B. Gross

Measurement of potential buildup and decay, surface charge density, and charging currents of corona‐charged polymer foil electrets

Conventional measurement techniques for the investigation of the behavior of corona‐charged polymer foil electrets allow one to determine the equivalent surface charge and the surface potential decay after termination of the charging process. This paper describes a technique which determines charges, currents, and potentials during the charging process. The application of this technique to Teflon and polyethylene foils show the usefullness of the method. The values of transit times and of the trap‐modulated mobility for polyethylene are obtained.

Short‐Circuit Currents in Charged Dielectrics and Motion of Zero‐Field Planes

When a previously charged dielectric is short circuited, discharge currents result due to the movement of charge carriers in the field of the space charge. It is shown for plane‐parallel geometry that the discharge current is given by the product of charge density at, and velocity of, the zero‐field plane. The resulting expression includes the displacement current. It can also be applied to the case where the dielectric is exposed to a beam of penetrating radiation; it then allows one to evaluate the effect of space charge on the external current.

Space‐charge‐limited currents in electron‐irradiated dielectrics

This paper develops the theory of steady‐state currents generated in a dielectric placed between positively or negatively biased electrodes and irradiated with a partially penetrating electron beam. The dielectric is divided into an irradiated region (IR), which extends from the electrode of incidence to the extrapolated range of the beam, and a nonirradiated region (NIR). In the IR the primary beam generates an electron‐hole plasma. Its end plane acts as a virtual electrode embedded in the dielectric. Currents are space‐charge limited in the NIR and Ohmic in the IR which is characterized by a uniform radiation‐induced conductivity. Depending on the polarity of the electrode bias, electrons or holes are drawn from the IR into the NIR. The theory correctly predicts an apparent threshold effect for the inset of steady‐state currents: the current amplitudes remain small as long as the electron range is smaller than half the sample thickness, and increase strongly only afterwards. Calculated current curves for different beam energies are in satisfactory agreement with experimental results. The role of the electron beam as a virtual electrode is discussed.This paper develops the theory of steady‐state currents generated in a dielectric placed between positively or negatively biased electrodes and irradiated with a partially penetrating electron beam. The dielectric is divided into an irradiated region (IR), which extends from the electrode of incidence to the extrapolated range of the beam, and a nonirradiated region (NIR). In the IR the primary beam generates an electron‐hole plasma. Its end plane acts as a virtual electrode embedded in the dielectric. Currents are space‐charge limited in the NIR and Ohmic in the IR which is characterized by a uniform radiation‐induced conductivity. Depending on the polarity of the electrode bias, electrons or holes are drawn from the IR into the NIR. The theory correctly predicts an apparent threshold effect for the inset of steady‐state currents: the current amplitudes remain small as long as the electron range is smaller than half the sample thickness, and increase strongly only afterwards. Calculated current curves fo...

Positive charging of fluorinated ethylene propylene copolymer (Teflon) by irradiation with low‐energy electrons

Starting with electron energies above a few keV, all materials show an increase in the number of secondary and backscattered primary electrons with decreasing primary energy. The number of secondary plus backscattered electrons becomes equal to the number of primaries at two energies called the first and second crossover points. Between these energies the number of backscattered primaries and of secondaries exceeds the number of incoming primaries. For Teflon FEP the higher‐energy crossover (second crossover) point occurs at about 1.6 keV. Between this energy and the first crossover (at a few hundred eV), holes are generated in the surface region of the material. This effect can be used to charge the surface of an insulating dielectric positively by negatively biasing the metallized rear during charging and subsequent discharging of the rear electrode after irradiation.

Transport of excess charge in electron‐irradiated dielectrics

Irradiation of dielectrics with nonpenetrating electron beams has been used for practical purposes, e.g., manufacturing of electrets, and as a research tool, e.g., the investigation of induced conductivity and transport properties. The present paper develops some aspects of the theory of charge transport and storage and radiation‐induced conductivity in irradiated dielectrics. It is assumed that the dielectric is kept in short circuit and that the range of the incident electron beam is smaller than half the sample thickness. The treatment applies only to dielectrics which have a very low dark conductivity and contain a great number of deep traps.

Constant current corona charging of PVF2

Conduction of polyvinylidene fluoride is investigated by the method of constant current corona charging. During poling the surface voltage approaches a constant value, proportional to the current, if this is sufficiently small. When the corona is turned off it decreases to zero. Measurements with single and double electroded samples, for both polarities, give identical results, seeming to indicate conduction due to preexisting charges. Measurements in air of different relative humidity and in argon show that the conductivity depends on the presence of water vapor in the surrounding gas. Results are tentatively analyzed in terms of an equivalent circuit in which the sample is represented by a parallel connection of a capacitor and a resistor.

Constant schubweg for hole transport in corona charged fluorethylenepropylene

Thin (25 μm) foils of Teflon® FEP are charged positively in a constant-current corona triode with currents between 1–6×10−9 A to a voltage of 3 kV. Experimental results give the voltage as a function of time during charging and during the discharge which occurs after the corona current has been turned off. Results can be interpreted in terms of a theory which introduces shallow surface and deep bulk traps and assumes that the field-induced carrier drift is characterized by a constant schubweg.

Electron emission from electron-irradiated dielectrics

Two methods for the measurement of the electron emission currents and the secondary electron yields for electron-irradiated dielectrics are discussed. Experimental results indicate that the dynamic measurement method for the secondary emission yield delta provides yield curves that are significantly lower than those obtained by conventional methods, at least in the region in which delta goes through a maximum. This effect is believed to be due to a recombination between the electrons undergoing emission and the positive surface charges which the dielectric acquires during the continuous irradiation to which it is exposed in the dynamic method. A modified method that replaces the continuous radiation by pulse radiation and eliminates the formation of charges on the dielectric is described. It is shown that, at beam energies well beyond the energy E/sub m/ of the maximum yield, both methods give similar results. The pulse method was used to determine the yield curves of a variety of polymers and to confirm that these curves can be represented in the form of a universal curve when delta / delta /sub m/ is plotted against E/E/sub m/. It was found that the positive and negative charges generated on the dielectric by the electron irradiation remain surface charges and do not drift into the bulk of the material. >

dc conduction and dielectric absorption in PVF2

Charging and discharging currents in PVF2 are found to depend on the nature of the gas (argon or air) in which the samples are kept during measuring and on the pressure and humidity of the air.

Induced Conductivity of Mylar and Kapton Irradiated by X-Rays

This paper presents a wide range of experimental results of prompt and delayed components of the conductivity induced by x-rays in Mylar (PET) and Kapton (Polyimide). Measurements were carried out under a vacuum at room temperature. They were made for a large range of parameters: electric field E (104 to 4.8X105 V/ cm); exposure rate X (85 to 550 R/ s); and thickness d of the sample (6 to 75 ¿m). Electrodes were made of evaporated aluminum, but in some measurements gold was used. Gold increased the exposure rate, due to its high er atomic number. Finally, a study of the recovery of the irradiated samples was made.