Rejuvenation of metallic glasses by non-affine thermal strain.

When a spatially uniform temperature change is imposed on a solid with more than one phase, or on a polycrystal of a single, non-cubic phase (showing anisotropic expansion-contraction), the resulting thermal strain is inhomogeneous (non-affine). Thermal cycling induces internal stresses, leading to structural and property changes that are usually deleterious. Glasses are the solids that form on cooling a liquid if crystallization is avoided--they might be considered the ultimate, uniform solids, without the microstructural features and defects associated with polycrystals. Here we explore the effects of cryogenic thermal cycling on glasses, specifically metallic glasses. We show that, contrary to the null effect expected from uniformity, thermal cycling induces rejuvenation, reaching less relaxed states of higher energy. We interpret these findings in the context that the dynamics in liquids become heterogeneous on cooling towards the glass transition, and that there may be consequent heterogeneities in the resulting glasses. For example, the vibrational dynamics of glassy silica at long wavelengths are those of an elastic continuum, but at wavelengths less than approximately three nanometres the vibrational dynamics are similar to those of a polycrystal with anisotropic grains. Thermal cycling of metallic glasses is easily applied, and gives improvements in compressive plasticity. The fact that such effects can be achieved is attributed to intrinsic non-uniformity of the glass structure, giving a non-uniform coefficient of thermal expansion. While metallic glasses may be particularly suitable for thermal cycling, the non-affine nature of strains in glasses in general deserves further study, whether they are induced by applied stresses or by temperature change.

Niveles plasmáticas de plaguicidas organocloradas en la población general / Plasmatic levels of organochlorine pesticides in the general population

Se determinó el nivel plasmático de plaguicidas organoclorados en personas no expuestas ocupacionalmente a los mismos, para averiguar el grado de impregnación que posee la población general. Se utilizó como metodología analítica la cromatografia en fase gaseosa, empleando no menos de tres columnas para la identificación de cada plaguicida. Se comprobó la presencia de alfa, ß y gama HCH; p,p'-DDT y p-p'-DDE; heptacloro y heptacloro expóxido en las muestras analizadas, correspondiendo al ß HCH el mayor valor medio y, en el 54% de las muestras analizadas que contenían HCH, mayores niveles individuales que las encontradas para los otros dos isómeros. El menor valor medio obtenido correspondió al epóxido del heptacloro