Determination of the Mechanical Response of Sintering Compacts by Cyclic Loading Dilatometry

Determination of the mechanical response of a powder compact during densification is critical in analyzing defect formation and macroscopic dimensional changes, particularly in systems where constrained sintering is involved. Cyclic loading dilatometry is proposed as a novel approach in evaluating the mechanical response of the sintering compact, including sintering pressure, elastic modulus, and viscosity. The advantage of this technique is that only one experiment is needed to determine the equilibrium elastic and viscous properties of a sintering material at any temperatures for any given heating schedule, and no interrupted tests are necessary. This methodology is demonstrated for sintering compacts of Al₂O₃, Ce-TZP, and their composite mixture. The loading dilatometric data showed that the compact behaved elastically prior to the onset of sintering and during the very initial stages of sintering, which was followed by a transition leading to a viscous behavior for the latter part of the densification. Application of different load levels, ranging from 0.25 to 1 MPa, showed that the compact viscosity is essentially linear within the applied stress range at temperatures greater than 1100°C.     

Stresses and Distortion Due to Green Density Gradients During Densification

Finite element analysis (FEA) was performed on a bi-layer cylindrical structure consisting of a low-density layer on top of a high-density layer. For this model, the layers used the shrinkage behavior, viscosity, and elastic properties of barium titanate determined for the 45% and 55% green densities. The stresses predicted by FEA showed good agreement with stresses predicted using analytical equations for a linear viscous bi-layer cylinder. The model was then extended to use more complex density gradients measured by X-ray computed tomography on a bi-layer compact. In this case, the shrinkage behavior and viscosity properties were extrapolated from the experimental data. In the subsequent simulation, the stresses and strains were predicted during sintering. For the bi-layer structure studied, a highly stressed region was identified on the free surface of the sintering compact and this was shown to lead to edge cracking during densification.     

Transgenic studies of peripheral and central glia

This paper reviews recent studies on the life cycle of Toxoplasma gondii. Tachyzoites, bradyzoites, and sporozoites are the three infectious stages of T. gondii. Humans and animals become infected mainly by ingesting bradyzoites or oocytes. After ingestion, both bradyzoites and sporozoites convert to tachyzoites inside tissues. The conversion of tachyzoites to bradyzoites and bradyzoites to tachyzoites is of biological and clinical significance because bradyzoites are less susceptible to chemotherapy and reactivation of bradyzoites to tachyzoites is considered the cause of fatal toxoplasmosis in AIDS patients. Of all the methods currently available to assess stage conversion of T. gondii, feeding infective stages to cats is the most reliable method. Felidae, the definitive hosts of T. gondii excrete oocysts 3-10 days after ingesting tissue cysts/bradyzoites, > or = 18 days after ingesting oocysts, and > or = 13 days after ingesting tachyzoites.     

(Reactive) Templated Grain Growth of Textured Sodium Bismuth Titanate (Na1/2Bi1/2TiO3-BaTiO3) Ceramics—I Processing

Textured (Na_1/2Bi_1/2)TiO₃-BaTiO₃ (5.5 mol% BaTiO₃) ceramics with <100>_pc (where pc denotes the pseudocubic perovskite cell) orientation were fabricated by Templated Grain Growth (TGG) and Reactive Templated Grain Growth (RTGG) using anisotropically shaped template particles. In the case of TGG, molten salt synthesized SrTiO₃ platelets were tape cast with a (Na_1/2Bi_1/2)TiO₃-5.5 mol%BaTiO₃ powder and sintered at 1200°C for up to 12 h. In the RTGG approach, Bi₄Ti₃O₁₂ (BiT) platelets were tape cast with a Na₂CO₃, Bi₂O₃, TiO₂, and BaCO₃ powder mixture and reactively sintered. The TGG approach using SrTiO₃ templates resulted in >90% texture along [001] whereas the RTGG approach using BiT templates resulted in 80% texture. The grain orientation distribution along the textured direction, as measured by X-ray rocking curve, showed a full width at half maximum of 8° and a texture fraction of 80%.     

Standing, sitting and associated working conditions in the Quebec population in 1998

Working posture is an important determinant of musculoskeletal and vascular health. Knowledge of the context and type of postures is necessary in order to examine their associations with health-related outcomes. This study describes self-reported usual working postures in a population and their associations with other working conditions and demographic variables. The 1998 Quebec Health and Social Survey is a population-based survey of 11,986 private households in the province of Quebec. It contained a self-administered questionnaire, including an extensive occupational health section. The analyses in this study were limited to respondents with paid employment who had at least 6 months seniority in their current job, comprising 9,425 subjects. The overall prevalence of usual work in a standing posture is 58%; it is more common among men, workers under 25 years, those in the two lowest educational quintiles and those with incomes under 20,000 Canadian dollars. Only one person in six who works standing reports being able to sit at will. Women and men differ in the types of usual standing and sitting postures at work. Those who work standing and/or who work in more constrained postures are more likely to be exposed to other physical work demands, such as handling heavy loads, repetitive work, forceful exertion and low job decision latitude. The association between decision latitude and constrained postures is an important link between psychosocial and physical stressors in the workplace. In epidemiological studies, exposure covariation and interactions should be considered in the generation and interpretation of the associations between work postures and musculoskeletal disorders.     

Growth of straight InAs-on-GaAs nanowire heterostructures

One of the main motivations for the great interest in semiconductor nanowires is the possibility of easily growing advanced heterostructures that might be difficult or even impossible to achieve in thin films. For III-V semiconductor nanowires, axial heterostructures with an interchange of the group III element typically grow straight in only one interface direction. In the case of InAs-GaAs heterostructures, straight nanowire growth has been demonstrated for growth of GaAs on top of InAs, but so far never in the other direction. In this article, we demonstrate the growth of straight axial heterostructures of InAs on top of GaAs. The heterostructure interface is sharp and we observe a dependence on growth parameters closely related to crystal structure as well as a diameter dependence on straight nanowire growth. The results are discussed by means of accurate first principles calculations of the interfacial energies. In addition, the role of the gold seed particle, the effect of its composition at different stages during growth, and its size are discussed in relation to the results observed.     

Fabrication of Oriented SiC-Whisker-Reinforced Mullite Matrix Composites by Tape Casting

Dense SiC-whisker-reinforced mullite composites with up to 50 vol% whiskers can be obtained by tape casting and hot pressing. The tape casting process results in high degrees of SiC whisker orientation as determined visually and by X-ray diffraction. The ability to achieve dense composites with as much as 50 vol % whiskers is attributed to the higher percolation threshold of aligned whiskers. The factors affecting the degree of whisker orientation during tape casting are described using a fluid dynamics model derived from Jeffery's equations and show that the orientation of anisometric particles is enhanced primarily by the casting rate and particle aspect ratio.     

An inhibitory fragment derived from protein kinase Cepsilon prevents enhancement of nerve growth factor responses by ethanol and phorbol esters

We have studied nerve growth factor (NGF)-induced differentiation of PC12 cells to identify PKC isozymes important for neuronal differentiation. Previous work showed that tumor-promoting phorbol esters and ethanol enhance NGF-induced mitogen-activated protein (MAP) kinase activation and neurite outgrowth by a PKC-dependent mechanism. Ethanol also increases expression of PKCdelta and PKCepsilon, suggesting that one these isozymes regulates responses to NGF. To examine this possibility, we established PC12 cell lines that express a fragment encoding the first variable domain of PKCepsilon (amino acids 2-144), which acts as an isozyme-specific inhibitor of PKCepsilon in cardiac myocytes. Phorbol ester-stimulated translocation of PKCepsilon was markedly reduced in these PC12 cell lines. In addition, phorbol ester and ethanol did not enhance NGF-induced MAP kinase activation or neurite outgrowth in these cells. In contrast, phorbol ester and ethanol increased neurite outgrowth and MAP kinase phosphorylation in cells expressing a fragment derived from the first variable domain of PKCdelta. These results demonstrate that PKCepsilon mediates enhancement of NGF-induced signaling and neurite outgrowth by phorbol esters and ethanol in PC12 cells.