495 research outputs found
Nucleation and crystallization process of silicon using Stillinger-Weber potential
We study the homogeneous nucleation process in Stillinger-Weber silicon in
the NVT ensemble. A clear first-order transition from the liquid to crystal
phase is observed thermodynamically with kinetic and structural evidence of the
transformation. At 0.75 T_m, the critical cluster size is about 175 atoms. The
lifetime distribution of clusters as a function of the maximum size their reach
follows an inverse gaussian distribution as was predicted recently from the
classical theory of nucleation (CNT). However, while there is a qualitative
agreement with the CNT, the free energy curve obtained from the simulations
differs significantly from the theoretical predictions, suggesting that the
low-density liquid phase found recently could play a role in the nucleation
process.Comment: 21 page
Mechanistic understanding of pore evolution enables high performance mesoporous silicon production for lithium-ion batteries
The cycling of silicon anodes within a lithium-ion battery (LIB) leads to degradation and capacity fade due to the 280% volume change of silicon. Many avenues of silicon synthesis have been explored to produce nanostructures which can withstand this change in volume. Magnesiothermic Reduction (MgTR) shows significant promise over other syntheses in scalability, economic and environmental aspects for producing porous silicon nanostructures. The problem with MgTR is a lack of understanding regarding the pore evolution of porous silicon based on reduction parameters and precursor material, which in turn limits predictive design for desired applications. Here we show that the pore structure of porous silicon is strongly related to the interconnectivity of silicon crystallites. We show that the MgTR is a thermodynamically driven equilibria which determines the purity of the silicon product. Higher temperatures also cause sintering of silicon nanocrystallites. We show that it is the interconnectivity of these crystallites that determine the pore size and distribution within porous silicon. These findings apply to a wide variety of porous silica precursors and we show this mechanism is true for the introduction of pores into nonporous quartz after MgTR. Further, we show that by exploiting this mechanism, mesoporous silicon can be produced which has excellent promise for LIB applications with a capacity of 2170 mAh/g after 100 cycles. The findings herein can be taken forward to design optimal materials for LIB applications. These results strongly support the potential for reduction in silicon costs for LIB in both economic and environmental terms as well as for a reverse engineering approach to design specific porous silicon for desired applications even beyond LIB
Nonmonotonic dependence of the absolute entropy on temperature in supercooled Stillinger-Weber silicon
Using a recently developed thermodynamic integration method, we compute the
precise values of the excess Gibbs free energy (G^e) of the high density liquid
(HDL) phase with respect to the crystalline phase at different temperatures (T)
in the supercooled region of the Stillinger-Weber (SW) silicon [F. H.
Stillinger and T. A. Weber, Phys. Rev. B. 32, 5262 (1985)]. Based on the slope
of G^e with respect to T, we find that the absolute entropy of the HDL phase
increases as its enthalpy changes from the equilibrium value at T \ge 1065 K to
the value corresponding to a non-equilibrium state at 1060 K. We find that the
volume distribution in the equilibrium HDL phases become progressively broader
as the temperature is reduced to 1060 K, exhibiting van-der-Waals (VDW) loop in
the pressure-volume curves. Our results provides insight into the thermodynamic
cause of the transition from the HDL phase to the low density phases in SW
silicon, observed in earlier studies near 1060 K at zero pressure.Comment: This version is accepted for publication in Journal of Statistical
Physics (11 figures, 1 table
An antisense oligodeoxynucleotide that depletes RI alpha subunit of cyclic AMP-dependent protein kinase induces growth inhibition in human cancer cells.
Enhanced expression of the RI alpha subunit of cyclic AMP-dependent protein kinase type I has been correlated with cancer cell growth. We provide evidence that RI alpha is a growth-inducing protein that may be essential for neoplastic cell growth. Human colon, breast, and gastric carcinoma and neuroblastoma cell lines exposed to a 21-mer human RI alpha antisense phosphorothioate oligodeoxynucleotide (S-oligodeoxynucleotide) exhibited growth inhibition with no sign of cytotoxicity. Mismatched sequence (random) S-oligodeoxynucleotides of the same length exhibited no effect. The growth inhibitory effect of RI alpha antisense oligomer correlated with a decrease in the RI alpha mRNA and protein levels and with an increase in RII beta (the regulatory subunit of protein kinase type II) expression. The growth inhibition was abolished, however, when cells were exposed simultaneously to both RI alpha and RII beta antisense S-oligodeoxynucleotides. The RII beta antisense S-oligodeoxynucleotide alone, exhibiting suppression of RII beta along with enhancement of RI alpha expression, led to slight stimulation of cell growth. These results demonstrate that two isoforms of cyclic AMP receptor proteins, RI alpha and RII beta, are reciprocally related in the growth control of cancer cells and that the RI alpha antisense oligodeoxynucleotide, which efficiently depletes the growth stimulatory RI alpha, is a powerful biological tool toward suppression of malignancy
Morphology and photoluminescence study of titania nanoparticles
Titania nanoparticles are prepared by solâgel chemistry with a poly(ethylene oxide) methyl ether methacrylate-block-poly(dimethylsiloxane)-block-poly(ethylene oxide) methyl ether methacrylate triblock copolymer acting as the templating agent. The solâgel componentsâhydrochloric acid, titanium tetraisopropoxide, and triblock copolymerâare varied to investigate their effect on the resulting titania morphology. An increased titania precursor or polymer content yields smaller primary titania structures. Microbeam grazing incidence small-angle X-ray scattering measurements, which are analyzed with a unified fit model, reveal information about the titania structure sizes. These small structures could not be observed via the used microscopy techniques. The interplay among the solâgel components via our triblock copolymer results in different sized titania nanoparticles with higher packing densities. Smaller sized titania particles, (âŒ13â20 nm in diameter) in the range of exciton diffusion length, are formed by 2% by weight polymer and show good crystallinity with less surface defects and high oxygen vacancies
Modification of the nanostructure of lignocellulose cell walls via a non-enzymatic lignocellulose deconstruction system in brown rot wood-decay fungi
Abstract Wood decayed by brown rot fungi and wood treated with the chelator-mediated Fenton (CMF) reaction, either alone or together with a cellulose enzyme cocktail, was analyzed by small angle neutron scattering (SANS), sum frequency generation (SFG) spectroscopy, Fourier transform infrared (FTIR) analysis, X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM). Results showed that the CMF mechanism mimicked brown rot fungal attack for both holocellulose and lignin components of the wood. Crystalline cellulose and lignin were both depolymerized by the CMF reaction. Porosity of the softwood cell wall did not increase during CMF treatment, enzymes secreted by the fungi did not penetrate the decayed wood. The enzymes in the cellulose cocktail also did not appear to alter the effects of the CMF-treated wood relative to enhancing cell wall deconstruction. This suggests a rethinking of current brown rot decay models and supports a model where monomeric sugars and oligosaccharides diffuse from the softwood cell walls during non-enzymatic action. In this regard, the CMF mechanism should not be thought of as a âpretreatmentâ used to permit enzymatic penetration into softwood cell walls, but instead it enhances polysaccharide components diffusing to fungal enzymes located in wood cell lumen environments during decay. SANS and other data are consistent with a model for repolymerization and aggregation of at least some portion of the lignin within the cell wall, and this is supported by AFM and TEM data. The data suggest that new approaches for conversion of wood substrates to platform chemicals in biorefineries could be achieved using the CMF mechanism with >75% solubilization of lignocellulose, but that a more selective suite of enzymes and other downstream treatments may be required to work when using CMF deconstruction technology. Strategies to enhance polysaccharide release from lignocellulose substrates for enhanced enzymatic action and fermentation of the released fraction would also aid in the efficient recovery of the more uniform modified lignin fraction that the CMF reaction generates to enhance biorefinery profitability
Aggregation and settling in aqueous polydisperse alumina nanoparticle suspensions
Nanoparticle suspensions (also called nanofluids) are often polydisperse and
tend to settle with time. Settling kinetics in these systems are known to be
complex and hence challenging to understand. In this work, polydisperse
spherical alumina (Al2O3) nanoparticles in the size range of ~10-100nm were
dispersed in water and examined for aggregation and settling behaviour near its
isoelectric point (IEP). A series of settling experiments were conducted and
the results were analysed by photography and by Small Angle X-ray Scattering
(SAXS). The settling curve obtained from standard bed height measurement
experiments indicated two different types of behaviour, both of which were also
seen in the SAXS data. But the SAXS data were remarkably able to pick out the
rapid settling regime as a result of the high temporal resolution (10s) used.
By monitoring the SAXS intensity, it was further possible to record the
particle aggregation process for the first time. Optical microscopy images were
produced on drying and dried droplets extracted from the suspension at various
times. Dried deposits showed the rapid decrease in the number of very large
particles with time which qualitatively validates the SAXS prediction, and
therefore its suitability as a tool to study unstable polydisperse colloids.
Keywords: Nanoparticles, nanofluids, polydisperse, aggregation, settling,
alumina, microscopy, SAX
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