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To explore the influence of dynamic impacts on the creep properties of underground pillars, an underground coal sample is selected as the research material, and the mechanical response characteristics under dynamic impact are explored by the split Hopkinson pressure bar and high-speed camera. The results show that (1) when the creep stress level is higher, the initial deformation of coal is greater, and the damage to the sample through long-term creep is more severe. (2) With increasing levels of static stress, the dynamic strength and elastic modulus of the coal sample first increase and then decrease, and the dynamic elastic modulus is positively correlated with the impact velocity. (3) The static stress level determines the process of crack propagation and the degree of fracture, while the external dynamic impact determines the propagation speed of the crack. The results of this research enrich the theory of control systems for the prevention of coal pillar failure and instability and provide a theoretical basis for safe and efficient mining of working faces. [ABSTRACT FROM AUTHOR]

The self-excited multi-scale mechanical vibrations, their sources and their mutual coupling of different regions on the forearms of supine subjects, are experimentally investigated, using a simple noncontact method, optical video microscopy, which provides 1 µm and 25 ms spatiotemporal resolutions. It is found that, in proximal regions far from the radial artery, the vibrations are the global vibrations of the entire forearm excited by remote sources, propagating through the trunk and the limb. The spectrum is mainly composed of peaks of very low frequency motion (down to 0.05 Hz), low frequency respiration modes, and heartbeat induced modes (about 1Hz and its harmonics), standing out of the spectrum floor exhibiting power law decay. The nonlinear mode-mode coupling leads to the cascaded modulations of higher frequency modes by lower frequency modes. The nearly identical waveforms without detectable phase delays for a pair of signals along or transverse to the meridian of regions far away from the artery rule out the detectable contribution from the propagation of Qi, some kind of collective excitation which more efficiently propagates along meridians, according to the Chinese medicine theory. Around the radial artery, in addition to the global vibration, the local vibration spectrum shows very slow breathing type vibration around 0.05 Hz, and the artery pulsation induced fundamental and higher harmonics with descending intensities up to the fifth harmonics, standing out of a flat spectrum floor. All the artery pulsation modes are also modulated by respiration and the very slow vibration. [ABSTRACT FROM AUTHOR]

We report ab initio calculations of electronic transport properties of heterostructure based on MoS2 nanoribbons. The heterostructure consists of edge hydrogen-passivated and non-passivated zigzag MoS2 nanoribbons (ZMoS2NR-H/ZMoS2NR). Our calculations show that the heterostructure has half-metallic behavior which is independent of the nanoribbon width. The opening of spin channels of the heterostructure depends on the matching of particular electronic orbitals in the Mo-dominated edges of ZMoS2NR-H and ZMoS2NR. Perfect spin filter effect appears at small bias voltages, and large negative differential resistance and rectifying effects are also observed in the heterostructure. [ABSTRACT FROM AUTHOR]

GaN thin films on Si(001) substrates are studied by infrared reflectance (IRR) spectroscopy at room temperature (RT). Variations in the IRR spectral line shape with the microstructure of GaN/Si(011) film are quantitatively explained in terms of a three-component effective medium model. In this model, the nominally undoped GaN film is considered to consist of three elementary components, i.e., single crystalline GaN grains, pores (voids), and inter-granulated materials (amorphous GaN clusters). Such a polycrystalline nature of the GaN/Si(001) films was confirmed by scanning electron microscopy measurements. It was demonstrated that based on the proposed three-component effective medium model, excellent overall simulation of the RT-IRR spectra can be achieved, and the fine structures of the GaN reststrahlen band in the measured RT-IRR spectra can also be interpreted very well. Furthermore, the volume fraction for each component in the GaN/Si(001) film was accurately deter! mined by fitting the experimental RT-IRR spectra with the theoretical simulation. These results indicate that IRR spectroscopy can offer a sensitive and convenient tool to probe the microstructure of GaN films grown on silicon. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

For effective use of a laser-produced-plasma (LPP) light source, an LPP is desired to emit a narrow spectral peak because the reflection spectrum of multilayer mirrors for guiding emission from the source is very narrow. While a Gd plasma has been studied extensively as an extreme ultraviolet (EUV) light source at around 6.8 nm, where La/B4C multilayer is reported to have a high reflectivity with a bandwidth of about 0.6%, all previous works using an Nd:YAG laser reported very broad spectra. This paper reports the first narrowing of the 6.8 nm peak in the case of using an Nd:YAG laser to generate a Gd plasma by using a pre-pulse. The best peak narrowing is observed when a pre-formed plasma is heated by a 1064 nm main laser pulse with a duration of 10 ns at the irradiation density of 4x 1011 W/cm² at a delay time of 50 ns after the pre-pulse irradiation. The observed spectral width of about 0.3 nm is about one fifth of the value for no pre-formed plasma. The peak wavelength of the 6.8 nm band shifted to a longer wavelength side and the peak was broadened both for lower and higher laser irradiation density. It is discussed that this robustness of the peak position of the 6.8 nm Gd peak against temperature change is suitable to achieve a narrow bandwidth from an LPP generated on solid. The observed spectra are compared with those previously reported in various conditions. [ABSTRACT FROM AUTHOR]

Under partial shading conditions (e.g., due to buildings, trees, and clouds), multiple peaks may exist on the power-voltage (P-V) characteristic curve of photovoltaic (PV) array, leading to the conventional maximum power point tracking methods fail to extract the global maximum power point (GMPP). In this paper, a mathematical model of PV array under partial shading conditions with a voltage calculated principle is established. The presented model is implemented with an m-file in MATLAB software, which is used as a tool to study the non-linear characteristics of current-voltage (I-V) and P-V curves of PV array, as well as quickly develop the GMPP tracking controllers. Besides, an adaptive random particle swarm optimization (ARPSO) algorithm is presented to accurately extract the GMPP under partial shading conditions. Five simulation cases with different partial shading patterns are used to evaluate the performance of the presented approach by comparing with the conventional PSO, perturbation and observation (P&O), incremental conductance (INC), and genetic algorithm (GA) methods. Simulation results show that the ARPSO algorithm can rapidly find the GMPP under different shading conditions compared with the conventional PSO algorithm. Furthermore, the presented algorithm can accurately extract the GMPP when the shading condition sharply changes, while the P&O and INC algorithms fail to track the GMPP, but only detect the rightmost MPP encountered either local or global and regardless of the course. Besides, the ARPSO can rapidly and accurately converge to the GMPP with smaller population size and higher convergence speed compared with the GA. [ABSTRACT FROM AUTHOR]

We proposed a method to realize three-dimensional (3D) gradient index (GRIN) devices requiring large refractive index (RI) range with broadband performance. By combining non-resonant GRIN woodpile photonic crystals structure in the metamaterial regime with a compound liquid medium, a wide RI range (1-6.32) was fulfilled flexibly. As a proof-of-principle for the low-loss and nondispersive method, a 3D Eaton lens was designed and fabricated based on 3D printing process. Full-wave simulation and experiment validated its omnidirectional wave bending effects in a broad bandwidth covering Ku band (12 GHz-18 GHz). [ABSTRACT FROM AUTHOR]

A droplet moving inside a grooved channel is studied by using a new lattice Boltzmann model for multiphase flows with large density ratio. A constant body force is applied to drive the droplet. Flows under different surface tensions, driving forces, density ratios, wall wettabilities, and groove geometries are investigated. It is found that the drag on the droplet and the flow pattern are strongly affected by the wall wettability and topography when the system scale is small. The effects of the driving force on the droplet are investigated through comparison of two different ways of applying it. Besides, the density ratio is varied over a wide range to assess its effects in the present setup. Special attention is paid to grooved hydrophilic walls which tend to enhance the droplet-wall contact. For such walls, two distinctive types of shape of the interface inside the groove are found and series of numerical investigations are carried out to find the critical wall contact angle, groove width and depth that determine which kind of shape the droplet assumes. Some typical cases are chosen for detailed analyses and compared to some other work. This study is expected to improve our understanding on the lotus effect and the physics of small scale flows near rough walls. [ABSTRACT FROM AUTHOR]

A free-space broadband carpet-cloak, designed based on transformations optics and quasi-conformal mapping, was realized with all-dielectric gradient index rod-connected diamond-structured photonic crystals (PCs) in metamaterial regime. Complex three-dimensional sample with smooth continuous changing unit cells was fabricated precisely by stereolithography (SL) using photo-curable resin. Thus, by gradually varying the unit cell constitutive parameters of the diamond-based PCs with nearly isotropic properties, the required complex spatial distribution of the refractive index profile was ideally achieved to reduce the scattering of the electromagnetic wave. The non-resonant property of the sub-wavelength PCs unit cell resulted in broad bandwidth and relatively low loss. [ABSTRACT FROM AUTHOR]

Threshold voltage instability measured by the pulse current-voltage technique has been recognized as the transient charging and discharging of the preexisting bulk traps in Hf-based high-k gate dielectrics, and these high-k traps or called border traps can instantly exchange charge carriers with the underlying Si substrate by tunneling through the thin interfacial oxide. Based on an elastic tunneling model through trapezoidal potential barriers, the spatial and energetic distribution of border traps in the HfO2/SiO2 high-k gate stack can be profiled as a smoothed, three-dimensional mesh by measuring the low-frequency capacitance-voltage characteristics of high-k metal-oxide-semiconductor capacitors with n-type Si substrate. [ABSTRACT FROM AUTHOR]