книги / Обучение аннотированию научных статей на английском языке
..pdfquake-resistant concrete, according to Rouzbeh Shahsavari, an assistant professor of civil and environmental engineering.
The Rice lab ran more than 20 computer simulations of how polymers and cement molecules come together at the nanoscale and what drives their adhesion. The researchers showed that the proximity of oxygen and hydrogen atoms is the critical factor in forming a network of weak hydrogen bonds that connects soft and hard layers. Common polyacrylic acid (PAA) proved best at binding the overlapping layers of cement crystals with an optimal overlap of about 15 nanometers.
"This information is important to make the best synthetic composites," said Shahsavari, who ran the project with Rice graduate student Navid Sakhavand. "A modern engineering approach to these materials will have a large impact on society, especially as we build new and replace aging infrastructure."
The lab's results appear in Applied Physics Letters.
While engineers understand that adding polymers improves cement by blocking the damaging effects of "aggressive" ions that invade its pores, details about how the materials interact at the molecular scale have remained unknown, Shahsavari said. To find out, the researchers modeled composites with PAA as well as polyvinyl alcohol (PVA), both soft matrix materials that have been used to improve cement.
They discovered that the two different oxygen atoms in PAA (as opposed to one in PVA) allowed it to receive and donate ions as it bonded with hydrogen in the crystals of tobermorite cement. Oxygen in PAA had eight ways to bond with hydrogen (six for PVA) and could also participate in salt bridging between the polymer and cement, which makes the bonding network even more complex.
The researchers tested their simulated structures by sliding layers of polymer and cement against each other and found that complexity allowed the bonds between PAA and cement to break and reconnect more frequently as the material was stressed, which significantly increases its toughness, the ability to deform without fracturing. This allowed the researchers to determine the optimum overlap between cement crystals.
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"In contrast to the common intuition that hydrogen bonds are weak, when the right number of them ‒ the optimum overlap ‒ cooperate, they provide sufficient connectivity in the composite to confer high strength and high toughness," Shahsavari said. "From an experimental standpoint, this can be done by carefully tuning and controlling the addition of the polymers with the right molecular weight while controlling cement mineral formation. Indeed, a recent experimental paper by our colleagues showed a proof of concept toward this strategy."
Rice University. "Weak hydrogen bonds key to strong, tough infrastructure: Rice University lab simulates polymer-cement composites to find strongest, toughest materials."
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BUILDING BLOCKS TO CREATE METAMATERIALS
Date:
January 18, 2018
Source:
California Institute of Technology
Engineers have created a method to systematically design metamaterials using principles of quantum mechanics. New design method could unlock the potential of materials that manipulate waves.
Engineers at Caltech and ETH Zürich in Switzerland have created a method to systematically design metamaterials using principles of quantum mechanics.
Their work could pave the way for wider use of metamaterials in more mainstream applications by creating a purpose-driven framework for their design.
Metamaterials are engineered materials that exploit the geometry of their internal structure to manipulate incoming waves. For example, a metamaterial that manipulates electromagnetic waves might bend light in an unusual way to create a cloaking device. Meanwhile, a wafer-thin acoustic metamaterial might reflect incoming sound waves to soundproof a room.
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This ability to control waves derives from how the material is structured, often on a microscopic scale. In 2010, Caltech researchers developed an optical metamaterial that uses a surface coated with three-dimensional structures to redirect light as desired. More recently, engineers at Caltech showed that flat surfaces coated with tiny pillars of silicon could focus light like a lens.
Picture a crystal ‒ a solid whose physical properties are determined by the way it is built from a repeating series of atomic structures. Carbon atoms structured in flat plates create crumbly graphite, while carbon atoms structured in tetrahedra create ultra-hard diamonds. Similarly, metamaterials are constructed from a repeating series of nanoand microscale structures that give them their unique properties.
Despite their promise and wide array of possible applications, metamaterials will not be used widely unless engineers can design them to have particular desired properties. While much progress has been made in the design of metamaterials that interact with electromagnetic waves, overall, the design of mechanical metamaterials ‒ those that influence mechanical waves, such as sound waves or seismic waves ‒ remains a scattershot affair, says Chiara Daraio, a professor of mechanical engineering and applied physics at Caltech.
"Before our work, there was no single, systematic way to design metamaterials that control mechanical waves for different applications," she says. "Instead, people often optimized a design to fulfill a specific purpose, or tried out new designs based on something they saw in nature, and then studied what properties would arise from repeated patterns."
To address this, a team led by Daraio and consisting of graduate students Marc Serra Garcia and Antonio Palermo, postdoctoral scholar Katie Matlack, and professor Sebastian Huber at ETH Zürich, turned to the field of quantum mechanics. On the surface, the choice was an unlikely one. Quantum mechanics governs the oftencounterintuitive behavior of subatomic particles, and would seem to have no bearing on the microand macro-scale designs of the metamaterials studied by Daraio's team.
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Quantum mechanics predicts the existence of certain exotic types of matter: among them, a "topological insulator" that conducts electricity across its surface while acting as an insulator in its interior. Daraio's team realized that they could build macro-scale versions of these exotic systems that could conduct and insulate against vibrations instead of electricity by using principles of quantum mechanics. In quantum mechanics, materials can sometimes be described as an ensemble of interacting particles. "Imagine that each particle is a tiny mass, connected to its neighbors by springs," she explains. "Each particle reacts to incoming waves in a unique way that is determined, in part, by the reaction of its neighbors. In our approach, we apply this mass-and-spring model to macroscopic, elastic materials, maintaining their characteristic properties."
Because metamaterials are built from arrays of geometrical structures (that can have building blocks at the nano-, micro-, or macro-scale) that are connected in repeating patterns, Daraio and her colleagues realized that, by representing each repeating structure as an ensemble of particles, it would be possible to design many different types of metamaterials, like waveguides, acoustic lenses, or vibration insulators.
When struck by an incoming wave, each repeating structure in a metamaterial has the potential to deform in a number of different ways. That deformation is governed not only by the geometry of that structure, but also by how the structures are connected and how the other structures around them are reacting. Treating this as a system of masses and springs, Daraio's team was able to predict how these systems would react, and then engineer them to react in desired ways.
It is complicated, but also predictable ‒ which is the important part. As a theoretical proof of concept, Daraio's team designed
metamaterials made from a series of rectangular millimeter-scale plates, each loosely connected to one another like a piece of a puzzle. By tuning the design of the plates and how well-connected the plates were, the team created a perfect acoustic lens that focuses sound without loss of signal. The plates also act as a waveguide that directs and slows the propagation of sound. The method could be used to design
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many other devices or sensors where high sensitivity, precision, or control are necessary, Daraio says. The work was published in Nature Materials on January 15, 2018.
Though Daraio's work is theoretical, validated using computer simulations, her coauthors at ETH used the method to design and build a 10 by 10-centimeter silicon wafer that consists of 100 small plates connected to each other via thin beams. When the wafer is stimulated using ultrasound, only the plates in the corners vibrate; the other plates remain still, despite their connections. The device could be used as a precise waveguide in a communications network. Their work was published in Nature on January 15, 2018.
The design process described can also be used to design optical metamaterials, antennas, and optical signal processing devices, says Daraio. Their paper is titled "Designing perturbative metamaterials from discrete models." This work was funded by ETH Zürich and the Swiss National Science Foundation.
Текст 3
NOVEL 3-D PRINTING TECHNIQUE YIELDS
HIGH-PERFORMANCE COMPOSITES
Date:
January 15, 2018
Source:
Harvard John A. Paulson School of Engineering and Applied Sciences
Arranging fibers just like nature does it.
A team of researchers has demonstrated a novel 3-D printing method that yields unprecedented control of the arrangement of short fibers embedded in polymer matrices. They used this additive manufacturing technique to program fiber orientation within epoxy composites in specified locations, enabling the creation of structural materials that are optimized for strength, stiffness, and damage tolerance.
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Nature has produced exquisite composite materials – wood, bone, teeth, and shells, for example - that combine light weight and density with desirable mechanical properties such as stiffness, strength and damage tolerance.
Since ancient civilizations first combined straw and mud to form bricks, people have fabricated engineered composites of increasing performance and complexity. But reproducing the exceptional mechanical properties and complex microstructures found in nature has been challenging.
Now, a team of researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) has demonstrated a novel 3D printing method that yields unprecedented control of the arrangement of short fibers embedded in polymer matrices. They used this additive manufacturing technique to program fiber orientation within epoxy composites in specified locations, enabling the creation of structural materials that are optimized for strength, stiffness, and damage tolerance.
Their method, referred to as "rotational 3D printing," could have broad ranging applications. Given the modular nature of their ink designs, many different filler and matrix combinations can be implemented to tailor electrical, optical, or thermal properties of the printed objects.
"Being able to locally control fiber orientation within engineered composites has been a grand challenge," said the study's senior author, Jennifer A. Lewis, Hansjorg Wyss Professor of Biologically Inspired Engineering at Harvard SEAS. "We can now pattern materials in a hierarchical manner, akin to the way that nature builds." Lewis is also a Core Faculty Member of the Wyss Institute for Biologically Inspired Engineering at Harvard.
The work, described in the journal PNAS, was carried out in the Lewis lab at Harvard. Collaborators included then-postdoctoral fellows Brett Compton (now Assistant Professor in Mechanical Engineering at the University of Tennessee, Knoxville), and Jordan Raney (now Assistant Professor of Mechanical Engineering and Applied Me-
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chanics at the University of Pennsylvania); and visiting PhD student Jochen Mueller from Prof. Kristina Shea's lab at ETH Zurich.
The key to their approach is to precisely choreograph the speed and rotation of a 3D printer nozzle to program the arrangement of embedded fibers in polymer matrices. This is achieved by equipping a rotational printhead system with a stepper motor to guide the angular velocity of the rotating nozzle as the ink is extruded.
"Rotational 3D printing can be used to achieve optimal, or near optimal, fiber arrangements at every location in the printed part, resulting in higher strength and stiffness with less material, "Compton said. "Rather than using magnetic or electric fields to orient fibers, we control the flow of the viscous ink itself to impart the desired fiber orientation."
Compton noted that the team's nozzle concept could be used on any material extrusion printing method, from fused filament fabrication, to direct ink writing, to large-scale thermoplastic additive manufacturing, and with any filler material, from carbon and glass fibers to metallic or ceramic whiskers and platelets.
The technique allows for the 3D printing of engineered materials that can be spatially programmed to achieve specific performance goals. For example, the orientation of the fibers can be locally optimized to increase the damage tolerance at locations that would be expected to undergo the highest stress during loading, hardening potential failure points.
"One of the exciting things about this work is that it offers a new avenue to produce complex microstructures, and to controllably vary the microstructure from region to region," Raney said. "More control over structure means more control over the resulting properties, which vastly expands the design space that can be exploited to optimize properties further."
"Biological composite materials often have remarkable mechanical properties: high stiffness and strength per unit weight and high toughness. One of the outstanding challenges of designing engineering materials inspired by biological composites is control of fiber orienta-
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tion at small length scales and at the local level," said Lorna J. Gibson, Professor of Materials Science and Engineering at MIT, who was not involved in the research. "This remarkable paper from the Lewis group demonstrates a way of doing just that. This represents a huge leap forward in the design of bio-inspired composites.
Текст 4
MAGNETIC LIQUIDS IMPROVE ENERGY
EFFICIENCY OF BUILDINGS
Date:
January 16, 2018
Source:
Friedrich Schiller University Jena
Climate protection and the reduction of carbon dioxide emissions have been on top of global development agendas. Accordingly, research and development projects have been conducted on national and international levels, which aim for the improvement of the CO2footprint in diverse processes. Apart from particularly energyintensive sectors of the industry, the building sector in particular is among the biggest CO2-emmitters: from residential homes, manufacturing facilities and storage depots to big commercial buildings, about 40 percent of the energy consumption within the EU are due to the heating, cooling, air conditioning and lighting of buildings.
Considering next-generation smart windows and façade devices, one aspect of this problem is addressed in the research project LargeArea Fluidic Windows (LaWin) which has been coordinated at the Friedrich Schiller University Jena, Germany, since 2015. A new type of such smart windows was now presented in the upcoming issue of 'Advanced Sustainable Systems'. In their paper 'Large-Area Smart Window with Tunable Shading and Solar-Thermal Harvesting Ability Based on Remote Switching of a Magneto-Active Liquid' the Jena materials researchers introduce prototypes of a window that changes
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its light permeability at the touch of a button, and, at the same time, can be used for solar-thermal energy harvesting.
Liquids in windows and facades
"Our project's key feature is the use of liquids in windows and façades, for example, as heat carriers or to enable additional functions," explains Lothar Wondraczek, the project's coordinator. "To this end we develop new glass materials, into which large-area channel structures are integrated. These are used for circulating functional fluids."
In latest prototypes, the liquid is loaded with the nanoscale magnetic iron particles. These can be extracted from the liquid with the help of a magnet. Vice versa, they can be re-suspended by simply switching-off the magnet. "Depending on the number of the iron particles in the liquid, the liquid itself takes on different shades of grey, or it will even turn completely black," Wondraczek explains. "Then, it becomes possible to automatically adjust the incidence of light, or to harvest solar heat which can then be put to further use within the building." The efficiency in terms of heat gain per area is comparable with that of state-of-the-art solar thermal facilities. But unlike those, the present system can be readily integrated in a vertical façade. Switching between on and off - the release or capture of particles - happens in a separate tank. An electrical connection at the windows is not necessary.
Indoor air conditioning, tunable shading and harvesting of solar heat
"The greatest advantage of large-scale fluidic windows is that they can substitute air conditioning systems, daylight regulation systems and for instance warm water processing," stresses Wondraczek, who holds the chair of Glass Chemistry at the University of Jena. Developing cost-effective large-size window glass modules is key. On the one hand the glass elements need to include the channels, on the other hand they maintain their performance over the whole lifespan of the building. Finally, they have to provide the ability for integration with standard window manufacture technologies in frames of double or triple glazings. With the present prototypes which were manufac-
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tured on a scale of around 200 square meters, the research consortium demonstrated that those requirements can be fulfilled.
Over the period of 2015-2017, the project received a grant of 5.9 million Euros from the European Union within the framework of the Horizon-2020-Programme for Industrial Leadership. A further 2.2 million Euro have been added by eleven industry partners who have been members of the consortium. After the end of the first funding period, commercialisation of first applications is planned for this year.
Текст 5
HIGHLY STRETCHABLE AQUEOUS BATTERIES
Date:
January 26, 2018
Source:
Ulsan National Institute of Science and Technology (UNIST) The current development of stretchable battery materials that
mimic the functions of nature has emerged as a highly interesting research area, necessary for the next wave of wearable electronics.
A recent study, affiliated with UNIST has presented a bioinspired Jabuticaba-like hybrid carbon/polymer (HCP) composite that was developed into a stretchable current collector using a simple and costeffective solution process. Using the HCP composite as a stretchable current collector, the research team has, for the first time, developed a highly stretchable rechargeable lithium-ion battery (ARLB) based on aqueous electrolytes.
This breakthrough has been led by Professor Soojin Park in the School of Energy and Chemical Engineering in collaboration with Professor Kwanyong Seo and Professor So Youn Kim in the School of Energy and Chemical Engineering at UNIST.
Stretchable electronic devices have recently attracted tremendous attention as next-generation devices due to their immense flexibility. The increasing interest and demand of flexible electronics has fueled the search for highly stretchable electrodes with high mechanical durability
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