Molecular dynamics modeling and simulation of water desalination through a double-walled carbon nanotube with moiré pattern

Freshwater scarcity has emerged as a major challenge of our time. Under this context, the importance of an efficient and energy-saving water desalination method is highlighted. In recent years, carbon nanotube (CNT) membrane characterizing with high permeability has attracted much attention in research, and it is regarded as a promising alternative to the conventional reverse osmosis technology. This work aims at numerically investigating the water desalination ability of a novel type of CNT membrane structure, namely the double-walled carbon nanotube (DWCNT) with Moiré pattern. After establishing the physical CNT models and running the molecular dynamics (MD) simulation of the water desalination system, it is found that both the single-walled carbon nanotube (SWCNT) and DWCNT can desalinate the seawater successfully while the water permeability of DWCNT is at least 18.9% higher than that of SWCNT within the same time. As far as the Moiŕe pattern adopted in this study is concerned, the water permeability of DWCNT without Moiŕe pattern is 18.6% higher than that with Moiré pattern.

Temperature and entropy in molecular system

The irreversibility, temperature, and entropy are identified for an atomic system of solid material. Thermodynamics second law is automatically satisfied in the time evolution of molecular dynamics (MD). The irreversibility caused by an atom spontaneously moves from a non-stable equilibrium position to a stable equilibrium position. The process is dynamic in nature associated with the conversion of potential energy to kinetic energy and the dissipation of kinetic energy to the entire system. The forward process is less sensitive to small variation of boundary condition than reverse process, causing the time symmetry to break. Different methods to define temperature in molecular system are revisited with paradox examples. It is seen that the temperature can only be rigorously defined on an atom knowing its time history of velocity vector. The velocity vector of an atom is the summation of the mechanical part and the thermal part, the mechanical velocity is related to the global motion (translation, rotation, acceleration, vibration, etc.), the thermal velocity is related to temperature and is assumed to follow the identical random Gaussian distribution for all of its [Formula: see text], [Formula: see text] and [Formula: see text] component. The [Formula: see text]-velocity (same for [Formula: see text] or [Formula: see text]) versus time obtained from MD simulation is treated as a signal (mechanical motion) corrupted with random Gaussian distribution noise (thermal motion). The noise is separated from signal with wavelet filter and used as the randomness measurement. The temperature is thus defined as the variance of the thermal velocity multiply the atom mass and divided by Boltzmann constant. The new definition is equivalent to the Nose–Hover thermostat for a stationary system. For system with macroscopic acceleration, rotation, vibration, etc., the new definition can predict the same temperature as the stationary system, while Nose–Hover thermostat predicts a much higher temperature. It is seen that the new definition of temperature is not influenced by the global motion, i.e., translation, rotation, acceleration, vibration, etc., of the system. The Gibbs entropy is calculated for each atom by knowing normal distribution as the probability density function. The relationship between entropy and temperature is established for solid material.

Optimal structure control for earthquake resistance

This work developed the optimal and active control algorithms applicable to structural control for earthquake resistance. [Lewis, F. L., Vrabie, D. and Syrmos, V. L. [2012] Optimal Control (John Wiley & Sons)] developed a rigorous and comprehensive procedure for the derivation of an optimal control strategy based on the calculus of variation. This work is an application of Lewis’ formulation to the control of a structure for earthquake resistance. We developed a computer software which can be used to generate a dynamic model to simulate a planar structure and to construct the control law. This model also includes the tendon driven actuators, sensors and true history of earthquake excitation. The control law has two parts: (I) the feedback control which depends on the estimate state variables (Kalman filter) and (II) the record of the realistic earthquake excitation. The optimal control problem eventually leads to a two-point boundary value problem whose solution hinges on the knowledge of the entire history of the earthquake excitation. We employ true records of earthquake excitation as input. This approach enables one to solve the Riccati equations rigorously. Then, from the simulation results, one may study the relations between the control algorithm design and the characteristics (frequency, amplitude and duration) of earthquake excitation.

Characterization of a soft magnetic composite for use in road-embedded wireless-charging systems

The ferrite magnetic core is an integral component of road-embedded wireless charging systems for electric vehicles. However, the brittleness of ferrite makes it susceptible to premature fracture due to cyclic wheel loading from vehicles. This has motivated the development of a soft magnetic composite (SMC) composed of a flexible polyurethane and crushed ferrite as an alternative. An experimental investigation was conducted into the trade-offs between mechanical, thermal and magnetic properties at ferrite volume fractions between 45.9[Formula: see text]vol% and 80.6[Formula: see text]vol%. A comparison was made between measured properties and predictions from analytical models in order to further investigate the characteristics of the composite. The investigation showed a trade-off between the increase in magnetic permeability and the reduction in strain-to-failure as ferrite volume fraction increased. In addition, a large increase in flexural modulus and thermal conductivity, along with a slight increase in flexural strength was observed. More importantly, the strain-to-failure of the composite was 20 times higher than that of ferrite even at the highest volume fraction, indicating that the SMC was successful in providing a more ductile and flexible alternative.

A study on the mechanical polishing technique by using shear thickening fluids

This paper reported the new polishing technique by using a shear thickening fluid (STF). In experiments, the steel workpiece was immersed into the STF under the static condition. When the workpiece started rotating at a certain speed, the surrounding STF became solidified due to the shear thickening effect. Consequently, the solidified STF held the abrasive particles and polished the surfaces of the workpiece. The surface roughness of the treated surfaces was clearly dependent on the size of the abrasive particles. Owing to the reversible phase transition between liquid and solid status for the STF, the polishing process can be conducted without the use of polishing pads. Moreover, the new polishing technique using the STF can polish some complex structures having the surfaces with different heights and/or orientations, which cannot be achieved by the traditional one-step polishing method.

Thermal residual stresses in thermoplastic CFRP-steel laminates: Modification and influence on fatigue life

Thermal residual stresses (TRS) in hybrid materials and structures occur by the mismatch of thermal expansion of different materials. Especially when combining metals with carbon fiber reinforced plastics (CFRP), a significant level of internal stresses can be reached. High processing temperatures and high stiffness of the constituents are also responsible for high stress levels. Laminates of thermoplastic CFRP (unidirectional carbon fiber reinforced polyamide 6) and stainless steel foils are a suitable material system to examine the TRS in detail. Since TRSs in the steel fraction are of tensile nature, these superpose to externally applied loads, resulting in higher efforts for the material and thus reduced lifetimes under cyclic fatigue loading. Therefore, a reduction of TRS is desired. Two methods for TRS reduction were applied, and its influence on fatigue lifetime was investigated. Firstly, specimens were stretched by a preloading to reduce TRS by yielding of the metal. Secondly, non-symmetric laminates were gradually cooled down after consolidation to compensate TRS formation by non-symmetric shrinkage. While preloading of materials and structures is known for TRS modification, the gradually cooling is not established, yet. Both modification principles were numerically investigated before experimental validation. A significant increase of lifetime was reached by TRS reduction.

The effects of printing directions on the compression behavior of the Re-entrant structure produced by 3D printing technology

In this work, a re-entrant structure having a negative Poisson’s ratio (NPRs) was designed and produced with polylactic acid (PLA) using 3D printing technology. A series of samples was prepared with the different printing directions, namely, printed following (PF) the structure orientation, at 0[Formula: see text] (PZ) and at 90[Formula: see text] (PN). Results showed that the printing direction plays a crucial role in determining the mechanical properties of the printed meta-materials. In particular, PF specimens achieved the highest energy absorption at break, which is [Formula: see text]2 times as high as PZ or PN samples. The PF specimens also showed the highest stiffness under compression. However, the Poisson’s ratio was less sensitive to the changes in printing directions. The measured Poisson’s ratios for PF, PZ and PN samples are −1.68, −1.87 and −1.70, respectively. Based on the experimental results, the effects of the printing direction and the geometry configuration of the structure on the deformation behavior of the printed meta-material were further discussed.

3D printing of metallic micro-gears for micro-fluidic applications

Micro-fluidic devices are essential to handle fluids on the micro-meter scale (micro-scale), making them crucial to biomedical applications, where micro-gear is the key component for active fluid mixing. Rapid and direct fabrication of micro-gears is preferred because they are usually custom-made to specific applications and iterative design is needed. However, conventional manufacturing (CM) techniques for micro-fluidic devices are labor-intensive and time-consuming as multiple steps are required. Three-dimensional (3D) printing, or formally known as additive manufacturing (AM) offers a promising alternative over CM techniques in producing near-net shape complex geometries, given the micro-scale fabrication process. In this work, two types of powder-bed fusion (PBF) AM techniques, namely laser-PBF (L-PBF) and electron beam-PBF (EB-PBF) are used to benchmark 3D-printed micro-gears from stainless steel 316L micro-granular powders. Results showcase the preeminence of L-PBF over EB-PBF in generating distinguishable micro-scale features on gear profiles and superior micro-hardness in mechanical property. Overall, PBF metal AM shows significant promise in advancing the otherwise tedious state of CM for micro-gears.

An overview of entropy principle

A brief overview of the development from classical linear irreversible thermodynamics to the modern rational thermodynamics with Coleman–Noll and Müller–Liu procedures is presented, emphasizing the basic assumptions and formulation details. The major arguments concerned are the improvement of physical assumptions and mathematical formulation differences. Extended thermodynamics is also briefly commented.

Experimental investigation of modified co-curing process for carbon fiber/epoxy-laminates

In this study, thermoset-based carbon fiber-reinforced polymer structures manufactured by the so-called modified co-curing process are analyzed and compared to well-established co-curing and co-bonding. The modified co-curing process allows manufacturing geometrically complex parts without traditional core technologies by producing laminates from a un-cured half and a pre-cured half in contrast to using two un-cured halves (co-curing) or a fully cured half plus an un-cured half (co-bonding). The interlaminar fracture toughness under Mode I loading, [Formula: see text], was determined in double cantilever beam (DCB) tests. [Formula: see text] displays a correlation of the degree of cure and the joint properties, with the co-curing laminates having 11% and 33% higher fracture toughness than the modified co-curing configurations. However, modified co-curing in all cases results is superior or equal to co-bonding. To assess the influence of surface properties for the bonding quality, different peel plies were compared with respect to the resulting joint properties. The results with up to 50% loss in [Formula: see text] values indicate the high importance of appropriate surface preparation. Subsequent tests also show that the negative influence of the peel ply on the joint properties can be reversed by abrasive surface treatment. It was found that at higher degrees of partial curing before co-curing, crack growth increasingly occurs in the interface of the bonded laminates. Therefore, the properties of the surface before joining were analyzed and modified to assess its relevance for the bonding properties and the potential for improvement.