The phase transition from the non-polar a-phase to the polar beta-phase of poly(vinylidene fluoride) (PVDF) has been investigated using micro-Raman spectroscopy, which is advantageous for being a non-destructive technique. Films of alpha-PVDF were subjected to stretching under controlled rates and at 80 degrees C, the transition to beta-PVDF being monitored by the decrease in the Raman band at 794 cm(-1) characteristic of the a-phase, with the concomitant increase in the 839 cm(-1) band characteristic of the beta-phase. Poling with negative corona discharge was found to affect the alpha-PVDF morphology improving the Raman bands related to this crystalline phase. This effect is minimized for films stretched to higher ratios. Significantly, corona-induced effects could not be observed with the other experimental techniques, viz. X-ray diffraction and infrared spectroscopy.
The very lively and stimulating discussion on the last day of the 7th International Symposium on Electrets was transcribed and edited from an audio tape in order to obtain a readable text. It was, however, attempted to preserve at least some of the original flavor of the discussion by not polishing the more colloquial style of the statements too much and by not eliminating all redundancies. If the results of this experiment are not totally satisfactory the editor (and not the discussion panelists or participants) is to be blamed.
A new method for the determination of the secondary emission yield of dielectrics is discussed. Results are given for Teflon, Aclar, Kapton, and Mylar. Surfaces of dielectrics irradiated with electrons of energies above 2 keV become negatively charged. For lower energies the number of back-scattered primaries plus backward emitted secondaries eventually exceeds that of the incoming primaries. Then the surface can be positively charged [1]. It was recognized that this effect might lead to a simple method for the determination of the total backscatter plus secondary emission yield [2]. This effect is used to determine the total emission yield curve in one run with a single beam energy.
Interfaces and surfaces are always involved with electrets and frequently, if not invariably, exert a controlling influence on their properties. Although classical featureless models have often been used to describe the behaviour, especially where contact to metal electrodes is involved, the situation is much more complex. The nature of interfaces and contacts and of charge transfer across them is reviewed using the concepts of modern molecular electronics and electrochemistry. The effect of blocking and non-blocking conditions and the special behaviour at free surfaces is considered. The roles of structure and local order especially for polymeric electrets and the influence of ionic and polar impurities in determining the magnitude, life-time and transport of charges in electret materials will be outlined.
A balance equation of the charge carrier transport process in the insulator regime (long-time process, time dependent injection, long-time stable space charges) is established. The equation describes the stochastic hopping process between localized levels, having random distribution in space and of energy. The specific aspect of the model is the stochastic treatment of the non-equilibrium dynamics of the space charge.
Electron injection and space charge relaxation are important time domain properties at PVDF/metal interfaces which can be investigated by free relaxation and AC-experiments as well as by contact electrification. The results of our free relaxation experiments show a field and time dependent interface barrier of Richard-son-Schottky type, which is active in a time domain up to 30 s in a temperature range from −40 to 90 °C. This type of rectifying contact leads to contact electrification phenomena depending on superimposed AC-voltage. Nonlinear dielectric properties can be discussed in terms of these results.
The use of pyroelectric thin-film electrets for calbrimetry of transient heating processes is explored. A pyroelectric calorimeter which combines a time resolution of nanoseconds with a sensitivity of nanocalories is described. The versatility and the unique properties of this new type of device are demonstrated with several applications in the thermal analysis of thin films and spectroscopy of adsorbates. Data are presented on the thermal diffusivity of thin films and on phase transitions during optical recording. In addition, results on spectroscopy of thin films and adsorbates are reported.
Part 1 of this paper describes aspects of research on dielectrics and electrets carried out by the author over the last 50 years, in an attempt to show the systematics of its development and logical relation between its successive steps. Starting with dielectric absorption, it covers open-circuit behavior of dielectrics, thermally stimulated currents, Paschen breakdown and its effects on the performance of the inductionplate method, and the superposition of free surface charge and dielectric volume polarization. The two-charge theory appears as the result of a straightforward application of the basic charge-and-field equations of electrostatics, without the introduction of a specific model. Theoretical results are developed on the basis of simple experiments, most of them performed with Carnauba wax. Part 2 reviews theoretical aspects of isothermal and thermally activated dielectric relaxation. It includes the theory of the dielectric relaxation function and its interpretation in terms of distributions of natural frequencies or of activation energies, establishment of universal expressions for the relaxation function related with the theory of cooperative processes, theories based on the diffusion equation and normal modes, and the failure of the theory of equivalence during thermally stimulated transients.
Unoriented, heat treated, compression molded 400–800μm thick sheets prepared from VF2/VF3 resins with mole ratios 65/35–80/20 were poled and their dielectric and piezoelectric properties studied. Both the permittivities and losses were reduced upon heat treatment before poling and further reduced after poling. Resulting piezoactivity is higher on heat treated material. The Curie transition temperatures and latent heats increase at each stage of treatment. Heat aging the poled material below Tc causes ε and tan δ to increase and piezoactivity to decrease when measured at room temperature; however, initial values are recovered after several days room temperature storage.
The ionic conduction in the polyester polymer doped with 0.5 wt% of rho-damine 6G is investigated. The doping increases the electric conductivity of the polymer by about two orders of magnitude. Above the glass transition temperature, the coloured cations form the near-electrode space charge which is used to determine the cation mobility. The temperature dependence of the cation mobility is determined.
Dielectric measurements have been made on poly (vinyl chloride: vinyl acetate: vinyl alcohol) in the frequency range 30 Hz-100 KHz and in the temperature range 77–410 K. A frequency dependent conductivity described by σ(ω) = A ω s , is observed where S < 1 and is independent of temperature upto 250 K but with further increase in temperature it decreases. Two relaxations, the α-and the β-relaxation, are observed having activation energies ∼ 0.92 and 0.42 eV, respectively. The α-relaxation is attributed to the segmental motion of the main chain consisting of the constituent groups of the terpolymer under dipole-dipole interaction and the β-relaxation is due to the C-C1 dipoles in the amorphous phase of the terpolymer.
The charging and the discharging current as well as the total charge transported in a circuit incorporating a PE or PET specimen were measured under different conditions. The results do not confirm the existence of an SCL regime in polymeric insulating materials.
A new form of analysis of thermally stimulated luminescence data is presented, enabling the form of the concentration vs. activation energy distribution function of the electron traps in a polymeric dielectric to be deduced. At the end of the irradiation the trapped electrons are divided into four groups, namely the fraction f 0 not re-trapped, and the fractions f 1 , f 2 and f 3 re-trapped once, twice and three times, respectively, before recombining with a luminescence centre. A distribution consisting of one or more sections of the form n(E)dE α exp (α(E — E)) is assumed, and the parameters adjusted to yield best agreement Between the experimental and synthesized data.
It has been said that the applications of electrets are unlimited. We will show that there is a great deal of truth, in this statement. The variety of applications is due first to the availability of three types of electrets: normal ones purely based on charge storage, piezo- and pyro-active ones and light-sensitive ones. Secondly, there are three ways of using electrets; one can use (a) its inductive power, (b) the forces it generates, and (c) sacrifice its charge, for instance for measuring radiation. Electrets can convert e.g. mechanical to electrical, electrical to mechanical, thermal to electrical and optical to electrical signals. They are or can be used in audio systems, for measuring and testing, in the medical and maritime field, for security and surveillance, etc. In this paper the emphasis will be on recent advances and developments, particularly those which have found large-scale application.
The TSD technique has been used to study the α-relaxation in Nylon terpolymer films. A well isolated peak observed at about 60 °C in the TSD spectrum suggests the onset of low frequency molecular motions in the polymer. Experimental results indicate that the induced polarization is due to dipole alingment. TSP experiments give a peak characterized by the same position and height as the TSD peak. T q determined by DTA lies in the region in which the α-peak occurs, a.c. measurements also reveal a loss peak in the region of T g . It is concluded that the relaxation is due to large-scale segmental motions at the glass transition.
Propagating arc discharges are the result of electrical breakdown in electron-beam-charged dielectrics. For incident electron accelerating voltages in the range of kilovolts to tens of kilovolts, the charge accumulation is primarily within a few micrometers of the exposed surface, so that the propagating subsurface arc frequently bursts through the surface, ejecting ionized debris and an appreciable fraction of the accumulated electrons. This blowoff current can reach a peak value of hundreds or even thousands of amperes and thus, when it occurs on spacecraft materials, severe interference to control systems can be the result. The present state of understanding of this phenomenon will be reviewed.
The dielectric spectrum of DNA was observed in the frequency range from 10 mHz to 100 kHz-Several dielectric absorption regions were found and their dependence on temperature and humidity was studied-There seems to be a conformational transition near 290 K and a collective behaviour of the whole system DNA-water-ions at low temperatures as if there were a glass transition near 238 K. Mechanisms of all dispersion regions are discussed and special attention is paid to the model of the jump-like movement of the DNA counter-ions accompanied by water rearrangement.
Electrically charged filter fibers, and, in particular, electret fibers, can be used in air filters. Planar electrets have a high internal and a low external electric field; but only external fields contribute to filtration, and so if electrets are used in filters, the fundamental problem is to increase the external field. In a conventional electret filter this problem is solved simply by reducing the width of the electret sheet by making it into fibers, but an alternative solution is to place conductors close to the surface of the electret. A filter with a low resistance to airflow can be produced by this means, but the level of charge that can be sustained on the electret is limited by dielectric breakdown in the air between dielectric and conductor. The total charge on such a filter has been measured microscopically by X-irradiation, and macroscopically with a probe; the two methods give similar results. The electric field inside the filter has been calculated by a self-consistent method. Specimen results for filtration efficiency are given.
When an electric field is applied to an insulating material three basic processes can take place: the dipoles tend to rotate, ions migrate and space charge can be injected at the interfaces, depending on such parameters as temperature or applied electric field. For many years, a direct analysis was impossible since the experimentally observable variables gave only an average of what was taking place in the samples under study. Many methods were developed over the years combining available measuring techniques with theoretical hypotheses. They led to phenomenological descriptions but it appeared that a direct measurement of space charge or polarization distributions in the materials would be the way for the understanding of the physical processes involved. In this paper we will present the evolution of these methods and will describe one which seems presently the most promising. It uses the propagation in the sample of a pressure wave which acts as a virtual probe sensitive to charge, field or potential. A very elegant technique to generate this pressure wave involves the use of short-duration laser pulses. It will be shown how this can find applications in a very large number of areas. Examples will be given in such different fields as electrets, high voltage insulation or transducer materials.
