Thermal Step Method Research Articles

Experimental validation of the use of thermal stimuli for space charge measurements in dielectric liquids: Application to electrical double layers

Electric charges in liquids are at the basis of numerous applications and may drive industrial hazards. The Thermal Step Method is applied to liquid/solid interfaces to validate the use of thermal stimuli techniques for space charge measurements in dielectric liquids. Experimental results obtained for different electrical double layers are presented and discussed. They show that thermal convection has no significant contribution to the signal compared to the response of the charges from the solid/liquid interface. The measured signals are in accordance with the classical theory of thermal methods in solids and are consistent with the behavior expected from numerical simulations.

Review of space charge measurements in high voltage DC extruded cables by the thermal step method

More than ever, controlling the amount of space charge that accumulates in high voltage DC (HVDC) extruded cables under electrical stress and a thermal gradient is a major challenge to ensure a good level of reliability of the cable and therefore a good availability of energy [1]. It is now clear that, although studies on space charge are absolutely necessary to understand the charge accumulation and relaxation phenomena, direct measurements on power cables are required to analyze and anticipate the local electric field. Thus, measurement of the electric field due to the presence of space charge within the insulation is essential for an optimal design of power cables [2].

Contributed Review: Review of thermal methods for space charge measurement.

The space charge accumulation phenomenon has garnered great interest over the last two decades because of the increased use of direct current in high voltage electrical systems. In this context, a significant relevance has been achieved by the thermal methods, used for solid dielectrics. This paper presents a review of this non-destructive measurement system used for the measurement of space charge. The thermal pulse method, the thermal step method, and the laser intensity modulation method are described. For each configuration, the principle of operation, the thicknesses analyzed, and the spatial resolution are described, reporting also the main related applications.

Space charge behavior of silicone rubber nanocomposites with thermal step method

In this paper, effects of the space charge behavior of silicone rubber (SiR) nanocomposites with SiO2, Al2O3, and MgO nanoparticles on charge transfer were clarified. To investigate the effects of SiO2, Al2O3, and MgO nanoparticles on space charge profiles, the space charge behavior was measured using a thermal step method system under a dc electric field of 2 to 4 kV/mm. In the case of SiR nanocomposites, a negative packet like charge injection with a peak charge density of more than several hundred C/m3 was measured. The effects of space charge behavior were discussed with the response current associated with the conductivity and permittivity affected by a thermal process. The obtained results showed that the relaxation processes had discrete energy differences with distinct peaks in the response currents of space charge affected by nanoparticles.

Study of electrical properties of polymethylmethacrylate treated in aqueous and saline environments

Polymethylmethacrylate (PMMA) is one of the most studied polymers in the last years due to its outstanding mechanical and physical-chemical properties. This material is used in various applications in which it can be subjected to different constraints. In this work, PMMA samples were submitted to thermal treatment (at 100 °C for 20 min) in aqueous and saline environments. We studied the hydrothermal effect as well as the saline environment effect on electrical properties of PMMA by means of thermal step method (TSM) and thermally stimulated depolarization current (TSDC) technique. It was found that polarization was significantly influenced by the hydrothermal treatment realized particularly in a saline medium. α - and ρ -relaxations were observed in all TSDC spectra. For each treated sample, ρ -relaxation peak is more intensive than the dipolar one and the corresponding releasing charge quantity increases.

Effect of additives on the performance of cross-linked polyethylene subjected to long term single and periodically reversed polarity DC voltage

The performance of cross-linked polyethylene with and without tree-retarding additives, aged with single and reversed polarity DC voltage was investigated. Under single polarity field of a 50 kV/mm, cross-linked polyethylene without the tree retardant (XLPE) showed significantly longer time to breakdown than the material with a tree retardant (TR-XLPE). This is attributed to the difference in how the two materials accumulate and retain space charge. In the case of polarity reversals, TR-XLPE showed better performance than XLPE. This seeming contradiction was studied by measuring the dynamics of space charge evolution with time. The Thermal Step Method (TSM) of space charge measurements was used. It showed that after the polarity reversal the TSM current rapidly inversed in the TR-XLPE but not in the XLPE. This implies more rapid charge dissipation in TR-XLPE. The retained charges cause the electric field enhancement in the material when the polarity is reversed. Thus, after each polarity reversal XLPE was subjected to higher local electric stress for a longer time and thus broke down sooner than TR-XLPE. For both materials the space charge decreased faster when the polarity was changed from negative to positive than vice versa. This could be explained by more efficient electron than holes injection. Since the control of the power flow in DC networks requires reversals of voltage polarity, their detrimental effect on XLPE-insulated equipment, such as cables, cannot be avoided. The effect could be mitigated by the application of carefully designed additives. However, as the present study indicates, additives that improve insulation performance under polarity reversals can be harmful under single polarity voltage.

Dielectric study of low glass transition temperature cycloaliphatic UV-curable epoxy networks

This work focuses on the dielectric properties of a UV-cured epoxy network with a low glass transition temperature obtained through photoinduced copolymerization of a cycloaliphatic diepoxy monomer, 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexanecarboxylate (CE), with 1,6-hexanediol diglycidyl ether (HDGE). The crosslinking density of the system has been tuned using different amounts of the low glass transition temperature HDGE comonomer (up to 60 wt.% HDGE). The structural morphology and space charge obtained with the Thermal Step Method were studied following various thermal treatments of the samples. Both the thermal properties and the dielectric responses (dielectric constant) for the UV-cured HDGE/CE systems turned out to be dependent on the structural morphology and composition of the polymer network. In particular, two relaxation modes, γ, β and α were observed, attributed to local molecular motions. Lastly, the potential effect of moisture and/or ions and the effect of space charge on the copolymers were also investigated.

A new calculation approach for dielectric space charge characterization from nano to macro-scales by the thermal step method

In this paper, a new calculation approach is developed and applied, for the first time, to different dielectric scales using the thermal step method (TSM). The thermal aspect of the approach concerns insulating materials from nano to macro-scales. The theoretical approach is based on the ballistic-diffuse transport taking into account the relaxation time of heat phonons and their effects on the trapped space charges. These effects are revealed on the temporal evolution of the induced image charges that constitute the electrical signal of the thermal step method. The temperature distributions and the TSM induced image charges are obtained by applying the numerical finite element method, the Newmark direct integration, the mean value theorem and the composite Simpson approximation. In a first step, the results of the temperature distributions are validated with those obtained by Boltzmann and Chen models for nano-polyethylene sample. In a second step, our approach is validated with TSM previous works using the Fourier's model for space charge characterizations in the micro and macro-dielectrics. The validated results of the TSM induced image charges for micro and macro-scales show good agreements, and prove that our approach is a consistent tool to be applied in space charge characterizations for different dielectric scales.

Analysis of Data Obtained Using the Thermal-Step Method on a MOS Structure—An Electrostatic Approach

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The thermal-step method (TSM) is a nondestructive technique for measuring electric charge, based on the acquisition of a capacitive current appearing when a thermal step of low amplitude is applied to an insulating structure. The electrostatic modeling of a metal–oxide–semiconductor (MOS) structure while crossed by a thermal wave is addressed in this paper, in view of analyzing the data obtained by TSM in conjunction with capacitance–voltage measurements. Applications to MOS samples with 120-nm-thick oxides are presented. </para>

Study of charge relaxations after thermal aging in poly (methyl methacrylate)

Effects of thermal aging on physico–chemical and electrical properties of polymethyl methacrylate (PMMA) are reported in this paper. PMMA samples are submitted to successive heat–cooling cycles (Tmin=20 ∘C and Tmax=90 ∘C) in the ambient air, each cycle lasts 12h. Different techniques are thus employed to investigate structural modifications, dielectric relaxations and conduction processes. These are the Fourier Transform Infrared (FTIR) spectroscopy, thermal step method (TSM), thermally stimulated depolarization currents (TSDC), dielectric spectroscopy (DS) and current–voltage technique. Results are discussed and a relationship between structural modifications and charge relaxations was emphasized. We demonstrated that thermal aging favors oxidation phenomenon. Three distinguishable charge relaxations (β, α and ρ) have been highlighted by TSDC.

Electric field and space charge measurements in thick power cable isulation

This paper describes the evolution of electric field and space charge in high voltage AC and DC cable polymeric insulation, followed using the thermal step method.

Effect of a few repeating stereosequences on electrical space charge behavior of poly(methyl methacrylate)

AbstractThree films of poly(methyl methacrylate) (PMMA) (samples X, Y, and Z) were prepared by compression molding of purified polymers as polymers having different tacticities. They were exhaustively characterized by 1H NMR spectroscopy so as to be able to evaluate the differences in content of the repeating stereosequences:…mmrm… (termini of isotactic sequences); ….rrrmr (breaking off syndiotactic sequences; pure heterotactic …mrmr… moistures; and atactic parts. By applying the so‐called thermal step method, the behavior in space charge showed a priority charge injection from both electrodes and some polarization at the cathode. Some strong differences between the samples were evidenced and they accurately agree with the variations in the above repeating stereosequences for the samples. The results allow to define every stereosequence as specific trap of the distinct sorts of space charge, namely injection from the anode or the cathode and polarization. These attributions are common with those proposed for poly(vinyl chloride) (PVC) and poly(propylene) (PP) in earlier work. Conversely, whether the priority charge is of injection or polarization type, depends markedly on the type of polymer. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 633–639, 2009

Determination of Electric Field and Space Charge in the Insulation of Power Cables With the Thermal Step Method and a New Mathematical Processing

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper proposes a technique for determining the distributions of the electric field and space charge in the insulation of power cables by using the data acquired with the thermal step method (TSM). The TSM consists of applying a low-temperature step to a short-circuited or dc-energized cable and of acquiring a transient capacitive current. The processing technique described in this paper is based on a series decomposition of the electric field, the coefficients of the series being identified via the measured current. The accuracy and the stability of the calculation method are evaluated by simulations performed using various distributions of the electric field and different noise levels. An application of the technique to space charge measurements in a dc conditioned power cable is then presented. </para>

Computation of the Electric Field in Cable Insulation in the Presence of Water Trees and Space Charge

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The presence of space charge changes locally the electric field distribution in power cable insulation and may play an important role in tree development, thus accelerating the dielectric breakdown. This paper is concerned with the computation of the electric field in polyethylene-insulated power cables affected by water trees which grow from the following: 1) the inner semiconducting layer; 2) the outer semiconducting layer; and 3) the inner and outer semiconducting layers, taking into account the space charge corresponding to the ions present in the treeing area. Space charge in plane samples where trees have been developed in an accelerated manner was estimated using the thermal step method. Average charge values given by space charge measurements were then used for the electric field computation in cable insulation with continuous or/and individual water trees. For the calculation of the electric field, an analytical and a numerical method have been used. This paper shows that the space charge changes the electric field distribution inside and outside the trees (the field increases in some areas and decreases in others) and that the field variations depend on the magnitude and on the polarity of the space charge, as well as on the dimensions of the water trees developed in the cable insulation. The obtained results show that, in the presence of water trees and space charge, the initiation of electric trees is more probable in the case of individual water trees than in the case of continuous water trees. </para>

Measurements and modelling of thermal step current in GaN/SiO 2/Si structure

In this work, we report results of measurement of space charge in GaN/SiO 2/Si structure by means of thermal step method (TSM) and capacitance–voltage ( C– V). TSM is a non destructive method for quantifying and localizing the electric charges in insulating materials. Its principle consists to apply a low thermal step to a short-circuited or dc-biased sample and to record a current response, which depends on the charge present in the device. The C– V characteristics show an almost metal-oxide-semiconductor (MOS) behaviour and retention of charges after forward bias sweeping. The space charge dynamics in the structure are followed under low applied dc voltages. Results show a significant inversion of the TSM currents above applied voltage of 200 mV. A theoretical model is then presented in order to estimate the amount of the trapped charges and to interpret the variation of the TS currents as a function of the applied gate voltages.

Application of the thermal step method to space charge measurements in inhomogeneous solid insulating structures: A theoretical approach

The thermal step method is a nondestructive technique for determining electric charge distribution across solid insulating structures. It consists in measuring and analyzing a transient capacitive current due to the redistribution of influence charges when the sample is crossed by a thermal wave. This work concerns the application of the technique to inhomogeneous insulating structures. A general equation of the thermal step current appearing in such a sample is established. It is shown that this expression is close to the one corresponding to a homogeneous sample and allows using similar techniques for calculating electric field and charge distribution.

Study of the thermal step signal of GaN grown on porous silicon substrate by MOVPE

In this work, we report the electric investigation of thin gallium nitride films by the thermal step method (TSM). The space charge dynamics was studied using the thermal step method with applied negative step (ΔT = −30 °C). The experimental TSM current indicates the presence of two peaks indicating the presence of two depletion widths in the silicon substrate and the GaN epilayer. The comparison between the theoretical and the measured TSM current indicates a good agreement for times less than 2 s. The divergence for times above 2 s is probably due to a thermoelectric current similar to Seebeck effect, due to the majority charge carriers’ contribution.

Study of thermal aging effect on space charge in poly(methyl methacrylate)

Charges evolution in poly(methyl methacrylate) (PMMA) samples under thermal aging effect has been studied by means of two complementary techniques, thermal step method (TSM) and thermally stimulated depolarization current (TSDC). For the first method, measurements reveal that injected charges, whose quantity is found depending on the number of applied temperature cycles, remain close to the surface sample. TSDC measurements have been carried out for different temperatures ranging from 25 °C to 140 °C. Three distinguishable dipolar relaxations ( β 1 , β 2 and α) have been highlighted. In the same way, the presence of polarization and injected charges has been confirmed. In support of electric characterization, X-ray reflectometry has been used. The obtained results equally emphasized the thermal aging effect on the material.

On the stereomicrostructure-based mechanisms of physical properties of poly(methyl methacrylate): 1. space charge behaviour and physical ageing

Three samples of poly(methyl methacrylate) (PMMA) were characterized after careful purification. Their composition in terms of some repeating stereosequences, especially mmr termini of isotactic sequences, was determined. Their space charge behaviour was analysed using the thermal step (TS) method. A correlation between the electrical current intensity, as given by the TS method, and the stereochemical microstructure content in each sample was evidenced. The results are explained in the light of previous data on the specific microstructures present in the samples at different contents. The physical ageing of the samples was studied, quantitatively, through differential scanning calorimetry measurements prior to and after equally performed thermal treatments of the samples. The corresponding values of the loss of enthalpy agree with the content of mmr tetrad termini of isotactic sequences. The results are analysed on the basis of the differences in stereochemical composition of the samples. The results give further evidence of the role of some definite stereosequences in physical properties as proposed in earlier works on poly(vinyl chloride) and polypropylene. Copyright © 2006 Society of Chemical Industry

On the use of the thermal step method as a tool for study of space charge in semiconductor gallium nitride: GaN

Thermal step method (TSM) is a non destructive technique which measures the distribution of electric field and the space charge density across a solid insulators. In this paper, we report the experimental results obtained by the Thermal step method (TSM) and The capacitive voltage (C-V) characterisations of the gallium nitride films grown by the Metal Organic Vapour Phase Epitaxy on silicon substrate. The C-V curves shows a MOS structure behaviours, this was attributed to the existence of an SiO2 insulator between silicon and GaN. Experimental results were in good accordance with TSM measurements. These where performed under different applied bias voltage, remnant saturation current indicate a sign inversion above -75 mV. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)