​ABSTRACT: Transmission electron microscopy (TEM) is an important tool for the characterization of materials as it can provide a clear understanding of the relationship between structure, properties and composition of nanomaterials. For this, the in-situ TEM analysis is performed and requires specially manufactured sample holders. In particular, those designed to carry out electrical biasing can be used to understand not just the I-V characteristics but also the failure mechanism, structure-property relationships, Joule heating dynamics, electromigration, field emission properties, etc. at the nanoscale.

The platforms holding the sample in most modern in-situ TEM holders rely on an insulating ceramic membrane which needs to be (almost) transparent to the imaging electron beam. Electrodes are defined through lithography and patterned on this membrane. Unfortunately, the presence of this membranes introduces several limitations such as electrostatic charging, reduction of image contrast and poor mechanical stability. To circumvent this issue it is necessary to fabricate a novel type of sample platform which does not rely on the presence of a membrane. 

In this work, novel membraneless sample-holding platforms were designed and manufactured using advanced microfabrication methods and tools. Besides fitting into an array of analytical tools, the novel platforms (or “chips”) can be subjected to thermal and/or chemical processing without compromising their function or structure. 

To test these, the electrical response of one-, two- and zero-dimensional nanoparticles were studied. Firstly, we investigated current-induced modifications in silver nanowires and expandable graphite flakes and studied various phenomenon involved. Along with these, corresponding ex-situ studies were also performed. Next, graphene oxide was explored as an alternative support platform for in-situ TEM. We successfully achieved temperature as high as 2000^o C by Joule heating of graphene oxide. Furthermore, this graphene oxide platform was used as a heater and chemical processing substrate for investigating thermal stability and synthesis of inorganic nanoparticles, respectively.

​Abstract:  Supramolecular self-assembly of a certain number of designer organic/inorganic building blocks by directional interactions such as metal coordination is the basis of elaborate molecular systems. Most molecular architectures are purpose-designed in light of the size, shape, and surface natures of building blocks as well as chemical environments. On the other hand, we have occasionally come across unexpected structures and functions as a result of more complicated self-assembly processes than expected, which have often opened a new frontier in chemistry. The present lecture will focus on complexity in coordination-driven self-assembly by taking some of our recent examples of artificial metallo-DNAs,1-4 metallo-containers,5,6 molecular gearing systems,7-9 and metal macrocycle framework (MMF).10-12

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• CONTACTLinda J. Sapolu
• EMAILLinda.Sapolu@kaust.edu.sa

Abstract:

Environmental issues enforce transportation sectors to limit their carbon dioxide emissions in various ways. Automotive manufacturers attempt to reduce carbon dioxide emissions by seeking various strategies, e.g., increasing aerodynamic efficiency, using more fuel-efficient engines, reducing friction and wear of transmission systems, and, most importantly, by using lightweight materials and structures. This dissertation is a contribution toward a lightweight design of structures by proposing numerical models suitable for damage prediction of thermoplastic composite materials.

In this dissertation, predictive damage models for two different length scales, namely micromechanics, and mesomechanics, were proposed.  Micromechanics is used to predict the nonlinear damage behavior of elementary thermoplastic composite ply, while the mesomechanics is used to predict the failure behavior of thermoplastic composite laminates (test coupon or plate scale).

For the micromechanics, a representative volume element (RVE) of such materials was rigorously determined using a geometrical two-point probability function and the eigenvalue stabilization of homogenized elastic tensor obtained by Hill-Mandel kinematic homogenization. We proposed a viscoelastic viscoplastic model for the polypropylene matrix to extend the capability of the micromechanics model in predicting the damage behavior of the composite ply at various loading cases and strain rates.

At the mesoscale, we improved the classical mesomechanics damage modeling in the off-axis direction by introducing the confinement effect. The pragmatic approach consists of separating the progressive damage into two parts, namely "diffuse damage regime" and "transverse-cracking regime", were described by two distinct damage parameters. We also enriched the mesomechanics model by proposing a viscoelastic and viscoplastic model to account for the rate-dependent behavior of the thermoplastic composites. We showed that the predictions given by the proposed micromechanics and mesomechanics models were in excellent agreement with the experimental results in terms of the global stress-strain curves, including the linear and nonlinear portion of the response and also the failure point, making it useful virtual testing tools for the design of thermoplastic composites.

Bio: 

Ditho obtained his B. Eng. from Bandung Institute of Technology, Indonesia (2010), and M.Sc. from Korea Advanced Institute of Science and Technology (KAIST), South Korea (2012). He joined KAUST in 2013 as a Ph.D. student in Computational Solid Mechanics Laboratory (CSML). In 2014, He started to work on the KAUST-SABIC thermoplastic project under the supervision of Prof. Gilles Lubineau (COHMAS Laboratory) to develop predictive micro- and meso-mechanics models for thermoplastic composites, which led him to his Ph.D. dissertation.

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• CONTACTEmmanuelle Sougrat