Pore shape effects during consolidation processing

Recent experimental studies [e.g. Y. Liu et al., Acta metall. mater.42(7), 2247 (1994)] have shown that final stage densification of alloy powders and metal-coated fibers occurs by the creep collapse of cusp-shaped voids at rates significantly higher than those predicted by the conventional spherical pore analyses of current consolidation models. In this paper, a strain rate potential is developed to describe the densification of a power-law creeping material containing aligned cylindrical pores (with axes perpendicular to the plane of deformation). The pores' cross-sections are randomly oriented (cusp-shaped) hypotrochoids, so the material is isotropic in the plane of deformation. The approach involves the computation of the incremental change in the potential energy of a linear creeping body due to an incremental increase in the porosity, the application of a differential self-consistent scheme to obtain a potential for a linear body with an arbitrary pore concentration and the use of a bounding principle to produce an estimate of the porous materials power-law creep potential. The results show that the presence of the cusps increases the predicted densification rates by a (sometimes large) factor that depends upon the cusp geometry, the applied load asymmetry and materials nonlinearity (i.e. its creep exponent).     

Constitutive potentials for dilutely voided nonlinear materials

Constitutive relations are derived for an incompressible, isotropic power-law matrix material containing a dilute concentration of spherical voids. The derivation is made for a nonlinearly viscous material used to characterize steady creep. However, the theory applies equally well to small strain nonlinear elasticity (deformation theory), and an extension to a rate-independent flow theory is also discussed. The starting point and key element in the formulation is the potential function for an isolated spherical void in an infinite block of power-law material. Approximate, but accurate, representations for this potential function are given. The overall constitutive relation governing the behavior of the dilutely voided solid is obtained simply and directly using the void potential. An assessment of the range of validity of the dilute concentration results is obtained using numerical solutions to the problem of a spherical void centered in a sphere of finite radius made of the power-law material. The potential function is also given for a dilute concentration of aligned penny-shaped cracks in the same power-law material.     

Disruption of spatial but not object-recognition memory by neurotoxic lesions of the dorsal hippocampus in rats.

Ischemia-induced cell loss in the CA1 region of the dorsal hippocampus results in severe deficits on delayed non-matching-to-sample (DNMS), whereas hippocampectomy produces little or no impairment, suggesting that partial hippocampal damage is more detrimental to DNMS performance than total ablation. To test this hypothesis, rats with or without preoperative DNMS training were given partial cytotoxic lesions of the dorsal hippocampus. When tested, neither group displayed any DNMS deficits despite widespread cell loss in the CA1 and other regions of the dorsal hippocampus. In the final experiments, rats tested previously on DNMS were found to be impaired on the Morris water maze. The finding that partial hippocampal lesions disrupt spatial memory while leaving object-recognition memory intact indicates a specialized role for the hippocampus in mnemonic processes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

PLZT Phases Near Lead Zirconate: 2. Determination by Capacitance and Polarization

The phases of PLZT, (Pb,La)(Zr,Ti)O₃, or L / Z / T for L <12 and T <10 were detected via determination of capacitance at varying temperatures at 0 field and via polarization at varying fields at room temperature and at selected elevated temperatures. It was possible to determine two different orthorhombic phases, and three rhombohedral (hexagonal) phases, one of which only exists at elevated temperatures. Two sets of phase diagrams were constructed with the phases as a function of temperature, and electric field, respectively. Temperature/field phase diagrams for 0/100/0, 2.95/5, and 4/95/5 were also constructed. A comparison was made between room-temperature and 0 field PLZT phase diagrams determined via X-ray diffraction, capacitance, and polarization.     

Development of a rare cell fractionation device: application for cancer detection

Isolating rare cells from biological fluids including whole blood or bone marrow is an interesting biological problem. Characterization of a few metastatic cells from cancer patients for further study is desirable for prognosis/diagnosis. Traditional methods have not proven adequate, due to the compositional complexity of blood, with its large numbers of cell types. To separate individual cells based on their mechanical characteristics, we have developed a series of massively parallel microfabricated sieving device. These devices were constructed with four successively narrower regions of channels numbering /spl sim/1800 per region. As cells traversed the device, they encountered each region and stopped at a gap width that prohibited passage due to their size. Cultured neuroblastoma cells, when mixed with whole blood and applied to the device, were retained in the 10-/spl mu/m-wide by 20-/spl mu/m-deep channels. All other cells migrated to the output. A derivative of the same device was utilized to characterize migration of whole blood. Adult white blood cells were retained at the 2.5-/spl mu/m-wide by 5-/spl mu/m-deep channels, while red blood cells passed through these channels. Devices designed to capture rare cells in peripheral circulation for downstream analysis will provide an important tool for diagnosis and treatment.     

Ischemia-induced object-recognition deficits in rats are attenuated by hippocampal ablation before or soon after ischemia.

The literature on the role of the hippocampus in object-recognition contains a paradox: Transient forebrain ischemia (ISC) produces hippocampal damage and severe deficits on the delayed nonmatching-to-sample (DNMS) task, yet hippocampal ablation (ABL) produces milder deficits. Experiment 1 confirmed that pretrained rats display severe DNMS deficits following ISC, but not ABL. Ischemia produced loss of CA1 neurons. but no obvious extrahippocampal damage. In Experiments 2 and 3, ISC rats from Experiment 1 received ABL. and ABL rats received ISC: neither treatment affected DNMS performance. In Experiment 4, rats that received ISC followed 1 hr later by ABL displayed only mild deficits. It is hypothesized that ISC-induced DNMS deficits are due to extrahippocampal damage produced by pathogenic processes that involve the hippocampus. (PsycINFO Database Record (c) 2010 APA, all rights reserved)