|This experimental setup was used by the team to measure the electrical output of a sample of solar cell material, under controlled conditions of varying temperature and illumination. The data from those tests was then used as the basis for computer modeling using statistical methods to predict the overall performance of the material in real-world operating conditions. Image, Riley Brandt|
The worldwide quest by researchers to find better, more efficient materials for tomorrow’s solar panels is usually slow and painstaking. Researchers typically must produce lab samples — which are often composed of multiple layers of different materials bonded together — for extensive testing.
Now, a team at MIT and other institutions has come up with a way to bypass such expensive and time-consuming fabrication and testing, allowing for a rapid screening of far more variations than would be practical through the traditional approach.
The new process could not only speed up the search for new formulations, but also do a more accurate job of predicting their performance, explains Rachel Kurchin, an MIT graduate student and co-author of a paper describing the new process that appears this week in the journal Joule. Traditional methods “often require you to make a specialized sample, but that differs from an actual cell and may not be fully representative” of a real solar cell’s performance, she says.
For example, typical testing methods show the behavior of the “majority carriers,” the predominant particles or vacancies whose movement produces an electric current through a material. But in the case of photovoltaic (PV) materials, Kurchin explains, it is actually the minority carriers — those that are far less abundant in the material — that are the limiting factor in a device’s overall efficiency, and those are much more difficult to measure. In addition, typical procedures only measure the flow of current in one set of directions — within the plane of a thin-film material — whereas it’s up-down flow that is actually harnessed in a working solar cell. In many materials, that flow can be “drastically different,” making it critical to understand in order to properly characterize the material, she says.
“Historically, the rate of new materials development is slow — typically 10 to 25 years,” says Tonio Buonassisi, an associate professor of mechanical engineering at MIT and senior author of the paper. “One of the things that makes the process slow is the long time it takes to troubleshoot early-stage prototype devices,” he says. “Performing characterization takes time — sometimes weeks or months — and the measurements do not always have the necessary sensitivity to determine the root cause of any problems.”
So, Buonassisi says, “the bottom line is, if we want to accelerate the pace of new materials development, it is imperative that we figure out faster and more accurate ways to troubleshoot our early-stage materials and prototype devices.” And that’s what the team has now accomplished. They have developed a set of tools that can be used to make accurate, rapid assessments of proposed materials, using a series of relatively simple lab tests combined with computer modeling of the physical properties of the material itself, as well as additional modeling based on a statistical method known as Bayesian inference.
The system involves making a simple test device, then measuring its current output under different levels of illumination and different voltages, to quantify exactly how the performance varies under these changing conditions. These values are then used to refine the statistical model.
“After we acquire many current-voltage measurements [of the sample] at different temperatures and illumination intensities, we need to figure out what combination of materials and interface variables make the best fit with our set of measurements,” Buonassisi explains. “Representing each parameter as a probability distribution allows us to account for experimental uncertainty, and it also allows us to suss out which parameters are covarying.”
The Bayesian inference process allows the estimates of each parameter to be updated based on each new measurement, gradually refining the estimates and homing in ever closer to the precise answer, he says.
In seeking a combination of materials for a particular kind of application, Kurchin says, “we put in all these materials properties and interface properties, and it will tell you what the output will look like.”
The system is simple enough that, even for materials that have been less well-characterized in the lab, “we’re still able to run this without tremendous computer overhead.” And, Kurchin says, making use of the computational tools to screen possible materials will be increasingly useful because “lab equipment has gotten more expensive, and computers have gotten cheaper. This method allows you to minimize your use of complicated lab equipment.”
The basic methodology, Buonassisi says, could be applied to a wide variety of different materials evaluations, not just solar cells — in fact, it may apply to any system that involves a computer model for the output of an experimental measurement. “For example, this approach excels in figuring out which material or interface property might be limiting performance, even for complex stacks of materials like batteries, thermoelectric devices, or composites used in tennis shoes or airplane wings.” And, he adds, “It is especially useful for early-stage research, where many things might be going wrong at once.”
Going forward, he says, “our vision is to link up this fast characterization method with the faster materials and device synthesis methods we’ve developed in our lab.” Ultimately, he says, “I’m very hopeful the combination of high-throughput computing, automation, and machine learning will help us accelerate the rate of novel materials development by more than a factor of five. This could be transformative, bringing the timelines for new materials-science discoveries down from 20 years to about three to five years.”
The research team also included Riley Brandt '11, SM '13, PhD '16; former postdoc Vera Steinmann; MIT graduate student Daniil Kitchaev and visiting professor Gerbrand Ceder, Chris Roat at Google Inc.; and Sergiu Levcenco and Thomas Unold at Hemholz Zentrum in Berlin. The work was supported by a Google Faculty Research Award, the U.S. Department of Energy, and a Total research grant.
David L. Chandler | MIT News Office
December 20, 2017
|MIT is on the path to achieve at least a 32 percent reduction in campus emissions of greenhouse gases by the year 2030.|
MIT has been forging ahead on strategies to implement the Institute’s Plan for Action on Climate Change, which was adopted in 2015. Two years after the plan was released, the Office of Sustainability and the Department of Facilities now confirm that MIT is on the path to achieve the plan’s call for at least a 32 percent reduction in campus emissions of greenhouse gases by the year 2030.
MIT’s greenhouse gas emissions have been reduced by 9 percent from 2016 levels, primarily due to electricity produced via a solar power purchase agreement last year with a solar farm in North Carolina, and by 16 percent from the 2014 baseline year – half of the minimum reduction called for. Without accounting for the solar energy purchase, MIT’s total greenhouse gas emissions in 2017 were flat compared to 2016 levels.
Using standard greenhouse-gas-accounting practices, MIT is able to reduce its carbon footprint by deducting the full amount of the purchased solar power from the amount of MIT’s grid-purchased electricity in Cambridge. Since the solar-generated electricity is considered to be carbon-free, the net impact is a reduction of greenhouse gas emissions associated with MIT’s greenhouse gas inventory. MIT’s purchase of power from the solar farm is equivalent to 40 percent of the Institute’s current electricity use.
While total campus emissions were flat, emissions associated with MIT’s academic buildings (comprising 94 percent of MIT’s total emissions) continued to decline in 2017. This is despite an increase in emissions from several other sources, including an increase in the use of specialty research gases, increases in the carbon-intensity of purchased grid electricity, more severe weather, and new campus buildings added to the academic building portfolio.
Energy efficiency measures have offset much of the emissions growth from other areas. In 2017, MIT documented an estimated savings of 581,000 kilowatt hours of electricity and 147,000 therms of thermal energy due to specific energy efficiency measures. This is equal to approximately 1,352 metric tons of carbon dioxide equivalents (MTCO2e) or 1 percent of total campus building emissions. Energy efficiency projects implemented included monitoring-based building commissioning, multibuilding utility pipe insulation, and utility water pumping reductions. MIT’s recently updated greenhouse gas inventory for 2017 can be read here.
MIT’s emissions reduction efforts are being informed by a recently completed report outlining plans for cutting campus greenhouse gas emissions. The report was developed collaboratively by the Department of Facilities, the Office of Sustainability, the Office of Campus Planning, and the Environment, Health and Safety Office. The report lays out a roadmap of strategies, and a timeline for implementing its highest-priority measures over the next five years, providing a clear pathway toward achieving the Institute’s near-term emissions reduction goals.
The campus greenhouse gas reduction plan outlined in the report centers on four key approaches: reducing the overall energy use on campus, reducing the use of fossil fuels in campus buildings and vehicles, increasing the use of renewable energy sources to meet campus needs, and minimizing the release of “fugitive” gases from campus operations such as specialty research gases in laboratory buildings. The plan seeks to implement the concept of using the campus as a kind of “living laboratory” to explore innovative and scalable ways of tackling the daunting challenges of climate change, and to use that living laboratory to enhance the educational experience of MIT’s students and provide new hands-on teaching opportunities for its faculty.
“Maximizing energy efficiency across our campus operations in both our existing and new buildings is our first priority,” the report says. In addition, reducing the use of traditional fossil fuels on campus, and developing increased sources of renewable energy, both on campus and in the region, are key components. Beyond reducing MIT’s own carbon footprint, the plan is strongly geared toward solutions that have the potential to be replicated or adapted by other institutions across the country and around the world to maximize their impact.
“A major enabling strategy in the plan is a Department of Facilities program to comprehensively upgrade building utility meters across campus,” says Bernie Richard, director of the Department of Facilities Systems Engineering Group. “This enhanced metering will allow for accurate and credible validation of energy efficiency measures, as well as help to identify appropriate priorities and opportunities.”
The plan also calls for a focus on advancing alternative fuel sources by MIT’s vehicles, potentially adding to rooftop solar panels, and possibly adding new purchases of off-campus renewable energy similar to the existing agreement for the Summit Farms solar plant in North Carolina, which was made possible by the commitment from MIT and its partners.
Another major project is the completion of a refurbishment of the campus Central Utility Plant, a cogeneration plant that currently supplies about 50 percent of the electricity to the main campus, as well as heat and chilled water to most of the central campus buildings. The project, which will take up to three years to complete, will increase its capacity and reduce greenhouse gas emissions by approximately 10 percent, by eliminating all use of fuel oil (except for emergencies) and installing newer, more efficient turbines. Overall, it will become both more efficient and cleaner in its output.
One new addition to the suite of actions to create greater progress toward MIT’s stretch goal of seeking carbon neutrality is a new class that will be offered in the spring 2018 semester, taught jointly by Director of Sustainability Julie Newman and professor of mechanical engineering Timothy Gutowski. Engaging diverse expertise from MIT’s faculty and staff, the class will explore what might be the challenges and potential solutions necessary to usher MIT into a climate-neutral future.
“I am grateful to the members of the greenhouse gas reduction working group for the effort and thought they put into producing this strategy,” says Maria T. Zuber, MIT’s vice president for research. “And I am thrilled that, through the new class being offered this spring, students will participate in a hands-on way in the effort to better understand how MIT can achieve its carbon neutrality aspiration. This is vitally important work, and we need the best ideas we can get.”
David L. Chandler | MIT News Office
November 21, 2017
|XiaoYu Wu pictured with the reactor his team used for the research. MIT researchers have developed a new system that could potentially be used for converting power plant emissions of carbon dioxide into useful fuels. The method may not only cut greenhouse emissions; it could also produce another potential revenue stream to help defray its costs. Image, Tony Pulsone|
MIT researchers have developed a new system that could potentially be used for converting power plant emissions of carbon dioxide into useful fuels for cars, trucks, and planes, as well as into chemical feedstocks for a wide variety of products.
The new membrane-based system was developed by MIT postdoc Xiao-Yu Wu and Ahmed Ghoniem, the Ronald C. Crane Professor of Mechanical Engineering, and is described in a paper in the journal ChemSusChem. The membrane, made of a compound of lanthanum, calcium, and iron oxide, allows oxygen from a stream of carbon dioxide to migrate through to the other side, leaving carbon monoxide behind. Other compounds, known as mixed ionic electronic conductors, are also under consideration in their lab for use in multiple applications including oxygen and hydrogen production.
Carbon monoxide produced during this process can be used as a fuel by itself or combined with hydrogen and/or water to make many other liquid hydrocarbon fuels as well as chemicals including methanol (used as an automotive fuel), syngas, and so on. Ghoniem’s lab is exploring some of these options. This process could become part of the suite of technologies known as carbon capture, utilization, and storage, or CCUS, which if applied to electicity production could reduce the impact of fossil fuel use on global warming.
The membrane, with a structure known as perovskite, is “100 percent selective for oxygen,” allowing only those atoms to pass, Wu explains. The separation is driven by temperatures of up to 990 degrees Celsius, and the key to making the process work is to keep the oxygen that separates from carbon dioxide flowing through the membrane until it reaches the other side. This could be done by creating a vacuum on side of the membrane opposite the carbon dioxide stream, but that would require a lot of energy to maintain.
In place of a vacuum, the researchers use a stream of fuel such as hydrogen or methane. These materials are so readily oxidized that they will actually draw the oxygen atoms through the membrane without requiring a pressure difference. The membrane also prevents the oxygen from migrating back and recombining with the carbon monoxide, to form carbon dioxide all over again. Ultimately, and depending on the application, a combination of some vaccum and some fuel can be used to reduce the energy required to drive the process and produce a useful product.
The energy input needed to keep the process going, Wu says, is heat, which could be provided by solar energy or by waste heat, some of which could come from the power plant itself and some from other sources. Essentially, the process makes it possible to store that heat in chemical form, for use whenever it’s needed. Chemical energy storage has very high energy density — the amount of energy stored for a given weight of material — as compared to many other storage forms.
At this point, Wu says, he and Ghoniem have demonstrated that the process works. Ongoing research is examining how to increase the oxygen flow rates across the membrane, perhaps by changing the material used to build the membrane, changing the geometry of the surfaces, or adding catalyst materials on the surfaces. The researchers are also working on integrating the membrane into working reactors and coupling the reactor with the fuel production system. They are examining how this method could be scaled up and how it compares to other approaches to capturing and converting carbon dioxide emissions, in terms of both costs and effects on overall power plant operations.
In a natural gas power plant that Ghoniem’s group and others have worked on previously, Wu says the incoming natural gas could be split into two streams, one that would be burned to generate electricity while producing a pure stream of carbon dioxide, while the other stream would go to the fuel side of the new membrane system, providing the oxygen-reacting fuel source. That stream would produce a second output from the plant, a mixture of hydrogen and carbon monoxide known as syngas, which is a widely used industrial fuel and feedstock. The syngas can also be added to the existing natural gas distribution network.
The method may thus not only cut greenhouse emissions; it could also produce another potential revenue stream to help defray its costs.
The process can work with any level of carbon dioxide concentration, Wu says — they have tested it all the way from 2 percent to 99 percent — but the higher the concentration, the more efficient the process is. So, it is well-suited to the concentrated output stream from conventional fossil-fuel-burning power plants or those designed for carbon capture such as oxy-combustion plants.
“It is important to use carbon dioxide to produce carbon monoxide for the conversion of sustainable thermal energies to chemical energy,” says Xuefeng Zhu, a professor of chemical physics at the Chinese Academy of Sciences, in Dalian, China, who was not involved in this work. “Using an oxygen-permeable membrane can significantly reduce the reaction temperature, from 1,500 C to less than 1,000 C, indicating a great energy saving compared to the traditional carbon dioxide decomposition process,” he says. “I think their work is important to the field of sustainable energy and membrane processes.”
The research was funded by Shell Oil and the King Abdullah University of Science and Technology.
David L. Chandler | MIT News Office
November 28, 2017
In large parts of the developing world, people have abundant heat from the sun during the day, but most cooking takes place later in the evening when the sun is down, using fuel — such as wood, brush or dung — that is collected with significant time and effort.
Now, a new chemical composite developed by researchers at MIT could provide an alternative. It could be used to store heat from the sun or any other source during the day in a kind of thermal battery, and it could release the heat when needed, for example for cooking or heating after dark.
A common approach to thermal storage is to use what is known as a phase change material (PCM), where input heat melts the material and its phase change — from solid to liquid — stores energy. When the PCM is cooled back down below its melting point, it turns back into a solid, at which point the stored energy is released as heat. There are many examples of these materials, including waxes or fatty acids used for low-temperature applications, and molten salts used at high temperatures. But all current PCMs require a great deal of insulation, and they pass through that phase change temperature uncontrollably, losing their stored heat relatively rapidly.
Instead, the new system uses molecular switches that change shape in response to light; when integrated into the PCM, the phase-change temperature of the hybrid material can be adjusted with light, allowing the thermal energy of the phase change to be maintained even well below the melting point of the original material.
This blue LED lamp setup is used to trigger the heat discharge from large-scale films of phase-change materials. (Melanie Gonick/MIT)
The new findings, by MIT postdocs Grace Han and Huashan Li and Professor Jeffrey Grossman, are reported this week in the journal Nature Communications.
“The trouble with thermal energy is, it’s hard to hold onto it,” Grossman explains. So his team developed what are essentially add-ons for traditional phase change materials, or, “little molecules that undergo a structural change when light shines on them.” The trick was to find a way to integrate these molecules with conventional PCM materials to release the stored energy as heat, on demand. “There are so many applications where it would be useful to store thermal energy in a way lets you trigger it when needed,” he says.
The researchers accomplished this by combining the fatty acids with an organic compound that responds to a pulse of light. With this arrangement, the light-sensitive component alters the thermal properties of the other component, which stores and releases its energy. The hybrid material melts when heated, and after being exposed to ultraviolet light, it stays melted even when cooled back down. Next, when triggered by another pulse of light, the material resolidifies and gives back the thermal phase-change energy.
“By integrating a light-activated molecule into the traditional picture of latent heat, we add a new kind of control knob for properties such as melting, solidification, and supercooling,” says Grossman, who is the Morton and Claire Goulder and Family Professor in Environmental Systems as well as professor of materials science and engineering.
The UV-activated thermal energy storage material shows the rapid crystallization and heat discharge upon visible light (blue LED) illumination. (Grossman Group at MIT)
The system could make use of any source of heat, not just solar, Han says. “The availability of waste heat is widespread, from industrial processes, to solar heat, and even the heat coming out of vehicles, and it’s usually just wasted.” Harnessing some of that waste could provide a way of recycling that heat for useful applications.
“What we are doing technically,” Han explains, “is installing a new energy barrier, so the stored heat cannot be released immediately.” In its chemically stored form, the energy can remain for long periods until the optical trigger is activated. In their initial small-scale lab versions, they showed the stored heat can remain stable for at least 10 hours, whereas a device of similar size storing heat directly would dissipate it within a few minutes. And “there’s no fundamental reason why it can’t be tuned to go higher,” Han says.
In the initial proof-of-concept system “the temperature change or supercooling that we achieve for this thermal storage material can be up to 10 degrees C (18 F), and we hope we can go higher,” Grossman says.
Under a dark-field microscope, the microscale environment shows the rapid crystal growth can easily be monitored. (Grossman Group at MIT)
Already, in this version, “the energy density is quite significant, even though we’re using a conventional phase-change material,” Han says. The material can store about 200 joules per gram, which she says is “very good for any organic phase-change material.” And already, “people have shown interest in using this for cooking in rural India,” she says. Such systems could also be used for drying agricultural crops or for space heating.
“Our interest in this work was to show a proof of concept,” Grossman says, “but we believe there is a lot of potential for using light-activated materials to hijack the thermal storage properties of phase change materials.”
“This is highly creative research, where the key is that the scientists combine a thermally driven phase-change material with a photoswitching molecule, to build an energy barrier to stabilize the thermal energy storage,” says Junqiao Wu, a professor of materials science and engineering at the University of California at Berkeley, who was not involved in the research. “I think the work is significant, as it offers a practical way to store thermal energy, which has been challenging in the past.”
The work was supported by the Tata Center for Technology and Design within MIT’s Energy Initiative.
David L. Chandler | MIT News Office
November 16, 2017
Interdisciplinary materials research holds the key to solving the e2xistential challenges facing humanity, former Sandia National Laboratories executive Julia M. Phillips told the annual MIT Materials Research Laboratory [MRL] Materials Day Symposium on Wednesday, Oct. 11, 2017. “What is both very exciting for us as materials researchers, also a little frustrating, is that the real impact of materials occurs when they turn into something that you actually carry around in your pocket or whatever,” Phillips said.
During the second half of the 20th century, many of the technological advances that we take for granted today, such as laptop computers and smart phones, came from fundamental advances in materials research and the ability to control and make materials, she noted. Phillips, who retired from Sandia National Laboratories as Vice President and Chief Technology Officer, also serves as chair of the MRL External Advisory Board and is a member of the National Science Board.
MRL formed from the merger of the Materials Processing Center and the Center for Materials Science and Engineering, effective Oct. 1, 2017. MRL Director Carl V. Thompson noted in his introductory remarks, the appointment of Associate Professor of Materials Science and Engineering Geoffrey S.D. Beach as co-director of the MRL and principal investigator for the National Science Foundation Materials Research Science and Engineering Center.
Fueled by industrial needs and government-funded research in the post-World War II era, “Materials research was undeniably an early model for interdisciplinary research,” Phillips said. With new tools such as scanning probe microscopes to understand the structure and properties of materials, materials scientists in the last half of the 20th Century created whole new classes of materials and products, ranging from super alloys that enabled larger and more reliable jet engines to strained layer superlattices that underlie modern magnetic recording,7 lasers and infrared detectors.
Future gains will come from the ability to synthesize and control increasingly complex materials, Phillips says, noting progress in areas such as high-temperature superconductors, porous solids like metal organic frameworks, and metamaterials that generate new properties from combining biological materials, organics, ceramics and metals at near molecular scale precision in ways not found in nature. “Somewhere in the fuzzy space between molecules and materials,” Phillips notes, these newer materials have very interesting properties that are still in the process of being fully explored, and they will be exploited in the years to come. “It’s very clear to many people that these also will be transformational as we move forward,” she says.
The materials research approach, which brings together researchers from across different science and engineering fields to solve complex problems, provides a model for solving 21st Century challenges in energy, environment and sustainability; health care and medicine; vulnerability to human and natural threats; and expanding and enhancing human capability and joy. “These are exemplars, but you can see materials written all over this list, and I would posit that any comparable list you might come up with would have materials written all over it,” Phillips said. “In order to address those grand challenges, we really need to be able to treat realistically complex systems that bring together all of these disciplines from the sciences, from engineering, from the social and behavioral sciences, and arguably even from the arts.”
Progress in scientific understanding and computational modeling are accelerating researchers’ ability to predict the structure and properties of new materials before actually making them, Phillips said.
MIT faculty members Antoine Allanore, Polina Anikeeva, A. John Hart, Pablo Jarillo-Herrero, Juejun Hu, and Jennifer Rupp presented research updates on their recent work which spans a range from ultra-thin layered materials for new electronic devices and cellular level probes for the brain and spinal cord to larger scale methods for 3D printing and metals processing.
Merging 2D materials with CMOS
Associate Professor of Physics Pablo Jarillo-Herrero stacks atomically thin, two-dimensional [2D] layers of different materials to discover new properties. Jarillo-Herrero’s lab demonstrated photodetectors, solar cells and the world’s thinnest LED. With materials such as tungsten selenide [WSe2], changing the number of layers also changes their electronic properties. Although graphene itself has no bandgap, closely aligning the lattices of graphene and boron nitride opens a 30-millivolt bandgap in graphene, he said.
“You have full electronic control with gate voltages,” Jarillo-Herrero said. Using bilayer molybdenum ditelluride, which is 10,000 times thinner than a silicon solar cell, he showed in work published in Nature Nanotechnology, a photodetector just 10 nanometers thick can be integrated on a silicon photonic crystal waveguide.
“You can just stack this at the very end of your CMOS [complementary metal oxide semiconductor] processing, and you don’t have to do any extra fabrication, any extra growth, you can just slap it on top,” Jarillo-Herrero explained. “It can be made as thin as 4 nanometers, so it’s still ultra thin, and you have a high degree of control in an ultra thin platform. The whole thing is semitransparent so we can see the light go in and out.” These new devices can be operated at telecommunications wavelengths by tuning the bandgap of the material.
Phase change materials
Juejun (JJ) Hu, the Merton C. Flemings Associate Professor of Materials Science and Engineering, is reducing power consumption, shrinking device size and ramping up processing speed with innovative combinations of materials that alternate between two different solid states, or phases, such as an alloy of germanium, antimony, selenium and tellurium. These materials are the basis for nonvolatile storage, meaning their memory state is preserved even when the power is turned off. Hu collaborated with MIT Professor Jeffrey C. Grossman and former postdoc Huashan Li to identify desirable materials for these alloys from first principles calculations, and graduate materials science and engineering student Yifei Zhang did much of the experimental work.
An earlier generation of devices based on germanium, antimony and tellurium [GST] suffers from losses to light absorption by the material. To overcome this problem, Hu substituted some of the tellurium with a lighter element, selenium, creating a new four-element structure of germanium, antimony, selenium and tellurium [GSST]. “We increase the bandgap to suppress short wavelength absorption, and we actually minimize any carrier mobility to mitigate the free carrier absorption,” he explained. Switching between amorphous and crystalline states can be triggered with a laser pulse or an electrical signal.
Although the structural state switching happens on the order of 100 nanoseconds, figuring out the techniques to accomplish it took a year of work, Hu said. Specifically, he found that using materials that switch between amorphous and crystalline states allows light to be directed over two different paths and reduces power consumption. He coupled this GSST optical phase change material with silicon nitride microresonators and waveguides to show this behavior. These switches based on phase change materials can be connected in a matrix to enable variable light control on a chip. Ultimately, Hu hopes to use this technology to build re-programmable photonic integrated circuits.
New tools for brain exploration
Class of 1942 Associate Professor in Materials Science and Engineering Polina Anikeeva works at the border between synthetic devices and the nervous system. Traditional electronic devices, with hardness like a knife, can trigger a foreign-body response from brain tissue, which typically is as soft as pudding or yogurt. Working with Prof. Yoel Fink and other MIT colleagues, Anikeeva developed soft polymer-based devices to stimulate and record activity of brain and spinal cord tissue borrowing from optical fiber drawing techniques.
An early version of their multi-functional fibers included three key elements: conductive polyethylene carbon composite electrodes to record brain cell activity; a transparent polycarbonate waveguide with cyclic olefin copolymer cladding to deliver light; and microfluidic channels to deliver drugs.
“Using this structure, for the first time, we were able to record, stimulate and pharmacologically modulate neural activity,” Anikeeva said. But the device recorded activity from clusters of neurons, not individual neurons. Anikeeva and her team addressed this problem by integrating graphite into the polyethylene composite electrodes, which increased their conductivity enough to shrink them into a structure that is as thin as a human hair. The device has six electrodes, an optical waveguide and two microfluidic channels.
Yet adding graphite increased the size and hardness of the glassy polycarbonate device, so her group turned to a new process using rubbery, stretchy polymers that they then coated with a conductive metal nanowire mesh. “This mesh of conductive metal nanowires can maintain low impedance even at 100 percent strain, and it maintains its structural integrity without any changes up to 20 percent strain, which is sufficient for us to operate in the spinal cord,” Anikeeva said.
Her students implanted these nanowire-mesh coated fibers in mice, which allowed them to stimulate and record neural activity in the spinal cord. A video showed a mouse moving its hindlimb when an optical signal delivered to the lumbar spinal cord traveled down the sciatic nerve to the gastrocnemius muscle. In these experiments, the device implanted in mice showed no decline in performance a year after surgery, Anikeeva said.
More recently, Anikeeva developed iron oxide-based nanoparticles that heat up in an applied magnetic field, which can trigger a response from neurons in the brain that express ion channels that are sensitive to heat such as capsaicin receptor, the same mechanism that is triggered when we eat hot peppers. Experimenting with mice, Anikeeva injected these tiny particles deep in the brain in a section that is associated with reward. “In our lab, we have started by modeling hysteresis in magnetic nanoparticles, synthesizing a broad range of these nanomaterials by engineering iron oxide with dopants and looking at different sizes and shapes, developing power electronics and a biological tool kit to assess this process,” Anikeeva explained. “In this case, there is no external hardwire, no wires, no implants, nothing is sticking out of the brain… however, they can now perceive magnetic field.” she said. To quantify their results, the researchers measured calcium ion influx into neurons. Work is now focused on shortening the response time to a few thousandths of a second by improving the heat output of the magnetic nanoparticles.
Ceramics for Solid-State Batteries, CO2 Sensors and Memristive Computing
Jennifer L. M. Rupp, the Thomas Lord Assistant Professor of Materials Science and Engineering, presented research showing a solid lithium garnet electrolyte can lead to batteries miniaturized on an integrated circuit chip.
Safety concerns regarding lithium batteries stem from their liquid component, which serves as the electrolyte and presents a risk of catching fire in air. Replacing the liquid electrolyte with a solid one could make batteries safer, Rupp explained. Her research shows that a ceramic material made of garnet, a material that is perhaps more familiar as a gemstone, can effectively pass lithium through a battery cell, but because it is solid, can be very safe for batteries and also have the opportunity to be miniaturized to thin film architectures. This garnet is a four-element compound of lithium, lanthanum, zirconium and oxygen. “The lithium is completely encapsulated; there is no risk of inflammation,” Rupp said.
In published research, Rupp showed that pairing a lithium titanium oxide anode with a ceramic garnet electrolyte and blurring the interface between the two materials allowed much faster battery charging time for large-scale cells. Lessons learned from applying these garnet materials pointed also to a new use for carbon dioxide sensing. “We can reconfigure the electrodes to have one electrode which simply goes as a reference, and another which undergoes a chemical reaction with carbon dioxide, and we use a tracker potential to track the effective change of carbon dioxide concentration in the environment based on bulk processing,” she explained. Rupp is also developing strained multi-layer materials to improve storage for memristive memory and computing elements.
Frontier for metals at high temperature
Associate Professor of Metallurgy Antoine Allanore pointed out that from 1980 to 2010, the world almost doubled its consumption of materials, with the fastest growth in metals and minerals. Such demand is due to the formidable low cost and high productivity of materials processing. The majority of such processes involve at some stage a high temperature operation and often the molten state of matter. Developing the science and engineering of the molten state brings huge opportunities, for example heat management in high-temperature processes such as metals extraction and glass making.
Steelmaking, for example, is already a highly efficient manufacturing process, turning out rebar, coil or wires of steel at a cost less than 32 cents per kilogram [about 15 cents per pound]. “Productivity is actually the key criteria to make materials processing successful and matter at the scale of the challenge of adding 2 billion people in the next 20 years,” he said.
Allanore’s group demonstrated that tin sulfide at high temperature, about 1,130 degrees Celsius [2,066 Fahrenheit], is an effective thermoelectric generator. “We have indications that the theoretical figure of merit for some sulfides, can be up to 1 at 1,130 [degrees Celsius]. For molten copper sulfide for example, we have estimates of the thermal conductivity, the melting point, and we have a cost that is a little bit high in my opinion, but that’s the nature of the research,” Allanore said. When his group looked at existing data, they found that for many molten compounds of sulfur and a metal, such as tin, lead or nickel, the thermoelectric figure of merit, as well as the compositional phases, had never been quantified, opening a frontier for new materials science research at high temperature. “It’s actually very difficult to know what are the true properties of the liquid,” Allanore said. “I need to know if that material will have semiconductivity. I need to know if it’s going to be denser or lighter than another liquid. … We don’t actually have computational methods to predict such property for liquids at high temperature.”
To address the problem, Allanore studied the relation in high-temperature melts between transport properties, including electrical conductivity and Seebeck coefficients, and a thermodynamic property called entropy. “We’ve put together a theoretical model that connects the transport property, like thermal power, and the thermodynamic property like entropy. This is important because it works for semiconductors, it works for metallic materials and more importantly it allows to find out regions of immiscibility in liquids,” Allanore said. Immiscibility means a material in the given condition will separate into two phases that do not mix together and remain separate.
Allanore has also developed a new method for observing molten compounds such as alumina, using a floating zone furnace, which is a transparent quartz tube located at the focal distance of four lamps. “If we can do that with oxides, we would really like to do that with sulfides,” he explained, showing a picture of molten tin sulfide sitting on a graphite plate in the floating zone furnace. The wide range of temperatures and properties of molten materials, “the ultimate state of condensed matter”, allows for better heat management, higher processing temperatures and electricity harvesting or electrical control of heat flow, he said.
3D printing a new manufacturing model
Traditional manufacturing requires economies of scale, in particular, large production volumes because of the fixed costs necessary to set up the production process, but 3D printing and other additive manufacturing technologies offer an alternative of high-performance, customizable products and devices, Associate Professor of Mechanical Engineering A. John Hart said.
Additive manufacturing is already a $6 billion a year business with reach from Hollywood special effects to high-tech jet engine nozzles. “Additive manufacturing already enables a diverse collection of materials, applications, and related processes – including by extrusion of plastics, melting metals, using lasers, and by coordinated chemical reactions that essentially are done with point wise control,” Hart explained.
“We can think of accessing new spaces in terms of the value of the products we create using additive manufacturing, also generally known as 3D printing. 3D printing is reshaping the axes by which we judge the economic viability of a manufacturing process, and allowing us to access new value spaces. For instance, we can think not only about production volume, but think about advantages in complexity of geometries, and advantages by customization of products to specific markets or even individuals. In these ways, 3D printing is influencing the entire product life cycle,” Hart said.
For instance, Hart’s group studied existing 3D printers to discover how to speed up the process from about 60 minutes to just 5 to 10 minutes to print a handheld mechanical part such as a gear. Former graduate student Jamison Go [SM, 2015] led this work, Hart said, building a desktop 3D printer about the size of a small microwave oven. The system features a control system for the printhead that moves the motors to the corner; an extrusion mechanism that drives the feedstock polymer filament like a screw; and a laser that penetrates and melts the polymer.
“By combining the fast motion control, the high heat transfer, and the high force, we can overcome the limits of the existing system,” Hart explained. The new design is three to 10 times faster in build rate than existing machines. “These kinds of steps forward can also change how we think about producing objects. If you can make something fast, you can think about how you might, or how others might, work differently,” he said. He mentioned, for instance, physicians who may need to 3D print a part for an emergency medical operation, or a repair technician who could use a 3D printer rather than hold inventory of many spare parts.
Hart’s group is currently working in collaboration with Oak Ridge National Lab on algorithms for optimization of 3D printing toolpaths, and adapting his innovations to large-scale 3d printers. “We can think about upscaling these principles to high productivity systems that are not only printing small things but printing big things,” Hart said. Hart has also worked with 3D printing of cellulose, which can be used for customization of consumer products and antimicrobial devices, and is the world’s most abundant natural polymer. He co-founded the company Desktop Metal with three other MIT faculty members and Ric Fulop SL ’06, who serves as Desktop Metal’s CEO. “The company is only two years old and will soon ship its first product which enables an entirely new approach to metal 3D printing,” Hart said.
– Denis Paiste, MIT Materials Research Laboratory
October 30, 2017
Coming up in November Newsletter: Materials Day Panel Discussion and Poster Session coverage
|“One of the biggest missing pieces” needed to make skyrmions a practical data-storage medium, Geoffrey Beach says, was a reliable way to create them when and where they were needed. “So this is a significant breakthrough.” Illustration by Moritz Eisebitt|
New research has shown that an exotic kind of magnetic behavior discovered just a few years ago holds great promise as a way of storing data — one that could overcome fundamental limits that might otherwise be signaling the end of “Moore’s Law,” which describes the ongoing improvements in computation and data storage over recent decades.
Rather than reading and writing data one bit at a time by changing the orientation of magnetized particles on a surface, as today’s magnetic disks do, the new system would make use of tiny disturbances in magnetic orientation, which have been dubbed “skyrmions.” These virtual particles, which occur on a thin metallic film sandwiched against a film of different metal, can be manipulated and controlled using electric fields, and can store data for long periods without the need for further energy input.
In 2016, a team led by MIT associate professor of materials science and engineering Geoffrey Beach documented the existence of skyrmions, but the particles’ locations on a surface were entirely random. Now, Beach has collaborated with others to demonstrate experimentally for the first time that they can create these particles at will in specific locations, which is the next key requirement for using them in a data storage system. An efficient system for reading that data will also be needed to create a commercializable system.
The new findings are reported this week in the journal Nature Nanotechnology, in a paper by Beach, MIT postdoc Felix Buettner, and graduate student Ivan Lemesh, and 10 others at MIT and in Germany.
The system focuses on the boundary region between atoms whose magnetic poles are pointing in one direction and those with poles pointing the other way. This boundary region can move back and forth within the magnetic material, Beach says. What he and his team found four years ago was that these boundary regions could be controlled by placing a second sheet of nonmagnetic heavy metal very close to the magnetic layer. The nonmagnetic layer can then influence the magnetic one, with electric fields in the nonmagnetic layer pushing around the magnetic domains in the magnetic layer. Skyrmions are little swirls of magnetic orientation within these layers, Beach adds.
Read more at the MIT News Office.
David Chandler | MIT News Office
October 2, 2017
|In this diagram, the atomic lattice of a crystal of barium oxide is depicted, with atoms of oxygen and barium represented by red and gray spheres. A neutral oxygen vacancy, a place where an oxygen atom should appear in the lattice but is instead replaced by two electrons, is represented by the yellow shape, which depicts the charge density of those electrons. At left, the crystal is seen with no electric field applied, and at right, with an applied field of 21.8 megavolts per centimeter. The distortions of the lattice reveal the effects of that applied electric field. Image, Felice Frankel|
Sometimes things that are technically defects, such as imperfections in a material’s crystal lattice, can actually produce changes in properties that open up new kinds of useful applications. New research from a team at MIT shows that such imperfections in a family of materials known as insulating metal oxides may be key to their performance for a variety of high-tech applications, such as nonvolatile memory chips and energy conversion technologies.
The findings are reported this week in the journal Physical Review Letters, in a paper by MIT Associate Professor Bilge Yildiz, Professor and Associate Provost Krystyn Van Vliet, and former postdoc Mostafa Youssef.
These metal oxide materials have been investigated by many researchers, Yildiz says, and “their properties are highly governed by the number and the kind of defects that are present.” When subjected to strong driving forces, such as strong electric fields, “the behavior of such defects had not been well-understood,” she says.
Researchers do have a well-established theoretical understanding of how perfectly structured versions of these insulating metal oxides function under a variety of conditions, such as in strong electric fields, but there was no such theory to describe the materials when they contain common types of defects, according to Yildiz. Understanding these effects quantitatively is important in order to develop this promising family of materials for potential applications including new types of low-energy computer memory and processing devices, electrically based refrigeration, and electro-catalytic energy-conversion devices such as fuel cells.
The team demonstrated a theoretical framework and showed how the stability and structure of a point defect is altered under strong electric fields.
Read more at the MIT News Office.
David L. Chandler | MIT News Office
Sept. 21, 2017
|Courtesy of The Engine|
The Engine, founded last year by MIT, has announced investments in its first group of seven startups that are developing innovations poised for transformative impact on aerospace, renewable energy, synthetic biology, medicine, and other sectors.
The founding startups were featured Sept. 19 at an event to celebrate the official opening of The Engine’s headquarters at 501 Massachusetts Ave. in Cambridge, Massachusetts, now renovated to include three floors of conference rooms, maker spaces, labs with cutting-edge equipment, computer stations, and other amenities.
“As we look at the first seven companies we have invested in, it is wonderful to see the breadth of tough-tech areas founders have leaned into,” says Katie Rae, president and CEO of The Engine. “We have been so gratified by the quality and passion of the founders that have come to us. These entrepreneurs are on a mission, and with our help they are going to change the world for the better.”
The seven startups are:
Read more at the MIT News Office.
Rob Matheson | MIT News Office
Sept. 19, 2017
2017 Materials Processing Center – Center for Materials Science and Engineering [MPC-CMSE] Summer Scholar Grace Noel explored the process of making and characterizing perovskite crystal materials for possible solar cell use in the lab of William A. Tisdale, ARCO Career Development Professor of chemical engineering. Noel synthesizes these lead bromide perovskite materials with different cations, including methyl ammonium, cesium and formamidinium.
“By changing the cation, you can change the properties of the perovskite,” Noel explains. “The word perovskite refers to a class of semiconductors that have a specific crystal structure, and they're an interesting area of research with applications in photovoltaics.”
MIT chemical engineering graduate student Nabeel Dahod, who studies thermal transport in nanostructured materials for his thesis project, is supervising Noel’s work in the Tisdale Lab. “Perovskites are a particularly new and exciting material with at this point undiscovered thermal transport properties, and this is where Grace's project this summer comes in,” Dahod says.
During a visit to the lab, Dahod and Noel demonstrate how these crystals are dried with a vacuum after wet synthesis. Noel explains that she caps her wet solution with tinfoil, perforated with a small hole in it, to slow the diffusion process. Dahod cautions Noel to set up a trap along with the vacuum so solvent vapors don’t harm the pump. Dahod prompts Noel to make sure the vacuum tube is firmly attached in order to be able pull vacuum through into the funnel.
Noel tests the vacuum process to make sure it is pulling out solvent using a pipette, which extracts a small volume. Noting that not all of the solid crystals made it into the filter, Dahod suggests scraping out the rest. Noel asks whether she should be worried about breaking any of the crystals. “No it's okay. They're pretty robust,” Dahod reassures. “Make sure to get as many of the orange ones as you can.”
View the embedded image gallery online at:
At a separate workbench, Noel displays formamidinium single crystals and methylammonium single crystals, which have crystallized for about four days. Noel observes that the methylammonium single crystals are slightly larger and that there is a color difference between the two. “The formamidinium are more red in color,” says Noel, who is a rising senior at Penn State University, where she majors in chemical engineering.
“My project is synthesizing these different perovskites in the two different forms, which are single crystals and microplates,” Noel explains. “Basically the single crystals are crystals that are millimeters in size, whereas the microplates are a lot smaller, so they're more like microns in size. But they should exhibit similar properties to the single crystals. This is advantageous because the single crystals have properties that aren't disturbed by things like defects in the material or grain boundaries. So we have the three different types of microplates with the different cations, and we want to see how the thermal properties of them change based on their composition.”
In CMSE’s Shared Experimental Facilities, Noel analyzes scanning electron microscope [SEM] images of the microplates. Speaking about images on a computer monitor, she notes, “These ones are formamidinium lead bromide perovskites, and they form these little plates that are about 1 to 3 microns. So I'm looking at the microplates to see the different sizes that they are, and looking to see if there are any defects or impurities.”
Noel’s internship is supported in part by NSF’s Materials Research Science and Engineering Centers program [grant DMR-14-19807]. Participants in the Research Experience for Undergraduates, co-sponsored by the Materials Processing Center and the Center for Materials Science and Engineering, presented their results at a poster session during the last week of the program. The program ran from June 15, 2017, to August 5, 2017, on the MIT campus in Cambridge, Mass.
– Denis Paiste, Materials Processing Center
September 25, 2017