Microstructures and microwave dielectric properties of Ba4LiNb3O12–BaWO4 composite ceramics

Composite ceramics of (1 ? x)Ba₄LiNb₃O₁₂–xBaWO₄ (x = 0.36–0.69) had been synthesized by co-firing the mixtures of Ba₄LiNb₃O₁₂ and BaWO₄ powders. The structures and microwave dielectric properties of the ceramics were studied by XRD, SEM, and TEM. These ceramics consisted of hexagonal Ba₄LiNb₃O₁₂ and tetragonal BaWO₄. The two phases co-existed well in the ceramics, and there was not obvious reaction at interfacial areas among grains. These ceramics had low sintering temperatures and excellent microwave dielectric properties, especially the small temperature coefficients of resonant frequency (τ_f) and high quality factor (Q × f) values. For composition at x = 0.69, the ceramic sintered at 1070 °C had Q × f value of 75,500 GHz and τ_f value of +8.7 ppm/°C.     

Microstructure and microwave dielectric properties of Ba(Mg1/3Nb2/3)O3–MgO composite ceramics with nano-silica added

Ba(Mg_1/3Nb_2/3)O₃–MgO composite ceramics were prepared by solid-phase method. The ceramic exhibited the 1:2 ordered structure. By adding a proper amount of MgO, the permittivity decreased rapidly compared with other Ba(Mg_1/3Nb_2/3)O₃-based ceramics. The Q?×?f values of the samples were greatly improved by nano-silica. Raman spectra showed that the permittivity and Q?×?f value showed a strong correlation with Raman shift and full width at half maximum of the A_1g(O) phonon mode, respectively. The Raman shift of A_1g(O) was consistent with the variation trend of permittivity. And the full width at half maximum of the A_1g(O) phonon mode had a negative correlation with the Q?×?f value. The results showed that upon adding 1.5 wt% nano-silica to the ceramics, the ceramics sintered at 1550 °C for 5 h had the lowest Raman shift and the narrowest full width at half maximum, achieving the best microwave dielectric properties: ε_r?=?22.22, Q?×?f?=?80,436 GHz, τ_f?=?–?5.89 ppm/°C.

     

Microwave dielectric properties of (1 − x)(Mg0.4Zn0.6)2SiO4–xCaTiO3 composite ceramics

Phase formation, microstructure and microwave dielectric properties of (1 ? x)(Mg_0.4Zn_0.6)₂SiO₄–xCaTiO₃ (MZS-C) composite ceramics synthesized by using the conventional solid-state method were systematically investigated. Three phase structure was observed in all samples by using X-ray diffraction and the back scattering electron images. Mg₂SiO₄ can form a solid solution with Zn₂SiO₄, which improved sinterability of the MZS-C composite ceramics. As the CaTiO₃ content was increased from 0.06 to 0.14, dielectric constant ε _r and temperature coefficient of resonant frequency τ _f values of the MZS-C ceramics sintered at 1,180 °C for 4 h increased from 6.74 to 8.35 and ?41.5 to ?6.46 ppm/°C, respectively. Zero τ _f value can be obtained by properly adjusting the x value of the (1 ? x)MZS–xC ceramics. With increasing content of CaTiO₃, densification temperatures of the composite ceramics were decreased. The composite ceramic with x = 0.14 sintered at 1,180 °C for 4 h exhibited excellent microwave dielectric properties of ε _r = 8.35, Q × f = 28,125 GHz and τ _f = ?6.46 ppm/°C.     

Phase structures and microwave dielectric properties of xCaTiO3–(1 − x)Sm0.9Nd0.1AlO3 ceramics

The xCaTiO₃–(1 ? x)Sm_0.9Nd_0.1AlO₃ (0.25 ≤ x ≤ 0.85) microwave dielectric ceramics were prepared by conventional solid state reaction method. The phase structures were characterized by X-ray diffraction, scanning electron microscope and Raman spectra. Solid solutions with the orthorhombic perovskite structure with octahedral tilting were formed from the x range of 0.25 to 0.85. Microwave dielectric properties of xCaTiO₃–(1 ? x)Sm_0.9Nd_0.1AlO₃ ceramics were investigated systematically. The optimum property of the ceramics was obtained for x = 0.65 sintered at 1,415 °C for 3 h: relative permittivity ε_r = 39.70, Q × f = 50,012 GHz, τ _f  = ?6.8 ppm/°C. xCaTiO₃–(1 ? x)Sm_0.9Nd_0.1AlO₃ ceramics provide a new promising material for application in the microwave technology with relative permittivity lying within the range from 39 to 44, high quality factor and near-zero τ _f .     

Microstructure and microwave dielectric properties of xSm(Mg0.5Ti0.5)O3–(1 − x)Ca0.8Sr0.2TiO3 ceramics

xSm(Mg_0.5Ti_0.5)O₃–(1 ? x)Ca_0.8Sr_0.2TiO₃ (x = 0.50–0.95) ceramics are prepared by a conventional solid-state ceramic route. The microstructure and microwave dielectric properties are investigated as a function of the x-value and sintering temperature. The single phase solid solutions were obtained throughout the studied compositional range. The variation of bulk density and dielectric properties are related with the x-value. Increasing sintering temperature can effectively promote the densification and dielectric properties of xSm(Mg_0.5Ti_0.5)O₃–(1 ? x)Ca_0.8Sr_0.2TiO₃ ceramic system. With the content of Sm(Mg_0.5Ti_0.5)O₃ increasing, the temperature coefficient of resonant frequency τ _f value decreased, and a near-zero τ _f could be obtained for the samples with x = 0.80. The optimal microwave dielectric properties with a dielectric constant ε _r of 30.1, Q × f of 115,000 GHz (at 8.0 GHz), and τ _f of 8.9 ppm/°C were obtained for 0.80Sm(Mg_0.5Ti_0.5)O₃–0.20Ca_0.8Sr_0.2TiO₃ sintered at 1,550 °C for 3 h, which showed high density and well-developed grain growth.     

Microstructure and dielectric properties of BaZr x Ti1−x O3 ceramics

The crystalline structure and dielectric properties of BaZr _x Ti_1−x O₃ ceramics with x = 0.05, 0.10, 0.15, and 0.20 were investigated. As zirconium increased, the a-axis lattice constant gradually increased, however, the c-axis lattice constant and c/a ratio gradually decreased. When x = 0.20, the crystal structures of the BZT ceramics are very close to cubic, different from the tetragonal structure when x < 0.20. The temperature dependence of the dielectric constant was studied and an enhanced diffuse phase transition behavior is found to be caused by the increased Zr content. The decreases of coercive electric field and remanent polarization were the result of increase of Zr/Ti ratio in BaZr _x Ti_1−x O₃.     

Microstructure and dielectric properties of Ba[Ti1 − x (Ln1/2Nb1/2) x ]O3 ceramics

We have studied the microstructure and dielectric properties of barium titanate-based ceramics containing niobium oxide and rare-earth (Nd, Sm, Gd, Dy, and Tm) oxide additions in a ratio needed for the formation of mixed perovskite solid solutions with the general formula Ba[Ti_{1 ? x} (Ln_1/2Nb_1/2) _x ]O₃. It was found that, after sintering at 1100–1120°C with the use of a zinc oxide-based sintering aid and manganese carbonate additions, the ceramics had a core-shell structure in which the core of the grains consisted of barium titanate and the shell consisted of a barium titanate-based solid solution. The average grain size of the major phase in the ceramics was within 0.7 μm. The ceramics contained additional phases in the form of inclusions which occasionally exceeded 5 μm in size. Their composition was determined. The Nd-, Sm-, and Gd-containing materials were shown to have the greatest potential as a base for the development of new engineering materials of stable groups with high dielectric permittivity for multilayer capacitors with electrodes containing 70% Ag and 30% Pd.     

Crystal structure and optimized microwave dielectric properties of (1 − x) LiZn0.5Ti1.5O4–xTiO2 ceramics for application in dielectric resonator

Microwave dielectric ceramics with the composition of (1?x) LiZn_0.5Ti_1.5O₄ (LZT)–xTiO₂ (0.05 ≤ x ≤ 0.4) were prepared by a solid-state reaction route. XRD patterns revealed that the samples consist of LiZn_0.5Ti_1.5O₄ and rutile TiO₂, and the amount of rutile TiO₂ phase increased with increasing the x values. The microwave measurements show that the dielectric properties of ceramics can be improved with increasing x values. When x = 0.1, the temperature coefficient of resonant frequency (τ _f ) of 0.9LZT–0.1TiO₂ ceramic can be adjusted to a near-zero value of ?1 ppm/°C, and permittivity (ε_r) and Q × f value are 26 and 45,000 GHz, respectively. These results indicate that 0.9LZT–0.1TiO₂ ceramic can be a candidate in microwave dielectric resonators.     

Microstructure and microwave dielectric characteristics of CaO–B2O3–SiO2 glass ceramics

CaO–B₂O₃–SiO₂ (CBS) glass powders are prepared by traditional glass melting method, whose properties and microstructures are characterized by Differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that the pure CBS glass ceramics possess excellent dielectric properties (ε _r = 6.5, tan δ = 5 × 10⁻³ at 10 GHz), but a higher sintering temperature (>900 °C) and a narrow sintering temperature range (about 10 °C). The addition of a low-melting-point CaO–B₂O₃–SiO₂ glass (LG) could greatly decrease the sintering temperature of CBS glass to 820 °C and significantly enlarge the sintering temperature range to 40 °C. The CBS glass ceramic with 30 wt% LG glass addition sintered at 840 °C exhibits better dielectric properties: ε _r ≈ 6, tan δ < 2 × 10⁻³ at 10 GHz, and the major phases of the sample are CaSiO₃, CaB₂O₄ and SiO₂.     

Microstructures and dielectric tunable properties of Ba0.5Sr0.5TiO3–MgO–Mg2TiO4 composite ceramics

Ba_0.5Sr_0.5TiO₃–MgO–Mg₂TiO₄ composite ceramics were prepared by a solid-state reaction method, and the dielectric tunable properties were investigated. It is observed that the addition of MgO–Mg₂TiO₄ into the Ba_0.5Sr_0.5TiO₃ forms ferroelectric (Ba_0.5Sr_0.5TiO₃)–dielectric (Mg₂TiO₄–MgO) composites. Increasing Mg₂TiO₄ content causes an increase of Curie temperature T_c towards room temperature and a decrease of dielectric constant peak ε_max. The dielectric constant and loss tangent of Ba_0.5Sr_0.5TiO₃–MgO–Mg₂TiO₄ composites have been reduced and the overall tunability is maintained at a sufficiently high level. With the increase of Mg₂TiO₄ content and the decrease of MgO content, the dielectric constant and tunability of Ba_0.5Sr_0.5TiO₃–MgO–Mg₂TiO₄ composite ceramics increase and the Q × f values decrease. Ba_0.5Sr_0.5TiO₃–Mg₂TiO₄–MgO composites have dielectric constant of 123.0–156.5 and tunability of 14.4–28.5 % at 10 kHz under 3.9 kV/mm, indicating that they are promising candidate materials for tunable microwave applications requiring a low dielectric constant.     

Temperature-stable and ultralow-loss (1 − x)CaSmAlO4–xSr2TiO4 microwave dielectric solid-solution ceramics

The (1???x)CaSmAlO₄–xSr₂TiO₄ (0.01?≤?x?≤?0.06) microwave dielectric ceramics were for the first time prepared by a conventional solid-state method. The phase composition, sintering behavior, microstructure and microwave dielectric properties were investigated as function of sintering temperature and composition. A single-phase solid solution with K₂NiF₄-type tetragonal structure was formed in the range of 0.02?≤?x?≤?0.06. All samples are well densified with very little pores. The addition Sr₂TiO₄ greatly improves dielectric properties originally by restraining the secondary phase and then deteriorates Q?×?f value because of decreasing tolerance factor. Moreover, the τ_f value dominated by the thermal expansion coefficient increases lineally as x rises. Finally, excellent microwave dielectric properties of ε_r?~?18.1, Q?×?f?~?140,433 GHz (8.5 GHz), and a near-zero τ_f?~??+?0.05 ppm/°C can be obtained in the x?=?0.05 ceramic sintered at 1425 °C.

     

Microstructure and microwave dielectric properties of MgNb2O6–ZnTa2O6 composite ceramics prepared by layered stacking method

ZnTa₂O₆/MgNb₂O₆/ZnTa₂O₆ layered ceramics with different volume fractions were designed and prepared to tailor the microwave dielectric properties of ZnTa₂O₆–MgNb₂O₆ composite ceramics. The crystal structure, microstructure and microwave dielectric properties of the composite ceramics were systematically investigated in this work. The formation of solid solutions at interface was confirmed by X-ray diffractometer. Scanning electron microscopy and energy dispersive spectroscopy were used to analysis the ionic diffusion behavior across the interface. The typical core–shell structure based on MgNb₂O₆ solid solution was clearly discerned in the composite ceramics. The ionic diffusion coefficients simulated by semi-infinite diffusion couple could give a reasonable explanation for the formation of the special structure. The tri-layered ZnTa₂O₆/MgNb₂O₆/ZnTa₂O₆ ceramic with the volume fraction of 6:1:6 had the optimum microwave dielectric properties of ε _r  = 31.98, Q × f = 82,778 GHz, and τ _f  = 0 ppm/°C, demonstrating a unique potential for low-loss microwave applications in resonators, filters and antenna substrates.     

Microstructure and properties of Ba1 − x La x TiO3 ceramics with submicron grain size

Submicron grain size Ba_{1 ? x} La _x TiO₃ (x = 0.025–0.05) barium titanate ceramics have been prepared by two-step heat treatment. The grain size of the ceramics was 200–400 nm at x = 0.025 and 300–700 nm at x = 0.05, and the ceramics had increased dielectric permittivity (?_max ?23 000 at x = 0.025 and ?_max ? 46 000 at x = 0.05) compared to large-grained (>1 μm) materials.