Peter H. F. Morshuis

Dielectric elastomers (DEs) assembled with compliant electrodes are widely utilized in polymer-based energy transducers. Under high DC voltage, space charge accumulation within DEs can result in significant electric field distortion inside the dielectrics and at the electrode/insulation interfaces. This phenomenon is recognized as a critical factor influencing dielectric breakdown, aging characteristics, and, consequently, the long-term operational reliability of energy conversion systems. The formation and transport of space charge are governed by charge injection and extraction processes at the electrode interfaces. Previous studies have demonstrated that doping polydimethylsiloxane (PDMS) with nanosilica particles can effectively suppress space charge accumulation and enhance breakdown strength. In this paper, the charge injection behavior of PDMS/SiO2 nanocomposites with varying filler concentrations was investigated based on simultaneous measurements of space charge and conduction current using a double-layer structure. The experimental results reveal a linear relationship between the square root of the electric field intensity and the logarithm of the injection current at the electrode/PDMS interface across all samples. From this relationship, key parameters, such as the injection barrier (Φ) and the field-enhancement coefficient (β), were extracted. Compared with neat PDMS, the nanocomposites exhibited lower injection barriers, indicating a reduced rate of charge injection due to the incorporation of silica nanoparticles, which partially explains the observed suppression of space charge under high DC fields. Moreover, nanocomposites containing surface-treated silica showed similar β values to the undoped PDMS while exhibiting lower injection barriers, which is beneficial for improving both conduction behavior and dielectric breakdown strength under high electric fields.

Dielectric elastomers (DEs) with compliant electrodes are widely employed in polymeric energy transducers. An example of the use of DEs is in ocean wave energy conversion, where the potential energy of the waves is directly converted into electrical energy. Inevitably, the DE materials will be exposed to sea water, which may hamper the mechanical and electrical properties and hence the service life. Moreover, when such a system is energized by a high DC voltage, space charge transport may distort the electric field distribution in the dielectric, with a potentially negative effect on breakdown performance. In this paper, polydimethylsiloxane (PDMS) and PDMS/SiO₂ nanocomposites were prepared using a commercial DE material. The nanocomposites contained a small weight percentage of nanosilica, surface-treated and non-surface-treated. The samples were put in a climate chamber with high humidity (97%) at 56.5°C to absorb moisture for different amounts of time. To accelerate the water uptake of the films, some samples were immersed in deionized water or prepared seawater. The effect of water absorption on the dielectric properties of PDMS and PDMS/SiO₂ nanocomposites is discussed and the space charge characters under different applied DC electric fields are analyzed using the pulsed electro-acoustic method (PEA). The results show that the introduction of nano silica improves the dielectric properties of PDMS, reducing charge accumulation and leading to very little distortion of the electric field distribution even after water uptake. In both PDMS and PDMS/SiO₂ nanocomposites, water absorption leads to increasing dielectric losses and conductivity, accompanied with a decrease of 50 Hz AC and DC breakdown strength. It is shown that an increasing charge carrier mobility results in a larger charge accumulation and a more distorted electric field in the wet samples.

Silicone rubbers, particularly poly(dimethylsiloxane) (PDMS), are popular across various industrial applications; however, notable issues arise from volatile cyclic siloxanes and their tendency to compromise performance. This study investigates the electrical performance (DC conductivity, dielectric spectroscopy, DC breakdown) of two commercial PDMS products, before and after a post‐curing treatment (200°C, 4 h). The tested materials, named PDMS A (RTV) and PDMS B (LSR), are chosen as they display distinct characteristics: they differ in terms of outgassing behavior and structural alterations during treatment. In particular, thermogravimetric analysis (TGA) reveals pronounced volatile removal in PDMS B, while Fourier transform infrared (FTIR) and Raman spectroscopy demonstrate pronounced structural alterations in PDMS A (crosslinking of unreacted long chains) promoted by post‐curing. As expected, the electrical insulation performance is enhanced in both post‐cured materials: the magnitude of this enhancement depends on the extent of the two abovementioned aspects. The DC conductivity reduction follows the volatiles removal pattern (larger scale in PDMS B), while the ε′ and ε″ values reduce following the structural alteration pattern (elimination of a relaxation peak in PDMS A). The DC breakdown strength enhancements are driven by structural variations rather than the removal of volatiles.

For the electronic components made of electroactive polymer materials, a breakdown between the compliant electrodes is the main cause of failure. Similar to high voltage capacitors, a self-clearing mechanism is needed to minimize the probability of a single defect leading to complete failure. In order to meet compliance and self-clearing needs, single-wall carbon nanotube (SWCNT) films are widely used. However, the complex and expensive process of manufacturing of large-area SWCNT film hinders its application in industry. To reduce the usage of SWCNT while maintaining self-clearing properties, a novel stretchable and self-clearing electrode configuration is proposed in this paper, using a patterned electrode consisting of carbon black (CB) connected by SWCNT line. A criterion was established for a successful self-clearing process of the SWCNT line electrode. Some key factors affecting the self-clearing properties were investigated. It was found that high breakdown voltage and thin and narrow SWCNT line electrodes are beneficial to the self-clearing success rate. Besides the self-clearing properties, the effect of the size and number of the carbon black patterns on capacitance retention was studied. After a self-clearing event, part of the active area of the dielectric is lost. The objective is to maximize capacitance retention after a self-clearing event, which was analyzed experimentally and theoretically. Finally, it is suggested how to select the number and area of the patterned carbon black, making optimal use of the electrode area. This study provides a theoretical and experimental basis for further mass production of carbon-based self-clearing electrodes.

The pulsed electroacoustic (PEA) technique for measuring space charge in electrical insulation systems requires a proper calibration procedure. Most calibration techniques require the test specimen to be space charge free when the calibration voltage is applied. In practice however, this requirement may be hard to fulfill. Either some unknown space charge may be present in the test specimen, or space charge is built up during the calibration procedure. Both situations lead to improper calibration. To overcome this problem, a technique was developed that discriminates between space charge and the electrode signal related to the calibration voltage. This technique makes use of the fact that, while part of the electrode signal follows a change of the calibration voltage almost instantaneously, the contribution of the space charge will follow with some delay. In the frequency domain, this results in an imaginary component in case space charge is present. This allows introducing a procedure that allows proper calibration, even in the presence of a changing space charge distribution during calibration. In this paper, the validity of the proposed method is verified by simulation and practical experiment, in which the calibration signal is acquired at high frequency where the imaginary component is almost zero and the signal can be seen as free from space charge.

This study discusses the space charge measurement in a scaled cable joint model as a move towards its application to full-scale HVDC cable joints. A prototype of the improved PEA cell is created using a high-sensitivity PEA cell with polymer acoustic coupler combined with a 16-channel piezoelectric sensor array. The measurement employs Pulsed Electro-acoustic (PEA) method on HVDC cable joint model with 6.6mm thick XLPE cable insulation and 11mm thick EPDM joint insulation. The feasibility of the PEA cell with a 16-channel sensor array was examined by applying a pulse voltage to the shield layer of the joint box and maintaining the cable screen at ground potential. The measurements obtained from 16-channels were deconvolved considering acoustic divergence, pulse field intensity and acoustic attenuation. The PEA cell dedicated to joints allowed the recording of interfacial charge buildup near to the triple junction at the deflector in a 2-D cross sectional profile.

For some applications of electroactive polymers (EAP), the applied voltage has a DC component and space charge may accumulate and distort the local electric field in the dielectric. The space charge transport and accumulation are considered linking with critical dielectric properties, such as breakdown, polarization and electrical ageing. Few studies can be found in the literature related to the space charge characters in EAP materials. In this paper, the space charge characters of a typical acrylic EAP, VHB 4910, are discussed and analyzed at different applied electric fields. The results show that even at an average electric field of 5 kV/mm, a large amount of heterocharge accumulation front of the cathode, resulting in an obvious increase of the electric field near the cathode. After the large charge accumulation, a positive charge packet apparently moves towards the anode which is far different from the charge behavior observed in polyethylene and epoxy. Generally, pre-strain enables larger electro-deformation in EAP, so the tests were performed on VHB 4910 with and without pre-strain. The results show that a pre-strain leads to an increase of the amount of heterocharge and the electric field distortion near the cathode. The strong enhancement of the local electric field may greatly influence the breakdown strength of the material. To further investigate the mechanism of the space charge transport property in VHB 4910, blocking layers were used to distinguish ionization from electrode injection and the space charge characters in multi-layer structure samples are analyzed. It illustrates that ionization accounts for the main part of charge accumulation below 20 kV/mm, related to the “apparent backwards moving” of a positive charge packet.

Dielectric elastomers (DE) used for energy conversion are often subjected to DC voltages or low frequency varying voltages where the influence of space charge accumulation should be considered. The accumulation of space charge distorts the local electric field in the dielectric, which is closely related to critical dielectric performance, such as breakdown and electrical ageing. Little research has been done on the effect of space charge in DE on the breakdown performance. In previous studies, it was shown that adding nano-sized silica may improve the breakdown strength, which is often ascribed to the traps created by the nano particles. Charge trapped in front of the electrode-dielectric interface is supposed to reduce the electric field and hence reduce charge injection and accumulation, resulting in a higher breakdown strength. In this paper, to test this hypothesis, PDMS samples were prepared with different concentrations of nano-silica, as well as reference samples without filler. These specimens were characterized through space charge measurements. Conduction current measurements were performed to obtain additional information on the injection current. To relate space charge accumulation to a change of the breakdown characteristics, different types of breakdown tests were performed under AC and DC voltage. First, AC and DC ramp voltages were used, with the hypothesis that any accumulated space charge will have an effect only on DC breakdown because AC stress at power frequency does not allow enough time for space charge accumulation. In an additional step, step-up DC voltage tests were performed with the duration of each step long enough to allow space charge accumulation. The hypothesis in this case was that, due to the effect of space charge, the DC breakdown voltage would be lower than under DC ramp tests. The results of these experiments show that in the reference specimens, the breakdown strength is indeed affected by a strong build-up of space charge. Further, the space charge can be mitigated by adding a small fraction of nano-silica. This mitigation of space charge accumulation was shown to lead to a significantly higher DC breakdown strength, whereas the AC breakdown strength was hardly affected.

PDMS-based composites such as silicone elastomers are commonly found in high-voltage engineering, especially in outdoor insulation as coatings or structural elements or at interfaces between network elements, such as cable sealing ends (CSE). They are also promising prospects for dielectric elastomer generators (DEG), which are retrieving electrostatic energy from large strain amplitudes. The upper limit of energy conversion from these transducers is determined by the dielectric breakdown strength (DBS). Therefore, developing reliable systems that operate under high electric fields and variable repeated strains requires a thorough understanding of the mechanisms behind electrical breakdown and its coupling to mechanical cycling. In this study, the effect of Mullins damage and mechanical fatigue on silicone elastomers has been investigated. An electro-mechanical instability model that considers cyclic softening allows for predicting the evolution of the breakdown strength depending on the loading history. The results highlight the importance of the “first cycle,” where up to a 30% reduction in the mean DBS was measured. However, subsequent mechanical fatigue only marginally contributes to the degradation, which is a promising perspective for the long-term performance of any silicone elastomer as long as the precise impact of the first cycle is known.

An epoxy based on diglycidyl ether of bisphenol A was reacted with a long-chain poly(oxypropylene diamine) hardener in the presence of an accelerator, resulting in a flexible epoxy network. Tensile properties were tested as a function of accelerator concentration. All systems exhibited high levels of extensibility, with strain at failure values in excess of 65%. Molecular dynamics in a formulation containing 10 phr of accelerator were then examined using dielectric spectroscopy over the temperature range of 103–433 K. At low temperatures, a molecular relaxation process (γ relaxation) was observed and shown to conform well to both the Arrhenius equation and activated tunnelling. A stronger relaxation appeared (203–303 K) just before the onset of charge transport, which dominated the behaviour at higher temperatures. The former takes an unusual bimodal form, which we consider a result of overlapping β and α relaxations, consequently termed αβ mode. Analysis of this mechanism revealed a Vogel–Fulcher–Tammann (VFT) behaviour. The temperature-dependent DC conductivity, σ_DC (deduced from the low-frequency charge transport contribution to ε_r″), also revealed VFT behaviour with an onset statistically equivalent to that of the αβ mode, therefore suggesting that charge transport, at this temperature regime, is strongly affiliated with cooperative molecular motion.