Yun-Hui Wu

Oxygen effects on plastic deformation of a Zr-based bulk metallic glass

Starting with Zr of two different purities, Zr-based bulk metallic glasses (BMGs) with a nominal composition of Zr62Cu15.5Ni12.5Al10 were prepared having oxygen concentrations of about 3900 and 920at.ppm, respectively. Uniaxial compression tests showed that the BMG containing the higher level of oxygen has a higher yield strength and is capable of undergoing much less plastic deformation than that containing the lower level of oxygen. It appears that oxygen suppresses the formation of multiple shear bands but leads to an inability to sustain shear-band propagation, thus, changing the failure mode from relatively ductile to catastrophic brittle fracture.

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.

Thermally stable polyimide nanocomposite films from electrospun BaTiO3 fibers for high-density energy storage capacitors

Barium titanate (BaTiO3, BT) fibers were prepared via electrospinning with a sol–gel precursor, followed by a calcination process. Polyimide (PI) nanocomposite films with the electrospun BT fibers were fabricated using an in situ dispersion polymerization method. The morphology and crystal structure of the BT fibers were analyzed through scanning electron microscopy and X-ray diffraction. It was found that their diameter and length were greatly dependent on the calcination temperature. Compared to the spherical BT nanoparticles, the introduction of one-dimensional BT fibers into the PI matrix gave rise to improved thermal stability. Besides, the dielectric behaviors of the PI/BT-fiber composite films were investigated over the frequency range from 102 Hz to 106 Hz and within a temperature range of 20–150 °C. The results demonstrated that the dielectric permittivity at 102 Hz of the PI nanocomposite films with 30 vol% BT fibers was improved up to ∼27, and the corresponding dielectric loss is relatively low (∼0.015). The dielectric permittivity of the PI/BT-fiber composite films exhibited a slight dependence on temperature, while it is highly dependent on the calcination temperature of the electrospun BT fibers. This work opens a new path to optimize the dielectric properties of thermosetting polymer composite films with high energy storage density.

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.

Enhanced positive temperature coefficient behavior of the high-density polyethylene composites with multi-dimensional carbon fillers and their use for temperature-sensing resistors

Positive temperature coefficient (PTC) materials usually suffer from the low intensity and poor reproducibility, which will limit their service time under harsh thermal control conditions. In this paper, both the functionalized carbon black (CB) and multi-walled carbon nanotubes (MWNT) were introduced into the high-density polyethylene (HDPE) matrix to achieve the improved PTC behaviors. The CB/MWNT/HDPE and CB/HDPE composites were respectively prepared through solution-melt mixing method, and their PTC behaviors were investigated. The results show that the HDPE composites filled with the modified CB exhibit better PTC effect than those filled with raw CB due to the anti-oxidation action of coupling agent. Moreover, it was found that the addition of a small amount of MWNT (0.7 wt%) into the HDPE composites with CB (18 wt%) could make larger intensity (∼6.5) and better reproducibility of PTC behavior. The synergistic effect of the modified CB and MWNT on improving the service time of PTC effect was further explored. The use of multi-dimensional carbon fillers was expected to provide a new route to fabricate high-performance polymeric PTC materials with a potential application as flexible temperature–resistivity sensor.

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.

Strain-controlled giant magnetoresistance of a spin valve grown on a flexible substrate

This paper studies the strain-controlled giant magnetoresistance (GMR) change of a top pinned spin valve with the stacking structure of Co90Fe10/Cu/Co90Fe10/IrMn fabricated on a flexible polyethylene terephthalate substrate. The strain transverse to the magnetic easy axis can manipulate the magnetism of the Co90Fe10 layer and the Co90Fe10/IrMn bilayer and results in a large reversible and monotonic GMR variation from 0.64% (tensile strain) to 2.08% (compressive strain), which almost remains the same after bending 500 times. According to the Stoner–Wohlfarth model, the magnetic anisotropy of the free and pinned layers can be manipulated by strain, which causes the GMR variation of the spin valve. The heterostructure could also be used to generate a rectangle or sawtooth wave GMR signal. These findings indicate an efficient way to design magnetoelectric devices based on strain-modulated GMR changes.

Preparation and mechanism of charge carrier transportation of MsO/LDPE nanocomposites

With the development of high-voltage direct current (HVDC) transmission, LDPE nanocomposites have attracted more attention for fabricating HVDC cables instead of oil-filled cables and mass impregnated cables. However, the HVDC cable insulating materials with high-performance electrical properties are urgent to realize and the mechanism of charge carrier transportation is still unclear. In this work, LDPE nanocomposites filled with multi-layer structure MgO was prepared by means of melt blending. As the MgO is considered as the ideal nanofillers owing to execellent electric properties in dc materials. Few studies have reported the special structure of nanofiller on suppressing space charge and mechanism of charge carriers transportation. The multi-layer structure MgO was prepared through a solvothermal method, which is low density, high aspect ratio. The microstructures, as well as the space charge distribution and thermally stimulated current (TSC) were studied. The mechanism of charge carrier injection and transportation in MgO/LDPE nanocomposites under HVDC transmission condition were also explored. Results showed that the multi-layer structure of MgO nanoparticles were homogeneously dispersed in LDPE matrix, which enhanced the overall electrical properties of the MgO/LDPE nanocomposites. Moreover, the hypothesis model of charge carrier transport in solid dielectrics coupling with experimental results were used to explore the mechanism of charge carrier injection and transportation under HVDC condition.