A molecular dynamics simulation on the effect of different parameters on thermal resistance of graphene-argon interface

Molecular dynamics simulations of argon molecules confined between two parallel graphene sheets are carried out to investigate the parameters affecting heat transfer and thermal properties. These parameters include wall–fluid interaction strength, fluid density and wall temperature. For constant wall temperature simulations, we show that the first two parameters have influence on near-wall fluid density. As a result, the heat transfer at wall–fluid interfaces and thus through argon molecules across the domain will change. Also, we demonstrate that variations in wall temperature rarely affects the density profiles of argon molecules next to the walls. Therefore, in these cases, the variations in thermal resistance at the interface is most dominantly due to wall temperature itself. To analyse the results, the density and temperature profiles and also other parameters including heat flux and temperature gradient of bulk of argon molecules, Kapitza length and argon thermal conductivity are considered. The Kapitza length describes thermal resistance at liquid–solid interface. According to the results, increasing wall–fluid interaction strength leads to greater molecular aggregation of argon molecules near the walls and, consequently, decreasing the Kapitza length. Furthermore, higher fluid density leads to greater thermal resistance at wall–fluid interactions and therefore greater temperature jumps are observed in temperature profiles.     

Molecular dynamics simulation of film size and vacancy defect effects on the thermal conductivities of argon thin films

It is well known that there is a size effect for the thermal conductivity of thin films and that vacancy defects in film reduce the film's thermal conduction. In this paper, the film size and vacancy defect effects on the thermal conductivities of argon thin films were studied by molecular dynamics simulations. The results show the existence of phonon boundary scattering. The results also confirm that the theoretical model based on the Boltzmann equation can accurately model the thermal conduction of thin argon films. Both the theoretical and MD results illustrate that, although, both the defect and the thickness of the thin film deduce the thermal conductivity, their physical mechanisms differ.     

Molecular dynamics study of the thermal conductivity of nanoscale argon films

Non-equilibrium molecular dynamics (MD) simulations were used to study the thermal conductivity of thin argon films. The MD simulations show that the argon film's thermal conductivity is affected by the thickness up to thickness of about 100 nm, which agrees with theoretical estimates. The results show that the MD method is very effective for modeling nanoscale thermal conduction. Besides pure argon films, the effect of vacancies on the argon film's thermal conductivity was also studied. The vacancies greatly reduce the thermal conductivity as a function of the vacancy concentration but not as a function of the vacancy distribution when the film's temperature is constant.     

Solvation Force and Confinement-Induced Phase Transitions of Model Ultra Thin Films

Abstract

The static (equilibrium) properties of atomically thin films confined between two surfaces are studied as a function of surface separation by Grand Canonical Monte Carlo and Molecular Dynamics simulations. A model was used, in which the fluid and wall species consist of two different Lennard-Jones rare gas atoms. This was designed to mimic the static SFA experiments in which it is known that epitaxy is not necessary for inducing an oscillatory solvation force in simple non polar liquids. We have been able to simulate, using this simple system, many aspects of the equilibrium properties observed in the experiments. The solvation force is an exponentially damped, periodic curve. All peaks of maximum amplitude in the solvation force correspond to solid-like structures. These structures melt in increasing the surface separation. A further increase in separation leads to the addition of a whole layer and the recrystallisation of the film. In addition this model displays an interesting phenomenon of confinement induced solid-solid phase transition. Two different stable packing (bcc and triclinic) can be observed in the bilayer film and a transition from one to the other occurs when the surface separation is changed. This phase change has been studied as a function of pressure and temperature. As compared to the simulations using a ‘commensurate’ model, in which the fluid and wall species are made of like atoms, the results obtained here are in much better agreement with experimental findings.     

Solid/Liquid Cluster Recognition in Heterogeneous Systems

Abstract

Criteria are presented which enable the differentiation between the extent of solid-like and liquid-like character in a heterogeneous system on a per atom basis. Such criteria are developed for two different interatomic potentials, the Stillinger-Weber model for group IV semiconductors and the Lennard-Jones model for insulators. For the Stillinger-Weber potential model, three criteria are presented: one based on the coordination number of nearest neighbors, one based on the three-body energy, and one based on the angular positions (spatial arrangements) of neighbors. For the Lennard-Jones model an angular criterion is used. The difficulties associated with the assignation of interfacial atoms (those with partial solid and partial liquid character) are discussed. The effectiveness of these criteria for both models is tested by application to the identification of solid-like nuclei in the melt.     

Molecular Dynamics Simulation of Self Diffusion in Microclusters

Abstract

Molecular dynamics simulation is carried out to study the mechanism of self diffusion which is characteristic of solid-like microclusters. A two-dimensional system with the Lennard-Jones potential is employed and the temperatures near the triple point of the two-dimensional bulk system are adopted for the simulation. The results show that: a) microclusters consist of two regions, i.e., solid-like cores and liquid-like surface regions, b) the size dependence of the diffusion coefficient for microclusters is weak, and the value of the solid-like core region is not much different from that of the bulk liquid, c) the activation energy of diffusion for microclusters is twenty to thirty times larger than that for the bulk liquid, d) the diffusion mechanism in the solid-like region involves the collective motion of small domains containing ten to twenty atoms which results in the formation of low density regions, sometimes even vacancy clusters, between them, and atoms in the low density regions change their positions to cause diffusion.     

The Effect of Annealing Temperature on the Plasma Edge in Reflectance Spectra of Al/Al2O3 Composites Synthesized by Thermal Oxidation of Aluminum Thin Films

In this study, the structural and optical properties of aluminum oxide thin films were investigated. Aluminum oxide thin films were prepared on silicon and glass substrate by DC magnetron sputtering of aluminum targets with subsequent thermal oxidation of the aluminum-deposited thin films. Important result obtained included the presence of a plasma edge for the individual aluminum atoms. In addition, the temperatures that resulted in the highest concentration of surface plasmons were determined. On other hand, the relationship between the plasma edge and the optical energy gap was investigated.

     

Cell–surface interactions involving immobilized magnetite nanoparticles on flat magnetic substrates

A new method to affect cells by cell–surface interaction is introduced. Biocompatible magnetic nanobeads are deposited onto a biocompatible magnetic thin layer. The particles are composed of small magnetite crystals embedded in a matrix which can be functionalized by different molecules, proteins or growth factors. The magnetic interaction between surface and beads prevents endocytosis if the setup is utilized for cell culturing. The force acting between particles and magnetic layer is calculated by a magnetostatic approach. Biocompatibility is ensured by using garnet layers which turned out to be nontoxic and stable under culturing conditions. The garnet thin films exhibit spatially and temporally variable magnetic domain configurations in changing external magnetic fields and depending on their thermal pretreatment. Several patterns and bead deposition methods as well as the cell–surface interactions were analyzed. In some cases the cells show directed growth. Theoretical considerations explaining particular cell behavior on this magnetic material involve calculations of cell growth on elastic substrates and bending of cell membranes.     

The phase diagram of the Lennard-Jones fluid using temperature dependent interaction parameters

The forces of interaction between argon atoms can be described by the Lennard-Jones potential model. It is hypothesised that the use of temperature dependent interaction parameters, instead of using temperature independent interaction parameters, may lead to improvement in the prediction of the vapour–liquid coexistence curve. Published second virial coefficient data were used to fit a simple two-parameter temperature dependent model for the collision diameter and well depth. Vapour–liquid coexistence curve for argon was simulated in the NVT Gibbs ensemble Monte Carlo technique. The simulations were carried out using each of the temperature independent and temperature dependent parameters in the temperature range: 110–148 K. The critical temperature and density were determined using the Ising-scaling model. The results using temperature dependent parameters produce, overall, a more accurate phase diagram compared to the diagram generated using temperature independent interaction parameters. The root mean square deviation is reduced by 42.1% using temperature dependent interaction parameters. Also, there was no significant difference between the results obtained using temperature dependent interaction parameters and the highly accurate and computationally demanding phase diagrams based on three body contributions.     

Precursor Decomposition,Microstructure, and Porosity of Yttria Stabilized Zirconia Thin Films Prepared by Aerosol‐Assisted Chemical Vapor Deposition

Microstructures of yttria stabilized zirconia thin films deposited by aerosol assisted chemical vapor deposition (AA‐CVD) are correlated with the thermal decomposition behavior of the corresponding metal precursors, zirconium and yttrium 2,4‐pentanedionate. Process conditions of AA‐CVD are investigated with the aim of producing dense and compact YSZ thin films for applications as gas‐tight electrolyte. Based on systematic cross sectional scanning transmission electron microscopy (STEM) investigations and conductivity measurements, the development of percolating nanoporosity is observed in samples prepared at temperatures between 350 °C and 600 °C at standard solution throughput. Compact columnar thin films with bulk conductivity are obtained at 600 °C by reducing the metal content of the precursor solution and at 450 °C by reducing the solution throughput.     

Specification of surface mating systems among conjugative drug resistance plasmids in Escherichia coli K-12

Representative plasmids for most incompatibility groups in Escherichia coli K-12 were transferred to a "bald" strain to compare transfer frequencies for liquid and solid media. Standard broth matings were used for a liquid environment, but for solid surface mating, conjugation was allowed to take place on nutrient plates before washing off the cells for transconjugant selection on plates containing appropriate drugs. Plasmids that determine rigid pili transferred at least 2,000x better on plates than in broth. Some plasmids that determine thick flexible pili transferred 45 to 470x better, whereas others transferred equally well in both environments, as did plasmids of the I complex, which determine thin flexible pili. These results clearly distinguished a number of surface mating systems where most plasmids were derepressed for transfer and determined conjugative pili constitutively. The temperature-independent IncH2 plasmid R831b transferred best on plates, but other IncH plasmids transferred equally well in broth. This inconsistency led to the reclassification of R831b as IncM.     

Determination of the Optical Thickness of sub 10-nm Thin Metal Films by SPR Experiments

We discuss the experimental data of surface plasmon resonance (SPR) occurring at the interface between air and single and bimetallic thin layers of Au and Ag prepared on glass substrates. The bilayer configuration allowed for the measurements of the optical constants of metallic films that are ultra thin; e.g., below 10 nm of thickness since SPR modes on such thin films in a single-layer configuration are shallow. We also discuss the effect of film thickness on SPR coupling. Thickness and refractive index of the films were determined by matching experimental SPR curves to the theoretical ones. Thickness and roughness of the films were also measured by atomic force microscopy. The results obtained by experimental measurements are in good agreement with AFM analysis.     

Simultaneously Enhancing the Thermal Stability,Mechanical Modulus,and Electrochemical Performance of Solid Polymer Electrolytes by Incorporating 2D Sheets

Solid state electrolytes are the key components for high energy density lithium ion batteries and especially for lithium metal batteries where lithium dendrite growth is an inevitable obstacle in liquid electrolytes. Solid polymer electrolytes based on a complex of polymers and lithium salts are intrinsically advantageous over inorganic electrolytes in terms of processability and film‐forming properties. But other properties such as ionic conductivity, thermal stability, mechanical modulus, and electrochemical stability need to be improved. Herein, for the first time, 2D additives using few‐layer vermiculite clay sheets as an example to comprehensively upgrade poly(ethylene oxide)‐based solid polymer electrolyte are introduced. With clay sheet additives, the polymer electrolyte exhibits improved thermal stability, mechanical modulus, ionic conductivity, and electrochemical stability along with reduced flammability and interface resistance. The composite polymer electrolyte can suppress the formation and growth of lithium dendrites in lithium metal batteries. It is anticipated that the clay sheets upgraded solid polymer electrolyte can be integrated to construct high performance solid state lithium ion and lithium metal batteries with higher energy and safety.     

Ultrastructural studies on the sporulation of oocysts of Toxoplasma gondii. II. Formation of the sporocyst and structure of the sporocyst wall

The ultrastructural changes observed during sporocyst formation and the structure of the sporocyst wall was examined in oocysts which had been allowed to sporulate for between 12 and 48 hours at 27 degrees C. As the spherical sporoblast developed into the sporocyst the cytoplasmic mass became ellipsoidal in shape although no change was noted in the organelle compliment, which cosisted of two nuclei plus a number of polysaccharide granules, lipid globules, mitochondria, Golgi bodies, and some rough endoplasmic reticulum. The sporocyst wall consisted of a thin outer layer (15-20 nm) which was formed from two limiting membranes of the sporoblast and an inner layer (40-50 nm) which was comprised of four curved plates. This inner layer was formed under the outer layer and, although no specific cytoplasmic organelle disappeared with its formation, some unit membranes were observed close to the plasmalemma during its formation. Each curved plate has a marginal swelling and an interposing strip of material is present between the margins of adjacent plates. The plates are joined to the interposing strip by a thin band of osmiophilic material. In oblique and tangential sections through the plates two types of cross banding were observed which differed in periodicity.     

Layer-by-layer assembly of multilayer films composed of avidin and biotin-labeled antibody for immunosensing

Protein multilayers composed of avidin and biotin-labeled antibody (bio-Ab) were prepared on gold surface by layer-by-layer assembly technology using the high specific binding constant (K(a): approximately 10(15) M(-1)) between avidin and biotin. The assembly process of the multilayer films was monitored by using real-time BIA technique based on surface plasmon resonance (SPR). The multilayer films were also characterized by electrochemical impedance spectroscopy (EIS) and reflection absorption Fourier transform infrared spectroscopy (FTIR). The results indicate that the growth of the multilayer is uniform. From response of SPR for each layer, the stoichiometry S for the interaction between avidin and bio-Ab is calculated to be 0.37 in the multilayer whereas 0.82 in the first layer. The protein mass concentration for each layer was also obtained. The schematic figure for the multilayer assembly was proposed according to the layer mass concentration and S value. The utility of the mutilayer films for immunosensing has been investigated via their subsequent interaction with hIgG. The binding ability of the multilayer increased for one to three layers of antibody, and then reach saturation after the fourth layer. These layer-by-layer constructed antibody multilayers enhance the binding ability than covalently immobilized monolayer antibody. This technology can be also used for construction of other thin films for immunosensing and biosensor.     

Graphene Multilayer Supported Gold Nanoparticles for Efficient Electrocatalysts Toward Methanol Oxidation

This study reports a simple method of integrating electroactive gold nanoparticles (Au NPs) with graphene oxide (GO) nanosheet support by layer‐by‐layer (LbL) assembly for the creation of 3‐dimensional electrocatalytic thin films that are active toward methanol oxidation. This approach involves the alternating assembly of two oppositely charged suspensions of Au NPs with GO nanosheets based on electrostatic interactions. The GO nanosheets not only serve as structural components of the multilayer thin film, but also potentially improve the utilization and dispersion of Au NPs by taking advantages of the high catalytic surface area and the electronic conduction of graphene nanosheets. Furthermore, it is found that the electrocatalytic activity of the multilayer thin films of Au NPs with graphene nanosheet is highly tunable with respect to the number of bilayers and thermal treatment, benefiting from the advantageous features of LbL assembly. Because of the highly versatile and tunable properties of LbL assembled thin films coupled with electrocatalytic NPs, we anticipate that the general concept presented here will offer new types of electroactive catalysts for direct methanol fuel cells.     

Mechanism of the Atomic Intermixing Process in Crystalline Microclusters

Abstract

The Mechanism of atomic intermixing process in crystalline microclusters is studied by molecular dynamics simulation for a two-dimensional system with the Lennard-Jones potential. Temperature is chosen so that a cluster consists of solid-like core region and the region of surface melting. It is found that atomic intermixing in the solid-like core region is caused by the motion of a dislocation through the cluster as well as the random walk of a vacancy in the cluster. Generation of a dislocation or a vacancy occurs at the interfacial region between the liquid-like surface and the solid-like core regions due to large scale fluctuation of the configuration of atoms in the region of surface melting and the opportune collective motion of atoms in the solid-like core region. The rate per atom of atomic intermixing, the basic quantity of our interest (for the definition see the text), in the solid-like core of the microcluster is three to four orders of magnitude larger than that in the bulk crystal.     

Measurement of the electric conductivity of tungsten in a continuous liquid-to-gas transition

A method is developed that makes it possible to investigate the transition of a metal from a condensed to a gaseous phase while maintaining almost uniform temperature and pressure distributions in the sample. The method consists in the pulsed Joule heating of a sample in the form of a thin foil strip placed between two relatively thick glass plates. This method is used to measure the conductivity of tungsten in a process during which the pressure in the sample is maintained at a level of 40–60 kbar and the density of the sample decreases from the normal solid density to a density 20 to 30 times lower. It is found that, along the 40-kbar isobar, the density dependence of the conductivity of tungsten changes radically at a certain density value, at which it has a pronounced kink. At the kink, the density of tungsten is approximately ten times lower than its characteristic solid density, and the internal energy is about two times the sublimation energy. The method makes it possible to carry out experiments with the almost isobaric heating of tungsten in the parameter range in which the effect in question takes place. No such effect is detected in nonisobaric processes.     

Technical device for obtaining Raman spectra of ultrathin films of phospholipids.

The association of a sample illumination apparatus for supporting ultra thin films with a microprobe (MOLE) spectral analysis system allowed us to obtain non-resonant Raman spectra of phospholipid films. Films as thin as 75 Å have yielded useful spectra.     

Autoradiographic Differentiation of Tritium and Another β-Emitter by A Combined Colour-Coupling and Double Stripping-Film Technique

This technique distinguishes cells labelled with ³H, with ₁₄C, or with both isotopes together, in the same histological preparation. The technique depends on the application of two layers of autoradiographic stripping film, separated by a thin layer of celloidin. The first layer (in contact with the tissue) records predominantly the distribution of ³H in the sample, the second exclusively that of ¹⁴C. The silver grains in one layer are coloured by dye-coupling, which enables the grains in the two layers to be differentiated without the need for separate focussing. The merits of stripping film over liquid emulsion are: rigid control of the thickness and uniformity of the film is assured; an inert celloidin layer of 0.1 μ or less can be applied between the two films; and the thickness of each film can be chosen to suit emission characteristics of the radioisotopes.