Modeling the effect of uniform and nonuniform dispersion of nanofillers on electrical tree propagation in polyethylene dielectric

A phase-field model is developed in this paper based on the similarity between mechanical fracture and dielectric breakdown.Electrical treeing is associated with the dielectric breakdown in solid dielectrics by the application of high voltages.Instead of explicitly tracing the propagation of conductive channel,this model initializes a continuous phase field to characterize the extent of damage.So far,limited research has been conducted for simulating the effect of nanofiller dispersion on electrical treeing.No study has modeled the effect of uniform and nonuniform dispersion of nanofillers with varying filler concentration on treeing.Since electrical treeing tends to decrease the breakdown strength of solid dielectrics therefore,nanofillers are widely used to distract the tree from a straight channel to distribute its energy in multiple paths.Diverting a straight treeing channel into mul-tiple paths reduces the chances of its propagation from live to dead-end hence,improving the breakdown strength.The physical and chemical nature of nanofillers has a crucial impact on increasing the resistance to treeing.In this paper,phase-field model is developed and used to simulate electrical treeing in polyethylene for varying concentrations of alumina nanofiller using COM-SOL Multiphysics.Tree inception time,tree-growth patterns,and corresponding changes in dielectric strength is studied for both dispersions.Electrical treeing under different concentrations of alumina nanofillers with uniform and nonuniform dispersion is investigated in polyethylene as a base material.It is observed that fillers with uniform dispersion increases the resistance to tree-ing and tree inception time.Highest resistance to treeing is observed by adding 1%nanoalumina uniformly in raw polyethylene.Moreover,in uniform dispersion the tree deflects into multiple branches earlier than nonuniform dispersion impeding the damage speed as well.

An Effective Approach for the Preparation of High Performance Thermal Conductive Polymer Composites Containing Liquid Metal

The preparation of high-performance thermal conductive composites containing liquid metals(LM)has attracted significant attention.However,the stable dispersion of LM within polymer solution and effective property contribution of liquid metals remains significant challenges that need to be overcome.Inspired by the properties of the dendritic structure of the tree root system in grasping the soil,“shear-induced precipitation-interfacial reset-reprotonation”processing strategy is proposed to prepare nanocomposites based on aramid micron fibers(AMFs)with hierarchical dendritic structure.Thanks to the combination of van der Waals force provided by hierarchical dendritic structure,electrostatic interaction between AMFs and LM,coordinative bonding of―NH to LM,together with interfacial re-setting and multi-step protonation,several features can be achieved through such strategy:conducive to the local filler network construction,improvement of interfacial interaction,improvement of the stability of filler dispersion in the solvent,and enhancement of mechanical and thermal properties of the films.The resulting AMFsp H=4/LM films demonstrate a thermal conductivity of 10.98 W·m^(-1)·K^(-1)at 70%filler content,improvement of 126.8%compared to ANFs/LM film;while maintaining a strength of~85.88 MPa,improvement of 77%compared to AMFs/LM film.They also possess insulation properties,enable heat dissipation for high power electronics.This work provides an effective strategy for the preparation of high performance polymer composites containing liquid metal.

A rheological method for selecting nano-kaolin powder as filler in SBR rubber

This paper shows that the stiffness ofstyrene-butadiene solid rubber with added kaolin powder is related to the yield stress of kaolin dispersion in liquid polybutadiene rubber up to the percolation threshold. For five kinds of kaolin powder, the value of τ° spans the range of 100-430 Pa, while the corresponding compressive elastic constant of SBR varies from 12 to 21 MPa. A relationship between τ° and △E^i^＊/ER^＊ is proposed. Critical examination of these data infers that kaolin powder dispersed in solid rubber matrix acts as an additive which decreases the random movement of the polybutadiene chains. Consequently, dispersions of nano-particles in liquid and in solid SBR are considered to be related, thus leading to a theological method for selecting nano-particles as fillers in solid SBR.

Facile Fabrication of High-performance Polyimide Nanocomposites with in situ Formed“Impurity-free”Dispersants

Polyimide/carbon black（PI/CB） nanocomposite films were fabricated via the direct ball-milling method with poly（amic acid）（PAA）, the precursor of PI, as an in situ formed impurity-free dispersant. FTIR and Raman spectral results reveal that, besides physical adsorption, chemical grafting of PAA chains onto the CB surface occurs during the ball-milling process. Comparative studies show that introduction of various commercial dispersants improves the dispersion of CB. However, the mixtures exhibit poor reproducibility, unstable electrical properties, and decreased tensile strength; these issues may be attributed to interfacial pollution brought about by differences in the chemical structures of the dispersant and the matrix. The impurity-free dispersant is effective not only in ensuring the uniform dispersion of CB particles but also in enhancing filler-matrix interfacial adhesion. High-molecular weight PAA chains are effective reagents for impurity-free modification and can therefore be used to improve the electrical and mechanical properties of the resultant composite.