Electrical treeing is a main cause of long term degradation of polymeric insulation and inorganic nanofillers have been added to improve the treeing resistance of polymers. In this work, a phase ...field model for the propagation of electrical tree in nanocomposites is presented, in which the damage status of insulation is described with a spatially and time dependent continuous variable. The evolution of damage phase is described with a modified Allen-Cahn equation, which is driven by the free energies originated from the phase separation, interface and electric field. The electric field distribution during growth of electrical tree is obtained by solving the iterative form of Poisson's equation with the spectral iterative perturbation method. Then the phase field model is applied to investigate the influence of nanofillers shape and distribution on propagation process of electrical tree. The results indicate that the phase field model can reproduce the dendritic shape of electrical tree, and the parallel nanosheet presents superior performance in hindering electrical tree than nanoparticle and random nanosheet.
•Machine learning methods were used to predict the thermal conductivity of polymers.•The training dataset was generated by large-scale molecular dynamics computations.•Machine learning models the ...polymers structure-thermal conductivity relationships.•Polymers containing chemical groups with strong bond strength give rise to high TC.•Polymer chains with well-ordered spatial structures usually present higher TC.
The ability to efficiently design new and advanced polymers with functional thermal properties is hampered by the high-cost and time-consuming experiments. Machine learning is an effective approach that can accelerate materials development by combining material science and big data techniques. Here, machine learning methods were used to predict the thermal conductivity of various single-chain polymers, and the relationship between molecular structures of polymer repeating units and thermal conductivity was also been investigated. The predict model starts from a benchmark dataset generated by large-scale molecular dynamics computations. In predict models, the polymers were ‘fingerprinted’ as simple, easily attainable numerical representations, which helps to develop an on-demand property prediction model. Further, potential quantitative relationship between molecular structures of polymer and thermal conductivity property was analyzed, and hypothetical polymers with ideal thermal conductivity were identified. The methods are shown to be general, and can hence guide the screening and systematic identification of high thermal conductivity.
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•A superhydrophobic coating with high DC flashover voltage was developed.•The ZnO and CNT introduce deep interfacial traps and enhance surface conductivity.•The coating significantly improves ...flashover voltage of various dielectrics.•The coating exhibits superhydrophobic, self-cleaning and good abrasive resistance.
In high voltage power equipment especially those for outdoor applications, the surface of dielectrics suffers from various problems such as flashover, raining, contamination, and ultraviolet illumination, which becomes the weak link in the whole insulating system. However, a high-throughput, flexible, and low-cost approach to simultaneously improve these properties of surface insulation was not reported yet. Here, a “binding polymer + nanofillers” based multifunctional nanocoating was fabricated by spray-coating of ZnO particles and multiwalled carbon nanotubes (MWCNT) dispersed in a poly(dimethylsiloxane) (PDMS) elastomer solution. The ZnO/MWCNT/PDMS coating plays multiple roles including inhibiting charge injection from electrodes due to the introduced deep interfacial traps, accelerating charge dissipation for the enhanced surface conductivity, and forming a micro-nanoscale hierarchical surface structure. As a result, the coatings endowed various dielectric materials such as epoxy resin, polymethyl methacrylate and silicone rubber with multifunctionality of high DC flashover strength (maximum 54% flashover voltage increase), superhydrophobic surface (water contact angle >150°), self-cleaning capability, good abrasive resistance, ultraviolet-resistance, and anti-icing performances. The ZnO/MWCNT/PDMS coating is well suitable for the surface modification of dielectrics in both indoor and outdoor HVDC insulating system. The present approach provides a new insight for simultaneously improving the multiple surface properties of dielectric insulation.
In this work, a phase-field model is presented to characterize the breakdown properties of polymers under alternating voltage. The breakdown status is described with a spatially and time dependent ...continuous variable, and the evolution of damage phase is driven by the electrostatic and thermal energy in the system. The relationship between breakdown strength and material properties such as permittivity and dielectric loss can be represented with the phase-field model. For thermoplastic polyolefin with low permittivity and dielectric loss, the electric breakdown is probably the dominant mechanism at room temperature. For other polymers, the breakdown characteristics may be dominated by the electrothermal mechanism. The dielectric loss of polymers increases with temperature, which intensifies the thermal energy and further reduces the breakdown strength. Dielectric loss of polymers with high glass transition temperature increases slowly, and they can keep high breakdown strength at high temperatures.
infection is mostly a family-based infectious disease. To facilitate its prevention and management, a national consensus meeting was held to review current evidence and propose strategies for ...population-wide and family-based
infection control and management to reduce the related disease burden.
Fifty-seven experts from 41 major universities and institutions in 20 provinces/regions of mainland China were invited to review evidence and modify statements using Delphi process and grading of recommendations assessment, development and evaluation system. The consensus level was defined as ≥80% for agreement on the proposed statements.
Experts discussed and modified the original 23 statements on family-based
infection transmission, control and management, and reached consensus on 16 statements. The final report consists of three parts: (1)
infection and transmission among family members, (2) prevention and management of
infection in children and elderly people within households, and (3) strategies for prevention and management of
infection for family members. In addition to the 'test-and-treat' and 'screen-and-treat' strategies, this consensus also introduced a novel third 'family-based
infection control and management' strategy to prevent its intrafamilial transmission and development of related diseases.
is transmissible from person to person, and among family members. A family-based
prevention and eradication strategy would be a suitable approach to prevent its intra-familial transmission and related diseases. The notion and practice would be beneficial not only for Chinese residents but also valuable as a reference for other highly infected areas.
In this work, the thermal pyrolysis process and produced chemical defects of crystalline polyethylene, taking the effects of oxygen into account, are explored by molecular dynamics simulation based ...on ReaxFF force field. The trap energies and sites of various types of chemical defects are calculated by density function theory (DFT), in which hybrid functional XC HSE06 is adopted. The results indicate that double bond C=C, conjugate double bond C=C-C=C, C=C=C and carbonyl C=O are the main products during thermal aging, and these defects are easier to formed at the breaking points of carbon backbone, which is different from the geometries generally used in the previous DFT computations. For electrons, chemical defects C=O and C-C=C-C at breaking points (BP) produce relative deep traps 1.810 and 1.054 eV, and C=C at BP shows shallower traps with energy of 0.445 eV. For holes, C=C-C=C at BP and C=C in the backbone generate deep traps 1.614 and 1.119 eV, and C=C at BP presents shallower traps with energy of 0.689 eV.
Micro-blogging is becoming an increasingly popular social media platform where users can discover interesting information about the real world and especially corporations are able to understand ...customers' demands. The fast diffusion of information and the convenience of micro-blogging have resulted in large audiences sharing their daily activities, exchanging opinions and establishing friendships with others. By analyzing the user-generated contents, one can explore users' potential interests, which helps micro-blogging provide users with better personalized information services. Users' behaviors are affected by opinions of their friends and changes in their interests over time. Based on these intuitions, in this paper we propose a temporal and social probabilistic matrix factorization model to predict users' potential interests in micro-blogging. By exploiting the matrix factorization technique to learn latent features of users and topics, our model analyzes the impacts of time information and users' activities, including posting of tweets and establishing friendships with others, on the latent feature space of users and topics of their interests. The proposed model provides a unified way to fuse the time information and the social network structure to predict users' future interests accurately. The experimental results on Sina-weibo, one of the most popular micro-blogging sites in China, demonstrate the efficiency and effectiveness of our proposed model.
► A new temporal and social PMF-based method is proposed to predict users' interests in micro-blogging. ► The model provides a unified way to fuse the time information and the social network structure. ► The experimental results demonstrate the efficiency and effectiveness of the model.
Van der Waals(vdW)layered two-dimensional(2D)materials,which may have high carrier mobility,valley polarization,excellent mechanical properties and air stability,have been widely investigated ...before.We ex-plore the possibility of producing a spin-polarized two-dimensional electron gas(2DEG)in the heterojunction composed of insulators MoSi2N4 and VSi2N4 by using first-principles calculations.Due to the charge trans-fer effect,the 2DEG at the interface of the MoSi2N4/VSi2N4 heterojunction is found.Further,for different kinds of stacking of heterojunctions,lattice strain and electric fields can effectively tune the electronic struc-tures and lead to metal-to-semiconductor transition.Under compressive strain or electric field parallel to c axis,the 2DEG disappears and band gap opening occurs.On the contrary,interlayer electron transfer enforces the system to become metallic under the condition of tensile strain or electric field anti-parallel to c axis.These changes are mainly attributed to electronic redistribution and orbitals'reconstruction.In addition,we reveal that MoSi2N4/VSi2N4 lateral heterojunctions of armchair and zigzag edges exhibit different electronic proper-ties,such as a large band gap semiconductor and a metallic state.Our findings provide insights into electronic band engineering of MoSi2N4/VSi2N4 heterojunctions and pave the way for future spintronics applications.
The shock wave released from physical explosion of a pressurized stratospheric airship can produce serious damage to the environment. Shock wave overpressure can measure the degree of damage that an ...explosion can cause to such things as buildings and the human body. To obtain the overpressure from an airship envelope explosion, explosion energy must first be conducted. Explosion energy is derived based on Brode’s equation, Brown’s equation, and Crowl’s equation. An equivalent TNT computational model is then applied to calculate the overpressure of the explosion energy. In order to verify the accuracy of the computational model, a ground test must be conducted. The experimental result shows that a computational model based on Crowl’s equation is more accurate than the other two. Finally, the effect of geometric scale ratio, pressure difference, and the gas of the explosion overpressure is discussed. This paper can provide a relatively effective calculation method for shock wave overpressure for an airship envelope explosion.
Rationally designing polymer composite structures, including physical parameters of nanofillers, nanofiller–matrix interface characteristics, and geometric distribution of nanofillers, is thought to ...be an effective approach to achieve the desired dielectric properties such as breakdown strength (E b), permittivity (εr), and energy density (U e) in wide applications. However, the work is difficult to complete through merely high-cost and time-consuming trial-and-error experiments. A machine learning (ML) driven approach, trained on hundreds of experimentally measured data, is presented to rationally design polymer composites with desired properties. The doping scheme of nanofillers is fingerprinted with a string of characters considering the physical parameters, shape, distribution of fillers, and shell properties in core–shell structures, and then the Gaussian process regression algorithm is trained to establish the linkage between the filler doping scheme and the dielectric properties. The dielectric properties of the randomly generated tens of millions of candidate composites are calculated with the resulting ML model, and representative composites with high E b, εr, and U e are presented. The results indicate that the effects of nanofiller permittivity and bandgap on E b and εr follow exactly the opposite trend, hence it is difficult to simultaneously improve E b and εr by choosing the type of nanofiller. Fortunately, the trade-off between E b and εr can be improved by tailoring the shape, orientation, and distribution of the nanofillers, for instance, by using horizontally orientated nanosheets and orthotropic nanowires with high permittivity.
