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A PT-SWCNTs self-assembled nanorobot is proposed to release residual H2 based on adhesion competition in a SWCNT system.

Ion selectivity of nanoporous graphene membranes is of great importance in separation technologies including ion sieving and seawater desalination. In this study, we put forward a method to produce...

We report the first experimental measurement on the thermal conductivity of strained monolayer graphene using optothermal Raman method. Biaxial tensile strain was achieved by applying a pressure difference across suspended monolayer graphene membranes. Experimental results demonstrate that, under 0.12%–0.27% strains, the in-plane thermal conductivity of monolayer graphene is sensitive to the applied biaxial strains. Under a 0.12% biaxial tensile strain, the thermal conductivity of suspended monolayer graphene drops approximately by 20% near 350K and 12% at about 500K. This study confirms the previous theoretical prediction that the thermal conductivity of monolayer graphene can be modified by applying tensile strains.

Polythiophene is considered as an effective dry adhesive and is promising to be a conductive adhesive due to its excellent properties. Here, we used steered molecular dynamics to investigate the interfacial strength between polythiophene and various semiconductors with similar structures including silicon, silicon carbide, and diamond. Energy decomposition was done to have a detailed insight into the adhesive mechanism. Particularly, we laid stress on the entropy difference of the polythiophene chain in different systems. Van der Waals interaction and electrostatic interaction both positively contributed to the adhesion between polythiophene and semiconductors, while the entropy change of polythiophene, including vibrational entropy change and conformational entropy change, weakened the adhesion to some extent. Our results indicated that the combined effect of these three factors made the adhesion between polythiophene and silicon carbide the strongest among the systems we studied. Additionally, it was found that such adhesion was scarcely influenced by temperature. This simple polythiophene-semiconductor interfacial study can help optimize the choice of the semiconductor when applying the polythiophene adhesive.

The effects of interatomic bonding strengths of graphite itself, in the in-plane (IP) and cross-plane (CP) direction, on the interfacial thermal conductance between graphite and substrate copper (Cu), are investigated by molecular dynamics simulations. It is found that the interfacial thermal conductance at the graphite/Cu interface monotonically decreases with respect to the increase of the IP bonding strength of graphite. However, the interfacial thermal conductance has a maximum with respect to the increase of the CP bonding strength of graphite. By extracting the acoustic iso-energy surfaces and phonon density of states of graphite under various bonding strength parameters, it is found that increasing the IP bonding strength decreases both the incident phonon group velocity component and average interfacial transmission coefficient. Increasing the CP bonding strength also decreases the incident phonon transmission coefficient. However, increasing the CP bonding strength increases the phonon group velocity component. To provide a practical method to enhance the interfacial thermal conductance, we compared the interfacial thermal conductance of porous graphite/Cu interface, which is higher than the graphite/Cu interface mainly due to the large reduction of the in-plane elasticity. This study provides important guidance in the active regulation of the interfacial thermal conductance between anisotropic materials and substrate.

The thermal conductivity (TC) of non-periodic Si/Ge superlattice nanowire (SLNW) is investigated by non-equilibrium molecular dynamics simulation (NEMD). It is found that the TC of non-periodic Si/Ge SLNW can be significantly reduced at room temperature. Compared to the minimum TC of periodic Si/Ge SLNWand pure Si nanowire, the TC of non-periodic Si/Ge SLNW is further decreased to be around 47.4%, 4.4% of that of periodic Si/Ge SLNW and pure Si nanowire respectively. By introducing 20% Ge atom doping, the TC of non-periodic Si/Ge SLNW with 10-unit cell (UC) thickness is reduced by 38%. The reduction of TC of non-period Si/Ge SLNW is first due to the destruction of phonon coherent transport. Additionally, the change of periodic length can cause shift of density of state (DOS), which makes the interface of Si/Ge with different lengths play roles to scatter the phonon with different wavelengths. Therefore, the distribution of thickness from atomic to nano scale can decrease the TC by scattering phonon with more wavelengths. The results provide an efficient way for future thermoelectric applications.

Underwater directed energy deposition (UDED) is a promising technology for on-site maintenance and repair of underwater structures. In this study, the damage zone on a Ti–6Al–4V plate was successfully repaired by UDED and no obvious internal defects were observed. The mechanisms of the special underwater thermal behaviors influencing the microstructural formation/evolution process and the resultant mechanical properties of Ti–6Al–4V repaired by UDED were systematically investigated by experimental and simulation methods. Compared with that prepared by in-air directed energy deposition (DED), the sample repaired by UDED presented the following differences: (1) The cooling rate was large and the heat accumulation of the sample was low during UDED due to the rapid heat dissipation by water and gas curtain gas. (2) The sample repaired by UDED was dominated by fine acicular martensite α′ with a high density of dislocations due to the weak intrinsic heat treatment (IHT), while the intensified IHT involved in DED provoked the significant decomposition of α′ into α and β film. (3) The weak IHT during UDED resulted in the formation of thin β films with a low vanadium content. The intensified IHT during DED promoted the diffusion of vanadium atoms and the coarsening of β films. (4) The average microhardness of the sample repaired by UDED (379 ± 15 HV) was higher than that repaired by DED (359 ± 11 HV). The sample repaired by UDED exhibited relatively poor ductility and low toughness due to the presence of acicular martensite α′. This work can provide an important foundation and useful guidance for tailoring the microstructure and properties of titanium alloys repaired by UDED in underwater environments.

The electrochemical intercalation of metal ions into layered materials is a useful strategy to reversibly tune thermal-transport properties, but the fundamental mechanisms are not well understood. In this study, we systematically investigated the effects of lithium-ion concentrations on the anisotropic thermal conductivity of intercalated graphite by molecular-dynamics simulations and a continuum-mechanics method. It was found that the in-plane thermal conductivity rapidly decreased to 34.3% of that of graphite in the same direction when the lithium-ion concentration increases to 0.1667. However, the cross-plane thermal conductivity first decreased to 23.7% of that of the graphite. Then, it surprisingly recovered to a thermal conductivity even higher than that of graphite when the lithium-ion concentration increased further. These two different trends and thermal-conductivity anisotropies were explained by extracting the phonon lifetimes and elastic constants of intercalated graphites with various lithium...

The ability to sieve ions through nanopores with high throughput has significant importance in seawater desalination and other separation applications. In this study, a plasma etching process has been demonstrated to be an efficient way to produce high-density nanopores on graphene membranes with tunable size in the sub-nanometer range. Besides the pore size, the nanopore density is also controllable through adjusting the exposure time of the sample to argon or oxygen plasma. The plasma-treated graphene membranes can selectively transport protons, Na+ and Cl− ions. Density function theory calculations uncover that the sp3 and vacancy-type defects construct different energy barriers for different ions, which allow the defected graphene membrane to selectively transport ions. Our study indicates that oxygen plasma etching can be used as a very convenient and efficient method for fabricating a monolayer filtration graphene membrane with tunable sub-nanometer pores.

The thermal conductivity of isotopically pure wurtzite GaN and the corresponding two-dimensional monolayer crystals are investigated based on first-principles calculations and phonon Boltzmann transport equation. It is found that the c-axis thermal conductivity of defect-free wurtzite GaN is 42–11% higher than that along the in-plane direction when temperature increases from 100 to 800 K. Our calculation also shows that the thermal conductivity of monolayer GaN with hexagonal lattice is higher than that with haeckelite lattice by a factor of four at room temperature. The phonon dispersion relation, phonon group velocity and the phonon scattering rate are extracted to uncover the underlying physical mechanism. The thermal conductivity accumulation function with respect to the phonon mean free path and the relative contribution of each phonon branch for the two kinds of monolayer GaN are further calculated as compared to that of graphene. These investigations would provide important guidance to develop GaN-based electronic and photonic device.

Interfacial thermal conductance as an accumulation function of the phonon mean free path is rigorously derived from the thermal conductivity accumulation function. Based on our theoretical model, the interfacial thermal conductance accumulation function between Si/Ge is calculated. The results show that the range of mean free paths (MFPs) for phonons contributing to the interfacial thermal conductance is far narrower than that for phonons contributing to the thermal conductivity. The interfacial thermal conductance is mainly contributed by phonons with shorter MFPs, and the size effects can be observed only for an interface constructed by nanostructures with film thicknesses smaller than the MFPs of those phonons mainly contributing to the interfacial thermal conductance. This is why most experimental measurements cannot detect size effects on interfacial thermal conductance. A molecular dynamics simulation is employed to verify our proposed model.

The thermal conductance across Au/graphene/Au and Au/hydrogenated graphene(H-graphene)/Au interfaces are measured using the time domain thermoreflectance (TDTR) technique. It is found that the thermal conductance decreases from 30 ± 5 MWm−2 K−1 to 14 ± 3 MWm−2K−1 when the graphene is hydrogenated. However, nonequilibrium molecular dynamics (NEMD) simulation results show that the thermal conductance can be decreased by no more than 30% as the graphene is hydrogenated if only phonons are considered as the heat energy carriers. Considering the difference between the experimental and simulated results, we infer that electrons are involved in heat conduction across the Au/Graphene/Au interface. Once the graphene is hydrogenated, the electron contribution channel is closed. It is estimated that the contribution from electrons accounts for approximately 53% of the total interfacial thermal conductance for the metal/graphene/metal interfaces.

The thermal conductivities of the newly synthesized all- sp 2 three-dimensional graphene are investigated by equilibrium molecular dynamics simulations in this work. It is found that the thermal conductivity parallel to the honeycomb axis direction ( k z ) is one order magnitude higher than that perpendicular direction ( k xy ). This anisotropy is explained by the direction-dependent effective elastic constants. For the size effects, the k xy is found to be independent of the hexagon size, while k z increases with it. Both k xy and k z are also validated by the nonequilibrium method. For the pressure effects, this study also reveals an unexpected k z reduction with increasing pressure. A critical pressure is found to be 0.65 GPa. Beyond this critical pressure, the three-dimensional graphene breaks its crystal symmetry, leading to the in-plane k xy becomes anisotropic and lower comparing to the three-dimensional graphene with no pressure. These investigations provide important guidance to develop all- sp 2 three-dimensional graphene for energy storage, catalysis, and sensor applications.

We describe here an Nd3+-sensitized upconversion fluorescent sensor for epirubicin (EPI) detection in aqueous solutions under 808 nm laser excitation. The upconversion fluorescence of nanoparticles is effectively quenched in the presence of EPI via a fluorescence resonance energy transfer mechanism. The dynamic quenching constant was 2.10 &times, 104 M&minus, 1. Normalized fluorescence intensity increased linearly as the EPI concentration was raised from 0.09 &mu, M to 189.66 &mu, M and the fluorometric detection limit was 0.05 &mu, M. The sensing method was simple, fast, and low-cost and was able to be applied to determine the levels of EPI in urine with spike recoveries from 97.5% to 102.6%. Another important feature of the proposed fluorescent sensor is that it holds a promising potential for in vivo imaging and detection due to its distinctive properties such as weak autofluorescence, low heating effect, and high light penetration depth.

We describe here an Nd3+-sensitized upconversion fluorescent sensor for epirubicin (EPI) detection in aqueous solutions under 808 nm laser excitation. The upconversion fluorescence of nanoparticles is effectively quenched in the presence of EPI via a fluorescence resonance energy transfer mechanism. The dynamic quenching constant was 2.10 × 104 M−1. Normalized fluorescence intensity increased linearly as the EPI concentration was raised from 0.09 μM to 189.66 μM and the fluorometric detection limit was 0.05 μM. The sensing method was simple, fast, and low-cost and was able to be applied to determine the levels of EPI in urine with spike recoveries from 97.5% to 102.6%. Another important feature of the proposed fluorescent sensor is that it holds a promising potential for in vivo imaging and detection due to its distinctive properties such as weak autofluorescence, low heating effect, and high light penetration depth.

The main task of the new engineering construction is to cultivate innovative talents for the new economic development characterized by new technologies, new industries, new formats and new models, and the course is the core element of talent cultivation. Considering that design courses play an important role in the training of mechanical undergraduate talents and industrial needs, the comprehensive ability and innovation ability of students can be improved to solve complex mechanical design problems through the construction of “golden course group of “Design Principles and Methods” project for the innovative talent cultivation in new engineering of machinery”.

A novel underwater directed energy deposition (UDED) technique was developed for the on-site repair of marine equipment in an underwater environment. A special drainage nozzle was integrated with a laser cladding head to form the underwater manufacturing tool which ensured the successful repair of the damage zone on an HSLA-100 steel plate. The influence mechanisms of inherently complex heat-treatment cycles and kinetics involved in the UDED process on the microstructural formation/evolution process were systematically investigated using both experimental and numerical approaches. The mechanical properties, including microhardness, nanohardness, tensile strength and Charpy impact toughness of the UDED repaired samples were investigated and the results were compared with those repaired by in-air directed energy deposition (DED). The experimental results show that the rapid cooling rate accelerated by the water quenching effect promoted the formation of columnar austenite and the subsequent fine martensite. Compared with the DED samples, the activation energy values for the nucleation and growth of the precipitates in UDED samples were much lower due to the existence of high-density crystal defects and large internal residual stresses. The numerical results indicate that compared with DED, the cooling rate significantly increased and the peak temperature decreased during UDED. The average microhardness of the samples prepared by UDED (315–334 HV) was higher than that prepared by DED (307 HV). The tensile properties and impact toughness of the as-repaired samples by UDED were equal or even better than those obtained by DED owing to the formation of fine martensite and high dislocation densities in the UDED samples. This work clarifies the difference in metallurgical processes in air and underwater environments, which can provide guidance for future structural manipulation of materials additively manufactured in underwater environments.

It is challenging to apply laser metal deposition (LMD) in underwater environment to realize on-site repair of marine equipment due to the potential impact of water. In the present work, we report an innovative underwater repair technique termed underwater laser metal deposition (ULMD) which can overcome the challenges. This new technique renders both technical and theoretical advancements from the following aspects: (1) A special in-house designed drainage nozzle was integrated with the laser cladding head to create local dry cavity which ensured the successful manufacturing of titanium alloy Ti-6Al-4V in underwater environment; (2) Unique microstructure formation/evolution mechanisms have been revealed for the ULMD process, which significantly differ from those of the in-air LMD process; (3) The hydrogen content has been well controlled during ULMD, which can effectively prevent the formation of hydrogen-induced cracks; (4) The mechanical properties of the ULMD Ti-6Al-4V parts were equal or even better than that fabricated by in-air LMD or SLM technique. In the meantime, a systematically parametric study was performed for this new technique and the experimental results showed that a high laser power (1600 W), a reasonable scanning velocity (800 mm/min) and a cross-hatching strategy were essential for achieving minimum metallurgical defects and full densification, which can provide very helpful guidance for the future research in this area. This research work opens a new avenue which makes underwater 3D-printing an applicable tool when coping with the fabrication and repair of customized components or complex shape parts in underwater environment.

Severe stress concentration and bad sealing performance are encountered in a non-metallic engineering part during an interference fit assembly process. Both numerical calculation and experimental test are employed to analyze the causes resulting in the assembly failure. Based on the finite element method (FEM), commercial computational software, ANSYS, is first used to simulate the whole assembly process with different boundary conditions. By comparing simulation results of the assembly process with various boundary conditions, it is found that deformation energy and friction force contribute differently to the reaction force at varying assembly depths. In virtue of these simulation results, an improved engineering part is designed and fabricated. Experimental test results show that stress concentration and sealing performance problems are basically solved compared to those in the original model. Moreover, reaction forces calculated from numerical simulation and measured from experimental tests agree reasonably with each other during the interference fit progress. This work is beneficial to the understanding of the interference fit process in engineering application and the avoiding of part failure resulting from inappropriate design.

The precise thermal expansion coefficient (TEC) of single layer graphene (SLG) is of vital importance to the preparation and application of graphene-based devices. In this work, a comprehensive Raman investigation on the TECs of SLG was conducted by reducing the effects of defects, native strains, and substrate doping. The TECs of graphene with few defects extracted from Raman spectra remained negative without the appearance of a positive transition in the temperature range of 303–503 K. After eliminating the influence of native strains and substrate doping on the TEC acquisition by excluding the first thermal cycle, a more reliable TEC, (−3.68 ± 0.49) × 10−6 /K, was obtained at room temperature, which was much closer to the theoretical predictions than those results in previous experimental reports. This investigation provides a more precise approach to measure the TECs of two-dimensional materials by Raman spectroscopy.

The electrochemical intercalation of metal ions into layered materials is an innovative approach to actively and reversely regulate the thermal transport properties of layered materials. In this work, the molecular dynamics (MD) simulation and lattice dynamics calculation are performed to investigate the thermal boundary conductance (TBC) between metal aluminum (Al) and intercalated graphite with various lithium-ion (Li-ion) concentrations. Both the thermal relaxation simulation and the non-equilibrium MD simulation show that the TBC in the intercalated graphene/Al interface is significantly high as compared to that in pristine graphite/Al interface, by a factor of 5.5. Besides, the TBC presents a slightly increasing trend with increasing Li-ion concentration. This is because increasing the Li-ion concentration increases the out-of-plane phonon group velocity of the intercalated graphite, leading to a sharp increase of phonon irradiation heat flux. By calculating the phonon interfacial transmission coefficient that includes the inelastic scattering effects, it is found that the transmission from the intercalated graphite to Al significantly decreases with increasing Li-ion concentration. The competition between the increased phonon irradiation heat flux and the decreased phonon interfacial transmission coefficient results in the slight increase of the TBC at high Li-ion concentration. This study provides important guidance to develop novel electrode materials and structures for thermal management in Li-ion batteries.

Experiments and numerical simulations are conducted to investigate fluid release from micro- to nanoscale rectangular channels. It is found that the release pressure can be flexibly adjusted by varying the channel size or the inclined angle. A critical inclined angle is found for wetting fluids, below which spontaneous release is achieved. A modified Young–Laplace equation for predicting the release pressure of rectangular channels is developed, which works well down to the nanoscale. The experimental results and theoretical predictions have also been verified by numerical simulations.

The effects of ultrasonic impact peening (UIP) and laser shock peening (LSP) on 316L stainless steel were compared in terms of surface morphologies, microstructural evolutions and mechanical properties. The grain refinement mechanisms by mechanical and laser shock wave were subsequently analyzed. Experimental results showed that both UIP and LSP produced micro-grooves with the same depth (~48 μm) at the surface of 316L. The nano-grain size induced by double UIP treatment (10–90 nm) was much smaller than that by triple LSP treatment (>70 nm) because the impact numbers and total impact energy of UIP were much higher. The mechanical twinning was almost complete absence in the sample by UIP. On the contrary, the mechanical twinning was frequently observed in samples by LSP. The magnitude of peak pressure determined the transition from dislocation-dominated mechanism (~680 MPa for UIP) to twinning-dominated mechanism (~2200 MPa for LSP). The resultant dislocation cell size by UIP was much smaller than that by LSP due to the difference of dislocation density caused by different shock wave speed and impact numbers. Additionally, the compressive residual stress on the surface by UIP was higher than that by LSP in both measuring direction. Furthermore, both grain refinement and high dislocation density induced by UIP contributed to a significant increase in the hardness (~433 HV) and yield strength (~447 MPa). By contrast, the LSP induced mechanical twins which can act as dislocation blockers significantly improved the yield strength (~423 MPa).

The non-equilibrium Green's function method was employed to investigate the pressure effects on the interfacial thermal resistance of few-layer graphene. It is found that a compressive pressure of 10 GPa along the cross-plane direction can reduce the interfacial resistance by approximately 4 times, when compared with no applied pressure. As pressure is applied along both in-plane and cross-plane directions, the effects of the in-plane direction pressure on the cross-plane thermal transport are much weaker than those of the cross-plane pressure. Our results indicate that the interlayer interfacial thermal resistance of graphite can be modulated to a large extent by external pressure.

The out-of-plane thermal conductivity of silicon thin film doped with germanium is calculated by non-equilibrium molecular dynamics simulation using the Stillinger-Weber potential model. The silicon thin film is doped with germanium atoms in a random doping pattern with a doping density of 5% and 50% respectively. The effect of silicon thin film thickness on its thermal conductivity is investigated. The simulated thicknesses of silicon thin film doped with germanium range from 2.2 to 10.9 nm at an average temperature 300K. The simulation results indicate that the out-of-plane thermal conductivity of the silicon thin film doped with germanium decreases linearly with the decreasing film thickness. As for the film thickness of 9.8nm and the average temperature ranging from 250 to 1000 K, the investigation shows that the temperature dependence of the film thermal conductivity is not sensitive.

Polymers are usually considered thermal insulators, because the amorphous arrangement of the molecular chains reduces the mean free path of heat-conducting phonons. The most common method to increase thermal conductivity is to draw polymeric fibres, which increases chain alignment and crystallinity, but creates a material that currently has limited thermal applications. Here we show that pure polythiophene nanofibres can have a thermal conductivity up to ∼ 4.4 W m(-1) K(-1) (more than 20 times higher than the bulk polymer value) while remaining amorphous. This enhancement results from significant molecular chain orientation along the fibre axis that is obtained during electropolymerization using nanoscale templates. Thermal conductivity data suggest that, unlike in drawn crystalline fibres, in our fibres the dominant phonon-scattering process at room temperature is still related to structural disorder. Using vertically aligned arrays of nanofibres, we demonstrate effective heat transfer at critical contacts in electronic devices operating under high-power conditions at 200 °C over numerous cycles.

The thermal conductivity of silicon nanowire doped with Germanium in different geometrical distributions was investigated with nonequilibrium molecular dynamics method. Five different geometrical arrangements of the impurities that influence the thermal conductivity properties of silicon nanowires were investigated, which include centralization doping, uniform doping, non-uniform doping, random doping and cubic pattern doping. The results demonstrate that the uniformity of the impurity positions has a substantial influence on the thermal conductivity of silicon nanowires under the same impurity concentration.

Interfacial thermal conductance plays a significant role in the heat transfer efficiency of nanoscale systems. The thermal conductance across Al/SiO2 interfaces, which is subjected to the change in concentration of sandwiched Rhodamine 6G solution, is measured with time domain transient thermoreflectance technique. The thermal conductance of the interface between Al and SiO2 logarithmically decreases with an increase in the concentration of the Rhodamine 6G solution. This study reveals that heat transport efficiency across an interface can be conveniently manipulated according to the demand of thermal engineering by introducing organic moleculars between both sides of the interface.

Thermal conductance at the interface between metal and non-metal materials in the presence or absence of an inserted graphene layer is measured using a time domain transient thermoreflectance technique. The insertion of a single layer graphene between thermal evaporation Al film and Si substrate enhances the interfacial thermal conductance, because the graphene works as a mask to prevent the metal atoms diffusing into the substrate and causes the reduction of the intermixing layer thickness. Conversely, for the Al/Si interface with the Al film prepared by magnetron sputtering, the insertion of a single layer graphene increases the number of interface and leads to the decrease of the interfacial thermal conductance.