Molecular dynamics simulations of cold welding of nanoporous amorphous alloys: effects of welding conditions and microstructures.

Nanoscale cold welding is a promising method in the bottom-up fabrication of nanodevices. Herein, cold welding mechanisms of Cu50Zr50 nanoporous amorphous alloys (NPAAs) are investigated by molecular dynamics simulations, along with the mechanical properties of the welded products. Effects of welding conditions and microstructural parameters are considered. Our results demonstrate that the welded joint has superior mechanical properties. The ultimate strength of the welded NPAAs can be as high as 94-99% that of the original NPAAs but 62-75% for the yield strength and elastic modulus. Voronoi analysis declares that the changes in atomic clusters of NPAAs caused by cold welding are mild. The welding conditions do not have remarkable influences on the mechanical responses of the welded structure. The NPAAs with smaller ligament sizes are more suitable for cold welding, benefiting from the size effect of amorphous alloys. We also successfully use cold welding to fabricate gradient NPAAs and repair fractured NPAAs. It is found that the ultimate tensile strength of the NPAAs changes very little with each successful cold welding. After ten fracture-welding cycles, the ultimate strength of the as-welded specimen is slightly lower than that of the raw materials.

Nanoindentation characteristics of nanocrystalline B2 CuZr shape memory alloy via large-scale atomistic simulation.

Nanoindentation tests are performed by molecular dynamics simulation to explore the mechanical properties of nanocrystalline B2 CuZr shape memory alloys with average grain sizes ranging from 6 to 18 nm. Some paramount aspects are monitored, including indentation force-depth curve, hardness, yield strength, and elastic recovery. The results demonstrate an inverse Hall-Petch effect, i.e., the hardness decreases with the decrease in grain size. For the single crystalline B2 CuZr, dislocation nucleation and propagation are the major plastic mechanisms. However, grain cleavage, grain boundary compression, and grain rotation prevail over the plastic behaviors of nanocrystalline B2 CuZr alloys. The elastic recovery becomes stronger with the increase in grain size. Besides, the effects of temperature, indenter size, and indenter speed on the nanoindentation responses are evaluated quantitively.

Characterization of the deformation behaviors under uniaxial stress for bicontinuous nanoporous amorphous alloys.

In this paper, the deformation behaviors of Cu50Zr50 bicontinuous nanoporous amorphous alloys (BNAMs) under uniaxial tension/compression are explored by molecular dynamics simulations. Scaling laws between mechanical properties and relative density are investigated. The results demonstrate that the bending deformation of the ligament is the main elastic deformation mechanism under tension. Necking and subsequent fracture of ligaments are the primary failure mechanism under tension. Under tensile loading, shear bands emerge near the plastic hinges for the BNAMs with large porosities. The typical compressive behaviors of porous structure are observed in the BNAMs with large porosities. However, for small porosity, no distinguished plateau and densification are captured under compression. The tension-compression asymmetry of modulus increases with increasing porosity, whereas the BNAMs can be seen as tension-compression symmetry of yield strength. The modulus and yield strength are negatively correlated with temperature, but a positive relationship between the tensile ductility and temperature is shown. This work will help to provide a useful understanding of the mechanical behaviors of the BNAMs.

Mechanical properties and clamping behaviors of snow crab claw.

The high-performing biomimetic behaviors of crustaceans are the optimal results of long-time wise adaption to their living environment. One outstanding prototype is crab claw, which has the combining advantages of lightweight and high strength. To promote relevant engineering applications, it is imperative to explore its mechanical behaviors and structural characteristics. In this work, mechanical test and finite element analysis (FEA) are performed to reveal the fundamental mechanical properties and clamping behaviors of snow crab (Chionoecetes opilio) claw, respectively. A lightweight modeling method, parametric lofting modeling, for the 3D modeling of the claw is employed, which is compared with the traditional reverse engineering modeling method based on tomography image. Our results demonstrated that the hardness and modulus of the regions near the top of the claw are larger than those of the regions near of bottom of the claw. Moisture is a critical factor in controlling the tensile behavior of the claw and the wet specimens exhibit higher modulus and strength under tensile loading. Besides, The parametric lofting method is highly flexible and efficient in generating 3D geometrical model. The investigation of clamping behaviors provides not only insights into mechanical behaviors and intrinsic mechanisms but also a practical guide for their potential applications, such as designing high-performing artificial clamping muscles for clinical operations, aerospace applications, and robotics.

A dissolving and glucose-responsive insulin-releasing microneedle patch for type 1 diabetes therapy.

Diabetes and its complications have become crucial public health challenges worldwide. In this study, we aim to develop a dissolving and glucose-responsive insulin-releasing microneedle (MN) patch system, for minimally invasive and glucose-responsive insulin delivery for type 1 diabetes therapy. The MNs were composed of dissolving and biodegradable gelatin and starch materials, which encapsulated glucose-responsive insulin-releasing gold nanocluster (AuNC) nanocarriers. The fabricated MNs had a complete and uniform structure, consisting of an array of 11 × 11 conical needles, with a needle height of 756 µm, a bottom diameter of 356 µm, a tip diameter of 10 µm, and a tip-to-tip distance of 591 µm. The encapsulated AuNC nanocarriers as additives in the MNs enhanced the mechanical strength of the MNs, and facilitated the penetration of the MNs into the skins of mice. Moreover, the AuNC nanocarrier drugs in the MNs enabled MN patches with a glucose-responsive insulin releasing behavior. With one transdermal application of MN patches on the dorsal skin of mice, the MN patches effectively regulated the BG levels of mice in normoglycemic ranges for 1 to 2 days, and effectively alleviated the diabetic symptoms in type 1 diabetic mice. This dissolving and glucose-responsive insulin-releasing MN patch system realized a closed-loop administration of insulin with minimal invasion, providing great potential applications for type 1 diabetes therapy.

Strengthening mechanisms of graphene in copper matrix nanocomposites: A molecular dynamics study.

To clarify the strengthening mechanism of coated/embedded graphene in metal matrix nanocomposites, nanoindentation responses of graphene-coated/embedded copper nanocomposites are investigated using molecular dynamics simulations, with the consideration of indentation force-displacement relation, stress distribution, evolution of microstructure and dislocation, and elastic recovery. Results show that two mechanisms, graphene layer bearing surface tensile stress disperses the contact stress and blocks the propagation of dislocations, contribute to the enhanced hardness and improved load bearing capacity, but one is often dominant for different nanocomposites. The former dominates in graphene-coated structure while the latter dominates in graphene-embedded structure, and the reinforcement is more obvious in the coated structure. The graphene delays the plastic deformation of matrix, and its elastic recovery is boosted due to the stress homogenization effect. The embedded graphene promotes the stress concentration and accelerates the plastic deformation of up Cu film, weakening its width elastic recovery. The observations will provide a practical guide for the mechanical optimization and design of metal-graphene nanocomposites.

Nanoscale Assembly of Copper Bearing-Sleeve via Cold-Welding: A Molecular Dynamics Study.

A bearing is an important component in contemporary machinery and equipment, whose main function is to support the mechanical rotator, reduce the friction coefficient during its movement, and guarantee the turning accuracy. However, assembly of a nanoscale bearing and sleeve is a challenging process for micro-nano mechanical manufacturers. Hence, we show the cold-welding mechanism of a copper nanobearing-nanosleeve via molecular dynamic simulations. We demonstrate that it is feasible to assemble a bearing and sleeve at the nanoscale to form a stable mechanism. The effect of temperature in the range of 150 to 750 K is investigated. As the temperature rises, the mechanical strength and the weld stress of the welded structures markedly decrease, accompanied by the observation of increasing disorder magnitude. This comparison study is believed to facilitate future mechanical processing and structural nano-assembly of metallic elements for better mechanical performance.

Effects of Cold Rolling and Annealing Prior to Dealloying on the Microstructure of Nanoporous Gold.

The properties of nanoporous gold (NPG) were known to be dependent on the microstructure of NPG. In this study, the effects of cold rolling and annealing of the original Ag0.7Au0.3 alloy on the microstructure of NPG produced by dealloying under free corrosion condition were investigated. Ag0.7Au0.3 alloy samples were cold-rolled to different strain levels/thickness reductions up to 98% and annealed at 900 °C for 3 h before dealloying. It was found that cold rolling and annealing of the original alloy can lead to reduced ligament and pore sizes of NPG. Moreover, post-deformation annealing of the original alloy was found to facilitate the formation of a homogeneous and continuous NPG structure. The minima of pore and ligament sizes (both being ~8 nm) with uniform distribution were obtained in the annealed sample with a thickness reduction of 60% for a dealloying time of 7 h. The present study indicated the significant effect of a pre-dealloying treatment of the original alloy (by plastic deformation and annealing) on the formation and optimization of the NPG microstructure produced by dealloying.

Molecular dynamics study on cold-welding of 3D nanoporous composite structures.

Nanoporous metals are a class of novel nanomaterials with potential applications in many fields. Herein, we demonstrate the cold-welding mechanism of nanoporous metals with various combinations using molecular dynamics simulations. This study shows that it is possible to cold-weld two nanoporous metals to form a novel composite material. The influence of temperature, in the range of 300-900 K, on the mechanical properties of the resultant composite material was investigated. With an increase in temperature, the weld stress and the mechanical strength of the nanoporous structures significantly decreased as an increase in disorder magnitude was observed. These results could lead to bottom-up nanofabrication and nanoassembly of combined nanoporous metals for high mechanical performance.

Nanoindentation Investigation of Temperature Effects on the Mechanical Properties of Nafion® 117.

Operating temperature can be a limiting factor in reliable applications of Proton Exchange Membrane (PEM) fuel cells. Nanoindentation tests were performed on perfluorosulfonic acid (PFSA) membranes (Nafion® 117) in order to study the influence of the temperature condition on their mechanical properties. The hardness and reduced modulus of Nafion® 117 were measured within a certain temperature range, from 10 to 70 °C. The results indicate that both hardness and elastic modulus show non-monotonic transition with the increase of the test temperature, with reaching peak values of 0.143 and 0.833 GPa at 45 °C. It also found that the membranes have a shape memory effect and a temperature dependent shape recovery ratio.

The Role of Computer Simulation in Nanoporous Metals-A Review.

Nanoporous metals (NPMs) have proven to be all-round candidates in versatile and diverse applications. In this decade, interest has grown in the fabrication, characterization and applications of these intriguing materials. Most existing reviews focus on the experimental and theoretical works rather than the numerical simulation. Actually, with numerous experiments and theory analysis, studies based on computer simulation, which may model complex microstructure in more realistic ways, play a key role in understanding and predicting the behaviors of NPMs. In this review, we present a comprehensive overview of the computer simulations of NPMs, which are prepared through chemical dealloying. Firstly, we summarize the various simulation approaches to preparation, processing, and the basic physical and chemical properties of NPMs. In this part, the emphasis is attached to works involving dealloying, coarsening and mechanical properties. Then, we conclude with the latest progress as well as the future challenges in simulation studies. We believe that highlighting the importance of simulations will help to better understand the properties of novel materials and help with new scientific research on these materials.

Synthesis of novel thiazolyl-pyrimidines and their anticancer activity in vitro.

A series of novel compounds 7-43 were prepared via the condensation of enaminones 4a-h and the guanidines carbonate 6a-f. The structures of these newly synthesized compounds were confirmed by (1) H-NMR, MS, EA and IR. All the compounds were tested for their cytotoxic activity in vitro against human cancer cell lines including Ishikawa, A549, BEL-7404, SPC-A-01 and SGC-7901. Most of them showed moderate cytotoxic against the tested cell lines. Among them, the most potent compounds 9 and 30 exhibited more efficient activity against Ishikawa, A549.

Surface effects on the mechanical properties of nanoporous materials.

Using the theory of surface elasticity, we investigate the mechanical properties of nanoporous materials. The classical theory of porous materials is modified to account for surface effects, which become increasingly important as the characteristic sizes of microstructures shrink to nanometers. First, a refined Timoshenko beam model is presented to predict the effective elastic modulus of nanoporous materials. Then the surface effects on the elastic microstructural buckling behavior of nanoporous materials are examined. In particular, nanoporous gold is taken as an example to illustrate the application of the proposed model. The results reveal that both the elastic modulus and the critical buckling behavior of nanoporous materials exhibit a distinct dependence on the characteristic sizes of microstructures, e.g. the average ligament width.

A new wetting mechanism based upon triple contact line pinning.

The classical Wenzel and Cassie models fail to give a physical explanation of such phenomenon as the macroscopic contact angle actually being equal to the Young's contact angle if there is a spot (surface defect) inside the droplet. Here, we derive the expression of the macroscopic contact angle for this special substrate in use of the principle of least potential energy, and our analytical results are in good agreement with the experimental data. Our findings also suggest that it is the triple contact line (TCL) rather than the contact area that dominates the contact angle. Therefore a new model based upon the TCL pinning is developed to explain the different wetting properties of the Wenzel and Cassie models for hydrophilic and hydrophobic cases. Moreover, the new model predicts the macroscopic contact angle in a broader range accurately, which is consistent with the existing experimental findings. This study revisits the fundamentals of wetting on rough substrates. The new model derived will help to design better superhydrophobic materials and provide the prediction required to engineer novel microfluidic devices.

Correlation of the thermal and electrical conductivities of nanoporous gold.

We measured the thermal and electrical conductivities of nanoporous Au thin foils in the temperature range 93-300 K. Resulting from the nanoscale microstructure, the two types of conductivities are both temperature dependent and significantly lower than those of bulk Au. However, the corresponding Lorenz number is strikingly similar to that of bulk Au, indicating that the Wiedemann-Franz law holds perfectly well for nanoporous metals in this temperature range. Compared to the bulk value, the Debye temperature of nanoporous Au is decreased. We predict the theoretical Debye temperature of nanoporous Au by its relation to the elastic constants. The present results indicate that the nanoporous Au foils should be comprised of macroscopic, single-crystalline porous grains rather than nanocrystals.