Low-frequency Dielectric Constant Research Articles

Optical parameters of ZnTe determined using continuous-wave terahertz radiation

The optical parameters of three ZnTe crystal wafers of different thicknesses were determined using transmittance measurements of continuous-wave terahertz radiation from a two-color photomixing source. The parameters are extracted by fitting the transmittance data with theoretical curves generated using a Drude-Lorentz dielectric model of the crystal and a bootstrap statistical analysis of the fits. It was found at room temperature that the low and high frequency dielectric constants are ϵ(0)=9.8±0.2 and ϵ(∞)=7.3±0.6, respectively. The transverse optical phonon frequency was found to be νTO=6.0±1.3 THz. Sample specific properties such as the plasma, collision, and phonon damping frequencies were determined and used for an approximate calculation of carrier concentration. The results are compared with a comprehensive review of earlier values from the literature. Our results are consistent with previous work, falling within the spread of accepted values, and demonstrate that this method is particularly suited for determining the low and high frequency dielectric constants of semiconductor samples.

Room temperature stabilized cubic zirconia nanocrystal: a giant dielectric material

A giant low-frequency dielectric constant (∼105) has been observed in room temperature stabilized cubic zirconia nanocrystal of size range 4–8 nm. Dielectric constant strongly depends on the crystallite size. Electrical behaviour of nanocrystallites has been described by an equivalent circuit to obtain grain and grain boundary effects. Mixed valency of Zr (Zr4+, Zr3+) ions gives rise to electronic conduction. Giant dielectric constant has been explained by interfacial polarization because of Maxwell–Wagner type relaxation effect.

Quantifying the dielectric constant of thick insulators using electrostatic force microscopy

Quantitative measurement of the low-frequency dielectric constants of thick insulators at the nanoscale is demonstrated utilizing ac electrostatic force microscopy combined with finite-element calculations based on a truncated cone with hemispherical apex probe geometry. The method is validated on muscovite mica, borosilicate glass, poly(ethylene naphthalate), and poly(methyl methacrylate). The dielectric constants obtained are essentially given by a nanometric volume located at the dielectric-air interface below the tip, independently of the substrate thickness, provided this is on the hundred micrometer-length scale, or larger.

Synthesis, magnetic and electrical properties of Fe-containing SiO 2 nanocomposite

Nanocrystalline Fe powders were synthesized by transmetallation reaction and embedded in silica to form Fe–SiO 2 nanocomposite. Thermomagnetic study of the as-prepared Fe sample indicates the presence of Fe 3O 4 and Fe particles. Oxidation studies of Fe and Fe–SiO 2 show an increased thermal stability of Fe–SiO 2 nanocomposite over pure Fe. The Fe–SiO 2 shows an enhanced oxidation temperature (i.e., 780 K) and a maximum saturation magnetization value of (135 emu/g) with 64 wt.% of Fe content in silica. Electrical and dielectric behaviour of the Fe–SiO 2 nanocomposite has been investigated as a function of temperature and frequency. Low frequency ac conductivity and dielectric constants were found to be influenced by desorptions of chemisorbed moisture. High saturation magnetization, thermal stability, frequency-dependent conductivity and low power loss make Fe–silica a promising material for high frequency applications.

Calculations of the Spontaneous Polarizations and Dielectric Constants for AlN, GaN, InN, and SiC

Within the scope of the Harrison’s bond orbital model the spontaneous polarization, high- and low frequency dielectric constants are obtained in an analytical form. Theoretical results are in a reasonable agreement with the experimental data available and the numerical calculations based on the ab initio methods.

Determining the structural relaxation times deep in the glassy state of the pharmaceuticalTelmisartan

By using the dielectric relaxation method proposed recently by Casalini and Roland(2009 Phys. Rev. Lett. 102 035701), we were able to determine the structuralα-relaxationtimes deep in the glassy state of the pharmaceutical, Telmisartan. Normally, deep in the glassy stateτα is so long that it cannotbe measured but τβ, which is usually much shorter, can be directly determined. Themethod basically takes advantage of the connection between theα-relaxation andthe secondary β-relaxation of the Johari–Goldstein kind, including a relation between their relaxation timesτα andτβ, respectively.Thus, τα of Telmisartan were determined by monitoring the change of the dielectricβ-loss,ε′′,with physical aging time at temperatures well below the vitrification temperature. The values ofτα were compared with those expected by the coupling model (CM). Unequivocalcomparison cannot be made in the case of Telmisartan because itsβ-loss peak is extremely broad, and the CM predicts only an order ofmagnitude agreement between the primitive relaxation frequency and theβ-peak frequency. We also made an attempt to analyze all isothermal and agingsusceptibility data after transformation into the electric modulus representation. Theτα found in the glass state by using the method of Casalini and Roland in themodulus representation are similar to those obtained in the susceptibilityrepresentation. However, it is remarkable that the stretching parameterβKWW − M = 0.51 in the electric modulus representation gives more precise fitsto the aging data than in the susceptibility representation withβKWW = 0.61. Our results suggest that the electric modulus representation may be useful as analternative to analyze aging data, especially in the case of highly polar glassformers havinga large ratio of low frequency and high frequency dielectric constants, such as theTelmisartan studied.

Dielectric constant in the incommensurate SDW phase of (TMTTF) 2Br

The low-frequency dielectric constant was measured in the pressure induced incommensurate SDW phase of (TMTTF) 2Br in order to investigate the dynamics of the SDW. Under the pressure above 0.5 GPa, the huge dielectric constant attributed to the motion of the SDW were observed. The temperature dependence of the real part of the dielectric constant ɛ ′ whose amplitudes grow with decreasing frequency shows the peak behavior in the incommensurate SDW phase. In contrast of the case of (TMTSF) 2PF 6, the peak temperature of ɛ ′ is constant, irrespective of the frequency. These peak temperature T P are scaled as T P / T SDW ∼ 0.3 irrespective of pressure for the pressure larger than 0.8 GPa and is associated with the sub-phase transition in the incommensurate SDW proposed by the NMR measurement. The dielectric properties are probably connected with the co-existence of the CDW component proposed for high temperature sub-phase.

Dielectric properties of polylactides and their nanocomposites with montmorillonite

The influence of the preparation method on the dielectric properties of two polylactides and their nanocomposites with an organoclay (Cloisite® 30B) is investigated. The samples obtained by melt-mixing followed by hot pressing and by solution casting are compared. Dielectric properties of the nanocomposites show a weak influence of the nanoclay on segmental ( α S) and local ( β) relaxations in PLA. Above T g a strong increase of dc conductivity attributed to ionic species in the clay is observed. It gives rise also to the Maxwell–Wagner–Sillars interfacial polarization and both real and imaginary parts of ε ∗ ( ω) strongly increase. In the temperature dependence of low frequency dielectric constant an additional maximum around 80–90 °C is observed due to cold crystallization of PLA which is significantly different for the two kinds of PLA. After cold crystallisation conductivity and α-relaxation decrease while MWS relaxation only shifts to higher temperatures. Dielectric spectra of solvent-cast and melt-blended nanocomposites suggest better dispersion in the latter case.

Relation between electric properties and water saturation for hematitic sandstone with frequency

This paper focuses on the effect of water saturation on A. C. electrical conductivity and dielectric constant of fully and partially saturated hematitic sandstone sample (Aswan area, Egypt). The saturation of the sample was changed from partial to full saturation. Complex resistivity measurements at room temperature (~16°C), were performed in the frequency range from 0.1 Hz to 100 KHz. Experimental electrical spectra indicate, generally, that the electrical conductivity and dielectric constant vary strongly with water saturations and frequency. The low frequency electrical conductivity and dielectric constant are mainly controlled by surface conduction and polarization of the electrical double layer. The behaviour of the electrical conductivity and dielectric constant, with increasing water content, were argued to the orientational polarization of bound water for very low saturations, displacement of the excess surface charges for relatively low saturations, and free exchange of excess ions in double layer with the bulk electrolyte and generation of transient diffusion potentials which lag behind the applied field for high saturations.

Dielectric response of tantalum oxide subject to induced ion bombardment during oblique sputter deposition

We describe the deposition of insulating tantalum oxide thin films under conditions of controlled ion bombardment, which can be achieved using reactive sputtering on 90° off-axis substrates with an applied substrate bias. Capacitive measurements of Ta2O5 deposited on unbiased off-axis substrates indicate low frequency dielectric constants as high as εr∼300. Low frequency loss tangents are high, tan δ>0.5, and have a pronounced frequency dependence. Deposition of the film off-axis with sufficient applied rf bias to the substrate (negative bias >−70 V) recovers the on-axis properties typical of Ta2O5, e.g., εr∼22 and tan δ∼0.02. The recovery of normal dielectric behavior is attributed to the ion bombardment of the growing film under substrate bias, similar to on-axis depositions but absent from depositions on off-axis substrates with no applied substrate bias. We suggest that insufficiently bombarded films develop a Maxwell–Wagner type polarization along columnar voids. The void structure and the associated dielectric response vary with distance from the sputtering source due to variations in ion density and angle from the sputtering source. A similar dielectric response is observed in depositions on on-axis substrates as a function of angle from the central sputter gun axis. Our results suggest that ion bombardment is necessary for good quality sputtered dielectric films but that a controlled Ar+ flux is essentially equivalent to the uncontrolled O2−/O2− flux of on-axis reactive sputtering.

Influence of dc bias on the dielectric relaxation in Fe-substituted CaCu3Ti4O12 ceramics: grain boundary and surface effects

CaCu3Ti4−xFexO12 (CCTFO, 0 ⩽ x ⩽ 0.2) ceramics were fabricated by the solid-state reaction method and the dielectric properties have been systematically studied. All ceramics showed the single cubic perovskite structure, and there was no secondary phase. The average grain size increased and the grain boundaries became thin and dense with increasing x. With increasing Fe doping content, the resistivity of grains and grain boundaries increased, which led to a decrease in ε′ in the frequency range from 1 kHz to 1 MHz. A new relaxation process II was discovered by means of dielectric properties spectra measurement at room temperature. Based on the internal barrier layer capacitance model, the dielectric relaxation process I originates from interfacial polarization at grain boundaries. The relaxation process II is sensitive to surface treatment and, in contrast to the relaxation process I, is also strongly affected by the dc bias. As the dc bias voltage increased up to 40 V, the low-frequency dielectric constant and resistivity decreased dramatically in the CCTFO (x = 0.01) ceramic. After polishing the surfaces of the CCTFO (x = 0.01) ceramic, ε′ decreased from 105 to 5 × 104 without the dc bias, but remained essentially unchanged with increasing dc bias voltage at low frequencies. The dependence of the dielectric properties on dc bias voltage and surface characteristics reveals that the relaxation process II originates from the interfacial polarization at the surface layers.

Light-induced ionic polarization in CdZnTe:V semiconductor crystals as a source of giant enhancement of nonlinear effects

We report on experimental observation of a remarkable light-induced increase in the low-frequency dielectric constant in doped CdZnTe:V (CZT:V) semiconductor crystals and show that this increase is due to electric dipoles forming under illumination. Our findings provide strong evidence that CZT:V undergoes a photoinduced phase transition at room temperature from a nonpolar to polar phase. A related symmetry-breaking effect is responsible for giant light-induced enhancement of the nonlinear effects (the electro-optic effect and electrostriction) previously observed in these crystals.

Giant optical anisotropy in an infinite-layer iron oxide SrFeO2: An ab initio investigation

Based on density functional theory, we study the electronic structure, magnetic structure, and linear optical response in SrFeO2. For its infinite-layer structure with iron square-planar coordination, the band structure, magnetic exchange interaction, and linear dielectric function show significant anisotropic behavior. In particular, giant optical anisotropy is found. The absorption edge differs by 0.5 eV between xx and zz components, and the low frequency dielectric constant is found to be 4.26 and 5.11 for εxx and εzz, respectively. These results provide evidence of potential application of this unique structure.

Homogenization of Dissipative Photonic Crystals

We consider propagation of the H- and E-modes in a periodic arrangement of dissipative dielectric cylinders. Using the plane-wave expansion method, we derive analytical formulas for the imaginary part of the low-frequency efiective dielectric constants. The formula for the anti-Hermitian part of the efiective dielectric tensor is valid for arbitrary Bravais lattice and arbitrary fllling fractions.

Synthesis, Characterization, and Dielectric Properties of Phthalocyanines with Ester and Carboxylic Acid Functionalities

Monomeric phthalocyanine pentylester derivatives containing Cu and Zn were synthesized via the cyclotetramerization of 4,5-dipentylphthalonitrile. Conversion of the pentylester derivatives by hydrolysis yielded the octacarboxylic acid phthalocyanine derivatives in high purity, without any contamination by oligomers. The dielectric properties of pressed pellets were investigated as a function of frequency. When exposing the phthalocyanine powders to water vapor, a high increase in the dielectric constant was found for the carboxylic acid derivatives, whereas the esters showed no effect. For Cu containing octacarboxylic acid phthalocyanine derivatives, the low-frequency dielectric constant of the pellets was raised from 12 to >1 × 105 when going from a water content of 6.4 to 14.7 wt %, respectively. The concomitant sharp rise in conductivity was attributed to the protons released from the acid groups after water uptake. The present work clearly demonstrates that water uptake and not oxygen is the major fac...

Dielectric-constant measurement of thin insulating films at low frequency by nanoscale capacitance microscopy

We demonstrate a method for quantitatively probing the local low-frequency dielectric constant of thin insulating films by nanoscale capacitance microscopy. The calibrated capacitance-distance curves are measured on the dielectric film and analyzed by using a tip-sample capacitance model here proposed. Applied to SiO2 films as small as 1×1μm2 area and 20–30nm thickness, the method gives a dielectric constant on the submicron scale in agreement with the value determined on the large scale. The observed precision is set by the capacitance noise level of the instrument and the tip radius.

Stable electro-optic striation grating produced by programed periodic modulation of the growth temperature

Electrically controlled Bragg gratings implemented by periodic striations that were produced during the crystal growth are demonstrated in potassium lithium tantalate niobate crystals. The striations were generated by blowing air with periodic flow at the flux surface. The gratings were investigated by measurements of the diffraction efficiency versus the applied electric field. It was found that the composition grating induced correlated gratings of the refractive index and the low frequency dielectric constant. The latter, under the application of a uniform electric field, produced an electrically controlled birefringence grating through the quadratic electro-optic effect.

Grain-boundary and subgrain-boundary effects on the dielectric properties of CaCu3Ti4O12 ceramics

CaCu3Ti4O12 ceramic samples sintered at different temperatures were studied by measurements of microstructural and dielectric properties. Electron microscopy for thermally-etched samples confirmed a microstructure with subgrains (SG) buried in a crystalline grain. The detailed dielectric properties were shown to be conformable with the double-barrier-layer capacitance model. The complex electric modulus analysis revealed unambiguously each contribution of the SG interiors, SG boundaries and grain boundaries to the dielectric constant. Our result clarified how the dielectric properties change with increasing sintering temperature. It was concluded that grain boundaries rather than SG boundaries played the key role in the low-frequency giant dielectric constant in CCTO ceramics.

Dielectric dispersion study of coexisting phases of aqueous polymeric solution: Poly(vinyl alcohol) + poly(vinyl pyrrolidone) two-phase systems

The complex dielectric constant ɛ ∗ ( ω ) = ɛ ′ − j ɛ ″ , electric modulus M ∗ ( ω ) = M ′ + j M ″ , impedance Z ∗ ( ω ) = Z ′ − j Z ″ and ac conductivity dispersion behaviour of 5, 10 and 15 wt% concentration aqueous solutions of poly(vinyl alcohol) (PVA) (14 000 and 77 000 g mol −1) and poly(vinyl pyrrolidone) (PVP) (24 000, 40 000 and 360 000 g mol −1) and their binary mixtures were investigated in the frequency range 20 Hz to 1 MHz at 25 °C. Analysis of dielectric constant values confirms that hydrophilic effect of PVA in aqueous solution increases the real part of dielectric constant ɛ′, whereas for aqueous PVP solutions the hydrophobic effect masks the hydrophilic effect, which reduces ɛ′ values below 10 kHz. Low-frequency dielectric constant of these aqueous polymeric systems is sensitive to ionic conduction and electrode polarization. The monotonous change in various electrical properties with change in volume percentage of mixture constituents indicates that individual aqueous polymer system retains their own electrical properties in the aqueous two-phase polymeric system.

Simple model of the dielectric response of disordered substances: an explanation of ‘nearlyconstant loss’ and giant dielectric constant

A new model of dielectric response of disordered substances is proposed. According to thismodel, the dielectric response is determined by the Drude drift currents localized in minimaof the random electrostatic potential. The drift of a charge carrier with mobilityμ in local parabolicpotential φ(x) = φmin + kx2/2 under an external alternating field results in the Debye-type response with ‘relaxation frequency’ωr = μk. The use ofdistribution functions G(k) for values k of local potential wells of a disordered material allows us to describe quantitativelyboth the spectral domains of the ‘nearly constant loss’ and the cases ofthe giant contribution to a low-frequency dielectric constant. In disorderedsubstance the response depends mainly on the width but not the shape ofG(k).