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Xiaofeng Li,Yaping Tao,Ziyu Hu,Shengli Zhang 한국물리학회 2015 Current Applied Physics Vol.15 No.9
The mechanical and thermodynamic properties, chemical bonding characteristics and electronic structure of Nb2MB2 (M = Mo, W, Re or Os) with a new tetragonal U3Si2-type superstructure (space group P4/ mnc, no. 128) were studied by means of density functional theory calculations. All Nb2MB2 structures studied were demonstrated to be thermodynamically and mechanically stable. The bulk, shear and Young's moduli, Poisson's ratio, Debye temperature and anisotropy factors were derived for ideal polycrystalline Nb2MB2 aggregates. Among these compounds, Nb2WB2 was found to have the highest shear modulus and hardness. The electronic densities of state and electronic localization function analysis revealed the metallicity and strong covalent BeB, NbeB and MB bonding in Nb2MB2. Moreover, these results reveal that the covalence between Nb 4d, M nd (n = 4 for Mo and 5 for W, Re and Os) and B 2p states is the cause of the relatively higher elastic modulus and hardness of the Nb-based compounds. Finally, thermodynamic properties, including the bulk modulus, heat capacity and thermal expansion coefficient of Nb2WB2 were obtained systematically under high temperature and pressure.
Female migrants in China: Second child fertility behavior and employment stability
Li Hu,Dong Chenyu,Wang Xiaofeng 이화여자대학교 아시아여성학센터 2022 Asian Journal of Women's Studies(AJWS) Vol.28 No.1
Researchers have found a negative relationship between female fertility and employment. China’s floating population comprises 244 million as of 2017. This migrant population has relevance in view of China’s hukou (household registration) system, with the proportion of females increasing annually. This study is grounded in social role theory and employs the 2016 China Migrants Dynamic Survey (CMDS) to examine the impact of second child fertility behavior on the employment stability of female migrants in China, with respect to urban and rural differences. Binary logistic regression results indicate that bearing second children negatively affects the employment stability of female migrants. Also, its impact is stronger for female migrants of urban origin than their rural counterparts. In addition, family migration is noted to mitigate the conflict between childbearing and employment stability for them as they continue to face the dilemma of having children or seeking employment stability, especially if family and social support is weak or absent. We discuss the implications of this study on policy changes to alleviate the conflict female migrants experience in choosing between fertility and employment.
Li, Hongjia,Xu, Xiaodong,Hu, Dan,Tao, Xiaofeng,Zhang, Ping,Ci, Song,Tang, Hui The Korea Institute of Information and Commucation 2011 Journal of communications and networks Vol.13 No.6
In order to control interference and improve spectrum efficiency in the femtocell and macrocell overlaid system (FMOS), we propose a joint frequency bandwidth dynamic division, clustering and power control algorithm (JFCPA) for orthogonal-frequency-division-multiple access-based downlink FMOS. The overall system bandwidth is divided into three bands, and the macro-cellular coverage is divided into two areas according to the intensity of the interference from the macro base station to the femtocells, which are dynamically determined by using the JFCPA. A cluster is taken as the unit for frequency reuse among femtocells. We map the problem of clustering to the MAX k-CUT problem with the aim of eliminating the inter-femtocell collision interference, which is solved by a graph-based heuristic algorithm. Frequency bandwidth sharing or splitting between the femtocell tier and the macrocell tier is determined by a step-migration-algorithm-based power control. Simulations conducted to demonstrate the effectiveness of our proposed algorithm showed the frequency-reuse probability of the FMOS reuse band above 97.6% and at least 70% of the frequency bandwidth available for the macrocell tier, which means that the co-tier and the cross-tier interference were effectively controlled. Thus, high spectrum efficiency was achieved. The simulation results also clarified that the planning of frequency resource allocation in FMOS should take into account both the spatial density of femtocells and the interference suffered by them. Statistical results from our simulations also provide guidelines for actual FMOS planning.
Impedance Interaction Modeling and Analysis for Bidirectional Cascaded Converters
Yanjun Tian,Fujin Deng,Zhe Chen,Xiaofeng Sun,Yanting Hu 전력전자학회 2015 ICPE(ISPE)논문집 Vol.2015 No.6
For the cascaded converter system, the output impedance of source converter interacts with the input impedance of load converter, and the interaction may cause the system instability. In bidirectional applications, when the power flow is reversed, the impedance interaction also varies, which brings more uncertainty to the system stability. An investigation is performed here for showing that the forward and reverse interactions are prominently different in terms of dynamics and stability even though the cascaded converter control remains unchanged. An important guideline has been drawn for the control of the cascaded converter. That is when voltage mode converter working as the load converter; the constant power mode converter as the source converter, the system is more stable. The concluded findings have been verified by simulation and experimental results.
Impedance Coordinative Control for Cascaded Converter in Bidirectional Application
Yanjun Tian,Fujin Deng,Zhe Chen,Xiaofeng Sun,Yanting Hu 전력전자학회 2015 ICPE(ISPE)논문집 Vol.2015 No.6
Two stage cascaded converters are widely used in bidirectional applications, but the negative impedance may cause system instability. Actually the impedance interaction is much different between forward power flow and reversed power flow, which will introduce more uncertainty to the system stability. This paper proposes a control method for the constant power controlled converter in cascaded system, and consequently it can change the negative impedance of constant power converter into resistive impedance, which will improve the cascaded system stability, as well as merge the impedance difference between forward and reversed power flow. This paper addresses the analysis with the topology of cascaded dualactive - bridge converter (DAB) with inverter, and the proposed control method can also be implemented in unidirectional applications and other general cascaded converter system. The effectiveness has been validated by both simulation and experimental results.
Hongjia Li,Xiaodong Xu,Dan Hu,Xiaofeng Tao,Ping Zhang,Song Ci,Hui Tang 한국통신학회 2011 Journal of communications and networks Vol.13 No.6
In order to control interference and improve spectrumef-ficiency in the femtocell and macrocell overlaid system (FMOS), we propose a joint frequency bandwidth dynamic division, clustering and power control algorithm (JFCPA) for orthogonal-frequencydivision-multiple access-based downlink FMOS. The overall system bandwidth is divided into three bands, and the macro-cellular coverage is divided into two areas according to the intensity of the interference from the macro base station to the femtocells, which are dynamically determined by using the JFCPA. A cluster is taken as the unit for frequency reuse among femtocells. We map the problem of clustering to the MAX k-CUT problem with the aim of eliminating the inter-femtocell collision interference, which is solved by a graph-based heuristic algorithm. Frequency bandwidth sharing or splitting between the femtocell tier and the macrocell tier is determined by a step-migration-algorithm-based power control. Simulations conducted to demonstrate the effectiveness of our proposed algorithm showed the frequency-reuse probability of the FMOS reuse band above 97.6% and at least 70% of the frequency bandwidth available for the macrocell tier, which means that the co-tier and the cross-tier interference were effectively controlled. Thus, high spectrum efficiency was achieved. The simulation results also clarified that the planning of frequency resource allocation in FMOS should take into account both the spatial density of femtocells and the interference suffered by them. Statistical results from our simulations also provide guidelines for actual FMOS planning.
Tang, Wei,Chen, Zhongxin,Tian, Bingbing,Lee, Hyun-Wook,Zhao, Xiaoxu,Fan, Xiaofeng,Fan, Yanchen,Leng, Kai,Peng, Chengxin,Kim, Min-Ho,Li, Meng,Lin, Ming,Su, Jie,Chen, Jianyi,Jeong, Hu Young,Yin, Xuesong American Chemical Society 2017 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.139 No.29
<P>Sulfur is an attractive cathode material for next-generation lithium batteries due to its high theoretical capacity and low cost. However, dissolution of its lithiated product (lithium polysulfides) into the electrolyte limits the practical application of lithium sulfur batteries. Here we demonstrate that sulfur particles can be hermetically encapsulated by leveraging on the unique properties of two-dimensional materials such as molybdenum disulfide (MoS<SUB>2</SUB>). The high flexibility and strong van der Waals force in MoS<SUB>2</SUB> nanoflakes allows effective encapsulation of the sulfur particles and prevent its sublimation during <I>in situ</I> TEM studies. We observe that the lithium diffusivities in the encapsulated sulfur particles are in the order of 10<SUP>–17</SUP> m<SUP>2</SUP> s<SUP>–1</SUP>. Composite electrodes made from the MoS<SUB>2</SUB>-encapsulated sulfur spheres show outstanding electrochemical performance, with an initial capacity of 1660 mAh g<SUP>–1</SUP> and long cycle life of more than 1000 cycles.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2017/jacsat.2017.139.issue-29/jacs.7b05371/production/images/medium/ja-2017-05371c_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja7b05371'>ACS Electronic Supporting Info</A></P>