MSE Research Areas
Biomaterials underlie the tissue engineering and drug delivery research in MSE. Novel polymer and inorganic materials are being developed to replace bone and blood vessels. Surface engineering is accomplished using peptides designed to improve cell attachment and ultimately function. Drug delivery research from biodegradable polymers involves processing, controlling degradation rate via microstructure, and release rate of drug. Novel ceramic-peptide composites are also used to control drug release.
Carbon nanotubes are tiny cylinders with nanoscale diameters (1-50 nm) and microscopic lengths (0.5 - 20 (m). They are grown catalytically from hot carbon vapor or by thermal decomposition of a carbon-containing gas or liquid. Different methods yield tubes with one or several nested cylinders and different degrees of perfection. Chemical reactions inside or on the tube surface can be exploited for energy storage and drug delivery. The spectacular mechanical, electronic and thermal properties suggest applications in molecular electronics, high-strength composites, heat pipes etc. Many inorganic semiconductors and oxides can be synthesized as quasi-one dimensional wires. Electrical and optical properties are modified with respect to their 3D bulk equivalents, similar to nanotubes. Breakthroughs in synthesis will lead to nanoscale arrays of high performance electro-optic devices, FET's and sensors.
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Ceramics are covalently and ionically bonded materials that have relatively high melting temperatures. They have unique properties useful for advanced applications, from high fidelity dielectric oxides for wireless communication, high temperature superconducting layered compounds for loss-free power lines, high efficiency cells for direct fuel-electricity conversion, heat-resistant silicon nitrides for high speed machining, ferroelectric perovskites for ultrasonic imaging, to magnetoresistive oxides for novel spintronic gate devices. Active research to develop and understand all of the above materials is being conducted in the department. Processing and atomistic design are an integral part of this effort.
The "Chemistry of Materials" is concerned with understanding and controlling functional condensed matter from a chemical perspective. Opportunities for the Materials Chemist range from using substances to build devices and novel structures to understanding the chemistry of solid state materials. Our program includes research on the chemistry of polymer-based systems, manipulation of the chemistry of carbon-based nano-materials, the synthesis and solid state chemistry of high performance ceramics, and the use of molecular precursors to prepare bio-composites. Our course work is designed to provide students from a chemistry background with the tools necessary to apply their expertise to materials systems, and equip those from other fields with a solid foundation in the chemical aspects of materials.
Modern electron microscopes rely upon the high charge/mass ratio of the electron to probe the local properties of materials with extraordinary resolution. In the department, we have a long tradition of investing in, and applying, the very latest in electron microscope technology to the study of materials. Today, the Electron Microscopy Facility within the Penn Regional Nanotechnology Facility has five scanning and transmission electron microscopes that can be used to study the structure, microstructure, chemistry and electronic bonding of materials. Studies of materials at levels of detail down to single atoms and molecules are now routinely accomplished by undergraduate and graduate students, and post-doctoral fellows using these instruments. Within the facility, regular training is provided to enable students to integrate electron microscopy studies in their research programs on an as-needed basis
The electronic and optical properties exhibit a large variation (10 to 20 orders of magnitude) from metals to ceramics. They arise from the electronic structure and interatomic bonding of materials. This wide property variation allows electrical and optical probes of materials, such as AC and DC electrical transport measurements, scanning tunneling microscopy, and optical spectroscopy such as transmission and reflection measurements, to be used as scientific probes of materials phenomena and structure/property relations. Metals such as gold or copper are characterized by free electron bonding, and therefore are very good conductors and are optically opaque and shiny/reflective. Semiconductors such as silicon are opaque and dull optically, covalently bonded, exhibit a band gap, and have intermediate conductivity that can be manipulated by atomic dopants. Insulating materials such as ceramics, optical materials, and polymers have larger band gap energies, can be ionically or covalently bonded, and have very low conductivity. They are typically transparent and/or colored materials. The design of materials for technologically important electronic and optical applications is a major area of scientific and industrial research.
The inexorable growth in the global demand for energy is raising fundamental problems in resource limitations and environmental pollution. Electricity -generation potentially provides a sustainable means for meeting the world's growing energy needs. New materials are essential in the development of new energy generation and storage technologies that use fossil fuels more efficiently with minimum emissions. Two major examples are fuel cells (with polymer or solid-oxide membranes) for direct electrochemical conversion of fossil fuels to electricity, and advanced batteries that can efficiently store the electricity. Fuel cells using fossil fuels can double the efficiency, with emissions of only H2O and ~ 50% less amounts of CO2. Materials advances are essential to increase the cell efficiency and lifetimes, as well as reducing cell -fabrication costs. Advanced batteries (e.g. lithium) with new materials are necessary to increase the storage capacity and lifetimes. Porous carbons with tuneable pore size can be obtained by "burning" metal carbides in chlorine. The resulting materials can be optimized for hydrogen storage, high power density Li ion battery anodes, and supercapacitors.
Atomic level modeling of the structure and properties of materials has become widespread owing to immense advances in fundamental physical understanding of bonding and the enormous increase of computing power. Currently, modeling and related theoretical studies in this Department encompass: (i) Structure of interfaces, surfaces, dislocations and other crystal defects controlling physical and mechanical properties of transition metals, intermetallic compounds and semiconductors; (ii) Structure and properties of metallic glasses; (iii) Structure and behavior of carbon nanotubes and related structures. The atomic level modeling is carried out using a variety of methods, ranging from calculations based on the density functional theory, through tight-binding based approaches to empirical potentials. The atomic level calculations are closely linked with high-resolution studies of local structure and composition by x-ray, neutron and electron diffraction, electron microscopy, atom-probe microscopy and scanning tunneling microscopy. In parallel, modeling on the continuum level links the atomistic studies with mechanical behavior on macroscopic scale as well as with phenomena such as brittle-to-ductile transition or strain relaxation in thin films that involve statistical cooperative behavior of crystal defects.
There is active research in a number of areas, including diffusion-controlled intergranular brittle fracture caused by surface-adsorbed embrittling elements, the relationship between atomic-level and continuum properties of interfaces, the deformation behavior of intermetallic compounds and other ordered alloys, including Laves phases and Ti-Al alloys, and a theoretical study of the ductile-brittle transition in metallic materials using methods of statistical physics. These programs involve atomistic and continuum modeling and experiments that utilize mechanical testing and high-resolution studies of local structure and composition by electron microscopy, atom-probe microscopy, x-ray diffraction, and microstructural manipulation by processing.
Metals are critically important materials for use in transportation and infrastructure. The goal of the research is to learn how to develop new high strength, high toughness metals based on novel alloy systems. For example, we are currently working on intermetallic compound-based systems, such as TiAl and Molybdenum silicides. These materials are particularly challenging because, while they commonly offer high strength and low densities, properties highly sought after for aerospace applications, they also tend to have limited ductility and toughness. Much of our research focuses on the reasons for this low ductility in these promising materials and involves a combination of theory, experiment and computer modelling. The experimental work is focused on single crystalline materials, while the theoretical and modelling efforts are focused on the structure and properties of dislocations, grain boundaries and other defects. The structure and properties of interfaces are an especially important part of this work because the mechanical properties of some of these materials are largely controlled by closely spaced periodic interfaces.
The basic building blocks in many new material systems are based on structural units with dimensions in the nm range. Tubes and wires with 1 nm widths, particles with 30-100 nm diameters, biological molecules with 3-30 nm dimensions, films with 2-100 nm thicknesses, membranes with 10 nm widths, and solids with grain sizes < 500 nm are now routinely produced. This diversity allows new classes of materials to be explored. Polymer-drug composites with particle sizes on the order of 100 nms are being developed for drug delivery using a variety of scheme for extended delivery. Hybrid structures that contain organic/biomolecular as well as inorganic structural units are being assembled for bioelectronic applications. Carbon nanotubes are the focus of much research due to potential applications in display technology, molecular electronics, sensors, and as reinforcements in structural materials. Nanodomains in complex oxide compounds are being controlled to induce new property combinations.
New materials sometimes possess physical properties that stretch the credibility of "standard" theoretical models, for example by exhibiting phenomena which are so far off the charts that new concepts are required to understand their behavior. Examples include a) cuprate and fullerene superconductors for which the extraordinarily high transition temperatures are hard to reconcile with the otherwise highly successful Bardeen-Cooper-Schrieffer (BCS) theory; and b) nanoscale materials in which novel quantum phenomena dominate the electrical, optical and thermal properties. Concerning the well-worked topic of mechanical behavior, we are beginning to learn how to distinguish situations in which continuum approaches suffice to those in which atomic-scale approaches are required. Thus materials scientists are often driven to go beyond the classic MSE triad of synthesis, characterization and modeling to introduce entirely new concepts into the fundamental knowledge base with which we correlate material properties to structure and composition.
Plastics, rubbers, proteins, epoxies, networks, and such belong to the broad class of materials called polymers, because all of these materials have many ("poly") small repeat units ("mers") covalently bonded together. Polymers have unique physical properties due to their considerable size, numerous conformations and chemical variety. Here at Penn, the nanoscale morphologies of self-assembled polymers, including block copolymers and ion-containing polymers, are under investigation along with efforts to correlate these morphologies to polymer chemistry and processing, as well as diffusion and mechanical properties. The surfaces of polymers and polymer blends are being controlled to promote adhesion to non-polymer materials or to direct cell and tissue growth. Electro-active polymers and their composites are being processed to achieve enhanced ferroelectric performance and thermal-mechanical stability. Finally, polymers are being used to assemble carbon nanotubes in composites for mechanical, electrical and thermal applications.
Scanning probe microscopy/spectroscopy allows the localized measurement of surface structure and electronic, magnetic, optical, and mechanical properties. The techniques are used to study atomistic processes at surfaces, transport in electronic materials, self assembled nanostructures, block copolymers, ferroelectric and magnetic domain interactions, crack tip plasticity, cell adhesion on surfaces, and nanotube properties. The physics of probe tip-surface interactions are also investigated, yielding new methods of nanocharacterization. Scanning Impedance Microscopy for the study of losses at interfaces and defects and Multiple Modulation Magnetic Force Microscopy for the detection of electromagnetic fields in current carrying devices are two recent examples. A Scanning Probe Facility within the Penn Regional Nanotechnology Facility houses a variety of scanning probe microscopes.
Semiconductors, such as silicon, germanium or gallium arsenide, are the class of materials that fall between metals, which exhibit no interband gap, and insulators, which have large band gaps. Semiconductors have a small band gap, and exhibit moderate carrier densities such that the number of free electrons present in the material can be easily controlled by extrinsic factors such as the addition of atomic dopants. The ability to flexibly design the electrical behavior of semiconductors gives rise to their importance in electronics and integrated circuits. Silicon technology is the basis of integrated circuits, electro-optical devices, and micro-electro-mechanical machines (MEMS). Semiconductor manufacturing relies on photolithographic patterning, deposition of oxides, nitrides, and metals, and their selective removal by wet or dry (RIE) etching. These semiconductor manufacturing and chip fabrication process steps require constant research for new lithographic materials for photomasks and photoresists, and new silicon wafer level materials for MOS field effect transistors and interconnect wiring of the chip.
The surface and interface properties of polymers, ceramics, superconductors, metals and nanotubes are investigated using state-of-the art experimental and modelling tools. Using powerful tools to "see the surface," scientists understand the orientation of magnetic domains in superconductors and placing individual molecules in precise patterns to control biomaterials. High resolution microscopes are used to image nanoscale ionic aggregates in polymers or subnanometer features in nanotubes. Atomic scale modeling is used to interpret experimental results and direct future experiments. A Surface and Thin Film Analysis Facility is housed within the Penn Regional Nanotechnology Facility.
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Understanding the relationship between the structure and properties is the central purpose of materials science. But, "structure" does not necessarily mean the crystal structure. Often micro- or nano-structure and local atomic structure influence the properties more acutely. At Penn MSE various groups use x-ray and neutron scattering creatively to study such important structural details. Many of such studies are done using synchrotron radiation and pulsed or reactor neutron sources, and students travel to national laboratories to use these advanced facilities. The LRSM and the faculty own several beamline facilities for both elastic and inelastic scattering. MAXS (that is the Multiple Angle X-ray Scattering instrument) is the newest x-ray scattering apparatus in the department.