Categorie: Materiale noi cu structura compozita

Graphena albă face ceramică multifuncțională(White graphene makes ceramics multifunctional)

profesor asistent de inginerie civilă și de mediu, a sugerat că încorporarea foilor superioare de nitrurați de bor (hBN) între straturile de silicat de calciu ar face un cristal dublu interesant cu proprietăți multifuncționale. Acestea ar putea fi potrivite pentru construcții și materiale refractare și aplicații în industria nucleară, petrol și gaz, industria aerospațială și alte domenii care necesită compozite de înaltă performanță.Graphenul alb de culoare albă (stratul de mijloc), combinat cu silicat de calciu, creează un ceramică multifuncțională cu rezistență ridicată și duritate

Posted: Jan 11, 2018

White graphene makes ceramics multifunctional

(Nanowerk News) A little hBN in ceramics could give them outstanding properties, according to a Rice University scientist.
Rouzbeh Shahsavari, an assistant professor of civil and environmental engineering, suggested the incorporation of ultrathin hexagonal boron nitride (hBN) sheets between layers of calcium-silicates would make an interesting bilayer crystal with multifunctional properties. These could be suitable for construction and refractory materials and applications in the nuclear industry, oil and gas, aerospace and other areas that require high-performance composites.
Bilayer white graphene (middle layer) combined with calcium-silicate creates a multifunctional ceramic
Bilayer white graphene (middle layer) combined with calcium-silicate creates a multifunctional ceramic with high strength and toughness, according to a Rice University lab. The material may be suitable for construction and refractory materials and applications in the nuclear industry, oil and gas, aerospace and other areas that require high-performance composites. (Image: Rouzbeh Shahsavari)
Combining the materials would make a ceramic that’s not only tough and durable but resistant to heat and radiation. By Shahsavari’s calculations, calcium-silicates with inserted layers of two-dimensional hBN could be hardened enough to serve as shielding in nuclear applications like power plants.
The research appears in the American Chemical Society journal ACS Applied Materials and Interfaces („Intercalated Hexagonal Boron Nitride/Silicates as Bi-layer Multifunctional Ceramics”).
Two-dimensional hBN is nicknamed white graphene and looks like graphene from above, with linked hexagons forming an ultrathin plane. But hBN differs from graphene as it consists of alternating boron and nitrogen, rather than carbon, atoms.
“This work shows the possibility of material reinforcement at the smallest possible dimension, the basal plane of ceramics,” Shahsavari said. “This results in a bilayer crystal where hBN is an integral part of the system as opposed to conventional reinforcing fillers that are loosely connected to the host material.
“Our high-level study shows energetic stability and significant property enhancement owing to the covalent bonding, charge transfer and orbital mixing between hBN and calcium silicates,” he said.
The form of ceramic the lab studied, known as tobermorite, tends to self-assemble in layers of calcium and oxygen held together by silicate chains as it dries into hardened cement. Shahsavari’s molecular-scale study showed that hBN mixes well with tobermorite, slips into the spaces between the layers as the boron and oxygen atoms bind and buckles the flat hBN sheets.
This accordion-like buckling is due to the chemical affinity and charge transfer between the boron atoms and tobermorite that stabilizes the composite and gives it high strength and toughness, properties that usually trade off against each other in engineered materials, Shahsavari said. The explanation appears to be a two-phase mechanism that takes place when the hBN layers are subjected to strain or stress.
Shahsavari’s models of horizontally stacked tobermorite and tobermorite-hBN showed the composite was three times stronger and about 25 percent stiffer than the plain material. Computational analysis showed why: While the silicate chains in tobermorite failed when forced to rotate along their axes, the hBN sheets relieved the stress by first unbuckling and then stiffening.
When compressed, plain tobermorite displayed a low yield strength (or elastic modulus) of about 10 gigapascals (GPa) with a yield strain (the point at which a material deforms) of 7 percent. The composite displayed yield strength of 25 GPa and strain up to 20 percent.
“A major drawback of ceramics is that they are brittle and shatter upon high stress or strain,” Shahsavari said. “Our strategy overcomes this limitation, providing enhanced ductility and toughness while improving strength properties.
“As a bonus, the thermal and radiation tolerance of the system also increases, rendering multifunctional properties,” he said. “These features are all important to prevent deterioration of ceramics and increase their lifetime, thereby saving energy and maintenance costs.”
When the material was tested from other angles, differences between the pure tobermorite and the composite were less pronounced, but on average, hBN improved significantly the material’s properties.
“Compared with one-dimensional fillers such as conventional fibers or carbon nanotubes, 2-D materials like hBN are two-sided, so they have twice the surface area per unit mass,” Shahsavari said. “This is perfect for reinforcement and adhesion to the surrounding matrix.”
He said other 2-D materials like molybdenum disulfide, niobium diselenide and layered double hydroxide may also be suitable for the bottom-up design of high-performance ceramics and other multifunctional composite materials.
Source: By Mike Williams, Rice University

Graphene spumă poate acționa ca un dispozitiv eficient de captură și conversie termică solare(Graphene foam can act as efficient solar thermal capture and conversion device)

După cum sa raportat în ACS Nano ( „Convertor de energie solară autonomă bazat pe grafenie pentru purificarea și purificarea apei” ), materialul de 3D-grafit (3DG) încrucișat cu polimeri 3D poate fi utilizat direct într-un mod ușor și scalabil ca o apă curată generator cu eficiență ridicată în lumina soarelui normală și chiar mai slabă (până la 0.25 soare).
Noul material 3DG are o rată specifică de producție a apei extrem de ridicată de 2,6 kg h -1 m -2 g -1 și 87% eficiență energetică sub 1 radiație solară a soareluiMai mult, diferit de alte materiale de desalinizare, acest nou material ușor și elastic este, de asemenea, foarte portabil pentru punctul de utilizare și ar fi ideal atât pentru aplicații personale, cât și industriale.
„Acest material de carbon usor de folosit ar putea oferi multe posibilitati pentru recoltarea energiei solare cu un randament ridicat”, concluzioneaza autorii.

Posted: Jan 10, 2018

Graphene foam can act as efficient solar thermal capture and conversion device

(Nanowerk News) Researchers have demonstrated that a simple sheet of standalone graphene foam can act as efficient solar thermal capture and conversion device.
As reported in ACS Nano („Graphene-Based Standalone Solar Energy Converter for Water Desalination and Purification”), the 3D cross-linked polymer-like graphene material (3DG) material can be used directly in an easy and scalable manner as a clean water generator with high efficiency under normal and even weaker sunlight (down to 0.25 sun).
The new 3DG material has an extremely high specific water production rate of 2.6 kg h-1 m-2 g-1and 87% energy efficiency under 1 sun solar radiation.
Solar thermal performance of the 3D graphene material
Solar thermal performance of the 3D graphene material. (a) Absorption spectrum of 3DG. It shows a blackbody-like property with ~97% absorption across 200-2500 nm. Left part of the inset is the SEM image showing its porous morphology. Such morphology benefits from efficient light absorption through the multiscattering effect and water transportation through the interconnected pores. Right part of the inset is the contact angle showing it is superhydrophilic. (b) Left is the evaporation mass of water with/without 3DG under 1 kW m-2 solar irradiation and the case without light; the middle is the schematic illustration of 3DG for the vapor generation and the temperature distribution (cross section); the right part is the IR image of vertical and side view under 1 kW m-2. The sample is smaller than the water/air interface to show that such a material can effectively prevent the heat conduction surrounding water. (c) Specific water production rate (water production rate/weight of 3DG) compared with some highest performing bulk converters reported in literature. (d) Solar thermal efficiency of the evaporation process by the 3DG under a range of optical concentrations. Error bars represent standard deviation for the same repeated measurements. (e) Performance of the 3DG with different water quantities under 10 kW m-2. The device was in the same diameter but different heights of water at 32 mm (red), 22 mm (green), and 12 mm (blue). (f) Reusability of 3DG under 1 and 10 kW m-2 solar irradiation for 20 cycles with a NaCl solution, mimicking the seawater. Each cycle was tested for 2 h. (© American Chemical Society) (click on image to enlarge)
The researchers point out that the high efficiency is achieved due to the combination of multiple properties simultaneously exhibited by this as-prepared bulk 3D crosslinked honeycomb graphene material, including its intrinsic blackbody-like high absorption, extremely low thermal loss to environment, hydrophilicity, and automatic water transportation to the absorber by capillary force through the microchannels present in the structure.
With these results, an efficient and scalable desalination (4 orders of salinity decrement) and sewage treatment device was demonstrated. A model for this converter has been used to understand the high efficiency and mechanism.
Furthermore, different from other desalination materials, this novel lightweight and elastic material is also highly portable for point of use and would be ideal for both personal and industrial application.
„This easy-to-use carbon material could provide many possibilities for high efficient solar energy harvesting,” the authors conclude.
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny.

Cum să optimizați nanomaterialele pentru catodii cu celule de combustibil(How to optimize nanomaterials for fuel-cell cathodes)

Simulările efectuate de oamenii de știință de la Universitatea Rice arată că nanomaterialele de carbon pot fi optimizate pentru a înlocui platina scumpe în catozi pentru celulele de combustie care generează electricitate.

Posted: Jan 05, 2018

How to optimize nanomaterials for fuel-cell cathodes

(Nanowerk News) Nitrogen-doped carbon nanotubes or modified graphene nanoribbons may be suitable replacements for platinum for fast oxygen reduction, the key reaction in fuel cells that transform chemical energy into electricity, according to Rice University researchers.
The findings are from computer simulations by Rice scientists who set out to see how carbon nanomaterials can be improved for fuel-cell cathodes. Their study reveals the atom-level mechanisms by which doped nanomaterials catalyze oxygen reduction reactions (ORR).
The research appears in the Royal Society of Chemistry journal Nanoscale („Mechanisms of the oxygen reduction reaction on B- and/or N-doped carbon nanomaterials with curvature and edge effects”).
optimizing nanomaterials for fuel-cell cathodes
Simulations by Rice University scientists show how carbon nanomaterials may be optimized to replace expensive platinum in cathodes for electricity-generating fuel cells. (Image: Yakobson Research Group)
Simulations by Rice University scientists show how carbon nanomaterials may be optimized to replace expensive platinum in cathodes for electricity-generating fuel cells. Courtesy of the Yakobson Research Group
Simulations by Rice University scientists show how carbon nanomaterials may be optimized to replace expensive platinum in cathodes for electricity-generating fuel cells. Courtesy of the Yakobson Research Group
Theoretical physicist Boris Yakobson and his Rice colleagues are among many looking for a way to speed up ORR for fuel cells, which were discovered in the 19th century but not widely used until the latter part of the 20th. They have since powered transportation modes ranging from cars and buses to spacecraft.
The Rice researchers, including lead author and former postdoctoral associate Xiaolong Zou and graduate student Luqing Wang, used computer simulations to discover why graphene nanoribbons and carbon nanotubes modified with nitrogen and/or boron, long studied as a substitute for expensive platinum, are so sluggish and how they can be improved.
Doping, or chemically modifying, conductive nanotubes or nanoribbons changes their chemical bonding characteristics. They can then be used as cathodes in proton-exchange membrane fuel cells. In a simple fuel cell, anodes draw in hydrogen fuel and separate it into protons and electrons. While the negative electrons flow out as usable current, the positive protons are drawn to the cathode, where they recombine with returning electrons and oxygen to produce water.
The models showed that thinner carbon nanotubes with a relatively high concentration of nitrogen would perform best, as oxygen atoms readily bond to the carbon atom nearest the nitrogen. Nanotubes have an advantage over nanoribbons because of their curvature, which distorts chemical bonds around their circumference and leads to easier binding, the researchers found.
The tricky bit is making a catalyst that is neither too strong nor too weak as it bonds with oxygen. The curve of the nanotube provides a way to tune the nanotubes’ binding energy, according to the researchers, who determined that “ultrathin” nanotubes with a radius between 7 and 10 angstroms would be ideal. (An angstrom is one ten-billionth of a meter; for comparison, a typical atom is about 1 angstrom in diameter)
They also showed co-doping graphene nanoribbons with nitrogen and boron enhances the oxygen-absorbing abilities of ribbons with zigzag edges. In this case, oxygen finds a double-bonding opportunity. First, they attach directly to positively charged boron-doped sites. Second, they’re drawn by carbon atoms with high spin charge, which interacts with the oxygen atoms’ spin-polarized electron orbitals. While the spin effect enhances adsorption, the binding energy remains weak, also achieving a balance that allows for good catalytic performance.
The researchers showed the same catalytic principles held true, but to lesser effect, for nanoribbons with armchair edges.
“While doped nanotubes show good promise, the best performance can probably be achieved at the nanoribbon zigzag edges where nitrogen substitution can expose the so-called pyridinic nitrogen, which has known catalytic activity,” Yakobson said.
“If arranged in a foam-like configuration, such material can approach the efficiency of platinum,” Wang said. “If price is a consideration, it would certainly be competitive.”
Source: Rice University

Nanocatalizatorul curata nitratii toxici din apa potabila(Nanocatalyst cleans toxic nitrates from drinking water)

Center au gasit un catalizator care curata nitrati toxici din apa de baut prin transformarea lor in aer si apa.am descoperit ca expunerea catalizatorilor la solutii care contin nitrati determina oxidarea indiului. Dar cand am adaugat apa cu hidrogen saturata, paladiul a determinat ca unii oxigen sa se lege cu hidrogenul si sa formeze apa, ceea ce a dus la mentinerea indiului intr-o stare redusa in care este liber sa spargi mai multe nitrati „.

Posted: Jan 05, 2018

Nanocatalyst cleans toxic nitrates from drinking water

(Nanowerk News) Engineers at Rice University’s Nanotechnology Enabled Water Treatment (NEWT) Center have found a catalyst that cleans toxic nitrates from drinking water by converting them into air and water.
The research is available online in the American Chemical Society journal ACS Catalysis („Insights into Nitrate Reduction over Indium-Decorated Palladium Nanoparticle Catalysts”).
Map of U.S. nitrate and nitrite pollution
Many areas of the United States are at risk for nitrate and nitrite contamination of drinking water due to overuse of agricultural fertilizers. (Image: USGS)
“Nitrates come mainly from agricultural runoff, which affects farming communities all over the world,” said Rice chemical engineer Michael Wong, the lead scientist on the study. “Nitrates are both an environmental problem and health problem because they’re toxic. There are ion-exchange filters that can remove them from water, but these need to be flushed every few months to reuse them, and when that happens, the flushed water just returns a concentrated dose of nitrates right back into the water supply.”
Wong’s lab specializes in developing nanoparticle-based catalysts, submicroscopic bits of metal that speed up chemical reactions. In 2013, his group showed that tiny gold spheres dotted with specks of palladium could break apart nitrites, the more toxic chemical cousins of nitrates.
“Nitrates are molecules that have one nitrogen atom and three oxygen atoms,” Wong explained. “Nitrates turn into nitrites if they lose an oxygen, but nitrites are even more toxic than nitrates, so you don’t want to stop with nitrites. Moreover, nitrates are the more prevalent problem.
“Ultimately, the best way to remove nitrates is a catalytic process that breaks them completely apart into nitrogen and oxygen, or in our case, nitrogen and water because we add a little hydrogen,” he said. “More than 75 percent of Earth’s atmosphere is gaseous nitrogen, so we’re really turning nitrates into air and water.”
Nitrates are toxic to infants and pregnant women and may also be carcinogenic. Nitrate pollution is common in agricultural communities, especially in the U.S. Corn Belt and California’s Central Valley, where fertilizers are heavily used, and some studies have shown that nitrate pollution is on the rise due to changing land-use patterns.
Both nitrates and nitrites are regulated by the Environmental Protection Agency, which sets allowable limits for safe drinking water. In communities with polluted wells and lakes, that typically means pretreating drinking water with ion-exchange resins that trap and remove nitrates and nitrites without destroying them.
From their previous work, Wong’s team knew that gold-palladium nanoparticles were not good catalysts for breaking apart nitrates. Co-author Kim Heck, a research scientist in Wong’s lab, said a search of published scientific literature turned up another possibility: indium and palladium.
“We were able to optimize that, and we found that covering about 40 percent of a palladium sphere’s surface with indium gave us our most active catalyst,” Heck said. “It was about 50 percent more efficient than anything else we found in previously published studies. We could have stopped there, but we were really interested in understanding why it was better, and for that we had to explore the chemistry behind this reaction.”
In collaboration with chemical engineering colleagues Jeffrey Miller of Purdue University and Lars Grabow of the University of Houston, the Rice team found that the indium speeds up the breakdown of nitrates while the palladium apparently keeps the indium from being permanently oxidized.
“Indium likes to be oxidized,” Heck said. “From our in situ studies, we found that exposing the catalysts to solutions containing nitrate caused the indium to become oxidized. But when we added hydrogen-saturated water, the palladium prompted some of that oxygen to bond with the hydrogen and form water, and that resulted in the indium remaining in a reduced state where it’s free to break apart more nitrates.”
Wong said his team will work with industrial partners and other researchers to turn the process into a commercially viable water-treatment system.
“That’s where NEWT comes in,” he said. “NEWT is all about taking basic science discoveries and getting them deployed in real-world conditions. This is going to be an example within NEWT where we have the chemistry figured out, and the next step is to create a flow system to show proof of concept that the technology can be used in the field.”
Source: Rice University

Tehnica nouă produce nanofibre puternice și rezistente pentru multe aplicații(New technique produces strong, resilient nanofibers for many applications)

Cercetatorii de la MIT au dezvoltat un proces care poate produce fibre ultrafine – al caror diametru este masurat in nanometri sau miliarde de metru – care sunt extrem de puternice si dure. Aceste fibre, care ar trebui să fie ieftine și ușor de realizat, ar putea fi materiale alese pentru multe aplicații, cum ar fi armura protectoare și nanocompozite.

Posted: Jan 05, 2018

New technique produces strong, resilient nanofibers for many applications

(Nanowerk News) Researchers at MIT have developed a process that can produce ultrafine fibers — whose diameter is measured in nanometers, or billionths of a meter — that are exceptionally strong and tough. These fibers, which should be inexpensive and easy to produce, could be choice materials for many applications, such as protective armor and nanocomposites.
The new process, called gel electrospinning, is described in a paper by MIT professor of chemical engineering Gregory Rutledge and postdoc Jay Park. The paper appears online and will be published in the February edition of the Journal of Materials Science („Ultrafine high performance polyethylene fibers”).
nanofibers
New ultra-fine fibers created by the MIT team are seen in a Scanning Electron Microscope (SEM) image. (Image: MIT)
In materials science, Rutledge explains, “there are a lot of tradeoffs.” Typically researchers can enhance one characteristic of a material but will see a decline in a different characteristic. “Strength and toughness are a pair like that: Usually when you get high strength, you lose something in the toughness,” he says. “The material becomes more brittle and therefore doesn’t have the mechanism for absorbing energy, and it tends to break.” But in the fibers made by the new process, many of those tradeoffs are eliminated.
“It’s a big deal when you get a material that has very high strength and high toughness,” Rutledge says. That’s the case with this process, which uses a variation of a traditional method called gel spinning but adds electrical forces. The results are ultrafine fibers of polyethylene that match or exceed the properties of some of the strongest fiber materials, such as Kevlar and Dyneema, which are used for applications including bullet-stopping body armor.
“We started off with a mission to make fibers in a different size range, namely below 1 micron [millionth of a meter], because those have a variety of interesting features in their own right,” Rutledge says. “And we’ve looked at such ultrafine fibers, sometimes called nanofibers, for many years. But there was nothing in what would be called the high-performance fiber range.” High-performance fibers, which include aramids such as Kevlar, and gel spun polyethylenes like Dyneema and Spectra, are also used in ropes for extreme uses, and as reinforcing fibers in some high-performance composites.
“There hasn’t been a whole lot new happening in that field in many years, because they have very top-performing fibers in that mechanical space,” Rutledge says. But this new material, he says, exceeds all the others. “What really sets those apart is what we call specific modulus and specific strength, which means that on a per-weight basis they outperform just about everything.” Modulus refers to how stiff a fiber is, or how much it resists being stretched.
Compared to carbon fibers and ceramic fibers, which are widely used in composite materials, the new gel-electrospun polyethylene fibers have similar degrees of strength but are much tougher and have lower density. That means that, pound for pound, they outperform the standard materials by a wide margin, Rutledge says.
In creating this ultrafine material, the team had aimed just to match the properties of existing microfibers, “so demonstrating that would have been a nice accomplishment for us,” Rutledge says. In fact, the material turned out to be better in significant ways. While the test materials had a modulus not quite as good as the best existing fibers, they were quite close — enough to be “competitive,” he says. Crucially, he adds, “the strengths are about a factor of two better than the commercial materials and comparable to the best available academic materials. And their toughness is about an order of magnitude better.”
The researchers are still investigating what accounts for this impressive performance. “It seems to be something that we received as a gift, with the reduction in fiber size, that we were not expecting,” Rutledge says.
He explains that “most plastics are tough, but they’re not as stiff and strong as what we’re getting.” And glass fibers are stiff but not very strong, while steel wire is strong but not very stiff. The new gel-electrospun fibers seem to combine the desirable qualities of strength, stiffness, and toughness in ways that have few equals.
Using the gel electrospinning process “is essentially very similar to the conventional [gel spinning] process in terms of the materials we’re bringing in, but because we’re using electrical forces” and using a single-stage process rather than the multiple stages of the conventional process, “we are getting much more highly drawn fibers,” with diameters of a few hundred nanometers rather than the typical 15 micrometers, he says. The researchers’ process combines the use of a polymer gel as the starting material, as in gel spun fibers, but uses electrical forces rather than mechanical pulling to draw the fibers out; the charged fibers induce a “whipping” instability process that produces their ultrafine dimensions. And those narrow dimensions, it turns out, led to the unique properties of the fibers.
These results might lead to protective materials that are as strong as existing ones but less bulky, making them more practical. And, Rutledge adds, “they may have applications we haven’t thought about yet, because we’ve just now learned that they have this level of toughness.”
Source: By David L. Chandler, MIT

Nanostructura pintenilor de urchin de mare inspiră un ciment mai bun(Nanostructure of the sea urchin spines inspires better cement)

a sintetizat cu succes cimentul la nano-nivel în conformitate cu acest principiu de „cărămidă și mortar”. În timpul acestui proces, au fost identificate macro-molecule care preluau funcția de mortar, fixând blocurile cristaline unul la celălalt pe scara nano, blocurile se asamblează într-un mod ordonat. Scopul este de a face cimentul mai durabil.Pe baza acestor calcule, cimentul optimizat a atins o valoare de 200 megapascali. Prin comparație: Cojile de mucegai, care sunt standardul de aur în rezistență la rupere, ajung la o valoare de 210 megapascali, care este doar puțin mai mare. Betonul utilizat în prezent în prezent are o valoare de două până la cinci megapascali

Posted: Nov 29, 2017

Nanostructure of the sea urchin spines inspires better cement

(Nanowerk News) The sea urchin spines are mostly made of calcite, usually a very brittle and fragile material. In the case of the sea urchin, however, the spines are much more durable than the raw material alone. The reason for its strength is the way that nature optimises materials using a brick wall-style architecture.
The research team Physical Chemistry, headed by Professor Helmut Cölfen, successfully synthesised cement at the nano-level according to this “brick and mortar principle”. During this process, macro-molecules were identified that take on the function of mortar, affixing the crystalline blocks to each other on the nano-scale, with the blocks assembling themselves in an ordered manner. The aim is to make cement more durable.
The study’s results are published in the 1 December 2017 issue of Science Advances(„Mesocrystalline calcium silicate hydrate: A bioinspired route toward elastic concrete materials”).
ending experiment on elastic cement in a scanning electron microscope
Bending experiment on elastic cement in a scanning electron microscope, enlarged 2,000 times Bar-shaped micro-structure precision-cut from nano-structured cement using an ion beam, bends under the pressure of a micro-manipulator without breaking (micro-manipulator left in the picture). (Image: Dr Zhaklina Burkhard)
“Our cement, which is significantly more fracture-resistant than anything that has been developed thus far, provides us with completely new construction possibilities”, Cölfen adds. A pillar made of this cement could be built 8,000 metres high, or ten times as high as the current tallest building in the world, before the material at its base would be destroyed by its weight. Normal steel, which has a value of 250 megapascals, could only reach 3,000 metres in height.
In nanoscience, brick wall-style architecture can be compared to the work of a mason: each layer of brick that is laid is held in place by mortar. The guiding principle is to layer hard, then soft, hard, then soft materials. This is exactly the principle nature uses to make sea urchin spines so resilient. When force is applied to the brittle calcite, its crystalline block does crack, however, the energy is then transferred to a soft disordered layer. Since this material has no cleavage planes to tear, it prevents further cracking. A thin section of sea urchin spine reveals this structural principle: crystalline blocks in an orderly structure are surrounded by a softer amorphous area. In the sea urchin’s case, this material is calcium carbonate.
Mussel shells or bones are constructed in much the same way. “Our goal is to learn from nature”, says Helmut Cölfen. The researcher has been honoured numerous times for his trailblazing results in the field of crystallisation, with, for example, the 2013 Academy Prize from the Berlin-Brandenburg Academy of Sciences and Humanities. Bionics or biomimetics is the term used for employing natural phenomena to inspire technical developments.
Cement itself has a disordered structure – each component sticks to all the others. This means: in order for cement to truly profit from the increased stability provided by brick and mortar construction, its structure will have to be reorganised at the nano-level. Helmut Cölfen describes the process as “encoding fracture-resistance at the nano-level”. In this case, it means identifying a material that bonds only with cement nanoparticles and nothing else in the cement. About ten negatively charged peptide combinations were identified that both adhere to and bond materials well.
In collaboration with the University of Stuttgart, the team was able to use an ion beam under an electron microscope to cut a bar-shaped micro-structure out of the nanostructured cement that was three micrometers in size. This micro-structure was then bent using a micro-manipulator. As soon as it was released, the micro-structure returned to its original position. Mechanical values could be calculated based on the elastic deformation of the micro-structure.
Based on these calculations, the optimised cement achieved a value of 200 megapascals. By comparison: Mussel shells, which are the gold standard in fracture-resistance, reach a value of 210 megapascals, which is only slightly higher. The concrete commonly used today has a value of two to five megapascals.
See urchin spines and mussel shells are made of calcite, because large quantities of calcium are available in water. Helmut Cölfen explains: “People have much better construction materials than calcite. If we succeed in designing the structures of materials and reproduce nature’s blueprints, we will also be able to produce much more fracture-resistant materials – high-performance materials inspired by nature”.
Source: Universität Konstanz

Producerea energiei termoelectrice la temperatura camerei: În curând?(Thermoelectric power generation at room temperature: Coming soon?)

Recent, cercetătorii de la Universitatea Osaka, în colaborare cu Hitachi, Ltd., au dezvoltat un nou material TE cu un factor de putere îmbunătățit la temperatura camerei. Studiul lor, publicat în Physica Status Solidi RRL ( „YtSi 2″ ): un material termoelectric promițător cu un factor de putere ridicat la temperatura camerei ” , ar putea ajuta aceste materiale să iasă din nișa la temperaturi înalte și în curentul principal.

Posted: Dec 26, 2017

Thermoelectric power generation at room temperature: Coming soon?

(Nanowerk News) Thermoelectric (TE) materials could play a key role in future technologies. Although the applications of these remarkable compounds have long been explored, they are mostly limited to high-temperature devices.
Recently, researchers at Osaka University, in collaboration with Hitachi, Ltd., developed a new TE material with an improved power factor at room temperature. Their study, published in Physica Status Solidi RRL („Ytterbium silicide (YbSi2): A promising thermoelectric material with a high power factor at room temperature”), could help bring these materials out of the high-temperature niche and into the mainstream.
Three-dimensional crystal structure of YbSi2
Fig.1. (a) Three-dimensional crystal structure of YbSi2, (b) view along the a-axis, and (c) along the c-axis.
TE materials display the thermoelectric effect: apply heat on one side, and an electric current starts to flow. Conversely, run an external current through the device, and a temperature gradient forms; i.e., one side becomes hotter than the other. By interconverting heat and electricity, TE materials can be used as either power generators (given a heat source) or refrigerators (given a power supply).
The ideal TE material combines high electrical conductivity, allowing the current to flow, with low thermal conductivity, which prevents the temperature gradient from evening out. The power generation performance mainly depends on the “power factor,” which is proportional to both electrical conductivity and a term called the Seebeck coefficient.
“Unfortunately, most TE materials are often based on rare or toxic elements,” according to study co-author Sora-at Tanusilp. “To address this, we combined silicon – which is common in TE materials – with ytterbium, to create ytterbium silicide (YbSi2). We chose ytterbium over other metals for several reasons. First, its compounds are good electrical conductors. Second, YbSi2 is non-toxic. Moreover, this compound has a specific property called valence fluctuation that make it a good TE material at low temperatures.”
Temperature dependences of (a) the Seebeck coefficient S, (b) electrical conductivity ?, (c) power factor S2? for YbSi2
Fig.2. Temperature dependences of (a) the Seebeck coefficient S, (b) electrical conductivity σ, (c) power factor S2σ for YbSi2. The reported data for Bi2Te3-based alloy are shown as solid lines for comparison. (click on image to enlarge)
The first advantage of YbSi2 is that the Yb atoms occupy a mixture of valence states, both +2 and +3. This fluctuation, also known as Kondo resonance, increases the Seebeck coefficient with keeping metal-like high electrical conductivity at low temperature, and therefore the power factor.
Second, YbSi2 has an unusual layered structure. While the Yb atoms occupy crystal planes similar to pure Yb metal, the Si atoms form hexagonal sheets between those planes, resembling the carbon sheets in graphite. This blocks the conduction of heat through the material, and therefore keeps the thermal conductivity down, preserving the temperature gradient. The researchers believe that heat conduction is further suppressed by controlling the structure in nanoscale and traces of impurities and other defects.
The result is an encouragingly high power factor of 2.2 mWm-1K-2 at room temperature. This is competitive with conventional TE materials based on bismuth telluride.
As corresponding author of this study Ken Kurosaki explains, “The use of Yb shows we can reconcile the conflicting needs of TE materials through carefully selecting the right metals. Room-temperature TEs, with moderate power, can be seen as complementary to the conventional high-temperature, high-power devices. This could help unlock the benefits of TE in everyday technology.”
Source: Osaka University

„Grafenă 3D” netedă „(Electronically-smooth ‘3D graphene’)

Cercetătorii au descoperit că materialul topologic bismutură trisodică (Na 3 Bi) poate fi fabricat ca fiind „electronic neted” ca alternativa bazată pe grafen cu cea mai bună calitate, menținând în același timp mobilitatea ridicată a electronilor grafenului.
Na 3 Bi este un Semimetal Dirac Topologic (TDS), considerat un echivalent 3D al grafenului prin aceea că arată aceeași mobilitate extraordinar de mare a electronilor.

Posted: Dec 22, 2017

Electronically-smooth ‘3D graphene’

(Nanowerk News) Researchers have found that the topological material trisodium bismuthide (Na3Bi) can be manufactured to be as ‘electronically smooth’ as the highest-quality graphene-based alternative, while maintaining graphene’s high electron mobility.
Na3Bi is a Topological Dirac Semimetal (TDS), considered a 3D equivalent of graphene in that it shows the same extraordinarily high electron mobility.
In graphene, as in a TDS, electrons move at constant velocity, independent of their energy.
This high electron mobility is highly desirable in materials investigated for fast-switching electronics. The flow of electrons in graphene can be, theoretically, 100 times as fast as in silicon.
However in practice there are limitations to graphene’s remarkable electron mobility, driven by the material’s two-dimensional nature.
Although graphene itself can be extremely pure, it is far too flimsy to use as a standalone material, and must be bound with another material. And because graphene is atomically thin, impurities in that substrate are able to cause electronic disorder within the graphene.
Such microscopic inhomogeneities, known as ‘charge puddles’, limit the mobility of charge carriers.
In practice, this means that graphene-based devices must be painstakingly constructed with a graphene sheet laid upon a substrate material that minimises such electronic disorder. Hexagonal boron-nitride (h-BN) is commonly used for this purpose.
But now, researchers at Australia’s FLEET research centre have found that trisodium bismuthide (Na3Bi) grown in their labs at Monash University are as electronically smooth as the highest-quality graphene/h-BN.
It’s a significant achievement, says lead researcher Dr Mark Edmonds. „This is the first time a 3D Dirac material has been measured in such a way,” Dr Edmonds says. „And we are excited to have found such a high degree of electronic smoothness in this material.”
The discovery will be critical for advancement of the study of this new topological material, which could have wide applications in electronics. „It’s impossible to know how many fields of research this could open,” says Dr Edmonds. „The same finding in graphene/h-BN sparked considerable supplementary studies in 2011.”
With electronic-smoothness of Na3Bi now demonstrated, an array of other research possibilities open up. There have been many studies into the relativistic (high mobility) flow of electrons in graphene since it was discovered in 2004. With this latest study, similar studies into Na3Bi can be expected.
Na3Bi offers a number of interesting advantages over graphene.
As well as avoiding the difficult construction methods involved in bi-layer graphene/h-BN devices, Na3Bi can be grown on a millimetre scale or larger. Currently, graphene-h-BN is limited to only a few micrometres.
Another significant advantage is the potential to use Na3Bi as the conducting channel in a new generation of transistors – one built upon the science of topological insulators. The study was published in Science Advances in December 2017 („Spatial charge inhomogeneity and defect states in topological Dirac semimetal thin films of Na3Bi”).

Next steps & topological transistors
„The discovery of electronically-smooth, thin films of TDS are an important step towards switchable topological transistors,” says FLEET Director Prof Michael Fuhrer.
„Graphene is a fantastic conductor, but it can’t be ‘switched off’, or controlled,” says Prof Fuhrer. „Topological materials, such as Na3Bi, can be switched from conventional insulator to topological insulator by the application of voltage or magnetic field.”
Topological insulators are novel materials that behave as electrical insulators in their interior, but can carry a current along their edges. Unlike a conventional electrical path, such topological edge paths can carry electrical current with near-zero dissipation of energy, meaning that topological transistors can switch without burning energy.
Topological materials were recognised in last year’s Nobel Prize in Physics.
Topological transistors would ‘switch’, just as a traditional transistor. The application of a gate potential would switch the edge paths in a Na3Bi channel between being a topological insulator (‘on’) and a conventional insulator (‘off’).

The bigger picture: energy use in computation
The overarching challenge is the growing amount of energy used in computation and information technology (IT).
Each time a transistor switches, a tiny amount of energy is burnt, and with trillions of transistors switching billions of times per second, this energy adds up. Already, the energy burnt in computation accounts for 5 per cent of global electricity use, and it’s doubling every decade.
For many years, the energy demands of an exponentially growing number of computations was kept in check by ever-more efficient, and ever-more compact computer chips – an effect related to Moore’s Law. But as fundamental physics limits are approached, Moore’s Law is ending, and there are limited future efficiencies to be found.
„For computation to continue to grow, to keep up with changing demands, we need more-efficient electronics,” says Prof Michael Fuhrer. „We need a new type of transistor that burns less energy when it switches.”
„This discovery could be a step in the direction of topological transistors that transform the world of computation.”
Source: ARC Centre of Excellence in Future Low-Energy Electronics Technologies
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Microstructura adezivă în formă de perete biomimetic pentru atașarea indusă de forfecare(Biomimetic wall-shaped adhesive microstructure for shear-induced attachment)

efectele unghiului de tragere și deplasarea preliminară;Până în prezent, o mână de adezivi asemănători gecko-ului inspirat de firele de atașare în formă de spatula au fost sugerate pe baza geometriei pană și a clapetei elementelor de contact

Interface

Drumul umed la baterii rapide și stabile(The wet road to fast and stable batteries)

a descoperit un material cu baterii anodice cu incarcare superficiala si operare stabila pe mai multe mii de cicluri.Echipa de cercetatori a urmarit modul in care compozitia si structura materialului s-au schimbat atunci cand au fost incalzite folosind diferite tehnici avansate de caracterizare, incluzand difractia de raze X furnizata de Advanced Photon Source, o Office DOE Office of Science Facility situat la Argonne. Atunci când analizează datele de caracterizare combinate, echipa a raportat că apa captată din materialul anodului a îmbunătățit performanța prin promovarea diversității structurale și formării nanostructurilor. Videoclipul de mai sus arată modificarea compoziției și a structurii, deoarece materialul de pornire este încălzit și apa este expulzată pentru a forma o nouă structură stratificată (LS), apoi o nanostructură hidratată dorită (HN), apoi dincolo de structura dorită, până la un nivel complet descompunerea nanostructurii (DN).

Posted: Dec 13, 2017

The wet road to fast and stable batteries

(Nanowerk News) An international team of scientists — including several researchers from the U.S. Department of Energy’s (DOE) Argonne National Laboratory — has discovered an anode battery material with superfast charging and stable operation over many thousands of cycles.
Over the last several decades, a multidisciplinary team of battery scientists at Argonne helped launch the modern energy storage race by conducting extensive research on a host of materials for lithium-ion batteries with applications in hybrid and all-electric vehicles, as well as stationary energy storage for buildings and the electric grid.
By heating the anode material to a much lower temperature, scientists could remove the water near the surface, but retain the water in the bulk of the material, which enhanced its characteristics
By heating the anode material to a much lower temperature (less than 260°C), scientists could remove the water near the surface, but retain the water in the bulk of the material, which enhanced its characteristics. (Image: Nature Communications and study authors)
One of Argonne’s latest contributions to innovative new materials for lithium-ion batteries is a water-bearing compound, “lithium titanate hydrate,” developed in collaboration with researchers from Tsinghua University in Beijing and the Massachusetts Institute of Technology. This material, discussed in a recent Nature Communications article („Lithium titanate hydrates with superfast and stable cycling in lithium ion batteries”), could replace the graphite anode commonly used in lithium-ion batteries.
As Jun Lu, Argonne battery scientist and co-author, explained, past research had identified lithium titanate as a promising anode material because of its potential for fast charging and long cycle life, as well as safer operation compared with graphite. In synthesizing this material, researchers used a water-based process that involved a final step of heating the anode material to very high temperature (above 500°C) to drive out the water completely. This step was needed because, during battery operation with this material, the water would react with the electrolyte and degrade performance.
Argonne Distinguished Fellow Khalil Amine, who also co-authored the study, noted that the heating to such a high temperature caused unwanted coarsening and clumping of the structure. The international team found that, by heating the anode material to a much lower temperature (less than 260°C), they could remove the water near the surface, but retain the water in the bulk of the material without coarsening and clumping of the structure. When the scientists tested the material in the laboratory, cycling stability improved and capacity degraded only slightly over 10,000 cycles. The material also charged very quickly — within less than two minutes — the team found. As noted by Jun Lu, “Most of the time, water is bad for non-aqueous lithium-ion batteries. But in this case, it can be downright good.”
The video shows the change in composition and structure as the starting material is heated and water is expelled to form a new layered structure (LS), then the desired hydrated nanostructure (HN), then beyond the desired structure all the way to a completely dehydrated nanostructure (DN).
The research team tracked how material composition and structure changed when heated by using various advanced characterization techniques, including x-ray diffraction provided by the Advanced Photon Source, a DOE Office of Science User Facility located at Argonne. When analyzing the combined characterization data, the team reported that the trapped water in the anode material improved performance by promoting structural diversity and forming nanostructures. The video above shows the change in composition and structure as the starting material is heated and water is expelled to form a new layered structure (LS), then the desired hydrated nanostructure (HN), then beyond the desired structure all the way to a completely dehydrated nanostructure (DN).
Looking to the future, Jun Lu observed that, because water is everywhere in nature and common in chemical synthesis, the fabrication approach reported in this research could open the door to discovery of other high-performance electrode materials.
Source: Argonne National Laboratory

Oamenii de stiinta sugereaza nitrura de titan in loc de aur in optoelectronica(Scientists sug­gest ti­ta­ni­um nit­ri­de in­ste­ad of gold in op­to­e­lec­tro­nics)

Nitridatul de titan are proprietăți excelente anti-coroziune și stabilitate termică, este netoxic și sintetizat ușor și ieftin, iar acest lucru este extrem de important pentru aplicarea sa practică – spre deosebire de aurul și argintul obișnuit

Posted: Dec 12, 2017

Scientists sug­gest ti­ta­ni­um nit­ri­de in­ste­ad of gold in op­to­e­lec­tro­nics

(Nanowerk News) An international team of scientists from Russia, Sweden and the USA suggested replacing gold and silver, which are used in optoelectronic devices, with an inexpensive material of titanium nitride. The results of the study are published in the journal Applied Physics Letters(„Refractory titanium nitride two-dimensional structures with extremely narrow surface lattice resonances at telecommunication wavelengths”).
„Titanium nitride has excellent anti-corrosion and thermal stability properties, it is non-toxic and synthesized easily and cheaply, and this is extremely important for its practical application – unlike commonly used gold and silver”, – says one of the scientists, Ilya Rasskazov from the University of Illinois at Urbana-Champaign.
In order to make optoelectronic devices faster and more accurate, for example, lasers and biosensors for medicine, plasmon resonance is used, in which an electromagnetic wave generated by the action of light spreads all over metal surface.
Plasma resonance can be obtained on noble metals, but they do not allow this effect to be obtained in the telecommunications wavelength range, which is used in most of the digital technology.
„The vast majority of digital technology functions in the telecommunications frequency range, but gold and silver, widely used in the field of plasmonics, don’t provide such an effect”, – said Sergei Polyutov, head of the research at SFU.
Siberian scientists proposed to use titanium nitride instead of gold and silver. Titanium nitride is a substance that is used for gilding church domes, for instance.
Studies of this material have shown that it produces a plasmon resonance with a Q-factor several thousand times greater than gold. This means that it preserves energy better and the wave oscillations do not fade in it longer.
Now the interest of scientists is more practical – introducing the material into the production of optoelectronic devices. In the future, it will be possible to build ultrasensitive sensors for medicine, in infrared vibration spectroscopy.
Source: Siberian Federal University