Zirconia-Stabilized Cubic Europia

The system Eu₂O₃-ZrO₂ was studied, concentrating attention on the region 0 to 33 mol% ZrO₂, which is of interest for fast-reactor neutron absorber applications. The addition of ∼20% ZrO₂ to Eu₂O₃ resulted in stable cubic phases. Results are compared for coprecipitated powders and pellets prepared from mechanically mixed powders fired at 1300°C and 1S50°C. The thermal stability of cubic structures at 600°C and 800°C for 8000 h was also demonstrated.     

Effects of Temperature and Microstructure on the Elastic Properties of Selected Eu2O3-HfO2 Compositions

Europium sesquioxide is of interest as a control material for reactors because it has several large-cross-section isotopes which are formed successively on neutron capture. Elastic properties of Eu₂O₃ and selected Eu₂O₃-rich Eu₂O₃-HfO₂ compositions were investigated by the sonic technique from room temperature to 1500°C. Sintered monoclinic Eu₂O₃ was found to have anomalously low room temperature moduli and to exhibit hysteresis on thermal cycling. Fine-grained hot-pressed Eu₂O₃, as well as sintered specimens that contained at least 6 mol% HfO₂ did not exhibit these anomalous characteristics.     

Phase Transformation of Hydrous-Zirconia Fine Particles Containing Cerium Hydroxide

Monoclinic hydrous-zirconia fine particles that contained cerium(IV) hydroxide (Ce(OH)₄) were heated from 200°C to 600°C, to investigate the phase transformation to CeO₂-doped tetragonal ZrO₂. Both ZrOCl₂·8H₂O and CeCl₃·7H₂O were dissolved in aqueous solutions and then boiled to prepare the hydrous-zirconia particles. The Ce(OH)₄-containing hydrous-zirconia particles were prepared by adding aqueous ammonia into the boiled solutions. The monoclinic-to-tetragonal ( m right arrow t ) phase transformation of the Ce(OH)₄-containing hydrous zirconias was observed at 300°C using X-ray diffraction (XRD). XRD and Brunauer-Emmett-Teller (BET) specific surface area measurements revealed that the Ce(OH)₄-containing hydrous zirconias had a tendency to transform from the monoclinic phase to the tetragonal phase at lower temperatures as the primary particle size of the hydrous zirconia decreased and the Ce(OH)₄ content increased. These tendencies for the m right arrow t phase transformation agree with the conclusions that have been derived from thermodynamic and kinetic considerations.     

Stability of ZrO2 Phases in Ultrafine ZrO2-Al2O3 Mixtures

Mixtures of ultrafine monoclinic zirconia and aluminum hydroxide were prepared by adding NH₄OH to hydrolyzed zirconia sols containing varied amounts of aluminum sulfate. The mixtures were heat-treated at 500° to 1300°C. The relative stability of monoclinic and tetragonal ZrO₂ in these ultrafine particles was studied by X-ray diffractometry. Growth of ZrO₂ crystallites at elevated temperatures was strongly inhibited by Al₂O₃ derived from aluminum hydroxide. The monoclinic-to-tetragonal phase transformation temperature was lowered to ∼500°C in the mixture containing 10 vol% Al₂O₃, and the tetragonal phase was retained on cooling to room temperature. This behavior may be explained on the basis of Garvie's hypothesis that the surface free energy of tetragonal ZrO₂ is lower than that of the monoclinic form. With increasing A1₂O₃ content, however, the transformation temperature gradually increased, although the growth of ZrO₂ particles was inhibited; this was found to be affected by water vapor formed from aluminum hydroxide on heating. The presence of atmospheric water vapor elevates the transformation temperature for ultrafine ZrO₂. The reverse tetragonal-to-monoclinic transformation is promoted by water vapor at lower temperatures. Accordingly, it was concluded that the monoclinic phase in fine ZrO₂ particles was stabilized by the presence of water vapor, which probably decreases the surface energy.     

Studies in Lithium Oxide Systems: XII, Li2O-B2O3-Al2O3

Tentative phase relations in the binary system BnOa-A1₂O₃ are presented as a prerequisite to the understanding of the system Li₂O-B₂O₃-Al₂O₃. Two binary compounds, 2A1₂O₃.B₂O₃ and 9A1₂O₃.-2B₂O₃, melted incongruently at 1030° f 7°C and about 144°C, respectively. Two ternary compounds were isolated, 2Li₂O.A1₂O₃.B₂O₃ and 2Li₂O. 2AI₂O₃. 3B₂0₃. The 2:1:1 compound gave a melting reaction by differential thermal analysis at 870°± 20° C, but the exact nature of the melting behavior was not determined. The 2:2:-3 compound melted at 790°± 20° C to Li_zO.-5Al₂O₃ and liquid. X-ray diffraction data for the compounds are presented and compatibility triangles are shown.     

Phase Relations and Ordering in the Systems MgO-Y2O3-ZrO2 and CaO-MgO-ZrO2

The phase relations in the systems MgO-Y₂O₃-ZrO₂ and CaO-MgO-ZrO₂ were established at 1220° and 1420°C. The system MgO-Y₂O₃-ZrO₂ possesses a much-larger cubic ZrO₂ solid solution phase field than the system CaO-MgO-ZrO₂ at both temperatures. The ordered δ phase (Zr₃Y₄O₁₂) was found to be stable in the system ZrO₂-Y₂O₃ at 1220°C. Two ordered phases φ₁ (CaZr₄O₉) and φ₂ (Ca₆Zr₁₉O₄₄) were stable at 1220°C in the system ZrO₂-CaO. At 1420°C no ordered phase appears in either system, in agreement with the previously determined temperature limits of the stability for the δ, φ₁, and φ₂ phases. The existence of the compound Mg₃Y_zO₆ could not be confirmed.     

Phase Relations in the System ZrO2-Y2O3 at Low Y2O3 Contents

Subsolidus phase relations in the low-Y₂O₃ portion of the system ZrO_2-Y₂O₃ were studied using DTA with fired samples and X-ray phase identification and lattice parameter techniques with quenched samples. Approximately 1.5% Y₂O₃ is soluble in monoclinic ZrO₂, a two-phase monoclinic solid solution plus cubic solid solution region exists to ∼7.5% Y₂O₃ below ∼500°C, and a two-phase tetragonal solid solution plus cubic solid solution exists from ∼1.5 to 7.5% Y₂O₃ from ∼500° to ∼1600°C. At higher Y₂O₃ compositions, cubic ZrO₂ solid solution occurs.     

Single-Calcination Synthesis of Pyrochlore-Free 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 and Pb(Mg1/3Nb2/3)O3 Ceramics Using a Coating Method

A coating approach for synthesizing 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃ (0.9PMN–0.1PT) and PMN using a single calcination step was demonstrated. The pyrochlore phase was prevented by coating Mg(OH)₂ on Nb₂O₅ particles. Coating of Mg(OH)₂ on Nb₂O₅ was done by precipitating Mg(OH)₂ in an aqueous Nb₂O₅ suspension at pH 10. The coating was confirmed using optical micrographs and zeta-potential measurements. A single calcination treatment of the Mg(OH)₂-coated Nb₂O₅ particles mixed with appropriate amounts of PbO and PbTiO₃ powders at 900°C for 2 h produced pyrochlore-free perovskite 0.9PMN–0.1PT and PMN powders. The elimination of the pyrochlore phase was attributed to the separation of PbO and Nb₂O₅ by the Mg(OH)₂ coating. The Mg(OH)₂ coating on the Nb₂O₅ improved the mixing of Mg(OH)₂ and Nb₂O₅ and decreased the temperature for complete columbite conversion to ∼850°C. The pyrochlore-free perovskite 0.9PMN–0.1PT powders were sintered to 97% density at 1150°C. The sintered 0.9PMN–0.1PT ceramics exhibited a dielectric constant maximum of ∼24 660 at 45°C at a frequency of 1 kHz.     

The System Zirconia-Scandia

The system zirconia-scandia was investigated using X-ray diffraction analysis, differential thermal analysis, metallographic analysis, and melting point studies. Results reveal the monoclinic α₁ phase (0 to 2 mol% Sc₂O₃), the tetragonal α₂'phase (5 to 8% Sc₂O₃), the rhombohedral β phase (9 to 13% Sc₂O₃), the rhombohedral γ phase (15 to 23% Sc₂O₃), the rhombohedral δ phase (24 to 40% Sc₂O₃), and the cubic % phase (77.5 to 100% Sc₂O₃). The monoclinic α₁ phase and the tetragonal α₂'phase were found to transform to the tetragonal α₂ phase over a wide temperature range depending on composition. The β, γ, and α phases transformed to a cubic phase at temperatures of %600^%, 1100^%, and 1300^%C, respectively. A maximum melting point of %2870^%C was found at %10% Sc₂O₃ and a eutectic at %2400^%C at 55% Sc₂O₃.     

Degradation During Aging of Transformation-Toughened ZrO2-Y2O3 Materials at 250°C

The detrimental aging phenomenon observed in ZrO₂-Y₂O₃ materials, which causes tetragonal ZrO₂ to transform to its monoclinic structure at temperatures between 150 and 400°C, was investigated with respect to the gaseous aging environment and the Y₂O₃ and SiO₂ content of the material. It is shown that the aging phenomenon is caused by water vapor and that inter-granular silicate glassy phases play no significant role. Transmission electron microscopy of thin foils, before and after aging, showed that the water vapor reacted with yttrium in the ZrO₂ to produce clusters of small (20 to 50 nm) crystallites of α-Y(OH)₃. It is hypothesized that this reaction produces a monoclinic nucleus (depleted of Y₂O₃) on the surface of an exposed tetragonal grain. Monoclinic nuclei greater than a critical size grow spontaneously to transform the tetragonal grain. If the transformed grain is greater than a critical size, it produces a microcrack which exposes subsurface tetragonal grains to the aging phenomenon and results in catastrophic degradation. Degradation can be avoided if the grain size is less than the critical size required for microcracking.     

Subsolidus Phase Equilibria and Ordering in the System ZrO2-Y2O3

The subsolidus phase relations in the entire system ZrO₂-Y₂O₃ were established using DTA, expansion measurements, and room- and high-temperature X-ray diffraction. Three eutectoid reactions were found in the system: ( a ) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y₂O₃ and ∼490°C, ( b ) cubic zirconia solid solutiow→δ-phase Y₄Zr₃O₁₂+hexagonalphase Y₆ZrO₁₁ at 45 mol% Y₂O₃ and ∼1325°±25°C, and ( c ) yttria C -type solid solution→wcubic zirconia solid solution+ hexagonal phase Y₆ZrO₁₁ at ∼72 mol% Y₂O₃ and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y₂O₃ with ideal formula Y₄Zr₃O₁₂, and another, a new hexagonal phase, at 75 mol% Y₂O₃ with formula Y₆ZrO₁₁. They decompose at 1375° and >1750°C into cubic zirconia solid solution and yttria C -type solid solution, respectively. The extent of the cubic zirconia and yttria C -type solid solution fields was also redetermined. By incorporating the known tetragonal-cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO₂-Y₂O₃.     

Characterization of Alkoxy-Derived Yttria-Stabilized Hafnia

High-purity submicron HfO₂−Y₂O₃ powders were prepared by the simultaneous hydrolytic decomposition of hafnium and yttrium alkoxides. Compositions of 1 to 7 mol% Y₂O₃ in HfO₂ were studied by high-temperature X-ray diffraction, electron microscopy, BET surface area measurements, emission spectrographic analysis, TGA, and DTA. X-ray diffraction and atomic absorption were used to determine the Y₂O₃ concentration. Hot-pressing or cold-pressing and sintering of the powders resulted in nearly theoretically dense bodies with uniform microstructure containing grains 1 to 5 μm in size. Surface reflection ir spectra were obtained for monoclinic, tetragonal, and cubic specimens. Fully cubic HfO₂ with 7 mol% Y₂O₃ additions was obtained at a relatively low temperature. The hot-pressed specimens, initially slightly oxygen-deficient or gray HfO_{2- x} , were reoxidized at 1000°C with no deleterious effects; thin sections were translucent to incident light.     

Subsolidus Phase Equilibria of the CaO-B2O3-BaO System

The "subsolidus" phase relations at room temperature in the system CaO-B₂O₃-BaO are investigated. Specimens of various compositions were prepared from appropriate ratios of CaCO₃, B₂O₃, and BaCO₃, and fired from 780° to 1040°C according to their melting points. There are three ternary compounds in this system. The crystal structures of these compounds were determined by X-ray diffraction (XRD). CaBa₂(BO₃)₂ and Ca₅Ba₂B₁₀O₂₂ are monoclinic structures. The lattice constants a = 14.221 Å, b = 4.569 Å, c = 11.926 A, β= 99.947°, and V = 763.4 å³ for CaBa₂(BO₃)₂ and a = 15.714 å, b = 6.184 å, c = 10.204 å, β= 93.954°, and V = 989.29 å₃ for Ca₅Ba₂B₁₀O₂₂ are obtained. The third compound, CaBa₂(B₃O₆)₂, is isostructural with the high form of BaB₂O₄ with lattice constants a = 7.167 å and c = 35.298 å. Powder second harmonic generation efficiencies of these ternary compounds were measured using a homemade apparatus.     

Particle Size Comparison of Hydrothermally Synthesized Cobalt and Zinc Aluminate Spinels

Nanosized CoAl₂O₄ and ZnAl₂O₄ powders with particle sizes of 67 and 6.5 nm are synthesized under hydrothermal conditions at 245°C for 20 h. The precursors are reacted at different temperatures to provide intermediate phase transformations using X-ray diffraction (XRD), infrared (IR) spectra, and thermal gravity and differential thermal analysis (TG-DTA). XRD patterns and IR spectra demonstrate that the CoAl layered double hydroxide structure (CoAl-LDHs) is more stable than ZnAl layered double hydroxide structure (ZnAl-LDHs) when they are hydrothermally treated. The different thermal stability of the CoAl- and ZnAl-LDHs results in the different aluminum source, e.g., β-Al(OH)₃ for ZnAl₂O₄ vs γ-AlO(OH) for CoAl₂O₄, when the aluminate spinels are formed. The different aluminum sources lead to the particle size difference. The phenomenon is reasonably expounded based on the nucleation theory from the microscopic scale.     

Mechanical-Activation-Triggered Gibbsite-to-Boehmite Transition and Activation-Derived Alumina Powders

Mechanical activation of monoclinic gibbsite (Al(OH)₃) in nitrogen led to the formation of nanocrystalline orthorhombic boehmite (AlOOH) at room temperature. The boehmite phase formed after merely 3 h of mechanical activation and developed steadily as the mechanical-activation time increased. Forty hours of mechanical activation resulted in essentially single-phase boehmite, together with α-alumina (α-Al₂O₃) nanocrystallites 2–3 nm in size. The sequence of phase transitions in the activation-derived boehmite was as follows: boehmite to γ-Al₂O₃ and then to α-Al₂O₃ when flash-calcined at a heating rate of 10°C/min in air. γ-Al₂O₃ formed at 520°C, and flash calcination to 1100°C led to the formation of an α-Al₂O₃ phase, which exhibited a refined particle size in the range of 100–200 nm. In contrast, the gibbsite-to-boehmite transition in the unactivated gibbsite occurred over the temperature range of 220°–330°C. A flash-calcination temperature of 1400°C was required to complete the conversion to α-Al₂O₃ phase, with both δ-Al₂O₃ and θ-Al₂O₃ as the transitional phases. The resulting alumina powder consisted of irregularly shaped particles 0.4–0.8 μm in size, together with an extensive degree of particle agglomeration.     

Preparation by a Sol-Gel Process of Borosilicate Glasses Containing Microcrystals of Tellurium and Zinc Telluride

Borosilicate glasses, 5B₂O₃· 95SiO₂ (mol%), containing TeO₂ and ZnO nominally equivalent to 10 wt% Te and ZnTe were prepared by a solgel method from Si(OC₂H₅)₄, B(OCH₃)₃, H₆TeO₆, and Zn(NO₃)₂. A study by electron spectroscopy for chemical analysis (ESCA) showed that glasses heated at high temperature (450°C) in air contained both Te⁶⁺ and Te⁴⁺ ions on the surface layer, but that mainly Te⁴⁺ ions occurred inside the bulk glass. When solgel-derived borosilicate glasses containing the TeO₂ compound were reduced at elevated temperature in a hydrogen atmosphere, Te crystallites ranging in size from 4 to 15 nm were produced at a lower temperature, between 200° and 250°C. The absorption edge moved from the infrared to the visible wavelength region as the particle size decreased to about 4 nm. For glasses containing both TeO₂ and ZnO, ZnTe crystallites formed at high temperature—over 300°C—and existed along with the Te phase.     

Synthesis and Characterization of Nanocrystalline Lanthanide Oxysulfide via a La(OH)3 Gel Solvothermal Route

A La(OH)₃ gel solvothermal process has been developed for the preparation of nanocrystalline lanthanide oxysulfide (La₂O₂S) in polar solvents at 300°C through a reaction between a La(OH)₃ gel and K₂S. X-ray powder diffraction (XRD) indicated that the product was hexagonal La₂O₂S with cell parameters a = 4.046 Å and c = 6.951 Å. Transmission electronic microscopy (TEM) showed that different morphology nanocrystallites were formed, including particles with diameters of about 10 nm and nanorods about 10 nm in diameter and 300 nm in length, depending on the solvent.     

Preparation of Strontium Ferrite Particles by Spray Pyrolysis

Crystalline, submicrometer strontium ferrite powders, including SrFeO_2.97, SrFe₂O₄, Sr₂FeO₄, Sr₃Fe₂O_6.16, and SrFe₁₂O₁₉, were prepared by spray pyrolysis of an aqueous solution of mixed metal nitrates. The Sr:Fe mole ratio in the precursor solution was retained in the final products. Phase-pure materials were typically obtained only at the highest temperatures investigated (>1100°C) and powders prepared at lower temperatures frequently contained crystalline Fe₂O₃. The as-prepared particles were unagglomerated, polycrystalline, and hollow at lower temperatures, but densified in the gas phase at higher temperatures to give solid particles. The strontium ferrite (SrFe₁₂O₁₉) system was studied in detail as a representative example of the Sr-Fe-O system. At temperatures of 1200°C, dense, phase-pure magnetoplumbite-structure material, SrFe₁₂O₁₉, was obtained, while at lower temperatures, small amounts of Fe₂O₃ were observed. The particles prepared at 800° and 1100°C were 0.1-1.0 μm in diameter, and consisted of crystallites <100 nm, and were nearly solid. The difficulty in forming phase-pure SrFe₁₂O₁₉ was the different thermal decomposition temperatures of Sr(NO₃)₂ (725°C) and Fe(NO₃)₃9H₂O (125°C) as demonstrated by thermogravimetric analysis in the SrFe₁₂O₁₉ system.     

High-Temperature Zirconium Phosphates

The existence of two zirconium phosphate compounds has been established. The "normal" phosphate, ZrP₂O₇, has a reversible inversion at 300 °C. and dissociates to the zirconyl compound, (ZrO)₂P₂O₇, at 1550°C., with the simultaneous loss of P₂O₅ as vapor. The room-temperature form of ZrP₂O₇ is cubic with refractive index 1.657 ± 0.003. The zirconyl compound has a stable existence to temperatures around 1600°C. and is characterized by a very low linear thermal-expansion coefficient.     

Phase Relations and Ordering in the System ZrO2-MgO

The Phase relationships in the system ZrO₂-MgO were reinvestigated over a wide range of temperatures and compositions. The extent of the cubic solid solution field was determined with precise lattice parameter measurements and a high-temperature X-ray furnace using analyzed samples. DTA results show that the addition of MgO to ZrO₂ decreases the transition temperature for monoclinic ⇌ tetragonal ZrO₂ and 1 mol% of MgO is soluble in the monoclinic zirconia at ∼1070°C.The invariant eutectiod point is at 13.5 ± 0.3 mol% MgO at 1406°± 7°C, which is in fair agreement with previous results by Grain. The ordered phase Mg₂Zr₅O₁₂ (δ-phase) can form metastably in cubic solid solutions at temperatures as low as 800°C after prolonged annealing. Evidence for the existence of the ordered phase MgZr₆O₁₃(γ-phase) was obtained by electron diffraction technique. Conditions for the formation of this phase are described. The ordered phases in this system are metastable and their formation is an intermediate step in the eutectoid decomposition of the cubic phase.