Impedance spectroscopy,B-site cation ordering and structure–property relations of (1 − x) La[Al0.9(Mg0.5Ti0.5)0.1]O3–x CaTiO3 ceramics for 5G dielectric waveguide filters

Dense (1 ? x) La[Al_0.9(Mg_0.5Ti_0.5)_0.1]O₃–x CaTiO₃ ceramics were synthesized via solid-state reaction. The crystal structure and microwave dielectric properties of the ceramics were systematically investigated. Rietveld refinement revealed that when x ≤ 0.2, the ceramics had a rhombohedral structure with an R-3c space group. When x ≥ 0.5, the ceramics had an orthorhombic structure with a Pbnm space group. Selected area electron diffraction and Raman spectroscopy analyses proved that the microwave dielectric ceramics had a B-site order, which accounted for the great improvement in microwave dielectric properties. The content of oxygen vacancies was identified through X-ray photoelectron spectroscopy, and the change rule of Q × f was closely related to oxygen vacancy content. The perturbation of A-site cations had an important influence on dielectric constant. Specifically, with the increase in Ti⁴⁺ content, the perturbation effect of the A-site cations was enhanced and dielectric constant increased. When x = 0.65, the temperature coefficient of resonant frequency of the (1 ? x) La[Al_0.9(Mg_0.5Ti_0.5)_0.1]O₃–x CaTiO₃ microwave dielectric ceramics was near zero. The optimal microwave dielectric properties of 0.35LaAl_0.9(Mg_0.5Ti_0.5)_0.1O₃–0.65CaTiO₃ were ε_r = 44.6, Q × f = 32,057 GHz, and τ_f = +2 ppm/°C.     

Sintering behavior and microwave dielectric properties of 0.67CaTiO3–0.33LaAlO3 ceramics modified by B2O3–Li2O–Al2O3 and CeO2

A low temperature sintering method was used to avoid the relatively high sintering temperatures typically required to prepare 0.67CaTiO₃–0.33LaAlO₃ (CTLA) ceramics. Additionally, CeO₂ was introduced into the CTLA ceramics as an oxygen-storage material to improve their microwave dielectric properties. 0.67CaTiO₃–0.33LaAlO₃ ceramics co-doped with B₂O₃–Li₂O–Al₂O₃ and CeO₂ were prepared by a conventional two-step solid-state reaction process. The sintering behavior, crystal structure, surface morphology, and microwave dielectric proprieties of the prepared ceramic samples were studied, and the reaction mechanism of CeO₂ was elucidated. CTLA+0.05 wt% BLA+3 wt% CeO₂ ceramics sintered at 1360 °C for 4 h exhibited the optimal microwave dielectric properties: dielectric constant (ε_r)=45.02, quality factor (Q×f)=43102 GHz, and temperature coefficient of resonant frequency (τ_f)=2.1 ppm/°C. The successful preparation of high-performance microwave dielectric ceramics provides a direction for the future development and commercialization of CTLA ceramics.     

Temperature stable dielectric properties of Mg2TiO4–MgTiO3–CaTiO3 Ceramics Over a Wide Temperature range

The zero resonant frequency temperature coefficient (τ_f) of microwave dielectric ceramics (MWDCs) at high and low temperature have attracted great attention in the development of microwave communication equipment. In this work, the Mg₂TiO₄–MgTiO₃–CaTiO₃ (MMC) ceramics with meeting the application requirements of 5G communication were prepared by traditional solid-phase sintering after investigating the relationship among phase compositions of xMg₂TiO₄-(0.931-x)MgTiO₃-0.069CaTiO₃ and 0.34Mg₂TiO₄-0.591MgTiO₃-yCaTiO₃, sintering process, and dielectric properties in detail. The results show that the dielectric properties of MMC ceramics are strongly affected by the phase relative contents of MgTiO₃, Mg₂TiO₄ and CaTiO₃. For instance, MMC ceramics with approximate τ_f = 0 is contributed by mutual compensation of Mg₂TiO₄ and MgTiO₃, in which the Mg₂TiO₄ phase plays an important role in decreasing the τ_f value; and the increase of CaTiO₃ will greatly increase the ε_r value for MMC ceramics, while has a negative effect in the Q × f value. After three-phase regulation, the 0.32Mg₂TiO₄-0.611MgTiO₃-0.069CaTiO₃ microwave dielectric ceramic has a better dielectric temperature stability, associated with dielectric properties of ε_r = 19.7, Q × f = 55,400 GHz (at 8.43 GHz), τ_f- = 4.5 ppm/°C (?40 °C–25 °C), and τ_f+ = ?5.1 ppm/°C (25 °C–90 °C).     

Phase relation and microwave dielectric properties of xCaTiO3–(1 − x)TiO2–3ZnTiO3 multiphase ceramics

Microwave dielectric ceramics of xCaTiO₃–(1 − x)TiO₂–3ZnTiO₃ (x = 0.05–1.00) were prepared by the solid-state reaction method. The phase relations were investigated using X-ray powder diffraction. In all the studied range, the sintered ceramic was multiphase, which was also verified by scanning electron microscopy (SEM) observation, as well as the energy-dispersive X-ray spectroscopy (EDX) analysis. With the increase of x from 0.05 to 0.25, the amount of rutile phase decreases due to the formation of new Ca₂Zn₄Ti₁₆O₃₈ polytitanates. And with x increasing from 0.25 to 1.00, rutile phase disappears while CaTiO₃ phase increases, accompanying with a slight decrease of Ca₂Zn₄Ti₁₆O₃₈. Thus, it is considered that the preferential chemical reaction in the system enhanced the formation of the Ca₂Zn₄Ti₆O₃₈ compound, CaTiO₃ and rutile phases in the ceramics. Moreover, the microwave dielectric properties of the ceramics were investigated. The simulated dielectric properties of the ceramics were also calculated based on the empirical model. The simulated results and the experimental ones have similar trends, which show that the change of microwave dielectric properties is related to the change of the phase composition in the multiphase ceramics.     

Microwave Dielectric Properties of Novel Glass‐Free Low‐Firing Li2CeO3 Ceramics

Novel glass–free low temperature firing microwave dielectric ceramics Li₂CeO₃ with high Q prepared through a conventional solid‐state reaction method had been investigated. All the specimens in this paper have sintering temperature lower than 750°C. XRD studies revealed single cubic phase. The microwave dielectric properties were correlated with the sintering conditions. At 720°C/4 h, Li₂CeO₃ ceramics possessed the excellent microwave dielectric properties of ε_r = 15.8, Q × f = 143 700 (GHz), and τ_f  = ?123 ppm/°C. Li₂CeO₃ ceramics could be excellent candidates for glass‐free low‐temperature co‐fired ceramics substrates.     

Low-temperature sintered MgWO4–CaTiO3 ceramics with near-zero temperature coefficient of resonant frequency

The phases, microstructure, composition analysis and microwave dielectric properties of (1 ? x)MgWO₄–xCaTiO₃ ceramics with Li₂CO₃–4H₃BO₃ additions prepared by solid-state reaction method have been investigated by using X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy and advantest network analyzer. The τ_f of (1 ? x)MgWO₄–xCaTiO₃ were dependent on phase constitutions. The microwave dielectric properties of 0.91MgWO₄–0.09CaTiO₃ ceramics with Li₂CO₃–4H₃BO₃ were characterized, the results indicated that the ?_r and Q × f were associated with the sintering temperature and amount of Li₂CO₃–4H₃BO₃. The sintering temperature of ceramics was reduced to 950 °C from 1150 °C and τ_f was modified to 0 ppm/°C with good Q × f. Addition of 5.0 wt% Li₂CO₃–4H₃BO₃ in 0.91MgWO₄–0.09 CaTiO₃ ceramics sintered at 950 °C showed excellent dielectric properties of ?_r = 15.5, Q × f = 20,780 GHz (f = 7.1 GHz) and τ_f ～ 0 ppm/°C. The material has a chemical compatibility with silver, making it a very promising candidate materials for LTCC applications.     

Structure refinement,defects evolution and calcium doping of Sr2CeO4 microwave dielectric ceramics with high quality factor

Sr_2-2xCa_2xCeO₄ (x = 0, 0.025, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8) ceramics were synthesized through cold isostatic pressing and solid-state reaction. The microstructure, defects, microwave dielectric properties, and the effect of Ca²⁺ doping of Sr₂CeO₄ ceramics were systematically investigated. As the sintering temperature increased, the densities of Sr₂CeO₄ ceramics rose, the content of oxygen vacancies increased, and Ce⁴⁺ reduction would be enhanced. In addition, the Sr₂CeO₄ structure had poor compatibility with Ca²⁺. The major phase could be kept unchanged only when x ≤ 0.1. The reason was that the doping of Ca²⁺ intensified the distortion of the CeO₆ octahedron and induced the structural transformation of the common edges (Sr₂CeO₄) to the common angles (SrCeO₃). With the increase of dopant, the densities of Sr_2-2xCa_2xCeO₄ ceramics increased significantly, while the content of oxygen vacancies also increased. The microwave dielectric properties were mainly influenced by the density, structural symmetry, defects, and the second phase SrCeO₃. The dielectric permittivity (ε_r) of 13.4–15, the quality factor (Qf) of 118,580–52,170 GHz, and the temperature coefficient of resonant frequency (τ_f) of ?58.3 ～ ?47.5 ppm/°C were obtained for Sr_2-2xCa_2xCeO₄ ceramics When x ≤ 0.1. This work has provided a foundation for further research on cerate microwave dielectric ceramics.     

Effect of ZnO doping on the microstructure and microwave dielectric properties of 0.2CaTiO3-0.8(Li0.5Sm0.5)TiO3 ceramics

0.2CaTiO₃-0.8(Li_0.5Sm_0.5)TiO₃-xZnO(x = 0, 0.3, 0.6, 0.9, 1.2 wt%, 0.2CT-0.8LST-xZnO) with orthogonal perovskite structure were fabricated by the solid state method. The effects of ZnO additives on the microwave dielectric properties of 0.2CT-0.8LST ceramics were systematically investigated. With increasing the dopant (x) concentration, the dielectric constant (ε_r) and the temperature co-efficient of resonance frequency (τ_f) decreased, however, the Q × f values increased. The relationship between vibration mode and microwave dielectric properties was studied using Raman spectroscopy. The Q × f value of ceramics was related to the half-height width of Raman scattering. Narrower Raman scattering peaks corresponded to longer microwave energy propagation decay times and higher Q × f value. Based on X-ray photoelectron spectroscopy (XPS), the addition of Zn²⁺ ions limited the reduction of Ti⁴⁺ cations. The excellent dielectric properties were obtained when x = 1.2 wt% with ε_r = 100.25, Q × f = 6525 GHz, and τ_f = ?12.12 ppm/°C.     

Effects of CaTiO3 addition on the densification and microwave dielectric properties of BiSbO4 ceramics

In this study, the effects of CaTiO₃ addition on the sintering characteristics and microwave dielectric properties of BiSbO₄ were investigated. Pure BiSbO₄ achieved a sintered density of 8.46 g/cm³ at 1100 °C. The value of sintered density decreased with increasing CaTiO₃, and sintering at a temperature higher than 1100 °C led to a large weight loss (>2 wt%) caused by the volatile nature of the compound. Samples either sintered above 1100 °C or with a CaTiO₃ content exceeding 3 wt% showed poor densification. SEM micrographs revealed microstructures with bimodal grain size distribution. The size of the smaller grains ranged from 0.5 to 1.2 μm and that of the larger grains between 3 and 7 μm. The microwave dielectric properties of the (1−x) BiSbO_4−x CaTiO₃ ceramics are dependent both on the x value and on the sintering temperature. The 99.0 wt% BiSbO₄–1.0 wt% CaTiO₃ ceramic sintered at 1100 °C reported overall microwave dielectric properties that can be summarized as ε_r≈21.8, Q×f≈61,150 GHz, and τ_f≈−40.1 ppm/°C, all superior to those of the BiSbO₄ ceramics sintered with other additives.     

Dielectric properties of B2O3-doped 0.98CeO2–0.02CaTiO3 ceramics at microwave frequency

Microwave dielectric properties and microstructure of 0.98CeO₂–0.02CaTiO₃ ceramics with B₂O₃ additions prepared with the conventional solid-state route have been investigated. 0.98CeO₂–0.02CaTiO₃ ceramics can be sintered at 1290 °C for 4 h due to the sintering aid effect resulting from the B₂O₃ additions. At sintering temperature of 1380 °C for 4 h, 0.98CeO₂–0.02CaTiO₃ ceramics with 0.25 wt% B₂O₃ addition possess a dielectric constant (?_r) of 21.3, a Q × f value of 60,000 (at 8 GHz) and a temperature coefficient of resonant frequency (τ_f) of −41 ppm/°C.     

Effect of nonstoichiometry on the structure and microwave dielectric properties of Ba(Co1/3Nb2/3)O3 ceramics

The effect of B-site cation deficiency on the structure and microwave dielectric properties of Ba(Co_1/3Nb_2/3)O₃ (BCN) was investigated. Stoichiometric and co-deficient compositions based on Ba(Co_1/3−xNb_2/3)O₃ [x = 0.0, 0.01, 0.02, 0.03 and 0.04] were prepared using the conventional mixed oxide route. Small amounts of V₂O₅ (0.1 wt%) were added to promote densification. The dielectric loss is very sensitive to the composition; it was found that co-deficiency degraded the microwave dielectric properties. The stoichiometric formulation (x = 0) exhibited the best microwave properties. The improvements in the microwave dielectric properties were achieved by increasing the degree of 1:2 cation ordering. The highly ordered, stoichiometric BCN ceramics showed a relative permittivity (ɛ_r) of 32, quality factor (Q × f) of 66,500 GHz and a negative temperature coefficient of resonant frequency (τ_f) of −10 ppm/°C at 4 GHz.     

Property optimization of Ba0.4Sr0.6TiO3–BaMoO4 composite ceramics for tunable microwave applications

(1 ? x)Ba_0.4Sr_0.6TiO₃–xBaMoO₄ ceramics with x = 5, 10, 20, 30, 40 and 60 wt% were prepared by traditional solid-state reaction method. Two crystalline phases, a cubic perovskite structure Ba_0.4Sr_0.6TiO₃ (BST) and a tetragonal scheelite structure BaMoO₄ (BM) were obtained by XRD analysis. The microwave dielectric properties of Ba_0.4Sr_0.6TiO₃–BaMoO₄ composite ceramics were investigated systematically. The results show that the composite ceramics exhibited promising microwave properties. The dielectric constant can be adjusted in the range from 900 to 78, while maintaining relatively high tunability from 27.3% to 12.8% under a direct current electric field of 60 kV/cm and Q values from 619 to 67 in the gigahertz frequency region.     

Structure and microwave dielectric properties of 1:1 ordered Nd[(Mg1-xZnx)1/2Ti1/2]O3 complex perovskite ceramics

Nd[(Mg_1-xZn_x)_1/2Ti_1/2]O₃ perovskite ceramics (x = 0, 0.2, 0.4, 0.6, 0.8) are prepared by the solid-state reaction method. The effects of Zn²⁺ substitution on the structure, microstructure, especially the B-site 1:1 cation ordering and microwave dielectric properties have been investigated. Sintered Nd[(Mg_1-xZn_x)_1/2Ti_1/2]O₃ ceramics all adopt dense microstructure, along with increased dimensional uniformity as Zn²⁺ substitution. All the ceramics are confirmed to have B-site 1:1 ordered monoclinic perovskite structure with P2₁/n space group. Atomic mass difference of B-site elements might be an important factor affecting the B-site 1:1 cation ordering. HRSTEM observation suggest that the doped Zn²⁺ cations have roughly entered the Mg²⁺ sites to promote 1:1 cation ordering. The degree of the 1:1 cation ordering can be negatively reflected by the full width at half maximum (FWHM) of F_2g(B) mode at 372 cm^?1 in Raman spectra. With Zn²⁺ doping, the degree of the 1:1 cation ordering first increases then decreases, and reaches its maximum at x = 0.6. Meanwhile the best combination of microwave dielectric properties is obtained, as ε_r = 31.4, Q × f = 74,000 GHz, τ_f = ?44 ppm/°C. It is found that the long-range ordering not only decreases the dielectric loss but also affects the dielectric constant, providing a theoretical foundation to understand further the correlation between ionic configuration and microwave dielectric properties.     

Structure and Microwave Dielectric Characteristics of Ca[(Ga1/2Nb1/2)1−xTix]O3 Ceramics

The microwave dielectric characteristics of Ca[(Ga_1/2Nb_1/2)_1?xTi_x]O₃ ceramics were investigated together with the structure evolution. The excellent microwave dielectric characteristics were achieved by forming solid solution between Ca(Ga_1/2Nb_1/2)O₃ and CaTiO₃ in the present ceramics. The solid solutions in space group Pbnm with antiphase and inphase tilting were determined for all compositions where minor secondary phase was detected for x = 0–0.47, whereas no B‐site ordering was detected. Owing to the structural modification, the dielectric constant (ε_r) increased with increasing x, and the temperature coefficient of resonant frequency (τ_f) could be tuned from negative to positive, while the decrease of Qf value was acceptable. The best combination of microwave dielectric properties was obtained at x = 0.47: ε_r = 51.6, Qf = 34 100 GHz and τ_f = ?0.3 ppm/°C.     

Microwave dielectric characteristics and finite element analysis of MgTiO3–CaTiO3 layered dielectric resonators

MgTiO₃ and CaTiO₃ ceramics were stacked in different schemes to yield the layered dielectric resonators, and the microwave dielectric characteristics were evaluated with TE_0 1 1 resonant mode. With increasing the thickness fraction of CaTiO₃, the measured resonant frequency and Q_f value decreased, while the effective dielectric constant and temperature coefficient of resonant frequency increased. The stacking scheme also had significant effect on the microwave dielectric properties. Finite element method was used to predict the microwave dielectric characteristics of the layered dielectric resonators, and the predicted results indicated good agreements with the experimental ones. The temperature-stable resonators could be attained by adjusting the thickness fractions of CaTiO₃ since MgTiO₃ and CaTiO₃ had reverse temperature coefficients of resonant frequency.     

Broad sintering temperature range and stable microwave dielectric properties of Mg2TiO4–CeO2 composite ceramics

There has been extensive research on microwave dielectric materials considering their application in 5G and 6G communication technologies. In this study, the sintering temperature range of Mg₂TiO₄–CeO₂ (MT-C) ceramics was broadened using a composite of CeO₂ and Mg₂TiO₄ ceramics, and their microwave dielectric performance was stabilized. Low-loss MT-C composite ceramics were prepared using the solid-phase reaction method, and their microwave dielectric properties, microscopic morphologies, and phase structures were investigated. The proposed MT-C ceramics contained Mg₂TiO₄ and CeO₂ phases; their average grain size was maintained at 2–4 μm in the sintering temperature range of 1275–1425 °C, and the samples were uniformly dense without porosity. The cross-distribution of Mg₂TiO₄ and CeO₂ grains in the samples inhibited the growth of ceramic grains, providing uniform and dense surfaces. The dielectric loss of MT-C ceramics remained constant in the temperature range of 1300–1425 °C at 9 × 10^?4 (8.45 ≤ f ≤ 8.75 GHz). As opposed to the base material, MT-C ceramics are advantageous owing to their wide sintering temperature range and the stable microwave dielectric properties, and there are suitable substrate materials for further industrial applications.     

Effects of CaTiO3 addition on microstructures and electrical properties of Na0.52K0.48NbO3 lead-free piezoelectric ceramics

In this article, various amounts of CaTiO₃ (CT) were added into (Na_0.52K_0.48)NbO₃ (NKN) ceramics using conventional oxide-mixing method for improving NKN's properties. The experimental results show that the (1?x)(Na_0.52K_0.48)NbO₃–xCaTiO₃ (x=0～0.07) solid solution system can be successfully synthesized. Addition of CaTiO₃ not only effectively prevents materials from deliquescence, but also improves the density and the electrical properties of the ceramics. The dielectric constant–temperature (ε_r?T) curves exhibit that the temperatures of the Curie point (T_c) and the phase transition from tetragonal to orthorhombic (T_O?T) are decreasing monotonously as the amount of CT addition is increased. A morphotropic phase boundary (MPB) can be found in the (1?x)NKN?xCT solid solution system as the doping amount of x=0.03, and the 0.97NKN–0.03CT ceramics, with a high bulk density, 98% theoretical density, and an appropriate grain size of about 1～2 μm, present a superior domain switching ability and the optimum properties: d₃₃=117 pC/N, k_p=0.39, P_r=21 μC/cm², and T_c=333 °C.     

Effect of Ca substitution on the microwave properties and the inhibition of Ag diffusion in Ba1-xCaxMoO4 ceramics

Ba_1-xCa_xMoO₄ (0 ≤ x ≤ 0.20) ceramics were prepared from powders to form solid solutions by a solid-state reaction sintering process. The influence of the Ca²⁺ content on the microstructure, sintering, densification, microwave dielectric properties and chemical stability of BaMoO₄-based ceramics with Ag metal was discussed in detail. The sintering temperatures of the Ba_1-xCa_xMoO₄ ceramics were effectively reduced to less than 950 °C by the formation of the solid solutions. Structural analysis indicates that the Ba_1-xCa_xMoO₄ ceramics belong to the class of tetragonal scheelites. The crystal grain size begins to decrease and become more regular as x increases from 0 to 0.12. However, as x continues to increase, a liquid phase begins to appear, and the grain boundaries are no longer clear. The ε_r value increases from approximately 8.6 to 9.8 as x increases from 0 to 0.2. The Ba_0.92Ca_0.08MoO₄ sample possesses the best microwave dielectric performance, namely an ε_r = 9.3 and the maximum Q × f value of 33593 GHz. The addition of 15 wt% TiO₂ or 10 wt% CaTiO₃ can effectively change the τ_f values of the Ba_1-xCa_xMoO₄ ceramics to approximately 0. The Ba_1-xCa_xMoO₄ ceramic samples can coexist with silver during the LTCC cofiring process.     

Ultra-high quality factor and low dielectric constant of (Zn0.5Ti0.5)3+ co-substituted MgAl2O4 ceramic

In this study, MgAl₂O₄-based ceramics with high quality factor (Qf) and low dielectric constant (ε_r ≤ 10) were obtained by fabricating MgAl_2-x(Zn_0.5Ti_0.5)_xO₄ (x = 0–0.5) ceramics via conventional solid-state reaction method. Excellent microwave dielectric properties were achieved for samples at x = 0.5 and sintered at 1550 °C, i.e., ε_r = 9.86, Qf = 263 900 GHz (five times better than that for x = 0 sample) and τ_f = ?92 ppm/°C. The X-ray diffraction (XRD) patterns displayed characteristic peaks of MgAl₂O₄ with spinel structure. MgTi₂O₅ and MgTiO₃ were considered as secondary phases. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and relative density analysis indicated that ultra-high Qf values were dominated by dense microstructure, secondary phase and cation vacancies; whereas ε_r values were mainly affected by secondary phase and ionic polarizability. MgAl_2-x(Zn_0.5Ti_0.5)_xO₄ ceramics with excellent microwave dielectric properties have potential application in millimeter-wave communication, dielectric filters, dielectric antennas and resonators.     

The structure and properties of 0.95MgTiO3–0.05CaTiO3 ceramics co-doped with ZnO–ZrO2

The (Mg_0.93Ca_0.05Zn_0.02)(Ti_1?xZr_x)O₃ ceramics were prepared by conventional solid-state route. The dielectric properties and structure of (Mg_0.93Ca_0.05Zn_0.02)(Ti_1?xZr_x)O₃ ceramics were investigated. It has been found that MgTiO₃ and CaTiO₃ are the main phases and a second phase CaZrTi₂O₇ appeared in 95MCT ceramics co-doped with Zn–Zr. With Zn–Zr additive, the sintering temperature of 95MCT ceramics can be reduced to 1300 °C, and adjust the temperature coefficient of dielectric constant. With the increasing of Zr content, dielectric constant ?_r decrease from 22.6 to 19.91 and the temperature coefficient of dielectric constant α_c from 5.93 to 2.52 ppm/°C when x = 0.01, 0.02, 0.03 and 0.04 mol respectively. The 95MCT ceramics with x = 0.02 has a dielectric constant ?_r of 22.02, a dielectric loss of 2.78 × 10^?4 and a temperature coefficient of dielectric constant α_c value of 2.98 ppm/°C.