Structure of Crystalline and Glassy Materials Flashcards
What is a metal?
Metals and alloys composed of one or more metallic elements and often small amounts of non-metallic elements.
Atoms are arrayed in an order manner and metals are relatively dense compared to polymers and ceramics.
Relatively stiff and strong but are ductile and resistant to fracture.
Large number of delocalised electrons.
What is a polymer?
Organic compounds chemically based on C, H and other non-metallic elements. Very large molecular structure. Low densities. Generally very chemically inert. Often soften or decompose at low temps.
What is a ceramic?
An inorganic non-metallic solid.
What is a semiconductor?
Have an electrical conductivity higher than insulators but lower than conductors.
Electrical conductivity increases rapidly with temperature.
Electrical conductivity can increase with dopants.
What are glasses?
Non-crystalline amorphous solids.
What are the primary chemical bonds in solids?
Ionic, covalent and metallic.
Give an example of a secondary type of bonding in solids.
Van der Waals
What are the several types of attraction bond forces between atoms?
Non-directional: metallic (strong), VdW (weak).
Covalent
Ionic (non-directional)
Repulsion forces between atoms are due to what?
Atoms being pushed together causing the energy of electrons to rise.
Electrostatic repulsion of overlapping charges.
Pauli Principle – no more than two electrons per state, so some are pushed to higher states when atoms become close together.
Materials with large bond energies have what kind of melting temperatures?
High Tm.
What is the key characteristic of bonds in ceramics and semiconductors?
Bonding is directional.
What is a directional bond?
A bond between specific atoms that may only exist in the direction of the two atoms.
The actual covalent bond is the sum of possible atomic energy states available so orbital combinations are…
hybrids.
p and d orbitals represent a directed what?
Distribution of electron density which is important for directionality of bonding.
How do covalent bonds form?
When two atoms are sufficiently close their electron orbitals/wave functions change. The bonding state has a lower energy than the energies of the separated atoms.
Hybridisation is what?
The promotion of electrons in an atom to allow hybrid bonds to form allowing the bonded product to take occupy a less energetic state.
Why are ionic bonds non-directional?
The magnitude of the bond is equal in all directions around ion.
In many ceramics and semiconductors, is bonding ionic or covalent?
It is an intermediate where it is partially ionic and partially covalent, the amount of each depends on the electronegativity of the elements.
Pauling’s empirical formula for bond character (electronegativity difference):
p = 16|Xa-Xb|+3.5|Xa-Xb|^2
An electronegativity difference of what corresponds to a 50% ionic character?
About 2.1
Why aren’t ionic substances malleable?
A displacement of an atomic unit will result in stronger reclusive forces between anions and cations.
Where are VdW present?
In solid inert gases or as intermolecular bonds between atoms or groups of atoms which themselves are joined by primary (intramolecular) bonds.
K/σ where K is thermal conductivity and σ is electrical conductivity is similar for what type of materials?
Metals.
Thermal conductivity by the vibration of atoms equation:
K = Cvl/3
Where
C is heat capacity per unit volume
v is phonon velocity
l is mean free path between collisions