Recently, the development of flexible electret based electrostatic actuator has been widely
discussed. The devices was shown to have high sound quality, energy saving, flexible
structure and can be cut to any shape. However, achieving uniform charge on the electret
diaphragm is one of the most critical processes needed to have the speaker ready for
large-scale production. In this paper, corona discharge equipment contains multi-corona
probes and grid bias was set up to inject spatial charges within the electret diaphragm. The
optimal multi-corona probes system was adjusted to achieve uniform charge distribution of
electret diaphragm. The processing conditions include the distance between the corona
probes, the voltages of corona probe and grid bias, etc. We assembled the flexible electret
loudspeakers first and then measured their sound pressure and beam pattern. The uniform
charge distribution within the electret diaphragm based flexible electret loudspeaker provided
us with the opportunity to shape the loudspeaker arbitrarily and to tailor the sound distribution
per specifications request. Some of the potential futuristic applications for this device such
as sound poster, smart clothes, and sound wallpaper, etc. were discussed as well.
Electret-based electrostatic devices have been used in the
electro-acoustic field for decades. Recently, the
improvement of its charge retention has been of interest for application to the field of smart materials. Hence, the
flexible electret-based loudspeaker has become an important research topic for futuristic applications such as 3C
(computers, communications and consumer electronics) and smart curtains. The volume velocity and the on-axis sound
pressure level (SPL) of an electret loudspeaker are the key parameters of interest. To study the vibration characteristics
of an electret diaphragm, a finite element analysis (FEA) was introduced to facilitate the design. To validate the finite
element analysis (FEA) model, an out-of plane full-field
non-destructive optical detection method which incorporates
electronic speckle pattern interferometry (ESPI), was applied to determine the vibration mode shape of thin film. By
driving the electret loudspeaker at different frequencies, a corresponding vibration mode of interest was detected with the
ESPI set-up. Both the simulations and the experimental results obtained on the measurement platform are detailed in this
paper.
Recently, power harvesting technologies for low-power electronic devices have attracted much interest. In this paper,
the design and fabrication methods of a micro-electrostatic power generator is presented. This power generator
comprises a stator developed using an electret film for charge storage and a rotor covered by an interdigital electrode for
electric power generation. The newly developed electret material is made from mixing two solutions. The first solution
was made by blending polystyrene (PS) and cycloolefin copolymer (COC). The second solution was obtained by an
additive process as polar molecule was added into COC. This unique two solution electret method can easily be
integrated and adopted to the micro fabrication process. The charge storage capability of this new electret material was
investigated and results showed that low concentration of polystyrene in the blended material will not only have more
stable but also higher electrostatic charge than that of pure COC. In addition, the polar molecular additives also improve
the electret properties of COC due to micro-cavities formation and the interactions between molecules and polymer. Our
newly developed blended electret material has excellent mechanical properties and is easy to use when compared to
using Teflon Fluorinated Ethylene Propylene (FEP) and polypropylene (PP). A feasibility study of a micro electrostatic
power generator based on our blended electret material was performed. Experimental results demonstrate the feasibility
and effectiveness of this new type of micro electrostatic power generator.
A solution made from blending cycloolefin copolymer (COC) and polystyrene (PS) was proposed to create a
double-sided electret polymer film. This electret polymer was then sandwiched to form an electret-metal-electret
structure by using the MEMS processes. The upper and lower polymer layers were found to both enhance charge
storage capacity significantly and to improve the machining property. It was identified that lower concentration of PS
led to sphere-like morphology distributed uniformly within the COC/PS blends, which created better electret properties
than that of pure COC or pure PS polymers. In addition, it was also found that these COC/PS blends have better
adhesion to both metal and polymers. A series of processes developed to optimize this line of new electret polymers for
actuator development are detailed. The recipe of this new material and the associated fabricating process to develop an
electret loudspeaker are also detailed. In comparison, the pros and cons of this speaker system versus a typical
electrostatic loudspeaker or a headphone, which require both bulky and expensive DC-to-DC converters, are detailed as
well.
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