monitoring and environmental remediation solutions. However, their high thermal conductivity (κ) is an obstacle to practical TE applications. Herein, we present a novel method to reduce the κ of CNT veils, by introducing defects, while preserving their Seebeck coefficient and electrical conductivity. Solid-state drawing of a CNT veil embedded within two polycarbonate films generates CNT veil fragments of reducing size with increasing draw ratio. A successive heat treatment, at above the polycarbonate glass-to-rubber transition temperature, spontaneously reconnects the CNT veils fragments electrically but not thermally. Stretching to a draw ratio of 1.5 and heat repairing at 170 °C leads to a dramatic 3.5-fold decrease in κ (from 46 to 13 W m-1 K-1), in contrast with a decrease in electrical conductivity of only 26% and an increase in Seebeck coefficient of 10%. To clarify the mechanism of reduction in thermal conductivity, a large-scale mesoscopic simulation of CNT veils under uniaxial stretching has also been used. This work shows that defect engineering can be a valuable strategy to optimize TE properties of CNT veils and, potentially, other thermoelectric materials.

Strength Analysis and Stress-Strain Deformation Behavior of 3 mol% Y-TZP and 21 wt.% Al2O3-3 mol% Y-TZP.

The mechanical behavior of 3 mol% Y2O3-ZrO2 ceramic and 21 wt.% Al2O3-3 mol% Y2O3-ZrO2 ceramic composite with submicron grain size was studied. Mechanical properties, such as hardness, Young's modulus, four-point bending strength, and fracture toughness of both materials were measured. Linear stress-strain deformation behavior of both 3 mol% Y2O3-ZrO2 and 21 wt.% Al2O3-3 mol% Y2O3-ZrO2 was observed in flexure, both at room temperature and at 400 °C. A small deviation from linear elastic deformation was detected in 21 wt.% Al2O3-3 mol% Y2O3-ZrO2 ceramic composite when loaded above a stress of 1500 MPa. Therefore, it was concluded that only elastic deformation occurred at low stresses upon loading, which exclude the presence of domain switching in zirconia upon bending under the loading conditions of this study.

Enhancing the Thermoelectric Performance of Calcium Cobaltite Ceramics by Tuning Composition and Processing.

Calcium cobaltite (Ca3Co4O9) is a promising p-type thermoelectric oxide material. Here, we present an approach to optimize the thermoelectric performance of Ca3Co4O9 by controlling the chemical composition and fabrication process. Ca3-xBixCo3.92O9+δ (0.1 ≤ x ≤ 0.3) and Ca2.7Bi0.3CoyO9+δ (3.92 ≤ y ≤ 4.0) ceramics were prepared by Spark Plasma Sintering (SPS). Stoichiometric mixtures of raw materials were combined and calcined at 1203 K for 12 h, followed by SPS at 1023 K for 5 min at 50 MPa. The samples were subsequently annealed at 1023 or 1203 K for 12 h in air. XRD and HRTEM analyses confirmed the formation of the cobaltite misfit phase with minor amounts of secondary phases; SEM-EDS showed the presence of Bi-rich and Co-rich secondary phases. After annealing at 1203 K, the secondary phases were significantly reduced. By controlling the cobalt deficiency and level of bismuth substitution, the electrical conductivity was enhanced without degrading Seebeck coefficients, promoting a high power factor of 0.34 mW m-1 K-2 at 823 K (parallel to the ab planes, //ab). Due to enhanced phonon scattering, the thermal conductivity was reduced by 20%. As a result, a highly competitive ZT(//ab) of 0.16 was achieved for Ca2.7Bi0.3Co3.92O9+δ ceramics at 823 K.

Magnetic Field-Induced Alignment of Nanofibrous Supramolecular Membranes: A Molecular Design Approach to Create Tissue-like Biomaterials.

A molecular design approach to fabricate nanofibrous membranes by self-assembly of aromatic cationic peptides with hyaluronic acid (HA) and nanofiber alignment under a magnetic field is reported. Peptides are designed to contain a block composed of four phenylalanine residues at the C-terminus, to drive their self-assembly by hydrophobic association and aromatic stacking, and have a positively charged domain of lysine residues for electrostatic interaction with HA. These two blocks are connected by a linker with a variable number of amino acids and the ability to adopt distinct conformations. Zeta potential measurements and circular dichroism confirm their positive charge and variable conformation (random coil, ß-sheet, or α-helix), which depend on the pH and sequence. Their self-assembly, examined by fluorescence spectroscopy, small-angle X-ray scattering, and transmission electron microscopy, show the formation of fiberlike nanostructures in the micromolar range. When the peptides are combined with HA, hydrogels or flat membranes are formed. The molecular structure tunes the mechanical behavior of the membranes and the nanofibers align in the direction of magnetic field due to the high diamagnetic anisotropy of phenylalanine residues. Mesenchymal stem cells cultured on magnetically aligned membranes elongate in direction of the nanofibers supporting their application for soft tissue engineering.

The grain size effect on the properties of Aurivillius phase Bi3.15Nd0.85Ti3O12 ferroelectric ceramics.

Aurivillius phase, bismuth layer structured ferroelectric Bi(3.15)Nd(0.85)Ti(3)O(12) (BNdT) ceramics with average grain sizes from 90 nm and high densities (>97%) were fabricated by spark plasma sintering. Decreasing grain size produced a diffuse ferro-paraelectric phase transition and a decrease in the Curie point. Compared with BNdT ceramics with grain sizes of micrometre scale, nanograined BNdT ceramics exhibit a depression of the dielectric maximum at the Curie point, enhanced dielectric constant from room temperature to 350 degrees C and dramatically decreased losses. Although ferroelectric switching was greatly inhibited in nanograined ceramics, both ferroelectric and piezoelectric measurements still clearly showed that BNdT ceramics with 90 nm average grain sizes are ferroelectrically switchable. This is the first reported evidence that nanoscale Aurivillius phase ceramics are ferroelectrically active.

Strength enhancement and slip behaviour of high-entropy carbide grains during micro-compression.

Bulk polycrystalline high-entropy carbides are a newly developed group of materials that increase the limited compositional space of ultra-high temperature ceramics, which can withstand extreme environments exceeding 2000 °C in oxidizing atmospheres. Since the deformability of grains plays an important role in macromechanical performance, in this work we studied the strength and slip behaviour of grains of a spark-plasma sintered (Hf-Ta-Zr-Nb)C high-entropy carbide in a specific orientation during micropillar compression. For comparison, identical measurements were carried out on the monocarbides HfC and TaC. It was revealed that (Hf-Ta-Zr-Nb)C had a significantly enhanced yield and failure strength compared to the corresponding base monocarbides, while maintaining a similar ductility to the least brittle monocarbide (TaC) during the operation of [Formula: see text] slip systems. Additionally, it was concluded that the crystal orientation and stress conditions determine the operation of slip systems in mono- and high-entropy carbides at room temperature.

Oxidation Protective Hybrid Coating for Thermoelectric Materials.

Two commercial hybrid coatings, cured at temperatures lower than 300 °C, were successfully used to protect magnesium silicide stannide and zinc-doped tetrahedrite thermoelectrics. The oxidation rate of magnesium silicide at 500 °C in air was substantially reduced after 120 h with the application of the solvent-based coating and a slight increase in power factor was observed. The water-based coating was effective in preventing an increase in electrical resistivity for a coated tethtraedrite, preserving its power factor after 48 h at 350 °C.

Ultrahigh ß-phase content poly(vinylidene fluoride) with relaxor-like ferroelectricity for high energy density capacitors.

Poly(vinylidene fluoride)-based dielectric materials are prospective candidates for high power density electric storage applications because of their ferroelectric nature, high dielectric breakdown strength and superior processability. However, obtaining a polar phase with relaxor-like behavior in poly(vinylidene fluoride), as required for high energy storage density, is a major challenge. To date, this has been achieved using complex and expensive synthesis of copolymers and terpolymers or via irradiation with high-energy electron-beam or γ-ray radiations. Herein, a facile process of pressing-and-folding is proposed to produce ß-poly(vinylidene fluoride) (ß-phase content: ~98%) with relaxor-like behavior observed in poly(vinylidene fluoride) with high molecular weight > 534 kg mol-1, without the need of any hazardous gases, solvents, electrical or chemical treatments. An ultra-high energy density (35 J cm-3) with a high efficiency (74%) is achieved in a pressed-and-folded poly(vinylidene fluoride) (670-700 kg mol-1), which is higher than that of other reported polymer-based dielectric capacitors to the best of our knowledge.

Topotactic anion-exchange in thermoelectric nanostructured layered tin chalcogenides with reduced selenium content.

Anion exchange has been performed with nanoplates of tin sulfide (SnS) via "soft chemical" organic-free solution syntheses to yield layered pseudo-ternary tin chalcogenides on a 10 g-scale. SnS undergoes a topotactic transformation to form a series of S-substituted tin selenide (SnSe) nano/micro-plates with tuneable chalcogenide composition. SnS0.1Se0.9 nanoplates were spark plasma sintered into phase-pure, textured, dense pellets, the ZT of which has been significantly enhanced to ≈1.16 from ≈0.74 at 923 K via microstructure texturing control. These approaches provide versatile, scalable and low-cost routes to p-type layered tin chalcogenides with controllable composition and competitive thermoelectric performance.

High coercivity, anisotropic, heavy rare earth-free Nd-Fe-B by Flash Spark Plasma Sintering.

ABSTARCT: In the drive to reduce the critical Heavy Rare Earth (HRE) content of magnets for green technologies, HRE-free Nd-Fe-B has become an attractive option. HRE is added to Nd-Fe-B to enhance the high temperature performance of the magnets. To produce similar high temperature properties without HRE, a crystallographically textured nanoscale grain structure is ideal; and this conventionally requires expensive "die upset" processing routes. Here, a Flash Spark Plasma Sintering (FSPS) process has been applied to a Dy-free Nd30.0Fe61.8Co5.8Ga0.6Al0.1B0.9 melt spun powder (MQU-F, neo Magnequench). Rapid sinter-forging of a green compact to near theoretical density was achieved during the 10 s process, and therefore represents a quick and efficient means of producing die-upset Nd-Fe-B material. The microstructure of the FSPS samples was investigated by SEM and TEM imaging, and the observations were used to guide the optimisation of the process. The most optimal sample is compared directly to commercially die-upset forged (MQIII-F) material made from the same MQU-F powder. It is shown that the grain size of the FSPS material is halved in comparison to the MQIII-F material, leading to a 14% increase in coercivity (1438 kA m-1) and matched remanence (1.16 T) giving a BHmax of 230 kJ m-3.

Large-Scale Surfactant-Free Synthesis of p-Type SnTe Nanoparticles for Thermoelectric Applications.

A facile one-pot aqueous solution method has been developed for the fast and straightforward synthesis of SnTe nanoparticles in more than ten gram quantities per batch. The synthesis involves boiling an alkaline Na2SnO2 solution and a NaHTe solution for short time scales, in which the NaOH concentration and reaction duration play vital roles in controlling the phase purity and particle size, respectively. Spark plasma sintering of the SnTe nanoparticles produces nanostructured compacts that have a comparable thermoelectric performance to bulk counterparts synthesised by more time- and energy-intensive methods. This approach, combining an energy-efficient, surfactant-free solution synthesis with spark plasma sintering, provides a simple, rapid, and inexpensive route to p-type SnTe nanostructured materials.

Ultrafast-Contactless Flash Sintering using Plasma Electrodes.

This paper presents a novel derivative of flash sintering, in which contactless flash sintering (CFS) is achieved using plasma electrodes. In this setup, electrical contact with the sample to be sintered is made by two arc plasma electrodes, one on either side, allowing current to pass through the sample. This opens up the possibility of continuous throughput flash sintering. Preheating, a usual precondition for flash sintering, is provided by the arc electrodes which heat the sample to 1400 °C. The best results were produced with pre-compacted samples (bars 1.8 mm thick) of pure B4C (discharge time 2s, current 4A) and SiC:B4C 50 wt% (3s at 6A), which were fully consolidated under a heating rate approaching 20000 °C/min. For the composite a cylindrical volume of 14 mm(3) was sintered to full density with limited grain growth.

Investigating the highest melting temperature materials: A laser melting study of the TaC-HfC system.

TaC, HfC and their solid solutions are promising candidate materials for thermal protection structures in hypersonic vehicles because of their very high melting temperatures (>4000 K) among other properties. The melting temperatures of slightly hypostoichiometric TaC, HfC and three solid solution compositions (Ta1-xHfxC, with x = 0.8, 0.5 and 0.2) have long been identified as the highest known. In the current research, they were reassessed, for the first time in the last fifty years, using a laser heating technique. They were found to melt in the range of 4041-4232 K, with HfC having the highest and TaC the lowest. Spectral radiance of the hot samples was measured in situ, showing that the optical emissivity of these compounds plays a fundamental role in their heat balance. Independently, the results show that the melting point for HfC0.98, (4232 ± 84) K, is the highest recorded for any compound studied until now.

Dimethylformamide: an effective dispersant for making ceramic-carbon nanotube composites.

Carbon nanotube (CNT) and alumina dispersions were prepared separately in dimethylformamide (DMF) and ethanol by ultrasonication. The colloidal stability of the dispersions was monitored and a particle size analysis was performed to evaluate the size range of the agglomerates after different times. DMF was found to be a much more effective dispersant than ethanol for making stable, homogeneous CNT and composite dispersions. Alumina-CNT (4.65 vol%) nanocomposites were sintered in a spark plasma sintering (SPS) furnace. DMF dispersions produced homogeneously distributed and agglomerate-free CNT-alumina nanocomposites with higher electrical conductivity as compared to nanocomposites prepared using ethanol.