Polar-Oriented Crystallization of Fresnoite (Ba2TiSi2O8) on Glass Surface Due to Ultrasonic Treatment with Suspensions

Improved polar ( c -axis) oriented growth of fresnoite (Ba₂TiSi₂O₈) was observed for surface crystallization of a glass 33.3BaO· 16.7TiO₂· 50SiO₂ (in mol%) due to ultrasonic surface treatment with an aqueous suspension of Ba₂TiSi₂O₈, and an oriented film more than 50 μm in thickness was prepared. The effect of ultrasonic treatment on the polar orientation depends on the suspending particles; of these, foreign particles of a phase other than the crystallizing Ba₂TiSi₂O₈ give poorer efficiency than the Ba₂TiSi₂O₈ particles. It is assumed that fresnoite has a tendency toward preferred polar growth on the glass, and seeding of a large number of fine particles through ultrasonic bombardment realizes the tendency. The polar-oriented growth kinetics of Ba₂TiSi₂O₈ were also discussed.     

Orientation Relationships of Reactively Grown Ba6Ti17O40 and Ba2TiSi2O8 on BaTiO3 (001) Determined by X-ray Diffractometry

Fresnoite grows at 700° and 800°C, and Ba₆Ti₇O₄₀ grows at 1200°C with definite orientations, which are determined by X-ray diffraction pole figure analysis. Partially textured fresnoite is formed at higher temperatures. The SiO₂ films react with the BaTiO₃ crystals, forming the phases Ba₂TiSi₂O₈ (fresnoite) and Ba₆Ti₁₇O₄₀. At 700° and 800°C, both phases grow with definite orientations, which are determined by X-ray diffraction pole figure analysis. Partially textured polycrystalline phases are formed at higher temperatures.     

Raman Study of Glasses of Ba2TiSi2O8 and Ba2TiGe2O8

Raman spectra are reported for fresnoite (Ba₂Ti(Si,Ge)₂O₈ glasses, and comparison is made between the Raman spectra of the corresponding crystalline powders and glasses of Ba₂TiSi₂O₈ and Ba₂TiGe₂O₈. The Ba₂TiGe₂O₈ glass spectra show correspondence with the Ba₂TiGe₂O₈ crystalline Raman spectra; the v _s(Ge–O–Ge) mode occurs at 518 cm⁻¹ in the glass and at 521 cm⁻¹ in the crystalline material. Five-fold coordinated titanium is the majority species present in the Ba₂TiGe₂O₈ glass as revealed by a strong band at 824 cm⁻¹ in the I glass spectrum. The Ba₂TiSi₂O₈ glass spectra are similar to the Ba₂TiSi₂O₈ crystalline spectrum; the strongest band is found at 836 cm⁻¹ in the I glass spectrum. Through comparison with the previous Raman data of other titania silicate glasses, we conclude that the Ba₂TiSi₂O₈ glass has a structure similar to the crystalline phase.     

Energetics of La2O3–HfO2–SiO2 Glasses

A series of La₂O₃–HfO₂–SiO₂ glasses, approximately along the join 0.73SiO₂–0.27( x HfO₂–(1− x )La₂O₃), 0< x <0.3), was prepared using containerless processing techniques (aerodynamic levitation combined with laser heating in oxygen). The enthalpy of formation and enthalpy of vitrification at 25°C were obtained from drop solution calorimetry of these glasses and appropriate crystalline compounds in a molten lead borate (2PbO–B₂O₃) solvent at 702°C. The enthalpy of formation from crystalline oxides was exothermic and became less exothermic with increasing HfO₂ content. Heat contents were measured by transposed temperature drop calorimetry and depended linearly on the HfO₂ content. Differential scanning calorimetry showed that both the onset glass transition and the onset crystallization temperature of these glasses increased with increasing HfO₂ content. Upon slow cooling in air, the glasses crystallized to a mixture of baddeleyite, cristobalite, lanthanum disilicate, and hafnon.     

Crystallization Kinetics and Dielectric Properties of Fresnoite BaO–TiO2–SiO2 Glass–Ceramics

Nucleation and crystallization kinetics of fresnoite (Ba₂TiSi₂O₈) crystals in BaO–TiO₂–SiO₂ glasses have been explored for dielectric applications. The volume fractions crystallized at different temperatures and times were tracked by XRD analysis. The activation energy of crystallization was estimated from DTA results to be about 528 kJ/mol, which is consistent with the value obtained by XRD results. The Avrami parameter values calculated at different temperatures from DTA results were found to be between 3.2 and 3.9, indicating that the growth is three dimensional and the mechanism of growth is interface-controlled. Additionally, because of compositional similarities, the dielectric contrast between the glass (ɛ_r∼15) and the resulting glass–ceramic (ɛ_r∼18) was minimal.     

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.     

Effect of Al2O3 and Bi2O3 on the Formation Mechanism of Sn-Doped Ba2Ti9O20

High-performance Ba₂Ti₉O₂₀ ceramics are attracting great attention, but their formation mechanism still is somewhat unclear. The present investigation shows that the formation of Ba₂Ti₉O₂₀ can be promoted strikingly by the participation of Bi₂O₃ and Al₂O₃. The effect of Bi₂O₃ on the formation of Ba₂Ti₉O₂₀ is attributed to the fact that migration of the involved reactants is accelerated by liquid which forms from the melting of Bi₂O₃ above 830°C. This migration, however, is not the only rate-limiting factor. A high potential-energy barrier, resulting from stress that arises along the crystal-structured layers, also heavily restricts the formation of Ba₂Ti₉O₂₀. The participation of Al₂O₃, on the other hand, can reduce the height of this potential-energy barrier and effectively improve the kinetics of the formation of Ba₂Ti₉O₂₀ by causing the formation of BaAI₂Ti₆O₁₆ crystals; these crystals intergrow with Ba₂Ti₉O₂₀ crystals and result in decreased stress.     

The Crystallization of Ba-Substituted CsTiSi2O6.5 Pollucite Using CsTiSi2O6.5 Seed Crystals

Barium (Ba)-substituted CsTiSi₂O_6.5 materials of two types, Cs _x Ba_{1− x} TiSi₂O_{(7− x /2)} and Cs _x Ba_{(1− x )/2}TiSi₂O_6.5 were synthesized with the pollucite structure with 1≥ x ≥0.6. When the Ba-substituted precursor materials were heat treated to 850°C for 4 h, a mixture of amorphous and unidentifiable phases formed. However, with the addition of 10 wt% of crystalline CsTiSi₂O_6.5 to the Ba-containing precursors, nearly single-phase pollucite was obtained after 20 h at 750°C for x ≥0.6. The added crystalline CsTiSi₂O_6.5 particles act as nuclei that allow the Ba-containing materials to crystallize into the pollucite phase and to avoid the formation of unwanted phases that would otherwise nucleate and grow. These new materials can be used to study the stability of CsTiSi₂O_6.5 as a durable ceramic waste form, which could accommodate with time both Cs and its decay product, Ba.     

Thermochemistry of Framework Titanosilicate A2TiSi6O15 (A=K, Rb, Cs)

A new family of framework titanosilicates, A₂TiSi₆O₁₅ (A=K, Rb, Cs) (space group Cc ), has recently been synthesized using the hydrothermal method. This group of phases can potentially be utilized for storage of radioactive elements, particularly ¹³⁷Cs, due to its high stability under electron radiation and chemical leaching. Here, we report the syntheses and structures of two intermediate members in the series: KRbTiSi₆O₁₅ and RbCsTiSi₆O₁₅. Rietveld analysis of powder synchrotron X-ray diffraction data reveals that they adopt the same framework topology as the end-members, with no apparent Rb/K or Rb/Cs ordering. To study energetics of the solid solution series, high-temperature drop-solution calorimetry using molten 2PbO·B₂O₃ as the solvent at 975 K has been performed for the end-members and intermediate phases. As the size of the alkali cation increases, the measured enthalpies of formation from the constituent oxides (     ) and from the elements (     ) become more exothermic, suggesting that this framework structure favors the cation in the sequence Cs⁺, Rb⁺, and K⁺. This trend is consistent with the higher melting temperatures of A₂TiSi₆O₁₅ phases with increase in the alkali cation size.     

Comment on "Effect of Al2O3 and Bi2O3 on the Formation Mechanism of Sn-Doped Ba2Ti9O20"

in a recent article of the Journal , Yu et al .¹ reported their experimental results on the effect of Al₂O₃ and Bi₂O₃ on the formation mechanism of Sn-doped Ba₂Ti₉O₂₀. They claimed that both Al₂O₃ and Bi₂O₃ can dramatically assist the formation of Sn-doped Ba₂Ti₉O₂₀ but are based on different mechanisms. They concluded that first, Bi₂O₃ melts above 830°C and accelerates the migration of the involved reactants to form Ba₂Ti₉O₂₀; second, Al₂O₃ can reduce the height of the potential energy barrier of the formation of Ba₂Ti₉O₂₀ due to the intergrowth of BaAl₂Ti₆O₁₆ phase. They explained their results from a point of view that the formation of Ba₂Ti₉O₂₀ is controlled by (1) the migration of reactants to the interfaces and (2) the height of the potential-energy barrier of the reaction at the interfaces. However, based on their results, we feel their conclusions are incautious and may be misleading, as will be discussed later.     

Influence of SiO2 Addition on the Dielectric Properties and Microstructure of (Ba0.96Ca0.04)(Ti0.85Zr0.15)O3 Ceramics

In this study, the effect of SiO₂ doping on the sintering behavior, microstructure, and dielectric properties of BaTiO₃-based ceramics was investigated. Silica was added to (Ba_0.96Ca_0.04)(Ti_0.85Zr_0.15)O₃ (BCTZ) powder prepared using the solid-state method. SiO₂-doped BCTZ ceramics with a high density and a uniform grain size were obtained and sintered at 1220°C in a reducing atmosphere. A second phase (BaTiSiO₅) existed in samples when SiO₂ was added in excess of 1%. The amount of the second phases was observed to increase as the number of SiO₂ additives increased. It was found that BCTZ ceramics sintered with SiO₂ are helpful in reducing the sintering temperature for a typical thick film and MLCC applications. However, there were disadvantageous effects on the dielectric properties with mere addition of SiO₂ addition (3% and 5%) due to higher formation of BaTiSiO₅. Doping with a small amount of silica can improve the sintering and dielectric properties of BCTZ ceramics. In addition, to understand the effect of the BaTiSiO₅ phase on the dielectric properties of BCTZ ceramics, the BaTiSiO₅ composition was synthesized from individual BaCO₃, TiO₂, and SiO₂ powders using conventional solid-state methods. X-ray diffraction results show the presence of mainly the crystalline phase, BaTiSiO₅, in the sintered ceramics.     

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 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.     

High-Temperature Transmission Electron Microscopy and X-Ray Powder Diffraction Studies of Polymorphic Phase Transitions in Ba4Nb2O9

Polymorphic phase transitions in Ba₄Nb₂O₉ were studied by thermal analyses, high-temperature transmission electron microscopy and X-ray powder diffractometry. Two stable polymorphs were isolated, low-temperature α-modification and high-temperature γ-modification, with the endothermic phase transition at 1176°C. The α→γ transformation is accompanied by the formation of a 120° domain structure, which is a consequence of hexagonal→orthorhombic unit cell reconstruction. Reheating the presintered γ-Ba₄Nb₂O₉ results in the formation of a metastable γ'-modification (formerly known as β-polymorph) in the temperature range between 360° and 585°C, before the γ→α transformation at 800°C. Above ∼490°C Ba₄Nb₂O₉ becomes moderately sensitive to a loss of BaO. In air the surface of Ba₄Nb₂O₉ grains decomposes to nanocrystalline Ba₅Nb₄O₁₅ and BaO, which instantly reacts with atmospheric CO₂ to form BaCO₃. Surface reaction delays γ→α transformation up to 866°C in air. In vacuum the loss of BaO is even more enhanced and consequently the formation of minor Ba₃Nb₂O₈ phase is observed above 1150°C.     

Formation Mechanism and Synthesis of Apatite-Type Structure Ba2+xLa8−x(SiO4)6O2−δ

The formation process of Ba₂La₈(SiO₄)₆O₂ was clarified using thermogravimetry–differential thermal analysis (TG-DTA) and a high-temperature powder X-ray diffraction (HT-XRD) method. Phase changes identified from the HT-XRD data surprisingly corresponded to the weight loss and/or endothermic peaks observed in the TG-DTA curves. Raw material with the composition Ba₂La₈(SiO₄)₆O₂ was completely reacted at 1400°C and produced only an apatite-type compound without a secondary phase. Moreover, the synthesis of Ba_{2+ x} La_{8− x} (SiO₄)₆O_2−δ crystals with x = 0–2 was attempted using a solid-state reaction.     

Ba2Ti9O20 Phase Equilibria

The heterogeneous phase distribution found in Ba₂Ti₉O₂₀ ceramic resonators results from a temperature-dependent phase boundary and slow reaction kinetics. When sintered at 1350°C or higher in oxygen the Ba₂Ti₉O₂₀ phase becomes slightly reduced and barium-rich. Thus a stoichiometric composition forms rutile and "Ba₂Ti₉O₂₀'phase. On slow cooling the excess barium diffuses to the oxygen-rich surface where it reacts to form an envelope of rutile-free material surrounding a core containing a small amount of rutile.     

Synthesis of Celsian (BaAl2Si2O8) from Solid Ba-Al-Al2O3-SiO2 Precursors: I, XRD and SEM/EDX Analyses of Phase Evolution

An intimate Ba-Al-Al₂O₃-SiO₂ powder mixture, produced by high-energy milling, was pressed to 3 mm thick cylinders (10 mm diameter) and hexagonal plates (6 mm edge-to-edge width). Heat treatments conducted from 300° to 1650°C in pure oxygen or air were used to transform these solid-metal/oxide precursors into BaAl₂Si₂O₈. Barium oxidation was completed, and a binary silicate compound, Ba₂SiO₄, had formed within 24 h at 300°C. After 72 h at 650°C, aluminum oxidation was completed, and an appreciable amount of BaAl₂O₄ had formed. Diffraction peaks consistent with hexagonal BaAl₂Si₂O₈, BaAl₂O₄, β-BaSiO₃, and possibly β-BaSi₂O₅ were detected after 24 h at 900°C. Diffraction peaks for BaAl₂O₄ and BaAl₂Si₂O₈ were observed after 35 h at 1200°C, although SEM analyses also revealed fine silicate particles. Further reaction of this silicate with BaAl₂O₄ at 1350° to 1650°C yielded a mixture of hexagonal and monoclinic BaAl₂Si₂O₈. The observed reaction path was compared to prior work with other inorganic precursors to BaAl₂Si₂O₈.     

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.     

Syntheses and Unit-Cell Determination of Ba3V4O13 and Low-and High-Temperature Ba3P4O13

The syntheses and the results of unit-cell determinations ofBa₃V₄O₁₃ and the two forms (low- and high-temperature) of Ba₃P₄O₁₃ are presented. Ba₃V₄O₁₃ crystallizes in the monoclinic system, space group Cc or C2/c with unit-cell dimensions a=16.087, b=8.948, c=10.159 (x10nm), β=114.52° Low-Ba₃P₄O₁₃ crystallizes in the triclinic system, space group P1 or P1 with unit-cell dimensions a=5.757, b=7.243, c=8.104 (x10 nm) α=82.75°, β=73.94°, γ=70.71°. Low-Ba₃P₄O₁₃ transforms at 870°C into high-Ba₃P₄O₁₃ which crystallizes in the orthorhombic system, space group Pbcm (No. 57) (or Pbc2, No. 29) with unit-cell dimensions a =7.107, b=13.883, c=19.219 (x10 nm). No relations have been found between the structures of the tribarium tetravanadate and the tribarium tetraphosphate.     

Preparation and Unit-Cell Parameters of Single Crystals of Ba2Ti9020

Ba₂Ti₉O₂₀ crystallizes in the monoclinic system with α= l.4818(5) nm, b = 1.4283(6), and c = 0.7109(2) with β = 98.37°±0.07°. The most likely space group is P 2₁/ m , Z = 4 with a calculated density 4.58 g/cm³. The powder pattern was indexed. The Ba₂Ti₉O₂₀ crystals form as stellated groups when melts of BaCl₂+ 20 to 50% TiO₂ cool from 1275°C.