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Tuesday, December 13th, 2016
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| 12:00a |
Measuring radiation damage on the fly Materials exposed to a high-radiation environment such as the inside of a nuclear reactor vessel can gradually degrade and weaken. But to determine exactly how much damage these materials suffer generally requires removing a sample and testing it in specialized facilities, a process that can take weeks.
An analytical method developed by researchers in the Department of Chemistry at MIT and applied by members of MIT’s Mesoscale Nuclear Materials Laboratory could change that, potentially allowing for continuous monitoring of these materials without the need to remove them from their radiation environment. This could greatly speed up the testing process and reduce the preventive replacement of materials that are in fact safe and usable.
The findings are being reported this week in the journal Physical Review B, in a paper by graduate student Cody Dennett, assistant professor of nuclear science and engineering Michael Short, and six others.
When it comes to measuring radiation damage in materials, Short says, “most of the current ways are slow and expensive.” For example, the method considered to be the gold standard for such testing, transmission electron microscopy (TEM), produces comprehensive data on many of the defects in the material that are responsible for changes in its properties. But not all the defects that affect the material’s properties can be seen in the TEM, so the test does not provide complete data.
“We’re not just interested in how many voids or vacancies you have,” Short says, referring to places where one or more atoms are missing from the material’s crystal lattice. “What we really want to know is how the material properties are changing.”
The team found the answer in a technique called transient grating spectroscopy. Essentially, this is a way of measuring the thermal and elastic properties of materials by inducing and monitoring acoustic waves on the material’s surface. Though the system only “sees” the outer surface of the materials, those acoustic vibrations are affected by subsurface defects in the material’s structure. The effect is similar to the way geologists can construct a picture of Earth’s interior layers by studying the way seismic waves propagate in different directions.
The system creates these acoustic oscillations by using two pulsed laser beams aimed at the sample in such a way that the light waves of the two beams cause an interference pattern. This interference pattern causes heating at the sample surface, generating a standing acoustic wave. The motion of the surface caused by this wave can be monitored by another set of lasers. “We create rippling acoustic waves,” Short says, “and measure how fast they move and how quickly they decay,” without physically contacting the material in any way.
The team’s work initially faced some skepticism. “People said ‘how do you know [this technique] is sensitive enough?’” Short says. But with careful experiments that “almost perfectly” matched theoretical simulations, they proved the necessary sensitivity, he says. “Those critical questions were important for us to hear, and motivated us to conduct this study.”
For one test, the team compared two batches of aluminum samples that were composed of perfect single crystals with different surface orientations. Though the internal atomic arrangement was different, “they looked identical to the eye or in the microscope,” he says. “We put them all in our device, and we were able to sort them all out.”
To follow up on their initial work, the researchers are now working to prove their technique’s sensitivity to tiny defects in a material’s structure. “We’re creating simple defects and then measuring the signals, to predict the impact,” Short says. “We want to show how sensitive we can get.”
The team used different materials in their tests but focused mostly on single-crystal aluminum. They chose that material because it was one of the most challenging, Short explains. “As you rotate the sample, its acoustic response changes” because of the different alignment of the crystal structure to the laser-induced surface acoustic waves. “But it changes very little. So if we can sense those subtle changes in wave speed in aluminum, then we’re well-prepared to measure radiation effects” in other materials. The results of those tests showed that their device is sensitive enough to detect changes in acoustic wave speed as small as one-tenth of 1 percent. And it can provide its answers “in seconds, versus months or years” for existing methods.
The method the researchers developed to directly simulate transient grating spectroscopy is as important as the measurements themselves, they say. Using careful molecular dynamics simulations, the researchers were able to accurately predict the expected response of copper and aluminum, and confirm this prediction with measurements. “The most powerful implication for these simulations,” Short says, “is that we can create new structures in the computer and predict their signals. Some defects are too complex for us to predict their signals using theory alone. That is where simulation comes in.” The ability to use simulation to explain experimental measurements on the atomic scale is also “extremely enlightening,” he says.
“Now, we can take a data point about every five minutes, where usually you would get a few data points per month,” he says. That speedier testing could be crucial in enabling the development of new generations of cladding material for nuclear fuel for advanced new reactors, he says. “Now, the biggest drawback to deploying new reactors is materials, and the biggest drawback to that is testing. If we can go from months to seconds, we can get around that bottleneck.”
Although their initial tests were done with larger laboratory setups, Short says it should be quite straightforward to reproduce those functions in a small, portable device that could be carried around for field tests or permanently mounted in strategic monitoring points within a reactor vessel.
“This is a great piece of work with a nice combination of experimental and modeling work,” says Felix Hoffman, an associate professor of engineering science at Oxford University in the U.K., who was not involved in this work.
“Transient Grating (TG) methods provide a great new alternative to traditional techniques of measuring radiation damage as they are rapid, nondestructive, and don’t require much in the way of sample preparation other than a polished surface,” he says. “This is in stark contrast to TEM, atom probe, or micromechanics that require long sample preparation. ... If the system can be miniaturized and made sufficiently portable to allow in situ measurements, this would open up tremendous possibilities for probing material property evolution due to irradiation.”
“The authors have demonstrated a significant and versatile advance in monitoring and quantifying point defects in mesoscale volumes,” says Steven Zinkle, chair of the department of nuclear engineering at the University of Tennessee, who also was not involved in this work. “With further refinement,” he says, “the newly developed TG spectroscopy technique could lead to improved understanding of real-time defect evolutions that occur in a wide range of pure materials and engineering alloys during exposure to ion beam processing or neutron bombardment during energy production in nuclear reactors.”
The research team also included Penghui Cao, an MIT postdoc; Sara Ferry, an MIT graduate student; Alejandro Vega-Flick of the CINVESTAV Unidad Merida in Mexico; Alexei Maznev, a research scientist in MIT’s Department of Chemistry; Keith Nelson, the Haslam and Dewey Professor of Chemistry at MIT; and Arthur Every at the University of Witwatersrand in South Africa. The work was supported by the National Science Foundation, Transatomic Power, Inc., and the U.S. Nuclear Regulatory Commission. | | 4:20p |
3Q: Seeking concrete solutions to an environmental issue Concrete is all around us, from buildings and sidewalks to bridges and roads, but graduate student Steven Palkovic sees it as more than just the most-used human-made material in the world; he sees it as an environmental problem. The production of concrete contributes up to 10 percent of the world’s carbon dioxide, and Palkovic hopes to reduce this environmental impact through his research of cement paste, the matrix of concrete materials that binds everything together.
Palkvoic studies cement paste at multiple scales, spanning from atoms to millimeters in length. His computational framework, developed with Professor Oral Buyukozturk of the Department of Civil and Environmental Engineering (CEE), promises to have a wide impact on the influence of changes in structure and properties caused by additives at multiple length scales for concrete mixes. These additives can be used as replacement for Portland cement, the largest contributor to carbon dioxide emissions during concrete productions.
Palkovic is an active member of CEE, serving as a teaching assistant for a summer fieldwork program on Materials in Art, Archeology and Architecture (ONE-MA³) and playing intramural sports. A lifelong Red Sox fan, Palkovic moved from New York to Boston for his undergraduate degree in civil engineering at Northeastern University, which he received in 2012. He earned his master’s degree in CEE in 2014 before pursuing a PhD in the department. Palkovic recently spoke with CEE about his work.
Q: What are the real world implications of your research?
A: The biggest implication of my research is being able to reduce our dependence on Portland cement, a generic name for limestone and other raw materials that are processed and mixed with water to form the glue for cementitious materials, in concrete and cement paste design. Due to the huge quantities used in construction, the production of Portland cement contributes 5 to 10 percent of the world’s greenhouse gases, so it is important to address that environmental impact.
I am looking at how we can investigate cement paste with different modeling techniques, and transfer this information seamlessly between length scales. When we understand what features promote improved performance, we can design new mixes that replace Portland cement with other suitable materials. Specifically, my group is studying volcanic ash, an additive that’s already reactive and readily available. This way, you don’t have to make as much Portland cement and use it in your mix, and that’s much more sustainable.
Since I work on such small scales, simulations can be used as a computational microscope for understanding what’s going on. My thesis is developing a computational framework that’s basically modeling cementitious materials from atoms up to the size of a dozen hairs. When we change the material by incorporating additives, my models need to be flexible enough to account for associated changes in chemistry, interactions and morphology throughout the material.
Ultimately, I hope that my research will contribute to a better understanding of how to use, how to activate, and how to proportion different additives within the material to get properties that you want. This would enable performance based material design.
Right now the industry progresses through trial-and-error, which is inefficient. You put in different amounts of an additive within the mix, add water, wait 28 days for it to harden, measure it with laboratory tests, and you evaluate “How good is this?” to determine the properties of the resulting concrete. The big goal of the industry has been to determine how we could design concrete to achieve specific properties and quality ahead of time based on the available raw materials, and this framework will contribute to filling that void.
Q: What opportunities have you had to delve deeper into your research?
A: MIT has given me a lot of opportunities to travel. This past summer, I travelled to Italy as a teaching assistant for ONE-MA³, a new CEE summer fieldwork program led by CEE Assistant Professor Admir Masic.
We travelled to Priverno and Pompeii and looked at the materials the Romans used for their structures. Similar to my research, the Romans also used volcanic ash as an additive in their concrete. This experience really affirmed that, like the Romans, we can use locally available materials to achieve building materials that can survive for millennia.
The trip also allowed me to obtain a perspective on the importance of materials in cultural heritage. We met world experts in preservation and restoration who taught us how local materials and different processing techniques could be used to create mortar for buildings or pigments in a fresco. The class emphasized how modern technology can be applied to study these ancient materials to look for secrets that explain their incredible durability.
Q: What’s the next step for you?
A: I became really interested in research as an undergraduate at Northeastern, and that led me to MIT, where I have had amazing research opportunities. CEE has changed my focus from a structural engineer that studied buildings to a material scientist investigating the world’s most widely used civil engineering material. Looking towards the future, I want to apply my research to private industry.
Understanding how materials behave at different scales has given me a better appreciation of mechanics and design. I’d like to apply these skills in the industry for developing novel ways to characterizing material properties at different length scales. | | 5:30p |
Student product ideas range from lifesaving to just plain fun The theme for this year’s student projects in the mechanical engineering class called Product Engineering Processes was “rough, tough, and messy,” but the student teams’ product ideas were much more pleasant and positive than that may sound: All eight were designed to be life-saving or health-enhancing, or at least lots of fun.
Three of the products introduced Monday night, in a raucous, enthusiastic, and music-filled set of final class presentations in MIT’s Kresge Auditorium, were designed to assist people with medical conditions or physical limitations, and two others were intended to be protective equipment for workers. The remaining three products were: a device to prevent ice dams on roofs, a fun new interactive arcade game for kids, and a musical device.
Now in its 21st year, the class, known by its course number, 2.009, is led by professor of mechanical engineering David Wallace and a large team of assistants and mentors. It is designed to give its students — 140 of them this term — a strong sense of what’s involved in taking a product all the way from brainstorming ideas, through preliminary design and testing, to a final, fully functional product design and a basic business plan. Along the way, students build a strong sense of teamwork and learn to collaborate effectively.
The device one of the teams came up with has the potential to be a lifesaver for many people, especially student-athletes. It is a simple, light, low-cost defibrillation device called Revive that could fit in a coach’s backpack and deliver heart-starting shocks using just the power of a smartphone battery. Meanwhile, the app controlling the device would have a built-in electrocardiograph to monitor the patient’s heartbeat and determine whether a defibrillation shock is actually needed, and if so exactly when and for how long. The app also coaches those using the device on exactly what to do, step by step. The device would cost about 30 percent less than existing defibrillation systems, and it would be more compact and lightweight, the team says.
During their presentation, David Hesslink described the experience of losing a 15-year-old friend while in high school; the friend died of cardiac arrest after colliding with another player on a school baseball field, and might have been saved if the needed equipment had been available in a timely way. Over 300,000 people each year undergo cardiac arrest in the U.S., and only 10 percent of those survive it, Hesslink says; this new device might help to increase that survival rate.
Two other assistive devices were also inspired by personal experiences, as described in the teams’ presentations. One is a new kind of walker, called Stryde, that improves the safety and ease-of-use of conventional walkers. When someone using a walker is on a sloping surface, they must be sure to hold on tightly to the device to make sure it doesn’t roll away downhill, leaving them stranded and unsupported. To prevent that, this innovative walker features built-in brakes that are always on by default and only release when the user leans on the device. The walker was inspired by the needs of an individual the team members had met, who described to them the problems he has experienced when using a conventional walker on sloping surfaces.
Another device was inspired by Burt Pusch, a wheelchair user who described the challenge of using an electrified wheelchair outdoors, where any risk of being caught out in the rain could not only could get the user wet but also cause a dangerous short circuit in the chair’s mechanisms. To alleviate that and allow wheelchair users more freedom to spend time outside in uncertain weather, the team designed a rain canopy that could be deployed above the chair within seconds, at the press of a button. It could then be retracted with another button press. Pusch took part in the team’s demonstration at Kresge, driving his Dryve-equipped wheelchair through a drenching “rainfall” that was provided onstage by a set of sprinklers.
Two teams addressed problems experienced by workers using protective gear. One solution is a system that allows people in chemical labs to remove their protective gloves without having to touch them. According to studies, more than half the people who use such gloves remove them improperly, creating a risk of serious chemical contamination. The new device is equipped with smooth, plastic, shoehorn-like protrusions that hook under the gloves and allow their removal without the need to touch them, potentially drastically reducing the risk of such contamination.
The other device is an intercom system built into a respirator of the kind used by painters and other construction workers. Since it’s very hard to speak or hear with the respirators on, people often pull them off when they need to communicate — potentially exposing themselves to dangerous airborne particles. The built-in communication system could eliminate the need to pull off the mask in order to talk or listen.
People in New England are all too familiar with the hazard of ice dams on roofs, but one team came up with an automated solution. They devised a pipe that runs along the roof and dispenses a de-icing solution automatically when the conditions are right for ice dams to form, creating open channels for the meltwater to run safely off the roof.
Finally, two teams demonstrated designs that were just plain fun. One is a way for guitar players to automatically record the notes they play, without having to remember the notes and put down the instrument to record them. And the other is a new kind of arcade game: a moving rock-climbing wall for kids, which moves steadily downward as the child climbs, showing the pathway to follow using lighted handholds. Because the wall is moving, no matter how much the users may climb, they’re always within a few feet of a cushioned landing pad for safety.
At the evening’s conclusion, as in years past, the students in the class expressed their appreciation for Wallace’s inspiring teaching by presenting him with gifts during the confetti-bedecked finale: a bottle of maple syrup in recognition of his Canadian origins (which he sipped heartily on the spot) and a pancake-making 3-D printer to provide him with an endless supply of automated breakfasts. |
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