Fabrication and Tunable Dielectric Properties of (Ba0.7Sr0.3)TiO3-Glass-Based Thick-Film Capacitors

Ferroelectric glass–ceramics of composition 0.90 (Ba_0.7Sr_0.3) TiO₃–0.10(B₂O₃:SiO₂) (0.90 BST:0.10 BS) synthesized by sol–gel method have been used for the preparation of dielectric thick-film inks. The particle dispersion of the glass–ceramic powders in the thick-film ink formulations have been studied through rheological measurements for fabricating thick-film capacitors by screen printing technique. The thick films derived from such glass–ceramics are found to sinter at considerably lower temperatures than the pure ceramic, and exhibit good dielectric characteristics with a tunability of 32% at 1 MHz under a dc bias field of 35 kV/cm.     

Effects of Heating Rate on the Sintering Behavior and the Dielectric Properties of Ba0.7Sr0.3TiO3 Ceramics Prepared by Boron-Containing Liquid-Phase Sintering

The effects of heating rate on the sintering behavior and the dielectric properties of Ba_0.7Sr_0.3TiO₃ ceramics prepared by boron-containing liquid-phase sintering were investigated. When 0.5 wt% B₂O₃ was added to Ba_0.7Sr_0.3TiO₃, sintering was achieved at ∼1150°C, and the overdoped B₂O₃ did not form an adequate amount of liquid phase or volatilize; it remained in the samples and formed a secondary phase. A transition broadening was observed as the heating rate increased. As the heating rate increased, the Curie temperature increased and the maximum dielectric constant ( k _max) at the Curie temperature decreased. This result is attributable to a decrease in the diffuseness parameter (δ) and the tetragonality ( c / a ).     

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.     

Effects of B2O3 on the Phase Stability of Ba2Ti9O20 Microwave Ceramic

Preparation of dense and phase-pure Ba₂Ti₉O₂₀ is generally difficult using solid-state reaction, since there are several thermodynamically stable compounds in the vicinity of the desired composition and a curvature of Ba₂Ti₉O₂₀ equilibrium phase boundary in the BaO–TiO₂ system at high temperatures. In this study, the effects of B₂O₃ on the densification, microstructural evolution, and phase stability of Ba₂Ti₉O₂₀ were investigated. It was found that the densification of Ba₂Ti₉O₂₀ sintered with B₂O₃ was promoted by the transient liquid phase formed at 840°C. At sintering temperatures higher than 1100°C, the solid-state sintering became dominant because of the evaporation of B₂O₃. With the addition of 5 wt% B₂O₃, the ceramic yielded a pure Ba₂Ti₉O₂₀ phase at sintering temperatures as low as 900°C, without any solid solution additive such as SnO₂ or ZrO₂. The facilities of B₂O₃ addition to the stability of Ba₂Ti₉O₂₀ are apparently due to the eutectic liquid phase which accelerates the migration of reactant species.     

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.     

Tungsten Bronze Field and Melt Growth of Crystals in the Na2O-BaO-Nb2O5 System

The extents of the liquidus and solidus fields were determined for tungsten bronze-type solid solutions in the Na₂O-BaO-Nb₂O₅ system by DTA and melt crystal growth experiments. Bronze-type solid solutions exist to 7.1Na₂O-34.9BaO-58Nb₂O₅ in the Nb₂O₅-rich region and from 12Na₂O-38BaO-50Nb₂O₅ to 4.6Na₂O-45.4BaO-50Nb₂O₅ along the NaNbO₂-BaNb₂O₆ join, which includes NaBa₂Nb₅O₁₅=10Na₂O-40BaO-50Nb₂O₆. There is little, if any, solid solubility of compositions with a deficiency of Nb₂O₅. Curie temperatures decline rapidly and dielectric constant peaks broaden with Nb₂O₅ substitution because the Nb:O ratio becomes greater than the octahedral 1:3 ratio. Useful ferroelectrics exist along the NaNbO₃-BaNb₂O₆ join where the Nb:O ratio is 1:3. Large striae-free crystals, with less optical scattering than Czochralski-grown crystals, were grown from unseeded Na₂O-rich melts (e.g. 15Na₂O-37.5BaO-47.5Nb₂O₅) cooled from 1520° to 1300°C at 2°C/h. Annealing effects on these crystals whose compositions lie on the NaNbO₃-BaNb₂O₆ join are discussed.     

Effect of Glass Additions on BaO–TiO2–WO3 Microwave Ceramics

The effect of glass addition on the properties of BaO–TiO₂-WO₃ microwave dielectric material N-35, which has Q = 5900 and K = 35 at 7.2 GHz for samples sintered at 1360°C, was investigated. Several glasses including B₂O₃, SiO₂, 5ZnO–2B₂O₃, and nine other commercial glasses were selected for this study. Among these glasses, one with a 5 wt% addition of B₂O₃ to N-35, when sintered at 1200°C, had the best dielectric properties: Q = 8300 and K = 34 at 8.5 GHz. Both Q and K increased with firing temperature as well as with density. The Q of N-35, when sintered with a ZnO–B₂O₃ glass system, showed a sudden drop in the sintering temperature to about 1000°C. The results of XRD, thermal analysis, and scanning electron microscopy indicated that the chemical reaction between the dielectric ceramics and glass had a greater effect on Q than on the density. The effects of the glass content and the mixing process on the densification and microwave dielectric properties are also presented. Ball milling improved the densification and dielectric properties of the N-35 sintered with ZnO–B₂O₃.     

Effects of Glass Additions on (Zr,Sn)TiO4 for Microwave Applications

The effects of glass additions on the properties of (Zr,Sn)TiO₄ as a microwave dielectric material were investigated. The (Zr,Sn)TiO₄ ceramics with no glass addition sintered at 1360°C gave Q = 4900 and K = 37 at 7.9 GH_z. Several glasses, including SiO₂, B₂O₃, 5ZnO–2B₂O₃, and nine commercial glasses, were tested during this study. Among these glasses, (Zr,Sn)TiO₄ sintered with ZnO-B₂O₃–SiO₂ (Corning 7574) showed more than 20% higher density than that of pure (Zr,Sn)TiO₄ sintered at the same temperature. A 5-wt% addition of SiO₂, to (Zr,Sn)TiO₄, when sintered at 1200°C, gave the best Q : Q = 2700 at 9 GHz. Results of XRD analysis and scanning electron microscopy and the effect of glass content are also presented.     

Formation and Transformation of TiO2 (Anatase) Solid Solution in the System TiO2—Al2O3

In the system TiO₂—Al₂O₃, TiO₂ (anatase, tetragonal) solid solutions crystallize at low temperatures (with up to ∼ 22 mol% Al₂O₃) from amorphous materials prepared by the simultaneous hydrolysis of titanium and aluminum alkoxides. The lattice parameter a is relatively constant regardless of composition, whereas parameter c decreases linearly with increasing Al₂O₃. At higher temperatures, anatase solid solutions transform into TiO₂ (rutile) with the formation of α-Al₂O₃. Powder characterization is studied. Pure anatase crystallizes at 220° to 360°C, and the anatase-to-rutile phase transformation occurs at 770° to 850°C.     

DTA and X-Ray Analysis Study of Nucleation and Crystallization of MgO-Al2O3-SiO2 Glasses Containing ZrO2, TiO2, and CeO2

Crystallization sequences of glasses with compositions in the tridymite primary phase field of the MgO-Al₂O₃-SiO₂ system were studied by DTA, X-ray diffraction, and other techniques. Crystallization was catalyzed by the addition of 7 wt% of either ZrO₂ or TiO₂. Up to 10 wt% CeO₂ was also added to some glasses. Metastable solid solutions with the high-quartz structure exhibiting varying lattice parameters commonly occurred at low temperatures, transforming into a high cordierite at higher temperatures. Depending on the composition and heat treatment, other phases also appeared, e.g. Ce₂Ti₂O₄ (Si₂O₇). The rate of crystallization was markedly dependent on the catalyst. Colloidal precipitation of the catalyst accompanied by bulk crystallization of the glass was observed with ZrO₂, but no crystalline TiO₂ was detected. In the presence of CeO₂, TiO₂ was a more effective catalyst than ZrO₂. Although CeO₂ lowered the melting temperatures of the glass-ceramics, it increased the stability of the glasses and inhibited volume nucleation, causing coarse structures to form on crystallization.     

Dielectric Constant and Optical Reflectance of Ceramic Filler-Added BaO–ZnO–B2O3–P2O5 Glass Composites

This study examined the influence of the addition of ceramic fillers (up to 20 wt% of TiO₂, Al₂O₃, and ZnO, respectively) to a BaO–ZnO–B₂O₃–P₂O₅ glass matrix on the dielectric and optical properties with the aim of using this material as the barrier ribs in plasma display panels. The modification of the dielectric constant by the fillers was related to the formation of secondary phases, the changes in the glass composition by the partial dissolution of the fillers, and the presence of pores. The reflectance of the composites ranged from 60% to 80% with the addition of 20 wt% filler.     

Direct-Current Field Dependence of Dielectric Properties in Alumina-Doped Barium Strontium Titanate

The dielectric properties of (Ba_0.6Sr_0.4)TiO₃ and Al₂O₃-doped (Ba_0.6Sr_0.4)TiO₃ have been characterized. The grain size of the specimen is maximum for (Ba_0.6Sr_0.4)TiO₃ that has been doped with 1 wt% Al₂O₃. The density and the real part of the relative dielectric constant each decrease as the Al₂O₃ content increases. The loss factor is minimum for (Ba_0.6Sr_0.4)TiO₃ that has been doped with 2 wt% Al₂O₃. The dielectric constant of the specimens decreases as the applied dc field increases. The influence of the dc field on the loss factor is much less than that on the dielectric constant. The tunability is ∼24% for (Ba_0.6Sr_0.4)TiO₃ that has been doped with 1 wt% Al₂O₃.     

Thermodynamic Activities in B2O3-SiO2 Melts at 1475 K

Eight glass samples in the B₂O₃-SiO₂ system with compositions from 20 to 90 mol% B₂O₃ were prepared. The equilibrium vaporization was studied by Knudsen effusion mass spectrometry at temperatures between 1450 and 1500 K. B₂O₃ ( g ) was the most abundant boron-containing species in the vapor; no silicon-containing gaseous species were detected. Thermodynamic activities of B₂O₃ in the liquid were determined at 1475 K. Thermodynamic activities of SiO₂ and integral excess Gibbs energies were estimated from the thermodynamic activities of B₂O₃. The thermodynamic data support the results obtained by other methods indicating the existance of a miscibility gap in the metastable liquid.     

Co-Additions of TiO2 and SiO2 Crystalline Fillers to Tailor the Properties of BaO–ZnO–B2O3–SiO2 Glass for Application to Barrier Ribs of Plasma Display Panels

The influence of co-additions of crystalline TiO₂ and SiO₂ fillers (10 wt% addition in total) to BaO–ZnO–B₂O₃–SiO₂ glass on resultant properties was investigated from the viewpoint of applying the material to the barrier ribs of plasma display panels. The substitution of SiO₂ for TiO₂ reduced the dielectric constant significantly, while it maintained high optical reflectance and appropriate coefficient of thermal expansion (CTE) in the case when TiO₂ alone was used. A 5–7.5 wt% SiO₂ addition with 2.5–5 wt% TiO₂ under the constraint of 10 wt% total fillers demonstrated an optical reflectance of about 55%, a CTE of about 8.3 × 10⁻⁶ K⁻¹ (compatible with glass panels), and a dielectric constant of about 7.5, which are promising properties for the barrier rib application.     

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-Temperature Sintering and Microwave Dielectric Properties of the Zn2SiO4 Ceramics

B₂O₃ was added to nominal composition Zn_1.8SiO_3.8 (ZS) ceramics to decrease their sintering temperature for application to low-temperature cofired ceramic (LTCC) devices. B₂O₃ reacted with SiO₂ to form a liquid phase containing SiO₂ and B₂O₃. The composition and melting temperature of the liquid phase depended on the sintering temperature and the B₂O₃ content. The specimen containing 20.0 mol% of B₂O₃ sintered at 900°C exhibited high microwave dielectric properties of Q × f =53 000 GHz, ɛ _r=5.7, and τ_f=−16 ppm/°C, confirming the promising potential of the B₂O₃-added ZS ceramics as candidate materials for the LTCC devices.     

Studies in Lithium Oxide Systems: V, Li2O-Li20 B2O3

Nine compositions containing 40 to 68% B₂O₃ were used to study the high-lithia portion of the system Li₂O-B₂O₃ by quenching and differential thermal analysis methods. The compounds 3Li₂O 2B₂O₃ and 3Li₂O B₂O₃ melted incongruently at 700°± 6°C, and 715°± 15°C., respectively. The compound 2Li₂O B₂O₃ is assumed to dissociate slightly below 650°± 15° C., although the data could also be interpreted as in-congruent melting. Below 600°± 6°C. it does dissociate to the 3:2 and 3:1 compounds. In this narrow temperature interval the 2:1 compound had an inversion at 618°± 6°C. Both forms of the 2:1 compound could be quenched to room temperature. X-ray diffraction data for the compounds are tabulated, and the complete phase diagram for the system Li₂O-B₂O₃ is presented.     

Microstructural Development on Crystallizing Hot-Pressed Pellets of Cordierite Melt-derived Glass Containing B2O3 and P2O5

Comparing the crystallization mechanism of stoichiometric and B₂O₃ and P₂O₅ containing glass reveals that the additives extend the gap between the glass transition and crystallization temperatures and suppress formation of μ, cordierite while promoting direct crystallization of α cordierite. Detailed TEM analysis of nucleation and growth of crystals in hot-pressed pellets of B₂O₃/P₂O₅-containing glass particles shows that nucleation occurs on unidentified heterogeneous nuclei at the sites of the previous particle surfaces. Growth of α cordierite with a cellular morphology or μ cordierite with a dendritic morphology is most likely controlled by the glass composition directly ahead of the growth front.     

Phase Separation and Crystallization of BiSrCaCu2Al0.5Ox Glass

Al₂O₃ addition to the melt of a BiSrCaCu₂O _x composition was found by TEM observation to cause the liquid-liquid phase separation of the melt-quenched glass, and to result in preferential precipitation of superconducting Bi₂Sr₂Ca₁Cu₂O _x crystals from the melt during the cooling process.     

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.