Yewen Zhang

Omnidirectional gap and defect mode of one-dimensional photonic crystals containing negative-index materials

We show theoretically that a one-dimensional photonic crystal containing a negative-index material has an omnidirectional gap, owing to the mechanism of zero (volume) averaged refractive index. In contrast to the Bragg gap, the edge of such a zero-n gap is insensitive to incident angle and polarization. When an impurity is introduced, a defect mode appears inside the zero-n gap with a very weak dependence on incident angle and invariant with scaling.

Tunneling modes of photonic heterostructures consisting of single-negative materials

We show theoretically that heterostructures consisting of single-negative materials can possess tunneling modes inside forbidden gaps, owing to the resonant coupling of the evanescent-wave-based interface modes. The tunneling modes appear when the heterostructure becomes nihility. They are independent of incident angles and polarizations and have zero phase delay, which can be utilized to design zero-phase-shift omnidirectional filters.

Subwavelength Substrate-Integrated Fabry-Pérot Cavity Antennas Using Artificial Magnetic Conductor

This paper presents the high-gain low-profile subwavelength substrate-integrated Fabry-Pérot (FP) cavity antennas with artificial magnetic conductor (AMC) sheets. A partially reflective planar AMC sheet and a ground plane are used as the two reflectors of the FP-type resonant cavity for an ultra-thin planar design. The cavity is fully filled with dielectric substrate for further reduction of thickness of the antenna and easy integration. A microstrip patch antenna is embedded into the cavity as a feed. As design examples, the antennas are designed to operate at 10 GHz with a fixed overall thickness of λ0/9 (where λ0 is the operating wavelength in free space) and an aperture of 2λ0 × 2λ0. The losses caused by both dielectric and conductors are analyzed, which are critical for a fully dielectric substrate antenna design. The via-walls surrounding the radiating aperture are introduced to improve radiation patterns and gain by suppressing the surface waves, which are another critical loss for a thin fully dielectric substrate antenna design. Measured results show that such dielectric-integrated subwavelength cavity antennas feature the high gain of 12.5 dBi, low profile, easy integration into circuit board, and mechanical robustness, which makes them suitable for low-cost mass production.

Suppression effect of surface fluorination on charge injection into linear low density polyethylene

To suppress charge injection from electrodes, direct fluorination using fluorine gas was used for linear low density polyethylene (LLDPE) since it is one of the most effective methods of the polymer surface modification. Surface fluorination of the LLDPE plates was obtained as indicated by attenuated total reflection infrared spectroscopy. Remarkable suppression of charge injection by the surface fluorination was observed by space charge distribution measurements using the pressure wave propagation method. Comparing with the remarkable bipolar charge distribution in bulk of the original LLDPE, there is less space charge in bulk and it mostly exists in the fluorinated surface layers. The possible mechanisms of the charge injection suppression are discussed, one of which, the effect of fluorination on the charge traps in surface layer was investigated by the thermally stimulated discharge technique. The results indicate that fluorination has charge traps in the surface layer remarkably deepened and charges ...

Rigid interpenetrating polymer network foams prepared from a rosin-based polyurethane and an epoxy resin

A series of rigid interpenetrating polymer network (IPN) foams, based on a rosin-based polyurethane and an epoxy resin, were prepared by a simultaneous polymerization technique. The changes in the chemical structure, dynamic mechanical properties, and morphology of the rigid IPN foams were investigated by Fourier transform infrared (FTIR) spectroscopy, dynamic mechanical thermal analysis, and scanning electron microscopy. The FTIR analysis showed clearly that the cure rate of the rosin-based rigid polyurethane foam and the epoxy resin were different and, as a result, these two networks formed sequentially in the final rigid IPN foams. All of the rigid IPN foams exhibited a single, broad glass transition that shifted to lower temperature as the epoxy resin content increased. The experimental composition dependence of Tgs of the rigid IPN foams showed slight positive deviation from the Fox equation for homogeneous polymer systems. No phase separation was observed from the scanning electron microscopy investigation. It could be concluded that these two component networks were compatible in the final rigid IPN foams. This compatibility could be attributed to a graft structure in the polyurethane and the epoxy resin networks arising from the reaction of the hydroxyl groups of the epoxy resin with the isocyanate groups of MDI, and from the reaction of the hydroxyl groups of the polyols with the epoxide groups of the epoxy resin, as suggested by FTIR analysis.

Morphology, mechanical properties, and thermal stability of polyurethane–epoxide resin interpenetrating polymer network rigid foams

A series of rigid interpenetrating network foams (IPNFs) based on a rosin-based polyurethane (PU) and a crosslinked epoxide resin (ER) were prepared by a simultaneous polymerization technique. The morphology, mechanical properties, thermal stability, and changes in the chemical structure during the thermal degradation process of the rigid IPNFs were investigated by scanning electron microscopy (SEM), compressive testing, thermogravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR). The SEM micrographs showed that the cell structure of the rigid IPNFs became less homogeneous with increasing ER content. The brittleness of the cell walls increased as the ER content and the cure time of the rigid IPNFs increased. The compressive strength of the rigid PU/ER IPNFs increased to a maximum value and then decreased with further increase in the ER content. Similar behavior was observed for the elastic modulus. This behavior was related to the nonhomogeneous cells and more brittle cell walls for the rigid IPNFs with high ER content. The TGA data showed that the thermal stability of the rigid PU foam increased with the addition of increasing levels of ER, due to the better thermal stability of the ER compared to that of the PU. With the exception of the ER alone, a two-stage weight-loss process was observed for all these rigid IPNFs and for the PU foam alone. The FTIR analysis suggested that the first stage of weight loss was due to the degradation of the polyol–derived blocks of the PU, and the second weight loss stage was governed by both the degradation of the ER component and that of the isocyanate-derived blocks of the PU.

Influence of oxyfluorination time on space charge behavior in polyethylene

Linear low density polyethylene (LLDPE) samples were surface oxyfluorinated for different times to investigate the influences of oxyfluorination time on charge injection from carbon black loaded poly(ethylene-co-vinyl acetate) (EVA) electrode and charge accumulation in bulk. Oxyfluorination led to the substantial variations in chemical composition, forming large numbers of various polar groups in the surface layers, depending on oxyfluorination time, as indicated by attenuated total reflection infrared (ATR-IR) analyses. Space charge measurements based on the pressure wave propagation method revealed obvious dependence of the charge injection and accumulation on oxyfluorination time when the samples were submitted to an average direct current field of about 50 kV/mm at 40°C. Suppression of the charge injection and accumulation by the oxyfluorinated surface layer was enhanced with extending oxyfluorination time. The total amounts of the positive and negative charges within the samples oxyfluorinated for 1, 6, and 13 h were calculated to be around 67, 50, and 29% of the total amount in the original sample, respectively. Diffusion of the volatile byproducts within the EVA electrode into LLDPE was evidenced by the ATR-IR analyses or indirectly by open-circuit thermally stimulated discharge current measurements. The diffusion influenced the charge injection and accumulation by changing charge traps in the surface layer and the field assisted ionization of the diffused volatile byproducts, and was influenced by the surface oxyfluorination depending on the treatment time. The influences of oxyfluorination on charge injection and accumulation were attributed to the remarkable increase in surface layer permittivity and the changes in its charge trap and barrier property to the diffusion of volatile by-products. The change in surface layer permittivity was indirectly evaluated by surface energy calculations.

Compact high-Q filters based on one-dimensional photonic crystals containing single-negative materials

A mechanism to design compact high-Q filters is proposed in this paper, based on the properties of a defect mode in a one-dimensional (1D) photonic crystal composed of alternating layers of negative-permeability material and negative-permittivity material. The eigenfrequency equation for the defect mode is derived by means of the transfer-matrix method, and then, the dependence of the eigenfrequency on the thicknesses of the two host layers and the defect layer is calculated. In contrast to the case for a filter based on a conventional 1D photonic crystal (with positive refractive indices), we find that the quality factor Q of the filter involved in this paper can be boosted noticeably while the volume of the filter decreases.

Significant suppression of surface charge accumulation on epoxy resin by direct fluorination

Prepared epoxy sheets were surface fluorinated in a laboratory vessel using a F2/N2 mixture with 12.5% F2 by volume at 50 oC and 0.1 MPa (1000 mbar) for 10 min to suppress surface charge accumulation on the epoxy sheet. Attenuated total reflection infrared analyses indicate that the fluorination led to substantial changes in chemical composition and structure of the sheet surface layer. The thickness of the fluorinated layer was determined to be 0.42 μm by SEM observation of the cross-section of the fluorinated sheet, and its SEM image shows that the fluorination also resulted in an increase in surface roughness. As a result, the deposited corona charge cannot be stored on the fluorinated surface even at room temperature, compared with a stable surface charge of the nonfluorinated (original) epoxy sample which has deep surface charge traps as indicated by the open-circuit thermally stimulated discharge current measurement. The measurements of surface conductivity and contact angle and the calculation of surface energy reveal that the fluorination gave rise to dramatic increases in surface conductivity and surface wettability and polarity. A very likely substantial decrease in depth of charge traps in the fluorinated surface layer and the adsorbed water on the fluorinated surface are responsible for the high surface conductivity of the fluorinated epoxy sheet. Surface charging current measurements further show a large steady state current flowing along the fluorinated surface during corona charging, compared with the almost zero steady state current of the original sample. This therefore suggests a lower steady state surface potential and a smaller dynamic surface charge accumulation of the fluorinated sample during the charge.

Rapid potential decay on surface fluorinated epoxy resin samples

Epoxy resin samples were surface fluorinated using a F2/N2 mixture with 12.5% F2 by volume at 50 °C and 0.1 MPa for different times of 10, 30, and 60 min. Surface potential measurements at room temperature and different relative humidity levels of 20% to 60% on the surface fluorinated epoxy samples charged by corona discharge showed a low initial surface potential and a rapid potential decay, depending on the ambient humidity and fluorination time, in comparison with the charged unfluorinated epoxy sample. Surface conductivity measurements at the different relative humidity levels further indicated a higher surface conductivity of the fluorinated samples than the unfluorinated sample by over three orders of magnitude and an increase or decrease in surface conductivity with the ambient humidity or fluorination time, in accordance with the results of surface potential measurements. Attenuated total reflection infrared analyses and scanning electron microscope surface and cross section observations on the un...