jeanflop/post_ocr_correction2 huggingface dataset:
la tension appliquée. Si ces deux paramètres restent proportionnels, alors aucune charge d'espace ne s'est développée. Au contraire, toute déviation de la dépendance linéaire est associée au développement d'une polarisation inhomogène ou d'une charge d'espace. Sur la figure 2b la dépendance est linéaire, ce qui indique qu'à 25°C, pendant le premier quart de cycle, il n'y a pas eu d'injection ou de déplacement notable de charges dans l'échantillon. CONCLUSION. — Les expériences présentées montrent qu'il est maintenant possible de mesurer en temps réel un champ électrique interne ou une charge d'espace à l'intérieur d'un diélectrique sous tension alternative appliquée. Cette nouvelle méthode sera particulièrement utile pour l'étude des processus d'accumulation ou de transfert de charge aux interfaces. Elle devrait permettre d'aider à la définition de matériaux ou de structures pour les hautes tensions. Cette méthode est particulièrement bien adaptée aux systèmes de mesure dans lesquels l'élément sensible est une onde de pression qui se propage dans l'échantillon, produite soit à partir de l'extérieur par des transducteurs (matériaux piézoélectriques ou, comme présenté ci-dessus, laser) soit de l'intérieur par application d'une impulsion de tension de temps de montée court. Dans ce cas la détection de l'onde de pression est faite par des transducteurs extérieurs. Les auteurs souhaitent exprimer tous leurs remerciements à la Fondation Langlois pour le soutien apporté à la réalisation de ce travail. INTRODUCTION. When a voltage is applied to an insulating material many processes can take place: dipoles are oriented in the electric field, ions are displaced or charges can be transferred at the interfaces. These effects depend on various parameters: nature of the material and of its electrodes, applied electric field, temperature. Contrary to what is being made in metals or semiconductors, insulators cannot be described using classical solid state physics. This is particularly true for polymer insulators. In such materials the periodicity is not extending over long distances and simple models with authorized and forbidden bands are not applicable. The deviation from pure periodic systems leads, among other things, to the presence of energy levels in the forbidden band. Moreover, small quantities of ionizable impurities can have
transducteurs extérieurs. Les auteurs souhaitent exprimer tous leurs remerciements à la Fondation Langlois pour le soutien apporté à la réalisation de ce travail. INTRODUCTION. When a voltage is applied to an insulating material many processes can take place: dipoles are oriented in the electric field, ions are displaced or charges can be transferred at the interfaces. These effects depend on various parameters: nature of the material and of its electrodes, applied electric field, temperature. Contrary to what is being made in metals or semiconductors, insulators cannot be described using classical solid state physics. This is particularly true for polymer insulators. In such materials the periodicity is not extending over long distances and simple models with authorized and forbidden bands are not applicable. The deviation from pure periodic systems leads, among other things, to the presence of energy levels in the forbidden band. Moreover, small quantities of ionizable impurities can have a very large influence on the behavior of the material. This is clear by looking at the orders of magnitudes of the effects involved. A concentration of impurities of the order of 1 part per million corresponds to about 1017 impurities per cubic centimetre. If one out of a hundred of such impurities is ionized, it creates a space charge of 10-4C/cm3, which is huge in terms of the electric field that such a space charge may produce. Various methods have been developed over the last 15 years to measure directly this space charge distribution [1-9]. They use the diffusion of heat [1-3] or the propagation of a pressure wave [4-9] to create a non destructive perturbation of the system, enabling a measurement of the charges encountered by the diffusion edge of the propagating wave front. In the first case it is possible to correlate the space charge distribution to the signal measured on adjacent electrodes during the diffusion of the perturbation, for instance heat. In the second case a pressure wave propagating in the sample generates a signal on the electrodes which is very simply related to the electric parameters inside the material. So far, these methods, which are
of such impurities is ionized, it creates a space charge of 10-4C/cm3, which is huge in terms of the electric field that such a space charge may produce. Various methods have been developed over the last 15 years to measure directly this space charge distribution [1-9]. They use the diffusion of heat [1-3] or the propagation of a pressure wave [4-9] to create a non destructive perturbation of the system, enabling a measurement of the charges encountered by the diffusion edge of the propagating wave front. In the first case it is possible to correlate the space charge distribution to the signal measured on adjacent electrodes during the diffusion of the perturbation, for instance heat. In the second case a pressure wave propagating in the sample generates a signal on the electrodes which is very simply related to the electric parameters inside the material. So far, these methods, which are described in more detail in references [10] and [11], have allowed the measurement of space charge distributions under quasi-static conditions or under a DC electric stress. However, time-varying systems are of great interest. It has been assumed [12] that, in such systems, charges can be transferred in non-symmetrical ways leading also to the build-up of space charge distributions. Presently the only way to detect such charges is by making a very delicate differential current measurement ([13]-[14]). In this paper, we present a new method which makes it possible to determine the space charge or electric field distributions in dielectrics under alternating electric stress. In the next paragraphs, we will describe the principle of the method, the experimental setup, and preliminary results. PRINCIPLE OF THE METHOD. As mentioned above, it is now possible to measure the charge, field, or potential distributions in an insulator. In the following, we only consider the method using the propagation of a pressure wave in the sample, for a reason which will be given below. In this case, the wave front of a short rise time pressure wave acts as a virtual probe sensitive to charge or field distributions. It has been shown [10] that during
also to the build-up of space charge distributions. Presently the only way to detect such charges is by making a very delicate differential current measurement ([13]-[14]). In this paper, we present a new method which makes it possible to determine the space charge or electric field distributions in dielectrics under alternating electric stress. In the next paragraphs, we will describe the principle of the method, the experimental setup, and preliminary results. PRINCIPLE OF THE METHOD. As mentioned above, it is now possible to measure the charge, field, or potential distributions in an insulator. In the following, we only consider the method using the propagation of a pressure wave in the sample, for a reason which will be given below. In this case, the wave front of a short rise time pressure wave acts as a virtual probe sensitive to charge or field distributions. It has been shown [10] that during the propagation of the pressure wave p(z, t), the current I(t) measured across the sample is related to the electric field distribution E(z, 0) by: In this equation, ZI is the position of the wave front and A a coefficient which depends on the mechanical properties of the sample and its geometry. If the wave is a very short duration pulse, this expression reduces to (2) I(t) = B p(zf, 0) whereas in the case of a pressure step, one has: (3) I(t) = CE(zf, 0) where B and C are two coefficients. In the first case, the variation in time of the current is a direct measurement of the space charge encountered by the wave front during its propagation. In the second case, it is an image of the local electric field. It is important to note that, contrary to what happens in the methods using the diffusion of heat, the time of measurement t1 in this technique is quite small. Typically, if one considers a 1 mm thick sample and a velocity of sound of 5,000 m/s, this time is of the order of 200 ns. This observation opens the way to the new proposed method. Under AC
its geometry. If the wave is a very short duration pulse, this expression reduces to (2) I(t) = B p(zf, 0) whereas in the case of a pressure step, one has: (3) I(t) = CE(zf, 0) where B and C are two coefficients. In the first case, the variation in time of the current is a direct measurement of the space charge encountered by the wave front during its propagation. In the second case, it is an image of the local electric field. It is important to note that, contrary to what happens in the methods using the diffusion of heat, the time of measurement t1 in this technique is quite small. Typically, if one considers a 1 mm thick sample and a velocity of sound of 5,000 m/s, this time is of the order of 200 ns. This observation opens the way to the new proposed method. Under AC stress, the applied electric field varies with a period which can typically be of 20 ms. It is then possible to sample successive situations, for instance by making measurements every millisecond or less, with a measuring time which is negligible as compared to the time of variation of the applied field. This is illustrated in figure 1 a. In this figure, one can see the variations in time of the applied voltage or electric field (upper curve) and the sampling time for the measurements (lower curve). As can be observed, the measuring time t1 is very small as compared to the time of variation of the voltage or field. In other words, this makes it possible to carry out many measurements within a time producing a small variation of the observed variables. EXPERIMENTAL. — In order to test the above principles, we have developed an experimental setup in which a short rise time laser pulse is used to generate the pressure waves in the sample. The signal detected is amplified and then digitized by a fast Analog to Digital Converter. The resulting message is transferred to a computer. Near the electrodes, during the penetration of the pressure pulse in the
field. This is illustrated in figure 1 a. In this figure, one can see the variations in time of the applied voltage or electric field (upper curve) and the sampling time for the measurements (lower curve). As can be observed, the measuring time t1 is very small as compared to the time of variation of the voltage or field. In other words, this makes it possible to carry out many measurements within a time producing a small variation of the observed variables. EXPERIMENTAL. — In order to test the above principles, we have developed an experimental setup in which a short rise time laser pulse is used to generate the pressure waves in the sample. The signal detected is amplified and then digitized by a fast Analog to Digital Converter. The resulting message is transferred to a computer. Near the electrodes, during the penetration of the pressure pulse in the insulator or during its exit, relation (3) must be used: the current peak Io is proportional to the electric field which can be monitored in real time. During the propagation of the pressure pulse in the bulk of the material, relation (2) must be used: the current is directly proportional to the space charge [11]. These three behaviors are visible on part b of the figure 1. Each recording is stored and the analysis of the successive signals can be made either at a given depth in the sample, in order to obtain an image of what happens at this position as a function of time, or at various times in order to observe the general evolution in the sample as a function of time. Fig. 1. (a) Variations in time of the applied voltage (upper curve), compared to the sampling time for the measurements (lower curve). (b) Typical signals recorded at each measurement. The time scale in part a is in the range of a fraction of a second, whereas in part b it is in the range of a fraction of a microsecond. Fig. 1. (a) Variations en fonction du temps de la tension appliquée (en haut). comparée
to the space charge [11]. These three behaviors are visible on part b of the figure 1. Each recording is stored and the analysis of the successive signals can be made either at a given depth in the sample, in order to obtain an image of what happens at this position as a function of time, or at various times in order to observe the general evolution in the sample as a function of time. Fig. 1. (a) Variations in time of the applied voltage (upper curve), compared to the sampling time for the measurements (lower curve). (b) Typical signals recorded at each measurement. The time scale in part a is in the range of a fraction of a second, whereas in part b it is in the range of a fraction of a microsecond. Fig. 1. (a) Variations en fonction du temps de la tension appliquée (en haut). comparée au temps d'échantillonnage des mesures (en bas). (b) Exemple de signal obtenu à chaque mesure. L'échelle des temps dans la partie (a) est une fraction de seconde, tandis que dans la partie b c'est une fraction de microseconde. Fig. 2. (a) Evolution of the signal during the rise of the applied voltage, from 0 to 40 kV. Thickness of the sample: 1 mm. The axis on the right refers to the number of the measurement in the sequence. (b) Variation of the interfacial field (proportional to the peak amplitude of the current Io), at the cathode, as a function of the applied voltage. Fig. 2. (a) Evolution du signal pendant la montée de la tension, de 0 à 40 kV. Épaisseur de l'échantillon: 1 mm. L'axe de coordonnées à droite indique le nombre de la mesure dans la séquence d'enregistrements (b) Variation du champ d'interface (proportionnel à l'amplitude du courant I0), à la cathode, en fonction de la tension appliquée. This is shown in figure 2 a. A 1 mm thick polyethylene plate was subjected to a varying voltage of peak amplitude 40 kV. One can see the evolution of the signal during the first quarter of cycle corresponding to
applied voltage, from 0 to 40 kV. Thickness of the sample: 1 mm. The axis on the right refers to the number of the measurement in the sequence. (b) Variation of the interfacial field (proportional to the peak amplitude of the current Io), at the cathode, as a function of the applied voltage. Fig. 2. (a) Evolution du signal pendant la montée de la tension, de 0 à 40 kV. Épaisseur de l'échantillon: 1 mm. L'axe de coordonnées à droite indique le nombre de la mesure dans la séquence d'enregistrements (b) Variation du champ d'interface (proportionnel à l'amplitude du courant I0), à la cathode, en fonction de la tension appliquée. This is shown in figure 2 a. A 1 mm thick polyethylene plate was subjected to a varying voltage of peak amplitude 40 kV. One can see the evolution of the signal during the first quarter of cycle corresponding to the rise of the applied voltage. It can be observed that the interfacial fields near the anode (entrance peak) and the cathode (exit peak) are following the applied voltage and that no space charge has yet appeared in the bulk of the material. If space charge develops it can be analyzed in two ways: either by looking at the signal during the propagation of the pressure wave in the sample or by plotting as is done in figure 2b the interfacial field as a function of applied voltage. If these two parameters remain proportional it means that there is no space charge build-up. On the contrary, any deviation from pure proportionality is associated with the development of a non-homogeneous polarization or space charge. In figure 2b, the amplitude of the current peak I0 near the cathode is plotted as a function of applied voltage. The dependence is quite linear up to 40 kV which means that, at 25°C, during this first quarter cycle, no significant space charges have been injected or displaced in the sample. Conclusion. The possibility to measure directly, in a non-destructive way, space charge build-up in real time under A.C. stress opens the way to a completely
analyzed in two ways: either by looking at the signal during the propagation of the pressure wave in the sample or by plotting as is done in figure 2b the interfacial field as a function of applied voltage. If these two parameters remain proportional it means that there is no space charge build-up. On the contrary, any deviation from pure proportionality is associated with the development of a non-homogeneous polarization or space charge. In figure 2b, the amplitude of the current peak I0 near the cathode is plotted as a function of applied voltage. The dependence is quite linear up to 40 kV which means that, at 25°C, during this first quarter cycle, no significant space charges have been injected or displaced in the sample. Conclusion. The possibility to measure directly, in a non-destructive way, space charge build-up in real time under A.C. stress opens the way to a completely new approach to the study of insulating materials. The experiments which have been presented show that internal electric field or space charge can now be directly measured, in real time. This new type of measurement will be particularly useful for the study of charge build-up or charge transfer at interfaces. It will probably be also of great interest in the definition of materials and structures for high voltage technology. This method is particularly well suited for measuring systems in which the sensitive element is a pressure wave propagated in the sample, either from the outside by transducer means (piezoelectric material or laser for instance) or from the inside, by the application of a short rise time voltage pulse and the detection of the pressure wave by outside means (transducer bonded on the sample for instance). The authors would like to express their thanks to Fondation Langlois for the support provided in the realization of this work. Note remise le 17 octobre 1992, acceptée le 7 novembre 1992. REFERENCES [1] R. E. COLLINS, Appl. Phys Lett., 26, 1975, pp. 675-677. [2] A. S. DE REGGI, C. M. GUTTMAN, F. I. MOPSIK, G. T. DAVIS and M. G. BROADHURST, Phys. Rev. Lett.,
It will probably be also of great interest in the definition of materials and structures for high voltage technology. This method is particularly well suited for measuring systems in which the sensitive element is a pressure wave propagated in the sample, either from the outside by transducer means (piezoelectric material or laser for instance) or from the inside, by the application of a short rise time voltage pulse and the detection of the pressure wave by outside means (transducer bonded on the sample for instance). The authors would like to express their thanks to Fondation Langlois for the support provided in the realization of this work. Note remise le 17 octobre 1992, acceptée le 7 novembre 1992. REFERENCES [1] R. E. COLLINS, Appl. Phys Lett., 26, 1975, pp. 675-677. [2] A. S. DE REGGI, C. M. GUTTMAN, F. I. MOPSIK, G. T. DAVIS and M. G. BROADHURST, Phys. Rev. Lett., 40, 1978, pp. 413-416. [3] S. B. LANG and D. K. DAS-GUPTA, Ferroelectrics, 39, 1981, pp. 1249-1252. [4] P. LAURENCEAU, J. BALL, G. DREYFUS and J. LEWINER, C. R. Acad. Sci. Paris, 283, Séries B, 1976, pp. 135-138. [5] P. LAURENCEAU, G. DREYFUS and J. LEWINER, Phys. Rev. Lett., 38, 1977, pp. 46-49. [6] C. ALQUIE, G. DREYFUS and J. LEWINER, Phys. Rev. Lett., 47, 1981, pp. 1483-1487. [7] G. M. SESSLER, J. E. WEST and R. GERHARD-MULTHAUPT, Phys. Rev. Lett., 48, 1982, pp. 563-566. [8] E. EISENMENGER and M. HAARDT, Solid St. Comm., 41, 1982, pp. 917-920. [9] T. TAKADA and T. SAKAI, IEEE Trans. Elec. Ins., EI-18, 1983, pp. 619. [10] J. LEWINER, IEEE Trans. Elec. Ins., EI-21, 1986, p. 351-360. [11] J. LEWINER, Proceedings of the 23rd Symposium on Electrical Insulating Materials, Tokyo, October 1990, pp. 1-11. [12] H. R. ZELLER and W. R. SCHNEIDER, J. Appl. Phys., 56, 1984, pp. 455-459. [13] T. HIBMA and H. R. ZELLER, J. Appl. Phys., 59, 1986, pp. 1614-1620. [14] C. LAURENT, C. MAYOUX and S. NOEL, J. Appl. Phys., 58, 1985, pp. 4346-4353. Laboratoire d'Électricité Générale, Ecole Superieure de Physique et de Chimie Industrielles de la Ville de Paris,
1977, pp. 46-49. [6] C. ALQUIE, G. DREYFUS and J. LEWINER, Phys. Rev. Lett., 47, 1981, pp. 1483-1487. [7] G. M. SESSLER, J. E. WEST and R. GERHARD-MULTHAUPT, Phys. Rev. Lett., 48, 1982, pp. 563-566. [8] E. EISENMENGER and M. HAARDT, Solid St. Comm., 41, 1982, pp. 917-920. [9] T. TAKADA and T. SAKAI, IEEE Trans. Elec. Ins., EI-18, 1983, pp. 619. [10] J. LEWINER, IEEE Trans. Elec. Ins., EI-21, 1986, p. 351-360. [11] J. LEWINER, Proceedings of the 23rd Symposium on Electrical Insulating Materials, Tokyo, October 1990, pp. 1-11. [12] H. R. ZELLER and W. R. SCHNEIDER, J. Appl. Phys., 56, 1984, pp. 455-459. [13] T. HIBMA and H. R. ZELLER, J. Appl. Phys., 59, 1986, pp. 1614-1620. [14] C. LAURENT, C. MAYOUX and S. NOEL, J. Appl. Phys., 58, 1985, pp. 4346-4353. Laboratoire d'Électricité Générale, Ecole Superieure de Physique et de Chimie Industrielles de la Ville de Paris, 10, rue Vauquelin, 75005 Paris, France. Magnetism Development of crystalline and self-reversed magnetizations in pyrrhotite rocks; implications in palaeomagnetism and for the interpretation of magnetic anomalies Mansour BINA and Lucien DALY Abstract A thermomagnetic study performed on samples of pyrrhotite-bearing rocks showed that an oxygen pressure as weak as 300 Pa is sufficient at 500°C to start the transformation into magnetite. The quantity of magnetite produced is a function of the oxidation time during which the temperature is maintained. The remanent magnetization acquired by the magnetite during its formation in the ambient field is a chemical remanent magnetization (CRM). In the case of a partial oxidation, the close coexistence of two ferrimagnetic phases in a single grain may produce in the pyrrhotite an interactive thermoremanent magnetization (ITRM) reversed with respect to that of the magnetite (self reversal). If the initial pyrrhotite is antiferromagnetic and hexagonal, oxidation is necessary to obtain the transformation into the monoclinic variety, in which an ITRM can then appear. These secondary magnetizations potentially have considerable consequences in palaeomagnetism and could open up new prospects for the interpretation of some magnetic anomalies. Abridged English Version Because of their common occurrence in rocks, the iron sulphides have always