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Material science book summary
- Vak
- Material Science
- Instelling
- Rijksuniversiteit Groningen (RuG)
Material science book summary
[Meer zien]Voorbeeld 3 van de 19 pagina's
Voorbeeld 3 van de 19 pagina's
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Voorbeeld van de inhoud
Materials science and engineeringChapter 2 Atomic Structure and Interatomic Bonding
Axis, X, Y & Z
Quantum numbers Table 2.1
Pauli exclusion principle, which stipulates that each electron state can hold no more than
two electrons that must have opposite spins.
Ground state of an atom.
The electron configuration represents the manner in which the energy states are occupied.
Valence electrons occupy the outermost shell.
Most of the elements really come under the metal classification, these are sometimes
termed electropositive elements, indicating they are capable of giving up their few valance
electrons to become positively charged.
The elements on the right side of the periodic table are electronegative, meaning they
readily accept electrons.
Attractive force Fa & repulsive force Fr, the magnitude of each depends on the separation or
interatomic distance r.
The net force Fn = Fa + Fr
When Fa and Fr are equal in magnitude but opposite in sign, there is no net force -> Fa+Fr= 0
and a state of equilibrium exists.
E= ∫F dr
The bonding energy Eo, corresponds to the energy at this minimum point; it represents the
energy required to separate these two atoms to an infinite separation.
The mechanical stiffness of a material is dependent on the shape of its force-versus-
interatomic separation curve. The slope for a relatively stiff material will be quite steep;
slopes are shallower for more flexible materials.
How much a material expands upon heating or contracts upon cooling is related to the
shape of its E-versus-r curve.
Ionic bonding
The attractive bonding forces are coulombic that is, positive and negative ions, by virtue of
their net electrical charge, attract one another. Coulombic attraction between positive and
negative ions. There is a balance of attractive and repulsive terms, its most stable at the
minimum energy.
Ionic materials are hard and brittle, they are electrically and thermally insulative.
En = -(A:r)
Er (repulsive energy) = B(r^n)
Ionic bonding is termed nondirectional that is, the magnitude of the bond is equal in all
directions around an ion.
1
,Covalent bonding is found in materials whose atoms have small differences in
electronegativity. They share electrons. This bond is directional, meaning it may exist only in
the direction between one atom and another that participates in electron sharing. Bonds
determined by valance electrons.
Hybridization – the mixing of two or more atomic orbitals with the result that more orbital
overlap during bonding results.
Metallic bonding, the final primary bonding type, is found in metals and their alloys. The
valance electrons are not bound to any particular atom in the solid and are more or less free
to drift throughout the entire metal. Forming a sea of electrons.
Secondary bonds, or van der Waals bonds, are weak in comparison to the primary or
chemical bonds. It exists between virtually all atoms or molecules, but its presence may be
obscured if any of the primary bonding types is present. Secondary bonding arise from
atomic or molecular dipoles.
Hydrogen bonding is a special type of secondary bonding.
Permanent dipole moments exist in some molecules by virtue of an asymmetrical
arrangement of positively and negatively charged regions, such molecules are termed polar
molecules.
Coulombic forces also exist between adjacent polar molecules.
Adhesives, surfactants, emulsifiers & desiccants
Bonding tetrahedrion – a three-dimensional tetrahedrion with one of these ‘extreme’ types
located at each vertex. Figure 2.25
Coefficient of thermal expansion
2
, Chapter 4 the structure of crystalline solids
Face-centred cubic (FCC) crystal structure. With atoms at each of the corners and the
centres of all the cube faces. Unit cell edge length for FCC -> A=2rsquarerootof2.
N=Ni + Nf:2 + Nc:8
Ni = number of interior atoms Nf = number of face atoms Nc = number of corner atoms
Coordination number and the atomic packing factor (APF) for metals, each atom has the
same number of nearest-neighbour or touching atoms, which is the coordination number.
APF = volume of atoms in a unit cell: total unit cell volume
Body-centred cubic (BCC) metallic crystal structure of a cubic unit cell with atoms located at
all eight corners and a single atom at the cube centre. A = 4R: therootof3
Simple cubic (SC) crystal structure is a unit cell of atoms only at the corners of a cube.
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