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.     

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.     

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 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.     

Low Loss Dielectrics in the Ca(B'1/2Nb1/2)O3 [B'=Lanthanides, Y] System

The Ca(B'_1/2Nb_1/2)O₃ [B'=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, and In] complex perovskites have been prepared by conventional solid-state ceramic route. The structure and microstructure of the ceramics have been characterized by X-ray diffraction and scanning electron microscopy methods. The ceramics have dielectric constant (ɛ_r) in the range 23–32, normalized Q -factor ( Q _u× f ) 11 000–38 000 GHz and temperature coefficient of resonant frequency (τ_f) −43–5.2 ppm/°C. The microwave dielectric properties of Ca(B'_1/2Nb_1/2)O₃ ceramics are found to depend on the ionic radii of B'-site elements and tolerance factor ( t ). The substitution of Ba²⁺ and Sr²⁺ for Ca²⁺ resulted a phase transition in Ca(B'_1/2Nb_1/2)O₃ ceramics. The (Ca_0.05Ba_0.95) (Y_1/2Nb_1/2)O₃ has τ_f close to zero (1.2 ppm/°C) with ɛ_r=35 and Q _u× f =48 500 GHz and is proposed as a useful material for base station applications. Dielectric properties of the Ca(B'_1/2Nb_1/2)O₃ ceramics were tailored by the addition of TiO₂ and CaTiO₃.     

Formation and Microwave Dielectric Properties of the Mg2V2O7 Ceramics

The formation process and microwave dielectric properties of the Mg₂V₂O₇ ceramics were investigated. The MgV₂O₆ phase that was formed at around 450°C interacted with remnant MgO above 590°C to form a homogeneous monoclinic Mg₂V₂O₇ phase. Finally, this monoclinic Mg₂V₂O₇ phase was changed to a triclinic Mg₂V₂O₇ phase for the specimen fired at 800°C. Sintering at 950°C for more than 5 h produced high-density triclinic Mg₂V₂O₇ ceramics. In particular, the Mg₂V₂O₇ ceramics sintered at 950°C for 10 h exhibited the good microwave dielectric properties of ɛ_r=10.5, Q × f =58 275 GHz, and τ_f=−26.9 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.     

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 Li2CO3 Addition on the Sintering Temperature and Microwave Dielectric Properties of Mg2V2O7 Ceramics

Li₂CO₃ was added to Mg₂V₂O₇ ceramics in order to reduce the sintering temperature to below 900°C. At temperatures below 900°C, a liquid phase was formed during sintering, which assisted the densification of the specimens. The addition of Li₂CO₃ changed the crystal structure of Mg₂V₂O₇ ceramics from triclinic to monoclinic. The 6.0 mol% Li₂CO₃-added Mg₂V₂O₇ ceramic was well sintered at 800°C with a high density and good microwave dielectric properties of ɛ _r=8.2, Q × f =70 621 GHz, and τ_f=−35.2 ppm/°C. Silver did not react with the 6.0 mol% Li₂CO₃-added Mg₂V₂O₇ ceramic at 800°C. Therefore, this ceramic is a good candidate material in low-temperature co-fired ceramic multilayer devices.     

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 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.     

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.     

Low-Temperature Synthesis and Growth of Bulk Ba2Ti9O20 Ceramics on Conducting Substrates for Applications in Integrated Dielectric Resonator Devices

This paper describes the synthesis of Ba₂Ti₉O₂₀ ceramics at extremely low temperatures (∼150°C) and the subsequent growth of bulk resonators on silicon substrates by hydrothermal processing of their sol–gel composites. X-ray diffraction analysis shows excellent crystallinity, while scanning electron microscopy evidenced densification and development of bridging structures at the grain boundaries and interfaces. Transmission electron micrographs further confirmed the development of sol–gel-derived crystalline interfaces between sol–gel-derived material and powder particles. The dielectric properties of a resonator measured in the 5–6 GHz microwave frequency range were ɛ_r=38 and Q _u× f =12,000 at 5.6 GHz and τ_f=+6 ppm/°C. The density, dielectric properties, and mechanical strength at the bulk ceramic–thin film interface are enhanced by a hydrothermally induced dissolution–crystallization process, which leads to interparticle bridges. The novel low-temperature ceramic process has high potential for the growth of ceramic resonators on integrated circuits and is demonstrated by the fabrication of an integrated dielectric resonator antenna for system-on-chip applications.     

Synthesis, Characterization, and Microwave Dielectric Properties of ATiO3 (A=Co, Mn, Ni) Ceramics

The ATiO₃ (A=Co, Mn, Ni) dielectric ceramics have been synthesized by the conventional solid-state ceramic route. The structure and microstructure of these ceramic samples have been studied using powder X-ray diffraction and scanning electron microscopy. The microwave dielectric properties such as relative permittivity (ɛ_r), quality factor ( Q _u× f ), and coefficient of temperature variation of resonant frequency (τ_f) of the ceramics have been measured in the frequency range 4–6 GHz using resonance methods. The dielectric constant of ATiO₃ (A=Co, Mn, Ni) varies from 19 to 25 and τ_f close to −50 ppm/°C. The ceramics have high-quality factors ( Q _u× f ) of 62 500 GHz (at 5.42 GHz) for CoTiO₃, 15 200 GHz (at 5.22 GHz) for MnTiO₃, and 13 900 GHz (at 5.24 GHz) for NiTiO₃, respectively.     

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.     

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.     

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.     

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.     

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.