Jeong Ho You

Low frequency acoustic energy harvesting using PZT piezoelectric plates in a straight tube resonator

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 W cm 3 , respectively. (Some figures may appear in colour only in the online journal)

Electron scattering due to threading edge dislocations in n-type wurtzite GaN

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 ...

Effect of screw dislocation density on optical properties in n-type wurtzite GaN

The effect of open-core screw dislocations on photoluminescence in n-doped wurtzite GaN epilayer is studied computationally and compared with experimental data. A k•p Hamiltonian calculation domain is set up to contain a dipole of open-core screw dislocations, and its size is varied according to the desired dislocation density. Using the finite element method, energy levels and wave functions for conduction and valence states are determined in three-dimensional real space; the emission spectrum is then evaluated. The void associated with the dislocation core and the deformation potential due to the strain surrounding the core perturb the density of states and reduce the photoluminescence (PL) spectrum intensity accordingly. For dislocation densities below a transition density of around 108cm−2, the deformation potential effect dominates in reducing the PL intensity; above this dislocation density the effect of the missing material at the core dominates. The calculated photoluminescence results agree with ...

Experimental study of the diffracted sound field around jagged edge noise barriers

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 th...

Acoustic Energy Harvesting Using Quarter-Wavelength Straight-Tube Resonator

Although there have been significant efforts in harvesting environmental energy, our environment is still full of wasted and unused energy. As clean, ubiquitous and sustainable energy source, acoustic energy is one of the wasted energies and is abundant in our life. Therefore, it is of great interest to investigate acoustic energy harvesting mechanism as an alternative to existing energy harvesters. In this study, in order to harvest acoustic energy, piezoelectric cantilever beams are placed inside a quarter-wavelength straight-tube resonator. When the straight-tube resonator is excited by an incident wave at its acoustic eigenfrequency, an amplified acoustic resonant wave is developed inside the tube and drives the vibration motion of the piezoelectric beams. The piezoelectric beams have been designed to have the same structural eigenfrequency as the acoustic eigenfrequency of the tube resonators to maximize the amount of the harvested energy. With a single beam placed inside the tube resonators, the harvested voltage and power become the maximum near the tube open inlet where the acoustic pressure gradient is at the maximum. As the beam is moved to the tube closed end, the voltage and power gradually decrease due to the decreased acoustic pressure gradient. Multiple piezoelectric beams have been placed along the centerline of the tube resonators in order to increase the amount of harvested energy. Due to the interruption of acoustic air particle motion caused by the beams, it is found that placing piezoelectric beams near the closed tube end is not beneficial. The output voltage of the piezoelectric beams increases linearly as the incident sound pressure increases.Copyright

Electron transport at interface of LaAlO3 and SrTiO3 band insulators

Electron transport properties at n-type LaAlO3/SrTiO3 interfaces have been investigated numerically. Carrier distributions, band structures, and sheet density have been calculated by solving Schrodinger equations with Poisson equation in a self-consistent manner for various LaAlO3/SrTiO3 interfaces with and without atomic interdiffusions. It was found that the interface with A-site atom interdiffusion has the critical thickness of 4 unit cells below which it remains insulating. Most electrons are localized within 10 nm from the interface forming two-dimensional electron gas and multi-subbands are occupied indicating the multi-channel conduction. Electron mobility along the A-site atom interdiffused interface has been calculated using the linearized Boltzmann transport equation including scattering mechanisms of acoustic phonon, polar optical phonon, interface roughness, and net charged layers. At low temperature, the mobility is limited by the interface roughness and net charged layers. At room temperatur...

Effects of mechanical contact stress on magnetic properties of ferromagnetic film

Mechanical and magnetic degradation of ferromagnetic films under contact stress was systematically investigated through novel experiments and analytical simulations. Permalloy (Ni80Fe20) film was deposited onto silicon substrate, and two different thicknesses of permalloy film (50 nm for sample A and 300 nm for sample B) were examined in this study. Magnetic properties were obtained from B-H loop tracer hysteresis measurement, while the mechanical properties (i.e., hardness and elastic modulus) were measured using nanoindentation techniques. It was observed that the 50 nm thick permalloy film showed weaker magnetic strength (lower coercivity and saturation magnetic flux values) and lower hardness than the 300 nm thick permalloy film. To apply mechanical contact stress on the permalloy film samples, nanoscratch experiments were performed using ramp and constant loading scratch profiles. Then, the resulting mechanical degradation (surface physical damage) of the two samples was determined from atomic force ...

Harvesting ambient acoustic energy using acoustic resonators.

Sound is abundant in our everyday life, especially in urban environments. Despite the prevalence of sound, it is difficult to harvest acoustic energy in practical applications due to its low power density. In this study, we conduct numerical calculations to maximize the stored energy in piezoelectric beam arrays placed inside a Helmholtz resonator. The shape of the Helmholtz resonator is optimized to be a tube in order to increase the sound pressure amplification factor and lower the eigenfrequency to ~400 Hz. When the tube resonates by an external sound, the piezoelectric beams vibrate by amplified standing wave resulting in generating electrical energy. The simulation results show that a single beam is able to store ~0.0569 μJ strain energy when it is placed near the tube inlet with the incident sound pressure level of 100 dB. Using nine piezoelectric beams increases the total strain energy to 0.382 μJ.

Atomistically Informed Electrostatic Model of an Edge Dislocation in a Complex Crystalline Material

Dislocations are not only mechanical features of crystalline materials, but also complex electrostatic features; this has important implications for understanding electrical and optical properties of real semiconductor materials. An edge dislocation in a semiconductor material becomes electrically charged when free electrons migrate to the dangling bonds along the core; the line charge along the core is then partially screened by the background free carrier concentration. In this work we consider the atomistic structure of an edge dislocation in a technologically important compound semiconductor material, GaN, and develop a complete continuum model of the electrostatic structure of the dislocation. The atomistic analysis is used to determine the maximum charged state of the dislocation core according to first principles electronic structure analysis; the maximum charged state is then expressed as a continuum electrostatic potential. By formulating an energy balance model as a function of the incremental filling of electron acceptor sites, the equilibrium electrostatic state of the dislocation is then determined. This electrostatic state can then be used as the basis for predictive models of electrical scattering or optical absorption.

Chapter 3 Effect of Dislocations on Electrical and Optical Properties in GaAs and GaN

Publisher Summary This chapter presents a review of electrical and optical effects of dislocations in semiconductors, and the effects of dislocations in gallium nitride (GaN) and gallium arsenide (GaAs). A dislocation is a line defect along which the lattice is locally distorted by misalignments of atoms. There are two basic types of dislocations: edge and screw. For edge dislocations, charge distributions are calculated to identify the positions of electron acceptors. Based on charge distributions, the filling fraction is calculated for an edge dislocation in wurtzite GaN and electron mobilities are calculated and compared with experimental results. To review dislocation effects on optical properties, a multi-subband Schrodinger equation method is formulated in real space and solved using the finite element method (FEM). Finally, the effects of dislocations on optical properties in GaN and GaAs are presented and a comparison is made between the effects of edge dislocations in GaN and GaAs.