Metamaterials: Researchers Design Mystifying Materials

Engineerblogger
May 27, 2012

Adilson E. Motter

It’s not magic, but new materials designed by two Northwestern University researchers seem to exhibit magical properties. Some contract when they should expand, and others expand when they should contract.

When tensioned, ordinary materials expand along the direction of the applied force. The new metamaterials (artificial materials engineered to have properties that may not be found in nature) do the opposite when tensioned -- they contract. Other materials designed by the researchers expand when compressed.

“Materials are networks of connected constituents, and when you apply tension or pressure, they can respond in surprising ways,” said Adilson E. Motter, the Harold H. and Virginia Anderson Professor of Physics and Astronomy at Northwestern’s Weinberg College of Arts and Sciences.

“Think of a piece of rod that you tension by pulling its ends with your fingers,” he said. “It would normally get longer, but for these materials it will get shorter.”

Motter and Zachary G. Nicolaou applied network concepts to design the new materials, all of which exhibit negative compressibility transitions. Their results are published this week in Nature Materials. Nicolaou, an undergraduate physics student at Northwestern when the work was done, now is a first-year graduate student at Caltech.

Different types of metamaterials already have led to interesting applications such as superlenses, visibility cloaks and acoustic shields. But no existing material or metamaterial was previously shown to exhibit negative compressibility transitions.

These metamaterials may enable new applications, including the development of new protective mechanical devices and actuators (a type of assembly for operating or controlling a system), and the enhancement of microelectromechanical systems.

The materials also exhibit force amplification, a phenomenon in which a small increase in deformation leads to an abrupt increase in the response force. The latter can be useful for the design of micro-mechanical controls, ratchets and force amplifiers.

All known materials deform along the direction of a constant applied force by expanding when they are tensioned and contracting when they are compressed. Owing to stability considerations, such contraction of a material in the same direction of an applied tension (in response to tension) cannot occur continuously. Possibly because of this, most people would intuitively expect that contraction in response to tension would be impossible.

The important point of the Northwestern study is that such a counter-intuitive response can occur discontinuously, namely, through something known by physicists as a phase transition. A familiar form of phase transition is the transformation of water into ice or vapor. Phase transitions allow for abrupt changes in the physical properties of a material. Yet, all conventional materials are such that phase transitions will lead to ordinary compressibility.

“This research shows that new materials, in fact, can be created to exhibit a phase transition during which the material undergoes contraction when tensioned or expansion when pressured,” Motter said. “We refer to such transformations as ‘negative compressibility transitions.’”

Materials with such properties have not been discovered in nature, but they can be constructed as metamaterials. Metamaterials are engineered materials that gain their properties from structure rather than composition. The relevant building blocks of such materials are not necessarily microscopic, atomic-sized objects, but may in fact be composed of a large number of atoms and hence be mesoscopic or macroscopic in size.

A key step for the discovery of the materials in this study was the representation of the material as a network of interacting particles.

“We were inspired by the observation that the realized equilibrium is not necessarily optimal in a decentralized network,” Motter said. “A conceptual precedent to this is the now 45-year-old insight from German mathematician Dietrich Braess that adding a road to a traffic network may increase rather than decrease the average travel time.”

Analogous effects also have been identified in physical networks, including an increase of current upon the removal of an intermediate conductor in electric networks. These are examples in which the equilibrium realized by the system can be brought closer to the optimum by constraining the structure of the network.

“Our materials are devised such that an analogous phenomenon occurs spontaneously, in response to a change in the external force rather than in the structure of the network,” Motter said.

Motter also is a faculty member in the department of engineering sciences and applied mathematics at the McCormick School of Engineering and Applied Science and an executive committee member of the Northwestern Institute on Complex Systems (NICO).

The Materials Research Science and Engineering Center at Northwestern University and the National Science Foundation supported the research.


Source: Northwestern University

Additional Information:  

• Mechanical Metamaterials With Negative Compressibility Transitions

Taking Solar Technology up a Notch: New Solar Cell Shines with Potential

Engineerblogger
May 27, 2012

Robert Chang

The limitations of conventional and current solar cells include high production cost, low operating efficiency and durability, and many cells rely on toxic and scarce materials. Northwestern University researchers have developed a new solar cell that, in principle, will minimize all of these solar energy technology limitations.

In particular, the device is the first to solve the problem of the Grätzel cell, a promising low-cost and environmentally friendly solar cell with a significant disadvantage: it leaks. The dye-sensitized cell’s electrolyte is made of an organic liquid, which can leak and corrode the solar cell itself.

Grätzel cells use a molecular dye to absorb sunlight and convert it to electricity, much like chlorophyll in plants. But the cells typically don’t last more than 18 months, making them commercially unviable. Researchers have been searching for an alternative for two decades.

At Northwestern, where interdisciplinary collaboration is a cornerstone, nanotechnology expert Robert P. H. Chang challenged chemist Mercouri Kanatzidis with the problem of the Grätzel cell. Kanatzidis’ solution was a new material for the electrolyte that actually starts as a liquid but ends up a solid mass. Thus, the new all solid-state solar cell is inherently stable.

“The Grätzel cell is like having the concept for the light bulb but not having the tungsten wire or carbon material,” said Kanatzidis, of the need to replace the troublesome liquid. “We created a robust novel material that makes the Grätzel cell concept work better. Our material is solid, not liquid, so it should not leak or corrode.”

Postdoctoral fellow In Chung in the Kanatzidis group worked closely with graduate student Byunghong Lee in the Chang group to develop the new cells, achieving performance gains that amounted to approximately 1 percent per month.

In the Northwestern cell, a thin-film compound made up of cesium, tin and iodine, called CsSnI3, replaces the entire liquid electrolyte of the Grätzel cell. Details of the new solar cell -- an efficient, more stable and longer lasting cell -- will be published May 24 by the journal Nature.

Kanatzidis, the Charles E. and Emma H. Morrison Professor of Chemistry in the Weinberg College of Arts and Sciences, and Chang, a professor of materials science and engineering at the McCormick School of Engineering and Applied Science, are the two senior authors of the paper.

“This is the first demonstration of an all solid-state dye-sensitized solar cell system that promises to exceed the performance of the Grätzel cell,” Chang said. “Our work opens up the possibility of these materials becoming state of the art with much higher efficiencies than we’ve seen so far.”

The Northwestern cell exhibits the highest conversion efficiency (approximately 10.2 percent) so far reported for a solid-state solar cell equipped with a dye sensitizer. This value is close to the highest reported performance for a Grätzel cell, approximately 11 to 12 percent. (Conventional solar cells made from highly purified silicon can convert roughly 20 percent of incoming sunlight.)

Unlike the Grätzel cell, the new solar cell uses both n-type and p-type semiconductors and a monolayer dye molecule serving as the junction between the two. Each nearly spherical nanoparticle, made of titanium dioxide, is an n-type semiconductor. Kanatzidis’ CsSnI3 thin-film material is a new kind of soluble p-type semiconductor.

“Our inexpensive solar cell uses nanotechnology to the hilt,” Chang said. “We have millions and millions of nanoparticles, which gives us a huge effective surface area, and we coat all the particles with light-absorbing dye.”

A single solar cell measures half a centimeter by half a centimeter and about 10 microns thick. The dye-coated nanoparticles are packed in, and Kanatzidis’ new material, which starts as a liquid, is poured in, flowing around the nanoparticles. Much like paint, the solvent evaporates, and a solid mass results. The sunlight-absorbing dye, where photons are converted into electricity, lies right between the two semiconductors.

Chang chose to use nanoparticles approximately 20 nanometers in diameter. This size optimizes the device, he said, increasing the surface area and allowing enough space between the particles for Kanatzidis’ material to flow through and set.

Technically, this new cell is not really a Grätzel cell since the hole-conducting material CsSnI3 is itself light absorbing. In fact, the material absorbs more light over a wider range of the visible spectrum than the typical dye used in Grätzel cells. In the Kanatzidis-Chang cell, the CsSnI3 plays an additional role in the operation of the cell that is not played by the liquid electrolyte couple, and that role is light absorption.

“This is only the beginning,” Chang said. “Our concept is applicable to many types of solar cells. There is a lot of room to grow.”

The lightweight thin-film structures are compatible with automated manufacturing, the researchers point out. They next plan to build a large array of the solar cells.

The paper is titled “All-Solid-State Dye-Sensitized Solar Cells With High Efficiency.” In addition to Kanatzidis, Chang, Chung and Lee, the other author of the paper is Jiaqing He, from Northwestern.

The National Science Foundation, the U.S. Department of Energy and the Initiative for Energy and Sustainability at Northwestern (ISEN) supported the research.
Source:  Northwestern University

Additional Information:

• All-solid-state dye-sensitized solar cells with high efficiency

Microreactors to produce explosive materials

Engineerblogger
May 27, 2012

Microreactors can e.g. be used to produce explosive materials much more safely. © Fraunhofer ICT

The larger the reaction vessel, the quicker products can be made – or so you might think. Microreactors show just how wrong that assumption is: in fact, they can be used to produce explosive materials – nitroglycerine, for instance – around ten times faster than in conventional vessels, and much more safely as well. At the ACHEMA trade fair, held June 18-22 in Frankfurt, researchers will demonstrate microreactors they use for a very broad range of chemical processes.

If the task is to tunnel through a mountain, workers turn to explosives: the 15-kilometer-long Gotthard Tunnel, for instance, was created by blasting through the rock with explosive gelatin made largely out of the nitroglycerine – better-known as dynamite. Producing these explosives calls for extreme caution. After all, no one wants a demonstration of explosive force in the lab. Because the production process generates heat, it must proceed slowly: drop for drop, the reagents are added to the agitating vessel that holds the initial substance. A mixture that heats up too suddenly can cause an explosion. The heat generated cannot be permitted to exceed the heat dissipated.

Researchers at the Fraunhofer Institute for Chemical Technology ICT in Pfinztal have developed a method for safer production of nitroglycerine: a microreactor process, tailored to this specific reaction. What makes the process safer are the tiny quantities involved. If the quantities are smaller, less heat is generated. And because the surface is very expansive compared to the volume involved, the system is very easy to cool. Another benefit: the tiny reactor produces the explosive material considerably faster than in agitating vessels. Unlike a large agitating vessel filled before the slow reaction proceeds, the microreactor works continuously: the base materials flow through tiny channels into the reaction chamber in “assembly-line fashion“. There, they react with one another for several seconds before flowing through other channels into a second microreactor for processing – meaning purification. This is because the interim product still contains impurities that need to be removed for safety reasons. Purification in the microreactor functions perfectly: the product produced meets pharmaceutical specifications and in a modified form can even be used in nitro capsules for patients with heart disease. “This marks the first use of microreactors in a process not only for synthesis of a material but also for its subsequent processing,“ observes Dr. Stefan Löbbecke, deputy division director at ICT. The microreactor process is already successfully in use in industry.

When developing a microreactor, researchers match the reactors to the reaction desired: how large may the channels be to ensure that the heat generated can be dissipated effectively? Where do researchers need to build impediments into the channels to ensure that the fluids are well blended and the reaction proceeds as planned? Another important parameter is the speed with which the liquids flow through the channels: on the one hand, they need enough time to react with one another, while on the other the reaction should come to an end as soon as the product is formed. Otherwise, the result might be too many unwanted by-products.

While microreactors suggest themselves for explosive materials, this is not the only conceivable application: researchers at ICT build reactors for every chemical reaction conceivable – and each is tailored to the particular reaction involved. Just one of numerous other examples is a microreactor that produces polymers for OLEDs. OLEDs are organic light-emitting diodes that are particularly common in displays and monitors. The polymers of which the OLEDs are made light up in colors. Still, when they are produced – synthesized – imperfections easily arise that rob the polymers of some of their luminosity. “Through precise process management, we are able to minimize the number of these imperfections,“ Löbbecke points out. To accomplish this, researchers first analyzed the reaction in minute detail: When do the polymers form? When do the imperfections arise? How fast does the process need to be? “As it turns out, many of the reaction protocol that people are familiar with from batch processes are unnecessary. Often, the base materials don‘t need to boil for hours at a time; in many cases all it takes is a few seconds,“ the researcher has found. Long periods spent boiling can cause the products to decompose or generate unwanted byproducts.

To develop and perfect a microreactor for a new reaction, the researchers study the ongoing reaction in real time – peering into the reactor itself, so to speak. Various analytical procedures are helpful in this regard: some, such as spectroscopic techniques, reveal which kinds of products are created in the reactor – and thus how researchers can systematically increase yields of the desired product, possibly even preventing by products from forming in the first place. Other analytical methods, such as calorimetry, provide scientists with information about the heat released over the course of a reaction. This measurement method tells them how quickly and completely the reaction is proceeding. It also provides an indication of how the process conditions need to be selected to ensure that the reaction proceeds safely. Researchers will be presenting a variety of microreactors, microreactor processes and process-analytical techniques at the ACHEMA trade fair from June 18-22 in Frankfurt.

Source: Fraunhofer-Gesellschaft

Bright future for solar power in space

Engineerblogger
May 19, 2012

Dr Massimiliano Vasile

Solar power gathered in space could be set to provide the renewable energy of the future thanks to innovative research being carried out by engineers at the University of Strathclyde, Glasgow.


Researchers at the University have already tested equipment in space that would provide a platform for solar panels to collect the energy and allow it to be transferred back to earth through microwaves or lasers.

This unique development would provide a reliable source of power and could allow valuable energy to be sent to remote areas in the world, providing power to disaster areas or outlying areas that are difficult to reach by traditional means.

Dr Massimiliano Vasile, of the University of Strathclyde’s Department of Mechanical and Aerospace Engineering, who is leading the space based solar power research, said: “Space provides a fantastic source for collecting solar power and we have the advantage of being able to gather it regardless of the time of the day or indeed the weather conditions.

“In areas like the Sahara desert where quality solar power can be captured, it becomes very difficult to transport this energy to areas where it can be used. However, our research is focusing on how we can remove this obstacle and use space based solar power to target difficult to reach areas.

“By using either microwaves or lasers we would be able to beam the energy back down to earth, directly to specific areas. This would provide a reliable, quality source of energy and would remove the need for storing energy coming from renewable sources on ground as it would provide a constant delivery of solar energy.

“Initially, smaller satellites will be able to generate enough energy for a small village but we have the aim, and indeed the technology available, to one day put a large enough structure in space that could gather energy that would be capable of powering a large city.”

Last month, a team of science and engineering students at Strathclyde developed an innovative ‘space web’ experiment which was carried on a rocket from the Arctic Circle to the edge of space.

The experiment, known as Suaineadh – or ‘twisting’ in Scots Gaelic, was an important step forward in space construction design and demonstrated that larger structures could be built on top of a light-weight spinning web, paving the way for the next stage in the solar power project.

Dr Vasile added: “The success of Suaineadh allows us to move forward with the next stage of our project which involves looking at the reflectors needed to collect the solar power.

“The current project, called SAM (Self-inflating Adaptable Membrane) will test the deployment of an ultra light cellular structure that can change shape once deployed. The structure is made of cells that are self-inflating in vacuum and can change their volume independently through nanopumps.

“The structure replicates the natural cellular structure that exists in all living things. The independent control of the cells would allow us to morph the structure into a solar concentrator to collect the sunlight and project it on solar arrays. The same structure can be used to build large space systems by assembling thousands of small individual units.”

The project is part of a NASA Institute for Advanced Concepts (NIAC) study led by Dr John Mankins of Artemis Innovation. The University of Strathclyde represents the European section of an international consortium involving American researchers, and a Japanese team, led by Professor Nobuyuki Kaya of the University of Kobe, a world leader in wireless power transmission.

The NIAC study is demonstrating a new conceptual design for large scale solar power satellites. The role of the team at the University of Strathclyde is to develop innovative solutions for the structural elements and new solutions for orbit and orbit control.

Source: University of Strathclyde

Additional Information:

• Advanced Space Concepts Laboratory
• Centre for Future Air-Space Transportation Technology (cFASTT)
• Suaineadh

Prosthetic retina offers simple solution to restoring sight

Engineerblogger
May 19, 2012

Credit: University of Strathclyde

A device which could restore sight to patients with one of the most common causes of blindness in the developed world is being developed in an international partnership.

Researchers from the University of Strathclyde and Stanford University in California are creating a prosthetic retina for patients of age related macular degeneration (AMD), which affects one in 500 patients aged between 55 and 64 and one in eight aged over 85.

The device would be simpler in design and operation than existing models. It acts by electrically stimulating neurons in the retina, which are left relatively unscathed by the effects of AMD while other ‘image capturing’ cells, known as photoreceptors, are lost.

It would use video goggles to deliver energy and images directly to the eye and be operated remotely via pulsed near infra-red light- unlike most prosthetic retinas, which are powered through coils that require complex surgery to be implanted.

The prosthetic retina is a thin silicon device that converts pulsed near infra-red light to electrical current that stimulates the retina and elicits visual perception. It requires no wires and would make surgical implantation simpler.

The device has been shown to produce encouraging responses in initial lab tests and is reported in an article published in Nature Photonics. The technology is now being developed further.

Credit: University of Strathclyde

Dr Keith Mathieson, now a Reader in the Institute of Photonics at the University of Strathclyde in Glasgow, was one of the lead researchers and first author of the paper. He said: “AMD is a huge medical challenge and, with an aging population, is continuing to grow. This means that innovative, practical solutions are essential if sight is to be restored to people around the world with the condition.

“The prosthetic retina we are developing has been partly inspired by cochlear implants for the ear but with a camera instead of a microphone and, where many cochlear implants have a few channels, we are designing the retina to deal with millions of light sensitive nerve cells and sensory outputs.

“The implant is thin and wireless and so is easier to implant. Since it receives information on the visual scene through an infra-red beam projected through the eye, the device can take advantage of natural eye movements that play a crucial role in visual processing.”

The research was co-authored by Dr. Jim Loudin of Stanford and led by Professor Daniel Palanker, also of Stanford, and Professor Alexander Sher, of the University of California, Santa Cruz.

Professor Palanker said: "The current implants are very bulky, and the surgery to place the intraocular wiring for receiving, processing and power is difficult. With our device, the surgeon needs only to create a small pocket beneath the retina and then slip the photovoltaic cells inside it."

Dr Mathieson was supported through a fellowship from SU2P, a venture between academic institutions in Scotland and California aimed at extracting economic impact from their joint research portfolio in photonics and related technologies.

Strathclyde leads the collaboration, which also includes Stanford, the Universities of St Andrews, Heriot-Watt and Glasgow and the California Institute of Technology. SU2P was established through funding from Research Councils UK- as part of its Science Bridges awards- the Scottish Funding Council and Scottish Enterprise.

The research links to Photonics and Health Technologies at Strathclyde- two of the principal themes of the University’s Technology and Innovation Centre (TIC), a world-leading research and technology centre transforming the way universities, business, and industry collaborate.

Through Health Technologies at Strathclyde, academics work with industry and the health sector to find technologies for earlier, more accurate disease detection and better treatments, as well as life-long disease prevention.

Source: University of Strathclyde 

Additional Information:

• Read the article in Nature Photonics
• Institute of Photonics
• SU2P

DARPA Collaboration Offers Hope to People with Tetraplegia

Engineerblogger
May 19, 2012

Momentum builds behind upper-limb prosthetic technology as researchers demonstrate direct neural control of an advanced prosthetic arm

DARPA launched the Revolutionizing Prosthetics program in 2006 to advance the state of upper-limb prosthetic technology with the goals of improving quality of life for service-disabled veterans and ultimately giving them the option of returning to duty. Since then, Revolutionizing Prosthetics teams have developed two anthropomorphic advanced modular prototype prosthetic arm systems, including sockets, which offer increased range of motion, dexterity and control options. Through DARPA-funded work and partnerships with external researchers, the arm systems and supporting technology continue to advance.

In a recent development reported in the May 17 issue of Nature, researchers at Providence VA Medical Center, Brown University and Massachusetts General Hospital demonstrated the ability to control an advanced prosthetic arm using a direct neural interface system in humans with brainstem stroke. The BrainGate research team was led by Drs. John Donoghue and Leigh Hochberg, VA researchers and professors at Brown and Brown/Harvard respectively, through funding from the Department of Veterans Affairs. This project featured collaboration with the National Institutes of Health, who provided additional funding, and the Defense Advanced Research Projects Agency, who provided a Generation 2 advanced prosthetic arm developed by DEKA under DARPA’s Revolutionizing Prosthetics program. Microelectrode arrays were implanted in the motor cortex of the brains of two tetraplegic patients. With minimal prior use, the patients were able to control the arm in three-dimensional space and perform reach and grasp tasks.

Source: Defense Advanced Research Projects Agency(DARPA)

Additional Information:

• Revolutionizing Prosthetics

Direct Digital: Novel Casting Process Could Transform How Complex Metal Parts Are Made

Engineerblogger
May 19, 2012

Image shows a collection of molds made through the large area maskless photopolymerization (LAMP) technology and airfoil components produced using them. Credit: Gary Meek

A Georgia Tech research team has developed a novel technology that could change how industry designs and casts complex, costly metal parts. This new casting method makes possible faster prototype development times, as well as more efficient and cost-effective manufacturing procedures after a part moves to mass production.

Suman Das, a professor in the George W. Woodruff School of Mechanical Engineering, has developed an all-digital approach that allows a part to be made directly from its computer-aided design (CAD). The project, sponsored by the Defense Advanced Research Projects Agency (DARPA), has received $4.65 million in funding.

“We have developed a proof-of-concept system which is already turning out complex metal parts, and which fundamentally transforms the way that very high-value castings are made,” said Das, who directs the Direct Digital Manufacturing Laboratory in Georgia Tech’s Manufacturing Research Center (MaRC). “We’re confident that our approach can lower costs by at least 25 percent and reduce the number of unusable waste parts by more than 90 percent, while eliminating 100 percent of the tooling.”

The approach being utilized by Das and his team focuses on a technique called investment casting, also known as lost-wax casting. In this process, which dates back thousands of years, molten metal is poured into an expendable ceramic mold to form a part.

The mold is made by creating a wax replica of the part to be cast, surrounding or “investing” the replica with a ceramic slurry, and then drying the slurry and hardening it to form the mold. The wax is then melted out – or lost – to form a mold cavity into which metal can be poured and solidified to produce the casting.

Today, Das explained, most precision metal castings are designed on computers, using computer-aided design software. But the next step – creating the ceramic mold with which the part is cast – currently involves a sequence of six major operations requiring expensive precision-machined dies and hundreds of tooling pieces.

“The result is a costly process that typically produces many defective molds and waste parts before a useable prototype is achieved,” Das said. “This trial-and-error development phase often requires many months to cast a part that is accurate enough to enter the next stage, which involves testing and evaluation.”

By contrast, Das’s approach involves a device that builds ceramic molds directly from a CAD design, completing the task much faster and producing far fewer unusable parts. Called Large Area Maskless Photopolymerization (LAMP), this high-resolution digital process accretes the mold layer by layer by projecting bitmaps of ultraviolet light onto a mixture of photosensitive resin and ceramic particles, and then selectively curing the mixture to a solid.

The technique places one 100-micron layer on top of another until the structure is complete. After the mold is formed, the cured resin is removed through binder burnout and the remaining ceramic is sintered in a furnace. The result is a fully ceramic structure into which molten metal – such as nickel-based superalloys or titanium-based alloys – are poured, producing a highly accurate casting.

“The LAMP process lowers the time required to turn a CAD design into a test-worthy part from a year to about a week,” Das said. “We eliminate the scrap and the tooling, and each digitally manufactured mold is identical to the others.”

A prototype LAMP alpha machine is currently building six typical turbine-engine airfoil molds in six hours. Das predicts that a larger beta machine – currently being built at Georgia Tech and scheduled for installation at a PCC Airfoils facility in Ohio in 2012 – will produce 100 molds at a time in about 24 hours.

Although the current work focuses on turbine-engine airfoils, Das believes the LAMP technique will be effective in the production of many types of intricate metal parts. He envisions a scenario in which companies could send out part designs to digital foundries and receive test castings within a short time, much as integrated-circuit designers send CAD plans to chip foundries today.

Moreover, he said, direct digital manufacturing enabled by LAMP should allow designers to create increasingly sophisticated pieces capable of achieving greater efficiency in jet engines and other systems.

“This process can produce parts of a complexity that designers could only dream of before,” he said. “The digital technique takes advantage of high-resolution optics and precision motion systems to achieve extremely sharp, small features – on the order of 100 microns.”

Das also noted that the new process not only creates testable prototypes but could also be used in the actual manufacturing process. That would allow more rapid production of complex metal parts, in both low and high volumes, at lower costs in a variety of industries.

“When you can produce desired volumes in a short period without tooling,” he said, “you have gone beyond rapid prototyping to true rapid manufacturing.”

The project depicted in this article is sponsored by the Defense Advanced Research Projects Agency(DARPA).

Source: Georgia Institute of Technology