Preparation of ZnAl2O4-based microwave dielectric ceramics and GPS antenna by aqueous gelcasting

A novel forming method-aqueous gelcasting was used to prepare 90 wt.% (0.75ZnAl₂O₄-0.25TiO₂)-10 wt.% MgTiO₃ (ZTM) microwave dielectric ceramics and GPS antenna. The effects of aqueous gelcasting and dry pressing on the phase compositions, microstructures and microwave dielectric properties of ZTM ceramics were investigated. The samples’ cracking problem happening in the process of drying and binder removal was successfully overcome. The phase compositions are completely the same no matter what forming method is adopted, but the ZTM ceramics prepared by aqueous gelcasting are denser than that prepared by dry pressing. Fewer pores and more uniform microstructures are observed in the ZTM ceramics prepared by aqueous gelcasting. Therefore, much better microwave dielectric properties are obtained in the ZTM ceramics prepared by aqueous gelcasting. Finally, a GPS antenna was successfully fabricated by aqueous gelcasting using the ZTM material, which meets the requirements of GPS application.     

Effect of MgWO4 content on properties of Ba0.5Sr0.5TiO3 composite ceramics for tunable microwave applications

xMgWO₄-(1 − x) Ba_0.5Sr_0.5TiO₃ (x = 0.0, 5.0, 15.0, 25.0 and 35.0 wt%) composite ceramics were prepared via solid state reaction processing. Their structural and dielectric properties were systematically characterized. A significant increase in grain size was observed with increasing MgWO₄ content, which was accompanied by obvious variations in dielectric properties of the composite ceramics. It is found that the permittivity peaks of the samples gradually shifted to low temperatures with increasing MgWO₄ content. At the same time, tunabilities of the composite ceramics decreased, but their Q values increased. The sample with 35 wt% MgWO₄ possesses a high tunability of 16.8% (∼10 kHz), a low permittivity of 65 and an appropriate Q value of 309 (∼4.303 GHz), which meet the requirements of high power and impedance matching, thus making it a promising candidate for applications as electrically tunable microwave devices.     

Influence of Bi2O3 and CuO addition on low-temperature sintering and dielectric properties of Ba0.6Sr0.4TiO3 ceramics

In this study, we tried to lower the sintering temperature of Ba_0.6Sr_0.4TiO₃ (BST) ceramics by several kinds of adding methods of Bi₂O₃, CuO and CuBi₂O₄ additives. The effects of different adding methods on the microstructures and the dielectric properties of BST ceramics have been studied. In the all additive systems, the single addition of CuBi₂O₄ was the most effective way for lowering the sintering temperature of BST. When CuBi₂O₄ of 0.6 mol% was mixed with starting BST powders and sintered at 1100 °C, the derived ceramics demonstrated dense microstructure with a low dielectric constant (? = 4240), low dielectric loss (tan δ = 0.0058), high tunability (Tun = 38.3%) and high Q value (Q = 251). It was noteworthy that the sintering temperature was significantly lowered by 350 °C compared with no-additive system, and the derived ceramics maintained the excellent microwave dielectric properties corresponding to pure BST.     

Comparative investigation of structure and dielectric properties of Pb(Mg1/3Nb2/3)O3-PbTiO3 (65/35) and 10% PbZrO3-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 (65/35) ceramics prepared by a modified precursor method

Relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (65/35) and 10% PbZrO₃-doped Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (65/35) ceramics were both prepared by a modified precursor method, which was based on the high-temperature synthesis of an oxide precursor that contained all the B-site cations for the consideration of B-site homogeneity. The dielectric properties of Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (65/35) ceramic was more of normal ferroelectric behavior, but the high dielectric constant (?_m = 34,200 at 1 kHz) and piezoelectric constant (d₃₃ = 709 pC/N) were observed for this composition close to the morphotropic phase boundary. Comparatively, introduction of 10% PbZrO₃ into Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (65/35) ceramics enhanced the diffuse phase transition as well as the rhombohedral to tetragonal phase transition temperature, while it also kept the high dielectric constant (?_m = 29,600 at 1 kHz) and piezoelectric constant (d₃₃ = 511 pC/N).     

(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.     

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.     

Dielectric and pyroelectric anisotropy in melt-quenched BaBi2(Nb1 − xVx)2O9 ceramics

The (0 0 l) textured BaBi₂(Nb_1 − xV_x)₂O₉ (where x = 0, 0.03, 0.07, 0.1 and 0.13) ceramics were fabricated via the conventional melt-quenching technique followed by high temperature heat-treatment (800-1000 °C range). The influence of vanadium content and sintering temperature on the texture development and relative density were investigated. The samples corresponding to the composition x = 0.1 sintered at 1000 °C for 10 h exhibited the maximum orientation of about 67%. The Scanning electron microscopic studies revealed the presence of platy grains having the a-b planes perpendicular the pressing axis. The dielectric constant and the pyroelectric co-efficient values in the direction perpendicular to the pressing axis were higher. The anisotropy in the dielectric constant is about 100 (at 100 kHz) at the dielectric maximum temperature and anisotropy in the pyroelectric co-efficient is about 50 μC cm⁻² °C⁻¹ in the vicinity of pyroelectric anomaly for the sample corresponding to the composition x = 0.1 sintered at 1000 °C. Higher values of the dielectric loss and electrical conductivity were observed in the direction perpendicular to the pressing axis which is attributed to the high oxygen ion conduction in the a-b planes.     

Processing, dielectric properties and impedance characteristics of Na0.5Bi0.5Cu3Ti4O12 ceramics

Na_0.5Bi_0.5Cu₃Ti₄O₁₂ (NBCTO) ceramics were prepared by conventional solid-state reaction method. The phase structure, microstructure and dielectric properties of NBCTO ceramics sintered at various temperatures with different soaking time were investigated. Pure NBCTO phase could be obtained with increasing the temperature and prolonging the soaking time. High dielectric permittivity (13,495) and low dielectric loss (0.031) could be obtained when the ceramics were sintered at 1000 °C for 7.5 h. The ceramics sintered at 1000 °C for 7.5 h also showed good temperature stability (−4.00 to −0.69%) over a large temperature range from −50 to 150 °C. Complex impedances results revealed that the grain was semiconducting and the grain boundaries was insulating. The grain resistance (R_g) was 12.10 Ω cm and the grain boundary resistance (R_gb) was 2.009 × 10⁵ Ω cm when the ceramics were sintered at 1000 °C for 7.5 h.     

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.     

Dielectric and piezoelectric properties of Y2O3 doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free piezoelectric ceramics

Y₂O₃ doped lead-free piezoelectric ceramics (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (0-0.7 wt%) were synthesized by the conventional solid state reaction method, and the effect of Y₂O₃ addition on the structure and electrical properties was investigated. X-ray diffraction shows that Y₂O₃ diffuses into the lattice of (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ to form a solid solution with a pure perovskite structure. The temperature dependence of dielectric constant of Y₂O₃ doped samples under various frequencies indicates obvious relaxor characteristics different from typical relaxor ferroelectric and the mechanism of the relaxor behavior was discussed. The optimum piezoelectric properties of piezoelectric constant d₃₃ = 137 pC/N and the electromechanical coupling factor k_p = 0.30 are obtained at 0.5% and 0.1% Y₂O₃ addition, respectively.     

Modified tunable dielectric properties by addition of MgO on BaZr0.25Ti0.75O3 ceramics

Phase composition, microstructure and tunable dielectric properties of (1 − x)BaZr_0.25Ti_0.75O₃-xMgO (BZTM) composite ceramics fabricated by solid-state reaction were investigated. It was found Mg not only existed in the matrix as MgO, there was also trace amount of Mg²⁺ ions dissolved in the BZT grains, which led to Curie temperature of the BZTM composites ceramics shifting to below −100 °C. Dielectric permittivity of the BZTM composite ceramics was reduced from thousands to hundreds by manipulating the content of MgO. Johnson's phenomenological equation based on Devonshire's theory was used to describe the nonlinear dielectric permittivity of the ceramics with increasing applied DC field. With increasing content of MgO, anharmonic constant α_(T) increased monotonously. Dielectric permittivity was 672, while dielectric tunability was as high as 30.0% at 30 kV/cm and dielectric loss was around 0.0016 for the 0.6BaZr_0.25Ti_0.75O₃-0.4MgO sample at 10 kHz and room temperature.     

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.     

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 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.     

Influence of sintering temperature on piezoelectric properties of (K0.5Na0.5)NbO3-LiNbO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ have been synthesized by traditional ceramics process without cold-isostatic pressing. The effect of the content of LiNbO₃ and the sintering temperature on the phase structure, the microstructure and piezoelectric properties of (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ ceramics were investigated. The result shows that the phase structure transforms from the orthorhombic phase to tetragonal phase with the increase of the content of LiNbO₃, and the orthorhombic and tetragonal phase co-exist in (K_0.5Na_0.5)NbO₃-LiNbO₃ ceramics when the content of LiNbO₃ is about 0.06 mol. The sintering temperature of (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ decreases with the increase of the content of LiNbO₃. The optimum composition for (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ ceramics is 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃. The optimum sintering temperature of 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃ ceramics is 1080 °C. Piezoelectric properties of 0.94 (K_0.5Na_0.5)NbO₃-0.06LiNbO₃ ceramics under the optimum sintering temperature are piezoelectric constant d₃₃ of 215 pC/N, planar electromechanical coupling factor k_p of 0.41, thickness electromechanical coupling factor k_t of 0.48, the mechanical quality factor Q_m of 80, the dielectric constant of 530 and the Curie temperature T_c = 450 °C, respectively. The results indicate that 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃ piezoelectric ceramics is a promising candidate for lead-free piezoelectric ceramics.     

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.     

Tunable and microwave dielectric properties of Ba0.5Sr0.5TiO3-BaWO4 composite ceramics doped with Co2O3

Co₂O₃ doped BaWO₄-Ba_0.5Sr_0.5TiO₃ composite ceramics, prepared by solid-state route, were characterized systematically, in terms of their phase compositions, microstructure and microwave dielectric properties. Doping of Co₂O₃ promoted grain growth, reduced Curie temperature and broadened phase-transition temperature range of BaWO₄-Ba_0.5Sr_0.5TiO₃, which were attributed mainly to the substitution of Co³⁺ for Ti⁴⁺ at B site in the perovskite lattice. Dielectric diffusion behaviors of the composite ceramics were discussed. The composite ceramics all had dielectric tunability of higher than 10% at 30 kV/cm and 10 kHz, with promising microwave dielectric properties. Specifically, the sample doped with 0.2 wt.% Co₂O₃ exhibited a tunability of 20%, permittivity of 225 and Q of 292 (at 1.986 GHz), making it a suitable candidate for applications in electrically tunable microwave devices.     

Structure and electrical properties of (1 − x)Bi0.5Na0.5TiO3-xBi0.5K0.5TiO3 ceramics near morphotropic phase boundary

The binary lead-free piezoelectric ceramics with the composition of (1 − x)Bi_0.5Na_0.5TiO₃-xBi_0.5K_0.5TiO₃ were synthesized by conventional mixed-oxide method. The phase structure transformed from rhombohedral to tetragonal phase in the range of 0.16 ≤ x ≤ 0.20. The grain sizes varied with increasing the Bi_0.5K_0.5TiO₃ content. Electrical properties of ceramics are significantly influenced by the Bi_0.5K_0.5TiO₃ content. Two phase transitions at T_t (the temperature at which the phase transition from rhombohedral to tetragonal occurs) and T_c (the Curie temperature) were observed in all the ceramics. Adding Bi_0.5K_0.5TiO₃ content caused the variations of T_t and T_c. A diffuse character was proved by the linear fitting of the modified Curie-Weiss law. Besides, the ceramics with homogeneous microstructure and excellent electrical properties were obtained at x = 0.18 and sintered at 1170 °C. The piezoelectric constant d₃₃, the electromechanical coupling factor K_p and the dielectric constant ?_r reached 144 pC/N, 0.29 and 893, respectively. The dissipation factor tan δ was 0.037.     

Low loss Sr1−xCaxLa4Ti5O17 microwave dielectric ceramics

Microwave dielectric ceramics in the Sr_1−xCa_xLa₄Ti₅O₁₇ (0 ≤ x ≤ 1) composition series were prepared through a solid state mixed oxide route. All the compositions formed single phase ceramics within the detection limit of in-house X-ray diffraction when sintered in the temperature range 1450-1580 °C. Theoretical density and molar volume decreased due to the substitution of Ca²⁺ for Sr²⁺ which was associated with a decrease in the dielectric constant (?_r) and temperature coefficient of resonant frequency (τ_f) but an increase in quality factor, Qf_o. Optimum properties were achieved for Sr_0.4Ca_0.6La₄Ti₅O₁₇ which exhibited, ?_r ∼ 53.7, Qf_o ∼ 11,532 GHz and τ_f ∼ −1.4 ppm/°C.     

Improved microwave dielectric properties of B2O3-doped Nd(Co1/2Ti1/2)O3 ceramics with near zero temperature coefficient of resonant frequency

The microwave dielectric properties and the microstructures of Nd(Co_1/2Ti_1/2)O₃ ceramics prepared by conventional solid-state route have been studied. The prepared Nd(Co_1/2Ti_1/2)O₃ exhibited a mixture of Co and Ti showing 1:1 order in the B-site. It is found that low-level doping of B₂O₃ (up to 0.75 wt.%) can significantly improve the density and dielectric properties of Nd(Co_1/2Ti_1/2)O₃ ceramics. Nd(Co_1/2Ti_1/2)O₃ ceramics with additives could be sintered to a theoretical density higher than 98.5% at 1320 °C. Second phases were not observed at the level of 0.25-0.75 wt.% B₂O₃ addition. The temperature coefficient of resonant frequency (τ_f) was not significantly affected, while the dielectric constants (?_r) and the unloaded quality factors Q were effectively promoted by B₂O₃ addition. At 1320 °C/4 h, Nd(Co_1/2Ti_1/2)O₃ ceramics with 0.75 wt.% B₂O₃ addition possesses a dielectric constant (?_r) of 27.2, a Q × f value of 153,000 GHz (at 9 GHz) and a temperature coefficient of resonant frequency (τ_f) of 0 ppm/°C. The B₂O₃-doped Nd(Co_1/2Ti_1/2)O₃ ceramics can find applications in microwave devices requiring low sintering temperature.