Optimized microwave dielectric properties of Co- and Ca-substituted Mg0.6Zn0.4TiO3

The system of (1 − y)(Mg_0.6Zn_0.4)_1−xCo_xTiO₃-yCaTiO₃ was investigated to optimize its microwave dielectric properties by adopting appropriate contents of Co and Ca and by controlling sintering conditions. The effect of Co substitution was to enhance densification and Qf value, while the addition of CaTiO₃ resulted in increases of dielectric constant and TCF. As an optimal compositions, 0.93(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃-0.07CaTiO₃ successfully demonstrated a dielectric constant of 23.04, a Qf of 79,460 GHz and a TCF value of +1.4 ppm/°C after firing at a relatively lower sintering temperature of 1200 °C. The increase of sintering temperature beyond 1200 °C tended to degrade overall microwave dielectric properties presumably due to Zn volatilization as evidenced by the presence of a Zn-deficient phase (MgTi₂O₅) at 1400 °C. An attempt to establish the correlation between microstructure characteristics and dielectric properties was made in this dielectric system where the extensive range of firing temperature up to 1400 °C was evaluated.     

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.     

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.     

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.     

Dielectric properties of (1 − x)(Mg0.95Zn0.05)TiO3-x(Na0.5Nd0.5)TiO3 ceramic system at microwave frequencies

Ceramics in the system (1 − x)(Mg_0.95Zn_0.05)TiO₃-x(Na_0.5Nd_0.5)TiO₃ were prepared by the conventional mixed oxide route. It shows a two-phase system of an ilmenite structured (Mg_0.95Zn_0.05)TiO₃ and a perovskite structured (Na_0.5Nd_0.5)TiO₃, which were confirmed by XRD and EDX. In addition, (Mg_0.95Zn_0.05)Ti₂O₅ was identified as a second phase. It was also responsible for a rapid drop in the Q × f value. The temperature coefficient of resonant frequency was a function of compositional ratio. Specimen with x = 0.16 possessed an excellent combination of microwave dielectric properties: ε_r ~ 24.27, Q × f ~ 82,000 GHz (at 9 GHz) and τ_f ~ 0 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 of (1 − x)Ca2/5Sm2/5TiO3-xLi1/2Nd1/2TiO3 ceramic powder via ethylenediaminetetraacetic acid precursor

(1 − x)Ca_2/5Sm_2/5TiO₃-xLi_1/2Nd_1/2TiO₃ (CSLNT) ceramic powder was prepared by a liquid mixing method using ethylenediaminetetraacetic acid (EDTA) as the chelating agent. TG, DTA, XRD and TEM characterized the precursors and derived oxide powders. When x = 0.3, perovskite CSLNT was synthesized at 1000 °C for 3 h in air. The CSLNT (x = 0.3) ceramics sintered at 1200 °C for 3 h show excellent microwave dielectric properties of ?_r = 99, Q_f = 6200 GHz and τ_f = 9 × 10⁻⁶ °C⁻¹.     

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.     

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.     

Microstructure and microwave dielectric characteristics of (1 − x)Ba(Zn1/3Ta2/3)O3-xBaTi4O9 ceramics

(1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ (0.1 ≤ x ≤ 0.85) composites are prepared by mixing 1150 °C-calcined BaTi₄O₉ with 1150 °C-calcined Ba(Zn_1/3Ta_2/3)O₃ powders. The crystal structure, microwave dielectric properties and sinterabilites of the (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics have been investigated. X-ray diffraction patterns reveal that BaTi₄O₉, ordered and disordered Ba(Zn_1/3Ta_2/3)O₃ phases exist independently over the whole compositional range. The sintering temperatures of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics are about 1240 - 1320 °C and obviously lower than those of Ba(Zn_1/3Ta_2/3)O₃ ceramics. The dielectric constants (?_r) and the temperature coefficient of resonant frequency (τ_f) of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics increase with the increase of BaTi₄O₉ content. Nevertheless, the bulk densities and the quality values (Q × f) of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics increase with the increase of Ba(Zn_1/3Ta_2/3)O₃ content. The results are attributed to the higher density and quality value of Ba(Zn_1/3Ta_2/3)O₃ ceramics, the better grain growth, and the densification of sintered specimens added a small BaTi₄O₉ content. The (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramic with x = 0.1 sintered at 1320 °C exhibits a ?_r value of 31.5, a maximum Q × f value of 68500 GHz and a minimum τ_f value of 4.1 ppm/°C.     

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.     

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.     

(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.     

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.     

A new dielectric material system using (1 − x)(Mg0.95Co0.05)2TiO4–xCa0.8Sm0.4/3TiO3 at microwave frequencies

The microwave dielectric properties of the (1 − x)(Mg_0.95Co_0.05)₂TiO₄–xCa_0.8Sm_0.4/3TiO₃ ceramic system prepared by mixed oxide route have been investigated. The crystal structures and the microstructures of the ceramics were characterized by means of X-ray and SEM, respectively. The microwave dielectric properties are strongly related to the density and the matrix of the specimen. Combining (Mg_0.95Co_0.05)₂TiO₄ (spinel structure) and Ca_0.8Sm_0.4/3TiO₃ (perovskite structure) forms a two-phase system and leads to a near-zero τ_f. With increasing x, the Q × f of the specimen decreased, whereas its ?_r increased due to a compositional variation. A new microwave dielectric 0.8(Mg_0.95Co_0.05)₂TiO₄–0.2Ca_0.8Sm_0.4/3TiO₃, possessing excellent microwave dielectric properties with a dielectric constant (?_r) of 20.84, a Q × f of 112,600 GHz (where f = 9.1 GHz, is the resonant frequency) and a τ_f value ∼0.8 ppm K⁻¹ at 1573 K/4 h, is proposed as a candidate material for GPS patch antennas and 3G passive components.     

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.     

Phase evolution and microwave dielectric characteristics of Ti-substituted Mg2SiO4 forsterite ceramics

In the present paper, the phase evolution and microstructures of Ti-substituted Mg₂SiO₄ forsterite ceramics with nominal composition Mg₂(Si_1 − xTi_x)O₄ were investigated together with their microwave dielectric characteristics. MgSiO₃ secondary phase was observed in Mg₂SiO₄ ceramics, and it could be suppressed by Ti-substitution. However, Mg₂TiO₄ and MgTi₂O₅ appeared gradually with increasing Ti-substitution amount. The dielectric constant slightly increased from 6.8 to 8.1 with Ti-substitution, and the Qf value was improved significantly and reached the maximum at x = 0.1 where the optimum combination of microwave dielectric characteristics were achieved: ε_r = 7.4, Qf = 73,760 GHz at 15 GHz, τ_f = − 60 ppm/°C.     

A new low-loss microwave dielectric using (Ca0.8Sr0.2)TiO3-doped MgTiO3 ceramics

The microwave dielectric properties and the microstructures of the (1-x)MgTiO₃-x(Ca_0.8Sr_0.2)TiO₃ ceramic system prepared by the conventional solid-state route were investigated. (Ca_0.8Sr_0.2)TiO₃ was employed as a τ_f compensator and was added to MgTiO₃ to achieve a temperature-stable material. Ilmenite-structured MgTiO₃ and perovskite-structured (Ca_0.8Sr_0.2)TiO₃ were coexisted and the two-phase system was confirmed by the X-ray diffraction patterns and the energy-dispersive X-ray analysis. Although the ε_r of the specimen could be boosted by increasing amount of (Ca_0.8Sr_0.2)TiO₃, it would instead render a decrease in the Q × f. The τ_f value is strongly correlated to the compositions and can be controlled through the existing phases. In fact, τ_f could be adjusted to a near-zero value by mixing 94 mole% MgTiO₃ and 6 mole% (Ca_0.8Sr_0.2)TiO₃. A dielectric constant (ε_r) of 21.42, a high Q × f value of 83,700 GHz (at 9 GHz) and a temperature coefficient of resonant frequency (τ_f) of − 1.8 ppm/°C were obtained for 0.94MgTiO₃-0.06(Ca_0.8Sr_0.2)TiO₃ sintered at 1300 °C for 4 h. It is proposed as a low-loss and low-cost dielectric material for microwave and millimeter wave applications.     

Synthesis and microwave dielectric properties of Sr3Zn1−xMgxNb2O9 phases

Oxides of the type, Sr₃Zn_1−xMg_xNb₂O₉ (0≤x≤1) have been obtained by the ceramic method. These oxides crystallize in the hexagonal cell corresponding to ordered triple perovskites. Sintered disks show nearly frequency-independent dielectric constant for all the compositions. Compositions sintered at 1425°C yield dielectric constant of 20-22 at ∼6 GHz, with quality factor ranging from 1300 to 1500. Sr₃Zn_0.5Mg_0.5Nb₂O₉ shows a very low temperature coefficient of resonant frequency (τ_f) of +4 ppm/°C.     

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.