Search Results (22)

Lithium niobate (LiNbO₃) has remarkable ferroelectric properties, and its unique crystal structure allows it to undergo significant spontaneous polarization. Lithium niobate plays an important role in the fields of electro-optic modulation, sensing and acoustics due to its excellent electro-optic and piezoelectric properties. Thin-film LiNbO₃ (TFLN) has attracted much attention due to its unique physical properties, stable properties and easy processing. This review introduces several main preparation methods for TFLN, including chemical vapor deposition (CVD), molecular beam epitaxy (MBE), pulsed laser deposition (PLD), magnetron sputtering and Smartcut technology. The development of TFLN devices, especially the recent research on sensors, memories, optical waveguides and EO modulators, is introduced. With the continuous advancement of manufacturing technology and integration technology, TFLN devices are expected to occupy a more important position in future photonic integrated circuits. Full article

The insulation of high-voltage direct-current (HVDC) cables experiences a short period of voltage polarity reversal when the power flow is adjusted, leading to sever field distortion in this situation. Consequently, improving the insulation performance of the composite insulation structure in these cables has become an urgent challenge. In this paper, SiC-SR (silicone rubber) and TiO₂-SR nanocomposites were chosen for fabricating HVDC cable accessories. These nanocomposites were prepared using the solution blending method, and an electro-acoustic pulse (PEA) space charge test platform was established to explore the electron transfer mechanism. The space charge characteristics and field strength distribution of a double-layer dielectric composed of cross-linked polyethylene (XLPE) and nano-composite SR at different concentrations were studied during voltage polarity reversal. Additionally, a self-built breakdown platform for flake samples was established to explore the effect of the nanoparticle doping concentration on the breakdown field strength of double-layer composite media under polarity reversal. Therefore, a correlation was established between the micro electron transfer process and the macro electrical properties of polymers (XLPE/SR). The results show that optimal concentrations of nano-SiC and TiO₂ particles introduce deep traps in the SR matrix, significantly inhibiting charge accumulation and electric field distortion at the interface, thereby effectively improving the dielectric strength of the double-layer polymers (XLPE/SR). Full article

In the design and manufacturing of epoxy resin insulation components, complex structures can be achieved through multiple pours, thereby forming the structure of interface of laminated epoxy resin. This type of interface structure is often considered a weak link in performance which can easily accumulate charges and cause electric field distortion. However, research on the interlayer interface of epoxy resin has received little attention. In this study, epoxy samples with and without interlayer interfaces were prepared, and the space charge accumulation characteristics and trap characteristics of the samples were analyzed via pulsed electro-acoustic (PEA) and thermally stimulated depolarization current (TSDC) methods. The experimental results indicate that the Maxwell–Wagner interface polarization model cannot fully explain the charge accumulation at the interface. Due to the influence of the secondary curing, the functional groups in the post-curing epoxy resin can move and react with the partially reacted functional groups in the prefabricated epoxy resin layer, resulting in a weak cross-linking network at the interface. With the increase in temperature, the molecular chain segments in the weak cross-linked region of the interface become more active and introduce deep traps at the interface, thereby exacerbating the accumulation of interface charges. In addition, due to the influence of interface polarization and weak cross-linking, the ability of the interface charges to cause field strength distortions decreases with the increase in applied field strength. This research study can provide a theoretical reference for the interfacial space charge transport characteristics of epoxy-cured cross-linked layers and provide ideas for regulating interfacial cross-linking to suppress interfacial charge accumulation. Full article

Solid-film electrets and cellular electrets are defined as promising insulating dielectric materials containing permanent electrostatic and polarizations. High-performance charging methods are critical for electret transducers. Unlike dielectric barrier discharge (DBD) charging, the soft X-ray charging method, with its strong penetration ability, has been widely used in electrets after packaging and has even been embedded in high-aspect-ratio structures (HARSs). However, the related charging model and the charging effect of the soft X-ray irradiation remain unclear. In this study, the charge carrier migration theory and the one-dimensional electrostatic model were employed to build the soft X-ray charging models. The influence of soft X-ray irradiation under deferent poling voltages was investigated theoretically and experimentally. The conducted space charge measurement based on a pulsed electro-acoustic (PEA) system with a soft X-ray generator revealed that soft X-ray charging can offer higher surface charge densities and piezoelectricity to cellular electrets under the critical poling voltage lower than twice the breakdown voltage. Full article

The homo-crosslinked-polyethylene (H-XLPE) bilayer simplifies the returned insulation structure of the factory joint in submarine cables, and its dielectric property is key to the reliability of the power transmission system. In this paper, we investigated the charge accumulation phenomenon in a secondary thermocompression H-XLPE bilayer using the pulse electro-acoustic method. The charge accumulation reduces its overall breakdown strength when compared with XLPE. According to X-ray diffraction measurement and thermal analysis results, the specimen forms a homo-junction region between the bilayers, which has overlapping spherulites with a thick lamella, high crystallinity, and high surface free energy. The charge accumulation can be ascribed to fused lamellas and the crystal imperfection of the homo-junction region, which restricts the charge transport process and exhibits a higher number of deep traps. This study emphasizes the importance of the homo-junction region in the H-XLPE bilayer, which should be considered in the design and operation of factory joint insulation. Full article

Low density polyethylene (LDPE) is a good insulating material which is widely used in cable materials due to its excellent insulation and processability. However, in the DC high voltage environment, pure polyethylene materials still face many problems, the most serious of which is space charge accumulation. The cable will inevitably be subjected to tensile stress during production, installation and operation. Therefore, it is of great significance to study the effect of stretching on the microstructure and space charge characteristics for polymers and their composites. In this paper, MMT/LDPE micro-composites, SiO₂/LDPE nano-composites and MMT-SiO₂/LDPE micro-nano-composites were prepared by melt blending. Mechanical stretching was carried out on pure LDPE materials and the above three kinds of composite materials. Each material was stretched according to four stretching ratios, which are 0%, 5%, 10% and 20%. The crystal morphology was observed by polarizing microscope (PLM), the crystallization perfection was tested by differential scanning calorimetry (DSC), and the space charge distribution inside each sample was measured by pulsed electro-acoustic (PEA) method. At the same time, the average charge density and apparent charge mobility for samples during depolarization were calculated and analyzed. The experimental results show that when the pure low density polyethylene sample is not stretched, its crystal structure is loose. Tensile stress can make the loose molecular chains align in LDPE and improve its crystalline structure, which is helpful to restrain the accumulation of space charge inside the sample. For MMT/LDPE, SiO₂/LDPE and MMT-SiO₂/LDPE composites, their internal crystal structure is compact. Stretching will destroy their original crystal structure at first, and then disorder molecular chains inside the three composite materials. With the increase of stretching ratio, the molecular chains begin to orient along the direction of force, the crystallization tends to be perfect gradually, and the space charge accumulation in samples also decreases. From the calculation results of apparent charge mobility for each sample, with the increase of stretching ratio, the trap depth and trap density inside samples firstly increased and then decreased. Full article

In this paper, we studied the space charge phenomena of a solid polymer under thermal and electrical stresses with different frequencies and waveforms. By analyzing the parameter selection method of a protection capacitor and resistor, the newly built pulsed electro-acoustic (PEA) system can be used for special electrical stresses under 500 Hz, based on which the charge phenomena are studied in detail under positive and negative DC and half-wave sine and rectangular wave voltages. Experimental results show that the charge accumulated in the polyimide polymer under DC conditions mainly comes from the grounded electrode side, and the amount of charge accumulated with electric field distortion becomes larger in a high-temperature environment. At room temperature, positive charges tend to accumulate in low-frequency conditions under positive rectangular wave voltages, while they easily appear under high-frequency situations of negative ones. In contrast, the maximum electric field distortion and charge accumulation under both half-wave sine voltages occur at 10 Hz. When the measurement temperature increases, the accumulated positive charge decreases, with a more negative charge appearing under rectangular wave voltages, while a more positive charge accumulates at different frequencies of half-wave sine voltages. Therefore, our study of the charge characteristics under different voltage and temperature conditions can provide a reference for applications in the corresponding environments. Full article

Space charges are one of the main challenges facing the constantly increasing use of extruded high voltage direct current (HVDC) cables. The Pulsed Electro-Acoustic (PEA) method is one of the most common procedures for space charge measurements of insulation. One issue with the PEA method is distortion due to the crosstalk between the applied voltage pulse and the acoustic sensor. This work analyzed two factors involved in the reduction in this distortion: the influence of the exposed semiconductor distance between the injection electrodes and PEA test cell, and the influence of adding a reactance at the grounding circuit of the PEA test cell. The interaction of these two factors with the distortion was analyzed through a series of experimental testing. Moreover, the performance regarding distortion after applying a developed coaxial injection was compared with the standard non-coaxial injection configuration. It was observed that these two factors had a direct impact on distortion and can be utilized for the reduction in distortion arising from the crosstalk of the applied pulsed voltage. The results can be utilized for the consideration of practical aspects during the construction of a PEA test setup for the measurement of full-size HVDC cables. Full article

Oil-impregnated insulation paper is an important part of transformers; its performance seriously affects the life of power equipment. It is of significance to study the aging characteristics and mechanism of oil-impregnated insulation paper under thermal stress for transformer status detection and evaluation. In the work, the accelerated thermal aging was carried out at 120 °C, and DP1490, DP787, and DP311 samples were selected to represent the new, mid-aging, and late-aging status of the transformer, respectively. The space charge distribution within the specimens was measured by the pulsed electro-acoustic (PEA) method and the trap parameters were extracted based on the measurement curves. Further, the aging mechanism was studied by molecular simulation technology. A typical molecular chain defect model was constructed to study the motion of cellulose molecules under thermal stress. The experimental results show that the corresponding trap energy levels are 0.54 eV, 0.73 eV, and 0.92 eV for the new specimen, the mid-aging specimen, and the late aging specimen, respectively. The simulation results show that the trapped energy at the beginning of aging is mainly determined by the loss of H atoms. The changes in trap energy in the middle stage of aging are mainly caused by the absence of some C atoms, and the trap energy level at the end of aging is mainly caused by the breakage of chemical bonds. This study is of great significance to reveal the aging mechanism of oil-impregnated insulation paper and the modification of insulation paper. Full article

Pockels cells used as electro-optical modulators in high-power high-repetition lasers suffer from piezoelectric ringing phenomenon due to piezoelectric properties of the crystals. A new method for active suppression of the piezoelectric ringing in Pockels cells is proposed in this work. It is based on symmetric control of Pockels cell using burst of short positive and negative voltage pulses with the same amplitude instead of a single long pulse for light polarization modulation. Rising and falling edges of pulses of the burst induce symmetrical acoustic waves of the opposite phase and cancel the piezoelectric ringing of the crystal. A new high voltage driver capable of generating positive and negative pulses of tens of nanoseconds of 3 kV magnitude was developed for this purpose. The amplitude of laser beam intensity pulsations caused by the piezoelectric ringing can be reduced up to five times when active suppression method is used for the deuterated potassium dihydrogen phosphate (DKDP) Pockels cell. Such crystals like DKDP, LiNbO₃, and LiTaO₃ may benefit from the proposed method and find new use in lasers of high repetition rate where piezoelectric ringing is a major limiting factor. Full article

One of the main issues that affect the development of high-voltage direct-current (HVDC) cable insulation is the accumulation of space charge. The load operation of an HVDC cable leads to the formation of a radially distributed temperature gradient (TG) across the insulation. In this study, the space charge accumulation in a cross-linked polyethylene (XLPE) cable is measured under a DC electric field and TG using the pulsed electro-acoustic (PEA) method, and the effect of the TG on the space charge behavior is investigated. In addition, the bipolar charge transport (BCT) model and the conductivity model based on an improved cylindrical geometry are used to simulate the charge behavior in the HVDC XLPE cable under TG, and the experimental and simulated results are compared. The results show that the higher temperature of the cable conductor promotes the accumulation of homocharge near the side of high temperature. Additionally, with the increase of the TG, not only does more heterocharge accumulates adjacent to the side of low temperature, but more space charge also extends into the bulk of the cable insulation. More attention should be paid to the conductor shield layer and the insulation shield layer in HVDC cables. Moreover, the BCT model can more accurately describe the experimental results than the conductivity model. Full article

Nowadays, with the widespread use of High Voltage Direct Current (HVDC) cables in power systems, the measurements of space charges in full-size cables are becoming more relevant. One of the most common methods used for space charge measurements is the Pulsed Electro-Acoustic (PEA) method. This paper analyzes two factors that influence the electromagnetic interference on the piezoelectric signal. These factors are the connection of the injected pulsed voltage at the PEA test cell and the grounding of the PEA test cell. The influence was analyzed by means of experimental tests to compare different configurations and the electromagnetic distortion created in each one of them. It was observed that the physical location of the pulsed voltage at the electrode has a very important impact on the magnitude of the electromagnetic distortion. Moreover, it is shown that the physical connection of the grounding and the existence of a parasitic capacitance at the PEA test cell are also an important source of distortion. Full article

On-site installation of accessories on extruded polymeric high voltage cables in a common practice. The procedure requires the shaping of the physical interface between the cable insulation surface and the pre-molded accessory body. On such interfaces, rough surfaces should be avoided in order to limit space charge accumulation in the insulation, which affects the cable performance by reducing insulation life-time, creating conditions for local field enhancement, and, respectively, the formation of possible breakdown path e.g. by electrical treeing. Space charge measurements on cable insulation peelings were undertaken to assess the space charge injection and accumulation on interfaces with varying degrees of surface roughness in order to improve understanding on this subject. The results of the measurements confirm the hypothesis regarding the enhancement of charge injection from rough surfaces when electric field strength exceeds a certain level. The accumulated charge density in the material is shown to strongly depend on the field strength and temperature in both polarization and subsequent depolarization measurements. These results emphasize that a bipolar charge transport model that incorporates field and temperature dependencies of charge injection, trapping, detrapping, and recombination processes needs to be adopted to accurately describe the observed electric conduction phenomena. Full article

The semi-conductive layer located between the wire core and the insulation layer in high voltage direct current (HVDC) cable plays a vital role in uniform electric field and affecting space charges behaviors. In this work, the research idea of adding ionic conductive particles to semi-conductive materials to improve the conductive network and reduce the energy of the moving charge inside it and to suppress charge injection was proposed. Semi-conductive composites doped with different La_0.8Sr_0.2MnO₃ (LSM) contents were prepared. Resistivity at different temperatures was measured to investigate the positive temperature coefficient (PTC) effect. Pulse electro-acoustic (PEA) method and thermal-stimulation depolarization currents (TSDC) tests of the insulation layers were carried out. From the results, space charge distribution and TSDC currents in the insulation samples were analyzed to evaluate the inhibitory effect on space charge injection. When LSM content is 6 wt. %, the experimental results show that the PTC effect of the specimen and charge injection are both being suppressed significantly. The maximum resistivity of it is decreased by 53.3% and the insulation sample has the smallest charge amount, 1.85 × 10⁻⁷ C under 10 kV/mm—decreased by 40%, 3.6 × 10⁻⁷ C under 20 kV/mm—decreased by 45%, and 6.42 × 10⁻⁷ C under 30 kV/mm—decreased by 26%. When the LSM content reaches 10 wt. %, the suppression effect on the PTC effect and the charge injection are both weakened, owing to the agglomeration of the conductive particles inside the composites which leads to the interface electric field distortion and results in charge injection enhancement. Full article

This paper describes the effects of α-Al₂O₃ nanosheets on the direct current voltage breakdown strength and space charge accumulation in crosslinked polyethylene/α-Al₂O₃ nanocomposites. The α-Al₂O₃ nanosheets with a uniform size and high aspect ratio were synthesized, surface-modified, and characterized. The α-Al₂O₃ nanosheets were uniformly distributed into a crosslinked polyethylene matrix by mechanical blending and hot-press crosslinking. Direct current breakdown testing, electrical conductivity tests, and measurements of space charge indicated that the addition of α-Al₂O₃ nanosheets introduced a large number of deep traps, blocked the charge injection, and decreased the charge carrier mobility, thereby significantly reducing the conductivity (from 3.25 × 10⁻¹³ S/m to 1.04 × 10⁻¹³ S/m), improving the direct current breakdown strength (from 220 to 320 kV/mm) and suppressing the space charge accumulation in the crosslinked polyethylene matrix. Besides, the results of direct current breakdown testing and electrical conductivity tests also showed that the surface modification of α-Al₂O₃ nanosheets effectively improved the direct current breakdown strength and reduced the conductivity of crosslinked polyethylene/α-Al₂O₃ nanocomposites. Full article