（精）材料科学基础(清华大学)-3.ppt

材料科学基础Fundamental of Materials §2.1 Space Lattice Ⅰ.Crystals versus non-crystals 1. Classification of functional materials Crystalline: The materials atoms are arranged in a periodic fashion. Amorphous: The material’s atoms do not have a long-range order (0.1～1nm). Ⅱ.Space lattice 1. Definition: Space lattice consists of an array of regularly arranged geometrical points, called lattice points. The (periodic) arrangement of these points describes the regularity of the arrangement of atoms in crystals. A lattice may be one , two, or three dimensional two dimensions Three dimensions Ⅲ.Unit cell and lattice constants Unit cell is the smallest unit of the lattice. The whole lattice can be obtained by infinitive repetition of the unit cell along it’s three edges. The space lattice is characterized by the size and shape of the unit cell. How to distinguish the size and shape of the deferent unit cell ? The six variables , which are described by lattice constants —— a , b , c ; α, β, γ §2.2 Crystal System Lattice Types If a rotation around an axis passing through the crystal by an angle of 360o/n can bring the crystal into coincidence with itself, the crystal is said to have a n-fold rotation symmetry. And axis is said to be n-fold rotation axis. We identify 14 types of unit cells, or Bravais lattices, grouped in seven crystal systems. Ⅰ.Seven crystal systems Seven Crystal Systems Ⅱ.14 types of Bravais lattices 1. Derivation of Bravais lattices Bravais lattices can be derived by adding points to the center of the body and/or external faces and deleting those lattices which are identical. 2. 14 types of Bravais lattice Tricl: simple (P) Monocl: simple (P). base-centered (C) Orthor: simple (P). body-centered (I). base-centered (C). face-centered (F) Tetr: simple (P). body-centered (I) Cubic: simple (P). body-centered (I). fa