机械工程材料ch02-Structure of Materials.pdf

Mechanical Engineering Materials (Bilingual Course) Chapter Two Outline 1、Atomic Scale Structure in Materials 2、Crystal Structure in Materials 3、Notification of the crystal plane and crystal orientation in cubic crystal system 4、Crystal Defects Atomic Scale Structure in Materials  The substance is made up of atoms. The combination C 60 and arrangement of atoms determine properties of the substance.  Binding forces among atoms, ions and molecules are called bonds. Their combination status is called structure. Atomic Scale Structure in Materials The atoms in materials are held together by bonds, which dictate the structure of materials. There are four important bonding mechanisms found in solids: ionic, covalent, metallic and van der Waals bonds. The first three bonds are relatively strong and are known as primary bonds.  Ionic Bonding(离子键)-NaCl solid as an example Atomic Scale Structure in Materials An ionic bond is created between two unlike atoms with different electronegativities. When sodium donates its valence electron to chlorine, each becomes an ion; attraction occurs, and the ionic bond is formed. They are excellent insulators and the electrons are so tightly held inside the bond that they can’t participate in the conduction process.  Covalent Bonding(共价键)-Silicon solid as an example The covalent boding is typically observed between atoms with small differences in their electronegativities. In covalent bonding, stable electron configurations are formed by the sharing of valence electrons among two or more atoms. Atomic Scale Structure in Materials