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
When spraying metal coatings, melting hurts rather than helps, MIT research reveals.
|Micrographs of a metal surface after impact by metal particles. Craters are formed due to melting of the surface from the impact. Courtesy of the researchers|
When bonding two pieces of metal, either the metals must melt a bit where they meet or some molten metal must be introduced between the pieces. A solid bond then forms when the metal solidifies again. But researchers at MIT have found that in some situations, melting can actually inhibit metal bonding rather than promote it.
The surprising and counterintuitive finding could have serious implications for the design of certain coating processes or for 3-D printing, which both require getting materials to stick together and stay that way. The research, carried out by postdocs Mostafa Hassani-Gangaraj and David Veysset and professors Keith Nelson and Christopher Schuh, was reported in two papers, in the journals Physical Review Letters and Scripta Materialia.
Schuh, who is the Danae and Vasilis Salapatas Professor of Metallurgy and head of the Department of Materials Science and Engineering, explains that one of the papers outlines “a revolutionary advance in the technology” for observing extremely high-speed interactions, while the other makes use of that high-speed imaging to reveal that melting induced by impacting particles of metal can impede bonding.
The optical setup, with a high-speed camera that uses 16 separate charged-coupled device (CCD) imaging chips and can record images in just 3 nanoseconds, was primarily developed by Veysset. The camera is so fast that it can track individual particles being sprayed onto a surface at supersonic velocities, a feat that was previously not possible. The team used this camera, which can shoot up to 300 million frames per second, to observe a spray-painting-like process similar to ones used to apply a metallic coating to surfaces in many industries.
While such processes are widely used, until now their characteristics have been determined empirically, since the process itself is so fast “you can’t see it, you can’t tell what’s happening, and no one has ever been able to watch the moment when a particle impacts and sticks,” Schuh says. As a result, there has been ongoing controversy about whether the metal particles actually melt as they strike the surface to be coated. The new technology means that now the researchers “can watch what’s happening, can study it, and can do science,” he says.
The new images make it clear that under some conditions, the particles of metal being sprayed at a surface really do melt the surface — and that, unexpectedly, prevents them from sticking. The researchers found that the particles bounce away in much less time than it takes for the surface to resolidify, so they leave the surface that is still molten.
If engineers find that a coating material isn’t bonding well, they may be inclined to increase the spray velocity or temperature in order to increase the chances of melting. However, the new results show the opposite: Melting should be avoided.
|The top row of photos shows a particle that melts the surface on impact and bounces away without sticking. The bottom row shows a similar particle that does not melt and does stick to the surface. Arrows show impact sprays that look like liquid, but are actually solid particles. Courtesy of the researchers|
It turns out the best bonding happens when the impacting particles and impacted surfaces remain in a solid state but “splash” outward in a way that looks like liquid. It was “an eye-opening observation,” according to Schuh. That phenomenon “is found in a variety of these metal-processing methods,” he says. Now, it is clear that “to stick metal to metal, we need to make a splash without liquid. A solid splash sticks, and a liquid one doesn’t.” With the new ability to observe the process, Hassani-Gangaraj says, “by precise measurements, we could find the conditions needed to induce that bond.”
The findings could be relevant for processes used to coat engine components in order to reuse worn parts rather than relegating them to the scrap-metal bin. “With an old engine from a large earth-moving machine, it costs a fortune to throw it away, and it costs a fortune to melt and recast it,” Schuh says. “Instead, you can clean it off and use a spray process to renew the surface.” But that requires that the sprayed coating will remain securely bonded.
In addition to coatings, the new information could also help in the design of some metal-based additive manufacturing systems, known as 3-D printing. There, as with coatings, it is critical to make sure that one layer of the printing material adheres solidly to the previous layer.
“What this work promises is an accurate and mathematical approach” to determining the optimal conditions to ensure a solid bond, Schuh says. “It’s mathematical rather than empirical.”
The work was supported by the U.S. Army through MIT’s Institute for Soldier Nanotechnologies, the U.S. Army Research Office, and the U.S. Office of Naval Research.
David L. Chandler | MIT News Office
November 22, 2017
Stonehill College meeting puts laser focus on enhancing regional integrated photonics training.
|MIT AIM Photonics Academy Executive Lionel Kimerling speaks during a meeting at Stonehill College in Easton, Mass., on Nov. 14, 2017. “With the help of the state, Massachusetts can be the Silicon Valley for the growth of ultra-high performance communications systems using integrated photonics,” Kimerling said. Photo, Rich Morgan|
MIT’s AIM Photonics Academy helped organize a gathering of more than 60 people at Stonehill College in Easton, Mass., on Nov. 14, 2017, to explore opportunities in integrated photonics, and discuss possibilities for a large investment to create a Lab for Education & Application Prototypes (LEAP) in integrated photonics at the college. Attendees came from companies, colleges and universities, the Massachusetts Manufacturing Extension Program, Massachusetts Technology Collaborative and aides to U.S. Rep. Joseph P. Kennedy III, D-Mass.
Integrated photonics uses complex optical circuits to process and transmit signals of light, similar to the routing of electrical signals in a computer microchip. In contrast to the electrical transmission in a microchip, a photonic integrated circuit can transmit multiple information channels simultaneously using different wavelengths of light with minimal interference and energy loss to enable high-bandwidth, low-power communications.
“Students need to be prepared for the jobs that are coming,” said Dr. Cheryl Schnitzer, associate professor of chemistry at Stonehill College. “It’s our obligation to teach them about the exploding field of photonics and integrated photonics.”
MIT’s AIM Photonics Academy is the education and workforce development arm of the AIM Photonics Institute, one of 14 Manufacturing USA institutes launched as part of a federal initiative to revitalize American manufacturing. The federal government has committed $110 million to the AIM Photonics Institute over five years. At the same time, the state of Massachusetts will spend $100 million on projects related to colleges and industry within the state, including $28 million to help launch AIM Photonics projects such as LEAP facilities.
|Anu Agarwal, MIT Principal Research Scientist, speaks during an AIM Photonics Academy meeting at Stonehill College in Easton, Mass., on Nov. 14, 2017. Stonehill is considering creation of a Lab for Education & Application Prototypes (LEAP) in integrated photonics at the college. Photo, Rich Morgan|
MIT received funding for the first LEAP facility, with a focus on packaging. The MIT Lab for Education & Application Prototypes is currently housed in Building 35, and will relocate to the fifth floor of MIT.nano in June 2018. A second LEAP site is in its final stages of planning at Worcester Polytechnic Institute, and it will also serve Quinsigamond Community College. AIM Photonics Academy and the Commonwealth of Massachusetts are in discussions to build four more LEAP Labs, including one at Stonehill College to serve the southeastern corner of the state. Once up and running, these labs will form a training network that helps Massachusetts become a major hub for photonics technology.
The meeting at Stonehill College, which also included the NextFlex Flexible Hybrid Electronics manufacturing innovation institute, generated many plans. The college has already connected with Bridgewater State and Bristol Community Colleges about creating photonic tracks in their programs. A team from AIM Photonics Academy, Stonehill College and MassTech will begin visiting companies to follow up on how they might get engaged in a LEAP Lab at Stonehill.
Companies were enthusiastic about the opportunity to expand in these areas, as well. “Any time you add high-tech education to an area, you are going to incubate high-tech companies,” noted John Lescinskas of Brockton Electro-Optics. “You’re planting a seed. It can lead to a tree, or even a forest.”
Massachusetts is an optimal location for this initiative to take place. Integrated photonics “is a technology that originated in Massachusetts, at MIT,” said AIM Photonics Academy Executive Lionel Kimerling. “With the help of the state, Massachusetts can be the Silicon Valley for the growth of ultra-high performance communications systems using integrated photonics,” Kimerling said.
– Julie Diop, Program Manager, AIM Photonics Academy
November 27, 2017
Materials Day poster presenters give two-minute introductions to their research during annual symposium.
Materials Day Poster Session presenters capped off the annual Materials Day Symposium with brief highlights of research ranging from solar energy and alternative fuels to spinal cord injury and neural networks for artificial intelligence.
Postdoc Grace Han, in Prof. Jeffrey Grossman’s group, Department of Materials Science and Engineering, described progress in creating materials which absorb photons from sunlight and convert them into heat energy through the charging and discharging cycle of organic photo switching molecules. “This is quite different from just heating water or concrete block by solar radiation in that we can actually store the energy and release energy by triggering,” Han said. These organic coatings can be integrated onto car windshields for deicing, fabrics for personal heating, or building materials for temperature control. Han’s poster also described a new process to harness waste heat from industrial furnaces, and store it for later release.
Janille Maragh, a graduate student in Professor Admir Masic’s lab, Department of Civil and Environmental Engineering, presented her work on sustainable construction materials. To study ancient Roman concrete from an archaeological site in Italy, she used Energy Dispersion Spectroscopy and Raman spectroscopy to map centimeter scale samples at microscopic resolution. “What we are trying to do is understand exactly what our sample is made of so can we understand this phenomenal material. … So we understand not only the bulk composition of our material but also their fracture surface.”
“Carbon monoxide is responsible for more than half of all fatal poisonings worldwide,” Vera Schroeder, a graduate student in Professor Timothy Swager’s lab, said. “Exposure to this odorless, colorless and tasteless gas is very difficult to detect for humans, which is compounded by the fact that the initial symptoms of poisoning – headache, dizziness, and confusion are non-specific.” Schroeder is developing bio-inspired carbon monoxide sensors that use a transistor-based design to activate a chemical change in iron atoms to detect carbon monoxide, even in air. “This new mode of sensor allows us to have a voltage activated, enhanced and highly specific response and we can detect carbon monoxide in air with much higher sensitivity than we detect CO2, oxygen or water,” she said.
- Repairing spinal cord damage Repairing spinal cord damage
- Making artificial axons Making artificial axons
- Fluid-solid interface on graphene Fluid-solid interface on graphene
- Organic photo switching molecules Organic photo switching molecules
- Examining hydrogen solubility Examining hydrogen solubility
Alfonso Juan Carrillo, a postdoc in the lab of Jennifer L. M. Rupp, the Thomas Lord Assistant Professor of Materials Science and Engineering, presented results of work on perovskite materials for solar-driven transformation of CO2 and water into fuels. Carillo selected the best candidate perovskite materials, synthesized these perovskites, analyzed their microstructure, and tested them in a fixed bed high-temperature reactor. “We use what are called thermochemical cycles,” Carillo said. As the perovskite absorbs oxygen, it can transform water and carbon dioxide into hydrogen and carbon monoxide.
Minghui Wang, a postdoc in Professor Karen Gleason’s lab, is creating thin-film microporous polymers for gas separation using chemical vapor deposition. Gas separation is important for industrial gas needs and carbon capture but heat-based methods are energy intensive,” he said. “One challenge is that you need to achieve both high flux and high gas selectivity for membrane materials. To do so, usually you need a rigid and microporous structure and also you need to fabricate very thin films, but to do both of them is kind of difficult. In our lab, we use chemical vapor deposition to deposit pinhole-free thin films by using this technique and using porphyrin as a monomer.” He achieved high selectivity for carbon dioxide and nitrogen separation using polymerized porphyrin on a flexible substrate.
Andrew Dane, a graduate student in Professor Karl Berggren’s group, discussed progress on improving speed and efficiency in superconducting nanowire single photon detectors. Two competing available materials tilt toward either speed or efficiency. “We changed the material deposition and made some devices and showed that we kind of combined the best of both worlds,” Dane said. “There is a quantum phase transition in the material that we’re working with and a lot of other interesting things.”
About a million Americans undergo hernia repair surgery each year and for one in four or them, hernia will re-occur. About half will experience some degree of chronic pain, said Sebastian Pattinson, a postdoc in the lab of Associate Professor of Mechanical Engineering A. John Hart. The surgical mesh used to mechanically reinforce the tissue as it heals causes many of these complications. Pattinson described a new 3D printing process that allows local customization of mechanical response in a surgical mesh and in particular allows for non-linear mechanical response in a way that mimics tissue. “We hope that these meshes will help alleviate the complications suffered by many patients all around the world,” Pattinson said.
Chemistry postdoc Zhou Lin, in Professor Troy A. Voorhis’ group, presented research on a process to double electric current in organic solar cells by splitting single excitons into pairs, a process that is called singlet fission. “We can generate two electric currents out of one high-energy photon so we can promote the efficiency of organic photovoltaics, that’s what we want,” Lin said. “Based on our electronic structure theory calculations, we are able to reproduce the experimental trend for the fission rate using three different isomers that can undergo this intramolecular singlet fission,” she said.
Yukio Cho, a graduate student in Prof. Harry L. Tuller’s lab, is working on mixed ionic and electronic conductor [MIEC] cathode materials for solid oxide fuel cells. Using electrochemical methods, Cho and colleagues synthesized n-type cathode material to improve the surface exchange. “We control the defects, control the electronic defects, and for the current result, we successfully synthesized n-type materials,” Cho said. “The expected good surface exchange capability is also confirmed through transfer diffusion measurements.”
Frank McGrogan, a graduate student in Professor Krystyn J. Van Vliet’s lab, presented his work with the Chemomechanics of Far-From-Equilibrium Interfaces [COFFEI] group on all-solid electrolytes in lithium ion battery systems. “One of the main sticking points is we have this problem of lithium metal unevenly plating the electrodes, crossing the electrolyte and shorting the cell. Our group has been treating this as a fracture issue. … We’ve demonstrated experimentally that fracture is indeed a mechanism for this lithium plating and shorting problem.”
“We’ve gone ahead and measured some mechanical properties including fracture properties of several important solid electrolytes and used these inputs in simulations to predict damage evolution,” McGrogan said. “I think that the way that our group has approached this problem and how we’re getting to the mechanism is going to change the way our field thinks about failure in all solid-state lithium ion batteries.”
Postdoc Dena Shahriari, who works with Professors Yoel Fink and Polina Anikeeva, shared an update on efforts to repair spinal cord damage by optically stimulating and guiding the growth of injured neurons. “We’re using a thermal drawing process, which is a high throughput technique which will allow us to create kilometer-long fibers in just one experiment,” Shahriari said. These highly flexible probes deliver light to the lesion of the spinal cord, and record at multiple sites of these neurons.
“For the tissue engineering part we needed to bridge the nerve gap, we needed to create porosity into these scaffolds, and for that we need to add a twist to this thermal drawing process that will allow us to not only create, but also control, porosity in that,” Shahriari said.
Gerald Wang, a graduate student in mechanical engineering under Prof. Nicolas Hadjiconstantinou, invited attendees to learn more about his poster by arranging it so that the first letters of each line spelled out “C-O-M-E.” He is exploring the fluid-solid interface atop a sheet of graphene. “It turns out when you put fluid in this environment under the right conditions, it will spontaneously arrange into a layered structure that mimics the solid below it. This layered fluid structure, practically indistinguishable from tiramisu or the layered cake of your preference, imparts upon the fluid remarkable fluid properties including enhanced heat transfer, remarkably long slip lengths and highly modified surface diffusivities very different from the bulk fluid.”
“It’s a very exciting story with some of the great actors and actresses of today including Van der Waals, high through-put simulation and molecular self-assembly. So there’s something for everybody whether you’re an experimentalist, a theorist, a computationalist, or you just like a good scaling relation, you should make like the letters and come on by,” Wang said.
Mary Elizabeth Wagner, a graduate student in the group of Associate Professor of Metallurgy Antoine Allanore, is working on a sustainable way to refine precious metals from nature and from recycled materials. “The problem is these expensive elements, silver, gold, platinum, are found in very, very tiny amounts, comparatively to copper, but they make up so much of the cost,” Wagner said.
“My idea in my research focuses on one system that can host electrochemistry for gold, for silver, and for platinum group metals,” Wagner said. Molten sulfide electrolytes are one promising system. “We should be able to treat all of these metals in one go, which should be able to provide an environmentally sustainable as well as a cost-effective way to treat these metals,” she said.
Vrindaa Somjit, a graduate student under Prof. Bilge Yildiz, is examining the effect of dopants on hydrogen solubility in alumina using a computational, first principles approach. Hydrogen may become a fuel of the future, but one of the main problems in making this a reality is the storage and transport of hydrogen. Hydrogen can penetrate steel and cause it to fail.
“One way to mitigate this problem of hydrogen embrittlement is by the use of permeation barrier coatings, and alpha-alumina is a promising candidate,” Somjit said. She set out to determine if dopants, extra chemical elements added to a compound, could improve the performance of alpha-alumina in resisting hydrogen penetration. “What we found is that actually dopants do not help in decreasing the hydrogen solubility because alpha-alumina itself lies at the bottom of the hydrogen solubility valley,” Somjit said.
Graduate student Chang Sub Kim, in Professor Harry Tuller’s group, conducts research to electrochemically pump oxygen in and out of a thin film of layered cuprate, which has potential as a cathode material. “An interesting fact is that it can accommodate both oxygen vacancies as well as interstitials. So in this study, I show you that I can control the region where I can access oxygen-access and also oxygen-deficient regions, and then show that I can simultaneously measure different materials properties such as oxygen surface reaction kinetics as well as in-plane conductivity, which agrees very well with the expected defect chemistry.”
Postdoc Yuming Chen in Professor Ju Li’s group, spoke about a project to develop a sodium-ion battery anode using nitrogen-doped carbon. Chen introduces nitrogen atoms into the structure of hollow carbon tubes to create larger spacing that allows sodium to penetrate the carbon tube and yield higher performance. These carbon tubes can be used as freestanding electrodes with long cycling life.
Ananya Balakrishna, a postdoc in Professor W. Craig Carter’s group, developed theoretical and computational models to investigate the link between material properties and microstructure. “In my research, I probe questions like what determines microstructural patterns, can we engineer microstructures to control macroscopic material properties,” she said. Her poster featured two projects describing microstructure in ferroelectric materials and in lithium battery electrodes.
“In lithium batteries, microstructures form during a typical charge/discharge cycle. In these microstructures, the underlying lattice symmetry has an effect on material properties, for example, certain lattice arrangements facilitate the faster propagation of diffusion of lithium ions and certain lattice arrangements cause non-uniform expansion of electrodes,” Balakrishna said. She is working on a phase field crystal model that couples lattice symmetry with the concentration field to describe electrode microstructure.
Menghsuan Sam Pan, a graduate student in Professor Yet-Ming Chiang’s group, focuses on using water-based sulfur batteries for low-cost energy storage. “It’s one of the lowest cost per stored charge in any electrochemically active materials, only behind water and oxygen,” Pan said. “When we work in soluble electrodes, we found that the sulfur can only be reversibly cycled between a di-sulfide and a tetra-sulfide regime, and with this we did some technical economic modeling to see the installed costs of the electrode. What surprised us is that the component that’s used to hold the electrode is more costly than the active material itself.”
Experiments showed these sulfide species cycle reversibly, precipitating into the electrode and then dissolving very well when they are cycled back, Pan said. “We cycled for more than 1,600 hours, more than two months,” he said, noting a 30 percent cost reduction in terms of cost per stored capacity.
Working under Professor Jeehwan Kim, graduate student Scott Tan is developing hardware for neural networks for artificial intelligence. He makes silicon-germanium cross-bar arrays with a reversible silver conductance channel to toggle the conductance state of these synaptic devices. “We’ve also used these devices in a simulation and showed that they can perform handwriting recognition with accuracy up to 95 percent,” Tan said.
Mechanical engineering graduate student Nicholas T. Dee presented work in Professor A. John Hart’s group on scalable roll-to-roll graphene production for membrane applications. “We’ve developed a roll-to-roll CVD reactor for this process that is unique in that it has two different zones, one specifically for annealing the substrate and catalyst and one zone for growth of the graphene,” Dee said. The researchers tuned the gas composition to achieve full coverage of monolayer graphene and explored the tradeoff between production rate and quality of the graphene. “We have demonstrated using our graphene produced in this high-throughput manner to produce nano-porous, atomically thin membranes for potential desalination applications,” he said.
Brad R. Nakanishi, a graduate student in Professor Antoine Allanore’s group, introduced his research on high-temperature materials chemistry in refractory metals. “What we’ve done, where experiment by conventional methods or prediction by first principles prove very complex and challenging, we’ve basically modified a floating zone furnace which has provided us with enhanced experimental throughput and also very unique ability to see and probe the properties of these refractory liquids,” Nakanishi said. His poster showed an image of the first direct electrolytic decomposition of aluminum oxide to oxygen gas and aluminum metal. “We’ve been using this approach to make fundamental thermodynamic property measurements like chemical potential,” he said. This work has implications for discovery of new materials for applications from aerospace to nuclear as well as discovery of new processes for materials extraction.
Chosen by guests who attended the Materials Day Poster Session, this year's Poster Session prize winners were Postdoc Dena Shahriari, electrical engineering and computer science; graduate student Vera Schroeder, chemistry; and Postdoc Sebastian Pattinson, mechanical engineering.
The annual MIT Materials Research Laboratory [MRL] Materials Day Symposium and Poster Session were held on Wednesday, Oct. 11, 2017.
Related: A magical dimension
MIT researchers demonstrate a new electrochemical method to study thermodynamic processes in an ultra-high temperature molten oxide
|Video of the operating cell shows oxygen bubbles forming within the cell as the alumina decomposes into pure aluminum at the cathode and pure oxygen at the iridium anode. Video, Bradley R. Nakanishi.|
The thermodynamic properties of compounds such as aluminum oxide, which are known as refractory materials because they melt at temperatures above 2,000 degrees Celsius [3,632°F], have been difficult to study because few vessels can withstand the heat to contain them and those that do often react with the melt, in effect contaminating the melt.
Now MIT researchers show a container-less electrochemical method to study the thermodynamic properties of these hot melts in a paper published in the Journal of The Electrochemical Society.
“We have a new technique which demonstrates that the rules of electrochemistry are followed for these refractory melts,” says senior author Antoine Allanore, Associate Professor of Metallurgy. “We have now evidence that these melts are very stable at high temperature, they have high conductivity.”
Adapting a thermal imaging (or arc imaging) furnace more commonly used for floating zone crystal growth, MIT graduate student Brad Nakanishi melted an alumina [aluminum oxide] rod and contacted the liquid pendant droplet that it formed with electrodes, creating an electrochemical cell that allowed decomposition of pure, alumina electrolyte to oxygen gas and aluminum alloy by electrolysis for the first time. The aluminum oxide itself serves as the electrolyte in this electrochemical cell, which operates similarly to water electrolysis.
“Decomposition voltage measurements give us direct access to the quintessential thermodynamic property that is chemical potential, also called Gibbs energy,” Nakanishi explains. “We’ve shown we make electrochemical measurements in a new class of electrolytes, the molten refractory oxides.” The change in this Gibbs energy, or chemical potential, with respect to temperature is known as entropy. “At high temperatures, entropy is really important and very challenging to predict, so having ability to measure entropy in these systems is key,” he says.
A hanging droplet
Using this technique, four reflected xenon lamps hone in on the tip of the sample, melting a liquid droplet, which is held to the rod by surface tension and quickly solidifies after the lights are turned off. While the droplet is liquefied, the electrodes are raised into the droplet to complete an electrical circuit, with the liquid alumina itself functioning as the electrolyte. “That’s something that we have not seen done otherwise, as well, doing electrochemistry in a suspended droplet above 2000°C,” Nakanishi says.
The hanging droplet has a high surface tension relative to its density. “The concentration of the light energy, hot zone, and large thermal gradients present, allows us in a very controlled way to create a situation for stable droplet and electrode contact,” Nakanishi says. “It sounds challenging, but the method we’ve refined is straightforward and rapid to operate in practice thanks, in part, to a camera enabling continuous observation of the droplet and electrodes during the experiment.”
The stability of the liquid aluminum oxide and a smart choice of electrode materials allow measurement of well-defined energy levels, Allanore says. “The paper shows that we can now measure fundamental thermodynamic properties of such a melt,” Allanore says. “In the case of molten alumina, we’ve actually been able to study the property of the cathode product. As we decompose aluminum oxide, to oxygen on one side [anode] and aluminum on the other side [cathode], then that liquid aluminum interacts with the electrode, which was iridium in that case,” Allanore says.
Video of the operating cell shows oxygen gas bubbles forming within the cell as the alumina decomposes into aluminum at the cathode [the negatively charged electrode] and pure oxygen at the iridium anode [the positively charged electrode]. The aluminum does interact with the iridium cathode, which is confirmed by partial melting and post-experiment images of the microstructure showing an aluminum-iridium alloy deposit.
“We can now calculate the thermodynamic property of that alloy, of that interaction, which is something that was never measured before. It was calculated and predicted. It was never measured. Here in this paper we confirm actually predictions from computation using our method,” Allanore says.
New predictive powers
For key industrial questions, such as how hot a turbine engine can run, engineers need thermodynamic data on both the solid and liquid states of metal alloys, in particular, the transition zone at which a solid melts. “We’re not so great on the liquid state, and at high temperature we also have a lot of trouble measuring Gibbs energy in the liquid state,” Nakanishi says.
“Here we’re adding experimental data,” he says. “We’ve created a method that you can measure the Gibbs free energy of a liquid, so now combined with our ability in a solid, we can start informing things like these transition temperatures among other thermodynamic questions, which are related to material stability.” The melt is ionic, containing a mix of both negatively charged oxygen anions and neutral oxygen atoms as well as positively charged aluminum cations and neutral aluminum atoms.
“The key significance of Bradley Nakanishi and Antoine Allanore’s research findings is the capability to determine thermodynamic parameters (e.g., thermodynamic activity) at temperatures greater than 1600°C from the electrochemical measurements for molten oxides, as well as the applicability to a broader electrolyte from a molten oxide to a molten salt,” says University of Texas at El Paso Professor of Mechanical Engineering Arturo Bronson, who was not involved in this research. “In addition, a possible relation of the oxygen partial pressure to the doubly-charged, free oxygen ion will characterize its effect on the associated cations and anions within the molten oxide to explain thermodynamic behavior between the liquid metal and liquid oxide.”
“The quality of the research is a world-class approach developed for difficult experimental studies of ultra-high temperature reactions of liquid metals and liquid oxides, especially with inclusion of electrochemical impedance spectroscopy,” Bronson says. However, a limitation of the study is the uncertainty of the temperature measurements within a range of plus or minus 10 degrees Celsius. “The uncertainty of the measured parameters will ultimately depend on the accuracy of the measured temperature (already at ± 10 kelvins), because the electrochemical parameters (i.e., voltage and current) will clearly depend on the temperature uncertainty,” Bronson explains.
More electrolyte possibilities
Electrochemistry is one of the most selective processing technologies, Allanore notes, “but to date it was very challenging to study the electrochemistry with these high temperature melts.” Electrolyte selection is key for designing new processes for the electrochemical extraction of reactive metals, and the new work demonstrates that more electrolytes are available for extracting metals. “We can now study the solubility of ores containing refractory metal oxides in these melts. So we are basically now adding at least 3 or 4 candidate electrolytes that could be used for metal extraction, in particular for what we call reactive metals such as aluminum, niobium, titanium, or the rare earths,” Allanore adds. This research was funded by the U.S. Office of Naval Research.
Future work will focus on applying these high-temperature electrochemical techniques to investigate the potential for selectively separating the rare earth oxides. Though required in only relatively small quantities usually, the individual rare earth elements are essential for high-tech applications, including cell phones and electric vehicles. Well-established methods to concentrate rare earth oxides from their ore produce a mixture of the 14 rare earth oxides, Allanore notes. “If we were using such a rare earth oxide mixture as our electrolyte, we could potentially selectively separate one rare earth metal from the 13 others,” he says.
New, stable materials such as rare earth oxides that can withstand high temperatures are needed for uses as varied as building faster airplanes and extending the lifetime of nuclear power plants. But one country, China, holds a near monopoly over rare earth element production. “The separation of rare earths from each other is the key challenge in making rare earth metals extraction more sustainable and economically feasible,” Nakanishi says.
While the newly published paper examines a single component electrolyte, aluminum oxide by itself, “Our aim is to extend this approach so that we can measure chemical potentials, Gibbs energy, in multi-component electrolytes,” Nakanishi explains. “This opens up the door to many more candidates for electrolytes that we can use to extract metals, and also make oxygen,” Nakanishi says. This ability to exhaust oxygen as a byproduct rather than carbon monoxide or carbon dioxide has potential to reduce greenhouse gas emissions and global warming.
– Denis Paiste, Materials Research Laboratory
Updated December 11, 2017