Electret Discharge Research Articles

First Review of Conductive Electrets for Low-Power Electronics

This is the first review of conductive electrets (unpoled carbons and metals), which provide a new avenue for low-power electronics. The electret provides low DC voltage (μV) while allowing low DC current (μA) to pass through. Ohm’s Law is obeyed. The voltage scales with the inter-electrode distance. Series connection of multiple electret components provides a series voltage that equals the sum of the voltages of the components if there is no bending at the connection between the components. Otherwise, the series voltage is below the sum. Bending within the component also diminishes the voltage because of the polarization continuity decrease. The electret originates from the interaction of a tiny fraction of the carriers with the atoms. This interaction results in the charge in the electret. Dividing the electret charge by the electret voltage V’ provides the electret-based capacitance C’, which is higher than the permittivity-based capacitance (conventional) by a large number of orders of magnitude. The C’ governs the electret energy (1/2 C’V’2) and electret discharge time constant (RC’, where R = resistance), as shown for metals. The discharge time is promoted by a larger inter-electrode distance. The electret discharges occur upon short-circuiting and charge back upon subsequent opencircuiting. The discharge or charge of the electret amounts to the discharge or charge of C’.

Dynamics of the electric polarization and depolarization of graphite

The dynamics of direct-current-induced electric polarization/depolarization of a conductive material (graphite) are studied. Forward-current polarization and reverse-current depolarization cause the apparent resistivity to increase by ≤ 2.2% and ≤1.0%, respectively. Polarization-related carrier-atom interaction engagement occurs more readily than depolarization-related disengagement. The number n of carriers that participate increases with the number NI of injected electrons; the rate of the n increase decreases with increasing NI. The efficiency of carrier-atom interaction (indicated by n/NI, ≤1.2 × 10−17) decreases with increasing NI (≤1.2 × 1020). For the same NI, the fraction of carriers that participate (n/(NI + Nc), where Nc = number of inherent carriers) increases with increasing current (rate of electron injection). At a particular current, this fraction increases and then decreases with increasing NI. The polarization/depolarization rate decreases as polarization/depolarization proceeds. At a given polarization/depolarization time, the rate and degree of polarization are higher than those of depolarization. The above fraction is higher for polarization (≤8.5 × 10−18) than depolarization (≤4.7 × 10−18). The difference between polarization and depolarization agrees with the apparent-resistivity asymmetry and true electret character indicated by the open-circuited DC electric field of 1.0 × 10−5 V/m. The above fraction is lower than the fraction of carriers that participate in the electret discharge (4.9 × 10−4) by 14 orders of magnitude.

Light-induced electrostatic lithography: selective discharge of electrets by utilizing photothermal conversion of Ti3C2Tx MXene

A novel light-induced electrostatic lithography is developed by combining in situ photothermal conversion with selective thermally stimulated discharging.

Micro-fabricated Liquid Encapsulated Energy Harvester with Polymer Barrier Layer as Liquid Electret Interface

This paper addresses the electret discharge issue for liquid based electret energy harvesters. An interface structure of PDMS/PTFE polymer barrier layer between liquid and electrets is introduced, achieving 75% charge retain rate over 100h, compared with 0% without the proposed layer over 100h. Further, the PDMS/PTFE layer is introduced into liquid encapsulated energy harvester (LEEH) and is compatible with micro-fabrication process. The retain rate of device voltage is about 47%~65% over 100h. At 100h after corona charging, the device generates maximally 3.7V, 0.55μW @1Hz rotation.

Electret effect in Pb5Ge3O11 crystals

The short-circuit thermally stimulated depolarization current (TSDC) through lead germanate single crystals has been measured as a function of temperature (100–800 K) and time. The results indicate that poling of the crystals at elevated temperatures (t ≥ 150°C) produces an electret state related to the spacecharge mechanism of polarization. Relatively mild poling conditions (E = 0.75 kV/cm, t = 150°C) produce a well-defined, long-lived electret with an abnormally large electret charge (1400 μC/cm2). The electret discharge leads to heating of the crystal, in the form of a peak in the range 520–570 K, whose height exceeds that of the pyroelectric peak near T C by two to three orders of magnitude. From the TSDC data, we evaluated the stored electret charge, its density, and the activation energy of traps for electret charges, and made preliminary conclusions as to the nature of the traps. We demonstrate that neglect of the electret discharge current (which depends on a number of factors) when determining the pyroelectric coefficient p σ above 280 K leads to a significant scatter in data.

ELECTRETS TO MEASURE ION CONCENTRATION IN AIR

Positive and negative ions are produced in air, mainly due to radon and terrestrial/cosmic radiation sources. Measuring ion concentration in air indirectly provides a measure of these sources. Electrets (electrically charged pieces of Teflon), when exposed in the environment, collect ions of opposite sign leading to a measurable decrease in charge, depending upon the exposure time and ion concentration. This work describes a method of correlating electret discharge rate to the ion concentration as measured by a calibrated ion density meter. Once calibrated, electrets can then be used to measure ion concentration of either sign. The ion concentration in ambient air was measured to be about 200 ions mL(-1), measured over several hours. Both positive and negative ion concentrations were similar. In a typical room, negative ion concentration was about 3,500 ions mL(-1), and, surprisingly, there were no positive ions at all in that room. Being an integrating passive device, the method provides the unique possibility of measuring low or high concentrations of positive or negative ions over extended periods, which is difficult to do with other ion concentration measuring instruments.

Thermally stimulated electron emission from charged dielectric surfaces

Abstract Electron emission from charged surfaces of spontaneously polarized dielectrics has been observed. TGS, a non-linear dielectric-ferroelectric crystal and the linear pyroelectrics lithium sulphate monohydrate and resorcinol have been studied. An electret state is formed at the free surface under the unscreened pyroelectric field. A subsequent thermally activated relaxation process of the electret charge gives rise to the electron emission which can be considered as an electret discharge current into vacuum. It is shown that, for TGS crystals, electron emission in the nonpolar, paraelectric phase occurs due to a space-charge electret effect. A dipole electret polarization formed by reoriented dipolar water molecules in lithium sulphate monohydrate is also sufficient to produce electron emission.

Origin of the αc relaxation in poly(ethylene-terephthalate) by thermally stimulated currents

The thermally stimulated depolarization currents of poly (ethylene-terephthalate) electrets with a reduced degree of crystallinity (≃4%), corona-charged at polarization temperatures between 65 and 100 °C, show a heteropolar α* relaxation placed between the α relaxation (heteropolar) and the ρ relaxation (homopolar). This α* relaxation is associated with a uniform mechanism and has been observed in the discharge of electrets with shorted evaporated electrodes, which have been formed using the windowing polarization method at polarization temperatures between 76 and 79 °C. The intensity and position of α* depend on the degree of crystallinity and the morphology. The application of the method of thermal stimulation by steps, which leads to a gradual crystallization of the sample, shows that the αc relaxation of crystalline PET has its origin in α*.

Analysis of the Debye model of electret discharge

The characteristics of electret discharge under open circuit conditions are investigated theoretically. The use of the Debye equation makes it possible to analyse the interaction between depolarization and the transport of space charge injected during the preparation of the sample. An approximate analytical solution is obtained and is verified by numerical calculations. It is shown that the effect of the space charge electric field on the relaxation of non-equilibrium polarization leads to a reduction of the maximum value of the voltage across the sample.

Optical studies of chain orientation by thermal treatment and mechanical stretching of Teflon PFA foil

AbstractThe effect of thermal treatment and uniaxial stretching on the temperature dependence of optical birefringence, degree of crystallinity, thermally stimulated depolarization, and isothermal discharge of electrets obtained of Teflon PFA foil has been studied. The increase in stability of charge is related to changes in the polymer structure.

Glass transition in semicrystalline polycarbonate

AbstractThe intensity of the glass transition in semicrystalline polycarbonate was measured by differential scanning calorimetry and by thermally stimulated discharge of electrets. Solution‐cast and bulk‐crystallized samples possessing widely varying crystallinities and morphologies were investigated. It is shown that the intensity of the glass transition is governed by the extent of primary crystallization and is a linear combination of intensities from the bulk amorphous regions and from noncrystalline polymer within semicrystalline aggregates such as spherulites. The intensity of the glass transition within spherulites is about 0.1–0.3 as great as that in bulk amorphous regions. A three‐phase model incorporating two distinct types of noncrystalline polycarbonate is proposed to account for the properties of this polymer.

A reply to “comment on a model of electret discharge”

A reply to “comment on a model of electret discharge”

“A model of electret discharge” — A comment

“A model of electret discharge” — A comment

A model of electret discharge

It is well established that there exist two relaxation processes in electrets: (1) the decay of persistent polarization and (2) the disappearance of free charge injected into the bulk during the preparation of the electret sample. Both processes have been separately studied in detail. But the treatment of these two relaxation processes together, when one affects the other, is a more complicated problem, which has been investigated insufficiently. In the models previously suggested, the electric field inside the bulk of the electret is assumed to decay due to the ohmic conductivity of the sample. On the other hand, it is well-known that most electrets are very good insulators with negligible ohmic conductivity. For this reason, the injection relaxation mechanism suggested in this paper seeme more reasonable. This mechanism is based on the concept of space-charge conductivity of injected carriers against the background of persistent polarization decay. The results obtained are in agreement with experimental work where the inversion of the voltage polarity and the change of dipole moment sign, as well as the observation that both the voltage and dipole moment pass through a minimum, have been discovered.

UV dosimetry by ZnS electret discharge

A method of UV dosimetry is described which depends on electret depolarization in single crystal and powder specimens of ZnS by the incoming radiation. The residual polarization can be measured by optical or thermal readout procedures. In both cases, the mechanism is electronic. The dosimetric characteristics of these systems are described and comparisons made with corresponding dosimetric procedures based on pyroelectricity and current-glow. Measurements of the type described also yield information on trap distributions in insulators.