Checa-Fernández, B.L. (Blanca Luna)

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    Effect of temperature on particle shape, size, and polycrystallinity of Nd-Fe-B powders obtained by hydrogen decrepitation
    (2023) Sarriegui-Estupiñan, G.C. (Gabriela Carolina); Checa-Fernández, B.L. (Blanca Luna); Burgos, N. (Nerea); Martín, J.M. (José Manuel)
    This work presents a detailed study of hydrogen decrepitation (HD) to obtain monocrystalline Nd-Fe-B powder. The effect of decrepitation temperature has been investigated to optimize both particle size and shape. Differential scanning calorimetry was applied to analyze the hydrogenation kinetics of Nd2Fe14B and Nd-rich phases in the range of 25 to 300 C. Thermogravimetry and X-ray diffraction allowed determining the hydrogen absorption of the whole alloy and the matrix phase, respectively. While scanning electron microscopy (SEM) was used to visualize particle shape and size, dynamic image analysis was applied to evaluate quantitatively these properties. The high monocrystallinity of the powder was confirmed by electron backscattering diffraction. The partial pressure of hydrogen required to initiate the hydrogenation reactions decreases when the temperature increases. The hydrogen absorbed by the whole alloy and, in particular, by the Nd2Fe14B phase decreases with temperature. Below 150 C, the hydrogen absorbed by the Nd2Fe14B phase produces a significant transgranular cracking that is undesirable for particle shape. At 300 C, the fast and limited absorption of hydrogen by the Nd-rich phase causes insufficient intergranular fracture and, hence, polycrystallinity. Between 150 and 300 C, the controlled fragmentation resulted in monocrystalline particles with a more equiaxial shape, which is a suitable precursor to develop anisotropic ultrafine powders by the hydrogenation, disproportionation, desorption, recombination (HDDR) process.
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    Study and optimization of a recycling process for sintered Nd-Fe-B magnets at the end of their useful life.
    (Servicio de Publicaciones. Universidad de Navarra., 2023-10) Checa-Fernández, B.L. (Blanca Luna); Martín-García, J.M. (José Manuel); Burgos-García, N. (Nerea)
    Nowadays the scarcity of rare earths is a problem of significant importance due to the indispensable role of Nd-Fe-B permanent magnets in numerous technological applications. The international reliance on rare earth elements and the costly manufacturing processes involved in producing these magnets pose today both environmental and economic challenges. As the demand for rare earth permanent magnets grows, concerns about resource scarcity, energy consumption, and waste accumulation become more pressing. Developing new recycling routes for Nd-Fe-B permanent magnets is crucial to address these challenges. By establishing efficient methods to recover and reuse rare earth containing elements from discarded or obsolete devices could significantly reduce the need for mining new resources, helping to mitigate the environmental impact associated with mining and processing. For that reason, the problem of permanent magnets underscores the necessity of finding alternative and sustainable ways to produce, utilize, and recover these crucial materials. The development of novel recycling routes is a key step towards addressing the challenges posed by resource scarcity, environmental concerns, and economic fluctuations, contributing to a more sustainable future, and creating new business opportunities in the green technology sector. This PhD thesis aims to study new recycling routes for rare earth magnets, based on the use of gas atomization technique to melt scrap magnets and produce Nd-Fe-B recycled powders. This powder can be used to manufacture a new magnet. Gas atomization is a highly interesting industrial production technique with numerous advantages, including high productivity and lower cost compared to other production techniques. The limitation of the gas-atomized powders lies in their low magnetic properties. For that reason, with the aim to enhance their magnetic properties, different alternatives of heat treatments and reactions in hydrogen atmosphere such as hydrogen decrepitation (HD) and the hydrogenation-disproportionation-desorption-recombination (HDDR) process will be analyzed. Moreover, the effect of some commonly used additives in Nd- Fe-B permanent magnets, such as Nb and Ga will be analyzed. The ultimate motivation is to enhance their magnetic properties, transforming them into a useful material to produce both isotropic and anisotropic bonded magnets.