A novel and practical acoustic energy harvesting mechanism to harvest traveling sound at low audible frequency is introduced and studied both experimentally and numerically. The acoustic energy harvester in this study contains a quarter-wavelength straight tube resonator with lead zirconate titanate (PZT) piezoelectric cantilever plates placed inside the tube. When the tube resonator is excited by an incident sound at its acoustic resonance frequency, the amplified acoustic pressure inside the tube drives the vibration motions of piezoelectric plates, resulting in the generation of electricity. To increase the total voltage and power, multiple PZT plates were placed inside the tube. The number of PZT plates to maximize the voltage and power is limited due to the interruption of air particle motion by the plates. It has been found to be more beneficial to place the piezoelectric plates in the first half of the tube rather than along the entire tube. With an incident sound pressure level of 100 dB, an output voltage of 5.089 V was measured. The output voltage increases linearly with the incident sound pressure. With an incident sound pressure of 110 dB, an output voltage of 15.689 V and a power of 12.697 mW were obtained. The corresponding areal and volume power densities are 0.635 mW cm −2 and 15.115 µW cm −3 , respectively.

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The effect of electrically active VGa–ON threading edge dislocations on drift and Hall mobilities in n-type epitaxial wurtzite (WZ) GaN is investigated theoretically. The charge distribution along the dislocation core is first obtained by means of a density-functional theory atomistic calculation; the two N atoms near the missing Ga atom at the dislocation core are found to be electron acceptors. An accurate analytical expression for dislocation electrostatic strength is then derived for the case of up to −2q charge per structural unit of the threading dislocation core. This strength factor is determined by minimizing the total increase of free energy per site of the partially charged dislocation line. Two different models of scattering potentials for charged dislocation lines are then used to determine the dislocation effect on in-plane electron mobility, and closed-form solutions for the dislocation contribution to drift and Hall mobilities are derived for the more accurate potential. By estimating the effects of other scattering mechanisms, the total mobility is then compared with available experimental data. It is found that for free-carrier concentrations higher than 1016cm−3, reducing dislocation density below ndis=108cm−2 has little beneficial effect on total mobility for typical WZ GaN samples.

Resonant tunnelling is a quantum mechanical process that has long been attracting both scientific and technological attention owing to its intriguing underlying physics and unique applications for high-speed electronics. The materials system exhibiting resonant tunnelling, however, has been largely limited to the conventional semiconductors, partially due to their excellent crystalline quality. Here we show that a deliberately designed transition metal oxide superlattice exhibits a resonant tunnelling behaviour with a clear negative differential resistance. The tunnelling occurred through an atomically thin, lanthanum d-doped SrTiO 3 layer, and the negative differential resistance was realized on top of the bipolar resistance switching typically observed for perovskite oxide junctions. This combined process resulted in an extremely large resistance ratio (B10 5 ) between the high and low-resistance states. The unprecedentedly large control found in atomically thin d-doped oxide superlattices can open a door to novel oxide-based high-frequency logic devices.

Noise barriers are commonly used to protect communities from transportation noise. In the present study, three types of barriers, modeled as half planes, have been tested in the laboratory: a conventional rigid barrier with a straight top edge, a straight top edge barrier covered with sound absorbing material, and a rigid barrier with a jagged top edge. Measurements were taken not just behind the barriers, but around them on a plane perpendicular to their top edge. Measured signals were compared against theoretical predictions contributing to further validation of a theoretical model. The sound absorbing material was found to affect the diffracted field more in the front of the barrier than behind it. The diffracted field in front of a jagged edge barrier, similar to the field behind it, was found to depend on the geometry of the edge in the area where the shortest diffraction path intersects the edge profile. Last, the performance of the three barriers was compared with one another in all areas around the barrier. It was found that the jagged edge barrier provides shielding similar to the sound absorbing barrier but at a fraction of the cost.