106 research outputs found
Modeling electromechanical properties of layered electrets: Application of the finite-element method
We present calculations on the deformation of two- and three-layer electret
systems. The electrical field is coupled with the stress-strain equations by
means of the Maxwell stress tensor. In the simulations, two-phase systems are
considered, and intrinsic relative dielectric permittivity and Young's modulus
of the phases are altered. The numerically calculated electro-mechanical
activity is compared to an analytical expression. Simulations are performed on
two- and three-layer systems. Various parameters in the model are
systematically varied and their influence on the resulting piezoelectricity is
estimated. In three-layer systems with bipolar charge, the piezoelectric
coefficients exhibit a strong dependence on the elastic moduli of the phases.
However, with mono-polar charge, there is no significant piezoelectric effect.
A two-dimensional simulation illustrated that higher piezoelectricity
coefficients can be obtained for non-uniform surface charges and low Poisson's
ratio of phases. Irregular structures considered exhibit low piezoelectric
activity compared to two-layer structures.Comment: To be appaer in J Electrostatic
Electric field profiles in electron-beam-charged polymer electrets
A recently developed method, which uses piezoelectrically generated pressure steps for the determination of electric-field profiles in dielectrics, has been applied to electron-beam-charged polyfluoroethylenepropylene (FEP) and polyethyleneterephthalate (PETP) electrets. The results indicate that the technique can be employed to study volume charge effects in thin dielectrics. If properly calibrated, the method provides a quantitative measure of charge-integral functions or electric-field distributions in polymer foils
Origin of temperature dependent conductivity of -polyvinylidene fluoride
The conductivity of -polyvinylidene fluoride (-PVDF) is
obtained from dielectric measurements performed in the frequency domain at
several temperatures. At temperatures above the glass-transition, the
conductivity can be interpreted as an ionic conductivity, which confirms
earlier results reported in the literature. Our investigation shows that the
observed ionic conductivity is closely related to the amorphous phase of the
polymer
Numerical calculations of effective elastic properties of two cellular structures
Young's moduli of regular two-dimensional truss-like and eye-shape-like
structures are simulated by using the finite element method. The structures are
the idealizations of soft polymeric materials used in the electret
applications. In the simulations size of the representative smallest units are
varied, which changes the dimensions of the cell-walls in the structures. A
power-law expression with a quadratic as the exponential term is proposed for
the effective Young's moduli of the systems as a function of the solid volume
fraction. The data is divided into three regions with respect to the volume
fraction; low, intermediate and high concentrations. The parameters of the
proposed power-law expression in each region are later represented as a
function of the structural parameters, unit-cell dimensions. The presented
expression can be used to predict structure/property relationship in materials
with similar cellular structures. It is observed that the structures with
volume fractions of solid higher than 0.15 exhibit the importance of the
cell-wall thickness contribution in the elastic properties. The cell-wall
thickness is the most significant factor to predict the effective Young's
modulus of regular cellular structures at high volume fractions of solid. At
lower concentrations of solid, eye-like structure yields lower Young's modulus
than the truss-like structure with the similar anisotropy. Comparison of the
numerical results with those of experimental data of poly(propylene) show good
aggreement regarding the influence of cell-wall thickness on elastic properties
of thin cellular films.Comment: 7 figures and 2 table
Synchronization of organ pipes: experimental observations and modeling
We report measurements on the synchronization properties of organ pipes.
First, we investigate influence of an external acoustical signal from a
loudspeaker on the sound of an organ pipe. Second, the mutual influence of two
pipes with different pitch is analyzed. In analogy to the externally driven, or
mutually coupled self-sustained oscillators, one observes a frequency locking,
which can be explained by synchronization theory. Further, we measure the
dependence of the frequency of the signals emitted by two mutually detuned
pipes with varying distance between the pipes. The spectrum shows a broad
``hump'' structure, not found for coupled oscillators. This indicates a complex
coupling of the two organ pipes leading to nonlinear beat phenomena.Comment: 24 pages, 10 Figures, fully revised, 4 big figures separate in jpeg
format. accepted for Journal of the Acoustical Society of Americ
Elastic properties of highly anisotropic thin poly(propylene) foams
In this letter, elastic properties of highly anisotropic cellular
poly(propylene) films are reported. The material shows peculiar elastic
properties compared to other foams in the literature. The data is displayed as
the relative Young's modulus versus relative density .
Almost all the data from the literature are located on the region
with . The introduced material on the
other hand have lower relative Young's modulus at high relative densities,
Figure of merit comparison of PP-based electret and PVDF-based piezoelectric polymer energy harvesters
The harvesting of mechanical strain and kinetic energy has received great attention over the past two decades in order to power wireless electronic components such as those used in passive and active monitoring applications. Piezoelectric ceramics, such as PZT (lead zirconate titanate), constitute the most commonly used electromechanical interface in vibration energy harvesters. However, there are applications in which piezoelectric ceramics cannot be used due to their low allowable curvature and brittle nature. Soft polymer PVDF (polyvinylidene fluoride) is arguably the most popular non-ceramic soft piezoelectric energy harvester material for such scenarios. Another type of polymer that has received less attention is PP (polypropylene) for electret-based energy harvesting using the thickness mode (33-mode). This work presents figure of merit comparison of PP versus PVDF for off-resonant energy harvesting in thickness mode operation, revealing substantial advantage of PP over PVDF. For thickness-mode energy harvesting scenarios (e.g. dynamic compression) at reasonable ambient vibration frequencies, the figure of merit for the maximum power output is proportional to the square of the effective piezoelectric strain constant divided by the effective permittivity constant. Under optimal conditions and for the same volume, it is shown that PP can generate more than two orders of magnitude larger electrical power as compared to PVDF due to the larger effective piezoelectric strain constant and lower permittivity of the former
Light valve technology for HDTV-state of the art
After an introduction, in which liquid crystal and electron beam addressed light-valve projectors are briefly reviewed, the main light-valve technologies are explained in some detail. Transmissive and reflective liquid-crystal light valves with various types of addressing are described first; the trend toward active-matrix addressing and high resolution is demonstrated. Commercial light-valve projectors with electro-optic and oil-film control layers are briefly introduced before more recently developed technologies such as metallised viscoelastic layers, deformable micromechanical mirrors, and polymer-encapsulated liquid crystals are discussed. Finally, in view of its importance for the field, some aspects of active-matrix addressing are briefly examined
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