Showing papers on "Dielectric loss published in 1997"

Planar microwave integrated phase-shifter design with high purity ferroelectric material

Abstract: Ferroelectric materials (FEM's) are very attractive because their dielectric constant can be modulated under the effect of an externally applied electric field perpendicular to the direction of propagation of a microwave signal. FEM may be particularly useful for the development of a new family of planar phase shifters which operate up to X-band. The use of FEM in the microwave frequency range has been limited in the past due to the high losses of these materials; tan /spl delta/=0.3 at 3 GHz is typical for commercial BaTiO/sub 3/ (BTO) and due to the high electric field necessary to bias the structure in order to obtain substantial dielectric constant change. In this paper, a significant reduction in material losses is demonstrated. This is achieved by using a new sol-gel technique to produce barium modified strontium titanium oxide [Ba/sub 1-x/Sr/sub x/TiO/sub 3/ (BST)], which has ferroelectric properties at room temperature. Also demonstrated is how the use of thin ceramics reduces the required bias voltage below 250 V, with almost no power consumption required to induce a change in the dielectric constant. A phase shift of 165/spl deg/ was obtained at 2.4 GHz, with an insertion loss below 3 dB by using a bias voltage of 250 V. Due to the planar geometry and light weight of the device, it can be fully integrated in planar microwave structures.

Novel ferroelectric materials for phased array antennas

Abstract: Various composites of barium strontium titanium oxide (BSTO) combined with other nonelectrically active oxide ceramics have been formulated for application in phased array antennas. In general, the composites have adjustable electronic properties which can be tailored for use in phased array antennas and other phase shifting devices. The dielectric constant and the loss tangents have been reduced to enhance the overall impedance matching and thereby lowering the insertion loss of the device. In addition, the overall tunability, the change in the dielectric constant with applied voltage, is maintained at a sufficiently high level. In order to address a broad frequency range in the microwave region, the composites have been fabricated in bulk ceramic, thick film, and thin film form. This article discusses the processing, material characterization, and electronic properties of the composites in MHz and GHz frequencies.

Complete FDTD analysis of microwave heating processes in frequency-dependent and temperature-dependent media

Abstract: It Is well known that the temperature rise in a material modifies its physical properties and, particularly, its dielectric permittivity. The dissipated electromagnetic power involved in microwave heating processes depending on /spl epsi/(/spl omega/), the electrical characteristics of the heated media must vary with the temperature to achieve realistic simulations. In this paper, we present a fast and accurate algorithm allowing, through a combined electromagnetic and thermal procedure, to take into account the influence of the temperature on the electrical properties of materials. First, the temperature dependence of the complex permittivity ruled by a Debye relaxation equation is investigated, and a realistic model is proposed and validated. Then, a frequency-dependent finite-differences time-domain ((FD)/sup 2/TD) method is used to assess the instantaneous electromagnetic power lost by dielectric hysteresis. Within the same iteration, a time-scaled form of the heat transfer equation allows one to calculate the temperature distribution in the heated medium and then to correct the dielectric properties of the material using the proposed model. These new characteristics will be taken into account by the EM solver at the next iteration. This combined algorithm allows a significant reduction of computation time. An application to a microwave oven is proposed.

Effect of oxygen to argon ratio on properties of (Ba,Sr)TiO3 thin films prepared by radio-frequency magnetron sputtering

Abstract: Thin films of (Ba,Sr)TiO3 on Pt/SiO2/Si substrates were deposited using rf magnetron sputtering at various substrate temperatures and O2/(Ar+O2) mixing ratios (OMR). The crystallinity of the films improved significantly as the OMR increased. The dielectric constant increased with increasing OMR and reached a maximum value at 50% OMR. The leakage current density decreased with increasing oxygen flow, but had a minimum value at 40% OMR. The results for the dielectric constant and the leakage current were interpreted in terms of polarization effect and loss theory. The film deposited at 450 °C and 50% OMR exhibited good surface morphology and had a dielectric constant of 375, a tangent loss of 0.074 at 100 kHz, a leakage current density of 7.35×10−9 A/cm2 at an electric field of 100 kV/cm with a delay time of 30 s, and a charge storage density of 49 fC/μm2 at an applied field of 150 kV/cm. The 10 yr lifetime of time-dependent dielectric breakdown studies indicate that a 50% OMR sample has a longer lifetime than the 0% OMR sample.

Dielectric relaxation spectroscopy of polyethylene terephthalate (PET) films

Abstract: The dielectric properties of biaxially stretched polyethylene terephthalate (PET) films of thickness and 68% degree of crystallinity were investigated by means of dielectric relaxation spectroscopy in the frequency range Hz and the temperature range C. Differencial scanning calorimetry (DSC), in the range C, was employed to investigate the thermal properties of the PET samples. Besides measuring the relaxation associated with the glass transition and the secondary relaxation, special attention has been paid to the investigation of DC-conductivity-related effects. They give rise to high dielectric permittivity values and dielectric losses at low frequencies and high temperatures. The results are analysed within the complex permittivity formalism and discussed in terms of interfacial Maxwell - Wagner - Sillars polarization, the peak, conductivity relaxation, space-charge polarization, electrode polarization and DC conductivity. DC conductivity values determined from frequency plots of the AC conductivity follow the Vogel - Tamman - Fulcher equation at temperatures higher than the glass transition temperature, indicating that the charge-carrier transport mechanism is governed by the motion of the polymeric chains. On the basis of the temperature dependence of the DC conductivity PET is classified as a fragile system.

Microwave absorption in single crystals of lanthanum aluminate

Abstract: A very sensitive dielectric resonator technique is employed to measure loss tangent tan δ and relative permittivity er of lanthanum aluminate (LaAlO3) single crystals at 4–300 K and 4–12 GHz A variety of single crystals grown by different techniques and purchased from different suppliers are considered For T>150 K the loss tangent tan δ is almost sample independent with linear frequency dependence and monotonous temperature variation from 8×10−6 at 300 K to 25×10−6 at 150 K and 41 GHz In this temperature range the experimental data are explained by a model based on lifetime broadened two-phonon difference processes The loss tangent below 150 K is characterized by a peak in tan δ(T) at about 70 K The height of this peak is frequency and strongly sample dependent This leads to a variation of the loss tangent from 10−6 to 15×10−5 at 77 K and 86 GHz, the lowest values are generally achieved with Verneuil grown crystals and approach the intrinsic lower limit predicted by the phonon model The peak is

Frequency dependence of the complex impedances and dielectric behaviour of some Mg-Zn ferrites

Abstract: Complex impedances of some magnesium-zinc ferrites, MgxZn1-xFe2O4 (x=04, 05, 06 and 07), in the frequency range of 1 mHz-10 MHz were measured at 300 K The complex-plane impedance spectrum from each sample indicates that the capacitive and the resistive properties of the materials are mainly attributed to the processes that associated with the bulk (grain) and the grain boundary A low-frequency dispersion (LFD) and a negative capacitance regions are also observed at low frequencies A small series resistance of about the same value for all samples is also observed The bulk and the grain boundary components are analysed by assuming a two-layer leaky capacitor The simulated complex impedances using the Cole-Cole expression are in agreement with the measured values The dielectric properties of the materials are attributed to the interfacial polarization and LFD at low frequencies and the orientational polarization at high frequencies These properties result in the dielectric loss which composed of a Debye-like loss peak at high frequency end and a strong LFD below 1 Hz Both regions exhibit universality with regard to the power-law relation between the dielectric loss (e″r or χ″) and frequency as given by e″r∝ωn-1, where n=014 and n=002, respectively The exponent is marginally negative, n=−003, in the region of negative capacitance below 3–4 mHz The variations of the above properties and the characteristic parameters for different compositions of the Mg-Zn ferrites are discussed

A new approach to estimate complex permittivity of dielectric materials at microwave frequencies using waveguide measurements

Abstract: In this paper, a simple waveguide measurement technique is presented to determine the complex dielectric constant of a dielectric material. The dielectric sample is loaded in a short-circuited rectangular waveguide. Using a network analyzer, the reflection coefficient of the waveguide is measured. Using the finite-element method (FEM) the exact reflection coefficient of this configuration is determined as a function of the dielectric constant. The measured and calculated values of the reflection coefficient are then matched using the Newton-Raphson method to estimate the dielectric constant of a material. A comparison of estimated values of the dielectric constant obtained from simple waveguide modal theory and the FEM approach is presented. Numerical results for dielectric constants of Teflon and Plexiglas measured at the X- and Ku-bands are presented. Numerical inaccuracies in the estimate of the dielectric constant due to: 1) the presence of airgaps between sample and sample holder waveguide surfaces and 2) inaccuracy in the sample dimensions are also discussed.

Electrode and Interface Polarizations in Nematic Liquid Crystal Cells

Abstract: Low-frequency dielectric properties were measured for study of the transport properties of impurity ions in nematic liquid crystal (NLC: 4-cyano-4'-pentyl biphenyl) cells with and without polyimide alignment layers. It was found that the dielectric behavior of the NLC cells without polyimide alignment layers is due to the electrode polarization. The diffusion constant and the concentration of the impurity ions are determined from the dielectric behavior. The NLC cells with polyimide alignment layers exhibit interface polarization between the polyimide alignment layers and the NLC slab. The ion concentration is estimated from the frequency at the dielectric loss peak of the interface polarization. The ion concentration is much higher in the NLC cells with polyimide alignment layers than in the cells without polyimide alignment layers, indicating that a considerable amount of impurity ions are dissolved from the polyimide layers. The temporal variations of the dielectric properties of the NLC cells with and without polyimide alignment layers after the cell preparation were also measured.

Relaxations and fast dynamics of the plastic crystal cyclo-octanol investigated by broadband dielectric spectroscopy

Abstract: The dielectric loss of cyclo-octanol has been investigated in a very broad frequency window of more than 17 decades. The relaxational response was traced in the plastic and the supercooled plastic phase, which are governed by the same relaxational dynamics. Experimental evidence is provided for two additional relaxational processes below the GHz region which contribute to the high-frequency wing of the \ensuremath{\alpha} relaxation peaks. Near 100 GHz a loss minimum shows up which cannot be ascribed to a simple transition from the \ensuremath{\alpha} relaxation to the infrared bands. As in supercooled liquids, additional fast processes seem to contribute to the dielectric response at these high frequencies.

Performance characteristics of dielectrics in the presence of space charge

Abstract: The short and long-term dielectric behavior of a number of representative electrical insulating systems is compared in the presence of space charge. Dielectric materials, used both in the communications and power application areas, are considered. In this overview, particular attention is given to thin inorganic films, organic solid-liquid and solid polymer systems as regards to the manner in which space charge affects their dielectric loss, voltage breakdown, treeing and electrical aging characteristics.

Properties of epitaxial ferroelectric PbZr0.56Ti0.44O3 heterostructures with La0.5Sr0.5CoO3 metallic oxide electrodes

Abstract: PbZr056Ti044O3 (PZT) epitaxial ferroelectric thin films on a LaAlO3 (100) substrate, covered by a metallic oxide electrode La05Sr05CoO3 (LSCO) are demonstrated in this work The films are fabricated by the sol–gel method and effort was focused on thermal processing to obtain the desired epitaxial heterostructure The dielectric and ferroelectric properties of PZT thin films were measured and it was found that they are as good as in the films deposited by other thin-film methods The dielectric constant and the dissipation factor of the PZT films are, respectively, about 500 and 006 below 20 kHz The remanent polarization Pr is about 27 μC/cm2 and the coercive field Ec is about 50 kV/cm It was found that the ferroelectric properties were significantly influenced by the microstructure of the PZT layers The present study also showed that the fatigue characteristics of the epitaxial heterostructure with LSCO electrodes under a reversed electrical field are far superior to those obtained with a polycrys

A numerical Kramers-Kronig transform for the calculation of dielectric relaxation losses free from Ohmic conduction losses

Abstract: A numerical Kramers-Kronig transform is described which allows the calculation of dielectric relaxation losses from dielectric constant data measured at a limited set of frequencies differing by a factor of 2. Conversion formulas for both the central frequencies and for frequencies near the edges of the experimental frequency window are derived. The approach used can be extended easily to measurement frequencies with a different logarithmic spacing. Using this conversion, relaxation and dissipative, conduction losses can be separated. In this way Ohmic conduction processes and simultaneously occurring relaxation processes like dipole or space-charge relaxations can be analysed independently. The results of some simulations and of calculations on experimental data for poly(vinyl-chloride) are used to illustrate the potentials of the ɛ′ to ɛ″ conversion.

Optimization of SrTiO/sub 3/ for applications in tunable resonant circuits

Abstract: A series of SrTiO/sub 3/ (STO) films have been grown at a wide range of deposition conditions in order to determine the optimal growth parameters to maximize ferroelectric tuning while maintaining the lowest dielectric losses. The deposition pressure of the ambient O/sub 2/ (40 mTorr a(bulk) at higher pressures (P(dep)>65 mTorr). The dielectric constant (/spl epsiv//sub r/) and loss tangent (tan/spl delta/) were determined as a function of applied field at room temperature (300 K) and at liquid nitrogen temperature (77 K). The low frequency dielectric properties of the STO films were found to be a weak function of the strain of the ferroelectric film.

Dynamics of polybutadienes with different microstructures. 2. Dielectric response and comparisons with rheological behavior

Abstract: A series of 1,2-1,4-polybutadienes with varying 1,2 vinyl content was investigated using time-domain dielectric spectroscopy. The time range was 10 Ϫ5-300 s, which can be converted by Laplace transform to a frequency range of 10 Ϫ3-6000 Hz. The samples were the same as those used in a previous rheological study from these laboratories. Therefore, a direct comparison of dielectric and mechanical responses was possible. Within experimental uncertainty, the ␣ relaxation observed by both methods shows the same temperature dependence but there is an offset between the characteristic times of both methods, which increases with increasing vinyl content. This result can be qualitatively understood from the difference of the size of the dipolar groups, viz. cis and vinyl monomeric units, in the context of the DiMarzio-Bishop model. In addition, the question of time-temperature superposition was studied using the dielectric data. In the cases of a vinyl content у0.53 no deviations from time-temperature superposition were detected. Only for the sample with the lowest vinyl content 0.07 does the attempt to construct a master curve from the dielectric loss data fail. In this case a fit with a combination of a Havriliak-Negami and a Cole-Cole functions suggests that this deviation from time-temperature superposition is an intrinsic feature of the ␣ relaxation rather than an effect of its merging with the ␤ relaxation. The absence of indications for such a deviation in the rheological study can be explained by the smaller frequency range of the latter. This stresses the necessity of a large dynamic range in experiments aimed at the examination of the time-temperature superposition principle.

IR absorption and dielectric properties of Li - Ti ferrite

Abstract: Samples of the system (x = 0, 0.1,..., 0.7) were prepared by the usual ceramic technique. IR absorption spectra were used for analysing the compositions. The AC conductivity and dielectric properties (loss tangent tan , dielectric constant and dielectric loss ) have been measured at various frequencies and temperatures. Three bands were observed in the IR spectra. The band at around and another one at around were assigned to the tetrahedral and octahedral complexes, respectively. A small band at around indicated the octahedral divalent metal-oxygen bond in these complexes. The threshold frequency for the electronic transition seems to increase with increasing content, namely with decreasing concentration. At room temperature (RT), the AC conductivity exhibits dispersion in the frequency range from Hz to Hz. The variation of tan with frequency at RT shows a peak in the range Hz. The electrical conduction mechanism was explained in terms of the electron hopping model. Both effects (temperature and frequency) tend to increase and . The composition dependence of the dielectric properties is divided into two regions. The first one is for x<0.5 (for which long-range order is predominant) and the second one is for (for which short-range order is predominant).

Dipolar dynamics of low-molecular-weight organic materials in the glassy state

Abstract: We have measured the dielectric relaxation of several low-molecular-weight glass-forming materials for temperatures ranging from 25 K to and for values of the loss tangent down to . For the materials under study (salol, glycerol, N-methyl--caprolactam, 2-methyltetrahydrofuran, 3-methylpentane, and o-terphenyl) a -relaxation appearing in quenched samples can be suppressed effectively by annealing at , thereby facilitating the study of the dipole dynamics in the absence of the secondary process. As a general behaviour of the remaining dielectric losses at f = 1 kHz we find a variation of signalling the increasing constraint in the angular degree of freedom as the temperature is lowered and a further increase or peak of in the range 25 K < T < 50 K. According to a comparison with the crystalline counterparts, the latter effects are characteristic features of the disordered solid state.

The role of the dielectric loss of dispersed material in the electrorheological effect

Abstract: A set of electrorheological (ER) suspensions composed of solid materials with different dielectric losses and dielectric constants were examined by means of dielectric and rheological methods. We found that the dielectric loss of dispersed material, which was only slightly stressed in previous works, plays a considerable role in ER response; a large dielectric loss value is necessary for a clear ER effect. Two processes, the turning of randomly distributed microparticles toward the direction of an applied external electric field, probed recently by an x-ray diffraction method, and a coalescing of ordered microparticles, widely observed in ER fluids under a microscope, are suggested for understanding the ER mechanism. The first process is dominated by the dielectric loss of dispersed material, and the second one by the dielectric constant. Our result can be used to understand why certain materials of high dielectric constant but low dielectric loss would have shown an obvious ER effect according to the polarization and the conduction models, yet only displays weak or even no detectable ER effect experimentally. Our findings help to further understand the ER mechanism and also offer a clear implication on how to design high performance ER fluids.

Accurate equivalent circuits for unloaded piezoelectric resonators

Abstract: Equivalent circuit models for unloaded piezoelectric vibrators are presented. The new circuit model includes terms which take into account the dielectric and piezoelectric loss as well as the mechanical loss found in the standard Van Dyke's model. The new model contains two branches, the motional branch and the static branch, like the Van Dyke's model. However there is no resistance element in the motional branch and the losses (mechanical, dielectric, piezoelectric) associated with the vibrator are represented as imaginary components of the remaining circuit elements (C/sub 0/, C/sub 1/, L/sub 1/). The model produced impedance curves that very closely matched the impedance calculated by using the equation derived from vibration theory and data from lead zirconate titanate and PVDF copolymer samples. The calculation of the circuit parameters C/sub 0/, C/sub 1/, and L/sub 1/ from the complex elastic, dielectric and piezoelectric material constants is straightforward and the model accurately represents the impedance spectra about the fundamental resonance even when the mechanical Q of the resonator is as low as 2. Conversely, If the circuit parameters are known, the material constants including losses can be derived by straightforward calculations without the loss of any information.

Deposition of self-polarized PZT films by planar multi-target sputtering

Abstract: A planar multi target sputtering approach was used to deposit self polarized PZT films on TiO2/Pt bottom electrodes for the use in thin film pyroelectric IR detector arrays By using elevated substrate temperatures of about 450°C “in situ” growth of tetragonal PZT could be achieved The films exhibited pyroelectric currents without poling The pyroelectric coefficient was 2×10−4 C/m2K, the dielectric constant was 300 and dielectric loss tan δ was 001 The self polarization disappears after heating the sample to 600°C Stresses were studied in the thin film processing for the bottom electrode and the PZT film The TiO2/Pt electrode is under high tensile stress of 900 MPa after preparation PZT has a small compressive stress of -60 MPa, the whole TiO2/Pt/PZT stack has a tensile stress of +80 MPa This low stress level together with the self polarization and the good electrical properties makes the films suitable for the use in pyroelectric detector arrays

Application of relaxation current measurements to on-site diagnosis of power transformers

Abstract: Results of relaxation current measurements on impregnated pressboard samples and on multi-layer arrangements of pressboards in series with oil ducts are presented. The measurements have been performed dependent on the material properties of oil and pressboard and the volume ratio of these materials. Calculated values of related dielectric quantities such as tan/spl delta/ versus frequency, based on equivalent circuits obtained from relaxation currents are also shown. The relaxation currents of multi-layer test objects are calculated from the dielectric properties of the individual components and their geometrical capacitances. Finally, it is shown that relaxation currents of power transformers can easily be measured and their evolution can be described well by basic relations of linear dielectric theory.

Structural, electrical, and low-temperature dielectric properties of sol–gel derived SrTiO3 thin films

Abstract: Strontium titanate sol was prepared using strontium ethyl haxanoate and titanium isopropoxide. The sol was then spin coated on fused silica, p-type single-crystal silicon wafers (100) and stainless-steel substrates and annealed to give polycrystalline, transparent, and crack-free films. The surface morphology and structural properties of the films were studied using scanning electron microscopy and x-ray diffraction, respectively, and differential thermal analysis was used to observe structural transition. The dielectric measurements were conducted on films with metal–insulator–metal and metal–insulator–semiconductor configurations. Capacitance–voltage (C–V) measurements were carried out and the effect of the annealing temperature was studied. The dielectric constant and loss tangent at 1 MHz at room temperature were found to be 105 and 0.02, respectively, for 1.1 μm thick films. These measurements were also carried out at low temperatures down to 20 K. There are indications for a phase transition from a ...

Passive artificial molecule realizations of dielectric materials

Abstract: The design of dielectric materials with artificial molecules formed by electrically small dipole antennas loaded with passive electrical circuit elements is considered. Variations in the antenna loads lead to known and generalizations of known dielectric material models. These artificial dielectrics are characterized in terms of their equivalent susceptibilities and polarization vectors both in the frequency and time domains. With suitable choices in the antenna loads one can design the loss and dispersion properties of the resultant materials.

Electrical and mechanical properties of ferroelectric thin films laser ablated from a Pb0.97Nd0.02(Zr0.55Ti0.45)O3 target

Abstract: Ferroelectric Nd-modified lead-zirconate-titanate (PZT) thin films were fabricated as capacitor structures with platinum electrodes using pulsed laser ablation deposition. Single-crystal MgO (100) and thermally oxidized Si (100) were used as substrates. The ablation processes were carried out at room temperature in a pressure of 4×10−5 mbar in a vacuum chamber. A pulsed XeCl excimer laser with the wavelength of 308 nm was used for the ablation of both platinum and Pb0.97Nd0.02(Zr0.55Ti0.45)O3 targets. For the PZT films with thicknesses between 300 and 600 nm, a laser-beam fluence of 1.0 J/cm2 was used. Amorphous PZT films were postannealed at 675 and 650 °C in the cases of MgO and silicon substrates, respectively. The dielectric constant and the loss angle were measured at room temperature as a function of the film thickness. On the MgO substrate the dielectric constant of the films increased from 400 to 600 with the increasing film thickness, while in the films on the silicon substrate the dielectric con...

Dynamics of poly(oxyethylene) melts: Comparison of 13C nuclear magnetic resonance spin-lattice relaxation and dielectric relaxation as determined from simulations and experiments

Abstract: Molecular dynamics simulations of poly(oxyethylene) (POE) melts have been performed using a previously derived quantum-chemistry-based force field. Local chain dynamics for POE melts in the form of 13C nuclear magnetic resonance (NMR) spin-lattice relaxation times (T1) and NOE values and dielectric relaxation behavior have been determined from molecular dynamics simulations and compared with experimental measurements. 13C NMR T1 and NOE values for the methyl, α, and interior carbons from molecular dynamics simulations are in good agreement with experimental values over a wide range of temperatures. Similar agreement is seen for the dielectric relaxation strength and maximum dielectric loss frequency as a function of temperature. Non-Arrhenius behavior is seen for the correlation times of the P2CH orientational autocorrelation function (OACF), the molecular dipole moment OACF, and the torsional autocorrelation function (ACF). Very close correspondence between the torsional ACF and the molecular dipole mome...