The notion of a locally resonant metamaterial—widely applied to light and sound—has recently been introduced to heat, whereby the thermal conductivity is reduced primarily by intrinsic localized atomic vibrations rather than scattering mechanisms. This article reviews and analyzes this new emerging concept, termed nanophononic metamaterial (NPM), and contrasts it with the competing concept of a nanophononic crystal (NPC) in which thermal conductivity reduction is realized primarily via nanoscale Bragg scattering. Both the NPM and NPC core mechanisms require the presence of a sufficient level of wave behavior, which is possible when there is a relatively wide distribution of the phonon mean free path (MFP). Silicon serves as a perfect material to form NPMs and NPCs given its relatively large average phonon MFP. This offers a unique opportunity considering silicon's abundance and mature fabrication technology. It is shown in this comparative study that while both the NPM and NPC nanosystems may be rendered to serve as extreme insulators of heat, an NPM may do so without excessive reduction in the minimum feature size–which is key to keeping the electrical properties intact. This trait makes a silicon‐based NPM poised to serve as a low‐cost thermoelectric material with exceptional performance.

No abstract

No abstract

The present study proposes a wideband two-dimensional Luneburg lens using graded phononic crystals (GPCs). We present a method to broaden the range of operation frequency of the Luneburg lens based on GPCs. The GPCs is composed of circular cylindrical rods with triangular lattice air background. In long wavelength limit assumption, periodic structure behaves like a homogeneous medium since dispersion relations is almost linear. The effective refractive index can be tuned by adjusting the filling ratio. The plan wave expansion method is utilized to calculate the band structures of the locally GPCs and the effective refractive index of the locally GPCs with different filling ratio. Finite element simulations was employed to confirm the acoustic properties of designed device. Our numerical simulations demonstrate that the operation frequency range of GPCs of circular cylindrical rods arranged as triangular array is much wider than those of square array. A wideband acoustic Luneburg lens can be achieved by using graded phononic crystals of triangular array.

In this paper, we proposed an asymmetry structure of printed dipole antenna which acquires wideband impedance match delivered by a double-side and center-feed-design and an open slot in the small radiator to expand the high resonant frequency band and harvest an antenna that can reach dual and wide band requirements as we proposed here. The frequency bands of this proposed antenna cover 2.31-3.83 GHz and 4.88-6.0 GHz, which can be applied to WLAN and WiMax operations and the fractional bandwidths reach 49.51% and 20.59%. We minimize the antenna size to 20u u47 mm 2 by using the feed lines as design parameters to achieve the dual-band and wideband design. This proposed antenna structure can be included in a WLAN/WiMax array antenna as a structural element. In this paper, we will present simulation and implemented results of antenna structure, frequency bands, gain value and the radiation patterns.