Chapter 3 Structure of Crystalline Solids Flashcards
energy and packing
why do atoms assemble in ordered/ crystal structures
crystalline materials
atoms pack in periodic arrays
- metals many ceramics, some polymers
single crystal
atoms are in a repeating/ periodic array over the entire material
polycrystalline
comprised of many single-crystal grains
noncrystalline (amorphous) materials
atoms have no periodic packing
- occurs for complex structures, rapid cooling
unit cell
is a structural unit/ building block that repeated in 2D or 3D, generates the crystal structure
7 types of crystal systems
In total there are seven crystal systems:
triclinic
monoclinic
orthorhombic
tetragonal
trigonal
hexagonal
cubic
A crystal family is determined by lattices and point groups.
* quick reference *
crystal systems bravais lattices
crystal structure lattice
A crystal lattice is the arrangement of these atoms, or groups of atoms, in a crystal.
*quick reference *
crystal structure lattice
metallic bonding
nondirectional, ion cores glued together by an electron cloud, hence can be arranged in dense packing schemes
three types of unit cells
face-centered cubic (FCC)
body-centered cubic(BCC)
hexagonal close-packed (HCP)
* quick reference *
BCC
*quick reference *
BCC part 2
atomic packing factor
atomic packing factor, packing efficiency, or packing fraction is the fraction of volume in a crystal structure that is occupied by constituent particles
atomic packing factor BCC
FCC lattice characteristics
closed packed structure
refers to the most tightly packed or space-efficient composition of crystal structures (lattices)
FCC stacking sequence
hexagonal close-packed structure
stacking sequence
theoretical density
is defined as the maximum density of a material or element that could be achieved assuming there are no voids or contaminants in the overall material
theoretical density example
densities of material classes
metals
ceramics
polymers
composites
metals - have close packing (metallic bonding) (often large atomic masses)
ceramics - have less dense packing (often lighter elements )
polymers - have a low packing density ( often amorphous, lighter elements )
composites - have intermediate values
single crystals vs polycrystals
single crystals - properties vary with direction (anisotropic)
polycrystals - properties may/may not vary with direction
isotropic - if grains are randomly oriented
anisotropic - if grains are textured
anisotropic vs isotropic
Isotropic materials show the same properties in all directions.
Anisotropic materials show different properties in different directions.
examples of isotropic materials
Glass, crystals with cubic symmetry, diamonds, metals
examples of anisotropic materials
Wood, composite materials, all crystals (except cubic crystal)
* quick reference *
polycrystals (iso vs aniso )
polymorphism vs allotropy
Polymorphism is defined as the ability of a solid material to exist in more than one form or crystal structure, whereas allotropy is defined as the property of some chemical elements to exist in two or more different forms
point coordinates
crystallographic directions
Refers to directions in the various crystal systems that correspond with the growth of the mineral and often with the direction of one of the faces of the original crystal itself
linear density of an atom
Miller indices
group of three numbers that indicates the orientation of a plane or set of parallel planes of atoms in a crystal.
miller indices example
crystallographic planes algorithm
1) read of intercepts of the plane with axes in terms a, b, c
2) take reciprocals of intercepts
3) reduce to the smallest integer values
4) enclose in parentheses no commas (hkl)
crystallographic plane example
planar density
is a measure of packing density in crystals.
planar density example
diffraction
is a powerful nondestructive technique for characterizing crystalline materials
X-ray diffraction
x-ray diffraction pattern
Summary of this chapter
