Thermal Coalescence Research Articles

Atomic-Scale Time-Resolved Imaging of Krypton Dimers, Chains and Transition to a One-Dimensional Gas.

Single-atom dynamics of noble-gas elements have been investigated using time-resolved transmission electron microscopy (TEM), with direct observation providing for a deeper understanding of chemical bonding, reactivity, and states of matter at the nanoscale. We report on a nanoscale system consisting of endohedral fullerenes encapsulated within single-walled carbon nanotubes ((Kr@C60)@SWCNT), capable of the delivery and release of krypton atoms on-demand, via coalescence of host fullerene cages under the action of the electron beam (in situ) or heat (ex situ). The state and dynamics of Kr atoms were investigated by energy dispersive X-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS). Kr atom positions were measured precisely using aberration-corrected high-resolution TEM (AC-HRTEM), aberration-corrected scanning TEM (AC-STEM), and single-atom spectroscopic imaging (STEM-EELS). The electron beam drove the formation of 2Kr@C120 capsules, in which van der Waals Kr2 and transient covalent [Kr2]+ bonding states were identified. Thermal coalescence led to the formation of longer coalesced nested nanotubes containing more loosely bound Krn chains (n = 3-6). In some instances, delocalization of Kr atomic positions was confirmed by STEM analysis as the transition to a one-dimensional (1D) gas, as Kr atoms were constrained to only one degree of translational freedom within long, well-annealed, nested nanotubes. Such nested nanotube structures were investigated by Raman spectroscopy. This material represents a highly compressed and dimensionally constrained 1D gas stable under ambient conditions. Direct atomic-scale imaging has revealed elusive bonding states and a previously unseen 1D gaseous state of matter of this noble gas element, demonstrating TEM to be a powerful tool in the discovery of chemistry at the single-atom level.

Analysis of Non‐Isothermal Viscous Flow Coalescence at Micro Scale

The non‐isothermal coalescence of two spherical bodies caused by capillary‐induced viscous flow was analyzed. Based on this analysis, a new dimensionless number (K number) was introduced for defining thermal coalescence regimes. Based on the value of this number, coalescence may or may not be affected by thermal effects in different cases. To make this clearer, the conventional coalescence models of Frenkel‐Eshelby and Pokluda were modified by assuming viscosity as a temperature dependent variable. This was conducted by considering the effects of temperature on the viscosity of the involved material through evaluating different expressions including linear and Reynolds and Williams‐Landel‐Ferry (WLF) equations. The results of the modified models for the bridge growth rate show that temperature changes significantly affect the kinetics of coalescence, particularly when the characteristic times of coalescence and heat conduction are in the same order, i.e., moderate K numbers. This analysis is applicable for diverse situations since viscous flow coalescence occurs in various physical and industrial applications of particles or droplets.

Synthesis, characterization and applications of low temperature melting glasses belonging to P2O5CaO Na2O system

Abstract This study details the relationship between the structure, chemical composition and thermal properties of a series of calcium sodium phosphate glasses belonging to xP2O5-yCaO-(100−x−y)Na2O system, which have been prepared by melt-quenching method. The glass compositions were limited by fixing the P2O5 content (x) to 40, 45, 50, 55 and 60 mol%, and varying the CaO mol% (y) at 10, 15, 20, 25, 30 mol%. Glass transition (Tg) and melting temperatures (Tm) show a clear relationship with composition. The polymerization of the glass structure increases the higher the content of P2O5 leading to an increase in Tg and Tm temperatures. Higher contents of calcium improve the phosphate network strength increasing Tg and Tm temperatures. Sodium ions, displaying lower charge-to-size ratio than calcium ions, decrease the phosphorous condensation reducing the Tg and Tm temperatures of glasses. The results obtained in this investigation show the feasibility to develop extremely low melting calcium sodium phosphate glasses with P2O5 content of 45 mol%. Those results laid the foundation of employing these calcium sodium phosphate glasses as amorphous low melting temperature materials for further applications involving: a) melting temperature adjustment, such as fire-retardant additive in polymer composites; b) nanoparticle binder, due to its low melting temperature, nanoparticles can be safely embedded into this glass avoiding thermal coalescence. Examples of such applications are described in this work.

Numerical investigation of the influence of process conditions on the temperature variation in fused deposition modeling

An adaptable three-dimensional transient mathematical model of temperature variation with respect to space and time, during and after the fused deposition modeling (FDM), is proposed and applied with a boundary-adjusting finite difference method. This model enables researchers to study the influence of almost all other parameters on the temperature field and gradient variation when constructing any cuboid at fixed raster angle of 0(90)° and filling ratio of 100%. Some predicted but significant conclusions are reached: reheating of a deposited raster by a newly deposited one is universal and happens mainly in the layer thickness direction; temperature settings are the primary and direct factors determining temperature field variation, while layer thickness and printing speed can still exert their own influence; printing speed is predicted to be positively correlated to mechanical properties of FDM constructed components through the mechanism of thermal coalescence; strict energy management is required if a FDM printer of higher resolution is to be invented and applied. The technique of such mathematical model can provide immediate insights in the understanding of FDM process from the perspective of energy balance, many of which have never been revealed from previous reports yet.

Rapid synthesis of Zn 3 (VO 4 ) 2 phosphor film on quartz substrate by RF magnetron sputtering and rapid thermal processing

Abstract The near-ultraviolet excited Zn 3 (VO 4 ) 2 phosphor film on a quartz substrate was fabricated by RF magnetron sputter deposition of the film at 150 °C or room temperature and subsequent rapid thermal calcination at 600–900 °C. The phosphor film exhibited a broad emission spectrum with a peak at 567 nm and a FWHM of 186 nm. The highest internal quantum efficiency of 45% was obtained after 800 °C calcination of the 150 °C-deposited film with a Zn/V molar ratio of 1.9. The 800 °C calcination promoted a thermal coalescence and effectively reduced surface traps of Zn 3 (VO 4 ) 2 particles. Zn 3 (VO 4 ) 2 was thermally decomposed into Zn 4 V 2 O 9 and Zn 2 V 2 O 7 at 900 °C, and the quantum efficiency decreased drastically as a consequence.

Trapping of xenon gas in closed inner spaces of carbon nanomaterials for stable gas storage under high-vacuum condition

Xe gas can be trapped in the closed inner spaces of glassy carbon derived from C60 fullerene by thermal coalescence of C60 in Xe atmosphere and in cap-opened carbon nanotubes (CNTs) covered with an ionic liquid by soaking Xe-adsorbing CNTs in an ionic liquid. The trapped Xe gas is detected by energy dispersive X-ray spectrometry using a spectrometer mounted on an analytical transmission electron microscope. The closed inner spaces store gas molecules even under high-vacuum condition (∼10−5 Pa).

Aerosol Flow in the Vitrocell 24/48 Exposure System: Flow Mixing and Aerosol Coalescence

In vitro exposure systems are increasingly used in modern toxicology assessment studies. An understanding of the functioning of these complex systems and their characterization ensures well-controlled delivery of substances to cell cultures and reliable biological response of cells to the given external conditions. In this work, we present the flow-mixing efficiency of the dilution unit in the Vitrocell® 24/48 (VC24/48) exposure system, together with the role of thermal coalescence. We adopted a computational fluid dynamics approach with integrated aerosol dynamics to analyze the dilution system using high-resolution simulations. We performed simulations for three total flow rates through the system: 3, 6, and 12 L/min, to investigate the influence of turbulent mixing and thermal coalescence on the aerosol characteristics. We found that for all simulated flow rates, instantaneous and localized turbulent mixing is induced by the two opposing dilution jets. We found the aerosol to be uniformly mixed and the flow to be laminarized upstream of the location of the first aerosol sampling trumpet. In summary, the design of the double-tee junction, serving as the dilution unit of the VC24/48 system, guarantees efficient and localized mixing/dilution of the aerosol, while still delivering laminar flow toward deposition wells, when operated under these high flow-rate conditions. In addition, we showed that thermal coalescence effects do not influence the aerosol droplet size distribution during the dilution process for the studied simulation conditions.

Nuclear matter symmetry energy at0.03⩽ρ/ρ0⩽0.2

Measurements of the density dependence of the free symmetry energy in low density clustered matter have been extended using the NIMROD multi-detector at Texas A&M University. Thermal coalescence models were employed to extract densities, $\rho$, and temperatures, $T$, for evolving systems formed in collisions of 47 $A$ MeV $^{40}$Ar + $^{112}$Sn,$^{124}$Sn and $^{64}$Zn + $^{112}$Sn, $^{124}$Sn. Densities of $0.03 \leq \rho/\rho_0 \leq 0.2$ and temperatures in the range 5 to 10 MeV have been sampled. The free symmetry energy coefficients are found to be in good agreement with values calculated using a quantum statistical model. Values of the corresponding symmetry energy coefficient are derived from the data using entropies derived from the model.

Laboratory Tests of Low Density Astrophysical Nuclear Equations of State

Clustering in low density nuclear matter has been investigated using the NIMROD multidetector at Texas A&M University. Thermal coalescence modes were employed to extract densities, ρ, and temperatures, T, for evolving systems formed in collisions of 47A MeV (40)Ar+(112)Sn, (124)Sn and (64)Zn+(112)Sn, (124)Sn. The yields of d, t, (3)He, and (4)He have been determined at ρ=0.002 to 0.03 nucleons/fm(3) and T=5 to 11 MeV. The experimentally derived equilibrium constants for α particle production are compared with those predicted by a number of astrophysical equations of state. The data provide important new constraints on the model calculations.

Electrodeposition of thermally stable gold and silver nanoparticle ensembles through a thin alumina nanomask

Hemispheric gold or silver nanoparticle (Au and AgNP) ensembles were electrodeposited on a smooth ITO electrode through a thin Al(2)O(3) nanomask. The nanomask reduced the deviation in the particle size and interparticle distance. The absorption peak based on localized surface plasmon resonance (LSPR) of the AuNP ensemble redshifted with increasing environmental refractive index, suggesting that the ensemble would be used as a LSPR sensor for chemical analysis and bioanalysis. The Al(2)O(3) nanomask prevented the Au and AgNPs from thermal coalescence even at 500 degrees C, and consequently, it improved thermal stabilities of nanoparticle ensembles. The ensembles exhibit LSPR-based absorption peak in the visible region, even after annealing. The nanomask allowed AgNPs, which are thermally and chemically less stable than AuNPs, to be coated with sintered TiO(2). The ITO/AgNP/TiO(2) electrode thus obtained functions as a photocathode on the basis of photoinduced electron transfer from silver nanoparticles to TiO(2).

Deuteron and triton production in Pb+Pb collisions at 158 A·GeV.

NA44 has measured the invariant cross section of deuterons and tritons at non zero p t in 158 A·GeV lead on lead collisions at CERN SPS. Normalized transverse mass spectra and coalescence parameters versus p t have been calculated showing a significant transverse flow. Radius parameters have been extracted using a simple thermal coalescence model. Results from RQMD+coalescence calculations are compared to the data.

Deuteron and Triton production in Pb+Pb collisions at 158 AaGeV

NA44 has measured the invariant cross section of deuterons and tritons at non zero pt in 158 A·GeV lead on lead collisions at CERN SPS. Normalized transverse mass spectra and coalescence parameters versus pt have been calculated showing a significant transverse flow. Radius parameters have been extracted using a simple thermal coalescence model. Results from RQMD+coalescence calculations are compared to the data.

Coalescence of C60 Molecules in Fullerite by Intense Femtosecond Laser Pulses: An Ab–Initio Simulation

ABSTRACTWe present an ab–initio molecular dynamics study of the modifications induced on fullerite by the irradiation with ultrashort very intense laser pulses. We identified the threshold for the non–thermal cage–opening of the C60 molecules which is the analogue of the laserinduced non–thermal melting observed in semiconductors and graphite. The new phase formed by the non–thermal coalescence of the C60 molecules is fluid–like and mainly formed by small chains of 2–fold coordinated carbon atoms.

Evolution processes in island films grown from single-component melts: The last stage

A theory of initial stages in island film growth from single-component melts is presented. A hypothesis on “thermal coalescence (coarsening)”, i.e. the Ostwald ripening of an island ensemble of a new-phase growing from single-component melts on a solid substrate surface, is advanced. The physical sense of the latter involves the fact that under specific conditions (at a late stage of the process) there is a common thermal field in the melt layer containing an ensemble of islands adjacent to a substrate surface. Islands with a radius below the critical value are dissolved in the field, whereas those with a radius exceeding the critical value grow. In the course of their growth and dissolution, heat of crystallization is evolved and absorbed. Supercooling of the melt drops accordingly and the islands interact with each other, similar to the case in which there is Ostwald ripening of growing islands due to the arrival of a diffusion substance. A complete set of equations describing the above process is derived and solved, and asymptotic relationships are found.