Technique could prevent overheating of laptops, mobile phones, and other electronics.
|Engineers at MIT have developed a polymer thermal conductor — a plastic material that, however counterintuitively, works as a heat conductor, dissipating heat rather than insulating it. Image, Chelsea Turner/MIT|
Plastics are excellent insulators, meaning they can efficiently trap heat — a quality that can be an advantage in something like a coffee cup sleeve. But this insulating property is less desirable in products such as plastic casings for laptops and mobile phones, which can overheat, in part because the coverings trap the heat that the devices produce.
Now a team of engineers at MIT has developed a polymer thermal conductor — a plastic material that, however counterintuitively, works as a heat conductor, dissipating heat rather than insulating it. The new polymers, which are lightweight and flexible, can conduct 10 times as much heat as most commercially used polymers.
“Traditional polymers are both electrically and thermally insulating. The discovery and development of electrically conductive polymers has led to novel electronic applications such as flexible displays and wearable biosensors,” says Yanfei Xu, a postdoc in MIT’s Department of Mechanical Engineering. “Our polymer can thermally conduct and remove heat much more efficiently. We believe polymers could be made into next-generation heat conductors for advanced thermal management applications, such as a self-cooling alternative to existing electronics casings.”
Xu and a team of postdocs, graduate students, and faculty, have published their results March 30, 2018, in Science Advances. The team includes Xiaoxue Wang, who contributed equally to the research with Xu, along with Jiawei Zhou, Bai Song, Elizabeth Lee, and Samuel Huberman; Zhang Jiang, physicist at Argonne National Laboratory; Karen Gleason, associate provost of MIT and the Alexander I. Michael Kasser Professor of Chemical Engineering; and Gang Chen, head of MIT’s Department of Mechanical Engineering and the Carl Richard Soderberg Professor of Power Engineering.
If you were to zoom in on the microstructure of an average polymer, it wouldn’t be difficult to see why the material traps heat so easily. At the microscopic level, polymers are made from long chains of monomers, or molecular units, linked end to end. These chains are often tangled in a spaghetti-like ball. Heat carriers have a hard time moving through this disorderly mess and tend to get trapped within the polymeric snarls and knots.
And yet, researchers have attempted to turn these natural thermal insulators into conductors. For electronics, polymers would offer a unique combination of properties, as they are lightweight, flexible, and chemically inert. Polymers are also electrically insulating, meaning they do not conduct electricity, and can therefore be used to prevent devices such as laptops and mobile phones from short-circuiting in their users’ hands.
Several groups have engineered polymer conductors in recent years, including Chen’s group, which in 2010 invented a method to create “ultradrawn nanofibers” from a standard sample of polyethylene. The technique stretched the messy, disordered polymers into ultrathin, ordered chains — much like untangling a string of holiday lights. Chen found that the resulting chains enabled heat to skip easily along and through the material, and that the polymer conducted 300 times as much heat compared with ordinary plastics.
But the insulator-turned-conductor could only dissipate heat in one direction, along the length of each polymer chain. Heat couldn’t travel between polymer chains, due to weak Van der Waals forces — a phenomenon that essentially attracts two or more molecules close to each other. Xu wondered whether a polymer material could be made to scatter heat away, in all directions.
Xu conceived of the current study as an attempt to engineer polymers with high thermal conductivity, by simultaneously engineering intramolecular and intermolecular forces — a method that she hoped would enable efficient heat transport along and between polymer chains.
The team ultimately produced a heat-conducting polymer known as polythiophene, a type of conjugated polymer that is commonly used in many electronic devices.
Hints of heat in all directions
Xu, Chen, and members of Chen’s lab teamed up with Gleason and her lab members to develop a new way to engineer a polymer conductor using oxidative chemical vapor deposition (oCVD), whereby two vapors are directed into a chamber and onto a substrate, where they interact and form a film. “Our reaction was able to create rigid chains of polymers, rather than the twisted, spaghetti-like strands in normal polymers.” Xu says.
In this case, Wang flowed the oxidant into a chamber, along with a vapor of monomers — individual molecular units that, when oxidized, form into the chains known as polymers.
“We grew the polymers on silicon/glass substrates, onto which the oxidant and monomers are adsorbed and reacted, leveraging the unique self-templated growth mechanism of CVD technology," Wang says.
Wang produced relatively large-scale samples, each measuring 2 square centimeters — about the size of a thumbprint.
“Because this sample is used so ubiquitously, as in solar cells, organic field-effect transistors, and organic light-emitting diodes, if this material can be made to be thermally conductive, it can dissipate heat in all organic electronics,” Xu says.
The team measured each sample’s thermal conductivity using time-domain thermal reflectance — a technique in which they shoot a laser onto the material to heat up its surface and then monitor the drop in its surface temperature by measuring the material’s reflectance as the heat spreads into the material.
“The temporal profile of the decay of surface temperature is related to the speed of heat spreading, from which we were able to compute the thermal conductivity,” Zhou says.
On average, the polymer samples were able to conduct heat at about 2 watts per meter per kelvin — about 10 times faster than what conventional polymers can achieve. At Argonne National Laboratory, Jiang and Xu found that polymer samples appeared nearly isotropic, or uniform. This suggests that the material’s properties, such as its thermal conductivity, should also be nearly uniform. Following this reasoning, the team predicted that the material should conduct heat equally well in all directions, increasing its heat-dissipating potential.
Going forward, the team will continue exploring the fundamental physics behind polymer conductivity, as well as ways to enable the material to be used in electronics and other products, such as casings for batteries, and films for printed circuit boards.
“We can directly and conformally coat this material onto silicon wafers and different electronic devices” Xu says. “If we can understand how thermal transport [works] in these disordered structures, maybe we can also push for higher thermal conductivity. Then we can help to resolve this widespread overheating problem, and provide better thermal management.”
This research was supported, in part, by the U.S. Department of Energy — Basic Energy Sciences and the MIT Deshpande Center.
– Jennifer Chu | MIT News Office
March 30, 2018
Design principles could point to better electrolytes for next-generation lithium batteries.
|Diagram illustrates the crystal lattice of a proposed battery electrolyte material called Li3PO4. The researchers found that measuring how vibrations of sound move through the lattice could reveal how well ions – electrically charged atoms or molecules – could travel through the solid material, and therefore how they would work in a real battery. In this diagram, the oxygen atoms are shown in red, the purple pyramid-like shapes are phosphate (PO4) molecules. The orange and green spheres are ions of lithium. Image: Sokseiha Muy|
A new approach to analyzing and designing new ion conductors — a key component of rechargeable batteries — could accelerate the development of high-energy lithium batteries, and possibly other energy storage and delivery devices such as fuel cells, researchers say.
The new approach relies on understanding the way vibrations move through the crystal lattice of lithium ion conductors and correlating that with the way they inhibit ion migration. This provides a way to discover new materials with enhanced ion mobility, allowing rapid charging and discharging. At the same time, the method can be used to reduce the material’s reactivity with the battery’s electrodes, which can shorten its useful life. These two characteristics — better ion mobility and low reactivity — have tended to be mutually exclusive.
The new concept was developed by a team led by W.M. Keck Professor of Energy Yang Shao-Horn, graduate student Sokseiha Muy, recent graduate John Bachman PhD ’17, and Research Scientist Livia Giordano, along with nine others at MIT, Oak Ridge National Laboratory, and institutions in Tokyo and Munich. Their findings were reported in the journal Energy and Environmental Science.
The new design principle has been about five years in the making, Shao-Horn says. The initial thinking started with the approach she and her group have used to understand and control catalysts for water splitting, and applying it to ion conduction — the process that lies at the heart of not only rechargeable batteries, but also other key technologies such as fuel cells and desalination systems. While electrons, with their negative charge, flow from one pole of the battery to the other (thus providing power for devices), positive ions flow the other way, through an electrolyte, or ion conductor, sandwiched between those poles, to complete the flow.
Typically, that electrolyte is a liquid. A lithium salt dissolved in an organic liquid is a common electrolyte in today’s lithium-ion batteries. But that substance is flammable and has sometimes caused these batteries to catch fire. The search has been on for a solid material to replace it, which would eliminate that issue.
A variety of promising solid ion conductors exist, but none is stable when in contact with both the positive and negative electrodes in lithium-ion batteries, Shao-Horn says. Therefore, seeking new solid ion conductors that have both high ion conductivity and stability is critical. But sorting through the many different structural families and compositions to find the most promising ones is a classic needle in a haystack problem. That’s where the new design principle comes in.
The idea is to find materials that have ion conductivity comparable to that of liquids, but with the long-term stability of solids. The team asked, “What is the fundamental principle? What are the design principles on a general structural level that govern the desired properties?” Shao-Horn says. A combination of theoretical analysis and experimental measurements has now yielded some answers, the researchers say.
“We realized that there are a lot of materials that could be discovered, but no understanding or common principle that allows us to rationalize the discovery process,” says Muy, the paper’s lead author. “We came up with an idea that could encapsulate our understanding and predict which materials would be among the best.”
|Diagram illustrates the crystal lattice of a proposed battery electrolyte material called Li3PO4. The researchers found that measuring how vibrations of sound move through the lattice could reveal how well ions – electrically charged atoms or molecules – could travel through the solid material, and therefore how they would work in a real battery. In this diagram, the purple pyramid-like shapes are phosphate (PO4) molecules. The orange and green spheres are ions of lithium.|
The key was to look at the lattice properties of these solid materials’ crystalline structures. This governs how vibrations such as waves of heat and sound, known as phonons, pass through materials. This new way of looking at the structures turned out to allow accurate predictions of the materials’ actual properties. “Once you know [the vibrational frequency of a given material], you can use it to predict new chemistry or to explain experimental results,” Shao-Horn says.
The researchers observed a good correlation between the lattice properties determined using the model and the lithium ion conductor material’s conductivity. “We did some experiments to support this idea experimentally” and found the results matched well, she says.
They found, in particular, that the vibrational frequency of lithium itself can be fine-tuned by tweaking its lattice structure, using chemical substitution or dopants to subtly change the structural arrangement of atoms.
The new concept can now provide a powerful tool for developing new, better-performing materials that could lead to dramatic improvements in the amount of power that could be stored in a battery of a given size or weight, as well as improved safety, the researchers say. Already, they used the method to find some promising candidates. And the techniques could also be adapted to analyze materials for other electrochemical processes such as solid-oxide fuel cells, membrane based desalination systems, or oxygen-generating reactions.
The team included Hao-Hsun Chang at MIT; Douglas Abernathy, Dipanshu Bansal, and Olivier Delaire at Oak Ridge; Santoshi Hori and Ryoji Kanno at Tokyo Institute of Technology; and Filippo Maglia, Saskia Lupart, and Peter Lamp at Research Battery Technology at BMW Group in Munich. The work was supported by BMW, the National Science Foundation, and the U.S. Department of Energy.
– David L. Chandler | MIT News Office
March 25, 2018
With an atomic structure resembling a Japanese basketweaving pattern, “kagome metal” exhibits exotic, quantum behavior.
|An illustration depicting a kagome metal — an electrically conducting crystal, made from layers of iron and tin atoms, with each atomic layer arranged in the repeating pattern of a kagome lattice. Images by Felice Frankel; Illustration overlays by Chelsea Turner|
A motif of Japanese basketweaving known as the kagome pattern has preoccupied physicists for decades. Kagome baskets are typically made from strips of bamboo woven into a highly symmetrical pattern of interlaced, corner-sharing triangles.
If a metal or other conductive material could be made to resemble such a kagome pattern at the atomic scale, with individual atoms arranged in similar triangular patterns, it should in theory exhibit exotic electronic properties.
In a paper published March 19, 2018, in Nature, physicists from MIT, Harvard University, and Lawrence Berkeley National Laboratory report that they have for the first time produced a kagome metal — an electrically conducting crystal, made from layers of iron and tin atoms, with each atomic layer arranged in the repeating pattern of a kagome lattice.
When they flowed a current across the kagome layers within the crystal, the researchers observed that the triangular arrangement of atoms induced strange, quantum-like behaviors in the passing current. Instead of flowing straight through the lattice, electrons instead veered, or bent back within the lattice.
This behavior is a three-dimensional cousin of the so-called Quantum Hall effect, in which electrons flowing through a two-dimensional material will exhibit a “chiral, topological state,” in which they bend into tight, circular paths and flow along edges without losing energy.
“By constructing the kagome network of iron, which is inherently magnetic, this exotic behavior persists to room temperature and higher,” says Joseph Checkelsky, assistant professor of physics at MIT. “The charges in the crystal feel not only the magnetic fields from these atoms, but also a purely quantum-mechanical magnetic force from the lattice. This could lead to perfect conduction, akin to superconductivity, in future generations of materials.”
To explore these findings, the team measured the energy spectrum within the crystal, using a modern version of an effect first discovered by Heinrich Hertz and explained by Einstein, known as the photoelectric effect.
“Fundamentally, the electrons are first ejected from the material’s surface and are then detected as a function of takeoff angle and kinetic energy,” says Riccardo Comin, an assistant professor of physics at MIT. “The resulting images are a very direct snapshot of the electronic levels occupied by electrons, and in this case they revealed the creation of nearly massless ‘Dirac’ particles, an electrically charged version of photons, the quanta of light.”
The spectra revealed that electrons flow through the crystal in a way that suggests the originally massless electrons gained a relativistic mass, similar to particles known as massive Dirac fermions. Theoretically, this is explained by the presence of the lattice’s constituent iron and tin atoms. The former are magnetic and give rise to a “handedness,” or chirality. The latter possess a heavier nuclear charge, producing a large local electric field. As an external current flows by, it senses the tin’s field not as an electric field but as a magnetic one, and bends away.
The research team was led by Checkelsky and Comin, as well as graduate students Linda Ye and Min Gu Kang in collaboration with Liang Fu, the Biedenharn Associate Professor of Physics, and postdoc Junwei Liu. The team also includes Christina Wicker ’17, research scientist Takehito Suzuki of MIT, Felix von Cube and David Bell of Harvard, and Chris Jozwiak, Aaron Bostwick, and Eli Rotenberg of Lawrence Berkeley National Laboratory.
“No alchemy required”
Physicists have theorized for decades that electronic materials could support exotic Quantum Hall behavior with their inherent magnetic character and lattice geometry. It wasn’t until several years ago that researchers made progress in realizing such materials.
“The community realized, why not make the system out of something magnetic, and then the system’s inherent magnetism could perhaps drive this behavior,” says Checkelsky, who at the time was working as a researcher at the University of Tokyo.
This eliminated the need for laboratory produced fields, typically 1 million times as strong as the Earth’s magnetic field, needed to observe this behavior.
“Several research groups were able to induce a Quantum Hall effect this way, but still at ultracold temperatures a few degrees above absolute zero — the result of shoehorning magnetism into a material where it did not naturally occur,” Checkelsky says.
|Assistant professor of physics at MIT Joe Checkelsky (left to right), graduate students Linda Ye and Min Gu Kang, and assistant professor of physics at MIT Riccardo Comin. Image, Takehito|
At MIT, Checkelsky has instead looked for ways to drive this behavior with “instrinsic magnetism.” A key insight, motivated by the doctoral work of Evelyn Tang PhD ’15 and Professor Xiao-Gang Wen, was to seek this behavior in the kagome lattice. To do so, first author Ye ground together iron and tin, then heated the resulting powder in a furnace, producing crystals at about 750 degrees Celsius — the temperature at which iron and tin atoms prefer to arrange in a kagome-like pattern. She then submerged the crystals in an ice bath to enable the lattice patterns to remain stable at room temperature.
“The kagome pattern has big empty spaces that might be easy to weave by hand, but are often unstable in crystalline solids which prefer the best packing of atoms,” Ye says. “The trick here was to fill these voids with a second type of atom in a structure that was at least stable at high temperatures. Realizing these quantum materials doesn’t need alchemy, but instead materials science and patience.”
Bending and skipping toward zero-energy loss
Once the researchers grew several samples of crystals, each about a millimeter wide, they handed the samples off to collaborators at Harvard, who imaged the individual atomic layers within each crystal using transmission electron microscopy. The resulting images revealed that the arrangement of iron and tin atoms within each layer resembled the triangular patterns of the kagome lattice. Specifically, iron atoms were positioned at the corners of each triangle, while a single tin atom sat within the larger hexagonal space created between the interlacing triangles.
Ye then ran an electric current through the crystalline layers and monitored their flow via electrical voltages they produced. She found that the charges deflected in a manner that seemed two-dimensional, despite the three-dimensional nature of the crystals. The definitive proof came from the photoelectron experiments conducted by co-first author Kang who, in concert with the LBNL team, was able to show that the electronic spectra corresponded to effectively two-dimensional electrons.
“As we looked closely at the electronic bands, we noticed something unusual,” Kang adds. “The electrons in this magnetic material behaved as massive Dirac particles, something that had been predicted long ago but never been seen before in these systems.”
“The unique ability of this material to intertwine magnetism and topology suggests that they may well engender other emergent phenomena,” Comin says. “Our next goal is to detect and manipulate the edge states which are the very consequence of the topological nature of these newly discovered quantum electronic phases.”
Looking further, the team is now investigating ways to stabilize other more highly two-dimensional kagome lattice structures. Such materials, if they can be synthesized, could be used to explore not only devices with zero energy loss, such as dissipationless power lines, but also applications toward quantum computing.
“For new directions in quantum information science there is a growing interest in novel quantum circuits with pathways that are dissipationless and chiral,” Checkelsky says. “These kagome metals offer a new materials design pathway to realizing such new platforms for quantum circuitry.”
This research was supported in part by the Gordon and Betty Moore Foundation and the National Science Foundation.
– Jennifer Chu | MIT News Office
March 19, 2018
Summer Scholar Jennifer Coulter models how spinning colloidal particles move through a fixed array of obstacles.
|MPC_CMSE Summer Scholar Jennifer Coulter worked on modeling the behavior of ferromagnetic particles stimulated by a rotating magnetic field to spin in a passive cluster of non-magnetic particles. Image on computer in background shows map of the path traveled by individual active particles as they spin through the passive matrix, with colors denoting particle location at different times. Purple represents the particles’ positions at the start of the simulation, while red represents the particles’ positions at the end. Coulter interned under Alfredo Alexander-Katz, the Walter Henry Gale Associate Professor of Materials Science and Engineering at MIT. Photo, Denis Paiste, Materials Processing Center.|
Earlier this year, MIT Associate Professor Alfredo Alexander-Katz’s group demonstrated experimentally that ferromagnetic particles spinning under a rotating magnetic field in a milky suspension are attracted to each other across relatively long distances in a crowd of non-magnetic particles.
MPC-CMSE Summer Scholar Jennifer Coulter interned with Alexander-Katz, the Walter Henry Gale Associate Professor of Materials Science and Engineering at MIT, this summer on a project to develop a more generalized computer model for these active spinning particles in a passive colloidal mixture. “We’re studying the interactions of the spinners with the passive particles through simulation,” Coulter says. Describing this theoretical environment, Coulter explains, “Only the spinners move.” The non-magnetic particles are now fixed in a pattern without moving during the simulation.
This project fits into the Alexander-Katz lab’s work on a wide range of active soft matter challenges from understanding how neurotransmitters move from one neuron to another in the brain to how single-celled organisms sense each other at a distance. “The power of soft means [that] with very small stimuli we can actually have large or strong changes,” Alexander-Katz said during a presentation to Summer Scholars in June. Alexander-Katz is part of the Physics of Living Systems group.
For Coulter, a rising senior at Rutgers University whose prior work focused on high-energy physics, biophysics is new territory. “The way I’ve been working with him on this project has been an experience in using what I know in terms of computing and general physics knowledge to develop and explore a new system,” she says.
“I’ve actually found that a lot of the work I did in high-energy physics has been really useful because that’s also computational, even though it’s very different. So I had some skills coming in, but I’ve definitely had some time to work on them here,” Coulter says.
“I’m running larger scale things, where I have to deal with huge numbers and lots of data, so I need to consider things in Unix command lines,” she says. “I’m using different computers to run my code, because I can’t run it on my laptop. It would overheat or take days,” Coulter explains. She is writing much of the code for this project herself from scratch using Python and a mix of Unix tools including Bash and Shell scripts.
“I think that in terms of just general computing stuff, in Unix and other things, it’s been really good to spend more time working on that, because those are skills I hope to use in grad school. I’d like to go for computational physics, probably,” Coulter says.
Using this computer model, Coulter analyzed how changes in simulation specifications affect the end result. For example, she could alter the speed at which the magnetic field rotates, change the torque from hydrodynamic interactions and modify the attractive or repulsive force between spinners and passive particles.
|MIT researchers led by Alfredo Alexander-Katz, the Walter Henry Gale Associate Professor of Materials Science and Engineering at MIT, found long-range interaction between particles in a liquid medium based entirely on their motions. Video, Melanie Gonick, MIT.|
“The key thing that we’re going to vary is a parameter that’s going to help us describe the disorder of the way we’ve arranged the passive particles; so we want to study how disorder affects the transport of the spinners through the passive particles,” Coulter says. The simulations cover a range from a highly ordered system through a range of different distortions to the ordered system to see how spinners behave as disorder increases.
She hopes to learn under what conditions spinners move in a straight line versus a diffuse pattern, that is, scatter in different directions. “I think it would be cool if we could see really diffusive transport in relation to adding disorder to our system,” Coulter says. “The end goal is to compare the active matter system to a system that’s currently very popular in terms of topological materials and 2D materials and transport in those materials. So we would like to try to create this system as an analogue to that more difficult to study system.”
Coulter says Alexander-Katz has been an extremely involved advisor. “I think in terms of my personal growth, actually the best part of my experience here has been just working with Prof. Alexander-Katz,” she says. “It’s really nice to be able to talk to him for an hour or more at a time, several times a week. He’s really supported me and gives really good feedback, and I think in terms of my development as a scientist, a lot of what I’ve gained from this has just been in my experience working with him. I really appreciate his role as a mentor.”
Success for her project would be characterizing the disorder of the arrangement of passive particles, and how it changes the nature of transport for the spinners, Coulter suggests. “It’s actually, I think, something we’re pretty close to attaining, but since it was a smaller project, we are now starting to do some more final runs of the code. I’m about to get some of the last results soon ... so hopefully they’re the good kind. They’ve looked really promising up to this point.”
MPC and CMSE sponsor the nine-week National Science Foundation Research Experience for Undergraduates (NSF REU) internships with support from NSF’s Materials Research Science and Engineering Centers program (grant number DMR-1419807). The program runs from June 7 through Aug. 6, 2016.
– Denis Paiste, Materials Processing Center | Aug. 22, 2016
Summer Scholar Victoria Yao experiments with water-based, flow-driven battery concept in Brushett Lab.
|MPC-CMSE Summer Scholar Victoria Yao shows a mixture of sodium thiosulfate being stirred in a solution of copper hexacyanoferrate where its reacts to produce larger compounds that can be filtered out of the solution. These compounds are being developed for use in electrodes for a water-based flow-driven battery for grid level storage. Photo, Maria E. Aglietti, Materials Processing Center.|
A convection oven cooks more uniformly because a fan creates a steady flow of hot air around the food being cooked. MIT’s St. Laurent Career Development Professor of Chemical Engineering, Fikile R. Brushett, is applying that principle to a new water-based battery design that pumps a steady flow of charge-carrying ions through a battery to create a more uniform distribution of ions in the cell.
“We’re fabricating an aqueous [water-based] battery which is safe, cheap and scalable,” MPC-CMSE Summer Scholar Victoria Yao explains. A rising junior at Vanderbilt University, Yao is majoring in chemical engineering. This summer, she worked with graduate student Thomas J. Carney to develop a convection cell battery, in which electrolyte would flow through the electrode material, rather than remain static as in a conventional battery.
During her summer internship in the Brushett Research Group, Yao synthesized electrode materials in the same chemical family as Prussian blue, which is commonly used to dye fabric and to color makeup such as eye shadow. Prussian blue is the common name for ferric ferrocyanide, but unlike some highly poisonous cyanide compounds, Prussian blue is considered non-toxic and is even ingested to treat radiation poisoning.
Yao created the Prussian blue-like materials in a liquid solution of metal nitrates, adding potassium hexacyanoferrate to precipitate the desired compounds, such as copper hexacyanoferrate. Sodium thiosulfate is added to make the particles bigger and change the amount of energy stored in the material. She then separated out the particles with a filter or centrifuge. Once dried, the colorful particles were mixed with additives to make “inks” that can be precisely dripped on carbon paper to form thin electrodes for battery testing. “Immediately after you make the ink, you need to pipette it onto carbon paper, let it dry, and then remove any contaminants under vacuum,” Yao says.
These thin electrode cells were tested in a flow-through reactor – an arrangement of glass components in an “H” configuration – that runs for hours at a time, sometimes overnight, to gather sufficient data to measure their capacity and efficiency. The carbon-paper electrodes are placed in the upright glass tubes with a sodium chloride [table salt] solution connecting the two through the middle. After adding a reference electrode, Yao bubbled argon gas through the experimental setup.
“I’ve made eight different powders, and tested their energy storage properties with thin electrodes,” she says.
After making and testing thin electrodes, Yao plans to move to producing thick porous electrodes, which could yield higher energy density. These thick electrodes require more raw material, which is compressed into pellets.
In presenting this project, which he calls Convection Enhanced Electrochemical Energy Storage, to Summer Scholars in June, Brushett said, “We’ve got a proof of concept, and we’re really excited. ... Ideally we’d like to translate this proof-of-concept device into a little canister-like flow cell with thick porous electrodes and pump electrolyte through the reactor... and there we should have our convection battery.”
|MPC-CMSE Summer Scholar Victoria Yao holds a container with a thin electrode made of carbon with a copper hexacyanoferrate [Prussian blue] type active area at the top end. Photo, Maria E. Aglietti, Materials Processing Center|
“Prussian blue-related chemical compounds have an open framework that allows ions to move in and out of the two battery electrodes while remaining solvated (bound to water molecules) by the electrolyte solution which, in turn, enables fast reactions. When coupled with a flowing electrolyte we may be able to develop energy dense batteries with thick electrodes and tunable power output,” Brushett says. The project fits into the Brushett lab’s larger goal of helping to build a sustainable energy economy through innovative, new energy storage technologies.
“Victoria has been instrumental in synthesizing and characterizing Prussian blue analogue materials for use in convection-based energy storage devices this summer. The MPC-CMSE Summer Scholar’s program provides opportunities for talented undergraduates from across the country to work on cutting-edge research projects here at MIT,” Brushett says. “Not only do these students learn of the discipline, tenacity, and creativity required to perform fundamental research but our graduate students learn how to mentor enthusiastic but less experienced lab workers. Her dedication and aptitude have directly accelerated our research! We are very happy to have hosted her this summer.”
MPC and CMSE sponsor the nine-week National Science Foundation Research Experience for Undergraduates (NSF REU) internships with support from NSF’s Materials Research Science and Engineering Centers program (grant number DMR-14-19807). The program runs from June 7 through Aug. 6, 2016.
– Denis Paiste, Materials Processing Center | Aug. 22, 2016
Related: Going with the Flow Battery
Summer Scholar Justin Cheng explores process in Berggren group for making ordered metal nanostructures that display interesting new properties.
|Summer Scholar Justin Cheng holds an experimental sample of nanostructured gold on silicon that has potential for use in sensors and display technologies based on the selective light absorption properties of the material that arise from plasmonic excitations on the surface of the gold. Photo, Maria E. Aglietti.|
Ordered patterns of gold nanoparticles on a silicon base can be stimulated to produce collective electron waves known as plasmons that absorb only certain narrow bands of light making them promising for a wide array of sensors and display technologies in medicine, industry and science.
MPC-CMSE Summer Scholar Justin Cheng worked this summer in Professor of Electrical Engineering Karl K. Berggren’s Quantum Nanostructures and Nanofabrication Group to develop specialized techniques for forming these patterns in gold on silicon. “Ideally, we’d want to be able to get arrays of gold nanoparticles to be completely ordered,” Cheng says.
“My work deals with the fundamentals of how to write a pattern using electron-beam lithography, how to deposit the gold, and how to heat up the substrate so we can get completely regular arrays of particles,” Cheng explains.
Cheng wrote code to produce a pattern that will guide the dewetting of a thin gold film into nanoparticles, examined partially ordered grids with an electron microscope, and worked in a clean room to spin-coat polymer resist onto samples, develop the resist, and plasma clean samples. He is part of a team that includes graduate student Sarah Goodman, postdoctoral associate Mostafa Bedewy, and he was assisted by Research Specialist James Daley, who is the lab manager in MIT’s NanoStructures Laboratory, where this work was performed.
“Plasmons are collective oscillations of the free-electron density at the surface of a material, and they give metal nanostructures amazing properties that are very useful in applications like sensing, optics and various devices,” Goodman explained in a presentation to Summer Scholars in June. “Plasmonic arrays are very good for visible displays, for example, because their color can be tuned based on size and geometry.”
This multi-step fabrication process begins with spin coating hydrogen silsesquioxane [HSQ], which is a special electron-beam resist, or mask, onto a silicon substrate. Cheng worked on software used to write a pattern onto the resist through electron-beam lithography. Unlike some resists, HSQ becomes more chemically resistant as you expose it to electron beams, he says. The entire substrate is about 1 cm by 1 cm (0.39 inch x 0.39 inch), he notes, and the write area is about 100 microns (0.0039 inch) wide.
After the electron-beam lithography step, the resist is put through an aqueous [water-based] developer solution of sodium hydroxide and sodium chloride, which leaves behind an ordered array of posts on top of the silicon layer. “When we put the sample in the developer solution, all of the less chemically resistant areas of the HSQ mask come off, and only the posts remain,” Cheng says. Then, Daley deposits a gold layer on top of the posts with physical vapor deposition. Next, the sample is heat treated until the gold layer decomposes into droplets that self-assemble into nanoparticles guided by the posts.
A key underlying materials science phenomenon at work in this self-assembly, Cheng says, is known as solid-state dewetting. “Self-assembly is a process where you apply certain conditions to a material that allow it to undergo a transformation over a large area. So it’s a very efficient patterning technique,” Goodman explains.
|Summer Scholar Justin Cheng inserts a sample into a scanning electron microscope in the NanoStructures Laboratory. Cheng explored processes for making ordered metal nanostructures that display interesting new properties. Photo, Maria E. Aglietti.|
Because of repulsive interaction between the silicon and gold layers, the gold tends to form droplets, which can be coaxed into patterns around the posts. The Berggren group is working collaboratively with Carl V. Thompson, the Stavros Salapatas Professor of Materials Science and Engineering, who is an expert in solid-state dewetting. Thompson also is director of the Materials Processing Center.Using a scanning electron microscope, Cheng examines these patterns to determine their quality and consistency. “The gold naturally forms droplets because there is a driving force for it to decrease the surface area it shares with the silicon. It doesn’t look completely ordered but you can see beginnings of some order in the dewetting,” he says while showing an SEM image on a computer. “[In] other pictures you can clearly see the beginnings of patterning.”
“When we take the posts and we make them closer together, you can see that the gold likes to dewet into somewhat regular patterns. These aren’t completely regular in all cases, but for certain post sizes and spacings, we start to see regular arrays.
Our goal is to successfully fabricate a plasmonic array of ordered, monodisperse [equally sized] gold nanoparticles,” Cheng explains.
Goodman notes that the group of Carl V. Thompson, the Stavros Salapatas Professor of Materials Science and Engineering at MIT, has demonstrated exquisite control over dewetting in single crystalline films at the micron scale, but the Berggren group hopes to extend this control down to the nanoscale. “This will be a really key result if we’re able to bring this dewetting that’s beautifully controlled on the micro scale and enable that on the nanoscale,” Goodman says.
Cheng, a rising senior at Rutgers University, says that during his summer internship in Berggren’s lab, he learned to operate the scanning electron microscope and learned about nanofabrication processes. “I have learned a lot. Aside from the lab work that I’m doing, I’ve been scripting for the [LayoutEditor] CAD program that I use, and I’ve been using Matlab, too,” he says. “I actually learned a lot about image analysis because there are a lot of steps that go into image analysis. Since we have so much data and so many images to analyze, I’m doing it quantitatively and automatically to make sure I have repeatability.”
MPC and CMSE sponsor the nine-week National Science Foundation Research Experience for Undergraduates (NSF REU) internships with support from NSF’s Materials Research Science and Engineering Centers program (grant number DMR-14-19807). The program ran from June 7 through Aug. 6, 2016.
– Denis Paiste, Materials Processing Center | Aug. 22, 2016
Summer Scholar Alexandra Barth analyzes carbon monoxide resistance of core-shell nanoparticle catalysts in the Román Lab.
|MPC-CMSE Summer Scholar Alexandra Barth works at a hood where she makes carbide core-platinum shell nanoparticles for electrocatalytic applications such as fuel cells and electrolyzers in the lab of MIT Associate Professor of Chemical Engineering Yuriy Román-Leshkov. The process of making core-shell nanoparticles consists of many steps and takes three to five days to complete. Photo, Maria E. Aglietti, Materials Processing Center.|
In May the group of MIT Associate Professor of Chemical Engineering Yuriy Román-Leshkov published a study showing that an ultra-thin shell of platinum on a carbide core could catalyze hydrogen evolution and oxidation reactions as effectively as pure platinum at a fraction of the cost. MPC-CMSE Summer Scholar Alexandra T. Barth is helping to advance this work by studying the tunability of these core-shell materials and their performance in a number of electrocatalytic applications.
“What’s been interesting is we found even when we’re creating nanoparticles that are just coated with an atomically thin layer of platinum, they act as effectively as conventional platinum-only nanoparticle catalysts,” Barth explains.
Platinum is a key component in many traditional and emerging technologies, including automobile catalytic converters, oil reforming, fuel cells and electrolyzers. The goal of the project, Dr. Maria Milina, a postdoctoral associate in the Román group explains, is to design noble metal catalysts with significantly reduced metal loadings but improved activity and stability. “We tackle this challenge through the synthesis of core-shell nanoparticles, in which a cheap metal carbide core not only reduces the requirements for expensive platinum but also beneficially modifies its electronic properties,” says Milina.
Avoiding carbon monoxide poisoning
The research of Prof. Román shows that platinum-coated carbide nanoparticles can be used as catalysts for hydrogen evolution and hydrogen oxidation reactions that occur at the cathode of water electrolyzers and at the anode of fuel cells, respectively. Simultaneously they demonstrate remarkable resistance to carbon monoxide, a common catalyst poison. “You want to create a catalyst that will activate hydrogen even when carbon monoxide is present in fuel streams,” Barth says. Carbon monoxide is known to bind strongly to platinum and to block its ability to catalyze other reactions. “Metal carbide cores favorably modulate electronic properties of platinum through subsurface strain and ligand effect leading to the reduced carbon monoxide binding energy of platinum in a core-shell architecture,” explains Prof. Román.
Barth, a rising senior from Florida State University, is interning in the Román lab at MIT this summer. She is synthesizing core-shell nanoparticles with varying core and shell composition, examining their structure with techniques such as infrared spectroscopy and powder X-ray diffraction, and conducting electrocatalytic experiments to analyze their performance in hydrogen evolution/hydrogen oxidation reactions.
A multistep process
The process of making core-shell nanoparticles consists of many steps and takes three to five days to complete, Barth notes. “It’s interesting because the entire process was devised in this lab, so it’s like nothing that’s been done before,” she says. The process involves synthesizing the nanoparticles in a reverse microemulsion, heating the sample in a methane atmosphere to produce a carbide core, and separating the nanoparticles from their silica templates simultaneously dispersing them on a high surface area carbon support in a diluted hydrofluoric acid. The last step in the process, working with hydrofluoric acid, required special safety training.
After synthesis, Barth tests the core-shell nanoparticle catalyst in a three-electrode electrochemical cell. “We initially determine the hydrogen evolution and oxidation activity of the catalysts in a pure hydrogen atmosphere. Then we intentionally introduce carbon monoxide poison into the hydrogen stream and record how quickly catalyst deactivates and how high is the overpotential required to strip carbon monoxide from the platinum surface,” she says.
Infrared spectroscopy challenge
While characterization of solids by X-ray diffraction was a familiar skill from her work at FSU, Barth was facing a challenge with infrared spectroscopy. “We know what we’re expecting of this analysis. I firstly need to record a spectrum of a reduced in hydrogen catalyst, then I should saturate it with carbon monoxide and, after removal of physisorbed species, register another spectrum with the absorbances corresponding to platinum-carbon monoxide interactions. But the use of infrared spectroscopy for carbon-supported catalysts has been always a challenge due to the high opacity of these materials. So that’s been a work in progress,” she says.
“Back at FSU, I do radiochemistry research, so I make crystals with nuclear elements,” she explains. “This is out of my comfort zone because I’ve never done nanoparticle research before, and I’ve never done catalysis research before. But what I have realized through doing this summer project is that I could advance my current research at FSU by including new catalytic studies.” Barth is considering modifying her honors thesis to bridge radiochemistry and catalysis, taking her work from just making crystals to testing their catalytic properties.
Barth is pursuing a major in chemistry at Florida State and hopes to pursue a doctorate in inorganic chemistry.
MPC and CMSE sponsor the nine-week National Science Foundation Research Experience for Undergraduates (NSF REU) internships with support from NSF’s Materials Research Science and Engineering Centers program (grant number DMR-14-19807). The program runs from June 7 through Aug. 6, 2016.
– Denis Paiste, Materials Processing Center | July 27, 2016
Assistant professor in EECS and DMSE is developing materials with novel structures and useful applications, including renewable energy and information storage.
|Jennifer Rupp's current ceramics research applications range from battery-based storage for renewable energy, to energy-harvesting systems, to devices used to store data during computation. Photo courtesy of Jennifer Rupp.|
Ensuring that her research contributes to society’s well-being is a major driving force for Jennifer Rupp.
“Even if my work is fundamental, I want to think about how it can be useful for society,” says Rupp, the Thomas Lord Assistant Professor of Materials Science and Engineering and an assistant professor in the Department of Electrical Engineering and Computer Science (EECS) at MIT.
Since joining the Department of Materials Science and Engineering in February 2017, she has been focusing not only on the basics of ceramics processing techniques but also on how to further develop those techniques to design new practical devices as well as materials with novel structures. Her current research applications range from battery-based storage for renewable energy, to energy-harvesting systems, to devices used to store data during computation.
Rupp first became intrigued with ceramics during her doctoral studies at ETH Zurich.
“I got particularly interested in how they can influence structures to gain certain functionalities and properties,” she says. During this time, she also became fascinated with how ceramics can contribute to the conversion and storage of energy. The need to transition to a low-carbon energy future motivates much of her work at MIT. “Climate change is happening,” she says. “Even though not everybody may agree on that, it’s a fact.”
One way to tackle the climate change problem is by capitalizing on solar energy. Sunshine falling on the Earth delivers roughly 170,000 terawatts per year — about 10,000 times the energy consumed annually worldwide. “So we have a lot of solar energy,” says Rupp. “The question is, how do we profit the most from it?”
To help convert that solar energy into a renewable fuel, her team is designing a ceramic material that can be used in a solar reactor in which incoming sunlight is controlled to create a heat cycle. During the temperature shifts, the ceramic material incorporates and releases oxygen. At the higher temperature, it loses oxygen; at the lower temperature, it regains the oxygen. When carbon dioxide and water are flushed into the solar reactor during this oxidation process, a split reaction occurs, yielding a combination of carbon monoxide and hydrogen known as syngas, which can be converted catalytically into ethanol, methanol, or other liquid fuels.
While the challenges are many, Rupp says she feels bolstered by the humanitarian ethos at MIT. “At MIT, there are scientists and engineers who care about social issues and try to contribute with science and their problem-solving skills to do more,” she says. “I think this is quite important. MIT gives you strong support to try out even very risky things.”
In addition to continuing her work on new materials, Rupp looks forward to exploring new concepts with her students. During the fall of 2017, she taught two recitation sections of 3.091 (Introduction to Solid State Chemistry), a class that has given thousands of MIT undergraduates a foundation in chemistry from an engineering perspective. This spring, she will begin teaching a new elective for graduate students on ceramics processing and engineering that will delve into making ceramic materials not only on the conventional large-scale level but also as nanofabricated structures and small-system structures for devices that can store and convert energy, compute information, or sense carbon dioxide or various environmental pollutants.
To further engage with students, Rupp has proposed an extracurricular club for them to develop materials science comic strips. The first iteration is available on Instagram (@materialcomics) and it depicts three heroes who jump into various structures to investigate their composition and, naturally, to have adventures. Rupp sees the comics as an exciting avenue to engage the nonscientific community as a whole and to illustrate the structures and compositions of various everyday materials.
“I think it is important to create interest in the topic of materials science across various ages and simply to enjoy the fun in it,” she says.
Rupp says MIT is proving to be a stimulating environment. “Everybody is really committed and open to being creative,” she says. “I think a scientist is not only a teacher or a student; a scientist is someone of any age, of any rank, someone who simply enjoys unlocking creativity to design new materials and devices.”
This article appears in the Autumn 2017 issue of Energy Futures, the magazine of the MIT Energy Initiative.
Kelley Travers | MIT Energy Initiative
MIT News Office, February 9, 2018
Merger of the Materials Processing Center and the Center for Materials Science and Engineering melds a rich history of materials science and engineering breakthroughs.
|MIT MRL Director Carl V. Thompson. Photo, Denis Paiste, MIT MRL.|
The Materials Research Laboratory at MIT starts from a foundation of fundamental scientific research, practical engineering applications, educational outreach and shared experimental facilities laid by its merger partners, the Materials Processing Center and the Center for Materials Science and Engineering.
“We’re bringing them together and that will make communication both inside and outside MIT easier and will make it clearer especially to people outside MIT that for interdisciplinary research on materials, this is the place to learn about it,” says MRL Director Carl V. Thompson.
The Materials Research Laboratory serves interdisciplinary groups of faculty researchers, spanning the spectrum of basic scientific discovery through engineering applications and entrepreneurship to ensure that research breakthroughs have impact on society. The center engages with approximately 150 faculty members and scientists from across the Schools of Science and Engineering who are conducting materials science research. MRL will work with MIT.nano to enhance the toolset available for groundbreaking research as well as collaborate with the MIT Innovation Initiative and The Engine.
MRL will benefit from the long history of research breakthroughs under MPC and CMSE such as “perfect mirror” technology developed through CMSE in 1998 that led to a new kind of fiber optic surgery and a spinout company, OmniGuide Surgical, and the first germanium laser operating at room temperature, which is used for optical communications, in 2012 through MPC’s affiliated Microphotonics Center.
The Materials Processing Center brings to the partnership its wide diversity of materials research, funded by industry, foundations and government agencies, while the Center for Materials Science and Engineering brings its seed projects in basic science and Interdisciplinary Research Groups, educational outreach and shared experimental facilities, funded under the National Science Foundation Materials Research Science and Engineering Center program [NSF-MRSEC]. Combined research funding was $21.5 million for the fiscal year ended June 30, 2017.
MPC’s research volume more than doubled during the past nine years under Thompson’s leadership. “We do have a higher profile in the community both internal as well as external. We developed over the years a close collaboration with CMSE, including outreach. That will be greatly amplified through the merger,” he says. Thompson is the Stavros Salapatas Professor of Materials Science and Engineering at MIT.
Tackling energy problems
With industrial support, MPC and CMSE launched the Substrate Engineering Lab in 2004. MPC affiliates include the AIM Photonics Academy, the Center for Integrated Quantum Materials and the MIT Skoltech Center for Electrochemical Energy Storage. Other research includes Professor Harry L. Tuller’s Chemomechanics of Far-From-Equilibrium Interfaces (COFFEI) project, which aims to produce better oxide-based semiconductor materials for fuel cells, and Senior Research Scientist Jurgen Michel’s Micro-Scale Optimized Solar-Cell Arrays with Integrated Concentration (MOSAIC) project, which aims to achieve overall efficiency of greater than 30 percent.
The MPC kicked off the Singapore-MIT Alliance for Research and Technology Center’s program in Low Energy Electronic Systems [SMART-LEES] in January 2012, managing the MIT part of the budget. SMART-LEES, led by Eugene A. Fitzgerald, the Merton C. Flemings-SMA Professor of Materials Science and Engineering at MIT, was renewed for another five years in January 2017.
Shared experimental facilities, including X-Ray diffraction, scanning and transmission electron microscopy, probe microscopy, and surface analytical capabilities, are used by more than 1,100 individuals each year. “The amount of investment that needs to be made to keep state-of-the-art shared facilities at a university like MIT is on the order of 1 to 2 million dollars per year in new investment and new tools. That kind of funding is very difficult to get. It certainly doesn’t come to us through just NSF funding,” says TDK Professor of Polymer Materials Science and Engineering Michael F. Rubner, who is retiring after 16 years as CMSE director. “MIT.nano, in concert with MRL, will be able to work together to look at new strategies for trying to maintain state-of-the-art equipment and to find funding sources and to figure out ways to not only get the equipment in, but to have highly trained professionals running that equipment.”
Associate Professor of Materials Science and Engineering Geoffrey S.D. Beach succeeds Rubner as co-director of the MIT MRL and principal investigator for the NSF-MRSEC.
Spinning out jobs
|Merton C. Flemings, founding director [1980-82] of MIT Materials Processing Center and retired Toyota Professor of Materials Processing. Photo, Denis Paiste, MIT MRL.|
NSF-MRSEC-funded research through CMSE has led to approximately 1,100 new jobs through spinouts such as American Superconductor [superconductivity], OmniGuide Surgical [optical fibers] and QD Vision [quantum dots], which Samsung acquired in 2016. Many of these innovations began with seed funding, CMSE’s earliest stage of support, and evolved through joint efforts with MPC, such as microphotonics research that began with a seed grant in 1993, followed by Interdisciplinary Research Group funding a year later. In 1997, MIT researchers published two key papers in Nature and Physical Review Letters, won a two-year, multi-university award through DARPA for Photonic Crystal Engineering, and formed the Microphotonics Center. Further research led to the spinout in 2002 of Luminus Devices, which specializes in solid-state lighting based on light emitting diodes [LEDs].
“Our greatest legacy is bringing people together to produce fundamental new science, and then allowing those researchers to explore that new science in ways that may be beneficial to society, as well as to develop new technologies and launch companies,” Rubner says. He recalls that research in complex photonic crystal structures began with Francis Wright Davis Professor of Physics John D. Joannopoulos as leader. “They got funding through us, at first as seed funding and then IRG [interdisciplinary research group] funding, and over the years, they have continued to get funding from us because they evolved. They would seek a new direction, and one of the new directions they evolved into was this idea of making photonic fibers, so they went from photonic crystals to photonic fibers and that led to, for example, the launching of OmniGuide.” An outgrowth of basic CMSE research, the company’s founders included Professors Joannopolous, Yoel Fink, and Edwin L. [“Ned”] Thomas, who served as William and Stephanie Sick Dean of the George R. Brown School of Engineering at Rice University from 2011 to 2017.
Under Fink’s leadership, that work evolved into Advanced Functional Fabrics of America [AFFOA], a public-private Manufacturing Innovation Institute devoted to creating and bringing to market revolutionary fibers and textiles. The institute, which is a separate nonprofit organization, is led by Fink, while MIT on-campus research is led by Lammot du Pont Professor of Chemical Engineering Gregory C. Rutledge.
Susan D. Dalton, NSF-MRSEC Assistant Director, recalls the evolution of perfect mirror technology into life-saving new fiber optic surgery. “From an administrator’s point of view,” Dalton says, “it’s really exciting because day to day, things happen that you don’t know are going to happen. When you think about saving people’s lives, that’s amazing, and that’s just one example,” she says.
Government, industry partners
Through its Collegium and close partnership with the MIT Industrial Liaison Program (ILP), MPC has a long history of government and industrial partnerships as well as individual faculty research projects. Merton C. Flemings, who is MPC’s founding director [1980-82], and a retired Toyota Professor of Materials Processing, recalls that the early focus was primarily on metallurgy, but ceramics work also was important. “It’s gone way beyond that, and it’s a delight to see what’s going on,” he notes.
“From the time of initiation of the MPC, we had interdepartmental participation, and quite soon after its formation, we initiated an industrial collegium to share in research formulation and participate in research partnerships. I believe our collegium was the first to work collaboratively with the Industrial Liaison Program. It was also at a period in MIT history when working directly with the commercial sector was rare,” Flemings says.
Founded in February 1980, the Materials Processing Center won early support from NASA, which was interested in processing materials in space. A question being asked then was: “What would it be like when you’re in zero gravity and you try and purify a metal or make anything out there? Dr. John R. Carruthers headed this zero gravity materials processing activity in NASA, and as he considered the problem, he realized we didn’t really have much of a science base of materials processing on earth, let alone in space. With that in mind, at Carruthers’ instigation, NASA provided a very generous continuing grant to MIT that was essential to us starting in those early years,” Flemings explains.
Carruthers went on to become director of research with Intel and is now Distinguished Professor of Physics, at Portland [Oregon] State University. The two men – Flemings at MIT and Carruthers at the University of Toronto – had been familiar with each other’s work in the study of how metals solidify, before Carruthers joined NASA as director of its materials processing in space program in 1977. Both Flemings and Carruthers wanted to understand how the effects of gravitationally driven convection influenced the segregation processes during metals solidification.
|Dr. John R. Carruthers headed zero gravity materials processing activity in NASA, and provided critical early funding for MIT Materials Processing Center. Courtesy photo.|
“In molten metal baths, as the metal solidifies into ingots, the solidification process is never uniform. And so the distribution of the components being solidified is very much affected by fluid flow or convection in the molten metal,” Carruthers explains. “We were both interested in what would happen if you could actually turn gravity down because most of the convective effects were influenced by density gradients in the metal due to thermal and compositional effects. So, we were quite interested in what would happen given that those density gradients existed, if you could actually turn the effects of gravity down.”
“When the NASA program came around, they wanted to try to use the low gravity environment of space to actually fabricate materials,” Carruthers recalls. “After a couple of years at NASA, I was able to secure some block grant funding for the center. It subsequently, of course, has developed its own legs and outgrown any of the initial funding that we provided, which is really great to see, and it’s a tribute to the MIT way of doing research, of course, as well. I was really quite proud to be part of the early development of the center,” Carruthers says. “Many of the things we learned in those days are relevant to other areas. I’m finding a lot of knowledge and way of doing things is transferrable to the biomedical sciences, for example, so I’ve become quiet interested in helping to develop things like nanomonitors, you know, more materials science-oriented approaches for the biomedical sciences.”
Expanding research portfolio
From its beginnings in metals processing with NASA support, MPC evolved into a multi-faceted center with diverse sponsors of research in energy harvesting, conversion and storage; fuel cells; quantum materials and spintronics; materials integration for microsystems; photonic devices and systems; materials systems and sustainability; solid-state ionics; as well as metals processing, an old topic that is hot again.
MRL-affiliated MIT condensed matter physicists include experimentalists Raymond C. Ashoori, Joseph G. Checkelsky, Nuh Gedik, and Pablo Jarillo-Herrero, who are exploring quantum materials for next-generation electronics, such as spintronics and valleytronics, new forms of nanoscale magnetism, and graphene-based optoelectronic devices. Riccardo Comin explores electronic phases in quantum materials. Theorists Liang Fu and Senthil Todadri envision new forms of random access memory, Majorana fermions for quantum computing, and unusual magnetic materials such as quantum spin liquids.
In the realm of biophysics, Associate Professor Jeff Gore tests fundamental ideas of theoretical ecology and evolutionary dynamics through experimental studies of microbial communities. Class of 1922 Career Development Assistant Professor Ibrahim Cissé uses physical techniques that visualize weak and transient biological interactions to study emergent phenomena in live cells with single molecule sensitivity. On the theoretical front, Professor Thomas D. & Virginia W. Cabot Career Development Associate Professor of Physics Jeremy England focuses on structure, function, and evolution in the sub-cellular biophysical realm.
Alan Taub, Professor of Materials Science and Engineering at the University of Michigan, has become a member of the new Materials Research Laboratory External Advisory Board. Taub previously served in senior materials science management roles with General Motors, Ford Motor Co. and General Electric and served as chairman of the Materials Processing Center Advisory Board from 2001-2006. He notes that under Director Lionel Kimerling [1993-2008], MPC embraced the new area of photonics. “That transition was really well done,” Taub says. The MRL-affiliated Microphotonics Center has produced collaborative roadmapping reports since 2007 to guide manufacturing research and address systems requirements for networks that fully exploit the power of photonics. Taub also is chief technical officer of LIFT Manufacturing Innovation Institute, in which MIT Assistant Professor of Materials Science and Engineering Elsa Olivetti and senior research scientist Randolph E. [Randy] Kirchain are engaged in cost modeling.
From its founding, Taub notes, MPC engaged the faculty with industry. Advisory board members often sponsored research as well as offering advice. “So it was really the way to guide the general direction, you know, teach them that there are things industry needs. And remember, this was the era well before entrepreneurism. It really was the interface to the Fortune 500’s and guiding and transitioning the technology out of MIT. That’s why I think it survived changes in technology focus, because at its core, it was interfacing industry needs with the research capabilities at the Institute,” Taub says.
|Alan Taub, Professor of Materials Science and Engineering at the University of Michigan, has become a member of the new Materials Research Laboratory External Advisory Board. Courtesy photo.|
Susan Rosevear, who is the Education Officer for the NSF-MRSEC, is responsible for an extensive array of programs, including the Summer Scholars program, which is primarily funded through NSF’s Research Experience for Undergraduates (REU) program. Each summer a dozen or so top undergraduates from across the country spend about two months at MIT as lab interns working with professors, postdocs and graduate students on cutting edge research.
CMSE also conducts summer programs for community college students and teachers, middle and high school teachers, and participates in the Women’s Technology Program and Boston Area Girls' STEM Collaborative. “Because diversity is also part of our mission, part of what our mission from NSF is, in all we do, we try to broaden participation in science and engineering,” Rosevear says.
Teachers who participate in these programs often note how collaborative the research enterprise is at MIT, Rosevear notes. Several have replaced cookbook-style labs with open-ended projects that let students experience original research.
Confidence to test ideas
Merrimack [N.H.] High School chemistry teacher Sean Müller first participated in the Research Experience for Teachers program in 2000. “Through my experiences with the RET program, I have learned how to ‘run a research group’ consisting of my students. Without this experience, I would not have had the confidence to allow my students to research, develop, and test their original ideas. This has also allowed me to coach our school’s Science Olympiad team to six consecutive state titles, to mentor a set of students that developed a mini bio-diesel processor that they sold to Turner Biodiesel, and to mentor another set of students that took second place in Embedded Systems at I.S.E.F. [Intel International Science and Engineering Fair] last year for their ChemiCube chemical dispensing system,” Müller says.
Müller says he is always looking for new ideas and researching older ideas to develop lab activities in his classroom. “One year my students made light emitting thin films. We have grown beautiful bismuth crystals in our test furnace, and currently I am working out how to make glow-in-the-dark zinc sulfide electroluminescent by doping it with copper so that we can make our own electroluminescent panels,” he says. “Next year we are going to try to make the clear see-through wood that was in the news earlier this year. I am also bringing in new materials that they have not seen before such as gallium-indium eutectic. These novel materials and activities generate a very high level of enthusiasm and interest in my students, and students that are excited, interested, and motivated learn more efficiently and more effectively.”
Müller developed a relationship with Prof. Steve Leeb that has brought Müller back to MIT during past summers to present a brief background in polymer chemistry, supplemented by hands-on demonstrations and activities, for the Science Teacher Enrichment Program (STEP) and Women’s Technology program. “Last year I showed them how they could use their cell phone and a polarized film to see the different areas of crystallization in polymers when they are stressed,” Müller says. “I enjoy the presentation because it is more of a conversation with all of the teachers, myself included, asking questions about different activities and methods and discussing what has worked and what has not worked in the past.”
Looking back on his nine years as MPC director, Thompson says, “The MPC served a broad community, but many people at MIT didn’t know about it because it was in the basement of Building 12. So one of the things that I wanted to do was raise the profile of MPC so people better understood what the MPC did in order to better serve the community.” MPC rolled out a new logo and developed a higher profile Web page, for example. “I think that was successful. I think many more people understand who we are and what we do and that enables us to do more,” Thompson says. In 2014 MPC moved to Building 24 as the old Building 12 was razed to make way for MIT.nano. The new MRL is consolidating its offices in Building 13.
“Research breakthroughs by their very nature are hard to predict, but what we can do is we can create an environment that leads to research breakthroughs,” Thompson says. “The successful model in both MPC and CMSE is to bring together people interested in materials, but with different disciplinary backgrounds. We’ve done that separately, we’ll do it together, and the expectation is that we’ll do it even more effectively.”
– Denis Paiste, Materials Research Laboratory
October 10, 2017
Updated January 25, 2018
Study finds topological materials could boost the efficiency of thermoelectric devices.
|MIT researchers, looking for ways to turn heat into electricity, find efficient possibilities in certain topological materials.|
What if you could run your air conditioner not on conventional electricity, but on the sun’s heat during a warm summer’s day? With advancements in thermoelectric technology, this sustainable solution might one day become a reality.
Thermoelectric devices are made from materials that can convert a temperature difference into electricity, without requiring any moving parts — a quality that makes thermoelectrics a potentially appealing source of electricity. The phenomenon is reversible: If electricity is applied to a thermoelectric device, it can produce a temperature difference. Today, thermoelectric devices are used for relatively low-power applications, such as powering small sensors along oil pipelines, backing up batteries on space probes, and cooling minifridges.
But scientists are hoping to design more powerful thermoelectric devices that will harvest heat — produced as a byproduct of industrial processes and combustion engines — and turn that otherwise wasted heat into electricity. However, the efficiency of thermoelectric devices, or the amount of energy they are able to produce, is currently limited.
Now researchers at MIT have discovered a way to increase that efficiency threefold, using “topological” materials, which have unique electronic properties. While past work has suggested that topological materials may serve as efficient thermoelectric systems, there has been little understanding as to how electrons in such topological materials would travel in response to temperature differences in order to produce a thermoelectric effect.
In a paper published in the Proceedings of the National Academy of Sciences, the MIT researchers identify the underlying property that makes certain topological materials a potentially more efficient thermoelectric material, compared to existing devices.
“We’ve found we can push the boundaries of this nanostructured material in a way that makes topological materials a good thermoelectric material, more so than conventional semiconductors like silicon,” says Te-Huan Liu, a postdoc in MIT’s Department of Mechanical Engineering. “In the end, this could be a clean-energy way to help us use a heat source to generate electricity, which will lessen our release of carbon dioxide.”
Liu is first author of the PNAS paper, which includes graduate students Jiawei Zhou, Zhiwei Ding, and Qichen Song; Mingda Li, assistant professor in the Department of Nuclear Science and Engineering; former graduate student Bolin Liao, now an assistant professor at the University of California at Santa Barbara; Liang Fu, the Biedenharn Associate Professor of Physics; and Gang Chen, the Soderberg Professor and head of the Department of Mechanical Engineering.
A path freely traveled
When a thermoelectric material is exposed to a temperature gradient — for example, one end is heated, while the other is cooled — electrons in that material start to flow from the hot end to the cold end, generating an electric current. The larger the temperature difference, the more electric current is produced, and the more power is generated. The amount of energy that can be generated depends on the particular transport properties of the electrons in a given material.
Scientists have observed that some topological materials can be made into efficient thermoelectric devices through nanostructuring, a technique scientists use to synthesize a material by patterning its features at the scale of nanometers. Scientists have thought that topological materials’ thermoelectric advantage comes from a reduced thermal conductivity in their nanostructures. But it is unclear how this enhancement in efficiency connects with the material’s inherent, topological properties.
To try and answer this question, Liu and his colleagues studied the thermoelectric performance of tin telluride, a topological material that is known to be a good thermoelectric material. The electrons in tin telluride also exhibit peculiar properties that mimic a class of topological materials known as Dirac materials.
The team aimed to understand the effect of nanostructuring on tin telluride’s thermoelectric performance, by simulating the way electrons travel through the material. To characterize electron transport, scientists often use a measurement called the “mean free path,” or the average distance an electron with a given energy would freely travel within a material before being scattered by various objects or defects in that material.
Nanostructured materials resemble a patchwork of tiny crystals, each with borders, known as grain boundaries, that separate one crystal from another. When electrons encounter these boundaries, they tend to scatter in various ways. Electrons with long mean free paths will scatter strongly, like bullets ricocheting off a wall, while electrons with shorter mean free paths are much less affected.
In their simulations, the researchers found that tin telluride’s electron characteristics have a significant impact on their mean free paths. They plotted tin telluride’s range of electron energies against the associated mean free paths, and found the resulting graph looked very different than those for most conventional semiconductors. Specifically, for tin telluride and possibly other topological materials, the results suggest that electrons with higher energy have a shorter mean free path, while lower-energy electrons usually possess a longer mean free path.
The team then looked at how these electron properties affect tin telluride’s thermoelectric performance, by essentially summing up the thermoelectric contributions from electrons with different energies and mean free paths. It turns out that the material’s ability to conduct electricity, or generate a flow of electrons, under a temperature gradient, is largely dependent on the electron energy.
Specifically, they found that lower-energy electrons tend to have a negative impact on the generation of a voltage difference, and therefore electric current. These low-energy electrons also have longer mean free paths, meaning they can be scattered by grain boundaries more intensively than higher-energy electrons.
Going one step further in their simulations, the team played with the size of tin telluride’s individual grains to see whether this had any effect on the flow of electrons under a temperature gradient. They found that when they decreased the diameter of an average grain to about 10 nanometers, bringing its boundaries closer together, they observed an increased contribution from higher-energy electrons.
That is, with smaller grain sizes, higher-energy electrons contribute much more to the material’s electrical conduction than lower-energy electrons, as they have shorter mean free paths and are less likely to scatter against grain boundaries. This results in a larger voltage difference that can be generated.
What’s more, the researchers found that decreasing tin telluride’s average grain size to about 10 nanometers produced three times the amount of electricity that the material would have produced with larger grains.
Liu says that while the results are based on simulations, researchers can achieve similar performance by synthesizing tin telluride and other topological materials, and adjusting their grain size using a nanostructuring technique. Other researchers have suggested that shrinking a material’s grain size might increase its thermoelectric performance, but Liu says they have mostly assumed that the ideal size would be much larger than 10 nanometers.
“In our simulations, we found we can shrink a topological material’s grain size much more than previously thought, and based on this concept, we can increase its efficiency,” Liu says.
Tin telluride is just one example of many topological materials that have yet to be explored. If researchers can determine the ideal grain size for each of these materials, Liu says topological materials may soon be a viable, more efficient alternative to producing clean energy.
“I think topological materials are very good for thermoelectric materials, and our results show this is a very promising material for future applications,” Liu says.
This research was supported in part by the Solid-State Solar Thermal Energy Conversion Center, an Energy Frontier Research Center of U.S. Department of Energy; and the Defense Advanced Research Projects Agency (DARPA).
Jennifer Chu | MIT News Office
January 16, 2018