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     

Stable and Metastable Equilibria in the Systems Y2O2-Al2O3, and Gd2O3-Fe2O3

The phase equilibrium relations in the systems Y₂O₃-Al₂O₃ and Gd₂O₃-Fe₂O₃ were examined. Each system has two stable binary compounds. A 3:s molar ratio garnet-type compound exists in both systems. The 1:1 distorted perovskite structure is stable in the system Gd₂O₃-Fe₂O₃ but only metastable in the system Y₂O₃-AI₂O₃. This interesting example of metastable formation and persistence of a compound with ions of high Z/r values explains the discrepancies in the literature on the structure of the composition YA1O₃. A new 2:1 molar ratio cubic phase has been found in the system Y₂O₃-A1₂O₃. Since silicon can be completely substituted for aluminum in this compound, the aluminum ions are presumably in fourfold coordination.     

Elastic Properties of Polycrystalline Monoclinic Gd2O3

The elastic properties of polycrystalline monoclinic Gd₂O₃ were determined by the sonic-resonance method. Volume-fraction porosity varied from 0.025 to 0.367 and temperature from room temperature to 1400°C. The Young's and shear moduli are linear functions of volume-fraction porosity, but the rate of their decrease with increasing porosity is less than that expected. The moduli decreased more rapidly than expected with increasing temperature. The Debye temperature is 362°K. With increasing temperature, the first Grueneisen constant, γ, decreases, whereas the second Grueneisen constant, δ, increases.     

Phases in the Binary Systems PbO—La2O3, Gd2O3, and Sm2O3

In the binary system PbO–La_zO₃ only one compound, 4PbO.La₂O₃, exists; it is flanked by two eutectics. The structure of the compound, although of lower symmetry, is intimately related to the C modification of the rare earths. Below 800° to 1000°C, metastable solid solutions are formed from oxide mixtures coprecipitated from mixed solutions of the nitrates, the cubic parameter a = 5.66 A, if extrapolated to pure La₂O₃, corresponding to half the a parameter of the C form of La₂O₃. The solid solutions existing between the compositions La₂O₃–2Pb0 and pure La₂O₃ have a cubic face–centered lattice and obey Vegard's rule. The systems of PbO with Sm₂O₃ and Gd₂O₈ are quite similar to that with La₂O₃. The compound Sm₂O₃.4Pb0 decomposes at 1000°C with evaporation of PbO; Sm₂O₃ remains in the B modification.     

Effects of Sm2O3 and Gd2O3+ Nd2O3 on Electromechanical Properties of PbTiO3 Ceramics

The electromechanical properties of PbTiO₃ ceramics, modified by substitution of Sm or Gd + Nd (same average atomic radius as Sm) for Pb, were studied in the range of 6% to 14% substitution. The modified PbTiO₃ ceramics were stable, and the Curie temperature decreased linearly over this composition range. The 10% Sm composition had a large anisotropy in the coupling factor ratio, k _t / k _p , and a similar, but weaker, effect developed for 12% (1/2 Gd + 1/2 Nd). This indicates that other than average ion size may influence the electromechanical coupling factor ratio.     

Cubic-to-Tetragonal Displacive Transformation in Gd2O3–Bi2O3 Ceramics

Gd₂O₃-doped Bi₂O₃ polycrystalline ceramics containing between 2 and 7 mol% Gd₂O₃ were fabricated by pressureless sintering powder compacts. The as-sintered samples were tetragonal at room temperature. Hightemperature X-ray diffraction (XRD) traces showed that the samples were cubic at elevated temperatures and transformed into the tetragonal polymorph during cooling. On the basis of conductivity measurements as a function of temperature and differential scanning calorimetry (DSC), the cubic → tetragonal as well as tetragonal → cubic → teansition temperatures were determined as a function of Gd₂O₃ concentration. The cubic → tetragonal transformation appears to be a displacive transformation. It was observed that additions of ZrO₂ as a dopant, which is known to suppress cation interdiffusion in rare-earth oxide–Bi₂O₃ systems, did not suppress the transition, consistent with it being a displacive transition. Annealing of samples at temperatures 660°C for several hundred hours led to decomposition into a mixture of monoclinic and rhombohedral phases. This shows that the tetragonal polymorph is a metastable phase.     

Phase Equilibrium Studies in the System Iron Oxide-Y2O3–Gd2O3

Equilibrium data at liquidus temperatures are presented for compositions in the quaternary system Y-Gd-Fe-O in ambient atmospheres of oxygen gas (p_O2, = 760 mm Hg), air (P_O2, = 159 mm Hg), and CO₂ (p_O2 variable). Incongruent melting occurred in yttrium-iron and gadolinium-iron garnet phases and in all intermediate garnet solid solutions in the three oxygen pressure sections studied. Fractionation in the yttrium/gadolinium ratio between oxide liquid and crystalline garnet phases in the quaternary system was not observed experimentally, indicating that unzoned (Y,Gd)₃Fe₅O₁₂ crystals may be grown from a melt without special precautions to maintain a fixed Y/Gd ratio.     

Para-to-Ortho H2 Conversion on Gd, Y, Gd2O3, and Y2O3

The mechanism of parahydrogen conversion was studied on Gd₂O₃ and Y₂O₃ powders and on Gd and Y evaporated metal films at low and high temperatures (77° to 90°K and 298° to 418°K). Absolute rates of conversion are compared to theoretical values for 3 possible reaction mechanisms, and it is concluded that a paramagnetic vibrational mechanism is operative on Gd₂O₃, Gd, and Y. On Y₂O₃ the reaction rate is enhanced by additional surface paramagnetic sites. The portion of the surface which is active is ∼1 for the metals and ∼0.01 for the oxides.     

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.     

Oxygen Diffusion in MgO and α-Fe2O3

Oxygen-diffusion coefficients were determined in single crystals of MgO and α-Fe₂O₃ by exchanging the samples with ¹⁸O enriched gas at 1 atm and measuring ¹⁸O profiles using a proton activation technique. For MgO, in the temperature range 1580 to 1820 K, the diffusion coefficient is represented by:         

Ion-Probe Measurement of Oxygen Self-Diffusion in Single-Crystal Al2O3

Anion self-diffusion coefficients normal to (1102) were obtained for single-crystal Al₂O₃ in a 1.3 × 10 ³ N/m² (10⁻⁵ torr) vacuum at 1585° to 1840°C. Tracer was supplied from an initial 650 to 1300 A Al₂¹⁸O₃ layer produced by the oxidation of vapor-deposited Al metal films in an ¹⁸O₂ atmosphere at 520°C. Concentration gradients extended over depths of 3000 to 5000 A and were measured by mass spectrometry of material sputtered from the samples with a beam of Ar⁺ ions. Crystals which had not been preannealed to remove surface damage displayed enhanced diffusion. Diffusion coefficients from preannealed crystals may be described by D₀ =6.4×10⁵cm²/s, with an activation energy of 188 ± 7 kcal/mol. The diffusion is interpreted as an extrinsic vacancy mechanism.     

High-Temperature Deformation of Polycrystalline Fe2O3

The effects of stress, temperature, grain size, porosity, and O₂ partial pressure on the creep of polycrystalline Fe₂O₃ were studied in the range 770° to 1105°C by tests in 4-point bending and compression. Deformation rates are controlled by the stress-directed diffusion of either oxygen or iron. Diffusion coefficients computed from the Nabarro-Herring formula modified by including an empirical porosity-correction term are also consistent with the values reported for oxygen and iron.     

Nano-Blast Synthesis of Nano-size CeO2–Gd2O3 Powders

CeCl₃·7H₂O and GdCl₃·6H₂O that were dissolved in water were precipitated with urea (NH₂CONH₂) to produce matrix agglomerates for three-component nano-reactors. Mixing hexamethylenetetramine with dilute nitric acid resulted in the formation of well-dispersed nano-particles of cyclotrimetilene trinitramine (C₃H₆N₆O₆) (RDX) in the solvent. Nano-reactors were produced by impregnating the nano-C₃H₆N₆O₆ into the matrix agglomerates of an intermediate complex of cerium and gadolinium compounds. Blast initiation of the C₃H₆N₆O₆ resulted in extremely rapid detonation and gaseous products formation at temperatures of 2000°–5000°C, which were compressed into a volume nearly equal to the initial volume of each RDX nano-particle. Multiple "nano-blasts" occurred in the volume of each nano-reactor. The impact of the blast waves led to fragmentation of the surrounding matter. The evolution of a large volume of gaseous products dissipated the heat of the process and limited temperature increase, thus reducing the possibility of local sintering among the primary particles. The short-term high temperature generated during the blasts enhanced the solid solubility of the metal oxides. Uniform aggregates of 22∼74 nm consisting of 6∼14 nm crystallites of gadolinia in ceria solid solution were synthesized.     

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.     

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.