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The influence of interface temperature on the damage characteristics of C–Cu contacts’ interface plays a critical role in the current-carrying friction process which occurs between the contact pairs. In this paper, a method is proposed to adjust the interface temperature via settling an external heat source. The damage morphology and material transfer of C–Cu contacts are focused on when the range of interface temperature varies from the room temperature (25 °C) to 300 °C. Based on the experimental results, it can be found that high interface temperature can inhibit the surficial erosion of carbon materials, which tends to be obvious, especially along with the increment of current. Moreover, both the delamination wear of carbon surface and the copper-to-carbon transfer behavior decrease with the thermal surge of interface. This beneficial effect of interface temperature results in the reduction in friction coefficient by 14.3%, whereas the negative impacts brought from the high interface temperature are the surface cracking and the rapid recovery of wear rate of carbon materials, under high-current condition. This project is supported by the National Natural Science Foundation of China (No.51837009, 51807167, 51922090, U1966602, 52077182 and U19A20105), the Foundation of Sichuan Youth Science and Technology (2019JDJQ0019), the Fundamental Research Funds for the Central Universities (2682018CX17) and the funding of Chengdu Guojia Electrical Engineering Co., Ltd (No. NEEC-2018-B06).

Carbon material, because of its high thermal conductivity, ablation resistance and friction resistance, has been widely used in the field of electrical engineering. When confronted strong thermal shock, the interface structure of carbon material would cause thermal shock damage by the rapid temperature difference. In order to systematically and quantificationally study the thermal shock damage of carbon materials, an experimental system that can provide rapid temperature difference was built. The experimental results showed that undergoing a temperature difference of 485 °C, more micro-cracks and holes arose in the pure carbon material, resulting in an increase of porosity by 26.39%, an increase friction coefficient by 48.65%, and a decrease of compressive strength by 23.96%. It was also found the ultrasonic velocity linearly decreased with the increase of porosity and the decrease of the compressive strength caused by thermal shock, which meant the non-destructive ultrasonic testing can be used to conveniently and accurately quantify the thermal shock damage. This conclusion also applies when testing copper-impregnated carbon material, making it a potential method to be used in the maintenance of pantograph which can effectively avoid in rail transit accidents.

Carbon-carbon composites are widely used in thermal management, photocatalysis, aerospace, high-speed railway, and nuclear energy applications, owing to their low density, high temperature stabilit...

Background and aims Helicobacter pylori (HP) culture for diagnosing HP infection is time-consuming and technologically complex. This study evaluated the clinical significance of gastric mucosal gene chip technology in the rapid diagnosis of HP infection and detection of drug resistance in children. Methods Patients (between the age of 2.5 and 16.0 years old) manifesting gastrointestinal symptoms were enrolled in this study. HP culture of gastric mucosa and drug sensitivity test were performed. A gene chip of gastric mucosa was used to detect the presence of HP infection, some single nucleotide polymorphisms in HP drug resistance genes, or associated gene mutation. DNA sequencing was investigated and compared with the gene chip test results. Results Out of 267 cases, HP culture was positive in 169 cases and negative in 98 cases. HP detection by the gene chip method was positive in 208 cases and negative in 59 cases. The sensitivity, specificity, and accuracy of the gene chip technology for diagnosing HP infection were 96.1, 85.0, and 93.6%, respectively. HP resistance gene locus using the gene chip showed the main mutation locus of clarithromycin to be 2143A/G, levofloxacin at locus GyrA 91 and GyrA 87, and amoxicillin at PBP1 556ser. Concordance rates between gene chip and DNA sequencing for VacA-S/M, 16S rRNA, 23S rRNA, and GyrA were greater than 95%, and that of PBP1 was greater than 82%. Conclusion Gastric mucosal gene chip technology can be used for rapid diagnosis and drug resistance detection of HP infection in children.

The discharge plasma channel (DPC) evolution process is observed by taking time-series self-emission intensified charge-coupled device (iCCD) photos and simultaneous shadow images. By adding neutral density filters before iCCD, the subtle structure of the DPC self-emission is observed. It is interesting that a treelike structure is observed soon after the voltage breakdown. Long exposure time photos demonstrate that this structure existed for hundreds of nanoseconds. An inner homogeneously luminous column arises latterly, forming a two-layer self-emission structure. Then, this column gradually occupies the whole DPC. Shadow images show that the expanding DPC maintains good axial uniformity, manifesting the long-standing uneven treelike structure only has limited impact on the energy distribution and the expansion process. The calculated expansion rate reaches its maximum value (4.75 km/s) not at the power peak, but 0.24 $\mu \text{s}$ after that. Then, the expansion rate undergoes fast drop and slow decline. In addition, it seems that the distributions of related plasma parameters experience an evolution from nonuniform to uniform.

Electrical explosion of wires has caused the attention of researchers because of its rich physical phenomena and plenty applications. Microsecond electrical explosion of Cu wires in air under various parameters is conducted, and diverse discharge waveforms are displayed and analyzed. By statistically summarizing the experimental results, experimental parameter selection suggestions for specific applications are given. In addition, the experimental results show that the average axial breakdown field strength is greatly changed with the breakdown time. It drops linearly from ~13 to ~3 kV/cm when breakdown time varies from 2 to $6~\mu \text{s}$ , it keeps at a relatively low value about 1 kV/cm when the breakdown occurred after $8~\mu \text{s}$ . The effect of current density on the rising edge of the resistivity curve is confirmed by the experimental results. Based on the correlation between the breakdown time and breakdown field strength and the correlation between the maximum current density and specific action when resistivity reaches 200 $\mu \Omega \cdot \text {cm}$ , the specific-action model is improved empirically. The proposed “two-stage” model better reflects the actual physical process and has higher accuracy and a wider scope of application.

The use of carbon/copper (C/Cu) composites as electrical contact materials has developed rapidly owing to their superior mechanical strength, electrical conductivity, and self-lubricity. However, the non-wettability of carbon and copper restricts further improvement in their performance. In this work, a strategy based on copper-boron alloying is proposed, to improve C/Cu interfacial bonding, and Cu-B/sintered-carbon composites were successfully prepared using a gas pressure impregnation technique. The results show that the compressive and flexural strengths of the modified composites increased by 39% and 54%, respectively, and the C/Cu contact angle decreased from 123.6° to 21.3° in the case of 2.5 wt% B doping. A minimum electrical resistivity of 1.7 μΩ·m was achieved when a moderate boron content (1.2 wt%) was selected. The role of boron carbide was examined, and competitive mechanisms of C/Cu interfacial bonding level and electrical conductivity are proposed, based on aspects of interfacial resistance and scattering of conducting electrons.

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.

Electrical wire explosion has been of great interest because of its value for the basic scientific researches and diverse applications. Some simulation models have been built to study the process of electrical wire explosion, in the meanwhile, offering a way to forecast the discharge current and load voltage, which is meaningful for researchers to select the appropriate experiment parameters and have a general understanding of the experiment results before you doing them. However, the mechanism behind the electrical wire explosion is considerably complicated and classical models sometimes can be invalid. For instance, the specific action model developed by Tucker is powerless to model the current pause correctly 1. A few magnetohydrodynamics models which are competent to do this are complex and timeconsuming. In this manuscript, a model considering the breakdown process is built on the basis of Tucker’s model.

Underwater electrical explosions of Cu wires were carried out on a microsecond time scale to produce underwater shock waves. Experimental results show that the radial density distribution of the water flow after restrike contains several oscillations, observed as ∼1 mm-spaced layers in the backlit streak images and laser shadowgraphs. The phenomenon is attributed to the partial reheating of the exploding product (EP) by an interior restrike arc, which stimulates a compression wave propagating back and forth radially in the EP. Simulations are used to support the interior breakdown scenario and to demonstrate that each reflection of the compression wave at the EP–water interface launches a weak shock into the water, forming a multilayer structure. As the surrounding metallic vapor is ionized due to radiation and thermal conduction from the arc, the highly conductive plasma channel continues to extend radially and launches the main compression wave, which drowns out the multilayers when the power injection is sufficiently high.

Strong shock wave (SW) can be generated during underwater electrical wire explosion as the exploding wire rapidly expands and pushes the surrounding water. In this paper, a coupled model that describes the behaviours of the circuit, the exploding wire, and the evolution of SW was established. Wide range equation of state (EOS) data and conductivity data were used, making the calculation from solid state possible. The calculated discharge current, voltage across the wire, and trajectories of the wire radius and SW front were generally in good agreement with the experimental results, manifesting the validity of the model. Evolution of SW peak pressure, the process of energy conversion, and the effect of the discharge period were studied using the model. Simulation results showed that the radial attenuation of SW peak pressure is largely dependent on wire diameter and energy deposition process, while the efficiency of converting the deposited electrical energy to the mechanical energy of SW is not as sensitive, varying from 40% to 50% tens of microsecond after the current starts. Fast discharge tends to generate SW with higher initial SW peak pressure, while slow discharge enables higher initial energy storage within the limit of insulation and generates SW with slower radial attenuation. The presented model and numerical results could serve as a reference for parameter selection in related applications.

Background and Aim The infection of Helicobacter pylori (H. pylori) is acquired in childhood and the prevalence vary greatly in different countries and regions. The study aimed to investigate the characteristics of H. pylori infection among children with gastrointestinal symptoms in Hangzhou, a representative city of eastern China. Methods A systematic surveillance of H. pylori infection according to the 13C-urea breath test was conducted from January 2007 to December 2014 in the Children’s hospital, Zhejiang University School of Medicine. The demographic information and main symptoms of every subject were recorded. Results A total of 12,796 subjects were recruited and 18.6% children evaluated as H. pylori positive. The annual positive rates decreased from 2007 to 2014 (χ2 = 20.461, p

Background The decreasing eradication rate of Helicobacter pylori is mainly because of the progressive increase in its resistance to antibiotics. Studies on antimicrobial susceptibility of H. pylori in children are limited. This study aimed to investigate the resistance rates and patterns of H. pylori strains isolated from children. Materials and methods Gastric mucosa biopsy samples obtained from children who had undergone upper gastrointestinal endoscopy were cultured for H. pylori, and susceptibility to six antibiotics (clarithromycin, amoxicillin, gentamicin, furazolidone, metronidazole, and levofloxacin) was tested from 2012-2014. Results A total of 545 H. pylori strains were isolated from 1390 children recruited. The total resistance rates of H. pylori to clarithromycin, metronidazole, and levofloxacin were 20.6%, 68.8%, and 9.0%, respectively. No resistance to amoxicillin, gentamicin, and furazolidone was detected. 56.1% strains were single resistance, 19.6% were resistant to more than one antibiotic, 16.7% for double resistance, and 2.9% for triple resistance in 413 strains against any antibiotic. And the H. pylori resistance rate increased significantly from 2012-2014. There was no significant difference in the resistance rates to clarithromycin, metronidazole, and levofloxacin between different gender, age groups, and patients with peptic ulcer diseases or nonulcer diseases. Conclusions Antibiotic resistance was indicated in H. pylori strains isolated from children in Hangzhou, and it increased significantly during the 3 years. Our data strongly support current guidelines, which recommend antibiotic susceptibility tests prior to eradication therapy.