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Unbalanced graph cuts with minimum capacity

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Frontiers of Computer Science 2014年第4期8卷 676-683页

作者： peng zhang School of Computer Science and Technology Shandong University Jinan 250101 China

We systematically investigate minimum capacity unbalanced cut problems arising in social networks. Let k be an input parameter. A cut （A, B） is unbalanced if the size of its smaller side is at most k （called k-size） or exactly k （called Ek-size）. An s-t cut （A, B） is unbalanced if its s-side is either k-size or Ek-size. In the min k-size cut （s-t cut, resp.） problem, we want to find a k-size cut （s-t cut, resp.） with the minimum capacity. The corresponding min Ek-size cut （and s-t cut） problem is defined in a similar way. While the classical min s-t cut problem has been studied extensively, the minimum capacity unbalanced cut problem has only re- cently attracted the attention of researchers. In this paper, we prove that the min k-size s-t cut problem is NP-hard, and give O（log n）-approximation algorithms for the min k-size s-t cut problem, the min Ek-size s-t cut problem, and the min Eksize cut problem. These results, together with previous results, complete our research into minimum capacity unbalanced cut problems.

关键词： unbalanced cut min cut approximation algorithm social network combinatorial optimization

Electronic origin of structural degradation in Li-rich transition metal oxides:The case of Li_(2)MnO_(3)and Li_(2)RuO_(3)

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Journal of Semiconductors 2024年第4期45卷 82-86页

作者： peng zhang Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province College of Physics andOp toelec-tronic EngineeringShenzhen UniversityShenzhen 518060China

Li_(2)MnO_(3)and Li_(2)RuO_(3)represent two prototype Li-rich transition metal(TM)oxides as high-capacity cathodes for Li-ion batteries,which have similar crystal structures but show quite different cycling ***,based on the first-princi-ples calculations,we systematically studied the electronic structures and defect properties of these two Li-rich cathodes,in order to get more understanding on the structural degradation mechanism in Li-rich TM *** calculations indicated that the structural and cycling stability of Li_(2)MnO_(3)and Li_(2)RuO_(3)depend closely on their electronic structures,especially the energy of their highest occupied electronic states(HOS),as it largely determines the defect properties of these *** Li_(2)MnO_(3)with low-energy HOS,we found that,due to the defect charge transfer mechanism,various defects can form spontaneously in its host structure as Li ions are extracted upon delithiation,which seriously deteriorates its structural and cycling *** for Li_(2)RuO_(3),on the other hand,we identified that the high-energy HOS prevents it from the defect formation upon delithi-ation and thus preserve its cycling *** studies thus illustrated an electronic origin of the structural degradation in Li-rich TM oxides and implied that it is possible to improve their cycling performances by carefully adjusting their TM compo-nents.

关键词： defects Li-ion batteries first-principles calculations

Statistical Analysis of Reinforcement Characterization in SiC Particle Reinforced Al Matrix Composites

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Journal of Materials Science & Technology 2009年第6期25卷 807-813页

作者： peng zhang Fuguo Li School of Materials Science and Engineering Northwestern Polytechnical University Xi'an 710072 China

The characterization of reinforcement in 15% SiC particles reinforced AI matrix composites processed by powder metallurgy route was studied by statistical method. During the analysis, a new approach for the estimation of the characterization of reinforcement was presented. The mathematic software MATLAB was used to calculate the area and perimeter of reinforcement, in which the image processing technique was applied. Based on the calculation, the fractal dimension, shape factor, reinforcement size distribution and reinforcement distribution were investigated. The results show that the reinforcement shape is similar to rectangle; the reinforcement size distribution is broad with the＇ range of 1-12 μm; the topography of reinforcement is smooth; and the reinforcement distribution is inhomogeneous. Furthermore, the cell model based on the statistical characterization was established and tested.

关键词： Particle reinforced AI matrix composites Fractal dimension Shape factor Distribution Cell model

Excellent plasma electrolytic oxidation coating on AZ61 magnesium alloy under ordinal discharge mode

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Journal of Magnesium and Alloys 2022年第9期10卷 2460-2474页

作者： Weiyi zhang Yunhui Du peng zhang School of Mechanical Electronic and Control EngineeringBeijing Jiaotong UniversityBeijing 100044China

In the preparation of plasma electrolytic oxidation(PEO)coating,the rapid heating of freely-happened electron avalanche under traditional discharge(TD)mode inevitably results in a strong eruption of electric breakdown *** PEO coating is loose and invariably composed of a very thin inner dense layer and an outer loose layer,as a result of which its properties and application have been limited *** this work,for purpose of weakening the eruption of breakdown melt,thickening the inner dense layer,densifying the outer loose layer and improving the performance of PEO coating,ordinal discharge(OD)mode of PEO coating is developed by regulating the mass ratio of MgF_(2) to MgO(α)and voltage in the PEO investigation on AZ61 magnesium alloy in KF-KOH *** formation mechanism under different discharge mode,electrochemical corrosion and wear of PEO coatings are *** results show that the suitableαand voltage for effective OD are 1.3 and 130 V under which the freely-happened electron avalanche in MgF_(2) under TD mode can be restricted by the adequate adjacent *** with TD mode,the inner dense layer,in which the(10¯1)plane of MgF_(2) is parallel to the(111)plane of MgO at their well-knit semi-coherent interface,is thickened to 2.4∼7.2 times,the corrosion potential(E_(corr))improvement is enlarged to 3.6∼13.2 times and the corrosion current intensity(I_(corr))is reduced from 10.8∼9.499 to 0.433(10^(−6) A/cm^(2)).The outer loose layer is densified and the wear rate is lessened 65.5%∼89.8%by the evident melioration in surface porosity,impedance and *** work deepens the understanding about the discharge of PEO coating and provides an available OD mode for preparing excellent PEO coating.

关键词： Magnesium alloy Plasma electrolytic oxidation coating Ordinal discharge Microstructure Performance

Notch Effect of Materials: Strengthening or Weakening?

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Journal of Materials Science & Technology 2014年第6期30卷 599-608页

作者： Ruitao Qu peng zhang Zhefeng zhang Shenyang National Laboratory for Materials Science Institute of Metal ResearchChinese Academy of Sciences

Notch is a very important geometry with widespread applications in engineering structural components. Finding a universal equation to predict the effect of notch on strength of materials is of much significance for structural design and materials selection. In the present work, we tried to find this universal equation from experimental results of metallic glasses （MGs） and other materials as well as theoretical derivations based on a universal fracture criterion （Qu and zhang, Sci. Rep. 3 （2013） 1117）. Experimental results showed that the notch effect of the studied MG was affected by the notch geometry characterized by the stress concentration factor Kt. As Kt becomes smaller, the notch strength ratio （NSR, which is the ratio of nominal ultimate tensile strength （UTS） of the notched sample to UTS of the unnotched sample） increases. By comparing MGs with other materials like brittle ceramics and ductile for ductile metals but smaller for brittle effect on strength of materials： NSR = equation was found to be consistent with crystalline metals, we find that when Kt is same, the NSR is larger ceramics. Theoretically, we derived a universal equation for notch M/Kt, where M is a constant related to materials. This universal the experimental results.

关键词： Notch Strength Metallic glass Fracture Plastic deformation

Bio-inspired construction of electrocatalyst decorated hierarchical porous carbon nanoreactors with enhanced mass transfer ability towards rapid polysulfide redox reactions

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Nano Research 2021年第11期14卷 3942-3951页

作者： Shijie zhang Yongshang zhang Guosheng Shao peng zhang School of Materials Science and Engineering Zhengzhou UniversityZhengzhou450001China State Centre for International Cooperation on Designer Low-Carbon&Environmental Materials(CDLCEM) Zhengzhou University100 Kexue AvenueZhengzhou450001China Zhengzhou Materials Genome Institute Building 2ZhongyuanzhiguXingyang450100China

Li-S batteries are considered as a highly promising candidate for the next-generation energy storage system, attributing to their tremendous energy density. However, the two-dimensional island nucleation-growth process of lithium sulfide leads to a thick insulating film covering the electrode, inducing slow electrons transfer and mass-transfer of ions and liquid sulfur species in working Li-S cells. Here, we demonstrate a bio-inspired strategy of constructing ant-nest-like hierarchical porous ultrathin carbon nanosheet networks with the implants of metallic nanoparticles electrocatalysts (HPC-MEC) as efficient nanoreactors enabling rapid mass transfer, via a simple and green NaCl template. Such nanoreactors with a large active surface area could effectively anchor polysulfides for mitigating the shuttle effect, facilitating uniformly thin Li2S film, and promoting the mass transfer for fast sulfur species conversions. This helps contribute to a continuously high sulfur utilization in Li-S batteries with the HPC-MEC reactors. As a typical exhibition, cobalt embedded hierarchical porous carbon (HPC-Co) could realize to deliver a remarkably high specific capacity of 1,540.6 mAh·g−1, an excellent rate performance of 878.8 mAh·g−1 at 2 C, and high area capacity of 11.6 mAh·cm−2 at a high sulfur load of 10 mg·cm−2 and low electrolyte/sulfur ratio of 5 µL·mg−1.

关键词： sulfur species reactor hierarchical porous carbon network electrocatalyst high sulfur loading low electrolyte/sulfur ratio

Fatigue cracking criterion of high-strength steels induced by inclusions under high-cycle fatigue

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Journal of Materials Science & Technology 2023年第23期154卷 114-128页

作者： peng Wang peng zhang Bin Wang Yankun Zhu Zikuan Xu Zhefeng zhang Shi-changxu Innovation Center for Advanced Materials Institute of Metal Research Chinese Academy of Sciences Shenyang 110016 China School of Materials Science and Engineering University of Science and Technology of China Shenyang 110016 China

Fatigue properties of high-strength steels become more and more sensitive to inclusions with enhancing the ultimate tensile strength (UTS) because the inclusions often cause a relatively low fatigue strength and a large scatter of fatigue lives. In this work, four S–N curves and more than 200 fatigue fracture morphologies were comprehensively investigated with a special focus on the size and type of inclusions at the fatigue cracking origin in GCr15 steel with a wide strength range by different heat treatments after high-cycle fatigue (HCF). It is found that the percentage of fatigue failure induced by the inclusion including Al2 O3 and TiN gradually increases with increasing the UTS, while the percentage of failure at sample surfaces decreases conversely and the fatigue strength first increases and then decreases. Besides, it is interestingly noted that the inclusion sizes at the cracking origin for TiN are smaller than that for Al2 O3 because the stress concentration factor for TiN is larger than that for Al2 O3 based on the finite element simulation. For the first time, a new fatigue cracking criterion including the isometric inclusion size line in the strength-toughness coordinate system with specific physical meaning was established to reveal the relationship among the UTS, fracture toughness, and the critical inclusion size considering different types of inclusions based on the fracture mechanics. And the critical inclusion size of Al2 O3 is about 1.33 times of TiN. The fatigue cracking criterion could be used to judge whether fatigue fracture occurred at inclusions or not and provides a theoretical basis for controlling the scale of different inclusion types for high-strength steels. Our work may offer a new perspective on the critical inclusion size in terms of the inclusion types, which is of scientific interest and has great merit to industrial metallurgical control for anti-fatigue design.

关键词： High-strength steel High-cycle fatigue Critical inclusion size Inclusion types Tensile strength Fracture toughness Fatigue cracking criterion

Amine-Actuated Catalyst Switch for One-Pot Synthesis of Ether-Ester Type Block Copolymers

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Chinese Journal of Polymer Science 2024年第12期42卷 1925-1932,I0008页

作者： Hong Qiu peng-Fei zhang Jun-peng Zhao Faculty of Materials Science and Engineering South China University of TechnologyGuangzhou 510640China

Organocatalysis has shown special potency for simplifying the construction of complex polymer *** are reporting here a one-pot synthetic pathway using amine as a selectivity-switching agent in the two-component catalytic system consisting of triethylborane(Et_(3)B)and a phosphazene *** first modelled the interactions of a variety of amines with Et_(3)B by density functional theory *** results indicate that the aliphatic diamines comprising both primary and tertiary amino groups,capable of forming stable intramolecular hydrogen bonds,undergo the strongest complexation with Et_(3)***,experimental results demonstrate that the addition of such amines promptly actuates the in situ selectivity switch from Lewis pair-catalyzed ring-opening polymerization(ROP)of epoxide(propylene oxide,n-butylglycidyl ether,or glycidyl phenyl ether)to organobase-catalyzed ROP ofδ-valerolactone,allowing one-pot continuous synthesis of ether-ester type block *** thus exploited the noncovalent interaction between amine and Et_(3)B to refine the catalyst switch strategy by exempting it from loading of extra catalyst.

关键词： Ring-opening polmerization Block copolymers Organocatalysis

Twin boundary： Controllable interface to fatigue cracking

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Journal of Materials Science & Technology 2017年第7期33卷 603-606页

作者： Zhefeng zhang Linlin Li Zhenjun zhang peng zhang Shenyang National Laboratory for Materials Science Institute of Metal Research Chinese Academy of Sciences Shenyang 110016 China

Twin boundaries（TBs） are key factors influencing the mechanical properties of crystalline materials. We have investigated the intrinsic fatigue cracking mechanisms of TBs during the past decade. The effects of TB orientations on the fatigue cracking mechanisms were revealed via cyclic deformation of a series of grown Cu bicrystals with a sole TB. Furthermore, the combined effects of crystallographic orientation and stacking fault energy（SFE） on the fatigue cracking mechanisms were clarified through cyclic deformation of polycrystalline Cu and Cu alloys. Both developments were reviewed in this report which will provide implications to optimize the interfacial design for the improvement of fatigue performance of metallic materials.

关键词： Fatigue cracking Twin boundary Stacking fault energy Orientation