Voronoi Network Research Articles

Electrothermal characteristics of zinc oxide varistors with different aging states after multiple lightning strikes based on Voronoi network finite element model

Multiple lightning strikes significantly degrade the electrical and thermal properties of zinc oxide (ZnO) varistors, affecting their protection capability. In order to quantitatively analyze the hazard of multiple lightning strikes on ZnO varistors, simulations at the microscopic level are required to account for the changes in macroscopic parameters. Unlike other published research, this paper presents an analytical approach to simulate the micro-level performance of ZnO varistors in different aging states. The details of the model construction, experimental platform, and computational results are thoroughly investigated. First, a Voronoi network finite element model for ZnO varistors is developed, and the required model parameters are presented. Next, an experimental testing platform is used to physically simulate multiple lightning strikes and perform parametric testing. Finally, the microscopic electrical and thermal properties of ZnO varistors under different aging states and pulse numbers are analyzed and compared. The results show that with an equivalent amount of energy per impact, triple-strike aging causes severe degradation for ZnO varistors. Compared to single-strike aging, triple-impact aging results in a 15.21 times higher quality coefficient and a 7.67 times higher 10 min temperature rise ratio.

Two conjectures on 3D Voronoi structures: a toolkit with biomedical case studies.

3D Voronoi scaffolds are widely applied in the field of additive manufacturing as they are known for their light weight structural resilience and share many topological similarities to various natural (bone, tumours, lymph node) and synthetic environments (foam, functionally gradient porous materials). Unfortunately, the structural design features that promote these topological similarities (such as the number of vertices) are often unpredictable and require the trial and error of varying design features to achieve the desired 3D Voronoi structure. This article provides a toolkit, consisting of equations, based on over 12 000 3D Voronoi structures. These equations allow design features, such as the number of generating points (G), to be efficiently and accurately predicted based on the desired structural parameters (within ±3G). Based on these equations we are proposing, to the best of our knowledge, two new mathematical conjectures that relate the number of vertices or edges, and the average edge length to G in Voronoi structures. These equations have been validated for a wide range of parameter values and Voronoi network sizes. A design code is provided allowing any of over 12 000 structures to be selected, easily adjusted based on user requirements, and 3D printed. Biomedical case studies relevant to T-cell culturing, bone scaffolds and kidney tumours are presented to illustrate the design code.

Withstanding capability and aging mechanism of metal-oxide varistors under DC temporary overvoltage

With the development of low-voltage DC (LVDC) application scenarios, the application of Surge protective devices (SPDs) for the protection of LVDC systems from overvoltage requires urgent attention. Temporary overvoltage (TOV) withstanding capability is a critical parameter so that studying the withstanding capability and aging mechanism of metal-oxide varistor (MOV) which serves as the core component of SPD is significantly important. The numerical model of MOV is established in this paper to study the TOV performance in DC systems with different voltage levels. The 2D Voronoi network is employed to depict the microstructure of MOV. After validating the reliability of the microstructure model, the electrothermal characteristics and capability of the MOV to withstand TOV in LVDC systems at various voltage levels are investigated. The parameters for the DC TOV are obtained from the latest committee draft of IEC 61643-41. The aging mechanism based on double Schottky Barrier theory under DC TOV is discussed in detail, which can explain the initial rise followed by a subsequent decline in varistor voltage. The Voltage-Temperature-Time (V-T-t) curve of MOV is proposed for the first time which serves as a valuable theoretical reference for the development of overcurrent and overheat protection of DC SPD.

A pipe domain seepage model based on outsourcing Voronoi network with particle flow code

A pipe domain seepage model based on outsourcing Voronoi network with particle flow code

Heterogeneous mesoscopic structure modulation enhancing dielectric strength of polypropylene insulation

This paper focuses on the heterogeneous mesoscopic structure modulation enhancing dielectric strength of polypropylene insulation. α-nucleating agents are used to modulate the heterogeneous mesoscopic structure of polypropylene, and Voronoi Network models with similar structures are established to simulate the electric field distribution at the mesoscopic scale. The experimental results show that the addition of an α-nucleating agent effectively diminishes the spherocrystal size and increases its density. The high continuity and wide range electric field distortion caused by the too-large spherocrystal size and too-loose arrangement is thus suppressed. The improvement of electric field uniformity effectively inhibits the frequency and intensity of partial discharge. Compared with pure polypropylene, the partial discharge in polypropylene composite with α-nucleating agents at 0.1 wt% decreased by 2414 times, and its maximum amplitude decreased by 58.3%. The increase in the spherocrystal density also enhances its blocking effect on the electrical tree growth, and the length and width of it are reduced by 88.6% and 89.1%, respectively. The dispersive development of the electrical tree reduces the incidence of penetration channels along the electric field direction. The breakdown strength is therefore increased by approximately 20%. The simulation results demonstrate that the dielectric property difference between the crystal and amorphous regions within polypropylene at the mesoscopic scale is the main reason that triggers the electric field distortion at its interface and further leads to insulating degradation or even failure. In this study, heterogeneous mesoscopic structure modulation is proved to be an efficient means to enhance the dielectric strength of polypropylene insulation.

Time-domain response model of nonlinear dielectric properties for electrical field grading composites

The electrical field grading materials are expected to well response to arbitrary electrical excitation. In this paper, a modified Voronoi network model is established to simulate the response of electrical field grading materials under arbitrary excitation. The grain-boundary structure of ZnO microvaristors is described by the Voronoi polygons with a distorted hexagonal structure and the nonlinear conductivity is modeled based on the carrier transport mechanism. The network calculations replicate the resistive contacts between fillers and the micro-region breakdown phenomenon of insulating materials during the formation of the conduction path. The accuracy of this model is evaluated using AC and surge measurements for the ZnO/SiR samples. The model can manage the typical nonlinear time-domain responses and well reflect the effect of filler concentrations as well as the relationship between capacitive and resistive components of the responses. This work is helpful to design field grading materials to self-adapt high electrical fields of electronic and electrical devices and increase their stability and durability.

Research on electrothermal characteristics of metal oxide varistor based on multi‐physical fields

The electrothermal characteristics of metal oxide varistor (MOV), closely related to its microstructure, are important for studying its failure mechanism. This paper proposes an improved MOV microstructure model based on the Voronoi network for simulating the electrothermal behaviour of MOV. With the proposed model, not only the simulation accuracy under constant electrical stress can be improved, but also the simulation of MOV under time-dependent electrical stress (a.c. or impulse) can be realized. The simulation results are in good agreement with the experimental data. In addition, according to the electrothermal characteristics of MOV microstructure and the failure phenomenon of MOV in the experiments, the failure mechanism of MOV under different electrical stresses is analysed and discussed. It is found that the serious local heating caused by current concentration and thermal stress caused by huge temperature gradient are the main reasons for the MOV failure. This paper provides an effective method for the study of MOV electrothermal characteristics and theoretical reference for the performance improvement of MOV.

Thickness effect on threshold electric field of ZnO microvaristors/silicone rubber composite with nonlinear conductivity

As a functional material, ZnO microvaristors/silicone rubber composite, with nonlinear conductivity, will be manufactured to different thicknesses for different applications. However, few study focuses on the thickness effect of the composite. In this paper, ZnO/SiR composites with thickness from 50 μm to 2800 μm were prepared. The conductivity results show that the threshold electric field (Eb) of the composites increases from 40 V/mm to a stable value (more than 1000 V/mm) as the thickness increases. A simulation model based on the Voronoi network was carried out to explore the mechanism. For the thickness close to the diameter of fillers, the key element determining Eb is the microstructural nonuniformity of electrical characteristics of microvaristors. As the sample becomes thick, the increasingly tortuous conduction path is considered to trigger off the increase of Eb. The thickness effect discussed in this paper will be helpful in designing electrostatic discharge elements in electronic devices.

A geometry-based relaxation algorithm for equilibrating a trivalent polygonal network in two dimensions and its implications

ABSTRACT The equilibration of a trivalent polygonal network in two dimensions (2D) is a universal phenomenon in nature, but the underlying mathematical mechanism remains unclear. In this study, a relaxation algorithm based on a simple geometrical rule was developed to simulate the equilibration. The proposed algorithm was implemented using the Python programming language. The simulated relaxation changed the polygonal cell of the Voronoi network from an ellipse’s inscribed polygon toward the ellipse’s maximal inscribed polygon. Meanwhile, the Aboav-Weaire’s law, which describes the neighbouring relationship between cells, still holds statistically. The simulation results strongly supports the ellipse packing hypothesis that is proposed to explain the dynamic behaviours of a trivalent 2D structure. The simulated relaxation concentrated the interior angles around 120°, and increased the area and edge length of large cells but decreased the area and edge length of small cells. The area changes of cells with different edges due to relaxation could be described by the von Neumann-Mullins law. The results presented in this work can help to understand the mathematical mechanisms of the dynamic behaviours of trivalent 2D structures.

Numerical modelling of MOV with Voronoi network and finite element method

Numerical modelling of MOV with Voronoi network and finite element method

Supramolecular Recognition in Crystalline Nanocavities through Monte Carlo and Voronoi Network Algorithms

Computational screening of templating molecules enables the discovery of new synthesis routes for zeolites. Despite decades of work in molecular modeling of organic structure-directing agents (OSDAs), the development and benchmarking of algorithms for docking molecules in nanoporous materials has received scarce attention. Here, we introduce Voronoi Organic–Inorganic Docker (VOID), a method based on Voronoi diagrams to dock molecules in crystalline materials, and release it as a Python package. Benchmarks of the implementation show that it generates docked poses up to 95 times faster than the traditional Monte Carlo docking scheme. We then evaluate the algorithm by obtaining binding energies for about 120 zeolite–OSDA pairs of industrial relevance. The computed host–guest interactions qualitatively explain experimental outcomes for traditional synthesis routes from the literature. The results further suggest new OSDAs to synthesize known zeolites. Finally, we exemplify the generality of VOID by docking molecules inside a metal–organic framework and on a metal surface. The proposed method and software provide a low-cost computational approach for generating molecule–material interfaces.

AoI-Based Multicast Routing Over Voronoi Overlays With Minimal Overhead

The increasing pervasive and ubiquitous presence of devices at the edge of the Internet is creating new scenarios for the emergence of novel services and applications. This is particularly true for location- and context-aware services. These services call for new decentralized, self-organizing communication schemes that are able to face issues related to demanding resource consumption constraints, while ensuring efficient locality-based information dissemination and querying. Voronoi-based communication techniques are among the most widely used solutions in this field. However, when used for forwarding messages inside closed areas of the network (called Areas of Interest, AoIs), these solutions generally require a significant overhead in terms of redundant and/or unnecessary communications. This fact negatively impacts both the devices' resource consumption levels, as well as the network bandwidth usage. In order to eliminate all unnecessary communications, in this paper we present the MABRAVO (Multicast Algorithm for Broadcast and Routing over AoIs in Voronoi Overlays) protocol suite. MABRAVO allows to forward information within an AoI in a Voronoi network using only local information, reaching all the devices in the area, and using the lowest possible number of messages, i.e., just one message for each node included in the AoI. The paper presents the mathematical and algorithmic descriptions of MABRAVO, as well as experimental findings of its performance, showing its ability to reduce communication costs to the strictly minimum required.

Time-Space Domain Availability Analysis Under Reliability Impairments

Availability and reliability are two essential metrics for the design, deployment, and operation of future ultra-reliable low latency communication (URLLC) networks. Despite a vast amount of research efforts toward URLLC, very little attention has been made on the ultra-reliable communication (URC) aspect of URLLC from a dependability perspective. As an effort toward achieving anytime and anywhere communication, this letter consolidates a dependability theory-based availability concept for individual users by taking into account reliability impairments that affect URC in both spatial and temporal domains . To this end, we perform per-user availability analysis by considering channel status and user mobility patterns in a Poisson Voronoi network.

Modeling of current distribution in zinc oxide varistors using Voronoi network and finite element method

Nonuniform current distribution inside varistor ceramics is a key factor influencing its performance and failures. Therefore understanding, modeling and predicting of current distribution in varistor ceramics is of crucial significance. This paper proposes a numerical model for simulation of nonuniform electric current distribution inside zinc oxide varistors. A numerical model is based on physical modeling of the varistor's grain-structured geometry presented by Voronoi network using finite element method (FEM) simulation. The presented method is solving complete electric field inside the modeled geometry and therefore provides a more physically accurate approach for better understanding and predicting nonuniform current distribution in the varistor. In order to properly establish a FEM model a novel approach in defining grain boundary characteristic is proposed. Thus, a macroscopic model of the varistor microstructure has been developed and the grain micro-junction boundary characteristic has been derived. The simulation results of nonuniform current distribution in a varistor agree well with measurement results for a typical ZnO varistor. The presented model enables investigation of influences of varistor geometry (shapes, sizes) and material properties on the current distribution. A new mathematical expression for varistor I–V characteristic based on Lambert function is proposed.

A METHOD FOR THE SEAMLINES NETWORK AUTOMATIC SELECTION BASED ON BUILDING VECTOR

Abstract. In order to improve the efficiency of large scale orthophoto production of city, this paper presents a method for automatic selection of seamlines network in large scale orthophoto based on the buildings’ vector. Firstly, a simple model of the building is built by combining building’s vector, height and DEM, and the imaging area of the building on single DOM is obtained. Then, the initial Voronoi network of the measurement area is automatically generated based on the positions of the bottom of all images. Finally, the final seamlines network is obtained by optimizing all nodes and seamlines in the network automatically based on the imaging areas of the buildings. The experimental results show that the proposed method can not only get around the building seamlines network quickly, but also remain the Voronoi network’ characteristics of projection distortion minimum theory, which can solve the problem of automatic selection of orthophoto seamlines network in image mosaicking effectively.

Disease Risk Assessment Using a Voronoi-Based Network Analysis of Genes and Variants Scores.

Much effort has been devoted to assess disease risk based on large-scale protein-protein network and genotype-phenotype associations. However, the challenge of risk prediction for complex diseases remains unaddressed. Here, we propose a framework to quantify the risk based on a Voronoi tessellation network analysis, taking into account the disease association scores of both genes and variants. By integrating ClinVar, SNPnexus, and DISEASES databases, we introduce a gene-variant map that is based on the pairwise disease-associated gene-variant scores. This map is clustered using Voronoi tessellation and network analysis with a threshold obtained from fitting the background Voronoi cell density distribution. We define the relative risk of disease that is inferred from the scores of the data points within the related clusters on the gene-variant map. We identify autoimmune-associated clusters that may interact at the system-level. The proposed framework can be used to determine the clusters that are specific to a subtype or contribute to multiple subtypes of complex diseases.

Understanding the Percolation Characteristics of Nonlinear Composite Dielectrics.

Nonlinear composite dielectrics can function as smart materials for stress control and field grading in all fields of electrical insulations. The percolation process is a significant issue of composite dielectrics. However, the classic percolation theory mainly deals with traditional composites in which the electrical parameters of both insulation matrix and conducting fillers are independent of the applied electric field. This paper measured the nonlinear V-I characteristics of ZnO microvaristors/silicone rubber composites with several filler concentrations around an estimated percolation threshold. For the comparison with the experiment, a new microstructural model is proposed to simulate the nonlinear conducting behavior of the composite dielectrics modified by metal oxide fillers, which is based on the Voronoi network and considers the breakdown feature of the insulation matrix for near percolated composites. Through both experiment and simulation, the interior conducting mechanism and percolation process of the nonlinear composites were presented and a specific percolation threshold was determined as 33%. This work has provided a solution to better understand the characteristics of nonlinear composite dielectrics.

Towards Tuning the Mechanical Properties of Three-Dimensional Collagen Scaffolds Using a Coupled Fiber-Matrix Model.

Scaffold mechanical properties are essential in regulating the microenvironment of three-dimensional cell culture. A coupled fiber-matrix numerical model was developed in this work for predicting the mechanical response of collagen scaffolds subjected to various levels of non-enzymatic glycation and collagen concentrations. The scaffold was simulated by a Voronoi network embedded in a matrix. The computational model was validated using published experimental data. Results indicate that both non-enzymatic glycation-induced matrix stiffening and fiber network density, as regulated by collagen concentration, influence scaffold behavior. The heterogeneous stress patterns of the scaffold were induced by the interfacial mechanics between the collagen fiber network and the matrix. The knowledge obtained in this work could help to fine-tune the mechanical properties of collagen scaffolds for improved tissue regeneration applications.

Weighted network Voronoi Diagrams for local spatial analysis

Detection of spatial clusters among geographic events in a planar space often fails in real world practices. For example, events in urban areas often occurred on or along streets. In those cases, objects and their movements were limited to the street network in the urban area. This deviated from what a set of freely located points could represent. Consequently, many of the spatial analytic tools would likely produce biased results. To reflect this limitation, we developed a new approach, weighted network Voronoi diagrams, to modeling spatial patterns of geographic events on street networks whose street segments can be weighted based on their roles in the events. Using kernel density estimation and local Moran’s index statistics, the frequency of events occurring on a street segment can be used to produce a weight to associate with the street segment. The weights can then be normalized using a predefined set of intervals. The constructed weighted Voronoi network explicitly takes into account the characteristics of how events distribute, instead of being limited to assessing the spatial distribution of events without considering how the structure of a street network may affect the distribution. This approach was elaborated in a case study of Wuhan City, China. Constructing weighted network Voronoi diagrams of these partitioned networks could assist city planners and providers of public/private services to better plan for network-constrained service areas.

Molecular Design of Amorphous Porous Organic Cages for Enhanced Gas Storage

Porous molecular solids are garnering increasing attention with examples of high surface areas and applications in molecular separations. Recently, amorphous networks of molecular cages have shown increased porosity with respect to their crystalline counterparts. However, the structures of amorphous materials cannot be precisely elucidated by X-ray diffraction techniques, thus molecular simulations are vital to understanding their pore structures and the origin of porosity. Here, we use GPU-accelerated molecular dynamics simulations as an efficient methodology to construct representative amorphous network structures. We employ Voronoi network analysis of amorphous networks of seven previously reported cage molecules to provide insight into structure–property relationships. Accordingly, we apply this understanding to delineate synthetic design features that give rise to highly porous analogues of chemically robust cages constructed from carbon–carbon bonds.