北京化工大学材料导论PPT

Module Module 2 2
Nature Nature and and Structure Structure of of Materials Materials
2.2.5 Allotropy/Polymorphism 2.2.6 Volume Changes and Packing Factor 2.2.7 Crystal Imperfections 2.2.11 Crystal Defects 2.2.8 Crystal Impurities 2.2.9 Substitutional Solid Solutions 2.2.10 Interstitial Solid Solutions
Q2.8
p79
have the structures in which the order of atoms is limited to an atom’s nearest neighboring atoms. Examples: water, glass, polymers Amorphous materials have no regular atomic structure. Many materials that are amorphous in structure possess remarkable properties needed for some specific application.
2.2.6 Volume changes and packing factor (PF)
Every change in atomic structure brings changes in properties of the solid, such as volume. Density= mass/volume, unit: kg/m3 or g/cm3. Density stays constant provided a substance is nonallotropic.
Why adding the impurities (the zinc atoms) to the copper atom produces such differences in the properties between the pure metal and the alloy? A: The addition of zinc has a great effect on the hardness, strength, ductility, and conductivity of the pure copper.
Definitions of solution, phase, solvent, solutes, mixture, diffusion
p80, para 2
Definitions Phase and Mixture
Phase (相):
P80, para.2
Solution (溶液)
A homogeneous mixture of chemically distinct substances that forms a phase.
Cooling curve for pure iron. (Allotropic behavior of pure iron)
Polymorphic materials and allotropic materials ● Allotropic materials同素异形材料 After changing to one structure, a material can reverse the phenomenon and return to its previous structure. > ¼ of the elements, such as steel
Nature Nature and and Structure Structure of of Materials Materials
2.1 Nature
2.1.1 Internal structure 2.1.1.1 Atomic structure 2.1.1.2 Molecular Structure and Bonding 2.1.2 Solid State 2.2 Structure of Solid Materials 2.2.1 Unit Cells and Space Lattices 2.2.2 Crystal Systems 2.2.3 Crystallographic planes, miller indices, and crystal directions 2.2.4 Coordination Number (CN)
PF is not dependent on the radius of the spheres being packed if all the atoms are of the same size
The coordination number varies directly with the PF.
How the size of atoms will change during ionization?
•change to cations, atoms decrease in size •change to anions, atoms increase in size
1
p75, Table2-5
Allotropy 同素异形(现象)
the change in structure is reversible, then the polymorphic change is known as allotropy.
1399-1538C 910-1399C
As the difference between r & R decreases, higher CNs are possible. e.g.: r/R=1, CN=12
laboratory conditions
crystal imperfections Impurity atoms Disorder of the crystal structure
The lack of perfection in the microstructure of materials imperfections in the crystalline arrangement of atoms
Two factors: the type of bonding the relative size of the atoms or ions involved (1) Type of bonding Valence electrons determine the type of bonding & number of bonds e.g.: C in group IV: 4 covalent bonds, CN=4 Cl in group VII, 1 covalent bond, CN=1
Minimum radii ratios for CNs
CN 3 4 6 8 (bcc) 12 (cph or fcc) r/R 0.155 0.225 0.414 0.732 1.0
2.2.5 Allotropy/Polymorphism
p75, last para.1
The change in structure from one crystal structure to another is irreversible
The ratio of the volume of atoms present in a crystal (unit cell) to the volume of the unit cell
p78
Void fraction (空隙率) [or void factor (空隙因数)]
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void fraction (void factor):
2
2.2.7 Crystal Imperfections
晶体不完整性
long-range order
long-range-order materials
P79， para.3
Nothing is perfect
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长程有序材料 have the structures in which the orderly arrangement of atoms extends throughout the entire material, forming a regular gridlike lattice or pattern.
Factors influencing the number of nearest atoms
SC is 6
FCC is 12
BCC is 8
Factors influencing the CN
p74, para.2
Factors influencing the CN
Q3.8
p74, para.2
(2) Relative size of the atoms or ions involved The relative size of atoms determines how many neighboring atoms will touch another atom. which is described by the ratio of the size (radii) of the combined atoms or ions, being limiting factor The minimum ratios of atomic (ionic) radii produce various CNs Ionic bonding involves ions of different charges, hence, different sizes
Module Module 2 2
p73, last para.1
p73, last para.1
2.2.4 Coordination Number (CN) (配位数)
The coordination number is the number of neighboring atoms that each atom has immediately surrounding it.
desirable properties
short-range-order materials
短程有序材料
p79
2.2.8 Crystal impurities
Impurities are purposely added to improve a material’s properties and/or reduce its cost.
The difference between the PF and unity (1), i.e.: the fraction of void (unoccupied or empty) space in the unit cell. the void factor for fcc: PF=0.74, 1-0.74=0.26 sc PF 0.52 0.48 Void factor bcc 0.68 0.32 fcc 0.74 0.26 cph 0.74 0.26
A physically distinct material that has its own structure, composition, or both. p87 Mixture (混合物): A material that has no fixed composition and contains more than one phase. The components (substances) keep their individual identities
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p78
Polymorphic materials多形态材料
A material does not possess this reverse phenomenon.
bcc Iron will contract in volume as it is heated above 910ºC fcc
Atomic Packing Factor (APF)原子堆砌密度