(5 − x)BaO-xMgO-2Nb2O5 ceramics prepared using a reaction-sintering process

(5 − x)BaO-xMgO-2Nb₂O₅ (x = 0.5 and 1; 5MBN and 10MBN) microwave ceramics prepared using a reaction-sintering process were investigated. Without any calcinations involved, the mixture of BaCO₃, MgO, and Nb₂O₅ was pressed and sintered directly. MBN ceramics were produced after 2-6 h of sintering at 1350-1500 °C. The formation of (BaMg)₅Nb₄O₁₅ was a major phase in producing 5MBN ceramics, and the formation of Ba(Mg_1/3Nb_2/3)O₃ was a major phase in producing 10MBN ceramics. As CuO (1 wt%) was added, the sintering temperature dropped by more than 150 °C. We produced 5MBN ceramics with these dielectric properties: ?_r = 36.69, Qf = 20,097 GHz, and τ_f = 61.1 ppm/°C, and 10MBN ceramics with these dielectric properties: ?_r = 39.2, Qf = 43,878 GHz, and τ_f = 37.6 ppm/°C. The reaction-sintering process is a simple and effective method for producing (5 − x)BaO-xMgO-2Nb₂O₅ ceramics for applications in microwave dielectric resonators.     

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.     

Microwave dielectric properties of Mg1.8Ti1.1O4 ceramics

Low-loss Mg_1.8Ti_1.1O₄ ceramics were prepared by the conventional solid-state route and their microwave dielectric properties were investigated for the first time. The forming of tetragonal-structured Mg_1.8Ti_1.1O₄ main phase associated with a second phase MgTiO₃ were confirmed by the X-ray diffraction patterns. However, the presence of the second phase would cause no significant variance in the dielectric properties of the specimen because the second phase properties are very similar to that of the main phase. A fine combination of microwave dielectric properties (?_r ∼ 15.74, Q × f ∼ 141,000 GHz at 10.57 GHz, τ_f ∼ − 52.4 ppm/°C) was achieved for Mg_1.8Ti_1.1O₄ ceramics sintered at 1450 °C for 4 h.     

Enhancement microwave dielectric properties of La(Mg0.5Sn0.5)O3 ceramics by Substituting Mg for Co

The microwave dielectric properties of La(Mg_0.5−xCo_xSn_0.5)O₃ ceramics were examined with a view to exploiting them for mobile communication. The La(Mg_0.5−xCo_xSn_0.5)O₃ ceramics were prepared using the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the La(Mg_0.4Co_0.1Sn_0.5)O₃ ceramics revealed that La(Mg_0.4Co_0.1Sn_0.5)O₃ is the main crystalline phase, which is accompanied by small extent of La₂Sn₂O₇ as the second phase. Formation of this Sn-rich second phase was attributed to the loss of MgO upon ignition. Increasing the sintering temperatures seemed to promote the formation of La₂Sn₂O₇. An apparent density of 6.67 g cm⁻³, a dielectric constant (?_r) of 20.3, a quality factor (Q.F.) of 70,500 GHz, and a temperature coefficient of resonant frequency (τ_f) of −77 ppm °C⁻¹ were obtained for La(Mg_0.4Co_0.1Sn_0.5)O₃ ceramics that were sintered at 1550 °C for 4 h.     

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.     

Microwave dielectric properties of Mg4Al2Ti9O25 ceramics

Phase-singular Mg₄Al₂Ti₉O₂₅ ceramics with the pseudobrookite structure suitable for microwave devices such as antenna substrate have been prepared by gel-carbonate method with the dielectric permittivity of 24.7 (at 2-8 GHz), Q-values > 30,000 and temperature coefficients in permittivity (TCK) of less than + 17 ppm K^− 1. The dielectric characteristics are accountable in terms of ordering in the cation sub-lattice.     

Correlation of crystal structure and microwave dielectric properties for Zn(Ti1−xSnx)Nb2O8 ceramics

The correlation of crystal structure and microwave dielectric properties for Zn(Ti_1−xSn_x)Nb₂O₈ ceramics were investigated. The Zn(Ti_1−xSn_x)Nb₂O₈ ceramics contained ZnTiNb₂O₈ and an unknown Columbite-type phase. The columbite structure phase with increasing degree of ordering led to decrease of dielectric constant, increase of Qf and τ_f. The ZnTiNb₂O₈ with decreasing cation valence led to increase of τ_f. The typical values were: ? = 30.88, Qf = 43,500 GHz, τ_f = −54.32 × 10⁻⁶/ °C.     

Microwave dielectric properties of Ba2 − xSrxLa3Ti3NbO15 ceramics

High dielectric constant and low loss ceramics in the system Ba_2 − xSr_xLa₃Ti₃NbO₁₅ (x = 0-1) have been prepared by conventional solid-state ceramic route. Ba_2 − xSr_xLa₃Ti₃NbO₁₅ solid solutions adopted A₅B₄O₁₅ cation-deficient hexagonal perovskite structure for all compositions. The materials were characterized at microwave frequencies. They show a linear variation of dielectric properties with the value of x. Their dielectric constant varies from 48.34 to 43.03, quality factor Q_u × f from 20,291 to 39,088 GHz and temperature variation of resonant frequency from 8 to 1.39 ppm/°C as the value of x increases. These low loss ceramics might be used for dielectric resonator (DR) applications.     

Dielectric properties of a new ceramic system (1 − x)Mg4Nb2O9-xCaTiO3 at microwave frequency

The microstructures and the microwave dielectric properties of the (1 − x)Mg₄Nb₂O₉-xCaTiO₃ ceramic system were investigated. In order to achieve a temperature-stable material, CaTiO₃ (τ_f ∼ 800 ppm/°C) was chosen as a τ_f compensator and added to Mg₄Nb₂O₉ (τ_f ∼ −70 ppm/°C) to form a two phase system. It was confirmed by the XRD and EDX analysis. By appropriately adjusting the x-value in the (1 − x)Mg₄Nb₂O₉-xCaTiO₃ ceramic system, near-zero τ_f value can be achieved. A new microwave dielectric material, 0.5Mg₄Nb₂O₉-0.5CaTiO₃ applicable in microwave devices is suggested and possesses the dielectric properties of a dielectric constant ?_r ∼ 24.8, a Q × f value ∼82,000 GHz (measured at 9.1 GHz) and a τ_f value ∼−0.3 ppm/°C.     

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.     

Temperature stable high-Q microwave dielectric ceramics in (1 − x)BaTi4O9-xBaZn2Ti4O11 system

The microwave dielectric properties of (1 − x)BaTi₄O₉-xBaZn₂Ti₄O₁₁ ceramics were investigated by solid-state reaction technique for obtaining high-Q dielectric ceramics in BaO-ZnO-TiO₂ system. And they were strongly determined by the chemical composition. As x was increased from 0.05 to 0.50, BaZn₂Ti₄O₁₁ phase formed more and more. Therefore, the ε_r decreased from 37.3 to 32.8 and the Q × f values first raised from 45,300 GHz to 60,600 GHz (x = 0.30) and then started to decline to 58,700 GHz (x = 0.40), and the τ_f values varied gradually from 12 ppm/°C to − 13 ppm/°C. 0.7BaTi₄O₉-0.3BaZn₂Ti₄O₁₁ ceramics sintered at 1240 °C for 3 h had excellent comprehensive microwave dielectric properties: ε_r = 34.2, Q × f = 60,600 GHz and τ_f = − 2 ppm/°C.     

Synthesis, low-temperature sintering and the dielectric properties of the ZnO-(1 − x)TiO2-xSnO2 (x = 0.04-0.2)

ZnO-(1 − x)TiO₂-xSnO₂ (x = 0.04-0.2) ceramics were prepared by conventional mixed-oxide method combined with a chemical processing. Fine particle powders were prepared by chemical processing to activate the formation of compound and to improve the sinterability. One wt.% of V₂O₅ and B₂O₃ with the mole ratios of 3:1 were used to lower the sintering temperature of ceramics. The effect of Sn content on phase structure and dielectric properties were investigated. The results show that the substituting Sn for Ti accelerates the hexagonal phase transition to cubic phase, and an inverse spinel structure Zn₂(Ti_1−xSn_x)O₄ solid solution forms. The best dielectric properties obtained at x = 0.12. The ZnO-0.88TiO₂-0.12SnO₂ ceramics sintered at 900 °C exhibit a good dielectric property: ?_r = 29 and tan δ = 9.86 × 10⁻⁵. Due to their good dielectric properties, low firing characteristics, ZnO-(1 − x)TiO₂-xSnO₂ (x = 0.04-0.2) can serve as the promising microwave dielectric capacitor.     

Microwave dielectric properties of MgAl2O4–CoAl2O4 spinel compounds prepared by reaction-sintering process

MgAl₂O₄ spinel exhibits fascinating microwave dielectric properties, but the synthesis of dense MgAl₂O₄ ceramics requires high firing temperatures. In this study, Co is introduced into MgAl₂O₄ ceramics to improve their sinterability and microwave dielectric properties. An Mg_1−xCo_xAl₂O₄ solid solution of a spinel structure was observed in the MgAl₂O₄–CoAl₂O₄ system, and dense Mg_1−xCo_xAl₂O₄ ceramics were obtained by sintering at 1475–1500 °C in air for 2–6 h. Co addition is effective in lowering the sintering temperature to 1475 °C. Q × f of Mg_1−xCo_xAl₂O₄ ceramics was increased to 49,300 GHz with an increase in Co content to 0.2, but degraded with a further increase in Co content. The temperature coefficient of resonant frequency of Mg_1−xCo_xAl₂O₄ ceramics was sustained at between −73 and −23 ppm/°C to the variation of Co content.     

Effect of Pb(Fe1/2Nb1/2)O3 modification on dielectric and piezoelectric properties of Pb(Mg1/3Nb2/3)O3-PbZr0.52Ti0.48O3 ceramics

10 mol% Pb(Fe_1/2Nb_1/2)O₃ (PFN) modified Pb(Mg_1/3Nb_2/3)O₃-PbZr_0.52Ti_0.48O₃ (PMN-PZT) relaxor ferroelectric ceramics with compositions of (0.9 − x)PMN-0.1PFN-xPZT (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9) were prepared. X-ray diffraction investigations indicated that as-prepared ceramics were of pure perovskite phase and the sample with composition of x = 0.8 was close to morphotropic phase boundary (MPB) between rhombohedral and tetragonal phase. Dielectric properties of the as-prepared ceramics were measured, and the Curie temperature (T_c) increased sharply with increasing PZT content and could be higher than 300 °C around morphotropic phase boundary (MPB) area. At 1 kHz, the sample with composition of x = 0.1 had the largest room temperature dielectric constant ?_r = 3519 and maximum dielectric constant ?_m = 20,475 at T_m, while the sample with composition of x = 0.3 possessed the maximum dielectric relaxor factor of γ = 1.94. The largest d₃₃ = 318 pC/N could be obtained from as-prepared ceramics at x = 0.9. The maximum remnant polarization (P_r = 28.3 μC/cm²) was obtained from as-prepared ceramics at x = 0.4.     

Phase evolution, bond valence and microwave characterization of (Zn1 − xNix)Ta2O6 ceramics

(Zn_1 − xNi_x)Ta₂O₆ ceramics have been prepared via conventional mixed oxide route. The phase evolution and microstructure of (Zn_1 − xNi_x)Ta₂O₆ ceramics were investigated. The Raman spectroscopy was used to confirm the minor phase formation. The bond valence of (Zn_1 − xNi_x)Ta₂O₆ ceramics was calculated to evaluate the relation between bond valence and the microwave properties. The effects of Ni²⁺ ionic substitution on microwave dielectric properties of (Zn_1 − xNi_x)Ta₂O₆ ceramics were discussed. The dielectric constant and temperature coefficient of resonant frequency of (Zn_1 − xNi_x)Ta₂O₆ ceramics were depended upon phase composition and bond valence. The Q × ? was not significantly different for all levels of Ni²⁺ ionic concentration.     

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.     

Study on properties of BaxSmyTi7O20 ceramics with CaO-SiO2-B2O3 glass

The effect of CaO-SiO₂-B₂O₃ (CSB) glass addition on the sintering temperature and dielectric properties of Ba_xSm_yTi₇O₂₀ ceramics has been investigated using X-ray diffraction, scanning electron microscopy and differential thermal analysis. The CSB glass starts to melt at about 970 °C, and a small amount of CSB glass addition to Ba_xSm_yTi₇O₂₀ ceramics can greatly decrease the sintering temperature from about 1350 to about 1260 °C, which is attributed to the formation of liquid phase. It is found that the dielectric properties of Ba_xSm_yTi₇O₂₀ ceramics are dependent on the amount of CSB glass and the microstructures of sintered samples. The product with 5 wt% CSB glass sintered at 1260 °C is optimal in these samples based on the microstructure and the properties of sintering product, when the major phases of this material are BaSm₂Ti₄O₁₂ and BaTi₄O₉. The material possesses excellent dielectric properties: ?_r = 61, tan δ = 1.5 × 10⁻⁴ at 10 GHz, temperature coefficient of dielectric constant is −75 × 10⁻⁶ °C⁻¹.     

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.     

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.     

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.