Novel high dielectric constant and low loss PTFE/CNT composites

Spherical Ca_0.55Nd_0.3TiO₃ ceramic filled polytetrafluoroethylene composites (abbreviated as PTFE/CNT) with different filler volume fractions were prepared. The effects of filler volume fraction on microstructure, dielectric properties and thermal property were studied by scanning electron microscope, vector network analyzer and thermal dilatometer, respectively. The SEM results show that spherical particles are advantageous to reduce the porosity in the interphase which would increase the dielectric loss. Moreover, both the dielectric constant and dielectric loss increased with the increasing volume fraction of CNT microspheres. The high dielectric constant and low dielectric loss composite can be prepared when the ceramic volume fraction is 50?V%: ε_r =?12, tan?δ?=?8.5?×?10^?4 (at 10?GHz). Different models were used to predict the dielectric constant of composite, and the effective medium theory shows the least deviation from the experiment. The experimental coefficient of thermal expansions of composites with different volume fractions were less than theoretical data due to the change from loosely bound polymer chain to tightly bound polymer chain which would restrain the coefficient of thermal expansions of composites.     

A study on the interfacial reaction and dielectric properties of Ba0.88(Nd1.40Bi0.42La0.30)Ti4O12/alkali-borosilicate glass composites

This study investigated the effects of sintering parameters and the addition of alkali-borosilicate glass into the Ba_0.88(Nd_1.40Bi_0.42La_0.30)Ti₄O₁₂ B(NBL)T ceramic. The microstructure evolution, ionic exchange phenomenon at phase interfaces and the dielectric properties variation of composites were examined by XRD, EPMA, TEM, RF impedance analyzer and network analyzer, respectively. XRD patterns revealed that interactions between B(NBL)T ceramic and glass during sintering could have caused the change in the preferred orientation as well as the shifting of the crystals’ diffraction angles. EPMA mapping showed that the concentrations of Ba, and Bi decreased along the edge of the B(NBL)T ceramic that is closest to the glass phase, while the opposite trend was seen for Na and Ca. TEM and EDS analyses confirm that an ionic exchange took place during sintering with the glass phase wetting the B(NBL)T ceramic and was responsible for the change in the crystal plane and the variation in lattice parameters. The ionic exchange that occurred between the B(NBL)T ceramic and the glass phase resulted in a decrease in the electrical resistivity of the glass phase, which in turn reduced the dielectric loss.     

Investigation on the origin of the giant dielectric constant in CaCu3Ti4O12 ceramics through analyzing CaCu3Ti4O12-HfO2 composites

A full range of CaCu₃Ti₄O₁₂-HfO₂ (CCTO-HfO₂) composites were prepared by sintering mixtures of the two components at 1000 °C for 10 h. X-ray diffraction studies confirmed the two-phase nature of the composites. The evolution of the microstructure in the composites, in particular, the size distribution of CCTO grains, was examined by scanning electron microscopy. The studies showed that, as more HfO₂ was added, the abnormal grain growth of CCTO and coarsening of the microstructure were gradually suppressed. As a result, the average CCTO grain size was reduced from 50 to 1 μm. The measured dielectric constants agree well with the values calculated from Lichtenecker's logarithmic law, using only the dielectric constants of pure CCTO and HfO₂ as two end points. The agreement suggests to us that the dielectric constant of CCTO is dominated by domain boundaries within the grains rather than by grain boundaries between the grains.     

Low-temperature sintering and microwave dielectric properties of 3Z2B glass–Al2O3 composites

A potential low temperature co-fired ceramics system based on zinc borate 3ZnO–2B₂O₃ (3Z2B) glass matrix and Al₂O₃ filler was investigated with regard to phase development and microwave dielectric properties as functions of the glass content and sintering temperature. The densification mechanism for 3Z2B–Al₂O₃ composites was reported. The linear shrinkage of 3Z2B glass–Al₂O₃ composites exhibited a typical one-stage densification behavior. XRD patterns showed that a new crystalline phase, ZnAl₂O₄ spinel, formed during densification, indicating that certain chemical reaction took place between the 3Z2B glass matrix and the alumina filler. Meanwhile, several zinc borate phases, including 4ZnO·3B₂O₃, crystallized from the glass matrix. Both of the reaction product phase and crystallization phases played an important role in improving the microwave dielectric properties of composites. The optimal composition sintered at 850–950 °C showed excellent microwave dielectric properties: ?_r = ∼5.0, Q·f₀ = ∼8000 GHz, and τ_f = ∼−32 ppm/°C at ∼7.0 GHz.     

ZnLi2/3Ti4/3O4: A new low loss spinel microwave dielectric ceramic

A new low loss spinel microwave dielectric ceramic with composition of ZnLi_2/3Ti_4/3O₄ was synthesized by the conventional solid-state ceramic route. The ceramic can be well densified after sintering above 1075 °C for 2 h in air. X-ray diffraction data show that ZnLi_2/3Ti_4/3O₄ ceramic has a cubic structure [Fd-3m (227)] similar to MgFe₂O₄ with lattice parameters of a = 8.40172 Å, V = 593.07 Å³, Z = 8 and ρ = 4.43 g/cm³. The best microwave dielectric properties can be obtained in ceramic with relative permittivity of 20.6, Q × f value of 106,700 GHz and τ_f value of −48 ppm/°C. The addition of BaCu(B₂O₅) (BCB) can effectively lower the sintering temperature from 1075 °C to 900 °C and does not induce much degradation of the microwave dielectric properties. Compatibility with Ag electrode indicates that the BCB added ZnLi_2/3Ti_4/3O₄ ceramics are good candidates for LTCC applications.     

Hot-pressing sintered BN-SiO2 composite ceramics with excellent thermal conductivity and dielectric properties for high frequency substrate

High thermal conductivity and low dielectric constant are the more and more important properties for high-frequency substrate materials to enhance their heat radiation and reduce signal delay. In this work, a series of BN-SiO₂ composite ceramics for high frequency application were successfully synthesized by hot-pressing sintering method. And their structures, thermal and dielectric properties were systematically studied. According to the results, the excellent thermal conductivity with low dielectric constant and low dielectric loss has been obtained in the BN-SiO₂ ceramic. Compared to the pure SiO₂, the sample with 50?wt% BN addition sintered at 1650?℃ exhibited excellent physical properties, including a high thermal conductivity of 6.75?W/m?K which is almost five times higher than that of pure SiO₂ and a low dielectric constant of 3.73. The achieved high thermal conductivity and appropriate dielectric property of the BN-SiO₂ composite ceramic make it a promising candidate for high-frequency substrate application.     

Fabrication and piezoelectric properties of BaTiO3/BaTiO3-Bi(Mg1/2Ti1/2)O3-BiFeO3 composites

We fabricated xBaTiO₃ (BT)/(1-x)[BaTiO₃-Bi(Mg_1/2Ti_1/2)O₃-BiFeO₃] (BT-BMT-BF)?+?0.1?wt%MnCO₃ composites by spark plasma sintering and investigated the effect of BT content x, BT powder size, and BT-BMT-BF composition on piezoelectric properties. For xBT/(1-x)(0.3BT-0.1BMT-0.6BF) +?0.1?wt%MnCO₃ (x?=?0–0.75) composites with a 0.5-µm BT powder, the dielectric constant was increased with x, and the relative density was decreased at x?=?0.67 and 0.75, creating optimum BT content of x?=?0.50 with a piezoelectric constant d₃₃ of 107?pC/N. When a larger 1.5-µm BT powder was utilized for the composite with x?=?0.50, the d₃₃ value increased to 150?pC/N due to the grain size effect of the BT grains. To compensate for a compositional change from the optimum 0.3BT-0.1BMT-0.6BF due to partial diffusion between the BT and 0.3BT-0.1BMT-0.6BF grains, a 0.5BT/0.5(0.275BT-0.1BMT-0.625BF)?+?0.1?wt%MnCO₃ composite with the 1.5-µm BT powder was fabricated. We obtained an increased d₃₃ value of 166?pC/N. These results provided a useful composite design to enhance the piezoelectric properties.     

Effect of surface-modified Al2O3 on the thermomechanical properties of polybutylene succinate/Al2O3 composites

This study investigates the effect of the incorporation of alumina particles on the thermomechanical properties of polybutylene succinate (PBS)/Al₂O₃ composites. The alumina surface was modified with the carboxylic groups of maleic acid through simple acid-base and in situ polymerization reactions. Scanning electron microscope (SEM) results revealed the introduction of maleic acid treated alumina significantly affect the morphology of the PBS/Al₂O₃ composites as compared to the neat PBS. The thermal conductivity of the composite (0.411?W?m^?1 K^?1) was more than twice that of neat PBS. The composite containing polymerization-modified alumina showed a 50% increase in storage modulus compared with that of neat PBS. In addition, universal testing machine (UTM) and differential scanning calorimetry (DSC) measurements indicated an increase in the tensile strength and degree of crystallinity after the incorporation of modified alumina in the PBS/Al₂O₃ composite.     

Microstructures and dielectric properties of spark plasma sintered Ba0.4Sr0.6TiO3/CaCu3Ti4O12 composite ceramics

(1 − x)Ba_0.4Sr_0.6TiO₃/xCaCu₃Ti₄O₁₂ composite ceramics were prepared by spark plasma sintering. Sintering behavior, microstructures and dielectric properties of the composite ceramics were investigated by XRD, SEM, EDS and dielectric spectrometer. Dense composite ceramics consisting of Ba_0.4Sr_0.6TiO₃ phase and CaCu₃Ti₄O₁₂ phase were prepared at 800 °C for 0 min. The dielectric loss of the composite ceramic decreased with increasing amount of Ba_0.4Sr_0.6TiO₃, and the high dielectric constant were retained. Moreover, the better temperature stability of dielectric constant was obtained. These improvements of dielectric characteristics have great scientific significance for potential application.     

Dispersion studies of La substitution on dielectric and ferroelectric properties of multiferroic BiFeO3 ceramic

Lanthanum La-substituted multiferroic Bi_1−xLa_xFeO₃ ceramics with x = 0.0, 0.05, 0.10, 0.15, 0.20 and 0.25 have been prepared by solution combustion method. The effect of La substitution for the dispersion studies on dielectric and ferroelectric properties of Bi_1−xLa_xFeO₃ samples have been studied by performing x-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), density, dc resistivity and dielectric measurements as well as characterizing the polarization-field hysteresis loop. The results of prepared samples are compared with those of bismuth ferrite (BiFeO₃). In the measuring frequency of 10 KHz to 1 MHz, the dielectric constants and dielectric losses for samples x = 0.20, 0.25 are almost stable and exhibited lowest dielectric loss close to 0.1. The resistivity of Bi_1−xLa_xFeO₃ samples reaches a maximum value of 10⁹ ohm-cm, which is about three times higher than that for pure BiFeO₃. The results also show that stabilization of crystal structure and nonuniformity in spin cycloid structure by La substitution enhances the resistivity, dielectric and ferroelectric properties. Furthermore, the substitution of rare earth La for Bi helps to eliminate the impurity phase in BiFeO₃ ceramic.     

Low-temperature preparation and microwave dielectric properties of cold sintered Li2Mg3TiO6 nanocrystalline ceramics

This study aims to fabricate Li₂Mg₃TiO₆ ceramics with ultrafine grains using a novel cold sintering process combined with a post-annealing treatment at a temperature <?950?°C. In this study, phase composition, sintering behavior, microstructure evolution, and microwave dielectric properties of the resultant nanocrystalline ceramics were investigated for the first time. The as-compacted green pellets at 180?°C yielded a high relative density of ~ 90% and the ceramics that were post-sintered over a broad temperature range (800–950?°C) possessed highly dense microstructure with a relative density of ~ 96%. The average grain size varied from 100 to 1200?nm for the samples sintered at 800–950?°C. Furthermore, the quality (Q × f) values of the obtained specimens exhibited a strong positive dependency on the grain size, which increased from 17,790 to 47,960?GHz for grain sizes ranging between 100 and 1200?nm, while the dielectric permittivity (ε_r) and temperature coefficient of the resonant frequency (τ_f) values did not undergo any significant changes over this range of grain size.     

IBLC effect leading to colossal dielectric constant in layered structured Eu2CuO4 ceramic

Dielectric (ε_r′) studies of phase pure T′-type Eu₂CuO₄ ceramics of two markedly different grain sizes (D), prepared by (i) conventional powder mixing and (ii) citrate complexation-Pechini process, have been carried out in the frequency range 0.1 Hz to 1 MHz, and at temperatures −100 °C to 150 °C. ε_r′ is found to be highly grain size dependent. For the sample with coarse bar-like grains (D²~17×6 μm²) ε_r′ is >10³, and for the finer grain size sample with bimodal distribution (D₁~1 μm, D₂~3 μm) ε_r′ is ~10⁵; for both the samples, high ε_r′ value is nearly frequency independent over 500 Hz≤f<100 kHz and T≥30 °C. The impedance spectroscopy (IS) study has clearly shown that both, the coarse- and the fine-grained samples consist of semiconducting grains and insulating grain boundaries that primarily lead to an internal barrier layer capacitance (IBLC) effect. And thus, manifest colossal dielectric constant (ε_r′>10³) in Eu₂CuO₄ ceramics. The smaller grain size (Pechini) sample, with over an order higher number of grains and grain boundary network, showing over an order higher ε_r′ (~10⁵) compared to the coarse grained one, further endorses the IBLC effect.     

Low temperature sintering and microwave dielectric properties of Ce2(WO4)3 ceramics

Ce₂(WO₄)₃ ceramics have been synthesized by the conventional solid-state ceramic route. Ce₂(WO₄)₃ ceramics sintered at 1000 °C exhibited ?_r = 12.4, Qxf = 10,500 GHz (at 4.8 GHz) and τ_f = −39 ppm/°C. The effects of B₂O₃, ZnO–B₂O₃, BaO–B₂O₃–SiO₂, ZnO–B₂O₃–SiO₂ and PbO–B₂O₃–SiO₂ glasses on the sintering temperature and microwave dielectric properties of Ce₂(WO₄)₃ were investigated. The Ce₂(WO₄)₃ + 0.2 wt% ZBS sintered at 900 °C/4 h has ?_r = 13.7, Qxf = 20,200 GHz and τ_f = −25 ppm/°C.     

Microwave assisted sintering of CaCu3Ti4O12

CaCu₃Ti₄O₁₂ electroceramic was prepared by a microwave assisted solid-state reaction technique from CaCO₃, CuO and TiO₂ powders. Processing involved the preparation of raw material, mixing and milling, calcination, pellet forming and sintering processes. Conventional furnace and microwave assisted sintering processes were employed in order to improve phase structures, morphology and dielectric properties of CaCu₃Ti₄O₁₂ ceramics. Surface and fracture FESEM analysis showed that the microwave assisted sintered CaCu₃Ti₄O₁₂ produced better densification and more uniform grain size compared to the conventional sintered sample.     

Enhancement in dielectric and ferroelectric properties of lead free Bi0.5(Na0.5K0.5)0.5TiO3 ceramics by Sb-doping

Lead free piezoelectric Bi_0.5(Na_0.5K_0.5)_0.5TiO₃ (pure and 1 wt.%, 2 wt.%, 4 wt.% Sb-doped) ceramics were synthesized away from its MPB. The crystalline nature of the BNKT ceramic was studied by XRD and SEM. Depolarization temperature (T_d) and transition temperature (T_c) were observed through phase transitions in dielectric studies which were found to increase after Sb-doping, thus increasing its usable temperature range. In the study of relaxation behavior, the activation energy for relaxation was found to be 0.33, 0.43, 0.57 and 0.56 eV for pure and Sb-doped samples, respectively. All samples were found to exhibit normal Curie-Weiss law above their T_c. Doping of Sb was found to restrain the diffused character of the pure sample. In P-E loop, Sb-doping was found to increase the ferroelectric properties.Pure and Sb-doped BNKT ceramics exhibited high values of piezoelectric charge coefficient (d₃₃) as 115, 121, 129 and 100 pC/N, respectively.     

Microwave dielectric properties of multi-ions Ba(Zn,Ta)O3-based perovskite ceramics

In this study, Ba(Zn_1/3Ta_2/3)O₃-based complex perovskite compounds, including Ba(Zn_1/3Ta_2/3)O₃, Ba(Zn_1/3Ta_1/3Nb_1/3)O₃, Ba(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃, and Ba_1/2Sr_1/2(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃, were prepared and characterized. There was no second phase formation shown in the XRD patterns. Though it has been suggested that substitutions of multiple ions over A-site or B-site of the Ba(Zn_1/3Ta_2/3)O₃ ceramics may not be beneficial to their microwave dielectric properties, the Ba(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃ and Ba_1/2Sr_1/2(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃ ceramics in this study were found to perform in a fairly acceptable manner. The Ba(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃ ceramic (sintered at 1575 °C for 6 h) and the Ba_1/2Sr_1/2(Zn_1/6Co_1/6Ta_2/9Nb_2/9Sb_2/9)O₃ ceramic (sintered at 1550 °C for 6 h) reported the following characteristics after annealing at 1400 °C for 10 h: 24.9 and 27.0 for dielectric constants (?_r), 83,000 and 32,100 GHz for quality factors (Q × f) values and −12.8 and −22.6 ppm/°C for temperature coefficients of resonance frequency (τ_f).     

The influence of sintering aids for Nd(Mg0.5Ti0.5)O3 microwave dielectric ceramics properties

The influence of various sintering aids on the microwave dielectric properties and the structure of Nd(Mg_0.5Ti_0.5)O₃ ceramics were investigated systematically. B₂O₃, Bi₂O₃, and V₂O₅ were selected as liquid-phase sintering aids to lower the sintering temperature. The sintered Nd(Mg_0.5Ti_0.5)O₃ ceramics are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and microwave dielectric properties. The sintering temperature of Nd(Mg_0.5Ti_0.5)O₃ microwave dielectric ceramics is generally high, about 1500 °C. However, the sintering temperature was significantly lowered about 175 °C from 1500 °C to 1325 °C by incorporating in 10 mol% B₂O₃ and revealed the optimum microwave dielectric properties of dielectric constant (_r) value of 26.2, a quality factor (Q × f) value of 61,307 (at 9.63 GHz), and τ_f value of −45.5 ppm/°C. NdVO₄ secondary phase was observed at 10 mol% V₂O₅ addition in the sintering temperature range of 1300–1325 °C, which led the degradation in microwave dielectric properties. The microwave dielectric properties as well as grain sizes, grain morphology, and bulk density were greatly dependent on sintering temperature and various sintering aids. In this study, it is found that Nd(Mg_0.5Ti_0.5)O₃ incorporated with 10 mol% B₂O₃ with lower sintering temperature and excellent dielectric microwave properties may be suggested for application in microwave communication devices. The use of liquid-phase sintering, the liquid formed during firing normally remains as a grain boundary phase on cooling. This grain boundary phase can cause a deterioration of the microwave properties. Therefore, the selection of a suitable sintering aid is extremely important.     

Fabrication and electrical properties of Pb(Co1/3Nb2/3)O3 ceramics

Pb(Co_1/3Nb_2/3)O₃ (PCN) ceramics have been produced by sintering PCN powders synthesized from lead oxide (PbO) and cobalt niobate (CoNb₂O₆) with an effective method developed for minimizing the level of PbO loss during sintering. Attention has been focused on relationships between sintering conditions, phase formation, density, microstructural development, dielectric and ferroelectric properties of the sintered ceramics. From X-ray diffraction analysis, the optimum sintering temperature for the high purity PCN phase was found at approximately 1050 and 1100 °C. The densities of sintered PCN ceramics increased with increasing sintering temperature. However, it is also observed that at very high temperature the density began to decrease. PCN ceramic sintered at 1050 °C has small grain size with variation in grain shape. There is insignificant change of dielectric properties with sintering temperature. The P–E hysteresis loops observed at −70 °C are of slim-loop type with small remanent polarization values, which confirmed relaxor ferroelectric behavior of PCN ceramics.     

Microstructure and dielectric properties of Dy/Mn doped BaTiO3 ceramics

Dy/Mn doped BaTiO₃ with different Dy₂O₃ contents, ranging from 0.1 to 5.0 at% Dy, were investigated regarding their microstructural and dielectric characteristics. The content of 0.05 at% Mn was constant in all the investigated samples. The samples were prepared by the conventional solid state reaction and sintered at 1290°, and 1350 °C in air atmosphere for 2 h. The low doped samples (0.1 and 0.5 at% Dy) exhibit mainly fairly uniform and homogeneous microstructure with average grain sizes ranged from 0.3 μm to 3.0 μm. At 1350 °C, the appearance of secondary, abnormal, grains in the fine grain matrix and core–shell structure were observed in highly doped Dy/BaTiO₃. Dielectric measurements were carried out as a function of temperature up to 180 °C. The low doped samples sintered at 1350 °C, display the high value of dielectric permittivity at room temperature, 5600 for 0.1Dy/BaTiO₃. A nearly flat permittivity–temperature response was obtained in specimens with 2.0 and 5.0 at% additive content. Using a Curie–Weiss and modified Curie–Weiss low, the Curie constant (C), Curie like constant (C′), Curie temperature (T_C) and a critical exponent (γ) were calculated. The obtained values of γ pointed out the diffuse phase transformation in highly doped BaTiO₃ samples.     

Microwave dielectric characteristics of ceramics in Mg2SiO4–Zn2SiO4 system

(Mg_1−xZn_x)₂SiO₄ ceramics were prepared and characterized. The densification temperatures of the present ceramics are much lower than those for Mg₂SiO₄ and Zn₂SiO₄ end-members. Small solid solution limits of Zn in Mg₂SiO₄ and Mg in Zn₂SiO₄ are observed, and the bi-phase structure is confirmed in (Mg_1−xZn_x)₂SiO₄ ceramics with x = 0.1–0.9. Even though, it is clear that the Qf value of Zn₂SiO₄ ceramics can be significantly improved together with a suppressed temperature coefficient of resonant frequency τ_f by substituting Mg for Zn. (Mg_0.4Zn_0.6)₂SiO₄ ceramics indicate a good combination of microwave dielectric characteristics: _r = 6.6 Qf = 95,650 GHz, and τ_f = −60 ppm/°C.