Space-charge and dipole-polarization are (traditionally) undesirable in electrical-
Figure 8: Typical structure of an extruded electric power cable After Ref [44].
Electrical insulators are materials that inhibit the free flow of electric charge, crucial for telecommunication and power cable applications A typical high voltage power cable features a sophisticated design that meets economic and technical standards, including electrical, mechanical, and thermal requirements This design includes a metal conductor core, often made of aluminum or copper, surrounded by several layers: an inner semiconductor screen, a primary extruded insulation layer, and an outer semiconductor screen, along with additional sheaths for moisture protection and mechanical support Significant research and development efforts are focused on enhancing semiconductor and main insulation materials to accommodate the rising voltage demands of high voltage power cables, as global electricity generation continues to increase.
By 2040, global electricity consumption is projected to reach 37 trillion kWh, up from 22 trillion kWh in 2012, driven by increasing demand and urgent climate change concerns This surge necessitates the development of a global electricity network to optimize resource utilization, enhance renewable energy adoption, reduce reliance on fossil fuels, and lower CO2 and greenhouse gas emissions To facilitate this, long-distance high-voltage (HV) transmission lines, particularly high-voltage direct current (HVDC) systems, are essential for connecting intercontinental and offshore renewable energy farms.
By 2030, HVDC power cable rated voltage of 1 MV should be in needed [58] from current highest rated voltage of 640 kV [59].
The field of electrical insulation research focuses on enhancing the design of telecommunication and electric power cables, alongside the development of improved insulating materials Key milestones in this evolution include the cultivation of gutta-percha by John Tradescant the Younger in 1656, following his travels to the Far East This material, derived from the "Mazer Wood" tree, remained largely overlooked until its reintroduction to the West by William Montgomerie in 1834, marking a significant advancement in insulation technology.
The history of electrical insulation research is marked by several key milestones In 1847, the first telegraph cable was manufactured, followed by the introduction of the first DC transmission line using jute and bitumen insulation in 1882, coinciding with Thomas Edison's commercial DC lighting system in New York City The year 1890 signified the conclusion of the AC/DC current war, paving the way for the development of AC power systems The introduction of synthetic insulators began in 1925 with alkyd, followed by polyethylene in 1933, PVC in 1936, epoxy and polyester resin in 1945, and XLPE in 1963 The first polymer-extruded power cable with PVC was laid in Germany in 1944, and in 1998, the first HVDC power cable with extruded XLPE insulation was commissioned Since then, HVDC cables with XLPE insulation have proven effective for transmitting large amounts of electrical power over long distances, particularly for renewable energy integration and connecting discrete electricity networks.
When a dielectric is subjected to a high electric field, various electric charge processes occur, particularly at metal/insulator interfaces where electrons can be injected or extracted, forming charge traps or leading to charge annihilation Similar interactions can happen with holes at these interfaces Within the insulator's bulk, charge carriers can diffuse and drift, becoming trapped, while interfacial polarization and molecular dipole orientation contribute to electric polarization Additionally, charge-carrier generation and recombination can occur through light absorption or emission These processes may result in space-charge accumulation, which distorts the internal electric field, enhances local fields, accelerates aging, and reduces the lifespan of cable insulation systems.
In polymer dielectrics, the accumulation of space charge is influenced by both the intensity and frequency of the applied electric field As illustrated in Figure 11, the total accumulated space charge in XLPE decreases consistently with increasing frequency, with minimal charge accumulation observed at 50 Hz, where charge patterns closely resemble those under a DC electric field Additionally, the type of dielectric material significantly affects charge accumulation; for polymers not optimized for DC applications, substantial charge buildup can occur, as shown in Figure 12b A key feature of polymer dielectrics used in high-voltage power cables is "stress inversion," where the electric field gradient within the dielectric reverses after a certain duration of field application, as depicted in Figure 12a, resulting in a shift of maximum electric stress from the inner to the outer semiconductor layer.
Figure 10: Electric charge processes in an insulator or at an insulator-electrode interface Adapted from Ref [73] and by courtesy of Prof Gerhard.
Figure 11: Total space charge accummulation in the bulk of XLPE versus frequency of the applied electric field After Ref [74].
(a) DC XLPE cable, under DC voltage (b) AC XLPE cable, under DC voltage
Figure 12: (a) Electric field distribution in DC XLPE cable under DC voltage and (b) Electric field distri- bution in AC XLPE cable under DC voltage After Ref [75].
In HVAC power cable applications, the impact of alternating electric fields (50/60 Hz) reduces large space charge accumulation in insulation, yet charge injection and extraction at semiconductor/insulation interfaces can lead to mechanical fatigue and accelerated aging of the insulation structure Conversely, for HVDC power cables, space charge accumulation poses significant challenges that must be addressed in the development of new cable systems It is advisable for cable manufacturers to conduct space charge accumulation tests on full-sized cable models, as highlighted in CIGRE technical brochure TB 496 and IEEE Standard 1732-2017 The Pressure Wave Propagation (PWP) method is effective for measuring electric field distribution in dielectrics and is suitable for full-sized cable specimens, as specified in IEC technical report IEC TR 62836:2013 Additionally, Thermal-Pulse Tomography (TPT) shows promise, although its application has been limited to signal cables thus far.
Organization of the thesis
Electrets are unique dielectrics characterized by their quasi-permanent electric charge and dipoles, akin to magnets in the electric realm Their significance has evolved from a scientific curiosity to essential applications in both science and technology, particularly following the discovery of poly(tetrafluoroethylene) and the development of electret microphones The advancement of electret research is closely linked to the pursuit of new materials that enhance charge and dipole retention To function effectively, electrets must undergo a charging or poling process, which aligns electric charges or dipoles within their structure Understanding the spatial distribution of electric charge and dipole polarization post-deposition is vital for enhancing the stability of these dielectrics.
In electrical insulation applications, understanding space-charge accumulation and polarization profiling is crucial Traditionally, the presence of space-charge and significant dipole polarization in insulating dielectrics is viewed as detrimental, leading to dielectric loss and potential internal electric field distortion, which can shorten the lifespan of these materials However, advancements in high-voltage DC transmission and energy storage capacitors have shifted this perspective There is a crossover between the fields of electrets and electric insulation; while electrets require quasi-permanently trapped electric charge and large remanent dipole polarization, stable charge trapping in electrical insulation reduces charge transport and conductivity Controlled charge trapping not only prevents further charge accumulation but also serves to enhance field grading in insulating dielectrics, while significant dipole polarization can be effectively harnessed for energy storage applications.
Various techniques for measuring charge and dipole polarization in dielectrics can be categorized into destructive and nondestructive methods Destructive methods, such as sectioning, solvent diffusion, and compensation techniques, involve altering the dielectric material and thus hinder the ability to track charge evolution over time In contrast, since the late 1970s, nondestructive methods have emerged, primarily classified into acoustical methods, which use pressure waves as probes, and thermal methods, which rely on heat wave diffusion Comprehensive reviews, including Table 1 in Ref [9], detail these nondestructive techniques, highlighting their detection principles, resolutions, and sample thicknesses Recent advancements, particularly in acoustical methods like Electro-Acoustic Reflectometry (EAR), have enhanced spatial resolution to the submicrometer range, marking significant progress in the field.
This thesis explores the use of Piezoelectrically-generated Pressure Steps (PPSs) as a nondestructive technique to investigate electric-charge and dipole polarization distributions in various thin film polymer-based materials, including polypropylene (PP), low-density polyethylene/magnesium oxide (LDPE/MgO) nanocomposites, and poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) copolymer Surface-treated PP films with phosphoric acid exemplify 2-dimensional nanocomposites, while LDPE/MgO represents 3-dimensional nanocomposites with dispersed MgO nanoparticles Findings indicate that nanoparticles enhance charge trapping capacity, thereby improving the materials' suitability for electret and electrical insulation applications Additionally, the study highlights that the remanent spatial polarization distribution in P(VDF-TrFE) copolymers is significantly influenced by the poling method and its parameters, with homogeneous polarization poling and hysteresis cyclical poling being investigated to assess remanent polarization growth and spatial distribution.
This thesis explores the significant roles of space charge and dipole polarization in electrets and electrical insulation It discusses the space-charge profiling method's potential to enhance electret materials and optimize poling methods while mitigating undesirable physical processes in insulation systems A review of both destructive and nondestructive charge profiling techniques is included, along with details on the scope of the work, the Piezoelectrically-generated Pressure Steps (PPS) measurement method, and the polymer materials investigated The PPSs are utilized for charge and dipole polarization profiling, with a focus on their working principles and unique characteristics during signal acquisition Subsequent sections present findings from four peer-reviewed publications on various polymer materials, examining charge storage and transport in phosphoric acid-treated polypropylene, low-density polyethylene/magnesium oxide nanocomposites, and the spatial polarization profiling of poly(vinylidene fluoride-co-trifluoroethylene) using the Bauer cyclical poling method.
• Section 3 is based on Publication I: “Piezoelectrically-generated Pressure Steps (PPSs) for studying charge distributions on corona-charged polypropylene (PP) films”
• Section 4 is based on Publication II: “Depth profile and transport of positive and negative charge in surface (2-D) and bulk (3-D) nanocomposite film”
• Section 5 is based on Publication III: “LDPE/MgO nanocomposite dielectrics for electrical- insulation and ferroelectret-transducer applications”
• Section 6 is based on Publication IV: “Non-uniform polarization profiles in PVDF copolymers after cyclical poling”
General conclusions and future work will conclude the thesis in Section 7.
2 Probing charges and dipoles in polymer dielectrics with Piezo- electrically-generated Pressure Steps (PPSs) - The PPS method and its features
The Piezoelectrically-generated Pressure Steps (PPSs) method, developed by Professor Eisenmenger and colleagues in 1982, is a nondestructive testing technique designed to assess electric charge and dipole polarization in thin polymer films and slabs This method utilizes pressure waves to probe the dielectric bulk, allowing for the collection of information regarding electric charge or field distribution The foundational concept of using traveling pressure waves for detection was first proposed in 1977 by Collins and others Various implementations of the PPS method exist, differing primarily in their pressure wave generation techniques, which include shock-tubes, high-voltage sparks, and various laser systems such as ruby, CO2, and Nd:YAG lasers Notably, the Laser-Induced Pressure Pulse (LIPP) method produces short-duration pressure pulses with high spatial resolution, while the PPS method generates pressure steps that induce uniform deformation within the sample Additionally, an alternative approach known as the Piezoelectrically-generated Pressure Pulse (PPP) method has been proposed, which creates shorter pressure steps using a brief electric square pulse and a shorter cable length.
The PPS method operates through a setup that includes a step-voltage generator, a piezoelectric crystal, a preamplifier, and an oscilloscope The piezoelectric crystal, specifically an x-cut quartz crystal with a thickness of 3 mm and a diameter of 25 mm, is essential for generating alternating pressure steps when a square voltage pulse is applied This pulse, produced by a cable-discharge generator with a duration of approximately 100 ns and a rising time of less than 1 ns, is transmitted to the sample via a thin layer of silicon oil The silicon oil, squeezed to a thickness of 100-200 nm under moderate static pressure from a rubber electrode, enhances the pressure-step amplitude and mitigates issues related to dust particles, the softness of the dielectric film, and the interface between the piezoelectric crystal and the dielectric film.
The PPS method offers a significant advantage over the LIPP method, as it utilizes a pressure-step amplitude that is approximately ten times smaller, around 50 kPa This method benefits from enhanced reproducibility of pressure steps provided by the piezoelectric crystal, allowing for improved signal-to-noise ratios through sampling and signal averaging techniques The number of averages can be adjusted based on the signal amplitude of each measurement, typically ranging from 500 to 3,000, and in some cases, up to 10,000 averages are employed in this thesis.
Figure 15 shows diagram of a series of pressure steps travel back-and-forth in the quartz
Figure 13: The working principle of Piezoelectrically-generated Pressure Steps (PPSs) method. Adapted from Ref [102].
Figure 14: The layout of PPS method.
The diagram illustrates the pressure reflection in a quartz crystal and sample in response to a single square pulse voltage, highlighting the relationship between the voltage-step's rising and falling edges and the corresponding pressure-step For further details, refer to Figures 14 and 16.
Upon applying a single square voltage pulse to the quartz crystal, two pressure-step pulses initiate and propagate in opposite directions from both the front and back sides of the crystal The pressure-step at the front side immediately releases the first response signal into the sample, due to the initial static pressure from the amplifier and rubber electrode This occurs at time t0=0 A transit time of tQ allows the pressure-step from the back side to travel through the quartz thickness, coupling another pressure-step into the sample at time tQ The back-side pressure-step is reflected, returning to the back side at time 2tQ, where it is again reflected and sent back to the front side, creating another sample response signal at time 3tQ This cycle of reflection and transit continues, resulting in sample response signals at instances t0, tQ, 2tQ, 3tQ, 4tQ, and so forth, corresponding to the pressure-steps coupled to the sample from both the front and back sides of the quartz.
The step-voltage generator produces a square voltage pulse lasting 100 ns, as illustrated in Figure 16 This results in the generation of two additional traveling pressure steps at the front side of the quartz upon the falling edge of the pulse.
The analysis of pressure response signals reveals that the inverted signals corresponding to the rising and falling edges of pressure steps exhibit a delay of approximately 100 ns These pressure steps, originating from both the front and back sides of the quartz crystal, generate sample response signals at specific time intervals, influenced by the duration of the voltage square pulse As time progresses, the pressure steps become increasingly distorted and attenuated, diverging from the initial ideal waveform characterized by rapid rising and falling edges This distortion impacts the sample response signal, leading to further attenuation and deviation from the expected results.
The typical PPS signal, illustrated in Figure 16, reveals several groups of sample signals, with four distinct groups identified; notably, the first group is obscured by substantial noise Each signal group is spaced approximately 500 ns apart, corresponding to the pressure steps applied to the front and acoustically-free back side of the piezoelectric quartz crystal This response occurs during the rising and falling edges of the voltage step The 500 ns interval represents the transit time (tQ) for a pressure step across the 3 mm thick quartz crystal, where the longitudinal sound velocity in fused quartz is 5960 m/s.
The sample response signal, illustrated in Figure 17, demonstrates the timing between the rising and falling edges of the voltage-step, with two inverting response signals separated by approximately 100 ns Most of the pressure-pulse signal (PPS) responses analyzed in this thesis were recorded within a timeframe of around 1 µs, particularly in the third group of sample signals (refer to Figures 16 and 17) These responses were captured following the pressure-step of the voltage-step rising-edge, originating from the front side of the quartz crystal, and involved a complete cycle of travel within the crystal, resulting in a total transit time of 1006.6 ns.
The pressure step coupling from quartz to the sample generates a compression wave that travels through the sample at the speed of sound (cS) This inhomogeneous compression results in a temporary rearrangement of induced charges within the sample, leading to a short-circuit current detected at the sample electrodes This current serves as a direct representation of the electric field or polarization distribution within the sample In dielectric samples with charge-compensated polarization, the characteristics of the short-circuit current can be specifically defined.
Introduction
This study explores charge injection and transport in Polypropylene (OPP-TSS from Puetz-Folien) films, which were charged at room temperature to high initial surface potentials Some films underwent linear heating to higher temperatures to partially discharge corona-deposited charges The non-destructive Piezoelectrically generated Pressure Step (PPS) method was utilized for charge profiling It was found that no charge injection occurred in PP films charged for brief periods (approximately 15 seconds) using a positive corona discharge However, charge injection could be enhanced by employing negative corona discharge or through thermally-assisted charge injection from both film surfaces under the influence of the internal electric field.
Sample
This study utilized 50 µm thick polypropylene films from Puetz-Folien (OPP-TSS), which were metallized on one side with 10 nm of chromium and 100 nm of aluminum This metallization enhances electrode contact for improved performance in corona-charging, Thermally-Stimulated Discharge (TSD), and PPS measurements.
Experimental method
Corona-charging with corona triode
Samples were shaped into circular forms to fit into a sample holder made of a 5 mm thick aluminum disk and a 30 mm diameter metal outer ring These samples, placed in the holder, were charged using a corona discharge in air, with a corona voltage of ±12 kV for the needle and a control-grid voltage ranging from ±1 kV to ±5 kV.
Samples in the holder were heated linearly at a rate of 3 K/min from room temperature to various set temperatures between 40 and 120 °C for a partial discharge analysis The heating plate was controlled by a computer, and surface potential measurements were taken using a non-contacting electrostatic probe (TREK instrument model 341).
Piezoelectrically-generated Pressure Steps (PPSs)
Surface charges from the sample were initially eliminated using liquid ethanol, followed by the measurement of charge distributions through the PPS method The recorded signal I(t) reflects the electric field distribution within the sample, and the charge distribution was determined by calculating the derivative of I(t).
Result and Discussion
Charge injection in polypropylene samples was analyzed through their charge profiles during corona charging at room temperature Two scenarios were tested: a short charging duration of approximately 15 seconds and an extended charging period lasting several hours The charge profiles were measured using PPS techniques.
Figure 20 illustrates the charge profiles of a PP sample negatively charged at room temperature for approximately 15 seconds, with final electric field intensities ranging from 19 MV/m to 92 MV/m The data indicates that negative-corona charging at electric field intensities of 39 MV/m and above results in negative charge injection into the PP film, even with short charging durations The positive segments of the curves correspond to the positive image charge on the rubber electrode in the PPS setup Notably, as the charging electric field intensity increases, the amount of charge injected into the free surface of the PP also rises However, no positive charge injection was detected at the Al/Cr/PP interface, even at the maximum intensity of 92 MV/m Additionally, experiments involving positive-corona charging of PP samples did not reveal any charge injection into the PP bulk from either the free surface or the Al/Cr/PP interface.
When polypropylene (PP) film is subjected to an extended charging time of several hours, the charging behavior significantly changes Figure 21 illustrates the charge profiles of PP samples that were corona-charged at room temperature with an electric field intensity of approximately 40 MV/m, under both negative and positive polarities for about 17 to 18 hours The results demonstrate that electric charge was injected from both sides of the PP film regardless of the charging polarity.
The charge distributions in polypropylene (PP) samples subjected to negative corona charging at room temperature for approximately 15 seconds are illustrated in Figure 20, showcasing the final electric-field intensities for each sample Charges are injected from the free surface on the right side, while they are blocked at the aluminum/chromium electrode on the left side.
Charge distributions in polypropylene (PP) samples exhibit both negative (blue) and positive (red) charges at room temperature, influenced by specific time intervals under an electric field of approximately 40 MV/m Charges are introduced from both the electroded surface on the left and the free surface on the right.
The charge distributions in polypropylene (PP) samples subjected to negative corona charging at room temperature for approximately 15 seconds, with a final electric field of around 40 MV/m, were analyzed as they were linearly heated to specified temperature levels for partial discharging Each curve represents a distinct single sample, with charge injection occurring from both the electroded surface on the left and the free surface on the right.
The transport of corona-deposited charge on polypropylene (PP) film was enhanced by thermal energy during thermally stimulated discharge (TSD) measurements PP samples were negatively charged via corona discharge at room temperature for 15 seconds, subjected to an electric field intensity of approximately 40 MV/m Negative charges were retained on the non-metallized side, while positive charges accumulated on the Al/Cr/PP electrode side By heating the PP samples to temperatures ranging from 40 to 120 °C during TSD, the trapped charges were released and drifted into the bulk under the influence of an internal electric field After rapid cooling back to room temperature, these charges were retrapped within the bulk, observable through pulsed photoconductivity (PPS) experiments Prior to PPS measurement, the surface charge layer on the free surface of each PP film was eliminated by rinsing with ethanol to avoid any unintended electric field gradients.
The study examines the surface charge of polypropylene (PP) and the rubber electrode of polyphenylene sulfide (PPS), noting that the rubber electrode has a diameter of approximately 5 mm, significantly smaller than the sample area of about 30 mm The analysis begins at a temperature of 40 °C and continues upward.
Heating polypropylene (PP) to 90 °C increases the charge injection from both sides, resulting in deeper penetration of charge into the bulk material However, as the temperature rises further, particularly beyond 90 °C, the amplitude of charge peaks diminishes, likely due to the neutralization of opposing charge clouds At temperatures reaching 120 °C, the sample exhibits almost no bulk charges.
Conclusion
For Polypropylene film (OPP-TSS from Puetz-Folien) we find:
• Short-time charging (ca 15 sec), RT, positive corona (electric field of up to approx. 90MV/m): No charge injection from sample surfaces.
• Short time charging (ca 15 sec), RT, negative corona (electric field higher than ca.40MV/m): Charge injection from sample free surface only.
• Much longer charging (at ca 40MV/m) or charging and heating to high temperature levels: Charge injection from both sample surfaces.
Published in: Poster session of the IEEE 16th International Symposium on Electrets (ISE), 2017.
Further detail can be found in Appendix 1.
4 Publication 2 - Depth Profile and Transport of Positive and Negative Charge in Surface (2-D) and Bulk (3-D) Nanocom- posite Films
Quyet D Nguyen, Jingwen Wang, Dmitry Rychkov and Reimund Gerhard Institute of Physics and Astronomy, Faculty of Science, University of Potsdam
Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany
This study examines the charge distribution and transport within the thickness of 2D and 3D polymer nanodielectrics, focusing on chemically surface-treated polypropylene (PP) films and low-density polyethylene nanocomposite films.
In this study, 3 wt % magnesium oxide (LDPE/MgO) was utilized as a model for 2-D and 3-D nanodielectrics Surface charges were applied to the nonmetallized areas of one-side metallized polymer films, and upon thermal stimulation at elevated temperatures, these charges were found to penetrate deeper into the film's bulk The resulting space-charge profiles across the film thickness were analyzed using Piezoelectrically-generated Pressure Steps (PPSs) Additionally, the effects of chemical surface treatment on these profiles were investigated.
The incorporation of bulk nanoparticles from LDPE/MgO nanocomposites results in the formation of nano-structures that significantly enhance charge trapping within the polymer films while simultaneously reducing charge transport in the samples.
LDPE nanocomposites, MgO nanoparticles, Space charge, Charge transport, Charge sta- bility, Acoustic probing of electric-field profiles, Piezoelectrically generated Pressure Steps(PPSs)
Introduction
The reduction of electrical conductivity in various nanocomposites is believed to stem from the interface between nanoparticles and the polymer matrix, which creates deep traps for charge carriers, hindering charge injection and transport This study examines charge distribution and transport in "2-D nanocomposites" (PP films treated with phosphoric acid) and "3-D nanocomposites" (LDPE/MgO) Charge layers deposited on the non-metalized surfaces of the polymer films were observed to migrate into the bulk during thermal stimulation The charge profiles were generated using Piezoelectrically-generated Pressure Steps (PPSs), providing evidence that surface nanostructures contribute to enhanced space charge trapping and diminished charge transport in the nanocomposite films.
Samples and Measurement Methods
Oriented polypropylene (PP) films, with a nominal thickness of 50 µm, were supplied by Puetz Folien These films underwent a metalization process, first coated with 10 nm of chromium (Cr) and subsequently with 100 nm of aluminum (Al) on one side.
The other side of the film was left unmetallized On surface-treated films, the open sur- face was brought in contact with H3PO4solution (Carl Roth® 85 %) for 24 hours at 120 ◦
The polypropylene films were divided into two groups for the experiments: the first group consisted of as-received samples, which served as the reference group with no treatment, while the second group included chemically surface-treated samples Prior to conducting further experiments, all surface-treated samples were thoroughly washed with water and dried For additional details about the two groups of polypropylene films, please refer to Reference [43].
Low-density polyethylene/magnesium oxide (LDPE/MgO) nanocomposite films, approximately 75 µm thick, were produced through compression molding of LDPE/MgO pellets that included LDPE powder infused with 200 ppm of the antioxidant Irganox 1076 and 3 wt % of MgO nanoparticles, which have crystallite sizes around 10 nm These films were developed by the polymer technology group at the Royal Institute of Technology (KTH) in Stockholm, with contributions from Professor Gubanski at Chalmers University of Technology, Sweden For comparative analysis, commercial LDPE films from Goodfellow, measuring about 52 µm thick, were utilized as reference samples Both LDPE and LDPE/MgO films underwent an evaporation process, where 10 nm of chromium and 100 nm of aluminum were applied to the rear side, while the front side remained unmetallized Further details on the preparation and characteristics of the LDPE/MgO nanocomposite films can be found in the referenced study.
Polymer films were mounted in a sample holder made of a compact aluminum disk and an outer ring, with surface charges deposited on their non-metalized surfaces using a corona discharge in air at room temperature The charging process lasted 15 seconds, with a point electrode voltage of ±12 kV and a grid electrode voltage adjusted between ±1 and ±3 kV For the negative corona charging of polypropylene films, the grid voltage was set to -2 kV, while for LDPE/MgO films, it was adjusted to achieve an average electric field of approximately 26 kV/mm across the film.
In a study on thermally-stimulated surface-potential decay, a polymer sample was heated to 80 °C for four hours while its surface potential was monitored using a non-contact electrostatic voltmeter (TREK model 341), achieving an accuracy of ±5 V The investigation involved the partial discharging of corona-deposited charges on the polymer films, utilizing Thermally-Stimulated Discharge (TSD) combined with rapid quenching to room temperature The polymer sample was heated from room temperature to various elevated temperatures at a rate of 3 K/min, and upon reaching the target temperature, it was swiftly transferred to a nearby metal surface at room temperature for rapid cooling.
Piezoelectrically-generated Pressure Steps (PPSs) were utilized to investigate the electric-field profile within polymer films, although the PPS signal remains uncalibrated, resulting in measurements represented in arbitrary units This study employed two configurations for PPS measurements: a single-layer configuration for polypropylene (PP) films and a double-layer configuration for low-density polyethylene (LDPE) and magnesium oxide (MgO) films.
The investigation of PP film involved applying a rubber electrode (approximately 5 mm in diameter) to its surface (about 30 mm in diameter), which resulted in the destruction of any existing surface charge on the PP film To prevent unwanted electric field gradients, the PP film's surface charge was eliminated by thoroughly washing it with ethanol prior to PPS measurement In contrast, for the LDPE/MgO films, a double-layer configuration was employed, where a non-metalized commercial LDPE film was placed on top of the LDPE/MgO film's free surface before the rubber electrode application This setup enabled the examination of both bulk and surface charges within the LDPE/MgO films.
Result and Discussion
A Charge distribution and transport in as-received and in chemically treated PP films
Figures 23 a and b illustrate the charge profiles of as-received and chemically surface-treated polypropylene (PP) films, respectively, after being negatively charged in a corona and subsequently discharged at elevated temperatures When the as-received PP samples are heated, surface charges migrate from the surface and the metal/polymer interface into the polymer bulk In contrast, the chemically treated films retain negative charges at or near the surface, even when heated to 120 °C These deeply trapped negative charges prevent positive charge from the metal/polymer interface from moving closer to the surface, resulting in a broader distribution of positive charge within the bulk of the PP film.
B Charge distribution and transport in LDPE/MgO nanocomposite films
Unlike 2-dimensional nanocomposites of PP films treated with H3PO4, LDPE/MgO nanocomposites feature a genuine 3-dimensional structure with MgO nanoparticles integrated within the LDPE polymer matrix Isothermal Surface-Potential Decay (ISPD) curves illustrate the behavior of both as-received LDPE films and LDPE/MgO nanocomposite films, which were charged using positive and negative corona discharges, resulting in initial bulk electric fields of approximately 26 kV/mm Analysis of charge distribution in the nanocomposite films reveals that, aside from minor space-charge accumulation near the surface, the bulk of the nanocomposite remains largely free of charge.
Conclusion
This study investigates charge distribution and transport in 2D and 3D polymer nanodielectrics, specifically chemically surface-treated polypropylene (PP) and bulk nanocomposite low-density polyethylene/magnesium oxide (LDPE/MgO) The research reveals that confined electric charge layers on the non-metallized surfaces of polymer films expand during thermally-stimulated discharge It was observed that surface treatment of PP and the incorporation of nanoparticles in LDPE/MgO films enhance charge trapping while simultaneously reducing charge transport In PP films, chemical treatment improves negative charge retention at the surface, creating deep negative traps, whereas charges are injected from the metal/PP interface when heated Conversely, in LDPE/MgO films, heated charge layers remain close to the near-surface region, with no significant space charge developing within the nanocomposite bulk.
Quyet D Nguyen is indebted to Vietnam’s International Education Cooperation Depart- ment (VIED) for kindly providing funding towards his PhD project at the University of Pots-
The charge distribution of polypropylene films subjected to negative corona discharge in air at room temperature reveals a partial reduction of deposited charges when heated at a rate of 3 K/min to elevated temperatures, followed by rapid quenching back to room temperature.
PP films and (b) PP films chemically surface-treated with phosphoric acid.
LDPE/MgO pristine LDPE (GoodFellow) pristine LDPE (GoodFellow) LDPE/MgO
The Isothermal Surface-Potential Decay (ISPD) at 80 °C for 4 hours was analyzed for as-received LDPE and LDPE/MgO nanocomposite films The samples were charged using a corona discharge, resulting in an average electric field of approximately 26 kV/mm The thickness of the LDPE film was around 52 µm, while the LDPE/MgO film measured approximately 77 µm.
The analysis of LDPE/MgO nanocomposite films reveals the electric-field and charge distributions post-Isothermal Surface Potential Decay (ISPD) at 80 °C for 4 hours A protective layer of non-metalized LDPE film was applied to the charge layer on the non-metalized sample surface The figure illustrates the electric-field distribution on the left side for the metalized surface and the charge distribution on the right side for the free surface of the LDPE/MgO nanocomposite film, highlighting the relationship between the electric field and charge in these materials.
Published in: © [2019] IEEE Reprinted, with permission, from Q D Nguyen, J Wang,
In their 2019 paper presented at the 2nd IEEE International Conference on Electrical Materials and Power Equipment (ICEMPE), D Rychkov and R Gerhard explore the depth profile and transport mechanisms of both positive and negative charges within surface (2D) and bulk (3D) nanocomposite films, providing valuable insights into their electrical properties.
5 Publication 3 - LDPE/MgO Nanocomposite Dielectrics for Electrical- Insulation and Ferroelectret-Transducer Applications
Q D Nguyen and R Gerhard doanguyen@uni-potsdam.de Institute of Physics and Astronomy, Faculty of Science University of Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany
Recent studies indicate that LDPE/MgO nanocomposites (3 wt%) are promising candidates for effective electrical insulation materials This research also explores their potential as (ferro-)electret materials Isothermal surface potential decay measurements reveal that charged LDPE/MgO films maintain significant surface potentials even after being heated for 4 hours at 80 °C, demonstrating their ability to retain electric charges of both polarities Additionally, open tubular-channel ferroelectrets made from LDPE/MgO nanocomposite films exhibit notable piezoelectric properties, with d33 coefficients around 20 pC/N post-charging, and show stability at temperatures of at least 80 °C.
80 ◦ C Thus LDPE/MgO nanocomposites may become available as a new ferroelectret ma- terial To increase theird33coefficients, it is desirable to optimize the charging conditions and the ferroelectret structure.
Keywords: ferroelectrets, LDPE nanocomposites, electroacoustic probing, space-charge and polar- ization profiles, thermally stimulated discharge
Introduction
The geographical distance between electric power generation centers and high consumption areas necessitates long-distance power transmission Increasing environmental concerns and limitations on new overhead transmission lines have led to a growing reliance on electric power-cable transmission systems To minimize power loss, high transmission voltages are essential, driving the demand for advanced insulation materials capable of withstanding these conditions Low-density polyethylene (LDPE) has proven effective in high-voltage AC (HVAC) power-cable insulation, and there is a strong interest in its application for high-voltage DC (HVDC) systems Key requirements for HVDC insulation include high dielectric breakdown strength, low electrical conductivity, and minimal space-charge accumulation.
To develop HVDC power cables with a nominal operating voltage of 1 MV, it is essential that the new insulating materials exhibit electrical conductivity at least ten times lower than that of current cross-linked polyethylene (XLPE) Incorporating a small percentage of nanoparticles into the polymer matrix presents a promising strategy for creating advanced polymer composites that offer improved electrical, mechanical, and thermal properties.
Insulating materials with low electrical conductivity effectively capture and retain electric charge, leading to space-charge accumulation that distorts internal electric fields This distortion can enhance local electric fields, resulting in accelerated aging and potential premature failure of insulating systems.
Electrets are materials capable of retaining a quasi-permanent electric charge or polarization, which are crucial for transducer applications A new class of electrets, known as ferroelectrets, features a foam-like structure with cavities that hold electric charges of both polarities, forming macroscopic dipoles and exhibiting quasi-ferroelectric characteristics Current research in ferroelectrets focuses on discovering new materials with enhanced charge-retaining capabilities, improving existing materials' charge-trapping properties, and optimizing ferroelectret system designs for increased piezoelectricity and thermal stability Promising multifunctional materials are those that exhibit both electrical insulation and electret properties This study investigates films of an LDPE/MgO nanocomposite containing 3 wt% magnesium oxide (MgO) nanoparticles, highlighting its potential as both an electrically insulating material and a ferroelectret.
Samples and Experiments
Thermally-Stimulated Discharge (TSD)
LDPE and LDPE/MgO nanocomposite films were subjected to charging at room temperature using a corona triode system with a needle and grid electrode, applying voltages of ±12 kV for approximately 15 seconds The grid-electrode voltage was adjusted to establish initial average electric fields of around 26 kV/mm within the sample bulk For the thermally stimulated depolarization (TSD) measurement, the samples underwent linear heating from room temperature to 80 °C at a rate of approximately 0.67 °C/sec, followed by isothermal heating at 80 °C.
◦C for 4 hours (Isothermal surface-potential decay).
Piezoelectrically-generated Pressure Steps (PPSs)
After TSD, the LDPE/MgO nanocomposite samples were measured with the PPS method
To investigate charge distributions, a second non-metallized LDPE film, approximately 50 µm thick, was placed on top of the LDPE/MgO sample to safeguard its surface charge layer Pressure was applied from the electroded side of the LDPE/MgO nanocomposite film The longitudinal sound velocity of the LDPE/MgO nanocomposite was measured at 2.3 km/s, with a spatial resolution of about 3–4 µm for the PPS measurement.
Open-tubular-channel ferroelectrets
Ferroelectret samples were created using LDPE/MgO nanocomposite films through a heat fusing technique around a 100 µm thick perfluoroalkoxy alkane (PFA) template, followed by cutting and removing the template The samples were metallized with a layer of 10 nm Cr and 50 nm Al, featuring circular electrodes with a diameter of 1.5 cm on both sides, and subsequently charged using one of two methods.
• Contact charging at RT for 10 minutes Charging voltage is+5 kV.
Thermal ferroelectret charging involves applying a contact voltage of -5 kV at 80 °C for 15 minutes Following this charging process, the samples are cooled to room temperature while maintaining the applied voltage.
The piezoelectric properties of LDPE/MgO ferroelectret samples were assessed using a dynamic measurement method after charging To evaluate the thermal stability of the piezoelectric d33 coefficient, the nanocomposite ferroelectret samples were subjected to heating at 80 °C for durations of 1 or 2 hours, followed by a subsequent measurement of the d33 coefficient.
Result and Discussion
LDPE/MgO as electrical insulation
Incorporating MgO nanoparticles into an LDPE polymer matrix enhances the electrical and thermal properties of the resulting nanocomposite film The electrical bulk conductivity of LDPE/MgO nanocomposite films is approximately 1×10⁻¹⁵ S/m after 11 hours under a 32.5 kV/mm electric field at 60°C, specifically for the UN-MgO 3 wt% variant This conductivity is 30 times lower than that of standard LDPE, which measures 3×10⁻¹⁴ S/m, and is also less than the conductivity of pure MgO crystals, which ranges from 1×10⁻¹³ to 1×10⁻¹² S/m.
The LDPE/MgO nanocomposite exhibits a significant thermal degradation onset temperature shift of approximately +100 °C compared to pristine LDPE, rising from 250 °C to 350 °C This material, characterized by low conductivity, demonstrates excellent charge retention, an essential property for ferroelectrets However, a notable drawback is the increased water uptake associated with MgO nanoparticles, which can reach up to seven times that of standard LDPE This high moisture content poses challenges for using LDPE/MgO nanocomposites in high-voltage direct current (HVDC) power cable insulation unless effective sealing measures are implemented Additionally, the elevated water absorption adversely affects the long-term stability of the charge in ferroelectret applications, potentially reducing the service life of their transducing capabilities.
LDPE/MgO as ferroelectret transducer
TSD measurements of as-received LDPE and LDPE/MgO nanocomposites reveal that the surface potential decays significantly faster in as-received LDPE compared to LDPE/MgO films after being charged by positive and negative corona discharges Initial average bulk electric fields were approximately 26 kV/mm, and after 30 minutes at 80 °C, only about 30% of the initial surface potential remained in the LDPE, while the LDPE/MgO samples exhibited much better charge retention Specifically, positively charged LDPE/MgO samples showed negligible surface potential decay after 4 hours of heating, whereas negatively charged samples experienced an initial drop but stabilized thereafter Charge distribution analysis using the PPS method indicated that charges of both polarities penetrated approximately 5 µm into the LDPE/MgO samples, with induced image charges forming on the electrode However, due to measurement uncertainties and potential thickness reduction from Maxwell stress, it remains challenging to assess charge injection into the film bulk, suggesting minimal charge presence within the film itself.
LDPE/MgO pristine LDPE (GoodFellow) pristine LDPE (GoodFellow) LDPE/MgO
Figure 26: Thermally-Stimulated Discharge (TSD) measurement of LDPE/MgO nanocomposite and as-received LDPE from GoodFellow The thicknesses of the LDPE/MgO and LDPE films are ca 77 and
52àm, respectively Samples were charged to an initial surface potential that generates an average bulk electric field of approx 26 kV/mm.
The measurements of Piezoelectrically-generated Pressure Steps (PPSs) in LDPE/MgO nanocomposites were conducted following Thermally-stimulated Discharge (TSD) at 80 °C for 4 hours To protect the surface charge layer on the LDPE/MgO free surface, a non-metallized as-received LDPE sample was utilized The figure illustrates the metallized surface on the left and the protected free surface on the right The longitudinal sound velocity in the LDPE/MgO film was estimated at 2.3 km/s, with a spatial resolution of approximately 3−4 µm for the PPS measurements The upper plot displays the bulk electric field distribution, while the lower plot shows the charge distribution derived from the electric field distribution according to Poisson’s equation.
Table 2: Remaining piezoelectric coefficientd 33 [pC/N] of LDPE/MgO nanocomposite ferroelectrets. Heating time is presented in brackets.
Charging temperature After charging Charged and heated at 80 ◦ C
LDPE/MgO nanocomposites exhibit excellent charge retention for both polarities, indicating their potential as ferroelectret materials These composites have been utilized to create open-tubular-channel ferroelectrets, as outlined in previous research Preliminary thermal stability measurements of the quasi-static piezoelectric coefficient d33 demonstrate that the piezoelectric properties of the tubular-channel ferroelectret remain stable at temperatures up to 80 °C Therefore, LDPE/MgO nanocomposites show promise as effective electrets.
Conclusion
The following conclusions can be drawn:
Electrical-conductivity and thermogravimetry measurements indicate that LDPE/MgO nanocomposite films exhibit significantly enhanced electrical and thermal properties compared to standard LDPE samples Consequently, these nanocomposites show great potential as effective electrical insulation materials for high voltage DC or AC power cables.
• LDPE/MgO nanocomposites could also be potential (ferro-)electret materials as ev- idenced by TSD, PPS and piezoelectric-coefficient (d33) measurements.
One significant drawback of LDPE/MgO nanocomposites is their tendency to absorb higher amounts of water, which can negatively impact their effectiveness as electrical insulation or electret materials Addressing this issue is crucial for future research and development.
Quyet D Nguyen expresses gratitude to Vietnam’s International Education Department for funding his PhD research in Germany and acknowledges Professor Gubanski from Chalmers University of Technology in Sweden for supplying the LDPE/MgO nanocomposite films utilized in his study.
Published in: © [2018] IEEE Reprinted, with permission, from Q D Nguyen and R Gerhard,
“LDPE/MgO nanocomposite dielectrics for electrical-insulation and ferroelectret-transducer applications,” in 2018 IEEE 2nd International Conference on Dielectrics (ICD) IEEE, 2018,
6 Publication 4 - Non-uniform polarization profiles in PVDF copoly- mers after cyclical poling
Q D Nguyen, T Raman Venkatesan, W Wirges and R Gerhard
Institute of Physics and Astronomy, Faculty of Science University of Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany
Poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymers are widely recognized as essential electro-active polymers, known for their ferro-, pyro-, and piezoelectric properties achieved through high electric field poling For effective device applications, achieving uniform polarization profiles across the thickness of the material is crucial In this study, we focused on obtaining spatially uniform polarization profiles in commercial P(VDF-TrFE) copolymer films with varying compositions of 50/50, 72/28, and 75/25 mol% VDF/TrFE using cyclical hysteresis poling The polarization profiles were analyzed through Piezoelectrically generated Pressure Steps (PPSs), and the depth profiles were linked to the corresponding hysteresis curves.
Fourier-Transform Infrared (FTIR) spectroscopy was utilized to determine the β-phase fractions in P(VDF-TrFE) compositions The 75/25 mol% copolymer samples exhibited the most uniform polarization profiles and achieved the highest remanent polarization of 70 mC/m², as indicated by the hysteresis curves, which correlates with the highest β-phase fraction observed in the FTIR measurements.
P(VDF-TrFE) copolymer; ferroelectric polymer; cyclical hysteresis poling (a.k.a “Bauer pol- ing”); Piezoelectrically generated Pressure Steps (PPSs); spatial polarization profiles