Linear and Planar Atomic Densities - PowerPoint
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Linear and Planar Atomic Densities Linear Density: Directional equivalency is related to the atomic linear density in the sense that equivalent directions have identical linear densities. The direction vector is positioned so as to pass through atom centers. The fraction of line length intersected by these atoms is equal to the linear density. Planar Density: Crystallographic planes that are equivalent have the same atomic planar density. The plane of interest is positioned so as to pass through atom centers. Planar density is the fraction of total crystallographic plane area that is occupied by atoms. Linear and planar densities are one- and two-dimensional analogs of the atomic packing factor. Linear Density for BCC Calculate the linear density for the following directions: a.  b.  c.  Planar Density for BCC Calculate the planar density for the following BCC planes: a. (100) b. (110) FCC Calculate the planar density of the (110) plane for FCC. Crystalline and Non-Crystalline Materials Single Crystal: The periodic and repeated arrangements of atoms is perfect or extends throughout the entirety of the specimen without interruption. All unit cells interlock in the same way and have the same orientation. Single crystals exist in nature, but they may also produced artificially. They are ordinarily difficult to grow, because the environment must be carefully controlled. Several Single Crystals of Fluorite Single crystals are needed for modern technologies today. Electronic micro-chips uses single crystals of silicon and other semiconductors. Polycrystalline Materials Composed of a collection of many small crystals or grains. Anisotropy Physical properties of single crystals of some substances depend on the crystallographic direction in which measurements are made. This directionality of properties is termed anisotropy, and it is associated with the variance of atomic or ionic spacing with crystallographic direction. Substances in which measured properties are independent of the direction of measurement are isotropic. Diffraction Diffraction occurs when a wave encounters a series of regularly spaced obstacles that (1) Are capable of scattering the wave, and (2) Have spacings that are comparable to the wavelength. Furthermore, diffraction is a consequence of specific phase relationships that are established between two or more waves that have been scattered by the obstacles. Constructive and Destructive Interference Bragg’s Law 2d hkl Sin n Interplanar Spacing a d hkl . h k l 2 2 2 Diffraction Angle Polycrystalline α-iron BCC Sum of the indices: h + k + l = even Problem 3.56 Polycrystalline Cu FCC Additional condition for diffraction: h, k, and l must either odd or even. Problem 3.57 and 3.58, Page 65.