Effect of V2O5 additive to 0.4SrTiO3–0.6La(Mg0.5Ti0.5)O3 ceramics on sintering behavior and microwave dielectric properties

The effect of V₂O₅ addition on the microwave dielectric properties and the microstructures of 0.4SrTiO₃–0.6La(Mg_0.5Ti_0.5)O₃ ceramics sintered for 5 h at different sintering temperature were investigated systematically. It was found that the sintering temperature was effectively lowered about 200 °C by increasing V₂O₅ addition content. The grain sizes, bulk density as well as microwave dielectric properties were greatly dependent on sintering temperature and V₂O₅ content. The 4ST–6LMT ceramics with 0.25% V₂O₅ sintered at 1400 °C for 5 h in air exhibited optimum microwave dielectric properties of ɛ_r = 50.7, Q × f = 15049.6 GHz, T_f = −1.7 ppm/°C.     

Effect of B2O3 and CuO additives on the sintering temperature and microwave dielectric properties of Ba(Mg1/3Nb2/3)O3 ceramics

B₂O₃ added Ba(Mg_1/3Nb_2/3)O₃ (BBMN) ceramics cannot be sintered below 930 °C. However, when CuO was added to them, they were sintered even at 850 °C. The amount of the Ba₂B₂O₅ second phase, which was formed in the BBMN ceramics decreased with the addition of CuO. Therefore, the CuO additive is considered to react with the B₂O₃ inhibiting the reaction between B₂O₃ and BaO. A dense microstructure without pores developed with the addition of a small amount of CuO. The bulk density, dielectric constant (ɛ_r) and Q-value increased with the addition of CuO, but decreased when a large amount of CuO was added. Excellent microwave dielectric properties were obtained for the Ba(Mg_1/3Nb_2/3)O₃ + 2.0 mol% B₂O₃ + 10.0 mol% CuO ceramic sintered at 875 °C for 2 h, with values Q_xf = 21 500 GHz, ɛ_r = 31 and temperature coefficient of resonance frequency (τ_f) = 21.3 ppm/°C.     

Microwave dielectric properties of Sr2Ce2Ti5O16 ceramics

Sr₂Ce₂Ti₅O₁₆ dielectric ceramics were prepared by conventional solid-state ceramic route. The structure and microstructure of the ceramics were investigated by X-ray diffraction and scanning electron microscopic methods. The Sr₂Ce₂Ti₅O₁₆ has a psuedocubic structure. It has ɛ_r of 113, unloaded quality factor (Q_u × f) of 8000 GHz and temperature coefficient of resonant frequency of 306 ppm/°C. The effects of various dopants on the structure, microstructure and microwave dielectric properties of the material have been investigated. It is found that addition of small amount of dopants such as PbO, Al₂O₃, Nd₂O₃, MoO₃, CeO₂, La₂O₃, Fe₂O₃ and NiO improve the microwave dielectric properties of Sr₂Ce₂Ti₅O₁₆.     

Microwave dielectric properties of BaO–TeO2 binary compounds

A series of BaO–TeO₂ binary ceramic compounds were explored for microwave dielectric applications with ultra-low processing temperatures. During the calcination of mixed BaCO₃ and TeO₂ raw powders, BaTe₄O₉, BaTe₂O₆, BaTeO₃, and Ba₂TeO₅ phases were obtained through the sequential phase formations from Te-rich to Ba-rich phases at temperatures ranging from 500 to 850 °C. Sintering temperatures were as low as only 550 °C for the Te-rich phases. Barium tellurate ceramics exhibited excellent microwave dielectric properties with intermediate dielectric permittivities and high quality factors (Q). The dielectric properties at microwave frequencies were ε_r = 10–21, Q × f = 34,000–55,000 GHz, and TCf = − 51 to − 124 ppm/°C, depending on compositions.     

Microwave dielectric properties of xNd(Zn1/2Ti1/2)O3–(1 − x)CaTiO3 ceramics

The microwave dielectric properties of xNd(Zn_1/2Ti_1/2)O₃–(1 − x)CaTiO₃ have been investigated. The system has been prepared by a conventional solid state ceramic route. Nd(Zn_1/2Ti_1/2)O₃ (NZT) possesses a dielectric constant (ε_r) of 32, a high quality factor (Q × f) of 170,000 GHz and a temperature coefficient of resonant frequency (τ_f) of − 42 ppm/°C. In order to produce a temperature-stable material, the addition of CaTiO₃ leads to a near-zero temperature variation of resonant frequency. In general, the microwave quality factor (Q × f) decreased as x increased and the temperature coefficient of resonant frequency (τ_f) was approximately linearly proportional to permittivity. The dielectric constant decreases from 77 to 32 as x varies from 0.2 to 1.0. The dielectric constant (ε_r) of 45, Q × f value of 56,000 (at 6 GHz) and temperature coefficient of resonant frequency (τ_f) of 0 ppm/°C were obtained for 0.5Nd(Zn_1/2Ti_1/2)O₃–0.5CaTiO₃ ceramics sintered at 1300 °C for 4 h. As the content of x increases, the highest Q × f value of 136,200 GHz for x = 0.8 is achieved at the sintering temperature 1300 °C.     

Influence of V2O5 additions to Ba(Mg1/3Ta2/3)O3 ceramics on sintering behavior and microwave dielectric properties

The microstructures and the microwave dielectric properties of barium magnesium tantalate ceramics prepared by conventional mixed oxide route have been investigated. The prepared Ba(Mg_1/3Ta_2/3)O₃ exhibited a mixture of cubic perovskite and a hexagonal superstructure with Mg and Ta showing 1:2 order in the B-site. It is found that low level doping of V₂O₅ (up to 0.5 wt.%) can significantly improve densification of the specimens and their microwave dielectric properties. The density of doped Ba(Mg_1/3Ta_2/3)O₃ ceramics can be increased beyond 95% of its theoretical value by 1500 °C-sintering, which is caused by the liquid-phase effect of V₂O₅ addition. The detected second phase Ta₂O₅ was mainly the result of V⁵⁺ substitution in the ceramics. Dielectric constant (ε_r) and temperature coefficient of resonant frequency (τ_f) were not significantly affected, while the unloaded quality factors Q were effectively promoted by V₂O₅ addition due to the increase in B-site ordering. The ε_r value of 24.1, Q×f value of 149,000 (at 10 GHz) and τ_f value of 7.2 ppm/°C were obtained for Ba(Mg_1/3Ta_2/3)O₃ ceramics with 0.25 wt.% V₂O₅ addition sintered at 1500 °C for 3 h.     

New dielectric materials of xSrTiO3–(1 − x)Ca(Mg1/3Nb2/3)O3 ceramic system at microwave frequency

The crystal structures and the microwave dielectric properties of the xSrTiO₃–(1 − x)Ca(Mg_1/3Nb_2/3)O₃ perovskite ceramic system have been investigated. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. SrTiO₃ has dielectric properties of dielectric constant ε_r ∼ 205, Q × f value ∼ 4200 GHz and a large positive τ_f value ∼ 1700 ppm/°C. Ca(Mg_1/3Nb_2/3)O₃ possesses high dielectric constant (ε_r ∼ 28), high quality factor (Q × f value ∼ 58,000 at 7 GHz) and negative τ_f value (− 48 ppm/°C). As the x value varies from 0.2 to 0.8, the xSrTiO₃–(1 − x)Ca(Mg_1/3Nb_2/3)O₃ system has the dielectric properties as follows: 40 < ε_r < 123, 4600 < Q × f < 33,400 and − 23 < τ_f < 600. A new microwave dielectric material, 0.3SrTiO₃–0.7Ca(Mg_1/3Nb_2/3)O₃, applicable in microwave devices is suggested and possesses the dielectric properties of a dielectric constant ε_r ∼ 46, a Q × f value ∼ 29,300 GHz (at 6.8 GHz) and a τ_f value ∼− 2 ppm/°C. A near-zero τ_f value can be achieved by adjusting the x value of xSrTiO₃–(1 − x)Ca(Mg_1/3Nb_2/3)O₃ ceramics.     

Effects of CaF2 addition on sintering behavior and microwave dielectric properties of ZnTa2O6 ceramics

Microwave dielectric ceramics ZnTa₂O₆ were prepared by conventional mixed oxide route. The effects of CaF₂ addition on the microstructures and microwave dielectric properties of ZnTa₂O₆ ceramics were investigated. Formation of second phase can be detected at the high addition of CaF₂ (0.5–1.0 wt.%). Variation of grain shapes were observed with CaF₂ content increasing. The sintering temperature of CaF₂-doped ZnTa₂O₆ ceramics can be effectively lowered from 1400 °C to 1225 °C due to liquid phase effect. The microwave dielectric properties were affected by the amount of CaF₂ addition. At 1225 °C for 4 h, ZnTa₂O₆ ceramics with 0.25 wt.% CaF₂ possesses excellent microwave dielectric properties: ε_r = 31.32, Q × ? = 73600 GHz(6.8 GHz) and τ_? = ? 6.97 ppm/°C.     

Influence of CuO addition to BaSm2Ti4O12 microwave ceramics on sintering behavior and dielectric properties

Microwave dielectric ceramics of tungsten–bronze-type BaSm₂Ti₄O₁₂ were prepared by doping CuO (up to 2 wt.%) as the liquid-phase sintering aid. The effects of CuO additive on the densification, micro structure and dielectric properties were investigated. Due to the liquid-phase effect, the sintering temperature of BaSm₂Ti₄O₁₂ ceramics with 1 wt.% CuO addition can be effectively reduced to 1160 °C, about 200 °C lower than that of pure BaSm₂Ti₄O₁₂ ceramics, while good microwave dielectric properties of ɛ_r = 75.8, Q*f = 4914.6 GHz and τ_f = −7.65 ppm/°C were still achieved.     

Effect of BaCu(B2O5) on the sintering temperature and microwave dielectric properties of BaO–Ln2O3–TiO2 (Ln = Sm,Nd) ceramics

BaCu(B₂O₅) (BCB) ceramic powder was used to decrease the sintering temperatures of BaSm₂Ti₄O₁₂ (BST) and BaNd₂Ti₅O₁₄ (BNT) ceramics. The sintering temperature of the BST and BNT ceramics was reduced from approximately 1350 °C to 850 °C by the addition of BCB. The bulk density of the specimens increased and reached the saturated value with increasing BCB content. The variation of the dielectric constant (ɛ_r) was similar to that of the bulk density and, thus, the relative density plays an important role in determining the ɛ_r value of the specimens. The Q-value initially increased with the addition of BCB but decreased considerably when a large amount of BCB was added because of the presence of the liquid phase. Good microwave dielectric properties of Qxf = 4500 GHz, ɛ_r = 60 and τ_f = −30 ppm/°C were obtained for the 16.0 mol% BCB-added BST ceramics sintered at 875 °C for 2 h.     

Preparation,phase structure and microwave dielectric properties of CoLi2/3Ti4/3O4 ceramic

A new low loss microwave dielectric ceramic with composition of CoLi_2/3Ti_4/3O₄ was prepared by a conventional solid-state reaction method. The compound has a cubic spinel structure [Fd-3m (227)] similar to MgFe₂O₄ with lattice parameters of a = 8.3939 Å, V = 591.42 Å³, Z = 8 and ρ = 4.30 g/cm³. This ceramic has a low sintering temperature (～1050 °C) and good microwave dielectric properties with relative permittivity of 21.4, Q × f value of 35,000 GHz and τ_f value of ?22 ppm/°C. Furthermore, the addition of BaCu(B₂O₅) (BCB) can effectively lower the sintering temperature from 1050 °C to 900 °C and does not induce much degradation of the microwave dielectric properties. Compatibility with Ag electrode indicates that the BCB added CoLi_2/3Ti_4/3O₄ ceramics are good candidates for LTCC applications.     

Microwave dielectric properties of Ba3La2Ti2Nb2−xTaxO15 ceramics

High dielectric constant and low loss ceramics in the system Ba₃La₂Ti₂Nb_2−xTa_xO₁₅ (x = 0–2) have been prepared by conventional solid-state ceramic route. Ba₃La₂Ti₂Nb_2−xTa_xO₁₅ 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 49.8 to 45.1, quality factor Q_u × f from 22,000 to 31,040 GHz and temperature variation of resonant frequency from + 6.9 to − 13.4 ppm/°C as the value of x increases. These low loss ceramics might be used for dielectric resonator (DR) applications.     

Synthesis and microwave dielectric properties of Ba4Ti3P2O15 ceramics

Present work introduces a new kind of microwave dielectric ceramic, Ba₄Ti₃P₂O₁₅. Ba₄Ti₃P₂O₁₅ ceramic can be prepared by solid state reaction method and be well densified after being sintered at above 1175 °C for 4 h in air. All the XRD patterns can be fully indexed as single-phase structure. The best microwave dielectric properties can be obtained in ceramic sintered at 1200 °C for 4 h with permittivity about 20.7, Q × f about 42,210 GHz and TCF about 37 ppm °C^?1. Measurements of the microwave dielectric properties of Ba₄Ti₃P₂O₁₅ ceramic revealed the existence of a maximum in the temperature dependence of the dielectric loss because of the defect dipoles relaxation.     

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.     

Structure and microwave dielectric characteristics of lithium-excess Ca0.6Nd0.8/3TiO3/(Li0.5Nd0.5)TiO3 ceramics

Compositions based on (1−x)Ca_0.6Nd_8/3TiO₃−x(Li_1/2Nd_1/2)TiO₃ + yLi (CNLNTx + yLi, x = 0.30–0.60, y = 0–0.05), suitable for microwave applications have been developed by systematically adding excess lithium in order to tune the microwave dielectric properties and lower sintering temperature. Addition of 0.03 excess-Li simultaneously reduced the sintering temperature and improved the relative density of sintered CNLNTx ceramics. The excess Li addition can compensate the evaporation of Li during sintering process and decrease the secondary phase content. The CNLNTx (x = 0.45) ceramics with 0.03 Li excess sintered at 1190 °C have single phase orthorhombic perovskite structure, together with the optimum combination of microwave dielectric properties of ɛ_r = 129, Q × f = 3600 GHz, τ_f = 38 ppm/°C. Obviously, excess-Li addition can efficiently decrease the sintering temperature and improve the microwave dielectric properties. The high permittivity and relatively low sintering temperatures of lithium-excess Ca_0.6Nd_0.8/3TiO₃/(Li_0.5Nd_0.5)TiO₃ ceramics are ideal for the development of low cost ultra-small dielectric loaded antenna.     

Microwave dielectric properties of Ba(Mg1/3Ta(2−2x)/3Wx/3Tix/3)O3 ceramics

The microwave dielectric properties of ceramics based on Ba(Mg_1/3Ta_(2−2x)/3W_x/3Ti_x/3)O₃ is investigated as a function of x. The densification as well as dielectric properties deteriorate with increase in the substitution levels of (Ti_1/3W_1/3)^3.33+ at (Ta_2/3)^3.33+ site in Ba(Mg_1/3Ta_2/3)O₃. The τ_f is approaching zero between x = 0.1 and 0.15 in Ba(Mg_1/3Ta_(2−2x)/3W_x/3Ti_x/3)O₃ where quality factor is reasonably good (Q_u × f = 80,000–90,000 GHz). The Ba(Mg_1/3Ta_(2−2x)/3W_x/3Ti_x/3)O₃ with x = 1.0 has ɛ_r = 15.4, τ_f = −25.1 ppm/°C, Q_u × f = 35,400 GHz.     

Ba2La2TiNb2O12: A new microwave dielectric of A4B3O12-type cation-deficient perovskites

A new A₄B₃O₁₂-type cation-deficient perovskite Ba₂La₂TiNb₂O₁₂ was prepared by the conventional solid-state reaction route. The phase and structure of the ceramics were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM). The compound crystallizes in the trigonal system with unit cell parameter a = 5.6726(3) Å, c = 27.740(2) Å, V = 773.04(9) ų and Z = 3. The microwave dielectric properties of the ceramic were studied using a network analyzer, and it shows a high dielectric constant of 42.70, a high quality factor with Q × f of 31,130 GHz, and a small negative τ_f of − 4.2 ppm °C^− 1.     

Effect of bond valence on microwave dielectric properties of (1 − x)CaTiO3–x(Li0.5La0.5)TiO3 ceramics

The origins of microwave dielectric properties (1 ? x)CaTiO₃–x(Li_0.5La_0.5)TiO₃ (0.2 ≤ x ≤ 0.8) ceramics, prepared by a conventional solid-state reaction method, were investigated based on the theory of bond valence. The XRD and SEM results showed that complete solid solutions with orthorhombic perovskite structure were formed in the whole investigated compositional range. The dielectric constant (?_r), quality factor (Q × f) and temperature coefficient of resonant frequency (τ_f) were closely related to B-site, A-site and the difference between A-site and B-site bond valences of ABO₃ perovskite compounds, respectively. As x value increased from 0.2 to 0.8, the dielectric constant increased from 198.3 to 276.8, the Q × f value decreased from 4340 to 1880 GHz, and the τ_f value varied from +489.7 to ?178 ppm/°C. For practical applications, excellent microwave dielectric properties of ?_r = 245, Q × f = 2750 GHz and τ_f = +0.75 ppm/°C were obtained for 0.4CaTiO₃–0.6(Li_0.5La_0.5)TiO₃ ceramics.     

Powder synthesis and properties of LiTaO3 ceramics

The LiTaO₃ powders with sub micrometer grade grain size have been synthesized successfully using a molten salt method. Lithium tantalate began to form at 400 °C reaction temperature and transformed to pure phase without residual reactants when it was processed at 500 °C for 4 h in static air. The undoped LiTaO₃ ceramics with a Curie temperature about 663 °C were obtained by pressureless sintering at 1300 °C for 3 h. The relative dielectric constant (ɛ_r) increases from 50 to 375 at temperature ranging from 30 to 663 °C and then decreases quickly as the temperature increases above 663 °C. The ceramics shows a relative dielectric constant of 49.4, a dielectric loss factor (tan δ) of 0.007, a coercive field (Ec) of 28.66 kV/cm and a remnant polarization (Pr) of 32.48 μC/cm² at room temperature.     

Microwave dielectric properties of ZnAl2O4–TiO2 spinel-based composites

The microstructures, phase compositions and microwave dielectric properties of ZnAl₂O₄–TiO₂ spinel-based composites have been investigated. It is found that ZnAl₂O₄ cannot form a solid solution with TiO₂. As TiO₂ content increases, the ε_r and τ_f values increase gradually, while the Q · f values degrade by degrees. Under the same amount of TiO₂ content, the ε_r and Q · f values increase initially and then decrease slightly with increasing sintering temperature, while the τ_f values increase slowly. The optimal microwave dielectric properties are achieved in (1 − x)ZnAl₂O₄–xTiO₂ (x = 0.21) sintered at 1500 °C for 3 h with ε_r value of 11.4, Q · f value of 71,810 GHz (at about 6.5 GHz), and τ_f value of − 0.5 ppm/°C.