Welcome to the NTNU Nanomechanical Lab


2016-08-22

New article in Computational Materials Science by PhD candidate Kai Zhao

– Kai Zhao, Inga Gudem Ringdalen, Jianyang Wu, Jianying He, Zhiliang Zhang

 

Ductile mechanisms of metals containing pre-existing nanovoids

 

Computational Materials Science

 

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


2016-08-19

New article in Engineering Fracture Mechanics by PhD candidate Haiyang Yu

– Haiyang Yu, Jim Stian Olsen, Vigdis Olden, Antonio Alvaro, Jianying He, Zhiliang Zhang

 

 

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.

 


2016-07-08

New article in Applied Physics Letters: Electrical four-point probing of spherical metallic thin films

– Sigurd R. Pettersen, August Emil Stokkeland, Helge Kristiansen, John Njagi, Keith Redford, Dan V. Goia, Zhiliang Zhang, and Jianying He

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.

 


2016-07-07

New article in Nanoscale: Nanoscale Deicing by Molecular Dynamics Simulation

– Senbo Xiao, Jianying He, Zhiliang Zhang

 

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.


2016-06-28

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.


2016-06-16

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!


2016-06-04

Electromechanical characterization of individual micron-sized metal coated polymer particles

– Molly Bazilchuk, Sigurd Pettersen, Helge Kristiansen, Zhiliang Zhang, Jianying He*

 

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.


2016-03-22

Contact resistance and metallurgical connections between silver coated polymer particles in isotropic conductive adhesives

– Sigurd Pettersen, Helge Kristiansen, Shijo Nagao, Susanne Helland, John Njagi, Katsuaki Suganuma, Zhiliang Zhang, and Jianying He*

 

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.

 


2016-03-15

CuO/Cu based superhydrophobic and self-cleaning surfaces

– Zhiwei He, Zhiliang Zhang, Jianying He*

 

 

 

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.


2016-03-11

PhD position in Arctic Icephobic Materials (IVT-51/16)

PhD Position in Durable Arctic Icephobic Materials

NTNU Nanomechanical Lab at the Department of Structural Engineering is looking for 1 new PhD candidate to further strengthen our group in the field of nanotechnology for anti-icing. The position is for 3 years without duties. Applications with CV, possible publications and other scientific works, certified copies of transcripts and reference letters must be submitted electronically to www.jobbnorge.no

Project details can be found http://www.ntnu.edu/nml/aim

Position info can be found http://www.ntnu.edu/nml/open-positions


2016-02-21

Joint Work with Brno University of Technology on DFT calculations of multiaxial stress-strain response of NiTi alloy


 

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.


2016-02-08

Three New PhD Positions


2016-02-02

A Uniform Hydrogen Degradation Law for High Strength Steels Is Proposed

– Haiyang Yu, Jim Stian Olsen, Antonio Alvaro, Vigdis Olden, Jianying He, Zhiliang Zhang*

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. 

Keywords: hydrogen embrittlement; high strength steel; cohesive zone modelling; hydrogen degradation law; constant loading test


2016-02-01

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.

2016-01-27

Colleagues from NTNU Gjøvik visited Nanomechanical Lab to discuss collaboration


Professors Are Strandlie and Halvor Holtskog from NTNU Gørvik visited NTNU Nanomechanical Lab today to discuss the potential collaboration between the two groups; one of the specific topics discussed is the nano-enabled icephobicity. 


 

Zhiliang Zhang

Professor 

Fracture Mechanics and Nanomechanics

Zhiliang.zhang@ntnu.no

Tel: +47 73592530

 

Jianying He 

Associate professor 

Nanomechanics

jianying.he@ntnu.no

Tel: +47 73594686

 

Helge Kristiansen 

Adjunct Professor 

Nanomechanics

helge@conpart.no

 

Jim Stian Olsen

Adjunct Associate professor

Fracture Mechanics and Materials Technology

jim.stian.olsen@akersolutions.com

 

Bing Liu

Visiting Professor

Molecular simulation

 

Postdoc research fellow
Atomistic modeling
 

Jianyang Wu

Research fellow

Nanomechanics, Molecuar Simulation

Jianyang.wu@ntnu.no

 

Petr Sestak

Visiting Post-doc fellow

Nanomechanics, DFT Calculations

sestak@fme.vutbr.cz

 

Øyvind Othar Aunet Persvik

PhD student

Fracture and Fatigue-Measurement Method

oyvind.persvik@ntnu.no

 

Zhiwei He

PhD student

Nanomechanics

zhiwei.he@ntnu.no

 

Sigurd Rolland Pettersen

PhD student

Nanomechanics

sigurd.r.pettersen@ntnu.no

 

Mao Wang

PhD student

Nanomechanics

mao.wang@ntnu.no

 

Yi Gong

PhD student

Nanomechanics

 

PhD student

Fracture Mechanics

 
PhD student
Nanomechanics for EOR
 
PhD student
Atomistic modeling
 
PhD student
Fracture mechanics
 
PhD student
Nanomechanics
 
PhD student
Nanomechanics
 
PhD student
Fracture mechanics
 
PhD student
Nanomechanics
 

 

PhD student
Nanomechanics
 

Yizhi Zhuo
PhD student
Nanomechanics
 
Merete Falck
PhD student
Nanomechanics
 
PhD student
Nanomechanics
 
Visiting PhD student
Fracture mechanics
 
Huihui Zhi
Visiting PhD student
Fracture mechanics