High Frequency Dielectric Relaxation Research Articles

Probing molecular interactions of polysaccharides in the presence of water

Herein we report new dielectric relaxation processes at high frequency (ca. 1.2•108 Hz) in polysaccharides pellets (cellulose, chitin, chitosan, starch and amylopectin) that are traceable to strong OH–H2O interactions. Temperature dependence of high frequency dielectric relaxation behavior is investigated using dielectric spectroscopy. In situ temperature FTIR, XRD and TGA measurements are studied to assess relaxation processes related to vibration of OH groups. Upon heating below the glass transition there appears an Arrhenius-type dependence with negative activation energy; this is due to re-orientation of dipoles to lower energy. Upon further heating a glass transition is observed and then there appears another Arrhenius-type process with positive activation energy due to the increasing of random thermal motions of chains, water molecule evaporation and hydrogen bonding destruction. In dry samples the activation energies of this relaxation are practically zero because no exist water molecules and OH groups of polysaccharides are trapped in a potential well.

Polyelectrolyte polypeptide scaling laws via mechanical and dielectric relaxation measurements

Experimental results from mechanical viscoelastic as well as dielectric relaxation times were compared to theoretical expectations utilizing polymer scaling theory. Viscoelastic relaxation of a hydrogel at 33% relative humidity fabricated from co-poly-L-(glutamic acid4, tyrosine1) [PLEY(4:1)] crosslinked with poly-L-lysine scaled with concentration according to reptation dynamics. High frequency dielectric relaxation of aqueous copolymer PLEY(4:1) scaled with concentration as an ideal chain and aqueous Poly-L-glutamic acid scaled as an extended chain. The study shows that two seemingly different measurement methods can yield information about the state of polymer chain conformation in situ.

Dielectric properties of CaCu3Ti4O12 ceramics doped by La3+

Abstract XRD, SEM, EDS and XPS were used to study the influence of La3+ on phase structure, microstructure, cationic distribution and cationic oxidation state of CaCu3Ti4O12 ceramics. It was found that donor doping is formed by preferential substitution of La3+ for Ca2+. The concentration of O in the grain is increased and the segregation of Cu into the grain boundary is suppressed effectively by La3+ doping, leading to the decrease and uniform distribution of gain size. The influence of La3+ on dielectric properties of CaCu3Ti4O12 ceramics were investigated also by Novocontrol broadband dielectric spectrometer. It was found that oxygen vacancy-related dielectric relaxation at medium frequency region is suppressed and the corresponding dielectric loss is significantly reduced also. At the same time, high frequency dielectric relaxation is increased obviously, which results in a significant increase of dielectric constant from about 5 × 103 to 2 × 104. The study in this paper shows that donor doping is an effective way to modulate intrinsic point defects, microstructure and then dielectric properties of CaCu3Ti4O12 ceramics.

Electrical conduction mechanisms and dielectric relaxation in Al2O3 thin films deposited by thermal atomic layer deposition

In the present work, aluminum oxide (Al2O3) thin films were deposited on p-type silicon substrates by thermal atomic layer deposition technique. The structural properties of as-deposited Al2O3 thin films were characterized by grazing-incidence x-ray diffraction and it was determined that the films had an amorphous structure. Electrical transport mechanisms and dielectrical properties of the amorphous Al2O3 thin films were then investigated by fabricating and characterizing Al/Al2O3/p-Si metal–oxide-semiconductor (MOS) capacitors by performing the dc current-voltage and frequency-dependent dielectric measurements. As a function of the applied gate electric fields, the current conduction in low electric fields consists of Schottky emission, but in relatively high electric fields current conduction is dominated by space-charge limited conduction. From the frequency-dependent dielectric measurements, the dielectric loss showed two peaks at the frequencies of 10 kHz and 0.7 MHz which evidence of the dielectric relaxation. The low and high frequency dielectric relaxation behaviors were attributed to space charge polarization and orientation-polarization of dipoles, respectively. It was concluded that defects related localized states in the Al2O3 films may contribute to dielectric properties at high frequency.

Anomalously high dielectric strength and low frequency dielectric relaxation of a bent-core liquid crystal with a large kink angle

We investigated the dielectric dispersion property of a bent-core liquid crystal (BLC) with a large kink angle in the frequency range of 1.0 Hz–5.5 MHz in a planar aligned cell. Single dielectric dispersion was observed in the smectic A, nematic, and isotropic phase of the planar aligned sample. The dielectric strengths, relaxation frequencies, distribution parameters, and dc conductivity were measured as a function of temperature. The dielectric strength of the observed relaxation mode was anomalously high (∼70), whereas the relaxation frequency was low at ∼500 Hz. The relaxation mode observed in the planar aligned cell was attributed to the fluctuations in the polarization direction due to the cooperative motion of the molecules.

Investigation on Optical Properties of SnSe2 Thin Films Synthesized by Two – Stage Process

Tin diselenide (SnSe2) is a binary compound semiconductor used in wide range of opto-electronic applications. In the present study, SnSe2 thin films have been deposited by using a two – stage process that involves selenization of DC – magnetron sputtered tin metallic layers at different selenization temperatures (TS), which varied in the range, 300 - 400°C. The optical properties of SnSe2 thin films made using the optical transmittance and reflectance measurements have been investigated in detail for the first time. The optical absorption coefficient was evaluated from the transmittance versus wavelength data and was >104 cm-1 for all the samples. The band gap of the as-grown films was estimated by using the differential reflectance (dR/dλ) spectra and found to be varied in the range, 1.36 – 1.47 eV with respect to the selenization temperature. Further, the refractive index, extinction coefficient and other optical parameters such as dielectric constant, dielectric loss factor, high frequency dielectric constant and relaxation time were also determined. The electrical parameters such as conductivity, carrier concentration and carrier mobility of as-prepared layers were estimated from the optical data. From this analysis, it is inferred that the sample prepared at TS = 350°C exhibited better optical and electrical properties than the layers formed using other temperatures that are suitable for solar cell application.

The dimensional effect of dielectric performance in CaCu3Ti4O12 ceramics: Role of grain boundary

The relationship between dielectric performance and sample thickness in CaCu3Ti4O12 ceramics was investigated. The geometric dimensional effect, observed in gradually grinded CaCu3Ti4O12 samples, is referred to as the responsible parameter for the change in breakdown field and nonlinear coefficient with the variation in the thickness. An inflection point behavior of the breakdown electric field, analogous to ZnO based varistor, can be observed with reduced thickness. When the thickness was reduced from 2.2mm to 0.45mm, the low-frequency (<100Hz) dielectric permittivity and dielectric loss was increased while high frequency dielectric relaxation can be barely affected. Impedance spectroscopy results show that the grain resistance remained as ∼40Ωcm, while grain boundary resistance decrease sharply from 79MΩcm to 14MΩcm. Scanning electron micrographs illustrate that there exists great difference between microstructure of surface area and interior area along thickness direction, which can be possibly caused by heterogeneous diffusion of oxygen and liquid phase sintering of ceramics. It was proposed that grain boundary play a key role in the dimensional effect, which can also be verified by the results of as-sintered CaCu3Ti4O12 samples with varied thickness.

Reason for the High Solubility of Chemically Modified Poly(vinyl alcohol)s in Aqueous Solution

Partially chemically modified poly(vinyl alcohol) synthesized via random introduction of a diol-type monomer (mPVA) shows much higher solubility in water than normal homo poly(vinyl alcohol) (PVA). To clarify the reason for the high solubility of mPVA, we investigated the temperature dependencies of the hydration numbers per hydroxy group (mOH) for mPVA at various diol-type monomer contents from x = 0 (PVA) to 0.12 using extremely high frequency dielectric relaxation measurements up to 50 GHz over a temperature range from 10 to 70 °C. The values of mOH for mPVA in the temperature range below 20 °C looked constant and increased from 2.4 to 3.4 with increasing x. Above 20 °C, the values of mOH decreased with increasing temperature and approached a constant value close to unity above 70 °C irrespective of x. Although the random introduction of the diol-type monomer into the mPVA backbones effectively increased the mOH values, broad dehydration was commonly observed at approximately 40 °C irrespective of the x value. It is likely that the presence of the diol-type monomers in the polymer backbone considerably disturbs the highly developed sequential intramolecular hydrogen bonding between adjacent hydroxy groups in the normal homo PVA.

Comprehensive optical studies on SnS layers synthesized by chemical bath deposition

A simple non-vacuum and cost effective wet chemical technique, chemical bath deposition was used to prepare tin sulphide (SnS) layers on glass substrates. The layers were formed by varying bath temperature in the range, 40–80°C, keeping other deposition parameters as constant. An exhaustive investigation on their optical properties with bath temperature was made using the transmittance and reflectance measurements. The absorption coefficient was evaluated from the optical transmittance data utilizing Lambert’s principle and is >104cm−1 for all the as-prepared layers. The energy band gap of the layers was determined from the differential reflectance spectra that varied from 1.41eV to 1.30eV. Consequently, refractive index and extinction coefficient were obtained from Pankov relations and dispersion constants were calculated using Wemple–Didomenico method. In addition, other optical parameters such as the optical conductivity, dielectric constants, dissipation factor, high frequency dielectric constant and relaxation time were also calculated. Finally electrical parameters such as resistivity, carrier mobility and carrier density of as-prepared layers were estimated using optical data. A detailed analysis of the dependence of all above mentioned parameters on bath temperature is reported and discussed for a clean understanding of electronic characteristics of SnS layers.

Evidence of anti-parallel dimer formation of 4-cyano-4'-alkyl biphenyls in isotropic cyclohexane solution.

The formation of anti-parallel dimers of two liquid crystal forming 4-cyano-4'-alkyl biphenyls (nCB, n = 5 (pentyl) and n = 8 (octyl)) was confirmed in an isotropic cyclohexane solution (nCB-cH). In addition to high frequency dielectric relaxation (DR) measurements up to 50 GHz, fluorescence emission (FE) experiments in the wavelength range from 280 to 500 nm were carried out to investigate the molecular dynamics of the nCB molecules. The DR spectra of solutions at intermediate to high concentrations were comprised of two dynamic modes. A fast mode with a relaxation time of ca. 90 ps was assigned to the free rotations of monomeric nCB molecules. The slow mode with a relaxation time of ca. 400 ps was attributed to the dissociation process of the anti-parallel dimers ((nCB)2). The Kirkwood factor (g(K)), a measure of the orientational correlation between the dipole moments of the cyano groups, was markedly less than unity for the slow mode, which demonstrated the formation of anti-parallel dimers, (nCB)2. The equilibrium constant for anti-parallel dimer formation in an isotropic solution, i.e., for the reaction, 2nCB ↔ (nCB)2, which was determined via the DR data increased with an increase in the concentration of nCB. Under extremely dilute conditions, a sharp fluorescence emission signal attributed to the nCB monomer was observed at 325 nm in FE spectrum measurements. However, at the moderate to high concentrations used in the DR measurements where the slow mode was clearly observed, a broad FE signal at 390 nm was observed, which was assigned to excimer emission (including emission from the excited ground state dimers). The relative intensity of the excimer emission to the monomer emission significantly increased with an increase in the concentration. Moreover, the equilibrium constant for the excimer formation reasonably agreed with that for the anti-parallel dimer formation evaluated by the DR data. Consequently, the results of the FE measurements evidently revealed that excimers of nCB and the (nCB)2 dimers formed in isotropic solution are identical chemical species.

Epoxy-Based Hydrogels Investigated by High-Frequency Dielectric Relaxation Spectroscopy

Using high-frequency dielectric relaxation spectroscopy, nanophase-separated structures of epoxy-based hydrogels were investigated as a function of water content at 25 °C. The dielectric spectra resulting from the hydrogels were reasonably decomposed into two Debye-type and two Cole-Cole-type relaxation modes. The fastest Debye-type mode, found at 8.3 ps, was attributed to the rotational relaxation process of free water molecules in the bulk state. The other Debye-type mode, at ca. 20-34 ps, originates from the exchange process of water molecules that are hydrogen-bonded to the hydrophilic epoxy network portions for free bulk ones. The first Cole-Cole-type mode observed, at ca. 20-370 ps, was assigned to the complicated dynamics for electric dipole moments of the hydrophilic groups in the epoxy networks (mainly monomeric oxyethylene units). The slowest major Cole-Cole-type mode, at 5-29 ns, was attributed to the Maxwell-Wagner-Sillars polarization process and confirmed the presence of the nanophase-separated structures as revealed by the previous small-angle neutron scattering experiments.

Temperature-Dependent Hydration/Dehydration Behavior of Poly(ethylene oxide)s in Aqueous Solution

It has been well-known that aqueous solutions of poly(ethylene oxide) (or poly(ethylene glycol)) (PEO) are clear in a low temperature range but show clouding points due to phase separation behavior at rather high temperatures close to 100 °C slightly dependent on its average molar mass and concentration. We carried out extremely high frequency dielectric relaxation measurements up to 50 GHz over a temperature range from 10 to 70 °C to determine the temperature dependence of a hydration number per ethylene oxide monomer unit (mEO) in PEOs with the average molar masses higher than 3000. The obtained mEO was a constant value of ca. 4 in a temperature range lower than 30 °C and decreased rather gradually and approached ca. 2 at 70 °C. The temperature dependence of mEO, hydration/dehydration behavior, confirmed in this study should be the essential reason for the phase separation behavior observed at high temperatures. The hydration/dehydration behavior observed in this study was compared with that reported previously using ultrasonic velocity measurement techniques.

Analysis of optical and related properties of tin oxide thin films determined by Drude-Lorentz model

In this paper, optical parameters, such as refractive index n and extinction coefficient κ, of tin oxide thin films determined by fitting the measured and simulated optical reflectance spectra, are presented and analysed. Among the various classical dispersion relations for the dielectric function, we use the Drude model combined with the Lorentzian oscillators to get a good fit of R in the measured spectral range. Results on related parameters such as high frequency dielectric constant, plasma frequency, relaxation lifetime, electron density, film thickness and band gap are discussed. Pure tin oxide films deposited at different temperature exhibit the same reflectance behaviour in the entire range while antimony doped films exhibit a deep minimum of reflectance in the visible domain and a maximum in the infrared domain. The results show also that the refractive index and extinction coefficient of antimony-doped SnO2 films increase with the increase of the Sb/Sn ratios. The dispersion of κ of doped films is higher than that the undoped ones in a wide photon energy spectral range while the dispersion of n exhibit an opposite behaviour. The high frequency dielectric constant ε∞ decreased with increasing percentages of Sb in SnO2 and the optical band gap Eg depend on the doping amount. The deposited films present a great reflection in the NIR wavelength region which may originate from the fact that these layers contain many carriers resulting in a large nonstochiometry in the chemical composition. The optically determined film thickness agrees with that obtained by ellipsometry.

Dielectric Analysis of Water in Microcrystalline Cellulose

Water content and the states of water are important factors with respect to the physical, chemical and microbiological-growth characteristics of pharmaceutical materials. In this study, the interaction of water with microcrystalline cellulose (as a function of water content between 9–37.5% w/w added water) was investigated by high frequency (1–20 GHz) dielectric relaxation spectroscopy. A dielectric dispersion (centred on frequencies between 4 and 8 GHz) was observed and attributed to the dipolar relaxation of water. The results suggest that up to 35% w/w, water is adsorbed onto the surface of microcrystalline cellulose and at the surface of the water-soluble components, and there is no free water; up to 20% w/w, water is strongly associated as the first hydration layer; between 20–35% w/w, additional hydration layers form at the surface; and up to 37.5% w/w, the water in the system is affected in some way by the surface of the solid. It is proposed that this technique could be used for determining the water content, and states of water, in pharmaceutical materials and could therefore be of great value in gaining an understanding of water-excipient interactions. A number of potential applications for this technique are given.

Anomalous Dehydration Behavior of a Short Collagen Model Polypeptide, (l-Prolyl-l-ProlylGlycyl)5, in Aqueous Solution

High-frequency dielectric relaxation measurements from 1 MHz to 20 GHz were carried out on aqueous solutions of a short collagen model polypeptide, (l-prolyl-l-prolylglycyl)5 (PPG5), at several temperatures ranging from 10 to 40 °C. Experiments revealed that PPG5 behaved as a fully dissociated flexible zwitterionic polymer chain over a wide temperature range. However, its hydration number per PPG5 molecule was altered dramatically and stepwise at Tdh = 27 °C. This is close to the triple-helix-to-single-coil (TH−SC) transition temperature (Tt ∼ 30 °C) for PPG10, clearly showing a TH−SC transition like that of native collagens in aqueous media. Below the Tdh, PPG5 kept about 135 water molecules per molecule, while it retained less than 50 water molecules above Tdh. This anomalous hydration number change, observed at Tdh (∼Tt), should be a TH−SC transition trigger for the longer PPG10.

Hydration Structure and Dynamic Behavior of Poly(vinyl alcohol)s in Aqueous Solution

High-frequency dielectric relaxation measurements up to 20 GHz were conducted in aqueous solutions of poly(vinyl alcohol)s (PVAs) to examine their hydration structures and dynamic behavior. Dielectric relaxation spectra of aqueous PVA solutions decomposed well into three sets of the Debye-type relaxation modes. This was attributed to the rotational relaxation mode of free water molecules, an exchange mode of water molecules hydrated to hydroxy (OH) groups for bulk water, and local motions of PVA main chains. OH groups of PVAs keep ca. 2−2.2 hydrated water molecules in aqueous solution, in contrast to the hydration behavior of 1,4-cyclohexanedimethanol and 1,4-cyclohexanediol model molecules possessing OH groups that lack intramolecular hydrogen bond formation, which keep ca. 5 water molecules per OH group in aqueous solution. In the case of PVAs, the formation of intramolecular hydrogen bonding between mainly adjacent OH groups effectively diminishes their hydration numbers.

Surface characterization of clay particles via dielectric spectroscopy

This work deals with the high frequency dielectric relaxation of clay (sodium montmorillonite, or NaMt) suspensions. By high frequency it is meant that the permittivity will be determined in the region where the Maxwell-Wagner-O’Konski relaxation takes place, roughly, the MHz frequency range. The applicability of dielectric determinations for the characterization of the electrical properties of these complex systems is demonstrated. In fact, standard electrophoresis measurements only allow to detect that the charge of the particles becomes slightly more negative upon increasing pH. Much more information is obtained from the high frequency electric permittivity for different concentrations of solids and pHs. From the characteristic frequencies of the relaxation it is possible to detect separate processes for parallel and perpendicular orientations of the clay platelets, modelled as oblate spheroids with a high aspect ratio. In addition, using a suitable model the surface conductivity of the clay particles can be estimated. Our data indicate that this quantity is minimum around pH 7, which is admitted as representative of the isoelectric point of the edges of the clay platelets. Data are also obtained on the amplitude (value of the relative permittivity at low frequency minus that at high frequency) of the relaxation, and it is found that it depends linearly on the volume fraction of solids, and that it is minimum at pH 5. These results are considered to be a manifestation of the fact that particle interactions do not affect the electric conduction inside the electric double layer, while the special behaviour at pH 5 is related to the existence of aggregates at pH 5, which increase the effective size of the particles and provoke a reduction of their effective conductivity.

Dielectric relaxation behavior of colloidal suspensions of palladium nanoparticle chains dispersed in PVP/EG solution

Dielectric measurements were carried out on colloidal suspensions of palladium nanoparticle chains dispersed in poly(vinyl pyrrolidone)/ethylene glycol (PVP/EG) solution with different particle volume fractions, and dielectric relaxation with relaxation time distribution and small relaxation amplitude was observed in the frequency range from 10(5) to 10(7) Hz. By means of the method based on logarithmic derivative of the dielectric constant and a numerical Kramers-Kronig transform method, two dielectric relaxations were confirmed and dielectric parameters were determined from the dielectric spectra. The dielectric parameters showed a strong dependence on the volume fraction of palladium nanoparticle chain. Through analyzing limiting conductivity at low frequency, the authors found the conductance percolation phenomenon of the suspensions, and the threshold volume fraction is about 0.18. It was concluded from analyzing the dielectric parameters that the high frequency dielectric relaxation results from interfacial polarization and the low frequency dielectric relaxation is a consequence of counterion polarization. They also found that the dispersion state of the palladium nanoparticle chain in PVP/EG solution is dependent on the particle volume fraction, and this may shed some light on a better application of this kind of materials.

Solvation and Dynamic Behavior of Cyclodextrins in Dimethyl Sulfoxide Solution

High-frequency dielectric relaxation behavior up to 20 GHz was investigated for plain (alpha, beta, gamma) and (62 and 100%) methylated cyclodextrins, CDs, in dimethyl sulfoxide, DMSO, solution. Each hydrogen atom of OH groups of the CDs solvated a DMSO molecule for a residence time of 130-180 ps due to the hydrogen bond formation to an oxygen atom of DMSO, and a few DMSO molecules were included in cavities of the CDs for a while similar to the residence time. The overall rotational relaxation modes of solvated CDs were also observed depending on the effective sizes of the solvated CDs.

DNA in Nanopores: Negative Capacitance and &lt;I&gt;δ&lt;/I&gt;-Relaxation at High Frequency

We measured the high frequency dielectric relaxation behavior of DNA molecules confined in nanopores of polycarbonate membrane. The data revealed the existence of a critical frequency omega(c) approximately GHz at which the ac conductivity showed delta-relaxation. Interestingly, the DNA molecules also exhibited a crossover from positive to negative capacitance corresponding to omega(c). The negative capacitance at the critical frequency suggested a strong inductive behavior of DNA molecules in the high frequency regime. The results are interpreted in terms of the confined geometry of the DNA molecules in the nanopores. The interfacial water H-bonded to DNA played a crucial role in determining the high frequency relaxation of DNA molecules. The results indicated that the DNA in nanopores could be designed for application in high frequency bandpass/notch filters.