As a core component of ac drive electric locomotive, performance of inverter-fed motor directly influences the stability and reliability of locomotive running. However, insulation early failures occur frequently. Presently, polyimide film, as a special type of engineering plastic film, widely applied in turn to turn insulation and turn to ground insulation of inverter-fed motors is a kind of basic insulating material. This paper investigates charge transport mechanism and space charge characteristics of polyimide films based on measuring and comparing space charge accumulation threshold field between corona-resistant and common polyimide films and analyzing the effect of temperature on space charge distribution. The results reveal that space charge accumulation threshold field of corona-resistant polyimide film is higher than that of common polyimide film; the adding of nano-particles effectively increases dielectric properties of corona-resistant film. Additionally, raising temperature promotes the electrode injecting charge and increases the charge energy and conductivity which gradually enhances space charge number and extends trapping position into the bulk, closely related to polyimide film insulation aging and breakdown which is an important characteristic.
Polypropylene (PP) nanocomposites filled with the pristine multi-wall carbon nanotubes (CNTs) and the purified CNTs were prepared by melt blending. The microstructure and linear viscoelastic properties were investigated using rheological and morphological measurements. The results show that the purified CNTs disperse uniformly in the PP matrix. At low frequencies, frequency dependence of modulus weakens clearly with the addition of the CNTs, indicating that the long-range motion of the polymer chains is restrained by the presence of the CNTs. Percolation networks form when the loading levels achieve up to 3 and 1.5 wt% for the composites with the pristine CNTs (PPCNTs) and the purified CNTs (PPcCNTs), respectively. The linear relaxation modulus increases with increasing loading level. And for composites with loading levels above percolation concentration, the modulus appears to reach a plateau at long time scales due to the formation of percolation network. Tensile strength and impact strength are simultaneously improved with the addition of the CNTs. The better the dispersion of the CNTs, the greater the improvement of the tensile strength and the impact strength.
Polylactide (PLA) and calcium carbonate (CaCO 3 ) were melt blended using a twin-screw extruder. The morphology of PLA/CaCO 3 composites was observed by scanning electronic microscopy. The linear and nonlinear shear rheological behaviors of PLA/CaCO 3 melts were investigated by an advanced rheology expended system. The results show that the CaCO 3 particles are evenly dispersed in the PLA matrix. The incorporation of low CaCO 3 content (<20%) causes the reduction of the storage moduli, loss moduli, and dynamic viscosities whereas high CaCO 3 content (>30%) leads to the increase of the storage moduli, loss moduli, and dynamic viscosities. The composites with high CaCO 3 content show pseudo-solidlike behaviors at low frequency. High CaCO 3 content also results in a significant increase of flow activation energy and a dramatic decrease of flow index n, which is in consistent with the more serious shear-thinning tendency of high-filled PLA composites melts. The particular rheological responses might be attributed to the formation and destruction of the percolating network.
A novel tunable frequency selective surface (FSS) with dual-degrees of freedom (DOF) is presented, and firstly applied to broadband absorber. Based on a simple prototype unit cell resonator, an approach for achieving multi-resonances is studied. A unit cell pattern with gradient edges is discussed, and variable resistor and variable capacitor are introduced to fully utilize its characteristic of multi-resonances. Bias line is designed to provide bias voltage respectively for two variable devices and provide two operational DOF for FSS. Simulation and measurement results both show that the tunable FSS absorber with dual-DOF has wideband absorption with the reflectivity below -10 dB in 1-5 GHz and with a total thickness of about 10 mm.
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