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On dielectric absorption in plastic insulators
LETTER On Dielectric
Absorption
TO THE EDITORS
in Plastic Insulators
(Received 6 February
1957)
DIELECTRIC absorption in insulators is usually explained either by the MAXWELL-WAGNERmechanism (interfacial polarization), or by Debye’s orientational polarization. The following mechanism could perhaps add to the understanding of dielectric absorption in homogeneous dielectrics, especially in regarding the connection between the time dependence of dielectric and mechanical relaxation. Suppose a solid material exhibiting elastic aftereffect, which contains charges (ions) or polar groups (e.g. dipoles) firmly attached to complex structural elements (e.g. bridged chain molecules in a high polymer). An electric field exerts a pull on bound charges, or torsion on polar groups respectively. These, in turn exert a stress on the structural constituents to which they are attached. Elastic strain should result in the same manner as under ordinary light mechanical loading of the material. The strain ought to be accompanied by dielectric polarization. The time dependence of the deformation and the polarization should coincide, since there are not two separate processes, but only a single molecular process giving rise to different manifestations. Investigations of the elastic behaviour of high polymers have shown(‘) that changes in free energy AF of elastically strained material are due partly to entropy changes AS, i.e. random redistribution processes governed by thermal vibrations, and partly to changes in internal energy AU, i.e. storing and release of potential energy in the process of molecular deformation. In principle, then, the described model for every kind of electrically active groups (ions, dipoles, quadrupoles, etc.) should render two possible polarization mechanisms, with AF caused either by AU or by AS respectively. The cases of bound ions and permanent dipoles seem to be of greatest practical interest. The last of these cases has been treated, for example, by PONOMAREV. c2) The author has undertaken measurements of the relation between the time dependences of the
pronounced elastic and electric after-effects in the polyester resin Soredur M-10. The elastic aftereffect was measured in torsion. A cylindrical sample was held twisted at a given angle during a certain time interval, and the movement after release was recorded. The dielectric relaxation was determined in a similar manner by recording the depolarization current in a prismatic sample after application of a constant voltage during a certain time interval. A marked similarity of the time dependence of the two processes could be observed (Fig. 1, curves 1 and 2).
log t
min
FIG. 1. Time dependence of elastic after-effect (curve l), dielectric absorption (curve 2), and photocurrent in stress-optical experiments (curve 3) for Soredur M-10. Ordinates in arbitrary units. Logarithmic co-ordinates.
With the aim of demonstrating, if possible, a direct connection between dielectric polarization and elastic deformation of the material, a prismatic sample of Soredur M-10, which exhibits strong strain birefringence, was placed in a light beam between crossed Polaroids. Two electrodes were attached to opposite sides of the sample so as to permit application of a field perpendicular to the direction of light. A photomultiplier with amplifier and a galvanometer in series with a bucking voltage permitted observation of small intensity changes of the light beam transmitted by the sample. After connection of a d.c. voltage to the electrodes, a galvanometer deflection was observed which slowly approached a stable value. After release of the voltage, the galvanometer deflection decreased with a time dependence similar to that observed in earlier measurements 150
LETTER
TO
of the elastic after-effect and the dielectric absorption in the same material (see curve 3 of Fig. 1). This phenomenon which, to the knowledge of the author, has not been described in the literature before, is easily understood on the basis of the proposed mechanisms for slow dielectric polarization of insulators exhibiting elastic after-effect, in particular many high polymers. The phenomenon may, on the other hand, serve as an indication of the existence of such mechanisms. Research Laboratories, Allminna Suenska Elektriska Aktiebolaget, Vaster&, Sweden.
ANDREASKELEN
THE
EDITORS
151
REFERENCES
1. STAVERMANA. J. and SCHWARZL F. Linear formation Behaviour of High Polymers, Physik der Hochpolymeren Vol. 4, p. 53 ff. Springer (1956). 2. PONOMAREVL. T. J. Techn. Phys. USSR 10, (1940).
DeDie Ed. 588
Absorption
TO THE EDITORS
in Plastic Insulators
(Received 6 February
1957)
DIELECTRIC absorption in insulators is usually explained either by the MAXWELL-WAGNERmechanism (interfacial polarization), or by Debye’s orientational polarization. The following mechanism could perhaps add to the understanding of dielectric absorption in homogeneous dielectrics, especially in regarding the connection between the time dependence of dielectric and mechanical relaxation. Suppose a solid material exhibiting elastic aftereffect, which contains charges (ions) or polar groups (e.g. dipoles) firmly attached to complex structural elements (e.g. bridged chain molecules in a high polymer). An electric field exerts a pull on bound charges, or torsion on polar groups respectively. These, in turn exert a stress on the structural constituents to which they are attached. Elastic strain should result in the same manner as under ordinary light mechanical loading of the material. The strain ought to be accompanied by dielectric polarization. The time dependence of the deformation and the polarization should coincide, since there are not two separate processes, but only a single molecular process giving rise to different manifestations. Investigations of the elastic behaviour of high polymers have shown(‘) that changes in free energy AF of elastically strained material are due partly to entropy changes AS, i.e. random redistribution processes governed by thermal vibrations, and partly to changes in internal energy AU, i.e. storing and release of potential energy in the process of molecular deformation. In principle, then, the described model for every kind of electrically active groups (ions, dipoles, quadrupoles, etc.) should render two possible polarization mechanisms, with AF caused either by AU or by AS respectively. The cases of bound ions and permanent dipoles seem to be of greatest practical interest. The last of these cases has been treated, for example, by PONOMAREV. c2) The author has undertaken measurements of the relation between the time dependences of the
pronounced elastic and electric after-effects in the polyester resin Soredur M-10. The elastic aftereffect was measured in torsion. A cylindrical sample was held twisted at a given angle during a certain time interval, and the movement after release was recorded. The dielectric relaxation was determined in a similar manner by recording the depolarization current in a prismatic sample after application of a constant voltage during a certain time interval. A marked similarity of the time dependence of the two processes could be observed (Fig. 1, curves 1 and 2).
log t
min
FIG. 1. Time dependence of elastic after-effect (curve l), dielectric absorption (curve 2), and photocurrent in stress-optical experiments (curve 3) for Soredur M-10. Ordinates in arbitrary units. Logarithmic co-ordinates.
With the aim of demonstrating, if possible, a direct connection between dielectric polarization and elastic deformation of the material, a prismatic sample of Soredur M-10, which exhibits strong strain birefringence, was placed in a light beam between crossed Polaroids. Two electrodes were attached to opposite sides of the sample so as to permit application of a field perpendicular to the direction of light. A photomultiplier with amplifier and a galvanometer in series with a bucking voltage permitted observation of small intensity changes of the light beam transmitted by the sample. After connection of a d.c. voltage to the electrodes, a galvanometer deflection was observed which slowly approached a stable value. After release of the voltage, the galvanometer deflection decreased with a time dependence similar to that observed in earlier measurements 150
LETTER
TO
of the elastic after-effect and the dielectric absorption in the same material (see curve 3 of Fig. 1). This phenomenon which, to the knowledge of the author, has not been described in the literature before, is easily understood on the basis of the proposed mechanisms for slow dielectric polarization of insulators exhibiting elastic after-effect, in particular many high polymers. The phenomenon may, on the other hand, serve as an indication of the existence of such mechanisms. Research Laboratories, Allminna Suenska Elektriska Aktiebolaget, Vaster&, Sweden.
ANDREASKELEN
THE
EDITORS
151
REFERENCES
1. STAVERMANA. J. and SCHWARZL F. Linear formation Behaviour of High Polymers, Physik der Hochpolymeren Vol. 4, p. 53 ff. Springer (1956). 2. PONOMAREVL. T. J. Techn. Phys. USSR 10, (1940).
DeDie Ed. 588