Influence Of Dielectric Layer Research Articles

Influence of dielectric layers on performance of transmission mode frequency selective surface based linear to circular polarization transformers

AbstractIn this article, we investigate the electromagnetic behavior of freestanding and dielectric embedded frequency selective surface (FSS) based linear to circular polarization convertors. The two structures were designed to operate in transmission mode at normal incidence and yield the lowest value for axial ratio (AR) at frequencies around 10 GHz. In addition to providing mechanical robustness, a 72% increase in the 3 dB AR bandwidth (BW) is achieved when the 140‐μm thick perforated FSS sheet is inserted between two 5‐mm thick polytetrafluoroethylene (PTFE) slabs. At oblique incidence a downward shift in frequency is observed, however the BW is still significantly larger than the results obtained for the freestanding periodic array within a field of view of ±30° in the cardinal planes. Spectral transmission measurements presented for both anisotropic media verify the simulated results.

Influence of dielectric layers on estimates of diffusion coefficients and concentrations of ions from impedance spectroscopy.

We present the analysis of the impedance spectra for a binary electrolyte confined between blocking electrodes with dielectric layers. An expression for the impedance is derived from Poisson-Nernst-Planck equations in the linear approximation taking into account the voltage drop on the dielectric layer. The analysis shows that characteristic features of the frequency dependence of the impedance are determined by the ratio of the Debye length and the effective thickness of the dielectric layer. The impact of the dielectric layer is especially strong in the case of high concentrated electrolytes, where the Debye length is small and thus comparable to the effective thickness of the dielectric layer. To verify the model, measurements of the impedance spectra and transient currents in a liquid crystal 4-n-pentyl-4^{'}-cyanobiphenyl (5CB) confined between polymer-coated electrodes in cells of different thicknesses are performed. The estimates for the diffusion coefficient and ion concentration in 5CB obtained from the analysis of the impedance spectra and the transient currents are consistent and agree with previously reported data. We demonstrate that calculations of the ion parameters from the impedance spectra without taking into account the dielectric layer contribution lead in most cases to incorrect results. Application of the model to analyze violations of the low-frequency impedance scaling and contradictions in the estimates of the ion parameters recently found in some ionic electrolytes are discussed.

Influence of Dielectric Layers on Charge Transport through Diketopyrrolopyrrole-Containing Polymer Films: Dielectric Polarizability vs Capacitance.

Field-effect mobility of a polymer semiconductor film is known to be enhanced when the gate dielectric interfacing with the film is weakly polarizable. Accordingly, gate dielectrics with lower dielectric constant (k) are preferred for attaining polymer field-effect transistors (PFETs) with larger mobilities. At the same time, it is also known that inducing more charge carriers into the polymer semiconductor films helps in enhancing their field-effect mobility, because the large number of traps presented in such a disorder system can be compensated substantially. In this sense, it may seem that employing higher k dielectrics is rather beneficial because capacitance is proportional to the dielectric constant. This, however, contradicts with the statement above. In this study, we compare the impact of the two, i.e., the polarizability and the capacitance of the gate dielectric, on the transport properties of poly[(diketopyrrolopyrrole)-alt-(2,2'-(1,4-phenylene)bisthiophene)] (PDPPTPT) semiconductor layers in an FET architecture. For the study, three different dielectric layers were employed: fluorinated organic CYTOP (k = ∼2), poly(methyl methacrylate) (k = ∼4), and relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) (k = ∼60). The beneficial influence of attaining more carriers in the PDPPTPT films on their charge transport properties was consistently observed from all three systems. However, the more dominant factor determining the large carrier mobility was the low polarizability of the gate dielectric rather than its large capacitance; field-effect mobilities of PDPPTPT films were always larger when lower k dielectric was employed than when higher k dielectric was used. The higher mobilities obtained when using lower k dielectrics could be attributed to the suppressed distribution of the density of localized states (DOS) near the transport level and to the resulting enhanced electronic coupling between the macromolecules.

Ferrite-dielectric resonators made of uniaxial ferrites in before-the-resonance region

Theeffect of splitting resonant frequencies of azimuthally heterogeneousHE ±11δ oscillations of a disc ferrite-dielectric resonator made of barium hexaferrite is studied in external magnetic field on frequencies below 47 GHz. Applicability of the developed earlier analytical theory of electromagnetic oscillations in magneto-gyrotropic resonators to mode identification and calculation of field dependencies of magnetic oscillations’ resonant frequencies in the before-the-resonance region is demonstrated. Hysteresis effect is discovered in the field dependencies of magneto-dynamic oscillations’ eigenfrequencies in resonators made of uniaxial ferrite. The influence of dielectric layer between the resonator and the metal screen on frequency splitting of HE ±11δ oscillations is studied. The possibility of using ferrite-dielectric resonators in the self-magnetization mode is discussed.

The influence of dielectric layers on the magnetic properties of thin GdFeCo layers for in-plane MAMMOS/MSR media

The effect of adjacent dielectric layers on sputter deposited GdFeCo, of thickness 10–30nm, suitable as the readout layer for in-plane magnetic amplifying magneto-optic systems, was investigated by measuring the magnetic hysteresis using magneto-optical Kerr effect at room temperature and above. It was found that the magnetic properties were sensitive to the GdFeCo thickness and the presence of the dielectric layer material. Although alternative materials, ZnS and SiN, were also considered, the paper concentrates on the effects of reactively sputtered AlN, which was the only dielectric that produced the required properties for such thin GdFeCo layers of this type. The effect of reducing GdFeCo thickness was similar to that of reducing Gd content. The percentage of nitrogen used in the sputtering gas during AlN deposition was seen to be crucial to the magnetic properties and to the stability of the films. Films 10nm thick were particularly affected, the best results requiring less nitrogen than was needed for the thicker films.

The quest for fast phase change materials

Strategies are presented to identify phase change materials with fast transformation kinetics. A microscopic investigation of crystallization kinetics with a static tester and an atomic force microscope demonstrates that for different alloy compositions recrystallization proceeds either via growth from the crystalline rim or by nucleation and growth of critical nuclei. The influence of dielectric layers enables a further optimization of transformation kinetics.

Negative coronas: Low current mode pulse mode transition

In the paper the results of experimental study of negative corona in a point – plane gap in a dust-free air flow at atmospheric pressure are presented. Both the low current mode (LCM) of the discharge and Trichel pulses are externally triggered by UV light. Close to the inception voltage of Trichel pulses short-duration current spikes appear besides the steady component of the LCM which indicates to the switch-on of Fowler-Nordheim type emission mechanism. At the inception voltage spike – Trichel pulse transition is recorded. The influence of dielectric layers at the cathode on the discharge parameters is discussed.