Crystal Chemistry and Phase Relations in the System Li2O-Fe2O3-TiO2

Above 755°C, compounds along the spinel join LiFe₅O₈-Li₄Ti₅O₁₂ form a complete solid solution and below that temperature a two-phase region separates the ordered LiFe₅O₈ and the disordered spinel phase. At 800° and 900°C, cubic LiFeO₂ ( ss ) and monoclinic Li_zTi0₃ ( ss ) exist on the monoxide join LiFeO₂-Li₂TiO₃. The distributions of cations in both the spinel and monoxide structures were calculated as a function of equilibrium temperature and composition. Sub-solidus equilibria in the system Li₂O-Fe₂O₃-TiO₂ at 800° and 900°C were determined for compositions containing ∼50 mol% Li₂O.     

1000°C Phase Relations and Superconductivity in the (Nd-Ce-Cu)-O Ternary System

The phase relations involving the 24 K n -type Nd_{2- x} Ce _x CuO₄ superconductor were investigated at 1000°C in air. The terminal solid solubility was confirmed to be x = 0.2. This solid solution is the only ternary phase in the Nd₂O₃–CeO₂–CuO diagram. A binary (1 − y )CeO₂– y NdO_1.5 solid solution exists out to y = 0.4. Phase diagrams for NdO_1.5–CeO₂–CuO (1000°C) and NdO_1.5–CeO₂ (900° to 1500°C) are presented.     

Synthesis of Strontium Barium Niobate Thin Films through Metal Alkoxide

Highly oriented Sr_0.5Ba_0.5Nb₂O₆ (SBN50) thin films have been prepared using a sol-gel method. A homogeneous and stable strontium barium niobate (Sr_1-xBa_xNb₂O₆, SBN) precursor solution could be prepared via the reaction control of metal alkoxides. The SBN precursor was stabilized by the coordination of the 2-ethoxyethoxy group to metals. SBN thin films on MgO(100) crystallized to a mixture of orthorhombic and tetragonal phase at 700°C and then transformed completely to the tetragonal phase of tungsten bronze at 1000°C. Two crystal lattice planes of SBN were intergrown at an orientation of 18.5° on MgO(100). SBN50 thin films on Pt(100)/MgO(100) substrates exhibited the P-E hysteresis.     

Homogeneity Region of Strontium- and Magnesium-Containing LaGaO3 at Temperatures between 1100° and 1500°C in Air

Phase stability studies were performed within the quasi-ternary system LaGaO₃-SrGaO_2.5-"LaMgO_2.5". Emphasis was cast on the temperature dependence of the homogeneity region of La_{1− x} Sr _x Ga_{1− y} Mg _y O_3−δ perovskite solid solutions. Isothermal sections were determined at 1100°, 1250°, 1400°, and 1500°C in a static air atmosphere. The single-phase homogeneity region was found to considerably diminish with decreasing temperature, indicating a reduction of the solid solubility of Sr and Mg, and below 1100°C the doped perovskite becomes unstable. Consequently, the cubic perovskite phase was found to exist only at elevated temperatures and for high Sr and Mg amounts. Sample preparation was performed by the mixed-oxide process as well as by a modified combustion synthesis.     

The System HfO2-TiO2

The system HfO₂-TiO₂ was studied in the 0 to 50 mol% TiO₂ region using X-ray diffraction and thermal analysis. The monoclinic ( M ) ⇌ tetragonal ( T ) phase transition of HfO₂ was found at 1750°± 20°C. The definite compound HfTiO₄ melts incongruently at 1980°± 10°C, 53 mol% TiO₂. A metatectic at 2300°± 20°C, 35 mol% TiO₂ was observed. The eutectoid decomposition of HfO_2,ss) ( T ) → HfO_2,ss ( M ) + HfTiO_34,ssss occurred at 1570°± 20°C and 22.5 mol% TiO₂. The maximum solubility of TiO₂ in HfO_2,ss,( M ) is 10 mol% at 1570°± 20°C and in HfO_2,ss ( T ) is 30 mol% at 1980°± 10°C. On the HfO₂-rich side and in the 10 to 30 mol% TiO₂ range a second monoclinic phase M of HfO₂( M ) type was observed for samples cooled after a melting or an annealing above 1600°C. The phase relations of the complete phase diagram are given, using the data of Schevchenko et al. for the 50% to 100% TiO₂ region, which are based on thermal analysis techniques.     

Ceria–Yttria-Stabilized Zirconia Composite Ceramic Systems for Applications as Low-Temperature Electrolytes

Ceramic samples of Ce_{1− x} Gd _x O_{2− y} and yttria-stabilized zirconia (YSZ) were prepared using solid-state methods. Polished faces of disks of these materials were held in intimate contact in a reaction cell at temperatures ranging from 1000° to 1300°C for durations up to 72 h. XRD, SEM, and microprobe Raman techniques were used to analyze the resulting reactions and ion diffusion. No reaction was observed at 1000°C after 72 h between the 10-mol%- and 20-mol%-Gd-doped CeO₂ and the YSZ. However, at 1300°C, a mixing region 25 μ m wide occurred, resulting in a cubic phase, where Zr⁴⁺ ions diffused into the CeO₂.     

High-Temperature Phase Relations in the System Y2O3-Ta2O5

The phase relations for the system y₂o₃–Ta₂o₅ in the composition range 50 to 100 mol% Y₂O₃ have been studied by solid-state reactions at 1350°, 1500°, or 17000C and by thermal analyses up to the melting temperatures. Weberite-type orthorhombic phases (W2 phase, space group C222₁), fluorite-type cubic phases (F phase, space group Fm3m )and another orthorhombic phase (O phase, space group Cmmm )are found in the system. The W2 phase forms in 75 mol% Y₂O₃ under 17000C and O phase in 70 mol% Y₂O₃ up to 1700°C These phases seem to melt incongruently. The F phase forms in about 80 mol% Y₂O₃ and melts congruently at 2454° 3°C. Two eutectic points seem to exist at about 2220°C 90 mol% Y²O₃, and at about 1990°C, 62 mol% Y₂O₃. A Phase diagram including the above three phases were not identified with each other.     

The Calcia–Zirconia Phase Diagram Revisited: Stability of the Ordered Phases φ1 and φ2

Single crystals of CaO-stabilized ZrO₂ containing between 15.3 and 18.9 mol% CaO were heat-treated for 5000 h at 1200°C to study the stability of the ordered defect-fluorite phase, CaZr₄O₉ (φ₁). Subsequent TEM analysis of the equiaxed φ₁ domains in samples richer than approximately 18 mol% in CaO showed a random distribution of φ₁ variants, with no preferred interfacial habit planes. A critical review of the literature, combined with the new data, supports the Stubican-Hellmann-Hannon version of the phase diagram in the region 15 to 26 mol% CaO and 1000° to 1400°C and strongly suggests that φ₁ is a stable phase in the ZrO₂-CaO systems.     

Preparation of Strontium Barium Niobate by Sol–Gel Method

Crack-free SBN (Sr_xBa_1-xNb₂O₆) thin films have been prepared by a sol–gel method with metal alkoxides. A homogeneous and stable precursor solution was obtained from Sr and Ba metal and Nb(OEt)₅ in ethanol with a key additive of ethoxyethanol. SBN (where x = 0.5) powder crystallized to orthorhombic phase at 700°C, and then transformed completely to tetragonal phase at 1200°C. The formation of tetragonal SBN was observed on sapphire (R) substrates at 700°C, whereas the tetragonal phase began to appear in the powders at 1000°C. SBN films with highly preferred orientation were successfully synthesized on MgO (100) substrates at 670°C.     

High-Temperature Phase Relations in the Sc2O3-Ta2O5 System

The phase relations for the Sc₂O₃-Ta₂O₅ system in the composition range of 50-100 mol% Sc₂O₃ have been studied by using solid-state reactions at 1350°, 1500°, or 1700°C and by using thermal analyses up to the melting temperatures. The Sc_5.5Ta_1.5O₁₂ phase, defect-fluorite-type cubic phase (F-phase, space group Fm 3 m ), ScTaO₄, and Sc₂O₃ were found in the system. The Sc_5.5Ta_1.5O₁₂ phase formed in 78 mol% Sc₂O₃ at <1700°C and seemed to melt incongruently. The F-phase formed in ∼75 mol% Sc₂O₃ and decomposed to Sc_5.5Ta_1.5O₁₂ and ScTaO₄ at <1700°C. The F-phase melted congruently at 2344°± 2°C in 80 mol% Sc₂O₃. The eutectic point seemed to exist at ∼2300°C in 90 mol% Sc₂O₃. A phase diagram that includes the four above-described phases has been proposed, instead of the previous diagram in which those phases were not identified.     

High-Temperature Phase Relations in the System Gd2O3-Ta2O5

The Phase relations of the system Gd₂O₃-Ta₂O₅ in the composition range 50 to100 mol% Gd2O3 was studied by solidstate reactions at 1350°, 1500°, or 1700°C and by thermal analyses up to the melting temperatures. Weberite-type orthorhombic phase (W2 phase, space group C2221) with the composition of Gd3 TaO7 seems to melt incongruently; at about 2040°C, although this Gd₃TaO₇ Phase was previously reported to melt congruently. A new fluorite-type cubic phase (F phase, space group Fm3m ) was found for the first time above 1500°C in the system. It melts congruently with the composition of about 80mol% Gd2O3at 2318° 3°C. A phase diagram was proposed for the system Gd₂O3–Ta2O₅ in the Gd₂O₃–rich portion     

Solid-Liquid Equilibria in the System Si3N4-AlN-Si02-A12O3

Solid-liquid equilibria at 1750°C and subsolidus phase relations in the system Si₃N₄−AlN-SiO₂−Al₂O₃ were determined for the composition region bounded by the β-Si₃N₄ solid solution line and silica-alumina join X-ray diffraction and optical microscopy were used to determine the phases present in specimens cooled rapidly after equilibration. The extent of a single liquid-phase region and the tie lines for the βsolid solution + liquid field at 1750°C were established from quantitative X-ray diffractometry and lattice parameter measurements of βsolid solutions in equilibrium with liquid. The results were corroborated by optical microscopy and melting behavior observations. A new composition, Si₁₂Al₁₈O₃9N₈, is suggested for the x₁ phase. The lowest melting temperature in the system is ≅ 1480°C and the corresponding composition is 10 eq% Al-90 eq%O.     

Crystalline Phase Change in Yttria-Partially-Stabilized Zirconia by Low-Temperature Annealing

Changes in the phase composition and microstructure of yttria-partially-stabilized zirconia by low-temperature annealing were investigated at 100° to 500°C using bodies sintered from coprecipitated fine ZrO₂-Y₂O₃ powders at varied temperatures. Tetragonal zirconia on the surfaces of bodies sintered at <1500°C transformed to the monoclinic phase at 100° to 400°C. Transformation behavior was strongly affected by grain size.     

Pseudobinary Phase Relations of Li2Ti3O7

The Li₂O-TiO₂ pseudobinary phase diagram was determined from 50 to 100 mol% TiO₂ by DTA, microscopy, and X-ray analysis; Li₂Ti₃O₇ effectively melts congruently at 1300° and decomposes eutectoidally at 940°C. A solid solution based on Li₂TlO₃ from 50 to ∼65 mol% TiO₃ was observed to exist at >930°C. A new metastable phase was discovered with a composition of ∼75 mol% TiO₂ and with a hexagonal unit cell (8.78 by 69.86 × 10⁻¹nm). Discrepancies in the literature regarding some of these phase equilibria are reconciled.     

Investigation of the System Beryllium–Boron

Mixtures of beryllium and boron powders were reacted under argon at elevated temperatures. X-ray single-crystal and powder diffraction analyses, chemical analyses, and Knoop micro-hardness measurements were made. Melting-point ranges were established for compounds of reasonable purity. One compound was identified. Beryllium diboride (BeB₂) had a hexagonal unit cell. Its hardness was 3180 ± 100 (H_k100) and its melting point was greater than 1970°C. Beryllium hexaboride ("BeB₆") had a tetragonal unit cell, a hardness of 2577 ± 75 (H_k100), and a melting point between 2020°) and 2120°C. An anisotropic unknown phase existed whose Be: B ratio was between 6:1 and 2:1 and whose hardness ranged between 623 and 917 (H_k25), depending on orientation. A eutectic between beryllium and this unknown phase was observed at 972°± 15°C.     

Homogeneous Precipitation of TiO2 Ultrafine Powders from Aqueous TiOCl2 Solution

Crystalline TiO₂ powders were prepared by the homogeneous precipitation method simply by heating and stirring an aqueous TiOCl₂ solution with a Ti⁴⁺ concentration of 0.5 M at room temperature to 100°C under a pressure of 1 atm. TiO₂ precipitates with pure rutile phase having spherical shapes 200-400 nm in diameter formed between room temperature and 65°C, whereas TiO₂ precipitates with anatase phase started to form at temperatures >65°C. Precipitates with pure anatase phase having irregular shapes 2-5 µm in size formed at 100°C. Possibly because of the crystallization of an unstable intermediate product, TiO(OH)₂, to TiO₂ x H₂O during precipitation, crystalline and ultrafine TiO₂ precipitates were formed in aqueous TiOCl₂ solution without hydrolyzing directly to Ti(OH)₄. Also, formation of a stable TiO₂ rutile phase between room temperature and 65°C was likely to occur slowly under these conditions, although TiO₂ with rutile phase formed thermodynamically at higher temperatures.     

Sol-Gel Synthesis of a New Oxide-Ion Conductor Sr- and Mg-Doped LaGaO3 Perovskite

Sr- and Mg-doped LaGaO₃ powders were prepared from a salt acetate solution. The stable solution was peptized by reacting ammonium hydroxide with the precursor solution. Thermal analysis (DTA/TGA) was used to characterize first the dehydration and then the thermal decomposition of the organic ligands of the dried gel. The transformation from amorphous powders into a crystallized, homogeneous oxide phase corresponds to two endothermic peaks in the DTA curve; the first one at 150°C is related to the removal of water and is followed by a shoulder at 250°C. The second peak at 300°C corresponds to a superposition of two decomposition reactions: acetate salt into its oxycarbonate and this oxycarbonate into its oxide. Two subsequent exothermic peaks correspond to oxidation of evolved gases such as methane, hydrogen, and carbon monoxide. TEM observations show an average 10 nm particle size of the LaGaO₃powder after annealing at 600°C. X-ray diffraction patterns indicate a pure primitive-cubic phase is formed by 1300°C without formation of any SrLaGaO₃ impurity. The impedance spectroscopy on a 93%-dense sample exhibits no grain-boundary contribution and an ac conductivity σ= 0.11 Ω⁻¹·cm⁻¹ at 800°C.     

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.     

Phase Relations in the System Uranium—Carbon—Oxygen

The ternary system U-C-O was investigated and a diagram is given expressing the phase relations in specimens which were reacted at 1800°C in vacuum or in inert atmospheres and furnace cooled. A U(C,O) single-phase area exists in this diagram. The limit of substitutional solid solution of oxygen atoms for carbon atoms in UC is 12.5 at.%, which means that only 25% of the carbon atoms in UC can be replaced by oxygen atoms. The oxygen-saturated UC phase, U(C_0.75O_0.25), has a lattice parameter of 4.953 ± 0.001 A. The carbon monoxide partial pressures in equilibrium with this phase were determined for temperatures between 1705° and 1855°C.     

Production of Spherical UO2-UC2 for Nuclear Fuel Applications Using Thermochemical Principles

A process for the fabrication of uranium dioxide-uranium dicarbide microspheres for use as an advanced nuclear fuel is described. The uranium-carbon-oxygen phase diagram was used extensively in applying thermochemical principles to the combined process of uranium carbide synthesis and kernel sintering. Variation of the partial pressure of carbon monoxide during the carbothermic reduction of urania plus carbon allowed the kernel composition and density to be controlled. X-ray diffraction, microstructural examination, and detailed chemical analyses were used to identify the kernel composition. A procedure was developed to convert urania plus carbon microspheres produced by a wet-chemical gelation process to a highly dense UO₂−UC₂ product at 1550°C. Kernels were first treated at 1550°C in Ar-1% CO for 4 h to produce high-density microspheres with a composition of UO₂+UC_x:O_y, (x +y≤1.1). These kernels were then processed at 1550°C for an additonal 4 h in Ar-3% CO to shift the thermodynamic equilibrium from UO₂+UC_xO_y, to the desired UO₂+UC₂. Batches of material containing different initial amounts of carbon were processed to produce high-density microspheres having specific UC₂ contents.