1,721,521 research outputs found
Green and low temperature synthesis of nanocrystalline transition metal ferrites by simple wet chemistry routes
No full textCrystalline and nanostructured cobalt (CoFe2O4), nickel (NiFe2O4), zinc (ZnFe2O4) and manganese (MnFe2O4) spinel ferrites are synthesized with high yields, crystallinity and purity through an easy, quick, reproducible and low-temperature hydrothermal assisted route starting from an aqueous suspension of coprecipitated metal oxalates. The use of water as a reaction medium is a further advantage of the chosen protocol. Additionally, the zinc spinel is also prepared through an alternative route combining coprecipitation of oxalates from an aqueous solution with thermal decomposition under reflux conditions. The nanocrystalline powders are obtained as a pure crystalline phase already at the extremely low temperature of 75 °C and no further thermal treatment is needed. The structure and microstructure of the prepared materials is investigated by means of X-ray powder diffraction (XRPD), while X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analyses are used to gain information about the surface and bulk composition of the samples, respectively, confirming the expected stoichiometry. To investigate the effect of the synthesis protocol on the morphology of the obtained ferrites, transmission electron microscopy (TEM) observations are performed on selected samples. The magnetic properties of the cobalt and manganese spinels are also investigated using a superconducting quantum device magnetometer (SQUID) revealing hard and soft ferrimagnetic behavior, respectively
Rare Earth Magnets Recycling: Study of a Safer and Cost-Effective Technique
No full textRare Earth magnets are currently adopted in different industrial sectors, from household utilities to automotive applications, informatics to sensors, etc. Rare Earth magnets show the best magnetic performance, predominantly in the case of Neodymium magnets, and help in reducing the weight/volume ratio. On the other hand, the instability of the raw material market should be considered together with the location of Rare Earth materials in a few areas of the planet, with an expensive extraction process. For these reasons, recycling Neodymium-Iron-Boron magnets is considered a necessary and promising solution. Different techniques are available, but they are generally expensive or rather dangerous. Some methods to recover NdFeB powder from various devices are based on Hydrogen, such as Hydrogen decrepitation or hydrometallurgical methods, and alternatively chemical processes. This work proposes a new method based on a mechanical technique without using Hydrogen, resulting in a safer, less complicated and cheaper process. Based on the vacuum impact mill, the mechanical process of grinding the magnets recovered from hard disk drives has been tested. The obtained powder shows NdFeB magnetic characteristics and can be used again to produce permanent magnets
Chemical state of arsenic and copper in enargite: evidences from EPR and X-ray absorption spectroscopies, and SQUID magnetometry
No full textThe chemical state of elements in enargite, Cu(3)AsS(4), has been traditionally interpreted assigning the monovalent (+1) state to copper, pentavalent (+5) to arsenic, and divalent (-2) to sulphur. However, there are some evidences that make this interpretation not fully satisfying. These uncertainties reflect on the understanding of the oxidation process of enargite, that has important implications for both environmental issues and mineral processing. To clarify the issue, our study involves the combined use of X-ray absorption (XAS) and electron paramagnetic resonance (EPR) spectroscopies, and SQUID magnetometry. XANES spectra at the Cu K-edge for enargite and other copper sulphides show the expected affinities between the spectra of enargite and chalcopyrite (although the pre-edge feature typical of chalcopyrite is absent in enargite), reflecting the structural similarities of these two minerals; the spectra are, on the other hand, fairly similar to tetrahedrite, whose structure is different. The spectral shapes of bornite, chalcocite, and covellite are distinctly different from enargite. The edge position of enargite is consistent with the presence of monovalent copper, in spite of some small differences with respect to other sulphides where Cu is considered monovalent. At the As K-edge, the most striking feature is the clear difference in the edge position of enargite with respect to As(V) compounds such as calcium arsenate, and the similarity with As(III) compounds such as sodium arsenite. On the other hand, the edge position for enargite occurs at higher energies than those of tennantite, where As is regarded as trivalent, and of realgar (As(4)S(4)), thus supporting the view that in enargite arsenic is in a higher valence state. Enargite does not show a recognizable EPR spectrum in the X band region; however, the mineral shows a rather strong bulk magnetisation, equivalent (in the hypothesis of divalent Cu) to 0.05 unpaired electron per formula unit. The most likely explanation of this behaviour is the occurrence of small amounts of clustered paramagnetic impurities (e. g., divalent Fe). In summary, to the extent that nominal valence states can describe complex chemical bonding such as in sulphides, the results of this study rule out the occurrence of a significant fraction of Cu(II) in enargite, and support the "traditional" assignment of a monovalent state to Cu, and pentavalent to As
X-ray magnetic circular dichroism investigation of the superparamagnetic transition-metal ion-cluster r-Mn12Bz
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Magnetic properties and cation ordering in two synthetic bornite samples, Cu5(Fe, Mn)S4
No full textThe magnetic properties of a synthetic bornite, Cu5FeS 4, and of a Mn-bearing synthetic bornite were investigated using Superconducting Quantum Interference Device (SQUID) magnetometry and room temperature X-band Electron Paramagnetic Resonance (EPR) spectroscopy. Samples were synthesised from the elements using conventional dry methods. Chemical and phase compositions of the samples were confirmed by means of Electron Probe Micro-Analysis (EPMA) and by powder X-Ray Diffraction (XRD). Experimental results, interpreted using spectral simulations, highlight the superexchange nature of ionic interactions between paramagnetic centres and the role played by partial intervalence charge transfer between the Fe(III) and Cu(I) species in natural and synthetic bornite. The cation distribution in synthetic samples was shown to be different from the natural sample, in spite of the close similarity in chemical composition and structure
Addressing the Influence of Localized Plasmon Resonance on the Magneto-Optical Properties of Cobalt Ferrite Nanoparticles
No full textThe optical and magneto-optical (MO) properties of magneto-plasmonic nanocomposite films made up of a transparent polymer with a dispersion of cobalt ferrite (CFO) nanoparticles (NPs) and different concentrations of Au NPs are investigated. The volumetric concentrations of CFO and Au NPs, around 3%, and below 7‰ respectively, are below the percolation limit, and hence the nanocom-posite films constitute models for investigating the influence of the electromagnetic field generated at the surface plasmon resonance of Au NPs on the magneto-optical properties of CFO NPs. The plasmon resonance is present in these magneto-plasmonic composites, red-shifted with respect to the bare Au NPs and covering the spectral region where charge-transfer and crystal field MO transitions can be excited. Moreover, the magneto-optical hysteresis loops were measured in the whole spectral region. We observe that the hysteresis loops shape is a fingerprint of the different MO transitions of the CFO NPs. The strength of the MO peak around 750 nm, corresponding to the Crystal Field transition is damped respect to the corresponding peak of the CFO NPs. The strength of this peak evolves non-monotonically with the Au NPs concentration. On the other hand, the MO band around 550 nm, excited by Charge Transfer transitions, changes sign when Au NPs are present. In addition, a second MO contribution is observed. Our results demonstrate that the interactions between plasmon resonance and MO effects are not only determined by the stronger local electromagnetic fields at the resonance but they depend on the type MO transition that is involved in these oxides. This study helps to understand and design the magneto plasmonic nano-structures and applications, for example in biomedicine and sensing, in which random and weak dipolar interparticle interactions between plasmonic and magnetic nanostructures are present
The Implementation Of A Novel Approach To The Rare Earth Magnets Recycling
No full textRare Earth magnets have been used in different industrial sectors: household utilities, automotive applications, informatics sensors, etc. Rare Earth magnets show the best magnetic performance, predominantly in the case of Neodymium magnets. However, the economic aspect concerning the raw magnetic materials affects many of the magnet devices, mainly considering the instability of the raw material market. For these reasons, recycling NdFeB magnets is considered a promising solution. Different techniques are available, but they are generally expensive or very dangerous. This work proposes a new approach to recycling the NdFeB sintered magnets using a particular mechanical technique without using Hydrogen, resulting in a safer, less complicated and cheaper process than chemical methods. Based on an impact mill, the process has been performed to grind the magnets recovered from the hard disks. The operation was conducted in a vacuum. Finally, some bonded magnets with recycled powder have been prepared and characterized
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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