Ab-Initio Study on the Hard Magnetic Properties of MnBi

We have studied the hard magnetic properties of the low-temperature phase of MnBi with first principle calculations based on the density functional theory. The calculations have been carried out on two distinct unit cell configurations MnBi and BiMn with the element in the unit cell origin named first. Our results show that these configurations are not equivalent and that MnBi describes the system better near T = 0K and the BiMn configuration describes the system better for T > 300K. The magnetic moments of both configurations agree well with experimental measurements considering both spin and orbital contributions. At high temperatures the magneto-crystalline anisotropy energy increases with increasing unit cell volume and reaches a maximum of 2:3MJ=m3 and a c=a ratio of 1:375.Comment: Presented at 20th International Conference on Magnetism (ICM2015) in Barcelona, accepted for publication. 5 pages, 2 figures, 4 table

Synthesis and Structure of the Double-Layered Sillén-Aurivillius Perovskite Oxychloride La2.1Bi2.9Ti2O11Cl as a Potential Photocatalyst for Stable Visible Light Solar Water Splitting.

Exploring photocatalysts for solar water splitting is a relevant step toward sustainable hydrogen production. Sillén-Aurivillius-type compounds have proven to be a promising material class for photocatalytic and photoelectrochemical water splitting with the advantage of visible light activity coupled to enhanced stability because of their unique electronic structure. Especially, double- and multilayered Sillén-Aurivillius compounds [An-1BnO3n+1][Bi2O2]2Xm, with A and B being cations and X a halogen anion, offer a great variety in material composition and properties. Yet, research in this field is limited to only a few compounds, all of them containing mainly Ta5+ or Nb5+ as cations. This work takes advantage of the outstanding properties of Ti4+ demonstrated in the context of photocatalytic water splitting. A fully titanium-based oxychloride, La2.1Bi2.9Ti2O11Cl, with a double-layered Sillén-Aurivillius intergrowth structure is fabricated via a facile one-step solid-state synthesis. A detailed crystal structure analysis is performed via powder X-ray diffraction and correlated to density functional theory calculations, providing a detailed understanding of the site occupancies in the unit cell. The chemical composition and the morphology are studied using scanning and transmission electron microscopy together with energy-dispersive X-ray analysis. The capability of the compound to absorb visible light is demonstrated by UV-vis spectroscopy and analyzed by electronic structure calculations. The activity toward the hydrogen and the oxygen evolution reaction is evaluated by measuring anodic and cathodic photocurrent densities, oxygen evolution rates, and incident-current-to-photon efficiencies. Thanks to the incorporation of Ti4+, this Sillén-Aurivillius-type compound enables best-in-class photoelectrochemical water splitting performance at the oxygen evolution side under visible light irradiation. Thus, this work highlights the potential of Ti-containing Sillén-Aurivillius-type compounds as stable photocatalysts for visible light-driven solar water splitting

A Combined TEM/STEM and Micromagnetic Study of the Anisotropic Nature of Grain Boundaries and Coercivity in Nd-Fe-B Magnets

The nanoanalytical high resolution TEM/STEM investigation of the intergranular grain boundary phase of anisotropic sintered and rapidly quenched heavy rare earth-free Nd-Fe-B magnet materials revealed a difference in composition for grain boundaries parallel (large Fe-content) and perpendicular (low Fe content) to the alignment direction. This behaviour vanishes in magnets with a high degree of misorientation. The numerical finite element micromagnetic simulations are based on the anisotropic compositional behaviour of GBs and show a decrease of the coercive field with an increasing thickness of the grain boundary layer. The magnetization reversal and expansion of reversed magnetic domains primarily start as Bloch domain wall at grain boundaries parallel to the c-axis and secondly as Néel domain wall perpendicular to the c-axis into the adjacent hard magnetic grains. The increasing misalignment of grains leads to the loss of the anisotropic compositional behaviour and therefore to an averaged value of the grain boundary composition. In this case the simulations show an increase of the coercive field compared to the anisotropic magnet. The calculated coercive field values of the investigated magnet samples are in the order of μ0HcJ=1.8 T–2.1 T for a mean grain boundary thickness of 4 nm, which agrees perfectly with the experimental data

Reversibly compressible and freestanding monolithic carbon spherogels

We present a versatile strategy to tailor the nanostructure of monolithic carbon aerogels. By use of an aqueous colloidal solution of polystyrene in the sol-gel processing of resorcinol-formaldehyde gels, we can prepare, after supercritical drying and successive carbonization, freestanding monolithic carbon aerogels, solely composed of interconnected and uniformly sized hollow spheres, which we name carbon spherogels. Each sphere is enclosed by a microporous carbon wall whose thickness can be adjusted by the polystyrene concentration, which affects the pore texture as well as the mechanical properties of the aerogel monolith. In this study, we used monodisperse polystyrene spheres of approximately 250 nm diameter, which result in an inner diameter of the final hollow carbon spheres of approximately 200 ± 5 nm due to shrinkage during the carbonization process. The excellent homogeneity of the samples, as well as uniform sphere geometries, are confirmed by small- and angle X-ray scattering. The presence of macropores between the hollow spheres creates a monolithic network with the benefit of being reversibly compressible up to 10% linear strain without destruction. Electrochemical tests demonstrate the applicability of ground and CO2 activated carbon spherogels as electrode materials

Minimal information for studies of extracellular vesicles 2018 (MISEV2018):a position statement of the International Society for Extracellular Vesicles and update of the MISEV2014 guidelines

The last decade has seen a sharp increase in the number of scientific publications describing physiological and pathological functions of extracellular vesicles (EVs), a collective term covering various subtypes of cell-released, membranous structures, called exosomes, microvesicles, microparticles, ectosomes, oncosomes, apoptotic bodies, and many other names. However, specific issues arise when working with these entities, whose size and amount often make them difficult to obtain as relatively pure preparations, and to characterize properly. The International Society for Extracellular Vesicles (ISEV) proposed Minimal Information for Studies of Extracellular Vesicles (“MISEV”) guidelines for the field in 2014. We now update these “MISEV2014” guidelines based on evolution of the collective knowledge in the last four years. An important point to consider is that ascribing a specific function to EVs in general, or to subtypes of EVs, requires reporting of specific information beyond mere description of function in a crude, potentially contaminated, and heterogeneous preparation. For example, claims that exosomes are endowed with exquisite and specific activities remain difficult to support experimentally, given our still limited knowledge of their specific molecular machineries of biogenesis and release, as compared with other biophysically similar EVs. The MISEV2018 guidelines include tables and outlines of suggested protocols and steps to follow to document specific EV-associated functional activities. Finally, a checklist is provided with summaries of key points

Nanocompositional Electron Microscopic Analysis and Role of Grain Boundary Phase of Isotropically Oriented Nd-Fe-B Magnets

Nanoanalytical TEM characterization in combination with finite element micromagnetic modelling clarifies the impact of the grain misalignment and grain boundary nanocomposition on the coercive field and gives guidelines how to improve coercivity in Nd-Fe-B based magnets. The nanoprobe electron energy loss spectroscopy measurements obtained an asymmetric composition profile of the Fe-content across the grain boundary phase in isotropically oriented melt-spun magnets and showed an enrichment of iron up to 60 at% in the Nd-containing grain boundaries close to Nd2Fe14B grain surfaces parallel to the c-axis and a reduced iron content up to 35% close to grain surfaces perpendicular to the c-axis. The numerical micromagnetic simulations on isotropically oriented magnets using realistic model structures from the TEM results reveal a complex magnetization reversal starting at the grain boundary phase and show that the coercive field increases compared to directly coupled grains with no grain boundary phase independently of the grain boundary thickness. This behaviour is contrary to the one in aligned anisotropic magnets, where the coercive field decreases compared to directly coupled grains with an increasing grain boundary thickness, if Js value is > 0.2 T, and the magnetization reversal and expansion of reversed magnetic domains primarily start as Bloch domain wall at grain boundaries at the prismatic planes parallel to the c-axis and secondly as Néel domain wall at the basal planes perpendicular to the c-axis. In summary our study shows an increase of coercive field in isotropically oriented Nd-Fe-B magnets for GB layer thickness > 5 nm and an average Js value of the GB layer < 0.8 T compared to the magnet with perfectly aligned grains

The role of Yb doped ZnO in the charge transfer process and stabilization

This work aims to investigate the role of Yb dopant in the ZnO photoactivity. A new material based on ZnO doped with a small amount of Yb (1% molar) has been synthesized, characterized and tested respect to an enhancement of the photoactivity via Electron Paramagnetic Resonance spectroscopy. The prepared material has been characterized via X Ray diffraction, UV visible spectroscopy and in particular with the transmission electron microscopy. The morphological analysis with high resolution TEM allowed us to identify the formation of a new species not visible with XRD and assigned to isolated Yb2O3 nanoparticles supported on the surface of larger particles of the more abundant zinc oxide phase. The as obtained material exhibits a band gap corresponding to UV light (≈3.3 eV), but nevertheless it shows a relevant photo-activity under irradiation with photons with λ > 420 nm (visible light). This phenomenon has been investigated by means of several methods. A specific EPR-based approach allowed to monitor and quantify the charge separation following the formation of holes in the valence band (VB) of the two materials

Minerals / Low Temperature Synthesis of Aegirine NaFeSi2O6: Spectroscopy (57Fe Mössbauer, Raman) and Size/Strain Analysis from X-ray Powder Diffraction

Using a low temperature synthesis protocol, it was possible to obtain phase-pure synthetic aegirine (NaFeSi2O6) at temperatures as low as 130 C, albeit only with rather long synthesis times of 200 h; at 155 C, a nano-crystallite shaped phase-pure material is formed after 24 h. These are, to the best of our knowledge, the lowest temperatures reported so far for phase-pure aegirine synthesis. Powder X-ray diffraction (PXRD) was used to characterize phase purity, structural state and microstructural properties (size and strain) of the as-synthesized (130230 C) and heat treated (300900 C) samples, via Rietveld analysis of powder patterns. Melting was observed at 999 C. With increasing synthesis temperature, crystallite size linearly increased from 10 nm to 30 nm at 230 C, while unit cell parameters decreased. The microstrain was very small. Additional heat treatment of as synthesized samples showed that the crystallite size remained rather unaffected up to 700 C. The lattice parameters, however, already changed at low temperatures and successively became smaller, indicating increasing ordering towards more regular arrangements of building units. This was confirmed by 57Fe Mössbauer spectroscopy, where a distinct decrease of the quadrupole splitting with increasing synthesis temperature was found. Finally, Raman spectroscopy showed that some weakly-developed pre-ordering effects were present in the samples, which appeared to be amorphous in PXRD, while well-resolved spectra appeared as soon as the long-range ordered crystalline state could be found with X-ray diffraction.(VLID)439456

Always cubes: A comparative evaluation of gas phase synthesis methods and precursor selection for the production of MgO nanoparticles

MgO nanocubes are well-suited for the study of defects and interfaces inside metal oxide powders and ceramics. We evaluated the synthesis of MgO nanocubes via flame spray pyrolysis (FSP) and compared the structural and spectroscopic particle properties with materials obtained by chemical vapor synthesis (CVS). Characterization of the as-synthesized and vacuum annealed powder samples revealed slightly larger particle sizes for the FSP material that is absolutely comparable to CVS powders in terms of the particles structural definition, morphology and crystallinity. Moreover, optical absorption and photoluminescence emission studies show fingerprints, which are characteristic of MgO nanocrystals with a high abundance of grain corners, kinks and edges. The opportunity to synthesize MgO nanoparticles of high structural definition via FSP at significantly larger production rates and lower costs compared to CVS carries great potential to upscale powder production to generate MgO based functional oxide nanocomposites for ceramics and catalysis