Nanoscale Energy Conversion Research Articles

Report on 6th U.S.–Japan Joint Seminar on Nanoscale Transport Phenomena—Science and Engineering

The objective of this U.S.–Japan joint seminar series is to provide a cross-disciplinary and international forum for discussing and identifying outstanding science and technology issues in the area of nanoscale thermophysics and energy conversion and to foster collaboration among researchers in these areas. The first of this seminar series, championed by the late Professors Chang-Lin Tien and Kunio Hijikata, was held in Kanazawa, Japan, in June of 1993. Subsequent meetings have been held every three years, alternating venues between the United States and Japan. The Sixth U.S.–Japan Joint Seminar on Nanoscale Transport Phenomena—Science and Engineering was held in Boston, Massachusetts, July 13–16, 2008, and was organized by Professors Gang Chen from MIT, Fushinobu Kazuyoshi from Tokyo Institute of Technology, Shigeo Maruyama from Tokyo University, and Pamela Norris from University of Virginia. Nearly 100 scientists participated in the seminar. (The agenda of the seminar is attached at the end at this report[14].) The seminar included keynote sessions and invited sessions, as well as a dedicated poster session of selected presentations from an open call for papers. All papers presented in the regular sessions, the invited sessions, were upon invitation by the organizers. Invited sessions used a mixed form of communication: each speaker gave a 5-minute summary of his work followed by a 30-minute poster session of just the papers summarized orally, and then these speakers came back to the podium, serving as panelists to answer questions regarding their papers and session themes. This format offered good opportunities for the presenters to discuss their work with the participants. Reports for each session were summarized by session chairs. Following is a brief summary of the sessions.

Limits of size confinement in silicon thin films and wires

Physically confined structures such as thin films and nanowires are becoming increasingly important in nanoscale energy conversion and nanoelectronics. The main focus of this work is to determine the size threshold below which the volumetric specific heat and group velocity of one- and two-dimensionally confined silicon nanostructures begin to differ significantly with respect to bulk silicon and to quantify these changes. The dynamical matrix approach subject to free-standing boundary conditions is employed to determine the phonon normal modes of vibration of the structures. The environment-dependent interatomic potential under the harmonic approximation is used to model interatomic forces. We find that above 10nm thickness, silicon [111]-films yield specific heats and group velocities which exhibit size-invariant behavior; for [111]-silicon nanowires, the limit is approximately 5nm. Moreover, we show that computed phonon group velocities using the dynamical matrix approach are affected by geometry-specific modes beyond confinement, and that size effects are mainly manifested through the volumetric specific heat at low temperatures. Furthermore, we conclude that confinement effects, when present, are only relevant at low temperatures, below 100K, when the predominant phonon wavelength becomes larger than the confined dimension.

Nanoscale energy transport and conversion: a parallel treatment of electrons, molecules, phonons, and photons

This is a graduate level textbook in nanoscale heat transfer and energy conversion that can also be used as a reference for researchers in the developing field of nanoengineering. It provides a comprehensive overview of microscale heat transfer, focusing on thermal energy storage and transport. Chen broadens the readership by incorporating results from related disciplines, from the point of view of thermal energy storage and transport, and presents related topics on the transport of electrons, phonons, photons, and molecules. This book is part of the MIT-Pappalardo Series in Mechanical Engineering.

Nanoscale radiation heat transfer for silicon at different doping levels

Heat transfer between surfaces at close vicinity has important applications in nanoscale energy conversion devices and near-field scanning thermal microscopy. The present paper describes a comprehensive investigation of the radiation energy transfer between two semi-infinite parallel plates at different temperatures, involving silicon with varying dopant concentrations, when the distance of separation is from 10 μm down to 1 nm. The net radiation heat flux is calculated by means of the fluctuational electrodynamics. The dielectric function of silicon is modeled using a Drude model, considering the effects of temperature and doping level on the carrier concentrations and scattering times. The calculated results show that the dopant concentration strongly affects the radiation heat flux when the two media are separated at nanometer distances. For heavily doped silicon plates separated at a distance of 1 nm, the present study predicts a radiation energy flux of over five orders of magnitude greater than that between two blackbodies placed far apart. Furthermore, the radiation energy flux can be more than ten times larger than the conduction heat flux of air at the atmospheric pressure, and the radiation heat transfer coefficient may exceed 1 MW m −2 K −1. The theoretical understanding gained from the present research will facilitate the design of experiments that utilize near-field radiation to enhance heating or cooling at the nanoscale for applications such as thermal control in nanoelectronics, energy conversion, and nanothermal probing and manufacturing.