Equivalent directions have identical linear densities(LD), and plans having the same planer density (PD)are also equivalent .
LD = No. of atoms centered on direction vector / length of direction vector
Note also that , in general , LD is equal to the reciprocal of the repeat distance(r) ; between adjacent atoms:
PD = No.of atoms centered on a plan / area of plan
Single Crystals and Polycrystalline Materials :
Single crystal: atoms are in a repeating or periodic array over the entire extent of the material
Polycrystalline material: comprised of many small crystals or grains. The grains have different crystallographic orientation. There exist atomic mismatch within the regions where grains meet. These regions are called grain boundaries.
Anisotropy :
Different directions in a crystal have a different packing. For instance, atoms along the edge of FCC unit cell are more separated than along the face diagonal. This causes anisotropy in the properties of crystals, for instance, the deformation depends on the direction in which a stress is applied.
In some polycrystalline materials, grain orientations are random, so bulk material properties are isotropic
Some polycrystalline materials have grains with preferred orientations (texture), so properties are dominated by those relevant to the texture orientation and the material exhibits anisotropic properties
Non-Crystalline (Amorphous) Solids :
In amorphous solids, there is no long-range order. But amorphous does not mean random, in many cases there is some form of short-range order.
Schematic picture of amorphous SiO2 structure
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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