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.
This strain sensor, made with carbon nanotubes using aerosol printing technology, was developed by professor Chuck Zhang of the School of Industrial and Systems Engineering to help the aerospace industry improve the quality of parts made with composite materials, while also lowering production costs and ensuring long-term structural integrity. (Click image for high-resolution version. Credit: Rob Felt)
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.
David McDowell, a Regents’ Professor in the School of Mechanical Engineering, with a joint appointment in the School of Materials Science and Engineering, is executive director of the Georgia Tech Institute for Materials (IMat). IMat was launched recently to foster materials-related research at Georgia Tech. (Click image for high-resolution version. Credit: Gary Meek)
“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.
Professors Naresh Thadhani (left) and David Bucknall, both from the School of Materials Science and Engineering, examine samples of polymers and metals that were fired against a steel target from the gas gun behind them. They use a high-speed camera to monitor the dynamic deformation of each sample as it hits. (Click image for high-resolution version. Credit: Gary Meek)
“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.
Preet Singh, a professor in the School of Materials Science and Engineering, is studying environments such as pipelines to judge how well metallic alloys stand up to corrosive environments and stresses. (Click image for high-resolution version. Credit: Gary Meek)
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 School of Aerospace Engineering, designs metamaterials, which are synthetic composite structures with properties not found in materials derived from nature. Here he demonstrates a structure that could help reduce structural fatigue caused by continual flexing in aircraft wings. (Click image for high-resolution version. Credit: Rob Felt)
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 Rimoli, an assistant professor in the Guggenheim School of Aerospace Engineering, focuses on computational solid mechanics, which investigates the behavior of any solid material – including metals, ceramics, polymers, composites and metamaterials – through advanced modeling and computational techniques. (Click image for high-resolution version. Credit: Rob Felt)
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.
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. (Click image for high-resolution version. Credit: Rob Felt)
“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.
Nanotechnology is an essential element in many of the materials research projects taking place at Georgia Tech. Professor Kimberly Kurtis of the School of Civil and Environmental Engineering is using titanium dioxide nanoparticles to enhance the ubiquitous but essential material known as concrete. (Click image for high-resolution version. Credit: Gary Meek)
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.
Researchers Brent Wagner (left) and Zhitao Kang of the Georgia Tech Research Institute display several glass-matrix nanocomposite samples. The computer monitor on the right shows photonic crystal structures fabricated on glass. These materials convert incoming radiation into light using the process of scintillation. (Click image for high-resolution version. Credit: Gary Meek)
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.
Satish Kumar, a professor in the School of Materials Science and Engineering, leads a DARPA project to improve composite materials that are based on carbon fibers by using nanotechnology in the form of carbon nanotubes. Here he views magnified carbon fibers. (Click image for high-resolution version. Credit: Gary Meek)
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. (Click image for high-resolution version. Credit: Rob Felt)
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.
Robert Speyer, a professor in the School of Materials Science and Engineering, displays ceramic body armor panels produced at his Atlanta-based company, Verco Materials LLC., using patented processes he developed at Georgia Tech. Behind him is a 1,700-ton uniaxial hydraulic granulated powder compaction press. (Click image for high-resolution version. Credit: Gary Meek)
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.
Krista Walton (left), an associate professor in the School of Chemical and Biomolecular Engineering, and Ph.D. student Michael Dutzer, discuss surface area analysis for a metal organic framework. Walton and her team design materials for gas-mask-type devices and larger filters that protect against toxic industrial chemicals. (Click image for high-resolution version. Credit: Gary Meek)
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.
Kenneth Sandhage, a professor in the School of Materials Science and Engineering, uses tiny diatoms – a type of single-celled algae – to make unique materials. Here he points to an electron microscope image of a silica glass microshell produced by the Coscinodiscus diatom; the container to his left contains a culture of diatoms that can reproducibly generate such microshells. (Click image for high-resolution version. Credit: Gary Meek)
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.
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. (Click image for high-resolution version. Credit: Rob Felt)
“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