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Unformatted text preview: 9/3/10 1 MEMS 1054 Materials Science 1 or Structure of Crystals and Diffraction Week 1-2 - Crystalline State 1 Professor Jrg Wiezorek, PhD, Materials Science and Metallurgy, University of Cambridge, UK, 1994. Meets in room G37 BEH, Mon/Wed/Fri, 1-1:50PM For low temperatures ( k B T<<bonding energy) the attractive interatomic forces (repulsive at distances smaller than atomic diameter!) acting between atoms and molecules of simple geometries typically result in solid aggregates with high-density, spatially periodic arrangements that fill space effectively. The resulting material structures have translational symmetry (LRO), the hallmark of the crystalline state . Crystals, crystalline phases and constituents form for all classes of materials, including metals, ceramics, polymers and biological materials (e.g. proteins). Understanding of DESCRIPTORS of the crystalline state is a fundamental component of the core knowledge for materials scientists and engineers. Crystallography, the field of science addressing the theory of spatially periodic, perfectly long-range ordered patterns, provides the most basic and quantitative DESCRIPTORS of the structure of crystalline materials. The Crystalline State 9/3/10 2 is important because Crystallography provides the common language for the description of structure of crystalline matters and facility with crystallography is a primary skill for communication in MSE and the broader field of materials research. Crystral Structure provides important constraints on the physical properties of crystals . Thus, cubic crystals (those exhibiting cubic symmetry) will generally have anisotropic elastic moduli, but will always have isotropic electrical conductivity and isotropic optical properties. The relationship between crystal structure and diffraction physics enables structure determination by various experimental techniques (XRD, electron and neutron diffraction and by certain microscopy imaging methods using photons, electrons and scanning contact probes). Rigorous description of the perfect structure or state of the crystalline materials is the reference state used to describe quantitatively imperfections in these materials , which often are critically responsible for the materials properties . Crystallography is a rigorous science in it s own right (see IUCR as search term on Google), providing methods relevant for theory and experiments in other fields of condensed-matter science , e.g. solid-state physics. CRYSTALLOGRAPHY Statements: Every crystal is a periodic assembly of atoms or molecules arranged to Fll space and are described by the underlying crystal structure ....
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- Fall '10