Surface Charge Of Electret Research Articles

Ultrasonic/electrical dual stimulation response nanocomposite bioelectret for controlled precision drug release

Electret materials have attracted extensive attention because of their permanent polarization and electrostatic effect. However, it is one of problem that needs to be solved in biological application to manipulate the change of surface charge of electret by external stimulation. In this work, a drug-loaded electret with flexibility and no cytotoxicity was prepared under relatively mild conditions. The electret can release the charge through stress change and ultrasonic stimulation, and the drug release can be accurately controlled with the help of ultrasonic and electric double stimulation response. Here, the dipoles like particles of carnauba wax nanoparticles (nCW) are fixed in the matrix based on the interpenetrating polymer network structure, and "frozen" oriented dipolar particles that are treated by thermal polarization and cooled at high field strength. Subsequently, the charge density of the prepared composite electret can reach 101.1​nC/m2 at the initial stage of polarization and 21.1​nC/m2 after 3 weeks. In addition, the stimulated change of electret surface charge flow under cyclic tensile stress and cyclic compressive stress can generate a current of 0.187​nA and 0.105​nA at most. The ultrasonic stimulation results show that when the ultrasonic emission power was 90% (Pmax​=​1200​W), the current of 0.472​nA can be generated. Finally, the drug release characteristics and biocompatibility of the nCW composite electret containing curcumin were tested. The results showed that it not only had the ability to accurately control the release by ultrasound, but also triggered the electrical effect of the material. The prepared drug loaded composite bioelectret provides a new way for the construction, design and testing of the bioelectret. Its ultrasonic and electrical double stimulation response can be accurately controlled and released as required, and it has broad application prospects.

Effects of Fabrication Conditions on the Partial Discharge Mitigation Performance of Electrets for Power Electronic Driven Systems

In our previous studies, we have shown that electrets can be designed to mitigate partial discharge (PD) in power electronic converter (PEC)-driven systems by neutralizing locally-enhanced harmful electric fields. Surface charge density is one of the main factors that determines the PD mitigation performance of electrets. In this study, we fabricate electrets from polyvinylidene fluoride (PVDF) films under various charging conditions including voltage magnitude, charging duration, voltage polarity, and temperature using triode-corona discharge method. The surface charge density and PD mitigation performance of the fabricated electret films are assessed by surface potential measurements and PD experiments. <styled-content style="color:black">The effects of fabrication temperature on electret surface charge stability, and the impacts of atmospheric conditions on electret surface charge density and PD magnitude are discussed.</styled-content> This study provides knowledge that is essential for establishing the fabrication conditions of electrets that can effectively mitigate PD in PEC-driven systems.

Sensitivity change with practical use of electret condenser microphone

Dr. Sessler and Dr. West invented the electret condenser microphone (ECM) in 1966. It has since been applied in various ways as a sound input device. The ECM has become an important component as a microphone for communications because of its stable sensitivity frequency characteristic. Materials and production methods have been improved continually up to the present. In particular, the ECM reliability is based on the electret’s stability. For that reason, the electret surface charge decay is the main factor in ECM sensitivity degradation. This study analyzed the changes of an ECM preserved for 28 years in the laboratory and actually used for an outdoor interphone unit for 29 years. The change of diaphragm stiffness and electret surface voltage were compared with the evaluation result of a heat-acceleration test and verified. A degradation estimate of sensitivity change of ECM was performed. Regarding the life of the electret predicted in the report of former study [K. Miura and Y. Yasuno, J. Acoust. Soc. Jpn. (E) 18(1), 29–35 (1997)], the validity was verified using actual data from this long-term observation.

Theoretical Model for Response Calculation of Electret Ionisation Chambers for Radon Dosimetry

The application of electret chambers for radon measurements requires the exact determination of their response. Therefore, a theoretical model has been established in order to calculate the radon response for any electret ionisation chamber and to quantify the physical processes responsible for the decrease of response at lower electret voltages. Moreover, a deeper understanding of the processes inside the chamber is helpful for the discussion of possible uncertainties of measurements. The model is based on a numerical calculation of the electric field inside the chamber and in the electret reader arrangement. It is shown that the main reasons for the decrease of the radon response with decreasing electret voltage are the growing recombination loss due to a lower field strength and the influence of changes in the electret surface charge distribution on the reader characteristics. The results of the calculation are compared with experimental data.

Radiation-induced conductivity of Teflon in electret ionization chambers

Studies of radiation-induced conductivity (RIC) of 254 mu m thick Teflon polyfluoroethylene propylene (FEP) film used in an electret ionization chamber (EIC) for charging and discharging of the electret are discussed. The EIC can be used to measure air-kerma from the amount of electret surface charge that has been neutralized by ions created in the chamber's sensitive volume. RIC permits the trapped surface charge to migrate through the polymer and recombine at the collecting electrode, which results in a reduced EC charging efficiency and an overestimation of air-kerma. A semi-empirical equation was developed to calculate time-dependent RIC as a function of air-kerma rate and electric field in the Teflon (FEP) film for steady-state and non-steady-state conditions. It was found that the air-kerma rate (dose rate) exponent is constant while the electric-field exponent is time dependent. It is found that RIC in Teflon (FEP) of this thickness causes air-kerma as measured with EICS to be very air-kerma rate dependent at low air-kerma rates. A discussion on the limitation of EICS due to this problem is given.

Electrets as a cascade impactor substrate

A theoretical investigation has shown that electret fiber substrates can be used to reduce particle bounce and blow-off by increasing the force of attraction of particles to the substrate. Uncharged particles are attracted to the fibers by induced dipole and van der Waals forces while charged particles experience an additional Coulombic force. The magnitudes of these forces are dependent on the particle size, electret surface charge density, the dimensions of the electret fibers, and the particle's net charge. These attractive forces dominate over different particle size ranges. In a strong electric field gradient, such as exists with an electret fiber, the induced dipole force is predicted to be the strongest force on large particles ( d p > 1–2 μm) and van der Waals attraction will be the dominant force on small particles ( d p < 1 μm).

Dynamics of radiation-induced charging and discharging of foil electrets

The time dependence of the polarization and depolarization current densities, the effective electric field in the electret chamber, and the electret surface charge densities are presented for the radiation-induced foil electret. With the use of the hyperbolic dependence of the ionization-chamber current density on the effective electric field, one obtains excellent agreement between calculated and measured electret polarization and depolarization current densities.

Discussion of “Effect of Low Pressure on Surface Charge of Electrets” [R. A. Draughn and A. Catlin (pp. 391–394, Vol. 115, No. 4)

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Effect of Low Pressure on Surface Charge of Electrets

Electrets prepared from Mylar and a polystyrene material were exposed to low pressure. The effective surface charge of the electrets decreased as the pressure was lowered. It is proposed that the charge decrease is due to either desorption of charge sources from the electret surfaces or spark breakdown on the electret surface.

Charge Stability of Electret Microphones

The sensitivity of foil-electret microphones depends largely on the surface charge of the electret; thus, the temporal stability of this charge under various environmental conditions is of interest. Since the charge decay is resistive, foils with large volume resistivity, such as fluorocarbon, are necessary for systems with high lifetime. The stability was determined by measuring both the sensitivity of microphones and the surface charge of the foil electrets over extended periods of time. Measurements at room temperature show the sensitivity of fluorocarbon-electret microphones to be constant over periods of the order of years. Extrapolation of results obtained at elevated temperatures indicates a time constant of the sensitivity of fluorocarbon microphones of 10–103 yr at room temperature, depending on the preconditioning (temperature cycling) of the systems and, to a smaller degree, on the relative humidity. Even at 50°C and high relative humidity, the time constant of a preconditioned system is still of the order of 10 yr. Typical sensitivities of electret microphones are between −50 and −60 dBV for 1 μbar. Preconditioning of the systems results in a sensitivity loss of about 10 dB.