Preparation, structure and dielectric properties of Ba4LaMNb3O15 (M = Ti, Sn ) ceramics

Two new cation-deficient hexagonal perovskites Ba₄LaMNb₃O₁₅ (M = Ti, Sn) ceramics were prepared by high temperature solid-state reaction route. The phase and structure of the ceramics were characterized by X-ray diffraction, scanning electron microscopy (SEM). The microwave dielectric properties of the ceramics were studied using a network analyzer. The Ba₄LaTiNb₃O₁₅ has high dielectric constant of 52, high quality factors (Q) 3500 (at 4.472 GHz), and temperature variation of resonant frequency (τ_f) +93 ppm °C⁻¹ at room temperature; Ba₄LaSnNb₃O₁₅ has dielectric constant of 39 with high Q value of 2510 (at 5.924 GHz), and τ_f −29 ppm °C⁻¹.     

Microwave dielectric properties and microstructures of CuO- and ZnO-doped LaAlO3 ceramics

The microwave dielectric properties and the microstructures of 0.25 wt.% CuO-doped LaAlO₃ ceramics with ZnO additions have been investigated. The sintered LaAlO₃ ceramics are characterized by X-ray diffraction spectra and scanning electron microscopy (SEM). Tremendous reduction in sintering temperature can be achieved with the addition of sintering aids CuO and ZnO. The ceramic samples show that dielectric constants (ε_r) of 22−24 and Q×f values of 33,000−57,000 (at 9.7 GHz) can be obtained at low sintering temperatures 1340−1460°C. The temperature coefficient of resonant frequency varies from −24 to −48 ppm/°C. At the level of 0.25 wt.% CuO and 1 wt.% ZnO additions, LaAlO₃ ceramics possesses a dielectric constant (ε_r) of 23.4, a Q×f value of 57,000 (at 9.7 GHz) and a τ_f value of −38 ppm/°C at 1400°C for 2 h.     

Improved high-Q microwave dielectric resonator using CuO-doped MgNb2O6 ceramics

The microwave dielectric properties and the microstructures of MgNb₂O₆ ceramics with CuO additions (1-4 wt.%) prepared with conventional solid-state route have been investigated. The sintered samples exhibit excellent microwave dielectric properties, which depend upon the liquid phase and the sintering temperature. It is found that MgNb₂O₆ ceramics can be sintered at 1140 °C due to the liquid phase effect of CuO addition. At 1170 °C, MgNb₂O₆ ceramics with 2 wt.% CuO addition possesses a dielectric constant (ε_r) of 19.9, a Q×f value of 110,000 (at 10 GHz) and a temperature coefficient of resonant frequency (τ_f) of −44 ppm/°C. The CuO-doped MgNb₂O₆ ceramics can find applications in microwave devices requiring low sintering temperature.     

Effect of glass fillers in Cu2ZnNb2O8 ceramics for advanced microwave applications

The effect of glass addition on sintering temperature and microwave dielectric properties of Cu₂ZnNb₂O₈ (CZN) is investigated for possible low temperature co-fired ceramic (LTCC) application. The CZN ceramic was prepared by the solid-state ceramic route. The phase formation, microstructure and elemental composition of the ceramics were studied using X-Ray Diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy and Energy Dispersive Analysis. The CZN sintered at 975 °C/4 h has ?_r = 15.2, tan δ = 0.0007 (at 5.1 GHz) and τ_f of −98 ppm °C⁻¹ and CTE = 1.9 ppm °C⁻¹. The addition of LBS and LMZBS glasses lowered the sintering temperature of CZN to below the melting point of silver. The 1 wt% LBS added CZN sintered at 935 °C/4 h had ?_r = 14.7, tan δ = 0.001 (at 5.1 GHz), τ_f = −19 ppm °C⁻¹ and CTE = −0.6 ppm °C⁻¹. The addition of 0.7 wt% LMZBS to CZN and sintered at 935 °C/4 h had ?_r = 14.8, tan δ = 0.002 (at 5.1 GHz), τ_f = −39 ppm °C⁻¹ and CTE = −0.9 ppm °C⁻¹.     

Low-temperature sintering of (Zn0.8 Mg0.2)2SiO4-TiO2 ceramics

Effect of Li₂O-B₂O₃-SiO₂ (LBS) glass on the sintering behavior and the microwave dielectric properties of (Zn_0.8 Mg_0.2)₂SiO₄-TiO₂ (ZMST) ceramics were investigated. The Li₂O-B₂O₃-SiO₂ glass lowered the sintering temperature of ZMST ceramics effectively from 1250 to 870 °C. The unknown second phase, which was formed in the ZMST ceramics increased with the addition of LBS glass. With increasing the LBS glass content, the bulk density, dielectric constant (ε_r) and the maximum Q × f value decreased, and the temperature coefficient of resonant frequency (τ_f) shifted to a negative value. (Zn_0.8 Mg_0.2)₂SiO₄-TiO₂ ceramics with 3 wt.% Li₂O-B₂O₃-SiO₂ glass sintered at 870 °C for 2 h shows excellent dielectric properties: ε_r = 8.48, Q × f = 11500 GHz, and τ_f = 0 ppm/°C.     

Dielectric properties of (1−x)(Mg0.95Co0.05)TiO3-xCaTiO3 ceramic system at microwave frequency

The microwave dielectric properties and the microstructures of the (1−x)MgTiO₃-xCaTiO₃ ceramic system were investigated. With partial replacement of Mg by Co, dielectric properties of the (1−x)(Mg_0.95Co_0.05)TiO₃-xCaTiO₃ ceramics can be promoted. The microwave dielectric properties are strongly correlated with the sintering temperature. At 1275°C, the 0.95(Mg_0.95Co_0.05)TiO₃-0.05CaTiO₃ ceramics possesses excellent microwave dielectric properties: a dielectric constant ε_r of 20.3, a Q×f value of 107 000 ( at 7 GHz) and a τ_f value of −22.8 ppm/°C. By appropriately adjusting the x value in the (1−x)(Mg_0.95Co_0.05)TiO₃-xCaTiO₃ ceramic system, zero τ_f value can be achieved. With x=0.07, a dielectric constant ε_γ of 21.6, a Q×f value of 92 000 (at 7 GHz) and a τ_f value of −1.8 ppm/°C was obtained for 0.93(Mg_0.95Co_0.05)TiO₃-0.07CaTiO₃ ceramics sintered at 1275°C for 4 h.     

Preparation,phase structure and microwave dielectric properties of a new low cost MgLi2/3Ti4/3O4 compound

A novel Li-based spinel compound with the composition of MgLi_2/3Ti_4/3O₄ was synthesized by the conventional solid-state reaction method. The phase structure was studied by X-ray diffraction (XRD) technique. When the calcination temperature was over 1050 °C, a single phase compound which has a cubic structure [Fd-3m (227)] with cell parameters of a = 8.4057 Å, V = 593.91 Å³, ρ = 3.51 g cm⁻³ and Z = 8 was obtained. MgLi_2/3Ti_4/3O₄ ceramic could be well densified after sintering above 1125 °C. The microwave dielectric properties were measured using a microwave vector network analyzer in the frequency range of 7–9 GHz MgLi_2/3Ti_4/3O₄ ceramic sintered at 1125 °C for 2 h showed microwave dielectric properties of ?_r = 20.2, Q × f = 62,300 GHz, and τ_f = −27.1 ppm °C⁻¹. Furthermore, 0.95MgLi_2/3Ti_4/3O₄–0.05CaTiO₃ ceramic sintered at 1200 °C for 2 h exhibited good properties of ?_r = 22.6, Q × f = 48,000 GHz, and τ_f = −2.3 ppm °C⁻¹.     

Low-temperature sintered ZnNb2O6–CaTiO3 ceramics with near-zero τf

The phases, microstructure and microwave dielectric properties of ZnNb₂O₆–xCaTiO₃ ceramics with BaCu(B₂O₅) glass additions prepared by solid state reaction method were charactered by using X-ray diffraction, Scanning electron microscopy and Advantest network analyzer. The τ_f of ZnNb₂O₆ was modified to near 0 ppm °C⁻¹ by incorporating CaTiO₃ with opposite τ_f values on the basis of Lichtenecker empirical rule. The microwave dielectric properties of ZnNb₂O₆–xCaTiO₃ (x = 8.0 wt.%) samples with BaCu(B₂O₅) glass additives sintered in 900–1000 °C were investigated, and the results indicated that the behaviors of the ε_r and Q × f were associated with the sintering temperature and the amount of BaCu(B₂O₅) glass. The sintering temperature of the ceramics was reduced to 950 °C from 1175 °C. Addition of 5.0 wt.% BaCu(B₂O₅) glass in ZnNb₂O₆–xCaTiO₃ (x = 8.0 wt.%) ceramics sintered at 950 °C showed excellent dielectric properties of ε_r = 20.2, Q × f = 14,100 GHz (f = 7.3 GHz) and τ_f = 0 ppm °C⁻¹. Moreover, the material had a chemical compatibility with silver, which represented a promising candidate materials for low-temperature-co-fired ceramics applications.     

Microwave dielectric properties of La1−xBix(Mg0.5Sn0.5)O3 ceramics

The La_1−xBi_x(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the La_0.97Bi_0.03(Mg_0.5Sn_0.5)O₃ ceramics revealed no significant variation of phase with sintering temperatures. An apparent density of 6.50 g cm⁻³, a dielectric constant (?_r) of 20.2, a quality factor (Q × f) of 58,100 GHz and a temperature coefficient of resonant frequency (τ_f) of −84.2 ppm °C⁻¹ were obtained for La_0.97Bi_0.03(Mg_0.5Sn_0.5)O₃ ceramics that were sintered at 1550 °C for 4 h.     

Phase relations and microwave dielectric properties of vanadium modified Ba5Nb4O15 ceramics

Microwave dielectric ceramics of Ba₅Nb_4−xV_xO₁₅ (x = 0-1) were prepared by a solid-state reaction method. Vanadium substitution can markedly lower the sintering temperature of Ba₅Nb₄O₁₅ from 1450 to 1100 °C. The X-ray powder diffraction analysis reveals the multiphase nature of this system. A hexagonal-to-orthorhombic phase transition was also observed for the BaNb₂O₆ secondary phase. The microwave dielectric properties, such as τ_f, ε_r and Q × f value, decreased with increasing vanadium content for samples sintered at 1100 °C. There was an apparent increase in τ_f and Q × f value for samples (x ≥ 0.5) sintered at 1200 °C due to the hexagonal-to-orthorhombic phase transition of the BaNb₂O₆ phase. These results suggested that the microwave dielectric properties of multiphase ceramics strongly depended on the phase compositions and the phase transitions.     

New dielectric material system of x(Mg0.95Zn0.05Ti)O3-(1 − x)Ca0.8Sm0.4/3TiO3 at microwave frequency

The microstructures and the microwave dielectric properties of the x(Mg_0.95Zn_0.05)TiO₃-(1 − x) Ca_0.8Sm_0.4/3TiO₃ ceramic system were investigated. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. Ca_0.8Sm_0.4/3TiO₃ has dielectric properties of dielectric constant ε_r ~ 120, Q × f value ~ 13,800 GHz and a large positive τ_f value ~ 400 ppm/°C. (Mg_0.95Zn_0.05)TiO₃ possesses high dielectric constant (ε_r ~ 16.21), high quality factor (Q × f value ~ 210,000 at 9 GHz) and negative τ_f value (− 59 ppm/°C). Sintering at 1300 °C with x = 0.9, 0.9(Mg_0.95Zn_0.05Ti)O₃ − 0.1 Ca_0.8Sm_0.4/3TiO₃ has a dielectric constant (ε_r) of 22.7, a Q × f value of 124,000 GHz and a temperature coefficient of resonant frequency (τ_f) of − 6.3 ppm/°C.     

Sintering and dielectric properties of 0.88Al2O3-0.12TiO2 microwave ceramics by glass addition

The microwave characteristics and the microstructures of 0.88Al₂O₃-0.12TiO₂ with various amounts of MgO-CaO-SiO₂-Al₂O₃ (MCAS) glass sintered at different temperatures have been investigated. The sintering temperature can be lowered to 1300 °C by the addition of MCAS glass. The densities, dielectric constants (ε_r) and quality values (Q×f) of the MCAS-added 0.88Al₂O₃-0.12TiO₂ ceramics decrease with the increase of MCAS glass content. The temperature coefficients of the resonant frequency (τ_f) are shifted to more negative values as the MCAS content or the sintering temperatures increase. The change of the crystalline phases of Al₂TiO₅ phase and rutile-TiO₂ phase has profound effects on the microwave dielectric properties of the MCAS-added Al₂O₃-TiO₂ ceramics. As sintered at 1250 °C, 0.88Al₂O₃-0.12TiO₂ ceramics with 2 wt.% MCAS glass addition exists a ε_r value of 8.63, a Q×f value of 9578 and a τ_f value of +5 ppm/°C.     

Dielectric properties of B2O3-doped (1 − x)LaAlO3-xSrTiO3 ceramic system at microwave frequency

The effects of B₂O₃ addition on the microwave dielectric properties and the microstructures of (1−x)LaAlO₃-xSrTiO₃ ceramics prepared by conventional solid-state routes have been investigated. Doping with 0.25 wt.% B₂O₃ can effectively promote the densification and the microwave dielectric properties of (1−x)LaAlO₃-xSrTiO₃ ceramics. It is found that LaAlO₃-SrTiO₃ ceramics can be sintered at 1400°C due to the liquid phase effect of a B₂O₃ addition observed by scanning electronic microscopy (SEM). The dielectric constant as well as the Q×f value decreases with increasing B₂O₃ content. At 1460°C, 0.46LaAlO₃-0.54SrTiO₃ ceramics with 0.25 wt.% B₂O₃ addition possesses a dielectric constant (ε_r) of 35, a Q×f value of 38,000 (at 7 GHz) and a temperature coefficients of resonant frequency (τ_f) of −1 ppm/°C.     

Synthesis and properties of Ba(Zn1/3Ta2/3)O3 for microwave and millimeter wave applications

High dielectric materials have gained an important position in microwave electronics by reducing the size and cost of components for a wide range of applications from mobile telephony to spatial communications. Ba(Zn_1/3Ta_2/3)O₃ (BZT) is an A(B′B″)O₃ type perovskite material, showing ultra high values of the quality factor Q. Ceramic-based BZT dielectric materials were prepared by solid state reaction. The samples were sintered at temperatures in the range 1400 ÷ 1600 °C for 4 h. Compositional, structural and morphological characterization were performed by using XRD, SEM and EDX analysis. The dielectric properties were measured in the microwave range (6 ÷ 7 GHz). An additional annealing at 1400 °C for 10 h has improved some dielectric parameters. For samples sintered at temperatures higher than 1500 °C, the permittivity values were obtained in the interval 30 ÷ 35 and almost do not change the value after the annealing. The Q × f product substantially increases up to about 135,000 GHz, exhibiting a low temperature coefficient of the resonant frequency (τ_f) in microwaves. The best parameters were obtained for the samples sintered at 1600 °C with additional annealing. The achieved high values of the Q × f product recommend these materials for microwave and millimeter wave applications.     

Microwave dielectric properties of Li2TiO3 ceramics sintered at low temperatures

Li₂TiO₃ ceramics were prepared at the sintering temperatures from 1050 to 1250 °C. The optimal microwave dielectric properties were ?_r = 23.29, Q × f = 15,525 GHz (5.9 GHz), and τ_f = 35.05 ppm/ °C for the sample sintered at 1200 °C. The microwave dielectric properties were improved obviously when the Li₂TiO₃ ceramics were sintered at low temperatures with small additions of H₃BO₃ (B₂O₃ in the form of H₃BO₃). Only monoclinic Li₂TiO₃ was found in the pure or H₃BO₃-doped Li₂TiO₃ ceramics. About 1.0 wt.% H₃BO₃ addition aided the sintering of Li₂TiO₃ ceramics effectively while excessive H₃BO₃ (≥2.5 wt.%) was not favorable. Typically the best microwave dielectric properties were ?_r = 23.28, Q × f = 37,110 GHz (6.3 GHz), and τ_f = 30.43 ppm/ °C for the 1.0 wt.% H₃BO₃-doped Li₂TiO₃ ceramic sintered at 920 for 3 h, which is promising for LTCC applications.     

Effect of B2O3-Bi2O3-SiO2-ZnO glass on the sintering and microwave dielectric properties of 0.83ZnAl2O4-0.17TiO2

The 0.83ZnAl₂O₄-0.17TiO₂ (ZAT) ceramics were synthesized by solid state ceramic route. The effect of 27B₂O₃-35Bi₂O₃-6SiO₂-32ZnO (BBSZ) glass on the microwave dielectric properties of ZAT was investigated. The crystal structure and the microstructure of the ceramic-glass composites were studied by X-ray diffraction and scanning electron microscopic techniques. The low frequency dielectric loss was measured at 1 MHz. The dielectric properties of the sintered samples were measured in the microwave frequency range by the resonance method. Addition of 0.2 wt% of BBSZ improved the dielectric properties with quality factor (Q_u × f) > 120,000 GHz, temperature coefficient of resonant frequency (τ_f) = −7.3 ppm/°C and dielectric constant (?_r) = 11.7. Addition of 10 wt% of BBSZ lowered the sintering temperature to about 950 °C with Q_u × f > 10,000 GHz, ?_r = 10 and τ_f = −23 ppm/°C. The reactivity of 10 wt% BBSZ added ZAT with silver was also studied. The results show that ZAT doped with suitable amount of BBSZ glass is a possible material for low-temperature co-fired ceramic (LTCC) application.     

Microwave dielectric properties and microstructures of the 11Li2O-3Nb2O5-12TiO2 ceramics with B2O3 addition

The effects of B₂O₃ addition, as a sintering agent, on the sintering behavior, microstructure and microwave dielectric properties of the 11Li₂O-3Nb₂O₅-12TiO₂ (LNT) ceramics have been investigated. With the low-level doping of B₂O₃ (≤2 wt.%), the sintering temperature of the LNT ceramic could be effectively reduced to 900 °C. The B₂O₃-doped LNT ceramics are also composed of Li₂TiO₃ss and “M-phase” phases. No other phase could be observed in the 0.5-2 wt.% B₂O₃-doped ceramics sintered at 840-920 °C. The addition of B₂O₃ induced no obvious degradation in the microwave dielectric properties but increased the τ_f values. Typically, the 0.5 wt.% B₂O₃-doped ceramics sintered at 900 °C have better microwave dielectric properties of ?_r = 49.2, Q × f = 8839 GHz, τ_f = 57.6 ppm/°C, which suggest that the ceramics could be applied in multilayer microwave devices requiring low sintering temperatures.     

Crystal structure and microwave dielectric properties of (1 − x)NdAlO3-xCaTiO3 ceramics

Crystal structure and microwave dielectric properties of (1−x)NdAlO₃-xCaTiO₃ ceramics have been investigated. Crystal structure of the specimens changed with the composition. Rhombohedral structure was found for the specimens with x≤0.1. When 0.3≤x≤0.7, the specimens had the tetragonal structure and it changed to the orthorhombic structure as x exceeded 0.7. Two types of the second phases were observed in (1−x)NdAlO₃-xCaTiO₃ ceramics. For the specimens with x≤0.5, Nd₄Al₂O₉ phase was observed and Al-rich phase was found in the specimens with x≥0.7. The dielectric constant (ε_r) and the temperature coefficient of the resonant frequency (τ_f) increased with the increase of x. The Q×f value of the specimen increased with x and exhibited the maximum value when x=0.5. The microwave dielectric properties of Q×f=45,000 GHz, ε_r=45 and τ_f=−1.5 ppm/°C were obtained for 0.3NdAlO₃-0.7CaTiO₃ ceramics.     

Low temperature sintering and microwave dielectric properties of ZnTiNb2O8 ceramics with BaCu(B2O5) additions

The phases, microstructure and microwave dielectric properties of ZnTiNb₂O₈ ceramics with BaCu(B₂O₅) additions prepared by solid-state reaction method have been investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The pure ZnTiNb₂O₈ ceramic shows a high sintering temperature of about 1250 °C. However, it was found that the addition of BaCu(B₂O₅) lowered the sintering temperature of ZnTiNb₂O₈ ceramics from above 1250 °C to 950 °C due to the BCB liquid-phase. The results showed that the microwave dielectric properties were strongly dependent on densification, crystalline phases and grain size. Addition of 3 wt% BCB in ZnTiNb₂O₈ ceramics sintered at 950 °C afforded excellent dielectric properties of ?_r = 32.56, Q × f = 20,100 GHz (f = 5.128 GHz) and τ_f = −64.87 ppm/°C. These represent very promising candidates for LTCC dielectric materials.     

Temperature compensated Sr2Al2SiO7 ceramic for microwave applications

The Sr–Gehlenite (Sr₂Al₂SiO₇) ceramic has been prepared by the conventional solid-state ceramic route. Phase pure Sr₂Al₂SiO₇ (SAS) ceramic sintered at 1525 °C for 4 h has ?_r = 7.2 and Q_u × f = 33,000 GHz. The SAS showed large negative τ_f of −37.0 ppm/ °C. A low value of τ_f was achieved by preparing SAS–CaTiO₃ composite. The composite with 0.04 volume fractions (V_f) CaTiO₃ sintered at 1500 °C for 4 h showed good microwave dielectric properties: ?_r = 8.6, Q_u × f = 20,400 GHz and τ_f = +8.5 ppm/°C.