Metal coated polymer particles can be used in conductive adhesives for electronic packing technology. In the cooperative project between NTNU and TU Delft, we announce two PhD positions in the relevant field.

PhD 1: PhD position in Modeling and Characterization of Metal and Polymer Interface, NTNU Nanomechanical Lab at the Department of Structural Engineering, Faculty of Engineering (IV), Norwegian University of Science and Technology (NTNU), Norway. Application deadline Sept. 15, 2017. More info

PhD 2: PhD position in Direct-Write Manufacturing of Nanoparticle-based Structures, The Department of Precision and Microsystems Engineering (PME), Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, The Netherlands. Application deadline Oct. 15, 2017. More info

We present the anisotropic optical properties of 1D nanobelts of 6,13-dichloropentacene (DCP). High-quality large-area well-aligned DCP nanobelt arrays were readily obtained through self-assembly utilizing the strong π-π interaction between the molecular cores by simple solution processing method. The comparison of absorption and emission spectra of DCP in solution and DCP nanobelt indicated the co-existence of intramolecular and intermolecular excitons in the aggregation state of DCP. The photoluminescence (PL) from individual DCP nanobelt exhibited strong anisotropic property and the measured polarization ratio is on average 0.92±0.05, superior to that of the prior-art organic semiconductors. Beyond that, the angle-dependent photoluminescence clearly verified that the emission arose only from the relaxation of intramolecular exciton in spite of the strong electronic coupling along the π-π stacking direction. We believe these findings will enrich our knowledge of the exciton behaviour in 1D π-π stacking organic semiconductors and demonstrate DCP’s great potential for low-cost large-scale organic optoelectronic.

A novel tensile testing method is proposed, and a  ‘magic’ specimen with a special notch geometry has been identified. By using this special notched tensile specimen, material’s flow stress-strain curve can be DIRECTLY obtained from the recorded load versus diameter reduction curve and no Bridgman correction is needed.

Imbibition in porous media is ubiquitous and has important application in oil fields. Understanding the fundamental imbibition mechanism for nanofluids is very crucial to enhanced oil recovery (EOR) by nanoparticles. As it is difficult to disentangle the specific role of different interfaces in imbibition process by experimental trials, atomistic and molecular simulations hold the key to explore the migration mechanism of nanofluids into porous media and identify the dominating driving force for nanoparticles application in EOR.

In this study, we employ molecular dynamics simulations to study the spontaneous water imbibition into ultraconfined reservoir channels influenced by nanoparticles. By combining the dynamic process of imbibition, the water contact angle in capillary and the relationship of displacement (l) and time (t), a competitive mechanism of nanoparticle effects and fluid properties on spontaneous imbibition is proposed. Our findings provide new physical insights into the roles of nanoparticles in fluid imbibition, which is the core process in a number of technologies, including enhanced oil recovery.

Small systems are known to deviate from the classical thermodynamic description, among other things due to their large surface area to volume ratio compared to corresponding big systems. As a consequence, extensive thermodynamic properties are no longer proportional to the volume, but are instead higher order functions of size and shape. We investigate such functions for second moments of probability distributions of fluctuating properties in the grand-canonical ensemble, focusing specifically on the volume and surface terms as proposed by Hadwiger [Hadwiger, Springer, 1957]. We resolve the shape dependence of the surface term and show, using Hill’s nanothermodynamics [Hill, J. Chem. Phys., 1962, 36, 3182], that the surface satisfies the thermodynamics of a flat surface as described by Gibbs [Gibbs, Ox Bow Press, 1993, Vol. 1]. The Small System Method (SSM), first derived by Schnell et al. [Schnell et al., J. Phys. Chem. B, 2011, 115, 10911], is extended and used to analyze simulation data on small systems of water. We simulate water as an example to illustrate the method, using the TIP4P/2005 and other models, and compute the isothermal compressibility and thermodynamic factor. We are able to retrieve the experimental value of the bulk phase compressibility within 2 %, and show that the compressibility of nanosized volumes increases by up to a factor of two as the number of molecules in the volume decreases. The value for a tetrahedron, cube, sphere, polygon, etc. can be predicted from the same scaling law, as long as second order effects (nook and corner effects) are negligible. Lastly, we propose a general formula for finite reservoir correction to fluctuations in subvolumes.

New paper published at Materials Science & Engineering A.

Our recent results show that hydrogen embrittlement is present at sub-zero temperatures, causing an increase in fracture toughness reference temperature T₀ and a small decrease in deformation capability. The relationship between the T₀ and the impact toughness transition temperature T_28J, which, in the case of ultra-high-strength steel, deviates from that observed for lower strength steels, is proposed to be affected by the hydrogen content.

New paper published in Scientific Reports.

Our results show that low ice adhesion strength does not correlate well with water contact angle and its variants, surface roughness and hardness. Low elastic modulus does not guarantee low ice adhesion, however, surfaces with low ice adhesion always show low elastic modulus. Low ice adhesion (below 60 kPa) of commercial surfaces uniquely associates with small water adhesion force.

The flash diffusivity method/laser flash analysis (LFA) is one of the most popular methods for finding the thermal conductivity of a large range of materials, including polymer composites for thermal and electronic interconnects. With standardized, commercial instruments available, it has become common practice even in peer-reviewed journal publications to only state the instrument model and manufacturer, and then give the estimated thermal conductivity as an absolute value without discussing the intermediate factors. In this paper, we show that both the absolute values and temperature-dependent behavior of the specific heat capacity of polymer composite materials varies significantly with the three most common methods used to estimate this input factor for the LFA method, and that this further has a significant impact on the estimated thermal conductivity. We also give a systematic theoretical overview of the methods used in the manuscript, as this to our best knowledge has not before been published in one single paper. We expect that this paper can be of large value for researchers interested in investigating thermal properties of polymer composites, and as a general starting point for researchers interested in using the LFA method.

Evaluation committee

The first opponent: Prof. James Morris, Department of Electrical & Computer Engineering, Portland State University, Oregon, USA

The second opponent: Chief Scientist Maaike Margrete Visser Taklo, Department of Instrumentation, SINTEF ICT

The third opponent: Prof. Kjell Magne Mathisen, Dep. of Structural Engineering, NTNU

Evaluation committee

The first opponent: Associate Professor Shijo Nagao, Osaka University, Japan

The second opponent: Associate Professor Caroline Laforte, Université du Québec á Chicoutimi, Canada

The third opponent: Associate Professor Hilde Lea Lein, Department of Materials Science and Engineering, NTNU

Postdoctoral position in Multi-scale modelling of CO2 condensation (IVT-126/16)

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.

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.

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

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 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.

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.

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.

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!

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.

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.