材料科学与工程基础：第2章 材料的结构1

1、Ch.2 Structures of Materials （材料的结构）Content2.1 Introduction (简介)2.2 Fundamental Concepts (基本概念)2.3 Unit cells (单胞)2.4 Metallic crystal Structures (金属的晶体结构)2.5 Density Computations- Metals (金属的密度计算)2.6 Ceramic Crystal Structures (陶瓷的晶体结构)2.7 Density Computations-Ceramics (陶瓷的密度计算)2.8 Silicate Ceramic

2、s(硅酸盐陶瓷）2.9 Polymer Structures(聚合物结构)Ch.2 Structures of Materials （材料的结构） The most concerned about materials for people are their properties. The properties of materials are related to their composition (成分)、 crystal structures (晶体结构) and the processing (加工).2.1 Introduction (简介)Ch.2 Structures of M

3、aterials （材料的结构）Ch.2 Structures of Materials （材料的结构）Eg：Mg (hexagonal close-packed, 六方密堆结构) -brittle fewer slip planeAu (face-centered cubic，面心立方结构) -ductile more slip planeNoncrystalline (otically transparent 透明) Crystalline (Opaque 不透明, or Translucent 半透明)Ch.2 Structures of Materials （材料的结构）Crystal

4、 Structure The different properties between same classification of materials are caused by their Atomic arrangement in the solid state.the manner in which atoms, ions, or molecules are spatially arranged.the manner in which atoms, ions, or molecules are spatially arranged.p31Crystal Structures(晶体结构)

5、2.2 Fundamental Concepts (基本概念) Crystal (晶体) It is solid. The arrangement of atoms in the crystal is periodic. In crystalline structure, the atoms display both short-range and long-range order. Eg: metals, many ceramics, and even some polymers.晶 体：原子(团)沿三维空间呈周期性长程有序 (long range order)排列的固体物质（金属，大多陶瓷

6、及一些聚合物） 非晶体：原子(团)无周期性长程有序排列的物质 （包括气体，液体和部分固体） 晶体的性质 熔点确定 有自发形成规则多面体外形的能力 稳定性 （晶体能量最低） 各向异性（物理性质不同） 均匀性（周期小，宏观连续）Crystal Structures(晶体结构) Lattice（点阵,晶格） An infinite array of points in space, in which each point has identical surroundings to all others. A three dimensional array of points coinciding w

7、ith atom positions (or atomic sphere center).p32点阵： 晶体结构的微观特征 某种结构单元（基元）在三维空间作周期性规则排列基元：原子、分子、离子或原子团 (组成、位形、取向均同)Crystal Structures(晶体结构)Crystal Structures(晶体结构) Crystal Structure（晶体结构） It can be described by associating each lattice point with a group of atoms. 抽象为 基元 几何点 抽象为基元的三维空间周期排列 空间点阵 晶体结构空间

8、点阵结构基元(原子,分子或集团) 点阵反映晶体结构的平移对称 点阵是抽象的几何图形 点阵中每个阵点的周围环境均相同Crystal Structures(晶体结构)2.3 Unit cells (晶胞) The smallest component of the crystal, which when stacked together with pure translational repetition reproduces the whole crystal. The basic structure unit of the crystal structure and defines the s

9、tructure by virtue of its geometry and the atom positions within. 晶胞：代表晶体内部结构的基本重复单位（平行六面体） 晶胞的基本要素： A大小和形状 B各原子坐标位置Crystal Structures(晶体结构) Lattice Dimensions (晶格参数) a, b and c are the unit cell edge lengths. , , and , are the angles. between b and c between c and a between a and bp49按晶格常数的不同组合可将晶胞

10、分为7种类型，对应7个晶系 (Crystal system)DeterminationX-ray diffraction transmission electron microscope Crystal Structures(晶体结构)2.4 Metallic crystal Structures (金属的晶体结构)Three crystal structures are found in most metals: Face-centered cubic (面心立方); Body-centered cubic (体心立方);Hexagonal-close-packed (六方密堆).p33金属

11、键； 无方向性；原子呈圆球状密堆积Metallic crystal Structures (金属的晶体结构) Face-centered cubic (FCC) crystal structure (面心立方结构) FCC crystal structure has a unit cell of cubic geometry, with atoms located at each of the corners and centers of all the cube face.除8个顶点外，每个面心上有一个阵点. a=b=c, =90oCu, Al, Au, Agp33Metallic crys

12、tal Structures (金属的晶体结构)Metallic crystal Structures (金属的晶体结构) The relationship between cube edge length a and the atomic radius R : Corner and face positions are really equivalent (顶角和面心的位置是完全相等的) For FCC structure, a total of four atoms are assigned to a given unit cell (对FCC结构来说,每个单胞有4个完整的原子)81/ 8

13、（角）+ 61/ 2（面）p33Metallic crystal Structures (金属的晶体结构)The length of a face diagonal 4R,The cell edge length aFor FCC, it is a cube. So, there has a right triangle on the face:a2+a2=(4R)2Unit cell volume of FCC VC is:Solution: aa4RRMetallic crystal Structures (金属的晶体结构) Coordination number (配位数) For ea

14、ch atom, coordination number is the number of nearest-neighbor or touching atoms.The face atom , has four corner nearest-neighbor , four face atoms from behind , and other four equivalent face atoms from the front.Coordination number=4 + 4 + 4 p34Metallic crystal Structures (金属的晶体结构) Atomic packing

15、factor (APF) (原子堆积系数) For FCC structure, APF=0.74Solution:p34Metallic crystal Structures (金属的晶体结构) Body-centered cubic (BCC) crystal structure (体心立方结构) BCC crystal structure has a unit cell of cubic geometry, with atoms located at eight corners and a single atom at cube center.除8个顶点外，体心上还有一个阵点.a=b=c

16、, =90oCr, a-Fe,W,p34Metallic crystal Structures (金属的晶体结构)BCC crystal structure (体心立方结构) Unit cell length a and atomic radius R are related as: .Each BCC unit has 2 atoms: single center atom + one atom from the eight corner.Corner and center atom positions are equivalent.The coordination number is 8.

17、Atomic packing factor is 0.68.p34Metallic crystal Structures (金属的晶体结构) Hexagonal close-packed (HCP) crystal structure (六方密堆结构) HCP: The top and bottom faces of the unit cell consist of six atoms that form regular hexagons and surround a single atom in the center. Another plane provides three additio

18、nal atoms to the unit cell.a=bc,a=g=90o, b=120oCo Cd Mg Zn Tip34Metallic crystal Structures (金属的晶体结构)HCP crystal structure (六方密堆结构) Unit cell length a and atomic radius R are related as:Each HCP unit has 6 atoms: (1/6) 12+(1/2) 2+3Atomic edge length c=1.633a (ideal value). The coordination number is 12.Atomic packing factor is 0.74.结 构 特 征结 构 类 型 体 心 立 方 （bcc） 面 心 立 方 （fcc） 密 排 六 方 （hcp）点 阵 类 型体心立