Finite Element Analysis and Computational Fluid Dynamics Verification of Molten Pool Characteristics During Selective Laser Melting of Ti-6Al-4V Plates.

The finite element (FE) method is used to characterize the thermal gradient, solidification rate, and molten pool sizes of Ti-6Al-4V plates in the process of selective laser melting (SLM). The results are verified by using the computational fluid dynamics (CFD) simulation. The proposed FE model contains a series of toolpath information that is directly converted from a G-code file, including hatch spacing, laser power, layer thickness, dwell time, and scanning speed generated by using Slic3r software from a CAD file. A proposed multi-layer, multi-track FE model is used to investigate the influence of the laser power, scanning speed, and scanning path on the microstructure in the Ti-6Al-4V plate built via SLM. The processing window is also determined based on the proposed FE model. The FE results indicate that, with a decrease in the laser power and an increase in the scanning speed, the morphology of the crystal grains, showing fully columnar crystals, gradually deviates from the fully equiaxed region. The formed grains are dependent on the laser power, scanning speed, and deposition position, but they are not sensitive to the scanning path, and with the deposition from the bottom layer to the top layer, the size of the formed grains is gradually increasing, which shows a good agreement with the experimental results.

Self-assembly for preparing nanotubes from monolayer graphyne ribbons on a carbon nanotube.

Graphyne nanotube (GNT), as a promising one-dimensional carbon material, attracts extensive attention in recent years. However, the synthesis of GNT is still challenging even in the laboratory. This study reveals the feasibility of fabricating a GNT by self-assembling a monolayer graphyne (GY) ribbon on a carbon nanotube (CNT) via theoretical and numerical analysis. Triggered by the van der Waals force from the CNT, a GY ribbon near the tube first winds upon the tube and then conditionally self-assembles to form a GNT. The self-assembly process and result are heavily influenced by the ambient temperature, which indicates the thermal vibration of the nanosystem. Molecular dynamic simulation results address the temperature range conducive to successful self-assembly. Different types of GNTs, e.g.α-,ß-, andγ-GNTs with specified chirality (armchair, zigzag, and chiral), length, and radius, can be obtained via self-assembly by controlling the geometry of the GY ribbons and temperature. The present theoretical understanding is helpful for fabricating GNTs with predefined morphology.

Optimized XGBoost Model with Small Dataset for Predicting Relative Density of Ti-6Al-4V Parts Manufactured by Selective Laser Melting.

Determining the quality of Ti-6Al-4V parts fabricated by selective laser melting (SLM) remains a challenge due to the high cost of SLM and the need for expertise in processes and materials. In order to understand the correspondence of the relative density of SLMed Ti-6Al-4V parts with process parameters, an optimized extreme gradient boosting (XGBoost) decision tree model was developed in the present paper using hyperparameter optimization with the GridsearchCV method. In particular, the effect of the size of the dataset for model training and testing on model prediction accuracy was examined. The results show that with the reduction in dataset size, the prediction accuracy of the proposed model decreases, but the overall accuracy can be maintained within a relatively high accuracy range, showing good agreement with the experimental results. Based on a small dataset, the prediction accuracy of the optimized XGBoost model was also compared with that of artificial neural network (ANN) and support vector regression (SVR) models, and it was found that the optimized XGBoost model has better evaluation indicators such as mean absolute error, root mean square error, and the coefficient of determination. In addition, the optimized XGBoost model can be easily extended to the prediction of mechanical properties of more metal materials manufactured by SLM processes.

A method for designing tunable chiral mechanical carbon networks for energy storage.

A method is proposed for designing tunable chiral nano-networks using partly hydrogenated graphene ribbons and carbon nanotubes (CNTs). In the network, the hydrogenated graphene ribbons (HGRs) act as basic components, which connect each other via CNT joints. Each component contains two HGR segments and an internal graphene joint (G-J2) or CNT joint (CNT-J2). Since the two HGR segments are hydrogenated at opposite surfaces, they may wind in chiral about the internal joint to form a scroll (G-J2-scroll or CNT-J2-scroll) or about the two end joints to form CNT-J4-scrolls. In general, a G-J2-scroll is formed more easily than both a CNT-J4-scroll and a CNT-J2-scroll. Because of scrolling, the surface energy is reduced. This reduction is converted to and stored as deformation potential energy. By means of molecular-dynamics simulations, we studied the final configurations of two types of networks from the same components, the maximum shrinkage, and their capacity of energy storage for potential application of energy storage or as large-deformable components in a nano-device. The results indicate that the network reaches a stable state when the shrinkage reaches 70% of the two in-plane dimensions.

Bonding few-layered graphene via collision with high-speed fullerenes.

Graphene, as a typical two-dimensional material, is popular in the design of nanodevices. The interlayer relative sliding of graphene sheets can significantly affect the effective bending stiffness of the few-layered graphene. For restricting the relative sliding, we adopted the atomic shot peening method to bond the graphene sheets together by ballistic C60 fullerenes from its two surfaces. Collision effects are evaluated via molecular dynamics simulations. Results obtained indicate that the fullerenes' incident velocity has an interval, in which the graphene sheet can be bonded after collision while no atoms on the fullerenes escaping from the graphene ribbon after collision. The limits of the interval increase with the layer number. Within a few picoseconds of collision, a stable carbon network is produced at an impacted area. The graphene sheets are bonded via the network and cannot slide relatively anymore. Conclusions are drawn to show the way of potential applications of the method in manufacturing a new graphene-based two-dimensional material that has a high out-of-plane bending stiffness.

Thermal shrinkage and stability of diamondene nanotubes.

By curving a rectangular diamondene, an sp 2/sp 3 composite carbon film, a diamondene nanotube (DNT) can be formed when the two straight edges are sewn together. In this study, thermal stabilities of DNTs are investigated using molecular dynamics simulation approaches. An interesting thermal shrinkage of damaged DNTs is discovered. Results indicate that DNTs have critical temperatures between 320 K and 350 K. At temperatures higher than the critical value, the interlayer bonds, i.e., the sp 3-sp 3 bonds, may break. The broken ratio of the interlayer bonds mainly depends on the temperature. For the DNT with a high broken ratio of interlayer bonds, it has thermal shrinkage in both the cross section and tube axis. The sp 2-sp 3 bonds in either the inner or the outer surface are much more stable. Even at 900 K, only a few sp 2-sp 3 bonds break. These properties can be used in the design of metamaterials.

Thermal Vibration-Induced Rotation of Nano-Wheel: A Molecular Dynamics Study.

By bending a straight carbon nanotube and bonding both ends of the nanotube, a nanoring (or nano-wheel) is produced. The nanoring system can be driven to rotate by fixed outer nanotubes at room temperature. When placing some atoms at the edge of each outer tube (the stator here) with inwardly radial deviation (IRD), the IRD atoms will repulse the nanoring in their thermally vibration-induced collision and drive the nanoring to rotate when the repulsion due to IRD and the friction with stators induce a non-zero moment about the axis of rotational symmetry of the ring. As such, the nanoring can act as a wheel in a nanovehicle. When the repulsion is balanced with the intertubular friction, a stable rotational frequency (SRF) of the rotor is achieved. The results from the molecular dynamics simulation demonstrate that the nanowheel can work at extremely low temperature and its rotational speed can be adjusted by tuning temperature.

Modelling and Characterization of Effective Thermal Conductivity of Single Hollow Glass Microsphere and Its Powder.

Tiny hollow glass microsphere (HGM) can be applied for designing new light-weighted and thermal-insulated composites as high strength core, owing to its hollow structure. However, little work has been found for studying its own overall thermal conductivity independent of any matrix, which generally cannot be measured or evaluated directly. In this study, the overall thermal conductivity of HGM is investigated experimentally and numerically. The experimental investigation of thermal conductivity of HGM powder is performed by the transient plane source (TPS) technique to provide a reference to numerical results, which are obtained by a developed three-dimensional two-step hierarchical computational method. In the present method, three heterogeneous HGM stacking elements representing different distributions of HGMs in the powder are assumed. Each stacking element and its equivalent homogeneous solid counterpart are, respectively, embedded into a fictitious matrix material as fillers to form two equivalent composite systems at different levels, and then the overall thermal conductivity of each stacking element can be numerically determined through the equivalence of the two systems. The comparison of experimental and computational results indicates the present computational modeling can be used for effectively predicting the overall thermal conductivity of single HGM and its powder in a flexible way. Besides, it is necessary to note that the influence of thermal interfacial resistance cannot be removed from the experimental results in the TPS measurement.

Critical conditions for escape of a high-speed fullerene from a BNC nanobeam after collision.

For a resonator-based nano-balance, the capability of capturing a nanoparticle is essential for it to measure the mass of the particle. In the present study, a clamped-clamped nanobeam from a Boron-Nitride and Carbon (BNC) nanotube acts as the nano-balance, and a fullerene, e.g., C60, is chosen as the particle, and the capturing capability is quantitatively estimated by the minimal escape velocity (MEV) of the fullerene from the nanobeam after collision. When centrally colliding with the nanobeam, the escape of fullerene depends on both incidence of fullerene and temperature of the system. When the colliding in the Boron-Nitride (BN) area of the beam surface, the nanoball escapes easier than that at the carbon area. The MEV of the nanoball is lower at higher temperature. As the nanoball sometimes slides for a few pica-seconds on the beam surface before being bounced out, the nanoball can escape only when the beam surface can provide the nanoball enough kinetic energy to overcome the van der Waals interaction between them. The capturing capability of the nano-balance can, thus, be improved by reducing the initial kinetic energy of the system.

Experimental study of time response of bending deformation of bone cantilevers in an electric field.

Bone is a complex composite material with hierarchical structures and anisotropic mechanical properties. Bone also processes electromechanical properties, such as piezoelectricity and streaming potentials, which termed as stress generated potentials. Furthermore, the electrostrictive effect and flexoelectric effect can also affect electromechanical properties of the bone. In the present work, time responses of bending deflections of bone cantilever in an external electric field are measured experimentally to investigate bone's electromechanical behavior. It is found that, when subjected to a square waveform electric field, a bone cantilever specimen begins to bend and its deflection increases gradually to a peak value. Then, the deflection begins to decrease gradually during the period of constant voltage. To analyze the reasons of the bending response of bone, additional experiments were performed. Experimental results obtained show the following two features. The first one is that the electric polarization, induced in bone by an electric field, is due to the Maxwell-Wagner polarization mechanism that the polarization rate is relatively slow, which leads to the electric field force acted on a bone specimen increase gradually and then its bending deflections increase gradually. The second one is that the flexoelectric polarization effect that resists the electric force to decrease and then leads to the bending deflection of a bone cantilever decrease gradually. It is concluded that the first aspect refers to the organic collagens decreasing the electric polarization rate of the bone, and the second one to the inorganic component influencing the bone's polarization intensity.

Friction effect of stator in a multi-walled CNT-based rotation transmission system.

The rotation transmission system (RTS) made from co-axial multi-walled nanotubes (MWNTs) has the function of regulating the input rotation from a nanomotor. The mechanism for the regulation is that the friction among the tubes during rotation governs the rotation of the rotors in the nanosystem. By integrating a rotary nanomotor and a nanobearing into an MWNT-based RTS, it is discovered that the stator (outer tube) provides relatively greater friction on the rotors by penetrating the motor tube, which has a higher stable rotational frequency. And the output rotation of the rotors in the system depends significantly on the temperature of the system, as the rotor tubes are slightly longer than the motor tube. Briefly, at low temperatures, say 8 K, the rotors rotate synchronously with the motor. However, at high temperatures, the rotors rotate slower than the motor with a bigger difference between their rotational frequencies. Hence, the output rotational frequencies can be adjusted by changing the temperature as well as the input rotational frequency.

Self-assembly of a parallelogram black phosphorus ribbon into a nanotube.

A nanotube from single-layer black phosphorus (BP) has never been discovered in experiments. The present study proposed a method for the fabrication of a BP nanotube (BPNT) from a parallelogram nanoribbon self-assembled on a carbon nanotube (CNT). The nanoribbon has a pair of opposite sides along the third principal direction. According to the numerical simulation via molecular dynamics approach, we discover that a wider BP nanoribbon can form into a series of chiral nanotube by self-assembly upon CNTs with different radii. The radius of a BPNT from the same ribbon has a wide range, and depends on both geometry of the ribbon and the CNT. One can obtain a BPNT with the specified radius by placing the ribbon nearby a given CNT. The method provides a clue for potential fabrication of BPNTs.

Self-assembly of a nanotube from a black phosphorus nanoribbon on a string of fullerenes at low temperature.

A string of fullerenes is used for generating a nanotube by self-assembly of a black phosphorus (BP) nanoribbon at a temperature of 8 K. Among the fullerenes in the string, there are at least two fixed fullerenes placed along the edge of the BP ribbon for keeping its configuration stability during winding. By way of molecular dynamics simulations, it is found that successful generation of a BP nanotube depends on the bending stiffness of the ribbon and the attraction between the fullerenes and the ribbon. When the attraction is strong enough, the two edges (along the zigzag direction) of the BP ribbon will be able to bond covalently to form a nanotube. By the molecular dynamics approach, the maximum width of the BP ribbon capable of forming a nanotube with a perfect length is investigated in three typical models. The maximum width of the BP ribbon becomes larger with the string containing more fullerenes. This finding reveals a way to control the width of the BP ribbon which forms a nanotube. It provides guidance for fabricating a BP nanotube with a specified length, the same as to the width of the ribbon.

Conditions for escape of a rotor in a rotary nanobearing from short triple-wall nanotubes.

In a short nanobearing system made from carbon nanotubes, the rotor with high rotational frequency may escape from the stator, which may cause a stability problem to the system of a nanodevice with such a nanobearing. In the present work, nanobearings with tri-walled nanotubes are investigated to reveal the conditions for the moving away of the free inner tube from the high-speed rotating middle tube. Experimental results show that the escape happens when the radii difference between the two rotors is larger than 0.34 nm and the rotational frequency of the middle tube is higher than a critical value. And before the escape occurs, the rotational frequency of the inner tube is lower than this critical value. Due to the radii difference being larger than 0.34 nm, the two rotors are non-coaxial, and the centrifugal force of the inner tube results in strong radial and axial interactions between the edges of the two rotors. When the relative sliding speed is relatively high, an edge of the inner rotor will pass through the potential barrier at the adjacent edge of the middle rotor, and further escape from the middle rotor occurs. The selection of a longer middle rotor with smaller radius can increase the critical rotational frequency of the middle rotor.

Robust rotation of rotor in a thermally driven nanomotor.

In the fabrication of a thermally driven rotary nanomotor with the dimension of a few nanometers, fabrication and control precision may have great influence on rotor's stability of rotational frequency (SRF). To investigate effects of uncertainty of some major factors including temperature, tube length, axial distance between tubes, diameter of tubes and the inward radial deviation (IRD) of atoms in stators on the frequency's stability, theoretical analysis integrating with numerical experiments are carried out. From the results obtained via molecular dynamics simulation, some key points are illustrated for future fabrication of the thermal driven rotary nanomotor.

Buckling behaviour of composites with double walled nanotubes from carbon and phosphorus.

Due to weak interactions among phosphorus atoms in black phosphorene, a nanotube obtained by curling single-layer black phosphorus is not as stable as a carbon nanotube (CNT) at finite temperature. In the present work, we recommend a new 1D composite material with a double-walled nanotube (DWNT) from a black phosphorus nanotube (BPNT) and a CNT. The dynamic response of the composite DWNTs is simulated using a molecular dynamics approach. Effects of the factors including temperature, slenderness and configurations of DWNTs on dynamic behavior of the composite are discussed. Compared with a single-walled BPNT, the composite DWNTs under uniaxial compression show some unique properties. When a BPNT is embedded in a CNT which will not only isolate the BPNT from the ambient conditions, but also improve the capability of axial deformation of the BPNT, the system will not collapse rapidly even if the BPNT has been buckled.

A nanoengine governor based on the end interfacial effect.

A conceptual design is presented for a nanoengine governor based on the end interfacial effect of two rotary nanotubes. The governor contains a thermal-driven rotary nanomotor made from double-walled carbon nanotubes (DWCNTs) and a coaxially laid out rotary nanotube near one end of the nanomotor rotor. The rotation of the rotor in the nanomotor can be controlled by two features. One is the stator (the outer tube of DWCNTs) which has some end atoms with inward radial deviation (IRD) on the stator. The other is the relative rotation of the neighboring rotary tube of the rotor. As the configuration of the stator is fixed, the end interfacial interaction between the two rotors will govern the dynamic response of the rotor in the nanomotor system. The obtained results demonstrate that the relative rotational speed between the two rotors provides friction on the rotor in the nanomotor system. In particular, higher relative rotational speed will provide lower friction on rotor 1, which is opposite to that between neighboring shells in DWCNTs.

Effects of size and surface on the auxetic behaviour of monolayer graphene kirigami.

Graphene is an active element used in the design of nano-electro-mechanical systems (NEMS) owing to its excellent in-plane physical properties on mechanical, electric and thermal aspects. Considering a component requiring negative Poisson's ratio in NEMS, a graphene kirigami (GK) containing periodic re-entrant honeycombs is a natural option. This study demonstrates that a GK with specific auxetic property can be obtained by adjusting the sizes of its honeycombs. Using molecular dynamics experiments, the size effects on the auxetic behaviour of GK are investigated. In some cases, the auxetic difference between the hydrogenated GK and continuum kirigami (CK) is negligible, in which the results from macro CK can be used to predict auxetic behaviour of nano kirigami. Surface effect of GK is demonstrated from two aspects. One is to identify the difference of mechanical responses between the pure carbon GK and the hydrogenated GK at same geometry and loading condition. Another is from the difference of mechanical responses between the GK model and the CK model under same loading condition and geometric configuration. Generally, surface energy makes the GK possess higher variation of auxetic behaviour. It also results in higher modulus for the GK as comparing with that of the CK.

Thermal stability of a free nanotube from single-layer black phosphorus.

Similar to a carbon nanotube fabricated from a graphene sheet, a black phosphorus nanotube (BPNT) can also be theoretically produced by curling rectangular single-layer black phosphorus (SLBP). In the present study, the effect of the thermal vibration of atoms on the failure of a BPNT is investigated using molecular dynamics simulations. Two types of double-shell BPNTs obtained by curling the SLBP along its armchair/pucker and zigzag directions respectively are involved in simulation. At finite temperature, a bond on the outer shell of the tube is under tension due to both the curvature of the tube and the serious thermal vibration of the atoms. As the length of a bond with such elongation approaches its critical value, i.e. 0.279 nm, or the smallest distance between two nonbonding phosphorus atoms is over 0.389 nm caused by a great variation of the bond angle, the tube fails quickly. The critical stable states of either an armchair or a zigzag BPNT at finite temperature are calculated and compared. To achieve a stable BPNT with high robustness, the tube should have a higher radius or should work at a lower temperature. Only when the BPNT has structural stability does it have the potential application as a nanowire in a future nano electro-mechanical system.

Effect of Regulatory T Cells on Promoting Apoptosis of T Lymphocyte and Its Regulatory Mechanism in Sepsis.

With both in vivo and in vitro experiments, the present study was conducted to investigate the effect of regulatory T cell (Treg) on promoting T-lymphocyte apoptosis and its regulatory mechanism through transforming growth factor-beta (TGF-ß1) signaling in mice. A murine model of polymicrobial sepsis was reproduced by cecal ligation and puncture (CLP); PC61 and anti-TGF-ß antibodies were used to decrease counts of CD4(+)CD25(+) Tregs and inhibit TGF-ß activity, respectively. Splenic CD4(+)CD25(+) Tregs and CD4(+)CD25(-) T cells were isolated. Phenotypes, including cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), forkhead/winged helix transcription factor p3 (Foxp3), and TGFß1(m+), as well as the apoptotic rate of CD4(+)CD25(-) T cell, were analyzed by flow cytometry. Real-time reverse transcription-polymerase chain reaction was performed to determine mRNA expression of TGF-ß1, and the expressions of Smad2/Smad3, Bcl-2 superfamily members of Bcl-2/Bim, cytochrome C, the mitochondrial membrane potential, and caspases in CD4(+)CD25(-) T cells were simultaneously determined. After treatment with PC61 or anti-TGF-ß antibody, CTLA-4, Foxp3, and TGFß1(m+) expressions of CD4(+)CD25(+) Tregs were markedly decreased in comparison to that of the CLP group and the apoptosis rate of CD4(+)CD25(-) T cells was significantly positively correlated with the expression of TGF-ß1. Meanwhile, levels of P-Smad2/P-Smad3, proapoptotic protein Bim, cytochrome C, and activity of caspase-3, -8, -9 were downregulated, whereas the mitochondrial membrane potential and antiapoptotic protein Bcl-2 expression were restored. Taken together, our data indicated that the TGF-ß1 signal could be partly involved in the apoptosis of CD4(+)CD25(-) T cells promoted by CD4(+)CD25(+) Tregs, therefore inhibition of TGF-ß1 expression may provide a novel strategy for the improvement of host immunosuppression following sepsis.