Low-Temperature Sintering of (Ba0.6Sr0.4)TiO3

An addition of just 0.4 wt% Li₂O to (Ba_0.6Sr_0.4)TiO₃ powder was able to reduce the sintering temperature to ≤900°C and produce ceramics with a relative density of 97%. Small amounts of two secondary phases were formed during this process: Li₂TiO₃ and Ba₂TiO₄. The addition of Li₂O depresses the ferroelectric character of the (Ba_0.6Sr_0.4)TiO₃ and, as a result, reduces the permittivity, improves the temperature coefficient of permittivity, and reduces the dielectric losses. The tunability shows no significant variation with Li₂O concentration and remains between 16.5% and 13.5%. A low-temperature sintering mechanism was proposed. The mechanism involves the intermediate formation of BaCO₃, its melting and the incorporation of Li⁺ into the BST. The sintering mechanism can be characterized as reactive liquid-phase sintering.     

Microstructure–Dielectric Properties Relationship in Ba0.6Sr0.4TiO3–Mg2SiO4–Al2O3 Composite Ceramics

0.60Ba_0.6Sr_0.4TiO₃(BST)–(0.40− x )Mg₂SiO₄(MS)– x Al₂O₃ ( x =0, 0.5, 3, 5wt%) composite ceramics exhibit excellent characteristics suitable for tunable device applications. With increasing amount of Al, the dielectric peak can be quantitatively broadened and suppressed; the "phase transition temperature" T _c or ( T _m) shifts to a lower temperature. Meanwhile, the tunability is still high in a wider temperature range. Far from T _c, pyroelectric effects are observed by using the Byer and Roundy technology and Slim polarization hysteresis loops are observed under high ac dielectric field at 10Hz. These proved the existence of spontaneous polarization in certain possible orientations in a broad temperature range above T _c in the paraelectric medium and reveal why 0.60BST–(0.40− x )MS– x Al₂O₃ have such remarkable dielectric nonlinearity.     

Space Charge Signature and Its Effects on Ionic Transport in Heterogeneous Solids

The Arrhenius plots of heterogeneous solids comprising of lithium aluminum titanium phosphate (LATP) glass–ceramic, Al₂O₃, and Ba_0.6Sr_0.4TiO₃ (0.6 BST) exhibit an inflection or peak around 27°C, which is interpreted as a signature of space charge. The space charge is formed by the adsorption of lithium ions onto the dielectric phases below 27°C. The space charge is also a source of an electric field, which influences the transport of conducting ions. The adsorbed ions are desorbed above 27°C, resulting in a reduction or elimination of the space charge effect. A high dielectric constant phase, 0.6 BST, retains the space charge effect even at temperatures greater than 27°C.     

Tunable Microwave Properties of Barium Titanate-Based Ferroelectric Glass-Ceramics

BaTiO₃-based ferroelectric glass-ceramics with the composition 0.65(Ba_{1− X} Sr _X )TiO₃·0.27SiO₂·0.08Al₂O₃ ( X = 0.2–0.6) were fabricated, and their tunable dielectric properties were measured at microwave frequency. Major crystalline phases that precipitated during thermal treatment up to 1000°C were (Ba,Sr)TiO₃, Ba₂TiSi₂O₈, and BaAl₂Si₂O₈. The Curie temperatures of the heated bulk samples were successfully tuned near room temperature at the composition between X = 0.2 and 0.3. A thick-film sample with X = 0.3 showed 27% tunability at 5 GHz under 10 kV/cm bias voltage.     

Low-Temperature Sintering (Ba0.6Sr0.4)TiO3 Thick Film Prepared by Screen Printing

(Ba_0.6Sr_0.4) TiO₃ thick films doped with glass slurry were fabricated by the screen-printing technique. The dielectric properties and the sintering mechanism were investigated. The films can be sintered at 600°C. The dielectric constant is 88 and the dielectric loss is 0.002 with a tunability of 23.86% under 100 kV/cm. Higher dielectric constant and tunability were obtained in the samples sintered at higher temperatures. The highest tunability is 61.12% under 150 kV/cm in the sample sintered at 800°C. The low sintering temperature and dielectric loss of the glass-doped thick films make them potential candidates for LTCC and microwave tunable devices.     

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.     

Processing and Characterization of BaTi4O9

BaTi₄O₉ powder prepared by calcining BaCO₃ and TiO₂ powders was sintered to over 97% of theoretical density. Less than 5% Ba₂Ti₉O₂₀ occurred as a second phase in "pure" BaTi₄O₉, and Al₂O₃ impurities from processing formed isolated hollandite (∼BaAl₂Ti₆O₁₆) grains, which were identified by fringes in bright-field TEM images. For pure BaTi₄O₉ at 1 MHz, a dielectric loss (tan δ) of 5 × 10⁻⁴ and dielectric constant of 39 were recorded. Hollandite impurities were found to increase tan δ by 2 orders of magnitude, whereas firing in oxygen decreased tan δ by an order of magnitude.     

Role of Liquid Phase in PTCR Characteristics of (Ba0.7Sr0.3)TiO3 Ceramics

The role of liquid phase in the enhancement of the PTCR (positive temperature coefficient of resistance) effect in (Ba_0.7Sr_0.3)TiO₃ (BST) with the addition of AST (4Al₂O₃· 9SiO₂· 3TiO₂) is investigated in this paper. The AST–BST samples were characterized with optical microscopy, transmission electron microscopy, energy-dispersive spectroscopy, and impedance spectroscopy. Microscopic observations showed that slower cooling might facilitate the precipitation of the (Ba,Sr)TiO₃ phase from the liquid phase on matrix grains since the amount of liquid phase was reduced with a decreasing cooling rate. Impedance spectroscopy indicated that this variation accompanied the change in the intrinsic properties of grain boundaries, which could not be explained by well-known oxidation effects. With the aid of a brick-layer model and high-resolution transmission electron microscopy (HRTEM), it appeared that the change in electrical characteristics of grain boundaries with decreasing cooling rate originated from the precipitation of (Ba,Sr)TiO₃. Finally, the effect of precipitated (Ba,Sr)TiO₃ on the PTCR characteristics is discussed in terms of the acceptor-state density and the polarization state at grain boundaries.     

Effects of the Addition of Different Types of Fillers on the Properties of BaO–ZnO–B2O3–SiO2 Glass Composites for Application to Barrier Ribs of Plasma Display Panels

Various types of crystalline ceramic fillers (TiO₂, ZrO₂, Al₂O₃, MgO, and cordierite) were added to BaO–ZnO–B₂O₃–SiO₂ (BZBS) glass (5–20 wt%), and the resultant dielectric constant, coefficient of thermal expansion (CTE), and optical reflectance were investigated for the application of the composites to the barrier ribs in plasma display panels. All the investigated fillers were partially dissolved into the glass at the fabrication temperature (575°C), and the residual fillers were aligned along the boundaries of sintered glass frits. By considering all aspects of the properties, the addition of TiO₂ fillers of about 10 wt% to BZBS glass was the most desirable of the types of fillers investigated. The addition of TiO₂ filler (10 wt%) yielded 61% in optical reflectance, 8.3 × 10⁻⁶ K^–1 in coefficient of thermal expansion, and 15.5 in dielectric constant, which were properties comparable with the currently used Pb-based barrier ribs.     

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.     

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.     

Effects of Ba6Ti17O40 on the Dielectric Properties of Nb-Doped BaTiO3 Ceramics

The dielectric properties, including the DC breakdown strength, of 1 mol% Nb⁵⁺-doped BaTiO₃ ceramics with different quantities of excess TiO₂ have been investigated. The breakdown strength was found to decrease with increasing TiO₂ content, but could not be readily explained by relative density and grain size effects. The decrease in the breakdown strength from a stoichiometric BaTiO₃ composition to samples with excess TiO₂ is believed to be due to the field enhancement effect (up to a factor of 1.40) at the BaTiO₃ matrix because of the presence of a Ba₆Ti₁₇O₄₀ second phase. The thermal expansion coefficient mismatch between the BaTiO₃ matrix phase and the Ba₆Ti₁₇O₄₀ phase may also result in a low breakdown strength. The dielectric properties of the pure Ba₆Ti₁₇O₄₀ phase were also investigated and are reported herein.     

The Structure and Dielectric Tunable Properties of Fine-Grained Ba0.6Sr0.4TiO3 Ceramics Prepared by Spark Plasma Sintering

Barium strontium titanate is a promising material for microwave-phased array applications. ^1,2 In this study, highly dense and fine-grained Ba_0.6Sr_0.4TiO₃ ceramics were prepared using the spark plasma sintering (SPS) technique. The structure and dielectric tunable properties of the samples were investigated. The "distorted nano-region" emerged in the interior of the grains of SPS samples, and resulted in the deterioration of the dielectric tunable properties of Ba _x Sr_{1− x} TiO₃. This phenomenon indirectly testified to the assumption of the "polar nano-region" mechanism. After the SPS samples were annealed, the "distorted nano-region" disappeared and better dielectric tunable properties were obtained. The dielectric constant was decreased to 1048, and the K value (Commutation Quality Factor) reached 7089.     

Subsolidus Phase Relationships in the System Fe2O3–Al2O3–TiO2 between 1000° and 1300°C

Subsolidus phase equilibria in the system Fe₂O₃–Al₂O₃–TiO₂ were investigated between 1000° and 1300°C. Quenched samples were examined using powder X-ray diffraction and electron probe microanalytical methods. The main features of the phase relations were: (a) the presence of an M₃O₅ solid solution series between end members Fe₂TiO₅ and Al₂TiO₅, (b) a miscibility gap along the Fe₂O₃–Al₂O₃ binary, (c) an α-M₂O₃( ss ) ternary solid-solution region based on mutual solubility between Fe₂O₃, Al₂O₃, and TiO₂, and (d) an extensive three-phase region characterized by the assemblage M₃O₅+α-M₂O₃( ss ) + Cor( ss ). A comparison of results with previously established phase relations for the Fe₂O₃–Al₂O₃–TiO₂ system shows considerable discrepancy.     

Use of Titanates to Achieve a Temperature-Stable Low-Temperature Cofired Ceramic Dielectric for Wireless Applications

A low-loss and near-zero temperature coefficient of resonant frequency ( T _f) low-temperature cofired ceramic (LTCC) host dielectric was developed for portable consumer wireless device applications. The low T _f was realized by compensating the Al₂O₃-filled-glass dielectric with admixtures of TiO₂ (negative temperature coefficient of dielectric constant ( T _ɛ)) in the starting formulation. XRD data indicated a portion of the TiO₂ in the starting formulation dissolved into the glass, and extensive formation of crystalline titanium compounds was observed via a nucleation and growth mechanism. The dissolution of TiO₂ in the glass and subsequent formation of titanium compounds was believed to result in the relatively small amount of TiO₂ required to achieve a near-zero T _f in the final sintered structure.     

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.     

Microstructure and Properties of Cr2O3-Doped Ternary Lead Zirconate Titanate Ceramics

The effects of Cr₂O₃ doping (0 to 0.6 wt%) on the microstructure and electrical properties of ternary Pb_0.97Sr_0.03- [Zr_0.458Ti_0.452 (Mn_1/3Nb_2/3) _0.09]O₃ piezoelectric ceramics have been studied. Abnormal grain growth (grain sizes of 2.8 to 28.5 μm) and densification (porosities of 2% to 12%) are found, and the tetragonality c/a nearly keeps constant wherease c and a slightly decrease in the doping range. The results for the temperature dependence of electric conductivity and the dielectric constant show that electric conductivity increase continuously over 0.1 wt% content, activation energies are all close to 0.92 eV in the paraelectric state, and the Curie temperature T_c decreases with increasing doping content. Minor additives of ≤0.1 wt% improve the dielectric and piezoelectric properties, but with more additives of ≥0.4 wt% electrical properties appear to deteriorate. Thus, these phenomena can be ascribed mainly to anomalous developed microstructure.     

Effect of Titania and Lithia Doping on the Boundary Migration of Alumina under an Electric Field

Boundary migration under an electric field was investigated for pure, TiO₂-doped, and Li₂O-doped Al₂O₃ specimens. Boundary migration rates in TiO₂-doped and Li₂O-doped Al₂O₃ specimens were much faster compared with that of pure Al₂O₃. In all specimens, the migration rate was observed to depend on the applied bias direction. Compared with pure Al₂O₃, the dependence of boundary migration on bias direction became more pronounced in TiO₂-doped Al₂O₃ but less pronounced in Li₂O-doped Al₂O₃. The results were explained in terms of the variation of grain sizes, mobility, and electrostatic potential of boundaries because of doping.