Categorie: Materiale noi cu structura compozita

Dezvoltarea unui nou „vas” de nanomateriale de carbon ( Development of a novel carbon nanomaterial ‘pot’

Din analiza detaliata a suprafetei, s-au gasit grupuri hidroxil hidrofile grupate de-a lungul suprafetei exterioare a corpului de nanopot de carbon”, a spus Associate Prof. Yokoi. „Grapena este de obicei hidrofobă, totuși, dacă grupările hidroxil sunt împachetate dens pe suprafața exterioară a corpului, zona respectivă va fi hidrofilă. Cu alte cuvinte, nanopotul de carbon ar putea fi un nanomaterial unic, cu caracteristici hidrofobe și hidrofile. Procesul de realizare a unei analize de suprafață mai sofisticate pentru a obține acea asigura

Posted: Aug 05, 2016

Development of a novel carbon nanomaterial ‘pot’

(Nanowerk News) A novel, pot-shaped, carbon nanomaterial developed by researchers from Kumamoto University, Japan is several times deeper than any hollow carbon nanostructure previously produced. This unique characteristic enables the material to gradually release substances contained within and is expected to be beneficial in applications such as drug delivery systems.
Transmission Electron Microscope Image of Carbon Nanopots
Carbon nanopots are made of several different laminated graphene layer structures as labeled by the yellow text. The magnified image (inset) reveals the area indicated by the dashed line in the main image. The numerical value in the inset is the average value of layer spacing, and the arrows indicate graphene edges exposed on the outer surface of the body. (© Materials Research Society, Dr. Eng. Hiroyuki Yokoi)
Carbon is an element that is light, abundant, has a strong binding force, and eco-friendly. The range of carbon-based materials is expected to become more widespread in the eco-friendly society of the future. Recently, nanosized (one-billionth of a meter) carbon materials have been developed with lengths, widths, or heights below 100 nm. These materials take extreme forms such as tiny grained substances, thin sheet-like substances, and slim fibrous substances. Example of these new materials are fullerenes, which are hollow cage-like carbon molecules; carbon nanotubes, cylindrical nanostructures of carbon molecules; and graphene, one-atom thick sheets of carbon molecules.
Why are these tiny substances needed? One reason is that reactions with other materials can be much larger if a substance has an increased surface area. When using nanomaterials in place of existing materials, it is possible to significantly change surface area without changing weight and volume, thereby improving both size and performance. The development of carbon nanomaterials has provided novel nanostructured materials with shapes and characteristics that surpass existing materials.
Now, research from the laboratory of Kumamoto University’s Associate Prof. Yokoi has resulted in the successful development of a container-type carbon nanomaterial with a much deeper orifice than that found in similar materials (Journal of Materials Research, „Novel pot-shaped carbon nanomaterial synthesized in a submarine-style substrate heating CVD method”).
Transmission Electron Microscope Image of Carbon Nanopot fiber
The black arrow indicates the end of the opening of carbon nanopot. A structural schematic of the carbon nanopot showing hydroxyl groups bonded to the edges of the graphene layers near the closed end of the nanopot is also indicated (not to scale). (© Materials Research Society)
To create the new material, researchers used their own, newly developed method of material synthesis. The container-shaped nanomaterial has a complex form consisting of varied layers of stacked graphene at the bottom, the body, and the neck areas of the container, and the graphene edges along the outer surface of the body were found to be very dense. Due to these innovate features, Associate Prof. Yokoi and colleagues named the material the „carbon nanopot.”
The carbon nanopot has an outer diameter of 20 ~ 40 nm, an inner diameter of 5 ~ 30 nm, and a length of 100 ~ 200 nm. During its creation, the carbon nanopot is linked to a carbon nanofiber with a length of 20 ~ 100 µm meaning that the carbon nanopot is also available as a carbon nanofiber. At the junction between nanopots, the bottom of one pot simply sits on the opening of the next without sharing a graphene sheet connection. Consequently, separating nanopots is very easy.
„From a detailed surface analysis, hydrophilic hydroxyl groups were found clustered along the outer surface of the carbon nanopot body,” said Associate Prof. Yokoi. „Graphene is usually hydrophobic however, if hydroxyl groups are densely packed on the outer surface of the body, that area will be hydrophilic. In other words, carbon nanopots could be a unique nanomaterial with both hydrophobic and hydrophilic characteristics. We are currently in the process of performing a more sophisticated surface analysis in order to get that assurance.”
Since this new carbon nanopot has a relatively deep orifice, one of its expected uses is to improve drug delivery systems by acting as a new foundation for medicine to be carried into and be absorbed by the body.
Source: Kumamoto University

Read more: Development of a novel carbon nanomaterial ‘pot’

Metoda de întărire a nanotuburilor de carbon ar putea face ramele avionului mai ușoare, mai rezistente la deteriorări(Method to reinforce carbon nanotubes could make airplane frames lighter, more damage-resistant)

Inginerii aerospațiali MIT au găsit o modalitate de a lega straturile compozite, producând un material care este substanțial mai puternic și mai rezistent la deteriorare decât alte compozite avansate

14 sept

Suprafața de schimbare îmbunătățește marginile nanofibrelor de grafen

Multe studii au prezis proprietățile nanofibrelor de grafen cu marginile zigzag”, a declarat Guangyu Zhang, autor principal al studiului. „Dar în experimente este foarte greu să faci acest material.”
Anterior, cercetatorii au incercat sa faca nanofibri de grafen prin plasarea foilor de grafen intr-un strat de silice si folosind hidrogenul atomic pentru a etchiza benzi cu marginile zig-zag, un proces cunoscut sub numele de gravura anizotropica. Aceste margini sunt esențiale pentru a modula proprietățile nanoribbonului.
Dar această metodă a funcționat doar bine pentru a face panglici care aveau două sau mai multe straturi de grafen. Neregulile din silicele create de vârfurile și văile electronice scrâșnesc suprafața sa, astfel încât să se creeze margini precise de zigzag pe monostraturile de grafen, a fost o provocar

Posted: Aug 01, 2016

Swapping substrates improves edges of graphene nanoribbons

(Nanowerk News) It is now feasible to make a prized material for spintronic devices and semiconductors – monolayer graphene nanoribbons with zigzag edges.
Miniscule ribbons of graphene are highly sought-after building blocks for semiconductor devices because of their predicted electronic properties. But making these nanostructures has remained a challenge. Now, a team of researchers from China and Japan have devised a new method to make the structures in the lab. Their findings appear in the current issue of Applied Physics Letters („Patterning monolayer graphene with zigzag edges on hexagonal boron nitride by anisotropic etching”).
etched Graphene
Graphene etched with an underlying silica substrate produces uneven edges (left) but forms precise edges when placed on boron nitride (right). (Image: Guangyu Zhang)
„Many studies have predicted the properties of graphene nanoribbons with zigzag edges,” said Guangyu Zhang, senior author on the study. „But in experiments it’s very hard to actually make this material.”
Previously, researchers have tried to make graphene nanoribbons by placing sheets of graphene over a layer of silica and using atomic hydrogen to etch strips with zigzag edges, a process known as anisotropic etching. These edges are crucial to modulate the nanoribbon’s properties.
But this method only worked well to make ribbons that had two or more graphene layers. Irregularities in silica created by electronic peaks and valleys roughen its surface, so creating precise zigzag edges on graphene monolayers was a challenge. Zhang and his colleagues from the Chinese Academy of Sciences, Beijing Key Laboratory for Nanomaterials and Nanodevices, and the Collaborative Innovation Center of Quantum Matter teamed up with Japanese collaborators from the National Institute for Materials Science to solve the problem.
They replaced the underlying silica with boron nitride, a crystalline material that’s chemically sluggish and has a smooth surface devoid of electronic bumps and pits. By using this substrate and the anisotropic etching technique, the group successfully made graphene nanoribbons that were only one-layer thick, and had well-defined zigzag edges.
„This is the first time we have ever seen that graphene on a boron nitride surface can be fabricated in such a controllable way,” Zhang explained.
The zigzag-edged nanoribbons showed high electron mobility in the range of 2000 cm2/Vs even at widths of less than 10nm – the highest value ever reported for these structures – and created clean, narrow energy band gaps, which makes them promising materials for spintronic and nano-electronic devices.
„When you decrease the width of the nanoribbons, the mobility decreases drastically because of edge defects,” said Zhang. „Using standard lithography fabrication techniques, studies have seen mobility of 100 cm2/Vs or even lower, but our material still exceeds 2000 cm2/Vs even at the sub-10 nanometer scale, demonstrating that these nanoribbons are of very high quality.”
In future studies, extending this method to other kinds of substrates could enable the quick large scale processing of monolayers of graphene to make high-quality nanoribbons with zigzag edges.
Source: American Institute of Physics

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Borul stimulează sensibilitatea grafenului la gazele nocive

Cercetatorii au descoperit o modalitate de imbunatatire semnificativa a performantei sale prin tratarea foilor de grafenie de inalta calitate cu impuritati de bor. Comparativ cu grafenul original, aceste foi modificate, de exemplu, grafenul dopat de bor, au fost de 27 de ori mai sensibile la detectarea dioxidului de azot și de 105 ori mai sensibile la detectarea amoniacului („Detectarea ultrasensibilă a gazului în grafenul dopat de bor cu suprafață mare”).

Posted: Jul 29, 2016

Boron boosts graphene’s sensitivity to noxious gases

(Nanowerk News) Detecting noxious gases, such as those released from power plants and other sources that can harm the environment, is something graphene does well, but it could be even better.
Researchers discovered a way to significantly improve its performance by peppering high-quality graphene sheets with boron impurities. Compared to pristine graphene, these modified sheets, a.k.a. boron-doped graphene, were 27 times more sensitive at detecting nitrogen dioxide and 105 times more sensitive at detecting ammonia („Ultrasensitive gas detection of large-area boron-doped graphene”).
boron-doped graphene
Experimental observations (left) confirm theory-based simulations (right) of boron-doped graphene. Boomerang-shaped atomic features revealed in a centimeter-wide sheet of boron-doped graphene are associated with enhanced sensitivities for nitrogen dioxide and ammonia detection.
Ultrasensitive gas detectors based on this proof-of-principle experiment could monitor environmental health and safety. Sheets of doped graphene with useful electronic and magnetic properties could also find application in field-effect transistors, hydrogen storage, and lithium-ion batteries.
Theory predicts that graphene could be much better at detecting noxious gases if impurities, or dopants, were incorporated into its rigid framework of one-atom-thick carbon. Experimental progress, however, has been limited by a lack of high-quality boron-doped graphene.
Here, scientists grew highly ordered, centimeter-wide sheets of boron-doped graphene and observed, for the first time, boomerang-shaped features corresponding to dopant atoms embedded within graphene’s hexagonal matrix. They characterized the material’s electronic properties, which depend on how impurity atoms incorporate into graphene’s honeycomb lattice and create defects that change the way gases chemically interact with graphene, which in turn alters current flow.
To discover the atomic-level details of how boron doping changes graphene’s properties, the researchers went to the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at Oak Ridge National Laboratory, for advanced instrumentation and expertise in low-voltage electron microscopy and scanning tunneling microscopy.
Results showed that, compared with pristine graphene, boron doping increases the gas sensing capability of graphene to parts per billion for nitrogen dioxide and parts per million for ammonia.
Source: Department of Energy, Office of Science

Read more: Boron boosts graphene’s sensitivity to noxious gases

Atomii individuali de aur pot ajuta la generarea unui combustibil curat(Individual gold atoms can help generate clean fuel)

Dioxidul de ceriu (ceria, CeO 2 ) este un material de suport utilizat pe scară largă pentru multe procese industriale importante; nanoparticulele metalice susținute pe ceria au prezentat activități înalte pentru aplicații, inclusiv convertoare catalitice auto, sinteză de alcool și pentru producția de hidrogen. Există două atribute cheie ale ceriei care o fac un excelent material de suport activ: capacitatea de stocare a oxigenului și capacitatea de eliberare și capacitatea sa de stabilizare a particulelor mici de metal în condiții de reacție.

Posted: Jul 22, 2016

Individual gold atoms can help generate clean fuel

(Nanowerk News) A combined experimental and theoretical study comprising researchers from the Chemistry Department and LCN, along with groups in Argentina, China, Spain and Germany, has shed new light on the behaviour of individual gold atoms supported on defective thin cerium dioxide films – an important system for catalysis and the generation of clean hydrogen for fuel.
Catalysis plays a vital role in our world; an estimated 80% of all chemical and materials are made via processes which involve catalysts, which are commonly a mixture of metals and oxides. The standard motif for these heterogeneous catalysts (where the catalysts are solid and the reactants are in the gas phase) is of a high surface area oxide support that is decorated with metal nanoparticles a few nanometres in diameter.
Cerium dioxide (ceria, CeO2) is a widely used support material for many important industrial processes; metal nanoparticles supported on ceria have displayed high activities for applications including car catalytic converters, alcohol synthesis, and for hydrogen production. There are two key attributes of ceria which make it an excellent active support material: its oxygen storage and release ability, and its ability to stabilise small metal particles under reaction conditions.
A recent system that has been the focus of much interest has been that of gold nanoparticles and single atoms with ceria, which has demonstrated high activity towards the water-gas-shift reaction, (CO + H2O –> CO2 + H2) a key stage in the generation of clean hydrogen for use in fuel cells.
The nature of the active sites of these catalysts and the role that defects play are still relatively poorly understood; in order to study them in a systematic fashion, the researchers prepared model systems which can be characterised on the atomic scale with a scanning tunnelling microscope.
STM images of CeO2-x(111) ultrathin films before and after the deposition of Au single atoms
STM images of CeO2-x(111) ultrathin films before and after the deposition of Au single atoms at 300 K. The bright lattice is from the oxygen atoms at the surface – vacancies appear as dark spots.
These model systems comprised well-ordered, epitaxial ceria films less than 2 nm thick, prepared on a metal single crystal, upon which single atoms and small clusters of gold were evaporated onto under ultra-high-vacuum (essential to prevent contamination of the surfaces). Oxygen vacancy defects – missing oxygen atoms in the top layer of the ceria – are relatively common at the surface and appear as dark spots in the STM images. By mapping the surface before and after the deposition of gold, it is possible to analyse the binding of the metal atoms, in particular there does not appear to be any preference for binding in the vacancy sites at 300 K.
Publishing their results in Physical Review Letters („Diffusion Barriers Block Defect Occupation on Reduced CeO2(111)”), the researchers combined these experimental results with theoretical studies of the binding energies and diffusion rates across the surface. They showed that kinetic effects governed the behaviour of the gold atoms, prohibiting the expected occupation of the thermodynamically more stable oxygen vacancy sites.
They also identified electron transfer between the gold atoms and the ceria, leading to a better understanding of the diffusion phenomena that occur at this scale, and demonstrated that the effect of individual surface defects may be more minor than is normally imagined.
Related articles:
„Spillover Reoxidation of Ceria Nanoparticles” (2016) and „Spectromicroscopy of a Model Water–Gas Shift Catalyst: Gold Nanoparticles Supported on Ceria” (2014) both in The Journal of Physical Chemistry
Source: London Centre for Nanotechnology

Împrumutându-se de la bucătari de patiserie, inginerii creează compozite nanolayered(Borrowing from pastry chefs, engineers create nanolayered composites)

materiale compozite care să conțină sute de straturi care sunt doar atomi grosime, dar care acoperă toată lățimea materialului . Descoperirea ar putea deschide posibilități largi de proiectare a compozitelor noi și ușor de fabricat pentru dispozitive optice, sisteme electronice și materiale de înaltă tehnologie.Echipa a găsit, de asemenea, o modalitate de a face fibre structurate din grafen, care să permită crearea de fire și țesături cu funcții electronice încorporate, precum și o altă clasă de compozite. Metoda folosește un mecanism de forfecare, oarecum asemănător cu un dispozitiv de tăiere a brânzei, pentru a îndepărta straturile de grafen într-un mod care le face să se rostogolească într-o formă asemănătoare scroll-ului, cunoscută din punct de vedere tehnic ca o spirală arhimedeană.O caracteristică neașteptată a noilor compozite stratificate, spune Strano, este că straturile de grafeni, care sunt extrem de conductive din punct de vedere electric, își mențin continuitatea pe tot parcursul eșantionului lor compozit fără a se scurtcircuita la straturile adiacente

Posted: Jul 21, 2016

Borrowing from pastry chefs, engineers create nanolayered composites

(Nanowerk News) Adapting an old trick used for centuries by both metalsmiths and pastry makers, a team of researchers at MIT has found a way to efficiently create composite materials containing hundreds of layers that are just atoms thick but span the full width of the material. The discovery could open up wide-ranging possibilities for designing new, easy-to-manufacture composites for optical devices, electronic systems, and high-tech materials.
The work is described this week in a paper in Science („Layered and scrolled nanocomposites with aligned semi-infinite graphene inclusions at the platelet limit”) by Michael Strano, the Carbon P. Dubbs Professor in Chemical Engineering; postdoc Pingwei Liu; and 11 other MIT students, postdocs, and professors.
The process of making a stack of parallel sheets of graphene
CThe process of making a stack of parallel sheets of graphene starts with a chemical vapor deposition process (I) to make a graphene sheet with a polymer coating; these layers are then stacked (II), folded and cut (III) and stacked again and pressed, multiplying the number of layers. The team used a related method the team to produce scroll-shaped fibers. (Image: Courtesy of the researchers) (click on image to enlarge)
Materials such as graphene, a two-dimensional form of pure carbon, and carbon nanotubes, tiny cylinders that are essentially rolled-up graphene, are „some of the strongest, hardest materials we have available,” says Strano, because their atoms are held together entirely by carbon-carbon bonds, which are „the strongest nature gives us” for chemical bonds to work with. So, researchers have been searching for ways of using these nanomaterials to add great strength to composite materials, much the way steel bars are used to reinforce concrete.
The biggest obstacle has been finding ways to embed these materials within a matrix of another material in an orderly way. These tiny sheets and tubes have a strong tendency to clump together, so just stirring them into a batch of liquid resin before it sets doesn’t work at all. The MIT team’s insight was in finding a way to create large numbers of layers, stacked in a perfectly orderly way, without having to stack each layer individually.
Although the process is more complex than it sounds, at the heart of it is a technique similar to that used to make ultrastrong steel sword blades, as well as the puff pastry that’s in baklava and napoleons. A layer of material – be it steel, dough, or graphene – is spread out flat. Then, the material is doubled over on itself, pounded or rolled out, and then doubled over again, and again, and again.
With each fold, the number of layers doubles, thus producing an exponential increase in the layering. Just 20 simple folds would produce more than a million perfectly aligned layers.
Now, it doesn’t work out exactly that way on the nanoscale. In this research, rather than folding the material, the team cut the whole block – itself consisting of alternating layers of graphene and the composite material – into quarters, and then slid one quarter on top of another, quadrupling the number of layers, and then repeating the process. But the result was the same: a uniform stack of layers, quickly produced, and already embedded in the matrix material, in this case polycarbonate, to form a composite.
In their proof-of-concept tests, the MIT team produced composites with up to 320 layers of graphene embedded in them. They were able to demonstrate that even though the total amount of the graphene added to the material was minuscule – less than 1/10 of a percent by weight – it led to a clear-cut improvement in overall strength.
„The graphene has an effectively infinite aspect ratio,” Strano says, since it is infinitesimally thin yet can span sizes large enough to be seen and handled. „It can span two dimensions of the material,” even though it is only nanometers thick. Graphene and a handful of other known 2-D materials are „the only known materials that can do that,” he says.
The team also found a way to make structured fibers from graphene, potentially enabling the creation of yarns and fabrics with embedded electronic functions, as well as yet another class of composites. The method uses a shearing mechanism, somewhat like a cheese slicer, to peel off layers of graphene in a way that causes them to roll up into a scroll-like shape, technically known as an Archimedean spiral.
That could overcome one of the biggest drawbacks of graphene and nanotubes, in terms of their ability to be woven into long fibers: their extreme slipperiness. Because they are so perfectly smooth, strands slip past each other instead of sticking together in a bundle. And the new scrolled strands not only overcome that problem, they are also extremely stretchy, unlike other super-strong materials such as Kevlar. That means they might lend themselves to being woven into protective materials that could „give” without breaking.
One unexpected feature of the new layered composites, Strano says, is that the graphene layers, which are extremely electrically conductive, maintain their continuity all the way across their composite sample without any short-circuiting to the adjacent layers. So, for example, simply inserting an electrical probe into the stack to a certain precise depth would make it possible to uniquely „address” any one of the hundreds of layers. This could ultimately lead to new kinds of complex multilayered electronics, he says.
Source: By David L. Chandler, MIT