Effects of Transition-Metal Substitution on the Catalytic Properties of Barium Hexaaluminogallate

The effects of the substitution of transition-metal ions and/or reductant gases on the catalytic properties of barium hexaaluminogallate were investigated. Transition-metal-substituted hexaaluminogallates (BaM(Al,Ga)₁₁O₁₉, M = transition metal, Al/Ga = 9/3) were synthesized from aqueous metal nitrates and ammonium carbonate by the coprecipitation followed by crystallization at 1100°C. The direct NO _x reduction was observed over BaM(Al,Ga)₁₁O₁₉ to be around 10%. The NO _x removal activity of BaM(Al,Ga)₁₁O₁₉ powders was improved by addition of C₃H₆ as a reductant gas. Co-, Ni- and Cu-substituted BaM(Al,Ga)₁₁O₁₉ catalysts exhibited about 40% NO _x reduction with C₃H₆ in excess oxygen at a high space velocity of 10 000 h⁻¹. The NO _x reduction on Mn- and Fe-substituted BaM(Al,Ga)₁₁O₁₉ catalysts was less than 10% even in the presence of C₃H₆. The temperature of the effective NO _x reduction on BaM(Al,Ga)₁₁O₁₉ catalysts could be adjusted from 350° to 500°C by the selection of the transition-metal substitution in the catalysts. The catalysts hold high activities for NO _x reduction even at 500°C in water vapor produced in the combustion system of reductant gases.     

Kinetics of Barium Titanate Synthesis

Reaction curves were obtained at various temperatures and concentrations for the formation of BaTiO₃ from particulate titania in Ba(OH)₂ solution. Kinetic analyses were performed by constructing mathematical models which took into account the particle size distribution of the reactant titania for both the topochemically-rate-controlled and the diffusion-rate-controlled reactions. At [Ba(OH)₂] > ca. 0.1 M the rate-controlling step is the Ba reaction with TiO₂ at the interface. The measured activation energy is 105.5 kJ/mol. The rates are independent of Ba(OH)₂ concentration, indicating that the TiO₂ interface is saturated. At [Ba(OH)₂] < ca. 0.1 M the rate-determining step shifts to diffusion through the product BaTiO₃ layer, the rates are concentration dependent, and the BaTiO₃ particle sizes are inversely proportional to the Ba(OH)₂ concentrations used.     

Synthesis and Processing of Barium Hexaaluminogallates

Barium hexaaluminogallate was synthesized by the mixed-oxide method and the coprecipitation method. Barium hexaaluminate and barium hexagallate were found to form barium hexaaluminogallate, Ba(Al,Ga)₁₂O₁₉, at 1400°C. BaCo(Al,Ga)₁₁O₁₉ of magnetoplumbite structure was formed from a mixture of metal oxides at 1200°C for 6 h. Co was successfully introduced to barium hexaaluminogallate, although the synthesis of BaCoAl₁₁O₁₉ is quite difficult via solid-state reaction of oxide powders. Anisotropic BaCo(Al,Ga)₁₁O₁₉ particles crystallized at 1100°C for 2 h through the coprecipitation method using metal nitrates and ammonium carbonate. BaCo(Al,Ga)₁₁O₁₉ supported on a cordierite honeycomb had the ability to reduce NO _x with methane over 500°C in the presence of excess oxygen.     

Dislocation-Containing Planar Faults in Metastably Retained Hexagonal Barium Titanate

Planar faults, designated F ₁– F ₆ and F _a , with intermixed dislocations in metastably retained hexagonal-BaTiO₃ ceramics, were found and analyzed by transmission electron microscopy. Only faults with one, two, and four extra c -layers parallel to the basal plane have been identified. Fault vectors R_F1=R_Fa=1/6[02 2 1], R_F2=R_F3=1/3[0 1 11], R_F4=1/3[01 1 2], and R_F'1=R_F5= R_F6=R_F'a=1/6[0 2 21] were determined adopting the 2πg·R=0 (or 2 n π) criteria in combination with high-resolution imaging. Further, the embedded dislocations were half-partials with Burgers vectors b=1/3〈10 1 0〉 determined by the g·b=0 effective invisibility criteria in conjunction with the eligible fault vectors. A rotation by 60° about the c -axis was found between fault segments with R_F1=1/6[02 2 1] and R_F'1=1/6[20 2 1] located on either side of a partial. Basal dislocations with b_B=1/3〈1 2 10〉 have dissociated into two prism-plane Shockley half-partials with b_Pr=1/3〈1 1 00〉 by glide in the fault plane (0002) according to 1/3〈1 2 10〉→1/3〈1 1 00〉+1/3〈0 1 10〉. The fault segment F'₁ encompassed by two half-partials is an extrinsic complex stacking fault.     

Synthesis of Lead Barium Niobate Powders and Thin Films by the Sol-Gel Method

Crack-free, dense, and transparent Pb_0.6Ba_0.4Nb₂O₆ (PBN60) thin films have been prepared by a sol-gel method with metal alkoxides and metal acetate. A homogeneous and stable precursor solution was obtained from Ba metal, Pb(CH₃COO)₂, and Nb(OEt)₅ in 2-methoxyethanol. PBN60 powder crystallized to the hexagonal phase at 600°C and then completely transformed to the orthorhombic phase of the tungsten bronze structure at 1250°C. The hexagonal phase was formed on SiO₂ glass, MgO(lOO), and sapphire(R) substrate at 600°C, while the orthorhombic phase was only on a sapphire(C) substrate. Orthorhombic PBN60 films with c -axis preferred orientation were successfully synthesized on sapphire(C) substrates at 600°C.     

Thermal Analysis of Precursors of Barium Titanate Prepared by Coprecipitation

Thermal analysis was performed on coprecipitated materials and on individual components. The detailed decomposition schemes of coprecipitates and individual components are proposed and discussed. According to the proposed decomposition schemes, the values of the observed weight loss are in good agreement with those of the theoretical values of the coprecipitated materials and individual components. The results indicate that barium titanyl oxalate is an inserting compound, i.e., a structure of distorted barium hydrogen oxalate hydrate being inserted by Ti(OH)₃⁺. The results also verify that copreciptation of barium and titanium ions in an oxalate aqueous solution at pH 7 is a mixture of BaC₂O₄· 0.5H₂O and TiO(OH)₂· 1.5H₂O and coprecipitation of barium and titanium ions using the process of Yamamura et al. is a mixture of Ba(NO₃)₂ and Ti(OH)₂C₂O₄.     

Phase-Transformation-Induced Anti-Phase Boundary Domains in Barium Cerate Perovskite

Anti-phase boundary (APB) domains resulting from the orthorhombic ( o ₁, Ibmm (No. 74)) to orthorhombic ( o ₂, Pbnm (No. 62)) phase transformation at ∼290°C in pressureless-sintered barium cerate (BaCeO₃) ceramics have been investigated by transmission electron microscopy and large-angle convergent beam electron diffraction (LACBED). APB exhibiting symmetric π-fringe patterns in both bright-field and dark-field images, lying in {011) with the fault vector R =1/2〈111〉, were determined adopting the invisibility criteria of 2π g · R =0 or 2 n π. The curved boundaries due to symmetry change can be related to the low symmetry phase by a pure translation upon phase transition. The displacement vector, R , relating the domains is the translational symmetry element from the origin to the lattice point that was lost upon transition. The formation of such domains induced by the o ₁→ o ₂ phase transition is discussed in terms of change in the Bravais lattice and loss of lattice point upon the transition. The identification has also confirmed the existence of a higher symmetry orthorhombic o ₁ phase along the transition sequence of R 3 c → Ibmm → Pbnm . The increased spatial frequency of APB domains is attributed to A-site ordering due to vacancies created by BaO loss and/or disordering of the B-site substitutional defects generated extrinsically when doped with either a Y³⁺-acceptor or a Nb⁵⁺-donor.     

Alternative Route for Synthesis of Barium Titanyl Oxalate: Molecular Precursor for Macrocrystalline Barium Titanate Powders

An important molecular precursor to barium titanate, namely, barium titanyl oxalate [BaTiO(C₂O₄)₂.4H₂O], has been synthesized by an alternative route. An alcoholic solution containing 1 mol of butyl titanate monomer [(C₄H₉O)₄Ti] is reacted with alcoholic solution containing 2 mol of oxalic acid (H₂C₂O₄:2H₂O) to form an intermediate soluble oxalotitanic acid [H₂TiO(C₂O₄)₂.nH₂O]. The oxalotitanic acid in alcoholic medium is subjected to cation exchange reaction with aqueous solution containing equimolar barium acetate to form an insoluble barium titanyl oxalate (BTO) in yields of 80–85% at room temperature. The pyrolysis of BTO in air at T .750°C/5 h produced barium titanate (BT) powders.     

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.     

Incorporation of Yttrium in Barium Titanate Ceramics

In this work, X-ray diffractometry and scanning electron microscopy techniques were used to study the incorporation of yttrium in BaTiO₃, with the following nominal compositions: (Ba_{1- x} Y _x )TiO₃(0.015 ≤ to x ≤ to 0.08), (Ba_{1- x} Y _x )Ti_{1- x /4}-(□_Ti) _{x /4}O₃(0.005 ≤ to x ≤ to 0.1), and Ba(Ti_{1- y} Y _y )O_3-delta(0.028 ≤ to y ≤ to 0.258). The phase assemblage and the lattice parameters indicated a slight solubility (∼1.5 at.%) of yttrium at the Ba sites at 1440°C but a high solubility (∼12.2 at.%) of yttrium at the Ti sites at 1515°C. When BaTiO₃was heavily doped with yttrium at the Ti sites (a yttrium concentration ( y ) of <0.059), the crystallographic structure was tetragonal, whereas for y greater than equal to 0.059, the crystallographic structure was cubic.     

Fabrication of Optically Transparent and Electrooptic Strontium Barium Niobate Ceramics

The tetragonal tungsten bronze (TTB) type of Sr _x Ba_{1− x} ^m Nb₂O₆ (SBN) (0.5 ≤ x ≤ 0.75) was synthesized by a mixed-oxide route. Using the two-step densification process of pressureless sintering followed by oxygen HIPing, transparent ceramics were successfully fabricated. Microstructural, dielectric, optical, and electrooptic properties were investigated. All single-phase TTB-type SBN showed the characteristics of relaxor ferroelectrics. The extinction coefficients of the sample with the highest transparency were calculated from transmission spectra to be 30.4, 5.8, and 2.4 cm⁻¹ at 600, 1450, and 2000 nm, respectively. The linear electrooptic coefficients for SBN55, r ₃₃ and r ₁₃, were 46 × 10⁻¹² and 21 × 10⁻¹² m/V at 633 nm, respectively.     

Comparison of the Formation of Calcium and Barium Phosphates by the Conversion of Borate Glass in Dilute Phosphate Solution at near Room Temperature

The formation of phosphates of calcium or barium at near room temperature by the direct conversion of borate glass in dilute phosphate solution was investigated. Borate glass particles (150–300 μm), with the composition 20Na₂O·20AO·60B₂O₃ (mol%), where A is the alkali-earth metal Ca or Ba, were prepared by conventional processing, and immersed in 0.25 M K₂HPO₄ solution at 37°C and with a starting pH value of 9.0 or 12.0. The effects of the borate glass composition and the solution pH on the rate of formation, the chemical composition, and the structure of the phosphate products formed in the conversion reaction were investigated using weight loss and pH measurements, X-ray diffraction, X-ray fluorescence, scanning electron microscopy, and Brunauer–Emmett–Teller surface area. At both pH values, the calcium borate glass particles were completely converted to hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂, with a high surface area (160–170 m²/g). At pH=9.0, the barium borate glass particles converted completely to a barium phosphate product that was isostructural with BaHPO₄, whereas at pH=12.0, the barium phosphate product was isostructural with Ba₃(PO₄)₂, with both products having much lower surface areas (4–8 m²/g). The formation of the phosphate products was pseudomorphic, retaining the external shape of the original glass particles.     

The {111} Growth Twins in Tetragonal Barium Titanate

The {1 1 1} twins frequently observed in pressureless-sintered BaTiO₃ ceramics have been analyzed by the X-ray diffractometry, scanning and transmission electron microscopy. Both the single twins and double lamellar twins are growth (or annealing) twins. The twins lying in the {1 1 1} mirror planes, which is not one of the symmetry elements of the (basic) crystal lattice's but that of the superlattice's, are therefore superlattice twins. The {1 1 1} twins, particularly the double twins, were found more frequently from samples sintered in an Ar atmosphere of lower oxygen partial pressure (pO₂). Further decreasing of pO₂ using the Ar-5% H₂ mixture has rendered the sintered samples entirely of hexagonal BaTiO₃, the 6H-polytype. The formation of such twins is attributed to changing of the corner-sharing TiO₆ octahedra to Ti₂O₉ face-sharing octahedra, which accommodates for local oxygen deficiency in tetragonal BaTiO₃. The stacking sequence alters accordingly from c-layer (constituting the 3C-polytype, treating tetragonal pseudo-cubic) to h-layer (as in (chc)₁(chc)₂ of 6H).     

Thermal Decomposition of Some Substituted Barium Titanyl Oxalates and Its Effect on the Semiconducting Properties of the Doped Materials

Thermal analysis has been performed on BaTiO(C₂O₄)₂.4H₂O, Ba_0.6Sr_0.4TiO(C₂O₄)₂.4H₂O, Sr(TiO(C₂O₄)₂.4H₂O, Ba_0.9Pb_0.1TiO(C₂O₄)₂.4H₂O, and BaTi_0.9Zr_0.1O(C₂O₄)₂.4H₂O. It was observed that the strontium compound decomposes differently than the others. Previous investigators have proposed conflicting mechanisms for the pyrolysis of the barium salt and these results are discussed in comparison with this work. The electrical resistivity and temperature coefficient of fired lanthanum-doped materials were found to vary with the calcination temperature. Maximum conductivity was observed in samples calcined at 900°C whereas maximum positive temperature coefficient was observed for materials calcined at 1050°C. Particle sizes of the calcined material were compared with grain sizes in the fired pieces and correlated with the electrical properties. A cursory examination was made on the effects of fabrication pressure, 1.25 to 15 tsi, on the electrical conductivity. Both the conductivity and positive temperature coefficient were found to increase with decreasing fabrication pressure.     

Solid Solutions Based on Ferroelectric PbNb2,O6

An X-ray and dielectric study of three series of PbNb₂O₆-type solid solutions was carried out. The PbNbZOs-type phase was present up to the following limits: (1) (Pb_1−xBi₄/_3x)(Ti_xNb₂-_x.)O₆, ×= 0.2; (2) (Pb_l-_xBi₄3_x)(Ti₂Nb_l—2)₂0₆, ×= 0.2; and (3) (Pb_1−xBi₂/_3x)Nb₂O₆×= 0.3. Orthorhombic distortion b/a and Curie temperature decreased with increasing x. Bi_4/3-Ti₂0₆ was most effective in this respect and Bi₂, ₃-Nb₂O₆ had the least influence. The important role played by ions of different valency located on crystallographically equivalent sites is pointed out.     

Thermal Analysis of Some Barium and Strontium Titanyl Oxalates

The thermal decompositions of BaTiO(c₂O₄)₂.- 4H₂O, BaTiO(OH)₂C₂O₄.2H₂O, SrTiO(C₂O₄)₂.- 4H₂O, and SrTiO(OH)₂C₂O₄.H₂O were investigated using TGA, DTA, and effluent gas analysis. The stoichiometry of the decompositions is discussed and it is proposed that a reduced state of titanium is formed as an intermediate.     

The Effect of Texture and Microstructure on the Macroscopic Properties of Polycrystalline Piezoelectrics: Application to Barium Titanate and PZN–PT

The effects of crystallographic texture and microstructure are analyzed for polycrystalline tetragonal BaTiO₃, pseudotetragonal PZN–PT, and cubic BaTiO₃. For tetragonal BaTiO₃ and pseudotetragonal PZN–PT, we demonstrate that a high anisotropy of the single-crystal properties induces an apparent enhancement in the macroscopic piezoelectric response. For tetragonal BaTiO₃, the predicted macroscopic piezoelectric constants d ₃₁ and d ₃₃ are enhanced with respect to its single-crystal value at the expense of the spatial contributions from d ₁₅. For samples possessing fiber texture, an optimal response is predicted for samples that are not perfectly textured. Similarly, an apparent enhancement of the macroscopic value of d ₁₅ is predicted for PZN–PT. For cubic BaTiO₃, the low anisotropy of the underlying crystal properties induces a uniform decrease of the macroscopic electrostrictive constant, Q ₁₁, with decreasing texture. A completely random polycrystal provides 0.85±0.05 times its single-crystal response.     

Investigation of Rare-Earth Doped Barium Titanate

In this study the effect of additions of 0.0015 to 0.0030 mole fraction of rare-earth oxides on the d.-c. resistivity of sintered barium titanate was investigated. The substitution may be represented by ( X ₂O₃)_M (BaTiO₃)_1-M where X is the rare earth. The rare earths samarium, gadolinium, and holmium were introduced singly into the titanate, and the resistivity was measured as a function of temperature from −170° to +330°C. An anomalous increase near the tetragonal - cubic transition temperature at 120°C. occurred which in some cases amounted to an increase in the resistivity of 4000 times the value in the tetragonal phase. The thermoelectric power of the material changed sign at the Curie temperature. The tetragonal phase exhibited n -type behavior whereas the cubic phase was p -type. The rhombohedra1 and orthorhombic phases exhibited conduction activation energies of the order of 0.2 e.v. whereas that in the tetragonal phase was approximately 0.1 e.v.     

Hydrothermal Synthesis of Tetragonal Barium Titanate from Barium Hydroxide and Titanium Dioxide under Moderate Conditions

Tetragonal BaTiO₃ powders were prepared hydrothermally, using Ba(OH)₂·8H₂O and TiO₂ (anatase), in the absence of anions such as chloride ions, at a temperature of 220°C for several days. Characterization via X-ray diffractometry, scanning electron microscopy, and differential scanning calorimetry confirmed that increasing the Ba:Ti molar ratios (from 1:1 to 4:1) and alkaline concentrations (from 1.0 M to 3.0 M ) promotes the formation of tetragonal BaTiO₃.     

Modified Lead Calcium Titanate Ceramics with a Relatively Large Dielectric Constant for Hydrophone Applications

Modified lead titanate of the base composition (Pb,Ca)Ti-(Co,W)O₃+ 0.01 mol% MnO was doped with 2 to 6 mol% strontium and barium to increase the dielectric constant, K , while preserving a high piezoelectric anisotropy ( d ₃₃ /d ₃₁). K was increased above 500 and a d ₃₃ /d ₃₁ ratio of 34/1 was achieved when the composition Pb_0.65Ca_0.31Sr_0.04Ti_0.09-(CO_0.5W_0.5)_0.06O₃+ 0.01 mol% MnO was fabricated. The large d ₃₃ /d ₃₁ resulted in an impressive hydrostatic piezoelectric coefficient ( d _h= 81 × 10_—12 m₂/V). The relatively low transition temperature ( T _c= 120°C) did not result in an increased aging rate when samples were tested at 40°C and 99% humidity for 120 h after poling. A small tetragonality ratio (1.008) enabled saturation of ceramic properties at poling fields as low as 30 kV/cm at 100°C.