Microwave Characteristics of BaO-TiO2 Ceramics Prepared via a Citrate Route

Microwave dielectrics of the TiO₂-rich BaO-TiO₂ system (BaTi₄O₉ and Ba₂Ti₉O₂₀) were prepared by the citrate route. Pure and well-crystallized BaTi₄O₉ and Ba₂Ti₉O₂₀ particles of nanometer size (30–50 nm) could be obtained by thermal decomposition of citrate gel precursors. After sintering at 1200°–1350°C (for 2–10 h), dense compounds with >90% of theoretical density could be obtained. Dielectric properties of disk-shaped sintered specimens, in the microwave frequency region, were measured in the TE_01δ mode. They were found to have excellent microwave characteristics: for BaTi₄O₉, ε_r= 36, Q = 4900 at 10.3 GHz, and τ_f= 16 ppm/°C; and for Ba₂Ti₉O₂₀, ε_r= 37, Q = 5300 at 10.7 GHz, and τ_f=−6.0 ppm/°C.     

Preparation of BaTi4O9 from Oxalates

A barium titanate precursor with a barium:titanium ratio of 1:4 was prepared by controlled coprecipitation of mixed barium and titanium species with an ammonium oxalate aqueous solution at pH 7. The results of thermal analysis and IR measurement show that the obtained precursor is a mixture of BaC₂O₄·0.5H₂O and TiO(OH)₂·1.5H₂O in a molar ratio of 1:4. Crystallized BaTi₄O₉ was obtained by the thermal decomposition of a precipitate precursor at 1300°C for 2 h in air. The dimensions of the powder calcined at 1000°C are between 100 and 300 nm. The grain dimensions of the sintered sample for 2 h at 1300°C are of the order of 10 to 30 μm. Dielectric properties of disk-shaped sintered specimens in the microwave frequency region were measured using the TE₀₁₁ mode. Excellent microwave characteristics for BaTi₄O₉—ɛ= 38 ± 0.5, Q = 3800–4000 at 6–7 GHz and τ _f = 11 ± 0.7 ppm/°C—were found.     

Ultra-Fine Ba2Ti9O20 Powders Synthesized by Inverse Microemulsion Processing and their Microwave Dielectric Properties

A double–inverse microemulsion (IME) process is used for synthesizing nano-sized Ba₂Ti₉O₂₀ powders. The crystallization of powders thus obtained and the microwave dielectric properties of the sintered materials were examined. The IME-derived powders are of nano-size (∼21.5 nm) and possess high activity. The BaTi₅O₁₁, intermediate phase resulted when the IME-derived powders were calcined at 800°C (4 h) in air. However, high-density Ba₂Ti₉O₂₀ materials with a pure triclinic phase (Hollandite like) can still be obtained by sintering such a BaTi₅O₁₁ dominated powders at 1250°C/4 h. The phase transformation kinetics for the IME-derived powders were markedly enhanced when air was replaced by O₂ during the calcinations and sintering processes. Both the calcination and densification temperatures were reduced by around 50°C compared with the process undertaken in air. The microwave dielectric properties of sintered materials increase with the density of the samples, resulting in a large dielectric constant ( K ≅39) and high-quality factor ( Q × f ≅28 000 GHz) for samples possessing a density higher than 95% theoretical density, regardless of the sintering atmosphere. Overfiring dissociates Ba₂Ti₉O₂₀ materials and results in a poor-quality factor.     

Sol–Gel Preparation of BaTi4O9 and Ba2Ti9O20

BaTi₄O₉ and Ba₂Ti₉O₂₀ precursors were prepared via a sol–gel method, using ethylenediaminetetraacetic acid as a chelating agent. The sol–gel precursors were heated at 700°–1200°C in air, and X-ray diffractometry (XRD) was used to determine the phase transformations as a function of temperature. Single-phase BaTi₄O₉ could not be obtained, even after heating the precursors at 1200°C for 2 h, whereas single-phase Ba₂Ti₉O₂₀ (as determined via XRD) was obtained at 1200°C for 2 h. Details of the synthesis and characterization of the resultant products have been given.     

Low-Fire Processing of Microwave BaTi4O9 Dielectric with BaO–ZnO–B2O3 Glass

The effects of BaO—ZnO–B₂O₃ (BZB) glass addition on the densification and dielectric properties of BaTi₄O₉ (BT4) have been investigated. With increasing BaO content in the BZB glass, the softening and melting points of the resulting BZB glass decrease, but the wetting between BZB and BT4 improves cosiderably. Although the densification temperature is reduced from 1300°C for pure BT4 to 925°C for BT4+BZB dielectric ceramics, the enhancement in densification becomes less significant with increasing BaO content in the BZB glass. The above result is attributed to a chemical reaction taking place at the interface of BZB/BT4 during firing, which becomes less extensive with increasing BaO content in the BZB glass. For the BZB glass with a BaO content in the range of 0–20 mol%, the resulting 90 vol% BT4+10 vol% BZB microwave dielectric has a dielectric constant of 28–33, and a product ( Q × f _r) of quality factor ( Q ) and a resonant frequency ( f _r) of 15 000–20 000 GHz at 6.6 GHz.     

Microwave Dielectric Properties and Low-Temperature Cofiring of BaTe4O9 with Aluminum Metal Electrode

Polycrystalline BaTe₄O₉ ceramic compound was investigated as a promising microwave dielectric compound for low-temperature cofired ceramics (LTCC) applications. The binary phase BaTe₄O₉ was synthesized and subsequently densified over the temperature range of only 500°–550°C, which allows for low-temperature cofiring with aluminum metal. The dielectric properties of BaTe₄O₉ ceramics sintered at 550°C for 2 h were determined in the microwave region of 12–14 GHz. The dielectric constant and Q × f product obtained were 17.5 and 54 700 GHz at 12 GHz, respectively. The temperature coefficient of resonance frequency showed a negative value of −90 ppm/°C. In terms of its evaluation for LTCC, the BaTe₄O₉ composition was found to be chemically compatible and successfully cofired with highly conductive aluminum electrode, while maintaining good electrical performance.     

A New BaO-TiO2 Compound with Temperature-Stable High Permittivity and Low Microwave Loss

The dielectric properties of ceramics in the TiO₂-rich region of the BaO-TiO₂ system were investigated. In the composition range between BaTi₄O₉ and TiO₂, another compound, Ba₂Ti₉O₂₀, can be obtained when calcining and sintering conditions are controlled carefully. When dense and single-phase, this ceramic has excellent dielectric and physical properties. At 4 GHz, the dielectric K = 39.8, Q = 8000, and τ _K (temperature coefficient of dielectric constant) =−24 ± 2 ppm/°C.     

BaO-TiO2-WO3 Microwave Ceramics and Crystalline BaWO4

Microwave ceramic resonators composed of BaO-TiO₂-WO₃ were developed. The effect of WO₃ addition on the system of BaO·xTiO₂·(1+x)yWO₃ (x=4 and 4.5, y=0 to 0.04) was studied. The ceramics of this system are composed of crystallines including Ba₂Ti₉O₂₀, BaTi₄O₉, BaWO₄, and TiO₂. At y=0.02, the BaO·4TiO₂·0.1WO₃ ceramic was found to have excellent microwave properties such as ε=35, Q=8400 at 6 GHz, and nearly 0 ppm/°C of τ_f.     

Structural Variation of the BaTi4O9 Thin Films Grown by RF Magnetron Sputtering

BaTi₄O₉ thin films were grown on a Pt/Ti/SiO₂/Si substrate using rf magnetron sputtering and the structure of the thin films were then investigated. For the films grown at low temperature (≤350°C), an amorphous phase was formed during the deposition, which then changed to the BaTi₅O₁₁ phase when the annealing was conducted below 950°C. However, when the annealing temperature was higher than 950°C, a BaTi₄O₉ phase was formed. On the contrary, for the films grown at high temperature (>450°C), small BaTi₄O₉ grains were formed during the deposition, which grew during the annealing. The homogeneous BaTi₄O₉ thin films were successfully grown on Pt/Ti/SiO₂/Si substrate when they were deposited at 550°C and subsequently rapid thermal annealed at 900°C for 3 min.     

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.     

Polymorphism and Dielectric Properties of Nb-Doped BaTiO3

A working subsolidus phase diagram for the system BaTiO₃–Ba₅Nb₄O₁₅ has been determined by firing sol–gel-synthesized samples over a range of temperatures. The main difference from previous diagrams is the greater extent of the Nb-doped BaTiO₃ cubic solid solutions, BaTi_{1−5 x} Nb_{4 x} O₃, at lower temperatures with x extending to 0.09 at 900°C, but only 0.05 at 1400°C. Electrical property measurements show that compositions with large x ( x ≥0.0025) are highly insulating for pellets sintered at 1300°C in air, followed by a slow cool. Compositions with low x , however, exhibit a residual semiconducting grain core and are not fully reoxidized readily. Composition dependence of the dielectric properties shows a continuous and smooth transition from classic ferroelectric behavior with pure BaTiO₃ to normal dielectric response with a temperature-independent relative permittivity of approximately 22–24 for x >∼0.08. At intermediate compositions, ranges of both relaxor ferroelectric and quasi-ferroelectric behavior are observed. Possible reasons for an observed anomalous increase in value of the permittivity at the ferroelectric transition temperature at low x , which is superposed on an overall decrease in permittivity with increasing x , are discussed.     

Phase Equilibria in the System BaO–TiO2

The system BaO-TiO₂ was investigated using quenching, strip-furnace, and thermal techniques. Five compounds were found to exist in the system: Ba₂TiO₄, BaTiO₃, BaTi₂O₅, BaTi₃O₇, and BaTi₄O₉. Of these, only barium metatitanate (BaTiO₃) melts congruently (at 1618°C.). The dititanate melts incongruently at 1322° C. to yield BaTiO₃ and liquid; the trititanate melts at 1357°C. to yield BaTi₄O₉ and liquid; the tetra-titanate melts to TiO₂ and liquid at 1428° C. The nature of melting of the orthotitanate could not be determined accurately because of the high temperature involved and the rapid reaction with platinum. The two eutectics in the system occur between Ba₂TiO₄ and BaTiO₃ at 1563°C. and between BaTi₂O₅ and BaTi₃O₇ at 1317°C. The temperature of the cubic-hexagonal transition in barium metatitanate was determined as 1460°C. and the transition has been shown to be reversible. The transition temperature is raised sharply by the addition of a small percentage of TiO₂ although the extent of solid solution is quite limited. Some applications to the manufacture of titanate bodies and to the growth of single crystals of barium metatitanate are discussed.     

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.     

A New Compound with Ultra Low Dielectric Loss at Microwave Frequencies

A new ultra low loss microwave dielectric ceramic, Mg(Sn_0.05Ti_0.95)O₃ (MSnT), was found and investigated. The compounds were prepared by the conventional solid-state route, and sintered at 1360°–1480°C for 2–6 h. The investigations show that the MgTi₂O₅ secondary phase was observed. Moreover, the dielectric properties were correlated with the formation second phase. The excellent microwave dielectric properties of Q × f =322 000 (GHz), ɛ_r=17.4, and τ_f=−54 ppm/°C were obtained from the new MSnT ceramics sintered at 1390°C for 4 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.     

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.     

Effect of V2O5 Doping on the Sintering and Dielectric Properties of M-Phase Li1+x−yNb1−x−3yTix+4yO3 Ceramics

The effect of the addition of V₂O₅ on the structure, sintering and dielectric properties of M -phase (Li_{1+ x − y} Nb_{1− x −3 y} Ti _x +4 _y )O₃ ceramics has been investigated. Homogeneous substitution of V⁵⁺ for Nb⁵⁺ was obtained in LiNb_{0.6(1− x )}V_{0.6 x} Ti_0.5O₃ for x ≤ 0.02. The addition of V₂O₅ led to a large reduction in the sintering temperature and samples with x = 0.02 could be fully densified at 900°C. The substitution of vanadia had a relatively minor adverse effect on the microwave dielectric properties of the M -phase system and the x = 0.02 ceramics had [alt epsilon]_r= 66, Q × f = 3800 at 5.6 GHz, and τ_f= 11 ppm/°C. Preliminary investigations suggest that silver metallization does not diffuse into the V₂O₅-doped M -phase ceramics at 900°C, making these materials potential candidates for low-temperature cofired ceramic (LTCC) applications.     

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 Low Firing Temperature Bi2W2O9 Ceramics

Phase stability, sinterability, and microwave dielectric properties of Bi₂W₂O₉ ceramics and their cofireability with Ag, Cu, and Au electrodes have been investigated. Single-phase Bi₂W₂O₉ powder was synthesized by solid-state reaction in air at 800°C for 3 days. X-ray powder diffraction data show Bi₂W₂O₉ to have an orthorhombic crystal structure described by the noncentrosymmetric space group Pna 2₁, with lattice parameters a =5.4401(8), b =5.4191(8), c =23.713(4) Å. Ceramics fired at temperatures up to 865°C remain single-phase but above this temperature ferroelectric Bi₂WO₆ appears as a secondary phase. The measured relative permittivity of Bi₂W₂O₉ ceramics increases continuously from 28.6 to 40.7 for compacts fired between 860° and 885°C. The bulk relative permittivity of Bi₂W₂O₉ corrected for porosity was calculated as 41.3. Bi₂W₂O₉ ceramics fired up to 875°C exhibit moderate quality factors, Q × f _r, ∼7500–7700 GHz and negative temperature coefficient of resonant frequency, ∼−54 to −63 ppm/°C. Chemical compatibility experiments show Bi₂W₂O₉ ceramics to react with both Ag and Cu electrodes, but to form good contacts with Au electrodes.     

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.