Patrick Wins ASME Bergles-Rohsenow Young Investigator Award in Heat Transfer

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

 

 

Ordering effects on the thermal transport mechanisms in metallic alloys: Paper published in Scientific Reports – Congrats Ash!

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.

Paper published AND recipient of a NDSEG Fellowship: CONGRATS JEFF

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.

Thermal storage and transport in organic/inorganic superlattices; 2 papers published in PRB – Congrats Ash!

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.

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.