Thermochemistry and Aqueous Durability of Ternary Glass Forming Ba-Titanosilicates: Fresnoite (Ba2TiSi2O8) and Ba-Titanite (BaTiSiO5)

Barium titanosilicates are possible oxide forms for the immobilization of short-lived fission products in radioactive waste. Ba₂TiSi₂O₈ (fresnoite) and BaTiSiO₅ (Ba-titanite) samples were prepared by a solid-state synthesis. The enthalpies of formation of Ba₂TiSi₂O₈ crystal and glass at 25°C and of BaTiSiO₅ glass were obtained from drop solution calorimetry in a molten lead borate (2PbO–B₂O₃) solvent at 701°C. The enthalpy of formation for fresnoite composition samples from constituent oxides was exothermic and became more exothermic with increasing crystallinity. Differential scanning calorimetry revealed that the crystallization rate of the fresnoite glasses increased with increasing devitrification. A modified Product Consistency Test-Procedure B (PCT-B) was used to collect solubility data on the fresnoite and titanate phases. The tests suggest that both glassy and crystalline fresnoite exhibit favorable aqueous stability and should be explored further as radioactive waste forms for long-term storage.     

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 Spectroscopy of Heat-Treated B2O3-SiO2 Glasses

Raman spectra were measured on B₂O₃-SiO₂ glasses heat-treated at different temperatures and for different periods of time. It was found that the intensity ratio of the 810- and 475-cm⁻¹ bands depended on the thermal history and composition of glass. The intensity vs heat-treatment temperature curve exhibited a maximum in the glass-transition temperature range. High-temperature Raman spectra were measured on 0.5B₂O₃-0.5SiO₂ glass, both with increasing and decreasing temperature, between room temperature and 600°C. Above 300°C with increasing temperature, the intensity of the 810-cm⁻¹ band decreased, whereas the intensity of the 475-cm⁻¹ band increased. The changes reversed with decreasing temperature. The reversible change of the intensity of the two bands was interpreted as a reversible formation and decomposition of boroxy rings with temperature. The reaction becomes very sluggish below the glass transition so that the intensity ratio stayed constant below 300°C.     

Infrared Spectra of GeO2-P4O10-V2O5 Glasses and Their Relation to Structure and Electronic Conduction

Infrared spectra of binary and ternary compositions in the glassy state and in the devitrified crystalline state in the system GeO₂-P₄O₁₀-V₂O₅ were studied and compared with infrared spectra of crystalline spinels, which are known to contain V⁵⁺ ions in sixfold coordination. Results indicated that the fundamental vibration frequency of the V—O bond occurred at wave number 1015 cm⁻¹. The spectra also provided evidence that the V⁵⁺ ion existed in sixfold coordination in the glassy state, as well as in the devitrified crystalline state, and that the VO₆ octahedron retained its identity even at low concentrations and melting temperatures of 1450°C. Conductivity measurements indicated that, as the concentration of V₂O₅ decreased below the critical range of 5 to 10 mole %, there was an abrupt loss in the electronic conductivity of the glasses; the conductivity decreased with increasing concentration of the lower valence states of vanadium. A mechanism of conduction compatible with the structure of glasses in the system is suggested to explain these observations.     

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.     

Effect of Sb2O3 on Thermal Properties of Glasses in Bi2O3–B2O3–SiO2 System

Glasses with compositions 50Bi₂O₃– x Sb₂O₃–10B₂O₃–(40– x ) SiO₂ ( x =0, 1, 3, 5, 8, 10) have been prepared by conventional melt quench technique. Substitution of Sb₂O₃ for SiO₂ exerted an obvious effect on properties of glasses, especially, increased glass transition temperature ( T _g) and crystalline temperature ( T _c) greatly. Results of infrared transmission spectra attributed the effect to the formation of new bridging bonds of Sb–O–B and Sb–O–Si in glass network.     

Depolymerization of GeO2 and B2O3-GeO2 Network Glasses by Sb2O3

Binary Sb₂O₃-GeO₂ glasses containing 45 mol% Sb₂O₃ and ternary Sb₂O₃-B₂O₃-GeO₂ glasses containing 50 mol% GeO₂ were prepared. Their densities (volumes), refractive indices, and infrared spectra were determined, and their colors and high-temperature viscosities were estimated visually. Small amounts of Sb₂O₃ (∼10 mol%) appear to perturb neither the Ge-O-Ge network nor those B-O-Ge networks with small B/Ge ratios (∼0.2). The B-O-Ge networks with larger B/Ge ratios (∼1.0) depolymerize in the presence of even less Sb₂O₃. Amounts of Sb₂O₃ >10 mol% appear to depolymerize the Ge-O-Ge and Ge-O-B networks progressively, possibly with the formation of chains. A structurally sensitive ir isofrequency contour technique developed for ternary glass systems was applied successfully to these Sb₂O₃-B₂O₃-GeO₂ glasses. These contours can thus readily detect significant network depolymerization in the absence of the usual network modifiers.     

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.     

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.     

Ba8ZnTa6O24: A New High Q Dielectric Perovskite

The hexagonal perovskite, Ba₈ZnTa₆O₂₄, was prepared in single-phase form and was found to be a stable secondary phase, formed as a result of the loss of ZnO from Ba(Zn_1/3Ta_2/3)O₃ microwave dielectrics. The experimental and calculated X-ray patterns of Ba₈ZnTa₆O₂₄ indicate it is isostructural with Ba₈Ta₆NiO₂₄ with an 8H (cchc)₂ close-packed BaO₃ stacking sequence and the lattice parameters, a =10.0825(14), c =19.0587(38)Å. High-density ceramics of Ba₈ZnTa₆O₂₄ could be prepared at temperatures considerably lower (1400°C) than those used to sinter pure Ba(Zn_1/3Ta_2/3)O₃, and exhibit very good microwave dielectric properties with ɛ=30.5, Q _f=62 300, and τ_f=+36 ppm/°C at 8.9 GHz.     

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.     

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.     

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.     

Atmospheric Dependence on Dielectric Loss of 1/6Ba5Nb4O15· 5/6BaNb2O6 Ceramics

Samples of 1/6Ba₅Nb₄O₁₅·5/6BaNb₂O₆ along with the pure end members, Ba₅Nb₄O₁₅ and BaNb₂O₆, were sintered under low oxygen partial pressure. The degradation mechanisms of dielectric loss in this reducing atmosphere have been studied. We found that the degradation occurred primarily due to the formation of oxygen vacancies caused by the reduction of Nb⁵⁺. This was determined by measuring the electrical conductivity, and through X-ray photoelectron spectroscopy. More importantly, the dielectric loss of 1/6Ba₅Nb₄O₁₅·5/6BaNb₂O₆ samples with higher temperature stability was further decreased on sintering in a reducing atmosphere. This observation has been explained by considering the increased porosity and formation of a reduced second phase, Ba_0.65NbO₃.     

The Influence of Sb2O3 Addition on the Properties of Low-melting ZnO–P2O5 Glasses

The influence of 0–16 mol% Sb₂O₃ substitution for P₂O₅ on the properties of ZnO–P₂O₅ glasses has been investigated. It was shown that Sb₂O₃ could participate in the glass network and thermal stability of the glasses decreased with increasing Sb₂O₃ content. Glass transition temperature T _g, softening temperature T _s, and water durability all decreased firstly (up to 6 mol% Sb₂O₃ added) and then increased. Substitution of 12 mol% Sb₂O₃ led to a 16°C decrease in T _g and 30°C decrease in T _s, and weight loss of the glass was only 0.42 mg/cm², which is ∼11 times lower than that of the glass without Sb₂O₃ after immersion in deionized water at 90°C for 1 day. The glass containing 12 mol% Sb₂O₃ might be a substitute for Pb-based glasses in some applications.     

Sol-Gel Process for the Preparation of Ba2Ti9O20 and BaTi5O11

Barium titanate precursors with Ba/Ti ratio 2:9 and 1:5 were prepared by first hydrolyzing titanium alkoxide and then mixing the resulting titania sol with a barium alkoxide-methanol solution. After drying, the xerogels of the precursors of barium titanates were sintered at temperatures from 700°C (4 h) to 1200°C (110 h or longer). Characterization of the product was performed using X-ray diffraction and laser Raman spectroscopy. At 700°C, BaTi₅O₁₁ was formed from the 1:5 precursor and a two-phase mixture of BaTi₂O₅ and BaTi₅O₁₁ was formed from the 2:9 precursor. After prolonged heating at 1200°C, the latter mixture converted to a single-phase material, Ba₂Ti₉O₂₀.     

Grain-Oriented Glass-Ceramics for Piezoelectric Devices

Grain-oriented glass-ceramics of Li₂Si₂o₅, fresnoite (Ba₂TiSi₂O₈), and its isomorphs Sr₂TiSi₂O₈, and Ba₂TiGe₂O₈, were prepared by recrystallizing glasses in a temperature gradient. Electromechanical and hydrostatic piezoelectric properties of these glass-ceramics were measured. Piezoelectric voltage coefficients g₃₃ and hydrostatic voltage coefficient g_h of these glass-ceramics are comparable to those of polyvinylidene fluoride and an order of magnitude higher than the corresponding values of lead zirconate-titanate. These glass-ceramics seem to be attractive candidate materials for hydrophones and several piezoelectric devices. Hydrostatic piezoelectric properties of Ba₂TiSi₂O₈ and Ba₂TiGe₂O₈ single crystals were also measured. The unusually high values of g_h in fresnoite single crystals and glass-ceramics are supposed to be due to positive d₃₁ in these materials. A composite model is proposed to explain the positive sign of d₃₁ in fresnoite based on its crystal structure and internal Poisson's ratio stress.     

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.     

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.