Wenhu Yang

Mechanism of high dielectric performance of polymer composites induced by BaTiO3-supporting Ag hybrid fillers

BaTiO3-supporting Ag hybrid particles (BT-Ag) with varied fraction of Ag were synthesized by reducing silver nitrate in the glycol solution containing BaTiO3 (BT) suspensions. The Ag nano particles with a size of about 20 nm were discretely grown on the surface of the BT. The dielectric performance of the composites containing the BT-Ag as fillers in the matrix of polyvinylidene fluoride (PVDF) was investigated. The relative permittivity (er) of the BT-Ag/PVDF composites increased prominently with the increase of BT-Ag loading amount, and the typical conductive path of the conductor/polymer system was not observed even with a high loading of BT-Ag. The er at 100 Hz for the three BT-(0.31, 0.49, 0.61)Ag/PVDF composites at room temperature were 283, 350, and 783, respectively. The er of the composites was enhanced by more than 3 times compared with that of the composite containing untreated BT nanoparticles at frequencies over 1 kHz and the loss tangent (tan δ) was less than 0.1 which should be attributed to the low conductivity of the composites. Theoretical calculations based on the effective medium percolation theory model and series-parallel model suggested that the enhanced permittivity of BT-Ag/PVDF composites should arise from the ultrahigh permittivity of BT-Ag fillers, which was over 104 and associated with the content of Ag deposited on the surface of BT.BaTiO3-supporting Ag hybrid particles (BT-Ag) with varied fraction of Ag were synthesized by reducing silver nitrate in the glycol solution containing BaTiO3 (BT) suspensions. The Ag nano particles with a size of about 20 nm were discretely grown on the surface of the BT. The dielectric performance of the composites containing the BT-Ag as fillers in the matrix of polyvinylidene fluoride (PVDF) was investigated. The relative permittivity (er) of the BT-Ag/PVDF composites increased prominently with the increase of BT-Ag loading amount, and the typical conductive path of the conductor/polymer system was not observed even with a high loading of BT-Ag. The er at 100 Hz for the three BT-(0.31, 0.49, 0.61)Ag/PVDF composites at room temperature were 283, 350, and 783, respectively. The er of the composites was enhanced by more than 3 times compared with that of the composite containing untreated BT nanoparticles at frequencies over 1 kHz and the loss tangent (tan δ) was less than 0.1 which should be attributed t...

Electrical modulus analysis on the Ni/CCTO/PVDF system near the percolation threshold

A type of Ni/CCTO/PVDF three-phase percolative composite was prepared, in which the filler content (volume fraction) of Ni and CCTO was set at 60?vol%. The dependence of permittivity, electrical modulus and ac conductivity on the concentration of Ni and CCTO fillers near the percolation threshold was investigated in detail. The permittivity of the composites dramatically increased as the Ni content approached 24?vol%. This unique physical mechanism was realized as the formation of conductive channels near the percolation threshold. Analysis on the electrical modulus showed that the conductive channels are governed by three relaxation processes induced by the fillers (Ni, CCTO) and PVDF matrix, which are the interfacial polarization derived from the interfaces between fillers (Ni, CCTO) and PVDF matrix, and the polarization of CCTO ceramic filler and PVDF matrix. The conductivity behaviour with various Ni loadings and temperature suggested that the transition from an insulating to a conducting state should be induced by charge tunnelling between Ni?Ni particles, Ni?CCTO fillers and Ni?PVDF matrix. These findings demonstrated that the tunnelling conduction in the composite can be attributed to the unique physical mechanism near the percolation threshold.

Tuneable cellular-structured 3D graphene aerogel and its effect on electromagnetic interference shielding performance and mechanical properties of epoxy composites

Microstructure design is a crucial factor in determining the performances of graphene/polymer composites. In this study, controllable cellular-structured graphene aerogel–epoxy composites (GA–EP) were fabricated by an infiltration method using porous GA as a framework; GA with different pore sizes and morphologies was tuned by employing precursor graphene oxide (GO) sheets with varied sizes. The structural morphology, electromagnetic interference shielding (EMI) performance and mechanical properties as well as the thermal stability of GA–EP composites were comprehensively and comparatively investigated. It was found that GA which originated from large-sized GO sheets after compounding with epoxy (LGA–EP) possesses superior EMI properties and compressive strength than those of GA assembled by small-sized GO sheets after integrating with epoxy (SGA–EP). LGA–EP with a thickness of 3 mm and loaded with ∼1.0 wt% filler exhibits an EMI shielding effectiveness of around 30 dB in the measured frequency range of 10–20 GHz and compressive strength of 56.01 ± 1.57 MPa. Both SGA–EP and LGA–EP exhibit excellent electrical conductivity, while the interconnected network with large graphene sheets in LGA–EP provides more effective and fast channels for electron transport than that of SGA–EP. The electrical conductivity of LGA–EP is up to 9.28 S m−1 at only 1.0 wt% filler content. On the contrary, SGA–EP can obtain a higher storage modulus, glass transition temperature and thermal stability than those of LGA–EP. These results indicate that the macro-properties including EMI shielding, conductivity and mechanical properties of GA–polymer composites can be tailored by using precursor GO sheets with different sizes, and the GA–EP composites with excellent EMI performance have great potential applications in various fields such as electronics, automobiles and aircraft.

Enhancement of dielectric performance upto GHz of the composites with polymer encapsulated hybrid BaTiO3–Cu as fillers: multiple interfacial polarizations playing a key role

Cu nanoparticles with diameters of 15–25 nm were grown discretely on the surface of BaTiO3 (about 100 nm) via a hydrothermal method, and a polyethylene glycol 4000 layer was coated on the surface of the obtained BT–Cu hybrid particles. The PEG layer will serve as a robust interface layer to suppress the mobilization of charge carriers and protect Cu from oxidation. The BT–Cu particles were loaded as fillers in the matrix of polyvinylidene fluoride (PVDF) to fabricate the BT–Cu/PVDF composites. Microstructure and dielectric performance have been investigated. The results showed that the relative permittivity (er) of the composites increased prominently with the loading amount and meanwhile the dielectric loss tangent was suppressed at a low level. For instance, the permittivity of BT–Cu/PVDF with the volume fraction of 53.7% reached 150 with a low loss of 0.16 at 1 kHz. The permittivities maintained high values of over 55 and the dielectric loss was less than 0.05 upto 1 GHz. Investigation on the polarization mechanisms has been conducted and the interfacial polarization between different phases should account for the high dielectric permittivity upto GHz. The energy storage characteristics were also studied.

A Compact Low-Pass Filter Based on the Fe3O4@SiO2-CCTO-Epoxy Composite Film

A compact low-pass filter (LPF) with defect ground structure (DGS) is presented based on the Fe3O4@SiO2-CCTO (CaCu3Ti4O12)-epoxy three-phase composite film. The transmission performances of the LPF were investigated through EM simulation. The results show that the frequency of the attenuation poles is related with the electromagnetic property of the film. The LPF with DGS shows good suppression at the high frequency and little ripple in the passband. The higher permeability and permittivity, as well as the DGS, will result in the attenuation poles moving to the low frequency. These results demonstrated that the dielectric–magnetic–polymer composites film with high permittivity and permeability coupled with the DGS is an effective way for the LPF miniaturization.

Influence of grid and target radius and ion–neutral collisions on grid-enhanced plasma source ion-implantation process

Grid-enhanced plasma source ion implantation (GEPSII) is a newly proposed technique for inner surface modification of materials with cylindrical geometry. In this paper, a collisional fluid model is used to investigate the ion sheath dynamics between the grid electrode and the inner surface of a cylindrical bore during the GEPSII process. Assuming the initial ion density along the radial direction is not uniform but determined by diffusion mechanisms, the effects of grid electrode radius, target radius and ion-neutral collisions on the ion dose and impact energy are investigated by solving fluid equations for ions coupled with Boltzmann assumption for electrons and Poissons equation. The results show that small gap distance between grid electrode and target is favourable to increase the ion dose and impact energy on the target. In addition, ion-neutral collisions can reduce both the ion dose and impact energy.

ZnO-decorated carbon nanotube hybrids as fillers leading to reversible nonlinear I-V behavior of polymer composites for device protection

Overvoltage protection is becoming increasingly important because of miniaturization and multifunctionality of electronic devices. Flexible, easily processable materials with nonlinear and reversible I-V behavior are highly desired. In this study, hybrid nanoparticles of ZnO-decorated carbon nanotubes (CNT-ZnO) were synthesized via a sol-gel hydrothermal process employed in an epoxy matrix to prepare composites. Microstructure analysis demonstrated that ZnO nanoparticles were well-bonded to the surface of CNT. The CNT-ZnO/epoxy composites exhibited nonlinear I-V behavior under increasingly applied voltage with a nonlinear coefficient of 5.01 (10 wt % filler loading). More importantly, the composites possessed excellent reversibility from dielectric to conductor and vise versa in the recycling of increase and decrease of applied electric field, in contrast to the poor recoverability of pure CNT-filled epoxy. The mechanism of the nonlinear I-V behavior and reversibility was investigated and discussed. A simple circuit was fabricated, which verified well the protection function of the CNT-ZnO/polymer composites.

Triple band-notched UWB antenna with tapered microstrip feed line and slot coupling for bandwidth enhancement

A new kind of triple band-notched antenna for ultrawideband (UWB) application is proposed, which can reject potential interference with the existing wireless systems including WiMAX, WLAN and X-Band communication. All the three notched bands are obtained by etching a bent and folded slot and a dipole-like resonator slots on the antenna radiating patch. A tapered microstrip feedline is introduced for good impedance matching. Furthermore, slot coupling is introduced to achieve a steep lower band characteristic. The antenna shows band-notched characteristics at the frequencies of 3.55 GHz (WiMAX), 5.75 GHz (WLAN) and 8.27 GHz (X-Band upper band). Good agreement between the simulation and measured results is observed, suggesting that the proposed antenna is suitable for the UWB communication application.

The nonlinear voltage-current characteristics of three-phase SiC/CNTs/epoxy composite

The nonlinear I-V behavior of the three-phase SiC/CNTs/epoxy composites for high voltage field grading application was investigated, and a composite bulk hopping mechanism was proposed. It is hypothesized that nearest-neighbor electronic hopping occurs through thin epoxy layers between CNTs and SiC particles, and an asymmetric rectangular potential barrier structure is the mechanism governing the nonlinear electric response of the three-phase SiC/CNTs/epoxy composites. Compared with CNTs/epoxy, the I-V behavior of the three-phase SiC/CNTs/epoxy composites exhibited much better reproducibility.

Development and application of polymer-based nanocomposites dielectrics

Polymeric composites are one of the most important materials in electronic packaging technology and much attention has been paid to its dielectric and energy storage behaviors in recent years. With the purpose of achieving nanocomposites with high dielectric constant, low loss and high breakdown strength, our group has focused on the microstructure construction of nanoparticles and grafting modification of polymer matrix. Various hybrids of metal nanoparticles randomly deposited on the surface of ceramic particles were prepared, including BaTiO<;sub>3<;/sub>-supported Ag (BT-Ag), BaTiO<;sub>3<;/sub>-suported Cu with a capping layer (BT-Cu), and BaTiO<;sub>3<;/sub> nano fiber-supported Ag (BTnf-Ag). The hybrid particles serving as filler in composites have effectively improved the dielectric constant and suppressed the increase of dielectric loss. For example, A high dielectric constant of 160 at 1 kHz (16 times larger than that of PVDF) was obtained for BT-Ag/PVDF composites, with low conductivity (σ <;10<;sup>-5<;/sup> S m<;sup>-1<;/sup>) and low dielectric loss (tanδ=0.11).In order to enhance the dielectric performance of the polymer matrix, polyethylene glycol (PEG) and chrome acetylacetonate (Cr-Acac) were employed to modify epoxy resin. The dielectric constant of the graft-modified epoxy was increased by 30%. A high dielectric constant of 37.75 and a low tangent loss of 0.022 were achieved at the frequency of 1 kHz when 100nm BaTi03 was used as filler and the modified epoxy as matrix. Film capacitor prototypes using some of the above hybrid particles and modified matrix were fabricated, and the dielectric characteristics were analyzed.