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By the end of 2020, 45 cities in the Chinese mainland operated 244 urban rail transit lines with a total length of 7,969.7 km, Urban rail transit in Chinese cities witnessed a steady growth both in operating scale and passenger traffic in 10 years. Recent studies have explored the environmental and social effects of urban rail transit; however lack in-depth discussion on economic growth. As a quasi-natural experiment, this paper empirically tests the effect of the opening of urban rail transit on urban economic growth based on the panel data of 286 prefecture-level cities in China from 2008 to 2020 and PSM-DID Model. Analysis results show that, rail transit drives urban economic growth. This effect has scale heterogeneity and regional heterogeneity. The findings of this study can provide a valuable reference for the government when it plans the layout of urban rail transit for construction.

Gas concentrations of different sections in return airway of working face were tested, and the gas distribution contours of roadway section were made for studying gas concentration distribution. Conclusions: gas distribution in different sections of roadway is uneven, and gas will be fully homogeneous with air after moving 100 m in front of working face; goaf gas pouring out through the upper corner has a greater impact on distribution of gas concentration. Recommendations for layout of gas monitoring points and sensors in return air of working face were provided, namely gas sensors in return airway should be arranged in central line of the roadway less than 300 mm away from the roof, and a sensor measuring point should be arranged at the place 10 m away from the upper corner.

In order to study the surface electric potential signals during the coal adsorbing gas, designed the experiment system of the coal adsorbing gas and tested the electric potential signal generated during the coal adsorbing gas under different gas pressure. Experiment results show that surface potential signals can be generated in the process of adsorbing gas, and they increases gradually along with the growth of adsorption time. For the same coal samples, different gas pressure from the coal surface potential signal to the first adsorption gas pressure of coal potential signal has a memory effect in the process of absorption. When the adsorption gas pressure is higher than the first adsorption gas pressure, the coal induces stronger surface potential signals. And when the adsorption gas pressure is lower than the first adsorption gas pressure, the coal induce weaker surface potential signals. The main reasons why surface potential signals can be generated from coal adsorbing gas are that the gas seepage in coal pores produces flow potential and free charge is generated when the gas damages micro cracks and crack of coal. [ABSTRACT FROM AUTHOR]

An investigation was conducted on the characteristics of surface current induced by coal under uniaxial compression. An interpretation of the mechanisms governing the electrical behavior of the coal structures under uniaxial stress tests was presented in this paper. The experimental results show that the current signal is generated during the deformation and fracture process of coal samples. When the load is applied, the electric current makes its initial appearance and increases rapidly after a considerable time lag at 8C, until after the stress reaches a value δc ≈0.75δmax where δmax is the maximum applied stress before sample fracture. There is a good correlativity between the current and the load, and the correlation coefficient between them can reach up to 0.8 to 0.9. The current peaks occur under sudden loading and unloading. It suggests that the current is closely related to the loading. The status of coal loading level may be determined by the current signal. The intensive study on coal surface current can provide a way for monitoring the stability of coal and rock. [ABSTRACT FROM AUTHOR]

The experiment was made on the rules of surface potential of coal or rock during friction process. The mechanism producing surface potential of coal or rock friction was discussed initially. The results show that surface potential can be produced during coal or rock friction process, and it has a better corresponding relationship with load. The coal or rock friction process can be divided into two stages: static friction stage and kinetic friction stage. The surface potential signal in kinetic friction stage was stronger than that in static friction stage. During rock friction process, the polarity of surface potential signal was basically bidirectional mutation, and the polarity of surface potential on both sides of friction surface had obvious symmetry. While the polarity of surface potential signal was basically unidirectional mutation in coal friction process, and the polarity of surface potential on both sides of friction surface was not in significant symmetry. The changes of surface potential during coal or rock friction process can be explained by mechanism of electricity produced by friction, thermionic emission and field electron emission. [ABSTRACT FROM AUTHOR]

In order to study the relationship between surface potential distribution and destruction localization of the coal sample, the distribution law of strain field and surface potential of coal samples were tested under load. The relationship between surface potential field and strain localization & fracture of coal samples was studied and analyzed. Results show that the coal samples show obvious strain localization characteristics in deformation and failure process, the strain localization concentrated area is the severe damage and fracture part of coal samples, and the strain localization zones is usually the fracture location of coal samples. The distribution character of surface potential field of coal samples during deformation and fracture is similar to the strain field, the potential field will be centralized in the damage localization area, the connection between the points of high potential and low potential reflects the rupture section of coal samples. [ABSTRACT FROM AUTHOR]

The controllable key factors in magnetorheological finishing device were studied to determine their influence on efficiency of magnetorheological finishing ( MRF ) and surface of MRF, as well as interaction between efficiency and surface. Based on theoretical and experimental research, the law of material removal was explored and a new process variable based material removal model (PVMR) was proposed. The experimental findings demonstrate that PVMR reveals the law of the material removal with introduction of three concepts and makes a material removal function z (y i) where the magnetorheological finishing process parameters are considered since they are easy to control and adjust. So the material function of this model is quadratic curve function which is readily suitable for stability and online control magnetorheological finishing. [ABSTRACT FROM AUTHOR]

In order to analyze the failure mechanism of coal under the load, multi-fractal characteristics of surface potential of coal sample under uniaxial compression was studied. The surface potential was analyzed using multi-fractal theory to calculate the multi-fractal spectrum. The change rules of characteristic parameters of multi-fractal spectrum with the loading process were studied. The results show that the surface potential represents multi-fractal feature, and keeps scale invariance in the wide range. The shape of the multi-fractal spectrum of different size specimens shows right hook. The characteristic parameters of multi-fractal spectrum, Δα and Δf, vary with the process of deformation and fracture of coal sample with load. The changes of trend that Δα and Δf increase in a long time, then decrease rapidly can be used as a precursory factor of coal-rock failure to forecast the instability of the samples. The results are helpful to understand the inherent law in the coal specimen failure process and forecast the dynamic disaster using the potential. [ABSTRACT FROM AUTHOR]

Acoustic emission signals generated in process of methane sorption and seepage in coal were observed, and the dynamic characteristic of damage evolution and the seepage process were analyzed based on the characteristic of acoustic emission. The experiments indicated that the strength of acoustic emission decreased with the adsorbed methane increase. And the acoustic emission showed paroxysmal in time domain. The results give implication that the methane seepage in coal layer by layer. In its seepage process, methane shows destructive seepage while the gas pressure gradient was bigger than the threshold value. Then, pressure of the seepage face decreased and the methane shows undestructive seepage. Sorption process tends to equilibrium with the gas pressure increase. [ABSTRACT FROM AUTHOR]

An efficient metal-free, (NH 4 ) 2 S 2 O 8 mediated oxidative dearomatization of indoles for the construction of C2-quaternary indolinones was disclosed. A series of C2-quaternary indolinones derivatives with good functional group tolerance were obtained in moderate to excellent yields. This methodology provides an alternative approach for the direct generation of all-carbon quaternary centers at the C2 position of indoles. This catalytic approach represents a step-economic and convenient strategy for the oxidative dearomatization of indoles., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)