Manipulating the polarizations of electroagnetic waves by flexible and diverse means is desirable for myriad microwave systems. More recently, metasurfaces have emerged as promising alternatives to conventional polarization manipulation components because the flexibility of their geometry means that they can be arbitrarily customized. In this context, a bilayered metasurface is presented to simultaneously manipulate the polarized states of reflected and transmitted microwaves. Regardless of whether an incident electromagnetic wave is x-polarized or y-polarized, the reflected and transmitted waves are converted into their orthogonal waves at the operating frequency. The designed metasurface has a high polarization conversion rate, above 90%, for both normal and oblique incidences. Experimental results verify the correctness of the simulated results. Finally, the axial ratio and surface current distributions are employed to reveal the physics of the polarization manipulation. The proposed metasurface will be beneficial in the design of flexible and versatile polarization converters, has great potential for applications in polarization-controlled devices and is believed to be extendable to higher frequency regimes.
Metamaterial absorbers have been widely studied in the past decade and their performances have been incessantly improved in the practical applications. In this paper, we present a broadband terahertz metamaterial absorber based on graphene-polyimide composite structure, and the structure consists of a metal substrate and graphene layers with different sizes separated by two polyimide dielectric layers. The simulation results show that the absorptance of the absorber is greater than 90% in 0.86–3.54 THz with the fractional bandwidth of 121.8%. The absorptance can be adjusted by changing the chemical potential of graphene. In addition, the absorber is insensitive to polarization and still has robust tolerance for the oblique incidence. The equivalent circuit model based on transmission line is introduced to analyze the physics of the designed absorber and the results are in good agreement with the simulations. We believe that the designed absorber is a potential competitive candidate in terahertz energy harvesting and thermal emission.
In order to explore the mechanism of coal pillar rock burst in the overlying coal body area, taking W1123 working face of Kuangou Coal Mine as the engineering background, the full mining stage of W1123 is simulated by FLAC3D. It is found that the high stress concentration area has appeared on both sides of the coal pillar when W1123 does not start mining. With the advance of the working face, the high stress concentration area forms X-shaped overlap. There is an obvious difference in the stress state between the coal pillar under the solid coal and the coal pillar under the gob in W1123. The concrete manifestation is that the vertical stress of the coal pillar below the solid coal is greater than the vertical stress of the coal pillar below the gob. The position of the obvious increase of the stress of the coal pillar in the lower part of the solid coal is ahead of the advancing position of the working face, and the position of the obvious increase of the stress of the lower coal pillar in the gob lags behind the advancing position of the working face. At the same time, in order to accurately reflect the true stress environment of coal pillars, the author conducted a physical similarity simulation experiment in the laboratory to study the local mining process of the W1123 working face, and it is found that under the condition of extremely thick and hard roof, the roof will be formed in the gob, the mechanical model of roof hinged structurer is constructed and analyzed, and the results show that the horizontal thrust of roof structure increases with the increase of rotation angle. With the development of mining activities, the self-stable state of the high stress balance in the coal pillar is easily broken by the impact energy formed by the sudden collapse of the key strata. Therefore, the rock burst of coal pillar in the overlying coal body area is the result of both static load and dynamic load. In view of the actual situation of the Kuangou Coal Mine, the treatment measures of rock burst are put forward from the point of view of the coal body and rock mass.
The research analyzes and improves upon the concept of ventilation safety subregions in coal mines and proposes a partition method based on the breadth-first search algorithm for assessing the quality of the ventilation. This involves the analysis of the function of ventilation subregions and of ventilation gas-monitoring data. Then, using the so-called ventilation sensitivity matrix as the analysis method, we confirm the consistency of considering safety subregions, which are associated with air consumption places as the core concept and the objective positioning of the safety subregions within the ventilation analysis. This allows us to establish the validity and practicability of employing regional characteristic information of mine ventilation and gas concentrations in the mine’s air. Finally, based on the characteristics of the ventilation subregions, ventilation air quantity network maps with safety subregions are proposed and their application is demonstrated. The advantages and characteristics of using these maps to replace ventilation network maps for ventilation analysis are demonstrated.
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