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.     

Mechanical Properties of Magnesia-Doped Lanthanum Chromite versus Temperature

Magnesia-doped lanthanum chromite is a potential material for use in solid oxide fuel cells as an interconnector due to its resistance to oxidation and reduction. The strength and toughness of La(Cr _0.9 Mg _0.1 )O₃ were measured from 25° to 1000^deg;C in the as-fired reduced state and after oxidation. The as-fired samples showed a peak in toughness of approximately 3.9 MPa.m^1/2 at 125^deg;C which decreased to approximately 1.4 MPa.m^1/2 at 600^deg;C and 2.8 MPa.m^1/2 at room temperature. This peak in toughness is hypothesized to be due to the rhombohedral-to-orthorhombic phase transition by a toughening mechanism that is currently being investigated. The strength was also affected by the phase transition for both the as-fired reduced and the oxidized samples.     

High-Temperature Mechanical Behavior of Stoichiometric Magnesium Spinel

The elastic and mechanical behavior, from room temperature up to 1300^deg;C, of Stoichiometric polycrystalline magnesium aluminum spinel is studied. Elastic modulus, fracture toughness, and modulus of rupture measurements and observations of polished and fracture surfaces have been performed. Two well-differentiated regions of fracture behavior as a function of temperature have been found. In the low-temperature region, this material behaves elastically, whereas in the high-temperature (>800^deg;C) region, plastic phenomena take place.     

Microstructural Stability of Nicalon at 1000deg;C in Air after Exposure to Salt (NaCl) Water

When either uncoated or BN-coated Nicalon fibers are exposed to water saturated with NaCl before being annealed in air at 1000^deg;C, accelerated degradation of the structure of the fiber occurs. The fiber surface oxidizes to tridymite instead of vitreous silica, and the crystallites of SiC in Nicalon begin to grow. These findings suggest that prior exposure to salt water may cause appreciable debit in the mechanical strength of Nicalon fibers with time at 1000^deg;C in air .     

Effect of Carbon and Boron on the High-Temperature Oxidation of Silicon Carbide

Silicon carbide has good oxidation resistance, due to the formation of a protective silica layer. Although amorphous silica is an excellent oxygen barrier, it is very sensitive to impurity elements, which affect its viscosity, oxygen diffusivity, and crystallization kinetics. This paper compares the oxidation rates of CVD SiC, sintered α-SiC, and CVD SiC- coated graphite in 1 atm oxygen at 1500^deg;C to determine the effects of small additions of boron and carbon. The formation of bubbles in the silica scale formed on sintered α-SiC in oxygen between 1230° and 1550^deg;C is also discussed.     

Thermal Expansion of YBa2Cu3O7_xas Determined by High-Temperature X-ray Diffraction under Controlled Oxygen Partial Pressures

The high-temperature phase relationship and thermal expansion coefficient of YBa₂Cu₃O_7-x under constant oxygen nonstoichiometry, x, were determined by high-temperature powder X-ray diffraction analysis under controlled oxygen partial pressure at temperatures up to 800^deg;C. The results are discussed based on reported nonstoichiometry data. The present study showed an orthorhombic-to-tetragonal transition near the composition x = 0.5. The lattice parameter c, perpendicular to the Cu-O plane, showed a maximum at around x = 0.7 to 0.8. In the ortho-rhombic phase, the lattice parameters a and b along the Cu-O plane were essentially constant for x < 0.2. For 0.2 < x < ∼ 0.5, a increased and b decreased with x. In the tetragonal phase, with x < ∼ 0.5, the lattice parameter a decreased with x. The thermal expansion coefficient, α, along the c-axis ranged from 19 × 10^-6 to 25 × 10^-6-K^-1, whereas a along the a- and b-axes ranged from 12 × 10^-6 to 22 × 10^-6-K^-1 at 400° to 800^deg;C, and these values were very small below 400^deg;C. It was found that a, b, and α along the a- and b-axes are smaller when the oxygen content along the respective axes is less, while the area of the ab plane and its thermal expansion coefficient are larger when the deviation of the oxygen content from the stoichiometric compositions of YBa₂Cu₃O₇ or YBa₂Cu₃O₆ is larger. Changes of x and temperature affected c more strongly than a and b.     

Na2SO4-lnduced Corrosion of Si3N4 Coated with Chemically Vapor Deposited Ta2O5

Hot isostatically pressed Si₃N₄ was coated with chemically vapor-deposited Ta₂O₅, and subjected to oxidative and corrosive environments to determine the feasibility of using a Ta₂O₅ coating for protecting Si₃N₄ from hot corrosion. The coated structure was relatively stable at 1000^deg;C in pure O₂. However, the Ta₂O₅-Si₃N₄ system became unstable in an environment containing Na₂SO₄ and O₂ at 1000^deg;C because (1) Ta₂O₅ and Na₂SO₄ reacted rapidly to form NaTaO₃ and (2) subsequently NaTaO₃ interacted destructively with the underlying Si₃N₄ substrate to form a molten phase.     

In Situ Annealing Studies of Sol-Gel Ferroelectric Thin Films by Spectroscopic Ellipsometry

Spectroscopic ellipsometry was utilized to follow in situ the annealing of sol-gel Pb(Zr,Ti)O₃ films on sapphire and platinum-coated silicon substrates. Low-temperature processes, such as pyrolysis of organics and film densification, could be identified readily. Crystallization of the perovskite phase was initiated between 500° and 600^deg;C for the film on sapphire. This was coincident with the roughening of the film surface. Identification of higher-temperature processes in the film on platinum-coated silicon was complicated by temperature-dependent changes in the substrate. In situ annealing studies on the substrate alone confirmed that, for the lengthy annealing profiles utilized in these experiments, substantial and irreversible changes in the effective substrate dielectric function occurred at temperatures >550^deg;C. In addition, the role of extended, high-temperature annealing on the optical frequency dielectric properties of the films was investigated .     

VAMAS Round Robin on Fracture Toughness of Silicon Nitride

Eight laboratories in Germany, Japan, U.K., and U.S. participated in the VAMAS round robin. The fracture toughness of silicon nitride at room temperature and at 1200^deg;C was measured by three methods: the single-edge V-notched beam (SEVNB), single-edge precracked beam (SEPB), and chevron notched beam (CNB). The obtained values show hardly any crosshead speed dependence, irrespective of test temperature and atmosphere. Results may have been influenced by a small amount of slow crack growth, but distinct R -curve behavior could not be detected within the scope of the tests. The values at 1200^deg;C in N₂ can be measured by the SEVNB and SEPB methods with small scatters. The oxidation of silicon nitride, caused by heating in air, increases the SEVNB and SEPB values. The CNB values are free from the effects of test temperature and atmosphere, but they show a large scatter between laboratories. However, the chevron V-notched beam (CVNB) method, which is an improved CNB method, shows values with a small scatter, irrespective of the measurement conditions. The SEVNB and SEPB measurements in N₂ and the CVNB measurement under any conditions are recommended for the measurement of high-temperature fracture toughness.     

Molten Glass Corrosion Resistance of Immersed Combustion-Heating Tube Materials in E-Glass

The corrosion resistance of molybdenum, molybdenum disilicide, and a SiC_{( p )}/Al₂O₃ composite to molten E-glass at 1550^deg;C was studied. Mo showed no tendency to oxidize as it was immersed in soda-lime silicate glass in a parallel study. MoSi₂ was corroded by soluble molecular oxygen, leaving a Mo₅Si₃ interface behind. The SiC_{( p )}/Al₂O₃ composite was corroded at a more rapid rate wherein the SiC component was oxidized to form amorphous silica and CO bubbles. Based on these results, the activity of soluble molecular oxygen in E-glass was determined to be in the range of 2.4 × 10^-14 to 2.0 × 10^-8.     

Zinc Sulfide/Gallium Phosphide Composites by Chemical Vapor Transport

Chemical vapor transport was employed to prepare ZnS/ GaP composite powders. Spherical, submicrometer, nano- porous ZnS powders were used as a substrate, where GaP was deposited via a transport reaction involving iodine. XRD examinations of the powders with various GaP contents indicated that the powders consisted mainly of solid solutions with small amounts of GaP and ZnS. The powders were then densified to greater than 98% relative densities by cold isostatic pressing followed by hot isostatic pressing at 900^deg;C under 207 MPa. Vickers hardness improved from 1.38 GPa for pure ZnS to 2.82 GPa (13.7 wt% GaP) and fracture toughness increased from 0.50 MPa.m^1/2 for pure ZnS to 0.89 MPa.m^1/2 (21.0 wt% GaP).     

Homogeneous Distribution of Sintering Additives in Liquid-Phase Sintered Silicon Carbide

The addition of sintering additives to silicon carbide particles by electrostatic adsorption of colloidal A1₂O₃ and Y₂O₃ sols has been studied as a way to achieve an optimum homogeneity in the microstructure. The adsorption behavior of the sol particles was examined by electrophoretic measurements and X-ray fluorescence analysis. Both A1₂O₃ and Y₂O₃ sols could simultaneously be adsorbed on the SiC particle surfaces. Viscosity measurements showed that the colloidal sol particles had a stabilizing effect on the slip, and hence slips with relatively high solid loadings could be prepared without adding extra dispersing agent. Liquid-phase-sintered silicon carbide materials (LPS-SiC) with 2 wt% A1₂O₃ and 1 wt% Y₂O₃ were prepared by freeze granulation/ pressing and sintering at 1880^deg;C for 4 h. The homogeneity of the green compacts was quantified using a spot analysis technique in an electron probe microanalyzer. It was clearly shown that the addition of sols gave a more homogeneous microstructure than the reference sample with Y₂O₃ and A1₂O₃ added as powders. The addition of sintering additives as sols also enhanced the sintering behavior.     

Growth of CeO2 Films on Sapphire and MgO by rf Magnetron Sputtering

pitaxial CeO₂ films on (1102) sapphire and (100) MgO were grown by rf magnetron sputtering. Substrate temperature, total pressure, and oxygen-to-argon mole ratio were varied to explore the optimal deposition condition. The X-ray diffraction spectra indicate that the degree of crystallinity of the deposited CeO₂ films depends on the oxygen- to-argon mole ratio and the substrate temperature. Atomic force microscopy images of the films on sapphire and MgO showed that substrate temperature and total pressure affect surface roughness. The best film surface is smooth with a 0.89 nm root-mean-square roughness. The quality of the films on MgO showed a strong dependence on substrate pretreatments. Epitaxial CeO₂ films could be grown on pre- annealed or pre-etched MgO if substrate temperatures reached higher than 790^deg;C. Additionally, the effect of ion bombardment at low total pressures on the crystallinity of the films was examined by growing the films outside the plasma region. Experimental results indicate that the ion bombardment does not prevent the films from preferred orientation.     

Pyrolysis of Titanium-Metal-Filled Poly(siloxane) Preceramic Polymers: Effect of Atmosphere on Pyrolysis Product Chemistry

Pyrolysis of unfilled as well as Ti-metal-filled poly(siloxane) preceramic polymers, heated at 1350^deg;C in N₂, Ar, or NH₃, was evaluated in terms of the effect of atmosphere on pyrolysis product chemistry. Pyrolysis of the unfilled poly(siloxane) in N₂ or Ar resulted in the formation of a metastable, amorphous SiO_xC_y phase with a residual turbostratic carbon phase, with the evolution of CH₄ and H₂ as the main pyrolysis products. However, pyrolysis of the unfilled poly (siloxane) in NH₃ resulted in a partial substitution of nitrogen from the atmosphere for network carbon to form a binary mixture of amorphous SiO_xC_y and SiO_xN_y phases. Pyrolysis of the Ti-filled poly(siloxane) in NH₃ resulted in the reaction of the Ti particles with the atmosphere to form nearly stoichiometric TiN. In Ar, the Ti particles react with either gaseous hydrocarbon, i.e., CH₄, or the carbon pyrolysis products of the poly(siloxane), to form slightly nonstoichiometric TiC along with a partial reduction of the SiO_xC_y matrix. Finally, in N₂, the Ti particles react with both carbon from the poly(siloxane) and nitrogen from the atmosphere to form a solid solution of TiC and TiN.     

Low-Temperature Synthesis of Hexagonal Anorthite via Hydrothermal Processing

Hexagonal anorthite (CaAl₂Si₂O₈) has been prepared by hydrothermal processing of monocalcium aluminate and quartz at temperatures as low as 200°C. The successful development of this phase is dependent upon several processing parameters, including the hydration of the calcium aluminate precursor material to the hydrogarnet phase (Ca₃Al₂O₆·6H₂O) prior to hydrothermal treatment and the use of quartz as opposed to amorphous sources of SiO₂. Quartz has partial solubility in the hydrogarnet lattice for additions up to 40 wt%. Increased SiO₂ substitution has been shown to reduce the conversion of hydrogarnet to Ca₄Al₆O₁₃·3H₂O, thereby increasing its thermal stability and improving its strength characteristics at temperatures greater than 200°C. Quartz additions greater than 43 wt% lead to the formation of CaAl₂Si₂O₈ as the sole reaction product. The moderate temperatures involved in forming this anhydrous material are an order of magnitude lower than those necessary to form this phase by melt crystallization, making it a true chemically bonded ceramic. The reaction can form a bonded matrix with strengths up to 40000 psi (280 MPa). Strengths are limited due to density changes during anorthite formation, but the matrix is thermally stable up to 1000°C.     

Crystallization and Properties of a Si-Y-Al-O-N Glass-Ceramic

Glasses have been prepared in the Si -Y-Al O-N system by melting mixtures of silica, alumina, yttria, and silicon nitride. One particular glass containing 17 equiv% N has been investigated to Observe the crystallization at temperatures in the range 1050^o-1300^oC. The major crystalline phase observed is Y₂Si₂O₇, existing as the α form below 1200^oC and the β form above this temperature. Properties of the glass-ceramic, including thermal expansion coefficient, hardness, electrical resistivity, and creep have been assessed.     

Preparation and Properties of Hydrothermally Stable γ-Alumina-Based Composite Mesoporous Membranes

We report here on our thermal and hydrothermal investigations comparing mesoporous γ-Al₂O₃ membrane with single and double dopant membranes prepared by the sol–gel method. Improvements in the hydrothermal stability of mesoporous γ-Al₂O₃ by transition (Ga³⁺) or rare-earth (La³⁺) cations are discussed, along with the effectiveness of double dopant (Ga³⁺–La³⁺). The amounts of Ga₂O₃ oxide used varied between 0.0 and 30 mol% and those of La₂O₃ between 6 and 15 mol%. The thermal and hydrothermal (up to 75% steam) stability of nine types of membranes fabricated on an asymmetric porous α-Al₂O₃ support by means of a dip-coating process was characterized by H₂ gas permeation at 500°C. By conducting tests with wide variations in dopant concentrations, material characterizations, and gas permeance performance, we have been able to optimize the key parameters for hydrothermally stable systems.     

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.     

Chemical Instability of Metal-Deficient Hexagonal Ferrites with W Structure

The hexagonal ferrite W=BaFe²⁺₂Fe³⁺₁₆O₂₇, which is in equilibrium with oxygen under 0.015 atm at 1350^°C, is oxidized in the range 250^° to 1100^°C in the same atmosphere to BaFe³⁺_16+2x Fe²_2-3x> _x O₂₇ with 2–3 x <0.03. The metal-deficient W structure is chemically unstable and decomposes after longer annealing at the same temperature. This difference in oxidation and decomposition rate is attributed to a difference in the diffusion rate of barium and iron ions.     

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.