Crystal nucleation in a glass during relaxation well below Tg.

Until quite recently, in almost all papers on crystal nucleation in glass-forming substances, it was assumed that nucleation proceeds in a completely relaxed supercooled liquid and, hence, at constant values of the critical parameters determining the nucleation rate for any given set of temperature, pressure, and composition. Here, we analyze the validity of this hypothesis for a model system by studying nucleation in a lithium silicate glass treated for very long times (up to 250 days) in deeply supercooled states, reaching 60 K below the laboratory glass transition temperature, Tg. At all temperatures in the considered range, T < Tg, we observed an enormous difference between the experimental number of nucleated crystals, N(t), and its theoretically expected value computed by assuming the metastable state of the relaxing glass has been reached. Analyzing the origin of this discrepancy, we confirmed that the key parameters determining the nucleation rates change with time as a result of the glass relaxation process. Finally, we demonstrate that, for temperatures below 683 K, this particular glass almost fully crystallizes prior to reaching the ultimate steady-state nucleation regime (e.g., at 663 K, it would take 176 years for the glass to reach 99% crystallization, while 2600 years would be needed for complete relaxation). This comprehensive study proves that structural relaxation strongly affects crystal nucleation in deeply supercooled states at temperatures well below Tg; hence, this phenomenon has to be accounted for in any crystal nucleation model.

Elemental and cooperative diffusion in a liquid, supercooled liquid and glass resolved.

The diffusion mechanisms controlling viscous flow, structural relaxation, liquid-liquid phase separation, crystal nucleation, and crystal growth in multicomponent glass-forming liquids are of great interest and relevance in physics, chemistry, materials, and glass science. However, the diffusing entities that control each of these important dynamic processes are still unknown. The main objective of this work is to shed some light on this mystery, advancing the knowledge on this phenomenon. For that matter, we measured the crystal growth rates, the viscosity, and lead diffusivities in PbSiO3 liquid and glass in a wide temperature range. We compared our measured values with published data covering 16 orders of magnitude. We suggest that above a certain temperature range Td (1.2Tg-1.3Tg), crystal growth and viscous flow are controlled by the diffusion of silicon and lead. Below this temperature, crystal growth and viscous flow are more sluggish than the diffusion of silicon and lead. Therefore, Td marks the temperature where decoupling between the (measured) cationic diffusivity and the effective diffusivities calculated from viscosity and crystal growth rates occurs. We reasonably propose that the nature or size of the diffusional entities controlling viscous flow and crystal growth below Td is quite different; the slowest is the one controlling viscous flow, but both processes require cooperative movements of some larger structural units rather than jumps of only one or a few isolated atoms.

A Literature Investigation about Electrospinning and Nanofibers: Historical Trends, Current Status and Future Challenges.

The development of new fibrilar materials based on electrospinning (ES) technique has a notable history of nearly four centuries of discoveries and results. The eletrospinning manufacturing is one of the most widely reported methods for nanofiber (NF) manufacturing, providing security, high quality and productivity. In spite of the first patent about electrospinning has been applied in April 5(th), 1900 by John Francis Cooley, a historical perspective (since 1600s) about this amazing discovery represents an important step for future applications. Nanofibers have been considered one of the top interesting fundamental study objects for academicians, and greatest intriguing business materials for modern industry. As a consequence, lucrative organizations and companies have explored the relevance of nanofibers. In this paper, the quantity of published manuscripts and patent inventions is presented and the correlation of research activities to the production of new electrospinning materials is shown. China and the United States have been leading in electrospinning and nanofibers development. The company triumph is mostly dependent on applications improvement relevant for broader business society. A dramatic rise of interest in nanofibers produced by electrospinning technique has been confirmed due to the publication data, author's affiliation, keywords, and essential characterization procedures. Is has been shown that the number of publications on electrospinning and nanofibers researches from academic institutions is higher than industrial laboratories. More than 1,891 patents using the term "electrospinning" and 2,960 with the term "nanofibers" according to the European Patent Office at title or abstract have been filed around the world up to 2013. These numbers just continue to increase along with worldwide ES-related sales. Curiously, for the same period 11,973 electrospinning documents and 18,679 nanofibers-related (mainly manuscripts) were published considering the Scopus database with the same terms in the title, abstract or using keywords. Thus, statistically, there are more published manuscripts worldwide than patents for both keywords.

Diffusion coefficients for crystal nucleation and growth in deeply undercooled glass-forming liquids.

We calculate, employing the classical theory of nucleation and growth, the effective diffusion coefficients controlling crystal nucleation of nanosize clusters and the subsequent growth of micron-size crystals at very deep undercoolings, below and above Tg, using experimental nucleation and growth data obtained for stoichiometric Li2O.2SiO2 and Na2O.2CaO.3SiO2 glasses. The results show significant differences in the magnitude and temperature dependence of these kinetic coefficients. We explain this difference showing that the composition and/or structure of the nucleating critical clusters deviate from those of the stable crystalline phase. These results for diffusion coefficients corroborate our previous conclusion for the same glasses, based on different experiments, and support the view that, even for the so-called case of stoichiometric (polymorphic) crystallization, the nucleating phase may have a different composition and/or structure as compared to the parent glass and the evolving macroscopic crystalline phase. This finding gives a key to explain the discrepancies between calculated (by classical nucleation theory) and experimentally observed nucleation rates in these systems, in particular, and in deeply undercooled glass-forming liquids, in general.