Shell Formation Time Research Articles

Osaka Feedback Model. II. Modeling Supernova Feedback Based on High-resolution Simulations

Feedback from supernovae (SNe) is an essential mechanism that self-regulates the growth of galaxies, and a better model of SN feedback is still needed in galaxy-formation simulations. In the first part of this paper, using an Eulerian hydrodynamic code Athena++, we find the universal scaling relations for the time evolution of momentum and radius for a superbubble, when the momentum and time are scaled by those at the shell-formation time. In the second part of this paper, we develop a SN feedback model based on the Athena++ simulation results utilizing Voronoi tessellation around each star particle, and implement it into the GADGET3-Osaka smoothed particle hydrodynamic code. Our feedback model was demonstrated to be isotropic and conservative in terms of energy and momentum. We examined the mass/energy/metal loading factors and find that our stochastic thermal feedback model produced galactic outflow that carries metals high above the galactic plane but with weak suppression of star formation. Additional mechanical feedback further suppressed star formation and brought the simulation results into better agreement with the observations of the Kennicutt–Schmidt relation, with all the results being within the uncertainties of observed data. We argue that both thermal and mechanical feedback are necessary for the SN feedback model of galaxy evolution when an individual SN bubble is unresolved.

Osaka Feedback Model II: Modeling Supernova Feedback Based on High-Resolution Simulations

AbstractFeedback from supernovae (SNe) is an essential mechanism that self-regulates the growth of galaxies. We build an SN feedback model based on high-resolution simulations of superbubble and SN-driven outflows for the physical understanding of the galaxy–CGM connection. Using an Eulerian hydrodynamic code Athena++, we find universal scaling relations for the time evolution of superbubble momentum, when the momentum and time are scaled by those at the shell-formation time. We then develop an SN feedback model utilizing Voronoi tessellation, and implement it into the GADGET3-Osaka smoothed particle hydrodynamic code. We show that our stochastic thermal feedback model produces galactic outflow that carries the metals high above the galactic plane but with weak suppression of star formation. Additional mechanical feedback further suppresses star formation. Therefore, we argue that both thermal and mechanical feedback is necessary for the SN feedback model of galaxy evolution when an individual SN bubble is unresolved.

Aqueous synthesis of colloidal CdSexTe1-x – CdS core–shell nanocrystals and effect of shell formation parameters on the efficiency of corresponding quantum dot sensitized solar cells

In this research CdSe0.4Te0.6 NCs were synthesized in aqueous solution through a cheap, simple, and modified hot injection chemical precipitation method. The reflux time was altered in a wide range for the synthesis of these NCs with different size and bandgap energies. Then they were applied as a co-sensitizing layer together with CdS NCs film in the photoanod of quantum dot sensitized solar cells (QDSCs). It was shown that the QDSC with TiO2 NCs/CdSeTe(7 h)/CdS/ZnS photoelectrode demonstrated the maximum power conversion efficiency (PCE) of 4.2% in AM1.5 solar irradiation. The selected CdSeTe NCs prepared in 7 h of the reflux time were utilized in a second growth process to form core–shell CdSeTe-CdS NCs. The shell formation time was widely altered and NCs were applied in the photoanod of corresponding QDSCs. It was demonstrated that the cell with TiO2 NCs/CdSeTe(7 h)-CdS(100 min)/CdS/ZnS photoelectrode revealed the best efficiency of 4.6% in the experiments. This was due to the surface passivation of the sensitizing NCs and better charge collection efficiency.

A mechanistic model to predict droplet drying history and particle shell formation in multicomponent systems

Spray drying allows tuning the physical properties of the resulting powders widely. However, targeted process design is complicated by the interplay between process characteristics and the non-ideal physical properties exhibited by multicomponent mixtures, such as pharmaceutical formulations. This work presents a mechanistic model describing the drying of single droplets. The model includes heat and mass balances, non-ideal vapour-liquid equilibria, and population balances describing the evolution of particulates within the drying droplet. The model is applied to (up to) ternary mixtures of solvents, polymers and solutes, and predicts properties of the drying droplet such as the time of shell formation and the size of the particle obtained after drying. A comparison with experimental data from single droplet drying experiments carried out at defined relative vapour saturation and temperature shows that the model can be used to predict shell formation (as well as other properties) for the systems studied here.

Aqueous synthesis of CdTe-CdS core shell nanocrystals and effect of shell-formation process on the efficiency of quantum dot sensitized solar cells

In this research CdTe nanocrystals (NCs) were synthesized in aqueous solution through a hot-injection chemical precipitation method. Then a shell formation process was carried out to form CdTe-CdS NCs with different shell thickness. Thioacetamide was applied as a thermo-sensitive source of S and the shell making time was altered in the range of 0–120 min. different core-shell CdTe-CdS NCs were utilized for co-sensitization of the photoanode of the quantum dot sensitized solar cells (QDSCs). These photoelectrodes were pre-sensitized by CdS NCs deposited on a transparent mesopous TiO2 scaffold with specific thickness. The effect of shell formation time on the photovoltaic (PV) performance of the corresponding cells was investigated. The photovoltaic parameters were quite reduced for the QDSCs sensitized by CdTe-CdS NCs prepared in short shell-formation times. Then the PV performance was considerably improved and reached to its maximum for the cells with sensitizing QDs with higher shell thicknesses. The corresponding performance was finally decreased for the co-sensitization with core-shell NCs synthesized in longer shell-making times than 100 min. A maximum power conversion efficiency of 4.2% was achieved for the co-sensitized QDSC with CdTe-CdS NCs prepared in 80 min of the shell-formation process. This efficiency was enhanced about 91% compared to the reference QDSC with TiO2/CdS/ZnS photoanode. The trend of changes in PV performance was explained based on the flat-band energy diagram of the QDSCs and permitted transitions for the electrons and holes in different shell thicknesses.

Synthesis of highly luminescent CdTe/CdS core-shell nanocrystals by optimization of the core and shell growth parameters

CdTe NCs were grown in aqueous solution through a hot injection method. The size of NCs was controlled by changing the refluxing time in a wide range of 1–20 h. The corresponding photoluminescence quantum yield was quite different for different sizes of nanocrystals and was saturated in long refluxing times. Different sizes of CdTe NCs within 2.5–4 nm and PL quantum yields between 28 and 50% were applied for the synthesis of core/shell CdTe/CdS nanostructures. The CdS shell was thermally over grown on the surface of different CdTe NCs for the luminescence improvement. The shell formation time was also changed in a wide range of 10–120 min to get the highest photoluminescence efficiencies. The range of changes and the maximum value of the PL quantum yield of the CdTe/CdS NCs were quite different for various sizes of core CdTe nanoparticles. A maximum PL quantum yield of 64% was achieved for the core-shell nanocrystals with cores prepared in 6 h of the refluxing time and CdS shell formed in 60 min of heating. Besides, there was a similar oscillating behaviour for the absorbance and PL quantum yields of the core-shell NCs during the shell formation process. This was qualitatively investigated and attributed to the successive formation of the perfect CdS shells on the NCs surface. The XRD, TEM and XPS analyses were performed to confirm the shell formation.

Formation process of core‐shell microparticles by solute migration during drying of homogenous composite droplets

Particle formation process during spray drying profoundly impacts particle morphology and microstructure. This study experimentally investigated the formation of core‐shell‐structured microparticles by one‐step drying of composite droplets made of Eudragit® RS (a polycationic acrylic polymer in nanoparticle form) and silica sol. The formation of an incipient surface shell was monitored using single droplet drying technique, and the freshly formed shell was recovered for subsequent analyses. Adding silica to RS precursor increased the shell formation time and altered the properties of the incipient shell from water‐dispersible to nondispersible. The incipient shell formed from RS/silica droplets with mass ratios of 1.5%:1.5% and 0.5%:2.5% showed ingredient segregation with a preferential accumulation of RS, similar to the shell region of dried microparticles. After shell formation, excess silica sol in the liquid phase could flow inward, producing a dense core. This mobility‐governed solute migration would be useful for constructing core‐shell microparticles using other precursor systems. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3297–3310, 2017

MOMENTUM INJECTION BY SUPERNOVAE IN THE INTERSTELLAR MEDIUM

Supernova (SN) explosions deposit prodigious energy and momentum in their environments, with the former regulating multiphase thermal structure and the latter regulating turbulence and star formation rates in the interstellar medium (ISM). However, systematic studies quantifying the impact of SNe in realistic inhomogeneous ISM conditions have been lacking. Using three-dimensional hydrodynamic simulations, we investigate the dependence of radial momentum injection on both physical conditions (considering a range of mean density n=0.1-100) and numerical parameters. Our inhomogeneous simulations adopt two-phase background states that result from thermal instability in atomic gas. Although the SNR morphology becomes highly complex for inhomogeneous backgrounds, the radial momentum injection is remarkably insensitive to environmental details. For our two-phase simulations, the final momentum produced by a single SN is given by 2.8*10^5 M_sun*km/s n^{-0.17}. This is only 5% less than the momentum injection for a homogeneous environment with the same mean density, and only 30% greater than the momentum at the time of shell formation. The maximum mass in hot gas is quite insensitive to environmental inhomogeneity. Strong magnetic fields alter the hot gas mass at very late times, but the momentum injection remains the same. Initial experiments with multiple spatially-correlated SNe show a momentum per event nearly as large as single-SN cases. We also present a full numerical parameter study to assess convergence requirements. For convergence in the momentum and other quantities, we find that the numerical resolution dx and the initial size of the SNR r_init must satisfy dx, r_init<r_sf/3, where the shell formation radius is given by r_sf = 30 pc n^{-0.46} for two-phase models (or 30% smaller for a homogeneous medium).

Elevated CO2 Levels do not Affect the Shell Structure of the Bivalve Arctica islandica from the Western Baltic

Shells of the bivalve Arctica islandica are used to reconstruct paleo-environmental conditions (e.g. temperature) via biogeochemical proxies, i.e. biogenic components that are related closely to environmental parameters at the time of shell formation. Several studies have shown that proxies like element and isotope-ratios can be affected by shell growth and microstructure. Thus it is essential to evaluate the impact of changing environmental parameters such as high pCO2 and consequent changes in carbonate chemistry on shell properties to validate these biogeochemical proxies for a wider range of environmental conditions. Growth experiments with Arctica islandica from the Western Baltic Sea kept under different pCO2 levels (from 380 to 1120 µatm) indicate no affect of elevated pCO2 on shell growth or crystal microstructure, indicating that A. islandica shows an adaptation to a wider range of pCO2 levels than reported for other species. Accordingly, proxy information derived from A. islandica shells of this region contains no pCO2 related bias.

Preparation of hollow submicrocapsules with a mesoporous carbon shell

The preparation of carbon submicrocapsules with size up to 800nm and a mesoporous shell by hard silica templating is reported. Washing and template synthesis conditions were varied to promote porosity and avoid deformation of the microcapsules. The silica template synthesis conditions analyzed were: silica nucleus formation time (0.25–6h), octadecyltrimethoxysilane/tetraethylorthosilicate volume ratio for silica shell formation (0.2–0.6) and silica shell formation time (1–24h). The samples were characterized by 77K nitrogen adsorption/desorption, mercury porosimetry and electron microscopy. Under all the washing conditions tested the carbon submicrocapsules were deformed due to the large size of the hollow core and the thickness of the shell. Changes in the silica template synthesis conditions did not result in substantial improvement of the strength of the microcapsules. The synthesis of a silica template with a double shell allowed us to obtain thick shell carbon submicrocapsules without significant deflation and with higher porosity. The characterization of these microcapsules showed that they have a BET surface area of 1541m2/g and a pore size distribution with peaks centered at 0.75, 0.86, 1.0nm in the micropore range and 3.5nm in the mesopore range. The pore volume in the 2–80nm range was 1.7cm3/g.

Carbon isotopes in mollusk shell carbonates

Mollusk shells contain many isotopic clues about calcification physiology and environmental conditions at the time of shell formation. In this review, we use both published and unpublished data to discuss carbon isotopes in both bivalve and gastropod shell carbonates. Land snails construct their shells mainly from respired CO2, and shell δ13C reflects the local mix of C3 and C4 plants consumed. Shell δ13C is typically >10‰ heavier than diet, probably because respiratory gas exchange discards CO2, and retains the isotopically heavier HCO3 −. Respired CO2 contributes less to the shells of aquatic mollusks, because CO2/O2 ratios are usually higher in water than in air, leading to more replacement of respired CO2 by environmental CO2. Fluid exchange with the environment also brings additional dissolved inorganic carbon (DIC) into the calcification site. Shell δ13C is typically a few ‰ lower than ambient DIC, and often decreases with age. Shell δ13C retains clues about processes such as ecosystem metabolism and estuarine mixing. Ca2+ ATPase-based models of calcification physiology developed for corals and algae likely apply to mollusks, too, but lower pH and carbonic anhydrase at the calcification site probably suppress kinetic isotope effects. Carbon isotopes in biogenic carbonates are clearly complex, but cautious interpretation can provide a wealth of information, especially after vital effects are better understood.

A numerical model of the partitioning of trace chemical solutes during drop freezing

Partitioning of volatile chemicals among the gas, liquid, and solid phases during freezing of liquid water in clouds can impact trace chemical distributions in the troposphere and in precipitation. We describe here a numerical model of this partitioning during the freezing of a supercooled liquid drop. Our model includes the time-dependent calculation of the coupled processes of crystallization kinetics, heat transport, and solute mass transport, for a freezing hydrometeor particle. We demonstrate the model for tracer partitioning during the freezing of a 1000 μm radius drop on a 100 μm ice substrate, under a few ambient condition scenarios. The model effectively simulates particle freezing and solute transport, yielding results that are qualitatively and quantitatively consistent with previous experimental and theoretical work. Results suggest that the ice shell formation time is governed by heat loss to air and not by dendrite propagation, and that the location of ice nucleation is not important to freezing times or the effective partitioning of chemical solutes. Even for the case of nucleation at the center of the drop, we found that dendrites propagated rapidly to form surface ice. Freezing then proceeded from the outside in. Results also indicate that the solid-liquid interfacial surface area is not important to freezing times or the effective partitioning of chemical solutes, and that the rate aspects of trapping are more important than equilibrium solid-liquid partitioning to the effective partitioning resulting from freezing.

Thin shell formation in radiative shocks. 1: Supernova remnants in low-density media

This paper explores the onset of thin-shell formation in interstellar shocks associated with supernova explosions. We outline a simple but useful scheme that indicates the time at which thin shell formation begins for supernova remnants (SNRs) evolving in a range of interstellar environments, extending the previous analytical models to arbitrary power-law density media. The result depends on the gas cooling properties and the shock velocity and radius. This is then applied to the specific case of SNRs in low-density media. The procedure for defining the time for the onset of shell formation, t(sub sf), equates the value of the adiabat, kappa = p/rho(exp gamma), to zero using the known time dependence of the shock radius and velocity. For the case of a power-law density ambient medium of the form rho(r) = Br(exp -omega), it is found that shell formation can be prevented when the ambient density drops faster than a critical rate. For a cooling function of the form Lambda = Lambda(sub 0) tau(exp beta), with beta = -0.5 (appropriate for line cooling), shell formation never occurs for omega greater than or equal to 9/5. The shell formation time is then computed for spherical shocks in a power-law density medium. For omega = 0, the onset of shell formation is found to be at t(sub sf) approx. equal to 2.87 x 10(exp 4) E51(exp 3/14) n(sub 0 exp -4.7) yr, which agrees well with previous estimates derived by other means. We compare the analytical shell formation time with the results of detailed numerical models for omega = 0 and three different ambient densities and find good agreement. The extension of the criterion for the onset of thin shell formation using the ratio of cooling to swept-up column density is also described. This method provides a useful approximation for cases when the exact solution is not known.

Seasonal and Life-long Variation of Sr/Ca Ratio in Shells of Mya arenaria from Grays Harbor (Washington)—an Ancillary Criterion in Demographic Studies

Mya arenaria was introduced to Grays Harbor (Washington) during the 1880s. Maximum size of members of the extant population is much smaller than that of clams dead in situ in preceding decades and nowadays found forming extensive 'death assemblages'. This could reflect differences in growth rate and/or maximum age (i.e. survival rate). Major 'growth marks' (seen in shell sections as translucent bands) are formed in the shell of extant clams during winter, when temperature and salinity are at their minima. Interpretation of the optical pattern observed in shells from the extinct population is difficult with criteria developed for the extant population, and cannot be validated experimentally. To assess the possible use of strontium/calcium ratios as an objective criterion for age estimation, profiles of Sr/Ca ratio and luminance were measured across thin sections of shell chondrophores of clams from the extinct ( N=4) and extant ( N=3) populations. Peaks in the Sr/Ca ratio always corresponded with translucent bands in shells from the two populations. Cross-correlation between vectors of slopes for two consecutive measurements of Sr/Ca and luminance was positive and significant for five of the seven clams. Cycles of high and low Sr/Ca ratios matched seasonality in the environment and are most probably governed by temperature, allowing validation of ages estimated for shells from the extinct population. There was a positive, significant correlation between mean Sr/Ca ratio and estimated age at the time of shell formation. This ontogenetic trend could be related to age, growth rate or both. Profiles of Sr/Ca constitute a useful ancillary criterion for age validation. Results support the hypothesis that the observed difference in maximum size is due to higher longevity in the extinct population.

The variations in limestone and flint grit consumptions by laying birds

AbstractAn experiment has been performed to find the quantities of limestone and flint grit consumed (a) by different birds, (6) on different days and (c) between two‐hourly periods within the day. The influence of two different feeding regimes was also considered. It was found that with limestone grit there were differences between birds and also differences between days, but with flint grit there were no such differences. It was possible to show that the need for calcium created by shell deposition did not cause a greater consumption of limestone grit in any one period directly related to a specific time of shell formation nor over the whole period of shell formation. The period of greatest consumption of limestone grit was always immediately before dusk. Most flint grit was also consumed at this time. Grit balances and an examination of the contents of the alimentary tract showed that there was a very great wastage of both limestone and flint grit but that the amount of limestone dissolved was probably adequate for the needs of the bird.