Copper (Cu) and its composites are widely employed in the electrical manufacturing industry. Among them, graphene (Gr)/Cu composites have emerged as a promising material system for the development of lightweight conductors with high electrical conductivity. In this work, based on a balance between cost and practicality, a new preparation method is proposed. Gr layers were first grown on Cu surfaces via chemical vapor deposition (CVD). The Cu foils with grown Gr layers were then stacked and consolidated by vacuum hot pressing, followed by annealing, to fabricate Gr/Cu laminated composites. As a result, an electrical conductivity of up to 106.4% IACS was achieved. The proposed process offers a straightforward and practical route for the fabrication of Gr/Cu composites. Furthermore, a simulation model of Gr/Cu laminates was established to investigate the effects of Gr stacking configuration and layer number on interfacial strength and electrical transport behavior. The results reveal that monolayer Gr, which is located between Cu-Cu layers, provides the optimal balance between interfacial strength and electrical conductivity. These findings highlight the importance of controlling the Gr layer number in order to simultaneously achieve high performance and cost-effective fabrication of Gr/Cu laminated composites.

The valve-side bushing of the converter transformer used in ultrahigh voltage direct current (UHVdc) systems is an important component of dc transmission networks, where the operational reliability of the bushing fundamentally determines system-level safety performance. However, research on bushings rarely considers the anisotropic bulk conductivity of epoxy resin-impregnated paper (RIP). Accordingly, this study systematically characterizes the anisotropic bulk conductivity and temperature-dependent behavior of the RIP composites. A transient multiphysics simulation of electric-thermal-fluid coupling is carried out, with anisotropic bulk conductivity parameters established for the valve-side bushing of a +/- 800 kV converter transformer. The numerical results indicate that anisotropic bulk conductivity induces bipolar charge segregation: The epoxy core's outer surface exposed to SF6 gas maintains positive charge accumulation, while the air gap and arc-corner surfaces develop negative charge densities of up to -14.82 pC/mm(2). Under isotropic bulk conductivity, the air gap termination region maintains a uniform electric field below 0.1 kV/mm. In contrast, anisotropic bulk conductivity induces field distortions characterized by tangential/normal components reaching 0.5/1.8 kV/mm. These distortions elevate the risk of surface flashover on the bushing because of air gap discharge.

Vanadium doping effectively modulates carrier transport and enhances breakdown voltage in 4H-SiC, yet its underlying microscopic mechanisms remain incompletely understood. The lack of clarity regarding doping-induced carrier behavior in both conducting and blocking states impedes rational carrier selection and limits device performance optimization. This study employs a multiscale simulation framework integrating first-principles calculations with Boltzmann transport theory, incorporating electro-thermal coupling effects, to systematically investigate the influence of vanadium doping on the electrical and thermal properties of 4H-SiC. Results demonstrate that vanadium introduces new defect energy levels, enhances carrier-phonon scattering, and significantly lowers carrier mobility, which is identified as the dominant contribution to the improved breakdown voltage. Meanwhile, the doping-induced lattice distortion markedly suppresses phonon transport and reduces thermal conductivity. The combined effect of these competing electrical-thermal processes ultimately strengthens the material's voltage withstand capability. This work establishes a comprehensive defect-carrier-phonon coupling model, providing critical theoretical guidance for the performance optimization of high-power 4H-SiC devices through defect engineering.

With the increasing application of cable accessories in extreme temperature environments, interface breakdown faults have become a critical concern for power system reliability. This study investigates the effect of temperature on interface breakdown characteristics of 15 kV cold-shrink silicone rubber cable accessories under different expansion ratios. Breakdown tests were conducted using a needle-plate electrode system across a temperature range of $-40^{\circ} \mathrm{C}$ to 90° C and expansion ratios of 134.33%, 157.17%, and 181.76%. The results show that both expansion ratio and temperature significantly influence breakdown performance. At $\mathbf{2 5}^{\boldsymbol{\circ}} \mathbf{C}$, as the expansion ratio increases from 134.33% to 181.76%, the scale parameter of breakdown voltage rises from 20.795 kV to 23.807 kV, accompanied by increasing shape parameter, indicating reduced data scatter. Under constant expansion ratio conditions, the scale parameter gradually increases with rising temperature, exhibiting a positive correlation. However, the reduction in breakdown voltage at low temperatures is substantially greater than the increase at high temperatures. At $-\mathbf{4 0}{ }^{\boldsymbol{\circ}} \mathbf{C}$ and $\mathbf{1 8 1. 7 6 \%}$ expansion ratio, the scale parameter decreases by 6.187 kV (25.981%) compared to room temperature. The shape parameter peaks at 25°C and decreases as temperature deviates, with greater scatter observed at low temperatures. For actual cable accessories, temperature-induced pressure variation due to polymer entropic elasticity further complicates the breakdown mechanism. The decrease in interface pressure at low temperatures leads to significant reduction in breakdown voltage, explaining the concentrated failures observed during winter. This research provides crucial insights for ensuring electrical reliability of cable accessories under complex temperature environments.

The recurrent failures of cable accessories under low-temperature environments highlight critical uncertainties in their interfacial mechanical reliability. To address this issue, this study elucidates the influence mechanism of low-temperature environments on the mechanical properties of silicone rubber (SiR) used for accessory insulation. The crystallization behavior at low temperature is investigated using differential scanning calorimetry (DSC). The stress relaxation behaviors under constant temperatures and variable temperatures are tested. The effects of temperature and tensile ratio on the stress relaxation of SiR are analyzed. A calculation formula for the entropy-elasticity stress of SiR at low temperature is established utilizing the rubber state equation. The results show that, at constant temperatures, the stress variation in the stretched SiR is exclusively governed by stress relaxation effect. The higher the tensile ratio and the temperature, the faster the relaxation rate. When the temperature decreases, the stress in the stretched SiR changes linearly, essentially resulting from the entropic elastic stress variation induced by the combined effects of thermal expansion, the Gough-Joule effect, and stress relaxation. The stress in the stretched SiR under low tensile ratio of 110% is mainly dominated by the thermal expansion effect, exhibiting a gradual increase. At tensile ratios between 125% and 400%, the Gough-Joule effect becomes the predominant mechanism, driving progressive stress decrease with the decrease of temperature. Moreover, it is precisely the reason for the unreliability of the interface pressure between cable and accessory under low-temperature environments. At extremely low temperatures, the strain-induced crystallization (SIC) effect in the stretched SiR can lead to a sharp increase in stress and an elevated crystallization temperature. These results can be used for evaluating the mechanical reliability of cable accessories in the cold environments.

Valve-side bushings for ultra-high voltage (UHV) converter transformers serve as critical components in UHV direct current transmission systems. Epoxy resin-impregnated paper (RIP), employed as the dry-type insulation of bushings, endures prolonged exposure to high-voltage and high-current conditions, leading to significant thermal aging. In this study, valve-side bushings of a ±800 kV converter transformer were investigated through thermal aging experiments conducted on the RIP specimens, which were sectioned from the bushing insulation along the radial and axial directions, respectively. The results showed that the RIP insulation exhibits anisotropic bulk conductivity due to its inherent structural anisotropy. This anisotropy intensified with aging time, with the initial anisotropy ratio of 5 to 25 rising to 6 to 38 after thermal aging at 115°C. The simulation results of the ±800 kV bushing revealed that under anisotropic conditions, substantial negative charges accumulate at the bushing end, thereby enhancing the electric field within the SF6 gas gap. This phenomenon is further intensified after thermal aging, resulting in an average electric field strength of 2.31 kV/mm and giving rise to both significant tangential and normal electric field components. Such field distortion elevates the risk of surface flashover initiated by partial discharges in the gas gap, suggesting progressive insulation degradation over the operational lifespan of the bushings.

This study investigates a ±800 kV converter transformer valve-side bushing. By experimentally characterizing the anisotropic material parameters and performing coupled electro-thermal simulations, the work systematically examines (i) how dielectric harmonic losses contribute to bushing temperature rise, and (ii) how resulting temperature gradients modulate the electric field distribution at the gas–solid interface under anisotropic bulk conductivity. The findings reveal that both excessively steep and shallow radial temperature gradients impair insulation performance in distinct spatial regions. Specifically, enhanced radial temperature gradients driven by high-frequency harmonic losses exacerbate radial electric field nonuniformity. As a result, the peak radial electric field increases by 3.5 kV/mm, and the bulk dielectric strength between the conductor and flange is degraded. Conversely, weaken radial temperature gradients, even when nearly reaching thermal homogenization, redirect electric field concentration toward the inner core layer. Under anisotropic condition, this phenomenon intensifies the electric field within these regions and promoting significant negative charge accumulation at the gas–solid interface. Under this condition, the maximum negative surface charge density reaches −33.26 pC/mm², and the interfacial electric field peaks at 3.48 kV/mm, which indicates a potential compromise in surface insulation reliability at the gas–solid interface.

The UHVDC converter transformer valve-side bushing is a critical component in DC transmission systems. However, the anisotropic properties of bushing insulating materials have been overlooked in previous studies. This paper investigated on the $\pm \mathbf{8 0 0 ~ k V}$ converter transformer valve-side bushing by measuring the bulk conductivity of epoxyimpregnated paper flake specimens cut from an actual bushing both parallel and perpendicular to the insulation paper surface. The study revealed that the bulk conductivity in the perpendicular direction was significantly higher than that in the parallel direction. A steady-state coupled electrical-thermalfluid simulation model for the $\pm 800 \text{kV}$ converter transformer valve-side bushing was further established. Simulations were conducted under both isotropic and anisotropic bulk conductivity conditions, and the results were comparatively analyzed. The study revealed that on the bushing epoxy core airgap surface near the shield, isotropic bulk conductivity resulted in minor negative charge accumulation, whereas anisotropic bulk conductivity induced significant negative charge accumulation. Compared to isotropic bulk conductivity, anisotropic conditions reduced the air-gap terminal potential by 14 kV and increased the normal/tangential components of the electric field on the SF6 side of the air-gap surface by $1.9 \text{kV} / \text{mm}$ and $0.51 \text{kV} / \text{mm}$, respectively, degrading the insulation performance of the air-gap region.

ABSTRACT The interface pressure between cables and accessories plays a crucial role in the long‐term reliable operation of cable systems. However, the change in interface pressure at high temperature and its influencing mechanism are still inconclusive. In this study, the changes of interface pressure at high temperature mainly caused by three influencing factors were investigated through theoretical analysis and experimental measurements. The results revealed that the thermal expansion effect will result in a decrease in interface pressure, while the Gough–Joule effect of rubber materials at high temperature will lead to an increase in interface pressure as the temperature rises. Additionally, the stress relaxation effect of rubber‐like materials at high temperature will accelerate the decline in interface pressure. Finally, a calculation method for interface pressure based on the comprehensive effect of these factors is proposed. The results showed that the interface pressure increases as the temperature rises, indicating that the Gough–Joule effect of rubber materials dominates the change in interface pressure. The error between the theoretical calculation and actual measurement of interface pressure was < 5%. Therefore, this method can feasibly be used to evaluate the interface pressure in the medium‐voltage (MV) and even high‐voltage (HV) integral prefabricated cable accessories as the temperature changes.

The interface pressure between cables and accessories plays a crucial role in the long-term reliable operation of cable systems. However, the mechanism of interface pressure at variable temperature is still inconclusive. In this study, the changes of interface pressure at variable temperature mainly caused by three influencing factors were investigated through theoretical analysis and experimental measurements. The results revealed that the thermal expansion effect will result in a decrease in interface pressure, while the Gough-Joule effect of rubber materials at high temperature will lead to an increase in interface pressure as the temperature rises. Additionally, the stress relaxation effect of rubber-like materials at higher temperature will accelerate the decline in interface pressure. Finally, the interface pressure at variable temperature is calculated based on the comprehensive effect of the three factors. The results showed that the interface pressure increases as the temperature rises, indicating that the Gough-Joule effect of rubber materials dominates the change in interface pressure. The error between the theoretical calculation and actual measurement of interface pressure was $<5 \%$. Therefore, this method can feasibly be used to predict the interface pressure as the temperature changes.

Due to silicon carbide (SiC) power devices' superior performance in terms of switching frequency, loss, and so on, they are widely used in high-power and high-frequency electronics. While SiC devices are in blocking mode, the transport and accumulation of charge carriers in SiC may lead to significant degradation and failure of semiconductor materials, which is the primary factor limiting their application in high-voltage and large-current fields. In addition, transition metal vanadium is often used to compensate for the defect levels in SiC, leading to more complex carrier transport dynamics behaviors. In this research, the effect of vanadium doping on the withstand voltage properties of SiC was explored. Firstly, characterization experiments, including x-ray diffraction, Raman spectroscopy, and Kelvin probe force microscopy on SiC before and after vanadium doping, were performed. Secondly, the transport dynamics of charge carriers in SiC were studied. Based on the influence of vanadium on the trap characteristics, the effect mechanism of vanadium doping on carrier transport dynamic behaviors was elucidated. Finally, the relationship between microscopic carrier transport and macroscopic dielectric properties was discussed. The results show that vanadium doped SiC introduces deep energy level traps, which are able to capture carriers and cause charge accumulation, inhibit carrier migration. This can also affect conductance current. Therefore, vanadium doping can effectively improve the voltage withstand capability of SiC in blocking mode by regulating the trap characteristics and carrier transport behaviors.

The interface pressure between cable and accessory is key to ensuring the safe operation of the cable terminal of electric locomotives, and temperature is the main cause of changes in interface pressure. This article studies the effect of temperature on the silicone rubber (SiR) used in cable accessories and then on the interface pressure between cable and accessory using the operation temperature range of the electric locomotive cable terminal. First, the low-temperature crystallization behavior of the SiR insulation is analyzed using differential scanning calorimetry (DSC) analysis and dynamic mechanical analysis (DMA). Second, the mechanical behavior of SiR at variable temperatures is analyzed using uniaxial mechanical tensile tests and stress relaxation tests. Finally, the changes in interface pressure between the cable and the flexible accessory in a wide temperature range are measured by building a thin-film piezoresistive measurement system. The results show that interface pressure between cable and accessory positively correlates with temperature. When the temperature rises from 25 degrees C to 90 degrees C, the interface pressure increases from 0.220 to 0.333 MPa, while the temperature drops to -20 degrees C, the interface pressure decreases to 0.195 MPa. The results indicate that the interface pressure between cable and accessory cannot be simply simulated by building the SiR material elastic mechanics equations, which are obtained through uniaxial stress-strain curves of SiR at variable temperatures. The changes in interface pressure under variable temperatures are caused by the combined effects of thermal expansion, entropy elasticity, and stress relaxation. The strain-induced crystallization (SIC) effect of SiR at low temperatures also decreases interface pressure.

Reliable interface pressure between cables and accessories is closely associated with the safe operation of a power cable system. Using an external optical fiber Bragg grating (FBG) temperature-compensating curvature sensor, a new method for online measurement of the interface pressure between a cable and an accessory at high temperatures is proposed. On the basis of the sensing principle of an FBG, a mathematical pure bending model, and a model of a thick-walled cylinder, an FBG temperature-compensating curvature sensor is fabricated using the Dow Corning Sylgard-184 silicone rubber. The interface pressures of 10-kV cable accessories with different expansion rates at different temperatures are measured. The results of the measurements show that the temperature-compensated deviation of wavelength measurements made by the fiber grating curvature sensor ranges from −2.59% to 5.50%. The interface pressure between the cable and the accessory gradually increases with an increase in temperature from 30 °C to 60 °C. The results are consistent with the results of measurements made using a traditional piezoelectric sensor. This method is expected to evaluate the long-term reliability of the interface pressure of cable accessories in actual complex-temperature environments.

The influence of laser energy, irradiation position, photoelectric material, and anode voltage on the trigger characteristics of a pseudospark switch triggered by a 532-nm nanosecond focused laser is studied. The parameters of the seed electrons induced by the interaction between the focused laser and metal are measured. It is found that the increase of laser energy and anode voltage can increase the current density of seed electrons, reduce the time of electrons diffusion to the cathode hole, and thus reduce the trigger delay. When the irradiation position is close to the cathode hole, the surface electric field is larger, which is conducive to achieving a smaller trigger delay and jitter under low laser energy. The seed electron current induced by the focused laser has two peaks. The first peak mainly corresponds to the thermal emission, with a short time delay and narrow pulsewidth, contributing to the trigger. The second peak mainly corresponds to the electrode surface ablation, with a long time delay and large pulsewidth, contributing little to the trigger. In this case, the electrode surface temperature is more important than the work function, so Cu has better electron emission characteristics than Mg. With the increase of laser energy, the number of electrons in high-energy components increases.

挤塑型交联聚乙烯(XLPE)高压直流电缆绝缘中空间电荷的积聚会造成局部场强畸变,导致材料的绝缘性能下降.电缆内导体的热效应在绝缘层产生的温度梯度会进一步影响电荷行为,纳米颗粒改性是抑制空间电荷的一种有效措施,但抑制作用具体如何实现,尤其是对于微观层面载流子输运过程的影响规律还需深入分析.试样内空间电荷的数值仿真可以探究各种微观粒子之间的相互作用和演化过程,因此文中基于载流子抽出受限的双极性电荷输运模型,对温度梯度下的电荷行为,深陷阱与浅陷阱对于载流子迁移过程的影响进行了研究.结果表明:低温侧会因抽出受限而积聚异极性电荷,深陷阱会限制载流子输运且深陷阱作用存在瓶颈,随着迁移率增大,电荷分布由同极性变为异极性分布,当迁移率足够大时,异极性电荷不再增长甚至开始降低.

目的 探讨MRI不同Dixon技术对成人非酒精性脂肪性肝病(NAFLD)肝脏脂肪定量的可行性.方法 选取符合纳入标准的NAFLD组54例,对照组21例.两组患者均于上腹部CT检查1周内行3.0T MR VIBE Dixon检查.测量并记录CT肝/脾比值、2E-VIBE Dixon(双回波磁共振水脂分离技术)脂肪分数HFF值、6E-VIBE Dixon(6回波磁共振水脂分离技术)质子密度脂肪分数PDFF值、R2*值.对两位医师测量的一致性和可重复性、2E-VIBE Dixon和6E-VIBE Dix-on 对各组肝脏脂肪定量组间及组内的差异性等进行统计学分析.结果 在肝脏脂肪定量测量者之间,2E-VIBE Dixon和6E-VIBE Dixon均有高度的一致性(ICC:0.946～0.993)和可重复性(LOA:1.43％～3.72％),其中 6E-VIBE Dixon 更好.2E-VIBE Dixon、6E-VIBE Dixon肝脏脂肪定量在各组间和组内均具有统计学差异(P＜0.05);2E-VIBE Dixon所测HFF均高于 6E-VIBE Dixon 所测 PDFF(P＜0.05).结论 2E-VIBE Dixon、6E-VIBE Dixon 对 NAFLD 肝脏脂肪定量均具有较高的一致性和可重复性,均可用于肝脏脂肪定量.与2E-VIBE Dixon相比,6E-VIBE Dixon所测肝脏脂肪分数可重复性更高,结果更加准确.

The interface pressure between cable and cable accessory can indicate and greatly affect the safe operation of cable accessory. In this paper, a new temperature compensation method is proposed to eliminate the influence of high temperature on the measurement results, based on the existing method of using fiber grating curvature sensor to measure the interface pressure of cable accessories online at room temperature. First, the fiber Bragg grating curvature sensor using this temperature compensation method was prepared using Dow Corning Sylgard-184 silicone rubber, and then the interface pressure values of 10KV cable accessories at room temperature and high temperature under different expanding ratios were measured. The measurement results show that the measurement deviation of the fiber grating temperature compensation curvature sensor is less than ± 5%, and the accuracy is improved compared with the original temperature compensation method. The interface pressure between cable and accessory gradually increases with the increase of temperature (from $30^{\circ}C$ to $60^{\circ}C)$, which are consistent with the measurement results of traditional piezoelectric sensors. It shows that this method is hopeful to be applied to the on-line measurement of the interface pressure of cable accessories under working conditions.

In direct current (dc) transmission lines, power cables are susceptible to the invasion of switching overvoltage or lightning overvoltage. This causes the accumulation of space charge in the insulation due to the higher transient composite electric stress. Space charge is an important factor in dielectric breakdown, especially under high electric stress. However, until now, there are few reports on space charge characteristics under the composite stresses of dc voltage and impulse voltage. In this article, a modified pulsed electroacoustic space charge measurement system used for dc voltage superimposed by impulse voltage is developed by reasonably choosing resistor and capacitor elements in the measuring system. Then, space charge characteristics in cross-linked polyethylene (XLPE) are measured under dc voltage, impulse voltage, and dc voltage superimposed by impulse voltage. The test results show that hetero-charges dominated in the XLPE bulk under a positive dc voltage, whereas homo-charge injection occurred from the semiconductor (SC) electrode under a negative dc voltage. However, a continuous impulse voltage with high amplitude can cause a large degree of homo-charge injection from both electrodes. Moreover, greater homo-charge injection appeared under the negative impulse voltage than that under the positive impulse voltage. Under a dc voltage superimposed by an impulse voltage, the superposition of voltages with the same polarities facilitated homo-charges injection from both electrodes to a greater extent than did the superposition of voltages with opposite polarities. Furthermore, regardless of the polarity of the applied dc voltage, greater space charge accumulation appeared in XLPE when a negative impulse voltage is superimposed.

近年来,高速机车用柔性电缆终端在极端环境温度下事故频发,严重影响了铁路运输安全性.为此,根据实际机车运行的极端运行环境温度(-40～90℃),通过对比与分析电缆附件用硅橡胶材料在高温、低温及高低温循环过程中的弹性模量变化及其扩张态下的应力松弛特性,探讨了极端运行环境温度对电缆与附件绝缘界面压力的影响规律.研究结果表明:扩张态的硅橡胶材料在高温90℃下的面压松弛率近30％,在低温-40℃下的面压松弛率约10％～20％,而在高低温循环(-40～90℃)120h后,面压松弛率高达50％.因此,极端环境温度下电缆与附件绝缘界面压力的大幅度下降或将对机车电缆终端的绝缘性能产生显著影响.

The conduction loss of the pseudospark switch (PSS) can be reduced by connecting the magnetic switch (MS) and PSS in series to form the magnetically delayed pseudospark switch (MDPSS). In this paper, a 2D electrostatic Particle in Cell/Monte Carlo Collision simulation model of MDPSS coupled with the external circuit is established, and the discharge process and characteristics are studied. It is found that the forward conduction process of the MDPSS can be divided into four stages. The first stage is characterized by the rapid drop of anode voltage, and the discharge mechanism is mainly the collision ionization of seed electrons. In the second stage, the anode voltage increases slowly, which is mainly maintained by secondary electrons emitted by ions impacting the cathode. The third stage marks the beginning of MS saturation, accompanied by the rapid rise of anode voltage and loop current, as well as the rapid strengthening of the sheath electric field in the cavity, thus inducing the fourth stage, that is, the complete conduction of PSS. The duration of hollow cathode discharge will be prolonged by increasing the number of magnetic cores, thus further reducing the total conduction loss of the switch.