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Search Results: 1 - 10 of 104025 matches for " ZHANG Yijun "
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Design and Measurement of Microwave Absorbers Comprising Resistive Frequency Selective Surfaces  [PDF]
Meng Zhang, Tian Jiang, Yijun Feng
Journal of Electromagnetic Analysis and Applications (JEMAA) , 2014, DOI: 10.4236/jemaa.2014.68020
Abstract:

Frequency selective surfaces (FSSs) have been successfully used in constructing microwave absorbers which demonstrate the ability of modifying and improving its absorbing performances. In this paper, microwave absorber based on ferromagnetic nano-films has been proposed and investigated with a structure similar to that of the Salisbury screen except that the resistive sheet is replaced by FSS of square patch array of the nano-film. We have explored the FSS absorber from three aspects: equivalent circuit model, electromagnetic full-wave simulation and the actual sample measurement. By carefully tuning the patch size, the thickness of the dielectric spacer and the specification of the ferromagnetic nano-film, we obtain the optimized performance of broadband microwave absorbing. Due to the light weight of the nano-films, the proposed absorbers can achieve an ultra-low surface density less than 0.25 kg/m2.

Non-coding RNAs and the acquisition of genomic imprinting in mammals
YiJun Zhang,LiangHu Qu
Science China Life Sciences , 2009, DOI: 10.1007/s11427-009-0035-2
Abstract: Genomic imprinting, representing parent-specific expression of alleles at a locus, is mainly evident in flowering plants and placental mammals. Most imprinted genes, including numerous non-coding RNAs, are located in clusters regulated by imprinting control regions (ICRs). The acquisition and evolution of genomic imprinting is among the most fundamental genetic questions. Discoveries about the transition of mammalian imprinted gene domains from their non-imprinted ancestors, especially recent studies undertaken on the most ancient mammalian clades — the marsupials and monotremes from which model species genomes have recently been sequenced, are of high value. By reviewing and analyzing these studies, a close connection between non-coding RNAs and the acquisition of genomic imprinting in mammals is demonstrated. The evidence comes from two observations accompanied with the acquisition of the imprinting: (i) many novel non-coding RNA genes emerged in imprinted regions; (ii) the expressions of some conserved non-coding RNAs have changed dramatically. Furthermore, a systematical analysis of imprinted snoRNA (small nucleolar RNA) genes from 15 vertebrates suggests that the origination of imprinted snoRNAs occurred after the divergence between eutherians and marsupials, followed by a rapid expansion leading to the fixation of major gene families in the eutherian ancestor prior to the radiation of modern placental mammals. Involved in the regulation of imprinted silencing and mediating the chromatins epigenetic modification may be the major roles that non-coding RNAs play during the acquisition of genomic imprinting in mammals.
Non-coding RNAs and the acquisition of genomic imprinting in mammals

YiJun Zhang,LiangHu Qu,

中国科学C辑(英文版) , 2009,
Abstract: Genomic imprinting, representing parent-specific expression of alleles at a locus, is mainly evident in flowering plants and placental mammals. Most imprinted genes, including numerous non-coding RNAs, are located in clusters regulated by imprinting control regions (ICRs). The acquisition and evolution of genomic imprinting is among the most fundamental genetic questions. Discoveries about the transition of mammalian imprinted gene domains from their non-imprinted ancestors, especially recent studies undertaken on the most ancient mammalian clades — the marsupials and monotremes from which model species genomes have recently been sequenced, are of high value. By reviewing and analyzing these studies, a close connection between non-coding RNAs and the acquisition of genomic imprinting in mammals is demonstrated. The evidence comes from two observations accompanied with the acquisition of the imprinting: (i) many novel non-coding RNA genes emerged in imprinted regions; (ii) the expressions of some conserved non-coding RNAs have changed dramatically. Furthermore, a systematical analysis of imprinted snoRNA (small nucleolar RNA) genes from 15 vertebrates suggests that the origination of imprinted snoRNAs occurred after the divergence between eutherians and marsupials, followed by a rapid expansion leading to the fixation of major gene families in the eutherian ancestor prior to the radiation of modern placental mammals. Involved in the regulation of imprinted silencing and mediating the chromatins epigenetic modification may be the major roles that non-coding RNAs play during the acquisition of genomic imprinting in mammals. Supported by National Natural Science Foundation of China (Grant No. 30830066), the Ministry of Education of China and Natural Science Foundation of Guangdong Province (Grant No. IRT0447, NSF-05200303) and National Key Basic Research and Development Program of China (Grant No. 2005CB724600)
Simulation study of discharge processes in thunderstorm
Yijun Zhang,Muhong Yan,Xinsheng Liu
Chinese Science Bulletin , 1999, DOI: 10.1007/BF02884930
Abstract: The discharge characteristics in thunderstorm have been calculated by using a numerical cloud model. The results indicate that discharge processes occur when simulated thunderstorm developed at the stage of 30—45 min and initial locations of discharge mainly dlstributes between 4.4—4.8 km and 6.4—6.8 km above the ground (corresponding to ambient temperature of - 10°C and - 25°C). In simulated thunderstorm with a tripole electric structure, 10% of discharges occur between upper positiie charge area and middle negative charge area and propagate upwards (from middle negative charge area to upper positive charge area); while, 90% of discharges occur between lower positive charge area and middle negative charge area and propagate downwards (from negative charge area to lower positive charge area). There is a positive correlation between updraft and discharge. The occurrence of discharge process needs hydrometer with a specified concentration and size in cloud (corresponding to 5 mm/h for precipitation on the ground). The maximum value of precipitation lags the peak of discharge number by a period of about several minutes. The discharge and its variation in thunderstorm are an important factor for monitoring and forecasting thunderstorm.
Polarity inverted intracloud discharges and electric charge structure of thunderstorm
Yijun Zhang,P. R. Krehbiel,Xinsheng Liu
Chinese Science Bulletin , 2002, DOI: 10.1007/BF03183317
Abstract: The three-dimensional spatial and temporal development of impulsive VHF radiation events during lightning discharges has been analyzed based on the data measured by the lightning mapping array (LMA) system with high space and time resolution. The results indicate that intracloud discharges in the thunderstorm with a normal tripole charge structure occur not only between the upper positive and middle main negative charge region, but also with inverted polarity between the lower positive and middle main negative charge region. The polarity-inverted discharge originates from the middle negative charge and propagates downward to the lower positive region, where then the discharge develops horizontally. The characteristics of the discharge are similar to that of normal intracloud discharge except the polarity opposite. The results further confirmed the existence of the lower positive charge region involved in the lightning discharge. It was also found that the inverted charge structure opposite to the normal polarity appears in some storms or at a certain stage of the storm development. In the storm with a polarity inverted charge structure, a main positive charge region is located at the middle part and a negative charge at the upper part of the storm. The intracloud discharges occurred between them are of inverted polarity, showing the existence of a polarity inverted charging mechanism in storms and the complexity of storm charge structure.
A comparative analysis on two solar proton events
GuiMing Le,YanBen Han,YiJun Zhang
Chinese Science Bulletin , 2007, DOI: 10.1007/s11434-007-2220-7
Abstract: This paper presents a comparative analysis on the two Solar Proton Events (SPE), which occurred on 14 July 2000 (Bastille Day) and 28 October 2003 (28OCT03) respectively. It is found that although the peak flux of the latter seemed to be greater than that of the former based on geostationary observations, the maximum intensities of the energetic protons (>10 MeV and 30 MeV) during the Bastille Day event were all higher than those of the 28OCT03 event according to the interplanetary observations. Further analysis indicated that the quantity of the seed particles, which could be accelerated to the energies exceeding 10 and 30 MeV by the Coronal Mass Ejection (CME)-driven shock on 14 July 2000, was far larger than that of the 28OCT03 event. In the Bastille Day case, when the CME approached to the height around 14 R⊙, the CME-driven shock would reach its maximum capacity in accelerating the solar energetic protons (>100 MeV). In contrast, on 28 October 2003, when CME approached to the height about 58R⊙, the CME-driven shock reached its highest potential in accelerating the solar energetic protons of the same category. At this moment, the peak flux (>100 MeV) was about 155 pfu, which was much lower than 355 pfu measured on 14 July 2000. This demonstrated that in the Bastille Day event, the quantity of the seed particles, which could be accelerated to the energy beyond 100 MeV, was significantly larger than its counterpart in the 28OCT03 case. Therefore, the peak flux of an SPE event depends not only on the interplanetary intensity of the solar energetic particles, but also on the velocity of the associated CME-driven shock, and the quantity of the seed particles as well as on the interplanetary magnetic environment. This paper also reveals that the magnetic sheath associated with ICME on 28 October 2003 captured a large number of solar energetic protons, including those having energy greater than 100 MeV.
Owner Risk Averse and Enterprise Technological Innovation Mode: Focus on Major Equipment Manufacturing Industry
Guofeng ZHANG,Yijun YUAN,Lin HU
International Journal of Economics and Finance , 2012, DOI: 10.5539/ijef.v4n11p144
Abstract: Major equipment manufacturing industry is the strategic industry of national defense construction. In the 1990s, China began implementing the "market for technology" strategy, but the strategy did not achieve the desired results, for the important technology and equipment is still highly dependent on imports. The paper analyses the risk aversion characteristics of the owners to purchase, and points out that the risk aversion characteristics comes from the product attributes of important equipment manufacturing industry and asymmetric information. Then it elaborates a dynamic game model to study on the choices of the enterprise’s technological innovation modes in the major equipment manufacturing industry. Finally, the paper concludes that under the strong risk aversion constraints, rational companies will choose collaborative innovation instead of independent innovation.
Frequency Specificity of Regional Homogeneity in the Resting-State Human Brain
Xiaopeng Song, Yi Zhang, Yijun Liu
PLOS ONE , 2014, DOI: 10.1371/journal.pone.0086818
Abstract: Resting state-fMRI studies have found that the inter-areal correlations in cortical networks concentrate within ultra-low frequencies (0.01–0.04 Hz) while long-distance connections within subcortical networks distribute over a wider frequency range (0.01–0.14 Hz). However, the frequency characteristics of regional homogeneity (ReHo) in different areas are still unclear. To examine the ReHo properties in different frequency bands, a data-driven method, Empirical Mode Decomposition (EMD), was adopted to decompose the time series of each voxel into several components with distinct frequency bands. ReHo values in each of the components were then calculated. Our results showed that ReHo in cortical areas were higher and more frequency-dependent than those in the subcortical regions. BOLD oscillations of 0.02–0.04 Hz mainly contributed to the cortical ReHo, whereas the ReHo in limbic areas involved a wider frequency range and were dominated by higher-frequency BOLD oscillations (>0.08 Hz). The frequency characteristics of ReHo are distinct between different parts of the striatum, with the frequency band of 0.04–0.1 Hz contributing the most to ReHo in caudate nucleus, and oscillations lower than 0.02 Hz contributing more to ReHo in putamen. The distinct frequency-specific ReHo properties of different brain areas may arise from the assorted cytoarchitecture or synaptic types in these areas. Our work may advance the understanding of the neural-physiological basis of local BOLD activities and the functional specificity of different brain regions.
Study on the Role of Supersonic Nozzle in Fiber Laser Cutting of Stainless Steel  [PDF]
Yijun Zhou, Jilan Kong, Jin Zhang
Materials Sciences and Applications (MSA) , 2017, DOI: 10.4236/msa.2017.81006
Abstract: Striation-free laser cutting, especially for thick section steel, is hard to obtain due to several factors. The inside shape of the gas nozzle is considered to be one of the most vital factors in striation-free fiber laser cutting. 0.8 mm normal nozzle and a supersonic nozzle are used to cut 0.8 mm AISI316L stainless steel (022Cr17Ni12Mo2) separately. The orthogonal experiment takes nozzle standoff distance, cutting speed, Laser power and gas pressure as its impacting factors. The same orthogonal table is adopted in different condition, using normal nozzle and using supersonic nozzle. In the mean time, Ar gas is used as assisted cutting gas in the experiment. The data from this experiment show that supersonic nozzle seems to be a strong helper for fiber laser cutting. Feed rate’s effect seems stable and inconspicuous under the condition of using supersonic nozzle.
Design of a Novel Semi-Automatic Coconut Tree Climbing Device  [PDF]
Yongfeng Liu, Jie Gong, Yanyun Lin, Fan Zhang, Hui Huang, Wen Xia, Yijun Liu, Yunfei Fu
World Journal of Engineering and Technology (WJET) , 2018, DOI: 10.4236/wjet.2018.63B002
Abstract:
The aim of this study is to present a novel semi-automatic coconut tree climbing device and validate its strength based on actual working conditions. Firstly, the structural composition and operational principal of this device is introduced. After that, the finite element model of this device is established with CATIA. Finally, the strength analysis is executed by means of CATIA. In this study, the equivalent stress, principal stress, and displacement contours are obtained. The simulation results show that the maximum stress of this device is below 355 MPa, which is the value of the yield strength. Therefore, this device is capable of satisfying practical application requirements.
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