Dielectric loss caused by oxygen vacancies in titania ceramics

Undoped TiO₂ exhibited deterioration in microwave (MW) dielectric loss as it reached its maximum density due to the reduction of Ti⁴⁺ to Ti³⁺ causing oxygen vacancies at high sintering temperatures. By adding small amounts of acceptor dopants with ionic radii between 0.5 and 0.95 Å, reduction during sintering was suppressed. The upper limit of ionic radius was discrete with almost no observed effect using dopants >0.96 Å ionic radius. In addition, the microwave dielectric loss of undoped TiO₂ could be improved by annealing at 1500 °C for 10 h in air, presumably as a result of re-oxidation. High loss samples exhibited a dark ‘core’ to the naked eye which was absent in low loss ceramics. Transmission electron microscopy revealed that grains in the dark core contained planar defects attributed to the condensation of O vacancies onto specific crystallographic planes, in a manner similar to that observed in Magnelli phases.     

Chemical Bonding and Physical Interaction by Attached Chains at the Fiber-Matrix Interface

Molecular chains with different chemical structures were attached to the surface of aramid engineering fiber, and their effect on the fiber's adhesion to epoxy matrix was measured. Adhesive performance increases up to 65% were achieved, depending on the structure of the attached chains. Increases were attributed to chemical bonding between the terminal reactive group of the chain and the epoxide molecule used in the matrix, and to a length-related physical interaction between the chain and epoxy matrix.     

Use of the Blister Test to Study the Adhesion of Brittle Materials. Part II. Application

Using a modified form of the blister test, where the adhesive layer was between the substrate and a massive base, instead of as a continuous sheet on top of the substrate, we determined the interfacial fracture energy F for a series of interfaces where a brittle material (ice) was adhering to various substrates. Fracture energies obtained were compared with work of adhesion values measured for water on the same substrates. Fracture energy, which contains within it both a reversible contribution due to intermolecular interactions across the interface (work of adhesion) and an irreversible contribution due to collective dissipative processes, was found to rise rapidly with modest increases in work of adhesion. The observed relation suggests that the irreversible contribution to fracture energy is influenced strongly by the intermolecular interactions at the interface.     

Relation between work of adhesion and work of fracture for simple interfaces

A study was conducted of the relation between work of adhesion and work of fracture for adhesive-substrate systems exhibiting fracture at the interface. Materials and test conditions were selected to eliminate contributions from irreversible, energy-consuming processes in the bulk of the adhesive or substrate. Values for W _a were determined from contact angle measurements made at room temperature with the adhesive in the liquid state; values for F were determined from inverted blister tests conducted at temperatures low enough for the adhesive to be in the solid state. The independent variable, W _a, ranged from about 40 mJ/m² to 144 mJ/m². The dependent variable, F, was found to range from 0.12 J/m² to 20 J/m², with most under 5 J/m². The excess of F over W _a was found to increase exponentially with W _a, and was proof of the occurrence of irreversible processes in specimens as they were loaded and fractured. The exponential behavior of (F – W _a) with W _a suggested that the irreversible process was orientation hardening. The absence of detectable permanent deformation of any kind in the bulk substrates or at the fracture surfaces, plus the incapability of the adhesives to sustain significant irreversible processes, led to the conclusion that the orientation hardening must have taken place in the substrate, within a thin layer (and small volume) adjacent to the interfacial plane.     

Use of the Blister Test to Study the Adhesion of Brittle Materials. Part I. Test Modification and Validation

In the traditional configuration of the blister test, where the adhesive sheet is mounted on top of the substrate of interest, a brittle or fragile adhesive exhibits early cohesive failure during pressurization, preventing the measure of the interfacial fracture energy F. To permit the determination of this important materials system parameter in ice-substrate systems, we modified the blister test to an inverted configuration, where the substrate is the continuous top sheet, and the brittle adhesive (ice) is in the form of a thin interlayer under the substrate, but mounted on a massive, inert base. Several aspects of the modified test were examined, and the test was found to be valid within the range evaluated.     

Kinetic and microscopic studies of reductive transformations of organic contaminants on goethite

Reactions mediated by iron mineral surfaces play an important role in the fate of organic contaminants in both natural and engineered systems. As such reactions proceed, the size, morphology, and even the phase of iron oxide minerals can change, leading to altered reactivity. The reductive degradation of 4-chloronitrobenzene and trichloronitromethane by Fe(II) associated with goethite (alpha-FeOOH) was examined by performing sequential-spike batch experiments. The particle size and size distribution of the pre- and postreaction particles were quantified using transmission electron microscopy (TEM). Results demonstrate that the degradation reactions result in goethite growth in the c-direction. Furthermore, pseudo-first-order reaction rate constants for the degradation of 4-chloronitrobenzene and trichloronitromethane and for the loss of aqueous Fe(II) decrease dramatically with each subsequent injection of organic compound and Fe(II). This result indicates that the newly formed material, which TEM and X-ray diffraction results confirm is goethite, is progressively less reactive than the original goethite. These results represent an important step toward elucidating the link between mineral surface changes and the evolving kinetics of contaminant degradation at the mineral-water interface.