•We studied MS waveforms prior to rock burst by HHT and FFT.•Main frequency declines before rock burst.•HHT is simple and reliable to analysis MS waveforms.
This study provides a new research idea ...concerning rock burst prediction. The characteristics of microseismic (MS) waveforms prior to and during the rock burst were studied through the Hilbert–Huang transform (HHT). In order to demonstrate the advantage of the MS features extraction based on HHT, the conventional analysis method (Fourier transform) was also used to make a comparison. The results show that HHT is simple and reliable, and could extract in-depth information about the characteristics of MS waveforms. About 10days prior to the rock burst, the main frequency of MS waveforms transforms from the high-frequency to low-frequency. What’s more, the waveforms energy also presents accumulation characteristic. Based on our study results, it can be concluded that the MS signals analysis through HHT could provide valuable information about the coal or rock deformation and fracture.
Frequent mining disturbance or periodic rupture of far-field roof in coal seam mining will produce multiple vibration loads, which have an important influence on the micro pore-fissure structure and ...macro mechanical behavior of coal. In order to explore the evolution characteristics of pore-fissure structure of coal under vibration load, the SHPB test system was used to carry out a multiple vibration load impact test on bituminous coal. With the help of low-field nuclear magnetic resonance analyzer, the T2 spectrum of coal after each impact was tested, and MRI was performed to analyze the law of pore distribution and evolution, and the damage evolution characteristics of coal pore-fissure structure were studied. The results demonstrate that the peak stress and dynamic elastic modulus of coal show a linear decline trend with the increase of vibration load, and the impact effect of vibration load significantly weakens the bearing capacity and deformation resistance of coal, so it is necessary to conduct in-dept
An efficient asymmetric allylic alkylation reaction with respect to 3-alkylidene oxindoles and racemic Morita-Baylis-Hillman carbonate has been achieved by using a chiral biscinchona alkaloid ...catalyst, which delivers the γ-substituted alkylideneoxindoles with a chiral tertiary center in moderate to good yields (up to 92%) and very good enantioselectivities (up to >99% ee).
An efficient asymmetric allylic alkylation reaction with respect to 3-alkylidene oxindoles and racemic MBH carbonate has been achieved.
Background Restless leg syndrome (RLS) is a type of sensorimotor disorder, and clinical workers have made some recent developments in its pathological mechanism and treatment. Epidemiological studies ...have verified that RLS is notably associated with neuro-psychological diseases, kidney disease, cerebro-cardiovascular diseases, nutritional and metabolic diseases, and pregnancy disorders. The diagnostic rate of RLS is low due to lack of specific clinical symptoms, clear genetic and biological markers, and polysomnography (PSG) diagnostic criteria. There are several sleep centers in foreign countries where the suggested immobilization test (SIT) has been applied in clinical treatment and research about RLS. Objective To evaluate the clinical significance of SIT in RLS diagnosis. Methods The study enrolled 220 patients who visited the PKU-UPenn Sleep Center, Peking University International Hospital with a main complaint of discomfort in bilateral lower limbs during daytime rest or before sleep, and 150 healthy vo
The purpose of this study is to experimentally investigate the dynamic deformation and fragmentation of coal subjected to impact loading. The experiments were carried out using a Split Hopkinson ...pressure bar (SHPB) system, and the characteristics of dynamic deformation and fragmentation were analyzed based on the stress-strain curves and fragmentation distribution. Experimental results show that 1) the dynamic compressive strength of coal is positively related to the impact velocity and it has a certain degree of dispersion, which is the most significant difference between coal and rock materials in terms of mechanical features; 2) The dynamic stress-strain curve of coal shows typical characteristics of stage evolution, which includes compaction stage, elastic stage, yield stage, strain softening stage, and failure stage. The yield stage and strain softening stage occupy the majority of the stress-strain curve, and thus constitute the main processes of inelastic deformation; 3) The coal fragmentation caused by impact loads results in a fractal distribution, and the fractal dimension ranges from 2.1 to 2.5, which is higher than that under quasi-static tests, this indicate that coal is more fragile under impact loads than that under quasi-static loads.
Microseismic (MS) technology has been widely adopted for monitoring coal and rock dynamic disasters. Insights into MS wave characteristics contribute to the accurate prediction of these disasters. In ...this study, MS wave characteristics were analysed from three aspects: the signal spectra, wavelet packet energy and fractal features. It is shown that prior to the rock burst, the MS wave main frequency decreased following a power law, the amplitude linearly increased, the wavelet packet energy tended to become concentrated on the low frequency bands, and the correlation dimension decreased. When the rock burst occurred, the MS wave main frequency, wavelet packet energy and correlation dimension declined to their lowest levels. Meanwhile, the amplitude rose to a maximum. Therefore, the MS wave characteristics in this study were found to effectively identify and extract precursor information of value for predicting rock dynamic disasters.
Upconverting ion-selective nanoparticles that emit light at the near-infrared region are prepared here. The transport of calcium ions induces the deprotonation of the incorporated chromoionophore ...(P6) through ion exchange resulting in an increase in the emission of UCNPs for the detection of intracellular calcium ions.
Upconverting ion-selective nanoparticles that emit light at the near-infrared region are prepared here.
4-hydroxymethyl phenylboronic acid (4-HMPBA), phenol and formaldehyde were for the first time to synthesize boron containing novolac phenolic resins (Novolac-4-HMPBA). Its corresponding carbon fibers ...(CF) composites were also fabricated via the lamination followed by hot-compression. Results revealed that Novolac-4-HMPBA was successfully synthesized. Compared to common novolac phenolic resin (NPR), the corresponding decomposition temperature of 5% weight loss (
T
5
) and char yield at 800 °C (
C
800
) of cured Novolac-4-HMPBA-30 were increased to 369.1 °C and 69.7%, increased by 41.3% and 26.3%, respectively. The CF/Novolac-4-HMPBA-20 composites presented relatively higher interlaminar shear strength (ILSS) of 35.9 MPa, increased by 50.2% compared to that of CF/NPR composites. After the treatment at 800 °C for one hour under Ar atmosphere, the corresponding ILSS value of CF/Novolac-4-HMPBA-20 composite was decreased to 7.8 MPa, but still higher than that of CF/NPR composites (3.0 MPa).
RSM was utilized to optimize and model influential parameters on fluoride removal. Regression models involving independent variables and main response were developed. Interactive effects and optimum ...of process factors were illuminated and determined. Fluoride removal efficiency of 99.69% was observed in optimal process conditions.
Response surface methodology was utilized to model and optimize the operational variables for defluoridation using an electrodialysis process as the treatment of secondary effluent of the graphite industry. Experiments were conducted using a Box-Behnken surface statistical design in order to evaluate the effects and the interaction of the influential variables including the operational voltage, initial fluoride concentration and flow rate. The regression models for defluoridation and energy consumption responses were statistically validated using analysis of variance (ANOVA); high coefficient of determination values ( R 2 = 0.9772 and R 2 = 0.9814; respectively) were obtained. The quadratic model exhibited high reproducibility and a good fit of the experimental data. The optimum values of the initial fluoride concentration, voltage and flow rate were found to be 13.9 mg/L, 13.4 V, 102.5 L/h, respectively. A fluoride removal efficiency of 99.69% was observed under optimum conditions for the treatment of the secondary effluent of the graphite industry.
