Showing papers on "Dielectric loss published in 2009"

High dielectric loss and its monotonic dependence of conducting-dominated multiwalled carbon nanotubes/silica nanocomposite on temperature ranging from 373 to 873 K in X-band

Abstract: The dielectric properties of multiwalled carbon nanotubes/silica (MWNTs/SiO2) nanocomposite with 10 wt % MWNTs are investigated in the temperature range of 373–873 K at frequencies between 8.2 and 12.4 GHz (X-band). MWNTs/SiO2 exhibits a high dielectric loss and a positive temperature coefficient (PTC) of dielectric effect that complex permittivity increases monotonically with increasing temperature. The PTC effect on the dielectric constant is ascribed to the decreased relaxation time of interface charge polarization, and the PTC effect on the dielectric loss is mainly attributed to the increasing electrical conductivity. The loss tangent strongly supports the dominating contribution of conductance to the dielectric loss.

A Thin Electromagnetic Absorber for Wide Incidence Angles and Both Polarizations

Abstract: A design for planar electromagnetic absorbers is presented. The performance of this absorber is maintained over a wide incidence angles and for both TE and TM polarization. The absorber is composed of a high-impedance surface comprising an array of patches over a grounded lossy dielectric slab perforated with metallic vias (wire medium). The main contribution of the paper is to demonstrate and practically use the presence of an additional resonance when the plasma frequency of the wire medium in the dielectric substrate is close to the original resonance of the high-impedance surface. The presence of the vias between FSS and the ground plane is discussed both for the case of a high-permittivity absorber and for a low permittivity one, through the derivation of simple and efficient analytical expressions, specifically derived for the problem at hand. We show that the presence of the vias influences the oblique incidence TM absorption, and when properly designed, the insertion of the vias into the absorber structure results in a bandwidth enlargement and higher absorption performance.

Colossal dielectric constant up to gigahertz at room temperature

Abstract: The applicability of recently discovered materials with extremely high (“colossal”) dielectric constants, required for future electronics, suffers from the fact that their dielectric constant e′ only is huge in a limited frequency range below about 1 MHz. In the present report, we show that the dielectric properties of a charge-ordered nickelate, La15/8Sr1/8NiO4, surpass those of other materials. Especially, e′ retains its colossal magnitude of >10 000 well into the gigahertz range.

Dielectric loss caused by oxygen vacancies in titania ceramics

Abstract: Undoped TiO2 exhibited deterioration in microwave (MW) dielectric loss as it reached its maximum density due to the reduction of Ti4+ to Ti3+ causing oxygen vacancies at high sintering temperatures. By adding small amounts of acceptor dopants with ionic radii between 0.5 and 0.95 A, reduction during sintering was suppressed. The upper limit of ionic radius was discrete with almost no observed effect using dopants >0.96 A ionic radius. In addition, the microwave dielectric loss of undoped TiO2 could be improved by annealing at 1500 °C for 10 h in air, presumably as a result of re-oxidation. High loss samples exhibited a dark ‘core’ to the naked eye which was absent in low loss ceramics. Transmission electron microscopy revealed that grains in the dark core contained planar defects attributed to the condensation of O vacancies onto specific crystallographic planes, in a manner similar to that observed in Magnelli phases.

Synthesis, magnetic and electromagnetic wave absorption properties of porous Fe3O4/Fe/SiO2 core/shell nanorods

Abstract: Porous Fe3O4/Fe/SiO2 core/shell nanorods were fabricated, in which the diameter of the pores was 5–30 nm. The magnetic and electromagnetic properties were investigated. The temperature dependent magnetic measurements showed that these nanorods were ferromagnetic with a Verwey temperature of 129 K. The electromagnetic data indicated that effective complementarities between the dielectric loss and the magnetic loss were realized, suggesting that they have excellent electromagnetic wave absorption properties. Thus the porous core/shell nanorods could be used as a kind of candidate absorber.

A polaronic stacking fault defect model for CaCu3Ti4O12 material: an approach for the origin of the huge dielectric constant and semiconducting coexistent features

Abstract: This paper proposes a polaronic stacking fault defect model as the origin of the huge dielectric properties in CaCu3Ti4O12 (CCTO) materials. The model reconciles the opposing views of researchers on both sides of the intrinsic versus extrinsic debate about the origin of the unusually high values of the dielectric constant measured for CCTO in its various forms. Therefore, by considering stacking fault as the origin of the high dielectric constant phenomena, it was shown that the internal barrier layer capacitance mechanism is enhanced by another similar, but different in nature, mechanism that operates in the nanoscale range due to polaron defects associated with stacking fault, a mechanism that was referred to as nanoscale barrier layer capacitance (NBLC). The NBLC approach explains the origin of the CCTO's huge dielectric constant coexisting with semiconducting features.

Electrical and magnetic properties of Cr3+ substituted nanocrystalline nickel ferrite

Abstract: The magnetic and electrical properties of Cr3+ substituted nickel ferrite synthesized by wet chemical route have been studied Particle size measured from x-ray diffraction and from transmission electron microscopy images confirms the nanosize dimension of prepared particles Magnetic parameters such as coercivity and saturation magnetization are measured from vibrating sample magnetometer Magnetization, ac susceptibility, electrical resistivity, and Mossbauer measurements were carried out Electrical properties such as ac resistivity as a function of frequency and dc resistivity as a function of temperature were studied for various Cr3+ substitutions in nickel ferrite The dielectric properties such as dielectric constant (e′) and dielectric loss (e″) were also studied The dielectric constant and dielectric loss obtained for the ferrites prepared through wet chemical route posses a value lower than that of the ceramically prepared samples of the same composition The resistivity obtained is higher than

Do Grain Boundaries Affect Microwave Dielectric Loss in Oxides

Abstract: Reducing loss in microwave dielectrics is critical to improving performance in wireless communications systems. Grain boundaries in polycrystalline microwave dielectric ceramics have long been suspected of increasing dielectric loss. They are often cited as the main contributor to the observed difference in dielectric losses between single crystals and polycrystalline ceramics. The exact configuration of grain boundaries is problematic to quantify in practice and their influence on the dielectric loss difficult to distinguish from other defects such as porosity, oxygen vacancies, impurities, and dislocations. Here we measure the sensitivity of a single grain boundary in a magnesium oxide bi-crystal to the polarization of an applied microwave field as a function of temperature.

High-Temperature Dielectrics in the BiScO3–BaTiO3–(K1/2Bi1/2)TiO3 Ternary System

Abstract: Perovskite solid solution in the (1−x)[0.4BiScO3–0.6BaTiO3]+x (K1/2Bi1/2)TiO3 [BSBT–KBTx] system was synthesized using conventional sintering and hot-isostatic pressing. Dielectric properties of BSBT ceramics with different dopant levels of KBT were characterized as a function of temperature and frequency for potential use of high-temperature capacitors. The BSBT ceramics with KBT exhibited high dielectric permittivities (ɛr) (>1700 at RT) and low dielectric loss over the temperature range from 100° to 300°C, with flat temperature coefficients of permittivity (TCɛs). In addition, BSBT ceramics with increasing KBT were observed to possess dielectric relaxation characteristics at temperatures (>RT) as observed in lead-based relaxors. Furthermore, high energy densities, being on the order of 4.0 J/cm3 at 220 kV/cm was observed for the BSBT–KBT20 ceramics from the electric-field polarization behavior.

Dielectric response of high permittivity polymer ceramic composite with low loss tangent

Abstract: The present communication investigates the dielectric response of the Sr9Ce2Ti12O36 ceramics loaded high density polyethylene and epoxy resin. Sr9Ce2Ti12O36 ceramic filled polyethylene and epoxy composites were prepared using hot blending and mechanical mixing, respectively. 40 vol % ceramic loaded polyethylene has relative permittivity of 12.1 and loss tangent of 0.004 at 8 GHz, whereas the corresponding composite using epoxy as matrix has permittivity and loss tangent of 14.1 and 0.022, respectively. The effective medium theory fits relatively well for the observed permittivity of these composites.

Phase structure, dielectric properties, and relaxor behavior of (K0.5Na0.5)NbO3-(Ba0.5Sr0.5)TiO3 lead-free solid solution for high temperature applications

Abstract: The (1−x)(K0.5Na0.5)NbO3–x(Ba0.5Sr0.5)TiO3 (KNN-BST) solid solution has been synthesized by conventional solid-state sintering in order to search for the new lead-free relaxor ferroelectrics for high temperature applications. The phase structure, dielectric properties, and relaxor behavior of the (1−x)KNN-xBST solid solution are systematically investigated. The phase structure of the (1−x)KNN-xBST solid solution gradually changes from pure perovskite phase with an orthorhombic symmetry to the tetragonal symmetry, then to the pseudocubic phase, and to the cubic phase with increasing addition of BST. The 0.90KNN-0.10BST solid solution shows a broad dielectric peak with permittivity maximum near 2500 and low dielectric loss (<4%) in the temperature range of 100–250 °C. The result indicates that this material may have great potential for a variety of high temperature applications. The diffuse phase transition and the temperature of the maximum dielectric permittivity shifting toward higher temperature with increasing frequency, which are two typical characteristics for relaxor ferroelectrics, are observed in the (1−x)KNN-xBST solid solution. The dielectric relaxor behavior obeys a modified Curie–Weiss law and a Vogel–Fulcher relationship. The relaxor nature is attributed to the appearance of polar nanoregions owing to the formation of randon fields including local electric fields and elastic fields. These results confirm that the KNN-based relaxor ferroelectrics can be regarded as an alternative direction for the development of high temperature lead-free relaxor ferroelectrics.

Influence of filler loading on dielectric properties of epoxy-ZnO nanocomposites

Abstract: Experimental investigations into the dielectric properties of epoxy-ZnO nanocomposites at different filler loadings reveal few unique behaviors (at certain filler loadings) and also advantageous characteristics in contrast to the properties obtained for the corresponding microcomposites. Results demonstrate that in nanocomposites, it is possible to achieve lower values of permittivity and tandelta with respect to unfilled epoxy over a wide frequency range. Analysis of the results attributes this interesting observation to the interaction dynamics between the epoxy chains and the ZnO nanoparticles at the interfacial area. The dc volume resistivities and ac dielectric strengths of nanocomposites were also experimentally determined in the present study and the obtained characteristics are found to be different as compared to the results obtained for microcomposites. The volume fraction and nature of the interfaces in the bulk of the composites seem to influence this difference in the examined dielectric properties of the nanocomposites.

Characterization of Hard Piezoelectric Lead-Free Ceramics

Abstract: K4CuNb8O23 doped K0.45Na0.55NbO3 (KNN-KCN) ferroelectric ceramics were found to exhibit asymmetrical polarization hysteresis loops, related to the development of an internal bias field. The internal bias field is believed to be the result of defect dipoles of acceptor ions and oxygen vacancies, which lead to piezoelectric "hardening" effect, by stabilizing and pinning of the domain wall motion. The dielectric loss for the hard lead-free piezoelectric ceramic was found to be 0.6%, with mechanical quality factors Q on the order of >1500. Furthermore, the piezoelectric properties were found to decrease and the coercive field increased, when compared with the undoped material, exhibiting a typical characteristic of "hard" behavior. The temperature usage range was limited by the polymorphic phase transition temperature, being 188degC. The full set of material constants was determined for the KNN-KCN materials. Compared with conventional hard PZT ceramics, the lead-free possessed lower dielectric and piezoelectric properties; however, comparable values of mechanical Q, dielectric loss, and coercive fields were obtained, making acceptor modified KNN based lead-free piezoelectric material promising for high-power applications, where leadfree materials are desirable.

Dielectric properties and electric breakdown strength of a subpercolative composite of carbon black in thermoplastic copolymer

Abstract: We investigate the dielectric properties and electric breakdown strength of subpercolative composites of conductive carbon black particles in a rubber insulating matrix A significant increase in the permittivity in the vicinity of the insulator to conductor transition was observed, with relatively low increases in dielectric loss; however, a rapid decrease in electric breakdown strength was inevitable A steplike feature was ascribed to agglomeration effects The low ultimate values of the electric field strength of such composites appear to prohibit practical use

The effect of Mn substitution on the magnetic and dielectric properties of cobalt ferrite synthesized by an autocombustion route

Abstract: Co1.2−xMnxFe1.8O4 (0≤x≤0.4) compositions were synthesized by the autocombustion route by keeping the oxidizer to fuel ratio (Φe) at 1. Thermogravimetric analysis (TGA) shows the stable phase formation takes place at a temperature above 600 °C. Structural characterization of all the samples was carried out by the x-ray diffraction technique. Room temperature magnetization measurements showed that, for the substitution of Co by Mn, there is an initial increase in the saturation magnetization (Ms) for lower concentrations (i.e. x = 0.1 and 0.2); and then the magnetization decreases for higher concentrations (i.e. x = 0.3 and 0.4). Also, it is observed that the coercivity (Hc) goes on decreasing with the substitution of Mn content, except for x = 0.3 which shows a slight increase in coercivity as compared to x = 0.4. Room temperature dielectric properties, namely relative dielectric permittivity (e'), dielectric loss (tanδ) and ac conductivity (σac), for all the samples were studied as a function of applied frequency in the range from 20 Hz to 1 MHz. These studies indicate that the relative dielectric permittivity goes on increasing with the increase of Mn content in Co ferrite and also all the samples show the usual dielectric dispersion which is due to the Maxwell–Wagner-type interfacial polarization. The ac conductivity measurement suggests that the conduction is due to small polaron hopping.

Microwave-absorbing properties of shape-optimized carbonyl iron particles with maximum microwave permeability

Abstract: The microwave-absorbing properties for different shapes of carbonyl-iron particles prepared by the high-energy planetary ball milling with 40 vol% in epoxy resin matrix have been investigated. Higher value of magnetic permeability and permittivity can be obtained in the composites for thin flake carbonyl iron than spherical powders. The results are attributed to reduction of eddy current loss, orientation of magnetic moment and space–charge polarization with the shape change from spherical powders to thin flake particles. As the iron flakes with 0.4 μm in thickness as the absorbent fillers, the minimum RL value of −6.20 dB was observed at 4.57 GHz with thickness of 1 mm. The minimum reflection loss (RL) shifts to lower frequency and the value declines with change from spherical powders to thin flakes. It results from the considerable dielectric loss in the absorbing materials.

Comparisons of microwave dielectric property measurements by transmission/reflection techniques and resonance techniques

Abstract: This review provides a general comparison of the two most commonly used techniques for measurement of complex permittivity at microwave frequencies: transmission/reflection and resonance. The transmission/reflectance techniques are analyzed using distributed and lumped impedance models. The resonance techniques are analyzed using both dielectric and cavity resonance models. The analysis, combined with experimental results, enables us to illustrate the advantages and disadvantages of the various techniques and provide guidance on which techniques to use under particular circumstances. In general, transmission/reflection techniques can be used over a broad band of frequencies, and are suitable for loss measurements on high loss materials. Resonance techniques do not have swept frequency capability, but have higher accuracy for measurement of the real part of permittivity and can measure the loss tangent of low loss materials with high resolution.

Mechanical and dielectric properties of porous Si3N4-SiO2 composite ceramics

Abstract: In order to prepare a structural/functional material with not only higher mechanical properties but also lower dielectric constant and dielectric loss, a novel process combining oxidation-bonding with sol–gel infiltration-sintering was developed to fabricate a porous Si 3 N 4 –SiO 2 composite ceramic. By choosing 1250 °C as the oxidation-bonding temperature, the crystallization of oxidation-derived silica was prevented. Sol–gel infiltration and sintering process resulted in an increase of density and the formation of well-distributed micro-pores with both uniform pore size and smooth pore wall, which made the porous Si 3 N 4 –SiO 2 composite ceramic show both good mechanical and dielectric properties. The ceramic with a porosity of 23.9% attained a flexural strength of 120 MPa, a Vickers hardness of 4.1 GPa, a fracture toughness of 1.4 MPa m 1/2 , and a dielectric constant of 3.80 with a dielectric loss of 3.11 × 10 −3 at a resonant frequency of 14 GHz.

Dielectric properties and energy storage capability of antiferroelectric Pb0.92La0.08Zr0.95Ti0.05O3 film-on-foil capacitors

Abstract: Antiferroelectric (AFE) Pb0.92La0.08Zr0.95Ti0.05O3 (PLZT) films were grown on nickel foils with lanthanum nickel oxide buffer by chemical solution deposition. We observed field-induced AFE-to-ferroelectric (FE) phase transition. The electric field for the AFE-to-FE phase transition (EAF ≈ 270 kV/cm) and that for the reverse phase transition (EFA ≈ 230 kV/cm) were measured at room temperature on samples with PLZT films of ≈1-µm thickness. Relative permittivity of ≈560 and dielectric loss of 5000 h when the capacitors are operated at room temperature with an applied field of ≈300 kV/cm.