Microwave Dielectric Properties of Low-Fired Ba5Nb4O15

The effect of B₂O₃ on the sintering temperature and microwave dielectric properties of Ba₅Nb₄O₁₅ has been investigated using X-ray powder diffraction, scanning electron microscopy, and a network analyzer. Interactions between Ba₅Nb₄O₁₅ and B₂O₃ led to formation of second phases, BaNb₂O₆ and BaB₂O₄. The addition of B₂O₃ to Ba₅Nb₄O₁₅ resulted in lowering the sintering temperature from 1400° to 925°C. Low-fired Ba₅Nb₄O₁₅ could be interpreted by measuring changes in the quality factor ( Q × f ), the relative dielectric constant (ɛ_r), and the temperature coefficient of resonant frequency (τ_f) as a function of B₂O₃ additions. More importantly, the formation of BaNb₂O₆ provided temperature compensation. The microwave dielectric properties of low-fired Ba₅Nb₄O₁₅ had good dielectric properties: Q × f = 18700 GHz, ɛ_r= 39, and τ_f= 0 ppm/°C.     

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.     

Origin of Microwave Dielectric Loss in ZnNb2O6-TiO2

(1− x )ZnNb₂O₆· x TiO₂ ceramics were prepared using both anatase and rutile forms of TiO₂. At a composition of x = 0.58, a mixture region of ixiolite (ZnTiNb₂O₈) and rutile was observed and the temperature coefficient of resonant frequency (τ_f) was ∼0 ppm/°C. We found that although ɛ_r and τ_f were comparable, the quality factor ( Q × f , Q ≈ 1/ tan δ, f = resonant frequency) of 0.42ZnNb₂O₆·0.58TiO₂ prepared from anatase and rutile was 6000 and 29 000, respectively. The origin of the difference in Q × f of both samples was investigated by measuring electrical conductivity and by analysis of the anatase–rutile phase transition. The anatase-derived sample had higher conductivity, which was related to the reduction of Ti⁴⁺. It is suggested that the increase of dielectric loss originates from an increase in Ti³⁺ and oxygen vacancies due to an anatase–rutile phase transition.     

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.     

Synthesis and Microwave Dielectric Properties of Re3Ga5O12 (Re: Nd, Sm, Eu, Dy, Yb, and Y) Ceramics

Re₃Ga₅O₁₂ (Re: Nd, Sm, Eu, Dy, Yb, and Y) garnet ceramics were synthesized and their microwave dielectric properties were investigated for advanced substrate materials in microwave integrated circuits. The Re₃Ga₅O₁₂ ceramics sintered at 1350°–1500°C had a high-quality factor ( Q × f ) ranging from 40 000 to 192 173 GHz and a low-dielectric constant (ɛ_r) of between 11.5 and 12.5. They also exhibited a relatively stable temperature coefficient of resonant frequency (τ_f) in the range of −33.7 to −12.4 ppm/°C. In particular, the Sm₃Ga₅O₁₂ ceramics sintered at 1450°C exhibited good microwave dielectric properties of ɛ_r=12.4, Q × f =192 173 GHz, and τ_f=−19.2 ppm/°C.     

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.     

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.     

Low-Dielectric Loss Characteristics of Nd(Co1/2Ti1/2)O3 Ceramics at Microwave Frequencies

The microwave dielectric properties and the microstructures of Nd(Co_1/2Ti_1/2)O₃ (NCT) ceramics using starting powders of Nd₂O₃, CoO, and TiO₂ prepared by the conventional solid-state route have been researched. The dielectric constant values (ɛ_r) saturated at 24.8–27. Quality factor ( Q × f ) values of 37 900–140 000 (at 9 GHz) and the measured τ_f values ranging from −45 to −48 ppm/°C can be obtained when the sintering temperatures are in the range of 1410°–1500°C. The ɛ_r value of 27, the Q × f value of 140 000 (at 9 GHz) and the τ_f value of −46 ppm/°C were obtained for NCT ceramics sintered at 1440°C for 4 h. For applications of high selective microwave ceramic resonator, filter, and antenna, NCT is proposed as a suitable material candidate.     

Glass-Free Zn2Te3O8 Microwave Ceramic for LTCC Applications

A Zn₂Te₃O₈ ceramic was investigated as a promising dielectric material for low-temperature co-fired ceramics (LTCC) applications. The Zn₂Te₃O₈ ceramic was synthesized using the solid-state reaction method by sintering in the temperature range 540°–600°C. The structure and microstructure of the compounds were investigated using X-ray diffraction (XRD) and scanning electron microscopy methods. The dielectric properties of the ceramics were studied in the frequency range 4–6 GHz. The Zn₂Te₃O₈ ceramic has a dielectric constant (ɛ_r) of 16.2, a quality factor ( Q _u× f ) of 66 000 at 4.97 GHz, and a temperature coefficient of resonant frequency (τ_f) of −60 ppm/°C, respectively. Addition of 4 wt% TiO₂ improved the τ_f to −8.7 ppm/°C with an ɛ_r of 19.3 and a Q _u× f of 27 000 at 5.14 GHz when sintered at 650°C. The chemical reactivity of the Zn₂Te₃O₈ ceramic with Ag and Al metal electrodes was also investigated.     

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.     

Characterization and Microwave Dielectric Properties of M2+Nb2O6 Ceramics

This paper details the investigation of the quality factor ( Q ), dielectric permittivity (ɛ_r) and temperature coefficient of resonant frequency (τ_f) of the TE_01δ mode of the columbite binary niobate ceramics, with the formula MNb₂O₆ where M=²⁺ cation, in relation to their degree of sintering, microstructure and phase composition. The ceramics were made from a mixed oxide preparative route and fired over a range of temperatures from 800° to 1400°C, and most formed the columbite structure. A comprehensive study was made of the niobates containing the transition metal cations M=Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺, and the group II metal cations M=Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺. All columbite niobates were found to have ɛ_r between 17 and 22 and negative τ_f values between –45 and –76 ppm/°C, and ZnNb₂O₆, MgNb₂O₆, CaNb₂O₆, and CoNb₂O₆ had high Q f values of 84 500, 79 600, 49 600, and 41 700 GHz, respectively. The Q f of MgNb₂O₆ was found to rise to over 95 000 GHz when heated at 1300°C for 50 h.     

Structure–Property Relations in xBaTiO3–(1−x)La(Mg1/2Ti1/2)O3 Solid Solutions

The microwave dielectric properties and microstructures of compounds in the solid solution series x BaTiO₃–(1− x )La(Mg_1/2Ti_1/2)O₃ (BTLMT) have been investigated. The structural phase transitions that occur as a function of x have been studied and are related to changes in the dielectric properties. For compounds where x ≤ 0.1, X-ray diffraction (XRD) showed evidence of 1:1 ordering between Mg and Ti cations. For x ≤ 0.3, XRD and electron diffraction revealed that compounds were tilted in both antiphase and in-phase. However, for 0.3 < x < 0.7, only antiphase tilting was present. The temperature coefficient of resonant frequency (τ_f) vs the relative permittivity (ɛ_r) was linear until x = 0.5 at which point in the solid solution the transition to a nontilted structure resulted in nonlinear behavior. τ_f values close to zero (−2 ppm/°C) were achieved at x = 0.5 (ɛ_r∼ 60), which had a quality factor ( Q · f _o) of 9600 GHz.     

BiNbO4-Based Glass–Ceramic Composites for Microwave Applications

The effect of a bespoke glass sintering aid, 0.3Bi₂O₃–0.3Nb₂O₅–0.3B₂O₃–0.1SiO₂ (BN1), developed from the base ceramic composition, BiNbO₄ (BN), on the sinterability, microstructure, and microwave (MW) dielectric properties of BN ceramics has been investigated. Densities >97% theoretical could be achieved at 1020°C for samples with up to 15% BN1 additions. The resulting microstructure was composed of BN laths surrounded by a residual glass phase that contained small fibrous crystals. Some evidence of dissolution of BN crystals was observed. Optimum properties were exhibited for samples with 15 wt% of glass addition sintered for 4 h at 1020°C with a relative permittivity ɛ_r=38, a MW quality factor Q × f ₀=17 353 at 5.6 GHz, and a temperature coefficient of resonant frequency τ_f=−10 ppm/°C. The high Q × f ₀, ɛ_r, and low τ_f, coupled with a relatively low sintering temperature, suggest that the use of bespoke glass sintering aids of this type may have great potential for the fabrication of MW ceramics.     

Influence of Copper(II) Oxide Additions to Zinc Niobate Microwave Ceramics on Sintering Temperature and Dielectric Properties

The effect of CuO additions on the firing temperature of ZnNb₂O₆ ceramics was investigated using dilatometry, transmission electron microscopy, and X-ray diffractometry. A 5 wt% CuO addition to ZnNb₂O₆ ceramics significantly lowered the firing temperature from 1150° to ∼900°C. The presence of a CuO-rich intergranular phase in the specimen was observed and was evidence of the formation of a liquid phase during sintering. The composition of the liquid phase was (ZnCu₂)Nb₂O₈. In particular, the low-fired ZnNb₂O₆ ceramics had good microwave dielectric characteristics— Q × f = 59 500, ɛ_r= 22.1, τ_f=–66 ppm/^oC. These properties were correlated with the formation of a second phase, (ZnCu₂)Nb₂O₈.     

Sintering Behavior and Dielectric Properties of Bi3NbO7 Ceramics Prepared by Mixed Oxides and High-Energy Ball-Milling Methods

The sintering behavior and dielectric properties of Bi₃NbO₇ ceramics prepared by the high-energy ball milling (HEM) method and conventional mixed oxides method with V₂O₅ addition were investigated. All the samples were sintered between 840° and 960°C. For the ceramics prepared by the mixed oxides method, the pure tetragonal Bi₃NbO₇ phase formed without any cubic phase. With changing sintering temperature, the dielectric constant ɛ_r lies between 79 and 92, while the Q × f values are between 300 and 640 GHz. The samples sintered at 870°C have the best microwave dielectric properties with ɛ_r=79, Q × f =640 GHz, and the temperature coefficients of resonant frequency τ_f between 0 and −20 ppm/°C. For the ceramics prepared by the HEM, a pure cubic phase was obtained. The ɛ_r changes between 78 and 80 and Q × f were between 200 and 290 GHz.     

Dielectric Properties of (1−x)CaTiO3–xCa(Zn1/3Nb2/3)O3 Ceramic System at Microwave Frequency

The microwave dielectric properties of the (1− x )CaTiO₃– x Ca(Zn_1/3Nb_2/3)O₃ ceramic system have been investigated. The ceramic samples sintered at 1300°–1450°C for 4 h in air exhibit orthorhombic pervoskite and form a complete solid solution for different x value. When the x value increased from 0.2 to 0.8, the permittivity ɛ_r decreased from 115 to 42, the unloaded quality factor Q × f increased from 5030 to 13 030 GHz, and the temperature coefficient τ_f decreased from 336 to −28 ppm/°C. When x =0.7, the best combination of dielectric properties, a near zero temperature coefficient of resonant frequency of τ_f∼−6 ppm/°C, Q × f ∼10 860 GHz and ɛ_r∼51 is obtained.     

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.     

Dielectric Characteristics of Nd(Zn1/2Ti1/2)O3 Ceramics at Microwave Frequencies

The microwave dielectric properties and the microstructures of Nd(Zn_1/2Ti_1/2)O₃ (NZT) ceramics prepared by the conventional solid-state route have been studied. The prepared NZT exhibited a mixture of Zn and Ti showing 1:1 order in the B-site. The dielectric constant values (ɛ_r) saturated at 29.1–31.6. The quality factor ( Q × f ) values of 56 700–170 000 (at 8.5 GHz) can be obtained when the sintering temperatures are in the range of 1300°–1420°C. The temperature coefficient of resonant frequency τ_f was not sensitive to the sintering temperature. The ɛ _r value of 31.6, the Q × f value of 170 000 (at 8.5 GHz), and the τ_f value of −42 ppm/°C were obtained for NZT ceramics sintering at 1330°C for 4 h. For applications of high selective microwave ceramic resonators, filters, and antennas, NZT is proposed as a suitable material candidate.     

Microwave Dielectric Properties and Chemical Resistance of Low-Temperature-Sintered CaZrB2O6 Ceramics

Dolomite-type borate ceramics consisting of CaZrB₂O₆ were synthesized via a conventional solid-state reaction route; low-temperature sintering was explored using Bi₂O₃–CuO additives of 1–7 wt% for low-temperature co-fired ceramics applications. For several sintering temperatures, the microwave dielectric properties and chemical resistance of the ceramics were investigated. The CaZrB₂O₆ ceramics with 3 wt% Bi₂O₃–CuO addition could be sintered below 925°C, and the microwave dielectric properties of the low-temperature samples were ɛ_r=10.55, Q × f =87,350 GHz, and τ_f=+2 ppm/°C. The chemical resistance test result showed that both CaZrB₂O₆- and Bi₂O₃–CuO-added CaZrB₂O₆ ceramics were durable in basic solution but were degraded in acid solution.     

Microwave Dielectric Properties of La5CrTi3O15 and La4MCrTi3O15 (M=Pr, Nd, and Sm) Ceramics

The La₅CrTi₃O₁₅ and La₄MCrTi₃O₁₅ (M=Pr, Nd, and Sm) microwave dielectric ceramics were prepared by the conventional solid-state ceramic route. The structure and microstructure of the ceramics were studied by X-ray diffraction and scanning electron microscopy methods. The dielectric properties of the ceramics were measured in the microwave frequency region using a network analyzer by the resonance method. The ceramics show a dielectric constant (ɛ_r) in the range of 37 to 39.5, a quality factor ( Q _u× f _o) 17,300 to 34,000 GHz, and a temperature coefficient of resonant frequency (τ_f) in the range from −22 to −38 ppm/°C.