Topotaxy between BaTiO3 and Ba6Ti17O40

The orientational relationship between intergrown lamellae of tetragonal (t) BaTiO₃ and monoclinic (m) Ba₆Ti₁₇O₄₀ was found to be ( 1 1 1 )_t||(001)_m, (001)_t||( 1 0 1 )_m, and [110]_t||[0 1 0]_m. On the basis of this relationship a model of a topotactic boundary between these two phases was constructed.     

Crystallography of the Tetragonal to Monoclinic Transformation in MgO-Partially-Stabilized Zirconia

The structure of monoclinic ZrO₂ particles dispersed in MgO-partially-stabilized zirconia has been examined systematically using transmission electron microscopy and electron micro-diffraction. In both particles transformed athermally and those transformed under stress, the product of the martensitic tetragonal to monoclinic transformation comprises parallel variants of the monoclinic structure. The monoclinic domains extend either parallel or normal to the original precipitate habit plane and, to within the accuracy of the technique, adjacent pairs are twin related. For particles with domains parallel to the particle habit plane the boundary between variants is (001)_m and the orientation relationship between tetragonal and monoclinic lattices is such that (001)_m‖ (001)_t and [100]_m‖ [100]_t. In particles with transverse domains, the domain boundaries are parallel to (100)_m, and the orientation relationship is given by (100)_m‖ (100)_t and [001]_m‖ [001]_t. In each case the lattice correspondence implied between parent and product lattices is such that the c _m axis is parallel to the C _m axis. The microcracking associated with transformed particles appears closely related to the substructure adopted by the particles and the origin of this microcracking is briefly discussed.     

Interaction of BaCl2 and BaSO4 at Elevated Temperatures

The effect of adding BaCl₂ to BaSO₄ at elevated temperatures was studied. Differential thermal analysis and hot-stage microscopy revealed that a eutectic occurs at 865°C between BaCl₂ and 13.5 mol% BaSO₄. The microstructure in the reaction zone indicated that BaS04 recrystallizes from the eutectic melt. Thus, BaCl₂ additions can activate BaSO₄ at a temperature as low as 865°C.     

Sintering of ZnS with a Small Amount of Ba2ZnS3

The effects of alkali and alkali-earth chloride additives on the sintering of ZnS powder compacts in H₂S have been examined. BaCl₂ was found to promote the sintering of ZnS. BaCl₂ reacts with ZnS to form Ba₂ZnS₃, which acts as a flux for sintering.     

Stability of YBa2Cu3O7-x in Molten Chloride Salts

The decomposition products of YBa₂Cu₃O_7-x depend on the composition of the molten chloride salt for exposure at 1173 K in air. The presence of dichloride salts such as CuCl₂, CaCl₂, or MgCl₂ promote formation of CuO, Cu₂Y₂O₅, and loss of barium to the chloride salt as BaCl₂. Salts based on BaCl₂ or containing LiCl result in YBa₂Cu₃O_7-x decomposition products of Y₂BaCuO₅, CuO, and BaCl₂. High barium activity in the salt supports formation of the Y₂BaCuO₅ phase and reaction of CO₂ with the salt producing BaCO₃. Decomposition is most sluggish in binary NaCl-KCl salts where minimal amounts of reaction or decomposition products are observed.     

Modification of Piezoelectric Characteristics of the Pb(Mg,Nb)O3-PbZrO3-PbTiO3 Ternary System by Aliovalent Additives

The effects of donor and acceptor doping on the piezoelectric properties of 18Pb(Mg_1/3Nb_2/3)O₃·39PbZrO₃·43PbTiO₃ternary solid solution have been investigated. The dopants influence in a pronounced way both the crystal structure and microstructure of the materials. The materials are transformed from the rhombohedral to the tetragonal structure, and the grain size is reduced when Nb ⁵⁺ cations are added. The crystal structures are transformed from the rhombohedral to the cubic structure, and grain growth is enhanced, when Mg ²⁺ cations are included. The values of K _P and K are maximized for 1-mol%-Nb⁵⁺-doped samples (K _P= 0.69, K = 1152); the Q _m factor and E _c are optimized for 4-mol%-Mg²⁺-doped samples ( Q _m= 1534, E _c= 0.56).     

Interaction of BaCl2 and BaCO3 at Elevated Temperatures

The effect of adding BaCl₂ to BaCO₃ at elevated temperatures was studied. DTA and hot-stage microscopy revealed that a eutectic occurs at 840°C and 14.0 mol% BaCO₃. The microstructure in the reaction zone indicated that BaCO₃ recrystallizes from the eutectic melt. Thus, BaCl₂ additions can activate BaCO₃ at a temperature as low as 840°C .     

Piezoelectric Properties of Pb[Zr0.45Ti0.5-xLux(Mn1/3Sb2/3)0.05]O3 Ceramics

The piezoelectric properties of Pb[Zr_0.45Ti_{0.5- x} Lu _x (Mn_1/3-Sb_2/3)_0.05]O₃ ceramics, with 0 lessthan equal to x lessthan equal to 0.03, have been investigated. The partial substitution of Ti⁴⁺ with Lu³⁺ permitted improvement of the electromechanical coupling factor ( k _p), the dielectric constant (epsilon^T₃₃), and the piezoelectric constant ( d ₃₃), while the dielectric loss (tan delta) increased and the mechanical quality factor ( Q _m) decreased with an increase of x . A pertinent piezoelectric material for actuator applications was Pb[Zr_0.45Ti_0.48Lu_0.02(Mn_1/3Sb_2/3)_0.05]O₃, and the piezoelectric properties were k _p = (58.5 ± 0.5)%, epsilon^T₃₃ = 32 ± 25, d ₃₃ = (373 ± 6) 10^-12 C/N, Q _m = 714 ± 22, and tan delta = (0.98 ± 0.03)%.     

Characteristics and Formation Mechanism of BaTiO3 Powders Prepared by Twin-Fluid and Ultrasonic Spray-Pyrolysis Methods

Spherical fine (micrometer and submicrometer in size) homogeneous BaTiO₃ powders were synthesized from ethanol: water solutions of BaCl₂ and TiCl₄ using the spray-pyrolysis technique. Two different atomizers—twin-fluid and ultrasonic, with a resonant frequency of 2.5 × 10⁶ Hz—were used for mist generation. Hollow spherical particles containing a certain amount of unreacted BaCl₂ phase and having a mean particle diameter of 2.5 μm were obtained at 1173 K using a twin-fluid atomizing system. Decomposition of precursors and their transition to the cubic BaTiO₃ phase occurred, even at 973 K in the case of the ultrasonic atomizing system. For the initial droplet size of 2.2 μm and residence time of ∼60 s, spherical BaTiO₃ particles with the mean particle diameter of 0.53 μm were obtained. A BaTiO₃ formation mechanism has been proposed as a reaction between TiO₂ and BaCl₂ rather than a reaction of oxides.     

Effect of Small Additions of Al2O3 and Ga2O3 on the Immiscibility Temperature of Na2O-SiO2 Glasses

Alumina and gallia were substituted separately for Na₂O in amounts of 0.2, 0.5, 1.0, 1.5, 2.0, and 3.0 wt% in three Na₂O-SiO₂ glass compositions (82, 84, and 86 wt% SiO₂) within the immiscibility region. The immiscibility regions for each system extend to ∼1.5 mol% of the added oxide. In general, the addition reduced the immiscibility temperature ( T _m), but at the edge of the immiscibility region (82% SiO₂) the Na₂O loss effect initially increased T _m. A structural model of the miscibility of Al₂O₃ added to silicate glasses is presented.     

Concentrations of Residual Carbonate Ions in the Bi-Sr-Ca-Cu-O Glasses

It was found that the IR absorption bands appearing at 600 to 1600 cm⁻¹, which had been previously assigned to the fundamental vibrations of [BiO₃] or [BiO₆] polyhedra, are due to residual carbonate ions (CO²⁻₃) dissolved in Bi-Sr-Ca-Cu-O glasses. The concentrations of the remaining CO²⁻₃ in the Bi_2.2Sr₂Ca₁Cu₂O _x glasses that melted at 1100° and 1400°C are 170 × 10⁻⁵ mol/cm³ (3.3 mol%) and 3.2 × 10⁻⁵ mol/cm³ (0.25 mol%), respectively. The apparent activation energy for the dissociation of the carbonates was approximately 220 kJ/mol. The CO²⁻₃ content in the precursor glasses did not significantly affect the superconducting properties of the resulting glass–ceramics.     

Vibrational Spectra of Water- and Ammonium-Chloride-Related Residues and Upconversion Fluorescence of Er3+ in ZnCl2-Based Glasses

Chloride glasses of the ZnCl₂ and 20KCl-20BaCl₂-60ZnCl₂-0.5ErCl₃ systems were prepared using NH₄Cl as a dehydrating and chlorinating agent, under the melt-quenching method. Water- and ammonium-chloride-related residues in ZnCl₂ glasses were investigated by infrared and near-infrared absorption spectra. Water, Zn—OH, ClO, ClO₂⁻, Zn²⁺-coordinated water, free NH₃, NH₄⁺, and Zn²⁺-coordinated NH₃ were identified in ZnCl₂ glasses. 20KCl-20BaCl₂-60ZnCl₂-0.5ErCl₃ glasses prepared by melting at 500°C for 20 min, under reduced pressure, contained the smallest amounts of water, Zn—OH, and Zn²⁺-coordinated NH₃ and showed strong Er³⁺ upconversion fluorescence.     

125Te, 27Al, and 71Ga NMR Study of M2O3–TeO2 (M = Al and Ga) Glasses

The structures of M₂O₃–TeO₂ (M = Al and Ga) glasses have been investigated by means of ¹²⁵Te, ²⁷Al, and ⁷¹Ga NMR spectroscopies. The structural units of respective cations in M₂O₃–TeO₂ glasses were quantitatively analyzed. The fractions of TeO₄ trigonal bipyramid, AlO₆ and GaO₆ octahedra decreased and those of TeO₃ trigonal pyramid, AlO₄, AlO₅, and GaO₄ polyhedra increased with increasing M₂O₃ content. Based on the local structures around Te, Al, and Ga atoms, the structure models of M₂O₃–TeO₂ glasses were proposed.     

Structure Evolution in Hydrothermally Processed (<100°C) BaTiO3 Films

Thin films of cubic BaTiO₃ were processed hydrothermally at 40°–80°C by reacting thin layers of titanium organo metallic liquid precursors in aqueous solutions of either Ba(OH)₂ or a mixture of NaOH and BaCl₂. All films (thickness ∼1 μm) were polycrystalline with grain sizes ranging from nano- to micrometer dimensions. BaTiO₃ formation was facilitated by increasing [OH^-], [Ba²⁺], and the temperature. The film structure was related to the nucleation and growth behavior of the BaTiO₃ particles. Films processed at relatively low [OH^-], [Ba²⁺], and temperatures were coarse grain and opaque, but increasing [OH^-], [Ba²⁺], and temperature caused the grain size to decrease, resulting in transparent films.     

Mirror Constant for Tyranno® Silicon-Titanium-Carbon-Oxygen Fibers Measured in Situ in a Three-Dimensional Woven Silicon Carbide/Silicon Carbide Composite

The strength, S , of ceramic and glass fibers often can be estimated from fractographic investigation using the fracture mirror radius, r _m, and the relationship S = A _m/( r _m)^1/2, where A _mis the "mirror constant." The present work estimates the value of A _mfor Tyranno^® Si-Ti-C-O fibers in situ in a three-dimensional woven SiC/SiC-based composite to be 2.50 ± 0.09 MPa·m^1/2. This value is within the range of 2–2.51 MPa·m^1/2 previously obtained for nominally similar Nicalon^® Si-C-O fibers.     

Phase Transition and Ferroelectric Characteristics of Pb[(Mg1/3Nb2/3)1-xTix]O3 Ceramics Modified with La(Mg2/3Nb1/3)O3

The effects of 0–5 mol% addition of La(Mg_2/3Nb_1/3)O₃ (LMN) on the phase transition and ferroelectric behaviors of Pb[(Mg_1/3Nb_2/3)_1-xTi_x]O₃ (PMNT) ceramics with compositions near the morphotropic phase boundary (MPB) were studied. An evolution of structure from rhombohedral to tetragonal was found with increasing PbTiO₃ (PT) content across the MPB (at ∼32.5 mol% PT), and a coexistence of both rhombohedral and tetragonal phases was also found at the MPB. The dual-phase field extended toward the lower PT content side of the MPB, and, moreover, the rhombohedrality or tetragonality was reduced, especially for the compositions near the MPB, by the addition of La in PMNT. The ferroelectric transition was found to change from normal to diffuse as the La content increased and the compositions became more rhombohedral. In accordance with the structural evolution, the change of remanent polarization ( P _r) and coercive field ( E _c) also became gradually indistinct, and both P _r and E _c were reduced. For compositions near the MPB, both PMNT and La-modified PMNT had a similar electromechanical factor ( k _p) in a range around 0.55–0.60, but the mechanical quality factor ( Q _m) was significantly reduced for the La-modified PMNT. The piezoelectric coefficient ( d ₃₃), however, was largely improved with increasing La content in PMNT of compositions at MPB. A high value of d ₃₃∼ 815 pC/N was obtained for the 5-mol%-La-modified ceramics, but it was associated with a low value of Q _m.     

Synergistic Effects of Water Vapor and Alkali Chloride Vapors on the High-Temperature Corrosion of SiC-Based Ceramics

The high-temperature oxidation of SiC-based structural ceramics is accelerated in the presence of KCl vapors. The corrosion products are primarily alkali silicates and the rate of attack is approximately proportional to the vapor pressure of KCl in the atmosphere. When both KCl and H₂O are present in the oxidizing atmosphere, the rate of attack is further accelerated. The accelerated attack is related to the more negative free energy change of the fluxing reaction when the Cl can be removed as the more stable HCl molecule. Addition of Cl₂ to the environment reduces the rate of attack by keeping the alkali bound to the chlorine rather than forming alkali silicates.     

Evidence for 6-Coordinated Zirconium in Inactive Nuclear Waste Glasses

In inactive glasses used as models for French nuclear waste forms, Zr K - and L _2,3-edge X-ray absorption spectroscopy spectra indicate that Zr is only 6-coordinated, with d (Zr-O) distances of 2.08 ± 0.01 Å and with a small radial disorder. Beyond, the first coordination shell, next-nearest Si neighbors occur at about 3.4 Å. These distances exclude the presence of ^[7]Zr and/or ^[8]Zr and suggest ^[6]Zr-O-Si angles close to 130°, indicating corner-shared ZrO₆ and SiO₄ polyhedra. This structural configuration is similar to those observed in some crystalline zirconosilicates and is in agreement with the stability of Zr in borosilicate melts and glasses.     

Effect of MnO2 Addition on the Structure and Electrical Properties of Pb(Zn1/3Nb2/3)0.20(Zr0.50Ti0.50)0.80O3 Ceramics

Pb(Zn_1/3Nb_2/3)_0.20(Zr_0.50Ti_0.50)_0.80O₃ ceramics of pure perovskite structure were prepared by the two-stage method with the addition of 0–3.0 wt% MnO₂ and their piezoelectric properties were investigated systematically. The MnO₂ addition influences in a pronounced way both the crystal structure and the microstructure of the materials. The materials are transformed from the tetragonal to the rhombohedral structure, and the grain size is enhanced when manganese cations are added. The distortion of crystal structure for samples with MnO₂ addition can be explained by the Jahn–Teller effect. The values of electromechanical coupling factor ( k _p) and dielectric loss (tan δ) are optimized for 0.5-wt%-MnO₂-doped samples ( k _p= 0.60, tan δ= 0.2%) and the mechanical quality factor ( Q _m) is maximized for 1.0-wt%-MnO₂-doped samples ( Q _m= 1041), which suggests that oxygen vacancies formed by substituting Mn³⁺ and Mn²⁺ ions for B-site ions (e.g., Ti⁴⁺ and Zr⁴⁺ ions) in the perovskite structure partially inhibited polarization reversal in the ferroelectrics. The ceramics with 0.50–1.0 wt% MnO₂ addition show great promise as practical materials for piezoelectric applications.     

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.