Dielectric Properties of the Fluorite-like Bi2O3–Nb2O5 Solid Solution and the Tetragonal Bi3NbO7

Our analysis of the microwave dielectric properties of the δ-Bi₂O₃–Nb₂O₅ solid solution (δ-BN_ss) showed a continuous increase in permittivity and dielectric losses with an increasing concentration of Nb₂O₅. The only discontinuity was found for the temperature coefficient of resonant frequency, which is negative throughout the entire homogeneity range but reaches a minimum value for the sample with 20 mol% Nb₂O₅. At the same composition there is a discontinuity in the grain size of the δ-BN_ss ceramics. For the sample containing 25 mol% Nb₂O₅ two structural modifications were observed. A single-phase tetragonal Bi₃NbO₇, in the literature referred to as a Type-III phase, is formed in a very narrow temperature range from 850° to 880°C. A synthesis performed below or above this temperature range resulted in the formation of the end member of the δ-BN_ss homogeneity range. Compared with the δ-BN_ss the Bi₃NbO₇ ceramics exhibit lower microwave dielectric losses, an increased conductivity, and a positive temperature coefficient of resonant frequency.     

Electrical Properties of Sr0.7Ba0.3TiO3 Ceramics Doped with Nb2O5, 3Li2O · 2SiO2, and Bi2O3

The electrical properties of Sr_0.5Ba_0.3TiO₃ in the presence of Nb₂O₅ as a donor, 3Li₂O · 2SiO₂ as a sintering agent, and Bi₂O₃ as a dopant have been studied. When the compositions of the ceramics were 1 mol Sr_0.7Ba_0.3TiO₃+ 0.5 mol% Nb₂O₅+ 2 mol% 3Li₂O · 2SiO₂+ 0.2 mol% Bi₂O₃, the ceramics were sintered at 1100°C and exhibited the following characteristics: apparent dielectric constant ɛ, 25000; loss factor tan δ, 2%; insulating resistivity ρ_j, 10¹⁰Ω· cm; variation of dielectric constant with temperature Δɛ/ɛ (−25° to +85°C), +10%, −14%. ɛ and tan δ show only small changes with frequency. The study shows this ceramic can be used in multilayer technology.     

Single-Calcination Synthesis of Pyrochlore-Free 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 and Pb(Mg1/3Nb2/3)O3 Ceramics Using a Coating Method

A coating approach for synthesizing 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃ (0.9PMN–0.1PT) and PMN using a single calcination step was demonstrated. The pyrochlore phase was prevented by coating Mg(OH)₂ on Nb₂O₅ particles. Coating of Mg(OH)₂ on Nb₂O₅ was done by precipitating Mg(OH)₂ in an aqueous Nb₂O₅ suspension at pH 10. The coating was confirmed using optical micrographs and zeta-potential measurements. A single calcination treatment of the Mg(OH)₂-coated Nb₂O₅ particles mixed with appropriate amounts of PbO and PbTiO₃ powders at 900°C for 2 h produced pyrochlore-free perovskite 0.9PMN–0.1PT and PMN powders. The elimination of the pyrochlore phase was attributed to the separation of PbO and Nb₂O₅ by the Mg(OH)₂ coating. The Mg(OH)₂ coating on the Nb₂O₅ improved the mixing of Mg(OH)₂ and Nb₂O₅ and decreased the temperature for complete columbite conversion to ∼850°C. The pyrochlore-free perovskite 0.9PMN–0.1PT powders were sintered to 97% density at 1150°C. The sintered 0.9PMN–0.1PT ceramics exhibited a dielectric constant maximum of ∼24 660 at 45°C at a frequency of 1 kHz.     

Low-Fire Processing of ZrO2–SnO2–TiO2 Ceramics

Effects of a liquid-phase-sintering aid, BaCuO₂+ CuO (BCC), on densification and microwave dielectric properties of (Zr_0.8Sn_0.2)TiO₄ (ZST) ceramics have been investigated. The densification kinetics of ZST are greatly enhanced with the presence of 2.5–5 wt% BCC, but become retarded when the amount of BCC increases further. At a given BCC content, moreover, slower densification kinetics are observed with a larger particle size of ZST. The above results are attributed to a chemical reaction taking place at the interface of BCC/ZST during firing. The ZST dissolves into BCC, forming crystalline phases of ZrO₂, SnO₂, CuO, and BaTi₈O₁₆ which reduce the amount of BCC flux available for liquid-phase sintering. The crystallization kinetics become more significant, compared with densification kinetics, with increasing the amount of BCC and the particle sizes of ZST. For samples with 2.5–5 wt% BCC, a high relative sintered density is obtained at 1000°C and the resulting microwave ceramics have a dielectric constant and a value of Q at 7 GHz in the ranges of 35–38 and 2800–5000, respectively.     

Synthesis of BaCu(B2O5) Ceramics and their Effect on the Sintering Temperature and Microwave Dielectric Properties of Ba(Zn1/3Nb2/3)O3 Ceramics

BaCu(B₂O₅) ceramics were synthesized and their microwave dielectric properties were investigated. BaCu(B₂O₅) phase was formed at 700°C and melted above 850°C. The BaCu(B₂O₅) ceramic sintered at 810°C had a dielectric constant (ɛ_r) of 7.4, a quality factor ( Q × f ) of 50 000 GHz and a temperature coefficient of resonance frequency (τ_f) of −32 ppm/°C. As the BaCu(B₂O₅) ceramic had a low melting temperature and good microwave dielectric properties, it can be used as a low-temperature sintering aid for microwave dielectric materials for low temperature co-fired ceramic application. When BaCu(B₂O₅) was added to the Ba(Zn_1/3Nb_2/3)O₃ (BZN) ceramic, BZN ceramics were well sintered even at 850°C. BaCu(B₂O₅) existed as a liquid phase during the sintering and assisted the densification of the BZN ceramic. Good microwave dielectric properties of Q × f =16 000 GHz, ɛ_r=35, and τ_f=22.1 ppm/°C were obtained for the BZN+6.0 mol% BaCu(B₂O₅) ceramic sintered at 875°C for 2 h.     

Effect of Additives on the Electromechanical Properties of Pb(Zr,Ti)O3–Pb(Y2/3W1/3)O3 Ceramics

Dielectric and piezoelectric properties of 0.02Pb(Y_2/3W_1/3)O₃ 0.98Pb(Zr0.52Ti0.48)O3 ceramics doped with additives (Nb₂O₅, La₂O₃, MnO₂, and Fe₂O₃) were investigated. The grain sizes of these ceramics decreased with increasing amounts of additives. For additions of MnO₂ and Fe₂O₃, dielectric losses decreased, while for Nb₂O₅ and La₂O₃, these values increased. The maximum values of the mechanical quality factor Q_m were found to be 956 and 975 for additions of 0.9 wt% Fe₂O₃ and 0.7 wt% MnO₂, respectively, but donor dopants (Nb₂O₅ and La₂O₃) did not change the values of Q_m . On the other hand, the piezoelectric constant d₃₃ and the electromechanical coupling factor k_p decreased with additions of MnO₂ and Fe₂O₃, but improved with additions of Nb₂O₅ and La₂O₃.     

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.     

High Q Microwave Dielectric Ceramics in (Ni1−x Znx)Nb2O6 System

(Ni_{1− x} Zn _x )Nb₂O₆, 0≤ x ≤1.0, ceramics with >97% density were prepared by a conventional solid-state reaction, followed by sintering at 1200°–1300°C (depending on the value of x ). The XRD patterns of the sintered samples (0≤ x ≤1.0) revealed single-phase formation with a columbite ( Pbcn ) structure. The unit cell volume slightly increased with increasing Zn content ( x ). All the compositions showed high electrical resistivity (ρ_dc=1.6±0.3 × 10¹¹Ω·cm). The microwave (4–5 GHz) dielectric properties of (Ni_{1− x} Zn _x )Nb₂O₆ ceramics exhibited a significant dependence on the Zn content and to some extent on the morphology of the grains. As x was increased from 0 to 1, the average grain size monotonically increased from 7.6 to 21.2 μm and the microwave dielectric constant (ɛ'_r) increased from 23.6 to 26.1, while the quality factors ( Q _u× f ) increased from 18 900 to 103 730 GHz and the temperature coefficient of resonant frequency (τ_f) increased from −62 to −73 ppm/°C. In the present work, we report the highest observed values of Q _u× f =103 730 GHz, and ɛ'_r=26.1 for the ZnNb₂O₆-sintered ceramics.     

Microwave Dielectric Properties and Low-Temperature Sintering of Ba0.60Sr0.40TiO3 Ceramics

In the present work, the sintering behaviors and dielectric properties of Ba_0.60Sr_0.40TiO₃ (BST) ceramics with the addition of BaCu(B₂O₅) were investigated in detail. The results indicated that the addition reduced the sintering temperature of BST by about 500°C. It was suggested that a liquid phase BaCu(B₂O₅) assisted the densification of BST ceramics at lower temperatures. For a low-level BaCu(B₂O₅) addition (2.0 mol%), the BST sample sintered at 950°C for 5 h displayed good dielectric properties, with a moderate dielectric constant (ɛ=2553) and a low dielectric loss (tan δ=0.00305) at room temperature and at 10 kHz. The sample showed 45.9% tunability at 10 kHz under a dc electric field of 30 kV/cm. At the frequency of 0.984 GHz, BST-added 2.0 mol% BaCu(B₂O₅) possessed a dielectric constant of 2204 and a Q value of 146.7.     

Investigation on the Microstructure and Dielectric Properties of BaTiO3–Nb2O5–Fe2O3 Materials Doped with La2O3

The influence of La₂O₃ doped on the microstructure and dielectric properties, including the phase structure, temperature dependence of permittivity, and the hysteresis loop of BaTiO₃–Nb₂O₅–Fe₂O₃ (BTNF) materials has been investigated in X-ray diffraction, SEM, and LCR analyzer, respectively. Experiments revealed that incorporation of proper content of La₂O₃ basically soluted in the lattice of BaTiO₃ and can control the grain-growth, reduce the dielectric loss of the BTNF materials. The development of microstructure promoted by the additives can result in the improvement of the dielectric constant. When the doping concentration of La₂O₃ was 3.846 wt%, the relative dielectric constant of the sample sintered at 1280°C only for 2 h could reach 4308, and improve the dielectric-temperature characteristics markedly. As a result, a novel Y5P can be achieved in the BTNF ceramics, which is very promising for practical use in Y5P multilayer ceramic capacitors.     

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.     

Structural and Dielectric Properties of Ag(Nb0.8Ta0.2)1−xSbxO3 (x≤0.08) Ceramics

Sb₂O₅ were selected to substitute (Nb_0.8Ta_0.2)₂O₅ and the effects of Sb substitution on the dielectric properties of Ag(Nb_0.8Ta_0.2)O₃ ceramics were studied. The perovskite Ag(Nb_0.8Ta_0.2)_{1− x} Sb _x O₃ ceramics showed no obvious change with x value being no more than 0.08, and the pseudoperovskite unit cell parameters a = c , b and monoclinic angle β decrease with Sb concentration increasing. The dielectric properties of Ag(Nb_0.8Ta_0.2)_{1− x} Sb _x O₃ ceramics were found to be affected greatly by the substitution of Sb for Nb/Ta. The ɛ value of Ag(Nb_0.8Ta_0.2)_{1− x} Sb _x O₃ ceramics sintered at their densified temperature increased from 480 to 825 with x from 0 to 0.08, the tan δ value decreased sharply from 0.0065 to 0.0023 (at 1 MHz) with x increasing from 0 to 0.04, and then kept a stable lower tan δ value ∼0.0024 with x to 0.08. The temperature coefficient of capacitance values continuously decreased from a positive value of 1450 ppm/°C for x =0 to a negative value of −38.52 ppm/°C for x =0.08.     

Effects of B2O3 Addition on the Dielectric and Ferroelectric Properties of Ba0.7Sr0.3TiO3 Ceramics

The effects of B₂O₃ addition on the sintering behavior and the dielectric and ferroelectric properties of Ba_0.7Sr_0.3TiO₃ (BST) ceramics were investigated. The dielectric and ferroelectric properties of a BST sample with 0.5 wt% B₂O₃ sintered at <1150°C were as good as those of undoped BST sintered at 1350°C, and the dielectric loss was better. When >1.0 wt% B₂O₃ was added to BST, the overdoped B₂O₃ did not form a liquid phase or volatilize; it remained in the samples and formed a secondary phase that lowered the sintering behavior and the dielectric and ferroelectric properties of the BST.     

Grain-Boundary Migration and Dielectric Properties of Semiconducting SrTiO3 in the SrTiO3-BaTiO3-CaTiO3 System

Chemically induced grain-boundary migration and its effects on the interface and dielectric properties of semiconducting SrTiO₃ have been investigated. Strontium titanate specimens that had been doped with 0.2 mol% of Nb₂O₅ were sintered in 5H₂/95N₂. The sintered specimens were diffusion annealed at 1400°C in 5H₂/95N₂ with BaTiO₃ or 0.5BaTiO₃-0.5CaTiO₃ (mole fraction) packing powder. The grain boundaries of the annealed specimens were oxidized in air. In the case of BaTiO₃ packing, grain-boundary migration occurred with the diffusion of BaTiO₃ along the grain boundary. The effective dielectric constant of the specimen decreased gradually as the temperature increased but showed two peaks, possibly because of barium enrichment at the grain boundary and an oxidized Sr(Ba)TiO₃ layer. In the case of 0.5BaTiO₃-0.5CaTiO₃ packing, although barium and calcium were present at the grain boundary of the specimen, no boundary migration occurred, as in a previous investigation. With the diffusion of barium and calcium, the resistivity of the specimen increased and the variation of the effective dielectric constant with temperature was much reduced, in comparison to those without solute diffusion. These enhanced properties were attributed to the solute enrichment and the formation of a thin diffusional Sr(Ba,Ca)TiO₃ layer at the grain boundary.     

Effects of La2O3/ZnO Additives on Microstructure And Microwave Dielectric Properties of Zr0.8Sn0.2TiO4 Ceramics

Dielectric ceramics of Zr_0.8Sn_0.2TiO₄ containing La₂O₃ and ZnO as sintering aids were prepared and investigated for microstructure and microwave dielectric properties. Low-level doping with La₂O₃ and ZnO (up to 0.30 wt%) is good for densification and dielectric properties. These additives do not affect the dielectric constant and the temperature coefficient. Dielectric losses increase significantly at additive levels higher than 0.15 wt%. The combined additives La₂O₃ and ZnO act as grain growth enhancers. With 0.15 wt% additives, a ceramic having a dielectric constant, a quality factor, and a temperature coefficient of frequency at 4.2 GHz of 37.6, 12 800, and –2.9 ppm/°C, respectively, was obtained. The quality factor was considerably improved by prolonged sintering.     

Influence of V2O5 Additions to 5Li2O–1Nb2O5–5TiO2 Ceramics on Sintering Temperature and Microwave Dielectric Properties

The effects of the addition of V₂O₅ on the sintering behavior, microstructure, and microwave dielectric properties of 5Li₂O–1Nb₂O₅–5TiO₂ (LNT) ceramics have been investigated. With low-level doping of V₂O₅ (≤3 wt%), the microstructure of the LNT ceramic changed from a special two-level intergrowth structure into a two-phase composite structure with separate grains. And the sintering temperature of the LNT ceramics could be lowered to around 900°C by adding a small amount of V₂O₅ without much degradation in microwave dielectric properties. Typically, better microwave dielectric properties of ɛ_r=41.7, Q × f =7820 GHz, and τ _f =45 ppm/°C could be obtained for the 1 wt% V₂O₅-doped ceramics sintered at 900°C.     

Effect of Bi2O3 Addition on the Sintering Temperature and Microwave Dielectric Properties of Zn2SiO4 Ceramics

Bi₂O₃ was added to a nominal composition of Zn_1.8SiO_3.8 (ZS) ceramics to decrease their sintering temperature. When the Bi₂O₃ content was <8.0 mol%, a porous microstructure with Bi₄(SiO₄)₃ and SiO₂ second phases was developed in the specimen sintered at 885°C. However, when the Bi₂O₃ content exceeded 8.0 mol%, a liquid phase, which formed during sintering at temperatures below 900°C, assisted the densification of the ZS ceramics. Good microwave dielectric properties of Q × f =12,600 GHz, ɛ_r=7.6, and τ_f=−22 ppm/°C were obtained from the specimen with 8.0 mol% Bi₂O₃ sintered at 885°C for 2 h.     

Microwave Dielectric Properties of 5Li2O–0.583Nb2O5–3.248TiO2 Ceramics with V2O5

The effects of V₂O₅ addition on the sintering behavior, microstructure, and the microwave dielectric properties of 5Li₂O–0.583Nb₂O₅–3.248TiO₂ (LNT) ceramics have been investigated. With addition of low-level doping of V₂O₅ (≤2 wt%), the sintering temperature of the LNT ceramics could be lowered down to around 920°C due to the liquid phase effect. A secondary phase was observed at the level of 2 wt% V₂O₅ addition. The addition of V₂O₅ does not induce much degradation in the microwave dielectric properties but lowers the τ_f value to near zero. Typically, the excellent microwave dielectric properties of ɛ_r=21.5, Q × f =32 938 GHz, and τ_f=6.1 ppm/°C could be obtained for the 1 wt% V₂O₅-doped sample sintered at 920°C, which is promising for application of the multilayer microwave devices using Ag as an internal electrode.     

Microstructure and Nonlinear Properties of Microwave-Sintered ZnO-V2O5 Varistors: I, Effect of V2O5 Doping

The modification of the densification behavior and the grain-growth characteristics of the microwave-sintered ZnO materials, caused by the incorporation of V₂O₅ additives, have been systematically studied. Generally, the addition of V₂O₅ markedly enhances the densification rate, such that a density as high as 97.9% of the theoretical density and a grain size as large as 10 µm can be attained for a sintering temperature as low as 800°C and a soaking time as short as 10 min. Increasing the sintering temperature or soaking time does not significantly change the sintered density of the ZnO-V₂O₅ materials but it does monotonously increase their grain size. Varying the proportion of V₂O₅ in the range of 0.2-1.0 mol% does not pronouncedly modify such behavior. The leakage current density ( J _L) of these high-density and uniform-granular-structure samples is still large, which is amended by the incorporation of 0.3 mol% of Mn₃O₄ in the ZnO materials, in addition to 0.5 mol% of the V₂O₅ additives. Samples that are obtained using such a method possess good nonohmic characteristics (α= 23.5) and a low leakage current density ( J _L= 2.4 10^-6 A/cm²).     

Effect of Nb2O5 Alloying on Thermal Expansion Anisotropy of 2 mol% Y2O3-Stabilized Tetragonal ZrO2

Tetragonal ZrO₂ ( t -ZrO₂) solid solutions were prepared with addit ons of 2 mol% Y₂O₃ plus up to 0.45 mol% Nb₂O₅. The thermal expansion coefficients in both the a- and c -axis lattice directions increased with Nb₂O₅ alloying and the thermal expansion in the c -axis direction was greater than that in the a -axis direction over the entire composition range. This anisotropic thermal expansion behavior was related to the 4-fold coordination of Nb⁵⁺ with oxygen ions in t -ZrO₂ solid solutions in the system ZrO₂–Y₂O₃–Nb₂O₅. The fracture toughness continuously increased with Nb₂O₅ alloying and suggested that the c/a axial ratio is a more significant factor than the internal stress that arises from the thermal expansion anisotropy, in the determination of the transformability of t -ZrO₂ in this system.