Thursday, 18 January 2018 11:28

New exotic phenomena seen in photonic crystals

Researchers observe, for the first time, topological effects unique to an “open” system.
MIT Fermi Arcs PRESS Web
A drawing illustrates the unusual topological landscape around a pair of features known as exceptional points (red dots), showing the emergence of a Fermi arc (pink line at center), and exotic polarization contours that form a Mobius-strip-like texture (top and bottom strips). Courtesy of the researchers

Topological effects, such as those found in crystals whose surfaces conduct electricity while their bulk does not, have been an exciting topic of physics research in recent years and were the subject of the 2016 Nobel Prize in physics. Now, a team of researchers at MIT and elsewhere has found novel topological phenomena in a different class of systems — open systems, where energy or material can enter or be emitted, as opposed to closed systems with no such exchange with the outside.

This could open up some new realms of basic physics research, the team says, and might ultimately lead to new kinds of lasers and other technologies.

The results are being reported in the journal Science, in a paper by recent MIT graduate Hengyun “Harry” Zhou, MIT visiting scholar Chao Peng (a professor at Peking University), MIT graduate student Yoseob Yoon, recent MIT graduates Bo Zhen and Chia Wei Hsu, MIT Professor Marin Soljačić, the Francis Wright Davis Professor of Physics John Joannopoulos, the Haslam and Dewey Professor of Chemistry Keith Nelson, and the Lawrence C. and Sarah W. Biedenharn Career Development Assistant Professor Liang Fu.

In most research in the field of topological physical effects, Soljačić says, so-called “open” systems — in physics terms, these are known as non-Hermitian systems — were not studied much in experimental work. The complexities involved in measuring or analyzing phenomena in which energy or matter can be added or lost through radiation generally make these systems more difficult to study and analyze in a controlled fashion.

But in this work, the team used a method that made these open systems accessible, and “we found interesting topological properties in these non-Hermitian systems,” Zhou says. In particular, they found two specific kinds of effects that are distinctive topological signatures of non-Hermitian systems. One of these is a kind of band feature they refer to as a bulk Fermi arc, and the other is an unusual kind of changing polarization, or orientation of light waves, emitted by the photonic crystal used for the study.

Photonic crystals are materials in which billions of very precisely shaped and oriented tiny holes are made, causing light to interact in unusual ways with the material. Such crystals have been actively studied for the exotic interactions they induce between light and matter, which hold the potential for new kinds of light-based computing systems or light-emitting devices. But while much of this research has been done using closed, Hermitian systems, most of the potential real-world applications involve open systems, so the new observations made by this team could open up whole new areas of research, the researchers say.

Fermi arcs, one of the unique phenomena the team found, defy the common intuition that energy contours are necessarily closed curves. They have been observed before in closed systems, but in those systems they always form on the two-dimensional surfaces of a three-dimensional system. In the new work, for the first time, the researchers found a Fermi arc that resides in the bulk of a system. This bulk Fermi arc connects two points in the emission directions, which are known as exceptional points — another characteristic of open topological systems.

The other phenomenon they observed consists of a field of light in which the polarization changes according to the emission direction, gradually forming a half-twist as one follows the direction along a loop and returns back to the starting point. “As you go around this crystal, the polarization of the light actually flips,” Zhou says.

This half-twist is analogous to a Möbius strip, he explains, in which a strip of paper is twisted a half-turn before connecting it to its other end, creating a band that has only one side. This Möbius-like twist in light polarization, Zhen says, could in theory lead to new ways of increasing the amount of data that could be sent through fiber-optic links.

The new work is “mostly of scientific interest, rather than technological,” Soljačić says. Zhen adds that “now we have this very interesting technique to probe the properties of non-Hermitian systems.” But there is also a possibility that the work may ultimately lead to new devices, including new kinds of lasers or light-emitting devices, they say.

The new findings were made possible by earlier research by many of the same team members, in which they found a way to use light scattered from a photonic crystal to produce direct images that reveal the energy contours of the material, rather than having to calculate those contours indirectly.

“We had a hunch” that such half-twist behavior was possible and could be “quite interesting,” Soljačić says, but actually finding it required “quite a bit of searching to figure out, how do we make it happen?”

“Perhaps the most ingenious aspect of this work is that the authors use the fact that their system must necessarily lose photons, which is usually an obstacle and annoyance, to access new topological physics,” says Mikael Rechtsman, an assistant professor of physics at Pennsylvania State University who was not involved in this work. “Without the loss … this would have required highly complex 3-D fabrication methods that likely would not have been possible.” In other words, he says, the technique they developed “gave them access to 2-D physics that would have been conventionally thought impossible.”

The work was supported by the Army Research Office through the Institute for Soldier Nanotechnologies; S3TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy; the U.S. Air Force; and the National Science Foundation. 

Read more at the MIT News Office.

David L. Chandler | MIT News Office
January 11, 2018

Approach could bypass the time-consuming steps currently needed to test new photovoltaic materials.
MIT Assessing PVs Web
This experimental setup was used by the team to measure the electrical output of a sample of solar cell material, under controlled conditions of varying temperature and illumination. The data from those tests was then used as the basis for computer modeling using statistical methods to predict the overall performance of the material in real-world operating conditions. Image, Riley Brandt

The worldwide quest by researchers to find better, more efficient materials for tomorrow’s solar panels is usually slow and painstaking. Researchers typically must produce lab samples — which are often composed of multiple layers of different materials bonded together — for extensive testing.

Now, a team at MIT and other institutions has come up with a way to bypass such expensive and time-consuming fabrication and testing, allowing for a rapid screening of far more variations than would be practical through the traditional approach.

The new process could not only speed up the search for new formulations, but also do a more accurate job of predicting their performance, explains Rachel Kurchin, an MIT graduate student and co-author of a paper describing the new process that appears this week in the journal Joule. Traditional methods “often require you to make a specialized sample, but that differs from an actual cell and may not be fully representative” of a real solar cell’s performance, she says.

For example, typical testing methods show the behavior of the “majority carriers,” the predominant particles or vacancies whose movement produces an electric current through a material. But in the case of photovoltaic (PV) materials, Kurchin explains, it is actually the minority carriers — those that are far less abundant in the material — that are the limiting factor in a device’s overall efficiency, and those are much more difficult to measure. In addition, typical procedures only measure the flow of current in one set of directions — within the plane of a thin-film material — whereas it’s up-down flow that is actually harnessed in a working solar cell. In many materials, that flow can be “drastically different,” making it critical to understand in order to properly characterize the material, she says.

“Historically, the rate of new materials development is slow — typically 10 to 25 years,” says Tonio Buonassisi, an associate professor of mechanical engineering at MIT and senior author of the paper. “One of the things that makes the process slow is the long time it takes to troubleshoot early-stage prototype devices,” he says. “Performing characterization takes time — sometimes weeks or months — and the measurements do not always have the necessary sensitivity to determine the root cause of any problems.”

So, Buonassisi says, “the bottom line is, if we want to accelerate the pace of new materials development, it is imperative that we figure out faster and more accurate ways to troubleshoot our early-stage materials and prototype devices.” And that’s what the team has now accomplished. They have developed a set of tools that can be used to make accurate, rapid assessments of proposed materials, using a series of relatively simple lab tests combined with computer modeling of the physical properties of the material itself, as well as additional modeling based on a statistical method known as Bayesian inference.

The system involves making a simple test device, then measuring its current output under different levels of illumination and different voltages, to quantify exactly how the performance varies under these changing conditions. These values are then used to refine the statistical model.

“After we acquire many current-voltage measurements [of the sample] at different temperatures and illumination intensities, we need to figure out what combination of materials and interface variables make the best fit with our set of measurements,” Buonassisi explains. “Representing each parameter as a probability distribution allows us to account for experimental uncertainty, and it also allows us to suss out which parameters are covarying.”

The Bayesian inference process allows the estimates of each parameter to be updated based on each new measurement, gradually refining the estimates and homing in ever closer to the precise answer, he says.

In seeking a combination of materials for a particular kind of application, Kurchin says, “we put in all these materials properties and interface properties, and it will tell you what the output will look like.”

The system is simple enough that, even for materials that have been less well-characterized in the lab, “we’re still able to run this without tremendous computer overhead.” And, Kurchin says, making use of the computational tools to screen possible materials will be increasingly useful because “lab equipment has gotten more expensive, and computers have gotten cheaper. This method allows you to minimize your use of complicated lab equipment.”

The basic methodology, Buonassisi says, could be applied to a wide variety of different materials evaluations, not just solar cells — in fact, it may apply to any system that involves a computer model for the output of an experimental measurement. “For example, this approach excels in figuring out which material or interface property might be limiting performance, even for complex stacks of materials like batteries, thermoelectric devices, or composites used in tennis shoes or airplane wings.” And, he adds, “It is especially useful for early-stage research, where many things might be going wrong at once.”

Going forward, he says, “our vision is to link up this fast characterization method with the faster materials and device synthesis methods we’ve developed in our lab.” Ultimately, he says, “I’m very hopeful the combination of high-throughput computing, automation, and machine learning will help us accelerate the rate of novel materials development by more than a factor of five. This could be transformative, bringing the timelines for new materials-science discoveries down from 20 years to about three to five years.”

The research team also included Riley Brandt '11, SM '13, PhD '16; former postdoc Vera Steinmann; MIT graduate student Daniil Kitchaev and visiting professor Gerbrand Ceder, Chris Roat at Google Inc.; and Sergiu Levcenco and Thomas Unold at Hemholz Zentrum in Berlin. The work was supported by a Google Faculty Research Award, the U.S. Department of Energy, and a Total research grant.

David L. Chandler | MIT News Office
December 20, 2017

State visit includes meeting with MIT president and award for MIT physicist Marin Soljačić.

 

MIT Croatian Press01 Boverman Web
The president of Croatia, Kolinda Grabar-Kitarović, visited MIT on Monday, Dec. 4. From left: MIT physicist Marin Soljačić; MIT President L. Rafael Reif; Croatian president Kolinda Grabar-Kitarović; and the dean of the MIT School of Science, Michael Sipser. Photo, Allegra Boverman

Croatian president Kolinda Grabar-Kitarović visited MIT on Monday, Dec. 5, 2017, discussing research and innovation policy, and bestowing two medals upon MIT Professor Marin Soljačić.

The state visit began in the office of MIT President L. Rafael Reif, who greeted Grabar-Kitarović and engaged with her in a discussion, partly about the need to have more women pursue careers in science, engineering, and mathematics. Reif also exchanged formal gifts with Grabar-Kitarović.

While in the MIT president’s office, Grabar-Kitarović presented Soljačić with two medals. One, for his special contributions to science, is the Order of the Croatian Morning Star, bearing the image of Ruder Bošković (a noted 18th-century physicist and astronomer from Dubrovnik). The other medal, the Order of the Croatian Interlace, is for contributions to Croatia’s development, reputation, and the welfare of its citizens.

Soljačić is a physicist and native of Zagreb, Croatia, whose work in photonics has produced a variety of new applications in fields ranging from solar energy to novel types of lasers.

Grabar-Kitarović, who was elected president of Croatia in 2015, called the visit to MIT “very inspiring” when she met later with a panel of Croatian students — mostly graduate students and postdocs — who are enrolled at MIT. Soljačić and Michael Sipser, the dean of the School of Science at MIT, were also part of the discussion forum.

Sipser presented an overview of basic research in science at MIT, from astronomy and physics to neuroscience and biology, and noted some of the recent fundamental advances in which MIT has played a role — such as the observation of gravitational waves, for which MIT physicist Rainer Weiss recently received the Nobel Prize.

“In many cases I’ve found that what attracts people to come into science are these fundamental discoveries,” Sipser said.

Soljačić, who runs the Photonics and Modern Electro-Magnetics Group at MIT, presented the forum with an overview of his research group’s advances — which apply to a wide range of topics and technologies, including more efficient solar cells, wireless power transfer, optical neural networks, and privacy for touch screens.

During the forum, Soljačić said he was “very honored” to have received the awards from Grabar-Kitarović and the government of Croatia; for her part, Grabar-Kitarović said it was “an honor for me too” to have presented them.

Grabar-Kitarović solicited input from the students at the panel about ways of encouraging Croatian students to stay in the country — or return to it, after a spell of study elsewhere — and to help add to the country’s intellectual ecosystem. She also noted that her country needed to address the “lack of opportunity” that some talented students can face when it comes to gaining a foothold in academic research.

“I hope with the help of the wonderful young people around the table, we will be able to help by bringing more science and technology [research] to Croatia,” Grabar-Kitarović said.

Grabar-Kitarović is the first female president in Croatia’s history. (The country declared independence from the former Yugoslavia in 1991.) Before becoming president, she served in a variety of governmental positions, including a term as Croatia’s ambassador to the U.S., from 2008 to 2011.

The Croatian delegation also included Pjer Simunovic, Croatia’s current ambassador to the U.S.

Soljačić is one of four Croatian-born faculty members at MIT. The others are Tanja Bosak, an associate professor in the Department of of Earth, Atmospheric and Planetary Sciences; Silvija Gradečak, a professor in the Department of Materials Science and Engineering; and economist Drazen Prelec, a professor in the MIT Sloan School of Management and the Department of Economics.

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Peter Dizikes | MIT News Office
December 5, 2017

Putting limits on foreign students or technical publications would be counterproductive, write Deutch and Condoleezza Rice.
MIT 3Q Deutch Coveney Web
MIT Institute Professor John Deutch. Photo, Donna Coveney

MIT Institute Professor John Deutch, who has been on the MIT faculty since 1970, has served as a department head, dean of the School of Science, and provost, and has published over 160 technical publications as well as numerous publications on technology, energy, international security, and public policy issues. He served in the U.S. government as director of central intelligence from 1995 to 1996, as deputy secretary of defense from 1994 to 1995, and in other posts in the departments of Defense and Energy. He is a member of the nonpartisan Aspen Strategy Group, which is composed of current and former policymakers, academics, journalists, and business leaders whose aim is to explore foreign policy and national security challenges facing the United States. The group has just released its annual report, and it includes a chapter co-written by Deutch and former U.S. Secretary of State Condoleezza Rice, about how the U.S. should deal with the risk of losing important intellectual property rights regarding technological innovations, in the face of efforts by China to acquire such technology through underhanded means. MIT News asked Deutch to describe the potential risks and remedies for such actions that he and Rice outlined in their report.

Q: What was the challenge that you and Prof. Rice, now at Stanford Business School, were asked to address in this piece, and what conclusions did you reach?

A: This year the subject [of the Aspen Strategy Group’s annual report] was the future challenges we see for policy. There was a lot of talk about China and what its relationship with the United States is likely to be, and in the course of this there was a lot of discussion about national security and the tremendous emphasis in China's new five-year plan on technology, in key areas such as robotics, artificial intelligence, and machine learning. There also was a great deal of discussion about nefarious activities by some in China, including trying to get certain Chinese nationals who live here to provide information to the Chinese government to help them acquire this advanced technology. As a result of that, there's been a hint of a new set of proposals from some elements of the natonal security community to, first, control information in the United States from leaving the country, and, second, restrict Chinese nationals from participating in certain kinds of research projects. Condi and I decided to write a short piece about the danger of these proposals.

Basically our view was, yes, the Chinese are putting a greater emphasis on technology; they are growing very fast and they're increasingly competent, and so we should expect greater competition. And yes, they are performing illegal acts against the U.S., especially theft of intellectual property. The U.S. should do everything it can to push back on that effort and prevent it if possible. But the idea that we should respond to this threat by either restricting access to U.S. universities or keeping our ideas in the United States is completely wrong. We'll lose the tremendous advantage we have of an open university system if we do that. The only answer is for U.S. universities to do even more in pursuing their great record of being innovative and creative.

Q: Do you think it's possible to maintain academic freedom of information in the context of dealing with people who may not share our commitment to protecting intellectual property?

A: In such a situation, we need to recognize that we will have some losses. But there will be more severe effects on our innovative enterprise, which is the best in the world, if we start trying to stop these losses by applying restrictions. Universities aren't very good, first of all, at assessing the nature of the risk [of intellectual-property loss] and, second, at deciding what restrictive measures should be put in place. So, both my co-author Condi and I believe, keep the system open. Recognize that you will have some losses, but do what you do well.

Universities should make sure that our scholarly efforts and our educational efforts permit advances in key areas where fundamental research and practical application come together, in health, energy, and environment, including an emphasis on innovation. And we see that happening. By the way, much as the Chinese universities are improving, they do not have the kind of ecosystem that is so strong here, in terms of promoting innovation, creativity, and getting important things implemented in the private sector.

Q: So are there specific measures that universities should be taking to address these efforts to exploit U.S. innovations, or is your advice that they should avoid taking any special measures?

A: My answer is no, there are not specific measures they should take, but it is very important that the administrative leadership of the university understands the concerns in Washington, appreciates the risks, and doesn’t enter into joint projects that could really lead to a loss of sensitive technology.

The universities should try and explain to the government that we think the proper response here is better performance by U.S. universities, rather than trying to keep people out or keep our ideas in.

I think one should expect that the technical competence of China will continue to improve, because of the capabilities of its people and the significant amount of resources the Chinese are putting into technology leadership in a variety of fields. We should expect that. How much of an advantage is given to China by their quite sustained illegal efforts to acquire technology from both the United States and Europe? I think it is helpful but by no means the most important or the determining factor in their advance.

This short piece with Condi Rice is not so much directed to U.S. universities; rather it is directed to the government and the national security community, to say to them, be cautious here — don't throw the baby out with the bathwater.

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David L. Chandler | MIT News Office 
December 15, 2017

Forbes calls this year's 30 Under 30 lists an "encyclopedia of creative disruption."
mit forbes 30 under 30 2018 Web
At least 30 MIT faculty, research staff, and alumni are listed throughout Forbes’ seventh annual 30 Under 30 edition, featuring some of the world’s best young innovators.

Forbes calls its 2018 30 Under 30 lists an “encyclopedia of creative disruption featuring 600 young stars in 20 different industries.” So it should come as no surprise that these lists are heavily populated by recent MIT graduates and other members of the Institute community.

Similar to past years, at least 30 MIT faculty, research staff, and alumni are listed throughout Forbes’ seventh annual edition of the world’s best young innovators. Read about the MIT community members who made this year’s list below:

Omar Abudayyeh ’12 and Jonathan Gootenberg ’13 (health care), doctoral candidates at the Broad Institute of Harvard and MIT. “Abudayyeh and Gootenberg pioneered two advances: a new enzyme for editing genes and a new technique for editing RNA.”

David Bierman SM ’14, PhD ’17 (energy), founder of Marigold Power, Inc. “At MIT he helped to develop a thermophotovoltaic converter that absorbs sunlight and converts it to a form of light.”

Greg Brockman ’13 (enterprise technology), chief technology officer of OpenAI. “The boundary-breaking nonprofit is dedicated to building safe AI and ensuring AI’s benefits are widely and evenly distributed.”

Ritchie Chen SM ’13, PhD ’16 (science), postdoc at the MIT Institute for Medical Engineering and Science. “Chen’s research found that dysfunctional brain regions could be stimulated by metal nanoparticles powered by magnetic fields.”

Tiffany Chu ’10 (enterprise technology), cofounder of Remix. “Chu is cofounder of Remix, a public transit platform used by more than 200 agencies worldwide…that evaluates transit data and suggests improvements.”

Lisa Conn MBA ’17 (law and policy), strategic partner and manager at Facebook. “Conn joined Facebook to lead the civic leadership team in its community partnerships program. Previously at the MIT Media Lab, Conn was program manager of the Electome Project.”

Cody Daniel ’11 (science), director of research at 3Scan. “Instead of fewer than 10 slices, 3Scan’s … robotic microscope can turn a small tissue sample into up to 60,000 slices.”

Maher Damak SM ’15 and Karim Khalil SM ’14 (energy), cofounders of Infinite Cooling. “Infinite Cooling … recaptures 80 percent of the water vapor that normally escapes from cooling towers attached to big power plants.”

Karen Dubbin ’12 (manufacturing and industry), science director at Aether. “Dubbin is the science director at Aether, which builds 3-D-printers capable of creating living tissue. She’s responsible for creating the 'bio-inks' that Aether uses to build tissues.”

Gregory Falco (enterprise technology), graduate student in the MIT Department of Urban Studies ans Planning and cofounder of NeuroMesh. “NeuroMesh provides endpoint security for smart devices and re-engineers malware to become a vaccine for the Internet of Things.”

Alistair Johnson (health care), postdoc in the MIT Laboratory for Computational Physiology. “Johnson created a database of ICU records used by 4,000 researchers from 30 countries to conduct clinical research.”

Brent Keller PhD ’16 (manufacturing), cofounder of Via Separations. “Via Separations develops membrane materials for separation processes. Keller [is] part of MIT’s The Engine accelerator program.”

Weihua Li ’15, MEng ’16 and Arun Saigal ’13, MEng ’13 (consumer technology), cofounders of Thunkable. “Saigal and Li decided spin-out MIT’s App Inventor tool, the drag-and-drop service for building your own app.”

Karthish Manthiram (science), assistant professor in the MIT Department of Chemical Engineering. “Manthiram’s research is focused on providing farmers with fertilizer by manufacturing it out of thin air, literally, by using air, water, and solar power.”

Jess Newman MBA ’17 (energy), director of U.S agronomy at Anheuser Busch InBev. "Her team of 15 agronomists advise barley, rice, and hop farmers on how to become more efficient."

Christina Qi ’13 and Jonathan Wang ’13, MEng ’15 (finance), partners at Domeyard LP. “[Domeyard] is a small hedge fund that is using high-frequency strategies to trade U.S. equity futures and currencies.”

Ritu Raman (science), postdoc at MIT's Koch Institute for Integrative Cancer Research. “Raman’s research focuses on understanding the dynamic interactions between biological and synthetic materials and developing bio-hybrid systems to tackle different applications. ”

Yichen Shen PhD ’16 (energy), postdoc in the MIT Research Laboratory of Electronics. “Has contributed to nanophotonic breakthroughs that could shape the future of energy. Light-AI designs computer chips powered by light rather than electricity.”

Hao Sun (science), research affiliate in the MIT Department of Civil and Environmental Engineering and assistant professor at the University of Pittsburgh. “Sun’s research uses analytics and machine learning combined with internet-of-things enabled sensors to track the health of buildings.”

Scott Sundvor ’12 (consumer technology), cofounder of Nima. “Nima is a portable bluetooth-enabled device that tests foods for allergens before you eat. The company has raised more than $20 million between venture funding and government grants.”

Michael Tomovich SM ’14 (manufacturing), cofounder of Kuvee. “Kuvee has engineered a patented, smart wine bottle that prevents oxygen from reaching the wine inside, and has raised $10 million in venture funding to roll it out.”

Sin Wang SM ’15 (science), cofounder of TetraScience. “Wang is cofounder and CTO of TetraScience, which is bringing the internet of things to daily lab work. The company has raised over $10 million in venture capital.”

Andrew Warren PhD ’16 (health care), founding scientist at Glympse Bio. “Based on Andrew Warren’s PhD at MIT, Glympse Bio uses modular nanoparticle sensors to create diagnostics for diseases potentially including cancer. ”

You Wu SM ’14 (manufacturing), cofounder of Pipeguard Robotics. “Wu’s company, Pipeguard Robotics, manufactures a shuttlecock-shaped robot that travels through water pipes to detect leaks.”

Jenny Xu ’19 (games), founder of JCSoft Inc. “Xu has released nine mobile games that have been downloaded over 3.5 million times, including Can You Escape Fate.”

Julia Yu ’10 (finance), investment analyst at Millennium Management. “Emerging markets trader with big role on a large macro team at billionaire Israel Englander’s $34 billion hedge fund.”

A version of this article originally appeared on the Slice of MIT blog.

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Jay London | MIT Alumni Association 
November 21, 2017
MIT News 

Tuesday, 28 November 2017 12:33

How to get sprayed metal coatings to stick

When spraying metal coatings, melting hurts rather than helps, MIT research reveals.
MIT Melting Bonding 01
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.

MIT Melting Bonding 02
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.

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David L. Chandler | MIT News Office
November 22, 2017

 

Stonehill College meeting puts laser focus on enhancing regional integrated photonics training.

Lionel Kimerling AIM Photonics RMorgan Web
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 AIM Photonics RMorgan Web
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.

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Julie Diop, Program Manager, AIM Photonics Academy
November 27, 2017

Wednesday, 01 November 2017 17:55

Poster Highlights

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.

 

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.

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Related: A magical dimension

Interdisciplinary materials science model offers key to progress

 

 

Wednesday, 01 November 2017 17:43

A magical dimension

Engineering at the nanoscale opens new doors to control optical, electronic and magnetic behaviors of materials and enable new multi-functional devices

Materials Day Panel 9584 DP Web
MIT MRL External Advisory Board Chair Julia Phillips [far left] moderated the Materials Day Symposium panel on “Frontiers in Materials Research.” She was joined by [from second left] Professors Karen Gleason, Caroline Ross, Timothy Swager, and Vladimir Bulović. The session was held Wednesday, Oct. 11, 2017.

Newly discovered optical, electronic and magnetic behaviors at the nanoscale, multifunctional devices that integrate with living systems, and the predictive power of machine learning are driving innovations in materials science, a panel of MIT professors told the MIT Materials Research Laboratory [MRL] Materials Day Symposium.

“The development of new material sets is a key to the launch of new physical technologies,” Professor Vladimir Bulović, founding director of MIT.nano, said. “Once we get down to the nanoscale, we can start inducing quantum phenomena that were never quite accessible. So that scale between 1 nanometer, the typical size of a molecule, and on the order of, let’s say, 20 nanometers, that’s a magical dimension, where you can fine tune your optical, electronic and magnetic properties.”

Professor Caroline Ross, Associate Head of the Department of Materials Science and Engineering, cited a trend of harnessing nature to self assemble complex structures. “As we want to make things smaller and smaller, we need to have nature helping out,” she said. Ross noted progress on a range of new multi-functional materials that use, for example, extremely low voltage levels  to control magnetism or that use strain to control electronic properties. “All of these can enable new kinds of devices from those materials, so you can imagine devices which are smart that can have memory or logic functions, that can have analog instead of just digital type of behavior, that can work together to make smart circuits. … The difficulties of integrating those types of materials will be well paid for by the new sorts of functionality we can get from the devices we make.”

MIT MRL External Advisory Board Chair Julia Phillips moderated the Materials Day Symposium panel on Wednesday, Oct. 11, 2017. Phillips is a former Sandia National Laboratories executive.

Professor Timothy Swager, Director of the Deshpande Center, said the expectation that new medical devices, for example, are compatible with our bodies demands different requirements than previous generations of electronics. “Thinking about how we interface complex dynamic chemically reactive systems with a material is really a very important area that, I think, will continue to be of importance and many good discoveries are going to come about as result of the interest in that area,” he said.

Associate Provost and Professor Karen Gleason spoke of the growing influence of machine learning on materials advances and the potential for one-dimensional and two-dimensional materials to provide better computers and memory storage. “It’s going be incredible for materials discovery as we learn how to use machine learning to predict what materials are optimal, but there’s also a credible place for materials in making this technology grow. Now computational power and memory and databases have gotten large enough that the predictive power is actually great.”

“The biggest component is you need the data so you need all of these sensors for accurate positioning, for detection of gases, for health. People want wearables,” Gleason said. “So I think this is an enormous field with tremendous impact in many different ways that materials can play.”

Bulović said while it takes a lot of perseverance to implement a new idea on the nanoscale, “It’s important to highlight that the invention of an idea happens in a moment, that eureka moment, but to actually scale that idea up so a million people can hold it in their hands, that takes a decade sometimes, especially if it’s in the materials space. Recognition of that is important in order to support the evolution of the new ideas.”

The annual Materials Day Symposium was hosted for the first time by the MIT Materials Research Laboratory, which formed from the merger of the Materials Processing Center and the Center for Materials Science and Engineering, effective Oct. 1, 2017. The MIT MRL will work hand-in-hand with MIT.nano, the central research facility being built in the heart of the MIT campus due to open in June 2018. MIT will receive a $2.5 million gift from the Arnold and Mabel Beckman Foundation to help develop a state-of-the-art cryo-electron microscopy (cryo-EM) center to be housed at the MIT.nano facility.

“I don’t think we can underestimate the value of the tool sets in providing us the direction to what we need to do to advance life as we know it,” Bulović said. “I get struck by the example of DNA … It took 80-plus years to obtain the first inkling that there was something twisted inside our cells. Then we debated for another decade, is this thing really a twisted molecule inside our cells. If you add it all up, 80, 90 years of debate. Today that’s reduced to a couple of hours of work by one graduate student who can take a cell, pull out a nucleus, put it under a scanning tunneling microscope or cryo electron microscope and see a twisted molecule we call DNA now.”

Swager noted that biologists also will use MIT.nano. “They are going to be using the cryo-EM in the basement, so nano is not only for engineers and molecule builders. … I think that’s going to be really exciting and where that fusion leads us, who knows.”

Moderator Phillips asked the panelists what tool sets that would like to see in MIT.nano. Gleason said she would like to see chemical vapor deposition for thin polymer films. Ross said that MIT needs to be at the forefront for materials characterization tools. “We need to have the best tools to do the best work,” Ross said. She would like to see MIT.nano get the best possible electron microscope and advanced deposition tools for oxide molecular beam epitaxy and building up complex materials layer by layer. Swager said it is important for the shared facility to house tools for rapid prototyping and fabrication of devices.back to newsletter

Denis Paiste, Materials Research Laboratory
November 27, 2017

Related: Poster Highlights

Interdisciplinary materials science model offers key to progress

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.

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Denis Paiste, Materials Research Laboratory
Updated December 11, 2017

 

PAPER: Electrochemical Study of a Pendant Molten Alumina Droplet

and Its Application for Thermodynamic Property Measurements of Al-Ir

 

 

 

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