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 .     

Solid-State Preparation of BaTiO3-Based Dielectrics, Using Ultrafine Raw Materials

BaTiO₃ and Ba(Ti,Zr)O₃ dielectric powders have been prepared from submicrometer BaCO₃, TiO₂, and ZrO₂. By use of submicrometer BaCO₃ the intermediate formation of Ba₂TiO₄ second phase can be widely suppressed. Monophase perovskites of BaTiO₃ were already formed at 900°C and Ba(Ti,Zr)O₃ at 1050°C. Aggregates of very small subgrains could be easily disintegrated to particle sizes <0.5 μm.     

Influence of Barium Dissolution on the Electrokinetic Properties of Colloidal BaTiO3 in an Aqueous Medium

Hysteresis in the electrokinetic behavior of colloidal hydrothermal BaTiO₃ occurs during sequential acid and base titrations. Ba dissolution during acid titration results in an oxide-rich surface. When the acid-treated BaTiO₃ is titrated back to pH 10, dissolved Ba is specifically adsorbed and/or precipitated onto the particle surface. The combined effects of dissolution and subsequent adsorption–precipitation results in titration hysteresis. Most of the labile Ba can be removed by multiple acid treatments, which result in a TiO₂-like surface layer composition. Barium dissolution increases with decreasing pH but levels off below pH 4 due to diffusion through the surface oxide layer as predicted previously. A phenomenological model is offered to explain the electrokinetic behavior as a function of pH. It is suggested that inherent BaCO₃ contamination is not the primary source of dissolved Ba from hydrothermal BaTiO₃ in acidic solution.     

Solid-State Synthesis of Ultrafine BaTiO3 Powders from Nanocrystalline BaCO3 and TiO2

Barium titanate has been prepared by solid-state reaction of nanocrystalline TiO₂ (70 nm) with BaCO₃ of different particle size (650, 140, and 50 nm). The results give evidence of a strong effect of the size of BaCO₃ in the solid-state synthesis of barium titanate. The use of nanocrystalline BaCO₃ already leads to formation of the single-phase BaTiO₃ after calcination for 8 h at 800°C. The final powder consists of primary particles of ≈100 nm, has a narrow particle size distribution with d ₅₀=270 nm, and no agglomerates larger than 800 nm. For the coarser carbonate, 4 h calcination at 1000°C are required and the final powder is much coarser. Solid-state reaction of nanocrystalline BaCO₃ and TiO₂ represents an alternative to chemical preparation routes for the production of barium titanate ultrafine powders.     

Batch Rotary Kiln Calcination of YBaCuO Precursor Powders

Precursor powders of Y₁Ba₂Cu₃O_6+x (123) derived from spray roasting were calcined in a batch rotary kiln. The optimum conditions were 15 min in air at temperatures between 920° and 940°C for 1.5 vol% solid loading. Pure 123 was formed in a single calcination step from homogeneous, micrometer-sized precursor material containing Y₂O₃, CuO, and BaCO₃. Rotation speed did not affect the product quality, but longer calcination times are needed at higher loadings. Product agglomeration increased with increasing loading and decreasing rotation speed. In nitrogen, 123 decomposed so that a pure product could not be obtained. The BaCO₃ decomposition is the rate-limiting step in the calcination. The presence of Ba(NO₃)₂ in the precursor led to formation of intermediate phases.     

Reaction and Fired-Property Studies of Cordierite Compositions

Ceramic bodies approaching the theoretical composition of cordierite were prepared from mixtures of talc and clay; talc, clay, and varying amounts of MgCO₃; talc, clay, and alumina; and prochlorite and clay. Additions of BaCO₃ and PbSiO, were made. Differential thermal analysis and X-ray diffraction methods were used in reaction studies of each composition. Bodies had thermal expansions from 0.344 to 0.109% (25° to 900°C.). BaCO₃ and PbSiO₃ affected thermal expansion. Dielectric properties were improved by additions of BaCO₃ and were markedly improved by additions of both BaCO₃ and PbSiO_l. Cordierite started to crystallize at about 1250°. Additions of PbSiO₃ lowered this temperature about 50°C. With sufficient BaCO₃, the Ba-C phase, as described by Wisely, crystallized in place of cordierite. The crystallization temperature for the Ba-C phase was about 1200°C. With insufficient BaCO₃, both cordierite and Ba-C formed. Additions of PbSiO₃ reduced the crystallization temperature of the Ba-C phase by 50°C. and increased the glassy phase. Several of the compositions should haw considerable practical value.     

Barium Carbonate Phase on the Surfaces of Barium Titanate Particles and in Situ Transmission Electron Microscopy Observation of Its Decomposition

A BaCO₃ phase is found on the surfaces of hydrothermally synthesized BaTiO₃ particles; it occurs as aggregates or small protuberances. A small proportion of the phase decomposes to BaO crystallites when heated by a convergent electron beam in a transmission electron microscope. The BaO and BaCO₃ crystallites disappear when they are irradiated successively by the convergent electron beam. The BaO crystallites and the BaCO₃ phase sublimate and/or react with BaTiO₃ crystals whose surface layers are deficient in Ba²⁺ ions.     

Kinetics and Mechanism of Formation of Barium Zirconate from Barium Carbonate and Zirconia Powders

The formation of BaZrO₃ from very fine (70–90 nm) ZrO₂ powders and coarser (∼1 μm) BaCO₃ powders has been studied in dry and humid air up to 1300°C using TGA/DTA, XRD, SEM, TEM, and EDS microanalysis. In the temperature range 900°–1100°C, barium is rapidly transported at the surface of the ZrO₂ particles and reacts, forming BaZrO₃. The compound grows as a concentric layer with gradual consumption of the central ZrO₂ particle. The overall formation kinetics of BaZrO₃ is well described by a diminishing core model, and the most likely rate-determining step is a phase-boundary process at the ZrO₂–BaZrO₃ moving interface. The size and shape of the final particles is generally determined by the morphology of the starting ZrO₂ particles and not by that of the BaCO₃. The reaction is faster in humid air than in dry air, and the activation energy decreases from 294 kJ·mol⁻¹ (dry air) to 220 kJ·mol⁻¹ (humid air). When the fraction reacted is >80–90 mol%, the reaction rate rapidly decreases.     

Formation of BaTiO3 from BaCO3 and TiO2 in Air and in CO2

The formation of BaTiO₃ from equimolar BaCO₃ and TiO₂ (rutile) mixtures was studied in air and in CO₂. A small amount of BaTiO₃ is formed first directly from BaCO₃ and TiO₂ at the surface of contact. From then on it is a diffusion-controlled reaction, and both BaTiO₃ and Ba₂TiO₄ are produced, with Ba₂TiO₄ being formed in much larger amounts. In 1 atmosphere of CO₂, the intermediate Ba₂TiO₄ was suppressed up to a temperature of about 1100°C. in agreement with thermodynamic calculations. Ba₂TiO₄ reacts fast with 1 atmosphere of CO₂ below about 1100°C. to produce BaTiO₃and BaCO₃     

Preparation of Ba(Mg1/3Ta2/3)O3 Using Oxine

A solution containing Mg²⁺ and Ta⁵⁺ was added to an aqueous ammonia solution of oxine, resulting in a precipitate. After the precipitate was thermally decomposed and fired, it was mixed with BaCO₃ powder and fired again at high temperatures to obtain Ba(Mg_1/3Ta_2/3)O₃ (BMT). This method resulted in BMT formation at temperatures lower than those used in the conventional mixed-oxide method, and single-phase BMT formed directly at 1300°C without intermediates.     

Leachability of Molybdenum from Ternary Phosphate Glasses

With a view to use glasses as a source of molybdenum, which is one of the micronutrients required by plants, a systematic study of the leachability of molybdenum from ternary phosphate glasses having molar compositions of 25MoO₃-25X50P₂O_s (where X = Li₂O, Na₂O, K₂O, CaO, B₂O₃, or SiO₂) was carried out. Leaching of molybdenum ions has been studied as a function of pH, which shows that leaching increases with increasing pH of the solutions. The concentration of molybdenum ions in the leachate decreases with increasing bond strength between nonbridging oxygen ions and the cations in glasses. Leaching increases with an increase in leaching time and decreases with an increase in the particle sizes of the glasses. P₂O₅ was also leached along with MoO₃. The leaching characteristics of P₂O₅ from the glasses are also reported.     

Spectroscopic Properties of Er3+-Doped Na2O–La2O3–Al2O3–SiO2 Glasses

Er³⁺-doped sodium lanthanum aluminosilicate glasses with compositions of (90− x )(0.7SiO₂·0.3Al₂O₃)· x Na₂O·8.2La₂O₃· 0.6Er₂O₃·0.2Yb₂O₃·1Sb₂O₃ (in mol%) ( x = 12, 20, 24, 40, 60 mol%) were prepared and their spectroscopic properties were investigated. Judd–Ofelt analysis was used to calculate spectroscopic properties of all glasses. The Judd–Ofelt intensity parameter Ω _t ( t = 2, 4, 6) decreases with increasing Na₂O. Ω₂ decreases rapidly with increasing Na₂O while Ω₄ and Ω₆ decrease slowly. Both the fluorescent lifetime and the radiative transition rate increase with increasing Na₂O. Fluorescence spectra of the ⁴ I _13/2 to ⁴ I _15/2 transition have been measured and the change with Na₂O content is discussed. It is found that the full width at half-maximum decreases with increasing Na₂O.     

Analytical Electron Microscope Analysis of the Formation of BaO · 6Al2O3

The formation process of barium hexaaluminate (BaO 6Al₂O₃) from BaCO₃/γ-Al₂O₃ powders or hydrolyzed alkoxides was studied by analytical electron microscopy. Barium hexaaluminate is produced by a two-step solid-state reaction from BaCO₃ and Al₂O₃ via formation of BaO·Al₂O₃. Marked grain growth and inclusion of nonequilibrium phase were inevitable in this powder mixture process. However, in an alkoxide-derived precursor, homogeneous mixing of components is attained and hence the formation of BaO·6Al₂O₃ proceeds readily. Powders obtained by this latter route consisted of fine planar particles with a uniform size and retained a large surface area (20.2 m²/g) even after heating at 1300°C. Electron diffraction results implied that suppression of crystal growth along the c axis is the reason for the large surface area of BaO·6Al₂O₃.     

Stress in Leached Phase-Separated Glass

When a phase-separated glass is leached, stresses develop because of release of thermal stresses, creation of surface area, ion exchange, and hydration. Analyses are presented for the thermal stresses, including the portion that develops on cooling from the heat-treatment temperature to the setting temperature of the less viscous phase. During leaching, the interfacial energy of the residual phase increases, so that phase tends to contract. A more important effect is the contraction caused by removal of alkali and B₂O₃ from the residual phase during leaching. The extent of removal of B₂O₂ decreases with heat-treatment time, t_H , because the scale of the microstructure increases as t^1/3_H. The change in residual B₂O₂ content with t_H is shown to be consistent with diffusion-controlled ion exchange. The dependence of stress on t_H in partially leached glasses, measured by Drexhage and Gupta, results principally from the change in extent of ion exchange; the reduction in surface area with increasing t_H also has a significant effect on the stresses.     

Thermal Behavior of BaTiO3 Particles Synthesized by Plasma Chemical Vapor Deposition

The thermal behavior of nanoparticles BaTiO₃, prepared by a radio-frequency plasma chemical vapor deposition (RF-plasma CVD) method, was characterized by various analysis methods. The BaCO₃ phase was included in the powder as byproducts, which is also observed in hydrothermal BaTiO₃ powder. The BaCO₃ phase decomposed and disappeared by annealing at 873 K for 30 min. H₂O, N₂, CO₂ and H₂, were detected by a thermal desorption spectra measurement from BaTiO₃ powder. The annealed powder became well-crystallized particles without grain growth, although as-prepared powder included polycrystalline particles. We successfully observed in-situ grain growth for BaTiO₃ nanoparticles by thermal transmission electron microscope. At the initial step of normal grain growth, very fine particles with 40–60 nm diameters started to merge into the larger grains around 1083 K. The migration rate was measured by video images and a grain boundary diffusion coefficient D_gb was calculated.     

Preparation of Barium Titanates from Oxalates

The decomposition of mixed barium titanium oxalates was studied by means of various thermochemical as well as spectroscopic methods. Infrared (IR) spectra of the mixed oxalates indicate the existence of an octahedral complex with Ti chelated by oxalate groups. In general, the results of both IR and the thermochemical analyses suggest that the oxalates are first converted to unidentate carbonate, then to ionic carbonate, and finally to mixed oxides of perovskite structure. The decomposition processes were found to depend upon the atmosphere. In the presence of oxygen the stoichiometrically mixed oxalates decompose by forming TiO₂ and BaCO₃ as intermediates. Under vacuum, two routes of decomposition occur in parallel. In one route, TiO₂ and BaCO₃ are formed as intermediates; in the other, partially reduced TiO_x(x < 2) is formed, which further reacts with BaCO₃ to produce also BaTiO₃, CO₂ and CO.     

Dispersion Behavior of ZrB2 Powder in Aqueous Solution

Dispersion conditions of ZrB₂ powder in water were investigated using poly(ethyleneimine) (PEI) as a dispersant. Pulverization of ZrB₂ powder to submicrometer size was difficult and a substantial amount of large particles remained after an intensive planatery milling for 72 h. The isoelectric point (IEP) of ZrB₂ powder was measured to be pH 5.8 by electrophoresis, which shifted to pH 6.2 after milling. The application of PEI changed the IEP of the boride slurry to ∼pH 11. Well-dispersed aqueous ZrB₂ slurries with a high solid loading (up to 45 vol%) were fabricated at pH 6.5–7.5 by the application of 1.5 wt% PEI.     

Effect of Polyethylenimine on Hydrolysis and Dispersion Properties of Aqueous Si3N4 Suspensions

The roles of polyethylenimine (PEI) in the hydrolysis and dispersion properties of aqueous Si₃N₄ suspensions were studied in terms of the hydrolysis, adsorption, electrokinetic, and rheological measurements. It was found that the pH change of the suspensions in the acidic environment could be minimized in the presence of ≥0.5 dwb% PEI. The ammonia and oxygen measurements suggest that this phenomenon is primarily attributed to the buffer mechanism generated by the ionized PEI, instead of the protection mechanism. The constant pH enables the suspensions to retain a better stability with time at acidic pH. The adsorption of PEI on Si₃N₄ is a high-affinity type at highly basic pH, but is a low-affinity type at acidic pH. As the PEI amount increases, the adsorption shifts the isoelectric point (IEP) of Si₃N₄ from pH 5.9 to pH ∼11 until complete coverage is attained. The stability of Si₃N₄ suspensions is found to depend strongly on the saturated adsorption of PEI, which is as a function of the pH and PEI amount. Once the saturated adsorption limit is reached, the excess free PEI molecules become more detrimental to the stability with increased solid loading. The stabilization mechanisms of Si₃N₄ suspensions by PEI were discussed in detail.     

Intrinsic Kinetics of Carbon Dioxide Degradation of YBa2Cu3O7-x

The intrinsic kinetics, unaffected by diffusional and masstransfer effects, of the CO₂ degradation of superconducting particles have been determined using a nonisothermal technique. Below 900°C, the carbonization of YBa₂Cu₃O_{7- x} leads to formation of BaCO₃, Y₂Cu₂O₅, CuO, and Cu₂O. A further increase in temperature results in formation of BaCuO₂ from BaCO₃ and CuO. The carbonization rate shows the 1.5th-order dependence on the amount of unreacted YBa₂Cu₃O_{7- x} for the temperature range of 550° to 815°C. The activation energy of carbonization was determined to be 95.1 kJ · mol⁻¹.     

Formation and Properties of Barium Metaplumbate

The preparation and the physical and electrical properties of BaPbO₃ ceramics are described. The compound was formed by firing an equimolar mixture of BaCO₃ and Pb₃O₄ in oxygen at 880°C. To produce the stoichiometric compound on firing in air at 980°C it was necessary to use an excess of PbO (molar ratio, Pb/Ba = 1.278). A well sintered ceramic was obtained only with a stoichiometric composition with a sintered density of 7.78. Free BaCO₃ was found in the fired bodies including the PbO-rich compositions. The electrical resistively of BaPbO₃ was 8.3 × 10⁻⁴ ohm-cm at 25°C with a temperature coefficient of +0.15%/°C.