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CHEE 380 - Materials Science > 5: Order in Solids > Flashcards

5: Order in Solids Flashcards

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1
Q

in solids, atoms or molecules tend to have what kind of order? what is the structure called ?

A

tend to order in a closed packed structure that minimizes energy. this is called a crystal.

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2
Q

what are non-crystalline solids called

A

amorphous

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3
Q

describe amorphous solids

A

non dense, random packing

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4
Q

describe crystalline solids

A

dense, regular packing

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5
Q

amorphous or crystalline solids have lower energy ? why?

A

crystalline.
dense, regular packed structures have lower energy.

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6
Q

quartz is crystalline or amorphous ?

A

crystaline

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7
Q

glass is crystalline or amorphous

A

amorphous

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8
Q

amorphous solids generally have what type of bonds ? what type of material is common

A

directional bonds
ceramics are common

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9
Q

example of compounds that can be both crystalline and amorphous. why ?

A

sio2
crystalline quartz or amorphous glass.
solidification conditions (purity, pressure, cooling rate) decide

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10
Q

glass describes what

A

amorphous solids

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11
Q

single crystals can imply ___ range order

A

very long

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12
Q

poly crystals are

A

several crystals packed together

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13
Q

in solids what decides packing structure

A

the way that atoms can be arranged to minimize empty space
the most dense is generally the lowest energy and thus preferred

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14
Q

how do metallic crystal structures tend to be packed ? why ?

A

densely packed
reasons:
- typically only one element, so atoms have equal radius
- metallic bonding is not directional
- nearest neighbour distances tend to be small in order to lower bond energy
- electron cloud shields cores from each other

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15
Q

what type of material has the simplest crystal structure

A

metals

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16
Q

describe unit cell

A

smallest repetitive volume which contains the complete pattern of a crystal

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17
Q

metallic cube unit cell is characterized but that parameters

A

a: cube edge length (lattice constant)
R: atomic radius

atomic packing factor (APF)
APF = vol atoms in unit cell/total unit cell volume

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18
Q

coordination number

A

number of first touching neighbours in a hard sphere model (for pure metals all same radius)

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19
Q

name common metallic crystal structures

A
  • simple cubic
  • body entered cubic
  • face centred cubic
  • hexagonal close packed
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20
Q

review unit cell from notes for all structures

A
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21
Q

FCC vs HCP

A

depends on the position of the third layer of atoms.
ABCABC = fcc
(stacking planes are oriented in plane (111))

ABABAB = hcp

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22
Q

voids in simple cubic unit cell

A

two interpenetrating simple cubic

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23
Q

FCC voids

A

two types of voids: 4 octahedral (CN = 6), 8 tetrahedral (CN = 4)
(in a unit cell)

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24
Q

describe rock salt structure

A

FCC structure with all octahedral voids occupied by the other atom. the structure is symmetrical and can be described as two interpenetrating FCC structures.

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25
compounds w rock salt structure
LiF, LiCl, NaF, NaBr, KCl, MgO
26
Zincblende structure
FCC structure with half of the tetrahedral voids occupied by the other atom. named after ZnS, also this is diamond structure w all atoms being C
27
theatrical density
28
polymorphism
some metals and non-metals can have more than one crystal structure note, at any given temperature and pressure there is one thermodynamically favoured structure.
29
allotropy
some metals and non-metals can have more than one crystal structure, this is called polymorphism. in elemental solids, the word allotropy is used.
30
example of polymorphic materials
carbon: diamond, graphite, fullerene family tin heating and cooling iron wire. structure changes with temp
31
crystal systems, # crystal lattices. theyre related to what
7 crystal systems, related to overall geometry - a, b, c = lattice constants 14 crystal lattices, related to position of the atoms within the geometry - bravais lattices
32
study crystallographic points, miller indices
33
bragg's law slides
34
anistropic in single crystals
properties vary with direction
35
anisotropy polycrystals
grains are oriented
36
polycrystal: grains are randomly oriented
isotropic
37
amorphous solids
solids without a long range order or crystallinity ex, glasses, polymers
38
amorphous solids occur why
when fast solidification does not allow time to organize crystal structure
39
appearance amorphous solids
solid with liquid like appearance
40
important parameter for amorphous materials
glass transition temperature (Tg)
41
crystalline solids before/after melting vs at melting
before/after: - atomic vibration increases with temp - volume expansion (+ void spaces above Tm) at melting: - crystal formation - high APF (less 1st neighbours) - sudden volume increase theres a distinct transition
42
transitions with temperature in amorphous solids
- no sharp density transition from liquid to glass. at Tm it becomes a supercooled liquid, at TG it becomes glass
43
in amorphous solids, below Tg is there rearrangement of atoms ?
below Tg there is no further arrangement of atoms. (volume change results from reduced thermal vibrations) thermal exp coef is then similar to the one for the crystal
44
if T
material is hard and brittle
45
if T>Tg (in polymer and ceramic)
material is flexible, rubbery
46
view structural disorder scales
47
what do vacancies promote
diffusion
48
vacancy
vacant atomic sites in a structure causes distortion of planes
49
self-interstitials
extra atoms positioned between atomic sites (less common than vacancies)
50
types of point defects
vacancies self-interstitials
51
point defects induce _____
local strain energy
52
eval number vacancies in notes
53
effects of vacancies in a material - thermal expansion
effect can be observed by looking at the specific volume changing w temp. changes are mainly due to thermal expansion (asymmetry in the bond potential) but there is also a small effect resulting from the increased number of vacancies with increasing temperature ie at higher temp, the length of the material increases faster. the increasing number of vacancies with T affects the total length, but not the size a of the unit cell
54
effects of vacancies in a material - diffusion
vacancies promote diffusion, so they follow same trend in T. smaller atoms diffuse quicker
55
dislocations
linear defect around which some atoms are misaligned
56
Burgers vector, b
measure of lattice distortion
57
edge dislocation
b is perpendicular to dislocation line extra half-plane of atoms inserted in a crystal structure
58
screw dislocation
b is parallel to dislocation line spiral planar ramp resulting from shear deformation
59
linear defects
one-demential defects around which atoms are misaligned dislocation leads to plastic deformation
60
grain boundaries + in notes
61
for simple cubic, what is: CN, #atoms/unti cell, a wrt R, APF
CN = 6 1 atoms/unit cell a = 2R APF = 0.52
62
for body entered cubic, what is: CN, #atoms/unti cell, a wrt R, APF
CN = 8 2 atoms/unit cell a = 4R/sqrt(3) APF = 0.68
63
for face centred cubic, what is: CN, #atoms/unti cell, a wrt R, APF
CN = 12 4 atoms/unit cell a = 2Rsqrt(2) APF = 0.74
64
for hexagonal close packed, what is: CN, #atoms/unti cell, APF
CN = 12 6 atoms/hcp cell APF = 0.74

CHEE 380 - Materials Science (6 decks)

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