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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
Q

compounds w rock salt structure

A

LiF, LiCl, NaF, NaBr, KCl, MgO

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26
Q

Zincblende structure

A

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
Q

theatrical density

A
28
Q

polymorphism

A

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
Q

allotropy

A

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
Q

example of polymorphic materials

A

carbon: diamond, graphite, fullerene family
tin
heating and cooling iron wire. structure changes with temp

31
Q

crystal systems, # crystal lattices. theyre related to what

A

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
Q

study crystallographic points, miller indices

A
33
Q

bragg’s law slides

A
34
Q

anistropic in single crystals

A

properties vary with direction

35
Q

anisotropy polycrystals

A

grains are oriented

36
Q

polycrystal: grains are randomly oriented

A

isotropic

37
Q

amorphous solids

A

solids without a long range order or crystallinity
ex, glasses, polymers

38
Q

amorphous solids occur why

A

when fast solidification does not allow time to organize crystal structure

39
Q

appearance amorphous solids

A

solid with liquid like appearance

40
Q

important parameter for amorphous materials

A

glass transition temperature (Tg)

41
Q

crystalline solids before/after melting vs at melting

A

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
Q

transitions with temperature in amorphous solids

A
  • no sharp density transition from liquid to glass. at Tm it becomes a supercooled liquid, at TG it becomes glass
43
Q

in amorphous solids, below Tg is there rearrangement of atoms ?

A

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
Q

if T<Tg (in polymer and ceramic)

A

material is hard and brittle

45
Q

if T>Tg (in polymer and ceramic)

A

material is flexible, rubbery

46
Q

view structural disorder scales

A
47
Q

what do vacancies promote

A

diffusion

48
Q

vacancy

A

vacant atomic sites in a structure
causes distortion of planes

49
Q

self-interstitials

A

extra atoms positioned between atomic sites (less common than vacancies)

50
Q

types of point defects

A

vacancies
self-interstitials

51
Q

point defects induce _____

A

local strain energy

52
Q

eval number vacancies in notes

A
53
Q

effects of vacancies in a material - thermal expansion

A

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
Q

effects of vacancies in a material - diffusion

A

vacancies promote diffusion, so they follow same trend in T.
smaller atoms diffuse quicker

55
Q

dislocations

A

linear defect around which some atoms are misaligned

56
Q

Burgers vector, b

A

measure of lattice distortion

57
Q

edge dislocation

A

b is perpendicular to dislocation line
extra half-plane of atoms inserted in a crystal structure

58
Q

screw dislocation

A

b is parallel to dislocation line
spiral planar ramp resulting from shear deformation

59
Q

linear defects

A

one-demential defects around which atoms are misaligned
dislocation leads to plastic deformation

60
Q

grain boundaries + in notes

A
61
Q

for simple cubic, what is:
CN, #atoms/unti cell, a wrt R, APF

A

CN = 6
1 atoms/unit cell
a = 2R
APF = 0.52

62
Q

for body entered cubic, what is:
CN, #atoms/unti cell, a wrt R, APF

A

CN = 8
2 atoms/unit cell
a = 4R/sqrt(3)
APF = 0.68

63
Q

for face centred cubic, what is:
CN, #atoms/unti cell, a wrt R, APF

A

CN = 12
4 atoms/unit cell
a = 2Rsqrt(2)
APF = 0.74

64
Q

for hexagonal close packed, what is:
CN, #atoms/unti cell, APF

A

CN = 12
6 atoms/hcp cell
APF = 0.74

CHEE 380 - Materials Science (6 decks)

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