Phase Equilibria in the System CaF2-AlF3

The phase diagram of the system CaF₂-AlF₃ was established from microscopic, powder X-ray diffraction, quench, and DTA data obtained from samples encapsulated in sealed Pt tubes and either reacted in the solid state or melted. Two compounds, CaAlF₅ and Ca₂AlF₇, melted incongruently at 873^°° 3^° and 845^°°3^°C, respectively. Previously unreported Ca₂AlF₇ was successfully indexed as orthorhombic with α₀= 18.22 Å, b ₀=9.06 Å, and c ₀= 7.11 Å. The only eutectic in the system exists at 836^°° 3^°C and 37.5 mol% AlF₃.     

Preparation and Properties of Oxynitride Glasses Made from Potassium Metaphosphate

Potassium phosphate oxynitride (KPON) glasses were made by heating crystalline KPO₃ at 702^° to 775^°C in dry ammonia. The softening temperature, thermal expansion coefficient, refractive index, and dissolution rate in water were measured as a function of nitrogen content and compared with the properties of oxynitride glasses made from LiPO₃ and NaPO₃.     

Calculated Thermodynamic Data and Metastable Immiscibility in the System SiO2-Al2O3

Thermodynamic data on activities, activity coefficients, and free energies of mixing in SiO₂-Al₂O₃ solutions were calculated from the phase diagram. Positive deviations from ideal mixing in the thermodynamic data suggest a tendency for liquid immiscibility in both SiO₂- and Al₂O₃-rich compositions. The calculated data were used to estimate regions of liquid-liquid immiscibility. A calculated metastable liquid miscibility gap with a consolute temperature of ∼1540^°C at a critical composition of ∼36 mol% Al₂O₃ was considered to be thermodynamically most probable; the gap extended from ∼11 to °49 mol% Al₂O₃ at 1100^°C. SiO₂-rich glass compositions showed evidence of glass-in-glass phase separation when examined by direct transmission electron microscopy.     

Electrical Resistivity of Magnetite and Nickel Ferrous Ferrite Above 300°K

The electrical resistivity of Fe²⁺_0.95Fe³⁺_2.05O²⁺₄, Ni²⁺_0.51Fe²⁺_0.46Fe_1.95,³⁺O²⁻₄, and Ni_0.89,²⁺Fe²⁺_0.13Fe³⁺_2.11O²⁻₄ single crystals was determined at >300°K. The dc resistivity up to 800°K shows a monotonic decrease with increased temperature, with an average activation energy of 0.065 eV independent of temperature and Fe^z+ concentration. The ac resistivity, at frequencies of 540 kHz to 34 MHz up to 450°K, shows temperature and frequency dependences of the relative dielectric constant similar to that found when true dielectric relaxation occurs, with the activation energy depending slightly on frequency and ferrous ion content.     

Microwave Characteristics of A(B3+1/2B5+1/2)O3 Ceramics (A = Ba, Ca, Sr; B3+= La, Nd, Sm, Yb; B5+= Nb, Ta)

Dielectric properties of A(B³⁺_1/2B⁵⁺_1/2)O₃ (A = Ba, Ca, Sr; B³⁺= La, Nd, Sm, Yb; B⁵⁺= Nb, Ta) ceramics have been investigated at microwave frequencies. Sr(B³⁺_1/2B⁵⁺_1/2)O₃ and Ca(B³⁺_1/2B⁵⁺_1/2)O₃ ceramics have relative dielectric constants (ε _r ) above 20 and negative temperature coefficients of resonant frequency (T _f ). In the group of Ba(B³⁺_1/2B⁵⁺_1/2)O₃ ceramics, T _f changes from + 118 ppm/° to nearly zero according to the kinds of B-site ions. Among the ceramics investigated, Sr(Sm_1/2Ta_1/2)O₃ ceramics have the highest Q values at microwave frequencies. For Sr(Sm_1/2Ta_1/2)O₃ ceramics Q = 7000, ε _r = 27.7, and T _f =−62.5 ppm/° at 8.5 GHz. The microstructure of Sr(Sm_1/2Ta_1/2)O₃ ceramics is composed of a matrix of the ternary compound (Sr-Sm-Ta-O system) and secondary phase grains of the binary compound (Sm-Ta-O system).     

Synthesis and Characterization of Sr2Ce2Ti5O16 in the System SrO-CeO2-TiO2

The ternary system SrO-CeO₂-TiO₂ was investigated using X-ray diffractometry. The formation of a new compound, Sr₂Ce₂Ti₅O₁₆, was established, and its compatibilities with SrO, SrCeO₃, and SrTiO₃ were studied. The results revealed the existence of a series of compounds Sr_6–12xCe_6xTi₅O₁₆ and solid solutions Sr_2+nCe₂Ti_5+nO_16+3n ( n ≤ 6).     

Mechanism and Kinetics for the Formation of Uranium Mononitride by the Reaction of Uranium Dioxide with Carbon and Nitrogen

The mechanism and kinetics of UN formation by reaction of a pellet of mixed UO₂ and C with N₂ were studied for temperatures of 1420° to 1750°C. The reaction followed the first-order rate equation; the activation energy was 83 kcal/mol. Only UN_1−xC _x was produced. The lattice parameter variation of UN_1−xC _x had a minimum and a maximum during reaction; at the maximum, UN_1−xC _x + C + N₂ were in equilibrium. The overall reaction was divided into four stages: (1) formation of UN_1−xC _x from UO₂, (2) decarburization of UN_1−xC _x , (3) formation of UN_1−xC _x with the equilibrium composition, and (4) pure UN formation. The lowest reaction rate was in stage (4).     

Internal Oxidation of Ce3+-BaTiO3 Solid Solutions

The reoxidation process in highly Ce³⁺-doped BaTiO₃ ceramics was studied using TEM. Samples of two different types of solid solutions, Ba_1−XCe³⁺ _X Ti_1−X/4( V _Ti) X/4 O₃ and Ba_1−XCe³⁺ _X Ti⁴⁺_{1− X} Ti³⁺ _X O₃, were prepared by sintering oxide mixtures in air and in a reducing atmosphere, respectively. The solid solutions were reoxidized by annealing in air at high temperatures (1000°—1100°C). As a result of internal oxidation of Ce³⁺ and Ti³⁺, fluorite CeO₂ and monoclinic Ba₆Ti₁₇O₄₀ phases were precipitated in the perovskite matrix. In Ba_1−XCe³⁺ _X Ti_1−X/4( V _Ti)X/4O3 solid solution precipitates nucleate heterogeneously at grain boundaries and at extended defects inside the grains, whereas in Ba_1−XCe³⁺_XTi⁴⁺_1−XTi³⁺_XO₃ solid solution precipitates are nucleated mainly homogeneously inside reoxidized perovskite grains. The form of the precipitates and their orientational relationship with the matrix, as well as the mechanism of internal oxidation, are discussed.     

Crystal Chemistry and Phase Equilibrium Studies of the BaO(BaCO3)–½R2O3–CuOx Systems in Air: VI, R = Neodymium

Crystal chemistry and subsolidus phase equilibrium studies of the Ba-Nd-Cu-O system near the CuO and Nd₂O₃ corners have been carried cut at 950°C in air. Two solid-solution series have been identified in the Ba-Nd-Cu-O system. The first series involves the high- T _c superconductor phase, and has the formula Ba_2–xNd_1+xCu₃O_6+z, where × < ≅ 0.7. At the ideal compound stoichiometry of Ba₂NdCu₃O_6+z, the transformation from the high- T _c orthorhombic to tetragonal phase occurs at 550°–575°C in air. This temperature varies as a function of composition, and at x ≅ 0.2 to 0.3 it occurs at 950°C. The second solid solution is the non-superconducting "brown phase" represented by Ba_2+2x-Nd_4–2xCu_2–xO_10–2z 0 ≤ x ≤ 0.1. Preliminary phase diagrams of the BaO–Nd₂O₃ and Nd₂O₃–CuO_x systems are also presented. Standard X-ray diffraction patterns of BaNd₂–CuO₅ and (Nd_1.9Ca_0.1)CuO_4–z are provided.     

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.     

Phase Studies Concerning Sintering in Aluminas Doped with Ti4+

Previous solid state analyses of sintering in Ti⁴⁺-doped-commercial alumina are shown to be in error because a liquid phase exists in the appropriate region of the Al₂O₃−TiO₃−Na₂Ophase diagram at least by 1350^°C, a temperature lower than that at which "solid state" studies were conducted. It is suggested that liquid-phase sintering is a much more common occurrence than was realized formerly.     

Volatility of UO2±x and Phase Relations in the System Uranium—Oxygen

The volatility of UO_2±x and the phase relations in the system uranium-oxygen were studied using thermogravimetric techniques. Chemical reactions describing the loss of uranium from UO_2±x at temperatures between 1100° and 2200°C in oxygen pressures between approximately 10² and 10⁻⁶ torr are proposed. Results were obtained requiring the consideration of UO₄(g) as the uranium-bearing vapor species above UO_2±x. Evidence supporting the existence of UO₄(g) included the volatilization of material with an oxygen-to-uranium ratio of 4 during the decomposition of UO_2+x(0.Z > × > 0) to near-stoichiometric UO₂ in vacuum above 1500°C and the dependence of the evaporation rate of the uranium dioxide on the oxygen pressure between 1200° and 1500°C. The equilibrium oxygen pressures over compositions between UO_2.02 and UO_2.63 in the UO_2+x and U₃O_8-y regions and over the boundary between these phases were measured between 1000° and 1600°C. The equilibrium oxygen-to-uranium ratio of UO_2±x was less than 2 above 1700°C in vacuum.     

Raman Spectrometric Determination of the Oxygen Self-Diffusion Coefficients in Oxides

New methods of determining the oxygen self-diffusion coefficients (D*_o) in oxides have been developed using Raman spectroscopy combined with the ¹⁶O–¹⁸O exchange technique. From the depth-profiles of the ¹⁸O concentration in the ¹⁶O–¹⁸O exchanged oxides, which was measured by Raman microscope with a spatial resolution of 5 μm, D *_o was determined for 2.8 mol% Y₂O₃-containing tetragonal zirconia polycrystall (the depth-profile method). Thus-obtained results are expressed as D *_O,D-P= 1.82(^+0.41_−0.40) × 10⁻¹·exp{−(139.3 ± 0.2) [kJ/mol]/ RT } [cm²/s] in the temperature range of 700–950^°C. We also determined D *_o for the same sample from the Raman spectrometric monitoring of the ambient gas during the ¹⁶O–¹⁸O exchange reaction (the gas-monitoring method). Thus-obtained results are expressed as D *_O,G-M= 1.14(^+0.05_−0.04) × 10⁻² exp{−(117.5 ± 0.4) [kJ/mol]/ RT } [cm²/s] in the temperature range of 700–1165°C. The results obtained from the above two different methods virtually agree with each other, indicating that reliable D *_o can be obtained by either of these two methods. We demonstrate that Raman spectroscopy is a useful tool for determining D *_o in oxides.     

Structure and Properties of (Ba,Ca) TiO3 Ceramics Prepared Using (Ba,Ca)TiO3 Ceramics I, Crystallographic and Microstructural Studies

Ba_1–xCa_xTiO₃ powders have been prepared using a novel route involving solid-state reaction in a mixture of Ba_1–xCa_xCO₃ and TiO₂. The Ba_1–xCa_xCO₃ precursors used in this method were prepared by a chemical coprecipitation route to ensure a uniform supply of Ba and Ca ions during thermochemical reaction with TiO₂. The compositional homogeneity of Ba_1–xCa_xTiO₃ powder prepared by such a semiwet route is compared with those prepared by the conventional dry route, involving thermochemical reaction in a mixture of BaCO₃, CaCO₃, and TiO₂, using principles of X-ray line broadening. It is shown that the powders obtained by the semiwet route possess better compositional homogeneity, over a length scale of 1800 Å corresponding to the coherently scattering domain size, than those prepared by the conventional dry route. Microstructural studies have revealed grain sizes of the order of 1 μm and several micrometers, respectively, for ceramics prepared by the semiwet and conventional dry routes. The solid solubility limit of Ca in Ba_1–xCa_xTiO₃ ceramics fired at 1300°C is 16 mol% for samples prepared by the semiwet route while conventionally processed ceramics had a Ca solid Solubility limit of 12 mol% at the same temperature.     

Dielectric, Piezoelectric, and Pyroelectric Properties of Lead Zirconate—Lead Zinc Niobate Ceramics

New piezoelectric and pyroelectric ceramics consisting of antiferroelectric lead zirconate (PZ) and relaxor ferroelectric lead zinc niobate (PZN) are studied from an application view-point of the field-induced antiferroelectric-to-ferroelectric phase transition. An antiferroelectric-ferroelectric phase boundary exists in PbZr_x(Zn_1/3Nb_2/3)_1−xO₃ (PZZN-1000x) close to x = 0.93 to 0.94 at room temperature. A new ferroelectric rhombohedral phase change, F_α–F'_α, at low temperature is found and studied by the temperature dependence of the pyroelectric coefficient. Electrical poling in these ceramics is easy, and the coercive field E_c∼8 to kV/cm is rather low. Samples with compositions in the range PZZN-86 to PZZN-92 have a large electromechanical coupling constant, k (k_t and k₁₅∼50% to 60%), and a low dielectric constant, ɛ_s (ɛ^T₃₃/ɛ₀= 260 to 320, ɛ^T₁₁/ɛ₀= 380). PZZN ceramics appear to be potential candidates for high-frequency ultrasonic transducers used in the thickness shear mode. The pyroelectric figure of merit (F_v) of these ceramics is comparable to the values published for the PZT-based or PbTiO₃-based materials.     

Phase Equilibria of the La2O3-SrO-CuO System at 950°C and 10 kbar

Phase equilibria of the La₂O₃-SrO-CuO system have been determined at 950°C and 10 kbar (1 GPa). Stable phases at the apices of the ternary phase diagram are CuO, La₂O₃, and SrO. Stable intermediate phases are La₂CuO₄ in the LaO_1.5-CuO binary and Sr₂CuO₃, SrCuO₂, and Sr₁₄Cu₂₄O₄₁ in the CuO-SrO binary. The La_2-xSr _x CuO_4-δ solid solution is stable where 0.0 ≤ x ≤ 1.3, the La_2-xSr_1+xCu₂O_6+δ solid solution is stable where 0.0 ≤ x ≤ 0.2, the La_8-xSr _x Cu₈O_20-δ solid solution is stable where 1.3 ≤ x ≤ 2.7, the La _x Sr_14-x-Cu₂₄O₄₁ solid solution is stable where 0 ≤ x ≤ 6, and the La_1+xSr_2-xCu₂O_5.5+δ phase is stable where 0.04 ≤ x ≤ 0.16. The La₂O₃-SrO-CuO phase diagram at 950°C and 10 kbar is almost identical to that determined by other authors at 950°C and 1 atm, in terms of phase stability and solid-solution ranges.     

Phase Equilibria of the Calcium Oxide-Copper Oxide System in Oxygen at 1 atm

Phase equilibria in the CaO-CuO system have been determined at 1 atm pressure in oxygen over the temperature range 800° to 1300^deg;C. CaO is the stable phase at the calcium-rich end of the system. Two intermediate crystalline phases, namely Ca₂CuO₃ and Ca₃Cu₇O₁₀, form. Ca_zCuO₃ is stable up to 1085°± 3^deg;C, where it melts incongruently to CaO + liquid (peritectic liquid has ε82% CuO). Ca₃Cu₇O₁₀ becomes stable at 977°± 3^deg;C by reaction between Ca₂CuO₃ and CuO. Ca₃Cu₇O₁₀ melts incongruently at 1046°± 3^deg;C to Ca₂CuO₃+ liquid (peritectic liquid has ε83% CuO). CuO is the stable copper oxide phase up to 1061°± 3^deg;C at the copper-rich end of the system; at higher temperatures, Cu₂O is stable until the liquidus is reached at 1121^deg;C. The binary eutectic is at 1045°± 5^deg;C, in which a liquid with 83% CuO coexists with Ca₃Cu₇O₁₀ and CuO.     

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.     

Feedback-Controlled Firing of Reaction-Bonded Aluminum Oxide

The reaction-bonded aluminum oxide (RBAO) process utilizes the oxidation of intensely milled aluminum/alumina powder compacts that are heat treated in air to make alumina-based ceramics. RBAO samples are typically oxidized in a furnace which is heated at 1^°C/min to 1100^°C. Heat-treating samples with a characteristic dimension >1 mm, without adjusting the furnace temperature program, usually results in a cracked ceramic. Cracking is caused by the excessive thermal and chemical stresses that result from steep temperature gradients (>30^°C/mm) and compositional gradients (>5000 mol·(m³·mm)⁻¹), which develop under the deleterious ignition and shrinking core reaction regimes. While adjustments to the furnace temperature program based on continuum models have had some success, the use of feedback-controlled firing is investigated as a means to avoid the furnace temperature program design step and to decrease the firing time. Feedback-controlled firing is shown to improve yields and significantly reduce the time required to completely oxidize the aluminum. For example, a 16 g sample with a characteristic dimension of 7.56 mm, which previously took >100 h to oxidize completely, was successfully oxidized crack free in 18.3 h using feedback control. Using the typical heat-treatment cycle, a 1 mm sample was fired in 18 h. With feedback-controlled firing, the same sized sample was fired in only 5 h.     

Formation of Grain-Boundary Carbon-Containing Phase During Annealing of YBa2Cu3O6+x

In situ annealing studies of YBa₂Cu₃O_6+x performed in an optical hot stage revealed that, at temperatures ∧450°, localized melting occurred. On subsequent cooling, a discrete second phase was observed at the YBa₂Cu₃O_6+x grain boundaries. Quantitative chemical analysis using X-ray wavelength dispersive spectroscopy indicated that the second phase was composed of a barium oxycarbonate. The source of the carbon in the second phase was identified to be CO₂ in the atmosphere.