Frequency Dependent Dielectric Measurements Research Articles

Temperature and frequency dependent dielectric properties of electrically conducting oxidatively synthesized polyazomethines and their structural, optical, and thermal characterizations

Three azomethine diol monomers were synthesized by condensing with methanolic solution of aromatic aldehydes with ethylenediamine. These monomers were oxidatively polymerized using NaOCl as an oxidant. The structures of the monomers and polymers were confirmed by various spectroscopic techniques. Spectral results showed that the repeating units are linked by CC and COC couplings. The polyazomethines have fluorescent property with high stokes shift. Solid state electrical conductivity of polymers both in I2 doped and undoped states, temperature and frequency dependent dielectric measurements were made by two probe method. The electrical conductivities of polyazomethines were compared based on the charge densities on imine nitrogens obtained from Huckel calculation. The conductivity of polymers increases with increase in iodine vapour contact time. Among the synthesized polymers PHNAE has shown high dielectric constant at low applied frequency of 50 Hz at 393 K due the presence of bulky naphthalene unit in polymer chain.

Electrical conduction and dielectric relaxation properties of AlN thin films grown by hollow-cathode plasma-assisted atomic layer deposition

In this study, aluminum nitride (AlN) thin films were deposited at 200 °C, on p-type silicon substrates utilizing a capacitively coupled hollow-cathode plasma source integrated atomic layer deposition (ALD) reactor. The structural properties of AlN were characterized by grazing incidence x-ray diffraction, by which we confirmed the hexagonal wurtzite single-phase crystalline structure. The films exhibited an optical band edge around ∼5.7 eV. The refractive index and extinction coefficient of the AlN films were measured via a spectroscopic ellipsometer. In addition, to investigate the electrical conduction mechanisms and dielectric properties, Al/AlN/p-Si metal-insulator-semiconductor capacitor structures were fabricated, and current density–voltage and frequency dependent (7 kHz–5 MHz) dielectric constant measurements (within the strong accumulation region) were performed. A peak of dielectric loss was observed at a frequency of 3 MHz and the Cole–Davidson empirical formula was used to determine the relaxation time. It was concluded that the native point defects such as nitrogen vacancies and DX centers formed with the involvement of Si atoms into the AlN layers might have influenced the electrical conduction and dielectric relaxation properties of the plasma-assisted ALD grown AlN films.

Influence of moisture on dielectric condition of high power Insulated Gate Bipolar Transistor modules

Moisture in a high power Insulated Gate Bipolar Transistor (IGBT) module raises serious concern on component and system level reliability. Employing diagnostic test methods on high power IGBT modules might provide an opportunity in detection and estimation of moisture, however has not received wide attention. This paper investigates on understanding the influence of moisture in high power IGBT modules through time and frequency dependent dielectric response measurements and partial discharge analysis. The moisture in the main insulation of an IGBT module under DC excitation produces a sharp rise in its response current introducing deviation in their respective time dependent dielectric properties. Alternatively, the frequency dependent dielectric response measurements under wide frequencies provide more information than their DC counterpart. This method enables discrimination of various defects such as moisture, physical defects, etc., thereby revealing a clear picture about the actual status. In particular, the lower frequencies reveal longer range ion movements, hence could be used for more accurate estimation of moisture. The higher frequencies though not accurate in moisture estimation reveal deviation in the material properties. These deviations alter the apparent dielectric characteristics such as loss factor, permittivity, etc., of the IGBT samples which becomes clear through simple comparative analysis. In conclusion, the findings in this paper might help in understanding the influence of moisture over the dielectric integrity of a high power IGBT module. Additionally, the permissible or acceptable level of moisture which might act as a threshold to initiate necessary precautionary measures could be subsequently developed.

Nd doped lead titanate crystals for microelectronic memory device applications

Using high energy ball milling technique the nanocrystalline Pb1−3x/2NdxTiO3 (where x = 0.01), abbreviated as PNT, ceramics with tetragonal shape has been synthesized and analyzed through X-ray diffraction and scanning electron microscope analysis. The frequency dependent ac conductivity and dielectric constant measurements of the PNT ceramics sintered at 800, 900, 1000 and 1100 °C has been performed in the 100–525 °C temperature range and the density of states at the Fermi level evaluated. The minimum hopping length and dielectric constants in PNT has been explained successfully using the correlated barrier hopping model. With first order correction to energy the quantum theory of ferroelectric has been applied to explain the dielectric behavior in the paraelectric phase. The explanation for density of states at morphotropic phase boundary and the conduction mechanism of charge transport were found to be in good agreement with the experimental observations.

Dielectric studies of multiferroic orthochromites Ho0.9(RE)0.1CrO3 (where RE=Gd and Yb)

Dielectric studies have been made on two orthochromite systems, Ho0.9Gd0.1CrO3 and Ho0.9Yb0.1CrO3. In the known multiferroic system of HoCrO3, Ho is substituted by one lower and one higher atomic number rare earth element (Gd and Yb respectively). Solid state diffusion method has been used to synthesize these polycrystalline samples. X-ray diffraction patterns for both the samples show a distorted orthorhombic perovskite phase. Temperature and frequency dependent dielectric measurements on the Gd and Yb substituted samples show ferroelectric nature with ferroelectric to paraelectric (FE–PE) transitions occurring at Curie temperature (Tc)∼530(5)K and ∼490(5)K (at excitation frequency of 1kHz) respectively. The two studied systems are non-Debye relaxor ferroelectrics and the FE–PE transitions exhibit high diffusiveness.

The origin of thermally stimulated depolarization currents in multiferroic CuCrO2

We have measured the thermally stimulated depolarization currents (TSDC) of multiferroic CuCrO2. We observe a sharp peak near the antiferromagnetic ordering temperature TN ∼ 24 K, below which the material becomes ferroelectric. In addition, we observe three other peaks above TN at ∼50, 120, and 150 K, when the poling is done at a higher temperature than TN. These peaks are not related to exotic kinds of ferroelectricity. Using the poling field dependence of TSDC, the origins of the first two peaks are ascribed to the relaxation of defect dipoles and to space charge relaxation due to the release of trapped charges, respectively. Upon polishing, the peaks observed at 120 and 150 K disappear, suggesting a surface defect origin. Moreover, using temperature and frequency dependent dielectric measurements, we find Maxwell–Wagner type dielectric relaxation. In connection with the mechanism of one of the TSDC peaks, we suggest a Schottky barrier formation to explain the dielectric relaxation.

Dielectric relaxation in YMnO3 single crystals

We have investigated the origin of the dielectric relaxation in YMnO3 single crystals. Two distinct dielectric relaxation features were observed at low (200⩽T⩽373K) and high (300⩽T⩽450K) temperatures. Analysis of our detailed frequency, electrode and thickness dependent dielectric measurements and ac conductivity data as well as the use of single crystals allow us to get a comprehensive picture of these relaxations. The low temperature relaxation is attributed to the Maxwell–Wagner type effects originating from the dipoles at the surface while the high temperature one is suggested to originate from hopping of charge carriers resulting from the second ionization of oxygen vacancies.

Magnetic frustration and dielectric relaxation in insulating Nd2NiMnO6 double perovskites

We have investigated structural, dielectric, and magnetic properties of polycrystalline double perovskite Nd2NiMnO6 compound. The compound crystallizes in monoclinic P21∕n symmetry and is partially B-site disordered depending on the synthesis conditions. It undergoes second-order ferromagnetic transition at 192 K and shows glassy behaviour at low temperature. The glassy phase is due to anti-site disorder within the homogeneous sample. Temperature and frequency dependent dielectric measurements reveal colossal values of dielectric constant and is best interpreted using Maxwell-Wagner interfacial polarization model. Impedance spectroscopy has been used to analyse the intrinsic dielectric response. This enabled us to differentiate the conduction process at the grain and grain boundaries. Arrhenius behaviour is favoured at the grain boundary, while variable range hopping mechanism is considered most suitable within the grain region. dc conductivity measurements corroborate variable range hopping conduction.

Effect of alumina nanoparticles on dielectric permittivity, electrical conductivity, director relaxation frequency, threshold and switching voltages of a nematic liquid crystalline material

In the present work, alumina (Al2O3) nanoparticles (Al-NPs) were dispersed in a room temperature nematic liquid crystal 4-pentyl-4ʹ-cyanobiphenyl (5CB) at the concentrations of 0.2 and 0.6 wt%. Differential scanning calorimetry studies suggest decrease in clearing temperature of the composite materials as compared to the pure material. Dielectric studies show that the presence of the Al-NPs decreases the effective longitudinal component () of the dielectric permittivity while has no effect on the transverse component (). Ionic conductivity decreases for low concentration of the nanoparticles. In the case of homeotropic aligned samples, a dielectric relaxation mode has been detected in the MHz frequency region. Presence of the Al-NPs decreases the relaxation frequency corresponding to flip-flop motion of molecules about their short axes. From the frequency-dependent dielectric measurements, important dielectric parameters have been determined. Electro-optical experiments show that the threshold voltage decreases and the steepness of the transmission–voltage curve improves due to dispersion of Al-NPs.

Structural characteristics, electrical conduction and dielectric properties of gadolinium substituted cobalt ferrite

Gadolinium (Gd) substituted cobalt ferrites (CoFe2−xGdxO4, referred to CFGO) with variable Gd content (x=0.0–0.4) have been synthesized by solid state reaction method. The crystal structure, surface morphology, chemistry, electrical conduction and dielectric properties of CFGO compounds have been evaluated. X-ray diffraction measurements indicate that CFGO crystallize in the inverse spinel phase. The CFGO compounds exhibit lattice expansion due to substitution of larger Gd ions into the crystal lattice. Gd-substitution induced smooth microstructure and particle size reduction is evident in electron microscopy analyses. Frequency dependent dielectric measurements at room temperature obey the modified Debye model with a relaxation time of ∼10−4s and a spreading factor of 0.244–0.616. The frequency (f=20Hz–1MHz) and temperature (T=30–900°C) dependent dielectric constant analyses indicate that pure CFO exhibits two dielectric relaxations in the frequency range of 1–10kHz while Gd substituted CFO compositions exhibit only single relaxation at 1kHz. The dielectric constant of CFGO is temperature independent up to ∼550°C. The dielectric constant increases with T>550°C. Dielectric constant of CoFe2−xGdxO4 ceramics is also enhanced compared to pure CoFe2O4 due to the lattice distortion upon Gd incorporation. The tanδ (loss tangent)–T data reveals the typical behavior of relaxation loses in CFGO. Activation energy of the dielectric relaxation calculated employing Arrhenius equation varies from 0.564 to 0.668 (±0.003)eV with increasing x values from 0.0 to 0.4. Thermally activated small polaron hopping mechanism is evident in temperature dependent electrical properties of CFGO. The effect of Gd-substitution in CFO is remarkable on the resistivity and, hence, activation energy; both increases with increasing Gd content. A two-layer heterogeneous model consisting of semiconducting grains separated by insulating grain boundaries was able to account for the observed temperature and frequency dependent electrical properties in CFGO ceramics. The results demonstrate that the crystal structure, microstructure, electrical and dielectric properties can be tailored by tuning Gd-content in the CFGO compounds.

Influence of formic acid on electrical, linear and nonlinear optical properties of potassium dihydrogen phosphate (KDP) crystals

In present investigation 0.5 and 1mol% formic acid (FA) added potassium dihydrogen phosphate (KDP) crystals have been grown by a slow evaporation technique. The cell parameters of the grown crystals were determined using single crystal X-ray diffraction analysis. The presence of different functional groups has been qualitatively analyzed by the FT-IR spectral analysis. The optical transparency and optical constants were assessed employing UV–visible studies in the range of 200–900nm. The wide optical band gap of 1mol% FA added KDP has been found to be 5eV. The frequency dependent dielectric measurements were studied for pure and KDP added FA crystals. The enhanced second harmonic generation (SHG) efficiency of grown crystals was determined by a classical Kurtz–Perry powder technique. The encouraging third order nonlinear properties were examined employing a Z-scan technique using He–Ne laser, at 632.8nm. The effective negative index of refraction and high figure of merit (FOM) essential for laser stabilization were determined for grown crystals.

Dielectric relaxations and alternating current conductivity in manganese substituted cobalt ferrite

Manganese (Mn) substituted cobalt ferrites (CoFe2-xMnxO4, referred to CFMO) have been synthesized by the solid state reaction method and their dielectric properties and ac conductivity have been evaluated as a function of applied frequency and temperature. X-ray diffraction measurements indicate that CFMO crystallize in the inverse cubic spinel phase with a lattice constant ∼8.38 Å. Frequency dependent dielectric measurements at room temperature obey the modified Debye model with relaxation time of 10−4 s and spreading factor of 0.35(±0.05). The frequency (20 Hz–1 MHz) and temperature (T = 300–900 K) dependent dielectric constant analyses indicate that CFMO exhibit two dielectric relaxations at lower frequencies (1–10 kHz), while completely single dielectric relaxation for higher frequencies (100 kHz–1 MHz). The dielectric constant of CFMO is T-independent up to ∼400 K, at which point increasing trend prevails. The dielectric constant increase with T > 400 K is explained through impedance spectroscopy assuming a two-layer model, where low-resistive grains separated from each other by high-resistive grain boundaries. Following this model, the two electrical responses in impedance formalism are attributed to the grain and grain-boundary effects, respectively, which also satisfactorily accounts for the two dielectric relaxations. The capacitance of the bulk of the grain determined from impedance analyses is ∼10 pF, which remains constant with T, while the grain-boundary capacitance increases up to ∼3.5 nF with increasing T. The tan δ (loss tangent)-T also reveals the typical behavior of relaxation losses in CFMO.

Structural modification of poly (methyl methacrylate) due to electron irradiation

We have studied the changes in various properties of Poly (methyl methacrylate) (PMMA) after irradiation with electron beam using several techniques and tried to understand the correlation of modification with electron beam energy and current. This work was carried out with an aim to fabricate 3D structures in freestanding PMMA thin films by lithography. Chemical, structural and electrical alterations in PMMA thin film after irradiation with different doses were monitored using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and frequency dependent dielectric measurements, respectively. All the measurements invariably showed that the irradiation induced degradation of PMMA resulted in breaking of parental chain and associated changes. FTIR indicated increase in un-saturation (CC bonds) and formation of carbon dioxide in irradiated samples with increase in electron beam energy and dose. The radial distribution function (RDF) analysis showed a change in the short range order with increase in dose, ascribed to the increase in the breaking of polymer chain. Frequency dependent dielectric measurements revealed that the conductivity was more for un-irradiated PMMA. All the measurements invariably showed that the amount of damage increased with increase in electron energy and dose.

Investigation of structural and electrical properties of vanadium substituted disordered pyrochlore-type Ho2−xVxZr2O7 nanostructure

Disordered pyrochlore system with composition Ho2−xVxZr2O7 (where x=0, 0.25, 0.50, 0.75 and 1) has been synthesized by the normal microemulsion route to examine the effect of vanadium substitution on structural and electrical properties. The prepared compounds are characterized by several techniques including X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, energy dispersive X-ray fluorescence (ED-XRF), energy dispersive spectra (EDS), scanning electron microscopy (SEM), temperature dependent electrical and frequency dependent dielectric measurements. The XRD analysis confirms the formation of disordered pyrochlore phase with crystallite size 7–30nm while a second phase is also observed in the highly substituted materials. The increase in resistivity is attributed to the removal of low energy pathway due to cation disordering. The dielectric constant decreases due to lowering of dipole moment with substitution and its resonance behavior shifted toward higher frequencies. The electrical and dielectric measurements suggest that materials are suitable for high frequency electronic devices, such as oscillators, resonators and frequency filters.

Induced modifications in the properties of Sr doped BiFeO3 multiferroics

Multiferroics exhibit unique combination of ferroic properties, simultaneously. For instance, in BiFeO3, magnetic and electric properties co-exist. In this work, BiFeO3 and Sr-doped BiFeO3 samples with general formula, Bi1−xSrxFeO3 (x=0.00, 0.05, 0.10, 0.20, and 0.30) were synthesized by sol–gel auto-combustion technique, in order to investigate these ferroic properties. The samples were confirmed to have perovskite type rhombohedral structure, characteristic of BiFeO3. A dilute phase of Bi2Fe4O9 was also found in all the Sr-doped samples. The micrographs of the palletized samples revealed that minutely doped Sr might not have any effect on the morphology of the samples. Frequency dependent dielectric measurements were carried out at room temperature for all the samples from 100Hz to 1MHz. The dielectric constant of un-doped sample at low frequency was 52 which decreased with increasing Sr doping. An enhancement of magnetic properties was observed with increasing the Sr contents. Pure BiFeO3 material was observed to have the least value of remanent magnetization. As the Sr2+ ions were doped in BiFeO3, its magnetization and remanence were increased to 0.867emu/g and 0.175emu/g, respectively, at x=0.30.

Radiation Stimulated Permanent Alterations in Structural and Electrical Properties of Core-Shell Mn-Zn Ferrite Nanoparticles

A series of core-shell type nanoparticle samples of MnxZn1-xFe2O4were prepared by auto-combustion technique with x=0.3, 0.5, 0.7, 1.0. Subsequent to the structural and electrical property investigation the samples were irradiated with gamma radiation for different time intervals using60Co source. A significant decrease in lattice parameter and particle size was observed in gamma irradiated samples. Electrical resistivity of irradiated samples was found to decrease with the increase in γ-radiation dose. Frequency and temperature dependent dielectric constant and dissipation constant measurements carried out showed multifold enhancement of dielectric constant with pragmatic increase in dielectric loss, in γ-irradiated samples.

Frequency dependent dielectric measurements of Cd2+ doped Mn-Zn nano ferrites prepared by sol gel and coprecipitation methods

Ferrite powders of Mn-Zn doped with Cd2+ (Mn0.5CdxZn0.5-xFe2O4) were prepared by the sol gel auto- combustion and coprecipitation methods. The effect of Cd2+ doping on the crystalline phase formation was investigated by X-ray diffraction which confirms the spinel structure of prepared samples. Crystallite size found to be lower for samples prepared by coprecipitation method. The samples prepared using coprecipitation, found to be denser as compared to sol gel method. SEM analysis confirmed the nano structure of the prepared samples. Dielectric properties were studied using impedance analyzer (20 Hz – 5 MHz) and RF material analyzer (1 MHz – 3 GHz) respectively. DC electrical measurements showed highly resistive nature of the prepared samples and values are high for samples prepared from sol gel method. Impedance spectroscopic analysis was carried out in the frequency range from 20 Hz to 5 MHz for the prepared samples, showing resistance of grain boundary is higher than grains. Impedance plane plots gave information about the resistive and reactive parts of impedance. The presence of semicircular arcs at lower frequency side, confirms that the high resistance of samples is due to increase in the number of grain boundaries in the nano synthesised samples.

Effect of Co substitution on the structural, electrical, and magnetic properties of Bi0.9La0.1FeO3 by sol-gel synthesis

Cobalt (Co)-doped Bi0.9La0.1FeO3 multiferroics were synthesized by a sol-gel method based on the autocombustion technique. As-synthesized powder was examined using various characterization techniques to explore the effect of Co substitution on the properties of Bi0.9La0.1FeO3. X-ray diffraction reveals that Co-doped Bi0.9La0.1FeO3 preserves the perovskite-type rhombohedral structure of BiFeO3, and the composition without Co preserves the original structure of the phase; however, a second-phase Bi2Fe4O9 has been identified in all other compositions. Surface morphological studies were performed by scanning electron microscopy. Temperature-dependent resistivity of the samples reveals the characteristic insulating behavior of the multiferroic material. The resistivity is found to decrease with the increase both in temperature and Co content. Room temperature frequency-dependent dielectric measurements were also reported. Magnetic measurements show the enhancement in magnetization with the increase in Co content.

Structure and dielectric properties of sputtered bismuth magnesium niobate thin films

The Bi1.5MgNb1.5O7 (BMN) thin films were prepared on platinum coated sapphire by rf magnetron sputter deposition. Effects of substrate temperature, sputter pressure and O2/(O2+Ar) mixing ratio on phase structures and dielectric properties of thin films were investigated. The results indicated that sufficiently high substrate temperature and low sputter pressure would facilitate the formation of cubic pyrochlore in BMN thin films. Meanwhile, the appropriate O2/(O2+Ar) mixing ratio of sputter atmosphere was required. The deposited Bi1.5MgNb1.5O7 cubic pyrochlore thin films with (222) oriented texture exhibited large tunability of ~50% at a maximum applied bias field of 1.5MV/cm, with low dielectric loss of ~0.007. The temperature and frequency dependent dielectric measurements indicated that no noticeable dielectric dispersion was detected in BMN cubic pyrochlore thin films.

Activation energy of polarization due to electrical conductivity and dipole rotation in purified ca-bentonite

We determined the activation energy for dipole rotation and electrical conductivity by analyzing the frequency dependent dielectric permittivity measurements of purified Ca-Bentonite. The measurements were performed at four different temperatures between 296K and 353K and at frequencies between 5Hz to 5MHz. We used the Havriliak–Negami (HN) relation to represent the polarization due to dipole orientations and the Maxwell–Wagner–Sillars (MWS) relation with a power law term to represent the interfacial polarization. We obtained expressions for the real and imaginary parts of electric modulus M*(ω).We determined the relaxation times for HN and MWS contributions from a fit of the experimental data. Subsequently, the activation energies for HN and MWS contributions were obtained from an Arrhenius plot. The activation energy for the HN and MWS processes were found to be 22.3kJ/mol and 18.2kJ/mol, respectively.