Resistivity in Formation and Decomposition of Natural Gas Hydrate in Porous Medium

A new one-dimensional system for resistivity measurement for natural gas hydrate(NGH)exploitation is designed,which is used to study the formation and decomposition processes of NGH.The experimental results verify the feasibility of the measurement method,especially in monitoring the nucleation and growth of the NGH. Isovolumetric formation experiment of NGH is performed at 2°C and 7.8 MPa.Before the NGH formation,the initial resistivity is measured to be 4-7Ω·m,which declines to the minimum value of 2-3Ω·m when NGH begins to nucleate after the pressure is reduced to 3.3 MPa.As the NGH grows,the resistivity increases to a great extent,and finally it keeps at 11-13Ω·m,indicating the completion of the formation process.The NGH decomposition experiment is then performed.When the outlet pressure decreases,NGH begins to decompose,accordingly,the resistivity declines gradually,and is at 5-9Ω·m when the decomposition process ends,which is slightly higher than the resistivity value before the formation of NGH.The occurrence and distribution uniformity of NGH are determined by the distribution and magnitude of the resistivity measured on an one-dimensional sand-packed model.This study tackles the accurate estimation for the distribution of NGH in porous medium,and provides an experimental basis for further study on NGH exploitation in the future.

Electric Parameter Measurement System Based on Virtual Instrument

In this paper, virtual instnnnent technology is applied in the development of an electric parameter measta＇ement system for mobile power stations. Virtual inset technology is used to lmasure and analyze electric parameters of mobile power stations. The whole measurement system is mmpmed of computer, data acquisition card and LabView virtual instnnnent platform. It can be used to monitor, record, and analyze electric parameters of sinusoidal alternating ciroait These parameters include voltage, effective value of ctaxent, phase differences, power, and etc. Experimental results show that this virtual measuring device satisfy the requirement of mobile power station montoring anti dynamic property analysis. Hardware and software design of the system will be discussed in detail.

Partial Discharge Measurement of OCP-Tapes in Rotating Electrical Machines

Nowadays, PD （partial discharge） measurements are a crucial part of the preventive maintenance of electrical equipment within high voltage engineering. Especially for electrical machines, both the supplier and the user are interested in the results of PD measurements. However, PDs hardly represent the cause of the failure, more likely they are claimed as the outcome of a failure. This paper deals with the insulation of a 6 kV electrical machine, whereas PD measurements were carried out at a single stator from wound coils. During manufacturing, these coils were equipped with different materials for the OCP （outer corona protection）. Using different PD measurement systems and different bandwidths, investigations of the PD behavior of the coils were carried out. Additionally, the surface resistivity of the corona protection was determined. As a result, conclusions for the correlations between the resistance of the OCP as well as the PD behavior are stated. Furthermore, the influence of using different measurement systems, different measuring circuits, and different bandwidths is shown.

Realization of low contact resistance close to theoretical limit in graphene transistors

Realizing low contact resistance between graphene and metal electrodes remains a well-known challenge for building high-performance graphene devices. In this work, we attempt to reduce the contact resistance in graphene transistors and further explore the resistance limit between graphene and metal contacts. The Pd/graphene contact resistance at room temperature is reduced below the 100 Ω·μm level both on mechanically exfoliated and chemical-vapor-deposition graphene by adopting high-purity palladium and high-quality graphene and controlling the fabrication process to not contaminate the interface. After excluding the parasitic series resistances from the measurement system and electrodes, the retrieved contact resistance is shown to be systematically and statistically less than 100 Ω·μm, with a minimum value of 69 Ω·μm, which is very close to the theoretical limit. Furthermore, the contact resistance shows no clear dependence on temperature in the range of 77-300 K; this is attributed to the saturation of carrier injection efficiency between graphene and Pd owing to the high quality of the graphene samples used, which have a sufficiently long carrier mean-free-path.