jeanflop/post-ocr-correction huggingface dataset:
c'est la relation qui doit être utilisée. Le courant est alors directement proportionnel à la charge d'espace rencontrée. Ces trois phases sont visibles sur la partie b de la figure 1. Chaque enregistrement est mis en mémoire et l'analyse des différents signaux peut être faite de deux manières : soit on échantillonne à une certaine profondeur dans l'échantillon, et l'on obtient l'évolution des variations (charges ou champ) à cette position en fonction du temps, soit on observe l'évolution globale qui se produit dans l'échantillon à des temps successifs. Ceci est visible sur la figure 2a qui montre l'évolution globale du
visible sur la figure 2a qui montre l'évolution globale du signal durant le premier quart d'un cycle correspondant à la montée de la tension appliquée aux bornes d'une plaque de polyéthylène de 1 mm d'épaisseur, soumise à une tension variable d'amplitude de crête 40 kV. On peut observer que le champ près de l'anode (pic d'entrée) et de la cathode (pic de sortie) suit l'évolution de la tension appliquée et qu'aucune charge d'espace n'est visible au cœur du matériau. Une telle charge d'espace pourrait être analysée de deux manières, soit en observant un signal pendant la propagation de l'onde de
en observant un signal pendant la propagation de l'onde de pression dans l'échantillon ou, comme cela est présenté sur la figure 2b, en portant le champ d'interface en fonction de 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. —
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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.
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
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
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.
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
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.
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.
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
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
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
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 been considered closely in magnetic prospecting for the interpretation of anomalies ([1], [2]). In spite of its low Curie temperature and thermal instability, the ferrimagnetic variety forming the monoclinic pyrrhotite, being a remanence carrier, has been studied from both the rock magnetism ([3], [4], [5]) and physico-chemical [6] points of view. A previous study [7] showed that magnetite was sometimes
view. A previous study [7] showed that magnetite was sometimes already present, at least in small amounts, in pyrrhotite-bearing rocks although generally characteristic of a reducing environment. The question of the minimal oxidizing conditions of temperature, oxygen pressure and time then arose. The transformation of pyrrhotite to magnetite by heating could appear as a potential source of magnetic anomalies. Further studies [8] showed that this transformation starts in an oxygen pressure as low as 300 Pa and that the remanent magnetization so induced in the ambient field was a crystalline one (CRM). It was then interesting to examine in more
one (CRM). It was then interesting to examine in more detail this CRM in considering the palaeomagnetic implications. New thermomagnetic experiments have therefore been performed on two different types of natural sample of pyrrhotite rocks. Note présentée par Jean-Louis LE MOUËL. 0764-4450/93/03160163 $ 2.00 © Académie des Sciences The first samples noted K 15 have been taken from a rock cored at a depth of about 3,060 m in the southern part of the Paris Basin near Sancerre (Deep Geology of France Program). To obtain a homogeneous oxidation, samples were made with the magnetic grains separated by magnet from crushed
made with the magnetic grains separated by magnet from crushed rock of about 0.5 mm size. An initial thermomagnetic analysis showed that a small fraction of magnetite is associated with the monoclinic pyrrhotite. The corresponding hysteresis cycle (fig. 1 a) is that of ferrimagnetism with low coercive force (4 mT). The second samples noted D 430,9 are coming from the tungsten mine of Salau situated in Hautes-Pyrénées, France. From initial susceptibility measurements, polished section observation and hysteresis cycle (fig. 2a), it was deduced that both some fine-grained hematite and principally hexagonal antiferromagnetic pyrrhotite grains are present. Thermomagnetic experiments have been
hexagonal antiferromagnetic pyrrhotite grains are present. Thermomagnetic experiments have been performed using a translation inductometer including a furnace and a field coil. Samples can be heated in air, in a moderate vacuum (1 Pa), or under a pressure of oxygen, argon or air controlled by means of an accurate leak valve system. Figures 3 show the heating and cooling thermomagnetic curves of the induced magnetization and the cooling remanent magnetization curve drawn after an oxidation at 500°C under 300 Pa of oxygen pressure for a given time. At each selected temperature, the magnetization has been measured under vacuum, in zero
temperature, the magnetization has been measured under vacuum, in zero and in 0.5 mT direct or inverse induction. Figure 3 a relates to Sancerre samples. An oxidation at 500°C for 10 min. in 0.5 mT produces a strong increase of the induced and remanent magnetizations (fig. 1 b). As the new crystallized grains of magnetite reach their blocking volume, a CRM is developing. To eliminate the possible [9], [10] superposed viscous remanent magnetization (VRM), the sample was maintained at 500°C in zero field for a time of the order of a minute which was calculated using the Néel theory [11],
a minute which was calculated using the Néel theory [11], [12]. The cooling remanent curve shows the development of a remarkable reversed magnetization. Self-reversal in pyrrhotite has already been observed [13] but without proof of the presence of magnetite. An interactive remanent magnetization (ITRM) seems to be induced in the local inverse field of the small grains of magnetite now crystallized in the large grains of pyrrhotite. In figure 3b, relating to the Salau mine samples, we can observe a similar behaviour concerning the ITRM but with an oxidation time reduced here to 2 min. An important new result concerns
reduced here to 2 min. An important new result concerns the transformation of the hexagonal pyrrhotite into its monoclinic variety. Figure 2b shows the combined effects of magnetite and monoclinic pyrrhotite. This transformation, from observations of thermomagnetic curves, needs oxidizing conditions. It was then easy to verify that during heating and cooling in vacuum, the susceptibility did not change. Figures 3c and d show the curves obtained for different oxidizing times. At a given temperature, the time seems to be a very sensitive parameter. In conclusion, the pyrrhotite and more generally the iron sulphides are able, according to heating and
generally the iron sulphides are able, according to heating and oxidizing conditions, to give different mineralogical phases [6], [14]. It is well known that pyrite gives pyrrhotite on moderate heating in reducing conditions. At 500°C, an oxygen pressure as low as 300 Pa is sufficient to start the magnetite production. In natural conditions, that is to say in the ambient terrestrial field, the developed remanence is a CRM on which a high-temperature VRM may be superposed. The transformation of the pyrrhotite into its monoclinic ferrimagnetic variety seems to be only possible in an oxidizing environment; this is a new fact
possible in an oxidizing environment; this is a new fact arising from our experiments. If the oxidation duration or the oxygen pressure increase, the transformation may develop hematite. If the pyrrhotite transformation is incomplete, an ITRM may be induced in the local field of the CRM of the small grains of magnetite crystallized in the large grains of pyrrhotite. In certain conditions, this magnetization is the most important. The conditions of these different transformations and the mechanisms of acquisition of the CRM and of the ITRM must be studied in detail because of the great number of geophysical implications. Concerning
detail because of the great number of geophysical implications. Concerning the interpretation of the magnetic anomalies, the increase of the induced magnetization due to the magnetite development and the existence of the competitive CRM and ITRM, the first one increasing and the second one reducing or even inverting the effect of the induced magnetization can no longer be ignored. Moreover, in the different palaeomagnetic applications of the remanent magnetism, these secondary magnetizations which could have appeared in a more or less recent geological past, will have to be taken into account. In particular, they seem to be possible complications in