吉林大学《材料科学与工程基础》chapter 3.pptVIP

Graphite is more stable than diamond at ambient temperature and pressure. ? The fourth bonding electron participates in a weak van der Waals type of bond between the layers. Diamond 金刚石 (Sp3) Graphite 石墨 (Sp2+范力) Fullerenes 富勒烯 C60 (Sp2) Carbon-Onion 洋葱碳 (Sp2 +范力) Bucky- Diamond B-金刚石 (Sp3+Sp2) Carbon- Nanotube 碳纳米管 (Sp2) Carbon family at the nanoscale纳米 The ability of a solid material to exist in more than one form or crystal structure. The possibility of existence of two or more different crystal structures for a substance (generally an elemental solid) 3.10 Polymorphism and allotropy Polymorphism (多形性, 多态现象) Allotropy (同素异构) Familiar examples: carbon graphite is the stable polymorph at ambient conditions, whereas diamond is formed at extremely high pressures. pure iron (纯铁) BCC FCC crystal structure 912?C Polymorphic transformation晶型转变: Generate modification of the density and other physical properties 3.11 Crystal systems (晶系) Lattice(点阵) Lattice points(结点) surrounding of each point are identical (每个结点是等同的) Basis or motif(基元) A group of atoms associated with a lattice point Crystal structure = Lattice + Basis For examples: Lattice: FCC basis: NaCl In this section, we deal with the type of lattice only without regard to the atomic positions in the cell. Many different possible crystal structures are divided into groups according to unit cell configurations. ? A scheme by which crystal structures are classified according to unit cell geometry. ? This geometry (parallelepiped平行六面体 ) is specified in terms of the different possible combinations of lattice parameters参数: a, b, c, and ?, ?, ?. ? There are seven different crystal systems. Crystal system (晶系) Fig 3.19 A unit cell with x, y, and z coordinate axes, showing axial lengths (a, b, and c) and interaxial angles (?, ?, and ?). x, y, and z coordinate system is established with its origin at one of the unit cell corner. x, y, and z axes c