Influence of synthesis process on the dielectric properties of B-doped SiC powders

Fine powders (～0.7 μm) of SiC doped with 3 mol% and 10 mol% B were successfully produced by mechanical activation assisted self-propagating high-temperature synthesis (MASHS). The experimental results showed that the presence of B caused a reduction in the combustion temperature, shrinkage of the crystal lattice, an increase in the tendency of the grains to be crystallized, and a decrease in the dielectric properties in the frequency range between 8.2 and 12.4 GHz, specifically the real (?′) and the imaginary parts (?″) of complex permittivity and the loss tangent (tan δ). Analysis of the results suggests that B ions should be preferably accommodated in the Si sites of the SiC lattice and cause a reduction in the number of defects (V_Si, V_C, and C_Si), which results in a decrease in the dielectric properties. Comparison of the experimental results of this study with results reported in similar earlier studies reveals that the influence of B on the dielectric properties of the B-SiC powders depends strongly on the synthesis process.     

Ferroelectric hysteresis behavior and dielectric properties of 1–3 lead zirconate titanate–cement composites

Lead zirconate titanate (PZT) ceramic was mixed with Portland cement (PC) to form 1–3 connectivity PZT–PC composite using a dice-and-fill technique. Ferroelectric hysteresis behavior and dielectric properties of these composites were investigated using PZT volume content of 60%, 70% and 80%. The results showed that the dielectric constant of the composite materials increased with PZT content and the dielectric constant (?_r) value is 781 for 80% PZT composite at 1 kHz. The dielectric loss tangent (tan δ) was found to decrease with increasing PZT content and the tan δ value of 80% PZT composite is 0.06. Parallel and series models were also compared to the dielectric measurement results. For the hysteresis measurements, the ferroelectric hysteresis loops can be seen for all composites. The “instantaneous” remnant polarization (P_ir) was found to increase with increasing PZT content from 3.20 to 4.28 μC/cm² at 90 Hz when PZT volume content used was 60% and 80% respectively.     

Effects of MgO addition on the microwave dielectric properties of high thermal-conductive silicon nitride ceramics sintered with ytterbia as sintering additives

1 mol% of MgO was added together with 7 mol% of Yb₂O₃ as sintering additives to silicon nitride powder to fabricate advanced silicon nitride ceramics with both high thermal conductivity and low dielectric loss at 2 GHz. The mixed powder was CIPed at a pressure of 120 MPa and was gas-pressure sintered at 1900 °C to >98% of theoretical density. The sintered Si₃N₄ sample exhibited a high thermal conductivity of ～100 W m^?1 K^?1 and a loss tangent (tan δ) of ～4 × 10^?4, concurrently. The tan δ was further reduced by half after the heat treatment at 1300 °C for 24 h. The improvement in tan δ due to the annealing was explained from the point of crystallization of the intergranular glassy phase.     

Microstructural design of BaTiO3-based ceramics for temperature-stable multilayer ceramic capacitors

In order to satisfy EIA X8R specification, a new type of BaTiO₃-based ceramic with hierarchical structure in a formula scheme “a ferroelectric ABO₃ + another ferroelectric ABO₃”, was designed. There were (Ba, Bi)TiO₃ and Ba(Ti, Zr)O₃ phases with different Curie temperatures coexisting in the grains from inside to outside, prepared by wet chemical method under 100 °C. The hierarchical structure of the ceramic grains was proved by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The dielectric constant of (Ba, Bi)TiO₃–Ba(Ti, Zr)O₃ ceramic was ～6000, the ΔC/C_20 °C was ?12.0%, 14.1%, and ?8.3% at ?55 °C, 130 °C, and 160 °C, respectively, and the dielectric loss is less than 0.1, which is obviously superior to (Ba, Bi)TiO₃ and Ba(Ti, Zr)O₃. The results of this work showed that the formula scheme “a ferroelectric ABO₃ + another ferroelectric ABO₃” for solid solutions is a promising approach to prepare high performance temperature-stable capacitor materials.     

Permittivity measurements at millimeter wave frequencies using dielectric rod resonator excited by NRD-guide

A method of measuring the relative complex permittivity (ɛ_r = ɛ′ – jɛ″, tan δ = ɛ″/ɛ′) for low-loss dielectric materials at millimeter wave frequencies has been developed, using a dielectric rod resonator excited by the nonradiative dielectric waveguide (NRD-guide). Relative permittivity (ɛ′) and loss factor (tan δ) of the rod specimen are determined by the resonant frequency (f₀) and unloaded Q-factor (Q_u) of a TE_0m1 mode resonator. The effective conductivity (σ) of conducting plates for short-circuiting the rod resonator is determined using TE₀₂₁ and TE_02δ mode sapphire resonators. Temperature dependence of ɛ′ and tan δ of sapphire and cordierite ceramics were evaluated at 60 GHz. This method has been adopted as the Japanese Industrial Standard (JIS R 1660-3) and is being prepared for the IEC international standard. Several standardized specifications are presented.     

Low loss microwave ferroelectric ceramics for high power tunable devices

In this paper we report the influence of the composition and concentration of Mg-containing additions such as Mg₂TiO₄, MgO and a mixture of Mg₂TiO₄–MgO on ceramics based on a mixture of BaTiO₃/SrTiO₃. Phase relations, crystal structure, microstructures, microwave dielectric properties (?, tan δ) and DC tunability have been studied over a wide range of frequencies. The temperature dependence of the dielectric properties has been measured as well. Among the compositions synthesized were low loss bulk ferroelectrics with dielectric constants in the range 150–800 and relatively high DC tunabilities (up to 1.49 under a DC electric field of 40 kV/cm). These materials can be used for high power tunable microwave devices.     

Microwave sintered sol–gel derived BaTiO3 and Ba0.95La0.05TiO3 ceramic samples for capacitor applications

Sol–gel derived BaTiO₃ and Ba_0.95La_0.05TiO₃ powders were calcined at 700 °C for 40 min and sintered at 1100 °C for 1 h in a microwave furnace to obtain single-phase perovskite ceramic samples. About 98% of the theoretical density was obtained in the sintered BT ceramic samples. Room temperature (RT) dielectric constant (ε_r) and dielectric loss (tan δ) at 1 kHz frequency of the BLT ceramic samples were found to be ~2220 and 0.005, respectively. High value of ε_r, low value of tan δ and negligible temperature coefficient of capacitance from RT to 60 °C suggested the suitability of BLT ceramic samples for multi-layer capacitor applications.     

Microwave dielectric properties of BaTi4O9 thin film

BaTi₄O₉ thin films have been prepared by RF magnetron sputtering on the Pt/Ti/SiO₂/Si substrates and the dielectric properties of the BaTi₄O₉ film have been investigated at microwave frequency range. The homogeneous BaTi₄O₉ thin film was obtained when the film was grown at 550 °C and rapid thermal annealed (RTA) at 900 °C for 3 min. The circular-patch capacitor (CPC) was used to measure the microwave dielectric properties of the film. The dielectric constant (ɛ_r) and the dielectric loss (tan δ) were successfully measured up to 6 GHz. The ɛ_r of the BaTi₄O₉ thin film slightly increased with the increase of the film thickness. However, the tan δ decreased with increasing the thickness of the film. The ɛ_r of BaTi₄O₉ thin film was similar to that of the BaTi₄O₉ ceramics, which is about 36–39. The tan δ of the film with 460 nm thickness was very low approximately, 0.0001 at 1–3 GHz. Since BaTi₄O₉ film has a high ɛ_r and a low tan δ, the BaTi₄O₉ film can be used as the microwave devices.     

Design of microwave dielectric resonator antenna using MZTO–CSTO composite

This paper presents the design of a cylindrical dielectric resonator antenna (DRA) using a nearly temperature stable dielectric composite material. By the combination of compounds with positive and negative temperature coefficient, a dielectric composite series (1 ? x)(Mg_0.95Zn_0.05)TiO₃ ? x(Ca_0.8Sr_0.2)TiO₃ has been developed. The structural and morphological properties of the grown samples have been characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray (EDX) spectroscopy analysis. Microwave dielectric properties of the composite samples have been investigated using the TE_01δ dielectric resonator method. Relative permittivity (?_r) of 21.9, dielectric loss tan δ of 0.0002 and temperature coefficient of resonant frequency (τ_f) of ?0.15 ppm/°C have been obtained for the x = 0.08 sample, which was used for designing the DRA. The DRA resonates at 4.6 GHz frequency and offers the bandwidth of 315 MHz. The characteristics of proposed DRA have been simulated using the Ansoft high frequency structure simulator (HFSS). Comparison between the simulated and measured results shows a reasonably good agreement.     

Fabrication and properties of composites from BST and polypropylene-graft-poly(styrene-stat-divinylbenzene)

In this work, ceramic–polymer composites were fabricated from barium strontium titanate powder (BST) and polypropylene-graft-poly(styrene-stat-divinylbenzene) (ER) using a twin-screw extruder. The compounding process was characterized by rheological measurements. The effects of volume loading of BST on dielectric and mechanical properties were investigated. The dielectric properties were measured as a function of frequency and BST loading. For example, the relative permittivity and loss tangent (tan δ) of the BST–ER composites at 1 GHz were gradually increased from 2.4 and 0.0001 to 28.5 and 0.0085, respectively, as the loading was increased from 0 to 50.5 vol.%. Stearic acid (StA) was used as a surface-modifier of the BST. With an approximate surface coverage of 83%, an improvement in processability and a slight increase of the permittivity was observed, while tan δ remained low. The excellent dielectric characteristics of these composites, with high permittivity and low tan δ, make them attractive novel electronic materials for high frequency applications.     

Studies on NO2 gas sensing properties of sprayed Co1−xMnxFe2O4 (0≤x≤0.5) spinel ferrite thin films

The Co_1−xMn_xFe₂O₄ (0≤x≤0.5) spinel ferrite thin films were deposited on quartz substrates by chemical spray pyrolysis technique. The effect of Mn substitution on to the structural, electrical, dielectric and NO₂ gas sensing properties of cobalt ferrite thin films was studied. The X-ray diffraction analysis reveals that deposited films exhibit spinel cubic crystal structure. The lattice constant increases with the increase in Mn²⁺ content. The decrease in resistivity with increase in temperature suggests that the films have a semiconducting nature. The room temperature dielectric properties such as dielectric constant (ε′), loss tangent (tanδ), dielectric loss (ε′′) and AC conductivity have been studied in the frequency range 20 Hz–1 MHz. The film shows the highest sensor response at moderately low (150 °C) operating temperature. The effect of operating temperature, gas concentration, film selectivity and substitution of Mn on to gas response is carefully studied. The manganese substituted cobalt ferrite films are extremely selective towards NO₂ with a 20 times gas response compared with other gases. The gas response achieved nearly 92% of its initial value after 150 days, indicating good stability of the films.     

Structural,chemical and dielectric properties of ceramic injection moulded Ba(Zn1/3Ta2/3)O3 microwave dielectric ceramics

We report the development of a ceramic injection moulding (CIM) process to produce complex-shaped structures using high-performance microwave ceramic materials. In particular, we describe the synthesis methods and the structural, chemical and dielectric properties of Ba(Zn_1/3Ta_2/3)O₃ (BZT) doped with Ni and Zr ceramics produced using ceramic injection moulding. Sintering the ceramic injection moulded Ba(Zn_1/3Ta_2/3)O₃ to a relative density of ∼94% was possible at a temperature of 1680 °C and a time of 48 h. The best samples to date exhibit a dielectric constant, ɛ_r, of ∼30, a Q value, of ∼31,250 (i.e. tan δ < 3.2 × 10⁻⁵) at 2 GHz, and a temperature coefficient of resonance frequency, τ_f, of 0.1 ppm/°C.     

Frequency dependence of dielectric properties of metallodielectric SrTiO3–Pt composites

The frequency dependence of dielectric properties of SrTiO₃–Pt (platinum) composites has been investigated. The SrTiO₃–Pt composites sintered at 1300 °C for 2 h was prepared by using conventional ceramic fabrication method. The dense metallodielectric composites without chemical reaction between SrTiO₃ and Pt during sintering process were obtained. The relative permittivity (ɛ_r) of SrTiO₃–Pt composites was increased with increase of amount of Pt. The maximum ɛ_r of 2150 at 1 MHz was obtained for the composite of SrTiO₃–27 vol.% Pt. Moreover, the SrTiO₃–Pt composites exhibited high values of ɛ_r at low frequencies than at high frequencies due to space charge polarization. The loss tangent (tan δ) of SrTiO₃–Pt composites increased with increasing amount of Pt, and decreased with increasing frequency up to 1 MHz. However, the tan δ of SrTiO₃–Pt composites was significantly increased at microwave frequency due to abrupt increase of ac conductivity of composite.     

Dielectric properties and tunability of Ba(ZrxTi1−x)O3 ceramics under high DC electric field

Ba(Zr_xTi_1−x)O₃ (BZT) (x = 0.25, 0. 3, 0.35, 0.4) ceramics were prepared by the traditional ceramic processing and their structural, surface morphological, dielectric properties, tunable properties as well as the mechanism of their nonlinear dielectric constant under DC electric field were systemically examined. The Zr ions substitution of Ti ions has a strong effect on the dielectric properties and the grain sizes. The results show Ba(Zr_xTi_1−x)O₃ (x = 0.25, 0.3, 0.35) ceramics to be promising candidates for the DC electric field tunable materials for microwave electronics application, because they exhibit high tunability (27.6%, 26.3%, 19.4%, respectively) as the strength of electric field is up to 2 kV/mm, low dielectric loss (0.001–0.002, 0.001–0.002, 0.004–0.005, respectively) at 10 kHz at room temperature and low temperature coefficient of capacitance.     

Electrical transport properties of Co–Zn–Y–Fe–O system

Co–Zn substituted nanoferrites having stoichiometric composition Co_1?xZn_xY_0.15Fe_1.85O₄ (x = 0.0–1.0, step: 0.2) were synthesized by chemical co-precipitation method. Analysis of the XRD patterns confirms the formation of cubic spinel phase as main phase along with few traces of secondary phase. The lattice constant was found to increase from 8.378 Å to 8.438 Å with zinc contents which can be explained on the basis of difference in ionic radii. SEM micrographs indicate nearly uniform distribution of grains. The average crystal size was found to decrease from 38.41 nm to 14.25 nm with the increase of Zn contents. The physical density increases with the increase of Zn contents from 3.95 g/cm³ to 4.42 g/cm³. It was found that the resistivity decreases with the increase of Zn contents from 9.20 × 10⁷ Ω cm to 5.26 × 10⁶ Ω cm which may be attributed to the increase in the number of Fe²⁺/Fe³⁺ ions pairs at B-sites. The transition temperature of the samples with substitution level x = 0.6, 0.8, 1.0 changes at 373, 333 and 313 K, respectively. The transition temperature of the sample with x = 1.0 is close to the room temperature. This may be the Curie temperature. Low Curie temperature material can be used for the preparation of temperature sensitive ferrofluid. Dielectric loss tangent (tan δ) has been observed to increase with the increase of zinc contents. This can be attributed to the decrease in resistivity which in turn increases the dielectric loss tangent.     

Temperature-stable and high-ɛ dielectric ceramics based on Ag (Nb1−xTax)O3

The dielectric properties of novel dielectric system AgNb_1−xTa_xO₃ (ANT) have been studied in this paper. In this system, the temperature coefficient of capacitance (TCC) can be adjusted to 0 ± 30 × 10⁻⁶/°C by choosing proper molar ratio of Nb⁵⁺ to Ta⁵⁺. When 2 wt% glass is added to the ceramics, the sintering temperature is reduced to 960 °C, which restrains Ag⁺ decomposition in ambient atmosphere. It is noted that the dielectric loss reduces further after adding 2.5 wt% Sb₂O₅. The dielectric properties of the resultant samples are as follows: dielectric constant ɛ ≈ 512, loss tangent tan δ ≈ 5.2 × 10⁻⁴, and TCC ≈ 10 × 10⁻⁶/°C.     

The effects of deposition temperature on structure and dielectric properties of Ba0.6Sr0.4TiO3 thin films produced by pulsed laser deposition

The effects of deposition temperature on orientation, surface morphology and dielectric properties of the thin films for Ba_0.6Sr_0.4TiO₃ thin films deposited on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition were investigated. X-ray diffraction patterns revealed a (2 1 0) preferred orientation for all the films. With rising substrate temperature from 650 °C to 700 °C, the crystallinity and crystal grain size of the films increase, the relative dielectric constant increases, but the dielectric losses have not obvious difference. The film deposited at 350 °C and annealed at 700 °C has strongly improved roughness and dielectric permittivity compared with the film only deposited directly at 700 °C. Three distinct relaxation processes within tan(δ) were found for the Ba_xSr_1?xTiO₃ film: a broadened process of the film relaxation, an intermediate peak which originates from Maxwell–Wagner–Sillars polarization, and an extremely slow process ascribed to leak current. The complex dielectric permittivity and loss can be fitted by an improved Cole–Cole model corresponding to a stretched relaxation function.     

Influence of cobalt and sintering temperature on structure and electrical properties of BaZr0.05Ti0.95O3 ceramics

Pure and Co-modified BaZr_0.05Ti_0.95O₃ ceramics were fabricated by the traditional solid state reaction technique. The influence of cobalt and sintering temperature on structure, dielectric, ferroelectric properties and diffuse phase transition of BZT ceramics were investigated systematically. 1300 °C was the optimal sintering temperature for BZT ceramics. The solid solubility limit of Co ions in BZT matrix was determined to be 0.4 mol%. The introduction of a moderate amount of Co ions was believed to benefit the microstructure development and make the grain size more uniform. Compared with undoped counterparts, 0.4 mol% Co-modified ceramics showed equivalent ferroelectric properties with a high remnant polarization (P_r=9.6 μC/cm²) and a low coercive field (E_c=0.21 kV/mm). Besides these, a relative high dielectric coefficient (ε_r=2030) and a low dielectric loss (tan δ=1.85%) were also obtained on this composition. The degree of diffuse phase transition was enhanced by the addition of Co ions. The related mechanism of the diffused phase transition behavior was discussed.     

Structural characteristics and electrical properties of copper doped manganese ferrite

A series of Mn_1?xCu_xFe₂O₄ ferrite samples with 0.2 ≤ x ≤ 0.5 were prepared using the co-precipitation method. X-ray analysis confirmed the formation of single phase cubic spinel structure for all concentrations. Rietveld refinement revealed that the Mn_1?xCu_xFe₂O₄ with all concentrations of x belongs to normal spinel structure. The lattice parameters decrease leading to the increase in the X-ray density with increasing the copper concentration and this may be due to the difference in the ionic radii between Mn²⁺ and Cu²⁺. The decrease in the crystallite size with increasing the copper content is attributed to the higher formation temperature. The IR absorption spectra analyses were used for the detection and confirmation of the chemical bonds in spinel ferrites. The AC electrical conductivity, real part of the dielectric constant and the loss tangent tan δ were studied as a function of the applied frequency and temperature. It was found that the AC electrical conductivity increased with increasing temperature, this increase may be related to the increase in the drift mobility of the charge carriers, which are localized at ions or vacant sites. The AC conductivity increases with increasing copper concentration which may be ascribed to the decrease in hopping length. The dielectric constant ?′ and dielectric loss showed a decrease with increasing frequency and increase with increasing temperature for all compositions. The dielectric behavior is explained by using the mechanism of polarization process.