Search the repository

A+ | A- | | SLO | ENG

Abstract: The green transition has spiked demand for high-performance sintered Nd-Fe-B permanent magnets, necessitating advanced powder consolidation technologies to enhance production efficiency. This study explores the rapid sintering methodology for an Nd-Fe-B powder using a radiation-assisted sintering approach. The case study material is an industrially used powder, prepared through strip-casting, hydrogen decrepitation, and jet milling, with a mean particle size of 5.5 µm. The powder is sintered to full density in a modified Spark Plasma Sintering furnace, achieving pressureless conditions and eliminating electrical currents in the sample to preserve grain alignment and prevent decomposition of the hard-magnetic phase. Fully-dense samples are obtained with heating rates ranging from 10 to 200 °C/min and up to 5 minutes of dwell time at 1100 °C. Rapid heating results in grain size and microstructure comparable to conventionally sintered magnets prepared from the same powder, without compromising magnetic performance after post-sinter annealing at 520 °C for 120 minutes. This sintering method contributes to a novel strategy for optimizing magnet production by utilizing efficient thermal-radiation heat transfer. The combination of rapid heating and pressureless sintering drastically reduces heat-up and dwell times, providing a fundamental advantage over slow conventional sintering.
Keywords: rapid sintering, finite-element modeling
Published in DiRROS: 28.03.2025; Views: 94; Downloads: 30
Full text (2,89 MB)

Abstract: Demagnetization is an essential step for the demounting and safe handling of end-of-life NdFeB. Thermal demagnetization in air is a straightforward option to demount adhesive-fixed or segmented magnets. However, this process is suspected to increase the uptake of contaminants like O, C and Zn from coatings and adhesives, potentially degrading the recyclate quality. This study tests the effects of thermal demagnetization in air at 400 °C for 15 to 240 min on variously coated samples with different initial oxidation levels. Furthermore, the possible reversal of the contaminant uptake is explored. Samples with low previous oxidation levels showed significant uptake in oxygen with a minimal diffusion depth, while the uptake depended on the used coating. The best protectiveness was achieved with NiCuNi with an increase in oxygen of only around 30%. Epoxy (up to ~130% O uptake) and Zn coatings (up to ~80% O uptake) disintegrated during the treatment and offered less protection but still made a difference compared to uncoated samples (up to ~220% O uptake). Samples with high initial oxidation levels show no clear tendency towards further oxygen uptake and the carbon uptake is generally low, likely due to contemporary epoxy coatings featuring a passivation underneath as a barrier layer. Zn infiltration, which carried organic debris, was observed. Short demagnetization times proved to be favorable for limiting the depth of the diffusing contaminants. Mechanical coating removal after thermal demagnetization in air can mitigate the contaminant uptake, producing clean, recyclable end-of-life material.
Keywords: magnetic scrap, thermal demagnetization, organic contamination
Published in DiRROS: 28.03.2025; Views: 128; Downloads: 44
Full text (2,76 MB)
This document has many files! More...

Keywords: visoko šolstvo, univerze, Slovenija, zgodovinski pregledi
Published in DiRROS: 28.03.2025; Views: 99; Downloads: 26
Full text (11,89 MB)

Abstract: A benchmark activity on two-fluid simulations of high-pressure boiling upward flows in a pipe is performed by 12 participants using different MCFD (Multiphase Computational Fluid Dynamics) codes and closure relationships. More than 30 conditions from DEBORA experiment conducted by CEA are considered. Each case is characterised by the flow rate, inlet temperature, wall heat flux and outlet pressure. High-pressure Freon (R12) at 14 bar and 26 bar is boiled in a 19.2 mm pipe heated over 3.5 m. Flow rates range from 2000 kg m−2 s −1 to 5000 kg m−2 s −1 and exit quality � ranges from single-phase conditions to � = 0.1 which leads to a peak void fraction of � = 70%. In these high pressure conditions, bubbles remain small and there is no departure from the bubbly flow regime (François et al., 2011; Hösler, 1968). However, different kind of bubbly flows are observed: wall-peak, intermediate peak or core-peak, depending on the case considered. Measurements along the pipe radius near the end of the heated section are compared to code predictions. They include void fraction, bubble mean diameter, vapour velocity and liquid temperature. The benchmark covered two phases. In the first phase of the benchmark activities, experimental data were given to the participants, allowing to compare the simulation results and to develop, to select or to adjust the models in the CMFD codes. The second phase included blind cases where the participants could not compare to the measurements. In between the two phases, possible additional model adjustments or calibrations were performed. Overall, the benchmark involved very different closures and a wide range of models’ complexity was covered. Yet, it is extremely difficult to have a robust closure for all conditions considered, even knowing experimental measurements. The wall-to-core peak transition is not captured consistently by the models. The degree of subcooling and the void fraction level are also difficult to assess
Published in DiRROS: 28.03.2025; Views: 77; Downloads: 42
Full text (2,59 MB)

Published in DiRROS: 28.03.2025; Views: 146; Downloads: 71
Full text (1,40 MB)
This document has many files! More...