Activity–Composition Relations in FeCr2O4–FeAl2O4 and MnCr2O4–MnAl2O4 Solid Solutions at 1500° and 1600°C

Activity–composition relations of FeCr₂O₄–FeAl₂O₄ and MnCr₂O₄–MnAl₂O₄ solid solutions were derived from activity–composition relations of Cr₂O₃–Al₂O₃ solid solutions and directions of conjugation lines between coexisting spinel and sesquioxide phases in the systems FeO–Cr₂O₃–Al₂O₃ and MnO–Cr₂O₃–Al₂O₃. Moderate positive deviations from ideality were observed.     

Activity–Composition Relations in NiAl2O4─MnAl2O4 Solid Solutions and Stabilities of NiAl2O4 and MnAl2O4 at 1300° and 1400°C

Activities of NiO were measured in the oxide and spinel solutions of the system MnO–NiO–Al₂O₃ at 1300° and 1400° C with the aim of deriving information on the thermodynamic properties of the spinel phases. Synthetic samples in selected phase assemblages of the system were equilibrated with metallic nickel and a gas phase of known oxygen partial pressures at a total pressure of 1 atm. The data on NiO activities and directions of conjugation lines between coexisting oxide and spinel phases were used to establish the activity–composition relations in spinel solid solutions at 1300° and 1400°C. The MnAl₂O₄–NiAl₂O₄ solid solutions exhibit considerable negative deviations from ideality at these temperatures. The free energy of formation of MnAl₂O₄ from its oxide components (MnO + Al₂O₃) at 1300° and 1400°C is calculated to be −24.97 and −26.56 kJ. mol⁻¹, respectively. The activities determined in the stoichiometric spinel solid solutions are more negative as compared with those predicted from cation distribution models.     

Stability of CaNiSi2O6 ("Niopside") and Activity–Composition Relations of CaMgSi2O6– CaNiSi2O6 Solid Solutions at 1350°C

A complete solid-solution series exists between diopside (CaMgSi₂O₆) and its nickel analogue, "niopside"(CaNiSi₂O₆). Activity–composition relations within this solid solution, and the stability of the end member CaNiSi₂O₆, have been determined by equilibrating CaNiSi₂O₆ with SiO₂, CaSiO₃, and metallic Ni in atmospheres of known oxygen pressures. Within limits of accuracy of the experiments, the solution is ideal at 1350°C. From the experimental data obtained in the present investigation, the standard free energy (Δ G °) of formation of CaNiSi₂O₆ according to the equation CaO + NiO + 2SiO₂= CaNiSi₂O₆ is calculated to be Δ G °=−165862 + 42.40 T J. Experiments in the system CaO–NiO–SiO₂ have shown that the nickel analogue of the phase pseudo-enstatite (MgSiO₃) is unstable with respect to SiO₂ and nickel olivine (Ni₂SiO₄), and the nickel analogues of the phases akermanite (Ca₂MgSi₂O₇) and monticellite (CaMgSiO₄) are unstable relative to the phase assemblage pseudo-wollastonite (CaSiO₃) plus NiO. In the system CaO–MgO–NiO–SiO₂, however, substitution of Ni for Mg in these phases was observed. The percentage substitution of Ni for Mg in the phases is given in parentheses: diopside (100%), olivine (100%), enstatite (18%), akermanite (20%), and monticellite (57%).     

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.     

Oxygen lon Mobility in Cubic Zr0.85Ca0.15O1.85

Oxygen ion mobility in the cubic fluorite-structure phase having the composition Zr_0.85Ca_0.15O_1.85 has been directly determined by exchange measurements employing the stable isotope O¹⁸ and mass spectrometer analysis. Measurements of electrical conductivity have been carried out as a function of both temperature and oxygen pressure. Results indicate that the conductivity is wholly ionic with a transfer number near unity for the oxygen ion.     

Oxygen Ion Mobility in Cubic Zr0.85Ca0.15O1.85

Oxygen ion mobility in the cubic fluorite-structure phase having the composition Zr_0.85 Ca_0.15 O_1.85 has been directly determined by exchange measurements employing the stable isotope O¹⁸ and mass spectrometer analysis. Measurements of electrical conductivity have been carried out as a function of both temperature and oxygen pressure. Results indicate that the conductivity is wholly ionic with a transfer number near unity for the oxygen ion.     

Equilibrium Oxygen Potential-Composition Relations in U0.75Pu0.25O2-x

Variations in the composition of U_0.75Pu_0.25O_2-x as a function of the oxygen potential were determined by a thermogravimetric technique over the interval 950° to 1400°C. Oxygen potentials ranging from -100 to -155 kcal/mol were established by means of H₂O-H₂-O₂ equilibria. The partial molar enthalpy and entropy of solution of O² in the mixed oxide fuel were independent of temperature over the interval investigated but varied with the composition of the fuel.     

Oxygen Nonstoichiometry and Defects in Mn-Doped Gd2Ti2O7+x

The oxygen nonstoichiometry in Mn-doped Gd₂Ti₂O₇, Gd₂(Ti_0.975Mn_0.025)₂O_7+x, was measured electrochemically, as a function of temperature and oxygen partial pressure, with the aid of an oxygen titration cell. The analysis of the data shows that the defect equilibrium can be described by considering the dominant point defects to be neutral oxygen interstitials, doubly charged oxygen vacancies, and trivalent and quadrivalent Mn ions substituted in the Ti sites. The enthalpies for the formation of neutral oxygen interstitials and trivalent Mn are determined.     

High-Temperature Mechanical Properties and Chemical Stability of Ba1+xZr4P6–2xSi2XO24 Low-Thermal-Expansion Ceramics

The NaZr₂P₃O₁₂ (NZP) family of materials is attracting increasing attention due to its low-thermal-expansion behavior. The system Ba_1+xZr₄P_6–2xSi_2xO₂₄ (0 ≤ x ≤ 1), belonging to the NZP family, shows ultralow thermal expansion over a wide temperature range. It also shows anisotropy in its lattice thermal expansion. This causes microcracking as the sintered specimens are cooled, which results in degradation of the mechanical properties. In this work, the chemical stability, strength, and Young's modulus of Ba_1+xZr₄P_6–2xO₂₄ ( X = 0.25 and 0.5) ceramics at high temperatures have been determined. An attempt has been made to correlate the mechanical properties to the thermal expansion anisotropy.     

Solubility Limit of MgO in Al2O3 at 1600°C

The solubility of Mg in alumina was measured using wavelength-dispersive spectroscopy mounted on a scanning electron microscope. Careful calibration of the microscope's working conditions was performed in order to optimize the detection limit and accuracy. Measurements were conducted on water-quenched and furnace-cooled samples, without any thermal or chemical etching to avoid alteration of the bulk concentration. The results indicate the solubility limit of Mg in alumina to be 132±11 ppm at 1600°C.     

Formation of Calcium Phosphate Whiskers in Hydrogen Peroxide (H2O2) Solutions at 90°C

A novel, one-pot technique of synthesizing calcium phosphate whiskers was developed. Commercially available β-tricalcium phosphate (β-Ca₃(PO₄)₂) powders were aged in unstirred 30% H₂O₂ solutions at 90°C for 48 h in ordinary glass media bottles. Resultant samples consisted of whiskers (200 nm wide and 5 μm-long) of a biphasic mixture of octacalcium phosphate (OCP: Ca₈H₂(PO₄)₆·5H₂O) and carbonated apatitic (apatite-like) calcium phosphate (Ap-CaP). As-formed whiskers possessed a Ca/P molar ratio of 1.46 and a BET surface area of 8 m²/g. Upon soaking these whiskers in a Tris-HCl-buffered SBF solution of 27 mM HCO₃⁻ for 6 days, Ca/P molar ratio and surface area values were increased to 1.60 and 52 m²/g, respectively. The technique, owing to its simplicity, may prove useful in providing large amounts of biocompatible short whiskers for numerous technology sectors.     

The Systems ZnO-Fe2O3-SnO2 and MgO-Fe2O3-SnO2 at 1060°C

Zn₂SnO₄, an inverse spinel, and ZnFe₂O₄, a normal spinel, form a complete series of solid solutions in the system ZnO-Fe₂O₃3nO₂. The variation of cell dimensions with composition varies from 0.8439 nm (ZnS₂nO₄) to 0.8660 nm (ZnS₂nO₄) and exhibits a positive deviation from a linear relationship. Mg₂SnO₄ and MgFe₂O₄, both predominantly inverse in nature, have only an incomplete series.     

Phase Equilibria in the System CaO-MgO-B2O3 at 900°C

Phase equilibria in the system CaO-MgO-B₂O₃ were investigated at 900°C using X-ray powder diffraction techniques. With the exception of MgO-B₂O₃, the binary phases reported previously were confirmed, but no ternary phases were found. Solid solution effects were investigated for the binary phases by comparison of patterns, whereas for CaO and MgO, accurate lattice parameters were compared. No solid solutions were detected. As a result, the isothermal equilibrium diagram at 900°C reduces to three phase triangles. X-ray powder diffraction data for the calcium berates are included.     

Synthesis and Thermal Expansion Properties of the Ca(1+x)/2Sr(1+x)/2Zr4P6−2xSi2xO24 System

Ca_{(1+ x )/2}Sr_{(1+ x )/2}Zr₄P_{6−2 x} Si_{2 x} O₂₄( x ≤ 0.37) compositions, which belong to the [NZP] family of low-thermal-expansion materials, were synthesized using the solid-state reaction method. The lattice thermal expansion of members of this system was determined up to 1000°C by high-temperature X-ray diffractometry. The bulk thermal expansion and the microcracking during cooling also were investigated. These properties depended on the composition, on the synthesis method, and on the sintering conditions.     

Structure of B2O3 and Binary Aluminoborate Melts at 1600°C

Viscosity and density data were obtained up to 1700°C for a series of binary aluminoborate melts that contained as much as 15 mole% (∼21 wt%) Al₂O₃ and up to 1620°C for pure molten B₂O₃. Large expansion coefficient decreases and a slight activation energy increase for B₂O₃ above 1400°C suggested a tightening of its structure. The addition of Al₂O₃ reduced viscosity and increased activation energy. The decreased compositional dependence of molar volume (compared to SiO₂ additions) and the increased expansion coefficients accompanying Al₂O₃ additions suggested a loosening of the O—B—O structure at 1600°C. Molar volume deviations from ideality were similar to but smaller than those for SiO₂ and GeO₂ additions at 1300°C. Microclustering of aluminum-bearing polyhedra appeared to occur at slightly higher boron atom contents than with SiO₂ and GeO₂ additions.     

Creep of Polycrystalline MgO-FeO-Fe2O3 Solid Solutions

Steady-state creep experiments were performed on hot-pressed polycrystalline MgO doped with Fe. Dead-load 4-point bend creep tests were conducted at stresses of 26 to 270 kg/cm², at temperatures of 1250° to 1450°C, in O₂ partial pressures of 1 to 10⁻⁹ atm, on specimens with grain sizes of 10 to 65 μm. Viscous steady-state creep was always observed when the grain size was stable. Experiments at variable P _O2's and temperatures were used to identify regimes of high (117 ± 10 kcal/mol) and low (81 ± 5 kcal/mol) activation energy. In the latter, creep rates were nearly independent of Fe dopant concentration and P _O2, whereas in the former creep rates were enhanced by increasing P _O2's and Fe dopant levels. The high- and low-activation-energy regimes were interpreted as diffusional creep controlled primarily by Mg lattice diffusion and O grain-boundary diffusion, respectively.