Granular and Lamellar Thermoelectric Oxides Consolidated by Spark Plasma Sintering

Using the spark plasma sintering (SPS) technique, dense nanostructured Ca_0.95Sm_0.05MnO₃ (n-type) and textured Ca₃Co₄O₉ (p-type) ceramics were prepared. Nanoceramic powders of doped n-type were synthesized using two routes: coprecipitation and solid-state reaction. The SPS method has been used to control the samples’ densification and grain growth. Microstructural investigations reveal that the SPS technique results in high bulk density and homogeneous morphology for Ca_0.95Sm_0.05MnO₃, and grain alignment for Ca₃Co₄O₉. The thermoelectric and mechanical properties were investigated, showing a dependence on the starting grain size and being governed by the SPS conditions.     

Progress in Processing of Thin Wall HTc Bulk Superconducting Cryomagnets

We report on our progress in the preparation of single domain of high temperature superconducting (HTS) bulk YBCO and GdBCO cryomagnets with multiple holes. Three mains steps have been explored and discussed in this study:

1. Gd₂BaCuO₅ (Gd211) and Y₂BaCuO₅ (Y211) and thin wall preforms shaping by the extrusion technique.
2. GdBa₂Cu₃O₃ (Gd123) YBa₂Cu₃O₃ (Y123) single domain obtained from Y211 using the Seed Infiltration Growth (SIG) process.
3. Annealing under high oxygenation pressure in order to improve the superconducting properties.

The thin wall samples with larger specific areas offer some advantages as better thermal exchange, shorter oxygen diffusion paths, possibility to reinforce the material to overcome the mechanically stresses during magnetization. The process and the microstructural features at various stages of processing, with particular emphasis on the characteristics of Gd211 and Y211 inclusions are discussed. Properties like the magnetic trapped field at different temperatures, the superconducting transition temperatures, and critical current densities of the resulting composites are presented and compared with their values in samples processed by other variants of the melt textured growth process.     

Development of multilayer textured Ca3Co4O9 materials for thermoelectric generators: Influence of the anisotropy on the transport properties

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 ${\rho }^c/{\rho }^{ab}$ of 13.5 and 8.8 at 300 and 900 K, 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 ${\kappa }^{ab}/{\kappa }^c=2.7$ at 900 K. The resulting figure-of-merit ZT is then noticeably anisotropic, with ratio $Z{T}^{ab}/Z{T}^c=4.6$. ZT^ab was found 2 times larger than ZT value of the conventional sintered 349 materials often used for TE modules fabrication.     

Ca3Co4O9 ceramics consolidated by SPS process: Optimisation of mechanical and thermoelectric properties

Ca₃Co₄O₉ (349) thermoelectric (TE) oxide ceramics were successfully prepared by Spark Plasma Sintering process. The effects of the uniaxial pressure (30-100 MPa), the dwell temperature (700-900 °C) and the cooling rate were investigated. Microstructure analyses have revealed strong enhancements of the bulk density as the pressure level and the applied temperature during the SPS process are increased. Mechanical properties were investigated by using instrumented nanoindentation and three point bending tests. Hardness, elastic modulus, strength and fracture toughness were shown to improve drastically and depend on the processing parameters. Thermal expansion measurements reveal a noticeable anisotropy induced by unidirectional hot pressing. The mechanical, thermal and thermoelectric properties were correlated to the microstructure and crystallographic texture of the resulting ceramics.     

Oxidation Behavior of the Skutterudite Material Yb0.2Co4Sb12

Metallurgical and Materials Transactions A - The skutterudite materials belonging to the CoSb3 family are widely studied for thermoelectric applications. They are typically used under vacuum, but...     

An Effective Approach for the Development of Reliable YBCO Bulk Cryomagnets with High Trapped Field Performances

Widespread use of YBa₂Cu₃O_7‐δ (Y123) bulk superconductors as source of strong magnetic fields requires development of high‐performance materials sufficiently reliable with improved thermal transfer ability. An effective approach based primarily on the growth of bulk Y123 single domains comprising a holes‐network to diminish the oxygen diffusion paths is reported here, as well as their progressive annealing at high temperature under oxygen pressure to reduce undue stresses and processing time. Finely, it aims to stimulate the thermal exchange inside the superconductor and compensate for induced magnetic stresses during the field‐trapping process. The approach brings considerable time and energy savings, and turns out to knock down barriers having stymied hitherto the use of Y123 bulk superconductors for engineering applications. Indeed, it enables the achievement of a pore‐free and crack‐free microstructure yielding marked fracture toughness and promoting large size persistent current loops, thereby boosting the trapped field performances. The fostering of the internal thermal exchange leads the maximum trapped field B_max to shift to higher temperatures by up to 14 K. A value B_max of 6.34 T is attained at 17 K on ≈16 mm‐diameter reinforced pellet (disk area s = 1.99 cm²), resulting in an outstanding field density B_max/s=3.19 Tcm^?2.