Power processing unit for hall-effect thrusters on "Meteor-M №3 spacecraf"

The development results of power processing unit (PPU-M) for hall-effect thrusters on "Meteor-M №3" spacecraft are considered. The structure, weight, dimensions and main technical characteristics of the system in the paper are presented. The work peculiarity of the system is unstable input voltage of both power bus and control bus that increases the ripple voltages and currents at the input and the output and causes the additional requirements to the circuit design. A comparative analysis of the system characteristics and European analogs was carried out, and then a conclusion on the basis of available data that the characteristics of the system are not inferior to European analogs was mad

Thermoelectric properties of Ca0.9Yb0.1MnO3-x prepared by spark plasma sintering in air atmosphere

In this paper we report the densification of Ca0.9Yb0.1MnO3-x ceramics by spark plasma sintering (SPS) in different atmospheres and using different tools. The thermoelectric properties of these ceramics are reported and compared to those of ceramics obtained by conventional sintering. This study clearly demonstrated the possibility for processing oxide materials by SPS under air atmosphere and overcomes the problem of reduction due to the low oxygen partial pressure (graphitic environment, dynamic vacuum or neutral atmosphere) commonly encountered

Measurement of the magnetic field inside the holes of a drilled bulk high-Tc superconductor

We use macroscopic holes drilled in a bulk YBCO superconductor to probe its magnetic properties in the volume of the sample. The sample is subjected to an AC magnetic flux with a density ranging from 30mT to 130mT and the flux in the superconductor is probed by miniature coils inserted in the holes. In a given hole, three different penetration regimes can be observed: (i) the shielded regime, where no magnetic flux threads the hole; (ii) the gradual penetration regime, where the waveform of the magnetic field has a clipped sine shape whose fundamental component scales with the applied field; and (iii) the flux concentration regime, where the waveform of the magnetic field is nearly a sine wave, with an amplitude exceeding that of the applied field by up to a factor of two. The distribution of the penetration regimes in the holes is compared with that of the magnetic flux density at the top and bottom surfaces of the sample, and is interpreted with the help of optical polarized light micrographs of these surfaces. We show that the measurement of the magnetic field inside the holes can be used as a local characterization of the bulk magnetic properties of the sample

The role of a-axis grains in the transition to the normal state of YBa2Cu3O7−δ films and of 2G-coated conductors when induced by high electrical current densities

The influence of surface defects, in particular of a-axis grains, on the transition to the normal state induced by high current densities in YBa2Cu3O7−δ (YBCO) thin films and in a commercial 2G-coated conductor is investigated. For that purpose, the surface of the samples is observed by scanning electron microscopy and isothermal current-voltage curves are measured at different temperatures with pulsed currents up to the quenching value I*. The results show that the ratio of I* to the critical current is large if a-axis grains are not visible at the surface of the YBCO films, while it is much lower if the surface includes a-axis grains as this is the case for the coated conductor. The connection between the transition onset and the vortex dynamics, as well as the role of the a-axis grains in this process are discussed. The relation between the I* values obtained from thermal calculations and those resulting from vortex dynamics considerations is also discussed, as well as the possible consequences suggested by this work for the different applications of the coated conductors

Overview of Spark Plasma Texturing of Functional Ceramics

This work reports the progress in the preparation of superconducting and thermoelectric lamellar compounds processed by the unconventional Spark Plasma Sintering (SPS). The SPS equipment was modified with the aim of obtaining the textured and dense superconductor Bi2Sr2Ca2Cu3O10,p-type oxide thermoelectric bulk as Ca3Co4O9 and Ca3-xAgxCo4O9/Ag composites respectively. The new process is referred to as Spark Plasma Texturing (SPT). During SPT, the bulk material can freely deform. As a result, inter-grain preferential crystallographic orientation is created. The series of sintered and textured samples using the same Ag content were processed respectively. From the results, we can evidence: (i) the magnetic and/or structural transition around 350 °C, for both series of samples. (ii) The electrical resistivity (ρ) decreases with increasing Ag-substituted or Ag-added. (iii) The Seebeck coefficient (S) of the textured series is higher than that of the sintered series. In the case of the Ag-substituted, S, decreases with Ag content. The optimized composite is found to be Ca2.6Ag0.4Co4O9/8wt% Ag. We can note the remarkable reduction of ρ, and the improvement of power factor values up to 360 μW.m−1.K−2.The superconducting properties of single phased Bi2Sr2Ca2Cu3O10 (Bi2223) consolidated using SPS and SPT will also be discussed

High Critical Current Density of Nanostructured MgB₂ Bulk Superconductor Densified by Spark Plasma Sintering

In situ MgB2 superconducting samples were prepared by using the spark plasma sintering method. The density of the obtained bulks was up to 95% of the theoretical value predicted for the material. The structural and microstructural characterizations of the samples were investigated using X-ray diffraction and SEM and correlated to their superconducting properties, in particular their critical current densities, Jc, which was measured at 20 K. Extremely high critical current densities of up to 6.75 × 105 A/cm2 in the self-field and above 104 A/cm2 at 4 T were measured at 20 K, indicating that vortex pinning is very strong. This property is mainly attributed to the sample density and MgB2 nanograins in connection to the presence of MgO precipitates and areas rich in boron

Mechanical properties of Ca3Co4O9 bulk oxides intended to be used in thermoelectric generators

International audienceThe lamellar thermoelectric (TE) oxides Ca3Co4O9 (Ca349) are promising candidates for energy conversion in a temperature range of 300–1200 K in air. To be well-suited for being integrated in TE devices, Ca349 bulk materials must show high mechanical reliability to withstand the assembly constrains and in-service conditions. In the aim of optimizing TE performances of these materials, specimens were elaborated by using Hot-Pressing (HP) and Spark Plasma Sintering (SPS). Indentation measurements were operated on these ceramics using both micro-hardness testing and depth-sensing nano-indentation. Fracture characteristics were assessed by 3 point bending tests. Nano-hardness (nH), elastic modulus (E), strength (σR) and fracture toughness (KIc) were shown to drastically enhance versus the pressures PHP and PSPS applied during HP and SPS treatments, respectively, which is ascribed to higher densification and, to a lesser extent, to the texture strengthening and grain boundary density decrease in the direction perpendicular to the pressing axis. The contribution to micro-hardness (µH) of both later factors was estimated to ~30% for the hot-pressed sample under PHP=30 MPa which depicted (nHxy=2.1±0.4 GPa, Exy=56±4 GPa) and (nHz=2.3±0.2 GPa, Ez=85±5 GPa) in its respective planes perpendicular and parallel to the pressing axis, revealing an anisotropy of the elastic modulus. It presented σR=251±12 MPa and KIc=2.3±0.4 MPa m1/2. Although the lower size of the intrinsic flaws was found for the HP materials, the largest mechanical characteristics were achieved by SPS under PSPS≥50 MPa. The elastic recovery H/E of Ca349 ceramics was found among the highest ratios reported for oxides and one order of magnitude larger compared to the half-Heusler or skutterudites potential TE materials