Publications

Results 51–75 of 123
Skip to search filters

Phonon considerations in the reduction of thermal conductivity in phononic crystals

Applied Physics A: Materials Science and Processing

Hopkins, Patrick E.; Phinney, Leslie M.; Rakich, Peter T.; Olsson, Roy H.; El-Kady, I.

Periodic porous structures offer unique material solutions to thermoelectric applications. With recent interest in phonon band gap engineering, these periodic structures can result in reduction of the phonon thermal conductivity due to coherent destruction of phonon modes characteristic in phononic crystals. In this paper, we numerically study phonon transport in periodic porous silicon phononic crystal structures. We develop a model for the thermal conductivity of phononic crystal that accounts for both coherent and incoherent phonon effects, and show that the phonon thermal conductivity is reduced to less than 4% of the bulk value for Si at room temperature. This has substantial impact on thermoelectric applications, where the efficiency of thermoelectric materials is inversely proportional to the thermal conductivity. © 2010 Springer-Verlag.

More Details
TYPE Conference YEAR 2011
ScopusOSTI

Manipulation of thermal phonons: A phononic crystal route to High-ZT thermoelectrics

Proceedings of SPIE - The International Society for Optical Engineering

El-Kady, I.; Su, Mehmet F.; Reinke, Charles M.; Hopkins, Patrick E.; Goettler, Drew; Leseman, Zayd C.; Shaner, Eric A.; Olsson, Roy H.

Phononic crystals (PnCs) are acoustic devices composed of a periodic arrangement of scattering centers embedded in a homogeneous background matrix with a lattice spacing on the order of the acoustic wavelength. When properly designed, a superposition of Bragg and Mie resonant scattering in the crystal results in the opening of a frequency gap over which there can be no propagation of elastic waves in the crystal, regardless of direction. In a fashion reminiscent of photonic lattices, PnC patterning results in a controllable redistribution of the phononic density of states. This property makes PnCs a particularly attractive platform for manipulating phonon propagation. In this communication, we discuss the profound physical implications this has on the creation of novel thermal phenomena, including the alteration of the heat capacity and thermal conductivity of materials, resulting in high-ZT materials and highly-efficient thermoelectric cooling and energy harvesting. © 2011 SPIE.

More Details
TYPE Conference YEAR 2011
ScopusOSTI

Silicon carbide phononic crystal cavities for micromechanical resonators

Proceedings of the IEEE International Conference on Micro Electro Mechanical Systems (MEMS)

Ziaei-Moayyed, M.; Su, M.F.; Reinke, Charles M.; El-Kady, I.; Olsson, Roy H.

This paper demonstrates silicon carbide phononic crystal cavities for RF and microwave micromechanical resonators. We demonstrate design, fabrication, and characterization of Silicon Carbide/air phononic crystals used as Bragg acoustic mirrors to confine energy in a lateral SiC cavity. Aluminum nitride transducers drive and sense SiC overtone cavities in the 2-3GHz range with fxQ products exceeding 3×1013 in air. This approach enables decoupling of the piezoelectric AlN material from the SiC cavity, resulting in high Q resonators at microwave frequencies. The SiC cavities are fabricated in a CMOS-compatible process, enabling integration with wirelesss communication systems.

More Details
TYPE Conference YEAR 2011
ScopusOSTI

NanoFIBrication of a two-dimensional phononic crystal in a free standing membrane

El-Kady, I.; Olsson, Roy H.

A two-dimensional phononic crystal (PnC) that can operate in the GHz range is created in a freestanding silicon substrate using NanoFIBrication (using a focused ion beam (FIB) to fabricate nanostructures). First, a simple cubic 6.75 x 6.75 ?m array of vias with 150 nm spacing is generated. After patterning the vias, they are backfilled with void-free tungsten scatterers. Each via has a diameter of 48 nm. Numerical calculations predict this 2D PnC will generate a band gap near 22 GHz. A protective layer of chromium on top of the thin (100 nm) silicon membrane confines the surface damage to the chromium, which can be removed at a later time. Inspection of the underside of the membrane shows the vias flaring out at the exit, which we are dubbing the 'trumpet effect'. The trumpet effect is explained by modeling the lateral damage in a freestanding membrane.

More Details
TYPE Conference YEAR 2010
OSTI
Results 51–75 of 123
Results 51–75 of 123