"Voltaglue" sticks in the wet and hardens when voltage is applied

 

NTU Asst Prof Terry Steele (right) with his researcher Gao Feng have developed a glue that hardens when an electrical voltage is applied A glue that performs at a high-level in wet environments could bring about all sorts of possibilities in areas like surgical care and ship maintenance. A somewhat common approach to this problem has been trying to replicate the freakish ability of mussels to bind themselves to boats and jetties, but a team from Singapore's Nanyang Technological University is coming at it from a slightly different angle by developing a glue that hardens when an electrical charge is applied. As the team's lead scientist Professor Terry Steele points out, most glues don't work when they're wet, in the same way that sticky tape won't stick to a wet surface because the adhesive will stick to the water rather than the surface. Steele and his team have been at work for more than a year, crafting a new form of adhesive that can perform its job in wet conditions, such as underwater or in the human body. They used hydrogels comprising carbon molecules known as carbenes, which are grafted onto tree-shaped plastic surfaces called dendrimers. Applying an electrical charge kicks the carbenes into action and sees them hook onto any nearby surfaces. A particularly promising aspect of this approach is that the length of time the voltage is applied to the gel dictates how many of these hooks the carbenes create. This means by controlling the charge you control the hardness of the glue, and in turn, its suitability for different applications. The team call this process "electrocuring." "For example, if we are gluing metal panels underwater, we want it hard enough to stick for a long time," says Steele. "However, for medical applications, we want the glue to be more rubber-like so it wouldn’t cause any damage to the surrounding soft tissues." Another attribute that could prove a huge plus of the glue, which the team has nicknamed "Voltaglue," may be the ability to reverse the process. That is, to cancel out the glue's adhesive properties to allow for simple dismantling of ship parts, for example, negating the need for nuts and bolts. Steele and his team are now working to reduce the time it takes for the glue to harden, from around half a minute down to just a few seconds. They will also conduct further research into the possibilities of making it "reversible." The research was published in the journal Nature Communications. Source: Nanyang Technological University   http://www.gizmag.com/voltaglue-adhesive-underwater-electricity-voltage/39107/

Compound discovery sets stage for speedier electronic devices

 

Electronics can get faster and better more quickly thanks to the discovery that niobium phosphide has an especially high magnetoresistance – a phenomenon illustrated here in which electrons are deflected from their original direction of flow (green arrow) by a magnetic field (black arrows), increasing electric resistance (Credit: Yulin Chen) A discovery at the Max Planck Institute for Chemical Physics of Solids could pave the way for further leaps forward in the speed of electronic systems. The scientists, who worked in collaboration with colleagues at Helmholtz-Zentrum Dresden-Rossendorf and Radbound University, found that a material called niobium phosphide, which is a compound of transition metal niobium and phosphorus, dramatically increases its resistance in a magnetic field. The material could find use in faster, higher-capacity hard drives and other electronic components. Electronic components such as hard disks typically use layers of different materials in filigree structure (tiny beads and threads of metal soldered onto the surface) to exploit a phenomenon known as magnetoresistance to develop a high electric resistance, which allows for higher density of data and thus greater storage capacity. What happens here is that a tiny amount of electricity causes the charge carriers to deflect via a phenomenon called the Lorentz force, and then that causes electrons to flow in the "wrong" direction – thereby increasing electric resistance and allowing a very precise read of the data that's magnetically stored in a given location. "The faster the electrons in the material move, the greater the Lorentz force and thus the effect of a magnetic field," explains study lead author Binghai Yan. The electrons in this material, niobium phosphide, travel very quickly. Niobium phosphide contains superfast charge carriers, or relativistic electrons, that move at 300 km/s (186 mi/s), which is one-thousandth the speed of light. And that extreme speed allows the resistance to increase by a factor of 10,000. The researchers believe that niobium phosphide has "enormous potential for future applications in information technology" – not only in hard drives but also in many other electronic components that use magnetoresistance to function. A paper describing the study was published in the journal Nature Physics. Source: Helmholtz-Zentrum Dresden-Rossendorf

The secrets of secretion

 

Mon, 06/22/2015 - 11:45am Harvard University. This image shows the self-assembly of the system through phase separation. The liquid droplets appear as bright fluorescent spots. Image: Harvard Paulson School of Engineering and Applied Sciences. Anything you can do, nature can do better. Chemical delivery systems, self-healing cells, non-stick surfaces—nature perfected those long ago. Now, researchers at Harvard Univ. have hacked nature's blueprints to create a new technology that could have broad-reaching impact on drug delivery systems and self-healing and anti-fouling materials. The secret is secretion. Living tissues rely on their ability to package, transport and secrete liquid, where and when it's needed. Nature's secretion system is responsive, self-regulatory, and intrinsically linked with its surroundings but synthetic systems haven't been able to replicate that complexity until now. The new system, described in Nature Materials, was developed in the lab of Joanna Aizenberg, the Amy Smith Berylson Professor of Materials Science at the Harvard John A. Paulson School of Engineering and Applied Science (SEAS). Aizenberg is also professor in the Dept. of Chemistry and Chemical Biology, co-director of the Kavli Institute for Bionano Science and Technology, and a core faculty member at Harvard's Wyss Institute for Biologically Inspired Engineering, leading the Adaptive Materials Technologies platform there. The new system, self-assembled through phase separation, consists of liquid droplets inside a supramolecular polymer gel with a thin layer of liquid on its surface. When the surface liquid is removed or depleted, the droplets spontaneously release only enough fluid to replace what is lost on the surface. "Current fluid secretion technologies are generally designed with one-time only release mechanisms. Once the fluid is released, it continues flowing, at a consistent pace, until the supply is exhausted, regardless of the needs of its surroundings," said Aizenberg. "These kinds of triggered releases aren't responsive to the consumption of fluid. Our system ties fluid secretion to fluid consumption and controls for when and how much liquid is secreted at a time." The feature is an advance for material application, especially the non-stick, slippery material known as SLIPS (Slippery Liquid-Infused Porous Surfaces) developed in the Aizenberg lab. "This system opens the way to create dynamic designer polymers that are capable of self-relubrication and highly regulated and long-lasting anti-fouling behavior," said Jiaxi Cui, a postdoctoral fellow in Aizenberg's lab and a lead author of the paper. Instability is the key to this system's success. Just as in nature, the secretion system developed by Aizenberg and her team is out of equilibrium, unstable enough to adapt and respond to its surroundings. The supramolecular polymers are reversibly bonded to each other, meaning they can come apart to allow the liquid to filter through the matrix, then stitch themselves back together and adjust to the shrinking liquid reserves. This is a major step forward in the design of self-healing materials. "There is a whole class of self-healing polymers out there, but most can't stitch themselves together if there is a big gash down the middle," said Daniel Daniel, a graduate student in Aizenberg's lab and coauthor on the paper. "Polymers can't fly through the air but they can swim through liquid." When this system is cut down the middle, the newly exposed polymer surface signals droplets to secrete liquid, quickly filling in the crack and bridging the ends of the broken polymers. Over time, the polymer strands will swim through the liquid and stitch themselves back together. Like the gauge on a gas tank, the system can self-report its liquid levels. As the fluid is secreted, the gel becomes more transparent. The system could also be used to improve drug delivery systems. Certain drugs, such as those used to treat cancer, are harmful if released all at one. A system that could not only tell a chemical where and when to release, but how quickly to release, could have a major impact on certain treatments. The Aizenberg lab is at the forefront of this kind of adaptable and responsive material science. "For years, material scientists created static materials and then tried to figure out how to get them to change as an afterthought," Aizenberg said. "Now, the direction of material science is to look at nature and recognize that nature has already invented sophisticated responsive, dynamical material systems, and we can take inspiration from that." Source: Harvard University

Natural rubber from dandelions

 

Mon, 06/08/2015 - 10:59am Fraunhofer-Gesellschaft   Left to right: Dr. Christian Schulze Gronover, Dr. Carla Recker (Continental Reifen Deutschland GmbH) and Prof. Dirk Prüfer make use of the Russian dandelion to obtain natural rubber for subsequent use in the manufacture of car tires. Image: Dirk Mahler/FraunhoferDandelions are modest plants that are an excellent alternative source for a raw material of high demand: natural rubber, the fundamental ingredient in rubber products. Fraunhofer researchers have established the basis for the large-scale production of high quality rubber with Russian dandelion. Approximately 40,000 products of everyday life contain natural rubber. It's the material that provides extreme elasticity, tensile strength and low-temperature flexibility in products from mattresses and gloves to adhesive tape and tires. As yet, it has no artificial replacement. However, researchers from the Fraunhofer Institute for Molecular Biology and Applied Ecology IME were able to identify a cost-effective and eco-friendly alternative to the natural rubber tree: the dandelion. Currently, all our natural rubber comes from Hevea brasiliensis, a tree that grows under subtropical climate. Increasing demands and potential problems with a devastating fungus have made natural rubber into a valuable resource. Southeast Asia accounts for 95% of global production. In order to meet growing demands, producers turn rainforest into agricultural land. Now Prof. Dirk Prüfer and his colleague Dr. Christian Schulze Gronover from Fraunhofer IME in Münster are developing Taraxacum kok-saghyz, also known as Russian dandelion, as an efficient replacement for the natural rubber tree. "The plant is extremely resilient, able to grow in moderate climates and even in soil that is not or just barely suited for the cultivation of food and feed crops," explains Christian Schulze Gronover. "Dandelions also have the advantage of growing annually. The natural rubber tree takes between seven and ten years to deliver the first harvest." Dirk Prüfer decided to investigate the dandelion after a sudden insight on a day out. "I was sitting in a meadow in the Sauerland region in Germany, and it was absolutely covered with dandelions. Having plucked the flower off one of them, I was wondering if the expelling white latex contains rubber." However, Germany's native dandelions don't produce sufficient quantities of rubber for being industrially viable. That's why the researchers subsequently turned their attention to the Russian dandelion, which produces large amounts of natural rubber. With the help of precision breeding, the researchers were quickly able to double the amount of natural rubber in the Russian dandelion. This was achieved without genetic modification; instead, Dirk Prüfer and Christian Schulze Gronover analyzed the dandelion's genome and identified suitable DNA markers. These genetic tools could tell already in a very early stage of plant development if a given plant will possess an efficient rubber production. Extraction of natural rubber from the plant was another challenge. To this end, the scientists developed an eco-friendly technique whereby only the roots are pulverized because the leaves contain very little rubber. At the end of the process, water is used to separate the resource from the other substances. New natural rubber successfully undergoes practical testing The performance of tires made of dandelion natural rubber has already proven in action, and manufacturer Continental has tested a first version. "The dandelion natural rubber has ideal material properties. The tires are equivalent to those made from Hevea natural rubber," says Dr. Carla Recker of Continental. Since natural rubber is critical to the quality of many rubber products, industrialized nations in particular regard it as a strategically important resource. Natural rubber obtained from dandelions could reduce the dependence on imports from Asia. However, if the entire world production will be based on dandelion rubber, one would need the size of Austria for its cultivation. Thus, Dirk Prüfer points out that rubber from dandelion will not replace the actual source, but will compensate the additional demand in the future. Source: Fraunhofer-Gesellschaft

Could electric clay be the world's next wonder-material?

 

A fan of conductive clay? (Photo: Drexel University) Image Gallery (8 images) Researchers at Drexel University have hit upon a conductive clay which they claim is an "exceptionally viable candidate" to one day replace the electrode materials used in batteries and supercapacitors. Sure, another day another super material, but MXene, as it's called, does boast some rather intriguing properties. Why's it good, then? You don't beat about the bush, do you? Very well. The three main properties of MXene that caught our eye are: 1. It's hydrophilic. That means, unlike graphene, it loves water. And that's good news because it means... 2. It's very malleable. You can mold it into complicated forms, or roll or press it very flat – both of which are potentially very handy for a material with conductivity supposedly is up there with metals. 3. The material has a very healthy capacitance of 900 F/cm3 – and that's basically at the first attempt. The team has done nothing to improve that performance. But perhaps more intriguing still is the claim that MXene lost no capacitance after more than 10,000 charge cycles. How do you say MXene? The researchers pronounce it mex-een. And this is a new material? Yes and no. MXene was first discovered in 2011. It's a material composed of two-dimensional titanium carbide derived from MAX phases. What's new is the researchers' ability to make it as a clay, a form which is not only useful, but much quicker and safer than the old method, and using readily-available materials. It used to take the researchers a day to make a MXene electrode. Now they can do it in 15 minutes. What does it look like under an electron microscope? This: So we can expect clay batteries from tomorrow, then? No. This is absurdly early stages for the material. And so far we've only read the press release, which tend not to emphasize any shortcomings or problems that may exist. We'll track down a copy of the paper and will update this article if we spot any major caveats. All of that said, the researchers do seem rather excited. "We’re talking about quite a special lump of clay here," said doctoral student Maria Lukatskaya.   Source: Drexel University

Next-Gen Concrete Will Make Buildings That Last 16K Years

 

Concrete is one of the world’s most popular building materials, with over 20 billion tons produced annually around the world. The production of concrete alone accounts for 5 to 10 percent of the world’s overall atmospheric CO2, so cutting down on its production could have major environmental benefits. Civil engineers at MIT have finally discovered what causes “creep,” the deformation of concrete that occurs over time. This discovery is the first step toward prolonging the life of concrete by slowing the rate at which creep occurs.

The MIT team discovered that creep is caused by tiny nano-scale rearrangements of particles. The discovery means that the engineers can use this new understanding of concrete creep to create incredibly long-lasting buildings and infrastructure objects. Until now, concrete structures have had to be pretty thick in order to remain stable over the years. Thin concrete succumbs to creep quickly, so lightweight concrete buildings have always been impossible.

In the future, concrete structures could be lightweight and thin while using far less material than current generations. And because they would be far stronger and longer-lasting than our current concrete structures, they will require fewer repairs and will last longer than any other concrete buildings. In theory, this nano approach to concrete engineering could result in concrete that lasts for thousands of years.

Glowing Starpath Uses Solar Energy to Brighten the Night

 

Pre-dawn jogging is about to get (a little) safer. A UK Company called PRO-TEQ has developed a spray-on coating for pavement that absorbs light during the day and then glows like starlight in the dark.

Starpath can be applied to just about any paved surface. The company touts it as a low-cost alternative to electric lighting. On a path or sidewalk, the glowing material can help prevent injuries by clearly outlining the walking surface even at night.

The Starpath spray is also non-slip and water resistant, further cutting down on injury risk. A number of materials can be mixed with it to provide different effects: rubber crumbs to resurface a playground, or maybe stone to create a fairytale-like pathway.

PRO-TEQ believe that their product is an attractive alternative to outdoor lighting, though we might argue that it isn’t quite bright enough to replace electric lights. It could very well allow for fewer lights to be used, however, which would save public sites money on utilities and provide a kind of magical environment.

Aerogel: See-Through, Strong as Steel & Ligher than Air

 

Despite its incredibly low density, aerogel is one of the most powerful materials on the planet. It can support thousands of times its own weight, block out intense heat, cold and sound – yet it is 1,000 times less dense than glass, nearly as transparent and is composed of %99.8 air. The lowest-density silica-based aerogels are even lighter than air.

Despite its fragility in certain regards and its incredible lack of density, aerogel has amazing thermal, acoustical and electrical insulation properties as illustrated by the images here. A single one-pound block can also support half a ton of weight. NASA continues to find new space-based applications for this incredible material.

An aerogel window one inch thick has the effective insulative capacity of a ten-inch thick glass window system. While it is still expensive and has other limitations, this material – originally developed nearly a century ago but still undergoing experimentation – could prove to be one of the most influential materials of the 21st Century.

Aside from its other capabilities, aerogel also has amazing absorbing abilities. Some speculate it could be the future solution to oil spills. It is also being tested as a possible slow-release drug deliver system for potential human patients.

Scientists come closer to the industrial synthesis of a material harder than diamond

 

 

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Diamond anvils malformed during synthesis of ultrahard fullerite. Note the dent in the center.

Researchers from the Technological Institute for Superhard and Novel Carbon Materials in Troitsk, MIPT, MISiS, and MSU have developed a new method for the synthesis of an ultrahard material that exceeds diamond in hardness. An article recently published in the journal Carbon describes in detail a method that allows for the synthesis of ultrahard fullerite, a polymer composed of fullerenes, or spherical molecules made of carbon atoms.

In their work, the scientists note that diamond hasn't been the hardest material for some time now. Natural diamonds have a hardness of nearly 150 GPa, but ultrahard fullerite has surpassed diamond to become first on the list of hardest materials with values that range from 150 to 300 GPa.

All materials that are harder than diamond are called ultra hard materials. Materials softer than diamond but harder than boron nitride are termed superhard. Boron nitride, with its cubic lattice, is almost three times harder than the well-known corundum.

Fullerites are materials that consist of fullerenes. In their turn, fullerenes are carbon molecules in the form of spheres consisting of 60 atoms. Fullerene was first synthesized more than 20 years ago, and a Nobel Prize was awarded for that work. The carbon spheres within fullerite can be arranged in different ways, and the material's hardness largely depends on just how interconnected they are. In the ultrahard fullerite discovered by the workers at the Technological Institutefor Superhard and Novel Carbon Materials (FSBITISNCM), C ₆₀ molecules are interconnected by covalent bonds in all directions, a material scientists call a three-dimensional polymer.

However, the methods providing production of this promising material on an industrial scale are not available yet. Practically, the superhard carbon form is of primary interest for specialists in the field of metals and other materials processing: the harder a tool is, the longer it works, and the more qualitatively the details can be processed.

What makes synthesizing fullerite in large quantities so difficult is the high pressure required for the reaction to begin. Formation of the three-dimensional polymer begins at a pressure of 13 GPa, or 130,000 atm. But modern equipment cannot provide such pressure on a large scale.

The scientists in the current study have shown that adding carbon disulfide (CS ₂ ) to the initial mixture of reagents can accelerate fullerite synthesis. This substance is synthesized on an industrial scale, is actively used in various enterprises, and the technologies for working with it are well-developed. According to experiments, carbon disulfide is an end product, but here it acts as an accelerator. Using CS ₂ , the formation of the valuable superhard material becomes possible even if the pressure is lower and amounts to 8GPa. In addition, while previous efforts to synthesize fullerite at a pressure of 13 GPa required heating up to 1100K (more than 820 degrees Celsius),in the present case it occurs at room temperature.

"The discovery described in this article (the catalytic synthesis of ultrahard fullerite) will create a new research area in materials science because it substantially reduces the pressure required for synthesis and allows for manufacturing the material and its derivatives on an industrial scale," explained Mikhail Popov, the leading author of the research and the head of the laboratory of functional nanomaterials at FSBI TISNCM.

 

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Story Source:

The above story is based on materials provided by Moscow Institute of Physics and Technology. Note: Materials may be edited for content and length.

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Journal Reference:

1. M. Popov, V. Mordkovich, S. Perfilov, A. Kirichenko, B. Kulnitskiy, I. Perezhogin, V. Blank. Synthesis of ultrahard fullerite with a catalytic 3D polymerization reaction of C60. Carbon, 2014; 76: 250 DOI: 10.1016/j.carbon.2014.04.075

New family of materials for energy-efficient information storage and processing

 

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A schematic illustration of the crystal structure of h-RFeO3. The arrows on the Fe sites indicate the atomic magnetic moments. The coexisting spontaneous electric polarization (P) and magnetic polarization (M) are both along the same crystal direction.

Switching the polarity of a magnet using an electric field (magnetoelectric memory [MEM] effect), can be a working principle of the next-generation technology for information processing and storage. Multiferroic materials are promising candidates for the MEM effect, due to the coexistence of electric and magnetic orders. On the other hand, the coexistence of spontaneous electric and magnetic polarizations is rare in known materials, which hinders the application potential of the MEM effect.

This article briefly reviews a new family of multiferroic materials -- hexagonal rare earth ferrites -- that have been demonstrated ferroelectric and ferromagnetic simultaneously by experiments. Both the ferroeletricity and ferromagnetism in hexagonal ferrites originate indirectly from structural distortions, resulting in so-called improper ferroelectric and ferromagnetic orders. Naturally, structural distortions may mediate the coupling between the electric and magnetic polarizations in hexagonal rare earth ferrites, causing the MEM effect, as predicted by theory.

The possible MEM effect in rare earth hexagonal ferrites is particularly useful for information storage and processing because the non-volatile nature of the magnetic polarization avoids the energy cost of constant memory refreshing and a constant flow of current. The polarity of magnets are used to store information, for example, in the hard disk of computers. The information is modified by "writing" the polarity using a magnetic field, which requires a flow of current that costs significant amount of energy. If the polarity can be switched using an electric field (the MEM effect), the energy-efficiency will be greatly improved, because the generation of the electric field intrinsically needs less power than for generating a magnetic field. The fact that the electric field can be easily localized also suggests application in miniaturized devices.

This research was supported in part by Nebraska EPSCoR.

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Story Source:

The above story is based on materials provided by World Scientific. Note: Materials may be edited for content and length.

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Journal Reference:

1. Xiaoshan Xu, Wenbin Wang. Multiferroic hexagonal ferrites (h-RFeO3,R=Y,Dy-Lu): a brief experimental review. Modern Physics Letters B, 2014; 28 (21): 1430008 DOI: 10.1142/S0217984914300087

A Super-Strong and Lightweight New Material

 

Nanostructured ceramics could be used to build lighter, stronger airplanes and batteries.

• By Katherine Bourzac on September 11, 2014

Why It Matters

The energy efficiency of aircraft, cars, and batteries depends heavily on the weight of the materials used to make them.

Tiny trusses: A scanning electron microscope image of the new material reveals its ceramic nano-lattices.

A new type of material, made up of nanoscale struts crisscrossed like the struts of a tiny Eiffel Tower, is one of the strongest and lightest substances ever made.

If researchers can figure out how to make the stuff in large quantities, it could be used as a structural material for making planes and trucks, as well as in battery electrodes.

Researchers led by Caltech materials scientist Julia Greer found that by carefully designing nanoscale struts and joints, they could make ceramics, metals, and other materials that can recover after being crushed, like a sponge. The materials are very strong and light enough to float through the air like a feather. The work is published today in the journal Science.

In conventional materials, strength, weight, and density are correlated. Ceramics, for example, are strong but also heavy, so they can’t be used as structural materials where weight is critical—for example, in the bodies of cars. And when ceramics fail, they tend to fail catastrophically, shattering like glass.

But at the nanoscale the same rules do not apply. In this size range, the structural and mechanical properties of ceramics become less tied to properties such as weight, and they can be altered more precisely.

“For ceramics, smaller is tougher,” says Greer, who was named one of MIT Technology Review’s 35 Innovators Under 35 in 2008 for her work on nanoscale mechanics. This means that nanoscale trusses made from ceramic materials can be both very light—unsurprising, since they are mostly air—and extremely strong.

In 2011, working with researchers at HRL Laboratories, a private engineering research company, Greer created one of the lightest materials ever made, a microlattice of hollow metal tubes. She later chose to take on the greater challenge of making ceramics with similar properties. This required fine-tuning structures at the nanoscale, meaning the materials are even more difficult to produce.

To make the ceramic nano-trusses, Greer’s lab uses a technique called two-photon interference lithography. It’s akin to a very low-yield 3-D laser printer.

First they use this method to create the desired structure, a lattice, out of a polymer. The polymer lattice is then coated with a ceramic such as alumina. Oxygen plasma etches out the polymer, leaving behind a lattice of hollow ceramic tubes.

Greer’s lab showed that by changing the thickness of the tube walls, it’s possible to control how the material fails. When the walls are thick, the ceramic shatters under pressure as expected. But trusses with thinner walls, just 10 nanometers thick, buckle when compressed and then recover their shape.

“You don’t expect these materials to recover—you expect them to be brittle and to fracture,” says Christopher Spadaccini, an engineer who specializes in materials manufacturing at the U.S. Department of Energy’s Lawrence Livermore National Laboratory in California.

The new materials might be particularly interesting for use in batteries, notes Nicholas Fang, a mechanical engineer at MIT who is also working on nanostructured ceramics. Nanostructures have a very high surface area and are lightweight, a combination that could make for a fast-charging battery that stores a lot of energy in a convenient package. In fact, Greer says she is collaborating with German electronics company Bosch to apply her designs to lithium-ion batteries.

Ultra-stiff and lightweight: Carbon-fiber epoxy honeycombs mimic material performance of balsa wood

 

Like other manufactured products that use sandwich panel construction to achieve a combination of light weight and strength, turbine blades contain carefully arrayed strips of balsa wood from Ecuador, which provides 95 percent of the world's supply.

For centuries, the fast-growing balsa tree has been prized for its light weight and stiffness relative to density. But balsa wood is expensive and natural variations in the grain can be an impediment to achieving the increasingly precise performance requirements of turbine blades and other sophisticated applications.

As turbine makers produce ever-larger blades -- the longest now measure 75 meters, almost matching the wingspan of an Airbus A380 jetliner -- they must be engineered to operate virtually maintenance-free for decades. In order to meet more demanding specifications for precision, weight, and quality consistency, manufacturers are searching for new sandwich construction material options.

Now, using a cocktail of fiber-reinforced epoxy-based thermosetting resins and 3D extrusion printing techniques, materials scientists at the Harvard School of Engineering and Applied Sciences (SEAS) and the Wyss Institute for Biologically Inspired Engineering have developed cellular composite materials of unprecedented light weight and stiffness. Because of their mechanical properties and the fine-scale control of fabrication, the researchers say these new materials mimic and improve on balsa, and even the best commercial 3D-printed polymers and polymer composites available.

A paper describing their results has been published online in the journal Advanced Materials.

Until now, 3D printing has been developed for thermo plastics and UV-curable resins -- materials that are not typically considered as engineering solutions for structural applications. "By moving into new classes of materials like epoxies, we open up new avenues for using 3D printing to construct lightweight architectures," says principal investigator Jennifer A. Lewis, the Hansjörg Wyss Professor of Biologically Inspired Engineering at Harvard SEAS. "Essentially, we are broadening the materials palette for 3D printing."

"Balsa wood has a cellular architecture that minimizes its weight since most of the space is empty and only the cell walls carry the load. It therefore has a high specific stiffness and strength," explains Lewis, who in addition to her role at Harvard SEAS is also a Core Faculty Member at the Wyss Institute. "We've borrowed this design concept and mimicked it in an engineered composite."

Lewis and Brett G. Compton, a former postdoctoral fellow in her group, developed inks of epoxy resins, spiked with viscosity-enhancing nanoclay platelets and a compound called dimethyl methylphosphonate, and then added two types of fillers: tiny silicon carbide "whiskers" and discrete carbon fibers. Key to the versatility of the resulting fiber-filled inks is the ability to control the orientation of the fillers.

The direction that the fillers are deposited controls the strength of the materials (think of the ease of splitting a piece of firewood lengthwise versus the relative difficulty of chopping on the perpendicular against the grain).

Lewis and Compton have shown that their technique yields cellular composites that are as stiff as wood, 10 to 20 times stiffer than commercial 3D-printed polymers, and twice as strong as the best printed polymer composites. The ability to control the alignment of the fillers means that fabricators can digitally integrate the composition, stiffness, and toughness of an object with its design.

"This paper demonstrates, for the first time, 3D printing of honeycombs with fiber-reinforced cell walls," said Lorna Gibson, a professor of materials science and mechanical engineering at the Massachusetts Institute of Technology and one of world's leading experts in cellular composites, who was not involved in this research. "Of particular significance is the way that the fibers can be aligned, through control of the fiber aspect ratio -- the length relative to the diameter -- and the nozzle diameter. This marks an important step forward in designing engineering materials that mimic wood, long known for its remarkable mechanical properties for its weight."

"As we gain additional levels of control in filler alignment and learn how to better integrate that orientation into component design, we can further optimize component design and improve materials efficiency," adds Compton, who is now a staff scientist in additive manufacturing at Oak Ridge National Laboratory. "Eventually, we will be able to use 3D printing technology to change the degree of fiber filler alignment and local composition on the fly.

The work could have applications in many fields, including the automotive industry where lighter materials hold the key to achieving aggressive government-mandated fuel economy standards. According to one estimate, shedding 110 pounds from each of the 1 billion cars on the road worldwide could produce $40 billion in annual fuel savings.

3D printing has the potential to radically change manufacturing in other ways too. Lewis says the next step will be to test the use of thermosetting resins to create different kinds of architectures, especially by exploiting the technique of blending fillers and precisely aligning them. This could lead to advances not only in structural materials, but also in conductive composites.

Previously, Lewis has conducted groundbreaking research in the 3D printing of tissue constructs with vasculature and lithium-ion microbatteries.

Primary support for the cellular composites work came from the BASF North American Center for Research on Advanced Materials at Harvard.

Additional support was provided by the Materials Research Science and Engineering Center at Harvard, funded by the National Science Foundation (DMR 0820484).

Video: https://www.youtube.com/watch?v=pnGPYwNM4rE

New ultrastiff, ultralight material developed

 

This microscope image shows a single unit of the structure developed by the team, called a stretch-dominated octet truss unit cell, made from a polymer using 3-D microstereolithography.

What's the difference between the Eiffel Tower and the Washington Monument? Both structures soar to impressive heights, and each was the world's tallest building when completed. But the Washington Monument is a massive stone structure, while the Eiffel Tower achieves similar strength using a lattice of steel beams and struts that is mostly open air, gaining its strength from the geometric arrangement of those elements.

Now engineers at MIT and Lawrence Livermore National Laboratory (LLNL) have devised a way to translate that airy, yet remarkably strong, structure down to the microscale -- designing a system that could be fabricated from a variety of materials, such as metals or polymers, and that may set new records for stiffness for a given weight.

The new design is described in the journal Science by MIT's Nicholas Fang; former postdoc Howon Lee, now an assistant professor at Rutgers University; visiting research fellow Qi "Kevin" Ge; LLNL's Christopher Spadaccini and Xiaoyu "Rayne" Zheng; and eight others.

The design is based on the use of microlattices with nanoscale features, combining great stiffness and strength with ultralow density, the authors say. The actual production of such materials is made possible by a high-precision 3-D printing process called projection microstereolithography, as a result of the joint research collaboration between the Fang and Spadaccini groups since 2008.

Normally, Fang explains, stiffness and strength declines with the density of any material; that's why when bone density decreases, fractures become more likely. But using the right mathematically determined structures to distribute and direct the loads -- the way the arrangement of vertical, horizontal, and diagonal beams do in a structure like the Eiffel Tower -- the lighter structure can maintain its strength.

A pleasant surprise

The geometric basis for such microstructures was determined more than a decade ago, Fang says, but it took years to transfer that mathematical understanding "to something we can print, using a digital projection -- to convert this solid model on paper to something we can hold in our hand." The result was "a pleasant surprise to us," he adds, performing even better than anticipated.

"We found that for a material as light and sparse as aerogel [a kind of glass foam], we see a mechanical stiffness that's comparable to that of solid rubber, and 400 times stronger than a counterpart of similar density. Such samples can easily withstand a load of more than 160,000 times their own weight," says Fang, the Brit and Alex d'Arbeloff Career Development Associate Professor in Engineering Design. So far, the researchers at MIT and LLNL have tested the process using three engineering materials -- metal, ceramic, and polymer -- and all showed the same properties of being stiff at light weight.

"This material is among the lightest in the world," LLNL's Spadaccini says. "However, because of its microarchitected layout, it performs with four orders of magnitude higher stiffness than unstructured materials, like aerogels, at a comparable density."

Light material, heavy loads

This approach could be useful anywhere there's a need for a combination of high stiffness (for load bearing), high strength, and light weight -- such as in structures to be deployed in space, where every bit of weight adds significantly to the cost of launch. But Fang says there may also be applications at smaller scale, such as in batteries for portable devices, where reduced weight is also highly desirable.

Another property of these materials is that they conduct sound and elastic waves very uniformly, meaning they could lead to new acoustic metamaterials, Fang says, that could help control how waves bend over a curved surface.

Others have suggested similar structural principles over the years, such as a proposal last year by researchers at MIT's Center for Bits and Atoms (CBA) for materials that could be cut out as flat panels and assembled into tiny unit cells to make larger structures. But that concept would require assembly by robotic systems that have yet to be developed, says Fang, who has discussed this work with CBA researchers. This technique, he says, uses 3-D printing technology that can be implemented now.

Martin Wegener, a professor of mechanical engineering at Karlsruhe Institute of Technology in Germany who was not involved in this research, says, "Achieving metamaterials that are ultralight in weight, yet stiffer than you would expect from usual scaling laws for elastic solids, is of obvious technological interest. The paper makes an interesting contribution in this direction."

The work was supported by the U.S. Defense Advanced Research Projects Agency and LLNL.