Nanoscale Materials and Phenomena

            In recent years, there has been an explosion of research on nanoscale or nanostructured materials because of the new phenomena and novel properties displayed by these materials, and the unprecedented opportunities that exist for extending the boundaries of these further. For instance, the substantial increases in hardness and strength of bulk consolidated nanostructured materials over that of conventional coarse-grained materials, because of reduction in grain size to the nanometer range, is now well known in pure metals and mechanically milled/alloyed materials, as well as in thin film multilayers of pure metals or inorganic compounds. There are, however, many important and daunting challenges in nanoscale materials for structural and functional applications, because one has to build fully dense components with length scales much greater (MEMS or >meter level) than of the individual nanoentities comprising them. As dimensions shrink or grow for both levels, new issues related to synthesis, processing, stability, functionality, structural integrity and reliable performance will no doubt arise. Thus, development of synthesis and processing methods and establishing a scientific understanding of their relation with phase transformations, microstructure and properties of the materials is a key problem area that must be addressed to enable the advanced applications that are foreseen. My current and planned projects are aimed at addressing the fundamentals of these aspects.

A.Nanoscale Aluminum Alloys (previously AFOSR supported, new proposal planned; Student:s Jixiong Han, Marty Pluth).

            Engineering new nanoscale alloys and composites may be the only way to meet the ever-demanding applications for structural components in transportation, propulsion, space, energy and other systems. This project combines novel synthesis and processing methods, with detailed atomic-scale characterization and quantitative analysis in a study of phase transformations, precipitation behavior and mechanical properties of age-hardenable nanoscale materials systems, using Al alloys as the model materials. New alloys in nanoscale powder form have been synthesized by novel methods, and processing methods to obtain fully dense, bulk materials, without compromising the nanoscale grain size are being explored. Studies are being conducted to establish the science-based understanding and modeling of the evolution of structure and chemistry of the fundamental nanoscale blocks, the interfaces between them, as well as of the structural, physical and mechanical behavior of the consolidated bulk pieces. In particular, we address basic science issues related to the precipitation behavior when the supersaturated parent phase grain size approaches the same nanoscale (1-100nm) dimensions as critical nuclei and transition phases like GP zones, aspects that no one has studied, in either individual nanoscale powders or in bulk pieces fabricated from these entities. The issue of whether spinodal decomposition, leading to GP zone formation, can occur at these “nano” length scales, along with effects of variation in the grain or particle size on this process, is being studied. Binary age-hardenable Al-Cu and Al-Zn alloys, as well as commercial alloys based on these (2024, 7075) are being studied. Processing of Al nanoparticles to bulk structures has also been studied.

B. Nanoscale Structures for High Temperature Resistance (Cummins Inc. supported; Student: Jixiong Han).

One part of this project area that is currently funded by Cummins will combine atomic-scale characterization and detailed quantitative analysis in a study of the microstructure of new aluminum alloys for high–temperature diesel engine applications. Candidate Al alloys reinforced with a high volume fraction of nanoscale aluminum oxide particles are being investigated in detail. An arsenal of diffraction and analytical and high-resolution electron microscopy techniques are being utilized in the characterization work. A continuation project about to be funded will involved studies of the creep behavior of these novel composites. The results of these studies are expected to lead to valuable new insight into the microstructures of these new alloys and into the factors and mechanisms that give rise to their enhanced high temperature mechanical properties.

C.Fundamental Studies of Mixed Metal Oxide Catalysts for Propane Ammoxidation (supported by DoE; Students: Bala Swaminathan, C. Neelakandan).

This interdisciplinary project, which is supported by DoE jointly with Prof. Vadim Guliants, will combine novel methods of synthesis with detailed atomic scale characterization and theoretical modeling in a study of model nanoscale mixed metal oxide catalytic technology for one-step transformation of propane into oxygenates, i.e. acrolein and acrylic acid. Through these methods we will gain fundamental insights into the behavior of these materials and associated phenomena, and advance the science of catalysis to replace the current fluid catalytic cracking of hydrocarbons to propylene intermediate, which is a major source of environmental pollution. These advances will, in turn, lead to a quantum leap in the design and development of new environmentally benign petrochemical technologies with very significant economic advantages over the existing technology. This research program will be conducted by the two co-PIs with unique and complementary expertise in heterogeneous catalysis, molecular modeling and microstructural characterization, and also involves international (Netherlands, Japan, Spain), US national laboratory (ANL) and industrial (Rohm & Haas, BP) collaborations. The specific objectives of this proposed research are: (1) Develop synthesis methods for model V-Mo-Te-(Nb)-O oxides; (2) Establish the bulk and surface molecular structure-reactivity/selectivity relationships for propane oxidation to oxygenates over model V-Mo-Te-(Nb)-O oxides; and (3) Develop molecular models for propane oxidation to oxygenates over model V-Mo-Te-(Nb)-O oxides.

D.  Phase Equilibria and Transformations in Magnetic Nanoparticle Systems (DoE-BES, in preparation).

            This project is based on work that we have been performing in collaboration with scientists (Jeff Eastman, Jim Vetrone) in the Materials Science Division at ANL on the synthesis, structure and properties of magnetic nanoparticles for biomedical applications. In our previous work, iron and iron oxide nanoparticles were synthesized by chemical vapor decomposition of n-butylferrocene precursor gas in a hot-walled deposition system. The effects of variations in reactor chamber pressure, temperature, precursor flow-rate, and oxygen:nitrogen supply gas ratio on the structure, composition, size and size distribution of the particles were studied and the particles were characterized by x-ray and electron diffraction, nanoprobe energy-dispersive x-ray spectroscopy, and HRTEM.

The reduction in length scale of materials to the nanometric range brings about fundamental changes that lead to remarkable new phenomena, very different from their coarse-grained counterparts. Examples relevant to solid-solid phase transformations within small grains or particles include the size-dependent extension of solid solubility and alteration of phase boundaries and phase equilibria; suppression of spinodal decomposition, precipitation, long range ordering and martensitic reactions, to name a few. These size-dependent changes often lead to novel properties, for example, super-paramagnetism, high coercivity, high field irreversibility, and high saturation field or shifted loops after field cooling in magnetic materials. In this context, magnetic FePt and FePd nanoparticles with the tetragonal, long-range ordered (LRO) L1₀ structure are of enormous interest not only for serving as model systems for studies of fundamental magnetic properties, but also because of their possible applications in memory devices with a high packing density, magnetic recording media, hard magnets and magnetic refrigeration. Because of their intrinsically high magneto-crystalline anisotropy (K_u), ordered FePt (K_u~10⁷ J/m³) and FePd (K_u~3x10⁶ J/m³) nanoparticles can overcome thermal fluctuations and be ferromagnetic even when a few nanometers in size. Indeed, a well-defined array of such particles is expected to lead to the next generation of magnetic storage devices with recording densities exceeding 1 Tb/in².

There are two great challenges limiting advances in this area. One relates to the ability to produce particles of these materials with controllable size, chemistry and structure, and their assembly into functional architectures. The second, which is the principal focus of the proposed project, relates to the fact that at very small particle sizes dramatic changes occur in the thermodynamics, phase transformations and phase equilibria in these systems that are poorly understood presently. As-synthesized FePt and FePd nanoparticles have a chemically disordered f.c.c. (A1) structure, whereas the LRO L1₀ structure, which is produced by subsequent thermal annealing, is essential for attaining the best magnetic properties. The ordering process (or lack of) is dramatically affected not only by particle size, but also by composition and temperature. For instance, the disorder-order critical temperature (and correspondingly the degree of LRO) is lowered from ~1300°C in bulk, coarse-grained FePt to less than 600°C for 4 nm diameter particles. Since the latter temperature is close to the Curie temperature, it offers an opportunity to tailor the microstructures by thermo-magnetic annealing. However, limited efforts have been made toward mapping out and obtaining a fundamental understanding of particle size effects on ordering and phase equilibria and incorporation of coupled size and composition effects into thermodynamic descriptions for predicting phase equilibria and transformations.

The proposed research program, which also includes Yunzhi Wang from OSU and Jeff Eastman and Xiao-Min Lin from ANL, will involve: 1) Development of CVC and solution-chemistry based methods for synthesis of magnetic Fe-Pt and Fe-Pd nanoparticles with controlled sizes and compositions across the relevant portions of the phase diagram.; 2) development of thermodynamic models that incorporate size effects on interfacial energy and coherency strain energy and hence phase equilibrium.; 3) development of a basic understanding of ordering, phase transformations and structural changes in the nanoparticles during ex situ and in situ annealing.; and 4) experimental determination and computer simulation of the structure, composition and magnetic properties of the particles as a function of synthesis/processing conditions, particle size and composition, temperature and time.

E Instrumentation for Synthesis of Nanoscale Materials (Ohio Board of Regents – Ohio Center for Advanced Propulsion and Power).

            Synthesis is one of the great challenges relating to the development and application of nanoscale materials. We are currently in the process of acquiring a versatile pulsed electron deposition system that will be capable of synthesizing a wide range of nanoscale materials with uniform composition in either nanoscale powder or thin film forms within one unit, in quantities adequate for fundamental scientific studies on a laboratory scale. The emphasis is on the synthesis of metallic alloys, ceramics, and composites in nanoscale powder form, carbon and boron nitride nanotubes, and novel nanostructured thin films. The development of the proposed system will also involve close collaborations with, and significant technical support from, Neocera, Inc., a world leader in research and development of pulsed laser and electron deposition (PLD) systems.

            The overall objective of research in these projects is to promote “atomic level engineering” and “quantitative understanding” through improved knowledge of the relationship between synthesis, alloy chemistry, microstructural length scales and interfaces on the one hand and properties on the other. It is anticipated that the knowledge gained through these efforts will act as a bridge between theory and experiment by providing experimental verification of predictions made from first-principle or other types of calculations.

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