Joint publication with China University of Petroleum (East China) in Physical Chemistry Chemical Physics
The reverse micelles (RMs) in supercritical CO2 (scCO2) are promising alternatives for organic solvents, especially for both polar and non-polar components are involved. Fluorinated surfactants, particularly the double-chain fluorocarbon surfactants, are appropriate to form well-structured RMs in scCO2. The mechanisms inherent to the self-assembly of the surfactants in scCO2 are still subject to discussion. In this study, molecular dynamics simulations were performed to investigate the self-aggregation behavior of di-CF4 based RM in scCO2 and a stable and spherical RM is formed. The dynamics process and the self-assembly structure in the RM reveal a three-step mechanism to form the RM, that is, small RMs, rod-like RMs and the fusion of rod-like RMs. The Hydrogen-bonds between headgroups and water molecules, and the salt-bridges linking Na+, headgroups and water molecules enhance the interfacial packing efficiency of the surfactant. The result shows the di-CF4 molecule has the high surfactant coverage at the RM interface, implying the high CO2-philicity. Ths mainly results from the bend of the short chain (C-COO-CH2-(CF2)3-CF3) due to the flexible carboxyl group. The microscopic insight provided in this study is helpful to understand the surfactant self-assembly phenomena and design new CO2-philic surfactants.
New article in RSC Advances by PhD candidate Yi Gong
The behavior of hydrophobic-core/hydrophilic-shell structured microgels at an interface: from Mickering emulsion to colloidosomes with dual-level controlled permeability
Colloidosomes have attracted great attention due to its special structure and broad applications, and the permeability is one of the key parameter of colloidosomes. In the manuscript, an effective and straightforward approach for fabricating novel colloidosomes from pH-responsive core-shell microgels is presented. One-pot surfactant-free synthesis of the microgels with hydrophobic core and hydrophilic shell is developed. The Model drug release results show that the permeability of colloidosomes can be coarsely controlled by pH and fine-tuned by the ratio of shell to core in microgels.
Compare to the works reported previously, the synthesis of microgels in one-pot process could avoid the preparation, isolation and purification of raw products, while microgels with well-defined core-shell structure are still obtained. Moreover, the core and shell of microgels can be tailor-designed by choosing different monomers; thus the method can be commonly used to integrate various functional materials into colloidosomes. The methodology revealed in the study not only provides a unique technique to control the permeability of colloidosomes but also opens a platform pathway to integrate multi-functionalities to the colloidosomes.
Delegation from Delft University of Technology to NTNU Nanomechanical Lab
NML Group seminar at Lofoten
New article in Computational Materials Science by PhD candidate Kai Zhao
The void growth in monocrystalline Cu and Fe are investigated by molecular dynamics simulations to reveal the ductile mechanisms based on dislocation emission and propagation. The results show that the void growth in Cu is governed by the collective interaction of stacking faults along four (111) planes. Three dominant mechanisms of void growth in Fe are identified: (i) for small voids, nucleation of twinning boundaries; (ii) for intermediate voids, emission of shear loops; (iii) for large voids, stacking faults nucleate at the void surface and then degenerate into shear loops. The slip-twinning transition rate of Fe at room temperature calculated according to Zerrili-Armstrong model is in the range measured by our atomistic simulations. Vacancy generation which promotes void growth results from the intersection of more than two stacking faults in Cu, while in Fe it is attributed to the jog dragging of screw dislocations. An analytical model based on nudged elastic band calculation is developed to include the strain rate dependence of the nanovoid-incorporated incipient yielding. This new model demonstrates that the critical radius of shear loop in Cu under a strain rate of 108 s-1 is on the order of Burgers vector. For both metals, the dislocation density has been calculated to elucidate the plastic hardening coupled with void growth. This work sheds new lights in exploring the atomistic origins of the void size and strain rate dependent mechanisms associated with dislocation activities close to void surface
New article in Engineering Fracture Mechanics by PhD candidate Haiyang Yu
Viscous regularization for cohesive zone modelling under constant displacement: an application to hydrogen embrittlement simulation
The convergence problem during the cohesive zone modelling of hydrogen embrittlement in constant displacement scenario is attributed to the numerical instability which is studied analytically in the present work. The property of numerical stability is directly associated with the number of solutions for the controlling equations from the failure initiation point. It is shown that all the cases with a non-unique solution are numerically unstable thereby having convergence problem. Linear elastic and elasto-plastic material models are considered in the derivation, and the convergence properties for both models are proved essentially the same. The viscous regularization proposed by Gao and Bower proves effective in solving the convergence problem with good accuracy under constant displacement, provided that the viscosity is small enough. This is further supported by a pipeline engineering case study where the viscosity regularized cohesive zone approach is applied to the hydrogen embrittlement simulation. The stabilizing mechanism of the viscous regularization is attributed to its capacity to enforce a single solution by modifying the controlling equations. The influence of viscous regularization on symmetry modelling is also discussed.
New article in Applied Physics Letters: Electrical four-point probing of spherical metallic thin films
New article published in Applied Physics Letters. In this manuscript, we are presenting a novel method for performing four-point electrical measurements on spherical thin films with micron-scale diameters. Such measurements have not been reported previously, as four-point measurements are normally performed on flat surfaces and with complete symmetry in probe positions. The method takes advantage of recent advances in commercially available micro-robots, which allows the positioning of four separately controlled electrical probes on very fine structures. Using finite element models to obtain geometric corrections factors yields the opportunity to estimate the resistivity of materials and structures where symmetric probe positioning is difficult to achieve. By this method, we show that the intrinsic resistivities of spherical thin films are higher than that of bulk metal. The findings are of large significance to the electronic packaging industry, where cost-efficiency and getting large gain from the consumed amount of precious metals are of large importance.
New article in Nanoscale: Nanoscale Deicing by Molecular Dynamics Simulation
Excessive icing is a general problem to human activities in low temperature environment. The aim of creating anti-icing materials, surfaces and applications rely on understanding the fundamental nanoscale ice adhesion mechanics. As increasing experimental trials on manufacturing anti-icing coatings have been carried out, theoretical knowledge on the atomistic determinants of ice adhesion is in urgent need. In this study, we employ all-atom modeling and molecular dynamics simulations to study ice adhesion, detaching and shearing on smooth silicon and graphene surfaces, aiming to decipher the basis of ice adhesion strength. We also study the mechanical effects of an aqueous water layer that sandwiched between ice and substrate, giving results to support previous experiments. Our results for the first time provide atomistic view on the key events of nanoscale deicing processes, and supply strong theoretical references for further anti-icing studies.
Verner Håkonsen winner of NTNU NanoLab Image contest 2016
PhD candidate Verner Håkonsen has won the NTNU NanoLab Image contest 2016. We congratulate Verner!
Image description: Self-assembled magnetic nanocubes into superstructured tubes, which again have self-assembled into "leaf-like" micropatterns. Instrument used: Hitachi S5500 S(T)EM.
Molly Bazilchuk, Best Student Talk Award
PhD candidate Molly Bazilchuk received the Best Student Talk Award from the 7th annual workshop of The Norwegian PhD Network on Nanotechnology for Microsystems. We congratulate Molly!
Electromechanical characterization of individual micron-sized metal coated polymer particles
New paper published in Journal of Applied Physics. In the paper we present a method of simultaneous electrical resistance and compression measurements of single micron-size metal coated polymer particles. The method allows fundamental physical insight into the mechanisms of electrical resistance in the interconnect where such particles are applied.
Contact resistance and metallurgical connections between silver coated polymer particles in isotropic conductive adhesives
Recently, there has been an increasing interest in silver thin film coated polymer spheres as conductive fillers in isotropic conductive adhesives (ICAs). Such ICAs yield resistivities similar to conventional silver flake based ICAs while requiring only a fraction of the silver content. In this work, effects of the nanostructure of silver thin films on inter-particle contact resistance were investigated. The electrical resistivity of ICAs with similar particle content was shown to decrease with increasing coating thickness. Scanning electron micrographs of ion milled cross-sections revealed that the silver coatings formed continuous metallurgical connections at the contacts between the filler particles after adhesive curing at 150 °C. The electrical resistivity decreased for all samples after environmental treatment for three weeks at 85 °C /85 % relative humidity. It was concluded that after the metallurgical connections formed, the bulk resistance of these ICAs were no longer dominated by the contact resistance, but by the geometry and nanostructure of the silver coatings. A figure of merit (FoM) was defined based on the ratio between bulk silver resistivity and the ICA resistivity, and this showed that although the resistivity was lowest in the ICAs containing most silver, the volume of silver was more effectively utilized in the ICAs with intermediate silver contents. This was attributed to a size effect due to smaller grains in the thickest coating.
CuO/Cu based superhydrophobic and self-cleaning surfaces
CuO/Cu based superhydrophobic surfaces with ordered micro/nanostructures have been prepared via a solution-immersion process combining with photolithography and argon ion beam etching. CuO nanoneedles grow only inside microholes on copper substrate due to delaying effect of both residual photoresist and carbon layer produced during Ar etching. The hierarchical structures and surfaces show a water contact angle of 152⁰, a contact angle hysteresis of 3⁰, and a low water adhesion force of 15 μN, indicating a good superhydrophobicity. The obtained surfaces also keep itself clean from carbon black, chalk dust and water, enabling a great potential in self-cleaning and anti-fouling applications.
Joint publication with Brno University of Technology
Abstract: Present ab initio study was focussed on a response of NiTi martensite to a superposition of shear and tensile or compressive stresses acting normally to the shear planes. The theoretically predicted base-centered orthorhombic (BCO) ground-state structure was found unstable under uniaxial compression and two transformations, one from orthorhombic to a monoclinic symmetry and the other back from monoclinic to orthorhombic symmetry, were observed in the computational model. The former transformation shows that the uniaxial compressive stress of about 4GPa destabilizes the BCO structure by reducing its symmetry to the experimentally observed monoclinic one. However, superposition of small shear stresses remarkably lowers the compressive stress necessary for this destabilization. The latter transformation then draws the crystal lattice to the B19 structure. The theoretical shear strength of NiTi martensite was subsequently computed as a function of the normal stress. The results obtained show that the e_ect of the normal stress is surprisingly opposite to that calculated for NiTi austenite and other cubic metals, i.e., that the shear strength is lowered by the compressive normal stress and vice versa.
A Uniform Hydrogen Degradation Law for High Strength Steels Is Proposed
Abstract: The degrading effect of hydrogen on high strength steels is well recognized. The hydrogen degradation is dependent not only on hydrogen content, but also on geometric constraints or equivalently, level of stress triaxiality, which means the hydrogen degradation locus is not likely to be a unique material property. Experimental data on notched tensile tests reported by Wang et al. are analyzed via cohesive zone modelling, and a cohesive strength based uniform hydrogen degradation law is proposed upon normalization of hydrogen degradation loci with different specimen geometries. Since the e ects of hydrogen content and geometric constraints are decoupled during normalization, the proposed law is applicable to all the specimen geometries as a material property. This law is subsequently applied to simulate the constant loading tests performed on the same material. Excellent agreement is observed between the simulation and test results in terms of incubation time for fracture initiation and highest permissible initial hydrogen content. The inconsistency observed in one of the cases is discussed, suggesting that the e ects of strain rate and stress relaxation need to be taken into account in order to improve the transferability of the degradation law calibrated from tensile tests to constant loading situations.
Two FRINATEK projects signed with Research Council of Norway
NTNU Nanomechanical Lab received two projects from Research Council of Norway through the highly competitive FRIPRO program. One project titled "Engineering Metal-Polymer Interface for Enhanced Heat Transfer (HEFACE)” is led by Jianying He and funded by the FRINATEK Young Research Talents program. Another FRINATEK project “Towards Design of Super-Low Ice Adhesion Surfaces (SLICE)” is led Zhiliang Zhang. Contracts for both projects have been signed.
Colleagues from NTNU Gjøvik visited Nanomechanical Lab to discuss collaboration
Deformation and fracture of nano-sized metal-coated polymer particles: A molecular dynamics study
The mechanical behavior of Ni-coated polyethylene (PE) nanoparticles subjected to compression loading is systemically investigated by classical molecular dynamics simulation. Results show that the Ni coatings on PE nanoparticles lead to a densification of particle surface and remarkably enhance the compression strength. The particle size-effect and the coating thickness effect on the compression responses in terms of compressive strength and particle burst are observed. Burst of Ni-coated PE nanoparticles initiates at the high stress-concentrated grain-boundaries zone of polycrystalline Ni-shell, and propagates along the compression direction to the flattened contact surface, resulting in release of core PE molecules.
Inverse Hall-Petch and Hall-Petch Behaviour First Time Found for Gas Hydrates
Nature Communications, 6: 8743. 2015 Nov 2.
GEMINI News: Uncovering secrets of ice that burns
GEMINI News: Den brennende isens hemmeligheter
Materials Today News: “Fire ice” an opportunity and a threat
Sediment-hosted gas hydrates have profound impacts on global energy sources and climate change. Their mechanical properties play a crucial role in gas recovery and understanding their evolution in nature, however, the deformation mechanisms of gas hydrates have not yet been elucidated owing to the difficulties in experimental measurements. Here we report direct molecular dynamics simulations of the material instability of monocrystalline and polycrystalline methane hydrates under mechanical loading. The results show dislocation-free brittle failure in monocrystalline hydrates and an unexpected ductile ultimate strength as a result of crossover from grain-size strengthening to weakening in polycrystals. Upon uniaxial depressurization, strain-induced hydrate dissociation accompanied by grain-boundary decohesion and sliding destabilizes the polycrystals. In contrast, upon compression, appreciable solid-state structural transformation dominates the response. These findings provide molecular insights into the destabilization mechanisms of gas hydrates caused by deformation beyond the conventionally thermodynamic instability.
Effects of Loading Path on the Fracture Loci in a 3D Space
Axi-symmetric and 3D unit cell analyses with continuous non-proportional loading paths are performed to investigate the path dependence of the fracture loci in a 3D space. The loading pattern utilized is the generalization of a number of non-proportional paths recorded in real tests. Failure of the unit cell is predicted when localization of plastic flow occurs, and the failure strains are plotted against the strain history averaged stress triaxiality and Lode parameter to construct fracture loci in a 3D space. The fracture locus with a non-proportional loading path deviates from that with a proportional loading path along the axis of stress triaxiality and becomes non-monotonic in high triaxiality regime. Meanwhile, such deviation occurs only when a certain level of triaxiality is reached. Agreement with the proportional locus as well as monotonicity maintains over a large range of stress triaxiality that covers most cases in reality, as long as the non –proportionality of the loading path is su_ciently low. This provides the rationale for utilizing the average triaxiality based fracture locus as an acceptable approximation in practice. Deviations of the non-proportional loci along the axis of Lode parameter are also observed. Further study on the Lode history dependence suggests using the final value of Lode parameter instead of the averaged one as the Lode axis in the fracture loci, which can alleviate the severity of path dependence for the loading patterns concerned. Based on these results, the e_ectiveness of the average stress state based fracture loci reported in the literature is discussed.
A SERS Study on the Assembly Behavior of Gold Nanoparticles at the Oil/Water Interface
Herein, the assembly behavior of gold nanoparticles (AuNPs) at oil/water interface is studied by surface-enhanced Raman scattering (SERS) spectroscopy. Two selected chemicals (1-dodecanethiol (DDT) and tetramethylammonium ion (TMA+)) are applied to tune the surface properties of AuNPs and the corresponding assembly behaviors at oil/water interface are thoroughly investigated. Various AuNPs films, namely sparse 2D film, perfect monolayer, and multilayers are obtained. The SERS spectra analyses show that the surface composition of AuNPs is strongly dependent on the chemical environment around AuNPs and results in different morphologies of AuNPs film at oil/water interface. Accordingly, we propose a rational relationship between AuNPs assembly behavior at oil/water interface and their surrounding chemical environment, and thus reveal the physical mechanism underlying the nanoparticle assembly.
Structural Instability and Mechanical Properties of MoS2 Toroidal Nanostructures
Molybdenum disulfide (MoS2) nanostructures have received considerable research attentions due to their outstanding physical and chemical properties. Recently, a form of MoS2 ring structure exhibiting unique transport properties has been experimentally identified. Herein, we present the first report describing direct molecular dynamics (MD) simulations of structural instability and mechanical properties of hypothetical MoS2 nanotube (NT) toroidal nanostructures. Nanorings with small MoS2 NTs' diameter retain their circular shape because of higher bending stability of NTs, while for those with large diameter of MoS2 NTs buckling/kinking and displacive phase transformation appear to effectively reduce bending stress as a mechanism for stabilizing the nanorings. However, the nanorings which have to polygonize maintain a circular shape as thick multi-walled inner nanorings are presented. Furthermore, mechanical responses of various nanoweaves (nanochains, nanomailles, and nanochainmailles) by linking nanorings together are also studied. Results show that Young’s modulus, stretchability and tensile strength of such nanoweaves depend not only on the helicity of MoS2 NTs but also the woven pattern. For example, nanostructures with 4-in-1 weave of nanorings exhibit much higher tensile strength and stiffness but lower extensibility than those with 2-in-1 weave. The finding suggests that MoS2 NT nanorings and their woven hierarchical structures may be used in the development of new flexible, light-weight electromechanical and optoelectronic nanodevices.
Czech-Norwegian cooperation in atomistic simulation
During October, 2015 we have started an international cooperation with CEITEC - Brno Univ. of Technology from the Czech Republic within the frame of the Norway Grants (proj. num. NF-CZ07-ICP-3-199-2015). The aim of this project is an international collaborative work between the Nanomechanical lab and the group of Advanced Metallic Materials and Metal Based Composites at the CEITEC in Brno. The main project objectives are focused to a collective work on ongoing research projects at the Nanomechanical lab, high quality scientific publications and knowledge exchange in atomistic modelling of materials properties using Ab initio and MD calculations. The project period is planned for twelve months and, at its beginning, Dr. Petr Šesták from CEITEC (the fifth person from the left site) has visited the Nanomechanical lab.
Selective growth of metallic nanostructures on microstructured copper substrate in solution
Selective growth of metallic micro/nanostructures on desired micropatterns was achieved via a simple solution-immersion process. Interestingly, metallic micro/nanostructures directly grow only inside the hollow copper micropatterns due to the different surface properties. Furthremore, these hierarchical micro/nanostructured Cu/CuO surfaces possess superhydrophobicity, low water adhesion forces and self-cleaning properties.
Extraordinary mechanical properties of smallest carbohelicene springs
A joint work with Brno University of Technology, Czech Republic and Xiamen University, China has been published at Phys. Chem. Chem. Phys. 2015, v17 (28) pp18684-18690, DOI: 10.1039/c5cp02043c
Abstract: The extraordinary deformation and loading capacity of nine different [∞]carbohelicene springs under uniaxial tension up to their fracture were computed using the density functional theory. The simulations comprised either the experimentally synthetized springs of hexagonal rings or the hypothetical ones that contained irregularities (defects) as, for example, pentagons replacing the hexagons. The results revealed that the presence of such defects can significantly improve mechanical properties. The maximum reversible strain varied from 78% to 222%, the maximum tensile force varied in the range of 5 nN to 7 nN and, moreover, the replacement of hexagonal rings by pentagons or heptagons significantly changed the location of double bonds in the helicenes. The fracture analysis revealed two different fracture mechanisms that could be related to the configurations of double and single bonds located at the internal atomic chain. Simulations performed with and without van der Waals interactions between intramolecular atoms showed that these interactions played an important role only in the first deformation stage.
Delegation from Singapore to NTNU Nanomechanical Lab to discuss the cooperation in Arctic Icephobic Coatings
A delagation from Singapore Polytechnic visited NTNU Nanomechanical Lab to discuss the cooperation in Arctic Icephobic Coatings.
Master students became the winners of NTNU NanoLab Image contest 2015
MSc student Emil Stokkeland developed a four wire measurement setup to investigate the electric properties of a silver coated polymer sphere, Ø 30 µm, with a 150 nm silver coating. Copyringht©Emil Stokkeland.
MSc student Jon Oddvar Kolnes developed Carbon nanotube arrays for anti-icing applications. Cross-sectional edge image of a desired structure: Carbon nanotubes, grown with an iron catalyst on a silicon substrate, with an aluminium oxide barrier layer. Copyringht©JOn Oddvar Kolnes.
Contact Resistance Measurement of Metal-coated Polymer Particles by Using miBots
Contact resistance measurement of nano-coated polymer particles. Experiment conducted by master student August Emil Stokkeland and PhD student Sigurd Pettersen.
NML logo carved on a polymer particle by FIB
The NTNU Nanomechanical Lab logo made on a polymer parictcle by using the Focused Ion Beam. Image created by PhD student Sigurd Pettersen.
A New Project on Hydrogen Induced Fracture
A NTNU-SINTEF-University of Oslo cooperation project titled "Hydrogen-induced degradation of offshore steels in ageing infrastructure - models for prevention and prediction(HIPP)" received 17 mkr from the Norwegian Research Council of Norway. The project is led by Zhiliang Zhang and will start from Spring 2014 and end Des 2017.
The primary objective of the HIPP project is to develop a model framework which describes and couples environment-assisted hydrogen degradation mechanisms at different length and time scales towards a predictive mechanism-based integrity assessment approach for oil and gas steel infrastructure.
A New Research Project to Start Spring 2014
A new KPN research project "Wettability alteration and improved flow transport by engineered nanoparticles for petroleum application" led by Professor Jianying He received 7.2 mkr funding from the Research Council of Norway's NANO2021 and PETROMAKS II programmes, the industrial partners Det norske oljeselskap ASA and Wintershall Holding GmbH. The project involves two PhD positions and will start spring 2014.
Thermal Conductivity of Carbon Nanocoils
In our previous research published at SMALL and JACS, it was demonstrated that defects incorporated in a proper manner can enhance the mechanical properties of helical CNTs. However, in our paper recently published at Applied Physics Letters we found that the thermal conductivity of the same type HCNTs will be dramatically reduced by the presence of defects. Details see the paper.
A Novel Constitutive Model
In a recent paper published at Journal of Mechanics and Physics of Solids (JMPS) a novel constitutive model was presented.
The traditional model of Alexander and Haasen poses several limitations. We introduce in this work a novel constitutive model for covalent single crystals and its implementation into a rate-dependent crystal plasticity framework. It is entirely physically based on the dislocation generation, storage and annihilation processes taking place during plastic flow.
Tougher Than the Toughest Materials!
We show in a recent paper published at the high impact jounral Small that helical nanotubes could be tougher than the toughest materials.
Helical carbon nanotubes with intentionally incorporated non-hexagonal defects have unexpectedly high toughness and plasticity, in addition to the well-recognized extreme elasticity. The obtained toughness approaches 5000 J g-1 with decreasing spring radius. The high toughness originates from the plastic nanohinge formation as a result of distributed partial fractures. A strong spring size effect, contradictory to the continuum solution, is precisely described by an atomistic bond-breaking model.
Tue, 26 Jan 2016 03:24:36 +0100
Fracture and Negative Poisson’s Ratio of Novel Spanned-Fullerenes Nanotube Networks under Tension
Carbon-based nanomaterials have attracted significant attention due to their unique optical, electrical, thermal and mechanical properties. In this study, various multi-dimensional graphitic architectures are constructed by spanning fullerenes with carbon nanotube (CNT) super-bonds. The mechanical properties of these novel architectures are systematically investigated by full atomistic simulations. Surprising negative Poisson's ratio observed in 2D and 3D networks is revealed to originate as a result of curvature-flattening or rigid mechanical model. The magnitude of Poisson's ratio is strongly dependent on the level of strain, CNT length as well as temperature. The insight on the deformation mechanism of these periodic graphitic nanostructures will facilitate the integration of low-dimensional materials towards high-dimensional organized structures to realize targeted multi-functional properties.
New project with Aker Solutions on Materials Technology for Future Demands
Aker Solutions is co-funding a research programme together with the Norwegian University of Science and Technology on the subjects of materials technology and mechanical engineering. The aim of the programme is to understand and assess high-performance materials for use in high pressure, high temperature and corrosive environments. A project agreement between Aker Solutions and NTNU was signed on the 12th of Oct 2013. The project involves two PhD positions.
Photo: professor Zhiliang Zhang (NTNU), Jim Stian Olsen (Aker Solutions), Kjartan Pedersen (Aker Solutions), Krista Amato (Aker Solutions), professor Roy Johnsen (NTNU).
Crosslinking effect on the deformation and fracture of monodisperse polystyrene-co-divinylbenzene particles
It has been found that the crosslinking density significantly influences the fracture property as well as the failure morphology. Slightly crosslinked particles become permanently deformed after compression, while highly crosslinked ones are entirely fragmented once a critical strain is reached.
Giant Stretchability and Reversibility of Helical Carbon Nanotubes
Our recent work has been published at the premier Journal - the Journal of the American Chemical Society (JACS).
There is a surging interest in 3D graphitic nanostructures which possess outstanding properties enabling them to be prime candidates for a new generation of nanodevices and energy-absorbing materials. Here we study the stretching instability and reversibility of tightly-wound helical carbon nanotubes (HCNTs) by atomistic simulations. The inter-coil vdW interaction-induced flattening of HCNT walls prior to loading is constrained by the defects coordinated for the curvature formation of helices. The HCNTs exhibit extensive stretchability in the range from 400% to 1000% as a result of two distinct deformation mechanisms depending on HCNT size. For small HCNTs tremendous deformation is achieved by domino-type partial fracture events, whereas for large HCNTs this is accomplished by stepwise buckling of coils. The formation and fracture of edge-closed graphene ribbons occur at lower temperatures while at elevated temperatures the highly distributed fracture realizes a phenomenal stretchability. Results of cyclic stretching-reversing simulations of large HCNTs display pronounced hysteresis loops, which produce large energy dissipation via full recovery of buckling and vdW bondings. This study provides physical insights into the origins of high ductility and superior reversibility of hybrid CNT structures
Role of Five-fold Twin Boundary on the Enhanced Mechanical Properties of fcc Fe Nanowires
In this work, the role of five-fold twin boundary on the structural and mechanical properties of fcc Fe nanowires is explored by classical molecular dynamics simulation with well-established embedded atom method (EAM) potential. Size-dependent mechanical properties of nanowires attributed to the surface tension in the literature. We identified, however, a new origin to the remarkable size effect on Young's modulus and tensile strength upon loss of elasticity of the novel five-fold twinned Fe nanowires. The inhomogeneous intrinsic stress due to the built-in five-fold twin structure contribute to the monatomic Young's modulus – elastic response per atom – distributed over the cross section, and results in size dependent Young's modulus. The five-fold twin boundaries strengthen the smaller nanowires by prohibiting dislocation nucleations in the fcc structure. Instead, phase transformation of fcc → bcc occurs at the elastic limit, and creates multi-grain structure that exhibits ductility. Yielding by dislocation nucleation is exclusively observed for larger nanowires at elevated temperature, followed simultaneously by the fcc → bcc phase transformation. Because of the five-fold twin structure, the central region of the nanowires experiences local expansion under elastic tension (negative Poisson's ratio). The critical stress/strain upon los of elasticity displays U-type temperature dependence.
Molecular-Dynamics Study Examines Effect of Nanoparticles on Oil/Water Flow
A paper highlighted at the widely distributed Journal of Petroleum Technology
Our SPE paper (156995) "Effect of Nanoparticles on Oil-Water Flow in a Confined Nanochannel: a Molecular Dynamics Study" presented at the SPE International Oilfield Nanotechnology Conference and Exhibition held in Noordwijk, The Netherlands, 12–14 June 2012, has recently been highlighted at the Journal of Petroleum Technology, February 2013, 148-151.