Sandia research team measures thermoelectric behavior by ‘Tinkertoy’ materials
by Patti Koning
Sandia researchers have made the first measurements of thermoelectric behavior by a nanoporous metal-organic framework (MOF), a development that could lead to an entirely new class of materials for such applications as cooling computer chips and cameras and energy harvesting.
“These results introduce MOFs as a new class of thermoelectric materials that can be tailored and optimized,” says Sandia physicist François Léonard (8342). “This discovery brings us a step closer to realizing the potential of MOFs in practical applications.”
The results were published in “Thin Film Thermoelectric Metal–Organic Framework with High Seebeck Coefficient and Low Thermal Conductivity,” which appeared April 28 online in Advanced Materials. This work builds on previous research in which the Sandia team realized electrical conductivity in MOFs by infiltrating the pores with a molecule known as tetracyanoquinodimethane, or TCNQ, as described in a 2014 article in Science.
“The fact that a TCNQ-filled MOF conducts electricity quite well made us hopeful that we’d also see thermoelectricity, but it was by no means a given,” says Sandia senior scientist Mark Allendorf (8300). “We found that not only is the material thermoelectric but also the efficiency of its temperature conversion approaches that of the best conducting materials like bismuth telluride.”
Thermoelectric devices convert heat to electricity and have no moving parts, making them extremely attractive for cooling and energy harvesting applications. Thermoelectric MOFs could take these advantages a step further with improved performance, smaller size, and flexible designs.
The researchers also gained a fundamental understanding of the charge transport properties of these novel materials that furthers the long-range goal of molding MOFs into electronic and optoelectronic devices.
Guest@MOF concept described
Described as “molecular tinker toys,” MOFs have a crystalline structure that resembles molecular scaffolding, consisting of rigid organic molecules linked by metal ions. Those organic molecules are the sticks and the metal ions are the balls.
The hybrid of inorganic and organic components produces an unusual combination of properties: nanoporosity, ultralarge surface areas, and remarkable thermal stability, which are attractive to chemists seeking novel materials. The empty space framed by the organic molecules and metal ions is what truly sets apart MOFs — empty space that can be filled with practically any small molecule a chemist chooses.
“We describe this concept as Guest@MOF, with the guest being practically any molecule small enough to fit in the MOF pores,” says materials scientist Alec Talin (8342). “The great thing about chemistry is you can synthesize a wide variety of molecules to be inserted inside a MOF to change its properties. In optimizing materials, this gives you a lot of knobs to turn.”
MOF-enabled efficient energy conversion
The researchers had to devise a method to measure the thermoelectric properties of TCNQ@MOF, where TCNQ was the guest molecule. MOFs are so new — they were discovered in 1999 — that researchers often find themselves on the frontier of science with few established tools or even a clear understanding of the material’s fundamental properties.
François, Alec, and Kristopher Erickson, a former Sandia postdoctoral fellow, created a thermoelectric device by connecting Peltier heaters and coolers to each end of a thin film of TCNQ@MOF to generate a tiny temperature gradient. They accurately measured the temperature gradient with an infrared camera while simultaneously measuring the generated voltage. From these data they obtained the voltage per unit of temperature change, known as the Seebeck coefficient.
Patrick Hopkins, an assistant professor of mechanical engineering at the University of Virginia, and his graduate student Brian M. Foley used a laser technique to measure the thermal conductivity.
The resulting measurements showed great promise. TCNQ@MOF has a high Seebeck coefficient and low thermal conductivity, two important properties for efficient thermoelectricity. The Seebeck coefficient was in the same range as bismuth telluride, one of the top solid state thermoelectric materials.
“The next step is how do we make it better?” says Mark. “The energy conversion is not competitive yet with solid state materials, but we think we can improve that with better electrical conductivity.”
Measurements yield fundamental understanding of electronic structure
The measurements also captured data that has advanced the team’s fundamental understanding of the TCNQ@MOF electronic structure. Sandia physicist Catalin Spataru (8342) and materials scientist Mike Foster (8223) conducted detailed electronic structure calculations of TCNQ@MOF and Sandia materials scientist Reese Jones (8256) performed thermal conductivity simulations.
“We were trying to understand the role of the guest molecule, TCNQ in this case, when it infiltrates the pore of an MOF. Finding a representative configuration for the combined TCNQ@MOF system via computer simulations was particularly challenging, as we don’t expect guest molecules to form an ordered structure,” says Catalin.
The simulations allowed the researchers to verify the source of the charge transport and establish that TCNQ@MOFs is a p-type material. Applications such as transistors and diodes require semiconductors of both p-type and n-type.
“We’re now looking for a molecule that in combination with a MOF creates an n-type semiconductor with similar properties to TCNQ@MOF,” says François. “Once we find that, we’ll be at the early stage of creating a full thermoelectric device.”
MOFs in space, smartphones, and cars
Once thermoelectric MOFs realize sufficient energy conversion efficiency, they could begin replacing existing cooling methods in devices where compactness and weight are priorities. Cameras mounted on satellites, which require constant cooling to function properly, are one example. Replacing the fans in computer chips with thermoelectric MOFs could reduce the weight of laptop computers, smartphones, and other portable electronics and the number of moving parts that will eventually wear out.
Energy-harvesting thermoelectric devices capitalize on wasted heat to draw power. A thermoelectric device near a car engine or exhaust system could transfer that wasted heat into a power source for the car’s electronics. Thermoelectric devices are also used to provide localized cooling for passenger comfort.
“Another potential application is using temperature gradients in the ground to power sensors in remote areas,” says François. “Thermoelectrics could be quite ideal for this application, as you could set up a device and leave it to run for long periods of time.”
Future MOF work seeks to improve efficiency
The researchers are now improving the thermoelectric efficiency of TCNQ@MOF. One avenue is to change the MOF films from the polycrystalline structures used in the initial research to single-crystal.
“A unified structure should conduct electricity better,” says Sandia chemist Vitalie Stavila (8341), who grew the MOF thin films. “However, we believe the interfaces between the polycrystal grains contribute to the low thermal conductivity. So the best energy conversion efficiency will likely be achieved by balancing these two parameters.”
The researchers also are turning their thermoelectric measurement technique to other MOFs and materials, such as carbon nanotube thin films.
“This is a very exciting time to be working on MOFs,” says Mark. “Fundamental science is only beginning to catch up with these new applications, which are advancing at a rapid pace. The improved understanding we’re beginning to get will help us extend MOFs into many exciting but challenging new areas.”
-- Patti Koning
Sandia hosts annual Robot Rodeo for bomb squads
Bomb squads from across the country saddled up their robots and duked it out at the ninth annual Western National Robot Rodeo and Capability Exercise May 11-15 at Sandia. The five-day event offered a challenging platform for civilian and military bomb squad teams to practice defusing dangerous situations with robots’ help.
The rodeo was designed with elaborate props to model the atmosphere of real-life emergencies in a low-risk, competitive environment. Robots are life-saving tools for the nation’s hazardous device teams, providing a buffer between danger and first responders.
“Our underlying goal is that we want to make good robot operators into great robot operators,” says Jake Deuel (6532), a Robot Rodeo coordinator and Sandia manager. “We design problems and scenarios that take our state and local bomb squad teams way outside their comfort zones, outside the known techniques and procedures to see how they can handle it.”
UAVs introduced at Robot Rodeo
This year, Sandia introduced unmanned aerial vehicles (UAVs) into the competition. Sandia’s David Novick (6533), a pilot and robotics engineer, flew the UAVs at the event.
“It’s exciting to be able to show off a new technology to this group,” David says. “This technology is a game changer. It’s a stable, highly intelligent vehicle with controls similar to an airplane. Emergency responders can use these small, portable vehicles to get a bird’s eye view of a situation to help them get out of a tight spot.”
Scenarios at the rodeo change every year and grow in difficulty, which brings competitors back. The event prepares officers for the types of situations they may face on the job.
“The only time we get to simulate the level of complexity that we face in real life is at the Robot Rodeo,” says Albuquerque Police Sgt. Carlos Gallegos. “Robots are saving officers’ lives and have been critical to our SWAT teams.” Challenges have included managing suicide bombers, operating in darkened buildings, responding to roadside bombs, navigating obstacle courses, and rescuing first responders.
Sandia partners with Los Alamos National Laboratory on the annual event, which allows teams to practice emergency scenarios where robots are life-saving tools.
The winning bomb squad team was Doña Ana County Sherriff’s Office. Participants included the Albuquerque Police Department, Kirtland Air Force Base Explosive Ordnance Disposal Team, Holloman Air Force Base Explosive Ordnance Disposal Team, Los Alamos Police Department, Riverside County Sheriff’s Office from California, two US Army teams from Fort Carson, Colorado, and a team from the British army.
-- Rebecca Brock
Sandia robotics scientist wins prestigious UNM dissertation award
by Nancy Salem
Aleksandra Faust’s family got lucky as civil war raged in their native Serbia. They were tapped in 1995 for US permanent residency in what is known as the green card lottery, which makes thousands of resident visas available each year to natives of countries with low rates of immigration to the United States.
Sandra’s father, mother, and brother quickly fled the fighting but she stayed behind to finish a bachelor’s degree in mathematics at the University of Belgrade. “Education is important to me,” Sandra (5563) says. “The war situation was really bad. I knew that a degree was critical to getting a job and succeeding in the United States. It was a risk to stay behind but it was a bigger risk to quit and start over.”
Two years later, degree in hand, Sandra rejoined her family in Michigan where she worked in industry a few years. Education called again, and she enrolled in the University of Illinois at Urbana-Champaign, earning a master’s degree in computer science in 2004.
Sandia hired Sandra in 2006 and in the fall of 2012 she entered the Labs’ Doctoral Studies Program to pursue a PhD in computer science at the University of New Mexico. She graduated last summer.
Sandra left UNM on a high note. The university announced recently that she won the 2015 Tom L. Popejoy Dissertation Prize recognizing the highest level of academic excellence among doctoral students.
“This is a prestigious award, extremely competitive, juried by tenured faculty members and the dean of Graduate Studies,” says Margaret Gonzales of the UNM Graduate Studies Department. “It’s the only dissertation award provided by UNM that is university-wide.”
How to program a robot
Sandra’s dissertation, “Reinforcement Learning and Planning for Preference Balancing Tasks,” centered on robotics and started with the problem of how to control an unmanned aerial vehicle that carries a suspended load (see abstract at right). “Robots are becoming ubiquitous in our daily lives, from cleaning homes to building cars. Advances are being made, but there are challenges,” says Sandra’s UNM adviser Lydia Tapia. “It is often a difficult and manual process to program a robot to perform a task.”
Sandra’s thesis put forth new learning-based algorithms to program complex, high-dimensional robotic systems. She also provided mathematically rigorous stability guarantees of robots’ behavior, and application to several complex robotic tasks.
She developed a framework, called PEARL, that addresses three major challenges of robots learning motion-based tasks: efficiency, safety, and adaptability. Her thesis led to a provisional patent and publication of numerous technical papers.
Sandra recently began a Laboratory Directed Research and Development project in geosciences using methods developed in her thesis to adaptively change seismic data processing parameters on the fly for more accurate seismic event characterization and detection. And she is working on the Hardware Acceleration of Adaptive Neural Algorithms, or HAANA, Grand Challenge. HAANA is developing a data analysis platform that uses neural-inspired algorithms and hardware to rapidly identify, learn, and track the evolution of threats.
Dissertation with technical, logical, and social impacts
Gonzales says the Popejoy Award jury was extremely impressed with Sandra’s dissertation. “They said it was well written and exhibited a balance between theory and application,” she says. “They concluded the research has deep technical, logical, and societal impacts.”
Sandra says she chose UNM for her doctorate so she could work closely with both the university and Sandia. “My goal was to bridge the collaboration between the Labs and UNM,” she says. “UNM maintains a high level of research and is in the top 100 computer science graduate programs in the country. And I received tremendous help and support from within Sandia all along the way.”
She says UNM provided more than just a technical education. “I didn’t expect to get such a great mentoring experience in terms of career, writing, and presenting myself,” she says. “It was really great to work one-on-one with a mentor.”
Sandra, who received the Popejoy Award at UNM’s spring commencement, says she was overwhelmed at the honor. “It is an endorsement of the research I developed,” she says. “I am excited to apply what I learned to challenges facing the Labs.”
And she says the award took her back to the commitment to education she made in war-torn Serbia. “It is a confirmation that years of hard work do pay off, and that in this country one does not need to be privileged to succeed,” she says. “I immigrated to the United States to escape wars in the Balkans. I arrived with a diploma in math, two suitcases, and a desire to live a dignified life, working hard. I worked three jobs during my first year in the States. Over the years, I invested in furthering my education. I worked on what I am passionate about and expanded my personal and professional network. This award is the crowning achievement of that effort.”
-- Nancy Salem