Interaction of BaCl2 and BaCO3 at Elevated Temperatures

The effect of adding BaCl₂ to BaCO₃ at elevated temperatures was studied. DTA and hot-stage microscopy revealed that a eutectic occurs at 840°C and 14.0 mol% BaCO₃. The microstructure in the reaction zone indicated that BaCO₃ recrystallizes from the eutectic melt. Thus, BaCl₂ additions can activate BaCO₃ at a temperature as low as 840°C .     

Defect Structure of TiTa2O7 at Elevated Temperatures

The electrical conductivity of polycrystalline TiTa₂O₇ was measured at 1000° and 1050°C as a function of oxygen partial pressure from 10⁻¹ to 10⁻²¹ MPa. In the apparent n-type region, the value of m in σ_n∝PO₂^−1/m was found to be ∼6 in the region >10⁻¹⁷ MPa and ∼4from 10⁻¹⁶ to 10⁻⁹ MPa. The conductivity appeared to be p-type for P₀₂<10⁻⁵ MPa. The measured data are explained on the basis of the presence of small amounts of acceptor impurities in the undoped sample.     

Fracture Mechanisms of a Coarse-Grained, Transparent MgAl2O4 at Elevated Temperatures

The fracture of a transparent, large-grain-sized MgAl₂O₄ spinel has been studied through temperatures of 1400°C. Fracture toughness values, falling between about 1.3 and 2.3 MPa.m^12;1/2, behaved sigmoidally with temperature, with a lower shelf transition appearing near 800°C. Rising R -curves, displaying a run-arrest character, were found at both the lowest and the highest temperatures, while only minimal values of d K _R/dΔ a were produced at intermediate temperatures of 800°C. This fracture character is ascribed largely to finite concentrations of a residual LiF pressing aid identified on the fracture surface, while additional influences associated with the lower shelf region at the highest temperatures may include the increased fraction of intergranular crack path and the onset of plasticity. This cubic monolithic ceramic displayed strong nonlinear fracture behavior in both temperature regimes. In both cases the fracture character is linked directly to an active toughening mechanism in the wake region, which depends upon crack face bridging.     

Fracture Resistance of SiC-Fiber-Reinforced Si3N4 Composites at Ambient and Elevated Temperatures

The crack growth behavior in unidirectional SiC-fiber-rein-forced Si₃N₄-matrix composites fabricated in our laboratories was investigated as a function of fiber volume fraction and temperature. Both the stress-intensity factor and an energy approach were adopted in the characterization of the crack growth behavior. Crack resistance increased with crack extension ( R -curve or T -curve) as a result of bridging effects associated with the intact fibers. Large-scale bridging was observed, and was considered in the determination of the R -curves. Temperature and fiber volume fraction affected the crack propagation behavior. At room temperature a single crack was initiated at the notch tip; it then branched and delaminated upon further loading. In contrast, at 1200°C, little crack branching was observed. Increasing fiber volume fraction increased the degree of crack branching. Temperature and fiber volume fraction also affected the R -curve behavior. Raising the temperature to 1200°C did not significantly degrade the room-temperature R -curve effect. Increasing the fiber volume fraction from 14% to 29% substantially enhanced the toughening effect and the R -curve behavior.     

Fracture Toughness of Fused SiO2 and Float Glass at Elevated Temperatures

The fracture toughnesses of fused SiO₂ and float glass were measured at high temperatures. In both glasses, low-temperature regions of elastic fracture were identified and correlated with the elastic moduli and their temperature dependence. A viscous flow contribution to the fracture toughness was identified in the fused SiO₂ at T >800°C. Similar indications of viscous flow were also noted in the float glass, although at much lower temperatures.     

Cyclic Fatigue-Crack Growth and Fracture Properties in Ti3SiC2 Ceramics at Elevated Temperatures

The cyclic fatigue and fracture toughness behavior of reactive hot-pressed Ti₃SiC₂ ceramics was examined at temperatures from ambient to 1200°C with the objective of characterizing the high-temperature mechanisms controlling crack growth. Comparisons were made of two monolithic Ti₃SiC₂ materials with fine- (3–10 μm) and coarse-grained (70–300 μm) microstructures. Results indicate that fracture toughness values, derived from rising resistance-curve behavior, were significantly higher in the coarser-grained microstructure at both low and high temperatures; comparative behavior was seen under cyclic fatigue loading. In each microstructure, Δ K _th fatigue thresholds were found to be essentially unchanged between 25° and 1100°C; however, there was a sharp decrease in Δ K _th at 1200°C (above the plastic-to-brittle transition temperature), where significant high-temperature deformation and damage are first apparent. The substantially higher cyclic-crack growth resistance of the coarse-grained Ti₃SiC₂ microstructure was associated with extensive crack bridging behind the crack tip and a consequent tortuous crack path. The crack-tip shielding was found to result from both the bridging of entire grains and from deformation kinking and bridging of microlamellae within grains, the latter forming by delamination along the basal planes.     

Deformation of UO2 at High Temperatures

The effects of temperature, strain rate, and grain size on the mechanical properties of UO₂ were investigated using the four-point bending technique. Strain rates were varied by two orders of magnitude, and test temperatures up to 1800°C were used. Data are presented on the ultimate tensile stress, yield stress, and plastic strain-to-fracture. Below the brittle-to-ductile transition temperature, T_c , the material fractured in a brittle manner, with no macroscopic plastic deformation. Between T_c and a second transition at a higher temperature, T_t , a small amount of plastic deformation was measured before fracture. Beyond T_t , the strength of UO₂ decreased continuously, and extensive plasticity was observed. This high-temperature plasticity was characterized by a thermally activated rate-controlling process; this behavior is consistent with observations of creep behavior under high stresses. The following phenomenological equations for the strain rate fit the data for the material with 8-μm grain size above T_t :
and
where σ_p and σ_88f are the proportional limit and steady-state flow stress, respectively, and temperature T is in °K.     

Tribological Behavior of Ti2SC at Ambient and Elevated Temperatures

The tribological properties of Ti₂SC were investigated at ambient temperatures and 550°C against Ni-based superalloys Inconel 718 (Inc718) and alumina (Al₂O₃) counterparts. The tests were performed using a tab-on-disk method at 1 m/s and 3N (≈0.08 MPa). At room temperature, against the superalloy, the coefficient of friction, μ, was ∼0.6, and at ∼8 × 10⁻⁴ mm³·(N·m)⁻¹ the specific wear rate (SWRs), was high. However, against Al₂O₃, at ∼5 × 10⁻⁵ mm³·(N·m)⁻¹ and ∼0.3, the SWRs and μ were significantly lower, which was presumably related to more intensive tribo-oxidation at the contact points. At 550°C, the Ti₂SC/Inc718 and Al₂O₃ tribocouples demonstrated comparable μ's of ∼0.35–0.5 and SWRs of ∼7–8 × 10⁻⁵ mm³·(N·m)⁻¹. At 550°C, all tribosurfaces were covered by X-ray amorphous oxide tribofilms. At present, Ti₂SC is the only member of a family of the layered ternary carbides and nitrides (MAX phases) that can be used as a tribo-partner against Al₂O₃ in the wide temperature range from ambient to 550°C.     

Sublimation of Cr2O3 at High Temperatures

The sublimation of chromic oxide, Cr₂O₃, has been observed in vacuum by the Langmuir technique using induction and solar heating. Extensive sublimation did not yield any new phases on the basis of X-ray powder studies, and condensates of Cr₂O₃ were always obtained. Flash vaporization and flow experiments in CO or O₂ atmospheres and in vacuum indicated no appreciable differences in rates of sublimation. Weight-loss experiments showed that the rate of sublimation was slightly higher than predicted for decomposition to the elements and suggested that small amounts of complex molecules, e.g. CrO and CrO₂, were also present in the equilibrium vapor.     

Failure Modes of SiC-Fiber/Si3N4-Matrix Composites at Elevated Temperatures

An investigation of composite failure modes as a function of temperature and fiber-volume fraction was carried out in SiC-fiber-reinforced Si₃N₄-matrix composites fabricated in our laboratories. Mechanical testing was carried out at temperatures from 25° to 1500°C. Matrix-cracking stress and ultimate strength of the composites were measured from load-displacement curves. They were both found to decrease with increasing temperature, but their temperature sensitivity decreased with increasing fiber-volume fraction. The tendency for noncatastrophic failure increased with fiber-volume fraction, while the tendency for catastrophic failure increased with temperature. The failure mode was demonstrated experimentally to be determined by the fiber bundle strength, S_fb, and the matrix cracking stress, σ_c. These two parameters, in turn, were shown to be controlled by the fiber-volume fraction, f , and the temperature. Failure at various temperatures was noncatastrophic when S_fb > σ_c, and catastrophic when S_fb < σ_c. Transition in composite failure mode between noncatastrophic and catastrophic failure was controlled via the variation of fibervolume fraction and testing temperature. Catastrophic failure at high temperatures was found to be mainly a result of fiber strength loss.     

Oxidation and Corrosion of SiC(particulate)/Al/Al2O3 Composites in Sodium Silicate at Elevated Temperatures

Ceramic-matrix composites (CMCs) fabricated by the directed metal oxidation process (Dimox^TM) may have applications in heat exchangers in high-temperature corrosive environments such as those in the glass industry. The oxidation and corrosion properties of such CMCs with and without preformed metal-free surface layers have been investigated in the temperature range of 1000–1300°C. The untreated CMCs experienced rapid oxidation in air leading to mass increases of 100 to 140 mg/cm² in less than 1 h. This occurred by oxidation of residual metal in the composite to form Al/Al₂O₃ deposits on the surface. After the initial formation of the oxidation product, there is little further reaction during up to 300-h exposures to oxidizing atmospheres. Experimental composite coupons with metal-free surfaces were resistant to oxidation except for localized events associated with flaws. Small amounts of sodium silicate (2 to 40 mg/cm²) painted on the surfaces produced no corrosive effects on any of the specimens. Dynamic corrosion experiments, in which a continuous mist of sodium silicate was sprayed onto the surfaces, produced corrosion at 1300°C.     

Microwave Dielectric Properties of Ti-Substituted Bi2 (Zn2/3Nb4/3)O7 Pyrochlores at Cryogenic Temperatures

Monoclinic pyrochlore ceramic Bi₂Zn_{2/3− x /3}Nb_{4/3−2 x /3}Ti _x O₇ (M–BZN) with x =0–0.4 is synthesized and the structure and microwave cryogenic properties are scrutinized. The dielectric constant (ɛ') and loss tangent (tanδ) of these ceramics are measured at a frequency of 3 GHz and temperature range of 15–300 K. With an increase in x value from 0 to 0.4, the dielectric constant and dielectric loss tangent of the investigated materials increase from 70 to 114 and 0.009 to 0.061, respectively. The Ti-substituted ceramics show an increase in dielectric constant with temperature, and the loss tangent shows a peak around 200 K. The peak in the dielectric loss tangent becomes more prominent with an increase of Ti content. The temperature where the dielectric loss tangent peak appears is found to be decreasing slightly with an increase of titanium doping. The observed dielectric characteristics of the titanium-doped M–BZN ceramics are attributed to the presence of the relaxation in these materials, originating from the disorder caused by the Ti⁴⁺ substitution.     

The System AI2SiO5 at High Temperatures and Pressures

Experiments on the system Al₂SiO₅ at high temperatures and pressures with the belt apparatus indicate that kyanite melts incongruently above about 1500°C at 25,000 bars to Al₂O₃ (corundum) plus liquid. The pressure-temperature curves obtained by starting with either a 1/1 Al₂O₃/SiO₂ gel or with kyanite are essentially identical but differ considerably from the results with andalu-site and sillimanite. The structure of the starting material has considerable influence on the kinetics of the reaction and the metastable formation of corundum in this system. An "equilibrium" curve based on the andalusite-sillimanite data is described by P = 33.8 × 10-³T - 26.4 (Pin kbars, Tin°C).     

Defect Equilibria and Transport in YBa2Cu3O7-x at Elevated Temperatures: II, Nonstoichiometry

Aspects of nonstoichiometry for the Y-Ba-Cu (1: 2: 3) system are considered. The general formula YBa₂Cu₃O_7-x has been assumed for considerations of nonstoichiometry in 1: 2:3 oxide cuprates. Assuming that copper ions may occupy different lattice positions (independently of their valency), the equilibrium constants for oxygen intercalation were determined:     

Stability of Bi2Sr2Ca1Cu2O8 ±δ Thick Films at Elevated Oxygen Pressures and Temperatures

Bi₂Sr₂Ca₂Cu₂O_8±δ-type compound thick films were exposed to oxygen-argon-gas mixtures (1% to 20% oxygen gas) at elevated pressures (up to 207 MPa) and temperatures (500° to 940°C) for times ranging from 5 to 96 h. At a sufficiently high oxygen fugacity and temperature, Bi₂Sr₂Ca₁Cu₂O_8±δ decomposed via a solid-state reaction. Room-temperature X-ray diffractometry and electron probe microanalysis of decomposed films revealed the presence of Bi₂(Sr,Ca)₂-Cu₁O_6±θ ro-type compound, Bi₂Sr₂,Ca₁O_8±δ-type compound, and CuO. Bi₂Sr₂Ca₁Cu₂O_8±δ decomposition was accompanied by a modest weight gain, which was consistent with an oxidation reaction. The solid-state decomposition reaction could be reversed by heat treatment of decomposed films at 860°C in pure, flowing oxygen at ambient pressure.     

Effect of Oxidation on Mechanical Properties of Fibrous Monolith Si3N4/BN at Elevated Temperatures in Air

The oxidation behavior and its effect on the mechanical properties of fibrous monolith Si₃N₄/BN after exposure to air at temperatures ranging from 1000° to 1400°C for up to 20 h were investigated. After exposure at 1000°C, only the BN cell boundary was oxidized, forming a B₂O₃ liquid phase. With increasing exposure temperature, the Si₃N₄ cells began to oxidize, forming crystalline Y₂Si₂O₇, SiO₂, and silicate glass. However, in this case, a weight loss was observed due to extensive vaporization of the B₂O₃ liquid. After exposure at 1400°C, large Y₂Si₂O₇ crystals with a glassy phase formed near the BN cell boundaries. The oxidation behavior significantly affected the mechanical properties of the fibrous monolith. The flexural strength and work-of-fracture decreased with increasing exposure temperature, while the noncatastrophic failure was maintained.     

Spheroidization of Tubular Voids in Al2 O3 Crystals at High Temperatures

Cracks introduced into single-crystal sapphire broke up after annealing, first into channels of cylindrical voids and ultimately into rows of spherical pores, with the thicker gap spacings in the original crack remaining open. Breakup of the cylindrical voids on subsequent annealing conformed to the models for surface-diffusion-controlled material transport. At the temperatures of measurement, the magnitudes of the calculated surface diffusivities agree well with values reported previously.     

On the Formation of Fe2+ in the System MgO-Fe2O3-MgFe2O4 at High Temperatures

The formation of ferrous iron in the system MgO-Fe₂O₃MgFe₂O₄ is of interest in connection with its deleterious effect on the microwave performance of magnesium ferrite, MgFe₂O₄. The partial reduction of ferric iron in the system at relatively low temperatures is discussed in terms of the preferential diffusion of iron, and its partial stabilization as ferrous iron from lattice energy considerations.     

Equilibria of SiF4 with SiO2, Be2SiO4, and BeO in Molten Li2BeF4

Equilibrium partial pressures of SiF₄ were measured for the reactions 2SiO₂( c )+2BeF₂( d )⇋SiF₄( g )+Be₂SiO₄( c ) (log P _siF4(mm) = [8.790 - 7620/ T ] ±0.06(500°–640°C)) and Be₂SiO₄( c ) +2BeF₂( d )⇋SiF₄( g ) +4BeO( c )(log P _siF4(mm) = [9.530–9400/T] ±0.04 (700°–780°C)), wherein BeF₂ was present in solution with LiF as molten Li₂BeF₄. The solubility of SiF₄ was low (∼0.04 mol kg^-1 atm^-1) in the melt. The results for the first equilibrium were combined with available thermochemical data to calculate improved Δ H_f and Δ G_f values for phenacite (–497.57 ±2.2 and –470.22±2.2 kcal, respectively, at 298°K). The few measurements above 700°C for the second equilibrium are consistent with the temperature of the subsolidus decomposition of phenacite to BeO and SiO₂ and with the heat of this decomposition as determined by Holm and Kleppa. Below 700°C, the pressures of SiF₄ generated showed an increasing positive deviation from the expression given for the equilibrium involving Be₂SiO₄ and BeO. This deviation might have been caused by the formation of an unidentified phase below 700°C which replaced the BeO; it more likely resulted from a metastable equilibrium involving BeO and SiO₂.     

Phase Equilibria at Liquidus Temperatures in the System MgO-FeO-Fe2O3-SiO2

Liquidus temperatures are presented for mixtures in the system MgO-FeO-Fe₂O₃-SiO₂. The standard quenching technique was modified for work under controlled atmospheres of varying O₂ pressures. Data were obtained for the temperature range 1159° to 1775°C., and with O₂ pressures ranging from 1 to 10^-8.⁹ atm. Approximate compositions of crystalline phases were determined, and paths of equilibrium crystallization were derived for selected mixtures under idealized conditions. Application of the phase diagrams to steel-plant refractories problems is indicated.