O cale pentru valurile de centrifugare

Spin este termenul pe care oamenii de știință îl folosesc pentru a descrie unghiul angular al electronilor care se rotesc în jurul axei proprii. Aceasta face particulele electrice să se comporte ca niște magneți extrem de mici. Acesta este motivul pentru care se aliniază într-o manieră paralelă în materiale feromagnetice. „Dacă cineva conduce o rotație într-o direcție diferită, acest lucru va avea un impact și asupra rotirilor învecinate”, explică Schultheiß. „Aceasta creează un val de spin care călătorește prin corpul solid. Poate fi folosit pentru a transporta și procesa informații, la fel ca purtătorii de încărcături care curg”. Cu toate acestea, electronii înșiși nu se mișcă în acest caz. „Ei nu se ciocnesc cu nimic și, prin urmare, nu generează aproape nici unul

Posted: Feb 01, 2016

A highway for spin waves

(Nanowerk News) The success story of information processing by way of moving electrons is slowly coming to an end. The trend towards more and more compact chips constitutes a major challenge for manufacturers, since the increasing miniaturization creates partly unsolvable physical problems. This is why magnetic spin waves could be the future: they are faster than electronic charge carriers and use less power. Researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and TU Dresden have developed a method for controlling the propagation of these information carriers at the nanolevel in a targeted and simple way; so far, this required a lot of power. They have thus created a basis for nanocircuits that use spin waves (Nature Nanotechnology,„Magnetic domain walls as reconfigurable spin-wave nanochannels”).
Domain Wall
The spin wave remains trapped in the domain wall, which is formed in the middle between the differently oriented magnetizations. Researchers at the HZDR could thus control its propagation purposefully. (Image: HZDR / H. Schultheiß)
„Our current information processing is based on electrons,” explains Dr. Helmut Schultheiß from the HZDR’s Institute of Ion Beam Physics and Materials Research. „These charged particles flow through the wires, creating electric currents. Yet in the process they collide with atoms and lose energy, which escapes into the crystal lattice in the form of heat. This means that chips get all the warmer, the closer the elements on them are grouped together. Eventually they fail, because the heat cannot be conveyed anymore.” This is why Schultheiß, head of an Emmy Noether Junior Research Group, pursues a different approach: information transport via spin waves, also known as magnons.
The magnetic moment of electrons
Spin is the term scientists use to describe the angular momentum of electrons revolving around their own axis. It makes the electric particles behave like extremely small magnets. This is why they align in a parallel manner in ferromagnetic materials. „If one guides a spin in a different direction, this will have an impact also on the neighboring spins,” Schultheiß explains. „This creates a spin wave that travels through the solid body. It can be used to transport and process information just like flowing charge carriers.” However, the electrons themselves do not move in this case. „They do not collide with anything and therefore generate hardly any heat.”
Yet in order to prevail in the race for future information processing methods, systems are needed that allow for controlling the propagation of spin waves at the nanolevel. „So far, approaches to a solution were based either on geometrically predefined conductor paths or on the permanent use of external magnetic fields,” says Schultheiß, explaining the current state of research. „In the case of the first solution, the propagation path cannot be changed; however this is necessary for the development of flexible circuits. The second method solves that problem, yet at the price of an enormous increase in power consumption.”
Controlled propagation path
The scientists have successfully developed a new procedure for the targeted steering of spin waves by utilizing basic magnetic characteristics: remanence, that is, the residual magnetism that a solid body retains after the removal of a magnetic field, and the formation of so-called domain walls. „This term denotes the area in solid bodies where differently aligned magnetic domains meet,” Schultheiß explains.
The HZDR researchers created such a domain wall within a nickel-iron alloy nanostructure in an experiment. They then triggered a spin wave using microwaves. As their tests have shown, the spin waves of a certain frequency got stuck in the domain wall, because the different magnetic domains act as barriers. „In a sense, one could say that we created a road with a crash barrier along which the spin waves travel in a controlled manner,” Schultheiß cheerfully describes the result.
However, the Dresden physicists were able to celebrate yet another success. They manipulated the course of the domain wall by way of small external magnetic fields of far below one millitesla, about one hundred times weaker than a commercial horseshoe magnet. In doing so, they likewise manipulated the propagation of the spin waves. „This could be the basis for a design of reconfigurable nanocircuits that uses magnons,” Schultheiß says, sizing up the options. Even so, the researcher thinks that several years are likely to pass before application. „We are still in the basic research phase. However, our results reveal that we are onto a good thing.”
Source: Helmholtz-Zentrum Dresden-Rossendorf

Read more: A highway for spin waves

Picăturile lichide arată capacitatea de a răci suprafețe extrem de fierbinți(A heat-seeking slingshot: Liquid droplets show ability to cool extremely hot surfaces)

Aceasta împiedică picăturile lichidului să facă contact fizic cu suprafața, determinându-le, în schimb, să se hrănească deasupra suprafeței. Și cauzează picăturile să se fiarbă mai încet decât ar face-o pe o suprafață cu o temperatură mai scăzută care este încă deasupra punctului de fierbere al lichidului.

Posted: Feb 01, 2016

A heat-seeking slingshot: Liquid droplets show ability to cool extremely hot surfaces

(Nanowerk News) Turn on a skillet and let it heat up until it is well above the boiling point of water. Then sprinkle a teaspoon of water on the skillet and watch. Water droplets will bounce up, form spheres and scurry across the surface.
What you have just observed is an example of the Leidenfrost effect, named for Johann Gottlob Leidenfrost, an 18th-century German physician and scientist. The phenomenon occurs when a liquid, upon approaching an object that is much hotter than the liquid’s boiling point, produces a vapor which insulates the liquid from the surface of the object.
Researchers in Hong Kong and at Lehigh University recently demonstrated that it is possible to exploit the Leidenfrost effect to control the direction and destination of liquid droplets on a surface and thus to cool it more efficiently. They achieved this by lithographically patterning a surface with microscale features that convert excess surface tension into a kinetic energy that propels droplets to „hot spots” on the surface.
The discovery, say Zuankai Wang of the City University of Hong Kong and Manoj Chaudhury of Lehigh, has the potential to improve technologies that involve microfluidics, heat transfer, heat exchange, micro-heat exchange, water management and thermal management.
„Many applications, such as power plant reactors, require the management and control of the movement of water droplets at very high temperatures,” says Wang, an associate professor of mechanical and biomedical engineering at City University. „Typically, the cooling of extremely hot surfaces has been accomplished with spray cooling. You spray a lot of water droplets onto a surface and as they boil, they take away the heat.
„At a high temperature, however, this doesn’t work because the Leidenfrost effect prevents the droplets from making sufficient contact with the surface to cool it. Thus it takes too long to cool a surface by boiling off water.”
Wang, Chaudhury and their colleagues reported their results today (Feb. 1) in Nature Physics(„Directional transport of high-temperature Janus droplets mediated by structural topography”). The article’s lead author is Jing Li, a Ph.D. candidate in the department of mechanical and biomedical engineering at City University.
Contrasting topographies
Scientists in the last 20 years have learned to control the movement of liquid droplets on a solid surface by breaking the wetting symmetry that results from the impact of a droplet on a surface. They have accomplished this by harnessing gradients of surface energy and by utilizing light, temperature, electric force and mechanical vibration.
Chaudhury, the Franklin J. Howes Jr. Distinguished Professor of Chemical and Biomolecular Engineering at Lehigh, for example, has published articles with his students in Science and Langmuir describing their successful efforts to direct the movement of water droplets on surfaces.
But scientists have not yet achieved this control on surfaces heated to Leidenfrost temperatures and above, or on surfaces with extremely hot local spots.
Two years ago, Wang came up with the idea of creating topographical contrasts on a silicon wafer by etching the wafer surface with micropillars and arranging the pillars in zones that vary according to the density of the pillars and the contact angle of the pillars with the surface.
„The Leidenfrost Effect has been extensively studied for drag reduction, while the presence of the undesired vapor layer also prevents efficient heat transfer,” says Wang. „Thus, we came up with the idea of creating an asymmetric surface to control droplet motion at high temperatures.”
In their Nature Physics article, the researchers reported that their experiments, which were conducted in Hong Kong, showed that „judicious control of the structural topography and operating temperature range of the solid substrate” served to break the wetting symmetry of droplets.
The group also reported a „new physical phenomenon in which two concurrent wetting states–Leidenfrost and contact-boiling–can be designed in a single droplet [heated] above its boiling point.”
The droplet, the researchers wrote in Nature Physics, „exhibits a contrasting (or Janus) thermal state with a lower contact angle in the boiling region, but a higher angle in the Leidenfrost region.” This contrast generates „a gradient of curvature, and thus a gradient of Laplace pressure.”
„As the [droplet’s] viscous dissipation is minimal,” the researchers wrote, „the resulting excess surface energy of the droplet is converted to kinetic energy, naturally causing it to dislodge from the surface and take flight into the air. The droplet eventually gets deposited in the contact-boiling region.”
A similar phenomenon in nature
The researchers liken this phenomenon to the action of a slingshot and note that something similar occurs with a filamentous mushroom called Basidiomycota. Each spore of the fungus is part hydrophobic, with a shape like a thin film, and part hydrophilic, with a shape like a sphere. When the two regions make contact, they coalesce, and the tension between hydrophobic and hydrophilic regions creates a force that carries the entire spore into the air.
„This is a chemical effect that nature produces,” says Shuhuai Yao, an associate professor of mechanical and aerospace engineering at the Hong Kong University of Science and Technology. „At the point of coalescence, there is an asymmetry but the desire for symmetry causes a transitory state and generates a force that propels the spore.”
The contrasting topographies of the micropillars on Wang’s silicon wafer create a similar phenomenon. At a high temperature, as one part of the droplet is boiling and one is non-boiling, an asymmetry is created. But as with the Basidiomycota, the natural tendency toward symmetry creates the slingshot effect that propels the droplet.
There is, however, a critical difference between the two phenomena, the researchers say. While nature makes no effort to guide a spore but instead merely to release it, a droplet on a bio-inspired surface can be targeted to a specific place and made to land on a hot spot and boil off.
Source: Lehigh University

Read more: A heat-seeking slingshot: Liquid droplets show ability to cool extremely hot surfaces

Testarea nedistructivă: Privind în interiorul compozitelor

ehnica de testare nedistructivă trimite undele sonore în materialul compozit, returnând datele cu fiecare lovire a sondei. SANDIA a început să studieze compozite în urmă cu câțiva ani pentru a vedea dacă materialele ușoare ar putea fi folosite în aplicațiile de securitate națională. În timp ce un compozit dintr-un telefon mobil trebuie să dureze doar câțiva ani, „materialele tipice pentru aplicațiile de securitate națională trebuie să supraviețuiască timp de decenii. Acest lucru te face sa te gandesti altfel cu privire la unde si de ce ai folosi un material „, a spus Moore, care lucreaza in departamentul Structural Dynamics si X-ray / Nondestructive. „Trebuie să studiem ciclul de viață al unei componente. Avem tendința de a ne gândi profund la consecințele fracturii sau deformării și cum putem verifica ce sa întâmplat „. Lucrarea susține multe aspecte ale misiunii naționale de securitate a Sandia, inclusiv eficiența energetică și îmbunătățiri ale performanței vehiculelor ușoare sau a lamelor cu turbine eoliene, a declarat Briggs of Sandia Structura usoara a structurilor usoare din California. Compozitele combina materiale separate cu caracteristici diferite. Ele adesea constau dintr-un soft

Posted: Feb 01, 2016

Nondestructive testing: Looking inside composites

(Nanowerk News) Researcher David Moore holds a rectangle of hard carbon composite material, smooth with a faint woven pattern on its surface. The sample shows normal wear and tear until he turns it over to reveal a circular impact mark with cracks radiating from it.The question for Moore, his Sandia National Laboratories colleague Timothy Briggs in California and their teams is whether the impact caused significant, hidden damage inside the composite. They’re developing nondestructive ways to detect damage in composites, using traditional medical inspection techniques such as X-rays and sonograms and advanced methods including infrared imaging, ultrasonic spectroscopy and computed tomography.Sandia National Laboratories technologist Andrew Lentfer passes a roller probe over a composite as researcher David Moore checks data on a screenSandia National Laboratories technologist Andrew Lentfer passes a roller probe over a composite as researcher David Moore checks data on a screen. The nondestructive testing technique sends sound waves into the composite material, returning data with each swipe of the roller probe.Sandia began studying composites several years ago to see whether the lightweight materials could be used in national security applications. While a composite in a cell phone needs to last only a couple of years, “typically materials for national security applications must survive for decades. This makes you think differently about where and why you would use a material,” said Moore, who works in the Structural Dynamics and X-ray/Nondestructive Evaluation department. “We need to study the lifecycle of a component. We tend to think deeply about the consequences of fracture or deformation and how we can verify what happened.”The work supports many aspects of Sandia’s national security mission, including energy efficiency and performance improvements in lightweight vehicles or wind turbine blades, said Briggs of Sandia California’s Lightweight Structures organization.Composites join together separate materials with different characteristics. They often consist of a soft polymer matrix with reinforcing fibers like carbon, Kevlar or glass. Composites can use bundles of thinner-than-a-hair carbon strands that yield a high strength-to-weight ratio. The final shape and strength is obtained after heating the part in an industrial oven that sets the polymer resin and yields the qualities necessary for a structural component.Composites are increasingly important in aerospace and other industries because they’re strong and weigh less than metals. Most can be bonded to metal for such uses as aircraft wings, making planes lighter and less expensive to fly.Outside surface of a composite doesn’t hint at what’s inside“We have a rich history of understanding metals and their failure mechanisms,” Moore said. “Composite materials are very different.”If a service truck backs into a composite aircraft fuselage, an examination of the impact site might not detect damage under the surface. That highlights the reason for nondestructive techniques that can fully evaluate how composites react in various circumstances.The research team is assessing the accuracy of nondestructive methods and how they could be used on a production floor. “You have to know what could go wrong in the processing steps and how to circumvent those, and then you want to make sure if you’re going to make one or a hundred or a thousand that you’re making them the same way all the time,” Moore said.“Once we establish the limits of detectability, the threshold of good, bad and questionable, we’ll be able to say, ‘We want this composite bonded to this material with a defined quality and it shall be inspected with this technique,’” he said.Sandia’s Lightweight Structures Lab defines and consolidates materials to study, using particular stack sequences of composite material layers to tailor strength and stiffness. It works in concert with the National Security Campus in Kansas City, Missouri, in everything from developing process methods to building prototypes to qualifying designs for particular applications.After making composites, the fabrication lab cuts out specimens for instrumented experiments — abusing the carefully made sample to study deformation, fracture and damage growth. Briggs “pulls, stretches, torques and crushes them. He performs these mechanical experiments so we can understand the fracture mechanisms around failure,” Moore said. “Then we try to detect some of those failure modes.”Characterizing material properties helps computer modeling, simulation“The fundamental characterization of composites measures material properties and structural characteristics, which in turn provides information to validate computer modeling and simulation,” Briggs said.Data from these destructive tests is correlated with nondestructive evaluations from Moore’s team to understand what caused the material to respond the way it did, Briggs said. The effort hinges on close collaboration with Sandia’s materials characterization groups and modeling and simulation colleagues to help validate their computer-based simulations.Composites must support a particular weight and size. “Anyone can build a structure to carry the load, but we have to design our structures to fit within a geometric envelope and be lightweight,” Briggs said. “We cannot simply over-engineer to unrealistic levels. We have to be very smart and efficient with our designs, yet provide enough margin for long-term reliability.”Materials bonded in an oven or autoclave often have different thermal expansion rates — aluminum expands more than fiber-reinforced plastics, for example. Once a composite cools after curing, residual stresses can build up inside, particularly at interfaces. If the composite can’t handle those stresses, the bonds can fail.Sandia is developing advanced techniques to complete sample inspections in less than 5 minutes in some cases. “This is a way to gain a lot of information very quickly about the quality of the bonds,” Moore said.Team studies specific nondestructive techniquesHis team uses the deliberately mistreated composites to assess such inspection techniques as advanced ultrasonics, flashed or active thermography and computed tomography.Ultrasonic testing has been around for 60 years, but computers and other improvements now allow study of more complex applications. Technologist Andrew Lentfer demonstrates, scanning a piece of composite with a handheld ultrasonic roller that resembles a small paint roller with a hollow, water-filled barrel. As he scans the composite’s layers, a computer screen maps them in color: Yellow-green is OK; blue indicates weakness. Rollers can scan curved surfaces, even large ones like airplanes.Fibers and interfaces in a composite scatter ultrasonic waves moving through the material. Moore compares it to ocean waves: “If a wave hits a rock face in the ocean it moves around it; if a wave washes up on the sand it gets absorbed; and if it hits a seawall the wave energy is redirected quickly. Those are the same fundamentals we investigate: ultrasonic energy moving through a composite matrix.”Knowledge gained from characterizing materials helps develop new nondestructive techniques “so when we establish an inspection criteria, we have a better feel for what we can detect and what we cannot,” he said.Flashed thermography, commercially available for more than 20 years, flashes very high-energy light onto a surface for 15 microseconds, then an infrared camera watches how the surface cools. The process takes only minutes.“It’s very fast, but you have to understand the fundamentals of heat flow and how the material surface either gives off heat to its surroundings or transfers heat within itself,” Moore said.Studies use good, not-so-good composites to test differencesThe research uses composites with high-quality bonds and others deliberately made with weaker bonds. Differences in results help the team improve detection of defects or damage. A computer screen shows light or dark spots indicating possible problems, and an overlaid graph ties the depth of the potential problem to the time indicated on the image.“The question becomes, is that a concern? Is that a crack or not?” Moore said. “We’ll be able to answer those questions. If the defect propagates deep into the material, we may not detect it. It’s wise to understand the capabilities of the technique and then perform the math and science behind it.”Computed tomography systems are efficient for finding small defects. The technique rotates a sample 360 degrees while taking 1,000 images, similar to a medical CT scan, and generates an image from each thin slice of the object. Since each image taken is two dimensional, computer algorithms reconstruct, calculate, locate and display everything to represent the object in three dimensions.“Once the 3-D image is reconstructed, you look at the front surface and then start moving through the thickness to view what is below the surface,” Moore said. “This technology gives us a knowledge baseline and validates how the other techniques are performing.”Sandia must ensure designs meet requirements by specifying and qualifying an inspection technique, and now is writing inspection procedures for the National Security Campus, Moore said. “Once we establish criteria and limits of acceptability, the product definition can be established,” he said.

Source: Sandia National Laboratories

Read more: Nondestructive testing: Looking inside composites

Simțiți spații cu „pălăria de proximitate”(Feeling spaces with the „proximity hat” )

Senzorul ultrasonic, bateriile și tampoanele de presiune sunt părți ale noului sistem care poate fi pus pe cap ca o pălărie sau bandă de susținere

101 sept

Polar vortex observate în feroelectrice

Materialele feroase prezintă proprietăți electrice sau magnetice unice – sau ambele în cazul multiferroizelor. De exemplu, câmpul electric al unui material feroelectric poate fi polarizat în favoarea unei încărcări pozitive sau negative cu aplicarea unui câmp electric extern. Într-un material feromagnetic, aplicarea unui câmp magnetic extern aliniază rotația particulelor încărcate, rezultând materialul devenind un magnet permanent. În ultimii ani, sa descoperit că aplicarea unui câmp magnetic extern poate produce, de asemenea, cicloane cu dimensiuni atomice de cercuri, care acționează ca particule de baryon și pot fi deplasate coerent pe distanțe macroscopice. Aceste proprietăți fac Skyrmions candidați excelenți pentru aplicații spintronice.

Posted: Jan 29, 2016

Polar vortices observed in ferroelectric

(Nanowerk News) The observation in a ferroelectric material of „polar vortices” that appear to be the electrical cousins of magnetic skyrmions holds intriguing possibilities for advanced electronic devices. These polar vortices, which were theoretically predicted more than a decade ago, could also „rewrite our basic understanding of ferroelectrics” according to the researchers who observed them.
A team of scientists with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley have recorded the first ever observations of rotating topologies of electrical polarization that are similar to the discrete swirls of magnetism known as „skyrmions.” If these smoothly rotating vortex/anti-vortex topologies prove to be electrical skyrmions, they could find potential applications in ultracompact data storage and processing, and could also lead to the production of new states of matter and associated phenomena in ferroic materials.
polar vortices in a ferroelectic material
The first ever observations of polar vortices in a ferroelectic material could find potential applications in ultracompact data storage and processing and the production of new states of matter.
„It has long been thought that rotating topological structures are confined to magnetic systems and aren’t possible in ferroelectric materials, but through the creation of artificial superlattices, we have controlled the various energies of a ferrolectric material to promote competition that lead to such new states of matter and polarization arrangements,” says Ramamoorthy Ramesh, Berkeley Lab’s Associate Laboratory Director for Energy Technologies and the co-principal investigator for this study. He also holds UC Berkeley’s Purnendu Chatterjee Endowed Chair in Energy Technologies.
„Ferroelectric materials such as the materials used in this work have produced a number of exciting emergent properties over the years, but these smoothly-rotating polar vortex structures really are different,” says Lane Martin, a faculty scientist with Berkeley Lab’s Materials Sciences Division and Associate Professor in UC Berkeley’s Department of Materials Science and Engineering, who is this study’s co-principal investigator. „I think if you surveyed the community many would shake their heads in disbelief at such structures, but it turns out there really is a tendency for vortex states to form in nature even in these polar systems. And, when one looks more broadly, vortex structures can occur across huge length scales – from galaxies and weather systems all the way down to 10s of atoms as in our case.”
Ramesh and Martin are the corresponding authors of a Nature paper describing this study in detail („Observation of polar vortices in oxide superlattices”).
Ferroic materials display unique electrical or magnetic properties – or both in the case of multiferroics. For example, the electrical field of a ferroelectric material can be polarized in favor of either a positive or negative charge with the application of an external electrical field. In a ferromagnetic material, the application of an external magnetic field aligns the spin of their charged particles, resulting in the material becoming a permanent magnet. In recent years, it was discovered that the application of an external magnetic field can also produce atom-sized cyclones of skyrmions, which act like baryon particles and can be moved coherently over macroscopic distances. These properties make skyrmions excellent candidates for spintronic applications.
Vortex structures occur in nature across a broad range of length scales
Vortex structures occur in nature across a broad range of length scales, from galaxies and weather systems all the way down to the atomic scale of polar vortices in ferroics.
„We believe the polar vortices we observed in ferroelectrics, when fully explored, have the potential to be topological states of matter that are similar to magnetic skyrmions,” Ramesh says. „The fact that our polar vortices can display emergent behavior in their electronic, optical, magnetic and other properties suggests that heretofore unexplored applications and functionalities could be possible.”
Ramesh, Martin and their collaborators worked with what has become a canonical system in the community, ultrafine layered structures built from lead titanate and strontium titante compounds controlled down to a few unit cells each, in which each unit cell is approximately 0.4 nanometers thick. They created superlattices that harbored a three-way competition between elastic, electrostatic and gradient energies within the layers of lead titanate and strontium titanate. This unique three-way competition gives rise to the polar vortices.
„As we tune the period lengths of our superlattices, we can tune the relative importance of these three energy scales,” Martin says. „Although rather exotic things can occur if one changes the superlattice period to be both smaller and bigger than we studied here, we really found the ‘sweet-spot’ in this work that produced these polar vortices which are an entirely new phenomenon.”
A combination of scanning transmission electron microscopy (STEM) and X-ray diffraction studies were used observe and characterize the polar vortices. The STEM work was carried out at Berkeley Lab’s Molecular Foundry, a DOE Office of Science User Facility, on TEAM 0.5, the world’s most powerful transmission electron microscope. The X-ray diffraction work was carried out at the Advanced Photon Source, another DOE Office of Science User Facility, which is hosted by DOE’s Argonne National Laboratory.
„Our study is really indicative of how DOE-funded research programs can bring together a diverse range of expertise, including atomically-controlled materials synthesis and cutting-edge research facilities, and materials theory to enable foundational discoveries that really change the way we think about exotic materials and the possibilities for using them,” says Ramesh.
„This is just the beginning for the study of polar vortices in ferroelectric materials,” Martin says. „We’re observing a new state of matter and we have our work cut out for us in mapping and understanding how it evolves. We can imagine adding a magnetic spin component to similar superlattices and thus potentially paving a pathway to fundamentally demonstrate electric-field control of magnetism.”
Source: By Lynn Yarris, Lawrence Berkeley National Laboratory
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Read more: Polar vortices observed in ferroelectric

Tehnica de creștere nanosheet ar putea revoluționa producția de nanomateriale

Datorită proprietăților chimice, agentul tensioactiv se asamblează într-un singur strat la suprafața lichidului, cu ioni sulfați încărcați negativ în direcția lichidului. Acești ioni sulfați extrag ionii de zinc încărcați pozitiv din interiorul lichidului la suprafață și în decurs de câteva ore sugerează suficient ioni de zinc pentru a forma nanopreți de oxid de zinc continuu doar câteva straturi atomice groase.

Posted: Jan 29, 2016

Nanosheet growth technique could revolutionize nanomaterial production

(Nanowerk News) After six years of painstaking effort, a group of University of Wisconsin-Madison materials scientists believe the tiny sheets of the semiconductor zinc oxide they’re growing could have huge implications for the future of a host of electronic and biomedical devices.
The group – led by Xudong Wang, a UW-Madison professor of materials science and engineering, and postdoctoral researcher Fei Wang – has developed a technique for creating nearly two-dimensional sheets of compounds that do not naturally form such thin materials. It is the first time such a technique has been successful.
The researchers described their findings in the journal Nature Communications („Nanometre-thick single-crystalline nanosheets grown at the water–air interface”).
nanosheet
The new nanoscale manufacturing process draws zinc to the surface of a liquid, where it forms sheets just a few atoms thick. (Image: Xudong Wang)
Essentially the microscopic equivalent of a single sheet of paper, a 2-D nanosheet is a material just a few atoms thick. Nanomaterials have unique electronic and chemical properties compared to identically composed materials at larger, conventional scales.
„What’s nice with a 2-D nanomaterial is that because it’s a sheet, it’s much easier for us to manipulate compared to other types of nanomaterials,” says Xudong Wang.
Until now, materials scientists were limited to working with naturally occurring 2-D nanosheets. These natural 2-D structures include graphene, a single layer of graphite, and a limited number of other compounds.
Developing a reliable method to synthesize and manufacture 2-D nanosheets from other materials has been a goal of materials researchers and the nanotechnology industry for years.
In their technique, the UW-Madison team applied a specially formulated surfactant – a detergent-like substance – onto the surface of a liquid containing zinc ions.
Due to its chemical properties, the surfactant assembles itself into a single layer at the surface of the liquid, with negatively charged sulfate ions pointed in the direction of the liquid. Those sulfate ions draw the positively charged zinc ions from within the liquid to the surface, and within a couple hours enough zinc ions are drawn up to form continuous zinc oxide nanosheets only a few atomic layers thick.
Xudong Wang first had the idea for using a surfactant to grow nanosheets during a lecture he was giving in a course on nanotechnology in 2009.
„The course includes a lecture about self-assembly of monolayers,” he says. „Under the correct conditions, a surfactant will self-assemble to form a monolayer. This is a well-known process that I teach in class. So while teaching this, I wondered why we wouldn’t be able to reverse this method and use the surfactant monolayer first to grow the crystalline face.”
After five years of trial and error with different surfactant solutions, the idea paid off.
„We are very excited about this,” says Xudong Wang. „This is definitely a new way to fabricate 2-D nanosheets, and it has great potential for different materials and for many different applications.”
Already, the researchers have found that the 2-D zinc oxide nanosheets they’ve grown are able to function as semiconductor transistors called a p-type, the opposite electronic behavior of naturally occurring zinc oxide. Researchers have for some time attempted to produce zinc oxide with reliable p-type semiconductor properties.
Zinc oxide is a very useful component of electronic materials, and the new nanosheets have potential for use in sensors, transducers and optical devices.
But the zinc oxide nanosheets are only the first of what could be a revolution in 2-D nanomaterials. Already, the UW-Madison team is applying its surfactant method to growing 2-D nanosheets of gold and palladium, and the technique holds promise for growing nanosheets from all sorts of metals that wouldn’t form them naturally.
„It brings a lot of new functional material to this 2-D material category,” Wang says.
Source: University of Wisconsin-Madison

Read more: Nanosheet growth technique could revolutionize nanomaterial production

Patriot Ordnance Factory POF-SUA P-300 (de asemenea scris P300). 300 Win.Mag (300WM) Piston Tactic AR Rifle / Carbine debutează la SHOT Show 2016!(Patriot Ordonance Factory POF-USA P-300(also written P300). 300 Win.Mag(300WM)Tactical Piston AR Rifle/Carbine Debuts at SHOT Show 2016!)

Înțelegerea lui DR este că POF-SUA P-300 utilizează în prezent un polimer Nemo Arms 300WM și va veni cu unul dintre declanșatoarele excelente ale puilor POF, în funcție de preferințele utilizatorilor finali

99 sept

O alternativă la platină: compuși ai azotului de fier ca catalizatori în grafen(An alternative to platinum: iron-nitrogen compounds as catalysts in graphene)

au produs un material catalitic eficient din punct de vedere al costurilor pentru celulele de combustie folosind un nou proces de preparare pe care l-au analizat în detaliu. Se compune din complexe de fier-azot încorporate în insulele mici de grafen cu numai câțiva nanometri în diametru

Posted: Jan 27, 2016

An alternative to platinum: iron-nitrogen compounds as catalysts in graphene

(Nanowerk News) Teams at Helmholtz Zentrum Berlin (HZB) and TU Darmstadt have produced a cost-effective catalyst material for fuel cells using a new preparation process which they analysed in detail. It consists of iron-nitrogen complexes embedded in tiny islands of graphene only a few nanometres in diameter. It is only the FeN4 centres that provide the excellent catalytic efficiency – approaching that of platinum. The results are interesting for solar fuels research as well and have been published in the Journal of the American Chemical Society („On an easy way to prepare Metal-Nitrogen doped carbon with exclusive presence of MeN4-type sites active for the ORR”).
Nano-island of graphene in which iron-nitrogen complexes are embedded
Nano-island of graphene in which iron-nitrogen complexes are embedded. The FeN4 complexes (shown in orange) are catalytically active. (Image: S. Fiechter/HZB)
Fuel cells convert the chemical energy stored in hydrogen (H2) into electrical energy by electrochemically “combusting” hydrogen gas with oxygen (O2) from the air into water (H2O), thereby generating electricity. As a result, future electric automobiles might be operated quite well with fuel cells instead of with heavy batteries. But for “cold” combustion of hydrogen and oxygen to function well, the anode and cathode of the fuel cell must be coated with extremely active catalysts. The problem is that the platinum-based catalysts employed for this contribute about 25 per cent of the total fuel-cell costs.
However, iron-nitrogen complexes in graphene (known as Fe-N-C catalysts) have been achieving levels of activity comparable to Pt/C catalysts for several years already. “Systematic investigation of Fe-N-C catalysts was difficult though, since most approaches for preparing the materials lead to heterogeneous compounds. These contain various species of iron compounds such as iron carbides or nitrides besides the intended FeN4 centres”, explains Sebastian Fiechter of HZB.
High density of catalytically active centres
“We had already developed a new preparation method at HZB a few years ago to produce an inexpensive catalyst material from organometallic compounds such as iron or cobalt porphyrin”, reports Peter Bogdanoff, HZB. Ulrike Kramm and Iris Herrmann-Geppert improved the process for producing it as part of their doctoral studies at HZB. As a result, the metal-N-C catalysts developed at HZB held the world record for the highest density of catalytically active centres of various nitro-metallic compounds up to about 2011. However, it remained unclear as to which inorganic compounds influenced the catalytic efficiency. The team was now able to determine this.
Purification process removes interfering compounds
The highlight in the current work is a purification process (a combination of thermal treatment with a subsequent etching step) by which the proportion of metallic compounds that interfere with catalytic activity can be substantially reduced, even for catalysts that are highly heterogeneous. The interesting thing here is that the activity increases enormously! Ulrike Kramm, who has since become a junior professor at TU Darmstadt, was successful in purifying several catalysts to such an extent that all the iron present in the graphene layers was exclusively in form of complexes made of iron and four nitrogen atoms (FeN4). The scientists thereby disproved the hypothesis debated among experts by which improvement in the activity of the FeN4 centres only resulted from promoters, as they are known, such as iron nanoparticles.
Now verified: FeN4 centres provide the high catalytic efficiency even without promoters
“To check this hypothesis, we employed numerous complex measurement techniques like Mößbauer spectroscopy, electron paramagnetic resonance spectroscopy and X-ray absorption spectroscopy at BESSY II. These enabled us to precisely survey the atomic structure of the catalytic centres”, Ulrike Kramm reports.
“The purification process enables us now to create catalysts having exclusively FeN4 centres. This allows us to subsequently select compounds to be added afterwards as promoters that further improve the activity level or stability of these catalysts”, as Ulrike Kramm summarises her research approach at TU Darmstadt.
Sebastian Fiechter and Peter Bogdanoff are continuing their research at HZB on novel catalysts, especially in regard to hydrogen generation using sunlight. “We can also use the insights into how these metal-N-C catalysts work in our on-going development of catalysing materials for solar-based hydrogen production at HZB,” says Fiechter.
Together, the research activities at HZB and TU Darmstadt could enable the development of a complete regenerative energy cycle, using solar hydrogen in low cost fuel cells, thus producing electricity without climate gas emission.
Source: Helmholtz Zentrum Berlin