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Sudden Drop (sudden + drop)

Distribution by Scientific Domains

Chemistry	50%

Selected Abstracts

The continuous cooling transformation (CCT) as a flexible tool to investigate polymer crystallization under processing conditions

ADVANCES IN POLYMER TECHNOLOGY, Issue 2 2009
V. Brucato
Abstract An experimental route for investigating polymer crystallization over a wide range of cooling rates (from 0.01 to 1000°C/s) and pressures (from 0.1 to 40 MPa) is illustrated, using a method that recalls the approach adopted in metallurgy for studying structure development in metals. Two types of experimental setup were used, namely an apparatus for fast cooling of thin films (100,200 ,m thick) at various cooling rates under atmospheric pressure and a device (based on a on-purpose modified injection molding machine) for quenching massive samples (about 1,2 cm3) under hydrostatic pressure fields. In both cases, ex situ characterization experiments were carried out to probe the resulting structure, using techniques such as density measurements and wide-angle x-ray diffraction (WAXD) patterns. The cooling mechanism and temperature distribution across the sample thickness were analyzed. Results show that the final structure is determined only by the imposed thermal history and pressure. Experimental results for isotactic polypropylene (iPP), poly(ethylene terephthalate) (PET), polyamide 6 (PA6), and syndiotactic polystyrene (sPS) are reported, showing the reliability of this experimental approach to assess not only quantitative information but also a qualitative description of the crystallization behavior of different classes of semicrystalline polymers. The present study gives an opportunity to evaluate how the combined effect of the cooling rate and pressure influences the crystallization kinetics for various classes of polymer of commercial interest. An increase in the cooling rate translates into a decrease in crystallinity and density, which both experience a sudden drop around the specific "crystallizability" (or "critical cooling rate") of the material examined. The exception is sPS where competition among the various crystalline modifications determines a minimum in the plot of density vs. cooling rate. As for the effect of pressure, iPP exhibits a "negative dependence" of crystallization kinetics upon pressure, with a decrease of density and degree of crystallinity with increasing pressure, owing to kinetic constraints. PA6 and PET, on the other hand, due to thermodynamic factors resulting in an increase in Tm with pressure, exhibits a "positive dependence" of crystallization kinetics upon pressure. Finally, recent original results concerning sPS have shown that the minimum in the density vs. cooling rate curve shifts toward larger cooling rates upon increasing pressure. © 2009 Wiley Periodicals, Inc. Adv Polym Techn 28:86,119, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20151 [source]

Synthesis of hafnium tungstate by a CO2 laser and its microstructure and Raman spectroscopic study

JOURNAL OF RAMAN SPECTROSCOPY, Issue 7 2008
E. J. Liang
Abstract Densely packed hafnium tungstate blocks were synthesized by rapid solidification with a CO2 laser. It is shown that the optimum synthesis conditions for HfW2O8 are around 700 W laser power and 1 mm/s scan speed. Scanning electron microscopy (SEM) observation shows that the blocks consist of oriented nano-threads/rods that grew horizontally on the surface region and vertically in the interior. The orientations of the nanostructures are governed by the heat transfer directions on the surface and in the interior. Raman spectroscopic and X-ray diffraction studies show that the samples solidified in the cubic structure with minor contents of the orthorhombic phase. This is explained by a compressive stress induced during the rapid solidification process due to a sudden drop of temperature of the molten pool to the ambient. The stress is estimated to be about 0.6 GPa by comparison with high-pressure Raman study. Some specific Raman bands appear in the samples synthesized with the laser synthetic route but not in the sample by solid-state reaction. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Evolution of 1612-MHz maser emission in expanding circumstellar shells

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2005
M. D. Gray
ABSTRACT Observations show that 1612-MHz masers of OH/IR stars can fade on a time-scale of a decade. This fading is probably associated with the switch from rapid mass loss, which is ultimately linked with an internal He-shell flash, to the much slower mass loss supported by more quiescent conditions. We study the observed maser decay with a composite computational model, comprising a time-dependent chemical model of the envelope, and a radiation transfer model which provides the maser pumping. Our combined model is able to reproduce the rapid decay of maser intensity, following a sudden drop in the stellar mass-loss rate. The explanation for the rapid fall in maser emission is not a fall in the OH number density, or the kinetic temperature in the inverted layers, but the loss of a radiative pump route which carries population from levels 1 to 4 via levels 16, 15 and 11. The loss of these pump routes is a result, in turn, of a greatly reduced energy density of 53-,m radiation. [source]

Collapse as Cultural Revolution: Power and Identity in the Tiwanaku to Pacajes Transition

ARCHEOLOGICAL PAPERS OF THE AMERICAN ANTHROPOLOGICAL ASSOCIATION, Issue 1 2004
John Wayne Janusek
Inherent foundations of power are often made explicit in state collapse and ethnogenesis, among the most problematic processes tackled by archaeologists. Recent research on collapse globally indicates that conventional models prioritizing external change (e.g., environmental shift, immigration) fail to address the historical intricacies of and human agency involved in state fragmentation. Some recent models treat collapse as a sudden drop in political complexity, and most fail to elaborate how state collapse influenced postcollapse sociopolitical and cultural patterns. Synthesizing substantial recent research on Tiwanaku (A.D. 500,1150) and post-Tiwanaku Pacajes (A.D. 1150,1450) polities in the south-central Andes, I suggest that state collapse involved a fateful conjunction of sociopolitical and environmental transformations. Drought conditions descended upon a centralized yet highly fragile sociopolitical landscape that had become increasingly volatile during Tiwanaku's apogee. Collapse involved rapid transformation as well as slow, cumulative shifts and enduring continuities. It was a cultural revolution that began during Tiwanaku hegemony and drew heavily on existing practices and ideals. Grounded in practice theory, this case study finds human agency squarely in the center of macroprocesses such as collapse and situates Andean foundations of power in the matrix of local ideals, practices, and identities from which hegemonic regimes such as Tiwanaku were forged. [source]