Congrats to Ash Giri for his recent publication in Phys. Rev. B on the pronounced low-frequency vibrational thermal transport in $C_{60}$fullerite realized through pressure-dependent molecular dynamics simulations.

Giri, A., Hopkins, P.E., “Pronounced low-frequency vibrational thermal transport in C60 fullerite realized through pressure-dependent molecular dynamics simulations,” Physical Review B 96, 220303(R) (2017). PDF.

Abstract: Fullerene condensed-matter solids can possess thermal conductivities below their minimum glassy limit while theorized to be stiffer than diamond when crystallized under pressure. These seemingly disparate extremes in thermal and mechanical properties raise questions into the pressure dependence on the thermal conductivity of $C_{60}$ fullerite crystals, and how the spectral contributions to vibrational thermal conductivity changes under applied pressure. To answer these questions, we investigate the effect of strain on the thermal conductivity of $C_{60}$ fullerite crystals via pressure-dependent molecular dynamics simulations under the Green-Kubo formalism. We show that the thermal conductivity increases rapidly with compressive strain, which demonstrates a power-law relationship similar to their stress-strain relationship for the $C_{60}$ crystals. Calculations of the density of states for the crystals under compressive strains reveal that the librational modes characteristic in the unstrained case are diminished due to densification of the molecular crystal. Over a large compression range (0–20 GPa), the Leibfried-Schlömann equation is shown to adequately describe the pressure dependence of thermal conductivity, suggesting that low-frequency intermolecular vibrations dictate heat flow in the $C_{60}$ crystals. A spectral decomposition of the thermal conductivity supports this hypothesis.

We would like to thank the Army Research Office for support (Grant No. W911NF-16-1-0320).

Dr. Tina Rost’s work has recently appeared in Applied Physics Letters.

Rost, C.M., Braun, J.L., Ferri, K., Backman, L., Giri, A., Opila, E., Maria, J.-P., Hopkins, P.E., “Hafnium nitride films for thermoreflectance transducers at high temperatures: Potential based on heating from laser absorption,” Applied Physics Letters 111, 151902 (2017). PDF.

In a typical TDTR experiment, a thin metal transducer is deposited on top of a sample to measure the sample’s thermal properties.  Ultimately, TDTR can be limited by the stability of this transducer.  In this work, we have demonstrated the ability to extend TDTR measurements up to 1000 K using HfN as a metal transducer.  Note only does HfN demonstrate one of the highest thermoreflectance coefficients at 800 nm measured to date, but it’s high temperature phase stability make it attractive for use as a metal transducer at high temperatures.

Abstract

Time domain thermoreflectance (TDTR) and frequency domain thermoreflectance (FDTR) are common pump-probe techniques that are used to measure the thermal properties of materials. At elevated temperatures, transducers used in these techniques can become limited by melting or other phase transitions. In this work, time domain thermoreflectance is used to determine the viability of HfN thin film transducers grown on SiO2 through measurements of the SiO2 thermal conductivity up to approximately 1000 K. Further, the reliability of HfN as a transducer is determined by measuring the thermal conductivities of MgO, Al2O3, and diamond at room temperature. The thermoreflectance coefficient of HfN was found to be 1.4 10^-4 K^-1 at 800 nm, one of the highest thermoreflectance coefficients measured at this standard TDTR probe wavelength. Additionally, the high absorption of HfN at 400 nm is shown to enable reliable laser heating to elevate the sample temperature during a measurement, relative to other transducers.

We acknowledge the financial support from the Office of Naval Research MURI program (Grant No. N00014-15-1- 2863).

John Tomko’s works has recently appeared in Physical Review B.

Tomko, J.A., Giri, A., Donovan, B.F., Bubb, D.M., O’Malley, S.M., Hopkins, P.E., “Energy confinement and thermal boundary conductance effects on short-pulsed thermal ablation thresholds in thin films,” Physical Review B 96, 014108 (2017). PDF.

In this work,we demonstrated a direct correlation between thermal ablation of thin gold films and the thermal boundary conductance across the film/substrate interface.  We used high energy pulsed lasers to induce the film ablation and showed a dependence on substrate.  However, the ablation threshold did not trend with the substrate thermal properties, but instead the thermal boundary conductance across the film/substrate interface.

Abstract

Chet Szwejkowski’s work on the thermal boundary conductance across fullerene derivative/liquid interfaces has recently appeared in ACS Nano.

Szwejkowski, C.J., Giri, A., Kaehr, B., Warzoha, R.J., Donovan, B.F., Hopkins, P.E., “Molecular tuning of the vibrational thermal transport mechanisms in fullerene derivative solutions,” ACS Nano 11, 1389-1396 (2017).

In this work, we showed that the functional group on fullerene derivatives can be used to control the thermal boundary conductance across the molecule/liquid interfaces.  Based on the geometry of the functional group, the density of states of low frequency modes will change and impact the transmission of thermal energy across the molecule/liquid interface.  We demonstrated this by measuring the thermal boundary conductance across these interfaces in samples of dilute fullerene derivative suspensions.  The measurements were conducted using time domain thermotransmission.  Our experimental results are supported with molecular dynamic simulations.

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Patrick has been awarded the 2016 ASME Bergles-Rohsenow Young Investigator Award in Heat Transfer for significant heat transfer research that has produced experimental and analytical advancements in areas including thermal transport across interfaces, reduced thermal conductivity materials, electron-phonon coupling, and transport of electrons and phonons.

The Bergles-Rohsenow Young Investigator Award in Heat Transfer is given to a young engineer that is under 36 years of age and has received a Ph.D. or equivalent degree in Engineering. The individual must be committed to pursuing research in heat transfer, and must have demonstrated the potential to make significant contributions to the field of heat transfer. Such contributions may take the form of, but are not limited to, analytical/numerical methods, equipment/instrumentation, or experimentation – any of which should lead to peer-reviewed publications.

Established by the Heat Transfer Division in 2003, the award was funded through the efforts of Arthur Bergles and Warren Rohsenow who are well known for their accomplishments in heat transfer research and for their mentoring of young researchers.

The official announcement can be found here

https://www.asme.org/about-asme/participate/honors-awards/achievement-awards/bergles-rohsenow-young-investigator-award-in-heat

Ash Giri’s work on the electron and phonon thermal transport mechanisms in ordered and disordered metallic alloys has recently appeared in Scientific Reports.  In this work, we show, via both experimental measurements and molecular dynamics simulations, that the thermal conductivity of an ordered metallic alloy (FePt) can have the same thermal conductivity as a disordered alloy at high temperatures.  This is due to a decreasing phonon thermal conductivity in the ordered alloy driven by three phonon scattering events.  This work has great implications for the thermal management involved in heat assisted magnetic recording applications.

Abstract

We report on the out-of-plane thermal conductivities of tetragonal L1₀FePt (001) easy-axis and cubic A1 FePt thin films via time-domain thermoreflectance over a temperature range from 133 K to 500 K. The out-of-plane thermal conductivity of the chemically ordered L1₀ phase with alternating Fe and Pt layers is ~23% greater than the thermal conductivity of the disordered A1 phase at room temperature and below. However, as temperature is increased above room temperature, the thermal conductivities of the two phases begin to converge. Molecular dynamics simulations on model FePt structures support our experimental findings and help shed more light into the relative vibrational thermal transport properties of the L1₀ and A1 phases. Furthermore, unlike the varying temperature trends in the thermal conductivities of the two phases, the electronic scattering rates in the out-of-plane direction of the two phases are similar for the temperature range studied in this work.

P.E.H. and A.G. appreciate support from the Air Force Office of Scientific Research, Grant No. FA9550-15-1-0079.

Jeff Braun recently was awarded a NDSEG Fellowship to support his PhD work.  At the time of the announcement of this award, Jeff’s paper that experimentally demonstrates the size effects on the thermal conductivity of amorphous silicon appeared in Physical Review B (Phys. Rev. B 93, 140201(R) (2016)).  CONGRATS JEFF!!!!!!

Abstract of Paper

We investigate thickness-limited size effects on the thermal conductivity of amorphous silicon thin films ranging from 3 to 1636 nm grown via sputter deposition. While exhibiting a constant value up to ∼100 nm, the thermal conductivity increases with film thickness thereafter. The thickness dependence we demonstrate is ascribed to boundary scattering of long wavelength vibrations and an interplay between the energy transfer associated with propagating modes (propagons) and nonpropagating modes (diffusons). A crossover from propagon to diffuson modes is deduced to occur at a frequency of ∼1.8 THz via simple analytical arguments. These results provide empirical evidence of size effects on the thermal conductivity of amorphous silicon and systematic experimental insight into the nature of vibrational thermal transport in amorphous solids.

This work was funded by ONR Grant No. N00014-15-12769.

We have measured the vibrational heat capacity and thermal conductivity in organic/inorganic superlattices grown by a combination of atomic layer deposition and molecular layer deposition.  We have demonstrate unique aspects of thermal transport including nearly complete spectral phonon scattering at an inorganic/organic interface in these nanocomposites, and ballistic transport across the organic layer for molecular layer thicknesses that are less than the vibrational wavelength in the inorganic layer.  Furthermore, we have demonstrated an average vibrational heat capacity change that scales with the number of organic layers in the superlattice.

This study resulted in two recently publications that appeared in Physical Review B (Physical Review B 93, 024201 (2016) and Physical Review B 93, 115310 (2016)), and Ash Giri is first author on both papers.  Congrats Ash!

The work discussed in these aforementioned Physical Review B articles was supported by the Army Research Office (Grant No. W911NF-13-1-0378).  The abstracts for these two works appear below.

Abstract – Physical Review B 93, 024201 (2016)

We study the influence of molecular monolayers on the thermal conductivities and heat capacities of hybrid inorganic/organic superlattice thin films fabricated via atomic/molecular layer deposition. We measure the cross plane thermal conductivities and volumetric heat capacities of TiO2– and ZnO-based superlattices with periodic inclusion of hydroquinone layers via time domain thermoreflectance. In comparison to their homogeneous counterparts, the thermal conductivities in these superlattice films are considerably reduced. We attribute this reduction in the thermal conductivity mainly due to incoherent phonon boundary scattering at the inorganic/organic interface. Increasing the inorganic/organic interface density reduces the thermal conductivity and heat capacity of these films. High-temperature annealing treatment of the superlattices results in a change in the orientation of the hydroquinone molecules to a 2D graphitic layer along with a change in the overall density of the hybrid superlattice. The thermal conductivity of the hybrid superlattice increases after annealing, which we attribute to an increase in crystallinity.

Abstract – Physical Review B 93, 115310 (2016)

Nanomaterial interfaces and concomitant thermal resistances are generally considered as atomic-scale planes that scatter the fundamental energy carriers. Given that the nanoscale structural and chemical properties of solid interfaces can strongly influence this thermal boundary conductance, the ballistic and diffusive nature of phonon transport along with the corresponding phonon wavelengths can affect how energy is scattered and transmitted across an interfacial region between two materials. In hybrid composites composed of atomic layer building blocks of inorganic and organic constituents, the varying interaction between the phononic spectrum in the inorganic crystals and vibronic modes in the molecular films can provide a new avenue to manipulate the energy exchange between the fundamental vibrational energy carriers across interfaces. Here, we systematically study the heat transfer mechanisms in hybrid superlattices of atomic- and molecular-layer-grown zinc oxide and hydroquinone with varying thicknesses of the inorganic and organic layers in the superlattices. We demonstrate ballistic energy transfer of phonons in the zinc oxide that is limited by scattering at the zinc oxide/hydroquinone interface for superlattices with a single monolayer of hydroquinone separating the thicker inorganic layers. The concomitant thermal boundary conductance across the zinc oxide interfacial region approaches the maximal thermal boundary conductance of a zinc oxide phonon flux, indicative of the contribution of long wavelength vibrations across the aromatic molecular monolayers in transmitting energy across the interface. This transmission of energy across the molecular interface decreases considerably as the thickness of the organic layers are increased.

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.”

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.

Abstract