Phase Structure, Microstructure, and Electrical Properties of Sb-Modified (K, Na, Li) (Nb, Ta) O3 Piezoelectric Ceramics

Lead-free (K_0.44Na_0.52Li_0.04)(Nb_{0.80− x} Ta_0.20Sb _x )O₃ piezoelectric ceramics were prepared by the ordinary sintering method. The much higher Pauling electronegativity of Sb compared with Nb makes the ceramics more covalent. By increasing x from 0.00 to 0.06, the phase structure of the ceramics changed from the tetragonal to the pseudocubic phase, and both the bands in the Raman scattering spectra shifted to lower frequency numbers. The grain growth of the ceramics was improved by substituting Sb⁵⁺ for Nb⁵⁺. By increasing x , the dielectric properties were optimized and the variation of dielectric constants before and after poling became smaller. Only the tetragonal–cubic phase transition was observed above room temperature in all the ɛ_r– T curves. The degree of diffuseness increased from 1.29 at x =0.00 to 1.96 at x =0.06, indicating that the ceramics at x =0.06 changed to an approximate ideal relaxor ferroelectric. The temperature dependences of f _r and k _p became better by increasing x properly. Significantly, the ceramics with x between 0.00 and 0.04 had high density and outstanding electrical properties ( d ₃₃=241–272pC/N, k _p=0.42–0.52, ɛ_r=1258–1591, tan δ=0.015–0.025, T _c=280°–355°C, E _c=10.62–12.60 kV/cm, and P _r=16.19–20.13 μC/cm²). Besides, the underlying mechanism for variations of the electrical properties due to Sb⁵⁺ substitution was explained in this work.     

Microwave Dielectric Properties of CaTiO3-CaAl1/2Nb1/2O3 Ceramics Doped with Li3NbO4

The microwave dielectric properties of CaTi_{1− x} (Al_1/2Nb_1/2) _x O₃ solid solutions (0.3 ≤ x ≤ 0.7) have been investigated. The sintered samples had perovskite structures similar to CaTiO₃. The substitution of Ti⁴⁺ by Al³⁺/Nb⁵⁺ improved the quality factor Q of the sintered specimens. A small addition of Li₃NbO₄ (about 1 wt%) was found to be very effective for lowering sintering temperature of ceramics from 1450–1500° to 1300°C. The composition with x = 0.5 sintered at 1300°C for 5 h revealed excellent dielectric properties, namely, a dielectric constant (ɛ_r) of 48, a Q × f value of 32 100 GHz, and a temperature coefficient of the resonant frequency (τ_f) of −2 ppm/K. Li₃NbO₄ as a sintering additive had no harmful influence on τ_f of ceramics.     

Microwave Dielectrics in the (La1/2Na1/2)TiO3–Ca(Fe1/2Nb1/2)O3 System

Dielectric properties of the system (1 − x)(La_1/2Na_1/2)TiO₃– _x Ca(Fe_1/2Nb_1/2)O₃, where 0.4 # x # 0.6, have been investigated at microwave frequencies. The temperature coefficient of resonant frequency (τ_f), nearly 0 ppm/°C, was realized at x = 0.58. These ceramics had perovskite structure and showed relatively low dielectric losses. A new dielectric material applicable to microwave devices having Q · f of 12000–14000 GHz and a dielectric constant (ε_r) of 59–60 has been obtained at 1300–1350°C for 5–15 h sintering.     

Phase Structure and Electrical Properties of (K0.48Na0.52)(Nb0.95Ta0.05)O3-LiSbO3 Lead-Free Piezoelectric Ceramics

(1− x )(K_0.48Na_0.52)(Nb_0.95Ta_0.05)O₃– x LiSbO₃ [(1− x )KNNT− x LS] lead-free piezoelectric ceramics were prepared by the conventional solid-state sintering method. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal phases was identified in the composition range of 0.03< x <0.05. The ceramics near the MPB exhibit a strong compositional dependence and enhanced electrical properties. The (1− x )KNNT– x LS ( x =0.04) ceramics exhibit good electrical properties ( d ₃₃=250 pC/N, k _p=45.1%, k _t =46.3%, T _c=348°C, T _{o − t} =74°C, P _r=25.9 μC/cm², E _c=10.7 kV/cm, ɛ_r∼1352, tan δ∼3%). These results show that (1− x )KNNT– x LS ceramic is a promising lead-free piezoelectric material.     

Effects of Ca/Ti Cosubstitution upon Microwave Dielectric Characteristics of CaSmAlO4 Ceramics

(Ca_{1+ x} Sm_{1− x} )(Al_{1− x} Ti _x )O₄ (0≤ x ≤0.4) ceramics were synthesized by solid-state reaction method and their microstructures and microwave dielectric properties were investigated. X-ray diffraction analysis and energy-dispersive X-ray analysis indicated that the matrix phase was a solid solution with a composition represented by the chemical formula (Ca_{1+ x} Sm_{1− x} ) (Al_{1− x} Ti _x )O₄ and minor amount of (Ca,Sm)(Al,Ti)O₃ secondary phase was detected. Ca/Ti cosubstitution could significantly improve the microwave dielectric characteristics of CaSmAlO₄ ceramics, and the excellent microwave dielectric characteristics were obtained in the modified ceramics as ɛ_r=19–23, Q × f =49 100–118 700 GHz, and τ_f=−15–15 ppm/°C.     

Microstructure and Microwave Dielectric Properties of (1−x)Ca(Mg1/3Ta2/3)O3/xCaTiO3 Ceramics

Dense (1− x )Ca(Mg_1/3Ta_2/3)O₃/ x CaTiO₃ ceramics (0.1≤ x ≤0.9) were prepared by a solid-state reaction process. The crystal structures and microstructures were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Single-phase solid solutions were obtained in the entire composition range. Long-range 1:2 ordering of B-site cations and oxygen octahedra tilting lead to the monoclinic symmetry with space group P 2₁/ c for x =0.1. For x above 0.1, the long-range ordering was destroyed and the crystal structure became the orthorhombic with space group Pbnm . The microwave dielectric properties showed a strong dependence on the composition and microstructure. The dielectric constant and temperature coefficient of resonant frequency increased nonlinearly as the CaTiO₃ content increased while the Qf values decreased approximately linearly. Good combination of microwave dielectric properties was obtained at x =0.45, where ɛ_r=45.1, Qf =34 800 GHz, and τ_f=17.4 ppm/°C.     

Microstructure and Electrical Properties of [(K0.50Na0.50)0.95−xLi0.05Agx](Nb0.95Ta0.05)O3 Lead-Free Ceramics

[(K_0.50Na_0.50)_{0.95− x} Li_0.05Ag _x ](Nb_0.95Ta_0.05)O₃ (KNLNANT- x ) lead-free piezoelectric ceramics were prepared by normal sintering. Effects of the Ag content on the microstructure and electrical properties of KNLNANT- x ceramics were systematically investigated. It is found that the ceramics with x =0.03 exhibit relatively good electrical properties along with high Curie temperature: ( d ₃₃∼252 pC/N, T _c∼438°C, k _p∼45.4%, P _r∼30.1 μC/cm², E _c∼13.8 kV/cm, ɛ_r∼1030, and tan δ∼2.6%). The related mechanism for enhanced electrical properties of the ceramics was also discussed. These results show that KNLNANT-0.03 ceramic is a promising candidate material for high temperature lead-free piezoelectric ceramics.     

Modification of MgAl2O4 Microwave Dielectric Ceramics by Zn Substitution

MgAl₂O₄ microwave dielectric ceramics were modified by Zn substitution for Mg, and their dielectric characteristics were evaluated, along with their structures. Dense (Mg_{1− x} Zn _x )Al₂O₄ ceramics were obtained by sintering at 1550°–1650°C in air for 3 h, and the (Mg_{1− x} Zn _x )Al₂O₄ solid solution was determined in the entire composition range. With Zn substitution for Mg, the dielectric constant ɛ of MgAl₂O₄ just varied from 7.90 to 8.56, while the Q × f value had significantly improved up to a maximal value of 106 000 GHz at x =1.0. Moreover, the τ_f of MgAl₂O₄ ceramics had declined from −73 to −63 ppm/°C.     

Effect of Cation Substitution and Non-Stoichiometry on the Microwave Dielectric Properties of Sr(B'0.5Ta0.5)O3 [B'=Lanthanides] Perovskites

The Sr(B'_0.5Ta_0.5)O₃ ceramics where B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, and Yb have been prepared by the conventional solid-state ceramic route and their microwave dielectric properties have been investigated. The structure and microstructure of the ceramics have been characterized by X-ray diffraction and scanning electron microscope techniques. The relative permittiviy (ɛ_r) varies linearly with B'-site ionic radii, except for La, and the temperature coefficient of resonant frequency (τ_f) varies linearly with the tolerance factor. The Sr(B'_0.5Ta_0.5)O₃ ceramics have ɛ_r in the range 25.9–32, Q _u× f =4500–54 300 GHz, and τ_f=−79 to −42 ppm/°C. A slight deviation from stoichiometry affects the dielectric properties of these double perovskites. Partial substitution of Ba for Sr could tune the dielectric properties. Addition of rutile (TiO₂) lowered the sintering temperature and improved the dielectric properties of Sr(B'_0.5Ta_0.5)O₃ ceramics.     

K/Na Ratio Dependence of the Electrical Properties of [(KxNa1−x)0.95Li0.05](Nb0.95Ta0.05)O3 Lead-Free Ceramics

[(K _x Na_{1− x} )_0.95Li_0.05](Nb_0.95Ta_0.05)O₃ (K _x NLNT) ( x= 0.40–0.60) lead-free piezoelectric ceramics were prepared by conventional solid-state sintering. The effects of K/Na ratio on the dielectric, piezoelectric, and ferroelectric properties of the K _x NLNT ceramics were studied. The experimental results show that the electrical properties strongly depend on the K/Na ratio in the K _x NLNT ceramics. The K _x NLNT ( x =0.42) ceramics exhibit enhanced properties ( d ₃₃∼242 pC/N, k _p∼45.7%, k _t∼47%, T _c∼432°C, T _o−t =48°C, ɛ_r∼1040, tanδ∼2.0%, P _r∼26.4 μC/cm², E _c∼10.3 kV/cm). Enhanced electrical properties of the K _x NLNT ( x =0.42) ceramics could be attributed to the polymorphic phase transition near room temperature. These results show that the K _x NLNT ( x =0.42) ceramic is a promising lead-free piezoelectric material.     

Reduced Dielectric Loss of Modified ZnNb2O6 Ceramics by Substituting Nb5+ with Ta5+

The effects of substituting Nb⁵⁺ with Ta⁵⁺ on the microwave dielectric properties of the ZnNb₂O₆ ceramics were investigated in this study. The forming of Zn(Nb_{1− x} Ta _x )₂O₆ ( x =0–0.09) solid solution was confirmed by the measured lattice parameters and the EDX analysis. By increasing x , not only could the Q × f of the Zn(Nb_{1− x} Ta _x )₂O₆ ( x =0–0.09) solid solution be tremendously boosted from 83 600 GHz at x =0 to a maximum 152 000 GHz at x =0.05, the highest ɛ_r∼24.6 could also be achieved simultaneously. It was mainly due to the uniform grain morphology and the highest relative density of the specimen. A fine combination of microwave dielectric properties (ɛ_r∼24.6, Q × f ∼152 000 GHz at 8.83 GHz, τ_f∼–71.1 ppm/°C) was achieved for Zn(Nb_0.95Ta_0.05)₂O₆ solid solution sintered at 1175°C for 2 h.     

Tailoring the Microwave Dielectric Properties of MgNb2O6 and Mg4Nb2O9 Ceramics

The columbites MgNb₂O₆, MgTa₂O₆, and corundum-type Mg₄Nb₂O₉ ceramics were prepared by the conventional solid-state ceramic route. The structure and microstructure of the sintered samples were investigated by X-ray diffraction and scanning electron microscopic techniques. The microwave dielectric properties of the samples were measured by the resonance method in the frequency range 4–6 GHz. The dielectric properties have been tailored by forming a solid solution between MgNb₂O₆ and MgTa₂O₆ and by the substitution of TiO₂ for Nb₂O₅ in both MgNb₂O₆ and Mg₄Nb₂O₉ ceramics. The Mg(Nb_0.7Ta_1.3)O₆ has ɛ_r=29, Q _u× f =67 800 GHz, and τ_f=0.8 ppm/°C and the MgO–(0.4)Nb₂O₅–(1.5)TiO₂ composition has ɛ_r=34.5, Q _u× f =81 300 GHz, and τ_f=−2 ppm/°C.     

Structure and Microwave Dielectric Properties of Hexagonal Ba[Ti1−x(Ni1/2W1/2)x]O3 Ceramics

Microwave dielectric ceramics with the composition of Ba[Ti_{1− x} (Ni_1/2W_1/2) _x ]O₃ ( x =0.4–0.6) were prepared by a solid-state reaction method. The evolution of the crystalline phases was investigated by X-ray powder diffraction analysis. A cubic-to-hexagonal phase transition occurred between 1000° and 1300°C. The phase transition is irreversible; thus, the hexagonal phase remains stable at room temperature. The X-ray powder diffraction data for x =0.5 were refined using the Rietveld method. It was identified as a h -BaTiO₃-type hexagonal perovksite with the space group of P 6₃/ mmc . It also reveals that random occupancy of Ti⁴⁺ and W⁶⁺ ions occurs in the B-site substructures, whereas Ni²⁺ ions exclusively occupy the octahedral site in the corner-sharing octahedron. The dielectric properties of dense-sintered ceramics were characterized at microwave frequencies. With an increase in x from 0.4 to 0.6, the Q × f value increased from 26 700 to 42 000 GHz, whereas ɛ_r decreased from 29.8 to 20.0, and τ_f from +6.5 to −9.9 ppm/°C.     

Microwave Dielectric Properties of Low-Fired ZnNb2O6 Ceramics with BiVO4 Addition

The BiVO₄ additive was found effective for low-temperature firing of ZnNb₂O₆ polycrystalline ceramics below 950°C in air without a serious degradation in their microwave dielectric properties. Dense BiVO₄-doped ZnNb₂O₆ samples of a relative sintered density over 95% could be prepared even at 925°C. An optimally processed specimen exhibited excellent microwave dielectric properties of Q · f = 55000 GHz, ɛ_r= 26, and τ_f=−57 ppm/°C. With increasing BiVO₄ addition up to 20 mol% relative to ZnNb₂O₆, while the quality factor Q · f was gradually decreased, the relative dielectric constant, ɛ_r, was linearly increased and the temperature coefficient of resonant frequency, τ_f, was slightly increased. The variations in Q · f and ɛ_r are surely attributable to the residual BiVO₄ in the ZnNb₂O₆ matrix. An unexpected slight increase in τ_f is probably due to the formation of the Bi(V,Nb)O₄-type solid solution.     

Structure and Microwave Dielectric Characteristics of Ca1+xNd1−xAl1−xTixO4 Ceramics

CaNdAlO₄ microwave dielectric ceramics were modified by Ca/Ti co-substitution, and their dielectric characteristics were evaluated along with their structure and microstructures. Ca_{1+ x} Nd_{1− x} Al_{1− x} Ti _x O₄ ( x =0, 0.025, 0.05, 0.10, 0.15, 0.20) ceramics with the relative density of over 95% theoretical density were obtained by sintering at 1400°–1450°C in air for 3 h, where the K₂NiF₄-type solid solution single phase was determined from the compositions of x <0.20, while a small amount of CaTiO₃ secondary phase was detected for x =0.20. With Ca/Ti co-substitution in CaNdAlO₄ ceramics, the dielectric constant (ɛ_r) increased with increasing x , and the temperature coefficient of resonant frequency (τ_f) was adjusted from negative to positive, while the Q × f ₀ value increased significantly at first and reached an extreme value at x =0.025 and the maximum at x =0.15. The best combination of microwave dielectric characteristics were achieved at x =0.15 (ɛ_r=19.5, Q × f ₀=93 400 GHz, τ_f=−2 ppm/°C). The improvement of the Q × f ₀ value primarily originated from the reduced interlayer polarization with Ca/Ti co-substitution, while the decreased tolerance factor, the subsequent increased interlayer stress, and the appearance of CaTiO₃ secondary phase brought negative effects upon the Q × f ₀ value.     

Phase Transitions and Electrical Properties of (Na1−xKx)(Nb1−ySby)O3 Lead-Free Piezoelectric Ceramics With a MnO2 Sintering Aid

Lead-free piezoelectric ceramics (Na_{1− x} K _x )(Nb_{1− y} Sb _y )O₃+ z mol% MnO₂ have been prepared by a conventional solid-state sintering technique. Our results reveal that Sb⁵⁺ diffuses into the K_0.5Na_0.5NbO₃ lattices to form a solid solution with a single-phase orthorhombic perovskite structure. The partial substitution of Sb⁵⁺ for B-site ion Nb⁵⁺ decreases the paraelectric cubic-ferroelectric tetragonal phase transition ( T _c) and the ferroelectric tetragonal-ferroelectric orthorhombic phase transition ( T _O–F), and retains strong ferroelectricity. A small amount of MnO₂ is enough to improve the densification of the ceramics. The co-effects of MnO₂ doping and Sb substitution lead to significant improvements in ferroelectric and piezoelectric properties. The ceramics with x =0.45–0.525, y =0.06–0.08, and z =0.5–1 exhibit excellent ferroelectric and piezoelectric properties: d ₃₃=163–204 pC/N, k _P=0.47–0.51, k _t=0.46–0.52, ɛ=640–1053, tan δ=1.3–3.0%, P _r=18.1–22.6 μC/cm², E _c=0.72–0.98 kV/mm, and T _C=269°–314°C.     

The Effect of Dopants on the Dielectric Properties of Ba(B'1/2Ta1/2)O3 (B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Yb, and In) Microwave Ceramics

Low-loss dielectric ceramics based on Ba(B'_1/2Ta_1/2)O₃ (B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Yb, and In) complex perovskites have been prepared by the solid-state ceramic route. The dielectric properties (ɛ_r, Q _u, and τ_f) of the ceramics have been measured in the frequency range 4–6 GHz by the resonance method. The resonators have a relatively high dielectric constant and high quality factor. Most of the compounds have a low coefficient of temperature variation of the resonant frequencies. The microwave dielectric properties have been improved by the addition of dopants and by solid solution formation. The solid solution Ba[(Y_{1− x} Pr _x )_1/2Ta_1/2]O₃ has x =0.15, with ɛ_r=33.2, Q _u× f =51,500 GHz, and τ_f≈0. The microwave dielectric properties of Ba(B'_1/2Ta_1/2)O₃ ceramics are found to depend on the tolerance factor ( t ), ionic radius, and ionization energy.     

High-Q Microwave Dielectrics in the (Mg1−xCox)2TiO4 Ceramics

The microwave dielectric properties and the microstructures of (Mg_{1− x} Co _x )₂TiO₄ ceramics prepared by the conventional solid-state route were investigated. Lattice parameters were also measured for specimens with different x . The formation of solid solution (Mg_{1− x} Co _x )₂TiO₄ ( x =0.02–0.1) was confirmed by the X-ray diffraction patterns, energy dispersive X-ray analysis, and the lattice parameters measured. By increasing x from 0 to 0.05, the Q × f of the specimen can be tremendously boosted from 150 000 GHz to a maximum of 286 000 GHz. A fine combination of microwave dielectric properties (ɛ_r∼15.7, Q × f ∼286 000 GHz at 10.4 GHz, τ_f∼−52.5 ppm/°C) was achieved for (Mg_0.95Co_0.05)₂TiO₄ ceramics sintered at 1390°C for 4 h. Ilmenite-structured (Mg_0.95Co_0.05)TiO₃ was detected as a second phase. The presence of the second phase would cause no significant variation in the dielectric properties of the specimen because it possesses compatible properties compared with that of the main phase. In addition, only a small deviation in the dielectric properties was monitored for specimens with x =0.04–0.05 at 1360°–1420°C. It not only provides a wide process window but also ensures an extremely reliable material proposed as a very promising dielectric for low-loss microwave and millimeter wave applications.     

Electric and Dielectric Properties of Nb-Doped CaCu3Ti4O12 Ceramics

Pure and Nb-substituted CaCu₃Ti_{4− x} Nb _x O_{12+ x /2} (CCTO, x =0, 0.02, 0.1, 0.2, 0.4) ceramics were prepared by a conventional solid-state sintering, and their electric and dielectric properties were investigated using an impedance analyzer. A single-phase CCTO was obtained up to x =0.2 Nb substitution and the lattice parameter increased with Nb substitution concentration. While the grain size decreased with Nb substitution, the resistivity of the grain boundary decreased. The dielectric constant increased with Nb substitution, and the highest value of ∼420 000 was observed in the x =0.2 Nb-substituted specimen at 10 kHz. The obtained electric and dielectric properties in Nb-substituted CCTO were discussed in terms of the internal barrier layer capacitor model, particularly focusing on a ratio of thickness of the grain boundary region to grain size.     

Microwave Dielectric Properties of La6Mg4A2W2O24 [A=Ta and Nb] Ceramics

The microwave dielectric properties of two A-site-deficient perovskite-type ceramics in the La₆Mg₄A₂W₂O₂₄ [A=Ta and Nb] system were investigated. The compounds were synthesized by the solid-state ceramic route. The structure and microstructure were analyzed using X-ray diffraction and scanning electron microscopy techniques. The dielectric properties were measured in the microwave frequency range [4–6 GHz] by the resonance method. La₆Mg₄Ta₂W₂O₂₄ had Q _u× f =13 600 GHz, ɛ_r=25.2, and τ_f=−45 ppm/°C and La₆Mg₄Nb₂W₂O₂₄ had Q _u× f =16 400 GHz, ɛ_r=25.8, and τ_f=−56 ppm/°C.