材料科学基础第2课

1. metallic bond;
2. covalent bond; 3. ionic bond;
4. Van der Waals bond.
FN FA FR
FN : net force FA : attractive force FR : repulsive force
Bonding energies:




H
oxygen
H
Eind＝－
2 1
r6
2

3. Dispersion force (London forces) If the interactions are between two dipoles that are induced in atoms or molecules, we refer to them as London forces.
Lesson two
1.2 Atomic Bonding
1.2.1 Bonding forces and energies
There are four important mechanisms by which atoms are bonded in engineered materials. These are:
• high strength
1.2.3 Secondary bonding （Van der Waals bonding)
The driving force for secondary bonding is the attraction of the electric dipoles contained in atoms or molecules. An electric dipole moment is created when two equal and opposite charges are separated.
chemical absorption
attractive force
physic absorption chemical absorption
1.2.5 Bonding and properties
Some properties can be related to the force-distance and energy-distance.
Interionic forces
Z1Z 2e 2 Fattractive 40 a 2
nb Frepulsive n1 a
Z1Z 2e 2 nb Fnet n1 2 40 a a
Z1, Z2＝number of electrons removed or added from the atoms during the ion formation
Attractive energy Repulsive energy
Feature of Ionic Bonding
• no directivity
• large CN numbers • high degree of hardness
• high strength
• low • high fragility
The characters of covalent bonding are:
• saturation • directivity • low CN numbers
The materials with covalent bonding have:
• high strength • high hardness
材料科学基础
Fundamental of Materials Science
Prof: Tian Min Bo
Tel: 62795426 ，62772851 E-mail: tmb@ Department of Material Science and Engineering Tsinghua University. Beijing 100084

Fluctuating dipole bonds These bonding forces arise because the asymmetrical distribution of electron charge distribution in these atoms creates electric dipoles.
• high melt point
• high brittleness
3. Metallic bonding Metallic bonding occurs in solid metals. In metals in solid state, atoms are packed relatively close together in a systematic pattern or crystal structure.
The characters of metallic bonding are non-directivity, high CN numbers .
• high electric conduction • high thermal conduction • good plasticity
• reflection ( non-transparency )
• insulation
2. Covalent bonding
Materials with covalent bonding are characterized by bonds that are formed by sharing of valence electrons among two or more atoms.



1 2 r 6

E I1 E I 2 3 Edis 2 E I1 E I 2
3 2 EI 4 r6
For the same kind of molecular formula, simplified to :
Edis
1.2.4 Mixed bonding in materials
dipole moment
q magnitude of electric charge d separation distance between the charge centers
q
d
q
1. Electrostatic force (Keesom forces) If the interactions are between molecules that are permanently polarized, we refer to these as Keesom interaction.
e ＝electron charge
a ＝interionic separation distance
ε0＝permittivity of free space＝8.85×10-12C2/(N· m2)
Interionic energies
Z1Z 2e 2 b Enet n 40 a a
E Fdr
For atomic systems:
EN EA ER
1.2.2 Primary interatomic bonds
1. Ionic bonding
Ionic bonding is always found in compounds that are composed of both metallic and nonmetallic elements, elements that are situated at the horizontal extremities of the periodic table.
1. Mixed bonding

Ionic-covalent mixed bonding
% ionic character (1 e
( 1 / 4)( X A X B ) 2
)(100%)
XA and XB are the electronegativities of the atoms A and B in the compound
H
oxygen
H H
Hຫໍສະໝຸດ oxygen
Ees＝－
1 2
r
3
f
f 2 cos1 cos2 sin 1 sin 2 cos(1 2 )
1
: dipole moment
r : distance between two molecules
1
1
2 2
For example:
However, the yield strength is quite different, and it is a microstructure sensitive. That is say, it is depend strongly on the microstructure compared to the E and α.

a) b) Electron charge cloud distribution in a noble-gas atom. a) Idealized symmetrical electron charge cloud distribution b) Real case with unsymmetrical electron charge cloud distribution which changes with time, creating a “fluctuating electric dipole”.
Metallic-covalent mixed bonding Metallic-ionic mixed bonding