Conference Programme

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I-04: Mechanically Coupled Phenomena and Testing Methods in Small Scales
Tuesday, 20/Jun/2017:
1:30pm - 3:30pm

Session Chair: Arief Budiman, Singapore University of Technology & Design
Session Chair: Alfonso Ngan, The University of Hong Kong
Location: Rm 305

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1:30pm - 2:00pm

Novel Redox Actuators Made from Nickel Hydroxide

Alfonso NGAN1, Kenneth KWAN1, Yuqi ZHANG1, Wang PINGYU1, Chuan CHENG1,2

1Department of Mechanical Engineering, The University of Hong Kong, Hong Kong S.A.R. (China); 2Institute of Materials Physics and Technology, Hamburg University of Technology, Germany

In this paper, we report a novel electrochemical actuating property of nickel hydroxide, with the actuation mechanism mainly due to reversible faradic redox reactions. Benefiting from the stable Ni(II)/Ni(III) redox couples in an electrolyte medium, highly reversible and stable actuation is observed under triggering voltages of less than 1 volt. By appropriate design of the actuating material with respect to the passive materials in a device, high actuation strains that can rival conducting polymers or even human muscles can be achieved. Also, by conditioning the structure of the actuator into a nanowire network morphology that facilitates ion transport, record high strain response time in the order of 0.1 second can be obtained, which is more than two orders faster than the current metallic based actuators.

To model the actuation mechanism, a multi-scale, multi-field simulation approach is used to model the above electrochemical actuation behavior. Specifically, molecular dynamics simulations with reactive force-field potentials and a modified charge-equilibrium (QEq) method are used to calculate the surface stress built up in Ni(100) surface in contact with water electrolyte due to a voltage applied across the interface, as a result of capacitive charging of the double layer in the contacting electrolyte as well as redox reaction of the Ni surface. The calculated surface stress is then used as input in a meso-scale finite-element (FE) model to compute the actuating stress set up in a single unit cell of a Ni nano-porous structure. The single-unit actuating stress is eventually used in a continuum FE model at a larger scale, to calculate the bending of an entire bilayered cantilever which replicates experimental conditions. This is the first successful attempt to simulate the electrochemical actuation of a real-sized, nano-porous metallic structure in an electrolytic environment.

2:00pm - 2:30pm

Reliable Soft Electronic Conductors using PEDOT:PSS based Organogels

Yoo-Yong LEE, Seung-Min LIM, Gwang Mook CHOI, Jeong-Yun SUN, Young-Chang JOO

Seoul National University, South Korea

One of the promising strategy to solve mechanical issues in current wearable electronics is using soft gel-type conductors. Due to their natural softness and compatibility with human skin, reliable transport of electrical signals upon severe mechanical deformation can be obtained, which have a great potential as application of skin attachable electronic devices and soft robotics.

We first developed a new class of soft-gel electronic conductors based on PEDOT:PSS based organogel. By using closely packed PEDOT:PSS sheets as a conducting component, an electrical polymeric path was effectively formed inside the gels. Thus, the polymeric conducting path was well maintained during stretching up to large strains (>300 %) strain, and even a strain insensitive resistance change was achieved up to 50% strain with fatigue cycle. Moreover, by using ethylene glycol as a liquid constituent for the organogel, purely electrical conduction was enabled without use of any electrochemical reactions successfully resulting in long-term environmental stability.

Furthermore, to improve the electrical percolation network in gel system, we implement the Ag nanowire/PEDOT:PSS hybrid nanocomposites as a conducting component. By introducing AgNW in PEDOT:PSS matrix, the electrical durability of organogel during the cyclic tensile deformation was highly enhanced, which demonstrates an excellent electrical and mechanical reliability of soft gel-type conductors.

2:30pm - 3:00pm

In-situ TEM observation of Grain Boundaries/Dislocation Interaction in Oxides

Shun KONDO1,3, Eita TOCHIGI1, Naoya SHIBATA1, Yuichi IKUHARA1,2,3

1The University of Tokyo, Japan; 2Japan Fine Ceramics Center, Japan; 3Kyoto University, Japan

It has been reported that several oxide crystals can be plastically deformed even at R.T. by dislocation slip like metals. So far, many experimental investigations have been tried for understanding the dislocation-grain boundary interaction, but these experiments were mostly carried out statically, and the fundamental processes are still not well understood yet. In this study, the nanoindentation experiments were conducted for SrTiO3 and Al2O3 crystals and their bicrystals inside Stransmission electron microscopy (TEM). Several kinds of TEM specimens for in situ nanoindentation experiments were prepared, that are single crystals and bicrystals including various types of GBs. The SrTiO3 single crystals were indented with the sharp diamond tip along the [001] direction and successfully observed the dislocation dynamics. From the detail ex situ analysis of the dislocations, the introduced dislocations have the slip system of {110}<110>, which is consistent with the primary slip system of SrTiO3 at room temperature. In the case of the GBs, the interaction between the introduced lattice dislocations and the GBs were directly observed. The dislocation-GB interaction and its dependence on the GB characters will be discussed in detail.


[1] S. Kondo, N. Shibata, T. Mitsuma, E. Tochigi, Y. Ikuhara, Appl. Phys.Lett., 100(18), 181906(2012)

[2] S. Kondo, T.Mistuma, N. Shibata, Y. Ikuhara, Sci.Adv., 2: e1501926 (2016)

3:00pm - 3:15pm

A Hybrid Study on the Deformation Behavior of Gamma Phase in TiAl Alloys using In-situ Transmission Electron Microscopy Experiments and Molecular Dynamics

Seong-Woong KIM1,2, Seung-Hwa RYU3, Jaemin KIM3, Young-Sang NA1, Seung-Eon KIM1, Andrew MINOR1

1Korea Institute of Materials Science, South Korea; 2Lawrence Berkeley National Laboratory, Berkeley, United States; 3Korea Advanced Institute of Science and Technology, South Korea

We present a new TiAl alloy with a record high ductility (0.78 total strain) at room temperature, solely achieved by casting. A combined in-situ transmission electron microscopy experiment and molecular dynamics simulation study is conducted in order to understand the underlying mechanism on the room temperature ductility. From in-situ straining transmission electron microscopy experiments, we found that the dominant deformation mode is different for the TiAl alloys with and without room temperature ductility. The difference in deformation mode was explained by the generalized stacking fault energy of the TiAl alloys which significantly depends on the composition of TiAl alloys. Furthermore, the role of lamellar orientation on deformation behavior is explained by considering Schmid factor. Finally, we proposed a set of important microstructural factors to form TiAl alloys with room temperature ductility.

3:15pm - 3:30pm

In Situ SEM Observation of Crack Growth in Metal-Metal Nanolayered Composites during Clamped Beam Bending

Ihor RADCHENKO, Hashina Parveen ANWAR ALI, Arief BUDIMAN

Singapore University of Technology and Design, Singapore

Depending on their individual layer thickness, layer crystallography and interface structure, various deformation mechanisms may be present in metal-metal nanolayered composites (NC). Some of these mechanisms include dislocation pile up based on Hall-Petch scaling law, confined layer slip, interface shear and interface crossing of a single dislocation. These deformation mechanisms can largely affect its subsequent fracture behaviour. Recent studies observed the influence of the nanolayer thickness on the crack propagation in NC, however there is not much evidence highlighting the effect of interface structure on the crack behaviour in NC especially for incoherent interfaces. In this study, we have investigated crack propagation across the layers of fcc-bcc nanolayered composites by the in situ nanofracture testing in a SEM technique. The clamped beam bending geometry was adopted to perform the mechanical fracture testing. It was found that the crack propagation through the nanolayers may be significantly different depending on interface structure and distance from sample surface due to dislocation deformation mechanisms that might lead to crack deflection during fracture.

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