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
Two factors: the type 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
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.
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.
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.
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
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.
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.
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
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.
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)]
●
void fraction (void factor):
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
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
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
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
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
●
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
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.
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.
How the size of atoms will change during ionization?