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We present a combined molecular dynamics (MD) and classical nucleation theory (CNT) approach to address many issues regarding the nucleation of inorganic aerosols. By taking parameters from MD simulations, we find the CNT predicts fairly reasonable free-energy profiles for the hygroscopic nucleation of aerosols. Moreover, we find that the ionization of sulfates can play a key role in stabilizing aqueous clusters and that both the size of the critical nucleus and the nucleation barrier can be significantly lowered by the H2SO4 and NH4HSO4, whereas the effect of NH3 on nucleation is negligible. NH4HSO4 provides stronger enhancement effect to aerosol formation than H2SO4. In view of the consistency between the theoretical prediction and experimental observation, the combination of MD simulation and CNT appears to be a valuable approach to gain deeper understanding of how aerosol nucleation is affected by different chemical species.

Introducing carbon nanotubes or graphene sheets into polymer matrices has received lots of scientific and technological attention. For the first time, we report a new kind of polymer nanocomposite (PNC) by means of employing anisotropic nanoparticles (NPs) as netpoints (referred to as an end-linked system), namely with NPs acting as netpoints to chemically connect the dual end-groups of each polymer chain to form a network. By taking advantage of this strategy, the anisotropic NPs can be uniformly distributed in the polymer matrix, with the NPs being separated via the connected polymer chains. And the separation distance between NPs, the stress-strain behavior and the dynamic hysteresis loss (HL) can be manipulated by varying the temperature and the polymer chain flexibility. Meanwhile, the physically mixed system is investigated by changing the interaction strength between polymer and NPs, and the temperature. It is emphasized that compared to the physically mixed system, the end-linked system which employs carbon nanotubes or graphene as netpoints possesses good thermal stability because of its thermodynamically stable morphology, exhibiting both excellent static and dynamic mechanical properties. These results help us to design and fabricate high performance and multi-functional PNCs filled with carbon nanotubes or graphene, facilitating the potentially large industrial application of these nanomaterials.

A concentration gradient along a fluid-fluid interface can cause flow. On a microscopic level, this so-called Marangoni effect can be viewed as being caused by a gradient in the pressures acting on the fluid elements, or as the chemical-potential gradients acting on the excess densities of different species at the interface. If the interfacial thickness can be ignored, all approaches should result in the same flow profile away from the interface. However, on a more microscopic scale, the different expressions result in different flow profiles, only one of which can be correct. Here we compare the results of direct non-equilibrium Molecular Dynamics simulations with the flows that would be generated by pressure and chemical potential gradients. We find that the approach based on the chemical potential gradients agrees with the direct simulations, whereas the calculations based on the pressure gradients do not., 5 pages, 4 figures

We use periodic density functional theory to investigate the adsorption properties of O and OH on different Pt–Ni alloy systems, including Pt(1 1 1), Pt(1 1 1) with surface Ni (surface for short), PtNi3(1 1 1), PtNi(1 1 1), Pt3Ni(1 1 1), Pt(1 1 1)-skin-Pt3Ni, Pt(1 1 1) with subsurface Ni (subsurface for short), and Ni(1 1 1). It is found that the adsorption strength of both O and OH displays the order of subsurface

Using the first-principles calculations, we investigated adsorption of CO and NO gas molecules on the Pd- and Pt-decorated single-walled carbon nanotube (SWNT). The metal-decorated SWNTs exhibit strong affinity toward the gas molecules. Our results reveal that the CO and NO gas molecules can be chemisorbed on the Pd or Pt atom, accompanying with the large binding energy and significant charge transfer. Adsorption of these gases would affect the electronic conductance of the materials, which can serve as a signal of gas sensor. In particular, adsorption of NO generates the magnetic properties to the metal-decorated SWNT, which can also serve as a sensitive signal for chemical sensors. After adsorption of CO and NO, the changes in binding energy, charge transfer and conductance may lead to the different response in the metal-doped CNT-based sensors. It is expected that these results could provide helpful information for the design and fabrication of the CO and NO sensing devices.

Using the first-principles calculation, we investigate the adsorption of CO and NO on (8, 0) silicon nanotubes (SiNTs). The detailed analysis of the structural and electronical properties of various optimized configurations is performed. The results show that CO molecule can be chemisorbed on SiNT with the C atom bonding with the Si atom of the tubular surface when CO is located on the top site, accompanying with the binding energy of 1.559 eV and charge transfer of 0.658|e|, which are larger than the results of other configurations. For the SiNT-NO systems, there exist four strong chemical adsorption configurations. The most stable configuration is the N atom bonding with two Si atoms on the bridge site. The binding energy is 2.135 eV and charge transfer is 2.064|e|. In addition, it is found that both the C–O and N–O bonds are elongated when CO and NO are chemisorbed on SiNT. Compared to carbon nanotubes (CNTs) or silicon carbon nanotubes (SiCNTs), the SiNTs have stronger interaction with the CO and NO a...

By employing an idealized model of a polymer network and filler, we have investigated the stress-strain behavior by tuning the filler loading and polymer-filler interaction in a broad range. The simulated results indicate that there actually exists an optimal filler volume fraction (between 23% and 32%) for elastomer reinforcement with attractive polymer-filler interaction. To realize this reinforcement, the rubber-filler interaction should be slightly stronger than the rubber-rubber interaction, while excessive chemical couplings are harmful to mechanical properties. Meanwhile, our simulated results qualitatively reproduce the experimental data of Bokobza. By introducing enough chemical coupling between the rubber and the filler, an upturn in the modulus at large deformation is observed in the Mooney-Rivlin plot, attributed to the limited chain extensibility at large deformation. Particularly, the filler dispersion state in the polymer networks is also characterized in detail. It is the first demonstration via simulation that the reinforcement mechanism stems from the nanoparticle-induced chain alignment and orientation, as well as the limited extensibility of chain bridges formed between neighboring nanoparticles at large deformation. The former is influenced by the filler amount, filler size and filler-rubber interaction, and the latter becomes more obvious by strengthening the physical and chemical interactions between the rubber and the filler. Remarkably, the reason for no obvious reinforcing effect in filled glassy or semi-crystalline matrices is also demonstrated. It is expected that this preliminary study of nanoparticle-induced mechanical reinforcement will provide a solid basis for further insightful investigation of polymer reinforcement.

In this paper, a triple resonance pulse transformer based on a magnetic core transformer is developed, which uses the first peak of the output voltage to charge the load capacitor. A compact magnetic core pulse transformer is developed, which uses an innovative bi-conical specific geometry. Based on this magnetic core pulse transformer, the triple resonance circuit is built by adding a tuning capacitor and a tuning inductor between this transformer and a pulse forming line (PFL). The tuning capacitor is designed to be an irregular coaxial capacitor with a capacitance of 70 pF and the tuning inductor is made as a single-layer air core cylindrical inductor. The experimental results indicate that the peak output voltage of the triple resonance pulse transformer is about 530 kV and the rise time is about 790 ns. The peak voltage across PFL is 1.6 times the peak voltage across the magnetic core transformer. It is feasible to develop a magnetic core pulse transformer into a triple resonance pulse transformer.

Currently, developing nonprecious-metal catalysts to replace Pt-based electrocatalysts in fuel cells has become a hot topic because the oxygen reduction reaction (ORR) in fuel cells often requires platinum, a precious metal, as a catalyst, which is one of the major hurdles for commercialization of the fuel cells. Recently, the newly emerging metal-organic frameworks (MOFs) have been widely used as self-sacrificed precursors/templates to fabricate heteroatom-doped porous carbons. Here, the recent progress of MOF-derived, heteroatom-doped porous carbon catalysts for ORR in fuel cells is systematically reviewed, and the synthesis strategies for using different MOF precursors to prepare heteroatom-doped porous carbon catalysts, including the direct carbonization of MOFs, MOF and heteroatom source mixture carbonization, and MOF-based composite carbonization are summarized. The emphasis is placed on the precursor design of MOF-derived metal-free catalysts and transition-metal-doped carbon catalysts because the MOF precursors often determine the microstructures of the derived porous carbon catalysts. The discussion provides a useful strategy for in situ synthesis of heteroatom-doped carbon ORR electrocatalysts by rationally designing MOF precursors. Due to the versatility of MOF structures, MOF-derived porous carbons not only provide chances to develop highly efficient ORR electrocatalysts, but also broaden the family of nanoporous carbons for applications in supercapacitors and batteries.

We systematically investigated the structural and dynamic properties of nanofilled elastomer on the basis of the idealized model of elastomer and nanoparticle. The simulated results indicate that the introduced nanoparticles induce more efficient chain packing. Meanwhile, a mobility gradient of polymer chains is found to exist approaching the nanoparticles. For the dynamic properties, we find for the first time that both the time−concentration and time−temperature superposition principles (TCSP and TTSP) are applicable at the chain length scale, while both break down at the segmental length scale for the filled system. However, the TTSP still holds at the segmental length scale for the pure system. Furthermore, the time−temperature−concentration superposition principle (TTCSP) applies well for the terminal relaxation of polymer nanocomposites. This scaling behavior has the underlying implication that the rheological properties (i.e., steady-state shear viscosity) of polymer nanocomposites possess thermorh...

A general synthesis strategy to prepare metal nanostructures by self-assembly was proposed by molecular dynamics (MD) simulation. In this simulating synthesis strategy, the metal nanostructures were generated by the self-assembly of the amorphous nanoparticles with the attractive forces of the nanoparticle−nanoparticle interactions by the annealing MD method at high temperatures, and finally, the resulting amorphous metal nanostructures were cooled to 10 K, which could resemble the nanoparticle self-assembly in experiment. By using the simulating synthesis, we obtained the full atomistic models of the shape-controlled metal nanostructures, including Au, Ag−Au, and beaded Ag−Cu nanorods, triangular and hexagonal Ag nanoplates, triangular Ag−Au and hexagonal Au, cubic hollow Fe, and Ag−Au nanoframes. It is found that these models are in good agreement with the experimental results. Moreover, we predicted a new metal nanostructure, Au nanoporous framework architecture, which has not been reported in experime...

Grand-canonical Monte Carlo (GCMC) simulations were performed to investigate the adsorption behavior of methane and hydrogen on a highly ordered carbon molecular sieve CMK-1 material. The rod-aligned slitlike pore (RSP) model was used to emphasize the grooved structure of the material, and the pore size distribution (PSD) was introduced to characterize the geometrical heterogeneity of the materials quantitatively. The PSD determined from adsorption isotherms of N2 at 77 K indicates that the CMK-1 adsorbent is a mesoporous material. By combining the GCMC and PSD techniques, adsorption isotherms of CH4 at 303 K and H2 at 303 and 77 K in the CMK-1 materials were obtained. The simulated isotherms are in an excellent agreement with experimental data, suggesting that it is necessary and efficient to use the PSD to characterize the materials. The GCMC predictions demonstrate that gravimetric uptakes of CH4 and H2 in the CMK-1 material at 30 MPa and 303 K are 31.23 and 1.19 wt %, respectively. Although a greater ...

We predict by using atomistic simulations that there exists a new kind of multishell onion-ring structures of bimetallic nanowires and clusters for three systems (A/B = Pd/Pt, Ag/Au, and Ag/Pd). In the structures of multishell onion-ring bimetallic nanowires and clusters, A atoms and B atoms occupy alternately the layers of the nanowires and clusters, thus forming the onion-ring morphology. The simulation results show that the multishell onion-ring structures can be found for the bimetallic nanowires and clusters at 100, 300, and 500 K. In summary, this investigation suggests a new possible morphology for the bimetallic systems at the nanoscale level, and the morphology can be tailored by controlling composition and atomic ordering of two metals. It is expected that the multishell onion-ring structure for bimetallic nanowires and clusters may provide potential applications for the design of catalytic and optical materials.

Application of lattice Monte Carlo method to the surfactant−inorganic−solvent system under a spherical confinement reveals that the confined self-assembly leads to a rich variety of morphologies. Many of the ordered structures are observed in our simulations, such as the core−shell spherical network structure and spiral structure with a core network, which have not been found for the bulk surfactant or copolymer systems and in experiments. The simulated results suggest that the occurrence of these novel structures depends on the pore diameter of the spherical cavity and the surfactant architecture.

On the basis of the all-atom model, cohesive energies per carbon atom between two single-walled carbon nanotubes (SWNTs) of uniform and different diameters were calculated, and structural stability of SWNT bundles with differing diameters was analyzed. It is found that the cohesive energy per carbon atom exhibits a minimum when the diameter of one tube is a half of that of the other. So we predict that the SWNT bundle formed by two species of tubes with the diameters of D and D / 2 is a novel stable structure. In particular, an energy of 17.44 meV per carbon atom is needed for the separation of a single tube of 2.713 nm from the stable arrangement, while only 15.88 meV per carbon atom for the separation of a single tube of 2.713 nm from the hexagonal array of SWNTs of uniform size, which suggests that the special arrangement structure is more stable than the hexagonal array. It is expected that the calculated cohesive energies per carbon atom may provide valuable information for separation and physical properties of the SWNT bundles with differing diameters.

A Monte Carlo method has been performed to simulate the thermal evolution of an icosahedral Pt55 cluster encapsulated in the (15, 15) and (20, 20) single wall carbon nanotubes (SWNTs), using the second-moment approximation of the tight-binding potentials for metal−metal interactions. The metal−carbon interactions are modeled by the Lennard-Jones potential, and the carbon atoms on the SWNTs are considered to be fixed. The melting-like structural transformation is found for the icosahedral clusters encapsulated in SWNTs. The melting-like transformation temperatures of the icosahedral clusters encapsulated in SWNTs are estimated from the fluctuations of the total potential energy, which are 280 and 320 K, respectively. The simulations indicate that the melting-like transformation temperature for the encapsulated icosahedral clusters increases with the pore size of SWNTs. At higher temperatures, a stacked structure in layers is found for the encapsulated icosahedral Pt55 clusters. Simulation results reveal th...

Molecular dynamics simulations were performed for CO2 dissolved in water near silica surfaces to investigate how the hydrophilicity and crystallinity of solid surfaces modulate the local structure of adjacent molecules and the nucleation of CO2 hydrates. Our simulations reveal that the hydrophilicity of solid surfaces can change the local structure of water molecules and gas distribution near liquid-solid interfaces and thus alter the mechanism and dynamics of gas hydrate nucleation. Interestingly, we find that hydrate nucleation tends to occur more easily on relatively less hydrophilic surfaces. Different from surface hydrophilicity, surface crystallinity shows a weak effect on the local structure of adjacent water molecules and on gas hydrate nucleation. At the initial stage of gas hydrate growth, however, the structuring of molecules induced by crystalline surfaces are more ordered than that induced by amorphous solid surfaces.

We have developed a high-current laser ion source for induction accelerators. A copper target was irradiated by a frequency-quadrupled Nd:YAG laser (266 nm) with relatively low intensities of 10(8) W/cm(2). The laser-produced plasma supplied a large number of Cu(+) ions (∼10(12) ions/pulse) during several microseconds. Emission spectra of the plasma were observed and the calculated electron temperature was about 1 eV. An induction voltage adder extracted high-current ion beams over 0.5 A/cm(2) from a plasma-prefilled gap. The normalized beam emittance measured by a pepper-pot method was smaller than 1 π mm mrad.

We have investigated segregation phenomena in Cu–Au bimetallic clusters with decahedral structures at 100 K and 300 K, based on the second-moment approximation of the tight-binding (TB-SMA) potentials by using Monte Carlo method. The simulation results indicate that there are three regions (split, three-shell onion-like and core-shell region) at 100 K and two regions (split and core-shell) at 300 K with the structure of decahedral clusters, as the chemical potential difference Δμ changes. It is found that the structure of decahedral clusters undergoes a division into smaller clusters in the split region. In the core-shell structure, Au atoms are enriched in surface and Cu atoms occupy the core of the clusters because of the different surface energy of Cu and Au. The Au atoms are enriched in the surface shell, and the Cu atoms are in the middle shell, while a single Au atom is located in the center to form the three-shell onion-like structure. The structure and binding energy of smaller clusters after splitting are also discussed. The Au atoms generally lie on the surface of the smaller clusters after splitting.

The melting behaviour of Palladium (Pd) isolated shell-symmetric cubooctahedron and icosahedron nanoclusters, both consisting of 309 atoms, were simulated by Molecular Dynamics (MD) simulation, using the Sutton-Chen many-body potential (SC) for the interaction between the Pd atoms. The thermal, structural and dynamic properties were calculated for the Pd nanoclusters along the heating process. The cubooctahedron nanocluster melts around 1040 K, much lower than the melting point of bulk Pd system (1828.05 K). The icosahedron nanocluster melts around 1070 K. Furthermore, structural and dynamic evidence is found for the pre-melting of the nanoclusters. The outer two shells of the shell-symmetric nanocluster melt prior to their homogeneous melting of the whole nanoclusters.

Potassium dihydrogen phosphate (KDP) crystal was grown from solutions in the presence of doping ethylenediamine tetra acetic acid (EDTA) in the concentration range of 0–100 ppm. Its Raman spectra were measured at room temperature with different geometrical configurations. On the basis of the group theory and lattice dynamics theory, all the Raman peaks were assigned, and a correlation between thermal conductivity and the Raman scattering strength was presented. The calculated thermal conductivity enhancements agree well with experimental enhancement of laser damage threshold, which indicates that the laser damage is a result of local thermal damage of KDP crystals.

A novel double-template route to synthesize silica hollow spheres with mesostructured walls using nano-sized calcium carbonate particles as inorganic templates and cationic surfactant as organic templates at room temperature was presented. X-ray diffraction (XRD), nitrogen adsorption, and transmission electron microscopy (TEM) were employed to characterize the morphologies and mesostructures of the as-prepared silica hollow spheres. The effect of inorganic template/surfactant ratio on the pore structure of the hollow spheres was studied. Experimental results indicated that the hollow spheres had an average diameter of nearly 40 nm with a surface area of ∼766–1050 m 2 /g.

The adsorption storage of methane on triangular arrays of single-walled carbon nanotubes (SWNT) at room temperature was investigated by the grand canonical Monte Carlo (GCMC) method. In the simulat...

In this paper the experimental adsorption isotherm and density functional theory (DFT) are used to characterize a sample of single-walled carbon nanotube (SWNT) arrays. Because the adsorption in SWNT arrays is rather complicated, and will occur both inside and outside isolated nanotubes as well as in adsorption sites due to aggregations of nanotubes, it is necessary to introduce adsorption models with different mechanisms. Four adsorption models have been introduced and compared in this work. The pore size distributions, which correspond to the models adopted, are determined by minimizing the deviations between the experimental isotherm and the DFT calculation. Our results indicate that by introducing enough flexibility in model IV proposed, in which adsorption takes place in both the outer surface and inside of the tubes separately or simultaneously, can give a good fit to the experimental isotherm.

The Raman spectra of neodymium-doped yttrium orthovanadate Nd:YVO4 (NYV) and neodymium-doped gadolinium orthovanadate Nd:GdVO4 (NGV) have been measured at room temperature with different geometrical configurations. On the basis of the group theory and lattice dynamics theory, all the Raman peaks are assigned, and a correlation between the thermal conductivity and the Raman scattering strength is presented. The thermal conductivity of Nd:GdVO4 and Nd:YVO4 crystals along [1 1 0] and [1 0 1] directions are calculated and a good agreement with experimental results is obtained. The higher thermal conductivity of NGV crystal is mainly attributed to the fact that the effective radius of Gd3+ ion in NGV crystal is close to that of Nd3+ ion, which in return, enhances the crystal field strength of the laser crystal.

The grand canonical Monte Carlo (GCMC) method is used to simulate adsorption recovery of CCl4 in activated carbon and layered pillared material at ambient temperature, T = 300 K. In the simulation, the activated carbon is modeled as a slit pore. CCl4 is described as a spherical Lennard-Jones molecule, and Steele's 10–4–3 potential is used to represent the interaction between a CCl4 molecule and a solid carbon wall. For the layered pillared pore, the site–site interaction is used to calculate the interaction between CCl4 and the pillars. In addition, an effective model is proposed by introducing the binary interaction parameters kfw to the ideal layered pillared pore model. The adsorption isotherms, local density profiles and snapshots of CCl4 in slit and layered pillared pores with pore widths H = 1.70 and 2.38 nm at temperature T = 300 K are obtained. Capillary condensation of CCl4 arises at a width of 2.38 nm for both pores. However, the layered pillared pore gives higher uptake, and the capillary condensation transition takes place at lower pressure, compared with the slit pore of the same width, 2.38 nm.

The grand canonical Monte Carlo method is used to simulate adsorption of linear two center ethane molecules in single walled carbon nanotube square arrays. Endohedral and exohedral adsorption isotherms of ethane molecules in the arrays of 2.719 and 4.077 nm tubes are obtained. Layering and capillary condensations are observed inside the tubes at 180 K. The results indicate that the exohedral adsorption in the arrays is significant, comparable with that inside the tubes. Temperature effects on the adsorption at 180 and 300 K are also discussed. The local density profiles and angle distributions simulated for the ethane molecules are used to explore microscopic and orientational structures of the adsorbate inside the tubes and in the interstices.

Removal of carbon dioxide is an essential step in many energy-related processes. Here we report a novel slurry concept that combines specific advantages of metal-organic frameworks, ion liquids, amines and membranes by suspending zeolitic imidazolate framework-8 in glycol-2-methylimidazole solution. We show that this approach may give a more efficient technology to capture carbon dioxide compared to conventional technologies. The carbon dioxide sorption capacity of our slurry reaches 1.25 mol l−1 at 1 bar and the selectivity of carbon dioxide/hydrogen, carbon dioxide/nitrogen and carbon dioxide/methane achieves 951, 394 and 144, respectively. We demonstrate that the slurry can efficiently remove carbon dioxide from gas mixtures at normal pressure/temperature through breakthrough experiments. Most importantly, the sorption enthalpy is only −29 kJ mol−1, indicating that significantly less energy is required for sorbent regeneration. In addition, from a technological point of view, unlike solid adsorbents slurries can flow and be pumped. This allows us to use a continuous separation process with heat integration., Common approaches for carbon dioxide capture include absorption in amine solutions or adsorption in porous materials. Here, the authors combine these two strategies and report carbon dioxide capture in a slurry of metal-organic frameworks in glycol-2-methylimidazole, which has advantages of both techniques.

A compact generator producing 2.2-ns 1.5 GW high-voltage pulses was developed. The generator employed a 27.6 Ω, 0.9 ns pulse-forming-line (PFL), which was charged by an iron core transformer with a turn ratio of 2:33.5 and a coefficient of 0.94. A 1.2 μF, 20 kV capacitor and a hydrogen thyratron were used in the primary circuit. When the thyratron closed at 14.5 kV, 3.4% of the energy stored in the capacitor was delivered to the PFL in 850 ns, producing a peak voltage of up to ∼500 kV. In addition, the principle of triple resonance transformation was employed by adding a 50 pF tuning capacitor and a 1.15 mH inductor between the transformer and the PFL, which led to a significant reduction of the duration and peak value of the transformer voltage without reducing that in the PFL. Meanwhile, an adjustable self-break oil switch was applied. By using transmission lines with impedance overmatched to that of the PFL, the generator delivered a 512 kV pulse across an electron beam diode, generating radiation with a dose of 20 mR/pulse at 20 cm ahead of the diode. The generator provides an excellent ultra-short radiation pulse source for the studies on radiation physics.

The ultrafine amorphous Fe 56 Zr 5 B 39 and Fe 68 B 32 powders prepared by borohydride reduction were investigated using the Mossbauer spectroscopy for a comparative study. The hyperfine field distribution P ( H ) of the Fe 56 Zr 5 B 39 powder shows a double-maxima function of H with a high-field component at 210 kOe and a low-field component at 48 kOe. The EXAFS investigation on the Fe 56 Zr 5 B 39 powders revealed the short-range order structure in the powders, which coincides with the behavior of P ( H ).

Using first-principles calculations, we have systematically studied the quantum size effects of ultrathin Pb(111) films on the adsorption energies and diffusion energy barriers of oxygen atoms. For the on-surface adsorption of oxygen atoms at different coverages, all the adsorption energies are found to show bilayer oscillation behaviors. It is also found that the work function of Pb(111) films still keeps the bilayer-oscillation behavior after the adsorption of oxygen atoms, with the values being enlarged by 2.10 to 2.62 eV. For the diffusion and penetration of the adsorbed oxygen atoms, it is found that the most energetically favored paths are the same on different Pb(111) films. And because of the modulation of quantum size effects, the corresponding energy barriers are all oscillating with a bilayer period on different Pb(111) films. Our studies indicate that the quantum size effect in ultrathin metal films can modulate a lot of processes during surface oxidation.

Polymer nanocomposites (PNCs) often exhibit excellent mechanical, thermal, electrical and optical properties, because they combine the performances of both polymers and inorganic or organic nanoparticles. Recently, computer modeling and simulation are playing an important role in exploring the reinforcement mechanism of the PNCs and even the design of functional PNCs. This report provides an overview of the progress made in past decades in the investigation of the static, rheological and mechanical properties of polymer nanocomposites studied by computer modeling and simulation. Emphases are placed on exploring the mechanisms at the molecular level for the dispersion of nanoparticles in nanocomposites, the effects of nanoparticles on chain conformation and glass transition temperature (T(g)), as well as viscoelastic and mechanical properties. Finally, some future challenges and opportunities in computer modeling and simulation of PNCs are addressed.

By using the first-principles DFT calculations, we design a novel hydrogen storage material, Li(12)Si(60)H(60) composite, and validate its geometric stability. It is found that the adsorbed Li atoms do not cluster on the Si(60)H(60) fullerene unlike other metals such as Ti, owing to the relatively low Li-Li binding energy and the inhibition of Si-H bonds. Our results show that the Li-doping enhances the hydrogen adsorption ability of Si(60)H(60) significantly, owing to the charge transfer from the doped Li atoms to the host material and the polarization of the adsorbed H(2) molecules. By combining the first-principles calculation and grand canonical Monte Carlo simulation, we further investigate the hydrogen storage capacity of the simulation-synthesized exohedral Li(12)Si(60)H(60) composite at T = 77 K. As the vdW gap (i.e., the separation between the surfaces of two Li(12)Si(60)H(60) fullerenes) is equal to 8.2 A, the total hydrogen uptake of the square-arranged Li(12)Si(60)H(60) array reaches 12.83 wt % at p = 10 MPa, while the excess hydrogen uptake shows a maximum of 7.46 wt % at p = 6 MPa. Impressively, at T = 298 K and p = 10 MPa, the Li(12)Si(60)H(60) array still exhibits a total hydrogen uptake of 3.88 wt % at the vdW gap of 8.2 A. These results clearly indicate that the composite, Li(12)Si(60)H(60) fullerene, is a promising candidate for hydrogen storage.

In this work, we propose a method to stabilize a nucleus in the framework of lattice density-functional theory (LDFT) by imposing a suitable constraint. Using this method, the shape of critical nucleus and height of the nucleation barrier can be determined without using a predefined nucleus as input. As an application of this method, we study the nucleation behavior of vapor-liquid transition in nanosquare pores with infinite length and relate the observed hysteresis loop on an adsorption isotherm to the nucleation mechanism. According to the dependence of hysteresis and the nucleation mechanism on the fluid-wall interaction, $w$, in this work, we have classified $w$ into three regions ($wg0.9$, $0.1\ensuremath{\le}w\ensuremath{\le}0.9$, and $wl0.1$), which are denoted as strongly, moderately, and weakly attractive fluid-wall interaction, respectively. The dependence of hysteresis on the fluid-wall interaction is interpreted by the different nucleation mechanisms. Our constrained LDFT calculations also show that the different transition paths may induce different nucleation behaviors. The transition path dependence should be considered if morphological transition of nuclei exists during a nucleation process.

We propose a density-functional theory for inhomogeneous polyatomic fluids with complex architecture by introducing a different representation for the polymers. This representation gives an efficient hierarchical algorithm to calculate the direct bonding connectivity integral for polymers with complex architecture, such as linear, star, branched, and dendritic structures. A comparison with the available simulated data for linear and star polymers confirms the accuracy of the present theory in reproducing the density profiles of the two types of polymer in the slits. By using the proposed algorithm, we also explore partitioning coefficients of polymers of different architectures in a slit, and find that the partitioning coefficients of branched, star, and dendrimer forms of 22-mers decrease to a minimum at extremely low packing fraction, and then increase monotonically with packing fraction. Moreover, it is found that it is more difficult for a linear polymer of 22-mers to enter the slit than for branched, star, and dendritic polymers. In addition, we also investigate the self-assembly of diblock copolymers with different tails in a slit. It is found that the linear copolymer self-assembles into a trilayer film structure, while copolymers with branched and dendritic tails self-assemble into a five-layer film structure. Interestingly, the copolymer with a star tail self-assembles into a trilayer film structure, and then the trilayer structure evolves into a five-layer structure with increase of the bulk packing fraction in the case studied.

A common computational method for the characterization of porous materials is to calculate the adsorption isotherm of fluids in the materials from the preassumed wall–fluid potential. If the wall–fluid potential is unknown, the common computational method becomes invalid. In a realistic experiment, however, it is common to know the experimental adsorption isotherm of nitrogen and not to know the wall–fluid potential. Here we propose a stepwise approximation for modeling wall–fluid potential under conditions where only the adsorption isotherm of nitrogen is measured experimentally. Based on the modeled wall–fluid potential, we can characterize the porous materials and predict the adsorption of other adsorbates on the materials. It is expected that the approach would provide a powerful means for the characterization of novel materials under conditions where only the experimental adsorption isotherm is available.

The meltinglike transition of the ${\mathrm{Cu}}_{1}{\mathrm{Au}}_{54}$ and ${\mathrm{Cu}}_{12}{\mathrm{Au}}_{43}$ clusters is investigated by canonical Monte Carlo simulations, based on the second-moment approximation of the tight-binding potentials. The structures of both the ${\mathrm{Cu}}_{1}{\mathrm{Au}}_{54}$ and ${\mathrm{Cu}}_{12}{\mathrm{Au}}_{43}$ clusters, shown to be icosahedral, are obtained from the so-called semi-grand-canonical ensemble Monte Carlo simulation at $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A core-shell structure is found in ${\mathrm{Cu}}_{1}{\mathrm{Au}}_{54}$, with a single Cu atom in the center and 54 Au atoms on the surface and in interior shells of the cluster. On the other hand, ${\mathrm{Cu}}_{12}{\mathrm{Au}}_{43}$ possesses a three-shell onionlike structure, with a single Au atom located in the center, 12 Cu atoms in the middle shell, and 42 Au atoms occupying the surface shell of the cluster. Melting characteristics are observed by the changes in the caloric curve, heat capacity, root-mean-square bond-length fluctuation, and deformation parameter. It is found that doping of ${\mathrm{Au}}_{55}$ with a single Cu atom can sharply raise the melting point of the cluster. In addition, the three-shell onionlike structure can be transformed into the core-shell structure at the higher temperatures after melting. It is also found that surface segregation of Au atoms in ${\mathrm{Cu}}_{1}{\mathrm{Au}}_{54}$ and ${\mathrm{Cu}}_{12}{\mathrm{Au}}_{43}$ occurs in the liquid phase. In addition, the simulation results show that the melting point increases with the concentration of Cu in the 55-atom Cu-Au bimetallic cluster.

We report a theoretical study of nanobubble stabilization on a substrate by contact line pinning. In particular, we predict the magnitude of the pinning force required to stabilize a nanobubble and the threshold values of the pinning force that the substrate can provide. We show that the substrate chemistry and the local structures of substrate heterogeneity together determine whether or not surface nanobubbles are stable. We find that for stable nanobubbles, the contact angles are independent of substrate chemistry as its effects are cancelled out by the pinning effect. This prediction is in agreement with available experimental data.

Selective epitaxial growth (SEG) of silicon-germanium (SiGe) films on patterned-oxide silicon substrates, using a tubular hot-wall low pressure chemical vapor deposition (LPCVD) system, is demonstrated in this study. This conventional system is proposed as a low cost alternative for SiGe epitaxial growth. Three process improvements needed to achieve quality growth are discussed. First, the hydrogen bake process is modified to eliminate Ge-outgassing. Secondly, a Si SEG buffer layer is deposited prior to SiGe SEG. Finally, a small flow of dichlorosilane is introduced during the temperature ramp-down period prior to SiGe SEG. The growth results are discussed in terms of growth selectivity, thickness uniformity, growth rate, defect density, SiGe film composition, and electrical properties.

The chemical doping method is often adopted to obtain metal-free conducting materials. To date, it is still a great challenge to controllably prepare metal-free semiconducting and conducting materials by tuning the inherent structure of a material. In this work, a class of novel one-dimensional (1D) covalent-organic polymer (COP) has been designed, whose cross-sections are triangular, tetragonal, pentagonal and hexagonal, and their electronic properties are explored. The tetragonal 1D COP exhibits unique phenomena in electronic properties, i.e. the tetragonal COPs with mono- or trilayer defects (odd defects) show semiconducting properties, while they become conductors for the two cases of non- or bilayer defects (even defects). This observation indicates that they comply with the characteristics of semiconducting and conducting switches induced by the odd-even defects. Therefore, we infer that for the tetragonal configuration, the odd-even defects could potentially manipulate the electrical behavior of the COP material. The discovery provides a new direction for the targeted synthesis of semiconducting and conducting materials by tuning the inherent structure of materials, which is entirely different from the chemical doping method yielding metal-free conducting materials.

Computational modelling is a powerful tool for the study of gas–solid interactions, and can be used both to complement experiment and design new materials. For the modelling of gas adsorption by nanoporous media, a multiscale approach can be used, in which the molecular force fields required for Grand Canonical Monte Carlo (GCMC) simulations are derived from first-principles calculations. This can result in significantly enhanced accuracy, in comparison with conventional empirical force field-based GCMC methods. In this article, we review the application of this multiscale approach to the simulation of the adsorption of hydrogen, methane and carbon dioxide in Porous Coordination Frameworks (PCFs) for the purpose of gas storage for energy transportation and Carbon Capture and Storage (CCS) technology. We also define a scheme for the design of new materials with improved adsorption performance for the storage of these gases through the combination of multiscale simulation and experimental work, and discuss some of the issues regarding gas adsorption measurement accuracy in the context of the validation of simulation results using experimental data.

Nano-strengthening by employing nanoparticles is necessary for high-efficiency strengthening of elastomers, which has already been validated by numerous researches and industrial applications, but the underlying mechanism is still an open challenge. In this work, we mainly focus our attention on studying the variation of the tensile strength of nanofilled elastomers by gradually increasing the filler content, within a low loading range. Interestingly, the percolation phenomenon is observed in the relationship between the tensile strength and the filler loading, which shares some similarities with the percolation phenomenon occurring in rubber toughened plastics. That is, as the loading of nanofillers (carbon black, zinc oxide) increases, the tensile strength of rubber nanocomposites (SBR, EPDM) increases slowly at first, then increases abruptly and finally levels off. Meanwhile, the bigger the particle size, the higher the filler content at the percolation point, and the lower the corresponding tensile strength of rubber nanocomposites. The concept of a critical particle-particle distance (CPD) is proposed to explain the observed percolation phenomenon. It is suggested that rubber strengthening through nanoparticles is attributed to the formation of stretched straight polymer chains between neighbor particles, induced by the slippage of adsorbed polymer chains on the filler surface during tension. Meanwhile, the factors to govern this CPD and the critical minimum particle size (CMPS) figured out in this work are both discussed and analyzed in detail. Within the framework of this percolation phenomenon, this paper also clearly answers two important and intriguing issues: (1) why is it necessary and essential to strengthen elastomers through nanofillers; (2) why does it need enough loading of nanofillers to effectively strengthen elastomers. Moreover, on the basis of the percolation phenomenon, we give out some guidance for reinforcement design of rubbery materials: the interfacial interactions between rubber and fillers cannot be complete chemical bonding, and partial physical absorption of macromolecular chains on the filler surface is necessary, otherwise the formation of stretched straight chains would be seriously hindered. There should exist such an optimum crosslinking density for a certain filler reinforced rubber system, and as well an optimum filler loading for rubber strengthening. Additionally, the different percolation behaviors of Young's modulus, the tensile strength and the electrical conductivity are compared and analyzed in our work. Lastly, molecular simulation indicates that it is not possible to strengthen glassy or hard polymer matrices by incorporating spherical nanoparticles. In general, by providing substantial experimental data and detailed analyses, this work is believed to promote the fundamental understanding of rubber reinforcement, as well provide better guidance for the design of high-performance and multi-functional rubber nanocomposites.

Segregation phenomena of Ag–Cu–Au trimetallic clusters with icosahedral structure are investigated by using a Monte Carlo method based on the second-moment approximation of the tight-binding (TB-SMA) potentials. We predict that the Ag atoms segregate to the surface of the Ag–Cu–Au trimetallic icosahedral clusters. The Ag concentrations in the surface layer of the clusters are about 11–29 at.% higher than the overall Ag concentration in all the cases studied. The simulation results also indicate that the Au atoms are mainly distributed in the middle shell and the Cu atoms are located in the center for the 147-, 309-, 561- and 923-atom clusters at 300 K. The segregation phenomena of the Ag, Au and Cu atoms in the Ag–Cu–Au trimetallic clusters are mainly due to the different surface energies of the Ag, Au and Cu atoms. It is found that the size and composition have little effect on the segregation phenomena of Ag, Au and Cu atoms in the Ag–Cu–Au trimetallic cluster.