Lecture 2 Flashcards
Amorphous
(non-crystalline) materials: atoms bonded to each other but there is no repeating pattern (short range order) e.g. water, glasses
Crystalline
atoms bonded together in a regular 3-D pattern known as lattice with a long range order
Doffference between amorphouse and crystalline
Armorphouse is not dense packed and short ranged random packing
Crystalline is densely packed with both short and long range and packing is ordered
Packing
Difference between crystalline and amorphous
First is packed in a 3d pattern while the other has no periodic packing
Many properties of material are determined by
The materials crystal structure
Most metals are
Crystalline and they can be
FCC bcc and hcp
The crystal structure atom arrange,ent depends on what
Pressure and temperature
What’s unit cell
Smallest repetitive volu,e which contains the complete lattice pattern of a crystal
Smallest u it of the element or compound that will exhibit the same physical and chemical behavior
Why are metals. Crystal structures densely packed
Because non directional therefore minimum , restriction on number of neighboring atoms and same atom radii and nearest bond distance small to lower bond energy
Densisities of materials and expand
Metals >ceramics>polymers
Metals have…
• close-packing
(metallic bonding)
• often large atomic masses
Ceramics have…
• less dense packing
• often lighter elements
Polymers have…
• low packing density
(often amorphous)
• lighter elements (C,H,O)
Composites have…
• intermediate values
Polymorphism
The ability of an element tone isn’t in multiple crystal forms and when it’s an elemental solid that’s called allotropy
Close packed crystal structured
FCC and hcp
Stacking
FCC abc
Hcp abab
Types of crystalline structures
Single crystal vs poly (most materials)
Single crystal
When periodic and repeated pattern of the atom arrange,ent perfectly extends through out in all directions
Difficult to grow
Unit cells aligned all in the same direction
Polycrystalline
Many different crystals with different orientations joined creating a distortion of pattern
Most materials are that way
Grain boundaries
Boundaries where different unit cells meet
ANISTROPY
when properties depend on the direction of the crystal. anistropic depends on the direction pf crystal. isotropic doesnt depend on the direction of crystal
single crystal iso or aniso
aniso
polycrystalline iso or aniso
depends. if material prefers a specific orientation of grains then aniso if doesnt then iso
what is the process of xray diffraction
to find out type of crystal structure.
beam is directed on the sample at different angles.
diffrscted beam coming off the sample is collected by a detector
go bsck and review indices detsails of fcc bcc hcp and denisty etc
