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Nanostructured Materials

(Robin Drew, Raynald Gauvin, Pat Kambhampati, Larry Lessard, James Nemes, Sasha Omanovic, Martin Ostoja-Starzewski, and Srikar Vengallatore)

Micro and nano structured materials are comprised of juxtapositions of discrete particles of the corresponding scale. The material is uniform at the scale of the particle, and approximately uniform at a macroscopic scale, at least in a statistical sense. Its response and behavior, however, depends on the detailed response of the microstructure to external stresses, and therefore can exhibit novel and complex properties. Attempts at a consistent formulation of response or constitutive laws of these materials regularly encounter micro structural randomness and complexity at many disparate spatial and temporal scales. As a result, the analysis of response and properties has to be cast in the framework of a statistical theory of micro mechanics, and calls for a range of techniques lying at the intersection of mechanics, materials science and applied/stochastic mathematics. Theoretical research at MIAM encompasses composite materials, polycrystals, granular media, functionally graded materials, and biomaterials. Methods involve classical and non-classical continuum mechanics, stochastic mechanics/dynamics, computational mechanics, and mesoscopic theoretical modeling and computation [M. Ostoja-Starzewski, “Towards stochastic continuum thermodynamics”, J. Nonequil. Thermo. 27, 335 (2002)]. A particular class of applications involves the development of models to describe the response of materials to high-rate loading for the design of aerospace components and structures. Advanced plasticity theories are being used to improve our understanding of several manufacturing processes, including sheet forming, rolling and heading. Viscoplastic and damage theories are being used to describe creep deformation and fracture of aircraft engine components fabricated from high-temperature nickel alloys. Micromechanical models are being developed to aid in the design of highly formable, high-strength multi-phase steels [G. Bande and JA Nemes, “A new approach for single crystal materials analysis: theory and application to initial yielding”. Trans. ASME 127, 119 (2005)].

At the scale of an individual nano crystal, the size of the particle itself determines many of its properties such as the electronic absorption and emission spectra, vibrational spectrum, or its melting temperature. The ultimate origin of these properties arises from quantum confinement effects - the particle's size approaches the characteristic length scale of the electronic wavefunction. For this reason, nano structured materials are under intense investigation due to their promise in a wide range of applications ranging from quantum computing to medical imaging [D.H. Son, P. Kambhampati, et al., “Femtosecond multicolor pump-probe study of ultrafast electron transfer of [(NH3)(5)(RuNCRuII)-N-III(CN)(5)](-) in aqueous solution”. J. Phys. Chem. A 106, 4591 (2002)]. Furthermore, ordered assemblies of nano crystals lead to novel structures such as photonic crystals and quantum dot super lattices. In these particular arrangements, the nanocrystal ensemble is periodicity in one or more dimensions. As a consequence, they are neither as disordered as a liquid nor as ordered as a crystalline solids, and the special hierarchical spatial order leads to novel interparticle interactions and dynamics. When the periodicity approaches the wavelength of visible light, a particular type of optical response is created. Just as a periodic potential in a crystal results in electronic bands and bandgaps in semiconductors, a periodic refractive index modulation will result in photonic bands and photonic bandgaps in these materials, with concomitant applications in the field of optoelectronics.

Moving up to the microscale, research is being conducted in microdevices (MEMS). Although the conventional definition of a microstructured material does not encompass microdevices per se, current miniaturization and functionalization of components has reached the micron scale, and therefore it is possible to embed devices in materials much in the same way that particles form composites, or grains lead to polycrystals. The integration of materials with different properties and functions into complex, microscale, systems has the potential to bear a revolutionary impact on many important technologies such as sensing, actuation, portable power generation, information storage, and medicine. The performance and reliability of microdevices are dictated by materials and structures whose critical dimensions range from a few nanometers to several micrometers. Research at MIAM centers around the design, synthesis, characterization, and integration of advanced materials for microdevices [S. Vengallatore, “Gorsky damping in nanomechanical structures”. Scripta Mater. 52, 1265 (2005)]. Examples of recent work include: analysis of thermoelastic dissipation in nanocrystalline silicon microresonators; development of microscale stress measurement techniques; synthesis and characterization of nanostructured surfaces using colloidal self-assembly techniques; and the design, manufacture and testing of structural components for microscale fuel cell devices [C.D. Baertsch, S. Vengallatore, et al., “Fabrication and structural characterization of self-supporting electrolyte membranes for a micro solid-oxide fuel cell”. J. Mater. Res. 19, 2604 (2004)].

Research is also conducted in more classical areas of advanced materials in metallurgy, ceramics, and composites. Research in ceramics encompasses the fundamentals of metal-ceramics interaction [M Brochu, R. Drew et al., “Application of electroless coating for processing and joining of advanced materials”. Mat. Sci. Forum 426-4, 2491 (2003)], their processing and sintering properties, and applications in ferroelectric ceramics, metal extraction of oxide fiber ceramics, and the development of metal and ceramic coatings [M. Brochu, R. Drew et al., “Fabrication of a composite powder and its application as an active brazing alloy”. J. Mat. Sci. 40, 1485 (2005)]. Research in composites involves new analytic techniques to predict damage initiation and growth in composite materials [R. Hosseinzadeh, L. Lessard et al., “Parametric study of automotive composite bumper beams subjected to low-velocity impacts”. Composite Structures 68, 419 (2005)]. The concept of progressive damage modeling is used coupled with finite element methods to attain failure prediction capabilities for composites. Finally, research into the structure and properties of materials necessitates advanced diagnostic tools which focus on the characterization of the microstructure of complex materials by electron microscopy. Current studies range from improvements to our understanding of the processes underlying electron scattering in solids and Monte Carlo simulations of the scattering amplitude given energy loss models of the material [H. Demers and R. Gauvin, “X-ray microanalysis of a coated nonconductive specimen: Monte Carlo simulation”. Microscopy and Microanalysis 10, 776 (2004)] to low voltage X-Ray analysis of nanomaterials by using field emission scanning electron microscopy, the characterization of non-conductive materials by using charge contrast imaging in variable pressure scanning electron microscope [K. Robertson, R. Gauvin, et al., “Application of charge contrast imaging in mineral characterization”. Min. Eng. 18, 343 (2005)], and the development and characterization of nanocomposite materials.

Finally, research is conducted on the properties of charged materials as it pertains to the field of electrochemistry. Nano structured electro catalysts are being investigated specifically for hydrogen generation to aid in the development of sustainable energy production. One of the main obstacles with the large scale commercial application of polymer electrolyte fuel cells is the cost due to the use of noble metals (Pt, Ir, Ru) as electro active catalysts. Research at MIAM focuses on developing new gas diffusion cathodes for hydrogen evolution, which are based on nanostructured, poly metallic electro catalysts, and that are highly active, efficient, stable, and durable. This non noble catalysts would offer low over potentials for the hydrogen evolution reaction, and would significantly lower catalyst loading, hence leading to lower power usage and operation closer to the reversible conditions of hydrogen generation. Electrochemical oxidation is also being investigated for its potential in wastewater treatment. In particular, the development of new electro catalytically active anodes could lead to efficient mineralization of selected organic compounds, or even to convert them into desired products suitable for either further biological treatment, or as chemical precursors in organic synthesis. Electro chemistry also finds application in biotechnology and medicine. On the one hand, there is increasing pressure to develop new environmentally friendly technologies. Electrochemistry, being a clean engineering discipline, offers advantageous routes for the processing involved in a wide range of new biotechnologies, such as for example the regeneration of the coenzyme Nicotinamide Adenine Dinucleotide [A. Azem, S. Omanovic et al., “Direct regeneration of NADH on a ruthenium modified glassy carbon electrode”. J. Molec. Catalysis A-Chemical 219, 283 (2004)]. Another example concerns the development of electrochemical biosensors, with research at MIAM focusing on mechanisms of charge transfer between solid electrode surfaces, electron mediators, and enzyme prosthetyc groups. On the other hand, we find electro chemical process with direct applications in medicine. They include the development of electrochemically-based biosensors to monitor various disease markers, such as an acute phase serum protein, C-reactive protein (CRP), and neurotransmitters. Research is also conducted on fundamental surface-electrochemistry processes on metallic biomaterial surfaces to improve their corrosion resistance and biocompatibility by surface modification, and on the interaction of proteins, lipids and drug molecules with metal surfaces in the modification of implant surfaces for the controlled drug release in metals used to produce stents [J. Wright, S. Omanovic, et al., “L-Phenylalanine adsorption on Pt: electrochemical impedance spectroscopy and quartz crystal nanobalance studies”. J. Eletroanalytical Chem. 550, 41 (2003)].