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Before putting into operation, ultra-high voltage transformers need to undergo strict handover tests to ensure their safe and reliable operation. The grading capacity of the grading ring on the transformer will affect the accuracy of the handover test. When there are problems with high altitude environments or grading ring structures, their grading ability will change. It is particularly important to study and design grading rings that can have good grading ability in different situations. This article first uses COMSOL software to build a three-dimensional model of the withstand voltage test circuit of a 500 kV transformer. Secondly, the influence of changes in the pipe diameter and ring spacing of the equalizing ring on the equalizing capacity of the equalizing ring is studied. Finally, the variation of equalizing capacity of the equalizing ring in a rough state is studied. It is found that as the diameter of the equalizing pipe and the ring diameter increase or the distance between the rings decreases, the equalizing ability of the equalizing ring will be enhanced, with the greatest impact brought by the change in pipe diameter. When the grading ring has a rough surface, its grading ability will significantly decrease. Among them, the grading ability decreases most significantly when adhering to conical particles, and decreases least when concave, indicating that the smoother the surface of the grading ring, the better the grading ability. This study can provide some assistance for the design of grading rings for high altitude ultra-high voltage transformers.

Temperature plays a crucial role in influencing the mechanical properties of materials. Accurately measuring the temperature of devices is essential for understanding the evolution of their mechanical properties under stress and evaluating their health and lifespan. However, traditional methods encounter challenges in measuring transient temperatures and lack sufficient time-resolution capability, particularly when it comes to the rapid temperature changes at the solder interface during the switching process of power devices. In this paper, based on the close correlation between the intensities of the characteristic spectral lines of the laser-induced elements and the temperatures, a method of measuring the surface temperatures with the time-resolved capability of the order of microsecond is proposed, and a quantitative relationship between the surface temperatures of the sample and the spectral characteristics is established. The findings demonstrate that an increase in the surface temperature of the material results in enhanced intensity and signal-to-noise ratio of laser-induced plasma spectra. This enhancement is influenced by the spectral acquisition delay and gate width. To establish a quantitative relationship between surface temperature and spectral properties, back propagation-artificial neural network (BP-ANN) and partial least squares (PLS) are employed for fitting and calibration. The fitted models can achieve linear correlation coefficient indexes exceeding 0.99. Notably, the BP-ANN fitted model exhibites a small fitting bias, with a root mean squared error (RMSE) of 2.582 and a correctness rate of 98.3%. The method provides an effective means for transient temperature measurement of objects and gives a strong support for the assessment of the health status of the soldering interface of power devices.

Grafting nanomaterials on the carbon fiber (CF) surface is considered an effective strategy for enhancing the interfacial properties of CF-reinforced polymer composites (CFRPs). However, the mechanical properties of the CFs are often compromised during treatment. A new method for in situ growing silver nanoparticles (AgNPs) on the CF surface is proposed in this study. The CFs are first immersed in a low-viscosity silver nitrate solution to form a thin liquid film on the surface. Subsequently, using the abundant active particles in the plasma, the silver ions are reduced to silver atoms and grown into AgNPs on the CF surface. The tensile strength of CF@Ag was 38.74% greater than that of untreated CF, potentially due to the reparative action of AgNPs on defects in CF. The CF showed an evident improvement in surface wettability because of the AgNPs. Furthermore, the interfacial properties were noticeably improved, with the interfacial shear strength of CF@Ag increasing to 91.59 MPa, which was about twice that of pristine CF. Thus, the mechanical properties of composites are significantly improved (flexural strength increased by 190.74%). This study presents a non-destructive and convenient method for growing nanoparticles onto CF to establish a robust interface in CFRPs. [ABSTRACT FROM AUTHOR]

We studied positive streamers with a 5 mm gap under 20–101 kPa pressure and 1%–31% O2 concentrations conditions using a 2D axisymmetric fluid model based on local field approximation. As the pressure decreases from 101 kPa to 20 kPa, the axial reduced electric field, the mean electron energy and the electron diffusion coefficient increase, which leads to the acceleration of the streamer propagation velocity and the increase of the streamer channel radius. The opposite change of ionization cross section and gas molecular density caused by the decrease of pressure leads to the non-monotonic change of the peak of net ionization rate. At medium-low pressure, there is a wider ionization region at the streamer head. As the O2 concentration decreases from 31% to 1% in N2/O2, the streamer propagation velocity decreases. When the O2 concentration drops to 1%, the streamer velocity decreases with a descent gradient of nearly 4 times, compared to 11% O2. Based on the space charge effect and chemical reaction rate, a possible mechanism is proposed to explain the abrupt change in the streamer velocity. [ABSTRACT FROM AUTHOR]

The development of composite materials with excellent thermal conductivity is attracting extensive attention to meet the increasing heat concentration challenges in both power and electronic equipment. Previous strategies for improving the thermal conductivities usually accompanied by the cost of mechanical strength. Herein, we propose a new strategy for developing both mechanically strong and thermally conductive composite films. The aramid nanofibers (ANF) are well prepared with introducing Ag nanoflakes and graphene oxide (GO) via vacuum-assisted filtration and hot-pressing treatment. Results indicate the synergistic effects of low dimension materials have resulted in a tightly arranged phonon network construction. The in-plane thermal conductivity of the prepared ANF/Ag/GO film reaches 9.84 W m−1 K−1, about 371% higher than that of pure ANF. Meanwhile, the strong hydrogen bond formed between the functional group of GO and the amide group of ANFs plays a synergistic strengthening and toughening role at the complex interface, and the tensile strength reaches 231 MPa. We believe that these findings shed some light on the design and fabrication of multifunctional materials for thermal management applications. [ABSTRACT FROM AUTHOR]

Pretreatment using O2 plasma is a commonly used method today to increase the surface activity of CFs and enable more functional groups to be grafted onto them. Although the oxidation treatment of the CF surface is considered a key procedure for further grafting with functional groups, few researchers have investigated how the oxidation treatment influences the grafting processes. In this paper, the effects of the degree of oxidation on the grafting of amino functional groups onto CFs were experimentally investigated. The experiment showed that the oxidation process developed in steps including initial oxidation, saturation oxidation, and peroxidation. The results demonstrated that the degree of oxidation was positively correlated with the grafting efficiency within a certain range. Furthermore, the groups (unsaturated C, C-OH, and O=C-OH) produced after oxidation determined the results of grafting with amino (C-N, C-NH2, and O=C-NH2) groups. When the O2 plasma treatment time was increased from 0 min to 3 min, the IFSS of CF@HA increased by 1.07 times and the flexural strength of CFRP increased by 1.26 times. This work proves that the ammonia grafting of CF can be controlled by adjusting its earlier oxidation process. [ABSTRACT FROM AUTHOR]

Thermal defects widely occur in electrical equipment and affect its stable operation. This paper proposes a novel detection method based on derivatives for thermal defects of electrical equipment. First, the equivalent models of the equipment are established to analyze the surface temperature formed by different defects. Second, the theoretical foundation of the derivative-based detection method is described, and numerical calculations on thermal defect detection based on derivative characteristic curves are carried out. The results show that the method can detect the position effectively and calculate the depth with an error of 1.6%. Then, the thermal defects in different situations are analyzed, and the results indicate that the method is effective in all cases. Finally, the experiment is conducted by artificially preparing samples with thermal defects, and the achieved calculation error of the experimental results is within 3.3%, which indicates the validity of this method. [ABSTRACT FROM AUTHOR]

Graphite matrix/copper composites have found significant potential uses in numerous areas. Until now, a high ambient gas pressure has to be applied to overcome the non-wettability between the copper melt and graphite matrix, which has restricted the technological and economic efficiencies, as well as the composite performance. In this paper, we have proposed a pressureless infiltration strategy to realize the preparation of highly conductive graphite matrix/copper composites. A tungsten carbide network is first in situ constructed in the graphite matrix, transforming the capillary force from the resistance mode to the impetus mode. Results show that the contact angle of graphite/copper decreased from 138.5° to 23° due to the constructed tungsten carbide network. The composites exhibit good interfacial bonding, and the electrical conductivity reaches 15.1 × 105 S/m, far higher than the requirement of the industry standard. [ABSTRACT FROM AUTHOR]

Aluminum/graphite composites are receiving much attention due to the unique features and diverse possibilities in different fields. Generally, the aluminum melt into the inner pores of the graphite matrix requires an ambient prompting with high temperature and gas pressure. The infiltration dynamics of aluminum melt, which highly depends on the ambient temperature, would have a significant effect on the final composite's performance. In this work, we have studied the temperature effect of aluminum melt during the infiltration process into the porous graphite matrix based on numerical simulation and experimental verification. Results show that the moderate temperature increase would effectively promote the infiltration of aluminum melt, achieving a higher filling rate with a shorter filling time. However, the promotion effects of the temperature increase would come to a certain limit due to the opposing effects of capillary porous and residual gas. The detailed influences of the temperature on the infiltration would be presented, and the final composite performance, including mechanical strength and thermal management capability, would be examined and discussed. [ABSTRACT FROM AUTHOR]

A method based on LIBS technology for measuring transient surface temperatures with a time resolution of several μs was proposed in this study, which holds great significance for fast sliding friction processes in linear electromagnetic propulsion, gun barrels, and high-speed trains. The effects of surface temperature on laser-induced plasma emission spectra were studied, and it was found that both the spectral intensity and signal-to-noise ratio increased with increasing temperature. The mechanism of the surface temperature effect was revealed by interferometry and spectral analysis techniques, which were mainly related to the processes of plasma ablation and surface air heating. Finally, three algorithms including SPF, PLS, and BP-ANN were used to predict the surface temperature, and the results showed that BP-ANN performed best in the 1 μs, 2 μs, and 3 μs exposure time. Under an exposure time of 3 μs, the regression coefficient (R2) and root mean square error (RMSE) could reach 0.9992 and 2.466, respectively. This research would provide a new measurement technology for transient temperature in high-speed physical processes. [ABSTRACT FROM AUTHOR]