determination of atomic arrangement and electronic structure at and near the surface of materials. Solid-state nuclear magnetic resonance allows determination of chemical makeup in complex polymer systems. Electron microscopy can show atomic arrangements and chemical compositions at near-atomic resolution; at lower magnifications, it allows the determination of maps of chemical inhomogeneity on larger scales. A host of spectroscopies enable the chemical characterization of surfaces. High-intensity neutron beams from reactors and photon beams from synchrotron sources have made possible a vast array of techniques for chemical and structural characterization.

This new instrumentation has increased understanding but brings with it major concerns for the field of materials science and engineering. The high cost of characterization has become an issue demanding care in the balance of allocations by funding agencies and has, in many instances, become a limiting factor in the progress of research. Furthermore, the availability of large characterization facilities in only a few geographic locations has led to changes in the way research is carried out for many materials scientists and engineers. Rather than do these experiments in their own laboratories, they may now travel thousands of miles to do their research at a major national facility in the midst of the exciting intellectual ferment present at such facilities.

Concurrent with the development of techniques for characterizing the structure and composition of materials has been the development of analytical and modeling techniques to explain the origins of these observations, for example, quantum calculations to describe electronic structure and crystal structure stability; equilibrium and nonequilibrium thermodynamics to describe multiphase materials; and hydrodynamics and instability analysis to explain the development of microstructures in crystalizing metals and polymers.

Historically, the development of materials has involved many key discoveries made at the macroscopic level (such as continuum behavior or mechanical properties). In recent decades, there has also been increased emphasis on the microscopic or atomic level, both in research and in education. New generations of electrical engineers, ceramists, metallurgists, polymer chemists, and condensed-matter physicists, who have been trained in the basic interactions of atoms and molecules, understand the fundamental concepts underlying and unifying previously disparate classes of materials. Opportunities to increase fundamental understanding in this area continue to occur. Three years ago, for example, the apparently well-defined science of geometric crystallography was jolted by the discovery of icosahedral symmetry in solids. These so-called quasi-crystals possess orientational order without translational periodicity. Study of these quasi-crystals promises to lead to a deeper understanding of the conditions leading to different atomic arrangements in solids. Figure 4.3, a picture of icosahedral Al6Li3Cu, shows the triacontahedral faceting that occurs upon slow quench of the phase. Figure 4.4



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