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    Nanoscale transistors could enable more efficient electronics

    Silicon transistors, which are used to amplify and switch signals, are a critical component in most electronic devices, from smartphones to automobiles. But silicon semiconductor technology is held back by a fundamental physical limit that prevents transistors from operating below a certain voltage.This limit, known as “Boltzmann tyranny,” hinders the energy efficiency of computers and other electronics, especially with the rapid development of artificial intelligence technologies that demand faster computation.In an effort to overcome this fundamental limit of silicon, MIT researchers fabricated a different type of three-dimensional transistor using a unique set of ultrathin semiconductor materials.Their devices, featuring vertical nanowires only a few nanometers wide, can deliver performance comparable to state-of-the-art silicon transistors while operating efficiently at much lower voltages than conventional devices.“This is a technology with the potential to replace silicon, so you could use it with all the functions that silicon currently has, but with much better energy efficiency,” says Yanjie Shao, an MIT postdoc and lead author of a paper on the new transistors.The transistors leverage quantum mechanical properties to simultaneously achieve low-voltage operation and high performance within an area of just a few square nanometers. Their extremely small size would enable more of these 3D transistors to be packed onto a computer chip, resulting in fast, powerful electronics that are also more energy-efficient.“With conventional physics, there is only so far you can go. The work of Yanjie shows that we can do better than that, but we have to use different physics. There are many challenges yet to be overcome for this approach to be commercial in the future, but conceptually, it really is a breakthrough,” says senior author Jesús del Alamo, the Donner Professor of Engineering in the MIT Department of Electrical Engineering and Computer Science (EECS).They are joined on the paper by Ju Li, the Tokyo Electric Power Company Professor in Nuclear Engineering and professor of materials science and engineering at MIT; EECS graduate student Hao Tang; MIT postdoc Baoming Wang; and professors Marco Pala and David Esseni of the University of Udine in Italy. The research appears today in Nature Electronics.Surpassing siliconIn electronic devices, silicon transistors often operate as switches. Applying a voltage to the transistor causes electrons to move over an energy barrier from one side to the other, switching the transistor from “off” to “on.” By switching, transistors represent binary digits to perform computation.A transistor’s switching slope reflects the sharpness of the “off” to “on” transition. The steeper the slope, the less voltage is needed to turn on the transistor and the greater its energy efficiency.But because of how electrons move across an energy barrier, Boltzmann tyranny requires a certain minimum voltage to switch the transistor at room temperature.To overcome the physical limit of silicon, the MIT researchers used a different set of semiconductor materials — gallium antimonide and indium arsenide — and designed their devices to leverage a unique phenomenon in quantum mechanics called quantum tunneling.Quantum tunneling is the ability of electrons to penetrate barriers. The researchers fabricated tunneling transistors, which leverage this property to encourage electrons to push through the energy barrier rather than going over it.“Now, you can turn the device on and off very easily,” Shao says.But while tunneling transistors can enable sharp switching slopes, they typically operate with low current, which hampers the performance of an electronic device. Higher current is necessary to create powerful transistor switches for demanding applications.Fine-grained fabricationUsing tools at MIT.nano, MIT’s state-of-the-art facility for nanoscale research, the engineers were able to carefully control the 3D geometry of their transistors, creating vertical nanowire heterostructures with a diameter of only 6 nanometers. They believe these are the smallest 3D transistors reported to date.Such precise engineering enabled them to achieve a sharp switching slope and high current simultaneously. This is possible because of a phenomenon called quantum confinement.Quantum confinement occurs when an electron is confined to a space that is so small that it can’t move around. When this happens, the effective mass of the electron and the properties of the material change, enabling stronger tunneling of the electron through a barrier.Because the transistors are so small, the researchers can engineer a very strong quantum confinement effect while also fabricating an extremely thin barrier.“We have a lot of flexibility to design these material heterostructures so we can achieve a very thin tunneling barrier, which enables us to get very high current,” Shao says.Precisely fabricating devices that were small enough to accomplish this was a major challenge.“We are really into single-nanometer dimensions with this work. Very few groups in the world can make good transistors in that range. Yanjie is extraordinarily capable to craft such well-functioning transistors that are so extremely small,” says del Alamo.When the researchers tested their devices, the sharpness of the switching slope was below the fundamental limit that can be achieved with conventional silicon transistors. Their devices also performed about 20 times better than similar tunneling transistors.“This is the first time we have been able to achieve such sharp switching steepness with this design,” Shao adds.The researchers are now striving to enhance their fabrication methods to make transistors more uniform across an entire chip. With such small devices, even a 1-nanometer variance can change the behavior of the electrons and affect device operation. They are also exploring vertical fin-shaped structures, in addition to vertical nanowire transistors, which could potentially improve the uniformity of devices on a chip.“This work definitively steps in the right direction, significantly improving the broken-gap tunnel field effect transistor (TFET) performance. It demonstrates steep-slope together with a record drive-current. It highlights the importance of small dimensions, extreme confinement, and low-defectivity materials and interfaces in the fabricated broken-gap TFET. These features have been realized through a well-mastered and nanometer-size-controlled process,” says Aryan Afzalian, a principal member of the technical staff at the nanoelectronics research organization imec, who was not involved with this work.This research is funded, in part, by Intel Corporation. More

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    Study finds mercury pollution from human activities is declining

    MIT researchers have some good environmental news: Mercury emissions from human activity have been declining over the past two decades, despite global emissions inventories that indicate otherwise.In a new study, the researchers analyzed measurements from all available monitoring stations in the Northern Hemisphere and found that atmospheric concentrations of mercury declined by about 10 percent between 2005 and 2020.They used two separate modeling methods to determine what is driving that trend. Both techniques pointed to a decline in mercury emissions from human activity as the most likely cause.Global inventories, on the other hand, have reported opposite trends. These inventories estimate atmospheric emissions using models that incorporate average emission rates of polluting activities and the scale of these activities worldwide.“Our work shows that it is very important to learn from actual, on-the-ground data to try and improve our models and these emissions estimates. This is very relevant for policy because, if we are not able to accurately estimate past mercury emissions, how are we going to predict how mercury pollution will evolve in the future?” says Ari Feinberg, a former postdoc in the Institute for Data, Systems, and Society (IDSS) and lead author of the study.The new results could help inform scientists who are embarking on a collaborative, global effort to evaluate pollution models and develop a more in-depth understanding of what drives global atmospheric concentrations of mercury.However, due to a lack of data from global monitoring stations and limitations in the scientific understanding of mercury pollution, the researchers couldn’t pinpoint a definitive reason for the mismatch between the inventories and the recorded measurements.“It seems like mercury emissions are moving in the right direction, and could continue to do so, which is heartening to see. But this was as far as we could get with mercury. We need to keep measuring and advancing the science,” adds co-author Noelle Selin, an MIT professor in the IDSS and the Department of Earth, Atmospheric and Planetary Sciences (EAPS).Feinberg and Selin, his MIT postdoctoral advisor, are joined on the paper by an international team of researchers that contributed atmospheric mercury measurement data and statistical methods to the study. The research appears this week in the Proceedings of the National Academy of Sciences.Mercury mismatchThe Minamata Convention is a global treaty that aims to cut human-caused emissions of mercury, a potent neurotoxin that enters the atmosphere from sources like coal-fired power plants and small-scale gold mining.The treaty, which was signed in 2013 and went into force in 2017, is evaluated every five years. The first meeting of its conference of parties coincided with disheartening news reports that said global inventories of mercury emissions, compiled in part from information from national inventories, had increased despite international efforts to reduce them.This was puzzling news for environmental scientists like Selin. Data from monitoring stations showed atmospheric mercury concentrations declining during the same period.Bottom-up inventories combine emission factors, such as the amount of mercury that enters the atmosphere when coal mined in a certain region is burned, with estimates of pollution-causing activities, like how much of that coal is burned in power plants.“The big question we wanted to answer was: What is actually happening to mercury in the atmosphere and what does that say about anthropogenic emissions over time?” Selin says.Modeling mercury emissions is especially tricky. First, mercury is the only metal that is in liquid form at room temperature, so it has unique properties. Moreover, mercury that has been removed from the atmosphere by sinks like the ocean or land can be re-emitted later, making it hard to identify primary emission sources.At the same time, mercury is more difficult to study in laboratory settings than many other air pollutants, especially due to its toxicity, so scientists have limited understanding of all chemical reactions mercury can undergo. There is also a much smaller network of mercury monitoring stations, compared to other polluting gases like methane and nitrous oxide.“One of the challenges of our study was to come up with statistical methods that can address those data gaps, because available measurements come from different time periods and different measurement networks,” Feinberg says.Multifaceted modelsThe researchers compiled data from 51 stations in the Northern Hemisphere. They used statistical techniques to aggregate data from nearby stations, which helped them overcome data gaps and evaluate regional trends.By combining data from 11 regions, their analysis indicated that Northern Hemisphere atmospheric mercury concentrations declined by about 10 percent between 2005 and 2020.Then the researchers used two modeling methods — biogeochemical box modeling and chemical transport modeling — to explore possible causes of that decline.  Box modeling was used to run hundreds of thousands of simulations to evaluate a wide array of emission scenarios. Chemical transport modeling is more computationally expensive but enables researchers to assess the impacts of meteorology and spatial variations on trends in selected scenarios.For instance, they tested one hypothesis that there may be an additional environmental sink that is removing more mercury from the atmosphere than previously thought. The models would indicate the feasibility of an unknown sink of that magnitude.“As we went through each hypothesis systematically, we were pretty surprised that we could really point to declines in anthropogenic emissions as being the most likely cause,” Selin says.Their work underscores the importance of long-term mercury monitoring stations, Feinberg adds. Many stations the researchers evaluated are no longer operational because of a lack of funding.While their analysis couldn’t zero in on exactly why the emissions inventories didn’t match up with actual data, they have a few hypotheses.One possibility is that global inventories are missing key information from certain countries. For instance, the researchers resolved some discrepancies when they used a more detailed regional inventory from China. But there was still a gap between observations and estimates.They also suspect the discrepancy might be the result of changes in two large sources of mercury that are particularly uncertain: emissions from small-scale gold mining and mercury-containing products.Small-scale gold mining involves using mercury to extract gold from soil and is often performed in remote parts of developing countries, making it hard to estimate. Yet small-scale gold mining contributes about 40 percent of human-made emissions.In addition, it’s difficult to determine how long it takes the pollutant to be released into the atmosphere from discarded products like thermometers or scientific equipment.“We’re not there yet where we can really pinpoint which source is responsible for this discrepancy,” Feinberg says.In the future, researchers from multiple countries, including MIT, will collaborate to study and improve the models they use to estimate and evaluate emissions. This research will be influential in helping that project move the needle on monitoring mercury, he says.This research was funded by the Swiss National Science Foundation, the U.S. National Science Foundation, and the U.S. Environmental Protection Agency. More

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    Liftoff: The Climate Project at MIT takes flight

    The leaders of The Climate Project at MIT met with community members at a campus forum on Monday, helping to kick off the Institute’s major new effort to accelerate and scale up climate change solutions.“The Climate Project is a whole-of-MIT mobilization,” MIT President Sally Kornbluth said in her opening remarks. “It’s designed to focus the Institute’s talent and resources so that we can achieve much more, faster, in terms of real-world impact, from mitigation to adaptation.”The event, “Climate Project at MIT: Launching the Missions,” drew a capacity crowd to MIT’s Samberg Center.While the Climate Project has a number of facets, a central component of the effort consists of its six “missions,” broad areas where MIT researchers will seek to identify gaps in the global climate response that MIT can help fill, and then launch and execute research and innovation projects aimed at those areas. Each mission is led by campus faculty, and Monday’s event represented the first public conversation between the mission directors and the larger campus community.“Today’s event is an important milestone,” said Richard Lester, MIT’s interim vice president for climate and the Japan Steel Industry Professor of Nuclear Science and Engineering, who led the Climate Project’s formation. He praised Kornbluth’s sustained focus on climate change as a leading priority for MIT.“The reason we’re all here is because of her leadership and vision for MIT,” Lester said. “We’re also here because the MIT community — our faculty, our staff, our students — has made it abundantly clear that it wants to do more, much more, to help solve this great problem.”The mission directors themselves emphasized the need for deep community involvement in the project — and that the Climate Project is designed to facilitate researcher-driven enterprise across campus.“There’s a tremendous amount of urgency,” said Elsa Olivetti PhD ’07, director of the Decarbonizing Energy and Industry mission, during an onstage discussion. “We all need to do everything we can, and roll up our sleeves and get it done.” Olivetti, the Jerry McAfee Professor in Engineering, has been a professor of materials science and engineering at the Institute since 2014.“What’s exciting about this is the chance of MIT really meeting its potential,” said Jesse Kroll, co-director of the mission for Restoring the Atmosphere, Protecting the Land and Oceans. Kroll is the Peter de Florez Professor in MIT’s Department of Civil and Environmental Engineering, a professor of chemical engineering, and the director of the Ralph M. Parsons Laboratory.MIT, Kroll noted, features “so much amazing work going on in all these different aspects of the problem. Science, engineering, social science … we put it all together and there is huge potential, a huge opportunity for us to make a difference.”MIT has pledged an initial $75 million to the Climate Project, including $25 million from the MIT Sloan School of Management for a complementary effort, the MIT Climate Policy Center. However, the Institute is anticipating that it will also build new connections with outside partners, whose role in implementing and scaling Climate Project solutions will be critical.Monday’s event included a keynote talk from Brian Deese, currently the MIT Innovation and Climate Impact Fellow and the former director of the White House National Economic Council in the Biden administration.“The magnitude of the risks associated with climate change are extraordinary,” Deese said. However, he added, “these are solvable issues. In fact, the energy transition globally will be the greatest economic opportunity in human history. … It has the potential to actually lift people out of poverty, it has the potential to drive international cooperation, it has the potential to drive innovation and improve lives — if we get this right.”Deese’s remarks centered on a call for the U.S. to develop a current-day climate equivalent of the Marshall Plan, the U.S. initiative to provide aid to Western Europe after World War II. He also suggested three characteristics of successful climate projects, noting that many would be interdisciplinary in nature and would “engage with policy early in the design process” to become feasible.In addition to those features, Deese said, people need to “start and end with very high ambition” when working on climate solutions. He added: “The good thing about MIT and our community is that we, you, have done this before. We’ve got examples where MIT has taken something that seemed completely improbable and made it possible, and I believe that part of what is required of this collective effort is to keep that kind of audacious thinking at the top of our mind.” The MIT mission directors all participated in an onstage discussion moderated by Somini Sengupta, the international climate reporter on the climate team of The New York Times. Sengupta asked the group about a wide range of topics, from their roles and motivations to the political constraints on global climate progress, and more.Andrew Babbin, co-director of the mission for Restoring the Atmosphere, Protecting the Land and Oceans, defined part of the task of the MIT missions as “identifying where those gaps of knowledge are and filling them rapidly,” something he believes is “largely not doable in the conventional way,” based on small-scale research projects. Instead, suggested Babbin, who is the Cecil and Ida Green Career Development Professor in MIT’s Program in Atmospheres, Oceans, and Climate, the collective input of research and innovation communities could help zero in on undervalued approaches to climate action.Some innovative concepts, the mission directors noted, can be tried out on the MIT campus, in an effort to demonstrate how a more sustainable infrastructure and systems can operate at scale.“That is absolutely crucial,” said Christoph Reinhart, director of the Building and Adapting Healthy, Resilient Cities mission, expressing the need to have the campus reach net-zero emissions. Reinhart is the Alan and Terri Spoon Professor of Architecture and Climate and director of MIT’s Building Technology Program in the School of Architecture and Planning.In response to queries from Sengupta, the mission directors affirmed that the Climate Project needs to develop solutions that can work in different societies around the world, while acknowledging that there are many political hurdles to worldwide climate action.“Any kind of quality engaged projects that we’ve done with communities, it’s taken years to build trust. … How you scale that without compromising is the challenge I’m faced with,” said Miho Mazereeuw, director of the Empowering Frontline Communities mission, an associate professor of architecture and urbanism, and director of MIT’s Urban Risk Lab.“I think we will impact different communities in different parts of the world in different ways,” said Benedetto Marelli, an associate professor in MIT’s Department of Civil and Environmental Engineering, adding that it would be important to “work with local communities [and] engage stakeholders, and at the same time, use local brains to solve the problem.” The mission he directs, Wild Cards, is centered on identifying unconventional solutions that are high risk and also high reward.Any climate program “has to be politically feasible, it has to be in separate nations’ self-interest,” said Christopher Knittel, mission director for Inventing New Policy Approaches. In an ever-shifting political world, he added, that means people must “think about not just the policy but the resiliency of the policy.” Knittel is the George P. Shultz Professor and professor of applied economics at the MIT Sloan School of Management, director of the MIT Climate Policy Center, and associate dean for Climate and Sustainability.In all, MIT has more than 300 faculty and senior researchers who, along with their students and staff, are already working on climate issues.Kornbluth, for her part, referred to MIT’s first-year students while discussing the larger motivations for taking concerted action to address the challenges of climate change. It might be easy for younger people to despair over the world’s climate trajectory, she noted, but the best response to that includes seeking new avenues for climate progress.“I understand their anxiety and concern,” Kornbluth said. “But I have no doubt at all that together, we can make a difference. I believe that we have a special obligation to the new students and their entire generation to do everything we can to create a positive change. The most powerful antidote to defeat and despair is collection action.” More

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    Mission directors announced for the Climate Project at MIT

    The Climate Project at MIT has appointed leaders for each of its six focal areas, or Climate Missions, President Sally Kornbluth announced in a letter to the MIT community today.Introduced in February, the Climate Project at MIT is a major new effort to change the trajectory of global climate outcomes for the better over the next decade. The project will focus MIT’s strengths on six broad climate-related areas where progress is urgently needed. The mission directors in these fields, representing diverse areas of expertise, will collaborate with faculty and researchers across MIT, as well as each other, to accelerate solutions that address climate change.“The mission directors will be absolutely central as the Climate Project seeks to marshal the Institute’s talent and resources to research, develop, deploy and scale up serious solutions to help change the planet’s climate trajectory,” Kornbluth wrote in her letter, adding: “To the faculty members taking on these pivotal roles: We could not be more grateful for your skill and commitment, or more enthusiastic about what you can help us all achieve, together.”The Climate Project will expand and accelerate MIT’s efforts to both reduce greenhouse gas emissions and respond to climate effects such as extreme heat, rising sea levels, and reduced crop yields. At the urgent pace needed, the project will help the Institute create new external collaborations and deepen existing ones to develop and scale climate solutions.The Institute has pledged an initial $75 million to the project, including $25 million from the MIT Sloan School of Management to launch a complementary effort, the new MIT Climate Policy Center. MIT has more than 300 faculty and senior researchers already working on climate issues, in collaboration with their students and staff. The Climate Project at MIT builds on their work and the Institute’s 2021 “Fast Forward” climate action plan.Richard Lester, MIT’s vice provost for international activities and the Japan Steel Industry Professor of Nuclear Science and Engineering, has led the Climate Project’s formation; MIT will shortly hire a vice president for climate to oversee the project. The six Climate Missions and the new mission directors are as follows:Decarbonizing energy and industryThis mission supports advances in the electric power grid as well as the transition across all industry — including transportation, computing, heavy production, and manufacturing — to low-emissions pathways.The mission director is Elsa Olivetti PhD ’07, who is MIT’s associate dean of engineering, the Jerry McAfee Professor in Engineering, and a professor of materials science and engineering since 2014.Olivetti analyzes and improves the environmental sustainability of materials throughout the life cycle and across the supply chain, by linking physical and chemical processes to systems impact. She researches materials design and synthesis using natural language processing, builds models of material supply and technology demand, and assesses the potential for recovering value from industrial waste through experimental approaches. Olivetti has experience building partnerships across the Institute and working with industry to implement large-scale climate solutions through her role as co-director of the MIT Climate and Sustainability Consortium (MCSC) and as faculty lead for PAIA, an industry consortium on the carbon footprinting of computing.Restoring the atmosphere, protecting the land and oceansThis mission is centered on removing or storing greenhouse gases that have already been emitted into the atmosphere, such as carbon dioxide and methane, and on protecting ocean and land ecosystems, including food and water systems.MIT has chosen two mission directors: Andrew Babbin and Jesse Kroll. The two bring together research expertise from two critical domains of the Earth system, oceans and the atmosphere, as well as backgrounds in both the science and engineering underlying our understanding of Earth’s climate. As co-directors, they jointly link MIT’s School of Science and School of Engineering in this domain.Babbin is the Cecil and Ida Green Career Development Professor in MIT’s Program in Atmospheres, Oceans, and Climate. He is a marine biogeochemist whose specialty is studying the carbon and nitrogen cycle of the oceans, work that is related to evaluating the ocean’s capacity for carbon storage, an essential element of this mission’s work. He has been at MIT since 2017.Kroll is a professor in MIT’s Department of of Civil and Environmental Engineering, a professor of chemical engineering, and the director of the Ralph M. Parsons Laboratory. He is a chemist who studies organic compounds and particulate matter in the atmosphere, in order to better understand how perturbations to the atmosphere, both intentional and unintentional, can affect air pollution and climate.Empowering frontline communitiesThis mission focuses on the development of new climate solutions in support of the world’s most vulnerable populations, in areas ranging from health effects to food security, emergency planning, and risk forecasting.The mission director is Miho Mazereeuw, an associate professor of architecture and urbanism in MIT’s Department of Architecture in the School of Architecture and Planning, and director of MIT’s Urban Risk Lab. Mazereeuw researches disaster resilience, climate change, and coastal strategies. Her lab has engaged in design projects ranging from physical objects to software, while exploring methods of engaging communities and governments in preparedness efforts, skills she brings to bear on building strong collaborations with a broad range of stakeholders.Mazereeuw is also co-lead of one of the five projects selected in MIT’s Climate Grand Challenges competition in 2022, an effort to help communities prepare by understanding the risk of extreme weather events for specific locations.Building and adapting healthy, resilient citiesA majority of the world’s population lives in cities, so urban design and planning is a crucial part of climate work, involving transportation, infrastructure, finance, government, and more.Christoph Reinhart, the Alan and Terri Spoon Professor of Architecture and Climate and director of MIT’s Building Technology Program in the School of Architecture and Planning, is the mission director in this area. The Sustainable Design Lab that Reinhart founded when he joined MIT in 2012 has launched several technology startups, including Mapdwell Solar System, now part of Palmetto Clean Technology, as well as Solemma, makers of an environmental building design software used in architectural practice and education worldwide. Reinhart’s online course on Sustainable Building Design has an enrollment of over 55,000 individuals and forms part of MIT’s XSeries Program in Future Energy Systems.Inventing new policy approachesClimate change is a unique crisis. With that in mind, this mission aims to develop new institutional structures and incentives — in carbon markets, finance, trade policy, and more — along with decision support tools and systems for scaling up climate efforts.Christopher Knittel brings extensive knowledge of these topics to the mission director role. The George P. Shultz Professor and Professor of Applied Economics at the MIT Sloan School of Management, Knittel has produced high-impact research in multiple areas; his studies on emissions and the automobile industry have evaluated fuel-efficiency standards, changes in vehicle fuel efficiency, market responses to fuel-price changes, and the health impact of automobiles.Beyond that, Knittel has also studied the impact of the energy transition on jobs, conducted high-level evaluations of climate policies, and examined energy market structures. He joined the MIT faculty in 2011. He also serves as the director of the MIT Climate Policy Center, which will work closely with all six missions.Wild cardsThis mission consists of what the Climate Project at MIT calls “unconventional solutions outside the scope of the other missions,” and will have a broad portfolio for innovation.While all the missions will be charged with encouraging unorthodox approaches within their domains, this mission will seek out unconventional solutions outside the scope of the others, and has a broad mandate for promoting them.The mission director in this case is Benedetto Marelli, the Paul M. Cook Career Development Associate Professor in MIT’s Department of Civil and Environmental Engineering. Marelli’s research group develops biopolymers and bioinspired materials with reduced environmental impact compared to traditional technologies. He engages with research at multiple scales, including nanofabrication, and the research group has conducted extensive work on food security and safety while exploring new techniques to reduce waste through enhanced food preservation and to precisely deliver agrochemicals in plants and in soil.As Lester and other MIT leaders have noted, the Climate Project at MIT is still being shaped, and will have the flexibility to accommodate a wide range of projects, partnerships, and approaches needed for thoughtful, fast-moving change. By filling out the leadership structure, today’s announcement is a major milestone in making the project operational.In addition to the six Climate Missions, the Climate Project at MIT includes Climate Frontier Projects, which are efforts launched by these missions, and a Climate HQ, which will support fundamental research, education, and outreach, as well as new resources to connect research to the practical work of climate response. More

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    AI method radically speeds predictions of materials’ thermal properties

    It is estimated that about 70 percent of the energy generated worldwide ends up as waste heat.If scientists could better predict how heat moves through semiconductors and insulators, they could design more efficient power generation systems. However, the thermal properties of materials can be exceedingly difficult to model.The trouble comes from phonons, which are subatomic particles that carry heat. Some of a material’s thermal properties depend on a measurement called the phonon dispersion relation, which can be incredibly hard to obtain, let alone utilize in the design of a system.A team of researchers from MIT and elsewhere tackled this challenge by rethinking the problem from the ground up. The result of their work is a new machine-learning framework that can predict phonon dispersion relations up to 1,000 times faster than other AI-based techniques, with comparable or even better accuracy. Compared to more traditional, non-AI-based approaches, it could be 1 million times faster.This method could help engineers design energy generation systems that produce more power, more efficiently. It could also be used to develop more efficient microelectronics, since managing heat remains a major bottleneck to speeding up electronics.“Phonons are the culprit for the thermal loss, yet obtaining their properties is notoriously challenging, either computationally or experimentally,” says Mingda Li, associate professor of nuclear science and engineering and senior author of a paper on this technique.Li is joined on the paper by co-lead authors Ryotaro Okabe, a chemistry graduate student; and Abhijatmedhi Chotrattanapituk, an electrical engineering and computer science graduate student; Tommi Jaakkola, the Thomas Siebel Professor of Electrical Engineering and Computer Science at MIT; as well as others at MIT, Argonne National Laboratory, Harvard University, the University of South Carolina, Emory University, the University of California at Santa Barbara, and Oak Ridge National Laboratory. The research appears in Nature Computational Science.Predicting phononsHeat-carrying phonons are tricky to predict because they have an extremely wide frequency range, and the particles interact and travel at different speeds.A material’s phonon dispersion relation is the relationship between energy and momentum of phonons in its crystal structure. For years, researchers have tried to predict phonon dispersion relations using machine learning, but there are so many high-precision calculations involved that models get bogged down.“If you have 100 CPUs and a few weeks, you could probably calculate the phonon dispersion relation for one material. The whole community really wants a more efficient way to do this,” says Okabe.The machine-learning models scientists often use for these calculations are known as graph neural networks (GNN). A GNN converts a material’s atomic structure into a crystal graph comprising multiple nodes, which represent atoms, connected by edges, which represent the interatomic bonding between atoms.While GNNs work well for calculating many quantities, like magnetization or electrical polarization, they are not flexible enough to efficiently predict an extremely high-dimensional quantity like the phonon dispersion relation. Because phonons can travel around atoms on X, Y, and Z axes, their momentum space is hard to model with a fixed graph structure.To gain the flexibility they needed, Li and his collaborators devised virtual nodes.They create what they call a virtual node graph neural network (VGNN) by adding a series of flexible virtual nodes to the fixed crystal structure to represent phonons. The virtual nodes enable the output of the neural network to vary in size, so it is not restricted by the fixed crystal structure.Virtual nodes are connected to the graph in such a way that they can only receive messages from real nodes. While virtual nodes will be updated as the model updates real nodes during computation, they do not affect the accuracy of the model.“The way we do this is very efficient in coding. You just generate a few more nodes in your GNN. The physical location doesn’t matter, and the real nodes don’t even know the virtual nodes are there,” says Chotrattanapituk.Cutting out complexitySince it has virtual nodes to represent phonons, the VGNN can skip many complex calculations when estimating phonon dispersion relations, which makes the method more efficient than a standard GNN. The researchers proposed three different versions of VGNNs with increasing complexity. Each can be used to predict phonons directly from a material’s atomic coordinates.Because their approach has the flexibility to rapidly model high-dimensional properties, they can use it to estimate phonon dispersion relations in alloy systems. These complex combinations of metals and nonmetals are especially challenging for traditional approaches to model.The researchers also found that VGNNs offered slightly greater accuracy when predicting a material’s heat capacity. In some instances, prediction errors were two orders of magnitude lower with their technique.A VGNN could be used to calculate phonon dispersion relations for a few thousand materials in just a few seconds with a personal computer, Li says.This efficiency could enable scientists to search a larger space when seeking materials with certain thermal properties, such as superior thermal storage, energy conversion, or superconductivity.Moreover, the virtual node technique is not exclusive to phonons, and could also be used to predict challenging optical and magnetic properties.In the future, the researchers want to refine the technique so virtual nodes have greater sensitivity to capture small changes that can affect phonon structure.“Researchers got too comfortable using graph nodes to represent atoms, but we can rethink that. Graph nodes can be anything. And virtual nodes are a very generic approach you could use to predict a lot of high-dimensional quantities,” Li says.“The authors’ innovative approach significantly augments the graph neural network description of solids by incorporating key physics-informed elements through virtual nodes, for instance, informing wave-vector dependent band-structures and dynamical matrices,” says Olivier Delaire, associate professor in the Thomas Lord Department of Mechanical Engineering and Materials Science at Duke University, who was not involved with this work. “I find that the level of acceleration in predicting complex phonon properties is amazing, several orders of magnitude faster than a state-of-the-art universal machine-learning interatomic potential. Impressively, the advanced neural net captures fine features and obeys physical rules. There is great potential to expand the model to describe other important material properties: Electronic, optical, and magnetic spectra and band structures come to mind.”This work is supported by the U.S. Department of Energy, National Science Foundation, a Mathworks Fellowship, a Sow-Hsin Chen Fellowship, the Harvard Quantum Initiative, and the Oak Ridge National Laboratory. More

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    Study finds health risks in switching ships from diesel to ammonia fuel

    As container ships the size of city blocks cross the oceans to deliver cargo, their huge diesel engines emit large quantities of air pollutants that drive climate change and have human health impacts. It has been estimated that maritime shipping accounts for almost 3 percent of global carbon dioxide emissions and the industry’s negative impacts on air quality cause about 100,000 premature deaths each year.Decarbonizing shipping to reduce these detrimental effects is a goal of the International Maritime Organization, a U.N. agency that regulates maritime transport. One potential solution is switching the global fleet from fossil fuels to sustainable fuels such as ammonia, which could be nearly carbon-free when considering its production and use.But in a new study, an interdisciplinary team of researchers from MIT and elsewhere caution that burning ammonia for maritime fuel could worsen air quality further and lead to devastating public health impacts, unless it is adopted alongside strengthened emissions regulations.Ammonia combustion generates nitrous oxide (N2O), a greenhouse gas that is about 300 times more potent than carbon dioxide. It also emits nitrogen in the form of nitrogen oxides (NO and NO2, referred to as NOx), and unburnt ammonia may slip out, which eventually forms fine particulate matter in the atmosphere. These tiny particles can be inhaled deep into the lungs, causing health problems like heart attacks, strokes, and asthma.The new study indicates that, under current legislation, switching the global fleet to ammonia fuel could cause up to about 600,000 additional premature deaths each year. However, with stronger regulations and cleaner engine technology, the switch could lead to about 66,000 fewer premature deaths than currently caused by maritime shipping emissions, with far less impact on global warming.“Not all climate solutions are created equal. There is almost always some price to pay. We have to take a more holistic approach and consider all the costs and benefits of different climate solutions, rather than just their potential to decarbonize,” says Anthony Wong, a postdoc in the MIT Center for Global Change Science and lead author of the study.His co-authors include Noelle Selin, an MIT professor in the Institute for Data, Systems, and Society and the Department of Earth, Atmospheric and Planetary Sciences (EAPS); Sebastian Eastham, a former principal research scientist who is now a senior lecturer at Imperial College London; Christine Mounaïm-Rouselle, a professor at the University of Orléans in France; Yiqi Zhang, a researcher at the Hong Kong University of Science and Technology; and Florian Allroggen, a research scientist in the MIT Department of Aeronautics and Astronautics. The research appears this week in Environmental Research Letters.Greener, cleaner ammoniaTraditionally, ammonia is made by stripping hydrogen from natural gas and then combining it with nitrogen at extremely high temperatures. This process is often associated with a large carbon footprint. The maritime shipping industry is betting on the development of “green ammonia,” which is produced by using renewable energy to make hydrogen via electrolysis and to generate heat.“In theory, if you are burning green ammonia in a ship engine, the carbon emissions are almost zero,” Wong says.But even the greenest ammonia generates nitrous oxide (N2O), nitrogen oxides (NOx) when combusted, and some of the ammonia may slip out, unburnt. This nitrous oxide would escape into the atmosphere, where the greenhouse gas would remain for more than 100 years. At the same time, the nitrogen emitted as NOx and ammonia would fall to Earth, damaging fragile ecosystems. As these emissions are digested by bacteria, additional N2O  is produced.NOx and ammonia also mix with gases in the air to form fine particulate matter. A primary contributor to air pollution, fine particulate matter kills an estimated 4 million people each year.“Saying that ammonia is a ‘clean’ fuel is a bit of an overstretch. Just because it is carbon-free doesn’t necessarily mean it is clean and good for public health,” Wong says.A multifaceted modelThe researchers wanted to paint the whole picture, capturing the environmental and public health impacts of switching the global fleet to ammonia fuel. To do so, they designed scenarios to measure how pollutant impacts change under certain technology and policy assumptions.From a technological point of view, they considered two ship engines. The first burns pure ammonia, which generates higher levels of unburnt ammonia but emits fewer nitrogen oxides. The second engine technology involves mixing ammonia with hydrogen to improve combustion and optimize the performance of a catalytic converter, which controls both nitrogen oxides and unburnt ammonia pollution.They also considered three policy scenarios: current regulations, which only limit NOx emissions in some parts of the world; a scenario that adds ammonia emission limits over North America and Western Europe; and a scenario that adds global limits on ammonia and NOx emissions.The researchers used a ship track model to calculate how pollutant emissions change under each scenario and then fed the results into an air quality model. The air quality model calculates the impact of ship emissions on particulate matter and ozone pollution. Finally, they estimated the effects on global public health.One of the biggest challenges came from a lack of real-world data, since no ammonia-powered ships are yet sailing the seas. Instead, the researchers relied on experimental ammonia combustion data from collaborators to build their model.“We had to come up with some clever ways to make that data useful and informative to both the technology and regulatory situations,” he says.A range of outcomesIn the end, they found that with no new regulations and ship engines that burn pure ammonia, switching the entire fleet would cause 681,000 additional premature deaths each year.“While a scenario with no new regulations is not very realistic, it serves as a good warning of how dangerous ammonia emissions could be. And unlike NOx, ammonia emissions from shipping are currently unregulated,” Wong says.However, even without new regulations, using cleaner engine technology would cut the number of premature deaths down to about 80,000, which is about 20,000 fewer than are currently attributed to maritime shipping emissions. With stronger global regulations and cleaner engine technology, the number of people killed by air pollution from shipping could be reduced by about 66,000.“The results of this study show the importance of developing policies alongside new technologies,” Selin says. “There is a potential for ammonia in shipping to be beneficial for both climate and air quality, but that requires that regulations be designed to address the entire range of potential impacts, including both climate and air quality.”Ammonia’s air quality impacts would not be felt uniformly across the globe, and addressing them fully would require coordinated strategies across very different contexts. Most premature deaths would occur in East Asia, since air quality regulations are less stringent in this region. Higher levels of existing air pollution cause the formation of more particulate matter from ammonia emissions. In addition, shipping volume over East Asia is far greater than elsewhere on Earth, compounding these negative effects.In the future, the researchers want to continue refining their analysis. They hope to use these findings as a starting point to urge the marine industry to share engine data they can use to better evaluate air quality and climate impacts. They also hope to inform policymakers about the importance and urgency of updating shipping emission regulations.This research was funded by the MIT Climate and Sustainability Consortium. More

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    “They can see themselves shaping the world they live in”

    During the journey from the suburbs to the city, the tree canopy often dwindles down as skyscrapers rise up. A group of New England Innovation Academy students wondered why that is.“Our friend Victoria noticed that where we live in Marlborough there are lots of trees in our own backyards. But if you drive just 30 minutes to Boston, there are almost no trees,” said high school junior Ileana Fournier. “We were struck by that duality.”This inspired Fournier and her classmates Victoria Leeth and Jessie Magenyi to prototype a mobile app that illustrates Massachusetts deforestation trends for Day of AI, a free, hands-on curriculum developed by the MIT Responsible AI for Social Empowerment and Education (RAISE) initiative, headquartered in the MIT Media Lab and in collaboration with the MIT Schwarzman College of Computing and MIT Open Learning. They were among a group of 20 students from New England Innovation Academy who shared their projects during the 2024 Day of AI global celebration hosted with the Museum of Science.The Day of AI curriculum introduces K-12 students to artificial intelligence. Now in its third year, Day of AI enables students to improve their communities and collaborate on larger global challenges using AI. Fournier, Leeth, and Magenyi’s TreeSavers app falls under the Telling Climate Stories with Data module, one of four new climate-change-focused lessons.“We want you to be able to express yourselves creatively to use AI to solve problems with critical-thinking skills,” Cynthia Breazeal, director of MIT RAISE, dean for digital learning at MIT Open Learning, and professor of media arts and sciences, said during this year’s Day of AI global celebration at the Museum of Science. “We want you to have an ethical and responsible way to think about this really powerful, cool, and exciting technology.”Moving from understanding to actionDay of AI invites students to examine the intersection of AI and various disciplines, such as history, civics, computer science, math, and climate change. With the curriculum available year-round, more than 10,000 educators across 114 countries have brought Day of AI activities to their classrooms and homes.The curriculum gives students the agency to evaluate local issues and invent meaningful solutions. “We’re thinking about how to create tools that will allow kids to have direct access to data and have a personal connection that intersects with their lived experiences,” Robert Parks, curriculum developer at MIT RAISE, said at the Day of AI global celebration.Before this year, first-year Jeremie Kwapong said he knew very little about AI. “I was very intrigued,” he said. “I started to experiment with ChatGPT to see how it reacts. How close can I get this to human emotion? What is AI’s knowledge compared to a human’s knowledge?”In addition to helping students spark an interest in AI literacy, teachers around the world have told MIT RAISE that they want to use data science lessons to engage students in conversations about climate change. Therefore, Day of AI’s new hands-on projects use weather and climate change to show students why it’s important to develop a critical understanding of dataset design and collection when observing the world around them.“There is a lag between cause and effect in everyday lives,” said Parks. “Our goal is to demystify that, and allow kids to access data so they can see a long view of things.”Tools like MIT App Inventor — which allows anyone to create a mobile application — help students make sense of what they can learn from data. Fournier, Leeth, and Magenyi programmed TreeSavers in App Inventor to chart regional deforestation rates across Massachusetts, identify ongoing trends through statistical models, and predict environmental impact. The students put that “long view” of climate change into practice when developing TreeSavers’ interactive maps. Users can toggle between Massachusetts’s current tree cover, historical data, and future high-risk areas.Although AI provides fast answers, it doesn’t necessarily offer equitable solutions, said David Sittenfeld, director of the Center for the Environment at the Museum of Science. The Day of AI curriculum asks students to make decisions on sourcing data, ensuring unbiased data, and thinking responsibly about how findings could be used.“There’s an ethical concern about tracking people’s data,” said Ethan Jorda, a New England Innovation Academy student. His group used open-source data to program an app that helps users track and reduce their carbon footprint.Christine Cunningham, senior vice president of STEM Learning at the Museum of Science, believes students are prepared to use AI responsibly to make the world a better place. “They can see themselves shaping the world they live in,” said Cunningham. “Moving through from understanding to action, kids will never look at a bridge or a piece of plastic lying on the ground in the same way again.”Deepening collaboration on earth and beyondThe 2024 Day of AI speakers emphasized collaborative problem solving at the local, national, and global levels.“Through different ideas and different perspectives, we’re going to get better solutions,” said Cunningham. “How do we start young enough that every child has a chance to both understand the world around them but also to move toward shaping the future?”Presenters from MIT, the Museum of Science, and NASA approached this question with a common goal — expanding STEM education to learners of all ages and backgrounds.“We have been delighted to collaborate with the MIT RAISE team to bring this year’s Day of AI celebration to the Museum of Science,” says Meg Rosenburg, manager of operations at the Museum of Science Centers for Public Science Learning. “This opportunity to highlight the new climate modules for the curriculum not only perfectly aligns with the museum’s goals to focus on climate and active hope throughout our Year of the Earthshot initiative, but it has also allowed us to bring our teams together and grow a relationship that we are very excited to build upon in the future.”Rachel Connolly, systems integration and analysis lead for NASA’s Science Activation Program, showed the power of collaboration with the example of how human comprehension of Saturn’s appearance has evolved. From Galileo’s early telescope to the Cassini space probe, modern imaging of Saturn represents 400 years of science, technology, and math working together to further knowledge.“Technologies, and the engineers who built them, advance the questions we’re able to ask and therefore what we’re able to understand,” said Connolly, research scientist at MIT Media Lab.New England Innovation Academy students saw an opportunity for collaboration a little closer to home. Emmett Buck-Thompson, Jeff Cheng, and Max Hunt envisioned a social media app to connect volunteers with local charities. Their project was inspired by Buck-Thompson’s father’s difficulties finding volunteering opportunities, Hunt’s role as the president of the school’s Community Impact Club, and Cheng’s aspiration to reduce screen time for social media users. Using MIT App Inventor, ​their combined ideas led to a prototype with the potential to make a real-world impact in their community.The Day of AI curriculum teaches the mechanics of AI, ethical considerations and responsible uses, and interdisciplinary applications for different fields. It also empowers students to become creative problem solvers and engaged citizens in their communities and online. From supporting volunteer efforts to encouraging action for the state’s forests to tackling the global challenge of climate change, today’s students are becoming tomorrow’s leaders with Day of AI.“We want to empower you to know that this is a tool you can use to make your community better, to help people around you with this technology,” said Breazeal.Other Day of AI speakers included Tim Ritchie, president of the Museum of Science; Michael Lawrence Evans, program director of the Boston Mayor’s Office of New Urban Mechanics; Dava Newman, director of the MIT Media Lab; and Natalie Lao, executive director of the App Inventor Foundation. More

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    Students research pathways for MIT to reach decarbonization goals

    A number of emerging technologies hold promise for helping organizations move away from fossil fuels and achieve deep decarbonization. The challenge is deciding which technologies to adopt, and when.MIT, which has a goal of eliminating direct campus emissions by 2050, must make such decisions sooner than most to achieve its mission. That was the challenge at the heart of the recently concluded class 4.s42 (Building Technology — Carbon Reduction Pathways for the MIT Campus).The class brought together undergraduate and graduate students from across the Institute to learn about different technologies and decide on the best path forward. It concluded with a final report as well as student presentations to members of MIT’s Climate Nucleus on May 9.“The mission of the class is to put together a cohesive document outlining how MIT can reach its goal of decarbonization by 2050,” says Morgan Johnson Quamina, an undergraduate in the Department of Civil and Environmental Engineering. “We’re evaluating how MIT can reach these goals on time, what sorts of technologies can help, and how quickly and aggressively we’ll have to move. The final report details a ton of scenarios for partial and full implementation of different technologies, outlines timelines for everything, and features recommendations.”The class was taught by professor of architecture Christoph Reinhart but included presentations by other faculty about low- and zero-carbon technology areas in their fields, including advanced nuclear reactors, deep geothermal energy, carbon capture, and more.The students’ work served as an extension of MIT’s Campus Decarbonization Working Group, which Reinhart co-chairs with Director of Sustainability Julie Newman. The group is charged with developing a technology roadmap for the campus to reach its goal of decarbonizing its energy systems.Reinhart says the class was a way to leverage the energy and creativity of students to accelerate his group’s work.“It’s very much focused on establishing a vision for what could happen at MIT,” Reinhart says. “We are trying to bring these technologies together so that we see how this [decarbonization process] would actually look on our campus.”A class with impactThroughout the semester, every Thursday from 9 a.m. to 12 p.m., around 20 students gathered to explore different decarbonization technology pathways. They also discussed energy policies, methods for evaluating risk, and future electric grid supply changes in New England.“I love that this work can have a real-world impact,” says Emile Germonpre, a master’s student in the Department of Nuclear Science and Engineering. “You can tell people aren’t thinking about grades or workload — I think people would’ve loved it even if the workload was doubled. Everyone is just intrinsically motivated to help solve this problem.”The classes typically began with an introduction to one of 10 different technologies. The introductions covered technical maturity, ease of implementation, costs, and how to model the technology’s impact on campus emissions. Students were then split into teams to evaluate each technology’s feasibility.“I’ve learned a lot about decarbonization and climate change,” says Johnson Quamina. “As an undergrad, I haven’t had many focused classes like this. But it was really beneficial to learn about some of these technologies I hadn’t even heard of before. It’s awesome to be contributing to the community like this.”As part of the class, students also developed a model that visualizes each intervention’s effect on emissions, allowing users to select interventions or combinations of interventions to see how they shape emissions trajectories.“We have a physics-based model that takes into account every building,” says Reinhart. “You can look at variants where we retrofit buildings, where we add rooftop photovoltaics, nuclear, carbon capture, and adopting different types of district underground heating systems. The point is you can start to see how fast we could do something like this and what the real game-changers are.”The class also designed and conducted a preliminary survey, to be expanded in the fall, that captures the MIT community’s attitudes towards the different technologies. Preliminary results were shared with the Climate Nucleus during students’ May 9 presentations.“I think it’s this unique and wonderful intersection of the forward-looking and innovative nature of academia with real world impact and specificity that you’d typically only find in industry,” Germonpre says. “It lets you work on a tangible project, the MIT campus, while exploring technologies that companies today find too risky to be the first mover on.”From MIT’s campus to the worldThe students recommended MIT form a building energy team to audit and retrofit all campus buildings. They also suggested MIT order a comprehensive geological feasibility survey to support planning regarding shallow and deep borehole fields for harvesting underground heat. A third recommendation was to communicate with the MIT community as well as with regulators and policymakers in the area about the deployment of nuclear batteries and deep geothermal boreholes on campus.The students’ modeling tool can also help members of the working group explore various decarbonization pathways. For instance, installing rooftop photovoltaics now would effectively reduce emissions, but installing them in a few decades, when the regional electricity grid is expected to be reducing its reliance on fossil fuels anyways, would have a much smaller impact.“When you have students working together, the recommendations are a little less filtered, which I think is a good thing,” Reinhart says. “I think there’s a real sense of urgency in the class. For certain choices, we have to basically act now.”Reinhart plans to do more activities related to the Working Group and the class’ recommendations in the fall, and he says he’s currently engaged with the Massachusetts Governor’s Office to explore doing something similar for the state.Students say they plan to keep working on the survey this summer and continue studying their technology areas. In the longer term, they believe the experience will help them in their careers.“Decarbonization is really important, and understanding how we can implement new technologies on campuses or in buildings provides me with a more well-rounded vision for what I could design in my career,” says Johnson Quamina, who wants to work as a structural or environmental engineer but says the class has also inspired her to consider careers in energy.The students’ findings also have implications beyond MIT campus. In accordance with MIT’s 2015 climate plan that committed to using the campus community as a “test bed for change,” the students’ recommendations also hold value for organizations around the world.“The mission is definitely broader than just MIT,” Germonpre says. “We don’t just want to solve MIT’s problem. We’ve dismissed technologies that were too specific to MIT. The goal is for MIT to lead by example and help certain technologies mature so that we can accelerate their impact.” More