Relationships between crystal structure and microwave dielectric properties of Li2(Mg1−xCox)3TiO6 (0 ≤ x ≤ 0.4) ceramics

Li₂(Mg_1−xCo_x)₃TiO₆ (x = 0, 0.1, 0.2, 0.3, 0.4) ceramics were prepared using the conventional solid-state reaction method. Effects of Co-substitution on the crystal structure, sintering characteristics, micro–structures, microwave dielectric properties and Raman spectra were investigated. Co-substitution would affect the crystal structure and further influence the microwave dielectric properties. On the basis of the chemical bond theory and Rietveld refinement, some intrinsic factors such as the lattice energy, bond ionicity were calculated to explain the variations of the microwave dielectric properties. Ti–O and Li–O bonds played major role in influencing intrinsic factors and correlations between intrinsic factors of Ti–O, and Li–O bonds and microwave dielectric properties were established. With the increase of x values, the dielectric constant (ε_r) gradually increased, which could be explained by the variations of the polarizability, bond ionicity of Ti–O bonds, and Raman vibration modes. The decrease of the quality factor (Q·f) could be predicted by the decrease of the bond valence, packing fraction and lattice energy of Ti–O bonds. The temperature coefficient of resonant frequency (τ_ƒ) significantly correlated with the bond energy of Li–O bonds and thermal expansion coefficient of Ti–O bonds.     

Structures and microwave dielectric properties of Ca(1−x)Nd2x/3TiO3 ceramics

Ca_(1?x)Nd_2x/3TiO₃ microwave dielectric ceramics were prepared by the mixed oxide route; powders were calcined at 1100 °C and sintered at 1450–1500 °C. High density, single phase products were obtained for all compositions. Grain sizes ranged from 1 μm to 100 μm. There was evidence of significant discontinuous grain growth in mid range compositions; all ceramics were characterised by complex domain structures. With increasing Nd content there was a evidence of a transition from an orthorhombic Pbnm structure to a monoclinic C2/m structure. This was accompanied by a decrease in relative permittivity (?_r) from 180 to 78, and decrease in the temperature coefficient of resonant frequency (τ_f) from +770 ppm K^?1 to +200 ppm K^?1. The product of dielectric Q value and resonant frequency (Q × f) varied in a grossly non-systematic way, exhibiting a peak at 13,000 GHz in Ca_0.7Nd_0.2TiO₃.     

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.     

Dielectric properties of Ba1−xSrxTiO3 ceramics prepared by microwave sintering

A comparative study on the dielectric properties of Ba_1?xSr_xTiO₃ (x=0.1–0.6) ceramics prepared by microwave sintering (MS) and conventional sintering (CS) has been done. It was found that MS samples need lower temperature and much shorter time than CS samples to obtain the same degree of densification. Compared with CS samples, MS samples possessed smaller grain size, better densification and more uniform grain growth. The dielectric properties of the samples were measured as a function of temperature. It was observed that the dielectric constant was higher for MS samples compared with that of CS samples especially in the ferroelectric phase.     

SrCo1−xSbxO3−δ cathode materials prepared by Pechini method for solid oxide fuel cell applications

In this study, SrCo_1?ySb_yO_3?δ powders were prepared by a modified Pechini method. According to the study results, the cubic Pm3m phase of the SrCo_1?ySb_yO_3?δ ceramics was obtained as 10% of cobalt ions were substituted by antimony ions. Doping of Sb³⁺ ions appeared both to stabilize the Pm3m phase of the SrCo_1?ySb_yO_3?δ ceramics and to enhance densification and retard grain growth. The coefficient of thermal expansion of the SrCo_1?xSb_xO_3?δ ceramics increased with the content of the antimony ions, ranging from 10.17 to 15.37 ppm/°C at temperatures lower than the inflection point (ranging from 450 °C to 550 °C) and from 22.16 to 29.29 ppm/°C at higher temperatures. For the SrCo_0.98Sb_0.02O_3?δ ceramic, electrical conductivity reached a maximum of 507 S/cm at 450 °C. The ohmic and polarization resistances of the single cell with the pure SrCo_0.98Sb_0.02O_3?δ cathode at 700 °C read respectively 0.298 Ω cm² and 0.560 Ω cm². The single cell with the SrCo_0.98Sb_0.02O_3?δ-SDC composite cathode appeared to reduce the impedances with the R₀ and R_P at 700 °C reading respectively 0.109 Ω cm² and 0.127 Ω cm². Without microstructure optimization and measured at 700 °C, the single cells with the pure SrCo_0.98Sb_0.02O_3?δ cathode and the SrCo_0.98Sb_0.02O_3?δ-SDC composite cathode, demonstrated maximum power densities of 0.100 W/cm² and 0.487 W/cm². Apparently, SrCo_1?ySb_yO_3?δ is a potential cathode for use in IT-SOFCs.     

Structure and microwave dielectric properties of Ba1−xSrxMg2V2O8 ceramics

The Ba_1−xSr_xMg₂V₂O₈ (0≤x≤0.4) microwave dielectric ceramics were fabricated by a standard solid-state reaction method. The formation of a continuous solid solution within the whole composition range was identified. The ceramic samples could be well densified in the temperature range of 885–975 °C in air for 4 h. The permittivity ε_r was found to increase with increasing ionic polarizabilities. The Q×f values were believed to be closely related with packing fraction and grain refinement. The Sr²⁺ substitution contributed to a monotonous increase of the A-site bond valence, such that the τ_f value experienced a considerable variation from negative to positive values. The optimum microwave dielectric properties of an ε_r of 13.3, a high Qxf of 86,640 GHz (9.6 Hz) and a near-zero τ_f of −6 ppm/°C could be yielded in the x=0.15 sample when sintered at 915 °C for 4 h.     

High-temperature dielectric properties and microwave absorption abilities of Bi1−xMgxFeO3 nanoparticles

BiFeO₃ (BFO) multiferroic nanoparticles have attracted increasing attention owing to the coexistence of ferroelectric and ferromagnetic properties. In this work, Bi_1−xMg_xFeO₃ (x = 0.05, 0.1, 0.15) multiferroic nanoparticles were synthesized by the sol-gel method. The electromagnetic properties and microwave absorption performance in the temperature range of 323–723 K at X-band were investigated. The qualified bandwidth (absorption intensity < −10 dB) of the Mg-doped BFO materials covers the whole X-band at 673 K, suggesting promising candidates as high-temperature electromagnetic absorbers.     

Intrinsic factors affecting the microwave dielectric properties of Mg2Ti1−x(Mg1/3Sb2/3)xO4 ceramics

The effect of crystal structure on the microwave dielectric properties of Mg₂Ti_1−x(Mg_1/3Sb_2/3)_xO₄ (0.025≤x≤0.15) ceramics was investigated. A single phase having a cubic inverse spinel structure formed over the entire range of compositions, in specimens sintered at 1450 °C for 4 h. The structural characteristics of these ceramics were quantitatively evaluated by applying the Rietveld refinement method to the X-ray diffraction data. The largest bond strength between the cation and the oxygen ion and hence the highest quality factor (Qf) of the specimens were obtained at x=0.05. Although the ionic polarizability (3.29 Å³) of (Mg_1/3Sb_2/3)⁴⁺ was larger than that (2.93 Å³) of Ti⁴⁺, the dielectric constant (K) of the specimens decreased owing to the decrease of rattling effect with increasing x. In addition, the temperature coefficient of resonant frequency (TCF) decreased with decreasing K values. Typically, a high Qf value of 229,000 GHz was obtained for the specimens with x=0.05.     

Microstructures and microwave dielectric properties of (1 − y)La1−xSmx(Mg0.5Sn0.5)O3–yCa0.8Sm0.4/3TiO3 ceramics

The (1 ? y)La_1?xSm_x(Mg_0.5Sn_0.5)O₃–yCa_0.8Sm_0.4/3TiO₃ ceramics were prepared by the conventional solid-state method. The X-ray diffraction patterns of the La_1?xSm_x(Mg_0.5Sn_0.5)O₃ ceramics revealed that La_1?xSm_x(Mg_0.5Sn_0.5)O₃ is the main crystalline phase, which is accompanied by a little La₂Sn₂O₇ as the second phase. An apparent density of 6.59 g/cm³, a dielectric constant (?_r) of 19.9, a quality factor (Q × f) of 70,200 GHz, and a temperature coefficient of resonant frequency (τ_f) of ?77 ppm/°C were obtained when the La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃ ceramics were sintered at 1500 °C for 4 h. The temperature coefficient of resonant frequency (τ_f) increased from ?77 to +6 ppm/°C as y increased from 0 to 0.6 when the (1 ? y)La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃–yCa_0.8Sm_0.4/3TiO₃ ceramics were sintered at 1500 °C for 4 h. 0.425La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃–0.575Ca_0.8Sm_0.4/3TiO₃ ceramic that was sintered at 1500 °C for 4 h had a τ_f of ?3 ppm/ °C.     

Preparation and microwave dielectric properties of Ca0.6La0.8/3(SnxTi1−x)O3 ceramics

Ca_0.6La_0.8/3(Sn_xTi_1−x)O₃ ceramics were prepared via a conventional solid state reaction method, and the effect of Sn doping on their crystal phase structure and microwave dielectric properties was investigated. Results showed that Sn doping could hinder the formation of the rutile TiO₂ detrimental phase of Ca_0.6La_0.8/3TiO₃ ceramic. Also, the Q×f₀ value was enhanced and the τ_f value was lowered by Sn doping. The best microwave dielectric properties, i. e. ε_r=113 and Q×f₀=8487 GHz were obtained for a Sn doping content of 0.02. The mechanism of the improved properties deriving by Sn doping is discussed.     

Structural characteristics and microwave dielectric properties of Zn1−xBixVxW1−xO4-based ceramics for LTCC applications

Herein, the improvement of the microwave dielectric properties and sintering characteristics of Zn_1?xBi_xV_xW_1?xO₄(x = 0–0.15)-based ceramics is reported. The results showed that an appropriate amount of doping could not only reduce the optimum sintering temperature from 1100° to 900°C, but also enhance the densification of the microstructures and increase the Q×f value from 5351 to 42525 GHz. Additionally, various structural parameters including the phase composition, crystal structure, vibrational and chemical bond characteristics that are correlated with the dielectric properties were systematically investigated. By considering the chemical bond characteristics, the first-principles calculations and the acquired Raman spectra, the interaction between W-O is stronger than Zn-O in the ZnWO₄ structure, while the interaction between V-O is stronger than Bi-O in BiVO₄. Interestingly, when the Zn_0.97Bi_0.03V_0.03W_0.97O₄-based ceramics were sintered at 900 °C, improved microwave dielectric properties were acquired (ε_r =18.32, Q×f=42525 GHz, τ_f=?67.51 ppm/°C), which provides a promising candidate in low-temperature co-fired ceramics technology.     

Effect of ZnO on Mg2TiO4–MgTiO3–CaTiO3 microwave dielectric ceramics prepared by reaction sintering route

ZnO-doped Mg₂TiO₄–MgTiO₃–CaTiO₃ microwave dielectric ceramics were successfully prepared by the reaction sintering route. The compact samples consisted of MgTiO₃, Mg₂TiO₄ and CaTiO₃, which was confirmed by X-ray diffraction and energy-dispersive spectra. ZnO efficiently lowered the sintering temperature and promoted the densification, as well as the improvements in the dielectric constant and the quality factor. At the level of ZnO?=?1 wt-%, the ceramics exhibited optimum microwave dielectric properties: a dielectric constant of 20.3, a high quality factor of 64,740 GHz (at 9.9 GHz) and a near-zero temperature coefficient of resonant frequency (–1.3 ppm/^oC) after sintering at 1320^oC for 4 h.     

Phase composition,crystal structure and microwave dielectric properties of Mg2−xCuxSiO4 ceramics

In this work, the Mg_2-xCu_xSiO₄(x = 0–0.40) microwave dielectric ceramics were prepared using solid-state reaction method. Compared with the Mg₂SiO₄ sample, the Cu-substituted Mg samples could be sintered at a lower temperature. The Mg_2?xCu_xSiO₄ ceramics exhibit the composite phases of Mg₂SiO₄ and a small quantity of MgSiO₃. The Cu²⁺ ion presented a solid solution with the Mg₂SiO₄ phase and preferentially occupy Mg(1) site. The distortion of MgO₆ octahedron was modified by Cu²⁺ ions, resulting in a positive change in the temperature coefficient of resonance frequency (τ_f) values. Excellent microwave dielectric properties of ε_r = 6.35, high Qf of ～ 188,500 GHz and near zero τ_f = ?2.0 ppm/°C were achieved at x = 0.08 under sintering at 1250 °C for 4 h. Thus, the fabricated ceramics were considered as possible candidates for millimeter-wave device applications.     

Structural evolution,dielectric and energy storage properties of Na(Nb1−xTax)O3 ceramics prepared by spark plasma sintering

Phase pure NaNb_1−xTaxO₃ ceramics have been successfully prepared by spark plasma sintering (SPS) process at 950–1150 °C. The structural evolution, dielectric and energy storage properties as a function of Ta-doping level were studied by X-ray diffraction (XRD) Rietveld refinement, X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), Raman spectroscopy, scan electron microscopy (SEM) and ferroelectric analyzer. It was found that small level of Ta-doping induced the stabilization of ferroelectric phase from antiferroelectric NaNbO₃, while high level of doping induced further transition from ferroelectric to paraelectric phase. The doping of Ta led to the decrease in Tc and dielectric loss. The NTN4 and NTN6 composition exhibited enhanced dielectric response due to the coexistence of ferroelectric/antiferroelectric or paraelectric phases. The presence of oxygen vacancies in the sintered body were confirmed, which gave rise to the loss at high temperature. The doping of Ta decreased the concentration of oxygen vacancies. Maximum energy density of ∼0.9 J/cm³ could be obtained for x = 0.6 composition with the BDS of ∼160 kV/cm and efficiency of ∼87%.     

Microstructure and microwave dielectric properties of Li2Ti1−x(Zn1/3Nb2/3)xO3 ceramics

Li₂Ti_1?x(Zn_1/3Nb_2/3)_xO₃ (0≤x≤0.5) ceramics were prepared by a solid state ceramic route, and the phase purity, microstructure, and microwave dielectric properties were investigated. The XRD results suggest the formation of solid solutions for all studied compositions (0≤x≤5). The dielectric properties are strongly dependent on the compositions, the densifications and the microstructures of the samples. The Q×f value increases with x up to x=0.2 and then decreases with the further increase of x. The best microwave dielectric properties of ε_r=20.5, Q×f =75,257 GHz, and τ_f =15.4 ppm/°C could be obtained when x=0.2.     

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.     

Crystal structure and microwave dielectric properties of Li2Mg0.6− xCoxZn0.4SiO4 ceramic for LTCC applications

In this work, Li₂Mg_0.6−xCo_xZn_0.4SiO₄ ceramics (x = 0–0.4) added with 3 wt% Li₂O–B₂O₃–Bi₂O₃–SiO₂ (LBBS) glass were synthesised using the solid-state reaction method. The effects of substituting Co²⁺ for Mg²⁺in Li₂Mg_0.6−xCo_xZn_0.4SiO₄ ceramics on crystal structure, microstructure, densification, crystallisation and microwave dielectric properties were investigated. X-ray diffraction patterns showed that monoclinic Li₂MgSiO₄, monoclinic Li₂ZnSiO₄ and orthorhombic Li₂CoSiO₄ formed finite solid solution in Li₂Mg_0.6−xCo_xZn_0.4SiO₄ ceramics. Clear grain boundaries were observed via scanning electron microscopy. The substitution of Co²⁺ for Mg²⁺ increased grain size, densification, crystallinity degree and dielectric constant; it also reduced the dielectric loss of the ceramics to a certain extent. The absolute values of τ_f were positively related to the crystallinity degree. Li₂Mg_0.55Co_0.05Zn_0.4SiO₄ ceramic added with 3 wt% LBBS and sintered at 900 °C exhibited considerable microwave dielectric properties of ε_r = 5.8, Q × f = 47,518 GHz and τ_f = −74.8 ppm/°C. Therefore, the ceramic is considered a candidate low-temperature co-fired ceramic material for substrate and filter applications.     

Microstructure and microwave dielectric properties of (1 − x)Ca0.6La0.267TiO3–xCa(Mg1/3Nb2/3)O3 ceramics

(1 ? x)Ca_0.6La_0.267TiO₃–xCa(Mg_1/3Nb_2/3)O₃ ceramics were prepared by a conventional solid-state ceramic route. The microstructure and microwave dielectric properties were investigated as a function of composition and sintering temperature. As the content of Ca(Mg_1/3Nb_2/3)O₃ increased, the temperature coefficient of resonant frequency (τ_f) value decreased gradually. By appropriately adjusting the x value in the present ceramic system, a near-zero τ_f value could be achieved. The appropriate increase of sintering temperature could significantly improve Q·f value by influencing the grain growth. The optimal microwave dielectric properties with a dielectric constant (?_r) of 52.4, Q·f of 36,428 GHz (at 5.8 GHz), and τ_f of 3.4 ppm/°C were obtained for the specimen 0.5Ca_0.6La_0.267TiO₃–0.5Ca(Mg_1/3Nb_2/3)O₃ sintered at 1490 °C for 4 h.