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High temperature superconductors (HTSs) are enablers of extensive electrification for aircraft propulsion. Indeed, if used in electrical machines, HTS materials can drastically improve their performance in terms of the power-to-weight ratio. Among the different topologies of superconducting electrical machines, a flux modulation machine based on HTS bulks is of interest for its compactness and light weight. Such a machine is proposed in the FROST (Flux-barrier Rotating Superconducting Topology) project led by Airbus to develop new technologies as part of their decarbonization goals driven by international policies. The rotor of the machine will house large ring-segment-shaped HTS bulks in order to increase the output power. However, the properties of those bulks are scarcely known and have barely been investigated in the literature. In this context, the present work aims to fill out partially this scarcity within the framework of FROST. Thus, a thorough characterisation of the performances and homogeneity of 11 large REBaCuO bulks was carried out. Ten of the bulks are to be utilized in the machine prototype, originally keeping the eleventh bulk as a spare. A first set of characterisation was conducted on the eleven bulks. For this set, the trapped field mapping and the critical current were estimated. Then, a series of in-depth characterisations on the eleventh bulk followed. It included critical current measurement, X-ray diffraction, and scanning electron microscopy on different millimetre-size samples cut out from the bulk at various locations. The X-ray diffraction and scanning electron microscopy showed weakly oxygenated regions inside the bulk explaining the local drop or loss in superconducting properties. The objective was to determine the causes of the inhomogeneities found in the trapped field measured on all the bulks, sacrificing one of them, here the spare one. To help obtain a clearer picture, a numerical model was then elaborated to reproduce the field map of the eleventh bulk using the experimental data obtained from the characterisation of its various small samples. It is concluded that further characterisations, including the statistics on various bulks, are still needed to understand the underlying reasons for inhomogeneity in the trapped field. Nonetheless, all the bulks presented enough current density to be usable in the construction of the proposed machine. [ABSTRACT FROM AUTHOR]

Two MgB2 samples were prepared using the spark plasma sintering (SPS) technique at different temperatures—950 °C (S1) and 975 °C (S2)—for 2 h under 50 MPa pressure to study the influence of preparation temperature on different facets, namely those perpendicular (PeF) and parallel (PaF) to the compression direction of uniaxial pressure during the SPS of MgB2 samples. We analyzed the superconducting properties of the PeF and PaF of two MgB2 samples prepared at different temperatures from the curves of the critical temperature (TC), the curves of critical current density (JC), the microstructures of MgB2 samples, and the crystal size from SEM. The values of the onset of the critical transition temperature, Tc,onset, were around 37.5 K and the transition widths were about 1 K, which indicates that the two samples exhibit good crystallinity and homogeneity. The PeF of the SPSed samples exhibited slightly higher JC compared with that of the PaF of the SPSed samples over the whole magnetic field. The values of the pinning force related to parameters h0 and Kn of the PeF were lower than those of the PaF, except for Kn of the PeF of S1, which means that the PeF has a stronger GBP than the PaF. In low field, the most outstanding performance was S1-PeF, whose critical current density (JC) was 503 kA/cm2 self-field at 10 K, and its crystal size was the smallest (0.24 µm) among all the tested samples, which is consistent with the theory that a smaller crystal size can improve the JC of MgB2. However, in high field, S2-PeF had the highest JC value, which is related to the pinning mechanism and can be explained by grain boundary pinning (GBP). With an increase in preparation temperature, S2 showed a slightly stronger anisotropy of properties. In addition, with an increase in temperature, point pinning becomes stronger to form effective pinning centers, leading to a higher JC. [ABSTRACT FROM AUTHOR]

Tin selenide (SnSe) has attracted widespread attention due to the high thermoelectric figure of merit (ZT≥ 2) in its single crystals, while polycrystalline SnSe shows moderate thermoelectric performance due to its electrical properties. In this study, a hot deformation process was applied to prepare a dense and textured Na-doped SnSe polycrystalline sample in a hot-press. The hot deformed samples exhibit excellent grain alignment, which increases their electrical conductivity to 148 S cm−1 at 823 K, leading to a maximum power factor of up to 10.2 μW cm−1 K−2 at 823 K. Moreover, the hot deformation procedure tends to increase the grain size, which could induce a high thermal conductivity. A peak ZT of ∼1.3 was obtained at 793 K for hot deformed Na0.03Sn0.97Se compared with that of 0.7 for the sample processed by the conventional hot pressing method at a similar temperature. The present study indicates that the texture modulation by hot deformation can lead to a great enhancement in the electrical performance in polycrystalline SnSe compounds. [ABSTRACT FROM AUTHOR]

Bi2Te3‐based materials possess excellent thermoelectric (TE) properties near room temperature. The microstructure's orientations of these materials have a great influence on their properties owing to the lamellar structure. Herein, a process for the preparation of bismuth telluride, named hot‐pressing texture (HPT) where the hot‐pressed (HP) samples are preformed by powder molding, leads to the fine alignment of grains. The orientation factor of the HPT sample is improved by 20%, compared with that of the HP sample. After further carrier concentration optimization, the improved ZT of 1.1 at 350 K is achieved, with an average ZT of 0.9 between 300 and 500 K. Herein, it is shown that the proposed preparation process is an effective strategy for the fabrication of layered materials. [ABSTRACT FROM AUTHOR]

The misfit-layered Ca3Co4O9 oxide is being seen as a potential thermoelectric (TE) candidate for high-temperature power generation in air. Given the very small size and low strength exhibited by single crystals, grain-oriented Ca3Co4O9 ceramics are worth elaborating to capitalize on their anisotropy. However, the usual textured pellets are too thin to probe the TE properties along their principal crystallographic directions. In this paper, we report on the anisotropy of TE properties in the 350–860 K range within thick textured Ca3Co4O9 ceramics fabricated by moderately pressing at 1173 K stacks of pellets primarily textured using spark plasma sintering (SPS), spark plasma texturing (SPT), and hot pressing (HP). The texture was quantitatively assessed, and the influent microstructural parameters were identified, particularly the grain boundary density parallel (GBDc) and perpendicular (GBDab) to the mean c*-axis. We found that the edge-free processing fostered material texturing and (a,b) plane grain growth, thereby dropping GBDab and increasing GBDc. This resulted in a resistivity ρab reduction, leading to a marked enhancement in power factor PFab, which reached 520 μW·m−1·K−2 at 800 K for the HP sample. The anisotropy ρc/ρab was substantially promoted as the texture was reinforced and the GBDc/GBDab ratio increased, with ρc/ρab (HP) > ρc/ρab (SPT) > ρc/ρab (SPS). The Seebeck coefficient S also revealed an anisotropic behavior, with a ratio Sc/Sab >1 for the SPS-processed materials. This behavior was reversed (Sc/Sab <1) for the more textured SPT and HP specimens. It therefore resulted in a PF anisotropy PFc/PFab (HP) < PFc/PFab (SPT) < PFc/PFab (SPS). The PFab/PFc ratio attained 13.6 at 800 K for the thick HP sample, which is the largest ratio recorded thus far on undoped Ca3Co4O9 ceramics. [ABSTRACT FROM AUTHOR]

An abstract of the article regarding the processing of ceramic materials with microwave energy by David Crossin, Sylvain Marinel and Jacques-Guillaume Noudem is presented.

Abstract: Multilayer Ca3Co4O9 (349) thick thermoelectric (TE) materials were fabricated by hot-pressing stacked dense and strongly textured single-layer samples. Microstructure and volume quantitative texture investigations were undertaken by using scanning electron microscopy and neutron diffraction techniques, respectively. The results show a bulk density similar to single-layer samples, but remarkable texture strength reinforcement. The electrical resistivity, ρ, and Seebeck coefficient, S, were reproducibly measured in directions parallel (ρ c and S c ) and perpendicular (ρ ab and S ab ) to the mean c-axis. ρ showed a high anisotropy ratio of 13.5 and 8.8 at 300 and 900K, respectively, and ρ ab kept the same values whereas ρ c decreased in the multilayer samples. S ab and S c unexpectedly revealed different values. The thermal conductivity also displayed a significant anisotropy, with ratio at 900K. The resulting figure-of-merit ZT is then noticeably anisotropic, with ratio . ZT ab was found 2 times larger than ZT value of the conventional sintered 349 materials often used for TE modules fabrication. [Copyright &y& Elsevier]

Abstract: Ca3Co4O9 thermoelectric (TE) ceramics have been processed using conventional sintering (CS), hot-pressing (HP) and spark plasma sintering (SPS). Microstructure investigations have revealed that HP processing is effective for obtaining highly textured 349 ceramics, with a degree of crystallite orientation showing a texture index of 6mrd2, and that SPS affords strong densified 349 ceramics, typically 98% of the theoretical X-ray density. Investigations of mechanical and thermal properties were undertaken for a reliable integration of these materials in practical devices. In detail, these properties were assessed by microindentation, nanoindentation and three point bending tests. Hardness, elastic modulus, strength and fracture toughness of the HP and SPS samples were drastically improved, compared with the CS specimen. The thermal expansions were measured and shown, for the samples processed by HP and SPS, different between the directions parallel and perpendicular to the stress axis. Thermoelectric properties were investigated from 5 to 850K and a remarkable power factor value of 550μWm−1 K−2 was obtained at 850K. Mechanical and TE properties were correlated to the microstructure and texture features. [Copyright &y& Elsevier]