Approaching Ferrite-Based Exchange-Coupled Nanocomposites as Permanent Magnets

During the past decade, CoFe2O4 (hard)/Co-Fe alloy (soft) magnetic nanocomposites have been routinely prepared by partial reduction of CoFe2O4 nanoparticles. Monoxide (i.e., FeO or CoO) has often been detected as a byproduct of the reduction, although it remains unclear whether the formation of this phase occurs during the reduction itself or at a later stage. Here, a novel reaction cell was designed to monitor the reduction in situ using synchrotron powder X-ray diffraction (PXRD). Sequential Rietveld refinements of the in situ data yielded time-resolved information on the sample composition and confirmed that the monoxide is generated as an intermediate phase. The macroscopic magnetic properties of samples at different reduction stages were measured by means of vibrating sample magnetometry (VSM), revealing a magnetic softening with increasing soft phase content, which was too pronounced to be exclusively explained by the introduction of soft material in the system. The elemental compositions of the constituent phases were obtained from joint Rietveld refinements of ex situ high-resolution PXRD and neutron powder diffraction (NPD) data. It was found that the alloy has a tendency to emerge in a Co-rich form, inducing a Co deficiency on the remaining spinel phase, which can explain the early softening of the magnetic material

Catalytic reactor for operando spatially resolved structure–activity profiling using high-energy X-ray diffraction

In heterogeneous catalysis, operando measurements probe catalysts in their active state and are essential for revealing complex catalyst structure–activity relationships. The development of appropriate operando sample environments for spatially resolved studies has come strongly into focus in recent years, particularly when coupled to the powerful and multimodal characterization tools available at synchrotron light sources. However, most catalysis studies at synchrotron facilities only measure structural information about the catalyst in a spatially resolved manner, whereas gas analysis is restricted to the reactor outlet. Here, a fully automated and integrated catalytic profile reactor setup is shown for the combined measurement of temperature, gas composition and high-energy X-ray diffraction (XRD) profiles, using the oxidative de­hydrogenation of C$_{2}$H$_{6}$ to C$_{2}$H$_{4}$ over MoO$_{3}$/γ-Al$_{2}$O$_{3}$ as a test system. The profile reactor methodology was previously developed for X-ray absorption spectroscopy and is here extended for operando XRD. The profile reactor is a versatile and accessible research tool for combined spatially resolved structure–activity profiling, enabling the use of multiple synchrotron-based characterization methods to promote a knowledge-based optimization of a wide range of catalytic systems in a time- and resource-efficient wa

The chemistry of ZnWO₄ nanoparticle formation

The need for a change away from classical nucleation and growth models for the description of nanoparticle formation is highlighted. By the use of in situ total X-ray scattering experiments the transformation of an aqueous polyoxometalate precursor mixture to crystalline ZnWO$_{4}$ nanoparticles under hydrothermal conditions was followed. The precursor solution is shown to consist of specific Tourné-type sandwich complexes. The formation of pristine ZnWO$_{4}$ within seconds is understood on the basis of local restructuring and three-dimensional reordering preceding the emergence of long range order in ZnWO$_{4}$ nanoparticles. An observed temperature dependent trend in defect concentration can be rationalized based on the proposed formation mechanism. Following nucleation the individual crystallites were found to grow into prolate morphology with elongation along the unit cell c-direction. Extensive electron microscopy characterization provided evidence for particle growth by oriented attachment; a notion supported by sudden particle size increases observed in the in situ total scattering experiments. A simple continuous hydrothermal flow method was devised to synthesize highly crystalline monoclinic zinc tungstate (ZnWO$_{4}$) nanoparticles in large scale in less than one minute. The present results highlight the profound influence of structural similarities in local structure between reactants and final materials in determining the specific nucleation of nanostructures and thus explains the potential success of a given synthesis procedure in producing nanocrystals. It demonstrates the need for abolishing outdated nucleation models, which ignore subtle yet highly important system dependent differences in the chemistry of the forming nanocrystals

Projetos de Aprendizagem Mediados por Ambientes Virtuais no Ensino de EngenhariaEl trica

As experiências relacionadas à metodologia de projetos de aprendizagem têm sua origem nas formulações de John Dewey no início do século XX. A concepção de projeto de aprendizagem defendida nesta pesquisa alia às formulações de Dewey as propostas da professora Lea Fagundes. Esta metodologia tem o aluno como responsável pela sua aprendizagem e reflete os conceitos construtivistas de Piaget. A integração e a construção de conhecimento via projetos é facilitada com o uso dos ambientes virtuais para apoio à aprendizagem que são softwares que permitem interações síncronas ou assíncronas além de possibilitar o registro de todo o caminho percorrido pelo estudante e de todas as atividades de uma disciplina. A partir de dados coletados, no período de 2003 a 2007, em atividades escolares de disciplinas do terceiro período letivo do curso de Engenharia Elétrica da Universidade Federal do Espírito Santo pretende-se analisar (a) em que medida os ambientes virtuais de aprendizagem e a apropriação das tecnologias de informação e comunicação acrescentam na formação do estudante de engenharia, em relação às práticas pedagógicas presenciais e (b) em que medida os projetos de aprendizagem contribuem para o enfrentamento dos problemas que temos, hoje, no ensino de engenharia elétric

Weak-signal extraction enabled by deep-neural-network denoising of diffraction data

Removal or cancellation of noise has wide-spread applications for imaging and acoustics. In every-day-life applications, denoising may even include generative aspects which are unfaithful to the ground truth. For scientific applications, however, denoising must reproduce the ground truth accurately. Here, we show how data can be denoised via a deep convolutional neural network such that weak signals appear with quantitative accuracy. In particular, we study X-ray diffraction on crystalline materials. We demonstrate that weak signals stemming from charge ordering, insignificant in the noisy data, become visible and accurate in the denoised data. This success is enabled by supervised training of a deep neural network with pairs of measured low- and high-noise data. This way, the neural network learns about the statistical properties of the noise. We demonstrate that using artificial noise (such as Poisson and Gaussian) does not yield such quantitatively accurate results. Our approach thus illustrates a practical strategy for noise filtering that can be applied to challenging acquisition problems.Comment: 8 pages, 4 figure

Impact of sulfur addition on the structure and dynamics of Ni-Nb alloy melts

We investigated the change in the structure and dynamics of a Ni–Nb bulk metallic glass upon sulfur addition on both microscopic and macroscopic scales. With the sulfur concentration of 3 at. %, where the composition Ni58Nb39S3 exhibits the best glass forming ability in the investigated sulfur concentration range, both the equilibrium and undercooled melt dynamics remain almost unchanged. Only in the glassy state does sulfur seem to result in mass transport less decoupled to the viscosity of the undercooled liquid, where the measured Ag tracer diffusion coefficient is slower in the ternary alloy. With the structural disorder introduced by the alloying sulfur, the improved glass forming ability is attributed to geometrical frustration, where crystal nucleation requires a depletion of sulfur and hence long range diffusion, as long as no primary sulfur-containing crystalline phase is involved

Analysis reveals new crystal structure in "gum metal"

A phase transition observed for the first time in a titanium alloy could pave the way for new structural materialsScientists from the Max-Planck-Institut für Eisenforschung (MPIE) in Düsseldorf have observed a new phase transformation in a titanium alloy at DESY. The mechanism they discovered could further our understanding of some surprising properties of certain alloys and be used to develop new materials. The team around main author Jian Zhang of MPIE presents its findings in the journal Nature Communications.The scientists used DESY’s X-ray source PETRA III to examine the inner structure of a special alloy consisting of the (transition) metals titanium, niobium, tantalum and zirconium. This titanium alloy displays some unusual mechanical properties which have earned it the name “gum metal”. When mechanical stress is applied to the alloy, it behaves in a very interesting way: “On being deformed, it does not become harder or brittle, the way metals usually do, but instead it bends, almost like honey. In scientific terms, it has a very low elastic stiffness and very high ductility,” explains Dierk Raabe, director at MPIE, who co-authored the paper.This makes the alloy extremely attractive for various industrial applications. In the aerospace industry, for example, it can be used as a kind of crash absorber. “When an aircraft’s turbine is damaged by hail or a bird strike, there is a risk that individual parts may shatter and damage the fuselage too. If parts of the protective casing around a turbine were made of this type of ‘gum metal’, they could capture the flying debris because the impact would not destroy but only deform them,” says Raabe.It is not yet quite clear why this alloy can be deformed to such a high degree. Various experiments have revealed peculiarities in its nanostructure, but have not yet shown a direct connection with its properties. Titanium alloys normally occur in two different phases, whereby the term phase refers to the crystal structure in which the atoms are arranged. At room temperature, the atoms are usually found in the so-called alpha phase, at high temperatures they switch to the beta phase. The metals display different properties, depending on which phase they occur in. Gum metals primarily consist of the beta phase, which is stable at room temperature in the case of these alloys.The researchers at MPIE have now discovered a new mechanism during the phase transformation. The team of Jian Zhang has observed a new structure, which forms when the beta phase is transformed into the alpha phase: the omega phase. At DESY’s X-ray source PETRA III, the scientists were able to examine the crystal structure of the alloy in great detail during the transition. “When you shine X-rays onto a sample, the radiation is reflected by the crystal lattice. This produces a distinct pattern of reflections, a so-called diffractogram, from which we are able to deduce the relative positions of the atoms, in other words the crystal structure that they adopt,” explains DESY co-author Ann-Christin Dippel, who supervised the experiments technically and scientifically at the measuring station P02.1.If the beta phase is cooled down rapidly from a high temperature, some of the atoms change position to adopt the energetically more favourable arrangement of the alpha phase. The movements of these atoms lead to mechanical stress along the phase boundary, almost as if the different phases were tugging on each other. When this stress exceeds a critical value, a new arrangement is adopted, the so-called omega phase.“This newly discovered structure only arises when sheer stress is generated at the phase boundary, and it facilitates the transformation of the alpha into the beta phase. It can only exist between two other phases because it is stabilised by them,” reports Raabe. When the stress drops below the critical value because of the new layer, a new alpha phase layer is formed bordering on an omega phase. This results in a microstructure consisting of lots of layers, some of them on an atomic scale, each having a different structure. This transition also occurs when static forces are applied and is completely reversible. The scientists are now hoping that the newly discovered structure will help them to better understand the properties of this material and later to develop new, improved varieties of titanium alloys.Xi'an Jiaotong University in China and the Massachusetts Institute of Technology in the USA were also involved in the research

New mechanism reveals the secrets of a Ti alloy transformation

A new phase transformation mechanism has been described for a variation of the gum metal, an oxygen-free, β-titanium (Ti) alloy, with niobium (Nb), tantalum (Ta), and zirconium (Zr) as alloying elements. The scientists behind the discovery believe that their findings will serve as a guide for future developments of new and improved varieties of Ti-alloys. Their work is published in Nature Communications.Pure titanium undergoes a crystallographic transformation at 882°C, from the alpha (α) phase, where the atoms are in a hexagonal lattice structure (hcp), to the beta (β) phase, which exhibits a body centered cubic crystalline structure (bcc). Depending on the mixture of chemical elements that are used for the alloying, this transformation temperature can be altered significantly. Gum metals, in which the β-phase is stabilized at room temperature, are a very special class of β-titanium alloys, with very low elastic stiffness and nearly hardening-free plasticity. Contrary to other metals, gum metals do not become harder or brittle when deformed, but easily change their shape and “bend almost like honey” as Dirk Raabe, director at the Max-Planck-Institut for Iron Research (MPIE) and co-author of the article, describes. This exceptional mechanical behavior makes them very important for biomedical and aerospace applications.Understanding the underlying transformation mechanisms in Ti-alloys is considered crucial for designing gum metals or gum-metal-like materials for desired microstructures and mechanical properties.Experts from MPIE and DESY Research Center in Germany, the State Key Laboratory for Mechanical Behavior of Materials in China, and the Massachusetts Institute of Technology, led by Jian Zhang of MPIE (currently at the State Key Laboratory), have unravelled the mechanism behind a new transformation phenomenon that may explain why and how the gum metal can be deformed to such a high degree. The phase transformation appears upon fast cooling (quenching) the Ti-23Nb-0.7Ta-2Zr at.% alloy from the β-phase region.The transformation unfolds in four structural steps and is characterized as martensitic since an αꞌꞌ martensite phase with orthorhombic crystal structure is involved. The detailed study of the individual steps also uncovered a new structure confined and stabilized in the interface of the adjacent αꞌꞌ and β phases, the omega (ω) planar complexion. A planar complexion refers to a metastable phase confined and stabilized in the interfaces of the adjacent phases. It was shown that the new ω-complexion plays a significant role in the transformation, as it mediates the β-to-αꞌꞌ transition The formation of the ω-complexion is induced by a diffusionless transformation in which the structural changes occur by coordinated movement of atoms toward the energetically more favorable arrangement of the αꞌꞌ phase. The mechanical stresses induced by the atomic movements along the phase boundaries have a significant influence on the morphology of the resulting phase: thus when the stress rises above a critical value, ω-complexion emerges before the decrease of the stress value causes the formation of a new αꞌꞌ layer.This mechanism leads to a final nanostructure of many layers of αꞌꞌ martensite, alternating with ω-planar complexions. Confirming the microstructure in the bulk sample was one of the most challenging parts, according to Zhang. The in situ synchrotron x-ray diffraction (SXRD) heating/cooling measurements and the decoding of the complex SXRD pattern were performed with the assistance of A.-C. Dippel’s research group at the PETRA III ring accelerator in the DESY facilities in Hamburg, Germany. The results demonstrate the co-existence of αꞌꞌ and ω phases, and a small volume of remaining β-phase.A great finding for the team was the way the nanostructure of the bulk sample fully mirrors its microscopic structure. To Zhang’s astonishment the micrographs from the scanning electron microscope with an integrated back-scattered electron detector showed the nanolaminate composite microstructure expanding to the macroscopic scale. David Dye, a professor at Imperial College London, an expert on design of titanium and nickel/cobalt superalloys who was not involved in the study, describes the idea that the high-pressure ω phase of Ti is formed at the interfaces of the orthorhombic stress-induced martensite αꞌꞌ by the interface strain and confinement as “very beautiful.”“Showing it so clearly in the TEM [transmission electron microscope] is also very nice work,” Dye says, adding that “previously, the variety of shear-related products—superelastic martensite, omega, twins—in these alloys has been very mysterious, and this paper helps explain the picture.”The researchers are now interested in finding out how the microstructure of the alloy will evolve during a cold deformation procedure. “It is not yet clear to us how this phase transformation will contribute to the final properties of the material; this is something we are trying to figure out,” Zhang says. The team also wants to understand why the β-phase in the gum metal system is unstable toward both the αꞌꞌ and ω phases. According to Zhang, this is the reason why the transformation from β to αꞌꞌ phase can induce the transformation from β to ω phase at the interphase. The researchers believe that this may be the key for designing new materials that could be even better than the gum-like titanium alloys