William L Johnson, Ph.D - US grants

This research project is directed in two major thrust areas (i) processing of metal alloys and (ii) processing of semiconductor materials. A component connecting both of these areas is simulation and theory work aimed at predicting materials properties. The metal processing research employs ion beam mixing, shock consolidation, mechanical alloying, and synthesis of layered and metastable materials. The semiconductor processing research employs epitaxial growth on Si and GaAs, ion implantation, nucleation and growth on new phases such as amorphous carbon. The use of sophisticated electron microscopy apparatus is central to the research projects. Better understanding of the influence of interfaces, surfaces and grain boundaries on materials properties is a primary goal of this reserach.

Harry Atwater Abstract The acquistion of a quantitative high-resolution video imaging system is proposed. The system will be used in conjunction with existing transmission electron microscopes for (i) atomic-scale imaging of thin films and nanoparticles, (ii) energy-filtered imaging of thin films and nanoparticles, (iii) quantitative electron diffraction nmeasurements,a nd (iv) facilitating the teaching of electron microscopy and microanalyis of materials. ***

This research involves an experimental investigation of the thermodynamics of nanophase materials. Mechanical attriting is the technique employed to process various nanophase metallic systems. Associated effects of environmental factors during attriting are determined. Both positive and negative heats of mixing are represented in the materials' systems proposed. Grain boundary segregation tendencies are noted and related to the potential for grain growth during low temperature annealing and consolidation. Several experimental characterization methods are proposed to examine the chemistry on a nanoscale, including calorimetry, electron energy loss spectrometry (EELS), Mossbauer spectrometry, and small angle X-ray scattering (SAXS). %%% Nanostructured materials are a class of new materials whose properties are expected to be markedly changed by the consequences of the fine structure. This research explores factors affecting the stability of such materials.

0076512
Johnson

This an award is for the acquisition of a modern sample preparation equipment to prepare with minimum damage specimens of complex materials for transmission electron microscopy (TEM). The equipment will provide new opportunities and advance the current capability of the TEM facility at Caltech acquired earlier with NSF support. The new sample preparation equipment will allow reliable and controlled specimen preparation, and the reduction of specimen contamination in the electron microscope. It will facilitate investigations of micromechanisms of deformation in bulk metallic glasses, interfacial phenomena in composites and other fundamental properties of a wide range of materials. The equipment will see heavy use for a broad range of materials investigations currently and for materials research at Caltech over the next decade.
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This an award is for the acquisition of a modern sample preparation equipment to prepare with minimum damage specimens of complex materials for transmission electron microscopy (TEM). The equipment will provide new opportunities and advance the current capability of the TEM facility at Caltech acquired earlier with NSF support. The new sample preparation equipment will allow reliable and controlled specimen preparation, and the reduction of specimen contamination in the electron microscope. It will facilitate investigations of micromechanisms of deformation in bulk metallic glasses, interfacial phenomena in composites and other fundamental properties of a wide range of materials. The equipment will see heavy use for a broad range of materials investigations currently and for materials research at Caltech over the next decade.
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[unreadable] DESCRIPTION (provided by applicant): My proposed research seeks to understand the complex relationship between the neural signals that activate the muscles of the legs, and the global forces that the legs produce to accomplish locomotion. A model of this relationship will be invaluable in the development of future functional electrical stimulation (FES) and neuroprosthesis devices intended to augment or replace the impaired portion of a neuromusculoskeletal system. In the human, such models exist but ethical considerations limit the development and testing of FES devices. In the rat, more freedom exists to test applications of the model, but there has been little work done toward developing the model itself. Therefore, I propose to develop a computational model of a rat hindlimb, which will precisely characterize the mathematical relationship between the neural input to the leg as reflected in electromyographic (EMG) recordings of the muscle activation, and its dynamic behavior as output. As the first part of this project, I will measure the morphological parameters that govern force output (i.e. joint torques and forces at the foot/ground interface), namely muscle paths, angles, and moment arms at the joints. These data will be useful not only in this project, but in any investigation of rat hindlimb dynamics, and are not currently found in the literature. The physiological muscle properties required for development of the model are also not well known in the rat. As the second part of model development, I will measure the physiological properties that govern local force generation by individual muscles. Again, these data alone will be a significant contribution to the utility of the rat model in biomechanics research. The third and final part of the project is to combine the morphological and physiological data with a Hill type muscle force generation model. The combined model will ease future FES and neuroprosthesis development in the rat model in several ways. Perhaps most significantly, in the most successful mechanical control schemes the controller uses an estimate of the system state - found using the model of the system - to determine the appropriate input. Thus with the model I will develop, established control theory will be much more easily applied to FES and neuroprosthesis development. This project draws on biomechanics, physiology, and control theory to develop a model of the rat hindlimb. The model is an important step toward creating prostheses that allow spinal cord injury patients to regain use of the affected muscles and limbs. [unreadable] [unreadable] [unreadable]

NON-TECHNICAL DESCRIPTION:

Metallic glasses are high-strength metallic alloys that have a random, non-crystalline structure. Like all glasses, they behave like a frozen liquid with flow characteristics determined by its viscosity, or resistance to flow, which varies with temperature. Understanding how flow varies with temperature is a key component both to unlocking the physics behind the underlying structure of metallic glasses, and knowing how its properties influence, and are influenced by, processing. There are three different behaviors: At low temperature the glass behaves as a solid; at moderate temperature, in which the glass flows like a thick liquid, and at high temperature the glass is fully melted. Viscosity measurement of the low- and high-temperature regimes is readily accessible through existing methods, but to date no methods are available to measure viscosity in the thick liquid regime. The focus of this research is to employ newly developed methods to measure viscosity of the thick liquid by rapidly heating the glass while measuring their elastic response. The data collected will be used to identify the fundamental parameters that control flow, develop and refine appropriate models for describing flow behavior, and improve processing techniques need for creating viable commercial products. This collaborative effort will be supported by faculty and graduate students at California State University, Northridge, and California Institute of Technology. Graduate students will mentor underrepresented summer high school and college interns, expanding their representation in the professional community and exciting them to pursue STEM careers.

TECHNICAL DESCRIPTION:

This proposal describes interdisciplinary and collaborative research addressing a conspicuous data gap in the rheological properties of bulk metallic glass alloys (BMGs). BMGs, with their outstanding hardness, toughness, strength and processability, show promise to combine the strengths of metals with the ease of plastic processing, and the potential for energy efficient, environmentally clean manufacturing. The gap in the rheological data exists in the supercooled liquid (SCL) region, between the glass transition and the crystallization temperatures. This data is pivotal in advancing the theory of glassy liquids, and directly impacts processability. We propose to measure the viscosity, and other thermodynamic properties, of a series of metallic glass compositions in this range using rapid temperature excursions driven by ohmic heating (rapid discharge forming, RDF). High-speed time-dependent deformation measurements of a sample under load will be used to determine viscosity variation with temperature. In separate experiments, the ultrasonic pulse-echo technique, coupled with high-speed infrared pyrometry, will be used to measure the elastic shear wave velocity variation with temperature. Data will be incorporated in existing glass rheology models, and new models will be developed as required. Application of the knowledge gained through these studies will be applied to the proof-of-concept processing of metallic glasses, making net-shape objects through thermoplastic forming, and further understanding of the microstructure/processing/properties relationship in glasses and glassy metals.