Patrick wins a PECASE!

The White House’s press release

CHARLOTTESVILLE, VA – President Barack Obama announced today that Associate Professor Patrick Hopkins of the University of Virginia School of Engineering and Applied Science will receive the highest honor the U.S. government bestows on science and engineering professionals in the early stages of their research careers: the Presidential Early Career Award for Scientists and Engineers.
Hopkins, a mechanical engineer who earned his undergraduate and graduate degrees from UVA, specializes in nanoscale energy transport. Hopkins was nominated by the Office of Naval Research, part of the Department of Defense, and his award comes with a $1 million, five-year grant.

“I am not only honored, I am humbled,” Hopkins said. “Being recognized as one of the top young scientists in the country motivates me to be the best researcher I can be, and to contribute to ensuring that the University of Virginia is doing the best possible research. Because of the grant that comes with this award, we will be able to build experiments that push the limits of what people understand about heat transfer on the atomic scale.”

This is not the first time Hopkins has been singled out for outstanding achievement. After completing his doctorate at UVA, he received a Harry S. Truman Postdoctoral Fellowship to conduct research at Sandia National Laboratories in Albuquerque, NM. Subsequently, the Air Force Office of Scientific Research and Office of Naval Research each presented him with Young Investigator Awards.

UVA School of Engineering Dean Craig Benson said, “Professor Hopkins is an exceptionally talented scholar whose work shows great promise for advancing our nation’s defense capabilities. He also epitomizes UVA Engineering’s historical commitment to research that addresses society’s biggest challenges. We are extraordinarily proud of him.”

Hopkins’ success reflects the UVA School of Engineering’s ability to recognize and nurture young talent.  Professor Pamela Norris, now the School’s Executive Associate Dean for Research, recognized his abilities while he was an undergraduate. She invited him into her Microscale Heat Transfer Laboratory and later directed his dissertation. “Having worked with Patrick since his undergraduate years, I have so enjoyed watching him mature into a true scholar,” Norris said.  “He is never satisfied with the status quo and relentlessly asks ‘why.’”

The research that Hopkins will conduct with his Presidential Early Career Award grant has a number of novel aspects. He is interested in the exchange of energy that occurs at the interface between different states of matter, for instance when liquids or gases encounter a solid surface. The general energy exchange mechanism is well understood, but Hopkins wants to take it a step further, discovering how to manipulate the transfer on the atomic level by accounting for surface geometry and chemistry.  This advance would open the door to new methods of maximizing energy exchange and using selective energy exchange as the basis for a new generation of sensors.

The driver behind the Office of Naval Research’s interest in this research is the Navy’s plan to convert its fleet to ships that rely on electricity for propulsion, as well as for defense, radar, and sensors. In this situation, a unified, efficient electric power source is a more flexible approach than having separate power plants for different functions.  A major obstacle to this transition is heat dissipation.

“The more circuits you have, the more heat you produce,” Hopkins said. “Creating more effective heat exchangers is critical to realizing this vision.”

Increasing the selectivity of the heat exchange and energy conversion, as well as its effectiveness, sets the stage for exquisitely sensitive power sources and sensors. In order to realize this goal, Hopkins’ work is developing a process that operates at two very different length and time scales. He intends to identify phenomena occurring at the nanoscale and picosecond—like a single molecule encountering a surface and changing its thermal and energy state—tracked over surfaces at the micrometer and millisecond scale.

“Our objective is to make a device that can identify specific molecules in the air or water at parts per billion, while harvesting their energy,” Hopkins said.  “This would provide early sensing and targeting of biological and chemical species like anthrax and sarin gas, while at the same time providing an avenue to improve the recycle of wasted energy.”

“Patrick’s research on energy transfer is at the core of critical technologies that address national and societal needs,” said Professor Eric Loth, the chair of the Department of Mechanical and Aerospace Engineering. “In addition, he is an excellent teacher and a celebrated scholar who strives to collaborate with students and faculty throughout the University.”

According to the White House’s press release, Hopkins was among 106 researchers President Obama named as recipients of the Presidential Early Career Awards for Scientists and Engineers. The winners will receive their awards at a Washington, D.C., ceremony this spring.

“These early-career scientists are leading the way in our efforts to confront and understand challenges from climate change to our health and wellness,” President Obama said in the release. “We congratulate these accomplished individuals and encourage them to continue to serve as an example of the incredible promise and ingenuity of the American people.”

The release further stated, “The Presidential Early Career Awards highlight the key role that the Administration places in encouraging and accelerating American innovation to grow the economy and tackle the country’s greatest challenges.

“This year’s recipients are employed or funded by the following departments and agencies: Department of Agriculture, Department of Commerce, Department of Defense, Department of Education, Department of Energy, Department of Health and Human Services, Department of the Interior, Department of Veterans Affairs, Environmental Protection Agency, National Aeronautics and Space Administration, National Science Foundation, and the Intelligence Community. These departments and agencies join together annually to nominate the most meritorious scientists and engineers whose early accomplishments show the greatest promise for assuring America’s preeminence in science and engineering and contributing to the awarding agencies’ missions.

“The awards, established by President Clinton in 1996, are coordinated by the Office of Science and Technology Policy within the Executive Office of the President. Awardees are selected for their pursuit of innovative research at the frontiers of science and technology and their commitment to community service as demonstrated through scientific leadership, public education, or community outreach.”

Vacancies mediate thermal conductivity in doped CdO – Congrats Brian Donovan!!

We have demonstrated that electron and vacancies can mediate the thermal conductivity of Dy doped CdO.  More specifically, our results suggest that the enhancement in thermal conductivity in CdO at low Dy concentrations is dominated by an increase in the electron mobility due to a decrease in oxygen vacancy concentration. Furthermore, we find that at intermediate doping concentrations, the subsequent decrease in thermal conductivity can be ascribed to a large reduction in phononic thermal transport due to both point defect and cation- vacancy scattering. With these results, we gain insight into the complex dynamics driving phonon scattering and resulting thermal transport in functional oxides.  This work, in which Brian Donovan was the first author, was recently published in Applied Physics Letters (Applied Physics Letters 108, 021901 (2016)), and was in collaboration with Professor J.P. Maria’s group at N.C. State University.

We appreciate the funding from the Office of Naval Research under Grant No. N00014-15-12769.

 

Abstract

Understanding the impact and complex interaction of thermal carrier scattering centers in functional oxide systems is critical to their progress and application. In this work, we study the interplay among electron and phonon thermal transport, mass-impurity scattering, and phonon- vacancy interactions on the thermal conductivity of cadmium oxide. We use time domain thermore- flectance to measure the thermal conductivity of a set of CdO thin films doped with Dy up to the saturation limit. Using measurements at room temperature and 80 K, our results suggest that the enhancement in thermal conductivity at low Dy concentrations is dominated by an increase in the electron mobility due to a decrease in oxygen vacancy concentration. Furthermore, we find that at intermediate doping concentrations, the subsequent decrease in thermal conductivity can be ascribed to a large reduction in phononic thermal transport due to both point defect and cation- vacancy scattering. With these results, we gain insight into the complex dynamics driving phonon scattering and resulting thermal transport in functional oxides.

Thermal boundary conductance in amorphous superlattices – Congrats Ash!

We have shown that the thermal boundary conductance across solid interfaces can change based on whether the materials comprising the interfaces are amorphous or crystalline.  Via a series of MD simulations, we show that mode distribution in amorphous materials can lead to an increase in thermal boundary conductance as compared to their crystalline counterparts.  This work, in which Ash Giri was the first author, was recently published in Journal of Applied Physics (Journal of Applied Physics 118, 165303 (2015)), and was in collaboration with Dr. John Duda from Seagate Technology.

 

Abstract

We report on the thermal conductivities of amorphous Stillinger-Weber and Lennard-Jones superlattices as determined by non-equilibrium molecular dynamics simulations. Thermal conductivities decrease with increasing interface density, demonstrating that interfaces contribute a non-negligible thermal resistance. Interestingly, Kapitza resistances at interfaces between amorphous materials are lower than those at interfaces between the corresponding crystalline materials. We find that Kapitza resistances within the Stillinger-Weber based Si/Ge amorphous superlattices are not a function of interface density, counter to what has been observed in crystalline superlattices. Furthermore, the widely used thermal circuit model is able to correctly predict the interfacial resistance within the Stillinger-Weber based amorphous superlattices. However, we show that the applicability of this widely used thermal circuit model is invalid for Lennard-Jones based amorphous superlattices, suggesting that the assumptions made in the model do not hold for these systems.

 

Diffusive thermal transport in phosphonic acid interfacial monolayers: Congrats Dr. John Gaskins

We have shown conclusive evidence of diffusive thermal transport in a single molecule of a phosponic acid (henicosafluorododecyl-phosphonic acid (F21PA)) between various metals and a sapphire substrate.  thermal transport across single molecules is typically assumed ballistic, where the thermal resistance is dominated by the molecular contacts.  We show that the large molecular weight phosphonic acid F21PA, vibron scattering in the molecule can lead to additional thermal resistance at metal/PA/sapphire interfaces, indicative of diffusive thermal transport.  This work, in which Dr. John Gaskins was first author, was recently published in The Journal of Physical Chemistry C (The Journal of Physical Chemistry C 119, 20931-20939 (2015)), and was in collaboration with Dr. Sam Graham from Ga Tech.

Abstract

The influence of planar organic linkers on thermal boundary conductance across hybrid interfaces has focused on the organic/inorganic interaction energy rather than on vibrational mechanisms in the molecule. As a result, research into interfacial transport at planar organic monolayer junctions has treated molecular systems as thermally ballistic. We show that thermal conductance in phosphonic acid (PA) molecules is ballistic, and the thermal boundary conductance across metal/PA/sapphire interfaces is driven by the same phononic processes as those across metal/sapphire interfaces without PAs, with one exception. We find a more than 40% reduction in conductance across henicosafluorododecyl-phosphonic acid (F21PA) interfaces, independent of metal contact, despite similarities in structure, composition, and terminal group to the variety of other PAs studied. Our results suggest diffusive scattering of thermal vibrations in F21PA, demonstrating a clear path toward modification of interfacial thermal transport based on knowledge of ballistic and diffusive scattering in single monolayer molecular interfacial films.

Acknowledgements

P.E.H. is grateful for support from the Office of Naval Research Young Investigator Program (N00014-13-4-0528).

Plasma-based chemical functionalization of graphene independently controls both electrical and thermal interface resistances – Congrats Brian Foley for this publication in Nano Letters!

We have demonstrated the ability to independently control the electrical contract resistance and thermal boundary resistance at Au/graphene interfaces by functionalizing the graphene via an atmospheric plasma-based approach.   The modification of surface energy either oxygen, nitrogen or fluorine adsorbates leads to varying interactions at the Au/graphene interface.  Where the bonding at this interface affects thermal transport, the change in surface energy and resulting electrical interactions lead to modifications in electrical transport.  This work, in which Brian Foley was first author, was recently published in Nano Letters (Nano Letters 15, 4876-4882 (2015)), and was in collaboration with Dr. Scott Walton from the Naval Research Labs.

Abstract

The high mobility exhibited by both supported and suspended graphene, as well as its large in-plane thermal conductivity, has generated much excitement across a variety of applications. As exciting as these properties are, one of the principal issues inhibiting the development of graphene technologies pertains to difficulties in engineering high-quality metal contacts on graphene. As device dimensions decrease, the thermal and electrical resistance at the metal/graphene interface plays a dominant role in degrading overall performance. Here we demonstrate the use of a low energy, electron-beam plasma to functionalize graphene with oxygen, fluorine, and nitrogen groups, as a method to tune the thermal and electrical transport properties across gold-single layer graphene (Au/SLG) interfaces. We find that while oxygen and nitrogen groups improve the thermal boundary conductance (h_K) at the interface, their presence impairs electrical transport leading to increased contact resistance (rho_C). Conversely, functionalization with fluorine has no impact on h_K, yet rho_C decreases with increasing coverage densities. These findings indicate exciting possibilities using plasma-based chemical functionalization to tailor the thermal and electrical transport properties of metal/2D material contacts.

Acknowledgements

P.E.H. appreciates funding from the Office of Naval Research Young Investigator Program (N00014-13-4-0528). B.M.F is grateful for support from the Army Research Office (W911NF- 13-1-0378) and the ARCS Foundation Metro Washington Chapter. This work was partially supported by the Naval Research Laboratory Base Program. This work was performed in part at the Center for Atomic, Molecular, and Optical Science(CAMOS) at the University of Virginia.

Congratulations Dr. Ramez Cheaito on a successful PhD Defense!

Congrats to the new doctor! Dr. Ramez Cheaito successfully defended his PhD, which is focused on size effects on the electron and phonon thermal conductivity of alloy, superlattice and multilayer thin films.  The title of his dissertation is “The Role of Size Effects on the Thermal Conductivity of Thin Film Alloys and Superlattices”.  At the time of his PhD defense, Dr. Cheaito’s work at U.Va. resulted in 13 journal publications and 123 citations.  Dr. Cheaito’s dissertation abstract is copied below.  CONGRATS DR. CHEAITO!!

 

Abstract

Advancements in modern technologies have relied primarily on the miniaturization of electronic devices. As the dimensions a these devices are reduced to hundreds of nanometers, thermal management becomes a challenge. Performances are now dictated by the amount of power a device can dissipate before surpassing the temperature set by reliability requirements. Understanding thermal transport in thin film nanostructures is a key element in manufacturing devices with long lifetimes and better energy efficiencies.
The role of size effects on the behavior of heat carriers in thin film structures and across interfaces have been the focus of numerous studies over the past few decades. However, discrepancies among studies on phonon behavior obstruct the understanding of the fundamental processes governing phonon transport. On the other hand, the lack of data on electron thermal transport across interfaces and in periodic structures motivates more research in this direction. This dissertation presents thermal conductivity measurement results on four different material systems of sample thicknesses spanning three orders of magnitude to provide a deep understanding into the processes of phonon and electron thermal transport in thin film alloys and superlattices. Measurements were performed using time-domain thermoreflectance, a non-contact, optical method for the thermal characterization of bulk and thin film materials.

The effect of boundary scattering of long mean free path phonons on the thermal conductivity of thin film SiGe alloys and AlAs-GaAs superlattices is thoroughly discussed in light of the spectral contribution of these phonons to thermal transport. The interplay between short and long range boundary scattering in AlAs-GaAs superlattices is studied by systematically varying the period and film thicknesses. Phonon coherence in epitaxially grown strontium titanate – calcium titanate superlattices is demonstrated by showing a minimum in the thermal conductivity as a function of period thickness. For electrons, the interplay between electron characteristic length and the materials’ intrinsic properties is studied via measurements of the thermal interface conductance in Cu-Nb multilayers.

A major result of this dissertation is demonstrating the possibility of achieving a desired thermal conductivity by prescribing both the period and sample thickness of a superlattice, a result that has important implications on thermal management and thermal engineering applications.

TDTR to characterize radiation-induced damage in materials – Congrats Ramez!

We have recently reported on the measurement of thermal conductivity in a series of ion irradiated materials via time domain thermoreflectance (TDTR), demonstrating TDTR as an effective means for the characterization of radiation induced damage.  Due to the relatively high modulation frequencies and time domain experimental data in a TDTR experiment, this measurement technique offers the unique ability to characterize radiation-induced damage of materials at varying depths under the surface via quantification of the thermal conductivity.  This work, in which Ramez Cheaito was the first author, was recently published in the Journal of Materials Research (J. Mat. Res30 1403 – 1412 (2015)) – Congrats Ramez!!

 

Abstract

The progressive build up of fission products inside different nuclear reactor components can lead to significant damage of the constituent materials. We demonstrate the use of time-domain thermoreflectance (TDTR), a nondestructive thermal measurement technique, to study the effects of radiation damage on material properties. We use TDTR to report on the thermal conductivity of optimized ZIRLO, a material used as fuel cladding in nuclear reactors. We find that the thermal conductivity of optimized ZIRLO is 10.7 +/- 1.8 W/m/K at room temperature. Furthermore, we find that the thermal conductivities of copper– niobium nanostructured multilayers do not change with helium ion irradiation doses of 1015 cm2 and ion energy of 200 keV, demonstrating the potential of heterogeneous multilayer materials for radiation tolerant coatings. Finally, we compare the effect of ion doses and ion beam energies on the measured thermal conductivity of bulk silicon. Our results demonstrate that TDTR can be used to quantify depth dependent damage.

This work was performed in part at the Center for Atomic, Molecular, and Optical Science (CAMOS) at the University of Virginia. P. E. H. recognizes support from the Naval Research Young Investigator Program (Grant No. N00014-13-4-0528). Authors acknowledge Evans Analytical Group for TEM data. We are appre- ciative of funding through Sandia National Laboratories. Sandia National Laboratories is a multiprogram labora- tory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corpo- ration, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE- AC04-94AL85000.