Thermal Step Method Research Articles

Study of space charge in gallium nitride by the thermal step method

In this paper, we report the electric investigation of thin nitride gallium films by the capacitance voltage technique and the thermal step method (TSM). The C– V analysis at 1 MHz of Au/GaN diode reveals MOS behaviour and shows strong capacitance hysteresis. This may be due to the presence of trapped charge in this structure. The space charge dynamics is studied by thermal step method at different applied voltages. The TS currents are reverted from negative ones to positive ones above inversion threshold of +0.2 V. This change corresponds to charge modulation from accumulation to the inversion one, in good agreement with the C– V characteristics. The stored charge in this sample is related to the nature of gallium nitride and to the manufacturing processes. The results confirm the possibility to apply the TSM for the measurement of the space charge in the semiconductor materials.

Structural changes in epoxy resin polymer after heating and their influence on space charges

AbstractInfrared spectroscopy was used to characterize changes in physicochemical properties of an epoxy‐amine resin as a function of temperature. The thermal degradation of the system was studied by means of thermogravimetric analysis. The effect of heat on evolution of space charges was investigated by the thermal step method and the thermally stimulated depolarization currents method. It was found that heat induced ring opening of epoxide groups and promoted formation of carbonyl groups. Space charge density was found to increase with increasing temperature while dipolar relaxation decreased. Copyright © 2003 Society of Chemical Industry

A new numerical technique of electric field determination within dielectric materials plate and cable using the TSM method

In the frame of the thermal step method (TSM) used to characterize the space charge in dielectric materials, we present an original numerical technique for determining the electric field distribution in the bulk of a dielectric plate or cable. The first stage of our technique is the application of the finite element method (FEM) in order to find instantaneous distributions of temperature profiles. The calculation of the electric field distribution is based on these obtained profiles. During the stage of the determination, our mathematical treatment is based on the TSM charge q(t) in order to avoid numerical instabilities on the temperature derivatives. Therefore, we have transformed the integral equation for the TSM current I(t) used in previous works to an integral equation for the TSM charge q(t). The control of the instantaneous propagation of the thermal wave front, produced by submitting one face of the dielectric to a thermal step, and the application of Simpson's method to the integral equation of the TSM charge q(t), allow to determine the electric field distribution. The results of the electric field distribution are validated with those obtained in previous works. A good agreement and an improvement near the dielectric thickness boundaries are observed on these results. The numerical space charge density within the material is obtained by numerical derivation of the field according to Poisson's equation.

Stereochemical microstructure dependence of physical properties of poly(vinylchloride) (PVC). Principles and some recent correlations

AbstractAs described in earlier work and surveyed herein, stereospecific substitution reactions on PVC produce controlled changes in stereochemical microstructure. The corresponding changes in Tg are analyzed for each of the reactants utilized. A linear correlation is found between Tg and the ratio between the mmr tetrad reacting through its central mm triad and the mmmr pentad reacting through its mr triad plus the rrmr pentad reacting through its mr triad. The proposed explanation lies in the nature of the conformational changes which each of these reactions involves. Interestingly, the absolute value of the microstructure/Tg correlation is independent of the nucleophile, the nature of which can, however, determine the sign of that correlation. In addition, the evolution of both the amount and nature of the space charges that originate when applying an electrical field to the polymer films has been systematically studied by the thermal step method. The correlations obtained justify the mmr tetrad as being the main trap of either heterocharges or homocharges, depending upon whether mmr takes the GTTG TT or the GTGTTT conformation, respectively. This result agrees with the results of a tentative attempt with the stretched films. Some novel approaches toward understanding the physical behavior are proposed.

Influence of heat treatment on the space charge within an epoxy resin polymer material

AbstractThe influence of heat treatment on the appearance and the evolution of the space charge repartition within an epoxy resin polymer material is investigated. The space charge measurements were made using the thermal step method (TSM) and the thermally stimulated depolarization currents (TSDC) method. The results obtained show the behaviour of the space charge density after heat treatment and, in particular, it can be concluded that the charges are stabilized in deep levels.© 2001 Society of Chemical Industry

Thermal step method for space charge measurements under applied dc field

The thermal step method (TSM) is a nondestructive technique which measures the distribution of the electric field and space charge density across solid insulating materials. This work first reviews the principle and the basic equations of the TSM when used on a short circuited flat insulating structure (film or plate). An evolution of the method, which allows the determination of space charge distribution in an insulator while submitted to an external dc field, is then described. The fundamentals, the experimental setup and the validation of this technique on flat samples and on power cables are presented and discussed.

A new method for space charge measurements in dielectric films for power capacitors

Space charge measurements form a recognized test for the characterization of insulating materials. The present paper is concerned with the determination of space charge in dielectric films for capacitors. We develop a new measuring technique inspired by thermal step method (TSM). Its originality is the use of an alternative thermal excitation so that the technique can be called alternative thermal wave method (ATWM). The equipment employed to generate alternative thermal excitations (sine or triangle) is described. The upper limit of the efficient frequency range is 10 Hz. Signal processing is made easier and more accurate because of the periodicity of the measured phenomena.

Study of space charge accumulation in polyolefins submitted to ac stress

The aim of the present study was to follow space charge formation under ac field (50 Hz) in polyolefins. The thermal step method was used, and charge accumulation was observed in crosslinked polyethylene (XLPE), very low density polyethylene (VLDPE), isotactic polypropylene (PP) and a PE /PP copolymer, the temperature varying from 45 to 90/spl deg/C. It was found that the amount of charge accumulated under ac conditions may reach a maximum value at a given temperature, while the highest space charge was found in the polyolefin with the lowest crystallinity ratio. The conduction current characteristics of XLPE specimens show the existence of traps in the polymer at a level deeper than the potential barrier electrode/dielectric, explaining the accumulation of injected charge in the material. The injection and conduction mechanisms observed were interpreted by Schottky and Poole-Frenkel processes and used to explain the maximum accumulation of charge occurring at certain temperatures.

Effect of stretching on space charge behaviour of poly(vinyl chloride) as analysed by the thermal step method and the thermally stimulated depolarization currents technique

Effect of stretching on space charge behaviour of poly(vinyl chloride) as analysed by the thermal step method and the thermally stimulated depolarization currents technique

Space charge measurements by the thermal step method in epoxidic polymer materials

To improve the performance of insulators, space charge measurements and localisation are of fundamental importance. For this purpose we have utilised the thermal step method (TSM) which allows measurement of the space charge density within the material. The object of this study is to characterise the electric state of epoxidic polymer material used in electrical insulation, and to study the influence of different factors involving the appearance and the evolution of space charge. The influence of poling temperature as well as the manufacturing process of the material on the remaining space charge distribution is also studied. Measurements show that the space charge behaviour was influenced by two factors: the poling temperature and the manufacturing process. © 2000 Society of Chemical Industry

Effect of stretching on space charge behaviour of poly(vinyl chloride) as analysed by the thermal step method and the thermally stimulated depolarization currents technique

The space charge behaviour of poly(vinyl chloride) (PVC) films after stretching to various degrees has been studied by the thermal step (TS) and the thermally stimulated discharge current (TSDC) methods, the aim being to show the influence of the two likely conformations GTTG TT and GTGTTT at the mmr local configuration terminal of any isotactic sequence on the space charge density and nature which appear when applying an electric field. It was demonstrated in previous work that stretching causes the conformational change GTTG TT ⇒ GTGTTT to occur irreversibly, so that the relative proportions of both configurations may be controlled by stretching. TS and TSDC results show that the polar orientation decreases as the degree of stretching increases. The results, when analysed in the light of previous findings, strongly suggest that GTTG - TT and GTGTTT conformations are sensitive to polarization changes and to injection charges from the electrode respectively.

Study of poly(methyl methacrylate) relaxations by thermally stimulated depolarization currents and the thermal step method

A combined study of β′ and α relaxations in poly(methyl methacrylate) (PMMA) by thermally stimulated depolarization currents and thermal step methods has been made. The use of null width polarization windows to form the electrets allows us to isolate the β′ peak. The charge distribution profiles allow us to assign a polar orientational mechanism for the β′ relaxation and in the case of the α peak a background polar mechanism over which a temperature-dependent injection and a conduction processes are overlapped. The charge distribution profiles observed by the thermal step method indicate that the increase of the conductivity observed in this material at temperatures just below the glass transition is mainly due to the positive carriers.

Exploration of the space charge behaviour of polyethylene using measurements of thermally stimulated discharge currents coupled with the determination of space charge distributions by the thermal step method in hydrogenated poly(vinyl chloride)

By using the thermally stimulated discharge currents (TSDC) technique and coupling it with the thermal step (TS) method, which allows accurate measurement of the space charge distribution (an outstanding electrical property), both the relaxation and the relative contribution of the intrinsic polarization charges and the extrinsic trapped charges have been analysed for a series of poly(vinyl chloride) (PVC) materials after partial hydrogenation as a precursor structure of polyethylene (PE). The α relaxation related to the release of frozen-in dipole orientation, and the ρ relaxation which is due to the trapping of extrinsic charges have been found to vary, both in intensity and in temperature, with the percentage of ethylene units. On the other hand, the TS measurement of the space charge distribution prior to and after destroying the polarization charges according to the TSDC behaviour, allows one to correlate, as a first approximation, those types of space charge with the molecular microstructure, of either PVC or PE parts. Even if further work remains to be done, the results, as discussed in the light of earlier work on tacticity-driven microstructure–PVC property relationships and on space charge distribution of PVC after hydrogenation, studied by the TS method, provide an original although still tentative interpretation of the space charge behaviour of PE.

Experimental results on the correlation between cross-linking, polarization and space charge in PMMA

The influence of cross-linking upon polarization and space charge is investigated. PMMA samples of different cross-linking degrees were prepared. The samples were poled under several DC fields and temperatures. Space-charge measurements were made by the thermal-step method. The ‘cleaning’ technique was used to separate dipolar phenomena and injected charge. The results show that cross-linking affects the state of polarization of the samples and could favour space-charge accumulation. © 1999 Society of Chemical Industry.

The thermal step technique: an advanced method for studying the properties and testing the quality of polymers

It is well known that the physical properties of a polymer alter with time. This phenomenon—ageing—is strongly related to the external factors acting upon the polymer and to the manufacturing process. An appropriate process could prevent the premature and undesired changes in the characteristics of a polymeric material. For this reason, considerable efforts are made to understand the mechanism of polymer degradation. Past studies have shown that the existence of electrical charges in the bulk of polymers can affect their properties significantly. These charges—usually called space charges or space charge—were first observed in polymers used for electrical insulation and therefore submitted to high electric fields. Some recent studies, particularly those presented in this paper, proved that space charge exists in almost every polymer. It has been shown that space charge can be present within the material from the very manufacturing process, without previous submission to stress. The increase of the amount of the space charge seems strongly related to the deterioration of the physical properties of polymers. Space charge measurements can therefore be considered as a test of quality for polymers. This paper is dedicated to a method for measuring space charge: the Thermal Step Method (TSM). The physical basis of the technique, as well as its applications, are presented. Results obtained on various polymers are presented and discussed. The results show that the features of this technique, particularly its high sensitivity, make it an appropriate tool for the characterization of a wide variety of materials. The TSM could also be associated with other physical, chemical or physico-chemical techniques. © 1998 SCI.

The thermal step technique: an advanced method for studying the properties and testing the quality of polymers

It is well known that the physical properties of a polymer alter with time. This phenomenon—ageing—is strongly related to the external factors acting upon the polymer and to the manufacturing process. An appropriate process could prevent the premature and undesired changes in the characteristics of a polymeric material. For this reason, considerable efforts are made to understand the mechanism of polymer degradation. Past studies have shown that the existence of electrical charges in the bulk of polymers can affect their properties significantly. These charges—usually called space charges or space charge—were first observed in polymers used for electrical insulation and therefore submitted to high electric fields. Some recent studies, particularly those presented in this paper, proved that space charge exists in almost every polymer. It has been shown that space charge can be present within the material from the very manufacturing process, without previous submission to stress. The increase of the amount of the space charge seems strongly related to the deterioration of the physical properties of polymers. Space charge measurements can therefore be considered as a test of quality for polymers. This paper is dedicated to a method for measuring space charge: the Thermal Step Method (TSM). The physical basis of the technique, as well as its applications, are presented. Results obtained on various polymers are presented and discussed. The results show that the features of this technique, particularly its high sensitivity, make it an appropriate tool for the characterization of a wide variety of materials. The TSM could also be associated with other physical, chemical or physico-chemical techniques. © 1998 SCI.

Molecular microstructure dependence of space charges in polymers. Study by the thermal step method of hydrogenated poly(vinyl chloride) as a precursor of polyethylene

The space charge distribution, an important electrical property, has been studied by means of the thermal step (TS) method for some poly(vinyl chloride) (PVC) samples containing polyethylene (PE) sequences, as the result of hydrogenation reaction of PVC with H 4LiAl to conversions ranging from 1 to 12%. Both the preparation and the characterization of the samples by 13C n.m.r. are to be published elsewhere. The aim of the present work is to investigate the effect of the changes in molecular microstructure, whether relevant to the tacticity of PVC or to that occurring when CH 2 groups are substituted for CHCl groups, on the nature and distribution of space charges which are formed upon applying an electrical field. The contribution of the PE structure to the overall space charge distribution could be evidenced by comparing the results obtained to those extensively published on tacticity-microstructure/space charge relationships for PVC. Since hydrogenated PVC may be considered as a precursor of PE, the results provide novel information about the space charge behaviour of the latter polymer.

Influence of the tacticity dependent microstructure on the space charge distribution of polypropylene. Part 1. A first tentative approach

Space charge trapping has been studied by the thermal step (TS) method for four samples of polypropylene (PP) of different tacticities, prepared by selective extractions from an additive-free commercial polymer with mixtures of a non-polar/polar pair of solvents of various compositions. The experimental currents derived from the application of the TS method to the samples after submitting them to an electrical field are proved to depend both on the isotactic content and on the electrode. In particular, the current intensity increases markedly as the isotactic content decreases, without diverging from non-bernoullian isotacticity. Similarly, the space charge distribution obtained through the deconvolution of the measured current curves usually utilized in the TS method, exhibits a decrease of the whole space charge in the material, as the isotactic content increases. It is of the type obtained when heterocharges and homocharges accumulate in the anode and the cathode, respectively. The results are discussed on the basis of the tacticity based molecular microstructure, as defined in earlier work, in particular the frequency of occurrence of isotactic mmr tetrads associated with isotactic sequences. In addition, they are the first indication that the space charge/tacticity-microstructure relationships found for poly(vinylchloride) in earlier work, apply to PP. © 1998 SCI.

Influence of the tacticity dependent microstructure on the space charge distribution of polypropylene. Part 1. A first tentative approach

Space charge trapping has been studied by the thermal step (TS) method for four samples of polypropylene (PP) of different tacticities, prepared by selective extractions from an additive-free commercial polymer with mixtures of a non-polar/polar pair of solvents of various compositions. The experimental currents derived from the application of the TS method to the samples after submitting them to an electrical field are proved to depend both on the isotactic content and on the electrode. In particular, the current intensity increases markedly as the isotactic content decreases, without diverging from non-bernoullian isotacticity. Similarly, the space charge distribution obtained through the deconvolution of the measured current curves usually utilized in the TS method, exhibits a decrease of the whole space charge in the material, as the isotactic content increases. It is of the type obtained when heterocharges and homocharges accumulate in the anode and the cathode, respectively. The results are discussed on the basis of the tacticity based molecular microstructure, as defined in earlier work, in particular the frequency of occurrence of isotactic mmr tetrads associated with isotactic sequences. In addition, they are the first indication that the space charge/tacticity-microstructure relationships found for poly(vinylchloride)in earlier work.

On the molecular microstructure dependence of space charge trapping in polymers

The space charges trapped after applying a strong electrical field have been measured using the thermal step (TS) method for a series of poly(vinylchloride) (PVC) samples differing in their tacticity-induced molecular microstructure. This has been controlled chemically (stereospecific substitution reactions) or physically (stretching of films at temperatures near Tg). From some of the results obtained, most of which are still to be published, the space charge quantity and distribution are inferred to depend strongly on the content of the isotactic tetrad mmr and on the two conformations, whether GTTG-TT or GTGTTT, which are probably in it. © 1998 SCI.