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.     

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.     

Scanning Electron-Beam Dielectric Microscopy for the Investigation of the Temperature Coefficient Distribution of Dielectric Ceramics

Studies on scanning electron-beam dielectric microscopy are reported. This microscopy technique is used for determining the temperature coefficient distribution of dielectric materials using an electron beam as a heat source instead of a light beam as in photothermal dielectric microscopy. This microscopy technique, which has the ability to simultaneously observe SEM images and the material composition by EPMA, has a resolution better than that of photothermal dielectric microscopy. To demonstrate the usefulness of this technique, the two-dimensional image of a two-phase composite ceramic composed of TiO₂ and Bi₂Ti₄O₁₁ is measured.     

The System TiO2-Bi2Ti4O11

The system TiO₂-Bi₂Ti₄O₁₁ was examined by Raman spectroscopy and X-ray diffraction to determine whether TiO₂ is soluble in Bi₂Ti₄O₁₁. The Raman spectral data obtained from preparations made at ∼ 1050°C and cooled to room temperature led us to conclude that TiO₂ is not soluble in the "high-temperature" form of Bi₂Ti₄O₁₁. It was also found that extensive grinding of the phase identified as the "high-temperature" form converts it to the "low-temperature" form, stable below 250°C.     

Factors Affecting the Formation of Ba2Ti9O20

The phase development sequence based on a composition equivalent to Ba₂Ti₉O₂₀ during heating is found to be in the following order: BaTi₅O₁₁ > BaTi₄O₉ > Ba₂Ti₉O₂₀. The lowest rate of formation of Ba₂Ti₉O₂₀ is caused by its high surface energy and interface energy, which result in a low nucleation rate. The existence of BaTi₅O₁₁ in calcined powder helps to form Ba₂Ti₉O₂₀ in sintered compacts. The effect of BaTi₅O₁₁ on Ba₂Ti₉O₂₀ formation can be explained by their similar oxygen packing and by reduced volume change during transformation. The amount of BaTi₅O₁₁ formed during heating depends greatly on the compositional homogeneity of powders. The addition of SnO₂ aids the formation of Ba₂Ti₉O₂₀ by reduced strain energy at transformation and reduced surface energy.     

Phase Equilibria in the TiO2-Rich Region of the System BaO-TiO2

Phase relations in the system BaO-TiO₂ from 67 to 100 mol% TiO₂ were investigated at 1200° to 1450°C in O₂. Data were obtained by microstructural, X-ray, and thermal analyses. The existence of the stable compounds Ba₆Ti₁₇O₄₀, Ba₄Ti₁₃O₃₀, BaTi₄O₉, and Ba₂Ti₉O₂₀ was confirmed. The compound BaTi₂O₅ is unstable and either forms as a reaction intermediate below the solidus or crystallizes from the melt. The compounds Ba₆Ti₁₇O₄₀ and Ba₄Ti₁₃O₃₀ decompose in peritectic reactions, and BaTiO₃ and Ba₆Ti₁₇O₄₀ react to form a eutectic. Special conditions are required for the formation of Ba₂Ti₉O₂₀, which decomposes in a peritectoid reaction at 1420°C. The new phase diagram is presented.     

Microstructure and Dielectric Properties of a LaAlO3–TiO2 Diffusion Couple

Near-field scanning microwave microscopy was applied to investigate the dielectric properties and microstructure in a polycrystalline LaAlO₃–TiO₂ diffusion couple, which included three regions containing different phases and microstructures. Relatively low (La₂Ti₄Al₁₈O₃₈), high (α-La_2/3TiO₃), and intermediate (La₄Ti₉O₂₄) dielectric constant phases were distinguished at the inter-diffusion interface in optical, backscattered electron scanning electron microscopy, and scanning microwave microscopy (SMM) images. The relative ranking of dielectric constants based on SMM examination was as follows: TiO₂>α-La_2/3TiO₃>La₄Ti₉O₂₄>LaAlO₃>La₂Ti₄Al₁₈O₃₈. La_2/3TiO₃ and LaAlO₃ will form solid solutions in the LaAlO₃-rich region. The reaction paths leading to phase development are discussed.     

Ba2Ti9O20 as a Microwave Dielectric Resonator

Microwave measurements of Ba₂Ti₉O₂₀ show that this ceramic is uniquely suited for dielectric resonators. (Suitable ceramics should have a high dielectric constant K , a low dielectric loss (high Q ), and a low temperature coefficient of resonant frequency, τ.) At 4 GHz, Ba₂Ti₉O₂₀ resonators have Q >8000, K = 39.8, and τ=2 ppm/°C. Measurements of Q and τ were made on unmetallized ceramic resonator disks positioned in a waveguide; K was measured using a dielectric post resonator technique. From 4 to 10 GHz, Q approaches that for a copper waveguide cavity, whereas the temperature coefficient is typically 8 times lower.     

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.     

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.     

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.     

Microwave Properties of Ba2Ti9O20 Doped with Zirconium and Tin Oxides

The effects of solid-solution additives, their concentration, and the thermal processing schedule on the microstructure evolution and microwave properties of Ba₂Ti₉O₂₀ were studied. The solubility of tin in Ba₂Ti₉O₂₀ was higher than that of zirconium. Both elements facilitated the formation of phase-pure Ba₂Ti₉O₂₀ resonators. Ba₂Ti₉O₂₀ formed most easily with low dopant concentrations (0.82 mol%) (most impressively for ZrO₂ substitutions). Extended heat treatment (16 h versus 6 h at a temperature of 1390°C) resulted in volatilization of the grain-boundary liquid phase, which leads to more-porous resonators that have correspondingly lower permittivities. Increasing the dopant concentration resulted in minor increases in the quality factor; doping with zirconium led to slightly higher values (a maximum of 13900 at a frequency of 3 GHz). Increasing the measurement temperature degraded the quality factor (most precipitously for BaTi₄O₉). The temperature coefficient decreased as the ZrO₂ substitution increased but was largely unaffected by the SnO₂ concentration. Excess TiO₂ in a batch with no other dopants demonstrated degraded microwave properties.     

A New Microwave Dielectric Ceramic for LTCC Applications

A new low-sintering temperature microwave dielectric ceramic, the Li₂TiO₃ solid solution, was found and investigated in the Li₂O–Nb₂O₅–TiO₂ system. The compound with the composition of Li_2.081Ti_0.676Nb_0.243O₃ crystallizes as a monoclinic structure. This new microwave dielectric ceramic shows a relatively low permittivity (∼20), high Q × f values up to 50 000 (7.8 GHz), and near-zero temperature coefficients (13 ppm/°C), which were obtained via sintering at 1100°C. The addition of ≤2 wt% B₂O₃ was very effective in lowering the sintering temperature ( T _s), and dense ceramics could be obtained at T _s≤900°C. The addition of B₂O₃ does not induce apparent degradation in the microwave properties but lowers the τ_f value to near zero. It is obvious that the ceramics could be promising candidates for multilayer low-temperature co-fired ceramics applications.     

Citrate Route to Sn-Doped BaTi4O9 with Microwave Dielectric Properties

Highly reactive and nanometer-sized (30–50 nm) Sn-doped BaTi₄O₉ (BaTi_{4- x} Sn _x O₉; x = 0.0–0.03) powders have been prepared by the citrate-precursor method. The effect of Sn substitution on the crystallization and microwave dielectric properties has also been investigated on the basis of microstructure and crystal structure. Addition of a small amount of SnO₂ resulted in a lowering of the sintering temperature of BaTi₄O₉, and at 1250–1300°C for 2–5 h, dense compounds with a theoretical density up to 99% could be obtained. The Sn-doped BaTi₄O₉ materials were found to have excellent microwave dielectric properties with epsilon_r = 34–37, Q = 8300–8900 at 11 GHz and tau_f = 3.6–16.1 ppm/°C.     

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.     

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.     

Microwave Dielectric Properties of Gallium-Doped Hexagonal Barium Titanate Ceramics

High-permittivity and low-loss ceramics with composition BaTi_0.92Ga_0.08O_2.96 have been prepared in the BaO–Ga₂O₃–TiO₂ system using the mixed-oxide route. This compound forms as the hexagonal polymorph (6 H ) of BaTiO₃ with the space group P 6₃/ mmc . The dielectric properties of dense ceramics have been studied, at microwave frequencies, with the ceramics fired at 1450°C under flowing oxygen gas; the results are a relative permittivity, ɛ_r, of ∼74 and a quality factor, Q · f _r, of ∼7815 at 5.5 GHz. The quality factor is strongly influenced by the sintering conditions (temperature and atmosphere), whereas the relative permittivity is not influenced significantly by ceramic processing for pellets ≥93% of the theoretical X-ray density. To our knowledge, this is the first report of microwave dielectric resonance in a perovskite-type BaTiO₃-based ceramic.     

Modified Phase Diagram for the Barium Oxide–Titanium Dioxide System for the Ferroelectric Barium Titanate

The ferroelectric phase transition behavior in BaTiO₃ was investigated for various annealing times, temperatures, and Ba/Ti ratios by means of a differential scanning calorimeter. Coupling these observations with powder X-ray diffraction and transmission electron microscopy allowed new insights into the barium oxide (BaO)–titanium dioxide (TiO₂) phase diagram. The transition temperature was varied systematically with the Ba/Ti ratio at annealing temperatures from 1200° to 1400°C in air. The transition temperature decreased with increasing concentrations of BaO and TiO₂ partial Schottky defects, and showed a discontinuous change at the phase boundaries. Beyond the solubility region, two peritectoid reactions were confirmed and revised; first around 1150°C for Ba_1.054Ti_0.946O_2.946→Ba₂TiO₄+BaTiO₃ and second 1250°C for BaTi₂O₅→Ba₆Ti₁₇O₄₀+BaTiO₃, respectively. All other regimes of the BaO–TiO₂ were found to be consistent with the reported diagrams in the literature.     

Second-Phase Development in Ba-Doped Rutile

During slow cooling, Ba-doped TiO₂ ceramics exsolve barium which migrates to the surface where it forms barium-rich second phases. Two phases, BaTL₄O₉ and Ba₂Ti₉O₂₀, were identified and the occurrence of each determined by the doping level. The driving force for exsolution and migration is the relaxation of the elastic energy asSociated with Ba substitution in rutile. The growth kinetics for BaTi₄O₉ indicate rapid surface and lattice diffusion of barium through rutile. The apparent lattice diffusivity for the large Ba ion at 1070°C is 2x10⁻⁹ cm²/s, which is much greater than that of either Ti or 0 self-diffusivity (<10⁻¹¹ cm²/s).     

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.