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).     

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.     

Stability of SrFeO3-Based Materials in H2O/CO2-Containing Atmospheres at High Temperatures and Pressures

The chemical stability of SrFeO₃-based perovskites in H₂O- and CO₂-containing atmospheres at high temperatures and pressures has been examined. The extent of reaction as a function of p CO₂, p H₂O, temperature, and time has been determined. Either strontium carbonate or Sr(OH)₂·H₂O was observed on sample surfaces after exposure. Observation of two different reaction-rate behaviors could be explained by the formation of different products. The stability of the perovskite has been found to increase when the activity of Sr is decreased. Chemical stability in H₂O/CO₂ is important to understand in order to use these membrane materials for syngas production.     

Effect of Oxygen Contamination on Densification of TiB2

In order to identify the mechanisms behind the adverse effect of oxygen contamination on the densification of TiB₂ ceramics, densification and grain growth behavior during pressureless sintering (1700° to 2050°C) were examined as a function of oxygen content of the powder and compared with those observed during hot-pressing (1400° to 1700°C). Marked difference in the grain size-density relations was observed between the two processing methods. Specific roles of oxygen in various kinetic processes were analyzed and the likely oxygen-bearing species were also identified.     

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.     

Phase Equilibrium in the System MgO-H2O at High Temperatures and Very High Pressures

Phase equilibrium in the system MgO-H₂O was investigated from 500° to 1450° C at 5 to 40 kbars by a quenching technique using a piston-cylinder apparatus. Brucite decomposed to periclase plus vapor at 810° and 10 kbars, 945° and 20 kbars, 1000° and 32 kbars, and 975°C and 40 kbars. The slope of the decomposition P-T curve became steeper with increasing pressure, infinite at 32^- kbars and 1000C, and then negative above 32 kbars. The extrapolation of this curve to lower pressures agrees well with data of other workers who used hydrothermal techniques.     

Formation of Nanometric TiB2 from TiO2

Titanium diboride can be produced by ball-milling a mixture of TiO₂, B₂O₃, and Mg metal for between 10 and 15 h. The reaction was found to be completed during the milling with no evidence of residual Mg. The unwanted phase, MgO, was readily removed by leaching in acid. The leached powder obtained after 15 h milling had a particle size of <200 nm and was highly faceted. The particle size decreased to ∼50 nm after 100 h milling and seemed to be relatively monodisperse. Scherrer calculation of the crystallite size showed that the product particles were probably single crystal.     

Effect of Microstructure on the Properties of TiB2 Ceramics

Samples of a TiB₂ ceramic containing 0 to 10 wt% Ni were fabricated by hot-pressing. Several properties, including fracture strength, indentation fracture toughness, and thermal expansion (between 25° and 1000°C) were measured. Resulting data were correlated with sample microstructure and composition.     

Wear Mechanisms of TiB2 and TiB2–TiSi2 at Fretting Contacts with Steel and WC–6 wt% Co

Unlubricated fretting wear tests on TiB₂ and TiB₂–5 wt% TiSi₂ ceramics against two different mating materials (bearing grade steel and WC–6 wt% Co balls) were performed with a view to understand the counterbody-dependent difference in friction and wear properties. The fretting experiments were conducted systematically by varying load (2–10 N) at an oscillating frequency of 4 Hz and 100 μm linear stroke, for a duration of 100,000 cycles. Adhesion, abrasion, and three-body wear have been observed as mechanisms of material damage for both the TiB₂/steel and TiB₂/WC–Co tribosystems. The third body is predominantly characterized as tribochemical layer for TiB₂/steel and loose wear debris particles for TiB₂/WC–Co tribocouple. An explanation on differences in tribological properties has been provided in reference to the counterbody material as well as microstructure and mechanical properties of flat materials.     

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.     

Mechanical Properties of Hot-Pressed TiB2-ZrO2 Composites

The use of monoclinic ZrO₂ as an additive improves the mechanical properties of TiB₂-based composites without the use of stabilizers. In particular, TiB₂-30% ZrO₂ compacts exhibited a transverse rupture strength of 800 MN/m², few pores, and a K_{I c} of 5 MPa·m^1/2. The high strength and toughness are thought to result mainly from the presence of partially stabilized tetragonal ZrO₂ and from solid solution of (TiZr)B₂ formed in sintering.     

Synthesis of TiB2 by High-Dose Implantation of Boron Ions in Titanium Films

Boron ions were implanted at room temperature in Ti films at a high dose (7.1 × 10^I7 and 2.3 × 10¹⁸ ions/cm²), The formation of TiB₂ films was confirmed by X-ray diffraction. Boron concentration profiles in implanted films were studied by secondary-ion mass spectrometry.     

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.     

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.     

Effect of SiC on Interfacial Reaction and Sintering Mechanism of TiB2

Compacts of TiB₂ with densities approaching 100% are difficult to obtain using pressureless sintering. The addition of SiC was very effective in improving the sinterability of TiB₂. The oxygen content of the raw TiB₂ powder used in this research was 1.5 wt%. X-ray photoelectron spectroscopy showed that the powder surface consisted mainly of TiO₂ and B₂O₃. Using vacuum sintering at 1700°C under 13–0.013 Pa, TiB₂ samples containing 2.5 wt% SiC achieved 96% of their theoretical density, and a density of 99% was achieved by HIPing. TEM observations revealed that SiC reacts to form an amorphous phase. TEM-EELS analysis indicated that the amorphous phase includes Si, O, and Ti, and X-ray diffraction showed the reaction to be TiO₂+ SiC → SiO₂+ TiC. Therefore, the improved sinterability of TiB₂ resulted from the SiO₂ liquid phase that was formed during sintering when the raw TiB₂ powder had 1.5 wt% oxygen.     

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 .     

Interaction of BaCl2 and BaSO4 at Elevated Temperatures

The effect of adding BaCl₂ to BaSO₄ at elevated temperatures was studied. Differential thermal analysis and hot-stage microscopy revealed that a eutectic occurs at 865°C between BaCl₂ and 13.5 mol% BaSO₄. The microstructure in the reaction zone indicated that BaS04 recrystallizes from the eutectic melt. Thus, BaCl₂ additions can activate BaSO₄ at a temperature as low as 865°C.