Scientific Publications
Nano-patterning using ultra-thin alumina membranes
Claudia Fernández-González, Sandra Ruiz-Gómez, Ana Arché-Núñez, Lucas Pérez, Célia Tavares de Sousa
Abstract
With the mass production of well-controlled and low-cost nanostructures on the horizon, considerable attention has been given to porous anodic alumina (PAA) templates to assist in the fabrication of both individual and ordered nanostructured objects – particles, rods, wires, and holes – with applications in electronics, data storage, bioengineering, and nanomedicine. The fabrication of free-standing PAA templates, several microns thick, as well as their applications, have been largely described in the literature. In recent years, research has focused on the synthesis of ultra-thin anodic alumina membranes (UTAMs), making them compatible with top-down fabrication and large-scale production. The ability to obtain these nanostructures on different surfaces, including glass, silicon wafers, or flexible substrates, extends their range of applications, enabling the integration of nanostructured materials on top of thin layers and allowing for the precise tuning of the physical and chemical properties of the materials. This review focuses on this new and promising nanopatterning approach to fabricate large areas of ordered nanostructures using UTAMs as patterning masks. We report the most recent advances in the synthesis of UTAMs, focusing on two different approaches: in-situ anodization of thin aluminum films on various substrates and deterministic transfer of UTAMs onto a desired substrate. In the first case, we collect information regarding substrates, buffer layers, growth of Al films, anodization, and the post-treatment of the UTAMs. In the second case, we focus the review on the synthesis of UTAMs and, especially, on the transfer process to the substrate. For both methods, we compare the results regarding the nanostructure’s self-organization and the control of size, shape, and spacing. Finally, we will review several applications in which the use of UTAMs plays a key role in the performance of nanostructured devices.
Growth of MnWO4 nanowires on W(110) by high-temperature oxygen-assisted molecular beam epitaxy
Kalina Fornal, Clara Gutierrez-Cuesta, Adolfo del Campo, Anna Mandziak, Pawel Nita, Jose´ Emilio Prieto, Jose´ F. Marco and Juan de la Figuera
Abstract
We describe the growth of synthetic hübnerite (MnWO4) by high-temperature oxygen-assisted molecular beam epitaxy on W(110). The hübnerite nanowires have widths of hundreds of nanometers, heights of tens of nanometers and lengths in the range of millimeters. The growth was followed in real time by low-energy electron microscopy (LEEM). The nanowires were characterized in situ by low-energy electron microscopy, X-ray absorption and X-ray photoelectron spectroscopy in photoemission microscopy, as well as ex situ by atomic force microscopy, optical microscopy and Raman spectroscopy. Hübnerite can be grown on W(110) by dosing only manganese in a molecular oxygen environment, likely due to the formation of highly mobile WOx species with diffusion lengths of the order of hundreds of micrometers. These species can react with the deposited Mn and be efficiently incorporated into the wolframite structure of hübnerite. The strongly anisotropic growth observed may stem from the inherent anisotropy of the wolframite lattice. We propose that this method may be applicable to the growth of other tungstates as well.
Thermally driven rock-salt to spinel transition in high-entropy (CoFeMnNiZn)xOy oxides: Effects on magnetic and mechanical properties
Hasmik Kirakosyan, Ani Sargsyan, Yeva Grigoryan, Alex T. Sheardy, Khachatur Manukyan, Marieta Zakaryan, Harutyun Gyulasaryan, Adrian Quesada, Cecilia Granados-Miralles, Sofiya Aydinyan, Suren Kharatyan,
Abstract
High-entropy (CoFeMnNiZn)xOy oxides with rock-salt and spinel structures were synthesized by a one-step solution combustion synthesis method employing metal nitrates as oxidizers and glycine as fuel. Two distinct combustion regimes were established by adjusting the fuel-to-oxidizer ratio. Fuel-lean conditions favored the formation of a single-phase rock-salt (CoFeMnNiZn)O, whereas fuel-rich conditions led to the formation of spinel (CoFeMnNiZn)3O4 through the re-oxidation of reduced intermediate phases. Thermal analysis and spark plasma sintering (SPS) revealed a reversible transformation between the rock-salt and spinel structures driven by the redox environment, i.e. oxygen uptake and release processes. SPS enabled densification up to ~97 % of the theoretical density and promoted grain growth while preserving nanoscale crystallinity, which is important for structural reversibility. The structural evolution was tracked using X-ray diffraction, Raman spectroscopy, and electron microscopy. Entropy calculations confirmed the high-entropy character of both phases. The sample retaining partial spinel content showed higher hardness and magnetization, while the fully transformed counterpart exhibited higher coercivity. These insights offer a tunable platform for designing functional high-entropy oxides with broad technological relevance.
Recovery of Rare Earths from End-of-Life NdFeB Permanent Magnets from Wind Turbines
Lorena Alcaraz, Olga Rodríguez-Largo, Gorka Barquero-Carmona, Alba Berja, Adrián Quesada, Félix A. López
This work aims to recover rare earths from wind turbines NdFeB magnets through pyrometallurgical and hydrometallurgical techniques. First, a NdFeB hydride powder is obtained by decrepitation with hydrogen. Subsequently, this powder was subjected to a chlorination roasting process and successive leaching with water to bring the metals into solution. This was followed by a liquid-liquid extraction to remove the iron and purify the rare earth solution. For this purpose, Aliquat 336 diluted in Solvesso was selected as the iron selective extraction agent. As a single extraction was not enough for complete iron removal, a second Fe extraction step was carried out. This second extraction step was performed using the restored organic phase. This restoration was achieved by treating the organic phase with Na2SO3 and then washing it with a 3 M HCl solution. In this way, the process was achieved more sustainably. Finally, the rare earths contained in the final solution were precipitated using oxalic acid to obtain mixed rare earth oxalates.
Growth and magnetic domain imaging of barium hexaferrite thin films with a Co overlayer
Guiomar Delgado Soria, Eduardo García-Martín, Sandra Ruiz-Gómez, Clara Gutiérrez-Cuesta, José Francisco Marco, Cecilia Granados-Miralles, Eva María Trapero, Santiago Sánchez, Michael Foerster, Lucía Aballe, Juan de la Figuera, Adrián Quesada, José Emilio Prieto
BaFe12O19 (BFO) thin films have been grown on Si(100) substrates by magnetron sputtering from previously synthesized ceramic BFO targets and have been compositionally and structurally characterized. Films grow with the c-axis orientation and magnetization direction parallel to the sample plane. In addition, the magnetic coupling between the BFO film and a deposited cobalt overlayer was studied. Images of X-ray magnetic circular dichroism in photoemission microscopy show magnetic regions in the BFO layer with domain sizes of several micrometers and others without magnetic contrast, the latter attributed to the presence of hematite. Magnetic domains in the Co overlayer show no significant correlations with those in the BFO film, pointing to a negligible magnetic coupling.
Self-Propagating High-Temperature Synthesis of High-Entropy Composite in a Ti–Cr–Mn–Co–Ni–Al–C System
Alina Zurnachyan, Abraam Ginosyan, Roman Ivanov, Irina Hussainova and Sofiya Aydinyan
High-entropy materials have emerged as promising candidates for high-temperature structural, magnetic, and electrochemical applications due to their unique combination of compositional complexity, thermal stability, and tailored functionality. In this study, self-propagating high-temperature synthesis (SHS) was employed to fabricate high-entropy composite in a Ti–Cr–Mn–Co–Ni–Al–C multicomponent system with a focus on elucidating the effect of titanium content on the combustion parameters, as well as on the phase and structure formation patterns of the resulting materials. In situ profiling enables evaluating the maximum combustion temperature of 1560 °C, combustion wave propagation velocity ranging from 0.22 to 4.3 mm/s depending on titanium content, and heating and cooling rates of 300–2000 °C/s and 3 °C/s during synthesis. The synthesized powders exhibited a bimodal particle size distribution, with ~90% of particles below 25 μm and a D50 of 5.38 μm. Post-synthesis densification via spark plasma sintering (SPS) at 1250 °C under 45 MPa yielded dense bulk samples, which exhibited a high relative density and high Vickers microhardness of 1270 ± 35 HV10 attributed to fine TiC dispersion and secondary carbide formation. Thermogravimetric analysis performed under air flow with a heating rate of 20 °C/min showed enhanced thermal stability for both the powder and the sintered bulk. These findings demonstrate the efficacy of SHS for rapid, energy-efficient fabrication of high-entropy composites and underscore the critical role of composition in tailoring their structural and mechanical properties.
Defining magnetic properties required for rare earth element free permanent magnets for electric vehicles
The use of novel permanent magnets in motors is evaluated by finding the lower limits for magnetic properties, through optimization considering torque density and permanent demagnetization. An electrical machine optimization methodology is combined with a new design paradigm of accepting some degree of permanent demagnetization to achieve higher performance. Different number of poles is compared and a sensitivity study regarding allowed demagnetization is performed. Results show an expected increase in torque when the remanence is increasing. However, for low intrinsic coercivity, the torque decreases substantially as the magnet is not optimally shaped. The results demonstrate that magnets with a remanence as low as 0.6 T can be used in a competitive electric car motor with a torque density comparable to conventional motors. It can also be concluded that the knee point of the demagnetization curve should be in the third quadrant to utilize the magnets properly. In addition, from a demagnetization point of view, a high remanence can be negative for a magnet with low intrinsic coercivity. Experimental results from an electric vehicle motor with commercially available ferrite magnets are presented, both to demonstrate the use of ferrites in electric motors and to experimentally verify the finite element method simulations.
A simple and industrially scalable process for recycling hexaferrite ceramic magnets
The demand for hexaferrite (BaFe12O19/SrFe12O19) permanent magnets is expected to rise in the next 5–10 years due to their potential as a sustainable alternative to rare-earth magnets. Currently, less than 1 % of recycling of permanent magnets occurs worldwide. This study presents a successful method for recycling strontium ferrite magnets from end-of-life household appliances, fabricating recycled bonded magnets as a first step to implement a circular economy in the value chain. This industrially scalable method optimizes comminution and annealing of recovered ceramic magnets, yielding powders with particle sizes below 2 µm. Thermal treatment at 900–1000 °C recovers competitive magnetic properties. A pilot batch of recycled hexaferrite bonded magnets, produced via injection moulding, exhibited excellent mechanical and magnetic properties, with coercivity Hcj of 190.1 kA/m, remanent polarization Jr of 234.3 mT, and maximum energy product (BH)max of 10.4 kJ/m³, comparable to commercial ferrite bonded magnets.
Efficient recycling of End-of-Life ceramic ferrite hard magnets into new permanent ceramic magnets
Within current sustainable and circular economy approaches, recycling hexaferrite magnets has emerged as a strategic alternative to rare-earth-based magnets. This study proposes an efficient recycling and fabrication protocol for sintered ferrite magnets by merging annealing and sintering into a single step, reducing energy consumption and emissions. Rietveld analysis revealed temperature-dependent variations in ferrite and hematite fractions, with reduced hematite content at a sintering temperature of 1250°C. Magnetic characterization indicated decreasing coercive field, Hc, and remanent magnetization, Mr, with increasing sintering temperature, while a density of 93 % was achieved at 1250°C for 30 min. X-ray diffraction, FE-SEM, and particle size analysis confirmed that the reduced magnetic properties of recycled magnets, compared to commercial counterparts, are due to hematite formation and sintering-induced grain growth. These findings highlight the importance of thermal optimization to balance densification and magnetic performance. This work demonstrates the first successful fabrication of fully recycled strontium ferrite magnets from End-of-Life materials.