Hollandite, [BaxCSy][Ti, Al)3+2x+y Ti4+8−2x+y]O16, has been subjected to aqueous chemical attack at 75 and 150 °C. The resultant surface alterations have been monitored by solution analysis, SEM/TEM investigations, and surface analysis. The evolution of the solid/solution system can be described by the following mechanisms: There is a near-instantaneous release of Cs and Ba from reactive surface sites, followed by relatively rapid dissolution of the first few monolayers. The release of Al drives the pH to low values as a result of Al-OH hydroxylation and precipitation of AIO(OH) and/or AI(OH)3 species. The rate of dissolution is reduced by an order of magnitude or more by the formation of a thin and continuous Al(Ti)-rich surface layer. The layer remains stable and protective even under extreme hydrothermal conditions. This description has implications for the use of hollandite as a ceramic phase for disposal of high level nuclear wastes.

Raman microprobe spectroscopy was employed to measure stresses in ceria partially stabilized zirconia/alumina (Ce-TZP/Al2O3) composites toughened by transformation zone shielding. The near crack-tip stresses in in situ loaded compact specimens were measured by measuring stress-induced shifts in the frequency of a Raman peak corresponding to the tetragonal zirconia phase. The peak shift as a function of the applied stress was separately calibrated using a ball-on-ring flexure test. Both analytical function-fitting methods and numerical methods that determine centroids were evaluated for measuring the shift in the frequency with applied stress. A method based on locating the centroid of the intensity peak with a moving range of frequencies was selected because it gave the best correlation between the frequency shift and the applied stress. The stresses measured within an inner core of the transformation zone (∼30 μm), where the volume fraction of the transformed monoclinic phase was essentially constant, were used to estimate a local crack-tip stress intensity (Kl). The amount of total crack shielding (ΔKs) was then estimated from the applied stress intensity (Ka) and the estimated local crack-tip stress intensity. Comparisons with the predictions of two different theories of zone shielding indicated that transformation toughening models can account for 80 to 89% of the measured crack shielding, depending upon the value of the crack-tip stress intensity assumed in the models.

Refined mode I and mode II stress intensity factors for short cracks perpendicular to a free surface driven by a Hertzian indenter have been calculated for the case where the sphere and substrate are made of the same material. Computer simulations of Hertzian indentation tests for a variety of random surface flaw distributions show that the ring crack is expected to form outside the contact radius in all cases and that for spheres of small radii, the fracture load is expected to be proportional to the sphere radius (Auerbach's law). Reconstruction of the surface flaw distribution using the “searched area” concept is described. A useful analytical approximation for the stress intensity factors is also presented.

Cobalt phthalocyanine (CoPc) adsorbed on a carbon black support (Vulcan XC-72) and pyrolyzed at various temperatures is a potential catalyst for the reduction of oxygen in solid polymer electrolyte fuel cells. This paper reports the results of the microstructural characterization of β-Co particles that are formed after pyrolysis at temperatures of 700, 900, and 1050 °C. Transmission electron microscopy (TEM) indicated that (i) for a pyrolysis temperature of 700 °C, the size distribution of the Co particles is bell-shaped with an average value of 4 nm and mean deviation of 1 nm; (ii) for a pyrolysis temperature of 900 °C, the Co particle size distribution skews toward larger particle sizes. The most probable particle size is about 6 nm, and the average particle size is 13 nm. By comparison with the TEM results, the particle size estimated from a spectroscopic method like x-ray absorption is underestimated, while from x-ray diffraction is overestimated. The TEM images show that Co particles act as heterogeneous nucleation sites for the graphitization of amorphous carbon. It is shown that (i), at least for pyrolysis temperature of 900 °C and above, most of the β-Co particles are surrounded by a shell of graphitic carbon layers that appears to protect the particles from corrosion in acidic media; (ii) for pyrolysis temperature of 1050 °C, graphite strings also appear throughout the amorphous carbon support in areas where Co particles are not detected. This behavior was not observed after pyrolysis of as-received carbon support at 1050 °C. These results allow for a better understanding of the behavior of the pyrolyzed catalysts immersed in an acidic solution or in a solid polymer fuel cell.

This is a study of the form and structure of particles and particle aggregates of an experimental high surface area carbon black (HSA) using small-angle neutron scattering and the method of contrast variation. Contrast variation was effected by studying suspensions of the carbon black in cyclohexane containing different fractions of deuterocyclohexane. We find that the approximately 29 nm diameter USA particles are arranged as small, linear aggregates with an average aggregation number between 4 and 6. The structure averaged over the particle population is best represented by a prolate ellipsoid of revolution with semiaxes 14.5 and 76.4 nm. The surface of the aggregates appears smooth over length scales larger than 1 nm, which places an upper limit on the surface roughness observed by other methods. The internal structure of the aggregates is described by a shell-core model, with the shell density being consistent with a graphitic structure and the core being of lower density, more like amorphous carbon. Some fraction of the core volume (0.1 to 0.2) is taken up by voids that are not accessible to the solvent. An estimate of the shell thickness gives 1 to 2 nm along the ellipsoid minor axis and 6 to 10 nm along the major axis. The particles of the aggregate appear to be fused so that the less dense amorphous core is continuous through the inner parts of the aggregate. The information that can be obtained on the internal structure using contrast variation is limited by heterogeneity in the chemical composition of carbon black aggregates.