Vaporization Studies of the La2O3–Ga2O3 System

The vaporization of the samples of the compositions Ga₂O₃+ LaGaO₃, LaGaO₃+ La₄Ga₂O₉, and La₄Ga₂O₉+ La₂O₃ was investigated using Knudsen effusion mass spectrometry in the temperature range 1494–1937 K. The partial pressures of the gaseous species O₂, Ga, GaO, Ga₂O, and LaO were determined over the samples investigated. The equilibrium partial pressures were used for the calculation of the thermodynamic activities of the components at 1700 K. Gibbs energies of formation of LaGaO₃( s ) and La₄Ga₂O₉( s ) at 1700 K from the component oxides were derived from the thermodynamic activities as −46.4 ± 4.7 and −99.2 ± 7.9 kJ·mol⁻¹, respectively. The results were compared with the literature data obtained using other methods.     

CrO2-Cr2O3 Phase Boundary Under High O2 Pressures

The phase boundary between CrO₂ and Cr₂O₃ was reinvestigated under high O₂ pressures by using a new type of gas compressor. The boundary curve can be represented as log Po₂= 7.16-(3579/ T ). Using the observed data, Δ G °, Δ H °, and Δ S ° for the reaction 2CrO₂⇋Cr₂O₃+½O₂ were calculated to be: Δ G °= -(1.55/100) T +7.60 kcal/mol, Δ H °= -8.19 kcal/mol, and Δ S °= (-15.8/ T )+0.0155 kcal/mol.     

Thermodynamic Properties of the System Indium-Oxygen

The free-energy change for the reaction 2In( l )+3NiO( s ) = In₂O₃( s )+3Ni( s ) was determined from 550° to 800°C from emf measurements on solid-oxide galvanic cells. The results were used to develop an equation for the standard molar free energy of formation of In₂O₃, i.e. ΔG°_ln2O₃= -215,550+72.63 T ±450 cal/ mol. The standard molar enthalpy and entropy of formation of In₂O₃ at 298°K were calculated to be -214,000±1500 cal/mol and -69.03±o0.22 eu, respectively, using the available thermo-chemical data. The absolute entropy of In₂O₃ at 298°K was calculated to be 32.23±0.22 eu. The free-energy results of this study were used in conjunction with literature data to calculate partial pressures of the gaseous species over In₂O₃ for different experimental conditions.     

Oxygen Potential in the System UMoO6/UMoO5 by the Solid Oxide Electrolyte Emf Method

The emf of the galvanic cell Pt|UMoO₆UMoO₅|ZrO₂-Y₂O₃|O₂(air, Po₂=0.21 atm)|Pt, measured from 776 to 1127 K, was determined to be E = 751.7−0.4909 ±4.3 mV. Using the standard Gibbs energy of formation, ^fΔG°, for UMoO₆ reported in the literature from transpiration studies, the ^fΔG° for UMoO₅ is calculated to be ^fΔG°〈UMoO₅〉=−1816.9+0.3748T±6.0 U-mol⁻¹ The magnitudes of the standard entropies of formation, ^fΔS°, for UMoO₆ and UMoO₅ were evaluated from those values reported for the binary oxides which constitute the ternary compounds.     

Subsolidus Relations in the La2O3─CuO─CaO Phase Diagram and the La2O3─CuO Binary Join

The phase diagram for the CuO-rich part of the La₂O₃─CuO join was redetermined. La₂Cu₂O₅ was found to have a lower limit of stability at 1002°± 5°C and an incongruent melting temperature of ∼1035°C. LagCu₇O₁₉ had both a lower (1012°± 5°C) and an upper (1027°± 5°C) limit of stability. Subsolidus phase relations were studied in the La₂O₃─CuO─CaO system at 1000°, 1020°, and 1050°C in air. Two ternary phases, La_1.9Ca_1.1Cu₂O_5.9 and LaCa₂Cu₃O_8.6, were stable at these temperatures, with three binary phases, Ca₂CuO₃, CaCu₂O₃, and La₂CuO₄. La₂Cu₂O₅ and La₈Cu₇O₁₉ were stable only at 1020°C, and did not support solid-solution formation.     

Heat of Vaporization of EuN and Its Standard Heat of Formation

The vaporization of EuN was studied by Knudsen effusion mass spectrometry with Mo Knudsen cells of 0.010 and 0.020 in. orifice diameter. Europium nitride has a narrow N-deficient homogeneity range and vaporizes congruently by dissociation into Eu vapor and N₂. No gaseous EuN was observed. The second-law enthalpy change for the reaction EuN_{1- x} ( s ) = Eu(g) + [(1- x )/2]N₂(g), where x is very small, was Δ H ⁰₂₉₈= 94±6 kcal (393.3±25.1 kJ). In conjunction with the heat of vaporization of Eu, the standard heat of formation of EuN, Δ H ⁰_f.298=–52±6 kcal mol⁻¹ (217.6±25.1 kJ mol⁻¹), was obtained.     

Energetics of Defect Fluorite and Pyrochlore Phases in Lanthanum and Gadolinium Hafnates

Aerodynamic levitation combined with laser heating was used to prepare melts in the HfO₂–La₂O₃ (–Gd₂O₃) systems. All melts crystallized upon quenching in oxygen. Hf₂La₂O₇ pyrochlore and Gd_0.5Hf_0.5O_1.75 fluorite phases were identified. Gd_0.5Hf_0.5O_1.75 fluorite was transformed into the pyrochlore structure by annealing at 1450°C. Pyrochlore that crystallized from HfO₂- La₂O₃ melts contained 31.6–34.2 mol% La₂O₃. The unit cell parameter increased linearly with La content from 10.736 to 10.789 Å. Drop solution calorimetric experiments were performed in 3Na₂O·4MoO₃ melt at 702°C. The enthalpies of formation from the oxides for pyrochlore phases are −107.0±5.0 kJ/mol for Hf₂La₂O₇ and −48.8±4.7 kJ/mol for Hf₂Gd₂O₇. The enthalpy of the pyrochlore–fluorite phase transition in Hf₂Gd₂O₇ is 23.6±3.1 kJ/mol.     

Vacuum Vaporization in the System MgO-Cr2O3

The vaporization of the system MgO-Cr₂O₃ was studied in a vacuum of 10⁻⁵ torr (10⁻³N/m²) at 1500° to 1700°C using the Langmuir and Knudsen methods. It was found that the phases in the system vaporize nearly congruently and the logarithm of the vaporization coefficient, α, of MgCr₂O₄ increases linearly with increasing reciprocal temperature. Alpha tends to unity at a temperature near the melting point (2525±23°C). The additivity rule can be applied to the Langmuir vaporization rates on the basis of the surface area ratios of the phases in the 2-phase system MgO-Cr₂O₃. The enthalpies of vaporization of MgCr₂O₄ were 695.0 and 549.2 kcaVmol for activated and equilibrium processes, respectively.     

Effect of (La0.8Sr0.2)CrO3 Coating on Carbon Deposition onto a Stainless-Steel (SUS430) Substrate

In this study, a dense strontium-doped lanthanum chromite (La_0.8Sr_0.2CrO₃, LSC) thin layer was designed to protect a stainless-steel (SUS430) substrate from carbon deposition. The LSC layer was coated onto an SUS430 substrate by a dipping technique from a precursor solution of La, Sr and Cr nitrates, acetylacetone (acac), and 2-methoxyethanol. The effect of AcAc on the phase behavior and microstructure evolution of the LSC thin films was investigated. After being heat-treated at 800°C in air, the thin film was found to consist of perovskite LaCrO₃, Mn_1.5Cr_1.5O₄, and Cr₂O₃ phases. The addition of a chelating agent, acac, to the precursor solution led to a reduction in the formation of the strontium chromite (SrCrO₄) phase. As a consequence, a thin film having a dense microstructure could be obtained. It was confirmed by Fourier-tranform Raman spectroscopic analysis and FESEM observations that the carbon deposited on the uncoated SUS430 substrate was amorphous with a spherical morphology. The LSC thin film thus obtained was found to be very effective at preventing carbon deposition when it was heat-treated under a dry hydrocarbon atmosphere.     

Immiscibility and the System Lanthanum Oxide–Boric Oxide

The phase equilibrium diagram for the system La₂O₃-B₂O₃ has been determined experimentally. The compounds La₂O₃-3B₂O₃and La₂O₃-B₂O₃ melt congruently at 1141°± 5°C. and 1660°± 15°C, respectively. At 1488°± 5°C, La₂O₃-B₂O₃ inverts from the aragonite-type structure to a high-temperature form. Trilanthanum borate, 3La₂O₃ B₂O₃, melts incongruently at 1386°± 5°C. to give liquid and La₂O₃. No solid solutions exist in the system. A region of liquid immiscibility exists in the system and extends at 1136°± 5°C. from almost pure B₂O₃ to 21.5 mole % La₂O₃. The experimental value for the extent of immiscibility agrees with that calculated from theoretical considerations. A second method for estimating immiscibility in the system is demonstrated, which requires experimentally only the determination of the index of refraction of the modifier-rich liquid. Principles governing immiscibility are discussed.     

Solid Phase Relations at 950°C in La-Ba-Cu-O

Subsolidus phase relations in the La₂O₃–BaO–CuO system were studied at 950°C. Three previously reported binary compounds exist (La₂CuO₄, BaLa₂O₄, and BaCuO₂) and five previously reported ternary phases occur (La_2-xBa_xCuO_4-(x/2)+δ, La_4-2xBa_2+2xCu_2-xO_10-2x, La_2-xBa_1+xCu₂O_6-(x-2), La_3-xBa_3+xCu₆O_14±δ, and La₄BaCu₅O_13+δ). Of the seven phases in the diagram, all but BaLa₂O₄, BaCuO₂, and La₄BaCu₅O_13+δ were shown to exhibit significant ranges of solubility. The diagram is important in that both >30 K (La_2-xBa_xCuO_4-(x/2)+δ) and >90 K (La_3-xBa_3+xCu₆O_14+δ, x=1) superconductors occur.     

Thermodynamic Stability of UMoO6 by the Transpiration Method

The equilibrium vaporization of UMoO₆(s) in dry air was studied by the transpiration method in the temperature interval 1110°≤ T (K) ≤ 1250°. Apparent pressure of the trimer measured over the two-phase mixture UMoO₆+ U₃O₈, with dry air as the carrier gas, was used to calculate the partial pressure of the trimer, (MoO₃)₃(g). In accordance with the vaporization reaction (UMoO₆) = 1/3(U₃O₈) + 1/6(MoO₃)₃ (g) + 1/6O₂ (g), the free energy of formation of UMoO₆)(s) is given by (δ G° in kJ-mol⁻¹) δ G° (UMoO₆) = (−1962 ± 10) + (0.463 ± 0.008) T     

The System Manganese Oxide–Cr2O3 in Air

The quenching technique has been used to determine equilibrium relations in the system manganese oxide-Cr₂O₃ in air in the temperature range 600° to 1980°C. The following isobaric invariant situations have been determined: At 910°± 5°C tetragonal Mn₃O₄ solid solution, cubic Mn₃O₄ solid solution (=spinel), Mn₂O₃ solid solution, and gas coexist in equilibrium. Cubic Mn₃O₄ solid solution, Cr₂O₃ solid solution, liquid, and gas are present together in equilibrium at 1970°± 20°C. The invariant situation at which cubic Mn₃O₄ solid solution, Mn₂O₃ solid solution, Cr₂O₃ solid solution, and gas exist together in equilibrium is below 600°C.     

High-Temperature Creep Behavior of Mixed Conducting La0.5Sr0.5Fe1−xCoxO3-δ (0.5≤x≤1) Materials

Steady-state compressive creep rate of La_0.5Sr_0.5Fe_0.5Co_0.5O_3−δ (LSFC) and La_0.5Sr_0.5CoO_3−δ (LSC) is reported in the temperature region 900°–1050°C and stress range 5–28 MPa. The stress exponents for the two materials were 1.71±0.18 and 1.24±0.15, respectively. The activation energy for creep was considerably higher for LSC (619±56 kJ/mol) than for LSFC (392±28 kJ/mol). The grain size exponent for LSC was 1.28±0.14. Considerably higher creep rates were observed for both materials in N₂ compared with air. Relaxation by creep of chemical-induced stresses in oxygen-permeable membranes is addressed, especially at low partial pressure of oxygen.     

Formation, Characterization, and Hot Isostatic Pressing of Cr2O3-Doped ZrO2 (0,3 mol% Y2O3) Prepared by Hydrazine Method

In the ZrO₂-Cr₂O₃ system, metastable t -ZrO₂ solid solutions containing up to 11 mol% Cr₂O₃ crystallize at low temperatures from amorphous materials prepared by the hydrazine method. The lattice parameter c decreases linearly from 0.5149 to 0.5077 nm with increased Cr₂O₃ content, whereas the lattice parameter a is a constant value ( a = 0.5077 nm) regardless of the starting composition. At higher temperatures, transformation (decomposition) of the solid solutions proceeds in the following way: t (ss)→ t (ss) + m + Cr₂O₃→ m + Cr₂O₃. Above 11 mol% Cr₂O₃ addition, c-ZrO₂ phases are formed in the presence of Cr₂O₃. The t -ZrO₂ solid solution powders have been characterized for particle size, shape, and surface area. They consist of very fine particles (15–30 nm) showing thin platelike morphology. Dense ZrO₂(3Y)-Cr₂O₃ composite ceramics (∼99.7% of theoretical) with an average grain size of 0.3 μm have been fabricated by hot isostatic pressing for 2 h at 1400°C and 196 MPa. Their fracture toughness increases with increased Cr₂O₃ content. The highest K _Ic value of 9.5 MPa·;m^1/2 is achieved in the composite ceramics containing 10 mol% Cr₂O₃.     

Knudsen Effusion Study of the Vaporization of CdO(s)

Knudsen effusion studies were conducted on CdO(s) in fused-SiO₂ Knudsen cells from 889 to 1110 K. The orifice size was varied; the largest cell orifice yielded a calculated pressure significantly lower than could be accounted for by experimental error. Average-slope- and average-free-energy-function-based values of Δ H ^c₂₉₈ were 86.9 ± 1.1 and 88.8 ± 0.5 kcal/mol, respectively. The data from the present work were combined with those of previous investigators using a method discussed by McCreary and Thorn; a Δ H ^c₂₉₈ of 88.2 ± 0.2 kcal/mol was obtained. Using this combined value for Δ H ^c₂₉₈ and ΔS^c₂₉₈= 51.40 ± 0.1 eu (obtained from the literature), the equilibrium constant for the reaction CdO(s)= Cd(g)+ 1/2 O₂(g) at 900 to 1380 K was calculated to be log K = 10.6-18,800(I/T).     

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.     

Preparation of Porous Cr3C2 Grains with Cr2O3

Porous Cr₃C₂ grains (∼300 to 500 μm) with ∼10 wt% of Cr₂O₃ were prepared by heating a mixture of MgCr₂O₄ grains and graphite powder at 1450° to 1650°C for 2 h in an Al₂O₃ crucible covered by an Al₂O₃ lid with a hole in the center. The porous Cr₃C₂ grains exhibited a three-dimensional network skeleton structure. The mean open pore diameter and the specific surface area of the porous grains formed at 1600°C for 2 h were ∼3.5 (μm and ∼6.7 m²/g, respectively. The present work investigated the morphology and the formation conditions of the porous Cr₃C₂ grains, and this paper will discuss the formation mechanism of those grains in terms of chemical thermodynamics.     

Oxidation/Vaporization Kinetics of Cr2O3

The weight loss of Cr₂O₃ in oxidizing environments (Po₂= 1 to 10⁻³ atm) at 1200°C was measured. Both hot-pressed and sintered Cr₂O₃ pellets were investigated in O₂/Ar gas mixtures, and the dependence of the weight loss on the O₂ partial pressure, the gas flow rate, and the total pressure was determined independently. The experimentally determined O₂ partial pressure dependence (rate ∝ PO₂^3/4) corresponds to that expected for the reaction Cr₂O₃(s)+3/2O₂⇌2CrO₃(g). The flow rate and total pressure dependencies show that mass transport through a gaseous boundary layer is the rate-controlling step in the oxidation/vaporization of Cr₂O₃. Evaporation coefficients for the loss of CrO₃(g) under the experimental conditions were <0.01.     

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.