High-throughput, combinatorial synthesis of multimetallic nanoclusters

Multimetallic nanoclusters (MMNCs) offer unique and tailorable surface chemistries that hold great potential for numerous catalytic applications. The efficient exploration of this vast chemical space necessitates an accelerated discovery pipeline that supersedes traditional “trial-and-error” experimentation while guaranteeing uniform microstructures despite compositional complexity. Herein, we report the high-throughput synthesis of an extensive series of ultrafine and homogeneous alloy MMNCs, achieved by 1) a flexible compositional design by formulation in the precursor solution phase and 2) the ultrafast synthesis of alloy MMNCs using thermal shock heating (i.e., ∼1,650 K, ∼500 ms). This approach is remarkably facile and easily accessible compared to conventional vapor-phase deposition, and the particle size and structural uniformity enable comparative studies across compositionally different MMNCs. Rapid electrochemical screening is demonstrated by using a scanning droplet cell, enabling us to discover two promising electrocatalysts, which we subsequently validated using a rotating disk setup. This demonstrated high-throughput material discovery pipeline presents a paradigm for facile and accelerated exploration of MMNCs for a broad range of applications

Evolution of geometrically necessary dislocations in plastically strained precipitation hardened materials

There has recently been a strong interest in modelling size-dependent plasticity in metals based on the concept of geometrically necessary dislocations (GNDs). However, the precise manner by which geometrically necessary dislocations interact with second-phase particles remains elusive. Nevertheless no detailed materials characterisation has been performed to quantify these relationships. In the present paper, the evolution of GNDs in the presence of different precipitate morphologies is investigated. Room temperature tensile deformation experiments were performed on the aged specimens of an aluminium alloy and the evolving microstructure was compared with the mechanical response. The precipitate morphologies were characterised using transmission electron microscopy and the dislocation structure was analysed using orientation imaging of deformed specimens. It was observed that structural evolution was a function of the precipitate characters. In general, the dislocation cell size and misorientation angle between dislocation cells evolves systematically with deformation at relatively small strain levels. Copyright © 2011 Inderscience Enterprises Ltd

Graphene as cathode material in molten carbonate fuel cells

Copyright © 2014 by ASME. The molten carbonate fuel cell (MCFC) is considered one of the best technologies for stationary power. This is due to its high efficiency, medium-high operating temperature, and low emissions. The MCFC operates at a temperature range from 600°C to 700°C and normally is combined with the gas turbine (GT) as a topping cycle. This work investigates the impact of Platinum/Graphene (Pt/G) on a combined cycle of MCFC-GT by applying the first and second laws of thermodynamics. The maximum work output of the hybrid cycle is ultimately calculated to be 1350 kW. The overall exergy efficiency achieved is 59.82%. Our findings reveal that there is an average 23% gain in the maximum work output, energy and exergy efficiencies when Pt/G is used as the cathode material compared to other materials such as Platinum/Carbon (Pt/C) and Platinum/Carbon cloth (Pt/CC)

Advanced experimental techniques for multiscale modeling of materials

From a scientific viewpoint, direct comparison between mechanical tests and computational simulations on a one-to-one basis has the potential to lead to substantial development in the concept of virtual testing of materials. Successful application of virtual testing methodology in our daily life basis requires the use of high-fidelity computational models that are being validated through accurate characterization techniques. The content of this chapter is prepared to cover some of the most recent developments in the area of materials characterizations with great potential for virtual testing and modeling applications. During the last decade, atomic force microscopy (AFM) has evolved into an essential tool for direct measurements of intermolecular forces that can be employed for verification of first-principle and molecular dynamic models. Novel techniques in the area of in situ electron microscopy have been developed in the last decade for investigating the structure-mechanical property relationship of advanced materials. X-ray ultra-microscopy (XuM) and microelectromechanical systems (MEMS) are among the two newest in situ microscopy developments. These techniques provide an excellent platform for direct correlation between structure and properties of nanoscale materials. These systems contain a limited number of atoms and possible equilibrium configurations, which can be identified in real time by means of in situ electron microscopy techniques. In addition, because of the limited number of atoms, these systems can be atomistically modeled within the reach of currently available computational power. This chapter provides a comprehensive review on the above-mentioned characterization techniques that can be used to validate computational models at nanometer length scales. © 2009 Springer-Verlag US

Effect of deformation on electrical properties of carbon fibers used in gas diffusion layer of proton exchange membrane fuel cells

In proton exchange membrane fuel cells, the stacks of anode, cathode, and membrane layers including gas diffusion layer (GDL) are held together by a compressive force applied through a bipolar plate. In this work, we studied the electrical properties of a carbon fiber of a GDL under deformation using four-point measurement methods inside a scanning electron microscope (SEM). We found out that through bending deformation the electrical resistivity of carbon fibers will be reduced. The drop in resistance during deformation may be the result of increasing conduction channels in the carbon fiber and parallel transport through them. Our finding offers a new insight on the effect of deformation on tuning the electrical properties of GDL materials. © 2009 Elsevier B.V

Gradient of nanomechanical properties in the interphase of cellulose nanocrystal composites

The nanoscale transitional zone between a nanofiber and surrounding matrix (interphase) defines the ultimate mechanical characteristics in nanocomposite systems. In spite of this importance, one can hardly find quantitative data on the mechanical properties of this transitional zone in the cellulose-nanofiber composites. In addition, most of the theoretical models to predict the mechanical properties of interphase are developed with the assumption that this transitional zone is independent of the nanofiber size. In the current study, we show that the mechanical properties of interphase in cellulose nanocrystal (CNC) composites can be quantitatively characterized and the correlation with the size of CNCs can be mapped. The peak force tapping mode in atomic force microscope (AFM) was used to characterize deformation, adhesion, and modulus gradient of the interphase region in poly(vinyl alcohol) (PVA)-poly(acrylic acid) (PAA)-cellulose nanocrystal (CNC) composites. In comparison to the polymer matrix, the adhesion force of CNC was lower. The average elastic modulus in the interphase varied from 12.8. GPa at the interface of CNC to 9.9. GPa in PVA-PAA matrix. It was observed that the existence of PAA increased the gradient of mechanical and adhesion properties of the interphase zone. This occurs due to the variation in the ester linkage density from the CNC interface to the polymer matrix. Finally, it is shown that interphase thickness is higher for CNCs with larger diameter. © 2011 Elsevier Ltd

Finite size effect on the piezoelectric properties of ZnO nanobelts: A molecular dynamics approach

The size scale effect on the piezoelectric response of bulk ZnO and ZnO nanobelts has been studied using molecular dynamics simulation. Six molecular dynamics models of ZnO nanobelts are constructed and simulated with lengths of 150.97 Å and lateral dimensions ranging between 8.13 and 37.37 Å. A molecular dynamics model of bulk ZnO has also been constructed and simulated using periodic boundary conditions. The piezoelectric constants of the bulk ZnO and each of the ZnO nanobelts are predicted. The predicted piezoelectric coefficient of bulk ZnO is 1.4 C m -2, while the piezoelectric coefficient of ZnO nanobelts increases from 1.639 to 2.322 C m -2 when the lateral dimension of the ZnO NBs is reduced from 37.37 to 8.13 Å. The changes in the piezoelectric constants are explained in the context of surface charge redistribution. The results give a key insight into the field of nanopiezotronics and energy scavenging because the piezoelectric response and voltage output scale with the piezoelectric coefficient. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Evolution of in-grain orientation gradient in plastically strained particulate materials

The in-grain orientation gradient and grain fragmentation are essential features of substructure development under cold deformation up to large strains. In the previous study [P. Trivedi, D.P. Field, H. Weiland, Alloying effects on dislocation substructure evolution of aluminum alloys, Int. J. Plasticity 20 (2004) 459-476], we showed that the Mg and Si elements can influence the development of dislocation structures. The present study extends the above-mentioned work to the materials where effective hardening particles are distributed throughout the polycrystalline structure, and the interaction of dislocation substructures and these particles is significant. We focus upon the small strain regime and compare the in-grain orientation gradient of a precipitation hardened aluminum alloy as a function of precipitate morphologies during deformation. The precipitate morphologies were characterized using transmission electron microscopy, and the dislocation structure was analyzed using electron backscatter diffraction analysis of deformed specimens. The results clearly show that the deformation response was a function of the precipitate characteristics. Although, grain fragmentation increased during deformation, the effect of precipitates on the in-grain orientation gradient, and dislocation substructure evolution was more pronounced in the presence of semi-coherent β″ precipitates. This investigation offers motivation to include the precipitate parameters in the deformation framework of physics based computational modeling of crystals containing hardening particles. © 2009

Revealing the 3D internal structure of natural polymer microcomposites using X-ray ultra microtomography

Properties of composite materials are directly affected by the spatial arrangement of reinforcement and matrix. In this research, partially hydrolysed cellulose microcrystals were used to fabricate polycaprolactone microcomposites. The spatial distribution of cellulose microcrystals was characterized by a newly developed technique of X-ray ultra microscopy and microtomography. The phase and absorption contrast imaging of X-ray ultra microscopy revealed two-dimensional and three-dimensional information on CMC distribution in polymer matrices. The highest contrast and flux (signal-to-noise ratio) were obtained using vanadium foil targets with the accelerating voltage of 30 keV and beam current of \u3e 200 nA. The spatial distribution of cellulose microcrystals was correlated to the mechanical properties of the microcomposites. It was observed that heterogeneous distribution and clustering of cellulose microcrystals resulted in degradation of tensile strength and elastic modulus of composites. The utilization of X-ray ultra microscopy can open up new opportunities for composite researchers to explore the internal structure of microcomposites. X-ray ultra microscopy sample preparation is relatively simple in comparison to transmission electron microscopy and the spatial information is gathered at much larger scale. © 2011 The Authors Journal of Microscopy © 2011 The Royal Microscopical Society

On dislocation-based artificial neural network modeling of flow stress

Computational design of materials processes has received great interests during the past few decades. Successful designs require accurate assessment of material properties, which can be influenced by the internal microstructure of materials. This work aim to develop a novel computational model based on dislocation structures to predict the flow stress properties of metallic materials. To create sufficient training data for the model, the flow stress of a precipitation-hardening aluminum alloy was measured by characterizing the dislocation structure of specimens from interrupted mechanical tests using a high resolution electron backscatter diffraction technique. The density of geometrically necessary dislocations was calculated based on analysis of the local lattice curvature evolution in the crystalline lattice. For three essential features of dislocation microstructures - substructure cell size, cell wall thickness, and density of geometrically necessary dislocations - statistical parameters of their distributions were used as the input variables of the predictive model. An artificial neural network (ANN) model was used to back-calculate the in situ non-linear material parameters for different dislocation microstructures. The model was able to accurately predict the flow stress of aluminum alloy 6022 as a function of its dislocation structure content. In addition, a sensitivity analysis was performed to establish the relative contribution of individual dislocation parameters in predicting the flow stress. The success of this approach motivates further use of ANNs and related methods to calibrate and predict inelastic material properties that are often too cumbersome to model with rigorous dislocation-based plasticity models. © 2010 Elsevier Ltd