By Rick Robinson
When scientists and engineers use the word materials, they mean any naturally occurring substance manipulated by humans to make things. Beginning with the first metals, discovered by trial and error thousands of years ago, the drive to develop materials that better serve human needs has played a central role in the rise of complex societies.
Modern researchers have moved past haphazard experimentation. Today they examine materials at every level – from the nanoscale to the visible and tangible macroscale – to understand why a material behaves as it does.
At Georgia Tech, investigators unite research capabilities with powerful new tools to develop and characterize novel materials. By pinpointing the complex physical and chemical interactions that control performance, they are creating materials with unique properties.
The White House recently stressed the economic importance of materials expertise when it launched the Materials Genome Initiative, aimed at speeding the pace with which advanced materials move from discovery to industry applications. Georgia Tech is well positioned with the Institute for Materials (IMat), established in 2013 as one of nine interdisciplinary research institutes on campus.
Interdisciplinary collaboration is a critical concept at Georgia Tech, explained David McDowell, a Regents’ Professor who is founding executive director of the new institute. Accordingly, IMat is emphasizing collaboration throughout campus and beyond.
“At Georgia Tech we have some 200 faculty who focus on materials research,” said McDowell, who is the Carter N. Paden Jr. Distinguished Chair in Metals Processing in the Woodruff School of Mechanical Engineering, with a joint appointment in the School of Materials Science and Engineering. ”They tackle a broad range of areas including materials for electronics, infrastructure, energy, environment, transportation, biotechnology, aerospace and defense. The very breadth of that research makes multidisciplinary collaboration both possible and desirable.”
The campus is home to numerous interdisciplinary materials groups – including the Materials Research Science and Engineering Center, the Center for Organic Photonics and Electronics, and the Institute for Electronics and Nanotechnology – that bring together dozens of faculty researchers to focus on core problems.
Materials research at Georgia Tech addresses every type of material, including metals, ceramics, polymers, textiles, composites, nanomaterials, bio-molecular solids – even familiar yet indispensable concrete. And cutting-edge structures that combine very different materials can offer unique capabilities – as in the case of spider silk and graphene oxide, which yield a light, flexible material stronger than steel.
“In the past, materials progress was highly empirical, based largely on trial and error,” said professor Naresh Thadhani, chair of the School of Materials Science and Engineering, which is the largest single locus of materials research at Georgia Tech, with 37 full-time faculty and 20 courtesy faculty appointments. “That approach is now widely regarded as excessively slow and costly.”
Instead, Thadhani explained, researchers are using microstructural tools, including optical and electron microscopes and neutron and X-ray scattering techniques, combined with time-resolved experimentation, mathematical and numerical modeling and computational simulations, to characterize materials. The aim is to predict how they’ll perform in real world applications, to accelerate the pace from discovery to deployment.
The ability to develop new materials for advanced manufacturing is essential to the United States, said Stephen E. Cross, executive vice president for research at Georgia Tech. In the new global economy, novel materials will be a key to the nation remaining competitive.
“From the day it opened, Georgia Tech has stressed support for industry, and interdisciplinary research is something we believe in very strongly as well,” said Cross. “I’m confident that our broad materials research capability, fostered by our Institute for Materials, can deliver innovations that will promote economic growth for both the state of Georgia and the nation.”
This article presents an overview of materials work at Georgia Tech, focusing on a few of the many innovative research projects underway.
Improving Materials for Extreme Conditions
Ensuring Engine Dependability
Everyone wants to be confident that jet engines are completely dependable. Richard W. Neu, a professor in the Woodruff School of Mechanical Engineering, studies the details of exactly this issue.
With funding from the Department of Energy and several multinational corporations, Neu has focused on fatigue and fracture of metallic alloy systems for nearly two decades. He specializes in high temperature fatigue and fracture behavior – how the microstructure of highly stressed metal parts changes over time.
“We’re developing models to capture the evolution of gas turbine engine parts over time, so we can predict how that microstructure will change with operational conditions,” said Neu, who directs the Mechanical Properties Research Laboratory at Georgia Tech.
Neu and his research team are studying gas turbine engines for both aerospace use and for land-based power generation. In both cases, gases in excess of 1,400 degrees Celsius – higher than the melting point of most metals – require active cooling strategies and parts made of special alloys to survive these harsh conditions.
In such demanding environments, the high temperatures and stress always take a toll, Neu said. “Among other investigations, we’ve taken a used engine blade, in service for about three years, and compared its microstructure to an unused blade,” said Neu, who also teaches in the School of Materials Science and Engineering. “And I can tell you, they’re vastly different.”
What’s more, he said, the differences are not uniform. The microstructure of engine parts can vary dramatically depending on the combined temperature and stress cycles – meaning exactly how and when the parts encountered temperature and stress. Neu simulates these complex thermomechanical cycles in the laboratory to characterize the degradation of the material under operational conditions.
The materials that Neu tests are typically nickel-based superalloys, which are widely used in gas turbine engines. More recently, he’s been studying promising new high temperature materials such as gamma titanium aluminides, which are so lightweight that they could be revolutionary for aerospace applications.
Understanding Pipeline Degradation
How long a material will last in a given application is always a major concern. Preet Singh, a professor in the School of Materials Science and Engineering (MSE), is pursuing a number of studies to see how well metallic alloys stand up to various corrosive environments and stresses.
Singh and his team are looking at the performance of conventional carbon steel pipelines used to transport fuel products such as gasoline. In work sponsored by the Department of Transportation and the pipeline industry, he’s addressing the role of corrosion and stresses in the environmental degradation of steel, which can potentially lead to pipeline failure.
Among other things, he’s studying whether pipeline integrity could be affected by new biofuels.
“Biofuels such as ethanol, bio-diesel, or bio-oils like pyrolosis oil are becoming increasingly important, so there is concern about how they may affect pipeline interior surfaces,” he said. “We are examining the interactions between these chemicals and steel pipelines – studying factors including stress, internal environment and the alloy composition – to understand the possible issues and the ways to mitigate them.”
The problem is a complex one, Singh explains. The iron oxides – rust – that form when steel begins to corrode may also create a passive film that can help protect the pipeline interior from being further damaged by chemicals flowing through.
At the same time, different types of iron oxides display very different characteristics. For example, if iron oxide molecules clump rather than dispersing smoothly and continuously, or make a defective surface film, then surface protection is greatly reduced.
High flow velocities can injure protective films as well. Damage can also come from the stresses placed on steel as sequential fuel batches pressurize and then depressurize the pipeline, which causes low frequency fatigue in these structures.
Results from Singh’s research have shown that a small amount of impurities such as water, or chlorides in biofuels, can actively affect the extent and mode of corrosion in pipelines as well. Working with MSE associate professor Seung Soon Jang, Singh is studying how very small differences in the ratio of water and ethanol can have a big effect on the corrosion taking place inside a pipe.
Exploiting Microstructure Data
Advanced metal alloys have become indispensable in various emerging technologies – especially where extreme conditions demand new levels of performance and lighter weight. But developing novel alloys is difficult without a thorough understanding of metal microstructure.
“We can no longer afford to depend on the element of luck in developing materials,” said Surya Kalidindi, a professor in the Woodruff School of Mechanical Engineering. “Today, interdisciplinary research has enabled us to capture materials knowledge that makes it much easier for the designer or manufacturing engineer to understand the microstructure – and this knowledge lets them deploy new technology much faster.”
In projects funded by the Department of Defense, Kalidindi and his team are researching ways to improve lightweight structural metals used in the transportation sector. The goal is to increase operating temperatures in service, which would translate to higher efficiency and major fuel savings.
But developing new alloys requires more than familiarity with the relevant metals chemistry, Kalidindi explained. The materials designer needs to understand how the crystals within a metal alloy fit together at the micron level, an interaction that has far-reaching effects on properties.
The solution is a computer database containing in-depth information on the internal makeup of many different materials, he said. The data on these properties are derived from experiments conducted by Kalidindi and many other researchers.
“In some ways you can compare this approach to a fingerprint database, where you can quickly compare a new print coming in to similar ones based on its characteristics,” he said. “We have developed techniques that allow us to represent each microstructure’s characteristics three-dimensionally, so we can look at a new material and see how it is similar to structures on which we already have detailed information.”
In two National Science Foundation-funded projects, Kalidindi is studying development of lighter weight automobile parts made of either high-strength steels or new types of magnesium alloys. Among the challenges is the need to find technologies that can reduce vehicle weight yet cost no more than current techniques.
Engineering Adaptive Metamaterials
In materials research, investigators often alter structures at or near the atomic scale to change behavior at the macroscale.
Massimo Ruzzene, a professor in the Guggenheim School of Aerospace Engineering (AE), takes a different approach. He designs metamaterials, which are artificial composite structures that combine two or more components in patterns that give them properties not found in materials derived from nature.
In a metamaterial, the geometry of the constituent parts lets it react to incoming wave energy – such as electromagnetic, sound or shock waves – in unusual ways. For example, traditional materials expand in one direction when compressed in another direction; a metamaterial could be designed to adapt to the force in a unique way, such as compressing in both directions.
“From my standpoint, structures and materials are becoming the same thing,” said Ruzzene, who directs AE’s Vibration and Wave Propagation Laboratory. “We work on what you might call atomically inspired structures. Rather than manipulating things at the molecular level, we look at molecules for design ideas – for concepts we can use at the larger scales to design artificial composite materials with geometries that give them unique properties.”
For instance, molecules at the smaller scales often realign under a stimulus, such as heat. In a metamaterial, that realignment might be imitated to improve the macroscale functioning of, say, a lattice structure that’s good at dissipating incoming energy but has poor strength.
To achieve this, researchers could add in elements – such as aluminum, rubber or simply air – which are carefully placed into the lattice geometry. These inclusions would enable the structure to change dramatically when exposed to a given type of stress, altering overall behavior and changing the directionality of incoming stress waves.
In one federally funded project, Ruzzene is developing a structure with both high stiffness and high damping – a demanding task because these properties conflict. Ruzzene and his team decided to decouple the two requirements, creating a structure that is stiff on the outside but uses resonating structures inside to damp out problem frequencies.
This approach could be useful in reducing structural fatigue caused by continual flexing in aircraft. The research team has developed an aluminum beam that fits inside an aircraft wing. The metamaterial design lets it carry a load and stiffen the wing, while also drastically reducing vibrations by means of damping in the critical range of 8 to 10 hertz.
Modeling Materials Behavior
Tests that show when a material will fail are critical to reliable engineering applications. The problem is that such tests are generally complex and expensive. They’re also time-consuming, slowing the insertion of new material designs into real world applications.
Julian J. Rimoli, an assistant professor in the Guggenheim School of Aerospace Engineering (AE), works in the field of computational solid mechanics, which investigates the behavior of any solid material – including metals, ceramics, polymers, composites and metamaterials – through advanced modeling and computational techniques. In particular, he is interested in the formulation of models that can dependably predict the life of materials in extreme environments.
“Traditionally, engineering models of degradation, wear, damage, and failure of materials are phenomenological. This phenomenological approach implies that models are formulated to fit experimental observations,” he said.
While this approach is good enough in many situations, he added, it is inherently not predictive. In addition, this approach does not provide any physical insight on why a material may have certain properties.
Rimoli specializes in the formulation of physics-based predictive multiscale models that link microstructure to mechanical behavior. His research aims to design new classes of materials that are more resistant to extreme conditions.
In one Air Force-sponsored project, Rimoli is trying to understand the leading erosion mechanisms in plasma thrust engines, which can be used to propel satellites.
His research shows that there are more erosion mechanisms than previously thought, such as mesoscale formation of inter- and intra-granular thermal cracks that play a prominent role in the premature wear of such components. These models are currently being used to tailor the microstructure of families of heterogeneous ceramic compounds to better withstand the demands of a plasma environment.
Materials Reliability in Structures, Infrastructures and Energy
Professor Min Zhou of the Woodruff School of Mechanical Engineering (ME) studies the effects of mechanical, thermal and chemical loading on the behaviors and reliability of structural, infrastructural and functional materials, such as metals, ceramics, semiconductors and composites. One focus involves high strain rate mechanical loading, which can come from several causes, including high-speed machining, impact, penetration, and the explosion of energetic materials.
As part of a federally funded project, Zhou has built a laboratory in the Georgia Tech Manufacturing Institute to investigate how ship structures respond to the effects of underwater explosions. Using a special gas-driven gun, he generates high-pressure waves through water and uses the impulsive loads to analyze the resulting fluid-solid interactions with high-speed digital cameras and laser interferometers.
The goal is to develop new materials for ship construction. Under special consideration are sandwich structures, which are polymer-based composites that are lightweight, inexpensive and highly corrosion resistant.
But such materials must also be highly resistant to heavy weather, encounters with reefs and other threats. Using both experiments and computer simulations, Zhou is designing composite structures that could meet these requirements.
In another project, sponsored by the Army and the Department of Homeland Security, Zhou is working with ME professor David McDowell on infrastructure materials that could offer increased protection against earthquakes, as well as terrorist attacks. The team is using both large-scale experiments and computational modeling techniques to study ultra-high-performance concrete designs that use novel metal fibers for added strength.
Zhou also studies a range of issues related to materials in energy applications. In a project sponsored by the National Research Foundation of Korea, he is addressing problems surrounding the use of silicon to replace graphite in next-generation high-capacity rechargeable lithium ion batteries.
Silicon is a highly desirable replacement for traditional graphite as anodes in lithium-ion batteries, because of its much higher lithium storing capacity. However, it is more prone to mechanical failure through cracking due to large volume changes during charge and discharge. Zhou is developing models that outline approaches for improving the reliability of silicon-based anodes by taking advantage of the size dependence of coupled mechanical chemical diffusional processes in the materials.
Novel Next-Generation Composites
Increasing Composite Material Integrity
Composites such as carbon fiber reinforced polymers are impressively light and strong, but they don’t have the track record of older materials like steel. Chuck Zhang, a professor in the Stewart School of Industrial and Systems Engineering, is working with aerospace companies to increase the quality of composite parts while lowering production costs and ensuring structural integrity long term.
“Unlike steel parts, which can be stamped, composites generally require time-consuming molding and curing processes,” Zhang said. “We are researching methods for shortening the composites’ manufacturing time while improving the quality of finished parts – and also adding a self-sensing capability that can perform structural health monitoring.”
Detecting problems and flaws during composite manufacturing and service is critical because such flaws can go unseen and lead to sudden failure. To guard against such flaws, as well as long-term structural fatigue problems, Zhang is working on novel methods for making composites with tiny built-in sensors that could monitor both the manufacturing process and composite structural integrity during service.
Conventional strain sensors – usually thin films of metal – would constitute a foreign body within the polymer composite itself, he explained. Their presence could affect integrity and lead to adverse delamination of composite layers.
Zhang uses special aerosol jet printing equipment to fabricate tiny sensors directly on composites using conductive inks comprised of carbon nanotubes, graphene or metal particles. These sensors – with feature sizes of about 10 microns – are far smaller than conventional strain sensors. They have more choices for ink materials and can be printed on substrates of various materials and shapes, which allow them to be more conformal, versatile and easily embedded. Their tiny size could let manufacturers build large numbers of them into polymer composites without disturbing structural integrity.
In other research, Zhang is working on a prosthetics-related project with Ben Wang, who is executive director of the Georgia Tech Manufacturing Institute (GTMI). The researchers are participating in the Socket Optimized for Comfort with Advanced Technology (SOCAT), a $4.4 million Department of Veterans Affairs contract led by Florida State University.
The effort addresses prosthetics shortcomings to benefit those who have lost limbs to injury or disease. The GTMI team is developing tiny printed sensor devices to monitor health- and comfort-related conditions in the socket where a patient’s limb connects to a prosthesis.
Zhang is also collaborating with researchers Xiaojuan (Judy) Song and Jud Ready of the Georgia Tech Research Institute to develop innovative sensors and photovoltaic devices.
Enhancing a Universal Material
Kimberly Kurtis, a professor in the School of Civil and Environmental Engineering, is pursuing multiple research projects involving a ubiquitous composite material: concrete.
Her research involves studies that range from chemistry and structure at the nanoscale to appraising massive structures such as dams and buildings at the macroscale.
“Our work is very multiscale, and like other materials researchers, we’re constantly trying to better define the relationship between structure and properties,” said Kurtis. “To do that, we study the broader class of all cement-based materials – not just concrete but anything that contains a mineral, non-biological cement – to link the chemistry of various cements with their structural performance.”
In one National Science Foundation (NSF)-sponsored project, Kurtis and her team studied the use of titanium dioxide nanoparticles as partial replacements for cement. They found the material significantly alters the way that the cement reacts, reducing the time it takes to cure, and potentially reducing the amount of cement needed to build a structure.
The team is also studying the role of titanium dioxide and concrete’s nanostructure in potentially reducing nitrogen oxide effects. Nitrogen oxides, a group of compounds that are major byproducts of vehicle emissions, can damage human health. Tailoring the interactions between concrete and its environment could lead to new approaches for improving air quality.
Among several other projects, Kurtis is working with NSF support to develop better statistical and probabilistic descriptors of concrete and its constituents, with a focus on nanoscale and micron-scale porosity. Concrete is heterogeneous, she explained, and its composition varies on multiple scales, from coarse aggregate to paste. Data on these related factors can be used in computer models to predict performance.
“An exciting thing about being at Georgia Tech is that you’ve always got one foot in science and one foot in practice,” Kurtis said. “You want to make sure that what you’re doing is relevant to the broader needs of society.”
Improving Medical Imaging
At the Georgia Tech Research Institute (GTRI), a composite developed for radioactive materials surveillance is being adapted for medical imaging applications. The goal is a new technology – the transparent nanophotonic scintillator for X-ray imaging – that exposes patients to less radiation while producing higher resolution images.
The basic technology development was led by GTRI researchers Brent Wagner and Bernd Kahn with Department of Homeland Security funding. The team created a unique composite made of nanoparticles of rare earth materials dispersed evenly in a silica matrix. The glasslike material detects gamma rays by converting them to visible light via a phenomenon known as scintillation.
A similar approach is now being developed under a National Institutes of Health-funded project led by GTRI senior research engineer Zhitao Kang, a member of Wagner’s research group. Kang is using the same basic scintillator material – nanoparticles in a glass matrix – to produce a clearer image with far less light scattering than conventional X-ray imaging scintillators.
To improve the technology further, Kang and his team have been working with professor emeritus Christopher Summers of the School of Materials Science and Engineering to add a layer of photonic crystals to the scintillator’s surface. The photonic crystals – basically a pattern of tiny holes tuned to a specific light frequency – help direct light out of the scintillator and thus increase light output.
“Our scintillator – the nanoparticles in glass – gives us high resolution, while the photonic crystals increase the light collection efficiency, which means we get more light out of the X-ray,” Kang said. “These are the two properties you want – a better image, along with high efficiency so you don’t need to use so many X-rays.”
Kang pointed to an added benefit of the nanophotonic approach: GTRI’s glass-like scintillator materials could be made in large sheets, just like industrial glass. That would decrease manufacturing overhead and make the technology less costly.
Kang and his team are also collaborating with Oak Ridge National Laboratory and a German national laboratory to modify GTRI’s scintillator so that it can detect neutrons. The researchers are adding neutron-detecting materials – varieties of lithium and boron – that can absorb incoming neutron energy and convert it to light via the scintillation process.
Advancing Carbon Fibers
Carbon fibers are stronger and lighter than steel, and composite materials based on carbon fiber reinforced polymers are used in an ever-expanding range of applications. The Boeing 787 aircraft employs carbon fiber materials extensively in its fuselage, wings, tail and other sections. Carbon fiber composites are utilized in civil engineering and construction, and in many consumer products.
Yet today’s carbon fiber materials have a long way to go before they achieve their full potential, said Satish Kumar, a professor in the School of Materials Science and Engineering. He is leading a four-year, $9.8 million project sponsored by the Defense Advanced Research Projects Agency (DARPA) to improve these materials using nanotechnology in the form of carbon nanotubes.
“It’s likely that carbon fiber materials could be about 10 times stronger than they are presently, so there is tremendous room for further improvement in their tensile and other structural properties,” Kumar said. “By using carbon nanotubes to reinforce carbon fibers, our objective is development of a next-generation carbon fiber with double the tensile strength of today’s strongest carbon fibers.”
In an advanced laboratory established for the current project, Kumar and his team are optimizing techniques for converting polymeric materials into high-strength carbon fiber, using a multi-stage process.
Untreated polymers contain carbon, hydrogen, oxygen and nitrogen, Kumar explained. They can be made into carbon fiber via a selective treatment process called pyrolysis, in which a polymer mix is gradually subjected to both heat and stretching. This treatment eliminates large quantities of hydrogen, oxygen and nitrogen, leaving an increased amount of carbon that makes the fiber stronger.
Kumar modifies this process by adding carbon nanotubes – about one percent by weight – to the polymer mixture before pyrolysis. Among the challenges is finding the best methods for dispersing the carbon nanotube solution uniformly in the polymer mix.
“If the mixing process is fully successful, the carbon nanotubes will reorient the crystals within the polymer in a uniform direction,” he said. “The altered molecular structure has the potential to make the resulting carbon fiber much stiffer and stronger.”
Materials for National Defense and Homeland Security
Deployable Chemical Sensing
Carbon nanomaterial-based chemiresistors are useful for environmental monitoring and agricultural applications.
Xiaojuan (Judy) Song, a senior research scientist at the Georgia Tech Research Institute (GTRI), has developed sensing technology that uses functionalized carbon nanotubes to detect minute amounts of chemicals in ambient air. Combined with radio frequency identification (RFID) electronics, this material could be used to make low-cost sensors that give advance warning of threats.
“We are using carbon nanotubes (CNT) that have been functionalized for a particular gas or analyte, applied as a sensing film,” said Song, who is the principal investigator on the project. “Sensors based on these materials could be used in the field by the thousands to inform first responders about nearby hazards.”
Working with graduate student Christopher Valenta of the School of Electrical and Computer Engineering, Song has developed a prototype sensor array integrated with an RFID chip that is 10 centimeters square. The next step might be a prototype as small as a one centimeter square, with sensing tips that could be aerosol jet printed on paper or a flexible substrate.
The RFID-enabled CNT-based wireless sensors could also be valuable for monitoring air pollution, she said. Low-cost sensing systems that detect trace ammonia, nitrogen oxides and other targeted gases could also be fielded in large numbers for agricultural applications, such as providing information on fertilizer usage and early detection of plant disease.
Building Better Body Armor
Robert Speyer, a professor in the School of Materials Science and Engineering, performs extensive research on the body armor that protects U.S. troops.
He also builds it.
His Atlanta-based Georgia Tech spinoff company, Verco Materials LLC, produces ceramic armor made primarily from boron carbide. Using patented processes, Verco has for several years been producing armor for research and development, as well as for actual protective equipment. To date, Verco has received some $6 million in contracts to expand the company and its capabilities.
Verco recently started work to improve side armor plates, which are used by U.S. troops to augment the protection offered by the familiar front torso plates.
“The most important objective in ceramic body armor is to have high hardness, so that the armor will not flow out of the way of the projectile. Instead, the projectile is forced to dwell at the surface, collapsing on itself and mushrooming out as it loses its energy,” Speyer said. “Our armor is really impressive in that regard, which is allowing us to develop armor at reduced weight that still defeats armor piercing rounds.”
Verco now has two 6,000-square-feet manufacturing locations in Atlanta, not far from the Georgia Tech campus. One location includes a massive 1,700-ton press capable of making powder compacts of full torso armor plates.
Among the challenges that Verco has overcome is a need to find less expensive boron carbide powders to use in making armor plate. The team solved that problem by devising a different formulation with an even higher hardness.
“Our ballistics results are disruptively good,” Speyer said. “As we scale up, we’re focusing on the need to keep our costs competitive as well.”
Trapping Chemical Threats
Since World War I, the U.S. military has used protection equipment – including gas mask-type devices and larger filters – to protect against possible chemical agents. Krista Walton, an associate professor in the School of Chemical and Biomolecular Engineering, works to ensure that U.S. air purification technology is equal to any class of chemicals, novel or conventional.
Walton and her research team focus on designing materials that are effective against a broad class of compounds called toxic industrial chemicals (TICs). They have developed porous materials that are designed to adsorb incoming TICs, protecting personnel against their effects for extended periods of time.
“There are a number of materials that for decades have protected effectively against many different chemicals,” Walton said. “Our work centers on finding ways to enhance filtration devices, to be sure they can also handle any new air purification challenges that emerge.”
With funding from the Defense Threat Reduction Agency and the Army Research Office, Walton and her research group are developing nanostructured porous materials that can effectively capture additional toxic chemicals. The goal is to improve performance in devices that range from gas masks to filters that protect the air intake equipment used in buildings.
One of the group’s principal research efforts focuses on metal organic framework (MOF) technology. These hybrid materials, which use both inorganic and organic parts, are designed to trap specific molecules that could be hazardous.
In this approach, organic ligands – molecules that bind to metal atoms – are modified to target one specific incoming molecule but not others. Several different ligands can be mixed together to protect against a range of different chemicals.
Walton uses a variety of tools, including powder X-ray diffraction and gas adsorption analysis, to characterize the materials she develops. The aim is to pinpoint materials with the most promise, which are then selected for more extensive testing.
Materials Derived from the Natural World
Utilizing a Bio-Factory
Natural structures can be far more complex than anything developed synthetically. Kenneth Sandhage, who is the B. Mifflin Hood Professor in the School of Materials Science and Engineering (MSE), is using tiny diatoms – a type of single-celled algae – to make unique materials with a variety of potential applications.
In nature, there are an estimated 100,000 species of diatoms, ranging from a few micrometers to several hundred micrometers in size. Each species creates a unique three-dimensional frustule, or micro-shell, out of silica, a material also used to make glass.
Once researchers identify a diatom configuration that holds promise for a specific application, that species may be allowed to reproduce in a laboratory culture. In 80 reproduction cycles, one parent diatom can produce more than a septillion daughters of similar three-dimensional structure.
“It’s massively parallel self-assembly, under precise 3-D control, that can be accomplished in a wide variety of shapes by using different diatom species,” Sandhage explained. “There’s no man-made approach that can accomplish such massively parallel 3-D assembly in such a range of complex patterns under ambient conditions.”
To make useful structures, the next step involves synthetic chemical processes, as the complex but delicate silica shell is replaced with a more desirable functional material suitable for a particular application. Sandhage and his research team have made ceramic and polymer replicas of diatom frustules composed of, for example, titanium oxide, magnesium oxide, silicon carbide, carbon, and barium titanate. They’ve also made replicas from silicon and other elements such as copper, silver, gold, platinum and other metals.
In one project, Sandhage and his team have worked with Meilin Liu, a Regents’ Professor in the School of Materials Science and Engineering, to use a diatom-derived material in polymer electrolyte membrane fuel cells. To speed up the critical oxygen reduction reaction in the fuel cell electrode, they placed a catalytic material, consisting of nanometer scale platinum particles, onto and into a conductive substrate of carbon diatom replicas.
The platinum particles lodged into the fine pores of the carbon replica cell walls, and went on to catalytically outperform standard platinum-loaded carbon black, as well as platinum-loaded carbon derived from silicon carbide.
This superior performance can be traced to the hollow, thin walled 3-D shape derived from the diatoms, Sandhage said.
The oxygen can readily move inside the tiny hollow structure, so it doesn’t have to travel far to reach the platinum buried within the thin cell walls. The result is an electrode with far better performance.
Other potential applications for diatom-derived materials include tiny sensors, fast acting drug delivery capsules, rapid water or synthetic chemical purification, anti-counterfeiting, and hierarchically patterned electrodes for other energy devices.
“Someday, it may become possible to genetically modify the diatom and basically dial in the 3-D shape that we want, which would then allow us to tailor the shape as well as the chemistry for a particular application,” Sandhage said.
Mimicking Biological Nanostructures
Mohan Srinivasarao, a professor in the School of Materials Science and Engineering, wants to understand how the outer shells of some creatures, such as insects, create unusual optical effects such as iridescent colors. He is also investigating how those structures can be simulated to produce comparable effects.
“We are investigating nature-inspired colors and how to change those colors dynamically,” said Srinivasarao. “There are many biological systems that have liquid crystal-like structures on their bodies, and that lets them create colors by altering the frequency of the incoming light.”
The potential applications of this National Science Foundation-sponsored research are broad, he said. One involves camouflage that would vary with the background. Others might center on long lasting commercial materials that could produce a brilliant color, or a range of shifting colors, using nanostructures rather than dyes.
In explaining nanostructure-based coloring, Srinivasarao pointed to the case of a butterfly that is not green but can make itself appear so.
Green is an excellent color choice for an insect living in foliage, but it’s also a difficult color for many creatures to generate in the natural world. The butterfly in question achieves this protective hue by mixing yellow and blue wavelengths together.
In another instance, one type of beetle can produce both green and yellow by depositing chitin, a natural polymer that occurs in its exoskeleton, in the manner of a helix. The pitch of that helix – the width of one complete turn – is about 300 nanometers.
At the same time, the exoskeleton’s index of refraction – a measure of how light propagates through it – is approximately 1.5. The interaction between the pitch, the index of refraction and incoming light simulates the color green.
“There are no dyes, no pigments,” said Srinivasarao. “If you look at the 300-nanometer spacing in between these lines here on the beetle’s shell, that’s on the right order of magnitude to provide the green reflection.”
Developing Hybrid Nanomaterials
Vladimir Tsukruk, a professor in the School of Materials Science and Engineering, is studying ways to put organic and inorganic materials together to create new functionality. Specifically, he unites “soft” materials – biologically derived polymers and organics – with “hard” materials such as noble metals and other inorganic structures.
“Our approach involves developing what can be called bioinspired materials – based on examples from nature – that have unusual physical properties,” Tsukruk said. “Soft materials and hard materials have unique sets of properties, but by combining them you can get something much more intriguing.”
Tsukruk and his research team are studying ways to interface such disparate materials so that they function together productively. A host of problems – including clear mismatches in physical properties, molecular structure and other characteristics – make the work challenging, he said.
In one project, Tsukruk is combining genetically modified spider silk – one of the toughest materials in nature – with ultrathin films of graphene oxide to form a layered nanocomposite. By alternating layers of the two materials, 20 percent graphene and 80 percent silk, he aims to unite graphene’s strength with the toughness and elasticity of the silk.
A paper on this work, funded by the Air Force Office of Scientific Research, was published in April 2013 in the journal Advanced Materials. And in another study, recently published in the journal Angewandte Chemie, Tsukruk and a research team demonstrated a method for writing electrically conductive patterns on flexible silk-graphene biopaper.
Silk-graphene nanocomposites can have strength comparable to the best steel and the flexibility of conventional paper, Tsukruk said, while also offering flexibility and lighter weight. Such materials could be mass produced after certain issues are resolved, such as obtaining low-cost silks, which could be manufactured through the use of genetically modified bacteria.
Developing Materials for Energy Applications
Launching Energy Applications
A critical part of materials development involves moving technology from the laboratory to real-world applications. Jud Ready, a principal research engineer in the Georgia Tech Research Institute (GTRI), brings nanomaterials discoveries to bear on a variety of energy-related and other components, including solar cells, batteries, supercapacitors and field electron emitters.
“We research a variety of different ways to use electrons in a material, with the intention of making a useful device and then hopefully commercializing that device,” he said.
Ready and his team have developed a 3-D photovoltaic technology that uses micron-scale “towers” to capture nearly three times as much light as flat solar cells of the same materials. The technology – aimed at applications such as satellites, cell phones and military equipment where limited surface area is an issue – is now licensed to California-based Bloo Solar Inc.
The research team is presently readying another solar cell technology that could lower costs while maintaining a useful level of performance. Under this approach, the low-cost elements copper, zinc, tin and sulfur (CZTS) replace more costly elements – copper, indium, gallium and selenium (CIGS) – that have been used in photovoltaics.
“CZTS materials are virtually identical in crystal structure and manufacturing approaches to CIGS, which costs at least a thousand times more,” Ready said. “So even if CZTS efficiency is only 15 percent versus some 20 percent for CIGS, the CZTS raw material costs a penny as opposed to $10 for CIGS.”
GTRI’s CZTS technology is expected to be installed and tested on the International Space Station in December 2014. Commercial development of the technology is on the horizon as well; the researchers are working with VentureLab, a startup company incubator for Georgia Tech researchers.
Researching Longer Lasting Batteries
A battery that costs less and lasts significantly longer in laptops, cell phones or electric cars before recharging would be welcome to both consumers and industry. Gleb Yushin, an associate professor in the School of Materials Science and Engineering, is working with battery materials that could outperform the conventional lithium ion technology common today.
Yushin and his research team are studying several chemistries that hold promise for future battery technologies. In addition to ultra-high capacity materials for lithium ion cells, the team is studying magnesium ion and aluminum ion chemistries, which can carry more charge than lithium ions, as well as sodium ion chemistry which may offer reduced cost.
The problem is, the larger the amount of charge on an ion – which is an atom or molecule that carries an electrical charge – the greater the potential barriers within the battery’s charging system. The result is that it presently takes far longer to charge and discharge batteries built around the non-lithium chemistries. The Georgia Tech researchers are working on improving these materials to reach an acceptable rate of charge and discharge.
Yushin also studies supercapacitors, which are energy storage devices that can charge up in seconds and then deliver energy quickly. He has developed composite materials that charge at the same fast rates but store far more energy – a big advantage for applications such as wind farms, certain hybrid vehicles, military activities and others.
“These are difficult problems that require long term study to solve,” Yushin said. “To do this, we are examining the fundamentals of structure and properties at the nanoscale – to learn how the microstructure of these materials and their chemistry can impact the insertion and extraction of different metal ions.”
Yushin is conducting fundamental studies of ion transport to advance his work on batteries and supercapacitors. In collaboration with Oak Ridge National Laboratory scientists, he and his team recently carried out experiments that added to the basic understanding of ion activity.
The researchers used a technique called small angle neutron scattering to directly observe how ions behave in certain microporous materials. By directing a beam of high-energy neutrons on activated carbon electrodes during supercapacitor operation, they determined how electrolyte composition affected the average ion concentrations in pores of different sizes as a function of the applied potential.
“In these experiments we gained unique information about ion adsorption in sub-nanometer pores that nobody else had obtained previously,” Yushin said. “Understanding these processes better could lead to the development of improved energy storage, as well as advances in fields such as water purification, desalination systems and biological systems.”
Making Energy Safer, Less Costly
Faisal Alamgir, an associate professor in the School of Materials Science and Engineering, is tackling research challenges involving energy-related behaviors in applications that include batteries and solar cells.
In battery-related research, Alamgir and his team are studying ways to make lithium ion batteries safer and longer-lasting. Among other things, Alamgir wants to understand and optimize the behavior of various elements within a cycling lithium ion cell.
In one important study sponsored by the National Science Foundation Materials Research Science and Engineering Center, Alamgir has investigated the role of oxygen in the creation of electrons in lithium ion batteries. Among the key issues: whether the oxygen present in battery materials is participating in the electrochemical reaction as lithium cycles in and out. If so, that could help explain why fires have occurred in some large lithium-based batteries.
Alamgir and his team used X-ray absorption spectroscopy to look inside an operating battery. The work confirmed that oxygen is indeed being created under some conditions during the charging and discharging process in a lithium ion battery.
“Now we know that fires may start inside a lithium cell even if there was no puncture in the cell – because there is oxygen participating in the reaction,” Alamgir said. “If we want to make safer batteries, we must work in a voltage range where the oxygen is not as active – which varies with temperature – or we must come up with an alternative cathode material that keeps the oxygen from participating electrochemically.”
In the area of solar cells, Alamgir is examining the use of new materials in dye-sensitized solar cells (DSSCs). This type of solar cell promises lower costs compared to more traditional techniques.
In one approach, he is studying the use of low-cost titanium dioxide instead of conventional silicon as the light-absorbing semiconductor in solar cells. By also adding certain dyes that increase light absorption, he has designed a photo-electrochemical system that could be manufactured more easily and inexpensively than today’s silicon technology.
In other work, Alamgir is looking at materials and methods that could replace or reduce the use of costly platinum, the traditional choice as a counter electrode of DSSCs, as well as electrodes in proton exchange membrane (PEM) fuel cells.
One method seeks to limit platinum’s use to ultra-thin films, consisting of only a few monolayers of the element. Alamgir has shown dimension-dependent transitions in enhancement of properties in platinum when it is restricted to layers that are only a few atoms thick.
Complex Modeling of Nanomaterials
In today’s world, there’s a pressing need to find the most energy efficient materials. Seung Soon Jang, an associate professor in the School of Materials Science and Engineering, is using computational modeling to examine the relationship between structure and properties in these materials.
“We perform first principles atomistic modeling, which means we include all the details of a material’s atoms – its hydrogen, carbon, oxygen and others – without simplifying anything,” Jang said. “We can then use that highly detailed knowledge to design new materials.”
Today’s sophisticated observational tools are used by scientists to produce reams of experimental data. Jang and his team in the Computational NanoBio Technology Laboratory utilize these big-data troves to develop useful models of materials behavior at the smallest scales, exploiting powerful computers.
At the atomic scale, Jang and his team use quantum mechanical simulations derived from computational physics and chemistry to understand the distribution of electrons, which give a particular material many of its unique properties. At a slightly larger scale, he employs molecular dynamics simulations to understand how the grouping of molecules also determines a material’s behavior.
Jang is using these techniques to tackle projects in multiple areas including semiconductors, carbon nanotubes and graphene, biomaterials, and fuel cells, batteries and solar cells.
In one recent project sponsored in part by the National Science Foundation and Department of Energy, Jang and several collaborators investigated behavior in semiconductor materials at very small scales. In particular, they studied a phenomenon that takes place when the dimensions of certain semiconducting materials are reduced to nanoscale levels: non-metallic materials unexpectedly take on metallic properties.
In another project, sponsored principally by the Department of Energy and a major automotive company, Jang is studying the performance of polymer membranes in automotive fuel cells. The aim is to find new membrane designs that will improve the material’s performance at extreme temperatures.
Developing Nanostructured Energy Materials
Zhiqun Lin, an associate professor in the School of Materials Science and Engineering, is pursuing research on solar energy conversion. To increase solar cell efficiency, he is working with nanostructured functional materials, including conjugated polymers, nanocrystals, and nanocomposites made of conjugated polymers and nanocrystals.
Nanocrystals are nanoparticles with a crystalline structure – meaning their atoms are arranged in a regular, periodic way. In a given material, their crystalline form can give them special behaviors that don’t occur at larger size scales in the same material. Lin has been concentrating on producing functional nanocrystals that will support more-efficient solar cells.
In work funded by the Air Force Office of Scientific Research, Lin recently discovered a simple and robust approach to making a wide variety of functional nanocrystals with controllable sizes, compositions and architectures – including metallic, ferroelectric, magnetic, semiconducting and luminescent nanocrystals. The new technique – described in the June 2013 issue of the journal Nature Nanotechnology – targets nanoparticles for applications where tight control over size and structure promotes desirable properties.
Lin and his team are pursuing several projects involving solar cells. One effort involves hybrid solar cells, so called because they utilize both organic and inorganic semiconductor materials.
In this approach, conjugated organic polymers are coupled with inorganic semiconducting nanocrystals. Incoming photons are absorbed in the polymer, generating electrons that are then injected into the semiconducting nanocrystals to produce current. Advantages include low cost to manufacture and toughness that could facilitate solar cell installation.
In another project, Lin is developing photovoltaic cells using abundant, low cost and environmentally friendly elements: copper, zinc, tin and sulfur. Made into functional nanocrystals that can serve as the semiconductor, these elements could replace expensive noble metals such as platinum, as well as rare earth elements that can be hard to obtain, for use as counter electrodes in high-efficiency dye-sensitized solar cells.
Understanding Structures to Aid Materials Development
Exploring Thin Films
The use of thin films – layers with thicknesses in the nanoscale to micron-scale range – has become increasingly important in a number of technological applications. Samuel Graham, a professor in the Woodruff School of Mechanical Engineering, is focusing on methods for growing thin films, as well as studying their properties and reliability.
Such films can be used in applications that include optical coatings, batteries, solar cells, semiconductors and micro-electromechanical systems (MEMS). In addition, thin films can be used to protect other materials against degradation such as corrosion or harmful reactions with the environment.
One of Graham’s current research goals involves developing defect-free coatings using a method called atomic layer deposition (ALD), which deposits the films in a layer by layer fashion and gives unique control over film thickness and film composition. He is also researching layers that can be used to allow uniform growth of these films using ALD on virtually any material, including plastics, metals and organic electronics.
In work sponsored by the Department of Energy, Graham and his team are collaborating with professor Bernard Kippelen of the School of Electrical and Computer Engineering in the use of ALD to create barrier films that can protect flexible and organic electronics from being degraded by water vapor and oxygen in the ambient environment. They have found that materials including aluminum oxide and titanium oxide perform well in protecting the electronics underneath.
Testing these oxides includes measuring water vapor transport rates through very thin layers to gauge protection levels. Graham and his team are also looking at the use of these oxides in flexible electronics, testing how far these thin films can be bent or stretched before they develop cracks.
“One of the things we have found is that the thicker a film is, the less strain it takes to crack it, so the ultra-thin films are better,” Graham said. “That finding could benefit industry, because putting down a 20-nanometer thick film takes less time and material than producing a 50-nanometer thick film.”
Among other projects, Graham is researching the use of oxide films in the development of novel electrodes for organic electronics. The result could be more efficient and stable electrodes for organic solar cells, which is an emerging technology for future photovoltaic systems.
Developing Metal Foams
Antonia Antoniou, an assistant professor in the Woodruff School of Mechanical Engineering, studies the mechanics of materials at the nanoscale. She and her research team are synthesizing and studying metal foams, which are materials with nano-sized pores that behave like tiny sponges. The aim is to assess the unusual mechanical properties of these nanopore formations. Their intricate three-dimensional structure contains a large amount of surface area, along with a granular composition that offers myriad interfaces.
Such foams hold promise for applications including battery electrodes or supercapacitors, catalysts that increase chemical reactions, and tiny sensors.
Antoniou’s techniques enable structures to self-assemble at the nanoscale. She and her team start with a mixture of two or more metal elements, and then selectively use a corrosive environment to dissolve one or more of them. The result is an intricate porous network with special properties.
The researchers test the foams’ mechanical properties using a tiny probe called a nanoindenter. They also use electron microscopes to image the surface and view changes taking place at the nanoscale.
“The rules tend to change when you reach the atomic level,” Antoniou said. “Unlike a bulk alloy, these foams often have gigapascal strength, which is a very high level of strength for a metal and could enable certain challenging applications.”
Antoniou works with a variety of metals, especially platinum, copper and molybdenum. Collaborating with scientists at Georgia Tech and several other universities, she is investigating applying these foams to the needs of several industries.
In one National Science Foundation-sponsored project, the group is studying applications that could be useful to the nuclear industry. The foams’ innate strength lets them tolerate a significant amount of radiation, suggesting potential applications such as protective coatings.
“It’s important to remember that successful applications are based on understanding these materials at a fundamental level,” she said. “By synthesizing them, we exercise control over the structure, and then by testing them, we can discover unique behaviors.”
Advancing Smart Materials
Nazanin Bassiri-Gharb, an assistant professor in the Woodruff School of Mechanical Engineering, focuses her research on thin films and nanostructures made with ferroelectric materials – which have a spontaneous electric polarization that can be reversed by applying an external electric field.
“We call ferroelectrics smart materials, because they react to many different external fields – not only mechanical but also electrical and thermal – and they lend themselves to many applications including sensors, actuators and energy harvesting,” said Bassiri-Gharb, who has a joint appointment in the School of Materials Science and Engineering. “My group is specifically trying to understand the fundamental behavior of ferroelectric materials at the very small scale.”
Bassiri-Gharb and her research team are pursuing multiple research projects related to ferroelectrics. They’re working extensively on electromechanical response in piezoelectric materials, which produce a charge when mechanically stimulated. In a Small Business Innovation Research (SBIR) project funded by the Air Force, Bassiri-Gharb is working with a company to develop large-scale production of ferroelectric nanotubes to harvest energy from human stepping motion. The challenge is to distribute the material in a network of piezoelectric islands rather than in one piece. That configuration creates pathways that allow the maximized amount of the generated electric charge to flow to collection points such as batteries or capacitors.
In a collaborative project with Oak Ridge National Laboratory, Bassiri-Gharb and her team are working on understanding multiscale coupling of mechanical, electrical and chemical properties in oxide thin films – specifically as it applies to miniaturization of energy technologies including multilayer capacitors, batteries, and fuel cell devices.
“We’ve learned a great deal from this work, including the interaction between the piezoelectric thin film and its inactive silicon substrate” she said. “Our research shows that as we drastically reduce the thickness of the silicon, the piezoelectric response gets much larger.”
Probing Polymer Structure
Understanding how materials function at the smallest scales is key to modern materials science and engineering. David Bucknall, a professor in the School of Materials Science and Engineering, uses advanced techniques – including neutron scattering and X-ray scattering – to characterize polymers at the atomic and molecular levels.
“Using these scattering techniques, we can probe a material in situ – meaning in an application-related environment – allowing us to observe structural changes that occur during use or operation of the material,” he said. “By applying a number of complementary techniques, this allows us build up a three-dimensional understanding of the structure and gives us a picture of the complex interplay between a material’s microstructure and its efficiency or its robustness in a device.”
In the field of organic electronics, Bucknall and his team are working with colleagues in the Center for Organic Photonics and Electronics (COPE) to manipulate molecular-level surface interactions to achieve increased efficiency in organic photovoltaic materials. Neutron scattering is extremely useful in this Department of Energy-funded research, Bucknall said, because it provides one of the few methods to differentiate between the active materials in the organic photovoltaic devices, allowing determination of the interaction between the constituent materials.
Understanding this interaction between the materials allows COPE scientists to adjust the chemistry and synthesize new variations. The aim of the work is to build organic photovoltaic devices with much higher efficiencies than currently available.
Bucknall is also using X-ray scattering and neutron scattering techniques to characterize fracture and deformation mechanisms in polymer structures. In one project, he is working with a major energy company to determine the molecular origin of tear and puncture resistance in polyethylene.
In a related project, he is collaborating on a study of very high rate deformation of polymers with professor Naresh Thadhani, chair of the School of Materials Science and Engineering. The work involves propelling different polymeric materials at very high velocities at a solid block of steel and then recording the resulting shape changes in real time using high-speed imaging, spectroscopy, and interferometry techniques.
This work, Bucknall said, could lead to a deeper fundamental understanding of deformation in other polymers, such as those used in automotive vehicles and increasingly in aircraft.
Representing Structures Mathematically
The more accurately researchers can analyze a material’s structure at multiple scales – from the nanoscale to the micron scale and larger – the more fully they can explain that material’s properties and performance.
Understanding this complex relationship is a challenge. Arun Gokhale, a professor in the School of Materials Science and Engineering, uses mathematics and computer simulations to tackle such problems.
“My research focuses on how we mathematically represent a structure, how we simulate it, and how we bring that simulation into a model that explains the structure’s properties,” he said. “The microstructure of a material is almost always three-dimensional, and how those particles are distributed – how many are there, what is their size, what is their shape – dictates how it will behave.”
In one common approach, researchers obtain structural data by capturing multiple images of a microstructure. They sequentially remove very thin layers, imaging each with electron microscopes or other techniques.
This procedure produces an entire stack of sections, making possible a 3-D representation of the structure that reveals every particle, chain and cluster. By bringing geometrical and statistical processing to bear on this information, Gokhale and his team produce a features library – a mathematical description of the patterns that comprise a particular material.
The research team then uses this information to generate a computer simulation that closely represents the actual material. Using the simulation, researchers can vary the material parameters individually, allowing them to create virtual models of potential new materials.
“We can take these virtual materials, apply stress to them and see how they behave in different applications,” Gokhale said. “Simulations will never give you the exact answer – but you can narrow down the possibilities substantially.”
Gokhale has used this type of approach in multiple projects, including Department of Energy-funded research to design lighter weight vehicle components.
Harnessing Organic Electronics
In materials, the secret to obtaining desirable properties often lies in understanding the extreme details. Elsa Reichmanis, a professor in the School of Chemical and Biomolecular Engineering, is working to improve the performance of organic materials by understanding the complex relationship between how they’re made and how they perform in a device.
Organic polymers – a type of plastic – offer promise as flexible, lightweight semiconductors for applications that include solar cells, sensors, displays and other electronics. For one thing, organic polymers offer the potential for lower materials costs and for less expensive and demanding manufacturing processes.
With research funding from the National Science Foundation and the U.S. Air Force, Reichmanis and her research team are studying various organic polymers – amorphous, crystalline and semi-crystalline – to understand their microstructures. The researchers are seeking to identify how each material’s structure, process and device performance attributes correlate.
“We want to be able to more rationally design materials for a particular application,” Reichmanis said. “We’re also trying to build a knowledge base of fundamental insights that can be used to develop better materials and processes.”
To achieve this, Reichmanis and her team synthesize an organic polymer material in the lab, and then characterize its microstructure at the nanoscale. Finally, they prepare tiny lab-scale devices, 50 to 100 microns in size, by applying a thin film of an organic polymer to a silicon substrate. This configuration is used for testing purposes – an actual production device would likely be made entirely of organic polymers.
The team analyzes the complete material, from nanostructure to macrostructure, to understand how all the components work together, she explained. Microstructure regions tend to vary considerably, affecting performance; in some regions molecules will line up in desirable ways, others will be amorphous, and still others will form a complex mix.
“If we want a viable commercial technology, we have to be able to repeat – controllably and on a large scale – the microstructure that we want,” she said. “Only by understanding the fundamental side of things can we really affect control on the manufacturing side.”
Promoting Sustainability Through Materials
Working for Environmental Sustainability
Christopher W. Jones is studying carbon dioxide capture, a technology with obvious potential as CO2 builds up in Earth’s atmosphere.
Jones, who is the New-Vision Professor in the School of Chemical and Biomolecular Engineering (ChBE) and Georgia Tech’s associate vice president for research, is collaborating on the challenge of carbon dioxide with several ChBE faculty: assistant professor Ryan Lively; assistant professor Yoshiaki Kawajiri; professor William Koros, Roberto C. Goizueta Chair for Excellence in Chemical Engineering; professor and David Wang Sr. Fellow Matthew Realff, and professor David Sholl, Michael E. Tennenbaum Family Chair and a Georgia Research Alliance Eminent Scholar for Energy Sustainability.
The group is working on methods by which carbon dioxide, the principal agent in climate change, can be separated from other gases and prevented from being released into the atmosphere. Together, they are working on new techniques for capturing concentrated CO2 at major sources, such as coal and natural gas fired power plants.
In collaboration with Sholl, Jones is researching an even more challenging approach that involves pulling CO2 from ambient air anywhere on the planet. Georgia Tech is a leader in this technology, which could be useful environmentally, commercially and even tactically, said Jones, who is also an adjunct professor in the School of Chemistry and Biochemistry.
“We have developed a process that can capture CO2 anywhere – and do it almost as effectively as if we were capturing it at the flue of a power plant, where it’s 300 times more concentrated,” he said. “From a climate change perspective, this allows addressing CO2 from all sources – cars, trucks, planes – anywhere it’s being produced. In addition, the military could use it as a carbon source to make synthetic fuels in the field.”
The approach used by Jones and the team is based on the use of solid oxides functionalized with amines, which are strongly basic organic compounds that bind to carbon dioxide. Georgia Tech research has shown that varying the nature of these amines on the surface of an oxide produces new materials with tunable CO2 adsorption behavior.
A company called Global Themostat LLC has licensed some of these research findings, Jones said. The company is working with Georgia Tech on scaling up the process at a pilot plant in California.
Jones and a research team are also engineering catalytic materials with important potential uses in the production of energy-related products and other chemicals. In one effort, Jones is developing solid catalysts capable of making bulk chemicals like ethanol and 1-propanol at lower costs. In another study, he is working on enantioselective catalytic reactions to enable production of specialty organic chemicals or pharmaceuticals.
Developing Greener Semiconductors
For decades, transistors used in electronic devices have been growing ever smaller. Yet the amount of energy they demand has remained high – a fact obvious to anyone who’s worked with a hot laptop computer.
Eric Vogel, a professor in the School of Materials Science and Engineering, is working on a project that would help lower that energy need.
“As we move forward, the problem with silicon technology is not performance – we could actually get much more performance out of the transistors we have now,” said Vogel, who is also an adjunct professor in the School of Electrical and Computer Engineering. “The big problem is the amount of energy they need, so we’re focusing on new materials that would offer similar performance with much lower energy consumption.”
Vogel and his team are working within the Center for Low Energy Systems Technology (LEaST), one of six centers supported by STARnet, a Semiconductor Research Corp. program sponsored by the Microelectronics Advanced Research Corp. (MARCO) and DARPA.
The researchers are tackling the energy issue utilizing a variant of graphene technology. They’re using chemical vapor deposition to grow multiple thin layers of current-carrying graphene, separated by energy barriers, on a substrate.
This approach takes advantage of a technique called resonant tunneling of carriers to increase electron flow. Under this quantum mechanical concept, the wave-like behavior of an electron allows it to move readily through an energy barrier and appear on the other side in the next graphene layer. The result is increased efficiency in the energy-carrying electron flow.
Vogel and his team are working on the materials challenges involved in making these types of devices. An important issue centers on the role of the dielectric layer – the energy barrier – and how it affects electron tunneling.
One challenge involves the metal oxides used as dielectric layers, because they have defects that can hamper electron tunneling. The researchers have discovered that thinning a dielectric layer to one nanometer eliminates the tunneling difficulties.
The team is currently investigating why this happens, Vogel said. Understanding this phenomenon in depth could bring these graphene-based materials closer to real world use.
Building on basic research strengths in the materials domain, researchers at Georgia Tech are developing an innovation ecosystem that serves to translate transformative technologies into real world applications. Aligning with federal efforts such as the Materials Genome Initiative and the Advanced Manufacturing Partnership, Georgia Tech is continuing to establish productive and mutually beneficial relationships with government and industry.
John Toon also contributed to this article.
In addition to the main story providing highlights of Georgia Tech materials research, the Winter-Spring 2014 issue of Research Horizons also includes sidebars on the Center for Organic Photonics and Electronics (COPE), the Georgia Tech Institute for Materials (IMat), the Materials Research Science and Engineering Center (MRSEC), research into nanogenerators and piezotronics, and a study aimed at improving the durability of bridge infrastructure.
The Center for Organic Photonics and Electronics (COPE)
Formed by four faculty members in 2003, the Center for Organic Photonics and Electronics(COPE) performs research in a challenging but promising field: the use of organic materials to build photonic and electronic devices. These lightweight, low-cost plastics represent a new direction for a field dominated by devices based on silicon and a few other inorganic materials.
Today, COPE includes 36 Georgia Tech faculty members from seven different schools in a highly interdisciplinary approach to research, innovation and student training. Nearly 150 graduate students, undergraduates and postdoctoral researchers also work within COPE.
The center has extensive shared facilities for computing, chemical synthesis and materials characterization, along with device fabrication and testing. COPE has attracted more than $66 million in funding since its start; sponsors include the National Science Foundation, Department of Energy, Department of Agriculture, several Department of Defense agencies, the King Abdullah University of Science and Technology, the international chemical group Solvay, and numerous others.
“Once you can develop and validate organic semiconductors, you can build any solid-state device that could be made traditionally with inorganic semiconductors,” said Bernard Kippelen, a professor in the School of Electrical and Computer Engineering who is COPE’s director. “But to do all this, you need expertise that goes beyond the conventional disciplines of chemistry or physics or material science or electrical engineering.”
That, he added, is why COPE was interdisciplinary from the start. A physicist by training, Kippelen helped found the center along with three chemists – Jean-Luc Brédas, Seth Marder and Joseph Perry – who are professors in the School of Chemistry and Biochemistry. Today, COPE researchers are among the leaders in developing novel organic materials for 3-D microfabrication, photonic computing, solar cells and other applications.
Kippelen is also president of the Lafayette Institute at Georgia Tech-Lorraine in Metz, France. The Lafayette Institute, which benefits from 31 million Euros in financing from the French government, provides new opportunities for COPE in Europe due to its focus on technologies at the intersection of materials, optics, photonics, electronics and nanotechnology.
“At this point COPE, through its many different interactions, is literally part of a global network,” said Marder, a Regents’ Professor who was the center’s founding director and is now associate director. “That gives us not only a process for productive collaboration worldwide, it also provides a rich opportunity for my students to get real-world training that prepares them for a future in which research is becoming more and more interdisciplinary.”
The physical advantages of organic devices include light weight, flexibility, and puncture and shatter resistance. The manufacturing advantages include low cost; unlike conventional semiconductors, organic thin films can be processed at room temperature onto a variety of common materials using conventional large area coating and printing technologies.
- Organic materials can be used for semiconducting, insulating or conducting applications. Among those becoming commercially viable or in development are:
- Organic light-emitting diodes that are long lived, environmentally friendly and able to be used in flexible sheets over large areas. Such devices are starting to be used in cell phones and TV displays, as well as in solid-state lighting applications;
- Organic solar cells that are lightweight, flexible and shatterproof, making them easy to install and maintain; entire photovoltaic sheets could be readily recycled when worn out;
- Organic dielectrics and hybrid materials for high energy density electrical storage with fast charge and discharge times.
COPE research highlights include:
Nonlinear optical properties and materials
When light in the form of intense laser pulses hits certain materials, it produces a range of nonlinear effects. Marder and Perry, collaborating with Brédas and Kippelen, study ways to use nonlinear optical properties to fabricate novel three-dimensional structures at the nanoscale, and also to use those materials to pursue novel applications.
In one line of investigation, they’re collaborating with research teams from Georgia Tech and other universities to study how novel materials can advance photonic computing, a technology that uses light – photons – for interconnects and some all-optical computing functions. The aim is photonic computing capabilities that could offer greatly increased speed and bandwidth, along with much lower power consumption.
Professor John Reynolds, who recently joined the School of Chemistry and Biochemistry, the School of Materials Science and Engineering, and COPE, has developed a family of polymers that are electrochromic – electrically color-changing. A thin film of these plastics can be printed or sprayed onto a substrate, such as conductive glass or plastic; applying an electrical charge can then switch them instantly from clear to a specific color. The voltage of the applied charge dictates the color intensity.
Unlike other electrochromic techniques, this technology offers memory. That means the color remains when the charge is turned off, saving power. In addition, Reynolds’ technology is unique in offering any color needed, and has been licensed by the BASF Corp.
Georgia Tech Institute for Materials
The Georgia Tech Institute for Materials (IMat) was launched with one core mission: to foster materials-related research throughout the campus. Its June 2013 announcement came exactly two years after the White House launched its Materials Genome Initiative for Global Competitiveness.
The Materials Genome aids U.S. economic development by providing training and infrastructure to help U.S. innovators discover, develop and deploy advanced materials more quickly. The Georgia Tech move supports President Barack Obama’s call for faster movement of advanced materials from laboratory to application.
“Traditionally, it’s taken about 15 years to get a new materials discovery into an advanced product, but it only takes 18 to 36 months to design that new product on computers,” said David McDowell, IMat’s executive director and a Regents’ Professor in the Woodruff School of Mechanical Engineering. “There’s a big disconnect there, and we need to integrate materials design and development much more tightly with new product development.”
IMat is focusing on collaborative, interdisciplinary linkages to achieve new levels of cooperation. Its job involves linking materials-related research within Georgia Tech’s academic units and the Georgia Tech Research Institute (GTRI) to industry, government and academic research laboratories across the nation.
Through this collaborative network, IMat connects the expertise of investigative teams at Georgia Tech with the materials community outside, to help move research advances forward more rapidly. At the same time, it seeks to build bridges between Georgia Tech materials research and important application areas such as energy, manufacturing, nanotechnology, bioengineering and the biosciences.
IMat is one of nine Interdisciplinary Research Institutes (IRIs) under the leadership of Georgia Tech’s Executive Vice President for Research, Stephen E. Cross. Each IRI spans Georgia Tech units to bring together researchers working in a core area. In addition, the IRIs help government and industry navigate Georgia Tech’s myriad activities and connect with researchers, students and laboratory capabilities.
“The benefits of materials-based advances over the last 20 years are now a part of our everyday lives – lifesaving medical technologies, the computers and phones we can’t live without, our more efficient and safer vehicles, and much more,” said McDowell. “Materials research both discovers new materials and uses existing materials in new and enhanced ways. Our continued growth in a competitive global economy depends on performing effectively in these research areas and then applying those insights to real-world applications.”
The Materials Research Science and Engineering Center (MRSEC)
The Georgia Tech Materials Research Science and Engineering Center (MRSEC) studies primarily epitaxial graphene, a carbon-based material that can be grown in sheets as little as one atom thick. Because it’s an excellent electrical conductor, graphene promises advances in electronics technology. If its speed potential were realized, it could facilitate new and demanding computing applications. MRSEC launched in September 2008 thanks to a six-year, $8.1 million grant from the National Science Foundation. Georgia Tech leads the center, collaborating with the University of California-Riverside, University of Michigan and several European research teams.
“Silicon has fundamental limitations in its material properties that restrict its performance,” said MRSEC director Dennis Hess, who holds the Thomas C. DeLoach Jr. Chair in the School of Chemical and Biomolecular Engineering. “Silicon will always be around in basic devices, but for high-speed devices we either have to change the type of device we make or come up with a new material – and graphene is a contender for that role.”
At Georgia Tech, Hess explained, graphene research started in 2001 when Regents’ Professor Walt de Heer of the School of Physics determined that there might be better ways to make electronic devices than using cylindrical carbon nanotubes. As a result, de Heer, who directs MRSEC’s graphene interdisciplinary research group, turned to epitaxial graphene.
In principle, an electron can move in graphene 100,000 times faster than in silicon, making possible much higher speed devices, Hess said. In practice, attempts to achieve these electron speeds in a functional graphene device have been problematic.
De Heer has successfully pioneered methods for producing high-quality layers of epitaxial graphene on the surface of silicon carbide wafers. In addition, his technique for fabricating nanoribbons of epitaxial graphene has produced structures just 15 to 40 nanometers wide that conduct current with little resistance – offering the possibility of very high electrical performance.
But challenges remain, Hess said. They include devising methods for connecting graphene devices with conventional computing architectures, including how to incorporate contacts and dielectrics into such devices, how to input and output current, and how to generate patterns in the material.
“The fact is, if graphene can be successfully developed as a device platform, it should produce major advances in computing capability,” Hess said.
Zinc Oxide Nanostructures – Nanogenerators and Piezotronics
Zinc oxide, a familiar material used in plastics, paints, ointments, foods and many other things, can play significantly different roles at the nanoscale. A Georgia Tech research team led by Zhong Lin Wang, a Regents’ Professor and Hightower Chair in the School of Materials Science and Engineering, uses the unique properties of zinc oxide nanostructures to generate electrical energy that can power and control electronic devices.
Wang uses nanostructures to create a piezoelectric effect. In piezoelectrics, electrical energy is produced when charge-producing structures – in this case zinc oxide nanowires – are strained or flexed by some mechanical action, even a very minor one. The action can take the form of motion from many overlooked sources, such as the flow of fluids in the human body, vibration or the flexing of fabric in a shirt.
Wang and his research group have been studying zinc oxide nanostructures since 1999. They have increased the piezoelectric output from zinc oxide nanogenerators from negligible amounts to as much as 50 volts using sophisticated engineering design.
The researchers have also developed the field of piezotronics, which uses piezoelectric properties of zinc oxide nanostructures to control charge transport in an electronic device such as a semiconductor, offering an alternative to traditional CMOS technology.
Wang has also coined the term “piezo-phototronics” to describe techniques for using zinc oxide-based nanotechnology to control electro-optical processes in such devices as light-emitting diodes (LEDs) and solar cells to produce enhanced performance.
“People have never really harnessed this energy before, but its potential can be tremendous,” said Wang, a physicist by training. “Using these nanotechnologies, it is possible to have self-powered, maintenance-free biosensors, environmental sensors, nanorobotics, micro-electromechanical systems, and even portable and wearable electronics.”
Among the energy-harvesting strategies that Wang and his team have developed are running shoes that use a polymer based nanogenerator to create a charge as the user moves. He’s also built a “power shirt” that produces energy as the wearer moves, and he has employed piezo-phototronic technology to boost the performance of LEDs.
Some recent developments include:
Wang and his team have developed a sensor device that uses nanowires to convert mechanical pressure – from a signature or a fingerprint – directly into light signals that can be captured and processed optically. The research was reported in the journal Nature Photonics.
Beyond collecting signatures and fingerprints, the technique could also be used in biological imaging and micro-electromechanical (MEMS) systems. Ultimately, it could provide a new approach for human-machine interfaces.
Again using nanowires, the researchers have fabricated arrays of piezotronic transistors capable of converting mechanical motion directly into electronic controlling signals. The arrays could help give robots a more adaptive sense by making artificial skin smarter and more like human skin, allowing the skin to feel activity on the surface. The research was reported in the journal Science.
The arrays include more than 8,000 functioning piezotronic transistors, each of which can independently produce an electronic controlling signal when placed under mechanical strain. These touch-sensitive transistors – dubbed “taxels” – have sensitivity comparable to that of a human fingertip.
Branching out from zinc oxide, Wang and his team recently discovered yet another way to harvest small amounts of electricity from motion. They can now capture the electrical charge produced when two different kinds of plastic materials rub against one another.
Based on flexible polymer materials, this “triboelectric” generator could provide current from activities such as walking, offering an alternative to nanogenerators that produce current from flexing nanowires. An energy conversion efficiency of around 50 percent, providing output power density of 300 watts per square meter and 400 kilowatts per cubic meter, has been demonstrated, with the potential to harvest energy from body motion, engine vibration, wind, flowing water, raindrops and even ocean waves. Details of the discovery were reported in the journal Nano Letters.
Triboelectric generators can be made nearly transparent, so they could offer a new way to produce active sensors that might replace technology now used for touch-sensitive device displays.
Improving Infrastructure: Tougher Materials for Better Structures
Georgia Tech researchers have been working to help bridges and other coastal structures last longer, making them safer and less costly to maintain. Faculty from the School of Civil and Environmental Engineering (CEE) and the School of Materials Science and Engineering (MSE) have collaborated to develop a more robust design for a pile – a large post-like component used to support structures in water.
The goal is an improved design that could be applied to any concrete-and-steel structure that supports bridges, piers and the like. The work, sponsored by the Georgia Department of Transportation, is aimed at finding approaches that conform to a new state of Georgia directive requiring bridges and other infrastructure to last for 100 years, rather than the 40 to 50 years common today.
At a location on the Georgia coast near Savannah, a team including CEE professors Lawrence Kahn and Kimberly Kurtis and MSE professor Preet Singh examined concrete piles that had been in service for 37 years. The object was to pinpoint which environmental factors played the biggest role in the deterioration of the steel-reinforced structures.
The research team, which included several graduate students, studied what happened to the piles in the corrosive conditions like those along the seacoast.
“We saw damage that wasn’t surprising in a coastal environment, such as extensive corrosion,” Kurtis said. “But we also encountered unexpected factors, such as destruction to the piles due to attack by sulfate ions and by species of sponges that consume certain portions of the concrete.”
Working with industrial companies in Georgia and Tennessee, the team has developed a novel pile design. They’re using a more environmentally resistant type of high-performance marine concrete, which is reinforced using stainless steel rather than rust-prone carbon steel.
Singh, a corrosion expert, selected the most promising type of stainless steel from hundreds of available varieties. In a novel design move, he chose duplex-grade stainless steel for the pile’s pre-stressing strand.
The new pile formula being field-tested to measure its resistance to environmental deterioration. If successful, the design could become standard in the construction of bridges and other infrastructure throughout Georgia and elsewhere.
Research projects mentioned in this article are supported by sponsors that include the National Science Foundation (NSF), Office of Naval Research (ONR), Department of Energy (DOE), Department of Homeland Security (DHS), U.S. Air Force (USAF), Air Force Office of Scientific Research (AFOSR), U.S. Army (USA), Army Research Office (ARO), Oak Ridge National Laboratory (ORNL), Defense Threat Reduction Agency (DTRA) and the Defense Advanced Research Projects Agency (DARPA). Any opinions, findings, conclusions or recommendations expressed in this publication are those of the principal investigators and do not necessarily reflect the views of the NSF, ONR, DOE, DHS, USAF, AFOSR, USA, ARO, ORNL, DTRA or DARPA.