Abstract: Extreme near-field heat transfer between metallic surfaces is a subject of debate as the state-of-the-art theory and experiments are in disagreement on the energy carriers driving heat transport. In an effort to elucidate the physics of extreme near-field heat transfer between metallic surfaces, this paper presents a comprehensive model combining radiation, acoustic phonon, and electron transport across sub-10-nm vacuum gaps. The results obtained for gold surfaces show that in the absence of bias voltage, acoustic phonon transport is dominant for vacuum gaps smaller than ∼ 2 nm. The application of a bias voltage significantly affects the dominant energy carriers as it increases the phonon contribution mediated by the long-range Coulomb force and the electron contribution due to a reduced potential barrier. For a bias voltage of 0.6 V, acoustic phonon transport becomes dominant at a vacuum gap of 5 nm, whereas electron tunneling dominates at sub-nm vacuum gaps. The comparison of the theory against experimental data from the literature suggests that well-controlled measurements between metallic surfaces are needed to quantify the contributions of acoustic phonon and electron as a function of the bias voltage.

Abstract: The present article reports a comprehensive energy balance analysis of a photon-enhanced thermionic emission (PETE) device when it is used for concentrated solar power (CSP) generation. To this end, we consider a realistic PETE device composed of a boron-doped silicon emitter on glass and a phosphorus-doped diamond collector on tungsten separated by the interelectrode vacuum gap. Depth-dependent spectral solar absorption and its photovoltaic and photothermal energy conversion processes are rigorously calculated to predict the PETE power output and energy conversion efficiency. Our calculation predicts that when optimized, the power output of the considered PETE device can reach 1.6 W/cm2 with the energy conversion efficiency of ∼ 18% for 100× solar concentration, which is substantially lower than those predicted in previous works under ideal conditions. In addition, the photon enhancement ratio is lower than 10 and decreases with the increasing solar concentration due to the photothermal heating of the emitter assembly, suggesting that PETE should be more suitable for low-to-medium CSP below ∼ 100× concentration. These observations signify the importance of a rigorous energy balance analysis based on spectral and spatial solar absorption distribution for the accurate prediction of PETE power generation.

Abstract: This paper presents the comprehensive performance analysis of thermionic power generation when the interelectrode vacuum gap shrinks to the submicron range. Although reducing the vacuum gap has been suggested as an effective approach to mitigate space-charge accumulation in thermionic-energy conversion (TEC) devices, previous theoretical works have predicted the optimal gap distance in the single-digit micrometer range. However, we demonstrate that nanoscale charge and thermal interactions between thermionic electrodes, such as Schottky barrier lowering due to image charge perturbation and near-field enhanced radiative heat transfer, significantly affects the TEC performance within the submicron vacuum gap. Carefully conducted energy-balance analysis reveals that submicron-gap TEC at d ≈ 700 nm can produce an approximately fourfold increase in power output with a higher energy conversion efficiency than micron-gap TEC under the same operating condition. In addition, significant thermionic and nearfield radiative heating of the collector in the submicron-gap TEC system can be beneficially used to further enhance the power output and efficiency by combining with a bottom-cycle heat engine. We believe that the present work provides a theoretical framework for submicron-gap thermionic power generation as a promising energy recycling scheme for high-quality heat sources.

Abstract: This paper presents an in-depth review of ongoing experimental research efforts to fundamentally understand the strong near-field enhancement of radiative heat transfer and make use of the underlying physics for various novel applications. Compared to theoretical studies on near-field radiative heat transfer (NFRHT), its experimental demonstration has not been explored as much until recently due to technical challenges in precision gap control and heat transfer measurement. However, recent advances in micro-/nanofabrication and nanoscale instrumentation/control techniques as well as unprecedented growth in materials science and engineering have created remarkable opportunities to overcome the existing challenges in the measurement and engineering of NFRHT. Beginning with the pioneering works in 1960s, this paper tracks the past and current experimental efforts of NFRHT in three different configurations (i.e., sphere-plane, plane-plane, and tip-plane). In addition, future remarks on how to address current challenges in the experimental research of NFRHT are briefly discussed.

On August 23^rd, 2019, Professors Keunhan Park, Stephan LeBohec, Vikram Deshpande, and Yue Zhao obtained a formal membership into the LIGO Scientific Collaboration (LSC). The LIGO Scientific Collaboration (LSC) is an international community of scientists focused on the direct detection of gravitational waves to study and explore the physics of gravity, gravitational wave science, and astronomical phenomena. This is the first LSC membership that has ever been granted in the University of Utah.

The University of Utah LIGO group consists of two teams: the theoretical physics subgroup (led by Yue Zhao) and the optics subgroup (with Vikram Deshpande, Keunhan Park, and Stephan LeBohec.)

We asked this newly instated LIGO team to share how the achieved about this highly prestigious membership.

What is a LIGO Scientific Collaboration membership?

By having a group of its professors obtain LSC membership, the University of Utah became part of one of the greatest ongoing scientific adventures. Membership gives us access to LIGO internal documents, as well as current  analysis codes and communication channels. This membership will facilitate the development of ties and partnership to make our contribution to the project more efficient. With the membership, our group is recognized as one making significant contributions to the science investigated by the LIGO collaboration.

How will this type of membership impact both our University and Utah communities?

It makes the University of Utah part of the adventure, which is significant in itself. Practically, it provides undergraduate and graduate students with new areas of research to explore over a broad range of topics: astrophysics, gravitational wave, particle physics, material science, optics, electronics, computation. Undergraduate students are being trained to perform hands-on research and work in teams toward an important scientific problem.

In a year or so, we will be in a good position to develop a community outreach program conducted individually toward schools & local science clubs or organizations  more formally through the channels maintained by the College of Science.

In addition, direct observation of propagating gravitational wave is only the beginning of the newly born field called gravitational wave astronomy.  The LIGO collaboration is expected to continue making major scientific discoveries, which will both broaden and deepen our understanding of the universe. Being an active part of this collaboration by contributing it’s their scientific studies will drastically improve the research profile of the University of Utah.

What was the initial VP for Research seed grant project and how did it set your group on the path to acquiring this membership?

In total, our group obtained two VPR seed grants. One is for fundamental research, and the other for optics instrumentation. These grants had a twofold impact: first and most importantly it gave us the chance to get started on our research before defending our membership application. Thanks to the seed grant, the theory group was able to complete a study using LIGO data to look for dark photon dark matter. This nicely demonstrates that the theory group is fully committed to LIGO-related studies and is capable of carrying out promised work. Similarly, the optics group had made progresses in the development of a mirror coating testing technique that was sufficient to demonstrate the commitment of the experimental group to the LSC research program. The Seed Grants also demonstrated to the LSC that the Utah group is developing within a supportive research environment. By admitting our group of researchers within the collaboration, the LSC really admitted the University of Utah, and it is now much easier for other interested University of Utah researchers to join the collaboration.

What do each of you hope to accomplish in your future research?

Vikram, Keunhan, Stephan and their students are working on the design of mirrors to be used in gravitational wave detectors to improve their sensitivity. The detection of gravitational waves is achieved by recording tiny changes in the distance between mirrors separated by a few miles. A gravitational wave reveals itself by a brief variation of the distance at the level of 10^-18m (that is one thousand times smaller than the nucleus of an atom, which is itself one million times smaller than an atom, which is one billion times smaller than this dot: “.”). This is amazing but we believe we can do even better. One limitation comes from the fact the mirrors vibrate because of thermal energy. The mirror coatings play an important role in this transfer of energy between heat and vibration. We are trying to engineer and identify coatings that will be less susceptible to this problem. We plan to do this by characterizing cryogenically cooled samples; this approach is doubly significant because future generations of gravitational wave interferometers will use cryogenically cooled mirrors for increased sensitivity. By improving the sensitivity of gravitational wave detectors, we will increase the volume of the universe that can be explored through gravitational waves.

Yue’s group works on several gravitational wave related theoretical topics.  The goal of the group is to build a bridge between gravitational wave physics and particle physics, and to reveal the fundamental laws in nature. One of the biggest mysteries in our Universe is the identity of dark matter. It is widely accepted that dark matter exists in our Universe. The energy taken by dark matter is much larger than, about 5 times, that in baryonic sector of which stars and human beings are made. However, we have no idea on the mass of dark matter particles at all. It can be ultra-light, about 26 orders of magnitude smaller than an electron mass, or it can be extremely heavy, comparable to the mass of our Sun. Yue’s group study the possibility on utilizing LIGO gravitational wave detector to search for dark matter candidates on both extremes. Furthermore, the group also studies the gravitational waves produced at the very early time of the Universe. In general, people have very comprehensive understandings of the Universe one second after its creation. However, what happens before that one second remains largely unknown. Exotic phenomena, such as the production of cosmic strings or strong first order phase transitions, can naturally happen in many particle physics models and they would produce relic gravitational wave background. The LIGO gravitational wave detector provides one, often the only one, method to probe the existence of such processes happening during the earliest cosmological time. If observed, this will trigger revolutions on particle physics and significantly extend our knowledge of the fundamental laws of nature.

To be officially considered as admitted to candidacy in the Ph.D. program, applicants must pass a qualifying exam. The qualification exams are the first of three major steps in the Mechanical Engineering Ph.D. program at the University of Utah. Students select two subject areas which support their Ph.D. research. The topics cover fundamental material with the expectation of a graduate-level understanding of the subjects. Taufiq took his qualification exams in the subject areas of thermodynamics and heat transfer.

The investigation of thermal transport across extremely small vacuum gap distances is of both practical and fundamental significance. Previous theoretical studies have predicted that in the near-field, or when the emitter-receiver separation is less than the thermal wavelength, thermal radiation can exceed Planck’s blackbody limit by up to several orders of magnitude due to radiation tunneling of evanescent electromagnetic waves. This enhancement has been verified by several recent experimental efforts providing exciting opportunities for the development of thermophotovoltaic energy conversion, passive radiative cooling, and nanoscale thermal management. While experimental findings for gap distances above 10 nm have observed good agreement with near-field thermal radiation theory within the fluctuational electrodynamics (FE) framework, the underlying physics of thermal transport in the sub-10-nm (i.e., extreme near-field) gap regime is still in significant debate. The aforementioned knowledge gap poses a fundamental question: How does thermal radiation transition to phonon heat conduction at contact? To investigate this question, the finite dipole model is combined with FE to elucidate the magnitude of extreme near-field thermal radiation. Then, an experimental setup based on a custom-built high-vacuum scanning probe microscope and platinum nanoheaters is developed to enable heat transfer measurements across sub-10-nm vacuum gaps. By implementing this experimental setup, thermal transport between a silicon tip and platinum nanoheater separated by single-digit nanometer vacuum gaps is found to be more than three orders of magnitude larger than FE predictions. The atomistic Green’s function for phonon transport illustrates that acoustic phonon tunneling links the measured gap and contact thermal transport mechanisms. The results of this dissertation suggest a direct correlation between the tip-substrate force and acoustic phonon transmission, such that interfacial heat transfer may be engineered using external force stimuli.

Congratulations Dr. Jarzembski!!!