Si-Jiao Wang

Large plasticity and tensile necking of Zr-based bulk-metallic-glass-matrix composites synthesized by the Bridgman solidification

The microstructures of the in situ bulk-metallic-glass-matrix composites are usually controlled by changing the alloy compositions. In this paper, Zr-based bulk-metallic-glass-matrix composites containing dendrites with a fixed composition of Zr37.5Ti32.2Nb7.2Cu6.1Be17.0 are synthesized by the Bridgman solidification. The sizes and volume fractions of dendrites in the composites are controlled by adjusting the withdrawal velocities. A linear relationship between the spanning lengths of individual dendrites and the withdrawal velocities is established. Large plasticity and tensile necking can be obtained by only controlling the cooling condition.

Tuning of thermal and dielectric properties for epoxy composites filled with electrospun alumina fibers and graphene nanoplatelets through hybridization

Epoxy resin is widely used for electrical and electronics packaging in various forms due to its excellent adhesion, low cure shrinkage and good electrical insulation. However, the low thermal conductivity and mismatched dielectric properties limit its application in highly integrated circuits. In this work, alumina fibers (AFs) were firstly prepared via electrospinning with sol–gel precursor. Epoxy (EP) composites with graphene nanoplatelets (GNPs) and AFs were fabricated using a hot-pressing process. Microstructures, thermal conductivity and dielectric properties of EP hybrid composites were studied. Scanning electron microscopy images reveal that the modified AFs and GNPs were uniformly dispersed in the epoxy matrix and the thermal conductive reticular structures were formed. The AFs can not only link the GNPs and epoxy but also reduce the interfacial thermal resistance so that a high thermal conductivity of 1.62 W m−1 K−1 is realized in the EP–GNP–AF composite, which is about 8 times higher than pure EP. The decomposition temperature of the epoxy composites with 2 vol% GNP and 50 vol% AF loading was enhanced by about 100 degrees. Dielectric properties of EP composites have a strong dependence on frequency and a weak dependence on temperature, which gives rise to the potential in different electronic/electrical field applications.

Distinctive electrical properties in sandwich-structured Al2O3/low density polyethylene nanocomposites

The sandwich-structured Al2O3/low density polyethylene (Al2O3/LDPE) nanocomposite dielectrics consisting of layer-by-layer with different concentration Al2O3 loading were prepared by melt-blending and following hot pressing method. The space charge distribution from pulsed electro-acoustic method and breakdown strength of the nanocomposites were investigated. Compared with the single-layer Al2O3/LDPE nanocomposites, the sandwich-structured nanocomposites remarkably suppressed the space charge accumulation and presented higher breakdown strength. The charges in the sandwich-structured nanocomposites decayed much faster than that in the single-layer nanocomposites, which was attributed to an effective electric field caused by the formation of the interfacial space charges. The energy depth of shallow and deep traps was estimated as 0.73 eV and 1.17 eV in the sandwich-structured nanocomposites, respectively, according to the thermal excitation theoretical model we proposed. This work provides an attractive strategy of design and fabrication of polymer nanocomposites with excellent space charge suppression.

Preparation, microstructure and properties of polyethylene/alumina nanocomposites for HVDC insulation

In the recent decades, the phenomena of space charge accumulation in the high voltage direct current (HVDC) insulation have been attracted more attention. In this paper, low density polyethylene (LDPE) nanocomposites filled with alumina nanoparticles (nano-Al2O3) were prepared employing melting blend method. Morphologies of nanoparticles and LDPE/Al2O3 nanocomposites were performed by scanning electron microscopy (SEM). Electrical properties of the LDPE nanocomposites were also investigated. Results shown that the nano-Al2O3 particles modified with vinyl silane coupling can effectively enhance the breakdown strength of LDPE nanocomposites. With the nano-Al2O3 particles loading, the volume resistivity of the LDPE nanocomposites was increased, while dielectric permittivity of the nanocomposites was decreased. Space charge of the LDPE nanocomposites was measured by pulsed electro-acoustic (PEA) method. The charge profiles indicated that space charge suppression of the LDPE nanocomposites was better than that of pure LDPE. The excellent insulation properties of the LDPE nanocomposites were attributed to the better interfacial adhesion between the surface-treated nano-Al2O3 particles and the LDPE matrix.

A remarkable suppression on space charge in isotatic polypropylene by inducing the β-crystal formation

This letter reports a significant suppression on space charge in isotatic polypropylene (iPP) by inducing the growth of β-crystal doped with the nucleating agent N,N′-dicyclohexylterephthalamide (DCTH). The α- and β-crystals in iPP were analyzed, and their effect on space charge and distribution of trap level was studied. Results indicated that the doping of DCTH (0.1 wt. %) had great effect on the formation of the microcrystallite clusters of β-crystal, which makes the fraction of β microcrystallite (βc) markedly an increase from 0% to 83.2% in iPP. Compared to the dispersed microcrystallites of α-crystal, the growth of β-crystal effectively suppressed the space charge accumulation. It would attribute that the deep traps greatly weakened the mobility of charge carrier in iPP material.

Improved mechanical and electrical properties in electrospun polyimide/multiwalled carbon nanotubes nanofibrous composites

Highly aligned polyimide (PI) and PI/multi-walled carbon nanotubes (PI/MWCNTs) nanofibrous composites by incorporating poly(ethylene oxide) as the dispersing medium were fabricated using electrospinning technique. The morphology, mechanical, and electrical properties of the electrospun nanofibrous composites were investigated. Scanning electron microscope showed that the functionalized MWCNTs (f-MWCNTs) were well dispersed and oriented along the nanofiber axis. Analysis of electrical properties indicated a remarkable improvement on the alternating current conductivity by introduction of the aligned f-MWCNTs. Besides, with addition of 3 vol. % f-MWCNTs, the obvious enhancement of tensile modulus and strength was achieved. Thus, the electrospun PI/MWCNTs nanofibrous composites have great potential applications in multifunctional engineering materials.

Improvement of space charge suppression of polypropylene for potential application in HVDC cables

High-performance polypropylene (PP) plays an important role in electrical/electronic engineering fields. Especially in high voltage direct current (HVDC) cables it compares well to cross linked polyethylene (XLPE) for its potential application as an eco-friendly material without cross-linking. The space charge injection under high electric stress is main obstacle for the development of HVDC cables. Here we adopted chemical modification on PP with polar functional group to improve the electrical properties, whereby the molecular structure has been designed to obtain excellent insulating material. The space charge suppression, dielectric properties and crystal characteristics of PP with and without grafting with maleic anhydride (MAH) were investigated. Results demonstrated that the MAH was successfully grafted onto PP macromolecular chain. Compared to pure PP, the grafting with 2 wt% MAH can effectively suppress space charge injection and provide better stability in volume resistivity as temperature increases. Besides, their dielectric properties were studied, and the mechanism of space charge suppression was proposed. This provides a useful method to prepare the HVDC cable insulating materials.

Electrical properties of polypropylene/styrene-ethylene-butylene-styrene block copolymer/MgO nanocomposites

With the development of the long-distance DC electrical power transmission, high voltage direct current (HVDC) cables have attracted more attention as an important transmission medium. HVDC transmission systems offer the best technical and economical solutions for long distance transmission. Polypropylene (PP) has better prospect than cross-linked polyethylene (XLPE), which is attributed to its excellent performance as an eco-friendly material in HVDC cables. However, the main problem in restricting the development of HVDC cables is the space charge injection and accumulation which can cause the breakdown and aging of materials. In this paper, we prepared the nanocomposites with the modified MgO, PP and Styrene-Ethylene-Butylene-Styrene Block Copolymer (SEBS) through melt blending method. The space charge, breakdown strength and dielectric properties of the nanocomposites were also investigated. Results showed that the nanocomposites with 0.5 phr surface-treated MgO dispersed well in the matrix. The addition of MgO effectively suppressed the space charge accumulation compared to PP/SEBS blends and improved its breakdown strength. At last, we explained the mechanism of space charge suppression using the trap theory. This work could provide theoretical basis for PP used as an eco-friendly insulating material in HVDC cables.

Effect of nano-fillers distribution on the nonlinear conductivity and space charge behavior in SiC/PDMS composites

Flexible polymer based materials with excellent breakdown strength and weak space charge behavior have been widely used in the field of cable insulation and cable termination. The silicon carbide (SiC) is beneficial to homogenize the electric field distributions, suppress the space charge injection and enhance the dissipation in the polymer composite. In this work, the polydimethylsiloxane (PDMS) based composites filled with β-SiC nanoparticles were prepared by solution mixing and mechanical blending, respectively. Compared with the pure PDMS, the SiC/PDMS composites possess not only the increased nonlinear conductivity coefficient but also the largely enhanced capability of dissipation of space charge. Especially under high electric field environment, the impact caused by uneven distribution of the SiC fillers was significant. The results of the space charges measurement under high electric field revealed that the SiC nanoparticles could largely enhance the dissipation and accelerate the movement of space charge, which was due to less trap sites leading to the better dispersion of SiC nanoparticles.

Sandwiched structure effect on space charge characteristics of alumina/polyethylene nanocomposites

In the recent decades, the phenomena of space charge accumulation in the high voltage direct current (HVDC) insulation have been attracted more attention. In this paper, the novel sandwich-structured low-density polyethylene (LDPE) nanocomposites filled with nano-sized Al2O3 particles were prepared by layer-by-layer hot pressing method. For comparison, the homo-dispersed LDPE composites with nano-Al2O3 were also prepared. Morphology, breakdown strength, space charge distribution and electrical conductivity of the specimens were investigated. Morphologies of the composites were characterized by scanning electron microscopy (SEM), and their space charge profiles were measured by pulsed electro-acoustic (PEA) method. SEM images showed that there are no obvious interfaces between the adjacent layers. The breakdown strength of the sandwich-structured LDPE nanocomposites is higher than that of the homo-dispersed ones. Besides, the charge profiles indicated that space charge suppression of the sandwich-structured LDPE composites was better than that of homo-dispersed ones. Under the electrical field of 50 kV/mm at 70 °C, the conductivity of homo-dispersed nanocomposite almost approaches to 1.0 ×10−11 S/m, which is almost four-fold over that of 1%N/LDPE/1%N (i.e, 2.5 ×10−12 S/m). It was concluded that the excellent electrical insulating properties of the sandwich-structured nanocomposites were attributed to the traps existing on the interfaces, which could effectively inhibit the charge injection.