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Materials module 1 > Structure of crystalline solids > Flashcards

Structure of crystalline solids Flashcards

1
Q

What makes a crystal not dense?

A

the random packing

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2
Q

What makes a crystal dense?

A

ordered packing

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3
Q

Dense, ordered packed structures tend to have lower or higher energies?

A

Lower

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4
Q

What are properties of crystalline materials?

A

atoms pack in periodic, 3D arrays

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5
Q

What are properties of non-crystalline materials?

A

atoms have no periodic packing

  • complex structures
  • rapid cooling
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6
Q

What are amorphous materials?

A

non-crystalline

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7
Q

what are examples of crystalline materials?

A

metals
many ceramics
some polymers

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8
Q

What properties of metallic crystal structures?

A

densely packed

have the simplest crystal structures

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9
Q

Why are metallic crystal structures densely packed? (4)

A
  • Typically, only one element is present, so all atomic radii are the same.
  • 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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10
Q

What are the three simplest crystal structures?

A

Simple cubic structure
Body centered cubic structure
Face centered cubic structure

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11
Q

What are simple cubic structures? (2)

A

Rare due to low packing density (only Po has this structure)
Close-packed directions are cube edges

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12
Q

How many coordination (nearest neighbours) does it have? SC

A

6

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13
Q

What is the APF of a simple cubic structure?

A

0.52

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14
Q

How many atom/unit cell are there is a SC?

A

1 atom/unit cell

8 x 1/8 = 1

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15
Q

What are body centered cubic structures?

A

Atoms touch each other along cube diagonals

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16
Q

What are examples of BCC structures?

A

Cr, W, Fe(a), Tantalum, Molybdenum

17
Q

What is the coordination of a BCC?

A

8

18
Q

How many atoms are there per cell in a BCC?

A

1 center + 8 corneres x 1/8 = 2 atoms/unit cell

19
Q

What are the close-packed directions in a BCC?

A

length = 4R = √3 a

20
Q

What is the APF for a BCC?

A

0.68

21
Q

What is a face centred cubic structure?

A

Atoms touch each other along face diagonals

22
Q

What are examples of face centred cubic structure?

A

Al, Cu, Au, Pb, Ni, Pt, Ag, Si

23
Q

What is the coordination of a FCC?

A

12

24
Q

How many atoms are there per cell in a FCC?

A

6 face x 1/2 + 8 corners x 1/8 = 4 atoms/unit cell

25
Q

What is the APF for a face-centered cubic structure?

A

0.74 which is the maximum achievable APF

26
Q

What are the close-packed directions for a FCC?

A

length = 4R = √2a

27
Q

What is the Hexangonal close-packed structure?

A

6 then 3 then 6 stacking structure

28
Q

What is the coordination for the HCP?

A

12

29
Q

What is the APF for the HCP?

A

0.74

30
Q

How many atoms/unit cell are there in a HCP?

A

6

31
Q

What are examples of HCP?

A

Cd, Mg, Ti, Zn

32
Q

What is c/a in HCP

A

1.633

33
Q

Level these from highest to lowest density: metals, ceramics and polymers

A

ƿmetals > ƿceramics > ƿpolymers

34
Q

Why do metals have higher densities than ceramics and polymers? (2)

A
  • close-packing
    (metallic bonding)
  • often large atomic masses
35
Q

Why do ceramics have lower density than metals but a higher density than polymers? (2)

A
  • less dense packing

* often lighter elements

36
Q

Why do polymers have the lowest density?

A
  • low packing density (often amorphous)

* lighter elements (C,H,O)

37
Q

What are polycrystals?

A

Crystals that have single crystal grains that are randomly oriented, the overall competent properties are not directional

38
Q

What is the difference between single and polycrystals? (properties)

A

Single crystal properties vary with direction: anisotropic
Polycrystals properties may/may not vary with direction and are randomly oriented: isotropic, if grains are textured, anisotropic

39
Q

What is polymorphism?

A

Two or more distinct crystal structures for the same material (allotropy/polymorphism)

Materials module 1 (12 decks)

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