Dielectric properties of Sr3CuNb2O9 perovskite ceramics

The dielectric constant ? and loss tangent tanδ of Sr₃CuNb₂O₉ perovskite ceramics prepared by solid-state reactions have been measured at temperatures from 300 to 900 K and frequencies from 25 to 1 × 10⁶ Hz. The results demonstrate that the samples slowly cooled from the temperature of the final, high-temperature firing (1200°C) have relatively low permittivity (? ? 10) and dielectric losses (tanδ ? 0.005 at 1 kHz) at room temperature, with no strong dielectric dispersion and no prominent maxima in the temperature dependences of their permittivity and dielectric loss. The ceramics quenched from 1300°C exhibit a pronounced Debye-type low-frequency relaxation and strong dielectric dispersion in conjunction with high permittivity ? ? 2000 at low frequencies and/or high temperatures. The observed dielectric anomalies in the Sr₃CuNb₂O₉ ceramics can be understood in terms of Maxwell-Wagner relaxation at dielectric inhomogeneities associated with the quenching-induced difference in oxygen-vacancy concentration between the grain bulk and surface layer.     

Microstructure and properties of ZrO2-, TiO2-doped Ca–Si–B based ceramics

The microstructure, sintering and dielectric properties of ZrO₂-, TiO₂-doped Ca–Si–B based ceramics prepared by solid-phase process were investigated, and the effects of ZrO₂, TiO₂ content on these performances were analyzed. The Ca–Si–B based ceramics without additive (ZrO₂ or TiO₂) showed a high sintering temperature (1,100?°C) and had the dielectric properties: dielectric constant (ε_r) of 8.38, dielectric loss (tanδ) of 1.51?×?10^?3 at 1?MHz, and volume density of 2.47?g/cm³. The addition of ZrO_2, TiO₂ was revealed to lower the sintering temperature of Ca–Si–B based ceramics to 1,000?°C and enhance the sintering and dielectric properties: ρ?=?2.61?g/cm³, ε_r?=?5.85, tanδ?=?1.59?×?10^?4 (1?MHz) with ZrO₂ addition, and ρ?=?2.65?g/cm³, ε_r?=?6.12, tanδ?=?6.4?×?10^?4 (1?MHz) with TiO₂ addition, which are superior to the pure Ca–Si–B. The results show that ZrO₂, TiO₂ as nucleating agents, are conducive to the precipitation of crystals, thus decrease the sintering temperature and improve the dielectric properties of Ca–Si–B based ceramics.     

Morphological, microstructural and electrical examinations on ZnO film on p-Si wafer

This study reports not only main electrical and dielectric characteristics of Ag/ZnO/p-Si heterostructure with the aid of the experimental admittance measurements at room temperature and theoretical approaches but also the microstructure and surface morphology of the heterostructure by means of X-ray diffraction, scanning electron microscopy and atomic force microscopy measurements. The results obtained show that the sample, obtaining Wurtzite structure with the (002) preferred orientation, has a fine crystalline microstructure consisting of micro-sized hexagonal rods growing uniformly in large scale on the film surface. When the diameters of the rods are found to vary from 0.5?μm to 1.5?μm, thickness values are observed to be about 2?μm. Further, series resistance (R_s) and some other electronic parameters of the heterostructure are obtained by the capacitance–voltage (C–V), conductance–voltage (G–V) and C^?2–V measurements. Moreover, voltage (V) and frequency (f) dependence of dielectric parameters such as dielectric constant (ε′), dielectric loss (ε″), dielectric loss tangent (tanδ), real and imaginary parts of electric modulus (Μ′ and Μ″) are determined and discussed. It is found that both electrical and dielectric parameters of the heterostructure prepared in this work depend strongly on the applied bias voltage and frequency.     

Free-Space Permittivity Measurements on Dielectric Materials at Millimeter Wavelengths

A system is described for measuring the complex permittivities of rigid dielectric sheet materials at millimeter wavelengths, using free-space transmission techniques and Brewster angle determinations. A first-order analysis is given for the effect of multiple reflections in thick low-loss dielectric sheets, under conditions of oblique incidence. Values of ?r and tan ? are given for selected sheet materials at 35 GHz and 25°C, that are estimated to be accurate to ±5 percent.     

Dielectric relaxation and alternating current conductivity of lanthanum, gadolinium, and erbium-polyvinyl alcohol doped films

Fourier transform infrared (FTIR) spectrum dielectric constant, ε', loss tangent, tan(δ), electric modulus, M*, and ac conductivity, σ(ac), of pure polyvinyl alcohol (PVA) as well as La-, Gd-, and Er-PVA doped samples have been carried out. The dielectric properties have been studied in the temperature and frequency ranges; 300-450?K and 1?kHz-4?MHz, respectively. FTIR measurements reveal that La(3+), Gd(3+), and Er(3+) ions form complex configuration within PVA structure. Two relaxation processes, namely, ρ and α were observed in pure PVA sample. The first process is due to the interfacial or Maxwell-Wagner-Sillers polarization. The second one is related to the micro-Brownian motion of the main chains. For doped PVA samples, α-relaxation process splits into α(a) and α(c). This splitting is due to the segmental motion in the amorphous (α(a)) and crystalline (α(c)) phases of PVA matrix. Electric modulus analysis was discussed to understand the mechanism of the electrical transport process. The behavior of ac conductivity for all PVA samples indicates that the conduction mechanism is correlated barrier hopping.     

Structural investigation and giant dielectric response of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramic by Nd/Zr co-doping for energy storage applications

High performance dielectric materials are highly required for practical application for energy storage technologies. In this work, high-k pristine and modified calcium copper titanate having nominal formula Ca_0.95Nd_0.05Cu₃Ti_4?xZr_xO₁₂ (x?=?0.01, 0.03 & 0.10) were synthesized and characterized for structural and dielectric properties. Single phase formation of the synthesized compositions was confirmed by X-ray diffraction patterns and further analysed using Rietveld refinement technique. Phase purity of the synthesized ceramics was further confirmed by Energy-dispersive X-ray Spectroscopy (EDX) analysis. SEM images demonstrated that grain size of the modified CCTO ceramics was controlled by Zr⁴⁺ ions due to solute drag effect. Impedance spectroscopy was employed to understand the grain, grain boundaries and electrode contribution to the dielectric response. Nyquist plots were fitted with a 2R-CPE model which confirms the non-ideality of the system. Substitution of specific concentration of Nd and Zr improved the dielectric properties of high dielectric permittivity (ε′ ~?16,902) and minimal tanδ (≤?0.10) over a wide frequency range. The giant ε′ of the investigated system was attributed to internal barrier layer capacitance (IBLC) effect and reduced tanδ accredited to enhanced grain boundaries resistance due to substitution of Zr⁴⁺ ions at Ti⁴⁺ site.     

Styrene butadiene rubber/aluminum powder composites—mechanical,morphological and electrical behaviors

Different weight fractions of aluminum (Al) powder viz., 10, 20, 30, 40, 50, 60 and 70 phr were incorporated into styrene butadiene rubber (SBR) matrix. The Al powder filled and vulcanized SBR composites have been characterized for mechanical properties such as tensile strength, tensile modulus and surface hardness. A drastical improvement in tensile strength and tensile modulus with increase in filler content of the composites was noticed. The electrical properties such as dielectric constant, tan delta and dielectric loss were measured for all the four compositions. The effect of volume fraction (0–70 phr) of conducting filler, frequency (100 kHz–30 MHz), temperature (25–75°C) and relative humidity on dielectric constant, dielectric loss and tan delta values of the composites were studied.     

Preparation and dielectric properties of microwave ceramics based on doped zinc titanates

We have studied the dielectric properties of microwave ceramic materials in the binary system Zn₂TiO₄-TiO₂ and examined the effect of doping with Sn²⁺, Sb³⁺, Bi³⁺, and Mg²⁺ on the dielectric properties and microstructure of the ceramics. The doped Zn-Ti-O ceramics are thermally stable and have ? = 18–33 and tan δ from 0.0001 to 0.007. Our results demonstrate that the doped Zn-Ti-O ceramics are potentially attractive for microwave applications, in particular for use as ceramic components (resonator substrates) of miniature antennas.     

Dependence of dielectric properties on the crystallinity of ceramic/polymer composites

Effects of ZnNb(2)O(6) content and crystallinity of polymers on the dielectric properties of ZnNb(2)O(6)/polytetrafluoroethylene (PTFE), polypropylene (PP), and polystyrene (PS) composites were investigated at microwave frequencies. With increasing ZnNb(2)O(6) content, the dielectric constant (K) of the composites increased, whereas the dielectric loss (tanδ) and temperature coefficient of resonant frequency (TCF) decreased. The tanδ of the composites with amorphous PS was lower than those of the composites with semi-crystalline PP and PTFE. For the composites with semi-crystalline PTFE and PP, the tanδ was strongly dependent on the degree of crystallinity of composites. Several types of theoretical models were applied to predict the effective dielectric properties of the composites. Typically, K of 5.73, tan δ of 1.45 x 10(-3), and TCF of 2.66 ppm/°C were obtained for the PP composites with 0.5 volume fraction V(f) of ZnNb(2)O(6).     

Measurement of Dielectric Loss Using a Cut-Off Circular Waveguide Cavity

Based on a resonant cavity technique used by Hanfling and Botte, this paper describes an extension of their method for the determination of the loss tangent of dielectric materials by means of Q-factor measurements. The present method appears to be suitable for materials of low loss (tan ? < 0.001). Measurements made on Teflon samples using a modified copper cavity designed for the X-band are reported.     

Dielectric losses in a polyethylene insulated superconducting cable between 4 and 22 K

The dielectric losses in a 5 m length of a superconducting cable, insulated with lapped polyethylene tape, have been determined between 4.3 K and 22 K with an inductively coupled ratio arms bridge which gave a resolution in tan δ of about 10^?6. The cable sample was built to a specification appropriate for power transmission at 132 kV and 6 kA; it was impregnated with helium at 0.4 MPa and cooled by a flow of cold helium circulated from a refrigerator.In the temperature range 4 to 5 K, applicable to a cable with niobium conductors, the dielectric loss increased slightly with electric field, and the observed tan δ (relative to tan δ of the reference capacitor) was 20 × 10^?6 at the maximum test stress of 7 MV m^?1. It is thought that a significant part of this loss can be attributed to the bedding layers and electrostatic screens between the dielectric and conductors.Tan δ showed a minimum at 18 K, which is encouraging for future developments of superconducting cables which might use hydrogen as a coolant and dielectric impregnant. The observed variation of tan δ with temperature is consistent with the existence of a low temperature relaxation mechanism with low activation energy.     

Structural and dielectric properties of La substituted polycrystalline Ca(Ti0.5Fe0.5)O3

Polycrystalline Ca_1?x La _x (Ti_0.5Fe_0.5)O₃ were prepared by the standard solid state reaction technique. Pellet shaped samples prepared from each composition were sintered at 1573 K for 5 h. The X-ray diffraction analysis indicated the formation of a single-phase orthorhombic structure. The lattice parameters as well as densities increased but the average grain sizes decreased with the increase of La content. The dielectric measurements were carried out at room temperature as a function of frequency and composition. The experimental results revealed that dielectric constant (?′) decreased but dielectric loss (tan δ) and ac electrical conductivity (σ _ac ) increased as frequency increased. The composition dependence of ?′ and tan δ indicated that they decreased with the increase of La content. The σ _ac was derived from the dielectric measurements and it is concluded that the conduction in the present samples is due to mixed polarons hopping.     

Enhanced tunable characteristics of the Na0.5Bi0.5TiO3-NaTaO3 relaxor-type system

We have investigated the voltage-tunable characteristics of the Na(0.5)Bi(0.5)TiO(3)-NaTaO(3) homogeneity region, for which samples were prepared using a conventional solid-state reaction. The highest value of the relative tunability (n(r)) was obtained for the sample with 5 mol% of NaTaO(3), i.e., 47% at 1 MHz and a 70 kV/cm dc bias field. This sample also showed the highest value of the dielectric losses (tan delta) and temperature coefficient of the dielectric constant (tau(epsilon)), i.e., 0.05 and 4478 ppm/K, respectively. As the concentration of NaTaO(3) increased up to 90 mol% n(r), tan delta, and tau(epsilon) gradually decreased toward 22%, 0.0002 and -899 ppm/K, respectively. The dielectric constant of the samples varied in a similar manner between 662 and 130. At microwave frequencies, the dielectric losses of the samples substantially increased due to their relaxor-type nature. The lowest value was obtained for the samples with 90 mol% of NaTaO(3), i.e., 0.002. The tunable characteristics of the samples are related to the ferroelectric and dielectric properties, and it appears that the dielectric tunability of the Na(0.5)Bi(0.5)TiO(3)-NaTaO(3) system originates from its relaxor-type behavior.     

Structural and Magnetic Properties of Nanocrystalline Mg–Co Ferrites

Mg?CCo nano crystalline ferrites having the general formula Mg_1?x Co _x Fe₂O₄ (x=0, 0.05, 0.1, 0.15, 0.2, 0.25) were prepared by the sol?Cgel method. X-ray powder diffractometry (XRD) and Fourier transform infrared spectroscopy (FTIR) were carried out to investigate the structural properties of the samples. X-ray powder diffraction patterns indicated the formation of a spinel structure of the prepared compounds. Fourier Transform Infrared (FTIR) spectroscopy of the samples confirmed the XRD results. The crystallite size, lattice parameters and porosity of samples were calculated by XRD data analysis as a function of cobalt concentration. The dielectric constant (?? _r ), dielectric loss tangent (tan???) and ac electrical conductivity (?? _ac) of nanocrystalline Mg?CCo ferrites were investigated as a function of frequency and Co concentration. The frequency dependence of ?? _r , tan??? and ?? _ac is in accordance with the Maxwell?CWagner model. The effect of Co doping on dielectric and electric properties was explained on the basis of cations distribution in the crystal structure. The saturation magnetization M_S, remanent magnetization M_r and coercivity H_C of all samples were explained as a function of cobalt concentration on the basis of Néel??s two-lattice model.     

Frequency and gate voltage effects on the dielectric properties and electrical conductivity of Al∕SiO(2)∕p-Si metal-insulator-semiconductor Schottky diodes

The dielectric properties and electrical conductivity of Al∕SiO(2)∕p-Si (MIS) Schottky diodes (SDs) in the frequency range of 10 kHz to 10 MHz and the gate voltage range of -2 to 6 V have been investigated in detail using experimental C-V and G∕w-V measurements. Experimental results indicated that the voltage dependence of the real part of the dielectric constant (?') and loss tangent (tan δ) characteristics have a peak at each frequency. The values of ?' increase with decreasing frequency and tend to be frequency independent in the negative voltage region. However, the values of the dielectric loss (?″) increase with decreasing frequency at each voltage. In contrast, ?' and ?″ are almost found to decrease, and the ac electrical conductivity (σ(ac)) and the real part of the electric modulus (M') increase, with increasing frequency. In addition, the imaginary part of the electric modulus (M″) showed a peak that shifts to a higher frequency with increasing applied voltage. It can be concluded that interfacial polarization can more easily occur at low frequencies, and consequently the majority of interface states at the Si-SiO(2) interface contribute to the deviation of the dielectric properties of Al∕SiO(2)∕p-Si (MIS) SDs.     

Physicotechnical properties of wurtzitic AlN-based ceramics and composites with ceramic matrix

Wurtzitic aluminum nitride-based ceramic materials having the following high characteristics: structural ceramics with a Vickers hardness of HV = 16.8 GPa, fracture toughness K _Ic = 4.7?4.9 MPa·m^1/2, bending strength σ = 370–430 MPa; functional ceramics for heatsinks with a thermal conductivity of 140 W/(m·K) and for microwaves absorption with dielectric constant ? = 23, dielectric loss tangent tan δ = 0.025, coefficient of the electromagnetic energy attenuation L to 36.3 dB/cm (at a frequency of 3 GHz) have been obtained using pressureless sintering and hot pressing.     

Loss mechanisms in piezoelectrics: how to measure different losses separately

Losses in piezoelectrics are considered in general to have three different mechanisms: dielectric, mechanical, and piezoelectric losses. This paper deals with the phenomenology of losses first, then how to measure these losses separately in experiments. We found that heat generation at off-resonance is caused mainly by dielectric loss tan delta' (i.e., P-E hysteresis loss), not by mechanical loss, and that a significant decrease in mechanical Qm with an increase of vibration level was observed in resonant piezoelectric ceramic devices, which is due to an increase in the extensive dielectric loss, not in the extensive mechanical loss. We propose the usage of the antiresonance mode rather than the conventional resonance mode, particularly for high power applications because the mechanical quality factor QB at an antiresonance frequency is larger than QA at a resonance frequency.     

Structural and Electrical Properties of Ni-Co Nanoferrites Prepared by Co-precipitation Route

A series of Ni_1?x Co _x Fe₂O₄ (x=0.1, 0.2, 0.3, 0.4, 0.5) spinel ferrites have been synthesized successfully using the chemical co-precipitation route. The materials were characterized by X-rays powder diffractometry (XRD) and the electrical properties. The obtained crystallite size variation was within 15 to 33 nm using the Scherrer formula. The dc electrical resistivity was measured as a function of temperature. It is noticed that ?? _dc increases with a rise in temperature. The dielectric measurements were carried out at room temperature as a function of frequency and composition (x). The dielectric constant (????) and dielectric loss tangent (tan???) showed a decreasing trend with increasing field frequency. The ac electrical conductivity is calculated from the dielectric measurements; it increases with the rise in frequency.     

Sur de nouvelles phases oxyfluorees derivees du niobate de plomb: Etudes cristallographique et dielectrique

The solid solution Pb_2?xNa_xNb₂O_7?xF_x obtained by substitution of oxygen by fluorine in the ferroelectric oxide Pb₂Nb₂O₇ has the cubic pyrochlore structure for 0, 75 ? x ? 1, 85 and monoclinic deformations of this structure for other values of x. The temperature and frequency variations of the dielectric constant and of the dielectric losses have been studied. The high values of ε′_r have been attributed to a mechanism of ionic polarizability, and those of tan δ to the mobility of the F^? ions.     

基于界面结构调控硅粒子/聚偏氟乙烯复合材料介电性能

为降低硅粒子/聚偏氟乙烯(Si/PVDF)复合材料体系的介电损耗(tanδ)及提高其击穿强度(E_b),采用高温氧化及聚苯乙烯(PS)包覆法,制备出两种分别具有SiO₂单壳及SiO₂@PS双壳的Si@SiO₂和Si@SiO₂@PS核壳结构粒子。采用FTIR、XRD和TEM分析测试了核壳粒子的壳层结构。分析测试证明,Si粒子表面存在SiO₂和PS壳层。结果表明,相比未改性Si/PVDF复合材料,SiO₂外壳显著降低和抑制了Si@SiO₂/PVDF复合材料的tanδ和漏导电流;PS层改进了Si/PVDF复合材料的界面相容性,促进其在基体中均匀分散。双壳结构Si@SiO₂@PS/PVDF复合材料呈现出最低tanδ和最高E_b。Si@SiO₂/PVDF和Si@SiO₂@PS/PVDF复合材料介电性能的改善归因于Si表面SiO₂及SiO₂@PS绝缘界面层有效阻止了半导体Si粒子间的直接接触,极大抑制了损耗。此外,Si/PVDF复合材料相界面缺陷减少及界面相容性改善均有效降低了局部电场畸变,提高了体系的E_b。Si@SiO₂@PS/PVDF复合材料在1 kHz下介电常数高达48,tanδ低至0.07,E_b约为6 kV/mm,在微电子器件及电力设备领域具有潜在的应用价值。