Microwave dielectric properties and microstructures of Nd(Mg0.5Sn0.5−xTix)O3 ceramics

This study elucidates the microwave dielectric properties and microstructures of Nd(Mg_0.5Sn_0.5?xTi_x)O₃ ceramics with a view to their potential for microwave devices. The Nd(Mg_0.5Sn_0.5?xTi_x)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the Nd(Mg_0.5Sn_0.4Ti_0.1)O₃ ceramics revealed no significant variation of phase with sintering temperatures. A dielectric constant (?_r) of 21.1, a quality factor (Q × f) of 50,000 GHz, and a temperature coefficient of resonant frequency (τ_f) of ?60 ppm/°C were obtained for Nd(Mg_0.5Sn_0.4Ti_0.1)O₃ ceramics that were sintered at 1550 °C for 4 h.     

Microwave dielectric properties of (Ba1−xSrx)(Mg0.5W0.5)O3 ceramics

The densification, microstructural evolution and microwave dielectric properties of (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ ceramics with x=0, 0.25, 0.5 and 0.75 are investigated in this study. The sintering temperature of the (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ is significantly reduced from 1575 °C to 1400 °C as the x value increases from 0 to 0.25 and 0.50; this result is accompanied by the formation of the (Ba_1−ySr_y)WO₄ phase and a small quantity of second phase surrounding the grains. The grain size of the (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ ceramics is increased by raising the Sr²⁺ content, which significantly lowers the sintering temperature. The microstructure of the (Ba_0.75Sr_0.25)(Mg_0.5W_0.5)O₃ ceramic displays the smallest average grain size of approximately 0.8 μm, with a narrow grain size distribution. Without long annealing time, very high Q×f values are obtained for the (Ba_1−xSr_x)(Mg_0.5W_0.5)O₃ ceramics sintered at 1400–1575 °C for a duration of only 2 h. The (Ba_0.75Sr_0.25)(Mg_0.5W_0.5)O₃ ceramic sintered at 1400 °C results in the best microwave dielectric properties, including ε_r of 20.6, Q×f of 152,600 GHz and τ_f of +24.0 ppm/°C.     

In situ synthesis of Ba0.5Sr0.5TiO3–Mg3B2O6 composites and their dielectric response

Ba_0.5Sr_0.5TiO₃–Mg₃B₂O₆ (BST–MB) composites have been prepared in situ by a citrate gel process, and their structure and effective dielectric response have been investigated. The precursor with pH≥7 is suitable for in situ formation of the diphase structure consisting of BST and MB. Accordingly, MB particles homogeneously disperse in BST particles, accompanied by the formation of boron-rich grain boundary resulting from liquid phases sintering of B₂O₃. Related with the existence of boron-rich grain boundary and the incorporation of Mg²⁺ into BST lattice, permittivity decreases rapidly with increasing volume fraction of MB from 0.0 to 0.2 and then decreases slowly with further increasing, which coincides with theoretical prediction of the layered model.     

Effect of SrTiO3 concentration and sintering temperature on microstructure and dielectric constant of Ba1−xSrxTiO3

The Ba_1−xSr_xTiO₃ materials have received increased attention as one of the most important materials for electroceramic components, such as high dielectric ceramic capacitors, tunable phase shifters and PTCR. In this paper, the effect of SrTiO₃ concentration and sintering temperature on the microstructure and dielectric constant of Ba_1−xSr_xTiO₃ materials at the Curie temperature have been investigated. When Ba_1−xSr_xTiO₃ materials were sintered at 1350 °C, the peak value of the dielectric constant, ϵ_max, monotonically decreased with increasing SrTiO₃ concentration. At the sintering temperature of 1400 °C the dielectric constant maximum at the T_C increased with an increase in the x value, reaching the highest value at around x=0.4 and then decreased. As sintering temperature increased to 1450 °C, ϵ_max increased with increasing SrTiO₃ concentration up to x=0.6. The dielectric properties of Ba_1−xSr_xTiO₃ materials were discussed in terms of SrTiO₃ concentration and microstructure.     

High- and low-field dielectric responses and ferroelectric properties of (Bi0.5Na0.5)Zr1−xTixO3 ceramics

Bismuth sodium zirconate titanate (Bi_0.5Na_0.5)Zr_1?xTi_xO₃ with (x=0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) ceramics was fabricated by a conventional sintering technique at 850–1000 °C for 2 h. From X-ray diffraction study, three regions of different phases were observed in the ceramic system; i.e., orthorhombic phase region (0≤x≤0.2), mixed-phase region (0.3≤x≤0.4), and rhombohedral phase region (0.5≤x≤0.6). It was observed that the phase evolution from orthorhombic to rhombohedral symmetry resulted in a noticeable increase of the dielectric properties. The results from the high- and low-field dielectric responses indicated that the dielectric properties of both BNZ and BNZT ceramics were dominantly attributed to the reversible contribution. It was also noticed that grain size showed only partial influence on the increase of low-field dielectric constant in Ti-rich BNZT ceramic.     

Phase transitions and microwave dielectric behaviors of the (Bi1−xLi0.5xY0.5x)(V1−xMox)O4 ceramics

Microwave dielectric ceramics with intrinsic low sintering temperatures are potential candidates for low temperature co-fired ceramics technology. In the present work, the (Li_0.5Y_0.5)MoO₄ ceramic with tetragonal scheelite structures was selected to improve microwave dielectric properties of BiVO₄ ceramics. As proved by X-ray diffraction (XRD) results, scheelite structured solid-solution ceramics were formed with x value ≤0.1 in the (Bi_1−xLi_0.5xY_0.5x)(V_1−xMo_x)O₄. In situ XRD results further confirmed that the addition of (Li_0.5Y_0.5)MoO₄ also lowered transition temperature from distorted monoclinic to tetragonal scheelite structure. When x value increased further, zircon phase was detected by XRD. Room and high-temperature Raman spectra also supported the XRD results. Differences of thermal expansion coefficients of both monoclinic and tetragonal scheelite phases lead to an abnormality at phase transition temperature. Good microwave dielectric properties with permittivity above 70 and Qf (Q = quality factor = 1/dielectric loss and f = frequency) value above 8000 GHz were obtained in the (Bi_1−xLi_0.5xY_0.5x)(V_1−xMo_x)O₄ solid-solution ceramics with x value ≤0.1 sintered below 800°C. However, permittivity peak values at phase transition temperatures lead to large positive or negative temperature coefficient of resonant frequency, and this needs to be modified via composite technologies in the future.     

Microwave dielectric properties of novel temperature stable high Q Li2Mg1−xZnxTi3O8 and Li2A1−xCaxTi3O8 (A = Mg,Zn) ceramics

The Li₂Mg_1?xZn_xTi₃O₈ (x = 0–1) and Li₂A_1?xCa_xTi₃O₈ (A = Mg, Zn and x = 0–0.2) ceramics are synthesized by solid-state ceramic route and the microwave dielectric properties are investigated. The Li₂MgTi₃O₈ ceramic shows ?_r = 27.2, Q_u × f = 42,000 GHz, and τ_f = (+)3.2 ppm/°C and Li₂ZnTi₃O₈ has ?_r = 25.6, Q_u × f = 72,000 GHz, and τ_f = (?)11.2 ppm/°C respectively when sintered at 1075 °C/4 h. The Li₂Mg_0.9Zn_0.1Ti₃O₈ dielectric ceramic composition shows the best dielectric properties with ?_r = 27, Q_u × f = 62,000 GHz, and τ_f = (+)1.1 ppm/°C. The effect of Ca substitution on the structure, microstructure and microwave dielectric properties of Li₂A_1?xCa_xTi₃O₈ (A = Mg, Zn and x = 0–0.2) has also been investigated. The materials reported in this paper are excellent in terms of dielectric properties and cost of production compared to commercially available high Q dielectric resonators.     

Effects of Cr substitution and oxygen reduction on elastic anomaly and ultrasonic velocity in charge-ordered Nd0.5Ca0.5Mn1−xCrxO3−δ ceramics

Charge-ordered Nd_0.5Ca_0.5Mn_1?xCr_xO_3?δ ceramics have been investigated by electrical resistivity, AC susceptibility and ultrasonic velocity measurements to elucidate the effects of Cr substitution and oxygen reduction on charge ordering (CO). Resistivity and susceptibility measurements showed that the x=0 sample exhibits insulating behavior and an anti-ferromagnetic transition at 230 K as well as a CO transition at 280 K. The substitution of Cr induces a ferromagnetic–paramagnetic and metal–insulator (MI) transition as well as gradually suppressing the CO state due to weakening of the Jahn–Teller (JT) effect. Quenching to reduce the oxygen content of the x=0.05 sample caused the MI transition temperature to shift to lower temperatures most likely due to oxygen reduction. On the other hand, both longitudinal and shear velocities at 100 K increased significantly with Cr substitution indicating improvement in elastic properties. However, quenching the x=0.05 sample slightly deceased both velocities and related elastic moduli. A step-like longitudinal velocity anomaly characterized by a slope change suggests the existence of CO state for x=0, 0.02 and 0.05 samples. The step-like anomaly shifts to lower temperatures from 266 K (x=0) to 222 K (x=0.05) with Cr substitution indicating a weakening of the CO state. Absence of the step-like anomaly for the quenched x=0.05 sample suggests suppression of the CO state due to oxygen reduction. Analysis of the step-like anomaly using the mean-field theory suggests involvement of the JT effect which transforms from dynamic to static type with decreasing temperature. Cr substitution weakened the CO state as a result of weakening the JT effect while reducing the oxygen content suppresses the CO state as a result of oxygen reduction.     

The effect of morphotropic phase boundary on dielectric properties of Bi0.5Na0.5Ti1−xZrxO3 solid solutions

Lead-free bismuth sodium titanate zirconate (Bi_0.5Na_0.5Ti_1?xZr_xO₃ or BNTZ) solid solutions with varied composition of x=0.50, 0.55, 0.58, 0.60, 0.63, 0.65, 0.68, 0.70, 0.73, 0.75 and 0.78 mol fraction were obtained using a conventional mixed-oxide method. XRD analysis indicated that the increase in concentration of Zr led to compositions across morphotropic phase boundary region. A quantitative structural investigation was carried out using the X-ray powder diffraction data. The rhombohedral phase was found to dominate for x<0.68 with space group R3c. In the morphotropic phase boundary (MPB) region i.e. 0.68≤x≤0.75, it was demonstrated that coexistence of rhombohedral and orthorhombic phase was observed. For x=0.78, the phase was completely orthorhombic with space group Pmna. Furthermore, the dielectric properties showed some enhanced activity of dipole movement at MPB boundaries which supported the presence of MPB region in this material system.     

Structural,dielectric and ferroelectric properties of Ba1−x(Bi0.5Na0.5)xTiO3 ceramics

Lead free Ba_1?x(Bi_0.5Na_0.5)_xTiO₃ (x=0, 0.02, 0.04, 0.06, 0.08, 0.1) ferroelectric ceramics were synthesized by conventional solid state reaction technique. Sintering was done at 1200 °C for 2 h in air atmosphere. The final products have tetragonal symmetry with decreasing c/a ratio confirmed by X-ray diffraction analysis. The grain size varies between 300 nm to 1000 nm for x=0 to 0.1. With increase in Bi_0.5Na_0.5TiO₃ [BNT] content, the room temperature permittivity decreases whereas the Curie temperature (T_c) increases and its highest value was found to be 155 °C for 10 mol% of BNT addition. The ceramics show stable and low dielectric loss characteristics. The remnant polarization (P_r) and the coercive field (E_c) increases monotonously with increase in BNT content. The highest value of 2P_r (=17 μC/cm²) and 2E_c (=22 Kv/cm) was obtained for x=10 mol% BNT addition.     

Activation and deactivation of the commercial-type CuO–Cr2O3–Fe2O3 high temperature shift catalyst

This work elucidates the structural evolution of a commercial-type iron oxide-based high temperature water–gas shift (HT-WGS) catalyst during activation and deactivation stages. The findings highlight the importance of Cu–FeO _x interfaces. Based on the new insights, future improvement of commercial iron-based catalysts should focus on stabilization of the active Cu–FeO _x interface. Much effort has been devoted to understanding the structure, mechanism, and promotion of the commercial-type CuO–Cr₂O₃–Fe₂O₃ catalyst for the high temperature water–gas shift (HT-WGS) reaction. However, structural evolution of the catalyst during the activation and deactivation stages was rarely reported. Herein, catalyst characterization, temperature-programmed studies, and kinetic analysis were conducted on iron oxide-based HT-WGS catalysts. Addition of Cu was found to accelerate both the bulk (Fe₂O₃ → Fe₃O₄) and surface (active FeO _x–Cu interface) transformations during the catalyst activation stage. During catalyst deactivation, Cu accelerated both sintering of the Fe₃O₄ bulk phase and unfavorable encapsulation of the metallic Cu particles with a substantial FeO _x overlayer. The loss of the initial active Cu–FeO _x interfacial sites reversed the promotional effect of Cu.     

Phase-microstructure evolution and microwave dielectric characteristic of (1−x)(Sr0.5Ce0.5)TiO3+δ−xNdAlO3 solid solution

A perovskite solid solution (1−x)(Sr_0.5Ce_0.5)TiO_3+δ−xNdAlO₃, x = 0.1 to 0.4 was prepared by conventional solid state method. X-ray diffraction spectra revealed a single phase with tetragonal structure, indicating that doping of NdAlO₃ significantly stabilized the perovskite-like structure. The addition of NdAlO₃ facilitated the formation of large plate-like grains with porous microstructure. The dielectric constant (ε_r) decreased with increasing x because of the small ionic polarizability of NdAlO₃. The Q × f value was strongly dependent on the microstructure of these ceramics. The temperature coefficient of resonant frequency (τ_f) gradually shifted to near zero with a rise of x, which resulted from the decrease in tolerance factor (t). The solid solution with x = 0.4 sintered at 1550 °C for 4 h showed a good combination of dielectric properties: ε_r = 72, Q × f = 12052 GHz and τ_f = +5 pmm/°C.     

High dielectric permittivity and dielectric relaxation behavior in a Y2/3Cu3Ti4O12 ceramic prepared by a modified Sol−Gel route

In this work, Y_2/3Cu₃Ti₄O₁₂ ceramics were fabricated via a modified sol?gel route. Structural, dielectric, and electrical parameters were systematically investigated. The XRD results indicate that a CaCu₃Ti₄O₁₂ phase (JCPDS No. 75–2188) is present in every sintered sample. SEM images of Y_2/3Cu₃Ti₄O₁₂ ceramics disclose a fine-grained ceramic microstructure. Interestingly, high dielectric permittivity, ～6600–7600, with loss tangents of ～0.918–1.086 were achieved in the sintered Y_2/3Cu₃Ti₄O₁₂ samples. Density functional theory (DFT) calculations were used to investigate the most stable structure of the Y_2/3Cu₃Ti₄O₁₂ ceramics. Our DFT results reveal that two calcium vacancies (V_Ca) are isolated from each other. We also determined the lowest energy configuration of an oxygen vacancy (V_O) in the Y_2/3Cu₃Ti₄O₁₂ ceramics occurred during the sintering process. We found that the V_O is trapped close to the Y atom in this structure. Both computational and experimental studies specify that the oxygen vacancy is located close to the Y atom in the Y_2/3Cu₃Ti₄O₁₂ lattice and it might be a bivalent oxygen vacancy. As a result, due to charge balance, charge compensation of the transition ions, i.e., Cu and Ti ions, might take place. The charge compensation mechanisms in the Y_2/3Cu₃Ti₄O₁₂ lattice were verified using an XPS technique. Impedance spectroscopy confirms the presence of an inhomogeneous microstructure consisting of semiconducting grains and insulating grain boundaries in the sintered Y_2/3Cu₃Ti₄O₁₂ ceramics. This electrical result is consistent with the computational analysis, showing that a charge compensation mechanism might be involved in generation of the grains' semiconductive region due to the presence of a V_O. Consequently, high dielectric permittivity in Y_2/3Cu₃Ti₄O₁₂ may have originated from an internal barrier layer capacitor (IBLC) effect.     

A novel Na0.5K0.5NbO3–La(Zn0.5Ti0.5)O3 ceramics with excellent dielectric properties at wide temperature range

Ceramic-based dielectrics are considered as the best candidates for high temperature capacitors because of their outstanding mechanical and electrical properties. Nevertheless, conventional barium titanate-based capacitors show narrow operating temperature ranges owing to the low tetragonal-cubic phase transition temperature. In order to increase the working temperature and relative permittivity, a novel (1-x)Na_0.5K_0.5NbO₃- xLa(Zn_0.5Ti_0.5)O₃ (NKN-xLZT) ceramics were chosen to meet the targets in this work. The NKN-xLZT ceramics with sub-micrometer grains (0.2–0.4 μm) were synthesized via a conventional solid-state sintering route. A relative permittivity (ε’ = 1560 ± 15%) with low loss tangent over wide temperature range from 96 °C to 350 °C was obtained in the x = 0.02 ceramics. Additionally, the crystal structure distortion and conduction behaviors of the NKN-xLZT ceramics were systematically studied. The decrease of oxygen octahedron distortion induced a weak polarization, and the high resistance (9 × 10⁶ Ωcm at 400 °C) greatly suppressed the long-term migration of defective ions in the ceramics. Therefore, the low loss tangent and high permittivity were still stabilized at the high temperature. It believes that the NKN-xLZT ceramic system in this work will become one of the most promising candidates for high-temperature capacitor devices.     

The effect of B-site Y substitution on cubic phase stabilization in (Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ

The cubic phase mixed ionic-electronic conductor (Ba_0.5Sr_0.5)(Co_0.8Fe_0.2)O_3−δ (BSCF) is well-known for its excellent oxygen ion conductivity and high catalytic activity. However, formation of secondary phases impedes oxygen ion transport and consequentially a widespread application of BSCF as oxygen transport membrane. B-cation substitution by 1, 3 and 10 at.% Y was employed in this work for stabilization of the cubic BSCF phase. Secondary phase formation was quantified on bulk and powder samples exposed to temperatures between 640 and 1100°C with annealing time up to 44 days. The phase composition, cation valence states, and chemical composition of all samples were analyzed by high-resolution analytical electron microscopic techniques. Y doping effectively suppresses the formation of Ba_n+1Co_nO_3n+3(Co₈O₈) (n ≥ 2) and Co_xO_y phases which would otherwise act as nucleation centers for the highly undesirable hexagonal BSCF phase. This work validates for 10 at.% Y cation substitution perfect stabilization of the cubic BSCF phase at temperatures ≥800°C, while a negligible small volume fraction of the hexagonal BSCF phase was found at lower temperatures. A newly developed model describes the effect of Y doping on the formation of secondary phases and their effective suppression with increasing Y concentration.     

Phase evaluation during high temperature long heat treatments in the Y2O3–Al2O3–SiO2 system

The effect of high temperature long heat treatments on the microstructure of the eutectic glass composition in the Y₂O₃–Al₂O₃–SiO₂ system was examined. The qualitative and quantitative phase analyses were conducted by using scanning electron microscopy combined with energy dispersive analysis and electron microprobe. Simultaneous thermal analyses were carried out to determine the transition temperatures and enthalpies. The crystallization behavior of these glasses was monitored with X-ray powder diffraction. A needle like X-phase was observed in the structure of this eutectic composition after long heat treatment at 1350 °C.     

Structure and microwave dielectric properties of La(Mg0.5Ti0.5)O3–CaTiO3 system

(1−x)La(Mg_0.5Ti_0.5)O₃ (LMT)–xCaTiO₃ (CT) [0<x<1] ceramics were prepared from powder obtained by a nonconventional chemical route based on the Pechini method. The crystal structure of the microwave dielectric ceramics has been refined by Rietveld method using X-ray powder diffraction data. LMT and CT were found to form a solid solution over the whole compositional range. The 0.9LMT–0.1CT composition was refined using P2₁/n space group, which allows taking into account B-site ordering. The compounds having x⩾0.3 were found to be disordered and were refined using Pbnm space group. Microstructure evolution was also analysed. Dielectric characterization at microwave frequencies was performed on the LMT–CT ceramics. The permittivity and the temperature coefficient of resonant frequency of the solid solutions showed a non-linear variation with composition. The quality factor demonstrates a considerable decrease with the increase of CT content.     

Chemical stability and electrical properties of Nb doped BaCe0.9Y0.1O3−δ as a high temperature proton conducting electrolyte for IT-SOFC

BaCe_0.9?xNb_xY_0.1O_3?δ (where x=0, 0.01, 0.03 and 0.05) powders were synthesized by solid-state reaction to investigate the influence of Nb concentration on chemical stability and electrical properties of the sintered samples. The dense electrolyte pellets were formed from the powders after being uniaxially pressed and sintered at 1550 °C. The electrical conductivities determined by impedance measurements in temperature range of 550–750 °C in different atmospheres (dry argon and wet hydrogen) showed a decreasing trend with an increase of Nb content. For all samples higher conductivities were observed in the wet hydrogen than in dry argon atmosphere. The chemical stability was enhanced with increasing of Nb concentration. It was found that BaCe_0.87Nb_0.03Y_0.1O_3?δ is the optimal composition that satisfies the opposite demands for electrical conductivity and chemical stability, reaching 0.8×10^?2 S cm^?1 in wet hydrogen at 650 °C compared to 1.01×10^?2 S cm^?1 for undoped electrolyte.     

Improvement of the magnetoelectric response in NiFe2O4−Sr0.5Ba0.5Nb2O6 composites using LiNbO3 as sintering additive

Magnetoelectric (NiFe₂O₄)_0.3−(Sr_0.5Ba_0.5Nb₂O₆)_0.7 composites with addition of LiNbO₃ as sintering additive were prepared by a classical mixed-oxide method. XRD patterns of ceramics sintered between 1000 and 1200 °C show the desired Sr_0.5Ba_0.5Nb₂O₆ and NiFe₂O₄ phases. SEM investigations confirm the 0–3 connectivity of the composite ceramics. The addition of 10 and 20 mol% LiNbO₃ improves the densification of the composite ceramics and leads to an increase of the size of the Sr_0.5Ba_0.5Nb₂O₆ grains. Magnetic measurements show hystereses with low coercivities. Dielectric measurements were carried out depending on temperature and frequency. The samples with the LiNbO₃ addition show significantly higher resistivity values (σ_DC). Magnetoelectric measurements were carried out in dependence of the magnetic DC-field, temperature, and frequency. The maximum magnetoelectric coefficient (α_ME) rises with the addition of LiNbO₃ from 180 to 803 µV Oe⁻¹ cm⁻¹ (@900 Hz). Temperature dependent measurements show a continuously decreasing of α_ME with lower temperature.     

Ultra-fast charge-discharge and high energy storage density realized in NaNbO3–La(Mn0.5Ni0.5)O3 ceramics

Lead-free antiferroelectric (AFE) NaNbO₃ (NN) is one of promising materials for dielectric capacitors, but the recoverable energy-storage density and efficiency get restrained owing to huge remanent polarization and limited dielectric breakdown field strength. In this work, a variety of NN based lead-free bulk (1-x)NaNbO₃-xLa(Mn_0.5Ni_0.5)O₃ (abbreviated as (1-x)NN-xLMN, x = 0, 0.05, 0.10, 0.15, 0.20) ceramics were designed using a solid-state synthesis method. Remarkably, an ultra-fast charge-discharge speed 47 ns and an acceptable recoverable energy-storage density W_rec ~1.77 J/cm³ with a high efficiency η = 77% were obtained under the E_b of 200 kV/cm at x = 0.05. The superior energy storage performance is attributed to the regulation of domain size and voltage resistance by special ions substitution of A and B sites. This work not only proposes an efficient strategy to realize high recoverable energy-storage density and efficiency, but also provide an candidate material for application of advanced pulsed power capacitors.