Stoichiometry Control and Phase Selection in Hydrothermally Derived BaxSr1–xTiO3 Powders

Ba _x Sr_{1- x} TiO₃ (BST) powders were processed at temperatures <100°C by reacting nanosized TiO₂ powders in alkaline, aqueous solutions of BaCl₂, SrCl₂, and NaOH. The effects of processing variables (NaOH concentration, time, temperature, and the ratios of barium, strontium, and titanium initially in solution) on the resultant BST powder stoichiometry and solid solubility were examined. In all cases, strontium was more readily incorporated into the BST powders than barium, and the extent varied systematically with the processing variables. BST powders that were processed in solutions with a large initial excess of barium and strontium, relative to titanium, consisted of a single-phase solid solution. In contrast, BST powders that were processed in solutions with a small initial excess of barium and strontium, relative to titanium, contained a biphasic solid solution which corresponded to separate barium-rich and strontium-rich phases.     

Strontium Dialuminate SrAl4O7: Synthesis and Stability

Up to now, strontium dialuminate, SrAl₄O₇ (SA₂), could be synthesized only by solidification from the high-temperature liquid state. We describe its synthesis from a spray-dried amorphous precursor, and specify its stability domains. Its kinetics of formation is very low. It can be crystallized in the 900–1000°C temperature range either directly with a low heating rate or via two metastable solid solutions—hexagonal strontium monoaluminate (SrAl₂O₄ (SA)) and γ-alumina—by annealing at 950–1000°C. As the temperature is raised beyond 1100°C, SA₂ becomes metastable, its formation is no longer possible, and the crystallization of Sr₄Al₁₄O₂₅ (S₄A₇) is favored. The latter compound, whose composition is close to that of SA₂, is stable up to 1500°C. At higher temperature it decomposes into SA and SA₂, which in its turn decomposes into SA and SA₆ (SrAl₁₂O₁₉). There is again another stability domain for SA₂, restricted to a narrow temperature scale close to its melting point (∼1800°C). The behaviors at crystallization from amorphous precursors at low temperature and from liquid at very high temperature are symmetrical: low heating or cooling rates produce pure SA₂ while too rapid kinetics result in mixtures of phases.     

Studies in the System Li2O–Al2O3–Fe2O3-H2O

Subsolidus equilibrium relations in a portion of the system Li2O-Fe₂O₃-Al₂O3 in the temperature range 500° to 1400°C. have been determined near po₂ = 0.21. Of particular interest in this system is the LiFe₅O₈-LiAl₅O₈ join, which shows complete solid solution above 1180°C. Below this temperature the solid solution exsolves into two spinel phases. At 600°C. approximately 15 mole % of each compound is soluble in the other. The high-temperature solid solution and the low-temperature exsolution dome extend into the ternary system from the 1:5 join. There is no appreciable crystalline solubility of LiFeO₂ or of α-Fe₂O₃ in LiFe₅O₈. An attempt to confirm HFe₅O₈ as the correct formulation of the magnetic ferric oxide "γ-Fe₂O₃" was inconclusive, but in the absence of positive evidence, the retention of γ-Fe₂O₃ is recommended. All the metallic oxides of the Group IV elements increase the temperature of the monotropic conversion of -γ-Fe₂O₃ to α-Fe₂O₃. Silica and thoria have a greater effect on this conversion than does titania or zirconia.     

Effects of trace of Bi2O3 addition on the morphology of strontium ferrite particles

A strontium ferrite powder added with trace of Bi₂O₃ was prepared by the conventional high-temperature solid phase reaction. The effects of Bi₂O₃ addition on the morphology of Sr-ferrite particles fired at different temperatures and times were investigated. The results show that a small quantity of Bi₂O₃ addition accelerates the reaction of SrO and Fe₂O₃ to form SrM ferrite and obviously improves the morphology and size of the strontium ferrite particles. A possible mechanism was proposed to interpret the influence of trace of Bi₂O₃ addition on the morphology of strontium ferrite particles.     

Activity–Composition Relations in MnCr2O4–CoCr2O4 Solid Solutions and Stabilities of MnCr2O4 and CoCr2O4 at 1300°C

Phase equilibria in the system MnO–CoO–Cr₂O₃ were investigated at 1300°C under controlled oxygen partial pressures by using the gas equilibration technique. The CoO activities in various phase assemblages of the system were measured by determining the partial pressures of oxygen in the gas phase for coexistence with metallic cobalt. The activity data revealed that at 1300°C, MnO–CoO and MnCr₂O₄–CoCr₂O₄ solid solutions exhibit mild positive departures from ideal behavior. The activities in the stoichiometric spinel solutions were found to be in good agreement with those predicted from a model based on cation distribution equilibria. The standard free energy of formation of the compound CoCr₂O₄ from its oxide components at 1300°C was determined as −37 636 J/mol, while that for MnCr₂O₄ was found as −44 316 J/mol.     

Microstructural Characterization of C60-Doped Zirconia

Zirconia powders doped with C₆₀ molecules were prepared from an aqueous solution of zirconium oxynitrate dihydrate, C₆₀ and C₁₆TMA, and sintered at 600°C under 5.5 GPa for 2 h. C₆₀ was found to be retained in the sintered specimens by HRTEM, and carbon was observed to be uniformly dispersed by the SEM-EDX analysis. HRTEM observations of the sintered specimens exhibited the formation of ZrO₂ crystal grains covered with thin graphitic or amorphous carbon films.     

Preparation and Characterization of Alkoxy-Derived SrZrO3 and SrTiO3

High-purity strontium, zirconium, and titanium alkoxides were synthesized and characterized as precursors for complex oxides. Simultaneous hydrolytic decomposition either of strontium and zirconium alkoxides or of strontium and titanium alkoxides was used to obtain nearly stoichiometric, ideally mixed SrZrO₃ or SrTiO₃ powders of high surface activity. As-prepared helium-dried SrTiO₃ is crystalline before calcination. An ultraviolet radiation technique demonstrates the nucleation and growth of SrZrO₃ crystallites in the calcination temperature range to 350°C. The experimental results are supported by ir, TGA, and X-ray diffraction data. The high degree of control over purity, mixing uniformity, and crystallite size demonstrates the value of the alkoxide precursor approach for the solution of reproducibility problems encountered in the synthesis of electrical-quality ceramics.     

Stability of Mullite as Derived from Equilibria in the System CoO-Al2O3-SiO2

The free energy of reaction for the formation of mullite from its oxide components was derived from equilibrium studies in the system CoO-Al₂O₃-SiO₂. Within this system there appears, at solidus temperature in a certain composition area, the phase assemblage mullite + silica + spinel (= cobalt aluminate) + liquid. Determination of the oxygen pressure of a gas phase at which metallic cobalt precipitates from this phase assemblage and from the phase assemblage spinel (= cobalt aluminate) + corundum in the system CoO-Al₂O₃ permits calculation of ΔG° for the reaction 3Al₂O₃+ 2SiO₂= Al₆Si₂O₁₃. The value obtained at 1422°C is -5.8 kcal.     

Coarsening Behavior of Tricalcium Silicate (C3S) and Dicalcium Silicate (C2S) Grains Dispersed in a Clinker Melt

Microstructural evolution during the heat treatment of cement clinker was investigated. Two model specimens, which consisted of faceted tricalcium silicate (C₃S) and spherical dicalcium silicate (C₂S) grains dispersed in a liquid matrix, were prepared with 5 wt% of large seed particles. The seed particles of faceted C₃S grains grew extensively, whereas those of the spherical C₂S grains grew rather slowly, relative to the matrix grains. As a consequence, C₃S grains exhibited a bimodal size distribution that was typical of exaggerated grain growth, whereas C₂S grains retained a uniform and normal size distribution. These results suggest that the growth of faceted C₃S grains was controlled by the interface atomic attachment, such as two-dimensional nucleation, and that of spherical C₂S grains was controlled by diffusion through the liquid matrix. The dependence of growth mechanisms on grain morphology has been explained in terms of the atomistic structure of the solid/liquid interface.     

Variability of the Sr/Ti Ratio in SrTiO3

TiO₂ is observed as a second phase in SrTiO₃ having Sr/Ti ≤ 0.995 (≥0.5 mol% excess TiO₂). The effect of excess TiO₂ on the equilibrium electrical conductivity at 1000°C is consistent with a solubility of <0.1 mol% TiO₂ with the formation of unassociated oxygen vacancies. More Ti0₂ is retained in solid solution when samples are quenched from the sintering temperature rather than furnace-cooled. The effect of excess SrO on the equilibrium electrical conductivity also indicates some solid solubility.     

High-Quartz Solid Solution Phases from Thermally Crystallized Glasses of Compositions Li2O2,MgO).Al2O3,nSiO2

Thermally crystallized glasses of compositions (Li₂,O₂, MgO).Al₂O₃.nSiO₂ were studied by X-ray powder diffraction methods. High-quartz solid solution phases developed at relatively low temperatures and, for n 3.5, transformed at higher temperatures to keatite solid solution phases. Associated phases, if present, were Mg spinel and/or cordierite, or a few other trace phases. The a crystallographic axis (a₀) of high-quartz solid solutions decreased with increase of MgO and/or SiO₂. The c crystallographic axis (c₀) decreased with increasing MgO; it also decreased with increasing SiO₂, but only when MgO content was low. X-ray diffraction photographs of single crystals of high-quartz solid solutions of compositions LiaO.Al₂O₃.nSiO₃ demonstrated that the maintenance of a basic high-quartz structure is the basis of the solid solution relation. Three modifications of the high-quartz structure were recognized in the Li₂O-Al₂O₃−SiO₃ system. These modifications were based on the occurrence and positions of superlattice reflections. The high-quartz solid solution from Li₂O Al₂O₃−2SiO₂, showing streaky reflections in its precession photographs, suggested a defective structure. The term "high-quartz solid solution," with or without additional prefixes specifying the compositional series and modification, was considered the preferred nomenclature for these solid solution phases.     

Fabrication and Mechanical Properties of Al2O3-WC-Co Composites by Vacuum Hot Pressing

Al₂O₃-WC-Co composites were fabricated by vacuum hot-pressing mixtures of Al₂O₃, WC, and cobalt powders. The phases formed with WC additions of up to 40 wt% were α-Al₂O₃, WC, Co₃W₃C, and small amounts of f-Co (face-centered cubic cobalt) and carbon (graphite); no cobalt or carbon phases formed at >40 wt% WC. A more-uniformly distributed and connected WC matrix formed as the WC content increased. The 10Al₂O₃-80WC-10Co (in wt%) composite exhibited high bending strength (1250 MPa), fracture toughness (9 MPam^1/2), and hardness (20.6 ± 0.5 GPa) simultaneously. The high bending strength was mainly attributed to fewer fracture origins due to the uniformly distributed and connected WC matrix together with a lower porosity. Increased fracture toughness was caused mainly by crack deflection and crack bridging in a uniformly connected WC matrix. High hardness resulted from finer WC metallic compounds and Co₃W₃C precipitation in almost all ranges.     

Mechanical Properties of La1-xSrxCo0.2Fe0.8O3 Mixed-Conducting Perovskites Made by the Combustion Synthesis Technique

This paper examined the room-temperature mechanical properties of a mixed-conducting perovskite La_{1– x} Sr _x Co_0.2Fe_0.8O₃ ( x = 0.2–0.8). Powders were made by the combustion synthesis technique and sintered at 1250°C in air. Sintered density, crystal phase, and grain size were characterized. Young's and shear moduli, microhardness, indentation fracture toughness, and biaxial flexure strength were determined. The Young's and shear moduli slightly increased with increasing strontium content. Young's modulus of 151–188 GPa and shear modulus of 57–75 GPa were measured. Biaxial flexure strength of ∼160 MPa was measured for lower strontium content batches. Strength greatly decreased to ∼40 MPa at higher strontium concentrations ( x = 0.6–0.8) because of the formation of extensive cracking. Indentation toughness showed a higher value (∼1.5 MPa·m^1/2) for low strontium ( x = 0.2) content and a lower value (∼1.1 MPa·m^1/2) for the other batches ( x = 0.4–0.8). Materials with fine and coarse grain size were also tested at various indent loads and showed no dependence of toughness on crack size. In addition, fractography was used to characterize the critical flaw and fracture mode.     

In Situ Processing and High-Temperature Properties of Ti3Si(Al)C2/SiC Composites

Ti₃SiC₂ has many salient properties including low density, high strength and modulus, damage tolerance at room temperature, good machinablity, and being resistant to thermal shock and oxidation below 1100°C. However, the low hardness and poor oxidation resistance above 1100°C limit the application of this material. The poor oxidation resistance at temperatures above 1100°C was because of the absence of protective layer in the scale and the presence of TiC impurity phase. TiC impurity could be eliminated by adding a small amount of Al to form Ti₃Si(Al)C₂ solid solutions. Although the high-temperature oxidation resistance was significantly improved for the Ti₃Si(Al)C₂ solid solutions, the strength at high temperatures was lost. One important way to enhance the high-temperature strength is to incorporate hard ceramic particles like SiC. In this article, we describe the in situ synthesis and simultaneous densification of Ti₃Si(Al)C₂/SiC composites using Ti, Si, Al, and graphite powders as the initial materials. The effect of SiC content on high-temperature mechanical properties and oxidation resistance were investigated. The mechanisms for the improved high-temperature properties are discussed.     

Phase Relations in the System UO2-UO3– Y2O3

The phase relations in the system U02-U03-Yz03, particularly in the Y203-rich region, were examined by X-ray and chemical analyses of reacted powders heated at temperatures up to 1700°C in H₂, CO₂-CO₂ and air. Four phases were identified in the system at temperatures between 1000° and 1700°C: U308, face-centered cubic solid solution, body-centered cubic solid solution, and a rhombohedral phase of composition (U,Y)₇O₂ ranging from 52.5 to 75 mole % Y₂O₃. The rhombohedral phase oxidized to a second rhombohedral phase with a nominal composition (U,Y), at temperatures below 1000°C. This phase transformed to a face-centered cubic phase after heating in air above 1000° C. The solubility of UO, in the body-centered cubic phase is about 14 mole % between 1000° and 1700°C but decreases to zero as the uranium approaches the hexavalent oxidation state. The solubility of Yz03 in the face-centered cubic solid solution ranges from 0 to 50 mole % Y2O3 under reducing conditions and from 33 to 60 mole % Y2O3 under oxidizing conditions at 1000°C. At temperatures above 1000° C, the face-centered cubic solid solution is limited by a filled fluorite lattice of composition (U,Y)O₂. For low-yttria content, oxidation at low temperatures (<300°C) permits additional oxygen to be retained in the structure to a composition approaching (U,Y)O_2.25 A tentative ternary phase diagram for the system UO₂-UO₃-Y₂O₃ is presented and the change in lattice parameter and in cell volume for the solid-solution phases is correlated with the composition.     

Lattice Parameters, Ionic Conductivities, and Solubility Limits in Fluorite-Structure MO2 Oxide [M = Hf4+, Zr4+, Ce4+, Th4+, U4+] Solid Solutions

Changes in the lattice parameters of fluorite type MO₂ oxides (M = Hf⁴⁺, Zr⁴⁺, Ce⁴⁺, Th⁴⁺, U⁴⁺) due to the formation of solid solutions can be predicted by proposed empirical equations. The equations show the generalized relationship between dopant size and ionic conductivity in the binary systems of these oxides, illustrating that the smaller the difference between the dopant ionic radius and the critical dopant radius, the higher the conductivity. The solubility limit of the same periodic group elements in fluorite-structure MO₂ oxides decreaes linearly with the square of Vegard's slope for each solute as determined from the proposed equations.     

Mechanical Properties of Y2O3-Stabilized Tetragonal ZrO2 Polycrystals After Ageing at High Temperature

Yttria-partially-stabilized zirconia (Y-PSZ) materials containing 2.5, 3.0, and 5.0 mol% of Y₂O₃ prepared by hot pressing were subjected to ageing in air for >10 h at temperatures in the range of 800°C and 1200°C. The sintered materials were measured to determine the mechanical properties and microstructures and analyzed for trace elements. A sharp reduction in bending strength was observed after the ageing, the cause for which was suggested to be the formation of cavities produced by the oxidation of carbon.     

Solubility of KUO3 in BaUO3

The formation of a solid solution between cubic perovskne-type KUO₃ and pseudocubic BaUO₃ was investigated. The reaction begins at 550°C, and the solubility of KUO₃ reaches more than 30 mol% KUO₃ in BaUO₃ at 750°C. The region in which a single-phase solid solution exists was determined. The variation of the lattice parameter of the reacted samples was caused by solid solution formation and by oxygen absorption. The electrical conductivities of the samples varied with composition and showed a distinct maximum. The activation energy for electric conduction was very low compared to that for UO_z+x, or U₃O₈.     

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.     

Effect of Excess PbO on the Sintering Characteristics and Dielectric Properties of Pb(Mg1/3Nb2/3)O3–PbTiO3-Based Ceramics

Additions of excess PbO to the perovskite Pb[(Mg_1/3Nb_2/3)_0.92Ti_0.08]O₃ solid solution enhanced the formation of a liquid phase at 840°C, which served as a densification aid for the ceramics. The liquid phase allowed elimination of pores and promoted grain growth during sintering. With additions of 1 to 2 wt% excess PbO, densities in excess of 97% of theoretical were obtained at a sintering temperature of 950°C. The peak dielectric constants of the resulting ceramics were over 18 000 at 30°C and dissipation factors less than 1%. Additions of PbO in excess of 2 wt% resulted in inferior dielectric properties due mainly to the dilution of the ferroelectric phase.