Ductile Phase Toughened Brittle Materials

Toughening of brittle materials by the inclusion of ductile phases is governed by several important factors which include ceramic-ductile phase interfacial bond strength, physical and chemical compatibility between ceramic and ductile phase, geometry and mechanical properties of ductile phase. The present understanding of the effect of these factors on toughening is reviewed and clarified. Continuous ductile phases (network, fibre or plate) are found to be more efficient for the toughening of brittle materials than discrete ductile particles. However, ductile particle toughened brittle materials have the advantages of material homogeneity isotropy and particularly better high temperature properties. It has been demonstrated that the influence of interfacial bond strength is determined by the geometry of the ductile phase in the composites. For the comparatively continuous ductile phase, such as ductile network, fibre or plate, comparatively weak inteffocial bond strength can promote partial debonding of the brittle matrix-ductile phase intedece during crack propagation and is beneficial for toughening. For discrete particulate ductile phase toughened brittle materials, the small gauge length of the ductile phase often results in the ductile phase pull-out during crack propagation which is the main limitation to toughening.Thus strong bond strength is required to ensure the bridging of the crack by the ductile phase.The coefficient of thermal expansion (CTE) mismatch between matrix and ductile phase has also been correlated with the geometry of the ductile phase. In most of the ceramic/metal systems,the CTE of the ductile metal phase is greater than that of the ceramic matrix. In the case of ductile network, fibre or plate, the residual stress created by the CTE mismatch can contribute to toughening through its influence on the initial crack opening stress while the bridging of the crack by the ductile phase is still ensured. However, for discrete ductile particles, the residual stress created by CTE mismatch is liable to cause cracks to by-pass the spherical particles, limiting the efficient use of the inherent toughness of the ductile phase. Low-modulus ductile inclusions are beneficial for the bridging of cracks by the ductile phase. Softer, more ductile inclusions are more effective for the toughening of brittle materials by particulate ductile phase     

Sensory registration and informational persistence.

The view of iconic memory as a precategorical, high-capacity, quickly decaying visible memory has recently come under attack (e.g., M. Coltheart, 1980). Distinctions have been drawn between visible persistence, or the phenomenal trace of an extinguished stimulus, and informational persistence, knowledge about the visual properties of the stimulus. Two alternative conceptions of informational persistence were tested in 3 experiments with 13 university students and the present authors. One conception is that visual information persists in a visual memory that begins at stimulus offset and lasts for 150–300 msec, independently of exposure duration. The 2nd conception is that informational persistence arises from a nonvisual memory that contains spatial coordinates for displayed items along with identity codes for those items. In the experiments, 3?×?3 letter arrays were presented for durations ranging from 50 to 500 msec. A single character mask presented at varying intervals after array offset cued report of an entire row of the array. Comparison of the cued row's masked and unmasked letters revealed that spatially specific visual (i.e., maskable) information persisted after stimulus offset, regardless of exposure duration. This result favors the visual conception of informational persistence. There was also support, however, for the nonvisual conception: Accuracy increased and item intrusion errors decreased as stimulus duration increased. Implications for models of informational persistence and for transsaccadic integration during reading are discussed. (47 ref) (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Effect of NiO on the Phase Stability and Microstructure of Yttria-Stabilized Zirconia

Nickel oxide (NiO) was screen printed onto the surfaces of 3 and 8 mol% yttria-stabilized zirconia (YSZ) dense pre-fired substrates and then heat treated at temperatures from 1350° to 1550°C. The effect of NiO was dependent on the yttria content of the substrate. In 3 mol% YSZ, it was found to alter the phase composition from predominantly tetragonal with a small amount of cubic phase to one consisting of approximately equal amounts of cubic and monoclinic phase. The cubic grains were much larger than the monoclinic ones and contained more nickel. Furthermore, nickel was observed to migrate through the thickness of the tile, a distance of approximately 200 μm. In the 8 mol% YSZ substrates, the phase composition was unaltered, although large grains developed under the printed NiO layer and the nickel migration was confined to the extent of these large grains.     

Modification of maillard browning in a microwaved glucose/glycine model system by water-soluble natural antioxidants and foods containing them

Inhibition of pyrazine formation by natural antioxidants and the foods containing them was measured in a microwaved glucose/glycine model system. Inhibition of lipid oxidation by the same materials was assayed in both bulk and emulsion systems. Pyrazines were determined by solid-phase micro extraction followed by GC. Lipid oxidation volatiles were assayed by polyamide fluorescence produced by either a bulk oil display or a hematin- or 2,2′-azobis-(2-amidino=propane) dihydrochloride-accelerated lecithin or fish oil emulsion. It was shown that (i) the inhibition of pyrazine formation depends on high concentrations of water-soluble antioxidants; (ii) such antioxidants occur naturally in some foods and are usually polyphenols; (iii) during pyrazine inhibition, oxidized polyphenols show enhanced nonfluorescing browning similar to enzymic browning products; (iv) monophenols, which structurally cannot form quinone polymers on oxidation, inhibit pyrazines with less browning; (v) during the final pyrazine-forming phase of the Maillard reaction, polyphenolics and reducing agents such as glutathione and ascorbic acid are partially consumed with some nutritional loss; (vi) fruit powders of grape seed, grape skin, and red wine are highly pyrazine-inhibitory, steeped blueberry strongly so, but plum purees are moderately pro-pyrazine, and freeze-dried vegetables strongly pro-pyrazine; and (vii) black and green tea infusions are highly inhibitory, whereas spices have mixed effects.     

Selective effects of naltrexone on food pleasantness and intake

The effects of 50 mg naltrexone on both pleasantness and intake of 10 common food items were investigated using a double-blind placebo-controlled study with 16 male volunteers. Rated food pleasantness was reduced significantly in the naltrexone condition compared with both controls (placebo and baseline). However, pleasantness ratings were not affected uniformly across foods, with sweetened, fatty, and high-protein foods being most affected. Changes in rated unpleasantness generally mirrored those for pleasantness, but evaluations of saltiness and sweetness were unaffected by naltrexone. Although total intake was reduced in the naltrexone condition, this was not significant compared with placebo. However, fat and protein intakes were significantly less following naltrexone. The effect of naltrexone on intake was also food dependent, but in this case intake of sweet foods was spared relative to other food categories. The apparent discrepancy between liking and intake data with sweet foods could be interpreted in terms of the likely influence of normal eating styles on food selection during a buffet-style meal, and may explain some contradictions in previous studies of this kind. The implications for understanding opioid involvement in food acceptability are discussed.     

Activity pulses induce spontaneous flow reversals in viscoelastic environments

Complex interactions between cellular systems and their surrounding extracellular matrices are emerging as important mechanical regulators of cell functions, such as proliferation, motility and cell death, and such cellular systems are often characterized by pulsating actomyosin activities. Here, using an active gel model, we numerically explore spontaneous flow generation by activity pulses in the presence of a viscoelastic medium. The results show that cross-talk between the activity-induced deformations of the viscoelastic surroundings and the time-dependent response of the active medium to these deformations can lead to the reversal of spontaneously generated active flows. We explain the mechanism behind this phenomenon based on the interaction between the active flow and the viscoelastic medium. We show the importance of relaxation time scales of both the polymers and the active particles and provide a phase space over which such spontaneous flow reversals can be observed. Our results suggest new experiments investigating the role of controlled pulses of activity in living systems ensnared in complex mircoenvironments.     

Precipitous weakening of quartz at the α–β phase inversion

Crystalline quartz has long been identified as among the weakest of abundant crustal minerals. This weakness is particularly evident around the α–β phase inversion at 573°C, in which Si–O bonds undergo a displacive structural transformation from trigonal to hexagonal symmetry. Here we present data using indentation testing methodologies that highlight the precipitous extent of the transformational weakening. Although the indentations are localized over relatively small specimen contact areas, the data quantify the essential deformation and fracture properties of quartz in a predominantly (but not exclusively) compressive stress field, at temperatures and pressures pertinent to conditions in the earth's crust.     

Microstructure and fracture toughness of nickel particle toughened alumina matrix composites

Al₂O₃-Ni composite materials have been made by a hot pressing technique. Two composite microstructures, i.e. a dispersive distribution of nickel particles and a network distribution of nickel particles in an alumina matrix, have been produced. The fracture toughness of the composite materials has been measured by a double cantilever beam method. Both composites are tougher than the virgin alumina matrix. The fracture toughness of the composite with a network microstructure is much higher and has a more desirable R-curve behaviour than the composite with a microstructure of dispersed particles. For the particulate dispersion microstructure, the main limitation to toughening is the lack of plastic deformation of the ductile nickel due to the pull out of nickel particles, indicating weak bonding at the Al₂O₃/Ni interface. For the network microstructure composite, the gauge length of the ductile phase is much larger, allowing the ductile nickel to stretch to failure between the crack faces. A large extent of nickel plastic deformation has been observed, and the weak bonding at the Al₂O₃/Ni interface can promote partial debonding and contribute further to toughening.     

The identification of radiolytic decomposition products from tributyrin

In order to facilitate the detection of radiolytic products of triglycerides which may be of higher molecular weight than their precursor, a low molecular weight triglyceride, tributyrin, was selected as a model system, and gel permeation chromatography was used to effect their separation. The irradiation treatment was conducted under vacuum at 50 Mrad. Radiolytic products were collected by a precolumn technique for the highly volatile compounds; a combination of cold finger distillation and gel permeation for the less volatile fractions and finally gel permeation chromatography of the residue after distillation for the higher molecular weight compounds. A large number of compounds expected on the basis of previous work were identified in the present work from irradiated tributyrin. In addition, the techniques employed permitted the identification of several new compounds. These include butanetriol triesters, erythritol tetraesters, and other polyglycol polyesters.