Temperature Stability Of Dielectric Properties Research Articles

Thermally stable polymer–ceramic composites for microwave antenna applications

Polymer–ceramic composites were prepared by twin screw melt extrusion with high-density polyethylene (HDPE) as the matrix and polystyrene-coated BaO–Nd2O3–TiO2 (BNT) ceramics as the filling material. Interestingly, the incorporation of polystyrene (PS) by the coating route could significantly improve the thermal behaviors of the composites (HDPE–PS/BNT), besides the temperature stability of dielectric properties and thermal displacement. The microwave dielectric properties of the composites were investigated systematically. The results indicated that, as the volume fraction of BNT ceramic particles increased from 10 to 50 vol% in the composites, the dielectric constant increased from 3.54 (9.23 GHz) to 13.14 (7.20 GHz), which can be beneficial for the miniaturization of microwave devices; the dielectric loss tangent was relatively low (0.0003–0.0012); more importantly, the ratio of PS to HDPE increased accordingly, making the composite containing 50 vol% BNT ceramics have a low value of temperature coefficient of resonant frequency (τ f = −11.2 ppm/°C) from −20 to 60 °C. The GPS microstrip antennas were therefore designed and prepared from the HDPE–PS/BNT composites. They possessed good thermal stability (τ f = 23.6 ppm/°C) over a temperature range of −20 to 60 °C, promising to meet the requirements of practical antenna applications.

Duplex structure in K0.5Na0.5NbO3-SrZrO3 ceramics with temperature-stable dielectric properties

Temperature-stable dielectric properties have been developed in the 0.86K0.5Na0.5NbO3-0.14SrZrO3 solid solution system. High dielectric permittivity (ε′=2310) with low loss sustained in a broad temperature range (−55–201°C), which was close to that of the commercial BaTiO3-based high-temperature capacitors. Transmission electron microscopy with energy dispersive X-ray analysis directly revealed that submicron grains exhibited duplex core-shell structure. The outer shell region was similar to the target composition, whilst a slightly poor content of Sr and Zr presented in the core region. Based on Lichtenecker’s effective dielectric function analysis along with Lorentz fit of the temperature dependence of dielectric permittivity, a plausible mechanism explaining the temperature-stable dielectric response in present work was suggested. These results offer an opportunity to achieve the X8R specification high-temperature capacitors in K0.5Na0.5NbO3 based materials.

Enhanced dielectric relaxation in (1−x)BaTiO3–xBiYO3 ceramics

The polycrystalline (1−x)BaTiO3–xBiYO3 ((1−x)BT–xBY) (x=0.00–0.30) ceramics were fabricated via a conventional solid state route. The phase transition, microstructure and dielectric performances of the composites were investigated. X-ray powder diffraction patterns indicates tetragonal symmetry at x≤0.04, whilst the XRD patterns for x>0.04 are pseudocubic. A progressive decline in εrmax with the elevation of x leads to near temperature-stable dielectric properties over a wide temperature range. For x≥0.04, dielectric measurements indicates relaxor behavior. Dielectric temperature stability of BT ceramics have improved effectively by the increasing amount of BY content. It is found that x=0.30 can meet the X9R criterion, with Δε/Δε25≤±15% in the temperature range of −55 to 200°C .

BaTiO3–Bi(Mg2/3Nb1/3)O3 Ceramics for High‐Temperature Capacitor Applications

Solid solutions of (1−x)BaTiO3–xBi(Mg2/3Nb1/3)O3 (0 ≤ x ≤ 0.6) were prepared via a standard mixed‐oxide solid‐state sintering route and investigated for potential use in high‐temperature capacitor applications. Samples with 0.4 ≤ x ≤ 0.6 showed a temperature independent plateau in permittivity (εr). Optimum properties were obtained for x = 0.5 which exhibited a broad and stable relative εr ~940 ± 15% from ~25°C to 550°C with a loss tangent <0.025 from 74°C to 455°C. The resistivity of samples increased with increasing Bi(Mg2/3Nb1/3)O3 concentration. The activation energies of the bulk were observed to increase from 1.18 to 2.25 eV with an increase in x from 0 to 0.6. These ceramics exhibited excellent temperature stable dielectric properties and are promising candidates for high‐temperature multilayer ceramic capacitors for automotive applications.

Bi4Ti3O12 Addition in the Ultra-Broad Temperature Stability of BaTiO3-Based Ceramics

BaTiO3-Nb2O5-xBi4Ti3O12 ceramics are prepared by a conventional mixed oxide route. From the observations, the proverskite-type solid solutions are formed for x ≤ 0.075, and second phase BaBi4Ti4O15 is found for x > 0.2. The sintering temperature is reduced effectively to 1090°C. A progressive decline in ϵr max with increasing x led to near temperature-stable dielectric properties over a wide temperature range. For x = 0.15, ϵr max = 772(Δϵ/ϵ25 °C≤ 15%), from −55°C to 400°C, and tan δ≤ 0.02 from −55°C to 192°C. This work provides a roadmap to obtain a temperature-stable dielectric ceramic.

Control of coring effect in BaTi4O9 microwave dielectric ceramics by doping with Mn4+

In the present work, we have attempted to reduce the effect of coring effect in the titanate ceramic system BaTi4O9 (BT4) by doping it with Mn4+. The microwave dielectric BaTi4O9 ceramics doped with 0, 0.5 and 1.0mol% Mn4+ were synthesized by conventional ceramic processing route. The XRD studies confirmed a single phase crystalline structure for all the ceramic samples studied. The SEM micrographs of the ceramics reveal a microstructural change leading towards a more uniform grain size distribution as the Mn4+ content increases to 1.0mol%. In the low frequency region (100Hz to 1MHz), the temperature stability of dielectric properties exhibits a marked improvement with the increasing amount of Mn4+ in the ceramic system. In the microwave frequency region (9.3GHz), Q-factor increases from 11,625GHz to 46,500GHz for BaTi4O9 ceramic doped with 1.0mol% Mn4+. The present paper reveals that the commonly observed degradation of dielectric properties due to coring effect in the BaTi4O9 ceramic system can be controlled by doping it with an appropriate quantity of Mn4+.

Impact analysis of LTCC materials on microstrip filters’ behaviour up to 13 GHz

Purpose – The purpose of this paper is to consider the adequacy of various microstrip filters’ behaviour based on different low-temperature co-fired ceramic (LTCC) dielectrics in the high frequency (HF) area up to 13 GHz. Design/methodology/approach – Low pass, band pass and band stop filters for ultra-wideband radar systems were designed, simulated, fabricated and measured using three various dielectric substrates: Dupont GreenTape 951, Dupont GreenTape 9K7 and Murata LFC. Findings – It is not possible to unambiguously determine the most suitable LTCC dielectric for these filter design because, in general, all designed filters fulfilled requirements (attenuation, cut off frequencies) with minimal divergences, but temperature-stable dielectric and physical properties of Murata LFC make them a promising ceramic for HF application (repeatability of realised experiments). Originality/value – The novelty of this work lies in unconventional usage of LTCC as material with defined dielectric properties proper for HF applications.

Fabrication of submicron La2O3-coated BaTiO3 particles and fine-grained ceramics with temperature-stable dielectric properties

La2O3-coated BaTiO3 core–shell submicron particles are synthesized using chemical methods. The effects of the La2O3 coating characteristics, adjusted through pH control, on the microstructure and dielectric properties of the resulting composite BaTiO3 multilayer-capacitor ceramics are studied, and compared with bulk BaTiO3 and Ba0.97La0.02TiO3 ceramics. We obtained fine-grained ceramics with temperature-stable dielectric properties, a maximum permittivity of 2902 and a dielectric loss of less than 0.6% at room temperature.

Temperature-stable dielectric properties from −20°C to 430°C in the system BaTiO3–Bi(Mg0.5Zr0.5)O3

Abstract Ceramics in the solid solution series (1 − x)BaTiO3–Bi(Mg0.5Zr0.5)O3 are single-phase tetragonal for compositions x ≤ 0.05, and cubic for x ≥ 0.1. Plots of relative permittivity (ɛr), versus temperature show double peaks for x = 0.03 and x = 0.05, changing to a single, frequency-dependent peak for compositions, x ≥ 0.1. A progressive decline in ɛr max with increasing x leads to near temperature-stable dielectric properties over a wide temperature range. For x = 0.3, ɛr = 570 ± 15%, from −20 °C to 430 °C, and tan δ ≤ 0.02 from 30 °C to 420 °C. For x = 0.4, ɛr = 600 ± 15% from 25 °C to 420 °C, and tan δ ≤ 0.02 from 55 °C to 280 °C (at 1 kHz). Values of dc resistivity were ∼109 Ω m at 250 °C and ∼106 Ω m at 400 °C. A piezoelectric strain of ∼0.25% (at 40 kV/cm) was recorded for composition x = 0.03.

Robust dielectric properties of B-site size-disordered hexagonal Ln2CuTiO6 (Ln = Y, Dy, Ho, Er, and Yb)

Hexagonal Ln2CuTiO6 (Ln = Y, Dy, Ho, Er, and Yb) exhibits a rare combination of interesting dielectric properties, in the form of relatively large dielectric constants (ϵ′>30), low losses, and extremely small temperature and frequency dependencies, over large ranges of temperature and frequency [Choudhury et al., Appl. Phys. Lett. 96, 162903 (2010) and Choudhury et al., Phys. Rev. B 82, 134203 (2010)], making these compounds promising as high-k dielectric materials. The authors present a brief review of the existing literature on this interesting class of oxides, complimenting it with spectroscopic data in conjunction with first-principles calculation results, revealing a novel mechanism underlying these robust dielectric properties. These show that the large size differences in Cu2+ and Ti4+ at the B-site, aided by an inherent random distribution of CuO5 and TiO5 polyhedral units, frustrates the ferroelectric instability, inherent to the noncentrosymmetric P63 cm space group of this system, and gives rise to the observed relatively large dielectric constant values. Additionally, the phononic contributions to the dielectric constant are dominated primarily by mid-frequency (>100 cm−1) polar modes, involving mainly Ti4+ 3d0 ions. In contrast, the soft polar phonon modes with frequencies typically less than 100 cm−1, usually responsible for dielectric properties of materials, are found to be associated with non-d0 Cu2+ ions and to contribute very little, giving rise to the remarkable temperature stability of dielectric properties of these compounds.

Sol-Gel Synthesis and Characterization of Na0.5 Bi0.5 TiO3 -NaTaO3 Thin Films

Thin films with the composition 70 mol% Na0.5Bi0.5TiO3 + 30 mol% NaTaO3 were prepared by sol–gel synthesis and spin coating. The influence of the annealing temperature on the microstructural development and its further influence on the dielectric properties in the low- (kHz–MHz) and microwave-frequency (15 GHz) ranges were investigated. In the low-frequency range we observed that with an increasing annealing temperature from 550°C to 650°C the average grain size increased from 90 to 170 nm, which led to an increase in the dielectric permittivity from 130 to 240. The temperature-stable dielectric properties were measured for thin films annealed at 650°C in the temperature range between −25°C and 150°C. The thin films deposited on corundum substrates had a lower average grain size than those on Si/SiO2/TiO2/Pt substrates. The highest average grain size of 130 nm was obtained for a thin film annealed at 600°C, which displayed a dielectric permittivity of 130, measured at 15 GHz.

Phase diagram and structure-property relationships in the lead-free piezoelectric system: Na0.5K0.5NbO3-LiTaO3

A phase-diagram for the Na(0.5)K(0.5)NbO(3)-LiTaO(3) solid solution series (NKN-LT) is presented for compositions ≤ 10 mol% LT, based on the combined results of temperaturevariable X-ray powder diffraction and dielectric measurements. In addition to the reported orthorhombic and tetragonal polymorphs of NKN-LT, a monoclinic phase is revealed. Changes to electrical properties as a function of LT substitution are correlated to phase content. Increasing the LT content from 5 to 7 mol% LT led to improved temperature stability of piezoelectric properties because of the avoidance of the monoclinic-tetragonal polymorphic phase transition during thermal cycling (at >25°C). For 7 mol% LT samples: d(33) = 200 pC/N; T(c) = 440°C; ε(r) = 550 and tan δ = 0.02 (at 20°C). Modification of this composition by solid solution with BiScO(3) led to a decrease in d(33) values. Transmission electron microscopy of a sample of 0.95[0.93 NKN-0.07LT]-0.05BiScO(3) indicated a core-shell grain structure which led to temperature-stable dielectric properties.

Improved electrical properties of Pr-doped [formula omitted] films

Sr 1− x Pr x TiO 3 films were prepared by metal organic deposition method on (111) Pt/Ti/SiO 2/Si substrates. Pr-doping greatly improves the dc leakage behavior of the films. The samples with x = 0.075 show excellent electric field frequency and temperature stability of dielectric properties, while the x = 0.025 samples indicate an obvious dielectric relaxation behavior and better polarization versus applied electric field ( P – E ) hysteresis loops. These peculiar electrical properties can be explained mainly by the Pr-doping-induced changes in the free charge carriers, the lattice distortion, the charge transfer process and polar nanoregions. In addition, the variable valence of Pr ions may also have significant impacts.

Highly Temperature Stable Dielectric Properties of Nanograin Barium Strontium Titanate Thin Films Grown on Silicon Substrate

Temperature-insensitive dielectric properties of nanograin size Ba0.7Sr0.3TiO3 thin films were grown on LaNiO3/Si substrates by rf magnetron sputtering. Scanning electron microscopy and X-ray diffraction investigations show the grain size is about 25 nm. The thin films with nanograin size have much better temperature stability of dielectric properties. The dielectric constant and loss tangent varied slightly as temperature increased from −60° to 60°C. The tunabilities were stable around 15.5% at 50 kV/mm and the temperature coefficients of capacitances relative to the capacitance value at 0°C were all about 2 × 10−4/°C during this broad temperature range.

Preparation and dielectric tunability of bismuth-based pyrochlore dielectric thick films on alumina substrates

Metal–insulator–metal capacitors structures, employing the cubic pyrochlore Bi 1.5Zn 1.0Nb 1.5O 7 (BZN) thick films, were fabricated by screen-printing techniques on alumina substrates. Chemical compatibility and microstructure between layers was studied by EDS/SEM analysis. The dielectric properties of BZN thick films have been investigated as a function of temperature and frequency. The films exhibited the dielectric constant up to 130, and the dielectric loss less than 0.005 at 1 MHz. The dielectric tunabilities of the films have been compared at different temperature. The BZN thick film showed low tunability of about 1.8% at 10 kHz under 100 kV/cm dc bias voltage in temperature from −150 to 180 °C. The temperature stability of dielectric properties for BZN thick film should be useful for designing temperature stable frequency agile.

Calcium-doping for enhanced temperature stability of (Ba 0.6Sr 0.4) 1− xCa xTiO 3 thin film dielectric properties

Ternary (Ba 0.6Sr 0.4) 1− x Ca x TiO 3 (BSCT) ( x = 0, 0.1, 0.2, 0.3 and 0.4) thin films with thickness of around 500 nm were prepared on Pt(111)/TiO 2/SiO 2/Si substrates by sol–gel methods. BSCT forms the complete solid solutions in a single cubic perovskite structure. The lattice constant, dielectric constant, tan δ and tunability of BSCT decrease, whereas the temperature stability of dielectric properties increases with increasing the Ca concentration. From 25 to 100 °C, the decrease of tunability is about 11% for BSCT with 40 at.% of Ca. BSCT thin films exhibit the comparable tunability, low loss and enhanced temperature stability.

Effect of PbO-B<sub>2</sub>O<sub>3</sub> Glass Additive on the Dielectric Properties of Ba<sub>0.80</sub>Sr<sub>0.20</sub>TiO<sub>3</sub> /Pb<sub>0.88</sub>La<sub>0.12</sub>Ti<sub>0.97</sub>O<sub>3</sub> Composite Thick Films

BST/PLT composite thick films with various content of PbO-B2O3 glass additive from 5mol% to 25mol% were successfully prepared by screen-printing the paste onto alumina substrates. XRD, SEM and impedance meter were used to analyze the phase structures, morphologies and dielectric properties of the thick films. The results showed that the BST phase has no interaction with PbO-B2O3 glass in the composite thick films at 750 oC. Diffusion between PLT phase and PbO-B2O3 glass phase occurs and the c axis of PLT phase in thick films reduces slightly. The high infiltration between the glass phase and particles densify the composite thick films and the uniform microstructure can be obtained in the thick films with PbO-B2O3 glass additive content of 10mol%. Good temperature stability of dielectric properties is achieved with the 25mol% content of PbO-B2O3 glass additive and the variance of the relative dielectric constant in the temperature range between 0oC and 300oC is 15%.

Graded ferroelectric capacitors with robust temperature characteristics

Ferroelectric thin films offer the possibility of engineering the dielectric response for tunable components in frequency-agile rf and microwave devices. However, this approach often leads to an undesired temperature sensitivity. Compositionally graded ferroelectric films have been explored as a means of redressing this sensitivity, but experimental observations vary depending on geometry and other details. In this paper, we present a continuum model to calculate the capacitive response of graded ferroelectric films with realistic electrode geometries by accurately accounting for the polarization distribution and long-range electrostatic interactions. We show that graded c-axis poled BaxSr1−xTiO3 (BST) parallel plate capacitors are ineffective while graded a-axis poled BST coplanar capacitors with interdigitated electrodes are extremely effective in obtaining high and temperature-stable dielectric properties.

Microstructure and Electrical Properties of Tungsten-Doped Bismuth Titanate Ceramics

Bismuth-layer-structured Bi4Ti3-xWxO12+x (BiTW) ceramics, where x=0, 0.02, 0.04, 0.08, 0.10 and 0.20, have been prepared by a conventional sintering technique. The conducting, dielectric and ferroelectric properties of BiTW were investigated. The microstructure results revealed randomly oriented grains with a needle structure, and the grain size decreased gradually with increasing W6+ doping content. The conducting properties were studied in a direct-current circuit at various temperatures. W6+ donor doping enhanced the electrical resistivity markedly above 108 Ωcm at 400°C on the whole. BiTW with suitable W6+ replacement showed good temperature stability from the temperature dependence of dielectric constant. Tc decreased gradually with W doping. Ferroelectric hysteresis loops were also determined for various x values. Excellent properties could be found at x=0.04, i.e., the composition Bi4Ti2.96W0.04O12.04, which possessed a high electrical resistivity and a good temperature stability of dielectric properties than other composition materials.

Microstructure and electrical properties of niobium doped Bi 4Ti 3O 12 layer-structured piezoelectric ceramics

Layer-structured Bi 4Ti 3− x Nb x O 12+ x/2 (BiTN) ceramics, where x = 0.02–0.20, have been prepared by the conventional sintering technique. XRD results reveal the existence of an orthorhombic structure. The grain size decreases gradually and grain growth anisotropy is limited when x increases. The domain structures of BiTN ceramics differ on x. Nb 5+ donor doping decreases markedly electrical conductivity of the materials and T c shifts gradually to lower temperatures. In addition, low dielectric losses and good temperature stability of dielectric constant are obtained in a wide temperature range. Ferroelectric hysteresis loops of the materials are also determined on x. The best properties can be found in x = 0.08 and 0.11, indicating lower electrical conductivity, good temperature stability of dielectric properties and adequate piezoelectric properties.