Quantum mechanical criteria for choosing appropriate voltage stabilization additives for polyethylene

One of the major application fields for solid dielectric polymers is their use as insulating materials for power cables. Since the electrical aging of the insulating polymeric materials is one of the most important factors affecting the service lifetime of power cables, developing a model which can be used to design materials with an improved resistance to electrical degradation would be highly beneficial. We developed a model for the electrical field within the polymer material contaminated with a sharp conducting defect (a metallic needle) and defined a parameter characterizing the resistance of polymer to electrical treeing. The model was used to analyse data for the electrical degradation of polyethylene stabilized with polycyclic aromatic hydrocarbons. Based on quantum mechanically calculated electron affinities and ionisation potentials of the stabilizer molecules, we discovered that if a molecule is to be considered for voltage stabilization use it has to have a specific combination of the ionisation potential and adiabatic electron affinity. The model allows for a choice of appropriate voltage stabilizers based on theoretical calculations only and can help to facilitate any experimental study for choosing appropriate voltage stabilizer additives.     

The effect of photostabilizers on polymeric insulation used at high voltages

Polymeric insulation used in high voltage applications and subjected to divergent electric fields emits ultraviolet radiation due to electroluminescence. The chromophores present in the polymer initiate photo-degradation of the material and, in dry environment, this leads to electrical treeing which causes premature failure of the insulation. The role of photostabilizers which can limit the photodegradation caused by ultraviolet radiation is studied for LDPE containing the usual and decreased concentration of air. Degassing or impregnating the polymer with an inert gas decreases the concentration of oxygen and prolongs the time to tree inception because the probability of forming excited or anionic oxygen is reduced.     

毛细管电泳芯片的电特性研究

摘要芯片毛细管电泳技术是20世纪末发展起来的一项新兴分析技术.本文研究了毛细管电泳芯片的电特性.在一定的电压范围内,玻璃和有机玻璃芯片的伏安特性都有线性段区域,因此在此线性段内研究芯片的电特性可以将其简化为电阻模型.根据基尔霍夫电流定律建立了毛细管电泳芯片的等效电阻模型,研究了分离电压以及分离焦耳热的影响因素,为毛细管电泳芯片的优化设计提供了理论依据.     

Investigation on water treeing behaviors of thermally aged XLPE cable insulation

The power cable insulation is in permanence subjected to thermal aging during its operating service. Thermal aging may influence not only the electrical, physicochemical and other properties of the XLPE cable insulation, but also the initiation and propagation of water tree inside it. Our research on the influence of thermal degradation to the water treeing behavior of XLPE cable insulation shows that thermal oxidation is the most influential to the initiation and growth of water treeing from the surface of XLPE cable insulation among all the probable factors caused during thermal aging.     

Tailored side‐chain architecture of benzil voltage stabilizers for enhanced dielectric strength of cross‐linked polyethylene

The synthesis and physico‐chemical properties of seven benzil‐type voltage stabilizers are reported. The benzil core is substituted with alkyl chains of different length that are linked to the benzil core via an ester, ether, or tertiary amine group. All additives can be melt‐processed with low‐density polyethylene (LDPE). Fourier‐transform infrared spectroscopy confirms that benzil compounds are not affected by the LDPE cross‐linking reaction induced by dicumyl peroxide. Moreover, a combination of gel content measurements, thermal analysis, and small‐angle X‐ray scattering indicates that the presence of benzil voltage stabilizers does not significantly alter the microstructure of cross‐linked polyethylene (XLPE). Electrical tree inhibition experiments under high‐voltage alternating current conditions show that all investigated additives substantially enhance the dielectric strength of the insulating material at a concentration of only 10 mmol kg^?1. The highest improvement in dielectric strength, of more than 70% with respect to reference XLPE, is obtained with voltage stabilizers, which carry short (methyl) side chains that are linked to the benzil core via an ester or tertiary amine group. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1047–1054     

The effect of adding virgin material or extra stabilizer on the recyclability of polypropylene as studied by multi-cell imaging chemiluminescence and microcalorimetry

The effect of adding virgin material or new stabilizers on the recyclability of polypropylene (PP) was determined. Stabilized PP was subjected to oven ageing at 130 °C for 100, 250 or 500 h before and after upgrading with virgin material (0, 20, 50, 80 or 100%) or additional stabilizer during reprocessing. The effect of upgrading recycled PP with extra stabilizer or virgin material was determined by measuring the elongation at break, yellowness change, oxidation induction time (OIT) or total luminous intensity (TLI) by chemiluminescence (CL) techniques. Selected specimens were analysed by microcalorimetry (MC).It has been shown that upgrading recycled PP with virgin material was not effective. Adding 80% virgin material did not increase the lifetime more than adding 20% virgin material. This indicates that a small amount of recycled material can induce degradation by contamination. Adding extra stabilizers instead of virgin material was much more effective. Of all techniques used the OIT as determined with CL showed a clear difference in residual stability of differently treated materials and seems to be the most valuable analytical method to determine the recyclability of PP. A comparison between CL and MC shows that MC is sensitive enough for measuring the solid-state oxidation of stabilized PP. Slightly lower OITs were detected by MC probably associated with the static atmosphere used during the measurements. The TLI method is not suitable to determine the degradation state of to be recycled material.     

Influence of typical stabilizers on the aging behavior of EVA foils for photovoltaic applications during artificial UV-weathering

Ethylene vinyl acetate copolymer (EVA) is the most commonly used embedding material in crystalline silicon photovoltaics. It is responsible for fixing module components, electrical isolation and protecting cells against mechanical and environmental stresses. The degradation of EVA during weathering can cause adhesion loss, the so called delamination, and yellowing of the foil, resulting in a drop of module efficiency. In order to improve the long-term stability, several stabilizers including UV-absorber, hindered amine light stabilizer and phosphite are added to the polymer. However, the exact influence of the different stabilizers on failure mechanisms has not yet been identified in detail. Therefore, different EVA foils containing variable additive formulations were exposed to artificial UV-irradiation at 50 °C. As a result, delayed EVA degradation could be observed depending on stabilizers added. On the other hand, some of the tested additives were found to be involved in delamination and yellowing processes.     

Thermal stability of poly(vinyl chloride) with epoxidised soybean oil as primary plasticizer

Epoxidised soybean oil (ESBO) has been tested as a primary plasticizer in suspension PVC. The stabilization of the material with different traditional stabilizers has been evaluated and compared to a compound with PVC-ESBO only. Surprisingly, the addition of stabilizers seems to decrease the stability of PVC-ESBO. Traditional stabilizers, for instance Ca/Zn-stearate have been evaluated as well as metal carboxylates regarding yellowness index and UV-Vis absorption. Among the metal carboxylates, a decrease in initial discolouration, i.e. yellowness was observed with addition of Zn-stearate whereas the addition of Al-stearate improved the colour after ageing compared to PVC-ESBO without stabilizer.The stabilizing mechanism of ESBO itself, without the addition of stabilizers, has also been investigated. Analyses with ion chromatography of ESBO extracted from PVC samples without stabilizer revealed that the chlorine content of ESBO had increased when ageing the sample. MALDI analysis revealed that hydrochloric acid likely had attached to the ESBO. Reactions between ESBO and PVC were found through NMR analysis.     

Graphene quantum dots from chemistry to applications

Graphene quantum dots (GQDs) have been widely studied in recent years due to its unique structure-related properties, such as optical, electrical and optoelectrical properties. GQDs are considered new kind of quantum dots (QDs), as they are chemically and physically stable because of its intrinsic inert carbon property. Furthermore, GQDs are environmentally friendly due to its non-toxic and biologically inert properties, which have attracted worldwide interests from academic and industry. In this review, a number of GQDs preparation methods, such as hydrothermal method, microwave-assisted hydrothermal method, soft-template method, liquid exfoliation method, metal-catalyzed method and electron beam lithography method etc., are summarized. Their structural, morphological, chemical composition, optical, electrical and optoelectrical properties have been characterized and studied. A variety of elemental dopant, such as nitrogen, sulphur, chlorine, fluorine and potassium etc., have been doped into GQDs to diversify the functions of the material. The control of its size and shape has been realized by means of preparation parameters, such as synthesis temperature, growth time, source concentration and catalyst etc. As far as energy level engineering is concerned, the elemental doping has shown an introduction of energy level in GQDs which may tune the optical, electrical and optoelectrical properties of the GQDs. The applications of GQDs in biological imaging, optoelectrical detectors, solar cells, light emitting diodes, fluorescent agent, photocatalysis, and lithium ion battery are described. GQD composites, having optimized contents and properties, are also discussed to extend the applications of GQDs. Basic physical and chemical parameters of GQDs are summarized by tables in this review, which will provide readers useful information.     

Effects of organic-inorganic hybrid coatings on durability of cross-linked polyethylene

Cross-linked polyethylene (XLPE) films have been coated with nanostructured hybrid organic-inorganic coatings in order to improve their durability. For this purpose, bi- and mono-layer coatings containing different amount of silica and different organic polymers have been prepared through sol-gel reactions and applied to XLPE commercial films. The thermo-oxidative stability, electrical strength and conductivity of XLPE coated films have been investigated after ageing in air at temperatures above the on-service conditions, i.e. at 105 and 120 °C for 1900 and 600 h, respectively. The performed investigations (FT-IR, DSC, TGA and electrical properties) showed that all the coatings tested were able to strongly protect XLPE against oxidation, and that the coating with a PVOH/SiO₂ layer gave the best protection. The increase of thermal resistance induced by the coatings reflects on the electrical strength after ageing, which is higher for coated samples than for uncoated ones. Moreover, while ageing has only a slight effect on electrical conductivity in different coated samples, a strong increase of conductivity was observed after ageing for highly oxidated uncoated samples.     

Nanosecond Surface Corona Discharge for Different Types of Strip Electrodes and Ceramic or Polymer Dielectrics

Surface corona discharge characteristics from single-stripped and multi-stripped electrodes on the surface of thin dielectric plates of different kinds of ceramics and polymers are presented. Experimental results are obtained with voltage pulses of microsecond and nanosecond duration and they show significant difference in the values of the inception voltage, the discharge structure and current impulse amplitude for different barrier materials. Analysis of calculated and experimental values of the inception voltage is given, and it shows that beside electrical characteristics such as the relative permittivity and the specific resistance of the barrier material, the surface properties of the barrier, its thickness and its structure play a significant role in the discharge appearance and development. Nanosecond applied voltage pulses give a more dense and uniform discharge structure than microsecond ones, other conditions being equal.     

Non-Flammable Electrolyte Enables High-Voltage and Wide-Temperature Lithium-Ion Batteries with Fast Charging

Electrolyte design has become ever more important to enhance the performance of lithium-ion batteries (LIBs). However, the flammability issue and high reactivity of the conventional electrolytes remain a major problem, especially when the LIBs are operated at high voltage and extreme temperatures. Herein, we design a novel non-flammable fluorinated ester electrolyte that enables high cycling stability in wide-temperature variations (e.g., −50 °C–60 °C) and superior power capability (fast charge rates up to 5.0 C) for the graphite||LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) battery at high voltage (i.e., >4.3 V vs. Li/Li⁺). Moreover, this work sheds new light on the dynamic evolution and interaction among the Li⁺, solvent, and anion at the molecular level. By elucidating the fundamental relationship between the Li⁺ solvation structure and electrochemical performance, we can facilitate the development of high-safety and high-energy-density batteries operating in harsh conditions.     

A comparative study of effects of SEBS and EPDM on the water tree resistance of cross-linked polyethylene

In this work, we investigated the effects of an ethylene propylene diene monomer (EPDM) and poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) on the water tree resistance in cross-linked polyethylene (XLPE). The XLPE/EPDM and XLPE/SEBS blend samples were prepared by melting compounding and subsequent compression molding. It was found that SEBS could greatly increase the water tree resistance of XLPE and the resistance performance was improved with SEBS content within 15 phr, whereas EPDM did not show any improvement in the water tree resistance of XLPE. The frequency dependent behaviors of the water treeing phenomena and the effects of EVA on the water tree resistance of XLPE/EPDM and XLPE/SEBS blends were also investigated. The water treeing phenomena of the blends were interpreted from the viewpoints of electro-mechanical and electro-chemical mechanisms.     

Shape memory properties of polyethylene/ethylene vinyl acetate /carbon nanotube composites

The safe operation of electrical equipment relies on advanced polymer insulation to contain electrical pathways. Polymer sheath materials should be mechanically robust and chemically stable in order to protect the internal metal wiring from environmental attack. Polyethylene (PE) and ethylene vinyl acetate (EVA) have often been used as electrical cable jacket materials for electrical power industry. Partially crosslinked PE is able to shrink and wrap tightly around the metal wires upon stimulated by external heat, exhibiting shape memory behaviour. In this work, multiwalled carbon nanotubes (MWCNTs) were introduced to partially crosslinked linear medium density polyethylene (LMDPE) and EVA blend in order to enhance the shape memory performance at lower temperature by promoting the thermal transfer and antistatic properties of the polymer nanocomposite. The morphologies of the partially crosslinked and non-crosslinked composites are analysed. The MWCNTs preferentially resided in the EVA phase while the peroxide crosslinking process drastically altered the morphology and electrical properties. The addition of 3 wt% of MWCNTs resulted in a percolation transition and enhanced the alternating current (AC) conductivity by 10 orders of magnitude for non-crosslinked LMDPE/EVA and by 3 orders of magnitude for crosslinked LMDPE/EVA composites. LMDPE/EVA (80/20) containing 3 wt% MWCNTs possessed excellent shape recovery of 100% and shape fixing of 82%. The addition of MWCNTs can not only promote the shape memory efficiency of the polymer sheath material, but also introduce antistatic properties to avoid electrical shocking or sparking.     

Etude par spectroscopie de photoelectrons (XPS) de la surface de profiles a base de PVC

Variations of the surface composition (over a thickness of 50 Å) of stabilized and rigid PVC for outdoor applications and of pure moulded PVC resin have been investigated by photoelectron spectroscopy (ESCA or XPS) after extrusion, outdoor and accelerated ageing and photooxidation. Extrusion is accompanied by a pronounced migration of lead thermal stabilizers and of calcium stearate, used as an external lubricant, towards the outer surface. This treatment however, induces neither significant oxidation nor formation of a great quantity of ionic chloride. Natural and accelerated weathering were compared and were characterized by a large decrease in the lead surface concentration. The surface of this material is unaffected when exposed to air in a closed vessel and protected from external radiations. Finally, surface photooxidation by u.v. exposure has been evaluated and compared for the stabilized material and for the pure resin. This treatment involves formation of carbonyl and carboxylic groups. ESCA spectroscopy, a speedy and non-destructive surface analytical method, is well suited for study migration of stabilizers and lubricants.     

Effects of different compositions from magnetic and nonmagnetic dopants on structural and electrical properties of ZnO nanoparticles-based varistor ceramics

In the current study, 20 nm zinc oxide (ZnO) nanoparticles were used to manufacture high-density ZnO discs doped with Mn and Sn via the conventional ceramic processing method, and their properties were characterized. Results show that the dopants were found to have significant effects on the ZnO varistors, especially on the shape and size of grains, which are significantly different for both dopants. The strong solid-state reaction in the varistor from the 20 nm ZnO powder during the sintering process may be attributed to the high surface area of the 20 nm ZnO nanoparticles. Although Mn and Sn do not affect the well-known peaks related to the wurtzite structure of ZnO ceramics, a few of the additional peaks could be formed at high doping content (≥2.0) due to the formation of other unknown phases during the sintering process. Both additives also significantly affect the electrical properties of the varistor, with a marked changed in the breakdown voltage from 415 V to 460 V for Sn and from 400 V to 950 V for Mn. Interestingly, the electrical behaviors of the varistors, such as breakdown voltage, nonlinear coefficient, and barrier height, are higher for Mn- than Sn-doping samples, and the opposite behaviors hold for hardness, leakage currents, and electrical conductivities. Results show that the magnetic moment and valence state of the two additive dopants are responsible for all demonstrated differences in the electrical characteristics between the two dopants.     

In vivo electrical impedance measurements during and after electroporation of rat liver

Electroporation is used for in vivo gene therapy, drug therapy and minimally invasive tissue ablation. Applying electrical pulses across cells can have a variety of outcomes; from no effect to reversible electroporation to irreversible electroporation. Recently, it has been proposed that measuring the passive electrical properties of electroporated tissues could provide real time feedback on the outcome of the treatment. Here we describe the results from the impedance characterization (single dispersion Cole model) for up to 30 min of the electroporation process in in vivo rat livers (n=8). The electroporation sequence consisted of 8 pulses of 100 micros with a period of 100 ms. Half of the animals were subjected to field magnitudes considered to have reversible effects (R group, E=450 V/cm) whereas for the other half irreversible field amplitudes were applied (I group, E=1500 V/cm). As expected, there was an immediate increase of conductivity (R group Deltasigma/sigma(t=0)=9+/-3%; I group Deltasigma/sigma(t=0)=43+/-1%). However, the overall long term pattern of change in conductivity after electroporation is complex and different between reversible and irreversible groups. This suggests the superposition of different phenomena which together affect the electrical properties.     

A Conductive polylactic acid/polyaniline porous scaffold via freeze extraction for potential biomedical applications

This paper reports for the first time a simple yet effective method for fabricating a conductive and highly porous scaffold material made up of polylactic acid (PLA) and conducting polyaniline (PANI). The electrical percolation state was successfully obtained at 3 wt% of PANI inclusions and reached a conductivity level of useable tissue engineering applications at 4 wt%. In addition, preliminary bioactivity test results indicated that the protonating agent could form a chelate at the scaffold surface leading to good in-vitro apatite forming ability during biomimetic immersion. This new conductive scaffold has potential as a suitable biomedical material that requires electrical conductivity.     

Two-Dimensional Conjugated Metal-Organic Frameworks with Large Pore Apertures and High Surface Areas for NO2 Selective Chemiresistive Sensing

The emergence of two-dimensional conjugated metal–organic frameworks (2D c-MOFs) with pronounced electrical properties (e.g., high conductivity) has provided a novel platform for efficient energy storage, sensing, and electrocatalysis. Nevertheless, the limited availability of suitable ligands restricts the number of available types of 2D c-MOFs, especially those with large pore apertures and high surface areas are rare. Herein, we develop two new 2D c-MOFs (HIOTP-M, M=Ni, Cu) employing a large p-π conjugated ligand of hexaamino-triphenyleno[2,3-b:6,7-b′:10,11-b′′]tris[1,4]benzodioxin (HAOTP). Among the reported 2D c-MOFs, HIOTP-Ni exhibits the largest pore size of 3.3 nm and one of the highest surface areas (up to 1300 m² g⁻¹). As an exemplary application, HIOTP-Ni has been used as a chemiresistive sensing material and displays high selective response (405 %) and a rapid response (1.69 min) towards 10 ppm NO₂ gas. This work demonstrates significant correlation linking the pore aperture of 2D c-MOFs to their sensing performance.     

聚2,6—萘二甲酸乙二醇酯的光物理,光化学及电导

PEN-2,6的一些特殊性质(如刚性、耐热性和荧光性等)皆可归因于其芳香性和分子链的刚性。自1967年以来,人们采用多种方法、手段来研究这种功能材料,观察到了一些现象并作了相应的解释。本文所列举的关于PEN-2,6的工作主要集中在光物理、光化学和电导性能等方面。