Phonons Arise! Small electric voltage alters conductivity in key materials
by Neal Singer
Modern research has found no simple, inexpensive way to alter a material’s thermal conductivity at room temperature.
That lack of control has made it hard to create new classes of devices that use phonons — the agents of thermal conductivity — rather than electrons or photons to harvest energy or transmit information. Phonons — atomic vibrations that can transport heat energy at up to the speed of sound in solids — have proved hard to harness.
Now, using only a 9-volt battery at room temperature, a team led by Jon Ihlefeld (1816) has altered the thermal conductivity of the widely used material PZT (lead zirconate titanate) by as much as 11 percent at subsecond time scales. They did it without resorting to expensive surgeries like changing the material’s composition or forcing phase transitions to other states of matter.
PZT, either as a ceramic or a thin film, is used in a wide range of devices ranging from computer hard drives, push-button sparkers for barbecue grills, speed-pass transponders at highway toll booths, and many microelectromechanical (MEMs) designs.
“We can alter PZT’s thermal conductivity over a broad temperature range, rather than only at the cryogenic temperatures achieved by other research groups,” Jon says. “And we can do it reversibly: When we release our voltage, the thermal conductivity returns to its original value.”
The work was performed on materials with closely spaced internal interfaces — so-called domain walls — unavailable in earlier decades. The close spacing allows better control of the passage of phonons.
“We showed that we can prepare crystalline materials with interfaces that can be altered with an electric field. Because these interfaces scatter phonons,” says Jon, “by simply changing their concentration, we can actively change a material’s thermal conductivity. We feel this groundbreaking work will advance the field of phononics.”
The researchers, supported by Sandia’s Laboratory Directed Research and Development office, the Air Force Office of Scientific Research (FA9550-13-1-0067), and the National Science Foundation (CBET-1339436), used a scanning electron microscope and an atomic force microscope to observe how the domain walls of subsections of the material changed in length and shape under the influence of an electrical voltage. It is this change that controllably altered the transport of phonons within the material.
“The real achievement in our work,” says Jon, “is that we’ve demonstrated a means to control the amount of heat passing through a material at room temperature by simply applying a voltage across it. We’ve shown that we can actively regulate how well heat — phonons — conducts through the material.”
Jon points out that active control of electron and photon transport has led to technologies that are taken for granted today in computing, global communications, and other fields.
“Before the ability to control these particles and waves existed, it was probably difficult even to dream of technologies involving electronic computers and lasers,” he says. “And prior to our demonstration of a solid-state, fast, room-temperature means to alter thermal conductivity, analogous means to control the transport of phonons have not existed. We believe that our result will enable new technologies where controlling phonons is necessary.”
The work, published last month in Nano Letters, was coauthored by David A. Scrymgeour (1728), Joseph R. Michael (1800), Bonnie B. McKenzie (1819), and Douglas L. Medlin (8341); Brian M. Foley and Patrick E. Hopkins from the University of Virginia; and Margeaux Wallace and Susan Trolier-McKinstry from Penn State University.
The goal of future work is to better understand “what caused this effect to happen so efficiently,” Jon says.
-- Neal Singer
Sandia hosts 25th international nuclear material protection course
Sandia is playing host to 44 nuclear security students from 36 nations, who will return to their home countries well-versed in the latest techniques and technologies needed to protect nuclear materials.
Top officials from Sandia, NNSA, and the International Atomic Energy Agency (IAEA) welcomed the students to the 25th International Training Course (ITC) on the Physical Protection of Nuclear Material and Nuclear Facilities.
“Threats to nuclear materials and facilities often seem very abstract, but in this course we provide practical training that equips participants to implement the highest standards of nuclear security,” Anne Harrington, NNSA deputy administrator for Defense Nuclear Nonproliferation, said at an April 20 news conference on the opening day of the course.
Every 18 months for the past 37 years, Sandia has hosted the ITC, and so far has trained more than 800 people from 73 countries. This year’s course will conclude May 8.
Denis Flory, deputy director general and head of the IAEA Department of Nuclear Safety and Security, said the course is one of the most respected in the world. “It enables partnerships to initiate and operate proper physical security in their respective countries — in line with global regulations and recommendations — and to take part in a global effort to strengthen nuclear securities,” he said.
When Congress passed the Nuclear Non-Proliferation Act in 1978, it directed DOE to provide training in physical security techniques and technology to security specialists from the now 164 states that are members of the IAEA. DOE selected Sandia to fulfill the directive.
“This course showcases the unique partnership we have had for 37 years. Sandia offers the very best expertise in world in this subject,” said Harrington.
Harrington said the hands-on approach by instructors and practitioners in the field not only provides technical experience the students may not have, but helps to build connections between members in the program.
“[Sandia and our partners] are long-standing practitioners in the research, development, implementation, and refurbishment of physical security systems,” said Div. 6000 VP Jill Hruby, who was joined at the event
by Sandia President and Laboratories Director Paul Hommert. “Our collective goal in providing this course is to assist member states in the protection of nuclear materials across the life cycle, to include use, production, storage, transportation, and disposition.”
“Through this program, Sandia supports the NNSA and the IAEA so, together, we can achieve a goal we all share” in the physical protection of nuclear materials and facilities, Paul said.
Course has evolved over time
Since its inception, the course has evolved to reflect industry best practices and up-to-date physical protection technologies. The original guidelines for IAEA member states to use in establishing, implementing, and maintaining their national nuclear security regime was published in June 1977, Harrington noted.
“We now live in a world that changes at an astonishing speed compared to 1978. So, very appropriately, the guidelines that we apply to physical protection must change with the times as well,” she said.
The guidelines now offer a graded approach to protection, taking into account the assessed threat and potential consequences related to physical protection, and incorporating cybersecurity guidance.
“It is important that we not lose sight of the fact that cyber threats affect our work as well. In fact, a cybersecurity incident could lead to the theft of nuclear material, a catastrophic sabotage of a nuclear facility, or the falsification of safeguards information,” Harrington said.
ITC training focuses on a systems engineering, performance-based approach to requirements definition, design, and evaluation for physical protection systems. Participants learn a methodology for designing and evaluating physical protection systems for nuclear facilities and materials that are effective against the threats of radiological sabotage and theft.
“A strong security culture can only be built if the most senior people in the management pyramid are fully committed and demonstrate that commitment in meaningful ways,” Harrington said. “From my perspective, working with the IAEA to conduct the ITC is one of the most important contributions that NNSA makes to promoting the highest standards of nuclear security."-- Lindsey Kibler
KinBot: A tool to accelerate chemical descriptions of combustion
by Holly Larsen
Faced with the time-consuming and tedious task of characterizing the myriad reactions in a combustion process at the molecular level, Judit Zádor (8353) turned to computers — an understandable choice, given her role as a computational physical chemist at Sandia’s Combustion Research Facility (CRF). Her solution, a code called KinBot, has demonstrated it can predict the behavior of potential reactions in combustion for a given molecule. The key is KinBot’s ability to identify viable 3-D structures for critical intermediate species on the pathway between reactants and products.
With KinBot, combustion scientists can more quickly predict the speed of reactions, expressed as rate coefficients. This knowledge brings the community closer to the goal of understanding and predicting the combustion process using either conventional fuels or new fuels that can reduce pollutants, greenhouse gas emissions, and dependence on petroleum.
A puzzle with too many pieces
Identifying the rates at which relevant reactions take place is a challenge for scientists who want to understand combustion. A fundamental problem is the sheer number of reactions — possibly in the thousands — involved in the combustion of even a single-component fuel. Typical fuels may comprise hundreds of chemical components, so even managing the pieces of the puzzle is difficult.
Chemists have developed software to keep a running list of potential reactions during combustion. But today’s software is a partial solution. Judit says, “We also need to calculate the parameters that characterize these reactions. We need to know the branching to create each molecule in a reaction and the corresponding rate coefficients. To calculate these properties, we need to know the 3-D structure of the intermediates for the reaction in question.”
The connections and arrangements in space of a molecule’s atoms determine its key properties, such as energy and vibrational frequencies. These properties govern the rate at which molecules turn into one another. For instance, the greater the energy of a transition state connecting two stable intermediates, the slower the transformation of one intermediate into the other.
Teasing out this information for a single reaction can take months because a single reaction often has many pathways associated with several structures. Judit uses Legos to explain. “Starting with just a few blocks, you can make a surprisingly large number of arrangements, and it is daunting to try to make all possible configurations. A molecule can stretch out, curl up, or take many forms between these extremes; this ensemble of shapes determines the molecule’s reactivity on a macroscopic level. Many times we have to deal with dozens of shapes — we call them conformers — for each chemical species involved. It’s a lot of work!”
Despite their advantages — speed, absence of errors, and incapacity for boredom — computers lack the intuition possessed by human chemists. Without intuition, even computers cannot characterize combustion reactions in a reasonable amount of computation time.
Working with Habib Najm (8351) and supported by an Early Career Laboratory Directed Research and Development grant, Judit generated a series of generic, minimalist rules that crystalize a century of knowledge. Validated by experiments and theory, these rules enable computers to proceed like a chemist.
Judit explains, “KinBot first analyzes a structure and makes conclusions just as a human would: this is a radical structure or this is a cyclic structure, for instance. Then KinBot applies rules to break and make bonds and morphs the structures into shapes that are very close to the critical transition states for these processes.” The resulting geometries are sent to an electronic-structure code for refinement.
KinBot repeats the process for all possible pathways and configurations in a reaction. Some pathways can be dismissed because they require too much energy. But, says Judit, “You have to know all the pathways to identify the lowest-energy pathway. So it’s helpful to have a code that rapidly scans and proposes a large number of possibilities.” These options are then screened by KinBot’s rules to keep the number of proposed structures within the bounds of practicality.
The unexpected virtue of inexperience
Given the realities of time and funding, researchers often limit their searches to the pathways shown to be probable by their experience and experimental knowledge. Moreover, because chemists typically focus on a single molecule or molecule type, they may develop tunnel vision and overlook — or never hear about — reactions found in another molecule.
In contrast, KinBot’s rules are assembled from the work of many researchers, and demonstrates the virtue of inexperience. “KinBot might find that rules for carbonate also apply to ether-type molecules and uncover pathways that would otherwise be overlooked,” says Judit. KinBot may stumble upon exotic structures (e.g., a high-energy but stable structure with an unusual trivalent oxygen atom), put new twists on known reaction steps, or simply combine simple steps in an unexpected way.
KinBot’s rules imperfectly reflect reality; however, Judit is unfazed by this gap. “When I see a result that doesn’t fit the rules, I know that I need to think about that result more.” A recent example is KinBot’s discovery of a water-elimination pathway in alcohol combustion. Solving the problem underlying this pathway required sophisticated electronic-structure calculations; the results were later presented in a high-profile publication.
“Chemists like working on tough problems. We’re happy to let KinBot handle the routine work, so we have time to concentrate on the problems that require creative thinking,” says Judit.
Further KinBot development
KinBot is off to a strong start. It has been tested by Oliver Welz, a former CRF postdoc who now heads a chemical kinetics research group at the University of Duisburg-Essen in Germany, and a paper on KinBot garnered Judit the Distinguished Paper award in Reaction Kinetics at the 35th International Symposium on Combustion last summer.
This summer, Ruben van de Vijver will use KinBot to find missing parameters for chemical mechanisms developed at Belgium’s Ghent University. The results will help Judit improve KinBot’s code to encompass more molecules.
Xiaohu Li (8353), a CRF postdoc working with Judit, has proposed extending KinBot’s capabilities by incorporating molecular-dynamics simulations. Although these calculations are significantly more time-consuming than the present approach, they require far fewer assumptions.
“We hope that if we combine the current rule-based strategy with molecular dynamics in a clever way, we are going to be ahead of the game. But this is yet to be seen,” says Judit.
-- Holly Larsen